V. L. Vakula

Manifestation of Hubbard and covalent correlations in the absorption spectra of YBa2Cu3O6+x films

The absorption spectra of single-crystal YBa2Cu3O6+x films with various doping levels in the range from x≈0.35 to x≈0.9 are measured in the energy region 0.3–3 eV. An analysis of the spectral composition of the absorption is made with allowance for intraband and interband transitions and the local dd transitions in the Cu2+ ion. It is concluded that the dd band (the transition dxy→dx2−y2 at 1.5 eV) reflects the enhancement of the covalent bonding (pd hybridization) upon metallization and that the spectral feature at ≈1.8 eV carries information about the contribution of electronic correlations, since it is sensitive to the opening of a spin gap in the insulator and to antiferromagnetic fluctuations in the metal. Although the covalent (≈1.5 eV) and correlation (≈1.8 eV) absorption peaks compete with each other, the coexistence of these bands in the metal supports the validity of a model based on the correlation polaron — a charge carrier which creates a region of covalent bonding in a Hubbard matrix of anti...

Optical evidence for compatibility of antiferromagnetism and superconductivity in YBa2Cu3O6+x

The evolution of the spectral composition of the absorption in the 1.25–2.6 eV region for metallic films of YBa2Cu3O6+x with superconducting transition temperatures of Tc=51 and 74 K is measured as the films are cooled from 180 to 20 K. Particular attention is paid to the temperature effects in two absorption bands: the A band (≃1.8 eV), which reflects the appearance of holes dressed in antiferromagnetic (AFM) fluctuations, and the (A+J) band (≃2.15 eV), which reflects an additional (magnon) excitation of the short-range AFM order. It is found that the changes of these bands begin in the normal phase at T<T* in the temperature region corresponding to the opening of the pseudogap state, and the (A+J) magnon band arises in the pseudogap state even in the case when it is absent at room temperatures. At the superconducting transition the parameters of the bands stop changing, and the (A+J) magnon band is preserved in the superconducting state. The results are interpreted as evidence of a magnetic nature of th...

Antiferromagnetic correlations in superconducting YBa2Cu3O6+x samples from optical absorption data; comparison with the results of neutron and muon experiments

Narrow-band spectral features demonstrating a high sensitivity to the development of the pseudogap state are detected in metallic films of YBa2Cu3O6+x with Tc≈51 and 74 K. Attention is focused on the temperature behavior of the exciton–bimagnon band A+3J (≈2.15 eV) and the exciton–two-magnon band A+4J (≈2.28 eV); these bands arise as a result of phase separation into insulating and metallic regions. By comparing the optical results with published data on the temperature behavior of the muon depolarization rate and the integrated intensity of the (π,π) magnetic resonance it is shown for the first time that all three independent techniques give the same observed temperature dependence f(T/Tc) of measured quantities both in the normal and superconducting states. The established correlation of the optical, neutron, and muon data is analyzed from the standpoint of the formation of stripe ordering and the compatibility of antiferromagnetic order and superconductivity.

Optical spectroscopy of antiferromagnetic correlations and the stripe state in the superconductor YBa2Cu3O6+x

Results of measurements of the temperature dependence of the absorption spectra of YBa2Cu3O6+x films in the antiferromagnetic and metallic (superconducting, Tc=88 K) phases are presented. The temperature evolution of the absorption bands, which are diagnostic for the electron and spin correlations and the degree of metallization (oxygen–copper hybridization), is investigated in the energy region 1.3–2.6 eV. It is shown that in the metallic phase in the pseudogap state below T*=120±10 K there is a sharp rise of these absorption bands characteristic of the antiferromagnetic (AF) phase, including the electron–two-magnon (A+J) band, which reflects the development of strong AF correlations. In contrast to the insulator, here the (A+J) band has a fine structure due to the creation of both interacting and noninteracting magnons, and it coexists with the bands due to the strong metallization. The results demonstrate phase separation at Tc

Luminescence evidence for bulk and surface excitons in free xenon clusters

The cathodoluminescence spectra of free xenon clusters produced by condensation of xenon-argon gas mixtures in supersonic jets expanding into vacuum were studied. By varying the initial experimental parameters, including the 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 (

Observation of exciton luminescence from icosahedral xenon-argon clusters

\ensuremath{\approx}1500

Direct observation of free excitons in the luminescence spectra of xenon clusters

atoms). The cluster size and temperature (

A new approach to studying the luminescence spectra of free icosahedral and crystalline argon nanoclusters

\ensuremath{\approx}40\phantom{\rule{0.3em}{0ex}}\mathrm{K}

Spectroscopic observation of (N2)2dimers in free icosahedral N2and Ar-N2clusters

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. The presence of an argon shell causes extinction of the excitonic bands. In addition, some bands were found which have no analogs for bulk xenon cryosamples.

Optical evidence for the existence of a stripe structure in the normal and superconducting phases of YbA2Cu3O6+x

Exciton-impurity luminescence is observed for the first time in binary mixtures of solidified rare gases, in the xenon-argon system with argon as the impurity. An intense exciton-impurity emission band is observed in binary clusters having the structure of a multilayered icosahedron. The optical transition occurs from an energy level lying very close to the lowest bulk exciton level in bulk xenon samples. The results demonstrate the potential for probing exciton levels in disordered condensed rare gas media.