Th. Wagner

Electrical resistivity and superconductivity of LaB6 and LuB12

Abstract The electrical resistivity of LaB 6 and LuB 12 single crystals with high residual resistivity ratios has been measured down to 1.6 K. The temperature dependence below 30 K, caused by phonon scattering, shows for both materials a T n behaviour with n = 4.2 ± 0.1 for LaB 6 and n = 5.2 ± 0.1 for LuB 12 . From susceptibility measurements the superconducting transition temperature of LuB 12 was determined to be 0.44 K, for the LaB 6 sample with residual resistivity ratio of 160 no transition into the superconducting state was observed down to 5 mK.

Preparation of self‐aligned in‐line tunnel junctions for applications in single‐charge electronics

The self‐aligned in‐line technique has been applied to the preparation of ultrasmall low‐capacitance metallic tunnel junctions. By using e‐beam lithography the area of Al/AlOx/Al contacts has so far been reduced to less than 0.005 μm2. At low temperatures high‐ohmic double junctions with a small metallic island between them show the Coulomb blockade effect. The current through such a device could be modulated by a voltage applied to a gate electrode capacitively coupled to the island (single‐electron transistor). Both single‐charge phenomena have been observed at temperatures up to 1 K.

Generalized anisotropic ellipsometry applied to an organic single crystal: Potassium acid phthalate

The results of generalized anisotropic ellipsometry on a biaxial organic single crystal, namely, potassium acid phthalate, are discussed and analyzed to obtain the optical functions of the crystal along the different crystal directions. The dispersion of the real refractive indices nx, ny, and nz in the spectral range from 300 to 1400 nm, as well as the values of the extinction coefficient kx,y at the absorption edge are determined and modeled.

An aluminium heat switch made from cold-pressed Cu–Al composite

Abstract A superconducting heat switch was built from a commercially available, cold-pressed Al–Cu composite. The diffusion of Cu and Al at the interface of the cold pressed composite has been determined by SEM as 16.4 at% Al into Cu and 3.3 at% Cu into Al. In the temperature range from 5 to 100 mK the thermal conductivities have been measured in the normal and superconducting state. At 10 mK the normal conductivity was only 50 μW / cm K , while the superconducting thermal conductivity was found to be less than 2 nW/cm K below 30 mK. The low thermal conductivity in the normal state limits the applicability of such cold-pressed composite materials in low-temperature experiments.

Field dependence of the magnetization of adsorbed3He films at ultra low temperatures

We present measurements of the magnetization of pure3He films adsorbed on graphite at a density of p = 0.235 atoms/Å2, which corresponds to the 2D Heisenberg ferromagnetic regime. Different NMR frequencies (461.3kHz and 1.004 MHz) were used to study the magnetic field dependence of the nuclear magnetization. Measurements were performed on a Papyex sample to investigate the influence of the platelet size. The results are discussed in the context of theoretical models presented recently to describe these systems.

NMR-investigations on thallium-samples of high spin-polarization

The big indirect exchange interaction and the huge nuclear magnetic moments of the two Tl-isotopes makes Tl useful at low temperatures for non linear pulsed NMR studies of inter- and intra-spin interactions mediated by the conduction electrons. We extended earlier experiments to higher fields (0.1 T < Bo < 1 T) on a cylindrically shaped sample of 6N purity and found the proposed spin waves trapped in the skin depth of the sample which manifest themselves in line splitting with increasing tipping angles. We also found frequency shifts in partial agreement with the expectations. However, the spin susceptibility was enhanced above the paramagnetic value at temperatures below 2 mK, whereas the line width decreased by a factor of two between 5 and 1 mK. Thus, while solving the high spin-polarization puzzle of Tl (line splitting of already merged lines) we were left with two other unexplained observations.

Cylindrical pressure gauge

Abstract We have constructed and tested cylindrical capacitance pressure gauges. They are easy to build and have the advantages of good sensitivity and good linearity of pressure versus inverse capacitance over a wide pressure range. One gauge, made of epoxy, is transparent to RF fields, which allows simultaneous NMR and pressure measurements on the same sample.

NMR on gallium single crystals: not yet an absolute thermometer at very low temperatures

Abstract A twin crystal of Ga was investigated in the temperature and field ranges of 0.5 to 10 mK and 60 to 250 mT, respectively. In these fields the magnetic interaction is too large to be treated as a perturbation of the quadrupolar interaction. Therefore, the eigenvalues of the Schrodinger equation were calculated numerically, and then used to determine the crystalline axes by pulsed NMR on 69 Ga and 71 Ga. The temperature dependence of the NMR line intensities was measured using 195 Pt-NMR for thermometry. The observed intensities do not follow the theoretical expectations for a spin system of low nuclear polarization. In addition, a temperature-dependent frequency shift was observed. These results suggest that the effect of interactions between the spins, as well as more complicated spin dynamics, need to be considered in order to use gallium as an absolute thermometer at temperatures below 1 mK.

Self-aligned in-line tunnel junctions for single-charge electronics

Abstract The self-aligned in-line (SAIL) technique has been applied to the preparation of ultrasmall low-capacitance metallic tunnel junctions. By using e-beam lithography the area of Al/AIO x /Al contacts has so far been reduced to less than 0.005 μm 2 . At low temperatures high-ohmic double junctions with a small metallic island between them show the Coulomb blockade effect. The current through such a device could be modulated by a voltage applied to a gate electrode capacitively coupled to the island (single-electron transistor). Both single-charge phenomena have been observed at temperatures up to 1 K.

Spin-spin interaction and multiple spin-echoes in AuIn2

We investigated several AuIn2 samples of similar purity by spin-echo techniques. The temperatures and fields ranged from 0.5 to 20 mK and from 0.06 to 0.8 T, respectively. Our results are: The Knight-shift changed from sample to sample within about 5%. Spin-echoes could not be found in all samples. T2 ranged from about 40 μs in one sample, to a temperature and field independent T2 = 83 μs for another sample, to a field and temperature dependent T2 which varied for this sample from 2 ms at low B/Tn-values (∼5 T/K) to 0.3 ms at B/Tn ∼300. We have shown that the temperature of the conduction electron system is the relevant temperature in the problem, and not the nuclear spin-temperature of In. From the amplitudes of multiple spin-echo structures we conclude that demagnetizing fields are not the only origin for their occurrence.