Charge stripe order has recently been established as an important ingredient of the physics of cuprate high-T$_c$ superconductors. However, due to the complex interplay between competing phases and the influence of disorder, it is unclear how it emerges from the parent metallic state. Here we report on the discovery of an unconventional electronic ordered phase between charge-stripe order and (pseudogapped) metal in the cuprate La$_{2-x-y}$Eu$_{y}$Sr$_x$CuO$_4$ with $y = 0.2$ (LESCO). The peculiar properties of the intermediate phase are revealed through three complementary experiments: nuclear quadrupole resonance, nonlinear conductivity, and specific heat. We demonstrate that the order appears through a sharp phase transition, and exists in a dome-shaped region of the phase diagram, similar to charge stripes. A comparison to recent theoretical work shows that the order is a state without broken translational symmetry -- a charge nematic. We thus resolve the complex process of charge stripe development in cuprates, show that the nematic phase is unrelated to high-temperature pseudogap physics, and establish a link with other strongly correlated electronic materials where nematic order is prominent.
We present the first study of codoped iron-arsenide superconductors of the 122 family (Sr/Ba)_(1-x)K_xFe_(2-y)Co_yAs_2 with the purpose to increase the upper critical field H_c2 compared to single doped (Sr/Ba)Fe_2As_2 materials. H_c2 was investigated by measuring the magnetoresistance in high pulsed magnetic fields up to 64 T. We find, that H_c2 extrapolated to T = 0 is indeed enhanced significantly to ~ 90 T for polycrystalline samples of Ba_0.55K_0.45Fe_1.95Co_0.05As_2 compared to ~75 T for Ba_0.55K_0.45Fe_2As_2 and BaFe_1.8Co_0.2As_2 single crystals. Codoping thus is a promising way for th…
We derive the Ginzburg-Landau theory of unconventional singlet superconductors in the presence of a Zeeman field and impurities, in order to examine the resulting Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phases. We show that the behavior of the FFLO phases in unconventional superconductors in the presence of impurities is qualitatively different from that found for s-wave superconductors.
Impurity nuclear spin relaxation is studied theoretically. A single impurity generates a bound state localized around the impurity atom in unconventional superconductors. With increasing impurity potential, the relaxation rate $T_1^{-1}$ is reduced by the impurity potential. However, it has a peak at low temperatures due to the impurity bound state. The peak disappears at non-impurity sites. The impurity site NMR measurement detecting a local electronic structure just on the impurity atom is very useful for identifying the unconventional pairing states.
We consider the energy of the filled quasiparticle's Fermi sea of a macroscopic superconducting ring threaded by an hc/2e-vortex, when the material of the ring is of an unconventional pairing symmetry. The energy relative to the one for the hc/e-vortex configuration is finite, positive, and inversely proportional to ring's inner radius. We argue that the existence of this energy in unconventional superconductors removes the commonly assumed degeneracy between the odd and the even vortices, with the loss of the concomitant hc/2e periodicity in external magnetic field as a consequence. This macr…
Group theoretical technique for construction of two-electron wavefunctions with zero total momentum in solids based on the Mackey theorem on symmetrized squares is developed. These states correspond to singlet and triplet Cooper pairs. The nodal structure of these states is investigated group theoretically and compared with experimental data for unconventional superconductors. It is shown that when the Mackey theorem is applied twice, the main four-electron states in solids can be constructed. Possible connection of these states with experimental data is discussed.
We review the results of the microscopic approach to the calculation of the anisotropy in the specific heat in unconventional superconductors under rotated field. Treating vortex scattering on equal footing with the energy shift we find that the electronic specific heat may have minima or maxima when the field is aligned with the nodes, depending on the temperature and field range. We discuss the influence of the paramagnetic limiting and Fermi surface shape on the location of the inversion line.
A polycrystalline sample of YNiGe3, being a non-magnetic isostructural counterpart to the unconventional pressure-induced superconductor CeNiGe3, was studied by means of specific heat and electrical resistivity measurements at temperatures down to 360 mK and in magnetic fields up to 500 Oe. The compound was found to exhibit an ambient-pressure superconductivity below Tc = 0.46 K. The superconducting state in YNiGe3 is destroyed by magnetic field of the order of 500 Oe.
Field-angle dependent heat capacity of the non-magnetic borocarbide superconductor YNi2B2C reveals a clear fourfold oscillation, the first observation of its kind. The observed angular variations were analyzed as a function of magnetic field angle, field intensity and temperature to provide its origin. The quantitative agreement between experiment and theory strongly suggests that we are directly observing nodal quasiparticles generated along <100> by the Doppler effect. The results demonstrate that field-angle heat capacity can be a powerful tool in probing the momentum-space gap structure in…
Chains of magnetic atoms, placed on the surface of s-wave superconductors, have been established as a laboratory for the study of Majorana bound states. In such systems, the breaking of time reversal due to magnetic moments gives rise to the formation of in-gap states, which hybridize to form one-dimensional topological superconductors. However, in unconventional superconductors even non-magnetic impurities induce in-gap states since scattering of Cooper pairs changes their momentum but not their phase. Here, we propose a path for creating topological superconductivity, which is based on an un…
The main purpose of the paper is to discuss a possibility of the determination of the values of the coherence length and the Cooper-pair size in unconventional superconductors by using tunnelling spectroscopy. In the mixed state of type-II superconductors, an applied magnetic field penetrates the superconductor in the form of vortices which form a regular lattice. In unconventional superconductors, the inner structure of a vortex core has a complex structure which is determined by the order parameter of the superconducting state and by the pairing wavefunction of the Cooper pairs. In clean sup…
Tunneling conductance in ferromagnet / unconventional superconductor junctions is studied theoretically as a function of temperatures and spin-polarization in feromagnets. In d-wave superconductor junctions, the existence of a zero-energy Andreev bound state drastically affects the temperature-dependence of the zero-bias conductance (ZBC). In p-wave triplet superconductor junctions, numerical results show a wide variety in temperature-dependence of the ZBC depending on the direction of the magnetic moment in ferromagnets and the pairing symmetry in superconductors such as $p_{x}$, $p_{y}$ and …
Study of superconductivity in layered iron-based materials was initiated in 2006 by Hosono's group, and boosted in 2008 by the superconducting transition temperature, Tc, of 26 K in LaFeAsO1-xFx. Since then, enormous researches have been done on the materials, with Tc reaching as high as 55 K. Here, we review briefly experimental and theoretical results on atomic and electronic structures and magnetic and superconducting properties of FeAs-based superconductors and related compounds. We seek for clues for unconventional superconductivity in the materials.
It has gone almost unquestioned that superexchange in the t-J (or Hubbard) model, and not phonons, is responsible for the unconventional ("d-wave") pairing symmetry of cuprate superconductors. However a number of advanced numerical studies have not found superconductivity in the Hubbard model. On the other hand compelling experimental evidence for a strong electron-phonon interaction (EPI) has currently arrived. Here I briefly review some phonon-mediated unconventional pairing mechanisms. In particular the anisotropy of sound velocity makes the phonon-mediated attraction of electrons non-local…
Unconventional superconductors such as the high-transition temperature cuprates, heavy-fermion systems and iron arsenide-based compounds exhibit antiferromagnetic fluctuations that are dominated by a resonance, a collective spin-one excitation mode in the superconducting state. Here we demonstrate the existence of a universal linear relation, $Er \propto 2Δ$, between the magnetic resonance energy (Er) and the superconducting pairing gap ($Δ$), spanning two orders of magnitude in energy. This relation is valid for materials that range from being close to the Mott-insulating limit to being on th…
We use magnetic long range order as a tool to probe the Cooper pair wave function in the iron arsenide superconductors. We show theoretically that antiferromagnetism and superconductivity can coexist in these materials only if Cooper pairs form an unconventional, sign-changing state. The observation of coexistence in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ then demonstrates unconventional pairing in this material. The detailed agreement between theory and neutron diffraction experiments, in particular for the unusual behavior of the magnetic order below $T_{c}$, demonstrates the robustness of our…
The search for broken time reversal symmetry (TRSB) in unconventional superconductors intensified in the past year as more systems have been predicted to possess such a state. Following our pioneering study of TRSB states in Sr$_2$RuO$_4$ using magneto-optic probes, we embarked on a systematic study of several other of these candidate systems. The primary instrument for our studies is the Sagnac magneto-optic interferometer, which we recently developed. This instrument can measure magneto-optic Faraday or Kerr effects with an unprecedented sensitivity of 10 nanoradians at temperatures as low a…
We have investigated the Se isotope effect in layered bismuth chalcogenide (BiCh2-based) superconductor LaO0.6F0.4Bi(S,Se)2 with 76Se and 80Se. For all examined samples, the Se concentration, which is linked to the superconducting properties, is successfully controlled within x = 1.09-1.14 in LaO0.6F0.4BiS2-xSex. From the magnetization and electrical resistivity measurements, changes in Tc are not observed for the LaO0.6F0.4Bi(S,Se)2 samples with 76Se and 80Se isotopes. Our results suggest that pairing in the BiCh2-based superconductors is not mediated by phonons, and unconventional supercondu…
Microwave conductivity experiments can directly measure the quasiparticle scattering rate in the superconducting state. We show that this, combined with knowledge of the Fermi surface geometry, allows one to distinguish between closely related superconducting order parameters, e.g., d$_{x^2-y^2}$ and d$_{xy}$ superconductivity. We benchmark this method on YBa$_2$Cu$_3$O$_{7-δ}$ and, unsurprisingly, confirm that this is a d$_{x^2-y^2}$ superconductor. We then apply our method to $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br, which we discover is a d$_{xy}$ superconductor.
We analyze the influence of unconventional superconductivity on crystalline electric field (CEF) excitations of rare earth ions in novel superconductors. We show that resonant magnetic excitations of the conduction electrons that have been observed in these systems below T$_c$ may result in the formation of the bound state in the 4$f$-electron susceptibility. This occurs at energies well below the CEF excitation energy. The effect is discussed as a function of temperature and the strength of the coupling between the 4$f$ and $d$-electrons. We argue that these effects may be present in the laye…
A single vortex in the charged Bose gas (CBG) has a charged core and its profile different from the vortex in neutral and BCS superfluids. Lower and upper critical fields of CBG are discussed. The unusual resistive upper critical field, Hc2(T), of many cuprates and a few other unconventional superconductors is described as the Bose-Einstein condensation field of preformed bosons-bipolarons. Its nonlinear temperature dependence follows from the scaling arguments. Exceeding the Pauli paramagnetic limit is explained. Controversy in the determination of Hc2(T) of cuprates from kinetic and thermody…
We develop a theory of the coexistence of superconductivity (SC) and antiferromagnetism (AFM) in CeCoIn5. We show that in Pauli-limited nodal superconductors the nesting of the quasi-particle pockets induced by Zeeman pair-breaking leads to incommensurate AFM with the moment normal to the field. We compute the phase diagram and find a first order transition to the normal state at low temperatures, absence of normal state AFM, and coexistence of SC and AFM at high fields, in agreement with experiments. We also predict the existence of a new double-Q magnetic phase.
We report the results of $^{77}$Se--nuclear magnetic resonance (NMR) in $α$-FeSe, which exhibits a similar crystal structure to the LaFeAsO$_{1-x}$F$_x$ superconductor and shows superconductivity at 8 K. The nuclear-spin lattice relaxation rate $1/T_1$ shows $T^3$ behavior below the superconducting transition temperature $T_c$ without a coherence peak. The $T_1T=$ const. behavior, indicative of the Fermi liquid state, can be seen in a wide temperature range above $T_c$. The superconductivity in $α$-FeSe is also an unconventional one as well as LaFeAsO$_{1-x}$F$_x$ and related materials. The Fe…
Single crystals of the unconventional superconductor UTe$_2$ have been grown in various conditions which result in different superconducting transition temperature as well as normal state properties. Stoichiometry of the samples has been characterized by the single-crystal X-ray crystallography and electron microprobe analyses. Superconducting samples are nearly stoichiometric within an experimental error of about 1 \%, while non-superconducting sample significantly deviates from the ideal composition. The superconducting UTe$_2$ showed that the large density of states was partially gapped in …
Iron-base superconductors exhibits features of systems where the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) phase, a superconducting state with non-zero total momentum of Cooper pairs, is actively sought. Experimental and theoretical evidence points strongly to the FFLO phase in these materials above the Pauli limit. In this article we discuss the ground state of iron-base superconductors near the critical magnetic field and the full $h-T$ phase diagram for pnictides in case of intra-band pairing, in a three-band model with $s_{\pm}$ symmetry.
I consider the problem of the vortex contribution to quasiparticle transport in unconventional superconductors with line nodes, a nd argue that the magnetic field dependence of transport coefficients is fixed by the same scattering processes which limit low-temperature transport. I give no detailed calculations, but show that qualitatively correct results may be obtained in the limit of low temperatures and fields by simple physical arguments, based on estimates of the density of states and relaxation time in analogy with the zero-field case. I conclude with a brief discussion of the problem o…
The gap function in unconventional superconductors may vanish at points or lines in momentum space, permitting electronic excitations, termed nodal quasiparticles, to exist at temperatures well below the superconducting transition. In the vortex phase, the presence of nodal quasiparticles should be directly observable through the variation of the heat capacity with the angle between a magnetic field and the location of the zeroes of the gap. The heat capacity of candidate non-magnetic unconventional superconductors Lu(Y)Ni2B2C were found to exhibit fourfold oscillations with field angle, the f…
We review recent developments in our understanding of how impurities influence the electronic states in the bulk of superconductors. Our focus is on the quasi-localized states in the vicinity of impurity sites in conventional and unconventional superconductors and our goal is to provide a unified framework for their description. The non-magnetic impurity resonances in unconventional superconductors are directly related to the Yu-Shiba-Rusinov states around magnetic impurities in conventional s-wave systems. We review the physics behind these states, including quantum phase transition between s…
It was previously suggested that an odd-frequency pair amplitude exists in the vicinity of boundaries in unconventional superconductors. We develop this idea and quest for a novel superconducting order parameter with an odd-frequency dependence. For this purpose, we focus on p-wave superconductors and extend the quasi-classical theory to include the odd-frequency dependence in the order parameter. Both of the frequency and spacial dependences of the order parameter are determined self-consistently. Under a finite electron-phonon interaction, it is found that an odd-frequency order parameter is…
We present a microscopic approach to the calculations of thermal conductivity in unconventional superconductors for a wide range of temperatures and magnetic fields. Our work employs the non-equilibrium Keldysh formulation of the quasiclassical theory. We solve the transport equations using a variation of the Brandt-Pesch-Tewordt (BPT) method, that accounts for the quasiparticle scattering on vortices. We focus on the dependence of the thermal conductivity on the direction of the field with the respect to the nodes of the order parameter, and discuss it in the context of experiments aiming to …
Recent development in the physics of high-temperature cuprate superconductivity is reviewed, with special emphasis on the phenomena of unconventional and non-universal low-energy excitations of hole- and electron-type cuprate superconductors and the possible physical origin. A phenomenology based on coexisting competing orders with cuprate superconductivity in the ground state appears to provide consistent account for a wide range of experimental findings, including the presence (absence) of pseudogaps and Fermi arcs above the superconducting transition $T_c$ in hole-type (electron-type) cupra…
Orbital degrees of freedom of a Cooper pair play an important role in the unconventional superconductivity. To elucidate the orbital effect in the Kondo problem, we investigated a single magnetic impurity coupled to Cooper pairs with a $p_x +i p_y$ ($d_{x^2-y^2}+id_{xy}$) symmetry using the numerical renormalization group method. It is found that the ground state is always a spin doublet. The analytical solution for the strong coupling limit explicitly shows that the orbital dynamics of the Cooper pair generates the spin 1/2 of the ground state.
A novel singular gauge transformation is developed for quasiparticles in the mixed state of a strongly type-II superconductor which permits a full solution of the problem at low and intermediate fields. For a periodic vortex lattice the natural low-energy quasiparticle states are Bloch waves rather than Landau levels discussed in the recent literature. The new representation elucidates the physics considerably and provides fresh insights into the spectral properties of such systems.
Over the past two decades, unconventional superconductivity with gap symmetry other than s-wave has been found in several classes of materials, including heavy fermion (HF), high-T_c, and organic superconductors. Unconventional superconductivity is characterized by anisotropic superconducting gap functions, which may have zeros (nodes) along certain directions in the Brillouin zone. The nodal structure is closely related to the pairing interaction, and it is widely believed that the presence of nodes is a signature of magnetic or some other exotic, rather than conventional phonon-mediated, pai…
We propose a mechanism whereby disorder can enhance the transition temperature Tc 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 …
The Kondo effect in superconductors is frequently investigated using the local quasiparticle density of states as sole bath characteristics, i.e., the presence of anomalous propagators is ignored. Here we point out that this treatment is exact for a number of situations, including point-like impurities in d-wave superconductors. We comment on recent investigations [M. Matsumoto and M. Koga, J. Phys. Soc. Jpn. 70, 2860 (2001), cond-mat/0011256, and Phys. Rev. B 65, 024508 (2002), cond-mat/0103053] which reached different conclusions: while their numerical results are likely correct, their inter…
Field-angle dependent specific heat measurement has been done on the heavy-fermion superconductor CeCoIn5 down to ~ 0.29 K, in a magnetic field rotating in the tetragonal c-plane. A clear fourfold angular oscillation is observed in the specific heat with the minima (maxima) occurring along the [100] ([110]) directions. Oscillation persists down to low fields H << Hc2, thus directly proving the existence of gap nodes. The results indicate that the superconducting gap symmetry is most probably of dxy type.
There is compelling evidence for a strong electron-phonon interaction (EPI) in cuprate superconductors from the isotope effects on the supercarrier mass, high resolution angle resolved photoemission spectroscopies (ARPES), a number of optical and neutron-scattering measurements in accordance with our prediction of high-temperature superconductivity in polaronic liquids. A number of observations point to the possibility that high-Tc cuprate superconductors may not be conventional Bardeen-Cooper-Schrieffer (BCS) superconductors, but rather derive from the Bose-Einstein condensation (BEC) of real…
Ultrasound velocity measurements of the unconventional superconductor CeCoIn_5 with extremely large Pauli paramagnetic susceptibility reveal an unusual structural transformation of the flux line lattice (FLL) in the vicinity of the upper critical field. The transition field coincides with that at which heat capacity measurements reveal a second order phase transition. The lowering of the sound velocity at the transition is consistent with the collapse of the FLL tilt modulus and a crossover to quasi two-dimensional FLL pinning. These results provide a strong evidence that the high field state …
The thermoelectric coefficient $η(T)$ in unconventional superconductors is enhanced below $T_c$ by intermediate strength impurity scattering that is intrinsically particle-hole asymmetric. We compute $η(T)$ for a strong-coupling d-wave superconductor and investigate the effects of inelastic scattering originating from electron-boson interactions. We show that $η(T)$ is severely suppressed at temperatures just below $T_c$ by a particle-hole symmetric inelastic scattering rate. At lower temperatures inelastic scattering is frozen out and $η(T)$ recovers and regains its large amplitude. In the li…
The understanding of disorder has profoundly influenced the development of condensed matter physics, explaining such fundamental effects as, for example, the transition from ballistic to diffusive propagation, and the presence of quantized steps in the quantum Hall effect. For superconductors, the response to disorder reveals crucial information about the internal gap symmetries of the condensate, and thereby the pairing mechanism itself. The destruction of superconductivity by disorder is traditionally described by Abrikosov-Gor'kov (AG) theory, which however ignores spatial modulations and c…
The resistive upper critical field, Hc2(T) of cuprates, superconducting spin-ladders, and organic (TMTSF)2X systems is shown to follow a universal nonlinear temperature dependence in a wide range near Tc, while its low-temperature behaviour depends on the chemical formula and sample quality. Hc2(T) is ascribed to the Bose-Einstein condensation field of preformed pairs. The universality originates from the scaling arguments. Exceeding the Pauli paramagnetic limit is explained. Controversy in the determination of Hc2(T) from the kinetic and thermodynamic measurements is resolved in the framework…
Unconventional density waves (UDW) are one of the ground states in metallic crystalline solids and have been speculated already in 1968. However, more focused studies on UDW started only recently, perhaps after the identification of the low temperature phase in alpha-(BEDT-TTF)_2KHg(SCN)_4 as unconventional charge density wave (UCDW) in 2002. More recently, the metallic phase of Bechgaard salts (TMTSF)_2X with X=PF_6 and ReO_4 under both pressure and magnetic field appears to be unconventional spin density wave (USDW). The pseudogap regime of high T_c superconductors LSCO, YBCO, Bi2212 and the…
The fluctuation conductivity $σ_{\rm s}$ in bulk superconductors with non s-wave pairing and with nonmagnetic disorder of strength $D$ is studied at low $T$ and within the Gaussian approximation. It is shown by assuming a quasi two-dimensional (2D) electronic state that, only if the gap function $d_μ({\p})$ is, as in a 2D p-wave pairing state, linear in the in-plane (relative) momentum ${\p}_\perp$, the in-plane fluctuation conductivity on the line $D=D_c$ is weakly divergent in low $T$ limit. The present result may be useful in clarifying the true gap function of spin-triplet ${\rm Sr_2RuO_4}…
We study theoretically the proximity effect between ferromagnets (F) and superconductors (S) with broken time-reversal symmetry (${\cal T}$). A chiral $(p_x\pm ip_y)$-wave, and a $d_{x^2-y^2}$-wave superconductor, the latter of which can form ${\cal T}$-breaking surface state, i.e., $(d_{x^2-y^2} \pm is)$-state, are considered for the S side. The spatial variations of the superconducting order parameters and the magnetization are determined by solving the Bogoliubov de Gennes equation. In the case of a chiral $(p_x\pm ip_y)$-wave superconductor, a spontaneous spin current flows along the inter…
In this review article we present a survey of unconventional superconductors and their coexistence behaviour with magnetism. We focus on Ce- and U-based heavy fermion superconductors, in addition we discuss the rare earth borocarbide and skutterudite superconductors. We show that heavy fermion behaviour in Ce- and U- compounds should be described differently. While the Kondo-lattice model with localised 4f-electrons is appropriate for Ce-compounds, dual models including localised and itinerant 5f-electrons are necessary for U-compounds. We demonostrate that this leads to consistent results for…
Muon spin relaxation/rotation (muSR) is a vital technique for probing the superconducting gap structure, pairing symmetry and time reversal symmetry breaking, enabling an understanding of the mechanisms behind the unconventional superconductivity of cuprates and Fe-based high-temperature superconductors, which remain a puzzle. Very recently double layered Fe-based super-conductors having quasi-2D crystal structures and Cr-based superconductors with a quasi-1D structure have drawn considerable attention. Here we present a brief review of the characteristics of a few selected Fe- and Cr-based su…
UCoGe is reported as a weak ferromagnetic (FM) two-band superconductor (SC) with the critical temperature, $T_{\mathrm{SC}}\sim$ 0.7 K and the Curie temperature, $T_{\mathrm{C}}\sim$ 3 K at ambient pressure. We report exotic attributes of the SC and FM state in moderate magnetic fields. The observed phenomena clearly demonstrate that the SC regime is superior to the paramagnetic state in the vicinity of $T_{\mathrm{SC}}$. Above $T_{\mathrm{SC}}$, the zero-field state is characterized by a regime with strong FM spin fluctuations, suggesting proximity of the FM quantum critical point (FM-QCP). I…
We determine the quantum phase diagram of the one-dimensional Hubbard model with bond-charge interaction X in addition to the usual Coulomb repulsion U at half-filling. For large enough X and positive U the model shows three phases. For large U the system is in the spin-density wave phase already known in the usual Hubbard model. As U decreases, there is first a spin transition to a spontaneously dimerized bond-ordered wave phase and then a charge transition to a novel phase in which the dominant correlations at large distances correspond to an incommensurate singlet superconductor.
This Letter theoretically discusses quasiparticle states and nuclear spin relaxation rates $T_1^{-1}$ in a quasi-one-dimensional superconductor (TMTSF)$_2$PF$_6$ under a magnetic field applied parallel to the conduction chains. We study the effects of Josephson-type vortices on $T_1^{-1}$ by solving the Bogoliubov de Gennes equation for $p$-, $d$- or $f$-wave pairing interactions. In the presence of line nodes in pairing functions, $T_1^{-1}$ is proportional to $T$ in sufficiently low temperatures because quasiparticles induced by vortices at the Fermi energy relax spins. We also try to identi…
We have performed high-resolution angle-resolved photoemission spectroscopy on Fe-based superconductor LiFeAs (Tc = 18 K). We reveal multiple nodeless superconducting (SC) gaps with 2D/kBTc ratios varying from 2.8 to 6.4, depending on the Fermi surface (FS). We also succeeded in directly observing a gap anisotropy along the FS with magnitude up to ~30 %. The anisotropy is four-fold symmetric with an antiphase between the hole and electron FSs, suggesting complex anisotropic interactions for the SC pairing. The observed momentum dependence of the SC gap offers an excellent opportunity to invest…
Motivated by the recent development of terahertz pump-probe experiments, we investigate the short-time dynamics in superconductors with multiple attractive pairing channels. Studying a single-band square lattice model with spin-spin interaction as an example, we find the signatures of collective excitations of the pairing symmetries (known as Bardasis-Schrieffer modes) as well as the order parameter amplitude (Higgs mode) in the short-time dynamics of the spectral gap and quasiparticle distribution after an excitation by a pump pulse. We show that the polarization and intensity of the pulse ca…
We develop a fully microscopic theory for the calculations of the angle-dependent properties of unconventional superconductors under a rotated magnetic field. We employ the quasiclassical Eilenberger equations, and use a variation of the Brandt-Pesch-Tewordt (BPT) method to obtain a closed form solution for the Green's function. The equations are solved self-consistently for quasi-two-dimensional $d_{x^2-y^2}$ ($d_{xy}$) superconductors with the field rotated in the basal plane. The solution is used to determine the density of states and the specific heat. We find that applying the field along…
Recent findings of new Higgs modes in unconventional superconductors require a classification and characterization of the modes allowed by nontrivial gap symmetry. Here we develop a theory for a tailored nonequilibrium quantum quench to excite all possible oscillation symmetries of a superconducting condensate. We show that both a finite momentum transfer and quench symmetry allow for an identification of the resulting Higgs oscillations. These serve as a fingerprint for the ground state gap symmetry. We provide a classification scheme of these oscillations and the quench symmetry based on gro…
