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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Spectroscopic evidence for the direct involvement of local moments in the pairing process of the heavy-fermion superconductor CeCoIn 5

The microscopic mechanism for electron pairing in heavy-fermion superconductors remains a major challenge in quantum materials. Some form of magnetic mediation is widely accepted with spin fluctuations as a prime candidate. A novel mechanism, “composite pairing” based on the cooperative two-channel Kondo effect directly involving the f -electron moments, has also been proposed for some heavy-fermion compounds including CeCoIn 5 . The origin of the spin-resonance peak observed in neutron-scattering measurements on CeCoIn 5 is still controversial and the corresponding hump-dip structure in the tunneling conductance is missing. This is in contrast to the cuprate and Fe-based high-temperature superconductors, where both characteristic signatures are observed, indicating spin fluctuations are likely involved in the pairing process. In this study, we report results from planar tunneling spectroscopy along three major crystallographic orientations of CeCoIn 5 over wide ranges of temperature and magnetic field. The pairing gap opens at T p ~ 5 K , well above the bulk T c = 2.3 K , and its directional dependence is consistent with d x 2 – y 2 symmetry. With increasing magnetic field, this pairing gap is suppressed as expected but, intriguingly, a gaplike structure emerges smoothly, increasing linearly up to the highest field applied. This field-induced gaplike feature is only observed below T p . The concomitant appearance of the pairing gap and the field-induced gaplike feature, along with its linear increase with field, indicates that the f -electron local moments are directly involved in the pairing process in CeCoIn 5 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Fulde-Ferrell-Larkin-Ovchinnikov pairing induced by a Weyl nodal line in an Ising superconductor with a high critical field

Superconductivity and electron topology are two quantum phenomena that have attracted much interest, but no causal relationship between them has been reported because superconductivity is a many-body effect due to electron-electron interaction, while electron topology is a single-particle manifestation of electron states. In this work, we demonstrate that electron topology can induce Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing in Ising Bardeen-Cooper-Schrieffer (IBCS) superconductors. Specifically, we predict that the nonmagnetic metals of the MA 2 Z 4 family, including α 1 -TaSi 2 P 4 , α 1 -TaSi 2 N 4 , α 2 -TaGe 2 P 4 , α 1 -NbSi 2 P 4 , and α 2 -NbGe 2 P 4 monolayers, are all IBCS superconductors with a transition temperature ranging from a few to tens of degrees Kelvin. The intrinsic IBCS pairing alone will enhance the in-plane critical field B c to ~20–60 times the Pauli limit B p , and the extrinsic FFLO pairing evoked by topological Weyl nodal lines under a magnetic field can further double the B c /B p ratio. Our findings not only enrich the fundamental relationship between superconductivity and electron topology, but they also yield an effective approach to enhance the robustness of superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Piezosuperconductivity: Novel effects in noncentrosymmetric superconductors

Here we study effects in noncentrosymmetric superconductors arising from their unique coupling of Cooper-pair condensate and elasticity. We show that although the much discussed Lifshitz coupling is not observable in a uniform bulk state, it strikingly endows dislocations with a fractional magnetic flux. We also predict a generation of voltage-free strain by a DC current in a P- and T-breaking Josephson junction. Viewing superconductors through the lens of higher-form symmetries we identify the Lifshitz coupling as a chemical potential for the approximately conserved winding number, drawing an analogy with pyroelectric insulators.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Electronic states dressed by an out-of-plane supermodulation in the quasi-two-dimensional kagome superconductor CsV 3 Sb 5

CsV 3 Sb 5 has attracted much recent attention as the first quasi-two-dimensional (2D) kagome superconductor. While the kagome layers are 2D in nature, increasing evidence has pointed to the importance of out-of-plane correlation in this material. However, it remains unclear whether such correlation can change the fundamental electronic structure of the quasi-2D system. Here, we reveal this missing piece of information, using angle-resolved photoemission spectroscopy, complemented by scanning tunneling microscope measurements. The three-dimensional electronic structures in the high-temperature state are revealed, which agree well with density-functional theory calculations. Electron energy bands are observed in the low-temperature state that exhibit additional periodicities along the out-of-plane momentum. Furthermore, these results reveal a direct response to the out-of-plane electronic supermodulation in the single-particle spectral function of CsV 3 Sb 5 , thus establishing an electronic platform to examine emergent phenomena beyond 2D limit in kagome superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Understanding resistance oscillation in the CsV 3 Sb 5 superconductor

A recent demonstration of the periodic oscillation of resistance in the thin film of the CsV 3 ⁢Sb 5 superconductor with a hole in the film suggests that charge-4⁢ e and charge-6⁢⁢ e Cooper pairs may have condensed in this compound. While exciting, such interpretation calls for a precise determination of the effective area for the passage of Cooper pairs from one end of the lead to the other. Unlike the traditional Little-Parks effect where the rim around the hole is thin, the effective hole area is not obviously defined for the “thick-rim geometry” adopted in the experiment. Here, we note that the experiment was conducted in a regime where the superconductivity is strongly fluctuating, which motivates an analysis based on the spacetime formulation of the time-dependent Ginzburg-Landau theory. Further, we argue that under appropriate conditions, the optimal semiclassical path is not the geometrically shortest one, but the one that moves along the edge of the hole and takes advantage of the reduced fluctuations at the boundary of the hole. The condition for the crossover from the geometrically shortest path to the path that sticks to the wall is clarified. In such a scenario, the geometric area of the hole indeed emerges as the effective area for the flux, providing a theoretical justification to the interpretation given by J. Ge et al. (arXiv:2201.10352). The conclusion of our analysis may have implications for similar experiments in other superconductors where the geometry of the device is not obviously that of the Little-Parks experiment employing the thin wall, but of the thick-rim type such as used in the experiment.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Increased localization of Majorana modes in antiferromagnetic chains on superconductors

Magnet-superconductor hybrid (MSH) systems are a key platform for custom-designed topological superconductors. Ideally, the ends of a one-dimensional MSH structure will host Majorana zero-modes (MZMs), the fundamental unit of topological quantum computing. However, some experiments with ferromagnetic (FM) chains show a more complicated picture. Due to tiny gap sizes and hence long coherence lengths, MZMs might hybridize and lose their topological protection. Recent experiments on a niobium surface have shown that both FM and antiferromagnetic (AFM) chains may be engineered, with the magnetic order depending on the crystallographic direction of the chain. While FM chains are well understood, AFM chains are less so. Here, we study two models inspired by the niobium surface: A minimal model to elucidate the general topological properties of AFM chains and an extended model to more closely simulate a real system by mimicking the proximity effect. Here, we find that, in general, for AFM chains, the topological gap is larger than for FM ones, and thus, coherence lengths are shorter for AFM chains, yielding more pronounced localization of MZMs in these chains. While for some parameters AFM chains may be topologically trivial, we find in these cases that adding an adjacent chain can result in a nontrivial system, with a single MZM at each chain end.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Cascade of vestigial orders in two-component superconductors: Nematic, ferromagnetic, s -wave charge- 4 e , and d -wave charge- 4 e states

Electronically ordered states that break multiple symmetries can melt in multiple stages, similarly to liquid crystals. In a partially melted phase, known as vestigial phase, a bilinear made out of combinations of the multiple components of the primary order parameter condenses. Multicomponent superconductors are thus natural candidates for vestigial order since they break both the U⁡(1)-gauge and also time-reversal or lattice symmetries. Here, we use group theory to classify all possible real-valued and complex-valued bilinears of a generic two-component superconductor on a tetragonal or hexagonal lattice. While the more widely investigated real-valued bilinears correspond to vestigial nematic or ferromagnetic order, the little explored complex-valued bilinears correspond to a vestigial charge-4e condensate, which itself can have an underlying s-wave, d x 2 –y 2 -wave, or d xy -wave symmetry. To properly describe the fluctuating regime of the superconducting Ginzburg-Landau action and thus access these competing vestigial phases, we employ both a large-N and a variational method. We show that while vestigial order can be understood as a weak-coupling effect in the large-N approach, it is akin to a moderate-coupling effect in the variational method. Despite these distinctions, both methods yield similar results in wide regions of the parameter space spanned by the quartic Landau coefficients. Specifically, we find that the nematic and ferromagnetic phases are the leading vestigial instabilities, whereas the various types of charge-4⁢e order are attractive albeit subleading vestigial channels. The only exception is for the hexagonal case, in which the nematic and s-wave charge-4e vestigial states are degenerate. Finally, we discuss the limitations of our approach, as well as the implications of our results for the realization of exotic charge-4⁢e states in material candidates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Pair breaking in superconductors with strong spin-orbit coupling

Here we study the influence of symmetry-breaking perturbations on superconductivity in multiorbital materials, with a particular focus on an external magnetic field. We introduce the field-fitness function which characterizes the pair-breaking effects of the perturbation on a given superconducting state. For even-parity superconductors we find that this field-fitness function for an external magnetic field is one, implying that the paramagnetic response is controlled only by a generalized effective g factor. For odd-parity superconductors, the interplay of the effective g factor and the field-fitness function can lead to counterintuitive results. We demonstrate this for p-wave pairing in the effective j = $\frac{3}{2}$ electronic states of the Luttinger-Kohn model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Nondegenerate surface pair density wave in the kagome superconductor CsV 3 Sb 5 : Application to vestigial orders

On the Sb layer of the kagome superconductor CsV 3 ⁢Sb 5 , pair density wave states have been observed. When the high-temperature charge orderings are treated as static backgrounds, these pair density wave (PDW) states exhibit the same wave vector in the effective two-dimensional Brillouin zone. Interestingly, these PDW states break the same symmetry on the surface. Considering the presence of this nondegenerate PDW, we investigate the implications for the possible existence of a vestigial charge-4⁢ e phase with a nonzero center-of-mass momentum. To distinguish between different vestigial phases, we propose scanning tunneling microscopy experiments. Here we aim to provide insights into the nature of the vestigial phases and their distinct characteristics in CsV 3 ⁢Sb 5 . This research sheds light on the interplay between PDW states, charge orderings, and superconductivity of the kagome superconductor.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Impurity-Induced Anomalous Thermal Hall Effect in Chiral Superconductors

Chiral superconductors exhibit novel transport properties that rely on the topology of the order parameter, topology of the Fermi surface, the spectrum of bulk and edge Fermionic excitations, and the structure of the impurity potential. In the case of electronic heat transport, impurities induce an anomalous (zero-field) thermal Hall conductivity that is easily orders of magnitude larger than the quantized edge contribution. The effect originates from branch-conversion scattering of Bogoliubov quasiparticles by the chiral order parameter, induced by potential scattering. The former transfers angular momentum between the condensate and the excitations that transport heat. The anomalous thermal Hall conductivity is shown to depend to the structure of the electron-impurity potential, as well as the winding number ν of the chiral order parameter $Δ(p)=|Δ(p)|e^{iν\phi_p}$. The results provide quantitative formulas for interpreting heat transport experiments seeking to identify broken T and P symmetries, as well as the topology of the order parameter for chiral superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Terahertz lightwave control of non-equilibrium phases and collective modes in multi-band superconductors

A gauge-invariant density matrix approach is presented to describe the non-equilibrium dynamics of multiband superconductors after photo-excitation. The derived gauge-invariant Bloch equations extend the Anderson pseudo-spin precession model by fully incorporating the center-of-mass motion of Cooper pairs. We also describe lightwave propagation effects inside a superconducting film by including the self-consistent interaction of the photo-excited superconducting system with the propagating electromagnetic field inside the superconductor using Maxwell's equations.

Mootz, Martin↗
DOE OSTI2020

Zero-energy bound states in the high-temperature superconductors at the two-dimensional limit

Majorana zero modes (MZMs) that obey the non-Abelian statistics have been intensively investigated for potential applications in topological quantum computing. The prevailing signals in tunneling experiments “fingerprinting” the existence of MZMs are the zero-energy bound states (ZEBSs). However, nearly all of the previously reported ZEBSs showing signatures of the MZMs are observed in difficult-to-fabricate heterostructures at very low temperatures and additionally require applied magnetic field. Here, by using in situ scanning tunneling spectroscopy, we detect the ZEBSs upon the interstitial Fe adatoms deposited on two different high-temperature superconducting one-unit-cell iron chalcogenides on SrTiO 3 (001). The spectroscopic results resemble the phenomenological characteristics of the MZMs inside the vortex cores of topological superconductors. Our experimental findings may extend the MZM explorations in connate topological superconductors toward an applicable temperature regime and down to the two-dimensional (2D) limit.

Science & Technology - Other Topics↗
DOE OSTI2020

Evidence for an edge supercurrent in the Weyl superconductor MoTe 2

Edge supercurrents in superconductors have long been an elusive target. Interest in them has reappeared in the context of topological superconductivity. We report evidence for the existence of a robust edge supercurrent in the Weyl superconductor molybdenum ditelluride (MoTe 2 ). In a magnetic field B , fluxoid quantization generates a periodic modulation of the edge condensate observable as a “fast-mode” oscillation of the critical current I c versus B . The fast-mode frequency is distinct from the conventional Fraunhofer oscillation displayed by the bulk supercurrent. We confirm that the fast-mode frequency increases with crystal area as expected for an edge supercurrent. In addition, weak excitation branches are resolved that display an unusual broken symmetry.

Science & Technology - Other Topics↗
DOE OSTI2022

Magnetic Field Penetration Technique to Study High Field Shielding of Multilayered Superconductors

The SIS structure which consists of alternative thin layers of superconductors and insulators on a bulk niobium has been proposed to shield niobium cavity surface from high magnetic field and hence increase the accelerating gradient. The study of the behavior of multilayer super-conductors in an external magnetic field is essential to optimize their SRF performance. In this work we report the development of a simple and efficient technique to measure penetration of magnetic field into bulk, thin film and multilayer superconductors. Experimental setup contains a small superconducting solenoid which can produce a parallel surface magnetic field up to 0.5 T and Hall probes to detect penetrated magnetic field across the superconducting sample. This system was calibrated and used to study the effect of niobium sample thickness on the field of full magnetic flux penetration. We determined the optimum thickness of the niobium substrate to fabricate the multilayer structure for the measurements in our setup. This technique was used to measure penetration fields of Nb₃Sn thin films and Nb₃Sn/Al₂O₃ multi-layers deposited on Al₂O₃ wafers.

Senevirathne, I. H.↗
DOE OSTI2023

The Quick Rise and Fall of LK-99 as a Room Temperature Superconductor [Slides]

A swift determination of superconductivity vs non-superconductivity of LK-99 demonstrates the importance of reproducibility in science. This has benefited from 4 decades study of high-temperature cuprate mechanism. Holy grail of room-temperature ambient-pressure superconductor remains to be pursued. Caution is needed about DFT calculations. Crucial old experimental data should be referenced. With the existing amount of experimentally discovered superconductors, could data science make a stride?

36 MATERIALS SCIENCE↗
DOE OSTI2024

Search for 3D topological superconductors using laser-based spectroscopy (Final Technical Report)

The three-dimensional topological superconductor (3D TSC) is a novel quantum phase of matter that is predicted to exhibit exotic thermal and electrical properties, which may serve applications ranging from precision thermal management and sensing technologies to fault-tolerant quantum information processing. Over the past decade, there has been an intensive effort to realize 1D and 2D TSCs by artificially engineering them from combinations of already existing materials. For example, coupling conventional superconductors to spin-orbit coupled 1D nanowires or to 2D topological insulator surfaces by the proximity effect provides a route to realizing 1D and 2D TSCs respectively. However no artificial engineering strategy is available for a 3D TSC. Therefore, current searches are limited to intrinsically superconducting compounds whose topological properties are difficult to predict. The overarching goal of this project was to realize and to identify 3D TSCs in bulk single crystals using novel laser-based spectroscopic techniques. One major thrust was to develop ultralow temperature and ultra-high-resolution angle-resolved photoemission spectroscopy (ARPES) to directly measure the dispersion of Majorana excitations that are predicted to exist on the surfaces of 3D TSCs. Another major thrust was to develop a rational route to realize 3D TSCs by identifying their precursor phases. It is theoretically proposed that the critical fluctuations of certain ordered electronic phases can mediate Cooper pairing in odd-parity channels and lead to topological superconductivity. Therefore, 3D TSCs may potentially emerge upon suppressing these inversion symmetry broken precursor phases to a critical point with external perturbations such as pressure, strain or even light. To test this hypothesis, we studied candidate materials using a suite of symmetry-sensitive optical probes including rotational anisotropy optical second harmonic generation (RA-SHG) and time-resolved coherent phonon spectroscopy, all operable under high-pressure and low temperature environments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1971

Josephson effects in a bulk granular superconductor.

Experimental results are presented for the dc and ac Josephson effects found in a bulk granular superconductor comprised of about 10 to the 4th superconducting grains (less than 45 microns in dimension) packed in an insulator capsule. This new type of Josephson weakly coupled superconductor offers high sensitivity in response to microwave radiation with a noise-equivalent power as low as 10 to the minus 14th W/(Hz) one half.

Warman, J.↗
NASA NTRS1972

Voltages induced in superconductors in the absence of transport currents.

Experimental observations of the production of dc and harmonic voltages are described in superconductors subjected to superimposed dc and oscillatory magnetic fields. The attempt is made to account for these observations in a qualitative manner by relating them to the asymmetric flux flow induced by the magnetic fields in the superconductor samples. The observed effects are quite complex and depend on the values of the many parameters in the experiment. Regarding some of the observed phenomena, for which no satisfactory explanation is as yet available, further understanding is expected to come from detailed work on samples with asymmetric surfaces produced in a controlled fashion.

Ohmer, M. C.↗