Measurements of surface relaxation time of solid,liquid and adsorbed 3He in porous glasses

The surface relaxation time T_ls has been investigated in solid, liquid and adsorbed ³He in porous glasses for a wide frequency range by using pulsed SQUID-NMR. ALL T_ls agreed with each other and were proportional to the frequency at least at low frequencies. A model is proposed to explain the above results of T_ls.     

NMR investigations of dynamical processes in orientationally ordered and disordered NaCN,RbCN and CsCN

The nuclear spin lattice relaxation timeT ₁ of the²³Na,⁸⁵Rb,⁸⁷Rb,¹³³Cs,¹⁴N nuclei is measured in NaCN, RbCN and CsCN as a function of temperature below and above the ferroelastic phase transition temperatureT _c. BelowT _c the behaviour ofT ₁ of the alkali nuclei renders possible to determine the flip frequency of the CN molecules and its temperature dependence. AboveT _c from the¹⁴NT ₁ the correlation time τ_c of the rotational motions of the CN molecules and its temperature dependence is determined. An empirical rule is verified demonstrating that atT _c the correlation times take nearly the same values for all cyanides. For the high and low temperature phases one obtains atT _c about τ_c=5·10^?13s and τ_c=5·10^?11s, respectively. The results are discussed with respect to the mechanism of the phase transition.     

NMR study of one-dimensional ionic conductor with hollandite-type structure. II. Frequency dependence of spin-lattice relaxation time of 27Al

Frequency dependence of spin-lattice relaxation time T₁ of ²⁷Al in one-dimensional K⁺ ion conductor, K-Al-priderite, was measured at 45 K in the frequency range from 10.1 MHz to 55 MHz. It is found that T₁ is proportional to ω^1.49±0.05 and agrres well with the $\text{ω}{}^{\mathrm{<mtext>3</mtext><mtext>4</mtext>}}$ dependence derived by the continuum diffusion model. The intrinsic activation energy is determined to be 0.058 eV by doubling the slope E_NMR=0.029 eV of the d(ln T₁)/dT curve in the low temperature region. The frequency dependence of T₁ in the high temperature region measured in the frequency range from 11.5 MHz to 20.8 MHz shows a tendency that the frequency dependence becomes smaller than the $\text{ω}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ dependence as temperature is raised above 450 K.     

Nuclear magnetic relaxation of <Superscript>3</Superscript>He in contact with an aerogel above the Fermi temperature

The spin kinetics of ³He in an aerogel has been studied above the Fermi temperature. The magnetic relaxation times T ₁ and T ₂ of adsorbed, gaseous, and liquid ³He in a 95% silica aerogel at a temperature of 1.5 K have been determined as functions of frequency by means of pulse nuclear magnetic resonance. It has been found that the time T ₁ is linear in frequency in all three cases, whereas T ₂ is independent of frequency. To explain the observed behavior of the longitudinal relaxation rate, a theoretical model of relaxation in the adsorbed layer of ³He taking into account the filamentary structure of the aerogel is proposed.     

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.     

Proton spin-lattice relaxation study of the phase transition in TlH2AsO4

Measurements of the temperature and frequency dependence of the proton laboratory and dipolar frame spin-lattice relaxation rates in powdered TlH₂AsO₄ demonstrate that the ⁷⁵As quadrupole resonance frequency changes from v_Q ≈ 38 MHz at T ?T_c to v_Q 〈 10 MHz at T 〉 T_c. The structural phase transition at T_c = 251 K is thus connected with a disordering of the protons in the O–H … O bonds surrounding the AsO₄ groups.     

The sensitivity of the transition temperature to changes in α2 F(ω)

The transition temperature of a superconductor depends on α² F(ω), the spectral function of the effective interaction due to phonon exchange. We discuss how strongly the transition temperature is influenced by different frequency parts of α² F(ω). For this purpose the functional derivative δT _c /δα² F(ω) is calculated. It is shown that all frequency regions of α² F(ω) yield a positive contribution toT _c and that the most effective range covers frequencies, slightly above 2πT _c . The functional derivative is calculated numerically for several superconductors from their measured α² F(ω)-spectra. Finally, we discuss the change in transition temperature due to the softening of α² F(ω) which has been observed in amorphous superconductors.     

Rydberg states in the D layer of the atmosphere and the GPS positioning errors

The noncoherent radiation in the frequency range 0.8–8.0 (GHz) formed in the D layer of the ionosphere at high solar activity due to transitions between Rydberg states is considered. The emitting layer thickness located 80–110 km above ground surface is estimated. A complicated irregular behavior of the frequency dependence of the radiation intensity for different values of å electron concentration n _e and temperature T _e due to different characteristics of electron scattering on the nitrogen and oxygen molecules is revealed. The dependences of the flux power of UHF radiation from the D layer in the indicated frequency range on the concentration and temperature of free electrons are calculated. It is shown that, at a frequency of ν = 1.44 GHz, the UHF radiation spectrum features a characteristic waist point, the position of which is almost independent of the electron temperature T _e ; i.e., a one-parameter dependence of the power flux on the electron n _e density takes place. In the frequency range of 4.0–8.0 GHz, the radiation spectrum exhibits a family of curves that, for each value of n _e and a wide range of T _e , give rise to a relationship known as the “bottleneck.” It was found that, with increasing frequency, the bottleneck moves upwards along a curve described by a quadratic dependence on the radiation frequency. For a frequency of ～5 GHz, and a certain range of temperature T _e and electron concentration within 5 · 10³ cm^?3 < n _e < 2 · 10⁴ cm^?3, an almost linear dependence of the UHF radiation power on n _e is observed. A comparative analysis of GPS signal delays at frequencies ν _f ⁽¹⁾ = 1.57 and ν _f ⁽²⁾ ≈ 5 GHz for various states of the ionosphere is performed. It is shown that, under the same condition, the use of the second frequency is more advantageous and informative. The ways of further development of the theory and experiment in studying the role of quantum resonant properties in the distortion of global satellite positioning system signals and in solving the fundamental problem of their elimination are discussed.     

The soft mode properties in Raman spectra of improper ferroelastics Hg2Cl2 and Hg2Br2

The temperature dependence of the frequency and intensity of the soft mode line appearing in the Raman spectra of Hg₂Cl₂, Hg₂Br₂ below the critical point T_c was studied. A proportionality between the scattering cross section of the soft mode and the square of its frequency v_sm was established. A strong stress-induced increase of the intensity and frequency of the soft mode line was observed near T_c, the relative shift of v_sm being ～30% at ～0.5 kg/mm² stress and T_c?T=5°K.     

Site selection NMR study on electronic state in and near vortex core of YBa2Cu4O8

We report a site-selective ¹⁷O spin-lattice relaxation rate T₁⁻¹ in the vortex state of YBa₂Cu₄O₈. We found that T₁⁻¹ at the planar sites exhibits an unusual nonmonotonic NMR frequency dependence. Based on T₁⁻¹ in the vortex core region, we establish strong evidence that the local density of states within the vortex core is strongly reduced.     

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.     

Dielectric dispersion in the microwave range of the incommensurate phase (NH4)2BeF4

The complex dielectric constant of (NH₄)₂BeF₄ single crystals was measured in the frequency range from 0.6 to 300 MHz in the vicinity of the transition temperature T_c. It was found that, the relaxation frequency is about 1 × 10⁸Hz atT_c. Dielectric relaxation can be described by a polydispersive process.     

Theory for the nonlinear optical response at noble-metal surfaces with nonequilibrium electrons

We present a theory for the electron-temperature dependence T _el of optical second harmonic generation (SHG). Such an analysis is required to study the dynamics of metallic systems with many hot electrons not at equilibrium with the lattice. Using a tight-binding theory for the nonlinear susceptibility χ ⁽²⁾(ω,T _e1) and the Fresnel coefficients we present results for the SHG intensity I ⁽²⁾(ω,T _e1) and its dependence on T _el for Cu. Note, χ ⁽²⁾(ω,T _e1) rather than the Fresnel coefficients determines essentially this temperature dependence. Most interestingly we find frequency ranges where I ⁽²⁾(ω,T _e1) increases for small light intensities, while it decreases for large light intensities. Our theory yields also that SHG probes effects due to hot electrons more sensitively than linear optics. The results of our calculations are compared with recent experiments on Cu and Au.     

Coherent state and superconductivity in the free carrier-bipolaron interacting system

In this paper a model to describe the free carrier-bipolaron interacting system is proposed. Effective hopping of the bipolaron is studied in the slave-boson approach, and a characteristic temperature T¹ is obtained, below which the system enters a coherent state. The density of states in the normal state and the superconductivity of the system are discussed in a quasiparticle picture. The results show that the mixing between the free carrier and the bipolaron results in an enhancement of the effective mass of the quasiparticle and meanwhile the renormalized coupling interaction, arising from the negative correlation energy in the bipolaron region, enhances the effective superconducting coupling interaction. Under the most favourable conditions, the superconducting transition temperature T_c ∼ ω_c, where ω_c is the Debye frequency related with local electron-phonon coupling. In general we have T¹ > T_c ⪢ T_c0 (T_c0 is the superconducting transition temperature of a usual superconductor). Therefore the system will firstly enter a coherent state before becoming a high-T_c superconductor.     

Hindered rotation of CH3 groups and phenyl ring in clofibric acid (drug) studied by35Cl-NQR and1H-NMR spectroscopy

The³⁵C1-NQR frequency (V_Q), nuclear quadrupole spin-lattice relaxation time (T_1Q),¹H-NMR second moment (M ₂), nuclear magnetic spin-lattice relaxation time (T ₁) and spin-lattice relaxation time in rotating frame (T _1p ) were measured for polycrystalline clofibric acid (drug) as a function of temperature. Hindered rotation of two dynamically inequivalent methyl groups and the phenyl ring was detected, the relevant activation energies were determined. The rotations are discussed in detail.     

Domain configuration and dynamic processes in orthorhombic NaCN studied by 23Na NMR satellite lines and relaxation times

The orientational dependences of the ²³Na NMR spectra which were measured in a NaCN single crystal below T_c1 are related to the EFG tensor at the Na sites in different domains. The results are consistent with the assumption that 12 different domains may arise in the orthorhombic phase. Below T_c1 also the temperature dependences of the spin relaxation times T₁ and T_1ρ of the ²³Na nucleus were measured and related to dynamic processes. Particularly the activation energy and the frequency of the flip motions of the CN^--dipoles are determined.     

Static susceptibility and longitudinal resonance frequency of superfluid 3He B

The static susceptibility and the longitudinal resonance frequency of superfluid ³He B have been measured at 18.7 bar from T_c to 0.27 T_c using continuous wave transverse NMR.     

Frequency dependent magnetic susceptibility and spin glass freezing inPtMn alloys

We report measurements of the low-field complex magnetic susceptibility on Pt_1?x Mn _x forx=0.01, 0.025 and 0.05 and for frequencies ν between 10 and 4,000 Hz. A strong frequency dependence of the freezing temperatureT _f is observed: ΔT _f /T _f Δ lnv=0.025 (decade ν)^?1 for all three alloys. These results as well as previous other measurements are interpreted in terms of a phenomenological model.     

Dielectric relaxation and ferroelectricity in Cd2Nb2O7 pyrochlore

The dielectric dispersion of Cd₂Nb₂O₇ pyrochlore in a weak electric field was studied in a broad frequency range (100 Hz to 13 MHz) using the crystal samples slowly cooled (0.5 K/min) in the temperature interval from 300 to 80 K. As the temperature decreased down to T _c=196 K and T _max～190 K, the dielectric permittivity exhibited deviation from the Curie-Weiss law. It is suggested that this behavior is related to the development of a short-range correlation between microscopic polar regions formed at T→T _max ⁺ . The local order parameter q(T) ～ 〈P _i P _j 〉^1/2 was calculated using the permittivity ε′(T) measured at various frequencies. The variation of this parameter is compared to that of the spontaneous polarization P _s (T) determined from the measurements of a pyroelectric current in the external electric field E _dc =0.95 kV/cm. In the frequency range from 100 Hz to 13 MHz, the dispersion of the dielectric response in the temperature region of 180–192 K is characteristic of a relaxator ferroelectric featuring a glasslike behavior. The parameters of this state were determined, including the activation energy of the polarization fluctuations (E _a ≈0.01 eV), the relaxation rate at T → ∞ (f ₀=1.9×10¹² Hz), and the polarization fluctuation freezing temperature (T _f =183 K). In Cd₂Nb₂O₇ pyrochlore, in contrast to the known relaxator ferroelectrics of the PMN type studied previously, this state coexists with the normal ferroelectric state appearing at T _c.     

Hyperfine interactions at R and In sites in RNiIn (R = Gd, Tb, Dy, Ho) compounds measured by perturbed angular correlation spectroscopy

Perturbed gamma–gamma angular correlation (PAC) technique was used to measure the magnetic hyperfine field (mhf) in RNiIn (R = Gd, Dy, Tb, Ho) intermetallic compounds using the ¹¹¹In→¹¹¹Cd and ¹⁴⁰La→¹⁴⁰Ce probe nuclei. The PAC spectra for ¹¹¹Cd measured above magnetic transition temperature show a major fraction with a well defined quadrupole interaction for all compounds except GdNiIn where a single frequency was observed. PAC measurements below T _C showed a combined electric quadrupole plus magnetic dipole interaction for ¹¹¹Cd probe at In sites, and a pure magnetic interaction for ¹⁴⁰Ce at R sites. The temperature dependence of mhf measured with ¹⁴⁰Ce at R sites shows that the values of fields drop to zero at temperatures around the expected T _C for each compound. However, in the measurements with ¹¹¹Cd at In sites, the mhf values become zero at temperatures which are smaller than T _C . The difference between the temperatures at which mhf is zero for ¹⁴⁰Ce and ¹¹¹Cd probes correlates with T _C . For each compound this difference decreases with T _C . The results are discussed in terms of the RKKY model for magnetic interactions and the existence of two magnetic systems, with distinct exchange interaction energies due to different types of atomic layers in these compounds.