Pair-breaking Effect Research Articles

Evolution of the pseudogap temperature dependence in YBa2Cu3O7–δ films under the influence of a magnetic field

The evolution of the temperature dependence of the pseudogap Δ*(T) in optimally doped (OD) YBa2Cu3O7–δ (YBCO) films with the superconducting critical temperature Tc = 88.7 K under the influence of a magnetic field B has been studied in detail. It has been established that the shape of Δ*(T) for various B over the entire range from the pseudogap opening temperature T* to T01, below which superconducting fluctuations occur, has a wide maximum at the BEC-BCS crossover temperature Tpair, which is typical for OD films and untwinned YBCO single crystals. T* was shown to be independent on B, whereas Tpair shifts to the low-temperature region along with the increase in B, while the maximum value of Δ*(Tpair) remains practically constant regardless of B. It was revealed that as the field increases, the low-temperature maximum near the 3D-2D transition temperature T0 is blurred and disappears at B > 5 T. Moreover, above the Ginzburg temperature TG, which limits superconducting fluctuations from below, for B > 0.5 T, a minimum appears on Δ*(T) at Tmin, which becomes very pronounced with a further increase in B. As a result, the overall value of Δ*(T) decreases noticeably most likely due to the pair-breaking effect. A comparison of Δ*(T) near Tc with the Peters–Bauer theory shows that the density of fluctuating Cooper pairs actually decreases from ⟨n↑n↓⟩ ≈ 0.31 at B = 0 to ⟨n↑n↓⟩ ≈ 0.28 in the field of 8 T. The observed behavior of Δ*(T) around Tmin is assumed to be due to the influence of a two-dimensional vortex lattice created by the magnetic field, which prevents the formation of fluctuating Cooper pairs near Tc.

Superconducting state in Ba1−x Sr x Ni2As2 near the quantum critical point

In the phase diagram of the nickel-based superconductor Ba1−x Sr x Ni2As2, T c has been found to be enhanced sixfold near the quantum critical point (QCP) x = 0.71 compared with the parent compound. However, the mechanism is still under debate. Here, we report a detailed investigation of the superconducting properties near the QCP (x ≈ 0.7) by utilizing scanning tunneling microscopy and spectroscopy. The temperature-dependent superconducting gap and magnetic vortex state were obtained and analyzed in the framework of the Bardeen–Cooper–Schrieffer model. The ideal isotropic s-wave superconducting gap excludes the long-speculated nematic fluctuations while preferring strong electron–phonon coupling as the mechanism for T c enhancement near the QCP. The lower than expected gap ratio of Δ/(k B T c) is rooted in the fact that Ba1−x Sr x Ni2As2 falls into the dirty limit with a serious pair breaking effect similar to the parent compound.

Electromagnetic response of disordered superconducting cavities

We present the results for the resonant frequency shift and quality factor of disordered niobium (Nb) superconducting radio-frequency cavities driven out of equilibrium by the resonant microwave field. The theory is based on the non-equilibrium theory of superconductivity for the current response to the electromagnetic field at the vacuum–metal interface. We are able to accurately predict the observed frequency shifts with a precision of order fractions of kHz over the full temperature range 0 < T ≤ Tc, including the negative frequency shift anomalies that are observed very near Tc. The origin of these anomalies is shown to be the competition between the normal metal skin depth and the London penetration depth, which diverges as T→Tc−. An analytical approximation to the full current response, valid for |T − Tc|≪ Tc, accounts for the negative frequency shift near Tc. The non-monotonic dependence of the quality factor on the quasiparticle scattering rate is related to the pair-breaking effect of disorder on the superfluid fraction and, thus, the London penetration depth.

Effect of Anisotropic Impurity Scattering in D-Wave Superconductors

In d x 2–y 2 -wave superconductors, the effect of s-wave point disorder has been extensively studied in literature. We study the anisotropic disorder in the form of with fk = cos(2θ) (θ the azimuthal angle of k), as proposed to be caused by apical oxygen vacancies in overdoped La-based cuprate films, under the Born approximation. The disorder self-energy and d-wave pairing affect each other and must be solved simultaneously self-consistently. We find that the self-energy is reduced at low frequencies and thus weakens the pair-breaking effect. This frequency dependence vanishes in the dirty limit for which the disorder is well described by a scattering rate . One consequence of the disorder effect is that the gap-to-T c ratio 2Δ(0)/T c is greatly enhanced by the d-wave disorder, much larger than the s-wave disorder and the clean BCS value 4.28. Lastly, we generalize the d-wave scattering rate to a general form around each nodal direction θ 0. We find the density of states ρ(ω)–ρ(0)∝|ω| (ω 2) for all α ≥ 1 (α < 1) in the limit of ω → 0. As a result, the superfluid density ρ s exhibits two and only two possible scaling behaviors: ρ s(0)–ρ s(T)∝ T (T 2) for α ≥ 1 (α < 1) in the low temperature limit.

Bound states around impurities in a superconducting bilayer

We theoretically study the appearance of bound states around impurities in a superconducting bilayer. We focus our attention on $s$-wave pairing, which includes unconventional odd-parity states permitted by the layer degree of freedom. Utilizing numerical mean-field and analytical $T$-matrix methods, we survey the bound state spectrum produced by momentum-independent impurity potentials in this model. For even-parity $s$-wave pairing bound states are only found for impurities which break time-reversal symmetry. For odd-parity $s$-wave states, in contrast, bound states are generically found for all impurity potentials, and fall into six distinct categories. This categorization remains valid for nodal gaps. Our results are conveniently understood in terms of the ``superconducting fitness'' concept, and show an interplay between the pair-breaking effects of the impurity and the normal-state band structure.

Pair breaking in superconductors with strong spin-orbit coupling

We study the influence of symmetry-breaking perturbations on superconductivity in multiorbital materials, with a particular focus on an external magnetic field. We introduce the field-fitness function which characterizes the pair-breaking effects of the perturbation on a given superconducting state. For even-parity superconductors we find that this field-fitness function for an external magnetic field is one, implying that the paramagnetic response is controlled only by a generalized effective $g$ factor. For odd-parity superconductors, the interplay of the effective $g$ factor and the field-fitness function can lead to counterintuitive results. We demonstrate this for $p$-wave pairing in the effective $j=\frac{3}{2}$ electronic states of the Luttinger-Kohn model.

Unconventional superconductivity with preformed pairs in twisted bilayer graphene

We present a theory of superconductivity in magic-angle twisted bilayer graphene and analyze the superconducting phase diagram in presence of the magnetic field. Namely, we consider a model of a granular array hosting localized states, which are hybridized via the delocalized fermions in the inter-grain regions. We study a strong coupling situation when the interactions lead to an incoherent state with preformed Cooper pairs inside the grains. The Andreev scattering among different grains manifests itself through the global phase-coherent superconducting state at lower temperatures. We demonstrate that a new phase transition between the preformed Cooper pairing state and the Larkin-Ovchinnikov-Fulde-Ferrell state might be induced by the spin pair-breaking effect of in-plane magnetic field. The upper critical magnetic field is shown to be enhanced in the strong coupling case.

Electron–phonon interactions and superconductivity of β -Nb2N thin films

Niobium nitride (NbN) has attracted scientific interest due to its diverse physical properties and a variety of structural phases. The structure and superconductivity of the cubic δ-NbN phase are well established, but its hexagonal phases are not explored hitherto. In the present work, we report a simple synthesis route and a detailed study of hexagonal β-Nb2N thin films. Thermal annealing of sputtered grown α-NbN leads to a single phase β-Nb2N at 973 K as confirmed by x-ray diffraction and absorption spectroscopy. The electrical transport measurements revealed a dominance of electron–phonon interactions with a superconducting transition around 4.74 K and an upper critical field [HC2(0)] of 3.99 T. The estimated HC2(0) is well below the calculated Pauli limit, and the Maki parameter value (α &amp;lt; 1) indicates that HC2(0) is dominated by an orbital pair breaking effect. Finally, the obtained value of electron–phonon coupling constant (λ) is in excellent agreement with a weak coupling Bardeen–Cooper–Schrieffer value of conventional superconducting materials.

Strain disorder and gapless intervalley coherent phase in twisted bilayer graphene

Correlated insulators are frequently observed in magic angle twisted bilayer graphene at even fillings of electrons or holes per moir\'e unit-cell. Whereas theory predicts these insulators to be intervalley coherent excitonic phases, the measured gaps are routinely much smaller than theoretical estimates. We explore the effects of random strain variations on the intervalley coherent phase, which have a pair-breaking effect analogous to magnetic disorder in superconductors. We find that the spectral gap may be strongly suppressed by strain disorder, or vanish altogether, even as intervalley coherence is maintained. We discuss predicted features of the tunneling density of states, show that the activation gap measured in transport experiments corresponds to the diminished gap, and thus offer a solution for the apparent discrepancy between the theoretical and experimental gaps.

The Compliance of the Upper Critical Field in Magic-Angle Multilayer Graphene with the Pauli Limit.

The Pauli limiting field represents a fundamental magnetic field at which the superconducting state collapses due to the spin-paramagnetic Cooper pair-breaking effect. Cao et al. (Nature 2021, 595, 526) reported that the magic-angle twisted trilayer graphene (MATNG, N = 3) exhibits the upper critical field which exceeds the Pauli limiting field by two to three times. This observation was interpreted as a violation of the Pauli-limiting field in MAT3G. Similar conclusions were recently reported by the same research group in MATNG (N = 4, 5) superlattices (Park, J.M. et al. Nat. Mater.2022, 21, 877). Here, we point out that Cao et al. (Nature 2021, 595, 526) calculated the Pauli limiting field by the use of reduced form (to the weak-coupling limit) of full equation of the theory of the electron−phonon-mediated superconductivity. Considering that in the same paper, Cao et al. (Nature 2021, 595, 526) reported that MATNGs are strong coupled superconductors, we calculate the Pauli limiting field for a strong coupled case and show that the observed upper critical fields in MATNGs comply with the Pauli limit. This implies that there is no violation of the Pauli limiting field in the Moiré multilayer graphene superlattices.

Electronic properties of -Mn-type non-centrosymmetric superconductor Re5.5Ta under hydrostatic pressure

We have studied the effect of hydrostatic pressure on the superconducting and normal state properties of the non-centrosymmetric superconductor Re5.5Ta using magnetotransport, magnetization, and band structure studies. The superconducting transition temperature () reveals a modest decrease of ∼0.52 K as the external pressure is raised to 8 GPa. We found the Maki parameter, both at ambient and high pressures, suggesting the significant role of the Pauli pair breaking effect. Furthermore, the lower critical field was found to decrease with a rate of −1.74 mT GPa−1 as pressure increases to 1.05 GPa. Analysis of the thermally activated flux flow region of using the Arrhenius equation reveals the coexistence of single and collective vortex states. Further analysis of the critical current density indicates that the surface and volume pinning centers coexist and remain unaffected with varying pressure. Band structure calculations were performed using density functional theory and reveal a large bulk modulus, ∼ 289 GPa with ∼ 3.7, suggesting poor compressibility. In addition, we found that the density of states at the Fermi level barely reduces, even at pressures of up to 20 GPa. This is consistent with the experimental observation of robust superconductivity under compression.

Boundary conditions for the order parameter and the proximity influenced internal phase differences in double superconducting junctions

This paper gives an overview of the unconventional dependence of internal phase differences on the external phase difference in superconductor–normal metal–superconductor (SINIS) and superconductor–superconductor–superconductor (SISIS) tunnel double junctions. The results are obtained within the Ginzburg–Landau (GL) approach that includes boundary conditions for the superconductor order parameter in the presence of a Josephson coupling through interfaces. The boundary conditions are derived within the GL theory and substantiated microscopically. The absence of the one-to-one correspondence between external ϕ and internal χ1,2 phase differences in the junctions is shown to occur in two qualitatively different ways, both of which result in the range of χ1,2 reduced and prevent the 4π-periodic current–phase relation j=jcsinϕ2. In SINIS junctions, the effect of the supercurrent–induced phase incursion φ between the end faces of the central electrode of mesoscopic length L can play a crucial role. In SISIS junctions, there occurs the regime of interchanging modes, which is modified as L decreases. The proximity and pair breaking effects in the double junctions with closely spaced interfaces are addressed.

Nucleation of superconductivity in clean superconductor-ferromagnet hybrid structures with Rashba spin-orbit interaction

We study the influence of the inverse proximity effect on the nucleation of superconductivity in planar hybrid structures consisting of a thin superconducting film proximity coupled to a material with a strong exchange or Zeeman field and the Rashba spin-orbit interaction. Based on numerical simulations and analytical estimates within the framework of the Gor'kov equations, we find the superconducting transition temperature of the system and determine the spatial structure of the gap function. It is shown that the spin-orbit interaction partially compensates the pair-breaking effect of the exchange or Zeeman field and stabilizes modulated superconducting states, which can be traced to the Lifshitz invariant in the Ginzburg-Landau free-energy density. The suggested approach provides a microscopic justification of this invariant for a large class of superconducting hybrid systems and clarifies the validity range of the corresponding phenomenological models with terms linear in the spatial gradients of the superconducting order parameter.

Pressure-Induced Superconductivity up to 9 K in the Quasi-One-Dimensional KMn6Bi5

The Mn-based superconductor is rare owing to the strong magnetic pair-breaking effect. Here we report on the discovery of pressure-induced superconductivity in KMn6Bi5, which becomes the first ternary Mn-based superconductor. At ambient pressure, the quasi-one-dimensional KMn6Bi5 is an antiferromagnetic metal with TN = 75 K. By measuring resistivity and ac magnetic susceptibility under hydrostatic pressures up to 14.2 GPa in a cubic anvil cell apparatus, we find that its antiferromagnetic transition can be suppressed completely at a critical pressure of Pc = 13 GPa, around which bulk superconductivity emerges and displays a superconducting dome with the maximal Tconset = 9.3 K achieved at about 14.2 GPa. The close proximity of superconductivity to a magnetic instability in the temperature-pressure phase diagram of KMn6Bi5 and an unusually large {\mu}0Hc2(0) = 18.9 T exceeding the Pauli limit suggests an unconventional magnetism-mediated paring mechanism. In contrast to the binary MnP, the flexibility of the crystal structure and chemical compositions in the ternary AMn6Bi5 (A = alkali metal) can open a new avenue for finding more Mn-based superconductors.

Effect of alloying in monolayer niobium dichalcogenide superconductors

When sulfur and silicon are incorporated in monolayer 2H-NbSe2 the superconducting transition temperature, Tc, has been found to vary non-monotonically. This was assumed to be a manifestation of fractal superconductivity. Using first-principles calculations, we show that the nonmonotonic dependence of Tc is insufficient evidence for multifractality. A unifying aspect in our study are selenium vacancies in NbSe2, which are magnetic pair-breaking defects that we propose can be present in considerable concentrations in as-grown NbSe2. We show that sulfur and silicon can occupy the selenium sites and reduce the pair-breaking effect. Furthermore, when sulfur is incorporated in NbSe2, the density of states at the Fermi level and the proximity to magnetism in the alloy are both reduced compared to the parent compound. Based on our results, we propose an alternative explanation of the non-monotonic change in Tc which does not require the conjecture of multifractality.

Studying magnetic-polaron cooper-pair-breaking effects in Bi2Sr2CaCu2O8+δ by femtosecond photoexcitation

The relation between magnetism and superconductivity, whether magnetism is beneficial to superconductivity or they permit co-existence and competition, has been a long debate in the high-temperature superconducting field. Here, to explore this odd issue, we study the relaxation dynamics of photoinduced magnetic-polaron and superconductivity quasiparticles by an ultrafast time-resolved optical reflectivity technology in Bi2Sr2CaCu2O8+δ . When the pump fluence is less than 7 mW, time-resolved reflection spectrum ΔR(t)/R 0 is dominated by the superconducting component. It is noteworthy that a large of photoinduced magnetic-polaron component can cause the inhibition of the superconducting component when the variable pump fluence is more than the value 7 mW. It can be attributed to the strong photoinduced magnetic-polaron Cooper-pair-breaking mechanism. Further studies have shown that the photoinduced magnetic-polaron Cooper-pair-breaking mechanism is similar to the pair-breaking effect caused by the magnetic impurity Fe in the Bi2Sr2CaCu2O8+δ superconductor, although the forms of their action are different.

Theory of disordered superconductors with applications to nonlinear current response

Abstract I present a review of the theory and basic equations for charge transport in superconducting alloys starting from the Keldysh formulation of the quasiclassical transport equations developed by Eilenberger, Larkin and Ovchinnikov, and Eliashberg. This formulation is the natural extension of Landau’s theory of normal Fermi liquids to the superconducting state of strongly correlated metals. For dirty metals the transport equations reduce to equations for charge diffusion, with the current response given by the Drude conductivity at low temperatures. The extension of the diffusion equation for the charge and current response of a strongly disordered normal metal to the superconducting state yields Usadel’s equations for the nonequilibrium quasiclassical Keldysh propagator. The conditions for the applicability of the Usadel equations are discussed, the pair-breaking effect of disorder on the current response, including the nonlinear current response to an electromagnetic field in the dirty limit, τ ≪ ℏ/Δ, are reported. The same nonlinearity is shown to lead to source currents for photon generation and nonlinear Kerr rotation driven by the nonlinear response to excitation of the superconductor by a multi-mode electromagnetic field. The potential relevance of the nonlinear source currents to superconducting radio-frequency cavities as detectors of axion-like dark matter candidates is briefly discussed.

Full orbital decomposition of Yu-Shiba-Rusinov states based on first principles

We have implemented the Bogoliubov-de Gennes (BdG) equation in a screened Korringa-Kohn- Rostoker (KKR) method for solving, self-consistently, the superconducting state for 3d crystals including substitutional impurities. In this report we extend this theoretical framework to allow for collinear magnetism and apply it to fcc Pb with 3d magnetic impurities. In the presence of magnetic impurities, there is a pair-breaking effect that results in sup-gap Yu-Shiba-Rusinov (YSR) states which we decompose into contributions from the individual orbital character. We determine the spatial extent of these impurity states, showing how the different orbital character affects the details of the YSR states within the superconducting gap. Our work highlights the importance of the first principles based description which captures the quantitative details making direct comparisons possible with experimental findings.

Onset of a superconductor-insulator transition in an ultrathin NbN film under in-plane magnetic field studied by terahertz spectroscopy

Optical conductivity of a moderately disordered superconducting NbN film was investigated by terahertz time-domain spectroscopy in external magnetic field applied along the film plane. The film thickness of about 5 nm was comparable with the coherence length, so vortices should not form. This was confirmed by the fact that no marked difference between the spectra with terahertz electric field set perpendicular and parallel to the external magnetic field was observed. Simultaneous use of Maxwell-Garnett effective medium theory and the model of optical conductivity by Herman and Hlubina proved to correctly reproduce the terahertz spectra obtained experimentally in a magnetic field of up to 7 T. This let us conclude that the magnetic field tends to suppress the superconductivity, resulting in an inhomogeneous state where superconducting domains are enclosed within a normal-state matrix. The scattering rate due to pair-breaking effects was found to linearly increase with magnetic field.

The FFLO State in the Dimer Mott Organic Superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br

The superconducting phase diagram for a quasi-two-dimensional organic superconductor, κ-(BEDT-TTF)2Cu[N(CN)2]Br, was studied using pulsed magnetic field penetration depth measurements under rotating magnetic fields. At low temperatures, Hc2 was abruptly suppressed even by small tilts of the applied fields owing to the orbital pair-breaking effect. In magnetic fields parallel to the conducting plane, the temperature dependence of the upper critical field Hc2 exhibited an upturn and exceeded the Pauli limit field HP in the lower temperature region. Further analyses with the second derivative of the penetration depth showed an anomaly at 31–32 T, which roughly corresponded to HP. The origin of the anomaly should not be related to the orbital effect, but the paramagnetic effect, which is almost isotropic in organic salts, because it barely depends on the field angle. Based on these results, the observed anomaly is most likely due to the transition between the Bardeen-Cooper-Schrieffer (BCS) and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states. Additionally, we discuss the phase diagram and physical parameters of the transition by comparing them with other FFLO candidates.