arXiv subjects

Hiroaki Kusunose

Publications and source records attributed to Hiroaki Kusunose.

At least 73 records · Page 4Linked to original sources

arXiv2016

Competing Kondo Effects in Non-Kramers Doublet Systems

In non-Kramers Kondo systems with a quadrupolar degrees of freedom, an ordinary magnetic Kondo effect can compete with the quadrupolar Kondo effect. We discuss such competition keeping Pr$T_{2}$Zn$_{20}$ ($T$=Ir, Rh) and Pr$T_{2}$Al$_{20}$ ($T$=V, Ti) in mind, where the $Γ_{3}$ non-Kramers crystalline-electric-field (CEF) doublet ground state is realized in Pr$^{3+}$ ion with $(4f)^{2}$ configuration under cubic symmetry. The quadrupolar Kondo effect can be described by the two-channel Kondo model, which leads to the local non-Fermi-liquid (NFL) ground state, while the magnetic Kondo effect favors the ordinary local Fermi-liquid (FL) ground state. Based on the minimal extended two-channel Kondo model including the magnetic Kondo coupling as well, we investigate the competition and resulting thermodynamics, and orbital/magnetic, and single-particle excitation spectra by the Wilson's numerical renormalization-group (NRG) method. There exists the first-order transition between the NFL and FL ground states. In addition to these two states, the alternative FL state accompanied by a free magnetic spin appears in intermediate temperature range, which eventually reaches the true NFL ground state, as a consequence of stronger competition between the magnetic and quadrupolar Kondo effects. In this peculiar state, the magnetic susceptibility shows a Curie-like behavior, while the orbital fluctuation exhibits the FL behavior. Moreover, the single-particle spectra yield a more singular behavior. Implications to the Pr 1-2-20 systems are briefly discussed.

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arXiv2016

Anomalous Local Fermi Liquid in $f^{2}$-Singlet Configuration: Impurity Model for Heavy-Electron System UPt_3

It is shown by the Wilson numerical renormalization group method that a strongly correlated impurity with a crystalline-electric-field singlet ground state in the f^2-configuration exhibits an anomalous local Fermi liquid state in which the static magnetic susceptibility remains an uncorrelated value while the NMR relaxation rate is enhanced in proportion to the square of the mass enhancement factor. Namely, the Korringa-Shiba relation is apparently broken. This feature closely matches the anomalous behaviors observed in UPt_3, i.e., the coexistence of an unenhanced value of the Knight shift due to quasiparticles contribution (the decrease across the superconducting transition) and the enhanced relaxation rate of NMR. Such an anomalous Fermi liquid behavior suggests that the Fermi liquid corrections for the susceptibility are highly anisotropic.

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arXiv2015

Spontaneous multipole ordering by local parity mixing

Broken spatial inversion symmetry in spin-orbital coupled systems leads to a mixing between orbitals with different parity, which results in unusual electronic structures and transport properties. We theoretically investigate the possibility of multipole ordering induced by a parity mixing. In particular, we focus on the system in which the parity mixing appears in a sublattice-dependent form. Starting from the periodic Anderson model with such a local parity mixing, we derive an extended Kondo lattice model with sublattice-dependent antisymmetric exchange couplings between itinerant electrons and localized spins. By the variational calculation, simulated annealing, and Monte Carlo simulation, we show that the model on a quasi-one-dimensional zig-zag lattice exhibits an odd-parity multipole order composed of magnetic toroidal and quadrupole components at and near half filling. The multipole order causes a band deformation with the band bottom shift and a magnetoelectric response. The results suggest that unusual odd-parity multipole orders will be widely observed in multi-orbital systems with local parity mixing.

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arXiv2014

Toroidal order in a partially disordered state on a layered triangular lattice: implication to UNi4B

A partial disorder on a layered triangular lattice is theoretically investigated from a viewpoint of toroidal ordering and magnetoelectric effects. We consider an extended periodic Anderson model including a site-dependent antisymmetric spin-orbit coupling between conduction and localized electrons. We show that, by the mean-field approximation, the model exhibits a coplanar vortex-lattice-type magnetic order as observed in a hexagonal uranium compound UNi4B, in the parameter region with intermediate hybridization and electron correlation. This peculiar state accommodates a toroidal order, which leads to the linear magnetoelectric effect. We discuss the implications of our results to UNi4B, focusing on the possible source of the site-dependent antisymmetric spin-orbit coupling.

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arXiv2014

Competition between Quadrupole and Magnetic Kondo Effects in Non-Kramers Doublet Systems

We discuss possible competition between magnetic and quadrupole Kondo effects in non-Kramers doublet systems under cubic symmetry. The quadrupole Kondo effect leads to non-Fermi-liquid (NFL) ground state, while the magnetic one favors ordinary Fermi liquid (FL). In terms of the $j$-$j$ coupling scheme, we emphasize that the orbital fluctuation must develop in the vicinity of the NFL-FL boundary. We demonstrate a change of behavior in the f-electron entropy by the Wilson's numerical renormalization-group (NRG) method on the basis of the extended two-channel Kondo exchange model. We present implications to extensively investigated PrT$_{2}$X$_{20}$ (T=Ti, V, Ir; X=Al, Zn) systems that exhibit both quadrupole ordering and peculiar superconductivity. We also discuss the magnetic-field effect which lifts weakly the non-Kramers degeneracy. Our model also represents the FL state accompanied by a free magnetic spin as a consequence of stronger competition between the magnetic and the quadrupole Kondo effects.

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arXiv2014

Quantum spin Hall effect in a two-orbital model on a honeycomb lattice

The spin Hall effect is investigated in a two-orbital tight-binding model on a honeycomb lattice. We show that the model exhibits three topologically-different insulating phases at half filling, which are distinguished by different quantized values of the spin Hall conductivity. We analytically determine the phase boundaries, where the valence and conduction bands touch with each other with forming the Dirac nodes at the Fermi level. The results are discussed in terms of the effective antisymmetric spin-orbit coupling. The relation to the Kane-Mele model and implications for a magnetoelectric effect are also discussed.

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arXiv2014

Spontaneous Parity Breaking in Spin-Orbital Coupled Systems

Effects of spontaneous parity breaking by charge, spin, and orbital orders are investigated in a two-band Hubbard model on a honeycomb lattice. This is a minimal model in which the inter-orbital hopping, atomic spin-orbit coupling, and strong electron correlation give rise to fascinating properties, such as the magnetoelectric effects, quantum spin Hall effect, and spin or valley splitting in the band structure. We perform the symmetry analysis of possible broken-parity states and the mean-field analysis of their competition. We find that the model at 1/4 filling exhibits a spin-orbital composite ordered state and a charge ordered state, in addition to a paramagnetic quantum spin-Hall insulator. We show that the composite ordered phase exhibits two types of magnetoelectric responses. The charge ordered state shows spin splitting in the band structure, while the topological nature varies depending on electron correlations.

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arXiv2014

Toroidal order in metals without local inversion symmetry

Toroidal order, given by a composite of electric and magnetic orders, manifests itself not only in the peculiar magnetism but also in anomalous transport and magnetoelectric effect. We report our theoretical results on the influence and stability of a toroidal order in metals on the basis of a microscopic model. We consider an effective single-band Hubbard-type model with a site-dependent antisymmetric spin-orbit coupling, which is derived from a four-band tight-binding model including the atomic spin-orbit coupling, off-site hybridizations between orbitals with different parities, and odd-parity crystalline electric field. For this single-band model on a layered honeycomb lattice, we investigate the electronic structure, magnetotransport, and magnetoelectric effect in the toroidal ordered state with a vortex-like magnetic structure. The ferroic order of the microscopic toroidal moments acts as an effective gauge field for electrons, which modulates the electronic band structure with a shift of the band bottom in the momentum space. In addition, the site-dependent antisymmetric spin-orbit coupling gives rise to highly anisotropic Hall responses. The most salient feature is two different types of magnetoelectric responses: one is a magnetic order with net toroidal magnetization induced by an electric current perpendicular to the planes, and the other is a uniform transverse magnetization induced by an electric current within the planes. We examine the ground state of the effective model by the mean-field approximation, and show that the toroidal order is stabilized by strong electron correlations at low electron density. We also discuss the temperature dependence of the magnetoelectric effects associated with spontaneous toroidal ordering. Implications to experiments are also presented.

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arXiv2013

Theory of Self-Induced Vortex State in Ferromagnetic Superconductors

We have developed the quasi-classical formalism for a self-induced vortex state in ferromagnetic superconductors. By combining the spatially averaged approximation of the superconductivity with the phenomenological form of the ferromagnetism, we demonstrate the thermodynamic properties of the p-wave triplet equal-spin-pairing state with the spontaneous vortex lattice. The occurrence condition for the self-induced vortex state is discussed within this formalism. The comparison of the calculated quantities with the observed ones indicates that the self-induced vortex state indeed occurs in UCoGe, and its pairing symmetry seems to be the A2-type with a small spin difference in the gap magnitudes. In the vicinity of the ferromagnetic quantum critical point with a large uniform susceptibility, it is possible to exhibit the type-II to the type-I transition in the magnetization process.

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arXiv2013

Emergent Odd-Frequency Superconducting Order Parameter near Boundaries in Unconventional Superconductors

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 stabilized near the boundary and coexists with the even-frequency one. By analyzing the induced odd-frequency pair amplitude in terms of the superconducting quasi-particle wavefunction, it is found that the mid-gap bound state generates the emergent odd-frequency order parameter.

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arXiv2012

Strong-Coupling Superconductivity with Mixed Even- and Odd-Frequency Pairing

We investigate general structure of Landau free energy for stabilizing a novel superconducting state with both even-frequency and odd-frequency components in gap function. On the basis of the Luttinger-Ward functional, we elucidate an emergent mixing between different "parities" in time. The simplest case of the conventional s-wave singlet mixed with the odd-frequency triplet state under broken time-reversal symmetry is examined to demonstrate the anomalous structure of the free-energy functional. The induced odd-frequency component alters behaviors of physical quantities from those obtained by neglecting the odd-frequency component. The novel mixed state may also be relevant to strong-coupling superconductivity coexisting with ferromagnetism.

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arXiv2012

Coexistence of Even- and Odd-Frequency Superconductivities Under Broken Time-Reversal Symmetry

A novel superconducting state under the broken time-reversal symmetry is studied in conventional phonon-mediated superconductors. By solving the Eliashberg equation self-consistently with the mass renormalization effect, it is found that the even- and odd-frequency components of the order parameter coexist in the bulk system as a consequence of the broken time-reversal symmetry. This finding would direct more attention to the odd-frequency pairing that affects physical quantities, especially in strong coupling superconductors.

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arXiv2011

Magnetic Exciton Mediated Superconductivity in the Hidden-Order Phase of URu2Si2

We propose the magnetic exciton mediated superconductivity occurring in the enigmatic hidden-order phase of URu2Si2. The characteristic of the massive collective excitation observed only in the hidden-order phase is well reproduced by the antiferro hexadecapole ordering model as the trace of the dispersive crystalline-electric-field excitation. The disappearance of the superconductivity in the high-pressure antiferro magnetic phase can naturally be understood by the sudden suppression of the magnetic-exciton intensity. The analysis of the momentum dependence of the magnetic-exciton mode leads to the exotic chiral d-wave singlet pairing in the Eg symmetry. The Ising-like magnetic-field response of the mode yields the strong anisotropy observed in the upper critical field even for the rather isotropic 3-dimensional Fermi surfaces of this compound.

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arXiv2011

Effects of Impurities with Singlet-Triplet Configuration on Multiband Superconductors

Roles of multipole degrees of freedom in multiband superconductors are investigated in a case of impurities whose low-lying states consist of singlet ground and triplet excited states, which is related to the experimental fact that the transition temperature $T_{\rm c}$ is increased by Pr substitution for La in LaOs$_4$Sb$_{12}$. The most important contribution to the $T_{\rm c}$ increase comes from the inelastic interband scattering of electrons coupled to quadrupole or octupole moments of impurities. It is found that a magnetic field modifies an effective pairing interaction and the scattering anisotropy appears in the field-orientation dependence of the upper critical field $H_{{\rm c}2}$ in the vicinity of $T_{\rm c}$, although a uniaxial anisotropic field is required for experimental detection. This would be proof that the Pr internal degrees of freedom are relevant to the stability of superconductivity in (La$_{1-x}$Pr$_x$)Os$_4$Sb$_{12}$.

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arXiv2011

On the Hidden Order in URu$_{2}$Si$_{2}$ --- Antiferro Hexadecapole Order and its Consequences

An antiferro ordering of an electric hexadecapole moment is discussed as a promising candidate for the long standing mystery of the hidden order phase in URu$_{2}$Si$_{2}$. Based on localized $f$-electron picture, we discuss the rationale of the selected multipole and the consequences of the antiferro hexadecapole order of $xy(x^{2}-y^{2})$ symmetry. The mean-field solutions and the collective excitations from them explain reasonably significant experimental observations: the strong anisotropy in the magnetic susceptibility, characteristic behavior of pressure versus magnetic field or temperature phase diagrams, disappearance of inelastic neutron-scattering intensity out of the hidden order phase, and insensitiveness of the NQR frequency at Ru-sites upon ordering. A consistency with the strong anisotropy in the magnetic responses excludes all the multipoles in two-dimensional representations, such as $(O_{yz},O_{zx})$. The expected azimuthal angle dependences of the resonant X-ray scattering amplitude are given. The $(x^{2}-y^{2})$-type antiferro quadrupole should be induced by an in-plane magnetic field along $[110]$, which is reflected in the thermal expansion and the elastic constant of the transverse $(c_{11}-c_{12})/2$ mode. The $(x^{2}-y^{2})$-type [$(xy)$-type] antiferro quadrupole is also induced by applying the uniaxial stress along $[110]$ direction [$[100]$ direction]. A detection of these induced antiferro quadrupoles under the in-plane magnetic field or the uniaxial stress using the resonant X-ray scattering provides a direct redundant test for the proposed order parameter.

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arXiv2010

Possible Odd-Frequency Superconductivity in Strong-Coupling Electron-Phonon Systems

A possibility of the odd-frequency pairing in the strong-coupling electron-phonon systems is discussed. Using the Holstein-Hubbard model, we demonstrate that the anomalously soft Einstein mode with the frequency $ω_{\rm E}\llω_{c}$ ($ω_{c}$ is the order of the renormalized bandwidth) mediates the s-wave odd-frequency triplet pairing against the ordinary even-frequency singlet pairing. It is necessary for the emergence of the odd-frequency pairing that the pairing interaction is strongly retarded as well as the strong coupling, since the pairing interaction for the odd-frequency pairing is effective only in the diagonal scattering channel, $(ω_{n},-ω_{n})\to(ω_{n'},-ω_{n'})$ with $ω_{n'}=ω_{n}\gtrsim ω_{\rm E}$. Namely, the odd-frequency superconductivity is realized in the opposite limit of the original BCS theory. The Ginzburg-Landau analysis in the strong-coupling region shows that the specific-heat discontinuity and the slope of the temperature dependence of the superfluid density can be quite small as compared with the BCS values, depending on the ratio of the transition temperature $T_{c}$ and $ω_{c}$.

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arXiv2010

On the Puzzle of Odd-Frequency Superconductivity

Since the first theoretical proposal by Berezinskii, an odd-frequency superconductivity has encountered the fundamental problems on its thermodynamic stability and rigidity of a homogenous state accompanied by unphysical Meissner effect. Recently, Solenov {\it et al}. [Phys. Rev. B {\bf 79} (2009) 132502.] have asserted that the path-integral formulation gets rid of the difficulties leading to a stable homogenous phase with an ordinary Meissner effect. Here, we show that it is crucial to choose the appropriate saddle-point solution that minimizes the effective free energy, which was assumed {\it implicitly} in the work by Solenov and co-workers. We exhibit the path-integral framework for the odd-frequency superconductivity with general type of pairings, including an argument on the retarded functions via the analytic continuation to the real axis.

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arXiv2010

Effect of Impurities with Internal Structure on Multiband Superconductors - Possible Enhancement of Transition Temperature -

We study inelastic (dynamical) impurity scattering effects in two-band superconductors with the same ($s_{++}$ wave) or different ($s_\pm$ wave) sign order parameters. We focus on the enhancement of the superconducting transition temperature $T_{\rm c}$ by magnetic interband scattering with the interchange of crystal-field singlet ground and multiplet excited states. Either the $s_{++}$-wave or $s_\pm$-wave state is favored by the impurity-mediated pairing, which depends on the magnetic and nonmagnetic scattering strengths derived from the hybridization of the impurity states with the conduction bands. The details are examined for the singlet-triplet configuration that is suggestive of Pr impurities in the skutterudite superconductor LaOs$_4$Sb$_{12}$.

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