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1.
We report a measurement of the excitation spectrum omega(k) and the static structure factor S(k) of a Bose-Einstein condensate. The excitation spectrum displays a linear phonon regime, as well as a parabolic single-particle regime. The linear regime provides an upper limit for the superfluid critical velocity, by the Landau criterion. The excitation spectrum agrees well with the Bogoliubov spectrum in the local density approximation, even close to the long-wavelength limit of the region of applicability. Feynman's relation between omega(k) and S(k) is verified, within an overall constant. 相似文献
2.
Moorad Alexanian 《Physica A》1980,100(1):45-63
A model Hamiltonian describing bosons in one and two space dimensions interacting via a repulsive interparticle potential is shown to give rise to the phenomenon of superfluidity. The model has macroscopic occupation of infinitely many single-particle quantum states and the energy spectrum of the elementary excitations exhibits both phonon and roton features. Relations are found between the roton parameters, the speed of first sound and the condensate fraction where any two of them can be taken as independent variables. It is also shown that the type of condensate considered is not excluded by any rigorous proof. 相似文献
3.
Yang ZHANG 《理论物理通讯》1996,25(4):409-414
Using the Bogolu bov transformation method, we calculate excitation energy spectra for a charged relativistic Bose-Einstein condensate with λ(φφ+)2 interactions. There are two distinct spectra. In the long-wavelength limit one of the spectra approaches to the phonon spectrum for an uncharged nonrelativistic boson condensate, and the other to the nonrelativistic particle energy spectrum. Theground state is explicitly constructed as a coherent state which contains zero-momentum bosons and pairs of quanta with nonzero opposite momenta. 相似文献
4.
V. A. Khodel 《JETP Letters》2017,105(8):531-536
The diagrammatic technique elaborated by Belyaev for the theory of a Fermi liquid has been implemented to analyze the behavior of Fermi systems beyond the topological phase transition point, where the fermionic condensate appears. It has been shown that the inclusion of the interaction between the condensate and above-condensate particles leads to the emergence of a gap in the single-particle excitation spectrum of these particles even in the absence of Cooper pairing. Hence, the emergence of this gap in homogeneous electron systems of silicon field-effect structures leads to a metal–insulator phase transition rather than to superconductivity. It has been shown that the same interaction explains the nature of the Fermi arc structure in twodimensional electron systems of cuprates. 相似文献
5.
A. Minguzzi G. Ferrari Y. Castin 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,17(1):49-55
We describe the excitation spectrum of a two-component neutral Fermi gas with attractive interactions in the superfluid phase
at finite temperature by deriving a suitable Random-Phase approximation in the collisionless regime with the technique of
functional derivatives. The obtained spectrum for the homogeneous gas at small wavevectors contains the Bogoliubov-Anderson
phonon and is essentially different from the spectrum predicted by the static Bogoliubov theory, which instead shows an unphysically
large response. We adapt the results for the homogeneous system to obtain the dynamic structure factor of a harmonically confined
superfluid and we identify in the spectrum a unique feature of the superfluid phase.
Received 28 March 2001 相似文献
6.
In this paper, we introduce a quasi-one-dimensional S = 1 antiferromagnet Heisenberg model, and some physical properties of antiferromagnet L2BaNiO5 without frame of two-time Green's function theory. In a high temperature region, we calculate the correlation functions, and obtain excitation spectrum along Ni chains and the Haldane gap in this spectrum versus temperature. We find that the short-range correlation still exists at high temperature, which leads to the existence of Haldane gap in excitation spectrum. The increment of excitation energy in the spectrum along the Ni chain is found to be induced by the AF interaction between spins of rare-earth and Ni ions. Additionally, we also find that Haldane gap goes up with temperature increasing. 相似文献
7.
Exact diagonalization study of a rotating dilute Bose-Einstein condensate reveals that as the first vortex enters the system the degeneracy of the low-energy yrast spectrum is lifted and a large energy gap emerges. As more vortices enter with faster rotation, the energy gap decreases towards zero, but eventually the spectrum exhibits a rotonlike structure above the nu=1/2 Laughlin state without having a phonon branch despite the short-range nature of the interaction. 相似文献
8.
HAN Jiu-Rong ZHANG Hai-Chao WANG Yu-Zhu 《理论物理通讯》2005,44(5):863-866
We investigate the energy spectrum of ground state and quasi-particle excitation spectrum of hard-core bosons, which behave very much like spinless noninteracting ferrnions, in optical lattices by means of the perturbation expansion and Bogoliubov approach. The results show that the energy spectrum has a single band structure, and the energy is lower near zero momentum; the excitation spectrum gives corresponding energy gap, and the system is in Mort-insulating state at Tonks limit. The analytic result of energy spectrum is in good agreement with that calculated in terms of Green's function at strong correlation limit. 相似文献
9.
The density of phonon states at various temperatures has been calculated for crystals with the NaCl structure with equal and strongly differing weights of anions and cations. It has been shown that, in the crystal with a considerable difference in the weights of components, a wide band gap exists in the spectrum of phonon states, which leads to spontaneous excitation of nonlinear localized vibrational modes??gap discrete breathers having frequencies inside the band gap, if the temperature is sufficiently high. It has been found that the peak of the density of phonon states lying above the spectrum of linear vibrations appears at elevated temperatures, which can indicate the existence of discrete breathers with corresponding frequencies. It has been noted that, previously, the existence of gap discrete breathers in the NaI crystal at 555 K was shown experimentally. The presented results bring up the question of the theoretical justification and experimental observation of the breathers with frequencies above the phonon spectrum. 相似文献
10.
Time-Dependent Variational Approach to the Phonon Dispersion Relation of the Commensurate Quantum Frenkel-Kontorova Model 总被引:3,自引:0,他引:3
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The phonon dispersion relation of the commensurate quantum Frenkel-Kontorova model is studied by means of the time-dependent variational approach combined with a Hartree-type many-body trial wavefunction for the particles. The single-particle state is taken to be a frozen Jackiw-Kerman wavefunction. Under the condition of minimum uncertainty, equations of motion for the particle expectation values are derived to obtain the phonon dispersion relation. It is shown that the strength of the substrate potential and the phonon excitation gap are reduced due to the quantum fluctuations in comparison with those of the classical model. We also compare our results with those previously obtained by using the path-integral molecular dynamics. 相似文献
11.
P. Lavallard M. ChamarroJ. Pérez-Conde A.K. BhattacharjeeS.V. Goupalov A.A. Lipovskii 《Solid State Communications》2003,127(6):439-442
Photoluminescence of CdTe nanocrystals (NC) is excited resonantly in the lowest energy absorption peak. The spectrum shows a luminescence line shifted to a lower energy and acoustical and optical phonon replica. The Stokes shift between the luminescence and excitation lines is attributed to the electron-hole exchange energy in the nanocrystal. By tuning the laser line inside the absorption peak, we are able to measure the Stokes shift as a function of the excitation energy. Calculation of the absorption gap and the Stokes shift is done in a tight-binding theory. It allows us to determine the radius R of NC excited at a given wavelength and to compare the experimental and theoretical values of the exchange energy as a function of R. A very good agreement is obtained. The observed size dependence of the acoustical phonon mode energy provides a further confirmation of our analysis. 相似文献
12.
The structural factors for the single-and double-component Bose-Einstein condensates of neutral atoms in a spherically symmetric harmonic trap are derived. The approach adopted in this paper is applicable in the low excitation energy limit, where the well-known Stringari dispersion relation is valid. Our expression is compared with the quasi-classical one, which is shown to overestimate the structural factor. It is argued that the second component first splits the frequency of collective condensate oscillations, which corresponds to the energy transferred by scattering, into two branches and then multiplies the single-component structural factor by some coefficient that can be greater than unity. The analysis is used to estimate the properties of an experimentally implemented double-component condensate of 87Rb atoms. 相似文献
13.
测量了掺Ho3+离子的氟玻璃的吸收光谱,激发光谱、发射光谱,10K~660K不同温度下的激发态寿命及氟玻璃的拉曼光谱.根据Judd-ofelt理论计算了辐射跃迁几率,进而计算了不同温度下的无辐射联迁几率.利用Huang-Rhys多声子跃迁理论分析了无辐射跃迁几率与温度的关系.在分析中考虑到不同频率声子的参与,并考虑到黄昆因子,得到与实验很好的符合.由理论和实验的拟合中估计了黄昆因子的大小,讨论了黄昆因子在无辐射跃迁中的作用. 相似文献
14.
Patrick L. Garrity 《哲学杂志》2013,93(8):1001-1012
An experimental study of collective oscillations in Pd covering the region of very low energy and momentum transfers is reported. Through Dynamic Electron Scattering spectroscopy, structure factor spectra were measured from 80?K to 298?K on a bulk polycrystalline Pd sample. Here we report the first experimental evidence of damped acoustic plasmons and their evolution to the single-particle excitation continuum. The acoustic plasmons follow a linear dispersion and are experimentally shown to be a separate and distinct resonance mode from acoustic surface plasmons. Calculations of the dielectric function employed a model that incorporates complete mixing of two conduction bands with contributions from both interband and intraband transitions. The model was used in computational studies that focused on specific experimental results to aid the characterization and understanding of the plasmon behavior. We found that the Pd acoustic plasmon energy matched the longitudinal phonon anomaly that has sparked numerous theoretical reports on the possible energetic coupling of these modes. Further experimental evidence of plasmon and phonon dynamical processes are found in the linewidth analysis of the data. The primary decay mechanism of the plasmons is interpreted to be strong phonon-assisted interband transitions. Further spectral features and the plasmon velocity are also reported. 相似文献
15.
We investigate the energy spectrum of fermionized bosonic atoms, which behave very much like spinless noninteracting fermions, in optical lattices by means of the perturbation expansion and the retarded Green's function method. The results show that the energy spectrum splits into two energy bands with single-occupation; the fermionized bosonic atom occupies nonvanishing energy state and left hole has a vanishing energy at any given momentum, and the system is in Mott-insulating state with a energy gap.Using the characteristic of energy spectra we obtained a criterion with which one can judge whether the Tonks-Girardeau (TG) gas is achieved or not. 相似文献
16.
17.
Phase Transition and Superfluid of Photons and Photon Pairs in a Two-Dimensional Optical Microcavity
We analyze the ground-state properties and the excitation spectrum of Bose-Einstein condensates of photons and PPs in a two-dimensional optical microcavity. First, using the variational method, we discuss the ground-state phase transition of the two-component system. We also investigate the energy gap between the ground state and the first excited state. Moreover, by investigating the excitation spectrum, we also illustrate how the superfluid behavior of photons and PPs can be associated with the phase transition of the system. 相似文献
18.
Phase Transition and Superfluid of Photons and Photon Pairs in a Two-Dimensional Optical Microcavity
We analyze the ground-state properties and the excitation spectrum of Bose-Einstein condensates of photons and PPs in a two-dimensional optical microcavity. First, using the variational method, we discuss the ground-state phase transition of the two-component system. We also investigate the energy gap between the ground state and the first excited state. Moreover, by investigating the excitation spectrum, we also illustrate how the superfluid behavior of photons and PPs can be associated with the phase transition of the system. 相似文献
19.
本文采用荧光窄化技术,对低组分的GaAs1-xPx:N(x=0.76,0.65)混晶材料中Nx束缚激子的声子伴线进行了研究.在低温下,选择激发Nx带时,我们得到了与GaP:N低温发光谱中类似的声子伴线结构.根据实验结果,给出了混晶中各种声子的能量.另外,对组分为x=0.76的GaAs1-xPx:N混晶样品,我们还首次观察并分析了多声子重现光谱. 相似文献