Low Charge Carrier Density Research Articles

μSR studies of low charge carrier density heavy-fermion systems

We report on μSR studies in low charge carrier density heavy‐fermion systems with cubic symmetry. Whereas for Ce3Au3Sb_4 and for Sm3Te4 the μSR data point to the presence of some kind of spin‐glass‐like magnetic states at low temperature, for Yb4As3 preliminary μSR studies just show a slight increase of the depolarization rate which indicates a slight slowing down of the Yb‐moment fluctuations. Consequently for this latter system, the large linear term of the specific heat seems indeed to be connected to electronic excitations.

Nature and amount of carriers in Ce-doped Nd 2CuO 4 II: Low temperature transport and XAS characterisation

This paper discusses the electrical (conductivity) and spectroscopic (XAS) evidence of Ce-doped Nd 2CuO 4 samples in which the charge carrier density has been fixed to a known value by annealing in well known and well controlled conditions, and then quenching to room temperature. All the experimental findings presented here are consistent with a band structure where no impurity levels are present in the energy gap. In addition, the XAS results indicate that the unoccupied band of lower energy is originated by a Mott-Hubbard splitting. It has been found that both Ce-doping and reduction of the oxygen excess δ concur in giving rise to the charge carrier density. For low charge carrier density, the conductivity data indicate an Anderson localisation of the charge carriers, which is seemingly responsible of the semiconducting-like behavior. As the charge carrier density increases, conductivity becomes less dependent on temperature, and superconductivity (above 10 K) is found only when the charge carrier density is greater than the value of 0.082 electrons per copper atom.

Superconductivity in two-band model with low charge carrier density and colllective oscillations

Superconductivity theory of the two-band systems with all possible intra- and interband interactions, applicable at the arbitrary charge carrier densities up to values of chemical potential μ≈Δ is developed. Collective oscillations in these systems are studied. The presence of additional order parameterΔ 12 , besidesΔ 11 ,Δ 22 , in these system leads to the essential mixing of the fluctuations of the phases and amplitudes of order parameters from different bands. This mixing is a reason of arising the additional exciton-like collective mode besides two ones (acoustical Bogolyubov-Anderson one and exciton-like one caused by the fluctuations of phases of order parameters from different bands) which are also observed in the case μ≫Δ. This mixing leads also to the essential dependence of these two exciton-like modes on the charge carrier density.

Properties of superconducting p-n junctions

Possible band diagrams and electrical transport properties of aniso-type superconducting junctions - contacts in which the charge carriers are electrons in one electrode and holes in the other - are discussed. Special attention is given to p-n junctions between superconductors with a low density of mobile charge carriers, such as the high- T c cuprates. It is predicted that most of such contacts are damped Josephon tunnel junctions with low critical current-normal-state resistance products due to a depletion layer at the interface. With increasing current bias the characteristics of the junctions exhibit a gradual transition to Esaki-diode behavior. Similar depletion layers may result in low critical currents at high- T c-low- T c junctions.

Anomalous behaviour of the chemical potential in superconductors with a low density of charge carriers

We discuss the behaviour near T c of the chemical potential in superconductors with a dilute gas of interacting fermions. It is shown that at T c there exists an anomaly in the chemical potential that distinguishes superconductors with Δ and E F of comparable magnitude from ordinary BCS superconductors (Δ ⪡ E F). We predict the size of the anomaly for a number of existing superconductors and we argue, that if high- T c superconductors are indeed special in the above sense, the anomaly is well in the observable range and could be used as an experimental test of the various “exotic” models of the superconducting mechanism.

A theory of one and two holes in antiferromagnetic CuO2

The authors consider a low density of positive charge carriers (holes on oxygen ions) interacting with magnetic degrees of freedom (spins on copper ions) in a single CuO2 plane, assumed to be antiferromagnetic and electrically insulating. Analysing the interaction of a single hole with the spins, they find it to be a strongly coupled nontopological soliton carrying spin 1/2. Various energies, spin correlations and overlap parameters are calculated and tabulated. Pairs of solitons are determined to be weakly coupled; hence they conclude bi-solitons to be an improbable species.

Magnetic-breakdown coupling between orbits of a low-electron-density system.

We have studied by use of the de Haas--van Alphen effect a system of coupled orbits with low charge-carrier density: a single crystal of bromine-intercalated graphite. We have used a new method to determine the effective masses, as well as the de Haas--van Alphen amplitudes at zero temperature for a system with several close de Haas--van Alphen frequencies. We have shown that the Falicov-Stachowiak theory does not give quantitative agreement with our results. Nevertheless, it appears that the Pippard approximation of a network of coupled orbits with Landau numbers of about 15 still remains valid, but the probability of magnetic breakdown should be modified by taking into account the finite size of the magnetic-breakdown area.