A. G. Swanson

Direct Observation of Spin-Splitting of the Shubnikov- de Haas Oscillations in a Quasi-Two-Dimensional Organic Conductor (BEDT-TTF)2KHg(SCN)4

A clear splitting in the Shubnikov-de Haas oscillation has been directly observed for the first time in a quasi-two-dimensional (2D) organic conductor (BEDT-TTF) 2 KHg(SCN) 4 . Analysis of the oscillations yields a 2D nature of the cylindrical Fermi surface, corresponding to 16% of the first Brillouin zone, with a small warping along the direction normal to the 2D layer. The condition for observation of spin-splitting and the evolution of the splitting pattern with a tilted magnetic field are discussed.

Observation of “saw-tooth” de Haas-van alphen oscillation in a two-dimensional organic superconductor θ-(BEDT-TTF)2I3

Abstract A “saw-tooth” de Haas-van Alphen oscillation was observed in an organic superconductor θ-(BEDT-TTF) 2 I 3 under the magnetic field H >15 T at T =0.5 K. The fundamental frequency of the oscillation ( H 0 =4170 T) corresponds to the closed Fermi surface area of 50% of the first Brillouin zone, consistent with one positive charge carrier per I 3 - anion. An issue about the Fermi surface topology is resolved based on the analysis of the dH-vA oscillation frequencies. The “saw-tooth” variation of magnetization demonstrates that organic metals can provide an ideal example of highly two-dimensional and extraordinary clean electron system with a cylindrical Fermi surface along the direction perpendicular to the conducting plane.

Analysis of de Haas-van Alphen Oscillations and Band Structure of an Organic Superconductor, θ-(BEDT-TTF)2I3

Analysis of the temperature and field dependence of the de Haas-van Alphen oscillations in an organic superconductor θ-(BEDT-TTF) 2 I 3 is presented. The cyclotron masses are estimated to be 2.0 m e and 3.6 m e for slow and fast oscillations, respectively. The indication of magnetic breakdown effect is discussed in terms of the geometry of the Fermi surface. The result is compared with those of the infrared reflectance spectra and the empirical tight-binding band calculations.

Diaphragm magnetometer for dc measurements in high magnetic fields

We describe the design and operation of a force magnetometer with high sensitivity which can be easily used between room and sub‐kelvin temperatures. The device can accommodate a large range of sample masses (10–0.001 g) with a relative resolution of 10−5 emu. The maximum sample size is only restricted by the geometry of the cryogenic and magnetic environment. The design is particularly suited for high magnetic field measurements or where no gradient coil is available.

Macroscopic quantum tunneling in single domain magnetic particles

Since the 1950’s there have been a number of experimental observations involving small single domain (SD) magnetic particles (≲200 A diameter) which have been attributed to what we’d today call ‘‘macroscopic quantum tunneling’’ (MQT) of the magnetization in its switching from one easy axis direction to another. A comprehensive study of this phenomenon is not only of great interest for the sake of fundamental physics: It is of utmost practical importance in that MQT sets a limit on the lifetime of information storage on magnetic tapes and films. In this paper we present a brief summary of some of the latest results of our theoretical and experimental studies of this phenomenon.

Magnetic quantum oscillations in low-dimensional organic conductors (BEDT-TTF)2X

Abstract We report observations of various ‘saw-tooth’ dHvA oscillation waveforms in organic conductors. In θ-(BEDT-TTF)2I3 and α-(BEDT-TTF)2KHg(SCN)4, we have observed ‘saw-tooth’ and ‘inverse saw-tooth’ waveforms, respectively, both characteristic for two-dimensional electron systems. The difference between the two is discussed. In β-(BEDT- TTF)2AuBr2, we observed pseudo ‘saw-tooth’ magnetization jumps due to the magnetic interaction effect.

Fermi surface study of two-dimensional organic metals (BEDT-TTF)2X

Abstract Observation of Shubnikov-de Haas(SdH) and de Haas-van Alphen(dHvA) oscillations in organic metals (BEDT-TTF)2X, with X = KHg(SCN)4, θ-I3 and β″-AuBr2, are reported. In KHg(SCN)4 salt, in addition to the SdH oscillations with fundamental frequency of 670 T corresponding to about 16 % of the first Brillouin zone(FBZ), we observed splitting of each SdH peak which we ascribed to “spin-splitting”. We have also found that the ground state of this salt is not simple metal but has some magnetic character. In θ-I3 salt we have succeeded in an observation of dHvA oscillations for the first time. We observed a “saw-tooth” dHvA oscillation characteristic to a highly two-dimensional and extraordinary clean electronic system. In addition to the fundamental frequency of 4170 T corresponding to 50.5% of the FBZ and its higher harmonics, we the FBZ. A new Fermi surface topology for θ-I3 salt is proposed based on the analysis of the dHvA effect. In β″-AuBr2, we observed complex dHvA oscillations, which can be explained in terms of the mixing of two fundamental frequencies of 47 and 268 T, suggesting the presence of very small pockets corresponding to 0.6 and 2.9 % of the FBZ.

Fermi Surface Study of Organic Superconductors (BEDT-TTF)2X: A Wonderland of Two-Dimensional Molecular Metals

Organic charge transfer salts (BEDT-TTF)2X, where BEDT-TTF (or ET) is bis(ethylenedithio)tetrathiafulvalene and X stands for monovalent anions such as I3 −, 1 Br2 −, AuI2 −, Cu(SCN)2 − and so on, constitute the most promising family among organic superconductors at present. They provided us with plenty of organic metals, which are mostly characterized by layered crystal structure, two-dimensional (2D) electronic structure and ambient-pressure superconductivity with relatively high critical temperature, including the record high T c among organics. In order to reach an understanding of the superconductivity mechanism in organic metals, it is extremely important to study not only the superconducting characteristics but also the electronic band structure, in particular, the Fermi surface.

Transport and Magnetization Studies of β″(BEDT-TTF)2AuBr2

The de Haas van Alphen and Shubnikov de Haas frequencies of β″(BEDT — TTF)2AuBr2 indicate that the closed portion of the Fermi surface corresponds to 2.9% of the first Brillouin zone. The angular dependence of the data suggest a quasi three dimensional Fermi surface. The Hall measurements yield a hole like carrier concentration of ∼ 3 × 1019 cm−3, substantially less than the assumed carrier concentration of one carrier per unit cell.

Single Domain Magnetic Particles: Magnetic Alignment and Evidence for Verwey Transition from Low Temperature Measurements

In order to make a careful study of ferrofluids of single domain particles of magnetite at low temperatures it is essential to know whether the Verwey transition has taken place. We report the first clearcut evidence for its occurrence as exhibited by the magnetization of a sample cooled down from room temperature to 1K in the presence of an applied field of 8T. Removal of the field leads to a drop in the magnetization associated with a change in the direction of the easy axes in conjunction with the Verwey transition. Previous evidence, based upon the temperature dependence of Mossbauer spectra was quite weak.