Ranran Zhang

Pressure-induced superconductivity in a three-dimensional topological material ZrTe5

Significance Three-dimensional (3D) Dirac semimetals have attracted a lot of advanced research recently on many exotic properties and their association with crystalline and electronic structures under extreme conditions. As one of the fundamental state parameters, high pressure is an effective, clean way to tune lattice as well as electronic states, especially in quantum states, thus their electronic and magnetic properties. In this paper, by combining multiple experimental probes (synchrotron X-ray diffraction, low-temperature transport under magnetic field) and theoretical investigations, we discover the pressure-induced 3D Dirac semimetal to superconductor transition in ZrTe5. As a new type of topological materials, ZrTe5 shows many exotic properties under extreme conditions. Using resistance and ac magnetic susceptibility measurements under high pressure, while the resistance anomaly near 128 K is completely suppressed at 6.2 GPa, a fully superconducting transition emerges. The superconducting transition temperature Tc increases with applied pressure, and reaches a maximum of 4.0 K at 14.6 GPa, followed by a slight drop but remaining almost constant value up to 68.5 GPa. At pressures above 21.2 GPa, a second superconducting phase with the maximum Tc of about 6.0 K appears and coexists with the original one to the maximum pressure studied in this work. In situ high-pressure synchrotron X-ray diffraction and Raman spectroscopy combined with theoretical calculations indicate the observed two-stage superconducting behavior is correlated to the structural phase transition from ambient Cmcm phase to high-pressure C2/m phase around 6 GPa, and to a mixture of two high-pressure phases of C2/m and P-1 above 20 GPa. The combination of structure, transport measurement, and theoretical calculations enable a complete understanding of the emerging exotic properties in 3D topological materials under extreme environments.

Pressure-Induced New Topological Weyl Semimetal Phase in TaAs

TaAs as one of the experimentally discovered topological Weyl semimetal has attracted intense interests recently. The ambient TaAs has two types of Weyl nodes which are not on the same energy level. As an effective way to tune lattice parameters and electronic interactions, high pressure is becoming a significant tool to explore new materials as well as their exotic states. Therefore, it is highly interesting to investigate the behaviors of topological Weyl fermions and possible structural phase transitions in TaAs under pressure. Here, with a combination of ab initio calculations and crystal structure prediction techniques, a new hexagonal P-6m2 phase is predicted in TaAs at pressure around 14 GPa. Surprisingly, this new phase is a topological semimetal with only single set of Weyl nodes exactly on the same energy level. The phase transition pressure from the experimental measurements, including electrical transport measurements and Raman spectroscopy, agrees with our theoretical prediction reasonably. Moreover, the P-6m2 phase seems to be quenched recoverable to ambient pressure, which increases the possibilities of further study on the exotic behaviors of single set of Weyl fermions, such as the interplay between surface states and other properties.

Pressure-induced iso-structural phase transition and metallization in WSe2

We present in situ high-pressure synchrotron X-ray diffraction (XRD) and Raman spectroscopy study, and electrical transport measurement of single crystal WSe2 in diamond anvil cells with pressures up to 54.0–62.8u2009GPa. The XRD and Raman results show that the phase undergoes a pressure-induced iso-structural transition via layer sliding, beginning at 28.5u2009GPa and not being completed up to around 60u2009GPa. The Raman data also reveals a dominant role of the in-plane strain over the out-of plane compression in helping achieve the transition. Consistently, the electrical transport experiments down to 1.8u2009K reveals a pressure-induced metallization for WSe2 through a broad pressure range of 28.2–61.7u2009GPa, where a mixed semiconducting and metallic feature is observed due to the coexisting low- and high-pressure structures.

Superconductivity and Charge Density Wave in ZrTe3−xSex

Charge density wave (CDW), the periodic modulation of the electronic charge density, will open a gap on the Fermi surface that commonly leads to decreased or vanishing conductivity. On the other hand superconductivity, a commonly believed competing order, features a Fermi surface gap that results in infinite conductivity. Here we report that superconductivity emerges upon Se doping in CDW conductor ZrTe3 when the long range CDW order is gradually suppressed. Superconducting critical temperature Tc(x) in ZrTe3−xSex (0u2009≤u2009xu2009≤u20090.1) increases up to 4u2009K plateau for 0.04u2009≤u2009xu2009≤u20090.07. Further increase in Se content results in diminishing Tc and filametary superconductivity. The CDW modes from Raman spectra are observed in xu2009=u20090.04 and 0.1 crystals, where signature of ZrTe3 CDW order in resistivity vanishes. The electronic-scattering for high Tc crystals is dominated by local CDW fluctuations at high temperatures, the resistivity is linear up to highest measured Tu2009=u2009300u2009K and contributes to substantial in-plane anisotropy.

Pressure-induced Td to 1T′ structural phase transition in WTe2

WTe2 is provoking immense interest owing to its extraordinary properties, such as large positive magnetoresistance, pressure-driven superconductivity and possible type-II Weyl semimetal state. Here we report results of high-pressure synchrotron X-ray diffraction (XRD), Raman and electrical transport measurements on WTe2. Both the XRD and Raman results reveal a structural transition upon compression, starting at 6.0 GPa and completing above 15.5 GPa. We have determined that the high-pressure lattice symmetry is monoclinic 1T′ with space group of P21/m. This transition is related to a lateral sliding of adjacent Te-W-Te layers and results in a collapse of the unit cell volume by ∼20.5%. The structural transition also casts a pressure range with the broadened superconducting transition, where the zero resistance disappears.

Dy2−xYxTi2O7: phonon vibration and magnetization with dilution

In this paper, the dilution effects of non-magnetic Y ions on spin-ice compound Dy2Ti2O7 by infrared and Raman spectra and magnetization measurements were investigated. An anomalous phonon softening with temperature decreasing is found in both the parent and diluted compounds, and Y doping can relax the softening of phonons except that of the IR mode near 233xa0cm−1, indicating a strong phonon–phonon coupling in the spin-ice material. The magnetization measurements reveal that the non-magnetic impurities do not severely influence the spin-ice rules in the ground state when the level of dilution is not very high. However, a large amount of dilution enhance the disorder and break the spin-ice state because the collective spin-flip clusters are no longer available.

Effect of K-Dopant on the Electro-Magnetic Behaviors in Cu1-xKxIr2S4

The electro-magnetic behaviors of K-doped Cu1−xKxIr2S4 are investigated. It is found that the doping of K has rare effect on the crystal lattice, however, apparent influence on the chemical bonds is observed from the Raman spectroscopy. In addition, the doping of K has prominent impact on the electronic and magnetic behaviors. Although the phase transition temperature is hardly affected by the doping of K, the hysteresis of orbitally induced Peierls phase transition is widened, which may be attributed to the potential barriers caused by the dopant. Moreover, the ρ(T) relation in the insulating phase has the tendency to metallic behavior, which indicates the orbital hybridization between 3p6 electrons of K+ with that of S2− ions.

Pressure-induced enhancement of optoelectronic properties in PtS2

PtS2, which is one of the group-10 transition metal dichalcogenides, attracts increasing attention due to its extraordinary properties under external modulations as predicted by theory, such as tunable bandgap and indirect-to-direct gap transition under strain; however, these properties have not been verified experimentally. Here we report the first experimental exploration of its optoelectronic properties under external pressure. We find that the photocurrent is weakly pressure-dependent below 3 GPa but increases significantly in the pressure range of 3 GPa–4 GPa, with a maximum ~ 6 times higher than that at ambient pressure. X-ray diffraction data shows that no structural phase transition can be observed up to 26.8 GPa, which indicates a stable lattice structure of PtS2 under high pressure. This is further supported by our Raman measurements with an observation of linear blue-shifts of the two Raman-active modes to 6.4 GPa. The pressure-enhanced photocurrent is related to the indirect-to-direct/quasi-direct bandgap transition under pressure, resembling the gap behavior under compression strain as predicted theoretically.

Pressure induced re-emergence of superconductivity in superconducting topological insulator Sr0.065Bi2Se3

The recent-discovered Sr

Pressure-induced reemergence of superconductivity in topological insulator Sr 0.065 Bi 2 Se 3

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