Double-sided laser heating system for in situ high pressure–high temperature monochromatic x-ray diffraction at the esrf

A new double-sided laser heating system optimized for monochromatic X-ray diffraction at high pressure and high temperature has been developed at beamline ID27 of the European Synchrotron Radiation Facility (ESRF). The main components of this system including optimized focusing optics to produce a large and homogenous heated area, optimized mirror optics for temperature measurements and a state-of-the-art diffraction setup are described in details. Preliminary data collected at high pressure and high temperature on tungsten and iron are presented.     

Relaxation and ordering processes in “macroscopic Wigner crystals”

The equilibrium configurations of a macroscopic Wigner crystal (2D system of interacting charged balls, mechanically excited) and their evolution towards these equilibrium configurations are presented. In particular, the variations of the number of remaining dislocations at equilibrium according to the number of particles, confinement shape and temperature have been extensively explored. One important result is the exhibition of the rapid creation of an unique grain boundary and its shrinkage during the annealing.Received: 25 February 2004, Published online: 18 June 2004PACS: 68.65.-k Low-dimensional, mesoscopic, and nanoscale systems: structure and nonelectronic properties - 73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems     

Installation and cosmic ray test of the high voltage system of the BESⅢ drift chamber

After examination of the designed high voltage power supply system of the BESⅢ drift chamber in the beam test of the full length prototype of drift chamber, a full system covering all the channels of high voltage was installed. The system's training and the high voltage value adjustment were carried out in the cosmic ray test of the BESⅢ drift chamber. The cosmic ray test for the full system and its final installation on the BESⅢ drift chamber were reported. The full system of high voltage power supply works stably and reliably.     

Study of the electric field and wall voltage in a high pressure ac-PDP cell by laser induced fluorescence spectroscopy

The electric field in a surface discharge type ac-PDP cell with He or He/Xe(0.1%) mixture has been measured over a wide range of pressure (5－50 kPa) using laser induced fluorescence detection. The wall voltage was estimated from the measured electric field. The Stark manifolds of triplet atomic helium Rydberg state (2s^3S) with principal quantum numbers (n=8 and 9) have been used to measure the electric field, as the lifetime of 2s^3S is longer than the single atomic helium Rydberg state (2s^1S) in high pressure discharge. Comparison of the Stark manifolds between the n=9 and n=8 shows that the measurement accuracy of electric field can be increased by 10%. The maximum electric field strength during discharge and the wall voltage at the end of pulse decreases with the increase of pressure. The comparison of He and He/Xe(0.1%) discharge at 13 kPa showed that He/Xe gas mixture discharge can accumulate more wall charge on MgO surface and the electric field was somewhat higher than those of pure helium discharge during pulse off period under the same discharge conditions.     

Characterization of the high—voltage pulsed discharge plasma of ammonia by emission spectroscopy

We have used optical emission spectroscopy to characterize the high-voltage pulsed discharge of ammonia.Ammonia was highly dissociated in the discharge at low pressures.More atomic nitrogen was generated as compared to the discharge of nitrogen gas at the same pressure of 0.8kPa.We discuss the elimination of the oxygen impurity in the ammonia discharge,and we estimate the time-dependent atomic excitation temperature and the electron density from the measured spectra.     

Effects of discharge current and voltage on the high density of metastable helium atoms

Both hollow-cathode and Penning-type discharges were adopted to excite helium atoms to a metastable state. Experimental data indicate that Penning discharge is more suitable for generating high fractions of metastables in a low-density helium beam for laser-induced fluorescence technique in measuring electric fields at the edge of a plasma. The metastable density increases with increasing helium gas pressure in the range of 1.33×10^{-2}－66.7Pa. The highest metastable density of 3.8×10^{16}m^{-3} is observed at a static gas pressure of 66.7Pa. An approximately linear relationship between the density of metastable helium atoms and the plasma discharge current is observed. Magnetic field plays a very important role in producing a high density of metastable atoms in Penning discharge.     

Evidence of antiferroelectric-like ordering from the study of dielectric permittivity–voltage behavior in bent core liquid crystalline material showing B2 phase

Detailed dielectric relaxation behavior of a bent core liquid crystalline material showing B₂ phase has been investigated. Only one relaxation mode is detected in the dielectric relaxation spectrum which was attributed to the long-axis rotation process. Though the field-dependent dielectric study does not reveal any other relaxation mode, the appearance of two humps in the dielectric permittivity–voltage spectrum, similar to the antiferroelectric (AFLC) case, confirms the signature of weak AFLC ordering in the B₂ phase.     

Radiation loss in a compound plasma system with high and low temperature regions

A code has been developed for calculating the non-coronal radiation of impurity in a compound plasma system which consists of high and low temperature regions. The radiation loss of impurity carbon in this system is calculated and analysed. The cooling rate in this system is smaller than that in a homogeneous plasma system due to the particle exchange between the two regions of the system. The volumetric radiation is much bigger in the low temperature region than in the high temperature region because of the relatively low temperature and high electron density, despite the much smaller volume.     

Antiferromagnetic ordering in the double perovskites La2−xSrxCoRuO6

The electric transport and magnetic susceptibility of double perovskites La_2?xSr_xCoRuO₆ have been studied over a temperature range up to 800–1000 K. The crystal and magnetic structure has been determined by neutron diffraction on two samples of the series, x=0.6 and 1.4, which represent the electron- and hole-doped systems with respect to “ideal” single-valent insulator x=1. The study shows that spins in both the Co and Ru f. c. c. like sublattices exhibit a long-range ordering of the antiferromagnetic type II (T_N=60 K for x=0.6 and T_N=60–80 K for x=1.4).     

First-principles study on superconductivity of the gold–indium alloy under high pressure

The superconductivity of gold–indium alloys has been investigated using first-principles calculations based on the density functional theory. At ambient pressure, the calculated superconducting transition temperature (T _c) is 0.04 μ K in pure gold, but T _c dramatically increases by substituting indium atoms for gold atoms. The gold–indium alloy having 12.5 atomic percent indium (Au_0.875In_0.125) shows T _c of 0.1 K, and Au_0.75In_0.25 marks 1.7 K. The dramatic increase in T _c owing to the alloying effect is caused by the enhancement of the electron–phonon coupling. The superconductivity of gold is predicted to be drastically weakened with increasing pressure and virtually disappear at 10 GPa, but it continues up to at least 30 GPa by the inclusion of indium atoms.     

Core-loss reduction of Fe–Si–Cr crystalline alloy according to particle size in the high frequency band

We used different sizes of gas atomized Fe–Si–Cr alloy powder to produce soft magnetic composites (SMCs), this alloy has higher resistivity than existing materials used in SMCs. These powders were prepared by sieving raw materials which had an average size from less than 25 μm to over 63 μm. Our experiments show that as particle size decreases, the magnetic saturation tends to increase, the sample made from the powder with particles 25–38 μm in size recorded the highest magnetic saturation of 169.38 emu/g. Additionally, as particle size decreased, permeability increased. The sample made from powder with particles under 25 μm had a permeability of 20.7 H/m at 1 MHz. Also, the relationship between particle size and quality factor was found to be inversely proportional. Finally, the minimum core-loss was 187.26 kW/m³ at 1 MHz for the sample made from powder whose constituent particles are under 25 μm.     

On the existence of “low/high spin transitions” in coupled spin-phonon models

We study the model used recently (Takahashi, K.: Z. Phys. B — Condensed Matter71, 205 (1988)) to perform Monte Carlo simulations of low/high spin transitions in certain transition metal complex materials. We show analytically that the model, a special Ising model on a harmonic square lattice with spring constants depending on local spin configurations, indeed shown a phase transition of the expected kind.     

The influences of the second harmonics together with the asymmetries of junctions on the voltage–current characteristics of high TC DC SQUIDs

We consider a superconducting quantum interference device having two arbitrary different over damped junctions transporting different currents. By replacing the governed two-dimensional Fokker–Planck equation with two one-dimensional equations, two density probability currents are appeared which determine the statistical average of the time-averaged total voltage across the device. To obtain the density probability currents, two coupled integral equations are introduced. These equations together with two other equations coming from normalizing conditions, found one generalized formulation for the voltage–current characteristics of the device. Based on that, the voltage–current characteristics of large inductance asymmetric DC SQUIDs having first and second harmonics in their current-phase relations are obtained and some predictions are illustrated.     

A simple method of extracting the polarization charge density in the AlGaN/GaN heterostructure from current—voltage and capacitance—voltage characteristics

An Ni Schottky contact on the AlGaN/GaN heterostructure is fabricated. The flat-band voltage for the Schottky contact on the AlGaN/GaN heterostructure is obtained from the forward current-voltage characteristics. With the measured capacitance-voltage curve and the flat-band voltage, the polarization charge density in the AlGaN/GaN heterostructure is investigated, and a simple formula for calculating the polarization charge density is obtained and analyzed. With the approach described in this paper, the obtained polarization charge density agrees well with the one calculated by self-consistently solving Schrodinger's and Poisson's equations.