Thermolumineszenz von KCl und NaCl nach Röntgen- und UV-Bestrahlung bei Heliumtemperatur

Glow curves of luminescence are recorded in the range from 10° K to 300° K. One gets characteristic changes by prior annealing the single crystals in O₂ or HCl. Also an increase of the lattice disorder causes new glow bands. In KCl a strong glow band always appears at 40° K after irradiating with X-rays or ultraviolet light in the range of the exciton bands. It is ascribed to trapped excitons, which become mobile at that temperature. For X-ray irradiated KCl the glow curve of luminescence is compared with the electrical glow bands and with the concentration change of known defects. The half widthΔT of all glow bands is found proportional to the temperature of the maximum:ΔT=(0.08±0.02)T _m .     

Study of the paraelectric behavior of OH− ions in alkali halides with optical and caloric methods II.: Dynamics of the dipole alignment

The relaxation behavior of OH^? dipole centers in ten alkali halides was studied with electro-optical and electro-caloric methods as a function of applied field, temperature and host lattice system. The reorientation kinetics of the 〈100〉 oriented OH^? dipoles can be fitted successfully to a phonon-assisted (90°) orientational tunneling model. Below 5°K the observed time dependence of the optical dichroism and the T^?1 dependence of the relaxation time gives clear evidence for the predominance of one-phonon processes, while above 5°K the T^?4 dependence indicates multi-phonon processes. It could be demonstrated that in the one-phonon region of the dipole relaxation a population inversion among the dipole levels (with subsequent phonon emission) can be achieved after a fast reversal of the field polarity. An attempt is made to discuss the observed strong variation of the relaxation time (12 orders of magnitude) with the host lattice in terms of a tunneling matrix element renormalized due to the anisotropic lattice distortion around the defect.     

Absence of premelting phenomena in pure perfect gallium crystals revealed by Debye-waller-factor measurements using anomalous diffraction of X-rays

Premelting phenomena have been searched for in pure, perfect gallium crystals by using anomalous X-ray transmission. The temperature dependence of the integrated intensity under the Bragg peak does not reveal any deviations from its normal behaviour, i.e. an essentially linear increase of the mean square vibrational amplitude of the lattice atoms as a function of temperature between 10 °C and a temperature at least 10^?4 °C close to the melting temperatureT _m =29.75 °C. The temperature dependence of the integrated intensity of the 020-reflection can be described by a Debye temperatureΘ ₍₀₂₀₎=189±6 K. Premelting effects could not be detected.     

Effects of pressure on magnetic properties of F-centers in rubidium halides

Room temperature magnetic resonance measurements have been made on additively colored RbCl and RbBr and on X-irradiated RbH at pressures up to 8 kbar. In RbCl and RbH pressure shifts of the F-center isotropic coupling constant a₁ were obtained from changes in the resolved EPR hyperfine structure. Pressure shifts of first and second shell hyperfine coupling constants of F-centers in RbBr were found from high pressure ENDOR spectra. Effects of the pressure induced polymorphic phase transformations in RbCl and RbBr are discussed. No evidence was found for a structure transformation in RbH. Room pressure ENDOR measurements on additively colored RbI were made at 77°K. The first and second shell hyperfine coupling constants are reported. All results accord well with the behavior of F-centers in other alkali halides.     

Thermal expansion measurements for estimating vacancy concentration in X-ray irradiated KCl single crystals

A sensitive capacitance technique is used for measuring changes in length (Δl) of KCl single crystals with temperature in the region 30–300°C. These measurements have been taken on KCl in (i) as-cleaved (ii) X-ray irradiated (iii) quenched and X-ray irradiated conditions (X-ray irradiation was always done at room temperature (≈ 30°C). The linear coefficient of thermal expansion (α) of the as-cleaved sample is 40.8 × 10^-6°C^-1. Variation of Δl with temperature in X-ray irradiated crystal shows two regions: (a) 30–180°C where α is 48.1 × 10^-6°C^-1, (b) 180–300°C where α is 40.4 × 10^-6°C^-1. Similar behaviour is exhibited by quenched and later X-ray irradiated KCl the first region is up to 140°C, beyond which the second region takes over. From these data, concentration of vacancies in X-ray irradiated KCl at room temperature is calculated to be 3.4 × 10¹⁷ cm^-3 which is in fairly good agreement with the value obtained from F-band absorption measurements on the sample. An attempt has been made to understand these results.     

Local structure of bivalent copper centers in SrF2 crystals: An ENDOR spectroscopic study

Structural distortions of the SrF₂ crystal lattice near the bivalent copper impurity Jahn-Teller center are investigated by the ENDOR method (v=9.3 GHz, T=4.2 K). The approximate directions and the magnitudes of displacements of a Cu²⁺ impurity ion and its surrounding F^? ions are determined with respect to one of the anionic networks in the crystal matrix. The tensor components for the ligand hyperfine interaction (LHFI) with fluorine ions separated from the impurity by a distance R≤6.2 Å are obtained from the angular dependences of the location of the ENDOR resonance lines. It is found that the parameters of magnetic interactions between the impurity and these ligands contain the contributions determined by the covalence of bonds in the impurity complex and the polarization of electron shells of the ligands.     

Single-spin asymmetry for charged hadrons produced in proton-nucleus collisions at 40 GeV for c.m. production angles in the range 40°–79°

The transverse single-spin asymmetry for charged hadrons (π ^±, K ^±, p, $\bar p$ ) produced in proton-nucleus collisions was measured for c.m. production angles in the range 40°–79°. The measurements were performed with the FODS-2 setup by using a 40-GeV polarized proton beam originating from the accelerator of the Institute for High Energy Physics (Protvino) and hitting carbon and copper nuclear targets. The data in question were obtained in the polarized-proton-fragmentation region (0.0 < x _F < 0.7, 0.6 < p T < 2.5 GeV/c). In agreement with data obtained at other energies, the single-spin asymmetry for π ^± mesons is significant at high x _F. For the first time, a sizable analyzing power, which changes sign at x _F = 0.43, is observed for protons. The dependence of the analyzing power on the target-nucleus mass is insignificant.     

Enhancement of F-centre colouration during plastic deformation of alkali halides

Stimulated growth of F-centres was observed by irradiating alkali-halide single crystals during uniaxial deformation at controlled rates. Quantitative monitoring of the phenomena was performed for pure KCl at L.N.T. as well as in the range from R.T. to 106°C. Distinct forms for the kinetics of growth were obtained in the two regions. A correlation was also established between the growth of excess colouration and the evolution of stress, in the higher temperature range. The data on the kinetics, stress dependence and temperature dependence suggest that the production of excess F-centres arises from the radiolysis of vacancy-producing debris (possibly, dislocation dipoles) that are also effective agents of hardening. The process is limited by thermally activated annealing, with an activation energy appropriate for free-vacancy migrations. It is suggested that the limiting reaction is a recombination of the excess negative-ion vacancies with an antimorph defect, contained in a complementary type of dislocation debris.     

M-Zentrenbildung in röntgenbestrahlten KCl-Kristallen

The production of F- and M-centres in KCl by X-irradiation has been studied at temperatures between ?20 °C and 50 °C. The optical absorption measurements could be conducted without interrupting the X-irradiation. The results can be summarized as follows: 1. In Harshaw KCl crystals the number of F-centres created by the so-called fast coloration process was proportional to the height of the absorption band at 204 mμ prior to the irradiation. 2. The F-centres formed by the fast process did not contribute to the formation of M-centres. 3. In crystals with a strong absorption band at 204 mμ unstable M-centres were observed, which decayed rapidly after the cessation of the X-irradiation. Their concentration was found to be independent of the F-centre concentration. 4. At temperatures below 0 °C the relation between the concentration of the stable M-centres and the F-centre concentration could not any longer be represented by [M]=k ₁₂·[F₁]·[F₂]+k ₂₂·[F₂]², F₁ and F₂ referring to the F-centres created respectively by the fast and the slow coloration process. Except at very low F₂-centre concentrations however the relationship [M]=k ₀+k ₂·[F₂]² represented the experimental data at all temperatures between ?20 °C and 50 °C. At constant temperaturek ₂ varied withL, the X-ray energy absorbed per unit time and unit volume, according to 1/k ₂=a+bL+cL ². The temperature dependence ofa ^?1 b ^?1 andc ^?1 could be approximated by Boltzmann factors. The corresponding activation energies wereE _a=0.12 eV,E _b=0.53 eV,E _c=0.97 eV.     

Paramagnetische Relaxation von CeCl3

The paramagnetic relaxation in CeCl₃ was investigated in the temperature interval between 1.07°K and 4.21°K using a mutual inductance bridge at frequencies between 3 Hz and 3200 Hz. The dependence of the complex susceptibility on temperature below theλ point is given by a Debye function. Above this temperature, however, deviations occur. The temperature dependence of the relaxation time forT<T _λ can be described byτ～T ^?n where 1.82≦n≦2.35 for 470 Oe≦H≦3360 Oe. At the highest temperatures Orbach Processes occur over the first excited crystal field component which according to these measurements lies atE _II=k(56±10)°K. In the entire temperature range the relaxation processes are determined by further relaxation mechanisms in addition to the spin lattice relaxation. The nature of these could not, however, be determined.     

Gitterfehlordnung in KCl mit Farbzentren

Color centers were created in KCl single crystals by additive coloration and X-irradiation at room temperature. Change of lattice parameter was determined by an X-ray method and change of mass density by a hydrostatic flotation method. Combination of these results, together with optical absorption measurements, led to the following results: F centers in additively colored KCl are Schottky defects. They cause a lattice expansionΔ v/v=0.58 perF center.F centers in X-irradiated KCl are parts of Frenkel defects. During irradiation atoms are displaced to interstitial lattice sites. The volume expansion isΔ v/v=1,15 perF center.     

Non-invasive temperature mapping using MRI: comparison of two methods based on chemical shift and T1-relaxation

Purpose: To implement and evaluate the accuracy of non-invasive temperature mapping using MRI methods based on the chemical shift (CS) and T₁ relaxation in media of various heterogeneity during focal (laser) and external thermal energy deposition.Materials and Methods: All measurements were performed on a 1.5 T superconducting clinical scanner using the temperature dependence of the water proton chemical shift and the T₁ relaxation time. Homogeneous gel and heterogeneous muscle phantoms were heated focally with a fiberoptic laser probe and externally of varying degree ex vivo by water circulating in a temperature range of 20–50°C. Magnetic resonance imaging data were compared to simultaneously recorded fiberoptic temperature readings.Results: Both methods provided accurate results in homogeneous media (turkey) with better accuracy for the chemical shift method (CS: ±1.5°C, T₁: ±2.0°C). In gel, the accuracy with the CS method was ±0.6°C. The accuracy decreased in heterogeneous media containing fat (T₁: ±3.5°C, CS: +5°C). In focal heating of turkey muscle, the accuracy was within 1.5°C with the T₁ method.Conclusion: Temperature monitoring with the chemical shift provides better results in homogeneous media containing no fat. In fat tissue, the temperature calculation proved to be difficult.     

Modification of the CVD-graphene resistivity by post-processing sample annealing

In this paper, we present the results of an additional annealing effect on the temperature dependences of the resistivity for CVD-graphene samples of a large area. We found that an annealing in a Ar/H₂ mixture at different temperatures modifies both the value of the resistivity and the slope of its temperature dependence. The annealing effect on the resultant sample quality depends on the type of the ρ(T) dependence for the initial sample. For samples with a metallic-like ρ(T) dependence, a low-temperature annealing (at T = 250 °C) results in a slight decrease in the resistivity value and an increase of the ρ(T) curve slope. Increasing the annealing temperature up to T = 400 °C leads to a stronger increase in the ρ(T) curve slope but to an increase in the resistivity value. For samples with a semiconductor-like ρ(T) dependence, increasing the annealing temperature up to T = 750 °C results in a gradual suppression of the activation character of the resistivity behavior at low temperatures. The additional annealing is concluded to be accompanied by two processes: a cleaning of the graphene surface from adsorbed contaminations and an additional defect formation in the graphene structure. A relative role of these processes in dependence on the annealing temperature and the type of the ρ(T) dependence for the initial sample is discussed.     

Quadrupole interaction of117In in Zn metal

We have used the TDPAC method to measure the temperature dependence of the quadrupole interaction frequencyv _Q of¹¹⁷In in Zn lattice. The observedv _Q reduces from 187.4 (20) MHz at 77°K to 155.3 (20) MHz at 673°K. The variation ofv _Q is seen to be proportional toT ^3/2.     

19F nuclear magnetic resonance in CeF3

A ¹⁹F pulsed NMR investigation of single crystals of CeF₃ and CaF₂-doped CeF₃ has been undertaken in the temperature range 300–570K at a frequency of 14 and 60 MHz. It was found that the free-induction decay is non-linear and could be fit by a sum of exponentials using a nonlinear least square procedure to an appropriate functional form. The results of this analysis indicate that there are three distinct spin-spin relaxation times T₂, which is consistent with the existence of three inequivalent fluorine sublattices. At room temperature, fluorine ions on two of the sublattices were found to be in motion, with the fastest ion specified as the F3-type in the Oftedal structure.The temperature dependence of the spin-spin relaxation time T₂ for the fastest ions was found to undergo an inversion at 400 and 385K for the pure and doped samples, respectively. The inversion indicates an increase in the interchange rate of ions between two sublattices. Activation energies are 0.29 ± 0.02 eV in the low temperature region and 0.21 ± 0.01 eV in the high temperature region for the pure sample, with corresponding values of 0.24 ± 0.01 eV and 0.23 ± 0.02 eV for the doped sample.T₁ was approximately the same for both samples, and essentially temperature independent, indicating an exchange-dominated spin-lattice decay mechanism.     

Oxidizing effects of high temperature annealing in reducing atmosphere in Ca-doped YIG films

The existence of an oxidation mechanism is reported in Ca-doped YIG films, when high temperature annealing is carried out in N₂ atmosphere. The annealing were performed at successively increasing temperatures, and optical absorption, lattice parameter and thermoelectric power were measured at each step. Optical absorption was observed going down to a minimum (T = 450°C in one cycle of annealing, T = 400°C in a different cycle) and then rise again. The minimum in absorption corresponds to a maximum of the lattice parameter, while the thermoelectric power is always p-type. Analysis of the data leads to the conclusion that we are observing a reoxidation process, triggered both by temperature and oxygen vacancy concentration. This process is subject to exhaustion. Previous annealing experiments are analyzed in the light of these results.     

Etude par correlation angulaire γ-γ perturbee des aspects statiques et dynamiques de la structure du heptafluorohafniate de sodium

We have studied the temperature dependence of the perturbation of the differential γ-γ angular correlation of the ¹⁸¹Ta 133–482 keV cascade in sodium heptafluorohafnate. In the 20–300°C temperature region, we observed a drastic variation of the quadrupole interaction parameters when raising the temperature from 100 to 180°C. Such a variation is due to the existence of a phase transition corresponding to an orthorhombic-quadratic transformation of the lattice. In the low temperature phase (T < 120°C) the perturbation is static and the nuclear probe is located at two equally populated sites. In the high temperature phase (T > 180°C) the perturbation arises from the combination of static and dynamic interactions. The dynamic part of the interaction could be analysed within the isotropic perturbation model and was ascribed to jumps of the fluorine atoms onto the vertices of the cubic coordination polyhedra of the hafnium in the HfF₇^3? ion. The activation energy of the process is 0.13 ±0.03 eV. The idea of an order-disorder phenomena occuring in the course of the phase transition is consistently supported by these observations. Our results yield also valuable informations about the nature of the hafnium-fluor bond in this compound.     

Fluorescence induction changes in bean leaves after heat treatment

The regularities in the alterations of chlorophyll a fluorescence induction curves of bean leaves after short (15 min) heating in the range of temperatures from 24 to 45°C were determined. A dramatic decrease in the relative f luorescence quenching (F_P–F_T)/F_T, as well as the F_v/_Fm ratio, were observed after heating at temperatures above 38°C, which indicates a decrease in the photochemical activity of photosystem II. The effect of an increase in the resistance of the photosynthetic apparatus to the temperature of 43°C after preheating at 37°C was demonstrated.     

Subnanosecond multiphonon relaxation of divalent copper impurities in LiTaO3

The temperature dependence of the lifetime of the excited ²T₂ state of the Cu²⁺ ion in a LiTaO₃ crystalline host has been measured with picosecond optical pulses. Its value ranges from 450 psec at 22°K to 10 psec at 423°K. Consistent with this result is the observation of weak fluorescence at 1.75 μm having a low temperature quantum efficiency of ～ 10^?6. The experimental results are in reasonably good agreement with recent theoretical models based on strong coupling of the Cu²⁺ impurities to the host lattice.     

Temperature dependence of the electric quadrupole interaction in PbHfO3

The temperature dependence of the Electric Field Gradient (EFG) in PbHfO₃ was studied in the temperature range 25–225°C by the Differential Perturbed Angular Correlation method. In the two anti-ferroelectric phases below 215°C, not too close to either transition temperature, the EFG decreases as the temperature increases toward T_c. Just above T_c an abrupt rise of EFG was observed indicating a critically behaving contribution to EFG. The results are interpreted in terms of a model based on the local static as well as time-dependent changes of the electric environment, at a lattice site. In accord with the predictions of this model the results exhibit qualitatively the P²_s temperature dependence of EFG far from T_c, where P_s is the sub-lattice polarization, while in close proximity to T_c the dominant contribution to EFG is due to the susceptibility X_q connected with the soft-mode fluctuations. The derived critical exponents are in agreement with previous experimental results on related compounds and with theoretical predictions. A hitherto unobserved additional component of V_zz was established, behaving critically at the antiferro-paraelectric transition at 215°C. This new component is interpreted to originate in local fluctuations connected with the central mode.