Search

We investigate the dc resistivity and Hall effect of the quasi-two-dimensional organic materials κ–(ET)2X, where X=Ag2(CN)3 and B(CN)4 and compare them with the results for X=Cu2(CN)3. All three compounds are considered to be quantum-spin-disordered Mott insulators. Despite high similarities in chemical composition and crystal structure, large differences in the dc resistivity and Hall coefficient are found. While around room temperature the dc transport properties are dominantly determined by the strength of the electron correlations, upon reducing the temperature, dc transport happens by hopping due to inherent disorder. The most disordered compound with X=Cu2(CN)3 turns out to have the lowest dc resistivity and the highest charge carrier density, i.e., in the phase diagram it is located closest to the metal-insulator transition. The least disordered compound with X=B(CN)4 shows the highest resistivity and the lowest carrier density, i.e., lies farthest from the metal-insulator transition. We explain such counterintuitive behavior within the theory of Mott-Anderson localization as a consequence of disorder-induced localized states within the correlation gap.

α-(BEDT-TTF)2I3 is a prominent example of charge ordering among organic conductors. In this work, we explore the details of transport within the charge-ordered as well as semimetallic phase at ambient pressure. In the high-temperature semimetallic phase, the mobilities and concentrations of both electrons and holes conspire in such a way to create an almost temperature-independent conductivity as well as a low Hall effect. We explain these phenomena as a consequence of a predominantly interpocket scattering which equalizes mobilities of the two types of charge carriers. At low temperatures, within the insulating charge-ordered phase two channels of conduction can be discerned: a temperature-dependent activation, which follows the mean-field behavior, and a nearest-neighbor-hopping contribution. Together with negative magnetoresistance, the latter relies on the presence of disorder. The charge-ordered phase also features a prominent dielectric peak which bears a similarity to relaxor ferroelectrics. Its dispersion is determined by free-electron screening and pushed by disorder well below the transition temperature. The source of this disorder can be found in the anion layers which randomly perturb BEDT-TTF molecules through hydrogen bonds.

alpha-(BEDT-TTF)2I3 is a quasi-2D organic conductor most famous for its prominent charge order within molecular planes below TCO =136 K [1]. Due to the breaking of an inversion center, the response at the charge ordering transition should have a ferroelectric character [2, 3]. Puzzlingly, at the moment it is unclear whether the ferroelectric peak appears in the dielectric properties at all [4]. Furthermore, the low-lying excitations above the ground state seem to bring forth both a phason-like as well as a soliton-like relaxation, which have so far not been reported to appear together in similar systems [5]. In order to explore these open questions, we have carefully determined the direction-dependent (in-plane and out-of-plane) dielectric response in alpha-(BEDT-TTF)2I3 together with resistance and Hall effect measurements down to lowest temperatures. The temperature dependence of the Hall coefficient absolute value follows a similar behavior as the resistivity. An intriguing feature is found at TCO where the Hall coefficient changes sign. Another interesting property is a change in the in-plane transport activation energy around 60 K. On the other hand, dielectric measurements give strongly anisotropic and complex spectra. Our results are discussed within the wider context of strongly correlated systems with charge order. [1] T. Takahashi, Y. Nogami, and Kyuya Yakushi, J. Phys. Soc. Jpn. 75, 051008 (2006). [2] T. Kakiuchi, Y. Wakabayashi, H. Sawa, T. Takahashi, and T. Nakamura, J. Phys. Soc. Jpn. 76, 113702 (2007) [3] K. Yamamoto, A. A. Kowalska, and K. Yakushi, Appl. Phys. Lett. 96, 122901 (2010). [4] T. Ivek, B. Korin-Hamzić, O. Milat, S. Tomić, C. Clauss, N. Drichko, D. Schweitzer, and M. Dressel, Phys. Rev. Lett. 104, 206406 (2010) ; P. Monceau, private communication (2014). [5] T. Ivek, B. Korin-Hamzić, O. Milat, S. Tomić, C. Clauss, N. Drichko, D. Schweitzer, and M. Dressel, Phys. Rev. B 83, 165128 (2011).

Upon cooling the highly anisotropic organic conductors (TMTTF) 2 X undergo transitions to various ordered states in the charge and spin degrees of freedom and therefore turn out to be an excellent choice for exploring the phenomena of low-dimensional physics. In order to study the transport mechanism of these quasi-one-dimensional (1D) organic salts, the temperature-dependent Hall effect of (TMTTF) 2 X (X=BF 4 , ReO 4 , AsF 6 , SbF 6 ) has been measured by an AC compensation technique with current injection along the high-conducting a-axis. At elevated temperatures the Hall coefficient R H of all investigated compounds seems to be constant, positive and somehow enhanced compared to the calculated value for one hole per unit cell. Beyond the charge order phase transition the Hall coefficient follows similar exponential laws as the resistivity data. This indicates the behavior of a semiconductor, where the change in ρ(T) mainly originates from the change in the number of carriers. Here we discuss how the Hall coefficient is influenced by anisotropy, localization effects, anion symmetry and charge- as well as anion-ordering phenomena at low temperatures. We compare the results to resistivity studies and theoretical expectations.

We have in detail characterized the anisotropic charge response of the dimer Mott insulator κ−(BEDT-TTF)2Cu2(CN)3 by dc conductivity, Hall effect, and dielectric spectroscopy. At room temperature, the Hall coefficient is positive and close to the value expected from stoichiometry; the temperature behavior follows the dc resistivity ρ(T). Within the planes the dc conductivity is well described by variable-range hopping in two dimensions; this model, however, fails for the out-of-plane direction. An unusually broad in-plane dielectric relaxation is detected below about 60 K; it slows down much faster than the dc conductivity following an Arrhenius law. At around 17 K, we can identify a pronounced dielectric anomaly concomitantly with anomalous features in the mean relaxation time and spectral broadening. The out-of-plane relaxation, on the other hand, shows a much weaker dielectric anomaly; it closely follows the temperature behavior of the respective dc resistivity. At lower temperatures, the dielectric constant becomes smaller both within and perpendicular to the planes; also, the relaxation levels off. The observed behavior bears features of relaxorlike ferroelectricity. Because heterogeneities impede its long-range development, only a weak-tunneling-like dynamics persists at low temperatures. We suggest that the random potential and domain structure gradually emerge due to the coupling to the anion network.

The only published Hall effect measurements of TTF-TCNQ date back to 1977. They were performed in the high temperature metallic region above the phase transitions, and the results vary in their sign and values. The aim of the present work has been to extend the previous investigations to lower temperatures and higher magnetic fields and to compare the new data with the known results.

I will present our comparative study of the magnetotransport properties of organic conductors (TMTCF)2X (C= Se, S ; X=PF6, ReO4, AsF6..). Their phase diagram spanning from more anisotropic C = S to C = Se series shows the increasing interchain coupling that leads to the crossing-over of the dimensionality. Our results of the conductivity anisotropy, Hall effect and magnetoresistance enabled us to gain information about the spin density wave ground state at low temperature, Fermi liquid or Luttinger liquid behavior in the normal phase at high temperatures, possible dimensional crossover and phase transitions. The results are discussed in the framework of the existing theoretical models. I will also present our results of the Hall effect measurements of quasi-one-dimensional cuprates Sr14-xCaxCu24O41 ( 0 ≤ x ≤ 11.5) that have been conducted in order to determine the number of carriers participating in dc transport, and its relationship with doping.

We present Hall-effect measurements of two-leg ladder compounds Sr14− xCaxCu24O41 (0

We present Hall effect measurements of two-leg ladder quasi-one-dimensional copper oxides Sr14-xCaxCu24O41 (0 ≤ x ≤ 11.5). In these composite materials charge and spin ordering occurs within chain and ladder subsystems [1]. They have been investigated intensively during the last years due to their fascinating physical properties, especially after the discovery of superconductivity under pressure (for x > 10). They are the first superconducting copper oxides without the CuO2 planes, characteristic for high-temperature superconductors. The question of charge dynamics and hole distribution between the ladders and chains is still open, because the different experimental techniques have provided contradictory results [2-5]. Our Hall effect and electric resistivity results show typical semiconducting behavior for x < 10. We have determined the effective number of carriers and its dependence on composition (x) and temperature (T) and compared it to the number of holes in the ladders obtained by other experimental techniques [2-5]. Our results give around one carrier per formula unit for x=0, which increases slightly with x. The x=11.5 compound shows metallic behavior and our results match qualitatively Hall effect behavior found in HTC [6]. [1] T. Vuletić, et al., Phys. Rep. 428, 169 (2006). [2] T. Osafune, et al., Phys. Rev. Lett. 78, 1980 (1997). [3] N. Nücker, et al., Phys. Rev. B 62, 14384 (2000). [4] Y. Piskunov, et al., Phys. Rev. B 72, 064512 (2005). [5] A. Rusydi, et al., Phys. Rev. B 75, 104510 (2007). [6] For a review see: N. E. Hussey, J. Phys: Condens. Matter 20, 123201 (2008).

We report the observation of the large Hall effect in the incommensurate spin-density wave (SDW) phase of (TMTSF)_2NO_3 and negative magnetoresistances in the semiconducting state above and below the commensurate SDW phase transition of (TMTTF)_2Br.