Two-gap Superconductivity Research Articles

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.

Nodeless superconductivity in the kagome metal CsV3Sb5

The recently discovered kagome metal series AV3Sb5 (A=K, Rb, Cs) exhibits topologically nontrivial band structures, chiral charge order and superconductivity, presenting a unique platform for realizing exotic electronic states. The nature of the superconducting state and the corresponding pairing symmetry are key questions that demand experimental clarification. Here, using a technique based on the tunneling diode oscillator, the magnetic penetration depth Δλ(T) of CsV3Sb5 was measured down to 0.07 K. A clear exponential behavior in Δλ(T) with marked deviations from a T or T2 temperature dependence was observed at low temperatures, indicating an absence of nodal quasiparticles. Temperature dependence of the superfluid density and electronic specific heat can be described by two-gap s-wave superconductivity, consistent with the presence of multiple Fermi surfaces in CsV3Sb5. These results evidence nodeless superconductivity in CsV3Sb5 under ambient pressure, and constrain the allowed pairing symmetry.

Current density dependence on phase differences in superconducting tunnel junctions based on two-gap superconductors

The quasiclassical equations in the t-representation for the matrix Green’s functions with different arguments are derived in order to study the superconducting tunnel junctions based on two-gap superconductors. The current density expression in terms of the Green’s functions in the t-representation are derived. The problem is solved in the framework of the model with the piecewise constant order parameters. The current density through the junction is calculated by taking the boundary conditions for the matrix Green’s functions at the junction into account. The obtained analytical expression for the current-phase relation in the two-gap superconductor based tunnel junctions is used to analyze the pairing symmetries of the order parameters.

Inverted V-shaped evolution of superconducting temperature in SrBC under pressure* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11974194 and 11974207) and K. C. Wong Magna Fund in Ningbo University.

Based on density functional first-principles calculations and anisotropic Eliashberg equations, we have investigated the electronic structure, lattice dynamics, and phonon-mediated superconductivity in newly synthesized layered compound SrBC under pressure. Different from LiBC and MgB2, our calculations surprisingly reveal that SrBC is isotropic in compressibility, due to the accumulation of substantial electrons in the interstitial region. We find that the Sr phonons strongly couple with B-2p z orbital and the interstitial states, giving rise to a two-gap superconductivity in SrBC, whose transition temperature shows an inverted V-shaped dependence on pressure. The maximal transition temperature is about 22 K at 50 GPa. On both sides of 50 GPa, the transition temperature exhibits quasi-linear variation with positive and negative slopes, respectively. Such a variation of transition temperature is infrequent among phonon-mediated superconductors. The competition between enhanced electron–phonon matrix element and hardened phonons plays an essential role in governing the behavior of the critical temperature.

Dimensional crossover and band topology evolution in ultrathin semimetallic NiTe2 films

Nickel ditelluride (NiTe2), a recently discovered Type-II Dirac semimetal with topological Dirac fermions near the Fermi energy, is expected to exhibit strong thickness-mediated electronic tunability and intrinsic two-gap superconductivity in the single-layer limit. Realizing such intriguing phenomena requires the fabrication of ultrathin NiTe2 films and an understanding of the underlying physics that is still under debate. By conducting experimental band mappings of ultrathin films prepared with molecular beam epitaxy, we reveal spectroscopic evidence for the dimensionality crossover of single-crystalline ultrathin NiTe2 films as a function of film thickness. As the film thickness increases from one to five layers, the gap in the conical topological surface states closes. Comparisons of experimental to first-principles results also highlight difficulties in fabricating atomically smooth single-layer NiTe2 films. Our results not only provide further impetus for studying emergent phenomena in NiTe2 but also underscore the limitations of fabricating NiTe2 films for device applications.

Fermiology and electron-phonon coupling in the 2H and 3R polytypes of NbS2

We investigate the electronic structure of the 2H and 3R polytypes of NbS$_2$. The Fermi surfaces measured by angle-resolved photoemission spectroscopy show a remarkable difference in size, reflecting a significantly increased band filling in 3R-Nb$_{1+x}$S$_2$ compared to 2H-NbS$_2$, which we attribute to the presence of additional interstitial Nb which act as electron donors. Thus we find that the stoichiometry, rather than the stacking arrangement, is the most important factor in the difference in electronic and physical properties of the two phases. Our high resolution data on the 2H phase shows kinks in the spectral function that are fingerprints of the electron-phonon coupling. However, the strength of the coupling is found to be much larger for the the sections of bands with Nb 4$d_{x^2-y^2,xy}$ character than for the Nb 4$d_{3z^2-r^2}$. Our results provide an experimental framework for interpreting the two-gap superconductivity and "latent" charge density wave in 2H-NbS$_2$.

The Upper Critical Field, Anisotropy, and Critical Current Density of Superconducting LaO1-xBiS2 with x = 0.07

The slice-shaped LaO1-xBiS2 (x = 0.07) superconducting single crystal was grown by a high-temperature flux method. The X-ray diffraction analysis and symmetric M(H) curves along the H-axis indicate no any other impurities are involved in current LaO1-xBiS2 sample. The M(T) curves show the suppression of the Tconset and Tirr with increasing magnetic field applied parallel to the c-axis which is more noteworthy than that under magnetic field perpendicular to the c-axis. The Hc2(T) curves show upward curvature near Tc, which cannot be described by the one-gap model, and similar Hc2(T) curves are observed in REO1-xFxBiS2 and captured by the two-gap model, suggesting LaO1-xBiS2 may be a two-gap superconductor. The superconducting anisotropy parameter (2.8~5.58) of LaO1-xBiS2 (x = 0.07) is estimated by different methods. The structural distortion plays an important role in the smaller superconducting anisotropy parameter of LaO1-xBiS2. The critical current density in the self-field is about 5.9 × 103 Acm−2 and 6.7 × 103 Acm−2 for H//c and H⊥c, respectively.

One or two gaps in Mo8Ga41 superconductor? Local Hall-probe magnetometry study

The magnetization properties of the endohedral cluster superconductor Mo8Ga41 are studied by sensitive Hall-probe magnetometry. The temperature dependence of the lower critical magnetic field H c1 is obtained and compared to theoretical models accounting for single-gap and two-gap superconductivity. Data can be described by both models with minor differences. These results are confronted with our previous measurements which evidence that the system is inherently single-gap s-wave superconductor but minor additional phases are present also in seemingly perfect samples. We discuss how presence of such additional phases with varying size of the penetration depth and H c1 field is reflected in magnetization measurements which could lead to misinterpretation of the multigap superconductivity in Mo8Ga41.

Theoretical study on the magnetic properties of the superconductor FeSe

Identifying the form of superconducting order parameter is still a controversial problem for the iron-based superconductor FeSe. Based on anisotropic two-component Ginzburg–Landau theory, we study the temperature dependence of upper critical field and London penetration depth for FeSe. Without including the spin paramagnetic effect, all of our theoretical calculations fit the experimental data well in a broad temperature range. Our results thus show that FeSe is a two-gap s-wave superconductor. And the anisotropy of effective masses in the band with larger (or smaller) gap can be estimated as about 10 (or 2) respectively.

Evidences for pressure-induced two-phase superconductivity and mixed structures of NiTe2 and NiTe in type-II Dirac semimetal NiTe2-x (x = 0.38 ± 0.09) single crystals

Bulk NiTe2 is a type-II Dirac semimetal with non-trivial Berry phases associated with the Dirac fermions. Theory suggests that monolayer NiTe2 is a two-gap superconductor, whereas experimental investigation of bulk NiTe1.98 for pressures (P) up to 71.2 GPa do not reveal any superconductivity. Here we report experimental evidences for pressure-induced two-phase superconductivity as well as mixed structures of NiTe2 and NiTe in Te-deficient NiTe2-x (x = 0.38 ± 0.09) single crystals. Hole-dominant multi-band superconductivity with the P3¯m1 hexagonal-symmetry structure of NiTe2 appears at P ≥ 0.5 GPa, whereas electron-dominant single-band superconductivity with the P2/m monoclinic-symmetry structure of NiTe emerges at 14.5 GPa < P < 18.4 GPa. The coexistence of hexagonal and monoclinic structures and two-phase superconductivity is accompanied by a zero Hall coefficient up to ∼ 40 GPa, and the second superconducting phase prevails above 40 GPa, reaching a maximum Tc = 7.8 K and persisting up to 52.8 GPa. Our findings suggest the critical role of Te-vacancies in the occurrence of superconductivity and potentially nontrivial topological properties in NiTe2-x.

Струмовий шум у металевих контактах на основі двощілинних надпровідників

Струмовий шум у металевих контактах на основі двощілинних надпровідників

Three-dimensional Fermi surface and small effective masses in Mo8Ga41

We report Fermi surface characteristics of Mo8Ga41, a two-gap superconductor with a critical temperature of Tc∼ 10 K, obtained from quantum oscillation measurements. Four major frequencies have been observed with relatively small quasiparticle masses. The angular dependence of major frequencies indicates three-dimensional Fermi surface sheets. This argues for a relatively isotropic superconducting state and, given its relatively high Tc, shows that a search for materials in this class could be of interest for superconducting wire applications.

Unconventional superconductivity in the nickel chalcogenide superconductor TlNi2Se2

We present the results of a study of the vortex lattice (VL) of the nickel chalcogenide superconductor TlNi2Se2, using small angle neutron scattering. This superconductor has the same crystal symmetry as the iron arsenide materials. Previous work points to it being a two-gap superconductor, with an unknown pairing mechanism. No structural transitions in the vortex lattice are seen in the phase diagram, arguing against d-wave gap symmetry. Empirical fits of the temperature-dependence of the form factor and penetration depth rule out a simple s-wave model, supporting the presence of nodes in the gap function. The variation of the VL opening angle with field is consistent with earlier reports of of multiple gaps.

Emergent superconductivity in two-dimensional NiTe2 crystals

Despite growing interest in them, highly crystalline two-dimensional superconductors derived from exfoliated layered materials are few. Employing the anisotropic Migdal-Eliashberg formalism based on {\it ab initio} calculations, we find monolayer NiTe$_{2}$ to be an intrinsic superconductor with a $T_{\text c}\sim$5.7~K, although the bulk crystal is not known to superconduct. Remarkably, bilayer NiTe$_{2}$ intercalated with lithium is found to display two-gap superconductivity with a critical temperature $T_{\text{c}}\sim 11.3$~K and superconducting gap of $\sim$3.1~meV, arising from a synergy of electronic and phononic effects. The comparatively high $T_\text{c}$, substrate independence and proximity tunability will make these superconductors ideal platforms for exploring intriguing correlation effects and quantum criticality associated two-dimensional superconductivity.

MgB4 trilayer film: A four-gap superconductor

Since the discovery of two-gap superconductivity in bulk ${\mathrm{MgB}}_{2}$, many efforts have been devoted to searching for multigap superconductors. Recently, a monolayer of ${\mathrm{MgB}}_{2}$ and a trilayer film of ${\mathrm{AlB}}_{4}$ were predicted to be robust three-gap superconductors. Here we investigate using the fully anisotropic Migdal-Eliashberg theory the superconducting behaviors in a series of boron-based monolayers and trilayer films that are dynamically stable but not synthesized experimentally heretofore. Remarkably, we find that the trilayer film of ${\mathrm{MgB}}_{4}$ has an exciting four-gap superconducting nature with a high transition temperature ${T}_{c}\ensuremath{\sim}52$ K. We demonstrate that this high-${T}_{c}$ four-gap superconductivity is closely related to the Fermi surface exhibiting an unusually strong electron-phonon coupling with evident four-region distribution characteristics. The electron-lattice coupling is dominated by the acoustic modes of a softening kink and the high-energy optical phonons around the Brillouin-zone center, of which the former ones place the system on the verge of a latent charge density wave state. To our knowledge, no material with more than three superconducting gaps has been reported. At the same time, we also show that the ${\mathrm{GaB}}_{4}$ trilayer film has a robust three-gap nature with ${T}_{c}\ensuremath{\sim}51$ K, and the trilayer films of ${\mathrm{BeB}}_{4}$ and ${\mathrm{LiB}}_{4}$ are two-gap superconductors with ${T}_{c}$'s of $\ensuremath{\sim}39$ and 32 K, respectively.

Prediction of superconductivity in pressure-induced new silicon boride phases

The crystal structures and properties of boron-silicon (B-Si) compounds under pressure have been systematically explored using particle swarm optimization structure prediction method in combination with first-principles calculations. Three new stoichiometries, ${\mathrm{B}}_{2}\mathrm{Si}$, BSi, and ${\mathrm{BSi}}_{2}$, are predicted to be stable gradually under pressure, where increasing pressure favors the formation of silicon rich B-Si compounds. In the boron-rich compounds, the network of boron atoms changes from ${\mathrm{B}}_{12}$ icosahedron in the ambient phases to the similar buckled graphenelike layers in the high-pressure phases, which crystalize in the same $P\overline{3}m1$ symmetry but with different numbers of boron layers between adjacent silicon layers. Phonon calculations show that these structures might be retained to ambient conditions as metastable phases. Further electron-phonon coupling calculations indicate that the high-pressure phases of boron-rich compounds might superconduct at 1 atm, with the highest ${T}_{c}$ value of 21 K from the Allen-Dynes equation in $P\overline{3}m1$ ${\mathrm{B}}_{2}\mathrm{Si}$, which is much higher than the one observed in boron doped diamond-type silicon. Moreover, further fully anisotropic Migdal-Eliashberg calculations indicate that ${\mathrm{B}}_{2}\mathrm{Si}$ is a two-gap anisotropic superconductor and the estimated ${T}_{c}$ might reach up to 30 K at 1 atm. On the silicon-rich side, ${\mathrm{BSi}}_{2}$ is predicted to be stable in the ${\mathrm{CuAl}}_{2}$-type structure. Our current results significantly enrich the phase diagram of the B-Si system and will stimulate further experimental study.

Time-dependent Ginzburg–Landau equations for multi-gap superconductors**Project supported by the National Natural Science Foundation of China (Grant Nos. 11564003 and 11865005) and the Natural Science Foundation of Guangxi Province of China (Grant No. 2018GXNSFAA281024).

We numerically solve the time-dependent Ginzburg–Landau equations for two-gap superconductors using the finiteelement technique. The real-time simulation shows that at low magnetic field, the vortices in small-size samples tend to form clusters or other disorder structures. When the sample size is large, stripes appear in the pattern. These results are in good agreement with the previous experimental observations of the intriguing anomalous vortex pattern, providing a reliable theoretical basis for the future applications of multi-gap superconductors.

Effective action approach to the Leggett's mode in two-gap superconductors

When electrons of two electronic bands participate in superconducting phenomena, it is said to be two gap superconductor. There are two set of cooper pairs in different energy gap with different energy. The observation of Leggett’s mode in two band superconductor provides an additional information about superconductor. By using the effective action, the thermodynamic potential in the case of neutral and charged two gap superconductor are calculated. Using phase dependent action, we investigate a collective excitation (Leggett’s mode) corresponding to small fluctuations of the relative phase of two condensates in two band superconductor. We consider the possibility of observing Leggett’s mode in MgB2 superconductor and conclude that for the known values of two band model parameters for MgB2, Leggett’s mode rises above the two particle threshold.&#x0D; BIBECHANA 17 (2020) 74-79

Tunneling-Spectroscopy Evidence for Two-Gap Superconductivity in a Binary Mo-Re Alloy

Expectation of fundamentally new phenomena owing to multiband superconductivity stimulates a search for novel materials with, at least, two distinct gaps in the superconducting state. Recent temperature-dependent electronic heat capacity and lower critical field measurements for molybdenum-rhenium alloys have revealed signatures of two-gap superconductivity. In this letter, we report a direct evidence of two superconducting energy gaps in a Mo1-xRex alloy films probed by single-electron tunneling spectroscopy. Earlier, it was found that the superconducting transition temperature Tc in the alloys with very close atomic concentrations of Mo and Re is about an order of magnitude higher than Tc’s of the constituent elements. Our observation of two distinct gaps in this system testifies in favor of the scenario that relates this enhancement to the interband interaction rather than to specific modifications of the molybdenium phonon spectrum after alloying with rhenium, i.e., a heavy mass defect.

Зарядовый транспорт в гетероструктурах на основе двухщелевых сверхпроводников

Зарядовый транспорт в гетероструктурах на основе двухщелевых сверхпроводников