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arXiv2014

Quasiparticle generation efficiency in superconducting thin-films

Thin-film superconductors with thickness 30 to 500 nm are used as non-equilibrium quantum detectors for photons, phonons or more exotic particles. One of the most basic questions in determining their limiting sensitivity is the efficiency with which the quanta of interest couple to the detected quasiparticles. As low temperature superconducting resonators, thin-films are attractive candidates for producing quantum-sensitive arrayable sensors and the readout uses an additional microwave probe. We have calculated the quasiparticle generation efficiency eta_s for low energy photons in a representative, clean thin-film superconductor (Al) operating well-below its superconductingtransition temperature as a function of film thickness, within the framework of the coupled kineticequations described by Chang and Scalapino.[J. J. Chang and D. J. Scalapino, J. Low Temp. Phys. 31, 1 (1978)]. We have also included the effect of a lower frequency probe. We show that phonon loss from the thin-film reduces eta_s by as much as 40% compared to earlier models that considered relatively thick films or infinite volumes. We also show that the presence of the probe and signal enhances the generation efficiency slightly. We conclude that the ultimate limiting noise equivalent power of this class of detector is determined by the thin-film geometry.

cond-mat.supr-con↗

arXiv2014

Direct measurement of the temperature dependence of the magnetic penetration depth in Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ superconductors

The temperature dependence of the in-plane magnetic penetration depth $λ_{ab}$ of Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ single crystals is determined directly from the shielding magnetic susceptibility, measured in the Meissner region with the field parallel to the ab layers. The doping levels studied cover the underdoped, optimally-doped and overdoped regimes. At temperatures below 0.5Tc a well-defined power-law behavior $λ_{ab}(T)-λ_{ab}(0)=AT^n$ (with n approximately 2.5) is observed. At lower temperatures (T<0.3Tc) the data are still consistent with n=2 and $A\propto T_c^{-3}$, as predicted by the strong pair-breaking scenario proposed by Gordon et al., Phys. Rev. B 81, 180501(R) (2010). The temperature dependence of the superfluid density $ρ_s\proptoλ_{ab}^{-2}$ presents a marked positive curvature just below Tc which is a sign of two-gap superconductivity. The analysis of $ρ_s(T)$ in terms of a two-gap model allowed to estimate parameters like the in-band and inter-band couplings, the relative weight of each band, and their dependence with the doping level. A comparison with $ρ_s(T)$ data obtained by using other techniques in compounds with a similar composition is also presented.

cond-mat.supr-con↗

arXiv2014

Examination of the tradeoff between intrinsic and extrinsic properties in the optimization of a modern internal tin Nb3Sn conductor

In modern Nb3Sn wires there is a fundamental compromise to be made between optimizing the intrinsic properties associated with the superfluid density in the A15 phase (e.g. Tc, Hc, Hc2, all of which are composition dependent), maximizing the quantity of A15 that can be formed from a given mixture of Nb, Sn and Cu, minimizing the A15 composition gradients within each sub-element, while at the same time generating a high vortex pinning critical current density, Jc, by maximizing the grain boundary density with the additional constraint of maintaining the RRR of the Cu stabilizer above 100. Here we study these factors in a Ta-alloyed Restacked-Rod-Process (RRP) wire with ~70 microns diameter sub-elements. Consistent with many earlier studies, maximum non-Cu Jc(12T,4.2K) requires preventing A15 grain growth, rather than by optimizing the superfluid density. In wires optimized for 12T, 4.2K performance, about 60% of the non-Cu cross-section is A15, 35% residual Cu and Sn core, and only 5% a residual Nb7.5wt.%Ta diffusion barrier. The specific heat and chemical analyses show that in this 60% A15 fraction there is a wide range of Tc and chemical composition that does diminish for higher heat treatment temperatures, which, however, are impractical because of the strong RRR degradation that occurs when only about 2% of the A15 reaction front breaches the diffusion barrier. As this kind of Nb3Sn conductor design is being developed for sub-elements 1/2 the present size, it is clear that better barriers are essential to allowing higher temperature reactions with better intrinsic A15 properties. We present here multiple and detailed intrinsic and extrinsic evaluations because we believe that only such broad and quantitative descriptions are capable of accurately tracking the limitations of individual conductor designs where optimization will always be a compromise between inherently conflicting goals

cond-mat.supr-con↗

arXiv2014

Vortex pinning vs superconducting wire network: origin of periodic oscillations induced by applied magnetic fields in superconducting films with arrays of nanomagnets

Hybrid magnetic arrays embedded in superconducting films are ideal systems to study the competition between different physical (such as the coherence length) and structural length scales such as available in artificially produced structures. This interplay leads to oscillation in many magnetically dependent superconducting properties such as the critical currents, resistivity and magnetization. These effects are generally analyzed using two distinct models based on vortex pinning or wire network. In this work, we show that for magnetic dot arrays, as opposed to antidot (i.e holes) arrays, vortex pinning is the main mechanism for field induced oscillations in resistance R(H), critical current Ic(H), magnetization M(H) and ac-susceptibility Xac(H) in a broad temperature range. Due to the coherence length divergence at Tc, a crossover to wire network behavior is experimentally found. While pinning occurs in a wide temperature range up to Tc, wire network behavior is only present in a very narrow temperature window close to Tc. In this temperature interval, contributions from both mechanisms are operational but can be experimentally distinguished.

cond-mat.supr-con↗

arXiv2009

Unconventional superconductivity in YNi2B2C

We use the semi-classical (Doppler shift) approximation to calculate magnetic field angle-dependent density of states and thermal conductivity kappa_zz for a superconductor with a quasi-two-dimensional Fermi surface and line nodes along k_x=0 and k_y=0. The results are shown to be in good quantitative agreement with experimental results obtained for YNi2B2C.

cond-mat.supr-con↗

arXiv2013

Periodic dynamics of fermionic superfluids in the BCS regime

We study the zero temperature non-equilibrium dynamics of a fermionic superfluid in the BCS limit and in the presence of a drive leading to a time dependent chemical potential $μ(t)$. We choose a periodic driving protocol characterized by a frequency $ω$ and compute the fermion density, the wavefunction overlap, and the residual energy of the system at the end of $N$ periods of the drive. We demonstrate that the BCS self-consistency condition is crucial in shaping the long-time behaviour of the fermions subjected to the drive and provide an analytical understanding of the behaviour of the fermion density $n_{{\mathbf k}_F}$ (where ${\mathbf k}_F$ is the Fermi momentum vector) after a drive period and for large $ω$. We also show that the momentum distribution of the excitations generated due to such a drive bears the signature of the pairing symmetry and can be used, for example, to distinguish between s- and d-wave superfluids. We propose experiments to test our theory.

cond-mat.quant-gas↗

arXiv2013

Reentrant Superconductivity in Eu(Fe1-xIrx)2As2

The interplay between superconductivity and Eu$^{2+}$ magnetic ordering in Eu(Fe$_{1-x}$Ir$_{x}$)$_{2}$As$_{2}$ is studied by means of electrical transport and magnetic measurements. For the critically doped sample Eu(Fe$_{0.86}$Ir$_{0.14}$)$_{2}$As$_{2}$, we witnessed two distinct transitions : a superconducting transition below 22.6 K which is followed by a resistivity reentrance caused by the ordering of the Eu$^{2+}$ moments. Further, the low field magnetization measurements show a prominent diamagnetic signal due to superconductivity which is remarkable in presence of a large moment magnetically ordered system. The electronic structure for a 12.5% Ir doped EuFe$_{1.75}$Ir$_{0.25}$As$_{2}$ is investigated along with the parent compound EuFe$_{2}$As$_{2}$. As compared to EuFe$_{2}$As$_{2}$, the doped compound has effectively lower value of density of states throughout the energy scale with more extended bandwidth and stronger hybridization involving Ir. Shifting of Fermi energy and change in band filling in EuFe$_{1.75}$Ir$_{0.25}$As$_{2}$ with respect to the pure compound indicate electron doping in the system.

cond-mat.supr-con↗

arXiv2013

Role of kinetic inductance in transport properties of shunted superconducting nanowires

Recently transport measurements have been carried out in resistively shunted long superconducting nanowires [M. W. Brenner {\it et. al.}, Phys. Rev. B {\bf{85}}, 224507 (2012)]. The measured voltage-current ($V$-$I$) characteristics was explained by the appearance of the phase slip centers in the shunted wire, and the whole wire was modeled as a single Josephson junction. The kinetic inductance of the long nanowires used in experiments is generally large. Here we argue that the shunted superconducting nanowire acts as a Josephson junction in series with an inductor. The inductance depends on the length and the cross section of the wire. The inclusion of inductance in our analysis modifies the $V$-$I$ curves, and increases the rate of switching from the superconducting state to the resistive state. The quantitative differences can be quite large in some practical parameter sets, and might be important to properly understand the experimental results. Our proposed model can be verified experimentally by studying the shunted superconducting nanowires of different lengths and cross sections.

cond-mat.supr-con↗

arXiv2013

Superconductivity with Rashba Spin-Orbit Coupling and Magnetic Field

Two dimensional electron systems at oxide interfaces are often influenced by a Rashba type spin- orbit coupling (SOC), which is tunable by a transverse electric field. Ferromagnetism at the interface can simultaneously induce strong local magnetic fields. This combination of SOC and magnetism leads to asymmetric two-sheeted Fermi surfaces, on which either intra- or inter-band pairing is favored. The superconducting order parameters are derived within a microscopic pairing model realizing both, the Bardeen-Cooper-Schrieffer superconductor with inter-band pairing in the regime of dominating magnetic field, and a mixed-parity intra-band pairing state with finite-momentum pairing in the regime of dominating SOC. We present a phase diagram for the superconducting groundstates and analyze the density of states of the different phases. The results are discussed in the context of superconductivity and ferromagnetism at LaAlO3-SrTiO3 interfaces.

cond-mat.supr-con↗

arXiv2012

Layered pnictide-oxide Na$_2$Ti$_2$Pn$_2$O (Pn=As, Sb): a paradigm for spin density waves

From the first-principles calculations, we have studied the electronic and magnetic structures of compound Na$_2$Ti$_2$Pn$_2$O (Pn = As or Sb). We find that in the ground state Na$_2$Ti$_2$As$_2$O is a blocked checkerboard antiferromagnetic semiconductor with a small band gap of about 0.15 eV, in contrast, Na$_2$Ti$_2$Sb$_2$O is a bi-collinear antiferromagnetic semimetal, both with a small moment of about 0.5$μ_B$ around each Ti atom. We show that there is a strong Fermi surface nesting in Na$_2$Ti$_2$Pn$_2$O. And we verify that the blocked checkerboard and bi-collinear antiferromagnetic states both are the spin density waves induced by the Fermi surface nesting. A tetramer structural distortion is found in company with the formation of a blocked checkerboard antiferromagnetic order, in good agreement with the experimentally observed commensurate structural distortion but with space group symmetry retained after the anomaly happening. Further analysis and discussion in connection with experimental observations are given as well.

cond-mat.supr-con↗

arXiv2013

First-Principle Wannier function analysis of the electronic structure of PdTe: Weaker magnetism and superconductivity

We report a first-principles Wannier function study of the electronic structure of PdTe. Its electronic structure is found to be a broad three-dimensional Fermi surface with highly reduced correlations effects. In addition, the higher filling of the Pd $d$-shell, its stronger covalency resulting from the closer energy of the Pd-$d$ and Te-$p$ shells, and the larger crystal field effects of the Pd ion due to its near octahedral coordination all serve to weaken significantly electronic correlations in the particle-hole (spin, charge, and orbital) channel. In comparison to the Fe Chalcogenide e.g., FeSe, we highlight the essential features (quasi-two-dimensionality, proximity to half-filling, weaker covalency, and higher orbital degeneracy) of Fe-based high-temperature superconductors.

cond-mat.supr-con↗

arXiv2013

Spin fluctuations and pairing symmetry in A$_{x}$Fe$_{2-y}$Se$_{2}$: dual effect of the itinerant and the localized nature of electrons

We investigate the spin fluctuations and the pairing symmetry in A$_{x}$Fe$_{2-y}$Se$_{2}$ by the fluctuation exchange approximation. Besides the on-site interactions, the next-nearest-neighbor antiferromagnetic coupling $J_{2}$ is also included. We find that both the itinerant and the localized natures of electrons are important to describe the recent experimental results of the spin fluctuations and the pairing symmetry. In particular, a small $J_{2}$ coupling can change the pairing gap from the d-wave symmetry to the s-wave symmetry. We have also studied the real-space structures of the gap functions for different orbits in order to gain more insight on the nature of the pairing mechanism.

cond-mat.supr-con↗

arXiv2013

Characterization of the spontaneous symmetry breaking due to quenching of a one-dimensional superconducting loop

We study the final distribution of the winding numbers in a 1D superconducting ring that is quenched through its critical temperature in the absence of magnetic flux. The study is conducted using the stochastic time-dependent Ginzburg--Landau model, and the results are compared with the Kibble--Zurek mechanism (KZM). The assumptions of KZM are formulated and checked as three separate postulates. We find a characteristic length and characteristic times for the processes we study. Besides the case of uniform rings, we examined the case of rings with several weak links. For temperatures close or below $T_c$, the coherence length does not characterize the correlation length. In order to regard the winding number as a conserved quantity, it is necessary to allow for a short lapse of time during which unstable configurations decay. We found criteria for the validity of the 1D treatment. The is no lower bound for final temperatures that permit 1D treatment. For moderate quenching times $τ_Q$, the variance of the winding number obeys the scaling $< n^2>\propto τ_Q^{-1/4}$, as predicted by KZM in the case of mean field models; for $τ_Q\alt 10^5\hbar /k_BT_c$, the dependence is weaker. We also studied the behavior of the system when fluctuations of the gauge field are suppressed, and obtained that the scaling $< n^2>\propto τ_Q^{-1/4}$ is obeyed over a wider range.

cond-mat.stat-mech↗

arXiv2012

Flux jumps and vortex pinning in Ba$_{0.65}$Na$_{0.35}$Fe$_2$As$_2$ single crystals

In this work we present the results of the bulk magnetization measurements in a superconducting state of single crystals of Ba$_{0.65}$Na$_{0.35}$Fe$_2$As$_2$. The isothermal magnetic field ($H||c$ axis) dependent magnetization ($M$) loops exhibit a second peak (SP) or `fishtail effect', as well as remarkable flux jumps at low temperatures. The critical current density $J_c$ obtained from the $M(H)$ loops is rather high, of the order of 10$^6$ A/cm$^2$. The analysis of the temperature and field dependent $J_c$ implies that high $J_c$ is mainly due to collective (weak) pinning of vortices by dense microscopic point defects with some contribution from a strong pinning mechanism. Pronounced magnetic instabilities in terms of flux jumps depend strongly on temperature as well on the field sweep rate. The field for first flux jump as calculated from an adiabatic model, however, is much lower than the experimentally observed values, and this enhanced stability is attributed to a flux creep phenomenon. The analysis of field dependent magnetic relaxation data additionally supports a collective pinning model. The data further suggests that SP in M(H) is likely related to crossover in creep dynamics from elastic to plastic mechanism. We have constructed the vortex phase diagram on field-temperature plane.

cond-mat.supr-con↗

arXiv2013

Evidence for a vortex-glass transition in superconducting Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$

Measurements of magneto-resistivity and magnetic susceptibility were performed on single crystals of superconducting Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$ close to the conditions of optimal doping. The high quality of the investigated samples allows us to reveal a dynamic scaling behaviour associated with a vortex-glass phase transition in the limit of weak degree of quenched disorder. Accordingly, the dissipative component of the ac susceptibility is well reproduced within the framework of Havriliak-Negami relaxation, assuming a critical power-law divergence for the characteristic correlation time $τ$ of the vortex dynamics. Remarkably, the random disorder introduced by the Fe$_{1-x}$Co$_{x}$ chemical substitution is found to act on the vortices as a much weaker quenched disorder than previously reported for cuprate superconductors such as, e.g., Y$_{1-x}$Pr$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-δ}$.

cond-mat.supr-con↗

arXiv2013

Superconductivity in a new layered bismuth oxyselenide: LaO0.5F0.5BiSe2

We report superconductivity in a new layered bismuth oxyselenide LaO0.5F0.5BiSe2 with the ZrCuSiAs-type structure composed of alternating superconducting BiSe2 and blocking LaO layers. The superconducting transition temperature is TC = 2.6K, as revealed from DC magnetization, resistivity and muon spin rotation (muSR) experiments. DC magnetization measurements indicate a superconducting volume fraction of approximately 80%, which is at least twice higher in comparison to that found in corresponding sulphide LaO0.5F0.5BiS2. Importantly, the bulk character of superconductivity in LaO0.5F0.5BiSe2 was confirmed by muSR.

cond-mat.supr-con↗

arXiv2013

Oxygen-vacancy clustering and pseudogap behaviour at LaAlO3/SrTiO3 interface

The 2DEG at the LaAlO3/SrTiO3 interface promises to add a new dimension to emerging electronic devices due to its high tunability. Defects in the form of Oxygen vacancies in titanate surfaces and interfaces, on the other hand, play a key role in the emergence of novel phases. Based on an effective model, we study the influence of Oxygen vacancies on superconductivity and ferromagnetism at LaAlO3/SrTiO3 interface. Using the Bogoliubov-de Gennes formulation in conjunction with Monte-Carlo simulation, we find a clustering of the Oxygen vacancies at the interface that favours formation of coexisting ferromagnetic puddles spatially separated from superconductivity. We also find a carrier freeze-out at low temperatures, observed experimentally in wide-variety of samples. Sufficiently large amount of Oxygen vacancy leads to pseudo-gap like behaviour in the superconducting state.

cond-mat.supr-con↗

arXiv2013

Angle-resolved photoemission spectroscopy observation of anomalous electronic states in EuFe$_2$As$_{2-x}$P$_x$

We used angle-resolved photoemission spectroscopy to investigate the electronic structure of EuFe$_2$As$_2$, EuFe$_2$As$_{1.4}$P$_{0.6}$ and EuFe$_2$P$_2$. We observed doubled core level peaks associated to the pnictide atoms, which are related to a surface state. Nevertheless, strong electronic dispersion along the $c$ axis, especially pronounced in EuFe$_2$P$_2$, is observed for at less one band, thus indicated that the Fe states, albeit probably affected at the surface, do not form pure two-dimensional surface states. However, this latter material shows reduced spectral weight near the Fermi level as compared to EuFe$_2$As$_2$ and EuFe$_2$As$_{1.4}$P$_{0.6}$. An anomalous jump is also found in the electronic states associated with the Eu$^{2+}$ $f$ states in EuFe$_2$P$_2$.

cond-mat.supr-con↗