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Ambient Pressure Superconductivity in -(BEDTTTF)2Cu[N(CN)2]Cl Single Crystals
E.B. Yagubskii,a N.D. Kushch,a A.V. Kazakova,a L.I. Buravov, V.N. Zverev,b A.I. Manakov,b S.S. Khasanov,b R.P. Shibaevab
a

a

b

Institute of Problems of Chemical Physics RAS, Chernogolovka, Moscow region, 142432 Russia; e-mail: yagubski@icp. ac. ru Institute of Solid State Physics RAS, Chernogolovka, Moscow region, 142432 Russia

Single crystals of the -(BEDT-TTF)2Cu[N(CN)2]Cl radical cation salt

possessing metallic properties and showing a superconducting transition with Tc = 11.5 K at ambient pressure were first prepared.

PACS numbers: 61.10 Nz, 72.80 Le, 74.70. Kn 1. INTRODUCTION Isostr uctur a l ra dica l ca tion sa lts of t he -( BEDT -T T F )2Cu [N(C N) 2]X fa mily, wher e BEDT-TTF is an organic -donor, bis(ethylenedithio)tetrathiafulva lene, = r, Cl, I, Br 1-xClx, Br1-xIx, have intensely been studied over past years1-12. They demonstrate a wide variety of electron properties despite of their similar crystal structures. These compounds are layered materials built of conducting radical cation BEDT TTF la yer s, which a lter na te with dielectr ic ones composed of singlechar ged {Cu[N(CN)2]X}- anions1-5. The anion sheet consists of polymeric zig-zag chains extended along the dir ection, which involve a flat thr eecoor dina ted Cu + ion wit h t w o br idge d [ ( N C ) N( C N ) ] - di c yanami d e ( dc a ) gr ou p s a n d a t er mi n a l ha l o g e n X atom. The radical cation layer is formed by pairs of the BEDT-TTF molecules with average +0.5 charge per molecule, which are packed perpendicular to each other in a crystal. According to theor etica l calculations of band structure1-3, these mater ia ls ar e expected to be meta ls. It was found that ra dica l cation sa lts with =r, r 0.510.5 a nd r0.7l0.3 are ambient pressure organic superconductors with = 11. 6 1, 5, while salts with = 1 and Br0.9I0.1 undergo a superconducting transition at 0.3 kbar with c = 12.8 K and 3.5 , respectively4,5, and a salt with = I exhibits a superconducting transition at 1. 2 kba r wit h 8 6. A r a dica l ca tion sa lt wit h X = 1 (hereinafter denoted as "-1") retains its semiconducting properties below 100 K at a mb ient pr essur e, a nd nea r 40


E.B. Yagubskii, N.D. Kushch, A.V. Kazakova, L.I. Buravov, V.V. Zverev, A.I. Manakov, S.S. Khasanov, R.P. Shibaeva
it undergoes a dielectric transition identified as an antiferromagnetic one, and below 22 thes e cr ysta ls wer e fou nd to exhibit wea k f er r oma gnet is m4, 7,8 . When var ying pr essur e above severa l hundr eds of ba rs, the cr ystals of the -1 salt showed a rich phase diagra m with a paramagnetic dielectric, antiferroma gnetic dielectric, metallic and superconducting phases7,9-11. Moreover, the existence of two structural phase transitions at room temperature and high pr essure was found for these crystals by a n X-ray diffraction method: a r ever sible tr a nsition a ccomp a nied by s ymmetr y lower ing a t pr essur e of 8.8 kbar and the second transition at 12 kbar, characterized by a reversible disappearance of Bragg reflections consistent with pressur e-induced amorphization12. Here, we present new intriguing properties of the -1-salt: single crystals of the salt were prepared (hereinafter denoted as "'-1"), which in contrast to the described Mott insulator -Cl have metallic properties and undergo a superconducting transition with = 11.5 at ambient pressure. X-ray analysis of these crystals was perfor med and transport properties wer e studied. 2. RUSULTS AND DISCUSSION The -Cl crystals are conventionally synthesized using the method of electro-oxidation of BEDT-TTF in a 1,1,2-trichloroethane (TCE) ­ absolute ethanol (9:1) mixture in the pr esence of either two- or three-component electrolytes comprising Cu+ salts (CuCl or Cu(dca))4,13. In the course of our investigations related to the synthesis of BEDT-TTF salts with magnetic coordination anions of dicyana mide complexes of divalent transition metals (Mn2+, Cu2+, Co2+), we studied the electro-oxidation of BEDT-TTF in the presence of Mn(dca)2 a nd Cu(dca)2 in TCE containing 10 vol% of 96% ethanol at a current of 0.3-0.5 µA. For Mn dicya na mide, the crystals of the BEDT-TTF salt with a paramagnetic [Mn(dca)3]- anion, (BEDTTTF)2[Mn(dca)3], wer e obtained, wher eas in the case of Cu dicyana mide the crystals of '-Cl salt wer e for med together with the (BEDT-TTF)2CuCl2 ones14 as a minor product. The generation of Cu+ cations, which ar e necessary to for m the {Cu[N(CN)2]X}- anion, is probably due to the chemical oxidation of BEDT-TTF by Cu(dca)2. It is known that Cu2+ oxidizes BEDT-TTF15. To slow down the possible chemical oxidation, the BEDT-TTF and Cu(dca)2 wer e placed in differ ent compartments of a n electrochemical cell. The source of Cl- anions was TCE16,17. The main cr ystallographic parameters for '-1 are: rhombic symmetry, space group P n m a, a = 12.932 ( 2 ) е , b = 29 .877 ( 5 ) е , = = 8. 45 8( 1) е, Z = 4, V = 3267.8( 9 ) е 3 . 4567 independent reflections with I 2 (1 ) we r e col l e ct e d o n a Enraf-Nonius CAD4 diffractome ter. The structure was


solved by a direct method and refined by the least-square method in anisotropic approximation to R1 = 0.056. The crystals of '- 1 have almost the sa me structure as the Mott insulator -14. The analysis of the crystal structure of '-1 at room temperature revea led the following differences between '-1 and -1. 1. It is known that ther e is some disor der ing in the radica l cation layer in -1 crystals at room temperatur e, which is due to the equipr obable presence of two confor mations (eclipsed and stagger ed) of ter mina l ethylene groups of the BEDT-TTF molecule. Full ordering of these groups takes place at low temperature and the BEDT-TTF molecule has only eclipsed confor mation3. The ratio of eclipsed and stagger ed confor mations in '-1 crystals is 0.8:0.2 at room temperature. 2. The unit cell volume of '-1 crystals, V = 3267.8(9) е3, is less tha n that of -1 cr ysta ls4, V = 3299(1) е3, and is even less than the volume of the latter ones, V = 3285.2(8) е3, at 1.2 kbar pr essure12. Corr espondingly, all inter molecular S ...S conta cts ar e shor ter in a conducting la yer that r esults in enha nced inter molecular intera ctions a nd, as a consequenc e, to a wider conductivity ba nd. T his effect is equiva lent to c hemica l compr ession. 3. The r efinement of the structur e r evea led incomplete population of the position of a Cu atom (~ 5%) at completely populated positions of other atoms. T his ca n t estif y tha t Cu2+ is present in addition to Cu+ in the a nion sheets of '- 1 crystals. T he electror esista nce was measur ed using a four-probe technique by a lock-in detector at 20 Hz alternating current. T he s a mp l es w er e t h i n p la t es wit h 1 0.3 0.02 mm3 character istic sizes. The surface was or iented along conducting la yers (the plane). Two contacts were made to each of two opposite sample surfaces with conducting graphite paste. Sample resistance was measured when current was passed both parallel (J||(ac)) a nd perpendicular (J||b ) t o c ondu cting la yer s. T he va lu e of t he cur r ent (J ) r unning t hr ou gh t he sa mp le wa s fixed and was not higher than 10 µA. superconducting solenoid, which generated the field of up to 17 T was used for exper iments in ma gnetic fields. In test exper iments a dyna mic susceptibility was also studied at 100 kHz frequency. The resistivity anisotropy (b/) calculated by the modified Montgomer y method18 was found to be within 200-400 depending on the sample at = 300 and monotonically increased with the temperature decrease attaining the value of 1000-1400 at 15 (F ig. 1). T he temperatur e dependences ha d a positive der ivative in the whole temp eratur e range below 300 K for bot h longitudina l a nd transver se r esista nces (F ig.1). A sma ll portion with a negative der ivative was obser ved only at low temperatur e near a super conducting tra nsition a nd was mor e clear ly pr onounced for tra nsvers e

Ambient pressure superconductivity in -(BEDTTTF)2Cu[N(CN)2]Cl single crystals


E.B. Yagubskii, N.D. Kushch, A.V. Kazakova, L.I. Buravov, V.V. Zverev, A.I. Manakov, S.S. Khasanov, R.P. Shibaeva
resista nce. We tested severa l sa mples obta ined dur ing the sa me synthesis. The data obta ined for differ ent sa mples wer e similar. The data for one of these sa mples ar e pr esented below.

18 1200 16 14 10
(Ohm·cm)

b /ac

800

b b/ac

12 8 6 4 2 200 250 0

400

ac x100
0 0 50 100 150

longitudina l (ac) a nd transverse (b) resistivities a nd the resistivity a nisotr opy (b/ac).

Fig.1. Temperature dependences for

T (K)

Fig.2 shows the R(T) dependence at current running along conducting layers and the temperature dependenc e of dyna mic susceptibility at < 15 . It is seen tha t ther e is a super conducting transition in the R(T ) dep endenc e with the cr itica l temp er a tur e = 11. 5 det er mined at one ha lf of the nor ma l state r esista nce level. A dyna mic susceptibility transition starts approximately at 1 1 K in t he s a me s a mp l e.
0.4 0.3
(arb.un its)

1 0 -1

15
Re() Im()

R ()

C2

-3 -4

(T)

0.2 0.1 0.0

-2

10
H//b

H//(ac)

H

-5

5
5 10
T (K)

15

6

8

10
T (K)

12

14

0

0

2

4

6
T (K)

8

10

Fig.2. Superconducting transit ion in '-1 register ed by resistance and
dyna mic susceptibility (in insert).

Fig.3. Hc2(T) curves for the field both parallel and perpendicular to conducting layers.


The application of ma gnetic field r esulted in lower ing , and a n essentia l br oa dening of a super conducting transition took pla ce in J ||() geometr y, while at J (ac ) the tr a nsition shifted to low er temp er a t ur es a nd no not ic ea b le br oa dening was observed. For this reason, the 2() dependences presented above were plotted for transverse resistance measurements, i.e. when the current was passed along the nor mal direction to the conducting layers. The absence of a notable broadening in a magnetic field suggest that we deal with the dependence 2(T) rather than with an "irreversibility line". In Fig.3, the dependence 2(T) is presented for two orientations of the magnetic field. All the points except one were obtained from the R(T) cur ves at fixed . The last point in the lower curve was obtained from the R(H) curve at = 1.2 . Of special attention is a positive curvature of the c2() dependences a nd the fa ct tha t at ||b the dHc2 /dT der iva tive is close to zero at the point = (0) . W hen ma gnetic field is a long the () plane, this derivative is finite and equal to dHc2 /dT |Tc(0) = -1.36 T/K. The appearance of superconductivity in the '-1 crystals is probably due to the effect of "chemical compression". However, it should be noted that a possible presence of Cu2+ in the anion sheets can also result in the cha nges in a conductivity ba nd filling as it wa s found in the ca se of the super conducting '-(BEDT-TTF)2Cu2(CN)3 sa lt19. The study of the EPR spectra of '-1 can provide with the infor mation on the presence of u2+ in these crystals. The EPR investigations are in progr ess. 3. CONCLUSIONS For the first time the crystals of '-(BEDT-TTF )2Cu[ N (CN) 2 ]1 ('-1) radical cation salt, which exhibit metallic conductivity a nd are a mbient pressure superconductors, wer e synthesized. The crystals showed some distinctions in structure as compared to that of the Mott insulator -l: sma ller size of the unit cell - "chemical compr ession" and deficiency ( 5%) of the occupancy of copper positions. The appearance of superconductivity in the '-1 crystals is probably due to the effect of "chemical compression". A nontrivial temperature dependence of upper critical field was obser ved for '-1 ones: the 2() curves have positive curvature wit h field dir ected both in the conducting plane and perpendicular to it. ACKNOWLEDGEMENTS The work was partly supported by Russian Foundation for Basic Research: grants #03-02-16926, # 04-02-17358, # 05-02-16980, RFBR-DFG #03-02-04023, and the P-28 program of Presidium of RAS.

Ambient pressure superconductivity in -(BEDTTTF)2Cu[N(CN)2]Cl single crystals


E.B. Yagubskii, N.D. Kushch, A.V. Kazakova, L.I. Buravov, V.V. Zverev, A.I. Manakov, S.S. Khasanov, R.P. Shibaeva
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