Pair Breaking Research Articles

Fundamental dissipation due to bound fermions in the zero-temperature limit

The ground state of a fermionic condensate is well protected against perturbations in the presence of an isotropic gap. Regions of gap suppression, surfaces and vortex cores which host Andreev-bound states, seemingly lift that strict protection. Here we show that in superfluid 3He the role of bound states is more subtle: when a macroscopic object moves in the superfluid at velocities exceeding the Landau critical velocity, little to no bulk pair breaking takes place, while the damping observed originates from the bound states covering the moving object. We identify two separate timescales that govern the bound state dynamics, one of them much longer than theoretically anticipated, and show that the bound states do not interact with bulk excitations.

Dielectric response and Cole–Cole plot analysis for (Zn0.91Mn0.03Co0.06O)x/Cu0.5Tl0.5Ba2Ca2Cu3O10-δ diluted magnetic semiconductor/superconductor composites

Adding nanoparticles (NPs) inclusions in the superconducting matrix is usually a very effective way to improve many of the high temperature superconductors (HTS) critical parameters. In this study, the influence of adding Zn0.91Mn0.03Co0.06O NPs on the superconducting and the dielectric properties of Cu0.5Tl0.5Ba2Ca2Cu3O10-δ, CuTl-1223, superconducting phase was investigated. The wet chemical Co-precipitation method was employed to prepare Zn0.91Mn0.03Co0.06O NPs. Four samples of type (Zn0.91Mn0.03Co0.06O)x/CuTl-1223, with x = 0.00, 0.02, 0.06 and 0.10 wt. %, were prepared by the standard solid state reaction technique. XRD results indicated that the samples possess single phase of CuTl-1223 with tetragonal structure and that the addition of Zn0.91Mn0.03Co0.06O did not modify the structure of the phase. SEM micrographs showed apparent suppression of voids in CuTl-1223 samples after Zn0.91Mn0.03Co0.06O addition. Moreover, the randomly oriented plate-like morphology of the samples is reduced with NPs inclusions and transferred in to well-connected spherical grains. The electrical resistivity measurements showed that addition of Zn0.91Mn0.03Co0.06O NPs in minor content (x = 0.02 wt. %) enhances the superconducting transition temperature (Tc) of CuTl-1223 phase to 124.5 K. Moreover, the suppression in Tc for x > 0.02 wt. % was attributed to ferromagnetic nature of Zn0.91Mn0.03Co0.06O NPs that will initiate Cooper pair breaking. The real and imaginary dielectric constants (e' and e''), dissipation factor (tan Δ) and ac conductivity (σac) were investigated as a function of frequency (102–106 Hz) and NPs addition at room temperature. The results clarified that all these dielectric parameters were strongly dependent on both frequency and NPs addition and that the samples possess colossal dielectric constants with low dissipations in the low frequency regime. Cole–Cole plot of the complex permittivity was investigated for the prepared samples. With the aid of Cole–Cole plot, the static dielectric constant (es), optical dielectric constant (e∞), distribution parameter (α), mean relaxation time (τo) and molecular relaxation time (τ) were estimated as function of NP content.

Twofold role of columnar defects in iron based superconductors

We report on the introduction of columnar defects in Ba1−xKxFe2As2 and BaFe2(As1−xPx)2 single crystals via 1.2 GeV Pb irradiation. Scanning transmission electron microscopy analysis proves the formation of continuous defects along the ion tracks, with a diameter of about 3 nm, and a planar density compatible with the irradiation fluence. The twofold role of such defects, i.e. as pair breakers as well as pinning centers, is investigated by a microwave technique, allowing us to determine critical temperature Tc, surface impedance and penetration depth λL, and by magneto-optical imaging and superconducting quantum interference device magnetometry to evaluate the critical current density Jc. The decrease of Tc is quite modest and, together with λL modifications, testifies the increase of pair-breaking scattering following irradiation. The dependence of Jc on irradiation dose and temperature is due to the pinning landscape induced by the columnar defects, and shows the existence of an optimal irradiation dose to enhance the critical current.

Single crystal growth and effects of Ni doping on the novel 12442-type iron-based superconductor RbCa2Fe4As4F2

The recently discovered 12442-type iron-based superconductors (IBSs), ACa2Fe4As4F2 (A = K, Rb, Cs), are intrinsically self-hole doped stoichiometric compounds that exhibit superconductivity with Tc = 30–33.5 K. In this paper, single crystals of Ni doped RbCa2(Fe1−xNix)4As4F2 with 0 ⩽ x ⩽ 0.1 have been successfully grown for the first time using a RbAs flux method and characterized by energy dispersive x-ray spectroscopy (EDS), x-ray diffraction (XRD), electrical resistivity, magnetic susceptibility, and Hall effect measurements. EDS and XRD measurements suggest that the Ni dopants are successfully doped into the crystal lattice. Based on the electrical resistivity and magnetization data, we construct the Tc–x phase diagram. Furthermore, it is found that Ni dopants not only introduce extra electrons that modify the topology of Fermi surface, but also act as impurity scattering centers that contribute to the pair breaking effect, i.e., the superconducting transition temperature Tc is suppressed with a rate of ΔTc/Ni-1% = −2.7 K. Intriguingly, such suppression of Tc and those in other similar hole doped IBSs, such as Ba0.6K0.4Fe2As2, Ba0.5K0.5Fe2As2, and EuRbFe4As4 with multiple nodeless gaps, can be well scaled together. Combining with relevant experimental data reported so far, we speculate that the pairing symmetry in 12442 system is very likely to be nodeless s±-wave. Finally, doping evolution of the upper critical field and its anisotropy are investigated and discussed in detail. Upon Ni doping, the coherence length ξc(0) is gradually increased and becomes larger than the FeAs interbilayer distance when x > 0.07, indicating that the nature of superconductivity changes from quasi two-dimensional (2D) to three-dimensional (3D). The anisotropy of the upper critical field γH close to Tc shows a nonmonotonic dependence on doping, which first increases from 6.7 at the pristine sample to its maximum 8.1 at x = 0.03, and then decreases to 3.7 at x = 0.09.

Fulde-Ferrell states in unequally charged Fermi gases

Atoms with different internal states can exhibit different responses to an artificial magnetic field. An atomic gas mixture of two different components can therefore be interpreted as a mixture of two atomic gases carrying different synthetic charges. We consider the superfluid state of such unequally charged Fermi gases coupled to a magnetic field via the orbital effect and trapped in a torus geometry. The orbital coupling to the magnetic field favors an inhomogeneous superfluid state with optimum finite center-of-mass momentum pairing. The resulting population-balanced orbital Fulde-Ferrell (FF) state is robust against the magnetic field and does not undergo pair breaking unlike the conventional BCS and Fulde-Ferrell-Larkin-Ovchinnikov type pairing states under the Zeeman effect. We contrast the homogeneous and inhomogeneous cases emphasizing the advantages of the unequally charged systems and present their momentum distributions. We conclude that an unequally charged atomic Fermi gas system orbitally coupled to an artificial magnetic field provides an ideal candidate for experimental realization of the FF state.

Superconductivity and phonon self-energy effects in Fe1+yTe0.6Se0.4

The measurement of superconducting gaps on different Fermi surface pockets is necessary for elucidating the pairing mechanism in multi-band superconductors. Inelastic light scattering, or Raman spectroscopy, have been shown to provide the best bulk probe to study the Cooper pair breaking excitations. This paper presents polarization-resolved Raman spectroscopic study of the pair breaking excitations in FeTe0.6Se0.4 iron-based superconductor.

Possible signatures of mixed-parity superconductivity in doped polar SrTiO3 films

Superconductors that possess both broken spatial inversion symmetry and spin-orbit interactions exhibit a mix of spin singlet and triplet pairing. Here, we report on measurements of the superconducting properties of electron-doped, strained SrTiO3 films. These films have an enhanced superconducting transition temperature and were previously shown to undergo a transition to a polar phase prior to becoming superconducting. We show that some films show signatures of an unusual superconducting state, such as an in-plane critical field that is higher than both the paramagnetic and orbital pair breaking limits. Moreover, nonreciprocal transport, which reflects the ratio of odd versus even pairing interactions, is observed. Together, these characteristics indicate that these films provide a tunable platform for investigations of unconventional superconductivity.

Eliashberg equations for an electron–phonon version of the Sachdev–Ye–Kitaev model: Pair breaking in non-Fermi liquid superconductors

We present a theory that is a non-Fermi-liquid counterpart of the Abrikosov–Gor’kov pair-breaking theory due to paramagnetic impurities in superconductors. To this end we analyze a model of interacting electrons and phonons that is a natural generalization of the Sachdev–Ye–Kitaev-model. In the limit of large numbers of degrees of freedom, the Eliashberg equations of superconductivity become exact and emerge as saddle-point equations of a field theory with fluctuating pairing fields. In its normal state the model is governed by non-Fermi liquid behavior, characterized by universal exponents. At low temperatures a superconducting state emerges from the critical normal state. We study the role of pair-breaking on Tc, where we allow for disorder that breaks time-reversal symmetry. For small Bogoliubov quasi-particle weight, relevant for systems with strongly incoherent normal state, Tc drops rapidly as function of the pair breaking strength and reaches a small but finite value before it vanishes at a critical pair-breaking strength via an essential singularity. The latter signals a breakdown of the emergent conformal symmetry of the non-Fermi liquid normal state.

Superconducting gap and nematic resonance at the quantum critical point observed by Raman scattering in BaFe2(As1−xPx)2

We report comprehensive temperature and doping-dependences of the Raman scattering spectra for $\mathrm{BaFe_{2}}(\mathrm{As}_{1-x}\mathrm{P}_{x}\mathrm{)_{2}}$ ($x =$ 0, 0.07, 0.24, 0.32, and 0.38), focusing on the nematic fluctuation and the superconducting responses. With increasing $x$, the bare nematic transition temperature estimated from the Raman spectra reaches $T =$ 0 K at the optimal doping, which indicates a quantum critical point (QCP) at this composition. In the superconducting compositions, in addition to the pair breaking peaks observed in the $A_{\mathrm{1g}}$ and $B_{\mathrm{1g}}$ spectra, another strong $B_{\mathrm{1g}}$ peak appears below the superconducting transition temperature which is ascribed to the nematic resonance peak. The observation of this peak indicates significant nematic correlations in the superconducting state near the QCP in this compound.

Topological ultranodal pair states in iron-based superconductors

Bogoliubov Fermi surfaces are contours of zero-energy excitations that are protected in the superconducting state. Here we show that multiband superconductors with dominant spin singlet, intraband pairing of spin-1/2 electrons can undergo a transition to a state with Bogoliubov Fermi surfaces if spin-orbit coupling, interband pairing and time reversal symmetry breaking are also present. These latter effects may be small, but drive the transition to the topological state for appropriate nodal structure of the intra-band pair. Such a state should display nonzero zero-bias density of states and corresponding residual Sommerfeld coefficient as for a disordered nodal superconductor, but occurring even in the pure case. We present a model appropriate for iron-based superconductors where the topological transition associated with creation of a Bogoliubov Fermi surface can be studied. The model gives results that strongly resemble experiments on FeSe1−xSx across the nematic transition, where this ultranodal behavior may already have been observed.

Plasticity or elasticity? Relating elastic moduli with secondary structural features of mixed films of polypeptides at air/fluid and fluid/solid interfaces

Viscoelastic properties of molecular assemblies formed by mixing poly (l-lysine) (PLL) and poly (l-glutamic acid) (PGA) at pH = 7.5 have been studied using Quartz crystal microbalance with dissipation (QCM-D). The inter-molecular complex between PLL and PGA arising from strong electrostatic interaction, leads to local changes in secondary structure resulting in intra-molecular complexes. ATR-FTIR analysis of the Secondary structural features of PLL, suggest an abundance of anti-parallel beta sheet that causes a significant change in the morphology of the self-assembled structures. A combination of spectroscopy, Brewster angle microscopy (BAM), Transmission electron and scanning electron microscopy show that an inverse relationship exists between the elasticity of the different PLL + PGA mixed films and the % anti parallel beta sheet conformation. The elastic moduli for the mixtures change from about 0.913 ± 0.01 GPa for pure PLL to about 0.764 ± 0.01 GPa in the mixture when PGA increases. The localized breaking and reformation of the ion pairs in the complex control their sizes and show an inverse relationship with the elastic moduli. The rheological profiles of the films, elastic moduli, together with their surface morphology from microscopy (TEM) and (SEM) confirmed their increasing propensity to self-assemble in one dimension to form tapes, colloidal particles and their composite assemblies.

Transitional Flow Dynamics Behind a Micro-Ramp

Micro-ramps are popular passive flow control devices which can delay flow separation by re-energising the lower portion of the boundary layer. We compute the laminar base flow, the instantaneous transitional flow, and the mean flow around a micro-ramp immersed in a quasi-incompressible boundary layer at supercritical roughness Reynolds number. Results of our Direct Numerical Simulations (DNS) are compared with results of BiLocal stability analysis on the DNS base flow and independent tomographic Particle Image Velocimetry (tomo-PIV) experiments. We analyse relevant flow structures developing in the micro-ramp wake and assess their role in the micro-ramp functionality, i.e., in increasing the near-wall momentum. The main flow feature of the base flow is a pair of streamwise counter-rotating vortices induced by the micro-ramp, the so-called primary vortex pair. In the instantaneous transitional flow, the primary vortex pair breaks up into large-scale hairpin vortices, which arise due to linear varicose instability of the base flow, and unsteady secondary vortices develop. Instantaneous vortical structures obtained by DNS and experiments are in good agreement. Matching linear disturbance growth rates from DNS and linear stability analysis are obtained until eight micro-ramp heights downstream of the micro-ramp. For the setup considered in this article, we show that the working principle of the micro-ramp is different from that of classical vortex generators; we find that transitional perturbations are more efficient in increasing the near-wall momentum in the mean flow than the laminar primary vortices in the base flow.

Construction of a Range-Separated Dual-Hybrid Direct Random Phase Approximation.

Blending the good performance of the global hybrid PBE0 functional at short-range and the dual-hybrid dRPA75 functional at long range, we propose a new range-separated direct random phase approximation (dRPA75rs), which considerably improves on the accuracy of the calculated reaction energies and barrier heights compared to the parent approaches and provides a good description of noncovalent interactions without any dispersion correction. We also combine the new scheme with spin-component scaling (SCS-dRPA75rs), which enables the accurate calculation of energy differences for processes involving electron pair breaking, such as atomization. The new method scaling as the fourth power of the system size shows a balanced performance on a broad test set involving radicals, transition metal atoms, and heavy atoms, which makes it competitive with the best double-hybrid functionals based on the second-order perturbation theory. According to the results for the homogeneous electron gas, our dRPA75rs method expectedly gives errors for metallic systems similar to the dRPA approach with an additional error cancellation in the case of partial spin polarization.

Different seniority states of $^{119-126}$Sn isotopes: shell-model description

Abstract In the present work, available experimental data up to high-spin states of $^{119-126}$Sn isotopes with different seniority ($v$), including $v = 4$, 5, 6, and 7, are interpreted with the shell model, by performing shell-model calculations in the 50–82 valence shell composed of $1g_{7/2}$, $2d_{5/2}$, $1h_{11/2}$, $3s_{1/2}$, and $2d_{3/2}$ orbitals. The results are compared with the available experimental data. These states are described in terms of broken neutron pairs occupying the $h_{11/2}$ orbital. Possible configurations of seniority isomers in these nuclei are discussed. The breaking of three neutron pairs is responsible for generating high-spin states. The isomeric states $5^-$, $7^-$, $10^+$, and $15^-$ of even Sn isotopes, and isomeric states $19/2^+$, $23/2^+$, $27/2^-$, and $35/2^+$ of odd Sn isotopes, are described in terms of different seniority. For even Sn isotopes, the isomeric states $5^-$, $7^-$, and $10^+$ are due to seniority $v = 2$; the isomeric state $15^-$ is due to seniority $v = 4$, and, in the case of odd Sn isotopes, the isomeric states $19/2^+$, $23/2^+$, and $27/2^-$ are due to seniority $v = 3$, and the isomeric state $35/2^+$ in $^{123}$Sn is due to seniority $v = 5$. These are maximally aligned spins, which involve successive pair breakings in the $\nu (h_{11/2})$ orbital.

Measurements of Microwave Vortex Response in DC Magnetic Fields in Tl<inline-formula> <tex-math notation="LaTeX">$_2$</tex-math> </inline-formula>Ba<inline-formula> <tex-math notation="LaTeX">$_2$</tex-math> </inline-formula>CaCu<inline-formula> <tex-math notation="LaTeX">$_2$</tex-math> </inline-formula>O<inline-formula> <tex-math notation="LaTeX">$_{8+x}$</tex-math> </inline-formula> Films

There is a renewed interest in superconductors for high-frequency applications, leading to a reconsideration of already known low-$T_c$ and high-$T_c$ materials. In this view, we present an experimental investigation of the millimeter-wave response in moderate magnetic fields of Tl$_2$Ba$_2$CaCu$_2$O$_{8+x}$ superconducting films with the aim of identifying the mechanisms of the vortex-motion-induced response. We measure the dc magnetic-field-dependent change of the surface impedance, $\Delta Z_s(H) = \Delta R_s(H) + i\Delta X_s(H)$ at 48 GHz by means of the dielectric resonator method. We find that the overall response is made up of several contributions, with different weights depending on the temperature and field: a possible contribution from Josephson or Abrikosov-Josephson fluxons at low fields; a seemingly conventional vortex dynamics at higher fields; a significant pair breaking in the temperature region close to $T_c$. We extract the vortex motion depinning frequency $f_p$, which attains surprisingly high values. However, by exploiting the generalized model for relaxational dynamics we show that this result come from a combination of a pinning constant $k_p$ arising from moderate pinning, and a vortex viscosity $\eta$ with anomalously small values. This latter fact, implying large dissipation, is likely a result from a peculiar microscopic structure and thus poses severe limits to the application of Tl$_2$Ba$_2$CaCu$_2$O$_{8+x}$ in a magnetic field.

Detecting parity effect in a superconducting device in the presence of parity switches

We present a superconducting device showing a clear parity effect in the number of electrons, even when there is, on average, a single nonequilibrium quasiparticle present and the parity of the island switches due to quasiparticles tunneling in and out of the device at rates on the order of 100 Hz. We detect the switching by monitoring in real time the charge state of a superconducting island connected to normal leads by tunnel junctions. The quasiparticles are created by Cooper pairs breaking on the island at a rate of a few kHz. We demonstrate that the pair breaking is caused by the backaction of the single-electron transistor used as a charge detector. With sufficiently low probing currents, our superconducting island is free of quasiparticles 97% of the time.

AC Magnetic Susceptibility of Y3Ba5Cu8O18 Substituted by Nd3+ and Ca2+ Ions

Effect of Nd3+ and Ca2+ ion substitutions in Y3Ba5Cu8O18 (Y-358) was studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), electrical resistivity, and ac magnetic susceptibility measurements. Superconducting samples of type Y3 − xNdxBa5 − yCayCu8O18 (with 0 ≤ x, y ≤ 0.4) were synthesized by the standard solid-state reaction technique. XRD patterns revealed that all the samples were mainly composed of Y-358 phase with orthorhombic structure. SEM micrographs showed that Nd3+ and Ca2+ ion substitutions had the ability to alter the grain morphology of Y-358 to irregular randomly oriented grains with smaller average grain sizes. The temperature-dependent electrical resistivity curves indicated that the superconducting transition temperature Tc was decreased from 96.51 to 87.41 K as the substitution contents were increased from 0 to 0.4. Detailed AC magnetic susceptibility measurements were done using a quantum design physical property measurement system (PPMS) at different dc fields (B = 0.5–20 kG) and frequency of 1 kHz. A comparison between electrical resistivity and AC magnetic susceptibility data was performed. It was found that Tcs obtained from AC magnetic susceptibility were much lower than those obtained from the resistivity measurements, for all studied samples. This was discussed in terms of filamentary paths that could produce sharp pronounced drops in diamagnetism. Generally, the depression in Tc was attributed to the decrease in relative volume fraction of Y-358 or due to trapping of mobile holes or other mechanism connected with oxygen vacancy disorder. Werthamer–Helfand–Hohenberg (WHH) theory was employed to study the temperature dependence of the upper critical magnetic field Bc2(T) for the samples. The relative importance of both orbital and paramagnetic effects in the suppression of superconductivity was described with the aid of Maki parameter (κM). The estimated values of the upper critical magnetic field at 0 K, Bc2(0), were found to decrease from 37.3 to 13.5 T as Nd3+ and Ca2+ contents were increased from 0 to 0.4. This decrement was attributed to the weak electron–phonon coupling and to the orbital as well as spin pair breaking mechanisms. Furthermore, Ginzburg–Landau coherence length ξGL of Y3 − xNdxBa5 − yCayCu8O18 was found to increase with the increase of the Nd3+ and Ca2+ contents. Therefore, Nd3+ and Ca2+ ions play an important role in the pair-breaking mechanism and in suppressing superconductivity of the Y-358 phase.

Baltimore Reading and Eye Disease Study (BREDS): compliance and satisfaction with glasses usage

To assess the patterns and predictors of glasses wear in a 2-year school-based study. Second and third graders underwent an eye examination at school. Two pairs of glasses were provided if they met prescribing criteria. Replacements were provided as needed. Students received follow-up examinations and completed survey questionnaires during the same and the following academic year. Of the 197 students prescribed glasses who completed year 1 follow-up, 172 (87%), were observed to still be wearing glasses. However, less than two-thirds of students reported wearing glasses as prescribed (eg full-time if prescribed full-time). Most students, 175 (89%), reported being happy with their glasses and 135 (69%) reported improvement in vision. Thirty-nine students (20%) reported being teased about their glasses. Replacement glasses were required by 136 students (66%). Refractive error was not associated with likelihood of requiring replacement. Being observed wearing glasses correlated with parent (OR=4.2; 95% CI, 1.2-15.0) and teacher reminders (OR=6.4; 95% CI, 1.5-28.4) in year2. Most children continued to wear glasses during follow-up, yet not always as prescribed. A substantial proportion of students required replacements, underscoring the importance of school-based programs developing mechanisms to monitor eyeglasses usage and mechanisms to replace lost or broken pairs.

The Importance of Being Frank

AbstractThis essay reads several “hidden” scenes in Jane Austen's Emma. Willfully drawing attention to stray details—an impulsive trip to London to get his hair cut, the mysterious gift of a piano, and a broken pair of spectacles—Frank Churchill uses these incidents to turn the narrator's attention, and the town's gossip, away from his secret engagement to the impoverished Jane Fairfax. In these oft-ignored and easily forgotten scenes, Frank performs an astonishing ontological trespass: flaunting his plot only to better conceal it, the literary character seems nearly possessed by a shadow of omniscience itself, wielding a sort of all-pervasive knowledge of the plot that is usually reserved for the narrator's exclusive access. Although critics have interpreted free indirect discourse as an authorial elision of individual consciousness, a ventriloquizing of characters' thoughts that dispossesses the subjects of such discourses, this essay suggests that Frank Churchill's fluency in the language of plotting captures the double bind of power and dispossession nestled within social performance. Plotting, I argue, emerges as a form of social know-how, both encouraged and curbed by the aesthetic requirements of novelistic emplotment writ large. In this light, literary characters who engage in plotting stand at a site of conversion, where an aspect of fiction seen as narrowly aesthetic becomes a technique for representing structures of social self-consciousness cultivated under conditions of precarity.

Fermi-surface selective determination of the g -factor anisotropy in URu2Si2

The $g$-factor anisotropy of the heavy quasiparticles in the hidden order state of URu$_2$Si$_2$ has been determined from the superconducting upper critical field and microscopically from Shubnikov-de Haas (SdH) oscillations. We present a detailed analysis of the $g$-factor for the $\alpha$, $\beta$ and $\gamma$ Fermi-surface pockets. Our results suggest a strong $g$-factor anisotropy between the $c$ axis and the basal plane for all observed Fermi surface pockets. The observed anisotropy of the $g$-factor from the quantum oscillations is in good agreement with the anisotropy of the superconducting upper critical field at low temperatures, which is strongly limited by the paramagnetic pair breaking along the easy magnetization axis $c$. However, the anisotropy of the initial slope of the upper critical field near $T_c$ cannot be explained by the anisotropy of the effective masses and Fermi velocities derived from quantum oscillations.