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We analyze the critical behavior of the superfluid density ps in strongly disordered superconductors near the superconductor-insulator transition and compare it with the behavior of the spectral gap Δ for collective excitations. We show that, in contrast to conventional superconductors, the superconductors with preformed pairs display an unusual scaling relation ps ∞ Δ2 close to the superconductor-insulator transition. This relation has been reported in very recent experiments. [ABSTRACT FROM AUTHOR]

Recent experiments have unveiled important properties of the ground state of the elusive heavy fermion URu2Si2. While tetragonal symmetry-breaking was reported below the hidden-order (HO) transition at THO≈17.5K, time-reversal symmetry breaking was observed below the superconducting transition temperature Tc

We perform time-resolved photoelectron spectroscopy measurements of optimally doped Bi2Sr2CaCu2C>8+δ (Bi-2212) and Bi2Sr2-xLaxCuO6+δ (Bi-2201). The electron dynamics shows that inelastic scattering by nodal quasiparticles decreases when the temperature is lowered below the critical value of the superconducting phase transition. This drop in electronic dissipation is astonishingly robust and survives to photoexcitation densities much larger than the value sustained by long-range superconductivity. The unconventional behavior of quasiparticle scattering is ascribed to superconducting correlations extending on a length scale comparable to the inelastic path. Our measurements indicate that strongly driven superconductors enter in a regime without phase coherence but finite pairing amplitude. The latter vanishes near to the critical temperature and has no evident link to the pseudogap observed by angle-resolved photoelectron spectroscopy. [ABSTRACT FROM AUTHOR]

We report magnetic and superconducting properties of the modified spin-valve system CoOx/Fe1/Cu/Fe2/Cu/Pb. Introduction of a Cu interlayer between Fe2 and Pb layers prevents material interdiffusion process, increases the Fe2/Pb interface transparency, stabilizes and enhances properties of the system. This allowed us to perform a comprehensive study of such heterostructures and to present theoretical description of the superconducting spin-valve effect and of the manifestation of the long-range triplet component of the superconducting condensate. [ABSTRACT FROM AUTHOR]

In light of recent proposals to realize a topological superconductor on the surface of strong topological insulators, we study impurity and vortex scattering in two-dimensional topological superconductivity. We develop a theory of quasiparticle interference in a model of the surface of a three-dimensional strong topological insulator with a pairing term added. We consider a variety of different scatterers, including magnetic and nonmagnetic impurity as well as a local pairing order parameter suppression associated with the presence of a vortex core. Similar to the case of a surface of a three-dimensional topological insulator without pairing, our results for nonmagnetic impurity can be explained by the absence of back scattering, as expected for a Dirac cone structure. In the superconducting case, doping away from the Dirac point leads to a doubling of the contours of constant energy. This is in contrast to the unpaired case where the chemical potential simply adds to the bias voltage and shifts the energy. This doubling of contours results in multiplying the number of possible scattering processes in each energy. Interestingly, we find that some processes are dominant in the impurity case while others are dominant in the vortex case. Moreover, the two types of processes lead to a different dependence on the chemical potential. [ABSTRACT FROM AUTHOR]

We study quantum phase slips (QPS) in ultrathin superconducting wires. Starting from an effective one-dimensional microscopic model, which includes electromagnetic fluctuations, we map the problem to a (1+1)-dimensional gas of interacting instantons. We introduce a method to calculate the tunneling amplitude of quantum phase slips directly from Monte Carlo simulations. This allows us to go beyond the dilute instanton gas approximation and study the problem without any limitations of the density of QPS. We find that the tunneling amplitude shows a characteristic scaling behavior near the superconductor-insulator transition. We also calculate the voltage-charge relation of the insulating state, which is the dual of the Josephson current-phase relation in ordinary superconducting weak links. This evolves from a sinusoidal form in the regime of dilute QPS to more exotic shapes for higher QPS densities, where interactions are important. [ABSTRACT FROM AUTHOR]

Here we report a further investigation on the thermodynamic and transport properties, and an assessment of theoretical calculations, for the BiS2-layered Bi3O2S3 superconductor. The polycrystalline sample is synthesized with a superconducting transition temperature of Tconset=5.75K and Tczero=4.03K (≈Tcmap) that drops to 3.3 K by applying a hydrostatic pressure of 6 kbar. Density-of-states (DOS) calculations give substantial hybridization between Bi, O, and S, with Bi the largest component of DOS, which supports the idea that the BiS2 layer is relevant for producing electron-phonon coupling. An analysis of previously published specific heat data for Bi3O2S3 is additionally suggestive of a strong electron-phonon interaction in the Bi-O-S system. The analysis of the Seebeck coefficient results strongly suggests that Bi3O2S3 is a semimetal. In fact, the semimetallic or narrow band gap behavior may occur in certain other materials in the BiS2-layered class of materials, such as Bi4O4S3. [ABSTRACT FROM AUTHOR]

In the recently synthesized Lix (NH2)y(NH3)z Fe2Se2 family of iron chalcogenides, a molecular spacer consisting of lithium ions, lithium amide, and ammonia separates the layers of FeSe. It has been shown that upon variation of the chemical composition of the spacer layer, superconducting transition temperatures can reach Tc ~ 44 K, but the relative importance of the layer separation and effective doping to the Tc enhancement is currently unclear. Using state of the art band structure unfolding techniques, we construct eight-orbital models from ab initio density functional theory calculations for these materials. Within an RPA spin-fluctuation approach, we show that the electron doping enhances the superconducting pairing, which is of s± symmetry and explain the experimentally observed limit to Tc in the molecular spacer intercalated FeSe class of materials. [ABSTRACT FROM AUTHOR]

Charge fluctuations in the quasi-one-dimensional material Li0.9Mo6O17 are analyzed based on a multiorbital extended Hubbard model. A charge-ordering transition induced by Coulomb repulsion is found with a particular charge-ordering pattern. The metallic phase is characterized by the presence of a charge collective mode which softens signaling the proximity to the charge order transition. We argue that the scattering between electrons mediated by these charge fluctuations can lead to non-Fermi liquid behavior in a quasi-one-dimensional system consistent with the unconventional metallic behavior observed above the superconducting transition in Li0.9Mo6O17. [ABSTRACT FROM AUTHOR]

Most iron-based superconductors undergo a transition to a magnetically ordered state characterized by staggered stripes of parallel spins. With ordering vectors (;π,0) or (0,;π), this magnetic state breaks the high-temperature tetragonal symmetry of the system, which is manifested by a splitting of the lattice Bragg peaks. Remarkably, recent experiments in hole-doped iron arsenides reported an ordered state that displays magnetic Bragg peaks at (π,0) and (0,π) but remains tetragonal. Despite being inconsistent with a magnetic stripe configuration, this unusual magnetic phase can be described in terms of a double-Q magnetic structure consisting of an equal-weight superposition of the ordering vectors (;π,0) and (0,;π). Here we show that a noncollinear double-Q magnetic configuration, dubbed orthomagnetic, arises naturally within an itinerant three-band microscopic model for the iron pnictides. In particular, we find that strong deviations from perfect nesting and residual interactions between the electron pockets favor the orthomagnetic over the stripe magnetic state. Using an effective low-energy model, we also calculate the spin-wave spectrum of the orthomagnetic state. In contrast to the stripe state, there are three Goldstone modes, manifested in all diagonal and one off-diagonal components of the spin-spin correlation function. The total magnetic structure factor displays two anisotropic gapless spin-wave branches emerging from both (π,0) and (0,π) momenta, in contrast to the case of domains of stripe order, where only one gapless spin-wave branch emerges from each momentum. We propose that these unique features of the orthomagnetic state can be used to unambiguously distinguish it from the stripe state via neutron scattering experiments, and discuss the implications of its existence for the nature of the magnetism of the iron arsenides. [ABSTRACT FROM AUTHOR]

We present a detailed study of single crystalline K2Cr3As3 and analyze its thermodynamic and transport properties, anisotropic Hc2(T), and initial pressure dependence of Tc. In zero field, the temperature-dependent resistivity is metallic. Deviation from a linear temperature dependence is evident below 100 K and a T3 dependence is roughly followed from just above Tc (∼10K) to ∼40 K. Anisotropic Hc2(T) data were measured up to 140 kOe with field applied along and perpendicular to the rodlike crystals. For the applied field perpendicular to the rod, Hc2(T) is linear with a slope ∼ -70 kOe/K. For field applied along the rod, the slope is about -120 kOe/K below 70 kOe. Above 70 kOe, the magnitude of the slope decreases to ∼ - 70 kOe/K. The electronic specific heat coefficient γ, just above Tc, is 73 mJ/mol K2; the Debye temperature ΘD is 220 K. The specific heat jump at the superconducting transition ΔC ∼ 2.2γTc. Finally, for hydrostatic pressures up to ∼7 kbar, Tc decreases under pressure linearly at a rate of -0.034K/kbar. [ABSTRACT FROM AUTHOR]

We address the kinetic competition between charge striped order and superconductivity in La1.675Eu0.2Sr0.125Cu04. Ultrafast optical excitation is tuned to a midinfrared vibrational resonance that destroys charge order and promptly establishes transient coherent interlayer coupling in this material. This effect is evidenced by the appearance of a longitudinal plasma mode reminiscent of a Josephson plasma resonance. We find that coherent interlayer coupling can be generated up to the charge-order transition TCo ≈80 K, far above the equilibrium superconducting transition temperature of any single layer cuprate. Two key observations are extracted from the relaxation kinetics of the interlayer coupling. First, the plasma mode relaxes through a collapse of its coherence length and not its density. Second, two distinct kinetic regimes are observed for this relaxation, above and below spin-order transition Tso≈ 25 K. In particular, the temperature-independent relaxation rate observed below Tso is anomalous and suggests coexistence of superconductivity and stripes rather than competition. Both observations support arguments that a low temperature coherent stripe (or pair density wave) phase suppresses c-axis tunneling by disruptive interference rather than by depleting the condensate. [ABSTRACT FROM AUTHOR]

Following the discovery of superconductivity in quasi-one-dimensional K2Cr3As3 containing [(Cr3As3)2-]∞ chains (J. K. Bao et al., arXiv:1412.0067), we succeeded in synthesizing an analogous compound, Rb2Cr3As3, which also crystallizes in a hexagonal lattice. The replacement of K by Rb results in an expansion of the a axis by 3%, indicating a weaker interchain coupling in Rb2Cr3As3. Bulk superconductivity emerges at 4.8 K, above which the normal-state resistivity shows a linear temperature dependence up to 35 K. The estimated upper critical field at zero temperature exceeds the Pauli paramagnetic limit by a factor of 2. Furthermore, the electronic specific-heat coefficient extrapolated to zero temperature in the mixed state increases with √H, suggesting the existence of nodes in the superconducting energy gap. Hence Rb2Cr3 As3 manifests itself as another example of an unconventional superconductor in the Cr3As3-chain based system. [ABSTRACT FROM AUTHOR]

We have used scanning micro x-ray diffraction to characterize different phases in superconducting KxFe2-ySe2 as a function of temperature, unveiling the thermal evolution across the superconducting transition temperature (Tc ∼ 32 K), phase separation temperature (Tps ∼ 520 K), and iron-vacancy order temperature (Tvo ∼ 580 K). In addition to the iron-vacancy ordered tetragonal magnetic phase and orthorhombic metallic minority filamentary phase, we have found clear evidence of the interface phase with tetragonal symmetry. The metallic phase is surrounded by this interface phase below ∼300 K, and is embedded in the insulating texture. The spatial distribution of coexisting phases as a function of temperature provides clear evidence of the formation of protected metallic percolative paths in the majority texture with large magnetic moment, required for the electronic coherence for the superconductivity. Furthermore, a clear reorganization of iron-vacancy order around the Tps and Tc is found with the interface phase being mostly associated with a different iron-vacancy configuration, that may be important for protecting the percolative superconductivity in KA Fe2-ySe2. [ABSTRACT FROM AUTHOR]

The magnetization and magnetic torque of a high-quality single crystal of Sr2RuO4 have been measured down to 0.1 K under precise control of the magnetic-field orientation. When the magnetic field is applied exactly parallel to the ab plane, a sharp magnetization jump 4πδM of (0.74 ± 0.15) G at the upper critical field Hc2.ab ~ 15 kOe with a field hysteresis of 100 Oe is observed at low temperatures, evidencing a first-order superconducting-normal transition. A strong magnetic torque appearing when H is slightly tilted away from the ab plane confirms an intrinsic anisotropy Γ = ξaξc of as large as 60 even at 100 mK, in contrast with the observed Hc2 anisotropy of ~20. The present results raise fundamental issues in both the existing spin-triplet and spin-singlet scenarios, providing, in turn, crucial hints toward the resolution of the superconducting nature of Sr2RuO4. [ABSTRACT FROM AUTHOR]

We propose a hole-doped molybdenum disulfide (MoS2) superconducting spin valve (F/S/F) hybrid structure in which the Andreev reflection process is suppressed for all incoming waves with a determined range of the chemical potential in ferromagnetic (F) region and the cross-conductance in the right F region depends crucially on the configuration of magnetizations in the two F regions. Using the scattering formalism, we find that the transport is mediated purely by elastic electron cotunneling (CT) process in a parallel configuration and changes to the pure crossed Andreev reflection (CAR) process in the low-energy regime, without fixing of a unique parameter, by reversing the direction of magnetization in the right F region. This suggests both valley- and spin-switch effects between the perfect elastic CT and perfect CAR processes and makes the nonlocal charge current to be fully valley- and spin-polarized inside the right F region where the type of the polarizations can be changed by reversing the magnetization direction in the right F region. We further demonstrate that the presence of the strong spin-orbit interaction λ and an additional topological term (β) in the Hamiltonian of MoS2 result in an enhancement of the charge conductance of the CT and CAR processes and make them to be present for long lengths of the superconducting region. Besides, we find that the thermal conductance of the structure with a small length of the highly doped superconducting region exhibits linear dependence on the temperature at low temperatures, whereas it enhances exponentially at higher temperatures. In particular, we demonstrate that the thermal conductance versus the strength of the exchange field (h) in F region displays a maximum value at h < λ, which moves towards larger exchange fields by increasing the temperature. [ABSTRACT FROM AUTHOR]

The magnetic penetration depth (λ) as a function of applied magnetic field and temperature in SrPt3P (Tc 8.4 K) was studied by means of muon-spin rotation (μSR). The dependence of λ-2 on temperature suggests the existence of a single s-wave energy gap with the zero-temperature value Δo = 1.55(2) meV. At the same time λ was found to be strongly field dependent which is the characteristic feature of the nodal gap and/or multiband systems. The multiband nature of the superconducting state is further suggested by the upward curvature of the upper critical field. This apparent contradiction is resolved by SrPt3P being a particular two-band superconductor with equal gaps but different coherence lengths within the two Fermi surface sheets. [ABSTRACT FROM AUTHOR]

A Bose insulator composed of a low density of strongly localized Cooper pairs develops at the two-dimensional superconductor to insulator transition (SIT) in a number of thin film systems. Investigations of ultrathin amorphous PbBi films far from the SIT described here provide evidence that the Bose insulator gives way to a second insulating phase with decreasing film thickness. At a critical film thickness dc the magnetoresistance changes sign from positive, as expected for boson transport, to negative, as expected for fermion transport, signs of local Cooper-pair phase coherence effects on transport vanish, and the transport activation energy exhibits a kink. Below dc pairing fluctuation effects remain visible in the high-temperature transport while the activation energy continues to rise. These features show that Cooper pairing persists and suggest that the localized unpaired electron states involved in transport are interspersed among regions of strongly localized Cooper pairs in this strongly localized, low Cooper-pair density phase. [ABSTRACT FROM AUTHOR]

We investigate the pressure and temperature dependence of the lattice dynamics of the underdoped, stoichiometric, high-temperature superconductor YBa2Cu4O8 by means of Raman spectroscopy and ab initio calculations. This system undergoes a reversible pressure-induced structural phase transition around 10 GPa to a collapsed orthorhombic structure that is well reproduced by the calculation. The coupling of the B1g-like buckling phonon mode to the electronic continuum is used to probe superconductivity. In the low pressure phase, self-energy effects through the superconducting transition renormalize this phonon, and the amplitude of this renormalization strongly increases with pressure. Whereas our calculation indicates that this mode's coupling to the electronic system is only marginally affected by the structural phase transition, the aforementioned renormalization is completely suppressed in the high pressure phase, demonstrating that under hydrostatic pressures higher than 10 GPa, superconductivity in YBa2Cu4O8 is greatly weakened or obliterated. [ABSTRACT FROM AUTHOR]

The effects of hydrostatic pressure and partial Na substitution on the normal-state properties and the superconducting transition temperature (Tc) of K1-xNaxFe2As2 single crystals were investigated. It was found that a partial Na substitution leads to a deviation from the standard T² Fermi-liquid behavior in the temperature dependence of the normal-state resistivity. It was demonstrated that non-Fermi-liquid like behavior of the resistivity for K1-xNaxFe2As2 and some KFe2As2 samples can be explained by a disorder effect in the multiband system with rather different quasiparticle effective masses. Concerning the superconducting state our data support the presence of a shallow minimum around 2 GPa in the pressure dependence of Tc for stoichiometric KFe2As2. The analysis of Tc in K1-xNaxFe2As2 at pressures below 1.5 GPa showed that the reduction of Tc with Na substitution follows the Abrikosov-Gor'kov law with the critical temperature Tc0 of the clean system (without pair breaking), which linearly depends on the pressure. Our observations also suggest that Tc of K1-xNa Fe2As2 is nearly independent of the lattice compression produced by the Na substitution. Further, we theoretically analyzed the behavior of the band structure under pressure within the generalized gradient approximation (GGA). A qualitative agreement between the calculated and the recently measured--in de Haas-van Alphen experiments [T. Terashima et al., Phys. Rev. B 89, 134520 (2014)]--pressure dependencies of the Fermi-surface cross sections has been found. These calculations also indicate that the observed minimum around 2 GPa in the pressure dependence of Tc may occur without a change of the pairing symmetry. [ABSTRACT FROM AUTHOR]

The physics of strongly correlated quantum particles within a flat band was originally explored as a route to itinerant ferromagnetism and, indeed, a celebrated theorem by Lieb rigorously establishes that the ground state of the repulsive Hubbard model on a bipartite lattice with an unequal number of sites in each sublattice must have nonzero spin S at half filling. Recently, there has been interest in Lieb geometries due to the possibility of topological insulator, nematic, and Bose-Einstein condensed (BEC) phases. In this paper, we extend the understanding of the attractive Hubbard model on the Lieb lattice by using determinant quantum Monte Carlo to study real space charge and pair correlation functions not addressed by the Lieb theorems. Specifically, our results show unusual charge and charge transfer signatures within the flat band, and a reduction in pairing order at ρ = 2/3 and ρ = 4/3, the points at which the flat band is first occupied and then completely filled. We compare our results to the case of flat bands in the Kagome lattice and demonstrate that the behavior observed in the two cases is rather different. [ABSTRACT FROM AUTHOR]

Pairing in the Weyl semimetal appearing on the surface of a topological insulator is considered. It is shown that due to an "ultrarelativistic" dispersion relation there is a quantum critical point governing the zero-temperature transition to a superconducting state. Starting from the microscopic Hamiltonian with local attraction, we calculated using the Gor'kov equations, the phase diagram of the superconducting transition at arbitrary chemical potential, and its magnetic properties and critical exponents close to the quantum critical point. The Ginzburg- Landau (GL) effective theory is derived for small chemical potential, allowing us to consider effects of spatial dependence of order parameters in a magnetic held. The GL equations are very different from the conventional ones reflecting the chiral universality class of the quantum phase transition. The order-parameter distribution of a single vortex is found to be different as well. The magnetization near the upper critical held is found to be quadratic, not linear as usual. We discuss the application of these results to recent experiments in which surface superconductivity was found for some three-dimensional topological insulators, and we estimate feasibility of the phonon pairing. [ABSTRACT FROM AUTHOR]

The prospect of combining the dissipationless nature of superconducting currents with the spin polarization of magnetic materials is interesting with respect to exploring superconducting analogs of topics in spintronics. In order to accomplish this aim, it is pivotal to understand not only how such spin supercurrents can be created, but also how they interact dynamically with magnetization textures. In this paper, we investigate the appearance of a spin supercurrent and the resulting magnetization dynamics in a textured magnetic Josephson current by using three experimentally relevant models: (i) a superconductor [ABSTRACT FROM AUTHOR]

Measurements of the specific heat jump at the onset of superconducting transition in the iron-pnictide compounds revealed strong variation of its magnitude as a function of doping that is peaked near the optimal doping. We show that this behavior is a direct manifestation of the coexistence between spin-density wave and superconducting orders and the peak originates from thermal fluctuations of the spin-density waves near the end point of the coexistence phase-a tetracritical point. Thermal fluctuations result in a power-law dependence of the specific heat jump that is stronger than the contribution of mass renormalization due to quantum fluctuations of spin-density waves in the vicinity of the putative critical point beneath the superconducting dome. [ABSTRACT FROM AUTHOR]

Superconducting condensation energy U0int has been determined by integrating the electronic entropy in various iron pnictide/chalcogenide superconducting systems. It is found that U0int ∝ Tcn with n = 3-4, which is in sharp contrast to the simple BCS prediction ψ0BCS = 1/2NFΔs2, with NF the quasiparticle density of states at the Fermi energy and Δs the superconducting gap. A similar correlation holds if we compute the condensation energy through U0cal = 3YneffΔs2/4π2k2B, with Yneff the effective normal state electronic specific heat coefficient. This indicates a general relationship Yneff ∝ Tcm with m = 1-2, which is not predicted by the BCS scheme. A picture based on quantum criticality is proposed to explain this phenomenon. [ABSTRACT FROM AUTHOR]

Electronic nematics, an electron orientational order which breaks the underlying rotational symmetry, were observed in iron pnictide superconductors several years after their discovery. However, the universality of the doping dependence of this phase and its relation to other symmetry-breaking orders (such as superconductivity) in distinct families of iron pnictides remain outstanding questions. Here we use torque magnetometry as a probe to study the rotational symmetry breaking in EuFe2(As1-xPx)2 without introducing external pressure. The nematic phase is found to proliferate well above the structural transition and to persist into the superconducting regime at optimal doping, after which it becomes absent or very weak, in sharp contrast to the behavior observed in BaFe2(As1-xPx)2. These measurements suggest a putative quantum nematic transition near optimal doping under the superconducting dome. [ABSTRACT FROM AUTHOR]

We study the role of impurities in a two-band superconductor, and elucidate the nature of the recently predicted transition from the s± state to the s++ state induced by interband impurity scattering. Using a Ginzburg-Landau theory, derived from microscopic equations, we demonstrate that close to Tc this transition is necessarily a direct one, but deeper in the superconducting state an intermediate complex state appears. This state has a distinct order parameter, which breaks the time-reversal symmetry, and is separated from the s± and s++ states by continuous phase transitions. Based on our results, we suggest a phase diagram for systems with weak repulsive interband pairing, and discuss its relevance to iron-based superconductors. [ABSTRACT FROM AUTHOR]

We present the evolution of the initial (up to ~10 kbar) hydrostatic pressure dependencies of Tc and of the ambient pressure, and the jump in the heat capacity associated with the superconducting transition as a function of Na doping in the Ba1-xNaxFe2As2 family of iron-based superconductors. For Na concentrations 0.15⩽x⩽0.9, the jump in specific heat at Tc, ▵ Cp\Tcfollows the ▵Cp ∞ T3 (the so-called BNC scaling) found for most BaFe2As2-based superconductors. This finding suggests that, unlike the related Ba1-xKxFe2As2 series, there is no significant modification of the superconducting state (e.g., change in superconducting gap symmetry) in the Ba1-xNaxFe2As2 series over the whole studied Na concentration range. Pressure dependencies are nonmonotonic for x = 0.2 and 0.24. For other Na concentrations, Tc decreases under pressure in an almost linear fashion. The anomalous behavior of the x = 0.2 and 0.24 samples under pressure is possibly due to the crossing of the phase boundaries of the narrow antiferromagnetic tetragonal phase, unique for the Ba1-xNaxFe2As2 series, with the application of pressure. The negative sign of the pressure derivatives of Tc across the whole superconducting dome (except for x = 0.2) is a clear indication of the nonequivalence of substitution and pressure for the Ba1-xNaxFe2As2 series. [ABSTRACT FROM AUTHOR]

We discuss the phase diagram and phase transitions in U(1) x Z2 three-band superconductors with broken time reversal symmetry. We find that beyond mean-field approximation and for sufficiently strong frustration of interband interactions there appears an unusual metallic state precursory to a superconducting phase transition. In that state, the system is not superconducting. Nonetheless, it features a spontaneously broken Z2 time reversal symmetry. By contrast, for weak frustration of interband coupling the energy of a domain wall between different Z2 states is low and thus fluctuations restore broken time reversal symmetry in the superconducting state at low temperatures [ABSTRACT FROM AUTHOR]

The Berezinskii-Kosterlitz-Thouless (BKT) transition is a generic transition describing the loss of coherence in two-dimensional systems and has been invoked, for example, to describe the superconductor-insulator transition in thin films. However, recent experiments have shown that the BKT transition, driven by phase fluctuations, is not sufficient to describe the loss of superconducting order, and amplitude fluctuations must also be taken into account. The standard models that are extensively used to model two-dimensional superconductors are the Hubbard and XY models. Whereas the XY model allows only phase fluctuations, the Hubbard model has an extra degree of freedom: amplitude fluctuations. In this paper we compare two Hubbard models with the same critical temperature but with different interactions and deduce the role of the amplitude fluctuations in the superconducting transition. For this purpose, a novel approximation is presented and used. We derive an effective phase-only (XY) Hamiltonian, incorporating amplitude fluctuations in an explicit temperature dependence of the phase rigidity. We study the relation between amplitude fluctuations and coupling strength. Our results support existing claims about the suppression of phase rigidity due to amplitude fluctuations not present in the XV model. [ABSTRACT FROM AUTHOR]

We present measurements of the temperature-dependent conductance for series arrays of small-capacitance superconducting quantum interference devices (SQUIDs). At low-bias voltages, the arrays exhibit a strong Coulomb blockade, which we study in detail as a function of temperature and Josephson energy EJ. We find that the zero-bias conductance is dominated by a thermally activated hopping of Cooper pairs between neighboring superconductive islands with the activation energy of the order of ΛEC, where Λ is the charge screening length in the array and EC is the charging energy of a single SQUID. [ABSTRACT FROM AUTHOR]

Using a high-resolution differential technique we have determined the electronic specific heat coefficient γ(T)=Cel/T of Ba1-xKxFe2As2 with x=0 to 1.0, at temperatures (T) from 2 K to 380 K and in magnetic fields H=0 to 13 T. In the normal state γn(x,T) increases strongly with x at low temperature, compatible with a mass renormalization ~12 at x=1, and decreases weakly with x at high temperature. A superconducting transition is seen in all samples from x=0.2 to 1, with transition temperatures and condensation energies peaking sharply at x=0.4. Superconducting coherence lengths ξab~20 Å and ξc~3 Å are estimated from an analysis of Gaussian fluctuations. For many dopings we see features in the H and T dependencies of γs(T,H) in the superconducting state that suggest superconducting gaps in three distinct bands. A broad "knee" and a sharp mean-field-like peak are typical of two coupled gaps. However, several samples show a shoulder above the sharp peak with an abrupt onset at Tc,s and a T dependence γs(T)∝√1-T/Tc,s. We provide strong evidence that the shoulder is not due to doping inhomogeneity and suggest it is a distinct gap with an unconventional T dependence Δs(T)∝(1-T/Tc,s)0.75 near Tc,s. We estimate band fractions and T=0 gaps from three-band α-model fits to our data and compare the x dependencies of the band fractions with spectroscopic studies of the Fermi surface. [ABSTRACT FROM AUTHOR]

We present measurements of the superconducting coherence length ξ in thin (d≤100−Å) films of MoGe and Nb using a combination of linear and nonlinear mutual inductance techniques. As the alternating current in the drive coil is increased at a fixed temperature, we see a crossover from linear to nonlinear coupling to the pickup coil, consistent with the unbinding of vortex-antivortex pairs as the peak pair momentum nears ℏ/ξ and the unbinding barrier vanishes. We compare measurements of ξ performed by this mutual inductance technique to values determined from the films' upper critical fields, thereby confirming the applicability of a recent calculation of the upper limit on a vortex-free state in our experiment. [ABSTRACT FROM AUTHOR]

We study the influence of the band structure on the symmetry and superconducting transition temperature in the (solvable) weak-coupling limit of the repulsive Hubbard model. Among other results we find that (1) as a function of increasing nematicity, starting from the square-lattice (zero nematicity) limit where a nodal d-wave state is strongly preferred, there is a smooth evolution to the quasi-1D limit, where a striking near-degeneracy is found between a p-wave- and a d-wave-type paired states with accidental nodes on the quasi-one-dimensional Fermi surfaces―a situation that may be relevant to the Bechgaard salts. (2) In a bilayer system, we find a phase transition as a function of increasing bilayer coupling from a d-wave to an s±-wave state reminiscent of the iron-based superconductors. (3) When an antinodal gap is produced by charge-density-wave order, not only is the pairing scale reduced, but the symmetry of the pairs switches from dx2-y² to dxy in the context of the cuprates, this suggests that were the pseudogap entirely due to a competing CDW order, this would likely cause a corresponding symmetry change of the superconducting order (which is not seen in experiment). [ABSTRACT FROM AUTHOR]

The evolution with carrier concentration of the electrical properties of amorphous indium oxide (InOx) thin films has been studied using electric double-layer transistor configurations. Carrier variations of up to 7 × 1014 carriers/cm² were achieved using an ionic liquid as a gate dielectric. The superconductor-insulator transition was traversed, and the magnitude and position of the large magnetoresistance peak found in the insulating regime were modified. The systematic variation of the magnetoresistance peak with charge concentration was found to be qualitatively consistent with a simulation based on a model involving granularity. [ABSTRACT FROM AUTHOR]

Using scanning tunneling spectroscopy, we address the problem of the superconductor-insulator phase transition in homogeneously disordered ultrathin (2–15 nm) films of NbN. Samples thicker than 8 nm, for which the Ioffe-Regel parameter kFl≥5.6, manifest a conventional superconductivity: a spatially homogeneous BCS-like gap, vanishing at the critical temperature, and a disordered vortex lattice in magnetic field. Upon thickness reduction, however, while kFl lowers, the STS reveals striking deviations from the BCS scenario, among which a progressive decrease of the coherence peak height and small spatial inhomogeneities. In addition, the gap below TC develops on a spectral background, which becomes more and more “V-shaped” approaching the localization. The thinnest film (2.16 nm), while not being exactly at the superconductor-insulator transition (SIT) (TC≈0.4TCbulk), showed unconventional signatures such as the vanishing of the coherence peaks and the absence of vortices. This behavior suggests a weakening of long-range phase coherence, when approaching the SIT in this quasi-2D limit. [ABSTRACT FROM AUTHOR]

Superconducting transition under pressure (P) has been investigated for optimum-doped RFeAsO1-xFx (R=La, Ce-Sm) by dc magnetization measurements. For R=La, Tc is found to be pressure independent up to P~3.0 GPa and then shows a monotonic decrease with increasing pressure. The plateau width decreases for the system with smaller lattice constants and shrinks to almost zero for R=Sm. From the Tc(P) data, we construct the Tc evolution map on the height of the As atom from the Fe-plane hAs versus lattice-constant a or c plane. It is shown that the characteristic Tc variations under pressure for all compounds and the rapid decrease in Tc induced by changing the R element from Sm to La can be described by a common Tc (hAs, a or c) surface, suggesting that Tc is determined by hAs and lattice constant. It is also suggested that there exists an upper limit of the lattice constant aulm (or culm), above which dTc/da (or dTc/dc) changes the sign from positive to negative. [ABSTRACT FROM AUTHOR]

We measured the resistivity and the specific heat of CsFe2As2 single crystals, which were grown by using a self-flux method. The CsFe2As2 crystal shows a sharp superconducting transition at 1.8 K with a transition width of 0.1 K. The sharp superconducting transition and pronounced jump in specific heat indicate high quality of the crystals. A large ΔCp/Tc accompanying a very low Tc, nonexponential dependence of the lowest temperature ΔCp/ T and an H1/2 dependence of Δγ(H) are observed in the superconducting-state specific heat; Combined with the fitting results by the α model for the superconducting-state specific heat, a pairing symmetry with nodal gaps was suggested in CsFe2As2. [ABSTRACT FROM AUTHOR]

To investigate the origin of a phase transition at TQ=0.06 K simultaneously occurring with a superconducting transition in a cage compound PrRh2Zn20, we carried out ultrasonic measurements on a single-crystalline sample. The transverse modulus (C11-C12)/2 is intimately coupled to the non-Kramers ground doublet Γ3, and elastic softening is observed at low temperatures. Below TQ, the softening stops, suggesting the disappearance of quadrupole degrees of freedom. We clarified the negative quadrupole-quadrupole coupling constant and reentrant behavior of TQ(H) in a magnetic field H. These results reveal that the phase transition at TQ is antiferroquadrupolar ordering. The anisotropic magnetic field-temperature phase diagram is determined for H ∥ [100], [110], and [111]. A magnetic-field-induced phase transition is newly found at high fields in all three field directions. We also observed ultrasonic dispersion at around 50 K owing to the rattling motion of Zn atoms at the 16c site, and pointed out the strong electron-phonon coupling in PrRh2Zn20. [ABSTRACT FROM AUTHOR]

We have observed hysteresis in superconducting resistive transition curves of Ba0.07K0.93Fe2As2 (Tc ∼ 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° or more. The temperature and angle dependencies 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. [ABSTRACT FROM AUTHOR]

Occurrence of the Berezinskii-Kosterlitz-Thouless (BKT) transition is investigated by superfluid density measurements for two-dimensional (2D) disordered NbN films with disorder level very close to a superconductor-insulator transition (SIT). Our data show a robust BKT transition even near this 2D disorder-tuned quantum critical point. This observation is in direct contrast with previous data on deeply underdoped quasi-2D cuprates near the SIT. As our NbN films approach the quantum critical point, the vortex core energy, an important energy scale in the BKT transition, scales with the superconducting gap, not with the superfluid density, as expected within the standard 2D-.XT model description of BKT physics. [ABSTRACT FROM AUTHOR]

We investigate theoretically and experimentally the statistical properties of the inhomogeneous order-parameter distribution (OPD) at the verge of the superconductor-insulator transition (SIT). We find within two prototype fermionic and bosonic models for disordered superconductors that one can identify a universal reseating .of. the OPD. By performing scanning-tunneling microscopy experiments in three samples of NbN with increasing disorder we show that such a reseating also describes the experimental data with excellent accuracy. These results . can provide a breakthrough in our understanding of the SIT. [ABSTRACT FROM AUTHOR]

We investigated the annealing effects on the physical properties of SrFe2(AS1-xPx)2 (x = 0.35). The superconducting transition temperature (Tc) increased from 26 to 33 K by annealing. The x-ray diffraction measurement suggested that the annealed crystals have the shorter/longer a/c axes and the larger pnictogen height hpn. This must be linked to the Tc enhancement by annealing. Moreover, it was found that the postannealing decreased the electronic specific heat coefficient at T = 0 K, γ, and changed the magnetic field (H) dependence from sublinear γ, α H0.7 to H linear γ, α H. This can be attributed the electronic change from dirty to clean superconductors with a s± ga. [ABSTRACT FROM AUTHOR]

We present the evolution of the initial (up to ~10 kbar) hydrostatic pressure dependencies of Tc and of the ambient pressure jump in the heat capacity associated with the superconducting transition as a function of K doping in the Ba1-xKxFe2As2 family of iron-based superconductors. The pressure derivatives show a weak but distinct anomaly near x~0.7. In the same concentration region, ΔCp∣Tc deviates from the ΔCpαT3 scaling found for most BaFe2As2-based superconductors. These results are consistent with a possible significant modification of the superconducting state, occurring near x~0.7. [ABSTRACT FROM AUTHOR]

We report an angle-dependent study of the magnetic torque t(ϑ) within the vortex state of single-crystalline LaO0.9F0.1FeAs and SmO0.9F0.1FeAs as a function of both temperature T and magnetic field H. Sharp peaks are observed at a critical angle ϑc at either side of ϑ=90°, where ϑ is the angle between H and the interplanar c axis. ϑc is interpreted as the critical depinning angle where the vortex lattice, pinned and locked by the intrinsic planar structure, unlocks and acquires a component perpendicular to the planes. We observe a series of smaller replica peaks as a function of ϑ and as ϑ is swept away from the planar direction. These suggest commensurability effects between the period of the vortex lattice and the interplanar distance leading to additional kinked vortex configurations. [ABSTRACT FROM AUTHOR]

We use the tomographic density of states (TDOS), which is a measure of the density of states for a single slice through the band structure of a solid, to study the temperature evolution of the superconducting gap in the cuprates. The TDOS provides accuracy in determining both the superconducting pair-forming strength Δ and the pair-breaking rate Γ. In both optimally and underdoped Bi7Sr2CaCu2O8+8, we find the near-nodal &916; smoothly evolves through the superconducting transition temperature--clear evidence for the existence of preformed pairs. Additionally, we find the long-observed "filling" of the superconducting gap in the cuprates is due to the strongly temperature-dependent Γ. [ABSTRACT FROM AUTHOR]

We present investigations of the superconductor-to-insulator transition (SIT) of uniform a-Bi films using a technique sensitive to Cooper pair phase coherence. The films are perforated with a nanohoneycomb array of holes to form a multiply connected geometry and subjected to a perpendicular magnetic field. Film magnetoresistances on the superconducting side of the SIT oscillate with a period dictated by the superconducting flux quantum and the areal hole density. The oscillations disappear close to the SIT critical point to leave a monotonically rising magnetoresistance that persists in the insulating phase. These observations indicate that the Cooper pair phase coherence length, which is infinite in the superconducting phase, collapses to a value less than the interhole spacing at this SIT. This behavior is inconsistent with the gradual reduction of the phase coherence length expected for a bosonic phase-fluctuation-driven SIT. This result starkly contrasts with previous observations of oscillations persisting in the insulating phase of other films implying that there must be at least two distinct classes of disorder-tuned SITs. [ABSTRACT FROM AUTHOR]

We study a model for the transverse thermoelectric response due to quantum superconducting fluctuations in a two-leg Josephson ladder, subject to a perpendicular magnetic field B and a transverse temperature gradient. Assuming a weak Josephson coupling on the rungs, the off-diagonal Peltier coefficient (αxy) and the Nernst effect are evaluated as functions of B and the temperature T. In this regime, the Nernst effect is found to exhibit a prominent peak close to the superconductor-insulator transition, which becomes progressively enhanced at low T. In addition, we derive a relation to the diamagnetic response αxy = --M/T0, where M is the equilibrium magnetization and T0 a plasma energy in the superconducting legs. [ABSTRACT FROM AUTHOR]

The paper by G. R. Berdiyorov, M. V. Milosevic, and F. M. Peeters [Phys. Rev. B 81, 144511 (2010)] studies theoretically the dynamic properties of a superconducting loop. The authors allege that their consideration of an asymmetric loop relates to our experimental results in this field. We try to show that their theoretical results can not be used for interpreting the experimental results obtained by measuring of the quantum periodicity in the critical current, dc voltage, and resistance of superconducting loops. [ABSTRACT FROM AUTHOR]

The influence of the disorder-induced Anderson localization and electron-electron interaction on the superconductivity in two-dimensional systems is explored. We determine the superconducting transition temperature Tc, the temperature dependence of the resistivity, the phase diagram, and the magnetoresistance. The analysis is based on the renormalization group (RG) for a nonlinear sigma model. The derived RG equations are valid to the lowest order in disorder but for an arbitrary electron-electron interaction strength in a particle-hole and the Cooper channels. Systems with preserved and broken spin-rotational symmetry are considered, with both short-range and long-range (Coulomb) interactions. In the case of short-range interaction, we identify parameter regions where the superconductivity is enhanced by localization effects. Our RG analysis indicates that the superconductor-insulator transition is controlled by a fixed point with a resistivity Rc of the order of the quantum resistance Rq=h/4e². When a transverse magnetic field is applied, we find a strong nonmonotonous magnetoresistance for temperatures below Tc. [ABSTRACT FROM AUTHOR]