Effect of the translational velocity of active particles on the properties of a photon echo

The photon echo method was used to detect an increase in the relaxation rate in the vibrational-rotational transition of fluoromethane in ¹³CH₃F gas and a ¹³CH₃F-argon mixture upon an increase in the velocity of active particles. For a photon echo in the 0–1 transition in a Yb + Xe mixture, the dependence of the relaxation parameters on the direction of the translational velocity of Yb leads to qualitative changes and a collision photon echo arises.     

Adiabatic and vertical carbon 1s ionization energies in representative small molecules

Adiabatic and vertical carbon 1s ionization energies are reported for methane (CH₄), ethane (CH₃CH₃), ethene (CH₂CH₂), ethyne (HCCH), carbon monoxide (CO), carbon dioxide (CO₂), fluoromethane (CH₃F), trifluoromethane (CHF₃), and tetrafluoromethane (CF₄) with an absolute accuracy of about 0.03 eV. The results are in good agreement with earlier values but are measured with higher resolution and accuracy than has previously been available.     

Magnetic irreversibility and metastability in a CuMn alloy

Our magnetic measurements on a CuMn alloy reveal that T_m, the temperature of the susceptibility peak, is marked by a qualitative change in the magnetization-vs.-field behavior. The magnetization process at and above T_m is reversible at all fields, whereas below T_m an irreversibility sets in at a threshold field H₁ and persists up to a second threshold field H₂. The irreversibility results in an isothermal remanence whose saturation value (attained when the field removed exceeds H₂) equals the saturation value of the thermoremanence (attained after cooling from above T_m in a field larger than H₂). Both states of remenence are characterized by a magnetization which is completely reversed by a small reverse field but returns to its original polarity when the field is removed. The hysteresis loop remains thus displaced as long as the cycling field is less than H₁.     

On the Meissner effect in gauge theories

The phase structure of spontaneously broken scalar electrodynamics in an external electromagnetic field is analyzed. With no external field, the spectrum comprises a scalar boson of mass m_H and a vector boson of mass m_W. If m_H ≤ m_W, it is shown that in the tree approximation, as the external field is increased, a first order phase transition to a restored symmetry phase occurs, and the critical field strength is calculated. Below the critical point the external field is completely screened, this being the analogue of the Meissner effect in superconductivity. If m_H > m_W, a third phase, characterized by vortex solutions of the field equations, occurs. Quantum effects, such as pair production in an electric field, are considered at the one (and two) loop level in the massless theory (the Coleman-Weinberg model). The leading correction to the critical magnetic field strength is calculated, and it is shown that for an external electric field the phase transition does not exist.     

Classical electrodynamics with nonlocal constitutive equations

It is assumed that the coupling of the field quantities D^μv (x) and F _αβ (x) is nonlocal. This hypothesis leads to a theory of an electromagnetic field that has the following properties.(1) The source of the field F _αβ (x) exhibits a center of charge and a center of mass that do not coincide, in general.(2) The field componentF _0i=?c²E_i is regular at the origin.(3) In the first-order approximation the new field equations are equivalent to the conventional Maxwell field equations.(4) The conventional cutoff procedure in momentum space as practiced in the Maxwell-Lorentz theory is equivalent to the first-order approximation in terms of an invariant length ξ².(5) The gyromagnetic ratio of the source of F _αβ (x) is equal toc/mc for a quantum of chargee and massm.     

Effects of a magnetic field on a modulated phase

The effects of a magnetic field on a modulated phase are studied. A modulated phase is found to have two critical fields H₁ and H₂. For a large enough magnetic field, H₁ and H₂ can be approximated by a linear law. As a result, the minimum magnetic field needed to destroy a modulated phase is a constant. The minimum magnetic field also greatly depends on the order of a commensurate phase. A very high order commensurate phase and an incommensurate phase cannot survive a magnetic field. The behaviour of a magnetoelastic chain in a magnetic field can be described by a harmless devil's staircase. The inverse temperature is found to play a role similar to that of a special magnetic field. The deeper physics underlying these new phenomena is the breaking of the left-right symmetry of a phase diagram. As a result a controllable path to a modulated phase is found.     

Magnetization reversal dynamics in La0.65Sr0.28Mn1.07O3 ceramics and NixZn1−x Fe2O4 nickel-zinc ferrite placed in complicated pulsating fields

The magnetization reversal dynamics in La_0.65Sr_0.28Mn_1.07O₃ ceramics and Ni_xZn_1?x Fe₂O₄ nickel-zinc ferrite placed in a magnetic field is studied experimentally. The field is a superposition of quasi-static field H, rf field h _ω cosωt, and low-frequency field h _ΩcosΩt with H ∥ h _ω ∥ h _Ω. The low-frequency response is found to be nonlinear in h _Ω. A procedure discriminating between the linear and nonlinear components of this response is proposed. Unlike the linear component, the nonlinear component is an essentially anharmonic function of time. The results are analyzed in detail using the high-frequency and low-frequency magnetization reversal curves of the materials.     

Kinetic theory of runaway breakdown in inhomogeneous thundercloud electric field

Kinetic theory of runaway breakdown in inhomogeneous thunderstorm electric field is developed. The kinetic equation, boundary and initial conditions are formulated. Spectrum of X-ray emission generated by runaway electrons is calculated. It has quite a specific form: a sharp maximum at energies 50–60 keV and a rapid fall both to the smaller and higher energies. The intensity of the emission is growing effectively with the relation E_m/E_c (of maximal electric field E_m to critical field E_c) and falling down sharply with the distance near the point z where electric field E(z) is equal to critical field E_c.     

Study of the field dependence of the magnetocaloric effect in Nd1.25Fe11Ti: A multiphase magnetic system

The field dependence of magnetic entropy change ΔS_M(T,H) has been studied in the crystalline sample Nd_1.25Fe₁₁Ti, a multiphase system constituted by three phases: Fe₁₇Nd₂, Fe₇Nd and Fe₁₁TiNd. The magnetic entropy change has been calculated from the numerical derivative of magnetization curves M(T,H) with respect to temperature and subsequent integration in field. To determine the field dependence of the experimental ΔS_M, a local exponent n(T,H) can be calculated from the logarithmic derivative of the magnetic entropy change vs. field. In contrast with the results for single phase materials, where n at the Curie temperature T_C is field independent, it is shown that for a multiphase system n evolves with field both at the Curie temperature of the system and the Curie temperatures of the constituent phases. This is in agreement with numerical simulations using the Arrott-Noakes equation of state.     

Influence of eddy currents on magnetic hysteresis loops in soft magnetic materials

In this paper an attempt has been made to extend the Jiles and Atherton (J–A) quasi-static hysteresis model to describe magnetisation of a material with an alternating magnetic field. In low – industrial – and medium frequency of magnetic field it is possible to ignore the magnetic relaxation and resonance. The field penetration is assumed to be uniform through the material. The influence of eddy currents on the hysteresis loop could be considered and calculated using the method of successive reactions of eddy currents, where a reaction is an additional magnetic field, called reaction H_d, induced in the material by the eddy currents according to rot J=γ∂B/∂t where γ is the electrical conductivity. The reaction field H_d was added to the basic field H₀∝Iz_1, where I is the current intensity in the magnetising coil of z₁ number of windings. By solving the J–A equation for the magnetic field H_w=H₀+H_d it has achieved an extension of the hysteresis loop at an increased frequency of the current, caused by increased losses of the eddy currents. At the frequency f→0 Hz , the hysteresis loop approaches the shape of the quasi-static one.     

Field dependence of uniform magnon energies in lamellar CoCl2 and CoBr2 by AFMR experiments

AFMR experiments in the frequency range 77<v<600 GHz and in the magnetic field range 0<H<75 kG on antiferromagnetic CoCl₂ and CoBr₂ single crystals at 4.2 K are reported. The waves propagate along the c axis of the crystals and the steady magnetic field is in the plane of the layers, which is also the easy plane of the spins.The influence of a rotation of the field in the easy magnetization plane is investigated, showing the existence of antiferromagnetic domains.The results of the field dependence of uniform magnon energies are reported in detail for the two compounds CoCl₂ and CoBr₂. In CoBr₂, the two magnon modes at zero field are non degenerate, their frequencies are v₁(0) = 60 GHzandv₂(0) = 675 GHz, and the field required for saturation is H_s⊥ = 74.2 kG.     

μSR studies of heavy fermion systems

We have performed both zero field and high transverse field measurements at dilution refrigerator temperatures on a number of heavy electron systems, examining the superconducting and magnetic properties of these interesting materials. Among the materials studied to date are UBe₁₃, URu₂Si₂ and U₆Fe. The magnetic field penetration depth in the superconducting state of UBe₁₃ is greater than 10000 Å, as no increase in the transverse field relaxation rate is observed belowT _c . A sharp increase in the precession frequency is seen, starting atT _c . This frequency shift shows little temperature dependence at low temperature; we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu₂Si₂ show the weak antiferromagnetic transition at 17.5 K. Finally, we we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu₂Si₂ show the weak antiferromagnetic transition at 17.5 K. Finally, we have observed relaxation in high transverse field due to the formation of a flux lattice in U₆Fe, a material where the electron effective mass is rather lighter than in other heavy fermion systems. The relaxation exhibits a sharp onset atT _c=3.9 K, and is flat at low temperatures as expected for a conventional superconductor.     

Features of the magnetic and magnetoelectric properties of HoAl3(BO3)4

The main features of the magnetic and record magnetoelectric properties of a HoAl₃(BO₃)₄ aluminoborate single crystal have been studied experimentally and theoretically. It has been found that the electric polarization that was previously detected in HoAl₃(BO₃)₄ and is record for multiferroics is significantly larger, ΔP _ba (B _a ) ≈ ?5240 μC/m², with an increase in the magnetic field to 9 T at T = 5 K. The measured magnetic properties and revealed features have been interpreted within a united theoretical approach based on the molecular field approximation and on calculations in the crystal field model for a rare-earth ion. The experimental temperature (from 3 to 300 K) and field (up to 9 T) dependences of the magnetization have been described. The parameters of the crystal field of trigonal symmetry for a Ho³⁺ ion in HoAl₃(BO₃)₄ have been determined from the interpretation of the experimental data.     

The instability point H s of the Meissner state for large-κ superconductors

The critical field H _s corresponding to the emergence of vortices in a superconductor without a threshold is found near the transition temperature and in the limit as T→0 for an arbitrary value of the depairing factor Γ. In superconductors of the second kind, this field value coincides with the absolute instability point of the Meissner state. In large-κ superconductors, the order parameter tends to zero on the surface of the super-conductor if the external magnetic field reaches the value H _s. We obtain that H _s=H _cm (where H _cm is the thermodynamic critical field) for an arbitrary value of the depairing factor Γin the temperature region near T _c and at T=0.     

Magnetic properties of (NdxY1-x)Co2(1⩾ x ⩾ 0)

The concentration dependence of T_c and T_R (T_c magnetic ordering temperature, T_R spin reorientation temperature) of the pseudobinary system (Nd, Y)Co₂ is reported. Furthermore the influence of an external magnetic field on the spin reorientation and the magnetization is studied. The observed variation of the magnetization in the vicinity of the spin reorientation is compared with theoretical results. For the calculation a Hamiltonian with terms describing a molecular field, a cubic crystal field, and an external field is used.     

Field and temperature dependences of the specific heat of the La1.85Sr0.15CuO4 superconductor

This paper reports on the temperature and field dependences of the specific heat of high-quality La_1.85Sr_0.15CuO₄ single crystals carried out at low temperatures in magnetic fields of up to 8 T for two magnetic field orientations, namely, along the [100] and [110] crystallographic axes. The field dependence of the electronic density of states (DOS) was found to be anisotropic for different magnetic field orientations in the a–b plane, with the electronic density being the lowest along the a axis (for H ∥ [100]) and maximum for the field inclined at 45° to the a axis (for H ∥ [110]). Electronic specific heat in a magnetic field was observed to depend linearly on temperature T and nonlinearly on the magnetic field H: C _DOS=bTH ^1/2. In a zero field, the electronic specific heat grows quadratically with temperature as C _DOS=αT ². Estimation of the maximum superconducting gap width from the experimentally determined values of the α coefficient of T ² and of the electronic DOS in the normal state yields Δ ₀=300 K. The observed features indicate that La_1.85Sr_0.15CuO₄ is a superconductor with d symmetry of the order parameter.     

Dielectric and electromechanical properties of (1−x)PMN-xPZT ferroelectric ceramics

The dielectric and electromechanical characteristics of the (1?x)PMN-xPZT ferroelectric ceramics have been obtained at different temperatures, amplitudes, and frequencies of the measuring field and at different bias field strengths. It is shown that this ferroelectric ceramics at low and infralow frequencies possesses pronounced relaxor properties in a certain temperature range and ferroelectric properties in other temperature range. The temperature and amplitude ranges have been determined, in which the permittivity ?′ either only decreases or first increases and then decreases with an increase in the measuring field amplitude E ₀. The temperature ranges of existing the phases similar to the superparaelectric phase, dipole glass phase, and ferroelectric phase are evaluated from the temperature dependences of the coercive field E _c (T) and the remanent polarization P _r (T) and also from the reverse dependences of ?* and the electromechanical characteristics. The PZT concentration in the PMN-PZT system is determined, at which the electrostrictive constant M ₁₁ is maximum. It is demonstrated that, in the neighborhood of the temperature at a maximum of ?′, the strain S ₃ is quadratic in the field E ₌; that is, S ₃=M ₁₁ E ².     

A dipole approximation for a dielectric mixture based on the equal field energy method

In this paper, a dipole-energy approximation for calculating the electric field distribution and saturation charge of spheres in an infinitely large dielectric mixture has been carried out. The approximation rests on the assumption that the field energy of mixture calculated using two different methods have the same value. One way is considering that the mixture in a uniform field E₀ as a uniform object of effective permittivity ?_eff from a macroscopic point of view, where ?_eff is seen as the average characteristic parameter of the object. The other way is assuming that the spheres in mixture are in the equivalent external field E₀′, and E₀′ related to the dielectric mismatch and the ratio of the sphere radius to the average distance between neighboring spheres has been obtained. Based on dipole-energy model, the approximate formulas for calculating the maximum field strength and saturation charge of spheres are derived separately.     

Hysteresis phenomena in the course of polarization evolution in a sinusoidal electric field in lead magnesium niobate crystals

The processes of polarization evolution in single crystals of the PbMg_1/3Nb_2/3O₃ model ferroelectric relaxor in a sinusoidal electric field are investigated at temperatures near and above the temperature T _d 0 of destruction of the induced ferroelectric state upon heating in zero electric field. The polarization switching current loops are measured in the ac electric field applied along the 〈111〉 and 〈110〉 pseudocubic directions. The electroluminescence intensity loops are obtained under the combined action of ac and dc electric fields applied along the 〈100〉 direction. In a certain temperature range above T _d 0 and the freezing temperature T _f in lead magnesium niobate, there are electric current anomalies, that correspond to the dynamic formation and subsequent destruction of the ferroelectric macroregions throughout each half-cycle of the ac electric field. The measurements of electroluminescence hysteresis loops demonstrate that the observed depolarization delay (related to the ac electric field amplitude) increases with an increase in the dc electric field and decreases as the ac field amplitude increases. The nature of the observed phenomena is discussed.     

Deformation of nematic layers in crossed electric and magnetic fields

A parallel nematic layer will show a Fréedericksz-transition in an electric field when the voltage exceeds a threshold value U_c. If a magnetic field is applied parallel to the layer but perpendicular to the molecular orientation the threshold voltage U_c increases with increasing magnetic field field H above a threshold field H_c2. From a plot of U_c vs H all three elastic constants can be obtained. Theoretical results are compared with data of HBAB.