J. Phys. I France
Volume 6, Numéro 12, December 1996
Page(s) 1865 - 1873
DOI: 10.1051/jp1:1996195
J. Phys. I France 6 (1996) 1865-1873

Hall Effect Anomalies and Phase Transitions in the Organic Superconductors, $\mathsf {\kappa}$(BEDT-TTF) $\mathsf{_2}$Cu(NCS) $\mathsf{_2}$ and $\mathsf{\beta}$-(BEDT-TTF) $\mathsf{_2}$I $\mathsf{_3}$

Keizo Murata

Electrotechnical Laboratory, 1-1-4, Umezono, Tsukuba Ibaraki, 305 Japan

(Received 13 May 1996, revised 22 August 1996, accepted 5 September 1996)

The Hall coefficient, $R_{\rm H}(T)$ for organic conductors is strongly temperature dependent even if they are metallic. The temperature dependence of $R_{\rm H}(T)$ is found to exhibit one-to-one correspondence with the structure of the Fermi surface, hence can be called "standard" for each surface. It turned out that the simple temperature-independent $R_{\rm H}(T)$ is only realized in a material where the Fermi surface is round and simple for these low dimensional systems. The "standard" $R_{\rm H}(T)$ for each Fermi surface is obtained empirically by comparing $R_{\rm H}(T)$'s of relatively high pressure, where low temperature novel states or excitations are suppressed. In some cases, $R_{\rm H}(T)$ deviates abruptly from the standard $R_{\rm H}(T)$-behavior, which suggests a phase transition or its precursor. We show the examples of $\kappa$-(BEDT-TTF) 2Cu(NCS) 2 and of $\beta$-(BEDT-TTF) 2I 3 in which the anomaly in $R_{\rm H}(T)$ corresponds to a peak ( $\sim 60$ K) of antiferromagnetic fluctuation, and to 20 K-phase transition which is associated with low- $T_{\rm c}$ superconductivity, respectively. As seen in these two examples, the Hall effect is strongly influenced by the electronic state, and therefore, can be a useful tool for locating phase transitions.

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