T. Osada

Semiclassical Interpretation of the Angular-Dependent Oscillatory Magnetoresistance in Quasi-Two-Dimensional Systems

In an attempt to understand angular-dependent oscillatory magnetoresistance phenomena recently discovered in organic conductors, calculations of magnetoresistance in quasi-two-dimensional systems were carried out in the framework of the Boltzmann transport theory. Calculated magnetoresistance curves show the angular-dependent oscillations reminiscent of those found experimentally. It is argued that the essential physics underlying the resistance oscillations lies in the angular dependence of the high field asymptotic behavior (saturation vs divergence) of semiclassical magnetoresistance which arises from Fermi surface topology.

Torque magnetometry in pulsed magnetic fields with use of a commercial microcantilever

A miniature torquemeter for use in a pulsed magnetic field is developed by utilizing a commercially available piezoresistive microcantilever. The high sensitivity of ∼5×10−13 Am2 at 38 T is achieved. The response to faster signals were considerably improved due to high eigenfrequency of the cantilever (f∼250–300 kHz). In this article, we successfully demonstrate the magnetic torque data of samples of sub-μg order in pulsed magnetic fields up to 38 T with pulse duration of less than 60 ms. We also found that the de Haas–van Alphen data are in good agreement with the previous reports obtained in steady magnetic fields.

Negative Interlayer Magnetoresistance and Zero-Mode Landau Level in Multilayer Dirac Electron Systems

The interlayer magnetotransport has been considered in multilayer Dirac electron systems, in which two-dimensional electron layers with Dirac-cone dispersion stack with weak interlayer coupling. Under magnetic fields, one Landau level (zero-mode) always appears at the contact point of Dirac cones, and tunneling between the zero-modes on neighboring layers dominates interlayer transport in multilayer systems. The increase of zero-mode degeneracy causes strong negative magnetoresistance. Spin splitting of the zero-mode could change the negative magnetoresistance to the positive one in high magnetic fields and low temperatures. The magnetotransport features observed in an organic conductor α-(BEDT-TTF) 2 I 3 are discussed based on the present model.

Giant Negative Magnetoresistance Reflecting Molecular Symmetry in Dicyano(phthalocyaninato)iron Compounds

Materials containing Fe(Pc)(CN) 2 dicyano(phthalocyaninato)iron molecules show a giant negative magnetoresistance from the interaction between the conduction and the local moment. Under a magnetic field, the resistance becomes two orders of magnitude smaller than the zero-field resistance. The magnetic-field-angle dependence of the magnetoresistance reflects the symmetry of the Fe(Pc)(CN) 2 molecule. We discuss, according to the scaling relation, the correlation between the magnetoresistance and the molecular spin fluctuation.

Quantum oscillations of magnetoresistance in a new organic superconductor (BEDT-TTF)2(NH4)Hg(SCN)4

Abstract Magnetotransport studies have been carried out in a new organic superconductor (BEDT-TTF)2(NH4)Hg(SCN)4 [bis(ethylenedithiolo)-tetrathiafulvalenium mercuric ammonium thiocyanate] using pulsed high magnetic fields up to 35T. We observed both the large Shubnikovda Haas (SdH) oscillations and the angle-dependent quantum oscillations. The experimental results suggest the existence of a cylindrical Fermi surface with weak warping. Its cross-sectional area parallel to the conducting plane is estimated from the SdH period Δ(1/B) = 0.0018 T−1 as 13% of that of the first Brillouin zone. Compared to the isostructual compound (BEDT-TTF)2KHg(SCN)4 which shows anomalous magnetotransport features, this compound shows very normal behaviours as expected in typical quasi-two-dimensional systems.

Hofstadter butterfly and integer quantum hall effect in three dimensions.

For a three-dimensional (3D) lattice in magnetic fields we have shown that the hopping along the third direction, which normally smears out the Landau quantization gaps, can rather give rise to a Hofstadters butterfly specific to 3D when a criterion is fulfilled by anisotropic (quasi-one-dimensional) systems. In 3D the angle of the magnetic field plays the role of the field intensity in 2D, so that the butterfly can occur in much smaller fields. We have also calculated the Hall conductivity in terms of the topological invariant in the Kohmoto-Halperin-Wu formula, and each of sigma(xy),sigma(zx) is found to be quantized.

Charge disproportionation in highly one-dimensional molecular conductor TPP[Co(Pc)(CN)2]2

We investigated the ground state of a highly one-dimensional conductor, TPP[Co(Pc)(CN) 2 ] 2 (TPP = tetraphenylphosphonium and Pc = phthalocyanine), by the measurement of the X-ray diffraction, electron spin resonance, nuclear quadrupole resonance, and magnetoresistance. An increase of the magnetic fluctuations was observed below 20 K, where no structural deformation was detected. In the 59 Co nuclear quadrupole resonance, we found an asymmetric broadening of the spectra owing to the intrinsic inhomogeneity of the molecular charge. We propose that the ground state is characterized by a weak charge disproportionation with antiferromagnetic fluctuations due to the high one dimensionality. A large magnetoresistance was observed under a high magnetic field. Spin effects are dominant at low fields. The anisotropic magnetoresistance suggests a change in the ground-state nature above 10 T.

Oscillatory magnetoresistance in the field-induced SDW state in (TMTSF)2 ClO4

Abstract The transverse magnetoresistance of (TMTSF) 2 ClO 4 was measured in pulsed high magnetic fields up to 35 T at T = 1.8−-4.2K for studying the field-induced spin density wave (FISDW) phase. In addition to a structure corresponding to the threshold field B th ( T ) of the FISDW phase, we found two temperature dependent structures in higher field range at 15–17 and 22–25 T, respectively. Moreover, Shubnikov-de Haas type oscillations with smaller period were also observed superposing on the structures. It was found that the peak positions of the oscillation are independent of temperature, whereas their amplitude changes discontinuously at the structures.

Enhancement of Irreversibility Field in Carbon-substituted MgB2 Single Crystals

We report on a detailed study of the irreversibility field H irr of single crystals of Mg(B 1- x C x ) 2 ( x =0, 2, 3.5, 5 and 7.5%) in strong magnetic fields of up to 40 T. Highly sensitive torque measurements revealed that H irr is greatly enhanced by a factor of two (µ 0 H irr (0) ≃33 T with x =5%) by carbon substitution compared with that of the pristine MgB 2 , owing to a reduction in in-plane coherence length. We also observed the temperature-dependent H irr anisotropy for all carbon contents. This strongly suggests that the two-gap superconductivity of MgB 2 is less influenced by a small amount of carbon substitution. We discuss these results by focusing on the impact of carrier doping and impurity scattering on a two-gap superconductor.We report the detailed study of the irreversibility field {\hirr} of single crystals of Mg(B

New Megagauss Laboratory of ISSP at Kashiwa

_{1-x}