Analyses of the thermal shifts of spectral lines in Nd3+-doped LiYF4 laser crystal

The red-shift of spectral line E ₁(R ₁????Y ₂) and blue-shift of line E ₂(R ₁????X ₅) with temperature in Nd³⁺-doped LiYF₄ laser crystal are studied by considering both the static contribution due to lattice thermal expansion and vibrational contribution due to electron?Cphonon interaction. The study is based on the analyses of pressure and temperature dependences of these spectral lines. It is found that for both lines, the static and vibrational contributions result in the blue- and red-shift, respectively. So, the observed red-shift of line E ₁ and blue-shift of line E ₂ are due respectively to the static contribution being smaller and larger than the vibrational one. Also, we infer that the thermal shifts of lines E ₃(R ₁????Y ₅) and E ₄(R ₂????Y ₅) are very small because both contributions may be approximately canceled. When both the contributions are contained, whether the red-shift or blue-shift of a spectral line can be fitted with the almost same theoretical expression as that by including only the vibrational contribution used in red-shift in the previous papers if we change the expression concerning electron?Cphonon interaction coefficient.     

Polarization-modulated magnetoreflection in magnetic semiconductors: CdCr2S4 and EuSe

The polarization-modulated (PM) magnetoreflection technique has been used to study magnetic ordering effects in CdCr₂S₄ and EuSe. The energy gap for direct band-band transitions in CdCr₂S₄ has been measured to be E_G = 2.3 eV, and the exchange splitting of the valence band to be about 0.03 eV. Good agreement with thin film absorption measurements is obtained in the temperature dependence of spectral structure, observed at energies less than E_G, associated with crystal field and charge transfer transitions. No strongly blue-shifting peak is observed with magnetic order. PM magnetoreflection spectra of the E₁ peak of EuSe show a direct manifestation of the spin alignment from the ferrimagnetic to ferromagnetic state of this crystal at 2 K in external fields up to 16 kOe. An attempt is made to explain resonant Raman scattering in EuSe (observed by other workers) in terms of the field-induced shift of one of the polarized E₁ reflectivity components into coincidence with the 5145 Å argon-ion laser line.     

Photoluminescence studies of exciton-ionized donor complexes in high pusity epitaxial GaAs

We report the first observation of three radiative transitions associated with excitons bound to three different residual ionized donors in high purity undoped vapor phase epitaxial (VPE) GaAs at liquid helium temperature. The values of the localization energies (E_l) of excitons bounds to these ionized donors were measured. We also determine simultaneously, the ionization energies of these donors using excited state transitions of exciton-neutral donor complexes as reported earlier. The variation of the localization energy (E_l) as a function of the donor binding energy (E_D) is plotted and a linear dependence described by E_l = ? 2.22 + 0.72E_D is observed.     

Absorption spectrum of the Mn2+ ions in crystals of lanthanum manganese double nitrate and praseodymium manganese double nitrate

The optical absorption spectrum of the Mn²⁺ ions in the two double nitrates La₂Mn₃(NO₃)₁₂ ? 24H₂O and Pr₂Mn₃(NO₃)₁₂ ? 24H₂O were measured up to 30000 cm^?1. Uniaxial stress was applied to the crystals to determine the multiplicity of the observed zero phonon lines belonging to the transitions from the groundstate⁶ A ₁ (⁶ S) to the excited states⁴ E(⁴ G) and⁴ T ₂(⁴ D). The Zeeman effect of the states⁴ A ₁(⁴ G) and⁴ E(⁴ G) was measured with magnetic fields up to 6.4T. The effect of covalency and trigonal field, especially to the states⁴ A ₁ (⁴ G) and⁴ E(⁴ G), is briefly discussed.     

Resonant two-photon absorption in the Cr2O3 antiferromagnet

Two-photon absorption (TPA) spectra of the Cr₂O₃ model antiferromagnet have been studied for two polarization configurations at energies ranging from 2.5 to 3.55 eV. Several strong TPA peaks with the maximum coefficient β ～ 0.08 cm/MW are observed in the 2.7–3.1-eV range characterized by the ⁴ A ₂(F) → ⁴ T ₁(F) d-d one-photon transition of the Cr³⁺ ion. An analysis of the results obtained suggests that the general form of the TPA spectrum in this range is primarily determined by the resonance at the ² E(G) and ² T ₁(G) intermediate levels. At energies above 3.44 eV, the TPA coefficient sharply increases up to β ～ 0.1 cm/MW due to transitions between the valence and conduction bands.     

Resonance structure of strength functions for first-forbidden β +/EC transitions

Experimental data obtained by measuring the fine structure of the strength function S _?? (E) in spherical and deformed nuclei were analyzed. The use of modern nuclear-spectroscopy methods made it possible to reveal the nuclear-deformation-induced splitting of peaks in S _?? (E) for transitions of the Gamow-Teller type. For first-forbidden transitions, the resonance nature of S _?? (E) was proven experimentally both for spherical and for deformed nuclei. It is shown that, at some values of the excitation energy, the intensity of first-forbidden transitions in nuclei can be commensurate with the intensity of Gamow-Teller transitions.     

Phase transitions in orthorhombic oxyfluoride (NH4)2MoO2F4

(NH₄)₂MoO₂F₄ single crystals were grown and studied using polarization-optical methods, and the birefringence was measured in the temperature range 90–350 K. The following sequence of phase transitions is revealed: G ₀ ? G ₁ ? G ₂. It is established that the phase transition at T ₀₁ ≈ 267 K is of the first order and exhibits thermal hysteresis δT ₀₁ ≈ 0.6 K. A weak anomaly is found in Δn(T) at T ₀₂ ≈ 180 K. The crystals are shown to retain the orthorhombic symmetry during the phase transitions.     

I.R.-Absorption peculiarities of GaP:Fe crystals irradiated by electrons

Using photo-ionization transitions theory by Allen and Langer the existence of two kinds of transitions was demonstrated for GaP:Fe crystals, namely, transition from valence band to local level (E_{2 = 1.27eV}). The effect of that form local level to conduction band (E_{2 = 1.27eV}). The effect of electron irradiation (E_{el = 3 MeV}) on both intracenter absorption and absorption in the 0.7?1.6 eV spectral region was investigated and the conclusion was made that absorption E₁ is probably due to Fe²⁺+e → Fe³⁺ photoionization transition while E₂-absorption is due to Fe³⁺ → Fe⁴⁺ +e transition.     

Nuclear excitation and prompt fission in muonic238U

A study of muonic²³⁸U has been performed in a combined (μ ^?,γ f) and (μ ^?,γγ) coincidence experiment to investigate the role of non-radiative transitions and their fission probabilities. An augmentation of the outer fission barrier ofΔE _b =(0.6±0.1) MeV due to the presence of the muon is deduced. A significant contribution to the prompt fission yield not only results from the (2p→1s) and (3d→1s) non-radiative transitions, but also from other radiationless transitions. Specifically, the measured fission probabilities of the transitions (2p→1s), (3d→1s), and (3p→1s) are (1.5±0.4)%, (5.7±1.7)%, and (5.3±1.9)%, respectively.     

Electric susceptibility of ethyl chloride in the centimetre region

Electric susceptibility of ethyl chloride has been determined in the microwave region at 8,780 mc/s over a pressure range of 10 cm to 75 cm of mercury, and a temperature range of 263° K to 353° K. It has been found that the total susceptibility, at constant temperature, varies almost linearly with pressure. The contribution due to quantised transitions to total susceptibility has been calculated from the data on absorption measurements and it is found to be 0·45 × 10^?3. Since most of this contribution is due to zero frequency transitions, no measurable dispersion is observed in this region. It has been found possible to determine the values of dipole moment and the sum of atomic and electronic polarisations from the data on temperature variation of susceptibility. The values are μ=2·06 × 10^?18 e.s.u and (P _A +P _E )=22·70. These results are in fair agreement with those obtained earlier     

Spin density functional theory of the temperature-dependent spin susceptibility: Pd and Pt

The finite temperature spin density functional (SDF) formalism is used to derive a variational expression for the temperature-dependent spin susceptibility χ_p(T) of an inhomogeneous electron gas. The use of a simple trial function in the variational expression results in a Stoner form for χ_p(T), i.e., χ_p(T) ≥ χ_s(T)/[1 - I(T) χ_s (T)] where χ_s(T) is the single-particle spin susceptibility including exchange-correlation (XC) effects on the band structure within the framework of the density functional formalism and I(T) is the SDF analog of the Stoner parameter. It should be emphasized that this form for χ_p(T) is derived for a general XC free energy functional F_xc[n, m; T]. χ_s(T) has been calculated self-consistently, including relativi stic effects, using the local approximation for F_xc for Pd and Pt. These results have been used to investigate the temperature dependence of I(T) required to explain their experimental susceptibilities. It is found that when the spin-orbit interaction is included in the calculation of χ_s(T), the temperature variation of I(T) is stronger than that of χ_s(T).     

Electron and nuclear magnetic resonances associated with magnetoelectric and antiferroelectric interactions in the exchange-noncollinear (“square”) chiral antiferromagnet Nd<Subscript>2</Subscript>CuO<Subscript>4</Subscript> and their behavior in chiral phase transitions

Experiments with the tetragonal antiferromagnet Nd₂CuO₄ in the temperature range 1.5 K < T < T _N = 245 K show that the magnetic moments of Cu²⁺ possess an exchange-noncollinear magnetic structure of the “square” type, which has the form of an exchange doublet whose components exhibit different chiralities (Γ₄ and Γ₅ phases). Between these phases, consecutive phase transitions Γ₄ ? Γ₅ ? Γ₄ with a change in chirality take place at temperatures T₁ = 30 K and T₂ = 70 K. The electron and nuclear magnetic resonances (natural frequencies and susceptibilities) associated with excitation of magnons (due to the magnetoelectric and antiferroelectric interactions) by an ac electric field E(t), as well as a variable magnetic field H(t) applied in the case of a constant electric field E⁰, are calculated. It is predicted that nuclear magnetic resonance is excited by an ac electric field at frequencies determined by hyperfine fields of the sublattices. The change in the resonance frequencies upon the above chiral phase transitions are analyzed (being first-order phase transitions, these transitions possess a number of features associated with the chirality of the magnetic structures).     

Magnetic susceptibility of La<Subscript>2</Subscript>NiO<Subscript>4+δ</Subscript> single crystals with δ = 0.095 and 0.105. Ordering of holes and ferromagnetic heisenberg chains

The temperature dependence of the magnetic susceptibility of La₂NiO_4+δ single crystals with δ = 0.095 and 0.105 has been analyzed. The anomalous dependence of χ(T) characteristic of stripe structures previously found in La_1.8Sr_0.2NiO₄ has been found. Investigation of the dependence χ(T) on the impurity and charge disorder degrees makes it possible to separate the contribution associated with the ordering of holes and impurity oxygen. It has been shown that the contribution from the ordering of holes is well described by the model of ferromagnetic Heisenberg chains of spins S = 1/2. The length of the chains and intrachain interaction J _N have been determined from the temperature dependence χ(T).     

Features of the structure and dynamics of Me1−x (NH4) x SCN (Me = K, Rb) mixed crystals

Neutron scattering is used to study the structure and dynamics of Me_{1 − x} (NH₄) _x SCN (Me = K, Rb) mixed crystals along the concentration section of 0.0 < x < 1.0 at room temperature 10 and 290 K. Phase transitions in Me_{1 − x} (NH₄) _x SCN mixed crystals are analyzed by neutron powder diffraction. The measured spectra of inelastic incoherent neutron scattering from mixed crystals in a concentration range of 0.0 < x < 1.0 at 10 are transformed into the generalized phonon density of states G(E) in the one-phonon incoherent approximation. Using G(E), we determine the changes in ammonium ion dynamics during phase transitions. Low energy resonance and local translational (two bands) and librational (two bands) modes are observed in the disordered rhombic phase at 10 K. The low energy resonance mode is not found in the ordered monoclinic phase at 10 K, though the local translational mode in the form of two bands and the local librational mode in the form of four bands are present there. The low energy resonance mode appears due to hybridization of the phonon spectrum of the host crystal with rotational tunneling modes of the split librational ground state of the impurity’s molecular ammonium ion.     

Fluctuations of orientational order in a uniaxial nematic liquid crystal with biaxial molecules and its response to an external field

Homogeneous thermal fluctuations of the orientational order parameters S and G of biaxial molecules in a uniaxial nematic liquid crystal are investigated in the framework of the molecular-statistical theory. It is demonstrated that the molecular biaxiality significantly affects the order parameters S and G, their temperature dependences in the nematic phase, the amplitude and the temperature dependence of the order parameter fluctuations in the nematic and isotropic phases, and the character of the transition from the nematic phase to the isotropic liquid phase. It is established that the fluctuations of the parameters S and G in the nematic phase are related to the temperature dependences of S and G and the susceptibilities χ_S and χ_G of the nematic liquid crystal to external fields, which leads to a change in the parameters S and G at a fixed director orientation. Explanations are offered for the known experimental data on the orientational ordering of biaxial molecules under the action of external fields in the isotropic phase of nematic liquid crystals.     

Fundamental reflectivity spectra of Hg1−xMnxTe mixed crystals in the 1.7 to 3.5 eV energy range

The room temperature reflectivity coefficient R(E) for the mixed crystals Hg_1?xMn_xTe (x up to 0.57) in the energy range 1.7 to 3.5 eV was investigated. Two distinct maxima E₁ and E₁ + Δ₁ connected with the transitions in the critical points on the [111] direction of the Brillouin zone for the samples with x up to 0.3 and the more diffused structure of R(E) for the samples with x > 0.3 was observed. A quadratic dependence of E₁ and E₁ + Δ₁ transition energy vs alloy composition with x up to 0.3 was found, with bowing coefficient c = 1.21 ± 0.02 and 1.06 ± 0.02 respectively. The energy variation of an additional shoulder probably connected with the e₁ transitions at L point of the Brillouin zone is also reported.     

Magnetic behaviour of U(MnxAl1−x)2 compounds

The results of magnetic measurements performed on U(Mn_xAl_1−x)₂ compounds in the temperature range 4.2K < T < 800K are reported. In the low temperature range (T < 200K), UMn₂ shows a Pauli-type paramagnetism. Above 420K a Curie-Weiss behaviour is evidenced. The magnetic properties of U(Mn_xAl_1−x)₂ compounds were analysed assuming a superposition of a temperature dependent term on a Pauli-type contribution, χ_O. The effective moments as well as the χ_O values were determined both in the low (T < 200K) and high (T > 420K) temperature range. The experimental data were discussed considering changes in the band structure and/or quenching of spin fluctuations.     

ARC spectra from the reaction 115In(n, γ) and temperature dependence of the width of the giant magnetic resonance

The resonance-averaged spectra of primary γ transitions in the reaction ¹¹⁵In(n, γ) are analyzed at the average neutron energies of E _n=1.9, 24.3, and 134 keV. The temperature dependence of the width of the giant magnetic resonance is found by parametrizing the observed intensities of M1 transitions.