It is shown that the temperature of the resistive transition $T_r$ of a superconducting film can be increased by a thin superconducting or normal overlayer. For instance, deposition of a highly conductive thin overlayer onto a dirty superconducting film can give rise to an "anti-proximity effect" which manifests itself in an initial increase of $T_r(d_2)$ with the overlayer thickness $d_2$ followed by a decrease of $T_r(d_2)$ at larger $d_2$. Such a nonmonotonic thickness dependence of $T_r(d_2)$ results from the interplay of the increase of a net superfluid density mitigating phase fluctuations and the suppression of the critical temperature $T_c$ due to the conventional proximity effect. This behavior of $T_r(d_2)$ is obtained by solving the Usadel equations to calculate the temperature of the Berezinskii-Kosterletz-Thouless transition, and the temperature of the resistive transition due to thermally-activated hopping of single vortices in dirty bilayers. The theory incorporates relevant materials parameters such as thicknesses and conductivities of the layers, interface contact resistance between them and the subgap quasiparticle states which affect both phase fluctuations and the proximity effect suppression of $T_c$. The transition temperature $T_r$ can be optimized by tuning the overlayer parameters, which can significantly weaken vortex fluctuations and nearly restore the mean-field critical temperature. The calculated behavior of $T_r(d_2)$ may explain the nonmonotonic dependence of $T_r(d_2)$ observed on (Ag, Au, Mg, Zn)-coated Bi films, Ag-coated Ga and Pb films or NbN and NbTiN films on AlN buffer layers. These results suggest that bilayers can be used as model systems for systematic investigations of optimization of fluctuations in superconductors.
Theory of the Resistive Transition in Overdoped $Tl_2Ba_2CuO_{6+x}$: Implications for the angular dependence of the quasiparticle scattering rate in High-$T_c$ superconductors
Within the framework of the generalized time-dependent Ginzburg-Landau equations, we studied the influence of the magnetic self-field induced by the currents inside a superconducting sample driven by an applied transport current. The numerical simulations of the resistive state of the system show that neither material inhomogeneity nor a normal contact smaller than the sample width are required to produce an inhomogeneous current distribution inside the sample, which leads to the emergence of a kinematic vortex-antivortex pair (vortex street) solution. Further, we discuss the behaviors of the …
Crystal structure effects on vortex dynamics in superconducting MgB$_2$ thin films
University of Vienna（维也纳大学）；Technische Universität Braunschweig（布伦瑞克工业大学）；Institute Jean Lamour, Université de Lorraine-CNRS（洛林大学-法国国家科学研究中心让·拉穆尔研究所）；G.V. Kurdyumov Institute for Metal Physics of the NAS of Ukraine（乌克兰国家科学院G.V.库尔久莫夫金属物理研究所）；FLUXONICS—The European Foundry for Superconducting Electronics e.V.（FLUXONICS——欧洲超导电子铸造协会）
Transport Properties of Vortex Matter Governed by the Edge Inductance of Superconducting Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ Single Crystals
本文研究了超导Bi $_2$ Sr $_2$ CaCu $_2$ O $_8$晶体中的输运电流分布以及通过样品边缘的涡流。我们表明，$T_x$过渡是电动力学，而不是热力学性质，低于该涡旋动力学是由边缘电感，而不是电阻。这允许测量低至传输噪声两个数量级的电阻。通过照射电流接触的电阻阶跃在涡流熔化示出是由于c轴相关性的损失，而不是准长程序内的a-b平面的故障。
Vortex melting and decoupling transitions in YBa$_{2}$Cu$_{4}$O$_{8}$ single crystals
采用准磁通Transformer结构，研究了不同磁场下，高质量YBaPDMATHABMM CuPDMATHABMN O $_{8}$单晶中沿c轴方向的涡旋关联随温度T的变化.在初级和次级电压（V $_{top}$和V $_{bot}$）中观察到与涡旋晶格熔化相关的同时急剧下降。刚高于熔化温度时，涡旋形成三维线液体，其相关长度沿c方向$L_{c}\leq $ t，样品厚度。R $_{bot}$中出现电阻峰值的温度对应于去耦温度T $_{d}$，在去耦温度T下，涡旋沿c方向失去它们的相关性，并且它们溶解成二维饼状涡旋。得到了YBa $_{2}$ Cu $_{4}$ O $_{8}$单晶的H-T相图。
We report magneto-transport measurements on a superconducting molybdenum-germanium (MoGe) film of thickness $d$=50 nm in parallel magnetic fields and show evidence of a transition from a Meissner state to a resistive state of spontaneous perpendicular vortices generated by thermal fluctuations above a certain temperature $T>T_v(B)$. Here $T_v$ appears to match the vortex core explosion condition $d\approx 4.4ξ(T_v)$, where $ξ$ is the coherence length. For $T>T_v$, we observed that a nonlinear current-voltage ($IV$) response (Ohmic at low currents and the power law $V\propto I^β$ at higher $I$)…
Comment to the paper : Collapse of the vortex-lattice inductance and shear modulus at the melting transition in untwinned YBa$_2$Cu$_3$O$_7$, by Matl \QTR{em}{et al.}
First order phase transition of the vortex lattice in twinned YBa2Cu3O7 single crystals in tilted magnetic fields
我们提出了一个详尽的分析，在孪晶YBa 2 Cu 3 O 7单晶体中进行的传输测量，其中确立了涡旋固-液转变是一阶时，施加的磁场H在一个角度θ远离孪晶面的方向。我们发现电阻转变是滞后的，V-I曲线是非线性的，在熔化线Hm（T）处显示出特征的S形，其标度为θ Hm（T，θ）。当接近临界点H*（θ）时，这些特征逐渐丧失。在H*（θ）以上，V-I特性在实验可达的V-I窗口中显示出线性响应，并且转变变得可逆。最后，我们表明，一级相变发生在一个高度相关的涡旋液体在磁场方向和未知对称性的固体状态。因此，可用的数据支持涡线熔化的情况，而不是最近提出的涡升华[T.Sasagawa等人，PRL 80，4297（1998）]。
Guided and rectified motion of magnetic flux quanta are important effects governing the magneto-resistive response of nanostructured superconductors. While at low ac frequencies these effects are rather well understood, their manifestation at higher ac frequencies remains poorly investigated. Here, we explore the upper frequency limits for guided and rectified net motion of superconducting vortices in epitaxial Nb films decorated with ferromagnetic nanostripes. By combining broadband electrical spectroscopy with resistance measurements we reveal that the rectified voltage vanishes at a geometr…
The presence of magnetic fields and/or transport currents can cause penetration of vortices in superconductors. Their motion leads to dissipation and resistive state arises, which in turn strongly affects the performance of superconducting devices such as single-photon and single-electron detectors. Therefore, an understanding of the dissipation mechanisms in mesoscopic superconductors is not only of fundamental value but also very important for further technological advances. In the present work, we analyzed the contributions and interplay of the dissipative mechanisms due to the locally indu…
Grain Boundary Engineering Effect on Vortex Matter in Superconducting Films
Materials Genome Institute, Shanghai University（上海大学材料基因组研究院）；Department of Physics, Shanghai Normal University（上海师范大学物理系）；Department of Physics and Shanghai Key Laboratory for High Temperature Superconductors, Shanghai University（上海大学物理系及上海市高温超导重点实验室）
Collective Josephson Vortex Dynamics in Long Josephson Junction Stacks
我们研究了传统长约瑟夫森结（LJJ）堆和层状超导体中的集体相动力学，表现出本征LJJ行为。使用解释磁感应效应和局部电荷中性击穿的理论模型（即，电荷效应），我们发现在本征LJJ叠层如Bi $_2$ Sr $_2$ CaCu $_2$ O $_{8+y}$（BSCCO）中的Josephson涡旋的集体运动，包括Josephson等离子体模的色散和Swihart型速度，与常规LJJ叠层中的那些相比有显著的改变.在BSCCO中，充电效应$α$的强度小（即，$α\sim 0.1-0.4$），但它导致集体相动力学的显著变化，包括稳定性条件的变化。此外，我们表明，分裂的超导分支的电阻状态是由于集体运动的约瑟夫森涡旋。这些子分支在线性电流-电压状态下的扩展宽度取决于$α$，这表明了测量BSCCO中充电效应的另一种方法。
We present theoretical and numerical investigations of vortices driven by strong dc and ac currents in long Josephson junctions described by a nonlinear integro-differential equation which takes into account nonlocal electrodynamics of films, vortex bremsstrahlung and Cherenkov radiation amplified by the attraction of vortices to the edges of the junction. The work focuses on the dynamics of vortices in Josephson junctions in thin films where the effects of Josephson nonlocality dominate but London screening is negligible. We obtained an exact solution for a vortex driven by an arbitrary time-…
Dynamical behavior of vortices plays central roles in the quantum phenomena of two-dimensional (2D) superconductors. Quantum metallic state, for example, showing an anomalous temperature-independent resistive state down to low-temperatures, has been a common subject in recently developed 2D crystalline superconductors, whose microscopic origin is still under debate. Here, we unveil a new aspect of the vortex dynamics in a noncentrosymmetric 2D crystalline superconductor of MoS$_{2}$ through the nonreciprocal transport measurement. The second harmonic resistance $R^{2w}$ at low temperature with…
High-entropy alloy (HEA) is an attracting topic raising in materials science and condensed matter physics. Although several types of superconductors have been discovered in HEAs, the critical currents (Jc) of HEA superconductors remain uncharacterized up to now. Here, we systematically study the current-carrying ability of (TaNb)0.7(HfZrTi)0.5 HEA at various heat treatment conditions. We obtained the high upper critical field and large current carrying ability, which point to optimistic applications. Interestingly, the fishtail or second peak effect was found for the first time in HEA supercon…
We study the effect of an externally imposed rotation or magnetic field on frustrated multiband superconductors/superfluids. The frustration originates with multiple superconducting bands crossing the Fermi surface in conjunction with interband Josephson-couplings with a positive sign. These couplings tend to frustrate the phases of the various components of the superconducting order parameter. This in turn leads to an effective description in terms of a $\groupUZ$-symmetric system, where essentially only the $\groupU$-sector couples to the gauge-field representing the rotation or magnetic fie…
Vortex rivers and multiple voltage transitions in superconducting MgB$_2$ thin films
Technische Universität Braunschweig（布伦瑞克工业大学）；University of Vienna（维也纳大学）；Université de Lorraine-CNRS（洛林大学-法国国家科学研究中心）；FLUXONICS—The European Foundry for Superconducting Electronics e.V.（FLUXONICS——欧洲超导电子铸造协会）
Strontium titanate has resurfaced as a material prompting vigorous debate about the origin of its superconductivity in the extremely low carrier concentration regime. Here, we used simultaneous AC susceptibility and transport methods to explore the superconducting phase transition region in this material. We determined that strontium titanate is extremely sensitive to even small AC fields, which also influence the resistive transition; we suggest that extreme vortex sizes and mobilities contribute to this large effect. Our findings will be of importance for accurately determining transition te…
Vortex dynamics is strongly connected with the mechanisms responsible for the photon detection of superconducting devices. Indeed, the local suppression of superconductivity by photon absorption may trigger vortex nucleation and motion effects, which can make the superconducting state unstable. In addition, scaling down the thickness of the superconducting films and/or the width of the bridge geometry can strongly influence the transport properties of superconducting films, e.g. affecting its critical current as well as its switching current into the normal state. Understanding such instabilit…
