NMR and magnetic studies on 7 K anomaly in Tl3H(SO4)2

Measurements of the spin-lattice relaxation time, NMR absorption line and magnetization have been carried out on the Tl₃H(SO₄)₂ crystal below 50 K. The anomaly at around 7 K was: (1) the spin-lattice relaxation times of ¹H and ²⁰⁵Tl nuclei increase steeply with decreasing temperature below 7 K, (2) the NMR absorption lines below 7 K shift to the high-magnetic field side in comparison with that above 7 K, and (3) the ¹H NMR line width exhibits a drastic increase of the line width with decreasing temperature below 7 K. These results indicate that the magnetic dipole fluctuation of the proton changes at 7 K. On the other hand, there are no remarkable anomalies of magnetic susceptibility at around 7 K. From these results it is deduced that the anomaly at around 7 K is caused by the change in quantum mechanical process of the proton from proton tunneling to zero-point vibration of hydrogen in the hydrogen bond with the decrease of temperature.     

Influence of the doping level on the charge distribution among the inequivalent CuO2 layers in Tl2Ba2Ca2Cu3O10−δ : a NMR study

and NMR measurements in the normal and superconducting states of Tl₂Ba₂Ca₂Cu₃O_10−δ with different δ are reported. In the overdoped Tl2223 sample with T_c=117 K (T_c^opt=123 K) and δ₁<δ^opt different temperature dependencies of the Knight shift are revealed for inequivalent CuO₂ layers. For the inner CuO₂ layer with the square oxygen coordination of Cu the decrease of with temperature is more gradual. In going towards the underdoped Tl2223 with T_c=104 K and δ₂>δ^opt the changes of with temperature are found to be the same for both types of copper layers. The quadrupole coupling constants for copper and oxygen from different CuO₂ layers were obtained. From the variations with doping of the valence contribution to the electric field gradient at copper sites, we estimate both the hole numbers at Cu and oxygen sites and the real concentration of mobile hole carriers n_h in each of inequivalent CuO₂ layers. In the overdoped Tl2223 sample the charge density in the inner layer differs from the one in the outer plane (with five-fold oxygen coordination for Cu). Our results show that the inhomogeneity of the charge distribution disappears in the underdoped regime. The results are compared with calculations of the charge distribution among the CuO₂ planes in multilayered cuprates reported by Haines and Tallon [E.M. Haines, J.L. Tallon, Phys. Rev. B 45 (1992) 3127].     

Nuclear magnetic relaxation of 205Tl in TlClO4 in a rotating frame

The spin-lattice relaxation time T_1ϱ of ²⁰⁵Tl in TlClO₄ has been measured in a rotating frame. It was found that the temperature dependence of T_1ϱ shows three minima due to the cross relaxation and the random modulation of the dipole-dipole interaction between ²⁰⁵Tl and ¹⁷O of natural abundance (0.037%).     

Electronic structure of thallous centres and Tl+ - Vk recombination in KCl crystal

Parameters of impurity Tl ion required for the calculation of different thallous centres in alkali halides are obtained for the semiempirical method of the intermediate neglect of differential overlap (INDO). The electronic structure of Tl⁺, Tl²⁺ centres in KCl is calculated. The potential energy curves for the recombination of nearest Tl⁺, V_$\text{k}$ centres against the breathing vibrational mode of the V_$\text{k}$ centre are calculated. This recombination (hole trapping) is found to be nonradiative tunneling with small activation energy rather than radiative one. A number of experimental data is also discussed in the light of the present calculations.     

VUV excitation of luminescence in pure and Tl+ doped KI

Excitation spectra of pure and Tl⁺ doped KI have been measured at room temperature (RT) and liquid nitrogen temperature (LNT) between 13 and 30 eV. Energy transfer between the host lattice ions and the Tl⁺ impurities, and intrinsic recombinaton of electron-hole pairs, have been studied at different temperatures and excitation energies. Energy transfer has been observed only at RT while at LNT the intrinsic recombination, V_k centers-electrons, is the dominant process.     

Electrical conductivity and NMR studies in paraelastic phase of Tl2SeO4

We have carried out the electrical conductivity and NMR measurements and investigated the characteristic features of the electrical conductivity in a paraelastic phase of Tl₂SeO₄. It was found from electrical conductivity measurement that the activation energy in the paraelastic phase (above T_c(= 661 K)) is 0.98 eV. We also found that ²⁰⁵Tl-NMR line width drastically decreases above T_c and becomes approximately 0.5 Gauss. This result indicates that in the paraelastic phase the mobile Tl ions exist. Moreover this result is consistent with the existence of the anomalously large hopping motion of Tl ions observed in X-ray diffraction measurement. Furthermore the activation energy estimated from the motional narrowing of ²⁰⁵Tl-NMR absorption line is 0.92 eV and is in agreement with that obtained from the conductivity measurement. From these results, it is deduced that the mobile Tl ions play an important role in the appearance of electrical conductivity in the paraelastic phase.     

CW laser oscillation on transitions of Tl+ in a hollow-cathode Ne-Tl-halide discharge

CW laser oscillation on Tl⁺ transitions (594.9 nm and 695.1 nm) at low operating temperatures is obtained. As a source of Tl atoms the volatile salt TlCl is used. The compound is evaporated due to selfheating by the discharge current. Maximum laser intensity is obtained at neon pressure of 10–13 Torr and cathode temperature ≈ 320°C.     

Pre-resonant enhancement of the Raman scattering from KI:Tl+

E_g and T_2g spectra at room and nitrogen temperature at several laser wavelengths are reported for KI:Tl⁺. As resonance with Tl⁺ -absorption bands is approached the optic phonon parts of the spectra are enhanced relative to the acoustic phonon parts, and distortions occur within the acoustic and optic parts of the spectra. A model for the scattering is employed which involves electron-phonon coupling to nearest and nn neighbors. Exchange and crystal field contributions to the electron-phonon coupling are distinguished.     

Li3VO4-Li4TO4(T=Ge,Si)固溶体离子导体7Li的NMR研究

在150—573K温度范围内,研究了固溶体Li₃VO₄-Li₄TO₄(T=Ge,Si)系统不同成分的⁷Li的NMR谱。发现γ_II相固溶体室温⁷Li的NMR线宽和自旋晶格弛豫时间T₁的值都比Li₄GeO₄,Li₄SiO₄和Li₃VO₄小约一个数量级。这表明在γ_II相固溶体离子导体中,Li⁺离子运动有可能比固溶前有数量级增长。同时还发现⁷Li的电四极分裂伴线数随成分和温度而异,以及伴线强度百分比依赖于温度。这反映γ_II相的不同成分中,间隙Li⁺离子占有的不等价位置个数不同,而Li⁺离子在每个不等价位置上的占有率又随温度而变化。 关键词：     

Decay modes of the 3000 Å emission band in KCl: Tl+

The time behavior of the 3000 Å luminescence signals of KCl: Tl crystals is complex. Three different components can be resolved in the temperature range investigated (80–340K). These results that are typical for the A_x emission of phosphors like Kl:Tl⁺ and KCl:Pb²⁺, cannot be explained with the available theoretical models for KCl:Tl system.     

Thermal relaxation in liquid 3He/4He mixtures near the tricritical point

Very long relaxation times τ were observed in our earlier specific heat measurements on liquid ³He/⁴He mixtures in two-phase-states near the tricritical temperature T_O. It appear that 1/τ ∝ (T_O ? T).     

A study of particle-vibration multiplets in 203Tl using the 205Tl(p,t)203Tl, 208Pb(p,t)206Pb and 206Pb(p,t)204Pb reactions

The ²⁰⁵Tl(p, t)²⁰³Tl reaction has been used at 26.2 MeV to obtain additional information on the particle-vibration multiplets in ²⁰³Tl. Enhancement factors for the L = 0 and L = 2 transitions to the lower excited states of ²⁰³Tl were well accounted for by using enhancement factors from the ²⁰⁶Pb(p, t)²⁰⁴Pb reaction and the wave functions from the intermediate coupling calculations of Covello and Sartoris. Additional data on the ²⁰⁸Pb(p, t)²⁰⁶Pb reaction at 26.2 MeV were used along with data at other energies to examine the energy dependence of the zero-range enhancement factors.     

Impurity 55Mn NMR in Cu-Mn dilute alloys

We detected impurity ⁵⁵Mn NMR in Cu-45 ppm and -110 ppm Mn dilute alloys at temperatures of 150–330 K. The hyperfine field we obtained was 65±5 kOe/μ_B. From a temperature variation of linewidths we could estimate J_sd as 1.2±0.1 eV, using the reported theories of relaxation times T₁ and T₂.     

Anomalous nuclear spin relaxation in liquid 3He4He mixtures

The longitudinal relaxation time, T₁, was measured in liquid ³He⁴He mixtures with molar concentrations of ³He of 0.12, 0.20 and 0.45 under saturated vapor pressure. λ-anomaly was clearly observed in the temperature dependence of T₁.     

Inelastic processes in the 19F(3He,d)20Ne reaction

We have examined the role of inelastic processes in the ¹⁹F(³He, d)²⁰Ne reaction. By coupling three levels in both the entrance and exit channels it was found that the inelastic processes were able to account for both the magnitude and rather flat shape of the angular distribution for the 4.25 MeV 4⁺ level observed in the ¹⁹F(³He, d)²⁰Ne reaction at 16 MeV bombarding energy. In contrast the DWBA could not account for the data. The magnitude of the inelastic processes was found to be quite sensitive to some of the optical model parameters involved. The DWBA predictions for the 0⁺ and 2⁺ cross sections were modified by the inelastic processes requiring some adjustment of the spectroscopic amplitudes to account for the data.     

Nuclear magnetic resonance of 55Mn and 75As in MnAs

Four sets of NMR signals, two each, from ⁵⁵Mn and ⁷⁵As nuclei have been observed. The temperature dependences of ⁵⁵Mn resonances have been studied from 77 to 311 K and that of ⁷⁵As, from 77 K to about 250 K. The results show that there is a phase transition at T₁ ≈ 220 K. This transition may be due to introduction of a local spontaneous distortion in the region of the domain walls in the lattice, resulting in lowering of symmetry at low temperatures. Another possibility is the canting of spins which would lower the magnetic group symmetry. The observed resonances have been assigned to arise from the nuclei at the edge and the centre of the domain walls at temperatures T >T₁ and from two types of wall edges with inequivalent orientation of atomic spins at T < T₁. The isotropic hyperfine field at 0 K obtained by extrapolating the resonance frequencies are 227 and 285.1 kOe at ⁵⁵As nuclei, respectively. The anisotropy in the hyperfine field is nearly zero at ⁵⁵Mn nuclei and about 5.8 kOe at ⁷⁵As nuclei at 0 K.     

Co2+ spin-lattice relaxation narrowing of 19F NMR in KCoF3

The temperature dependence of the ¹⁹F NMR linewidth ΔH in KCoF₃ has been measured over the entire paramagnetic solid state region. The dramatic decrease in the hyperfine-broadened, exchange narrowed ΔH that occurs above 200 K is interpreted as arising from fast Co²⁺ single-ion, spin-lattice relaxation. A model theory of the temperature dependence of ΔH is given which incorporates the interplay of exchange and spin lattice relaxation effects on the decay of the spin autocorrelation function.     

Radiation induced defects in KBr:Tl+ crystals

The defects produced in KBr:Tl⁺ crystals during x-irradiation at 77 K were studied using thermoluminescence (TL), thermally stimulated currents (TSC), and absorption and emission spectra. Three main glow peaks at 165, 193 and 258 K were observed both in the TL and in the TSC curves. A variety of irradiation induced absorption bands were observed in the UV, visible and infrared up to about 2 microns. The 165 K TL peak was found to emit only the 440 nm band assigned to thallium dimers, while the peaks at 193 and 258 K exhibited the UV bands at 310 and 365 nm as well as the 440 nm band.The defects produced during the irradiation were the V_k hole center, the Tl° and the Tl⁺₂ electron centers. Smaller concentrations of Tl²⁺ and (Tl⁺)⁺₂ centers were also produced.An analysis of the results including measurements on lightly and heavily doped crystals enabled to draw conclusions on the nature of the defects and on the recombination processes involved. A close correlation has been found between the temperatures at which changes in the various absorption bands take place and the temperatures of the TL peaks. The analysis enabled also a full classification of the absorption bands.     

NMR study of the spin dynamics in full over-doped Tl2Ba2CuO y (T c=0 K)

The temperature dependence and the anisotropy of the Knight shift and the spin-lattice relaxation time (T ₁) at the Cu and the O sites of the full over-doped Tl₂Ba₂CuO _y (T _c=0 K: Tl2201L) were measured. The Knight shift values at the Cu and the o sites in the CuO₂ plane are larger in Tl2201L compared to those in Tl₂Ba₂CuO _y (T _c=85 K: Tl2201M). This indicates the increase of the spin part of the Knight shift due to the over-doping. Based on the comparison between theT ₁ in Tl2201L and Tl2201M, the enhancement of the antiferromagnetic correlation still remains in the whole over-doped region.     

Alignment of Anisotropic Particles by Magnetic Field as Seen by NMR

We applied ¹³C and ²⁰⁵Tl NMR for studying alignment of particles of graphene and high temperature superconductor (Tl_0.5Pb_0.5)(Ba_0.2Sr_0.8)₂Ca₂Cu₃O_y caused by magnetic field. These compounds have layered structure and reveal anisotropic magnetic susceptibility. We found that the field of 8 T causes minor alignment of powder graphene and somewhat better alignment of fluffy graphene particles. Herewith the effect of alignment is well pronounced in ²⁰⁵Tl spectra of the superconducting particles fixed in epoxy in the field of 8 T. This effect is reflected in the ²⁰⁵Tl line shape measured in a magnetic field of 1.17 T and becomes much more pronounced in measurements made in high magnetic field of 8 T. Spectra simulations allow determining the degree of the particles’ alignment.