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Journal of Crystal Growth 311 (2008) 218–221

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

�Corr

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journal homepage: www.elsevier.com/locate/jcrysgro

Single crystal growth and characterizations of Cu0.03TaS2 superconductors

X.D. Zhu a, Y.P. Sun a,b,�, X.B. Zhu a, X. Luo a, B.S. Wang a, G. Li a, Z.R. Yang a, W.H. Song a, J.M. Dai a

a Key Laboratory of Materials Physics, Institute of Solid State Physics, Hefei 230031, People’s Republic of Chinab High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China

a r t i c l e i n f o

Article history:

Received 21 August 2008

Received in revised form

6 October 2008

Accepted 10 October 2008

Communicated by R. Fornaribecomes a superconductor below 4.2 K. Besides, electrical resistivity and Hall effects results show that a

Available online 18 October 2008

PACS:

81.10.�h

81.10.Bk

74.70.�b

Keywords:

A1. Charge density wave

A2. Single crystal growth

A2. Chemical vapor transport

B2. Superconducting materials

48/$ - see front matter & 2008 Elsevier B.V. A

016/j.jcrysgro.2008.10.023

esponding author. Tel.: +86 551559 2757; fax

ail address: ypsun@issp.ac.cn (Y.P. Sun).

a b s t r a c t

Single crystal of Cu0.03TaS2 with low copper intercalated content was successfully grown via the

chemical vapor transport method. The structural characterization results show that the copper

intercalated 2H-Cu0.03TaS2 single crystal has the same structure of the CdI2-type structure as the parent

2H-TaS2 crystal. Electrical resistivity and magnetization measurements reveal that 2H-Cu0.03TaS2

charge-density-wave transition occurs at TCDW=50 K.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

Diverse physical properties such as charge-density-wave(CDW) and superconductivity in layered transition metal dichal-cogenides (TMDC) have been broadly studied [1,2]. TMDC have ageneral formula TX2, where T is usually a transition metal atomfrom groups of IVB, VB, and VIB of the periodic table of elementsand X is one of sulfur, selenium, or tellurium. The layered TMDCcan be regarded as stacking two-dimensional X–T–X sandwiches.The bonding within each sandwich is covalent, while the bondingbetween them is weak van der Waals type. The crystal structuresof the layered TMDC are usually described as 1T, 2H, 3R, 4Ha,4Hb, 6R phases [1,2]. The integer denotes the number of X–T–Xlayers per unit cell perpendicular to the layers, while T, H, and R

denote trigonal, hexagonal, and rhombohedral symmetries,respectively.

Introducing foreign atoms or molecules between the weakcoupling sandwiches or layers is the process of intercalation.Intercalation not only increases layer separation but also providesa powerful way to tune the electronic structures of the hostmaterials. Intercalated TMDC, graphite, and layered structuraltransition metal nitrides have received considerable attention due

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to their special structures and transport properties especially forsuperconductivities [2–4]. New interests have been aroused sincethe recent discoveries of superconductivities in YbC6, CaC6, andCuxTiSe2 [5–7].

2H-TaS2 is one of the layered TMDC [8]. Metallic 2H-TaS2

undergoes a CDW transition at 78 K, and becomes a super-conductor below 0.8 K [8]. Many inorganic atoms and organicmolecules have been intercalated into TaS2 [3,9–14]. Intriguingly,superconductivity has been found in Py1/2TaS2 [9], A0.33TaS2 (A=Li,Na, K, Rb, Cs) [14], 2H-FexTaS2 (x=0.05), etc. [15]. Recently, thecompetition driven by intercalation between superconductivityand CDW in NaxTaS2 has been discovered [16]. Copper intercala-tions in TaS2 have been achieved by the chemical vapor transport(CVT) method to produce Cu1/2TaS2 [17], by direct reaction fromcopper and TaS2 to produce single-phase CuxTaS2 (0.33oxo0.75)[18], and by the electro-chemical method [19]. However, super-conductivity has not been observed in any high copper intercala-tion CuxTaS2 [17–19]. Further, there is also no report about lowercopper intercalation in CuxTaS2. Thus, it is interesting to explorewhether or not low copper intercalation in CuxTaS2 can inducesuperconductivity . The low copper intercalation experimentswere carried out in order to search for possible superconductivityin CuxTaS2 (xo0.33). In this paper, we report the single crystalgrowth of Cu0.03TaS2 with low copper intercalated content via theCVT method. Electrical resistivity and magnetization measure-ments reveal that 2H-Cu0.03TaS2 becomes a superconductor below

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X.D. Zhu et al. / Journal of Crystal Growth 311 (2008) 218–221 219

4.2 K. Besides, resistivity and Hall effects results show that a CDWtransition occurs at TCDW=50 K.

2. Experimental procedure

Single crystal Cu0.03TaS2 was grown via the CVT method.Firstly, polycrystalline TaS2 powder was synthesized by solid-statereaction. Stoichiometric amounts of Ta and S powders were mixedand sealed in an evacuated quartz tube. The tube was heated at900 1C for 4 days. Secondly, Cu, TaS2 powders in a mol ratio of0.5:1, and 100 mg iodine were mixed and sealed in a 12-mm-diameter and 19-cm-long evacuated quartz tube. The tube wasinserted in a two-zone horizontal tube furnace with a source-zonetemperature of 1000 1C and a growth-zone temperature of 900 1Cfor 10 days. Both zones were decreased to 440 1C in several days,and then cooled to room temperature. Dark blue, mirror-likeplates with a typical size of 3�3�0.2 mm3 were obtained, andthe photograph is shown in Fig. 1. Gamble et al. [9] have reportedthat 2H-TaS2 single crystals grown via the CVT method were blackand somewhat wrinkled.

The grown Cu0.03TaS2 plates were cleaned by ultrasonication insupersaturated aqueous solutions of KI, de-ionized water, andalcohol, respectively. The composition of Cu0.03TaS2 single crystalwas characterized by energy dispersive X-ray spectroscopy (EDS)and inductively coupled plasma atomic emission spectrometry(ICP-AES, Atomscan Advantage). The crystal structure and phasepurity were examined by powder and single crystal X-raydiffraction pattern (XRD) using a Philips X’ pert PRO X-raydiffractometer with Cu Ka radiation at room temperature. Theelectrical resistivity was performed by the standard four-probemethod using a quantum design physical property measurementsystem (PPMS) (1.8 KpTp400 K, 0 TpHp9 T). The Hall effectsexperiments were also performed using PPMS. Magnetizationmeasurements as a function of temperature were performed in aquantum design superconducting quantum interference device(SQUID) system (1.8 KpTp400 K, 0 TpHp5 T).

Fig. 1. Photograph of Cu0.03TaS2 single crystal.

3. Results and discussions

3.1. Chemical analysis

The EDS pattern for the grown single crystal is shown in Fig. 2.The estimated mol ratio of Ta:S is about 1:2. Though the copperpeak is evident in the EDS spectrum, the accurate copper contentcannot be obtained because the copper content is too small. Thus,the copper content is determined by ICP-AES. The determined molratio of the Cu:Ta is 0.03:1.

3.2. Structural analysis

The XRD patterns for Cu0.03TaS2 and 2H-TaS2 single crystals areshown in Fig. 3a. It can be observed that the orientations of thecrystal surfaces are both (00 l) planes. The magnification plots of(0 0 6) peaks are shown in the inset of Fig. 3a. Obviously, the peakpositions of Cu0.03TaS2 shift to the lower 2y value. According to theBragg equation nl=2d sin y, copper intercalation leads toexpansion of the lattice constant c.

Several single crystals were crushed into powders, which wereused in the powder XRD experiment. The powder XRD patterns ofcrushed Cu0.03TaS2 and 2H-TaS2 crystals are shown in Fig. 3b. Allthe peaks can be well indexed to the 2H structure [20]. Thepresent Cu0.03TaS2 is single phase with no detectable secondaryphases. The lattice constants were obtained from powder XRDpatterns. The lattice constant of Cu0.03TaS2 is almost equal to thatof 2H-TaS2 (a=3.31 A), while the lattice constant c of Cu0.03TaS2 is12.13(7) A, which is slightly larger than that of 2H-TaS2

(c=12.08(0) A).

3.3. Co-existence of superconductivity and CDW

The temperature dependence of electrical resistivity in the ab

plane (rab–T) for 2H-Cu0.03TaS2 is plotted in Fig. 4. Obviously,there is a kink around T=50 K, which can be attributed to CDWtransition. The inset shows the enlarged view of the rab–T curvenear the superconducting region. The resistivity sharply drops tozero around T=4.2 K, which indicates the superconductivity. Theonset superconducting transition temperature is 4.2 K, and thetransition width (10–90%) is 0.2 K.

The temperature dependence of dc magnetic susceptibility forCu0.03TaS2 is plotted in Fig. 5. The magnetic field of 2 Oe wasapplied parallel to the ab plane of a Cu0.03TaS2 plate. A sharp dropof magnetization at 4.2 K was observed for both zero-field-cooling

Fig. 2. The EDS pattern for Cu0.03TaS2.

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Fig. 3. (a) The single crystal XRD patterns for Cu0.03TaS2 and 2H-TaS2; inset: the

magnification plot of single crystal XRD patterns. (b) The powder XRD patterns for

Cu0.03TaS2 and 2H-TaS2.

Fig. 4. Temperature dependence of the in-plane resistivity (rab) for Cu0.03TaS2. The

inset shows the rab–T curve near the superconducting region. The arrows show the

CDW transition temperature (TCDW) and the onset transition temperature (TConset).

Fig. 5. The temperature dependence of ZFC and FC susceptibility wg measured in

an applied field of 2 Oe parallel to the ab plane for Cu0.03TaS2: FC (filled circles);

ZFC (open circles).

Fig. 6. Temperature dependence of the Hall coefficients (RH) for Cu0.03TaS2. The

inset shows the measured Hall resistivity (rxy) versus H measured at different

temperatures.

X.D. Zhu et al. / Journal of Crystal Growth 311 (2008) 218–221220

(ZFC) and field-cooling (FC) measurements, which furtherconfirms the existence of superconductivity. The ZFC curveshows an almost perfect shielding effect, while the magneticflux exclusion fraction estimated from the FC curve is as low as�4% at 2 K, using �4pwv to estimate the volume fraction. Similarphenomena have been reported in other intercalated compoundssuch as Py1/2TaS2 [21], YbC6 and CaC6 [5,6], and LiPd3B2 [22].Prober et al. [23] suggested that the small FC magnetization wasdue to the complicated flux trapping effect in these intercalated

compounds. Thus, it is suggested that the bulk superconductivityis found in Cu0.03TaS2.

The temperature dependence of the Hall (RH) coefficient forCu0.03TaS2 is shown in Fig. 6. The RH versus T is determined fromthe Hall resistivity (rxy) results using RH=rxy/H. The inset in Fig. 6shows the magnetic field dependence of the rxy measured atdifferent temperatures. The RH increases from �2.8�10�4 cm3/Cabove 60 K to �5.5�10�4 cm3/C below 30 K. In contrast, the signof the RH of matrix 2H-TaS2 even changes from positive to negativenear the CDW transition, which was reported by Thompson et al.[21]. The change of the RH of Cu0.03TaS2 in the region of30 KoTo60 K confirms the CDW transition revealed by theresistivity results. Apparently, copper intercalation suppressesthe TCDW from TCDW�78 K (2H-TaS2) to TCDW�50 K. According tothe formula nH=�1/eRH, the reduction in carrier density isconsistent with the partial gapping of the Fermi surface due tothe occurrence of CDW transition.

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4. Conclusion

In summary, Cu0.03TaS2 single crystals with low copperintercalated content was successfully grown via CVT. Structureanalysis shows that the low copper intercalated Cu0.03TaS2 and theparent compound 2H-TaS2 share the same structure. Electricalresistivity and magnetization measurements reveal that2H-Cu0.03TaS2 becomes a superconductor below 4.2 K. Besides,electrical resistivity and Hall effects results show that a CDWtransition occurs at TCDW=50 K.

Acknowledgements

This work was supported by the National Key Basic Researchunder contract No. 2006CB601005, 2007CB925002, and theNational Nature Science Foundation of China under contractNo.10774146, 10774147 and Director’s Fund of Hefei Institutes ofPhysical Science, Chinese Academy of Sciences.

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