Session W58: Exotic Features of Superconductors

Aharonov-Casher effect with vortices in an amorphous superconductor

A trajectory of a magnetic moment in an electric field results in the acquisition of a topological phase called the Aharonov-Casher (A-C) phase, analogous to the A-B phase acquired by a charged particle in a magnetic field. In condensed matter, experiments demonstrating the A-C effect are relatively few, the earliest being for superconducting vortices traversing a fabricated Josephson-junction array. In that regular geometry, a periodic response to induced charge was observed. In an irregular geometry, in analogy with universal conductance fluctuations, a complex quasi-periodic respinse may be expected.
An amorphous superconductor close to a superconductor-insulator transition (SIT) provides such a complex multiply connected region for superconducting vortices to interfere, as indicated by recent studies. In this study with amorphous indium oxide close to a disorder driven SIT, we introduce vortices in the system by applying a perpendicular magnetic field. The motion of these vortices around superconducting islands generates a complex interference pattern in the form of a spontaneous voltage that is tunable with both gate voltage and magnetic field. This effect is a demonstration of the quantum behavior of Josephson vortices which are present in the system close to the SIT.   

IDEAL DIAMAGNETIC RESPONSE OF A PRISTINE GRAPHENE-n-HEPTANE-PERMALLOY SYSTEM

We explored magnetic screening of milligauss fields in a system comprised of a single-layer graphene (on various substrates) wetted by n-heptane in the presence of a thin Fe-Ni foil parallel to the graphene layer. Our initial task was to confirm the results and find the origins of ideal diamagnetic screening reported previously in [Y. Kawashima, arXiv:cond-mat/1801.09376]. Indeed, we obtained ideal diamagnetic response. Also, in some cases “paramagnetic Meissner effect” has been obtained which in case of superconducting origin of observed facts can be associated with Wohlleben effect. At the same time, a crucial dependence of humidity and possibly other atmospheric factors on results has been observed which can be explained by chemisorption described in e.g. [T.L. Burnett et.al., arXiv:1204.3323]. These factors completely inhibit both normal and negative diamagnetism. To minimize this effect experiments were performed in Ar atmosphere. This may or may not be a footprint of superconductivity at room temperature. In both cases understanding the signal under different conditions may spread a light on a yet unknown phenomenon in condensed matter physics.   

Odd Frequency Pairing in Supercodcuting Systems Using Renormalization Group Approach

We investigate odd frequency paring of electron-electron interaction on superconducting systems using the renormalization group method. Several experimental results induced by proximity effect have shown the feasibility of odd frequency pairing. Efforts have been made in the theoretical arena, since its first conception by Berezinskii, to understand this phenomenon, but many questions still remain unanswered. In our work, by considering a two-dimensional rotationally invariant Fermi surface and spinless system within the framework of renormalization group, we present a brief elucidation on the microscopic mechanism for odd frequency pairing, and show a self-contained derivation of the exact flow equation for the vertex functions and self energy that depend on the gradual evolution of the microscopic model action to the final effective action as a function of a continuously decreasing energy scale.

  

Local structure of the cuprate superconductors

The cuprate superconductors have long been known to be electronically inhomogeneous at the nanoscale, although the nature and role of the inhomogeneity remain debated. Recent studies of superconducting fluctuations in cuprates have uncovered a universal inhomogeneity-driven regime in multiple compounds [1], and similar findings for other perovskite-derived superconductors point to universal underlying structural inhomogeneity [2]. Here we present complementary X-ray and neutron diffuse scattering studies of several cuprates, with the real-space local structure determined by the 3D ΔPDF method [3]. The experiments reveal intricate short-range correlated structural distortions, even in HgBa₂CuO_4+δ, which features a simple tetragonal average structure. We discuss the doping dependence of the local structure and correlations with the superconducting T_c.

[1] D. Pelc et al., Nat. Commun. 9, 4327 (2018)
[2] D. Pelc et al., Nat. Commun. 10, 2729 (2019)
[3] M. J. Krogstad et al., Nat. Mater. (2019)   

Plasticity and stress engineering of cuprate superconductors

Ceramic materials, including the cuprate high-temperature superconductors, are generally thought to be brittle at low temperatures due to their rigid ionic and covalent bonding. We present here the surprising discovery of room-temperature plasticity in the cuprates, observed with compressive stress perpendicular to the copper-oxygen planes. We further show that the electronic properties, in particular superconductivity, can be manipulated via plastic deformation. We discuss the origin of these effects and correlate them with structural changes as investigated by neutron and X-ray diffuse scattering measurements.   

Thermodynamic Evidence for a Multi-component superconducting order parameter in Sr2RuO4

The unconventional superconductivity of Sr₂RuO₄ has remained a mystery for over 25 years, even though its normal metallic state is quite well-understood. Reaching a consensus on the bulk order parameter symmetry has been difficult primarily due to the contradictions that exist between several major experimental findings. We performed resonant ultrasound spectroscopy (RUS) measurements on Sr₂RuO₄, to measure its entire symmetry-resolved elastic tensor through T_C (∼1.43 K). Since different superconducting order parameters couple differently to shear and compressional sound modes, measurement of all elastic moduli places strong constraints on the order parameter symmetry, independent of microscopics. We observe a thermodynamic discontinuity in one of the shear moduli (c₆₆) at T_C, which requires a two-component superconducting order parameter (E representation of D_4h). We discuss leading E-symmetry order parameters, and suggest that it may be necessary to go beyond the conventional "singlet/triplet" paradigm to finally resolve the mystery that is Sr₂RuO₄. We also measure an anomalous attenuation peak in compressional sound below T_C, possibly arising due to sound absorption into order parameter collective modes.
  

Manipulating superconductivity in Sr2RuO4 thin films through epitaxial strain

Pushing the van Hove singularity in the γ band of Sr₂RuO₄ towards the Fermi level can potentially raise the transition temperature, T_c, of this unconventional superconductor. To test this concept, we explore the effect of biaxial strains from –0.9% to +1.0% imposed by commensurate epitaxy on (NdAlO₃)_0.39—(SrAl_1/2Ta_1/2O₃)_0.61 (NSAT), NdGaO₃, (LaAlO₃)_0.29—(SrAl_1/2Ta_1/2O₃)_0.71 (LSAT), LaGaO₃, and SrTiO₃ substrates on the superconducting T_c and upper critical field H_c2 of Sr₂RuO₄ thin films. These biaxial strain studies are distinct from and compliment the existing uniaxial strain studies on Sr₂RuO₄ single crystals. Mean free paths up to 144 nm are observed in Shubnikov-de Haas oscillation measurements on these thin films and T_cs range from 1 K to 1.8 K for films with similar residual resistivities. The measured mean free paths in combination with the coherence lengths, determined from upper critical field measurements, established that all films are superconducting in the clean limit.   

Spectroscopic study of doped strontium ruthenates compositions

Extraordinary properties of Sr₂RuO₄ have attracted much attention since the discovery of superconductivity in this material in [Y. Maeno et.al., Nature 372, 532 (1994)]. This material are isostructural to the high T_c cuprate superconductors which motivated extensive search of physically and chemically modified ruthenates [Gulian, et.al., Quantum Stud. Math. Found., 5, 161 (2018)]. One of the compositions Sr₂Ru_1-xRe_xO₃Se, revealed feature, which most naturally fitted into the pattern of presence of a small fraction of superconducting phase in a multiphase sample with the dominant ferromagnetic phase, almost masking out this superconductivity at ~30 K. We present compositional, morphological, structural, magnetotransport, heat capacity, magnetic susceptibility, Raman and terahertz spectroscopy data to facilitate theoretical analysis of metamagnetism vs. superconductivity in this material.
  

μSR study on the antiperovskite oxide supercondctor Sr3−xSnO

Antiperovskite (inverse perovskite) oxides are the materials crystallizing in the same structure as the ordinary perovskite oxides but with the inverted positions of the metal element and oxygen. Some of these materials are theoretically suggested to be topological crystalline insulators based on the first-principles calculations [1, 2].
We discovered the first superconductivity among this group of oxides in Sr_3−xSnO [3]. Reflecting the topology of the electronic band structure in the normal state, a topological superconductivity is theoretically proposed in this compound [3, 4]. In order to investigate the nature of the superconducting state, we performed the muon spin rotation (μSR) experiment. Under small magnetic fields, we successfully observed increase of the relaxation rate of μ⁺ below the transition temperature and confirmed the bulk superconductivity. The ratio between the relaxation rate and transition temperature σ/T_c is much smaller than those of the conventional superconductors and close to the values known for unconventional ones.

[1] T. Kariyado and M. Ogata, J. Phys. Soc. Jpn. 80, 083704 (2011).
[2] T. H. Hsieh et al., Phys. Rev. B 90, 081112(R) (2014).
[3] M. Oudah, A. Ikeda et al., Nat. Commun. 7, 13617 (2016).
[4] T. Kawakami et al., Phys. Rev. X 8, 041026 (2018).