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Johannes Lischner

Publications and source records attributed to Johannes Lischner.

At least 37 records · Page 2Linked to original sources
arXiv2023

Breakdown of phonon band theory in MgO

We present a series of detailed images of the distribution of kinetic energy among frequencies and wavevectors 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.

cond-mat.mtrl-sci↗
arXiv2023

Crystal Facet Effect in Plasmonic Catalysis

In the realm of plasmonic catalytic systems, much attention has been devoted to the plasmon-derived mechanisms, yet the influence of nanoparticles' crystal facets in this type of processes has been sparsely investigated. In this work, we study the plasmon-assisted electrocatalytic CO2 reduction reaction using three different shapes of plasmonic Au nanoparticles - nanocube (NC), rhombic dodecahedron (RD) and octahedron (OC) - with three different exposed facets: {100}, {110} and {111}, respectively. These particles were synthesized with similar sizes and LSPR wavelengths to reveal the role of the facet more than other contributions to the plasmon-assisted reaction. Upon plasmon excitation, Au OCs exhibited nearly a doubling in the Faradaic efficiency of CO (FE(CO)) and a remarkable threefold enhancement in the partial current density of CO (j(CO)) compared to the non-illuminated response, NCs also demonstrated an improved performance under illumination. In contrast, Au RDs showed nearly the same performance in dark or light conditions. Temperature-dependent experiments ruled out heat as the main factor in the enhanced response of Au OCs and NCs. Large-scale atomistic simulations of the nanoparticles' electronic structure and electromagnetic modeling revealed higher hot carrier abundance and electric field enhancement on Au OCs and NCs compared to RDs. Abundant hot carriers on edges facilitate molecular activation, leading to enhanced selectivity and activity. Thus, OCs with the highest edge/facet ratio exhibited the strongest enhancement in FE(CO) and j(CO) upon illumination. This observation is further supported by plasmon-assisted H2 evolution reaction experiments. Our findings highlight the dominance of low coordinated sites over facets in plasmonic catalytic processes, providing valuable insights for designing more efficient catalysts for solar fuels production.

physics.optics↗
arXiv2023

Valley-polarized Exitonic Mott Insulator in WS2/WSe2 Moiré Superlattice

Strongly enhanced electron-electron interaction in semiconducting moiré superlattices formed by transition metal dichalcogenides (TMDCs) heterobilayers has led to a plethora of intriguing fermionic correlated states. Meanwhile, interlayer excitons in a type-II aligned TMDC heterobilayer moiré superlattice, with electrons and holes separated in different layers, inherit this enhanced interaction and strongly interact with each other, promising for realizing tunable correlated bosonic quasiparticles with valley degree of freedom. We employ photoluminescence spectroscopy to investigate the strong repulsion between interlayer excitons and correlated electrons in a WS2/WSe2 moiré superlattice and combine with theoretical calculations to reveal the spatial extent of interlayer excitons and the band hierarchy of correlated states. We further find that an excitonic Mott insulator state emerges when one interlayer exciton occupies one moiré cell, evidenced by emerging photoluminescence peaks under increased optical excitation power. Double occupancy of excitons in one unit cell requires overcoming the energy cost of exciton-exciton repulsion of about 30-40 meV, depending on the stacking configuration of the WS2/WSe2 heterobilayer. Further, the valley polarization of the excitonic Mott insulator state is enhanced by nearly one order of magnitude. Our study demonstrates the WS2/WSe2 moiré superlattice as a promising platform for engineering and exploring new correlated states of fermion, bosons, and a mixture of both.

cond-mat.mes-hall↗
arXiv2023

Atomistic theory of hot carrier relaxation in large plasmonic nanoparticles

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 present an atomistic approach to predict the population of hot carriers under continuous wave illumination in large nanoparticles. For this, we solve the equation of motion of the density matrix taking into account both excitation of hot carriers as well as 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 both on 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 qualitative features of the hot-carrier population are robust.

cond-mat.mtrl-sci↗
arXiv2023

Accelerating GW calculations through machine learned dielectric matrices

The GW approach produces highly accurate quasiparticle energies, but its application to large systems is computationally challenging, which can be largely attributed 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. For this, an atomic decomposition of the DDRF is introduced as well as the neighbourhood 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. This technique is called the ML-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 included in the training set. These advances pave the way towards GW calculations of complex systems, such as disordered materials, liquids, interfaces and nanoparticles.

cond-mat.mtrl-sci↗
arXiv2023

Optical Properties of Charged Defects in Monolayer MoS$_2$

We present theoretical calculations of the optical spectrum of monolayer MoS$_2$ with a charged defect. In particular, we solve the Bethe-Salpeter equation based on an atomistic tight-binding model of the MoS$_2$ electronic structure which allows calculations for large supercells. The defect is modelled as a point charge whose potential is screened by the MoS$_2$ 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.

cond-mat.mtrl-sci↗
arXiv2023

Combining the $Δ$-Self-Consistent-Field and GW Methods for Predicting Core Electron Binding Energies in Periodic Solids

For the computational prediction of core electron binding energies in solids, two distinct kinds of modelling 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.

cond-mat.mtrl-sci↗
arXiv2023

Theory of hot-carrier generation in bimetallic plasmonic catalysts

Bimetallic nanoreactors in which a plasmonic metal is used to funnel solar energy towards 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 with acts as 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 modelling 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 hot-carrier generation rate on the size of the gap, the radius of the antenna nanoparticle and the direction of light polarization. Our insights pave the way towards a mechanistic understanding of hot-carrier generation in heterogeneous nanostructures for photocatalysis and other energy conversion applications.

cond-mat.mtrl-sci↗
arXiv2023

Dirac half-semimetallicity and antiferromagnetism in graphene nanoribbon/hexagonal boron nitride heterojunctions

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 that are seamlessly integrated into a two-dimensional insulator, thus holding promise for the development of carbon-based spintronics.

cond-mat.mtrl-sci↗
arXiv2023

Short vs. long range exchange interactions in twisted bilayer graphene

We discuss the effect of long-range interactions within the self-consistent Hartree-Fock (HF) approximation in comparison to short-range atomic Hubbard interactions on the band structure of twisted bilayer graphene (TBG) at charge neutrality for various twist angles. Starting from atomistic calculations, we determine the quasi-particle band structure of TBG with Hubbard interactions for various magnetic orderings: modulated anti-ferromagnetic (MAFM), nodal anti-ferromagnetic (NAFM) and hexagonal anti-ferromagnetic (HAFM). Then, we develop an approach to incorporate these magnetic orderings along with the HF potential in the continuum approximation. Away from the magic angle, we observe a drastic effect of the magnetic order on the band structure of TBG compared to the influence of the HF potential. Near the magic angle, however, the HF potential seems to play a major role on the band structure compared to the magnetic order. These findings suggest that the spin-valley degenerate broken symmetry state often found in HF calculations of charge neutral TBG near the magic angle should favour magnetic order, since the atomistic Hubbard interaction will break this symmetry in favour of spin polarization.

cond-mat.mes-hall↗
arXiv2023

The interplay of field-tunable strongly correlated states in a multi-orbital moiré system

The interplay of charge, spin, lattice, and orbital degrees of freedom leads to a wide range of emergent phenomena in strongly correlated systems. In heterobilayer transition metal dichalcogenide moiré systems, recent observations of Mott insulators and generalized Wigner crystals are well described by triangular lattice single-orbital Hubbard models based on K-valley derived moiré bands. Richer phase diagrams, mapped onto multi-orbital Hubbard models, are possible with hexagonal lattices in $Γ$-valley derived moiré bands and additional layer degrees of freedom. Here we report the tunable interaction between strongly correlated hole states hosted by $Γ$- and K-derived moiré bands in a monolayer MoSe$_2$ / natural WSe$_2$ bilayer device. To precisely probe the nature of the correlated states, we optically characterise the behaviour of exciton-polarons and distinguish the layer and valley degrees of freedom. We find that the honeycomb $Γ$-band gives rise to a charge-transfer insulator described by a two-orbital Hubbard model with inequivalent $Γ_\mathrm{A}$ and $Γ_\mathrm{B}$ orbitals. With an out-of-plane electric field, we re-order the $Γ_\mathrm{B}$- and K-derived bands energetically, driving an abrupt redistribution of carriers to the layer-polarized K orbital where new correlated states are observed. Finally, by fine-tuning the band-alignment, we obtain degeneracy of the $Γ_\mathrm{B}$ and K orbitals at the Fermi level. In this critical condition, stable Wigner crystals with carriers distributed across the two orbitals are observed until the Fermi-level reaches one hole per lattice site, whereupon the system collapses into a filled $Γ_\mathrm{B}$ orbital. Our results establish a platform to investigate the interplay of charge, spin, lattice, and layer geometry in multi-orbital Hubbard model Hamiltonians.

cond-mat.mes-hall↗
arXiv2023

Influence of atomic relaxations on the moiré flat band wavefunctions in antiparallel twisted bilayer WS$_{\text{2}}$

Twisting bilayers of transition metal dichalcogenides (TMDs) gives rise to a periodic moiré potential resulting in flat electronic bands with localized wavefunctions and enhanced correlation effects. In this work, scanning tunneling microscopy is used to image a WS$_{2}$ bilayer twisted approximately $3^{\circ}$ off the antiparallel alignment. Scanning tunneling spectroscopy reveals the presence of localized electronic states in the vicinity of the valence band onset. In particular, the onset of the valence band is observed to occur first in regions with a Bernal stacking in which S atoms are located on top of each other. In contrast, density-functional theory calculations on twisted bilayers which have been relaxed in vacuum predict the highest lying flat valence band to be localized in regions of AA' stacking. However, agreement with the experiment is recovered when the calculations are carried out 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.

cond-mat.mes-hall↗
arXiv2023

Effect of Coulomb impurities on the electronic structure of magic angle twisted bilayer graphene

In graphene, charged defects break the electron-hole symmetry and can even give rise to exotic collapse states when the defect charge exceeds a critical value which is proportional to the Fermi velocity. In this work, we investigate the electronic properties of twisted bilayer graphene (tBLG) with charged defects using tight-binding calculations. Like monolayer graphene, tBLG exhibits linear bands near the Fermi level but with a dramatically reduced Fermi velocity near the magic angle (approximately 1.1°). This suggests that the critical value of the defect charge in magic-angle tBLG should also be very small. We find that charged defects give rise to significant changes in the low-energy electronic structure of tBLG. Depending on the defect position in the moiré unit cell, it is possible to open a band gap or to induce an additional flattening of the low-energy valence and conduction bands. Our calculations suggest that the collapse states of the two monolayers hybridize in the twisted bilayer. However, their in-plane localization remains largely unaffected by the presence of the additional twisted layer because of the different length scales of the moiré lattice and the monolayer collapse state wavefunctions. These predictions can be tested in scanning tunnelling spectroscopy experiments.

cond-mat.mtrl-sci↗
arXiv2023

Phason-mediated interlayer exciton diffusion in WS2/WSe2 moiré heterostructure

Moiré potentials in two-dimensional materials have been proven to be of fundamental importance to fully understand the electronic structure of van der Waals heterostructures, from superconductivity to correlated excitonic states. However, understanding how the moiré phonons, so-called phasons, affect the properties of the system still remains an uncharted territory. In this work, we demonstrate how phasons are integral to properly describing and understanding low-temperature interlayer exciton diffusion in WS2/WSe2 heterostructure. We perform photoluminescence (PL) spectroscopy to understand how the coupling between the layers, affected by their relative orientation, impacts the excitonic properties of the system. Samples fabricated with stacking angles of 0° and 60° are investigated taking into account the stacking angle dependence of the two common moiré potential profiles. Additionally, we present spatially and time-resolved exciton diffusion measurements, looking at the photoluminescence emission in a temperature range from 30 K to 250 K. An accurate potential for the two configurations are computed via density functional theory (DFT) calculations. Finally, we perform molecular dynamics simulation in order to visualize the phasons motion, estimating the phason speed at different temperatures, providing novel insights into the mechanics of exciton propagation at low temperatures that cannot be explained within the frame of classical exciton diffusion alone.

cond-mat.mtrl-sci↗
arXiv2022

Unveiling and Manipulating Hidden Symmetries in Graphene Nanoribbons

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 $\textit{ab initio}$ 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.

cond-mat.mes-hall↗
arXiv2022

Imaging gate-induced molecular melting on a graphene field-effect transistor

Solid-liquid phase transitions are fundamental physical processes, but atomically-resolved microscopy has yet to capture both the solid and liquid dynamics for such a transition. We have developed a new technique for controlling the melting and freezing of 2D molecular layers on a graphene field-effect transistor (FET) that allows us to image phase transition dynamics via atomically-resolved scanning tunneling microscopy. Back-gate voltages applied to a F4TCNQ-decorated graphene FET induce reversible transitions between a charge-neutral solid phase and a negatively charged liquid phase. Nonequilibrium molecular melting dynamics are visualized by rapidly heating the graphene surface with electrical current and imaging the resulting evolution toward new equilibrium states. An analytical model has been developed that explains the observed equilibrium mixed-state phases based on spectroscopic measurement of both solid and liquid molecular energy levels. Observed non-equilibrium melting dynamics are consistent with Monte Carlo simulations.

cond-mat.mes-hall↗
arXiv2022

Combining time-dependent density functional theory and the $Δ$SCF approach for accurate core-electron spectra

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\nobreakdash-consistent field ($Δ$SCF) 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 $Δ$SCF. We apply this method to several small molecules and find encouraging agreement between calculated and measured spectra.

cond-mat.mtrl-sci↗
arXiv2022

Fully relativistic $GW$/Bethe-Salpeter calculations in BerkeleyGW: implementation, symmetries, benchmarking, and performance

Computing the $GW$ quasiparticle bandstructure and Bethe-Salpeter Equation (BSE) absorption spectra for materials with spin-orbit coupling has commonly been done by treating $GW$ corrections and spin-orbit coupling as separate perturbations to density-functional theory. However, accurate treatment of materials with strong spin-orbit coupling often requires a fully relativistic approach using spinor wavefunctions 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 bandstructures and optical absorption spectra of solids with different strengths of spin-orbit coupling, including Si, Ge, GaAs, GaSb, CdSe, Au, and Bi$_2$Se$_3$. The calculated quasiparticle band gaps of these systems are found to agree with experiment to within a few tens of meV. The absorption spectrum of GaSb calculated with the fully-relativistic $GW$-BSE captures the large spin-orbit splitting of peaks in the spectrum. For Bi$_2$Se$_3$, we find a drastic change in the low-energy bandstructure compared to that of DFT, with the fully-relativistic 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 performance compared to spinless $GW$/BSE calculations and perturbative approaches to SOC. This work aims to spur further development of spinor $GW$/BSE methodology in excited-state research software.

cond-mat.mtrl-sci↗