Low-lying Excitation Spectrum Research Articles

Matrix product study of spin fractionalization in the one-dimensional Kondo insulator

The Kondo lattice is one of the classic examples of strongly correlated electronic systems. We conduct a controlled study of the Kondo lattice in one dimension, highlighting the role of excitations created by the composite fermion operator. Using time-dependent matrix product state methods, we compute various correlation functions and contrast them with both large-N mean-field theory and the strong-coupling expansion. We show that the composite fermion operator creates long-lived, charge-e and spin-1/2 excitations, which cover the low-lying single-particle excitation spectrum of the system. Furthermore, spin excitations can be thought to be composed of such fractionalized quasiparticles with a residual interaction which tend to disappear at weak Kondo coupling. Published by the American Physical Society 2024

An improved microscopic core–quasiparticle coupling model for spectroscopy of odd-mass nuclei with octupole correlations

The microscopic core–quasiparticle coupling model for odd-A nuclei with octupole correlations is extended to include the monopole, quadrupole and octupole couplings between the core and the odd nucleon, based on the framework of covariant density functional theory. The model is tested in a study of low-lying excitation spectra for the odd-A nucleus [Formula: see text]Ra. Theoretical results, e.g., the parity doublets, interband [Formula: see text] and intraband [Formula: see text], are in very good agreement with the available data. In particular, the description of signature splitting in the ground-state band has been improved. This method provides a very useful tool for a systematic study of low-energy spectra in odd-mass actinides with octupole correlations.

Shape Phase Transitions in Even–Even 176–198Pt: Higher-Order Interactions in the Interacting Boson Model

Dynamical symmetry plays a dominant role in the interacting boson model in elucidating nuclear structure, for which group theoretical or algebraic techniques are powerful. In this work, the higher-order interactions required in describing triaxial deformation in the interacting boson model are introduced to improve the fitting results to low-lying level energies, B(E2) values and electric quadrupole moments of even–even nuclei. As an example of the model application, the low-lying excitation spectra and the electromagnetic transitional properties of even–even 176−198Pt are fitted and compared to the experimental data and the results of the consistent-Q formalism. It is shown that the results obtained from the model are better than those of the original consistent-Q formalism, indicating the importance of the higher-order interactions in describing the structure and the shape phase evolution of these nuclei.

Critical properties of quantum three- and four-state Potts models with boundaries polarized along the transverse field

By computing the low-lying energy excitation spectra with the density matrix renormalization group algorithm we show that boundaries polarized in the direction of the transverse field lead to scale-invariant conformal towers of states at the critical point of the quantum four-state Potts model - a special symmetric case of the Ashkin-Teller model. Furthermore, by direct comparison of the excitation spectra we phenomenologically establish the duality between the transverse-polarized and three-state-mixed boundary conditions at the four-state Potts critical point. Finally, for completeness, we verify that in the quantum three-state Potts model the "new" boundary conditions dual to the mixed ones can be realized by polarizing edge spins along the transverse field.

Study of nuclear low-lying excitation spectra with the Bayesian neural network approach

Nuclear low-lying excitation spectra are studied with the Bayesian neural network (BNN) approach by taking 02+, 21+, and 41+ states as examples. The BNN approach can well describe the low-lying excitation energies in a large energy scale from about 0.1 MeV to about several MeV, by including an input related to nuclear collectivity besides proton and neutron numbers and employing the logarithm of excitation energy as the output. Comparing with the sophisticated microscopic collective Hamiltonian model, the BNN approach significantly improves the description of nuclear low-lying excitation energies, which can generally reproduce the experimental data within about 1.12 times including those of transitional nuclei and magic nuclei. Taking Mg, Ca, Kr, Sm, and Pb isotopes as examples, it is found that the BNN approach well describes the isotopic trend of low-lying excitation energies, including those abrupt increases at magic numbers due to the shell effect, very low excitation energies of 02+ states due to the shape coexistence, and complex nuclear shape evolution due to the shape phase transition.

Weak binding effects on the structure of $$^{40}$$Mg

While the phenomenon of one- and two-neutron ground-state halo nuclei is well established, the effects of weak binding on nuclear excitation properties remain largely unexplored. Motivated by this question and by recent data in $^{40}$Mg we investigate the coupling of weakly bound (halo) valence neutrons to a core using the known properties of $^{40}$Mg to explore and illustrate possible particle-core coupling schemes and their impact on the low-lying excitation spectrum.

Emergence and Disruption of Spin-Charge Separation in One-Dimensional Repulsive Fermions.

At low temperature, collective excitations of one-dimensional (1D) interacting fermions exhibit spin-charge separation, a unique feature predicted by the Tomonaga-Luttinger liquid (TLL) theory, but a rigorous understanding remains challenging. Using the thermodynamic Bethe ansatz (TBA) formalism, we analytically derive universal properties of a 1D repulsive spin-1/2 Fermi gas with arbitrary interaction strength. We show how spin-charge separation emerges from the exact TBA formalism, and how it is disrupted by the interplay between the two degrees of freedom that brings us beyond the TLL paradigm. Based on the exact low-lying excitation spectra, we further evaluate the spin and charge dynamical structure factors (DSFs). The peaks of the DSFs exhibit distinguishable propagating velocities of spin and charge as functions of interaction strength, which can be observed by Bragg spectroscopy with ultracold atoms.

The excitation spectrum of Bose gases interacting through singular potentials

We consider systems of N bosons in a box of volume one, interacting through a repulsive two-body potential of the form \kappa N^{3\beta-1} V(N^\beta x) . For all 0 < \beta < 1 , and for sufficiently small coupling constant \kappa > 0 , we establish the validity of Bogolyubov theory, identifying the ground state energy and the low-lying excitation spectrum up to errors that vanish in the limit of large N .

Low-lying level structure of Λ hypernuclei and spin dependence of the ΛN interaction with antisymmetrized molecular dynamics

$\Lambda N$ spin-spin and spin-orbit splittings in low-lying excitation spectra are investigated for $p$-shell $\Lambda$ hypernuclei on the basis of the microscopic structure calculation within the antisymmetrized molecular dynamics, where the $\Lambda N$ $G$-matrix interaction derived from the baryon-baryon interaction model ESC (extended soft core) is used. It is found that the ground-state spin-parity is systematically reproduced in the $p$-shell $\Lambda$ hypernuclei by tuning the $\Lambda N$ spin-spin and spin-orbit interactions so as to reproduce the experimental data of $^{4}_\Lambda$H, $^{7}_\Lambda$Li and $^9_\Lambda$Be. Furthermore, we also focus on the excitation energies of the excited doublets as well as the energy shifts of them by the addition of a $\Lambda$ particle.

Unraveling the excitation spectrum of many-body systems from quantum quenches

Quenches are now routinely used in synthetic quantum systems to study a variety of fundamental effects, including ergodicity breaking, light-cone-like spreading of information, and dynamical phase transitions. It was shown recently that the dynamics of equal-time correlators may be related to ground-state phase transitions and some properties of the system excitations. Here, we show that the full low-lying excitation spectrum of a generic many-body quantum system can be extracted from the after-quench dynamics of equal-time correlators. We demonstrate it for a variety of one-dimensional lattice models amenable to exact numerical calculations, including Bose and spin models, with short- or long-range interactions. The approach also applies to higher dimensions, correlated fermions, and continuous models. We argue that it provides an alternative approach to standard pump-probe spectroscopic methods and discuss its advantages.

Dirac Spin Liquid on the Spin-1/2 Triangular Heisenberg Antiferromagnet.

We study the spin liquid candidate of the spin-1/2 J_{1}-J_{2} Heisenberg antiferromagnet on the triangular lattice by means of density matrix renormalization group (DMRG) simulations. By applying an external Aharonov-Bohm flux insertion in an infinitely long cylinder, we find unambiguous evidence for gapless U(1) Dirac spin liquid behavior. The flux insertion overcomes the finite size restriction for energy gaps and clearly shows gapless behavior at the expected wave vectors. Using the DMRG transfer matrix, the low-lying excitation spectrum can be extracted, which shows characteristic Dirac cone structures of both spinon-bilinear and monopole excitations. Finally, we confirm that the entanglement entropy follows the predicted universal response under the flux insertion.

Time-dependent generator-coordinate-method study of mass-asymmetric fission of actinides

Low-energy positive and negative parity collective states in the equilibrium minimum, and the dynamics of induced fission of actinide nuclei are investigated in a unified theoretical framework based on the generator coordinate method (GCM) with the Gaussian overlap approximation (GOA). The collective potential and inertia tensor, both at zero and finite temperature, are computed using the self-consistent multidimensionally constrained relativistic mean field model, based on the energy density functional DD-PC1. Pairing correlations are treated in the Bardeen-Cooper-Schrieffer approximation with a separable pairing force of finite range. A collective quadrupole-octupole Hamiltonian characterized by zero-temperature axially symmetric deformation energy surface and perturbative cranking inertia tensor, is used to model the low-lying excitation spectrum. The fission fragment charge distributions are obtained by propagating the initial collective states in time with the time-dependent GCM+GOA that uses the same quadrupole-octupole Hamiltonian, but with the collective potential and inertia tensor computed at finite temperature. The illustrative charge yields of $^{228}\mathrm{Th}$, $^{234}\mathrm{U}$, $^{240}\mathrm{Pu}$, $^{244}\mathrm{Cm}$, and $^{250}\mathrm{Cf}$ are in very good agreement with experiment, and the predicted mass asymmetry is consistent with the result of a recent microscopic study that has attributed the distribution (peak) of the heavier-fragment nuclei to shell-stabilized octupole deformations.

Itinerant surfaces with spin-orbit couplings, correlations and external magnetic fields: exact results

ABSTRACTWe analyze, in exact terms, multiband 2D itinerant correlated fermionic systems with many-body spin-orbit interactions, and in-plane external magnetic fields. Even if such systems with broad applicability in leading technologies are non-integrable, we set up an exact solution procedure for them, which is described in details. Casting the Hamiltonian in positive semidefinite form, the technique leads to the ground state, and also characterises the low lying excitation spectrum.

First Spectroscopy of the Near Drip-line Nucleus ^{40}Mg.

One of the most exotic light neutron-rich nuclei currently accessible for experimental study is ^{40}Mg, which lies at the intersection of the nucleon magic number N=28 and the neutron drip line. Low-lying excited states of ^{40}Mg have been studied for the first time following a one-proton removal reaction from ^{41}Al, performed at the Radioactive Isotope Beam Factory of RIKEN Nishina Center with the DALI2 γ-ray array and the ZeroDegree spectrometer. Two γ-ray transitions were observed, suggesting an excitation spectrum that shows unexpected properties as compared to both the systematics along the Z=12, N≥20 Mg isotopes and available state-of-the-art theoretical model predictions. A possible explanation for the observed structure involves weak-binding effects in the low-lying excitation spectrum.

Structure of even-even cadmium isotopes from the beyond-mean-field interacting boson model

The structure of even-even $^{108-116}$Cd isotopes is investigated based on the self-consistent mean-field approach. By mapping the quadrupole-$(\beta,\gamma)$ deformation energy surface, obtained from the constrained self-consistent mean-field calculations with a choice of the Skyrme force and pairing property, onto the Hamiltonian of the interacting boson model with configuration mixing, the strength parameters of the Hamiltonian are determined. The low-lying excitation spectra and electric quadrupole transition rates for the considered Cd nuclei are computed by the resultant Hamiltonian, and are compared in detail with the experimental data. Our semi-microscopic prediction identifies several intruder states as suggested empirically, and overall, provides a reasonable qualitative description of the experimental energy levels and transition rates.

The Inverse-Square Interaction Phase Diagram: Unitarity in the Bosonic Ground State

Ground-state properties of bosons interacting via inverse square potential (three dimensional Calogero-Sutherland model) are analyzed. A number of quantities scale with the density and can be naturally expressed in units of the Fermi energy and Fermi momentum multiplied by a dimensionless constant (Bertsch parameter). Two analytical approaches are developed: the Bogoliubov theory for weak and the harmonic approximation (HA) for strong interactions. Diffusion Monte Carlo method is used to obtain the ground-state properties in a non-perturbative manner. We report the dependence of the Bertsch parameter on the interaction strength and construct a Padé approximant which fits the numerical data and reproduces correctly the asymptotic limits of weak and strong interactions. We find good agreement with beyond-mean field theory for the energy and the condensate fraction. The pair distribution function and the static structure factor are reported for a number of characteristic interactions. We demonstrate that the system experiences a gas-solid phase transition as a function of the dimensionless interaction strength. A peculiarity of the system is that by changing the density it is not possible to induce the phase transition. We show that the low-lying excitation spectrum contains plasmons in both phases, in agreement with the Bogoliubov and HA theories. Finally, we argue that this model can be interpreted as a realization of the unitary limit of a Bose system with the advantage that the system stays in the genuine ground state contrarily to the metastable state realized in experiments with short-range Bose gases.

Microscopic description of triaxiality in Ru isotopes with covariant energy density functional theory

The triaxiality in nuclear low-lying states has attracted great interests for many years. Recently, the reduced transition probabilities for levels near the ground state in $^{110}$Ru have been measured and provided strong evidences for a triaxial shape of this nucleus. The aim of this work is to provide a microscopic study of low-lying states for the Ru isotopes with $A\sim100$ and to examine in detail the role of triaxiality, and the evolution of quadrupole shapes with the isospin and spin degrees of freedom. The low-lying excitation spectra and transition probabilities of even-even Ru isotopes are described at the beyond mean-field level by solving a five-dimensional collective Hamiltonian with parameters determined by constrained self-consistent mean-field calculations based on the relativistic energy density functional PC-PK1. The calculated energy surfaces, low-energy spectra, intraband and interband transition rates, as well as some characteristic collective observables, such as $E(4_{\rm g.s.}^+)/E(2^+_{\rm g.s.})$, $E(2^+_\gamma)/E(4^+_{\rm g.s.})$, $B(E2; 2^+_{\rm g.s.}\to 0^+_{\rm g.s.})$, and $\gamma$ band staggerings are in a good agreement with the available experimental data. The main features of the experimental low-lying excitation spectra and electric transition rates are well reproduced, and thus strongly support the onset of triaxiality in the low-lying excited states of the Ru isotopes around $^{110}$Ru.

Low-lying electronic excitations of a water-soluble BODIPY: from the gas phase to the solvated molecule

We present a description of the optical properties of a water-soluble BODIPY derivative in the gas phase and in water. Comparison with a hydrophobic BODIPY derivative with very similar structure but deprived of the hydrophilic groups, clarifies the effects of functionalization. The changes in solution are studied at different levels of modeling. In particular, we make use of Car–Parrinello molecular dynamics to take thermal motion into account, by generating a set of molecular configurations at about room temperature and embedding them in the polarizable continuum model (PCM) for the solvent. The theoretical scheme is that of time-dependent density functional theory which we apply using different approximations for the exchange-correlation functionals. Changes in the low-lying excitation spectrum (≈2.5–4.5 eV) of the solvated molecule in water relative to gas phase are shown to be due not only to the electrostatic interaction with the solvent but also to the thermal motion that induces a downward energy shift and broadening of the absorption lines. Addition of explicit water molecules to the PCM only affects spectral features at higher energies.

Entanglement, excitations, and correlation effects in narrow zigzag graphene nanoribbons

We investigate the low-lying excitation spectrum and ground-state properties of narrow graphene nanoribbons with zigzag edge configurations. Nanoribbons of comparable widths have been synthesized very recently [P. Ruffieux, \emph{et al.} Nature \textbf{531}, 489 (2016)], and their descriptions require more sophisticated methods since in this regime conventional methods, like mean-field or density-functional theory with local density approximation, fail to capture the enhanced quantum fluctuations. Using the unbiased density-matrix renormalization group algorithm we calculate the charge gaps with high accuracy for different widths and interaction strengths and compare them with mean-field results. It turns out that the gaps are much smaller in the former case due to the proper treatment of quantum fluctuations. Applying the elements of quantum information theory we also reveal the entanglement structure inside a ribbon and examine the spectrum of subsystem density matrices to understand the origin of entanglement. We examine the possibility of magnetic ordering and the effect of magnetic field. Our findings are relevant for understanding the gap values in different recent experiments and the deviations between them.

Infrared Behavior of Dipolar Bose Systems at Low Temperatures

We rigorously discuss the infrared behavior of the uniform three dimensional dipolar Bose systems. In particular, it is shown that low-temperature physics of the system is controlled by two parameters, namely isothermal compressibility and intensity of the dipole-dipole interaction. By using hydrodynamic approach we calculate the spectrum and damping of low-lying excitations and analyze infrared behavior of the one-particle Green's function. The low-temperature corrections to the anisotropic superfluid density as well as condensate depletion are found. Additionally we derive equations of the two-fluid hydrodynamics for dipolar Bose systems and calculate velocities of first and second sound.