Physica C 235-240 (1994)3363-3364 P H Y S ~ C ~ @ North-Holland

Electric field effects on transport properties in YBa2Cu307_6 T. Kawahara '~, T. Suzuki '~, K. Shinmra I', T. Terashima b and Y. Bando b

'~Department of Physics, Kyoto University, Kyoto 606-01, Japan.

bInstitnte for Chetnieal Research, Kyoto University, Uji 611, Japan

We made field effect transistor-like junctions of YBa~Cu:~OT_~ (YBCO). Thin "(13(10 films with 6 and 2 unit cell thicknesses are prepared on a (100) surface of SrTiO3. Insulating layer of BaTiO3 w~s made on top of YI~C(). The application of a negative gate voltage ~g,t~ lowers the resistance R linearly in ~,,,., with a positive ~'~,,t~ having the opposite effect. Near the onset temperature T~0 of superconductivity, AR/R is independent of temperature across T~o. This result indicates a small electric field effect on T~0 for YBCO with high T,-0(~ 80h').

1. I n t r o d u c t i o n 3. R e s u l t s

Recently, experimental studies on high temper- ature superconductors showed significant charg- ing effects[I-3]. A chain oxygen dynamics model was proposed for the charging effects in YBa.,Cu3OT_~(YBCO)[4]. This model was supported t)3' the measurement of critical cur- rent change due to electric field(E-field) in par- tially oxygen-depleted superconducting YBCO thin fihn[5].

We expect a weak E-field effect on YBCO with saturated behavior of onset tempera ture To0 of superconductivity. Here, we report the E-field effect on resistance near T~0 for ultra tltin YBCO films with high T~0.

2. E x p e r i m e n t a l P r o c e d u r e

Our samples were grown in situ epitaxially on a SrTiO3(STO) (100) substrate by an activated reactive evaporation method[6]. We deposited a PrBaeCu307 layer between YBCO and the sub- s t rate and a 1.5nm STO capping layer on YBCO. After that, we deposited a 200nm thick dielectric BaTiO3(BTO) insulation layer. A silver counter electrode was then deposited onto BTO layer so as to cover the part with the highest T~0 within the YBCO films for our convenience of masking. The thicknesses of YBCO films were 6(Sample A) and 2(Satnple B and Sample C) unit cells.

We used the ac four-terminal method for mea- suring resistance.

Figure 1 shows the tentperature dependet~ce of resistance normalized to that at 100K. Zero resis- tance tetnperatures are 69K,18K and 5K for Sant- ple A, B attd C, respectively. N)r all samples, T,-0 falls on about 80K.

The application of a negative gate voltage 1 ~,~,~ to the Ag electrode lowers the resistance of YBCO linearly in I g.~t~, while a positive 1 ~,~,~ having the opposite effect.

In Figure 2, we show the temperature depen- dence of AR/R , where AR is the resistance change at l~,~,~ = 1V, R is a resistance for Igat~=0. In the present experitnetlt, AR/R re- mains constant through To0 down to To at which the gradient dR/dT takes the maximum value. The temperatures To are 75, 60, 63K for Sample A, B, and C, respectively, as shown by horizontal arrows in Figure 1. For Sample A, AR/R is en- hanced at lower temperatures than To, while it is decreased with T in Sample B and C.

4. D i s c u s s i o n

Observed AR/.R in tile normal state, namely, AR,~/Rn and its thickness dependence can be ex- plained by the field induced charge(AN) in the areal density as discussed in Ref.[2]. Here, we evaluated A N by using a measured capacitance and an applied l/~te.

In the superconducting state, AR/R should contain a contribution of ATco/T¢o multiplied by

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3 3 6 4 77 Kawahara et al./12hysica C 235 240 (1994) 3363 3364

1

g

cE

8

(1)

g cxJ g o 0

Z I i i i I I i I i

0 50 100 T e m p e r a t u r e (K)

Figure 1. Temperature (teI)enden((, ()f R/I~I().. where Rl00 is the resistance a.t 100K. E) is in- dicated by an horizontal air()}v for each Saml)lo.

c r

-,-'2

× S a m p l e A • ~ S a m p l e B A z, S a m p l e C .:%w'-~ -:" -

'"J" ] win"

!

8~0 6'0 T e m p e r a t u r e (K)

Figur(~ 2. T('mp('raturc d(,l)('ndcn(-(' of _Xl~/I~. Ol)(ql an(t solid svmb()ls d(,m)w those for positiv( ~ and m'gativ(, gal(' v()lt.ag(',~.

an enhan(:ement factor ]t.. Oil the assumptioll that R / R . under E-field would be described by a uni- fied flmction of T/T,.o in each sample, whoro h must be divergent at T~,I where resistance b(~- COllies zero.

As se(m in Figure 2, 5 R / / ~ is ahnost ('(in- stant in the temperature region above T0. Thus AT,.o/T,.o must b(' negligibly small ('omt)ar('d with 2XR./]~,~ 1)ecause h has been enhanced there.

Below To, ,5I?/I? is enhanced up to ().6(~ in Samt)le A. However, ATco/T,,o is significantly slnall and at most 0.02% because h is ('xtromoly enhanced u I) to 200 reflecting a sharp transition. D)r Sample B and C, ,5/~/R decreases to zero b(-- low To. This decrease is apparent because an ap- t)lied E-field acts on the fihn part with the higlwst Tc0 in the YBCO channel under the gate eJe(:tro(le and thus resistance comes from the other inactiw' part of the channel. In this sense, we could not observe E-field effects on the critical current.

These decreases in ~ R / / ? for Sample B an(t C rather indicate those in AR. Thus, ~/7 goes to zero at about 30K and 40K for Sample B and C, respectively. These temperatures correspond to TKT for the YBCO films with the high Tc0 of 80K and are comparable with those rel)orted in Ref.[7].

5. S u m m a r y

We observed electric field effects on resistance

for ultra thin ~'B(?() films. Fl()m t('itq)oratur(, d( 1)('n(tcn('( ' (,f .5/2//'?. w(' r('('(,gniz('d th(' fiold (fffe(l ,m T,,. t() 1)(, w('ak('r f,~r high 7',() "VBCO ~han that ()n R.

A c k n o w l e d g m e n t

Th(' aulh()rs th;mk Pr()f. T._Niizusaki amt Prof. T.Tsunet() for th('ir kind sllgg('>li()ll>, ;111(1 diSt'llS- s i ( )I1%.

R E F E R E N C E S

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