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arXiv2013

High temperature superconductivity: Cooper pairs in trap

The tremendous efforts to unveil high temperature superconductivity (HTSC) have been devoted to the search of the mechanism underlying Cooper pairs which, however, remains a mysterious subject of vigorous debate, let alone many other mysteries like the pseudogap state, the peculiar Homes law, the unnegligible electron-phonon interaction, the stripe, the universal nodal Fermi velocity, etc. Most of subsequent works either bring in more controversies or swell the list of mysteries. Here I tentatively propose a whole new perspective on the basis of low-dimensional Bose-Einstein Condensation (LDBEC), which possibly makes lots of those mysteries correlated and understood in single picture.

cond-mat.supr-con↗

arXiv2013

Double criticality in the magnetic field-driven transition of a high-TC superconductor

Driving a two-dimensional superconductor normal by applying a high magnetic field may lead to Cooper pair localization. In this case, there should be a quantum critical point associated with specific scaling laws. Such a transition has been evidenced in a number of low critical temperature superconducting thin films and has been suggested to occur also in high temperature cuprate superconductors. Here we show experimental evidence for two distinct quantum critical regimes when applying perpendicular magnetic fields to underdoped La2-xSrxCuO4 thin films. At intermediate values of the magnetic field (18T-20T), a "ghost" QCP is observed, for which the values of the related critical exponents point towards a fermionic -as opposed to bosonic- scenario. At higher (about 37 T) magnetic field, another QCP is observed, which suggests the existence of either a 2D/3D or a clean/dirty temperature crossover.

cond-mat.supr-con↗

arXiv2013

Evidence for Intra-Unit-Cell magnetic order in the pseudo-gap state of high-Tc cuprates

The existence of the mysterious pseudo-gap state in the phase diagram of copper oxide superconductors and its interplay with unconventional {\it d-wave} superconductivity has been a long standing issue for more than a decade. There is now a growing number of experimental indications that the pseudo-gap phase actually corresponds to a symmetry breaking state. In his theory for cuprates, C. M. Varma proposes that the pseudo-gap is a new state of matter associated with the spontaneous appearance of circulating current loops within $\rm CuO_2$ unit cell. This intra-unit-cell order breaks time reversal symmetry, but preserves lattice translation invariance. Polarized elastic neutron scattering measurements provide evidence for an intra-unit-cell magnetic order inside the pseudo-gap state. This order could be produced by the orbital-like magnetic moments induced by the circulating current loops. The magnetic order displays the same characteristic features in $\rm HgBa_2CuO_{4+δ}$, $\rm YBa_2Cu_3O_{6+x}$ and $\rm Bi_2Sr_2CaCu_2O_{8+δ}$ demonstrating that this genuine phase is ubiquitous of the pseudo-gap of high temperature copper oxide materials. We review the main properties characterizing this intra-unit-cell magnetic order and discuss its interplay or competition with other spin and charge instabilities.

cond-mat.supr-con↗

arXiv2013

Dynamical revivals of the paired superfluidity and counterflow superfluidity

The quantum dynamics of two-species bosons in an optical lattice is studied within the mean-field theory. The quantum coherence experiences periodical collapses and revivals, which depends on the relative strength of the inter-and intra-species interactions. The paired superfluidity and the counterflow superfluidity are identified and are verified by exact diagonaliztion in the two-site Hubbard model. We suggest a dynamical method of implementation and detection for the paired superfluid and the counterflow superfluidity in the optical lattices.

cond-mat.quant-gas↗

arXiv2013

Periodic steps in the resistance vs. temperature characteristics of doped graphite and graphene: evidence of superconductivity?

We have observed periodically repeated steps in the resistance vs. temperature characteristics of doped Highly Oriented Pyrolytic Graphite and exfoliated doped multi-layer graphene. The observations consist of a series of regularly spaced steps in the resistance vs. temperature curves. The lowest step is observed at a temperature of from 50 to 60 K. Additional steps are observed at multiples of that basic temperature with the highest step temperature being at approximately 270 K. Quenching by a modest applied magnetic field has been observed. The sizes and widths of the observed steps appear to vary and may be related to some sort of aggregation reminiscent of flux vortex pinning. An additional argon ion implantation at reduced energy was done to see if additional defects in the material would yield additional structure. This was observed to yield much sharper resistance steps at temperatures in excess of 200 K in thin exfoliated peels from the implanted surface of the sample and would support the possibility of the steps being related to pancake vortex pinning in a layered superconductor. Unfortunately, as yet, there has been no direct measurement of either the superconducting energy gap nor has a definitive Meissner effect been observed.

cond-mat.supr-con↗

arXiv2013

Vortex penetration field of the multilayer coating model

The vortex penetration field of the multilayer coating model with a single superconductor layer and a single insulator layer formed on a bulk superconductor are derived. The same formula can be applied to a model with a superconductor layer formed on a bulk superconductor without an insulator layer.

physics.acc-ph↗

arXiv2013

Effects of carbon concentration and filament number on advanced internal-Mg-infiltration-processed MgB2 Strands

An advanced internal Mg infiltration method (AIMI) in this paper has been shown to be effective in producing superconducting wires containing dense MgB2 layers with high critical current densities. In this study, the in-field critical current densities of a series of AIMI-fabricated MgB2 strands were investigated in terms of C doping levels, heat treatment (HT) time and filament numbers. The highest layer Jc for our monofilamentary AIMI strands is 1.5 x 10^5 A/cm2 at 10 T, 4.2 K, when the C concentration was 3 mol% and the strand was heat-treated at 675 °C for 4 hours. Transport critical currents were also measured at 4.2 K on short samples and one-meter segments of eighteen-filament C-doped AIMI strands. The layer Jcs reached 4.3 x 10^5 A/cm2 at 5 T and 7.1 x 10^4 A/cm2 at 10 T, twice as high as those of the best PIT strands. The analysis of these results indicates that the AIMI strands, possessing both high layer Jcs and engineering Jes after further optimization, have strong potential for commercial applications.

cond-mat.supr-con↗

arXiv2013

Full superconducting gap in the doped topological crystalline insulator, Sn$_{0.6}$In$_{0.4}$Te

The thermal conductivity of the doped topological crystalline insulator, Sn$_{0.6}$In$_{0.4}$Te superconducting single crystal with $T_c =$ 4.1 K, was measured down to 50 mK. It is found that the residual linear term $κ_0/T$ is negligible in zero magnetic field. The $κ_0/T$ shows a slow field dependence at low magnetic field. These results suggest that the superconducting gap is nodeless, unless there exist point nodes with directions perpendicular to the heat current. Due to its high-symmetry fcc crystal structure of Sn$_{0.6}$In$_{0.4}$Te, however, such point nodes can be excluded. Therefore we demonstrate that this topological superconductor candidate has a full superconducting gap in the bulk. It is likely the unconventional odd-parity $A_{1u}$ state which supports a surface Andreev bound state.

cond-mat.supr-con↗

arXiv2013

On a parabolic operator of dissipative systems

A parabolic integro differential operator operator L suitable to describe many phenomena in various physical fields,is considered. By means of equivalence between L and the third order equation which describe the evolution inside an exponentially shaped Josephson junction (ESJJ), an asymptotic analysis for (ESJJ) is achieved, evaluating explicitly boundary contributions related to the Dirichlet problem.

math-ph↗

arXiv2013

Andreev Reflection Like Enhancement Above Bulk $T_c$ in Electron Underdoped Iron Arsenides

We use point contact spectroscopy (PCS) to probe the superconducting properties of electron doped $\rm{Ba(Fe_{1-x}Co_x)_2As_2}$ ($\rm{x = 0.05, 0.055, 0.07, 0.08}$) and hole doped $\rm{Ba_{0.8}K_{0.2}Fe_2As_2}$. PCS directly probes the low energy density of states via Andreev reflection, revealing two distinct superconducting gaps in both compound families. Apart from the electron underdoped $\rm{Ba(Fe_{1-x}Co_{x})_2As_2}$, the excess current due to Andreev reflection for the compounds follows the typical BCS temperature dependence. For underdoped $\rm{Ba(Fe_{1-x}Co_{x})_2As_2}$, the temperature dependence of the excess current deviates from that of BCS, developing a tail at higher temperatures and surviving above bulk $T_c$. Possible explanations for this anomalous behavior are explored.

cond-mat.supr-con↗

arXiv2013

Modeling the linewidth dependence of coherent terahertz emission from intrinsic Josephson junction stacks in the hot-spot regime

Recently it has been found that, when operated at large input power, the linewidth of terahertz radiation emitted from intrinsic Josephson junction stacks can be as narrow as some megahertz. In this high-bias regime a hot spot coexists with regions which are still superconducting. Surprisingly, the linewidth was found to decrease with increasing bath temperature. We present a simple model describing the dynamics of the stack in the presence of a hot spot by two parallel arrays of pointlike Josephson junctions and an additional shunt resistor in parallel. Heat diffusion is taken into account by thermally coupling all elements to a bath at temperature T_b. We present current-voltage characteristics of the coupled system and calculations of the linewidth of the radiation as a function of T_b. In the presence of a spatial gradient of the junction parameters critical current and resistance, the linewidth deceases with increasing T_b, similar to the experimental observation.

cond-mat.supr-con↗

arXiv2013

Polaron-bipolaron superconductivity and the intrinsic pairing symmetry in cuprate superconductors

We have calculated the tunneling conductance of a superconductor-insulator-superconductor junction based on the polaron-bipolaron theory of superconductivity. The predicted incoherent hump features are in quantitative agreement with tunneling spectra of optimally doped Bi2Sr2CaCu2O8+y and Bi2Sr2Ca2Cu3O10+y. We further show that angle-resolved photoemission spectra of underdoped cuprates are consistent with the Bose-Einstein condensation of inter-site bipolarons and that the superconducting gap symmetry is d-wave, which is determined by the anomalous kinetic process rather than by the pairing interaction. In the overdoped cuprates (BCS-like superconductors), the superconducting gap symmetry is the same as the pairing symmetry, which is found to be extended s-wave with eight line nodes in hole-doped cuprates and nodeless s-wave in electron-doped cuprates. The polaronic effect significantly enhances the density of states at the Fermi level and the effective electron-phonon coupling constant for low-energy phonon modes, which is the key to the understanding of high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2013

Hydrostatic pressures dependence of superconductivity in PrO0.5F0.5BiS2 superconductor

In this communication, we report the temperature dependence (3 to 300K) of the electrical resistivity of BiS2 based layered PrO0.5F0.5BiS2 superconductor at ambient and hydrostatic pressure of up to 3GPa. It is observed that Tc increases with pressure at the rate of dTc/dP=0.45/GPa for PrO0.5F0.5BiS2 compound. It is envisaged that one may increase the Superconducting transition temperature (Tc) of recently discovered PrO0.5F0.5BiS2 superconductor by applying hydrostatic external or internal chemical pressure via suitable on site substitutions.

cond-mat.supr-con↗

arXiv2013

An energy gap in the spectrum of atomic excitations systems

It is shown that atoms in the system can be treated as quantum particles localized in potential wells, created by atomic potentials of neighboring atoms. As a result, the state of atoms in the liquid is characterized by wave functions and the discrete energy spectrum resulting in formation of s and p - zones corresponding to the ground and excited states of helium atoms, respectively, separated by a gap. The width of the gap in system equals ~8.5 K at T=0. Formation of the gap in the energy spectrum of atomic excitations in helium systems allows us to draw the analogy between the physical mechanisms of superfluidity and classical superconductivity.

cond-mat.other↗

arXiv2013

Effect of a Pseudogap in Strong Coupling d-Wave Superconductivity

A phenomenological strong coupling model that has been used to analyze superconducting- insulator-superconducting (SIS) break junction experiments on optimal to overdoped Bi-2212 is modified to include a pseudogap. The calculated density of states and SIS conductances are compared with experimental data on the underdoped phase of Bi-2212

cond-mat.supr-con↗

arXiv2013

Microstructure and Structural Phase Transitions in Iron-Based Superconductors

Crystal structures and microstructural features, such as structural phase transitions, defect structures, chemical and structural inhomogeneities, are known to have profound effects on the physical properties of superconducting materials. Recently, a large amount of works on the structural properties of Fe-based high-Tc superconductors have been published. This review article will mainly focus on typical microstructural features in samples that have been well characterized by physical measurements. (a) Certain common structural features are discussed. In particular, crystal structural features for different superconducting families, local structural distortions in the Fe2Pn2 (Pn=P, As, Sb) or Fe2Ch2 (Ch=S, Se, Te) blocks, and structural transformations in the 122 system. (b) In FeTe(Se) (11-family), the superconductivity, chemical and structural inhomogeneities are investigated and discussed in correlation with superconductivity. (c) In K0.8Fe1.6+xSe2 system, we focus on typical compounds with emphasis on Fe-vacancy order and phase separations. The microstructural features in other superconducting materials are also briefly discussed.

cond-mat.supr-con↗

arXiv2013

Releasing half-quantum vortices via the coupling of spin polarization, charge- and spin-current

Motivated by recent experiments observing half-integer flux quanta in mesoscopic loops of superconducting Sr2RuO4, a theory is developed based on a "d-soliton", a topological defect of spin-triplet superconductors involving a change of the spin configuration. A phenomenological Ginzburg-Landau free energy is given which allows to describe such a soliton yielding a half-integer flux quantum inside a loop, assuming a chiral p-wave superconducting phase. The d-soliton couples to spin polarization perpendicular to the loop axis, which allows to reduce the energy of the d-soliton, fitting well to the experimental findings. The origin of this coupling lies in the combination of spin and charge current within the d-soliton generating a spin polarization analogous to a spin Hall effect. The effect of Fermi liquid corrections is discussed, in particular, in view of the spatial extension of the d-soliton and the energetics in comparison with a standard integer flux state.

cond-mat.supr-con↗

arXiv2013

Pressure effects and orbital characters in cuprate and carbon-based superconductors

Pressure effect is overviewed for the cuprates and carbon-based superconductors, with an emphasis on how their orbital characters are modified by pressure. For the high-Tc cuprates, we start from an observation for ambient pressure that, on top of the main orbital (dx2-y2), a hybridization with the second (dz2) orbital around the Fermi energy significantly affects Tc in the spin-fluctuation mediated pairing, where the hybridization is dominated by material parameters. We can then show that applying pressures along a, b axes enhances Tc while a c axis pressure suppresses Tc, where not only the dz2 hybridization but also Cu(4s) hybridization exert an effect. For the multi-layer cuprates, inter-layer pair hopping is suggested to be important, which may contribute to pressure effect. Pressure effect is also interesting in a recently discovered aromatic family of superconductors (picene, etc). There, we have again multi-band systems, which in this case derive from different molecular orbitals. The Fermi surface is an intriguing composite of different pockets/sheets having different dimensionalities arising from anisotropic transfers between the molecular orbitals, and pressure effects should be an important probe of these.

cond-mat.supr-con↗