We show that under an a.c. magnetic field excitation the vortex lattice in a superconductor with periodic array of holes can undergo a transition from a Mott-like state where each vortex is localized in a hole, to a metal-like state where the vortices get delocalized. The vortex dynamics is studied through the magnetic shielding response which is measured using a low frequency two-coil mutual inductance technique on a disordered superconducting NbN film having periodic array of holes. We observe that the shielding response of the vortex state is strongly dependent on the amplitude of the a.c. magnetic excitation. At low amplitude the shielding response varies smoothly with excitation amplitude, corresponding to elastic deformation of the vortex lattice. However, above a threshold value of excitation the response shows a series of sharp jumps, signaling the onset of the Mott to metal transition. Quantitative analysis reveals that this is a collective phenomenon which depends on the filling fraction of vortices in the antidot lattice.
The paradigmatic Mott insulator arises in strongly correlated systems, where strong local repulsion localizes interacting particles in underlying egg-holder-like potential. The corresponding Mott transition reflects delocalization of the charges either by varying parameters of the system and temperature, or by applied current, the latter being referred to as the dynamic Mott transition. Recently, the dynamic Mott transition was experimentally observed on the vortex system trapped by the periodic proximity array and described in terms of non-Hermitian theory of nonequilibrium processes. Here we…
We study the differential resistivity transition of two-dimensional superconducting arrays induced by an external driving current, in the presence of thermal fluctuations and a magnetic field corresponding to $f$ flux quantum per plaquette. Recent experiments have identified this transition as a dynamic vortex Mott insulator transition at vortex densities near rational values of $f$. The critical behavior is determined from a scaling analysis of the current-voltage relation near the transition, obtained by Monte Carlo simulations of a Josephson-junction array model in the vortex representation…
We study the dynamic vortex Mott transition in two-dimensional superconducting arrays in a magnetic field with $f$ flux quantum per plaquette. The transition is induced by external driving current and thermal fluctuations near rational vortex densities set by the value of $f$, and has been observed experimentally from the scaling behavior of the differential resistivity. Recently, numerical simulations of interacting vortex models have demonstrated this behavior only near fractional $f$. A fine-grid vortex model is introduced, which allows to consider both the cases of fractional and integer $…
We report measurements of the energy gap of granular aluminum films by THz spectroscopy. We find that as the grains progressively decouple, the coupling ratio $2Δ(0)/k_{B}T_{c}$ increases above the BCS weak coupling ratio $3.53$, and reaches values consistent with an approach to BCS-BEC crossover for the high resistivity samples, expected from the short coherence length. The Mattis-Bardeen theory describes remarkably well the behavior of $σ_{1,s}/σ_{1,n}$ for all samples up to very high normal state resistivities.
Vortex dynamics in superconductors have received a great deal of attention from both fundamental and applied researchers over the past few decades. Because of its critical role in the energy relaxation process of type II superconductors, vortex dynamics have been deemed a key factor for the emerging superconducting devices, but the effect of irradiation on vortex dynamics remains unclear. With the support of electrical transport measurements under external magnetic fields and irradiation, photon effect on vortex dynamics in amorphous MoSi (a MoSi) superconducting thin films are investigated in…
The glass-like vortex distribution in pulsed laser deposited YBCO thin films is observed by scanning superconducting quantum interference device microscopy and analysed for ordering after cooling in magnetic fields significantly smaller than the Earth's field. Autocorrelation calculations on this distribution show a weak short-range positional order, while Delaunay triangulation shows a near-complete lack of orientational order. The distribution of these vortices is finally characterised as an isotropic vortex glass. Abnormally closely spaced groups of vortices, which are statistically unlikel…
We extract the vortex lattice parameters using low-frequency two-coil mutual inductance measurements in a 20-nm-thick superconducting a-MoGe thin film. We fit the temperature dependence of ac penetration depth in the mixed state using a model developed by Coffey and Clem and demonstrate a procedure for extracting vortex lattice parameters such as pinning constant, vortex lattice drag coefficient, and pinning potential barrier. We show that the extracted parameters follow the magnetic field variation expected for a weakly pinned 2-dimensional vortex lattice.
The work reported in my doctoral thesis is an experimental study of vortex dynamics and phase transitions in thin films of type II superconductors using scanning tunneling spectroscopy, low frequency ac susceptibility measurements and complimentary transport measurements. Chapter I and II cover basics of superconductivity and methodologies used in the thesis. Chapter III discusses the effect of periodic pinning centers on the geometry and dynamics of vortex lattice in NbN thin films. Consequent study of dynamic transition of vortex Mott-like to vortex metal-like state is described. In Chapter …
We study vortex current distributions in narrow thin-film superconducting strips. If one defines the vortex core ``boundary" as a curve where the current reaches the depairing value, intriguing features emerge. Our conclusions based on the London approach have only qualitative relevance since the approach breaks down near the core. Still, the main observation which might be useful is that the core size near the strip edges is smaller than in the rest of the strip. If so, the Bardeen-Stephen flux-flow resistivity should be reduced near the edges. Moreover, at elevated temperatures, when the dep…
Superconductivity and vortex structure on Bi$_{2}$Te$_{3}$/FeTe$_{0.55}$Se$_{0.45}$ heterostructures with different thickness of Bi$_{2}$Te$_{3}$ films
By using the full 3D generalized time dependent Ginzbug-Landau equation we study a long superconducting film of finite width and thickness under an applied transport current. We show that, for sufficiently large thickness, the vortices and the antivortices become curved before they annihilate each other. As they approach the center of the sample, their ends combine, producing a single closed vortex. We also determine the critical values of the thickness for which the closed vortex sets in for different values of the Ginzburg-Ladau parameter. Finally, we propose a model of how to detect a close…
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）
Anisotropic response of the moving vortex lattice in superconducting Mo$_{(1-x)}$Ge$_{x}$ amorphous films
我们已经进行了磁化率测量Mo $_{(1-x)}$ Ge $_x$非晶薄膜偏置电流使用各向异性线圈。我们测试了改变偏置电流和磁化率线圈之间的相对方向的涡旋响应的对称性。在直流电流-温度相图中发现了一个动态涡旋结构各向异性的区域.在该区域中，由磁化率线圈测量的超导电流的屏蔽能力沿着涡旋运动方向与横向方向相比是不太有效的。这种各向异性的响应被发现在同一区域中的临界电流的峰值效应的开发。温度升高时，各向同性行为恢复。
An epitaxial NdFeAs(O,F) thin film of 90 nm thickness grown by molecular beam epitaxy on MgO single crystal with Tc = 44.2 K has been investigated regarding a possible vortex glass-liquid tran-sition. The voltage-current characteristics show excellent scalability according to the vortex-glass model with a static critical exponent ν of around 1.35 and a temperature-dependent dynamic exponent z increasing from 7.8 to 9.0 for the investigated temperature range. The large and non-constant z values are discussed in the frame of 3D vortex glass, thermally activated flux motion, and inhomogeneity bro…
