arXiv subjects

Cyrus E. Dreyer

Publications and source records attributed to Cyrus E. Dreyer.

At least 37 records · Page 2Linked to original sources

arXiv2022

Frequency splitting of chiral phonons from broken time reversal symmetry in CrI$_3$

Conventional approaches for lattice dynamics based on static interatomic forces do not fully account for the effects of time-reversal-symmetry breaking in magnetic systems. Recent approaches to rectify this involve incorporating the first-order change in forces with atomic velocities under the assumption of adiabatic separation of electronic and nuclear degrees of freedom. In this work, we develop a first-principles method to calculate this velocity-force coupling in extended solids, and show via the example of ferromagnetic CrI$_3$ that, due to the slow dynamics of the spins in the system, the assumption of adiabatic separation can result in large errors for splittings of zone-center chiral modes. We demonstrate that an accurate description of the lattice dynamics requires treating magnons and phonons on the same footing.

cond-mat.mtrl-sci↗

arXiv2022

Ultra Localized Optoelectronic Properties of Nanobubbles in 2D Semiconductors

The optical properties of transition metal dichalcogenides have previously been modified at the nanoscale by using mechanical and electrical nanostructuring. However, a clear experimental picture relating the local electronic structure with emission properties in such structures has so far been lacking. Here, we use a combination of scanning tunneling microscopy (STM) and near-field photoluminescence (nano-PL) to probe the electronic and optical properties of single nano-bubbles in bilayer heterostructures of WSe2 on MoSe2. We show from tunneling spectroscopy that there are electronic states deeply localized in the gap at the edge of such bubbles, which are independent of the presence of chemical defects in the layers. We also show a significant change in the local bandgap on the bubble, with a continuous evolution to the edge of the bubble over a length scale of ~20 nm. Nano-PL measurements observe a continuous redshift of the interlayer exciton on entering the bubble, in agreement with the band to band transitions measured by STM. We use self-consistent Schrödinger-Poisson (SP) simulations to capture the essence of the experimental results and find that strong doping in the bubble region is a key ingredient to achieving the observed localized states, together with mechanical strain.

cond-mat.mes-hall↗

arXiv2022

Electronic structure of the highly conductive perovskite oxide SrMoO$_3$

We use angle-resolved photoemission to map the Fermi surface and quasiparticle dispersion of bulk-like thin films of SrMoO$_3$ grown by pulsed laser deposition. The electronic self-energy deduced from our data reveals weak to moderate correlations in SrMoO$_3$, consistent with our observation of well-defined electronic states over the entire occupied band width. We further introduce spectral function calculations that combine dynamical mean-field theory with an unfolding procedure of density functional calculations and demonstrate good agreement of this approach with our experiments.

cond-mat.str-el↗

arXiv2022

Quantum embedding methods for correlated excited states of point defects: Case studies and challenges

A quantitative description of the excited electronic states of point defects and impurities is crucial for understanding materials properties, and possible applications of defects in quantum technologies. This is a considerable challenge for computational methods, since Kohn-Sham density-functional theory (DFT) is inherently a ground state theory, while higher-level methods are often too computationally expensive for defect systems. Recently, embedding approaches have been applied that treat defect states with many-body methods, while using DFT to describe the bulk host material. We implement such an embedding method, based on Wannierization of defect orbitals and the constrained random-phase approximation approach, and perform systematic characterization of the method for three distinct systems with current technological relevance: a carbon dimer replacing a B and N pair in bulk hexagonal BN (C$_{\text{B}}$C$_{\text{N}}$), the negatively charged nitrogen-vacancy center in diamond (NV$^-$), and an Fe impurity on the Al site in wurtzite AlN ($\text{Fe}_{\text{Al}}$). For C$_{\text{B}}$C$_{\text{N}}$ we show that the embedding approach gives many-body states in agreement with analytical results on the Hubbard dimer model, which allows us to elucidate the effects of the DFT functional and double-counting correction. For the NV$^-$ center, our method demonstrates good quantitative agreement with experiments for the zero-phonon line of the triplet-triplet transition. Finally, we illustrate challenges associated with this method for determining the energies and orderings of the complex spin multiplets in $\text{Fe}_{\text{Al}}$.

cond-mat.mtrl-sci↗

arXiv2022

Nonadiabatic Born effective charges in metals and the Drude weight

In insulators, Born effective charges describe the electrical polarization induced by the displacement of individual atomic sublattices. Such a physical property is at first sight irrelevant for metals and doped semiconductors, where the macroscopic polarization is ill-defined. Here we show that, in clean conductors, going beyond the adiabatic approximation results in nonadiabatic Born effective charges that are well defined in the low-frequency limit. In addition, we find that the sublattice sum of the nonadiabatic Born effective charges does not vanish as it does in the insulating case, but instead is proportional to the Drude weight. We demonstrate these formal results with density functional perturbation theory calculations of Al, and electron-doped SnS$_2$ and SrTiO$_3$.

cond-mat.mtrl-sci↗

arXiv2022

The Oscura Experiment

The Oscura experiment will lead the search for low-mass dark matter particles using a very large array of novel silicon Charge Coupled Devices (CCDs) with a threshold of two electrons and with a total exposure of 30 kg-yr. The R&D effort, which began in FY20, is currently entering the design phase with the goal of being ready to start construction in late 2024. Oscura will have unprecedented sensitivity to sub-GeV dark matter particles that interact with electrons, probing dark matter-electron scattering for masses down to 500 keV and dark matter being absorbed by electrons for masses down to 1 eV. The Oscura R&D effort has made some significant progress on the main technical challenges of the experiment, of which the most significant are engaging new foundries for the fabrication of the CCD sensors, developing a cold readout solution, and understanding the experimental backgrounds.

astro-ph.IM↗

arXiv2022

Photocatalytic water oxidation on SrTiO$_3$ [001] surfaces

SrTiO$_3$ is a highly efficient photocatalyst for the overall water splitting reaction under UV irradiation. However, an atomic-level understanding of the active surface sites responsible for the oxidation and reduction reactions is still lacking. Here we present a unified experimental and computational account of the photocatalytic activity at the SrO- and TiO$_2$- terminations of aqueous-solvated [001] SrTiO$_3$. Our experimental findings show that the overall water-splitting reaction proceeds on the SrTiO$_3$ surface only when the two terminations are simultaneously exposed to water. Our simulations explain this, showing that the photogenerated hole-driven oxidation primarily occurs at SrO surfaces in a sequence of four single hole transfer reactions, while the TiO$_2$ termination effects the crucial band alignment of the photocatalyst relative to the water oxidation potential. The present work elucidates the interdependence of the two chemical terminations of SrTiO$_3$ surfaces, and has consequent implications for maximizing sustainable solar-driven water splitting.

cond-mat.mtrl-sci↗

arXiv2021

Rotational $g$ factors and Lorentz forces of molecules and solids from density-functional perturbation theory

Applied magnetic fields can couple to atomic displacements via generalized Lorentz forces, which are commonly expressed as gyromagnetic $g$ factors. We develop an efficient first-principles methodology based on density-functional perturbation theory to calculate this effect in both molecules and solids to linear order in the applied field. Our methodology is based on two linear-response quantities: the macroscopic polarization response to an atomic displacement (i.e., Born effective charge tensor), and the antisymmetric part of its first real-space moment (the symmetric part corresponding to the dynamical quadrupole tensor). The latter quantity is calculated via an analytical expansion of the current induced by a long-wavelength phonon perturbation, and compared to numerical derivatives of finite-wavevector calculations. We validate our methodology in finite systems by computing the gyromagnetic $g$ factor of several simple molecules, demonstrating excellent agreement with experiment and previous density-functional theory and quantum chemistry calculations. In addition, we demonstrate the utility of our method in extended systems by computing the energy splitting of the low-frequency transverse-optical phonon mode of cubic SrTiO$_3$ in the presence of a magnetic field.

cond-mat.mtrl-sci↗

arXiv2021

First-principles study of the electronic, magnetic, and crystal structure of perovskite molybdates

The molybdate oxides SrMoO$_3$, PbMoO$_3$, and LaMoO$_3$ are a class of metallic perovskites that exhibit interesting properties including high mobility, and unusual resistivity behavior. We use first-principles methods based on density functional theory to explore the electronic, crystal, and magnetic structure of these materials. In order to account for the electron correlations in the partially-filled Mo $4d$ shell, a local Hubbard $U$ interaction is included. The value of $U$ is estimated via the constrained random-phase approximation approach, and the dependence of the results on the choice of $U$ are explored. For all materials, GGA+$U$ predicts a metal with an orthorhombic, antiferromagnetic structure. For LaMoO$_3$, the $Pnma$ space group is the most stable, while for SrMoO$_3$ and PbMoO$_3$, the $Imma$ and $Pnma$ structures are close in energy. The $R_4^+$ octahedral rotations for SrMoO$_3$ and PbMoO$_3$ are found to be overestimated compared to the experimental low-temperature structure.

cond-mat.mtrl-sci↗

arXiv2021

Correlation-Induced Octahedral Rotations in SrMoO$_3$

Distortions of the oxygen octahedra influence the fundamental electronic structure of perovskite oxides, such as their bandwidth and exchange interactions. Utilizing a fully ab-initio methodology based on density functional theory plus dynamical mean field theory (DFT+DMFT), we study the crystal and magnetic structure of SrMoO$_3$. Comparing our results with DFT+$U$ performed on the same footing, we find that DFT+$U$ overestimates the propensity for magnetic ordering, as well as the octahedral rotations, leading to a different ground state structure. This demonstrates that structural distortions can be highly sensitive to electronic correlation effects, and to the considered magnetic state, even in a moderately correlated metal such as SrMoO$_3$. Moreover, by comparing different downfolding schemes, we demonstrate the robustness of the DFT+DMFT method for obtaining structural properties, highlighting its versatility for applications to a broad range of materials.

cond-mat.str-el↗

arXiv2021

Interplay between breathing-mode distortions and magnetic order in rare-earth nickelates from $ab$ $initio$ magnetic models

We use density-functional theory calculations to explore the magnetic properties of perovskite rare-earth nickelates, $\mathcal{R}$NiO$_3$, by constructing microscopic magnetic models containing all relevant exchange interactions via Wannierization and Green's function techniques. These models elucidate the mechanism behind the formation of antiferromagnetic order with the experimentally observed propagation vector, and explain the reason previous DFT plus Hubbard $U$ calculations favored ferromagnetic order. We perform calculations of magnetic moments and exchange-coupling parameters for different amplitudes of the $R_1^+$ breathing mode distortion, which results in expanded and compressed NiO$_6$ octahedra. We find that the magnetic moment vanishes for the "short bond" nickels, i.e., the ones in the compressed octahedra. The inclusion of spin-orbit coupling demonstrates that the magnetic anisotropy is very small, while the magnetic moment of the short bond nickel atoms tend to zero even for the noncollinear case. Our results provide a clear picture of the trends of the magnetic order across the nickelate series and give insights into the coupling between magnetic order and structural distortions.

cond-mat.mtrl-sci↗

arXiv2020

Nonrad: Computing Nonradiative Capture Coefficients from First Principles

Point defects in semiconductor crystals provide a means for carriers to recombine nonradiatively. This recombination process impacts the performance of devices. We present the Nonrad code that implements the first-principles approach of Alkauskas et al. [Phys. Rev. B 90, 075202 (2014)] for the evaluation of nonradiative capture coefficients based on a quantum-mechanical description of the capture process. An approach for evaluating electron-phonon coupling within the projector augmented wave formalism is presented. We also show that the common procedure of replacing Dirac delta functions with Gaussians can introduce errors into the resulting capture rate, and implement an alternative scheme to properly account for vibrational broadening. Lastly, we assess the accuracy of using an analytic approximation to the Sommerfeld parameter by comparing with direct numerical evaluation.

cond-mat.mtrl-sci↗

arXiv2020

Radiative capture rates at deep defects from electronic structure calculations

We present a methodology to calculate radiative carrier capture coefficients at deep defects in semiconductors and insulators from first principles. Electronic structure and lattice relaxations are accurately described with hybrid density functional theory. Calculations of capture coefficients provide an additional validation of the accuracy of these functionals in dealing with localized defect states. We also discuss the validity of the Condon approximation, showing that even in the event of large lattice relaxations the approximation is accurate. We test the method on GaAs:$V_\text{Ga}$-$\text{Te}_\text{As}$ and GaN:C$_\text{N}$, for which reliable experiments are available, and demonstrate very good agreement with measured capture coefficients.

cond-mat.mtrl-sci↗

arXiv2020

Long-range quadrupole electron-phonon interaction from first principles

Lattice vibrations in materials induce perturbations on the electron dynamics in the form of long-range (dipole and quadrupole) and short-range (octopole and higher) potentials. The dipole Fröhlich term can be included in current first-principles electron-phonon ($e$-ph) calculations and is present only in polar materials. The quadrupole $e$-ph interaction is present in both polar and nonpolar materials, but currently it cannot be computed from first principles. Here we show an approach to compute the quadrupole $e$-ph interaction and include it in ab initio calculations of $e$-ph matrix elements. The accuracy of the approach is demonstrated by comparing with direct density functional perturbation theory calculations. We apply our method to silicon as a case of a nonpolar semiconductor and tetragonal PbTiO$_3$ as a case of a polar piezoelectric material. In both materials we find that the quadrupole term strongly impacts the $e$-ph matrix elements. Analysis of $e$-ph interactions for different phonon modes reveals that the quadrupole term mainly affects optical modes in silicon and acoustic modes in PbTiO$_3$, although the quadrupole term is needed for all modes to achieve quantitative accuracy. The effect of the quadrupole $e$-ph interaction on electron scattering processes and transport is shown to be important. Our approach enables accurate studies of $e$-ph interactions in broad classes of nonpolar, polar and piezoelectric materials.

cond-mat.mtrl-sci↗

arXiv2020

Piezoelectric Electron-Phonon Interaction from Ab Initio Dynamical Quadrupoles: Impact on Charge Transport in Wurtzite GaN

First-principles calculations of $e$-ph interactions are becoming a pillar of electronic structure theory. However, the current approach is incomplete. The piezoelectric (PE) $e$-ph interaction, a long-range scattering mechanism due to acoustic phonons in non-centrosymmetric polar materials, is not accurately described at present. Current calculations include short-range $e$-ph interactions (obtained by interpolation) and the dipole-like Fröhlich long-range coupling in polar materials, but lack important quadrupole effects for acoustic modes and PE materials. Here we derive and compute the long-range $e$-ph interaction due to dynamical quadrupoles, and apply this framework to investigate $e$-ph interactions and the carrier mobility in the PE material wurtzite GaN. We show that the quadrupole contribution is essential to obtain accurate $e$-ph matrix elements for acoustic modes and to compute PE scattering. Our work resolves the outstanding problem of correctly computing $e$-ph interactions for acoustic modes from first principles, and enables studies of $e$-ph coupling and charge transport in PE materials.

cond-mat.mtrl-sci↗

arXiv2019

Giant polarization charge density at lattice-matched GaN/ScN interfaces

Rocksalt ScN is a semiconductor with a small lattice mismatch to wurtzite GaN. Within the modern theory of polarization, ScN has a nonvanishing formal polarization along the [111] direction. As a result, we demonstrate that an interface between (0001) GaN and (111) ScN exihibts a large polarization discontinuity of $-$1.358 $\rm Cm^{-2}$. Interfaces between ScN and wurtzite III-nitrides will exhibit a high-density electron gas on the (000$\bar{1}$) GaN interface or a hole gas on the (0001) GaN interface, with carrier concentrations up to $8.5 \times 10^{14}$ cm$^{-2}$. The large polarization difference and small strain makes ScN a desirable choice for polarization-enhanced tunnel junctions within the III-nitride materials system. The large sheet carrier densities may also be useful for contacts or current spreading layers.

cond-mat.mtrl-sci↗

arXiv2018

Metric-wave approach to flexoelectricity within density-functional perturbation theory

Within the framework of density functional perturbation theory (DFPT), we implement and test a novel "metric wave" response-function approach. It consists in the reformulation of an acoustic phonon perturbation in the curvilinear frame that is comoving with the atoms. This means that all the perturbation effects are encoded in the first-order variation of the real-space metric, while the atomic positions remain fixed. This approach can be regarded as the generalization of the uniform strain perturbation of Hamann et al. [D. R. Hamann, X. Wu, K. M. Rabe, and D. Vanderbilt, Phys. Rev. B 71, 035117 (2005)] to the case of inhomogeneous deformations, and greatly facilitates the calculation of advanced electromechanical couplings such as the flexoelectric tensor. We demonstrate the accuracy of our approach with extensive tests on model systems and on bulk crystals of Si and SrTiO$_3$.

cond-mat.mtrl-sci↗

arXiv2018

Defect identification based on first-principles calculations for deep level transient spectroscopy

Deep level transient spectroscopy (DLTS) is used extensively to study defects in semiconductors. We demonstrate that great care should be exercised in interpreting activation energies extracted from DLTS as ionization energies. We show how first-principles calculations of thermodynamic transition levels, temperature effects of ionization energies, and nonradiative capture coefficients can be used to accurately determine actual activation energies that can be directly compared with DLTS. Our analysis is illustrated with hybrid functional calculations for two important defects in GaN that have similar thermodynamic transition levels, and shows that the activation energy extracted from DLTS includes a capture barrier that is temperature dependent, unique to each defect, and in some cases large in comparison to the ionization energy. By calculating quantities that can be directly compared with experiment, first-principles calculations thus offer powerful leverage in identifying the microscopic origin of defects detected in DLTS.

cond-mat.mtrl-sci↗