Pressure and temperature induced modification of luminescence from tetracene single crystals

Fluorescence spectra of crystalline tetracene have been recorded in the temperature range 120 to 300 K under hydrostatic pressure up to 600 MPa. From discontinuities in both emission spectra and spectral intensities it is concluded that two phase transitions occur. The room temperature phase is transformed to a low temperature phase/high pressure phase I at T^I_t (p = 0) = 182 K, the temperature coefficient being dT^I_t/dp = 0.395 K/MPa. The phase transition is induced by a decrease of the specific volume under pressure and/or upon cooling. Lack of a significant shift of the origin of the fluorescence band near T^I_t at constant pressure is an artifact resulting from the neglect of reabsorption effects. The Stokes shift is 260 cm^-1, independent of temperature and crystal modification. In accord with previous Raman data a second phase transition occurs at T^II_t (p) = 143 K, the pressure shift being dT^II_t/dp = 0.088 K/MPa.In addition, the shift of the triplet energy as a function of pressure as well as the pressure-dependence of the rate constants governing fission of a singlet exciton into a pair of triplets is discussed utilizing their magnetic field dependences.     

Muon Knight shift in palladium

The Knight shift at positive muons implanted in pure palladium has been measured as a function of temperature from 19.8 to 883 K. The Knight shift variation is strictly proportional to the Pd magnetic susceptibility with ΔK^μ/Δ_x=-(0.43±0.02) mole/emu=-(2.39±0.11)kG/μ_B. A temperature independent term K^μ(x=0)=+45±10 ppm is found. The results are discussed in terms of the electronic structure of H in Pd.     

Broadening and shift of neutral chromium absorption lines by various perturbing gases

The broadening and shift of the absorption lines (a⁷S-z⁷P°, a⁷S-y⁷P° and a⁵S-z⁵P°) of the neutral chromium atom were observed at various temperatures (1500–1800 K) and densities (0.24-3.2x 10¹⁹/cm³) of foreign perturbers (He, Ar, N₂, H₂). The dependence of width and shift on temperature and density was studied. The collisional broadening and shift rates were determined from the profile measurements. The results are interpreted in terms of the Lindholm-Foley interaction potential between radiating atom and perturbers. Semi-quantitative agreement of theory and experiment was found in some cases for the temperature dependence of width and shift and for the differences of attractive constants ΔC₆ between upper and lower states of chromium lines.     

Phonon spectrum and interaction between nanotubes in single-walled carbon nanotube bundles at high pressures and temperatures

The Raman spectra of single-walled carbon nanotubes at temperatures up to 730 K and pressures up to 7 GPa have been measured. The behavior of phonon modes and the interaction between nanotubes in bundles have been studied. It has been found that the temperature shift of the vibrational G mode is completely reversible, whereas the temperature shift of radial breathing modes is partially irreversible and the softening of the modes and narrowing of phonon bands are observed. The temperature shift and softening of radial breathing modes are also observed when samples are irradiated by laser radiation with a power density of 6.5 kW/mm². The dependence of the relative frequency Ω/Ω₀ for G ⁺ and G ^? phonon modes on the relative change A ₀/A in the triangular lattice constant of bundles of nanotubes calculated using the thermal expansion coefficient and compressibility coefficient of nanotube bundles shows that the temperature shift of the G mode is determined by the softening of the C-C bond in nanotubes. An increase in the equilibrium distances between nanotubes at the breaking of random covalent C-C bonds between nanotubes in bundles of nanotubes is in my opinion the main reason for the softening of the radial breathing modes.     

Mössbauer study of Fe57 and Co57 in Co1 + xV2−xO4

Mössbauer sources and absorbers, prepared by doping Fe⁵⁷ and radioactive Co⁵⁷ into samples of Co_1?xV_2?xO₄ (0 ? x ? 1), were studied in the temperature range of 80–500 K. The source and absorber spectra are very similar. The absence of any Fe²⁺ at the A site can be understood by partial covalent bond formation with an anion. However, the predominance of Fe³⁺ at the B site (with some Fe²⁺ for x = 1) cannot be explained by simple crystal-field or molecular-orbital theories. The x dependence of the isomer shift and of the Fe³⁺B-site quadrupole interaction can be related to changes in the lattice constant and the oxygen parameter. The temperature dependence of the Fe²⁺B-site quadrupole interaction can be fitted in the motional-averaging model. In the range of 0 ? x ? 0.5 the temperature dependence of the isomer shift shows effects of chemical bonding beyond the second-order Doppler shift.     

Raman study of charge-density-wave phase transition in quasi-one-dimensional conductor (TaSe4)2I

Polarized Raman spectra were obtained in the quasi-one-dimensional conductor (TaSe₄)₂I above and below the charge-density-wave (CDW) transition temperature (T_c=263 K). The Raman intensities of many peaks become intenser and two of the phonon peaks shift to higher frequency with decreasing temperature. Moreover a new broad peak at about 90 cm^?1 and a new peak around 166 cm^?1 appear in the low-temperature phase. The polarization characteristic shows that the former is assigned to totally symmetric mode. The damping constant of the phonon at 90 cm^?1 increases markedly with increasing temperature. The frequency shifts to higher frequency as the temperature increases and the coupling coefficient is approximately proportional to (T_c?T)^{$\text{1}\text{2}$}. This peak becomes Raman active owing to the CDW phase transition. The temperature dependence of the damping constant and the frequency shift may have a relation to the dynamical properties of the CDW phase transition.     

Effect of quantum fluctuations and isotopic substitution on the curie temperature of BaTiO3, PbTiO3, and KNbO3

The Hartree-Fock formalism and density functional theory are used in first-principles calculations of the Landau-Ginzburg free-energy expansion coefficients in the Devonshire-Barrett one-ion model for BaTiO₃, PbTiO₃, and KNbO₃. The Curie temperature T ₀ and the Curie-Weiss constant are calculated. The effect of quantum fluctuations on T ₀ is considered in the approximation of the first quantum correction to the free energy. The quantum shift of the Curie temperature ΔT ₀ ^quant for BaTiO₃, PbTiO₃, and KNbO₃ and the isotopic shift ΔT ₀ ^iso for barium titanate due to the substitution ⁴⁸Ti → ⁴⁶Ti or ⁴⁸Ti → ⁵⁰Ti are calculated.     

Analysis of the Mössbauer gamma-ray energy shift of 57Fe in natural iron

The temperature and pressure dependence of the Mössbauer γ-ray energy shift of ⁵⁷Fe in natural iron, has been analysed taking into account both the second order Doppler shift (S.O.D.) and the isomer shift (I.S.) contributions. The variation of the S.O.D. both with temperature and pressure, is calculated from the experimental phonon frequency distribution and that of the I.S. has been evaluated using the scaling with volume of the direct 4s electron density, |ψ_4s(O)|², as well as the indirect 3s electron density, |ψ_3s(O)|², at the nucleus; and including the electron transfer from the 4s to 3d band. Data from pressure stuides have been utilized to evaluate two parameters; the proportionality constant between the I.S. and |ψ(O)|² and the number of electrons transferred with change of volume from the 4s to 3d band. It appears that in the case of a magnetic lattice such as iron, the temperature dependence of the S.O.D. calculated from the phonon distribution is inadequate to completely explain the observed energy shift, that instead a contribution arising from magnetization seems important; as is to be expected theoretically. On including a term proportional to the magnetizations, the calculated temperature variation of the I.S. compares well with that determined by us from the pressure studies of the γ-ray energy shift.     

67Zn Mössbauer study of lattice-dynamical effects and hyperfine interactions in ZnF2

Using the high-resolution 93.3-keV transition in⁶⁷Zn we studied the temperature dependence of the Lamb-Mössbauer factor (LMF), the center shift, and the quadrupole interaction in polycrystalline ZnF₂ in the temperature range between 4.2 and 55.3 K. The LMF is anisotropic within the whole temperature range. The electric quadrupole tensor characterized byeV _zz Q/h=+(8.2±0.1) MHz and η=(0.15±0.02) is independent of temperature. The results on the isomer shift (relative to ZnO) and on the electric quadrupole tensor are in very good agreement with recent theoretical cluster calculations.     

Pressure broadening studies on the 33D1-2 3P0 helium line by saturated-absorption techniques

Broadening and shift measurements on the 3³D₁?2³P₀ component of the helium line at λ=587.5 nm are reported. The saturated-absorption method is used. Measurements are performed at room temperature and at liquid nitrogen temperature. Homogeneous broadening constants are deduced from an analysis taking into account the effect of weak velocity-changing collisions. Broadening and shift constants are interpreted in term of a Lennard-Jones interaction potential. The very small shifts observed show the predominance of the repulsive core effect in the interaction potential.     

27Al pulsed nuclear magnetic resonance study of CeAl2

The magnetic properties and the electronic structures of a rare-earth aluminum intermetallic compound CeAl₂ are investigated by magnetic susceptibility measurements and ²⁷Al pulsed nuclear magnetic resonance (NMR) techniques. The magnetic susceptibility is strongly temperature-dependent, following a Curie–Weiss law down to ∼12 K, and shows an antiferromagnetic transition at 4 K. The ²⁷Al NMR spectra show a typical powder pattern for a nuclear spin I of 5/2 with the second-order nuclear quadrupole interaction at high temperature and an additional large dipolar broadening between the 4f electron spins of cerium and the ²⁷Al nuclear spins at low temperature. The ²⁷Al NMR Knight shift follows the same temperature dependence as the magnetic susceptibility, suggesting that the ²⁷Al NMR Knight shift originates from the transferred hyperfine field of the Ce 4f electron spins with the hyperfine coupling constant of A = +5.7 kOe/μ_B. The spin-lattice relaxation rate 1/T₁ is roughly proportional to temperature, as with most non-magnetic metals at high temperature, and then strongly temperature-dependent, increasing rapidly with a peak near the antiferromagnetic transition temperature and decreasing at lower temperature. The temperature dependence of the Korringa ratio K, however, suggests that the antiferromagnetic spin fluctuation signature, which is an enhancement in the Korringa ratio, is washed out owing to the geometrical cancellation of Ce 4f fluctuations at the Al sites.     

Optical absorption in MnIn2S4 single crystals

Optical absorption in MnIn₂S₄ single crystals has been studied. Direct and indirect optical transitions are found to occur at photon energies of 1.90?C2.16 eV in the temperature range of 80?C342 K. The temperature dependence of the band gap is determined; its temperature coefficients E _gd and E _gi are found to be ?4.84 × 10^?4 and ?6.33 × 10^?4 eV/K, respectively. The electron-phonon interaction is the main mechanism of the temperature shift of the intrinsic-absorption edge. MnIn₂S₄ single crystals exhibit anisotropy in polarized light at the absorption edge in the temperature range of 90?C190 K; the nature of this anisotropy is explained.     

Li- and V-NMR study of LiVS2

⁷Li- and ⁵¹V-NMR have been measured to make clear the electronic state in a two-dimensional triangular lattice LiVS₂. Knight shift of both ⁷Li- and ⁵¹V-NMR is almost independent of temperature below the phase transition temperature T_c of about 310 K from the paramagnetic state to non-magnetic state. The ⁵¹V- spin-lattice relaxation rate 1/T₁ reveals an exponential temperature dependence below T_c, indicating a gap structure of electronic state. These results are consistent with a non-magnetic state with a trimer singlet of V³⁺ spins below T_c.     

Optical absorption spectrum of Ni2+ ion doped in synthetic lecontite

The optical absorption spectrum of Ni²⁺ ion doped in lecontite (sodium ammonium sulphate dihydrate) single crystal has been studied at room and liquid air temperatures. All the bands could be assigned assumingO _h symmetry for the Ni²⁺ ion in the crystal. The splitting of³ T _1g (F) band at liquid air temperature has been attributed to spin-orbit interaction. The crystal field and spin-orbit parameters derived areD _q=1000 cm^?1;B=740 cm^?1;C/B=4.27 and ζ=600 cm^?1. All the bands observed show a blue shift when the crystal was cooled to liquid air temperature.     

Temperature dependence of Knight shifts for 12 B in Pt

The Knight shift of a short lived β-emitting nucleus ¹²B (I = 1, T _1/2 = 20 ms) implanted into Pt has been measured as a function of temperature (140?600 K) by means of the β-NMR method. The relation between the Knight shift and the susceptibility for Pt was deduced, which shows the similar tendency to that for the case of ¹²B in Pd.     

Magnetic phase transitions in Fe80-xNixCr20 (14 ⩽ x ⩽ 30) alloy studied by using 57Fe Mössbauer spectroscopy

This paper reports ⁵⁷Fe Mössbauer spectroscopic studies of the polycrystalline samples of the substitutionally disordered, isostructural (fcc), ternary alloy system Fe_80-xNi_xCr₂₀ for x = 30, 26, 19 and 14 in the temperature range of 10–295 K. The data have been analyzed in terms of the magnetic phase transitions occurring in these alloys by examining the temperature dependence of the various Mössbauer parameters like line‐width, center shift, resonance area, distribution of hyperfine field, P(H), and the average hyperfine field 〈H〉. An estimate of the magnetic transition temperature T _C is made for the alloys with x = 30 and 26 and these results are compared with those previously obtained by magnetic measurements and neutron diffraction experiments. The data for the second order Doppler shift have been analyzed to estimate the Debye temperature ΘD for alloys with x = 30 and 26.     

Influence of magnetic ordering on the phonon Raman spectra in YCrO3 and GdCrO3

A_1g and B_2g phonon Raman lines with frequencies of ≈ 560 cm^-1 in YCrO₃ and GdCrO₃ show large blue shift from a little above the Néel temperature to lower temperatures. The contribution of the volume exchange striction to the frequency shift is estimated to be about 30 to 40 per cent for the A_1g and B_2g lines, respectively.     

Thermally induced optical bistability in CdHgTe

We report optical bistability in a room temperature, uncoated Cd_0.185Hg_0.815Te etalon using a cw CO₂ laser at 10.6 μm. The results are consistent with a band gap resonant, dispersive nonlinearity induced by a thermal shift of the band gap energy through interband and free carrier absorption. A refractive index temperature coefficient, dn/dT ～ ? 1 × 10^-3K^-1 was measured, and bistability was observed at incident powers of 20 mW.     

Shift measurements of Hα and Hβ lines in arc plasmas

Red shifts are reported for the H_α and H_β lines at electron densities of 10¹⁶–10¹⁷ cm^-3. The peak shift of the H_α line agrees with the ELC shift. For the H_β line, the dip shift exceeds the ELC shift systematically. The H_β dip shift is proportional to N_e/T.     

A multispectrum analysis of the ν2 band of HCN: Part II. Theoretical calculations of self-broadening, self-induced shifts, and their temperature dependences

A semiclassical theory based upon the Robert-Bonamy formalism has been developed to explain the experimental measurements of self-broadening, self-induced pressure shift coefficients in the ν₁,ν₂, 2ν₂ bands of H¹²C¹⁴N and the 2ν₁ band of H¹³C¹⁴N, as well as the temperature dependences of these parameters with special emphasis on the ν₂ band. Our calculations include only electrostatic interactions and neglect the vibrational dependence of the isotropic part of the intermolecular potential, which probably has a weak contribution to the HCN self-shifts for the bands investigated in this study. The agreement between theory and measurements is good in the cases of self-broadening coefficients and their variation with temperature, as well as the self-shift coefficients determined at room temperature. However, the observed temperature dependence of self-shift coefficients in the ν₂ band is different from that derived theoretically.