Low-energy Excitations Research Articles

Flat-band ferromagnetism in a correlated topological insulator on a honeycomb lattice

We study the flat-band ferromagnetic phase of a spinfull and time-reversal symmetric Haldane-Hubbard model on a honeycomb lattice within a bosonization formalism for flat-band Z$_2$ topological insulators. Such a study extend our previous one [L. S. G. Leite and R. L. Doretto, Phys. Rev. B {\bf 104}, 155129 (2021)] concerning the flat-band ferromagnetic phase of a correlated Chern insulator described by a Haldane-Hubbard model. We consider the topological Hubbard model at $1/4$ filling of its corresponding noninteracting limit and in the nearly flat band limit of its lower free-electronic bands. We show that it is possible to define boson operators associated with two distinct spin-flip excitations, one that changes (mixed-lattice excitations) and a second one that preserves (same-lattice excitations) the index related with the two triangular sublattices. Within the bosonization scheme, the fermionic model is mapped into an effective interacting boson model, whose quadratic term is considered at the harmonic approximation in order to determine the spin-wave excitation spectrum. For both mixed and same-lattice excitations, we find that the spin-wave spectrum is gapped and has two branches, with an energy gap between the lower and the upper bands at the $K$ and $K'$ points of the first Brillouin zone. Such a behavior is distinct from the one of the corresponding correlated Chern insulator, whose spin-wave spectrum has a Goldstone mode at the center of the first Brillouin zone and Dirac points at $K$ and $K'$ points. We also find some evidences that the spin-wave bands for the same-lattice excitations might be topologically nontrivial even in the completely flat band limit.

Hg178 and asymmetric fission of neutron-deficient pre-actinides

Fission at low excitation energy is an ideal playground to probe the impact of nuclear structure on nuclear dynamics. While the importance of structural effects in the nascent fragments is well-established in the (trans-)actinide region, the observation of asymmetric fission in several neutron-deficient pre-actinides can be explained by various mechanisms. To deepen our insight into that puzzle, an innovative approach based on inverse kinematics and an enhanced version of the VAMOS++ heavy-ion spectrometer was implemented at the GANIL facility, Caen. Fission of $^{178}$Hg was induced by fusion of $^{124}$Xe and $^{54}$Fe. The two fragments were detected in coincidence using VAMOS++ supplemented with a new SEcond Detection arm. For the first time in the pre-actinide region, access to the pre-neutron mass and total kinetic energy distributions, and the simultaneous isotopic identification of one the fission fragment, was achieved. The present work describes the experimental approach, and discusses the pre-neutron observables in the context of an extended asymmetric-fission island located south-west of $^{208}Pb. A comparison with different models is performed, demonstrating the importance of this "new" asymmetric-fission island for elaborating on driving effects in fission.

Excitation Spectra and Edge Singularities in the One-Dimensional Anisotropic Heisenberg Model for Δ = cos(π/n), n = 3,4,5

The T=0 excitation spectra of the antiferromagnetic (J>0) anisotropic Heisenberg chain of spins 1/2 are studied using the Bethe Ansatz equations for Δ=cos(π/n), n=3,4 and 5. The number of unknown functions is n−1 for Δ=cos(π/n) and can be solved numerically for a finite external field. The low-energy excitations form a Luttinger liquid parametrized by a conformal field theory with conformal charge of c=1. For higher energy excitations, the spectral functions display deviations from the Luttinger behavior arising from the curvature in the dispersion. Adding a corrective term of the form of a mobile impurity coupled to the Luttinger liquid modes corrects this difference. The “impurity” is an irrelevant operator, which if treated non-perturbatively, yields the threshold singularities in the one-spinwave particle and hole Green’s function correctly.

Anomalous low-energy carrier dynamics of stripe-type charge ordered Pr0.5Sr0.5MnO3 thin film revealed by magneto-terahertz spectroscopy

The perovskite manganites host a range of charge- and orbital-ordered phases wherein the intrinsic and extrinsic controls of magnetic field, doping, epitaxial strain, chemical pressure, etc., induce subtleties of phase transition. The Pr0.5Sr0.5MnO3 (PSMO) is one such system that possesses a unique stripe-type charge-order manifesting in anisotropic transport. It exhibits a Drude-type and charge-density-wave (CDW) type of low-energy phases; the magnetic field is contemplated as a key control for both these phases; however, it has not yet been explored. Here, we have investigated the magnetic-field dependence of Drude-type terahertz (THz) carriers dynamics and the CDW type of low-energy excitations along two non-identical orthogonal in-plane axes of the (110) epitaxial thin film in the energy range of 1–7 meV using magneto-THz time-domain spectroscopy in Faraday geometry. While THz Drude conductivity expectedly increases with the increasing magnetic field, the CDW type of resonance absorption peak anomalously strengthens with the field. The origin of this unusual field-induced stabilization of the CDW-type mode is expected to lie in the d(x2–y2) orbital ordering in the A-type antiferromagnetic ordered state and anisotropic charge ordering. This study brings out a unique facet of the effect of magnetic field on orbital ordering of anisotropic stripe-type charge-ordered PSMO, which has potential THz modulation and switching applications.

Carrier trapping centers in a two-dimensional silica bilayer: Strongly localized shallow gap states and resonances induced by oxygen vacancies

Structural and electronic properties of neutral native defects in a two-dimensional ${\mathrm{SiO}}_{2}$ bilayer (2D-${\mathrm{SiO}}_{2}$) are examined using the SIESTA ab initio approach. We identify scissor and rotation modes of oxygen atoms, in the middle of Si-O-Si chains, as low-energy structural excitations responsible for the response of the 2D-${\mathrm{SiO}}_{2}$ lattice to the formation of all native defects in the present study. Furthermore, we find that oxygen native defects (single vacancies and interstitials) should be the most abundant defect species in thermal-equilibrated samples. Regarding native-defect electronic states, single oxygen vacancies and interstitials are found to be amphoteric trapping centers in 2D-${\mathrm{SiO}}_{2}$. Silicon vacancies and interstitials introduce several strongly localized states spanning a large fraction of the gap. Generally, we identify a marked tendency for the appearance of strongly spatially localized defect states, be their energies shallow or deep within the band gap, as well as the emergence of strongly localized resonances in the valence and/or conduction bands. Strong spatial localization of defect states, a hallmark of systems displaying trapping and polaronic effects, is a result of quantum confinement and enhanced Coulombic effects in this 2D system. Carrier trapping and polaron formation are thus expected to be a common feature of defect states and added carriers in 2D-${\mathrm{SiO}}_{2}$.

Spectroscopic properties of Er3+/Yb3+ co-doped fluoro bismuth borate glasses for up-conversion luminescence

In this study, 24ZnF2-(47-x)B2O3-18Bi2O3-10ZnO-xEr2O3-1Yb2O3 (FBZEY) fluorine bismuth borate glass was prepared by a high-temperature melting method. XRD analysis showed that all glass samples were amorphous. The differential thermal analysis determined that ΔT of the glass was 107 °C, and it had good thermal stability. The role of doped rare earth ions in the matrix structure was studied using infrared spectroscopy. The near-infrared spectrum of the glass samples shows a wider peak corresponding to the 4I13/2→4I15/2 transition at 1530 nm at 980 nm excitation. When Er3+/Yb3+ is simultaneously located on 4I11/2, Yb3+ transfers energy to Er3+, converts it to 4F7/2 and returns itself to the ground state 4I15/2, thus greatly improving the optical properties of fluorescent glasses. By up-conversion spectra, it can be found that high energy emission photons can be obtained by using low energy excitation sources. Fluorescence attenuation analysis shows that the fluorescence lifetimes of FBZE1Y, FBZE3Y and FBZE5Y were 0.36 ms, 0.59 ms and 0.67 ms, respectively. The results show that the fluorine bismuth zinc glass sample has potential application value for enhancing up-conversion optical devices and amplifiers.

Polarized Raman spectroscopy on topological semimetal Co3Sn2S2

AbstractWe present polarized Raman spectroscopy of the topological semimetal Co3Sn2S2, which was recently shown to host a Weyl semimetal phase. Stokes Raman spectra were obtained with the incident light parallel to the c‐axis of Co3Sn2S2. Two major phonon Raman peaks were observed at 289 and 386 cm−1 over continuous background emission signals. The intensity of the low‐wavenumber (289 cm−1) peak showed no polarization dependence. The high‐wavenumber (386 cm−1) peak and the continuous background signal were strongly polarized in the incident light polarization direction. These responses were almost independent of the in‐plane crystal orientation to the incident polarization, as is the manifestation of the point group symmetry of the unit cell of Co3Sn2S2. According to the group theory and Raman tensor analyses, the low‐ and high‐wavenumber Raman signals are attributed to Γ point phonon modes with and symmetries, respectively. Furthermore, line shape analyses revealed that the high‐wavenumber mode exhibited asymmetric peak feature well described by the Breit–Wigner–Fano function. These results suggest the Fano resonance between the phonon scattering with the continuous electronic background associated with low energy excitations near the Fermi energy. The clarified phonon energies and symmetries, as well as the electronic contribution to the Raman scattering, will not only be useful as a fingerprint to readily verify the experimentally grown or theoretically calculated crystal structure but also suggest importance of Raman spectroscopy as an effective tool to study low energy excitations and their interactions in Co3Sn2S2.

Optical Properties and Concentration Quenching Mechanism of Er3+ Heavy Doped Gd2(MoO4)3 Phosphor for Green Light-Emitting Diode.

Upconversion materials capable of converting low-energy excitation photons into high-energy emission photons have attracted considerable interest in recent years. However, the low upconversion luminescence seriously hinders the application of upconversion phosphors. Heavy lanthanide doping without concentration quenching represents a direct and effective method to enhance the emission intensity. In this study, Er3+ heavy doped Gd2(MoO4)3 phosphor with a monoclinic phase was prepared by a sol–gel process. Under excitation at 976 nm, Gd2(MoO4)3:Er3+ phosphor emitted remarkably intense green emission, and Er3+ concentration up to 20 mol% did not cause concentration quenching. Here, we discuss the upconversion mechanism and concentration quenching. When the Er3+ concentration was in the range of 30–60 mol%, the concentration quenching was governed by the electric dipole–dipole interaction, and when the concentration was greater than 60 mol%, the concentration quenching was controlled by the exchange interactions. The result provides a schematic basis for identifying a phosphor host with heavy lanthanide doping.

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.

Low-energy spin excitations of the frustrated ferromagnetic J1−J2 chain material linarite PbCuSO4(OH)2 in applied magnetic fields parallel to the b axis

We report a study of the spin dynamics of the frustrated ferromagnetic $J_1$-$J_2$ chain compound linarite, PbCuSO$_4$(OH)$_2$, in applied magnetic fields up to field polarization. By means of an extreme-environment inelastic neutron scattering experiment, we have measured the low-energy spin excitations of linarite in fields up to 8.8 T for $\mathbf{H} \parallel b$ axis. We have recorded the spin excitation spectra along $h$, $k$ and $l$ for the field-induced magnetic phases IV, V and the field polarized state close to saturation. By employing first-principles calculations, we estimate the leading magnetic exchanges out of the $bc$ plane and model the dispersion relations using linear spin-wave theory. In this way, we find evidence for a (very weak) residual magnetic exchange coupling out of the $bc$ plane. Together with the previously established dominant intrachain couplings $J_1$ and $J_2$ and the interchain coupling $J_3$, we derive an effective set of exchange couplings for a microscopic description of linarite. Further, we find that the peculiar character of phase V manifests itself in the measured spin dynamics.

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.

Spin excitations of individual magnetic dopants in an ionic thin film

Individual magnetic transition metal dopants in a solid host usually exhibit relatively small spin excitation energies of a few meV. Using scanning tunneling microscopy and inelastic electron tunneling spectroscopy (IETS) techniques, we have observed a high spin excitation energy around 36 meV for an individual Co substitutional dopant in ultrathin NaCl films. In contrast, the Cr dopant in the NaCl film shows much lower spin excitation energy around 2.5 meV. Electronic multiplet calculations combined with first-principles calculations confirm the spin excitation induced IETS, and quantitatively reveal the out-of-plane magnetic anisotropies for both Co and Cr. They also allow reproducing the experimentally observed redshift in the spin excitations of Co dimers and ascribe it to a charge and geometry redistribution.

Quasimolecular electronic structure of the spin-liquid candidate Ba3InIr2O9

The mixed-valent iridate Ba3InIr2O9 has been discussed as a promising candidate for quantum spin-liquid behavior. The compound exhibits Ir$^{4.5+}$ ions in face-sharing IrO6 octahedra forming Ir2O9 dimers with three t2g holes per dimer. Our results establish Ba3InIr2O9 as a cluster Mott insulator. Strong intra-dimer hopping delocalizes the three t2g holes in quasi-molecular dimer states while inter-dimer charge fluctuations are suppressed by Coulomb repulsion. The magnetism of Ba3InIr2O9 emerges from spin-orbit entangled quasi-molecular moments with yet unexplored interactions, opening up a new route to unconventional magnetic properties of 5d compounds. Using single-crystal x-ray diffraction we find the monoclinic space group C2/c already at room temperature. Dielectric spectroscopy shows insulating behavior. Resonant inelastic x-ray scattering (RIXS) reveals a rich excitation spectrum below 1.5 eV with a sinusoidal dynamical structure factor that unambiguously demonstrates the quasi-molecular character of the electronic states. Below 0.3 eV, we observe a series of excitations. According to exact diagonalization calculations, such low-energy excitations reflect the proximity of Ba3InIr2O9 to a hopping-induced phase transition based on the condensation of a quasi-molecular spin-orbit exciton. The dimer ground state roughly hosts two holes in a bonding j=1/2 orbital and the third hole in a bonding j=3/2 orbital.

Aliphatic-Bridged Early Lanthanide Metal–Organic Frameworks: Topological Polymorphism and Excitation-Dependent Luminescence

Six new three-dimensional metal–organic frameworks based on early lanthanide(III) cations and trans-1,4-cyclohexanedicarboxylic acid (H2chdc) were obtained. Their crystal structures were determined by single-crystal X-ray diffraction analysis. The structure of [La2(H2O)4(chdc)3]·2DMF·H2O (1; DMF = N,N-dimethylformamide) contains one-dimensional infinite La(III)-carboxylate chains interconnected by cyclohexane moieties to form a highly porous polymeric lattice with 30% solvent accessible volume. Compounds [Ln2(phen)2(chdc)3]·0.75DMF (2Ln; Ln3+ = Ce3+, Pr3+, Nd3+ and Sm3+; phen = 1,10-phenanthroline) are based on binuclear carboxylate building blocks, which are decorated by chelate phenanthroline ligands and interconnected by cyclohexane moieties to form more dense isostructural coordination frameworks with primitive cubic pcu topology. Compound [Nd2(phen)2(chdc)3]·2DMF·0.67H2O (3) is based on secondary building units similar to 2Ln and contains a coordination lattice isomeric to 2Ln with a rare hexagonal helical snz topology. Thermal stability and luminescent properties were investigated. For 2Sm, a strong and nonmonotonous dependence of the luminescence color on the variation of excitation wavelength was revealed, changing its emission from pinkish red at λex = 340 nm to white at λex = 400 nm, and then to yellow at lower excitation energies. Such nonlinear behavior was rationalized in terms of the contribution of several different luminescence mechanisms. Thus, 2Sm is a rather rare example of a highly tunable monometallic lanthanide-based luminophore with possible applications in light-emitting devices and optical data processing.

Signatures of spin-liquid state in a 3D frustrated lattice compound KSrFe2(PO4)3 with S = 5/2

A quantum spin-liquid is a spin disordered state of matter in which spins are strongly correlated and highly entangled with low-energy excitations. It has been often found in two-dimensional S = ½, highly frustrated spin networks but rarely observed in three-dimensional (3D) frustrated quantum magnets. Here, KSrFe2(PO4)3, forming a complicated 3D frustrated lattice with a spin moment S = 5/2, is investigated by thermodynamic, neutron diffraction measurements and electronic structure calculations. Despite the relatively sizable Curie–Weiss temperature θCW = −70 K, a conventional magnetic long-range order is confirmed to be absent down to 0.19 K. The magnetic heat capacity data follow the power-law behavior at the lowest temperature region, supporting gapless excitations in a 3D spin-liquid state. Strong geometrical spin frustration responsible for the spin-liquid feature is understood as originating from the almost comparable five competing nearest-neighbor antiferromagnetic exchange interactions, which form the complicated 3D frustrated spin network. All these results suggest that the compound KSrFe2(PO4)3, representing a unique 3D spin frustrated network, could be a rare example of forming a gapless spin-liquid state even with a large spin moment of S = 5/2.

Influence of Triaxiality on the Description of Low-Energy Excitation Spectrum of 96Zr

The observed properties of the low-lying collective states of $^{96}$Zr are investigated within the geometrical collective model. The quadrupole-collective Bohr Hamiltonian with the potential having spherical and axially-symmetric deformed minima is applied. The role of triaxiality is investigated by rotating the potential in $\gamma$-degree of freedom so that the deformed minimum occurs at various axially asymmetric shapes. The change of excitation energies and reduced matrix elements of quadrupole transitions with increase of triaxiliaty is analyzed.

Imaging Vibrational Excitations in the Electron Microscope

Vibrational spectroscopy is a powerful tool for probing atomic motions in molecules and extended solids. Recent advances in aberration-corrected, monochromated-scanning transmission electron microscopy (STEM) now allow imaging of individual phonon modes at the nanoscale, overcoming the spatial limit imposed by diffraction-limited optical spectroscopy techniques. In this Perspective, we summarize the recent progress in high-resolution electron energy-loss spectroscopy (EELS), which now enables vibrational spectroscopy to be performed in the electron microscope. The high spatial and energy resolution of low-loss EELS can be used to directly image phonons inside or near inorganic and organic materials without considerable sample damage. Monochromated EELS can furthermore resolve isotope specific vibrational signatures and phonon modes arising from single-atom dopants. In parallel with these advances, we highlight the ability of spatial- and momentum-resolved EELS to measure the phonon dispersions at material interfaces, obtaining valuable knowledge about interfacial thermal and electrical transport phenomena. Before concluding, we illustrate how imaging infrared (IR) plasmons at high spatial resolution deepens our understanding of how plasmons interact with molecular vibrations. Taken together, these diverse studies illustrate the capability of monochromated STEM-EELS to image low-energy phonon and plasmon excitations at the nanoscale and the new insights from these studies can be leveraged to engineer the specific material properties needed for next-generation devices.

Theoretical Spectroscopic Study of Two Ketones of Atmospheric Interest: Methyl Glyoxal (CH3COCHO) and Methyl Vinyl Ketone (CH3COCH═CH2).

Two ketones of atmospheric interest, methyl glyoxal and methyl vinyl ketone, are studied using explicitly correlated coupled cluster theory and core–valence correlation-consistent basis sets. The work focuses on the far-infrared region. At the employed level of theory, the rotational constants can be determined to within a few megahertz of the experimental data. Both molecules present two conformers, trans/cis and antiperiplanar (Ap)/synperiplanar (Sp), respectively. trans-Methyl glyoxal and Ap-methyl vinyl ketone are the preferred structures. cis-Methyl glyoxal is a secondary minimum of very low stability, which justifies the unavailability of experimental data in this form. In methyl vinyl ketone, the two conformers are almost isoenergetic, but the interconversion implies a relatively high torsional barrier of 1798 cm–1. A very low methyl torsional barrier was estimated for trans-methyl glyoxal (V3 = 273.6 cm–1). Barriers of 429.6 and 380.7 cm–1 were computed for Ap- and Sp-methyl vinyl ketone. Vibrational second-order perturbation theory was applied to determine the rovibrational parameters. The far-infrared region was explored using a variational procedure of reduced dimensionality. For trans-methyl glyoxal, the ground vibrational state was estimated to split by 0.067 cm–1, and the two low excited energy levels (1 0) and (0 1) were found to lie at 89.588 cm–1/88.683 cm–1 (A2/E) and 124.636 cm–1/123.785 cm–1 (A2/E). For Ap- and Sp-methyl vinyl ketone, the ground vibrational state splittings were estimated to be 0.008 and 0.017 cm–1, respectively.

Synergistic interplay between photoisomerization and photoluminescence in a light-driven rotary molecular motor

Photoactuators and photoluminescent dyes utilize light to perform mechanical motion and undergo spontaneous radiation emission, respectively. Combining these two functionalities in a single molecule would benefit the construction of advanced molecular machines. Due to the possible detrimental interaction between the two light-dependent functional parts, the design of hybrid systems featuring both functions in parallel remains highly challenging. Here, we develop a light-driven rotary molecular motor with an efficient photoluminescent dye chemically attached to the motor, not compromising its motor function. This molecular system shows efficient rotary motion and bright photoluminescence, and these functions can be addressed by a proper choice of excitation wavelengths and solvents. The moderate interaction between the two parts generates synergistic effects, which are beneficial for lower-energy excitation and chirality transfer from the motor to the photoluminescent dye. Our results provide prospects towards photoactive multifunctional systems capable of carrying out molecular rotary motion and tracking its location in a complex environment.

Magnetically Hidden State on the Ground Floor of the Magnetic Devil's Staircase.

We investigated the low-temperature and high-field thermodynamic and ultrasonic properties of SrCu_{2}(BO_{3})_{2}, which exhibits various plateaux in its magnetization curve above 27T, called a magnetic Devil's staircase. The results of the present study confirm that magnetic crystallization, the first step of the staircase, occurs above 27T as a first-order transition accompanied by a sharp singularity in heat capacity C_{p} and a kink in the elastic constant. In addition, we observe a thermodynamic anomaly at lower fields around 26T, which has not been previously detected by any magnetic probes. At low temperatures, this magnetically hidden state has a large entropy and does not exhibit Schottky-type gapped behavior, which suggests the existence of low-energy collective excitations. Based on our observations and theoretical predictions, we propose that magnetic quadrupoles form a spin-nematic state around 26T as a hidden state on the ground floor of the magnetic Devil's staircase.