Quasi-two-dimensional kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K}$, Rb, and Cs) have attracted much recent interest due to exotic quantum phenomena such as unconventional superconductivity, topological charge order, and giant anomalous Hall effect. Here we report pressure-induced reemergent superconductivity in $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$ by electrical transport measurements under high pressures up to 47.9 GPa. We show that the superconducting critical temperature $T_c$ is first enhanced by pressure and reaches its first maximum ∼8.9 K at 0.8 GPa; then the $T_c$ is suppressed by pressure and cannot be detected above 7.5 GPa, forming a dome-shaped superconducting phase diagram. Remarkably, upon further compression above 16.5 GPa, a new superconducting state arises, of which $T_c$ is enhanced by pressure to a second maximum ∼5.0 K and the reemergent superconductivity keeps robust up to 47.9 GPa. Combined with high-pressure synchrotron x-ray-diffraction measurements that demonstrate the stability of the pristine hexagonal phase up to 43.1 GPa, we suggest that the reemergence of superconductivity in the V-based superconductor could be attributed to a pressure-induced Lifshitz transition.

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.

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 54.0–62.8 GPa. The XRD and Raman results show that the phase undergoes a pressure-induced iso-structural transition via layer sliding, beginning at 28.5 GPa and not being completed up to around 60 GPa. 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 1.8 K reveals a pressure-induced metallization for WSe₂ through a broad pressure range of 28.2–61.7 GPa, where a mixed semiconducting and metallic feature is observed due to the coexisting low- and high-pressure structures.

Among the family of transition metal dichalcogenides, ReS₂ occupies a special position, which crystalizes in a unique distorted low-symmetry structure at ambient conditions. The interlayer interaction in ReS₂ is rather weak, thus its bulk properties are similar to those of monolayer. However, how compression changes its structure and electronic properties is unknown so far. Here using ab initio crystal structure searching techniques, we explore the high-pressure phase transitions of ReS₂ extensively and predict two new high-pressure phases. The ambient pressure phase transforms to a “distorted-1T” structure at very low pressure and then to a tetragonal I4₁/amd structure at around 90 GPa. The “distorted-1T” structure undergoes a semiconductor–metal transition at around 70 GPa with a band overlap mechanism. Electron–phonon calculations suggest that the I4₁/amd structure is superconducting and has a critical superconducting temperature of about 2 K at 100 GPa. We further perform high-pressure electrical resistance measurements up to 102 GPa. Our experiments confirm the semiconductor–metal transition and the superconducting phase transition of ReS₂ under high pressure. These experimental results are in good agreement with our theoretical predictions.

Weyl semimetal defines a material with three-dimensional Dirac cones, which appear in pair due to the breaking of spatial inversion or time reversal symmetry. Superconductivity is the state of quantum condensation of paired electrons. Turning a Weyl semimetal into superconducting state is very important in having some unprecedented discoveries. In this work, by doing resistive measurements on a recently recognized Weyl semimetal TaP under pressures up to about 100 GPa, we show the concurrence of superconductivity and a structure transition at about 70 GPa. It is found that the superconductivity becomes more pronounced when decreasing pressure and retains when the pressure is completely released. High-pressure x-ray diffraction measurements also confirm the structure phase transition from I4₁md to P-6m2 at about 70 GPa. More importantly, ab-initial calculations reveal that the P-6m2 phase is a new Weyl semimetal phase and has only one set of Weyl points at the same energy level. Our discovery of superconductivity in TaP by high pressure will stimulate investigations on superconductivity and Majorana fermions in Weyl semimetals.

We report on the discovery of a pressure-induced topological and superconducting phase of SnSe, a material which attracts much attention recently due to its superior thermoelectric properties. In situ high-pressure electrical transport and synchrotron x-ray diffraction measurements show that the superconductivity emerges along with the formation of a CsCl-type structural phase of SnSe above around 27 GPa, with a maximum critical temperature of 3.2 K at 39 GPa. Based on ab initio calculations, this CsCl-type SnSe is predicted to be a Dirac line-node (DLN) semimetal in the absence of spin-orbit coupling, whose DLN states are protected by the coexistence of time-reversal and inversion symmetries. These results make CsCl-type SnSe an interesting model platform with simple crystal symmetry to study the interplay of topological physics and superconductivity.