High-pressure Electrical Transport Research Articles

Pressure-induced ferroelectric and electronic transitions in two-dimensional ferroelectric semiconductor of NbOCl2 up to 41.7 GPa

NbOCl2, a representative van der Waals ferroelectric (FE) semiconductor, has become the research frontier due to its peculiar appeal in both fundamental research studies and potential applications. In the present work, the high-pressure structural, vibrational, and electrical transport properties of NbOCl2 under different hydrostatic environments were systematically investigated over a wide pressure range of 1.7–41.7 GPa using a diamond anvil cell coupled with in situ Raman spectroscopy, electrical conductivity, and high-resolution transmission electron microscopy (HRTEM) observations. Upon non-hydrostatic compression, NbOCl2 underwent a FE-to-antiferroelectric phase transition at 3.4 GPa, followed by a semiconductor-to-metal transformation at 15.7 GPa. Under hydrostatic compression, the FE transformation and metallization of NbOCl2 were postponed by ∼2.0 and ∼4.0 GPa due to the effect of helium pressure-transmitting medium. Upon decompression, the phase transition was demonstrated to be reversible under different hydrostatic environments, which was well corroborated by HRTEM analyses. In addition, the linear relations between electrical current and sinusoidal voltage with the nonlinearity factors of ∼1.0 reflect the Ohmic response of NbOCl2 before and after the FE transition. Our findings on NbOCl2 provide a guideline for exploring other layered FE materials under high pressure and establishing a design paradigm for new generations of FE-based devices.

Robust T c in element molybdenum up to 160 GPa

Element superconductors with the single atoms provide clean and fundamental platforms for studying superconductivity. Although elements with d electrons are usually not favored by conventional BCS, the record superconducting critical temperature (T c) in element scandium (Sc) has further ignited the intensive attention on transition metals. The element molybdenum (Mo) with a half-full d-orbital is studied in our work, which fills the gap in the study of Mo under high pressure and investigates the pressure dependence of superconductivity. In this work, we exhibit a robust superconductivity of Mo in the pressure range of 5 GPa to 160 GPa via high-pressure electrical transport measurements, the T c varies at a rate of 0.013 K/GPa to 8.56 K at 160 GPa. Moreover, the superconductivity is evidenced by the T c shifting to lower temperature under applied magnetic fields, and the upper critical magnetic fields are extrapolated by the WHH equation and GL equation; the results indicate that the maximum upper critical magnetic field is estimated to be 8.24 T at 137 GPa. We further investigate the superconducting mechanism of Mo, the theoretical calculations indicate that the superconductivity can be attributed to the strong coupling between the electrons from the partially filled d band and the phonons from the frequency zone of 200–400 cm−1.

Pressure-induced superconductivity and robust Tc against external pressure in (Ge,Sn,Pb)Te

The robustness of superconducting transition temperature (Tc) against external pressure in medium entropy alloy (MEA) and high entropy alloy (HEA) -type compounds has attracted significant interest with regards to the realization of stable superconducting applications. In this study, we have synthesized Ge1/3Sn1/3Pb1/3Te belonging to MEA-type MTe, where the M site comprises only isovalent group-14 elements, to depict a critical factor for the robustness of Tc. High-pressure electrical transport measurements and structural analysis reveal that the high-pressure phase of CsCl-type cubic structure exhibits the robustness of Tc against external pressure. Molecular dynamics simulation and density functional theory calculation suggest that the glassy atomic vibration characteristic mainly contributes to the appearance of the robustness. This insight accelerates the further development of unique properties within HEA superconductors and their applications.

Pressure-Driven Structural and Electronic Transitions in a Two-Dimensional Janus WSSe Crystal.

In this work, we presented the first report on the high-pressure structural stability and electrical transport characteristics in WSSe under different hydrostatic environments through Raman spectroscopy, electrical conductivity, and high-resolution transmission electron microscopy (HRTEM) coupled with first-principles theoretical calculations. For nonhydrostatic conditions, WSSe endured a phase transition at 15.2 GPa, followed by a semiconductor-to-metal crossover at 25.3 GPa. Furthermore, the bandgap closure was accounted for the metallization of WSSe as derived from theoretical calculations. Under hydrostatic conditions, ∼ 2.0 GPa pressure hysteresis was detected for the emergence of phase transition and metallization in WSSe because of the feeble deviatoric stress. Upon depressurization, the reversibility of the phase transition was substantiated by those of microscopic HRTEM observations under different hydrostatic environments. Our high-pressure investigation on WSSe advances the insightful understanding of the crystalline structure and electronic properties for the Janus transition-metal dichalcogenide (TMD) family and boosts prospective developments in functional devices.

Pressure-induced transition from a Mott insulator to a ferromagnetic Weyl metal in La2O3Fe2Se2

The insulator-metal transition in Mott insulators, known as the Mott transition, is usually accompanied with various novel quantum phenomena, such as unconventional superconductivity, non-Fermi liquid behavior and colossal magnetoresistance. Here, based on high-pressure electrical transport and XRD measurements, and first-principles calculations, we find that a unique pressure-induced Mott transition from an antiferromagnetic Mott insulator to a ferromagnetic Weyl metal in the iron oxychalcogenide La2O3Fe2Se2 occurs around 37 GPa without structural phase transition. Our theoretical calculations reveal that such an insulator-metal transition is mainly due to the enlarged bandwidth and diminishing of electron correlation at high pressure, fitting well with the experimental data. Moreover, the high-pressure ferromagnetic Weyl metallic phase possesses attractive electronic band structures with six pairs of Weyl points close to the Fermi level, and its topological property can be easily manipulated by the magnetic field. The emergence of Weyl fermions in La2O3Fe2Se2 at high pressure may bridge the gap between nontrivial band topology and Mott insulating states. Our findings not only realize ferromagnetic Weyl fermions associated with the Mott transition, but also suggest pressure as an effective controlling parameter to tune the emergent phenomena in correlated electron systems.

High-pressure magnetic properties and electrical transport behaviors of half-metallic ferromagnet CrO2.

The magnetic properties and electrical transport behaviors of half-metallic ferromagnet chromium dioxide (CrO2) powders under high pressure have been investigated by in situ electrical resistivity, magneto-resistivity, and Hall-effect measurements. Our results reveal that the Hall coefficient, carrier concentration, and mobility all present discontinuous changes from 11.7 GPa to 14.9 GPa which can be attributed to the second-order structural transition from the rutile-type to CaCl2-type. However, the resistivity decreases monotonically from ambient pressure to 16.5 GPa. This is due to, first, the decreased carrier concentration and the increased carrier mobility canceling the effects of each other on the resistivity; second, according to the calculation results, the bandgap of CrO2 decreased gradually with the pressure, and the bandgaps of the rutile-type phase and the CaCl2-type phase are extremely similar. CrO2 exhibits a linear and negative magnetoresistance under the applied magnetic field (0∼ ± 15 kOe). As the pressure increases, the magnetoresistance remains negative, but it becomes nonlinear and less symmetric, suggesting that pressure has an appreciable impact on the double-exchange mechanism leading to ferromagnetism in CrO2.

Pressure-driven structural phase transitions and metallization in the two-dimensional ferromagnetic semiconductor CrBr3.

High-pressure structural, magnetic and electrical transport characteristics of CrBr3 were synthetically investigated using Raman scattering, electrical conductivity, high-resolution transmission electron microscopy (HRTEM) and first-principles theoretical calculations during compression and decompression under different hydrostatic conditions. Upon pressurization, CrBr3 underwent a second-order structural transition at 9.5 GPa, followed by the semiconductor-to-metal and magnetic switching at 25.9 GPa under non-hydrostatic conditions, whereas, an obvious pressure hysteresis of ∼3.0 GPa was detected in the occurrence of structural transitions and metallization under hydrostatic conditions due to the deviatoric stress. Upon decompression, the structural and electronic transitions of CrBr3 under different hydrostatic conditions were of good reversibility with a considerable pressure sluggishness of ∼5.0 GPa, which was corroborated well by the microstructural observation with HRTEM. Our systematic high-pressure investigation on CrBr3 not only reveals its underlying application in spintronic, magnetic and electronic devices but also advances the understanding of the physicochemical behaviors for 2D magnetic materials.

Suppressed charge-density-wave, robust ferromagnetism and Lifshitz transition in Sm2Ru3Ge5 crystal under high pressure

The recently discovered tetragonal Sm2Ru3Ge5 provides an ideal avenue to study the interplay between the charge-density-wave (CDW) transition and ferromagnetic (FM) order which appear at TCDW = 175 K and Tm = 8 K, respectively. By performing high-pressure structure and electrical transport measurements on the Sc2Fe3Si5-type Sm2Ru3Ge5 (space group: P4/mnc), it is found that the TCDW is gradually suppressed to low temperature while the Tm changes negligibly with the increase of pressure, likely suggesting a weak link between the two states. Besides, a likely rare example of temperature-induced Lifshitz transition is also observed around 93 K, which is gradually suppressed to low temperature upon increasing pressure and eventually becomes invisible at ∼ 8 GPa. Hall resistivity measurements and first-principles calculations provide evidence for the Lifshitz transition. The work covers a topic of high interest in complex quantum materials showing the interplay of ferromagnetism, charge density wave and lattice instability.

Variation of the critical temperature with the lattice parameter in K3C60

Alkali-doped fullerides are a fascinating system exhibiting the convergence of s-wave superconductivity and Mott physics. A direct relationship between the critical temperature Tc and lattice constant established by applying physical or chemical pressure in these compounds is generally believed to be the foundation for the identification of the microscopic electron pairing(s). Here the evolution of Tc with lattice compression for the first fulleride K3C60 is systematically studied via the high-pressure electrical transport and synchrotron x-ray diffraction measurements on the same well-characterized sample. The combination of the theoretically calculated density of states at the Fermi level, the electron-phonon coupling and mean phonon-frequency determined from high-pressure Raman spectra yields the Coulomb pseudopotential ranging from 0.15 to 0.32 with increasing pressure in terms of the McMillan formula. This indicates that the conventional phonon-mediated mechanism cannot account for the high-pressure evolution of Tc solely. However, the obtained pressure derivative of Tc and bulk modulus are used to reproduce well the experimentally observed isotope exponent in this superconductor within the theoretical framework of the non-phononic theory. These results and findings together demonstrate the important role of the study of the lattice effect on Tc played in determining the mechanisms of superconductivity in fullerides.

Pressure-induced superconductivity and nontrivial band topology in compressed γ-InSe

We performed high-pressure electrical transport and Raman measurements on a two-dimensional rhombohedral semiconductor \ensuremath{\gamma}-InSe. Our results confirm two structural phase transitions at high pressure. More interestingly, a domelike superconducting transition with maximum ${T}_{c}$ around 2.3 K is discovered when the compound transforms to the cubic CsCl phase above 40 GPa. Our first-principles calculations indicate that the high-pressure superconducting phase possesses nontrivial topological band structure in the vicinity of the Fermi level. These results show that the physical properties in this material strongly depend on its structure, which provides insights into the interplay between superconductivity and topological physics. Our work suggests promising emergent phenomena in this material and other related III-VI semiconductors under high-pressure conditions.

Double-dome superconductivity under pressure in the V-based kagome metals AV3Sb5 ( A=Rb and K)

We present high-pressure electrical transport measurements on the newly discovered V-based superconductors $A$V$_3$Sb$_5$ ($A$ = Rb and K), which have an ideal Kagome lattice of vanadium. Two superconducting domes under pressure are observed in both compounds, as previously observed in their sister compound CsV$_3$Sb$_5$. For RbV$_3$Sb$_5$, the $T_c$ increases from 0.93 K at ambient pressure to the maximum of 4.15 K at 0.38 GPa in the first dome. The second superconducting dome has the highest $T_c$ of 1.57 K at 28.8 GPa. KV$_3$Sb$_5$ displays a similar double-dome phase diagram, however, its two maximum $T_c$s are lower, and the $T_c$ drops faster in the second dome than RbV$_3$Sb$_5$. An integrated temperature-pressure phase diagram of $A$V$_3$Sb$_5$ ($A$ = Cs, Rb and K) is constructed, showing that the ionic radius of the intercalated alkali-metal atoms has a significant effect. Our work demonstrates that double-dome superconductivity under pressure is a common feature of these V-based Kagome metals.

Pressure-Induced Structural Phase Transition and Metallization of CrCl3 under Different Hydrostatic Environments up to 50.0 GPa.

High-pressure structural, vibrational, and electrical transport properties of CrCl3 were investigated by means of Raman spectroscopy, electrical conductivity, and high-resolution transmission electron microscopy under different hydrostatic environments using the diamond anvil cell in conjunction with the first-principles theoretical calculations up to 50.0 GPa. The isostructural phase transition of CrCl3 occurred at 9.9 GPa under nonhydrostatic conditions. As pressure was increased up to 29.8 GPa, CrCl3 underwent an electronic topological transition accompanied by a metallization transformation due to the discontinuities in the Raman scattering and electrical conductivity, which is possibly belonging to a typical first-order metallization phase transition as deduced from first-principles theoretical calculations. As for the hydrostatic condition, a ∼2.0 GPa pressure delay in the occurrence of two corresponding transformations of CrCl3 was observed owing to the different deviatoric stress. Upon decompression, we found that the phase transformation from the metal to semiconductor in CrCl3 is of good reversibility, and the obvious pressure hysteresis effect is observed under different hydrostatic environments. All of the obtained results on the structural, vibrational, and electrical transport characterizations of CrCl3 under high pressure can provide a new insight into the high-pressure behaviors of representative chromium trihalides CrX3 (X = Br and I) under different hydrostatic environments.

Pressure-Induced Structural Phase Transition and Metallization in Ga2Se3 Up to 40.2 GPa under Non-Hydrostatic and Hydrostatic Environments

A series of investigations on the structural, vibrational, and electrical transport characterizations for Ga2Se3 were conducted up to 40.2 GPa under different hydrostatic environments by virtue of Raman scattering, electrical conductivity, high-resolution transmission electron microscopy, and atomic force microscopy. Upon compression, Ga2Se3 underwent a phase transformation from the zinc-blende to NaCl-type structure at 10.6 GPa under non-hydrostatic conditions, which was manifested by the disappearance of an A mode and the noticeable discontinuities in the pressure-dependent Raman full width at half maximum (FWHMs) and electrical conductivity. Further increasing the pressure to 18.8 GPa, the semiconductor-to-metal phase transition occurred in Ga2Se3, which was evidenced by the high-pressure variable-temperature electrical conductivity measurements. However, the higher structural transition pressure point of 13.2 GPa was detected for Ga2Se3 under hydrostatic conditions, which was possibly related to the protective influence of the pressure medium. Upon decompression, the phase transformation and metallization were found to be reversible but existed in the large pressure hysteresis effect under different hydrostatic environments. Systematic research on the high-pressure structural and electrical transport properties for Ga2Se3 would be helpful to further explore the crystal structure evolution and electrical transport properties for other A2B3-type compounds.

Unusual competition of superconductivity and charge-density-wave state in a compressed topological kagome metal

Understanding the competition between superconductivity and other ordered states (such as antiferromagnetic or charge-density-wave (CDW) state) is a central issue in condensed matter physics. The recently discovered layered kagome metal AV3Sb5 (A = K, Rb, and Cs) provides us a new playground to study the interplay of superconductivity and CDW state by involving nontrivial topology of band structures. Here, we conduct high-pressure electrical transport and magnetic susceptibility measurements to study CsV3Sb5 with the highest Tc of 2.7 K in AV3Sb5 family. While the CDW transition is monotonically suppressed by pressure, superconductivity is enhanced with increasing pressure up to P1 ≈ 0.7 GPa, then an unexpected suppression on superconductivity happens until pressure around 1.1 GPa, after that, Tc is enhanced with increasing pressure again. The CDW is completely suppressed at a critical pressure P2 ≈ 2 GPa together with a maximum Tc of about 8 K. In contrast to a common dome-like behavior, the pressure-dependent Tc shows an unexpected double-peak behavior. The unusual suppression of Tc at P1 is concomitant with the rapidly damping of quantum oscillations, sudden enhancement of the residual resistivity and rapid decrease of magnetoresistance. Our discoveries indicate an unusual competition between superconductivity and CDW state in pressurized kagome lattice.

Electronic and structural properties of the honeycomb iridates A2IrO3 ( A=Na , Li) at elevated pressures

The honeycomb lattice iridates ${A}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ ($A=\mathrm{Na}$, Li) are spin-orbit assisted Mott insulators proximate to Kitaev's quantum spin liquid. The insulating state as well as the magnetic properties are believed to arise due to a delicate balance of several energy scales. We report on high-pressure electrical transport and x-ray-diffraction measurements on ${A}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ ($A=\mathrm{Na}$, Li) in an attempt to study their structural and electronic evolution with pressure. We found that while $\mathrm{L}{\mathrm{i}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ undergoes a structural phase transition into the dimerized state at a pressure of $P$ \ensuremath{\sim} 4 GPa, in $\mathrm{N}{\mathrm{a}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ the conservation of the original $C2/m$ structure up to at least 58 GPa is observed. In addition, $\mathrm{L}{\mathrm{i}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ undergoes a sluggish structural rearrangement at the pressure range 20--40 GPa coinciding with a significant decrease in resistance. Despite dissimilar structural evolution and different mechanisms of the electrical conductivity, Arrhenius conductivity for $\mathrm{N}{\mathrm{a}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ and Mott variable-range hopping in $\mathrm{L}{\mathrm{i}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$, both systems show a very similar $R$($P$) behavior. Namely, after a nonmonotonic decrease of the resistance $R$ and the charge gap \ensuremath{\Delta}, the \ensuremath{\Delta} stabilizes at about 45 GPa and even increases slightly with pressure; the $R$($T$) shows insulating behavior up to the highest pressure measured, 80 and 55 GPa, respectively. This resilient nonmetallic behavior of the studied iridates suggests a formation close to a localized-itinerant crossover of unusual electronic states, whose possible features are discussed. Unforeseeably, the $R$($P$) behavior is not dependent on the buffer element $A$, which seems essential for understanding the nature of the electrical conductivity in iridates.

Stabilization of antiferromagnetism in 1T- Fe0.05TaS2

1T-TaS$_2$ is a prototypical charge-density-wave (CDW) system with a Mott insulating ground state. Usually, a Mott insulator is accompanied by an antiferromagnetic state. However, the antiferromagnetic order had never been observed in 1T-TaS$_2$. Here, we report the stabilization of the antiferromagnetic order by the intercalation of a small amount of Fe into the van der Waals gap of 1T-TaS$_2$, i.e. forming 1T-Fe$_{0.05}$TaS$_2$. Upon cooling from 300~K, the electrical resistivity increases with a decreasing temperature before reaching a maximum value at around 15~K, which is close to the Neel temperature determined from our magnetic susceptibility measurement. The antiferromagnetic state can be fully suppressed when the sample thickness is reduced, indicating that the antiferromagnetic order in Fe$_{0.05}$TaS$_2$ has a non-negligible three-dimensional character. For the bulk Fe$_{0.05}$TaS$_2$, a comparison of our high pressure electrical transport data with that of 1T-TaS$_2$ indicates that, at ambient pressure, Fe$_{0.05}$TaS$_2$ is in the nearly commensurate charge-density-wave (NCCDW) phase near the border of the Mott insulating state. The temperature-pressure phase diagram thus reveals an interesting decoupling of the antiferromagnetism from the Mott insulating state.

Pressure-driven valence increase and metallization in the Kondo insulator Ce3Bi4Pt3

We report the results of high pressure x-ray diffraction, x-ray absorption, and electrical transport measurements of Kondo insulator Ce$_3$Bi$_4$Pt$_3$ up to 42 GPa, the highest pressure reached in the study of any Ce-based KI. We observe a smooth decrease in volume and movement toward intermediate Ce valence with pressure, both of which point to increased electron correlations. Despite this, temperature-dependent resistance data show the suppression of the interaction-driven ambient pressure insulating ground state. We also discuss potential ramifications of these results for the predicted topological KI state.

Structural and electrical transport properties of Dirac-like semimetal PdSn4 under high pressure**Project supported by the National Key Research and Development Program of China (Grant Nos. 2018YFA0305700 and 2016YFA0401804), the National Natural Science Foundation of China (Grant Nos. U1632275, 11574323, 11874362, 11704387, and 11804344), the Natural Science Foundation of Anhui Province, China (Grant Nos. 1908085QA18, 1708085 QA19, and 1808085MA06), the Major

We conducted in-situ high-pressure synchrotron x-ray diffraction (XRD) and electrical transport measurements on Dirac-like semimetal PdSn4 in diamond anvil cells with quasi-hydrostatic pressure condition up to 44.5 GPa–52.0 GPa. The XRD data show that the ambient orthorhombic phase (Ccca) is stable with pressures to 44.5 GPa, and the lattice parameters and unit-cell volume decrease monotonously upon compression. The temperature dependence of the resistance exhibits a metallic conduction and follows a Fermi-liquid behavior below 50 K, both of which keep unchanged upon compression to 52.0 GPa. The magnetoresistance curve at 5 K maintains a linear feature in a magnetic field range of 2.5 T–7 T with increasing pressure to 20.0 GPa. Our results may provide pressure-transport constraints on the robustness of the Dirac fermions.

Perovskites with a Twist: Strong In1+ Off-Centering in the Mixed-Valent CsInX3 (X = Cl, Br)

The search for new halide perovskites has recently expanded to the double perovskites A2M+M3+X6, which form in the 3D elpasolite structure with alternating M+ and M3+ octahedra. Here, we report the ternary mixed-valent indium compounds CsInX3 (X = Br, Cl). They adopt a tetragonal (space group I4/m) structure, which is a derivative of the charge-ordered double perovskite but with a twisted chain of octahedra along the c-axis. This twist is a result of the off-centering at the In+ site to accommodate its stereochemically active 5s2 lone pair, which induces a 45° rotation of all In3+ octahedra in the chain. This is a non-cooperative rotation that creates pentagonal pyramids in the structure and induces significant disorder, with extensive twinning, which results in partial occupation of different rotations of the same In3+ site. Temperature-dependent synchrotron X-ray diffraction reveals that both compounds form the cubic double perovskite structure at high temperature (Fm3̅m), thus demonstrating that the twist occurs on cooling from the melt. UV–vis spectroscopy reveals band gaps near 2.3–2.4 eV for the bromide and 3.0 eV for CsInCl3. High-pressure electrical transport measurements show significant enhancement in conductivity as pressure increases, indicating a narrowing of the band gap at high pressures. Temperature-dependent transport measurements at high pressure show uniformly semiconducting behavior, with no superconducting transition observed down to 2.4 K. The CsInX3 are unique inorganic halide double perovskites because they are based on a single metal, with CsInBr3 just the third bromide after the mixed metal compounds Cs2AgBiBr6 and Cs2AgTlBr6.

Pressure-Induced Metallization Accompanied by Elongated S–S Dimer in Charge Transfer Insulator NiS2**Supported by the National Key Research and Development Program of China under Grant Nos 2018YFA0305700 and 2016YFA0401804, the National Natural Science Foundation of China under Grant Nos 11574323, 11704387, 11874362, 11804344, 11804341, 61774136, 11605276 and U1632275, the Major Program of Development Foundation of Hefei Center for Physical Science and

The insulator-metal transition triggered by pressure in charge transfer insulator NiS2 is investigated by combining high-pressure electrical transport, synchrotron x-ray diffraction and Raman spectroscopy measurements up to 40–50 GPa. Upon compression, we show that the metallization firstly appears in the low temperature region at ∼3.2 GPa and then extends to room temperature at ∼8.0 GPa. During the insulator-metal transition, the bond length of S–S dimer extracted from the synchrotron x-ray diffraction increases with pressure, which is supported by the observation of abnormal red-shift of the Raman modes between 3.2 and 7.1 GPa. Considering the decreasing bonding-antibonding splitting due to the expansion of S–S dimer, the charge gap between the S-ppπ* band and the upper Hubbard band of Ni-3d eg state is remarkably decreased. These results consistently indicate that the elongated S–S dimer plays a predominant role in the insulator-metal transition under high pressure, even though the p-d hybridization is enhanced simultaneously, in accordance with a scenario of charge-gap-controlled type.