66 research outputs found
Luminescence evidence for bulk and surface excitons in free xenon clusters
Cathodoluminescence spectra of free xenon clusters produced by condensation
of xenon-argon gas mixtures in supersonic jets expanding into vacuum were
studied. By varying initial experimental parameters, including xenon
concentration, we could obtain clusters with a xenon core (300-3500 atoms)
covered by an argon outer shell as well as shell-free xenon clusters (about
1500 atoms). The cluster size and temperature (about 40 K for both cases) were
measured electronographically. Luminescence bands evidencing the existence of
bulk and surface excitons were detected for shell-free xenon clusters. The
emission from bulk excitons in small clusters is supposed to be due to
processes of their multiple elastic reflections from the xenon-vacuum
interface. A presence of an argon shell causes extinction of the excitonic
bands. In addition, some new bands were found which have no analogs for bulk
xenon cryosamples.Comment: The final modified version will be published in Phys. Rev. A 76
(2007
Quasiparticle undressing in a dynamic Hubbard model: exact diagonalization study
Dynamic Hubbard models have been proposed as extensions of the conventional
Hubbard model to describe the orbital relaxation that occurs upon double
occupancy of an atomic orbital. These models give rise to pairing of holes and
superconductivity in certain parameter ranges. Here we explore the changes in
carrier effective mass and quasiparticle weight and in one- and two-particle
spectral functions that occur in a dynamic Hubbard model upon pairing, by exact
diagonalization of small systems. It is found that pairing is associated with
lowering of effective mass and increase of quasiparticle weight, manifested in
transfer of spectral weight from high to low frequencies in one- and
two-particle spectral functions. This 'undressing' phenomenology resembles
observations in transport, photoemission and optical experiments in high T_c
cuprates. This behavior is contrasted with that of a conventional electron-hole
symmetric Holstein-like model with attractive on-site interaction, where
pairing is associated with 'dressing' instead of 'undressing'
Photoproduction of Long-Lived Holes and Electronic Processes in Intrinsic Electric Fields Seen through Photoinduced Absorption and Dichroism in Ca_3Ga_{2-x}Mn_xGe_3O_{12} Garnets
Long-lived photoinduced absorption and dichroism in the
Ca_3Ga_{2-x}Mn_xGe_3O_{12} garnets with x < 0.06 were examined versus
temperature and pumping intensity. Unusual features of the kinetics of
photoinduced phenomena are indicative of the underlying electronic processes.
The comparison with the case of Ca_3Mn_2Ge_3O_{12}, explored earlier by the
authors, permits one to finally establish the main common mechanisms of
photoinduced absorption and dichroism caused by random electric fields of
photoproduced charges (hole polarons). The rate of their diffusion and
relaxation through recombination is strongly influenced by the same fields,
whose large statistical straggling is responsible for a broad continuous set of
relaxation components (observed in the relaxation time range from 1 to about
1000 min). For Ca_3Ga_{2-x}Mn_xGe_3O_{12}, the time and temperature dependences
of photoinduced absorption and dichroism bear a strong imprint of structure
imperfection increasing with x.Comment: 20 pages, 10 figure
Superconductivity from Undressing. II. Single Particle Green's Function and Photoemission in Cuprates
Experimental evidence indicates that the superconducting transition in high
cuprates is an 'undressing' transition. Microscopic mechanisms giving
rise to this physics were discussed in the first paper of this series. Here we
discuss the calculation of the single particle Green's function and spectral
function for Hamiltonians describing undressing transitions in the normal and
superconducting states. A single parameter, , describes the strength
of the undressing process and drives the transition to superconductivity. In
the normal state, the spectral function evolves from predominantly incoherent
to partly coherent as the hole concentration increases. In the superconducting
state, the 'normal' Green's function acquires a contribution from the anomalous
Green's function when is non-zero; the resulting contribution to
the spectral function is for hole extraction and for hole
injection. It is proposed that these results explain the observation of sharp
quasiparticle states in the superconducting state of cuprates along the
direction and their absence along the direction.Comment: figures have been condensed in fewer pages for easier readin
Superconductivity from Undressing
Photoemission experiments in high cuprates indicate that quasiparticles
are heavily 'dressed' in the normal state, particularly in the low doping
regime. Furthermore these experiments show that a gradual undressing occurs
both in the normal state as the system is doped and the carrier concentration
increases, as well as at fixed carrier concentration as the temperature is
lowered and the system becomes superconducting. A similar picture can be
inferred from optical experiments. It is argued that these experiments can be
simply understood with the single assumption that the quasiparticle dressing is
a function of the local carrier concentration. Microscopic Hamiltonians
describing this physics are discussed. The undressing process manifests itself
in both the one-particle and two-particle Green's functions, hence leads to
observable consequences in photoemission and optical experiments respectively.
An essential consequence of this phenomenology is that the microscopic
Hamiltonians describing it break electron-hole symmetry: these Hamiltonians
predict that superconductivity will only occur for carriers with hole-like
character, as proposed in the theory of hole superconductivity
Physical origin of the buckling in CuO: Electron-phonon coupling and Raman spectra
It is shown theoretically that the buckling of the CuO planes in
certain cuprate systems can be explained in terms of an electric field across
the planes which originates from different valences of atoms above and below
the plane. This field results also in a strong coupling of the Raman-active
out-of-phase vibration of the oxygen atoms ( mode) to the electronic
charge transfer between the two oxygens in the CuO plane. Consequently,
the electric field can be deduced from the Fano-type line shape of the
phonon. Using the electric field estimated from the electron-phonon coupling
the amplitude of the buckling is calculated and found to be in good agreement
with the structural data. Direct experimental support for the idea proposed is
obtained in studies of YBaCuO and
BiSr(CaY)CuO with different oxygen and
yttrium doping, respectively, including antiferromagnetic samples. In the
latter compound, symmetry breaking by replacing Ca partially by Y leads to an
enhancement of the electron-phonon coupling by an order of magnitude.Comment: 12 pages, 4 figures, and 1 tabl
EXCITON - INDUCED PROCESSES IN SOLID MOLECULAR OXYGEN - NEON SOLUTIONS
Author Institution: B. Verkin Institute for Low Temperature Physics and Engineering, Ukrainian Academy of SciencesThe paper is concerned with the processes induced by the trapping of excitons of the neon matrix by impurity centres O and Xe. Binary and triple solutions are studied by a cathodoluminescence spectroscopy method. It is found that the exciton trapping by impurity particles results in an effective dissociation of O molecules, causes an accelerated migration of atoms through the the crystal and a desorption of O, initiates a defect formation in the vicinity of impurity centers and produces a formation of excimer compounds XeO in triple solutions Ne-O -Xe. The mechanisms of these processes associated with the transport of energy of the matrix electronic excitations to an impurity subsystem and its further relaxation are considered. It is shown that all the effects are based on the general principles conditioned by the energy structure and thermophysical characteristics of the Ne matrix- (i) the fast dissipation of exciton excess energy followed by lattice local heating and deformation; (ii) the repulsive behavior of the interaction between Rydberg’s excited impurity center and surrounding atoms of the matrix
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