arXiv subjects

Andrzej M. Oleś

Publications and source records attributed to Andrzej M. Oleś.

At least 19 recordsLinked to original sources

arXiv2025

Electronic and magnetic properties of the NdNiO$_2$/SrTiO$_3$ thin films

The hole-doped NdNiO$_2$ layer deposited on the SrTiO$_{3}$ surface exhibits unconventional superconductivity. Here, we present a systematic study of the electronic and magnetic properties of the NdNiO$_2$ superconductor using the density functional theory (DFT). The strong local Coulomb interactions in the Ni($3d)$ and Nd($4f$) states are included within the DFT+$U$ method. The effect of Sr doping on the electronic band structure and density of states was studied for the NdNiO$_2$ thin films deposited on the SrTiO$_3$ (001) surface. The results obtained for the uncapped thin films were compared with the calculations for the NdNiO$_2$ films capped by the SrTiO$_3$ layer. We have found significant changes in the electronic structure and magnetic properties of the thin films compared to the bulk crystal.

cond-mat.mtrl-sci↗

arXiv2025

Charge and magnetic orders in a two-band model with long-range interactions for infinite-layer nickelates NdNiO$_2$

We present an effective two-band model for infinite-layer nickelates NdNiO$_2$ that consisting of a $d$ band centered at Ni site and an interstitial $s$-like band centered at Nd site. To the large extent of the wave functions, we find intersite Coulomb interactions to be substantial. We then use the variational cluster approach together with mean-field theory to investigate magnetic and charge ordering. While tendencies towards charge modulation are found, they are weak and might be due to finite-size effects. Magnetic order is determined mostly by the filling of the $d$ band and hardly affected by including longer-ranged interactions. For a $d$-band density consistent with density-functional theory, magnetic ordering vanishes once quantum fluctuations are included to a sufficient spatial extent. Apart from self-doping, $d$ and $s$ bands remain largely uncoupled despite the presence of inter-orbital Coulomb interaction and (small) inter-orbital hopping.

cond-mat.str-el↗

arXiv2025

Lattice dynamics study of electron-correlation-induced charge density wave in antiferromagnetic kagome metal FeGe

Electron-correlation-driven phonon soft modes have been recently reported in the antiferromagnetic kagome FeGe compound and associated with the observed charge density wave (CDW). In this paper, we present a systematic investigation of the CDW origin in the context of the ab initio lattice dynamics study. Performing the group theory analysis of the mentioned soft mode, we found that the stable structure has the Immm symmetry and can be achieved by shifts of Ge atoms. Additionally, we show that the final structure realizes a distorted honeycomb Ge lattice as well as a non-flat kagome-like Fe net. For completeness, we present the electronic properties calculations. From the theoretical STM topography simulation, we indicate that the observed CDW occurs in the deformed honeycomb Ge sublattice.

cond-mat.mtrl-sci↗

arXiv2024

Topology of chalcogen chains

We investigate the topological properties of the helical atomic chains occurring in elemental selenium and tellurium. We postulate a realistic model that includes spin-orbit interaction and show this to be topologically non-trivial, with a topological invariant protected by a crystalline symmetry. We describe the end-states, which are orbitally polarized, with an orbital density modulation strongly peaked at the edge. Furthermore, we propose a simplified model that decomposes into three orbital chains, allowing us to define a topological invariant protected by a crystalline symmetry. We contrast this result with recent observations made for the orbital Su-Schrieffer-Heeger model containing a $p$-orbital zigzag chain.

cond-mat.mtrl-sci↗

arXiv2024

Electronic and dynamical properties of cobalt monogermanide CoGe phases under pressure

We present the pressure dependence of the electronic and dynamical properties of six different CoGe phases: orthorhombic Cmmm, hexagonal P6/mmm and P$\bar{6}$2m, monoclinic C2/m, cubic P2$_{1}$3, and orthorhombic Pnma. Using first-principles DFT calculations and the direct force-constants method, we study the dynamical stability of individual phases under external pressure. We show that the orthorombic Cmmm and hexagonal P6/mmm structures are unstable over a broad pressure range and most pronounced imaginary phonon soft mode in both cases leads to a stable hexagonal P$\bar{6}$2m structure of the lowest ground-state energy of all studied phases at ambient and low (below $\sim 3$ GPa) external pressure. Under these conditions, the cubic P2$_{1}$3 phase has the highest energy, however, together with monoclinic C2/m and orthorombic Pnma it is dynamically stable and all these three structures can potentially coexist as meta-stable phases. Above $\sim 3$ GPa, the cubic P2$_{1}$3 phase becomes the most energetically favorable. Fitting the Birch--Murnaghan equation of state we derive bulk modulus for all mentioned phases, which indicate relatively high resistance of CoGe to compression. Such conclusions are confirmed by band structure calculations. Additionally, we show that electronic bands of the hexagonal P$\bar{6}$2m phase reveal characteristic features of the kagome-like structure, while in the cubic P2$_{1}$3 phase spectrum, one can locate spin-1 and double Weyl fermions. In both cases, the external pressure induces the Lifshitz transition, related to the modification of the Fermi surface topology.

cond-mat.mtrl-sci↗

arXiv2023

Machine-learning-inspired quantum optimal control of nonadiabatic geometric quantum computation via reverse engineering

Quantum control plays an irreplaceable role in practical use of quantum computers. However, some challenges have to be overcome to find more suitable and diverse control parameters. We propose a promising and generalizable average-fidelity-based machine-learning-inspired method to optimize the control parameters, in which a neural network with periodic feature enhancement is used as an ansatz. In the implementation of a single-qubit gate by cat-state nonadiabatic geometric quantum computation via reverse engineering, compared with the control parameters in the simple form of a trigonometric function, our approach can yield significantly higher-fidelity ($>99.99\%$) phase gates, such as the $π/ 8$ gate (T gate). Single-qubit gates are robust against systematic noise, additive white Gaussian noise and decoherence. We numerically demonstrate that the neural network possesses the ability to expand the model space. With the help of our optimization, we provide a feasible way to implement cascaded multi-qubit gates with high quality in a bosonic system. Therefore, the machine-learning-inspired method may be feasible in quantum optimal control of nonadiabatic geometric quantum computation.

quant-ph↗

arXiv2023

Phononic drumhead surface state in distorted kagome compound RhPb

RhPb was initially recognized as one of a CoSn-like compounds with $P6/mmm$ symmetry, containing an ideal kagome lattice of $d$-block atoms. However, theoretical calculations predict the realization of the phonon soft mode which leads to the kagome lattice distortion and stabilization of the structure with $P\bar{6}2m$ symmetry [A. Ptok et al., Phys. Rev. B 104, 054305 (2021)]. Here, we present the single crystal x-ray diffraction results supporting this prediction. Furthermore, we discuss the main dynamical properties of RhPb with $P\bar{6}2m$ symmetry. The bulk phononic dispersion curves contain several flattened bands, Dirac nodal lines, and triple degenerate Dirac points. As a consequence, the phononic drumhead surface state is realized for the (100) surface, terminated by the zigzag-like edge of Pb honeycomb sublattice.

cond-mat.mtrl-sci↗

arXiv2023

Quantum many-body scars in spin-1 Kitaev chain with uniaxial single-ion anisotropy

To establish a solid-state-based framework for the coexistence of quantum many-body scars and quantum criticality, we investigate the spin-1 Kitaev chain with uniaxial single-ion anisotropy (SIA). In the subspace with uniform $\mathbb{Z}_2$ gauge fields, this model can be exactly mapped to the spin-1/2 effective detuned PXP Hamiltonian, where the SIA plays a role of the static detuning term. The quench dynamics starting from the product states is symmetric between positive and negative values of the SIA, while a quantum phase transition from the Kitaev spin liquid to the dimer phase only occurs at the critical point with a negative $D_c$, implying the spontaneous breaking of the translational symmetry. We find that the coherent oscillations of quantum fidelity and certain local observables are sustained against small SIA perturbations in a quantum quench from special initial states. While the oscillation amplitudes of these observables decay with time as the SIA strength is increased, the system completely thermalizes upon approaching the critical point. In contrast, the initial polarized state, which shows an absence of revivals of quantum fidelity, will exhibit long revivals for $D<D_c$. Finally, we investigate the evolution of phase boundaries of the Kitaev spin liquid and dimer phase by introducing Heisenberg interactions, which spoil the $\mathbb{Z}_2$ gauge fields. A complete phase diagram is given by the infinite time-evolving block decimation method and the ground state properties of each phase are accurately captured by various spin correlations. Our work opens the door to understanding exotic connections between many-body scars and quantum criticality in systems with higher spins.

cond-mat.str-el↗

arXiv2023

Density functional theory study of experimentally observed Au/Ge interfaces

In recent years, nanostructures with hexagonal polytypes of gold have been synthesised, opening new possibilities in nanoscience and technology. As bulk gold crystallizes in the \textit{fcc} phase, surface effects can play an important role in stabilizing hexagonal gold nanostructures. Here we investigate several hetero-structures with Ge substrate, including the \textit{fcc} and \textit{hcp} phases of gold that have been observed experimentally. We determine and discuss their interfacial energies and optimized atomic arrangements, comparing the theory results with available experimental data. Our calculations for the Au-\textit{fcc}(011)/Ge(001) show how the presence of defects in the interface layer can help to stabilize the atomic pattern consistent with microscopic images. The Au-\textit{hcp}/Ge interface with the significant mismatch between two surfaces reveals large atomic displacements, which might indicate that the (111) germanium substrate is not responsible for the formation of the \textit{hcp} phase of gold. Finally, analyzing the electronic properties, we demonstrate that Au/Ge systems have metallic character but covalent-like bonding states between interfacial Ge and Au atoms are also present.

cond-mat.mtrl-sci↗

arXiv2023

Spin-orbital mechanisms for negative thermal expansion in Ca2RuO$_4$

The phenomenon of negative thermal expansion (NTE) deals with the increase of the lattice parameters and the volume of the unit cell when the material is thermally cooled. The NTE is typically associated with thermal phonons and anomalous spin-lattice coupling at low temperatures. However, the underlying mechanisms in the presence of strong electron correlations in multi-orbital systems are not yet fully established. Here, we investigate the role of Coulomb interaction in the presence of lattice distortions in setting out the NTE effect, by focusing on the physical case of layered Ca$_2$RuO$_4$ with $d^4$ configuration at each Ru ion site. We employ the Slater-Koster parametrization to describe the electron-lattice coupling through the dependence of the $d-p$ hybridization on the Ru-O-Ru bond angle. The evaluation of the minimum of the free energy at finite temperature by fully solving the multi-orbital many-body problem on finite size cluster allows us to identify the regime for which the system is prone to exhibit NTE effects. The analysis shows that the nature of the spin-orbital correlations is relevant to drive the reduction of the bond angle by cooling, and in turn the tendency toward a NTE. This is confirmed by the fact that a changeover of the electronic and orbital configuration from $d^4$ to $d^3$ by transition metal substitution is shown to favor the occurrence of NTE in Ca$_2$RuO$_4$. This finding is in agreement with the experimental observations of a NTE effect which is significantly dependent on the transition metal substitution in the Ca$_2$RuO$_4$ compound.

cond-mat.str-el↗

arXiv2023

Influence of $f$ Electrons on the Electronic Band Structure of Rare-Earth Nickelates

Recently, superconductivity was discovered in the infinite layer of hole-doped nickelates NdNiO$_{2}$. Contrary to this, superconductivity in LaNiO$_{2}$ is still under debate. This indicates the crucial role played by the $f$ electrons on the electronic structure and the pairing mechanism of infinite-layer nickelates. Here we discuss the role of the electron correlations on the $f$ electron states and their influence on the electronic structure. We show that the lattice parameters are in good agreement with the experimental values, independent of the chosen parameters within the DFT+$U$ approach. Increasing Coulomb interaction $U$ tends to shift the $f$ states away from the Fermi level. Surprisingly, independently of the position of $f$ states with respect to the Fermi energy, these states play an important role in the electronic band structure, which can be reflected in the modification of the NdNiO$_{2}$ effective models.

cond-mat.str-el↗

arXiv2023

Long-range order in the XY model on the honeycomb lattice

Using the reflection positivity method we provide the rigorous proof of the existence of long range magnetic order for the XY model on the honeycomb lattice for large spins $S\ge 2$. This stays in contrast with the result obtained using the {\it same} method but on the square lattice -- which gives a stable long-range order for spins $S\ge 1$. We suggest that the difference between these two cases stems from the enhanced quantum spin fluctuations on the honeycomb lattice. Using linear spin-wave theory we show that the enhanced fluctuations are due to the overall much higher kinetic energy of the spin waves on the honeycomb lattice (with Dirac points) than on the square lattice (with good nesting properties).

cond-mat.str-el↗

arXiv2022

Orbital liquid in the $e_g$ orbital Hubbard model in $d=\infty$ dimensions

We demonstrate that the three-dimensional $e_g$ orbital Hubbard model can be generalized to arbitrary dimension $d$, and that the form of the result is determined uniquely by the requirements that (i) the two-fold degeneracy of the $e_g$ orbital be retained, and (ii) the cubic lattice be turned into a hypercubic lattice. While the local Coulomb interaction $U$ is invariant for each basis of orthogonal orbitals, the form of the kinetic energy depends on the orbital basis and takes the most symmetric form for the so-called complex-orbital basis. Characteristically, with respect to this basis, the model has two hopping channels, one that is orbital-flavor conserving, and a second one that is orbital-flavor non-conserving. We show that the noninteracting electronic structure consists of two nondegenerate bands of plane-wave real-orbital single-particle states for which the orbital depends on the wave vector. Due to the latter feature each band is unpolarized at any filling, and has a non-Gaussian density of states at $d=\infty$. The \textit{orbital liquid} state is obtained by filling these two bands up to the same Fermi energy. We investigate the $e_g$ orbital Hubbard model in the limit $d\to\infty$, treating the on-site Coulomb interaction $U$ within the Gutzwiller approximation, thus determining the correlation energy of the orbital liquid and the (disordered) para-orbital states. (...) We show that the orbital liquid is the ground state everywhere in the $(n,U)$ phase diagram except close to half-filling at sufficiently large $U$, where ferro-orbital order with real orbitals occupied is favored. The latter feature is shown to be specific for $d=\infty$, being of mathematical nature due to the exponential tails in the density of states.

cond-mat.str-el↗

arXiv2022

Hubbard subbands and superconductivity in the infinite-layer nickelate

An effective two-dimensional two-band model for infinite-layer nickelates consists of bands obtained from $d_{x^2-y^2}$ and $s$--like orbitals. We investigate whether it could be mapped onto a single-band Hubbard model and the filling of Hubbard bands. We find that both one-band physics and a Kondo-lattice regime emerge from the same two-orbital model, depending on the strength of electronic correlations and the filling of the itinerant $s$-band. Next we investigate one-particle excitations by changing the screening. First, for weak screening the strong correlations push electrons out of the $s$-band so that the undoped nickelate is similar to a cuprate. Second, for strong screening the $s$ and $d_{x^2-y^2}$ bands are both partly filled and weakly couple. Particularly in this latter regime mapping to a one-band model gives significant spectral weight transfer between the Hubbard subbands. Finally we point out that the superconducting phases may have either $d$-wave or $s$-wave symmetry.

cond-mat.str-el↗

arXiv2022

Screening in a two-band model for superconducting infinite-layer nickelate

Starting from an effective two-dimensional two-band model for infinite layered nickelates, consisting of bands obtained from $d$ and $s$--like orbitals, we investigate to which extend it can be mapped onto a single-band Hubbard model. We identify screening of the more itinerant $s$-like band as an important driver. In absence of screening one strongly-correlated band gives an antiferromagnetic ground state. For weak screening, the strong correlations push electrons out of the $s$-band so that the undoped nickelate remains a Mott insulator with half filled $d$ orbitals. This regime markedly differs from the observations in high-$T_c$ cuprates and pairing with $s$-wave symmetry would rather be expected in the superconducting state. In contrast, for strong screening, the $s$ and $d_{x^2-y^2}$ bands are both partly filled and couple only weakly, so that one approximately finds a self-doped $d$ band as well as tendencies towards $d$-wave pairing. Particularly in the regime of strong screening mapping to a one-band model gives interesting spectral weight transfers when a second $s$ band is also partly filled. We thus find that both one-band physics and a Kondo-lattice--like regime emerge from the same two-orbital model, depending on the strength of electronic correlations and the size of the $s$-band pocket.

cond-mat.str-el↗

arXiv2022

Quantum Many-Body Scars in Spin-1 Kitaev Chains

To provide a physical example of quantum scars, we study the many-body scars in the spin-1 Kitaev chain where the so-called PXP Hamiltonian is exactly embedded in the spectra. Regarding the conserved quantities, the Hilbert space is fragmented into disconnected subspaces and we explore the associated constrained dynamics. The continuous revivals of the fidelity and the entanglement entropy when the initial state is prepared in $\vert\mathbb{Z}_k\rangle$ ($k=2,3$) state illustrate the essential physics of the PXP model. We study the quantum phase transitions in the one-dimensional spin-1 Kitaev-Heisenberg model using the density-matrix renormalization group and Lanczos exact diagonalization methods, and determine the phase diagram. We parametrize the two terms in the Hamiltonian by the angle $ϕ$, where the Kitaev term is $K\equiv\sin(ϕ)$ and competes with the Heisenberg $J\equiv\cos(ϕ)$ term. One finds a rich ground state phase diagram as a function of the angle $ϕ$. Depending on the ratio $K/J\equiv\tan(ϕ)$, the system either breaks the symmetry to one of distinct symmetry broken phases, or preserves the symmetry in a quantum spin liquid phase with frustrated interactions. We find that the scarred state is stable for perturbations which obey $\mathbb{Z}_2$-symmetry, while it becomes unstable against Heisenberg-type perturbations.\\ \textit{Accepted for publication in Physical Review Research}

cond-mat.str-el↗

arXiv2021

Dynamical Study of the Origin of the Charge Density Wave in $A$V$_{3}$Sb$_{5}$ ($A=$K, Rb, Cs) Compounds

Systems containing the ideal kagome lattice can exhibit several distinct and novel exotic states of matter. One example of such systems is a recently discovered $A$V$_{3}$Sb$_{5}$ ($A$ = K, Rb, and Cs) family of compounds. Here, the coexistence of the charge density wave (CDW) and superconductivity is observed. In this paper, we study the dynamic properties of the $A$V$_{3}$Sb$_{5}$ systems in context of origin of the CDW phase. We show and discuss the structural phase transition from $P6/mmm$ to $C2/m$ symmetry that are induced by the presence of phonon soft modes. We conclude that the CDW observed in this family of compounds is a consequence of the atom displacement, from the high symmetry position of the kagome net, in low-temperature phase. Additionally, using the numerical {\it ab initio} methods, we discuss the charge distribution on the $A$V$_{3}$Sb$_{5}$ surface. We show that the observed experimental %$4\times 1$ stripe-like modulation of the surface, can be related to surface reconstruction and manifestation of the three dimensional $2 \times 2 \times 2$ bulk CDW. Finally, the consequence of realization of the $C2/m$ structure on the electronic properties are discussed. We show that the electronic band structure reconstruction and the accompanying modification of density of states correspond well to the experimental data.

cond-mat.mtrl-sci↗

arXiv2021

Character of Doped Holes in Nd$_{1-x}$Sr$_x$NiO$_2$

We investigate charge distribution in the recently discovered high-$T_c$ superconductors, layered nickelates. With increasing value of charge-transfer energy we observe the expected crossover from the cuprate to the local triplet regime upon hole doping. We find that the $d-p$ Coulomb interaction $U_{dp}$ plays a role and makes Zhang-Rice singlets less favorable, while the amplitude of local triplets is enhanced. By investigating the effective two-band model with orbitals of $x^2-y^2$ and $s$ symmetries we show that antiferromagnetic interactions dominate for electron doping. The screened interactions for the $s$ band suggest the importance of rare-earth atoms in superconducting nickelates.

cond-mat.str-el↗