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Manfred Sigrist

Publications and source records attributed to Manfred Sigrist.

At least 109 records · Page 6Linked to original sources
arXiv2012

Electronic and magnetic properties in strongly correlated heterostructures

We present a theoretical study of a model heterostructure for a Mott-insulator sandwiched between two band insulators, such as SrTiO3/LaTiO3. Particular emphasis is given on the interplay between magnetism and inhomogeneous charge distributions. Our mean-field analysis of the generalized Hubbard model displays numerous ordered phases in the ground-state phase diagram. In particular, we find a canted antiferromagnetic state near the interface when antiferromagnetic-ordering exists inside the Mott insulator. A checkerboard charge-ordering proposed previously is also stabilized for large long-range Coulomb interactions. Regarding its origin we also point out the importance of interlayer spin-mediated interactions. It is further shown that such a strong spin-charge coupling gives rise to pronounced magnetic/charge order phase transitions in external magnetic fields: a firstorder metamagnetic transition and a reentrant charge-order transition with checkerboard pattern. The mechanisms stabilizing these intriguing phases are explored through a detailed analysis of the physical quantities with special focus on the spin-charge interplay.

cond-mat.str-el↗
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

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

Coexistence of Ferromagnetism and Superconductivity in Noncentrosymmetric Materials with Cubic Symmetry

This is a model study for the emergence of superconductivity in ferromagnetically ordered phases of cubic materials whose crystal structure lacks inversion symmetry. A Ginzburg-Landau-type theory is used to find the ferromagnetic state and to determine the coupling of magnetic order to superconductivity. It is found that noncentrosymmetricity evokes a helical magnetic phase. If the wavelength of the magnetic order is long enough, it gives rise to modulations of the order parameter of superconductivity, both in modulus and complex phase. At magnetic domain walls the nucleation of superconductivity is found to be suppressed as compared to the interior of ferromagnetic domains.

cond-mat.supr-con↗
arXiv2011

Superconductivity and local non-centrosymmetricity in crystal lattices

Symmetry of the crystal lattice can be a determining factor for the structure of Cooper pairs in unconventional superconductors. In this study we extend the discussion of superconductivity in non-centrosymmetric materials to the case when inversion symmetry is missing locally, but is present on a global level. Concretely, we investigate the staggered non-centrosymmetricity within a regular sublattice structure, in some analogy to the discussion of superconductivity in antiferromagnetic systems. Three crystal structures are analyzed in detail as illustrative examples for the extended classification of Cooper-pairing channels. One of the cases may be relevant for the class of iron-pnictide superconductors.

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↗
arXiv2011

Antiferromagnetic Phases in the Fulde-Ferrell-Larkin-Ovchinnikov State of CeCoIn_5

The antiferromagnetic (AFM) order in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state is analyzed on the basis of a Ginzburg-Landau theory. To examine the possible AFM-FFLO state in CeCoIn_5, we focus on the incommensurate AFM order characterized by the wave vector Q = Q_{0} \pm q_inc with Q_0 =(π,π,π) and q_inc \parallel [110] or [1-10] in the tetragonal crystal structure. We formulate the two component Ginzburg-Landau theory and investigate the two degenerate incommensurate AFM order. We show that the pinning of AFM moment due to the FFLO nodal planes leads to multiple phases in magnetic fields along [100] or [010]. The phase diagrams for various coupling constants between the two order parameters are shown for the comparison with CeCoIn_5. Experimental results of the NMR and neutron scattering measurements are discussed.

cond-mat.supr-con↗
arXiv2010

Phase transition in the 3 Kelvin phase in the eutectic Sr2RuO4-Ru

The inhomogeneous 3-Kelvin (3K) phase of the eutectic Sr2RuO4 with Ru inclusions nucleates superconductivity at the interface between Ru and Sr2RuO4. The structure of the interface state and its physical properties are examined here. Two superconducting phases are identified between the transitions to the bulk phase at 1.5K and to the 3K phase. The nucleation of the 3K phase results in a state conserving time reversal symmetry, which generates an intrinsically frustrated superconducting network in samples with many Ru inclusions. At a lower temperature (>1.5K), a discontinuous (first order) transition to an interface state breaking time reversal symmetry is found leading to an unfrustrated network phase. It is shown that this phase transition located at a temperature between 1.5 and 3K would yield the anomalous property that the critical current in such a network depends on the sign of the current, reproducing recent experimental observations.

cond-mat.supr-con↗
arXiv2010

Influence of the domain walls on the Josephson effect in Sr$_2$RuO$_4$

A detailed theoretical interpretation of the Josephson interference experiment between Sr$_2$RuO$_4$ and Pb reported by Kidwingira \textit{et al} is given. Assuming chiral p-wave pairing symmetry a Ginzburg-Landau theory is derived in order to investigate the structure of domain walls between chiral domains. It turns out that anisotropy effects of the Fermi surface and the orientation of the domain walls are essential for their internal structure. Introducing a simple model for a Josephson junction the effect of domain walls intersecting the interface between Sr$_2$RuO$_4$ and Pb is discussed. It is shown that characteristic deviations of the Fraunhofer interference pattern for the critical Josephson current as a function of the magnetic field occurs in qualitative agreement with the experimental finding. Moreover the model is able also to account for peculiar hysteresis effects observed in the experiment.

cond-mat.supr-con↗
arXiv2010

Nucleation of Vortex State in Ru-inclusion in Eutectic Ruthenium Oxide Sr2RuO4-Ru

Eutectic samples of Sr2RuO4-Ru possess many micrometer-sized Ru-metal inclusions. Based on a Ginzburg-Landau formulation we analyze the interplay of the chiral p-wave state of Sr2RuO4 (Tc = 1.5 K) and the s-wave state of Ru metal (Tc,Ru = 0.5 K) for an inclusion of cylindrical geometry. As a consequence of the mismatch of the order parameter phase, the occurrence of a spontaneous flux distribution appears for T < Tc,Ru which evolves into a "Josephson vortex" on the Sr2RuO4-Ru interface. At a sufficiently low temperature a depinning transition can occur whereby the vortex moves to the center of the cylinder.

cond-mat.supr-con↗
arXiv2010

Effect of a staggered spin-orbit coupling on the occurrence of a nematic phase in Sr$_3$Ru$_2$O$_7$

Ultra-clean crystals of Sr$_3$Ru$_2$O$_7$ undergo a metamagnetic transition at low temperatures. This transition shows a strong anisotropy in the applied field direction with the critical field $H_c$ ranging from $\sim 5.1$T for $H\perp c$ to $\sim 8$T for $H\parallel c$. In addition, studies on ultra-pure samples revealed a bifurcation of the metamagnetic line for fields in $c$-direction and it is argued that a nematic phase emerges between the magnetization jumps. The aim of this study is to explain the field-direction anisotropy of these phenomena. Based on a microscopic tight-binding model, we introduce the metamagnetic transition by means of a van Hove singularity scenario. We show that the rotation of the O-octahedra around the c-axis expected for this material introduces a staggered spin-orbit coupling within the planes and naturally leads to an anisotropy of the magnetic response. We describe the low-temperature phase as a nematic state favored by forward scattering processes. The spin-orbit coupling shows an influence on both, the critical field $H_c$ and the occurrence of the nematic phase.

cond-mat.str-el↗
arXiv2010

Dynamical Unbinding Transition in a Periodically Driven Mott Insulator

We study the double occupancy in a fermionic Mott insulator at half-filling generated via a dynamical periodic modulation of the hopping amplitude. Tuning the modulation amplitude, we describe a crossover in the nature of doublon-holon excitations from a Fermi Golden Rule regime to damped Rabi oscillations. The decay time of excited states diverges at a critical modulation strength, signaling the transition to a dynamically bound non-equilibrium state of doublon-holon pairs. A setup using a fermionic quantum gas should allow to study the critical exponents.

cond-mat.quant-gas↗
arXiv2009

Probing the d_{x2-y2}-wave Pomeranchuk instability by ultrasound

Selection rules of ultrasound attenuation and sound velocity renormalization are analyzed in view of their potential application to identify Pomeranchuk instabilities (electronic nematic phase). It is shown that the transverse sound attenuation along [110] direction is enhanced by the Fermi surface fluctuations near a d_{x2-y2}-wave Pomeranchuk instability, while the attenuation along [100] direction remains unaffected. Moreover the fluctuation regime above the instability is analyzed by means of a self-consistent renormalization scheme. The results could be applied directly to Sr3Ru2O7 which is a potential candidate for a Pomeranchuk instability at its metamagnetic transition in strong magnetic fields.

cond-mat.str-el↗
arXiv2009

Antiferromagnetic Order and π-triplet Pairing in the Fulde-Ferrell-Larkin-Ovchinnikov State

The antiferromagnetic Fulde-Ferrell-Larkin-Ovchinnikov (AFM-FFLO) state of coexisting d-wave FFLO superconductivity and incommensurate AFM order is studied on the basis of Bogoliubov-de Gennes (BdG) equations. We show that the incommensurate AFM order is stabilized in the FFLO state by the appearance of the Andreev bound state localized around the zeros of the FFLO order parameter. The AFM-FFLO state is further enhanced by the induced π-triplet superconductivity (pair density wave). The AFM order occurs in the FFLO state even when it is neither stable in the normal state nor in the BCS state. The order parameters of the AFM order, d-wave superconductivity, and π-triplet pairing are investigated by focusing on their spatial structures. Roles of the spin fluctuations beyond the BdG equations are discussed. Their relevance to the high-field superconducting phase of CeCoIn_5 is discussed.

cond-mat.supr-con↗
arXiv2009

Electronic transport properties of graphene nanoribbons

We will present brief overview on the electronic and transport properties of graphene nanoribbons focusing on the effect of edge shapes and impurity scattering. The low-energy electronic states of graphene have two non-equivalent massless Dirac spectrum. The relative distance between these two Dirac points in the momentum space and edge states due to the existence of the zigzag type graphene edges are decisive to the electronic and transport properties of graphene nanoribbons. In graphene nanoribbons with zigzag edges, two valleys related to each Dirac spectrum are well separated in momentum space. The propagating modes in each valley contain a single chiral mode originating from a partially flat band at band center. This feature gives rise to a perfectly conducting channel in the disordered system, if the impurity scattering does not connect the two valleys, i.e. for long-range impurity potentials. On the other hand, the low-energy spectrum of graphene nanoribbons with armchair edges is described as the superposition of two non-equivalent Dirac points of graphene. In spite of the lack of well-separated two valley structures, the single-channel transport subjected to long-ranged impurities is nearly perfectly conducting, where the backward scattering matrix elements in the lowest order vanish as a manifestation of internal phase structures of the wavefunction. Symmetry considerations lead to the classification of disordered zigzag ribbons into the unitary class for long-range impurities, and the orthogonal class for short-range impurities. However, no such crossover occurs in armchair nanoribbons.

cond-mat.mes-hall↗
arXiv2008

Role of multiple subband renormalization in the electronic transport of correlated oxide superlattices

Metallic behavior of band-insulator/ Mott-insulator interfaces was observed in artificial perovskite superlattices such as in nanoscale SrTiO3/LaTiO3 multilayers. Applying a semiclassical perspective to the parallel electronic transport we identify two major ingredients relevant for such systems: i) the quantum confinement of the conduction electrons (superlattice modulation) leads to a complex, quasi-two dimensional subband structure with both hole- and electron-like Fermi surfaces. ii) strong electron-electron interaction requires a substantial renormalization of the quasi-particle dispersion. We characterize this renormalization by two sets of parameters, namely, the quasi-particle weight and the induced particle-hole asymmetry of each partially filled subband. In our study, the quasi-particle dispersion is calculated self-consistently as function of microscopic parameters using the slave-boson mean-field approximation introduced by Kotliar and Ruckenstein. We discuss the consequences of strong local correlations on the normal-state free-carrier response in the optical conductivity and on the thermoelectric effects.

cond-mat.str-el↗