Ph. Lerch

Stability of coronene at high temperature and pressure.

The infrared response of coronene (C(24)H(12)) under pressure and temperature conditions up to 10 GPa and 300 °C is examined in situ using a diamond anvil cell and synchrotron-source Fourier transform infrared (FTIR) spectroscopy. Coronene is a polycyclic aromatic hydrocarbon that is present in the interstellar medium and meteorites which may have contributed to the Earths primordial carbon budget. It appears to undergo a reversible phase transition between 2 and 3.2 GPa at ambient temperature; new intramolecular bonds in the region 840-880 cm(-1) result from compression. We document the shift of spectral features to higher wavenumbers with increasing pressure but find this change suppressed by increased temperature. By investigating the stability of coronene over a range of naturally occurring conditions found in a range of environments, we assess the survival of the molecule through various terrestrial and extraterrestrial processes. Coronene has previously been shown to survive atmospheric entry during Earth accretion; this can now be extended to include survival through geological processes such as subduction and silicate melting of the rock cycle, opening the possibility of extraterrestrial coronene predating terrestrial accretion existing on Earth.

Arrays of Josephson Junctions: Model Systems for Two-Dimensional Physics

Planar arrays of Josephson junctions provide excellent model systems to study a variety of phase transitions in two dimensions. Experiments are reported in which the critical behaviour of periodic arrays is studied by measuring the dynamic response of the system to a small oscillating field. Tuning of frustration occurs by adjusting the magnetic flux threading a unit cell of the array, while by varying the driving frequency it is possible to probe the critical behaviour of the system at different length scales. The dynamic response near the phase transition of an un frustrated array is well described by a model incorporating the vortex unbinding mechanism predicted by the Kosterlitz-Thouless theory. Some aspects of the dynamic response of frustrated arrays are also discussed.

Infrared study of hydrogen up to 310 GPa at room temperature

We observed a strong difference of the pressure dependence of the infrared (IR) active molecular vibron of hydrogen in phase IV in the 200–310 GPa pressure range in comparison with the Raman vibrons. While the Raman vibron strongly splits (∼250 cm−1) at the transition from phases III to IV at 220 GPa, the IR vibron nearly does not change. This small spitting of IR vibron is not described by the graphene-like structure proposed for phase IV. The combined pressure dependence of Raman and IR vibrons provides a sensitive test for further theoretical models of phase IV.

Inductive superconducting transition in artificial cuprate superlattices the effect of in-plane and interface disorder

Abstract Artificial c -axis oriented YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 (YBACO/PBCO) supperlattices were grown on MgO and SrTiO 3 (100) substrates by an in-situ laser ablation process. Samples with stackings of two unit cells of YBCO, alternating with 3, 4, 5 and 15 unit cells of PBCO, respectively, were prepared. Information on the in-plane orientation was obtained from X-ray pole figure measurements. The effective inverse kinetic inductance L K - 1 (T), proportional to the effective density of Cooper pairs in the sample, was determined with complex-impedance measurements. The data reveal that the transition to the superconducting state, deduced from the temperature dependence of L K - 1 , depends both on the in-plane order of the crystal structure and on the quality of the interfaces between YBCO and PBCO.

AC quantum interference in a two-dimensional array of superconducting weak links

Abstract The observation of quantum interference transitions in the dynamic resistance of a large two-dimensional array of proximity-effect junctions driven by superposed dc- and rf-currents is reported. It is shown that, under the synchronizing effect of the rf-field, the array behaves as a superposition of coherent quantum oscillators connected in series.

In-situ vibrational optical rotatory dispersion of molecular organic crystals at high pressures

Organic structures respond to pressure with a variety of mechanisms including degradation, intramolecular transformation and intermolecular bonding. The effects of pressure on chiral organic structures are of particular interest because of the potential steric controls on the fate of pressurized molecules. Despite representing a range of opportunities, the simultaneous study of high pressures on different forms of chiral structures is poorly explored. We have combined synchrotron-source vibrational optical rotatory dispersion, micro-Fourier transform infrared spectroscopy and the use of a diamond anvil cell to simultaneously monitor the effects of pressure on the two enantiomers of the simple amino acid, alanine.

The influence of proximity effect on the quasiparticle collection

Abstract Results from a comparison between current-voltage data measured on Nb/Al/AlO x /Al/Nb tunneling junctions to a proximity effect model proposed by Golubov et al. [J. Low Temp. Phys. 70 (1988) 83] are used to get an estimation of the quasiparticle collection as a function of the Al-film thickness (4 and 120 nm) in X-ray detectors.

Fabrication of Nb and Ta/Al/AlOx/Al/Nb tunneling junction X-ray detectors: a list of problems

Abstract A whole wafer fabrication procedure using selective etching of refractory materials to produce Nb- or Ta/Al/AlO x Al/Nb tunneling X-ray detectors is discussed. Fabrication aspects and preliminary results are presented.

Superconducting strip detectors as position sensitive particle detectors

Abstract The feasibility of using of current-biased superconducting strips for radiation detection is investigated. Narrow Ta strips are exposed to 5.5 MeV α-particle radiation and the rise-time of the induced voltage pulses is measured as function of temperature and bias current. The rise-time of the voltage signal strongly depends on the site on the strip which is hit by the α-particle. In order to determine the spatial resolution of a superconducting strip detector, position-sensitive measurements were performed. The maximum lateral resolution estimated so far is 25μm in a 7μm wide, 340 nm thick and 0.6 mm long Ta-strip.

A Nb/AlOx/Nb trilayer process for the fabrication of X-ray detectors

Abstract A new Nb/AlO x /Nb trilayer process for X-ray detector applications has been developed. The process employs a movable mechanical slit which, after the sputter deposition of the base Nb electrode followed by a thin layer of Al and its oxidation, is shifted to a second position where the Nb counter electrode is deposited. The overlap area of the two electrodes forms the junction are after final patterning by standard photolithography and reactive ion etching. The best junctions so far obtained have, at 4.2 K, typically a quality factor of 42 mV with a Josepshon current density of 1200 A/cm 2 , and a tunneling time τ tun in the range of 50 to 500 ns. We discuss several aspects of this technique in comparison with the selective niobium anodization process (SNAP) and the selective niobium etching process (SNEP).