J. Caulfield

Magnetotransport studies of the organic superconductor kappa -(BEDT-TTF)2Cu(NCS)2 under pressure: the relationship between carrier effective mass and critical temperature

Magnetotransport measurements have been carried out on the organic superconductor kappa -(BEDT-TTF)2Cu(NCS)2 at temperatures down to 500 mK and in hydrostatic pressures up to 16.3 kbar. The observation of Shubnikov-de Haas and magnetic breakdown oscillations has allowed the pressure dependences of the area of the closed pocket of the Fermi surface and the carrier effective masses to be deduced and compared with simultaneous measurements of the superconducting critical temperature Tc. The effective mass measured by the temperature dependence of the Shubnikov-de Haas oscillations is found to fall rapidly with increasing pressure up to a critical pressure Pc approximately=5 kbar. Above Pc a much weaker pressure dependence is observed; Tc also falls rapidly with pressure from 10.4 K at ambient pressure to zero at around Pc. This strongly suggests that the enhanced effective mass and the superconducting behaviour are directly connected in this organic superconductor. A simplified model of the band structure of kappa -(BEDT-TTF)2Cu(NCS)2 has been used to derive the bare band masses of the electrons from optical data. Comparisons of these parameters with cyclotron resonance data and the effective masses measured in the present experiments indicate that the greater part of the enhancement of the effective mass necessary for superconductivity in this material is due to quasiparticle interactions, with the electron-phonon interactions playing a secondary role. The dependence of Tc on the effective mass may be fitted satisfactorily to a suitably parametrized weak-coupling BCS expression, although this cannot be taken as a definitive proof of the nature of superconductivity in organic conductors.

Magnetic breakdown and quantum interference in the quasi-two-dimensional superconductor in high magnetic fields

Magnetic breakdown phenomena have been investigated in the longitudinal magnetoresistance of the quasi-two-dimensional (Q2D) superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2) in magnetic fields of up to 50 T, well above the characteristic breakdown field. The material is of great interest because its relatively simple Fermi surface, consisting of a closed Q2D pocket and an open Q1D band, is almost identical to the initial hypothetical breakdown network proposed by Pippard. Two frequencies are expected to dominate the magnetoresistance oscillations: the a frequency, corresponding to orbits around the closed pocket, and the beta frequency, corresponding to the simplest classical breakdown orbit. However, a beta - alpha frequency is in fact found to be the dominant high-frequency oscillation in the magnetoresistance. Numerical simulations, employing standard theories for calculating the density of states, indicate that a significant presence of the beta - alpha frequency (forbidden in the standard theories) can result simply from the frequency-mixing effects associated with the pinning of the chemical potential in a quasi-two-dimensional system. While this effect is able to account for the previous experimental observation of beta - alpha frequency oscillations of small amplitude in the magnetization, it cannot explain why such a frequency dominates the high-field magnetotransport spectrum. Instead we have extended the numerical simulations to include a quantum interference model adapted for longitudinal magnetoresistance in a quasi-two-dimensional conductor. The modified simulations are then able to account for most of the features of the experimental magnetoresistance data.

Organic ferromagnetism in the nitronyl nitroxides p-NPNN and 3-QNNN: MUSR, EPR and a.c. susceptibility studies

Following the discovery of organic ferromagnetism in p-NPNN, we report the observation of ferromagnetic ordering in a second nitronyl nitroxide radical system, 3-quinoyl nitroxide (3-QNNN), using the muon spin rotation (MUSR) technique. The onset of ferromagnetic order is indicated in the MUSR experiment by the appearance of a rotation signal in zero applied field for temperatures below ∼0.2 K. This signal corresponds to the precession of the muon spin in the internal field of the ferromagnetic state. We have measured the muon spin rotation and relaxation signals as a function of temperature, giving a measure of the magnetization and spin dynamical properties. The temperature dependence of the internal field is consistent with the S = 12 Weiss molecular field model with a Curie temperature of 0.21 K and a saturation internal field of 60 G. We also report additional studies of the magnetic properties of 3-QNNN and p-NPNN using EPR and a.c. susceptibility techniques. We find evidence for a canted ferromagnetic spin structure in 3-QNNN and a simpler uniform magnetic lattice in p-NPNN.

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.

Quantum oscillations near Bc2 in the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2

Abstract We report a study of the de Haas-van Alphen effect near the upper critical field in superconducting κ-(BEDT-TTF) 2 Cu(NCS) 2 . The experiments have been carried out for a range of sample orientation with respect to the magnetic field and temperatures down to 20 mK. On entering the vortex state, an attenuation of the dHvA signal is observed relative to an extrapolation of the standard Lifshitz-Kosevich formula, using band parameters measured in the normal state.

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 ).

Magnetic breakdown and quantum interference in the quasi-two-dimensional superconductor κ-(BEDT-TTF)2Cu(NCS)2

Abstract With the aid of numerical modelling and recent pulsed magnetic field experiments, we discuss the applicability of the established magnetic breakdown and quantum interference theories to κ-(BEDT-TTF)2Cu(NCS)2. We pay particular attention to the possible origin(s) of the ‘β-α’ frequency, which is forbidden in the classical breakdown picture.

De Haas-van Alphen oscillations near Bc2 in the organic superconductor κ-(ET)2Cu(NCS)2

Abstract We report a study of the de Haas-van Alphen effect near the upper critical field in the organic superconductor κ-(ET) 2 Cu(NCS) 2 . The experiments have been carried out for a range of sample orientations with respect to the magnetic field and of temperatures down to 20 mK. On entering the vortex state, an attenuation of the dHvA signal is observed relative to an extrapolation of the standard Lifshitz-Kosevich formula using band parameters measured in the normal state.

Fermi surface studies of low-dimensional organic conductors based on BEDT-TTF

Abstract This paper provides an introduction to charge-transfer salts of the ion bis(ethylenedithio)tetrathiafulvalene (ET) and their band-structure, and reviews some recent experiments on the salts involving high magnetic fields carried out by the Oxford group and coworkers.

Fermi surface studies of the pressure induced organic superconductor (ET)3Cl2.2H2O

Abstract The effects of temperature, pressure and magnetic field on the electrical transport of single crystal (ET)3Cl2.2H2O are reported. Increasing pressure gradually reduces the ordering temperature of a charge density wave ground state from ∼160 K at 1 bar to 6 K at 10.2 kbar. A superconducting state with TC>4 K is stabilised between 10.2 kbar and 13.5 kbar. Above 12.5 kbar, the observation of Shubnikov-de Haas oscillations allows the pressure dependences of the area of a closed Fermi surface pocket and the associated carrier effective mass to be deduced.