Condensed Matter

In-plane anisotropy of electrical resistivity was studied in samples of the hole-doped Ba 1−x K x Fe 2 As 2 in the composition range 0.21≤x≤0.26 where anisotropy changes sign. Low-temperature ( ∼ 20~K) irradiation with relativistic 2.5 MeV electrons was used to control the level of disorder and residual resistivity of the samples. Modification of the stress-detwinning technique enabled measurements of the same samples before and after irradiation, leading to conclusion of anisotropic character of predominantly inelastic scattering processes. Our main finding is that the resistivity anisotropy is of the same sign irrespective of residual resistivity, and remains the same in the orthorhombic C 2 phase above the re-entrant tetragonal transition. Unusual T -linear dependence of the anisotropy Δρ≡ ρ a (T)− ρ b (T) is found in pristine samples with x= 0.213 and x= 0.219, without similar signatures in either ρ a (T) or ρ b (T) . We show that this feature can be reproduced by a phenomenological model of R.~M.~Fernandes {\it et al.} Phys. Rev. Lett. {\bf 107},217002 (2011). We speculate that onset of fluctuations of nematic order on approaching the instability towards the re-entrant tetragonal phase contributes to this unusual dependence.

We explore the effects of sample dimensionality on vortex pinning in a type-II, low- T C , s-wave superconductor, NbN, in the presence of a perpendicular magnetic field, H . We find significant differences in the phase diagrams in the magnetic field--temperature plane between 3-dimensional (3D) and 2-dimensional (2D) NbN films. The differences are most striking close to the normal-superconductor phase transition. We establish that these variances have their origin in the differing pinning properties in two different dimensions. We obtain the pinning strength quantitatively in both the dimensions from two independent transport measurements performed in two different regimes of vortex-motion -- (i) thermally assisted flux-flow (TAFF) regime and (ii) flux flow (FF) regime. Both the measurements consistently show that both the pinning potential and the zero-field free-energy barrier to depinning in the 3D superconductor are at least an order of magnitude stronger than that in the 2D superconductor. Further, we probed the dynamics of pinning in both 2D and 3D superconductor through voltage fluctuation spectroscopy. We find that the mechanism of vortex pinning-depinning is qualitatively similar for the 3D and 2D superconductors. The voltage-fluctuations arising from vortex-motion are found to be correlated only in the 2D superconductor. We establish this to be due to the presence of long-range phase fluctuations near the Berezinskii-Kosterlitz-Thouless (BKT) type superconducting transition in 2-dimensional superconductors.

We investigated transport characteristics of superconducting Ta thin films with three configurations in rf radiation filters; no filter, only room-temperature filters, and low-temperature filters in addition to room-temperature filters. The transport properties near the transition temperature are strongly dependent on whether the room-temperature filter is installed or not. The entire transition is shifted to higher temperature with loading layers of the room-temperature filters. Once the zero-resistance state is achieved at B=0, no strong radiation effect is observed even with low-temperature filters installed. When magnetic field is turned on, the nonzero-resistance saturation at low temperatures is revealed without low-temperature filters, which has been considered to be magnetic-field-induced quantum metallic phase. However, the insertion of the additional low-temperature filter weakens the saturation of the resistance, the signature of the metallic behavior. This observation suggests that the previously reported anomalous metallic state in Ta films is mainly induced by the unfiltered radiation and, thus, the intrinsic metallic ground state should be limited to the narrow range of magnetic fields near the critical point, if exists.

We theoretically study ferromagnetic (FM) fluctuations that are experimentally observed in the heavily overdoped region of cuprate superconductors. To explore the origin of FM fluctuations, we evaluate the spin susceptibilities of a single-band Hubbard model within the fluctuation exchange approximation. Model parameters are derived using the Wannierization technique and the constrained random phase approximation method based on the maximally localized Wannier functions. The constrained random phase approximation calculations reveal that the on-site Coulomb interaction decreases with an increase in hole doping. By taking this reduction of the on-site Coulomb interaction into account, the emergence of FM fluctuations in heavily overdoped cuprates can be explained.

Starting with a minimal model for the CuO 2 planes with the on-site Hilbert space reduced to only three effective valence centers [CuO 4 ] 7−,6−,5− (nominally Cu 1+,2+,3+ ) with different conventional spin and different orbital symmetry we propose a unified non-BCS model that allows one to describe the main features of the phase diagrams of doped cuprates within the framework of a simple effective field theory.

In this paper are presented the effects of Lorentz violation in superconductivity. Constructing a Lorentz-Violating Ginzburg-Landau theory of superconductivity we discuss the influence of the Lorentz-Violating tensor k ^ i a in the London's depth penetration, in the coherence length and in the critical magnetic field. We also study the behavior of the magnetic field inside the superconductor for two different geometries, cylindrical and rectangular.

Among the many remarkable properties of diamond, the ability to superconduct when heavily doped with boron has attracted much interest in the carbon community. When considering the nanocrystalline boron doped system, the reduced dimensionality and confinement effects have led to several intriguing observations most notably, signatures of a mixed superconducting phase. Here we present ultra-high-resolution transmission electron microscopy imaging of the grain boundary and demonstrate how the complex microstructure leads to enhanced carrier correlations. We observe hallmark features of spin-orbit coupling (SOC) manifested as the weak anti-localization effect. The enhanced SOC is believed to result from a combination of inversion symmetry breaking at the grain boundary interfaces along with antisymmetric confinement potential between grains, inducing a Rashba-type SOC. From a pronounced zero bias peak in the differential conductance, we demonstrate signatures of a triplet component believed to result from spin mixing caused by tunneling of singlet Cooper pairs through such Rashba-SOC grain boundary junctions.

The recent discovery of high-T c superconductivity (HTS) in Sr-doped NdNiO 2 has sparked a renewed interest in investigating nickelates as cuprate counterparts. Parent cuprates [Cu 2+ : d 9 ] are antiferromagnetic charge-transfer insulators with the involvement of a single d x 2 − y 2 band around the Fermi level and strong p−d hybridization. In contrast, isoelectronic NdNiO 2 [Ni + : d 9 ] is metallic with a d x 2 − y 2 band self-doped by Nd-d states. Using first-principles calculations, we study the effect of Sr-doping in the electronic and magnetic properties of infinite-layer nickelates as well as the nature of the holes. We find that hole doping tends to make the material more cuprate-like as it minimizes the self-doping effect, it enhances the p−d hybridization, and it produces low-spin (S=0, non-magnetic) Ni 2+ dopants in analogy with the S=0 Zhang-Rice singlets that appear in cuprates.

In order to enhance the critical current density ( J c ) of superconductors, introduction of columnar defects (CDs) through swift-particle irradiation is effective. By dispersing the direction of CDs (splayed CDs), not only further enhancement of J c has been confirmed but also an anomalous peak effect (APE) in J c at a certain magnetic field determined by the irradiation dose was observed. It has been proposed that the APE arises from the suppression of kink motion and/or the effects of vortex entanglement in systems with splayed CDs. In this study, we measure J c properties of optimally K-doped Ba-122-type iron-based superconductor Ba 0.6 K 0.4 Fe 2 As 2 single crystals with splayed CDs that are introduced asymmetrically with respect to the c -axis by irradiating 2.6 GeV U ions. We discuss the significance of the average direction of splayed CDs to the APE. We also discuss the relationship between the APE and the entanglement of vortices.

We study the topological phase transitions of a Kitaev chain in the presence of geometric frustration caused by the addition of a single long-range hopping. The latter condition defines a legged-ring geometry (Kitaev tie) lacking of translational invariance. In order to study the topological properties of the system, we generalize the transfer matrix approach through which the emergence of Majorana modes is studied. We find that geometric frustration gives rise to a topological phase diagram in which non-trivial phases alternate with trivial ones at varying the range of the extra hopping and the chemical potential. Frustration effects are also studied in a translational invariant model consisting of multiple-ties. In the latter system, the translational invariance permits to use the topological bulk invariant to determine the phase diagram and bulk-edge correspondence is recovered. It has been demonstrated that geometric frustration effects persist even when translational invariance is restored. These findings are relevant in studying the topological phases of looped ballistic conductors.