Electro-optic behavior of lithium niobate at cryogenic temperatures

The unclamped relative permittivity, , and the Pockels coefficient, , of congruent lithium niobate at a frequency f = 5760 Hz have been determined at low temperatures (7 K < T < 300 K). A He cryostat setup mounted to one arm of an electronically phase-stabilized Michelson interferometer was utilized for the measurement of . A continuous decrease in both parameters was observed as T → 0 K with limiting values of and , respectively.     

First-principles study on the formation energies of intrinsic defects in LiNbO3

First-principles plane-wave ultrasoft pseudopotential method within local density approach (LDA) has been used to study three possible vacancy-defect models for non-stoichiometric lithium niobate (LiNbO₃): (1) the oxygen-vacancy model , (2) the niobium-vacancy model , and (3) the lithium-vacancy model . The corresponding formation energies are obtained via energy minimization of a supercell. In Nb-rich environment, the calculated defect formation energies, both under oxidation and reduction conditions, show little effect on the intrinsic defect structures. We find that the lithium vacancy model has the most stable configuration in the non-stoichiometric lithium niobate crystals. Our calculations also show that the formation of any type of neutral defects and Frenkel pairs in a Nb-rich environment is difficult.     

Effect of high temperature treatments on defect centers and impurities in hydrothermally grown ZnO

Deep level transient spectroscopy (DLTS) has been employed to study electron traps in hydrothermally grown n-type ZnO samples after thermal treatments up to 1500 °C. Schottky barrier contacts were formed by e-beam evaporation of Pd, followed by DLTS and secondary ion mass spectrometry (SIMS) measurements in order to investigate possible correlations between electron traps in the upper part of the band gap and the concentration of the most prominent impurities. The DLTS results show three different levels having energy positions of , , and (E_c denotes the conduction band edge). The SIMS results showed that the most pronounced impurities were Li, Al, Si, Mg, Fe, Mn, and Ni with concentrations up to . A decrease in the level is observed after temperature treatments above 1300 °C, and in the same temperature range the Li concentration drops from ∼10¹⁷ to . However, based on absolute concentration values an association between Li and the level can be ruled out. In contrast, the level, which is not stable above 1300 °C, may be associated with Li but further experimental data are needed to substantiate this assignment. The level occurred in selected samples and is presumably impurity-related but no correlation was found with the main impurities detected by SIMS. Except for Li, the concentration of all the impurities remained essentially constant as a function of heat treatment temperature.     

First-principles study of arsenic impurity clusters in molecular beam epitaxy (MBE) grown HgCdTe

The understanding of the microstructures of the arsenic tetramer , dimer , and singlet of HgCdTe is important to explain the high electrical compensation of molecular beam epitaxy (MBE) samples and the conversion to p-type behavior. The stable configurations were obtained from the first-principles calculations for the arsenic cluster defects [ (n=1, 2, and 4)] in as-grown HgCdTe. According to the defect formation energies calculated under Te-rich conditions, the most probable configurations of , , and have been established. For the optimized and the energy is favorable to combine in a nearest neighboring mercury vacancy , and the corresponding configurations can be used to explain the self-compensated n-type characteristics in as-grown materials. is likely to be more abundant than in as-grown materials, but arsenic atoms are more strongly bounded in than in , thus more substantial activation energy is needed for than that for . The atomic relaxations as well as the structural stability of the arsenic defects have also been investigated.     

Measurements of insulator band parameters using combination of single-electron and two-electron spectroscopy

We describe the application of low energy time-of-flight coincidence (e,2e) spectroscopy for measurements of the energy band parameters of a dielectric. The (e,2e) spectrometer can operate also in a single-electron mode by switching off coincidence conditions, and can be used for recording electron energy loss spectra (EELS). Thus, the combination of (e,2e) and EELS allows the measurement of energy gap E_g, valence bandwidth ΔE_val, electron affinity χ and excitonic levels position E_ex of a dielectric. The energy band parameters of LiF film deposited on Si(001) surface are measured: ΔE_val=      

Rotational spectrum of HCBr produced by 193-nm laser photolysis of bromoform

The pure rotational spectrum of bromomethylene (HCBr) was studied by kinetic microwave spectroscopy between 420 and 472 GHz. The HCBr radical was produced by 193-nm ArF laser photolysis of bromoform (CHBr₃). More than 130 rotational transitions for both and species in the ground vibrational state were measured involving 1?J?33 and 0?K_a?5. The spectra were well described by an S-reduced Watson Hamiltonian in the I^r representation including the nuclear quadrupole and spin-rotation hyperfine terms. Rotational, centrifugal distortion, nuclear quadrupole and spin-rotation coupling constants were derived for both and species in the ground vibrational state.     

Problems with power content beam parameter product

The M² beam propagation factor is widely used to characterize the quality of laser radiation and its propagation. When M² is defined by the second-moments, M² ? 1 holds in the paraxial approach. For many applications it is more convenient to use the power content values (normally η = 86.5%), also proposed by ISO. For the corresponding power content factor, it is often assumed that also holds. We have demonstrated previously that for a superposition of two coherent Gauss-Laguerre modes with radial symmetry, the 86.5% value of [6]. In recent years, has also been presented experimentally for a superposition of axially shifted Gaussian beams [7]. The problems with power content for axial superposition of Gaussian beams are discussed. In this paper it is shown that the 86.5% power content value can not be smaller than one for a coherent superposition of axially shifted Gaussian beams with radial symmetry presented in Ref. [7]. A superposition of two Gaussian beams with different waists and without shift is also discussed, and the corresponding of such beam can be smaller than one, depending strongly on the power content value η. For low power content values η and a large (or very small) ratio of the two different waists approaches zero. These investigations demonstrate that is not a suitable parameter to characterize laser radiation.     

Reverse Monte Carlo simulations and Raman scattering of an amorphous GeSe4 alloy produced by mechanical alloying

The short and intermediate range order of an amorphous GeSe₄ alloy produced by Mechanical Alloying were studied by Reverse Monte Carlo simulations of its X-ray total structure factor and Raman scattering. The simulations were used to compute the , and partial distribution functions and the , and partial structure factors. We calculated the coordination numbers and interatomic distances for the first and second neighbors. The data obtained indicate that the structure of the alloy has important differences when compared to alloys prepared by other techniques. There are a high number of Se-Se pairs in the first shell, and some of the tetrahedral units formed seemed to be connected by Se-Se bridges.     

From special unitary groups SU(2I+1) to nuclear spin statistical weights

Following a suggestion of S.R. Polo, Galbraith [J. Chem. Phys. 68 (1978) 1677] was the first to obtain nuclear spin statistical weights for molecules of the type , , and ) starting from special unitary groups. In the present contribution, an analogous but simpler method is presented allowing the calculation of nuclear spin statistical weights of rigid molecules belonging to all important molecular point groups. The molecules under consideration may contain more than one set of identical nuclei with spins up to 3. The method proposed can easily be extended to nonrigid molecules.     

Near-infrared diode laser spectroscopy of the HCSi radical

The , , and band spectra of HCSi radical were investigated by means of near-infrared diode laser spectroscopy to determine precise molecular constants for the and states. The detailed analysis of the rotationally resolved band spectra, studied for the first time in the present investigation, leads to the precise determination of molecular constants for the state associated with the Renner-Teller interaction. We obtained −0.15126663(53) and 495.00698(30) cm⁻¹ as the Renner-Teller parameter ε and the bending vibrational frequency ω₂, respectively. Based on the molecular constants for the and states, the rotational levels of the state were analyzed to obtain molecular constants and information on upper state perturbations. Using the available spectroscopic data, valence force fields for both the and states were estimated to aid in understanding the vibrational energy levels of the HCSi radical.     

Ground state electronic and magnetic properties of RCrSb3 (R=La, Ce, Nd, Gd and Dy): A first principles study

The electronic density of states (DOS) and magnetic moments of rare-earth antimonides (RCrSb₃) have been studied by the first principles full-potential linearized augmented plane wave (FP-LAPW) method based on density functional theory (DFT). For the exchange-correlation potential, the LSDA+U method is used. The effective moments of LaCrSb₃, CeCrSb₃, NdCrSb₃, GdCrSb₃, and DyCrSb₃ were found to be , , , and respectively. The exchange-splittings of Cr-3d state electrons and 4f-states of rare earth elements were analyzed to explain the magnetic nature of these systems. The Cr atom plays a significant role on the magnetic properties due to the hybridization between Cr-3d and Sb-5p state orbitals. The results obtained are compared and found to be in close agreement with the available data.