Electron diffraction and infrared study of the semimetallic layered compound Ti1−xVxSe2

The crystals of the system Ti_1?xV_xSe₂ (C ≤ x ≤ 0.05) undergo a second order structural phase transition. Electron diffraction studies show that the transition temperature decreases with progressive V-doping. The phase transition considerably affects the infrared reflectivity, measured at 300 and 77 K in the spectral range 40 cm^?1 to 4000 cm^?1.The presence of free carriers and the existence of optical infrared active E_u phonon modes $\text{(}\text{E}\text{⊥}\text{c}\text{)}$ confer their characteristic appearance to the spectra. At room temperature one phonon structure is measured at 143 cm^?1. At 77 K a new series of phonon peaks appears up to a V-concentration of 5 % as a direct consequence of superlattice formation.At room temperature the plasma-edge shifts towards higher frequencies as the vanadium concentration increases. This effect is caused by a large increase of $\text{N}\text{m}{}^1$, associated with the mixing of impurities. It is indicative of the small density of states at the Fermi level in semimetallic TiSe₂. Our results suggest a phase transition driven by lattice dynamical effects.     

Lattice parameters variation with temperature of Ti2O3 and (Ti0.98V0.02)2O3 from single crystal X-ray data

The lattice parameters of Ti₂O₃ and (Ti_0.98V_0.02)₂O₃ have been measured as a function of temperature (24–670°C for Ti₂O₃ and 24–440°C for V-doped Ti₂O₃) from single crystal X-ray data. The high temperatures were attained by blowing hot argon directly on the crystal mounted on an automatic Philips diffractometer. This experimental set-up gives standard deviations which are at least 10 times better than those of the previous measurements and allows to keep Ti₂O₃ as such well above the transition. The variations of a, c, $\text{c}\text{a}$ (hexagonal axes) for pure Ti₂O₃ are in agreement with the previous results. On the contrary we did not observe any transition in the unit cell volume. The V-doping seems to attenuate the transition which is visible only on the a vs T curve     

Cell dimensions and antiferromagnetism of lunar and terrestrial ilmenite single crystals

X-Ray diffraction and anisotropic magnetic measurements have been made on single crystals of lunar ilmenite and on terrestrial ilmenite from Bancroft, Ontario, Canada and the Ilmen Mountains, U.S.S.R. The elongated $\text{c}$-axis of lunar ilmenite, previously reported, is confirmed by new measurements. The shorter $\text{c}$-axis found in terrestrial specimens is ascribed to Fe³⁺ substitution for Ti⁴⁺ in the titanium layer. Magnetic measurements on the same specimens show that, in agreement with the Ishikawa-Shirane et al. model, the initial shortening of the $\text{c}$-axis by the above substitution of small amounts of Fe³⁺ (<8%) causes an increase in Fe²⁺?Fe²⁺ exchange coupling through Fe³⁺ in the titanium layer that lowers the Néel transition temperature. The Weiss temperatures and other magnetic parameters confirm this model proposed by Ishikawa and Shirane et al.Additional transitions found in one of the terrestrial specimens (Bancroft) have been ascribed to a small amount of an exsolved spinel phase, possibly a solid solution phase of magnetite-ülvospinel. The spinel phase is localized in hematite-rich blebs which exsolved from the host ilmenite-rich phase.     

The U(612) supersymmetric limit of the interacting Boson-Fermion Model

The $\text{U(}\text{6}\text{12}\text{)}$ dynamical supersymmetry of the Interacting Boson-Fermion Model, appropriate for nuclei where the odd nucleon can occupy the single-particle orbits $\text{J =}\text{1}\text{2}\text{,}\text{3}\text{2}\text{,}\text{and}\text{5}\text{2}$ is analyzed. The subgroup structure of $\text{U(}\text{6}\text{12}\text{)}$ is discussed and the generators and Casimir operators of these subgroups are given. In each of the three dynamical symmetries of $\text{U(}\text{6}\text{12}\text{)}$ the energy formulas, reduction rules, and wave functions are obtained and closed expressions for E2 transition rates and one-nucleon transfer intensities are derived. Examples of $\text{U(}\text{6}\text{12}\text{)}$ symmetries are presented.     

Formation energies of point defects in rutile (TiO2)

The formation energies for the point defect species Ti⁴⁺ interstitials, Ti_i⁴⁺, and oxygen vacancies with an effective charge of + 2e, Vo²⁺, have been calculated using the polarisable point ion shell model. The formation energy of Ti_i⁴⁺ was found to be a 10.8 eV for occupation of the (0, $\text{1}\text{2}$, $\text{1}\text{2}$) site. The energy of Ti_i⁴⁺ at other sites is somewhat higher than this figure, and allowed the migration energy of Ti_i⁴⁺ parallel to the c axis to be estimated as 2.9 eV. The formation energy of Vo²⁺ was found to be 10.1eV. The variation of these formation energies with dielectric constant was also calculated. It was found that the energy of formation of Ti_i⁴⁺ varied considerably with dielectric constant, while the formation energy of Vo²⁺ was almost independent of dielectric constant. The relevance of these results to the presence of crystallographic shear planes in reduced rutile is considered.     

Structure of high-spin states in 232Th, 234U and 236U

Multiple Coulomb excitation of ²³²Th, ²³⁴U and ²³⁶U by 5.3 $\text{MeV}\text{u}$²⁰⁸Pb ions has been studied using γ-ray spectroscopy. Excitation of ground-band levels is observed up to spin I^π = 26⁺ (tentatively 28 ⁺) in ²³²Th and ²³⁴U and I^π = 28⁺ (tentatively 30⁺)in ²³⁶U. High-spin levels of the K^π = 0^? octupole-vibrational bands are also observed in these nuclei. The measured transition energies between ground-band levels suggest that at $\text{I≈ 28}\text{h}\text{?}$ several units of angular momentum are carried by single particles aligned with the rotation axis.This result can be understood in terms of a super band built on aligned two-quasiparticle configurations which crosses the ground-state rotational band at a rotational frequency of $\text{h}\text{?}\text{ω ? 0.25}\text{MeV}\text{(I ? 28}\text{h}\text{?}\text{)}$. The E2 transition-matrix elements deduced from the experimental γ-yields agree within their errors with the rigid-rotor predictions up to the highest spins observed.The experimental results are discussed using the concept of rotation alignment and are compared with predictions of the rotation-vibration model and the interacting-boson model.     

A frequency-doubled frequency-stabilized cw ring dye laser for spectroscopy: A study of the λ = 293.3 nm transition in In1

In this letter a frequency-doubled frequency-stabilized CW ring dye laser for the wavelength region 292–305 nm is described. 500 μW U.V. power combined with a 2 MHz linewidth and a 7 GHz scanwidth has been obtained by intracavity frequency doubling in an ADA crystal. The set-up has been tested on the hyperfinestructure of the λ = 293.3 nm transition 5p²P_{$\text{3}\text{2}$} → 7s²S_{$\text{1}\text{2}$} in In1.     

The ft value of the superallowed Fermi decay 46V(β+)46Ti

The${}^{46}\text{Ti}\text{(p, n)}{}^{46}\text{V}$ reaction threshold has been measured as 8007.7 ± 1.8 keV, equivalent to an energy release of 6030.6 ± 1.8 keV in the beta decay${}^{46}\text{V}\text{(β}{}^{\mathrm{+}}\text{)}{}^{46}\text{Ti}$. The ${}^{46}\text{V}$ half-life has been measured as 422.28 + 0.23 ms. The corresponding ft value is reasonably consistent with those of other superallowed Fermi decays.     

Neutron diffraction study of UAs in high magnetic fields

Magnetization experiments have shown that at low temperature and in an applied magnetic field of ～90 kOe antiferromagnetic (AF) uranium arsenide transforms to a new ferrimagnetic state. The high-field transition produces a state with magnetic components $\text{parallel}$ to the applied field. A small ferromagnetic component (0.4 μ_B) is seen in magnetization experiments but with neutron diffraction we have observed directly the large AF component (～ 1.8 μ_B) which has a $\text{q}$ value of 0.58 ± 0.01 $\text{c}$¹ in contrast to the zero field type IA structure with $\text{q}\text{= (0,0,0.50)}$. This transition is quite different from that occuring in the conventional spin-flop transition.     

Infrared study of superlattice formation in Ti1+xSe2

Infrared reflectivity measurements have been made on two Ti_1+xSe₂ crystals of different stoichiometry over the range 200 cm^?1 to 4000 cm^?1 before and after the formation of a 2a_o × 2c_o superlattice. In both crystals at room temperature a heavily damped Drude edge is observed at about 1000 cm^?1. At liquid helium temperatures, below the phase transition, the Drude minimum of a non-stoichiometric crystal ($\text{x ? 0.02, T}{}_{\mathrm{c}}\text{= 155 K}$) is at 750 cm^?1 whereas the low temperature minimum of a more stoichiometric sample (T_c = 198 K) is below 350 cm^?1. Also, new absorption peaks are seen to develop below T_c at about 3700 cm^?1 and 3300 cm^?1 for the non- stoichiometric and stoichiometric-crystals respectively. The observed phenomena are related to the energy gaps which open up in the band structure below the phase transition temperature.     

The cooperative Jahn-Teller effect in uranium (4+) hydroxy-sulphate

The magnetic susceptibility measurements of orthorhombic U(OH)₂SO₄ within the temperature range 4.2–300 K have revealed a magnetic anomaly at T_D = 21 K associated with crystallographic transition induced by the cooperative Jahn-Teller effect. Above 21 K the magnetic susceptibility of the uranium (4+) ion corresponds to the electronic ground doublet ${}_{\mathrm{¡}}\text{M}{}_{\mathrm{J}}\text{= ± 2〉}$ confirming thus the antiprismatic symmetry (D_4d) of the crystal field at the uranium site. Below T_D the system of two singlets ($\text{1}\text{√2}$)|2〉 ± ($\text{1}\text{√2}$)|$\text{2}$〉 separated by δ(T) is the ground state of the uranium ion.     

Pressure dependence of the metal-semiconductor transition in TaS3

TaS₃ exhibits the metal-semiconductor transition at 218 K due to the Peierls instability. The electrical resistance was measured under pressure. It was found that both the transition temperature, Tp, and the activation energy at T=0 K, △(0), decrease with pressure at the rates of $\text{dTp}\text{dP}\text{?-1.3}\text{K}\text{kbar}$ and $\text{d△(0)}\text{dP}\text{?-4}\text{K}\text{kbar}$, respectively, while the ratio, $\text{△ (0)}\text{Tp}$, is independent of pressure.     

The antiferroelectric phase transition in TlH2PO4 and TlD2PO4

It is shown by X-ray diffraction measurements that the transitions at 230 K in TlH₂PO₄ and at ≈ 350 K in TlD₂PO₄ are structurally equivalent. In both cases, on cooling through the transition, new reflections appear at points ($\text{h +}\text{1}\text{2}$, $\text{k +}\text{1}\text{2}$, l) referred to the primitive monoclinic unit cell of the high-temperature phase. In TlH₂PO₄ some distinct but broadened scattering remains at the ($\text{h +}\text{1}\text{2}$, $\text{k +}\text{1}\text{2}$, l) points up to 10 K, or more, above the transition temperature.     

Radiation-induced paramagnetic centres in glass-ceramics derived from the MgO-Al2O3-SiO2-TiO2 system

MgO-Al₂O₃-SiO₂-TiO₂ glass-ceramics have been γ-irradiated and examined by electron spin resonance. The spectra observed arise from two principle sites; Ti⁴⁺ ions radiochemically reduced to Ti³⁺ and holes trapped at the π-type orbitals of oxygen ions bridging between SiO₄ and AlO₄ units. The Ti³⁺ line, although very similar to its form in the parent glasses, is in general a composite of two distinct lineshapes, each of which is associated with one of the two major crystalline phases Cordierite or Enstatite. The hole centre has a characteristic hyperfine interaction similar to that of the Boron Oxygen Hole Centre of Borate glasses. The Hamiltonian parameters of this centre are $\text{S =}\text{1}\text{2}$, $\text{I =}\text{5}\text{2}$, g₁ = 2.0023, g₂ = 2.0148, g₃ = 2.035 and |A₁| = |A₂| = |A₃| = 8.41 × 10^-4cm^-1.     

On the question of fine structure in νp and EK for 233U and 235U

The question of fine structure in the variation of the average number of prompt neutrons, $\text{ν}{}_{\mathrm{<mtext>p</mtext>}}$, emitted per fission with energy in the neutron fission of ²³³U and ²³⁵U has been examined. Consistent structure has been found in measurements of both $\text{ν}{}_{\mathrm{<mtext>p</mtext>}}\text{and}\text{E}{}_{\mathrm{<mtext>K</mtext>}}$ (the average total fission fragment kinetic energy) for ²³³U. Channel analysis of the neutron fission cross section of ²³³U allows the structure to be calculated quantitatively provided the collective energy at the second hump in the fission barrier is weakly coupled to the nuclear degrees of freedom at scission. A similar calculation for neutron fission of ²³⁵U supports the case for the absence of fine structure in $\text{ν}{}_{\mathrm{<mtext>p</mtext>}}\text{and}\text{E}{}_{\mathrm{<mtext>K</mtext>}}$ for this nucleus.     

Muon-induced prompt fission of uranium

The prompt-fission probabilities of muonic ²³⁸U in radiationless dipole and quadrupole transitions of the muon are calculated. The 3d → 1s transition is found to dominate, in agreement with experiment. A measured drastic enhancement in the $\text{Γ}{}_{\mathrm{<mtext>n</mtext>}}\text{Γ}{}_{\mathrm{<mtext>f</mtext>}}$ ratio compared to photofission data is reproduced in terms of the increase of the fission barrier in the presence of the muon.     

Phase transition in four-dimensional compact QED

The energy and the specific heat of the four-dimensional U(1) lattice gauge model is evaluated by Monte Carlo simulations on lattices of size L⁴, where L = 4, 5 and 6, evidence is presented for the occurence of a second-order phase transition. A finite size scaling analysis of our results gives the critical value of the coupling constant e²_c = 0.995 and a correlation length exponent $\text{v ≈}\text{1}\text{3}$.     

States in 163Dy and 169Er populated in the (t,p) reaction

Angular distributions of protons from the ¹⁶¹Dy(t, p)¹⁶³Dy and ¹⁶⁷Er(t, p)¹⁶⁹Er reactions were studied, using 15 MeV and 17 MeV tritons from the McMaster University tandem Van de Graaff accelerator. The reaction products were analyzed with a magnetic spectrograph and detected with nuclear emulsions. Since the ¹⁶¹Dy target ground state is the $\text{5}\text{2}$⁺[642] orbital, a strong L = 0 transition was observed to the $\text{5}\text{2}$⁺[642] bandhead in ¹⁶³Dy, which was previously assigned at 251 keV. Also transitions to the $\text{7}\text{2}$, $\text{9}\text{2}$ and $\text{11}\text{2}$ band members were observed. Similarly, a strong L = 0 transition was observed to the $\text{7}\text{2}$⁺[633] bandhead at 244 keV in ¹⁶⁹Er, with the other band members only weakly populated. The angular distributions to the various members of these two bands can be described when higher-order reaction processes are taken into account. In ¹⁶³Dy, surprisingly strong L = 0 transitions were observed to levels at 1831 keV, 1937 keV and 2053 keV, with strengths of 23%, 30% and 37% of that for the $\text{5}\text{2}$⁺[642] bandhead. In ¹⁶⁹Er, the 905 keV level was populated with an L = 0 transition that had 31% of the strength observed for the strong L = 0 transition to the 244 keV level. The nature of these states is at present not understood.