A theoretical study of dynamics of small thermal and electromagnetic perturbations in type-II superconductors in the flux creep regime with a nonlinear current-voltage characteristics is performed. For this purpose, the space-time evolution of temperature and electric field was calculated using the heat diffusion equation, coupled with Maxwell's equations and material law, assuming that the applied magnetic field is directed parallel to the surface of the sample. A threshold field $B_j$ for the occurrence of thermomagnetic instability has been found assuming that the heat flux diffusion is considerable faster than the magnetic flux diffusion in superconductor. The obtained stability criterion for the thermomagnetic flux jumps demonstrates the extremely high sensitivity of the threshold field on the values of the critical current density and external magnetic field sweep rate.
We consider the current density distribution function of a flux creep regime in type-II superconductors by mapping the flux creep process to the dynamics of a model with a self-organized criticality. We use an extremal Robin Hood type model which evolves to Been's type critical state to treat magnetic flux penetration into a superconductor and derive an analog of the current-voltage characteristics in the flux creep region.
We study theoretically the space-time evolution of the thermal and electromagnetic perturbation in a superconductor with a nonlinear current-voltage characteristics in the flux creep regime. On the basis of a linear analysis of a set of differential equations describing small perturbations of temperature and electromagnetic field, it is found that under some conditions a branching instability may occur in a superconductor sample.
Vortex-antivortex arrays that are created in a type-II superconducting film by lattices of ferromagnetic dots with perpendicular anisotropy have been investigated. The highest critical current is shifted to a nonzero value of the applied field, and current-voltage characteristics show different regimes of vortex motion even in zero-applied field due to the presence of the vortex-antivortex arrays. Creep of interstitial vortices is observed at low driving forces. This flux creep is strongly field-polarity dependent.
The influence of the antiferromagnetic order on the superconductor in the mixed state results in creation of spin-flop domains along the cores of the vortex lines. It is shown that this phenomenon makes possible crossover from quantum creep regime to thermal one, and vice versa, at constant temperature. To do this one needs to simply change the intensity or the direction of applied magnetic field in the basal ${\bf ab}$ plane of layered structure.
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 use Monte Carlo simulations of a coarse grained three dimensional model to demonstrate that the experimentally observed approximate temperature independence of the magnetic creep rate for a broad range of temperatures may be explained in terms of record dynamics, {\it viz.} the dynamical properties of the times at which a stochastic fluctuating signal establishes records.
We image vortex creep at very low temperatures using Scanning Tunneling Microscopy (STM) in the superconductor Rh$_9$In$_4$S$_4$ ($T_c$=2.25 K). We measure the superconducting gap of Rh$_9$In$_4$S$_4$, finding $Δ\approx 0.33$meV and image a hexagonal vortex lattice up to close to H$_{c2}$, observing slow vortex creep at temperatures as low as 150 mK. We estimate thermal and quantum barriers for vortex motion and show that thermal fluctuations likely cause vortex creep, in spite of being at temperatures $T/T_c<0.1$. We study creeping vortex lattices by making images during long times and show t…
Superconductors are excellent testbeds for studying vortices, topological excitations that also appear in superfluids, liquid crystals, and Bose-Einstein condensates. Vortex motion can be disruptive; it can cause phase transitions, glitches in pulsars, and losses in superconducting microwave circuits, and it limits the current carrying capacity of superconductors. Understanding vortex dynamics is therefore of fundamental and technological importance, and the competition between the effects of thermal energy and energy barriers defined by material disorder is not completely understood. In parti…
We consider the flux jump instability of the Bean's critical state arising in the flux creep regime in type-II superconductors. We find the flux jump field, $B_j$, that determines the superconducting state stability criterion. We calculate the dependence of $B_j$ on the external magnetic field ramp rate, $\dot B_e$. We demonstrate that under the conditions typical for most of the magnetization experiments the slope of the current-voltage curve in the flux creep regime determines the stability of the Bean's critical state, {\it i.e.}, the value of $B_j$. We show that a flux jump can be preceded…
The flux creep problem for a superconductor slab placed in a constant or time-dependent magnetic field is considered. Logarithmic dependence of the activation energy on the current density is assumed, U=U0 ln(J/Jc), with a field dependent Jc. The density B of the magnetic flux penetrating into the superconductor, is shown to obey a scaling law, i.e., the profiles B(x) at different times can be scaled to a function of a single variable. We found exact solution for the scaling function in some specific cases, and an approximate solution for a general case. The scaling also holds for a slab carry…
Two commonly accepted approaches for simulations of thermally-activated vortex motion in superconductors are compared. These are (i) the so-called flux creep approach based on the expression E=vB relating the electric field E to the velocity v of the thermally-activated flux motion and the local flux density B, and (ii) the approach employing a phenomenological nonlinear current-voltage curve, E(j). Our results show that the two approaches give similar but also distinctly different behaviors for the distributions of current and flux density in both a long slab and thin strip geometry. The diff…
Systematic ac susceptibility measurements have been performed on a MgB$_2$ bulk sample. We demonstrate that the flux creep activation energy is a nonlinear function of the current density $U(j)\propto j^{-0.2}$, indicating a nonlogarithmic relaxation of the current density in this material. The dependence of the activation energy on the magnetic field is determined to be a power law $U(B)\propto B^{-1.33}$, showing a steep decline in the activation energy with the magnetic field, which accounts for the steep drop in the critical current density with magnetic field that is observed in MgB$_2$. …
The glassy dynamics is a consequence of the elastic properties of the vortex matter, and in principle may occur in any superconductor. However, whereas a large amount of experimental evidence confirms glassiness in high Tc superconductors (HTS), the applicability of the whole framework developed to describe vortex matter in HTS to other superconducting materials is unclear. In this framework, the particular creep behavior of MgB2, larger than creep of conventional superconductor materials but much lower than HTS one, had precluded a complete understanding of the vortex regimes in this material…
In the mixed state of layered superconductor the antiferromagnetic order of magnetic ions can create the spin-flop domains along the phase cores of the Josephson vortices. The paper discusses how this feature affects the macroscopic quantum tunnelling of the Josephson vortices. It is shown that the action and hence the activation energy is rendered temperature dependent so that the quantum tunnelling rate becomes temperature dependent below the crossover temperature. It is also shown that in constant temperature thermal or quantum creep may occur, depending on the direction or intensity of the…
A systematic study of I-V characteristic curves for RuSr2GdCu2O8 [Ru-(1212)] is presented, with magnetic fields up to 3 T and 5 K<T<30 K, in the region of the superconducting transition. The activation energy E_{a}(H,T) for flux line depinning was determined by fitting the nonlinear region of the curves using the flux creep model. E_{a}(H,T) was found to vary linearly with temperature, while a power-law dependence on the magnetic field was observed up to H=0.1 T, where an abrupt reduction in its decreasing rate occurs. The extrapolated value, E_{a}(0,0)=50 meV, is twice the reported value for …
Flux-creep data was obtained for fields along the second magnetization peak observed in M(H) curves of BaFe{1.9}Ni{0.1}As2 for H||c-axis. H||ab-planes and H forming a 45 degrees angle with ab-planes. The M-H loops from the different field directions can be collapsed onto a universal curve with a scaling factor equivalent to the superconducting anisotropy, showing that the pinning is three dimensional, although with remarkable differences in the vortex-dynamics as a function of field orientation. The resulting relaxation rate, R, when plotted as a function of field and temperature does not show…
In type-II superconductors exposed to magnetic fields between upper and lower critical values, $H_{c1}$ and $H_{c2}$, penetrating magnetic flux forms a lattice of vortices whose motion can induce dissipation. Consequently, the magnetization $M$ of superconductors is typically progressively weakened with increasing magnetic field $B \propto n_v$ (for vortex density $n_v$). However, some materials exhibit a non-monotonic $M(B)$, presenting a maximum in $M$ at what is known as the second magnetization peak. This phenomenon appears in most classes of superconductors, including low $T_c$ materials,…
We investigated the vortex dynamics in the non-centrosymmetric superconductor Li_2Pt_3B in the temperature range 0.1 K - 2.8 K. Two different logarithmic creep regimes in the decay of the remanent magnetization from the Bean critical state have been observed. In the first regime, the creep rate is extraordinarily small, indicating the existence of a new, very effective pinning mechanism. At a certain time a vortex avalanche occurs that increases the logarithmic creep rate by a factor of about 5 to 10 depending on the temperature. This may indicate that certain barriers against flux motion are …
We simulated the creep motions of flux lines subject to randomly distributed point-like pinning centers. It is found that at low temperatures, the pinning barrier $U$ defined in the Arrhenius-type $v-F$ characteristics increases with decreasing force $U(F) \propto F^{-μ}$, as predicted by previous theories. The exponent $μ$ is evaluated as $0.28\pm 0.02 $ for the vortex glass and $μ\simeq 0.5\pm 0.02$ for the Bragg glass (BrG). The latter is in good agreement with the prediction by the scaling theory and the functional-renormalization-group theory on creep, while the former is a new estimate. …
A perturbed system relaxes towards an equilibrium given by a minimum in the potential energy landscape. This often occurs by thermally activated jumps over metastable states. The corresponding dynamics is named creep and follows Arrhenius' law. Here we consider the situation where the equilibrium position depends on temperature. We show that this effect occurs in the vortex lattice of the anisotropic superconductor $2\mathrm{H}$-$\mathrm{Nb}\mathrm{Se}_{2}$ when the magnetic field is tilted away from the principal axes, and that it leads to the peculiar appearance of creep when cooling the sam…
Magnetic flux jumps in textured Bi2Sr2CaCu2O(8+d) have been studied by means of magnetization measurements in the temperature range between 1.95 K and Tc, in an external magnetic field up to 9 T. Flux jumps were found in the temperature range 1.95 K - 6 K, with the external magnetic field parallel to the c axis of the investigated sample. The effect of sample history on magnetic flux jumping was studied and it was found to be well accounted for by the available theoretical models. The magnetic field sweep rate strongly influences the flux jumping and this effect was interpreted in terms of the…
Order-disorder phase transition and elastic-to-plastic vortex creep crossover in a triclinic iron pnictide superconductor (Ca0.85La0.15)10(Pt3As8)(Fe2As2)5
We present an extensive study of vortex dynamics in a high-quality single crystal of HgBa$_2$CuO$_{4+δ}$ (Hg1201), a highly anisotropic superconductor that is a model system for studying the effects of anisotropy. From magnetization $M$ measurements over a wide range of temperatures $T$ and fields $H$, we construct a detailed vortex phase diagram. We find that the temperature-dependent vortex penetration field $H_p(T)$, second magnetization peak $H_{smp}(T)$, and irreversibility field $H_{irr}(T)$ all decay exponentially at low temperatures and exhibit an abrupt change in behavior at high temp…
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…
The paper reviews a concept of induced spin-flop domain inside vortices in an antiferromagnetic superconductor. Such phenomenon may occur when an external magnetic field is strong enough to flip over magnetic moments in the core of the vortex from their ground state configuration. The formation of the domain structure inside vortices modifies the surface energy barrier of the superconductor. During this process the entrance of the flux is stopped and a newly created state exhibits perfect shielding. Such behavior should be visible as a plateau on the dependence of flux density as a function of…
Numerical studies of the flux creep in superconductors show that the distribution of the magnetic field at any stage of the creep process can be well described by the condition of spatial constancy of the activation energy $U$ independently on the particular dependence of $U$ on the field B and current $j$. This results from a self-organization of the creep process in the undercritical state $j<j_{c}$ related to a strong non-linearity of the flux motion. Using the spatial constancy of $U$, one can find the field profiles $B(x)$, formulate a semi-analytical approach to the creep problem and gen…
In this work we will introduce the A-V-A formulation based iterative algorithm method (IAM) for simulating the magnetization current of high temperature superconductors. This new method embedded in ANSYS can simulate the critical state model by forcing the trapped current density to the critical current density Jc for all meshed superconducting elements after each iterative load step, as well as simulate the flux creep model by updating the E-J power law based resistivity values. The simulation results of a disk-shaped ReBCO bulk during zero field cooling (ZFC) or field cooling (FC) magnetizat…
The magnetic levitation forces of a superconducting disk (SC) levitated by a cylindrical permanent magnet (PM) have been calculated from first principles for superconductors with finite thickness. The current $j(ρ,z)$ and field $B(ρ,z)$ profiles in the SC in the non-uniform magnetic field generated by the PM have been derived. The levitation force is observed to depend non-linearly on the critical current density $j_c$ and on the thickness of the SC. The flux creep is described by a current-voltage law $E(j)=E_c(j/j_c)^n$, from which we show that the levitation force depends on the speed at wh…
In this report, the temperature distribution in the critical state of hard superconductors is investigated in the quasi-stationary approximation. It is shown that the temperature profile can be essentially inhomogeneous in the sample, which affects the conditions of initiation of a magnetic flux jumps.
It is argued that the stress caused by vortex cores in the mixed state of superconductors may result in a field dependent contribution to the free energy and magnetization of measurable levels. For sufficiently strong stress dependence of the critical temperature, $\partial T_c/\partial p$, this contribution may result in the so-called "second peak" in the field dependence of the reversible magnetization, the effect often masked by vortex pinning and creep.
We analyse the thermally activated depinning of an elastic string (line tension $ε$) governed by Hall dynamics from a columnar defect modelled as a cylindrical potential well of depth $V_{0}$ for the case of a small external force $F.$ An effective 1D field Hamiltonian is derived in order to describe the 2D string motion. At high temperatures the decay rate is proportional to $F^{{5}/{2}}T^{-{1}/{2}} \exp{\left [{F_{0}}/{F}-{U(F)}/{T}\right ]},$ with $F_{0}$ a constant of order of the critical force and $U(F) \sim{\left ({εV_{0}})}^{{1}/{2}}{V_{0}/{F}}$ the activation energy. The results are a…
We revise the applicability of the theory of self-organized criticality (SOC) to the process of magnetic relaxation in type-II superconductors. The driving parameter of self-organization of vortices is the energy barrier for flux creep and not the current density. The power spectrum of the magnetic noise due to vortex avalanches is calculated and is predicted to vary with time during relaxation.
A model was proposed to account for a new kind of history effect in the transport measurement of a sample with inhomogeneous flux pinning coupled with flux creep. The inhomogeneity of flux pinning was described in terms of alternating weak pinning (lower jc) and strong pinning region (higher jc). The flux creep was characterized by logarithmic barrier. Based on this model, we numerically observed the same clockwise V-I loops as reported in references. Moreover, we predicted behaviors of the V-I loop at different sweeping rates of applied current dI/dt or magnetic fields Ba, etc. Electric trans…
The specific-heat jump $ΔC$ at a critical temperature $T_c$ in an anisotropic superconductor containing both potential and spin-flip scatterers is calculated within a weak-coupling mean-field approximation. It is shown that the presence of even a small amount of spin-flip scatterers in the sample leads to a drastic change in the dependence of $ΔC$ on $T_c$ in a disordered $(d+s)$-wave or a strongly anisotropic s-wave superconductor. The implications for experimental tests for the presence of an s-wave admixture in the superconducting order parameter of high-$T_c$ superconductors are discussed.
The specific heat jump at a normal-superconducting phase transition in an anisotropic superconductor with nonmagnetic impurities is calculated within a weak-coupling mean field approximation. It is shown that its dependence on the impurity concentration is remarkably different for d-wave and (d+s)-wave states. This effect may be used as a test for the presence of an s-wave admixture in the cuprates.
Within the weak-coupling BCS scheme we derive a general form of the coefficients in the Ginzburg-Landau expansion of the free energy of a superconductor for the case of a Fermi level close to a van Hove singularity (VHS). A simple expression for the influence of the VHS on the specific heat jump is then obtained for the case where gaps for different bands are distinct but nearly constant at the corresponding sheets of the Fermi surface.
We examine intrinsic interfaces separating crystalline twin domains of opposite spin-orbit coupling in a noncentrosymmetric superconductor such as CePt3Si. At these interfaces, low-energy Andreev bound states occur as a consequence of parity-mixed Cooper pairing, and a superconducting phase which violates time reversal symmetry can be realized. This provides an environment allowing flux lines with fractional flux quanta to be formed at the interface. Their presence could have strong implications on the flux creep behavior in such superconductors.
We show that some experimentally observed features of vortex matter in high temperature superconductors may be interpreted in simpler ways than it is usually done. In particular, we consider magnetic flux creep at low temperatures as well as the irreversibility line in the H-T phase diagram. We also discuss a new approach to the analysis of the equilibrium magnetization in the mixed state of type-II superconductors and we suggest an alternative configuration for the mixed state in magnetic fields close to the upper critical field.
A theoretical study of small thermal and electromagnetic perturbations in type-II superconductors is studied in the flux creep regime with a power-law voltage-current characteristics. The space-time evolution of temperature and electric field was calculated using the heat diffusion equation, coupled with Maxwell's equations and material law, assuming that heat diffusion is small that the magnetic diffusion. An explicit blow-up-like solution is obtained, which describes the time and space evolution of the electromagnetic field and temperature due to flux jump instabilities.
The flux jump dynamics in the flux flow regime of type II superconductors is investigated, analytically. It is found that under some conditions flux jump avalanche may occur in a superconductor sample, which takes into account an inertial properties of the vortex matter.
The influence of randomly distributed point impurities \emph{and} planar defects on the order and transport in type-II superconductors and related systems is considered theoretically. For planar defects of identical orientation the flux line lattice exhibits a new glassy phase dominated by the planar defects with a finite compressibility, a transverse Meissner effect, large sample to sample fuctuations of the susceptibility and an exponential decay of translational long range order. The flux creep resistivity for currents $J$ parallel to the defects is $ρ(J)\sim \exp-(J_0/J)^{3/2}$ . Strong di…
A trapped magnetic field of greater than 3 T has been achieved in a single grain GdBa2Cu3O7-δ (GdBaCuO) bulk superconductor of diameter 30 mm by employing pulsed field magnetisation (PFM). The magnet system is portable and operates at temperatures between 50 K and 60 K. Flux jump behaviour was observed consistently during magnetisation when the applied pulsed field, Ba, exceeded a critical value (e.g. 3.78 T at 60 K). A sharp dBa/dt is essential to this phenomenon. This flux jump behaviour enables the magnetic flux to penetrate fully to the centre of the bulk superconductor, resulting in full …
We have studied the influence of the patterning procedure on the critical current density of high-$T_c$ YBa$_2$Cu$_3$O$_{7-δ}$ thin rings using the nonlinear ac susceptibility method. At no applied dc magnetic field we have found that laser ablation degrades strongly the critical current density whereas ion beam etching has only a weak influence on it. From the measurements at different frequencies and dc magnetic fields we analyzed the influence of flux creep and obtained the field dependence of the flux creep exponent. Our data reconfirm the recently observed scaling relation for the nonline…
We investigate the influence of domain walls on the vortex dynamics in superconductors with multi-component order parameters. We show that, due to their complex structure domain walls can carry vortices with fractional flux quanta. The decay of conventional vortices into fractional ones on domain walls is examined. This decay presents an extraordinarily strong pinning mechanism for vortices and turns domain walls occupied with pinned fractional vortices into efficient barriers for the vortex motion. Therefore, domain walls can act as fences for the flux flow, preventing the decay of the remnan…
The impurity effects on the transition temperature Tc and the isotope effect are examined in multiband superconductors with magnetic and nonmagnetic impurities, where the effect of Coulomb repulsion is considered. It is shown that an internal transition of the superconducting state is induced by impurity doping, and that the transition is accompanied by a jump of the isotope-effect coefficient alpha. In particular, the transition is illustrated in a system with two electron bands. In some special cases, extended Abrikosov and Gor'kov (AG) equations for Tc and the expressions of the isotope-eff…
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…
Magnetic flux distribution across a high-temperature superconductor strip is measured using magneto-optical imaging at 15 K. Both the current-carrying state and the remanent state after transport current are studied up to the currents 0.97 Ic where Ic is the critical current. To avoid overheating of the sample current pulses with duration 50 ms were employed. The results are compared with predictions of the Bean model for the thin strip geometry. In the current-carrying state, reasonable agreement is found. However, there is a systematic deviation -- the flux penetration is deeper than theoret…
The intrinsic pinning in high-temperature superconductor with long range antiferromagnetic order of rare-earth ions confined to the isolating planes is described. The interaction of antiferromagnetic and superconducting subsystems in the mixed state may lead to the creation of spin-flop domain along the vortex core. It is shown that the behavior of several physical quantities such as magnetization, activation energy, current-voltage characteristic and flux creep changes when the direction of the external field changes in the basal a-b plane. It is also shown that the decay of the trapped flux …
It is shown that the specific heat jump at the superconducting transition temperature in pnictide and cuprate superconductors is produced by ferroelastic fluctuations just below the transition temperature. The amplitude of the corresponding lattice distortion is estimated. It is shown that a contribution from ferroelastic fluctuations to the specific heat jump is also present in low-temperature superconductors.
We have monitored new peculiarities of the dynamics of catastrophic avalanches of the magnetic flux in superconducting Nb, Nb-Ti, and YBaCuO samples: i) convergent oscillations of the magnetic flux; ii) a threshold for entering the huge flux avalanches in the shielding experiments; iii) a threshold for the exit of a residual flux in the trapping experiments. The observed phenomena are interpreted in terms of a theoretical model which takes into account the inertial properties of the vortex matter.
We report the numerical calculations of the current-voltage characteristics of intrinsic Josephson junctions in high- superconductors. The charging effect at superconducting layers is taken into account. A set of equations is used to study the non-linear dynamics of the system. In framework of capacitively coupled Josephson junctions model we obtain the total number of branches using fixed initial conditions for phases and their derivatives. The influence of the coupling constant αon the current-voltage characteristics at fixed parameter β(β^2=1/β_c, where β_c is McCumber parameter) and the in…
In the high-temperature superconductor BSCCO, local measurements of magnetic field at the surface of a crystal in the mixed state show sharp changes as a function of applied field or temperature. We show that if `intermediate state' effects are accounted for, a first-order transition leads to a sharp jump in the global magnetisation only in the case of samples that are significantly non-ellipsoidal in shape. We also investigate the relationship between a jump in magnetisation, M, and the associated change in the B-field immediately above the crystal surface and show that deltaM is expected to …
Standard superconductors display a ubiquitous discontinuous jump in the electronic specific heat at the critical superconducting transition temperature. In a growing class of unconventional superconductors, however, a second order parameter component may get stabilized and produce a second heat capacity jump at a lower temperature, typically associated with the spontaneous breaking of time-reversal symmetry. The splitting of the two specific heat discontinuities can be controlled by external perturbations such as chemical substitution, hydrostatic pressure, or uniaxial strain. We develop a the…
We derive the response of the magnetic superconductors in the vortex state to the ac Lorentz force, $F_L(t)=F_{\rm ac}\sin(ωt)$, taking into account the interaction of vortices with the magnetic moments described by the relaxation dynamics (polaronic effect). At low amplitudes of the driving force $F_{\rm ac}$ the dissipation in the system is suppressed due to the enhancement of the effective viscosity at low frequencies and due to formation of the magnetic pinning at high frequencies $ω$. In the adiabatic limit with low frequencies $ω$ and high amplitude of the driving force $F_{ac}$, the vor…
We show that for a thin superconducting strip placed in a transverse dc magnetic field - the typical geometry of experiments with high-Tc superconductors - the application of a weak ac magnetic field perpendicular to the dc field generates a dc voltage in the strip. This voltage leads to the decay of the critical currents circulating in the strip, and eventually the equilibrium state of the superconductor is established. This relaxation is not due to thermally activated flux creep but to the "walking" motion of vortices in the two-dimensional critical state of the strip with in-plane ac field.…
In this work we present the results of the bulk magnetization measurements in a superconducting state of single crystals of Ba$_{0.65}$Na$_{0.35}$Fe$_2$As$_2$. The isothermal magnetic field ($H||c$ axis) dependent magnetization ($M$) loops exhibit a second peak (SP) or `fishtail effect', as well as remarkable flux jumps at low temperatures. The critical current density $J_c$ obtained from the $M(H)$ loops is rather high, of the order of 10$^6$ A/cm$^2$. The analysis of the temperature and field dependent $J_c$ implies that high $J_c$ is mainly due to collective (weak) pinning of vortices by de…
The strong power law behavior of the specific heat jump $ΔC$ vs. $T_c$ ($ΔC/T_c \sim T_c ^α, α\approx 2$), first observed by Bud'ko, Ni, and Canfield (BNC)[1], has been confirmed with several families of the Fe-based superconducting compounds with a series of doping. We show here that this anomalous non-BCS behavior is an intrinsic property of the multiband superconducting state paired by a dominant interband interaction ($V_{inter} > V_{intra}$) reflecting the relation $\frac{Δ_h}{Δ_e} \sim \sqrt{\frac{N_e}{N_h}}$ near $T_c$, as in the $\pm$S-wave pairing state. Then this $ΔC$ vs. $T_c$ relat…
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…
We performed Langevin dynamics simulations for the \textit{ac} driven flux lines in a type II superconductor with random point-like pinning centers. Scaling properties of flux-line velocity with respect to instantaneous driving force of small frequency and around the critical \textit{dc} depinning force are revealed successfully, which provides precise estimates on dynamic critical exponents. From the scaling function we derive a creep law associated with the activation by the regular shaking. The effective energy barrier vanishes at the critical dc depinning point in a square-root way when th…
