二维约瑟夫森结阵列中的相变及噪声频谱研究

采用电阻分路(RSJ)模型，运用数值计算方法研究二维约瑟夫森结阵列的涡旋度,涡旋密度涨落随温度的变化，表明了超导阵列中存在KTB型相变.还研究了相变点附近的涡旋噪声频谱随温度、驱动电流变化的特性.计算结果与最近实验报导定性一致，并能用涡旋的运动图像来解释噪声频谱的变化规律. 关键词： 高温超导 噪声频谱 约瑟夫森结阵列 KTB相变     

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     

高温超导体中约瑟夫森涡旋流阻的振荡效应

通过数值计算耦合sine-Gordon方程组研究高温超导体中约瑟夫森涡旋的运动,得到约瑟夫森涡旋电压和流阻随平面磁场和驱动电流的变化规律.固定驱动电流,约瑟夫森涡旋电压和流阻随着磁场的增大出现周期性的振荡行为,振荡周期与每层约瑟夫森结中进入一个磁通量子相对应.分析和阐明了产生这种周期性振荡的原因. 关键词： 约瑟夫森涡旋 涡旋格子 高温超导     

BaNi_(0.1)Fe_(1.9)As_2单晶的钉扎机制及涡旋相图

本文中,我们通过磁-电阻测量研究了T_c=19.4K的BaNi_(0.1)Fe_(1.9)As_2单晶中的涡旋玻璃-涡旋液体相变.根据修正过的涡旋玻璃理论,并通过选择合适的临界指数s和H_0,我们对所有的ρ(H,T)电阻曲线进行了标度,并得到了很好的结果.基于涡旋玻璃温度T_g,涡旋维度转变温度T~*和上临界磁场H_(c2),我们得到了H⊥c和H//c两个磁场方向的涡旋相图.结果表明,磁场低于和高于5T,钉扎机制由单涡旋钉扎和集体蠕动共存转变为集体蠕动.     

磁性d波超导/铁磁/磁性d波超导结中的约瑟夫森效应

通过求解磁性d波超导中的能隙和磁交换能的自洽方程, 研究磁性d波超导/铁磁/磁性d波超导结中的约瑟夫森电流. 计算结果表明: 1)临界电流随中间的铁磁层厚度呈现出两种不同周期的振荡混合, 通过增强铁磁层中的磁交换能q₀和铁磁/磁性d波超导界面处的势垒强度z₀, 短周期分量可从长周期中分离出来, 反之, 通过降低q₀和z₀, 长周期分量可从短周期中分离出来; 2)在两边磁性d波超导的磁化方向取平行时, 在取一些特定的铁磁层厚度下, 磁性d波超导中的磁交换能可增强系统的临界电流. 关键词： 磁性d波超导体 铁磁体 约瑟夫森电流     

BaNi_(0.1)Fe_(1.9)As_2单晶的钉扎机制及涡旋相图北大核心CSCD

本文中,我们通过磁-电阻测量研究了T_c=19.4K的BaNi_(0.1)Fe_(1.9)As_2单晶中的涡旋玻璃-涡旋液体相变.根据修正过的涡旋玻璃理论,并通过选择合适的临界指数s和H_0,我们对所有的ρ(H,T)电阻曲线进行了标度,并得到了很好的结果.基于涡旋玻璃温度T_g,涡旋维度转变温度T~*和上临界磁场H_(c2),我们得到了H⊥c和H//c两个磁场方向的涡旋相图.结果表明,磁场低于和高于5T,钉扎机制由单涡旋钉扎和集体蠕动共存转变为集体蠕动.     

本征约瑟夫森结跳变电流分布的量子修正

在约瑟夫森结跳变电流统计分布的理论拟合过程中,通常考虑的是宏观量子隧穿与热激活这两种过程. 对Bi₂Sr₂CaCu₂O_8+δ表面本征约瑟夫森结的结果分析表明,在宏观量子隧穿与热激活的交界区域内,若考虑量子修正能使实验曲线与理论曲线符合得更好. 这种较为完整的拟合方法,对研究本征约瑟夫森器件中的宏观量子现象及其在超导量子比特中的应用具有积极的意义. 关键词： 约瑟夫森结 跳变电流分布 量子修正     

基于拓扑绝缘体纳米线约瑟夫森结的反常临界超流增强和半整数夏皮洛台阶

基于拓扑绝缘体材料的约瑟夫森结是寻找马约拉纳零能模的候选器件,因而受到拓扑量子计算研究领域的关注.这方面实验的关键之一,是制备具有优质结区的约瑟夫森器件.本工作在三维拓扑绝缘体Bi₂Te₃和Bi₂(Se_xTe_1-x)₃纳米线上制作了约瑟夫森结器件,研究了其结区的超导邻近效应、多重安德列夫反射和超流-相位关系,观测到了约瑟夫森结的临界超流随磁场增大而反常地增大、其交流约瑟夫森效应出现半整数的夏皮洛台阶的实验结果.本文还讨论了这些反常现象的可能来源,特别是与结区界面处超导电极的Ti缓冲层和拓扑绝缘体纳米线中的Te元素形成TiTe铁磁性合金层的关系.     

超导电性的交流磁化率研究

本文综述了交流磁化率在超导电性物理研究方面的进展.主要包括如何用交流磁化率确认超导电性,测定各种超导电参量(临界温度,临界磁场等),研究涡旋物质的强度(临界电流密度,不可逆场等),涡旋动力学(磁弛豫,钉扎能等).     

高温超导体霍尔电阻和霍尔角在涡旋玻璃相变附近的普适标度律及统一霍尔电阻方程

发现了一个钉扎效应影响霍尔电阻ρ_xy和霍尔角θ_H的普适标度律. 同时,根据纵向电阻ρ_xx的扩展幂律形式和对霍尔电导σ_xy的微观分析,给出了一个对有一次或多次霍尔反号的高温超导体都适用的霍尔电阻方程. 关键词： 高温超导体 涡旋玻璃相变 普适标度律 霍尔电阻方程     

Two-Dimensional Coulomb Glass as a Model for Vortex Pinning in Superconducting Films

A glass model of vortex pinning in highly disordered thin superconducting films in magnetic fields B ≪ H_c2 at low temperatures is proposed. Strong collective pinning of a vortex system realized in disordered superconductors that are close to the quantum phase transition to the insulating phase, such as InO_x, NbN, TiN, MoGe, and nanogranular aluminum, is considered theoretically for the first time. Utilizing the replica trick developed for the spin glass theory, we demonstrate that such vortex system is in non-ergodic state of glass type with a large kinetic inductance per square L_K. The distribution function of local pinning energies is calculated, and it is shown that it possesses a wide gap; i.e., the probability to find a weakly pinned vortex is extremely low.

     

Doping dependence of the vortex glass and sublimation transitions in the high-T<Subscript>c</Subscript> superconductor La<Subscript>2-x</Subscript>Sr<Subscript>x</Subscript>CuO<Subscript>4</Subscript> as determined from macroscopic measurements

Magnetization and ac-susceptibility measurements are used to characterize the mixed phase of the high-temperature cuprate superconductor La_2-xSr_xCuO₄ over a large range of doping (0.075 0.20). The first order vortex lattice phase transition line H_FOT(T), the upper critical field H_c2(T) and the second peak H_sp(T) have been investigated up to high magnetic fields (8 Tesla applied perpendicular to the CuO₂ planes). Our results reveal a strong doping dependence of the magnetic phase diagram, which can mainly be explained by the increasing anisotropy with underdoping. Within our interpretation, the first order vortex lattice phase transition is due to the sublimation (rather than melting) of the vortex lattice into a gas of pancake vortices, whereas the second peak is related to the transition to a more disordered vortex glass state.     

Critical points in the Bragg glass phase of a weakly pinned crystal of Ca3Rh4Sn13

New experimental data are presented on the scan rate dependence of the magnetization hysteresis width ΔM(H) (∞ critical current densityJ _c(H)) in isothermalMH scans in a weakly pinned single crystal of Ca₃Rh₄Sn₁₃, which displays second magnetization peak (SMP) anomaly as distinct from the peak effect (PE). We observe an interesting modulation in the field dependence of a parameter which purports to measure the dynamical annealing of the disordered bundles of vortices injected through the sample edges towards the destined equilibrium vortex state at a givenH. These data, in conjunction with the earlier observations made while studying the thermomagnetic history dependence inJ _c(H) in the tracing of the minor hysteresis loops, imply that the partially disordered state heals towards the more ordered state between the peak field of the SMP anomaly and the onset field of the PE. The vortex phase diagram in the given crystal of Ca₃Rh₄Sn₁₃ has been updated in the context of the notion of the phase coexistence of the ordered and disordered regions between the onset field of the SMP anomaly and the spinodal line located just prior to the irreversibility line. A multi-critical point and a critical point in the (H,T) region of the Bragg glass phase have been marked in this phase diagram and the observed behavior is discussed in the light of recent data on multi-critical point in the vortex phase diagram in a single crystal of Nb.     

Vortex glass phase and flux creep in YBa2Cu3O y epitaxial film

Nonlinear I–V characteristics of YBa₂Cu₃O _y epitaxial film at different temperatures in a magnetic field of up to 5T and angles between the direction of the field and the a–b plane of 0°, 30°, 45°, 60°, and 90° were measured. We find that there are the vortex glass phase and vortex liquid phase in the mixed state. The flux creep exists in the vortex glass phase, and it obviously appears in the area around the boundary between the vortex glass phase and the vortex liquid phase.     

Dynamical phase transition in two-dimensional fully frustrated Josephson junction arrays with resistively shunted junction dynamics

The dynamical phase transitions in two-dimensional fully frustrated Josephson junction arrays at zero temperature are investigated numerically with the resistively shunted junction model through the fluctuating twist boundary condition. The model is subjected to a driving current with nonzero orthogonal components i _x , i _y parallel to both axes of the square lattice. We find a roughly lattice size independent phase diagram with three dynamical phases: a pinned vortex lattice phase, a moving vortex lattice phase and a moving plastic phase. The phase diagram shows a direct transition from the pinned vortex to the moving vortex phase and the separation of the pinned vortex and the moving plastic phases. The time-dependent voltages v _x and v _y are periodic in the moving vortex lattice phase. But they are aperiodic in the moving plastic phase, resulting in non-monotonic characteristics and hysteresis in the current-voltage curves. It is found that the characteristic frequency is twice the time-averaged voltage in the moving vortex phase and around the time-averaged voltage in the plastic flow regime.Received: 29 May 2003, Published online: 2 October 2003PACS: 64.60.Ht Dynamic critical phenomena - 74.25.Sv Critical currents - 74.25.Fy Transport properties     

Magnetic ordering in Fe2−xZnxMoO4(X=0.1–1) spinel

We have studied the diluted Fe_2−xZn_xMoO₄ spinel ferrite which shows a frozen disordered magnetic state at low temperature. Magnetic properties are examined by DC magnetisation measurements as a function of temperature, field and time and AC susceptibility experiment. Our measurements show that this disordered magnetic system at low fields, shares many common features with spin glass or cluster glass like phases. Results suggest that the interaction gradually changes as the magnetic ion concentration decreases by the substitution of non-magnetic Zn on A site and causing a perturbation to the magnetically ordered spins and magnetic order decreases.     

Application of nano-scale Josephson junction as phase sensitive detector for analysis of vortex states in mesoscopic superconductors

We study phase shifts in a Josephson junction induced by vortices in superconducting mesoscopic electrodes. The position of the vortices are controlled by suitable geometry of a nano-scale Nb–Pt_1−xNi_x–Nb junction of the overlap type made by Focused Ion Beam (FIB) sculpturing. The vortex is kept outside the junction, parallel to the junction plane. From the measured Fraunhofer characteristics the entrance and exit of vortices are detected. By changing the bias current through the junction at constant magnetic field the vortices can be manipulated and the system can be switched between two consecutive vortex states which are characterized by different critical currents of the junction. A mesoscopic superconductor thus acts as a non-volatile memory cell in which the junction is used both for reading and writing information (vortex). Furthermore, we observe that the critical current density of Nb–Pt_1−xNi_x–Nb junctions decreases non-monotonously with increasing Ni concentration. It exhibits a minimum at ∼40 at.% Ni, which is an indication of switching into the π state.     

Self-localized carrier states in disordered ferroelectrics

A theory of self-localized states of free carriers near polarization fluctuations (fluctuons) in disordered ferroelectrics is developed. Calculations are carried out for the model disordered ferroelectric K_{1− x} Li_xTaO₃ (x≪0.05). The basic characteristics of the fluctuon — the energy and radius of the fluctuon state — are calculated as functions of the impurity dipole concentration and temperature. The theory predicts the appearence of stable fluctuon states in both the mixed ferroelectric-dipole-glass phase (a dipole glass is the electric analog of a spin glass) and the dipole-glass state of disordered ferroelectrics. The possible role of fluctuons in kinetic phenomena such as conductivity in these substances is discussed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 5, 425–429 (10 March 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Scaling analysis of phase transition in Hg-based superconductor

With the vibrating-reed technique, the internal friction (IF) Q⁻¹ is measured for sing-phase (Hg_0.66Pb_0.34)Ba₂Ca₂Cu₃O_8+x superconductor as a function of temperature at low applied magnetic field up to 0.5 T and as a function of frequency at normal state temperatures. An IF peak associated with flux motion can be found below T_C. The IF peak becomes higher and shifts towards lower temperature with increasing magnetic field. In addition an IF peak is found near 200 K. By scaling analysis we have demonstrated that the internal friction around the peak temperature can be collapsed into a single curve, indicating that the IF peak below T_C is originated from a phase transition associated with a vortex glass transition and a structural phase transition occurs at around 200 K in the superconductor.     

Bi2Sr2CaCu2O8单晶中的反常尖锋效应

使用牛津震动样品磁强计(VSM)研究了Bi₂Sr₂CaCu₂O₈单晶的磁滞回线.在20到40K温度之间发现了反常的尖锋效应,随样品O含量的增加,发生尖锋效应的外场也相应提高.可以认为在尖峰效应处发生了由涡漩物质的有序固态到无序固态的相变,在有少量点缺陷存在的BSCCO单晶相图上,B_sp线终止于20K温度处,在20K以下温区没有发生准格子到涡漩玻璃的相变,涡漩固相始终以准格子形式存在;可以认为尖峰效 关键词： 2Sr₂CaCu₂O₈单晶')" href="#">Bi₂Sr₂CaCu₂O₈单晶 磁滞回线 尖锋效应 相变