S-wave Superconductor Research Articles

Correlation of Yu-Shiba-Rusinov States and Kondo Resonances in Artificial Spin Arrays on an s-Wave Superconductor.

Mutually interacting magnetic atoms coupled to a superconductor have gained enormous interest due to their potential for the realization of topological superconductivity. Individual magnetic impurities produce states within the superconducting energy gap known as Yu–Shiba–Rusinov (YSR) states. Here, using the tip of a scanning tunneling microscope, we artificially craft spin arrays consisting of an Fe adatom interacting with an assembly of interstitial Fe atoms (IFA) on a superconducting oxygen-reconstructed Ta(100) surface and show that the magnetic interaction between the adatom and the IFA assembly can be tuned by adjusting the number of IFAs in the assembly. The YSR state experiences a characteristic crossover in its energetic position and particle–hole spectral weight asymmetry when the Kondo resonance shows spectral depletion around the Fermi energy. By the help of slave-boson mean-field theory (SBMFT) and numerical renormalization group (NRG) calculations we associate the crossover with the transition from decoupled Kondo singlets to an antiferromagnetic ground state of the Fe adatom spin and the IFA assembly effective spin.

Local magnetism induced by non-magnetic impurities in FeSe in proximity to s-wave superconductivity

The response of non-magnetic impurities to superconductivity is useful to characterize the superconducting pairing symmetry. The s-wave superconductivity is not affected by the presence of non-magnetic impurities. However, the unconventional superconductivity responds to the non-magnetic impurities, inducing in-gap states in the superconducting gap. However, this characterization fails if non-magnetic impurities could induce magnetic moments in superconductors. Here, we used scanning tunneling microscopy/spectroscopy to elucidate if non-magnetic impurities are irrelevant to magnetism in FeSe. To study this, we have grown FeSe films on the Pb(111) substrate. We find that the FeSe films are proximity-induced s-wave superconductors. By investigating various non-magnetic impurities and native defects of FeSe, we explicitly show that these impurities and defects can directly induce local magnetism in FeSe.

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.

S-Wave Superconductivity in the Dirac Line-Nodal Material CaSb2

We performed 121/123Sb-nuclear quadrupole resonance (NQR) measurements on the superconducting (SC) line-nodal material CaSb2 in order to investigate electronic properties in the normal and SC states from a microscopic point of view. In the normal state, the nuclear spin-lattice relaxation rate 1/T1 for the Sb(1) site, which is responsible for the line-nodal parts, is approximately proportional to temperature, indicating the conventional Fermi liquid state. From comparison with band structure calculations, it is considered that the NQR properties related to the line-nodal character are hidden because the conventional behavior originating from Fermi-surface parts away from the nodes is dominant. In the SC state, a clear coherence peak just below the transition temperature and an exponential decrease at lower temperatures were observed in 1/T1. These results strongly suggest that conventional s-wave superconductivity with a full gap is realized in CaSb2.

Tunable topological states hosted by unconventional superconductors with adatoms

Chains of magnetic atoms, placed on the surface of s-wave superconductors, have been established as a laboratory for the study of Majorana bound states. In such systems, the breaking of time reversal due to magnetic moments gives rise to the formation of in-gap states, which hybridize to form one-dimensional topological superconductors. However, in unconventional superconductors even non-magnetic impurities induce in-gap states since scattering of Cooper pairs changes their momentum but not their phase. Here, we propose a path for creating topological superconductivity, which is based on an unconventional superconductor with a chain of non-magnetic adatoms on its surface. The topological phase can be reached by tuning the magnitude and direction of a Zeeman field, such that Majorana zero modes at its boundary can be generated, moved and fused. To demonstrate the feasibility of this platform, we develop a general mapping of films with adatom chains to one-dimensional lattice Hamiltonians. This allows us to study unconventional superconductors such as Sr$_2$RuO$_4$ exhibiting multiple bands and an anisotropic order parameter.

Holographic p-wave superconductivity from higher derivative theory

We construct a holographic SU(2) p-wave superconductor model with Weyl corrections. The high derivative (HD) terms do not seem to spoil the generation of the p-wave superconducting phase. We mainly study the properties of AC conductivity, which is absent in holographic SU(2) p-wave superconductor with Weyl corrections. The conductivities in superconducting phase exhibit obvious anisotropic behaviors. Along y direction, the conductivity sigma _{yy} is similar to that of holographic s-wave superconductor. The superconducting energy gap exhibits a wide extension. For the conductivity sigma _{xx} along x direction, the behaviors of the real part in the normal state are closely similar to that of sigma _{yy}. However, the anisotropy of the conductivity obviously shows up in the superconducting phase. A Drude-like peak at low frequency emerges in Resigma _{xx} once the system enters into the superconducting phase, regardless of the behaviors in normal state.

S-Wave Superconductivity in Kagome Metal CsV3Sb5 Revealed by 121/123Sb NQR and 51V NMR MeasurementsSupported by the National Key Research and Development Program of China (Grant Nos. 2017YFA0302901, 2018YFE0202600, and 2016YFA0300504), the National Natural Science Foundation of China (Grant Nos. 11921004, 11634015, 11822412, and 11774423), the Beijing Natural Science Foundation (Grant No. Z200005), and the Strategic Priority Research Program and Key

We report 121/123Sb nuclear quadrupole resonance (NQR) and 51V nuclear magnetic resonance (NMR) measurements on kagome metal CsV3Sb5 with T c = 2.5 K. Both 51V NMR spectra and 121/123Sb NQR spectra split after a charge density wave (CDW) transition, which demonstrates a commensurate CDW state. The coexistence of the high temperature phase and the CDW phase between 91 K and 94 K manifests that it is a first-order phase transition. At low temperature, electric-field-gradient fluctuations diminish and magnetic fluctuations become dominant. Superconductivity emerges in the charge order state. Knight shift decreases and 1/T 1 T shows a Hebel–Slichter coherence peak just below T c, indicating that CsV3Sb5 is an s-wave superconductor.

Evidence of zero-point fluctuation of vortices in a very weakly pinned a -MoGe thin film

In a Type II superconductor, the vortex core behaves like a normal metal. Consequently, the single-particle density of states in the vortex core of a conventional Type II superconductor remains either flat or (for very clean single crystals) exhibits a peak at zero bias due to the formation of Caroli-de Gennes-Matricon bound state inside the core. Here we report an unusual observation from scanning tunneling spectroscopy measurements in a weakly pinned thin film of the conventional s-wave superconductor a-MoGe, namely, that a soft gap in the local density of states continues to exist even at the center of the vortex core. We ascribe this observation to rapid fluctuation of vortices about their mean position that blurs the boundary between the gapless normal core and the gapped superconducting region outside. Analyzing the data as a function of magnetic field we show that the variation of fluctuation amplitude as a function of magnetic field is consistent with quantum zero-point motion of vortices.

Large microwave inductance of granular boron-doped diamond superconducting films

Boron-doped diamond granular thin films are known to exhibit superconductivity with an optimal critical temperature of Tc=7.2 K. Here, we report the measured in-plane complex surface impedance of boron-doped diamond films in the microwave frequency range using a resonant technique. Experimentally measured inductance values are in good agreement with estimates obtained from the normal state sheet resistance of the material. The magnetic penetration depth temperature dependence is consistent with that of a fully gapped s-wave superconductor. Boron-doped diamond films should find application where high kinetic inductance is needed, such as microwave kinetic inductance detectors and quantum impedance devices.

Influence of the Fermi surface geometry on the Josephson effect between iron-pnictide and conventional superconductors

We study Josephson junctions between a multi-band iron-pnictide Ba1-xNaxFe2As2 and conventional s-wave superconductors Nb and Cu/Nb bilayer. We observe that junctions with a Cu interlayer exhibit much larger IcRn, despite a weaker proximity-induced superconductivity. This counterintuitive result is attributed to the difference in Fermi surface geometries of Nb and Cu, which leads to a selective one-band tunneling from Cu and a non-selective multi-band tunnelng from Nb. The latter leads to a mutual cancellation of supercurrents due to the sign-reversal s+- symmetry of the order parameter in the pnictide. Our results indicate that Fermi surface geometries play a crucial role for pnictide-based junctions. This provides a new tool for phase sensitive studies and paves a way to a conscious engineering of such junctions.

Superconductivity of centrosymmetric and non-centrosymmetric phases in antiperovskite (Ca,Sr)Pd3P

In the recently discovered antiperovskite phosphide (Ca,Sr)Pd3P, centrosymmetric (CS) and non-centrosymmetric (NCS) superconducting phases appear depending on the Sr concentration, and their transition temperatures (Tc) differ by as much as one order of magnitude. In this study, we investigated the superconducting properties and electronic band structures of CS orthorhombic (CSo) (Ca0.6Sr0.4)Pd3P (Tc = 3.5 K) and NCS tetragonal (NCSt) (Ca0.25Sr0.75)Pd3P (Tc = 0.32 K) samples with a focus on explaining their large Tc difference. Specific heat measurements indicated that CSo (Ca0.6Sr0.4)Pd3P was an s-wave superconductor in a moderate-coupling regime with a 2Δ0/kBTc value of 4.0. Low-lying phonons leading to the strong coupling in the structurally analogous SrPt3P were unlikely to be present in CSo (Ca0.6Sr0.4)Pd3P. Given that CSo (Ca0.6Sr0.4)Pd3P and NCSt (Ca0.25Sr0.75)Pd3P exhibited similar Debye temperatures (ΘD) of approximately 200 K, the large Tc difference could not be attributed to ΘD. Tc of each phase was accurately reproduced based on the Bardeen–Cooper–Schrieffer (BCS) theory using experimental data and the density of states of the Fermi level N(0) calculated from their band structures. We concluded that the considerable suppression of Tc in NCSt (Ca0.25Sr0.75)Pd3P can be primarily attributed to the decrease in N(0) associated with the structural phase transition without considering the lack of inversion symmetry.

Merohedral disorder and impurity impacts on superconductivity of fullerenes

Local quasiparticle states around impurities provide essential insight into the mechanism of unconventional superconductivity, especially when the candidate materials are proximate to an antiferromagnetic Mott-insulating phase. While such states have been reported in atom-based cuprates and iron-based compounds, they are unexplored in organic superconductors which feature tunable molecular orientation. Here we employ scanning tunneling microscopy and spectroscopy to reveal multiple forms of robustness of an exotic s-wave superconductivity in epitaxial Rb3C60 films against merohedral disorder, non-magnetic single impurities and step edges at the atomic scale. Yu-Shiba-Rusinov (YSR) states, induced by deliberately incurred Fe adatoms that act as magnetic scatterers, have also been observed. The YSR bound states show abrupt spatial decay and vary in energy with the Fe adatom registry. These results and a doping-dependent study of superconductivity point towards local electron pairing in which the multiorbital electronic correlations and intramolecular phonons together drive the high-temperature superconductivity of doped fullerenes.

Topological Shiba bands in artificial spin chains on superconductors

A major challenge in developing topological superconductors for implementing topological quantum computing is their characterization and control. It has been proposed that a p-wave gapped topological superconductor can be fabricated with single-atom precision by assembling chains of magnetic atoms on s-wave superconductors with spin-orbit coupling. Here, we analyze the Bogoliubov quasiparticle interference in atom-by-atom constructed Mn chains on Nb(110) and for the first time reveal the formation of multi-orbital Shiba bands using momentum resolved measurements. We find evidence that one band features a topologically non-trivial p-wave gap as inferred from its shape and particle-hole asymmetric intensity. Our work is an important step towards a distinct experimental determination of topological phases in multi-orbital systems by bulk electron band structure properties only.

Superconductivity in amorphous RexZr (x≈6) thin films

We report the growth, characterization and superconducting properties of thin films of a new amorphous superconductor, RexZr ( x≈6). Films were grown by pulsed laser deposition with the substrate kept at room temperature. Films with thicknesses larger than 40 nm showed a superconducting transition temperature (Tc) of 5.9 K. Superconducting properties were measured for films with varying thickness from 120 to 3 nm. The normal state resistance scaled linearly with the inverse of thickness. The transition temperature, critical field, coherence length, penetration depth and superconducting energy gap changed marginally with decreasing film thickness down to 8 nm. Scanning tunneling spectroscopy and penetration depth measurements provide evidence for a single gap strong coupling s-wave superconductor.

Anisotropic thermoelectric and superconducting properties of the bulk misfit-layered (SnSe)1.17(TaSe2) compound

We present the thermoelectric and superconducting properties of the bulk misfit-layered (SnSe)1.17(TaSe2) compound with the critical temperature (Tc ~ 3.8 K). From XRD and electrical resistivity, the anisotropic properties of the misfit-layered compound by the preferred orientation were observed. The zero-temperature-limit upper critical field Hc2(0) and coherence length ξ are obtained by Hc2(0) = 8.94 T and ξ = 6.1 nm. The electron-phonon coupling constant (λe-p = 0.735) and the specific heat jump (ΔCe/γTc ~ 1.4) imply that the (SnSe)1.17(TaSe2) compound is a weak coupled conventional s-wave superconductor. Density functional theory (DFT) calculation results show that some electronic charge transfers from the SnSe layers into the TaSe2 layers. In addition, the thermoelectric properties of the bulk misfit-layered (SnSe)1.17(TaSe2) compound show significant anisotropic properties.

Landau-Fermi liquidness and $$s$$-wave superconducting properties of pressurized gray phosphorus

We investigate the physical properties of compressed gray phosphorus through density functional plus dynamical mean-field theory, showing self-doping and s-wave electronic structure reconstruction. At commensurate electron density, the 3p spectrum is shown to be almost unaffected by electronic correlations, however upon self-doping largely the normal and superconducting states. These findings provide a microscopic understanding of pressure-induced hole carrier superconductivity on the normal state coherence of s-wave superconductors with well-defined Bogoliubov quasiparticles at low temperatures. Upon internal thermalization, the s-wave superconducting state loses its phase coherence.

Fabrication of Corner-Like Josephson Junctions Based on Pnictide Single crystals

In order to study fundamental properties of pnictide superconductors such as the pairing symmetry, we have fabricated corner-like Josephson devices. Their configuration contains two SNS* (superconductor/barrier/superconductor*) thin-film junctions in different crystal directions. We have used multi-band Co-doped Ba-122 single crystals as the base electrode (S), evaporated lead films as a counter electrode (S*) made of a conventional s-wave superconductor and normal-metal (N) layers as the barrier. The junctions exhibit symmetric I-V characteristics with IcRN values about 0.975 μV. Ic is the critical current, and RN is the normal-state resistance. The observed Fraunhofer pattern-like dependence of Ic on the magnetic field indicates homogeneity of the junctions studied.

Competing Energy Scales in Topological Superconducting Heterostructures.

Artificially engineered topological superconductivity has emerged as a viable route to create Majorana modes. In this context, proximity-induced superconductivity in materials with a sizable spin–orbit coupling has been intensively investigated in recent years. Although there is convincing evidence that superconductivity may indeed be induced, it has been difficult to elucidate its topological nature. Here, we engineer an artificial topological superconductor by progressively introducing superconductivity (Nb), strong spin–orbital coupling (Pt), and topological states (Bi2Te3). Through spectroscopic imaging of superconducting vortices within the bare s-wave superconducting Nb and within proximitized Pt and Bi2Te3 layers, we detect the emergence of a zero-bias peak that is directly linked to the presence of topological surface states. Our results are rationalized in terms of competing energy trends which are found to impose an upper limit to the size of the minigap separating Majorana and trivial modes, its size being ultimately linked to fundamental materials properties.

Inhomogeneous superconductivity in LuxZr1−xB12 dodecaborides with dynamic charge stripes

We have studied the normal and superconductive state characteristics (resistivity, Hall coefficient, heat capacity and magnetization) of model strongly correlated electronic systems LuxZr1-xB12 with cooperative Jahn-Teller instability of the boron rigid cage and with dynamic charge stripes. It was found that these metals are s-wave dirty limit superconductors with a small mean free path of charge carriers l = 5 - 140 A and with a Cooper pair size changing non-monotonously in the range 450 - 4000 A. The parent ZrB12 and LuB12 borides are type-I superconductors, and Zr to Lu substitution induces a type-I - type-II phase transition providing a variation of the Ginzburg-Landau-Maki parameter in the limits 0.65 < kappa1,2 < 6. We argue in favor of the two-band scenario of superconductivity in LuxZr1-xB12 with gap values Delta_1 ~ 14 K and Delta_2 ~ 6 - 8 K, with pairing corresponding to strong coupling limit (lambda_e-ph ~ 1) in the upper band, and to weak coupling (lambda_e-ph ~ 0.1 - 0.4) in the lower one. A pseudogap Delta_ps-gap ~ 60 - 110 K is observed in LuxZr1-xB12 above Tc. We discuss also the possibility of anisotropic single-band superconductivity with stripe-induced both pair-breaking and anisotropy, and analyze the origin of unique enhanced surface superconductivity detected in these model compounds.

Interface and bulk superconductivity in superconducting heterostructures with enhanced critical temperatures

We consider heterostructures obtained by stacking layers of two s-wave superconductors with significantly different coupling strengths, respectively in the weak- and strong-coupling regimes. The weak- and strong-coupling superconductors are chosen with similar critical temperatures for bulk systems. Using dynamical mean-field theory methods, we find an ubiquitous enhancement of the superconducting critical temperature for all the heterostructures where a single layer of one of the two superconductors is alternated with a thicker multilayer of the other. Two distinct physical regimes can be identified as a function of the thickness of the larger layer: (i) an inherently inhomogeneous superconductor characterized by the properties of the two isolated bulk superconductors where the enhancement of the critical temperature is confined to the interface and (ii) a bulk superconductor with an enhanced critical temperature extending to the whole heterostructure. We characterize the crossover between these regimes in terms of the competition between two length scales connected with the proximity effect and the pair coherence.