Non-ohmic electrical transport in the Peierls-Mott state of deuterated copper-DCNQI systems

1. Non-ohmic electrical transport in the Peierls-Mott state of deuterated copper-DCNQI systems T. Vuletić1, M.Pinterić1,2, M. Lončarić1, S. Tomić1, J. U. von Schütz3 contact e-mail: tvuletic@ifs.hr 1 Institute of Physics, Zagreb, Croatia 2 Faculty of Civil Engineering, University of Maribor, Slovenia 3 3.Physikalisches Institut, Universität Stuttgart, Germany d8 system: Cu[2,5(CD3)2-DCNQI]2 h8/d6 70%:30% system: Cu[(2,5(CH3)2)0.70(2,5(CD3)2)0.30-DCNQI]2 Since 1987 an important amount of research has been devoted to a novel class of charge-transfer salts [R1R2-DCNQI]2Cu where DCNQI stands for dicyanoquinonediimine and R1, R2 = CH3, CH3O, Cl, Br, I etc.), due to the possibility to control easily, by varying external (pressure, magnetic field) and internal (isotope substitution, doping) parameters, unique physical properties inside an extremely rich phase diagram [1,2,3]. Compared with conventional organic metals, the novel electronic states have emerged associated with the hybridization between p-orbitals of DCNQI molecule and d-orbitals of Cu ions [4]. We have used DC electrical transport measurements in order to investigate N=3 CDW in the Peierls-Mott insulating state of the fully deuterated d8 system and the partially deuterated h8/d6 70%:30% system. We correlate observed features of the electric conduction below and above threshold field with the temperature evolution of the N=3 CDW order as detected by the low-frequency dielectric measurements [5]. h8/d6 Single-particle transport The observed resistance Rxxvs. temperature Tcurves have reproduced the previous ones given in the literature [6]. For both systems resistance is normalized to metallic state values just above TC1warm. Due to deuteration two systems are representative in different temperature ranges, while the general behavior is similar. Experimental Technique The measurements were performed on single crystals with varying lengths of about 2 mm and cross sections of typically 0.01 mm2. All measured samples exhibited qualitatively the same behaviour. The RT conductivity sRT was in the range 800 - 1200 Scm-1. The DC resistance measurements were performed using a standard DC technique or Keithley 617 electrometer in V-I mode. d8 References [1] H.P. Werner, J.U. von Schütz, H.C. Wolf, R. Kremer, M. Gehrke, A. Aumüller, P. Erk and S. Hünig, Solid State Commun. 65 (1988) 809. [2] S. Tomić, D. Jérome, A. Aumüller, P. Erk, S. Hünig and J.U. von Schütz, J. Phys. C.: Solid State Phys. 21 (1988) L203. [3] R. Kato, H. Sawa, S. Aonuma, M. Tamura, M. Kinoshita and H. Kobayashi, Solid State Commun. 85 (1993) 831. [4] T. Ogawa and Y. Suzumura, Phys. Rev. B 53 (1996) 7085. [5] M.Pinterić, T. Vuletić, M. Lončarić, S.Tomić, J. U. von Schütz, to appear in Eur. Phys. J. B (2000). [6] D. Gomez, J.U. von Schütz, H.C. Wolf and S. Hünig, J. Phys. I France 6 (1996) 1655. [7] M. Pinterić, M. Miljak, N. Biškup, O. Milat, I. Aviani, S. Tomić, D. Schweitzer, W. Strunz and I. Heinen, Eur. Phys. J. B 11 (1999) 217. [8] S. Tomić, M.Pinterić, T. Vuletić, J. U. von Schütz, D. Schweitzer, these Proceedings [9] M. Pinterić, N. Biškup, S. Tomić, J. U. von Schütz, Synth. Metals 103 (1999) 2185. CDW -- nonlinear transport h8/d6 d8 d8 Electric field dependent conductivity h8/d6 As an illustration, we show the field-dependent conductivity normalized to its Ohmic value at a few selected temperatures, for both systems. We point out that the nonlinear effect is quite small, but the threshold field ET is clearly defined. The results given for the h8/d6 system are for the sample denoted Sample 1 in the next figure. Threshold field ETand (s2ET-s0)/s0, nonlinear effect (non-ohmic conductivity at twice the threshold field, normalized to its ohmic value), for . system and 70\%:30\% system, for which we compare two different single crystals. For the d8 system ET decreases below TC1warm, reaching a minimum value of about 400 mV/cm around 65K, and then increases again towards low temperatures. The nonlinear effect is constant in the broad T-range between 55 K and 75 K and starts to increase at lower temperature. For the h8/d6 70\%:30\% system there is equally pronounced increase of ET at low temperatures. However, the magnitude of this feature appears to be sample dependent. For this system we did not observe divergence of ET at temperatures close to TC1warm. Again, as in d8 system, the magnitude of nonlinear effect decreases towards TC1warm. h8 /d6 d8 Ohmic and Non-Ohmic conductivity We show the ohmic conductivity and non-ohmic conductivity at twice the threshold field vs.inverse teperature 1/T. The preferred choice for the fit (full lines) was Mott's variable range hopping (VRH) formula, which applies when the dominant conduction mechanism becomes the conduction by the carriers localized on impurities. The collective and single particle conductivities are closely related since they both obey the same temperature dependence. This behavior and the T-independent behavior of the mean relaxation time [5], are in accordance with an extremely low free electron density in the insulating state. Using the observed values of ET we estimated the CDW characteristic length to be 0.1-1 mm, for both systems. The collective and single particle conductivities are closely related since they both obey the same temperature dependence. CONCLUSION Electric-field-dependent measurements have revealed clear, albeit weak in magnitude, non-linear characteristics above large threshold fields of the order of 1 V/cm. At low temperatures the threshold field was found to increase substantially reaching values between 10 and 100 V/cm, concomitantly with the increase in the magnitude of nonlinear effect. It should be noted that this represents a feature not usually encountered in CDW and SDW. However, we have already reported a qualitatively same behavior in commensurate SDW of k-(BEDT-TTF)2Cu[N(CN)2]Cl [7]. A common aspect of these two systems is a domain structure of DW ground state [8]. On the other hand, for a DW in an incommensurate structure a rise of ET followed by a disappearance of the nonlinear effect, once the free-electron screening becomes uneffective, would be expected. Indeed, we have observed such a behavior in N=4 CDW of DCNQI-Li system [9]. We have suggested that metallic islands persisting below Peierls-Mott transition act as charged domain walls in the random domain commensurate structure characterized with broad dielectric response. This scenario we have correlated with lower threshold field and nonlinear effect observed there. Further, we correlate an important rise of the threshold field at low temperatures outside hysteretic region with the Debye relaxation, observed in the same temperature range, and we suggest that both are the manifestation of the N=3 CDW long range order established outside the hysteretic region. To our knowledge this might be the first opportunity to study experimentally the theoretically intriguing issue of the collective DC (and AC [5]) CDW dynamics in the random domain commensurate structure in an organic p - inorganic d hybrid conductor. d8 h8/d6