Many-body effects in the highly excited state of fullerenes

The highly excited state of fullerenes is characterised by a luminescence output which is dependent on the cube of the input intensity. This nonlinear emission is red shifted from the low-level emission and has a long, intensity dependent lifetime. Under similar irradiation conditions, the photoconductive response is seen to increase with the cube of the input intensity and the photocurrent in the highly excited state is observed to be largely independent of temperature, contrasting sharply with the thermally activated behaviour at low excitation densities. The degree of nonlinearity of the observed phenomena exclude an interpretation in terms of intra-molecular processes and the temperature dependence of the photoconductive response is suggestive of a Mott-like transition. The nonlinear behaviour is compared to that of indirect band-gap semiconductors in which the origin of similar nonlinear phenomena in the highly excited state luminescence and photoconductivity are explained in terms of electron-hole droplet formation. The similarities of the behaviours leads to a consideration of exchange and correlation energies in fullerenes, which are calculated according to a phenomenological model. Estimates of the contributions are consistent with a Mott-like transition at high excitation densities and an excess exchange/correlation energy in the highly excited state of 150 meV.     

Relaxed excited states and anomalous hyperfine structure of muonium centers in KBr

We report on the luminescence induced by positive muons implanted (with 4 MeV) into KBr crystal, which evidences a long-lived (lifetime=13.3 s excited state produced by muon radiolysis. The temperature dependence of the luminescence yield has a strong correlation with the amplitude of an anomalous muonium center: both are observed only below 50 K. This correlation strongly suggests that the muonium center is perturbed by the muon-induced excitons to cause the anomalous hyperfine structure. Moreover, the luminescence energy and decay time indicate that the observed luminescence is not associated with the intrinsic or impurity-related self-trapped excitons, but with a relaxed excited state specific to the muon(ium)-KBr system.     

The short-range correlations of a doped Mott insulator

This paper presents numerical studies of the single hole model that address the interplay between the kinetic energy of itinerant electrons and the exchange energy of local moments as of interest to doped Mott insulators. Due to this interplay, two different spin correlations coexist around a mobile vacancy. These local correlations provide an effective two-band picture that explains the two-band structure observed in various theoretical and experimental studies, the doping dependence of the momentum space anisotropic pseudogap phenomena and the asymmetry between hole and electron doped cuprates.     

Synthesis and luminescence of CdS quantum dots capped with a silica precursor

A novel synthesis method for efficiently luminescing CdS nanocrystals is reported using (3-mercaptopropyl)-trimethoxysilane (MPS) as a capping agent in THF or MeOH. The synthesis yields particles capped with a precursor for coating the particle with a silica shell and offers control over a wide range of particle sizes (2-) by varying the MPS concentration. The temperature dependence of the luminescence and the luminescence life time was studied and explained in terms of a donor-acceptor recombination process in which both electron and hole are trapped at low temperature. In addition to varying the emission color by controlling the particle size, nanocrystals were doped with Zn or Cu resulting in a blue shift or a red shift of the emission.     

Nucleon-content effect on the fission lifetime of excited nuclei

It was shown previously that the fission lifetime of a nucleus excited to about 100 MeV depends strongly and nonmonotonically on the initial value of its angular momentum L ₀. This result was obtained on the basis of a refined version of the combined dynamical and statistical model. The present study is devoted to a theoretical analysis of the dependence of the fission time on the nucleonic composition of the nucleus involved. The respective calculations were performed within the same model. The dependence of the average fission time 〈t _f 〉 on the initial fissility parameter (Z ²/A)₀ appears to be of a resonance type and is similar to its dependence on L ₀. This dependence of the average fission time on (Z ²/A)₀ stems both from statistical calculations and from a dynamical simulation of the fission mode with allowance for friction. The conditions under which the average fission time reaches a maximum are specified. The dependence of the average fission time on (Z ²/A)₀ remains nonmonotonic in the fusion-fission reaction as well, in which case the distribution of compound nuclei with respect to the initial angular momentum is broad.     

Infrared cathodoluminescence of AlN

Measurements were made of cathode-ray excited infrared luminescence band of AlN powder in the temperature range 110 K to 535 K. The form of the temperature-dependent luminescence band and the temperature dependence of the band half-width were used in the determination of the value of the Huang-Rhys factor (S 2) as well as of the frequency of phonons (441 cm^–1) interacting with electron transition. A discussion is presented of the possibility of interaction between an infrared luminescence centre and branch of natural transverse acoustic vibrations of the AlN lattice.Na Slovance 2, Praha 8, Czechoslovakia.The author is indebted to Dr. J. Pastrák for valuable advice and suggestions concerning the measurement as well as the evaluation. Thanks are also due to Mr. J. Hejduk and Mr. J. Veselý for the preparation of AlN powders, and to Dr. V. Kohl for the setting-up of the programme and the computer calculations.     

Momentum distributions in fragmentation reactions with relativistic heavy ions

The momentum distribution (or angular distribution) in inelastic heavy ion reactions is calculated by using a two-step model (abrasion and ablation). First, all nucleons in the volume element where projectile and target overlap spatially during the collision are sheared away. The remaining prefragment (the projectile minus the sheared off nucleons) has a recoil momentum proportional to the Fermi momentum. The prefragment is left in an excited state and emits nucleons, the recoil momentum given to the final fragment is proportional to the nuclear temperature. This two-step model reproduces the overall trend and the isotopic dependence for the widths of the experimental momentum distributions. Contrary to previous theoretical studies we find that surface and friction phenomena lead to an anisotropy: The momentum distributions in transverse direction are always broader than in the longitudinal direction by about 5 to 10%.     

Temperaturabhängigkeit der Lumineszenz der Kristalle von AgCl mit Beimengung von Tl

The temperature dependence of the luminescence of AgCl-Tl single crystals in the red region of the spectrum was measured. The curves have a characteristic beginning in the 50–70 K region, peaks at 70–100K and at 190K followed by a rapid fall. The dependence of the luminescence intensity on that of the exciting radiation is linear in the range measured. The experiments point to the complex character of the centre and the probability of energy transfer between the absorption and luminescence centres.

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Ich betrachte es gerne als meine Pflicht, an dieser Stelle den Herren Prof. Dr. O. Stasiw und Dr. Koswig meinen Dank auszusprechen für ihre Bereitwilligkeit, mit der sie mir ermöglichten, im Tieftemperaturlaboratorium des Instituts zu arbeiten, sowie für die wertvollen Diskussionen.     

Anti‐Stokes Luminescence of Cadmium Telluride Nanocrystals

For the first time, the antiStokes luminescence in colloidal solutions of CdTe nanocrystals on excitation below the absorption edge has been discovered. The maximum spectral shift to the shortwave region relative to the excitation energy E _con ^max = 319 meV is obtained for meansized nanocrystals (2.5 nm). The conversion efficiency of the absorbed radiant energy is 1.3·10^–2%. The rise in the antiStokes photoluminescence intensity with increasing temperature and the linear dependence on the exciting radiation intensity have been established. It is shown that the effect observed cannot be caused by twophoton excitation or by Auger recombination. It is assumed that the basic mechanism of the luminescence observed is the radiative recombination through the energy levels of the states attributable to the disturbance of the crystal structure in the nanocrystals.     

X‐Ray Luminescence of Thin Oxide Films Containing Lead and Bismuth

We obtained thin films of the Bi₄Ge₃O₁₂, Bi₄Si₂O₁₂, PbWO₄, and Bi₂WO₆ scintillators and investigated their luminescence properties on xray excitation. We measured the xray luminescence spectra and established the linear character for the dependence of the luminescence intensity on the power of the dose of xray irradiation within the limits of up to 3·10^–3 A/kg. We investigated the radiation stability of the films obtained. The possibility of their application for detecting ionizing radiation is investigated. The films based on Bi₄Ge₃O₁₂ with an admixture of the Ge and Si or Mn activator of up to 1.5 mol.% are most suitable.     

Current induced resistance change of magnetic tunnel junctions with ultra-thin MgO tunnel barriers

Ultra-thin magnetic tunnel junctions with low resistive MgO tunnel barriers are prepared to examine their stability under large current stress. The devices show magnetoresistance ratios of up to 110% and an area resistance product of down to . If a large current is applied, a reversible resistance change is observed, which can be attributed to two different processes during stressing and one relaxation process afterwards. Here, we analyze the time dependence of the resistance and use a simple model to explain the observed behavior. The explanation is further supported by numerical fits to the data in order to quantify the timescales of the involved phenomena.     

Spectral-Luminescent Properties of Deposited Rhodamine 6G Films

We have investigated the absorption and fluorescence spectra of Rhodamine 6G thin (20–100 nm) films vacuum-deposited on solid dielectric substrates: quartz and borosilicate glasses and polymer films. By the absorption spectra several molecular absorption centers — monomers, dimers, and complex associates — have been revealed. For the first time luminescence of vacuum-deposited Rhodamine 6G films has been obtained and investigated. We classified it as fluorescence of plane dimers and dimers of the house-of-cards type and also as luminescence of complex associates. The dependence of the spectral composition of the films on the excitation wavelength in the 293–353 K temperature range has been studied. It has been proposed that amorphous Rhodamine 6G films obtained by the vacuum deposition method contain crystalline zones and microclusters of adsorbed water molecules.     

Characterization of Mn2+ Coordination States in ZnS Nanocrystal By EPR Spectroscopy and Related Photoluminescence Properties

Coordination states of Mn²⁺ inside and near the surface of ZnS:Mn nanocrystal (NC) (ca. 1.8 nm in particle radius) coated with poly(acrylic acid) (PAA) were examined by the detailed analysis of electron paramagnetic resonance (EPR). The symmetry of both Mn²⁺ sites inside and near the surface of NC is lower than that of submicron particles (0.125 m in particle radius), because of larger lattice distortion and larger zero field splitting constant. Temperature dependence of line width ( H _pp) of EPR signals from Mn²⁺ inside and near surface sites of ZnS:Mn disappears when the particle radius of ZnS:Mn decreases from 0.125 m to 1.8 nm. These indicate increasing extent of d–d transition and stronger interaction between Mn²⁺ and ZnS as well as between Mn²⁺ and PAA in NC, leading to more effective energy transfer from ZnS and PAA to Mn²⁺. These phenomena explain the high luminescence intensity of ZnS:Mn nanocrystal coated with PAA.     

PbWO4闪烁晶体的发光动力学模型

在对PbWO₄闪烁晶体的光谱特性、发光衰减及其温度依赖以及热释光的研究基础上,并结合理论计算,提出了PbWO₄晶体发光的动力学模型,给出了PbWO₄晶体的基本能带结构及激子发光中心能态、陷阱能级在能隙中的位置。用此模型可以完整说明PbWO₄的发光过程,特别是导致室温下发光效率低的原因。最后还对其主发射成分蓝、绿发光中心的起源作了简要讨论。     

A study of the photoluminescence of crystals of the hexagonal modification of Β-Ga2As3

The excitation and radiation spectra of-Ga₂As₃ crystals have been studied. The radiation spectrum consists of two overlapping bands (1.44 and 1.61 eV) and a very weak band in the 2.8–2.9 eV region. The temperature dependence of the halfwidth of the luminescence bands was studied and the position of the levels responsible for the luminescence were determined: E_1,v = 0.84 eV and E_2,v = 0.59 eV. These luminescence bands in -Ga₂As₃ crystals undergo thermal quenching. The kinetics of the photoconductivity and the photoluminescence were studied and it was shown that the recombination of nonequilibrium carriers occurs by two channels: via fast s levels and slow r levels, the trapping cross section of the latter being .Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 72–75, September, 1972.     

Pyroelectric detection of a CO2 laser beam using organic copolymers

Pyroelectric detection of a CO₂ laser beam has been investigated by using organic copolymers such as VDF/TrFE- and VDCN/VAc copolymers, and compared with that of PVDF polymer. The voltage responsitiviesR _v are 40 mV/W for VDF/TrFE and 2 mV/W for VDCN/VAc at room temperature and at a chopping frequency of 200 Hz. (For comparison, the value of PVDF is 20 mV/W.) From the frequency dependence of the detected output, the electric time constant _E is evaluated to be about 6.4 ms and 2.6 ms for the former and latter copolymers with the detection area 3×3 mm², respectively. Some theoretical discussion is also given of the responsivity, frequency dependence and temperature dependence obtained.     

Orientational dependence of luminescence yield excited by fast electrons in NaCl crystals

An investigation is made of the orientational dependence of the integral luminescence yield when NaCl(T1) single crystals are irradiated by electrons of energy 0.8–2.5 MeV in the direction 100. It is shown that the luminescence yield at small excitation densities depends linearly on the specific ionizational energy losses of the bombarding particles. The ionizational energy losses are calculated for the case when electrons of energy E=1.0 MeV pass through an NaCl single crystal in the direction 100. The minimum observed in the luminescence yield is due to electron capture in canalization conditions and decrease in energy losses. The maximum in the calculated curve at small angles of disorientation is due to the focusing of fast electrons by atomic chains.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 75–79, March, 1979.It remains to thank A. M. Taratin for performing the computer calculations and S. A. Vorob'ev for supervising the work.