Martin Kupka

Current-voltage characteristic and differential conductance of a point contact and planar tunnel contact between an ordinary metal and superconductor in realistic three-dimensional geometry: comparison with a one-dimensional case☆

Abstract Current-voltage characteristic for a point or planar tunnel contact of the structure ordinary metal-finite-thickness barrier-ordinary superconductor was derived taking into account a realistic three-dimensional (3D) geometry of the problem. Ordinary and Andreev reflection coefficients were obtained by means of a so-called transmission matrix for regions with varying model parameters. The model with a sharp barrier-superconductor interface was investigated in detail. For contacts with a barrier transmissivity clearly dependent on the incident electron energy for electrons from the chemical potential vicinity, differential conductances are very different from the curves usually provided by the commonly used one-dimensional (1D) theories. Due to a relevant influence of an applied voltage on the barrier shape and height, the differential conductance as a function of voltage is a rough curve with a number of peaks at various voltage values. For contacts with a barrier transmissivity practically independent on the incident electron energy, expressions for ordinary and Andreev reflection coefficients are formally the same as in 1D theories. But in our approach the barrier character is described by an increasing function of an electron incidence angle instead of a single parameter (barrier strength Z ) in 1D cases. Due to this difference, the differential conductance calculated within the 3D approach reveals more tunnel character than the one obtained for the same barrier from a 1D theory.

Point contact spectroscopy of U2Zn17

Abstract We report on point contact measurements on the heavy fermion system U 2 Zn 17 . It is shown that the first derivative of the current-voltage characteristic of low resistance contacts is very sensitive to the magnetic phase transition. A point contact theory applicable to heavy fermion compounds is still missing. The results are discussed in the frame of the heating model, which includes electrical and thermal resistivity as well as thermopower effects.

Transmission coefficient calculation using the derived inelastic transport analogy of the transmission matrix method; application to tunnelling into a marginal Fermi liquid

An expression for the probability of transmission of an electron from an ordinary metal into a medium with a finite quasiparticle lifetime is derived. The derivation is based on using a certain matrix, analogous to the transmission matrix for the elastic transport case, assigned to each region of the sample being investigated. This matrix is determined by the functional shapes of the electronic reciprocal-effective-mass tensor, the one-particle potential energy and the retarded self-energy in a given sample region, as well as by the energy and direction of motion of the incident electron. As an example of an application, the differential conductance is calculated for a tunnelling contact between an ordinary metal and an anisotropic marginal-Fermi-liquid metal for various characters of the contact interlayer zone.

Modification of the Blonder, Tinkham and Klapwijk expressions for Andreev and normal reflection coefficients due to a finite-thickness barrier at the normal metal-superconductor interface

Abstract Amplitudes of normal and Andreev reflections for a normal metal-arbitrary potential barrier-superconductor system are derived with explicitly expressed contributions due to the superconductivity and due to the barrier at the interface. General results are illustrated by the example of a rectangular potential barrier. For somevalues of barrier parameters the results are expressively different from the ones predicted by the standard Blonder, Tinkham and Klapwijk model with a delta-functional barrier at the interface.

Coherently Precessing Spin Structure in 3He-B High-Resolution Quantum Amplifier

We present results of the study of surface oscillation modes of coherently precessing nuclear spins in the superfluid 3He-B known as a homogeneously precessing domain (HPD). HPD represents the first macroscopic experimental manifestation of magnetic superfluidity phenomenon. The measurements showed that HPD is extremely sensitive to longitudinal perturbations and it behaves as tuned (by magnetic field gradient) quantum amplifier that allows to measure very small changes of magnetic field with very high relative resolution, of the order of 10-7.

Differential conductance of a “tunneling” point contact between an ordinary metal and marginal-Fermi-liquid metal

Abstract We have calculated a differential conductance as a function of applied voltage for a “tunneling” point contact (i.e. point contact with an insulating barrier at the interface) between an ordinary metal and system described as marginal Fermi liquid. Obtained differential conductance is nearly proportional to the absolute value of the applied voltage for positive as well as negative bias (positive bias corresponds to the marginal Fermi liquid being positive), but with different proportionality factors. If the electronic “band” dispersion relation for a marginal Fermi liquid is chosen in a simple “particle” form, the slope of the differential conductance for positive bias is greater then the one for negative bias. Situation is opposite for the case when the “band” dispersion relation is chosen in a simple “hole” form. But for the both cases, the slope of the differential conductance increases with increasing coupling constant appearing in the one-particle self-energy of the marginal Fermi liquid.

The Effect of Corrosion in Hank's Solution on the Bending Stiffness of Bars from Sintered Iron-Manganese Alloys

Fe-Mn alloys represent promising degradable biomaterials for temporary implants. To investigate the effect of corrosion on their mechanical properties, iron powders were mixed with 25, 30 and 35 wt.% of a manganese powder, compressed into prismatic bars and sintered. Sintered bars were immersed in Hanks solution for 8 weeks. The bending stiffness of each bar before and after its exposure to electrolyte was examined. The higher the porosity of a bar was, the higher relative reduction in bending stiffness the bar exhibited. A likely explanation was that in a more porous bar Hank’s solution penetrated deeper and affected larger volume fraction of bar’s material.

The Effect of Graded Structure of Sintered Iron-Manganese Biomaterials on the Range and Distribution of Local Hardness Values

Powders comprised of Fe particles and 25, 30, 35wt.% of Mn particles were mixed, compacted and sintered to investigate the effect of Mn on the properties of sintered Fe-Mn alloys. It was found that the sample’s swelling, microstructure and distribution of local hardness values were strongly affected by the Mn content. The particles in Fe-25Mn and Fe-30Mn samples exhibited a distinct onion-like structure causing a considerable variability in local properties, while the particles in Fe-35Mn samples were at a glance more homogeneous, with a large volume fraction occupied by a nearly uniform material with almost constant properties.

On the Indentation Modulus of Sintered Materials

When the depth-sensing (nano)indentation is applied to sintered samples, measured properties, which are expected to represent the material of an individual grain, seem to depend on the overall porosity of the macroscopic sample. To understand such a result, it is assumed that while the nanoindenter penetrates into the surface grain and probes the properties of its material, the grain itself serves as another, larger indenter indenting the rest of sample and probing the properties that represent the bulk of material rather than individual grains. Load vs. displacement curve reflects the synergetic response of these two “indenters” and so it contains information about the sample’s mechanical properties at both microscopic and macroscopic scales. Obtained theoretical results agree qualitatively with the experimental data (the dependence of the indentation modulus on the porosity of sample; the indentation size effect).

The Influence of Cu-Coating versus Admixing on Mechanical Properties and Dimensional Change of Sintered Fe-Cu Parts

The differences in mechanical properties of pressed and sintered specimens made from mixtures of iron and copper powders or from copper-coated iron powders, produced by a cementation process, were studied. For this purpose different copper contents were used (3, 8 or 12wt%) and the oxygen content of the coated powders was measured. After sintering at 1120°C for 60 minutes in hydrogen flow, microgradient structures were observed. The samples were investigated by light optical microscopy and tested under bending and tensile loads. Young´s Moduli were calculated from resonance frequencies. The copper-coating of the iron powder results in an improvement of all properties, owing to a more homogeneous copper distribution and the absence of large secondary pores, compared to specimens made from mixtures of iron and copper powders. In the case of Fe-12Cu (coated), all determined properties tend to result in a maximum: highest sintered density (7,33 g/cm³), tensile strength (489 MPa), transverse rupture strength (1098 MPa) and apparent hardness (162 HV10). The Young´s Modulus (150 GPa) of coated Fe-12Cu is nearly the same as that of sintered iron (154 GPa).