Spin-singlet Superconducting State Research Articles

The study on temperature dependent superfluid density of anisotropic superconductors

The most of superconductors are highly anisotropic superconductors, then the measurement on the superfluid density properties of superconductors are complicated and depended on directions. In this study, the superfluid density of anisotropic superconductors was studied by semiclassical approach. We are interested in the spin-singlet superconducting state with the anisotropic spherical Fermi surface, and the anisotropic gap function with the spherical ellipse shape in weak-coupling limit. After some calculation, we can derive the temperature dependent formula of the superfluid density near zero temperature for the ab- and c- spatial components. The numerical calculation fit to the experimental data of CaAlSi superconductor was shown. We found that our model can fit well, then the CaAlSi superconductor shows the anisotropic superfluid density.

Superconductivity in a multiorbital model for the Γ3 crystalline electric field state

Superconductivity in a three-orbital model for the Γ3 crystalline electric field (CEF) ground state is investigated by applying the random phase approximation. In this model, we assume an antiferromagnetic interaction between the Γ7 and Γ8 orbitals to stabilize the Γ3 CEF state among f2 states. This interaction may induce spin-singlet pairing between electrons in these orbitals. If we consider the onsite pairing state composed of the Γ7 and Γ8 orbitals, it has the Eg symmetry. Indeed, we find that the Eg spin-singlet superconducting state is realized in a wide parameter region. Possible coexistence of the superconductivity and quadrupole order is also discussed.

Anisotropic Superconductivity Emerging from the Orbital Degrees of Freedom in a Γ3 Non-Kramers Doublet System

We study superconductivity in a three-orbital model for $f^2$ ions with the $\Gamma_3$ crystalline electric field (CEF) ground state. An antiferromagnetic interaction between the $\Gamma_7$ and $\Gamma_8$ orbitals is introduced to stabilize the $\Gamma_3$ CEF state. This interaction also works as an on-site attractive interaction for spin-singlet pairing between electrons in these orbitals. The interorbital pairing state composed of the $\Gamma_7$ and $\Gamma_8$ orbitals on the same site has the $E_g$ symmetry. Indeed, by applying the random phase approximation, we find that the $E_g$ spin-singlet superconducting state is realized over a wide parameter range.

Instability of three-band Tomonaga-Luttinger liquid: Renormalization group analysis and possible application toK2Cr3As3

Motivated by recently discovered quasi-one-dimensional superconductor K$_{2}$Cr$_{3}$As$_{3}$ with $D_{3h}$ lattice symmetry, we study one-dimensional three-orbital Hubbard model with generic electron repulsive interaction described by intra-orbital repulsion $U$, inter-orbital repulsion, and Hund's coupling $J$. As extracted from density functional theory calculation, two of the three atomic orbitals are degenerate ($E^{\prime}$ states) and the third one is non-degenerate ($A^{\prime}_1$), and the system is presumed to be at an incommensurate filling. With the help of bosonization, we have usual three-band Tomonaga-Luttinger liquid for the normal state. Possible charge density wave (CDW), spin density wave (SDW) and superconducting (SC) instabilities are analyzed by renormalization group. The ground state depends on the ratio $J/U$ and is sensitive to the degeneracy of $E^{\prime}$ bands. At $0<J<U/3$, spin-singlet SC state is favored, while spin-triplet superconductivity will be favored in the region $U/3<J<U/2$. The SDW state has the lowest energy only in the unphysical parameter region $J>U/2$. When the two-fold degeneracy of $E^{\prime}$ bands is lifted, SDW instability has the tendency to dominate over the spin-singlet SC state at $0<J<U/3$, while the order parameter of the spin-triplet SC state will be modulated by a phase factor $2\Delta k_F x$ at $U/3<J<U/2$. Possible experimental consequences and applications to K$_{2}$Cr$_{3}$As$_{3}$ are discussed.

Electronic orders in multiorbital Hubbard models with lifted orbital degeneracy

We study the symmetry-broken phases in two- and three-orbital Hubbard models with lifted orbital degeneracy using dynamical mean field theory. On the technical level, we explain how symmetry relations can be exploited to measure the four-point correlation functions needed for the calculation of the lattice susceptibilities. In the half-filled two-orbital model with crystal field splitting, we find an instability of the metallic phase to spin-orbital order with neither spin nor orbital moment. This ordered phase is shown to be related to the recently discovered fluctuating-moment induced spin-triplet superconducting state in the orbitally degenerate model with shifted chemical potential. In the three-orbital case, we consider the effect of a crystal field splitting on the spin-triplet superconducting state in the model with positive Hund coupling, and the spin-singlet superconducting state in the case of negative Hund coupling. It is demonstrated that for certain crystal field splittings the higher energy orbitals instead of the lower ones are relevant for superconductivity, and that $T_c$ can be enhanced by the crystal field effect. We comment on the implications of our results for the superconductivity in strontium ruthenates, and for the recently reported light-enhanced superconducting state in alkali-doped fullerides.

Spin-singlet superconductivity with a full gap in locally noncentrosymmetric SrPtAs

We report $^{195}\mathrm{Pt-NMR}$ and $^{75}\mathrm{As}$ nuclear quadrupole resonance measurements for the locally noncentrosymmetric superconductor SrPtAs where the As-Pt layer breaks inversion symmetry while globally the compound is centrosymmetric. The nuclear spin-lattice relaxation rate $1/{T}_{1}$ shows a well-defined coherence peak below ${T}_{c}$ and decreases exponentially at low temperatures. The spin susceptibility measured by the Knight shift also decreases below ${T}_{c}$ down to $T&lt;{T}_{c}/6$. These data, together with the penetration depth obtained from the NMR spectra, can be consistently explained by a spin-singlet superconducting state with a full gap. Our results suggest that the spin-orbit coupling due to the local inversion-symmetry breaking is not large enough to bring about an exotic superconducting state, or the interlayer hopping interaction is larger than the spin-orbit coupling.

Superconducting state of iron arsenide [formula omitted]: 57Fe and 75As NMR studies

We report 57Fe and 75As NMR measurements of the novel normal and superconducting-state characteristics of the iron-arsenide superconductor Ba0.6K0.4Fe2As2Tc=38K. In the normal state, the measured Knight shift and nuclear spin-lattice relaxation rate (1/T1) demonstrate the development of wave-number (q)-dependent spin fluctuations, except at q=0, which may originate from the nesting across the disconnected Fermi surfaces. In the superconducting state, the spin component in the 57Fe-Knight shift decreases down to zero with decreasing T, evidencing that a spin-singlet superconducting state is realized in Ba0.6K0.4Fe2As2.

Strong-Coupling Spin-Singlet Superconductivity with Multiple Full Gaps in Hole-Doped Ba0.6K0.4Fe2As2Probed by57Fe-NMR

We present 57 Fe-NMR measurements of the novel normal and superconducting-state characteristics of the iron-arsenide superconductor Ba 0.6 K 0.4 Fe 2 As 2 ( T c = 38 K). In the normal state, the measured Knight shift and nuclear spin-lattice relaxation rate (1/ T 1 ) demonstrate the development of wave-number ( q )-dependent spin fluctuations, except at q = 0, which may originate from the nesting across the disconnected Fermi surfaces. In the superconducting state, the spin component in the 57 Fe-Knight shift decreases to almost zero at low temperatures, evidencing a spin-singlet superconducting state. The 57 Fe-1/ T 1 results are totally consistent with a s ± -wave model with multiple full gaps in the strong coupling regime. We demonstrate that the respective 1/ T 1 data for Ba 0.6 K 0.4 Fe 2 As 2 and LaFeAsO 0.7 , which seemingly follow a T 5 - and a T 3 -like behaviors below T c , are consistently explained in terms of this model only by changing the size of the superconducting gap.

Evidence for multiband superconductivity in the heavy Fermion compound UNi2Al3.

Epitaxial thin films of the heavy fermion superconductor UNi2Al3 with Tc(max)=0.98 K were investigated. The transition temperature Tc depends on the current direction which can be related to superconducting gaps opening at different temperatures. Also the influence of the magnetic ordering at TN approximately 5 K on R(T) is strongly anisotropic, indicating different coupling between the magnetic moments and itinerant charge carriers on the multisheeted Fermi surface. The upper critical field Hc2(T) suggests an unconventional spin-singlet superconducting state.