SU(2) gauge theory of the Hubbard model: emergence of an anomalous metallic phase near the Mott critical point

We propose one possible mechanism for an anomalous metallic phase appearing frequently in two spatial dimensions, that is, local pairing fluctuations. Introducing a pair-rotor representation to decompose bare electrons into collective pairing excitations and renormalized electrons, we derive an SU(2) gauge theory of the Hubbard model as an extended version of its U(1) gauge theory. Since our effective SU(2) gauge theory admits two kinds of collective bosons corresponding to pair excitations and density fluctuations, respectively, an intermediate phase appears naturally between the spin liquid Mott insulator and Fermi liquid metal of the U(1) gauge theory, characterized by softening of density-fluctuation modes as the Fermi liquid, but gapping of pair-excitation modes. We show that this intermediate phase is identified with an anomalous metallic phase because there are no electronlike quasiparticles although it is metallic.     

Structure of the intersubband collective excitations in quasi-two-dimensional systems in a magnetic field

The spectrum of intersubband collective spin-and charge-density excitations is calculated for a system of quasi-two-dimensional electrons with ν≤10 (ν is the filling factor) in a magnetic field. The transitions both without changing the Landau level and with its change (Bernstein modes) are considered. All excitations are shown to have a multimode structure, the number of modes being determined by the filling factor. The dispersion and interaction of small-quasimomentum collective excitations are also considered. The possibility of observing the multimode structure is predicted.     

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.     

Collective microdynamics of liquid lithium: An inelastic neutron scattering study

A portion of the dispersion curve for collective modes in liquid lithium has been constructed from experimental data on inelastic scattering of slow neutrons obtained on the DIN-2PI neutron spectrometer (IBR-2 reactor, Joint Institute for Nuclear Research, Dubna, Russia). Measurements have been performed at a temperature of 500 K (T _m (Li) = 453.7 K). The coherent scattering component has been separated from the experimental spectra and analyzed. Information on the characteristics of collective excitations in liquid lithium has been derived.     

Collective excitations in double quantum wells with strong tunnel coupling

Spectra of collective and magnetic excitations in symmetric double quantum wells with strong tunnel coupling were studied by inelastic scattering. Gaps in the spectrum of collective and single-particle excitations associated with tunnel interwell splitting were measured. New excitation modes, namely, tunnel Bernstein modes, were detected in a magnetic field. Based on the dispersion and magnetic-field dependences of the energies of inelastic light scattering lines, a classification of excitations and magnetic excitations was given.     

Theory of resonant Raman scattering from low-energy collective excitations of interacting electrons in quantum wires

The Raman spectra of quantum wires in the region of electronic intra-band excitations are investigated using one- and two-band models based on the Luttinger approximation with spin. Structures related to charge and spin density modes are identified, and analyzed with respect to their behavior with photon energy and temperature. It is found that the low-energy peaks in the polarized spectra, close to resonance that are commonly assigned to “single particle excitations”, can be interpreted as the signature of spin density excitations. A broad structure in the resonant depolarized spectrum is predicted above the frequency of the spin density excitations. This is due to simultaneous but independent propagation of spin and charge density modes. The results, when compared with experiment, show, that the electronic collective excitations of quantum wires at low energies are characteristic for a non-Fermi liquid. Received: 25 March 1998 / Accepted: 3 June 1998     

Intersubband collective excitations in quasi-two-dimensional systems in a strong magnetic field

The spectrum of intersubband collective excitations of spin and charge density in a system of quasi-two-dimensional electrons is calculated in the strong magnetic field limit for filling factors υ≤4. For υ≤2 two new closely spaced modes of collective excitations are obtained. The modes obtained make it possible to give a new interpretation of the experimentally observed line, which is usually interpreted as being due to single-particle excitations. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 8, 543–547 (25 April 1999)     

Spin–orbit interaction effects on plasmons in double coupled quantum wires

We report on collective and single-particle excitations of quasi-one-dimensional (Q1D) electron gas, experiencing Rashba spin–orbit interaction (SOI), and embedded in semiconductor-based coupled symmetrical and asymmetrical biwires. We choose five different and most significant situations regarding the conditions of anisotropy and tunneling between the wires. As the Rashba SOI strength increases, we find extra undamped plasmon modes resulting from the SOI induced spin splitting of those modes appearing in the absence of SOI. We also find these modes remaining in the gap between the spin-down and spin-up SPE continua. We show similar effects occurring on the optical and acoustical plasmon modes; signatures of collective excitations appearing in biwires without tunneling. We find those plasmons related to the spin-down charge density fluctuations being of the most robust modes concerning the increase of the Rashba SOI strength.     

Amplitude-mode dynamics of polariton condensates

We study the stability of collective amplitude excitations in nonequilibrium polariton condensates. These excitations correspond to renormalized upper polaritons and to the collective amplitude modes of atomic gases and superconductors. They would be present following a quantum quench or could be created directly by resonant excitation. We show that uniform amplitude excitations are unstable to the production of excitations at finite wave vectors, leading to the formation of density-modulated phases. The physical processes causing the instabilities can be understood by analogy to optical parametric oscillators and the atomic Bose supernova.     

Spin-plasmons in topological insulator

Collective plasmon excitations in a helical electron liquid on the surface of strong three-dimensional topological insulator are considered. The properties and internal structure of these excitations are studied. Due to spin-momentum locking in helical liquid on a surface of topological insulator, the collective excitations should manifest themselves as coupled charge- and spin-density waves.     

Second harmonic generation of propagating collective excitations in Bose－Einstein condensates

We consider a possible second harmonic generation (SHG) of propagating collective excitations in a two-component Bose－Einstein condensate (BEC) with repulsive atom－atom interactions. We show that the phase-matching condition for the SHG can be fulfilled if the wave vectors and frequencies of the excitations are chosen adequately from different dispersion branches. We solve the nonlinear amplitude equations for the SHG derived using a method of multiple-scales and provide SHG solutions similar to those obtained for a SHG in nonlinear optical media. A possible experimental realization of the SHG for the propagating collective modes in a cigar-shaped two-component BEC is also discussed.     

Neutron scattering in superionic conductors

The coherent dynamical structure factor S(q,ω) of one-dimensional superionic conductors is studied in a model of Brownian particles with harmonic interactions moving in a periodic potential. We present results for the q-dependence of the halfwidth of the quasielastic peak, which reflects the commensurability ratios of the periodic potential and the molten sublattice. Furthermore the influence of the periodic potential on the collective excitations of the mobile ions is discussed.     

The Collective Excitation Spectra of σ, ω and π Mesons in Nuclear Matter

The recent progress on the study of the collective excitation in relativistic nuclear matter is reviewed. The collective excitation modes are derived by meson propagators in nuclear matter. The mesons we studied are σ, ω, γ and π mesons. For pion, we derived not only the relativistic particle-hole, delta-hole excitations but also antiparticle excitations, suchas particle-antiparticle, antidelta-particle, delta-antiparticle excitations. By calculating the dispersion relation and the spin-isospin-dependent response function, the effects of all these excitation are studied.     

Collective modes in a slab of interacting nuclear matter

We consider a slab of nuclear matter and investigate the collective excitations, which develop in the response function of the system. We introduce a finiterange realistic interaction among the nucleons, which reproduces the full G-matrix by a linear combination of gaussian potentials in the various spin-isospin channels. We then analyze the collective modes of the slab in theS=T=1 channel: for moderate momenta hard and soft zero-sound modes are found, which exhaust most of the excitation strength. At variance with the results obtained with a zero range force, new “massive” excitations are found for the vector-isovector channel.     

Collective excitations in fast ion conductor superlattices

Within the framework of the electromagnetic theory, the collective modes in the superlattice system composed of superionic conductors and ionic crystals are studied. The superionic conductor is described by the hydrodynamical model in which the anion cage is immersed in the cation liquid. The behavior of the modes are analysed in terms of the coupling strength between excitations pertaining to different layers. The coupling strength is controlled by varying the slab thicknesses. An interesting behavior in which the diffusion mode transforms to the relaxation mode when the coupling strength is varied from strong to weak is obtained. Also, the effect of the coupling strength on the acoustical and optical modes are shown.     

Collective magnetoplasma excitations in two-dimensional electron rings

The spectra of magnetoplasma excitations in two-dimensional electron disks and rings are studied by optical detection of resonance microwave absorption. For ring-shaped structures, two types of edge magnetoplasma modes localized along the inner and outer boundaries of the ring are observed. It is shown that the interaction between these modes leads to a strong modification of their magnetic-field dependences as compared to disks. In addition to the longitudinal edge magnetoplasma excitations, transverse plasma modes associated with the electron density oscillations along the ring radius are revealed. The spectra of magnetoplasma excitations are calculated in terms of the electrodynamic theory for both ring-shaped and disk-shaped structures. The classification of all modes of collective magnetoplasma excitations observed in the experiment is performed on the basis of the comparison between experimental and theoretical results.     

Charge-fluctuation contribution to the Raman response in superconducting cuprates

We calculate the Raman response contribution due to soft collective modes, finding a strong dependence on the photon polarizations and on the characteristic wave vectors of the modes. We compare our results with recent Raman spectroscopy experiments in underdoped cuprates, La2-xSrxCuO4 and (Y1.97Ca0.3)Ba2CuO6.05, where anomalous low-energy peaks are observed, which soften upon lowering the temperature. We show that the specific dependence on doping and on photon polarizations of these peaks can naturally arise from charge collective excitations at finite wavelength.     

Supersymmetry and electron-hole excitations in semiconductors at finite temperature

The fermionic and bosonic electron-hole low-lying excitations in a semiconductor are analyzed at finite temperature in a unified way following Nambu's quasi-supersymmetric approach for the BCS model of superconductivity. The effective lagrangian for the fermionic modes and for the bosonic low-lying collective excitations in the semiconductor is no longer supersymmetric in a conventional finite-temperature treatment. However, the bosonic excitations do not couple directly to the heat bath and as a result, quasi-supersymmetry is restored to the effective lagrangian when a redefinition of the coupling constant associated with the collective excitations is performed. Our result shows that although the mass and coupling parameters are now temperature dependent, the fermion and boson excited states pair together and can still be transmuted into one another.     

Search for acoustic and optic vibrational modes in Zr<Subscript>40</Subscript>Be<Subscript>60</Subscript> metallic glass

Dispersion of collective modes in metallic glass (Zr₄₀Be₆₀, composed of disparate mass particles) was measured at small-angle spectrometer BRISP at the ILL what enabled us to extend to lower momentum transfers unlike to high-angle spectrometer IN4 at the ILL. It was shown that the behavior of the optical mode in the metallic glass is similar to the behavior of optic modes in the other systems with non-sized atoms (liquid LiPb, inert gas mixtures with high density (He₆₅Ne₃₅)).