H.B. Brom

Metal-cluster compounds and universal features of the hopping conductivity of solids

Abstract In this paper we present an overview of data on the DC conductivity and on the frequency dependent (up to 1 MHz) dielectric properties of polynuclear metal cluster compounds. These materials can be viewed as model systems for identical clusters embedded in a dielectric matrix. Comparison is made with results on related narrow band materials, such as ceramic metals (cermets), various other metal-nonmetal composites, and doped or amorphous semiconductors. Remarkable similarities are observed in the behavior of these at first sight quite different systems. In order to understand this apparent universality an overview is made of the available predictions from theoretical models for the hopping conductivity in random composites and other disordered media. Although these models are often based on quite different assumptions regarding the microscopic transport mechanisms, the nature of the carriers, etc.., one finds the same similarities in the predicted behavior as found in the experiments. Thus, for a system of carriers hopping between sites separated by random barriers in a diffusive motion, the nature of carriers, sites and barriers does not appear to be very important, except that external variables such as temperature, strength and frequency of the electric field have to be properly scaled. From the observed dependences of the dielectric properties on these parameters, it is concluded that the behavior is well explained in terms of stochastic models for the hopping conductivity. The possible relevance of an extended Hubbard model for the description of these materials is discussed.

Physical properties of metal cluster compounds I: Magnetic measurements on high-nuclearity nickel and platinum carbonyl clusters

Abstract Metal-carbonyl cluster compounds are composed of macromolecules that consist of a core of metal atoms coordinated by a “shell” of CO ligands. We present susceptibility and high-field magnetization measurements on various high-nuclearity platinum and nickel clusters. The compounds display very unusual low-temperature magnetic behaviour, which we ascribe to quantum-size-effects arising from the discreteness of the electronic energy levels due to the small dimensions of the metal clusters (10–40 atoms). We compare our results with predictions from molecular orbital calculations.

Charge transport in doped polythiophene

Abstract The conductive properties of the conjugated polymer polythiophene doped with FeCl3 are studied as a function of frequency co/2n, electric field E, temperature T and dopant concentration c. The data show a transition from quasi-one-dimensional (for low doping levels) to three dimensional (for high doping) variable range hopping. At all doping levels the macroscopic carrier mobility μ is determined by inter-chain hopping.

Physical properties of metal cluster compounds II: Dc-conductivity of the high-nuclearity gold cluster compound Au55(PPh3)12Cl6

Abstract The title compound consists of (electrically neutral) Au55(PPh3)12Cl6 macromolecules, where each molecule is composed of a core of 55 Au-atoms in a cuboctahedral packing, surrounded by a shell of 12 phosphine and 6 chlorine ligands. We present dc-electrical resistance data in the range 70 to 350 K, which can be described by R ∝ exp ( T o / T ) 1 2 , with To = 5.7 × 104 K. This same temperature dependence has also been observed in granular metal systems and in some 1-dimensional conductors. Similar as in those systems, we observe strong non-ohmic effects in the conductivity, which become more pronounced as the temperature is lowered.

Physical properties of high-nuclearity metal cluster compounds

Metal cluster compounds are composed of large macromolecules, which consist of a metal core (cluster) containing a certain number (n=6–560) of metal atoms, to which core a shell of ligands is coordinated. They provide excellent model systems for an assembly of identical metal clusters, embedded in a dielectric matrix. We discuss a number of physical properties of these materials.

Measurement of the complex dielectric constant down to helium temperatures. I. Reflection method from 1 MHz to 20 GHz using an open ended coaxial line

The reflection off an open ended coaxial probe pressed against a material under test is used to determine the complex microwave (1 MHz–20 GHz) dielectric response of the material. A full-wave analysis of the aperture admittance of the probe, in terms of the dielectric properties of the backing material and the dimensions of the experimental geometry, is given. We discuss the calibration procedure of the setup and present the complex dielectric response of several materials determined from the measured reflection coefficient. The results obtained with the open ended coax interpolate well between data taken at lower and higher frequency bands using different experimental methods. We demonstrate that this method can be applied to perform dielectric measurements at cryogenic temperatures.

Physical properties of metal cluster compounds III: NMR study of platinum carbonyl clusters

Abstract The Pt NMR line shapes for [Pt38(CO)44H2] [N(PPh3)2]2 and [Pt26(CO)32H2] [PPh4]2 resemble those found in free Pt-particles of comparable size. Our analysis does not allow a discrimination between Knight or chemical shift. It is suggested that the Pt-relaxation rate, which normally gives another tool to check the mechanism of the shift, is determined by the presence of paramagnetic electronic spins as seen both in ESR and in the magnetic susceptibility.

Numerical simulations on two dimensional vortex lattices

Abstract We have studied the dynamics of a two-dimensional vortex lattice with the use of a molecular dynamics method for systems of up to 2000 vortices. The interaction potential is taken to be of the Pearl form and temperature is taken into account via a white Gaussian noise source. We find that the low temperature state is predominantly a triangular array of two-dimensional vortices. Long range order is absent. Defects are present at very low temperatures already, and possibly cause at higher temperatures Kosterlitz-Thouless (dislocation mediated) melting of the lattice. With a London penetration depth of 1250 A, we calculate the field distribution at zero temperature and the T -dependent magnetic field time-correlation function. The results compare well with NMR experiments on Tl in Tl 2 Ba 2 CaCu 2 O 8 in the temperature range below the melting temperature T m .

ESR in YBa2Cu3O7-δ single crystals

Reports ESR measurements in single crystals of the high-temperature superconductor YBa2Cu3O7- delta . Measurements were carried out both on a superconducting single crystal with Tc=89 K, and on a non-superconducting sample. No ESR signal was found in either sample at 300 K, nor at 4.2 K. These results show irrefutably that there are no unpaired localised spins in the 90 K superconductor YBa2Cu3O7- delta . It also implies that the earlier reported ESR signals in powders of the 90 K superconducting ceramics YBa2Cu3O7- delta come from an impurity phase.

Single crystal 205Tl NMR in the normal and superconducting state of the high-Tc superconductor Tl2Ba2CaCu2O8: A study of the Knight and chemical shift

Abstract We have performed 205 Tl NMR in c -axis aligned single crystals of Tl 2 Ba 2 CaCu 2 O 8 ( T c ≈101 K ) in a field of 4.7 T. There are two Tl-lines with, at room temperature, distinct intensities, chemical and Knight shifts. One line arises from the normal (N) Tl-sites in the Tl-double layers and mimics the Cu-nuclear spin. The other one arises from impurity (I) Tls replacing part of the Ca-sites within the Cu double layer, i.e. the same locations as for the Y-ions in YBa 2 Cu 3 O 7 . Because it appeared possible to follow the dependence of the shifts of the two lines on temperature ( T ) down to T / T c ≤0.1, (for H ∥ c and H ∥ a, b ) the chemical and the Knight shifts could be determined accurately. The T -dependences of the Knight shift of the N(I)-line in the present sample with about 10% Ca-replacement are analogous to those of Cu(Y) in YBa 2 Cu 3 O 6.6 : the shift starts to decrease already above T c without an abrupt change at T c . Only below T / T c ≈0.6 do the resonance lines start to broaden.