0000000001288138
AUTHOR
K. Gloos
Discrete saturation thickness and anomalous potential height of native ultrathin aluminum oxide tunnel barriers
We have investigated planar metal - insulator - metal tunnel junctions with aluminum oxide as dielectricum. These oxide barriers were grown on an aluminum electrode in pure oxygen at room temperature till saturation. We observed discrete barrier widths separated by \Delta s \approx 0.38 nm, corresponding to the addition of one layer of oxygen atoms. The minimum thickness of s_0 \approx 0.54 nm is due to a double layer of oxygen. We found a strong and systematic dependence of the barrier height \Phi_0 on the thickness s like \Phi_0 \approx 2.5 eV / s^2(nm), which nearly coincides with the kinetic electron energy E = h^2/2ms^2 for which the deBroglie wavelength matches the width of the barrie…
Production of zero energy radioactive beams through extraction across superfluid helium surface
A radioactive Ra-223 source was immersed in superfluid helium at 1.2-1.7 K. Electric fields transported recoiled Rn-219 ions in the form of snowballs to the surface and further extracted them across the surface. The ions were focussed onto an aluminium foil and alpha particle spectra were taken with a surface barrier spectrometer. This enabled us to determine the efficiency for each process unambiguously. The pulsed second sound wave proved effective in enhancing the extraction of positive ions from the surface. Thus we offer a novel method for study of impurities in superfluid helium and propose this method for production of zero energy nuclear beams for use at radioactive ion beam facilit…
Ground state properties of SmB6
Abstract We have studied the ground state properties of the intermediate valence narrow-gap semiconductor SmB 6 by means of point-contact (PC) spectroscopy and specific heat measurements. The density of states derived from PC tunneling spectra could be decomposed into two energy-dependent parts with E g =21 meV and E d =4.5 meV wide gaps, and a finite residual density of states at the Fermi level. The specific heat of SmB 6 is enhanced below about 2 K, more pronounced for the sample with less impurities. This behavior can be attributed to the formation of a coherent state within the residual density of states in the energy gap.
Cryogenic helium as stopping medium for high-energy ions
We have investigated the survival and transport efficiency of Ra-219 ions emitted by a Ra-223 source in high-density cryogenic helium gas, with ionisation of the gas induced by a proton beam. The combined efficiency of ion survival and transport by an applied electric field was measured as a function of ionisation rate density for electric fields up to 160 V/cm and for three temperature and density combinations: 77 K, 0.18 mg/cm(3), 10 K, 0.18 mg/cm(3) and 10 K, 0.54 mg/cm(3). At low beam intensity or high electric field, an efficiency of 30%, is obtained, confirming earlier results. A sharp drop in efficiency is observed at a "threshold" ionisation rate density which increases with the squ…
Andreev Reflection at Point Contacts with Heavy-FermionUBe13?
We comment on a recent Letter by Waelti et al. (PRL 84, 5616 (2000)) on 'Spectroscopic evidence for unconventional superconductivity in UBe13'. We show - by using the basic Wexler formula - that it is unlikely to observe Andreev reflection at point contacts between UBe13 and a normal metal. The large superconducting anomalies usually found for those contacts are mainly due to the resistive Maxwell resistance vanishing below Tc of UBe13.
Positive ion extraction across the superfluid-vapor helium interface
The extraction efficiency of positive (219)Rn ions across the superfluid-vapor helium interface above similar to 1.3 K indicates that extraction results from thermal activation across a barrier of about 20 K. Below similar to 1.3 K, the extraction efficiency is constant at about 0.7%. The evaporation of the superfluid surface by second sound pulses has a negative impact on the ion extraction, but not on the ions themselves. It takes 3.2( 6) s at 1.60 K and 15( 6) s at 1.15 K for the extraction process to recover from a disturbed state of yet unknown nature.
Electronic correlation effects and the Coulomb gap at finite temperature.
We have investigated the effect of the long-range Coulomb interaction on the one-particle excitation spectrum of n-type Germanium, using tunneling spectroscopy on mechanically controllable break junctions. The tunnel conductance was measured as a function of energy and temperature. At low temperatures, the spectra reveal a minimum at zero bias voltage due to the Coulomb gap. In the temperature range above 1 K the Coulomb gap is filled by thermal excitations. This behavior is reflected in the temperature dependence of the variable-range hopping resitivity measured on the same samples: Up to a few degrees Kelvin the Efros-Shkovskii ln$R \propto T^{-1/2}$ law is obeyed, whereas at higher tempe…
Andreev reflection at point contacts with heavy-fermion UBe13 ?
We comment on a recent Letter by Waelti et al. (PRL 84, 5616 (2000)) on 'Spectroscopic evidence for unconventional superconductivity in UBe13'. We show - by using the basic Wexler formula - that it is unlikely to observe Andreev reflection at point contacts between UBe13 and a normal metal. The large superconducting anomalies usually found for those contacts are mainly due to the resistive Maxwell resistance vanishing below Tc of UBe13.
EU dissemination of the provisional ultra-low-temperature scale, PLTS-2000
Following the introduction of the provisional low-temperature scale from 0.9 mK to 1K, PLTS-2000, there is a need for primary and secondary thermometers and fixed points, which can disseminate the scale to users. This paper reports on the progress, within the EU collaborative project ‘ULT Dissemination’, in the development and evaluation of several devices with associated instrumentation. Principal among them are a current-sensing noise thermometer, a CMN thermometer adapted for industrial use, a Coulomb blockade thermometer, a second-sound thermometer, a 3He melting pressure thermometer for a direct realisation of the PLTS-2000. A superconductive reference device has also been developed, a…
Wide-range thermometer based on the temperature-dependent conductance of planar tunnel junctions
The effect of the Fermi–Dirac distribution on the current through standard planar tunnel junctions is a suitable basis for thermometry in a wide temperature range. In particular, it extends the range spanned by Coulomb-blockade thermometers up to room temperature.
Energy gap of intermediate-valentSmB6studied by point-contact spectroscopy
We have investigated the intermediate valence narrow-gap semiconductor ${\mathrm{SmB}}_{6}$ at low temperatures using both conventional spear-anvil type point contacts as well as mechanically controllable break junctions. The zero-bias conductance varied between less than $0.01 \ensuremath{\mu}\mathrm{S}$ and up to 1 mS. The position of the spectral anomalies, which are related to the different activation energies and band gaps of ${\mathrm{SmB}}_{6},$ did not depend on the the contact size. Two different regimes of charge transport could be distinguished: Contacts with large zero-bias conductance are in the diffusive Maxwell regime. They had spectra with only small nonlinearities. Contacts…
European Dissemination of the Ultra-low Temperature Scale, PLTS-2000
The first phase of the EU collaborative project on sub‐kelvin thermometry, ‘ULT Dissemination’, is nearing completion, leading to the development of several thermometers and devices, and the instrumentation needed to disseminate the new Provisional Low Temperature Scale, PLTS‐2000, to users. Principal among these are a current‐sensing noise thermometer (CSNT), a CMN thermometer adapted for industrial use, a Coulomb blockade thermometer, a second‐sound acoustic thermometer and a superconductive reference device SRD‐1000. Several partners have set up 3He melting‐pressure thermometers to realise the PLTS‐2000, and will check it using Pt‐NMR, CMN and other thermometers. The scale, which was for…
Extraction of radioactive positive ions across the surface of superfluid helium: A new method to produce cold radioactive nuclear beams
Alpha-decay recoils 219Rn were stopped in superfluid helium and positive ions were extracted by electric field into the vapour phase. This first quantitative observation of extraction was successfully conducted using highly sensitive radioactivity detection. The efficiency for extraction across the liquid surface was 23 ± 4% at 1.60 K, the release time was 90 ± 10 ms at 1.50 K and the barrier for positive ions through a free superfluid-helium surface was 19.4 ± 4.5 K. The pulsed second sound proved to be effective in enhancing the extraction.
The energy gap of intermediate-valent SmB6 studied by point-contact spectroscopy
We have investigated the intermediate valence narrow-gap semiconductor SmB6 at low temperatures using both conventional spear-anvil type point contacts as well as mechanically controllable break junctions. The zero-bias conductance varied between less than 0.01 mikrosiemens and up to 1 mS. The position of the spectral anomalies, which are related to the different activation energies and band gaps of SmB6, did not depend on the the contact size. Two different regimes of charge transport could be distinguished: Contacts with large zero - bias conductance are in the diffusive Maxwell regime. They had spectra with only small non-linearities. Contacts with small zero - bias conductance are in th…