Critical Temperature Tc Research Articles

Enhancement of superconductivity in Zr2S2C arising from phonon softening on transition from bulk to monolayer

We report a new compound, Zr2S2C, belonging to the transition metal carbo-chalcogenide (TMCC) family. Through first-principles calculations, our analysis of phonon dispersion spectra indicates that the compound is dynamically stable in both bulk and monolayer forms. We systematically investigated the electronic structure, phonon dispersion, and electron–phonon coupling (EPC) driven superconducting properties in bulk and monolayer Zr2S2C. The results demonstrate the metallic character of bulk Zr2S2C, with a weak EPC strength (λ) of 0.41 and superconducting critical temperature ( Tc ) of ∼3 K. The monolayer Zr2S2C has an enhanced λ of 0.62 and Tc of ∼6.4 K. The increased λ value in the monolayer results from the softening of the acoustic phonon mode. We found that when biaxial strain is applied, the low energy acoustic phonon mode in monolayer becomes even softer. This softening leads to a transformation of the Zr2S2C monolayer from its initial weak coupling state (λ = 0.62) to a strongly coupled state, resulting in an increased λ value of 1.33. Consequently, the superconducting critical temperature experiences a twofold increase. These findings provide a theoretical framework for further exploration of the layered two-dimensional TMCC family, in addition to offering valuable insights.

Predicted hot superconductivity in LaSc2H24 under pressure

The recent theory-driven discovery of a class of clathrate hydrides (e.g., CaH6, YH6, YH9, and LaH10) with superconducting critical temperatures (Tc) well above 200 K has opened the prospects for "hot" superconductivity above room temperature under pressure. Recent efforts focus on the search for superconductors among ternary hydrides that accommodate more diverse material types and configurations compared to binary hydrides. Through extensive computational searches, we report the prediction of a unique class of thermodynamically stable clathrate hydrides structures consisting of two previously unreported H24 and H30 hydrogen clathrate cages at megabar pressures. Among these phases, LaSc2H24 shows potential hot superconductivity at the thermodynamically stable pressure range of 167 to 300 GPa, with calculated Tcs up to 331 K at 250 GPa and 316 K at 167 GPa when the important effects of anharmonicity are included. The very high critical temperatures are attributed to an unusually large hydrogen-derived density of states at the Fermi level arising from the newly reported peculiar H30 as well as H24 cages in the structure. Our predicted introduction of Sc in the La-H system is expected to facilitate future design and realization of hot superconductors in ternary clathrate superhydrides.

Infrared spectroscopic study on Nb-ion-irradiated MgB2 thin films

Magnesium diboride (MgB2) is a two-band superconductor with a high superconducting critical temperature (Tc) of approximately 39 K. Owing to the lack of vortex pinning centers, MgB2 exhibits an abrupt decline in the critical current density (Jc) in an applied magnetic field. Here, we prepared 1 MeV Nb ion-irradiated MgB2 thin-film samples with doses of 3×1013, 7×1013, and 9×1013 ions/cm2. Temperature-dependent magnetization and x-ray diffraction (XRD) measurements were performed to determine the Tc and c-axis lattice constant of each sample. Furthermore, a Fourier transform infrared (FTIR) spectroscopy was performed to obtain the infrared properties of the Nb-ion-irradiated MgB2 thin-film samples. The optical conductivity of each sample in the low-energy region was fitted with two (narrow and broad) Drude modes. We found that the spectral weight redistribution from the low-to high-frequency regions and the broadening of the narrow Drude mode caused by irradiation are closely related to the reduction in Tc.

Prediction of charge density wave, superconductivity and topology properties in two-dimensional Janus 2H/1T-WXH (X = S, Se)

The multiple properties in two-dimensional (2D) materials have attracted widespread attention. Utilizing first-principles calculations, we theoretically predict four 2D transition metal Janus sulfide hydrides, named 2H/1T-WXH (X = S, Se). The 2H-WSH and 2H–WSeH demonstrate superconductivity with critical temperatures (Tc) of 13.4 K and 11.4 K, respectively. Whereas 1T-WSH and 1T-WSeH display charge density wave (CDW) properties arising from electron-phonon coupling (EPC). It is noteworthy that the CDW can be suppressed through biaxial compressive strain, leading to the emergence of superconductivity with Tc of 12.2 K and 11.9 K for 1T-WSH (−2 %) and 1T-WSeH (−4 %), respectively. The superconductivity of the above materials originates from the coupling between W-3d electrons and the low-frequency vibrations of W. Interestingly, the 2H–WSeH and 1T-WSeH (−4 %) display nontrivial topological properties, as evidenced by topological invariant Z2 and the presence of edge states. Our research not only provides theoretical guidance for further exploring 2D Janus materials, but also provides ideas for the competition of multiple orders in 2D materials.

Effects of forward disorder on quasi-one-dimensional superconductors

We study the competition between disorder and singlet superconductivity in a quasi-one-dimensional (1D) system. We investigate the applicability of the Anderson theorem, namely that time-reversal conserving (nonmagnetic) disorder does not impact the critical temperature, by opposition to time-reversal breaking disorder (magnetic). To do so, we examine a quasi-1D system of spin 1/2 fermions with attractive interactions and forward scattering disorder using field theory (bosonization). By computing the superconducting critical temperature (Tc), we find that, for nonmagnetic disorder, the Anderson theorem also holds in the quasi-1D geometry. In contrast, magnetic disorder has an impact on the critical temperature, which we investigate by deriving renormalization group equations describing the competition between disorder and interactions. Computing the critical temperature as a function of disorder strength, we observe different regimes depending on the strength of interactions. We discuss possible platforms where this can be observed in cold atoms and condensed matter. Published by the American Physical Society 2024

The interplay between a pseudogap and superconductivity in a two-dimensional Hubbard model

Strongly correlated electrons systems may exhibit a variety of interesting phenomena, for instance, superconductivity and pseudogap, as is the case of cuprates and pnictides. In strongly correlated electron systems, it is considered essential to understand, not only the nature of the pseudogap, but also the relationship between superconductivity and the pseudogap. In order to address this question, in the present work, we investigated a one-band Hubbard model treated by the Green's function method within an n-pole approximation. In the strongly correlated regime, antiferromagnetic correlations give rise to nearly flat band regions in the nodal points of the quasiparticle bands. As a consequence, a pseudogap emerges at the antinodal points of the Fermi surface. The obtained results indicate that the same antiferromagnetic correlations responsible for a pseudogap, can also favor superconductivity, providing an increase in the superconducting critical temperature Tc.

Structural and superconducting properties in hard Mo2BC

The utilization of high-hardness superconducting materials enhances and broadens the applications of superconductors. In this study, a high-hardness Mo2BC superconductor is synthesized using the high-pressure and high-temperature (HPHT) method. Our results demonstrate that it exhibits zero-resistance and Meissner effect below a superconducting critical temperature (Tc) of 7.0 K. The upper critical magnetic field of 4.3 T and the lower critical magnetic field of 226 Oe were obtained by data fitting, respectively. Type-II superconducting properties were confirmed by critical states analysis and Ginzburg-Landau (GL) parameters. Specific thermal properties revealed a Debye temperature as 565.1 K, while an electron-phonon coupling (EPC) parameter λ of 0.588 indicated properly coupled BCS conventional superconductivity. Mo2BC exhibits a Vickers hardness of up to 18.2 GPa, significantly surpassing that of traditional superconducting ceramics. It can be attributed to the robust covalent boron-zigzag-chains coupled with appropriate covalent [Mo6C] octahedrons within a non-centrosymmetric structure. The coexistence of both superconductivity and high hardness enables the versatility of Mo2BC, thereby laying the way for the development of high-hardness superconductors.

Dual dome-like superconductivity in CuIr2Te3.85S0.15-xSex chalcogenides

In this paper, we report the superconductivity (SC) evolution on Se-substitution CuIr2Te3.85S0.15 telluride chalcogenide. The results of powder X ray-diffraction (PXRD) show that CuIr2Te3.85S0.15Se0.15-x (0≤x ≤ 0.15) series maintain a layered triangular structure and the space group are P3-m1 (No.164). Both the lattice constants a and c increase with increasing Se content. To systematically characterize the superconductivity properties, the electrical, magnetic, and thermal properties of the CuIr2Te3.85S0.15Se0.15-x (0≤x ≤ 0.15) series were tested. The resistivity and magnetic susceptibility measurements reveal that when Se gradually replaces S, CuIr2Te3.85S0.15Se0.15-x remains superconducting at low temperatures and does not disappear or transform into other electron ground states. In addition, With the increase of Se doping, the superconducting critical temperature (Tc) Rises and then falls twice, so the Tc value has two peaks in CuIr2Te3.85S0.15Se0.15-x (0≤x ≤ 0.15) series. Finally, we plot a phase plot of the superconducting transition temperature (Tc) versus the x content, which shows a "double dome" shape. CuIr2Te3.85S0.15Se0.15-x retains the inhibitory effect on CDW as the host CuIr2Te3.85S0.15. This peculiar change in superconducting transition temperature may be due to the change in the density of states at the Fermi surface caused by Se doping.

Pressure-induced novel phases with the high-Tc superconductivity in zirconium dihydride

The unprecedented structures in hydrides could be uncovered under high pressure, which probably possess unusual properties, such as high-Tc superconductivity. Here, the structural transition of ZrH2 at 0−500 GPa are comprehensively explored by using the evolutionary algorithm in the universal structure predictor combined with first-principles calculations. Three new phases are found: Pnma-ZrH2, P-3m1-ZrH2 and R-3m-ZrH2, with dynamic stability. Moreover, the sequence of phases of ZrH2 under pressure is I4/mmm → P4/nmm → Pnma → P-3m1 → R-3m with the zero-point energy effects. Structural feature, electronic properties, bonding characters and superconductivity of all the ZrH2 are analyzed in detail. More importantly, the superconducting critical temperature Tc values for P-3m1 at 440 GPa and R-3m at 500 GPa are 17.70–20.14 K and 26.68–30.03 K, respectively, which stems from the strong electron-phonon coupling.

Two-photon emission from a superlattice-based superconducting light-emitting structure

Superconductor-semiconductor hybrid devices can bridge the gap between solid-state-based and photonics-based quantum systems, enabling new hybrid computing schemes, offering increased scalability and robustness. One example for a hybrid device is the superconducting light-emitting diode (SLED). SLEDs have been theoretically shown to emit polarization-entangled photon pairs by utilizing radiative recombination of Cooper pairs. However, the two-photon nature of the emission has not been shown experimentally before. We demonstrate two-photon emission in a GaAs/AlGaAs SLED. Measured electroluminescence spectra reveal unique two-photon superconducting features below the critical temperature (Tc), while temperature-dependent photon-pair correlation experiments (g(2)(τ,T)) demonstrate temperature-dependent time coincidences below Tc between photons emitted from the SLED. Our results pave the way for compact and efficient superconducting quantum light sources and open new directions in light-matter interaction studies.

Data-driven design of high pressure hydride superconductors using DFT and deep learning

The observation of superconductivity in hydride-based materials under ultrahigh pressures (for example, H3S and LaH10) has fueled the interest in a more data-driven approach to discovering new high-pressure hydride superconductors. In this work, we performed density functional theory (DFT) calculations to predict the critical temperature ( Tc ) of over 900 hydride materials under a pressure range of (0–500) GPa, where we found 122 dynamically stable structures with a Tc above MgB2 (39 K). To accelerate screening, we trained a graph neural network (GNN) model to predict Tc and demonstrated that a universal machine learned force-field can be used to relax hydride structures under arbitrary pressures, with significantly reduced cost. By combining DFT and GNNs, we can establish a more complete map of hydrides under pressure.

Enhancing electronic, magnetic, and elastic properties of SmFe2 by Mn substitution at Fe sites: A first-principle computational study

In this comprehensive theoretical investigation, we present the effects of manganese (Mn) substitution for iron (Fe) in samarium iron (SmFe2) on its electronic, magnetic, elastic, and magnetostriction properties, utilizing state-of-the-art Density Functional Theory (DFT) within the Generalized Gradient Approximation (GGA) complemented by the Full Potential Linearized Augmented Plane Wave (FP-LAPW) method. Our investigations are further enriched by Monte Carlo simulations framed within the Ising model, providing a comprehensive understanding of the material's behavior under Mn doping. Our findings reveal a significant shift in magnetic anisotropy from the [100] direction in pristine SmFe2 to the [111] direction following Mn substitution. This transition enhances the material's magnetic favorability and introduces new electronic states at the Fermi level, increasing the number of states available for conduction and enhancing its metallic behavior. Consequently, this transition results in altered elastic constants and magnetostriction values, highlighting a delicate interplay between the material's structural and magnetic properties that warrants further exploration. By developing a Hamiltonian model, we delve into the intricate coupling among magnetic atoms within SmFe2, offering insights into the underlying mechanisms governing the observed phenomena. Monte Carlo simulations further corroborate the robust ferromagnetic ordering of SmFe2, evidenced by a high critical temperature (Tc = 651.3 K).

Raman scattering of TixV1‐xO2 thin films on (110) rutile TiO2 in the low and high temperature phase adjacent to the metal–insulator transition

AbstractVanadium dioxide (VO2) undergoes a reversible first‐order metal‐to‐insulator transition (MIT) from a high‐temperature metallic phase to a low‐temperature insulating phase at a critical temperature Tc of 68°C. The MIT is accompanied by a structural phase transition. In addition to the metallic high‐temperature rutile phase, several insulating phases may be involved depending on doping, interfacial stress, or external stimuli. Unambiguously identifying the crystal phases involved in the phase transition is of key interest from the point of view of application as well as fundamental science. We study the impact of Ti doping of VO2 thin films on (110) rutile TiO2 substrates. We conduct a careful analysis of structural properties by combining results of x‐ray diffraction, Raman spectroscopy, and transmission electron microscopy. The transition temperature Tc of the deposited thin films decreases with increasing Ti‐content. All our thin film samples undergo a structural phase transition from the monoclinic M1‐phase to the rutile R‐phase with increasing temperature without passing the intermediate monoclinic M2‐phase. A careful analysis of polarization and angle‐dependent Raman data reveals that, above Tc, the unit cell of the high‐temperature rutile TixV1‐xO2 phase is aligned with that of the rutile TiO2 substrate whereas, below Tc, 180°‐domains of the M1‐phase of TixV1‐xO2 are observed. The structural relationship between TiO2 substrate and the high respective low‐temperature phase of the TixV1‐xO2 determined by Raman spectroscopy is in excellent agreement with TEM results on these samples. Raman spectroscopy is a powerful tool for studying structural changes of VO2‐based samples in the vicinity of MIT.

Critical pressure (Pc) and critical temperature (Tc) of Midra shale

The critical pressure (Pc) and critical temperature (Tc) of shale gas depend on the characteristic pore size because of the importance of fluid–rock interactions in the matrix. This size dependency is neglected in highly permeable formations, where gas composition is only implemented because the fluid–fluid interactions are dominant. This study determines the critical properties by accounting for the characteristic pore size in the shale matrix and gas composition. The analyzed components are carbon dioxide, ethane, methane, n-butane, nitrogen, pentane, and propane. It shows that the bulk properties overestimate the actual critical properties. The overestimation varies between 15 and 26% in a uniform 5 nm conduit with a circular cross section, and it increases nonlinearly when decreasing the conduit size. Overestimation versus size is presented to provide a convenient tool for correcting the existing data. This study also determines the critical properties of Midra shale by accounting for the pore-throat size and pore-body size distributions. The former distribution is based on the mercury injection capillary pressure measurements of eight samples, whereas the latter is based on the nitrogen adsorption measurements of six samples. This study indicates that common bulk properties overestimate the critical properties of the studied shale between 5 and 22%. The results have applications in characterizing multiphase transport in shale gas reservoirs.

Assessment of superconducting and structural stability of advanced Y-123 and Y-358 ceramics with Tb/Y substitution in main matrices

In this study, the performances of Y1-x(Tb)xBa2Cu3O7-δ (Y-123) and Y3-x(Tb)xBa5Cu8O18-δ (Y-358) advanced ceramics prepared by sol-gel preparation route are investigated by the advanced characterization methods including X-ray diffraction (XRD), temperature dependent resistivity (ρ-T), vibrating sample magnetometry (VSM), scanning electron microscopy (SEM) and energy distribution spectroscopy (EDS) measurements and theoretical approaches. The EDS results illustrated that the Tb impurities were replaced successfully by the yttrium sites in the YBa2Cu3O7-y main matrix. Similarly, it was noted that the presence of excess impurities in the crystal structure led to the oxygenation problems. Electrical properties were determined by temperature dependent resistivity (ρ-T) measurements. ρ-T results indicated that Tb doping reduced the critical transition temperature (Tc) values for the transition to superconductivity for all samples in two YBCO phases. Although it appears to have negative effects of Tb ions on the number of mobile charge carrier concentrations in the σ antibonding in-plane Cu-O bonds, bipolaron formations in the polarizable lattices, oxidation states, and amplitude of the pair wave function of both phases, the other valuable properties (morphology, crystallinity quality, interaction between adjacent layers, critical current density, and flux pinning ability) were noted to improve remarkably in the case of the optimum amount in the Y-123 superconducting phase. In this context, the Y-358 superconducting phase was harshly affected by the substitution due to the change of the ortorombicity, localization problem, impurity phases, structure stabilization, crystal structure quality, crystallization system, homogeneities of oxidation states, oxygen ordering degree, and impurity scatterings. Besides, the analysis of the fluctuation induced conductivity indicated that the Y-123 ceramics with longer c-axis coherence length and less anisotropic nature exhibited well superconducting properties. Additionally, the doping ratio of x=0.01 led to the formation and distribution of more nucleation centers for the thermal fluxon motions of 2D pancake vortices. Similarly, the optimum Tb doped Y-123 system exhibited much durable to applied field strengths due to the best interaction quality between grains. Accordingly, this study recommended that the Y1-x(Tb)xBa2Cu3O7-δ advanced ceramic structure with new functionalities can find much more application areas in innovative, heavy-industrial technologies, and advanced engineering-related sectors.

Optimizing Ti/TiN Multilayers for UV, Optical and Near-IR Microwave Kinetic Inductance Detectors

Microwave Kinetic Inductance Detectors (MKIDs) combine significant advantages for photon detection like single photon counting, single pixel energy resolution, vanishing dark counts and µs time resolution with a simple design and the feasibility to scale up into the megapixel range. But high quality MKID fabrication remains challenging as established superconductors tend to either have intrinsic disadvantages, are challenging to deposit or require very low operating temperatures. As alternating stacks of thin Ti and TiN films have shown very impressive results for far-IR and sub-mm MKIDs, they promise significant improvements for UV, visible to near-IR MKIDs as well, especially as they are comparably easy to fabricate and control. In this paper, we present our ongoing project to adapt proximity coupled superconducting films for photon counting MKIDs. Some of the main advantages of Ti/TiN multilayers are their good control of critical temperature (Tc) and their great homogeneity of Tc even over large wafers, promising improved pixel yield especially for large arrays. We demonstrate the effect different temperatures during fabrication have on the detector performance and discuss excess phase noise observed caused by surface oxidization of exposed Si. Our first prototypes achieved photon energy resolving powers of up to 3.1 but turned out to be much too insensitive. As the work presented is still in progress, we also discuss further improvements planned for the near future.

Unconventional superconductivity without doping in infinite-layer nickelates under pressure

High-temperature unconventional superconductivity quite generically emerges from doping a strongly correlated parent compound, often (close to) an antiferromagnetic insulator. The recently developed dynamical vertex approximation is a state-of-the-art technique that has quantitatively predicted the superconducting dome of nickelates. Here, we apply it to study the effect of pressure in the infinite-layer nickelate SrxPr1−xNiO2. We reproduce the increase of the critical temperature (Tc) under pressure found in experiment up to 12 GPa. According to our results, Tc can be further increased with higher pressures. Even without Sr-doping the parent compound, PrNiO2, will become a high-temperature superconductor thanks to a strongly enhanced self-doping of the Ni dx2−y2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${d}_{{x}^{2}-{y}^{2}}$$\\end{document} orbital under pressure. With a maximal Tc of 100 K around 100 GPa, nickelate superconductors can reach that of the best cuprates.

Superconductivity above 105 K in Nonclathrate Ternary Lanthanum Borohydride below Megabar Pressure.

Hydrides are promising candidates for achieving room-temperature superconductivity, but a formidable challenge remains in reducing the stabilization pressure below a megabar. In this study, we successfully synthesized a ternary lanthanum borohydride by introducing the nonmetallic element B into the La-H system, forming robust B-H covalent bonds that lower the pressure required to stabilize the superconducting phase. Electrical transport measurements confirm the presence of superconductivity with a critical temperature (Tc) of up to 106 K at 90 GPa, as evidenced by zero resistance and Tc shift under an external magnetic field. X-ray diffraction and transport measurements identify the superconducting compound as LaB2H8, a nonclathrate hydride, whose crystal structure remains stable at pressures as low as ∼ half megabar (59 GPa). Stabilizing superconductive stoichiometric LaB2H8 in a submegabar pressure regime marks a substantial advancement in the quest for high-Tc superconductivity in polynary hydrides, bringing us closer to the ambient pressure conditions.

Large impact of phonon lineshapes on the superconductivity of solid hydrogen

Phonon anharmonicity plays a crucial role in determining the stability and vibrational properties of high-pressure hydrides. Furthermore, strong anharmonicity can render phonon quasiparticle picture obsolete questioning standard approaches for modeling superconductivity in these material systems. In this work, we show the effects of non-Lorentzian phonon lineshapes on the superconductivity of high-pressure solid hydrogen. We calculate the superconducting critical temperature TC ab initio considering the full phonon spectral function and show that it overall enhances the TC estimate. The anharmonicity-induced phonon softening exhibited in spectral functions increases the estimate of the critical temperature, while the broadening of phonon lines due to phonon-phonon interaction decreases it. Our calculations also reveal that superconductivity emerges in hydrogen in the Cmca − 12 molecular phase VI at pressures between 450 and 500 GPa and explain the disagreement between the previous theoretical results and experiments.

Imaging the Magnetic Anisotropy in Ultrathin Fe4GeTe2 with a Nitrogen-Vacancy Magnetometer.

Two-dimensional (2D) FenGeTe2, with n = 3, 4, and 5, has been realized in experiments, showing strong magnetic anisotropy with enhanced critical temperature (Tc). The understanding of its magnetic anisotropy is crucial for the exploration of more stable 2D magnets and its spintronic applications. Here, we report a quantitative reconstruction of the magnetization magnitude and its direction in ultrathin Fe4GeTe2 using nitrogen vacancy centers. Through imaging stray magnetic fields, we identified the spin-flop transition at approximately 80 K, resulting in a change of the easy axis from the out-of-plane direction to the in-plane direction. Moreover, by analyzing the thermally activated escape behavior of the magnetization near Tc in terms of the Ginzburg-Landau model, we observed the in-plane magnetic anisotropy effect and the formation capability of magnetic domains at ∼0.4 μm2 μT-1. Our findings contribute to the quantitative understanding of the magnetic anisotropy effect in a vast range of 2D van der Waals magnets.