Publications
85 results found
Jin H, Herran M, Cortés E, et al., 2023, Theory of Hot-Carrier Generation in Bimetallic Plasmonic Catalysts, ACS Photonics, ISSN: 2330-4022
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.
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 JJ, Prendergast D, Lischner J, 2023, Recent advances in modelling core-electron spectroscopy., Phys Chem Chem Phys, Vol: 25, Pages: 7572-7573
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.
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.
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
Och M, Anastasiou K, Leontis I, et al., 2022, Synthesis of mono- and few-layered n-type WSe2 from solid state inorganic precursors, NANOSCALE, Vol: 14, Pages: 15651-15662, ISSN: 2040-3364
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, In situ 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.
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.
Fischer A, Goodwin ZAH, Mostofi AA, et al., 2022, Unconventional superconductivity in magic-angle twisted trilayer graphene, npj Quantum Materials, Vol: 7, Pages: 1-10, ISSN: 2397-4648
Magic-angle twisted trilayer graphene (MATTG) recently emerged as a highly tunable platform for studying correlated phases of matter, such as correlated insulators and superconductivity. Superconductivity occurs in a range of doping levels that is bounded by van Hove singularities, which stimulates the debate of the origin and nature of superconductivity in this material. In this work, we discuss the role of spin-fluctuations arising from atomic-scale correlations in MATTG for the superconducting state. We show that in a phase diagram as a function of doping (ν) and temperature, nematic superconducting regions are surrounded by ferromagnetic states and that a superconducting dome with Tc ≈ 2 K appears between the integer fillings ν = −2 and ν = −3. Applying a perpendicular electric field enhances superconductivity on the electron-doped side which we relate to changes in the spin-fluctuation spectrum. We show that the nematic unconventional superconductivity leads to pronounced signatures in the local density of states detectable by scanning tunneling spectroscopy measurements.
Zauchner MG, Dal Forno S, Cśanyi G, et al., 2021, Predicting polarizabilities of silicon clusters using local chemical environments, Machine Learning: Science and Technology, Vol: 2, Pages: 1-16, ISSN: 2632-2153
Calculating polarizabilities of large clusters with first-principles techniques is challenging because of the unfavorable scaling of computational cost with cluster size. To address this challenge, we demonstrate that polarizabilities of large hydrogenated silicon clusters containing thousands of atoms can be efficiently calculated with machine learning methods. Specifically, we construct machine learning models based on the smooth overlap of atomic positions (SOAP) descriptor and train the models using a database of calculated random-phase approximation polarizabilities for clusters containing up to 110 silicon atoms. We first demonstrate the ability of the machine learning models to fit the data and then assess their ability to predict cluster polarizabilities using k-fold cross validation. Finally, we study the machine learning predictions for clusters that are too large for explicit first-principles calculations and find that they accurately describe the dependence of the polarizabilities on the ratio of hydrogen to silicon atoms and also predict a bulk limit that is in good agreement with previous studies.
Liou F, Tsai H-Z, Aikawa AS, et al., 2021, Imaging reconfigurable molecular concentration on a graphene field-effect transistor, Nano Letters: a journal dedicated to nanoscience and nanotechnology, Vol: 21, Pages: 8770-8776, ISSN: 1530-6984
The spatial arrangement of adsorbates deposited onto a clean surface under vacuum typically cannot be reversibly tuned. Here we use scanning tunneling microscopy to demonstrate that molecules deposited onto graphene field-effect transistors (FETs) exhibit reversible, electrically tunable surface concentration. Continuous gate-tunable control over the surface concentration of charged F4TCNQ molecules was achieved on a graphene FET at T = 4.5K. This capability enables the precisely controlled impurity doping of graphene devices and also provides a new method for determining molecular energy level alignment based on the gate-dependence of molecular concentration. Gate-tunable molecular concentration is explained by a dynamical molecular rearrangement process that reduces total electronic energy by maintaining Fermi level pinning in the device substrate. The molecular surface concentration is fully determined by the device back-gate voltage, its geometric capacitance, and the energy difference between the graphene Dirac point and the molecular LUMO level.
Wentink M, Gaberle J, Aghajanian M, et al., 2021, Substitutional tin acceptor states in black phosphorus, The Journal of Physical Chemistry C, Vol: 125, Pages: 22883-22889, ISSN: 1932-7447
Nominally pure black phosphorus (BP) is commonly found to be a p-type semiconductor, suggesting the ubiquitious presence of impurity species or intrinsic, charged defects. Moreover, scanning tunneling microscopy (STM) images of black phosphorus reveal the presence of long-range double-lobed defect features superimposed onto the surface atomic lattice. We show that both the p-type doping of BP and the defect features observed in STM images can be attributed to substitutional tin impurities. We show that black phosphorus samples produced through two common synthesis pathways contain tin impurities, and we demonstrate that the ground state of substitutional tin impurities is negatively charged for a wide range of Fermi level positions within the BP band gap. The localized negative charge of the tin impurities induces hydrogenic states in the band gap, and it is the 2p level that sits at the valence band edge that gives rise to the double-lobed features observed in STM images.
Kahk JM, Michelitsch GS, Maurer RJ, et al., 2021, Core electron binding energies in solids from periodic all-electron delta-self-consistent-field calculations, Journal of Physical Chemistry Letters, Vol: 12, Pages: 9353-9359, ISSN: 1948-7185
Theoretical calculations of core electron binding energies are required for the interpretation of experimental X-ray photoelectron spectra, but achieving accurate results for solids has proven difficult. In this work, we demonstrate that accurate absolute core electron binding energies in both metallic and insulating solids can be obtained from periodic all-electron Δ-self-consistent-field (ΔSCF) calculations. In particular, we show that core electron binding energies referenced to the valence band maximum can be obtained as total energy differences between two (N – 1)-electron systems: one with a core hole and one with an electron removed from the highest occupied valence state. To achieve convergence with respect to the supercell size, the analogy between localized core holes and charged defects is exploited. Excellent agreement between calculated and experimental core electron binding energies is found for both metals and insulators, with a mean absolute error of 0.24 eV for the systems considered.
Baek H, Brotons-Gisbert M, Campbell A, et al., 2021, Optical read-out of Coulomb staircases in a moiré superlattice via trapped interlayer trions, Nature Nanotechnology, Vol: 16, Pages: 1237-1243, ISSN: 1748-3387
Moiré patterns with a superlattice potential can be formed by vertically stacking two layered materials with a relative twist or lattice constant mismatch. In transition metal dichalcogenide-based systems, the moiré potential landscape can trap interlayer excitons (IXs) at specific atomic registries. Here, we report that spatially isolated trapped IXs in a molybdenum diselenide/tungsten diselenide heterobilayer device provide a sensitive optical probe of carrier filling in their immediate environment. By mapping the spatial positions of individual trapped IXs, we are able to spectrally track the emitters as the moiré lattice is filled with excess carriers. Upon initial doping of the heterobilayer, neutral trapped IXs form charged IXs (IX trions) uniformly with a binding energy of ~7 meV. Upon further doping, the empty superlattice sites sequentially fill, creating a Coulomb staircase: stepwise changes in the IX trion emission energy due to Coulomb interactions with carriers at nearest-neighbour moiré sites. This non-invasive, highly local technique can complement transport and non-local optical sensing techniques to characterize Coulomb interaction energies, visualize charge correlated states, or probe local disorder in a moiré superlattice.
Goodwin Z, Klebl L, Vitale V, et al., 2021, Flat bands, electron interactions and magnetic order in magic-angle mono-trilayer graphene, Physical Review Materials, Vol: 5, ISSN: 2475-9953
Starting with twisted bilayer graphene, graphene-based moiré materials have recently been established as a new platform for studying strong electron correlations. In this paper, we study twisted graphene monolayers on trilayer graphene and demonstrate that this system can host flat bands when the twist angle is close to the magic angle of 1.16∘. When monolayer graphene is twisted on ABA trilayer graphene, the flat bands are not isolated, but are intersected by a Dirac cone with a large Fermi velocity. In contrast, graphene twisted on ABC trilayer graphene (denoted AtABC) exhibits a gap between flat and remote bands. Since ABC trilayer graphene and twisted bilayer graphene are known to host broken-symmetry phases, we further investigate the ostensibly similar magic-angle AtABC system. We study the effect of electron-electron interactions in AtABC using both Hartree theory and an atomic Hubbard theory to calculate the magnetic phase diagram as a function of doping, twist angle, and perpendicular electric field. Our analysis reveals a rich variety of magnetic orderings, including ferromagnetism and ferrimagnetism, and demonstrates that a perpendicular electric field makes AtABC more susceptible to magnetic ordering.
Vitale V, Atalar K, Mostofi AA, et al., 2021, Flat band properties of twisted transition metal dichalcogenide homo- andheterobilayers of MoS2, MoSe2, WS2 and WSe2, 2D Materials, Vol: 8, ISSN: 2053-1583
Twisted bilayers of two-dimensional materials, such as twisted bilayergraphene, often feature flat electronic bands that enable the observation ofelectron correlation effects. In this work, we study the electronic structureof twisted transition metal dichalcogenide (TMD) homo- and heterobilayers thatare obtained by combining MoS$_2$, WS$_2$, MoSe$_2$ and WSe$_2$ monolayers, andshow how flat band properties depend on the chemical composition of the bilayeras well as its twist angle. We determine the relaxed atomic structure of thetwisted bilayers using classical force fields and calculate the electronic bandstructure using a tight-binding model parametrized from first-principlesdensity-functional theory. We find that the highest valence bands in thesesystems can derive either from $\Gamma$-point or $K$/$K'$-point states of theconstituent monolayers. For homobilayers, the two highest valence bands arecomposed of monolayer $\Gamma$-point states, exhibit a graphene-like dispersionand become flat as the twist angle is reduced. The situation is morecomplicated for heterobilayers where the ordering of $\Gamma$-derived and$K$/$K'$-derived states depends both on the material composition and also thetwist angle. In all systems, qualitatively different band structures areobtained when atomic relaxations are neglected.
Goebel A, Rubio A, Lischner J, 2021, Light-Induced Charge Transfer from Transition-Metal-Doped Aluminum Clusters to Carbon Dioxide, JOURNAL OF PHYSICAL CHEMISTRY A, Vol: 125, Pages: 5878-5885, ISSN: 1089-5639
Lu Q, Martins H, Kahk JM, et al., 2021, Layer-resolved many-electron interactions in delafossite PdCoO2 from standing-wave photoemission spectroscopy, Computer Physics Communications, Vol: 4, Pages: 1-8, ISSN: 0010-4655
When a three-dimensional material is constructed by stacking different two-dimensional layers into an ordered structure, new and unique physical properties can emerge. An example is the delafossite PdCoO2, which consists of alternating layers of metallic Pd and Mott-insulating CoO2 sheets. To understand the nature of the electronic coupling between the layers that gives rise to the unique properties of PdCoO2, we revealed its layer-resolved electronic structure combining standing-wave X-ray photoemission spectroscopy and ab initio many-body calculations. Experimentally, we have decomposed the measured VB spectrum into contributions from Pd and CoO2 layers. Computationally, we find that many-body interactions in Pd and CoO2 layers are highly different. Holes in the CoO2 layer interact strongly with charge-transfer excitons in the same layer, whereas holes in the Pd layer couple to plasmons in the Pd layer. Interestingly, we find that holes in states hybridized across both layers couple to both types of excitations (charge-transfer excitons or plasmons), with the intensity of photoemission satellites being proportional to the projection of the state onto a given layer. This establishes satellites as a sensitive probe for inter-layer hybridization. These findings pave the way towards a better understanding of complex many-electron interactions in layered quantum materials.
Klebl L, Goodwin Z, Mostofi AA, et al., 2021, Importance of long-ranged electron-electron interactions for the magnetic phase diagram of twisted bilayer graphene, Physical Review B, Vol: 103, Pages: 1-7, ISSN: 2469-9950
Electron-electron interactions are intrinsically long ranged, but many models of strongly interacting electrons only take short-ranged interactions into account. Here, we present results of atomistic calculations including both long-ranged and short-ranged electron-electron interactions for the magnetic phase diagram of twisted bilayer graphene and demonstrate that qualitatively different results are obtained when long-ranged interactions are neglected. In particular, we use Hartree theory augmented with Hubbard interactions and calculate the interacting spin susceptibility at a range of doping levels and twist angles near the first magic angle to identify the dominant magnetic instabilities. At the magic angle, mostly antiferromagnetic order is found, while ferromagnetism dominates at other twist angles. Moreover, long-ranged interactions significantly increase the twist angle window in which strong correlation phenomena can be expected. These findings are in good agreement with available experimental data.
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