Strong-coupling Superconductivity Research Articles

From weak- to strong-coupling superconductivity in the AlB2-type solid solution SrGa1−xAlxGe with honeycomb layers

We report on the structure and the superconducting properties of 9-electron 111 compounds with honeycomb layers, namely SrGaGe, SrAlGe, and the SrGa1-xAlxGe solid solution. By means of single-crystal x-ray diffraction we show that, on one hand, SrGaGe crystallizes into the centrosymmetricP6/mmmspace group (a= 4.2555(2) Å,c= 4.7288(2) Å) with statistical disorder in the [GaGe]62-honeycomb layers. On the other hand, we confirm that SrAlGe crystallizes in a non-centrosymmetric space group, namelyP6¯m2(a= 4.2942(1) Å,c= 4.7200(2) Å) with fully ordered [AlGe]62-honeycomb layers. By using magnetization and specific heat measurements, we show that the superconducting properties of SrGaGe and SrAlGe differ significantly from each other. SrGaGe is a superconductor with a critical temperature ofTc= 2.6 K falling into the weak coupling limit, while SrAlGe has aTc= 6.7 K and can be classified in the strong coupling limit. By realizing the SrGa1-xAlxGe solid solution, we were able to investigate the transition between the different crystal structures as well as the evolution of the electronic properties. We show that the transition from the weak-to strong-coupling superconductivity in this system is likely associated with the disorder-to-order transition of the honeycomb layer, along with the loss of the inversion center in the crystal structure.

Ultrafast dynamics evidence of strong coupling superconductivity in LaH10±δ

Recently, tremendous research interest has been aroused in clathrate superhydrides. However, their microscopic properties, especially the superconducting (SC) gap and electron-phonon coupling (EPC) strength, are largely unexplored experimentally. Here, we investigate the time-resolved ultrafast spectroscopy of a superconductor LaH10±δ under an ultrahigh pressure of 165 GPa. By analyzing the ultrafast dynamics of the quasiparticles, we experimentally obtain the SC gap Δ(0) = 53 ± 5 meV, revealing a gap ratio 2Δ(0)/kBTc = 5.6 and a gap parameter ϑ = 1.95. Significantly, we experimentally estimate λ⟨Ω2⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda \\langle {\\Omega }^{2}\\rangle$$\\end{document} = (2.4 ± 0.1) × 104 (meV)2, which corresponds to an EPC strength λ = 2.58 ± 0.11. These results together provide direct experimental evidence that strong EPC is responsible for the near-room-temperature superconductivity in clathrate superhydrides. Our investigation significantly advances the experimental exploration of superhydrides, and contributes to the ultrafast dynamics investigations of quantum materials under high pressures.

From weak to strong-coupling superconductivity tuned by substrate in TiN films

Abstract The interplay between substrates and superconducting thin films has attracted increasing attention. Here, we report an in-depth investigation on superconducting properties of the epitaxial TiN thin films grown on three different substrates by dc reactive magnetron sputtering. The TiN films grown on (0001) sapphire exhibit (111) crystal orientation, while that grown on (100) Si and MgO substrates exhibit (100) orientation. Moreover, the samples grown on Si reveal a relatively lower level of disorder, accompanied by the higher critical transition temperature Tc and smaller magnitude of upper critical field slope near Tc. Remarkably, we uncovered a rather high value of superconducting gap (with Δ0/kBTc = 3.05) in TiN film on Si indicating a very strong coupling superconductivity, in sharp contrast to the case using sapphires and MgO as the substrate which reveals a weak-coupling feature. The comprehensive analysis considering the scenarios of the three substrate shows that the grain size of the thin films may be an important factor influencing the superconductivity.

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.

Strong-coupling superconductivity with Tc above 70 K in Be-decorated monolayer T-graphene

Strong-coupling superconductivity with Tc above 70 K in Be-decorated monolayer T-graphene

Multiband strong-coupling superconductors with spontaneously broken time-reversal symmetry

Multiband strong-coupling superconductors with spontaneously broken time-reversal symmetry

Superconductivity of Ta-Hf and Ta-Zr alloys: Potential alloys for use in superconducting devices

The electronic properties relevant to superconductivity are reported for bulk Ta-Hf and Ta-Zr body centered cubic alloys, in a large part to determine whether their properties are suitable for potential use in superconducting qbits. The body centered cubic unit cell sizes increase with increasing alloying. The results of magnetic susceptibility, electrical resistivity, and heat capacity characterization are reported. While elemental Ta is a type I superconductor, the alloys are type II strong coupling superconductors. Although decreasing the electron count per atom is expected to increase the density of electronic states at the Fermi level and thus the superconducting transition temperature (${T}_{c}$) in these systems, we find that this is not sufficient to explain the significant increases in the superconducting ${T}_{c}$'s observed.

Mechanism for fluctuating pair density wave

In weakly coupled BCS superconductors, only electrons within a tiny energy window around the Fermi energy, EF, form Cooper pairs. This may not be the case in strong coupling superconductors such as cuprates, FeSe, SrTiO3 or cold atom condensates where the pairing scale, EB, becomes comparable or even larger than EF. In cuprates, for example, a plausible candidate for the pseudogap state at low doping is a fluctuating pair density wave, but no microscopic model has yet been found which supports such a state. In this work, we write an analytically solvable model to examine pairing phases in the strongly coupled regime and in the presence of anisotropic interactions. Already for moderate coupling we find an unusual finite temperature phase, below an instability temperature Ti, where local pair correlations have non-zero center-of-mass momentum but lack long-range order. At low temperature, this fluctuating pair density wave can condense either to a uniform d-wave superconductor or the widely postulated pair-density wave phase depending on the interaction strength. Our minimal model offers a unified framework to understand the emergence of both fluctuating and long range pair density waves in realistic systems.

Pressure-driven evolution of upper critical field and Fermi surface reconstruction in the strong-coupling superconductor Ti4Ir2O

We report pressure-driven evolution of the superconducting transition temperature (${T}_{\mathrm{c}}$) and upper critical field ${B}_{\mathrm{c}2}$ $(0)$ of strong-coupling superconductor ${\mathrm{Ti}}_{4}{\mathrm{Ir}}_{2}\mathrm{O}$ that possesses an unusually large ${B}_{\mathrm{c}2} (0)$ at ambient pressure (AP). Our results reveal an extremely low-pressure coefficients of ${T}_{\mathrm{c}}$, i.e., $d{T}_{c}/dP\ensuremath{\approx}\ensuremath{-}0.047$ K/GPa for $P<15$ GPa and \ensuremath{-}0.017 K/GPa for $15<P<50$ GPa, presumably associated with an inherent large bulk modulus of $\ensuremath{\sim}252$ GPa. Interestingly, we find that its ${B}_{\mathrm{c}2}$ $(0)$ undergoes a smooth crossover at 35.6 GPa from well beyond to less than the Pauli paramagnetic limit ${B}_{\mathrm{p}}^{\mathrm{BCS}}(0)=1.84\phantom{\rule{0.16em}{0ex}}{T}_{c}$; i.e., the ${B}_{\mathrm{c}2}(0)\ensuremath{\approx}18.2 \mathrm{T}=1.7\phantom{\rule{0.16em}{0ex}}{B}_{\mathrm{p}}^{\mathrm{BCS}} (0)$ at AP decreases to $\ensuremath{\sim}5.8 \mathrm{T}=0.88\phantom{\rule{0.16em}{0ex}}{B}_{\mathrm{p}}^{\mathrm{BCS}} (0)$ at 50 GPa. The density-functional calculations predict the possible occurrence of pressure-induced Fermi surface reconstruction for the energy bands near the $K$ point with strong spin-orbit coupling in 31--41 GPa. The present work sheds more light on this intriguing superconductor capable of resisting large external compression and strong magnetic fields.

Evidence for Band Renormalizations in Strong-Coupling Superconducting Alkali-Fulleride Films.

There has been a long-standing debate about the mechanism of the unusual superconductivity in alkali-intercalated fullerides. In this Letter, using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the electronic structures of superconducting K_{3}C_{60} thin films. We observe a dispersive energy band crossing the Fermi level with the occupied bandwidth of about 130meV. The measured band structure shows prominent quasiparticle kinks and a replica band involving the Jahn-Teller active phonon modes, which reflects strong electron-phonon coupling in the system. The electron-phonon coupling constant is estimated to be about 1.2, which dominates the quasiparticle mass renormalization. Moreover, we observe an isotropic nodeless superconducting gap beyond the mean-field estimation (2Δ/k_{B}T_{c}≈5). Both the large electron-phonon coupling constant and large reduced superconducting gap suggest a strong-coupling superconductivity in K_{3}C_{60}, while the electronic correlation effect is suggested by the observation of a waterfall-like band dispersion and the small bandwidth compared with the effective Coulomb interaction. Our results not only directly visualize the crucial band structure but also provide important insights into the mechanism of the unusual superconductivity of fulleride compounds.

The effect of critical coupling constants on superconductivity enhancement

In this study, we propose a phenomenological model to extend McMillan's results on a coupling strength equal to 2. We investigate possible strategies to enhance superconductivity by tuning the phonon frequency, carrier number, or pressure. In particular, we show that the critical coupling constants corresponding to the phonon frequency, carrier number, or pressure determine whether the variation of the critical temperature is positive or negative. These observations explain the contrasting behavior between weak and strong coupling superconductors and are consistent with experimental observations. We also demonstrate the dome observed in the carrier number effect and pressure effect. Additionally, these critical coupling constants systematically separate superconductivity into three regions: weak, intermediate, and strong coupling. We find that the enhancement strategies for weak and strong coupling regions are opposite, but both inevitably bring superconductivity into the intermediate coupling region. Finally, we propose general zigzag methods for intermediate coupling superconductors to further enhance the critical temperature.

Superconductivity nearby quantum critical region in hole-doped organic strange metal κ-(ET)4Hg3-δBr8, δ=11%

The hole-doped organic superconductor κ-(ET)4Hg3-δBr8, (κ-HgBr), where δ=11% and ET=bis(ethylenedithio)tetrathiafulvalene, has been the key to bridge the knowledge gap between half-filled organics and doped cuprate systems. Nonetheless, the isotropic triangular lattice of ET dimers of κ-HgBr is responsible for the magnetic susceptibility and its superconductivity. We have measured zero-field (ZF) muon spin relaxation-rotation (µ+SR) in κ-HgBr showing the ZF-µ+SR relaxation rate from temperature around 10 K down to 0.3 K is temperature-independent. This is consistent with a superconducting state that preserved time-reversal symmetry. There was almost no change in the maximally 100 Oe of transverse-field-µ+SR time spectra, at 0.3 K and above superconducting temperature, T c~4.6(3) K. This suggests that the in-plane London penetration depth, λbc, is longer than a μm order, while we estimate the lower limit of the lower critical field, H c1, to be 30 Oe, although, however, the measurement using another geometric setup is necessary to determine the absolute value of λbc. These could be an indication of a strong-coupling superconductor. A possible mechanism of preserved time-reversal Cooper pairing formation from strong-coupling non-FL metal with geometrical frustration is discussed.

Strong-coupling superconductivity and weak vortex pinning in Ta-doped CsV3Sb5 single crystals

By measuring magnetizations of pristine and Ta-doped ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ single crystals, we have carried out systematic studies on the lower critical field, critical current density, and equilibrium magnetization of this kagome system. The lower critical field has been investigated in the two typical samples, and the temperature-dependent lower critical field obtained in Ta-doped sample can be fitted by using the model with two $s$-wave superconducting gaps yielding the larger gap of $2{\mathrm{\ensuremath{\Delta}}}_{s1}/{k}_{\mathrm{B}}{T}_{\mathrm{c}}=7.9\phantom{\rule{0.28em}{0ex}}(\ifmmode\pm\else\textpm\fi{}1.8)$. This indicates a strong-coupling feature of the V-based superconductors. The measured magnetization hysteresis loops allow us to calculate the critical current density, which shows a very weak bulk vortex pinning. The magnetization hysteresis loops measured in these two kinds of samples can be well described by a recently proposed generalized phenomenological model, which leads to the determination of many fundamental parameters for these superconductors. Our systematic results and detailed analysis conclude that this V-based kagome system has features of strong-coupling superconductivity, relatively large Ginzburg-Landau parameter, and weak vortex coupling.

Titanium doped kagome superconductor CsV3−xTixSb5 and two distinct phases

The vanadium-based kagome superconductor CsV3Sb5 has attracted tremendous attention due to its unexcepted anomalous Hall effect (AHE), charge density waves (CDWs), nematicity, and a pseudogap pair density wave (PDW) coexisting with unconventional strong-coupling superconductivity. The origins of CDWs, unconventional superconductivity, and their correlation with different electronic states in this kagome system are of great significance, but so far, are still under debate. Chemical doping in the kagome layer provides one of the most direct ways to reveal the intrinsic physics, but remains unexplored. Here, we report, for the first time, the synthesis of Ti-substituted CsV3Sb5 single crystals and its rich phase diagram mapping the evolution of intertwining electronic states. The Ti atoms directly substitute for V in the kagome layers. CsV3−xTixSb5 shows two distinct superconductivity phases upon substitution. The Ti slightly-substituted phase displays an unconventional V-shaped superconductivity gap, coexisting with weakening CDW, PDW, AHE, and nematicity. The Ti highly-substituted phase has a U-shaped superconductivity gap concomitant with a short-range rotation symmetry breaking CDW, while long-range CDW, twofold symmetry of in-plane resistivity, AHE, and PDW are absent. Furthermore, we also demonstrate the chemical substitution of V atoms with other elements such as Cr and Nb, showing a different modulation on the superconductivity phases and CDWs. These findings open up a way to synthesise a new family of doped CsV3Sb5 materials, and further represent a new platform for tuning the different correlated electronic states and superconducting pairing in kagome superconductors.

Single crystal growth of iridates without platinum impurities

Iridates have attracted much interest in the last decade for their novel magnetism emerging in the limit of strong spin-orbit coupling and possible unconventional superconductivity. A standard for growing iridate single crystals has been the flux method using platinum crucibles. Here, we show that this widely used method compromises the sample quality by inclusion of platinum impurities. We find that Sr2IrO4 single crystals grown in iridium crucibles show remarkable differences from those grown in platinum crucibles in their sample characterizations using Raman spectroscopy, resistivity, magnetization, optical third harmonic generation, resonant X-ray diffraction, and resonant inelastic X-ray scattering measurements. In particular, we show that several peaks of sizable intensities disappear in the Raman spectra of samples free of platinum impurities, and a significantly larger activation energy is extracted from the resistivity data compared to previously reported values. Furthermore, we find no evidence of the previously reported glide symmetry breaking structural distortions and confirm the I41/acd space group of the lattice symmetry. Although the platinum impurities are not apparent in the magnetic properties and thus went unnoticed in the stoichiometric insulating phase for a long time, their effects can be much more detrimental to transport properties in chemically doped compounds. Therefore, our result suggests using growth methods that avoid platinum impurities for an investigation of intrinsic physical properties of iridates, and possible superconducting phases.

Strong Coupling Superconductivity in Ca-Intercalated Bilayer Graphene on SiC.

The metal-intercalated bilayer graphene has a flat band with a high density of states near the Fermi energy and thus is anticipated to exhibit an enhanced strong correlation effect and associated fascinating phenomena, including superconductivity. By using a self-developed multifunctional scanning tunneling microscope, we succeeded in observing the superconducting energy gap and diamagnetic response of a Ca-intercalated bilayer graphene below a critical temperature of 8.83 K. The revealed high value of gap ratio, 2Δ/kBTc ≈ 5.0, indicates a strong coupling superconductivity, while the variation of penetration depth with temperature and magnetic field indicates an isotropic s-wave superconductor. These results provide crucial experimental clues for understanding the origin and mechanism of superconductivity in carrier-doped graphene.

Strong-coupling superconductivity with Tc ∼ 10.8 K induced by P doping in the topological semimetal Mo5Si3

By performing P doping on the Si sites in the topological semimetal Mo5Si3, we discover strong-coupling superconductivity in Mo5Si3−xPx (0.5 ≤ x ≤ 2.0). Mo5Si3 crystallizes in the W5Si3-type structure with space group of I4/mcm (No. 140), and is not a superconductor itself. Upon P doping, the lattice parameter a decreases while c increases monotonously. Bulk superconductivity is revealed in Mo5Si3−xPx (0.5 ≤ x ≤ 2.0) from resistivity, magnetization, and heat capacity measurements. Tc in Mo5Si1.5P1.5 reaches as high as 10.8 K, setting a new record among the W5Si3-type superconductors. The upper and lower critical fields for Mo5Si1.5P1.5 are 14.56 T and 105 mT, respectively. Moreover, Mo5Si1.5P1.5 is found to be a fully gapped superconductor with strong electron-phonon coupling. First-principles calculations suggest that the enhancement of electron-phonon coupling is possibly due to the shift of the Fermi level, which is induced by electron doping. The calculations also reveal the nontrivial band topology in Mo5Si3. The Tc and upper critical field in Mo5Si3−xPx are fairly high among pseudobinary compounds. Both of them are higher than those in NbTi, making future applications promising. Our results suggest that the W5Si3-type compounds are ideal platforms to search for new superconductors. By examinations of their band topologies, more candidates for topological superconductors can be expected in this structural family.

Characterization of collective excitations in weakly coupled disordered superconductors

Isolated islands in two-dimensional strongly-disordered and strongly-coupled superconductors become optically active inducing sub-gap collective excitations in the ac conductivity. Here, we investigate the fate of these excitations as a function of the disorder strength in the experimentally relevant case of weak electron-phonon coupling. An explicit calculation of the ac conductivity, that includes vertex corrections to restore gauge symmetry, reveals the existence of collective sub-gap excitations, related to phase fluctuations and therefore identified as the Goldstone modes, for intermediate to strong disorder. As disorder increases, the shape of the sub-gap excitation transits from peaked close to the spectral gap to a broader distribution reaching much smaller frequencies. Phase-coherence still holds in part of this disorder regime. The requirement to observe sub-gap excitations is not the existence of isolated islands acting as nano-antennas but rather the combination of a sufficiently inhomogeneous order parameter with a phase fluctuation correlation length smaller than the system size. Our results indicate that, by tuning disorder, the Goldstone mode may be observed experimentally in metallic superconductors based for instance on Al, Sn, Pb or Nb.

Process for preparing catalyst for selective hydrogenation of acetylene to ethylene

In the present paper we report comprehensive analysis of the thermodynamic properties of novel hydrogen dominant potassium hydride superconductor (KH6). Our computations are conducted within the Eliashberg theory which yields quantitative estimations of the most important thermodynamic properties of superconducting phase. In particular, we observe that together with the increasing pressure all the thermodynamic properties decrease, e.g. TC∈〈72.91,55.50〉 K for p∈〈166,300〉 GPa. It is predicted that such decreasing behavior corresponds to the decreasing hydrogen lattice molecularization with increasing pressure value. Furthermore, by calculating the dimensionless thermodynamic ratios, familiar in the Bardeen–Cooper–Schrieffer (BCS) theory, it is proved that KH6 material is a strong-coupling superconductor and cannot be quantitatively described within the BCS theory.

Superconductivity Enhanced by Abundant Low-Energy Phonons in (Sr1−xCax)3Rh4Sn13

The effects of structural quantum criticality on the strong-coupling superconductivity of (Sr1−xCax)3Rh4Sn13 have been investigated via electrical resistivity and specific heat measurements. We demonstrate that the lattice specific heat at low temperatures considerably increases toward the structural quantum critical point (SQCP), xc ≈ 0.9. The superconducting gap increases with x in the exact same fashion as the low-temperature lattice specific heat, clearly indicating that the abundant low-energy phonons cause strong-coupling superconductivity. Despite the electron–phonon interaction, which is much more enhanced than the electron correlation, the low-temperature electrical resistivity in the normal state varies as ∝ T2 near the SQCP. This finding suggests that structural quantum fluctuation affects the electron–phonon scattering.