A15 Superconductors Research Articles

First-principles insights on the A15 phase in Ti-Se system

A pressure-composition phase diagram of Ti-Se system has been established after a comprehensive computational study at high pressure. As the pressure increases to 72 GPa, a new A15 Ti3Se structure is predicted and it can be stabilized dynamically at zero pressure. We further investigate the electronic and superconducting properties of A15 Ti3Se. Due to the significant contribution of transition metal d electrons at the Fermi level, the phonon properties of the Ti-lattice considerably enter into the calculation of electron-phonon interactions. The calculated robust superconducting critical temperature Tc (μ∗= 0.10-0.15) is 26-21.34 K under 80 GPa. Our results may provide theoretical guidance for the discovery of other titanides at high pressure, facilitating the exploration of A15 superconductors in the form of A3B.

Quantifying the Nonadiabaticity Strength Constant in Recently Discovered Highly Compressed Superconductors

Superconductivity in highly pressurized hydrides has become the primary direction for the exploration of the fundamental upper limit of the superconducting transition temperature, Tc, after Drozdov et al. (Nature 2015, 525, 73) discovered a superconducting state with Tc=203 K in highly compressed sulfur hydride. To date, several dozen high-temperature superconducting polyhydrides have been discovered and, in addition, it was recently reported that highly compressed titanium and scandium exhibit record-high Tc (up to 36 K). This exceeded the Tc=9.2 K value of niobium many times over, which was the record-high Tc ambient pressure metallic superconductor. Here, we analyzed the experimental data for the recently discovered high-pressure superconductors (which exhibit high transition temperatures within their classes): elemental titanium (Zhang et al., Nature Communications 2022; Liu et al., Phys. Rev. B 2022), TaH3 (He et al., Chinese Phys. Lett. 2023), LaBeH8 (Song et al., Phys. Rev. Lett. 2023), black phosphorous (Li et al., Proc. Natl. Acad. Sci. 2018; Jin et al., arXiv 2023), and violet (Wu et al., arXiv 2023) phosphorous to reveal the nonadiabaticity strength constant TθTF (where Tθ is the Debye temperature, and TF the Fermi temperature) in these superconductors. The analysis showed that the δ-phase of titanium and black phosphorous exhibits TθTF scores that are nearly identical to those associated with A15 superconductors, while the studied hydrides and violet phosphorous exhibit constants in the same ballpark as those of H3S and LaH10.

Superconducting properties of the A15 structure compound V[formula omitted]Ge

Applications of A15 superconductors have received considerable attention, resulting in a large body of reported research work on A15 superconductors. Nonetheless, studies on this material focusing on its superconducting parameters are scarce. This motivates new efforts toward bulk measurements in typical A15 material V3Ge, which is also the isomorph of V3Si with a superconducting transition temperature at 17 K. In this study, we determined the superconducting parameters of single-phase V3Ge crystals, which crystallize in the A15 structure, through measurements of magnetization, magnetic susceptibility, electrical resistivity, and heat capacity. Our results show that the temperature (Tc) of its superconducting transition is 6.2±0.4 K. Moreover, we conducted electrical resistivity and magnetic susceptibility measurements and found the lower and higher critical fields to be 559 Oe and 39.8 kOe, respectively. The Ginzburg–Landau parameter κGL is determined to be 9.78, which is much greater than 1/2, implying V3Ge Type-II superconductor. The Sommerfeld coefficient γ in the normal state is determined to be 31.2 mJ/K2 mol, which is 50∼100 folds higher compared to that in the normal metal, thereby indicating the enhancement of the density of state at the Fermi energy. The Debye temperature Θ is estimated to be 430 K, and the value of the drastic increase in the specific heat at Tc is approximately 1.33, which is 7% smaller than the weak coupling limit of 1.43 predicted by the Bardeen–Cooper–Schrieffer (BCS) theory. The electron–phonon coupling constant λe−ph is 0.61. These values suggest that V3Ge is a conventional superconductor with s-wave symmetry in the BCS framework, which implies that the high Debye temperature and large density of states at the Fermi energy contribute to the increase in Tc in V3Ge. The superconducting parameters determined in this study provide guidance for the development of a new metal superconductor with high-Tc.

Tuning of upper critical field in a vanadium-based A15 superconductor by the compositionally-complex-alloy concept

Tuning of upper critical field in a vanadium-based A15 superconductor by the compositionally-complex-alloy concept

Electron–phonon coupling of the Ti3Sb compound

Using the first-principles method based on density functional theory, we investigate the electronic, mechanical, phononic, superconducting, and topological properties of the A15 superconductor Ti3Sb with/without the inclusion of spin–orbit coupling (SOC). We find that the calculated elastic constants satisfy the Born stability criteria and the ductile nature of Ti3Sb. The result of phonon calculations reveals that the Pm3¯n structure is dynamically stable. Sb atoms are dominated in the low-frequency region due to the mass difference between Ti and Sb from partial phonon calculations. Our electronic structure calculations show that Ti3Sb has metallic band structures with Ti-d bands near the Fermi energy. The electron–phonon coupling constant (λ≃0.80) indicates that Ti3Sb has strong electron–phonon coupling. The critical temperature (Tcμ⋆=0.10) is found to be 6.24 and 6.56 K with and without the SOC effect, respectively. Due to Ti’s relatively small SOC strength, we do not find evidence of non-trivial topology in contrast to Ta compounds such as Ta3Sb.

Ab initio theory of the impact of grain boundaries and substitutional defects on superconducting Nb3Sn

Grain boundaries play a critical role in superconducting applications of Nb3Sn: in dc applications, grain boundaries preserve the material’s inherently high critical current density by pinning flux, while in ac applications grain boundaries can provide weak points for flux entry leading to significant dissipation. We present the first ab initio study to investigate the physics of different grain boundary types in Nb3Sn and their impact on superconductivity using density-functional theory. We identify an energetically favorable selection of high-angle tilt and twist grain boundaries of distinct orientations. We find that clean grain boundaries free of point defects reduce the Fermi-level density of states by a factor of two, an effect that decays back to the bulk electronic structure ∼1–1.5 nm from the boundary. We further calculate the binding free-energies of tin substitutional defects to multiple boundaries, finding a strong electronic interaction that extends to a distance comparable to that of the reduction of density of states. Associated with this interaction, we discover a universal trend in defect electronic entropies near a boundary. We then probe the effects of defect segregation on grain boundary electronic structure and calculate the impact of substitutional impurities on the Fermi-level density of states in the vicinity of a grain boundary. We find that titanium and tantalum defects have little impact regardless of placement, whereas tin, copper, and niobium defects each have a significant impact but only on sites away from the boundary core. Finally, we introduce a model for a local superconducting transition temperature and consider how grain boundary composition affects as a function of distance from the boundary plane. The methodology established in this manuscript can be applied to other A15 superconductors in addition to Nb3Sn.

Imaging Flux Avalanches in V$_3$Si Superconducting Thin Films

When developing superconducting devices patterned on thin films, one should bear in mind that flux avalanches might occur for some materials at a certain window of applied fields and temperatures. Although the A15 superconductors are well known and used in a variety of purposes, there are no studies about flux avalanches in V <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_3$</tex-math></inline-formula> Si thin films. In the present work, we report the observation of flux avalanches in films of V <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_3$</tex-math></inline-formula> Si, grown by pulsed laser deposition on a LaAlO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_3$</tex-math></inline-formula> substrate. The range of temperatures and applied magnetic fields at which the avalanches take place is determined using dc-magnetometry. Magneto-Optical Imaging is employed to visualize the occurrence and spatial distribution of such flux avalanches. Images of the flux penetrated into the samples indicate a clear anisotropic distribution, which is ascribed to a thickness gradient. The observation of flux avalanches reported here implies that attention to this feature must be given when films of V <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_3$</tex-math></inline-formula> Si are envisaged for possible applications.

Topological states in A15 superconductors

Superconductors with the A15 structure are prototypical type-II s-wave superconductors which have generated considerable interest in early superconducting material history. However, the topological nature of the electronic structure remains unnoticed so far. Here, using first-principles band structure calculations based on density functional theory, we show that the A15 superconductors (Ta$_3$Sb, Ta$_3$Sn, and Ta$_3$Pb) have nontrivial band topology in the bulk electronic band structures, leading to the formation of topological surface states near the Fermi energy. Due to the bulk superconductivity, the proximity effect in the topological surface states would induce topological superconductivity even without heterostructure of a topological insulator and an s-wave superconductor. Our results indicate that the A15 superconductors are promising candidates for the realization of the topological superconductivity and the Majorana fermion.

Giant intrinsic spin Hall effect in W3Ta and other A15 superconductors.

The spin Hall effect (SHE) is the conversion of charge current to spin current, and nonmagnetic metals with large SHEs are extremely sought after for spintronic applications, but their rarity has stifled widespread use. Here, we predict and explain the large intrinsic SHE in β-W and the A15 family of superconductors: W3Ta, Ta3Sb, and Cr3Ir having spin Hall conductivities (SHCs) of -2250, -1400, and 1210 , respectively. Combining concepts from topological physics with the dependence of the SHE on the spin Berry curvature (SBC) of the electronic bands, we propose a simple strategy to rapidly search for materials with large intrinsic SHEs based on the following ideas: High symmetry combined with heavy atoms gives rise to multiple Dirac-like crossings in the electronic structure; without sufficient symmetry protection, these crossings gap due to spin-orbit coupling; and gapped crossings create large SBC.

Ultrafast nonthermal terahertz electrodynamics and possible quantum energy transfer in the Nb3Sn superconductor

We report the low-energy electrodynamics of a moderately clean A15 superconductor (SC) following ultrafast excitation to understand and manipulate terahertz (THz) quasi--particle (QP) transport by tuning pump photoexcitation of from competing orders. Using 35-fs optical pulses, we observe a non-thermal enhancement in the low frequency conductivity, opposite to that observed for THz pump, which persists up to an additional critical temperature, above the SC one, from an electronic order in the Martensitic normal state. In the SC state, the fluence dependence of pair breaking kinetics together with an analytic model provides evidence for a `one photon-to-one pair' non-resonant energy transfer during the laser pulse. Such initial transfer of photon energy $\hbar\omega$ to QPs at the {\em quantum} limit, set by $2\Delta_{SC}/\hbar\omega$=0.33$\%$, is more than one order of magnitude smaller than in previously studied BCS SCs, which we attribute to strong electron--phonon coupling and possible influence of phonon condensation in A15 SCs.

A multi-scale point of view on the structure–property relationships of A15 superconductors

Nb3Sn and other A15 members have been widely applied in nuclear power, nuclear magnetic resonance, and high-energy particle accelerators for their high critical current density (Jc) and upper critical field (Bc2). There have been comprehensive and intensive studies on the applications, the fundamental lattice dynamic and electronic properties, etc., of A15 superconductors. Various reviews on the preparations, structures, and properties have already been written in the last few years. Nevertheless, on account of the large amount of existing facts and views, a coherent view on the relations between the structures and properties has not appeared to unify the facts. This article sketches a multi-scale point of view on the relations between the multi-scale structures and the corresponding properties.

A theory of the strain-dependent critical field in Nb3Sn, based on anharmonic phonon generation

We propose a theory to explain the strain dependence of the critical properties in A15 superconductors. Starting from the strong-coupling formula for the critical temperature, and assuming that the strain sensitivity stems mostly from the electron–phonon α2F function, we link the strain dependence of the critical properties to a widening of α2F. This widening is attributed to the nonlinear generation of phonons, which takes place in the anharmonic deformation potential induced by the strain. Based on the theory of sum- and difference-frequency wave generation in nonlinear media, we obtain an explicit connection between the widening of α2F and the anharmonic energy. The resulting model is fit to experimental datasets for Nb3Sn, and the anharmonic energy extracted from the fits is compared with first-principles calculations.

Strain sensitivity and superconducting properties of Nb3Sn from first principles calculations

Using calculations from first principles based on density-functional theory we have studied the strain sensitivity of the A15 superconductor Nb3Sn. The Nb3Sn lattice cell was deformed in the same way as observed experimentally on multifilamentary, technological wires subject to loads applied along their axes. The phonon dispersion curves and electronic band structures along different high-symmetry directions in the Brillouin zone were calculated, at different levels of applied strain, ε, on both the compressive and the tensile side. Starting from the calculated averaged phonon frequencies and electron–phonon coupling, the superconducting characteristic critical temperature of the material, Tc, has been calculated by means of the Allen–Dynes modification of the McMillan formula. As a result, the characteristic bell-shaped Tc versus ε curve, with a maximum at zero intrinsic strain, and with a slight asymmetry between the tensile and compressive sides, has been obtained. These first-principle calculations thus show that the strain sensitivity of Nb3Sn has a microscopic and intrinsic origin, originating from shifts in the Nb3Sn critical surface. In addition, our computations show that variations of the superconducting properties of this compound are correlated to stress-induced changes in both the phononic and electronic properties. Finally, the strain function describing the strain sensitivity of Nb3Sn has been extracted from the computed Tc(ε) curve, and compared to experimental data from multifilamentary, composite wires. Both curves show the expected bell-shaped behavior, but the strain sensitivity of the wire is enhanced with respect to the theoretical predictions for bulk, perfectly binary and stoichiometric Nb3Sn. An understanding of the origin of this difference might open potential pathways towards improvement of the strain tolerance in such systems.

Tensile strain dependence of critical current of RHQ-Nb3Al wires

Nb3Al superconducting wires produced by rapid heating and quenching (RHQ) have been developed for high-field accelerator magnets. It is known that critical currents of A15 superconductors (e.g., Nb3Al and Nb3Sn) have a dependence on stress/strain. It is thus important to clarify the stress/strain behavior of the RHQ-Nb3Al wires for the development of high-field accelerator magnets. We recently started experimentally investigating the strain dependence of the critical current of the RHQ-Nb3Al wires with a Ta interfilament by measuring their critical currents under longitudinal tensile strains. To evaluate the effect of residual strain in Nb3Al filaments induced by thermal contraction of the materials in the wire, neutron diffraction measurements were performed at room temperature.

R&amp;D Efforts Towards High Field Accelerator Magnets at KEK

KEK has emphasized efforts to develop the RHQ-Nb3Al superconductor and a 13 T sub-scale magnet towards the LHC upgrade in last years. In addition, relevant R&D regarding radiation resistance has been carried out. For realization of higher field magnets beyond 15 T, thorough understanding of strain behavior of HTS and A15 superconductors is truly desired. KEK has launched a new research subject on stress/strain sensitivity of HTS and A15 superconductors in collaboration with the neutron diffraction facility at J-PARC and the High Field Laboratory in Tohoku University.

Second superconducting energy gap ofNb3Snobserved by breakjunction point-contact spectroscopy

We report on investigations of the superconducting energy gap of the A15 superconductor Nb_3Sn by point-contact spectroscopy of breakjunctions. The voltage-dependent differential conductance dI/dV reveals features of a second energy gap besides the energy gap known from previous tunnel measurements with maxima at \Delta_1=3.92 +/- 0.16 meV and \Delta_2=0.85 +/- 0.17 meV as derived from a histogram summarizing the data of more than 60 contacts. These findings are the first spectroscopic evidence that Nb_3Sn belongs to the class of two-band superconductors and they are in line with low-temperature specific-heat measurements on Nb_3Sn.

$J_{C}(B,T,\varepsilon)$ Parameterization for the ITER ${\rm Nb}_{3}{\rm Sn}$ Production

A number of models for the critical surface of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn, and in general A15 superconductors, have been developed in the past years. This paper compares the most common parameterizations using consistent notation. Although the parameterizations appear dissimilar at first sight, they are in reality all based on a fit of the normalized pinning force vs. the reduced field, and have similar scalings for the critical field and critical temperature based on a Unified Scaling Law. In this paper we take the various parameterizations as a basis for a generic scaling proposed for the characterization and production follow-up of the ITER Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands. The accuracy of the scaling is estimated using the fitting residuals on various sets of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> (B,T,epsiv) data available in literature. We discuss the results, and give our view of the work towards a unified, practical parameterization.

The effects of Ti and Cr co-doping on the structure and superconductivity ofV3Si

V3Si is a well known A15 superconductor with a superconducting transition temperature(Tc) value of 17 K. Weprepared a series of V3−2xTixCrxSi (0≤x≤0.2) samples which have the same number of electrons per cell asV3Si in orderto study the doping effect. The formation of the single-phase samples was confirmed by x-ray diffraction whenx≤0.1. It was found thatthe lattice parameter a decreases slightly with increasing dopant concentration. TheTc value of theV3−2xTixCrxSi samples drops slowlywhen x<0.1 whereas asharp decline of Tc occurs when x>0.1. TheTc drop in the Tiand Cr co-doped V3Si is slower than that in the single-element Ti or Cr dopedV3Si with the same total dopant concentrations, indicating that the density of states at the Fermi level,N(EF), plays an important role in the change ofTc. The criticalcurrent density of V2.8Ti0.1Cr0.1Si is significantly enhanced to more than six times that of pureV3Si at 10 K in magnetic fields up to 4 T.

Radiation damaged MgB2: a comparison with A15 superconductors

In the present paper we discuss in detail experiments of radiation damage and their effects on MgB2 properties (Tc, resistivity, critical field, gap), making constant reference to similar experiments performed in the seventies and eighties on A15 superconductors (Nb3Sn, V3Si). We will show that the overall behaviour of MgB2 and A15s is surprisingly similar, pointing out that similar mechanisms (smearing of density of states) plays a role at high radiation doses, whereas, at low radiation, two-band effects mainly determine the MgB2 behaviuor.

Inelastic light scattering from correlated electrons

Inelastic light scattering is an intensively used tool in the study of electronic properties of solids. Triggered by the discovery of high-temperature superconductivity in the cuprates and by new developments in instrumentation, light scattering in both the visible (Raman effect) and x-ray part of the electromagnetic spectrum has become a method complementary to optical (infrared) spectroscopy while providing additional and relevant information. The main purpose of the review is to position Raman scattering with regard to single-particle methods like angle-resolved photoemission spectroscopy, and other transport and thermodynamic measurements in correlated materials. Particular focus will be placed on photon polarizations and the role of symmetry to elucidate the dynamics of electrons in different regions of the Brillouin zone. This advantage over conventional transport (usually measuring averaged properties) provides new insights into anisotropic and complex many-body behavior of electrons in various systems. Recent developments in the theory of electronic Raman scattering in correlated systems and experimental results in paradigmatic materials such as the A15 superconductors, magnetic and paramagnetic insulators, compounds with competing orders, as well as the cuprates with high superconducting transition temperatures are reviewed. An overview of the manifestations of complexity in the Raman response due to the impact of correlations and developing competing orders is presented. In a variety of materials, observations which may be understood and a summary of important open questions that pave the way to a detailed understanding of correlated electron systems, are discussed.