Pressure-induced Superconductivity Research Articles

Pressure-Induced Alloying and Superconductivity in GeTe

Pressure-Induced Alloying and Superconductivity in GeTe

Crystallographic parameters and their systematics within the CeCuAl3-related family of intermetallics

Rare-earth RTX3 compounds (T stands for a transition d-metal and X is a p-metal) form a large family of intermetallics, revealing a variety of intriguing physical properties. Among them, CeTX3 compounds represent a particularly studied group, due to their frequently complex magnetic structures, Kondo physics, pressure-induced superconductivity, or recently highlighted quantum quasi-bound states. We report on the structural properties and their systematics within the CeCuAl3-related intermetallics crystallising in the tetragonal structure of the BaNiSn3-type. Our synchrotron X-ray diffraction data of 14 selected alloys allow us to follow the evolution of crystallographic parameters of the CeCuAl3-based substitution series. Substitutions of R, T and X elements are shown to have significant impact on the unit cell volume, virtual lattice expansion and free fraction coordinates of individual Wyckoff sites. Structural parameters are discussed within the framework of physical properties of investigated compounds.

Strain-mediated phase crossover in Ruddlesden–Popper nickelates

Recent progress on the signatures of pressure-induced high-temperature superconductivity in Ruddlesden–Popper (RP) nickelates (Lan+1NinO3n+1) has attracted growing interest in both theoretical calculations and experimental efforts. The fabrication of high-quality single-crystalline RP nickelate thin films is critical for possible reducing the superconducting transition pressure and advancing applications in microelectronics in the future. In this study, we report the observations of an active phase transition in RP nickelate films induced by misfit strain. We found that RP nickelate films favor the perovskite structure (n = ∞) under tensile strains, while compressive strains stabilize the La3Ni2O7 (n = 2) phase. The selection of distinct phases is governed by the strain dependent formation energy and electronic configuration. In compressively strained La3Ni2O7, we experimentally determined the eg splitting energy is ~0.2 eV and electrons prefer to occupy in-plane orbitals. First-principles calculations unveil a robust coupling between strain effects and the valence state of Ni ions in RP nickelates, suggesting a dual driving force for the inevitable phase co-existence transition in RP nickelates. Our work underscores the sensitivity of RP nickelate formation to epitaxial strain, presenting a significant challenge in fabricating pure-phase RP nickelate films. Therefore, special attention to stacking defects and grain boundaries between different RP phases is essential when discussing the pressure-induced superconductivity in RP nickelates.

Tuning the Flat Band in Bi2O2Se by Pressure to Induce Superconductivity.

The discovery of superconductivity in twisted bilayer graphene has reignited enthusiasm in the field of flat-band superconductivity. However, important challenges remain, such as constructing a flat-band structure and inducing a superconducting state in materials. Here, we successfully achieved superconductivity in Bi2O2Se by pressure-tuning the flat-band electronic structure. Experimental measurements combined with theoretical calculations reveal that the occurrence of pressure-induced superconductivity at 30 GPa is associated with a flat-band electronic structure near the Fermi level. Moreover, in Bi2O2Se, a van Hove singularity is observed at the Fermi level alongside pronounced Fermi surface nesting. These remarkable features play a crucial role in promoting strong electron-phonon interactions, thus potentially enhancing the superconducting properties of the material. These findings demonstrate that pressure offers a potential experimental strategy for precisely tuning the flat band and achieving superconductivity.

Pressure-induced superconductivity in SnSb2Te4

Owing to their unique compositional and structural characteristics, layered van der Waals solids in binary and ternary chalcogenide families provide a fertile testbed for exploring novel exotic structures and states, e.g., topological insulators and superconductors. Herein, a comprehensive study on the structural variations and correlated electrical transport behavior of SnSb2Te4, a ternary member, has been carried out considering elevated pressures. Under 45.6 GPa, three distinct structural phase transitions have been observed, with strong evidence from the variations of high-pressure X-ray diffraction patterns. The onsets of phase II (monoclinic, C2/m ) at 6.3 GPa, phase III (monoclinic, C2/c ) at 15.5 GPa, and phase IV (body-centered cubic with substitutional disorder, Im-3m ) at 17.2 GPa have been observed owing to the emergence of new diffractions. Based on electrical measurements at low temperature and high pressure conditions, two pressure-induced superconducting states have been distinguished in SnSb2Te4. The first state occurs in the range of 12.3-17.1 GPa. The positive pressure dependence on Tc indicates that the aforementioned state is related to the monoclinic C2/m phase. At > 17.1 GPa, the second superconducting state emerges, with the negative pressure dependence on Tc . It relates to the body-centered cubic solid solution phase, which is characteristic of a substitutional disordered crystal structure. The discovery that the pressure-induced superconductivity in SnSb2Te4 is affected by structural phase transitions under pressure may help understand the universal relationship between the ambient condition topological insulating state and derived superconductivity. Ab initio theoretical calculations reveal that an electronic topological transition takes place at approximately 2.0 GPa, which is featured by the obvious changes in the distribution of electronic density of states near the Fermi level.

Distinct superconducting states in the pressure-induced metallic structures of topological heterostructure BiTe

The (Bi2)m(Bi2Te3)n homologous series possess natural multilayer heterostructure with intriguing physical properties at ambient pressure. Herein, we report the pressure-dependent evolution of the structure and electrical transport of the dual topological insulator BiTe, a member of the (Bi2)m(Bi2Te3)n series. With applied pressure, BiTe exhibits several different crystal structures and distinct superconducting states, which is remarkably similar to other members of the (Bi2)m(Bi2Te3)n series. Our results provide a systematic phase diagram for the pressure-induced superconductivity in BiTe, contributing to the highly interesting physics in this (Bi2)m(Bi2Te3)n series.

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-induced Lifshitz transition in the type-II Weyl semimetal WP2

We report evidences of a pressure-induced Lifshitz transition in the type-II Weyl semimetal WP2 via resistance, synchrotron X-ray diffraction, and Raman scattering measurements with the aid of density functional theory calculations up to 50 GPa. The resistance at 2 K and 300 K and residual resistance ratio rise precipitously at ca. 20 GPa. The pristine orthorhombic structure is robust against pressures up to 50 GPa. There occurs a discontinuity in the unit cell volume, lattice parameter, lattice parameter ratio and axial compressibility at ～20 GPa. Two Raman-active modes soften abruptly accompanied with appearance of a new Raman-active mode at ～15 GPa. Density functional theory calculations identify drastic changes in the Fermi surface topology at～30 GPa. Our findings stimulate further pursuit of pressure-induced superconductivity via a Lifshitz transition in WP2.

Superconductivity of barium with highest transition temperatures in metallic materials at ambient pressure

Pressure-induced superconductivity often occurs following structural transition under hydrostatic pressure (PHP) but disappears after the pressure is released. In the alkali-earth metal barium, superconductivity appears after structural transformation from body-centered cubic structure to hexagonal-close-packed (hcp) structure at PHP = 5 GPa, and the superconducting transition temperature (Tc) reaches a maximum of 5 K at PHP = 18 GPa. Furthermore, by stabilizing the low-temperature phase at PHP ~ 30 GPa, Tc reached a higher level of 8 K. Herein, we demonstrate a significantly higher Tc superconductivity in Ba even at ambient pressure. This was made possible through severe plastic deformation of high-pressure torsion (HPT). In this HPT-processed Ba, we observed superconductivity at Tc = 3 K and Tc = 24 K in the quasi-stabilized hcp and orthorhombic structures, respectively. In particular, the latter Tc represents the highest value achieved at ambient pressure among single-element superconducting metals, including intermetallics. The phenomenon is attributed to a strained high-pressure phase, stabilized by residual strains generated from lattice defects such as dislocations and grain boundaries. Significantly, the observed Tc far exceeds predictions from DFT calculations under normal hydrostatic compressions. The study demonstrates the importance of utilizing high-pressure strained phases as quasi-stable superconducting states at ambient pressure.

Pressure-induced superconductivity in a novel germanium allotrope

High-pressure studies on elements play an essential role in superconductivity research, with implications for both fundamental science and applications. Here we report the experimental discovery of surprisingly low pressure driving a novel germanium allotrope into a superconducting state in comparison to that for α-Ge. Raman measurements revealed structural phase transitions and possible electronic topological transitions under pressure up to 58 GPa. Based on pressure-dependent resistivity measurements, superconductivity was induced above 2 GPa and the maximum Tc of 6.8 K was observed under 4.6 GPa. Interestingly, a superconductivity enhancement was discovered during decompression, indicating the possibility of maintaining pressure-induced superconductivity at ambient pressure with better superconducting performance. Density functional theory analysis further suggested that the electronic structure of Ge (oP32) is sensitive to its detailed geometry and revealed that disorder in the β-tin structure leads to a higher Tc in comparison to the perfect β-tin Ge.

Pressure-induced hydrogen-dominant high-temperature superconductors

The century-old pursuit of room temperature superconductivity has finally been reached in highly compressed hydrogen-dominant compounds. Future efforts will be focused on understanding the high-pressure hydrogen physics and ambient-pressure applications.

Effects of pressure and doping on Ruddlesden-Popper phases Lan+1NinO3n+1

Recently, the discovery of superconductivity with a critical temperature Tc up to 80 K in Ruddlesden–Popper phases Lan+1NinO3n+1 (n = 2) under pressure has garnered considerable attention. Up to now, the superconductivity was only observed in La3Ni2O7 single crystal grown with the optical-image floating zone furnace under oxygen pressure. It remains to be understood the effect of chemical doping on superconducting La3Ni2O7 as well as other Ruddlesden–Popper phases. Here, we systematically investigate the effect of external pressure and chemical doping on polycrystalline Ruddlesden–Popper phases. Our results demonstrate that the application of pressure and doping effectively tunes the transport properties of Ruddlesden–Popper phases. We find pressure-induced superconductivity up to 86 K in La3Ni2O7 polycrystalline sample, while no signatures of superconductivity are observed in La2NiO4 and La4Ni3O10 polycrystalline samples under high pressure up to 50 GPa. Our study sheds light on the exploration of high-Tc superconductivity in nickelates.

Pressure-induced superconductivity in tantalum superhydrides

The quest for high-temperature superconductivity in pressure-stabilized hydrides has received great interests due to the recent findings of CaH6, LaH10, YH6, YH9 and H3S with superconducting critical temperature, Tc, above 200 K and at a pressure range of 155–201 GPa. In this work, we conduct extensive structure-searching simulations for the Ta–H system at a pressure range of 300–400 GPa. As a result, we predict several stable pressure-stabilized phases, such as TaH10, TaH11, TaH16 and TaH17. These predicted structures consist of molecular and atomic hydrogen units, where the latter holds the key to improve the superconductivity of the system. Among these high-Tc candidates, tetragonal TaH16 phase, with the large hydrogen content of ∼84 %, exhibits a high Tc value of 196 K at 400 GPa. Our current results could be helpful for the future design of high-temperature superconductivity in the system of transition metal superhydrides under high pressure.

Pressure-induced superconductivity and phase transitions in Bi2S3 under non-hydrostatic conditions

Despite extensive and systematic studies on pressure-induced phase transitions and superconductivity in sesqui-chalcogenide A2B3 (A=Sb, Bi; BS, Se, and Te) compounds, the detailed transporting properties of Bi2S3 have rarely been reported. Here, we discovered an unambiguous correlation between superconductivity and phase transitions in Bi2S3 under non-hydrostatic conditions. The rapid decrease in resistance below 10 GPa and the occurrence of superconductivity at approximately 25.5 GPa should be caused by the two unusual pressure-induced isostructural transitions. Subsequently, the superconducting transition temperature (TC) increased to around 7.3 K, which was accompanied by the amorphization of Bi2S3 above 43 GPa. Furthermore, divergence between the theoretical calculations and experimental measurements showed that the above phase transitions and pressure-induced charge transfer had a critical effect on the electronic properties of this compound. The results of this study indicated that high pressure could help to improve thermoelectric properties.

Two high-pressure superconducting phases in pressurized optical semiconductor GaP

Pressure engineering in semiconductors leads to a variety of novel physical phenomena and has recently received considerable attention. Here, we report on pressure-induced superconductivity in III–V gallium phosphide (GaP), a commercially important semiconductor that exhibits excellent optical performance. We show that the emergence of superconductivity is accompanied by the concurrence of piezochromic transition and metallization and can be correlated to a structural transition from the cubic to orthorhombic phase. In line with the structural origin of superconductivity, the critical temperature Tc monotonically decreases with increasing pressure up to ~50 GPa. Moreover, the superconductivity could be preserved toward ambient pressure because of the irreversibility of the structural transition. Nevertheless, the superconducting transition displays evident broadening associated with the presence of amorphization in the depressurized sample. The synchronous evolution of the structural and electronic properties not only shows a vivid structure-property relationship but also could facilitate the exploration of novel functionalities by means of pressure treatment.

Observation of low-frequency Raman peak in layered WTe2

WTe2 recently attracted considerable attention as a layered material exhibiting ferroelectricity, giant magnetoresistance, and pressure-induced superconductivity. In this study, we performed Raman spectroscopy on bulk WTe2, including the low-frequency region. An unfamiliar Raman peak (P0) was found at approximately 9 cm−1 in addition to the seven already known peaks (P1–P7). Furthermore, the angular and polarization dependence of the spectra revealed the peak P0 had A 1 symmetry. The symmetry is consistent with the experimental results and first-principles calculations by other groups. Our work paves the way for studying the symmetry of the low-frequency phonons in atomic-layer ferrolectric films using Raman spectroscopy.

Pressure-induced superconductivity in the nonsymmorphic topological insulator KHgAs

Recently, topological insulators (TIs) KHgX (X = As, Sb, Bi) with hourglass-shaped dispersion have attracted great interest. Different from the TIs protected by either time-reversal or mirror crystal symmorphic symmetry tested in previous experiments, these materials were proposed as the first material class whose band topology relies on nonsymmorphic symmetries. As a result, KHgX shows many exotic properties, such as hourglass-shaped electronic channels and three-dimensional doubled quantum spin Hall effects. To date, high-pressure experimental studies on these nonsymmorphic TIs are minimal. Here, we carried out high-pressure electrical measurements up to 55 GPa, together with high-pressure X-ray diffraction measurements and high-pressure structure prediction on KHgAs. We found a pressure-induced semiconductor-metal transition between ~16 and 20 GPa, followed by the appearance of superconductivity with a Tc of ~3.5 K at approximately 21 GPa. The superconducting transition temperature was enhanced to a maximum of ~6.6 K at 31.8 GPa and then slowly decreased until 55 GPa. Furthermore, three high-pressure phases within 55 GPa were observed, and their crystal structures were established. Our results showed the high-pressure phase diagram of KHgAs and determined the pressure-induced superconductivity in nonsymmorphic TIs. Thus, our study can be used to facilitate further research on superconductivity and topologically nontrivial features protected by nonsymmorphic symmetries.

Emergence of High-Temperature Superconducting Phase in Pressurized La3Ni2O7 Crystals

The recent report of pressure-induced structural transition and signature of superconductivity with T c ≈ 80 K above 14 GPa in La3Ni2O7 crystals has garnered considerable attention. To further elaborate this discovery, we carried out comprehensive resistance measurements on La3Ni2O7 crystals grown in an optical-image floating zone furnace under oxygen pressure (15 bar) using a diamond anvil cell (DAC) and cubic anvil cell (CAC), which employ a solid (KBr) and liquid (glycerol) pressure-transmitting medium, respectively. Sample 1 measured in the DAC exhibits a semiconducting-like behavior with large resistance at low pressures and gradually becomes metallic upon compression. At pressures P ⩾ 13.7 GPa we observed the appearance of a resistance drop of as much as ∼ 50% around 70 K, which evolves into a kink-like anomaly at pressures above 40 GPa and shifts to lower temperatures gradually with increasing magnetic field. These observations are consistent with the recent report mentioned above. On the other hand, sample 2 measured in the CAC retains metallic behavior in the investigated pressure range up to 15 GPa. The hump-like anomaly in resistance around ∼ 130 K at ambient pressure disappears at P ⩾ 2 GPa. In the pressure range of 11–15 GPa we observed the gradual development of a shoulder-like anomaly in resistance at low temperatures, which evolves into a pronounced drop of resistance of 98% below 62 K at 15 GPa, reaching a temperature-independent resistance of 20 μΩ below 20 K. Similarly, this resistance anomaly can be progressively shifted to lower temperatures by applying external magnetic fields, resembling a typical superconducting transition. Measurements on sample 3 in the CAC reproduce the resistance drop at pressures above 10 GPa and realize zero resistance below 10 K at 15 GPa even though an unusual semiconducting-like behavior is retained in the normal state. Based on these results, we constructed a dome-shaped superconducting phase diagram and discuss some issues regarding the sample-dependent behaviors on pressure-induced high-temperature superconductivity in the La3Ni2O7 crystals.

Pressure-Induced Superconductivity in the Charge-Density-Wave Compound LaTe2–x Sb x (x = 0 and 0.4)

Magnetic CeTe2 achieving superconductivity under external pressure has received considerable attention. The intermingling of 4f and 5d electrons from Ce raised the speculation of an unconventional pairing mechanism arising from magnetic fluctuations. Here, we address this speculation using a nonmagnetic 4f-electron-free LaTe2 as an example. No structural phase transition can be observed up to 35 GPa in the in situ synchrotron diffraction patterns. Subsequent high-pressure electrical measurements show that LaTe2 exhibits superconductivity at 20 Gpa with its T c (4.5 K) being two times higher than its Ce-counterpart. Detailed theoretical calculations reveal that charge transfer from the 4p orbitals of the planar square Te–Te network to the 5d orbitals of La is responsible for the emergence of superconductivity in LaTe2, as confirmed by Hall experiments. Furthermore, we study the modulation of q CDW by Sb substitution and find a record high K in LaTe1.6Sb0.4. Our work provides an informative clue to comprehend the role of 5d–4p hybridization in the relationship between charge density wave (CDW) and superconductivity in these RETe2 (RE = rare-earth elements) compounds.

Orbital-selective charge-density wave in TaTe4

TaTe4, a metallic charge-density wave (CDW) material discovered decades ago, has attracted renewed attention due to its rich interesting properties, such as pressure-induced superconductivity and candidate nontrivial topological phase. Here, using high-resolution angle-resolved photoemission spectroscopy and ab initio calculation, we systematically investigate the electronic structure of TaTe4. At 26 K, we observe a CDW gap as large as 290 meV, which persists up to 500 K. The CDW-modulated band structure shows a complex reconstruction that closely correlates with the lattice distortion. Inside the CDW gap, there exist highly dispersive energy bands contributing to the remnant Fermi surface and metallic behavior in the CDW state. Interestingly, our ab initio calculation reveals that the large CDW gap mainly opens in the electronic states with out-of-plane orbital components, while the in-gap metallic states originate from in-plane orbitals, suggesting an orbital texture that couples with the CDW order. Our results shed light on the interplay between electron, lattice, and orbital in quasi-one-dimensional CDW materials.