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Solid State Sciences 10 (2008) 1749e1751 www.elsevier.com/locate/ssscie

Comparative study of magnetotransport properties and structural features of the organic superconductors k0-(BEDT-TTF)2Cu[N(CN)2]X, where X ј Cl, Br
Vladimir Zverev a,*, Andrey Manakov a, Salavat Khasanov a, Rimma Shibaeva a, Natalia Kushch b, Anna Kazakova b, Eduard Yagubskii b
a

Institute of Solid State Physics RAS, Institutskaya, 2, Chernogolovka, Moscow Region 142432, Russia b Institute of Problems of Chemical Physics RAS, Chernogolovka, Moscow Region 142432, Russia Received 26 October 2007; received in revised form 9 January 2008; accepted 21 January 2008 Available online 2 February 2008

Abstract The crystal structure, the resistivity and the upper critical field of radical cation salts k0 -(BEDT-TTF)2Cu[N(CN)2]X, where X ј Cl, Br (k -X hereafter), have been studied. Both k0 -Br and k0 -Cl samples were superconductors with Tc lying in the interval 11.3e11.9 K and had the unit cell volume smaller than that for k-Br and k-Cl, respectively. The temperature dependence of interlayer resistivity rt for k0 -Br is characterized by the hump lying in the temperature interval 35e100 K. For k0 -Cl rt is considerably smaller and goes down monotonously with the temperature. The curves Hcr(T ) for all crystals demonstrate a non-BCS-behaviour for both in-plane and out-of-plane magnetic field orientations. с 2008 Elsevier Masson SAS. All rights reserved.
0

Keywords: Organic conductors; Superconductivity

A quasi two-dimensional systems k-(BEDT-TTF)2 Cu[N(CN)2]X are characterized by close proximity to the metaleinsulator transition and one can get either metal or Mott insulator depending on the kind of the halogen atom X. For example, the cation-radical salt with X ј Br is organic superconductor, while the salt with X ј Cl is dielectric at ambient pressure and undergoes a transition to a superconducting state at a pressure of 0.3 kbar with the critical temperature Tc ј 12.8 K. Recently a new approach to the synthesis of k-Cl salt was proposed [1,2], as a result the modified k0 -Cl salts with new unexpected properties were synthesized. In particular, this salt was found to be the ambient pressure superconductor. In this paper we present the results of comparative study of the crystal structure, the resistivity and the upper critical field

* Corresponding author. Tel.: Ч7 496 522 10 04; fax: Ч7 496 524 97 01. E-mail address: zverev@issp.ac.ru (V. Zverev). 1293-2558/$ - see front matter с 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.01.025

of radical cation salts k0 -(BEDT-TTF)2Cu[N(CN)2]X, where X ј Cl, Br (k0 -X hereafter). The crystals of k0 -Br salt were obtained by electrooxidation of BEDT-TTF (10ю3 M/l) in 1.1.2TCE-alcohol absolute (10 v/v%) medium in constant current conditions (I ј 0.3e0.5 mA) at 20 C. Mixture of the complex 18-cr-6, Na[N(CN)2] and CuBr in a 1:1:1 molar ratio (4.5 б 10ю3 M/l) with addition of M[N(CN)2]2 salts, M ј Ni, Mn, Zn (20 mol% of CuBr concentration) was used as an electrolyte. X-ray analysis of obtained crystals defined that k0 -Br crystals were growing on a Pt anode together with those of the superconductor k-Br. The k0 -Cl crystals were synthesized in the presence of only the Cu[N(CN)2]2 salt as an electrolyte [1,2]. For comparison, the deuterated d8-k-Br salts, synthesized by traditional method [3], were also studied. Both k0 Br and k0 -Cl samples were superconductors with Tc lying in the interval 11.3e11.9 K and had the unit cell volume smaller than that for k-Br and k-Cl. According to our X-ray analysis, k0 -Cl and k0 -Br crystals have the same structure as correspondent k-X salts, but there


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V. Zverev et al. / Solid State Sciences 10 (2008) 1749e1751

'-Br(Zn) 100 -Br(Ni) 50 '-Br(Ni) 10

0 0 20 40 60 80 100 120 140 160

0

T (K)
Fig. 1. Temperature dependences of the out-of plain resistivity for different kand k0 -salts.

Bcr(T ) for all crystals demonstrate a non-BCS-behaviour for both in-plane and out-of-plane magnetic field orientations. We would like to emphasize that Hcr values for all bromides including the deuterated samples are very close to each other and their temperature dependences are very similar. Moreover, Bcr(T ) dependences for k0 -Cl are also very similar to that for bromide samples. In conclusion, we have studied the crystal structure, the resistivity and the temperature dependences of the upper critical field of new radical cation salts k0 -(BEDTTTF)2Cu[N(CN)2]X, where X ј Cl, Br. We have found that both k0 -Cl and k0 -Br crystals have smaller unit cell volume ° (DVy15 A3) with respect to that of the correspondent k- salts, which is probably due to the deficiency in the copper positions. Unlike to traditionally synthesized k-chloride samples, k0 -Cl is ambient pressure superconductors with Tc ј (11.3e 11.9) K and exhibits metallic r(T ) dependence without ``hump'', which is characteristic for k- and k0 -bromides. At the same time, we have not found any considerable difference in transport properties of k- and k0 -bromides. Both k0 -Cl and k0 -Br samples demonstrate nontrivial temperature dependences of the upper critical field.
18 16 14 12 k'-Cl d8-k-Br k'-Br(Zn) k-Br(Ni)

B||(ac)

Bcr (T)

10 8 6 4 2 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 B||b

Table 1 Unit cell parameters for the single crystals of k- and k0 -(BEDTTT)2Cu[N(CN)2]Br at room temperature (Space Group: Pnma, Z ј 4) k-Br [4] a, b, c, V, ° A ° A ° A ° A 12.942(3) 30.016(4) 8.539(3) 3317(1) k0 -Br(Ni) 12.936(2) 29.930(4) 8.527(1) 3301.6(9) k0 -Br(Zn) 12.931(2) 29.964(3) 8.5182(8) 3300.4(6) d8-k-Br 12.947(2) 29.998(3) 8.5356(9) 3314.9(7)

T (K)
Fig. 2. Temperature dependences of the upper critical fields.

3





are two essential differences: (i) the deficiency (z7%) in the copper positions in k0 -Cl due to vacancies and (ii) smaller unit ° cell volume of both k0 -Cl and k0 -Br crystals (DV z 15 A3) (see 0 Ref. [2] for the unit cell parameters of k -Cl and the Table 1 for k0 -Br). Smaller unit cell volume of k0 -X salts with respect to k-X salts may be ascribed to some effective ``chemical'' pressure, which, according to our measurements [2], is of the order of 600 bar for k0 -Cl. The samples for transport measurements were thin plates with 1 б 0.3 б 0.02 mm3 characteristic sizes. The surface was oriented along conducting layers (the (ac) plane). The resistance was measured by a Lock-in detector at 20 Hz alternating current using a four-probe technique. Two contacts were prepared to each of two opposite sample surfaces with conducting graphite paste. Sample resistances Rk and Rt were measured by sending the current either parallel (Jk(ac)) or perpendicular (Jkb) to conducting layers, respectively. From these measurements two correspondent components rk and rt of the resistivity tensor were calculated. The value of the current running through the sample was fixed and was not higher than 10 mA. A superconducting solenoid, which generated the field of up to 17 T was used for the experiments in magnetic fields. The temperature dependence of interlayer resistivity for k0 Br and k0 -Cl samples is shown in Fig. 1. One can see that for all k- and k0 -bromides r(T ) dependences are characterized by the hump lying in the temperature interval 35e100 K, which is well known for traditionally prepared k-bromides. Usually the hump in k-Br is observed at the temperature about 90 K and is attributed to some orderedisorder transition in ET-ethylene groups (see, for example, Ref. [5]). Fig. 1 shows that the hump temperature may lie in the wide temperature region, which means that the hump nature is not still clear and requires further investigations. The resistivity of k0 -Cl is much smaller than that of k- and k0 -Br in the whole temperature range between room temperature and Tc and, unlike to kand k0 -bromides, the temperature dependence r(T ) for k0 -Cl is monotonous. The influence of magnetic field on the superconducting transition was studied for both in-plane (Bk(ac)) and out-ofplane (Bkb) orientations of magnetic field by measurements of Rt(T ) dependences at different fixed magnetic field values. In these experiments the superconducting transition temperature Tc, determined at one half of the normal state resistance level, was measured as function of magnetic field. As a result, we have got the temperature dependences of the upper critical field Hcr(T ), shown in Fig. 2. One can see that the curves

200

d8--Br

'-Cl 20

cm) for bromides

150

cm) for chloride


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Acknowledgments The work was supported by Russian Foundation for Basic Research: grants 05-02-16980, 06-02-16471 and P-03 of RAS. References
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[2] V.N. Zverev, A.I. Manakov, S.S. Khasanov, R.P. Shibaeva, N.D. Kushch, A.V. Kazakova, L.I. Buravov, E.B. Yagubskii, E. Canadell, Phys. Rev. B 74 (2006) 104504. [3] V.V. Metlushko, V.D. Kuznetsov, L.A. Epanchikov, E.F. Makarov, E.B. Yagubskii, N.D. Kushch, Sov. Phys. JETP 74 (1992) 1041. [4] A.V. Kini, U. Geiser, H.H. Wang, K.D. Carlson, J.M. Williams, W.K. Kwok, K.G. Vandervoort, J.E. Thompson, D.L. Stupka, D. Jung, M.-H. Whangbo, Inorg. Chem. 29 (1990) 2555. [5] Ch. Strack, C. Akinci, V. Pashchenko, B. Wolf, E. Uhrig, W. Assmus, M. Lang, J. Schreuer, L. Wiehl, J.A. Schlueter, J. Wosnitza, D. Schweitzer, J. Muller, J. Wykhoff, Phys. Rev. B 72 (2005) 054511.