Branches Of Excitations Research Articles

Massive triplet excitations in a magnetized anisotropic Haldane spin chain

Inelastic neutron scattering experiments on the Haldane-gap quantum antiferromagnet $\mathrm{Ni}({\mathrm{C}}_{5}{\mathrm{D}}_{14}{\mathrm{N}}_{2}{)}_{2}{\mathrm{N}}_{3}({\mathrm{PF}}_{6})$ are performed at mK temperatures in magnetic fields of almost twice the critical field ${H}_{c}$ applied perpendicular to the spin chains. Above ${H}_{c}$ a reopening of the spin gap is clearly observed. In the high-field N\'eel-ordered state the spectrum is dominated by three distinct excitation branches. A theoretical model consistently describing the experimental data is proposed.

Zeeman levels with exotic field dependence in the high field phase of an S=1 Heisenberg antiferromagnetic chain.

We have performed electron spin resonance measurements over a wide frequency and magnetic field range on a single crystal of the S=1 quasi-one-dimensional Heisenberg antiferromagnet Ni(C5H14N2)2N3(PF6). We observed gapped excitation branches above the critical field H(c) where the Haldane gap closes. These branches are analyzed by a phenomenological field theory using the complex-field phi(4) model. A satisfactory agreement between experiment and theory is obtained.

Resonance peak in underdoped cuprates

The magnetic susceptibility measured in neutron-scattering experiments in underdoped ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}y}$ is interpreted based on the self-consistent solution of the $t\ensuremath{-}J$ model of a Cu-O plane. The calculations reproduce correctly the frequency and momentum dependencies of the susceptibility and its variation with doping and temperature in the normal and superconducting states. This allows us to interpret the maximum in the frequency dependence---the resonance peak---as a manifestation of the excitation branch of localized Cu spins and to relate the frequency of the maximum to the size of the spin gap. The low-frequency shoulder well resolved in the susceptibility of superconducting crystals is connected with a pronounced maximum in the damping of the spin excitations. This maximum is caused by intense quasiparticle peaks in the hole spectral function for momenta near the Fermi surface and by the nesting.

Damped orbital excitations in the titanates

A possible mechanism for the removal of the orbital degeneracy in RTiO3 (where R=La, Y, ...) is considered. The calculation is based on the Kugel-Khomskii Hamiltonian for electrons residing in the t2g orbitals of the Ti ions, and uses a self-consistent pe rturbation expansion in the interaction between the orbital and the spin degrees of freedom. The latter are assumed to be ordered in a Neel state, brought about by delicate interactions that are not included in the Kugel-Khomskii Hamiltonian. Within our model calculations, each of the t2g bands is found to acquire a finite, temperature-dependent dispersion, that lifts the orbital degeneracy. The orbital excitations are found to be heavily damped over a rather wide band. Consequently, they do not participate as a separate branch of excitations in the low-temperature thermodynamics.e

Double-exchange mechanism forCrO2

Band-structure calculations predict a gap of 1.5 eV in the minority-spin density of states of the half-metallic ferromagnet ${\mathrm{CrO}}_{2}.$ The magnitude of ${T}_{C},$ the magnons, and the activation gap in the resistivity cannot be explained with this band picture. We propose a double-exchange mechanism for two electrons per Cr site invoking strong Hund's rule correlations and the distortion in the ${\mathrm{CrO}}_{6}$ octahedra, which localize one electron into the $\mathrm{xy}$ orbital at each Cr site, while the electrons in the $\mathrm{xz}$ and $\mathrm{yz}$ orbitals are itinerant. The effective exchange interaction naturally leads to a ferromagnetic ground state with low-energy magnon excitations and several branches of excitations separated by energy gaps from the ground state, reducing ${T}_{C}.$ Quantum fluctuations suppress the orbital long-range order, giving rise to an orbital resonant valence bond-type ground state.

Low-energy photon scattering experiments of151,153Eu,163Dy,and165Hoand the systematics of theM1scissors mode in odd-mass rare-earth nuclei

Nuclear resonance fluorescence experiments were performed on the rare-earth nuclei ${}^{151,153}\mathrm{Eu},$ and with considerably increased sensitivity on ${}^{163}\mathrm{Dy}$ and ${}^{165}\mathrm{Ho}$ to study the fragmentation of the $M1$ scissors mode in odd-mass nuclei, and to clarify the puzzle of the missing total $M1$ strength observed for odd-mass nuclei so far. Using the bremsstrahlung photon beam of the Stuttgart Dynamitron (end point energy 4.05 MeV) and high-resolution Ge $\ensuremath{\gamma}$-ray spectrometers, detailed information was obtained on excitation energies, decay widths, transition probabilities, and branching ratios. Whereas in ${}^{151}\mathrm{Eu}$ only 11 weak excitations were observed, 161 and 138 excitations could be detected in the heavier nuclei ${}^{163}\mathrm{Dy}$ and ${}^{165}\mathrm{Ho},$ respectively. The results are compared to those observed recently at the Stuttgart facility for the neighboring odd-mass nuclei ${}^{161}\mathrm{Dy},$ ${}^{155,157}\mathrm{Gd},$ and ${}^{159}\mathrm{Tb}.$ The measured total strengths increase with the mass number $A.$ Ascribing the same portion of the dipole strength to $M1$ excitations as measured in the neighboring even-even nuclei, the total $M1$ strength deduced from the most sensitive experiment on ${}^{163}\mathrm{Dy}$ is comparable to those found in the neighboring even-even nuclei. The results for ${}^{163}\mathrm{Dy}$ and ${}^{165}\mathrm{Ho}$ are compared with a fluctuation analysis of the photon scattering spectra to estimate the amount of still unresolved strength eventually hidden in the background due to the extreme fragmentation of the $M1$ scissors mode in odd-mass rare-earth nuclei. For ${}^{165}\mathrm{Ho},$ the total derived strength of $B(M1)\ensuremath{\uparrow}=2.9(5){\ensuremath{\mu}}_{N}^{2}$ agrees within error bars with an earlier analysis of a different measurement of the ${}^{165}\mathrm{Ho}(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ reaction. In ${}^{163}\mathrm{Dy}$ the method leads to an unphysical background shape, underlining the experimental observation of a significantly reduced fragmentation pattern of the dipole modes in this nucleus, which must be traced back to structure features of the Dy isotopes.

Observation of a new excitation in BCC [formula omitted] by inelastic neutron scattering

In a classical cubic crystal with one atom per unit cell, the only allowed excitations are three acoustic phonon branches. Recently, it was predicted that in a quantum crystal another optic-like excitation branch can exist. This additional excitation arises from the assumption that atomic zero-point motion in the crystal can be correlated. Here, we describe a neutron scattering experiment aiming to detect this excitation in BCC 4 He . In addition to the three phonon branches, we indeed observed a weakly dispersive, optic-like excitation branch along the [1 1 0] direction. One possible interpretation is that the new excitation is a quantum analogue of a point defect.

Spin ordering and coherent atomic motion in bcc solid [formula omitted

We propose a new model to describe the nuclear magnetism of bcc 3 He . According to this model, in bcc 3 He there is correlated zero-point motion of the nuclei, which gives rise to electric and magnetic polarizations of the electronic cloud. Our model describes the resulting modification of the phonon spectra and predicts new localized modes, which have been observed during the past year. The polarization of the electronic cloud leads to hyperfine magnetic interactions with the nuclear spin. Magnetic ordering results from an indirect spin–spin interaction. The model predicts correctly both the u2d2 symmetry of the ordered phase and the volume dependence of the magnetic interaction. We further predict the excitation branches (spin waves) of the ordered phase.

Intersubband collective excitations in a quasi-two-dimensional electron system in external magnetic field

The spectrum of collective excitations in a quasi-two-dimensional electron system was studied by the method of Raman scattering spectroscopy. In an applied magnetic field, such systems exhibit collective excitations related to the electron transitions between dimensionally quantized subbands with a change in the Landau level index (intersubband Bernstein modes). It is shown that these modes interact with the fundamental intersubband excitations of the charge and spin densities, the interaction energy being determined by the excitation quasimomentum. Interaction of the intersubband Bernstein modes and the fundamental intersubband excitations with quasi-two-dimensional LO phonons was studied. Behavior of the new branches of collective excitations in a quasi-two-dimensional electron system possessing more than one occupied Landau level was studied and the nature of these branches was determined.

Spin-wave spectrum of copper metaborate in the commensurate phase [formula omitted

We have investigated the spin-wave spectrum of copper metaborate, CuB 2O 4, by means of inelastic neutron scattering in the commensurate magnetic phase. We have found two branches of spin-wave excitations associated with the two magnetic sublattices Cu(A) and Cu(B), respectively. In the temperature regime 10 K⩽T⩽21 K , where only the Cu(A) magnetic moments are ordered, the interaction between the two sublattices is found to be negligible. With this approximation we have determined the ‘easy plane’ exchange parameters of the Cu(A) subsystem within standard spin-wave theory.

Dynamic properties in the bilayer quantum Hall ferromagnet

The spectral functions of pseudospin response functions in the bilayer quantum Hall system at $\ensuremath{\nu}=1$ are investigated numerically, where the pseudospin describes the layer degrees of freedom. In the pseudospin-ferromagnetic phase, the lowest-lying excitation branch is closely connected with the ground state through the fluctuations of pseudospin ${S}_{y}$ and ${S}_{z},$ and this pseudospin-wave excitation branch plays a significant role in the tunneling-current properties in the system.

Theory for low-lying excitations in the spin-Peierls systemCuGeO3

Using the bosonization technique and self-consistent quantum theory, we study two branches of low-lying excitation in the spin-Peierls system ${\mathrm{CuGeO}}_{3}$ with frustration. The energy gaps of triplet one-magnon excited state and singlet two-magnon bound state are calculated for a set of parameters appropriate to the compound ${\mathrm{CuGeO}}_{3}.$ The calculated results are consistent with experimental data of ${\mathrm{CuGeO}}_{3}.$ It is found that the energy gaps of the one- and two-magnon states increase with increasing either frustration or dimerization parameter. As the dimerization is increased, the binding energy of the singlet two-magnon bound state increases for smaller frustration, whereas it decreases for larger frustration. The power-law relation between the binding energy and the dimerization exists for small dimerization and at criticality $\ensuremath{\alpha}={\ensuremath{\alpha}}_{c}=0.2411.$

An origin of light induced centers in the visible range in ferroelectric oxides: possible role of the states of charge transfer vibronic excitons

A light induced absorption peculiarity connected with an absorption peak in the visible range for SBN/Ce was detected. The analysis of these data together with investigation results for a visible range center (VIS-center) absorption study in SBN/Ce and Cr, in nominally pure strontium barium niobate (SBN) as well as in Ba0.77Ca0.23TiO3 crystals give support to charge transfer vibronic excitons (CTVEs) as origin of the phenomenon. While the fast (with relaxation time ∼1ns at T∼300K) response can be connected with transitions between excited branches of the adiabatic potential of the CTVE phase, the slow (with relaxation time ∼50s at T∼230K for SBN/Cr) response is connected with transitions between ground and first excited CTVE phase branches. The additional contribution to the latter effect due to the absorption of recharged oxygen ions which are in the framework of charge transfer induced by the CTVE cannot be disregarded. The explanation of the main results of the experiments shows that the VIS-center phenomena can be induced by the CTVE-states.

Nonlinear Landau damping of a plasmino in the quark-gluon plasma

On the basis of the Blaizot-Iancu equations, which are a local formulation of the hard thermal loop (HTL) equations of motion for soft fluctuating quark and gluon fields and their induced sources, the coupled kinetic equations for plasminos and plasmons are obtained. The equality of matrix elements for nonlinear scattering of a plasmino by hard particles in covariant and temporal gauges is established by using effective Ward identities. The model problem of the interaction of two infinitely narrow packets with fermion and boson quantum numbers is considered. The kinematical relations between wave vectors of the plasmino and plasmon are derived, when the effective pumping over of the plasma excitation energy from the fermion branch of plasma excitations to the boson branch and vice versa occur. The expression for the nonlinear Landau damping rate of a plasmino at rest is found, and a comparison with a plasmino damping constant obtained within the framework of the hard thermal loop approximation is made. The nonlinear Landau damping rate for normal quark excitations is shown to diverge like $1/\sqrt{{q}^{2}}$ near the light cone where q is a four-momentum of excitations, and the improved Blaizot-Iancu equations removing this divergence are proposed.

Photochemical processes: potential energy surface topology and rationalization using VB arguments

The development of quantum chemical methods capable of treating excited and ground states of organic molecules in a balanced way has prompted many applications in the field of mechanistic organic photochemistry. In this paper, we review a few representative computational results which illustrate the currently emerging concept of a photochemical (and photophysical) reaction pathway. In particular, we focus on the shape (topology) of the potential energy surface along the excited state branch of the reaction path as well as on the shape and nature of the photochemical funnel where decay to the ground state occurs. The chemical effect of different topologies and their origin in terms of simple valence bond ideas are discussed.

Excitations of the field-induced quantum soliton lattice in CuGeO(3).

The incommensurate magnetic soliton lattice in the high-field phase of a spin-Peierls system results from quantum fluctuations. We have used neutron scattering techniques to study CuGeO(3), allowing us to obtain the first complete characterization of the excitations of the soliton lattice. Three distinct excitation branches are observed, all of which are gapped. The two highest energy modes have minimum gaps at the commensurate wave vector and correspond to the creation or annihilation of soliton pairs. The third mode is incommensurate and is discussed in relation to theoretical predictions.

BOUND STATES IN DIMERIZED AND FRUSTRATED HEISENBERG CHAINS

Using the Bond-Operator Technique (BOT), we have studied the low energy excitation spectrum of a frustrated dimerized antiferromagnetic Heisenberg chain. In particular, we have compared our analytical results with previous Exact Diagonalization (ED) data. Qualitatively, the BOT results are in good agreement with the ED data. And even a very good quantitative agreement is obtained in some parameter region. It is clearly shown that there is only one elementary excitation branch (lowest triplet branch) and that the two other well defined excitations which appear below the continuum, one singlet and one triplet, are bound states of two elementary triplets.

Origin of kinetic collective modes in pure and binary liquids

A generalized collective mode approach in combination with molecular dynamics are applied for the study of physical mechanisms responsible for the formation of a collective mode spectrum in pure and binary liquids. The origin of different branches of collective excitations in longitudinal and transverse dynamics is studied in detail. Analytical expressions for propagation gaps for shear waves, heat waves and optic-like excitations are derived within the two-variable approximation. The meaning of a purely relaxing kinetic mode, determined mainly by slow density fluctuations, is discussed.

Antiferromagnetic correlation in the half-filled strongly correlated electron models at nonzero temperature: A rigorous result

As an extension of our previous rigorous investigation on the spin correlations in the ground states of the half-filled Hubbard model and the periodic Anderson model [G. S. Tian, Phys. Rev. B 50, 6246 (1994)], in the present paper we study the behavior of these correlation functions at finite temperature. We show rigorously that, at any $T\ensuremath{\ne}0,$ the predominant spin correlations in these systems are antiferromagnetic. Furthermore, based on this result, we also show that a quasi-one (or two)-dimensional itinerant electron ferrimagnet must have a gapless branch of ferromagnetic excitations. This conclusion is consistent with the previous results derived by the spin-wave theory.

The analytic solution for the spectrum of surface spin waves for two-sublattice Heisenberg ferrimagnet

Using a simplified procedure we find the general analytic solution for the spectrum of the surface spin excitations in two-sublattice Heisenberg ferrimagnets in the closed form. We obtain simultaneously two bulk energy bands and also one surface excitation branch associated with each bulk band. These surface excitation branches could be either optical or acoustic, depending on the ratio of surface to bulk exchange integral η=JS/J. Our results include as a special case the results for the purely geometrical surface effect (η=1), previously known from the literature.