Publications
97 results found
Campbell AJ, Vitale V, Brotons-Gisbert M, et al., 2024, The interplay of field-tunable strongly correlated states in a multi-orbital moiré system, Nature Physics, ISSN: 1745-2473
The interplay of charge, spin, lattice and orbital degrees of freedom leads to a variety of emergent phenomena in strongly correlated systems. In transition-metal-dichalcogenide-based moiré heterostructures, recent observations of correlated phases can be described by triangular-lattice single-orbital Hubbard models based on moiré bands derived from the Brillouin-zone corners—the so-called K valleys. Richer phase diagrams described by multi-orbital Hubbard models are possible with hexagonal lattices that host moiré bands at the zone centre—called Γ valleys—or an additional layer degree of freedom. Here we report the tunable interaction between strongly correlated hole states hosted by Γ- and K-derived bands in a heterostructure of monolayer MoSe2 and bilayer 2H WSe2. We characterize the behaviour of exciton–polarons to distinguish the layer and valley degrees of freedom. The Γ band gives rise to a charge-transfer insulator described by a two-orbital Hubbard model. An out-of-plane electric field re-orders the Γ- and K-derived bands and drives the redistribution of carriers to the layer-polarized K orbital, generating Wigner crystals and Mott insulating states. Finally, we obtain degeneracy of the Γ and K orbitals at the Fermi level and observe interacting correlated states with phase transitions dependent on the doping density. Our results establish a platform to investigate multi-orbital Hubbard model Hamiltonians.
Coiana G, Lischner J, Tangney P, 2024, Breakdown of phonon band theory in MgO, Physical Review B: Condensed Matter and Materials Physics, Vol: 109, ISSN: 1098-0121
We present a series of detailed images of the distribution of kinetic energy among frequencies and wave vectors in the bulk of an MgO crystal as it is heated slowly until it melts. These spectra, which are Fourier transforms of mass-weighted velocity-velocity correlation functions calculated from accurate molecular dynamics (MD) simulations, provide a valuable perspective on the growth of thermal disorder in ionic crystals. We use them to explain why the most striking and rapidly progressing departures from a band structure occur among longitudinal optical (LO) modes, which would be the least active modes at low temperature (T) if phonons did not interact. The degradation of the LO band begins, at low T, as an anomalously large broadening of modes near the center of the Brillouin zone (BZ), which gradually spreads towards the BZ boundary. The LO band all but vanishes before the crystal melts, and transverse optical (TO) modes' spectral peaks become so broad that the TO branches no longer appear band-like. Acoustic bands remain relatively well defined until melting of the crystal manifests in the spectra as their sudden disappearance. We argue that, even at high T, the long wavelength acoustic (LWA) phonons of an ionic crystal can remain partially immune to disorder generated by its LO phonons; whereas, even at low T, its LO phonons can be strongly affected by LWA phonons. This is because LO displacements average out in much less than the period of an LWA phonon; whereas during each period of an LO phonon, an LWA phonon appears as a quasistatic perturbation of the crystal, which warps the LO mode's intrinsic electric field. LO phonons are highly sensitive to acoustic warping of their intrinsic fields because their frequencies depend strongly on them: They cause the large frequency difference between LO and TO bands known as LO-TO splitting. We calculate vibrational spectra from MD trajectories using a method that we show to be classically exact and therefore applicab
Lian Z, Meng Y, Ma L, et al., 2024, Valley-polarized excitonic Mott insulator in WS2/WSe2 moiré superlattice, Nature Physics, Vol: 20, Pages: 34-39, ISSN: 1745-2473
The strongly enhanced electron–electron interactions in semiconducting moiré superlattices formed by transition metal dichalcogenide heterobilayers have led to a plethora of intriguing fermionic correlated states. Meanwhile, interlayer excitons in a type II aligned heterobilayer moiré superlattice, with electrons and holes separated in different layers, inherit this enhanced interaction and suggest that tunable correlated bosonic quasiparticles with a valley degree of freedom could be realized. Here we determine the spatial extent of interlayer excitons and the band hierarchy of correlated states that arises from the strong repulsion between interlayer excitons and correlated electrons in a WS2/WSe2 moiré superlattice. We also find evidence that an excitonic Mott insulator state emerges when one interlayer exciton occupies one moiré cell. Furthermore, the valley polarization of the excitonic Mott insulator state is enhanced by nearly one order of magnitude. Our study demonstrates that the WS2/WSe2 moiré superlattice is a promising platform for engineering and exploring new correlated states of fermion, bosons and a mixture of both.
Molino L, Aggarwal L, Maity I, et al., 2023, Influence of Atomic Relaxations on the Moiré Flat Band Wave Functions in Antiparallel Twisted Bilayer WS2., Nano Lett, Vol: 23, Pages: 11778-11784
Twisting bilayers of transition metal dichalcogenides gives rise to a moiré potential resulting in flat bands with localized wave functions and enhanced correlation effects. In this work, scanning tunneling microscopy is used to image a WS2 bilayer twisted approximately 3° off the antiparallel alignment. Scanning tunneling spectroscopy reveals localized states in the vicinity of the valence band onset, which is observed to occur first in regions with S-on-S Bernal stacking. In contrast, density functional theory calculations on twisted bilayers that have been relaxed in vacuum predict the highest-lying flat valence band to be localized in regions of AA' stacking. However, agreement with experiment is recovered when the calculations are performed on bilayers in which the atomic displacements from the unrelaxed positions have been reduced, reflecting the influence of the substrate and finite temperature. This demonstrates the delicate interplay of atomic relaxations and the electronic structure of twisted bilayer materials.
João SM, Jin H, Lischner JC, 2023, Atomistic Theory of Hot-Carrier Relaxation in Large Plasmonic Nanoparticles., J Phys Chem C Nanomater Interfaces, Vol: 127, Pages: 23296-23302, ISSN: 1932-7447
Recently, there has been significant interest in harnessing hot-carriers generated from the decay of localized surface plasmons in metallic nanoparticles for applications in photocatalysis, photovoltaics, and sensing. In this work, we develop an atomistic method that makes it possible to predict the population of hot-carriers under continuous wave illumination for large nanoparticles of relevance to experimental studies. For this, we solve the equation of motion of the density matrix, taking into account both the excitation of hot-carriers and subsequent relaxation effects. We present results for spherical Au and Ag nanoparticles with up to 250,000 atoms. We find that the population of highly energetic carriers depends on both the material and the nanoparticle size. We also study the increase in the electronic temperature upon illumination and find that Ag nanoparticles exhibit a much larger temperature increase than Au nanoparticles. Finally, we investigate the effect of using different models for the relaxation matrix but find that the qualitative features of the hot-carrier population are robust. These insights can be harnessed for the design of improved hot-carrier devices.
Aghajanian M, Mostofi AA, Lischner J, 2023, Optical properties of charged defects in monolayer MoS₂, Electronic Structure, Vol: 5, ISSN: 2516-1075
We present theoretical calculations of the optical spectrum of monolayer MoS2 with a charged defect. In particular, we solve the Bethe–Salpeter equation based on an atomistic tight-binding model of the MoS2 electronic structure which allows calculations for large supercells. The defect is modelled as a point charge whose potential is screened by the MoS2 electrons. We find that the defect gives rise to new peaks in the optical spectrum approximately 100–200 meV below the first free exciton peak. These peaks arise from transitions involving in-gap bound states induced by the charged defect. Our findings are in good agreement with experimental measurements.
Jin H, Herran M, Cortés E, et al., 2023, Theory of hot-carrier generation in bimetallic plasmonic catalysts, ACS Photonics, Vol: 10, Pages: 3629-3636, ISSN: 2330-4022
Bimetallic nanoreactors in which a plasmonic metal is used to funnel solar energy toward a catalytic metal have recently been studied experimentally, but a detailed theoretical understanding of these systems is lacking. Here, we present theoretical results of hot-carrier generation rates of different Au-Pd nanoarchitectures. In particular, we study spherical core-shell nanoparticles with a Au core and a Pd shell as well as antenna-reactor systems consisting of a large Au nanoparticle that acts as an antenna and a smaller Pd satellite nanoparticle separated by a gap. In addition, we investigate an antenna-reactor system in which the satellite is a core-shell nanoparticle. Hot-carrier generation rates are obtained from an atomistic quantum-mechanical modeling technique which combines a solution of Maxwell's equation with a tight-binding description of the nanoparticle electronic structure. We find that antenna-reactor systems exhibit significantly higher hot-carrier generation rates in the catalytic material than the core-shell system as a result of strong electric field enhancements associated with the gap between the antenna and the satellite. For these systems, we also study the dependence of the hot-carrier generation rate on the size of the gap, the radius of the antenna nanoparticle, and the direction of light polarization. Overall, we find a strong correlation between the calculated hot-carrier generation rates and the experimentally measured chemical activity for the different Au-Pd photocatalysts. Our insights pave the way toward a microscopic understanding of hot-carrier generation in heterogeneous nanostructures for photocatalysis and other energy-conversion applications.
Mario Z, Horsfield A, Lischner J, 2023, Accelerating GW calculations through machine learned dielectric matrices, npj Computational Materials, Vol: 9, ISSN: 2057-3960
The GW approach produces highly accurate quasiparticle energies, but its application to large systems is computationally challenging due to the difficulty in computing the inverse dielectric matrix. To address this challenge, we develop a machine learning approach to efficiently predict density–density response functions (DDRF) in materials. An atomic decomposition of the DDRF is introduced, as well as the neighborhood density–matrix descriptor, both of which transform in the same way under rotations. The resulting DDRFs are then used to evaluate quasiparticle energies via the GW approach. To assess the accuracy of this method, we apply it to hydrogenated silicon clusters and find that it reliably reproduces HOMO–LUMO gaps and quasiparticle energy levels. The accuracy of the predictions deteriorates when the approach is applied to larger clusters than those in the training set. These advances pave the way for GW calculations of complex systems, such as disordered materials, liquids, interfaces, and nanoparticles.
Tepliakov NV, Ma R, Lischner J, et al., 2023, Dirac half-semimetallicity and antiferromagnetism in graphene nanoribbon/Hexagonal boron nitride heterojunctions, Nano Letters: a journal dedicated to nanoscience and nanotechnology, Vol: 23, Pages: 6698-6704, ISSN: 1530-6984
Half-metals have been envisioned as active components in spintronic devices by virtue of their completely spin-polarized electrical currents. Actual materials hosting half-metallic phases, however, remain scarce. Here, we predict that recently fabricated heterojunctions of zigzag nanoribbons embedded in two-dimensional hexagonal boron nitride are half-semimetallic, featuring fully spin-polarized Dirac points at the Fermi level. The half-semimetallicity originates from the transfer of charges from hexagonal boron nitride to the embedded graphene nanoribbon. These charges give rise to opposite energy shifts of the states residing at the two edges, while preserving their intrinsic antiferromagnetic exchange coupling. Upon doping, an antiferromagnetic-to-ferrimagnetic phase transition occurs in these heterojunctions, with the sign of the excess charge controlling the spatial localization of the net magnetic moments. Our findings demonstrate that such heterojunctions realize tunable one-dimensional conducting channels of spin-polarized Dirac fermions seamlessly integrated into a two-dimensional insulator, thus holding promise for the development of carbon-based spintronics.
Ramzan MS, Goodwin ZAH, Mostofi AA, et al., 2023, Effect of Coulomb impurities on the electronic structure of magic angle twisted bilayer graphene, npj 2D Materials and Applications, Vol: 7, Pages: 1-8, ISSN: 2397-7132
In graphene, charged defects break the electron-hole symmetry and can evengive rise to exotic collapse states when the defect charge exceeds a criticalvalue which is proportional to the Fermi velocity. In this work, we investigatethe electronic properties of twisted bilayer graphene (tBLG) with chargeddefects using tight-binding calculations. Like monolayer graphene, tBLGexhibits linear bands near the Fermi level but with a dramatically reducedFermi velocity near the magic angle (approximately 1.1{\deg}). This suggeststhat the critical value of the defect charge in magic-angle tBLG should also bevery small. We find that charged defects give rise to significant changes inthe low-energy electronic structure of tBLG. Depending on the defect positionin the moir\'e unit cell, it is possible to open a band gap or to induce anadditional flattening of the low-energy valence and conduction bands. Ourcalculations suggest that the collapse states of the two monolayers hybridizein the twisted bilayer. However, their in-plane localization remains largelyunaffected by the presence of the additional twisted layer because of thedifferent length scales of the moir\'e lattice and the monolayer collapse statewavefunctions. These predictions can be tested in scanning tunnellingspectroscopy experiments.
Liou F, Tsai H-Z, Goodwin ZAH, et al., 2023, Imaging field-driven melting of a molecular solid at the atomic scale, Advanced Materials, Pages: 1-8, ISSN: 0935-9648
Solid-liquid phase transitions are basic physical processes, but atomically resolved microscopy has yet to capture their full dynamics. A new technique is developed for controlling the melting and freezing of self-assembled molecular structures on a graphene field-effect transistor (FET) that allows phase-transition behavior to be imaged using atomically resolved scanning tunneling microscopy. This is achieved by applying electric fields to 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane-decorated FETs to induce reversible transitions between molecular solid and liquid phases at the FET surface. Nonequilibrium melting dynamics are visualized by rapidly heating the graphene substrate with an electrical current and imaging the resulting evolution toward new 2D equilibrium states. An analytical model is developed that explains observed mixed-state phases based on spectroscopic measurement of solid and liquid molecular energy levels. The observed nonequilibrium melting dynamics are consistent with Monte Carlo simulations.
Maity I, Mostofi AA, Lischner J, 2023, Electrons surf phason waves in moiré bilayers, Nano Letters: a journal dedicated to nanoscience and nanotechnology, Vol: 23, Pages: 4870-4875, ISSN: 1530-6984
We investigate the effect of thermal fluctuations on the atomic and electronic structure of a twisted MoSe2/WSe2 heterobilayer using a combination of classical molecular dynamics and ab initio density functional theory calculations. Our calculations reveal that thermally excited phason modes give rise to an almost rigid motion of the moiré lattice. Electrons and holes in low-energy states are localized in specific stacking regions of the moiré unit cell and follow the thermal motion of these regions. In other words, charge carriers surf phason waves that are excited at finite temperatures. We also show that such surfing survives in the presence of a substrate and frozen potential. This effect has potential implications for the design of charge and exciton transport devices based on moiré materials.
Kahk JM, Lischner J, 2023, Combining the Δ-self-consistent-field and GW methods for predicting core electron binding energies in periodic solids, Journal of Chemical Theory and Computation, Vol: 19, Pages: 3276-3283, ISSN: 1549-9618
For the computational prediction of core electron binding energies in solids, two distinct kinds of modeling strategies have been pursued: the Δ-Self-Consistent-Field method based on density functional theory (DFT), and the GW method. In this study, we examine the formal relationship between these two approaches and establish a link between them. The link arises from the equivalence, in DFT, between the total energy difference result for the first ionization energy, and the eigenvalue of the highest occupied state, in the limit of infinite supercell size. This link allows us to introduce a new formalism, which highlights how in DFT─even if the total energy difference method is used to calculate core electron binding energies─the accuracy of the results still implicitly depends on the accuracy of the eigenvalue at the valence band maximum in insulators, or at the Fermi level in metals. We examine whether incorporating a quasiparticle correction for this eigenvalue from GW theory improves the accuracy of the calculated core electron binding energies, and find that the inclusion of vertex corrections is required for achieving quantitative agreement with experiment.
Doiron B, Li Y, Bower R, et al., 2023, Optimizing hot electron harvesting at planar metal–semiconductor interfaces with titanium oxynitride thin films, ACS Applied Materials and Interfaces, Vol: 25, Pages: 30417-30426, ISSN: 1944-8244
Understanding metal-semiconductor interfaces is critical to the advancement of photocatalysis and sub-bandgap solar energy harvesting where electrons in the metal can be excited by sub-bandgap photons and extracted into the semiconductor. In this work, we compare the electron extraction efficiency across Au/TiO2 and titanium oxynitride (TiON)/TiO2-x interfaces, where in the latter case the spontaneously forming oxide layer (TiO2-x) creates a metal-semiconductor contact. Time-resolved pump-probe spectroscopy is used to study the electron recombination rates in both cases. Unlike the nanosecond recombination lifetimes in Au/TiO2, we find a bottleneck in the electron relaxation in the TiON system, which we explain using a trap-mediated recombination model. Using this model, we investigate the tunability of the relaxation dynamics with oxygen content in the parent film. The optimized film (TiO0.5N0.5) exhibits the highest carrier extraction efficiency (NFC ≈ 2.8 × 1019 m-3), slowest trapping, and an appreciable hot electron population reaching the surface oxide (NHE ≈ 1.6 × 1018 m-3). Our results demonstrate the productive role oxygen can play in enhancing electron harvesting and prolonging electron lifetimes, providing an optimized metal-semiconductor interface using only the native oxide of titanium oxynitride.
Rehr JJJ, Prendergast D, Lischner J, 2023, Recent advances in modelling core-electron spectroscopy, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 25, Pages: 7572-7573, ISSN: 1463-9076
Tepliakov NV, Lischner J, Kaxiras E, et al., 2023, Unveiling and manipulating hidden symmetries in graphene nanoribbons, Physical Review Letters, Vol: 130, Pages: 1-6, ISSN: 0031-9007
Armchair graphene nanoribbons are a highly promising class of semiconductors for all-carbon nanocircuitry. Here, we present a new perspective on their electronic structure from simple model Hamiltonians and ab initio calculations. We focus on a specific set of nanoribbons of width n=3p+2, where n is the number of carbon atoms across the nanoribbon axis and p is a positive integer. We demonstrate that the energy-gap opening in these nanoribbons originates from the breaking of a previously unidentified hidden symmetry by long-ranged hopping of π electrons and structural distortions occurring at the edges. This hidden symmetry can be restored or manipulated through the application of in-plane lattice strain, which enables continuous energy-gap tuning, the emergence of Dirac points at the Fermi level, and topological quantum phase transitions. Our work establishes an original interpretation of the semiconducting character of armchair graphene nanoribbons and offers guidelines for rationally designing their electronic structure.
Lischner J, 2023, Hot carrier generation in metallic nanoparticles
In this contribution, I will introduce a new modelling approach for calculating properties of hot carrier generation from the decay of localized surface plasmons in metallic nanoparticles for applications in photocatalysis.
Lischner J, 2023, Modelling hot carrier generation in large metallic nanoparticles
Localized surface plasmons in metallic nanoparticles give rise to very strong light absorption. The decay of these excitations results in the generation of energetic or "hot" electrons and holes which can be harvested and harnessed for applications in photovoltaics, photocatalysis and light sensing. To optimize hot carrier production in devices, a detailed theoretical understanding of the relevant microscopic processes, including light-matter interactions, plasmon decay and hot electron thermalization, is needed. In my talk, I will describe a material-specific theory of hot-carrier generation in metallic nanoparticles which combines a classical description of the electromagnetic radiation with large-scale atomistic quantum-mechanical simulations. I will present results for hot carrier distributions in spherical nanoparticles of gold, silver and copper and discuss the relative importance of interband and intraband transitions as function of nanoparticle size. Finally, I will describe results for more complex systems, such as core-shell nanoparticles or "reactor" systems in which small catalytic nanoparticles are adsorbed to a larger plasmonic nanoparticles.
Annegarn M, Kahk JM, Lischner J, 2022, Combining time-dependent density functional theory and the delta scf approach for accurate core-electron spectra, Journal of Chemical Theory and Computation, Vol: 18, Pages: 7620-7629, ISSN: 1549-9618
Spectroscopies that probe electronic excitations from core levels into unoccupied orbitals, such as X-ray absorption spectroscopy and electron energy loss spectroscopy, are widely used to gain insight into the electronic and chemical structure of materials. To support the interpretation of experimental spectra, we assess the performance of a first-principles approach that combines linear-response time-dependent density (TDDFT) functional theory with the Δ self-consistent field (ΔSCF) approach. In particular, we first use TDDFT to calculate the core-level spectrum and then shift the spectrum such that the lowest excitation energy from TDDFT agrees with that from ΔSCF. We apply this method to several small molecules and find encouraging agreement between calculated and measured spectra.
Campbell AJ, Brotons-Gisbert M, Baek H, et al., 2022, Exciton-polarons in the presence of strongly correlated electronic states in a MoSe2/WSe2 moiré superlattice, npj 2D Materials and Applications, Vol: 6, Pages: 1-8, ISSN: 2397-7132
Two-dimensional moiré materials provide a highly tunable platform to investigate strongly correlated electronic states. Such emergent many-body phenomena can be optically probed in moiré systems created by stacking two layers of transition metal dichalcogenide semiconductors: optically injected excitons can interact with itinerant carriers occupying narrow moiré bands to form exciton-polarons sensitive to strong correlations. Here, we investigate the behaviour of excitons dressed by a Fermi sea localised by the moiré superlattice of a molybdenum diselenide (MoSe2)/tungsten diselenide (WSe2) twisted hetero-bilayer. At a multitude of fractional fillings of the moiré lattice, we observe ordering of both electrons and holes into stable correlated electronic states. Magneto-optical measurements reveal extraordinary Zeeman splittings of the exciton-polarons due to exchange interactions in the correlated hole phases, with a maximum close to the correlated state at one hole per site. The temperature dependence of the Zeeman splitting reveals antiferromagnetic ordering of the correlated holes across a wide range of fractional fillings. Our results illustrate the nature of exciton-polarons in the presence of strongly correlated electronic states and reveal the rich potential of the MoSe2/WSe2 platform for investigations of Fermi–Hubbard and Bose–Hubbard physics.
Och M, Anastasiou K, Leontis I, et al., 2022, Synthesis of mono- and few-layered n-type WSe<sub>2</sub> from solid state inorganic precursors, NANOSCALE, Vol: 14, Pages: 15651-15662, ISSN: 2040-3364
Barker BA, Deslippe J, Lischner J, et al., 2022, Spinor GW/Bethe-Salpeter calculations in BerkeleyGW: Implementation, symmetries, benchmarking, and performance, Physical Review B, Vol: 106, Pages: 1-17, ISSN: 2469-9950
Computing the GW quasiparticle band structure and Bethe-Salpeter equation (BSE) absorption spectra for materials with spin-orbit coupling have commonly been done by treating GW corrections and spin-orbit coupling (SOC) as separate perturbations to density-functional theory. However, accurate treatment of materials with strong spin-orbit coupling (such as many topological materials of recent interest, and thermoelectrics) often requires a nonperturbative approach using spinor wave functions in the Kohn-Sham equation and GW/BSE. Such calculations have only recently become available, in particular for the BSE. We have implemented this approach in the plane-wave pseudopotential GW/BSE code BerkeleyGW, which is highly parallelized and widely used in the electronic-structure community. We present reference results for quasiparticle band structures and optical absorption spectra of solids with different strengths of spin-orbit coupling, including Si, Ge, GaAs, GaSb, CdSe, Au, and Bi2Se3. The calculated quasiparticle band gaps of these systems are found to agree with experiment to within a few tens of meV. SOC splittings are found to be generally in better agreement with experiment, including quasiparticle corrections to band energies. The absorption spectrum of GaAs is not significantly impacted by the inclusion of spin-orbit coupling due to its relatively small value (0.2 eV) in the Λ direction, while the absorption spectrum of GaSb calculated with the spinor GW/BSE captures the large spin-orbit splitting of peaks in the spectrum. For the prototypical topological insulator Bi2Se3, we find a drastic change in the low-energy band structure compared to that of DFT, with the spinorial treatment of the GW approximation correctly capturing the parabolic nature of the valence and conduction bands after including off-diagonal self-energy matrix elements. We present the detailed methodology, approach to spatial symmetries for spinors, comparison against other codes, and per
Kahk JM, Lischner J, 2022, Predicting core electron binding energies in elements of the first transition series using the Δ-self-consistent-field method., Faraday Discussions, Vol: 236, Pages: 364-373, ISSN: 1359-6640
The Δ-Self-Consistent-Field (ΔSCF) method has been established as an accurate and computationally efficient approach for calculating absolute core electron binding energies for light elements up to chlorine, but relatively little is known about the performance of this method for heavier elements. In this work, we present ΔSCF calculations of transition metal (TM) 2p core electron binding energies for a series of 60 molecular compounds containing the first row transition metals Ti, V, Cr, Mn, Fe and Co. We find that the calculated TM 2p3/2 binding energies are less accurate than the results for the lighter elements with a mean absolute error (MAE) of 0.73 eV compared to experimental gas phase photoelectron spectroscopy results. However, our results suggest that the error depends mostly on the element and is rather insensitive to the chemical environment. By applying an element-specific correction to the binding energies the MAE is reduced to 0.20 eV, similar to the accuracy obtained for the lighter elements.
Arrigo R, Ban L, Bartels-Rausch T, et al., 2022, Future directions: general discussion., Faraday Discuss, Vol: 236, Pages: 412-428
Arrigo R, Aureau D, Bhatt P, et al., 2022, <i>In situ</i> methods: discoveries and challenges: general discussion, FARADAY DISCUSSIONS, Vol: 236, Pages: 219-266, ISSN: 1359-6640
Lischner J, Jin H, Ferreira A, et al., 2022, Plasmon-induced hot carriers from interband and intraband transitions in large noble metal nanoparticles, Physical Review X, Vol: 1, Pages: 1-9, ISSN: 2160-3308
Hot electrons generated from the decay of localized surface plasmons in metallic nanostructureshave the potential to transform photocatalysis, photodetection and other optoelectronic applications.However, the understanding of hot-carrier generation in realistic nanostructures, in particular therelative importance of interband and intraband transitions, remains incomplete. Here we reporttheoretical predictions of hot-carrier generation rates in spherical nanoparticles of the noble metalssilver, gold and copper with diameters up to 30 nanometers consisting of more than one millionatoms obtained from an atomistic linear-scaling approach. As the nanoparticle size increases therelative importance of interband transitions from d-bands to sp-bands relative to surface-enabled spband to sp-band transitions increases. We find that the hot-hole generation rate is characterized bya peak at the onset of the d-bands, while the position of the corresponding peak in the hot-electrondistribution can be controlled through the illumination frequency. In contrast, intraband transitionsgive rise to hot electrons, but relatively cold holes. Importantly, increasing the dielectric constantof the environment removes hot carriers generated from interband transitions, while increasing thenumber of hot carriers from intraband transitions. The insights resulting from our work enable thedesign of nanoparticles for specific hot-carrier applications through their material composition, sizeand dielectric environment.
Cheung CTS, Goodwin ZAH, Vitale V, et al., 2022, Atomistic Hartree theory of twisted double bilayer graphene near the magic angle, Electronic Structure, Vol: 4, Pages: 1-11, ISSN: 2516-1075
Twisted double bilayer graphene (tDBLG) is a moiré material that has recently generated significant interest because of the observation of correlated phases near the magic angle. We carry out atomistic Hartree theory calculations to study the role of electron–electron interactions in the normal state of tDBLG. In contrast to twisted bilayer graphene, we find that such interactions do not result in significant doping-dependent deformations of the electronic band structure of tDBLG. However, interactions play an important role for the electronic structure in the presence of a perpendicular electric field as they screen the external field. Finally, we analyze the contribution of the Hartree potential to the crystal field, i.e. the on-site energy difference between the inner and outer layers. We find that the on-site energy obtained from Hartree theory has the same sign, but a smaller magnitude compared to previous studies in which the on-site energy was determined by fitting tight-binding results to ab initio density-functional theory (DFT) band structures. To understand this quantitative difference, we analyze the ab initio Kohn–Sham potential obtained from DFT and find that a subtle interplay of electron–electron and electron–ion interactions determines the magnitude of the on-site potential.
Aghajanian M, Mostofi A, Lischner J, 2022, Electronic structure of monolayer and bilayer black phosphorus with charged defects, Physical Review Materials, Vol: 6, Pages: 1-13, ISSN: 2475-9953
We use an atomistic approach to study the electronic properties of monolayer and bilayer black phosphorus in the vicinity of a charged defect. In particular, we combine screened defect potentials obtained from first-principles linear response theory with large-scale tight-binding simulations to calculate the wave functions and energies of bound acceptor and donor states. As a consequence of the anisotropic band structure, the defect states in these systems form distorted hydrogenic orbitals with a different ordering from that in isotropic materials. For the monolayer, we study the dependence of the binding energies of charged adsorbates on the defect height and the dielectric constant of a substrate in an experimental setup. We also compare our results with an anisotropic effective mass model and find quantitative and qualitative differences when the charged defect is close to the black phosphorus or when the screening from the substrate is weak. For the bilayer, we compare results for charged adsorbates and charged intercalants and find that intercalants induce more prominent secondary peaks in the local density of states because they interact strongly with electronic states on both layers. These insights can be directly tested in scanning tunneling spectroscopy measurements and enable a detailed understanding of the role of Coulomb impurities in electronic devices.
Kalha C, Ratcliff LE, Gutierrez Moreno JJ, et al., 2022, Lifetime effects and satellites in the photoelectron spectrum of tungsten metal, Physical Review B: Condensed Matter and Materials Physics, Vol: 105, Pages: 1-18, ISSN: 1098-0121
Tungsten (W) is an important and versatile transition metal and has a firm place at the heart of many technologies. A popular experimental technique for the characterization of tungsten and tungsten-based compounds is x-ray photoelectron spectroscopy (XPS), which enables the assessment of chemical states and electronic structure through the collection of core level and valence band spectra. However, in the case of tungsten metal, open questions remain regarding the origin, nature, and position of satellite features that are prominent in the photoelectron spectrum. These satellites are a fingerprint of the electronic structure of the material and have not been thoroughly investigated, at times leading to their misinterpretation. The present work combines high-resolution soft and hard x-ray photoelectron spectroscopy (SXPS and HAXPES) with reflected electron energy loss spectroscopy (REELS) and a multitiered ab initio theoretical approach, including density functional theory (DFT) and many-body perturbation theory (G0W0 and GW+C), to disentangle the complex set of experimentally observed satellite features attributed to the generation of plasmons and interband transitions. This combined experiment-theory strategy is able to uncover previously undocumented satellite features, improving our understanding of their direct relationship to tungsten's electronic structure. Furthermore, it lays the groundwork for future studies into tungsten-based mixed-metal systems and holds promise for the reassessment of the photoelectron spectra of other transition and post-transition metals, where similar questions regarding satellite features remain.
Maity I, Mostofi AA, Lischner J, 2022, Chiral valley phonons and flat phonon bands in moire materials, Physical Review B: Condensed Matter and Materials Physics, Vol: 105, Pages: 1-6, ISSN: 1098-0121
We investigate the chirality of phonon modes in twisted bilayer WSe2 and demonstrate distinct chiral behavior of the K/K′ valley phonons for twist angles close to 0∘ and close to 60∘. In particular, multiple chiral nondegenerate K/K′ valley phonons are found for twist angles near 60∘ whereas no nondegenerate chiral modes are found for twist angles close to 0∘. Moreover, we discover two sets of emergent chiral valley modes that originate from an inversion symmetry breaking at the moiré scale and find similar modes in moiré patterns of strain-engineered bilayers WSe2 and MoSe2/WSe2 heterostructures. At the energy gap between acoustic and optical modes, the formation of flat phonon bands for a broad range of twist angles is observed in twisted bilayer WSe2. Our findings are relevant for understanding electron-phonon and exciton-phonon scattering in moiré materials and also for the design of phononic analogues of flat band electrons.
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