arXiv subjects

Youichi Yanase

Publications and source records attributed to Youichi Yanase.

At least 109 records · Page 6Linked to original sources

arXiv2019

Multiorbital Ferroelectric Superconductivity in doped SrTiO3

SrTiO$_3$ is a unique example of a system which exhibits both quantum paraelectricity and superconductivity. Thus, it is expected that the superconducting state is closely related to the intrinsic ferroelectric instability. Indeed, recent experiments suggest existence of a coexistent phase of superconductivity and ferroelectricity in Ca-substituted SrTiO$_3$. In this paper, we propose that SrTiO$_3$ can be a platform of the ferroelectric superconductivity, which is characterized by a ferroelectric transition in the superconducting state. By analyzing a multiorbital model for $t_{2g}$ electrons, we show that the ferroelectric superconductivity is stabilized through two different mechanisms which rely on the presence of the spin-orbit coupling. First, the ferroelectric superconducting state is stabilized in the dilute carrier density regime due to a ferroelectricity-induced Lifshitz transition. Second, it is stabilized under a magnetic field independent of the carrier density. The importance of the multiorbital or multiband nature for the ferroelectric superconductivity is clarified. Then, we predict a topological Weyl superconducting state in the ferroelectric superconducting phase of SrTiO$_3$.

cond-mat.supr-con↗

arXiv2019

Efficient method to compute $\mathbb{Z}_4$-indices with glide symmetry and applications to Möbius materials, $\mathrm{CeNiSn}$ and $\mathrm{UCoGe}$

We propose an efficient method to numerically evaluate $\mathbb{Z}_4$-indices of Möbius/hourglass topological phases with glide symmetry. Our method directly provides $\mathbb{Z}_4$-indices in the lattice Brillouin zone while the existing method requires careful observations of momentum dependent Wannier charge centers. As applications, we perform systematic computation of $\mathbb{Z}_4$-indices for Möbius materials, $\mathrm{CeNiSn}$ and $\mathrm{UCoGe}$. In particular, our analysis elucidates that $\mathrm{UCoGe}$ shows strong Möbius superconductivity for the $A_u$- or $B_{3u}$-representation whose topology has not been fully characterized. Furthermore, obtained phase diagrams reveal novel topological gapless excitations in the bulk which are protected by nonsymmorphic glide symmetry. We observe these gapless excitations with glide symmetry by doping holes into the superconducting phase of the $B_{1u}$- or $B_{3u}$-representation.

cond-mat.str-el↗

arXiv2019

$Z_4$ Topological Superconductivity in UCoGe

Topological nonsymmorphic crystalline superconductivity (TNCS) is an intriguing phase of matter, offering a platform to study the interplay between topology, superconductivity, and nonsymmorphic crystalline symmetries. Interestingly, some of TNCS are classified into $Z_4$ topological phases, which have unique surface states referred to as a Möbius strip or an hourglass, and have not been achieved in symmorphic superconductors. However, material realization of $Z_4$ TNCS has never been known, to the best of our knowledge. Here we propose that the paramagnetic superconducting phase of UCoGe under pressure is a promising candidate of $Z_4$-nontrivial TNCS enriched by glide symmetry. We evaluate $Z_4$ invariants of UCoGe by deriving the formulas relating $Z_4$ invariants to the topology of Fermi surfaces. Applying the formulas and previous ab-initio calculations, we clarify that three odd-parity representations, out of four, are $Z_4$-nontrivial TNCS, while the other is also $Z_2$-nontrivial TNCS. We also discuss possible $Z_4$ TNCS in CrAs and related materials.

cond-mat.supr-con↗

arXiv2019

Classification of topological crystalline superconducting nodes on high-symmetry lines: Point nodes, line nodes, and Bogoliubov Fermi surfaces

Recent development in exact classification of a superconducting gap has elucidated various unconventional gap structures, which have not been predicted by the classification of order parameter based on the point group. One of the important previous results is that all symmetry-protected line nodes are characterized by nontrivial topological numbers. Another intriguing discovery is the gap structures depending on the angular momentum $j_z$ of normal Bloch states on threefold and sixfold rotational-symmetric lines in the Brillouin zone. Stimulated by these findings, we classify irreducible representations of the Bogoliubov-de Gennes Hamiltonian at each $\boldsymbol{k}$ point on a high-symmetry $n$-fold ($n = 2$, $3$, $4$, and $6$) axis for centrosymmetric and paramagnetic superconductors, by using the combination of group theory and $K$ theory. This leads to the classification of all crystal symmetry-protected nodes (including $j_z$-dependent nodes) on the axis that crosses a normal-state Fermi surface. As a result, it is shown that the classification by group theory completely corresponds with the topological classification. Based on the obtained results, we discuss superconducting gap structures in SrPtAs, CeCoIn$_5$, UPt$_3$, and UCoGe.

cond-mat.supr-con↗

arXiv2019

Magnetoelectric Response in Electric Octupole State: Possible Hidden Order in Cuprate Superconductors

Motivated by recent studies of odd-parity multipole order in condensed matter physics, we theoretically study magnetoelectric responses in an electric octupole state. Investigating the Edelstein effect and spin Hall effect in a locally noncentrosymmetric bilayer Rashba model, we clarify characteristic properties due to parity violation in the electric octupole state. Furthermore, a possible realization of electric octupole order in bilayer high-Tc cuprate superconductors is proposed. Our calculation of magnetic torque is consistent with recent experimental observation of a kink above the superconducting transition temperature. We also show significant enhancement of the in-plane anisotropy in spin susceptibility due to the superconductivity, and propose an experimental test by means of the nuclear magnetic resonance in the superconducting state. A spin-orbit coupled metal state in Cd2Re2O7 is also discussed.

cond-mat.str-el↗

arXiv2019

Theory of spin magnetic quadrupole moment and temperature-gradient-induced magnetization

We revisit a quantum-mechanical formula of the spin magnetic quadrupole moment (MQM) in periodic crystals. Two previous attempts were inconsistent with each other; one is gauge dependent, and the other is gauge invariant. Here we define the spin MQM by calculating the spin density in a nonuniform system. Our definition is analogous to that of the charge polarization, but the result is gauge invariant and coincides with the latter previous one. We also formulate what we call gravitomagnetoelectric (gravito-ME) effect, in which the magnetization is induced by a temperature gradient. Although the Kubo formula for the gravito-ME effect provides an unphysical divergence at zero temperature, we prove that the correct susceptibility is obtained by subtracting the spin MQM from the Kubo formula. It vanishes at zero temperature and is related to the ME susceptibility by the Mott relation. We explicitly calculate the gravito-ME susceptibility in a Rashba ferromagnet and show its experimental feasibility.

cond-mat.str-el↗

arXiv2018

Symmetry analysis of current-induced switching of antiferromagnets

Antiferromagnets are robust to external electric and magnetic fields, and hence are seemingly uncontrollable. Recent studies, however, realized the electrical manipulations of antiferromagnets by virtue of the antiferromagnetic Edelstein effect. We present a general symmetry analysis of electrically switchable antiferromagnets based on group-theoretical approaches. Furthermore, we identify a direct relation between switchable antiferromagnets and the ferrotoroidic order. The concept of the ferrotoroidic order clarifies the unidirectional nature of switchable antiferromagnets and provides a criterion for the controllability of antiferromagnets. The scheme paves a way for perfect writing and reading of switchable antiferromagnets.

cond-mat.str-el↗

arXiv2018

Group-theoretical classification of multipole order: emergent responses and candidate materials

The multipole moment is an established concept of electrons in solids. Entanglement of spin, orbital, and sublattice degrees of freedom is described by the multipole moment, and spontaneous multipole order is a ubiquitous phenomenon in strongly correlated electron systems. In this paper, we present group-theoretical classification theory of multipole order in solids. Intriguing duality between the real space and momentum space properties is revealed for odd-parity multipole order which spontaneously breaks inversion symmetry. Electromagnetic responses in odd-parity multipole states are clarified on the basis of the classification theory. A direct relation between the multipole moment and the magnetoelectric effect, Edelstein effect, magnetopiezoelectric effect, and dichromatic electron transport is demonstrated. More than 110 odd-parity magnetic multipole materials are identified by the group-theoretical analysis. Combining the list of materials with the classification tables of multipole order, we predict emergent responses of the candidate materials.

cond-mat.mtrl-sci↗

arXiv2018

Odd-parity multipole fluctuation and unconventional superconductivity in locally noncentrosymmetric crystal

A microscopic calculation and symmetry argument reveal superconductivity in the vicinity of parity-violating magnetic order. An augmented cluster magnetic multipole order in a crystal lacking local space inversion parity may break global inversion symmetry, and then, it is classified into an odd-parity multipole order. We investigate unconventional superconductivity induced by an odd-parity magnetic multipole fluctuation in a two-dimensional two-sublattice Hubbard model motivated by Sr$_2$IrO$_4$. We find that even-parity superconductivity is more significantly suppressed by a spin-orbit coupling than that in a globally noncentrosymmetric system. Consequently, two odd-parity superconducting states are stabilized by magnetic multipole fluctuations in a large spin-orbit coupling region. Both of them are identified as $Z_2$ topological superconducting states. The obtained gap function of inter-sublattice pairing shows a gapped/nodal structure protected by nonsymmorphic symmetry. Our finding implies a new family of odd-parity topological superconductors. Candidate materials are discussed.

cond-mat.supr-con↗

arXiv2018

Spin-orbit-coupled ferroelectric superconductivity

Motivated by recent studies on ferroelectric-like order coexisting with metallicity, we investigate ferroelectric (FE) superconductivity in which a FE-like structural phase transition occurs in the superconducting state. We consider a two-dimensional s-wave superconductor with Rashba-type antisymmetric spin-orbit coupling (ASOC). Assuming linear relationship between polar lattice displacement and strength of the ASOC, we treat the Rashba-type ASOC as a molecular field of FE-like order. It is shown that the FE-like order is induced by the magnetic field when the system is superconducting. Furthermore, we clarify the FE superconductivity in a low carrier density regime, which was recently discovered in doped SrTiO$_3$. It is demonstrated that the FE superconducting state can be stable in this regime in the absence of the magnetic field. Our results open a way to control the electric polarization by superconductivity, that is, superconducting multiferroics.

cond-mat.supr-con↗

arXiv2018

Theory of orbital magnetic quadrupole moment and magnetoelectric susceptibility

We derive a quantum-mechanical formula of the orbital magnetic quadrupole moment (MQM) in periodic systems by using the gauge-covariant gradient expansion. This formula is valid for insulators and metals at zero and finite temperature. We also prove a direct relation between the MQM and magnetoelectric (ME) susceptibility for insulators at zero temperature. It indicates that the MQM is a microscopic origin of the ME effect. Using the formula, we quantitatively estimate these quantities for room-temperature antiferromagnetic semiconductors BaMn$_2$As$_2$ and CeMn$_2$Ge$_{2 - x}$Si$_x$. We find that the orbital contribution to the ME susceptibility is comparable with or even dominant over the spin contribution.

cond-mat.mtrl-sci↗

arXiv2018

Unconventional superconducting gap structure protected by space group symmetry

Recent superconducting gap classifications based on space group symmetry have revealed nontrivial gap structures that were not shown by point group symmetry. First, we review a comprehensive classification of symmetry-protected line nodes within the range of centrosymmetric space groups. Next, we show an additional constraint; line nodes peculiar to nonsymmorphic systems appear only for primitive or orthorhombic base-centered Bravais lattice. Then, we list useful classification tables of 59 primitive or orthorhombic base-centered space groups for the superconducting gap structures. Furthermore, our gap classification reveals the $j_z$-dependent point nodes (gap opening) appearing on a 3- or 6-fold axis, which means that the presence (absence) of point nodes depends on the Bloch-state angular momentum $j_z$. We suggest that this unusual gap structure is realized in a heavy-fermion superconductor UPt$_3$, using a group-theoretical analysis and a numerical calculation. The calculation demonstrates that a Bloch phase contributes to $j_z$ as effective orbital angular momentum by site permutation. We also discuss superconducting gap structures in MoS$_2$, SrPtAs, UBe$_{13}$, and PrOs$_4$Sb$_{12}$.

cond-mat.supr-con↗

arXiv2018

Strong enhancement of the Edelstein effect in f-electron systems

The Edelstein effect occurring in systems with broken inversion symmetry generates a spin polarization when an electric field is applied, which is most advantageous in spintronics applications. Unfortunately, it became apparent that this kind of magnetoelectric effect is very small in semiconductors. We here demonstrate that correlation effects can strongly enhance the magnetoelectric effect. Particularly, we observe a strong enhancement of the Edelstein effect in $f$-electron systems close to the coherence temperature, where the $f$-electrons change their character from localized to itinerant. We furthermore show that this enhancement can be explained by a coupling between the conduction electrons and the still localized $f$-electrons.

cond-mat.str-el↗

arXiv2018

Anomalous change in the de Haas-van Alphen oscillations of CeCoIn$_5$ at ultra-low temperatures

We have performed de Haas-van Alphen (dHvA) measurements of the heavy-fermion superconductor CeCoIn$_5$ down to 2 mK above the upper critical field. We find that the dHvA amplitudes show an anomalous suppression, concomitantly with a shift of the dHvA frequency, below the transition temperature $T_{\rm n}=20$ mK. We suggest that the change is owing to magnetic breakdown caused by a field-induced antiferromagnetic (AFM) state emerging below $T_{\rm n}$, revealing the origin of the field-induced quantum critical point (QCP) in CeCoIn$_5$. The field dependence of $T_{\rm n}$ is found to be very weak for 7--10 T, implying that an enhancement of AFM order by suppressing the critical spin fluctuations near the AFM QCP competes with the field suppression effect on the AFM phase. We suggest that the appearance of a field-induced AFM phase is a generic feature of unconventional superconductors, which emerge near an AFM QCP, including CeCoIn$_5$, CeRhIn$_5$, and high-$T_{\rm c}$ cuprates.

cond-mat.str-el↗

arXiv2018

Generation and control of noncollinear magnetism by supercurrent

When superconductivity couples with noncollinear spin textures, rich physics arises; for instance, singlet Cooper pairs can be converted to triplet pairs, and topological superconductors can be realized. For their applications, the controllability of noncollinear magnetism is a crucial issue. Here, we propose that a supercurrent can induce and control noncollinear magnetic orders in a correlated metal on top of a singlet superconductor. We show that the magnetic instability in the correlated metal is enhanced by the proximity effect of supercurrents, which leads to phase transitions from a paramagnetic state to noncollinear magnetic phases with helical or vortexlike spin textures. Furthermore, these magnetic orders can be switched by the direction of the supercurrent. We also discuss the effect of the Rashba spin-orbit coupling and the experimental realization.

cond-mat.str-el↗

arXiv2018

Orbitally limited pair-density wave phase of multilayer superconductors

We investigate the magnetic field dependence of an ideal superconducting vortex lattice in the parity-mixed pair-density wave phase of multilayer superconductors within a circular cell Ginzburg-Landau approach. In multilayer systems, due to local inversion symmetry breaking, a Rashba spin-orbit coupling is induced at the outer layers. This combined with a perpendicular paramagnetic (Pauli) limiting magnetic field stabilizes a staggered layer dependent pair-density wave phase in the superconducting singlet channel. The high-field pair-density wave phase is separated from the low-field BCS phase by a first-order phase transition. The motivating guiding question in this paper is: what is the minimal necessary Maki parameter $α_M$ for the appearance of the pair-density wave phase of a superconducting trilayer system? To address this problem we generalize the circular cell method for the regular flux-line lattice of a type-II superconductor to include paramagnetic depairing effects. Then, we apply the model to the trilayer system, where each of the layers are characterized by Ginzburg-Landau parameter $κ_0$, and a Maki parameter $α_M$. We find that when the spin-orbit Rashba interaction compares to the superconducting condensation energy, the orbitally limited pair-density wave phase stabilizes for Maki parameters $α_M> 10$.

cond-mat.supr-con↗

arXiv2017

Symmetry-protected line nodes and Majorana flat bands in nodal crystalline superconductors

Line nodes in the superconducting gap are known to be a source of Majorana flat bands (MFBs) in time-reversal-invariant superconductors (SCs). Here, we extend this relation to all symmetry-protected line nodes where an additional constraint arising from a symmetry of the crystal destabilizes or hides the existence of MFBs. By establishing a one-to-one correspondence between group theoretical and topological classifications, we are able to classify the possible line-node-induced MFBs, including cases with (magnetic) non-symmorphic space groups. Our theoretical analysis reveals a new type of MFB, i.e., MFBs in antiferromagnetic SCs.

cond-mat.supr-con↗

arXiv2017

Magnetic hexadecapole order and magnetopiezoelectric metal in Ba$_{1-x}$K$_x$Mn$_2$As$_2$

We study an odd-parity magnetic multipole order in Ba$_{1-x}$K$_x$Mn$_2$As$_2$ and related materials. Although BaMn$_2$As$_2$ is a seemingly conventional Mott insulator with G-type antiferromagnetic order, we identify the ground state as a magnetic hexadecapole ordered state accompanied by simultaneous time-reversal and space-inversion symmetry breaking. A symmetry argument and microscopic calculations reveal the ferroic ordering of leading magnetic hexadecapole moment and admixed magnetic quadrupole moment. Furthermore, we clarify electromagnetic responses characterizing the magnetic hexadecapole state of semiconducting BaMn$_2$As$_2$ and doped metallic systems. A magnetoelectric effect and antiferromagnetic Edelstein effect are shown. Interestingly, a counter-intuitive currentinduced nematic order occurs in the metallic state. The electric current along the \textit{z}-axis induces the \textit{xy}-plane nematicity in sharp contrast to the spontaneous nematic order in superconducting Febased 122-compounds. Thus, the magnetic hexadecapole state of doped BaMn$_2$As$_2$ is regarded as a magnetopiezoelectric metal. The anomalous responses stem from the peculiar symmetry of the parity-violating magnetic order. Other candidate materials for magnetic hexadecapole order are proposed.

cond-mat.mtrl-sci↗