NMR spin lattice relaxation in dilute CuFe alloys with impurity interactions

Previous measurements by Wilkening and Hesse have shown, that the excess relaxation rate ΔT^-1₁ of the matrix nuclei in $\text{CuF}$e dilute alloys can be explained in terms of the LD-model with rapid spin diffusion. Measurements reported in this paper confirm the existence of an electric quadrupole diffusion barrier. It could be shown that the influence of the quadrupole barrier is coupled to large clusters within the alloy. The electron spin lattice relaxation time τ₁ behaves temperature independent in the range 30 K ? T ? 300 K. This can be understood if an effective correlation time τ is introduced, which results from a distrubution of temperature dependent times τ^cl(T) belonging to clusters of different size.     

Electrical resistivity measurements on the Cu2MnAl heusler alloy

Electrical resistivity measurements have been performed on the Cu₂MnAl Heusler alloy in the temperature range from 4.2 to 500°K. An AT⁵ functionality was found for 4.2°K ? = T ? 15°K, evidencing a Bloch-Grüneisen electron-phonon scattering mechanism. For 270°K ? T ? 500°K the resistivity may be described by the AT + BT² polynomial. The linear term is interpreted as due to the electron-phonon scattering process while the quadratic term may be ascribed to an electronic scattering due to a Spin disorder type relaxation process. The experimental results fail to provide evidences of s-d scattering of the conduction electrons.     

Infrared spectroscopy of the intermediate-valence semiconductor YbB12

The dynamic conductivity and permittivity spectra of the intermediate-valence compound YbB₁₂ are measured in the frequency range (6–10⁴) cm^?1 (quantum energy 0.75 meV-1.24 eV) at temperatures of 5–300 K. Analysis of the spectral singularities associated with the response of free charge carriers has made it possible for the first time to determine the temperature dependences of their microscopic parameters, viz., concentration, effective mass, relaxation frequency and time, mobility, and plasma frequency. It is shown that the relaxation frequency decreases upon cooling from 300 K to the coherence temperature T ^* = 70 K for YbB₁₂, which is mainly associated with the phonon mechanism of scattering of charge carriers. For cooling below the coherence temperature T ^* = 70 K, the temperature dependence of the relaxation frequency for charge carriers of the Fermi-liquid type is found to be γ ～ γ₀ + T ², while their effective mass and relaxation time increase, respectively, to m ^*(20 K) = 34m ₀ (m ₀ is the free electron mass) and τ(20 K) = 4 × 10^?13 s, indicating the establishment of coherent scattering of carriers from localized magnetic moments of the f centers. At a temperature of T = 5 K, the conductivity spectrum contains an absorption line at a frequency of 22 cm^?1 (2.7 meV); the origin of this line can be associated with the exciton-polaron bound state. Since such a state was observed earlier in other intermediate-valence semiconductors (such as SmB₆, TmSe_1?x Te, and (Sm, Y)S), it is probably typical of this class of compounds.     

Frequency response of the low-temperature ionic conductivity of single crystals R 1 − y M y F3 − y (R = La-Er; M = Ca, Sr, Ba, Cd)

The frequency response (10^?1–10^?7 Hz) of the ionic conductivity σ of R _{1 ? y} M _y F_{3 ? y} single crystals (R is a rare-earth element, and M stands for an alkaline-earth element and Cd) with a tysonite structure is studied over a wide temperature range (114–410 K), which includes (for the first time for these phases) the interval below room temperature. The dependences σ(ν) obtained are discussed within the hopping relaxation model. The characteristics of the relaxation and migration processes and the carrier concentration and mobility are determined.     

Dielectric relaxation in vanadium phosphate glasses

An asymmetric distribution of relaxation times has been inferred from an increase in the Cole-Cole distribution parameter α with increasing values of ωτ in 62% v₂O₅–38% P₂O₂ glass. The conventional Debye type relaxation loss peaks in the frequency range 10²–10⁵ Hz are observed in this sample above 85°K. The extrapolated values of dielectric constant and relaxation time below 100°K seem unexpectedly large while the high temperature extrapolated values of ?' are close to ?_∞ as expected. Probably the conventional dielectric loss peaks are observed only above a critical temperature at which the carriers gain sufficient energy to be excited to the conduction band edge. Below this temperature hopping of carriers within kT of the Fermi level may dominate and conventional Debye type dielectric loss peaks may lose their significance as envisaged in the models of frequency dependent ac conductivity.     

Non-exponential nuclear magnetic relaxation by solitary-waves in K2Pt(CN)4Cl0.303.2H2O

Pulsed NMR technique is used to study the ¹⁹⁵Pt nuclear spin relaxation in KCP (Cl) in the temperature range 44 ? T ? 112 K where solitary waves are expected to be the main excitations of this quasi one-dimensional system.A theoretical explanation of the strongly non-exponential magnetization recovery is presented based on hopping of solitons in connection with NMR relaxation.     

Central peak in the strontium titanate crystal near the tetragonal-to-cubic phase transition

Raman spectra of the SrTiO3 crystal have been measured in wide temperature (22?C316 K) and frequency (2?C1020 cm^?1) ranges. It has been shown that a central peak appears in low-frequency Raman spectra at temperatures above 70 K. In the spectral geometry with polarization rotation near the temperature T _c = 106 K of the cubic-to-tetragonal phase transition, the central peak exhibits properties of the order-disorder phase transition. Such a behavior of the central peak has been explained by the interaction of the low-frequency soft mode E _g with the relaxation mode near T _c .     

Spin conversion detected by Mössbauer spectroscopy and μSR on a 1D FeII paramagnetic chain

While magnetic properties of the 1D chain [Fe(hyetrz)₃](4-bromophenylsulfonate)₂ investigated over the temperature range from 300 K to 2 K show paramagnetic behavior, detailed ⁵⁷Fe Mössbauer and muon spin relaxation measurements reveal an unexpected spin conversion. Approximately ~14 % of the high-spin ions are found to convert to the low-spin state with a transition temperature T _1/2?~?120 K.     

H and K spin-lattice relaxation, ionic motion and Raman processes in the proton conducting KHSeO4 single crystal

The spin-lattice relaxation rates of ¹H and ³⁹K nuclei in KHSeO₄ crystals were studied in the temperature range 160-400 K. The spin-lattice relaxation recovery of ¹H nucleus in this crystal can be represented with a single exponential function, and the relaxation T₁⁻¹ curve of ¹H can be represented with the Bloembergen-Purcell-Pound (BPP) function. The relaxation process of ³⁹K with dominant quadrupole relaxation can be described by a linear combination of two exponential functions. T₁⁻¹ for the ³⁹K nucleus was found to have a very strong temperature dependence, T₁⁻¹=βT⁷. Rapid variations in relaxation rates are associated with critical fluctuations in the electronic spin system. The T⁷ temperature dependence of the Raman relaxation rate is shown here to be due to phonon-magnon coupling.     

Spin gap, phase separation and d-wave pairing as revealed by electron spin-lattice relaxation in 123 and 124 YBaCuO systems doped with Gd3+

With an original modulation technique, the Gd³⁺ electron spin-lattice relaxation has been investigated in normal and superconducting states of YBa₂Cu₃O_6+x (123) and YBa₂Cu₄O₈ (124) compounds doped with 1% Gd. In the 123 sample withx = 0.9T _c = 90 K), theT ₁ behavior within 50 <T< 200 K reveals the [1 ? tanh²(Δ/2kT)]/T dependence typical of a spin gap opening with Δ ≈ 240 K. Below 50 K, the exponential slowing down ofT ₁ is limited by the Korringa-like behaviorT ₁ T = const); the same Korringa-like law is found in the 123 sample withx = 0.59 (T _c = 56 K) within the total 4.2–200 K temperature range. This is interpreted in terms of microscopic separation of the normal and superconducting phases allowing for the electron spin cross-relaxation between them. In the 124 sample (T _c = 82 K), the Gd³⁺ relaxation rate below 60 K is found to obey a power lawT _n with an exponentn ≈ 3. Such a behavior (previously reported for nuclear spin relaxation) is indicative of the d-wave superconducting pairing. Additional paramagnetic centers characterized by relatively slow spin-lattice relaxation are found in both 123 and 124 systems. A well-pronounced change in theT ₁ temperature dependence atTT* ≈ 180–200 K is observed for these slowly relaxing centers as well as for the conventional, fast-relaxing Gd³⁺ ions, suggesting microscopic phase separation and a change in the relaxation mechanism due to electronic crossover related with the opening of the spin gap. This hypothesis is supported by some “180 K anomalies” previously reported by other authors.     

Relaxation effects and evidence for one dimensional ordering in KFeCl3 studied by Mössbauer spectroscopy

Mössbauer measurements of KFeCl₃ over the temperature range 4.2–293°K show a transition to a magnetically ordered phase at T_N ? 18.5°K and evidence for one-dimensional order above T_N. In the region 10–25°K striking relaxation effects appear. An approximate analysis of the quadrupole splitting data was used for the determination of the fine structure of the ⁵D levels below T_N which in turn was used for a theoretical reproduction of the relaxation spectra between 10–25°K.     

NMR spectral densities and two dimensional diffusion in TiS2(NH3)1.0 and TaS2(NH3)x

NMR measurements of proton spin-lattice relaxation times T₁ and T_1? in the layered intercalation compounds TiS₂(NH₃)_1.0 and TaS₂(NH₃)_x (x = 0.8, 0.9, 1.0) are reported as functions of frequency and temperature (100 K – 300 K). These observations probe the spectral density of magnetic fluctuations due to motions of the intercalated molecules at frequencies accessible to the T₁ (4–90 MHz) and T_1? (1–100 kHz) measurements. Since the average molecular hopping time (τ) can be changed by varying temperature, different regions of the spectral density can be examined. For T > 200 K, both T^?1₁ and T^?1_1? vary logarithmically with frequency, reflecting the two dimensional character of the molecular diffusion. The temperature dependence of T₁ suggests that a more accurate picture of the short time dynamics is required. No dependence of relaxation rate on vacancy concentration is found.     

Spin relaxation of conduction electrons in GaAs

We have used optical spin orientation techniques to measure T₁ of conduction electrons in GaAs (NinA ≈ 10¹⁷ cm^-3) for 4.7 K ? T ? 200 K. From Hall effect measurements we estimated the electron momentum relaxation time τ_p. For 50 K ? T ? 200 K, the product T₁τ_p agrees with our earlier order of magnitude estimate of the D'yakonov-Perel' mechanism, in which band structure induced precession is strongly narrowed by momentum relaxation. The Elliott mechanism is one to two orders of magnitude weaker.     

Paramagnetische Relaxation im Holmiumäthylsulfat

The paramagnetic relaxation of Ho _x Y_1?x (C₂H₅SO₄)₃·9H₂O (x=1; 0.14; 0.1) was investigated with a mutual inductance bridge with frequencies fromν=10 sec^?1 toν=10600 sec^?1 in the temperature range betweenT=1.14°K and 2.11°K. In the diluted samples there is a strong influence of cross relaxation processes between the hyperfine levels on both the paramagnetic relaxation and the paramagnetic susceptibility. One and two spin cross relaxation processes were found. The susceptibility measured as a function of frequency is compared with the susceptibility calculated from the known energy levels of Ho(C₂H₅SO₄)₃·9H₂O. Thus the way how the relaxation process takes place between the various systems (crystal field-, Zeeman-, dipol dipol coupling-, hyperfine structure-, lattice- and bath-system) can be deduced. Some preliminary measurements atν=24.10⁶ sec^?1 are reported and discussed.     

Calorimetric study of proton ordering in hexagonal ice catalyzed by hydrogen fluoride

Heat capacities of hexagonal ices doped with 2.6, 26 and 260 m mol dm^?3 HF were measured with an adiabatic calorimeter. The HF doping accelerated proton ordering which has been known to take place sluggishly around 100 K. The ice containing 26 m mol dm^?3 HF showed the largest excess entropy ((0.102±0.01) J K^?1 mol^?1) and the shortest relaxation time. The relaxation time at 90 K was about $\text{1}\text{30}$ of that of the pure ice I_h at the same temperature. The activation enthalpies obtained were the same for all of the doped ices, (23.5±2.0) J mol^?1, which is approximately equal to the activation energy of the mobility of the Bjerrum L-defect.     

Zero-bias tunneling anomaly in a two-dimensional electron system with disorder

The temperature dependence of a zero-bias anomaly in the tunneling conductance of an Al/δ-GaAs tunneling structure with a two-dimensional electron density in the δ-layer of 3.5 × 10¹² cm^?2 has been investigated. It has been shown that the respective drop Δρ(?, T) in the tunneling density of states ρ near the Fermi level E _F of the two-dimensional electron system depends logarithmically on the energy ? within the range of 2.7kT < |?| < ?/τ, where ? is measured with respect to E _F and τ is the momentum relaxation time of two-dimensional electrons. It has been found that the drop depth Δρ(0, T)/ρ is also proportional to ln(kT/?₀) in the temperature range T = 0.1–20 K and saturates below 0.1 K.     

Slow dynamics of supercooled liquid revealed by Rayleigh scattering of Mössbauer radiation method in time domain

Characteristic relaxation times τ of the microscopic correlation in o-terphenyl and glycerol were obtained by a time-domain interferometry method using nuclear resonant scattering of synchrotron radiation. From the temperature dependence of τ, a new decoupling temperature 278 K, which is below the conventional decoupling temperature 290 K, was obtained for o-terphenyl. Furthermore, we found a liquid-like relation between τ and q (momentum transfer) as $\tau \propto q^{-2}$ at 285 K in o-terphenyl and 260 and 265 K in glycerol. However, an anomalous q dependence of the slow β relaxation time, $\tau \propto q^{-2.9}$ and q ^?3.8, was observed in deeply supercooled (265 K) and glass (240 K) states, respectively, in o-terphenyl, indicating specially restricted dynamics of the slow β process.     

Oxidised polyethylene as a paraelectric

A new experimental study of a known low temperature dielectric relaxation in oxidised polyethylene confirms that it is a manifestation of paraelectricity with certain similarities to the doped alkali halide systems. The tunnel splitting parameter (half the minimum energy splitting) is found to be 3.9 μeV ± 10%, while the strain coefficient of the double-well asymmetry is ≈ 5 meV. The multiphonon regime takes over from the one-phonon range at characteristic temperature T_o = 7.2 ± 0.5 K. A rather well oxidised sample shows a relaxation rate which varies as T^0.4 when T < 1 K, while the spread becomes markedly broader than a Debye curve. Oxidation with O₂¹⁸ leads to a 4.2 K peak at a frequency 13–15% lower than oxidation with O₂¹⁶. When the oxidised polyethylene is vacuum annealed the relaxation intensity diminishes. The half life at 126 C is about 2 hr and the activation energy of the annealing process is 40 ± 5 kcal/mole. A contaminated specimen showed accelerated oxidation kinetics and also accelerated anneal effect. These results lead to a suggested attribution of the relaxation to the rotation of the dihedral angle of isolated hydroperoxide groups. It is suggested that other hydrocarbon type materials may exhibit paraelectricity.     

Phase diagram of YBa2Cu3O7−y at T<T c according to transverse nuclear relaxation data for Cu(2)

Two peaks are observed at T=35 and 47 K in the transverse relaxation rate for Cu(2) nuclei in YBa₂Cu₃O_7?y . A comparison of the relaxation rates for isotopes ⁶³Cu(2) and ⁶⁵Cu(2) at T=47 K indicates the magnetic nature of relaxation. The enhancement of local magnetic field fluctuations perpendicular to CuO₂ planes at T=47 K is associated with critical fluctuations of orbital currents. The peak at T=35 K is attributed to the emergence of an inhomogeneous superconducting phase. The obtained experimental results and the available data from the literature made it possible to propose a qualitatively new phase diagram of the superconducting state.     

Nuclear magnetic resonance study of the heavy-fermion system URu2Si2

Nuclear magnetic resonance (NMR) and relaxation studies on ²⁹Si have been carried out on the heavy Fermion system URu₂Si₂. Above the Kondo temperature of about 60 K, the nuclear relaxation time T₁ is nearly temperature independent, which is consistent with the occurrence of fluctuations of localized U moments. Below about 60 K T₁ is inversely proportional to temperature suggesting that the system behaves like a Fermi liquid. A sharp increase in T₁ occurs below 17 K which is probably associated with the opening of an energy gap at the Fermi surface due to the formation of a spin density wave state. Below about 10 K, T₁ reacquires the inverse temperature dependence observed in the 17 K ∼ 60 K temperature range.