arXiv subjects

Robert Peters

Publications and source records attributed to Robert Peters.

At least 55 records · Page 3Linked to original sources

arXiv2018

Exceptional rings in two-dimensional correlated systems with chiral symmetry

Emergence of exceptional points in two dimensions is one of the remarkable phenomena in non-Hermitian systems. We here elucidate the impacts of symmetry on the non-Hermitian physics. Specifically, we analyze chiral symmetric correlated systems in equilibrium where the non-Hermitian phenomena are induced by the finite lifetime of quasi-particles. Intriguingly, our analysis reveals that the combination of symmetry and non-Hermiticity results in novel topological degeneracies of energy bands which we call symmetry-protected exceptional rings (SPERs). We observe the emergence of SPERs by analyzing a non-Hermitian Dirac Hamiltonian. Furthermore, by employing the dynamical mean-field theory, we demonstrate the emergence of SPERs in a correlated honeycomb lattice model whose single-particle spectrum is described by a non-Hermitian Dirac Hamiltonian. We uncover that the SPERs survive even beyond the non-Hermitian Dirac Hamiltonian, which is related to a zero-th Chern number.

cond-mat.str-el↗

arXiv2018

Interplay between charge, magnetic and superconducting order in a Kondo lattice with an attractive Hubbard interaction

We investigate the competition between superconductivity, charge-ordering, magnetic-ordering, and the Kondo effect in a heavy fermion $s$-wave superconductor described by a Kondo lattice model with an attractive on-site Hubbard interaction. The model is solved using the real-space dynamical mean field theory. For this purpose, we develop a numerical renormalization group (NRG) framework in Nambu space, which is used to solve the superconducting impurity problem. This extended NRG scheme also allows for SU(2) spin symmetry broken solutions, enabling us to examine the competition or cooperation between $s$-wave superconductivity and incommensurate spin-density waves (SDWs). At half filling, we find an intriguing phase where the magnetic ordering of the $f$-electrons lifts the degeneracy between the charge density wave (CDW) state and the superconducting state, leading to a strong suppression of superconductivity. In addition, the system may also become a half metal in this parameter regime. Away from half filling, the CDWs vanish and are replaced by superconductivity combined with incommensurate SDWs up to moderate Kondo couplings to the $f$-electrons. We find that both CDWs as well as superconductivity enhance magnetic ordering due to the suppression of Kondo screening.

cond-mat.str-el↗

arXiv2018

Direct Evidence for the Existence of Heavy Quasiparticles in the Magnetically Ordered Phase of CeRhIn$_5$

It is a long-standing important issue in heavy fermion physics whether $f$-electrons are itinerant or localized when the magnetic order occurs. Here we report the {\it in situ} scanning tunneling microscopy observation of the electronic structure in epitaxial thin films of CeRhIn$_5$, a prototypical heavy fermion compound with antiferromagnetic ground state. The conductance spectra above the Néel temperature $T_N$ clearly resolve the energy gap due to the hybridization between local 4$f$ electrons and conduction bands as well as the crystal electric field excitations. These structures persist even below $T_N$. Moreover, an additional dip in the conductance spectra develops due to the antiferromagnetic order. These results provide direct evidence for the presence of itinerant heavy $f$-electrons participating in the Fermi surface even in the magnetically ordered state of CeRhIn$_5$.

cond-mat.str-el↗

arXiv2018

Breakdown of topological Thouless pumping in the strongly interacting regime

We elucidate the mechanism for instability of topological Thouless pumping in strongly interacting systems from a viewpoint of symmetry-protected topological phases. If the protecting symmetries of the underlying topological phases change between noninteracting fermions and a bosonic system in the strong coupling limit, the symmetry protection argument enforces a gap closing and thereby predicts a breakdown of the topological pumping. We also demonstrate that, even in the weakly interacting regime where the bulk topological pumping is still robust, the interaction effects manifest themselves in the edge density profiles, leading to a unique feature of the pumping in open boundary conditions. Furthermore, an extension of the above results indicates that an analog of an interaction-induced phase of Weyl semimetals can be realized in the setup of the topological pumping. Our results provide a systematic understanding for the stability of topological pumping against strong interactions, to which the conventional perturbative argument cannot be applied.

cond-mat.str-el↗

arXiv2018

Non-Hermitian perspective of the band structure in heavy-fermion systems

We analyze a two-dimensional Kondo lattice model with special emphasis on non-Hermitian properties of the single-particle spectrum, following a recent proposal by Kozii and Fu. Our analysis based on the dynamical mean-field theory elucidates that the single-particle spectral weight shows the exceptional points (EPs). Correspondingly, the spectral weight exhibits the band touching, resulting in a structure similar to the "Fermi arc". Furthermore, we find an intriguing phenomenon arising from the periodicity of the lattice. The EPs generated by two distinct Dirac points merge and change into a hybrid point which vanishes as the exchange interaction is increased. Accordingly, the paramagnetic phase in the low temperature region shows a significant difference from non-interacting fermions: the imaginary part of the self-energy yields the "Fermi loop" without any defective points.

cond-mat.str-el↗

arXiv2018

Magnetic states in a three-dimensional topological Kondo insulator

We theoretically study the magnetic phase diagram of a three-dimensional topological Kondo insulator by means of real-space dynamical mean field theory. We find that ferromagnetically ordered states become stable upon hole doping. Besides a wide ferromagnetic phase, we observe surface magnetism close to half-filling, which corresponds to an A-type antiferromagnetic state. We further study the impact of the magnetism on the symmetry protected surface states and find that depending on the surface and the magnetization direction, surface states are still protected by reflection symmetry present in our model. The symmetry protected surface states are shifted away by the magnetization from their original high symmetry momenta in the Brillouin zone. Remarkably, due to the magnetization, the surface states are deformed, resulting in the appearance of arcs in the momentum resolved spectrum.

cond-mat.str-el↗

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

In Situ STM Observation of Nonmagnetic Impurity Effect in MBE-grown CeCoIn$_5$ Films

Local electronic effects in the vicinity of an impurity provide pivotal insight into the origin of unconventional superconductivity, especially when the materials are located on the edge of magnetic instability. In high-temperature cuprate superconductors, a strong suppression of superconductivity and appearance of low-energy bound states are clearly observed near nonmagnetic impurities. However, whether these features are common to other strongly correlated superconductors has not been established experimentally. Here, we report the {$in$} {$situ$} scanning tunneling microscopy observation of electronic structure around a nonmagnetic Zn impurity in heavy-fermion CeCo(In$_{1-x}$Zn$_x$)$_5$ films, which are epitaxially grown by the state-of-the-art molecular beam epitaxy technique. The films have very wide atomically flat terraces and Zn atoms residing on two different In sites are clearly resolved. Remarkably, no discernible change is observed for the superconducting gap at and around the Zn atoms. Moreover, the local density of states around Zn atoms shows little change inside the $c$-$f$ hybridization gap, which is consistent with calculations for a periodic Anderson model without local magnetic order. These results indicate that no nonsuperconducting region is induced around a Zn impurity and do not support the scenario of antiferromagnetic droplet formation suggested by indirect measurements in Cd-doped CeCoIn$_5$. These results also highlight a significant difference of the impurity effect between cuprates and CeCoIn$_5$, in both of which $d$-wave superconductivity arises from the non-Fermi liquid normal state near antiferromagnetic instabilities.

cond-mat.str-el↗

arXiv2017

Reduction of topological $\mathbb{Z}$ classification in cold atomic systems

One of the most challenging problems in correlated topological systems is a realization of the reduction of topological classification, but very few experimental platforms have been proposed so far. We here demonstrate that ultracold dipolar fermions (e.g., $^{167}$Er, $^{161}$Dy, and $^{53}$Cr) loaded in an optical lattice of two-leg ladder geometry can be the first promising testbed for the reduction $\mathbb{Z}\to\mathbb{Z}_4$, where solid evidence for the reduction is available thanks to their high controllability. We further give a detailed account of how to experimentally access this phenomenon; around the edges, the destruction of one-particle gapless excitations can be observed by the local radio frequency spectroscopy, while that of gapless spin excitations can be observed by a time-dependent spin expectation value of a superposed state of the ground state and the first excited state. We clarify that even when the reduction occurs, a gapless edge mode is recovered around a dislocation, which can be another piece of evidence for the reduction.

cond-mat.str-el↗

arXiv2017

Competition of striped magnetic order and partial Kondo screened state in the Kondo lattice model

We analyze the magnetically ordered phases in the Kondo lattice model on a square lattice around quarter filling using the dynamical mean field theory. We find that close to quarter filling besides the paramagnetic phase, at least three magnetic phases compete. These phases are a ferromagnetic state, a partial Kondo screened state, which is a combination of charge order and magnetic order, and a striped magnetic phase, which is ferromagnetically ordered in one direction and antiferromagnetically in the other direction. We analyze the spectral properties of these states and show that the PKS state is insulating with a flat band above the Fermi energy, while the other states are metallic. Furthermore, we demonstrate that the energy gain by the Kondo effect is larger in the PKS state, while the striped magnetic state can gain more energy from the RKKY interaction. Thus, while the striped magnetic state is stable at weak coupling, the PKS state becomes stable at intermediate coupling.

cond-mat.str-el↗

arXiv2016

Magnetism in f electron superlattices

We analyze antiferromagnetism in $f$ electron superlattices. We show that the competition between the Kondo effect and the RKKY interaction in $f$ electron materials is modified by the superlattice structure. Thus, the quantum critical point which separates the magnetic phase and the Fermi liquid phase depends on the structure of the $f$ electron superlattice. The competition between the Kondo effect and the RKKY interaction is also reflected in the magnetic interlayer coupling between different $f$ electron layers. We demonstrate that in the case of weak Kondo effect the magnetic interlayer coupling behaves similar to other magnetic heterostructures without Kondo effect. However, close to the quantum phase transition, the dependence of the interlayer coupling on the distance between the $f$ electron layers is modified by the Kondo effect. Another remarkable effect, which is characteristic for $f$ electron superlattice, is that the magnetic interlayer coupling does vanish stepwise depending on the distance between different $f$ electron layers. As a consequence, the quantum critical point depends also stepwise on this distance.

cond-mat.str-el↗

arXiv2016

Coexistence of light and heavy surface states in a topological multi-band Kondo insulator

We analyze the impact of strong correlations on the surface states of a three-dimensional topological Kondo insulator. An important observation is that correlations are strongly increased at the surface, which leads to an energetical confinement of the surface $f$-electrons into a narrow window around the Fermi energy at low enough temperature. This correlation effect in combination with the nontrivial topology has two remarkable consequences: (i) coexistence of light and heavy surface states at low temperatures. While heavy surface states are formed directly in the surface layer, light surface states are formed in the next-nearest surface layer. Furthermore, (ii) with increasing the temperature, the heavy surface states become incoherent and only light surface states can be observed. The coexistence of light and heavy surface states is thereby a remarkable characteristic of the combination of strong correlations and nontrivial topology. We believe that these results are applicable to the candidate topological Kondo insulator SmB6.

cond-mat.str-el↗

arXiv2015

Restoration of topological properties at finite temperatures in a heavy-fermion system

We study how topological phases evolve in the Kane-Mele-Kondo lattice at finite temperatures and obtain the topological Doniach phase diagram. In particular, we find an intriguing crossover behavior induced by the interplay between the topological structure and electron correlations; the topological properties are restored by temperature effects. This restoration can be observed in the behavior of the bulk as well as the edge. In the bulk, we observe an increase of the spin-Hall conductivity at finite temperatures, while it is zero in the low temperature region. At the edge, we observe gapless edge modes emerging with increasing temperature.

cond-mat.str-el↗

arXiv2015

Large and Small Fermi-Surface Spin Density Waves in the Kondo Lattice Model

We demonstrate the existence of metallic spin density waves (SDWs) in the Kondo lattice model on a square lattice for a wide range of parameters by means of real space dynamical mean field theory. In these SDWs, the spin polarization as well as the charge density depend on the lattice site and are modulated along one direction of the square lattice. We show that within this phase of metallic SDWs the Fermi surface changes from small to large, when the coupling strength is increased. Furthermore, the transition between the large Fermi-surface SDW phase and the paramagnetic phase is of second order, while the transition between the small Fermi-surface SDW phase and the paramagnetic phase is of first order. A local quantum critical point is thus avoided in our calculations by undergoing a first order phase transition.

cond-mat.str-el↗

arXiv2015

Spin fluctuations and superconductivity in layered $f$-electron superlattices

We investigate magnetic and superconducting properties of layered $f$-electron superlattices within the fluctuation exchange approximation (FLEX). We show that spin fluctuations, which are characterized by the maximum value of the spin susceptibility in the 3-dimensional (3D) Brillouin zone, are strongly suppressed in $f$-electron superlattices. However, effective 2D spin fluctuations can be increased due to the spatial confinement of the $f$-electrons. Therefore, the tendency towards $d_{x^2-y^2}$-wave superconductivity, mediated by these spin fluctuations, can be strongly increased in $f$-electron-superlattices. This is in sharp contrast to superlattices composed of conventional $s$-wave superconductors, where superconductivity is generally suppressed.

cond-mat.supr-con↗

arXiv2015

Influence of Slip on the Plateau-Rayleigh Instability on a Fibre

The Plateau-Rayleigh instability of a liquid column underlies a variety of fascinating phenomena that can be observed in everyday life. In contrast to the case of a free liquid cylinder, describing the evolution of a liquid layer on a solid fibre requires consideration of the solid-liquid interface. In this article, we revisit the Plateau-Rayleigh Instability of a liquid coating a fibre by varying the hydrodynamic boundary condition at the fibre-liquid interface, from no-slip to slip. While the wavelength is not sensitive to the solid-liquid interface, we find that the growth rate of the undulations strongly depends on the hydrodynamic boundary condition. The experiments are in excellent agreement with a new thin film theory incorporating slip, thus providing an original, quantitative and robust tool to measure slip lengths.

physics.flu-dyn↗

arXiv2015

Local origin of the pseudogap in the attractive Hubbard model

We provide a new perspective on the pseudogap physics for attractive fermions as described by the three-dimensional Hubbard model. The pseudogap in the single-particle spectral function, which occurs for temperatures above the critical temperature $T_c$ of the superfluid transition, is often interpreted in terms of preformed, uncondensed pairs. Here we show that the occurrence of pseudogap physics can be consistently understood in terms of local excitations which lead to a splitting of the quasiparticle peak for sufficiently large interaction. This effect becomes prominent at intermediate and high temperatures when the quantum mechanical hopping is incoherent. We clarify the existence of a conjectured temperature below which pseudogap physics is expected to occur. Our results are based on approximating the physics of the three-dimensional Hubbard model by dynamical mean field theory calculations and a momentum independent self-energy. Our predictions can be tested with ultracold atoms in optical lattices with currently available temperatures and spectroscopic techniques.

cond-mat.supr-con↗

arXiv2014

Characterization of a topological Mott insulator in one dimension

We investigate properties of a topological Mott insulator in one dimension by examining the bulk topological invariant and the entanglement spectrum of a correlated electron model. We clarify how gapless edge states in a non-interacting topological band insulator evolve into spinon edge states in a topological Mott insulator. Furthermore, we propose a topological Mott transition, which is a new type of topological phase transition and never observed in free fermion systems. This unconventional transition occurs in spin liquid phases in the Mott insulator and is accompanied by zeros of the single-electron Green's function and a gap closing in the spin excitation spectrum.

cond-mat.str-el↗