arXiv subjects

Jeffrey B. Neaton

Publications and source records attributed to Jeffrey B. Neaton.

At least 73 records · Page 4Linked to original sources

arXiv2019

Resonant and Bound States of Charged Defects in Two-Dimensional Semiconductors

A detailed understanding of charged defects in two-dimensional semiconductors is needed for the development of ultrathin electronic devices. Here, we study negatively charged acceptor impurities in monolayer WS$_2$ using a combination of scanning tunnelling spectroscopy and large-scale atomistic electronic structure calculations. We observe several localized defect states of hydrogenic wave function character in the vicinity of the valence band edge. Some of these defect states are bound, while others are resonant. The resonant states result from the multi-valley valence band structure of WS$_2$, whereby localized states originating from the secondary valence band maximum at $Γ$ hybridize with continuum states from the primary valence band maximum at K/K$^{\prime}$. Resonant states have important consequences for electron transport as they can trap mobile carriers for several tens of picoseconds.

cond-mat.mes-hall↗

arXiv2019

Large spin-orbit splitting of deep in-gap defect states of engineered sulfur vacancies in monolayer WS2

Structural defects in 2D materials offer an effective way to engineer new material functionalities beyond conventional doping in semiconductors. Specifically, deep in-gap defect states of chalcogen vacancies have been associated with intriguing phenomena in monolayer transition metal dichalcogenides (TMDs). Here, we report the direct experimental correlation of the atomic and electronic structure of a sulfur vacancy in monolayer WS2 by a combination of CO-tip noncontact atomic force microscopy (nc-AFM) and scanning tunneling microscopy (STM). Sulfur vacancies, which are absent in as-grown samples, were deliberately created by annealing in vacuum. Two energetically narrow unoccupied defect states of the vacancy provide a unique fingerprint of this defect. Direct imaging of the defect orbitals by STM and state-of-the-art GW calculations reveal that the large splitting of 252 meV between these defect states is induced by spin-orbit coupling. The controllable incorporation and potential decoration of chalcogen vacancies provide a new route to tailor the optical, catalytic and magnetic properties of TMDs.

cond-mat.mtrl-sci↗

arXiv2019

Superlattice-induced ferroelectricity in charge-ordered La$_{1/3}$Sr$_{2/3}$FeO$_{3}$

Charge-order-driven ferroelectrics are an emerging class of functional materials, distinct from conventional ferroelectrics, where electron-dominated switching can occur at high frequency. Despite their promise, only a few systems exhibiting this behavior have been experimentally realized thus far, motivating the need for new materials. Here, we use density functional theory to study the effect of artificial structuring on mixed-valence solid-solution La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ (LSFO), a system well-studied experimentally. Our calculations show that A-site cation (111)-layered LSFO exhibits a ferroelectric charge-ordered phase in which inversion symmetry is broken by changing the registry of the charge order with respect to the superlattice layering. The phase is energetically degenerate with a ground-state centrosymmetric phase, and the computed switching polarization is 39 $μ$C/cm$^{2}$, a significant value arising from electron transfer between Fe ions. Our calculations reveal that artificial structuring of LSFO and other mixed valence oxides with robust charge ordering in the solid solution phase can lead to charge-order-induced ferroelectricity.

cond-mat.mtrl-sci↗

arXiv2019

Accelerating $GW$-Based Energy Level Alignment Calculations for Molecule-Metal Interfaces Using a Substrate Screening Approach

The physics of electronic energy level alignment at interfaces formed between molecules and metals can in general be accurately captured by the \emph{ab initio} $GW$ approach. However, the computational cost of such $GW$ calculations for typical interfaces is significant, given their large system size and chemical complexity. In the past, approximate self-energy corrections, such as those constructed from image-charge models together with gas-phase molecular level corrections, have been used to compute level alignment with good accuracy. However, these approaches often neglect dynamical effects of the polarizability and require the definition of an image plane. In this work, we propose a new approximation to enable more efficient $GW$-quality calculations of interfaces, where we greatly simplify the calculation of the non-interacting polarizability, a primary bottleneck for large heterogeneous systems. This is achieved by first computing the non-interacting polarizability of each individual component of the interface, e.g., the molecule and the metal, without the use of large supercells; and then using folding and spatial truncation techniques to efficiently combine these quantities. Overall this approach significantly reduces the computational cost for conventional $GW$ calculations of level alignment without sacrificing the accuracy. Moreover, this approach captures both dynamical and nonlocal polarization effects without the need to invoke a classical image-charge expression or to define an image plane. We demonstrate our approach by considering a model system of benzene at relatively low coverage on aluminum (111) surface. Although developed for such interfaces, the method can be readily extended to other heterogeneous interfaces.

cond-mat.mtrl-sci↗

arXiv2019

Emergence of topological electronic phases in elemental lithium under pressure

Lithium, a prototypical simple metal under ambient conditions, has a surprisingly rich phase diagram under pressure, taking up several structures with reduced symmetry, low coordination numbers, and even semiconducting character with increasing density. Using first-principles calculations, we demonstrate that some predicted high-pressure phases of elemental Li also host topological electronic structures. Beginning at 80 GPa and coincident with a transition to the Pbca phase, we find Li to be a Dirac nodal line semimetal. We further calculate that Li retains linearly-dispersive energy bands in subsequent predicted higher pressure phases, and that it exhibits a Lifshitz transition between two Cmca phases at 220 GPa. The Fd-3m phase at 500 GPa forms buckled honeycomb layers that give rise to a Dirac crossing 1 eV below the Fermi energy. The well-isolated topological nodes near the Fermi level in these phases result from increasing p-orbital character with density at the Fermi level, itself a consequence of rising 1s core wavefunction overlap, and a preference for nonsymmorphic symmetries in the crystal structures favored at these pressures. Our results provide evidence that under pressure, bulk 3D materials with light elements, or even pure elemental systems, can undergo topological phase transitions hosting nontrivial topological properties near the Fermi level with measurable consequences; and that, through pressure, we can access these novel phases in elemental lithium.

cond-mat.mtrl-sci↗

arXiv2019

Reproducibility in $G_0W_0$ Calculations for Solids

Ab initio many-body perturbation theory within the $GW$ approximation is a Green's function formalism widely used in the calculation of quasiparticle excitation energies of solids. In what has become an increasingly standard approach, Kohn-Sham eigenenergies, generated from a DFT calculation with a strategically-chosen exchange correlation functional ``starting point'', are used to construct $G$ and $W$, and then perturbatively corrected by the resultant $GW$ self-energy. In practice, there are several ways to construct the $GW$ self-energy, and these can lead to variations in predicted quasiparticle energies. For example, for ZnO and TiO$_2$, reported $GW$ fundamental gaps can vary by more than 1 eV. In this work, we address the convergence and key approximations in contemporary $G_0W_0$ calculations, including frequency-integration schemes and the treatment of the Coulomb divergence in the exact-exchange term. We study several systems,and compare three different $GW$ codes: BerkeleyGW, Abinit and Yambo. We demonstrate, for the first time, that the same quasiparticle energies for systems in the condensed phase can be obtained with different codes, and we provide a comprehensive assessment of implementations of the $GW$ approximation.

cond-mat.mtrl-sci↗

arXiv2019

Ferroelectricity in [111]-oriented epitaxially strained SrTiO$_3$ from first principles

We use first principles density functional theory calculations to investigate the effect of biaxial strain in the low-temperature structural and ferroelectric properties of [111]-oriented SrTiO$_3$. We find that [111] biaxial strain, achievable by coherent epitaxial growth along the [111] direction, induces structural distortions in SrTiO$_3$ that are not present in either bulk or [001]-oriented SrTiO$_3$. Under [111] biaxial strain, SrTiO$_3$ displays ferroelectricity at tensile strain, and paraelectricity at compressive strain. We compute the phonon spectrum and macroscopic polarization of SrTiO$_3$ as a function of [111] biaxial strain, and relate our results to the predictions of the free energy phenomenological model of Pertsev, Tagantsev and Setter [Phys. Rev. B 61, 825 (2000); Phys. Rev. B 65, 219901 (2002)].

cond-mat.mtrl-sci↗

arXiv2019

Observation of highly dispersive bands in pure thin film C$_{60}$

While long-theorized, the direct observation of multiple highly dispersive C$_{60}$ valence bands has eluded researchers for more than two decades due to a variety of intrinsic and extrinsic factors. Here we report a realization of multiple highly dispersive (330-520 meV) valence bands in pure thin film C$_{60}$ on a novel substrate--the three-dimensional topological insulator Bi$_{2}$Se$_{3}$--through the use of angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. The effects of this novel substrate reducing C$_{60}$ rotational disorder are discussed. Our results provide important considerations for past and future band structure studies as well as the increasingly popular C$_{60}$ electronic device applications, especially those making use of heterostructures.

cond-mat.mes-hall↗

arXiv2019

Topological Semimetal features in the Multiferroic Hexagonal Manganites

Using first-principles calculations we examine the band structures of ferromagnetic hexagonal manganites $\mathrm{YXO_3}$ (X=V, Cr, Mn, Fe and Co) in the nonpolar nonsymmorphic $P6_3/mmc$ space group. For $\mathrm{YVO_3}$ and $\mathrm{YCrO_3}$ we find a band inversion near the Fermi energy that generates a nodal ring in the $k_z=0$ mirror plane. We perform a more detailed analysis for these compounds and predict the existence of the topological "drumhead" surface states. Finally, we briefly discuss the low-symmetry polar phases (space group $P6_3cm$) of these systems, and show they can undergo a $P6_3/mmc \rightarrow P6_3cm$ transition by condensation of soft $K_3$ and $Γ_2^-$ phonons. Based on our findings, stabilizing these compounds in the hexagonal phase could offer a promising platform for studying the interplay of topology and multiferroicity, and the coexistence of real-space and reciprocal-space topological protection in the same phase.

cond-mat.mtrl-sci↗

arXiv2018

Exploring the Influence of Dynamic Disorder on Transport Gap in Solid Pentacene

We combine a GW approach and ab initio Molecular Dynamics (AIMD) simulations to study the impact of thermal effects on transport gap in solid pentacene (C22H14). The dynamic disorder induced by thermal fluctuations is simulated by AIMD, providing the ensemble-averaged density of states (DOS) near the band gap. The GW corrected DOS, averaged over hundreds of snapshots from AIMD simulation containing disordered structures indicates that the edge-to-edge transport gap is 2.1+/-0.04 eV, reduced by ~0.1 eV in contrast to the static 0 K GW calculation. The peak-to-peak gap is found to be 2.7eV in excellent agreement with experiment after corrections for the surface and the Frank-Condon effects and providing fully ab initio agreement with experiment where previous theory required ad hoc Gaussian broadening and temperature corrections.

physics.chem-ph↗

arXiv2018

Defect-induced modification of low-lying excitons and valley selectivity in monolayer transition metal dichalcogenides

We study the effect of point-defect chalcogen vacancies on the optical properties of monolayer transition metal dichalcogenides using ab initio GW and Bethe-Salpeter equation calculations. We find that chalcogen vacancies introduce unoccupied in-gap states and occupied resonant defect states within the quasiparticle continuum of the valence band. These defect states give rise to a number of strongly-bound defect excitons and hybridize with excitons of the pristine system, reducing the valley-selective circular dichroism. Our results suggest a pathway to tune spin-valley polarization and other optical properties through defect engineering.

cond-mat.mtrl-sci↗

arXiv2017

Prediction of $\mathrm{TiRhAs}$ as a Dirac Nodal Line Semimetal via First-Principles Calculations

Using first-principles calculations we predict that $\mathrm{TiRhAs}$, a previously synthesized compound, is a Dirac nodal line (DNL) semimetal. The DNL in this compound is found to be protected both by the combination of inversion and time-reversal symmetry, and by a reflection symmetry, in the absence of spin-orbit coupling (SOC). Our calculations show that band velocities associated with the nodal line have a high degree of directional anisotropy, with in-plane velocities $v_\perp$ perpendicular to the nodal line between $1.2-2.8\times10^5$ m/s. The crossings along the DNL are further found to exhibit a prominent and position-dependent tilt along directions perpendicular to the nodal line. We calculate $\mathbb{Z}_2$ indices based on parity eigenvalues at time-reversal invariant momenta and show that $\mathrm{TiRhAs}$ is topological. A tight-binding model fit from our first-principles calculations demonstrates the existence of two-dimensional drumhead surface states on the surface Brillouin zone. Based on the small gapping of the DNL upon inclusion of SOC and the clean Fermi surface free from trivial bands, $\mathrm{TiRhAs}$ is a promising candidate for further studies of the properties of topological semimetals.

cond-mat.mes-hall↗

arXiv2017

Cooperative gas adsorption without a phase transition in metal-organic frameworks

Cooperative adsorption of gases by porous frameworks permits more efficient uptake and removal than does the more usual non-cooperative (Langmuir-type) adsorption. Cooperativity, signaled by a step-like isotherm, is usually attributed to a phase transition of the framework. However, the class of metal-organic frameworks mmen-M$_2$(dobpdc) exhibit cooperative adsorption of CO2 but show no evidence of a phase transition. Here we show how cooperativity emerges in these frameworks in the absence of a phase transition. We use a combination of quantum and statistical mechanics to show that cooperativity results from a sharp but finite increase, with pressure, of the mean length of chains of CO2 molecules that polymerize within the framework. Our study provides microscopic understanding of the emergent features of cooperative binding, including the position, slope and height of the isotherm step, and indicates how to optimize gas storage and separation in these materials.

cond-mat.mtrl-sci↗

arXiv2017

Origins of singlet fission in solid pentacene from an ab initio Green's-function approach

We develop a new first-principles approach to predict and understand rates of singlet fission with an ab initio Green's-function formalism based on many-body perturbation theory. Starting with singlet and triplet excitons computed from a GW plus Bethe-Salpeter equation approach, we calculate the exciton--bi-exciton coupling to lowest order in the Coulomb interaction, assuming a final state consisting of two non-interacting spin-correlated triplets with finite center-of-mass momentum. For crystalline pentacene, symmetries dictate that the only purely Coulombic fission decay from a bright singlet state requires a final state consisting of two inequivalent nearly degenerate triplets of nonzero, equal and opposite, center-of-mass momenta. For such a process, we predict a singlet lifetime of 40 to 150 fs, in very good agreement with experimental data, indicating that this process can dominate singlet fission in crystalline pentacene. Our approach is general and provides a framework for predicting and understanding multiexciton interactions in solids.

cond-mat.mtrl-sci↗

arXiv2017

Detection of sub-MeV Dark Matter with Three-Dimensional Dirac Materials

We propose the use of three-dimensional Dirac materials as targets for direct detection of sub-MeV dark matter. Dirac materials are characterized by a linear dispersion for low-energy electronic excitations, with a small band gap of O(meV) if lattice symmetries are broken. Dark matter at the keV scale carrying kinetic energy as small as a few meV can scatter and excite an electron across the gap. Alternatively, bosonic dark matter as light as a few meV can be absorbed by the electrons in the target. We develop the formalism for dark matter scattering and absorption in Dirac materials and calculate the experimental reach of these target materials. We find that Dirac materials can play a crucial role in detecting dark matter in the keV to MeV mass range that scatters with electrons via a kinetically mixed dark photon, as the dark photon does not develop an in-medium effective mass. The same target materials provide excellent sensitivity to absorption of light bosonic dark matter in the meV to hundreds of meV mass range, superior to all other existing proposals when the dark matter is a kinetically mixed dark photon.

hep-ph↗

arXiv2017

Large bulk photovoltaic effect and spontaneous polarization of single-layer monochalcogenides

We use a first-principles density functional theory approach to calculate the shift current and linear absorption of uniformly illuminated single-layer Ge and Sn monochalcogenides. We predict strong absorption in the visible spectrum and a large effective three-dimensional shift current ($\sim$100 $μ$A/V$^2$), larger than has been previously observed in other polar systems. Moreover, we show that the integral of the shift-current tensor is correlated to the large spontaneous effective three-dimensional electric polarization ($\sim$1.9 C/m$^2$). Our calculations indicate that the shift current will be largest in the visible spectrum, suggesting that these monochalcogenides may be promising for polar optoelectronic devices. A Rice-Mele tight-binding model is used to rationalize the shift-current response for these systems, and its dependence on polarization, in general terms with implications for other polar materials

cond-mat.mes-hall↗

arXiv2017

Thermodynamic signatures for the existence of Dirac electrons in ZrTe5

We combine transport, magnetization, and torque magnetometry measurements to investigate the electronic structure of ZrTe5 and its evolution with temperature. At fields beyond the quantum limit, we observe a magnetization reversal from paramagnetic to diamagnetic response, which is characteristic of a Dirac semi-metal. We also observe a strong non-linearity in the magnetization that suggests the presence of additional low-lying carriers from other low-energy bands. Finally, we observe a striking sensitivity of the magnetic reversal to temperature that is not readily explained by simple band-structure models, but may be connected to a temperature dependent Lifshitz transition proposed to exist in this material.

cond-mat.mes-hall↗

arXiv2017

Observation of two-dimensional Fermi surface and Dirac dispersion in YbMnSb$_2$

We present the crystal structure, electronic structure, and transport properties of the material YbMnSb$_2$, a candidate system for the investigation of Dirac physics in the presence of magnetic order. Our measurements reveal that this system is a low-carrier-density semimetal with a 2D Fermi surface arising from a Dirac dispersion, consistent with the predictions of density functional theory calculations of the antiferromagnetic system. The low temperature resistivity is very large, suggesting scattering in this system is highly efficient at dissipating momentum despite its Dirac-like nature.

cond-mat.mtrl-sci↗