Multigap Superconductivity Research Articles

Unraveling the evolution of multigap superconductivity in layered Na–B–C films: An additional energy gap induced by the internal B–C layer

Multigap superconductors provide a platform to confirm rich new physics such as time-reversal symmetry breaking, giant paramagnetic response, and hidden criticality. However, an obstacle hindering the experimental validation of these phenomena lies in the lack of superconductors with three or more gaps and critical temperatures higher than the liquid nitrogen temperature. In this work, we predicted NaB2C2 and Na2B3C3 films with high-temperature superconductivity (beyond 90 K) using the fully anisotropic Migdal-Eliashberg theory. The multigap behaviors of Na–B–C films with three and five atomic layers were analyzed in detail, revealing two typical configurations: three-gap (NaB2C2) and four-gap (Na2B3C3) superconductors. Compared with the NaB2C2 film, the additional gap observed in the Na2B3C3 film originates from the in-plane covalent state of the internal B–C layer. This research offers valuable insights into the evolution of multigap superconductivity in layered B–C films.

Strong electron-phonon coupling and multigap superconductivity in 2H/1T Janus MoSLi monolayer.

Two-dimensional (2D) Janus transition metal dichalcogenides MXY manifest novel physical properties owing to the breaking of out-of-plane mirror symmetry. Recently, the 2H phase of MoSH has been demonstrated to possess intrinsic superconductivity, whereas the 1T phase exhibits a charge density waves state. In this paper, we have systematically studied the stability and electron-phonon interaction characteristics of MoSLi. Our results have shown that both the 2H and 1T phases of MoSLi are stable, as indicated by the phonon spectrum and the abinitio molecular dynamics. However, the 1T phase exhibits an electron-phonon coupling constant that is twice as large as that of the 2H phase. In contrast to MoSH, the 1T phase of MoSLi exhibits intrinsic superconductivity. By employing the abinitio anisotropic Migdal-Eliashberg formalism, we have revealed the two-gap superconducting nature of 1T-MoSLi, with a transition temperature (Tc) of 14.8K. The detailed analysis indicates that the superconductivity in 1T-MoSLi primarily originates from the interplay between the vibration of the phonon modes in the low-frequency region and the dz2 orbital. These findings provide a fresh perspective on superconductivity within Janus structures.

Features of the Multigap Superconductivity in Cobalt-Doped NaFeAs

Features of the Multigap Superconductivity in Cobalt-Doped NaFeAs

The Depairing Current Density of a Fe(Se,Te) Crystal Evaluated in Presence of Demagnetizing Factors

The effect of the demagnetizing factor, regarding the determination of the de-pairing current density Jdep, has been studied in the case of a Fe(Se,Te) crystal, using DC magnetic measurements as a function of a magnetic field (H) at different temperatures (T). First, the lower critical field Hc1(T) values were obtained, and the demagnetization effects acting on them were investigated after calculating the demagnetizing factor. The temperature behaviors of both the original Hc1 values and the ones obtained after considering the demagnetization effects (Hc1demag) were analyzed, and the temperature dependence of the London penetration depth λL(T) was obtained in both cases. In particular, the λL(T) curves were fitted with a power law dependence, indicating the presence of low-energy quasiparticle excitations. Furthermore, by plotting λL−2 as a function of T, we found that our sample behaves as a multigap superconductor, which is similar to other Fe-11 family iron-based compounds. After that, the coherence length ξ values were extracted, starting with the Hc2(T) curve. The knowledge of λL and ξ allowed us to determine the Jdep values and to observe how they are influenced by the demagnetizing factor.

The Superconducting Dome in Artificial High-Tc Superlattices Tuned at the Fano–Feshbach Resonance by Quantum Design

While the search for new high-temperature superconductors had been driven by the empirical “trials and errors” method for decades, we now report the synthesis of Artificial High-Tc Superlattices (AHTS) designed by quantum mechanics theory at the nanoscale. This discovery paves the way for engineering a new class of high-temperature superconductors, following the predictions of the Bianconi Perali Valletta (BPV) theory recently implemented in 2022 by Mazziotti et al. including Rashba spin-orbit coupling to create nanoscale AHTS composed of quantum wells. The high-Tc superconducting properties within these superlattices are controlled by a conformational parameter of the superlattice geometry, specifically, the ratio L/d which represents the thickness of La2CuO4 layers (L) relative to the superlattice period (d). Using molecular beam epitaxy, we have successfully grown numerous AHTS samples. These samples consist of initial layers of stoichiometric La2CuO4 units with a thickness L, doped by interface space charge, and intercalated with second layers of non-superconducting metallic material, La1.55Sr0.45CuO4 with thickness denoted as W = d − L. This configuration forms a quantum superlattice with periodicity d. The agreement observed between the experimental dependence Tc (the superconducting transition temperature) versus L/d ratio and the predictions of the BPV theory for AHTS in the form of the superconducting dome validates the hypothesis that the superconducting dome arises from the Fano–Feshbach or shape resonance in multigap superconductivity driven by quantum nanoscale confinement.

Multiband Superconductivity in High-Pressure Sulfur Hydrides

The temperature dependence of the two superconducting gaps in pressurised H3S at 155 GPa with a critical temperature of 203 K has been determined using a data analysis of the experimental curve of the upper critical magnetic field as a function of temperature in the framework of the two-band s-wave Eliashberg theory. Two different phonon-mediated intra-band Cooper pairing channels in a regime of moderate strong couplings have the key role of the pair-exchange interaction between the two gaps, giving the two non-diagonal terms of the coupling tensor, which are missing in the single-band s-wave Eliashberg theory. The results provide a prediction of the different temperature dependence of the small and large gaps as a function of temperature, which provides evidence of multigap superconductivity in H3S.

Surface structure and multigap superconductivity of V3Si (111) revealed by scanning tunneling microscopy

V3Si, a classical silicide superconductor with relatively high T_{mathrm{C}} (∼16 K), is promising for constructing silicon-based superconducting devices and hetero-structures. However, real space characterization on its surfaces and superconducting properties are still limited. Here we report the first low-temperature scanning tunnelling microscopy (STM) study on cleaned V3Si (111) single crystal surface. We observed a sqrt{3} times sqrt{3} superstructure which displays mirror symmetry between adjacent terraces, indicating the surface is V-terminated and reconstructed. The tunneling spectrum shows full superconducting gap with double pairs of coherence peaks, but has a relatively small gap size with comparing to bulk T_{mathrm{C}}. Impurity induced in-gap state is absent on surface defects but present on introduced magnetic adatoms. Upon applying magnetic field, a hexagonal vortex lattice is visualized. Interestingly, the vortex size is found to be field dependent, and the coherence length measured from single vortex at low field is significantly larger than estimated value from bulk H_{c2}. These results reflect V3Si is a multi-band, s-wave superconductor.

Observation of multigap and coherence peak in the noncentrosymmetric superconductor CaPtAs: As75 nuclear quadrupole resonance measurements

We present synthesis and $^{75}$As-nuclear quadrupole resonance (NQR) measurements for the noncentrosymmetric superconductor CaPtAs with a superconducting transition temperature $T_c$ of $\sim 1.5$ K. We discovered two different forms of CaPtAs during synthesis; one is a high-temperature tetragonal form that was previously reported, and the other is a low-temperature form consistent with the orthorhombic structure of CaPtP. According to the $^{75}$As-NQR measurement for superconducting tetragonal CaPtAs, the nuclear spin-lattice relaxation rate $1/T_1$ has an obvious coherence peak below $T_c$ and does not follow a simple exponential variation at low temperatures. These findings indicate that CaPtAs is a multigap superconductor and a large $s$-wave component.

Upper critical magnetic field in NbRe and NbReN micrometric strips.

Non-centrosymmetric superconductors have recently received significant interest due to their intriguing physical properties such as multigap and nodal superconductivity, helical vortex states, as well as non-trivial topological effects. Moreover, large values of the upper critical magnetic field have been reported in these materials. Here, we focus on the study of the temperature dependence of the perpendicular magnetic field of NbRe and NbReN films patterned in micrometric strips. The experimental data are studied within the Werthamer-Helfand-Hohenberg theory, which considers both orbital and Zeeman pair breaking. The analysis of the results shows different behavior for the two materials with a Pauli contribution relevant only in the case of NbReN.

Mo-Re alloy: A new benchmark two-band superconductor

Multigap superconductivity, emerging in metals with several bands crossing the Fermi level, favors exotic superconducting orders that have no equivalent in a single-band counterpart. In this context, it is important to search for new materials with well-established two (or more) gaps having distinctly different sizes. In this work, we confirm previous statements and present new evidence to support the claim that Mo-Re alloy with a comparable concentration of the components is a two-band/two-gap superconductor. The differential conductance spectra obtained in point-contact experiments demonstrate the presence of a bosonic, undamped collective mode and its harmonics associated with the superconducting state. Following previous works on MgB2, we have identified these features as manifestations of the so-called Leggett mode arising due to relative phase fluctuations between two superconducting order parameters.

Two-dimensional multigap superconductivity in bulk 2H−TaSeS

Superconducting transition metal dichalcogenides emerged as a prime candidate for topological superconductivity. This work presents a detailed investigation of superconducting and transport properties on 2H-TaSeS single crystals using magnetization, transport, and specific heat measurements. These measurements suggest multigap anisotropic superconductivity with the upper critical field, breaking Pauli limiting field in both in-plane and out-of-plane directions. The angle dependence of the upper critical field suggests 2-dimensional superconducting nature in bulk 2H- TaSeS.

Evidence of unconventional pairing in the quasi-two-dimensional CuIr2−xRuxTe4 superconductor

The CuIr$_{2-x}$Ru$_x$Te$_4$ superconductors (with a $T_c$ around 2.8 K) can host charge-density waves, whose onset and interplay with superconductivity are not well known at a microscopic level. Here, we report a comprehensive study of the $x$ = 0 and 0.05 cases, whose superconductivity was characterized via electrical-resistivity-, magnetization-, and heat-capacity measurements, while their microscopic superconducting properties were studied via muon-spin rotation and relaxation ($\mu$SR). In CuIr$_{2-x}$Ru$_x$Te$_4$, both the temperature-dependent electronic specific heat and the superfluid density (determined via transverse-field $\mu$SR) are best described by a two-gap (s+d)-wave model, comprising a nodeless gap and a gap with nodes. The multigap superconductivity is also supported by the temperature dependence of the upper critical field $H_\mathrm{c2}(T)$. However, under applied pressure, a charge-density-wave order starts to develop and, as a consequence, the superconductivity of CuIr$_2$Te$_4$ achieves a more conventional s-wave character. From a series of experiments, we provide ample evidence that the CuIr$_{2-x}$Ru$_x$Te$_4$ family belongs to the rare cases, where an unconventional superconducting pairing is found near a charge-density-wave quantum critical point.

Single-crystal growth and superconductivity in RbNi2Se2

We report the synthesis and characterization of RbNi$_2$Se$_2$, an analog of the iron chalcogenide superconductor Rb$_x$Fe$_2$Se$_2$, via transport, angle resolved photoemission spectroscopy, and density functional theory calculations. A superconducting transition at $T_{c}$ = 1.20 K is identified. In normal state, RbNi$_2$Se$_2$ shows paramagnetic and Fermi liquid behaviors. A large Sommerfeld coefficient yields a heavy effective electron mass of $m^{*}\approx6m_{e}$. In the superconducting state, zero-field electronic specific-heat data $C_{es}$ can be described by a two-gap BCS model, indicating that RbNi$_2$Se$_2$ is a multi-gap superconductor. Our density functional theory calculations and angle resolved photoemission spectroscopy measurements demonstrate that RbNi$_2$Se$_2$ exhibits relatively weak correlations and multi-band characteristics, consistent with the multi-gap superconductivity.

Study of superconducting gap in presence of antiferromagnetism in iron pnictide superconductors

A Hamiltonian model of the pnictide composite superconductors is presented to study the Superconducting(SC) gap. The total Hamiltonian has been solved by writing equations of motion for single particle Greens function of Zubarev type. Equations for appropriate single particle correlation functions are derived and the order parameters corresponding to superconductivity and antiferromagnetism are determined. The equations for superconducting gap and antiferromagnetic gap are solved self-consistently and numerically. The multi-gap superconductivity is studied for different values of interlayer coupling constant parameters and Debye frequencies.

High-temperature multigap superconductivity in two-dimensional metal borides

Using first-principles calculations in combination with the Eliashberg formalism, we systematically investigated phonon-mediated superconductivity in two-dimensional (2D) metal-boride crystals, consisting of a boron honeycomb network doped by diverse metal elements. Such 2D metal-boride compounds, named MBenes, are chemically exfoliable from single-crystalline layered ternary borides. First we identified the MBene layers with potential for superconductivity via isotropic Eliashberg calculations, considering a wide range of metal elements, with a focus on alkaline earth and transition metals. Subsequently, we performed a detailed analysis of the prominent superconducting MBenes by solving the anisotropic Eliashberg equations. The obtained high critical temperatures (up to 72 K), as well as the rich multigap superconducting behavior, recommend these crystals for further use in multifunctional 2D heterostructures and superconducting device applications.

Two-gap s± -wave superconductivity at an oxide interface

After half a century of debate, superconductivity in doped ${\mathrm{SrTiO}}_{3}$ has come to the fore again with the discovery of interfacial superconductivity in the ${\mathrm{LaAlO}}_{3} /{\mathrm{SrTiO}}_{3}$ heterostructures. While these interfaces share the interesting properties of bulk ${\mathrm{SrTiO}}_{3}$ , quantum confinement generates a complex band structure involving bands with different orbital symmetries whose occupancy is tunable by electrostating doping. Multigap superconductivity has been predicted to emerge in ${\mathrm{LaAlO}}_{3} /{\mathrm{SrTiO}}_{3}$ at large doping, with a Bose-Einstein condensation character at the Lifshtiz transition. In this article, we report on the measurement of the upper critical magnetic field ${\mathrm{H}}_{\mathrm{c}2}$ of superconducting (110)-oriented ${\mathrm{LaAlO}}_{3} /{\mathrm{SrTiO}}_{3}$ heterostructures and evidence a two-gap superconducting regime at high doping. Our results are quantitatively explained by a theoretical model based on the formation of an unconventional ${\mathrm{s}}_{\ifmmode\pm\else\textpm\fi{}}$-wave superconducting state with a repulsive coupling between the two condensates.

Nodal multigap superconductivity in the anisotropic iron-based compound RbCa2Fe4As4F2

The 12442 compounds are a recently discovered family of iron-based superconductors, that share several features with the cuprates due to their strongly anisotropic structure, but are so far poorly understood. Here, we report on the gap structure and anisotropy of RbCa2(Fe1−xNix)4As4F2 single crystals, investigated by a combination of directional point-contact Andreev-reflection spectroscopy and coplanar waveguide resonator measurements. Two gaps were identified, with clear signatures of d-wave-like nodal structures which persist upon Ni doping, well described by a two-band d − d state with symmetry-imposed nodes. A large London penetration depth anisotropy was revealed, weakly dependent on temperature and fully compatible with the d − d model.

Strain-induced multigap superconductivity in electrene Mo2N: a first principles study.

Superconductivity in low dimensional materials and 2D electrides are topics of great interest with possible applications in next generation electronic devices. Using density functional theory (DFT) associated with Migdal-Eliashberg approach and maximally localized Wannier functions this study shows how biaxial strain affects superconductivity in a monolayer of Mo2N. Results indicate that 2D Mo2N presents strong electron-phonon coupling with large anisotropy in the superconducting energy gap. It is also proposed that, at low temperatures, a single layer of Mo2N becomes an electride with localized electron gas pockets on the surface, resembling anions adsorbed on an atomic sheet. Calculations point to Tc = 24.7 K, a record high transition temperature for this class of material at ambient pressure. Furthermore, it is shown that when biaxial strain is applied to a superconducting Mo2N monolayer, a new superconductivity gap starts at 2% strain and is enhanced by continuum strain, opening additional coupling channels.

On the Superconducting Critical Temperature of Heavily Disordered Interfaces Hosting Multi-Gap Superconductivity

LaAlO3/SrTiO3 interfaces are a nice example of a two-dimensional electron gas, whose carrier density can be varied by top- and back-gating techniques. Due to the electron confinement near the interface, the two-dimensional band structure is split into sub-bands, and more than one sub-band can be filled when the carrier density increases. These interfaces also host superconductivity, and the interplay of two-dimensionality, multi-band character, with the possible occurrence of multi-gap superconductivity and disorder calls for a better understanding of finite-bandwidth effects on the superconducting critical temperature of heavily disordered multi-gap superconductors.

Three-gap superconductivity in two-dimensional InB2/InB4 films

In recent years, multigap superconductors have attracted much attention since the discovery of novel two-gap superconductivity with transition temperature ${T}_{c}\ensuremath{\sim}39\phantom{\rule{4pt}{0ex}}\text{K}$ in bulk ${\mathrm{MgB}}_{2}$. Based on the first-principles calculation and anisotropic Migdal-Eliashberg theory, we conduct a study on a series of two-dimensional (2D) boron-based materials with doped metal atoms to search for multigap superconductors. We find that ${\mathrm{InB}}_{2}$ monolayer films and ${\mathrm{InB}}_{4}$ trilayer films are dynamically stable but not synthesized experimentally yet. An evident three-gap superconductor with high ${T}_{c}\ensuremath{\sim}$ 41.5 K is obtained in a ${\mathrm{InB}}_{2}$ monolayer film. Similarly, ${\mathrm{InB}}_{4}$ trilayer film is a novel superconductor with three distinct superconducting gaps and a high ${T}_{c}\ensuremath{\sim}$ 53 K. The superconductivity in both 2D films originates mainly from the covalent-state-driven metallization. In addition, the effect of biaxial strain on the superconducting behavior of $\mathrm{In}{\text{B}}_{4}$ trilayer films is also involved. The ${\mathrm{InB}}_{4}$ trilayer films stay dynamically stable under biaxial tensile strain of $\ensuremath{-}3%--6%$, and the highest ${T}_{c}$ boosts to 64 K under the biaxial tensile strain of $\ensuremath{\sim}4%$. Meanwhile, we also find that the ${\mathrm{GeB}}_{4}$ and ${\mathrm{ZnB}}_{4}$ trilayer films are two-gap superconductors with high ${T}_{c}\ensuremath{\sim}$ 48.5 and 32 K, respectively.