Frontiers in frustrated magnetism

This article discusses at a qualitative level a number of issues at the forefront of current understanding and developments in frustrated quantum magnetism. The focal point of the presentation is the spin liquid, which is introduced in terms of (un)broken spin and lattice symmetries. An overview of the full spectrum of research activity in the field is obtained by considering selected examples from experimental approaches to realising spin-liquid states, from theoretical efforts which seek both to classify spin liquids according to their physical properties and to broaden the search for spin-liquid behaviour, and from numerical techniques which offer the prospect of qualitatively new insight into frustrated spin systems.     

Zeeman coherence in saturated absorption

By modulating the saturating beam polarization, it is possible to extract from the probe beam a signal which depends only on the Zeeman coherence Δm = 2 (transverse alignment), and to study velocity changing collisions which do not destroy the alignment. This type of collisions appears to be of small importance in neon, so that width of the saturated absorption curve from alignment is narrower than that from populations.     

Exploitation of inter-track interference in a shingled replay system

The expected increase in areal density in hard drives will require very narrow tracks. Tracks which are of a similar width to the read head and which are not separated by guardbands normally suffer from large inter-track interference (ITI) or crosstalk. Here, we show that it is possible to read from tracks which are not separated by guardbands and that are narrower than the head. In addition to the significant increase in areal density obtainable by reducing the unrecorded area of the disk and narrowing the tracks, such a system would also lead to a decrease in data retrieval times. We have identified across-track magnetization constraints for future coding across three adjacent tracks so that it will be possible to read from tracks which are only 73% as wide as the read head. Reading from tracks not separated by guardbands which have been written under these constraints yields an increase in track density of at least 47% greater than that possible in conventional drives.     

Probing states in the Mott insulator regime in the case of coherent bosons trapped in an optical lattice

We propose a method to probe states in the Mott insulator regime produced from a condensate in an optical lattice. We consider a system in which we create time-dependent number fluctuations in a given site by turning off the atomic interactions and lowering the potential barriers on a nearly pure Mott state to allow the atoms to tunnel between sites. We calculate the expected interference pattern and number fluctuations from such a system and show that one can potentially observe a deviation from a pure Mott state. We also discuss a method in which to detect these number fluctuations using time-of-flight imaging.     

The sound attenuation in an open-plan office

Attenuation curves have been measured in an open-plan office without furniture and with a number of different furniture arrangements. The furniture and screens were found to have little influence on the attenuation curves, which fluctuate in level out to distances of approximately 10 m from the source and tend to stay close to the 6 dB per distance doubling attenuation line beyond 10 m from the source.A theoretical model of the office was found to give predictions which agree closely with the above measurements. The same model does predict attenuation curves which deviate from the 6 dB line in the far field for certain floor and ceiling absorptions not found in the above-mentioned office.     

On the nondirect optical transition characteristics in the photon-enhanced field emission spectra

An analysis of the photon-enhanced field emission characteristics due to nondirect optical transitions is given. It is shown that the minima and maxima appearing in energy distributions which belong to levels from which and to which optical transitions occur, give information enabling one to discern nondirect transitions from direct ones.     

Spatial correlations in condensed phase reactions

Abstract

Spatial correlations among reacting species in condensed media result from an interplay of reaction (which tends to lead to negative correlations), replacement or dissociation (which tend to produce positive correlations), and diffusion (which tends to smooth correlations out). Where correlations are high associative reactions are generally rapid. Where they are low the associative reaction is generally diminished. These characteristics of condensed phase processes have important consequences in a wide range of situations. In this paper, I review several examples drawn from radiation-physics, -chemistry, and -biochemistry of how spatial correlations determine the kinetics of reaction. Utilizing this theme, I also present two new results. I discuss a model of defect production in collision cascades in which the collective excitations during the cooling of the cascade contribute to defect recombination. A quantitative comparison with experiment is made. Lastly, I show how the kinetics of a diffusion-limited reversible reaction differ from a model which tacitly assumes that the spatial correlations which prevail are those of the irreversible reaction. A connection is drawn between these models and the observed kinetics of thermal donor formation.     

Correlation entropy in a classical liquid

A statistical formula is presented for the entropy of a classical monatomic liquid, including effects of multiparticle correlations. This theory applies to the liquid state, which has small density fluctuations, and which is distinct from the gas state. The statistical entropy is found to agree well with the thermodynamic entropy for liquid sodium at temperatures from 378 to 823 K.     

Cosmic-ray backgrounds in infrared bolometers

Cosmic-ray events are produced in infrared detectors by energy impulses which result from the ionization trails of fast charged particles. Model calculations are compared to the pulse-amplitude spectrum observed in a balloon flight from Palestine, Texas, at an altitude of 38 km. The results are useful for understanding cosmic-ray backgrounds which occur in all applications of high sensitivity bolometers.     

Impurity effects in graphene

Polarization-independent and omnidirectional tunable bandpass filters have been proposed and demonstrated. It is seen that a bilayer period structure composed of single-negative materials can act as a tunable filter, which can be changed from a filter with two symmetrical passband to one with a single pass bandpass, or from one with a wide to one with a narrow pass band. Moreover, the proposed filter is insensitive to the incident angle and polarization of light, which is an omnidirectional, polarization-independent filter for incidence angles between 0° to 90°.     

介质阻挡放电丝结构转变中的缺陷研究

在介质阻挡放电中,对放电丝结构从四边形向四边形格子态结构转变中的缺陷进行了研究.实验发现,在四边形向四边形格子态结构的转化中,存在两种缺陷:角缺陷及错位缺陷.为了研究缺陷的形成机理,实验测量了四边形格子态结构中不同放电丝的发光信号,结果发现八边形晶胞的中心放电丝发光强度强于边缘放电丝发光强度.通过引入准势场,研究了放电丝之间的相互作用及其对放电丝结构转变的影响,并由此分析了在转变过程中出现的缺陷,结果与实验符合得很好. 关键词： 介质阻挡放电 角缺陷 错位缺陷 超四边格子态     

Gravitational radiation in quantum gravity

The effective field theory of quantum gravity generically predicts non-locality to be present in the effective action, which results from the low-energy propagation of gravitons and massless matter. Working to second order in gravitational curvature, we reconsider the effects of quantum gravity on the gravitational radiation emitted from a binary system. In particular, we calculate for the first time the leading order quantum gravitational correction to the classical quadrupole radiation formula which appears at second order in Newton’s constant.     

Galvanomagnetic effects in anisotropic metals

The aim of this article is to present an introductory review of those aspects of the isothermal galvanomagnetic effects in metals (which here means the electrical resistance, magnetoresistance and Hall effect) which are peculiar to the existence of an anisotropic electrical conductivity. This anisotropy may be intrinsic to the solid, such as that arising from a non-cubic crystal structure, or it may be induced by the application of the external magnetic field. Such induced anisotropy can itself be divided into two classes : that arising directly from the effect of the Lorentz force upon the itinerant electrons, and that in which the external field is merely an agent which makes evident on a macroscopic scale an anisotropy which exists microscopically, even in the field-free case. The article traces the usual phenomenological formulation of the galvanomagnetic effects, and then particularizes this to the commonly-encountered experimental situations used to study the consequences of both induced and intrinsic anisotropy. The final section consists of a comprehensive compilation of relevant literature, arranged under the headings for the individual elemental metals and semimetals.     

Secondary plasmon resonance in graphene nanostructures

The plasmon characteristics of two graphene nanostructures are studied using time-dependent density functional theory (TDDFT). The absorption spectrum has two main bands, which result from π and σ + π plasmon resonances. At low energies, the Fourier transform of the induced charge density maps exhibits anomalous behavior, with a π phase change in the charge density maps in the plane of the graphene and those in the plane 0.3 ? from the graphene. The charge density fluctuations close to the plane of the graphene are much smaller than those above and beneath the graphene plane. However, this phenomenon disappears at higher energies. By analyzing the electronic properties, we may conclude that the restoring force for the plasmon in the plane of the graphene does not result from fixed positive ions, but rather the Coulomb interactions with the plasmonic oscillations away from the plane of the graphene, which extend in the surface-normal direction. The collective oscillation in the graphene plane results in a forced vibration. Accordingly, the low-energy plasmon in the graphene can be split into two components: a normal component, which corresponds to direct feedback of the external perturbation, and a secondary component, which corresponds to feedback of the Coulombic interaction with the normal component.     

Percolation in a Voronoi Competition-Growth Model

We study a model in which two entities (e.g., plant species, political ideas,...) compete for space on a plane, starting from randomly distributed seeds and growing deterministically at possibly different rates. An entity which forms an infinite cluster is considered to dominate over the other (which then cannot percolate). We analyze the occurrence of such a form of domination in situations in which one entity starts from a much larger density of seeds than the other one, but the latter one grows at a much faster rate than the former one. The model studied here generalizes the problem of Voronoi percolation.     

Interionic potentials in alkali halides and their use in simulations of the molten salts

After an outline of work on rare-gas systems, which serves as a target for parallel work on alkali halides, and an initial brief survey of those parts of this parallel work for which results have been obtained, interionic potential models for alkali halides are considered in some detail. The rigid ion potentials of Fumi and Tosi are discussed and then a major part of the section is devoted to deriving a new set of polarizable ion potentials, which incorporate the ideas behind the lattice dynamical shell model. Extensions which include many-body terms in the potentials are considered briefly and finally the information which can be obtained from alkali halide diatomic molecules is discussed.

In the third section methods of computer simulation for ionic liquids are outlined, concentrating on the molecular dynamics method, and some of the properties which can be obtained by analysing the ion trajectories are listed. Results from simulations, including some new work on LiF, NaCl and RbI, are reviewed.     

Freezing Transitions in Non-Fellerian Particle Systems

Non-Fellerian particle systems are characterized by nonlocal interactions, somewhat analogous to non-Gibbsian distributions. They exhibit new phenomena that are unseen in standard interacting particle systems. We consider freezing transitions in one-dimensional non-Fellerian processes which are built from the abelian sandpile additions to which in one case, spin flips are added, and in another case, so called anti-sandpile subtractions. In the first case and as a function of the sandpile addition rate, there is a sharp transition from a non-trivial invariant measure to the trivial invariant measure of the sandpile process. For the combination sandpile plus anti-sandpile, there is a sharp transition from one frozen state to the other anti-state.     

Molekulare Schallabsorption in Benzol und Zyklohexan

The sound absorption in liquid benzene and cyclohexane measured in the frequency range from 3 to 150 Mc/s is proportional to the square of the frequency. In polycrystalline benzene and cyclohexane an additional grain boundary absorption was found, which can be neglected above 100 Mc/s. In all cases the absorption which is proportional to the square of the frequency is ascribed to a vibrational relaxation process. In each substance the extrapolated relaxation times of the solid and liquid states differ from each other very little.     

Tensor distinction of domains in ferroic crystals

Ferroic crystals contain two or more domains and may be distinguished by the values of components of tensorial physical properties of the domains. We have extended Aizu’s global tensor distinction by magnetization, polarization, and strain of all domains which arise in a ferroic phase transition to include distinction by toroidal moment, and from phases invariant under time reversal to domains which arise in transitions from all magnetic and non-magnetic phases. For determining possible switching of domains, a domain pair tensor distinction is also considered for all pairs of domains which arise in each ferroic phase transition.     

Multibubble sonoluminescence pulses from Na atoms in viscous liquid

Multibubble sonoluminescence pulses of Na and continuum emissions were measured from NaCl-ethylene glycol solution saturated with Xe at 28 kHz. The Na emission consisted of multiple-peak pulses and single pulses. The intrinsic pulse width estimated from single pulses was 0.37 ns, which differs from 10-165 ns obtained by previous work. High-speed shadowgraphs of bubble dynamics and high-speed movies (32000 fps) of sonoluminescence were observed. The observations suggest that the multiple-peak pulse is due to the superposition of single peaks resulting from bubbles fragmented from a characteristic bubble which repeats the fragmentation and coalescence. This phenomenon may be specific to viscous liquids.