STM imaging of flux line arrangements in the peak effect regime

We present the results of a study of vortex arrangements in the peak-effect regime of 2H-NbSe2 by scanning tunneling microscopy. By slowly increasing the temperature in a constant magnetic field, we observed a sharp transition from collective vortex motion to positional fluctuations of individual vortices at the temperature which coincides with the onset of the peak effect in ac susceptibility. We conclude that the peak effect is a disorder driven transition, with the pinning energy winning against the elastic energy.     

Dynamic phases and the peak effect in dirty type II superconductors

We study numerically and experimentally the dynamics of driven vortex matter. Our London-Langevin simulations find that the critical current exhibits a peak both across the Bragg glass to vortex glass transition and across the melting line. The peak is accompanied by a clear crossing of the I-V curves. We report transport measurements on untwinned YBCO crystals, in complete accordance with these findings. At higher drives disorder is averaged to reduced values, and in three dimensions the vortices reorder into a "moving solid." The effect of the disorder can be well represented with a "shaking temperature" which is inversely proportional to the velocity.     

Dynamical phase transition of a driven vortex lattice with disordered pinning

We have numerically solved the overdamped equation of vortex motion in a two-dimensional driven vortex lattice with disordered pinning, in which the driving Lorentz force, the pinning force due to point defects, the intervortex interacting force, and the thermal fluctuation force are taken into account. It is found that the vortex density and pinning strength are two important factors of affecting the melting transition of a vortex lattice. At low magnetic fields, there exist hysteresis loops of the average vortex velocity and the average pinning force vs. the driving force, from which the feature of a first-order melting transition of the vortex motion can be clearly seen. As the magnetic field is increased beyond a critical value, the hysteresis loops disappear and the melting transition is replaced by a second-order glass transition. We have also studied the influence of intervortex interactions on the vortex melting transition by comparing several forms of repulsive forces between the vortices.     

Static and dynamic coupling transitions of vortex lattices in disordered anisotropic superconductors

We use three-dimensional molecular dynamics simulations of magnetically interacting pancake vortices to study vortex matter in disordered, highly anisotropic materials such as BSCCO. We observe a sharp 3D-2D transition from vortex lines to decoupled pancakes as a function of relative interlayer coupling strength, with an accompanying large increase in the critical current reminiscent of a second peak effect. We find that decoupled pancakes, when driven, simultaneously recouple and order into a crystalline-like state at high drives. We construct a dynamic phase diagram and show that the dynamic recoupling transition is associated with a double peak in dV/dI.     

Local melting and drag for a particle driven through a colloidal crystal

We numerically investigate a colloidal particle driven through a colloidal crystal as a function of temperature. When the charge of the driven particle is larger or comparable to that of the colloids comprising the crystal, a local melting can occur, characterized by defect generation in the lattice surrounding the driven particle. The generation of the defects is accompanied by an increase in the drag force on the driven particle, as well as large noise fluctuations. We discuss the similarities of these results to the peak effect phenomena observed for vortices in superconductors.     

Evaporation of the pancake-vortex lattice in weakly coupled layered superconductors

We calculate the melting line of the pancake-vortex system in a layered superconductor, interpolating between two-dimensional (2D) melting at high fields and the zero-field limit of single-stack evaporation. Long-range interactions between pancake vortices in different layers permit a mean-field approach, the "substrate model, " where each 2D crystal fluctuates in a substrate potential due to the vortices in other layers. We find the thermal stability limit of the 3D solid, and compare the free energy to a 2D liquid to determine the first-order melting transition and its jump in entropy.     

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.     

磁通运动的电压噪声频谱分析和动力学相变

用Langevin分子动力学方法模拟磁通运动的纵向电压噪声谱随磁场和电流的变化.计算结果表明，外加磁场增大到磁通运动动力学相变场F_P，电压噪声 谱中低频宽带噪声减小而出现洛伦兹形高频窄带噪声.外加磁场增大到熔化场F_m附近，高频窄带噪声 峰值增高 ，峰值对应频率增大.在外加电流增强到磁通弹性运动区域，高频窄带噪声频谱呈现搓衣板 形式.搓衣板高频窄带噪声产生于磁通平移速度的周期性调制，它表明层状超导体中运动的 磁通格子存在有平移序的BG相. 关键词： 第Ⅱ类超导体 电压噪声 动力学模拟     

Vortex sublattice melting in a two-component superconductor

We consider the vortices in a superconductor with two individually conserved condensates in a finite magnetic field. The ground state is a lattice of cocentered vortices in both order parameters. We find two phase transitions: (i) a "vortex sublattice melting" transition where vortices in the field with lowest phase stiffness ("light vortices") lose cocentricity with the vortices with large phase stiffness ("heavy vortices"), entering a liquid state (the structure factor of the light vortices vanishes continuously; this transition is in the 3Dxy universality class); (ii) a first-order melting transition of the lattice of heavy vortices, in a liquid of light vortices.     

Depinning dynamics of driven colloids with random pinning： a numerical study

Using Langevin simulations, we have investigated numerically the depinning dynamics of driven two-dimensional colloids subject to the randomly distributed point-like pinning centres. With increasing strength of pinning, we find a crossover from elastic to plastic depinnings, accompanied by an order to disorder transition of state and a substantial increase in the depinning force. In the elastic regime, no peaks are found in the differential curves of the velocity－force dependence (VFD) and the transverse motion is almost none. In addition, the scaling relationship between velocity and force is found to be valid above depinning. However, when one enters the plastic regime, a peak appears in the differential curves of VFD and transverse diffusion occurs above depinning. Furthermore, history dependence is found in the plastic regime.     

Vortex flow and transverse flux screening at the bose glass transition

We study the vortex phase diagram in untwinned YBCO crystals with columnar defects. These randomly distributed defects are expected to induce a "Bose glass" phase of localized vortices exhibiting a vanishing resistance and Meissner effect for magnetic fields H( perpendicular) transverse to the columns. We directly observe the transverse Meissner effect using a Hall probe array. As predicted, the Meissner state breaks down at temperatures T(s) that decrease linearly as H( perpendicular) increases. However, T(s) lies far below the conventional melting temperature T(m) determined by a vanishing resistivity, suggesting a regime where vortices are effectively localized even when rotated off the columnar defects.     

无序超导体磁通运动的两次退钉扎效应和重新进入超导相

考虑平面内和不同平面磁通之间的相互作用力，计算了无序各向异性超导体中磁通运动的平均速度、微分电阻随驱动力F_l的变化规律，用层间关联函数C_z的值来判断2D塑性流动和3D关联流动的运动图像.观察到随着外驱动力的增大微分电阻出现两个尖峰，它对应着磁通运动存在两次退钉扎现象.在一定层间耦合条件下，在微分电阻双峰之间，可观察到重新进入微分电阻为零的钉扎相.这与最近实验上新发现的无序弱钉扎超导体有重新进入超导相的巨大峰值效应相吻合.同时，也可发现随着驱动电流的增大，磁通运动出现由2D塑性流动到3D弹性流动的相变，这一维度的变化对应着微分电阻dV/dI曲线中的二次峰位置. 并证明当层间耦合(即代表磁场的大小)在一定范围时，3D-2D相变对应的临界电流随磁场的增大而增大, 反映了第二磁化峰附近的磁通格子软硬度改变的微观图像. 关键词： 第Ⅱ类超导体 磁通线格子 钉扎 峰值效应     

Vortex states of rapidly rotating dilute Bose-Einstein condensates

We show that, in the Thomas-Fermi regime, the cores of vortices in rotating dilute Bose-Einstein condensates adjust in radius as the rotation velocity, Omega, grows, thus precluding a phase transition associated with core overlap at high vortex density. In both a harmonic trap and a rotating hard-walled bucket, the core size approaches a limiting fraction of the intervortex spacing. At large rotation speeds, a system confined in a bucket develops, within Thomas-Fermi, a hole along the rotation axis, and eventually makes a transition to a giant vortex state with all the vorticity contained in the hole.     

Direct observation of the washboard noise of a driven vortex lattice in a high-temperature superconductor, Bi2Sr2CaCu2O(y)

We studied the conduction noise spectrum in the vortex state of a high-temperature superconductor, Bi(2)Sr(2)CaCu(2)O(y), subject to a uniform driving force. Two characteristic features, a broad-band noise (BBN) and a narrow-band noise (NBN), were observed in the vortex-solid phase. The origin of the large BBN was determined to be plastic motion of the vortices, whereas the NBN was found to originate from the washboard modulation of the translational velocity of the driven vortices. We believe this to be the first observation of washboard noise of dc driven vortices in any superconductor.     

Effect of the temperature dependence of the viscosity of pseudoplastic lubricants on the boundary friction regime

The boundary friction regime appearing between two atomically smooth solid surfaces with an ultrathin lubricating layer between them is considered. The interrupted (stick-slip) regime of motion typical of the boundary lubrication is represented as a first-order phase transition between the structural states of the lubricant. The thermodynamic and shear melting is described. The universal dependence of the viscosity of high-molecular alkanes (lubricants) on the temperature and velocity gradient is taken into account. The dependence of the friction force on the lubricant temperature and the relative shear velocity of the interacting surfaces are analyzed. It is shown that the temperature dependence of the viscosity makes it possible to describe some experimentally observed effects. The possibility of prolonged damped oscillations after lubricant melting prior to the stabilization of the steady-state sliding mode is predicted. In the stick-slip regime in a wide range of parameters, a reversive motion is observed when the upper block moves in both directions after melting.     

Instabilities and disorder-driven first-order transition of the vortex lattice

Transport studies in a Corbino disk suggest that the Bragg glass phase undergoes a first-order transition into a disordered solid. This transition shows sharp reentrant behavior at low fields. In contrast, in the conventional strip configuration, the phase transition is obscured by the injection of the disordered vortices through the sample edges, which results in the commonly observed vortex instabilities and smearing of the peak effect in NbSe2 crystals. These features are found to be absent in the Corbino geometry in which the circulating vortices do not cross the sample edges.     

Crystal-lattice coupling to the vortex-melting transition in YBa(2)Cu(3)O(7-delta)

Distinct discontinuities in the thermal expansion of the crystal lattice are observed at the melting transition of the vortex lattice in a naturally untwinned reversible YBa(2)Cu3O(7-delta) single crystal using high-resolution dilatometry. This coupling between the vortex transition and the crystal lattice demonstrates that the crystal lattice is more than a mere host for the vortices, and it is attributed to a strong pressure dependence of the superconducting transition temperature and thus to the condensation energy at the vortex-melting temperature.     

Experimental adiabatic vortex ratchet effect in Nb films with asymmetric pinning trap

Nb films grown on top of an array of asymmetric pinning centers show a vortex ratchet effect. A net flow of vortices is induced when the vortex lattice is driven by fluctuating forces on an array of pinning centers without reflection symmetry. This effect occurs in the adiabatic regime and it could be mimiced only by reversible DC driven forces.     

受驱无序胶体动力学

利用Langevin分子动力学，本数值研究点钉扎中心随机分布的二维胶体动力学．随着钉扎中心强度的提高，我们发现了从弹性脱钉到塑性脱钉的渡越，并伴随临界钉扎力在渡越区的明显提高，类似于超导体中的峰值效应．另外，我们首次发现：当塑性流动发生时，高速运动胶体粒子感受到的平均钉扎力在从玻璃态到液态的转变过程中会出现峰值效应，并伴随有速度-驱动力曲线的交叠．     

Substrate-induced stress in silicon nanocrystal/SiO2 multilayer structures

Magnetoresistance in superconducting Nb films perforated with rectangular arrays of antidots (holes) is investigated at various temperatures and currents. Normally, the magnetoresistance increases with the increasing magnetic field. In this paper, we report a reverse behavior in a certain range of high fields after vortex reconfiguration transition, where the resistances at non-matching fields are smaller than those in the low field regime. This phenomenon is due to a strong caging effect, in which the interstitial vortices are trapped among the pinned multiquanta vortices. This effect is temperature and current dependent.