Evolution of Structural and Electronic Properties of TiSe2 under High Pressure

Structural phase transition and potential superconductivity initiated by pressure-driven in 1T-CrSe2

The exploration of the superconducting properties of antiferromagnetic parent compounds containing transition metals under pressure provides a unique idea for finding and designing superconducting materials with better performance. In this paper, the close relationship between the possible superconductivity and structure phase transition of the typical van der Waals layered material 1T-CrSe2induced by pressure is studied by means of electrical transport and x-ray diffraction for the first time. We introduce the possibility of pressure-induced superconductivity at 20 GPa, with a criticalTcof approximately at 4 K. The superconductivity persists up to the highest measured pressure of 70 GPa, with a maximumTc∼ 5 K at 24 GPa. We observed a structure phase transition fromP-3m1 toC2/mspace group in the range of 9.4-11.7 GPa. The results show that the structural phase transition leads to the metallization of 1T-CrSe2and the further pressure effect makes the superconductivity appear in the new structure. The material undergoes a transition from a two-dimensional layered structure to a three-dimensional structure under pressure. This is the first time that possible superconductivity has been observed in 1T-CrSe2.

Pressure-Induced Re-Entrant Superconductivity in Transition Metal Dichalcogenide TiSe2

Transition metal dichalcogenide TiSe2 exhibits a superconducting dome within a low pressure range of 2-4 GPa, which peaks with the maximal transition temperature Tc of ≈1.8 K. Here it is reported that applying high pressure induces a new superconducting state in TiSe2, which starts at ≈16 GPa with a substantially higher Tc that reaches 5.6 K at ≈21.5 GPa with no sign of decline. Combining high-throughput first-principles structure search, X-ray diffraction, and Raman spectroscopy measurements up to 30 GPa, It is found that TiSe2 undergoes a first-order structural transition from the 1T phase under ambient pressure to a new 4O phase under high pressure. Comparative ab initio calculations reveal that while the conventional phonon-mediated pairing mechanism may account for the superconductivity observed in 1T-TiSe2 under low pressure, the electron-phonon coupling of 4O-TiSe2 is too weak to induce a superconducting state whose transition temperature is as high as 5.6 K under high pressure. The new superconducting state found in pressurized TiSe2 requires further study on its underlying mechanism.

Lifshitz transition enabling superconducting dome around a charge-order critical point

Superconductivity often emerges as a dome around a quantum critical point (QCP) where long-range order is suppressed to zero temperature, mostly in magnetically ordered materials. However, the emergence of superconductivity at charge-order QCPs remains shrouded in mystery, despite its relevance to high-temperature superconductors and other exotic phases of matter. Here, we present resistance measurements proving that a dome of superconductivity surrounds the putative charge-density-wave QCP in pristine samples of titanium diselenide tuned with hydrostatic pressure. In addition, our quantum oscillation measurements combined with electronic structure calculations show that superconductivity sets in precisely when large electron and hole pockets suddenly appear through an abrupt change of the Fermi surface topology, also known as a Lifshitz transition. Combined with the known repulsive interaction, this suggests that unconventional s± superconductivity is mediated by charge-density-wave fluctuations in titanium diselenide. These results highlight the importance of the electronic ground state and charge fluctuations in enabling unconventional superconductivity.

Superconductivity in Quasi-One-Dimensional Ferromagnet CrSbSe3 under High Pressure

Nearly a decade has passed since the discovery of superconductivity in CrAs, but until now, the discovered structure types of chromium-based superconductors are still scanty. It is urgent to expand this family to decipher the interplay between magnetism and superconductivity penetratingly. Here, we report the observation of superconductivity in ferromagnet CrSbSe3 with a quasi-one-dimensional structure under high pressure. Under compression, CrSbSe3 undergoes an insulator-to-metal transition and sequential isostructural phase transitions accompanied by volume collapse. Superconductivity emerges at 32.8 GPa concomitant with metallization in CrSbSe3. A maximum superconducting transition temperature Tc of 7.7 K is achieved at 57.9 GPa benefiting from both the phonon softening and the enhanced p-d hybridization between Se and Cr in CrSbSe3. The discovery of superconductivity in CrSbSe3 expands the existing chromium-based superconductor family and sheds light on the search for concealed superconductivity in low-dimensional van der Waals materials.

Pressure-modulated lattice structural evolution in TiS2

Titanium disulfide (TiS2) has drawn considerable attention in materials, physics, and chemistry thanks to its potential applications in batteries, supercapatteries and thermoelectric devices. However, the simplified and controlled synthesis of high-quality TiS2 remains a great challenge. In this study, a straightforward widely accessible approach to the one-step chemical vapor transport (CVT) process is presented. Meanwhile, combining high-pressure (HP) Raman spectroscopy measurements and first-principles calculations, the pressure-induced phase transition of TiS2 from P3̄m1 phase (phase I) to C2/m phase (phase II) at 16.0 GPa and then to P6̄2m phase (phase III) at 32.4 GPa was disclosed. The discovery of HP being within the Weyl semi-metallic phase represents a significant advancement towards understanding the electronic topological states, discovering new physical phenomena, developing new electronic devices, and gaining insight into the properties of elementary particles.

Unveiling the structural, dynamical, elastic, and electronic properties of cuboid silver tetrathiotungstate by means ofab initiocalculations

We present for the first time a theoretical study of the structural stability and physical properties of the newly synthesized Ag₂WS₄. The study contributes to a better understanding of its electronic and vibrational properties, which is fundamental for the optimization of the technological applications of Ag₂WS₄. Calculations have been carried out by means of density-functional theory. The obtained results support that Ag₂WS₄is thermodynamically, mechanically, and dynamically stable in a tetragonal layered structure, in good agreement with experiments. Calculations have also been used to obtain phonon frequencies, their assignments, and the Raman scattering spectrum. Furthermore, we show that Ag₂WS₄has a brittle structure, that is governed by van der Waals interactions, which favors its exfoliation as a low-dimensional structure. Additionally, the results show that Ag₂WS₄has a band gap of 2.02 eV with a favorable band-edge diagram for water splitting as well as for optoelectronic applications.

Pressure-induced phase transitions, amorphization and alloying in Sb2S3

Despite the extensive and systematic studies of pressure-induced phase transitions in sesqui-chalcogenides, several puzzles still remain to be solved. Here, the complicated phase transitions, amorphization, and alloying behaviors of the binary semiconductor antimony trisulfide (Sb₂S₃) were observed by performing in situ high-pressure angle-dispersive x-ray diffraction, Raman spectroscopy and resistance measurements. Upon compression, two phase transitions are observed in Sb₂S₃ before it transforms into a high-density amorphous state (HDA). Notably, it is found that the pressure transmitting medium has a great effect on these changes. Then, Sb₂S₃ shows an irreversible process after full decompression and a low-density amorphous state (LDA) can be obtained. Unexpectedly, a site-disordered Sb-S alloy can be formed via recompressing LDA. These results indicate that the Sb³⁺ lone electron pair activity will be destroyed at high pressures, which may make Sb₂S₃ a promising thermoelectric material at high pressures.

The pressure-induced structural transitions, alloying and superconductivity in topological insulators Bi2Te2Se and Bi2Se2Te

Over the past decade, external pressure has been found to play an important role in modulating the structures and properties of sesqui-chalcogenides, but several key puzzles still remain to be...