Evolution of Structural and Electronic Properties in NbTe2 under High Pressure.

Transition metal dichalcogenides (TMDs) have attracted wide attention due to their quasi-two-dimensional layered structure and exotic properties. Plenty of efforts have been done to modulate the interlayer stacking manner for novel states. However, as an equally important element in shaping the unique properties of TMDs, the effect of intralayer interaction is rarely revealed. Here, we report a particular case of pressure-tuned re-arrangement of intralayer atoms in distorted 1T-NbTe2, which was demonstrated to be a new type of structural phase transition in TMDs. The structural transition occurs in the pressure range of 16-20 GPa, resulting in a transformation of Nb atomic arrangement from the trimeric to dimeric structure, accompanied by a dramatic collapse of unit cell volume and lattice parameters. Simultaneously, a charge density wave (CDW) was also found to collapse during the phase transition. The strong increase in the critical fluctuations of CDW induces a significant decline in the electronic correlation and a change of charge carrier type from hole to electron in NbTe2. Our finding reveals a new mechanism of structure evolution and expands the field of pressure-induced phase transition.

High-temperature concomitant metal-insulator and spin-reorientation transitions in a compressed nodal-line ferrimagnet Mn3Si2Te6.

Symmetry-protected band degeneracy, coupled with a magnetic order, is the key to realizing novel magnetoelectric phenomena in topological magnets. While the spin-polarized nodal states have been identified to introduce extremely-sensitive electronic responses to the magnetic states, their possible role in determining magnetic ground states has remained elusive. Here, taking external pressure as a control knob, we show that a metal-insulator transition, a spin-reorientation transition, and a structural modification occur concomitantly when the nodal-line state crosses the Fermi level in a ferrimagnetic semiconductor Mn3Si2Te6. These unique pressure-driven magnetic and electronic transitions, associated with the dome-shaped Tc variation up to nearly room temperature, originate from the interplay between the spin-orbit coupling of the nodal-line state and magnetic frustration of localized spins. Our findings highlight that the nodal-line states, isolated from other trivial states, can facilitate strongly tunable magnetic properties in topological magnets.

Superconductivity in In2Te3 under Compression Induced by Electronic and Structural Phase Transitions.

Indium telluride (In2Te3) is a typical layered material among III-IV families that are extremely sensitive to pressure and strain. Here, we use a combination of high-pressure electric transport, Raman, XRD, and first-principles calculations to study the electronic properties and structural evolution characteristics of In2Te3 under high pressure. Our results reveal the evidence of isostructure electronic transitions. First-principle calculations demonstrate that the evolution of phonon modes is associated with the transition from semiconductor to metal due to the increase in the density of states near the Fermi level. The pressure-induced metalization as a precursor monitors the structural phase transition, and then the superconductivity is produced. Further, in decompression, Tc slightly increased and remained at 3.0 GPa, and then the disorder is present and the superconductivity is suppressed. Our work not only perfects the superconducting phase of the In-Te system under pressure but also provides a reference for further superconducting research and applications.

Superconductivity Induced by Lifshitz Transition in Pristine SnS2 under High Pressure.

The importance of electronic structure evolutions and reconstitutions is widely acknowledged for strongly correlated systems. The precise effect of pressurized Fermi surface topology on metallization and superconductivity is a much-debated topic. In this work, an evolution from insulating to metallic behavior, followed by a superconducting transition, is systematically investigated in SnS2 under high pressure. In-situ X-ray diffraction measurements show the stability of the trigonal structure under compression. Interestingly, a Lifshitz transition, which has an important bearing on the metallization and superconductivity, is identified by the first-principles calculations between 35 and 40 GPa. Our findings provide a unique playground for exploring the relationship of electronic structure, metallization, and superconductivity under high pressure without crystal structural collapse.

Short-Range Crystalline Order-Tuned Conductivity in Cr2Si2Te6 van der Waals Magnetic Crystals.

Two-dimensional magnetic materials (2DMMs) are significant not only for studies on the nature of 2D long-range magnetic order but also for future spintronic devices. Of particular interest are 2DMMs where spins can be manipulated by electrical conduction. Whereas Cr2Si2Te6 exhibits magnetic order in few-layer crystals, its large band gap inhibits electronic conduction. Here we show that the defect-induced short-range crystal order in Cr2Si2Te6, on the length scale below 0.6 nm, induces a substantially reduced band gap and robust semiconducting behavior down to 2 K that turns to metallic above 10 GPa. Our results will be helpful in designing conducting states in 2DMMs and call for spin-resolved measurement of the electronic structure in exfoliated ultrathin crystals.

Pressure-induced structural and electronic transitions of thiospinel Fe3S4.

We report the investigations on the structural and electronic properties of an inverse spinel Fe3S4 at high pressures using synchrotron x-ray diffraction (XRD) and electrical transport measurements. Our XRD measurements at high pressures reveal an irreversible structural phase transformation on compression above ∼3 GPa from a cubic spinel (Fd-3m space group) into a monoclinic Cr3S4-type structure (I2/m space group). Electrical transport measurements suggest that the high pressure monoclinic phase has a semiconducting behavior. This semiconducting behavior is found to persist up to the highest pressure of measurement of ∼23 GPa. These results show that while Fe3S4 possesses similar high pressure structural properties with other thiospinels, the electronic properties under pressure show a rather strong similarity to its oxide counterpart, Fe3O4, at high pressures.

Pressure-induced enhancement in the superconductivity of ZrTe3.

We report the superconductivity enhancement of ZrTe3 on compression up to 33 GPa. The superconducting transition occurs above 4.1 GPa and the superconducting temperature (T C) increases with pressure in further compression, reaching a maximum of 7.1 K at ~28 GPa. An anomalous change of superconducting temperature is seen in the compression above 21 GPa. No structural phase transition is observed in the whole compression up to 36 GPa, but a subtle change in structural parameter is seen between 17-19 GPa, which seems relevant to the anomalous increase in the superconducting temperature. First-principle calculations reveal that the density of states at the Fermi level increases with pressure, which explains the enhancement of T C in ZrTe3 under compression.