arXiv subjects

Youichi Yanase

Publications and source records attributed to Youichi Yanase.

At least 145 records · Page 8Linked to original sources

arXiv2015

Chiral superconductivity in nematic states

We investigate chiral superconductivity which occurs in the electronic nematic state. A vortex state in a $c$-axis magnetic field is studied on the basis of the two-component Ginzburg-Landau model for nematic-chiral superconductors. It is shown that various vortex lattice structures are stabilized by nontrivial cooperation of nematicity and chirality in superconductors. In particular, the vortex lattice structural transition occurs when a square anisotropy parameter $ν$ is positive (negative) and the nematicity is induced along the [110] axis ([100] axis). We discuss nematic-chiral superconductivity in URu$_2$Si$_2$, Sr$_2$RuO$_4$, and UPt$_3$. An experimental test for the examination of nematic order and chiral superconductivity is proposed.

cond-mat.supr-con↗

arXiv2015

Spin-Triplet Pairing State of Sr2RuO4 in the c-Axis Magnetic Field

We investigate the spin-triplet superconducting state of Sr2RuO4 in the magnetic field along the c-axis on the basis of the four-component Ginzburg-Landau (GL) model with a weak spin-orbit coupling. We consider superconducting states described by the d-vector parallel to the ab-plane, and find that three spin-triplet pairing states are stabilized in the magnetic field-temperature phase diagram. Although a helical state is stable at low magnetic fields, a chiral II state is stabilized at high magnetic fields. A non-unitary spin-triplet pairing state appears near the transition temperature owing to the coupling of magnetic field and chirality. We elucidate synergistic and/or competing roles of the magnetic field, chirality, and spin-orbit coupling. It is shown that a fractional vortex lattice is stabilized in the chiral II phase owing to the spin-orbit coupling.

cond-mat.supr-con↗

arXiv2015

Multi-orbital Fulde-Ferrell-Larkin-Ovchinnikov State in SrTiO_3 Heterostructures

We study exotic superconducting states in SrTiO$_3$ heterostructures on the basis of the three-orbital model, which reproduces the band structure of two-dimensional electron gases. We show various Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states induced by the broken inversion symmetry and orbital degree of freedom. In particular, a novel orbital-dependent FFLO state is stabilized in a magnetic field along the [110]-axis. The field angle dependence of the FFLO state is clarified on the basis of the spin and orbital texture in the momentum space. It is shown that the orbital degree of freedom in Cooper pairs gives rise to in-plane anisotropy of the critical magnetic field. The carrier density dependence of the superconducting state is also discussed.

cond-mat.supr-con↗

arXiv2014

Electric Octupole Order in Bilayer Ruthenate Sr$_3$Ru$_2$O$_7$

Although the odd-parity multipole order barely occurs in crystals with high symmetry, it can be formed in locally noncentrosymmetric crystals. We illustrate the odd-parity electric octupole order generated in a bilayer structure. When the local electric quadrupole is alternatively stacked between layers, it is regarded as an electric octupole order from the viewpoint of symmetry. We show that the $p_y s_x + p_x s_y$ spin nematic order is induced by the spin-orbit coupling in the electric octupole state, and it results from the spontaneous inversion symmetry breaking leading to the $D_{2d}$ point group symmetry. We investigate the possible realization of the electric octupole order in the bilayer ruthenate Sr$_3$Ru$_2$O$_7$, assuming a local electric quadrupole arising from the Pomeranchuk instability and/or the orbital order. Effects of the lattice distortion and magnetic field are also clarified, and the nature of the "electronic nematic state" of Sr$_3$Ru$_2$O$_7$ is discussed. It is proposed that the asymmetric band structure is a signature of the electric octupole order in Sr$_3$Ru$_2$O$_7$. The odd-parity multipole order in other strongly correlated electron systems is discussed.

cond-mat.str-el↗

arXiv2014

Spin-orbit Coupling and Multiple Phases in Spin-triplet Superconductor Sr$_2$RuO$_4$

We study the spin-orbit coupling in spin-triplet Cooper pairs and clarify multiple superconducting (SC) phases in Sr$_2$RuO$_4$. Based on the analysis of the three-orbital Hubbard model with atomic LS coupling, we show some selection rules of the spin-orbit coupling in Cooper pairs. The spin-orbit coupling is small when the two-dimensional $γ$-band is the main cause of the superconductivity, although the LS coupling is much larger than the SC gap. Considering this case, we investigate multiple SC transitions in the magnetic fields for both H // [001] and H // [100] using the Ginzburg-Landau theory and the quasi-classical theory. Rich phase diagrams are obtained because the spin degree of freedom in Cooper pairs is not quenched by the spin-orbit coupling. Experimental indications for the multiple phases in Sr$_2$RuO$_4$ are discussed.

cond-mat.supr-con↗

arXiv2014

Vortex Core Structure in Multilayered Rashba Superconductors

We numerically study the electronic structure of a single vortex in two dimensional superconducting bilayer systems within the range of the mean-field theory. The lack of local inversion symmetry in the system is taken into account through the layer dependent Rashba spin-orbit coupling. The spatial profiles of the pair potential and the local quasiparticle density of states are calculated in the clean spin-singlet superconductor on the basis of the quasiclassical theory. In particular, we discuss the characteristic core structure in the pair-density wave state, which is spatially modulated exotic superconducting phase in a high magnetic field.

cond-mat.supr-con↗

arXiv2013

Magneto-Electric Effect in Three-dimensional Coupled Zigzag Chains

Stimulated by recent studies of quantum phases with broken local inversion symmetry, we study the magnetoelectric effect in locally noncentrosymmetric metals. We consider three-dimensional (3D) coupled zigzag chains and demonstrate that the antiferromagnetic moment is induced by the electric current through a staggered antisymmetric spin-orbit coupling. This current-induced magnetism is much larger than that in globally noncentrosymmetric metals. We provide an intuitive understanding of the current-induced antiferromagnetic moment by showing the inverse magnetoelectric effect, that is, the ferroic p-wave charge nematic order accompanied by the asymmetric band structure in the antiferromagnetic state. We also examine conduction electrons coupled to localized spins via Kondo exchange coupling and demonstrate a significant enhancement of the magnetoelectric effect. A possible experimental observation of the magnetoelectric effect in metals is discussed, with focus on LnM_2Al_10 compounds, such as NdRu_Al_10 and TbRu_2Al_10.

cond-mat.str-el↗

arXiv2013

Parity-Mixed Superconductivity in Locally Non-centrosymmetric System

We study the parity-mixed superconductivity in locally non-centrosymmetric systems. In multilayer systems an inhomogeneous Rashba spin-orbit coupling is induced by the local violation of inversion symmetry. Our previous study revealed that a pair-density wave (PDW) phase, with a sign-modulated order parameter, is stabilized in the spin-singlet multilayer superconductors owing to the spin-orbit coupling. In this letter, we show that the uniform spin-triplet superconductivity emerges through parity mixing in the PDW phase, taking into account a weak interaction in the spin-triplet channel, which was neglected in our previous study. The spin-triplet superconducting phase is nonunitary owing to field-induced parity mixing. The critical magnetic field is markedly increased by the emergence of spin-triplet superconductivity. We calculate the density of states and analyze the signature specific to this phase.

cond-mat.supr-con↗

arXiv2013

Multi-Orbital Superconductivity in SrTiO3 /LaAlO3 Interface and SrTiO3 Surface

We investigate the superconductivity in two-dimensional electron systems formed in SrTiO3 nanostructures. Our the- oretical analysis is based on the three-orbital model, which takes into account t2g orbitals of Ti ions. Because of the interfacial breaking of mirror symmetry, a Rashba-type antisymmetric spin-orbit coupling arises from the cooperation of intersite and interorbital hybridyzation and atomic LS coupling. This model shows a characteristic spin texture and carrier density dependence of Rashba spin-orbit coupling through the orbital degree of freedom. Superconductivity is mainly caused by heavy quasiparticles consisting of dyz and dzx orbitals at high carrier densities. We find that the Rashba spin-orbit coupling stabilizes a quasi-one-dimensional superconducting phase caused by one of the dyz or dzx orbitals at high magnetic fields along interfaces. This quasi-one-dimensional superconducting phase is protected against para- magnetic depairing effects by the Rashba spin-orbit coupling and realizes a large upper critical field Hc2 beyond the Pauli-Clogston-Chandrasekhar limit. This finding is consistent with an extraordinarily large upper critical field observed in SrTiO3 /LaAlO3 interfaces and its carrier density dependence. The possible coexistence of superconductivity and fer- romagnetism in SrTiO3 /LaAlO3 interfaces may also be attributed to this quasi-one-dimensional superconducting phase.

cond-mat.supr-con↗

arXiv2013

Spin Susceptibility in Non-centrosymmetric Superconductors with Topological Transition of Fermi Surfaces

The non-centrosymmetric superconductors Li2Pd3B and Li2Pt3B show different superconducting properties despite having the same crystal symmetry. Motivated by experimental results, we investigate the spin susceptibility of non-centrosymmetric superconductors accompanied by the topological transition of Fermi surfaces due to antisymmetric spin-orbit coupling, which is indicated by the first-principles band structure calculation for Li2Pt3B. We study three types of topological transition, namely, (A) the disappearance of the Fermi surface, (B) crossing the Dirac point, and (C) crossing the saddle point van-Hove singularity. The spin susceptibility in the superconducting state is increased by the topological transitions (A) and (C), while it is decreased by (B). We discuss the unusual magnetic properties observed in Li2Pt3B on the basis of these results.

cond-mat.supr-con↗

arXiv2013

Complex-Stripe Phases Induced by Staggered Rashba Spin-Orbit Coupling

We study superconducting phases in a quasi-two-dimensional multilayer system without local inversion symmetry. Broken local inversion symmetry induces layer-dependent Rashba-type spin-orbit couplings. We find that a complex-stripe phase, which is the intermediate phase between the Fulde-Ferrell (FF) phase and Larkin-Ovchinnikov (LO) phase, is realized in the magnetic field applied parallel to the layers. A crossover from the FF phase to the LO phase appears by tuning temperature and magnetic field. We show the local density of states that characterizes the complex-stripe phase. As a possible realization of the complex-stripe phase, we discuss the artificial superlattices of CeCoIn_5.

cond-mat.supr-con↗

arXiv2013

Spin-Orbit Coupling in Multilayer Superconductors with Charge Imbalance

In this study, we investigate the spin susceptibility in the superconducting state of a multilayer system with a layer-dependent Rashba spin-orbit coupling, representing a locally non-centrosymmetric superconductor. We show that the charge imbalance between different layers yields a strong layer dependence on the susceptibility, most significantly for a weak spin-orbit coupling, which is in contrast to a situation with an equally distributed charge. These results can be relevant for experimental test in multilayer high-Tc cuprates as well as in superconducting CeCoIn5/YbCoIn5 superlattices with more than two layers.

cond-mat.supr-con↗

arXiv2012

Pair-density wave states through spin-orbit coupling in multilayer superconductors

Spin singlet superconductors with quasi-two dimensional multilayer structure are studied in high magnetic fields. Specifically we concentrate on bi- and tri-layer systems whose layers by symmetry are subject Rashba-type spin-orbit coupling. The combination of magnetic field and spin-orbit coupling leads to a first order phase transition between different states of layer-dependent superconducting order parameters upon rising the magnetic field. In this context we distinguish the low-field Bardeen-Cooper-Schrieffer state where all layers have order parameters of the same sign and the high-field pair-density wave state where the layer-dependent order parameters change the sign at the center layer. We also show that progressive paramagnetic limiting effects yield additional features in the H-T phase diagram. As possible realizations of such unusual superconducting phases we consider artificial superlattices of CeCoIn_5, as well as some multilayer high-T_c cuprates.

cond-mat.supr-con↗

arXiv2012

Locally Non-centrosymmetric Superconductivity in Multilayer Systems

Although multilayer systems possess global inversion symmetry, some of the layers lack local inversion symmetry because no global inversion centers are present on such layers. Such locally non-centrosymmetric systems exhibit spatially modulated Rashba spin-orbit coupling. In this study, the superconductivity in multilayer models exhibiting inhomogeneous Rashba spin-orbit coupling is investigated. We study the electronic structure, superconducting gap, and spin susceptibility in the superconducting state with mixed parity order parameters. We show the enhancement of the spin susceptibility by Rashba spin-orbit coupling and interpret it on the basis of the crossover from a centrosymmetric superconductor to a non-centrosymmetric superconductor. It is also shown that the spin susceptibility is determined by the phase difference of the order parameter between layers and is nearly independent of the parity mixing of order parameters. An intuitive understanding is given on the basis of the analytic expression of superconducting order parameters in the band basis. The results indicate that not only a broken global inversion symmetry but also a broken local inversion symmetry leads to unique properties of superconductivity. We discuss the superconductivity in artificial superlattices involving CeCoIn5 and multilayer high-Tc cuprates.

cond-mat.supr-con↗

arXiv2011

Rotating Fulde-Ferrell-Larkin-Ovchinnikov state in cold Fermi gases

We study an effect of rotation on the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state of two component Fermi superfluid gases in a toroidal trap. We investigate a stability of the FFLO states in the quasi-one-dimensional regime on the basis of the Bogoliubov-de Gennes equation. We find that two novel FFLO phases, i.e., the half quantum vortex state and the intermediate state of Fulde-Ferrell (FF) state and Larkin-Ovchinnikov (LO) state, are stabilized by the rotation. The phase diagram for the FF state, LO state, intermediate state, and half quantum vortex state is shown in both T-P plane and T-h plane. We demonstrate characteristic features of these states, such as the order parameter, flux quantization, and local polarization. Several related works are discussed, and the advantages of cold Fermi gases are indicated.

cond-mat.quant-gas↗

arXiv2011

Superconductivity without Local Inversion Symmetry; Multi-layer Systems

While multi-layer systems can possess global inversion centers, they can have regions with locally broken inversion symmetry. This can modify the superconducting properties of such a system. Here we analyze two dimensional multi-layer systems yielding spatially modulated antisymmetric spin-orbit coupling (ASOC) and discuss superconductivity with mixed parity order parameters. In particular, the influence of ASOC on the spin susceptibility is investigated at zero temperature. For weak inter-layer coupling we find an enhanced spin susceptibility induced by ASOC, which hints the potential importance of this aspect for superconducting phase in specially structured superlattices.

cond-mat.supr-con↗

arXiv2011

Ginzburg-Landau Analysis for the Antiferromagnetic Order in the Fulde-Ferrell-Larkin-Ovchinnikov Superconductor

Incommensurate antiferromangetic (AFM) order in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductor is investigated on the basis of the Ginzburg-Landau theory. We formulate the two component Ginzburg-Landau model to discuss two degenerate incommensurate AFM states in the tetragonal crystal structure. Owing to the broken translation symmetry in the FFLO state, a multiple phase diagram of single-q phase and double-q phase is obtained under the magnetic field along [100] or [010] direction. Magnetic properties in each phase are investigated and compared with the neutron scattering and NMR measurements for a heavy fermion superconductor CeCoIn_5. An ultrasonic measurement is proposed for a future experimental study to identify the AFM-FFLO state. The field orientation dependence of the AFM order in CeCoIn_5 is discussed.

cond-mat.supr-con↗

arXiv2011

Magnetic structure of antiferromagnetic Fulde-Ferrell-Larkin-Ovchinnikov state

The properties of incommensurate antiferromagnetic (AFM) order in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is studied by solving the Bogoliubov-de-Gennes (BdG) equations. The relationship between the electronic structure and the magnetic structure is clarified. We find that the magnetic structure in the AFM-FFLO state includes three cases. (I) In the strongly localized case, the AFM staggered moment is confined into the FFLO nodal planes where the superconducting order parameter vanishes. (II) In the weakly localized case, the AFM staggered moment appears in the whole spatial region, and its magnitude is enhanced around the FFLO nodal planes. (III) In the extended case, the AFM staggered moment is nearly homogeneous and slightly suppressed in the vicinity of FFLO nodal planes. The structure of Bragg peaks in the momentum resolved structure factor is studied in each case. We discuss the possibility of AFM-FFLO state in the heavy fermion superconductor CeCoIn_5 by comparing these results with the neutron scattering data. Experimentally the magnetic structure and its dependence on the magnetic field orientation in the high field superconducting phase of CeCoIn_5 are consistent with the case (II).

cond-mat.supr-con↗