Liu Zhen-Xing

High-pressure synthesis of MgB2 superconductor with T-c above 39 K

We report on the high-pressure synthesis and superconductivity of MgB2 intermetallic compounds. The compounds have been obtained through high-pressure sintering of the mixtures of magnesium and boron fine powders under 5.0 GPa and at ~1000°C for 30 min. Magnetic measurements using a SQUID magnetometer show the sharp bulk superconducting transition above 39 K; the four-probe dc resistivity measurements indicate the highly-conductive normal state and sharp superconducting transition. The results highlight that high-pressure synthesis would be a promising way to promote the studies of this new kind of intermetallic superconductors.We report on the high-pressure synthesis and superconductivity of MgB2 intermetallic compounds. The compounds have been obtained through high-pressure sintering of the mixtures of magnesium and boron fine powders under 5.0 GPa and at similar to 1000 degreesC for 30 min. Magnetic measurements using a SQUID magnetometer show the sharp bulk superconducting transition above 39 K; the four-probe de resistivity measurements indicate the highly-conductive normal state and sharp superconducting transition. The results highlight that high-pressure synthesis would be a promising way to promote the studies of this new kind of intermetallic superconductors.

Effect of upward ion on field-aligned currents in the near-earth magnetotail

A 3-dimensional resistive MHD simulation was carried out to study the effect of the upward ions on the field-aligned currents (FACs) in the near-earth magnetotail. The simulation results show that the up-flow ions originating from the nightside auroral oval would drift into the center plasma sheet along the magnetic field lines in the plasma sheet boundary, and have an important effect on the field-aligned currents. The main conclusions include that: 1) the upward-ions mainly affect the field-aligned currents in the near-earth magnetotail (inside 15 Re); 2) the generated FACs in the near-earth region have two types, i.e., Region 1 FAC in the high-latitude and Region 2 FAC in the low-latitude; 3) FACs increase with the enhancement of the upward ion flux; 4) with the same flux of the upward ions, FACs enhance with the increase of the velocity of the up-flow ions; 5) the intensification of FACs is also closely related with the latitude of the upward ions, and the ions from the closed field line region generate larger FACs; 6) the generation of FACs is closely related with By created by the upward ions.

Bifurcated Current Sheet Structure in a Quiet Time by Cluster Spacecrafts

We analyse the vertical structure of the magnetotail current sheet for two time intervals during which the Cluster spacecrafts crossed the neutral sheet in a quiet time. In the intervals, the current sheet moved slowly, and the value of the AE index was relatively small, about 40–130 nT. We find two examples of current sheets, with the current density maximum at the magnetic equator (Bx = 0), as well as an example of off-centre or bifurcated current sheets. In the quiet time, without any fast plasma flow and without significant flapping motion, we also directly observed the bifurcated current sheet. The bifurcated current sheet is probably associated with instabilities in the current sheet. These may be important for researching the mechanism of current sheet bifurcation.

Acceleration of Ionospheric Out-Flowing Ions in the Substorm in Geo-magnetotail

Using dynamics equation, acceleration of out-flowing ion during dipolarization in a substorm in the magnetotail is simulated. The main results show that: (1) The ion distribution function that is initially exponentially decreasing with increasing speed is turned into a single peak distribution, and with time the peak moves towards higher speed. (2) The peak moves along V⊥ faster than that along V||, and the ion acceleration mainly occurs in the middle of the dipolarization. (3) The higher the initial energy, the faster the peak moves, and the more energy is obtained by the ions. The ion energy theoretically calculated is as high as about 102 keV, this is consistent with the observation.

Properties of Field Aligned Current in Plasma Sheet Boundary Layers in Magnetotail: Cluster Observation

Field aligned current (FAC) distribution in the plasma sheet boundary layers (PSBLs) in the magnetotail is studied statistically by analysing magnetic field data from the Cluster 4-point measurements. The results show that the FAC distribution on the dusk side is not the same as that on the dawn side in the magnetotail. On the each side earthward and tailward, FAC occurrences are different; occurrence and average current density of FACs in the northern hemisphere are different from those in the southern hemisphere. This implies that the FACs have dusk-dawn side asymmetry, polarity asymmetry and inter hemisphere difference in the magnetotail. The present results give a good observation evidence for study on the FAC mechanism.

Enhanced MgB2 Superconductivity Under High Pressure

We report on in situ high-pressure studies up to 1.0?GPa on the MgB2 superconductor which was high-pressure synthesized. The as-prepared sample is of high quality in terms of sharp superconducting transition (Tc) at 39?K from the magnetic measurements. The in situ high-pressure measurements were carried out using a Be-Cu piston-cylinder-type instrument with mixed oil as the pressure transmitting medium which warrants a quasi-hydrostatic pressure environment at low temperature. The superconducting transitions were measured using the electrical conductance method. It is found that Tc increases by more than 1?K with pressure in the low-pressure range, before the Tc value decreases with the further increase of the pressure.

Superconductivity of (R0.4Pr0.6)0.5Ca0.5Ba2Cu3O7-δ( R= La, Pr, Nd, Sm, Eu, Gd and Y) Prepared Under High Pressure

The nearly single phase (R0.4Pr0.6)(0.5)Ca0.5Ba2Cu3O7-delta compounds (R = La, Pr, Nd, Sm, Eu, Gd and Y) with T-c about 100K were prepared under high pressure and high temperature using the cubic-anvil apparatus. The T-c is much higher than that of the traditional R-123 superconductors without doping Ca on the rare-earth site. For various rare earths with different ionic radii, the (R0.4Pr0.6)(0.5)Ca0.5Ba2Cu3O7-delta compounds have almost the same critical temperature. This is different from the result of the ion-size effect of rare-earth ions in R1-xPrxBa2Cu3O7-delta compounds. Our results show that Pr has the same properties as other rare earths in the 123-phase compounds due to the doping of Ca.

Infrared spectra of the high-Tc TlBaCaCuO superconductors

Abstract The TlBaCaCuO superconductors with zero resistance at temperatures in the range of 100–123 K have been sintered by solid state reactions. Electron microscopy and X-ray diffraction studies reveal that they are multiphased, containing both Tl 2 Ba 2 CaCu 2 O 8 and Tl 2 Ba 2 Ca 2 Cu 3 O 10 superconducting phases. Infrared spectra have been measured at room temperature for the multiphase samples. The spectra display structures at ∼590 cm -1 ( P 1 ) and ∼530 cm -1 ( P 2 ) which seem characteristic to Cu-O stretching modes. The difference Δ P between P 1 and P 1 has been found to be related to the superconducting transition temperature. Smaller Δ P corresponds to a higher T c . The electron-like excitation of∼0.1 eV and the phonon-like structure at ∼850 cm -1 are also found to be important for high T c .

Field-Aligned Currents at the PSBL on 17 August 2001 Storm: Relationships with solar Wind Conditions

Using magnetic field and plasma data acquired with Cluster spacecrafts, we investigate the relationship between the field-aligned currents (FACs) at the plasma sheet boundary layer (PSBL) and solar wind dynamic pressure, as well as the interplanetary magnetic field (IMF) B(y) on 17 August 2001 storm. Our studies reveal that FAC density at the PSBL in the magnetotail in the storm time is controlled mainly by the solar wind dynamic pressure rather than IMF B(y) The FACs at the PSBL are associated with the low-altitude region-1 current and have the same polarity as region-1 current in the dawn sector. In the polar region, the footprints of the FACs at the PSBL expand equatorward. The data analysis also shows that a very strong FAC with a density over 40 nA.m(-2) appeared in this storm time when a substorm just occurred.

Clarification on Polarity of Bipolar Electric Field Solitary Structures in Space Plasmas with Satellite Observation

The bipolar electric field solitary (EFS) structures observed frequently in space plasmas by satellites have two different polarities, first positive electric field peak then negative (i.e., positive/negative) and first negative then positive peak (i.e., negative/positive). We provide the physical explanation on the polarity of observed bipolar EFS structures with an electrostatic ion fluid model. The results show that if initial electric field E0 > 0, the polarity of the bipolar EFS structure will be positive/negative; and if E0 < 0, the polarity of the bipolar EFS structure will be negative/positive. However, for a fixed polarity of the EFS, either positive/negative or negative/positive, if the satellite is located at the positive side of the EFS, the observed polarity should be positive/negative, if the satellite is located at the negative side of the EFS, the observed polarity should be negative/positive. Therefore, we provide a method to clarify the natural polarity of the EFS with observed polarity by satellites. Our results are significant to understand the physical process in space plasma with the satellite observation.