Possible vortex production by a vibrating wire in superfluid He–B

We observe spectacular features on the force–velocity characteristic of a vibrating wire resonator in superfluid ³He–B at ultralow temperatures. There are both plateaux and discontinuities in the characteristic. The plateaux seem to have two separate critical velocities where firstly some ‘event’ occurs, which causes the wire to lose energy and slow down, followed by a second critical velocity where the ‘event’ decouples. The shape of the pulses so created depends on the precise position on the particular plateau. At the low-velocity end of the plateau, the pulses show a sudden decrease in velocity followed by a gradual growth, whereas at the high-velocity end the pulses consist of a sudden increase in velocity followed by a gradual decrease. Eventually, the pulsing ceases indicating a saturation effect. We speculate that these events are due to vortex creation and annihilation/separation.     

Vibrating Wire for Profile Measurements of Thin Beams in Particle Accelerators: Preliminary Tests Using a Laser Beam

The possibility of using a vibrating wire as a target in the method of scanning transverse profiles of the thin beams in particle accelerators has been studied. In the case where the transverse dimensions of the beam are comparable to the amplitude of the wire oscillations, the vibrating wire can be used for a fast measurement of the transverse beam profile without moving the sensor. The scanning procedure is replaced by the use of the movement of the wire during its mechanical oscillations. The method is tested on a focused beam of a semiconductor laser with a spot size at the focus of ～0.1 mm. The reconstruction of the transverse profile is performed on the basis of the measurements of the photons reflected from the vibrating wire by using fast photodiodes.     

Voltage plateaus on <Emphasis Type="Italic">V</Emphasis>(<Emphasis Type="Italic">I</Emphasis>) curves of long quasi-one-dimensional superconducting wires (without microwave irradiation)

Segments of an almost constant voltage (plateaus) on the V(I) curves of long quasi-one-dimensional superconducting aluminum wires placed in a magnetic field are found slightly below T _c, which are unexpected at the parameters and geometry considered in this work. These plateaus are assumingly attributed to subharmonics of the superconducting gap and are due to multiple Andreev reflection and strong quasiparticle heating, which occur in the nonequilibrium region of a wire. The plateaus indicate the coexistence of superconductivity and dissipation in these wires. These results cannot be described by the existing theories.     

Analysis and experimental concepts of the vibrating wire alignment technique

The vibrating wire alignment technique is a method which, by measuring the spatial distribution of a magnetic field, can achieve very high alignment accuracy. The vibrating wire alignment technique can be applied to fiducializing magnets and the alignment of accelerator straight section components, and it is a necessary supplement to conventional alignment methods. This article gives a systematic summary of the vibrating wire alignment technique, including vibrating wire model analysis, system frequency calculation, wire sag calculation, and the relation between wire amplitude and magnetic induction intensity. On the basis of this analysis, this article outlines two existing alignment methods, one based on magnetic field measurement and the other on amplitude and phase measurements. Finally, some basic experimental issues are discussed.     

振动线数据采集和数据处理方案设计与验证

振动线理论提供了一种高精度准直方法,将振动线理论应用于共架磁铁准直,可大大提高共架磁铁间的准直精度。介绍了应用振动线准直共架磁铁的实施方案,从总体方案出发,以振动线理论为依据提出了一种振动线系统数据采集和数据处理设计方案,并对传感器工作点对准、导线电流测量、数据采集设备选取、磁感应强度计算、磁场中心计算等关键问题进行分析,给出了解决方案。最后通过实验验证了方案的可行性以及系统的测量精度。     

Decay of pure quantum turbulence in superfluid 3He-B

We describe measurements of the decay of pure superfluid turbulence in superfluid 3He-B, in the low temperature regime where the normal fluid density is negligible. We follow the decay of the turbulence generated by a vibrating grid as detected by vibrating wire resonators. Despite the absence of any classical normal fluid dissipation processes, the decay is consistent with turbulence having the classical Kolmogorov energy spectrum and is remarkably similar to that measured in superfluid 4He at relatively high temperatures. Further, our results strongly suggest that the decay is governed by the superfluid circulation quantum rather than kinematic viscosity.     

A device for measuring contact potential differences with high spatial resolution

A method is described for measuring local variations of the surface potential in one dimension on solid surfaces. The variations are determined by a contact potential difference (CPD) method utilizing a 6 μm thick wire as the vibrating reference electrode. The vibrating wire being placed at a distance of 15 μm above the surface can be moved under UHV conditions across the surface and the CPD variation is directly plotted on and XY-recorder. The spatial resolution amounts to about 50μm and the sensitivity for CPD changes is less than 20 mV. The method has been applied to studies of the surface diffusion of oxygen on a (110) tungsten single crystal plane.     

振动线准直技术的原理和研究概述

振动线准直技术是一种通过直接测量空间磁场分布情况来进行磁铁设备准直的方法,其原理完全不同于目前国内正在使用的基于磁铁机械结构进行准直的方法。振动线方法具有很高的准直精度和灵敏性,可以用于单个磁铁磁中心测量、磁铁准直标定和多块磁铁磁中心准直等。主要介绍了振动线准直技术的原理、起源和国际上的应用研究概况。     

Primarily nonlinear effects observed in a driven asymmetrical vibrating wire

The purpose of the work reported here is to further experimentally explore the wide variety of behaviors exhibited by driven vibrating wires, primarily in the nonlinear regime. When the wire is driven near a resonant frequency, it is found that most such behaviors are significantly affected by the splitting of the resonant frequency and by the existence of a "characteristic" axis associated with each split frequency. It is shown that frequency splitting decreases with increasing wire tension and can be altered by twisting. Two methods are described for determining the orientation of characteristic axes. Evidence is provided, with a possible explanation, that each axis has the same orientation everywhere along the wire. Frequency response data exhibiting nonlinear generation of transverse motion perpendicular to the driving direction, hysteresis, linear generation of perpendicular motion (sometimes tubular), and generation of motion at harmonics of the driving frequency are exhibited and discussed. Also reported under seemingly unchanging conditions are abrupt large changes in the harmonic content of the motion that sometimes involve large subharmonics and harmonics thereof. Slow transitions from one stable state of vibration to another and quasiperiodic motions are also exhibited. Possible musical significance is discussed.     

Low-field phase diagram of the spin Hall effect in the mesoscopic regime

When a mesoscopic two dimensional four-terminal Hall cross bar with spin-orbit interaction (SOI) is subjected to a perpendicular uniform magnetic field B, both integer quantum Hall effect (IQHE) and mesoscopic spin Hall effect (MSHE) may exist when disorder strength W in the sample is weak. We have calculated the low field "phase diagram" of MSHE in the (B,W) plane for disordered samples in the IQHE regime. For weak disorder, MSHE conductance G(sH) and its fluctuations rms(G(sH)) vanish identically on even numbered IQHE plateaus, they have finite values on those odd numbered plateaus induced by SOI, and they have values G(sH)=1/2 and rms(G(sH))=0 on those odd numbered plateaus induced by Zeeman energy. At larger disorders, the system crosses over into a regime where both G(sH) and rms(G(sH)) are finite, a chaotic regime, and finally a localized regime.     

Tunneling exponents sensitive to impurity scattering in quantum wires

We show that the scaling exponent for tunneling into a quantum wire in the "Coulomb Tonks gas" regime of impenetrable, but otherwise free, electrons is affected by impurity scattering in the wire. The exponent for tunneling into such a wire thus depends on the conductance through the wire. This striking effect originates from a many-body scattering resonance reminiscent of the Kondo effect. The predicted anomalous scaling is stable against weak perturbations of the ideal Tonks gas limit at sufficiently high energies, similar to the phenomenology of a quantum critical point.     

Critical velocity in 3He-B vibrating wire experiments as analog of vacuum instability in a slowly oscillating electric field

The Lancaster experiments [C. A. M. Castelijns, K. F. Coates, A. M. Guenault, S. G. Mussett, and G. R. Pickett Phys. Rev. Lett. 56, 69 (1986)] with a cylindrical wire moving in superfluid ³He-B are discussed, where the measured critical velocity of pair creation was much below the Landau critical velocity. The phenomenon is shown to be analogous to the instability of the electron-positron vacuum in an adiabatically alternating strong electric potential of both signs, where the positive-and negative-root levels cross and thus the instability threshold is half the conventional value in a single static potential well. Zh. éksp. Teor. Fiz. 115, 70–79 (January 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Top flavour signal in thep\bar p collider events—A detailed analysis with flavour creation and excitation models

The single jet electron events recently observed by the UA1 collaboration at the protonantiproton collider are analysed in terms of top flavour production using both flavour creation and excitation mechanisms. A top particle of mass ～ 35 GeV can naturally account for both the event topology (isolated electron coming back to back with a jet) and the neutrinop _T distribution relative to the electron, irrespective of the production mechanism. Each of the two production mechanisms gives an event rate any where between the observed rate and an order of magnitude lower, depending on the model parameters. They can be easily distinguished, however, by studying the jet coming back to back with the electron. It is predicted to be a fat jet containing a top particle for flavour creation and an ordinary narrow jet for flavour excitation. A detailed list of other predictions is given, which can be tested with the available or the forthcoming data. Some of these provide further tests of the top signal irrespective of the production mechanism, while others can discriminate between the two mechanisms as well as between different models for flavour excitation. It is also shown that not only are the available event characteristics naturally explained by top, but they cannot be explained by any other mechanism explored so far.     

Quantum Hall effect of massless dirac fermions in a vanishing magnetic field

The effect of strong long-range disorder on the quantization of the Hall conductivity sigma{xy} in graphene is studied numerically. It is shown that increasing Landau-level mixing progressively destroys all plateaus in sigma{xy} except the plateaus at sigma{xy}=-/+e{2}/2h (per valley and per spin). The critical state at the Dirac point is robust to strong disorder and belongs to the universality class of the conventional plateau transitions in the integer quantum Hall effect. We propose that the breaking of time-reversal symmetry by ripples in graphene can realize this quantum critical point in a vanishing magnetic field.     

振动颗粒物质“巴西果”分离效应实验和理论研究

本文通过实验和理论研究了影响"巴西果"分离的因素及其物理机理. 分析了振动加速度、大小颗粒尺寸和密度对分离时间的影响, 并利用流体模型对分离时间作估算, 对实验结果进行定性解释. 结果表明在振动频率固定时, 调节振动加速度是控制"巴西果"分离的一个主要手段. 振动加速度存在一个临界值, 当高于此临界值时, "巴西果"分离的主要物理机理由对流机理转变为几何填空机理, 且振动加速度对分离影响变小, 大颗粒尺寸对分离的影响增大. 可通过调节大颗粒的尺寸来改变分离效果. 当大、 小颗粒密度比为1时, 仍会出现"巴西果"分离现象. 增大小颗粒尺寸或密度可以促进"巴西果"分离.     

Fractional quantum conductance in gold nanowires

The conductance of a nanoscopic wire decreases in steps when it is stretched until it breaks. This is due to narrowing of the wire whereby quantized conductance channels sequentially close. However the conductance plateaus seldom occur at integral multiples of the quantum of conductance G₀ and most steps are much smaller than G₀. High speed precision measurements of the conductance steps reveal that the nanowires are composed of not one but several quantized conductors in series and that a step is caused by a quantized conductance change in one of the elements in the sequence. Resolved series conductance quanta up to 20 G₀ have been observed at room temperature. The effect is explained in terms of elastic electronic scattering and has important consequences for nano-electronic circuitry.     

Disorder effect on the quantum Hall effect in thin films of three-dimensional topological insulators

We numerically study the quantum Hall effect (QHE) in three-dimensional topological insulator (3DTI) thin film in the presence of the finite Zeeman energy g and the hybridization gap Δ under a strong magnetic field and disorder. For Δ = 0 but g ≠ 0, the Hall conductivity remains to be odd-integer quanti-zed σ _xy = ν(e ²/h) , where ν = 2? + 1 with ? being an integer. In the presence of disorder, the Hall plateaus can be destroyed through the float-up of extended levels toward the band center and the higher plateaus disappear first. The two central plateaus with ν = ± 1 around the band center are strongest against disorder scattering. With the increasing of the disorder strength, Hall plateaus are destroyed faster for the system with a weaker magnetic field. If g = 0 but Δ ≠ 0, there is a splitting of the central (n = 0) Landau level, yielding a new plateau with ν = 0, in addition to the original odd-integer plateaus. In the strong-disorder regime, the QHE plateaus can be destroyed due to the float-up of extended levels toward the band center. The ν = 0 plateau around the band center is strongest against disorder scattering, which eventually disappears. For both g ≠ 0 and Δ ≠ 0, the simultaneous presence of nonzero g and Δ causes the splitting of the degenerating Landau levels, so that all integer Hall plateaus ν = ? appear. The ν = 0,1 plateaus are the most stable ones. In the strong-disorder regime, all QHE states are destroyed by disorder, and the system transits into an insulating phase.     

Der Einfluß von Strom und Magnetfeld auf die Supraleitungseigenschaften von Nb3Sn-, V3Ga- und V3Si-Diffusionsschichten

For diffusion layers of Nb₃Sn, V₃Ga, and V₃Si on niobium and vanadium wires, respectively, the transition to superconductivity has been measured at various currents in longitudinal and transverse magnetic fields. Current and field shift the transition curve to lower temperatures, except near the onset of superconductivity where the core of the wire influences the transition. The critical current vs. temperature curves are linear at higher currents, the critical field curves are bended. The results are explained by local variations of the transition temperature due to inhomogeneities and by a displacement of the current paths under the influence of the Lorentz force. For very thin diffusion layers an influence of thickness on the transition temperature has been observed. In addition quenching curves of the diffusion layers have been measured. The shape is in accordance with the model ofKim and coworkers. The absolute values and deviations at higher fields are discussed. A deduction of the critical values at low temperatures from measurements near the transition is possible.     

高能同步辐射光源验证装置振动线准直系统结构设计

振动线准直技术是为了满足新一代同步辐射光源对超低发射度的要求而正在研究的准直技术。新光源要求磁铁具有极高的准直精度,同一支架上的磁铁间准直精度要求小于30 m。介绍了高能同步辐射光源验证装置振动线准直系统的设计,描述了振动线准直系统的具体机械结构方案、电路结构方案和运动控制与数据采集方案等。     

高能同步辐射光源验证装置振动线准直系统结构设计

振动线准直技术是为了满足新一代同步辐射光源对超低发射度的要求而正在研究的准直技术。新光源要求磁铁具有极高的准直精度,同一支架上的磁铁间准直精度要求小于±30μm。介绍了高能同步辐射光源验证装置振动线准直系统的设计,描述了振动线准直系统的具体机械结构方案、电路结构方案和运动控制与数据采集方案等。