Shuxia Zhang

Pressure-induced superconductivity in topological parent compound Bi2Te3

We report a successful observation of pressure-induced superconductivity in a topological compound Bi2Te3 with Tc of ∼3 K between 3 to 6 GPa. The combined high-pressure structure investigations with synchrotron radiation indicated that the superconductivity occurred at the ambient phase without crystal structure phase transition. The Hall effects measurements indicated the hole-type carrier in the pressure-induced superconducting Bi2Te3 single crystal. Consequently, the first-principles calculations based on the structural data obtained by the Rietveld refinement of X-ray diffraction patterns at high pressure showed that the electronic structure under pressure remained topologically nontrivial. The results suggested that topological superconductivity can be realized in Bi2Te3 due to the proximity effect between superconducting bulk states and Dirac-type surface states. We also discuss the possibility that the bulk state could be a topological superconductor.

Observation of room temperature saturated ferroelectric polarization in Dy substituted BiFeO3 ceramics

High quality Bi1−xDyxFeO3 (0u2009≤u2009xu2009≤u20090.15) ceramics have been fabricated by sintering Dy-doped BiFeO3 (BFO) precursor powders at a low temperature of 780u2009°C. The magnetic properties of BFO were improved by the introduction of Dy on the Bi-site. More importantly, well saturated ferroelectric hysteresis loops and polarization switching currents have been observed at room temperature. A large remnant polarization (2Pr) value of 62u2009μC/cm2 is achieved, which is the highest value reported so far for rare-earth-doped BFO ceramics. Moreover, mechanisms for improved multiferroic properties depending on chemical doping-caused structure evolutions have also been discussed.

Superconductivity in Topological Insulator Sb2Te3 Induced by Pressure

Topological superconductivity is one of most fascinating properties of topological quantum matters that was theoretically proposed and can support Majorana Fermions at the edge state. Superconductivity was previously realized in a Cu-intercalated Bi2Se3 topological compound or a Bi2Te3 topological compound at high pressure. Here we report the discovery of superconductivity in the topological compound Sb2Te3 when pressure was applied. The crystal structure analysis results reveal that superconductivity at a low-pressure range occurs at the ambient phase. The Hall coefficient measurements indicate the change of p-type carriers at a low-pressure range within the ambient phase, into n-type at higher pressures, showing intimate relation to superconducting transition temperature. The first principle calculations based on experimental measurements of the crystal lattice show that Sb2Te3 retains its Dirac surface states within the low-pressure ambient phase where superconductivity was observed, which indicates a strong relationship between superconductivity and topology nature.

Superconductivity in Topological Insulator Sb[subscript 2]Te[subscript 3] Induced by Pressure

Topological superconductivity is one of most fascinating properties of topological quantum matters that was theoretically proposed and can support Majorana Fermions at the edge state. Superconductivity was previously realized in a Cu-intercalated Bi2Se3 topological compound or a Bi2Te3 topological compound at high pressure. Here we report the discovery of superconductivity in the topological compound Sb2Te3 when pressure was applied. The crystal structure analysis results reveal that superconductivity at a low-pressure range occurs at the ambient phase. The Hall coefficient measurements indicate the change of p-type carriers at a low-pressure range within the ambient phase, into n-type at higher pressures, showing intimate relation to superconducting transition temperature. The first principle calculations based on experimental measurements of the crystal lattice show that Sb2Te3 retains its Dirac surface states within the low-pressure ambient phase where superconductivity was observed, which indicates a strong relationship between superconductivity and topology nature.

Superconductivity in Strong Spin Orbital Coupling Compound Sb2Se3

Recently, A2B3 type strong spin orbital coupling compounds such as Bi2Te3, Bi2Se3 and Sb2Te3 were theoretically predicated to be topological insulators and demonstrated through experimental efforts. The counterpart compound Sb2Se3 on the other hand was found to be topological trivial, but further theoretical studies indicated that the pressure might induce Sb2Se3 into a topological nontrivial state. Here, we report on the discovery of superconductivity in Sb2Se3 single crystal induced via pressure. Our experiments indicated that Sb2Se3 became superconductive at high pressures above 10u2005GPa proceeded by a pressure induced insulator to metal like transition at ~3u2005GPa which should be related to the topological quantum transition. The superconducting transition temperature (TC) increased to around 8.0u2005K with pressure up to 40u2005GPa while it keeps ambient structure. High pressure Raman revealed that new modes appeared around 10u2005GPa and 20u2005GPa, respectively, which correspond to occurrence of superconductivity and to the change of TC slop as the function of high pressure in conjunction with the evolutions of structural parameters at high pressures.

The comprehensive phase evolution for Bi2Te3 topological compound as function of pressure

The recently discovered three-dimensional topological insulator Bi2Te3 is studied as function of pressure in terms of crystal structures, resistance, and Hall coefficient. The superconductivity is found in phase I (ambient phase) Bi2Te3 with Tcu2009∼u20093u2009K, which is related to the topological features. The evolution of crystal structure with pressure is investigated by high pressure synchrotron radiation experiments that reveal structural transitions occurring at about 8u2009GPa, 13u2009GPa, and 16u2009GPa, respectively. Furthermore, the high pressure phases of Bi2Te3 are also superconducting but with much higher Tcu2009∼u20098u2009K. The superconducting transitions are compared with those for Bi, Te elements. A global phase diagram of Bi2Te3 as function of pressure up to 30u2009GPa is obtained.

Simultaneously improved magnetization and polarization in BiFeO3 based multiferroic composites

The multiferroic composites with nominal stoichiometry Bi0.8Dy0.2Fe1−yTiyO3+δ were fabricated by adding Ti to Bi0.8Dy0.2FeO3 through a two-stage solid-state reaction method. In the composites obtained, the major phase was isostructural to Bi0.8Dy0.2FeO3, and the secondary phase of Dy3Fe5O12 was also detected. Most interestingly, it was found that both the magnetization and electrical polarization were encouraged by the secondary phase Dy3Fe5O12. In addition to the improved ferroelectric property, a large saturation magnetization of 2.07 emu/g was reached for y≥0.03 samples. The simultaneously enhanced ferroelectric and magnetic properties achieved in the composites due to the existence of ferrimagnetic insulated Dy3Fe5O12 may have implications for further studies on BiFeO3 based materials.

Significantly Improved Multiferrioc Properties of BiFeO3/Pb(Zr0.52Ti0.48)O3 Bilayer Films by Magnetic Field Annealing

We report a novel method of synthesizing multiferroic BiFeO3/Pb(Zr0.52Ti0.48)O3 (BFO/PZT) bilayer films based on the use of a high magnetic field. Simultaneously enhanced magnetization and electric polarization were observed at room temperature in the films annealed under an external magnetic field. Compared with the control samples annealed at zero field, the saturated magnetization and double remanent polarization were increased by a factor of 6 at room temperature. These results demonstrate that the strong magnetic annealing method is an alternative way to fabricate high-performance BiFeO3 films.

Superconductivity in HfTe 5 across weak to strong topological insulator transition induced via pressures

Recently, theoretical studies show that layered HfTe5 is at the boundary of weak & strong topological insulator (TI) and might crossover to a Dirac semimetal state by changing lattice parameters. The topological properties of 3D stacked HfTe5 are expected hence to be sensitive to pressures tuning. Here, we report pressure induced phase evolution in both electronic & crystal structures for HfTe5 with a culmination of pressure induced superconductivity. Our experiments indicated that the temperature for anomaly resistance peak (Tp) due to Lifshitz transition decreases first before climbs up to a maximum with pressure while the Tp minimum corresponds to the transition from a weak TI to strong TI. The HfTe5 crystal becomes superconductive above ~5.5u2009GPa where the Tp reaches maximum. The highest superconducting transition temperature (Tc) around 5u2009K was achieved at 20u2009GPa. Crystal structure studies indicate that HfTe5 transforms from a Cmcm phase across a monoclinic C2/m phase then to a P-1 phase with increasing pressure. Based on transport, structure studies a comprehensive phase diagram of HfTe5 is constructed as function of pressure. The work provides valuable experimental insights into the evolution on how to proceed from a weak TI precursor across a strong TI to superconductors.

Enhanced Jc–B properties of MgB2 tapes by yttrium acetate doping

We report the effect of yttrium acetate addition on in situ powder-in-tube processed MgB2/Fe tapes. The amount of yttrium acetate was varied from 0 to 20?wt% and sintering was performed at 700?850??C for 1?h in a flowing Ar atmosphere. We found that a significant Jc?B enhancement was easily achieved in high field by yttrium acetate doping. At 4.2?K, the transport Jc for the best yttrium-acetate-added tapes (10?wt%) reached almost 104?A?cm ? 2 at 12?T. The improvement of Jc performance in the yttrium-acetate-doped samples can be attributed to the combination of the carbon substitution for B and stronger flux pinning.