arXiv subjects

Manfred Sigrist

Publications and source records attributed to Manfred Sigrist.

At least 91 records · Page 5Linked to original sources

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

Topological and edge state properties of a three-band model for Sr2RuO4

Modeling the spin-triplet superconductor Sr2RuO4 through a three-orbital tight-binding model we investigate topological properties and edge states assuming chiral p-wave pairing. In concordance with experiments the three Fermi surfaces consist of two electron-like and one hole-like one corresponding to the alpha-, beta- and gamma-band on the level of a two-dimensional system. The quasi-particle spectra and other physical quantities of the superconducting phase are calculated by means of a self-consistent Bogoliubov-de Gennes approach for a ribbon shaped system. While a full quasiparticle excitation gap is realized in the bulk system, at the edges gapless states appear some of which have linear and others nearly flat dispersion around zero energy. This study shows the interplay between spin-orbit coupling induced spin currents, chiral edge currents and correlation driven surface magnetism. The topological nature of the chiral p-wave state manifests itself in the gamma-band characterized by an integer Chern number. As the gamma-band is close to a Lifshitz transition in Sr2RuO4, changing the sign of the Chern number, the topological nature may be rather fragile.

cond-mat.supr-con↗

arXiv2013

Thermoelectric Effect of Correlated Metals - Band Structure Effects and the Breakdown of Mott's Formula

We study the thermoelectric effect of two-dimensional metals on a square lattice within semiclassical Boltzmann transport theory with particular focus on electron-electron scattering. We compute the electrical conductivity and the Seebeck coefficient as a function of band filling and temperature for generically chosen hopping parameters in a two-dimensional tight-binding model. The Boltzmann equation is solved numerically after computing the full collision integral, taking the angular and radial degrees of freedom into account. These degrees of freedom of the collision integral, neglected in the standard single-relaxation-time approximation, play an important role if the transport coefficients show unconventional features. Within our detailed numerical simulation, we show that the widely used Mott formula to compute the Seebeck effect is not sufficient to describe the thermoelectric effect in the presence of strong electron-electron scattering. Furthermore, we study the Seebeck coefficient and its temperature dependence in the vicinity of a Lifshitz transition and demonstrate that it shows remarkable parallels to transport features near a quantum critical point.

cond-mat.str-el↗

arXiv2013

Study of interface phenomena in a topological-insulator/Mott-insulator heterostructure

We theoretically investigate a two-dimensional heterostructure composed of a topological insulator (TI) and a Mott insulator (MI), and clarify what kind of electronic states can be realized at the interface. By using inhomogeneous dynamical mean-field theory, we confirm that the topological edge state penetrating into the MI region induces a heavy-fermion like mid-gap state. We further elucidate the nature of the spatially-modulated quasi-particle weight of the mid-gap state, and discuss the effects of local correlation in the TI region. The optical conductivity and the Drude weight are also computed with changing the electron tunneling near the interface.

cond-mat.str-el↗

arXiv2013

Quantum transport signatures of chiral edge states in Sr$_2$RuO$_4$

We investigate transport properties of a double quantum dot based Cooper pair splitter, where the superconducting lead consists of Sr$_2$RuO$_4$. The proposed device can be used to explore the symmetry of the superconducting order parameter in Sr$_2$RuO$_4$ by testing the presence of gapless chiral edge states, which are predicted to exist if the bulk superconductor is described by a chiral $p$--wave state. The odd orbital symmetry of the bulk order parameter ensures that we can realize a regime where the electrons tunneling into the double dot system come from the chiral edge states and thereby leave their signature in the conductance. The proposed Cooper pair splitter has the potential to probe order parameters in unconventional superconductors.

cond-mat.supr-con↗

arXiv2013

Releasing half-quantum vortices via the coupling of spin polarization, charge- and spin-current

Motivated by recent experiments observing half-integer flux quanta in mesoscopic loops of superconducting Sr2RuO4, a theory is developed based on a "d-soliton", a topological defect of spin-triplet superconductors involving a change of the spin configuration. A phenomenological Ginzburg-Landau free energy is given which allows to describe such a soliton yielding a half-integer flux quantum inside a loop, assuming a chiral p-wave superconducting phase. The d-soliton couples to spin polarization perpendicular to the loop axis, which allows to reduce the energy of the d-soliton, fitting well to the experimental findings. The origin of this coupling lies in the combination of spin and charge current within the d-soliton generating a spin polarization analogous to a spin Hall effect. The effect of Fermi liquid corrections is discussed, in particular, in view of the spatial extension of the d-soliton and the energetics in comparison with a standard integer flux state.

cond-mat.supr-con↗

arXiv2013

Spin Currents and Spontaneous Magnetization at Twin Boundaries of Noncentrosymmetric Superconductors

Twin boundaries are generic crystalline defects in noncentrosymmetric crystal structures. We study theoretically twin boundaries in time-reversal symmetric noncentrosymmetric superconductors that admit parity-mixed Cooper pairing. Twin boundaries support spin currents as a consequence of this parity mixing. If the singlet and triplet components of the superconducting order parameter are of comparable magnitude, the superconducting state breaks spontaneously the bulk time-reversal symmetry locally near the twin boundary. By self-consistently evaluating the Bogoliubov-de-Gennes equations and the gap functions we find two distinct phases: First, time-reversal symmetry breaking enhances the spin currents but does not lead to chiral supercurrents. A secondary phase transition then triggers a spin magnetization and an orbital supercurrent near the twin boundary.

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

Proximity effects in a topological-insulator/Mott-insulator heterostructure

We investigate proximity effects in a correlated heterostructure of a two-dimensional Mott insulator (MI) and a topological insulator (TI) by employing inhomogeneous dynamical mean-field theory. We show that the edge state of the TI induces strongly renormalized mid-gap states inside the MI region, which still have a remnant of the helical energy-spectrum. The penetration of low-energy electrons, which is controlled by the interface tunneling $V$, largely enhances the electron mass inside the MI and also splits a single Dirac-cone at edge sites into the spatially-separated two Dirac-cones in the strong $V$ region.

cond-mat.str-el↗

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↗

arXiv2013

Numerical Study of Charge Transport of Overdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ within Semiclassical Boltzmann Transport Theory

The in-plane resistivity of the high-temperature oxide superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$ [LSCO] shows a strong growth of a contribution linear in temperature as the doping is reduced in the overdoped region toward optimal. This linear term is a signature of non-Fermi liquid behavior. We find that the appearance of a linear term in the resistivity can arise in a semiclassical Boltzmann transport theory which uses renormalized quasiparticle scattering rates and an empirical band structure fitted to ARPES data on LSCO. The linearized Boltzmann equation is solved numerically by discretizing the Brillouin zone in a way that fits best to the Fermi surface geometry. The main trends in the development of the anomalous temperature dependence are well reproduced. There is a substantial underestimation of the magnitude of the resistivity which is expected in view of the moderate to weak values we chose for the onsite repulsion to stay within the one-loop renormalization group approximation. The analysis was extended to the Seebeck coefficient with similar agreement with the main trends in the data.

cond-mat.str-el↗

arXiv2013

Reply to "Comment on 'Z2-slave-spin theory for strongly correlated fermions' "

We show that the physical subspace in the Z2-slave-spin theory is conserved under the time evolution of the system. Thus, when restricted to the physical subspace, this representation gives a complete and consistent description of the original problem. In addition, we review two known examples from the existing literature in which the projection onto the physical subspace can be relaxed: (i) the non-interacting limit in any dimension at half filling and (ii) the interacting model in the infinite dimensional limit at half filling. In both cases, physical observables are correctly obtained without explicit treatment of the constraints which define the physical subspace. In these examples, correct results are obtained, despite the fact that unphysical states enter the solution.

cond-mat.str-el↗

arXiv2012

From the Cooper problem to canted supersolids in Bose-Fermi mixtures

We calculate the phase diagram of the Bose-Fermi Hubbard model on the 3d cubic lattice at fermionic half filling and bosonic unit filling by means of single-site dynamical mean-field theory. For fast bosons, this is equivalent to the Cooper problem in which the bosons can induce s-wave pairing between the fermions. We also find miscible superfluid and canted supersolid phases depending on the interspecies coupling strength. In contrast, slow bosons favor fermionic charge density wave structures for attractive fermionic interactions. These competing instabilities lead to a rich phase diagram within reach of cold gas experiments.

cond-mat.quant-gas↗

arXiv2012

Model for magnetic flux pattern induced by in-plane magnetic fields on spatially inhomogeneous superconducting interfaces of LaAlO_3-SrTiO_3

The effect of spatial inhomogeneity on the properties of a two-dimensional non-centrosymmetric superconductor in an in-plane magnetic field is investigated, as it can be realized in LaAlO_3-SrTiO_3 interfaces. We demonstrate that the spatial variation of Rashba spin-orbit coupling (RSOC) yields a local magnetic flux pattern due to the field-induced inhomogeneous helical phase. For sufficiently strong fields, vortices can nucleate at inhomogeneities of the RSOC.

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

Possible pairing symmetries in SrPtAs with a local lack of inversion center

We discuss possible pairing symmetries in the hexagonal pnictide superconductor SrPtAs. The local lack of inversion symmetry of the two distinct conducting layers in the unit cell results in a special spin-orbit coupling with a staggered structure. We classify the pairing symmetry by the global crystal point group D_3d, and suggest some candidates for the stable state using a tight-binding model with an in-plane, density-density type pairing interaction. We may have some unconventional states like s+f-wave and a mixture of chiral d-wave and chiral p-wave. The spin orbit coupling is larger than the interlayer hopping, and the mixing between spin-singlet and triplet states can be seen in spite of the fact that the system has a global inversion center.

cond-mat.supr-con↗

arXiv2012

Superfluid properties of one-component Fermi gas with an anisotropic p-wave interaction

We investigate superfluid properties and strong-coupling effects in a one-component Fermi gas with an anisotropic p-wave interaction. Within the framework of the Gaussian fluctuation theory, we determine the superfluid transition temperature $T_{\rm c}$, as well as the temperature $T_0$ at which the phase transition from the $p_x$-wave pairing state to the $p_x+ip_y$-wave state occurs below $T_{\rm c}$. We also show that while the anisotropy of the p-wave interaction enhances $T_{\rm c}$ in the strong-coupling regime, it suppresses $T_0$.

cond-mat.quant-gas↗

arXiv2012

Properties of edge states in spin-triplet two-band superconductor

Motivated by Sr2RuO4 the magnetic properties of edge states in a two-band spin-triplet superconductor with electron- and hole-like Fermi surfaces are investigated assuming chiral p-wave pairing symmetry. The two bands correspond to the alpha-beta-bands of Sr2RuO4 and are modeled within a tight-binding model including inter-orbital hybridization and spin-orbit coupling effects. Including superconductivity the quasiparticle spectrum is determined by means of a self-consistent Bogolyubov-de Gennes calculation. While a full quasiparticle excitation gap appears in the bulk, gapless states form at the edges which produce spontaneous spin and/or charge currents. The spin current is the result of the specific band structure while the charge current originates from the superconducting condensate. Together they induce a small spin polarization at the edge. Furthermore onsite Coulomb repulsion is included to show that the edge states are unstable against the formation of a Stoner-like spin polarization of the edge states. Through spin-orbit coupling the current- and the correlation-induced magnetism are coupled to the orientation of the chirality of the superconducting condensate. We speculate that this type of phenomenon could yield a compensation of the magnetic fields induced by currents and also explain the negative result in the recent experimental search for chiral edge currents.

cond-mat.str-el↗