Mechanically Versus Thermally Generated Quantum Turbulence of 4He Superflow

An experiment on thermally driven ⁴He superflow (Chagovets and Skrbek in Phys. Rev. Lett. 100:215302, 2008; J. Low Temp. Phys. 153:162, 2008) yielded very different steady state turbulence properties from a nominally identical recent realisation where the helium fountain drive was replaced by bellows. Here we argue that differences are most likely not due to fundamental reasons, but to incorrect velocity calculation in the fountain drive version. We show by a combination of direct velocity measurements and indirect analysis that the real velocity is significantly lower than predicted, and one main reason for this is unaccounted heat spent to enhance helium evaporation. The velocity correction brings the two experiments in reasonable agreement. We also suggest that the transition to a new state of superfluid turbulence claimed in Refs. (Chagovets and Skrbek in Phys. Rev. Lett. 100:215302, 2008; J. Low Temp. Phys. 153:162, 2008) may be a spurious effect of an observed nonlinearity occurring between fountain heat current and superflow velocity.     

Large anisotropic conductance and band gap fluctuations in nearly round-shape bismuth nanoparticles

Unlike their bulk counterpart, nanoparticles often show spontaneous fluctuations in their crystal structure at constant temperature [Iijima, S.; Ichihashi T. Phys. Rev. Lett.1985, 56, 616; Ajayan, P. M.; Marks L. D. Phys. Rev. Lett.1988, 60, 585; Ben-David, T.; Lereah, Y.; Deutscher, G.; Penisson, J. M.; Bourret, A.; Korman, R.; Cheyssac, P. Phys. Rev. Lett.1997, 78, 2585]. This phenomenon takes place whenever the net gain in the surface energy of the particles outweighs the energy cost of internal strain. The configurational space is then densely populated due to shallow free-energy barriers between structural local minima. Here we report that in the case of bismuth (Bi) nanoparticles (BiNPs), given the high anisotropy of the mass tensor of their charge carriers, structural fluctuations result in substantial dynamic changes in their electronic and conductance properties. Transmission electron microscopy is used to probe the stochastic dynamic structural fluctuations of selected BiNPs. The related fluctuations in the electronic band structure and conductance properties are studied by scanning tunneling spectroscopy and are shown to be temperature dependent. Continuous probing of the conductance of individual BiNPs reveals corresponding dynamic fluctuations (as high as 1 eV) in their apparent band gap. At 80 K, upon freezing of structural fluctuations, conductance anisotropy in BiNPs is detected as band gap variations as a function of tip position above individual particles. BiNPs offer a unique system to explore anisotropy in zero-dimension conductors as well as the dynamic nature of nanoparticles.     

Theoretical Analysis of Neutron and X-ray Scattering Data on 3He

X-ray scattering experiments on bulk liquid ³He (Albergamo et al. in Phys. Rev. Lett. 99:205301, 2007; Schmets and Montfrooij in Phys. Rev. Lett. 100:239601, 2008; Albergamo et?al. in Phys. Rev. Lett. 100:239602, 2008) have indicated the possibility of the existence of a sharp collective mode at large momentum transfers. We address this issue within a manifestly microscopic theory of excitations in a Fermi fluid that can be understood as proper generalization of the time-honored theory of Jackson, Feenberg, and Campbell (Jackson in Phys. Rev. A 8:1529, 1973; Feenberg in Theory of Quantum Fluids, 1969; Chang and Campbell in Phys. Rev. B 13:3779, 1976) of excitations in ⁴He. We show that both neutron and X-ray data can be well explained within a theory where the high momentum excitations lie in fact inside the particle-hole continuum. ??Pair fluctuations?? contribute a sharpening of the mode compared to the random phase approximation (RPA). When the theoretical results are convoluted with the experimental resolution, the agreement between theory and X-ray data is quite good.     

Space charge effects in a self-magnetically insulated pinch diode

Space charge effects in a self-magnetically insulated pinch diode have been studied numerically using the electromagnetic time-independent BFCPIC code (Particle-In-Cell code based on Boundary-Fitted Coordinates). The influence of the eigenfields on the electromagnetic system and in particular on the focusing properties will be discussed in detail. In the case of a bipolar flow of electrons and ions about 77% of the electrons reach the anode near the rotational axis. Due to accumulating space charge a potential hill is established in a region close to the rotational axis. The corresponding radial electric field slows down the electrons, causing them to lose most of their kinetic energy, before they are accelerated almost perpendicular to the anode. The angle between the normal to the anode and the current density of the electrons lies between 15 and 30° which is confirmed by experimental results from Maron [Phys. Rev. Lett. 45 (1980) 1849]. It will be demonstrated that the focusing properties of the pinch diode are influenced by this effect dramatically: As a consequence of the large electric field near the axis the ions from the lower region of the diode are defocused as experimentally observed by Goldstein et al. [Phys. Rev. Lett. 40 (1978) 1504].     

Cyclotron Resonance of Composite Fermions

The introduction of suitable fictitious entities occasionally permits to cast otherwise difficult strongly interacting many-body systems in a single particle form. We can then take the customary physical approach, using concepts and representations which formerly could only be applied to systems with weak interactions, and yet still capture the essential physics. A most notable recent example occurs in the conduction properties of a two-dimensional electron system (2DES), when exposed to a strong perpendicular magnetic field B. They are governed by electron–electron interactions, that bring about the fractional quantum hall effect (FQHE). S. Das Sarma and A. Pinczuk (eds.), Perspectives on Quantum Hall Effects (Wiley, New York, 1996). Composite fermions, that do not experience the external magnetic field but a drastically reduced effective magnetic field B*, were identified as apposite quasi-particles that simplify our understanding of the FQHE. J. K. Jain, Phys. Today, 39–45 (2000). J. K. Jain, Phys. Rev. Lett. 63, 199–202 (1989). They precess, like electrons, along circular cyclotron orbits, with a diameter determined by B* rather than B. B. I. Halperin, P. A. Lee, and N. Read, Phys. Rev. B 47, 7312–7343 (1993). O. Heinonen, (ed.), Composite Fermions (World Scientific, Singapore, 1998). R. R. Du, H. L. Stormer, D. C. Tsui, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. 70, 2944–2947 (1993). R. R. Du et al., Phys. Rev. Lett. 75, 3926–3929 (1995). R. L. Willett, R. R. Ruel, K. W. West, and L. N. Pfeiffer, Phys. Rev. Lett. 71, 3846–3849 (1993). V. J. Goldman, B. Su, and J. K. Jain, Phys. Rev. Lett. 72, 2065–2068 (1994). J. H. Smet, D. Weiss, R. H. Blick, G. Lütjering, and K. von Klitzing, Phys. Rev. Lett. 77, 2272–2275 (1996). The frequency of their cyclotron motion remained hitherto enigmatic, since the effective mass is no longer related to the band mass of the original electrons and is entirely generated from electron–electron interactions. Here, we demonstrate the enhanced absorption of a microwave field that resonates with the frequency of their circular motion. From this cyclotron resonance, we derive a composite fermion effective mass that varies from 0.7 to 1.2 times the electron mass in vacuum as their density is tuned from 0.6× 10¹¹/cm² to 1.2× 10¹¹/cm².     

Buoyancy driven convection in vertically shaken granular matter: experiment, numerics, and theory

Buoyancy driven granular convection is studied for a shallow, vertically shaken granular bed in a quasi 2D container. Starting from the granular Leidenfrost state, in which a dense particle cluster floats on top of a dilute gaseous layer of fast particles (Meerson et al. in Phys Rev Lett 91:024301, 2003; Eshuis et al. in Phys Rev Lett 95:258001, 2005), we witness the emergence of counter-rotating convection rolls when the shaking strength is increased above a critical level. This resembles the classical onset of convection—at a critical value of the Rayleigh number—in a fluid heated from below. The same transition, even quantitatively, is seen in molecular dynamics simulations, and explained by a hydrodynamic-like model in which the granular material is treated as a continuum. The critical shaking strength for the onset of granular convection is accurately reproduced by a linear stability analysis of the model. The results from experiment, simulation, and theory are in good agreement. The present paper extends and completes our earlier analysis (Eshuis et al. in Phys Rev Lett 104:038001, 2010).     

Implementation aspects of the bridging scale method and application to intersonic crack propagation

The major purpose of this work is to investigate the performance of the bridging scale method (BSM), a multiscale simulation framework for the dynamic, concurrent coupling of atomistics to continua, in capturing shear-dominant failure. The shear-dominant failure process considered in this work is intersonic crack propagation along a weak plane in an elastic material, similar to the seminal molecular dynamics (MD) simulations by Abraham and Gao (Phys. Rev. Lett. 2000; 84 (14):3113–3116). We show that the BSM simulations accurately capture the essential physics of the intersonic crack propagation, including the formation of a daughter crack and the sudden acceleration of the crack to a velocity exceeding the material shear wave speed. It is also demonstrated that the non-reflecting boundary condition can adequately dissipate the strongly localized wave formed by the Mach cone after the crack accelerates beyond the material shear wave speed. Finally, we provide the algorithm for our implementation of the BSM, as well as the code used to determine the damping kernels via a newly adopted technique which is less expensive than previous methods. Copyright © 2007 John Wiley & Sons, Ltd.     

Crack propagation in polymeric composites

The velocities of rapidly moving cracks in polymethylmethacrylate, an epoxy resin, a rubber modified epoxy resin, coupled and decoupled glass bead filled epoxies and randomly oriented glass fiber reinforced epoxies were measured with a crack propagation gage that was electrolissecally plated on the surfaces of the materials.

When fracture was initiated from a natural crack it was found that the velocity conformed to Mott's equation , while fracture initiated from a blunted notch resulted in a velocity that conformed to Dulany and Brace's equation . A general energy balance was used to show how one could develop these two equations as bounds to the velocity of catastrophic crack propagation.

The terminal crack velocity in the unfilled materials and the glass bead filled materials was , where E/ρ was the modulus to density ratio of the matrix phase at the macroscopic strain rate of the fracture test. The proportionality constant of 0.28 was independent of matrix type, temperature and degree of adhesion. Cracks in the rubber reinforced epoxies always tended to become blunt, resulting in breaking loads that were higher than that expected for materials possessing a natural crack. In addition, the average terminal velocity was less than 0.28√E/ρ, indicating the retardation effects of the rubber particles. These facts were used to explain the higher fracture toughness of these composites.

Fracture surface roughness was primarily a function of crack extension and breaking stress and was less sensitive to crack velocity. An empirical modification of the Mott energy balance was used to qualitatively explain this behavior.     

Influence of inhomogeneous broadening on group velocity in coherently pumped atomic vapour

Abstract

We studied experimentally the influence of thermal atomic motion on light propagation in a vacuum atomic vapour cell above room temperature. We found that atomic motion introduces sizable changes in the spectral properties of the medium, such as dispersion and absorption, if the conditions for coherent population trapping are fulfilled. In particular, studying the group delay of light in the atomic vapour, we confirm the theoretical predictions of Kocharovskaya et al. (2001, Phys. Rev. Lett., 86, 628) and demonstrate that a coherent atomic medium has light dragging abilities large enough to make feasible the realization of frozen light based on atomic motion. We also demonstrate that dragging can be observed in measurements of the electromagnetically induced transparency resonance width, as well as in group delay measurements.     

Combined modelling and experimental studies of rubber toughening in polymers

When researching the effect of rubber toughening in acrylonitrile-butadiene-styrene (ABS) copolymer and high-impact polystyrene (HIPS) and also toughening methods used in other materials, then, often comparisons are needed between materials with and without the toughening agents over the full crack velocity range. This is from the threshold stress to just maintain crack propagation up to the limiting crack velocity conditions. However, the toughening of materials can change other material properties including, for example, the onset of yield in the material under static stress. The result can be that a crack velocity versus stress curve cannot be obtained by experiment over the full crack velocity range for some toughened materials. However, as used in this study, combined experimental and computer simulations can provide for a meaningful and informative comparison between crack velocity versus stress curves for materials with and without toughening agents over the full crack velocity range.     

Vortex Dynamics in hcp Solid 4He

The peculiar features noted in (Penzev et al. in Phys. Rev. Lett. 101: 065301, 2008) we conjecture are evidence of a vortex fluid state in solid He. We suggest to analyze this state by means of the dynamics of quantized vortices, as used for the tangle of vortices in superfluid turbulence. We introduce parameters of the vortex tangle dynamics, e.g., relaxation time for the drift of lines in parallel to the torsional oscillation axis. We briefly discuss the transition from the supposed vortex fluid state into the supposed supersolid state (Shimizu et al. in Phys. Rev. Lett., arXiv:0903.1326).     

Numerical investigation of dynamic crack branching under biaxial loading

Dynamic crack growth and branching of a running crack under various biaxial loading conditions in homogeneous and heterogeneous brittle or quasi-brittle materials is investigated numerically using RFPA2D (two-dimensional rock failure process analysis)-Dynamic program which is fully parallelized with OpenMP directives on Windows. Six 2D models were set up to examine the effect of biaxial dynamic loading and heterogeneity on crack growth. The numerical simulation vividly depicts the whole evolution of crack and captured the crack path and the angles between branches. The path of crack propagation for homogenous materials is straight trajectory while for heterogeneous materials is curved. Increasing the ratio of the loading stress in x-direction to the stress in y-direction, the macroscopic angles between branches become larger. Some parasitic small cracks are also observed in simulation. For heterogeneous brittle and quasi-brittle materials coalescence of the microcracks is the mechanism of dynamic crack growth and branching. The crack tip propagation velocity is determined by material properties and independent of loading conditions.     

Yoffe-type moving edge crack in a piezoelectric block

Summary We consider an anti-plane edge moving crack problem with the constant velocity in a piezoelectric ceramic block. The far-field anti-plane shear mechanical and in-plane electrical loads are applied to the piezoelectric block. It is expressed to a Fredholm integral equation of the second kind. Expressions for the dynamic field intensity factors and the dynamic energy release rate are obtained. The dynamic stress intensity factor and the dynamic energy release rate depend on the crack propagation speed. Numerical results for several piezoelectric materials are also presented.     

Characterization of the energy bursts in vibrated shallow granular systems

We study the recently reported energy bursts that take place in a granular system confined to a vertically vibrated shallow box containing two types of grains of equal size but different mass (Rivas in Phys Rev Lett 106:088001?C1?C088001?C4, 2011). In a quasi one dimensional configuration, it is possible to characterize the propagating fronts. The rapid expansion and the subsequent compression of the energy bursts take place at roughly constant velocities. The expansion velocity is 40 times larger than the compression velocity. Starting from an initially segregated configuration it is possible to determine the instants at which the energy bursts begin and the mechanisms that trigger them. Two mechanisms are identified: an oblique collision of a heavy grain with a light one in contact with one of the horizontal walls and a slow destabilization produced by light grains that are surrounded by heavy ones.     

Problem of image superresolution with a negative-refractive-index slab

By means of the angular spectrum representation of wave fields, a discussion is given on the propagation and restoration of the wave-front structure in a slab of a left-handed medium (or negative-index medium) whose surface impedance matches that of vacuum, namely, one whose effective optical parameters are n = epsilon = mu = -1. This restoration was previously discussed [Phys. Rev. Lett. 85, 3866 (2000)] in regard to whether it may yield superresolved images. The divergence of the wave field in the slab, and its equivalence with that of the inverse diffraction propagator in free space, is addressed. Further, the existence of absorption, its regularization of this divergence, and the trade-off of a resulting limited superresolution are analyzed in detail in terms of its effect on the evanescent components of the wave field and hence on the transfer function width.     

裂纹稳态扩展下正交异性材料的动应力强度因子K_(Ⅲ)解答

研究了无限大正交异性材料中半无限长Ⅲ型裂纹的动态扩展问题。裂纹尖端附近的应力和位移被表达为解析复函数的形式,而复函数可以表达为幂级数的形式,幂级数的系数由研究问题的边界条件来确定。这样就给出了裂纹尖端附近的应力分量和位移分量的简单近似表达式,由推导出的动应力分量和动位移分量可以退化为其在各向同性材料静态断裂问题中的情况。最后,裂纹扩展特性由裂纹几何参数和裂纹扩展速度来反映出来,相同的几何参数情况下,裂纹扩展愈快,裂纹尖端附近的最大应力分量和最大位移分量愈大。     

Generation of induced Smith-Purcell radiation: free-electron laser in open system

We have used the framework of the dispersion equation to study coherent Smith–Purcell (SP) radiation induced by a relativistic magnetized electron beam in the absence of a resonator. As an important example of the application of the obtained results of our previous paper JMO v.57, 2060, (2010) the growth rate of SP FEL in the case with a rectangular grating was calculated. The growth rate of the instability is proportional to the square root of the electron beam current. The calculated results are consistent with the experimental data obtained by Urata et al. [Phys. Rev. Lett. 80, 516 (1998)].     

Quantized Vortex State in hcp Solid 4He

The quantized vortex state appearing in the recently discovered new states in hcp ⁴He since their discovery (Kim and Chan, Nature, 427:225–227, 2004; Science, 305:1941, 2004) is discussed. Special attention is given to evidence for the vortex state as the vortex fluid (VF) state (Anderson, Nat. Phys., 3:160–162, 2007; Phys. Rev. Lett., 100:215301, 2008; Penzev et al., Phys. Rev. Lett., 101:065301, 2008; Nemirovskii et al., arXiv:0907.0330, 2009) and its transition into the supersolid (SS) state (Shimizu et al., arXiv:0903.1326, 2009; Kubota et al., J. Low Temp. Phys., 158:572–577, 2010; J. Low Temp. Phys., 162:483–491, 2011). Its features are described. The historical explanations (Reatto and Chester, Phys. Rev., 155(1):88–100, 1967; Chester, Phys. Rev. A, 2(1):256–258, 1970; Andreev and Lifshitz, JETP Lett., 29:1107–1113, 1969; Leggett, Phys. Rev. Lett., 25(22), 1543–1546, 1970; Matsuda and Tsuneto, Prog. Theor. Phys., 46:411–436, 1970) for the SS state in quantum solids such as solid ⁴He were based on the idea of Bose Einstein Condensation (BEC) of the imperfections such as vacancies, interstitials and other possible excitations in the quantum solids which are expected because of the large zero-point motions. The SS state was proposed as a new state of matter in which real space ordering of the lattice structure of the solid coexists with the momentum space ordering of superfluidity. A new type of superconductors, since the discovery of the cuprate high T _c superconductors, HTSCs (Bednorz and Mueller, Z. Phys., 64:189, 1986), has been shown to share a feature with the vortex state, involving the VF and vortex solid states. The high T _c s of these materials are being discussed in connection to the large fluctuations associated with some other phase transitions like the antiferromagnetic transition in addition to that of the low dimensionality. The supersolidity in the hcp solid ⁴He, in contrast to the new superconductors which have multiple degrees of freedom of the Cooper pairs with spin as well as angular momentum freedom, has a unique feature of possessing possibly only the momentum fluctuations and vortex ring excitations associated with the possible low dimensional fluctuations of the subsystem(s). The high onset temperature of the VF state can be understood by considering thermally excited low D quantized vortices and it may be necessary to seek low dimensional sub-systems in hcp He which are hosts for vortices.     

A Conjecture to Derive An Equation of Motion For Dynamic Fracture

Crack propagation is hypothesized as being a discontinuous process and by decoupling in time the (macroscopic) dynamic energy release rate, Gd, from the (microscopic) J-integral a discrete non-linear equation is obtained having the form of a logistic map. Applying this equation to fracture in amorphous brittle materials (in particular PMMA) it shows that for an accelerating crack, propagation changes from a continuous process in time to a discontinuous one concomitant with instabilities in velocity related to macroscopic branching.     

Fatigue crack propagation after overloading and underloading at negative stress ratios

This paper intends to evaluate the influence of the intrinsic properties of the materials, namely plastic and cyclic plastic properties, on the overloading/underloading effect on crack propagation rate, at baseline negative stress ratios, under plane strain conditions. The importance of the negative loading part of the fatigue cycle on crack propagation rate has been shown by previous works of this same author. In those works has also been shown that under baseline negative stress ratios there exists negative open loads and crack propagation rate does not correlate properly with the crack closure concept. These features were shown to be strongly related to plastic properties and cyclic plastic properties of the materials. It has been concluded that the Bauschinger effect may be the explanation for the different sensitivity to negative loads. Thus, some materials may be very sensitive to negative loads and some others may not be so sensitive. Tensile overload and compression underload tests, at positive and negative baseline stress ratios were made in different materials, with different plastic properties, in order to predict their influence on crack propagation rate. The main emphasis in this paper is the importance of the compressive part of the loading cycle under negative baseline R ratios on overloads/underloads effect on crack propagation rate. Results will show that the effect of overloads and underloads on crack propagation rate, at baseline negative stress ratios, are not fully accounted for by crack propagation models and that the generalized accepted behaviour of OL/UL may not be the same at baseline negative stress ratios. It will be shown that Overloads may produce acceleration instead of the accepted retardation effect. A physical understanding on the effects of OL/UL is also provided in the paper.