Low-energy Excitations Research Articles

Dissociative excitation of acetone induced by electron impact

Abstract We report the results of an experimental study focused on the low-energy (10-100 eV) electron impact excitation of acetone. In the emission spectrum, the most prominent excited species were found to be the hydrogen Balmer series, the excited CH radical in the A and B electronic states, and atomic carbon. The presence of OH emission in the spectra confirms the tautomerization process induced by electron impact. The acetone emission spectra were measured at several different reaction energies, forming a unique 3D spectral electron energy map. Excitation-emission functions were determined for selected most prominent products.

Equivariant Variational Quantum Eigensolver to detect Phase Transitions through Energy Level Crossings

Abstract Level spectroscopy stands as a powerful method for identifying the transition point that delineates distinct quantum phases. Since each quantum phase exhibits a characteristic sequence of excited states, the crossing of energy levels between low-lying excited states offers a reliable mean to estimate the phase transition point. While approaches like the Variational Quantum Eigensolver are useful for approximating ground states of interacting systems using quantum computing, capturing low-energy excitations remains challenging. In our study, we introduce an equivariant quantum circuit that preserves the total spin and the translational symmetry to accurately describe singlet and triplet excited states in the $J_1$-$J_2$ Heisenberg model on a chain, which are crucial for characterizing its transition point. Additionally, we assess the impact of noise on the variational state, showing that conventional mitigation techniques like Zero Noise Extrapolation reliably restore its physical properties.

Unraveling the structure-property relationship of novel thiophene and furan-fused cyclopentadienyl chromophores for nonlinear optical applications.

Development of organic nonlinear optical materials has become progressively more important due to their emerging applications in new-generation photonic devices. A novel series of chromophores based on innovative thiophene and furan-fused cyclopentadienyl bridge with various powerful donor and acceptor moieties were designed and theoretically investigated for applications in nonlinear optics. To unravel the structure-property relationship between this new push-pull conjugated systems and their nonlinear optical property, multiple methods, including density of states analysis, coupled perturbed Kohn-Sham (CPKS) method, sum-over-states (SOS) model, the two-level model (TSM), hyperpolarizability density analysis, and the (hyper)polarizability contribution decomposition, were performed to comprehensively investigated the nonlinear optical and electronic properties of this new π-system. Due to excellent charge transfer ability of new bridge and distinctive structure of donor and acceptor, the designed chromophores exhibit deep HOMO levels, low excitation energy, high dipole moment difference and large hyperpolarizability, indicating the appealing air-stable property and remarkable electrooptic performance of them. Importantly, THQ-CS-A3 and PA-CS-A3 shows outstanding NLO response properties with βtot value of 6953.9 × 10-30 and 5066.0 × 10-30 esu in AN, respectively. The influence of the push-pull strength, the heterocycle and the π-conjugation of new bridge on the nonlinear optical properties of this novel powerful systems are clarified. This new series of chromophores exhibit remarkable electro-optical Pockels and optical rectification effect. More interestingly, PA-CS-A3 and THQ-CS-A2 also show appealing SHG effect. This study will help people understand the nature of nonlinear optical properties of innovative heteroarene-fused based cyclopentadienyl chromophores and offer guidance for the rational design of chromophores with outstanding electrooptic (EO) performance in the future.

Reflectance spectral studies of spark plasma sintered tungsten carbide pellet

Abstract We report the first spectral reflectance of tungsten carbide (WC) as potential solar selective absorber. We developed an optical measurement system for visible to mid-infrared spectroscopy, covering the range of 0.1 to 2.5 eV, to evaluate the solar selectivity. A polycrystalline WC was prepared using spark plasma sintering method. The measured spectral reflectance of WC exhibits a low-energy plasma excitation around 0.6 eV corresponding to the cutoff energy of sunlight, consistent with ab initio calculations, thus making it preferable for the solar selective absorber. We also discuss effects of the sample quality on the spectral reflectance.

Studying magnon band topology through low-energy magnon excitations: role of anisotropic Dzyaloshinskii-Moriya interaction.

In this work, we study topological properties of magnons via creating spin excitations in both ferromagnets and antiferromagnets in presence of an external magnetic field on a two-dimensional square lattice. It is known that Dzyaloshinskii-Moriya interaction (DMI) plays an important role in coupling between different particle (spin excitation) sectors, here we consider an anisotropic DMI and ascertain the role of the anisotropy parameter in inducing topological phase transitions. While the scenario, for dealing with ferromagnets, albeit with isotropic DMI is established in literature, we have developed the formalism for studying magnon band topology for the antiferromagnetic case. The calculations for the ferromagnetic case are included to facilitate a comparison between the two magnetically ordered systems. Owing to the presence of a two-sublattice structure of an antiferromagnet, a larger number of magnon bands participate in deciding upon the topological properties. However, in both the cases, an extended trivial region is observed even with the DMI to be non-zero, which is surprising since the DMI is the origin of the finite Berry curvature in presence of external magnetic field. The nature of the phases in both the cases and the phase transitions therein are characterized via computing the band structure, ascertaining the presence (or absence) of the chiral edge modes observed in a semi-infinite nano-ribbon geometry, and investigation of the thermal Hall effect. Moreover, the strength of the magnetic field is found to play a decisive role in controlling the critical point that demarcates various topological phases.

Symmetries as Ground States of Local Superoperators: Hydrodynamic Implications

Symmetry algebras of quantum many-body systems with locality can be understood using commutant algebras, which are defined as algebras of operators that commute with a given set of local operators. In this work, we show that these symmetry algebras can be expressed as frustration-free ground states of a local superoperator, which we refer to as a “super-Hamiltonian.” We demonstrate this for conventional symmetries such as Z2, U(1), and SU(2), where the symmetry algebras map to various kinds of ferromagnetic ground states, as well as for unconventional ones that lead to weak ergodicity-breaking phenomena of Hilbert-space fragmentation (HSF) and quantum many-body scars. In addition, we show that the low-energy excitations of this super-Hamiltonian can be understood as approximate symmetries, which in turn are related to slowly relaxing hydrodynamic modes in symmetric systems. This connection is made precise by relating the super-Hamiltonian to the superoperator that governs the operator relaxation in noisy symmetric Brownian circuits and this physical interpretation also provides a novel interpretation for Mazur bounds for autocorrelation functions. We find examples of gapped (gapless) super-Hamiltonians indicating the absence (presence) of slow modes, which happens in the presence of discrete (continuous) symmetries. In the gapless cases, we recover hydrodynamic modes such as diffusion, tracer diffusion, and asymptotic scars in the presence of U(1) symmetry, HSF, and a tower of quantum scars, respectively. In all, this demonstrates the power of the commutant-algebra framework in obtaining a comprehensive understanding of exact symmetries and associated approximate symmetries and hydrodynamic modes, and their dynamical consequences in systems with locality. Published by the American Physical Society 2024

Theoretical investigation of the structure-property relationships of [n]benzocyclobutadiene carbon nanobelt: Aromaticity, singlet fission, and (hyper)polarizability

We present a theoretical design for a series of multifunctional nanomaterials that exhibit superior performance in singlet fission (SF) as well as exceptional nonlinear optical (NLO) performance. Our study focuses on a systematic investigation into the aromaticity, diradical character, excited state transition and (hyper)polarizability of the [n]benzocyclobutadiene carbon nanobelt system. To provide further guidance for designing multifunctional nanomaterials, our quantum chemical calculations propose empirical rules regarding the influence of aromatics on SF and NLO: Through an exploration of SF properties, we observed significant diradical character with radical sites exclusively located at anti-aromatic positions. Furthermore, these anti-aromatic positions serve as transition sites for excited state transitions to achieve the necessary low triplet excited states energy for SF. Then, by conducting calculations on NLO properties, we ascertain that the main tensor components of first-order hyperpolarizability (βtotal) and second-order hyperpolarizability (γtotal) come from the anti-aromatic region. However, there exists a contrasting impact of anti-aromaticity on first- and second-order hyperpolarizability density. The positive and negative contributions to first-order hyperpolarizability density mainly come from both aromatic and anti-aromatic regions while for second-order hyperpolarizability density it is contradictory. This study elucidates the role of aromatic sites in multifunctional nanomaterials with respect to diradical sites, excited state transition regions, and (hyper)polarizability density distribution.

Chiral Virasoro algebra from a single wavefunction

Chiral edges of 2+1D systems can have very robust emergent conformal symmetry. When the edge is purely chiral, the Hilbert space of low-energy edge excitations can form a representation of a single Virasoro algebra. We propose a method to systematically extract the generators of the Virasoro algebra from a single ground state wavefunction, using entanglement bootstrap and an input from the edge conformal field theory. We corroborate our construction by numerically verifying the commutation relations of the generators. We also study the unitary flows generated by these operators, whose properties (such as energy and state overlap) are shown numerically to agree with our analytical predictions.

General theory of cavity-mediated interactions between low-energy matter excitations.

The manipulation of low-energy matter properties such as superconductivity, ferromagnetism, and ferroelectricity via cavity quantum electrodynamics engineering has been suggested as a way to enhance these many-body collective phenomena. In this work, we investigate the effective interactions between low-energy matter excitations induced by the off-resonant coupling with cavity electromagnetic modes. We extend a previous work by going beyond the dipole approximation accounting for the full polarization and magnetization densities of matter. We further include the often neglected diamagnetic interaction and, for the cavity, we consider general linear absorbing media with possibly non-local and non-reciprocal response. We demonstrate that, even in this general scenario, the effective cavity-induced interactions between the matter degrees of freedom are of electrostatic and magnetostatic nature. This confirms the necessity of a multimode description for cavity engineering of matter systems where the low-energy assumption holds. Our findings provide a theoretical framework for studying the influence of general optical environments on extended low-energy matter excitations.

Imaging Brain Tissue with Quantum Light at Low Power.

Light-induced tissue damage is a crucial limitation for traditional microscopy of the living brain, underscoring the need for new techniques that minimize exposure of samples to light. Here, we tested the hypothesis that quantum light, i.e., entangled photons, could detect brain structures at a lower excitation energy. In a proof of principle, we show microscopic images of fixed brain tissue in the hippocampus area created by fluorescence selective excitation in the process of entangled two-photon absorption in a scanning microscope. Quantum-enhanced entangled two-photon microscopy (TPM) had brain imaging capabilities at an unprecedented low excitation intensity of ∼3.6 × 107 photons/s, orders of magnitude lower than the excitation level for the classical two-photon fluorescence image obtained in the same microscope. The extremely low light probe intensity demonstrated in entangled TPM is of critical importance in the investigation of neural activity to minimize heating and photobleaching during repetitive imaging. It may have important functional implications in optogenetic technology, removing unintended heating and accumulated photodamage effects. This technology also opens avenues in spatially resolved brain tissue investigations with quantum light, providing new capabilities in local spectroscopy.

Charge-neutral electronic excitations in quantum insulators.

Experiments on quantum materials have uncovered many interesting quantum phases ranging from superconductivity to a variety of topological quantum matter including the recently observed fractional quantum anomalous Hall insulators. The findings have come in parallel with the development of approaches to probe the rich excitations inherent in such systems. In contrast to observing electrically charged excitations, the detection of charge-neutral electronic excitations in condensed matter remains difficult, although they are essential to understanding a large class of strongly correlated phases. Low-energy neutral excitations are especially important in characterizing unconventional phases featuring electron fractionalization, such as quantum spin liquids, spin ices and insulators with neutral Fermi surfaces. In this Perspective, we discuss searches for neutral fermionic, bosonic or anyonic excitations in unconventional insulators, highlighting theoretical and experimental progress in probing excitonic insulators, new quantum spin liquid candidates and emergent correlated insulators based on two-dimensional layered crystals and moiré materials. We outline the promises and challenges in probing and using quantum insulators, and discuss exciting new opportunities for future advancements offered by ideas rooted in next-generation quantum materials, devices and experimental schemes.

Mapping a dissipative quantum spin chain onto a generalized Coulomb gas

An XXZ spin chain at zero magnetization is subject to spatially correlated baths acting as dissipation. We show that the low-energy excitations of this model are described by a dissipative sine-Gordon field theory, i.e. a sine-Gordon action with an additional long-range interaction emerging from dissipation. The field theory is then exactly mapped onto a generalized Coulomb gas which, in addition to the usual integer charges, displays half-integer charges that originate from the dissipative baths. These new charges come in pairs linked by a charge independent logarithmic interaction. In the Coulomb gas picture, we identify a Berezinsky–Kosterlitz–Thouless-like phase transition corresponding to the binding of charges and derive the associated perturbative renormalization group equations. For superohmic baths, the transition is due to the binding of the integer charges, while for subohmic baths, it is due to the binding of the half-integer charges, thereby signaling a dissipation-induced transition.

A novel translationally invariant supersymmetric chain with inverse-square interactions: partition function, thermodynamics and criticality

Abstract We introduce a novel family of translationally invariant su ( m | n ) supersymmetric spin chains with long-range interaction not directly associated with a root system. We study the symmetries of this model, establishing in particular the existence of a boson–fermion duality characteristic of this type of system. Taking advantage of the relation of the new chains with an associated many-body supersymmetric spin dynamical model, we are able to compute their partition function in closed form for all values of m and n and for an arbitrary number of spins. When both m and n are even, we show that the partition function factorizes as the product of the partition functions of two supersymmetric Haldane–Shastry spin chains, which in turn leads to a simple expression for the thermodynamic free energy per spin in terms of the Perron eigenvalue of a suitable transfer matrix. We use this expression to study the thermodynamics of a large class of these chains, showing in particular that the specific heat presents a single Schottky peak at approximately the same temperature as a suitable k-level model. We also analyze the critical behavior of the new chains, and in particular, the ground-state degeneracy and the existence of low-energy excitations with a linear energy–momentum dispersion relation. In this way, we show that the only possible critical chains are those with m = 0 , 1 , 2 . In addition, using the explicit formula for the partition function we are able to establish the criticality of the su ( 0 | n ) and su ( 2 | n ) chains with even n, and to evaluate the central charge of their associated conformal field theory.

Magnetic Excitations in Ferromagnetically Coupled Spin-1 Nanographenes.

In the pursuit of high-spin building blocks for the formation of covalently bonded 1D or 2D materials with controlled magnetic interactions, -electron magnetism offers an ideal framework to engineer ferromagnetic interactions between nanographenes. As a first step in this direction, we explore the spin properties of ferromagnetically coupled triangulenes-triangular nanographenes with spin . By combining in-solution synthesis of rationally designed molecular precursors with on-surface synthesis, we successfully achieve covalently bonded triangulene dimers and trimers on Au(111). Starting with the triangulene dimer, we meticulously characterize its low-energy magnetic excitations using inelastic electron tunneling spectroscopy (IETS). IETS reveals conductance steps corresponding to a quintet-to-triplet excitation, and a zero-bias peak resulting from higher-order spin-spin scattering of the five-fold degenerate ferromagnetic ground state. The Heisenberg model captures the key parameters of inter-triangulene ferromagnetic exchange, and its successful extension to the larger system validates the model's accuracy. We anticipate that incorporating ferromagnetically coupled building blocks into the repertoire of magnetic nanographenes will unlock new possibilities for designing carbon nanomaterials with complex magnetic ground states.

Improving Optoelectronic Properties of Acceptor–Donor–Acceptor‐Type Non‐Fullerene Acceptors Containing Extended Fused Ring Donor Units for Efficient Organic Semiconductors

Developing efficient small molecule‐based non‐fullerene acceptors (NFAs) has gained huge attention in fabricating high‐efficiency and stable organic solar cells (OSCs). Herein, we designed and characterized eight new NFAs for OSCs. To investigate the potential of these newly designed NFAs series (IBH1–IBH8) for OSCs, various advanced quantum chemical simulation approaches are used and compared with the synthetic reference molecule IBH–R. Due to the extended donor cores, the IBH1–IBH8 molecules possess strong intramolecular and intermolecular interactions, which helps improve the thin‐film surface crystallinity. Moreover, the designed IBH1–IBH8 molecules present improved UV–visible absorption, narrower bandgaps, lower excitation, and binding energies, and improved photovoltaic characteristics. Furthermore, the impact on the intrinsic properties such as transition density matrix, density of state, electrostatic potential, distribution of frontier molecular orbitals, and reorganizational energies of holes and electrons are estimated. Additionally, the charge‐transfer phenomenon by establishing a donor:acceptor blend (PTB7–Th:IBH4) is analyzed, their geometric analyses are studied, and a good charge‐shifting process is found at the donor:acceptor interface. With these results, we demonstrated the enhancements in the optoelectronic and photovoltaic characteristics of OSCs by performing a simple end‐capped modulation. Hence, these molecules are recommended for the development of efficient and cost‐effective OSCs.

Continuum Excitations in a Spin Supersolid on a Triangular Lattice.

Magnetic, thermodynamic, neutron diffraction and inelastic neutron scattering are used to study spin correlations in the easy-axis XXZ triangular lattice magnet K_{2}Co(SeO_{3})_{2}. Despite the presence of quasi-2D "supersolid" magnetic order, the low-energy excitation spectrum contains no sharp modes and is instead a broad and structured multiparticle continuum. Applying a weak magnetic field drives the system into an m=1/3 fractional magnetization plateau phase and restores sharp spin wave modes. To some extent, the behavior at zero field can be understood in terms of spin wave decay. However, the presence of clear excitation minima at the M points of the Brillouin zone suggest that the spinon language may provide a more adequate description, and signals a possible proximity to a Dirac spin liquid state.

The puzzle of suppression of nuclear level density in N ≈ Z Zn isotopes compared to N > Z

A systematic investigation of isospin dependence of nuclear level density (NLD) is performed for the Zn isotopes. Experimental level densities of 67Zn and 61Zn have been determined by analyzing the spectra of evaporated neutrons emitted from the excited 68Zn and 62Zn nuclei populated via 4He + 64Ni and 4He + 58Ni reactions, respectively. At the low excitation energies, the neutron spectra are predominantly contributed by the first-chance decay, leading to 67Zn and 61Zn as daughters for the two cases. A comparison of the present experimental data along with previously measured NLDs of 66Zn, 64Zn, and 60Zn is performed with state-of-the-art microscopic calculations. While the experimental NLDs of stable isotopes (i.e., 67Zn, 66Zn, and 64Zn) align excellently with theoretical predictions, a compelling contrast emerges for the unstable isotopes 61Zn and 60Zn, exhibiting NLDs an order of magnitude lower than predicted. The possible origins of the observed suppression of NLDs for the N ≈ Z isotopes are discussed. A significant reduction of level density for nuclei situated away from the stability line could have critical implications for the calculation of astrophysical reaction rates pertinent to heavy-element nucleosynthesis.

Rare-Earth Chalcogenides: An Inspiring Playground For Exploring Frustrated Magnetism

Abstract The rare-earth chalcogenide $ARECh_{2}$ family ($A =$ alkali metal or monovalent ions, $RE =$ rare earth, $Ch =$ chalcogen) has emerged as a paradigmatic platform for studying frustrated magnetism on a triangular lattice. The family members exhibit a variety of ground states, from quantum spin liquid to exotic ordered phases, providing fascinating insight into quantum magnetism. Their simple crystal structure and chemical tunability enable systematic exploration of competing interactions in quantum magnets. Recent neutron scattering and thermodynamic studies have revealed rich phase diagrams and unusual excitations, refining theoretical models of frustrated systems. This review provides a succinct introduction to $ARECh_{2}$ research. It summarizes key findings on crystal structures, single-ion physics, magnetic Hamiltonians, ground states, and low-energy excitations. By highlighting current developments and open questions, we aim to catalyze further exploration and deeper physical understanding on this frontier of quantum magnetism.

Terahertz Crystal-Field Transitions and Quasi Ferromagnetic Magnon Excitations in a Noncollinear Magnet for Hybrid Spin-Wave Computation.

The complexity of interactions between the crystal-field and unusual noncollinear spin arrangement in nontrivial magnets demands novel tools to unravel the mystery underneath. In this work, we study such interaction dynamics of crystal-field excitations (CFE) and low-energy magnetic excitations in orthochromite TmCrO3 with controls of temperature and magnetic field using high-resolution magneto-terahertz (THz) time-domain spectroscopy. The THz energy spectrum spanning 0.5-10 meV possesses a low-frequency spin-excitation (magnon) mode and a multitude of CFE modes at 10 K, all of which uniquely embody a range of phenomena. For the magnon mode, a temperature dependence of peak frequency is induced by magnetic interactions between Tm and Cr subsystems. While a change from blue to red shift of peak frequency of this mode marks the magnetization reversal transition, the spin-reorientation temperature and change of magnetic anisotropy are depicted by different features of field and temperature dependent peak frequency dynamics. The modes corresponding to CFE are robust and laden with a multitude of submodes, which are attributes of nontrivial interactions across different transitions. These modes are suppressed only upon substitution of Tb3+ at the Tm3+ site, which suggests a dominant role of single-ion anisotropy in controlling entire THz excitation spectra. Overall, this remarkable range of phenomena seen through the unique lens of all-optical THz tools provides deeper insights into the origin of magnetic phases in systems with complex interactions between rare-earth and transition metal ions and provides a multitude of novel combinations of closely spaced modes for emerging hybrid spin-wave computation.

Lifting of Gap Nodes by Disorder in Tetragonal FeSe_{1-x}S_{x} Superconductors.

The observation of time-reversal symmetry breaking and large residual density of states in tetragonal FeSe_{1-x}S_{x} suggests a novel type of ultranodal superconducting state with Bogoliubov Fermi surfaces (BFSs). Although such BFSs in centrosymmetric superconductors are expected to be topologically protected, the impurity effect of this exotic superconducting state remains elusive experimentally. Here, we investigate the impact of controlled defects introduced by electron irradiation on the superconducting state of tetragonal FeSe_{1-x}S_{x} (0.18≤x≤0.25). The temperature dependence of magnetic penetration depth is initially consistent with a model with BFSs in the pristine sample. After irradiation, we observe a nonmonotonic evolution of low-energy excitations with impurity concentrations. This nonmonotonic change indicates a transition from nodal to nodeless, culminating in gapless with Andreev bound states, reminiscent of the nodal s_{±} case. This points to the accidental nature of the possible BFSs in tetragonal FeSe_{1-x}S_{x}, which are susceptible to disruption by the disorder.