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Andrzej M. Oles

Publications and source records attributed to Andrzej M. Oles.

At least 19 recordsLinked to original sources
arXiv2020

Evolution of Spin-Orbital Entanglement with Increasing Ising Spin-Orbit Coupling

Several realistic spin-orbital models for transition metal oxides go beyond the classical expectations and could be understood only by employing the quantum entanglement. Experiments on these materials confirm that spin-orbital entanglement has measurable consequences. Here, we capture the essential features of spin-orbital entanglement in complex quantum matter utilizing 1D spin-orbital model which accommodates SU(2)xSU(2) symmetric Kugel-Khomskii superexchange as well as the Ising on-site spin-orbit coupling. Building on the results obtained for full and effective models in the regime of strong spin-orbit coupling, we address the question whether the entanglement found on superexchange bonds always increases when the Ising spin-orbit coupling is added. We show that (i) quantum entanglement is amplified by strong spin-orbit coupling and, surprisingly, (ii) almost classical disentangled states are possible. We complete the latter case by analyzing how the entanglement existing for intermediate values of spin-orbit coupling can disappear for higher values of this coupling.

cond-mat.str-el↗
arXiv2020

How spin-orbital entanglement depends on the spin-orbit coupling in a Mott insulator

The concept of the entanglement between spin and orbital degrees of freedom plays a crucial role in understanding various phases and exotic ground states in a broad class of materials, including orbitally ordered materials and spin liquids. We investigate how the spin-orbital entanglement in a Mott insulator depends on the value of the spin-orbit coupling of the relativistic origin. To this end, we numerically diagonalize a 1D spin-orbital model with the 'Kugel-Khomskii' exchange interactions between spins and orbitals on different sites supplemented by the on-site spin-orbit coupling. In the regime of small spin-orbit coupling w.r.t. the spin-orbital exchange, the ground state to a large extent resembles the one obtained in the limit of vanishing spin-orbit coupling. On the other hand, for large spin-orbit coupling the ground state can, depending on the model parameters, either still show negligible spin-orbital entanglement, or can evolve to a highly spin-orbitally entangled phase with completely distinct properties that are described by an effective XXZ model. The presented results suggest that: (i) the spin-orbital entanglement may be induced by large on-site spin-orbit coupling, as found in the 5d transition metal oxides, such as the iridates; (ii) for Mott insulators with weak spin-orbit coupling of Ising-type, such as e.g. the alkali hyperoxides, the effects of the spin-orbit coupling on the ground state can, in the first order of perturbation theory, be neglected.

cond-mat.str-el↗
arXiv2019

From "Weak" to "Strong" Hole Confinement in a Mott Insulator

We study the problem of a single hole in an Ising antiferromagnet and, using the magnon expansion and analytical methods, determine the expansion coefficients of its wave function in the magnon basis. In the 1D case, the hole is "weakly" confined in a potential well and the magnon coefficients decay exponentially in the absence of a string potential. This behavior is in sharp contrast to the 2D square plane where the hole is "strongly" confined by a string potential and the magnon coefficients decay superexponentially. The latter is identified here to be a fingerprint of the strings in doped antiferromagnets that can be recognized in the numerical or cold atom simulations of the 2D doped Hubbard model. Finally, we attribute the differences between the 1D and 2D cases to the magnon-magnon interactions being crucially important in a 1D spin system.

cond-mat.str-el↗
arXiv2015

Influence of isolated and clustered defects on electronic and dielectric properties of wustite

The influence of intrinsic Fe defects in FeO (either single cation vacancies or prototypical 4:1 vacancy clusters) on the electronic and dielectric properties is studied within the density functional theory. The importance of local Coulomb interactions at Fe atoms is highlighted and shown to be responsible for the observed insulating Mott gap in FeO which is reduced by the presence of defects. We investigate nonstoichiometric configurations of Fe1-xO with x ranging from 3 to 9 % and find the aliovalent Fe cations in both the regular and interstitial lattice sites of the considered configurations. Furthermore, we show that the trivalent Fe ions are induced by both isolated and clustered Fe-vacancies and introduce the empty band states inside the insulating gap, which decreases monotonically with increasing cation vacancy concentration. The Fe1-xO systems with high defect content become metallic for small values of the Coulomb interaction U, yielding the increase in the dielectric functions and optical reflectivity at low energies in agreement with the experimental data. Due to the crystal defects, the infrared-active transverse optic phonons split and distribute over a wide range of frequencies clarifying the origin of the exceptionally large spectral linewidths of the dielectric loss functions observed for wustite in recent experiments.

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↗
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

Anharmonicity due to Electron-Phonon Coupling in Magnetite

We present the results of inelastic x-ray scattering for magnetite and analyze the energies and spectral widths of the phonon modes with different symmetries in a broad range of temperature 125<T<293 K. The phonon modes with X_4 and Delta_5 symmetries broaden in a nonlinear way with decreasing temperature when the Verwey transition is approached. It is found that the maxima of phonon widths occur away from high-symmetry points which indicates the incommensurate character of critical fluctuations. Strong phonon anharmonicity induced by electron-phonon coupling is discovered within ab initio calculations which take into account local Coulomb interactions at Fe ions. It (i) explains observed anomalous phonon broadening, and (ii) demonstrates that the Verwey transition is a cooperative phenomenon which involves a wide spectrum of phonons coupled to charge fluctuations condensing in the low-symmetry phase.

cond-mat.str-el↗
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

Valence Bond Crystal and possible orbital pinball liquid in a t2g model

We study a model for orbitally degenerate Mott insulators, where localized electrons possess t_2g degrees of freedom coupled by several, competing, exchange mechanisms. We provide evidence for two distinct strongly fluctuating regimes, depending on whether superexchange or direct exchange mechanism predominates. In the superexchange-dominated regime, the ground state is dimerized, with nearest neighbor orbital singlets covering the lattice. By deriving an effective quantum dimer model and analyzing it numerically, we characterize this dimerized phase as a valence bond crystal stabilized by singlet resonances within a large unit cell. In the opposite regime, with predominant direct exchange, the combined analysis of the original model and another effective model adapted to the local constraints, shows that subleading perturbations select a highly resonating ground state, with coexisting diagonal and off-diagonal long-range orbital orders.

cond-mat.str-el↗
arXiv2011

Dimensional crossover and the magnetic transition in electron doped manganites

We introduce a microscopic model for electron doped manganites that explains the mechanism of the observed transition from $G$-type antiferromagnetic ($G$-AF) to $C$-type antiferromagnetic ($C$-AF) order under increasing doping by double exchange mechanism. The model unravels the crucial role played by $e_g$ orbital degrees of freedom and explains the observed metal-to-insulator transition by a dimensional crossover at the magnetic phase transition. The specific heat and the spin canting angle found for the $G$-AF phase agree with the experimental findings. As a surprising outcome of the theory we find that spin canting is suppressed in the $C$-AF phase, in agreement with the experiment, due to the Fermi surface topology.

cond-mat.str-el↗
arXiv2011

Spin-orbital physics for p orbitals in alkali RO_2 hyperoxides --- generalization of the Goodenough-Kanamori rules

We derive a realistic spin-orbital model at finite Hund's exchange for alkali hyperoxides. We find that, due to the geometric frustration of the oxygen lattice, spin and orbital waves destabilize both spin and p-orbital order in almost all potential ground states. We show that the orbital order induced by the lattice overrules the one favoured by superexchange and that this, together with the large interorbital hopping, leads to generalized Goodenough-Kanamori rules. They (i) lift the geometric frustration of the lattice, and (ii) explain the observed layered C-type antiferromagnetic order in alkali hyperoxides. This is confirmed by a spin-wave dispersion with no soft-mode behavior presented here as a prediction for future experiments.

cond-mat.str-el↗
arXiv2011

Competing Pairing Symmetries in a Generalized Two-Orbital Model for the Pnictides

We introduce and study an extended "t-U-J" two-orbital model for the pnictides that includes Heisenberg terms deduced from the strong coupling expansion. Including these J terms explicitly allows us to enhance the strength of the (pi, 0)-(0, pi) spin order which favors the presence of tightly bound pairing states even in the small clusters that are here exactly diagonalized. The A1g and B2g pairing symmetries are found to compete in the realistic spin-ordered and metallic regime. The dynamical pairing susceptibility additionally unveils low-lying B1g states, suggesting that small changes in parameters may render any of the three channels stable.

cond-mat.supr-con↗
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

Spontaneous symmetry breaking in a generalized orbital compass model

We introduce a generalized two-dimensional orbital compass model, which interpolates continuously from the classical Ising model to the orbital compass model with frustrated quantum interactions, and investigate it using the multiscale entanglement renormalization ansatz (MERA). The results demonstrate that increasing frustration of exchange interactions triggers a second order quantum phase transition to a degenerate symmetry broken state which minimizes one of the interactions in the orbital compass model. Using boson expansion within the spin-wave theory we unravel the physical mechanism of the symmetry breaking transition as promoted by weak quantum fluctuations and explain why this transition occurs only surprisingly close to the maximally frustrated interactions of the orbital compass model. The spin waves remain gapful at the critical point, and both the boson expansion and MERA do not find any algebraically decaying spin-spin correlations in the critical ground state.

cond-mat.str-el↗
arXiv2010

Charge and orbital order in transition metal oxides

A short introduction to the complex phenomena encountered in transition metal oxides with either charge or orbital or joint charge-and-orbital order, usually accompanied by magnetic order, is presented. It is argued that all the types of above ordered phases in these systems follow from strong Coulomb interactions as a result of certain compromise between competing instabilities towards various types of magnetic order and optimize the gain of kinetic energy in doped systems. This competition provides a natural explanation of the stripe order observed in doped cuprates, nickelates and manganites. In the undoped correlated insulators with orbital degrees of freedom the orbital order stabilizes particular types of anisotropic magnetic phases, and we contrast the case of decoupled spin and orbital degrees of freedom in the manganites with entangled spin-orbital states which decide about certain rather exotic phenomena observed in the perovskite vanadates at finite temperature. Examples of successful concepts in the theoretical approaches to these complex systems are given and some open problems of current interest are indicated.

cond-mat.str-el↗
arXiv2010

Ferro-Orbitally Ordered Stripes in Systems with Alternating Orbital Order

We establish a novel mechanism of stripe formation in doped systems with alternating $t_{2g}$ orbital order --- the stripe takes the form of a ferro-orbitally ordered domain wall separating domains with staggered order and allowing for deconfined motion of holes along the stripe. At a finite level of hole concentration this gives rise to the stability of this solitonic type of stripes, while we show that the phase change of the staggered order by $π$ plays a minor role in orbitally ordered systems. These results shed new light on the physics of doped materials in which orbital degeneracy is present.

cond-mat.str-el↗
arXiv2009

Low-energy Mott-Hubbard excitations in LaMnO_3 probed by optical ellipsometry

We present a comprehensive ellipsometric study of an untwinned, nearly stoichiometric LaMnO_3 crystal in the spectral range 1.2-6.0 eV at temperatures 20 K < T < 300 K. The complex dielectric response along the b and c axes of the Pbnm orthorhombic unit cell, ε^b(ν) and ε^c(ν), is highly anisotropic over the spectral range covered in the experiment. The difference between ε^b(ν) and ε^c(ν) increases with decreasing temperature, and the gradual evolution observed in the paramagnetic state is strongly enhanced by the onset of A-type antiferromagnetic long-range order at T_N = 139.6 K. In addition to the temperature changes in the lowest-energy gap excitation at 2 eV, there are opposite changes observed at higher energy at 4 - 5 eV, appearing on a broad-band background due to the strongly dipole-allowed O 2p -- Mn 3d transition around the charge-transfer energy 4.7 eV. Based on the observation of a pronounced spectral-weight transfer between low- and high-energy features upon magnetic ordering, they are assigned to high-spin and low-spin intersite d^4d^4 - d^3d^5 transitions by Mn electrons. The anisotropy of the lowest-energy optical band and the spectral weight shifts induced by antiferromagnetic spin correlations are quantitatively described by an effective spin-orbital superexchange model. An analysis of the multiplet structure of the intersite transitions by Mn e_g electrons allowed us to estimate the effective intra-atomic Coulomb interaction, the Hund exchange coupling, and the Jahn-Teller splitting energy between e_g orbitals in LaMnO_3. This study identifies the lowest-energy optical transition at 2 eV as an intersite d-d transition, whose energy is substantially reduced compared to that obtained from the bare intra-atomic Coulomb interaction.

cond-mat.str-el↗