Vacancies in a strongly strained crystal at low temperatures

It is shown that an equilibrium vacancy concentration can be noticeable even at a temperature close to absolute zero. This will be the case if the volume deformation of a crystal is higher than a critical value, which is a second-order phase transition point.     

Nuclear quadrupole alignment in Zn,Lu, and Re and the quadrupole moments of69m Zn and177m Lu

We report low-temperature nuclear quadrupole alignment of^69m Zn,^{177m, g} Lu and^{186, 188}Re in the corresponding metal single crystal hosts. A comparison with data on these systems from other laboratories is given, and sources of systematic error discussed. In the Lu system, measurements on in situ irradiated and implanted samples are found to be in agreement. From the measured quadrupole frequencies, we derive the following quadrupole moments: Q(^69m Zn, 13.86 h, 9/2⁺)=?0.45(7)b,Q(^177m Lu, 161 d, 23/2^?) =4.23(67)b,Q(¹⁸⁶Re, 90h, 1^?)=0.57(9)b, andQ(¹⁸⁸Re, 16.7h, 1^?)=0.38(17)b.     

Nuclear spin-lattice relaxation of thallium at low temperatures

The nuclear spin-lattice relaxation time in thallium metal has been measured from 0.4 to 75 mK in magnetic fields from 18 to 1000 mT. The value of the Korringa constant к = T₁T is 4.37(8) ms K. It is independent of field and temperature over the ranges investigated and differs significantly the previously measured value.     

Magnetization process of single molecule magnets at low temperatures

We show that correlations established before quenching to very low temperatures later drive the magnetization process of systems of single molecule magnets, after a magnetic field is applied at t=0. We also show that in simple cubic lattices m proportional, variant sqrt[t], as observed in Fe8, but only for 1+2log((10)(h(d)/h(w)) time decades, where h(d) is a nearest neighbor dipolar magnetic field and a spin reversal can occur only if the field on it is within (-h(w),h(w)). However, the sqrt[t] behavior is not universal. For bcc and fcc lattices, m proportional, variant t(p), but p approximately 0.7. The value to which m finally levels off is also given.     

Nuclear quadrupole interaction in the low dimensional hf-trichalcogenides

We report on Time Differential Perturbed Angular Correlation measurements of the nuclear quadrupole interaction of¹⁸¹Hf(β⁻)¹⁸¹ and its temperature dependence in the low dimensional Hf- trichalcogenides HfS₃, HfSe₃ and HfTe₃. In HfS₃ the temperature dependence of the precession frequency is a slowly decreasing function with a negative curvature, the deviation from axial symmetry of the electric field gradient is practically temperature independent. In HfSe₃ the NQI shows a peculiar behaviour; between 10K and 300K the quadrupole frequency and the asymmetry parameter increase, between 300K and 400K the frequency decreases while the deviation from axial symmetry increases, indicating a structural phase transition near room temperature. In HfTe₃ the quadrupole frequency is practically temperature independent below 600K and increases linearely at higher temperatures, the asymmetry parameter increases steadily in the temperature range from 10K to 600K and then decreases. There is a strong correlation between the strength and the asymmetry of the electric field gradient so that the system depends on one control parameter only.     

Nonlinear acoustic transparency for longitudinal-transverse picosecond pulses in a paramagnetic crystal at low temperatures

Nonlinear propagation of longitudinal-transverse acoustic pulses of duration shorter than one oscillation period (video pulses) is studied theoretically in a system of paramagnetic centers with effective spin S=1. It is shown that, depending on the relationship between the magnitudes of the longitudinal and transverse strain components and on the detuning of their linear velocities, various regimes of propagation corresponding to different dynamics of the field and the medium can occur. In the case where the velocities of longitudinal and transverse hypersonic waves differ only slightly, an effect similar to self-induced transparency is analyzed. For substantial velocity detuning, propagation in the form of rational solitons is possible. If the transverse component is dominant, these solitons can produce full population inversion of Zeeman sublevels. In the opposite limit, the populations remain practically unchanged.     

Dipolar relaxation at low temperature of ice single crystal

The depolarization thermocurrent (D.T.C.) measurements of a single crystal of ice between gold electrodes allow observation of two relaxation mechanisms. One corresponds to the previously reported relaxation range which is ascribed to Bjerrum defects, the other is ascribed to ionic states.     

Zeeman nuclear quadrupole resonance study of a germanium tetrachloride single crystal

Zeeman studies of a single crystal of GeCl₄ have been carried out on a previously described pulsed FT NQR spectrometer. The technique for growing single crystals from liquid samples is described. The Zeeman-split spectrum for each of the resonances at μ₁ = 25.4493, μ₂ = 25.7133, μ₃ = 25.7354, and μ₄ = 25.7457 MHz reveals four quadruplets. The asymmetry parameters found lie in two groups of average values η = 0.035 and η = 0.078, much lower than the previously reported value of η = 0.35 obtained from a polycrystalline sample. The crystal of GeCl₄ is found to be orthorhombic with mmm or D_2h symmetry. From the directions of the field gradient axes the interbond angles have been calculated and show that the GeCl₄ molecule is a slightly distorted tetrahedron.     

Deformation of Cd-based alloys single crystals at very low temperatures

The temperature dependence of the critical resolved shear stress — CRSS on Cd-Ag andCd-Zn single crystals was studied at very low temperatures 1·5–80 K. The deformation experi-ments were made by a creep technique. The CRSS for Cd-Ag alloys was determined from re-constructed shear stress — shear strain curves, while the method of one sample was applied tothe determination of ₀-T dependence for Cd-Zn alloys. The difference in the temperaturedependences of ₀ for both Cd-based alloys can be caused by different methods for determiningthe CRSS.     

Deuteron NMR spectra of ND4ClO4 single crystal at low temperatures

²H NMR spectra of ND₄ClO₄ single crystal were obtained at ν₀ = 44 MHz. Orientation and temperature (1.9–75 K) dependences were measured. Fitting the spectra gives the effective quadrupole coupling constants for all deuterons and the ground torsional level structure. The isotope reduction of the (A-T) and (A-E) tunnelling splittings, i.e., the ratios of the respective splittings for NH₄⁺ and ND₄⁺, were found to be different. The splittings at T = 24 K are about 60% of the helium temperature values. The spectrum undergoes intermediate narrowing by reorientations between 26 and 34 K and tunnelling related features in the spectra are eradicated. After reaching the extreme narrowing limit, a doublet with gradually decreasing separation was observed, what was attributed to averaging by torsional oscillations of increasing amplitude. At high temperatures (T > 75 K), the narrow spectrum reflects fast multiaxial reorientation of the ammonium ion.     

锗单晶体变温霍尔效应实验数据的处理

采用微机控制和数字式数据采集系统,在变温环境下(77~420 K)对长为6.0 mm、宽为4.0 mm、厚为0.6 mm的锗样品薄片进行霍尔效应相关数据测量;通过对测量的霍尔电压作数据处理得到锗的霍尔系数RH(T)、电导率σ(T)和霍尔迁移率μH(T)与温度的依赖关系.该实验结果对学生理解半导体物理中的相关知识有重要意义.     

Nuclear electric quadrupole interaction of 182Re (64h) and 183Re recoil-implanted into graphite

Abstract

The technique of static low-temperature nuclear orientation has been used to investigate the nuclear electric quadrupole interaction of implanted ¹⁸²Re (64h) and ¹⁸³Re in thin foils of highly oriented pyrolytic graphite (HOPG) at temperatures down to 5 mK. The isotopes were recoil-implanted at a temperature of 700°C in order to limit the amount of lattice damage. The fraction of nuclei experiencing a large electric field gradient was considerably improved after two successive annealing sessions at 1200°C, from 1.0(0.6)% before annealing to 4.5(0.8)% after 6h and to 11.3(2.0)% after 162 h annealing. The electric field gradient for Re in HOPG is deduced to be V_zz = +5.4(2.6)·10²²V/m².     

Nuclear quadrupole interaction at99Tc in molybdenum compounds

The nuclear quadrupole interaction (NQI) at⁹⁹Tc on molybdenum sites in a series of molybdenum compounds was investigated by time differential perturbed angular correlation. The NQI is rather small for compounds with isolated oxygen/sulfur tetrahedra and fairly large for all compounds with condensed coordination polyhedra.On leave from Tanta University, Egypt     

XPS and UPS study of oxygen adsorption on Re(0001) at low temperatures

Oxygen adsorption was studied in the temperature range 50–300 K. Three adsorbed states were found: (i) an atomic state existing in the whole temperature range, (ii) a molecular state (physisorbed oxygen molecule) between 50 and 80 K, (iii) a peroxide state whose bond order between the two oxygen atoms is around one and which exists between 80 and 300 K.