Mode locking features of driven vortex matter in an amorphous MoGe film detected by rf impedance technique

We have investigated mode locking resonance of a driven vortex lattice in an amorphous MoGe film by means of rf impedance measurements. This technique allows us to detect directly the lattice motion in response to a rf current superimposed on top of a dc current. At low rf currents the resonance appears as jumps (dips) in real (imaginary) part of rf impedance Z. On increasing rf current, the jump height in the real part of Z at a given resonant condition decreases monotonically, whereas the imaginary part of Z exhibits a transformation from the dip to peak behaviors. Namely, the resonant feature changes from capacitive to inductive responses, on increasing rf current. This behavior implies variations in the phase of rf current relative to that of the lattice velocity modulated by the pinning potential.     

Mode locking of vortex matter driven through mesoscopic channels

We investigated the driven dynamics of vortices confined to mesoscopic flow channels by means of a dc-rf interference technique. The observed mode-locking steps in the IV curves provide detailed information on how both the number of vortex rows and the lattice structure in each flow channel change with magnetic field. Minima in flow stress occur when an integer number of rows is moving coherently, while maxima appear when the incoherent motion of mixed n and n+/-1 row configurations is predominant. Simulations show that the enhanced pinning at mismatch originates from quasistatic fault zones with misoriented edge dislocations induced by disorder in the channel edges.     

Dynamic melting of driven vortex matter below 1 K

We report on measurements of a mode-locking resonance in amorphous Mo_xGe_1?x films at different temperatures T down to 0.8 K, which is well below the superconducting transition (6 K). We observe dynamic ordering of driven vortex matter for all the temperatures studied. As the field exceeds a certain critical field B_c,dyn at fixed T, moving vortices do not exhibit dynamic ordering. At high T, this field B_c,dyn(T), so-called dynamic melting, nearly coincides with a characteristic field B_c(T) where the linear resistivity vanishes. At low T, however, B_c,dyn(T) is significantly suppressed compared to B_c(T), suggesting intrinsic quantum melting in the absence of pinning.     

Simulation study for the orientation of the driven vortex lattice in an amorphous superconductor

We investigate the orientation of the vortex lattice driven by an applied current by means of numerical simulations based on the time-dependent Ginzburg–Landau (TDGL) theory. A lattice order is restored by a current driving of vortices under the influence of random vortex pinnings. The orientation of the moving vortex lattice is different between the presence and the absence of vortex pinnings. We show results of TDGL simulations for these phenomena.     

Anomalous vortex motion in the quantum-liquid phase of amorphous MoxSi1-x films

We measure, in real time (t), the fluctuating component of the flux-flow voltage V(t), deltaV(t) identical withV(t)-V0, about the average V0 in the vortex-liquid phase of amorphous MoxSi1-x films. For the thick film, deltaV(t) originating from the vortex motion is clearly visible in the quantum-liquid phase, where the distribution of deltaV(t) is asymmetric, indicative of large velocity and/or number fluctuations of driven vortices. For the thin film the similar anomalous vortex motion is observed in nearly the same (reduced-)temperature regime. These results suggest that vortex dynamics in the low-temperature liquid phase of thick and thin films is dominated by common physical mechanisms, presumably related to quantum effects.     

Angular dependence of the magnetic-field driven superconductor-insulator transition in thin films of amorphous indium-oxide

A significant anisotropy of the magnetic-field driven superconductor-insulator transition is observed in thin films of amorphous indium-oxide. The anisotropy is largest for more disordered films which have a lower transition field. At higher magnetic fields the anisotropy reduces and even changes sign beyond a sample specific and temperature independent magnetic field value. The data are consistent with the existence of more than one mechanism affecting transport at high magnetic fields.     

Non-monotonic driven vortex noise in HTSC

We present noise measurements on YBCO thin films in different conditions of magnetic field and driving current. Noise spectra for non-driven and driven cases (in the flux-creep region) evidence deep differences in vortex dynamics between these two regimes. For the driven case, the effect of applying magnetic field is a reduction in noise, which can be explained by the increase in the fraction of vortices that undergo flux-flow. For the non-driven case, magnetic field has no significative influence on noise, probably due to the absence of Lorentz force that causes coherent movement of vortices. For all magnetic fields studied in this work (0-154 mT) the effect of increasing current is an increase of noise, which is in contrast to the results from other authors. This behavior can be explained by an increase of current induced vortex-antivortex annihilation events. We propose that driven noise has a non-monotonic behavior due to the competition between annihilation events and driving force which causes opposite effects on noise.     

Phases of atom-molecule vortex matter

We study ground state vortex configurations in a rotating atom-molecule Bose-Einstein condensate. It is found that the coherent coupling between the atomic and molecular condensates can render a pairing of atomic and molecular vortices into a composite structure that resembles a carbon dioxide molecule. Structural phase transitions of vortex lattices are also explored through different physical parameters including the rotational frequency of the system.     

Peak effect in polycrystalline vortex matter

We report a study of the peak-effect phase diagram of a strongly disordered type-II superconductor V-21 at. %Ti using ac magnetic susceptibility and small-angle neutron scattering (SANS). In this system, the peak effect appears only at fields higher than 3.4 T. The sample is characterized by strong atomic disorder. Vortex states with field-cooled thermal histories show that both deep in the mixed state, as well as close to the peak effect, there exist no long-range orientationally ordered vortex lattices. The SANS scattering radial widths reveal vortex states ordered in the sub-mum scale. We conjecture that the peak effect in this system is a remnant of the Bragg glass disordering transition, but occurs on submicron length scales due to the presence of strong atomic disorder on larger length scales.     

非晶中结构遗传性及描述

非晶态物质广泛存在于人们的日常生活和工业生产活动中,但人们对其原子结构及其结构与性能关系的认识还远不如对晶体材料那样充分.非晶态物质的原子结构不具备空间平移对称性,这使得传统针对晶体材料的实验技术和手段无法直接有效地应用到非晶态物质的结构分析中.用常规的衍射实验数据分析方法并不能直接地观察到非晶态物质的本征结构特征,但这些实验衍射数据往往隐含有极其重要的微观结构信息.本文简要综述了这些衍射数据背后所隐含的与金属玻璃中程序相关的结构信息.研究发现,非晶态物质中的一类隐含序与晶体结构中的球周期序紧密相关,意味着非晶态物质与晶体材料之间在原子结构上存在着非凡的同源性.进一步的研究结果还表明,不同隐含拓扑序之间纠缠的强弱与体系本身的玻璃形成能力存在明显的对应关系,这为衡量金属合金玻璃形成能力强弱的经验规律——混乱原理提供了微观结构上的理解,同时为进一步深入认识和理解非晶态材料衍射数据所隐含的微观结构信息提供了新的分析思路和方法.     

Dynamic melting of confined vortex matter

We study dynamic melting of confined vortex matter moving in disordered, mesoscopic channels by mode-locking experiments. The dynamic melting transition, characterized by a collapse of the mode-locking effect, strongly depends on the frequency, i.e., on the average velocity of the vortices. The associated dynamic ordering velocity diverges upon approaching the equilibrium melting line T(m,e)(B) as v(c) approximately (T(m,e)-T)(-1). The data provide the first direct evidence for velocity dependent melting and show that the phenomenon also takes place in a system under disordered confinement.     

Superconductivity in amorphous Cr films

Superconducting transition has been observed for the first time in amorphous Cr films prepared by co-sputtering of Cr and BN. Superconducting transition temperatures (Tc) up to 1.14 K have been observed with an orbital contribution (i.e.(dH_c2⊥/dT)_Tc) as large as 49 kOe K^-1. The electronic specific heat coefficient (γ) has been estimated to be about 1.6 times as large as that of amorphous Mo-BN (or Mo-metalloid) alloys. This finding, i.e. the low Tc with the large γ for amorphous Cr is inconsistent with data reported for amorphous alloys of 4d and 5d transition elements. This inconsistency implies a formation of local magnetic moments or spin fluctuation in the amorphous Cr film.     

用晶格玻尔兹曼方法研究颗粒在涡流中的运动

用晶格玻尔兹曼方法研究小颗粒在涡流中的运动.涡流由流经空腔的流体产生.用动量交换法和压力张量积分法计算颗粒在涡流中的运动轨迹、速度和角速度.最后用张量积分法计算两个不同半径的颗粒在涡流中的运动. 关键词： 晶格玻尔兹曼方法 涡流 颗粒     

Melting of heterogeneous vortex matter: The vortex ‘nanoliquid’

Disorder and porosity are parameters that strongly influence the physical behavior of materials, including their mechanical, electrical, magnetic and optical properties. Vortices in superconductors can provide important insight into the effects of disorder because their size is comparable to characteristic sizes of nanofabricated structures. Here we present experimental evidence for a novel form of vortex matter that consists of inter-connected nanodroplets of vortex liquid caged in the pores of a solid vortex structure, like a liquid permeated into a nanoporous solid skeleton. Our nanoporous skeleton is formed by vortices pinned by correlated disorder created by high-energy heavy ion irradiation. By sweeping the applied magnetic field, the number of vortices in the nanodroplets is varied continuously from a few to several hundred. Upon cooling, the caged nanodroplets freeze into ordered nanocrystals through either a first-order or a continuous transition, whereas at high temperatures a uniform liquid phase is formed upon delocalization-induced melting of the solid skeleton. This new vortex nanoliquid displays unique properties and symmetries that are distinct from both solid and liquid phases.     

Vortex line ordering in the driven three-dimensional vortex glass

Resistively shunted junction dynamics is applied to the three-dimensional uniformly frustrated XY model with randomly perturbed couplings, as a model for driven steady states in a type-II superconductor with quenched point pinning. For a disorder strength p strong enough to produce a vortex glass in equilibrium, we map the phase diagram as a function of temperature T and uniform driving current I. Using finite size analysis and averaging over many realizations of quenched randomness we find a first-order melting T(m)(I) from a vortex line smectic to an anisotropic liquid.     

Disorder driven melting of the vortex line lattice

The 3D XY model with random in-plane couplings is simulated to model the phase diagram of a disordered type II superconductor as a function of temperature T and randomness strength p for fixed applied magnetic field. As p increases to a critical p(c), the first order vortex lattice melting line turns parallel to the T axis, continuing down to low temperatures, rather than ending at a critical point. Above p(c) preliminary results suggest the absence of a phase coherent vortex glass.     

Effective temperature in driven vortex lattices with random pinning

We study numerically correlation and response functions in nonequilibrium driven vortex lattices with random pinning. From a generalized fluctuation-dissipation relation, we calculate an effective transverse temperature in the fluid moving phase. We find that the effective temperature decreases with increasing driving force and becomes equal to the equilibrium melting temperature when the dynamic transverse freezing occurs. We also discuss how the effective temperature can be measured experimentally from a generalized Kubo formula.