Pinning and thermal creep determine the response of numerous systems containing superstructures, e.g., vortices in type II superconductors, domain walls in ferroics, or dislocations in metals. The combination of drive and thermal fluctuations lead to the superstructure's depinning and its velocity $v$ determines the electric, magnetic, or mechanical response. It is commonly believed that pinning and creep collapse above the critical drive $F_c$, entailing a sharp rise in the velocity $v$. We challenge this perception by studying the effects of thermal fluctuations within the framework of strong vortex pinning in type-II superconductors. In fact, we show that pinning and thermal creep persist far beyond the critical force. The resulting force-velocity characteristic largely maintains its zero-temperature shape and thermal creep manifests itself by a downward renormalisation of the critical drive. Such characteristics is in agreement with Coulomb's law of dry friction and has been often observed in experiments.
Pinning and creep determine the current--voltage characteristic of a type II superconductor and thereby its potential for technological applications. The recent development of strong pinning theory provides us with a tool to assess a superconductor's electric properties in a quantitative way. Motivated by the observation of typical excess-current characteristics and field-scaling of critical currents, here, we analyze current--voltage characteristics measured on 2H-NbSe$_2$ and $a$-MoGe type II superconductors within the setting provided by strong pinning theory. The experimentally observed sh…
We study thermal effects on pinning and creep in type-II superconductors where vortices interact with a low density $n_p$ of strong point-like defects with pinning energy $e_p$ and extension $ξ$, the vortex core size. Defects are classified as strong if the interaction between a single pin and an individual vortex leads to the appearance of bistable solutions describing pinned and free vortex configurations. Extending the strong pinning theory to account for thermal fluctuations, we provide a quantitative analysis of vortex depinning and creep. We determine the thermally activated transitions …
We report the temperature, magnetic field and time dependences of magnetization in advanced Ba122 superconducting tapes. The sample exhibits a peculiar vortex creep behavior. Below 10 K, the normalized magnetization relaxation rate S=dln(-M)/dln(t) shows a temperature insensitive plateau with a value comparable to that of the low temperature superconductors, which can be explained within the framework of the collective creep theory. It then enters into a second collective creep regime when the temperature increases. Interestingly, the relaxation rate below 20 K tends to saturate with the incre…
We report the magnetic field -- temperature ($H-T$) phase diagram of Ca$_{10}$(Pt$_4$As$_8$)[(Fe$_{1-x}$Pt$_x$)$_2$As$_2$]$_5$ ($x\approx 0.05$) single crystals, which consists of normal, vortex liquid, plastic creep and elastic creep phases. The upper critical field anisotropy is determined by a radio frequency technique via the measurements of magnetic penetration depth, $λ$. Both, irreversibility line, $H_{irr}(T)$, and flux creep line, $H^{SPM}(T)$, are obtained by measuring the magnetization. We find that $H_{irr}(T)$ is well described by the Lindemann criterion with parameters similar to…
In this report, we comprehensively study the effect of H$^+$ irradiation on the critical current density, $J_c$, and vortex pinning in FeSe single crystal. It is found that the value of $J_c$ for FeSe is enhanced more than twice after 3-MeV H$^+$ irradiation. The scaling analyses of the vortex pinning force based on the Dew-Hughes model reveal that the H$^+$ irradiation successfully introduce point pinning centers into the crystal. We also find that the vortex creep rates are strongly suppressed after irradiation. Detailed analyses of the critical current dependent pinning energy based on the …
Applying the strong pinning formalism to the mixed state of a type II superconductor, we study the effect of thermal fluctuations (or creep) on the penetration of an ac magnetic field as quantified by the so-called Campbell length $λ_\textrm{C}$. Within strong pinning theory, vortices get pinned by individual defects, with the jumps in the pinning energy ($Δe_\mathrm{pin}$) and force ($Δf_\mathrm{pin}$) between bistable pinned and free states quantifying the pinning process. We find that the evolution of the Campbell length $λ_{\rm C}(t)$ as a function of time $t$ is the result of two competin…
Higher critical currents yet faster vortex creep in EuBa$_2$Cu$_3$O$_y$ films containing coherent artificial pinning centers
University of Washington（华盛顿大学）；Seikei University（圣心女子大学）；Japan Fine Ceramics Center（日本精细陶瓷中心）；National Institute of Advanced Industrial Science and Technology（产业技术综合研究所）
Flux-creep in the second magnetization peak of BaFe{1.9}Ni{0.1}As2 superconductor
获得了BaFe{1.9}Ni{0.1} As 2的M（H）曲线中沿第二磁化峰的磁场的磁通蠕动数据，H|| c轴。H|| ab-平面和与ab-平面形成45度角的H。来自不同场方向的M-H环可以塌缩到具有与超导各向异性等效的标度因子的通用曲线上，这表明钉扎是三维的，尽管作为场方向的函数的涡旋动力学具有显著差异。在第二磁化强度峰值磁场Hp附近，弛豫速率R没有任何特殊的变化，在磁场H_2远高于磁场Hp时，弛豫速率R出现最大值||c和H^* 处的最小值，对于H|| ab $ and H$ -45-ab。从磁通蠕变数据在几个不同的温度下获得的缩放激活能的等场图也没有显示任何证据的钉扎机制的变化，作为峰值字段是交叉的。在由此产生的相图中的Hp线似乎不符合集体塑性钉扎交叉的双折射项。
Directional pinning and anisotropy in YBa2Cu3O7-x with BaZrO3 nanorods: intrinsic and nanorods-induced anisotropy
研究了在倾斜磁场作用下，含BaZrO 3纳米棒的YBa 2 Cu 3 O 7-x薄膜在涡旋态下的微波复电阻率.给出了涡旋粘性$η$、钉扎常数k_p和蠕变因子χ_M的上限随角度的变化关系。我们发现，纳米棒的方向效应在η中是不存在的，这是由质量各向异性γ决定的。相比之下，钉扎介导的性质受到纳米棒的强烈影响。重要的是，受纳米棒影响的钉扎和蠕变在我们非常高的工作频率下也是可检测的，这意味着涡旋从其平衡位置的非常短的位移。
We demonstrate a geometrical effect on the depinning line (DL) of the flux line lattice of the Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ high-T$_c$ superconductor (HTSC) micrometer ring. The DL shifts to notably lower temperatures in comparison to bulk crystals and thin flakes of the same sample. The shift is attributed to a decrease in the overall pinning potential due to a double size effect, namely: a) the ring thickness $\sim 1~μ$m being smaller than the pinning correlation length, and b) the increase in the effective London penetration depth of the vortices (Pearl vortices). The large shift of …
Competition between the modification of intrinsic superconducting properties and the pinning landscape under external pressure in CaKFe$_4$As$_4$ single crystals
Current - voltage characteristics in magnetic field for layered superconductor with intrinsic pinning of Josephson vortices
M.N. Mikheev Institute of Metal Physics, Ural Branch, Russian Academy of Sciences（俄罗斯科学院乌拉尔分校 M.N. 米凯耶夫金属物理研究所）；National Research Nuclear University MEPhI（国立核研究大学莫斯科工程物理学院）
We report the influence of crystalline defects introduced by 6 MeV $^{16}$O$^{3+}$ irradiation on the critical current densities J$_c$ and flux creep rates in 1.3 $μ$m thick GdBa$_2$Cu$_3$O$_{7-d}$ coated conductor produced by co-evaporation. Pristine films with pinning produced mainly by random nanoparticles with diameter close to 50 nm were irradiated with doses between 2x10$^{13}$ cm$^{-2}$ and 4x10$^{14}$ cm$^{-2}$. At temperatures below 40 K with the magnetic field applied parallel (H//c) and at 45° (H//45°) to the c-axis, the in-field J$_c$ dependences can be significantly improved by ir…
A series of high quality BaFe$_{2-x}$Ni$_x$As$_2$ pnictide superconductors were studied using magnetic relaxation and isothermal magnetic measurements in order to study the second magnetization peak (SMP) and critical current behaviour in Ni-doped 122 family. The temperature dependence of the magnetic relaxation rate suggests a pinning crossover, whereas, it's magnetic field dependence hints a vortex-lattice structural phase-transition. The activation energy ($U$) estimated using the magnetic relaxation data was analyzed in detail for slightly-underdoped, slightly-overdoped and an overdoped sa…
