M. Sekania | University of Augsburg (original) (raw)

Papers by M. Sekania

Abstract. In order to clarify the physics of the crossover from a Peierls band insulator to a cor... more Abstract. In order to clarify the physics of the crossover from a Peierls band insulator to a correlated Mott-Hubbard insulator, we analyze ground-state and spectral properties of the one-dimensional halffilled Holstein-Hubbard model using quasi-exact numerical techniques. In the adiabatic limit the transition is connected to the band to Mott insulator transition of the ionic Hubbard model. Depending on the

Physical Review B

Local excitations as carriers of quantum information spread out in the system in ways governed by... more Local excitations as carriers of quantum information spread out in the system in ways governed by the underlying interaction and symmetry. Understanding this phenomenon, also called quantum scrambling, is a prerequisite for employing interacting systems for quantum information processing. The character and direction dependence of quantum scrambling can be inferred from the out-of-time-ordered commutators (OTOCs) containing information on correlation buildup and entanglement spreading. Employing OTOC, we study and quantify the directionality of quantum information propagation in oxide-based helical spin systems hosting a spin-driven ferroelectric order. In these systems, magnetoelectricity permits the spin dynamics and associated information content to be controlled by an electric field coupled to the emergent ferroelectric order. We show that topologically nontrivial quantum phases, such as chiral or helical spin ordering, allow for electric-field controlled anisotropic scrambling and a direction-dependent buildup of quantum correlations. Based on general symmetry considerations, we find that starting from a pure state (e.g., the ground state) or a finite temperature state is essential for observing directional asymmetry in scrambling. In the systematic numerical studies of OTOC on finite-size helical multiferroic chains, we quantify the directional asymmetry of the scrambling and verify the conjectured form of the OTOC around the ballistic wavefront. The obtained direction-dependent butterfly velocity v B (n) provides information on the speed of the ballistic wavefront. In general, our calculations show an early time power-law behavior of OTOC, as expected from an analytic expansion for small times. The long-time behavior of OTOC reveals the importance of (non)integrability of the underlying Hamiltonian as well as the implications of conserved quantities such as the z projection of the total spin. The results point to the potential of spin-driven ferroelectric materials for the use in solid-state-based quantum information processing.

arXiv: Strongly Correlated Electrons, 2020

We calculated the magnetic Compton profiles (MCPs) of Ni using density functional theory suppleme... more We calculated the magnetic Compton profiles (MCPs) of Ni using density functional theory supplemented by electronic correlations treated within dynamical mean-field theory (DMFT). We present comparisons between the theoretical and experimental MCPs. The theoretical MCPs were calculated using the KKR method with the perturbative spin-polarized T-matrix fluctuation exchange approximation DMFT solver, as well as with the full potential linear augmented planewave method with the numerically exact continuous-time quantum Monte Carlo DMFT solver. We show that the total magnetic moment decreases with the intra-atomic Coulomb repulsion UUU, which is also reflected in the corresponding MCPs. The total magnetic moment obtained in experimental measurements can be reproduced by intermediate values of UUU. The spectral function reveals that the minority X$_2$ Fermi surface pocket shrinks and gets shallower with respect to the density functional theory calculations.

arXiv: Strongly Correlated Electrons, 2001

We investigate the ground state phase diagram of the one-dimensional ionic Hubbard model with an ... more We investigate the ground state phase diagram of the one-dimensional ionic Hubbard model with an alternating potential at half-filling by the bosonization technique as well as by numerical diagonalization of finite systems with the Lanczos and DMRG methods. Our results support the existence of a single "metallic" transition point from a band to a correlated insulator with simultaneous charge and bond-charge order. In addition, we present results for the optical conductivity obtained by the dynamical DMRG method. The insulator-insulator phase transition scenario is discussed in detail including a critical review of existing approaches and results for the ionic Hubbard model.

Physica A: Statistical Mechanics and its Applications

The contribution of the valence electrons to the Compton profiles of the crystalline alkali metal... more The contribution of the valence electrons to the Compton profiles of the crystalline alkali metals is calculated using density functional theory. We show that the Compton profiles can be modeled by a q−Gaussian distribution, which is characterized by an anisotropic, element dependent parameter q. Thereby we derive an unexpected scaling behavior of the Compton profiles of all alkali metals.

High Performance Computing in Science and Engineering, Munich 2002, 2003

We analyze ground state and spectral properties of the one-dimensional half-filled Holstein Hubba... more We analyze ground state and spectral properties of the one-dimensional half-filled Holstein Hubbard model with respect to the Peierls-insulator to Mottinsulator transition, exploiting Lanczos diagonalization, density matrix renormalization, kernel polynomial expansion, and maximum entropy methods on the Hitachi SR8000-F1 supercomputer.

We compute the phonon dispersion, density of states, and the Grüneisen parameters of bulk palladi... more We compute the phonon dispersion, density of states, and the Grüneisen parameters of bulk palladium in the combined density functional theory (DFT) and dynamical mean-field theory (DMFT). We find good agreement with experimental results for ground state properties (equilibrium lattice parameter and bulk modulus) and the experimentally measured phonon spectra. We demonstrate that at temperatures T ≲ 20 K the phonon frequency in the vicinity of the Kohn anomaly, ω_T1( q_K), strongly decreases. This is in contrast to DFT where this frequency remains essentially constant in the whole temperature range. Apparently correlation effects reduce the restoring force of the ionic displacements at low temperatures, leading to a mode softening.

Physical Review B

We compute the phonon dispersion, density of states, and the Grüneisen parameters of bulk palladi... more We compute the phonon dispersion, density of states, and the Grüneisen parameters of bulk palladium in the combined density functional theory (DFT) and dynamical mean-field theory (DMFT). We find good agreement with experimental results for ground state properties (equilibrium lattice parameter and bulk modulus) and the experimentally measured phonon spectra. We demonstrate that at temperatures T 20 K the phonon frequency in the vicinity of the Kohn anomaly, ωT 1(qK), strongly decreases. This is in contrast to DFT where this frequency remains essentially constant in the whole temperature range. Apparently correlation effects reduce the restoring force of the ionic displacements at low temperatures, leading to a mode softening.

Physical Review B, 2016

Including on-site electronic interactions described by the multi-orbital Hubbard model we study t... more Including on-site electronic interactions described by the multi-orbital Hubbard model we study the correlation effects in the electronic structure of bulk palladium. We use a combined density functional and dynamical mean field theory, LDA+DMFT, based on the fluctuation exchange approximation. The agreement between the experimentally determined and the theoretical lattice constant and bulk modulus is improved when correlation effects are included. It is found that correlations modify the Fermi surface around the neck at the L-point while the Fermi surface tube structures show little correlation effects. At the same time we discuss the possibility of satellite formation in the high energy binding region. Spectral functions obtained within the LDA+DMFT and GW methods are compared to discuss non-local correlation effects. For relatively weak interaction strength of the local Coulomb and exchange parameters spectra from LDA+DMFT shows no major difference in comparison to GW .

Physical Review B

Quantum dynamics of magnetic order in a chiral multiferroic chain is studied. We consider two dif... more Quantum dynamics of magnetic order in a chiral multiferroic chain is studied. We consider two different scenarios: Ultrashort terahertz (THz) excitations or a sudden electric field quench. Performing analytical and numerical exact diagonalization calculations we trace the pulse induced spin dynamics and extract quantities that are relevant to quantum information processing. In particular, we analyze the dynamics of the system chirality, the von Neumann entropy, the pairwise and the many body entanglement. If the characteristic frequencies of the generated states are non-commensurate then a partial loss of pair concurrence occurs. Increasing the system size this effect becomes even more pronounced. Many particle entanglement and chirality are robust and persist in the incommensurate phase. To analyze the dynamical quantum transitions for the quenched and pulsed dynamics we combined the Weierstrass factorization technique for entire functions and Lanczos exact diagonalization method. For a small system we obtained analytical results including the rate function of Loschmidt echo. Exact numerical calculations for a system up to 40 spins confirm phase transition. Quenchinduced dynamical transitions have been extensively studied recently. Here we show that related dynamical transitions can be achieved and controlled by appropriate electric field pulses.

ABSTRACT We investigate the ground-state phase diagram of the one-dimensional "ionic&amp... more ABSTRACT We investigate the ground-state phase diagram of the one-dimensional "ionic" Hubbard model with an alternating periodic potential at half-filling by numerical diagonalization of finite systems with the Lanczos and density matrix renormalization group (DMRG) methods. We identify an insulator-insulator phase transition from a band to a correlated insulator with simultaneous charge and bond-charge order. The transition point is characterized by the vanishing of the optical excitation gap while simultaneously the charge and spin gaps remain finite and equal. Indications for a possible second transition into a Mott-insulator phase are discussed. The analises is extended to the 3/4-filling and alternating Coulomb interaction -- minimum model Hamiltonian for the crossover from the charge-transfer insulator to the Mott insulator.

We investigate the ground-state phase diagram of the one-dimensional "ionic" Hubbard model with a... more We investigate the ground-state phase diagram of the one-dimensional "ionic" Hubbard model with an alternating periodic potential at half-filling by the bosonization technique as well as by numerical diagonalization of finite systems with the Lanczos and density matrix renormalization group (DMRG) methods. Our results support the existence of a single "metallic" transition point from a band to a correlated insulator with simultaneous charge and bond-charge order. In addition, we present results for the optical conductivity obtained by the dynamical DMRG method. The insulator-insulator phase transition scenario is discussed in detail including a critical review of existing approaches and results for the ionic Hubbard model.

Physical Review B, 2001

The ground state phase diagram of the 1D Hubbard chain with pair-hopping interaction is studied. ... more The ground state phase diagram of the 1D Hubbard chain with pair-hopping interaction is studied. The analysis of the model is performed using the continuum-limit field theory approach and exact diagonalization studies. At half-filling the phase diagram is shown to consist of two superconducting states with Cooper pair center-of-mass momentum Q = 0 (BCS-η0 phase) and Q = π (ηπ-phase) and four insulating phases corresponding to the Mott antiferromagnet, the Peierls dimerized phase, the charge-density-wave (CDW) insulator as well as an unconventional insulating phase characterized by the coexistence of a CDW and a bond-located staggered magnetization. Away from half-filling the phase diagram consists of the superconducting BCS-η0 and ηπ phases and the metallic Luttingerliquid phase. The BCS-η0 phase exhibits smooth crossover from a weak-coupling BCS type to a strong coupling local-pair regime. The ηπ phase shows properties of the doublon (zero size Cooper pair) superconductor with Cooper pair center-of-mass momentum Q = π. The transition into the ηπ-paired state corresponds to an abrupt change in the groundstate structure. After the transition the conduction band is completely destroyed and a new ηπ-pair band corresponding to the strongly correlated doublon motion is created.

Physical Review B, 2009

Paramagnetic solutions of the ionic Hubbard model at half-filling in dimensions D > 2 indicate th... more Paramagnetic solutions of the ionic Hubbard model at half-filling in dimensions D > 2 indicate that the band and the Mott insulator phases are separated by a metallic phase. We present zero-temperature dynamical mean-field theory solutions, which include antiferromagnetic long-range order, and show that the one-particle spectral functions always possess an energy gap and therefore the system is insulating for all interaction strengths. The staggered charge density modulation coexists with antiferromagnetic long-range order of Néel type.

Journal of Physics: Condensed Matter, 2003

We investigate the ground-state phase diagram of the one-dimensional "ionic" Hubbard model with a... more We investigate the ground-state phase diagram of the one-dimensional "ionic" Hubbard model with an alternating periodic potential at half-filling by numerical diagonalization of finite systems with the Lanczos and density matrix renormalization group (DMRG) methods. We identify an insulatorinsulator phase transition from a band to a correlated insulator with simultaneous charge and bondcharge order. The transition point is characterized by the vanishing of the optical excitation gap while simultaneously the charge and spin gaps remain finite and equal. Indications for a possible second transition into a Mott-insulator phase are discussed.

The European Physical Journal B - Condensed Matter, 2003

In order to clarify the physics of the crossover from a Peierls band insulator to a correlated Mo... more In order to clarify the physics of the crossover from a Peierls band insulator to a correlated Mott-Hubbard insulator, we analyze ground-state and spectral properties of the one-dimensional halffilled Holstein-Hubbard model using quasi-exact numerical techniques. In the adiabatic limit the transition is connected to the band to Mott insulator transition of the ionic Hubbard model. Depending on the strengths of the electron-phonon coupling and the Hubbard interaction the transition is either first order or evolves continuously across an intermediate phase with finite spin, charge, and optical excitation gaps.

Abstract. In order to clarify the physics of the crossover from a Peierls band insulator to a cor... more Abstract. In order to clarify the physics of the crossover from a Peierls band insulator to a correlated Mott-Hubbard insulator, we analyze ground-state and spectral properties of the one-dimensional halffilled Holstein-Hubbard model using quasi-exact numerical techniques. In the adiabatic limit the transition is connected to the band to Mott insulator transition of the ionic Hubbard model. Depending on the

Physical Review B

Local excitations as carriers of quantum information spread out in the system in ways governed by... more Local excitations as carriers of quantum information spread out in the system in ways governed by the underlying interaction and symmetry. Understanding this phenomenon, also called quantum scrambling, is a prerequisite for employing interacting systems for quantum information processing. The character and direction dependence of quantum scrambling can be inferred from the out-of-time-ordered commutators (OTOCs) containing information on correlation buildup and entanglement spreading. Employing OTOC, we study and quantify the directionality of quantum information propagation in oxide-based helical spin systems hosting a spin-driven ferroelectric order. In these systems, magnetoelectricity permits the spin dynamics and associated information content to be controlled by an electric field coupled to the emergent ferroelectric order. We show that topologically nontrivial quantum phases, such as chiral or helical spin ordering, allow for electric-field controlled anisotropic scrambling and a direction-dependent buildup of quantum correlations. Based on general symmetry considerations, we find that starting from a pure state (e.g., the ground state) or a finite temperature state is essential for observing directional asymmetry in scrambling. In the systematic numerical studies of OTOC on finite-size helical multiferroic chains, we quantify the directional asymmetry of the scrambling and verify the conjectured form of the OTOC around the ballistic wavefront. The obtained direction-dependent butterfly velocity v B (n) provides information on the speed of the ballistic wavefront. In general, our calculations show an early time power-law behavior of OTOC, as expected from an analytic expansion for small times. The long-time behavior of OTOC reveals the importance of (non)integrability of the underlying Hamiltonian as well as the implications of conserved quantities such as the z projection of the total spin. The results point to the potential of spin-driven ferroelectric materials for the use in solid-state-based quantum information processing.

arXiv: Strongly Correlated Electrons, 2020

We calculated the magnetic Compton profiles (MCPs) of Ni using density functional theory suppleme... more We calculated the magnetic Compton profiles (MCPs) of Ni using density functional theory supplemented by electronic correlations treated within dynamical mean-field theory (DMFT). We present comparisons between the theoretical and experimental MCPs. The theoretical MCPs were calculated using the KKR method with the perturbative spin-polarized T-matrix fluctuation exchange approximation DMFT solver, as well as with the full potential linear augmented planewave method with the numerically exact continuous-time quantum Monte Carlo DMFT solver. We show that the total magnetic moment decreases with the intra-atomic Coulomb repulsion UUU, which is also reflected in the corresponding MCPs. The total magnetic moment obtained in experimental measurements can be reproduced by intermediate values of UUU. The spectral function reveals that the minority X$_2$ Fermi surface pocket shrinks and gets shallower with respect to the density functional theory calculations.

arXiv: Strongly Correlated Electrons, 2001

We investigate the ground state phase diagram of the one-dimensional ionic Hubbard model with an ... more We investigate the ground state phase diagram of the one-dimensional ionic Hubbard model with an alternating potential at half-filling by the bosonization technique as well as by numerical diagonalization of finite systems with the Lanczos and DMRG methods. Our results support the existence of a single "metallic" transition point from a band to a correlated insulator with simultaneous charge and bond-charge order. In addition, we present results for the optical conductivity obtained by the dynamical DMRG method. The insulator-insulator phase transition scenario is discussed in detail including a critical review of existing approaches and results for the ionic Hubbard model.

Physica A: Statistical Mechanics and its Applications

The contribution of the valence electrons to the Compton profiles of the crystalline alkali metal... more The contribution of the valence electrons to the Compton profiles of the crystalline alkali metals is calculated using density functional theory. We show that the Compton profiles can be modeled by a q−Gaussian distribution, which is characterized by an anisotropic, element dependent parameter q. Thereby we derive an unexpected scaling behavior of the Compton profiles of all alkali metals.

High Performance Computing in Science and Engineering, Munich 2002, 2003

We analyze ground state and spectral properties of the one-dimensional half-filled Holstein Hubba... more We analyze ground state and spectral properties of the one-dimensional half-filled Holstein Hubbard model with respect to the Peierls-insulator to Mottinsulator transition, exploiting Lanczos diagonalization, density matrix renormalization, kernel polynomial expansion, and maximum entropy methods on the Hitachi SR8000-F1 supercomputer.

We compute the phonon dispersion, density of states, and the Grüneisen parameters of bulk palladi... more We compute the phonon dispersion, density of states, and the Grüneisen parameters of bulk palladium in the combined density functional theory (DFT) and dynamical mean-field theory (DMFT). We find good agreement with experimental results for ground state properties (equilibrium lattice parameter and bulk modulus) and the experimentally measured phonon spectra. We demonstrate that at temperatures T ≲ 20 K the phonon frequency in the vicinity of the Kohn anomaly, ω_T1( q_K), strongly decreases. This is in contrast to DFT where this frequency remains essentially constant in the whole temperature range. Apparently correlation effects reduce the restoring force of the ionic displacements at low temperatures, leading to a mode softening.

Physical Review B

We compute the phonon dispersion, density of states, and the Grüneisen parameters of bulk palladi... more We compute the phonon dispersion, density of states, and the Grüneisen parameters of bulk palladium in the combined density functional theory (DFT) and dynamical mean-field theory (DMFT). We find good agreement with experimental results for ground state properties (equilibrium lattice parameter and bulk modulus) and the experimentally measured phonon spectra. We demonstrate that at temperatures T 20 K the phonon frequency in the vicinity of the Kohn anomaly, ωT 1(qK), strongly decreases. This is in contrast to DFT where this frequency remains essentially constant in the whole temperature range. Apparently correlation effects reduce the restoring force of the ionic displacements at low temperatures, leading to a mode softening.

Physical Review B, 2016

Including on-site electronic interactions described by the multi-orbital Hubbard model we study t... more Including on-site electronic interactions described by the multi-orbital Hubbard model we study the correlation effects in the electronic structure of bulk palladium. We use a combined density functional and dynamical mean field theory, LDA+DMFT, based on the fluctuation exchange approximation. The agreement between the experimentally determined and the theoretical lattice constant and bulk modulus is improved when correlation effects are included. It is found that correlations modify the Fermi surface around the neck at the L-point while the Fermi surface tube structures show little correlation effects. At the same time we discuss the possibility of satellite formation in the high energy binding region. Spectral functions obtained within the LDA+DMFT and GW methods are compared to discuss non-local correlation effects. For relatively weak interaction strength of the local Coulomb and exchange parameters spectra from LDA+DMFT shows no major difference in comparison to GW .

Physical Review B

Quantum dynamics of magnetic order in a chiral multiferroic chain is studied. We consider two dif... more Quantum dynamics of magnetic order in a chiral multiferroic chain is studied. We consider two different scenarios: Ultrashort terahertz (THz) excitations or a sudden electric field quench. Performing analytical and numerical exact diagonalization calculations we trace the pulse induced spin dynamics and extract quantities that are relevant to quantum information processing. In particular, we analyze the dynamics of the system chirality, the von Neumann entropy, the pairwise and the many body entanglement. If the characteristic frequencies of the generated states are non-commensurate then a partial loss of pair concurrence occurs. Increasing the system size this effect becomes even more pronounced. Many particle entanglement and chirality are robust and persist in the incommensurate phase. To analyze the dynamical quantum transitions for the quenched and pulsed dynamics we combined the Weierstrass factorization technique for entire functions and Lanczos exact diagonalization method. For a small system we obtained analytical results including the rate function of Loschmidt echo. Exact numerical calculations for a system up to 40 spins confirm phase transition. Quenchinduced dynamical transitions have been extensively studied recently. Here we show that related dynamical transitions can be achieved and controlled by appropriate electric field pulses.

ABSTRACT We investigate the ground-state phase diagram of the one-dimensional "ionic&amp... more ABSTRACT We investigate the ground-state phase diagram of the one-dimensional "ionic" Hubbard model with an alternating periodic potential at half-filling by numerical diagonalization of finite systems with the Lanczos and density matrix renormalization group (DMRG) methods. We identify an insulator-insulator phase transition from a band to a correlated insulator with simultaneous charge and bond-charge order. The transition point is characterized by the vanishing of the optical excitation gap while simultaneously the charge and spin gaps remain finite and equal. Indications for a possible second transition into a Mott-insulator phase are discussed. The analises is extended to the 3/4-filling and alternating Coulomb interaction -- minimum model Hamiltonian for the crossover from the charge-transfer insulator to the Mott insulator.

We investigate the ground-state phase diagram of the one-dimensional "ionic" Hubbard model with a... more We investigate the ground-state phase diagram of the one-dimensional "ionic" Hubbard model with an alternating periodic potential at half-filling by the bosonization technique as well as by numerical diagonalization of finite systems with the Lanczos and density matrix renormalization group (DMRG) methods. Our results support the existence of a single "metallic" transition point from a band to a correlated insulator with simultaneous charge and bond-charge order. In addition, we present results for the optical conductivity obtained by the dynamical DMRG method. The insulator-insulator phase transition scenario is discussed in detail including a critical review of existing approaches and results for the ionic Hubbard model.

Physical Review B, 2001

The ground state phase diagram of the 1D Hubbard chain with pair-hopping interaction is studied. ... more The ground state phase diagram of the 1D Hubbard chain with pair-hopping interaction is studied. The analysis of the model is performed using the continuum-limit field theory approach and exact diagonalization studies. At half-filling the phase diagram is shown to consist of two superconducting states with Cooper pair center-of-mass momentum Q = 0 (BCS-η0 phase) and Q = π (ηπ-phase) and four insulating phases corresponding to the Mott antiferromagnet, the Peierls dimerized phase, the charge-density-wave (CDW) insulator as well as an unconventional insulating phase characterized by the coexistence of a CDW and a bond-located staggered magnetization. Away from half-filling the phase diagram consists of the superconducting BCS-η0 and ηπ phases and the metallic Luttingerliquid phase. The BCS-η0 phase exhibits smooth crossover from a weak-coupling BCS type to a strong coupling local-pair regime. The ηπ phase shows properties of the doublon (zero size Cooper pair) superconductor with Cooper pair center-of-mass momentum Q = π. The transition into the ηπ-paired state corresponds to an abrupt change in the groundstate structure. After the transition the conduction band is completely destroyed and a new ηπ-pair band corresponding to the strongly correlated doublon motion is created.

Physical Review B, 2009

Paramagnetic solutions of the ionic Hubbard model at half-filling in dimensions D > 2 indicate th... more Paramagnetic solutions of the ionic Hubbard model at half-filling in dimensions D > 2 indicate that the band and the Mott insulator phases are separated by a metallic phase. We present zero-temperature dynamical mean-field theory solutions, which include antiferromagnetic long-range order, and show that the one-particle spectral functions always possess an energy gap and therefore the system is insulating for all interaction strengths. The staggered charge density modulation coexists with antiferromagnetic long-range order of Néel type.

Journal of Physics: Condensed Matter, 2003

We investigate the ground-state phase diagram of the one-dimensional "ionic" Hubbard model with a... more We investigate the ground-state phase diagram of the one-dimensional "ionic" Hubbard model with an alternating periodic potential at half-filling by numerical diagonalization of finite systems with the Lanczos and density matrix renormalization group (DMRG) methods. We identify an insulatorinsulator phase transition from a band to a correlated insulator with simultaneous charge and bondcharge order. The transition point is characterized by the vanishing of the optical excitation gap while simultaneously the charge and spin gaps remain finite and equal. Indications for a possible second transition into a Mott-insulator phase are discussed.

The European Physical Journal B - Condensed Matter, 2003

In order to clarify the physics of the crossover from a Peierls band insulator to a correlated Mo... more In order to clarify the physics of the crossover from a Peierls band insulator to a correlated Mott-Hubbard insulator, we analyze ground-state and spectral properties of the one-dimensional halffilled Holstein-Hubbard model using quasi-exact numerical techniques. In the adiabatic limit the transition is connected to the band to Mott insulator transition of the ionic Hubbard model. Depending on the strengths of the electron-phonon coupling and the Hubbard interaction the transition is either first order or evolves continuously across an intermediate phase with finite spin, charge, and optical excitation gaps.