Dielectric relaxation and magnetic characteristics of the Bi0.5La0.5MnO3 ceramics

The complex permittivity ?* of ceramics of bismuth-lanthanum manganite Bi_0.5La_0.5MnO₃ has been measured in ranges of temperatures T = 10–200 K and frequencies f = 10²–10⁶ Hz. Clearly pronounced regions of the non-Debye dielectric relaxation have been revealed at low temperatures (T < 90 K). To describe them, the possible mechanisms have been proposed and discussed. The temperature dependences of magnetization, the anomalous behavior of which can be associated with the phase transition from the paramagnetic phase into the ferromagnetic phase occurring at T ～ 40–80 K, have been measured in the temperature range T = 10–120 K.     

Complex impedance spectroscopy studies of (La0.70−xNdx)Sr0.30Mn0.70Cr0.30O3 (x≤0.30) perovskite compounds

The transport properties of Nd-doped perovskite materials (La_0.7−xNd_x)Sr_0.3Mn_0.7Cr_0.3O₃ (x≤0.30) were investigated using impedance spectroscopy techniques over a wide range of temperatures and frequencies. AC conductance analyses indicate that the conduction mechanism is strongly dependent on temperature and frequency. The DC conductance plots can be described using the small polaron hopping (SPH) model, with an apparent reduction of the polaron activation energy below the Curie temperature T_C. Complex impedance plots exhibit semicircular arcs described by an electrical equivalent circuit. Off-centered semicircular impedance plots show that the Nd-doped compounds obey to a non-Debye relaxation process. The conductivity of grains and grain-boundaries has been estimated. The activation energies calculated from the conductance and from time relaxation analyses are comparable. This indicates that the same type of charge carriers is responsible for both the electrical conduction and relaxation phenomena.     

Analysis of the AC electrical data in the Davidson-Cole dielectric representation

The representation of the AC electrical data in the complex plane reveal two major classifications of the relaxation processes known as ideal (Debye) and non-ideal (non-Debye) types. The non-Debye relaxation has been empirically observed via Cole-Cole (C-C), Davidson-Cole (D-C), and Havriliak-Negami (H-N) responses. Each of these non-Debye relaxations is visualized with an equivalent circuit similar to the ideal relaxation. Both ideal and C-C relaxations reveal semicircular behavior in the complex plane while D-C and H-N relaxations deviate from the usual semicircular loci known as skewed behavior. The extracted equivalent circuit elements are essentially non-Debye for both D-C and H-N relaxations possessing complexity in the relaxation time. The analytical method of extracting these elements in conjunction with the empirical parameters of the D-C relaxation is described using conventional (real) domain and complex domain. The curve fitting procedure provided extremely small error for the complex domain analysis. The behavior of the D-C relaxation function and the depression parameter β are also discussed using ωτ=1 and ωτ≠1 corresponding to the maximum of the imaginary part of the impedance (Z^*) or permittivity (ε^*).     

Immittance response of the SnO2-Bi2O3 based thick-films

The SnO₂-Bi₂O₃ based thick-film polycrystalline material is fabricated on alumina substrate via screen-printing technique. This material system is evaluated at various temperatures (35 °C≤T≤100 °C) using ac small-signal (immittance) measurements in the frequency range 10 Hz≤f≤10⁶ Hz. The simplistic analytical scenario for the immittance data employed the Cole-Cole empirical equation in conjunction with the estimation of the inspected input parameters. This is an alternate approach compared to the complex nonlinear least squares (CNLS) fitting procedure, and purely based on the appearance of the semicircular relaxation in the complex plane. It is found that the constituting components of the semicircular relaxation in the impedance plane are thermally activated indicating complexity in the grain boundary contributions despite the Debye and non-Debye relaxation responses. The possible degree of uniformity or non-uniformity in the grain boundary activity associated with its capacitance term observed via the Debye or non-Debye semicircular relaxation in the impedance (Z?) plane has been postulated.     

Vibrational relaxation of a molecule in strong interaction with a reservoir: Nonmonotonic temperature dependence

This theoretical study of the vibrational relaxation of a molecule in interaction with a reservoir uncovers a noteworthy temperature (T) dependence of the time evolution of the relaxation. Its rate increases with T in one interval but decreases in another. The feature arises not for a weak molecule-reservoir interaction but only for coupling strong enough to require polaronic dressing transformations. Our treatment, based on a recent generalization of the well-known Montroll–Shuler equation for relaxation and an explicit calculation of bath correlations from the microscopically specified Hamiltonian, could provide an alternative explanation of an “inverted” T-dependence of relaxation in an experimental report by Fayer and collaborators on W(CO)₆ dissolved in CHCl₃.     

Phase transition and ferroelastic property studied by using the Cs nuclear magnetic resonance in a CsHSO4 single crystal

The ¹³³Cs spin-lattice relaxation time in a CsHSO₄ single crystal was measured in the temperature range from 300 to 450 K. The changes in the ¹³³Cs spin-lattice relaxation rate near T_c1 (=333 K) and T_c2 (=415 K) correspond to phase transitions in the crystal. The small change in the spin-lattice relaxation time across the phase transition from II to III is due to the fact that during the phase transition, the crystal lattice does not change very much; thus, this transition is a second-order phase transition. The abrupt change of T₁ around T_c2 (II-I phase transition) is due to a structural phase transition from the monoclinic to the tetragonal phase; this transition is a first-order transition. The temperature dependences of the relaxation rates in phases I, II, and III are indicative of a single-phonon process and can be represented by T₁⁻¹=A+BT. In addition, from the stress-strain hysteresis loop and the ¹³³Cs nuclear magnetic resonance, we know that the CsHSO₄ crystal has ferroelastic characteristics in phases II and III.     

Relaxation function of a Heisenberg paramagnet at large wavevectors

An approach based on the Mori's theory to calculate the Kubo relaxation function is presented in order to explain the structure of neutron scattering cross section at large wavevectors for Heisenberg paramagnets. The limits of validity of the previous approximate theories are discussed. A low-frequency expansion for the third order memory function is performed: this corresponds to considering a more consistent long-time approximation for the second order memory function. The relaxation function has been computed for RbMnF₃ at T?T_N and T = 1.125T_N.     

Simultaneous measurement of total water content and myelin water fraction in brain at 3 T using a T2 relaxation based method

PurposeThis work demonstrates the in vivo application of a T₂ relaxation based total water content (TWC) measurement technique at 3 T in healthy human brain, and evaluates accuracy using simulations that model brain tissue. The benefit of using T₂ relaxation is that it provides simultaneous measurements of myelin water fraction, which correlates to myelin content.MethodsT₂ relaxation data was collected from 10 healthy human subjects with a gradient and spin echo (GRASE) sequence, along with inversion recovery for T₁ mapping. Voxel-wise T₂ distributions were calculated by fitting the T₂ relaxation data with a non-negative least squares algorithm incorporating B₁⁺ inhomogeneity corrections. TWC was the sum of the signals in the T₂ distribution, corrected for T₁ relaxation and receiver coil inhomogeneity, relative to either an external water standard or cerebrospinal fluid (CSF). Simulations were performed to determine theoretical errors in TWC.ResultsTWC values measured in healthy human brain relative to both external and CSF standards agreed with literature values. Simulations demonstrated that TWC could be measured to within 3–4% accuracy.ConclusionIn vivo TWC measurement using T₂ relaxation at 3 T works well and provides a valuable tool for studying neurological diseases with both myelin and water changes.     

Impedance and Modulus studies of the solid electrolyte system 20CdI2–80[xAg2O–y(0.7V2O5–0.3B2O3)], where 1 ≤x/y ≤ 3

In the present work, an evaluation of the transport properties of super ion conducting quaternary system 20CdI₂–80[xAg₂O–y(0.7V₂O₅–0.3B₂O₃)], where 1 ≤ x/y ≤ 3, in steps of 0.25, to study the effect of changing the modifier to former ratio on the conduction phenomena has been undertaken. Electrical conductivity measurements were made using complex impedance method. The electrical conductivity and conductivity relaxation of the system were studied in the temperature range from 303 K to 333 K and in the frequency range from 100 Hz to 10 MHz. The highest conductivity at room temperature is obtained for the system with modifier to former ratio 1.75. Impedance and modulus analyses had indicated the temperature independent distribution of relaxation times and the non-Debye behavior in these materials. The co-operative motion due to strong coupling between the mobile Ag⁺ ions is assumed to give rise to non-Debye type of relaxation. The silver ionic transport number (t_Ag+) obtained by the emf technique suggested the occurrence of silver ion conduction in the CdI₂-doped Ag₂O–V₂O₅–B₂O₃ system.     

MRICOM–MRI COntrast Modelling using 2D T1–T2 correlation spectra and relaxation signatures

Image contrast is calculated by inputting experimental 2D T₁–T₂ relaxation spectra into the ODIN software interface. The method involves characterising a magnetic resonance imaging pulse sequence with a “relaxation signature” which describes the sensitivity of the sequence to relaxation and is independent of sample parameters. Maximising (or minimising) the overlap between the experimental 2D T₁–T₂ relaxation spectra and the relaxation signature can then be used to maximise image contrast. The concept is illustrated using relaxation signatures for the echo planar imaging and Turbo spin-echo imaging sequences, together with in-vitro 2D T₁–T₂ spectra for liver and cartilage.     

Spin-lattice relaxation and magnetization transfer in intracranial tumors in vivo: Effects of Gd-DTPA on relaxation parameters

Spin-lattice relaxation time T₁ and relaxation parameters in magnetization transfer (MT) imaging were measured in 11 intracranial tumors before and after injection of Gd-DTPA at 0.1 T by using the inversion recovery method and the saturation transfer technique, respectively. Preinjection T₁ relaxation times of the tumors were longer than those of white matter, but after Gd-enhancement the relaxation times of most tumors were in the same range as those of white matter. Gd-DTPA shortened the apparent relaxation time in the presence of off-resonance saturation pulse (T_1α) due to marked shortening of the relaxation time of mobile water (T_1w). Gd-DTPA decreased the magnetization transfer contrast (MTC) but did not influence on the magnetization transfer rate (R_wm). The parameters MTC and R_wm differed clearly between Gd-enhanced tumors and normal brain, whereas the relaxation time T_1α was in many Gd-enhanced tumors in the same range as in normal brain.     

Non-Debye behaviours of heat capacities of cubic II–VI materials

On the basis of an appropriate four-oscillator version of a representative dispersion-related hybrid model we perform detailed analyses of isobaric heat capacity data available for cubic ZnS, ZnSe, ZnTe, CdTe, HgSe, and HgTe. Characteristic non-Debye behaviours of the C_p(T) data sets under study, which are manifested above all in form of non-monotonic dependences (maxima) of the respective C_p(T)/T³ curves in the cryogenic region, are described in terms of two Einstein oscillators for short-wave transversal acoustic (TA) phonons in combination with relatively weak components of Debye and non-Debye type due to long-wave acoustic phonons. This prominent non-Debye feature is represented alternatively in the form of non-monotonic dependences (minima) of conventional Debye temperature curves, Θ_D(T). The close correlation between the low-temperature asymptotic (decreasing vs. increasing) sections of Θ_D(T) vs. C_p(T)/T³ curves is described by simple algebraic formulae. The maxima positions of the latter are shown to be nearly proportional to the centre of gravity positions of the respective TA phonon spectra sections. The inherent non-Debye nature of the whole phonon density of states (PDOS) spectra is shown to find its global expression in characteristic snakelike shapes of the equivalent, moment-related phonon energy curves.     

Disentanglement of two-qubit system in the Bloch channel

Disentanglement of two qubits is studied in the relaxation process characterized by the longitudinal and transversal relaxation times, T₁ and T₂, which satisfy the inequality T₂ ≤ 2T₁ due to the complete positivity of the time-evolution. In the case of sufficiently low temperature, it is shown that whether the equality T₂ = 2T₁ is satisfied or not is of essential importance in the finite-time disentanglement. This is the characteristic feature of entanglement.     

Study of the diffusion of hydrogen in LaNi5+xH6 compounds by 1H NMR relaxation

The diffusion of hydrogen in LaNi_5+xH₆ (x=?0.2, 0.0, 0.2) has been investigated by NMR from 150 to 300 K. High-temperature data of the spin-spin relaxation time T₂ and the rotating frame spin lattice relaxation time T₁? are independent of stoichiometry but the data of the spin lattice relaxation time T₁ and low-temperature T₁? data are not, and they do not fit Torrey's relaxation model.     

Conductivity relaxation in Ag+ ion conducting PEO–PMMA–PEG polymer blends

In this paper, the electrical conductivity and relaxation studies in two different temperature regions (T?<?T _m and T?>?T _m) on plasticized PEO–PMMA blend polymer electrolyte system with AgNO₃ salt are reported. The polymer electrolyte system has been prepared by solution cast technique and characterized by X-ray diffraction and differential scanning calorimetry. The conduction and relaxation processes at various temperatures have been investigated in the framework of modulus formalism. The distribution of relaxation times is discussed using Argand plot. Variation in ionic conductivity of polymer blends is discussed with the increase of PEO as well as with temperature.     

Precise Determination of T1 Relaxation Values by a Method in Which Pairs of Nonequilibrium Magnetizations Are Measured across a Constant Relaxation Period

The constant-relaxation-period (CREPE) method for spin-lattice (T₁) relaxation measurement presented here offers a significant improvement over other methods currently in use. The CREPE approach involves the measurement of both the initial and the final magnetizations flanking a constant relaxation period; the relaxation rates can be extracted from the data by linear regression without the necessity of additional parameters to correct for off-resonance effects or errors in pulse-width settings. Computer simulations used to compare the accuracy of T₁ measurements made by the new method with those by the fast inversion-recovery (FIR) approach (generally considered to be the best general method for T₁ measurements) showed that the CREPE method provides about a twofold reduction in errors over a wide range of signal-to-noise in the input data. Experimental data collected by the two approaches showed that the CREPE method provides the same precision in T₁ values as the FIR method with one-fourth the data-collection time.     

Nuclear magnetic relaxation of 205Tl in TlClO4 in a rotating frame

The spin-lattice relaxation time T_1ϱ of ²⁰⁵Tl in TlClO₄ has been measured in a rotating frame. It was found that the temperature dependence of T_1ϱ shows three minima due to the cross relaxation and the random modulation of the dipole-dipole interaction between ²⁰⁵Tl and ¹⁷O of natural abundance (0.037%).     

Interaction of a nuclear spin with a soliton gas

We investigate the dynamics of a nuclear spin which is weakly coupled to a one-dimensional, thermalized, low-density gas of sine-Gordon solitons. Longitudinal (T₁) and transverse (T₂) relaxation times and the dynamic frequency shift are computed.     

Specific features of the central peak in the Raman spectra of a lithium niobate crystal

The low-frequency Raman scattering (RS) spectra of a LiNbO₃ ferroelectric crystal are studied in the temperature range 300–1423 K. The central peak characterizing the relaxation susceptibility of the crystal lattice is observed over the entire temperature range studied, including at temperatures much lower than the Curie temperature (T _c = 1470 K). Far from T _c, the characteristics of the central peak are shown to be unlike those expected in the framework of the standard approaches. (i) The central-peak width γ_R increases as the temperature increases to T ～ 1300 K, and the critical slowing down (γ_R ∝ T _c ? T) occurs only above this temperature. (ii) A central peak arises in the RS geometry where the scattering by “nonferroelectric” E phonons is allowed. The experimental results are interpreted with allowance for relaxation dynamics in local regions of the crystal lattice.     

Multi-component T1 relaxation and magnetisation transfer in peripheral nerve

We report here a study of longitudinal relaxation (T₁) and magnetisation transfer (MT) in peripheral nerve. Amphibian sciatic nerve was maintained in vitro and studied at a magnetic field strength of 3 T. A CPMG pulse sequence was modified to include either a saturation pulse to measure T₁ relaxation or an off-resonance RF irradiation pulse to measure MT. The resulting transverse relaxation (T₂) spectra yielded four components corresponding to three nerve compartments, taken to result from myelinic, axonal, and inter-axonal water, and a fourth corresponding to the buffer solution water in which the nerve sample was bathed. Each nerve component was analysed for T₁ relaxation and MT. All three nerve T₂ components exhibited unique T₁ relaxation and MT characteristics, providing further support for the assignment of the components to unique physical compartments of water. Numerical investigation of T_1sat measurements of each of the three nerve T₂ components indicates that while the two shorter-lived exhibit similar steady-state magnetisation transfer ratios (MTRs), their respective MT properties are quite different. Simulations demonstrate that mobile water exchange between these two components is not necessary to explain their similar steady-state MTR. In the context of the assignment of these two components to signal from myelinic and axonal water, this is to say that these two microanatomical regions of nerve may exhibit similar steady-state MTR characteristics despite possessing widely different MT exchange rates. Therefore, interpreting changes in MTR solely to reflect a change in degree of myelination could lead to erroneous conclusions.