arXiv subjects

Walter R. L. Lambrecht

Publications and source records attributed to Walter R. L. Lambrecht.

At least 37 records · Page 2Linked to original sources

arXiv2022

Calculated electron paramagnetic resonance $g$-tensor and hyperfine parameters for zinc vacancy and N related defects in ZnO

Various defects in ZnO, focused on substitutional N$_O$ and N$_2$ in various sites, O-site, interstitial and Zn-site are studied using first-principles calculations with the goal of understanding the electron paramagnetic resonance (EPR) center reported for N$_2$ in ZnO and substitutional N on the O-site. The $g$ tensors are calculated using the gauge including projector augmented wave (GIPAW) method and compared with experiments. The $g$-tensor of the free N$_2^+$ and N$_2^-$ radicals and their various contributions within the GIPAW theory are analyzed first to provide a baseline reference for the accuracy of the method and for understanding the N$_2$ behavior in ZnO. Previous controversies on the site location of N$_2$ in ZnO for this EPR center and on the shallow or deep nature and donor or acceptor nature of this center are resolved. We find that the N$_2$ on the Zn site is mostly zinc-vacancy like in its spin density and $g$-tensor, while for the O-site, a model with the N$_2$ axis lying in-the basal plane and the singly occupied $π_g$-orbital along the {\bf c} axis provides good agreement with experiment. For the interstitial location, if the N$_2$ is not strongly interacting with the surroundings, no levels in the gap are found and hence also no possible EPR center. The calculated $g$-tensors for N$_O$ and $V_ Zn$ are also found to be in good agreement with experiment. The effects of different functionals affecting the localization of the spin density are shown to affect the $g$-tensor values.

cond-mat.mtrl-sci↗

arXiv2022

Quasiparticle self-consistent $GW$ band structures and phase transitions of LiAlO$_2$ in tetrahedrally and octahedrally coordinated structures

A first-principles computational study is presented of various phases of LiAlO$_2$.The $β$ and $γ$ tetrahedral phases are found to be very close in energy with the $γ$ phase having the lowest energy. The octahedral $α$ phase is a high-pressure phase and the transition pressure from the $γ$ and $β$ phases to $α$ is determined to be about 1 GPa. The electronic band structures are determined using the quasiparticle self-consistent (QS) $GW$ method. The effective masses of the band edges and the nature of the band gaps are presented. The lowest energy $γ$ phase is found to have a pseudodirect gap of 7.69 eV. The gap is direct at $Γ$ but corresponds to a dipole forbidden transition. The imaginary part of the dielectric function and the absorption coefficient are calculated in the long-wavelength limit and the random phase approximation, without local field or electron-hole interaction effects for each phase and their anisotropies are discussed. Si doping on the Al site is investigated as a possible $n$-type dopant in $γ$-LiAlO$_2$ using a 128 atom supercell corresponding to 3.125 \% Si on the Al sublattice in the generalized gradient approximation and a smaller 16 atom cell with 25 \% Si in the QS$GW$ approximation. The Si is found to significantly perturb the conduction band and lower the gap but a clearly separated deep donor defect level is not found. However, the donor binding energy is still expected to be relatively deep, of order a few 0.1 eV in the hydrogenic effective mass approximation.

cond-mat.mtrl-sci↗

arXiv2022

Delocalization of dark and bright excitons in flat-band materials and the optical properties of V$_2$O$_5$

The simplest picture of excitons in materials with atomic-like localization of electrons is that of Frenkel excitons, where electrons and holes stay close together, which is associated with a large binding energy. Here, using the example of the layered oxide V$_2$O$_5$ , we show how localized charge-transfer excitations combine to form excitons that also have a huge binding energy but, at the same time, a large electron-hole distance, and we explain this seemingly contradictory finding. The anisotropy of the exciton delocalization is determined by the local anisotropy of the structure, whereas the exciton extends orthogonally to the chains formed by the crystal structure. Moreover, we show that the bright exciton goes together with a dark exciton of even larger binding energy and more pronounced anisotropy. These findings are obtained by combining first principles many-body perturbation theory calculations, ellipsometry experiments, and tight binding modelling, leading to very good agreement and a consistent picture. Our explanation is general and can be extended to other materials.

cond-mat.mtrl-sci↗

arXiv2022

Real space representation of the quasiparticle self-consistent $GW$ self-energy and its application to defect calculations

The quasiparticle self-consistent QS$GW$ approach incorporates the corrections of the quasiparticle energies from their Kohn-Sham density functional theory (DFT) eigenvalues by means of an energy independent and Hermitian self-energy matrix usually given in the basis set of the DFT eigenstates. By expanding these into an atom-centered basis set (specifically here the linearized muffin-tin orbitals) a real space representation of the self-energy corrections becomes possible. We show that this representation is relatively short-ranged. This offers new opportunities to construct the self-energy of a complex system from parts of the system by a cut-and-paste method. Specifically for a point defect, represented in a large supercell, the self-eneregy can be constructed from those of the host and a smaller defect containing cell. The self-energy of the periodic host can be constructed simply from a $GW$ calculation for the primitive cell. We show for the case of the As$_\mathrm{Ga}$ in GaAs that the defect part can already be well represented by a minimal 8 atom cell and allows us to construct the self-energy for a 64 cell in good agreement with direct QS$GW$ calculations for the large cell. Using this approach to an even larger 216 atom cell shows the defect band approaches an isolated defect level. The calculations also allow to identify a second defect band which appears as a resonance near the conduction band minimum. The results on the extracted defect levels agree well with Green's function calculations for an isolated defect and with experimental data.

cond-mat.mtrl-sci↗

arXiv2021

Calculated phonon modes, infrared and Raman spectra in orthorhombic $α$-MoO$_3$ and monolayer MoO$_3$

Orhorhombic $α$-MoO$_3$ is a layered oxide with various applications and with excellent potential to be exfoliated as a 2D ultrathin film or monolayer. In this paper, we present a first-principles computational study of its vibrational properties. Our focus is on the zone center modes which can be measured by a combination of infared and Raman spectroscopy. The polarization dependent spectra are simulated. Calculations are also performed for a monolayer form in which "double layers" of Mo$_2$O$_6$ which are weakly van der Waals bonded in the $α$-structure are isolated. Shift in phonon frequencies are analyzed.

cond-mat.mtrl-sci↗

arXiv2021

Optical response and band structure of LiCoO2 including electron-hole interaction effects

The optical response functions and band structures of LiCoO$_2$ are studied at different levels of approximation, from density functional theory (DFT) in the generalized gradient approximation (GGA) to quasiparticle self-consistent QS$GW$ (with $G$ for Green's function and $W$ for screened Coulomb interaction) without and with ladder diagrams (QS$G\hat W$) and the Bethe Salpeter Equation (BSE) approach. The QS$GW$ method is found to strongly overestimate the band gap and electron-hole or excitonic effects are found to be important. They lower the quasiparticle gap by only about 11~\% but the lowest energy peaks in absorption are found to be excitonic in nature. The contributions from different band to band transitions and the relation of excitons to band-to-band transitions are analyzed. The excitons are found to be strongly localized. A comparison to experimental data is presented.

cond-mat.mtrl-sci↗

arXiv2021

Ultrathin 2D-oxides: a perspective on fabrication, structure, defect, transport, electron and phonon properties

In the field of atomically thin 2D materials, oxides are relatively unexplored in spite of the large number of layered oxide structures amenable to exfoliation. There is an increasing interest in ultra-thin film oxide nanostructures from applied points of view. In this perspective paper, recent progress in understanding the fundamental properties of 2D oxides is discussed. Two families of 2D oxides are considered: (1) van der Waals bonded layered materials in which the transition metal is in its highest valence state (represented by V$_2$O$_5$ and MoO$_3$) and (2) layered materials with ionic bonding between positive alkali cation layers and negatively charged transition metal oxide layers (LiCoO$_2$). The chemical exfoliation process and its combinaton with mechanical exfoliation are presented for the latter. Structural phase stability of the resulting nanoflakes, the role of cation size and the importance of defects in oxides are discussed. Effects of two-dimensionality on phonons, electronic band structures and electronic screening are placed in the context of what is known on other 2D materials, such as transition metal dichalcogenides. Electronic structure is discussed at the level of many-body-perturbation theory using the quasiparticle self-consistent $GW$ method, the accuracy of which is critically evaluated including effects of electron-hole interactions on screening and electron-phonon coupling. The predicted occurence of a two-dimensional electron gas on Li covered surfaces of LiCoO$_2$ and its relation to topological aspects of the band structure and bonding is presented as an example of the essential role of the surface in ultrathin materials. Finally, some case studies of the electronic transport and the use of these oxides in nanoscale field effect transistors are presented.

cond-mat.mtrl-sci↗

arXiv2021

Experimental determination of the valence band offsets of $ZnGeN_2$ and $ZnGe_{0.94}Ga_{0.12}N_2$ with $GaN$

A predicted type-II staggered band alignment with an approximately $1.4 eV$ valence band offset at the $ZnGeN_2/GaN$ heterointerface has inspired novel band-engineered $III-N/ZnGeN_2$ heterostructure-based device designs for applications in high performance optoelectronics. We report on the determination of the valence band offset between metalorganic chemical vapor deposition grown $(ZnGe)_{1-x}Ga_{2x}N_2$, for $x = 0$ and $0.06$, and $GaN$ using X-ray photoemission spectroscopy. The valence band of $ZnGeN_2$ was found to lie $1.45-1.65 eV$ above that of $GaN$. This result agrees well with the value predicted by first-principles density functional theory calculations using the local density approximation for the potential profile and quasiparticle self-consistent GW calculations of the band edge states relative to the potential. For $(ZnGe)_{0.94}Ga_{0.12}N_2$ the value was determined to be $1.29 eV$, $~10-20\%$ lower than that of $ZnGeN_2$. The experimental determination of the large band offset between $ZnGeN_2$ and $GaN$ provides promising alternative solutions to address challenges faced with pure III-nitride-based structures and devices.

cond-mat.mtrl-sci↗

arXiv2020

Topological obstructed atomic limit by annihilating Dirac fermions

We show that annihilating a pair of Dirac fermions implies a topological transition from the critical semi-metallic phase to an Obstructed Atomic Limit (OAL) insulator phase instead of a trivial insulator. This is shown to happen because of branch-cuts in the phase of the wave functions, leading to non trivial Zak phase along certain directions. To this end, we study their Z$_2$ invariant and also study the phase transition using Entanglement Entropy. We use low energy Hamiltonians and numerical result from model systems to show this effect. These transitions are observed in realistic materials including strained graphene and buckled honeycomb group-V (Sb/As).

cond-mat.str-el↗

arXiv2020

Quasiparticle self-consistent $GW$ band structures and high-pressure phase transitions of LiGaO$_2$ and NaGaO$_2

Quasi-particle self-consistent $GW$ calculations are presented for the band structures of LiGaO2 and NaGaO2 in the orthorhombic $Pna2_1$ tetrahedrally coordinated crystal structures. Symmetry labeling of the bands near the gap is carried out and effective mass tensors are extracted for the conduction band minimum and crystal field split valence band maxima at $Γ$. The gap is found to be direct at $Γ$ and is 5.81 eV in LiGaO2 and 5.46 eV in NaGaO2. Electron-phonon coupling zero-point normalization is estimated to lower these gaps by about 0.2 eV. Optical response functions are calculated within the independent particle long wavelength limit and show the expected anisotropy of the absorption onsets due to the crystal field splitting of the VBM. The results show that both materials are promising candidates as ultrawide gap semiconductors with wurtzite based tetrahedrally bonded crystal structures. Direct transitions from the lowest conduction band to higher bands, relevant to n-type doped material and transparent conduction applications are found to start only above 3.9 eV and are allowed for only one polarization, and several higher band transitions are forbidden by symmetry. Alternative crystal structures, such as $R\bar{3}m$ and a rocksalt type phase with tetragonally distorted $P4/mmm$ spacegroup, both with octahedral coordination of the cations are also investigated. They are found to have higher energy but about 20 \% smaller volume per formula unit. The transition pressures to these phases are determined and for LiGaO2 found to be in good agreement with experimental studies. The $R\bar{3}m$phase also has a comparably high but slightly indirect band gap while the rocksalt type phase if found to have a considerably smaller gap of about 3.1 eV in LiGaO2 and 1.0 eV in NaGaO2.

cond-mat.mtrl-sci↗

arXiv2020

Electron microscopy and spectroscopic study of structural changes, electronic properties and conductivity in annealed Li$_x$CoO$_2$

Chemically exfoliated nanoscale few-layer thin Li$_x$CoO$_2$ samples are studied as function of annealing at various temperatures, using transmission electron microscopy (TEM) and Electron Energy Loss Spectroscopies (EELS), probing the O-K, Co-L$_{2,3}$ spectra along with low energy interband transitions. These spectra are compared with first-principles DFT calculations of -Im$[\varepsilon^{-1}(q,ω)]$ and O-2p Partial Densities of States weighted by dipole matrix elements with the core wavefunction and including the O-1s core-hole and with known trends of the L$_2$/L$_3$ peak ratio to average Co valence. Trends in these spectra under the annealing procedures are established and correlated with the structural phase changes observed from diffraction TEM and High Resolution TEM images. The results are also correlated with conductivity measurements on samples subjected to the same annealing procedures. A gradual disordering of the Li and Co cations in the lattice is observed starting from a slight distortion of the pure LiCoO$_2$ $R\bar{3}m$ to $C2/m$ due to the lower Li content, followed by a $P2/m$ phase forming at 200$^o$C indicative of Li-vacancy ordering, formation of a spinel type $Fd\bar{3}m$ phase around 250$^o$C and ultimately a rocksalt type $Fm\bar{3}m$ phase above 350$^o$C. This disordering leads to a lowering of the band gap as established by low energy EELS. The O-K spectra of the rocksalt phase are only reproduced by a calculation for pure CoO and not for a model with random distribution of Li and Co. This indicates that there may be a loss of Li from the rocksalt regions of the sample at these higher temperatures. The conductivity measurements indicate a gradual drop in conductivity above 200$^o$C, which is clearly related to the more Li-Co interdiffused phases, in which a low-spin electronic structure is no longer valid and stronger correlation effects are expected.

cond-mat.mtrl-sci↗

arXiv2020

Topological quantum switch and controllable quasi 1D wires in antimonene

Based on the recently found non-trivial topology of buckled antimonene, we propose the conceptual design of a quantized switch that is protected by topology and a mechanism to create configurable 1D wire channels. We show that the topologically required edge states in this system can be turned on and off by breaking the inversion symmetry (reducing the symmetry from $S_6$ to $C_3$), which can be achieved by gating the system. This is shown to create a field-effect quantum switch projected by topology. Secondly we show that by locally gating the system with different polarity in different areas, a soliton-like domain wall is created at their interface, which hosts a protected electronic state, in which transport could be accessed by gated doping.

cond-mat.mtrl-sci↗

arXiv2020

Buckled honeycomb group-$V$--$S_6$ symmetric $(d-2)$ higher order topological insulators

Higher Order Topological Insulators (HOTI) are $d$-spatial dimensional systems featuring topologically protected gap-less states at their $(d-n)$-dimensional boundaries. With the help of \textit{ab-initio} calculations and tight binding models along with symmetry considerations we show that monolayer buckled honeycomb structures of group-V elements (Sb,As), which have already been synthesized, belong in this category and have a charge fractionalization of $\frac{e}{2}$ at the corner states as well as weak topological edge states, all protected by their properties under the inversion operation which classify this system as a quadrupole topological insulator.

cond-mat.mtrl-sci↗

arXiv2020

Spin-polarized two-dimensional electron/hole gases on LiCoO$_2$ layers

First-principles calculations show the formation of a 2D spin polarized electron (hole) gas on the Li (CoO$_2$) terminated surfaces of finite slabs down to a monolayer of LiCoO$_2$ in remarkable contrast with the bulk band structure stabilized by Li donating its electron to the CoO$_2$ layer forming a Co-$d-t_{2g}^6$ insulator. By mapping the first-principles computational results to a minimal tight-binding models corresponding to a non-chiral 3D generalization of the quadripartite Su-Schriefer-Heeger (SSH4) model, we show that these surface states have topological origin.

cond-mat.str-el↗

arXiv2019

Topological band structure transitions in honeycomb antimonene as function of buckling

The electronic band topology of monolayer $β$-Sb (antimonene) is studied from the flat honeycomb to the equilibrium buckled structure using first-principles calculations and analyzed using a tight-binding model and low energy Hamiltonians. In flat monolayer Sb, the Fermi level occurs near the intersection of two warped Dirac cones, one associated with the $p_z$-orbitals, and one with the $\{p_x,p_y\}$-orbitals. The differently oriented threefold warping of these two cones leads to an unusually shaped nodal line, which leads to anisotropic in-plane transport properties and goniopolarity. A slight buckling opens a gap along the nodal line except at six remaining Dirac points, protected by symmetry. Under increasing buckling, pairs of Dirac points of opposite winding number annihilate at a critical buckling angle. At a second critical angle, the remaining Dirac points disappear when the band structure becomes a trivial semiconductor. Spin-orbit coupling and edge states are discussed.

cond-mat.mtrl-sci↗

arXiv2019

Questaal: a package of electronic structure methods based on the linear muffin-tin orbital technique

This paper summarises the theory and functionality behind Questaal, an open-source suite of codes for calculating the electronic structure and related properties of materials from first principles. The formalism of the linearised muffin-tin orbital (LMTO) method is revisited in detail and developed further by the introduction of short-ranged tight-binding basis functions for full-potential calculations. The LMTO method is presented in both Green's function and wave function formulations for bulk and layered systems. The suite's full-potential LMTO code uses a sophisticated basis and augmentation method that allows an efficient and precise solution to the band problem at different levels of theory, most importantly density functional theory, LDA+U, quasi-particle self-consistent GW and combinations of these with dynamical mean field theory. This paper details the technical and theoretical bases of these methods, their implementation in Questaal, and provides an overview of the code's design and capabilities.

cond-mat.mtrl-sci↗

arXiv2019

Understanding the crystallographic phase relations in alkali-trihalogeno-germanate perovskites in terms of ferroelectric or antiferroelectric arrangements of the tetrahedral GeX$_3$ units

The alkali-trihalogeno-germanates AGeX$_3$ with A a large single positive ion such as Rb, Cs, or organic radicals such as methyl ammonium (MA), and X a halogen (I, Br, Cl, F) along with the corresponding stannates (ASnX$_3$) and plumbates (APbX$_3$) exhibit a large variety of crystal structures, some of which are of the perovskite type. These materials, better known as "halide perovskites'' have recently gained worldwide attention as promising photovoltaic and more broadly opto-electronic materials. But their stability problems relative to the non-perovskite phases is a major issue. Here we show that the phase relations in these materials can be understood in terms of the relative orientation of the GeX$_3$ tetrahedral units, which is ferroelectric in the perovskite phase but antiferroelectric in the competing phases. This suggests that an applied electric field could be used to stabilize the desired phases and trigger a phase transition between two phases of the material with widely different optical and electronic properties.

cond-mat.mtrl-sci↗

arXiv2019

First-principles study of point defects in LiGaO2

The native point defects are studied in LiGaO2 using hybrid functional calculations. We find that the relative energy of formation of the cation vacancies and the cation antisite defects depends strongly on the chemical potential conditions. The lowest energy defect is found to be the Ga_Li^2+ donor. It is compensated mostly by V_Li^-1and in part by Li_Ga^-2 in the more Li-rich conditions. The equilibrium carrier concentrations are found to be negligible because the Fermi level is pinned deep in the gap and this is consistent with insulating behavior in pure LiGaO2. The V_Ga has high energy under all reasonable conditions. Both the Ga_Li and the V_O are found to be negative U centers with deep 2+/0 transition levels.

cond-mat.mtrl-sci↗