arXiv subjects

Peter Horsch

Publications and source records attributed to Peter Horsch.

At least 19 recordsLinked to original sources

arXiv2020

Orbital Rotations induced by Charges of Polarons and Defects in Doped Vanadates

We explore the competiton of doped holes and defects that leads to the loss of orbital order in vanadate perovskites. In compounds such as La$_{1-{\sf x}}$Ca$_{\,\sf x}$VO$_3$ spin and orbital order result from super-exchange interactions described by an extended three-orbital degenerate Hubbard-Hund model for the vanadium $t_{2g}$ electrons. Long-range Coulomb potentials of charged Ca$^{2+}$ defects and $e$-$e$ interactions control the emergence of defect states inside the Mott gap. The quadrupolar components of the Coulomb fields of doped holes induce anisotropic orbital rotations of degenerate orbitals. These rotations modify the spin-orbital polaron clouds and compete with orbital rotations induced by defects. Both mechanisms lead to a mixing of orbitals, and cause the suppression of the asymmetry of kinetic energy in the $C$-type magnetic phase. We find that the gradual decline of orbital order with doping, a characteristic feature of the vanadates, however, has its origin not predominantly in the charge carriers, but in the off-diagonal couplings of orbital rotations induced by the charges of the doped ions.

cond-mat.str-el↗

arXiv2019

Defect-induced orbital polarization and collapse of orbital order in doped vanadium perovskites

We explore mechanisms of orbital order decay in doped Mott insulators $R_{1-x}$(Sr,Ca)$_x$VO$_3$ ($R=\,$Pr,Y,La) caused by charged (Sr,Ca) defects. Our unrestricted Hartree-Fock analysis focuses on the combined effect of random, charged impurities and associated doped holes up to $x=0.5$. The study is based on a generalized multi-band Hubbard model for the relevant vanadium $t_{2g}$ electrons, and includes the long-range (i) Coulomb potentials of defects and (ii) electron-electron interactions. We show that the rotation of occupied $t_{2g}$ orbitals, induced by the electric field of defects, is a very efficient perturbation that largely controls the suppression of orbital order in these compounds. We investigate the inverse participation number spectra and find that electron states remain localized on few sites even in the regime where orbital order is collapsed. From the change of kinetic and superexchange energy we can conclude that the motion of doped holes, which is the dominant effect for the reduction of magnetic order in high-$T_c$ compounds, is of secondary importance here.

cond-mat.str-el↗

arXiv2017

Fingerprints of spin-orbital polarons and of their disorder in the photoemission spectra of doped Mott insulators with orbital degeneracy

We explore the effects of disordered charged defects on the electronic excitations observed in the photoemission spectra of doped transition metal oxides in the Mott insulating regime by the example of the $R_{1-x}$Ca$_x$VO$_3$ perovskites, where $R=$La,$\dots$,Lu. A fundamental characteristic of these vanadium $d^2$ compounds with partly filled $t_{2g}$ valence orbitals is the persistence of spin and orbital order up to high doping, in contrast to the loss of magnetic order in high-$T_c$ cuprates at low defect concentration. We demonstrate that the disordered electronic structure of doped Mott-Hubbard insulators can be obtained with high precision within the unrestricted Hartree-Fock approximation. In particular: (i) the atomic multiplet excitations in the inverse photoemission spectra and the various defect-related states and satellites are well reproduced, (ii) a robust Mott gap survives up to large doping, and (iii) we show that the defect states inside the Mott gap develop a soft gap at the Fermi energy. The soft defect states gap can be characterized by a shape and a scale parameter extracted from a Weibull statistical sampling of the density of states near the chemical potential. We demonstrate that charge defects trigger small spin-orbital polarons, with their internal kinetic energy responsible for the opening of the soft defect states gap. The small size of spin-orbital polarons is inferred by an analysis of the inverse participation ratio which explains the origin of the robustness of spin and orbital order. Using realistic parameters for La$_{1-x}$Ca$_x$VO$_3$, we show that its soft gap is well reproduced as well as the marginal doping dependence of the position of the chemical potential relative to the center of the lower Hubbard band.

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arXiv2015

Defects, disorder and strong electron correlations in orbital degenerate, doped Mott insulators

We elucidate the effects of defect disorder and $e$-$e$ interaction on the spectral density of the defect states emerging in the Mott-Hubbard gap of doped transition-metal oxides, such as Y$_{1-x}$Ca$_{x}$VO$_{3}$. A soft gap of kinetic origin develops in the defect band and survives defect disorder for $e$-$e$ interaction strengths comparable to the defect potential and hopping integral values above a doping dependent threshold, otherwise only a pseudogap persists. These two regimes naturally emerge in the statistical distribution of gaps among different defect realizations, which turns out to be of Weibull type. Its shape parameter $k$ determines the exponent of the power-law dependence of the density of states at the chemical potential ($k-1$) and hence distinguishes between the soft gap ($k\geq2$) and the pseudogap ($k<2$) regimes. Both $k$ and the effective gap scale with the hopping integral and the $e$-$e$ interaction in a wide doping range. The motion of doped holes is confined by the closest defect potential and the overall spin-orbital structure. Such a generic behavior leads to complex non-hydrogen-like defect states that tend to preserve the underlying $C$-type spin and $G$-type orbital order and can be detected and analyzed via scanning tunneling microscopy.

cond-mat.str-el↗

arXiv2015

Quantum entanglement in the one-dimensional spin-orbital SU(2)$\otimes XXZ$ model

We investigate the phase diagram and the spin-orbital entanglement of a one-dimensional SU(2)$\otimes XXZ$ model with SU(2) spin exchange and anisotropic $XXZ$ orbital exchange interactions and negative exchange coupling. As a unique feature, the spin-orbital entanglement entropy in the entangled ground states increases here linearly with system size. In the case of Ising orbital interactions we identify an emergent phase with long-range spin-singlet dimer correlations triggered by a quadrupling of correlations in the orbital sector. The peculiar translational invariant spin-singlet dimer phase has finite von Neumann entanglement entropy and survives when orbital quantum fluctuations are included. It even persists in the isotropic SU(2)$\otimes$SU(2) limit. Surprisingly, for finite transverse orbital coupling the long-range spin singlet correlations also coexist in the antiferromagnetic spin and alternating orbital phase making this phase also unconventional. Moreover we also find a complementary orbital singlet phase that exists in the isotropic case but does not extend to the Ising limit. The nature of entanglement appears essentially different from that found in the frequently discussed model with positive coupling. Furthermore we investigate the collective spin and orbital wave excitations of the disentangled ferromagnetic-spin/ferro-orbital ground state and explore the continuum of spin-orbital excitations. Interestingly one finds among the latter excitations two modes of exciton bound states. Their spin-orbital correlations differ from the remaining continuum states and exhibit logarithmic scaling of the von Neumann entropy with increasing system size.

cond-mat.str-el↗

arXiv2015

Entanglement Driven Phase Transitions in Spin-Orbital Models

To demonstrate the role played by the von Neumann entropy spectra in quantum phase transitions we investigate the one-dimensional anisotropic SU(2)$\otimes XXZ$ spin-orbital model with negative exchange parameter. In the case of classical Ising orbital interactions we discover an unexpected novel phase with Majumdar-Ghosh-like spin-singlet dimer correlations triggered by spin-orbital entanglement and having $k=π/2$ orbital correlations, while all the other phases are disentangled. For anisotropic $XXZ$ orbital interactions both spin-orbital entanglement and spin-dimer correlations extend to the antiferro-spin/alternating-orbital phase. This quantum phase provides a unique example of two coupled order parameters which change the character of the phase transition from first-order to continuous. Hereby we have established the von Neumann entropy spectral function as a valuable tool to identify the change of ground state degeneracies and of the spin-orbital entanglement of elementary excitations in quantum phase transitions.

cond-mat.str-el↗

arXiv2014

Exact treatment of magnetism-driven ferroelectricity in the one-dimensional compass model

We consider a class of one-dimensional compass models with antisymmetric Dzyaloshinskii-Moriya exchange interaction in an external magnetic field. Based on the exact solution derived by means of Jordan-Wigner transformation, we study the excitation gap, spin correlations, ground-state degeneracy, and critical properties at phase transitions. The phase diagram at finite electric and magnetic field consists of three phases: ferromagnetic, canted antiferromagnetic, and chiral. Dzyaloshinskii-Moriya interaction induces an electrical polarization in the ground state of the chiral phase, where the nonlocal string order and special features of entanglement spectra arise, while strong chiral correlations emerge at finite temperature in the other phases and are controlled by a gap between the nonchiral ground state and the chiral excitations. We further show that the magnetoelectric effects in all phases disappear above a typical temperature corresponding to the total bandwidth of the effective fermionic model. To this end we explore the entropy, specific heat, magnetization, electric polarization, and the magnetoelectric tensor at finite temperature. We identify rather peculiar specific-heat and polarization behavior of the compass model which follows from highly frustrated interactions.

cond-mat.str-el↗

arXiv2014

Hole propagation in the Kitaev-Heisenberg model: From quasiparticles in quantum Neel states to non-Fermi liquid in the Kitaev phase

We explore with exact diagonalization the propagation of a single hole in four magnetic phases of the t-J-like Kitaev-Heisenberg model on a honeycomb lattice: the Neel antiferromagnetic, stripe, zigzag and Kitaev spin-liquid phase. We find coherent propagation of spin-polaron quasiparticles in the antiferromagnetic phase by a similar mechanism as in the $t$-$J$ model for high-$T_c$ cuprates. In the stripe and zigzag phases clear quasiparticles features appear in spectral functions of those propagators where holes are created and annihilated on one sublattice, while they remain largely {\it hidden} in those spectral functions that correspond to photoemission experiments. As the most surprising result, we find a totally incoherent spectral weight distribution for the spectral function of a hole moving in the Kitaev spin-liquid phase in the strong coupling regime relevant for iridates. At intermediate coupling the finite systems calculation reveals a well defined quasiparticle at the $Γ$ point, however, we find that the gapless spin excitations wipe out quasiparticles at finite momenta. Also for this more subtle case we conclude that in the thermodynamic limit the lightly doped Kitaev liquid phase does not support quasiparticle states in the neighborhood of $Γ$, and therefore yields a {\it non-Fermi liquid}, contrary to earlier suggestions based on slave-boson studies. These observations are supported by the presented study of the dynamic spin-structure factor for the Kitaev spin liquid regime.

cond-mat.str-el↗

arXiv2014

Quantum phase transitions in exactly solvable one-dimensional compass models

We present an exact solution for a class of one-dimensional compass models which stand for interacting orbital degrees of freedom in a Mott insulator. By employing the Jordan-Wigner transformation we map these models on noninteracting fermions and discuss how spin correlations, high degeneracy of the ground state, and $Z_2$ symmetry in the quantum compass model are visible in the fermionic language. Considering a zigzag chain of ions with singly occupied $e_g$ orbitals ($e_g$ orbital model) we demonstrate that the orbital excitations change qualitatively with increasing transverse field, and that the excitation gap closes at the quantum phase transition to a polarized state. This phase transition disappears in the quantum compass model with maximally frustrated orbital interactions which resembles the Kitaev model. Here we find that finite transverse field destabilizes the orbital-liquid ground state with macroscopic degeneracy, and leads to peculiar behavior of the specific heat and orbital susceptibility at finite temperature. We show that the entropy and the cooling rate at finite temperature exhibit quite different behavior near the critical point for these two models.

cond-mat.str-el↗

arXiv2014

Dissipationless Spin Current between Two Coupled Ferromagnets

We demonstrate the general principle which states that a dissipationless spin current flows between two coupled ferromagnets if their magnetic orders are misaligned. This principle applies regardless the two ferromagnets are metallic or insulating, and also generally applies to bulk magnetic insulators. On a phenomenological level, this principle is analogous to Josephson effect, and yields a dissipationless spin current that is independent from scattering. The microscopic mechanisms for the dissipationless spin current depend on the systems, which are elaborated in details. A uniform, static magnetic field is further proposed to be an efficient handle to create the misaligned configuration and stabilize the dissipationless spin current.

cond-mat.mes-hall↗

arXiv2013

Hidden Quasiparticles and Incoherent Photoemission Spectra in Na2IrO3

We study two Heisenberg-Kitaev t-J-like models on a honeycomb lattice, focusing on the zigzag magnetic phase of Na$_2$IrO$_3$, and investigate hole motion by exact diagonalization and variational methods. The spectral functions are quite distinct from those of cuprates and are dominated by large incoherent spectral weight at high energy, almost independent of the microscopic parameters --- a universal and generic feature for zigzag magnetic correlations. We explain why quasiparticles at low energy are strongly suppressed in the photoemission spectra and determine an analog of a pseudogap. We point out that the qualitative features of the predominantly incoherent spectra obtained within the two different models for the zigzag phase are similar, and they have remarkable similarity to recently reported angular resolved photoemission spectra for Na$_2$IrO$_3$.

cond-mat.str-el↗

arXiv2013

Defect states and excitations in a Mott insulator with orbital degrees of freedom: Mott-Hubbard gap versus optical and transport gaps in doped systems

We address the role played by charged defects in doped Mott insulators with active orbital degrees of freedom. It is observed that defects feature a rather complex and rich physics, which is well captured by a degenerate Hubbard model extended by terms that describe crystal-field splittings and orbital-lattice coupling, as well as by terms generated by defects such as the Coulomb potential terms that act both on doped holes and on electrons within occupied orbitals at undoped sites. We show that the multiplet structure of the excited states generated in such systems by strong electron interactions is well described within the unrestricted Hartree-Fock approximation, once the symmetry breaking caused by the onset of magnetic and orbital order is taken into account. Furthermore, we uncover new spectral features that arise within the Mott-Hubbard gap and in the multiplet spectrum at high energies due to the presence of defect states and strong correlations. These features reflect the action on electrons/holes of the generalized defect potential that affects charge and orbital degrees of freedom, and indirectly also spin ones. The present study elucidates the mechanism behind the Coulomb gap appearing in the band of defect states and investigates the dependence on the electron-electron interactions and the screening by the orbital polarization field. As an illustrative example of our general approach, we present explicit calculations for the model describing three t_2g orbital flavors in the perovskite vanadates doped by divalent Sr or Ca ions, such as in La_(1-x)Sr_xVO_3 and Y_(1-x)Ca_xVO_3 systems. We analyze the orbital densities at vanadium ions in the vicinity of defects, and the excited defect states which determine the optical and transport gaps in doped systems.

cond-mat.str-el↗

arXiv2013

Flux Quantization Due to Monopole and Dipole Currents

By discussing field-induced quantum interference effects due to monopole moments and those due to dipole moments on equal footing, their similarities and differences are clarified. First, we demonstrate the general principle for flux quantization. For particles carrying a monopole moment, the interference causes monopole current to oscillate periodically with flux defined as inner product of field and area, whereas for particles carrying a fixed dipole moment, the dipole current oscillates periodically with flux vector defined as cross product of field and trajectory. Our analysis unifies the oscillation of monopole or dipole currents in various devices, such as SQUID and spin-FET, into the same physical picture. Second, we show that interference effects can also happen in open trajectory devices that transport dipole currents, such as spin Josephson effect, based on the non-gauge field nature of the interference effects of dipole moments. In addition, we propose that the interference effect of electric dipoles, known as He-McKellar-Wilkens effect, can be realized by the bilayer exciton condensates observed in semiconductor heterostructure and bilayer graphene.

cond-mat.mes-hall↗

arXiv2013

Defect states and spin-orbital physics in doped vanadates Y1-xCaxVO3

We present a model for typical charged defects in weakly doped Y1-xCaxVO3 perovskites and study how they influence the magnetic and orbital order. Starting from a multiband Hubbard model, we show that the charge carriers introduced by doping are bound to the Ca defects with large binding energy of about 1 eV at small doping, and give rise to the in-gap absorption band observed in the optical spectroscopy. The central position of a generic Ca defect with eight equidistant vanadium neighbors implies a partly filled defect band and permits activated transport due to Coulomb disorder. We explore the effect of bound charge carriers on the dynamics of the (yz,zx) orbital and spin degrees of freedom in the context of a spin-orbital t-J model. After deriving the superexchange interactions around the doped hole, we show that the transition from G-type to C-type antiferromagnetic (AF) order is triggered by the kinetic energy of doped holes via the double-exchange mechanism. The defect states lead to local modification of orbital correlations within ferromagnetic chains along the c axis; some of them contain hole defects, while the charge-orbital coupling suppresses locally (yz,zx) orbital fluctuations in the others. Thereby, Ca defects provide a physical mechanism for spin-orbital dimerization along the ferromagnetic bonds, suggesting that, in the C-AF phase of weakly doped Y1-xCaxVO3, dimerization increases with doping.

cond-mat.str-el↗

arXiv2012

Von Neumann Entropy Spectra and Entangled Excitations in Spin-Orbital Models

We consider the low-energy excitations of one-dimensional spin-orbital models which consist of spin waves, orbital waves, and joint spin-orbital excitations. Among the latter we identify strongly entangled spin-orbital bound states which appear as peaks in the von Neumann entropy (vNE) spectral function introduced in this work. The strong entanglement of bound states is manifested by a universal logarithmic scaling of the vNE with system size, while the vNE of other spin-orbital excitations saturates. We suggest that spin-orbital entanglement can be experimentally explored by the measurement of the dynamical spin-orbital correlations using resonant inelastic x-ray scattering, where strong spin-orbit coupling associated with the core hole plays a role.

cond-mat.str-el↗

arXiv2011

Effects of spin vacancies on magnetic properties of the Kitaev-Heisenberg model

We study the ground state properties of the Kitaev-Heisenberg model in a magnetic field and explore the evolution of spin correlations in the presence of non-magnetic vacancies. By means of exact diagonalizations, the phase diagram without vacancies is determined as a function of the magnetic field and the ratio between Kitaev and Heisenberg interactions. We show that in the (antiferromagnetic) stripe ordered phase the static susceptibility and its anisotropy can be described by a spin canting mechanism. This accounts as well for the transition to the polarized phase when including quantum fluctuations perturbatively. Effects of spin vacancies depend sensitively on the type of the ground state. In the liquid phase, the magnetization pattern around a single vacancy in a small field is determined, and its spatial anisotropy is related to that of non-zero further neighbor correlations induced by the field and/or Heisenberg interactions. In the stripe phase, the joint effect of a vacancy and a small field breaks the six-fold symmetry of the model and stabilizes a particular stripe pattern. Similar symmetry-breaking effects occur even at zero field due to effective interactions between vacancies. This selection mechanism and intrinsic randomness of vacancy positions may lead to spin-glass behavior.

cond-mat.str-el↗

arXiv2010

Compass-Heisenberg Model on the Square Lattice : Spin Order and Excitations

We explore the physics of the anisotropic compass model under the influence of perturbing Heisenberg interactions and present the phase diagram with multiple quantum phase transitions. The macroscopic ground state degeneracy of the compass model is lifted in the thermodynamic limit already by infinitesimal Heisenberg coupling, which selects different ground states with Z_2 symmetry depending on the sign and size of the coupling constants --- then low energy excitations are spin waves, while the compass states reflecting columnar order are separated from them by a macroscopic gap. Nevertheless, nanoscale structures relevant for quantum computation purposes may be tuned such that the compass states are the lowest energy excitations, thereby avoiding decoherence, if a size criterion derived by us is fulfilled.

cond-mat.str-el↗

arXiv2010

Orbital fluctuations in the $R$VO$_3$ perovskites

The properties of Mott insulators with orbital degrees of freedom are described by spin-orbital superexchange models, which provide a theoretical framework for understanding their magnetic and optical properties. We introduce such a model derived for $(xy)^1(yz/zx)^1$ configuration of V$^{3+}$ ions in the $R$VO$_3$ perovskites, $R$=Lu,Yb,$\cdots$,La, and demonstrate that $\{yz,zx\}$ orbital fluctuations along the $c$ axis are responsible for the huge magnetic and optical anisotropies observed in the almost perfectly cubic compound LaVO$_3$. We argue that the GdFeO$_3$ distortion and the large difference in entropy of $C$-AF and $G$-AF phases is responsible for the second magnetic transition observed at $T_{N2}$ in YVO$_3$. Next we address the variation of orbital and magnetic transition temperature, $T_{\rm OO}$ and $T_{N1}$, in the $R$VO$_3$ perovskites, after extending the spin-orbital model by the crystal-field and the orbital interactions which arise from the GdFeO$_3$ and Jahn-Teller distortions of the VO$_6$ octahedra. We further find that the orthorhombic distortion which increases from LaVO$_3$ to LuVO$_3$ plays a crucial role by controlling the orbital fluctuations, and via the modified orbital correlations influences the onset of both magnetic and orbital order.

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