arXiv subjects

Rossitza Pentcheva

Publications and source records attributed to Rossitza Pentcheva.

At least 37 records · Page 2Linked to original sources

arXiv2021

Theoretical description of optical and X-ray absorption spectra of MgO including many-body effects

Here we report the optical and x-ray absorption (XAS) spectra of the wide-band-gap oxide MgO using density functional theory (DFT) and many-body perturbation theory (MBPT). Our comprehensive study of the electronic structure shows that while the band gap is underestimated with the exchange-correlation functional PBEsol (4.58 eV) and the hybrid functional HSE06 (6.58 eV) compared to the experimental value (7.7 eV), it is significantly improved (7.52 eV) and even overcompensated (8.53 eV) when quasiparticle corrections are considered. Inclusion of excitonic effects by solving the Bethe-Salpeter equation (BSE) yields the optical spectrum in excellent agreement with experiment. Excellent agreement is observed also for the O and Mg K-edge absorption spectra, demonstrating the importance of the electron-hole interaction within MBPT. Projection of the electron-hole coupling coefficients from the BSE eigenvectors on the band structure allows us to determine the origin of prominent peaks and identify the orbital character of the relevant contributions. The real space projection of the lowest energy exciton wavefunction of the optical spectrum indicates a Wannier-Mott type, whereas the first exciton in the O K-edge is more localized.

cond-mat.other↗

arXiv2021

Correlated interface electron gas in infinite-layer nickelate versus cuprate films on SrTiO$_3$(001)

Based on first-principles calculations including a Coulomb repulsion term, we identify trends in the electronic reconstruction of $A$NiO$_2$/SrTiO$_3$(001) ($A=$ Pr, La) and $A$CuO$_2$/SrTiO$_3$(001) ($A=$ Ca, Sr). Common to all cases is the emergence of a quasi-two-dimensional electron gas (q2DEG) in SrTiO$_3$(001), albeit the higher polarity mismatch at the interface of nickelates vs. cuprates to the nonpolar SrTiO$_3(001)$ substrate (${3+}/0$ vs. ${2+}/0$) results in an enhanced q2DEG carrier density. The simulations reveal a significant dependence of the interfacial Ti $3d_{xy}$ band bending on the rare-earth ion in the nickelate films, being $20$-$30\%$ larger for PrNiO$_2$ and NdNiO$_2$ than for LaNiO$_2$. Contrary to expectations from the formal polarity mismatch, the electrostatic doping in the films is twice as strong in cuprates as in nickelates. We demonstrate that the depletion of the self-doping rare-earth $5d$ states enhances the similarity of nickelate and cuprate Fermi surfaces in film geometry, reflecting a single hole in the Ni and Cu $3d_{x^2-y^2}$ orbitals. Finally, we show that NdNiO$_2$ films grown on a polar NdGaO$_3(001)$ substrate feature a simultaneous suppression of q2DEG formation as well as Nd~$5d$ self-doping.

cond-mat.supr-con↗

arXiv2020

Chern insulating phases and thermoelectric properties of EuO/MgO(001) superlattices

The topological and thermoelectric properties of (EuO)$_{n}$/(MgO)$_{m}$(001) superlattices (SLs) are explored using density functional theory calculations including a Hubbard $U$ term together with Boltzmann transport theory. In (EuO)$_{1}$/(MgO)$_{3}$(001) SL at the lattice constant of MgO a sizable band gap of 0.51 eV is opened by spin-orbit coupling (SOC) due to a band inversion between occupied localized Eu $4f$ and $5d$ conduction electrons. This inversion between bands of opposite parity is accompanied by a spin reorientation in the spin-texture along the contour of band crossing surrounding the $Γ$ point and leads to a Chern insulator with $C$=-1, also confirmed by the single edge state. Moreover, this Chern insulating phase shows promising thermoelectric properties, e.g. a Seebeck coefficient between 400 and 800 $μ$VK$^{-1}$. A similar SOC-induced band inversion takes place also in the ferromagnetic semimetallic (EuO)$_{2}$/(MgO)$_{2}$(001) SL. Despite the vanishing band gap, it leads to a substantial anomalous Hall conductivity with values up to -1.04 $e^{2}/h$ and somewhat lower thermoelectric properties. Both systems emphasize the relation between non-trivial topological bands and thermoelectricity also in systems with broken inversion symmetry.

cond-mat.str-el↗

arXiv2020

Fundamental difference in the electronic reconstruction of infinite-layer vs. perovskite neodymium nickelate films on SrTiO$_3$(001)

Motivated by recent reports of superconductivity in Sr-doped NdNiO$_2$ films on SrTiO$_3$(001) [Nature (London) 572, 624 (2019)], we explore the role of the polar interface on the structural and electronic properties of NdNiO$_n$/SrTiO$_3$(001) ($n=2,3$) by performing first-principles calculations including a Coulomb repulsion term. For infinite-layer nickelate films ($n=2$), electronic reconstruction drives the surprising emergence of a two-dimensional electron gas (2DEG) at the interface involving a strong occupation of the Ti $3d$ states. This effect is more pronounced than in LaAlO$_3$/SrTiO$_3$(001) and accompanied by a substantial reconstruction of the Fermi surface: a depletion of the self-doping Nd $5d$ states and an enhanced Ni $e_g$ orbital polarization reaching up to $35\%$ at the surface, reflecting a single hole in the $3d_{x^2-y^2}$ states, i.e., cuprate-like behavior. In contrast, no 2DEG forms for perovskite films ($n=3$) or if a single perovskite layer persists at the interface. We show that the topotactic reaction from the perovskite to the infinite-layer phase is confined to the nickelate film, whereas the SrTiO$_3$ substrate remains intact.

cond-mat.supr-con↗

arXiv2020

Competition of defect ordering and site disproportionation in strained LaCoO$_{3}$ on SrTiO$_3$(001)

The origin of the $3 \times 1$ reconstruction observed in epitaxial LaCoO$_{3}$ films on SrTiO$_3(001)$ is assessed by using first-principles calculations including a Coulomb repulsion term. We compile a phase diagram as a function of the oxygen pressure, which shows that ($3 \times 1$)-ordered oxygen vacancies (LaCoO$_{2.67}$) are favored under commonly used growth conditions, while stoichiometric films emerge under oxygen-rich conditions. Growth of further reduced LaCoO$_{2.5}$ brownmillerite films is impeded by phase separation. We report two competing ground-state candidates for stoichiometric films: a semimetallic phase with $3 \times 1$ low-spin/intermediate-spin/intermediate-spin magnetic order and a semiconducting phase with intermediate-spin magnetic order. This demonstrates that tensile strain induces ferromagnetism even in the absence of oxygen vacancies. Both phases exhibit an intriguing ($3 \times 1$)-reconstructed octahedral rotation pattern and accordingly modulated La-La distances. In particular, charge and bond disproportionation and concomitant orbital order of the $t_{2g}$ hole emerge at the Co sites that are also observed for unstrained bulk LaCoO$_3$ in the intermediate-spin state and explain structural data obtained by x-ray diffraction at elevated temperature. Site disproportionation drives a metal-to-semiconductor transition that reconciles the intermediate-spin state with the experimentally observed low conductivity during spin-state crossover without Jahn-Teller distortions.

cond-mat.mtrl-sci↗

arXiv2019

Chern and $Z_{2}$ topological insulating phases in perovskite-derived $4d$ and $5d$ oxide buckled honeycomb lattices

Based on density functional theory calculations including a Coulomb repulsion parameter $U$, we explore the topological properties of (La$X$O$_3$)$_2$/(LaAlO$_3$)$_4$(111) with $X=$ $4d$ and $5d$ cations. The metastable ferromagnetic phases of LaTcO$_3$ and LaPtO$_3$ preserve P321 symmetry and emerge as Chern insulators (CI) with $C$=2 and 1 and band gaps of 41 and 38 meV at the lateral lattice constant of LaAlO$_3$, respectively. Berry curvatures, spin textures as well as edge states provide additional insight into the nature of the CI states. While for $X$=Tc the CI phase is further stabilized under tensile strain, for $X$=Pd and Pt a site disproportionation takes place when increasing the lateral lattice constant from $a_{\rm LAO}$ to $a_{\rm LNO}$. The CI phase of $X$=Pt shows a strong dependence on the Hubbard $U$ parameter with sign reversal for higher values associated with the change of band gap opening mechanism. Parallels to the previously studied ($X_2$O$_3$)$_1$/(Al$_2$O$_3$)$_5$(0001) honeycomb corundum layers are discussed. Additionally, non-magnetic systems with $X$=Mo and W are identified as potential candidates for $Z_2$ topological insulators at $a_{\rm LAO}$ with band gaps of 26 and 60 meV, respectively. The computed edge states and $Z_{2}$ invariants underpin the non-trivial topological properties.

cond-mat.str-el↗

arXiv2019

Role of the exchange-correlation functional on the structural, electronic and optical properties of cubic and tetragonal SrTiO$_3$ including many-body effects

SrTiO$_3$ is a model perovskite compound with unique properties and technological relevance. At 105 K it undergoes a transition from a cubic to a tetragonal phase with characteristic antiferrodistortive rotations of the TiO$_6$ octahedra. Here we study systematically the effect of different exchange correlation functionals on the structural, electronic and optical properties of cubic and tetragonal STO by comparing the recently implemented strongly constrained and appropriately normed (SCAN) meta-GGA functional with the generalized gradient approximation (PBE96 and PBEsol) and the hybrid functional (HSE06). SCAN is found to significantly improve the description of the structural properties, in particular the rotational angle of the tetragonal phase, comparable to HSE06 at a computational cost similar to GGA. The addition of a Hubbard $U$-term (SCAN+$U$, $U=7.45$ eV) allows to achieve the experimental band gap of 3.25 eV with a moderate increase in the lattice constant, whereas within GGA+$U$ the gap is underestimated even for high $U$ values. The effect of the exchange-correlation functional on the optical properties is progressively reduced from 1.5 eV variance in the onset of the spectrum in the independent particle picture to 0.3 eV upon inclusion of many-body effects within the framework of the $GW$ approximation (single-shot $G_0W_0$) and excitonic corrections by solving the Bethe-Salpeter equation (BSE). Moreover, a model BSE approach is shown to reproduce the main features of the optical spectrum at a lower cost compared to $G_0W_0$+BSE. Strong excitonic effects are found in agreement with previous results and their origin is analyzed based on the contributing interband transitions. Last but not least, the effect of the tetragonal distortion on the optical spectrum is discussed and compared to available experimental data.

cond-mat.str-el↗

arXiv2019

Inducing $n$- and $p$-type thermoelectricity in oxide superlattices by strain tuning of orbital-selective transport resonances

By combining first-principles simulations including an on-site Coulomb repulsion term and Boltzmann theory, we demonstrate how the interplay of quantum confinement and epitaxial strain allows to selectively design $n$- and $p$-type thermoelectric response in (LaNiO$_3$)$_3$/(LaAlO$_3$)$_1(001)$ superlattices. In particular, varying strain from $-4.9$ to $+2.9\%$ tunes the Ni orbital polarization at the interfaces from $-6$ to $+3\%$. This is caused by an electron redistribution among Ni $3d_{x^2-y^2}$- and $3d_{z^2}$-derived quantum well states which respond differently to strain. Owing to this charge transfer, the position of emerging cross-plane transport resonances can be tuned relative to the Fermi energy. Already for moderate values of $1.5$ and $2.8\%$ compressive strain, the cross-plane Seebeck coefficient reaches $\sim -60$ and $+100$ $μ$V/K around room temperature, respectively. This provides a novel mechanism to tailor thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2019

Dynamics of optical excitations in a Fe/MgO(001) heterostructure from time-dependent density functional theory

In the framework of real-time time-dependent density functional theory (RT-TDDFT) we unravel the layer-resolved dynamics of the electronic structure of a (Fe)$_1$/(MgO)$_3$(001) multilayer system after an optical excitation with a frequency below the band gap of bulk MgO. Substantial transient changes to the electronic structure, which persist after the duration of the pulse, are mainly observed for in-plane polarized electric fields, corresponding to a laser pulse arriving perpendicular to the interface. While the strongest charge redistribution takes place in the Fe layer, a time-dependent change in the occupation numbers is visible in all layers, mediated by the presence of interface states. The time evolution of the layer-resolved time-dependent occupation numbers indicates a strong orbital dependence with the depletion from in-plane orbitals (e. g., $d_{x^2-y^2}$ of Fe) and accumulation in out-of-plane orbitals ($d_{3z^2-r^2}$ of Fe and $p_z$ of apical oxygen). We also observe a small net charge transfer away from oxygen towards the Mg sites even for MgO layers which are not directly in contact with the metallic Fe.

cond-mat.mtrl-sci↗

arXiv2018

Interaction-driven spin-orbit effects and Chern insulating phases in corundum-based $4d$ and $5d$ oxide honeycomb lattices

Using density functional theory calculations with a Hubbard $U$, we explore topologically nontrivial phases in $X_2$O$_3$ honeycomb layers with $X=$ $4d$ and $5d$ cation inserted in the band insulator $α$-Al$_2$O$_3$ along the [0001]-direction. Several promising candidates for quantum anomalous Hall insulators (QAHI) are identified. In particular, for $X$=Tc and Pt spin-orbit coupling (SOC) opens a gap of 54 and 59 meV, respectively, leading to Chern insulators (CI) with $C$=--2 and --1. The nature of different Chern numbers is related to the corresponding spin textures. The Chern insulating phase is sensitive to the Coulomb repulsion strength: $X$=Tc undergoes a transition from a CI to a trivial metallic state beyond a critical strength of $U_c$ =2.5 eV. A comparison between the isoelectronic metastable FM phases of $X$=Pd and Pt emphasizes the intricate balance between electronic correlations and SOC: while the former is a trivial insulator, the latter is a Chern insulator. In addition, $X$=Os turns out to be a FM Mott insulator with an unpaired electron in the $t_{2g}$ manifold where SOC induces an unusually high orbital moment of 0.34 $μ_{\rm B}$ along the $z$-axis. Parallels to the $3d$ honeycomb corundum cases are discussed.

cond-mat.str-el↗

arXiv2018

Confinement- and strain-induced enhancement of thermoelectric properties in LaNiO$_3$/LaAlO$_3(001)$ superlattices

By combining ab initio simulations including an on-site Coulomb repulsion term and Boltzmann theory, we explore the thermoelectric properties of (LaNiO$_3$)$_n$/(LaAlO$_3$)$_n$(001) superlattices ($n=1,3$) and identify a strong dependence on confinement, spacer thickness, and epitaxial strain. While the system with $n=3$ shows modest values of the Seebeck coefficient and power factor, the simultaneous reduction of the LaNiO$_3$ region and the LaAlO$_3$ spacer thickness to single layers results in a strong enhancement, in particular of the in-plane values. This effect can be further tuned by using epitaxial strain as control parameter: Under tensile strain corresponding to the lateral lattice constant of SrTiO$_3$ we predict in- and cross-plane Seebeck coefficients of $\pm 600$ $μ$V/K and an in-plane power factor of $11$ $μ$W/K$^2$cm for an estimated relaxation time of $τ= 4$ fs around room temperature. These values are comparable to some of the best performing oxide systems such as La-doped SrTiO$_3$ or layered cobaltates and are associated with the opening of a small gap ($0.29$ eV) induced by the concomitant effect of octahedral tilting and Ni-site disproportionation. This establishes oxide superlattices at the verge of a metal-to-insulator transition driven by confinement and strain as promising candidates for thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2018

Unexpected Termination Switching and Polarity Compensation in LaAlO3/SrTiO3 heterostructures

Polar crystals composed of charged ionic planes cannot exist in nature without acquiring surface changes to balance an ever-growing dipole. The necessary changes can manifest structurally or electronically. An electronic asymetry has long been observed in the LaAlO3/SrTiO3 system. Electron accumulation is observed near the LaAlO3/TiO2-SrTiO3 interface, while the LaAlO3/SrO-SrTiO3 stack is insulating. Here, we observe evidence for an asymmetry in the surface chemical termination for nominally stoichiometric LaAlO3 films in contact with the two different surface layers of SrTiO3 crystals, TiO2 and SrO. Using several element specific probes, we find that the surface termination of LaAlO3 remains AlO2 irrespective of the starting termination of SrTiO3 substrate surface. We use a combination of cross-plane tunneling measurements and first principles calcula- tions to understand the effects of this unexpected termination on band alignments and polarity compensation of LaAlO3/SrTiO3 heterostructures. An asymmetry in LaAlO3 polarity compensation and resulting electronic properties will fundamentally limit atomic level control of oxide heterostructures.

cond-mat.mtrl-sci↗

arXiv2017

Confinement-driven electronic and topological phases in corundum-derived $3d$-oxide honeycomb lattices

Using density functional theory calculations including an on-site Coulomb term, we explore electronic and possibly topologically nontrivial phases in $3d$ transition metal oxide honeycomb layers confined in the corundum structure ($α$-Al$_2$O$_3$) along the [0001] direction. In most cases the ground state is a trivial antiferromagnetic Mott insulator, often with distinct orbital or spin states compared to the bulk phases. With imposed symmetry of the two sublattices the ferromagnetic phases of Ti, Mn, Co and Ni exhibit a characteristic set of four bands, two relatively flat and two with a Dirac crossing at K, associated with the single electron occupation of $e_{g}'$ (Ti) or $e_{g}$ (Mn, Co, Ni) orbitals. Our results indicate that the Dirac point can be tuned to the Fermi level using strain. Applying spin-orbit coupling (SOC) leads to a substantial anomalous Hall conductivity with values up to 0.94 $e^2/h$. Moreover, at $a_{Al_2O_3}$=4.81Å we identify a particularly strong effect of SOC with out-of-plane easy axis for ($Ti_2$O$_3$)$_1$/(Al$_2$O$_3$)$_5$(0001) which stabilizes dynamically the system. Due to the unusually high orbital moment of -0.88$μ_{\rm B}$ that nearly compensates the spin moment of 1.01$μ_{\rm B}$, this system emerges as a candidate for the realization of the topological Haldane model of spinless fermions. Parallels to the perovskite analogs (La$X$O$_3$)$_2$/(LaAlO$_3$)$_4$(111) are discussed.

cond-mat.mtrl-sci↗

arXiv2017

Ordering tendencies and electronic properties in quaternary Heusler derivatives

The phase stabilities and ordering tendencies in the quaternary full-Heusler alloys NiCoMnAl and NiCoMnGa have been investigated by in-situ neutron diffraction, calorimetry and magnetization measurements. NiCoMnGa was found to adopt the L2$_1$ structure, with distinct Mn and Ga sublattices but a common Ni-Co sublattice. A second-order phase transition to the B2 phase with disorder also between Mn and Ga was observed at 1160 K. In contrast, in NiCoMnAl slow cooling or low-temperature annealing treatments are required to induce incipient L2$_1$ ordering, otherwise the system displays only B2 order. Linked to this L2$_1$ ordering, a drastic increase in the magnetic transition temperature was observed in NiCoMnAl, while annealing affected the magnetic behavior of NiCoMnGa only weakly due to the low degree of quenched-in disorder. First principles calculations were employed to study the thermodynamics as well as order-dependent electronic properties of both compounds. It was found that a near half-metallic pseudo-gap emerges in the minority spin channel only for the completely ordered Y structure, which however is energetically unstable compared to the predicted ground state of a tetragonal structure with alternating layers of Ni and Co. The experimental inaccessibility of the totally ordered structures is explained by kinetic limitations due to the low ordering energies.

cond-mat.str-el↗

arXiv2017

Design of $n$- and $p$-type oxide thermoelectrics in LaNiO$_3$/SrTiO$_3(001)$ superlattices exploiting interface polarity

We investigate the structural, electronic, transport, and thermoelectric properties of LaNiO$_3$/SrTiO$_3(001)$ superlattices containing either exclusively $n$- or $p$-type interfaces or coupled interfaces of opposite polarity by using density functional theory calculations with an on-site Coulomb repulsion term. The results show that significant octahedral tilts are induced in the SrTiO$_3$ part of the superlattice. Moreover, the La-Sr distances and Ni-O out-of-plane bond lengths at the interfaces exhibit a distinct variation by about $7\,\%$ with the sign of the electrostatic doping. In contrast to the much studied LaAlO$_3$/SrTiO$_3$ system, the charge mismatch at the interfaces is exclusively accommodated within the LaNiO$_3$ layers, whereas the interface polarity leads to a band offset and to the formation of an electric field within the coupled superlattice. Features of the electronic structure indicate an orbital-selective quantization of quantum well states. The potential- and confinement-induced multiband splitting results in complex cylindrical Fermi surfaces with a tendency towards nesting that depends on the interface polarity. The analysis of the thermoelectric response reveals a particularly large positive Seebeck coefficient ($135~μ$V/K) and a high figure of merit ($0.35$) for room-temperature cross-plane transport in the $p$-type superlattice that is attributed to the participation of the SrTiO$_3$ valence band. Superlattices with either $n$- or $p$-type interfaces show cross-plane Seebeck coefficients of opposite sign and thus emerge as a platform to construct an oxide-based thermoelectric generator with structurally and electronically compatible $n$- and $p$-type oxide thermoelectrics.

cond-mat.mtrl-sci↗

arXiv2017

Modulations in martensitic Heusler alloys originate from nanotwin ordering

Heusler alloys exhibiting magnetic and martensitic transitions enable applications like magnetocaloric refrigeration and actuation based on the magnetic shape memory effect. Their outstanding functional properties depend on low hysteresis losses and low actuation fields. These are only achieved if the atomic positions deviate from a tetragonal lattice by periodic displacements. The origin of the so-called modulated structures is the subject of much controversy: They are either explained by phonon softening or adaptive nanotwinning. Here we used large-scale density functional theory calculations on the Ni2MnGa prototype system to demonstrate interaction energy between twin boundaries. Minimizing the interaction energy resulted in the experimentally observed ordered modulations at the atomic scale, it explained that a/b twin boundaries are stacking faults at the mesoscale, and contributed substantially to the macroscopic hysteresis losses. Furthermore, we found that phonon softening paves the transformation path towards the nanotwinned martensite state. This unified both opposing concepts to explain modulated martensite.

cond-mat.mtrl-sci↗

arXiv2016

Impact of lattice dynamics on the phase stability of metamagnetic FeRh: Bulk and thin films

We present phonon dispersions, element-resolved vibrational density of states (VDOS) and corresponding thermodynamic properties obtained by a combination of density functional theory (DFT) and nuclear resonant inelastic X-ray scattering (NRIXS) across the metamagnetic transition of B2 FeRh in the bulk material and thin epitaxial films. We see distinct differences in the VDOS of the antiferromagnetic (AF) and ferromagnetic (FM) phase which provide a microscopic proof of strong spin-phonon coupling in FeRh. The FM VDOS exhibits a particular sensitivity to the slight tetragonal distortions present in epitaxial films, which is not encountered in the AF phase. This results in a notable change in lattice entropy, which is important for the comparison between thin film and bulk results. Our calculations confirm the recently reported lattice instability in the AF phase. The imaginary frequencies at the $X$-point depend critically on the Fe magnetic moment and atomic volume. Analyzing these non vibrational modes leads to the discovery of a stable monoclinic ground state structure which is robustly predicted from DFT but not verified in our thin film experiments. Specific heat, entropy and free energy calculated within the quasiharmonic approximation suggest that the new phase is possibly suppressed because of its relatively smaller lattice entropy. In the bulk phase, lattice degrees of freedom contribute with the same sign and in similar magnitude to the isostructural AF-FM phase transition as the electronic and magnetic subsystems and therefore needs to be included in thermodynamic modeling.

cond-mat.mtrl-sci↗

arXiv2016

Tailoring Magnetic Frustration in Strained Epitaxial FeRh Films

We report on a strain-induced martensitic transformation, accompanied by a suppression of magnetic order in epitaxial films of chemically disordered FeRh. X-ray diffraction, transmission electron microscopy and electronic structure calculations reveal that the lowering of symmetry (from cubic to tetragonal) imposed by the epitaxial relation leads to a further, unexpected, tetragonal-to-orthorhombic transition, triggered by a band-Jahn-Teller-type lattice instability. The collapse of magnetic order is a direct consequence of this structural change, which upsets the subtle balance between ferromagnetic nearest-neighbor interactions arising from Fe-Rh hybridization and frustrated antiferromagnetic coupling among localized Fe moments at larger distances.

cond-mat.mtrl-sci↗