Ziqiao Wang

Physical origin of the ferromagnetic ordering above room temperature in GaMnN nanowires

We report a new method for large-scale production of GaMnN nanowires, by annealing manganese–gallium oxide nanowires in flowing ammonia at high temperature. Microstructure analysis indicates that the GaMnN nanowires have wurtzite GaN structure without Mn precipitates or Mn-related second phases. Magnetic measurements reveal that ferromagnetic ordering exists in the GaMnN nanowires, whose Curie temperature is above room temperature. A mean-field model based on the exchange coupling of the nondegenerate carrier-localized impurity band, together with the consideration of the superexchange antiferromagnetic interaction, is used to explain the physical origin of the observed ferromagnetic ordering. Theoretical calculations indicate that the Curie temperature increases with the increase of the hole density and reveal a small ratio of the hole to the magnetic dopant density in the samples. The low ratio of hole to Mn concentration and the superexchange antiferromagnetic interaction lead to the very concave temperature dependence of the magnetization curve in contrast to conventional ferromagnetic M–T behaviour. The theoretical calculation is in agreement with the experimental data.

Superconductivity in topologically nontrivial material Au 2 Pb

The search for nontrivial superconductivity in novel quantum materials is currently a most attractive topic in condensed matter physics and material science. The experimental studies have progressed quickly over the past couple of years. In this article, we report systematic studies of superconductivity in Au2Pb single crystals. The bulk superconductivity (onset transition temperature, Tconset= 1.3 K) of Au2Pb is characterized by both transport and diamagnetic measurements, where the upper critical field Hc2 shows unusual quasi-linear temperature dependence. The superconducting gap is revealed by point contact measurement with gold tip. However, when using tungsten (W) tip, which is much harder, the superconducting gap probed is largely enhanced as demonstrated by the increases of both Tconset and upper critical field (Hc2). This can be interpreted as a result of increase in density of states under external anisotropic stress imposed by the tip, as revealed by first-principles calculations. Furthermore, novel phase winding of the pseudospin texture along k-space loops around the Fermi energy is uncovered from the calculations, indicating that the observed superconductivity in Au2Pb may have nontrivial topology.

High-temperature superconductivity in one-unit-cell FeSe films

Since the dramatic enhancement of the superconducting transition temperature (T c) was reported in a one-unit-cell FeSe film grown on a SrTiO3 substrate (1-UC FeSe/STO) by molecular beam epitaxy (MBE), related research on this system has become a new frontier in condensed matter physics. In this paper, we present a brief review on this rapidly developing field, mainly focusing on the superconducting properties of 1-UC FeSe/STO. Experimental evidence for high-temperature superconductivity in 1-UC FeSe/STO, including direct evidence revealed by transport and diamagnetic measurements, as well as other evidence from scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES), are overviewed. The potential mechanisms of the enhanced superconductivity are also discussed. There are accumulating arguments to suggest that the strengthened Cooper pairing in 1-UC FeSe/STO originates from the interface effects, specifically the charge transfer and coupling to phonon modes in the TiO2 plane. The study of superconductivity in 1-UC FeSe/STO not only sheds new light on the mechanism of high-temperature superconductors with layered structures, but also provides an insight into the exploration of new superconductors by interface engineering.

Growth of HgBa2CaCu2Oy films with a buffer layer

Using YBa2 Cu3 O7 (YBCO) as a buffer layer high Tc superconducting HgBa2 CaCu2 Oy (Hg-1212) thin films with zero resistance temperature (Tc ) of 115-124 K and critical current density (Jc ) of 1 MA cm-2 at 77 K in zero magnetic field have been grown on (100)Y-ZrO2 substrates. The high Tc , Jc and good morphology indicate that YBCO is a good buffer for growing the Hg cuprate films. This novel buffer makes it possible to grow Hg cuprate films on many substrates, including some metallic tapes.

Resistivity variation in heteroepitaxial multilayers with different thicknesses of the layers

Resistivity measurements on (LCMO/GSMO) multilayers with different thicknesses of the LCMO layers are reported. The resistivity maximum temperature indicates that the metal-insulator transition decreases with reducing LCMO layer thickness. Data above show activated conduction behaviour with , the temperature dependence of the conductivity for small-polaron hopping. The activation energy increases with reducing LCMO layer thickness in the multilayers. The explanation of the properties of the LCMO/GSMO multilayers is twofold. For doped manganese oxides, these changes can be understood by (i) considering the effect of strain on the LCMO layers in the LCMO/GSMO multilayers and (ii) assuming that the ferromagnetic transition Curie temperature and depend on the Mn-O-Mn bond angle.

Transport properties of Pr0.5Ca0.5(Ba1−xSrx)2Cu3O7−y(x=0,0.1,0.2,0.3) epitaxial thin films

Pr 0.5 Ca 0.5 (Ba 1-x Sr x ) 2 Cu 3 O 7-y (PCBSCO)(x=0, 0.1, 0.2, 0.3) epitaxial thin films have been prepared by the pulsed laser deposition method. By 10% Sr substitution for Ba, the T C (R=0) of the PCBSCO films increases from 35K(x=0) to 39K(x=0.1). The resistance measurments show that PCBSCO (x=0.1) films have lower resistivity (p) compared with that of PCBSCO (x=0) films. Increasing the substitution concentration x further to 0.2 and 0.3, the resistivities of the films begin to increase and the superconductivity is gradually suppressed. The effects of the oxygen content on the superconductivity of PCBSCO have also been studied.