M. Doporto

The effects of open sections of the Fermi surface on the physical properties of 2D organic molecular metals

Abstract We report two cases in which the quasi-one-dimensional (Q1D) sections of Fermi surface (FS) in ET salts contribute to the low temperature behaviour of the magnetoresistance (MR). In κ-(ET)2Cu(NCS)2, breakdown orbits are observed between the Q2D and Q1D sections of FS; the MR oscillations contain a probable contribution from the Stark quantum interference effect. The results for the FS parameters are compared with theoretical calculations and the effects of many-body interactions are deduced. In α-(ET)2KHg(NCS)4, nesting of the Q1D open orbits gives a SDW ground state which considerably modifies the FS and leads to a field-induced transition known as the ‘kink’ between 22 and 23 T. Measurements of MR as a function of orientation of the crystal in the magnetic field reveal that the changes in FS at the ‘kink’ are rather subtle.

Magnetotransport studies of the organic metals (BEDT-TTF)2AuBr2 and (BEDT-TTF)2KHg(SCN)4

Abstract We report magnetoresistance measurements on ET 2 KHg(SCN) 4 and β″ET 2 AuBr 2 . Both show Shubnikov-de Haas oscillation frequencies additional to those expected from band structure calculations, and other magnetoresistance anomalies occur (e.g. hysteresis, “kink” structures). These effects are explained qualitatively by band structure modifications produced by magnetic ordering.

Anomalous negative magnetoresistance in κ-(BEDT-TTF)2Cu(SCN)2

Abstract The organic superconductor κ-(BEDT-TTF) 2 Cu(SCN) 2 shows a region of negative magnetoresistance (NM) just above the upper critical field. In order to gain further information about this effect, we have studied its dependence on a number of parameters. The magnetoresistance has been measured as a function of field direction, sample temperature and current configuration. The temperature dependence of the resistance has also been studied as a function of field applied at different angles and with different current configurations. The NM is found to be much more pronounced when the measuring current is in the inter-layer direction ( j  a ∗ ) and almost absent when the current is in the plane of molecular layers ( j ⊥ a ∗ ). This suggests that the NM is associated with carrier localisation in the inter-layer direction. However, for the former current orientation, the strongest NM is observed when the magnetic field is in the inter-layer direction (i.e. j ∥ B ).

Cyclotron resonance and electron-electron interactions in the organic metal (BEDT-TTF)2KHg(SCN)4

Abstract We report the first cyclotron resonance measurements on (BEDT-TTF) 2 KHg(SCN) 4 , which yield an effective mass of m ∗ cyc =(0.40±0.02)m e at 1.2 K; transport measurements give m ∗ tr ≅2.0 m e . Using Kohns theorem we propose that the enhancement of m ∗ tr over m ∗ cyc is the first direct gauge of the electron-electron interaction in a metallic charge transfer salt.

Magnetotransport studies of (BEDT-TTF)2Cu(SCN)2

Abstract Measurements of the normal state transport properties of (BEDT-TTF) 2 Cu(SCN) 2 are presented and discussed in relation to the band structure. The temperature dependence of both the Hall resistance and the resistance anisotropy within the bc plane and also the configuration dependence of the SdH frequency all point to the inadequacy of the relaxation time approximation and give strong evidence for the presence of scattering mechanisms with temperature dependent anisotropy.

High field magnetotransport studies of β″-(BEDT-TTF)2AuBr2

Abstract We have measured the high-field magnetoresistance (MR) of the quasi-two-dimensional organic metal β″-ET2AuBr2 over a wide range of angles and temperatures. Band structure calculations using the overlaps of Mori et al. yield a Fermi surface (FS) consisting of an open section and only a single closed hole pocket [1] for this material. However, Shubnikov-de Haas (SdH) frequencies are observed in the MR with fundamental fields of 39 T, 141 T, 180 T and 219 T plus harmonics, and so a detailed study of the temperature and angle dependence of the amplitude of the SdH oscillations was carried out in order to clarify the origin of the different frequencies. We propose the existence of a 2c spin density wave modulation driven by the nesting properties of the open section of the FS, and interpret the different SdH frequencies in terms of the modified band structure and the Shoenberg magnetic interaction.

Magnetoresistance studies of βμ-ET2AuBr2

Abstract Low-temperature transverse magnetoresistance (MR) measurements have been made on the organic metal βμ-ET2AuBr2. The MR was found to be highly anisotropic. A 2-band model was used to evaluate the carrier mobilities in the different sections of the Fermi surface. Shubnikov-de Haas (SdH) oscillations of two distinct frequencies were observed for current along the higher resistance direction. Analysis of the field and temperature dependence of these oscillations yields an effective mass of 4–5 me and a Dingle temperature of 0.3–0.5 K. An interpretation of the different oscillation frequencies gives an inter-planar bandwidth of 4tc∼2.2±0.3 meV.