PHYSICAL REVIEW 8 VOLUME 44, NUMBER 13 1 OCTOBER 1991-I Anisotropic thermopower of the organic superconductor a. -(BEDT-TTF)2Cu[N(CN)z]Br R. C. Yu Department of Physics and Material Research Laboratory, Uniuersity of Illinois, Urbana, Illinois 6180I J. M. Williams, H. H. Wang, J. E. Thompson, A. M. Kini, and K. D. Carlson Chemistry and Materials Science DiUisions, Argonne Rational Laboratory, Argonne, Illinois 60439 J. Ren and M. -H. Whangbo Department of Chemistry, North Carolina State Uniuersity, Raleigh, North Carolina 27695-8204 P. M. Chaikin Department of Physics, Princeton Uniuersity, Princeton, Netu jersey 08544 and Exxon Research and Engineering Company, Annandale, Rem Jersey 08801 (Received 15 March 1991; revised manuscript received 19 June 1991) The thermopower of the organic superconductor «. -(BEDT-TTF)zCu[N(CN}z]Br single crystals has been measured in two crystallographic directions a and c within the most conducting organic donor molecule plane. [Here BEDT-TTF represents his(ethylenethio)-tetrathiafulvalene. ] While the thermo- power in the a direction is positive, the thermopower in the c direction is negative. The drastic anisotro- py in thermopower reveals that the carriers in the a direction are holelike, whereas the carriers in the c direction are electronlike. A calculation based on the tight-binding electronic band structure is able to describe the temperature dependence of the anisotropic thermopower, but with a much reduced band dispersion. There is a renewed interest in the organic superconduc- tors stimulated by the discoveries of new organic super- conductors a-(BEDT- TTF)2X with superconduct- ing transition temperatures in the range 10 — 13 K (where BEDT-TTF represents his(ethylenedithio)- tetrathiafulvalene; X is the anion]; with X=Cu(SCN)2 Cu[N(CN)2]Br, and Cu[N(CN)2]C1, the transition temperatures are 10. 4, 11. 6, and 12. 8 K, respectively' ). While critical-field and transport studies have revealed the quasi-two-dimensionality of these salts, the anisot- ropy within the most conducting plane has been much less studied. In this paper we report our measurements and calculations of the thermop ower of Ir-(BEDT-TTF)2Cu[N(CN)2]Br single crystals in the a and c crystallographic directions. The thermopower in both directions was measured from 300 to 4 K. While the thermopower in the a direction is positive, the ther- mopower in the c direction is negative. These data reveal that the carriers in the a direction are holes, whereas the carriers in the c direction are electrons. A calculation of the anisotropic thermopower within the most conducting plane by utilizing a tight-binding model reproduced the general behavior of the thermopower, although with a much reduced band dispersion. The Ir-(BEDT-TTF)2Cu[N(CN)2]Br single crystal sam- ples were grown at Argonne National Laboratory by an electrochemical synthesis technique, which is described elsewhere. The typical geometric shape of the crystals is that of a rhombic plate with the longer diagonal axis be- ing the a direction and the shorter axis being the c direc- tion. The typical dimensions of the single crystals is 0.8 mm for each side of the rhombus and 0. 1 mm for the thickness. The thermopower of the single-crystal samples was measured by the following technique. One end of the sample was thermally anchored to a single-crystal quartz block attached to the temperature-controlled heat sink. The other end of the sample was suspended and attached with a thin-film heater with dimensions of 1 XO. 5 X0. 2 mm . Two junctions of a Cromel-Constantan difFerential thermocouple were attached to the two ends of the sam- ple by use of GE varnish. Two 12. 5-pm gold wires were attached to the two ends of the sample with silver paste, serving as the voltage leads. Another two gold leads may also be attached to the sample to allow four-terminal ac resistance measurements. The generation of a tempera- ture gradient of 0.5 K and the measurement cycle were computer controlled, and the digitally acquired data are averaged over several cycles. The estimated uncertainty in magnitude of thermopower is less than 15%. In Fig. 1 we show the temperature dependence of the resistance of one sample. When the temperature de- creases from 300 K, the resistance increases. It reaches a peak value 50% more than the room-temperature value at 100 K. When the temperature further decreases, the resistance decreases drastically by more than an order of magnitude before the sample superconducts at about 12 K. The nonmetallic behavior of the resistance above 100 K is reminiscent of the resistivity observed in lr-(BEDT- TTF) zCu(NCS) 2. ' In the inset of Fig. 1, we show the resistance of the 1991 The American Physical Society