Pair-breaking Effect Research Articles

Signatures of Bardasis-Schrieffer mode excitation in third-harmonic generated currents

We theoretically analyze the collective modes in unconventional superconductors focusing on Bardasis-Schrieffer (BS) mode and its contribution to the third harmonic generation currents. Starting from a model with competing superconducting pairing instabilities we add fluctuations of the fields beyond saddle point approximation and calculate their response to an applied pulsed electric field. To model phase fluctuations appropriately we take into account the effect of the long-range Coulomb interaction. While the phase mode is pushed into a plasmon frequency, as known from the literature, we show that the BS mode remains unaffected. Furthermore, it has a characteristic polarization dependence and, unlike the Higgs mode, generates a current in perpendicular direction to the applied field. We find that the Bardasis-Schrieffer excitations contribute a sizable signal to the third harmonic generated current, which is clearly distinguishable from the charge density fluctuations due to Cooper pair breaking effects and can be straightforwardly detected in experiment.

Investigation of the upper critical field in artificially engineered Nb/V/Ta superlattices

Artificially engineered superlattice offers highly flexible control of the quantized electronic state by manipulating its lattice structure. We fabricated artificially engineered superlattices [Nb/V/Ta] n as a new class of superconducting superlattices and investigated the upper critical field H c2 in terms of the structural difference. It is found that the out-of-plane upper critical field significantly increases compared with that of a Nb single layer film as the thickness of the constituent Nb, V, and Ta layers becomes thin. In the case of the superlattice [Nb/V/Ta] n , the is basically governed by the orbital pair-breaking effect rather than Pauli pair-breaking effect and increases only when the crystalline size of the superlattice decreases by the insertion of V layers. Thus, we conclude that the shortening of the coherence length of Cooper pairs due to the crystalline disorder predominantly contributes to the increase of the in the superlattice [Nb/V/Ta] n .

Fulde–Ferrell–Larkin–Ovchinnikov State in Perpendicular Magnetic Fields in Strongly Pauli-Limited Quasi-Two-Dimensional Superconductors

We examine the Fermi-surface effect called the nesting effect for the FFLO state in strongly Pauli-limited Q2D superconductors, focusing on the effect of 3D factors, such as interlayer electron transfer, interlayer pairing, and off-plane magnetic fields including those perpendicular to the most conductive layers. We examine the systems with a large Maki parameter so that the orbital pair-breaking effect is negligible, except for the locking of the direction of the FFLO vector q in the field direction.It is known that the nesting effect for the FFLO state can be strong in QLD systems in which the orbital pair-breaking effect is suppressed by applying the mag. field parallel to the layers. Hence, it has sometimes been suggested that the nesting effect may hardly enhance the stability of the FFLO state for perpendicular fields. We illustrate that, contrary to this view, the nesting effect can strongly stabilize the FFLO state for perpendicular fields as well as for parallel fields when tz is small so that the Fermi surfaces are open in the kz-direction, where tz denotes the interlayer transfer energy. In particular, the nesting effect in perpendicular fields can be strong in interlayer states. For example, in systems with cylindrical Fermi surfaces warped by tz /= 0, interlayer states with Dlt_k prop sin k_z exhibit mu_e Hc=1.65 Dlt_a0 for perpendicular fields, which is much larger than typical values for parallel fields, such as mu_e Hc=Dlt_s0 of the s-wave state and mu_e Hc = 1.28 Delta_d0 of the d-wave state in cylindrical systems with tz=0. The present result could potentially provide a physical reason why the areas in the phase diagrams occupied by the high-field phases for the perpendicular and parallel fields are of the same order in CeCoIn5 and FeSe.

Superconducting properties of Ni-substituted CuTl-1223 superconductor

Superconducting properties of Cu0.5Tl0.5Ba2Ca2(Cu3−xNix)O10−δ (x = 0, 0.5, 1, 1.5) have been investigated by XRD, dielectric measurements, excess conductivity analyses and resistivity measurements. These samples show orthorhombic crystal structure in which a-axis length suppresses but b-axis length increases with increased Ni concentration. These samples have shown metallic variations of resistivity from room temperature down to onset of superconductivity in which the onset temperature of superconductivity and its zero resistivity critical temperature suppresses with increased Ni concentration. Excess conductivity analyses of conductivity data has shown values of coherence length ξc(0) along c-axis, Fermi velocity of the carriers VF and inter-layer coupling J, suppress with increased Ni-substitution which shows that density of superconducting carriers is also suppressed. Lower values of real part (e′) of dielectric constant has confirmed lower density of carriers in Ni-doped samples which further shows that doped Ni-atoms induce pair breaking effects. The flux pinning characteristics of Ni substituted samples are, however, significantly improved in comparison with parent compound i.e. Cu0.5Tl0.5Ba2Ca2Cu3O10−δ, manifesting that spin carrying Ni-atoms behave like efficient pinning centers.

Superfluid density from magnetic penetration depth measurements in Nb\u2013Cu 3D nano-composite films

Superconductivity in 3D Nb–Cu nanocomposite granular films have been studied with varying thickness for two different compositions, Nb rich with 88 at% of Nb and Cu rich with 46 at% of Nb. For both compositions, the superconducting transition temperature (Tc) decreases with decreasing film thickness. For any thickness, doubling the Cu content in the films decreases the Tc by about 2 K. To explore if phase fluctuations play any role in superconductivity in these 3D films, the superfluid stiffness (JS) of the films was measured using low frequency two-coil mutual inductance (M) technique. Interestingly, the measurement of M in magnetic fields showed two peaks in the imaginary component of M for both Nb rich and Cu rich films. The two peaks were associated with the pair-breaking effect of the magnetic field on the intra and inter-granular coupling in these films consisting of random network of superconductor (S) and normal metal (N) nano-particles. Furthermore, JS was seen to decrease with decreasing film thickness and increasing Cu content. However, for all films studied JS remained higher than the superconducting energy gap (∆) indicating that phase fluctuations do not play any role in superconductivity in the film thickness and composition range investigated. Our results indicate that an interplay of quantum size effects (QSE) and superconducting proximity effect (SPE) controls the Tc with composition in these 3D nano-composite films.

Fully gapped superconductivity without sign reversal in the topological superconductor PbTaSe2

We investigate the superconducting gap function of topological superconductor PbTaSe$_2$. Temperature, magnetic field, and three-dimensional (3D) field-angle dependences of the specific heat prove that the superconductivity of PbTaSe$_2$ is fully-gapped, with two isotropic $s$-wave gaps. The pair-breaking effect is probed by systematically increasing non-magnetic disorders through H$^+$-irradiations. The superconducting transition temperature, $T_{\rm{c}}$, is found to be robust against disorders, which suggests that the pairing should be sign-preserved rather than sign-reversed.

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.

Effects of paramagnetic pair-breaking and spin-orbital coupling on multi-band superconductivity

The BCS picture of superconductivity describes pairing between electrons originating from a single band. A generalization of this picture occurs in multi-band superconductors, where electrons from two or more bands contribute to superconductivity. The contributions of the different bands can result in an overall enhancement of the critical field and can lead to qualitative changes in the temperature dependence of the upper critical field when compared to the single-band case. While the role of orbital pair-breaking on the critical field of multi-band superconductors has been explored extensively, paramagnetic and spin-orbital scattering effects have received comparatively little attention. Here we investigate this problem using thin films of Nd-doped SrTiO3. We furthermore propose a model for analyzing the temperature-dependence of the critical field in the presence of orbital, paramagnetic and spin-orbital effects, and find a very good agreement with our data. Interestingly, we also observe a dramatic enhancement in the out-of-plane critical field to values well in excess of the Chandrasekhar–Clogston (Pauli) paramagnetic limit, which can be understood as a consequence of multi-band effects in the presence of spin-orbital scattering.

Reentrant s-wave superconductivity in the periodic Anderson model with attractive conduction band Hubbard interaction

Spin-flip scattering from magnetic impurities has a strong pair-breaking effect in s-wave superconductors where increasing the concentration of impurities rapidly destroys superconductivity. For small Kondo temperature TK the destruction of superconductivity is preceded by the reentrant superconductivity at finite temperature range Tc2 < T < Tc1, while the normal phase reappears at T < Tc2 ∼ TK. Here we explore the superconducting phase in a periodic system modeled as the Anderson lattice with additional attractive on-site (Hubbard) interaction g acting on the conduction band electrons. We solve the equations using dynamical mean field theory which incorporates Kondo physics, while the pairing interaction is treated on the static mean-field level. For large coupling g we find reentrant superconductivity which resembles the case with diluted impurities. However, we find evidence that reentrant superconductivity is here not a consequence of many-body correlations leading to the Kondo effect, but it rather stems from a competition between the single-particle hybridization and superconducting pairing. An insight into the spectral functions with in-gap structures is obtained from an approximate noninteracting dual model whose solution interpolates between several exact limits.

Upper critical field and its anisotropy in RbCr3As3

The temperature dependence of the upper critical field $({H}_{c2})$ in ${\mathrm{RbCr}}_{3}{\mathrm{As}}_{3}$ single crystals $({T}_{c}\ensuremath{\approx}7.3$ K) has been determined by means of magnetoresistance measurements with temperature down to 0.35 K in static magnetic fields up to 38 T. The magnetic field was applied both for directions parallel $(H\ensuremath{\parallel}c,{H}_{c2}^{\ensuremath{\parallel}c})$ and perpendicular $(H\ensuremath{\perp}c,{H}_{c2}^{\ensuremath{\perp}c})$ to the Cr chains. The curves ${H}_{c2}^{\ensuremath{\parallel}c}(T)$ and ${H}_{c2}^{\ensuremath{\perp}c}(T)$ cross at $\ensuremath{\sim}5.5$ K. As a result, the anisotropy parameter $\ensuremath{\gamma}(T)={H}_{c2}^{\ensuremath{\perp}c}/{H}_{c2}^{\ensuremath{\parallel}c}(T)$ increases from 0.5 near ${T}_{c}$ to 1.6 at low temperature. Fitting with the Werthamer-Helfand-Hohenberg (WHH) model yields zero-temperature critical fields of ${\ensuremath{\mu}}_{0}{H}_{c2}^{\ensuremath{\parallel}c}(0)\ensuremath{\approx}27.2$ T and ${\ensuremath{\mu}}_{0}{H}_{c2}^{\ensuremath{\perp}c}(0)\ensuremath{\approx}43.4$ T, both exceeding the BCS weak-coupling Pauli limit ${\ensuremath{\mu}}_{0}{H}_{p}=1.84{T}_{c}=13.4$ T. The results indicate that the paramagnetic pair-breaking effect is strong for $H\ensuremath{\parallel}c$ but absent for $H\ensuremath{\perp}c$, which was further confirmed by the angle dependent magnetoresistance and ${H}_{c2}$ measurements.

Andreev reflection in ballistic normal metal/graphene/superconductor junctions

We report the study of ballistic transport in normal metal/graphene/superconductor junctions in edge-contact geometry. While in the normal state, we have observed Fabry-P\'{e}rot resonances suggesting that charge carriers travel ballistically, the superconducting state shows that the Andreev reflection at the graphene/superconductor interface is affected by these interferences. Our experimental results in the superconducting state have been analyzed and explained with a modified Octavio-Tinkham-Blonder-Klapwijk model taking into account the magnetic pair-breaking effects and the two different interface transparencies, \textit{i.e.}\,between the normal metal and graphene, and between graphene and the superconductor. We show that the transparency of the normal metal/graphene interface strongly varies with doping at large scale, while it undergoes weaker changes at the graphene/superconductor interface. When a cavity is formed by the charge transfer occurring in the vicinity of the contacts, we see that the transmission probabilities follow the normal state conductance highlighting the interplay between the Andreev processes and the electronic interferometer.

Field-dependent nonlinear surface resistance and its optimization by surface nanostructuring in superconductors

We propose a theory of nonlinear surface resistance of a dirty superconductor in a strong radio-frequency (RF) field, taking into account magnetic and nonmagnetic impurities, finite quasiparticle lifetimes, and a thin proximity-coupled normal layer characteristic of the oxide surface of many materials. The Usadel equations were solved to obtain the quasiparticle density of states (DOS) and the low-frequency surface resistance $R_s$ as functions of the RF field amplitude $H_0$. It is shown that the interplay of the broadening of the DOS peaks and a decrease of a quasiparticle gap caused by the RF currents produces a minimum in $R_s(H_0)$ and an extended rise of the quality factor $Q(H_0)$ with the RF field. Paramagnetic impurities shift the minimum in $R_s(H_0)$ to lower fields and can reduce $R_s(H_0)$ in a wide range of $H_0$. Subgap states in the DOS can give rise to a residual surface resistance while reducing $R_s$ at higher temperatures. A proximity-coupled normal layer at the surface can shift the minimum in $R_s(H_0)$ to either low and high fields and can reduce $R_s$ below that of an ideal surface. The theory shows that the behavior of $R_s(H_0)$ changes as the temperature and the RF frequency are increased, and the field dependence of $Q(H_0)$ can be very sensitive to the materials processing. Our results suggest that the nonlinear RF losses can be minimized by tuning pairbreaking effects at the surface using impurity management or surface nanostructuring.

Paramagnetic Pair-Breaking in Spin-Triplet Superconductors with Spin–Orbit Coupling: Application to Sr2RuO4

Paramagnetic pair-breaking effects by in-plane magnetic fields are studied in the presence of the spin–orbit coupling, based on the tight-binding three-orbital model for Sr2RuO4. The spin–orbit cou...

Magnetic Field Effect on s-wave Superconductor LaRu4P12 Studied by 31P-NMR

We have performed 31P-NMR measurements on the s-wave superconductor LaRu4P12 to investigate the magnetic field effect of the nuclear spin-lattice relaxation rate 1/T1 on a conventional full-gap superconductor. With increasing magnetic field, the Hebel-Slichter peak immediately below Tc in 1=T1 was suppressed, and the magnetic field dependence of 1/T1 at 0.8 K, well below Tc, was proportional to H2. These behaviors can be fully understood by the orbital pair-breaking effect in a single-band s-wave superconductor

Impact of electron-electron interactions on the superfluid density of dirty superconductors

Landau's theory of the Fermi liquid is adapted to analyze the impact of electron-electron ($e-e$) interactions on the deficit of the superfluid density $\rho_{s0}=\rho_s(T=0)$ in dirty superconducting electron systems in which the damping $\gamma$ of single-particle excitations exceeds the zero temperature BCS gap $\Delta_0$. In the dirty strong-coupling limit $\gamma/\Delta_0\gg 1,m^*/m_e\gg 1$, the formula derived for $\rho_{s0}$ is shown to coincide with the well-known empirical Uemura relation provided pair-breaking contributions are nonexistent. The roles of the crystal lattice and magnetic pair-breaking effects in the observed decline of the zero-temperature superfluid density $\rho_{s0}$ in overdoped LSCO compounds are also discussed. Our method is also applied to elucidation of results from the pioneering experimental studies performed recently by Bozovi\`c and collaborators in overdoped LSCO compounds.

Quantum criticality in the (Cr98.4Al1.6)100-yMoy alloy system

Indications of quantum critical behaviour (QCB), driven by electron-hole pair breaking effects through Mo doping of Cr98.4Al1.6 are reported. Previous studies on the electrical resistivity (ρ), Seebeck coefficient (S), Sommerfeld electronic specific heat coefficient (γ) and magnetic susceptibility (χ) as a function of temperature on the (Cr98.4Al1.6)100-yMoy alloy system point towards the existence of a quantum critical point (QCP) at yc ≈ 4.5. In the present study the Hall coefficient RH measurements and an analysis of the previous results are used to substantiate the presence of a QCP in the alloy system. The charge carrier density at 2 K, (qRH(2K))−1, of the alloy systems increases continuously with y from the antiferromagnetic (AFM) phase through yc ≈ 4.5 and into the paramagnetic (P) phase. Scaling relationships between the magnetic contribution to the electrical resistivity, Δρ2K, the magnetic contribution to the Sommerfeld electronic specific heat coefficient, Δγ, and (qRH(2K))−1, are furthermore used to support the presence of a QCP in the alloy system. dS/dT in the temperature limit T→ 2 K depicts an anomalous behaviour in the form of an upturn on decreasing y through the QCP, yc ≈ 4.5. The critical exponents in the vicinity of the QCP were found to be β = 0.5 ± 0.1 for the Δρ2K/ρ2K(y) curve and γ = 0.4 ± 0.1 for the TN(y) curve. The nature of the QCP in the present alloy system is classified as an incommensurate (I) spin-density-wave (SDW) type, driven mainly by electron-hole pair breaking effects.

Geometric quenching of orbital pair breaking in a single crystalline superconducting nanomesh network

In a superconductor Cooper pairs condense into a single state and in so doing support dissipation free charge flow and perfect diamagnetism. In a magnetic field the minimum kinetic energy of the Cooper pairs increases, producing an orbital pair breaking effect. We show that it is possible to significantly quench the orbital pair breaking effect for both parallel and perpendicular magnetic fields in a thin film superconductor with lateral nanostructure on a length scale smaller than the magnetic length. By growing an ultra-thin (2 nm thick) single crystalline Pb nanowire network, we establish nm scale lateral structure without introducing weak links. Our network suppresses orbital pair breaking for both perpendicular and in-plane fields with a negligible reduction in zero-field resistive critical temperatures. Our study opens a frontier in nanoscale superconductivity by providing a strategy for maintaining pairing in strong field environments in all directions with important technological implications.

Unconventional Multiband Superconductivity in Bulk SrTiO_{3} and LaAlO_{3}/SrTiO_{3} Interfaces.

Although discovered many decades ago, superconductivity in doped SrTiO_{3} remains a topic of intense research. Recent experiments revealed that, upon increasing the carrier concentration, multiple bands cross the Fermi level, signaling the onset of Lifshitz transitions. Interestingly, T_{c} was observed to be suppressed across the Lifshitz transition of oxygen-deficient SrTiO_{3}; a similar behavior was also observed in gated LaAlO_{3}/SrTiO_{3} interfaces. Such a behavior is difficult to explain in the clean theory of two-band superconductivity, as the additional electronic states provided by the second band should enhance T_{c}. Here, we show that this unexpected behavior can be explained by the strong pair-breaking effect promoted by disorder, which takes place if the interband pairing interaction is subleading and repulsive. A consequence of this scenario is that, upon moving away from the Lifshitz transition, the two-band superconducting state changes from opposite-sign gaps to same-sign gaps.

Raising the Critical Temperature by Disorder in Unconventional Superconductors Mediated by Spin Fluctuations

We propose a mechanism whereby disorder can enhance the transition temperature T_{c} of an unconventional superconductor with pairing driven by exchange of spin fluctuations. The theory is based on a self-consistent real space treatment of pairing in the disordered one-band Hubbard model. It has been demonstrated before that impurities can enhance pairing by softening the spin fluctuations locally; here, we consider the competing effect of pair breaking by the screened Coulomb potential also present. We show that, depending on the impurity potential strength and proximity to magnetic order, this mechanism results in a weakening of the disorder-dependent T_{c}-suppression rate expected from Abrikosov-Gor'kov theory, or even in disorder-generated T_{c} enhancements.

Pair-breaking quantum phase transition in superconducting nanowires

Quantum phase transitions (QPT) between distinct ground states of matter are widespread phenomena1–5, yet there are only a few experimentally accessible systems6,7 where the microscopic mechanism of the transition can be tested and understood. These cases are unique and form the experimentally established foundation for our understanding of quantum critical phenomena. Here we report that a magnetic-field-driven QPT in superconducting nanowires—a prototypical one-dimensional system (d=1)—can be fully explained by the critical theory8,9 of pair-breaking transitions characterized by a correlation length exponent v≈1 and dynamic critical exponent z≈2. We find that in the quantum critical regime, the electrical conductivity is in agreement with a theoretically predicted scaling function and, moreover, that the theory quantitatively describes the dependence of conductivity on the critical temperature, field magnitude and orientation, nanowire cross-sectional area, and microscopic parameters of the nanowire material. At the critical field, the conductivity follows a T(d–2)/z dependence predicted by phenomenological scaling theories10,11 and more recently obtained within a holographic framework12. Our work uncovers the microscopic processes governing the transition: the pair-breaking effect of the magnetic field on interacting Cooper pairs overdamped by their coupling to electronic degrees of freedom. It also reveals the universal character of continuous quantum phase transitions. Transport measurements performed on MoGe superconducting nanowires reveal a quantitative agreement with quantum critical behaviour driven by a pair-breaking mechanism.