Mechanisms of spin-lattice relaxation of vk and related centers

An experimental and theoretical investigation of the spin-lattice relaxation (SLR) mechanisms of V_K-centers in alkali and alkaline earth halides has been carried out. It has been shown that at low temperatures the main role in SLR is played by rapidly relaxing centers (RRC). In some cases, however, an intrinsic one-phonon SLR process due to modulation of both the isotropic and anisotropic hyperfine (HF) interaction seems to be important. At higher temperatures SLR can be explained by an anharmonic Raman process with the participation of the resonant molecular vibrational (RMV) mode of the V_K-center. In this SLR process, similarly to the low-temperature intrinsic process, the spin-phonon coupling is due to the modulation of the HF interaction. Deviations from the Debye phonon spectrum and the local anharmonicity of V_K-centers turn out to be important. The SLR of several other hole and interstitial type centers in alkali halides has also been investigated and their SLR mechanisms are discussed.     

Self-organization of nanostructures on the n-GaN(0001) surface in the Cs and Ba adsorption

A regular self-organized 2D nanostructure of a new type of nanocombs has been created in situ in an ultrahigh vacuum on the n-GaN(0001) surface. The nanostructure is formed as a result of multilayer adsorptions of Cs and Ba. The structure is highly regular in the microrange, arranged in the form of combs 60–70 nm in diameter with a wall height of about 7 nm. It has been revealed that the nanostructure has a quasi-metallic conduction, a low work function of about 1.4 eV, and a high quantum yield of photoemission under light excitation in the GaN transparency region. A self-organization model is proposed, which implies the formation of a surface 2D longperiod incommensurate phase interacting with the superstructure of the Cs⁺ and Ba²⁺ ion clusters with allowance for the polaron compensation on the GaN surface.     

Light-induced charge transfer and kinetics of the NIR absorption of Nb4+ polarons in SBN crystals at low temperatures

Sr_1-xBa_xNb₂O₆ (SBN) crystals with open tungsten-bronze structure show enhanced photorefractive properties with doping of impurities such as Ce, Cr, Rh etc. Under illumination with Kr⁺ laser (647 nm) or Ar⁺ laser light (488 nm or 514 nm) or UV light at low temperature, pure and doped SBN crystals show a broad polaron absorption band around 0.7 eV (6000 cm^-1). The first step of a theoretical model involves the excitation of electrons by illumination from Cr³⁺/Ce³⁺ to higher excited states or the conduction band. The excited electrons can then be trapped by Nb⁵⁺ to form Nb⁴⁺ polarons and further on can directly tunnel through or hop over the potential barrier (with a value Δ≈0.15±0.02 eV) to recombine with Cr⁴⁺/Ce⁴⁺ ions. The experimental intensity dependence, temperature dependence, and decay process of the light-induced Nb⁴⁺ polarons can be fitted with the help of this model. Small, but systematic, differences lead to the additional assumption of different recombination rates of polarons at distinct distances from the Cr⁴⁺/Ce⁴⁺ recombination centers and therefore many parallel decay channels are active where each decay channel obeys a monoexponential decay law. A stretched exponential decay function is employed to fit in this case the decay process of the Nb⁴⁺ polarons at different temperatures and under illumination with different intensities. Due to the high dielectric constant value (ε₃₃ and ε₁₁ have values in the 10²-10³ range) at low temperature, the long range Coulomb attraction (to Ce³⁺ _Sr/Ba) or repulsion (from Cr³⁺ _Nb) of the electronic polaron is suppressed. The leading role in the attraction and the following trapping of the electronic Jahn–Teller polaron, both on Cr³⁺ _Nb and Ce³⁺ _Sr/Ba centers, is played by the indirect dipole–dipole interaction via the soft TO-mode. Received: 27 November 1998 / Revised version: 22 January 1999 / Published online: 7 April 1999     

Electron paramagnetic resonance and optical spectroscopy of K3Na(CrO4)2 ferroelastics activated by MnO 4 2− molecular impurity ions

Crystals of a proper ferroelastic K₃Na(CrO₄)₂ containing molecular impurity ions MnO ₄ ^2? are studied using electron paramagnetic resonance (EPR) and optical spectroscopy. The EPR spectrum of the Mn⁶⁺ ion contained in the molecular impurity ion MnO ₄ ^2? is identified at low temperatures (T ≤ 20 K). The intensity of this spectrum decreases unusually fast as the temperature increases. A broad IR luminescence band with a vibronic structure well resolved at a temperature of 8 K is revealed. Theoretical treatment of the Mn⁶⁺ ion involved in the molecular impurity ions MnO ₄ ^2? of the K₃Na(CrO₄)₂ ferroelastic crystal suggests that an important role in this case is played by the pseudo-Jahn-Teller. The pseudo-Jahn-Teller effect offers an explanation for the appearance of a fine structure in the vibronic replicas in the luminescence spectrum, on the one hand, and accounts for the fast decrease in the intensity of the EPR spectrum of K₃Na(CrO₄)₂: MnO ₄ ^2? with increasing temperature, on the other.     

Photoluminescence and photoinduced charge transfer in KTaO3:Cr,Li

Kinetics and spectral composition of the luminescence of a KTaO₃:0.1%Cr,10%Li sample (molar charge percentages) have been studied within a broad temperature range (2–300 K). The luminescence spectra of KTaO₃:0.1%Cr,10%Li have been found to have a strong narrow R-line characteristic of Cr³⁺ ions, with a clearly pronounced vibronic wing. Whereas the R-line width in Li-codoped samples is only higher by 120% than that of crystals doped only with chromium, the R-luminescence intensity in the presence of lithium increases by several orders of magnitude. It is shown that the luminescence excitation, besides the intracenter mechanism, is dominated by a process involving photoinduced charge transfer, which results in specific kinetics of the photoluminescence. The luminescence excitation model considered in the work is based, in particular, on the substantial role played in this process by electronic polaron capture by the Li⁺ dipole center caused by the existence of a characteristic shallow electron trap. Fiz. Tverd. Tela (St. Petersburg) 39, 2172–2178 (December 1997)     

Vibronic Frenkel exciton in a soft-mode crystal

The pair electron-hole Jahn-Teller effect, which is realized on degenerate states of a vibronic Frenkel exciton (VFE) in soft-mode crystals, is considered. It is shown that the vibronic effect in a VFE can be significantly enhanced in crystals having soft lattice modes. The occurrence of strong local order-disorder fluctuations in this situation is predicted. The manifestations of VFEs localized at impurities in soft matrices (Cr³⁺ in SrTiO₃, Fe⁵⁺ in KTaO₃, and Mn⁵⁺ in TbVO₄ and TbPO₄) are considered. The possibility of fluctuation effects related to VFEs in nominally pure soft-mode crystals (SrTiO₃ and BaTiO₃) is discussed.     

Oscillations in the threshold photoemission spectra of GaN(0001) with submonolayer Cs coverages

It is found that Cs adsorption on the n-type GaN(0001) surface generates an unusual change in the electronic properties of the surface and the near-surface space-charge layer, which leads to the appearance of photoelectron emission upon excitation in the transparent region of GaN. It is established that the photoemission is due to the formation of quasimetallic states induced by Cs adsorption in the band-bending region near the surface. The behavior of the photoemission threshold upon excitation by s-polarized light is studied as a function of the Cs coverage. It is found that the minimum value of the threshold corresponds to ～1.4 eV at a concentration of Cs atoms of ～4.5×10¹⁴ atom/cm² in the submonolayer coverage. A new effect is revealed, namely, the appearance of oscillations in the spectral curves of threshold photoemission. A model is proposed for photocurrent oscillations that takes into account the formation of quasimetallic states in the near-surface layer of GaN band bending and the occurrence of interference in the GaN slab upon light irradiation in the transparent region.     

New type charge transfer states in ferroelectric oxides: Actual problems

Brief review of recent theoretical and experimental studies of Charge Transfer Vibronic Excitons and induced by defects, charge transfer - lattice ground states in ferroelectric oxides and analysis of actual problems is presented. Predictions of new effects related to such charge transfer states are made.     

Charge transfer vibronic excitons: Charge-transfer lattice-instability effects

Two aspects for investigating bipolaronic excitons [charge transfer vibronic exciton (CTVE)] in ionic-covalent solids are considered. These aspects are predictions of the charge-transfer lattice instability corresponding to charge transfer and lattice anharmonicity, as well as to repulsion between different types of CTVEs in the order parameter field. Oxide crystals (SrTiO₃, BaTiO₃, K₃Na(CrO₄)₂) are considered as an example.