Superconductivity up to 164 K in HgBa2Cam-1CumO2m+2+ delta (m=1, 2, and 3) under quasihydrostatic pressures

Coexistence of topological node surface and Dirac fermions in phonon-mediated superconductor YB2C2

The interaction between nontrivial topology and superconductivity in condensed matter physics has attracted tremendous research interest as it could give rise to exotic phenomena. Herein, based on first-principles calculations, we investigate the electronic structures, mechanical properties, topological properties, dynamic stability, electron-phonon coupling (EPC), and superconducting properties of the synthesized real material YB2C2. It is a tetragonal structure with P4/mbm symmetry and exhibits excellent stability. The calculated electronic band structures reveal that a zero-dimension (0D) Dirac point and two-dimensional (2D) nodal surface coexist near the Fermi level. A spin-orbit coupling (SOC) Dirac point with the topological Fermi arc is observed on the (001) surface. These nodal surfaces are protected by a two-fold screw axis and time-reversal symmetry. Based on the Bardeen-Cooper-Schrieffer theory, the superconducting transition temperature (Tc) in the range 1.25-4.45 K with different Coulomb repulsion constant μ* for YB2C2 is estimated to be consistent with previous experimental results. In addition, the EPC is mainly from the coupling between the dx2-y2 and dz2 orbitals of the Y atom and low-energy phonon modes. The presence of superconductivity and nontrivial topological surface state in YB2C2 suggests that it may be a candidate material for topological superconductors.

Cuprate superconducting materials above liquid nitrogen temperature from machine learning

The superconductivity of cuprates remains a challenging topic in condensed matter physics, and the search for materials that superconduct electricity above liquid nitrogen temperature and even at room temperature is of great significance for future applications. Nowadays, with the advent of artificial intelligence, research approaches based on data science have achieved excellent results in material exploration. We investigated machine learning (ML) models by employing separately the element symbolic descriptor atomic feature set 1 (AFS-1) and a prior physics knowledge descriptor atomic feature set 2 (AFS-2). An analysis of the manifold in the hidden layer of the deep neural network (DNN) showed that cuprates still offer the greatest potential as superconducting candidates. By calculating the SHapley Additive exPlanations (SHAP) value, it is evident that the covalent bond length and hole doping concentration emerge as the crucial factors influencing the superconducting critical temperature (T_c). These findings align with our current understanding of the subject, emphasizing the significance of these specific physical quantities. In order to improve the robustness and practicability of our model, two types of descriptors were used to train the DNN. We also proposed the idea of cost-sensitive learning, predicted the sample in another dataset, and designed a virtual high-throughput search workflow.

Temperature effects on the calculation of the functional derivative of Tc with respect to α2F(ω)

The functional derivative of the superconducting transition temperature Tc with respect to the electron-phonon coupling function [Formula: see text] permits identifying the frequency regions where phonons are most effective in raising Tc. This work presents an analysis of temperature effects on the calculation of the δTc/δα2F(ω) and μ* parameters. The results may permit establishing that the variation of the temperature in the δTc/δα2F(ω) and μ* parameter allows establishing patterns and conditions that are possibly related to the physical conditions in the superconducting state, with implications on the theoretical estimation of the Tc.

Modulations in Superconductors: Probes of Underlying Physics

The importance of modulations is elevated to an unprecedented level, due to the delicate conditions required to bring out exotic phenomena in quantum materials, such as topological materials, magnetic materials, and superconductors. Recently, state-of-the-art modulation techniques in material science, such as electric-double-layer transistor, piezoelectric-based strain apparatus, angle twisting, and nanofabrication, have been utilized in superconductors. They not only efficiently increase the tuning capability to the broader ranges but also extend the tuning dimensionality to unprecedented degrees of freedom, including quantum fluctuations of competing phases, electronic correlation, and phase coherence essential to global superconductivity. Here, for a comprehensive review, these techniques together with the established modulation methods, such as elemental substitution, annealing, and polarization-induced gating, are contextualized. Depending on the mechanism of each method, the modulations are categorized into stoichiometric manipulation, electrostatic gating, mechanical modulation, and geometrical design. Their recent advances are highlighted by applications in newly discovered superconductors, e.g., nickelates, Kagome metals, and magic-angle graphene. Overall, the review is to provide systematic modulations in emergent superconductors and serve as the coordinate for future investigations, which can stimulate researchers in superconductivity and other fields to perform various modulations toward a thorough understanding of quantum materials.

The breakdown of both strange metal and superconducting states at a pressure-induced quantum critical point in iron-pnictide superconductors

Here we report the first observation of the concurrent breakdown of the strange metal (SM) normal state and superconductivity at a pressure-induced quantum critical point in Ca₁₀(Pt₄As₈)((Fe_0.97Pt_0.03)₂As₂)₅ superconductor. We find that, upon suppressing the superconducting state, the power exponent (α) changes from 1 to 2, and the slope of the temperature-linear resistivity per FeAs layer (A^□) gradually diminishes. At a critical pressure, A^□ and superconducting transition temperature (T_c) go to zero concurrently, where a quantum phase transition from a superconducting state with a SM normal state to a non-superconducting Fermi liquid state occurs. Scaling analysis reveals that the change of A^□ with T_c obeys the relation of T_c ~ (A^□)^0.5, similar to what is seen in other chemically doped unconventional superconductors. These results suggest that there is a simple but powerful organizational principle of connecting the SM normal state with the high-T_c superconductivity.

La26Ge19M5O5 (M = Ag, Cu): Rare-Earth Metal Suboxide Superconductors with [La18O5] Cluster Units

Herein we report two reduced rare-earth metal-based superconductors, La₂₆Ge₁₉M₅O₅ (M = Ag, Cu), that feature an unprecedented [La₁₈O₅] cluster composed of five oxygen-centered [La₆O] octahedra condensed through shared faces and capped with [Ge₄] butterfly rings. The structure, determined by single-crystal X-ray diffraction, crystallizes in a tetragonal space group (P4/nmm), with a = 15.508(2) Å and c = 11.238(2) Å. Resistivity and magnetic susceptibility measurements show onsets of superconductivity at T_c = 5.4 and 6.4 K for the Ag and Cu compounds, respectively. Applying high pressures, up to 1.3 GPa, results in increased superconducting transition temperatures (T_c = 6.8 K for Ag and 7.2 K for Cu compounds), with no sign of saturation.

Brillouin Scattering and First-Principles Studies of BaMO3 (M = Ti, Zr, and Cu) Perovskites

Perovskite oxides with the general formula ABO₃ comprise a large number of families among the structures of oxide-based materials, and currently, several perovskite structures have been identified. From a variety of compositions and structures, various functions are observed in perovskite compounds, and therefore, they became very useful for various applications in the electronic and medical industries. One of the most puzzling issues for perovskite compounds is the understanding of the vibration and relaxation dynamics in the gigahertz range. In that sense, the micro-Brillouin scattering system is a very effective tool to probe the gigahertz dynamics, and also, first-principles calculations can be used to describe the phonon structure with different atomic contributions. The micro-Brillouin scattering system and first-principles calculations provide the fundamental information on a variety of vibration and relaxation processes related to structural phase transitions under different external conditions such as temperature, electric field, and pressure. This review article summarizes the Brillouin scattering and first-principles studies on BaMO₃ (M = Ti, Zr, and Cu). Through a detailed analysis of the existing results, we summarize the existing limitations and future perspectives in these research areas, which may propel the development of different perovskite ferroelectrics and extend their practical application areas.

Room-Temperature Superconductivity in Yb/Lu Substituted Clathrate Hexahydrides under Moderate Pressure

Room temperature superconductivity is a dream that mankind has been chasing for a century. In recent years, the synthesis of H₃S, LaH₁₀, and C-S-H compounds under high pressures has gradually made that dream become a reality. But the extreme high pressure required for stabilization of hydrogen-based superconductors limit their applications. So, the next challenge is to achieve room-temperature superconductivity at significantly low pressures, even ambient pressure. In this work, we design a series of high temperature superconductors that can be stable at moderate pressures by incorporating heavy rare earth elements Yb/Lu into sodalite-like clathrate hexahydrides. In particular, the critical temperatures (T_c) of Y₃LuH₂₄, YLuH₁₂, and YLu₃H₂₄ can reach 283 K at 120 GPa, 275 K at 140 GPa, and 288 K at 110 GPa, respectively. Their critical temperatures are close to or have reached room temperature, and minimum stable pressures are significantly lower than that of reported room temperature superconductors. Our work provides an effective method for the rational design of low-pressure stabilized hydrogen-based superconductors with room-temperature superconductivity simultaneously and will stimulate further experimental exploration.

Pressure-induced monotonic enhancement of Tc to over 30 K in superconducting Pr0.82Sr0.18NiO2 thin films

The successful synthesis of superconducting infinite-layer nickelate thin films with the highest T_c ≈ 15 K has ignited great enthusiasm for this material class as potential analogs of the high-T_c cuprates. Pursuing a higher T_c is always an imperative task in studying a new superconducting material system. Here we report high-quality Pr_0.82Sr_0.18NiO₂ thin films with T_c^onset ≈ 17 K synthesized by carefully tuning the amount of CaH₂ in the topotactic chemical reduction and the effect of pressure on its superconducting properties by measuring electrical resistivity under various pressures in a cubic anvil cell apparatus. We find that the onset temperature of the superconductivity, T_c^onset, can be enhanced monotonically from ~17 K at ambient pressure to ~31 K at 12.1 GPa without showing signatures of saturation upon increasing pressure. This encouraging result indicates that the T_c of infinite-layer nickelates superconductors still has room to go higher and it can be further boosted by applying higher pressures or strain engineering in the heterostructure films.

Lattice dynamics of BaFe2Se3

This paper presents a study of the lattice dynamics in BaFe₂Se₃. We combined first-principle calculations, infrared measurements and a thorough symmetry analysis. Our study confirms thatPnmacannot be the space group of BaFe₂Se₃, even at room temperature. The phonons assignment requiresPmto be the BaFe₂Se₃space group, not only in the magnetic phase, but also in the paramagnetic phase at room temperature. This is due to a strong coupling between a short-range spin-order along the ladders, and the lattice degrees of freedom associated with the Fe-Fe bond length. This coupling induces a change in the bond-length pattern from an alternated trapezoidal one (as inPnma) to an alternated small/large rectangular one. Out of the two patterns, only the latter is fully compatible with the observed block-type magnetic structure. Finally, we propose a complete symmetry analysis of the BaFe₂Se₃phase diagram in the 0-600 K range.

High-Temperature Superconducting Phase in Clathrate Calcium Hydride CaH_{6} up to 215 K at a Pressure of 172 GPa

The recent discovery of superconductive rare earth and actinide superhydrides has ushered in a new era of superconductivity research at high pressures. This distinct type of clathrate metal hydrides was first proposed for alkaline-earth-metal hydride CaH_{6} that, however, has long eluded experimental synthesis, impeding an understanding of pertinent physics. Here, we report successful synthesis of CaH_{6} and its measured superconducting critical temperature T_{c} of 215 K at 172 GPa, which is evidenced by a sharp drop of resistivity to zero and a characteristic decrease of T_{c} under a magnetic field up to 9 T. An estimate based on the Werthamer-Helfand-Hohenberg model gives a giant zero-temperature upper critical magnetic field of 203 T. These remarkable benchmark superconducting properties place CaH_{6} among the most outstanding high-T_{c} superhydrides, marking it as the hitherto only clathrate metal hydride outside the family of rare earth and actinide hydrides. This exceptional case raises great prospects of expanding the extraordinary class of high-T_{c} superhydrides to a broader variety of compounds that possess more diverse material features and physics characteristics.

Superconductivity in nickel-based 112 systems

Superconductivity has been discovered recently in infinite-layer nickel-based 112 thin films R_1−xA_xNiO₂ (R = La, Nd, Pr and A = Sr, Ca). They are isostructural to the infinite-layer cuprate (Ca,Sr)CuO₂ and are supposed to have a formal Ni 3d⁹ valence, thus providing a new platform to study the unconventional pairing mechanism of high-temperature superconductors. This important discovery immediately triggers a huge amount of innovative scientific curiosity in the field. In this paper, we try to give an overview of the recent research progress on the newly found superconducting nickelate systems, both from experimental and theoretical aspects. We mainly focus on the electronic structures, magnetic excitations, phase diagrams and superconducting gaps, and finally make some open discussions for possible pairing symmetries in Ni-based 112 systems.

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The infinite-layer nickel-based 112 thin films R_1−xA_xNiO₂ can host superconductivity up to 15 K

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R_1−xA_xNiO₂ is a multiband system, in which the short-range antiferromagnetic fluctuations can be detected

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R_1−xA_xNiO₂ has an unconventional superconducting pairing sate with a robust d-wave gap and a full gap without unified understanding

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The nickelate system provides a new platform for researching unconventional superconductivity

Future Study of Dense Superconducting Hydrides at High Pressure

The discovery of a record high superconducting transition temperature (Tc) of 288 K in a pressurized hydride inspires new hope to realize ambient-condition superconductivity. Here, we give a perspective on the theoretical and experimental studies of hydride superconductivity. Predictions based on the BCS-Eliashberg-Midgal theory with the aid of density functional theory have been playing a leading role in the research and guiding the experimental realizations. To date, about twenty hydrides experiments have been reported to exhibit high-Tc superconductivity and their Tc agree well with the predicted values. However, there are still some controversies existing between the predictions and experiments, such as no significant transition temperature broadening observed in the magnetic field, the experimental electron-phonon coupling beyond the Eliashberg-Midgal limit, and the energy dependence of density of states around the Fermi level. To investigate these controversies and the origin of the highest Tc in hydrides, key experiments are required to determine the structure, bonding, and vibrational properties associated with H atoms in these hydrides.

Oxide and Organic–Inorganic Halide Perovskites with Plasmonics for Optoelectronic and Energy Applications: A Contributive Review

The ascension of halide perovskites as outstanding materials for a wide variety of optoelectronic applications has been reported in recent years. They have shown significant potential for the next generation of photovoltaics in particular, with a power conversion efficiency of 25.6% already achieved. On the other hand, oxide perovskites have a longer history and are considered as key elements in many technological applications; they have been examined in depth and applied in various fields, owing to their exceptional variability in terms of compositions and structures, leading to a large set of unique physical and chemical properties. As of today, a sound correlation between these two important material families is still missing, and this contributive review aims to fill this gap. We report a detailed analysis of the main functions and properties of oxide and organic–inorganic halide perovskite, emphasizing existing relationships amongst the specific performance and the structures.

Superconductivity up to 243 K in the yttrium-hydrogen system under high pressure

The discovery of superconducting H3S with a critical temperature Tc∼200 K opened a door to room temperature superconductivity and stimulated further extensive studies of hydrogen-rich compounds stabilized by high pressure. Here, we report a comprehensive study of the yttrium-hydrogen system with the highest predicted Tcs among binary compounds and discuss the contradictions between different theoretical calculations and experimental data. We synthesized yttrium hydrides with the compositions of YH3, YH4, YH6 and YH9 in a diamond anvil cell and studied their crystal structures, electrical and magnetic transport properties, and isotopic effects. We found superconductivity in the Im-3m YH6 and P63/mmc YH9 phases with maximal Tcs of ∼220 K at 183 GPa and ∼243 K at 201 GPa, respectively. Fm-3m YH10 with the highest predicted Tc > 300 K was not observed in our experiments, and instead, YH9 was found to be the hydrogen-richest yttrium hydride in the studied pressure and temperature range up to record 410 GPa and 2250 K.

Charge Order and Suppression of Superconductivity in HgBa2CuO4+d at High Pressures

New insight into the superconducting properties of HgBa2CuO4 (Hg-1201) cuprates is provided by combined measurements of electrical resistivity and single crystal X-ray diffraction under pressure. The changes induced by increasing pressure up to 20 GPa in optimally doped single crystals were investigated. The resistivity measurements as a function of temperature show a metallic behavior up to ~10 GPa that gradually passes into an insulating state, typical of charge ordering, which totally suppresses superconductivity above 13 GPa. The changes in resistivity are accompanied by the apparition of sharp Bragg peaks in the X-ray diffraction patterns, indicating that the charge ordering is accompanied by a 3D oxygen ordering. Considering that pressure induces a charge transfer of about 0.02 at 10 GPa, our results are the first observation of charge order competing with superconductivity developed in the overdoped region of the phase diagram of a Hg-based cuprate.

Pressure-induced high-temperature superconductivity retained without pressure in FeSe single crystals

To raise the superconducting-transition temperature (T_c) has been the driving force for the long-sustained effort in superconductivity research. Recent progress in hydrides with T_cs up to 287 K under pressure of 267 GPa has heralded a new era of room temperature superconductivity (RTS) with immense technological promise. Indeed, RTS will lift the temperature barrier for the ubiquitous application of superconductivity. Unfortunately, formidable pressure is required to attain such high T_cs. The most effective relief to this impasse is to remove the pressure needed while retaining the pressure-induced T_c without pressure. Here, we show such a possibility in the pure and doped high-temperature superconductor (HTS) FeSe by retaining, at ambient pressure via pressure quenching (PQ), its T_c up to 37 K (quadrupling that of a pristine FeSe at ambient) and other pressure-induced phases. We have also observed that some phases remain stable without pressure at up to 300 K and for at least 7 d. The observations are in qualitative agreement with our ab initio simulations using the solid-state nudged elastic band (SSNEB) method. We strongly believe that the PQ technique developed here can be adapted to the RTS hydrides and other materials of value with minimal effort.

Chemistry in Superconductors

Superconductors with exotic physical properties are critical to current and future technology. In this review, we highlight several important superconducting families and focus on their crystal structure, chemical bonding, and superconductivity correlations. We connect superconducting materials with chemical bonding interactions based on their structure-property relationships, elucidating our empirically chemical approaches and other methods used in the discovery of new superconductors. Furthermore, we provide some technical strategies to synthesize superconductors and basic but important characterization for chemists needed when reporting new superconductors. In the end, we share our thoughts on how to make new superconductors and where chemists can work on in the superconductivity field. This review is written using chemical terms, with a focus on providing some chemically intuitive thoughts on superconducting materials design.

Strain Engineering at Heterointerfaces: Application to an Iron Pnictide Superconductor, Cobalt-Doped BaFe2As2

We propose a unique strategy to apply stronger strain at heterointerfaces than conventional epitaxial strain methods to extract hidden attractive physical/chemical properties in materials. This strategy involves precisely accounting for the epitaxial strain induced by lattice mismatch as well as the differences in the thermal expansion coefficients and compressibilities of epitaxial films and substrates. We selected optimally cobalt-doped BaFe₂As₂(Ba122:Co), an iron-based superconductor with a bulk critical temperature (T_c) of 22 K, as a model material and four types of single-crystal substrates. Ba122:Co was selected because its T_c is robust to hydrostatic pressure but sensitive to epitaxial strain (i.e., one of the anisotropic strains), and the selected substrates entirely cover the positive/negative lattice mismatches, thermal expansion coefficients, and compressibilities with respect to Ba122:Co. With strong anisotropic strain successfully induced by film growth, external hydrostatic pressurizing, and cooling processes, we observed unique carrier transport properties in Ba122:Co epitaxial films on CaF₂ and BaF₂ substrates including (i) upturn behavior in the temperature dependence of the longitudinal resistivity, (ii) negative magnetoresistance, (iii) large enhancement of anomalous Hall effects in the epitaxial films on CaF₂, and (iv) enhancement of T_c to 27 K in the epitaxial films on BaF₂. These results demonstrate the effectiveness of our strategy, and this approach can be further extended to other inorganic materials in thin-film form.

Straining quantum materials even further

A nanoscale membrane enables exploration of large tensile strains on complex oxides

Influence of pressure on the transport, magnetic, and structural properties of superconducting Cr0.0009NbSe2 single crystal

We investigate the superconducting critical current density (J _c), transition temperature (T _c), and flux pinning properties under hydrostatic pressure (P) for Cr_0.0009NbSe₂ single crystal. The application of P enhances T _c in both electrical resistivity (∼0.38 K GPa^-1: 0 ≤ P ≤ 2.5 GPa) and magnetization (∼0.98 K GPa^-1: 0 ≤ P ≤ 1 GPa) measurements, which leads to a monotonic increase in J _c and flux pinning properties. The field-dependent J _c at various temperatures under P is analyzed within the collecting pinning theory and it shows that δT _c pinning is the crossover to δl pinning above the critical pressure (P _c ∼0.3 GPa). Our systematic analysis of the flux pinning mechanism indicates that both the density of pinning centers and pinning forces greatly increase with the application of P, which leads to an enhancement in the vortex state. Structural studies using synchrotron X-ray diffraction under pressure illustrate a stable hexagonal phase without any significant impurity phase and lattice parameter reduction with P shows highly anisotropic nature.

Magnetic signatures of 120 K superconductivity at interfaces in La2CuO4+δ

In self-assembled vertically aligned nanocomposite (VAN) thin films of La2CuO4+δ + LaCuO3, we find from DC magnetic susceptibility measurements, weak signatures of superconductivity at ∼120 K. This compares to a maximum TC of 40 K in bulk La2CuO4+δ. The 120 K signature occurs only when both c-axis and a-axis oriented La2CuO4+δ grains are present in the films. The superconductivity was lost after 3 months of storage but was recovered by annealing in oxygen. From lattice parameter analyses undertaken close to the c/a grain boundaries, it was determined that expansion of the La perovskite block in c-La2CuO4+δ enables the differently oriented grains to join at the interface. This expansion is consistent with the higher TC interfacial region. The work shows a new direction for increasing TC in cuprates - namely careful strain engineering of the crystal structure independently in-plane and out-of-plane.

High Compression-Induced Conductivity in a Layered Cu-Br Perovskite

We show that the onset pressure for appreciable conductivity in layered copper-halide perovskites can decrease by ca. 50 GPa upon replacement of Cl with Br. Layered Cu-Cl perovskites require pressures >50 GPa to show a conductivity of 10^-4  S cm^-1 , whereas here a Cu-Br congener, (EA)₂ CuBr₄ (EA=ethylammonium), exhibits conductivity as high as 2×10^-3  S cm^-1 at only 2.6 GPa, and 0.17 S cm^-1 at 59 GPa. Substitution of higher-energy Br 4p for Cl 3p orbitals lowers the charge-transfer band gap of the perovskite by 0.9 eV. This 1.7 eV band gap decreases to 0.3 eV at 65 GPa. High-pressure X-ray diffraction, optical absorption, and transport measurements, and density functional theory calculations allow us to track compression-induced structural and electronic changes. The notable enhancement of the Br perovskite's electronic response to pressure may be attributed to more diffuse Br valence orbitals relative to Cl orbitals. This work brings the compression-induced conductivity of Cu-halide perovskites to more technologically accessible pressures.

Relation between Crystal Structure and Transition Temperature of Superconducting Metals and Alloys

Using the Roeser–Huber equation, which was originally developed for high temperature superconductors (HTSc) (H. Roeser et al., Acta Astronautica 62 (2008) 733), we present a calculation of the superconducting transition temperatures, T c , of some elements with fcc unit cells (Pb, Al), some elements with bcc unit cells (Nb, V), Sn with a tetragonal unit cell and several simple metallic alloys (NbN, NbTi, the A15 compounds and MgB 2 ). All calculations used only the crystallographic information and available data of the electronic configuration of the constituents. The model itself is based on viewing superconductivity as a resonance effect, and the superconducting charge carriers moving through the crystal interact with a typical crystal distance, x. It is found that all calculated T c -data fall within a narrow error margin on a straight line when plotting ( 2 x ) 2 vs. 1 / T c like in the case for HTSc. Furthermore, we discuss the problems when obtaining data for T c from the literature or from experiments, which are needed for comparison with the calculated data. The T c -data presented here agree reasonably well with the literature data.

High-temperature superconductivity in sulfur hydride evidenced by alternating-current magnetic susceptibility

The search for high-temperature superconductivity is one of the research frontiers in physics. In the sulfur hydride system, an extremely high T _c (∼200 K) has been recently developed at pressure. However, the Meissner effect measurement above megabar pressures is still a great challenge. Here, we report the superconductivity identification of sulfur hydride at pressure, employing an in situ alternating-current magnetic susceptibility technique. We determine the superconducting phase diagram, finding that superconductivity suddenly appears at 117 GPa and T _c reaches 183 K at 149 GPa before decreasing monotonically with increasing pressure. By means of theoretical calculations, we elucidate the variation of T _c in the low-pressure region in terms of the changing stoichiometry of sulfur hydride and the further decrease in T _c owing to a drop in the electron-phonon interaction parameter λ. This work provides a new insight into clarifying superconducting phenomena and anchoring the superconducting phase diagram in the hydrides.

Na-doping effects on structural evolution and superconductivity in (K1-x Na x )2Cr3As3 (x = 0-1)

In this report, we studied the effects of isovalent Na-doping on the recently discovered quasi-one-dimensional Cr-based unconventional superconductor K₂Cr₃As₃. A series of polycrystalline samples with nominal component (K_1-x Na _x )₂Cr₃As₃ (x  =  0-1) were synthesized by the solid state reaction method. From crystal structure and chemical phase characterizations, we found two distinct chemical phases with the same hexagonal structure but distinguished by different site occupancy of Na⁺ ions at the two kinds of K-site in the K₂Cr₃As₃ lattice structure. When x  ⩽  0.4, the doped samples form a continuous sosoloid phase of (K_1-x Na _x )₂Cr₃As₃ with the Na⁺ ions randomly doping at the K-sites (denoted as α-phase); when x  ⩾  0.5, a novel individual phase of (K_0.25Na_0.75)₂Cr₃As₃ emerges, in which the Na⁺ ions selectively occupy all the '3k' sites and the K⁺ ions occupy the '1c' sites (denoted as β-phase). No chemical phase of Na₂Cr₃As₃ was detected. Superconductivity in these samples was studied by electrical transport and magnetic susceptibility measurements, and it evolves in a much sophisticated manner. In the α-phase, the superconducting T _c decreases quickly upon Na-doping. All these α-phase samples have surprisingly low superconducting volume fraction and relatively low T _c compared with the undoped K₂Cr₃As₃. However, the β-phase has a clearly enhanced T _c up to 7.6 K which locates between the values of K₂Cr₃As₃ and Na₂Cr₃As₃, and exhibits a full superconducting shielding signal.

Stoichiometric evolutions of PH3 under high pressure: implication for high-T c superconducting hydrides

The superconductivity of hydrides under high pressure has attracted a great deal of attention since the recent observation of the superconducting transition at 203 K in strongly compressed H2S. It has been realized that the stoichiometry of hydrides might change under high pressure, which is crucial in understanding the superconducting mechanism. In this study, PH3 was studied to understand its superconducting transition and stoichiometry under high pressure using Raman, IR and X-ray diffraction measurements, as well as theoretical calculations. PH3 is stable below 11.7 GPa and then it starts to dehydrogenate through two dimerization processes at room temperature and pressures up to 25 GPa. Two resulting phosphorus hydrides, P2H4 and P4H6, were verified experimentally and can be recovered to ambient pressure. Under further compression above 35 GPa, the P4H6 directly decomposed into elemental phosphorus. Low temperature can greatly hinder polymerization/decomposition under high pressure and retains P4H6 up to at least 205 GPa. The superconductivity transition temperature of P4H6 is predicted to be 67 K at 200 GPa, which agrees with the reported result, suggesting that it might be responsible for superconductivity at higher pressures. Our results clearly show that P2H4 and P4H6 are the only stable P-H compounds between PH3 and elemental phosphorus, which is helpful for shedding light on the superconducting mechanism.

3D strain-induced superconductivity in La2CuO4+δ using a simple vertically aligned nanocomposite approach

A long-term goal for superconductors is to increase the superconducting transition temperature, T _C. In cuprates, T _C depends strongly on the out-of-plane Cu-apical oxygen distance and the in-plane Cu-O distance, but there has been little attention paid to tuning them independently. Here, in simply grown, self-assembled, vertically aligned nanocomposite thin films of La₂CuO_4+δ + LaCuO₃, by strongly increasing out-of-plane distances without reducing in-plane distances (three-dimensional strain engineering), we achieve superconductivity up to 50 K in the vertical interface regions, spaced ~50 nm apart. No additional process to supply excess oxygen, e.g., by ozone or high-pressure oxygen annealing, was required, as is normally the case for plain La₂CuO_4+δ films. Our proof-of-concept work represents an entirely new approach to increasing T _C in cuprates or other superconductors.

Higher superconducting transition temperature by breaking the universal pressure relation

By investigating the bulk superconducting state via dc magnetization measurements, we have discovered a common resurgence of the superconducting transition temperatures (Tcs) of the monolayer Bi2Sr2CuO6+δ (Bi2201) and bilayer Bi2Sr2CaCu2O8+δ (Bi2212) to beyond the maximum Tcs (Tc-maxs) predicted by the universal relation between Tc and doping (p) or pressure (P) at higher pressures. The Tc of underdoped Bi2201 initially increases from 9.6 K at ambient to a peak at 23 K at 26 GPa and then drops as expected from the universal Tc-P relation. However, at pressures above 40 GPa, Tc rises rapidly without any sign of saturation up to 30 K at 51 GPa. Similarly, the Tc for the slightly overdoped Bi2212 increases after passing a broad valley between 20 and 36 GPa and reaches 90 K without any sign of saturation at 56 GPa. We have, therefore, attributed this Tc resurgence to a possible pressure-induced electronic transition in the cuprate compounds due to a charge transfer between the Cu 3[Formula: see text] and the O 2p bands projected from a hybrid bonding state, leading to an increase of the density of states at the Fermi level, in agreement with our density functional theory calculations. Similar Tc-P behavior has also been reported in the trilayer Br2Sr2Ca2Cu3O10+δ (Bi2223). These observations suggest that higher Tcs than those previously reported for the layered cuprate high-temperature superconductors can be achieved by breaking away from the universal Tc-P relation through the application of higher pressures.

Scanning tunneling microscopic observation of enhanced superconductivity in epitaxial Sn islands grown on SrTiO3 substrate

Recent experimental and theoretical studies of single-layer FeSe film grown on SrTiO₃ have revealed interface enhanced superconductivity, which opens up a pathway to promote the superconducting transition temperature. Here, to investigate the role of SrTiO₃ substrate in epitaxial superconducting film, we grew a conventional superconductor β-Sn (bulk T_c ∼ 3.72 K) onto SrTiO₃ substrate by molecular beam epitaxy. By employing scanning tunneling microscope and spectroscopic measurements, an enhanced T_c of 8.2 K is found for epitaxial β-Sn islands, deduced by fitting the temperature dependence of the gap values using the BCS formula. The observed interfacial charge injection and enhanced electron-phonon coupling are responsible for this T_c enhancement. Moreover, the critical field of 8.3 T exhibits a tremendous increase due to the suppression of the vortex formation. Therefore, the coexistence of enhanced superconductivity and high critical field of Sn islands demonstrates a feasible and effective route to improve the superconductivity by growing the islands of conventional superconductors on perovskite-type titanium oxide substrates.

Prediction of Thermodynamically Stable Compounds of the Sc-N System under High Pressure

Materials under high pressure often exhibit unusual physical and chemical behaviors. We investigated the Sc-N system under high pressure in the range of 0-110 GPa using variable-composition methodology implemented in Universal Structure Predictor: Evolutionary Xtallograpgy (USPEX) in conjunction with Vienna Ab Initio Simulation Package (VASP). The calculation led to prediction of new thermodynamically stable compounds, Sc4N3, Sc8N7, ScN3, and ScN5, and also phase transition of ScN5 from triclinic to monoclinic at 104 GPa. These results are important to understand the structure of Sc-N compounds under high pressure and their elastic and electronic properties.

Journey to the centre of the Earth: Jules Vernes' dream in the laboratory from an NMR perspective

High pressure nuclear magnetic resonance is among the most challenging fields of research for NMR spectroscopists due to inherently low signal intensities, ultra-small samples that are barely accessible, and overall extremely harsh conditions in the sample cavity of modern high pressure vessels. This review aims to provide a comprehensive overview of the topic of high pressure research and its fairly young and brief relationship with NMR.

Simple analytical model of the effect of high pressure on the critical temperature and other thermodynamic properties of superconductors

Within the general conformal transformation method a simplified analytical model is proposed to study the effect of external hydrostatic pressure on low- and high-temperature superconducting systems. A single fluctuation in the density of states, placed away from the Fermi level, as well as external pressure are included in the model to derive equations for the superconducting gap, free energy difference, and specific heat difference. The zero- and sub-critical temperature limits are discussed by the method of successive approximations. The critical temperature is found as a function of high external pressure. It is shown that there are four universal types of the response of the system, in terms of dependence of the critical temperature on increasing external pressure. Some effects, which should be possible to be observed experimentally in s-wave superconductors, the cuprates (i.e. high-T_c superconductors) and other superconducting materials of the new generation such as two-gap superconductors, are revealed and discussed. An equation for the ratio \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\boldsymbol{ {\mathcal R} }}}_{{\bf{1}}}$$\end{document}ℛ1 ≡ 2Δ(0)/T_c, as a function of the introduced parameters, is derived and solved numerically. Analysis of other thermodynamic quantities and the characteristic ratio \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\boldsymbol{ {\mathcal R} }}}_{{\bf{2}}}$$\end{document}ℛ2 ≡ ΔC(T_c)/C_N(T_c) is performed numerically, and mutual relations between the discussed quantities are investigated. The simple analytical model presented in the paper may turn out to be helpful in searching for novel superconducting components with higher critical temperatures induced by pressure effects.

Electron-Lattice Coupling in Correlated Materials of Low Electron Occupancy

In correlated materials including transition metal oxides, electronic properties and functionalities are modulated and enriched by couplings between the electron and lattice degrees of freedom. These couplings are controlled by external parameters such as chemical doping, pressure, magnetic and electric fields, and light irradiation. However, the electron-lattice coupling relies on orbital characters, i.e., symmetry and occupancy, of t_2g and e_g orbitals, so that a large electron-lattice coupling is limited to e_g electron system, whereas t_2g electron system exhibits an inherently weak coupling. Here, we design and demonstrate a strongly enhanced electron-lattice coupling in electron-doped SrTiO₃, that is, the t_2g electron system. In ultrathin films of electron-doped SrTiO₃ [i.e., (La_0.25Sr_0.75)TiO₃], we reveal the strong electron-lattice-orbital coupling, which is manifested by extremely increased tetragonality and the corresponding metal-to-insulator transition. Our findings open the way of an active tuning of the charge-lattice-orbital coupling to obtain new functionalities relevant to emerging nanoelectronic devices.

High-pressure Raman study of Terephthalonitrile

The in situ high-pressure Raman spectra of Terephthalonitrile (TPN) have been investigated from ambient to 12.6GPa at room temperature. All the fundamental vibrational modes of TPN at ambient were assigned based on the first-principle calculations. A detailed Raman spectroscopy analysis revealed that TPN underwent a phase transition at ~5.3GPa. The frequencies of the TPN Raman peaks increase with increasing the pressure which can be attributed to the reduction in the interatomic distances and the escalation of effective force constants. The intensity of the C-C-C ring-out-plane deformation mode increases gradually as the frequency remains almost constant during the compression which can be explained by the existence of π-π interactions in TPN molecules. Additionally, the pressure-induced structural changes of TPN on the Fermi resonance between the C≡N out-of-plane vibration mode and the C-CN out-of-plane vibration mode have been analyzed.

High-pressure studies with x-rays using diamond anvil cells

Pressure profoundly alters all states of matter. The symbiotic development of ultrahigh-pressure diamond anvil cells, to compress samples to sustainable multi-megabar pressures; and synchrotron x-ray techniques, to probe materials' properties in situ, has enabled the exploration of rich high-pressure (HP) science. In this article, we first introduce the essential concept of diamond anvil cell technology, together with recent developments and its integration with other extreme environments. We then provide an overview of the latest developments in HP synchrotron techniques, their applications, and current problems, followed by a discussion of HP scientific studies using x-rays in the key multidisciplinary fields. These HP studies include: HP x-ray emission spectroscopy, which provides information on the filled electronic states of HP samples; HP x-ray Raman spectroscopy, which probes the HP chemical bonding changes of light elements; HP electronic inelastic x-ray scattering spectroscopy, which accesses high energy electronic phenomena, including electronic band structure, Fermi surface, excitons, plasmons, and their dispersions; HP resonant inelastic x-ray scattering spectroscopy, which probes shallow core excitations, multiplet structures, and spin-resolved electronic structure; HP nuclear resonant x-ray spectroscopy, which provides phonon densities of state and time-resolved Mössbauer information; HP x-ray imaging, which provides information on hierarchical structures, dynamic processes, and internal strains; HP x-ray diffraction, which determines the fundamental structures and densities of single-crystal, polycrystalline, nanocrystalline, and non-crystalline materials; and HP radial x-ray diffraction, which yields deviatoric, elastic and rheological information. Integrating these tools with hydrostatic or uniaxial pressure media, laser and resistive heating, and cryogenic cooling, has enabled investigations of the structural, vibrational, electronic, and magnetic properties of materials over a wide range of pressure-temperature conditions.

Quantitative analysis of nonadiabatic effects in dense H3S and PH3 superconductors

The comparison study of high pressure superconducting state of recently synthesized H₃S and PH₃ compounds are conducted within the framework of the strong-coupling theory. By generalization of the standard Eliashberg equations to include the lowest-order vertex correction, we have investigated the influence of the nonadiabatic effects on the Coulomb pseudopotential, electron effective mass, energy gap function and on the 2Δ(0)/T_C ratio. We found that, for a fixed value of critical temperature (178 K for H₃S and 81 K for PH₃), the nonadiabatic corrections reduce the Coulomb pseudopotential for H₃S from 0.204 to 0.185 and for PH₃ from 0.088 to 0.083, however, the electron effective mass and ratio 2Δ(0)/T_C remain unaffected. Independently of the assumed method of analysis, the thermodynamic parameters of superconducting H₃S and PH₃ strongly deviate from the prediction of BCS theory due to the strong-coupling and retardation effects.

Interface-induced superconductivity at ∼25 K at ambient pressure in undoped CaFe2As2 single crystals

Superconductivity has been reversibly induced/suppressed in undoped CaFe₂As₂ (Ca122) single crystals through proper thermal treatments, with T_c at ∼25 K at ambient pressure and up to 30 K at 1.7 GPa. We found that Ca122 can be stabilized in two distinct tetragonal (T) phases at room temperature and ambient pressure: PI with a nonmagnetic collapsed tetragonal (cT) phase at low temperature and PII with an antiferromagnetic orthorhombic (O) phase at low temperature, depending on the low-temperature annealing condition. Neither phase at ambient pressure is superconducting down to 2 K. However, systematic annealing for different time periods at 350 °C on the as-synthesized crystals, which were obtained by quenching the crystal ingot from 850 °C, reveals the emergence of superconductivity over a narrow time window. Whereas the onset T_c is insensitive to the anneal time, the superconductive volume fraction evolves with the time in a dome-shaped fashion. Detailed X-ray diffraction profile analyses further reveal mesoscopically stacked layers of the PI and the PII phases. The deduced interface density correlates well with the superconducting volume measured. The transport anomalies of the T-cT transition, which is sensitive to lattice strain, and the T-O transition, which is associated with the spin-density-wave (SDW) transition, are gradually suppressed over the superconductive region, presumably due to the interface interactions between the nonmagnetic metallic cT phase and the antiferromagnetic O phase. The results provide the most direct evidence to date for interface-enhanced superconductivity in undoped Ca122, consistent with the recent theoretical prediction.

The cold neutron chopper spectrometer at the Spallation Neutron Source-A review of the first 8 years of operation

The first eight years of operation of the Cold Neutron Chopper Spectrometer (CNCS) at the Spallation Neutron Source in Oak Ridge is being reviewed. The instrument has been part of the facility user program since 2009, and more than 250 individual user experiments have been performed to date. CNCS is an extremely powerful and versatile instrument and offers leading edge performance in terms of beam intensity, energy resolution, and flexibility to trade one for another. Experiments are being routinely performed with the sample at extreme conditions: T ≲ 0.05 K, p ≳ 2 GPa, and B = 8 T can be achieved individually or in combination. In particular, CNCS is in a position to advance the state of the art with inelastic neutron scattering under pressure, and some of the recent accomplishments in this area will be presented in more detail.