Vortex Pinning Effect Research Articles

Magnetoresistance hysteresis in the superconducting state of kagome CsV3Sb5

Abstract The hysteresis of magnetoresistance observed in superconductors is of great interest due to its potential connection with unconventional superconductivity. In this study, we perform electrical transport measurements on Kagome superconductor CsV3Sb5 nanoflakes and uncover unusual hysteretic behaviour of magnetoresistance in the superconducting state. This hysteresis can be induced by applying either a large DC or AC current at temperatures (T) well below the superconducting transition temperature (T c). As T approaches T c, similar weak hysteresis is also detected by applying a small current. Various scenarios are discussed, with particular focus on the effects of vortex pinning and the presence of time-reversal-symmtery-breaking superconducting domains. Our findings support the latter, hinting at chiral superconductivity in Kagome superconductors.

Minimum critical velocity of a Gaussian obstacle in a Bose-Einstein condensate

When a superfluid flows past an obstacle, quantized vortices can be created in the wake above a certain critical velocity. In the experiment by Kwon et al. [Phys. Rev. A 91, 053615 (2015)], the critical velocity ${v}_{c}$ was measured for atomic Bose-Einstein condensates (BECs) using a moving repulsive Gaussian potential, and ${v}_{c}$ was minimized when the potential height ${V}_{0}$ of the obstacle was close to the condensate chemical potential $\ensuremath{\mu}$. Here we numerically investigate the evolution of the critical vortex shedding in a two-dimensional BEC with increasing ${V}_{0}$ and show that the minimum ${v}_{c}$ at the critical strength ${V}_{0c}\ensuremath{\approx}\ensuremath{\mu}$ results from the local density reduction and vortex-pinning effect of the repulsive obstacle. The spatial distribution of the superflow around the moving obstacle just below ${v}_{c}$ is examined. The particle density at the tip of the obstacle decreases as ${V}_{0}$ increases to ${V}_{c0}$, and at the critical strength, a vortex dipole is suddenly formed and dragged by the moving obstacle, indicating the onset of vortex pinning. The minimum ${v}_{c}$ exhibits power-law scaling with the obstacle size $\ensuremath{\sigma}$ as ${v}_{c}\ensuremath{\sim}{\ensuremath{\sigma}}^{\ensuremath{-}\ensuremath{\gamma}}$ with $\ensuremath{\gamma}\ensuremath{\approx}1/2$.

Pinning effects in a two-dimensional cluster glass

We study numerically the glass formation and depinning transition of a system of two-dimensional cluster-forming monodisperse particles in presence of pinning disorder. The pairwise interaction potential is nonmonotonic, and is motivated by the intervortex forces in type-$1.5$ superconductors. Such systems can form cluster glasses due to the intervortex interactions following a thermal quench, without underlying disorder. We study the effects of vortex pinning in these systems. We find that a small density of pinning centers of moderate depth has limited effect on vortex glass formation, i.e., formation of vortex glasses is dominated by intervortex interactions. At higher densities pinning can significantly affect glass formation. The cluster glass depinning, under a constant driving force, is found to be plastic, with features distinct from non-cluster-forming systems such as clusters merging and breaking. We find that in general vortices with cluster-forming interaction forces can exhibit stronger pinning effects than regular vortices.

Tailored Flux Pinning in Superconductor-Ferromagnet Multilayers with Engineered Magnetic Domain Morphology From Stripes to Skyrmions

Superconductor/Ferromagnet (S/F) hybrid systems show interesting magneto-transport behaviors that result from the transfer of properties between both constituents. For instance, magnetic memory can be transferred from the F into the S through the pinning of superconducting vortices by the ferromagnetic textures. The ability to tailor this type of induced behavior is important to broaden its range of applications. Here we show that engineering the F magnetization reversal allows tuning the strength of the vortex pinning (and memory) effects, as well as the field range in which they appear. This is done by using magnetic multilayers in which Co thin films are combined with different heavy metals (Ru, Ir, Pt). By choosing the materials, thicknesses, and stacking order of the layers, we can design the characteristic domain size and morphology, from out-of-plane magnetized stripe domains to much smaller magnetic skyrmions. These changes strongly affect the magneto-transport properties. The underlying mechanisms are identified by comparing the experimental results to a magnetic pinning model.

Design Optimization of a Hybrid Trapped Field Magnet Lens (HTFML)

The concept of a hybrid trapped field magnet lens (HTFML) was recently proposed by the authors, which consists of a trapped field magnet (TFM) cylinder exploiting the “vortex pinning effect,” combined with a superconducting bulk magnetic lens exploiting the “diamagnetic shielding effect.” This HTFML can generate, within its bore, a magnetic field higher than the applied magnetic field, even after external field decreases to zero. In this paper, a design optimization of the inner GdBaCuO magnetic lens within the GdBaCuO TFM cylinder was carried out using numerical simulations based on the finite-element method, in order to maximize the concentrated magnetic field. The HTFML with an optimized shape and size achieved a concentrated magnetic field of B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> = 5.6 and 12.8 T at the center of the lens for applied magnetic fields of B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">app</sub> = 3 and 10 T, respectively. A maximum tensile stress of +135 MPa exists in the outer GdBaCuO TFM cylinder during the magnetizing process for B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">app</sub> = 10 T, which exceeds the fracture strength of the bulk. This result suggests that mechanical reinforcement is necessary to avoid mechanical fracture under such high magnetic field conditions.

Pinning effects in two-band superconductors

Recently, new high-temperature superconductors have been discovered. One such material, magnesium diboride, is an ideal candidate for superconducting technology due to its high critical temperature (39 K), high critical current, and relative inexpensiveness. Numerical simulations of vortex dynamics and vortex pinning in such materials can help investigate material properties that can be utilized in superconducting technology. Unfortunately, magnesium diboride comes with many odd properties such as multiple superconducting bands rendering previous numerical models of superconductivity unable to fully describe vortex behaviors. It is well known in one-band superconductors that impurities can pin vortices and thus reduce the resistance. Here, we propose a modified Ginzburg-Landau model to describe a two-band superconductor with normal inclusions. The new model is used to investigate the effects of vortex pinning in magnesium diboride with regards to passing a resistance-free current through the superconducting sample.

Vortex pinning and rectification effect in a nanostructured superconducting film with a square array of antidot triplets**Project supported by the National Natural Science Foundation of China (Grant Nos. 11702034, 11702218, and 11421062), Fundamental Research Funds for the Central Universities, China (Grant Nos. 310812171011 and G2016KY0305), and the National Key Project of Magneto-Constrained Fusion Energy Development Program, China (Grant No. 2013GB110002).

We study the stability of vortices pinning and dynamics in a superconducting thin strip containing a square array of antidot triplets by using the nonlinear Ginzburg–Landau (GL) theory. Compared with the regular square array of circular holes, the vortices are no longer pinned inside the circular holes, but instead stabilized at the center of the antidot triplets depending on the geometry parameters. Moreover, the influences of the geometry parameters and the polarity of the applied current on the current–voltage (I–V) characteristics are also studied. The critical current for the sample turning into a normal state becomes smaller when the hole diameter D is smaller and the spacing B between the holes is larger. Due to the asymmetric pinning sites, our numerical simulations demonstrate that the positive and negative rectified voltages appear alternately in the resistive state of the sample under an ac current of square pulses.

A new concept of a hybrid trapped field magnet lens

In this paper, a new concept of a hybrid trapped field magnet lens (HTFML) is proposed. The HTMFL exploits the ‘vortex pinning effect’ of an outer superconducting bulk cylinder, which is magnetized as a trapped field magnet (TFM) using field-cooled magnetization (FCM), and the ‘diamagnetic shielding effect’ of an inner bulk magnetic lens to generate a concentrated magnetic field higher than the trapped field from the TFM in the bore of the magnetic lens. This requires that, during the zero-field-cooled magnetization process, the outer cylinder is in the normal state (T> superconducting transition temperature, Tc) and the inner lens is in the superconducting state (T < Tc) when the external magnetizing field is applied, followed by cooling to an appropriate operating temperature, then removing the external field. This is explored for two potential cases: (1) exploiting the difference in Tc of two different bulk materials (‘case-1’), e.g. MgB2 (Tc = 39 K) and GdBaCuO (Tc = 92 K) or (2) using the same material for the whole HTFML, e.g., GdBaCuO, but utilizing individually controlled cryostats, the same cryostat with different cooling loops or coolants, or heaters that keep the outer bulk cylinder at a temperature above Tc to achieve the same desired effect. The HTFML is verified using numerical simulations for ‘case-1’ using an MgB2 cylinder and GdBaCuO lens pair and for ‘case-2’ using a GdBaCuO cylinder and GdBaCuO lens pair. As a result, the HTFML could reliably generate a concentrated magnetic field Bc = 4.73 T with the external magnetizing field Bapp = 3 T in the ‘case-1’, and a higher Bc = 13.49 T with higher Bapp = 10 T in the ‘case-2’, respectively. This could, for example, be used to enhance the magnetic field in the bore of a bulk superconducting NMR/MRI system to improve its resolution.

Vortex pinning by the point potential in topological superconductors: A scheme for braiding Majorana bound states

We propose theoretically an effective scheme for braiding Majorana bound states by manipulating the point potential. The vortex pinning effect is carefully elucidated. This effect may be used to control the vortices and Majorana bound states in topological superconductors. The exchange of two vortices induced by moving the potentials is simulated numerically. The zero-energy state in the vortex core is robust with respect to the strength of the potential. The Majorana bound states in a pinned vortex are identified numerically.

Magnetoresistivity plateau of graphene in proximity to superconductingNbSe2

The distribution and dynamic behavior of Abrikosov vortices in type-$\mathrm{II}$ superconductors could modulate the nearby magnetic field, thus enriching new physics in the proximal two-dimensional electron gas. Here we investigate the effects of vortex dynamics on the quantum transport properties of graphene in proximity to the superconducting single-crystalline ${\mathrm{NbSe}}_{2}$ thin flakes. Pronounced asymmetric magnetoresistivity plateaus emerge near zero field below the critical temperature of ${\mathrm{NbSe}}_{2}$, accompanied by hysteresis loops. In sharp contrast, only weak localization behavior is observed for graphene placed on ${\mathrm{SiO}}_{2}\text{/}\mathrm{Si}$ substrate. Such magnetoresistivity plateaus and hysteresis can be attributed to the vortex pinning effect at the edge defects of the single-crystalline ${\mathrm{NbSe}}_{2}$ flakes. Our findings may shed light on the realization of emergent states in the two-dimensional electron gases by manipulation of vortex dynamics and distribution in the proximal superconductors.

Effect of vortex pinning by point defects on the lower critical field in layered superconductors

The lower critical field Hc1 in layered superconductors is calculated under the assumption that vortex pinning by point defects is strong in these materials. We consider the case of a purely electromagnetic coupling of vortex pancakes and the case of both the electromagnetic and Josephson couplings of the pancakes in a vortex line. In the latter case, singularities in the temperature dependence of Hc1 are predicted at certain characteristic temperatures.

Pinning of hidden vortices in Bose-Einstein condensates

We study the vortex dynamics and vortex pinning effect in Bose-Einstein condensates in a rotating double-well trap potential and corotating optical lattice. We show that the vortex number does not diverge when the rotational frequency $\ensuremath{\Omega}\ensuremath{\rightarrow}1$ if the trap potential is of anisotropic double-well type. The critical rotational frequency as obtained from numerical simulations agrees very well with the value $\sqrt{l}/l$ for $l=4$, which supports the conjecture that surface modes with angular momentum $l=4$ are excited when the rotating condensate is trapped in a double-well potential. The vortex lattice structure in a rotating triple-well trap potential and its pinning show very interesting features. We show the existence and pinning of hidden vortices whose phase profile is similar to that of the visible vortices.

Vortex pinning and magnetic peak effect in Eu(Eu,Ba)2.125Cu3Ox

Eu–Ba–Cu–O composition was synthesized by solid state reaction technique. To determine optimum growth temperature, heat treatment was examined on the material at 880–1,100 °C. Microstructural evolution, phase formation and elemental distribution depending on heat treatments were examined by using X-ray diffraction, scanning electron microscope, energy dispersive X-ray spectroscope analysis. Optimum fabrication conditions were determined as 1,020 °C for 24 h under oxygen atmosphere and detailed characterization of corresponding compound was performed. The magnetization hysteresis loops are expounded to be the product of superconducting Eu-123 grains and magnetic Eu2+ ions. The peak effect on the magnetization curves was described by the extended critical state model. Scaling of the pinning force was found such that the peak position is proportional to the irreversibility field H irr and the maximum pinning force is proportional to H irr 2 .

Microwave absorption study of pinning regimes in Ba(Fe1−xCox)2As2 single crystals

Magnetic field dependent modulated microwave absorption (MMWA) measurements have been carried out to investigate vortex pinning effects in single crystals of the iron-based high-Tc superconductor Ba(Fe1−xCox)2As2 with three different cobalt doping levels of x = 0.07, 0.09 and 0.11. The dependence of the MMWA hysteresis loops on temperature, magnetic field and Co concentration have been measured and analyzed using a theoretical model of microwave absorption in superconductors. The analysis reveals that in the underdoped crystal (x = 0.07) the so called δTc-pinning due to magnetically ordered regions defines the temperature dependence of the critical current density, while in the optimally doped (x = 0.09) and overdoped (x = 0.11) samples the pinning is governed by structural imperfections due to the inhomogeneous distribution of the cobalt dopant and has the so called δl character.

Effects of Vortex Pinning on the Temperature Dependence of the Magnetic Field Distributions in Superconductors

Abstract The temperature and applied-magnetic-field dependence of the second moments of the magnetic-field distributions as measured by μ SR for YBCO and BSCCO have been fit for four different intrinsic-field-distribution models (d-wave, 2-fluid, empirical, and BCS). It is found that if a pinning potential becomes important at about 20 K, all of the models can fit the data reasonably well. The fits and the associated fitting parameters are presented.

Intrinsic Josephson junction characteristics in the stripe-ordered La1.6−xNd0.4SrxCuO4 bulk single crystals

The current-voltage (I-V) properties along the c axis of stripe-ordered La1.6−xNd0.4SrxCuO4 (LNSCO, x=0.10, 0.12, 0.15, and 0.18) bulk single crystals are studied. For all the samples, the I-V curves exhibit Josephson junctionlike characteristics, the voltage jumps at some critical currents and clear hysteresis without multiple branches appears below the superconducting transition temperatures. With increasing Sr doping level, the characteristics of intrinsic Josephson junctions in LNSCO change from a superconductor-insulator-superconductor type to a superconductor-normal metal-superconductor one. The field dependence of critical current Ic exhibits a periodical perturbation with a large scale of magnetic field. A fourfold symmetry of the angular dependent Ic for the rotation of the CuO2 plane in magnetic fields confirms the vortex pinning effect of the static stripes. The abrupt jump of voltage at a rather large Ic in LNSCO may be used as a high-power current restrictor or switch.

Temperature and Current Dependent Matching Fields in Superconducting NbN Film

Patterned superconducting thin films having a periodic array of submicrometric pinning centers have been of great interest due to their excellence for the studies of the vortex pinning mechanisms in the type-II superconductors. Square hole array has been fabricated over a micro-bridge 60 mm×60 mm of NbN thin film by electron beam lithography. Previous works have been carried out in Nb, Pb and Al thin films where the vortex pinning effect is assumed to be small. In this work, we study the matching pinning effect by the artificial hole array in superconducting NbN thin films. We observed the interplay between the vortex quantization and the artificial hole array. Magneto-resistance minima at integer matching fields up to five times of H 1 (the first matching field corresponding to one vortex inside each hole) and fractional matching fields at 1/2H 1, 3/2H 1 and 5/2H 1 have been observed.

RESISTANCE IN SUPERCONDUCTORS

In this pedagogical review, we discuss how electrical resistance can arise in superconductors. Starting with the idea of the superconducting order parameter as a condensate wave function, we introduce vortices as topological excitations with quantized phase winding, and we show how phase slips occur when vortices cross the sample. Superconductors exhibit non-zero electrical resistance under circumstances where phase slips occur at a finite rate. For one-dimensional superconductors or Josephson junctions, phase slips can occur at isolated points in space-time. Phase slip rates may be controlled by thermal activation over a free-energy barrier, or in some circumstances, at low temperatures, by quantum tunneling through a barrier. We present an overview of several phenomena involving vortices that have direct implications for the electrical resistance of superconductors, including the Berezinskii–Kosterlitz–Thouless transition for vortex-proliferation in thin films, and the effects of vortex pinning in bulk type II superconductors on the nonlinear resistivity of these materials in an applied magnetic field. We discuss how quantum fluctuations can cause phase slips and review the non-trivial role of dissipation on such fluctuations. We present a basic picture of the superconductor-to-insulator quantum phase transitions in films, wires, and Josephson junctions. We point out related problems in superfluid helium films and systems of ultra-cold trapped atoms. While our emphasis is on theoretical concepts, we also briefly describe experimental results, and we underline some of the open questions.

Anomalous enhancement of vortex pinning and microscopic phase separation in the overdoped regime of La2-xSrxCuO4

Our previous study of the magnetic susceptibility, χ, on zero-field and field cooling and magnetization-hysteresis-loop (MHL) for La2-xSrxCuO4 (LSCO) single crystals with x = 0.178 – 0.261 has suggested that a microscopic phase separation into superconducting (SC) and normal-state regions takes place in the overdoped regime of LSCO. In order to clarify the onset hole-concentration where the microscopic phase separation appears, we have carried out χ measurements on zero-field and field cooling and MHL measurements for LSCO single crystals with x = 0.125 and 0.147. From the χ measurements in a low magnetic field on field cooling, absolute values of χ at 2 K, |χ2k|, in x = 0.125 and 0.147 have been found to be smaller than that of x = 0.178, which can be understood in terms of the vortex-pinning effect around twin boundaries in a crystal. Neither two-step SC transition in χ vs- T in a moderate field on zero-field cooling nor second magnetization peak in MHL has been observed in x = 0.125, while both characteristics have been observed in x = 0.147. These results imply no enhancement of the vortex pinning in a moderate field-range and therefore suggest the existence of a homogeneous superconducting state in x = 0.125. Accordingly, it is concluded that the onset hole-concentration of the microscopic phase separation is located between x = 0.125 and 0.147.

Inhomogeneous superconductivity in both underdoped and overdoped regimes of high-Tc cuprates

Our recent experimental works on the inhomogeneity of superconductivity in both underdoped and overdoped regimes of La2-xSrxCuO4 are reviewed. In order to estimate the superconducting (SC) volume fraction in a sample, bulk-sensitive magnetic-susceptibility and specific-heat measurements have been carried out, using La2-xSrxCuO4 single crystals of good quality with x = 0.05−0.3. As a result, it has been found that the SC volume fraction is not 100% in both underdoped and overdoped regimes, suggesting the occurrence of a phase separation into SC and non-SC regions. Moreover, strong vortex-pinning effects have been observed in moderate magnetic fields in the overdoped regime, suggesting that the phase separation in the overdoped regime is not macroscopic but microscopic.