W. Hayes

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.

Theory of defects in conducting polymers. I. Theoretical principles and simple applications

The authors describe a method for solving simultaneously the Hartree-Fock equations of motion in the zero differential overlap approximation for the electronic structure of a molecule and the dynamical equations of motion for its atoms. The approach is similar to that of Car and Parrinello (1985) in that they optimize the electronic structure and the geometry simultaneously, but differs in that they make a chosen number of iterations towards electronic self-consistency at each geometry rather than treating the electron wavefunctions as dynamical variables. They give examples of the use of the method to calculate the equilibrium geometries, dipole moments, molecular polarizabilities and vibrational frequencies of small molecules. In a following paper they apply this approach to problems of defect processes in conducting polymers.

Raman scattering of the Li1+xTi2−xO4 superconducting system

We have measured Raman spectra of polycrystalline samples of the Li1+xTi2−xO4 superconducting system (0⩽x⩽13) at room temperature. For LiTi2O4 (x=0), which is a superconductor (Tc=13K), the frequencies of the detected phonon modes are 200, 339, 429, 494 and 628 cm−1, corresponding to the expected five zone-centre Raman active modes for the spinel-type structure. For Li[Li13Ti53]O4(x=13), which is an insulator, we observe five phonon peaks centred on 239, 274, 367, 427 and 675 cm−1. However for intermediate values of x, the scattering peaks broaden; in the range of 0.1 < x < 0.2, the Raman spectra can barely be resolved, consistent with chemical inhomogeneity of Li1+xTi2−xO4 due to a spinodal decomposition.

THEORY OF THE STRUCTURE OF THE SELF-TRAPPED EXCITON IN QUARTZ

Quartz is an insulator with an extremely wide band gap in the vacuum ultra-violet. However, under irradiation from high-energy electrons or X-rays, samples of high purity emit a luminescence band in the blue, corresponding to a Stokes shift of approximately 7 eV. This large Stokes shift has been ascribed to the self-trapping of an exciton in an otherwise perfect lattice owing to the distortion it induces; the authors review the evidence for this assignment, and describe electronic-structure calculations which reveal the structure of the distorted configuration and also explain various experimentally determined properties of the centre. The self-trapping process they postulate is a novel one as it is driven primarily by the electron component of the exciton.

Muon spin relaxation investigation of magnetic ordering in the hybrid organic-inorganic perovskites [(CH3)2NH2]M(HCOO)3 (M=Ni,Co,Mn,Cu)

Muon spin relaxation measurements are reported on samples of dimethylammonium metal formates containing magnetic divalent nickel, cobalt, manganese, and copper ions. These hybrid organic-inorganic perovskites exhibit weak ferromagnetism and are, apart from the copper system, multiferroics with well separated magnetic and antiferroelectric transitions. We use muons to follow the sublattice magnetization, observing the effect of the spin reorientation transitions below TN and the criticality approaching TN. The multiferroic samples have three-dimensional antiferromagnetic interactions, but the copper sample shows quasi-one-dimensional behavior due to its Jahn-Teller distorted structure, with a ratio of its inter- and intrachain exchange constants j/J=0.037.

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.

Raman and infrared determination of vibrational fundamentals of single-crystal C60 and derivatives and of C70

We describe results of Raman and infrared (IR) studies of single crystals of pure C60, CS2 solvated C60 (C601.5CS2), (BEDT-TIF)2 C60 (BEDT-TIF or ET is bis(ethylenedithio)tetrathiafulvalene) and C70. We find that changing the crystalline environment of C60 has little effect upon the vibrational frequencies of the intramolecular vibrations but has some effect on relative intensities of peaks. By comparing our IR and Raman spectra using the published inelastic neutron scattering and surface enhanced Raman data we have been able to identify the complete set of 46 fundamental intramolecular vibrational frequencies of C60 and 64 of the 122 fundamentals of C70. Theoretical calculations for C60 are not accurate enough to completely assign the symmetries of the identified fundamentals. We comment on effects of oxygen and laser power on the Raman spectrum of C60.

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.

A study of the magnetoresistance of the charge-transfer salt at hydrostatic pressures of up to 20 kbar: evidence for a charge-density-wave ground state and the observation of pressure-induced superconductivity

The magnetoresistance of single crystals of the quasi-two-dimensional (Q2D) organic conductor has been studied at temperatures between 700 mK and 300 K in magnetic fields of up to 15 T and hydrostatic pressures of up to 20 kbar. Measurements of the resistivity using a direct-current van der Pauw technique at ambient pressure show that the material undergoes a metal-to-insulator transition at ; below this temperature the resistivity increases by more than five orders of magnitude as the samples are cooled to 4.2 K. If the current exceeds a critical value, the sample resistivity undergoes irreversible changes, and exhibits non-ohmic behaviour over a wide temperature range. Below 30 K, either an abrupt increase of the resistivity by two orders of magnitude or bistable behaviour is observed, depending on the size and/or direction of the measurement current and the sample history. These experimental data strongly suggest that the metal - insulator transition and complex resistivity behaviour are due to the formation of a charge-density wave (CDW) with a well-developed domain structure. The magnetotransport data recorded under hydrostatic pressure indicate that pressure has the effect of gradually reducing the CDW ordering temperature. At higher pressures, there is a pressure-induced transition from the CDW state to a metallic, superconducting state which occurs in two distinct stages. Firstly, a relatively small number of Q2D carriers are induced, evidence for which is seen in the form of the magnetoresistance and the presence of Shubnikov - de Haas oscillations; in spite of the low carrier density, the material then superconducts below a temperature of . Subsequently, at higher pressures, the CDW state collapses, resulting in Q1D behaviour of the magnetoresistance, and eventual suppression of the superconductivity.