High-pressure Resistivity Measurements Research Articles

Coevolution of superconductivity and Hall coefficient with anisotropic lattice shrinkage in compressed KCa2Fe4As4F2

Abstract It is well known that the stability of superconductivity in superconductors is widely recognized to be determined by various factors, including charge, spin, orbit, lattice, and other related degrees of freedom. Here, we report our findings on the pressure-induced coevolution of superconductivity and Hall coefficient in KCa2Fe4As4F2, an iron-based superconductor possessing a hybrid crystal structure combining KFe2As2 and CaFeAsF. Our investigation, involving high-pressure resistance, Hall effect and X-ray diffraction (XRD) measurements, allows us to observe the connection of the superconductivity and Hall coefficient with the anisotropic lattice shrinkage. We find that its ambient-pressure tetragonal (T) phase presents a collapse staring at around 18 GPa, where the sign of the Hall coefficient (R H ) changes from positive to negative. Upon further compression, both superconducting transition temperature (Tc) and R H exhibit a monotonous decrease. At around 41 GPa, the superconductivity is completely suppressed (T c =0), where the parameter a begins to decline again and the Hall coefficient remain nearly unchanged. Our experiment results clearly demonstrate that the pressure-induced anisotropic lattice collapse plays a curial role in tuning the interplay among multiple degrees of freedom in the superconducting system and, correspondingly, the stability of the superconductivity.

From Insulator to Superconductor: A Series of Pressure-Driven Transitions in Quasi-One-Dimensional TiS3 Nanoribbons.

Transition metal trichalcogenides (TMTCs) offer remarkable opportunities for tuning electronic states through modifications in chemical composition, temperature, and pressure. Despite considerable interest in TMTCs, there remain significant knowledge gaps concerning the evolution of their electronic properties under compression. In this study, we employ experimental and theoretical approaches to comprehensively explore the high-pressure behavior of the electronic properties of TiS3, a quasi-one-dimensional (Q1D) semiconductor, across various temperature ranges. Through high-pressure electrical resistance and magnetic measurements at elevated pressures, we uncover a distinctive sequence of phase transitions within TiS3, encompassing a transformation from an insulating state at ambient pressure to the emergence of an incipient superconducting state above 70 GPa. Our findings provide compelling evidence that superconductivity at low temperatures of ∼2.9 K is a fundamental characteristic of TiS3, shedding new light on the intriguing high-pressure electronic properties of TiS3 and underscoring the broader implications of our discoveries for TMTCs in general.

Signature of Superconductivity in Pressurized La4Ni3O10

The discovery of high-temperature superconductivity near 80 K in bilayer nickelate La3Ni2O7 under high pressures has renewed the exploration of superconducting nickelate in bulk materials. The extension of superconductivity in other nickelates in a broader family is also essential. Here, we report the experimental observation of superconducting signature in trilayer nickelate La4Ni3O10 under high pressures. By using a modified sol-gel method and post-annealing treatment under high oxygen pressure, we successfully obtained polycrystalline La4Ni3O10 samples with different transport behaviors at ambient pressure. Then we performed high-pressure electrical resistance measurements on these samples in a diamond-anvil-cell apparatus. Surprisingly, the signature of possible superconducting transition with a maximum transition temperature (T c) of about 20 K under high pressures is observed, as evidenced by a clear drop of resistance and the suppression of resistance drops under magnetic fields. Although the resistance drop is sample-dependent and relatively small, it appears in all of our measured samples. We argue that the observed superconducting signal is most likely to originate from the main phase of La4Ni3O10. Our findings will motivate the exploration of superconductivity in a broader family of nickelates and shed light on the understanding of the underlying mechanisms of high-T c superconductivity in nickelates.

Superconductivity in the High-Entropy Ceramics Ti0.2 Zr0.2 Nb0.2 Mo0.2 Ta0.2 Cx with Possible Nontrivial Band Topology.

Topological superconductors have drawn significant interest from the scientific community due to the accompanying Majorana fermions. Here, the discovery of electronic structure and superconductivity (SC) in high-entropy ceramics Ti0.2 Zr0.2 Nb0.2 Mo0.2 Ta0.2 Cx (x=1 and 0.8) combined with experiments and first-principles calculations is reported. The Ti0.2 Zr0.2 Nb0.2 Mo0.2 Ta0.2 Cx high-entropy ceramics show bulk type-II SC with Tc ≈ 4.00 K (x=1) and 2.65 K (x=0.8), respectively. The specific heat jump (∆C/γTc ) is equal to 1.45 (x=1) and 1.52 (x=0.8), close to the expected value of 1.43 for the BCS superconductor in the weak coupling limit. The high-pressure resistance measurements show a robust SC against high physical pressure in Ti0.2 Zr0.2 Nb0.2 Mo0.2 Ta0.2 C, with a slight Tc variation of 0.3K within 82.5 GPa. Furthermore, the first-principles calculations indicate that the Dirac-like point exists in the electronic band structures of Ti0.2 Zr0.2 Nb0.2 Mo0.2 Ta0.2 C, which is potentially a topological superconductor. The Dirac-like point is mainly contributed by the d orbitals of transition metals M and the p orbitals of C. The high-entropy ceramics provide an excellent platform for the fabrication of novel quantum devices, and the study may spark significant future physics investigations in this intriguing material.

Signatures of superconductivity near 80 K in a nickelate under high pressure.

Although high-transition-temperature (high-Tc) superconductivity in cuprates has been known for more than three decades, the underlying mechanism remains unknown1-4. Cuprates are the only unconventional superconductors that exhibit bulk superconductivity with Tc above the liquid-nitrogen boiling temperature of 77 K. Here we observe that high-pressure resistance and mutual inductive magnetic susceptibility measurements showed signatures of superconductivity in single crystals of La3Ni2O7 with maximum Tc of 80 K at pressures between 14.0 GPa and 43.5 GPa. The superconducting phase under high pressure has an orthorhombic structure of Fmmm space group with the [Formula: see text] and [Formula: see text] orbitals of Ni cations strongly mixing with oxygen 2p orbitals. Our density functional theory calculations indicate that the superconductivity emerges coincidently with the metallization of the σ-bonding bands under the Fermi level, consisting of the [Formula: see text] orbitals with the apical oxygen ions connecting the Ni-O bilayers. Thus, our discoveries provide not only important clues for the high-Tc superconductivity in this Ruddlesden-Popper double-layered perovskite nickelates but also a previously unknown family of compounds to investigate the high-Tc superconductivity mechanism.

Pressure tuning of structure, magnetic frustration, and carrier conduction in the Kitaev spin liquid candidate Cu2IrO3

The layered honeycomb lattice iridate Cu$_2$IrO$_3$ is the closest realization of the Kitaev quantum spin liquid, primarily due to the enhanced interlayer separation and nearly ideal honeycomb lattice. We report pressure-induced structural evolution of Cu$_2$IrO$_3$ by powder x-ray diffraction (PXRD) up to $\sim$17 GPa and Raman scattering measurements up to $\sim$25 GPa. A structural phase transition (monoclinic $C2/c \: \rightarrow$ triclinic $P\bar{1}$) is observed with a broad mixed phase pressure range ($\sim$4 to 15 GPa). The triclinic phase consists of heavily distorted honeycomb lattice with Ir-Ir dimer formation and a collapsed interlayer separation. In the stability range of the low-pressure monoclinic phase, structural evolution maintains the Kitaev configuration up to 4 GPa. This is supported by the observed enhanced magnetic frustration in dc susceptibility without emergence of any magnetic ordering and an enhanced dynamic Raman susceptibility. High-pressure resistance measurements up to 25 GPa in the temperature range 1.4--300 K show resilient non-metallic $R$($T$) behaviour with significantly reduced resistivity in the high-pressure phase. The Mott 3D variable-range-hopping conduction with much reduced characteristic energy scale $T_0$ suggests that the high-pressure phase is at the boundary of localized-itinerant crossover. Using first-principles density functional theoretical (DFT) calculations, we find that at ambient pressure $\rm Cu_2IrO_3$ exists in monoclinic $P2_1/c$ phase which is energetically lower than $C2/c$ phase (both the structures are consistent with experimental XRD pattern). DFT reveals structural transition from $P2_1/c$ to $P\bar{1}$ structure at 7 GPa (involving dimerization of Ir-Ir bonds) in agreement with experimentally observed transition pressure.

Creating superconductivity in WB2 through pressure-induced metastable planar defects

High-pressure electrical resistivity measurements reveal that the mechanical deformation of ultra-hard WB2 during compression induces superconductivity above 50 GPa with a maximum superconducting critical temperature, Tcof 17 K at 91 GPa. Upon further compression up to 187 GPa, the Tcgradually decreases. Theoretical calculations show that electron-phonon mediated superconductivity originates from the formation of metastable stacking faults and twin boundaries that exhibit a local structure resembling MgB2 (hP3, space group 191, prototype AlB2). Synchrotron x-ray diffraction measurements up to 145 GPa show that the ambient pressure hP12 structure (space group 194, prototype WB2) continues to persist to this pressure, consistent with the formation of the planar defects above 50 GPa. The abrupt appearance of superconductivity under pressure does not coincide with a structural transition but instead with the formation and percolation of mechanically-induced stacking faults and twin boundaries. The results identify an alternate route for designing superconducting materials.

Pressure-induced superconductivity in the kagome single-crystal Pd3P2S8

The kagome lattice offers unique opportunities for the exploration of unusual quantum states of correlated electrons. Here, we report on the observation of superconductivity in a kagome single-crystal ${\mathrm{Pd}}_{3}{\mathrm{P}}_{2}{\mathrm{S}}_{8}$ when a semiconducting-to-metallic transition is driven by pressure. High-pressure resistance measurements show that the metallization and superconductivity are simultaneously observed at about 11 GPa. With increasing pressure, the superconducting critical temperature ${T}_{c}$ is monotonously enhanced from 2.6 K to a maximum 7.7 K at $\ensuremath{\sim}52\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Interestingly, superconductivity is retained when the pressure is fully released. Synchrotron x-ray diffraction and Raman experiments consistently evidence that the emergence of superconductivity is accompanied with an amorphization, and the retainability of superconductivity upon decompression can be attributed to the irreversibility of the amorphization.

Second harmonic AC calorimetry technique within a diamond anvil cell.

Tuning the energy density of matter at high pressures gives rise to exotic and often unprecedented properties, e.g., structural transitions, insulator-metal transitions, valence fluctuations, topological order, and the emergence of superconductivity. The study of specific heat has long been used to characterize these kinds of transitions, but their application to the diamond anvil cell (DAC) environment has proved challenging. Limited work has been done on the measurement of specific heat within DACs, in part due to the difficult experimental setup. To this end, we have developed a novel method for the measurement of specific heat within a DAC that is independent of the DAC design and is, therefore, readily compatible with any DACs already performing high pressure resistance measurements. As a proof-of-concept, specific heat measurements of the MgB2 superconductor were performed, showing a clear anomaly at the transition temperature (Tc), indicative of bulk superconductivity. This technique allows for specific heat measurements at higher pressures than previously possible.

Superconductivity above 80 K in polyhydrides of hafnium

Studies on polyhydrides are attracting growing attentions recently due to their potential high temperature superconductivity (SC). We here report the discovery of SC in hafnium polyhydrides at high pressures. The hafnium superhydrides are synthesized at high pressure and high temperature conditions using diamond anvil cell in combination with in-situ high pressure laser heating technique. The SC was investigated by in-situ high pressure resistance measurements in applied magnetic fields. A superconducting transition with onset Tc ∼83 K was observed at 243 GPa. The upper critical field μ0Hc2(0) was estimated to be 24 T by GL theory and the consequent superconducting coherent length ξ to be ∼37 Å. Our results suggest that the superconducting phase is from C2/m-HfH14. This is the first 5d transition metal polyhydride superconductor with Tc above the liquid nitrogen temperature.

Pressure-induced metallization in the absence of a structural transition in the layered ferromagnetic insulator Cr2Ge2Te6

We report the crystallographic and electrical transport properties of single crystals of the ferromagnetic two-dimensional (2D) material ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ under high pressure. In contrast to previous studies, our high-pressure single-crystal x-ray diffraction under hydrostatic conditions shows prominent anisotropic compressibility in the layered structure of the crystalline $R\overline{3}$ phase of ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ without any structural phase transitions up to 20 GPa. Our data confirm a distinct and irreversible crystalline-amorphous transformation in ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$. The loss of crystallinity starts at 20 GPa; however, the crystalline phase and amorphous state coexist even at the maximum pressure of 31.2 GPa. High-pressure powder x-ray diffraction data and electrical resistivity measurements of ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ using NaCl as the pressure-transmitting medium reveal an insulator-to-metal transition in the absence of a phase transition at \ensuremath{\sim}3.9 GPa; at a considerably lower pressure than the previously reported (7--14 GPa). Density functional theory calculations demonstrate the density of states around the Fermi level are primarily dominated by Cr $3d$ and Te $5p$ states. Hence the large reduction of Cr-Te bond lengths within the ${\mathrm{CrTe}}_{6}$ octahedra under compression is most likely responsible for the band-gap closure. This study clarifies that the phase stability and onset of metallization pressure in the ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ sample are sensitive to the hydrostatic environments and demonstrates how pressure can be used to tune the physical properties of 2D ferromagnetic compounds.

Temperature-Pressure Phase Diagram and Possible Pressure-Driven New Electronic Phase in the Polar Metal LiOsO3

LiOsO3 is a strongly correlated metal that undergoes a nonpolar to polar transition at the critical temperature (T s) of 140 K. Complementary to previous studies of structure, Raman, and resistivity under high pressure (P), here we map out a complete pressure evolution of T s(P) via high-pressure resistance measurements up to 18.5 GPa by using a low-temperature multianvil apparatus. Our results show that T s(P) first increases linearly with pressure at a large slope as reported and then levels off gradually at pressures above 10 GPa when approaching room temperature (295 K). Interestingly, we find that the resistance R(T) of LiOsO3 at 18.5 GPa in the polar R3c phase exhibits a distinct temperature profile in comparison with those at lower pressures, signaling the possible occurrence of pressure-driven new electronic phase. The critical pressure for this transition is determined to be P c ≈ 16.8(1) GPa based on the pressure-dependent resistance measurement at room temperature.

Strain induced electronic transition in 1T′ MoTe2: high pressure Raman, x-ray diffraction, resistivity measurements and first principles theoretical studies

A detailed high pressure study is carried out on 1T′ MoTe2 using x-ray diffraction (XRD) and Raman spectroscopy measurements up to about 30.5 GPa along with a room temperature resistivity measurement up to 14.3(4) GPa and density functional theory calculations. Though high-pressure XRD measurements show no structural transition, all the lattice parameters exhibit anomalous changes in the pressure region 8.4 to 12.7 GPa. The compressibility of the sample is found to be reduced by almost four times above 12.7 GPa with respect to that below 8.4 GPa. The anomalies in the Raman mode corresponding to the out of plane vibrations of Mo atoms sitting in the unit cell surface indicate a strong electron–phonon coupling possibly mediated by differential strain inside the unit cell. A rapid decrease in resistivity value up to about 7.0(2) GPa of pressure agrees well with the increase in the density of states (DOS) at the Fermi energy with pressure. Pressure evolution of band structure, as well as DOS at the Fermi level, shows an enhancement of the metallic character of the sample. First principle calculations show increased stress in the x and y directions compared to the z-direction with the application of pressure.

Single-Component Molecular Conductors — Multi-Orbital Correlated π-d Electron Systems

Abstract Traditional molecular conductors are composed of more than two chemical species. Two prerequisites for the design of molecular metals have long been considered to be 1) forming of the electronic band and 2) existence of charge carriers created by the intermolecular charge transfer between the molecules constructing the band and other chemical species. On the other hand, a single-component molecular metal, [Ni(tmdt)2] (tmdt = trimethylenetetrathiafulvalenedithiolate), was developed in 2001; it is a planar nickel complex coordinated by the extended-TTF dithiolate ligands, tmdt from both sides. Since then, various types of single-component molecular conductors with a variety of extended-TTF dithiolate ligands have been developed. In this account, we briefly describe the recent progress in research on single-component molecular conductors. First, single-component molecular conductors in isostructural systems, [M(tmdt)2] (M = Ni, Pd, Pt, Au, and Cu) are described. Recent orbital-selective 13C and 1H NMR experiments have genealogically elucidated the differences in the electronic states and physical properties of these systems, that is, their various unusual phenomena are produced from their multi-orbital correlated π or π-d electron systems. Next, we describe [Ni(hfdt)2] (hfdt = bis(trifluoromethyl)tetrathiafulvalenedithiolate), the first single-component molecular superconductor, which was revealed by high-pressure resistivity measurements with a diamond anvil cell (DAC). The superconducting transition occurred around 7.5–8.7 GPa with a maximum Tc (onset temperature) of 5.5 K. Recent theoretical calculation has revealed that [Ni(hfdt)2] will be a new molecular Dirac electron system. In the final section, we briefly introduce molecular Dirac electron systems. Recently, a new series of semimetals, [M(dmdt)2] (M = Pt and Ni; dmdt = dimethyltetrathiafulvalenedithiolate) was synthesized. They belong to a three-dimensional ambient-pressure molecular massless Dirac electron system. The first-principles band structure calculations of [M(dmdt)2] (M = Pt and Ni) revealed that Dirac cones emerge along the a* direction and form Dirac nodal lines.

High-pressure study of the low- Z rich superconductor Be22Re

With $T_c \sim 9.6~\mathrm{K}$, Be$_{22}$Re exhibits one of the highest critical temperatures among Be-rich compounds. We have carried out a series of high-pressure electrical resistivity measurements on this compound to 30 GPa. The data show that the critical temperature $T_c$ is suppressed gradually at a rate of $dT_c/dP = -0.05~\mathrm{K/GPa}$. Using density functional theory (DFT) calculations of the electronic and phonon density of states (DOS) and the measured critical temperature, we estimate that the rapid increase in lattice stiffening in Be$_{22}$Re overwhelms a moderate increase in the electron-ion interaction with pressure, resulting in the decrease in $T_c$. High pressure x-ray diffraction measurements show that the ambient pressure crystal structure of Be$_{22}$Re persists to at least 154 GPa. We discuss the relationship between low-Z Be-rich superconductors and the high-$T_c$ superhydrides.

High-pressure insulating phase of Mo4O11 with collapsed volume

We investigated the effect of pressure on the crystal structure and transport properties of the Magn\'eli phase \ensuremath{\eta}-${\mathrm{Mo}}_{4}{\mathrm{O}}_{11}$, which at ambient pressure undergoes two successive charge-density-wave (CDW) transitions at ${T}_{\mathrm{CDW}\text{\ensuremath{-}}1}\ensuremath{\approx}105\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{CDW}\text{\ensuremath{-}}2}\ensuremath{\approx}30\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, respectively. We find that \ensuremath{\eta}-${\mathrm{Mo}}_{4}{\mathrm{O}}_{11}$ exhibits a structural phase transition from the low-pressure monoclinic $P{2}_{1}/a$ phase to a high-pressure $P{2}_{1}$ phase at ${P}_{c}\ensuremath{\approx}3.5\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$. Around ${P}_{\mathrm{c}}$, the lattice parameters experience a sudden change with a large volume collapse of $\mathrm{\ensuremath{\Delta}}V/V=\ensuremath{-}8.1%$, while the room-temperature resistivity exhibits a sudden jump by two orders of magnitude, signaling a pressure-induced metal-to-insulator transition. For $P<{P}_{c}$, the high-pressure resistivity measurements revealed opposite pressure dependences of these two CDW transitions, i.e., ${T}_{\mathrm{CDW}\text{\ensuremath{-}}1}$ is enhanced gradually to \ensuremath{\sim}130 K while ${T}_{\mathrm{CDW}\text{\ensuremath{-}}2}$ is almost suppressed completely by the application of 2.6 GPa pressure. For $P\ensuremath{\ge}{P}_{c}$, the temperature dependence of resistivity changes to an insulating-like behavior, but the activation energy is reduced gradually upon further increasing pressure. We have rationalized the insulating ground state of the high-pressure phase in terms of the structural modifications and charge redistribution based on the refinement of single-crystal x-ray diffraction data at 8.9 GPa.

Anomalous charge transport of superconducting CuxPdTe2 under high pressure

By means of high-pressure resistivity measurements on single crystals, we investigate the charge transport properties of Cu$_x$PdTe$_2$, notable for the combination of topological type-II Dirac semimetallic properties with superconductivity up to $T_c = 2.5$ K. In both cases of pristine ($x = 0$) and intercalated ($x=0.05$) samples, we find an unconventional $T^4$ power law behavior of the low-temperature resistivity visible up to $\sim$40 K and remarkably stable under pressure up to 8.2 GPa. This observation is explained by the low carrier density $n$, which strongly reduces the $k$-region available for electron-phonon scattering, as previously reported in other low-$n$ two-dimensional systems, such as multilayer graphene and semiconductor heterostructures. Our data analysis complemented by specific heat measurements and supported by previous quantum oscillation studies and \textit{ab initio} calculations suggests a scenario of one-band charge transport. Within this scenario, our analysis yields a large value of transport electron-phonon coupling constant $\lambda_{tr} = 1.2$ at ambient pressure that appears to be strongly enhanced by pressure assuming a constant effective mass.

New Cage-Like Cerium Trihydride Stabilized at Ambient Conditions

New Cage-Like Cerium Trihydride Stabilized at Ambient Conditions

Pressure-induced superconductivity and structural transition in ferromagnetic CrSiTe3

Layered structural materials have been a fertile playground to investigate mechanisms of fundamental physics and explore potential applications. Here, we report investigations on ferromagnetic van der Waals $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_{3}$ via high-pressure synchrotron x-ray diffraction, electrical resistance, and magnetoresistance measurements. Under compression, $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_{3}$ undergoes an insulator-metal transition and a structural transition at $\ensuremath{\sim}7.5$ GPa. Concomitantly with the structural transition, the magnetoresistance changes sign, the negative Hall coefficient increases dramatically, and superconductivity emerges at 3 K. The superconductivity persists up to the highest measured pressure of 47.1 GPa with a maximum ${T}_{c}\ensuremath{\approx}$ 4.5 K at $\ensuremath{\sim}30$ GPa. Our results suggest that $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_{3}$ is paramagnetic in the pressure range of superconductivity. The discoveries of superconductivity and magnetic transition in ferromagnetic $\mathrm{Cr}\mathrm{Si}{\mathrm{Te}}_{3}$ under pressure provide new perspectives to explore the interplay between superconductivity and magnetism in Cr-based two-dimensional van der Waals materials.

Correlation between Fermi surface reconstruction and superconductivity in pressurized FeTe0.55Se0.45

Here we report the first results of the high-pressure Hall coefficient (RH) measurements, combined with the high-pressure resistance measurements, at different temperatures on the putative topological superconductor FeTe0.55Se0.45. We find the intimate correlation of sign change of RH, a fingerprint to manifest the reconstruction of Fermi surface, with structural phase transition and superconductivity. Below the critical pressure (PC) of 2.7 GPa, our data reveal that the hole - electron carriers are thermally balanced (RH=0) at a critical temperature (T*), where RH changes its sign from positive to negative, and concurrently a tetragonal-orthorhombic phase transition takes place. Within the pressure range from 1bar to PC, T* is continuously suppressed by pressure, while TC increases monotonically. At about PC, T* is indistinguishable and TC reaches a maximum value. Moreover, a pressure-induced sign change of RH is found at ~PC where the orthorhombic-monoclinic phase transition occurs. With further compression, TC decreases and disappears at ~ 12 GPa. The correlation among the electron-hole balance, crystal structure and superconductivity found in the pressurized FeTe0.55Se0.45 implies that its nontrivial superconductivity is closely associated with its exotic normal state resulted from the interplay between the reconstruction of the Fermi surface and the change of the structural lattice.