Low-energy Spectrum Research Articles

The Spectral Gap and Low-Energy Spectrum in Mean-Field Quantum Spin Systems

Abstract A semiclassical analysis based on spin-coherent states is used to establish a classification and novel simple formulae for the spectral gap of mean-field spin Hamiltonians. For gapped systems, we provide a full description of the low-energy spectra based on a second-order approximation to the semiclassical Hamiltonian, hence justifying fluctuation theory at zero temperature for this case. We also point out a shift caused by the spherical geometry in these second-order approximations.

Triplet states in the reaction center of Photosystem II.

In oxygenic photosynthesis sunlight is harvested and funneled as excitation energy into the reaction center (RC) of Photosystem II (PSII), the site of primary charge separation that initiates the photosynthetic electron transfer chain. The chlorophyll ChlD1 pigment of the RC is the primary electron donor, forming a charge-separated radical pair with the vicinal pheophytin PheoD1 (ChlD1+PheoD1-). To avert charge recombination, the electron is further transferred to plastoquinone QA, whereas the hole relaxes to a central pair of chlorophylls (PD1PD2), subsequently driving water oxidation. Spin-triplet states can form within the RC when forward electron transfer is inhibited or back reactions are favored. This can lead to formation of singlet dioxygen, with potential deleterious effects. Here we investigate the nature and properties of triplet states within the PSII RC using a multiscale quantum-mechanics/molecular-mechanics (QM/MM) approach. The low-energy spectrum of excited singlet and triplet states, of both local and charge-transfer nature, is compared using range-separated time-dependent density functional theory (TD-DFT). We further compute electron paramagnetic resonance properties (zero-field splitting parameters and hyperfine coupling constants) of relaxed triplet states and compare them with available experimental data. Moreover, the electrostatic modulation of excited state energetics and redox properties of RC pigments by the semiquinone QA- is described. The results provide a detailed electronic-level understanding of triplet states within the PSII RC and form a refined basis for discussing primary and secondary electron transfer, charge recombination pathways, and possible photoprotection mechanisms in PSII.

Structure of low-lying states in 158-168Hf nuclei

The characteristics of low-lying mixed-symmetry states in 158-168Hf isotopes have been investigated in the framework of the interacting boson model. The obtained results of the low energy spectra, B(E2) and mixing ratio δ(E2/M1) for all isotopes are compared with empirical values . The effect of the Majrona terms on the energy levels have been investigated. The 2ms+ and 1+ states are the lowest mixed-symmetry for vibrational 158-164Hf nuclei and for rotational 166,168Hf nuclei, respectively.

Observation of electronic structure replicas in photoemission spectra of graphite upon adsorption of tin phthalocyanine

Cone-type bands near the center of the surface Brillouin zone were observed in low-energy angle-resolved photoemission spectra of tin phthalocyanine adsorbed on graphite. Simulations in comparison with the experimental data show that the spectral features represent replicas of the electronic structure of graphite near resulting from high-order momentum transfer processes up to the fourth order owing to the interaction of substrate electrons with the long-range structural order of the adsorbate overlayer. The analysis of time-resolved photoemission data from one of the replicas yields a quantitative and very good agreement with previous studies on the excited carrier dynamics in the Dirac cones of graphite and graphene.

Method of difference-differential equations for some Bethe-ansatz-solvable models

In studies of one-dimensional Bethe ansatz solvable models, a Fredholm integral equation of the second kind with a difference kernel on a finite interval often appears. This equation does not generally admit a closed-form solution and hence its analysis is quite complicated. Here we study a family of such equations, concentrating on their moments. We find exact relations between the moments in the form of difference-differential equations. The latter results significantly advance the analysis, enabling one to practically determine all the moments from the explicit knowledge of the lowest one. As applications, several examples are considered. First, we study the moments of the quasimomentum distribution in the Lieb-Liniger model and find explicit analytical results. The latter moments determine several basic quantities, e.g., the $N$-body local correlation functions. We prove the equivalence between different expressions found in the literature for the three-body local correlation functions and find an exact result for the four-body local correlation function in terms of the moments of the quasimomentum distributions. We eventually find the analytical results for the three- and four-body correlation functions in the form of asymptotic series in the regimes of weak and strong interactions. Next, we study the exact form of the low-energy spectrum of a magnon (a polaron) excitation in the two-component Bose gas described by the Yang-Gaudin model. We find its explicit form, which depends on the moments of the quasimomentum distributions of the Lieb-Liniger model. Then, we address a seemingly unrelated problem of capacitance of a circular capacitor and express the exact result for the capacitance in the parametric form. In the most interesting case of short plate separations, the parametric form has a single logarithmic term. This should be contrasted with the explicit result that has a complicated structure of logarithms.

Predicting the Electronic Absorption Band Shape of Azobenzene Photoswitches

Simulations based on molecular dynamics coupled to excitation energy calculations were used to generate simulated absorption spectra for a family of halide derivatives of azobenzene, a family of photoswitch molecules with a weak absorption band around 400-600 nm and potential uses in living tissue. This is a case where using the conventional approach in theoretical spectroscopy (estimation of absorption maxima based on the vertical transition from the potential energy minimum on the ground electronic state) does not provide valid results that explain how the observed band shape extends towards the low energy region of the spectrum. The method affords a reasonable description of the main features of the low-energy UV-Vis spectra of these compounds. A bathochromic trend was detected linked to the size of the halide atom. Analysis of the excitation reveals a correlation between the energy of the molecular orbital where excitation starts and the energy of the highest occupied atomic orbital of the free halide atom. This was put to the test with a new brominated compound with good results. The energy level of the highest occupied orbital on the free halide was identified as a key factor that strongly affects the energy gap in the photoswitch. This opens the way for the design of bathochromically shifted variants of the photoswitch with possible applications.

Semiclassical 3D gravity as an average of large-c CFTs

A two-dimensional CFT dual to a semiclassical theory of gravity in three dimensions must have a large central charge c and a sparse low energy spectrum. This constrains the OPE coefficients and density of states of the CFT via the conformal bootstrap. We define an ensemble of CFT data by averaging over OPE coefficients subject to these bootstrap constraints, and show that calculations in this ensemble reproduce semiclassical 3D gravity. We analyze a wide variety of gravitational solutions, both in pure Einstein gravity and gravity coupled to massive point particles, including Euclidean wormholes with multiple boundaries and higher topology spacetimes with a single boundary. In all cases we find that the on-shell action of gravity agrees with the ensemble-averaged CFT at large c. The one-loop corrections also match in the cases where they have been computed. We also show that the bulk effective theory has random couplings induced by wormholes, providing a controlled, semiclassical realization of the mechanism of Coleman, Giddings, and Strominger.

Recognition of Core-Fucosylated Glycopeptides Based on the Y1+Fuc/Y1 Ratio in Low-Energy HCD Spectra.

Accurate identification of core fucosylation on N-glycopeptides remains challenging due to fucose migration during mass spectrometry analysis. Here, we introduce a simple and straightforward method for core-fucosylated glycopeptide recognition based on the relative intensities of Y1+Fuc ions compared with their corresponding Y1 ions (labeled as Y1+Fuc/Y1 or simply Y1F/Y1 ratio > 0.1) in low-energy HCD-based spectra. The method was first developed by systematically evaluating the influence of fucose migration on the Y1F ion from antenna fucoses based on the distribution of the Y1F/Y1 ratios in the MS/MS spectra of antenna-fucosylated glycopeptides from Fut8-/- mouse brain. The feasibility of the method was then confirmed by using two standard glycoproteins, comparison with glycopeptides in Fut8+/+ mouse brain with/without in silico core-fucosylation removal, and Y1F/Y1 ratio alterations under a lower HCD energy. This method will be applicable to the manual interpretation and software-based high-throughput analysis of core-fucosylated glycopeptides.

BQ-Chem: A Quantum Software Program for Chemistry Simulation Based on the Full Quantum Eigensolver Algorithm

We describe a quantum chemistry simulation software program BQ-Chem, which can calculate the low-energy spectrum and potential energy surface of molecules on a quantum computer. BQ-Chem is based on the full quantum eigensolver (FQE), which is implemented with a quantum gradient descent algorithm. Benefiting from FQE, BQ-Chem can perform all the calculations on a quantum computer. Compared with the classical optimization methods which encounter the optimization difficulty of high-dimensional and multivariable functions in dealing with multielectron orbitals of macromolecules, FQE provides an exponential speedup. FQE works fully on a quantum computer; thus, BQ-Chem can be smoothly transited to future large-scale quantum computers.

Implications of multiwavelength spectrum on cosmic-ray acceleration in blazar TXS 0506+056

Context.The MAGIC collaboration has recently analyzed data from a long-term multiwavelength campaign of theγ-ray blazar TXS 0506+056. In December 2018 it was flaring in the very high-energy (VHE;E > 100 GeV)γ-ray band, but no simultaneous neutrino event was detected.Aims.We modeled the observed spectral energy distribution (SED) using a one-zone leptohadronic emission.Methods.We estimated the neutrino flux through the restriction from the observed X-ray flux on the secondary radiation due to the hadronic cascade, initiated by protons with energyEp ≲ 0.1 EeV. We assumed that ultra-high-energy cosmic rays (UHECRs;E ≳ 0.1 EeV), with the same slope and normalization as the low-energy spectrum, are accelerated in the jet but escape efficiently. We propagate the UHE protons in a random turbulent extragalactic magnetic field (EGMF).Results.The leptonic emission from the jet dominates the GeV range, whereas the cascade emission from CR interactions in the jet contributes substantially to the X-ray and VHE range. The line-of-sight cosmogenicγ-rays from UHECRs produce a hardening in the VHE spectrum. Our model prediction for neutrinos from the jet is consistent with the 7.5-year flux limit by IceCube and shows no variability during the MAGIC campaign. Therefore, we infer that the correlation between GeV-TeVγ-rays and neutrino flare is minimal. The luminosity in CRs limits the cosmogenicγ-ray flux, which in turn bounds the RMS value of the EGMF to ≳10−5nG. The cosmogenic neutrino flux is lower than the IceCube-Gen2 detection potential for 10 yr of observation.Conclusions.Very high-energyγ-ray variability should arise from increased activity inside the jet; thus, detecting steady flux at multi-TeV energies may indicate UHECR acceleration. Upcomingγ-ray imaging telescopes, such as the CTA, will be able to constrain the cosmogenicγ-ray component in the SED of TXS 0506+056.

On the Theory of Magnetoelectric Coupling in Fe2Mo3O8.

In the last decade, Fe2Mo3O8 was recognized for a giant magnetoelectric effect, the origin of which is still not clear. In the present paper, we contribute to the microscopic theory of the magnetoelectric coupling in this compound. Using crystal field theory and the molecular field approximation, we calculated the low-lying energy spectrum for iron ions and their interaction with electric and magnetic fields. Classical ionic contribution to the electric polarization related to the ionic shifts is also estimated. It is found that the electronic and ionic contributions to the electric polarization are comparable and these mechanisms support each other at T<TN. The suggested electronic mechanism provides insight into the nature of huge jumps in polarization upon phase transitions from paramagnetic (PM) to antiferromagnetic (AFM) and then to ferrimagnetic (FRM) states under an applied external magnetic field as well as the large differential magnetoelectric coefficient.

Disordered graphene ribbons as topological multicritical systems

The low energy spectrum of a zigzag graphene ribbon contains two gapless bands with highly non-linear dispersion, $\epsilon(k)=\pm |\pi-k|^W$, where $W$ is the width of the ribbon. The corresponding states are located at the two opposite zigzag edges. Their presence reflects the fact that the clean ribbon is a quasi one dimensional system naturally fine-tuned to the topological {\em multicritical} point. This quantum critical point separates a topologically trivial phase from the topological one with the index $W$. Here we investigate the influence of the (chiral) symmetry-preserving disorder on such a multicritical point. We show that the system harbors delocalized states with the localization length diverging at zero energy in a manner consistent with the $W=1$ critical point. The same is true regarding the density of states (DOS), which exhibits the universal Dyson singularity, despite the clean DOS being substantially dependent on $W$. On the other hand, the zero-energy localization length critical exponent, associated with the lattice staggering, is not universal and depends on the topological index $W$.

Anomalous Transport Signatures in Weyl Semimetals with Point Defects.

We present the first theoretical study of transport properties of Weyl semimetals with point defects. Focusing on a class of time-reversal symmetric Weyl lattice models, we show that dilute lattice vacancies induce a finite density of quasilocalized states at and near the nodal energy, causing strong modifications to the low-energy spectrum. This generates novel transport effects, namely, (i)an oscillatory behavior of the dc conductivity with the charge carrier density in the absence of magnetic fields, and (ii) a plateau-shaped dissipative optical response for photon frequencies below the interband threshold, E_{F}≲ℏω≲2E_{F}. Our results provide a path to engineer unconventional quantum transport effects in Weyl semimetals by means of common point defects.

A comparison between the one- and two-step spin-orbit coupling approaches based on the ab initio density matrix renormalization group.

The efficient and reliable treatment of both spin-orbit coupling (SOC) and electron correlation is essential for understanding f-element chemistry. We analyze two approaches to the problem: the one-step approach, where both effects are treated simultaneously, and the two-step state interaction approach. We report an implementation of the ab initio density matrix renormalization group with a one-step treatment of the SOC effect, which can be compared to prior two-step treatments on an equal footing. Using a dysprosium octahedral complex and bridged dimer as benchmark systems, we identify characteristics of problems where the one-step approach is beneficial for obtaining the low-energy spectrum.

Ab initio no-core shell model study of neutron-rich 18,19,20C isotopes

We implement the ab initio no-core shell model approach to study neutron rich 18C, 19C and 20C isotopes. For this purpose, we employ charge-dependent Bonn 2000 (CDB2K), inside non-local outside Yukawa (INOY) and chiral next-to-next-to-next-to-leading order (N3LO) nucleon-nucleon interactions. Low-lying energy spectra, electromagnetic properties and point-proton radii for these nuclei up to basis space Nmax = 4 are calculated. Binding energies obtained with INOY interaction are in better agreement with the experimental values as compared to other ab initio interactions. We also show behavior of ground state energy and point-proton radii with the NCSM parameters, ħΩ and Nmax. We report a strong sensitivity of the B(E2) values from the first excited 2+ to ground state of 18C and 20C to the nuclear interaction. Shell model calculations with YSOX interaction are also performed, and corresponding results are compared with ab initio one.

Measured low-energy x-ray spectra of interest in diagnostic radiology and mammography

Objective. In cities situated at high-altitude, the measured x-ray spectra are required to calculate the absorbed dose to water in radiobiology and medical dosimetry because of the air-density effect. This work aims to measure, in a high-altitude city, a set of x-ray spectra from an x-ray tube with tungsten anode generated at potentials of the M-series x-ray beams from NIST in the range of 20 and 150 kV as well as other beam qualities that have been characterized and used for dosimetry study in our group called W-series. To also measure some spectra of mammography interest in the energy range of 25 and 35 kV using additional filtration of rhodium (Rh), molybdenum (Mo) and silver (Ag). Approach. A tungsten anode x-ray tube with potential between 10 to and 160 kV was used. A high-purity germanium (HPGe) detector associated with a DSPEC Jr 2.0TM digital signal processing module has been used for the spectra measurements. Prior the spectra measurements, the HPGe detector has been characterized and calibrated using several radioactive sources. Main results. The M80 spectrum was compared to a mathematically filtered beam from Physicalisch Technische Bundesanstalt corresponding to the same potential finding a difference of 0.7% in the average energy. So, the mathematically filtered beam from PTB matches very well our M80 beam. The beams M80, M100 and M120 were also compared with calculations for the corresponding kilovoltage from an executable that calculates x-ray spectra from tungsten anode x-ray tubes in the energy range between 40 to 300 kV called SpekCalc. The SpekCalc reproduces qualitatively the experimental spectra, but not quantitatively, mainly for M100 and M120 beams. Significance. These spectra can be used to simulate the patient dose as well as image quality using Monte Carlo (MC) codes or to evaluate the absorbed dose in dosimetry studies

Variational solutions to fermion-to-qubit mappings in two spatial dimensions

Through the introduction of auxiliary fermions, or an enlarged spin space, one can map local fermion Hamiltonians onto local spin Hamiltonians, at the expense of introducing a set of additional constraints. We present a variational Monte-Carlo framework to study fermionic systems through higher-dimensional (>1D) Jordan-Wigner transformations. We provide exact solutions to the parity and Gauss-law constraints that are encountered in bosonization procedures. We study thet-Vmodel in 2D and demonstrate how both the ground state and the low-energy excitation spectra can be retrieved in combination with neural network quantum state ansatze.

Quasiparticle Self-Consistent GW-Bethe-Salpeter Equation Calculations for Large Chromophoric Systems.

The GW-Bethe-Salpeter equation (BSE) method is promising for calculating the low-lying excitonic states of molecular systems. However, so far it has only been applied to rather small molecules and in the commonly implemented diagonal approximations to the electronic self-energy, it depends on a mean-field starting point. We describe here an implementation of the self-consistent and starting-point-independent quasiparticle self-consistent (qsGW)-BSE approach, which is suitable for calculations on large molecules. We herein show that eigenvalue-only self-consistency can lead to an unfaithful description of some excitonic states for chlorophyll dimers while the qsGW-BSE vertical excitation energies (VEEs) are in excellent agreement with spectroscopic experiments for chlorophyll monomers and dimers measured in the gas phase. Furthermore, VEEs from time-dependent density functional theory calculations tend to disagree with experimental values and using different range-separated hybrid (RSH) kernels does change the VEEs by up to 0.5 eV. We use the new qsGW-BSE implementation to calculate the lowest excitation energies of the six chromophores of the photosystem II (PSII) reaction center (RC) with nearly 2000 correlated electrons. Using more than 11,000 (6000) basis functions, the calculation could be completed in less than 5 (2) days on a single modern compute node. In agreement with previous TD-DFT calculations using RSH kernels on models that also do not include environmental effects, our qsGW-BSE calculations only yield states with local characters in the low-energy spectrum of the hexameric complex. Earlier works with RSH kernels have demonstrated that the protein environment facilitates the experimentally observed interchromophoric charge transfer. Therefore, future research will need to combine correlation effects beyond TD-DFT with an explicit treatment of environmental electrostatics.

The Synchrotron Low-energy Spectrum Arising from the Cooling of Electrons in Gamma-Ray Bursts

This work is a continuation of a previous effort (Panaitescu) to study the cooling of relativistic electrons through radiation (synchrotron and self-Compton) emission and adiabatic losses, with application to the spectra and light curves of the synchrotron gamma-ray burst (GRB) produced by such cooling electrons. Here, we derive the low-energy slope β LE of a GRB pulse-integrated spectrum and quantify the implications of the measured distribution of β LE. Radiative processes that produce soft integrated spectra can accommodate the harder slopes measured by CGRO/BATSE and Fermi/GBM only if the magnetic field lifetime t B is shorter than the time during which the typical GRB electrons cool to radiate below 1–10 keV, which is less than (at most) 10 radiative cooling timescales t rad of the typical GRB electron. In this case, there is a one-to-one correspondence between t B and β LE. To account for low-energy slopes β LE > −3/4, the adiabatic electron-cooling requires a similar restriction on t B . In this case, the diversity of slopes arises mostly from how the electron-injection rate varies with time (temporal power-law injection rates yield power-law low-energy GRB spectra) and not from the magnetic field timescale.

Constraining the local pulsar population with the cosmic-ray positron fraction

Observations of the TeV halos associated with nearby pulsars indicate that these objects inject significant fluxes of very high-energy electron-positrons pairs into the interstellar medium (ISM), thereby likely providing the dominant contribution to the cosmic-ray positron flux. In this paper, we use the cosmic-ray positron fraction as measured by the AMS-02 Collaboration to constrain the characteristics of the local pulsar population. For reasonable model parameters, we find that we can obtain good agreement with the measured positron fraction up to energies of Ee ∼ 300 GeV. At higher energies, the positron fraction is dominated by a small number of pulsars, making it difficult to reliably predict the shape of the expected positron fraction. The low-energy positron spectrum supports the conclusion that pulsars typically transfer approximately η ∼ 5–20% of their total spindown power into the production of very high-energy electron-positron pairs, producing a spectrum of such particles with a hard spectral index, α ∼ 1.5–1.7. Such pulsars typically spindown on a timescale on the order of τ ∼ 104 years. Our best fits were obtained for models in which the radio and gamma-ray beams from pulsars are detectable to 28% and 62% of surrounding observers, respectively.