Relaxation phenomena in nuclear orientation

A survey is given of relaxation phenomena relevant to nuclear orientation at temperatures below 1 K. Relaxation mechanisms, time dependence of the angular distribution of nuclear radiation and experimental methods for relaxation measurements are reviewed. Finally, some open questions related to simultaneous influence of rf fields and relaxation, the existence of a spin temperature and the role of domain walls are discussed shortly.     

Quantum valence criticality as an origin of unconventional critical phenomena

It is shown that unconventional critical phenomena commonly observed in paramagnetic metals YbRh2Si2, YbRh2(Si0.95Ge0.05)2, and β-YbAlB4 are naturally explained by the quantum criticality of Yb-valence fluctuations. We construct the mode-coupling theory taking account of local correlation effects of f electrons and find that unconventional criticality is caused by the locality of the valence fluctuation mode. We show that measured low-temperature anomalies such as divergence of uniform spin susceptibility χ～T(-ζ) with ζ～0.6 giving rise to a huge enhancement of the Wilson ratio and the emergence of T-linear resistivity are explained in a unified way.     

A nonconventional approach to describe classical-quantum crossover phenomena near criticality

A nonconventional renormalization group procedure, which involves fixed points and eigenvectors depending on temperature T, is proposed to describe classical-quantum crossover phenomena for quantum systems near criticality. In this new picture, T-dependent critical exponents occur, where T assumes the role of a natural crossover parameter.     

Cluster size distribution at criticality

Monte Carlo studies of the cluster size distribution for the site percolation problem on the triangular lattice are extended to lattices with up to 4 × 10¹¹ sites. Agreement with the predictions of scaling theory at p_c is excellent over a range of cluster sizes spanning five orders of magnitude.     

Relaxation phenomena in nucleon-induced reactions and preequilibrium angular distributions

The intranuclear relaxation process in nucleon-induced reactions is investigated in a Fermi gas model utilizing a statistical operator for the non-equilibrium state, which contains energy, particle number, and linear momentum as relevant observables. In the hydrodynamic stage the phase space is subdivided into subspaces, assumed to be in a quasi-equilibrium, which is characterized by a Fermi distribution function with time-dependent thermodynamic parameters. A set of coupled non-linear equations for the time development of the thermodynamic parameters is derived. For the case of two sybsystems, a numerical solution of these generalized transport equations is provided, with kinetic coefficients calculated microscopically. The initial conditions are fixed in accordance with the energy and angle distribution of both particles occupying states above the Fermi surface after the first collision event. From the results, a fast and a slow stage are established in the relaxation process, with equal relaxation times for all physical quantities under consideration. The dependence of the relaxation time on particle number and excitation energy is estimated. The particle emission from the precompound stage of the reaction is taken into account by using the principle of detailed balance to find expressions for the mean fluxes from the compound system to the outer space, which are included in the equations for the relaxation. From the time evolution of the occupation number for states above the nucleon binding energy, the precompound double-differential cross section cumulating up to a certain time is calculated. The results for the angular distributions in nucleon-induced precompound reactions are compared with measured data as well as with previous predictions from generalized exciton models. Satisfactory agreement with experimental data is obtained. Following the time development of the compound nucleus the consistency of the present model with the evaporation theory is demonstrated by investigating the mean nucleon decay width.     

Entanglement of fermions at quantum criticality

The study of entanglement properties of quantum critical many-particle systems has become a subject of intense interest. While the basic features of entanglement scaling for critical spin-1/2 systems (coupled qubits) are by now fairly well understood, entanglement properties of critical fermions (or bosons) are less well studied. In an effort to contribute to this problem, we have analyzed the single-site entanglement of a generic spin-1/2 lattice fermion system and found that (under certain provisos) this measure can be used as a reliable marker to identify and partly characterize a quantum critical point. We illustrate our findings by exact analytical results for the single-site entanglement at the magnetic and Mott-Hubbard transitions of the 1D Hubbard model and at the Mott-Hubbard transitions of the 1D Hubbard model with long-range hopping. The text was submitted by the authors in English.     

Relaxation phenomena in water - n-methylmorpholin n-oxide binary mixtures

Binary mixtures composed of water and N-methylmorpholin N-oxide (NMMNO) were studied in the concentration range 0.19 m to 26.5 m with shear and longitudinal waves. The dependence on temperature and molality of 1)the ultrasonic spectrum, 2)the real part of the complex mechanical impedance and 3)the ultrasonic velocity indicates equilibrium between bulk and solvated water with increasing NMMNO concentration and organisation of the structure of the system.     

Relaxation phenomena at accelaration of rotation of a spherical vessel with helium II and relaxation in pulsars

Measurements of the rotational speed of the PSR 0833 pulsar in Vela and PSR 0531 in the Crab nebula have shown that after a sudden speed-up of their rotation a considerably long relaxation process is observed. According to Pines [1] it is associated with the neutron star's superfluidity predicted by Migdal [2]. The present paper is devoted to an attempt of modelling these processes observing the relaxation motions of liquid helium after sharp changes in the rotational speed of a spherical vessel filled with superfluid liquid.     

Surface criticality and multifractality at localization transitions

We develop the concept of surface multifractality for localization-delocalization (LD) transitions in disordered electronic systems. We point out that the critical behavior of various observables related to wave functions near a boundary at a LD transition is different from that in the bulk. We illustrate this point with a calculation of boundary critical and multifractal behavior at the 2D spin quantum Hall transition and in a 2D metal at scales below the localization length.     

Relaxation phenomena induced by edge biasing experiments in the CASTOR tokamak

In this paper we present the first experimental results obtained in the CASTOR tokamak by using a segmented biasing electrode, which is composed of five segments radially separated by 3 mm. The basic idea of choosing such configuration was to allow a spatial distribution of the biasing voltage in the radial direction. In the described experiments, one or more selected segments can be biased up to +300V, while the remaining segments can be either grounded or floating. Two rake probes measure the edge radial profiles of the floating potential and the ion saturation current at the top and low field side of the torus. A Gundestrup probe, located at the top of the torus, measures the parallel and perpendicular Mach numbers together with the local electron temperature and density. A clear and reproducible transition to a regime with improved particle confinement is routinely observed, if the electrode is inserted deep enough into the plasma (r/a ≈ 0.5) and if one or two segments are biased up to +250V (the remaining segments are floating). The steepening of the radial profiles of the plasma density and potential demonstrate the formation of a transport barrier just inside the last closed flux surface. Fast relaxations of the edge profiles, with a frequency of about 10 kHz or higher, are observed when the value of the average radial electric field within the barrier reaches values of about 20 kV/m and the density gradient increases up to a factor 3 with respect to the ohmic phase. A detailed analysis of the spatial-temporal behavior of these relaxations is presented. Presented at the Workshop “Electric Fields, Structures and Relaxation in Edge Plasmas”, Tarragona, Spain, July 3–4, 2005.     

Dose assessment in the criticality accident at Tokai-mura

The present paper reviews a dose assessment carried out after the criticality accident that occurred on September 30, 1999 at JCO in Tokai-mura, Japan. In the accident, almost all doses were caused by external exposure to neutrons and γ-rays emitted upon the fission of uranium. By a joint effort of Japanese experts in radiation dosimetry, a dose assessment was performed for neighboring residents, JCO employees including 3 workers who were at the accident spot, and emergency response personnel. The dose assessment was carried out using records of dosimeters, radiation monitoring data in and around the site, analysis of biological specimens, and computer simulation techniques. It was concluded from the results of the dose assessment that deterministic effects are not expected, except for the 3 heavily exposed workers, and that the probability of stochastic effects is very small and will be undetectable.     

Relaxation at clean metal surfaces

An electrostatic model of the energy of the surface layers of a metal is shown to describe in detail complex phenomena of surface relaxation in clean metals. The model accounts for relaxation effects that go many layers deep, that have both parallel and perpendicular components and that show large variations from surface to surface of the same metal. The model adds a new physically plausible assumption to the simple electrostatic model previously proposed by Finnis and Heine, which increases the force binding each ion to its bulk position by an amount fixed empirically for each metal. The equilibrium configuration of surface layers is found by minimizing the energy with respect to rigid translations of ion nets in a fixed electronic background density. The many surface structure parameters thus determined fit low-energy electron diffraction data on six surfaces of bcc Fe and six of fcc Al well in almost all cases.     

Sensitive chemical compass and quantum criticality at finite temperature

We investigate the influence of a flip operation of the central spin on the quantum criticality of a radical pair system by employing the spin echo and its product yield. It is found that with echo control on the central spin, the critical behavior can be described by the product yield at very high temperatures. Moreover, we also study the short and long time behavior of the spin echo, and show that the decay factor of the short time evolution scales linearly. The long time evolution shows different statistics for varying chain lengths, temperature and external parameters of the Hamiltonian.     

Entanglement scaling in the one-dimensional Hubbard model at criticality

We derive exact expressions for the local entanglement entropy epsilon in the ground state of the one-dimensional Hubbard model at a quantum phase transition driven by a change in magnetic field h or chemical potential mu. The leading divergences of delta epsilon/delta h and delta epsilon/delta mu are shown to be directly related to those of the zero-temperature spin and charge susceptibilities. Logarithmic corrections to scaling signal a change in the number of local states accessible to the system as it undergoes the transition.     

Power-law scalings in weakly-interacting Bose gases at quantum criticality

Weakly interacting quantum systems in low dimensions have been investigated for a long time, but there still remain a number of open questions and a lack of explicit expressions of physical properties of such systems. In this work, we find power-law scalings of thermodynamic observables in low-dimensional interacting Bose gases at quantum criticality. We present a physical picture for these systems with the repulsive interaction strength approaching zero; namely, the competition between the kinetic and interaction energy scales gives rise to power-law scalings with respect to the interaction strength in characteristic thermodynamic observables. This prediction is supported by exact Bethe ansatz solutions in one dimension, demonstrating a simple 1/3-power-law scaling of the critical entropy per particle. Our method also yields results in agreement with a non-perturbative renormalization-group computation in two dimensions. These results provide a new perspective for understanding many-body phenomena induced by weak interactions in quantum gases.     

Relaxation phenomena due tos—d exchange interaction of dilute iron in molybdenum

It is demonstrated that the localized magnetic moments of Fe in Mo which undergo spin compensation at low temperatures vias—d exchange interaction (Kondo-effect) also exhibit, at temperaturesT>T _k , relaxation phenomena in the Mössbauer spectra due to the same exchange interaction. Utilizing the Korringa relation, the product of the exchange parameterJ and the density of statesp(? _F is estimated to be ¦Jp(? _F )¦=0.008. At low temperatures, however, deviations from this relation have been observed. The relaxation rate becomes temperature independent and decreases with increasing external magnetic field.