NMR Relaxometer Operating in a Continuous Mode upon Rapid Adiabatic Approach to Resonance

A nuclear magnetic resonance relaxometer operating in a continuous mode of the high-frequency low-power generation upon a rapid approach to resonance, and consequent “passage” through resonance for the time of the signal decay has been developed. In addition to the increased signal-to-noise ratio, the application of the periodic audio-frequency modulation of the magnetic field under adiabatic conditions has enabled us to obtain the absorption derivative, which, in turn, has allowed us to record the establishment of the thermodynamic equilibrium between spins and the lattice. Experimental data on the amplitude–relaxation response of the relaxometer are given. Spin–lattice relaxation times of some proton-containing substances have been determined as well.     

Coexistence of superconductivity and magnetic order

Experimental work on systems showing evidence of the coexistence of superconductivity and magnetic order is reviewed. The presently known systems are shown to be “superconducting spin glasses” rather than “ferromagnetic superconductors”. In those systems where the magnetic order is expected to be long-ranged (up to now) superconductivity vanishes as soon as the magnetic order appears. But it is not excluded that magnetic order with a correlation length larger than the superconductive coherence length can coexist with superconductivity.     

Flexible laser diode trace gas sensor based on cavity ringdown spectroscopy with an optical fiber-coupled high-finesse external-cavity diode laser

A flexible and portable trace nitrogen dioxide sensor based on cavity ringdown spectroscopy using an optical fiber-coupled high-finesse cavity was successfully demonstrated. Tailoring the spatial mode matching condition of the core of an optical fiber and high-finesse external cavity allows for effective optical feedback into an antireflection-coated laser diode for stable resonant enhancement of the external cavity. The external cavity, which works as a ringdown cavity, could be remotely located from the light source and receiver section by only a single mode optical fiber. The sensitivity was found to be 1.0×10⁻⁷ cm⁻¹ in a compact 1-cm³ ringdown cavity volume.     

Ferromagnetic resonance and bistability field in a uniaxial magnetic film

Peculiarities of ferromagnetic resonance (FMR) corresponding to bias along the “hard” magnetic axis of a film with 2D uniaxial anisotropy are studied based on numerical solution of magnetic moment dynamics equations. It is shown that an additional resonance peak is formed in the FMR spectrum in the vicinity of “bistability field” H _b . The dependence of this field on the amplitude of the microwave field and damping parameters is analyzed.     

Optical vortices in the scattering field of magnetic domain holograms

Experimental investigations and theoretical-model analyses have been made of the magnetooptic diffraction of light at ferrite garnet magnetic films with a banded domain structure which includes isolated magnetic grating defects in the form of “forks” and “breaks.” An analysis of the magnetic grating structure and the light diffraction field shows that in terms of its action on laser radiation, a banded domain grating is similar to a computer-synthesized phase hologram of an isolated pure screw-wavefront dislocation. It is shown that as a result of magnetooptic diffraction at a magnetic hologram, optical vortices may be reconstructed with a helicoidal wavefront carrying the topological charge l=±1,±2. Zh. Tekh. Fiz. 68, 54–58 (December 1998)     

Continuous-wave cavity-ringdown detection of stimulated Raman gain spectra

Cavity ringdown (CRD) spectroscopy, with its high sensitivity, provides a novel way to perform continuous-wave (cw) stimulated Raman gain (SRG) spectroscopy, rather than by conventional optically detected coherent Raman techniques. Tunable cw laser light at ∼1544 nm is used to probe ringdown decay from a rapidly-swept, high-finesse optical cavity containing a gas-phase sample of interest and itself located inside the cavity of a cw single-longitudinal-mode Nd:YAG ring laser operating at ∼1064.4 nm. This approach is used to measure cw SRG spectra of the ν ₁ fundamental rovibrational Raman band of methane gas at ∼2916.5 cm⁻¹. The resulting SRG-CRD resonances have ringdown times longer than in the off-resonance case, in contrast to the usual shorter ringdown times arising from absorption and other loss processes. Previously reported noise-equivalent sensitivities have been substantially improved, by using a second ringdown cavity to facilitate subtraction of infrared-absorption background signals. Moreover, by employing a ringdown cavity in the form of a ring, the SRG-pump and CRD-detected Stokes beams can co-propagate uni-directionally, which significantly reduces Doppler broadening.     

Prospects for the rapid detection of mealiness in apples by nondestructive NMR relaxometry

The potential of nuclear magnetic resonance (NMR) relaxometry for quantitative evaluation of apple mealiness has been investigated. The degree of “mealiness” was defined by several mechanical techniques, including penetration, compression and shear rupture as well as by the BRIX (soluble solids) and juiciness levels. These data were correlated with both magnetic resonance imaging (MRI) and NMR water proton transverse relaxation time measurements on a fruit-by-fruit basis. It was found that increasing mealiness caused a systematic increase in the transverse relaxation rate. The potential for rapid, on-line NMR/MRI detection of apple mealiness is discussed.     

A Novel Magnetic Resonance Phased-Array Coil Designed with FDTD Algorithm

Radio-frequency receiver phased-array coils in magnetic resonance imaging systems are used to pick up the signals emitted by the nuclei with high signal-to-noise ratio and a large region of sensitivity. Since the quality of obtained images strongly depends upon the correct choice of the coil geometry and position, array coils have to be designed by minimizing the mutual interaction among nearby coil elements and this is generally achieved by overlapping such adjacent elements. In this paper, we describe the use of a numerical solver based on finite-difference time-domain method to determine the optimal overlap distance, which guarantees the maximum decoupling level between the coil loops, for array coils constituted by various geometry elements. A novel array coil was designed, constituted by a couple of elliptical geometry elements in “folding” version around the animals’ spine curvature, for small animals’ imaging applications.     

Hindered decay of an unstable system: A quantum zeno effect

A direct test of the so-called “quantum Zeno effect” is proposed for a truly decaying system. It is suggested that the lifetime of an unstable atom can be extended by illuminating it with an intense laser beam at the frequency of another of its transitions. The “Zeno” time is also compared to the lifetime     

Kinetics and Optical Properties of the Strongly Driven Gas Medium of Interacting Atoms

This paper investigates stimulated emission and absorption near resonance for a driven system of interacting two-level atoms. Microscopic kinetic equations for the density matrix elements of N-atom states including atomic motion are built, taking into account atom-field and atom-atom interactions. Analytical solutions are given for the resulting macroscopic equations in different limits, for a system composed of a strong coherent “pump” field and a weak counter-propagating “probe” field. It was shown that the existence of a dipole-dipole (long-range) interaction between atoms separated by distance less than the pump wave-length can cause the formation of periodic polarization and population structures (gratings in time and space) in the pumped medium without a probe field. The magnitude of pump induced population grating can have a strong dependence on the relation between the pump field detuning and the polarization decay rate. The “interaction” between pump and probe induced polarization/population gratings through a dipole-dipole interaction mechanism causes the absorption line shape asymmetry. Under certain conditions, this asymmetry is revealed in increasing probe gain for the “red”-shifted (relative to pump) probe and suppressing the gain for the “blue”-shifted probe field when pump is “red”-shifted relative to the ensemble averaged resonant frequency. The theoretical results are consistent with experimental data for the probe gain or absorption as the function of frequency and the dependance of the gain on atomic density for sodium vapor when the pump laser is tuned near the D ₂ line. Here the dependance of gain on particle density was explained in the terms of the long-range interaction between the atoms.     

Thermodynamics of the HMF model with a magnetic field

We study the thermodynamics of the Hamiltonian mean field (HMF) model with an external potential playing the role of a “magnetic field”. If we consider only fully stable states, the caloric curve does not present any phase transition. However, if we take into account metastable states (for a restricted class of perturbations), we find a very rich phenomenology. In particular, the caloric curve displays a region of negative specific heat in the microcanonical ensemble in which the temperature decreases as the energy increases. This leads to ensembles inequivalence and to zeroth order phase transitions similar to the “gravothermal catastrophe” and to the “isothermal collapse” of self-gravitating systems. In the present case, they correspond to the reorganization of the system from an “anti-aligned” phase (magnetization pointing in the direction opposite to the magnetic field) to an “aligned” phase (magnetization pointing in the same direction as the magnetic field). We also find that the magnetic susceptibility can be negative in the microcanonical ensemble so that the magnetization decreases as the magnetic field increases. The magnetic curves can take various shapes depending on the values of energy or temperature. We describe first order phase transitions and hysteretic cycles involving positive or negative susceptibilities. We also show that this model exhibits gaps in the magnetization at fixed energy, resulting in ergodicity breaking.     

Physical principles and methodology for investigating the frequency selective properties of randomly inhomogeneous channels with discrete opaque inhomogeneities

The characteristic features of experimental and theoretical investigations of the pass band in channels having discrete mutually screening inhomogeneities are analyzed. A comparison is made of methods and methodologies for measuring the delay spectra and the angular energy spectra of a multibeam field at a point of observation in such channels and in channels having continuously distributed “transparent” inhomogeneities. It is shown that when estimating the pass band in the former, the delay spectrum must be analyzed in a moving system of coordinates whose origin is coincident with the time of arrival of the first “beam.” A corresponding theoretical analysis is performed for the case when the “beam” arrival times form a nonsteady-state Poisson stream. State University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 95–101, June, 1996.     

Formation of the pulsed electron-electron double resonance signal in the case of a finite amplitude of microwave fields

A theoretical study of the effect of microwave (MW) fields of finite amplitude on the process of pulsed electron-electron double resonance (PELDOR) signal formation is carried out. It is shown that the behavior of the experimentally observed values can be described by four vectors of partial magnetizations whose motion is reduced to precession in effective magnetic fields. In the case of strong spin-spin interaction, the PELDOR effect can be observed when a sufficiently powerful MW field is applied at pumping frequency to affect both components of the Pake doublet. The possibility of a “two-frequency” spin echo to appear under the action of two pulses with different carrier frequencies in the system where the inhomogeneous broadening of the electron spin resonance line contour is mainly determined by the dipole-dipole interaction is demonstrated.     

Nuclear magnetic relaxation in ferrimagnetic Y3Fe5−x SixO12 films

The effect of silicon impurities on the damping of spin-echo signals from the ⁵⁷Fe nuclei of tetrahedral Fe³⁺ ions in epitaxial yttrium-iron-garnet films was investigated. It was found that for silicon concentrations 0.015⩽x⩽0.037 the damping of the spin echo is a two-component process, which made it possible to separate nuclei into two types, differing by both the longitudinal and transverse magnetic relaxation times. For silicon concentrations 0.044⩽x⩽0.073 the decay of the echo can be described by one exponential and all nuclei in the sample have the same transverse relaxation times and the same longitudinal relaxation times. The experimental results are interpreted on the basis of the supposition that impurity “macromolecules” form around the Si⁴⁺ ions. The relaxation times of the iron nuclei in a “macromolecule” are much shorter than the relaxation times of iron nuclei belonging to the matrix ions. The radius of a “macromolecule” is estimated on the basis of percolation theory. Fiz. Tverd. Tela (St. Petersburg) 40, 1494–1497 (August 1998)     

Specific heat of magnetic clusters with S=2 YBa2Cu3O7−δ

An analysis is made of published data from measurements of the temperature-and-field dependence of the specific heat in YBaCuO high-temperature superconductors at temperatures below 10 K. It is shown that the “excess” contribution to the specific heat can be explained by spin splittings in copper-oxygen clusters with S=2. The magnitudes of the necessary splittings agree with the available magnetic resonance data. Fiz. Tverd. Tela (St. Petersburg) 39, 1320–1322 (August 1997)     

Water transbilayer diffusion in macroscopically oriented lipid bilayers as studied by pulsed field gradient NMR

Water self-diffusion in lipid bilayers macroscopically oriented on glass plates was studied by pulsed field gradient¹H nuclear magnetic resonance technique. Diffusion decays were multicomponent with a distribution of diffusion coefficients ranging from about 10⁻¹⁰ to about 10⁻¹³ m²/s. A number of measurements with variations of the sample orientation, diffusion time and the distance between the glass plates showed that the “fast” component of diffusion corresponds to water in the bilayer “cracks”. The “slow” component of diffusion corresponds to transbilayer water diffusion in the long-diffusion-time regime. For a more reliable separation of parts corresponding to fast and slow diffusion of water, a “component-resolved spectroscopy” method for the global analysis of correlated spectral data (P. Stilbs, K. Paulsen, P.C. Griffiths: J. Phys. Chem. 100, 8180, 1996) was applied.     

Superradiance by a system of highly polarized exchange-coupled nonequivalent spins

We propose a model of radiofrequency (rf) superradiance by a system of interacting nonequivalent spins in a point specimen. In contrast to the rf superradiance observed and described earlier, here spin-spin coupling acts as the interaction with the cavity. To be definite, we examine the spins of two isotopes of a metal that are coupled by the Ruderman-Kittel interaction. The analysis of such a system when the magnetization of one spin species is inverted shows that the system can have one resonance frequency and two different decay times, instead of two resonance frequencies and one decay time in the usual situation. When such “repulsion” of decay times occurs and the absolute values of the spin polarizations are large, transverse magnetization increases and exhibits features characteristic of superradiance. Finally, we calculate the parameters of this superradiance: the voltage across the terminals of an rf pickup coil, the pulse length, the delay time, and the superradiant intensity. Zh. éksp. Teor. Fiz. 112, 551–563 (August 1997)     

Destruction of the long-range antiferromagnetic order by defects of the “random local field” type

The behavior of diluted antiferromagnets in an external magnetic field has been considered. It has been shown that, because the “force” of induced defects of the “random local field” type depends on the magnetic induction, the destruction of the long-range antiferromagnetic order by these defects in space of dimension d ≥ 2 is impossible.     

Nonequilibrium properties of re-entrant spin glasses in a magnetic field

In this paper the authors discuss how the re-entrant spin-glass state arises in the disordered alloys Ni_100−x Mn_x (x=19, 21, 23), along with the nonequilibrium magnetic properties of these alloys. It is shown that near the Gabay-Toulouse phase line the time it takes the system to reach equilibrium is comparable to times required to perform static experiments (10¹–10⁴ s); cooling the sample into the region of crossover with the de Almeida-Thouless line causes these times to increase to astronomical values of more than 10¹⁶ s. A method is proposed for constructing magnetic phase diagrams of systems of this type in “magnetic field-temperature” coordinates. Fiz. Tverd. Tela (St. Petersburg) 41, 2028–2033 (November 1999)