High Critical Temperature Research Articles

Superconductivity Induced by Strong Electron-Exciton Coupling in Doped Atomically Thin Semiconductor Heterostructures

We study a mechanism to induce superconductivity in atomically thin semiconductors where excitons mediate an effective attraction between electrons. Our model includes interaction effects beyond the paradigm of phonon-mediated superconductivity and connects to the well-established limits of Bose and Fermi polarons. By accounting for the strong-coupling physics of trions, we find that the effective electron-exciton interaction develops a strong frequency and momentum dependence accompanied by the system undergoing an emerging BCS-BEC crossover from weakly bound s-wave Cooper pairs to a superfluid of bipolarons. Even at strong-coupling the bipolarons remain relatively light, resulting in critical temperatures of up to 10% of the Fermi temperature. This renders heterostructures of two-dimensional materials a promising candidate to realize superconductivity at high critical temperatures set by electron doping and trion binding energies. Published by the American Physical Society 2024

H$_{3}$Se in the $Im\overline{3}m$ Phase: A High-Pressure Superconductor with $T_{\text{c}}$ Reaching 200 K at 64 GPa Mediated by Anharmonic Phonons

Abstract Hydrogen-based compounds have attracted significant attention in recent years due to the discovery of conventional superconductivity with high critical temperature under high pressure, rekindling hopes for searching room temperature superconductor. In this work, we investigated systematically the vibrational and superconducting properties of H$_{3}$Se in $Im\overline{3}m$ phase under pressures ranging from 50 to 200 GPa. Our approach combines the stochastic self-consistent harmonic approximation with first-principles calculations to address effects from the quantum and anharmonic vibrations of ions. It turns out that these effects significantly modify the crystal structure, increasing the inner pressure by about 8 GPa compared to situations where they are ignored. The phonon spectra suggest that with these effects included, the crystal can be stabilized at pressures as low as about 61 GPa, much lower than the previously predicted value of over 100 GPa. Our calculations also highlight the critical role of quantum and anharmonic effects on the electron-phonon coupling properties. Neglecting these factors could result in a substantial overestimation of the superconducting critical temperature $T_{\text{c}}$, by approximately 25 K at 125 GPa, for example. With anharmonic phonons, the $T_{\text{c}}$ derived from the Migdal-Eliashberg equations, reaches 200 K ($\mu^\star=0.1, \lambda$=4.1) as the pressure decreases to 64 GPa, making the crystal a rare high-$T_{\text{c}}$ superconductor at moderate pressures.

Surface superconductivity in the topological Weyl semimetal t-PtBi2

Topological superconductivity is a promising concept for generating fault-tolerant qubits. Early experimental studies looked at hybrid systems and doped intrinsic topological or superconducting materials at very low temperatures. However, higher critical temperatures are indispensable for technological exploitation. Recent angle-resolved photoemission spectroscopy results have revealed that superconductivity in the type-I Weyl semimetal—trigonal PtBi2 (t-PtBi2)—is located at the Fermi-arc surface states, which renders the material a potential candidate for intrinsic topological superconductivity. Here we show, using scanning tunnelling microscopy and spectroscopy, that t-PtBi2 presents surface superconductivity at elevated temperatures (5 K). The gap magnitude is elusive: it is spatially inhomogeneous and spans from 0 to 20 meV. In particular, the large gap value and the shape of the quasiparticle excitation spectrum resemble the phenomenology of high-Tc superconductors. To our knowledge, this is the largest superconducting gap so far measured in a topological material. Moreover, we show that the superconducting state at 5 K persists in magnetic fields up to 12 T.

Superconducting memory and trapped magnetic flux in ternary lanthanum polyhydrides

Superconducting memory is a promising technology for data storage because of its speed, high energy efficiency, non-volatility, and compatibility with quantum computing devices. However, the need for cryogenic temperatures renders superconducting memory an extremely expensive and specialized device. Ternary lanthanum polyhydrides, due to their high critical temperatures of 240–250 K, represent a convenient platform for studying effects associated with superconductivity in disordered granular systems. In this work, we investigate trapped magnetic flux and memory effects in recently discovered lanthanum-neodymium (La,Nd)H10 and lanthanum-scandium (La,Sc)H12 superhydrides at a pressure of 175–196 GPa. We use a steady magnetic field of a few Tesla (T) and strong pulsed fields up to 68 T to create the trapped flux state in the compressed superhydrides. We find a clockwise hysteresis of magnetoresistance in cerium CeH9-10 and lanthanum-cerium (La,Ce)H10+x polyhydrides, a characteristic feature of granular superconductors. A study of the current-voltage characteristics and voltage-temperature curves of the samples with trapped magnetic flux indicates a significant memory effect in La-Sc polyhydrides already at 225–230 K.

Superconductivity in the pressurized nickelate La3Ni2O7 in the vicinity of a BEC–BCS crossover

Ever since the discovery of high-temperature superconductivity in cuprates, gaining microscopic insights into the nature of pairing in strongly correlated systems has remained one of the greatest challenges in modern condensed matter physics. Following recent experiments reporting superconductivity in the bilayer nickelate La3Ni2O7 (LNO) with remarkably high critical temperatures of Tc = 80 K, it has been argued that the low-energy physics of LNO can be described by the strongly correlated, mixed-dimensional bilayer t–J model. Here we investigate this bilayer system and utilize density matrix renormalization group techniques to establish a thorough understanding of the model and the magnetically induced pairing through comparison to the perturbative limit of dominating inter-layer spin couplings. In particular, this allows us to explain appearing finite-size effects, firmly establishing the existence of long-range superconducting order in the thermodynamic limit. By analyzing binding energies, we predict a BEC–BCS crossover as a function of the Hamiltonian parameters. We find large binding energies of the order of the inter-layer coupling that suggest strikingly high critical temperatures of the Berezinskii–Kosterlitz–Thouless transition, raising the question of whether (mixD) bilayer superconductors possibly facilitate critical temperatures above room temperature.

Wavelength dependence of linear birefringence and linear dichroism of Bi2−xPbxSr2CaCu2O8+δ single crystals

Copper oxide high-temperature superconductors, such as Bi2Sr2CaCu2O8+δ (Bi2212), have garnered extensive research interest due to their high critical temperatures (Tc) surpassing the Bardeen–Cooper–Schrieffer (BCS) limit. The two-dimensional CuO₂ plane is widely regarded as the most crucial element of high-Tc cuprate superconductors. The anisotropy of this CuO₂ layer remains a topic of ongoing interest. Although a few experimental results have reported strong optical anisotropy in both ab and ac-planes of Bi2212 through optical “reflectivity” measurements, there is a lack of studies focusing on the optical anisotropy of these materials using optical “transmittance” measurements by utilizing ultraviolet and visible light. Using a generalized high-accuracy universal polarimeter, we observed significant linear birefringence and linear dichroism peaks in the UV region at room temperature. To investigate the origin of the significant optical anisotropy, single crystals of Bi2−xPbxSr2CaCu2O8+δ with different lead contents (x = 0, 0.4, and 0.6) were grown using the floating zone method, and the wavelength dependencies of linear birefringence and linear dichroism along the c axis were measured. The insights gained into the optical anisotropy of Bi2−xPbxSr2CaCu2O8+δ from this study are significant for discussing its origin of the mechanism of high-Tc superconductivity.

Designing multicomponent hydrides with potential high Tc superconductivity

While hydrogen-rich materials have been demonstrated to exhibit high Tc superconductivity at high pressures, there is an ongoing search for ternary, quaternary, and more chemically complex hydrides that achieve such high critical temperatures at much lower pressures. First-principles searches are impeded by the computational complexity of solving the Eliashberg equations for large, complex crystal structures. Here, we adopt a simplified approach using electronic indicators previously established to be correlated with superconductivity in hydrides. This is used to study complex hydride structures, which are predicted to exhibit promisingly high critical temperatures for superconductivity. In particular, we propose three classes of hydrides inspired by the Fm[Formula: see text]m RH[Formula: see text] structures that exhibit strong hydrogen network connectivity, as defined through the electron localization function. The first class [RH[Formula: see text]X[Formula: see text]Y] is based on a Pm[Formula: see text]m structure showing moderately high Tc, where the Tc estimate from electronic properties is compared with direct Eliashberg calculations and found to be surprisingly accurate. The second class of structures [(RH[Formula: see text])[Formula: see text]X[Formula: see text]YZ] improves on this with promisingly high density of states with dominant hydrogen character at the Fermi energy, typically enhancing Tc. The third class [(R[Formula: see text]H[Formula: see text])(R[Formula: see text]H[Formula: see text])X[Formula: see text]YZ] improves the strong hydrogen network connectivity by introducing anisotropy in the hydrogen network through a specific doping pattern. These design principles and associated model structures provide flexibility to optimize both Tc and the structural stability of complex hydrides.

Emergence of superconductivity and superionicity in the electride [formula omitted]La under extreme conditions

Superconducting electrides, a unique type of conventional superconductors, have recently attracted considerable attention. Inspired by the discovery of high critical temperature (Tc) in lanthanum hydrides and the feasibility of forming electrides in lithium-based compounds, we have conducted a systematic investigation the phase diagram and superconductivity of Li-La alloys under high pressure. Using the crystal structure prediction method and first-principles calculations, we uncovered three pressure-stabilized compounds with stoichiometries LiLa3, LiLa2 and Li6La. All three compounds exhibit metallic behavior with electrons transferred from Li to La atoms. Notably, Li6La emerges as a prototypical superconducting electride with Tc of 11.6 K at 350 GPa, which increases to 16.3 K as pressure decreases to 200 GPa. Electron–phonon analysis reveal that the enhancement of superconductivity mainly originates from the coupling between the increased interstitial quasiatoms (ISQs) and low-frequencies phonons. Additionally, akin to lanthanum hydrides, Li6La is predicted to enter a superionic phase characterized by high lithium diffusion coefficients under high pressure and temperature. Our findings indicate the coexistence of superconductivity and superionicity in the electride Li6La, which may prompt further experimental investigations.

Nonlinear buckling and free vibration analysis of auxetic graphene origami composite beams under nonuniform thermal environment

This study examines the thermo-mechanical behavior of auxetic metamaterial beams enhanced by graphene origami (GOri) under spatially varying nonuniform temperature distributions (SVTD). Utilizing Timoshenko beam theory considering von-Kármánn type nonlinear strain–displacement relationship, GOri beams are modeled as layered structures. The Ritz method is employed to solve equilibrium equations, analyzing the impact of GOri distribution patterns, content, and folding degree on post-buckling and vibration paths. The effects of five SVTDs, three end conditions, and three GOri distribution patterns on buckling, post-buckling behavior, and nonlinear free vibration characteristics are explored. Findings reveal that the parabolic temperature distribution with peak temperatures at beam ends (P-MAE) results in higher critical temperatures and nonlinear free vibration frequencies. This research provides crucial insights into the design and optimization of GOri-enabled metamaterial structures in complex thermal environments, highlighting the significant influence of nonuniform temperature distributions along the beam’s length.

Experimental comparison of cycle modifications and ejector control methods using variable geometry and CO2 pump in a multi-evaporator transcritical CO2 refrigeration system

To reduce the direct global warming impact of refrigerants in HVAC&R applications, low-global warming potential (GWP) refrigerants, including natural refrigerants, have been extensively investigated as alternatives to hydrofluorocarbon (HFC) refrigerants. Among the natural refrigerants, Carbon Dioxide (CO2) offers several advantages, such as excellent transport and thermo-physical properties, being neither toxic nor flammable, and having a low price and high availability around the world. However, the high critical pressure and low critical temperature of CO2 often lead to transcritical operation, resulting in lower efficiency due to the additional compressor power necessary to achieve transcritical operation relative to subcritical HFC cycles. Therefore, a number of cycle modifications are used to enhance the coefficient of performance (COP) of transcritical CO2 cycles to meet or surpass those of HFC cycles. This paper provides a systematic experimental investigation of four such cycle architectures by employing the same multi-stage, two-evaporator CO2 refrigeration cycle test stand, 3 of these configurations in transcritical and 1 in subcritical conditions. The four cycles architectures included intercooling, open economization, an internal heat exchanger and two different ejector control approaches. Specifically, a variable-diameter motive nozzle and a variable-speed liquid CO2 pump located directly upstream of the ejector motive nozzle inlet were analyzed. Based on the experimental data, the maximum COP improvements are 4.64 % and 9.47 % when the ejector and the internal heat exchanger are used, respectively. The CO2 pump, once successfully stabilized, can control the ejector, increase its efficiency by up to 15 % and increase the cooling capacity to a maximum of 6.2 %. Nevertheless, a reduction in COP is measured when the pump is in use; however, unlike the other three different configurations, it was only analyzed under subcritical conditions.

Microscopic probing of the superconducting and normal state properties of Ta2V3.1Si0.9 by muon spin rotation

The two-dimensional kagome lattice is an experimental playground for novel physical phenomena, from frustrated magnetism and topological matter to chiral charge order and unconventional superconductivity. A newly identified kagome superconductor, Ta2V3.1Si0.9 has recently gained attention for possessing a record high critical temperature, TC = 7.5 K for kagome metals at ambient pressure. In this study we conducted a series of muon spin rotation measurements to delve deeper into understanding the superconducting and normal state properties of Ta2V3.1Si0.9. We demonstrate that Ta2V3.1Si0.9 is a bulk superconductor with either a s+s-wave or anisotropic s-wave gap symmetry, and has an unusual paramagnetic shift in response to external magnetic fields in the superconducting state. Additionally, we observe an exceptionally low superfluid density − a distinctive characteristic of unconventional superconductivity − which remarkably is comparable to the superfluid density found in hole-doped cuprates. In its normal state, Ta2V3.1Si0.9 exhibits a significant increase in the zero-field muon spin depolarisation rate, starting at approximately 150 K, which has been observed in other kagome-lattice superconductors, and therefore hints at possible hidden magnetism. These findings characterise Ta2V3.1Si0.9 as an unconventional superconductor and a noteworthy new member of the vanadium-based kagome material family.

Why Mg2IrH6 Is Predicted to Be a High‐Temperature Superconductor, But Ca2IrH6 Is Not

The X2MH6 family, consisting of an electropositive cation X and a main group metal M octahedrally coordinated by hydrogen, has been predicted to hold promise for high‐temperature conventional superconductivity. Herein, we analyze the electronic structure of two members of this family, Mg2IrH6 and Ca2IrH6, showing why the former may possess superconducting properties rivaling those of the cuprates, whereas the latter does not. Within Mg2IrH6 the vibrations of the IrH64‐ anions are key for the superconducting mechanism, and they induce coupling in the eg* set, which are antibonding between the H 1s and the Ir dx2−y2 or dz2 orbitals. Because calcium possesses low‐lying d‐orbitals, eg* → Ca d back‐donation is preferred, quenching the superconductivity. Our analysis explains why high critical temperatures were only predicted for second or third row X metal atoms and may hold implications for superconductivity in other systems where the antibonding anionic states are filled.

Why Mg2IrH6 Is Predicted to Be a High-Temperature Superconductor, But Ca2IrH6 Is Not.

The X2MH6 family, consisting of an electropositive cation Xn+ and a main group metal M octahedrally coordinated by hydrogen, have been identified as promising templates for high-temperature conventional superconductivity. Herein, we analyze the electronic structure of two members of this family, Mg2IrH6 and Ca2IrH6, showing why the former may possess superconducting properties rivaling those of the cuprates, whereas the latter does not. Within Mg2IrH6 the vibrations of the anions IrH6 4- anions are key for the superconducting mechanism, and they induce coupling in the set of orbitals, which are antibonding between the H 1s and the Ir or orbitals. Because calcium possesses low-lying d-orbitals, →Ca d back-donation is preferred, quenching the superconductivity. Our analysis explains why high critical temperatures were only predicted for second or third row X metal atoms, and may provide rules for identifying likely high-temperature superconductors in other systems where the antibonding anionic states are filled.

Theoretical investigations of two-dimensional intrinsic magnets derived from transition-metal borides M3B4 (M = Cr, Mn, and Fe)

ABSTRACT Two-dimensional (2D) magnetic materials with high critical temperatures (T C ) and robust magnetic anisotropy energies (MAE) hold significant potential for spintronic applications. However, most of 2D magnetic materials are derived from the van der Waals (vdW) layered bulks, which greatly limits the synthesis of 2D magnetic materials. Here, 2D M3B4 (M = Cr, Mn, and Fe; B = Boron), derived from hexagonal and orthorhombic M3AlB4 phases by selectively etching Al layers, was studied for its structural stability, electronic structure, and magnetic properties. By utilizing ab initio calculations and Monte Carlo simulations, we found that the orthorhombic Cr3B4 shows ferromagnetic (FM) metal and possesses an in-plane magnetic easy axis, while the remaining hexagonal and orthorhombic M3B4 structures exhibit antiferromagnetic (AFM) metals with a magnetic easy axis which is perpendicular to the two-dimensional plane. The critical temperatures of these 2D M3B4 structures are found to be above the 130 K. Notably, the ort-Mn3B4 possesses highest T C (~600 K) and strongest MAE (~220 µeV/atom) among these borides-based 2D magnetic materials. Our findings reveal that the 2D M3B4 compounds exhibit much better resistance to deformation compared to M2B2 MBenes and other 2D magnetic materials. The combination of high critical temperature, robust MAE, and excellent mechanical properties makes 2D Mn3B4 monolayer exhibits a favorable potential for spintronic applications. Our research also sheds light on the magnetic coupling mechanism of 2D M3B4, providing valuable insights into its fundamental characteristics.

Geothermal Nano-SiO2 Waste as a Supplementary Cementitious Material for Concrete Exposed at High Critical Temperatures.

The partial replacement effect of Portland cement by geothermal nano-SiO2 waste (GNSW) for sustainable Portland-cement-based concrete was investigated to improve the properties of concrete exposed at high critical temperatures. Portland cement was partially replaced by 20 and 30 wt.% of GNSW. The partial replacement effect on Portland-cement-based concrete subjected to 350, 550, and 750 °C was evaluated by measuring the weight changes, ultrasonic pulse velocity, thermogravimetric and differential thermal analysis, X-ray diffraction, surface inspection, and scanning electron microscopy under residual conditions. The ultrasonic pulse velocity results showed that the GNSW specimens maintained suitable stability after being heated to 350 °C. The SEM analysis revealed a denser microstructure for the 20 wt.% of partial replacement of Portland cement by GNSW specimen compared to the reference concrete when exposed to temperatures up to 400 °C, maintaining stability in its microstructure. The weight losses were higher for the specimens with partial replacements of GNSW than the reference concrete at 550 °C, which can be attributed to the pozzolanic activity presented by the GNSW, which increases the amounts of CSH gel, leading to a much denser cementitious matrix, causing a higher weight loss compared to the reference concrete. GNSW is a viable supplementary cementitious material, enhancing thermal properties up to 400 °C due to its high pozzolanic activity and filler effect while offering environmental benefits by reducing industrial waste.

Emergence of Superconductivity in Indium Triphosphate via Pressure‐Tuned Interlayer Bond Formation

Tuning interlayer interactions offer an alternative approach to access novel electronic structure and intriguing physical properties in layered materials. Here, the emergence of a new form of superconductivity in two‐dimensional (2D) binary phosphides by strengthening the interlayer coupling with lattice compression is reported. Electrical transport measurements show strong evidence of superconductivity in InP3 with the highest critical temperature (Tc) of 9.5 K at 45.1 GPa. Raman and X‐ray diffraction (XRD) measurements indicate that the interlayer interactions are dramatically modulated under compression, along with the deformation of local In–P bipyramid structure and reduction of the interlayer distances, which eventually results in the formation of In–P bonds between neighboring In–P bipyramids and a Rm to Cmcm structural transition. First‐principles density functional theory (DFT) calculations reveal that pressure enhances the interlayer interactions, which increases the density of states (DOS) near the Fermi surface (N(EF)) and strengthens the electron–phonon coupling. Consequently, this favors the occurrence of superconductivity in compressed InP3. This study not only introduces a new superconductivity phase with enhanced electron–phonon coupling in binary phosphides, but also provides a platform for exploring the pressure effect on interlayer interactions in material systems with corrugated layered structure.

Half-metallic ferromagnetism with high critical temperatures in Substitutionally Doped Rare-Earth 2D Germanene

This work delves deep into the exploration of Rare-earth (RE) replacement doping germanene monolayers, where RE elements stand out for their fascinating electronic and magnetic nature derived from the elusive f-orbitals. Employing density functional theory (DFT) calculations, we unveil the electronic and magnetic characteristics of doped germanene systems. With its two-dimensional honeycomb structure, germanene holds immense promise for a wide range of applications within group IV materials. Through the discussion, we deeply uncover how the substitution of two specific concentrations of RE-atoms, namely La, Ce, Pr, Nd, Sm and Pm, transforms semi-metallic non magnetic germanene to half-metallic ferromagnets with remarkably high critical temperatures, when Hubbard correction is considered. These revelations not only deepen our comprehension of integrating f-orbitals into germanene but also open up exciting avenues for advancements in spintronic devices and beyond.

Transcritical Cycles With CO2-based Mixtures for CSP Applications: An Overview of the SCARABEUS Findings

This work summarizes the methodology developed within the H2020 EU project SCARABEUS for the analysis of innovative CO2-based mixtures used as working fluid in transcritical cycles for CSP applications. By adding a specific quantity of carefully selected dopant to CO2 it is possible to reach a high mixture critical temperature, suitable for air cooled cycles at very high minimum temperature in hot environments with high efficiencies. Along the methodology, some results related to the design of the turbine and the heat exchangers for the innovative mixtures are presented, including a focus on the system integration between the power block and the solar plant: as such, these results can be considered as interesting research outcomes to widen the knowledge on innovative solutions for efficient power cycles. The new power cycles are foreseen to drastically increase the profitability and cut the specific cost of CSP systems with respect to the state-of-the-art

Investigations on cooling heat transfer of CO2-based mixtures in a novel airfoil fin mini-channel at supercritical pressure

Thermal-hydraulic characteristics of three supercritical CO2-based mixtures (CO2/H2S, CO2/Xe and CO2/propane) in an airfoil fin (AFF) mini-channel adopted in the printed circuit heat exchanger (PCHE) under cooling conditions are comprehensively investigated through numerical simulations. The results show that the CO2/H2S mixture, which has higher critical temperature and larger critical pressure than the pure CO2, exhibits the greatest heat transfer performance among three mixtures. This advantage is particularly pronounced at relatively large Reynolds number, with small mass fraction of the H2S and under near-critical conditions. The overall friction loss of mixtures with the additive mass fraction smaller than 0.3 is comparable to that of the pure CO2. The heat transfer coefficient of the mixtures presents fluctuations during the cooling processes, and this goes against the stable and safe operation of the PCHEs. These fluctuations could be mitigated at relatively small mass flow rate and with small mass fraction of the additive. Three widely-recognized buoyancy criteria are employed to predict the onset of the buoyancy effects in the AFF channel, and the superior heat transfer of the CO2/H2S mixture could be attributed to the fiercer buoyancy effects. Considering the buoyancy effects, modified correlations for the flow and heat transfer of pure CO2 and the CO2-based mixtures in the novel mini-channel are proposed. The maximum deviations of the modified Nusselt number and fanning friction factor correlations are ± 25 % and ± 20 %, respectively.

Quadrupling the depairing current density in the iron-based superconductor SmFeAsO1-xHx.

Iron-based 1111-type superconductors display high critical temperatures and relatively high critical current densities Jc. The typical approach to increasing Jc is to introduce defects to control dissipative vortex motion. However, when optimized, this approach is theoretically predicted to be limited to achieving a maximum Jc of only ∼30% of the depairing current density Jd, which depends on the coherence length and the penetration depth. Here we dramatically boost Jc in SmFeAsO1-xHx films using a thermodynamic approach aimed at increasing Jd and incorporating vortex pinning centres. Specifically, we reduce the penetration depth, coherence length and critical field anisotropy by increasing the carrier density through high electron doping using H substitution. Remarkably, the quadrupled Jd reaches 415 MA cm-2, a value comparable to cuprates. Finally, by introducing defects using proton irradiation, we obtain high Jc values in fields up to 25 T. We apply this method to other iron-based superconductors and achieve a similar enhancement of current densities.