arXiv subjects

Sohrab Ismail-Beigi

Publications and source records attributed to Sohrab Ismail-Beigi.

At least 37 records · Page 2Linked to original sources

arXiv2019

Causes of ferroelectricity in HfO$_{2}$-based thin films: An $\textit{ab initio}$ perspective

We present a comprehensive first principles study of doped hafnia in order to understand the formation of the ferroelectric orthorhombic [001] grains. Assuming that tetragonal grains are present during the early stages of growth, matching plane analysis shows that tetragonal [100] grains can transform into orthorhombic [001] during thermal annealing, when they are laterally confined by other grains. We show that among 0%, 2% and %4 Si doping, 4% doping provides the best conditions for the tetragonal [100] to orthorhombic [001] transformation. This also holds for Al doping. We also show that for Hf$_{x}$Zr$_{1-x}$O$_{2}$, where we have studied ${x}=1.00,0.75,0.50,0.25,0.00$, the value ${x}=0.50$ provides the most favorable conditions for the desired transformation. In order for this transformation to be preferred over the tetragonal [100] to monoclinic [100] transformation, out-of-plane confinement also needs to be present, as supplied by a top electrode. Our findings illuminate the mechanism that causes ferroelectricity in hafnia-based films and provide an explanation for common experimental observations for the optimal ranges of doping in Si:HfO$_{2}$, Al:HfO$_{2}$ and Hf$_{x}$Zr$_{1-x}$O$_{2}$. We also present model thin film heterostructure computations of Ir/HfO$_{2}$/Ir stacks in order to isolate the interface effects, which we show to be significant.

cond-mat.mtrl-sci↗

arXiv2019

Ferroelectric ZrO$_{2}$ monolayers as buffer layers between SrTiO$_{3}$ and Si

A monolayer of ZrO$_{2}$ has recently been grown on the Si(001) surface and shown to have ferroelectric properties, which signifies the realization of the lowest possible thickness in ferroelectric oxides [M. Dogan et al., Nano Lett., 18 (1) (2018)]. In our previous computational study, we reported on the multiple (meta)stable configurations of ZrO$_{2}$ monolayers on Si, and how switching between a pair of differently polarized configurations may explain the observed ferroelectric behavior of these films [M. Dogan and S. Ismail-Beigi, arXiv:1902.01022 (2019)]. In the current study, we conduct a DFT-based investigation of (i) the effect of oxygen content on the ionic polarization of the oxide, and (ii) the role of zirconia monolayers as buffer layers between silicon and a thicker oxide film that is normally paraelectric on silicon, e.g. SrTiO$_{3}$. We find that (i) total energy-vs-polarization behavior of the monolayers, as well as interface chemistry, is highly dependent on the oxygen content; and (ii) SrTiO$_{3}$/ZrO$_{2}$/Si stacks exhibit multiple (meta)stable configurations and polarization profiles, i.e. zirconia monolayers can induce ferroelectricity in oxides such as SrTiO$_{3}$ when used as a buffer layer. This may enable a robust non-volatile device architecture where the thickness of the gate oxide (here strontium titanate) can be chosen according to the desired properties.

cond-mat.mtrl-sci↗

arXiv2019

Theory of Ferroelectric ZrO$_{2}$ Monolayers on Si

We use density functional theory and Monte Carlo lattice simulations to investigate the structure of ZrO$_{2}$ monolayers on Si(001). Recently, we have reported on the experimental growth of amorphous ZrO$_{2}$ monolayers on silicon and their ferroelectric properties, marking the achievement of the thinnest possible ferroelectric oxide [M. Dogan et al. Nano Lett., 18 (1) (2018)]. Here, we first describe the rich landscape of atomic configurations of monocrystalline ZrO$_{2}$ monolayers on Si and determine the local energy minima. Because of the multitude of low-energy configurations we find, we consider the coexistence of finite-sized regions of different configurations. We create a simple nearest-neighbor lattice model with parameters extracted from DFT calculations, and solve it numerically using a cluster Monte Carlo algorithm. Our results suggest that up to room temperature, the ZrO$_{2}$ monolayer consists of small domains of two low-energy configurations with opposite ferroelectric polarization. This explains the observed ferroelectric behavior in the experimental films as a collection of crystalline regions, which are a few nanometers in size, being switched with the application of an external electric field.

cond-mat.mtrl-sci↗

arXiv2018

Suppression of the Spectral Weight of Topological Surface States on the Nanoscale via Local Symmetry Breaking

In topological crystalline insulators the topological conducting surface states are protected by crystal symmetry, in principle making it possible to pattern nanoscale insulating and conductive motifs solely by breaking local symmetries on an otherwise homogenous, single-phase material. We show using scanning tunneling microscopy/spectroscopy that defects that break local symmetry of SnTe suppress electron tunneling over an energy range as large as the bulk band gap, an order of magnitude larger than that produced globally via magnetic fields or uniform structural perturbations. Complementary ab initio calculations show how local symmetry breaking obstructs topological surface states as shown by a threefold reduction of the spectral weight of the topological surface states. The finding highlights the potential benefits of manipulating the surface morphology to create devices that take advantage of the unique properties of topological surface states and can operate at practical temperatures.

cond-mat.mtrl-sci↗

arXiv2018

Scalable GW software for quasiparticle properties using OpenAtom

The GW method, which can describe accurately electronic excitations, is one of the most widely used ab initio electronic structure technique and allows the physics of both molecular and condensed phase materials to be studied. However, the applications of the GW method to large systems require supercomputers and highly parallelized software to overcome the high computational complexity of the method scaling as $O(N^4)$. Here, we develop efficient massively-parallel GW software for the plane-wave basis set by revisiting the standard GW formulae in order to discern the optimal approaches for each phase of the GW calculation for massively parallel computation. These best numerical practices are implemented into the OpenAtom software which is written on top of charm++ parallel framework. We then evaluate the performance of our new software using range of system sizes. Our GW software shows significantly improved parallel scaling compared to publically available GW software on the Mira and Blue Waters supercomputers, two of largest most powerful platforms in the world.

physics.comp-ph↗

arXiv2018

Control of hidden ground-state order in NdNiO$_3$ superlattices

The combination of charge and spin degrees of freedom with electronic correlations in condensed matter systems leads to a rich array of phenomena, such as magnetism, superconductivity, and novel conduction mechanisms. While such phenomena are observed in bulk materials, a richer array of behaviors becomes possible when these degrees of freedom are controlled in atomically layered heterostructures, where one can constrain dimensionality and impose interfacial boundary conditions. Here, we unlock a host of unique, hidden electronic and magnetic phase transitions in NdNiO$_3$ while approaching the two-dimensional (2D) limit, resulting from the differing influences of dimensional confinement and interfacial coupling. Most notably, we discover a new phase in fully 2D, single layer NdNiO$_3$, in which all signatures of the bulk magnetic and charge ordering are found to vanish. In addition, for quasi two-dimensional layers down to a thickness of two unit cells, bulk-type ordering persists but separates from the onset of insulating behavior in a manner distinct from that found in the bulk or thin film nickelates. Using resonant x-ray spectroscopies, first-principles theory, and model calculations, we propose that the single layer phase suppression results from a new mechanism of interfacial electronic reconstruction based on ionicity differences across the interface, while the phase separation in multi-layer NdNiO$_3$ emerges due to enhanced 2D fluctuations. These findings provide insights into the intertwined mechanisms of charge and spin ordering in strongly correlated systems in reduced dimensions and illustrate the ability to use atomic layering to access hidden phases.

cond-mat.str-el↗

arXiv2018

Imaginary time, shredded propagator method for large-scale GW calculations

The GW method is a many-body approach capable of providing quasiparticle bands for realistic systems spanning physics, chemistry, and materials science. Despite its power, GW is not routinely applied to large complex materials due to its computational expense. We perform an exact recasting of the GW polarizability and the self-energy as Laplace integrals over imaginary time propagators. We then "shred" the propagators (via energy windowing) and approximate them in a controlled manner by using Gauss-Laguerre quadrature and discrete variable methods to treat the imaginary time propagators in real space. The resulting cubic scaling GW method has a sufficiently small prefactor to outperform standard quartic scaling methods on small systems (>=10 atoms) and also represents a substantial improvement over several other cubic methods tested. This approach is useful for evaluating quantum mechanical response function involving large sums containing energy (difference) denominators.

cond-mat.mtrl-sci↗

arXiv2017

How correlated is the FeSe/SrTiO$_3$ system ?

Recent observation of $\sim$ 10 times higher critical temperature in FeSe monolayer compared with its bulk phase has drawn a great deal of attention because the electronic structure in the monolayer phase appears to be different than bulk FeSe. Using a combination of density functional theory and dynamical mean field theory, we find electronic correlations have important effects on the predicted atomic-scale geometry and the electronic structure of the monolayer FeSe on SrTiO$_3$. The electronic correlations are dominantly controlled by the Se-Fe-Se angle either in the bulk phase or the monolayer phase. But the angle sensitivity increases and the orbital differentiation decreases in the monolayer phase compared to the bulk phase. The correlations are more dependent on Hund's J than Hubbard U. The observed orbital selective incoherence to coherence crossover with temperature confirms the Hund's metallic nature of the monolayer FeSe. We also find electron doping by oxygen vacancies in SrTiO$_3$ increases the correlation strength, especially in the $d_{xy}$ orbital by reducing the Se-Fe-Se angle.

cond-mat.supr-con↗

arXiv2017

Justifying quasiparticle self-consistent schemes via gradient optimization in Luttinger-Ward theory

The question of which non-interacting Green's function "best" describes an interacting many-body electronic system is both of fundamental interest as well as of practical importance in describing electronic properties of materials in a realistic manner. Here, we study this question within the framework of Baym-Kadanoff theory, an approach where one locates the stationary point of a total energy functional of the one-particle Green's function in order to find the total ground-state energy as well as all one-particle properties such as the density matrix, chemical potential, or the quasiparticle energy spectrum and quasiparticle wave functions. For the case of the Klein functional, our basic finding is that minimizing the length of the gradient of the total energy functional over non-interacting Green's functions yields a set of self-consistent equations for quasiparticles that is identical to those of the Quasiparticle Self-Consistent GW (QSGW) [1] approach, thereby providing an a priori justification for such an approach to electronic structure calculations. In fact, this result is general, applies to any self-energy operator, and is not restricted to any particular approximation, e.g. the GW approximation for the self-energy. The approach also shows that, when working in the basis of quasiparticle states, solving the diagonal part of the self-consistent Dyson equation is of primary importance while the off-diagonals are of secondary importance, a common observation in the electronic structure literature of self-energy calculations. Finally, numerical tests and analytical arguments show that when the Dyson equation produces multiple quasiparticle solutions corresponding to a single non-interacting state, minimizing the length of the gradient translates into choosing the solution with largest quasiparticle weight.

cond-mat.mtrl-sci↗

arXiv2017

Symmetry Breaking in Occupation Number Based Slave-Particle Methods

We describe a theoretical approach for finding spontaneously symmetry-broken electronic phases due to strong electronic interactions when using recently developed slave-particle (slave-boson) approaches based on occupation numbers. We describe why, to date, spontaneous symmetry breaking has proven difficult to achieve in such approaches. We then provide a total-energy based approach for introducing auxiliary symmetry breaking fields into the solution of the slave-particle problem that leads to lowered total energies for symmetry broken phases. We point out that not all slave-particle approaches yield to energy lowering: the slave-particle model being used must explicitly describe the degrees of freedom that break symmetry. Finally, our total energy approach permits us to greatly simplify the formalism used to achieve a self-consistent solution between spinon and slave modes while increasing numerical stability and greatly speeding up the calculations.

cond-mat.str-el↗

arXiv2016

Experimental verification of orbital engineering at the atomic scale: charge transfer and symmetry breaking in nickelate heterostructures

Epitaxial strain, layer confinement and inversion symmetry breaking have emerged as powerful new approaches to control the electronic and atomic-scale structural properties in complex metal oxides. Nickelate heterostructures, based on RENiO$_3$, where RE is a trivalent rare-earth cation, have been shown to be relevant model systems since the orbital occupancy, degeneracy, and, consequently, the electronic/magnetic properties can be altered as a function of epitaxial strain, layer thickness and superlattice structure. One such recent example is the tri-component LaTiO$_3$-LaNiO$_3$-LaAlO$_3$ superlattice, which exhibits charge transfer and orbital polarization as the result of its interfacial dipole electric field. A crucial step towards control of these parameters for future electronic and magnetic device applications is to develop an understanding of both the magnitude and range of the octahedral network's response towards interfacial strain and electric fields. An approach that provides atomic-scale resolution and sensitivity towards the local octahedral distortions and orbital occupancy is therefore required. Here, we employ atomic-resolution imaging coupled with electron spectroscopies and first principles theory to examine the role of interfacial charge transfer and symmetry breaking in a tricomponent nickelate superlattice system. We find that nearly complete charge transfer occurs between the LaTiO$_3$ and LaNiO$_3$ layers, resulting in a Ni$^{2+}$ valence state. We further demonstrate that this charge transfer is highly localized with a range of about 1 unit cell, within the LaNiO$_3$ layers. The results presented here provide important feedback to synthesis efforts aimed at stabilizing new electronic phases that are not accessible by conventional bulk or epitaxial film approaches.

cond-mat.mtrl-sci↗

arXiv2016

The role of double TiO2 layers at the interface of FeSe/SrTiO3 superconductors

We determine the surface reconstruction of SrTiO3 used to achieve superconducting FeSe films in experiments, which is different from the 1x1 TiO2 terminated SrTiO3 assumed by most previous theoretical studies. In particular, we identify the existence of a double TiO2 layer at the SrTiO3-FeSe interface that plays two important roles. First, it facilitates the epitaxial growth of FeSe. Second, ab initio calculations reveal a strong tendency for electrons to transfer from an oxygen deficient SrTiO3 surface to FeSe when the double TiO2 layer is present. As a better electron donor than previously proposed interfacial structures, the double layer helps to remove the hole pocket in the FeSe at the Γ point of the Brillouin zone and leads to a band structure characteristic of superconducting samples. The characterization of the interface structure presented here is a key step towards the resolution of many open questions about this novel superconductor.

cond-mat.str-el↗

arXiv2015

Generalized slave-particle method for extended Hubbard models

We introduce a set of generalized slave-particle models for extended Hubbard models that treat localized electronic correlations using slave-boson decompositions. Our models automatically include two slave-particle methods of recent interest, the slave-rotor and slave-spin methods, as well as a ladder of new intermediate models where one can choose which of the electronic degrees of freedom (e.g., spin or orbital labels) are treated as correlated degrees of freedom by the slave bosons. In addition, our method removes the aberrant behavior of the slave-rotor model at weak correlation strength by removing the contribution of unphysical states from the bosonic Hilbert space. The flexibility of our formalism permits one to separate and isolate the effect of correlations on the key degrees of freedom.

cond-mat.str-el↗

arXiv2015

The importance of anisotropic Coulomb interaction in LaMnO$_{3}$

In low-temperature anti-ferromagnetic LaMnO$_{3}$, strong and localized electronic interactions among Mn 3d electrons prevent a satisfactory description from standard local density and generalized gradient approximations in density functional theory calculations. Here we show that the strong on-site electronic interactions are described well only by using direct and exchange corrections to the intra-orbital Coulomb potential. Only DFT+U calculations with explicit exchange corrections produce a balanced picture of electronic, magnetic and structural observables in agreement with experiment. To understand the reason, a rewriting of the functional form of the +U corrections is presented that leads to a more physical and transparent understanding of the effect of these correction terms. The approach highlights the importance of Hund's coupling (intra-orbital exchange) in providing anisotropy across the occupation and energy eigenvalues of the Mn d states. This intra-orbital exchange is the key to fully activating the Jahn-Teller distortion, reproducing the experimental band gap and stabilizing the correct magnetic ground state in LaMnO$_{3}$. The best parameter values for LaMnO$_{3}$ within the DFT (PBEsol) +U framework are determined to be U = 8 eV and J = 1.9 eV.

cond-mat.mtrl-sci↗

arXiv2014

Dynamical control of orbital occupations via a ferroelectric-induced polar state in metallic manganites

The breaking of orbital degeneracy on a transition metal cation and the resulting unequal electronic occupations of these orbitals provide a powerful lever over electron density and spin ordering in metal oxides. Here, we show how to dynamically modulate the orbital populations on Mn atoms at ferroelectric/manganite interfaces by switching the ferroelectric polarization. The change in orbital occupation can be as large as 10\%, greatly exceeding that of bulk manganites. This flippable orbital splitting is in large part controlled by the propagation of ferroelectric polar displacements into the interfacial region, a structural motif absent in the bulk and unique to the interface. We use {\it ab initio} theory, epitaxial thin film growth, and scanning transmission electron microscopy to verify the predicted interfacial polar state and concomitant orbital splittings.

cond-mat.mtrl-sci↗

arXiv2014

Reversible modulation of orbital occupations via an interface-induced state in metallic manganites

The breaking of orbital degeneracy on a transition metal cation and the resulting unequal electronic occupations of these orbitals provide a powerful lever over electron density and spin ordering inmetal oxides. Here, we use ab initio calculations to show that reversibly modulating the orbital populations on Mn atoms can be achieved at ferroelectric/manganite interfaces by the presence of ferroelectric polarization on the nanoscale. The change in orbital occupation can be as large as 10%, greatly exceeding that of bulk manganites. This reversible orbital splitting is in large part controlled by the propagation of ferroelectric polar displacements into the interfacial region, a structural motif absent in the bulk and unique to the interface. We use epitaxial thin film growth and scanning transmission electron microscopy to verify this key interfacial polar distortion and discuss the potential of reversible control of orbital polarization via nanoscale ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2014

Imaging the buried MgO/Ag interface: formation mechanism of the STM contrast

Scanning tunneling microscopy (STM) provides real-space electronic state information at the atomic scale that is most commonly used to study materials surfaces. An intriguing extension of the method is attempt to study the electronic structure at an insulator/conductor interface by performing low-bias imaging above the surface of an ultrathin insulating layer on the conducting substrate. We use first-principles theory to examine the physical mechanisms giving rise to the formation of low-bias STM images in the MgO/Ag system. We show that the main features of the low-bias STM contrast are completely determined by the atoms on the surface of MgO. Hence, the low-bias contrast is formed by states at the Fermi level in the Ag that propagate evanescently through the lattice and atomic orbitals of the MgO on their way to the surface. We develop a number of analysis techniques based on an ab initio tight-binding representation that allow identification of the origin of the STM contrast in cases where previous approaches have proven ambiguous.

cond-mat.mtrl-sci↗

arXiv2013

Modifying the Electronic Orbitals of Nickelate Heterostructures Via Structural Distortions

We describe a general materials design approach that produces large orbital energy splittings (orbital polarization) in nickelate heterostructures, creating a two-dimensional single-band electronic surface at the Fermi energy. The resulting electronic structure mimics that of the high temperature cuprate superconductors. The two key ingredients are: (i) the construction of atomic-scale distortions about the Ni site via charge transfer and internal electric fields, and (ii) the use of three component (tri-component) superlattices to break inversion symmetry. We use {\it ab initio} calculations to implement the approach, with experimental verification of the critical structural motif that enables the design to succeed.

cond-mat.mtrl-sci↗