PHYSICAL REVIEW B 98, 165434 (2018)

Phase diagram of quantum Hall breakdown and nonlinear phenomenafor InGaAs/InP quantum wells

V. Yu,1,2 M. Hilke,1,* P. J. Poole,2 S. Studenikin,2 and D. G. Austing1,2,†1Department of Physics, McGill University, Montréal, Quebec, Canada H3A 2T8

2Emerging Technology Division, National Research Council of Canada, M50, 1200 Montreal Road, Ottawa, Ontario K1A 0R6, Canada

(Received 29 March 2018; revised manuscript received 15 June 2018; published 24 October 2018)

We investigate nonlinear magnetotransport in a Hall bar device made from a strained InGaAs/InP quantumwell: a material system with attractive spintronic properties. From extensive maps of the longitudinal differentialresistance (rxx) as a function of current and magnetic (B) field, phase diagrams are generated for quantum Hallbreakdown in the strong quantum Hall regime reaching filling factor ν = 1. By careful illumination, the electronsheet density (n) is incremented in small steps and this provides insight into how the transport characteristicsevolve with n. We explore in depth the energetics of integer quantum Hall breakdown and provide a simplepicture for the principal features in the rxx maps. A simple tunneling model that captures a number of thecharacteristic features is introduced. We present results on the quantum Hall transport diamonds, a spin-flipresonance at high current near ν = 1, instabilities at large B field and current, and a zero-current anomaly. Inaddition, parameters such as critical Hall electric fields and the exchange-enhanced g factors for odd-fillingfactors including ν = 1 are extracted. A detailed examination is made of the B-field dependence of the criticalcurrent as determined by two different methods and compiled for different values of n. A simple rescalingprocedure that allows the critical current data points obtained from rxx maxima for even filling to collapse ontoa single curve is demonstrated. Exchange-enhanced g factors for odd filling are extracted from the compileddata and are compared to those determined by conventional thermal-activation measurements. The exchange-enhanced g factor is found to increase with decreasing n.

DOI: 10.1103/PhysRevB.98.165434

I. INTRODUCTION

The quantum Hall effect occurs in a two-dimensionalelectron gas (2DEG) confined in a quantum well (QW) whensubjected to a high perpendicular magnetic field [1]. Bypassing a sufficiently high current through a Hall bar, thequantum Hall effect can be destroyed [2]. Current-inducedquantum Hall breakdown is a valuable tool to access nonlinearmagnetotransport phenomena in a 2DEG (see the extensivereview in Ref. [3] and references therein). Some examples ofthe rich variety of nonlinear phenomena that can be accessedin a strong magnetic field at and beyond current-inducedquantum Hall breakdown include the following: hysteresisarising from dynamic nuclear polarization through the inter-play of electron and nuclear spins via the hyperfine interac-tion near integer odd-filling factors [4,5], cyclotron emissionat terahertz frequency arising from nonequilibrium electrondistribution in Landau levels [6,7], electric instability leadingto resistance fluctuations and negative differential resistance[8–11], and reentrant quantum Hall states of the second Lan-dau level [12,13].

The above-mentioned phenomena are typically observedin widely studied GaAs/AlGaAs heterostructure 2DEGs withhigh mobility. Here we report on the general characteristicsof quantum Hall breakdown in an InGaAs/InP QW Hall bar

*hilke@physics.mcgill.ca†guy.austing@nrc-cnrc.gc.ca

device. Such indium-based QWs are of current interest forspintronic applications. As compared to widely employedGaAs/AlGaAs QWs, InGaAs/InP QWs offer larger electrong factors, stronger spin-orbit coupling, and the nuclear spinfor indium (9/2) is larger than for gallium (3/2) [14–20]. Weexamine in detail two-dimensional maps of the differentialresistance as a function of current and magnetic field. Thesemaps may be regarded as phase diagrams and provide a wealthof information about quantum Hall breakdown (including theconditions for, and the energetics of, quantum Hall break-down), and a number of other rarely studied phenomena forsheet current densities up to ∼1 A/m. We describe howquantum Hall breakdown and nonlinear phenomena evolvesystematically not only with increasing magnetic field (B) upto 9 T (equivalently decreasing filling factor ν) but also withincreasing electron sheet density from 1.6 × 1011 cm−2 upto 3.9 × 1011 cm−2. The capability to increment the electrondensity in small steps by careful illumination was particularlyvaluable and enabled us to compile sufficient data to examine,for example, trends in electronic g factors and to comparedifferent methods by which the breakdown current can beextracted. In contrast to an earlier study of an InGaAs/InP QWin Ref. [21], here we access the strong quantum Hall regime(ν = 1). We note that there are some reports of current-induced breakdown in other indium-based QW systems whereν = 1 is also attained (see Ref. [22] for InGaAs/InAlAs QWsand Ref. [23] for InSb/AlInSb QWs).

The outline of this paper is as follows. Section II describesdetails of the InGaAs/InP heterostructure and the Hall bar

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YU, HILKE, POOLE, STUDENIKIN, AND AUSTING PHYSICAL REVIEW B 98, 165434 (2018)

investigated, and transport characteristics of the 2DEG as afunction of the electron density. Section III explains how dif-ferential resistance color maps are generated and outlines theprincipal features that are revealed. Section IV describes howthe breakdown characteristics evolve with electron density.Section V provides a simple picture for the principal featuresin the differential resistance color maps and introduces asimple tunneling model that captures a number of charac-teristic features. Section VI describes parameters that can beextracted from the transport diamonds in the strong quantumHall regime: principally, the critical Hall electric fields forν = 1 and ν = 2 and an estimate of the g factor for ν = 1.Section VII examines in detail the B-field dependence of thecritical breakdown current as determined by two differentmethods compiled for different electron densities. The g

factors for odd-filling factors extracted from the compiled dataare compared to those determined by conventional thermal-activation measurements. Lastly, Sec. VIII discusses a curiousand as yet unexplained zero-current anomaly readily visible inthe experimental data over a wide B-field range.

II. EXPERIMENTAL DETAILS AND 2DEGCHARACTERISTICS

The investigated 2DEG is formed in the QW region ofa strained In0.76Ga0.24As/In0.53Ga0.47As/InP heterostructuregrown by chemical beam epitaxy. The conducting channelin the QW is an undoped 10 nm In0.76Ga0.24As layer. Ascompared to lattice matched In0.53Ga0.47As/InP QWs, the 0.76indium fraction is intended to reduce the effective mass andmaximize the mobility at the cost of introducing strain [24].Because of the mobility enhancement, InxGa1−xAs QWswith In fraction x of ∼0.75 have attracted attention over theyears [25–33]. Other details of the growth and heterostructureare given in Refs. [21,34]. All the transport measurements de-scribed in this report are for a 15 -μm-wide Hall bar preparedby standard optical lithography and wet etching [34]. Threepairs of potential probes separated by 50 μm are positionedalong the Hall bar and the potential probes have width 7.5 μmwhere they join the Hall bar. The Hall bar is ∼200 μm longand opens out at either end to avoid sharp corners on the entryto the source and drain regions. The Hall bar device design anddimensions were motivated by the work in Refs. [4,5]. Sol-dered indium beads that are subsequently annealed at 350 ◦Cin vacuum for 30 mins form Ohmic contacts. The device ismaintained at the base temperature of a top-loading 3He Janiscryostat equipped with a 9 T superconducting magnet.

Before discussing the nonlinear transport measurements,we describe the standard characteristics of the 2DEG. Asan alternative approach to changing the electron sheet den-sity (n) by applying a voltage to a front or back gate, wecould controllably increment n in small steps from ∼0.5 to∼4.0 × 1011 cm−2 by careful illumination of the Hall bar at∼0.3 K with a standard red-light-emitting diode passing asmall current up to a few tens of nanoamperes. Figure 1 showsthe 0.3 K values of n and (transport) mobility μ obtainedusing a standard lock-in technique with ac excitation currentof 100 nA at 13.5 Hz. We note that here, n is the Hallcarrier density. Data points were collected on three separatecooldowns. The dependence of mobility on electron density

FIG. 1. 0.3 K values of electron density n and mobility μ

attained by illumination over three separate cooldowns nos. 1–3.Estimated uncertainties in n and μ are included for each data point.Empirically, over the accessible density range, we find that themobility follows a power-law dependence μemp∼(n − n0)p , wheren0 = 0.50 ± 0.02 × 1011 cm−2 and p = 0.42 ± 0.02 (see dashedmagenta trace). Following the model in Ref. [35], we have alsofitted our data for ionized impurity scattering and short-range alloydisorder scattering. Included in the figure are the calculated mobilityvs density traces for background ionized impurity scattering (μB :green trace), remote ionized impurity scattering (μR: blue trace),short-range alloy disorder scattering (μA: red trace), and the totalmobility [μtot = (μ−1

B + μ−1R + μ−1

A )−1: black trace] [36]. For thefits, we excluded the anomalous outlying point near 4.0 × 1011 cm−2.

is reproducible from cooldown to cooldown except near thehighest density, ∼4.0 × 1011 cm−2, attained for maximumillumination that leads to saturation of the persistent photo-conductivity effect [37]. We note that n = 4.0 × 1011 cm−2 iswell below the electron density required to populate the first-excited subband of the QW (estimated to be 1.8 × 1012 cm−2

based on a simple calculation using appropriate materialparameters). Empirically, we find that the observed sublineardependence of mobility on electron density follows a power-law relationship, μemp ∼ (n − n0)0.42, where n0 is a constant;see magenta trace in Fig. 1. The observed dependence isqualitatively similar to those reported in earlier works onInGaAs QWs (see, for example, Refs. [27,37,38]). The de-pendence is consistent with theoretical models for disorderscattering mechanisms in 2DEG structures [35,38]. At lowelectron density, μ is expected to increase with n when limitedby ionized impurity scattering. At high electron density, μ

is expected to saturate in quantum wells [39] (or even de-crease with n in heterojunctions [40]), when limited by alloydisorder scattering. Following the model in Ref. [35] whichincludes screening, we have fitted our data incorporating theappropriate scattering mechanisms. Figure 1 includes calcu-lated mobility versus density traces for background ionizedimpurity scattering (μB), remote ionized impurity scattering(μR), short-range alloy disorder scattering (μA), and the totalmobility μtot = (μ−1

B + μ−1R + μ−1

A )−1 [36].

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For the experiments discussed below, we focus on datataken during the third cooldown (no. 3) for which n is inthe range 1.6–3.9 × 1011 cm−2. Over this range, from thedetermined values of the transport mobility, we find that thetransport mean free path (transport lifetime) increases from1.2 μm (4.8 ps) to 2.7 μm (7.1 ps). For the transport lifetime,we took the effective mass (m∗) to be 0.047m0, where m0 isthe free-electron mass [21]. From other measurements (datanot shown [34]), we also determined the quantum mobilityμq and, subsequently, the quantum lifetime τq by Dingleplot analysis of the low B-field Shubnikov–de Haas (SdH)envelope [21,41]. μq (τq) is found to be ∼12 000 cm2/Vs(∼0.31 ps) for n in the range 1.6–3.2 × 1011 cm−2, but risesto ∼19 000 cm2/Vs (∼0.51 ps) near 3.9 × 1011 cm−2 whenthe electron density is close to saturating. From τq , we canthen estimate the Landau- level broadening �. Defining thefull width at half maximum (FWHM) of a Lorentzian-shapedLandau level to be 2� [21], we find 2� = h/τq to be inthe range ∼1.2–2.0 meV. Note that for this Hall bar, thelongitudinal resistance becomes very large at low density, n =n0 ∼ 0.5 × 1011 cm−2, when the 2DEG is almost insulatingand the mobility collapses (see Fig. 1). This density (markingthe percolation threshold for transport) equates to a Fermienergy of ∼2 meV. From this observation, we conclude thatthere are effective random potential fluctuations of amplitude∼2 meV.

III. DIFFERENTIAL RESISTANCE COLOR MAPS

A map (or “phase diagram”) displaying how the nonlineartransport properties of quantum Hall breakdown depend oncurrent (I ) and B field can be built up by sweeping thecurrent and stepping the B field. Color maps of directlymeasured as opposed to numerically derived longitudinal andtransverse differential resistances rxx and rxy , respectively, areshown in Fig. 2 for n = 1.6 × 1011 cm−2. The four-point mea-surements of the differential resistances, rxx (Idc ) ≡ dV xx/dI

and rxy (Idc ) ≡ dV xy/dI , were performed simultaneously bydriving a combination of a dc current Idc and a small acexcitation current of 100 nA at 13.5 Hz through the Hall barand measuring the ac voltage component drop �Vxx and �Vxy

between appropriate potential probes of the Hall bar using astandard lock-in technique [21,42].

The distinctive dark-blue diamond-shaped regions in themap of rxx identify where rxx ∼ 0. These regions are theso-called transport diamonds [21]. In the map of rxy too,diamonds are visible within which the rxy is quantized. Thediamonds in rxx and rxy occupy the same region in the I -Bplane. We can now assign these transport diamonds to specificinteger filling factors from the value of rxy consistent with theexpected quantized values of the Hall resistance. We concludethat the two most prominent diamonds centered at 3 and 6 Tin Fig. 2 are related to filling factors ν = 2 and 1, respectively.

We note the following about the transport diamonds re-vealed in Fig. 2: (i) a horizontal cut through the diamondsin rxx near Idc ∼ 0 would essentially give a traditional plotof Rxx = Vxx/Idc with swept B field when measured in thelow current linear response regime (Rxx = rxx) and revealszeros in Rxx at integer filling factors at high B field in thequantum Hall regime and SdH minima at low B field; (ii) with

FIG. 2. Experimental color maps of (a) rxx and (b) rxy : n =1.6 × 1011 cm−2, μ = 170 000 cm2/Vs. Note that the regions of rxy

exceeding 26 k� are colored black. In both panels, the ν = 2, 3, and4 transport diamonds are outlined, and the high-current protrusionfeatures emerging from near the high current tips of the ν = 1transport diamond are identified. The color maps are built up withthree overlying data sets. The current sweep rate is typically a fewtens of nanoamperes per second. The B-field step at low (high) B

field is 25 mT (100 mT).

increasing B field, the diamonds become more pronouncedand bigger (most clearly reflected in the maximum width inthe current of the diamonds); (iii) on passing from insidea diamond to outside a diamond, the quantum Hall effectbreaks down; and (iv) beyond the diamonds, the quantum Halleffect has broken down and transport is generally nonlinear(Rxx �= rxx).

The color maps provide a wealth of information in thestrong quantum Hall regime beyond that reported in Ref. [21],which describes transport diamonds for an InGaAs/InP Hallbar but only for B fields up to 5 T and for n = 5.3 ×1011 cm−2, so filling factors no lower than ν = 5 were investi-gated. We found it insightful to capture color maps over a widerange of B field and for sheet current densities up to ∼1 A/m.The color maps in Fig. 2 together with those presented in thenext section for progressively higher electron density provideaccess to a wide range of filling-factor diamonds from ν = 1to at least 10. We note that a color map of Vxx showing theν = 2 diamond is presented in Ref. [43] for a GaAs/AlGaAsHall bar of width 10 μm, and color maps of differentialresistance between ν = 2 and 4 are presented in Refs. [12,13]for high-mobility GaAs/AlGaAs structures of width 500 μmor greater and for sheet current densities much less than1 A/m.

In particular, with the capability to measure the ν = 1transport diamond, we uncovered distinctive high-current fea-tures that emerge from near the high-current tips of thediamond. These “protrusions” (resonances) are clear in Fig. 2as peaks in rxx and valleys in rxy that track to lower current

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as the B field is reduced, and end near the high B-fieldside of the ν = 2 diamond. Note that the value of rxy inthe valley is ∼h/2e2 or lower. The protrusion features werealso seen in a second Hall bar device made from a simi-lar In0.76Ga0.24As/In0.53Ga0.47As/InP heterostructure (data notshown [34]). The protrusions are discussed further in Secs. IVand V.

IV. ELECTRON DENSITY DEPENDENCE OF QUANTUMHALL BREAKDOWN FEATURES

Figure 3 shows measured color maps of rxx for six elec-tron densities from the lowest (n = 1.6 × 1011 cm−2) to thehighest (n = 3.9 × 1011 cm−2). Several noteworthy trends areclear with increasing electron density:

(1) The transport diamonds shift systematically to higherB field, as expected. For example, the center of the ν = 2diamond shifts from ∼3 T at n = 1.6 × 1011 cm−2 to ∼7.5 Tat n = 3.9 × 1011 cm−2. Note that the ν = 1 diamond issubstantially in range only for the first two densities.

(2) The size of the transport diamonds increases. Forexample, the maximum critical current of the ν = 2 diamondincreases from ∼4 μA at n = 1.6 × 1011 cm−2 to ∼11 μA atn = 3.9 × 1011 cm−2. We will return to the subject of how thecritical currents for even- and odd-filling factors vary with B

field and electron density in Sec. VII.(3) The phenomenon of phase inversion of SdH oscillations

at high-filling factors discussed in Ref. [21] due to electronheating is observed at low B field and high current (see alsoRef. [23] for a discussion of electron heating at low B field forInSb/AlInSb QWs), and the phase inversion becomes morevisible at higher electron density. By phase inversion, wemean that the minima (maxima) of SdH oscillations in rxx

versus B field at zero current develop into maxima (minima)at high current (see also Fig. 4).

(4) High even-filling-factor (ν � 6) transport diamondsreported in Ref. [21] appeared to have straight edges. Thiswould seem to be the case too in Fig. 3, but closer inspectionreveals otherwise for low-filling-factor diamonds. For exam-ple, the ν = 4 diamond appears to have straight edges at lowdensity, but these edges become curved at higher density. Theedges of the ν = 1 and ν = 2 diamonds are clearly curved forall densities shown [44].

(5) Regarding odd-filling factors, at n = 1.6 × 1011 cm−2,only the ν = 1 transport diamond is well developed. At n =3.9 × 1011 cm−2, the ν = 3 and ν = 5 diamonds become cleartoo. The ν = 1 diamond is out of range beyond 9 T.

(6) Outside the ν = 1 transport diamond, features whererxx � 10 k� protrude towards lower B field are clear forthe lower densities. These protrusions always start from nearthe finite-current tips of the ν = 1 diamond and appear totrack towards zero current at 0 T. They also weaken andeventually terminate at a point located approximately mid-way along the high B-field sides of the ν = 2 diamond(for example, near 4.3 T for n = 1.9 × 1011 cm−2). We willdiscuss these protrusions later in Sec. V in connection toFig. 5.

(7) At higher density, the regions just outside the ν = 3 andν = 5 transport diamonds develop a distinctive double-peakstructure. These double-peak features where rxx � 5 k� areclearest at n = 3.9 × 1011 cm−2 (in the color map, the peaksare marked by asterisks on the positive current side). For boththe ν = 3 and ν = 5 diamonds, the leftmost peak is near thefinite-current tip of the diamond and the rightmost peak islocated on the high B-field side of the diamond. The originof the curious double-peak features is unknown.

FIG. 3. Experimental color maps of rxx for six electron densities in the range of n = 1.6 × 1011 cm−2 to n = 3.9 × 1011 cm−2 attainedafter controlled illumination. Note that for the left- (right-)side panels, the color scales and the current axis ranges are different. White dashedlines through the center of the protrusions are marked on the color map for n = 1.9 × 1011 cm−2. Asterisks identify distinctive “double-peak”features for positive current polarity in the vicinity of the ν = 3 and 5 transport diamonds in the color map for n = 3.9 × 1011 cm−2. The colormap for n = 3.9 × 1011 cm−2 is replotted in Fig. 4 with a different color scale to highlight certain features more clearly.

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FIG. 4. Color map of rxx for the highest electron density n = 3.9 × 1011 cm−2 replotted with a different color scale to emphasize certainfeatures. Phase inversion (PI) of SdH oscillations due to electron heating is observed at low B field (high filling factor). The left panelillustrates the PI effect. The black (red) rxx trace at 1.08 T (1.15 T) cutting through ν = 14 SdH minimum (neighboring SdH maximum) atzero current develops into a maximum (minimum) at ∼4 μA (∼3 μA). The zero-current anomaly (ZCA) “dip,” discussed further in Sec. VIII,is also clear in these two traces. Asterisks in the color map identify the distinctive “double-peak” features in the vicinity of the ν = 3 and 5transport diamonds. Regions exhibiting negative differential resistance appear white (rxx � 350 �) and become widespread beyond 3 T (see,for example, the crescent-shaped features marked I and II between 3 and 5 T. Concurrently, instability appearing as random “spots” of coloralso develops above 6 T near the ν = 2 diamond (see region marked III) and becomes very pronounced on the high B-field side of the diamond(see region marked IV).

(8) Starting at n = 2.7 × 1011 cm−2, close inspection ofthe color maps on the right side of Fig. 3 reveals that outsidethe transport diamonds (when the quantum Hall effect hasbroken down), regions appear where: (i) rxx approaches zeroor even becomes negative, i.e., negative differential resistanceis exhibited, and (ii) rxx is unstable. These features becomeeven more pronounced at higher density. For clarity, in Fig. 4we have replotted the color map for n = 3.9 × 1011 cm−2 witha color scale that enhances these features. Regions appearingwhite, where rxx � 0, become widespread beyond 3 T, andinstability develops near the ν = 2 diamond, especially on thehigh B-field side (see the random “spots” of color). We willexamine these features of electric instability in more detailelsewhere [45].

(9) Although integer filling-factor diamonds are clear inFig. 3, fractional filling-factor diamonds are not observedeven for n = 3.9 × 1011 cm−2 when the transport mobility ishighest (see Fig. 1). The absence of fractional filling-factordiamonds in our data contrasts with those observed betweenν = 2 and 4 in Refs. [12,13] for high-mobility GaAs/AlGaAsheterostructures (μ ∼ 2 × 107 cm2/Vs). Based on a simpleCoulomb interaction picture [46,47], we estimate the energygap for fractional states to be 0.56 meV (0.74 meV) at 4 T(7 T) [48]. However, the disorder strength in the measured In-GaAs/InP heterostructure is significant (h/τq ∼ 1.2–2.0 meV;see Sec. II) and is comparable to or exceeds the estimatedenergy gap for fractional states. This accounts for the ab-sence of fractions. Moreover, alloy disorder scattering in theIn0.76Ga0.24As QW channel is likely even more disruptive tothe fractional states [49].

V. SIMPLE PICTURE OF TRANSPORT DIAMONDS

We now present a simple picture of how we interpret keyfeatures related to quantum Hall breakdown that appear mostclearly in the color maps of rxx . We will use the color map ofrxx for n = 1.9 × 1011 cm−2 in Fig. 5 for this purpose.

It is nontrivial to directly relate current (y axis in the colormaps) to energy: a point that will be emphasized further inSec. VII. However, two features of the transport diamondsstand out. First, the diamond size generally increases withincreasing B field (for odd-filling and even-filling factorsseparately). Second, the diamond size alternates betweenrelatively small for odd-filling factors and relatively big foreven-filling factors. These observations strongly suggest thatthe width of the diamond in current is related to the Zeemanenergy splitting (|g∗|μBB) at odd filling and to the cyclotronenergy (hωc = heB/m∗) at even filling. Here, g∗, μB, h, ande, respectively, are the effective g factor, Bohr magneton,reduced Planck constant, and charge of an electron. For odd-filling factors, we would then expect the maximum width ofthe (2N + 1)th diamond (N = 0, 1, 2, . . . for ν = 1, 3, 5, . . .)to reflect the transition (N,↑) → (N,↓) where the Landau-level index N is conserved, but electron spin is flipped (byhyperfine or spin-orbit coupling [5]) from up to down. Foreven-filling factors, the maximum width of the (2N + 2)thdiamond (N = 0, 1, 2, . . . for ν = 2, 4, 6, . . .) could reflectthe transition (N,↓) → (N + 1,↓), where N is changed butelectron spin is conserved. More likely, it reflects the lower-energy transition (N,↓) → (N + 1,↑) where electron spinis flipped too. This transition has energy hωc − |g∗|μBB. InFig. 5, we have added white guide lines that separately bound

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FIG. 5. Experimental color map of rxx for n = 1.9 × 1011 cm−2. White guide lines bounding tips of even- (odd-)filling-factor diamonds areshown and reflect principally the growth in the cyclotron energy (Zeeman energy splitting) with B field. Cartoons (a)–(g) illustrate alignmentof Landau levels for the feature marked on the color map. See text for full description.

odd- and even-filling-factor diamonds and interpolate to zerocurrent at zero field. In Sec. VII, we will more carefullyexamine the magnetic field dependence of the critical currents.

The transitions that we picture as relevant are princi-pally tunneling transitions between Landau levels that aretilted in the y direction (across the Hall bar). These transi-tions are induced by the Hall electric field when current isapplied. This Zener-type tunneling mechanism is discussedin Refs. [50,51]. Inter-Landau-level tunneling is also referredto in the literature as quasielastic inter-Landau-level scatter-ing [50]. Figures 5(a)–5(g) illustrate alignment of spin-splitLandau subband energy levels for the features marked onthe color map. In each cartoon, there are two ladders ofenergy levels. Relevant levels are labeled in compact form, forexample, 0 ↑= (0,↑). Extended states in each level are shownas bars, and blue (white) identifies filled (empty) states. Theleft (right) ladder of levels is filled up to the local chemicalpotential μL (μR). L and R represent two points along the y

axis. L and R are defined such that μL �μR so that transitionsoccur from the left ladder to the right ladder for either currentpolarity. In the next section, we will specify further whatpoints ideally L and R represent and discuss the characteristiclength scales that could be associated with the separationbetween these two points.

The specific features marked in Fig. 5 are the following:(1) At ν = 2, the chemical potential is midway be-

tween (0,↓) and (1,↑). At the tips of the ν = 2 diamond[Fig. 5(a)], breakdown occurs when fully filled (0,↓) at L

aligns with empty (1,↑) at R [Fig. 5(a) depicts a resonant

inter-Landau-level transition with spin flip]. Similarly, at ν =1, the chemical potential is midway between (0,↑) and (0,↓).At the tips of the ν = 1 diamond [Fig. 5(f)], breakdownoccurs when fully filled (0,↑) at L aligns with empty (0,↓)at R [Fig. 5(f) depicts resonant transition between spin-splitsubband levels of the N = 0 Landau level].

(2) Figure 5(f) also corresponds to the high B-field endof the protrusion. Figures 5(e) and 5(c) depict alignment forpoints along the protrusion to lower B field for the same(0,↑) → (0,↓) resonant transition when μR approaches (0,↓)[Fig. 5(e)] and lies through (0,↓) [Fig. 5(c)].

(3) At zero current, the ν = 1 and ν = 2 diamonds touchnear 4.9 T [Fig. 5(b)] [52]. For small currents, in the vicinityof the point at Fig. 5(b), intrasubband transitions occur.

(4) Lastly, Figs. 5(d) and 5(g), respectively, representprocesses responsible for the edges of the ν = 1 diamondon the low (dashed pink line) and high (dashed green line)B-field side. In both cases, localized states in the Landau-levelsubband tails are important and the processes occur similarlyfor the other diamonds. Figure 5(d) depicts transitions fromlocalized states at L in the high- [low]-energy tail of (0,↑)[(0,↓)] to extended states in empty (0,↓) at R. Figure 5(g)depicts transitions from extended states in full (0,↑) at L tolocalized states at R in the high- [low]-energy tail of (0,↑)[(0,↓)]: extended states in (0,↑) at L are no longer all filled.

We are not aware of any calculation or model thatwould substantially reproduce, even qualitatively, many of thenotable features in the color map of rxx in Fig. 5 (and colormaps in Fig. 3). We have developed a toy model that does

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generate a number of general features observed in the experi-mental color maps. The model is a one-dimensional tunnelingmodel (see the Appendix for further details). The motivationfor this approach is that the measured color map in its entiretyprimarily reflects the B-field evolution of the Landau levelsand the broadening of the Landau levels. This is captured bythe model. The model though is not microscopic and it doesnot explicitly include details of spin-flip mechanisms, i.e., allenergetically allowed transitions are permitted whether or notelectron spin is flipped. The generated color maps are alsosensitive to the choice of the form of the density of states (seethe Appendix for an example of a color map generated withour model).

VI. TRANSPORT DIAMONDS IN STRONG QUANTUMHALL REGIME

Two metrics are frequently quoted in the literature to char-acterize breakdown: the critical current density Jc = Ic/w ata given filling factor ν (where Ic and w, respectively, are thecritical current and Hall bar width) and the correspondingcritical Hall electric field Ec = hI c/νe2w (where h is thePlanck constant). Although in the following section we willmore carefully discuss how one can define and extract Ic,from Fig. 5, we estimate that Ic ∼ 6 μA (∼5 μA) fromthe tips of the dark-blue colored ν = 2 (ν = 1) diamondat 3.9 T (7.8 T) for n = 1.9 × 1011 cm−2. Thus, Jc and Ec

are, respectively, ∼0.4 A/m and ∼5.2 kV/m (∼0.33 A/m and∼8.6 kV/m) at ν = 2 (ν = 1). We can compare these numberswith those compiled in Refs. [53,54] for GaAs/AlGaAs 2DEGheterostructures. Interestingly, the numbers we obtain for theInGaAs/InP QW here are quite similar in value. For example,from Refs. [53,54], we infer Ec is in the range of 2–6 kV/mat 4 T (∼6 kV/m at 8 T) for even-filling (odd-filling) factors.In our estimation of Jc and Ec, we took w to be the nominalwidth of the Hall bar (=15 μm) and neglected any reductiondue to wet etching and side-wall depletion.

Assuming the Landau levels are tilted uniformly across theHall bar, one would anticipate that one could make a crudeestimate of the minimum characteristic separation betweenpoints L and R (δ in the toy model described in the Appendix)required to induce the transitions depicted in Fig. 5. We shallexamine the estimated value for Ec of ∼5.2 kV/m at ν = 2(B = 3.9 T) for n = 1.9 × 1011 cm−2. For the pure Zener tun-neling mechanism represented in Fig. 5(a), we might expectEc ∼ �/eδ where � = hωc − |g∗|μBB is evaluated at 3.9 Twith appropriate values of 0.047m0 and 4.8, respectively,for m∗ and |g∗| [55]. Hence, a value for the characteristiclength scale δ ∼ �/eEc ∼ (8.5 mV)/(5.2 kV/m) of 1.6 μmis determined. Reasonably, δ w = 15 μm: we would notexpect direct tunneling from one side of the Hall bar to theother side. However, δ is two orders of magnitude larger thanthe distance over which we would expect direct inter-Landau-level tunneling to be significant. As discussed in Ref. [53], thetunneling distance can be related to the spatial extent of wavefunctions of electrons in the Landau levels. This is comparableto the magnetic length �B = 25.6 nm/

√B, with B in units

of Tesla. For B = 3.9 T, �B ∼ 13 nm δ. The values for Ec

that we determine experimentally are similar to those reportedin the literature for GaAs/AlGaAs 2DEGs and are up to two

orders of magnitude smaller than expected based on a simplepure Zener tunneling picture (also see Ref. [51]).

While a fully satisfactory answer to the longstanding prob-lem of accounting for the empirical values of Ec remainsto be found [53], we make the following comments. Theexperimentally derived critical Hall electric fields, equallythe critical current densities, should be regarded as averagequantities across the width of the Hall bar. Furthermore, theLandau levels are not tilted uniformly across the Hall bar.Two effects locally can significantly enhance the electric field.First, the Landau levels bend up strongly near the edges ofthe Hall bar [43,56,57]. Second, in reality, there are randompotential fluctuations in the 2DEG due to disorder [9,58–61].Lastly, although Zener tunneling is almost certainly involvedin quantum Hall breakdown, it is not the only factor. In theelectron heating model of Komiyama and Kawaguchi [51], therole of Zener tunneling is primarily to kickstart an avalanchemultiplication of excited carriers, the conditions for whichdepend upon the details of energy balance between gain andloss processes [62].

To finish this section, we describe how the ν = 1 and ν = 2transport diamonds in Fig. 5 can be used to make a crudeestimate of the g factor for ν = 1. n = 1.9 × 1011 cm−2 isthe highest electron density for which the ν = 1 diamondis substantially visible with the 9 T magnet available in theexperiments. Note that the energy gap associated with theν = 1 diamond is too large for the g factor to be determined byperforming thermal-activation measurements over the accessi-ble temperature range of 270 mK to 1.5 K. Instead, we startwith the observation that the maximum breakdown current forthe ν = 2 diamond is ∼6 μA at 3.9 T. We take this currentto be proportional to energy hωc − |g∗

e |μBB ∼ 8.5 meV. Thesubscript e is to emphasize that the g factor should be thatappropriate for an even-filling factor (we have used the valueof 4.8 estimated from separate measurements [55]). For theν = 1 diamond, the maximum breakdown current is ∼5 μAat 7.8 T. We take this current to be proportional to energy|g∗

o |μBB. The subscript o is to emphasize that the g factorshould be that appropriate for an odd-filling factor subjectto enhancement by many-body electron-electron exchangeinteractions [21,63–66]. Assuming the constant of proportion-ality for converting current to energy is the same for bothdiamonds, the 5 μA breakdown current for ν = 1 is equatedto the energy ∼7 meV, implying |g∗

o | ∼ 16. Our estimation of|g∗

o | here has neglected Landau-level broadening and assumedthat the breakdown currents quoted above for the ν = 1 and2 diamonds reflect the corresponding resonant transitions(effectively, method B to be described in Sec. VII). We willdiscuss these assumptions further in the following section.Nonetheless, recent calculations of the exchange enhance-ment of g factors in strained InGaAs/InP heterostructuressuggest that values of magnitude exceeding 10 are possible forν = 1 [66].

VII. B-FIELD DEPENDENCE OF CRITICALBREAKDOWN CURRENT

The maximum critical current Ic at integer filling (wherethe transport diamond is widest) and how it depends on themagnetic field with varying electron density have received

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considerable attention for a number of decades. Not only hasthis subject been of interest for metrological reasons, but, inprinciple, the dependence can shed light on the physical mech-anisms that drive quantum Hall breakdown; see, for example,Refs. [3,51,53,54,67]. Controlled illumination allows us tocompile sufficient data for different values of n to examinethe magnetic field dependence of Ic for even- and odd-fillingfactors for an InGaAs/InP QW 2DEG: a system other thanGaAs/AlGaAs which is usually investigated.

The choice of how one defines the onset of breakdowncan influence the value of Ic. The following two methods arereasonable and have been used in the literature. For methodA, Ic is taken to be the value at which rxx exceeds a smallthreshold. The value of the threshold though is influenced to adegree by the noise level in the measured rxx . Furthermore,deviation from rxx = 0 above a small threshold may occurfor a gentle prebreakdown increase or for multiple-step tran-sitions prior to abrupt breakdown [2,68]. For method B, Ic

is simply given by the position of the rxx maximum usuallyclear at the high-current tip of a transport diamond. Notethat this corresponds to where the rise in Vxx with current issteepest. This is the most direct method to apply except if rxx

exhibits multiple-peak features or becomes unstable, whichis sometimes the case, as noted in Sec. IV, for high electrondensity and high B field (low-filling factor) [68]. In general,the value of Ic determined by method A is smaller than thatdetermined by method B; however, these methods can givesimilar values if breakdown is very abrupt.

Figure 6 shows the critical current Ic determined bymethod A [Fig. 6(a)] and method B [Fig. 6(b)] for even (E)-and odd (O)-integer filling factors for six electron densities inthe range 1.6–3.9 × 1011 cm−2 [68]. For method A, since thenoise level in rxx near zero current for the largest transportdiamonds is a few ohms, we have set the threshold value tobe 10 �. From Fig. 6(a), for method A, we find that datapoints essentially fall on distinct common straight lines foreven- and odd-filling factors. The common line for even-filling factors is steeper than that for odd-filling factors. Thelines clearly do not extrapolate to the origin. The even- (odd-)filling-factor line intercepts zero current at ∼1.5 T (∼2.5 T).From Fig. 6(b), for method B, although the even- and odd-filling-factor families of lines are clear, there is considerablymore variation and scatter than for method A, i.e., data pointsfor different electron densities do not fall on a single com-mon line. Even-filling-factor points above ∼1.5 T for eachspecific electron density appear to have approximately lineartrends that would extrapolate to the origin, although there aredeviations from the linear dependence near 1 T. Also, withincreasing electron density, Ic for even-filling factors clearlychanges more rapidly with B field. Although less pronounced,a similar behavior is observed too for the odd-filling-factorpoints. The odd-filling-factor trends intercept zero currentnear 1 T.

We make the following comments. First, the linear de-pendence with the B field for Ic data points determined bymethod A in Fig. 6(a) is, to a certain degree, unexpected. ForGaAs/AlGaAs 2DEG heterostructures, it is often argued thatthe critical Hall electric field Ec ∝ Ic/ν is proportional to B3/2

(for example, see the discussion and plots of compiled datain Refs. [53,54] mostly extracted by method A). However,

models for breakdown generally predict values for Ec thatare larger than experimental values, and depend on factorssuch as the inclusion of higher-order tunneling processes,details of scattering (particularly the B-field dependence ofthe scattering rate), and Landau-level broadening induced bythe Hall electric field [51,53]. Others have also suggested thata linear B dependence fit of compiled data appears to be atleast as good as a B3/2 dependence fit [67]. For our data, wedo not find a power-law dependence for even- or odd-fillingfactors if we examine Ic/ν (data not shown). Consistent withour observations, we note that a linear dependence of theν = 1 and 2 breakdown currents with B field was also recentlyreported in Ref. [23] for InSb/AlInSb heterostructures withmobility exceeding ∼160 000 cm2/Vs. To emphasize thispoint, data points for ν = 1 and ν = 2 are circled in Fig. 6(a).Second, the characteristics of Ic determined by method A[Fig. 6(a)] are notably different from the characteristics ofIc determined by method B [Fig. 6(b)], especially regardingthe scatter of data points, i.e., method A data points appearto collapse onto two common lines but clearly method B datapoints do not. However, we stress that the two methods reflecttwo quite different features of Landau levels. Consistent withthe discussion in Sec. V, we expect method A to be sensitiveto how electrons tunnel between states in the tails of Landaulevels (when the tunnel barrier for inter-Landau-level transi-tions becomes sufficiently transparent), and method B to besensitive to how electrons tunnel resonantly between Landaulevels at higher Hall electric field. Since in the framework ofour simple picture for the transport diamonds method B offersa means to extract g factors for odd filling (crudely done forν = 1 in Sec. VI at n = 1.9 × 1011 cm−2), we now examinethis method more closely after introducing a simple procedureto account for the data scatter.

At first sight, the scatter in data points in Fig. 6(b) iscounter to our intuition. In particular, based on our simplepicture described in Sec. V, we take the individual Ic tracesfor even-integer filling in Fig. 6(b) to reflect the energyhωc − |g∗

e |μBB (see also Sec. VI). This energy is dominatedby the cyclotron energy and is not expected to have anysignificant n dependence. However, this expectation neglectselectrostatic details that determine the proportionality factorbetween current and energy. Since the Hall electric field is thedominant field and is proportional to I/n at fixed magneticfield, this prompted us to examine Ic/n instead of Ic (methodB rescaled); see Fig. 6(c) in which all data points of Fig. 6(b)are replotted. Noticeable immediately, even-integer fillingdata points now effectively collapse onto a single (solid) line.This universal behavior means Ic is proportional not just toB, but also n, i.e., Ic/n and not Ic should be equated withenergy hωc − |g∗

e |μBB. Regarding the odd-integer filling datapoints, rescaling reduces the scatter to a degree and theylie in a region bounded by the dashed line (with steeperslope) and the dash-dotted line (with shallower slope) [69].Interestingly, the order by slope of the rescaled odd-integerfilling data point traces is inverted with respect to those priorto rescaling [see Fig. 6(b)]. The linear B-dependence tracenow appears steepest (shallowest) for n = 1.6 × 1011 cm−2

(n = 3.9 × 1011 cm−2), i.e., there is a trend for the slope todecrease with increasing n. Values of Ic estimated from thetips of the dark-blue colored ν = 1 and ν = 2 diamonds for

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FIG. 6. Critical current Ic compiled for even (E)- and odd (O)-integer filling factors for electron densities in the range 1.6–3.9 × 1011 cm−2.The values are determined by two methods on examination of rxx (Idc) traces that are used to build up the color maps in Fig. 3. In (a), Ic is thecurrent at which rxx exceeds 10 � (method A), and in (b), Ic is the current position of the rxx maximum at the high-current tip of a transportdiamond (method B). Guidelines are shown joining two or more data points for even- (dashed) or odd- (solid) integer filling factors at a givenelectron density. In (a), data points for ν = 1 and ν = 2 are circled. In (c), the data points from (b) are replotted to show Ic/n instead of Ic

(method B rescaled). Even-integer filling data points effectively collapse onto a single (solid) line. Odd-integer filling data points lie in a regionbounded by the dashed line (with steeper slope for n = 1.6 × 1011 cm−2 data points) and the dash-dotted line (with shallower slope for n3.9 × 1011 cm−2 data points); see main text for further discussion. Although not strictly determined by method B, values of Ic estimated fromthe tips of the dark-blue colored ν = 1 and ν = 2 diamonds for n = 1.9 × 1011 cm−2 in Fig. 5 (see Sec. VI) and rescaled are also included(open red circles). In (d), the data points from (b) are replotted on a log-log scale and sorted by ν instead of n to show absolute values of Ic/ν(also effectively method B is rescaled since ν = nh/eB). The dashed black line with a ∼B2 dependence is included as a guide to the eye. Inthe legend, +ve I (−ve I ) means positive (negative) current polarity.

n = 1.9 × 1011 cm−2 in Fig. 5 (see Sec. VI) and rescaled arealso included in Fig. 6(c); see open red circles. Although notstrictly determined by method B, since breakdown is fairlyabrupt, these extra points nonetheless lie either very closeto data points extracted by method B at the correspondingdensity, in the case of ν = 1, or very close to the singlecommon line for even-filling factor points, in the case ofν = 2.

The rescaling procedure also implies that the critical Hallfield Ec is proportional to B2 when method B is applied

since, empirically, Ic ∝ Bn (Ec = hI c/νe2w = BIc/new).To corroborate this, in Fig. 6(d) all data points of Fig. 6(b)are replotted on a log-log scale and sorted by ν instead ofn to show absolute values of Ic/ν (also, effectively, methodB rescaled). The dependence we see for both even and oddfilling is closer to B2 than B or B3/2. Interestingly, this issimilar to observations for Hall-induced resistance oscilla-tions (HIROs) in high-mobility GaAs/AlGaAs 2DEGs at lowmagnetic fields (<0.5 T), where Zener tunneling betweenLandau levels takes place across distance 2Rc ∼ 1/B (the

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cyclotron diameter) [70]. If we equate the characteristic lengthscale δ for Zener tunneling introduced in Sec. VI to 2Rc, thiswould lead to a B2 dependence for Ec. However, in the strongquantum Hall regime at large magnetic fields, 2Rc is muchsmaller than the length of 1.6 μm at 3.9 T that we estimatedearlier. We stress the rescaling in Fig. 6(c) and Fig. 6(d)involves no fitting or knowledge of quantities other than n

or ν.Having demonstrated that data points of Ic/n for even-

integer filling collapse onto a single line which can be equatedto the energy hωc − |g∗

e |μBB, we proceed to estimate g

factors for odd filling from Fig. 6(c). We now expect the in-dividual Ic/n traces for odd-integer filling to be a measure ofthe energy |g∗

o |μBB (see also Secs. V and VI). By examiningthe ratio of the slopes of the single solid line for even fillingand the dashed and dash-dotted bound lines for odd filling,we estimate |g∗

o | to be in the range ∼15–21 if we take valuesof 0.047m0 and 4.8, respectively, for m∗ and |g∗

e |. The valuefor the g factor appears to be larger for lower density [71]. Incontrast to |g∗

e |, not only is a larger value for |g∗o | expected,

but an n dependency is reasonable too. The increase of |g∗o |

with decreasing n is significant and likely reflects an increasein the strength of the effective electron-electron interactions[63,65,72,73]. To investigate this point further, we end thissection by comparing the effective exchange-enhanced g fac-tors estimated for odd filling from the rescaled compiled datain Fig. 6(c) to those determined by conventional thermal-activation measurements over the same range of electrondensity.

At exact odd-integer filling, the Fermi level is midway be-tween two broadened spin-split Landau subbands (N,↑) and(N,↓). Close to equilibrium (Idc = 0 and small ac excitationcurrent), the energy to activate electrons from the Fermi levelto extended states in the broadened level (N,↓) is Egap/2,where Egap = |g∗

o |μBB − 2� (the effective energy gap be-tween (N,↑) and (N,↓) [21,63–66,74–76]). This activationenergy can be extracted from the temperature (T ) dependenceof rxx at odd-filling-factor minima, namely, from an Arrheniusplot if rxx ∼ exp(−Egap/2kBT ), where kB is the Boltzmannconstant. We examined the temperature dependence of rxx atIdc = 0 for odd-filling-factor minima as a function of B. Plotsof ln(rxx) vs 1/T for T in the range 0.3 to ∼1.6 K were foundto be close to linear for T � 1 K for electron densities in therange 1.6–3.9 × 1011 cm−2 (data not shown [34]). Fitting theplots at higher temperature where they are reasonably linear,Egap was extracted. The values of Egap are plotted in Fig. 7as a function of B (B field at which the rxx minima arelocated). From this plot, except for the lowest density, |g∗

o |can be estimated from the linear dependence on B [77], while� can be estimated from the zero B-field intercept [65,74–76,78]. For n = 1.9, 2.3, 2.7, 3.2, and 3.9 × 1011 cm−2, |g∗

o |is determined to be 9.8, 9.7, 7.6, 7.1, and 5.9, respectively. Theuncertainty in these numbers is estimated to be ±0.4. Thisnumber reflects uncertainty in the slopes of the fit lines forn = 2.7, 3.2, and 3.9 × 1011 cm−2 for which there are threedata points in each case.

The trend of increasing |g∗o | with decreasing n suggested

from the analysis of the rescaled method-B data in Fig. 6(c)is thus also seen in the thermal-activation data in Fig. 7.We note the similarity between our data in Fig. 7 with

FIG. 7. Values of Egap determined from the temperature depen-dence of rxx at the ν = 3, 5, 7, and 9 minima (Idc = 0) for differentelectron densities n as a function of B. The number next to a pointidentifies ν. The lines intercept the B axis in the range 1.2–1.4 T.Estimated values of |g∗| from the slopes are also tabulated. There isno value shown for n = 1.6 × 1011 cm−2 since only one data point(for ν = 3) could be extracted.

that in Ref. [65] for GaAs/AlGaAs 2DEGs: namely, for agiven electron density, an energy gap for odd-filling factors(ν > 1) extracted by thermal-activation measurements (forout-of-plane B field) which is approximately linear in theB field (and with a negative intercept at B = 0 reflectingthe level broadening that is effectively constant), and aneffective g factor determined from the slope that decreaseswith increasing n. The values of the exchange-enhanced g

factor determined by thermal-activation measurements hereare consistent with those extracted by the same techniqueand reported in earlier work for similar materials [21,66].Notable, however, is that the estimated values for |g∗

o | from therescaled method-B approach are a factor of ∼2–3 times higherthan those from the thermal-activation measurements [79].Currently we do not have a fully detailed understanding ofthe difference. We believe some of the discrepancy may arisebecause the compared values were estimated for differentdensity and filling-factor ranges, and Ref. [22] also reports ag factor for an In-based QW estimated by thermal activationthat is lower than the value determined by other techniques;see note in Ref. [79]. A number of other factors may berelevant. For example, thermal-activation measurements areinfluenced by the Landau-level broadening which can varyto a degree with magnetic field. Furthermore, the exchangeenergy also depends on the difference in population of spin-up and spin-down subbands. Under certain conditions anddependent on details of disorder and spin-flip scattering, thespin polarization may not be at a maximum and this can im-pact the measured dependence of the spin gap [64,65,73,74].Outside the scope of this paper, a more quantitative evaluationof the enhanced g factor requires involved self-consistentcalculations for the density of states that depend on detailsof scattering mechanisms for InGaAs/InP QWs. Lastly, from

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FIG. 8. rxx sections through color map for B fields near whereneighboring transport diamonds touch for electron density n = 3.2 ×1011 cm−2. See rxx color map in Fig. 3. The zero-current anomaly(ZCA) “dip” is labeled and is present in all traces shown.

the zero B-field intercepts in Fig. 7, we find 2� to be in therange ∼0.4–0.8 meV. These numbers are about a factor of 2lower than those given in Sec. II that were determined by themore well-established Dingle plot method. Our estimation of� from analysis of Fig. 7 though neglects any contributionto conduction by mechanisms such as variable-range hopping[63,80] and assumes that the boundary in energy betweenlocalized and extended states in a density-of-states peak iswell defined.

VIII. ZERO-CURRENT ANOMALY

Close inspection of the rxx color maps in Fig. 3 revealsa narrow “dip” in rxx in regions close to zero current, mostnotably where the transport diamonds touch. This dip is asignature of the zero-current anomaly (ZCA) described inRef. [21]. In fact, the ZCA can be observed over a widerange of B field provided that rxx is not zero in the vicinityof zero current over a current range exceeding ∼0.4 μA (seealso Fig. 4). To illustrate this, Fig. 8 shows rxx sections on alogarithmic scale for n = 3.2 × 1011 cm−2 that cut throughthe points close to where neighboring transport diamondstouch at zero current. At such points, encompassing the ν = 1to 8 transport diamonds, the ZCA is clear and has FWHM oftypically ∼0.4 μA [81].

Studenikin et al. [21] described basic properties of theZCA for InGaAs/InP Hall bars, but only for B fields up to∼5 T and for an electron density ∼5.3 × 1011 cm−2, so fillingfactors no lower than ν = 5 were investigated. The ZCAFWHM was found to be ∼2.1 μA (∼0.25 μA) for a 100 -μm(10- μm)-wide Hall bar, so this indicates, consistent with theFWHM 0.4 μA for the 15- μm-wide Hall bar measured here,that the FWHM is proportional to the Hall bar width [82].From the near linear increase in width of even-filling-factortransport diamonds with B field which principally reflects thegrowth in cyclotron energy (see discussion in Secs. V–VII),we can convert the FWHM of the ZCA into an energy set

by hωc − |g∗|μBB. For an effective mass of 0.047m0 andtaking |g∗

e | ∼ 4.8, we estimate that the FWHM ∼0.4 μA hereis equivalent to an energy ∼0.3 meV.

Reference [21] also reported the ZCA in higher-mobilityGaAs/AlGaAs Hall bar structures so the phenomenon ap-pears to reflect intrinsic properties of a 2DEG. The originof the ZCA is not understood and we are not aware of anytheoretical description on this subject. In Ref. [21], it isspeculated to originate from modifications to the density ofstates of interacting electrons in the presence of disorder and amagnetic field.

IX. CONCLUSIONS

We have investigated nonlinear magnetotrans-port in a Hall bar device made from a strainedIn0.76Ga0.24As/In0.53Ga0.47As/InP quantum-well heterostruc-ture (with mobility approaching 300 000 cm2/Vs forn ∼ 4.0 × 1011 cm−2 at 0.3 K). We have demonstratedthat maps of the differential resistance (rxx and rxy) overa wide range of sheet current density (up to ∼1 A/m) andmagnetic field (up to 9 T) provide a valuable insight intoquantum Hall breakdown and a number of other nonlinearphenomena (including a spin-flip resonance at high currentnear ν = 1, electric instability, phase inversion of SdHoscillations, and a zero-current anomaly). We have shownthat these maps (phase diagrams) give detailed informationabout the conditions for and the energetics of quantumHall breakdown. Compared to earlier work in Ref. [21],we extended our study to the strong quantum Hall regime(reaching filling factor ν = 1). Also, an additional perspectivewas gained by incrementing the electron density in smallsteps by careful illumination and tracking the systematicevolution of the transport characteristics with n. We presenteda simple picture for the principal features in the maps,namely, the distinctive transport diamonds and high-currentprotrusions (resonances) emerging from near the high-currenttips of the ν = 1 transport diamond, and introduced asimple tunneling model that qualitatively reproduces thesefeatures. Primary parameters such as critical current densitiesand critical Hall electric fields for ν = 1 and 2, and anexchange-enhanced g factor for ν = 1 were extracted. Thecritical Hall electric fields for the InGaAs/InP quantum wellwere found to be comparable to those reported for widelystudied GaAs/AlGaAs 2DEG heterostructures.

A detailed examination was made of the B-field depen-dence of the critical current Ic as determined by two differentmethods and compiled for different values of n. From the firstmethod (method A by which Ic is defined to be the currentwhen rxx exceeds a small threshold), we found Ic for botheven ν and odd ν has an approximate linear dependence withthe B field. This finding is similar to that for an investigationin another In-based quantum-well system [23], but does notfit with the often cited B3/2 dependence for the critical Hallfield Ec. From the second method (method B by which Ic isdefined from the current position of the rxx maxima at the tipsof the transport diamonds), we found Ic for both even ν andodd ν also has an approximate linear dependence with the B

field. While method B was easier to apply, there was (on initialinspection) seemingly more scatter in data points. However,

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FIG. 9. The local density of states (LDOS) including disorderbroadening in a quantizing magnetic field is shown in blue. A suf-ficiently large electric field E (typically the transverse Hall electricfield across the Hall bar) will lead to a Zener tunneling transitionbetween regions i and j separated by distance δ. Relevant forbreakdown at ν = 1, the dashed red arrow depicts the transition (0,↑)→ (0, ↓). The corresponding Landau levels are shifted in energy byμi − μj .

by simple rescaling, Ic/n data points for even ν were foundto collapse onto a single curve. This universal behavior un-covered on rescaling even-ν data points also implies that Ec

has a B2 dependence when method B is applied. Ic/n datapoints for odd ν do not all collapse onto a single curve and weattributed this to signs that the exchange-enhanced g factor,g∗

o (a quantity that can be estimated from the slopes of theIc/n traces for even and odd ν), increases with decreasing n.This trend was corroborated by examination of the effectiveenergy gap for odd ν determined from conventional thermal-activation measurements. By method B, we estimated |g∗

o | tobe in the range ∼15–20 at base temperature.

We have demonstrated that building up a map of quan-tum Hall breakdown provides access to numerous nonlinearphenomena for carriers confined in two dimensions that aredriven out of equilibrium. We have provided a perspective ofthe quantum Hall system from the quasiclassical regime atlow magnetic field to the strong quantum Hall effect regime athigh magnetic field by analyzing the entire phase diagram andpresenting a step-by-step guide toward the different elementsof that phase diagram. The measurement and analytical tech-niques we have described here can, in principle, be appliedto any material hosting a two-dimensional electron or holesystem. Furthermore, method B can complement existingtechniques such as the coincidence technique [64] and thethermal-activation technique for estimation of exchange en-hancement of spin splitting.

ACKNOWLEDGMENTS

Part of this work is supported by NSERC (Discovery GrantNo. 208201) and INTRIQ.

FIG. 10. Example of a color map of dI

dVgenerated using ex-

pression (A2) with B in units of 4ν−1n, taking n = 1.9 in unitsof 1011 cm−2 (compare the color map with the experimental colormap in Fig. 5). We have set hωc/gμBB = 3.75. This corresponds tog ≈ 11.5: a value between g∗

e and g∗o mentioned in Sec. VI. Since

we have assumed a single value for the g factor, we have neglectedmany-body interaction effects. We also used �DOS = hωc at 0.2 T,which is equivalent to ∼0.5 meV. V and dI/dV are in arbitrary units.

APPENDIX: TUNNELING DENSITY-OF-STATES MODEL

Here we formulate a single-particle model describing theinter-Landau-level tunneling processes exemplified in Fig. 9.

We will assume that the system can be described by asimple tunneling Hamiltonian, which is given by the standardexpression

I = e

h

∑ij

Tijni (1 − nj )δ(εj − εi ), (A1)

where Tij are the tunneling matrix elements. The local chemi-cal potentials across the tunneling region are determined bythe electric field E and the distance δ between i and j :μi = μ + eEδ/2 and μj = μ − eEδ/2. The uniform electricfield in the model is governed by the dc bias, V , applied to theHall bar (equivalent in the experiment to the dc current driventhrough the Hall bar, which causes the Landau levels to tiltacross the Hall bar). For simplicity, we will assume V = eEδ

and that the matrix elements Tij do not depend on energy,which leads at zero temperature to

I (V ) ∼∫ μ+eV/2

μ−eV/2dεD(ε − eV/2)D(ε + eV/2), (A2)

where D(ε) is the LDOS of the broadened Landau levels.The broadened Landau levels are typically rounded closeto the Landau-level energies (elliptic in the self-consistentBorn approximation) with exponential tails between Landaulevels. To mimic this behavior and in order to simulate theexperimental findings, we assume the form of the Landaulevels to be e−|ε/�DOS|2/3

in the tails and e−|ε/�DOS|4/3close to

the Landau levels (ε � 0), where ε is the energy relative tothe center of a Landau level and �DOS is the broadening. Thequalitative features will depend on the particular choice of

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Landau-level broadening. We tried different forms but choseone that reproduces well the main features observed.

The differential conductance is given by g ∼ dI/dV . Toapply this model to the four-terminal experimental configura-tion, we note that for large magnetic fields where σxx < σxy ,rxx � σxx/σ

2xy : here, σij are the magnetoconductivity tensor

elements. Hence we assume rxx ∼ dI/dV , using expression(A2), which we evaluate as a function of V and magnetic field.

An example of a calculated rxx color map is shown in Fig. 10.Although we do not expect our toy model to explain allaspects of the experimental rxx color maps, it does nonethelessreproduce a number of key features: (i) transport diamondsthat grow in size with B field and the alternating larger andsmaller transport diamonds for even- and odd-filling factors,(ii) transport diamonds with clearly curved edges, and (iii)resonant features beyond the transport diamonds.

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[36] For the fit calculation, the Bohr radius, effective mass, andstatic dielectric constant are, respectively, set to 17 nm, 0.047m0

(m0 is the free-electron mass), and 15. The Si-doped InPlayer above the QW is 5 nm wide and nominally doped to6.7 × 1017 cm−3 so we set ni = 3.35 × 1011 cm−2, where ni

is the effective 2D density of the remote ionized impurities (weassume the donors are fully ionized). The setback distance isset to 42.5 nm (this is the distance from the middle of the 10-nm-wide In0.76Ga0.24As channel to the middle of the Si-dopedInP layer). Fit parameters are μA = 349 000 ± 7000 cm2/Vs,and nB = (1.8 ± 0.2) × 1015 cm−3, where nB is the effectivedensity of the background ionized impurities. The value fornB is in line with expectations for the chemical beam epitaxysystem used to grow the heterostructure.

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YU, HILKE, POOLE, STUDENIKIN, AND AUSTING PHYSICAL REVIEW B 98, 165434 (2018)

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Phys. 16, 113071 (2014).[44] We have also examined the color maps of rxx replotted with

1/B on the x axis rather than B (data not shown [34]). Thediamond center positions are now periodic in 1/B as expected,but the edges of the low-filling-factor diamonds are still curvedafter the transformation. Other reports also show that thedecrease of the critical current with B field on moving awayfrom the maximum critical current is not linear (see, for exam-ple, Refs. [54,2]).

[45] V. Yu, M. Hilke, P. Poole, S. Studenikin, and D. G. Austing(unpublished).

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[48] The theoretical energy gap for fractions is given by �th =Ce2/4πε�B , where C depends on the filling factor and thedetails of the theory (typically 0.01 < C < 0.1), ε is the staticdielectric constant, and �B is the magnetic length. From ex-perimental data for the energy gap for fractional states �exp =�th − � shown in Ref. [46] for GaAs (ε = 13), we first estimateC = 0.075. Using this C for InGaAs and taking ε = 15, weevaluate �th = 0.56 meV (0.74 meV) at 4 T (7 T).

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[52] Note that the point where the ν = 1 and ν = 2 diamonds touch[in Fig. 5(b)] is not at ν = 1.5 but ν > 1.5.

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(2001).[55] From thermal-activation measurements in a 4He

cryostat on a second Hall bar device made from a similarIn0.76Ga0.24As/In0.53Ga0.47As/InP heterostructure withn = 1.5 × 1011 cm−2 (data not shown [34]), g∗ at ν = 2was estimated. For an effective electron mass m∗ = 0.047m0

and Landau-level broadening � ∼ 1 meV (see Sec. II), weevaluated |g∗| to be 4.8 ± 0.9. This value is consistent withthe theoretical estimate of 5.45 for the bare g factor from thenon-self-consistent calculation cited in Ref. [21]. The effectiveg factor here for the spin-unpolarized ν = 2 state is notsubject to strong enhancement by many-body electron-electronexchange interactions so is lower than the values of g∗

determined later in Secs. VI and VII at odd-filling factors (seealso Ref. [66]).

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[60] In Ref. [9], an estimate is made for the characteristic energy(the length scale corresponding to Landau-level wave-functionoverlap) of a simple model potential representing a nearbyionized impurity sufficient to create a locally enhanced electricfield to explain certain features of breakdown in the region ofν = 2 of the device studied when the average Hall electricfield is relatively small (with value ∼7 kV/m, which is smallcompared to that theoretically expected for inter-Landau-leveltunneling between uniformly tilted levels). The value for thecharacteristic energy (length scale) was estimated to be a fewmillielectronvolts (comparable to the magnetic field length).Our deduction given in Sec. II that the effective random po-tential fluctuation in the InGaAs QW studied has amplitude∼2 meV fits in with the picture that locally enhanced electricalfields are naturally present and are an important factor.

[61] A percolation picture of the breakdown in Ref. [83] examinestheoretically the abrupt formation of metallic conduction fil-aments across the Hall bar. The model requires the presenceof regions in the Hall bar with locally enhanced electric fieldsinduced in the calculations by a disorder potential to generateunstable energy-occupation configurations. In this work, onlyν = 2 is considered and the calculated values for the criticalelectric field are still larger than typically measured experimen-tal values.

[62] There is still no complete consensus on the quantum Hall break-down mechanism. References [53,67] include interesting ex-tended commentaries on whether leading models for breakdowncan be reconciled and describe the considerable challengesin interpreting available experimental data with these modelsgiven the wide variety of Hall bars studied (with different Hallbar layouts, channel widths, channel lengths, and mobilities).We note that under the assumption of a uniform Hall electricfield, application of the Zener tunneling mechanism even forwell-studied GaAs/AlGaAs structures, with commonly reportedbreakdown electric fields (such as those tabulated in Ref. [53]),would give a characteristic length δ of the order of a micron at4 T. Explicitly, from the compilation of data in Refs. [53,54],the average breakdown electric field is typically in the range2–6 kV/m at 4 T for even-filling factors, which correspondsto δ in the range 1.1–3.4 μm for a cyclotron energy of ∼1.7meV/T relevant to a GaAs QW: these δ values are significantlylarger than the corresponding magnetic field length ∼13 nm.The value of δ estimated for the InGaAs QW studied is thereforesimilar to that for GaAs/AlGaAs structures. Beyond the likelyimportant role of locally enhanced electric fields near the Hallbar edges and random potential fluctuations, Ref. [53] discussesthe possible influence of higher-order tunneling processes, andLandau-level broadening induced by the Hall electric field.Even with the widely cited bootstrap-type electron heatingmodel of Komiyama and Kawaguchi [51], which incorporatesZener tunneling as a component, the predicted breakdown Hallelectric fields still exceed those experimentally measured (seeFig. 6 in Ref. [51]). This model is subject to assumptionsabout the energy-relaxation mechanism (presumed to be dom-inated by intra-Landau-level acoustic-phonon scattering) anduncertainty in parameters. Lastly, in most detailed descriptionsand discussions of breakdown mechanisms, only breakdown ateven-filling factor is considered (Ref. [53] is one exception).

[63] T. Ando, A. B. Fowler, and F. Stern, Rev. Mod. Phys. 54, 437(1982).

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[67] O. Makarovsky, A. Neumann, L. Dickinson, L. Eaves, P.Main, M. Henini, S. Thoms, and C. Wilkinson, Physica E(Amsterdam) 12, 178 (2002).

[68] For method A, generally data points for even ν could beextracted above ∼1.5 T; however, above 6 T, determinationfor ν = 2 was not straightforward because of fluctuations ormultiple-step transitions prior to abrupt breakdown when n >

2.7 × 1011 cm−2. For odd ν, other than for ν = 1 which movedout of range when n > 1.9 × 1011 cm−2, only data points forν = 3 could be extracted over a wide range of n. For methodB, more data points could be extracted and this was generallystraightforward above ∼1 T. However, for even ν, determinationwas not possible or straightforward if the transport diamondwidth was close to or exceeded the current sweep range (forexample, ν = 2 at n = 1.9 × 1011 cm−2), or multiple-peakfeatures developed at breakdown (for example, ν = 2 at n =2.3 × 1011 cm−2; note that the data points for this density fallshort of the common lines in the lower two panels of Fig.6 for even-filling-factor points after rescaling), or widespreadinstability occurred beyond breakdown (ν = 2 beyond ∼5 T forn > 2.3 × 1011 cm−2).

[69] In Figs. 6(c) and 6(d), even after rescaling, there is more scatterin odd-integer filling data points than for even-integer filling-factor points. As discussed in Sec. VII, not only can this be asign of an exchange-enhanced g factor that varies with n, butalso of an exchange-enhanced g factor that varies with B fieldat constant n.

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[71] For n = 1.9 × 1011 cm−2, the estimate of |g∗o | for ν = 1 from

the slopes of the lines in Fig. 6(c) is higher than the crudeestimate of |g∗

o | for ν = 1 given in Sec. VI. For that estimate,however, Landau-level broadening was neglected and so the|g∗

o |μBB line bounding odd-ν diamonds was assumed to extrap-olate to the origin (see Fig. 5). However, Fig. 6(c) reveals thatodd-ν data points generally lie on lines that do not extrapolateto the origin, but instead intercept zero current near 1 T. As aconsequence, the slope and hence the extracted g factor will belarger.

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[77] We assume |g∗o | does not vary with odd ν at fixed electron den-

sity. This is reasonable since we analyze ν = 3, 5, 7, and 9 andnot ν = 1 (where exchange can be more strongly enhanced).The calculations in Ref. [66] show some weak dependence ofthe exchange-enhanced g factor as a function of odd ν.

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[79] We note that the g factors we estimate from thermal-activationmeasurements are determined with data points for odd-fillingfactors no lower than ν = 3 with electron densities greater than1.6 × 1011 cm−2 (see Fig. 7), whereas the upper limit for theg factors estimated from the transport diamonds principallyreflects the width of the ν = 1 transport diamond at the lowestelectron density (see Fig. 6) where the exchange enhancementis expected to be largest. See, also, Refs. [73,77]. Interestingly,Nachtwei et al. [22] report a g factor determined by thermalactivation for ν = 1 in an InGaAs/InAlAs QW that was signif-icantly lower (by factor ∼4) than the g factor found by anothertechnique.

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[81] The FWHM value of ∼0.4 μA is a typical value for the selectedsections shown in Fig. 8. Examination of other sections nearwhere neighboring transport diamonds touch gives values forthe FWHM as low as 0.2 μA.

[82] In Ref. [21], it is erroneously stated that the ZCA width isinversely proportional to the Hall bar width.

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