Dielectric relaxation near the antiferroelectric phase transition of squaric acid

The dielectric response of single crystals of squaric acid is studied around the antiferroelectric phase transition atT _c 100°C. The dielectric data between 64 and 400 GHz can be explained by a simple Debye-relaxation with a critical slowing-down of the relaxation frequency to about 55 Ghz on approachingT _c . The results in the paraelectric phase are discussed on the basis of weakly coupled one-dimensional Ising chains. Within the scope of this model we determine the intra and interchain coupling constants, the molecular dipolar moments and the noninteracting proton intrabond-jump time.     

Dielectric relaxation of mixed crystals of Rb1-x(NH4)xH2PO4 at microwave frequencies

Measurements of the clamped dielectric constant on mixed single crystals of Rb_1-x(NH₄)_xH₂PO₄,x=0.35, are reported up to 11 GHz. Between 24 and 60 K, the dielectric dispersion can be fitted to a Cole-Cole relaxation, the parameters of which indicate a temperature-dependent distribution of relaxation times consistent with Vogel-Fulcher freezing. Both the audio and the microwave measurements can be scaled up to 100 MHz, with a freezing temperature ofT _o8.4 K. In the GHz range, a relaxation process in addition to the low-frequency freezing mode contributes to the dielectric response.     

μSR studies of YFeMnO4

Muon spin relaxation (SR) studies were carried out on YFeMnO₄. Two interesting phenomena were found in measurements of longitudinal relaxation time spectra atT>T _cusp for a single crystal: (1) difference of the relaxation rates between the directions parallel with and perpendicular to thec-axis, and (2) suppression of local field fluctuations in a longitudinal external field. The spectra atT >T _cusp were reproduced with the function exp[-(₀ t)]. The results are discussed under a picture of the magnetic spin fluctuations of triangular antiferromagnets.     

Far-infrared spectrum and the soft mode of ferroelectric betaine phosphite

The far-infrared reflection spectra of betaine phosphite single crystals in the three crystallographic directions are investigated in the range from 5 to 600 cm^?1 at temperatures between 203 and 323 K using a dispersive Fourier spectrometer. The dielectric function is evaluated from the complex amplitude reflection coefficient measured. In the spectrum polarized along the ferroelectric direction a soft mode of relaxation type is observed which accounts for a large part of the relevant static dielectric constant at T_c = 210 K. Among the oscillator modes, which generally exhibit normal temperature dependence, there is a heavily overdamped mode at 58 cm^?1, polarized along the crystallographia a₁ axis, with anomalous damping behaviour. The damping constant decreases nearly linearly with temperature up to 323 K which is close to T = 355 K where a second phase transition is known to exist.     

Generalized Vogel law for glass-forming liquids

A model for non-Arrhenius structural and dielectric relaxation in glass-forming materials is based on defect clustering in supercooled liquids. Relaxation in the cold liquid is highly hindered, and assumed to require the presence of a mobile defect to loosen the structure near it. A mild distribution of free-energy barriers impeding defect hopping can generate a wide distribution of waiting times between relaxation events. When the mean waiting time is longer than the time of an experiment, no characteristic time scale exists. This case directly yields the Kohlrausch-Williams-Watts (KWW) relaxation law. A free-energy mismatch between defect and nondefect regions produces a defect-defect attraction, which can lead to aggregation. This may occur in defect-rich fragile liquids which also exhibit Vogel kinetics. Defect aggregation and correlation in the high-temperature region above the critical consolute temperatureT _c is described using the Ornstein-Zernike theory of critical fluctuations. For a defect correlation length divergence (T-T _c)^-/2, a generalized Vogel law for the structural relaxation time results: =₀exp[B./(T-T _c)^1.5] In the mean-field limit (=1) this provides as good an account of dielectric and structural relaxation in glycerol,n-propanol, andi-butyl bromide as does the original Vogel law, and for the mixed salt KNO₃–Ca(NO₃)₂ and B₂O₂ it also describes kinetics over their entire temperature ranges. A breakdown of the Vogel law in the immediate vicinity ofT _g is avoided, and the need to invoke extra low-temperature mechanisms to explain an apparent return to Arrhenius behavior is removed.This paper is dedicated to Prof. N. G. van Kampen on the occasion of his 67th birthday.     

μ +SR study of some magnetic superconductors

The relaxation of the ⁺SR signal has been investigated for the three compounds YRh₄B₄, ErRh₄B₄ and SmRh₄B₄. In the non-magnetic superconducting (T _c 11 K) YRh₄B₄, the data display a Kubo-Toyabe (gaussian) shape for zero (transverse) magnetic fields. ErRh₄B₄ (superconducting below 8.7 K and ferromagnetic below 1 K) shows a dominant signal with very slow relaxation. In contrast SmRh₄B₄ (superconducting below 2.7 K and antiferromagnetic-superconducting below 0.87 K) shows a change in relaxation from gaussian above 60 K to exponential between 1 K and 4 K to two exponential signals (fast and slow) belowT _N=0.9 K. In the region 0.9 K <T < 4.5 K, the relaxation time and the asymmetry both increase withT.supported by the U.S. Department of Energy.Supported by NSERC of Canada.supported by the U.S. Department of Energy.We are grateful to Drs. H. Umezawa and H. Matsumoto for interesting discussions regarding the persistent current screening and the results of self-consistent calculations.     

MD simulation of concentrated polymer solutions: Structural relaxation near the glass transition

We examine by molecular dynamics simulations the relaxation of polymer-solvent mixtures close to the glass transition. The simulations employ a coarse-grained model in which polymers are represented by bead-spring chains and solvent particles by monomers. The interaction parameters between polymer and solvent are adjusted such that mixing is favored. We find that the mixtures have one glass transition temperature T _g or critical temperature T _c of mode-coupling theory (MCT). Both T _g and T _c (> T _g decrease with increasing solvent concentration . The decrease is linear for the concentrations studied (up to = 25%. Above T _c we explore the structure and relaxation of the polymer-solvent mixtures on cooling. We find that, if the polymer solution is compared to the pure polymer melt at the same T, local spatial correlations on the length scale of the first peak of the static structure factor S(q) are reduced. This difference between melt and solution is largely removed when comparing the S(q) of both systems at similar distance to the respective T _c. Near T _c we investigate dynamic correlation functions, such as the incoherent intermediate scattering function (t), mean-square displacements of the monomers and solvent particles, two non-Gaussian parameters, and the probability distribution P(ln r;t) of the logarithm of single-particle displacements. In accordance with MCT we find, for instance, that (t) obeys the time-temperature superposition principle and has relaxation times which are compatible with a power law increase close (but not too close) to T _c. In divergence to MCT, however, the increase of depends on the wavelength q, small q values having weaker increase than large ones. This decoupling of local and large-length scale relaxation could be related to the emergence of dynamic heterogeneity at low T. In the time window of the relaxation an analysis of P(ln r;t) reveals a double-peak structure close to T _c. The first peak correponds to “slow” particles (monomer or solvent) which have not moved much farther than 10% of their diameter in time t, whereas the second occurs at distances of the order of the particle diameter. These “fast” particles have succeeded in leaving their nearest-neighbor cage in time t. The simulation thus demonstrates that large fluctuations in particle mobility accompany the final structural relaxation of the cold polymer solution in the vicinity of the extrapolated T _c.     

Temperature dependence of viscosity near the glass transition

Real glass transitions, where activated processes are included, are analysed near Whitney fold and cusp singularities. In particular the temperature dependence of the viscosity is studied. For sufficiently strongly coupled systems there is close to a fold singularity located atT=T _c , a crossover from an algebraic divergence (T–T _c )^– forT>T _c , to an Arrhenius dependence exp(E/k _B T) forT<T _c . If a parameter vector specifying the system also passes close to a cusp singularity, atT ₀<T _c , there may be a temperature region where the viscosity also shows Fogel-Fulcher like dependence. The results for the viscosity are intimately connected with the dynamics for the so-called -relaxation process, where relaxation occurs via a fractal time process in the delevant time region.     

Coulombic and non-Coulombic contributions to the criticality of ionic fluids. An experimental approach

The recent discovery of liquid-liquid phase separations in electrolyte solutions with critical points near room temperature enables the systematic study of the critical behavior of ionic fluids. Depending on the nature of the molecular interactions, either sharp mean-field or Ising behavior is obtained in the temperature range down tot=(T–T _c )/T _c =10^–4 or less. Mean-field-like criticality is obtained with systems which in the framework of a simple corresponding states model are fairly close to the critical point of the restricted primitive model (RPM) of equally-sized charged spheres in a dielectric continuum. In these cases the phase separation is driven by the Coulombic forces (so-calledCoulombic phase separations). This type of unmixing occurs for 11 electrolytes in solvents of low dielectric constant. Simple mechanisms for unmixing suggested in the literature are discussed in relation to the available data. Some evidence for departures from the simple RPM prediction is found. The presence of additional short-range interactions leads to sharp Ising behavior. Examples are solutions of tetraalkylammonium salts in water and other highly structured solvents, where phase separation results from the peculiar solvophobic nature of ions (solvophobic phase separations). Previous speculations that this type of unmixing shows the tendency toward closed loops are confirmed by the first direct observation of a lower consolute point in an aqueous solution of propyl-tributylammonium iodide. By light scattering studies and measurements of the coexistence curve near the upper and lower consolute points Ising criticality is confirmed. A new mechanism for phase separation is reported for the system ethylammonium nitrate+octanol, where ion pairs are stabilized by hydrogen bonding beyond what is expected from the RPM. This comparatively subtle additional interaction (so-calledstricky ions) already changes the behavior of otherwise RPM-like systems from mean-field to Ising criticality. The results are discussed with particular emphasis on their implications for possible scenarios for explaining a mean-field critical point or crossover from mean-field to Ising behavior beyond the accessible temperature range.     

Hebel-Slichter peak and superconducting energy gap in Rb3C60 observed by muon spin relaxation

Muon spin relaxation has been observed in both the normal and superconducting states of Rb₃C₆₀ (T _c=29.3K). The field dependence of theT ₁ spin relaxation rate is due to muonium undergoing spin-exchange scattering with conduction electrons, making this the first observation of muonium in a metal. The temperature dependence ofT ₁ ^–1 shows a Hebel-Slichter coherence peak just belowT _c which is not seen in¹³C spin relaxation. The peak can be fit assuming spin relaxation due to interaction with the quasiparticle excitations of a BCS superconductor provided the density of states is broadened relative to that of BCS. Such fits yield a value for the zero temperature energy gap, ₀/k _B , of 53(4)K, consistent with weak-coupling BCS.     

Relationship between low frequency dielectric dispersion and ferroelectric phase transition in pulsed-laser-deposited Bi<Subscript>4-x</Subscript>La<Subscript>x</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> thin film

The frequency and temperature dependence of the complex dielectric constant of Bi_4-xLa_xTi₃O₁₂ (BLT, x=0.9) ferroelectric thin film was studied in the frequency range of 10^-110⁶ Hz and the temperature range of 298673 K. A low frequency dielectric dispersion (LFDD) was found. A model was proposed to account for this observed phenomena. The complex dielectric constant data obtained in the measured frequency and temperature ranges have been found to fit very well to the dielectric dispersion relation: ^*=+i/₀+[B(i)^n-1]/₀. The knee in the log of the electrical conductivity versus the reciprocal temperature curve occurs at T_c. The activation energies associated with charge conduction are E_a,II=0.73 eV below T_c and E_a,I=0.95 eV above T_c. The occurrence of an anomaly in both the n and parameters near T_c indicates a coupling between charge carries and phonons. PACS 77.55.+f; 77.80.-e; 77.22.Jp     

Acoustic susceptibility of an insulating spin-glass

High frequency acoustic measurements have been performed at temperatures around the freezing pointT _c in a cobalt insulating spin-glass. Assuming a Waller coupling mechanism between the spins and the acoustic wave, the data are interpreted in the framework of a dynamical model which takes into account the modulation of the reaction field. As a result, at the freezing temperature the characteristic relaxation time has no anomaly while the relevant susceptibility presents a maximum. The peculiar frequency dependence (as ^1/2) atT _c of the complex elastic constant is experimentally put in evidence.Associated with the Centre National de la Recherche Scientifique (UA 789)     

Muon spin relaxation studies in frustrated and/or low-dimensional spin systems

An overview is given on muon spin relaxation (SR) measurements in frustrated and/or low dimensional spin systems. In the frustrated Kagomé lattice system SrCr₈Ga₄O₁₉, we observed dynamic spin fluctuations of 30 GHz, without any static frozen component even atT=0.1 K, much below the susceptibility-cusp temperatureT _g=3.5 K. This is in clear contrast with the case in dilute-alloy spin glassesCuMn andAuFe, where static order develops belowT _g. We also present the dimensionality dependence of the sub-lattice magnetization curves in 2-d Heisenberg systems, the remarkable suppression of the ordering temperature in a 1-d system Sr₂CuO₃, the observation of activation type spin dynamics in a 1-d Ising ferromagnet (DMeFc)(TCNE) aboveT _c, slow spin fluctuations ( 60 MHz) in Haldane-gap systems at low temperatures, and some results from organic 1-d and 2-d magnetic systems.     

The critical temperature in mode coupling theory and magnetic resonance data on molecular dynamics in glassy liquids

For different molecular glass-forming liquids the critical temperaturesT _c predicted by the mode coupling theory of glass transition are compared with the temperaturesT _b at which a single relaxation process seems to bifurcate into slow and faster relaxations. The latter temperatures are taken from the magnetic resonance data available in the literature. Both temperatures coincide within experimental accuracy.     

Bound-state solution in momentum space of three-particle equations with Coulomb interaction

We solve the bound-state Faddeev equations in the momentum representation for a system of three identical bosons interacting through Yamaguchi forces. Two of the particles are then given an electric charge. The choice of interaction parameters is inspired by the trinucleon systems. In this model we are able to compute the Coulomb energyE _c of³He without further numerical or analytical approximations by solving the corresponding equations of Veselova and Alt, Sandhas, and Ziegelmann. We then are able to test commonly made approximations: We find that all three possible types of diagrams involving the CoulombT matrixT _c contribute substantially. ReplacingT _c byV _c induces an error of a few percent inE _c , and simplifies the numerical computations by orders of magnitude. ReplacingV _c by itsS-wave projection induces only a small error.Presented at the symposium Mesons and Light Nuclei, Liblice, Czechoslovakia, June 1981.We are grateful to Drs. D. J. Struik and J. E. Holwerda for carrying out part of the numerical computations. This work is part of the project The Coulomb Potential in Quantum Mechanics and Related Topics.     

Inhomogeneous muonium diffusion in solid nitrogen

The spin dynamics of the muonium (Mu) atom diffusing quantum mechanically in solid nitrogen (s-¹⁴N₂) has been studied using the technique of Mu spin relaxation. A strong relationship between longitudinal (T ₁ ^–1 ) and transverse (T ₂ ^–1 ) relaxation rates (familiar in NMR) has been experimentally demonstrated for the first time for muonium relaxation. At low temperatures the results are inconsistent with diffusion models using a single correlation time _c; this is taken as evidence for the intrinsic inhomogeneity of the problem. The temperature dependence of theaverage Mu hop rate _c ^–1 gives clear evidence that Mu quantum diffusion ins-N₂ is governed by the two-phonon interaction.     

Spatially-resolved characterization of HTSC-films near T c by laser raster microscopy with thermal excitation

A method (laser raster microscopy with thermal excitation, LRMTE) for characterizing high-T _c thin-film superconductors (HTSC) with microscopic resolution is described. By means of spatially resolved laser excitation and subsequent monitoring of the time dependence of the film resistivity at a base temperature near the transition temperature T _c, spatial variations of the transition temperature (T _c), of the temperature coefficient of the resistivity (dQ/dT), of the heat conductivity and heat capacity of the film and of the heat conductivity between film and substrate can be detected with high spatial resolution (15 m have been achieved so far).     

Magnetic relaxation in a three-dimensional ferromagnet with weak quenched random-exchange disorder

Isothermal remanent magnetization decay,M _r(t), and ‘in-field’ growth of zero-field-cooled magnetization,M _ZFC(t), with time have been measured over four decades in time at temperatures ranging from 0.25T _c to 1.25T _c (whereT _c is the Curie temperature, determined previously for the same sample from static critical phenomena measurements) for a nearly ordered intermetallic compound Ni₃Al, which is an experimental realization of a three-dimensional (d = 3) ferromagnet with weak quenched random-exchange disorder. None of the functional forms ofM _r(t) predicted by the existing phenomenological models of relaxation dynamics in spin systems with quenched randomness, but only the expressions and closely reproduce such data in the present case. The most striking features of magnetic relaxation in the system in question are as follows: Aging effects are absent in bothM _r t andM _ZFC(t) at all temperatures in the temperature range covered in the present experiments. A cross-over in equilibrium dynamics from the one, characteristic of a pured = 3 ferromagnet with complete atomic ordering and prevalent at temperatures away from T_c, to that, typical of ad = 3 random-exchange ferromagnet, occurs asT → T_c. The relaxation times τ₁(T)(τ₁ ^′(T)) and τ₂(T)(τ₂ ^′(T)) exhibit logarithmic divergence at critical temperatures and ; and both increase with the external magnetic field strength,H, such that at any given field value, . The exponent characterizing the logarithmic divergence in τ ₁ ^′ (T) and τ ₂ ^′ T possesses a field-independent value of ≃16 for both relaxation times. Of all the available theoretical models, the droplet fluctuation model alone provides a qualitative explanation for some aspects of the present magnetic relaxation data     

Switching of spontaneous electric polarization in the DyMnO<Subscript>3</Subscript> multiferroic

For the DyMnO₃ multiferroic with a modulated magnetic structure, switching of its spontaneous electric polarization (P ‖ c axis) near the ferroelectric transition (T < T _FE ～ 20 K) is revealed by measuring the dielectric hysteresis loops. It is found that the coercive field strongly increases as the temperature decreases (up to 2.6 kV/mm at 17.6 K). The values obtained for the spontaneous polarization are found to agree well with the data obtained from pyroelectric measurements. In addition, anomalies are observed in the temperature dependences of the spontaneous polarization P _c , dielectric constant ? _c , and magnetic susceptibility x _b at T ～ 6 K; these anomalies are attributed to the antiferromagnetic ordering of the Dy³⁺ ions.