J.A.A.J. Perenboom

New opportunities in science, materials, and biological systems in the low-gravity (magnetic levitation) environment (invited)

We discuss new opportunities that present themselves with the advent of very high magnetic field resistive magnets with appreciable central bore access. A detailed description of the parameters of the magnetic force environment for the case of diamagnetic materials in a water-cooled Bitter-type resistive magnet is provided for the reader who may have an interest in low-gravity experiments. We discuss emerging research activities involving novel uses of magnetic forces in high field resistive magnets at the National High Magnetic Field Laboratory. Particular attention is given to the area of diamagnetic materials that allow a low or “zero” gravity state, i.e., magnetic levitation. These include studies involving plant growth, protein crystallization, and dynamics of single particles and granular materials. In the latter case, unique aspects of the magnetic force environment allow low gravity experiments on particulates that cannot be performed on the Space Shuttle due to the lack of a weak confining potent...

Argument for charge density wave sub-phases in the ground state of α-(BEDT-TTF)2KHg(SCN)4

Abstract A resistive anomaly at temperature T p in the title compound is associated with a Fermi surface reconstruction from a metallic to a (spin or charge) density wave state. At high magnetic fields a corresponding feature in the magnetoresistance above a field B K indicates the breaking of this state. We argue that T P indicates a second order phase line identical to that measured by specific heat methods and show that it decreases monotonically up to 30T. We find that Pauli (rather than orbital) effects, dominate the reduction in T p . We further argue that B K is a first-order transition between two subphases below T p . We compare the phase diagram with recent theoretical models for CDW and SDW ground states in high magnetic fields.

Uniaxial stress method for delicate crystals: Application to Shubnikov-de Haas and superconductivity studies in organic conductors

A method to measure the resistance of fragile single crystals under high uniaxial stress is reported. To compensate for its natural brittleness, the crystal is embedded in epoxy, whose shape is optimized to sustain the stress. This method was used to study the magnetoresistance of (BEDT‐TTF)2KHg(SCN)4 at low temperatures and high magnetic fields and the superconducting transition temperature of (BEDT‐TTF)2Cu(NCS)2 under uniaxial stress perpendicular to the planes of BEDT‐TTF molecules. The results obtained indicate a high degree of homogeneity of the stress in the crystal and comparison with previous hydrostatic pressure measurements reveal the true uniaxial nature of the stress applied.

High-field measurements on the high-Tc superconductors La1.85Sr0.15CuO4-δ and YBa2Cu3O7-δ

We have measured the superconductive properties of the high-T/sub c/ superconductors La/sub 1.85/Sr/sub 0.15/CuO/sub 4-//sub delta/ and YBa/sub 2/Cu/sub 3/O/sub 7-//sub delta/ in magnetic fields up to 25 tesla. Conservative estimates of the critical field at T = 0 yield respectively 45 +- 5 and 200 +- 25 tesla. The slope (dB/sub c//sub 2//dT) near T/sub c/ for the Y-Ba-Cu-O compound is about twice as large as in La-Sr-Cu-O. For both compounds the magnetoresistivity indicates a large anisotropy of the critical field. The Sommerfeld parameter ..gamma.. is estimated to be respectively 7 and 16 mJ(mole Cu)/sup -1/K/sup -2/, which indicates that the higher T/sub c/ of Y-Ba-Cu-O, when compared with La-Sr-Cu-O, might be primarily due to an increased density of states at the Fermi level.

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.

Angle dependence of the upper critical field in the layered organic superconductor kappa-(BEDT-TTF)2Cu(NCS)2 (BEDT-TTFequiv bis(ethylene-dithio)tetrathiafulvalene)

We have performed detailed studies of the angle- and temperature-dependent resistive upper critical fields in the layered organic superconductor -(BEDT-TTF)2Cu(NCS)2. With the magnetic field lying in the conducting planes, our measurements show an upper critical field which comfortably exceeds the Pauli-paramagnetic limit in this material. We find no azimuthal angle dependence of the critical field, in spite of recent evidence that this material has gap nodes characteristic of d-wave superconductivity. We propose that the large critical fields may be due to a Fulde-Ferrell-Larkin-Ovchinnikov state which can exist in exactly in-plane fields because of the nature of the Fermi surface of -(BEDT-TTF)2Cu(NCS)2.

Research in high magnetic fields: The installation at the University of Nijmegen

For research in the highest continuous and pulsed magnetic fields large, complex and powerful installations are needed. This paper describes the new 20 MW installation for continuous high magnetic fields that has been built at the University of Nijmegen. The ultra-low ripple power converter provides the capability to perform experiments up to 33 T with resistive magnets (up to 40 T with the hybrid magnet system under construction) and will be of great value for investigations in physics, chemistry and biology at the forefront of fundamental and applied research. Typically during experiments, the magnetic field is slowly varied or held constant for a period lasting from a few minutes to an hour. The cooling installation is designed to allow uninterrupted operation at maximum power for 3 hours, and when the magnetic field is being swept between zero and full field the cooling plant does not pose limits to the operation. When much higher fields are required, there is the option to go to pulsed magnetic fields with duration in the tens of milliseconds.

Phase boundary in the dimensionality of angle-dependent magnetoresistance oscillations in the charge-transfer salt ?

Angle-dependent magnetoresistance oscillations (AMROs) have been studied in the isostructural charge-transfer salts and (where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene) in steady fields of up to 30 T. The shapes of the approximately elliptical quasi-two-dimensional (Q2D) Fermi surfaces that these organic metals possess have been determined at 30 T and are found to be in broad agreement with recent band-structure calculations. The Fermi surface of the salt undergoes a reconstruction at low fields and temperatures, resulting in a change in the dimensionality of the AMROs from Q2D character to quasi-one-dimensional character. This change is associated with the kink transition that is observed in magnetic field sweeps and is attributed to the formation of a spin-density wave ground state. The phase boundary of the change in the AMRO dimensionality has been followed to both the low-temperature high-field (about 23 T) and low-field high-temperature (about 8 K) extremes. The data are compared with recently proposed models of the AMROs and Fermi surfaces for these materials.

Subband population and electron subband mobility for two interacting Si-δ-doping layers in GaAs

In this paper we present measurements of the subband population and quantum mobility in the various subbands of GaAs samples that contain two coupled Si-δ-layers and of GaAs samples that contain a single δ-doping layer which was increased in thickness by thermal annealing. The measured subband population will be compared with the subband population obtained from self-consistent solutions of the coupled Poisson and Schrodinger equation. The experimental results on both types of structures are compared and show that the population of the higher subbands is not sensitive to the charge distribution of the ionized donors in the center of the confining potential. The quantum mobility in the highest subbands on the contrary is sensitive to the distribution of the ionized donors.

Novel phases in the field-induced spin-density-wave state in (TMTSF)2PF6

Magnetoresistance measurements on the quasi-one-dimensional organic conductor