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arXiv2013

Effective interactions and fluctuation effects in spin-singlet superfluids

We derive and evaluate one-loop functional flow equations for the effective interactions, self-energy and gap function in spin-singlet superfluids. The flow is generated by a fermionic frequency cutoff, which is supplemented by an external pairing field to treat divergencies associated with the Goldstone boson. To parametrize the singular momentum and frequency dependences of the effective interactions, the Nambu interaction vertex is decomposed in charge, magnetic, and normal and anomalous pairing channels. The one-loop flow solves reduced (mean-field) models for superfluidity exactly, and captures also important fluctuation effects. The Ward identity from charge conservation is generally violated, but can be enforced by projecting the flow. Applying the general formalism to the two-dimensional attractive Hubbard model, we obtain detailed results on the momentum and frequency dependences of the effective interactions for weak and moderate bare interactions. The gap is reduced by fluctuations, with a stronger reduction at weaker interactions, as expected.

cond-mat.supr-con↗

arXiv2013

Understanding the re-entrant superconducting phase diagram of an iron-pnictide Ca$_4$Al$_2$O$_6$Fe$_2$(As$_{1-x}$P$_x$)$_2$

Recently, a very rich phase diagram has been obtained for an iron-based superconductor Ca4Al2O6Fe2(As1-xPx)2. It has been revealed that nodeless (x=0) and nodal (x = 1) superconductivity are separated by an antiferromagnetic phase. Here we study the origin of this peculiar phase diagram using a five orbital model constructed from first principles band calculation, and applying the fluctuation exchange approximation assuming spin fluctuation mediated pairing. Based on the calculation results, we propose a scenario where the frustration in momentum space degrades superconductivity in the intermediate x regime, while antiferromangetism takes place due to a very good nesting. In order to see whether the present theoretical scenario is consistent with the actual nature of the competition between superconductivity and antiferromagnetism, we also perform hydrostatic pressure experiment for Ca4Al2O6Fe2(As1-xPx)2. In the intermediate x regime where antiferromagnetism occurs at ambient pressure, applying hydrostatic pressure smears out the antiferromagnetic transition, but superconductivity does not take place. This supports our scenario that superconductivity is suppressed by the momentum space frustration in the intermediate x regime, apart from the presence of the antiferromangnetism.

cond-mat.supr-con↗

arXiv2013

Spatially Resolved Magnetization in the Bose-Einstein Condensed State of BaCuSi2O6: Evidence for Imperfect Frustration

In order to understand the nature of the two-dimensional Bose-Einstein condensed (BEC) phase in BaCuSi2O6, we performed detailed 63Cu and 29Si NMR above the critical magnetic field, Hc1= 23.4 T. The two different alternating layers present in the system have very different local magnetizations close to Hc1; one is very weak, and its size and field dependence are highly sensitive to the nature of inter-layer coupling. Its precise value could only be determined by "on-site" 63Cu NMR, and the data are fully reproduced by a model of interacting hard-core bosons in which the perfect frustration associated to tetragonal symmetry is slightly lifted, leading to the conclusion that the population of the less populated layers is not fully incoherent but must be partially condensed.

cond-mat.str-el↗

arXiv2013

Hysteretic superconducting resistive transition in Ba0.07K0.93Fe2As2

We have observed hysteresis in superconducting resistive transition curves of Ba$_{0.07}$K$_{0.93}$Fe$_2$As$_2$ ($T_c\sim$8 K) below about 1 K for in-plane fields. The hysteresis is not observed as the field is tilted away from the $ab$ plane by 20$^{\circ}$ or more. The temperature and angle dependences of the upper critical field indicate a strong paramagnetic effect for in-plane fields. We suggest that the hysteresis can be attributed to a first-order superconducting transition due to the paramagnetic effect. Magnetic torque data are also shown.

cond-mat.supr-con↗

arXiv2013

C-axis optical conductivity from the Yang-Rice-Zhang model of the underdoped cuprates

The c-axis optical response of the underdoped cuprates is qualitatively different from its in-plane counterpart. The features of the pseudogap show themselves more prominently in the c-axis than in-plane. We compute both the c-axis and in-plane optical conductivity using the Yang-Rice-Zhang model of the underdoped cuprates. This model combined with coherent interlayer tunnelling is enough to explain the qualitative differences between the in-plane and c-axis data. We show how pseudogap features manifest themselves in the infrared and microwave conductivity within this model.

cond-mat.supr-con↗

arXiv2013

Phase Stability in the Two dimensional Anisotropic Boson Hubbard Hamiltonian

The two dimensional square lattice hard-core boson Hubbard model with near neighbor interactions has a `checkerboard' charge density wave insulating phase at half-filling and sufficiently large intersite repulsion. When doped, rather than forming a supersolid phase in which long range charge density wave correlations coexist with a condensation of superfluid defects, the system instead phase separates. However, it is known that there are other lattice geometries and interaction patterns for which such coexistence takes place. In this paper we explore the possibility that anisotropic hopping or anisotropic near neighbor repulsion might similarly stabilize the square lattice supersolid. By considering the charge density wave structure factor and superfluid density for different ratios of interaction strength and hybridization in the $\hat x$ and $\hat y$ directions, we conclude that phase separation still occurs.

cond-mat.str-el↗

arXiv2012

Crystal Structure, Lattice Vibrations, and Superconductivity of LaO$_{1-x}$F$_{x}$BiS$_{2}$

Neutron scattering measurements have been performed on polycrystalline samples of the newly discovered layered superconductor LaO$_{0.5}$F$_{0.5}$BiS$_{2}$, and its nonsuperconducting parent compound LaOBiS$_{2}$. The crystal structures and vibrational modes have been examined. Upon F-doping, while the lattice contracts significantly along $c$ and expands slightly along $a$, the buckling of the BiS$_2$ plane remains almost the same. In the inelastic measurements, a large difference in the high energy phonon modes was observed upon F substitution. Alternatively, the low energy modes remain almost unchanged between non-superconducting and superconducting states either by F-doping or by cooling through the transition temperature. Using density functional perturbation theory we identify the phonon modes, and estimate the phonon density of states. We compare these calculations to the current measurements and other theoretical studies of this new superconducting material.

cond-mat.supr-con↗

arXiv2013

Multipair DC-Josephson Resonances in a biased all-superconducting Bijunction

An all-superconducting bijunction consists of a central superconductor contacted to two lateral superconductors, such that non-local crossed Andreev reflection is operating. Then new correlated transport channels for the Cooper pairs appear in addition to those of separated conventional Joseph- son junctions. We study this system in a configuration where the superconductors are connected through gate-controllable quantum dots. Multipair phase-coherent resonances and phase-dependent multiple Andreev reflections are both obtained when the voltages of the lateral superconductors are commensurate, and they add to the usual local dissipative transport due to quasiparticles. The two-pair resonance (quartets) as well as some other higher order multipair resonances are π-shifted at low voltage. Dot control can be used to dramatically enhance the multipair current when the voltages are resonant with the dot levels.

cond-mat.mes-hall↗

arXiv2013

Calorimetric study in single crystal of CsFe$_2$As$_2$

We measured resistivity and specific heat of high-quality CsFe$_2$As$_2$ single crystals, which were grown by using a self-flux method. The CsFe$_2$As$_2$ crystal shows sharp superconducting transition at 1.8 K with the transition width of 0.1 K. The sharp superconducting transition and pronounced jump in specific heat indicate high quality of the crystals. Analysis on the superconducting-state specific heat supports unconventional pairing symmetry in CsFe$_2$As$_2$.

cond-mat.supr-con↗

arXiv2013

Linewidth of the electromagnetic radiation from Josephson junctions near cavity resonances

The powerful terahertz emission from intrinsic Josephson junctions in high-Tc cuprate superconductors has been detected recently. The synchronization of different junctions is enhanced by excitation of the geometrical cavity resonance. A key characteristic of the radiation is its linewidth. In this work, we study the intrinsic linewidth of the radiation near the internal cavity resonance. Surprisingly, this problem was never considered before, neither for a single Josephson junction nor for a stack of the intrinsic Josephson junctions realized in cuprate superconductors. The linewidth appears due to the slow phase diffusion, which is determined by the dissipation and amplitude of the noise. We found that both these parameters are resonantly enhanced when the cavity mode is excited but enhancement of the dissipation dominates leading to the net suppression of diffusion and dramatic narrowing of the linewidth. The line shape changes from Lorentzian to Gaussian when either the Josephson frequency is shifted away from the resonance or the temperature is increased.

cond-mat.supr-con↗

arXiv2013

Kondo-like behavior near the metal-to-insulator transition of nano-scale granular aluminum

We show that the normal state transport properties of nano-scale granular Aluminum films, near the metal to insulator transition, present striking similarities with those of Kondo systems. Those include a negative magneto-resistance, a minimum of resistance R at a temperature Tm in metallic films, a logarithmic rise at low temperatures and a negative curvature of R(T) at high temperatures. These normal state properties are interpreted in terms of spin-flip scattering of conduction electrons by local magnetic moments, possibly located at the metal/oxide interfaces. Their co-existence with the enhanced superconductivity seen in these films is discussed.

cond-mat.supr-con↗

arXiv2013

Topological Superconductivity without Proximity Effect

Majorana Fermions, strange particles that are their own antiparticles, were predicted in 1937 and have been sought after ever since. In condensed matter they are predicted to exist as vortex core or edge excitations in certain exotic superconductors. These are topological superconductors whose order parameter phase winds non-trivially in momentum space. In recent years, a new and promising route for realizing topological superconductors has opened due to advances in the field of topological insulators. Current proposals are based on semiconductor heterostructures, where spin-orbit coupled bands are split by a band gap or Zeeman field and superconductivity is induced by proximity to a conventional superconductor. Topological superconductivity is obtained in the interface layer. The proposed heterostructures typically include two or three layers of different materials. In the current work we propose a device based on materials with inherent spin-orbit coupling and an intrinsic tendency for superconductivity, eliminating the need for a separate superconducting layer. We study a lattice model that includes spin-orbit coupling as well as on-site and nearest neighbor interaction. Within this model we show that topological superconductivity is possible in certain regions of parameter space. These regions of non-trivial topology can be understood as a nodeless superconductor with d-wave symmetry which, due to the spin-orbit coupling, acquires an extra phase twist of $2π$.

cond-mat.supr-con↗

arXiv2013

Dopant Clustering, Electronic Inhomogeneity, and Vortex Pinning in Iron-Based Superconductors

We use scanning tunneling microscopy to map the surface structure, nanoscale electronic inhomogeneity, and vitreous vortex phase in the hole-doped superconductor Sr$_{0.75}$K$_{0.25}$Fe$_2$As$_2$ with $T_c$=32 K. We find the low-$T$ cleaved surface is dominated by a half-Sr/K termination with $1\times 2$ ordering and ubiquitous superconducting gap, while patches of gapless, unreconstructed As termination appear rarely. The superconducting gap varies by $σ/\barΔ$=16% on a $\sim$3 nm length scale, with average $2\barΔ/k_B T_c=3.6$ in the weak coupling limit. The vortex core size provides a measure of the superconducting coherence length $ξ$=2.3 nm. We quantify the vortex lattice correlation length at 9 T in comparison to several iron-based superconductors. The comparison leads us to suggest the importance of dopant size mismatch as a cause of dopant clustering, electronic inhomogeneity, and strong vortex pinning.

cond-mat.supr-con↗

arXiv2013

Enhancing the Critical Current of a Superconducting Film in a Wide Range of Magnetic Fields with a Conformal Array of Nanoscale Holes

The maximum current (critical current) a type-II superconductor can transmit without energy loss is limited by the motion of the quantized magnetic flux penetrating into a superconductor. Introducing nanoscale holes into a superconducting film has been long pursued as a promising way to increase the critical current. So far the critical current enhancement was found to be mostly limited to low magnetic fields. Here we experimentally investigate the critical currents of superconducting films with a conformal array of nanoscale holes that have non-uniform density while preserving the local ordering. We find that the conformal array of nanoscle holes provides a more significant critical current enhancement at high magnetic fields. The better performance can be attributed to its arching effect that not only gives rise to the gradient in hole-density for pinning vortices with a wide range of densities but also prevent vortex channeling occurring in samples with a regular lattice of holes.

cond-mat.supr-con↗

arXiv2013

Possible realization of an antiferromagnetic Griffiths phase in Ba[Fe(1-x)Mn(x)](2)As(2)

We investigate magnetic ordering in metallic Ba[Fe(1-x)Mn(x)](2)As(2) and discuss the unusual magnetic phase, which was recently discovered for Mn concentrations x > 10%. We argue that it can be understood as a Griffiths-type phase that forms above the quantum critical point associated with the suppression of the stripe-antiferromagnetic spin-density-wave (SDW) order in BaFe2As2 by the randomly introduced localized Mn moments acting as strong magnetic impurities. While the SDW transition at x = 0, 2.5% and 5% remains equally sharp, in the x = 12% sample we observe an abrupt smearing of the antiferromagnetic transition in temperature and a considerable suppression of the spin gap in the magnetic excitation spectrum. According to our muon-spin-relaxation, nuclear magnetic resonance and neutron-scattering data, antiferromagnetically ordered rare regions start forming in the x = 12% sample significantly above the Néel temperature of the parent compound. Upon cooling, their volume grows continuously, leading to an increase in the magnetic Bragg intensity and to the gradual opening of a partial spin gap in the magnetic excitation spectrum. Using neutron Larmor diffraction, we also demonstrate that the magnetically ordered volume is characterized by a finite orthorhombic distortion, which could not be resolved in previous diffraction studies most probably due to its coexistence with the tetragonal phase and a microstrain-induced broadening of the Bragg reflections. We argue that Ba[Fe(1-x)Mn(x)](2)As(2) could represent an interesting model spin-glass system, in which localized magnetic moments are randomly embedded into a SDW metal with Fermi surface nesting.

cond-mat.supr-con↗

arXiv2013

Amplitude mode of the $d$-density wave state and its relevance to high-$T_c$ cuprates

We calculate the spectrum of the amplitude mode, the analog of the Higgs mode in high energy physics, for the $d$-density wave (DDW) state proposed to describe the pseudogap phase of the high $T_c$ cuprates. Even though the state breaks translational symmetry by a lattice spacing and is described by a particle-hole singlet order parameter at the wave vector $q=Q=(π,π)$, remarkably, we find that the amplitude mode spectrum can have peaks at both $q=(0,0)$ and $q=Q=(π,π)$; we shall lattice spacing to unity. In general, the spectrum is non-universal, and, depending on the microscopic parameters, can have one or two peaks in the Brillouin zone, signifying existence of two kinds of magnetic excitations. Our theory sheds important light on how multiple inelastic neutron peaks at different wave vectors can, in principle, arise even with an order parameter that condenses at $Q=(π,π)$.

cond-mat.supr-con↗

arXiv2013

Scaling, Finite Size Effects, and Crossovers of the Resistivity and Current-Voltage Characteristics in Two-Dimensional Superconductors

We revisit the scaling properties of the resistivity and the current-voltage characteristics at and below the Berezinskii-Kosterlitz-Thouless transition, both in zero and nonzero magnetic field. The scaling properties are derived by integrating the renormalization group flow equations up to a scale where they can be reliably matched to simple analytic expressions. The vortex fugacity turns out to be dangerously irrelevant for these quantities below Tc, thereby altering the scaling behavior. We derive the possible crossover effects as the current, magnetic field, or system size is varied, and find a strong multiplicative logarithmic correction near Tc, all of which is necessary to account for when interpreting experiments and simulation data. Our analysis clarifies a longstanding discrepancy between the finite size dependence found in many simulations and the current-voltage characteristics of experiments. We further show that the logarithmic correction can be avoided by approaching the transition in a magnetic field, thereby simplifying the scaling analysis. We confirm our results by large-scale numerical simulations, and calculate the dynamic critical exponent z, for relaxational Langevin dynamics and for resistively and capacitively shunted Josephson junction dynamics.

cond-mat.supr-con↗

arXiv2013

Signatures of quantum criticality in the thermopower of Ba(Fe(1-x)Co(x))2As2

We demonstrate that the thermopower (S) can be used to probe the spin fluctuations (SFs) in proximity to the quantum critical point (QCP) in Fe-based superconductors. The sensitivity of S to the entropy of charge carriers allows us to observe an increase of S/T in Ba(Fe(1-x)Co(x))2As2 close to the spin-density-wave (SDW) QCP. This behavior is due to the coupling of low-energy conduction electrons to two-dimensional SFs, similar to heavy-fermion systems. The low-temperature enhancement of S/T in the Co substitution range 0.02 < x < 0.1 is bordered by two Lifshitz transitions, and it corresponds to the superconducting region, where a similarity between the electron and non-reconstructed hole pockets exists. The maximal S/T is observed in proximity to the commensurate-to-incommensurate SDW transition, for critical x_c ~ 0.05, close to the highest superconducting T_c. This analysis indicates that low-T thermopower is influenced by critical spin fluctuations which are important for the superconducting mechanism.

cond-mat.supr-con↗