S. A. Uyutnov

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...

Charging effects in an electron bombarded Ar matrix and the role of chemiluminescence-driven relaxation.

The relaxation processes in pure and doped Ar films preirradiated by an electron beam are studied with the focus on charging effects. Correlated real time measurements have been performed applying current and optical activation spectroscopy methods. Thermally stimulated exoelectron emission and thermally stimulated luminescence are detected in the vacuum ultraviolet and visible range. An appreciable accumulation of electrons in the matrix is found, and prerequisites for negative space charge formation are ascertained. The part played by pre-existing and radiation-induced defects as well as dopants is considered and the temperature range of the electron trap stability is elucidated. It is shown that laser-induced electron detachment from O(-) centers results in an enhancement of electron detrapping via the chemiluminescence mechanism, viz. neutralized and thermally mobilized O atoms recombine. Formation of O(2)* results in the emission of visible photons. These photons act as a stimulating factor for electron release and transport, terminating in exoelectron emission and charge recombination. Chemiluminescence therefore plays an important role in the decay of charged centers.

Anomalous low-temperature "post-desorption" from solid nitrogen

Anomalous low-temperature post-desorption (ALTpD) from the surface of nominally pure solid nitrogen preliminary irradiated by an electron beam was detected for the first time. The study was performed using a combination of activation spectroscopy methods—thermally stimulated exoelectron emission (TSEE) and spectrally resolved thermally stimulated luminescence (TSL)—with detection of the ALTpD yield. Charge recombination reactions are considered to be the stimulating factor for the desorption from pre-irradiated α-phase solid nitrogen.

Formation of (Xe2H)* centers in solid Xe via recombination: nonstationary luminescence and internal electron emission

The formation of excimers (Xe2H)* in solid Xe doped with molecular hydrogen under irradiation by an electron beam is studied using the original two-stage technique of nonstationary (NS) cathodoluminescence (CL) in combination with current activation spectroscopy—thermally stimulated exoelectron emission (TSEE). Charged species are generated using a high-density electron beam. The species produced are probed with a low-density beam with sample gradually heated. The near-UV emission of the (Xe2H)* is used to monitor the neutralization process. It is found that the temperature behavior of the NS CL band of (Xe2H)* clearly correlates with the yield of TSEE measured after identical pre-irradiation of the sample. The fingerprints of the thermally stimulated detrapping of electrons—«internal electron emission»—-in the spectrum of NS CL point to the essential role of the neutralization reaction in the stability of protons solvated by rare-gas atoms.

Relaxation channels and transfer of energy stored by pre-irradiated rare gas solids

The processes of energy relaxation in rare gas solids pre-irradiated with an electron beam are discussed. The emission of exoelectrons and photons from rare gas solids is studied. Investigations are performed by a set of activation spectroscopy methods applied simultaneously to each sample. Photon-stimulated exoelectron emission from solid Ne is observed for the first time.

Electron traps in solid Xe

Correlated real-time measurements of thermally stimulated luminescence and exoelectron emission from solid Xe pre-irradiated with an electron beam are performed. The study enables us to distinguish between surface and bulk traps in solid Xe and to identify a peak related to electronically induced defects. The activation energy corresponding to annihilation of these defects is estimated by the following methods: the method of different heating rates, the initial-rise method, and the curve cleaning technique with fitting of the thermally stimulated luminescence glow curve.

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

Electron-stimulated desorption of excited atoms from solid nitrogen

The desorption of particles from the surface of solid nitrogen, stimulated by electrons having subthreshold energy, is studied in the atomic transition range using VUV fluorescent spectroscopy. Varying the energy of the electron beam allowed us to differentiate the inputs from the bulk and the sub-surface regions of the sample. Evidence of electron-stimulated desorption of excited nitrogen atoms and the mechanisms thereof are provided for the first time.

Optically stimulated desorption of “hot” excimers from pre-irradiated Ar solids

Electronically induced desorption from solid Ar pre-irradiated by a low-energy electron beam is investigated by activation spectroscopy methods —photon-stimulated exoelectron emission and photon-stimulated luminescence in combination with spectrally resolved measurements in the VUV range of the spectrum. Desorption of vibrationally excited argon molecules Ar2*(ν) from the surface of pre-irradiated solid Ar is observed for the first time. It is shown that desorption of “hot” Ar2*(ν) molecules is caused by recombination of self-trapped holes with electrons released from traps by visible-range photons. The possibility of optical stimulation of the phenomenon is evidenced.