Junkai Zhang

Electronic topological transition and semiconductor-to-metal conversion of Bi2Te3 under high pressure

Accurate high pressure in situ Hall-effect and temperature dependent electrical resistivity measurements have been carried out on Bi2Te3, a topological insulator. The pressure dependent electrical resistivity, Hall coefficient, carrier concentration, and mobility show the abnormal inflection points at 8, 12, and 17.8 GPa, indicating that the pressure-induced structural phase transitions of Bi2Te3 can result in a series of changes in the carrier transport behavior. In addition, the Hall coefficient shows a significant discontinuous change at 4 GPa, which is caused by the electronic topological transition. A sign inversion of Hall coefficient from positive to negative is found around 8 GPa. Furthermore, the temperature dependent electrical resistivity shows that the sample undergoes a semiconductor-to-metal conversion around 9.2 GPa, indicating that the insulating gap of Bi2Te3 becomes closed at this pressure. As the metallization occurs in the sample, the topological property of Bi2Te3 disappears.

Anomalous semiconducting behavior on VO2 under high pressure

High-pressure electrical transport properties of VO2 have been investigated by in situ resistivity, Hall-effect, and temperature dependence of resistivity measurements. The electrical transport parameters including resistivity, Hall coefficient, carrier concentration, and mobility varies significantly around 10.4 GPa, which can be attributed to the isostructural phase transition of VO2. Temperature dependence of resistivity indicates that the phase transition is a semiconductor-to-semiconductor transformation, not the pressure-induced metallization as previously reported by Raman and IR experiment observations. The dramatic increase of activation energy at 10.4 GPa indicates an increasingly insulating behavior of VO2 accompanied with the isostructural phase transition. The electrical transport properties, especially the carries transportation under compression open up a new possible basis for optimizing the performance of VO2 based applications under ambient or extreme conditions.

Semiconductor-to-metal transition of Bi2Se3 under high pressure

Pressure-induced electrical transport properties of Bi2Se3, including Hall coefficient, carrier concentration, mobility, and electrical resistivity, have been investigated under pressure up to 29.8 GPa by in situ Hall-effect measurements. The results indicate that the structural and electronic phase transitions of Bi2Se3 induce discontinuous changes in these electrical parameters. The significant anomaly in Hall coefficient at 5 GPa reveals an electronic topological transition deriving from the topological change of the band extremum (Van Hove singularity). Additionally, electrical resistivity measurements under variable temperatures show that the insulating state of Bi2Se3 becomes increasingly stable with an increase of pressure below 9.7 GPa. But above 9.7 GPa, Bi2Se3 enters into a fully metallic state. As the metallization occurs, the topological property of Bi2Se3 disappears.

Impurity level evolution and majority carrier-type inversion of Ag2S under extreme compression: Experimental and theoretical approaches

The ability to probe charge carriers transport behavior under pressure has the potential to unlock many key questions in photoconduction, resistive switch, solid ion transport, etc. We utilize the Hall-effect measurements and the first-principles calculations on Ag2S to reveal the pressure induced changes in its electrical transport parameters. Beyond 7.8 GPa, the donor level in modified monoclinic phase moves away from the conduction-band minimum as the pressure increases, and then Ag2S changes its dominant majority carriers from electrons to holes around 13.5 GPa. Additionally, increasing the pressure makes the electrical resistivity of Ag2S trend to decrease with temperature. At 15.8 GPa, Ag2S undergoes a transformation from metallic-like conduction to semiconductor conduction. These results are beneficial to achieve unique properties of high-pressure phases and develop Ag2S applications under ambient or extreme conditions.

Electrical properties and behaviors of cuprous oxide cubes under high pressure.

An accurate in situ electrical resistivity measurement of cuprous oxide cubes has been conducted in a diamond anvil cell at room temperature with pressures up to 25 GPa. The abnormal electrical resistivity variation found at 0.7-2.2 GPa is attributed to the phase transformation from a cubic to a tetragonal structure. Three other discontinuous changes in the electrical resistivity are observed around 8.5, 10.3, and 21.6 GPa, corresponding to the phase transitions from tetragonal to pseudocubic to hexagonal to another hexagonal phase, respectively. The first-principles calculations illustrate that the electrical resistivity decrease of the tetragonal phase is not related to band-gap shrinkage but related to a higher quantity of electrons excited from strain-induced states increasing in band gap with increasing pressure. The results indicate that the Cu(2)O cubes begin to crush at about 15 GPa and completely transform into nanocrystalline at 25 GPa.

Decompression‐Driven Superconductivity Enhancement in In2Se3

An unexpected superconductivity enhancement is reported in decompressed In2 Se3 . The onset of superconductivity in In2 Se3 occurs at 41.3 GPa with a critical temperature (Tc ) of 3.7 K, peaking at 47.1 GPa. The striking observation shows that this layered chalcogenide remains superconducting in decompression down to 10.7 GPa. More surprisingly, the highest Tc that occurs at lower decompression pressures is 8.2 K, a twofold increase in the same crystal structure as in compression. It is found that the evolution of Tc is driven by the pressure-induced R-3m to I-43d structural transition and significant softening of phonons and gentle variation of carrier concentration combined in the pressure quench. The novel decompression-induced superconductivity enhancement implies that it is possible to maintain pressure-induced superconductivity at lower or even ambient pressures with better superconducting performance.

Interlayer-glide-driven isosymmetric phase transition in compressed In2Se3

We report an anomalous phase transition in compressed In2Se3. The high-pressure studies indicate that In2Se3 transforms to a new isosymmetric R-3m structure at 0.8 GPa whilst the volume collapses by ∼7%. This phase transition involves a pressure-induced interlayer shear glide with respect to one another. Consequently, the outer Se atoms of one sheet locate into the interstitial sites of three Se atoms in the neighboring sheets that are weakly connected by van der Waals interaction. Interestingly, this interlayer shear glide changes the stacking sequence significantly but leaves crystal symmetry unaffected. This study provides an insight to the mechanisms of the intriguing isosymmetric phase transition.

Anomalous variation of electrical transport property and amorphization in dense Alq3

Herein, we report the intriguing electrical transport and structural properties of compressed Alq3, which is an extensively used electron transport material in OLEDs. The bulk resistance (Rb) of Alq3 increases with uploading pressure, but drops markedly when the uploading pressure is above 8.6 GPa. In contrast, the grain boundary resistance (Rgb) varies smoothly below 16.4 GPa. With further compression, both Rb and Rgb increase with the amorphization of Alq3. The pressure-induced amorphization is found to be reversible at a low density amorphous state, while it is irreversible at a higher density state. Interestingly, XRD measurements indicate no structural transition at ∼8.0 GPa. The variation of Rb is found to be synchronous with the blue-shift of the Al–oxine deformation mode, which rationalizes the anomalous changes of Rb. The Al–oxine interaction is believed to be also significant in the electrical transport properties of dense Alq3, which provides insight into the correlation between its structural changes and electrical transport properties.

An immutable array of TiO2 nanotubes to pressures over 30 GPa

We report the successful formation of an immutable array of α-PbO2 phase TiO2 nanotubes by compression of a TiO2 nanotube array in an anatase phase. During compression to 31.3 GPa, the TiO2 nanotubes started to directly transform from an anatase phase to a baddeleyite phase at 14.5 GPa and completed the transition at 30.1 GPa. Under decompression, the baddeleyite phase transformed to an α-PbO2 phase at 4.6 GPa, which was quenchable to ambient pressure. Notably the tubular array microstructure was retained after the application of ultra high pressure and undergoing a series of phase transformations. Measurements indicated that the nanotubes in the array possessed higher compressibility than in the bulk form. The highly aligned array structure is believed to reinforce the nanotubes themselves, giving exceptional stability. This, as well as the wall thickness, may also account for their different phase transition pathway.