We have studied the onset temperature of the superconductivity Tc_onset of the organic superconductor (TMTSF)2ClO4, by precisely controlling the direction of the magnetic field H. We compare the results of two samples with nearly the same onset temperature but with different scattering relaxation time tau. We revealed a complicated interplay of a variety of pair-breaking effects and mechanisms that overcome these pair-breaking effects. In low fields, the linear temperature dependences of the onset curves in the H-T phase diagrams are governed by the orbital pair-breaking effect. The dips in the in-plane field-angle phi dependence of Tc_onset, which were only observed in the long-tau sample, provides definitive evidence that the field-induced dimensional crossover enhances the superconductivity if the field direction is more than about 19-degrees away from the a axis. In the high-field regime for H//a, the upturn of the onset curve for the long-tau sample indicates a new superconducting state that overcomes the Pauli pair-breaking effect but is easily suppressed by impurity scatterings. The Pauli effect is also overcome for H//b' by a realization of another state for which the maximum of Tc_onset(phi) occurs in a direction different from the crystalline axes. The effect on Tc_onset of tilting the applied field out of the conductive plane suggests that the Pauli effect plays a significant role in determining Tc_onset. The most plausible explanation of these results is that (TMTSF)2ClO4 is a singlet superconductor and exhibits Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states in high fields.
We suggest theoretical explanation of the high upper critical magnetic field, perpendicular to conducting chains, Hc2, experimentally observed in the superconductor (TMTSF)2ClO4, in terms of singlet superconducting pairing. In particular, we compare the results of d-wave-like nodal, d-wave-like node-less, and s-wave scenarios of superconductivity. We show that, in d-wave-like nodal scenario, superconductivity can naturally exceed both the orbital upper critical magnetic field and Clogston-Shandrasekhar paramagnetic limit as well as reach experimental value, Hc2 = 6T, in contrast to d-wave-like…
We study the transition temperature Tc(H) of a quasi-one-dimensional (Q1D) superconductivity which is derived from the quantum effect of an electron motion in a strong magnetic field. We calculate Tc(H) of both isotropic and anisotropic superconductivity by taking account of the optimal momentum of the Cooper pairs and the effect of higher harmonic terms along second conducting axis in the tight-binding model. We find that, although Tc(H) of the spin-singlet superconductivity is suppressed strongly by the Zeeman effect, the suppression of Tc(H) is not very severe if we take the optimal pair-mo…
We report the magnetic field-amplitude and field-angle dependence of the superconducting onset temperature Tc_onset of the organic superconductor (TMTSF)2ClO4 in magnetic fields H accurately aligned to the conductive ab' plane. We revealed that the rapid increase of the onset fields at low temperatures occurs both for H // b' and H // a, irrespective of the carrier confinement. Moreover, in the vicinity of the Pauli limiting field, we report a shift of a principal axis of the in-plane field-angle dependence of Tc_onset away from the b' axis. This feature may be related to an occurrence of Fuld…
We present a study of the superconducting properties (Tc and Hc2) in the solid solution (TMTSF)2(ClO4)(1-x)(ReO4)x with a ReO-4 nominal concentration up to x = 6%. The dramatic suppression of Tc when the residual resistivity is increased upon alloying with no modification of the Fermi surface is the signature of non-conventional superconductivity . This behaviour strongly supports p or d wave pairing in quasi one dimensional organic superconductors. The determination of the electron lifetime in the normal state at low temperature confirms that a single particle Drude model is unable to explain…
The layered quasi-one-dimensional molecular superconductor (TMTSF)$_2$PF$_6$ is a very exotic material with a superconducting order parameter whose ground state symmetry has remained ill-defined. Here we present a pulsed NMR Knight shift (K) study of $^{77}$Se measured simultaneously with transport in pressurized (TMTSF)$_2$PF$_6$. The Knight shift is linearly dependent on the electron spin susceptibility $χ_s$, and is therefore a direct measure of the spin polarization in the superconducting state. For a singlet superconductor, the spin contribution to the Knight shift, K$_s$, falls rapidly o…
Using various transport and magnetotransport probes we study the coexistence of spin-density wave and superconductor states in (TMTSF)2ClO4 at various degrees of ClO4 anions ordering. In the two-phase complex state when both superconductivity and spin-density wave are observed in transport, we find prehistory effects, enhancement of the superconducting critical field, and strong spatial anisotropy of the superconducting state. These features are inconsistent with the conventional model of structural inhomogeneities produced by anion ordering transition. We reveal instead that superconductor an…
We report simultaneous NMR and electrical transport experiments in the pressure range near the boundary of the antiferromagnetic spin density wave (SDW) insulator and the metal-lic/superconducting (SC) phase in (TMTSF)2PF6. Measurements indicate a tricritical point separat-ing a line of second order SDW/metal transitions from a line of first order SDW/metal(SC) transi-tions with coexistence of macroscopic regions of SDW and metal(SC) order, with little mutual in-teraction but strong hysteretic effects. NMR results quantify the fraction of each phase.
The organic materials (TMTSF)2X are unique unconventional superconductors with archetypal quasi-one-dimensional (Q1D) electronic structures. Here, based on our comprehensive field-angle-resolved calorimetry of (TMTSF)2ClO4, we succeeded in mapping the nodal gap structure for the first time in Q1D systems, by discriminating between the Fermi wavevectors and Fermi velocities. In addition, the thermodynamic phase diagrams of (TMTSF)2ClO4 for all principal field directions are obtained. These findings, providing strong evidence of nodal spin-singlet superconductivity, serves as solid bases for fur…
The study of the anion ordered (TMTSF)\_2ClO\_4\_(1-x)ReO\_4\_x, solid solution in the limit of a low ReO\_4- substitution level (0<=x<=17%) has revealed a new and interesting phase diagram. Superconductivity is drastically suppressed as the effect of ReO\_4- non magnetic point defects increases following the digamma behaviour for usual superconductors in the presence of paramagnetic impurities. Then, no long range order can be stabilized above 0.1K in a narrow window of substitution. Finally, an insulating SDW ground state in ReO\_4- -rich samples is rapidly stabilized with the decrease of th…
We report the first comprehensive investigation of the organic superconductor (TMTSF)2PF6 in the vicinity of the endpoint of the spin density wave - metal phase transition where phase coexistence occurs. At low temperature, the transition of metallic domains towards superconductivity is used to reveal the various textures. In particular, we demonstrate experimentally the existence of 1D and 2D metallic domains with a cross-over from a filamentary superconductivity mostly along the c?-axis to a 2D superconductivity in the b?c-plane perpendicular to the most conducting direction. The formation o…
The symmetry of the superconductivity in Bechgaard salts is still unknown, though the triplet pairing has been established by Hc2 and NMR for (TMTSF)2PF6. The large upper critical field at T = 0K (Hc2 ~ 5 Tesla) both for H || a and H || b also indicates strongly the triplet pairing. Here we start with a low energy effective Hamiltonian and study the temperature dependence of the corresponding Hc2(T)'s. The present analysis suggests that one chiral f-wave superconductor should be the most likely candidate near the upper critical field.
Using a proper cooling procedure, a controllable amount of non-magnetic structural disorder can be introduced at low temperature in (TMTSF)2ClO4. Here we performed simultaneous measurements of transport and magnetic properties of (TMTSF)2ClO4 in its normal and superconducting states, while finely covering three orders of magnitude of the cooling rate around the anion ordering temperature. Our result reveals, with increasing density of disorder, the existence of a crossover between homogeneous defect-controlled d-wave superconductivity and granular superconductivity. At slow cooling rates, with…
The phase diagram of the organic superconductor (TMTSF)_2PF_6 has been revisited using transport measurements with an improved control of the applied pressure. We have found a 0.8 kbar wide pressure domain below the critical point (9.43 kbar, 1.2 K) for the stabilisation of the superconducting ground state featuring a coexistence regime between spin density wave (SDW) and superconductivity (SC). The inhomogeneous character of the said pressure domain is supported by the analysis of the resistivity between T_SDW and T_SC and the superconducting critical current. The onset temperature T_SC is pr…
We solve a long-standing problem about a theoretical description of the upper critical magnetic field, parallel to conducting layers and perpendicular to conducting chains, in (TMTSF)$_2$ClO$_4$ superconductor. In particular, we explain why the experimental upper critical field, $H^{b'}_{c2} \simeq 6 \ T$, is higher than both the quasi-classical upper critical field and Clogston paramagnetic limit. We show that this property is due to the coexistence of the hidden Reentrant and Larkin-Ovchinnikov-Fulde-Ferrell phases in a magnetic field in a form of three plane waves with non-zero momenta of t…
We consider a formation of the Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) phase in a quasi-one-dimensional (Q1D) conductor in a magnetic field, parallel to its conducting chains, where we take into account both the paramagnetic spin-splitting and orbital destructive effects against superconductivity. We show that, due to a relative weakness of the orbital effects in a Q1D case, the LOFF phase appears in (TMTSF)$_2$ClO$_4$ superconductor for real values of its Q1D band parameters. We compare our theoretical calculations with the recent experimental data by Y. Maeno's group [S. Yonezawa et al., Phy…
We have theoretically investigated the spin and charge fluctuations in the quasi-one dimensional organic superconductor (TMTSF)2ClO4. Using the extended multi-site Hubbard model, which contains four sites in a unit cell and the transfer energies obtained by the extended Hückel method, we calculate the linearized gap equation with the random phase approximation, to find novel order parameters of superconductivity due to several kinds of charge fluctuations induced by the anisotropic intersite repulsive interactions. For the singlet state, the order parameter with line nodes appears in the case …
The low field phase of the organic superconductor (TMTSF)2ClO4 is studied by muon spin rotation. The zero temperature limit of the magnetic penetration depth within the TMTSF layers is obtained to be lambda_{ab}(0) = 0.86(2) um. Temperature dependence of the muon spin relaxation shows no indication of gap nodes on the Fermi surface nor of any spontaneous fields due to time-reversal-symmetry breaking. The weight of evidence suggests that the symmetry of this low field phase is odd-frequency p-wave singlet; a novel example of odd-frequency pairing in a bulk superconductor.
We discuss a number of experiments that could detect the electron edge states in the organic quasi-one-dimensional conductors (TMTSF)2X and the inorganic quasi-two-dimensional perovskites Sr2RuO4. We consider the chiral edges states in the magnetic-field-induced spin-density-wave (FISDW) phase of (TMTSF)2X and in the time-reversal-symmetry-breaking triplet superconducting phase of Sr2RuO4, as well as the nonchiral midgap edge states in the triplet superconducting phase of $(TMTSF)2X. The most realistic experiment appears to be an observation of spontaneous magnetic flux at the edges of Sr2RuO4…
We study superconductivity in spin-orbit-coupled systems in the vicinity of inversion symmetry breaking. We find that due to the presence of spin-orbit coupling, fluctuations of the incipient parity-breaking order generate an attractive pairing interaction in an odd-parity pairing channel, which competes with the s-wave pairing. We show that applying a Zeeman field suppresses the s-wave pairing and promotes the odd-parity superconducting state. Our work provides a new mechanism for odd-parity pairing and opens a route to novel topological superconductivity.
Mineral calaverite AuTe2 is a layered compound with an incommensurately modulated structure. The modulation is characterized by the formation of molecular-like Te2 dimers. We have found that the breaking of Te2 dimers that occurs in Au1-xPtxTe2 results in the emergence of superconductivity at Tc = 4.0 K.
Roles of paramagnetic and diamagnetic pair-breaking effects in superconductivity in electric-field-induced surface metallic state are studied by Bogoliubov-de Gennes equation, when magnetic fields are applied parallel to the surface. The multi-gap states of sub-bands are related to the depth dependence and the magnetic field dependence of superconductivity. In the Fermi-energy density of states and the spin density, sub-band contributions successively appear from higher-level sub-bands with increasing magnetic fields. The characteristic magnetic field dependence may be a key feature to identif…
IrTe2, a layered compound with a triangular iridium lattice, exhibits a structural phase transition at approximately 250 K. This transition is characterized by the formation of Ir-Ir bonds along the b-axis. We found that the breaking of Ir-Ir bonds that occurs in Ir1-xPtxTe2 results in the appearance of a structural critical point in the T = 0 limit at xc = 0.035. Although both IrTe2 and PtTe2 are paramagnetic metals, superconductivity at Tc = 3.1 K is induced by the bond breaking in a narrow range of x > xc in Ir1-xPtxTe2. This result indicates that structural fluctuations can be involved in …
Using the quasi-classical formalism we provide the description of the temperature and field-direction dependence of the in-plane upper critical field in layered superconductors, taking into account the interlayer Josephson coupling and the paramagnetic spin splitting. We generalize the Lawrence-Doniach model for the case of high magnetic fields and show that the re-entrant superconductivity is naturally described by our formalism when neglecting the Pauli pair breaking effect. We demonstrate that in layered superconductors the in-plane anisotropy of the onset of superconductivity exhibits four…
We discuss superconductivity in a model on a cubic lattice for a $Γ_3$ non-Kramers system. In previous studies, it is revealed that $d$-wave superconductivity with $E_g$ symmetry occurs in a wide parameter range in a $Γ_3$ system. Such anisotropic superconductivity can break the cubic symmetry of the lattice. In a $Γ_3$ system, the quadrupole degrees of freedom are active and the effect of the cubic symmetry breaking should be important. Here, we investigate the coexisting states of the $d$-wave superconductivity and quadrupole order by a mean-field theory. In particular, we discuss possible c…
We consider mixed symmetry superconducting phases in Dirac materials in the odd parity channel, where pseudoscalar and vector order parameters can coexist due to their similar critical temperatures when attractive interactions are of finite range. We show that the coupling of these order parameters to unordered magnetic dopants favors the condensation of novel time-reversal symmetry breaking (TRSB) phases, characterized by a condensate magnetization, rotation symmetry breaking, and simultaneous ordering of the dopant moments. We find a rich phase diagram of mixed TRSB phases characterized by p…
The anisotropic superconductivity and the density wave have been investigated by applying the Kadanoff-Wilson renormalization group technique to the quasi-one-dimensional system with finite-range interactions. It is found that a temperature (T) dependence of response functions is proportional to exp(1/T) in a wide region of temperature even within the one-loop approximation. Transition temperatures are calculated to obtain the phase diagram of the quasi-one-dimensional system, which is compared with that of the pure-one-dimensional system. Next-nearest neighbor interactions (V_2) induce large …
Epitaxial bilayer films of Bi(110) and Ni host a time-reversal symmetry (TRS) breaking superconducting order with an unexpectedly high transition temperature $T_c = 4.1$ K. Using time-domain THz spectroscopy, we measure the low energy electrodynamic response of a Bi/Ni bilayer thin film from $0.2$ THz to $2$ THz as a function of temperature and magnetic field. We analyze the data in the context of a BCS-like superconductor with a finite normal-state scattering rate. In zero magnetic field, all states in the film become fully gapped, providing important constraints into possible pairing symmetr…
We investigate the nature of the time-reversal breaking pairing state in the hole-doped monolayer MoS$_{2}$ on the basis of the realistic three-orbital attractive Hubbard-like model with the atomic spin-orbit coupling. Due to the multi-band features arising from the Mo $d$ orbitals in the noncentrosymmetric crystal structure, the Lifshitz transition takes place upon hole doping. Across the Lifshitz transition point, the sign of the relative phase between the Cooper-pair components drastically changes, leading to the emergence of the time-reversal breaking phase with complex gap functions. It i…
There are strong experimental evidences of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state formation in layered organic superconductors in parallel magnetic field. We study theoretically the interplay between the orbital effect and the FFLO modulation in this case and demonstrate that the in-plane critical field anisotropy drastically changes at the transition to the FFLO state. The very peculiar angular dependence of the superconducting onset temperature which is predicted may serve for unambiguous identification of the FFLO modulation. The obtained results permit us to suggest the modulate…
The structural or electronic symmetry breaking of the host lattice is a recurrent phenomenon in many quantum materials, including superconductors. Yet, how these broken symmetry states affect the electronic pair wave function of superconductivity have been rarely elucidated. Here, using low-temperature scanning tunneling microscopy and first-principles calculations, we identify the broken rotational symmetry via stripe ordering on the (001) surface of perovskite BaBi3 films grown by molecular beam epitaxy, and show that it consequently leads to anisotropic superconductivity with twofold symmet…
The quasi-1D organic Bechgaard salt (TMTSF)$_2$PF$_6$ displays spin-density-wave (SDW) order and superconductivity in close proximity in the temperature-pressure phase diagram. We have measured its normal-state electrical resistivity $ρ_a(T)$ as a function of temperature and pressure, in the $T \to 0$ limit. At the critical pressure where SDW order disappears, $ρ_a(T) \propto T$ down to the lowest measured temperature (0.1 K). With increasing pressure, $ρ_a(T)$ acquires a curvature that is well described by $ρ_a(T) = ρ_0 + AT + BT^2$, where the strength of the linear term, measured by the $A$ …
Superconductors with broken time-reversal symmetry represent arguably one of the most promising venues for realizing highly sought-after topological superconductivity that is vital to fault-tolerant quantum computation. Here, by using extensive muon-spin relaxation and rotation measurements, we report that the isostructural silicide superconductors (Ta, Nb)OsSi spontaneously break time-reversal symmetry at the superconducting transition while surprisingly showing a fully-gapped superconductivity characteristic of conventional superconductors. The first-principles calculations show that (Ta, Nb…
The Th$_{7}$Fe$_{3}$ family of superconductors provides a rich playground for unconventional superconductivity. La$_7$Ni$_3$ is the latest member of this family, which we here investigate by means of thermodynamic and muon spin rotation and relaxation measurements. Our specific heat data provides evidence for two distinct and approximately isotropic superconducting gaps. The larger gap has a value slightly higher than that of weak-coupling BCS theory, indicating the presence of significant correlations. These observations are confirmed by transverse-field muon-rotation measurements. Furthermor…
As an extension of our previous work on the holon pairing instability in the t-J Hamiltonian [Phys. Rev. B {\bf 66}, 054427 (2002)], we examine the orbital symmetries of holon pairing order parameters in high $T_c$ superconductivity by examining the energy poles of t-matrix. We find that both $s$- and d-wave symmetries in holon pair order parameter occur at low lying energy states corresponding to the higher energy poles of t-matrix while only the s-wave symmetry appears at the lowest energy pole and that this results in the d-wave symmetry in the Cooper pair order which is a composite of the …
Zero-field muon spin relaxation experiments have been carried out in the Pr(Os_{1-x}Ru_x)_4Sb_12 and Pr_{1-y}La_yOs_4Sb_12 alloy systems to investigate broken time-reversal symmetry (TRS) in the superconducting state, signaled by the onset of a spontaneous static local magnetic field B_s. In both alloy series B_s initially decreases linearly with solute concentration. Ru doping is considerably more efficient than La doping, with a ~50% faster initial decrease. The data suggest that broken TRS is suppressed for Ru concentration x >~ 0.6, but persists for essentially all La concentrations. Our d…
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 extensive…
Recently observed 4π periodic Andreev bound states in three dimensional Dirac materials are attributed to convnetional superconducting pairing. Our alternative explanation in terms of a novel form of parity breaking pseudo-scalar superconducting order can be sharply diagnosed by nuclear magnetic resonance (NMR) relaxation rate. The left-right symmetry breaking of the pseudo-scalar superconductivity can be directly probed as an anti-peak structure below TC in sharp contrast to the conventional Hebel-Slichter peak.
The possible existence of a sign-changing gap symmetry in BaFe$_{2}$As$_{2}$-derived superconductors (SC) has been an exciting topic of research in the last few years. To further investigate this subject we combine Electron Spin Resonance (ESR) and pressure-dependent transport measurements to investigate magnetic pair-breaking effects on BaFe$_{1.9}M_{0.1}$As$_{2}$ ($M=$ Mn, Co, Cu, and Ni) single crystals. An ESR signal, indicative of the presence of localized magnetic moments, is observed only for $M=$ Cu and Mn compounds, which display very low SC transition temperature ($T_{c}$) and no SC,…
Superconductivity that spontaneously breaks time-reversal symmetry (TRS) has been found, so far, only in a handful of 3D crystals with bulk inversion symmetry. Here we report an observation of spontaneous TRS breaking in a 2D superconducting system without inversion symmetry: the epitaxial bilayer films of bismuth and nickel. The evidence comes from the onset of the polar Kerr effect at the superconducting transition in the absence of an external magnetic field, detected by the ultrasensitive loop-less fiber-optic Sagnac interferometer. Because of strong spin-orbit interaction and lack of inve…
We study a tight-binding model on the honeycomb lattice of chiral $d$-wave superconductivity that breaks time-reversal symmetry. Due to its nontrivial sublattice structure, we show that it is possible to construct a gauge-invariant time-reversal-odd bilinear of the pairing potential. The existence of this bilinear reflects the sublattice polarization of the pairing state. We show that it generates persistent loop current correlations around each lattice site and opens a topological mass gap at the Dirac points, resembling Haldane's model of the anomalous quantum Hall effect. In addition to the…
We report a $μ$SR investigation of a non-centrosymmetric superconductor (LaNiC$_2$) in single crystal form. Compared to previous $μ$SR studies of non-centrosymmetric superconducting polycrystalline and powder samples, the unambiguous orientation of single crystals enables a simultaneous determination of the absolute value of the magnetic penetration depth and the vortex core size from measurements that probe the magnetic field distribution in the vortex state. The magnetic field dependence of these quantities unambiguously demonstrates the presence of two nodeless superconducting energy gaps. …
We study the superconducting state of multi-orbital spin-orbit coupled systems in the presence of an orbitally driven inversion asymmetry assuming that the inter-orbital attraction is the dominant pairing channel. Although the inversion symmetry is absent, we show that superconducting states that avoid mixing of spin-triplet and spin-singlet configurations are allowed, and remarkably, spin-triplet states that are topologically nontrivial can be stabilized in a large portion of the phase diagram. The orbital-dependent spin-triplet pairing generally leads to topological superconductivity with po…
We discuss the temperature dependence of the Hall conductivity $σ_{xy}$ in the magnetic-field-induced spin-density-wave (FISDW) state of the quasi-one-dimensional Bechgaard salts (TMTSF)_2X. Electronic thermal excitations across the FISDW energy gap progressively destroy the quantum Hall effect, so $σ_{xy}(T)$ interpolates between the quantized value at zero temperature and zero value at the transition temperature T_c, where FISDW disappears. This temperature dependence is similar to that of the superfluid density in the BCS theory of superconductivity. More precisely, it is the same as the te…
The effects of chain disorder on superconductivity in YBa_2Cu_3O_{7-δ} are discussed within the context of a proximity model. Chain disorder causes both pair-breaking and localization. The hybridization of chain and plane wavefunctions reduces the importance of localization, so that the transport anisotropy remains large in the presence of a finite fraction $δ$ of oxygen vacancies. Penetration depth and specific heat measurements probe the pair-breaking effects of chain disorder, and are discussed in detail at the level of the self-consistent T-matrix approximation. Quantitative agreement with…
Conventional superconductivity is robust against the addition of impurities unless the impurities are magnetic in which case superconductivity is quickly suppressed. Here we present a study of the cuprate superconductor Bi$_2$Sr$_2$Ca$_1$Cu$_2$O$_{8+δ}$ that is intentionally doped with the magnetic impurity, Fe. Through the use of our Tomographic Density of States (TDoS) technique, we find that while the superconducting gap magnitude is essentially unaffected by the inclusion of iron, the onset of superconductivity, T$_{C}$, and the pair-breaking rate are strongly dependent and correlated. The…
We show that the nonuniform state (Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state) of the spin triplet superconductivity in noncentrosymmetric systems is stabilized by antisymmetric spin-orbit coupling even if the magnetic field is absent. The transition temperature of the spin triplet superconductivity is reduced by the antisymmetric spin-orbit coupling in general. This pair breaking effect is shown to be similar to the Pauli pair breaking effect due to magnetic field for the spin singlet superconductivity, in which FFLO state is stabilized near the Pauli limit (or Chandrasekhar-Clogston limit)…
Spontaneous rotational-symmetry breaking in the superconducting state of doped $\mathrm{Bi}_2\mathrm{Se}_3$ has attracted significant attention as an indicator for topological superconductivity. In this paper, high-resolution calorimetry of the single-crystal $\mathrm{Sr}_{0.1}\mathrm{Bi}_2\mathrm{Se}_3$ provides unequivocal evidence of a two-fold rotational symmetry in the superconducting gap by a \emph{bulk thermodynamic} probe, a fingerprint of nematic superconductivity. The extremely small specific heat anomaly resolved with our high-sensitivity technique is consistent with the material's …
We analyze a mean-field model of electrons on a square lattice with two types of interaction: forward scattering favoring a d-wave Pomeranchuk instability and a BCS pairing interaction driving d-wave superconductivity. Tuning the interaction parameters a rich variety of phase diagrams is obtained. If the BCS interaction is not too strong, Fermi surface symmetry breaking is stabilized around van Hove filling, and coexists with superconductivity at low temperatures. For pure forward scattering Fermi surface symmetry breaking occurs typically via a first order transition at low temperatures. The …
Recent theoretical work predicted emergence of chiral topological superconducting phase with spontaneously broken time reversal symmetry in a twisted bilayer composed of two high-$T_c$ cuprate monolayers, such as Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. Here we identify large intrinsic Hall response that can be probed through the polar Kerr effect measurement as a convenient signature of the $\mathcal{T}$-broken phase. Our modelling predicts the Kerr angle $θ_K$ to be in the range of 10-100 $μ$rad, which is a factor of $10^3-10^4$ times larger than what is expected for the leading chiral supercondutor c…
