The critical point indices of ferroelectric and ferromagnetic phase transitions

Magnetic transitions are described by the critical indices0,1/3,4/3 while some ferroelectric transitions appear to give0,R~1/2,1. It is pointed out that these two sets of values for the critical indices are allowed by the scaling laws and stability conditions near the phase transitions.The authors thank Prof. R. S. Krishnan for his encouragement and Mr. B. Viswanathan for some discussions. The financial assistance from DAE and CSIR is also acknowledged.     

Self interstitial trapping at Hf impurities in Niobium

After electron irradiation at 15 K of Niobium doped with¹⁸¹Hf the trapping of defects at the Hf impurities was observed by means of the perturbed angular correlation method. The results are interpreted as the formation of Hf-Nb mixed dumbbells which are formed at 5 K and are stable up to 80 K.     

A broadband photon drag detector for pulsed, high-power radiation detection

A broadband detector based on the photon drag effect in semiconductors has been used for radiation detection from 600 m to 1 cm wavelengths. The measured responsivity of the detector at the longer wavelengths is 3 V/W.     

Detecting torsion from massive electrodynamics

A new method of detecting torsion in the case of massive electrodynamics is proposed. The method is based on the study of spectral lines of hydrogen-like atoms placed in a torsion field, where the interaction energy between the torsion vector field Q and an electric dipole is given by P · Q. All the methods designed so far have been based on spinning test particles interacting with magnetic fields in which the energy splitting is given by S · B on a Stern-Gerlach type experiment. We arrive at an energy splitting of order of 10^–21erg 10^–9eV, which is within the frequency band of radio waves.     

Determination of the Coefficient of Laser Radiation Reflection from Dielectric Targets in Vacuum

We present the results of experimental determination of the coefficients of laser radiation reflection (₁ 10.6 m and ₂ 1.06 m) from dielectric targets of complex chemical composition in vacuum with allowance for a regime of developed plasma formation.     

Temperature dependence of the effective work function for the BaO - W system

The temperature dependence of the work function has been examined for the BaO-W system for degrees of coating less than 1 and about 1 in the range 700–1400 °K; it is found that a monomolecular film gives a negative sign of this coefficient in this temperature range ( /T 10^–4 –10^–3 eV/deg). At 1000–1200 °K, the temperature coefficient is almost zero, while at 1200–1400 °K there is a positive value. If the covering is less than monomolecular, the sign is positive throughout the entire temperature range (/T 10^–4 eV/deg).Deceased.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 9, pp. 65–70, September, 1971.We are indebted to N. I. Éngovatova for direct assistance with the measurements, and to V. Rumyantsev for advice.     

57Fe Mössbauer studies in a new cluster spin glass system RuxFeGa (0.5≦x≦1.5)

⁵⁷Fe Mössbauer and bulk magnetization studies have been carried out in a new magnetic system Ru_xFeGa for 0.5x1.5. The system is seen to freeze into a cluster spin glass phase at 70 K. The freezing temperature and the hyperfine field at Fe at the lowest measured temperature, 190 kOe, is essentially same for all values of x investigated.     

Cubic nonlinear tensor of a ferromagnet

The nonlinear properties of a ferromagnet are studied. Many-time retarded Green's functions are used to obtain an expression for the cubic nonlinearity tensor with allowance for spatial dispersion of a uniaxial ferromagnet. The components due to the dipole-dipole interaction of the spins and also due to the anisotropy energy are found. A comparative analysis is made of the different components of the cubic nonlinearity tensor in both the nonresonance case and for various resonances, in particular when ₀, 3 2w₀, 2 ₀, 3 ₀ for the case in tripling of the frequency. Here, is the frequency of the incident wave and ₀ is the frequency of uniform precession. It is shown that in the non-resonance case the largest components are those that are nonvanishing when no allowance is made for spatial dispersion; in the resonance cases the largest components are those due to the dipole-dipole interaction of the ferromagnetism spins.Translated from Izvestiya VUZ. Fizika, No. 12, pp. 53–58, December, 1973.     

Studies of laser-induced cavitation and tissue ablation in saline using a fibre-delivered pulsed HF laser

We describe studies of the interaction of 400 ns duration, fibre delivered, multiline (2.6–3.0 m) HF-laser pulses with cornea and retina samples in saline. In this wavelength region water exhibits strong absorption (beam penetration depth 1.6 m) and laser heating leads to the creation of a hot, high pressure, vapour cavity (laser-induced cavitation) at the fibre tip. The dynamics of vapour cavity growth have been investigated experimentally using the laser shadowgraph technique and theoretically by employing an equivalent spherical cavity model for an incompressible liquid. Measurements of ablation rates and transient acoustic pressures for cornea ablation in saline, together with scanning electron microscope evaluation of irradiated samples are used to assess the damage range and mechanisms for this mid-IR laser in a strongly absorbing fluid.     

ИЗЛУЧЕНИЕ ЭЛЕКТРИЧЕ СКОГО ДИПОЛЯ НАД ГРАН ИЦЕЙ ДВУХ СРЕД

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On the theory of ferromagnetic thin films

As is known, the second approximation in the calculation of the partition function by the traces method of ferromagnetic thin films gives wrong results for the coordination number equal to eight. In order to obtain correct results even for this case, the third order approximation of the partition function is developed and thus the magnetic properties of body-centred cubic iron thin films are studied. The dependence of the Curie temperature on the thickness, for different values of the ratio between the anisotropy constant and the exchange energy between two neighbours, is discussed. A value can be chosen for this ratio such that the thin film becomes ferromagnetic only for a thickness greater than a definite value.

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Hyperfine field of samarium in europium monoxide

The hyperfine magnetic field at samarium in europium monoxide at 4.2 K was measured using the time-differential perturbed angular correlation technique. The resultH _hf=147.9±9.7 T indicates a 4f contribution of 130±20 T which is 60% less than the free ion value for Sm³⁺. It is suggested that there exists crystal plus exchange field interactions causing admixtures of the first excited state of Sm³⁺ with its ground state.     

Decay of the Two-Point Function in One-Dimensional O(N) Spin Models with Long-Range Interactions

Using Griffiths and Lieb–Simon type inequalities, it is shown that the two-point function of ferromagnetic spin models with N components in one dimension decays like the interaction J(n)n ^– provided that 1N4 and T>T _c.     

Theory of percolation in fluids of long molecules

We use the reference interaction site model (RISM) integral equation theory to study the percolation behavior of fluids composed of long molecules. We examine the roles of hard core size and of length-to-width ratio on the percolation threshold. The critical density _c is a nonmonotonic function of these parameters exhibiting competition of different effects. Comparisons with Monte Carlo calculations of others are reasonably good. For critical exponents, the theory yields =2=2 for molecules of any noninfinite lengthL. WhenL is very large, the theory yields _cL ^–2. These predictions compare favorably with observations of the conductivity for random assemblies of conductive fibers. The threshold region where asymptotic scaling holds requires the correlation length (/ _c ) ^–v to be much larger thanL. Evidently, the range of densities in this region diminishes asL increases, requiring that density deviations from _c be no larger thanL ^–2. Otherwise, crossover behavior will be observed.     

Comments on transitions in bulk tetramethyltin Sn(CH3)4 and as adsorbate on a (0001) graphite plane

In the case of tetramethyltin, whether in bulk form or in the form of an adsorbate on (0001) graphite basal plane, it was found that anomalies in the Mössbauer or X-ray diffraction parameters occur at similar temperatures, viz: 83 K, 95 K, and 125 K. It is therefore suggested that these anomalies can be associated in some way with intramolecular rearrangements and be affected by intramolecular motions.Sponsored by the U.S.-Israel Binational Science Foundation     

On the interface stiffness of the random field ising model

Recent results of Grinstein, Ma, Villain and Binder on interface roughening incontinuum andlattice random field Ising models are related by introducing an effective interface stiffness function {ei247-1}. Ford3 dimensions the continuum theory is shown to be valid for non-zero random field strengthh for all temperatures and on a length scaleL>l _d (h,T) _d (h,T). Ford=2 and smallT a smeared spin-glass transition occurs at ₂(h,T)h. It is argued, that for 3<d<5 interface roughening occurs only forh larger than a critical field strengthh _R (T).     

Quasistatic magnetic permeability of an amorphous ferromagnet

Dependence of the differential magnetic permeability on the speed of magnetization reversal was found in ribbons of an amorphous ferromagnetic material. When the ribbon thickness is 10^–3 cm and the initial permeability is 10^–5, an increase in the frequency of the harmonic magnetization reversal causes a decrease of by a factor of 2–3; this takes place in the region around 100 Hz. The effect is explained by the influence of eddy currents on the magnetization distribution.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 58–62, January, 1986.     

A Mössbauer study of FeC2O4·2D2O below the néel temperature

From low-temperature Mössbauer measurement on FeC₂O₄·2D₂O the reported difference in quadrupole splitting from the simple dihydrate is inferred to be due to lattice effects. The Zeeman split spectrum has been analyzed taking into account the ambiguity problem and the hyperfine parameters were determined to be I.S.=1.22 mm/sec; Q.S.=1.93 mm/sec; =0.65 to 0.72; =90 to 83.1^o and =0 to 11.8^o. The principal electric field gradient axis lies along the crystal a-axis with V_YY and lying along the crystal b-axis. The crystal field parameters 10Dq, Ds and Dt have been determined to be 10500, 185 and 211 cm^–1, respectively.     

Operation of the quasi-optical gyrotron at harmonics of the gyrofrequency

This paper investigates the performance of a quasioptical gyrotron, when the electron beam interacts with the radiation fields at harmonics of the gyrofrequency. The nonlinear equations of motion are obtained in the slow-timescale. The expression for the linear gain is derived and the conditions for excitation are given (frequency threshold, optimal operating point, bean current and resonator quality thresholds). In the nonlinear regime, it is shown that maximum efficiencies comparable to those at the fundemental (50%) are possible, albeit at a prohitively high radiation field amplitude, while realistically feasible field amplitudes can give somewhat smaller, but nevertheless still high efficiencies (15%). Finally, the results are suplemented by empirical scaling laws, useful for experimental designs.     

Electromagnetic-Wave Scattering from Steep Mesoscale Wavelets: Interpolating the Results of the Perturbation Theory and the Geometrical Theory of Diffraction

We analyze the dependence of the microwave backscattering cross section on the height h of steep isolated mesoscale wavelets. The scattering cross section is calculated based on the Born approximation of the perturbation theory valid for small wavelet heights kh 1, where k is the wave number, and also within the framework of the geometrical theory of diffraction valid for kh 1. We propose a convenient interpolation formula which yields a plausible estimate of the scattering cross section for arbitrary values of kh, including the intermediate case kh 1 that cannot be described by the well-known theories.