Excitonic effects in nonlinear optical responses: Diagrammatic approach, exciton-state formalism and first-principles calculations

arXiv preprint arXiv:2402.15882

The integration of low-energy states into bottom-up engineered graphene nanoribbons (GNRs) is a robust strategy for realizing materials with tailored electronic band structure for nanoelectronics. Low-energy zero-modes (ZMs) can be introduced into nanographenes (NGs) by creating an imbalance between the two sublattices of graphene. This phenomenon is exemplified by the family of [n]triangulenes. Here, we demonstrate the synthesis of [3]triangulene-GNRs, a regioregular one-dimensional (1D) chain of [3]triangulenes linked by five-membered rings. Hybridization between ZMs on adjacent [3]triangulenes leads to the emergence of a narrow band gap, Eg = 0.7 eV, and topological end states that are experimentally verified using scanning tunneling spectroscopy (STS). Tight-binding and first-principles density functional theory (DFT) calculations within the local spin density approximation (LSDA) corroborate our experimental observations. Our synthetic design takes advantage of a selective on-surface head-to-tail coupling of monomer building blocks enabling the regioselective synthesis of [3]triangulene-GNRs. Detailed ab initio theory provides insight into the mechanism of on-surface radical polymerization, revealing the pivotal role of Au-C bond formation/breakage in driving selectivity.

Bulletin of the American Physical Society

B43. 00001: Novel Excited States in 2D van der Waals Structures and Moiré Superlattices*

Bulletin of the American Physical Society

M10. 00003: Directly layer-resolved observation of flat-band in phononic magic-angle twisted bilayer graphene*

Bulletin of the American Physical Society

Fabricating new 2D materials from molecular components is desirable because of the extraordinary flexibility of molecular building blocks. This has driven much growth in the fields of covalent organic frameworks (COFs) and metal organic frameworks (MOFs). A common problem with molecular materials, however, is that they are typically insulators and very difficult to electrically dope. Here we describe a new synthesis strategy for growing periodic 2D and 1D molecular networks that exhibit intrinsic metallicity. We achieved this through a unique covalent bonding motif that occurs between N-heterocyclic carbenes (NHCs) and transition metal atoms. A carbene is a molecule with two unpaired electrons (ie, radicals), and when two carbenes bond to a gold atom (ie, NHC-Au-NHC) then this results in frontier molecular orbitals (FMOs) that each contain a single unpaired electron. Periodic structures built from such FMOs …

arXiv preprint arXiv:2402.05456

The behavior of two-dimensional electron gas (2DEG) in extreme coupling limits are reasonably well-understood, but our understanding of intermediate region remains limited. Strongly interacting electrons crystalize into a solid phase known as the Wigner crystal at very low densities, and these evolve to a Fermi liquid at high densities. At intermediate densities, however, where the Wigner crystal melts into a strongly correlated electron fluid that is poorly understood partly due to a lack of microscopic probes for delicate quantum phases. Here we report the first imaging of a disordered Wigner solid and its quantum densification and quantum melting behavior in a bilayer MoSe2 using a non-invasive scanning tunneling microscopy (STM) technique. We observe a Wigner solid with nanocrystalline domains pinned by local disorder at low hole densities. With slightly increasing electrostatic gate voltages, the holes are added quantum mechanically during the densification of the disordered Wigner solid. As the hole density is increased above a threshold (p ~ 5.7 * 10e12 (cm-2)), the Wigner solid is observed to melt locally and create a mixed phase where solid and liquid regions coexist. With increasing density, the liquid regions gradually expand and form an apparent percolation network. Local solid domains appear to be pinned and stabilized by local disorder over a range of densities. Our observations are consistent with a microemulsion picture of Wigner solid quantum melting where solid and liquid domains emerge spontaneously and solid domains are pinned by local disorder.

Bulletin of the American Physical Society

M59. 00008: Moiré effect on the higher-energy excitonic states in WSe 2/WS 2 superlattice*

Bulletin of the American Physical Society

The combination of deep learning and ab initio calculation has shown great promise in revolutionizing future scientific research. However, designing neural network models that effectively incorporate symmetry requirements and a priori knowledge of physical systems remains a significant challenge. Here, we present an E (3)-equivariant deep-learning framework that models the density functional theory (DFT) Hamiltonian as a function of material structure [1, 2]. The neural network respects the Euclidean symmetry of material systems and leverages the locality property of electronic matter, allowing us to achieve sub-meV level accuracy in electronic structure calculations with small-sized training structures [1-3]. Our method scales linearly with system size and is applicable to materials with up to 10 4 atoms. Additionally, our method can be integrated with advanced computational techniques beyond the DFT level …

arXiv preprint arXiv:2404.16344

One-dimensional (1D) interacting electrons are often described as a Luttinger liquid1-4 having properties that are intrinsically different from Fermi liquids in higher dimensions5,6. 1D electrons in materials systems exhibit exotic quantum phenomena that can be tuned by both intra- and inter-1D-chain electronic interactions, but their experimental characterization can be challenging. Here we demonstrate that layer-stacking domain walls (DWs) in van der Waals heterostructures form a broadly tunable Luttinger liquid system including both isolated and coupled arrays. We have imaged the evolution of DW Luttinger liquids under different interaction regimes tuned by electron density using a novel scanning tunneling microscopy (STM) technique. Single DWs at low carrier density are highly susceptible to Wigner crystallization consistent with a spin-incoherent Luttinger liquid, while at intermediate densities dimerized Wigner crystals form due to an enhanced magneto-elastic coupling. Periodic arrays of DWs exhibit an interplay between intra- and inter-chain interactions that gives rise to new quantum phases. At low electron densities inter-chain interactions are dominant and induce a 2D electron crystal composed of phased-locked 1D Wigner crystal in a staggered configuration. Increased electron density causes intra-chain fluctuation potentials to dominate, leading to an electronic smectic liquid crystal phase where electrons are ordered with algebraical correlation decay along the chain direction but disordered between chains. Our work shows that layer-stacking DWs in 2D heterostructures offers new opportunities to explore Luttinger liquid physics.

Computer Physics Communications

We present a workflow of practical calculations of electron-phonon (e-ph) coupling with many-electron correlation effects included using the GW perturbation theory (GWPT). This workflow combines BerkeleyGW, ABINIT, and EPW software packages to enable accurate e-ph calculations at the GW self-energy level, going beyond standard calculations based on density functional theory (DFT) and density-functional perturbation theory (DFPT). This workflow begins with DFT and DFPT calculations (ABINIT) as starting point, followed by GW and GWPT calculations (BerkeleyGW) for the quasiparticle band structures and e-ph matrix elements on coarse electron k- and phonon q-grids, which are then interpolated to finer grids through Wannier interpolation (EPW) for computations of various e-ph coupling determined physical quantities such as the electron self-energies or solutions of anisotropic Eliashberg equations …

Bulletin of the American Physical Society

B11. 00007: Oral: Role of electron-phonon interaction in quasi-1D excitonic chalcogenide Ta 2 Ni (Se, S) 5*

Bulletin of the American Physical Society

Excitonic insulators are a strongly correlated phase of matter whose electronic ground state is a condensate of electron-hole pairs, which may be viewed as a BCS-like state in a certain limit. In this talk, we present an ab initio framework for determining the optical absorption spectrum for this phase. Here we start with the electronic band structure of the normal phase within the GW approximation, and then utilize an ab initio electron-hole interaction kernel for an iterative computation of the BCS gap function. Within this framework, the interaction kernel matrix elements are the same ones used to compute excitonic effects on the optical absorption of the normal phase in standard GW plus Bethe Salpeter Equation (GW-BSE) calculations. Finally, the excitonic insulator optical response is evaluated from the resultant excitonic insulator band structure.As a first study, this method is applied to a Kagome lattice 4-triangulene …

Bulletin of the American Physical Society

G04. 00007: Density Functional Theory-Derived General-Purpose Machine Learning Interatomic Potential for Monolayer and Bilayer Transition-Metal Dichalcogenides

Nano Letters

Topological phases in laterally confined low-dimensional nanographenes have emerged as versatile design tools that can imbue otherwise unremarkable materials with exotic band structures ranging from topological semiconductors and quantum dots to intrinsically metallic bands. The periodic boundary conditions that define the topology of a given lattice have thus far prevented the translation of this technology to the quasi-zero-dimensional (0D) domain of small molecular structures. Here, we describe the synthesis of a polycyclic aromatic hydrocarbon (PAH) featuring two localized zero modes (ZMs) formed by the topological junction interface between a trivial and nontrivial phase within a single molecule. First-principles density functional theory calculations predict a strong hybridization between adjacent ZMs that gives rise to an exceptionally small HOMO–LUMO gap. Scanning tunneling microscopy and …

Bulletin of the American Physical Society

D09. 00006: Nature of moiré excitons in rotationally aligned and misaligned MoSe 2/WS 2 heterobilayers.*

arXiv preprint arXiv:2401.17822

Despite the weak, van-der-Waals interlayer coupling, photoinduced charge transfer vertically across atomically thin interfaces can occur within surprisingly fast, sub-50fs timescales. Early theoretical understanding of the charge transfer is based on a noninteracting picture, neglecting excitonic effects that dominate the optical properties of such materials. Here, we employ an ab initio many-body perturbation theory approach which explicitly accounts for the excitons and phonons in the heterostructure. Our large-scale first-principles calculations directly probe the role of exciton-phonon coupling in the charge dynamics of the WS/MoS heterobilayer. We find that the exciton-phonon interaction induced relaxation time of photo-excited excitons at the valley of MoS and WS is 67 fs and 15 fs at 300 K, respectively, which sets a lower bound to the intralayer-to-interlayer exciton transfer time and is consistent with experiment reports. We further show that electron-hole correlations facilitate novel transfer pathways which are otherwise inaccessible to non-interacting electrons and holes.

Bulletin of the American Physical Society

The Franz-Keldysh effect refers to the change in the optical absorption with an electric field applied to a semiconductor. However, previous interpretations of this phenomenon often overlook the significant role played by excitonic effects (electron-hole interactions), especially near the absorption spectrum edge. Therefore, an accurate incorporation of electron-hole interactions (excitonic effects) is essential for a comprehensive understanding of the Franz-Keldysh phenomenon. In this work, based on an ab initio time-dependent adiabatic GW approach, we investigate the modifications in the optical absorption induced by a DC electric field in gallium nitride (GaN), which is a wide bandgap semiconductor with numerous practical applications, particularly in power devices. Our findings show that the prominent variations at the spectral edge of the optical absorption of GaN are dominated by excitonic effects, which cannot …

Moiré heterostructures of layered materials such as transition metal dichalcogenides enable periodic arrays of localized quasi-particles with long-range Coulomb interactions which can host a plethora of quantum phenomena. Depending on the lattice mismatch and twist angle across the individual layers, resulting moiré potential modulates the distribution of electronic states, significantly changing the landscape of moiré excitons and their characteristics. We employ simultaneous hyperspectral electron energy loss spectroscopy and annular dark field imaging in a scanning transmission electron microscope to investigate WS2/WSe2 heterostructures at the nanoscale. Through this technique, we present the mapping of intralayer moiré excitons within a moiré supercell, shedding light on the interplay between interlayer coupling and atomic reconstruction. Our observations provide valuable insights into the …

Bulletin of the American Physical Society

GW perturbation theory (GWPT)[1] is an ab initio linear-response method to compute electron-phonon (e-ph) coupling matrix elements that include many-electron self-energy effects at the GW level. However, large-scale or systematic studies using GWPT are highly demanding due to the computational expense of calculating the GW self-energy corrections to the numerous e-ph matrix elements that are needed in physical studies. In this talk, we present a method to reduce the cost of GWPT based on tensor rank decomposition, a widely used compression technique for high-dimensional data. This method allows us to obtain properties such as the e-ph coupling constant λ to high accuracy for much lower computational cost, as the full GW self-energy correction matrix is replaced by a low-rank approximation requiring only a small fraction of all terms to be computed.

Nature materials

Moiré superlattices provide a highly tuneable and versatile platform to explore novel quantum phases and exotic excited states ranging from correlated insulators to moiré excitons. Scanning tunnelling microscopy has played a key role in probing microscopic behaviours of the moiré correlated ground states at the atomic scale. However, imaging of quantum excited states in moiré heterostructures remains an outstanding challenge. Here we develop a photocurrent tunnelling microscopy technique that combines laser excitation and scanning tunnelling spectroscopy to directly visualize the electron and hole distribution within the photoexcited moiré exciton in twisted bilayer WS2. The tunnelling photocurrent alternates between positive and negative polarities at different locations within a single moiré unit cell. This alternating photocurrent originates from the in-plane charge transfer moiré exciton in twisted bilayer WS2 …

Bulletin of the American Physical Society

Weakly measuring a quantum critical system can dramatically alter both its long-distance correlations and entanglement structure. In this talk, I will present a roadmap towards experimentally realizing such measurement-altered quantum criticality in Rydberg atom arrays tuned to an Ising phase transition. We specifically envision a protocol wherein a critical two-chain array is adiabatically prepared into its ground state, and then one chain is projectively measured. Notably, this scheme eschews the need for unitary entangling gates commonly utilized for implementing weak measurements. We employ symmetry arguments bolstered by numerical simulations to study the resulting post-measurement wavefunction. The influence of experimental imperfections will also be discussed.

Bulletin of the American Physical Society

W56. 00001: On the use of physics in machine learning for imaging and quantifying complex processes*

Bulletin of the American Physical Society

G07. 00014: Thermal conductivity study of charge-neutral excitations in topological Kondo insulator YbB 12*

Bulletin of the American Physical Society

D09. 00007: Symmetric, off-diagonal, resistance from rotational symmetry breaking in graphene-WSe heterostructure: prediction for a large magic angle in a Moire system*

Bulletin of the American Physical Society

T14. 00009: Classical coupled parametric oscillators as an example of time crystals defined by order parameter dynamics*

Bulletin of the American Physical Society

We study the excitonic mechanism of superconductivity in monolayer-bilayer graphene system by using Eliashberg theory. The electrons of the monolayer interact with virtual excitons and plasmons from the bilayer, producing an attractive interaction at low frequency among the electrons, leading to the possibility of excitonic superconductivity. These virtual excitons and plasmons are contained in the polarization function. We derive the Eliashberg spectral function via the polarization functions of two layers in the random phase approximation. Migdal's adiabaticity condition for the validity of Eliashberg theory can be satisfied since the Fermi velocity of the two layers can be dramatically distinct. Finally, we show that the excitonic mechanism should produce superconductivity at a reasonable Tc through this purely electronic mechanism.

Bulletin of the American Physical Society

Z62. 00014: Generic magnetic field dependence of thermal conductivity in effective spin-1/2 magnetic insulators enabled by hybridization of acoustic phonons and spin-flip excitations*

Bulletin of the American Physical Society

Supersolids are a paradoxical quantum phase of matter intermediate between superfluids and crystals. I will show how the fundamental properties of supersolids can be explored in Bose-Einstein condensates of strongly dipolar atoms. I will in particular show how one of their key properties, the sub-unity superfluid fraction, can be measured by studying the Josephson effect that naturally exists in supersolids.

Bulletin of the American Physical Society

S60. 00008:(Ca, Ce)(Ti, Mn) O 3 Perovskites for Two-Step Solar Thermochemical H 2 Production*

Bulletin of the American Physical Society

Search for violation of fundamental symmetries provides a unique opportunity for testing the Standard Model. Atomic and molecular experiments offer a low energy and comparatively inexpensive alternative to high energy accelerator research in this field. As the observable effects (such as parity violation, PV) are expected to be very small, highly sensitive systems and extremely precise measurements are required for the success of such experiments. Atomic and molecular theory can provide crucial support for these experiments.An important task of theoretical research is to identify optimal molecular and atomic systems for measurements and to understand the mechanisms behind the enhanced sensitivity, which is strongly dependent on the electronic structure. Thus, accurate computational methods are needed in order to provide reliable predictions rather than estimates, and to obtain the various parameters that …

Bulletin of the American Physical Society

Biological systems ranging from genetic circuits to the human brain perform computations. Although extensive research has been done on the functional and behavioral traits accompanying these computations, the equally crucial question of quantifying the precise computation done in biological systems, or at a minimum, the amount of computation done, is under-investigated. Here we introduce a novel approach, and use it to quantify the amount of computation done in the joint dynamics of the neurons in a C. elegans. Our approach uses recent breakthroughs in neural stochastic differential equations to estimate the requisite latent dimensionality of the joint dynamics of the neurons, based on time series data sets of that dynamics. In turn, our time series data are found using fluorescent microscopy to acquire the whole brain activity of C. elegans. Our approach finds that high and low amounts of computation are …

Bulletin of the American Physical Society

Natural materials, such as spider silk, wood, and seed pods, are excellent models for the design of polymeric systems that respond to complex and interacting environments, and that exhibit controlled and modular mechanical behavior under low energy conditions and with a limited set of chemical building blocks. These bio-inspired strategies provide a rich landscape for innovation, mentorship, and outreach via multidisciplinary, collaborative team science initiated by NSF CAREER-funding in the POL division. In this context, I will highlight a few examples of how natural systems inspired the design of shape memory materials, injectable gels, and water-responsive composites utilizing principles, such as hierarchy, interfaces, orientation, and dynamics. Collectively, these vignettes offer a framework that provides insight of the interplay macromolecular design, molecular engineering, and robust manufacturing.

Bulletin of the American Physical Society

Variational quantum optimization algorithms show promise in solving hard combinatorial optimization problems, which could have significant impact on operational domains such as supply chains and logistics. However, their use is currently limited by the number of qubits on the existing hardware which have O (100) qubits, while heuristic classical algorithms can solve problems with O (1000) variables. We implement a qubit-efficient mapping of optimization problems that will enable larger problems to be mapped to currently available quantum computers. The qubit-efficient mapping uses a many-to-one map from classical variables to qubits, and stores the variables in an entangled wavefunction of fewer qubits. We quantify the performance of variational quantum circuits in solving Ising spin glass problems up to 1000 variables, and investigate the tradeoff between algorithm performance and the qubit-efficiency of …

Bulletin of the American Physical Society

Atomic systems have been used as highly precise quantum sensors in a number of applications, including magnetometry and gravimetry. In particular, Rydberg atoms exhibit remarkable sensitivity to electric fields and can form the basis of atomic electrometers. Despite the widespread use of warm vapor-based electrometers due to their convenience, these systems are limited by Doppler effects and constraints on interrogation time. We present progress towards a strontium clock electrometer, combining the state of the art precision of strontium optical lattice clocks with the high electric field sensitivity of Rydberg atoms. Off-resonant Rydberg dressing of the clock state causes the atomic clock frequency to depend on environmental electric fields from DC to THz. This shift can then be read out in clock Ramsey spectroscopy with extended and variable interrogation time at optimal sensitivity, providing enhanced signal …

Bulletin of the American Physical Society

Superfluidity phenomena in the context of thermodynamics, nonlinear, and out-of-equilibrium physics can all be studied with the spectroscopic tool offered by the excitation of sound waves in a Bose-Einstein condensate.In our experiment, we prepare a quasi-2D 23 Na Bose-Einstein condensate confined in a 2D box potential. We spatially modulate the condensate to create an inhomogeneous density profile, on which the dynamics of low-lying sound excitation is observed. We measure the speed of sound along and orthogonal to the 1D inhomogeneities; the case of a 1D lattice, studied in [1, 2], serves as a reference case. The anisotropic speed of sound reveals a reduction of the component of the superfluid density tensor along the modulation direction. Different inhomogeneities potentially change the superfluid density differently, and we compare each case to the result of Leggett [3].

Bulletin of the American Physical Society

L06. 00009: Use of Ion Beam Analysis (IBA) for Elemental Characterization and Multi-Variate Data Analysis of Household Dust Across Indiana*

Bulletin of the American Physical Society

B05. 00001: Superconductivity Mediated by Nematic Fluctuations in Tetragonal FeSe 1-x S x.*

Bulletin of the American Physical Society

S01. 00007: Visualizing superconductivity mediated by nematic fluctuations in the Fe-based superconductor FeSe 1-x S x: Part 1*

Bulletin of the American Physical Society

Quasiparticle interference (QPI) is observed in Scanning Tunneling Spectroscopy (STS) where an impurity causes oscillating patterns of the local density of states (LDOS)[1]. Green's function methods commonly used to simulate QPI [2-4] often show disagreement with experiments, especially when computing inter-band scattering. Motivated by our own STS data [5] on Fe-based superconductors we developed a new scheme to help experimentalists to qualitatively analyze the calculated QPI. It relies on Feynman parametrization (FP), a technique widely used in quantum field theory for loop evaluation. We show that the interband QPI problem can be represented by a collection of new'intermediate bands' possessing relatively easier geometries. We discuss two example applications. First, the QPI between two quadratic bands, where IBA predicts DOS divergences at momentum values that are confirmed by straight …

Bulletin of the American Physical Society

Phospholipid monolayers are a valuable model system to investigate the two-dimensional physics of soft thin film systems. The availability of enantiomers of Dipalmitoylphosphatidylcholine (DPPC) in purified form allows us to investigate the chiral nature of model lipid monolayers through fluorescence microscopy and traditional Langmuir thermodynamic techniques. These phospholipid monolayers are the primary lipid component of lung surfactant-necessary for proper respiration. We focus on mixtures of DPPC with cholesterol, hexadecanol (HD), and palmitic acid (PA), which have been previously studied and shown to form equilibrium morphologies over experimental time scales REF: Valtierrez et al., Sci. Adv. v: 8: 14, 2022. In this poster, we will assess the use of complementary image processing and analysis routines to measure the curvature of morphologies within these monolayers. The introduction of …