arXiv subjects

Hyowon Park

Publications and source records attributed to Hyowon Park.

49 records · Page 3Linked to original sources

arXiv2016

Influence of quantum confinement and strain on orbital polarization of four-layer LaNiO$_3$ superlattices: a DFT+DMFT study

Atomically precise superlattices involving transition metal oxides provide a unique opportunity to engineer correlated electron physics using strain (modulated by choice of substate) and quantum confinement (controlled by layer thickness). Here we use the combination of density functional theory and dynamical mean field theory (DFT+DMFT) to study Ni E$_g$ $d$-orbital polarization in strained LaNiO$_3$/LaAlO$_3$ superlattices consisting of four layers of nominally metallic NiO$_2$ and four layers of insulating AlO$_2$ separated by LaO layers. The layer-resolved orbital polarization is calculated as a function of strain and analyzed in terms of structural, quantum confinement, and correlation effects. The effect of strain is determined from the dependence of the results on the Ni-O bond-length ratio and the octahedral rotation angles; quantum confinement is studied by comparison to bulk calculations with similar degrees of strain; correlation effects are inferred by varying interaction parameters within our DFT+DMFT calculations. The calculated dependence of orbital polarization on strain in superlattices is qualitatively consistent with recent X-ray absorption spectroscopy and resonant reflectometry data. However, interesting differences of detail are found between theory and experiment. Under tensile strain, the two inequivalent Ni ions display orbital polarization similar to that calculated for strained bulk LaNiO$_3$ and observed in experiment. Compressive strain produces a larger dependence of orbital polarization on Ni position and even the inner Ni layer exhibits orbital polarization different from that calculated for strained bulk LaNiO$_3$.

cond-mat.str-el↗

arXiv2015

New class of planar ferroelectric Mott insulators via first principles design

Inorganic perovskite oxide ferroelectrics have recently generated substantial interest for photovoltaic applications; however, existing materials suffer from excessive electronic band gaps and insufficient electric polarization. The recent resurgence of hybrid perovskite ferroelectrics addresses the aforementioned deficiencies, but they are highly unstable against environmental effects and an inorganic resolution may still be optimal. Here we use first-principles calculations to design a low band gap, planar ferroelectric by leveraging the complexity of layered double perovskite oxides AA$^\prime $BB$^\prime$O$_6$. We exploit A$\ne$A$^\prime$ size mismatch, a nominally empty $d$-shell on the B-site, and A cations bearing lone-pair electrons to achieve a large ferroelectric polarization. Additionally, B$^\prime\ne$B is chosen to achieve full charge transfer and a Mott susceptible filling on the B$^\prime$-site, yielding a low band gap. These principles are illustrated in BaBiCuVO$_6$, BaBiNiVO$_6$, BaLaCuVO$_6$, and PbLaCuVO$_6$, which could be realized in layer-by-layer growth. This new class of materials could lead to stable, high-efficiency photovoltaic.

cond-mat.mtrl-sci↗

arXiv2015

Density functional versus spin-density functional and the choice of correlated subspace in multi-variable effective action theories of electronic structure

Modern extensions of density functional theory such as the density functional theory plus U and the density functional theory plus dynamical mean-field theory require choices, including selection of variable (charge vs spin density) for the density functional and specification of the correlated subspace. This paper examines these issues in the context of the "plus U" extensions of density functional theory, in which additional correlations on specified correlated orbitals are treated using a Hartree-Fock approximation. Differences between using charge-only or spin-density-dependent exchange-correlation functionals and between Wannier and projector-based definitions of the correlated orbitals are considered on the formal level and in the context of the structural energetics of the rare earth nickelates. It is demonstrated that theories based on spin-dependent exchange-correlation functionals can lead to large and in some cases unphysical effective on-site exchange couplings. Wannier and projector-based definitions of the correlated orbitals lead to similar behavior near ambient pressure, but substantial differences are observed at large pressures. Implications for other beyond density functional methods such as the combination of density functional and dynamical mean field theory are discussed.

cond-mat.str-el↗

arXiv2015

Charge transfer across transition metal oxide interfaces: emergent conductance and new electronic structure

We perform density functional theory plus dynamical mean field theory calculations to investi- gate internal charge transfer in an artificial superlattice composed of alternating layers of vanadate and manganite perovskite and Ruddlesden-Popper structure materials. We show that the elec- tronegativity difference between vanadium and manganese causes moderate charge transfer from VO2 to MnO2 layers in both perovskite and Ruddlesden-Popper based superlattices, leading to hole doping of the VO2 layer and electron doping of the MnO2 layer. Comparison of the perovskite and Ruddlesden-Popper based heterostructures provides insights into the role of the apical oxy- gen. Our first principles simulations demonstrate that the combination of internal charge transfer and quantum confinement provided by heterostructuring is a powerful approach to engineering electronic structure and tailoring correlation effects in transition metal oxides.

cond-mat.mtrl-sci↗

arXiv2014

Total energy calculations using DFT+DMFT: computing the pressure phase diagram of the rare earth nickelates

A full implementation of the $ab$ $initio$ density functional plus dynamical mean field theory (DFT+DMFT) formalism to perform total energy calculations and structural relaxations is proposed and implemented. The method is applied to the structural and metal-insulator transitions of the rare earth nickelate perovskites as a function of rare earth ion, pressure, and temperature. In contrast to previous DFT and DFT+$U$ theories, the present method accounts for the experimentally observed structure of $La$NiO$_3$ and the insulating nature of the other perovskites, and quantitatively reproduces the metal-insulator and structural phase diagram in the plane of pressure and rare earth element. The temperature dependence of the energetics of the phase transformation indicates that the thermal transition is driven by phonon entropy effects.

cond-mat.str-el↗

arXiv2014

Computing total energies in complex materials using charge self-consistent DFT+DMFT

We have formulated and implemented a fully charge-self-consistent density functional theory plus dynamical mean field theory methodology which enables an efficient calculation of the total energy of realistic correlated electron systems. The density functional portion of the calculation uses a plane wave basis set within the projector augmented wave method enabling study of systems with large, complex unit cells. The dynamical mean field portion of the calculation is formulated using maximally localized Wannier functions, enabling a convenient implementation which is independent of the basis set used in the density functional portion of the calculation. The importance of using a correct double counting term is demonstrated. A generalized form of the standard double counting correction, which we refer to as the $U^\prime$ form, is described in detail and used. For comparison the density functional plus U method is implemented within the same framework including the generalized double counting. The formalism is validated via a calculation of the metal-insulator and structural phase diagrams of the rare-earth nickelate perovskites as functions of applied pressure and A-site rare-earth ions. The calculated density functional plus dynamical mean field results are found to be consistent with experiment. The density functional plus U method is shown to grossly overestimate the tendency for bond-disproportionation and insulating behavior.

cond-mat.str-el↗

arXiv2014

Selectively Localized Wannier Functions

Since the seminal work of Marzari and Vanderbilt, maximally localized Wannier functions have become widely used as a real-space representation of the electronic structure of periodic materials. In this paper we introduce selectively localized Wannier functions which extend the method of Marzari and Vanderbilt in two important ways. First, our method allows us to focus on localizing a subset of orbitals of interest. Second, our method allows us to fix centers of these orbitals, and ensure the preservation of the point-group symmetry. These characteristics are important when Wannier functions are used in methodologies that go beyond density functional theory by treating a local subspace of the Hamiltonian more effectively. Application of our method to GaAs, SrMnO$_3$, and Co demonstrates that selectively localized Wannier functions can offer improvements over the maximally localized Wannier function technique.

cond-mat.str-el↗

arXiv2013

The failure of DFT-based computations for a stepped-substrate-supported correlated Co wire

Density functional theory (DFT) has been immensely successful in its ability to predict physical properties, and, in particular, structures of condensed matter systems. Here, however, we show that DFT qualitatively fails to predict the dimerized structural phase for a monatomic Co wire that is self-assembled on a vicinal, i.e. stepped, Cu(111) substrate. To elucidate the nature of this failure, we compute the energetics of a Co chain on a Cu surface, step, notch, and embedded in bulk. The results demonstrate that increasing Co coordination extinguishes the dimerization, indicating that the failure of DFT for Co on the Cu step arises from excessive hybridization, which both weakens the ferromagnetic correlations that drive the dimerization and increases the bonding that opposes dimerization. Additionally, we show that including local interactions via DFT+U or DFT+DMFT does not restore the dimerization for the step-substrate supported wire, though the Co wire does dimerize in DFT+DMFT for the isolated vacuum case. This system can serve as a benchmark for future electronic structure methods.

cond-mat.mtrl-sci↗

arXiv2012

Covalency, double-counting and the metal-insulator phase diagram in transition metal oxides

Dynamical mean field theory calculations are used to show that for late transition-metal-oxides a critical variable for the Mott/charge-transfer transition is the number of d-electrons, which is determined by charge transfer from oxygen ions. Insulating behavior is found only for a narrow range of d-occupancy, irrespective of the size of the intra-d Coulomb repulsion. The result is useful in interpreting 'density functional +U' and 'density functional plus dynamical mean field' methods in which additional correlations are applied to a specific set of orbitals and an important role is played by the 'double counting correction' which dictates the occupancy of these correlated orbitals. General considerations are presented and are illustrated by calculations for two representative transition metal oxide systems: layered perovskite Cu-based "high-Tc" materials, an orbitally non-degenerate electronically quasi-two dimensional systems, and pseudocubic rare earch nickelates, an orbitally degenerate electronically three dimensional system. Density functional calculations yield d-occupancies very far from the Mott metal-insulator phase boundary in the nickelate materials, but closer to it in the cuprates, indicating the sensitivity of theoretical models of the cuprates to the choice of double counting correction and corroborating the critical role of lattice distortions in attaining the experimentally observed insulating phase in the nickelates.

cond-mat.str-el↗

arXiv2012

Site-selective Mott transition in rare earth nickelates

A combination of density functional and dynamical mean field theory calculations are used to show that the remarkable metal-insulator transition in the rare earth nickelate perovskites arise from a site-selective Mott phase, in which the $d$-electrons on a half of the Ni ions are localized to form a fluctuating moment while the $d$-electrons on other Ni ions form a singlet with holes on the surrounding oxygen ions. The calculation reproduces key features observed in the nickelate materials, including an insulating gap in the paramagnetic state, a strong variation of static magnetic moments among Ni sites and an absence of "charge order". A connection between structure and insulating behavior is documented. The site-selective Mott transition may be a more broadly applicable concept in the description of correlated materials.

cond-mat.str-el↗

arXiv2012

Nature of magnetic excitations in superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$

Since the discovery of the metallic antiferromagnetic (AF) ground state near superconductivity in iron-pnictide superconductors, a central question has been whether magnetism in these materials arises from weakly correlated electrons, as in the case of spin-density-wave in pure chromium, requires strong electron correlations, or can even be described in terms of localized electrons such as the AF insulating state of copper oxides. Here we use inelastic neutron scattering to determine the absolute intensity of the magnetic excitations throughout the Brillouin zone in electron-doped superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ ($T_c=20$ K), which allows us to obtain the size of the fluctuating magnetic moment $ $, and its energy distribution. We find that superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ and AF BaFe$_2$As$_2$ both have fluctuating magnetic moments $ \approx3.2\ μ_B^2$ per Fe(Ni), which are similar to those found in the AF insulating copper oxides. The common theme in both classes of high temperature superconductors is that magnetic excitations have partly localized character, thus showing the importance of strong correlations for high temperature superconductivity.

cond-mat.supr-con↗

arXiv2011

The magnetic excitation spectra in BaFe$_{2}$As$_{2}$: a two-particle approach within DFT+DMFT

We study the magnetic excitation spectra in the paramagnetic state of BaFe$_{2}$As$_{2}$ from the \textit{ab initio} perspective. The one-particle excitation spectrum is determined within the combination of the density functional theory and the dynamical mean-field theory method. The two-particle response function is extracted from the local two-particle vertex function, also computed by the dynamical mean field theory, and the polarization function. This method reproduces all the experimentally observed features in inelastic neutron scattering (INS), and relates them to both the one particle excitations and the collective modes. The magnetic excitation dispersion is well accounted for by our theoretical calculation in the paramagnetic state without any broken symmetry, hence nematic order is not needed to explain the INS experimental data.

cond-mat.supr-con↗

arXiv2007

A dynamical mean-field theory study of Nagaoka ferromagnetism

We revisit Nagaoka ferromagnetism in the U=oo Hubbard model within the dynamical mean-field theory (DMFT) using the recently developed continuous time quantum Monte Carlo method as the impurity solver. The stability of Nagaoka ferromagnetism is studied as a function of the temperature, the doping level, and the next-nearest-neighbor lattice hopping t'. We found that the nature of the phase transition as well as the stability of the ferromagnetic state is very sensitive to the t' hopping. Negative t'=-0.1t stabilizes ferromagnetism up to higher doping levels. The paramagnetic state is reached through a first order phase transition. Alternatively, a second order phase transition is observed at t'=0. Very near half-filling, the coherence temperature T_{coh} of the paramagnetic metal becomes very low and ferromagnetism evolves out of an incoherent metal rather than conventional Fermi liquid. We use the DMFT results to benchmark slave-boson method which might be useful in more complicated geometries.

cond-mat.str-el↗