Pure Superconductivity Research Articles

Decoupling multiphase superconductivity from normal state ordering in CeRh2As2

${\mathrm{CeRh}}_{2}{\mathrm{As}}_{2}$ is a multiphase superconductor with ${T}_{\text{c}}=0.26$ K. The two superconducting (SC) phases, SC1 and SC2, observed for a magnetic field $H$ parallel to the $c$ axis of the tetragonal unit cell, have been interpreted as even- and odd-parity SC states, separated by a phase boundary at ${\ensuremath{\mu}}_{0}{H}^{*}=4$ T. Such parity switching is possible due to a strong Rashba spin-orbit coupling at the Ce sites located in locally noncentrosymmetric environments of the globally centrosymmetric lattice. The existence of another ordered state (phase I) below a temperature ${T}_{0}\ensuremath{\approx}0.4$ K suggests an alternative interpretation of the ${H}^{*}$ transition: It separates a mixed SC$+$I (SC1) and a pure SC (SC2) state. Here, we present a detailed study of higher-quality single crystals of ${\mathrm{CeRh}}_{2}{\mathrm{As}}_{2}$, showing much sharper signatures at ${T}_{\text{c}}=0.31$ K and ${T}_{0}=0.48$ K. We refine the $T\ensuremath{-}H$ phase diagram of ${\mathrm{CeRh}}_{2}{\mathrm{As}}_{2}$ and demonstrate that ${T}_{0}(H)$ and ${T}_{\text{c}}(H)$ lines meet at ${\ensuremath{\mu}}_{0}H\ensuremath{\approx}6$ T, well above ${H}^{*}$, implying no influence of phase I on the SC phase switching. A basic analysis with the Ginzburg-Landau theory indicates a weak competition between the two orders.

Competitive and cooperative electronic states in Ba(Fe1\u2212xTx)2As2 with T\u2009=\u2009Co, Ni, Cr

The electronic inhomogeneities in Co, Ni, and Cr doped BaFe2As2 single crystals are compared within three bulk property regions: a pure superconducting (SC) dome region, a coexisting SC and antiferromagnetic (AFM) region, and a non-SC region. Machine learning is utilized to categorize the inhomogeneous electronic states: in-gap, L-shape, and S-shape states. Although the relative percentages of the states vary in the three samples, the total volume fraction of the three electronic states is quite similar. This is coincident with the number of electrons (Ni0.04 and Co0.08) and holes (Cr0.04) doped into the compounds. The in-gap state is confirmed as a magnetic impurity state from the Co or Ni dopants, the L-shape state is identified as a spin density wave which competes with the SC phase, and the S-shape state is found to be another form of magnetic order which constructively cooperates with the SC phase, rather than competing with it.

Optimization of HTS Superconducting Solenoid Magnet Dimensions for Maximum Energy Density

Superconducting coil provides enormous amount of stored energy inside its magnetic field. Such a pure inductive superconducting (SC) coil can be designed for high power density or high energy density depending on coil dimensions and inductance based on the prerequisite of application. In this paper, a design procedure is developed to optimize dimensions of a high-temperature superconducting (HTS) coil to store maximum energy for a given length and critical properties of HTS tape. A minimum volume constraint (MVC) is developed to confine the variation of aspect ratios to keep the coil volume constant, thereby limiting the material, cryogenic and vacuum costs of HTS coil. A global optimum inner diameter is established by MVC, with which the optimum dimensions can be reached directly. Multiple constraints such as coil volume, stacked pancake arrangement, HTS tape length and mechanical properties are considered. The electrical, magnetic and mechanical parameters for different dimensions are then found by finite element method (FEM). The proposed method is applied to different lengths of 1G and 2G HTS tapes. The optimum dimensions of maximum stored energy are decided which gives a solenoid coil of maximum energy density.

Effect of the nonmonotonic d-wave superconducting gap on the electronic Raman scattering of electron-doped cuprate superconductors

ABSTRACT By considering a nonmonotonic d-wave superconducting (SC) gap effect in electron-doped cuprate superconductors, the energy and momentum dependence of the electronic Raman scattering is studied. The electronic Raman scattering of electron-doped cuprate superconductors has obvious differences with hole-doped case described by a pure d-wave SC gap, which can be attributed to the nonmonotonic d-wave SC gap in electron-doped cuprate superconductors. Three main different behaviours for the electronic Raman scattering spectra between electron-doped and hole-doped cuprate superconductors have been obtained, the intensity comparison of and spectra, the Raman peak positions as a function of energy for and spectra, and energy asymptotic behaviour of the low-energy spectrum. For electron-doped cuprate superconductors, the intensity of spectrum as a function of energy is larger than one, and the Raman peak position of spectrum appears at a higher energy than one, then the low-energy spectrum of as a function of energy increases linearly, not energy dependence. All these theoretical results of electron-doped cuprate superconductors are in qualitative agreement with the corresponding Raman scattering experimental data.

Group-theoretical classification of superconducting states of strontium ruthenate

The possible superconducting states of strontium ruthenate (Sr$_2$RuO$_4$) are organized into irreducible representations of the point group $D_{4h}$, with a special emphasis on nodes occurring within the superconducting gap. Our analysis covers the cases with and without spin-orbit coupling and takes into account the possibility of inter-orbital pairing within a three-band, tight-binding description of Sr$_2$RuO$_4$. No dynamical treatment if performed: we are confining ourselves to a group-theoretical analysis. The case of uniaxial deformations, under which the point group symmetry is reduced to $D_{2h}$, is also covered. It turns out that nodal lines, in particular equatorial nodal lines, occur in most representations. We also highlight some results specific to multiorbital superconductivity. Among other things, we find that odd inter-orbital pairing allows to combine singlet and triplet superconductivity whithin the same irreducible representation, that pure inter-orbital superconductivity leads to nodal surfaces and that the notion of nodes imposed by symmetry is not clearly defined.

Specific Heat Investigation in Iron-Based Superconductors

We have proposed a mean field theoretical model in the coexistence of superconductivity and Jahn–Teller (JT) effect for the study of specific heat in iron-based superconductors in the presence of an external magnetic field. The superconducting (SC) gap and lattice strain energy expressions are calculated using Zubarev’s technique of double-time electron Green’s function and solved numerically. The specific heat jump at the critical temperatures are observed. Keeping fixed the SC transition temperature at 28[Formula: see text]K for the iron pnictide [Formula: see text] like superconductors, the coefficient of electronic specific heat [Formula: see text] at low temperatures is determined to be 5.75[Formula: see text]mJ/mol[Formula: see text]K2 in the pure SC state and 4.76[Formula: see text]mJ/mol[Formula: see text]K2 in the coexistence state of SC and JT effect which are in agreement with some experimental results. The effect of the external magnetic field on the specific heat is studied.

Superconductivity switch from spin-singlet to -triplet pairing in a topological superconducting junction

The interedge coupling is the cardinal characteristic of the narrow quantum spin Hall (QSH) insulator, and thus could bring about exotic transport phenomena. Herein, we present a theoretical investigation of the spin-resolved Andreev reflection (AR) in a QSH insulator strip touching on two neighbouring ferromagnetic insulators and one s-wave superconductor. It is demonstrated that, due to the interplay of the interedge coupling and ferromagnetic configuration, there could be not only usual local ARs leading to the spin-singlet pairing with the incident electron and Andreev-reflected hole from different spin subbands, but also novel local ARs giving rise to the spin-triplet pairing from the same spin subband. However, only the latter exists in the absence of the interedge coupling, and therefore the two pairings in turn testify the helical spin texture of the edge states. By proper tuning of the band structures of the ferromagnetic layers, under the resonance bias voltage, the usual and novel local ARs of can be all exhibited, resulting in fully spin-polarized pure spin-singlet superconductivity and pure spin-triplet superconductivity, respectively, which suggests a superconductivity switch from spin-singlet to -triplet pairing by electrical control. The results can be experimentally confirmed by the tunneling conductance and the noise power.

Antiferromagnetism, charge density wave, and d-wave superconductivity in the extended t-J-U model: role of intersite Coulomb interaction and a critical overview of renormalized mean field theory

In the first part of the paper, we study the stability of antiferromagnetic (AF), charge density wave (CDW), and superconducting (SC) states within the t-J-U-V model of strongly correlated electrons by using the statistically consistent Gutzwiller approximation (SGA). We concentrate on the role of the intersite Coulomb interaction term V in stabilizing the CDW phase. In particular, we show that the charge ordering appears only above a critical value of V in a limited hole-doping range δ. The effect of the V term on SC and AF phases is that a strong interaction suppresses SC, whereas the AF order is not significantly influenced by its presence. In the second part, separate calculations for the case of a pure SC phase have been carried out within an extended approach (the diagrammatic expansion for the Gutzwiller wave function, DE-GWF) in order to analyze the influence of the intersite Coulomb repulsion on the SC phase with the higher-order corrections included beyond the SGA method. The upper concentration for the SC disappearance decreases with increasing V, bringing the results closer to experiment. In appendices A and B we discuss the ambiguity connected with the choice of the Gutzwiller renormalization factors within the renormalized mean filed theory when either AF or CDW orders are considered. At the end, we overview briefly the possible extensions of the current models to put descriptions of the SC, AF, and CDW states on equal footing.

Tailoring phase slip events through magnetic doping in superconductor-ferromagnet composite films.

The interplay between superconductivity (SC) and ferromagnetism (FM) when embedded together has attracted unprecedented research interest due to very rare coexistence of these two phenomena. The focus has been mainly put into the proximity induced effects like, coexistence of magnetism and superconductivity, higher critical current, triplet superconductivity etc. However, very little attention has been paid experimentally to the role of magnetic constituent on triggering phase slip processes in the composite films (CFs). We demonstrate that less than 1 at.% of magnetic contribution in the CFs can initiate phase slip events efficiently. Due to advanced state-of-the-art fabrication techniques, phase slip based studies have been concentrated mainly on superconducting nanostructures. Here, we employ wide mesoscopic NbGd based CFs to study the phase slip processes. Low temperature current-voltage characteristics (IVCs) of CFs show stair-like features originated through phase slip events and are absent in pure SC films. Depending on the bias current and temperature, distinct regions, dominated by Abrikosov type vortex-antivortex (v-av) pairs and phase slip events, are observed. The results presented here open a new way to study the phase slip mechanism, its interaction with v-av pairs in two dimensions and hence can be useful for future photonic and metrological applications.

Pairing by Kondo interaction and magnetic phases in the Anderson–Kondo lattice model: Statistically consistent renormalized mean‐field theory

AbstractWe start from the Anderson–Kondo lattice model derived by us earlier, in which both the Kondo interaction and the residual hybridization processes have been included in a systematic manner. In the present work we discuss a fairly complete phase diagram including magnetic, pure superconducting (SC), and coexistent antiferromagnetic (AF)–SC phases. Both intra‐ and interatomic hybridization cases have been considered, separately. The Kondo insulating state with completely compensated magnetic moments has been obtained as a reference state, from which either AF or SC or mixed AF + SC phases evolve when the metallic state becomes stable. The SC pairing is induced mainly by the Kondo interaction. The method of approach we use is the so‐called statistically consistent renormalized mean‐field theory (SC‐RMFT), encompassing essential corrections to the standard Gutzwiller‐ansatz approach.

Superconducting Phases in Potassium-Intercalated Iron Selenides

The ubiquitous coexistence of majority insulating 245 phases and minority superconducting (SC) phases in A(x)Fe(2-y)Se(2) (A = K, Cs, Rb, Tl/Rb, Tl/K) formed by high-temperature routes makes pure SC phases highly desirable for studying the intrinsic properties of this SC family. Here we report that there are at least two pure SC phases, K(x)Fe(2)Se(2)(NH(3))(y) (x ≈ 0.3 and 0.6), determined mainly by potassium concentration in the K-intercalated iron selenides formed via the liquid ammonia route. K(0.3)Fe(2)Se(2)(NH(3))(0.47) corresponds to the 44 K phase with lattice constant c = 15.56(1) Å and K(0.6)Fe(2)Se(2)(NH(3))(0.37) to the 30 K phase with c = 14.84(1) Å. With higher potassium doping, the 44 K phase can be converted into the 30 K phase. NH(3) has little, if any, effect on superconductivity. Thus, the conclusions should apply to both K(0.3)Fe(2)Se(2) and K(0.6)Fe(2)Se(2) SC phases. K(0.3)Fe(2)Se(2)(NH(3))(0.47) and K(0.6)Fe(2)Se(2)(NH(3))(0.37) stand out among known superconductors as their structures are stable only at particular potassium doping levels, and hence the variation of T(c) with doping is not dome-like.

Coexistence of Superconductivity and Antiferromagnetism and its Effects on Crossover as to Correlation Strength in Hubbard Model

Abstract Using a variational Monte Carlo method, the interplay between antiferromagnetism and superconductivity is studied for the two- dimensional Hubbard model with a diagonal transfer t’. It is found with a refined function that the optimized state is sensitive to the values of model parameters, U/t, t’/t, and δ (doping rate), and becomes a coexisting state or a pure superconducting (SC) state. SC properties in the coexisting state also undergo considerable changes around the crossover point Uco(~10t) from a fragile BCS type for U Uco, in which pairs are formed through local exchange or doublon-holon processes, but supercurrent flows separately with the doped holes.

Repulsion and Attraction in High T c Superconductors

The influence of repulsion and attraction in high-T c superconductors to the gap functions is studied. A systematic method is proposed to compute the gap functions using the irreducible representations of the point group. It is found that a pure s-wave superconductivity exists only at very low temperatures, and attractive potentials on the near shells significantly expand the gap functions and increase significantly the critical temperature of superconductivity. A strong on-site repulsion drives the A 1g gap into a B 1g gap. It is expected that superconductivity with the A 1g symmetry reaches a high critical temperature due to the cooperation of the on-site and the next-nearest neighbor attractions.

Superconductivity enhanced by d-density wave: A weak-coupling theory

Making a revision of mistakes in Ref. [19], we present a detailed study of the competition and interplay between the d-density wave (DDW) and d-wave superconductivity (DSC) within the fluctuation-exchange (FLEX) approximation for the two-dimensional (2D) Hubbard model. In order to stabilize the DDW state with respect to phase separation at lower dopings a small nearest-neighbor Coulomb repulsion is included within the Hartree–Fock approximation. We solve the coupled gap equations for the DDW, DSC, and π-pairing as the possible order parameters, which are caused by exchange of spin fluctuations, together with calculating the spin fluctuation pairing interaction self-consistently within the FLEX approximation. We show that even when nesting of the Fermi surface is perfect, as in a square lattice with only nearest-neighbor hopping, there is coexistence of DSC and DDW in a large region of dopings close to the quantum critical point (QCP) at which the DDW state vanishes. In particular, we find that in the presence of DDW order the superconducting transition temperature T c can be much higher compared to pure superconductivity, since the pairing interaction is strongly enhanced due to the feedback effect on spin fluctuations of the DDW gap. π-pairing appears generically in the coexistence region, but its feedback on the other order parameters is very small. In the present work, we have developed a weak-coupling theory of the competition between DDW and DSC in 2D Hubbard model, using the static spin fluctuation obtained within FLEX approximation and ignoring the self-energy effect of spin fluctuations. For our model calculations in the weak-coupling limit we have taken U/ t=3.4, since the antiferromagnetic instability occurs for higher values of U/ t.

New superconductivity induced by d-density wave in high-temperature superconductors

We present a self-consistent theory of the interplay between the d-density wave (DDW) and superconductivity on the basis of the fluctuation-exchange (FLEX) approximation for the two-dimensional (2D) Hubbard model with the nearest-neighbor hopping t and on-site Coulomb repulsion U. In order to stabilize the DDW state with respect to phase separation at lower dopings a small nearest-neighbor Coulomb repulsion V is included within the Hartree–Fock approximation. The competition between the DDW and superconductivity can generically result in the appearance of a new superconducting state, which has the π-pairing order parameter. We solve the gap equations for competing DDW, BCS, and π-pairing order parameters with d-wave symmetry, together with calculating the FLEX interaction self-consistently, and calculating the corresponding three transition temperatures T ⁎, T c , and T c ⁎ as a function of hole doping. The calculated phase diagram has a quantum critical point (QCP) separating a pure BCS superconducting state at large dopings δ> δ 0 from a coexisting DDW and π-pairing state at lower dopings δ< δ 0. Remarkable is the much higher value of the T c ⁎ for the π-pairing superconductivity as compared with the pure BCS superconductivity.

Coexistence of the antiferromagnetic and superconductingorder and its effect on spin dynamics in electron-doped high-Tc cuprates

In the framework of the slave-boson approach to thet–t′–t′′–J model, it is found that for electron-doped high-Tc cuprates, the staggered antiferromagnetic (AF) order coexists with the superconducting(SC) order in a wide doping level ranged from underdoped to nearly optimally doped at themean-field level. In the coexisting phase, it is revealed that the spin responseis commensurate in a substantial frequency range below a crossover frequencyωc forall dopings considered, and it switches to the incommensurate structure when the frequency is higherthan ωc. This result is in agreement with the experimental measurements. Comparison ofthe spin response between the coexisting phase and the pure SC phase with adx2−y2-wave pairing plus a higher harmonics term (DP+HH) suggests that the inclusion of the two-band effect is important to consistently account forboth the dispersion of the spin response and the non-monotonic gap behavior in theelectron-doped cuprates.

Stability of nodal quasiparticles in underdopedYBa2Cu3O6+yprobed by penetration depth and microwave spectroscopy

High resolution measurements of superfluid density and broadband quasiparticle conductivity have been used to probe the low energy excitation spectrum of nodal quasiparticles in underdoped YBCO. Penetration depth is measured to temperatures as low as 0.05 K. Microwave conductivity is measured from 0.1 to 20 GHz and is a direct probe of zero-energy quasiparticles. The data are compared with predictions for a number of theoretical scenarios that compete with or otherwise modify pure d-wave superconductivity, in particular commensurate and incommensurate spin and charge density waves; d + i s and d + i d superconductivity; circulating current phases; and the BCS--BEC crossover. We conclude that the data are consistent with a pure d-wave state in the presence of a small amount of strong scattering disorder, and are able to rule out most candidate competing states either completely, or to a level set by the energy scale of the disorder, ~ 4 K. Commensurate spin and charge density orders, however, are not expected to alter the nodal spectrum and therefore cannot be excluded.

Magnetic properties of the superconducting state of iron-based superconductors

We show that features of the dynamical spin susceptibility can unambiguously distinguish between different pairing symmetries of the superconducting state in iron-based superconductors. A magnetic resonance is a feature for the extended ${s}_{{x}^{2}{y}^{2}}$-wave $\text{cos}\text{ }{k}_{x}\text{ }\text{cos}\text{ }{k}_{y}$ pairing symmetry. It is present in the pure superconducting (SC) state but weakens in the mixed SC and magnetically ordered state. We calculate the random-phase approximation correction to the NMR spin-relaxation rate $1/{T}_{1}$ and the Knight shift in the above states and show a good agreement with experimental results. Moreover, we argue that the energy dispersion of the magnetic resonance along the $c$ axis observed in neutron scattering experiments is also an indirect evidence supporting the ${s}_{{x}^{2}{y}^{2}}$ pairing symmetry.

Interplay between antiferromagnetism and superconductivity in two-dimensional Hubbard model

Using a variational Monte Carlo method, we discuss the interplay between antiferromagnetism (AF) and superconductivity (SC) in a Hubbard model on a square lattice with a diagonal transfer t ′ . We simultaneously introduce the following improvements into the trial function: (1) coexistence of AF and d-wave singlet gaps that allows a continuous description of their interplay, (2) band renormalization effect in the range of fourth-neighbor hopping, and (3) refined doublon–holon correlation factors. For t ′ / t = - 0.3 , appropriate to hole-doped cuprates, it is found that AF ordered state is stabilized near half filling, especially, for U ∼ band width, by retrieving the nesting condition. As the doping rate increases, AF is weakened and a pure d-wave SC phase evolves for large U. The coexisting state of SC and AF orders appears only in a narrow range of U ( ∼ band width) and electron density n ( = 0.8 ).

Quantum condensates classified in superconductivity with topology in the Minkowski space-time

A substantial problem in the macroscopic theory of pure superconductivity has been left forgotten for a long time since London and London in 1935. An impression survived that the Meissner effect is more substantial than the zero-resistivity. But, the London equation [I], the Newtonian equation of motion, was abandoned, whereas the London equation [II], derived from the Maxwell equations, was postulated. The London equation [II] included the logical gap [ α ] in real time, whereas the London equation [I] has been ignored without even noting the logical gap [ β ] in space. Microscopically, after the publication of F. London's book and the discovery of the isotope effect in 1950, the success of the Bardeen--Cooper--Schrieffer (BCS) theory in 1957 was likely to have finally given the definitive explanation on superconductivity by proving only the London equation [II] that claimed the coherent condensation of Cooper pairs in the momentum space. Since then, these arguments have been regarded to be a standard among various preceding theories. Meanwhile, the London equation [I] has faded away and has been long-forgotten. But we must not abandon the London equation [I], and, rather, retrieve it. We later recognized also that the DC-component of a persistent current can never be determined by using the Fourier transform analysis, because of its singularity at ω = 0 and q = 0 with huge differences of space-time domain. Quite recently, in 2003, we first recognized a proper and harmonious view to simultaneously account for (i) the zero-resistivity in an open system with (i-c) the resultant persistent current in a closed system, and (ii) the perfect diamagnetism at T ≅ 0 K in the space-time aspects in terms of the gauge field theory. Here, we further clarify where and how we have lost and found a properly perspective view of the superconductivity. Here, we eliminate two logical gaps [ α ] and [ β ] by using the gauge field theory for further clarifying a position of the previous and present works. We especially classify superconductors with topology which eventually leads us such as (ii-2D) magnetic flux quantization in a ring. By projecting the 3-dimensional BCS-theory with the concept of ‘coherence’ among an enormous number of Bosons like Cooper pairs onto the (1 + 3)-dimensional Minkowski space-time [β = (v/c) = 0], we clarify responses of the ground state Ψ macro at T ≅ 0 K in a set of the basic equations, for (i) the zero-resistivity, [E K − qφ( R )] = 0 at ω = 0 and (ii) the perfect diamagnetism [ℏK − qA ( R )] = 0 at q = 0 as an inevitable consequence at the gauge fields in the proper theory of superconductivity.