Strong Vortex Pinning Research Articles

Anisotropy of the Critical Current and Abrikosov Vortex Pinning in Magnetic Superconductor EuCsFe<sub>4</sub>As<sub>4</sub>

In this study, for the first time, we present a systematic study of the critical current densityJcat two magnetic field orientationsB∥abandB∥cin a single crystal of the superconducting pnictide EuCsFe4As4, which shows a magnetic ordering belowTcand belongs to the 1144 family. The anisotropy ofJcand its temperature dependence are determined. The exponentsαof theJc(B)dependences from 0.1 to 1 T are in a good agreement with the theoretical predictions for strong Abrikosov vortex pinning, which is also confirmed by the values of the pinning strength estimated using the Dew-Hughes model.

Magnetic flux trapping in hydrogen-rich high-temperature superconductors

Recent discoveries of superconductivity in various hydrides at high pressures have shown that a critical temperature of superconductivity can reach near-room-temperature values. However, experimental studies are limited by high-pressure conditions, and electrical transport measurements have been the primary technique for detecting superconductivity in hydrides. Here we implement a non-conventional protocol for the magnetic measurements of superconductors in a SQUID magnetometer and probe the trapped magnetic flux in two near-room-temperature superconductors H3S and LaH10 at high pressures. Contrary to traditional magnetic susceptibility measurements, the magnetic response from the trapped flux is almost unaffected by the background signal of the diamond anvil cell due to the absence of external magnetic fields. The behaviour of the trapped flux generated under zero-field-cooled and field-cooled conditions proves the existence of superconductivity in these materials. We reveal that the absence of a pronounced Meissner effect is associated with the very strong pinning of vortices inside the samples. This approach can also be a tool for studying multiphase samples or samples that have a low superconducting fraction at ambient pressure.

Vortex pinning and magnetic phase diagram of EuRbFe4As4 iron-based superconductor

We performed systematic AC susceptibility and magnetic moment measurements to investigate the vortex dynamics and pinning in single crystals as a function of temperature, frequency, and DC magnetic field. The vortex solid-liquid line was determined, and it can be fitted well with using β = 1.74 ± 0.1–1.91 ± 0.1, where Tp is the peak position on an imaginary part of the AC susceptibility plot (), for H ab. It indicates a rather high pinning strength of the vortex system. The activation energy U0 was determined from thermally activated flux creep theory and reached 6700 ± 1100 K at low fields, suggesting strong vortex pinning. A field dependence of U0(H ab) ∼ Ha with a = 0.47(4) suggests thermally activated plastic pinning or caused by planar defects. Magnetic moment measurements also confirmed strong pinning in the superconductor, and the superconducting response gives the main contribution to the M(H) hysteresis. Additionally, we show long-range ordering of the Eu2 + sublattice according to the AC susceptibility measurement data.

Proximity coupling of superconducting nanograins with fractal distributions

We explore the electrical and magnetic properties of a fractal assembly of Josephson junctions with transparent interfaces. For this purpose, we employ an $\mathrm{Mg}/\mathrm{MgO}/\mathrm{Mg}{\mathrm{B}}_{2}$ nanocomposite with \ensuremath{\sim}16 vol. % of $\mathrm{Mg}{\mathrm{B}}_{2}$ nanograins, which are distributed in a fractal manner in the normal matrix. Irrespective of the low volume fraction of $\mathrm{Mg}{\mathrm{B}}_{2}$ nanograins, the nanocomposite behaves as a bulk-like superconductor, i.e., zero resistivity, perfect diamagnetism, and strong vortex pinning. Thus, a global Josephson phase coherence is achieved in the nanocomposite. The lower (${H}_{\mathrm{c}1\mathrm{J}}$) and higher (${H}_{\mathrm{c}2\mathrm{J}}$) critical fields of the Josephson network are exceptionally high (${H}_{\mathrm{c}1\mathrm{J}}=96\phantom{\rule{0.16em}{0ex}}\mathrm{Oe}$ and ${H}_{\mathrm{c}2\mathrm{J}}=83.5\phantom{\rule{0.16em}{0ex}}\mathrm{kOe}$) as compared to those reported previously for granular superconductors. This will give an example of robust macroscopic superconducting coherence derived from long-range proximity coupling among fractally distributed superconducting nanograins through quantum interference of Andreev quasiparticles. Transverse-field muon spin rotation measurements reveal that the mean internal field in the superconducting mixed state increases with decreasing temperature below which the Josephson phase coherence sets in, opposite to the diamagnetic response observed in magnetization measurements. This unusual behavior implies a highly disordered and fluctuating nature of the Josephson vortices in the present superconducting nanocomposite.

Persistence of pinning and creep beyond critical drive within the strong pinning paradigm

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.

Generating mixed morphology BaZrO3 artificial pinning centers for strong and isotropic pinning in BaZrO3–Y2O3 double-doped YBCO thin films

High concentration artificial pinning centers (APCs), such as BaZrO3 nanorods (BZO 1D APCs) aligned along the c-axis of the high temperature superconductor YBa2Cu3O7 (YBCO) can provide strong pinning of magnetic vortices and are desirable for applications in high magnetic fields. Unfortunately, in YBCO films with single-doping (SD) of BZO 1D APCs, a monotonic decreasing superconducting Tc and critical current density Jc(H) with BZO doping has been observed due to strain field overlap at high-concentration perfectly c-axis aligned BZO 1D APCs. In order to resolve this issue, double-doping (DD) of 2–6 vol% BZO 1D APCs and 3.0 vol% Y2O3 nanoparticles (Y2O3-NPs) in YBCO films has been explored to promote BZO-NR orientation misalignment from the c-axis. Remarkably, a monotonic increasing Jc(H) with BZO 1D APCs concentration has been obtained in the BZO DD samples. Such a microstructure change is evidenced in the much smaller c-lattice parameter expansion of 0.103% in the DD samples as opposed to 0.511% in the SD counterparts and reduced c-axis alignment of the BZO 1D APCs as revealed in TEM. This yields a mixed 1D + 2D + 3D APC morphology and enhanced isotropic pinning with respect to the orientation of the H-field in the BZO DD samples.

Vortex dynamics in type-II superconductors under strong pinning conditions

We study effects of pinning on the dynamics of a vortex lattice in a type II superconductor in the strong-pinning situation and determine the force--velocity (or current--voltage) characteristic combining analytical and numerical methods. Our analysis deals with a small density $n_p$ of defects that act with a large force $f_p$ on the vortices, thereby inducing bistable configurations that are a characteristic feature of strong pinning theory. We determine the velocity-dependent average pinning-force density $\langle F_p(v)\rangle$ and find that it changes on the velocity scale $v_p \sim f_p/\eta a_0^3$, where $\eta$ is the viscosity of vortex motion and $a_0$ the distance between vortices. In the small pin-density limit, this velocity is much larger than the typical flow velocity $v_c \sim F_c/\eta$ of the free vortex system at drives near the critical force-density $F_c = \langle F_p(v=0)\rangle \propto n_p f_p$. As a result, we find a generic excess-force characteristic, a nearly linear force--velocity characteristic shifted by the critical force-density $F_c$; the linear flux-flow regime is approached only at large drives. Our analysis provides a derivation of Coulomb's law of dry friction for the case of strong vortex pinning.

Vortex pinning and irreversibility fields in FeS1–xSex (x = 0, 0.06)

We report strong vortex pinning and large irreversibility fields in single crystals of tetragonal FeS1–xSex (x = 0, 0.06). Vortex dynamics is characterized by crossover in field dependence of the depinning energy U0, indicative of single flux surface pinning to the region of collective flux pinning on point-like defects. The close proximity of the irreversibility lines to the upper critical field (Hc2) is consistent with strong pinning in FeS and FeS0.94Se0.06, pointing that new materials with building-blocks of FeS4 tetrahedra are likely to host high critical currents.

A route for a strong increase of critical current in nanostrained iron-based superconductors

The critical temperature Tc and the critical current density Jc determine the limits to large-scale superconductor applications. Superconductivity emerges at Tc. The practical current-carrying capability, measured by Jc, is the ability of defects in superconductors to pin the magnetic vortices, and that may reduce Tc. Simultaneous increase of Tc and Jc in superconductors is desirable but very difficult to realize. Here we demonstrate a route to raise both Tc and Jc together in iron-based superconductors. By using low-energy proton irradiation, we create cascade defects in FeSe0.5Te0.5 films. Tc is enhanced due to the nanoscale compressive strain and proximity effect, whereas Jc is doubled under zero field at 4.2 K through strong vortex pinning by the cascade defects and surrounding nanoscale strain. At 12 K and above 15 T, one order of magnitude of Jc enhancement is achieved in both parallel and perpendicular magnetic fields to the film surface.

Strong pinning of vortices by antiferromagnetic domain boundaries in CeCo(In1−xCdx)5

We have studied the isothermal magnetization of CeCo( Cdx)5 with x = 0.0075 and 0.01 down to 50 mK. Pronounced field-history dependent phenomena occur in the coexistence regime of the superconducting and antiferromagnetic phases. At low-fields, a phenomenological model of magnetic-flux entry well explains implying the dominance of bulk pinning effect. However, unless crystallographic quenched disorder is hysteretic, the asymmetric peak effect (ASPE) which appears at higher fields cannot be explained by the pinning of vortices due to material defects. Also, the temperature dependence of the ASPE deviates from the conventional scenario for the peak effect. Comparison of our thermodynamic phase diagrams with those from previous neutron scattering and magnetoresistance experiments indicates that the pinning of vortices takes place at the field-history dependent antiferromagnetic domain boundaries.

Vortex dynamics in phase separated Tl0.58Rb0.42Fe1.72Se2 crystals

Abstract We report the critical current density J c and the vortex dynamics in phase-separated Tl 0.58 Rb 0.42 Fe 1.72 Se 2 crystals by performing magnetization measurements. Structural investigation reveals micro- and nanoscopic phase separation between 122 (superconducting) and 245 (not superconducting) phases. Micrometric phase separation refers to 245 islands with typical diameters of 2 µm embedded in a multiply-connected 122 superconducting network. Nanoscopic phase separation refers to 245 nanoprecipitates embedded in the 122 superconducting paths. The 245 nanoprecipitates with size comparable to the coherence length produce strong vortex pinning. It was observed that the temperature dependence of the flux creep rate presents a peak at intermediate temperatures and magnetic fields lower than 0.5 T. The peak is systematically suppressed as the magnetic field is increased, and it could be related with relaxation generated by double-kink excitations. Double-kinks are low-energy depinning excitations usually associated with strong pinning produced by correlated disorder.

Anomalous superconducting phase diagram in UBe13

Surface impedance measurements Zs, where Zs consists of surface resistance Rs and surface reactance Xs, were carried out for a single crystal UBe13. Rs exhibits unusual field dependence with a significant hump around the H*, which is nearly equal to 0.6Hc2. This behavior indicates that the strong vortex pinning occurs around H*.

Vortices at nanoscale: Still some room at the bottom

Vortices as topological defects existon all scales of Nature from cosmicstrings to a dense quark matter.In macroscopic quantum systemslike superconductors, superfluidsand Bose-Einstein condensatesthe vortices are of special interest.There, they are quantized having“normal” cores where the superfluidorder parameter is zero and itsphase usually changes by 2

Dopant clustering, electronic inhomogeneity, and vortex pinning in iron-based superconductors

We use scanning tunneling microscopy to map the surface structure, nanoscale electronic inhomogeneity, and vitreous vortex phase in the hole-doped superconductor Sr${}_{0.75}$K${}_{0.25}$Fe${}_{2}$As${}_{2}$ with ${T}_{c}=32$ K. We find that the low-$T$ cleaved surface is dominated by a half Sr/K termination with $1\ifmmode\times\else\texttimes\fi{}2$ ordering and ubiquitous superconducting gap, while patches of gapless, unreconstructed As termination appear rarely. The superconducting gap varies by $\ensuremath{\sigma}/\overline{\ensuremath{\Delta}}=16%$ on a $\ensuremath{\sim}$3 nm length scale, with average $2\overline{\ensuremath{\Delta}}/{k}_{B}{T}_{c}=3.6$ in the weak-coupling limit. The vortex core size provides a measure of the superconducting coherence length $\ensuremath{\xi}=2.3$ nm. We quantify the vortex lattice correlation length at 9 T in comparison to several iron-based superconductors. The comparison leads us to suggest the importance of dopant size mismatch as a cause of dopant clustering, electronic inhomogeneity, and strong vortex pinning.

Dynamical Aspects of Strong Pinning of Magnetic Vortices in Type-II Superconductors

We determine the current-voltage characteristic of type-II superconductors in the presence of strong pinning centers. Focusing on a small density of defects, we derive a generic form for the characteristic with a linear flux-flow branch shifted by the critical current (excess-current characteristic). The details near onset, a hysteretic jump (for κ>>1) or a smooth velocity turn-on (κ→1), depend on the Labusch parameter κ characterizing the pinning centers. Pushing the single-pin analysis into the weak pinning domain, we reproduce the collective pinning results for the critical current.

Irreversibility Lines and Anomalous Meissner Effect in Ba(Fe1-xCox)2As2 Superconducting Crystals

Magnetization curves were measured in Ba(Fe 1-x Co x ) 2 As 2 crystals grown by an indium flux method. A very different vortex dynamics behavior was found when comparing results for crystals in the underdoped and slightly above the optimal doped regions. The irreversibility line is closer to the upper critical field line in the latter case, which indicates a much strong pinning of vortices for the near optimally doped crystal. Also, field cooled magnetization curves revealed an anomalous Meissner effect, where a diamagnetic response increases with the applied field, in both crystals. Possible mechanisms involving vortex pinning at interfaces or by local magnetic ions are presented.

Anisotropy of the coherence length from critical currents in the stoichiometric superconductor LiFeAs

Miniature Hall-probe arrays were used to measure the critical current densities for the three main directions of vortex motion in the stoichiometric LiFeAs superconductor. These correspond to vortices oriented along the c-axis moving parallel to the ab-plane, and to vortices in the ab-plane moving perpendicular to, and within the plane, respectively. The measurements were carried out in the low-field regime of strong vortex pinning, in which the critical current anisotropy is solely determined by the coherence length anisotropy parameter, {\epsilon}_{\xi}. This allows extraction of {\epsilon}_{\xi} at magnetic fields far below the upper critical field B_c2. We find that increasing magnetic field decreases the anisotropy of the coherence length.

The effect of BZO doping concentration and thickness dependent properties of YBCO films grown by PLD on buffered NiW substrates

The effect of BaZrO 3 (BZO) doping is systematically studied in YBa 2Cu 3O 6+ x (YBCO) thin films deposited by pulsed laser deposition (PLD) on buffered NiW substrates. Based on the structural and magnetic properties, the optimal BZO doping concentration is obtained to vary between 4 and 7.5 wt.%, depending mainly on applied magnetic field. This relatively high optimal concentration is linked to the nanograined target material and metal substrate that cause low-angle grain-boundaries and in-plane spread of YBCO crystals on NiW. Thickness dependent analysis of undoped and BZO-doped YBCO films predicts differences in growth mechanisms where early growth next to the substrate interface is 2D-type in BZO-doped films. This leads to the situation where crystallographic structure as well as superconducting properties are improved when the film develops and the thickness is increased. Therefore from the resistivity measurements a threshold thicknesses where reasonable properties occur are determined for both set of films. Measurements in thermally activated flux-flow regime (TAFF) indicate that above the threshold thickness relatively strong and isotropic vortex pinning is realized in BZO-doped YBCO films. Generally, this paper demonstrates that especially for thin film applications on NiW substrates even more compatible buffer layer structures should be utilized.

Vortex imaging and vortex lattice transitions in superconducting Sr2RuO4single crystals

Scanning Hall probe microscopy has been used to study vortex structures in very-high-quality single crystals of the unconventional superconductor Sr${}_{2}$RuO${}_{4}$ $({T}_{c}\ensuremath{\cong}1.5$ K). In none of our samples do we find credible evidence for the existence of the spontaneous fields or chiral domains predicted for the expected time-reversal symmetry-breaking order parameter. Even in our highest-quality samples we observe very strong vortex pinning and anomalous broadening of vortex profiles. The best samples also exhibit a clear field-driven triangular to square vortex lattice transition at low fields, as predicted by extended London theory calculations. In stark contrast, slightly less well-ordered samples exhibit pronounced vortex chaining/banding that we tentatively attribute to an extrinsic source of disorder.

Visualization of vortex motion in FeAs-based BaFe1.9Ni0.1As2 single crystal by means of magneto-optical imaging

Superconductivity has been found in newly discovered iron-based compounds. This paper studies the motion of magnetic vortices in BaFe1.9Ni0.1As2 single crystal by means of the magneto-optical imaging technique. A series of magneto-optical images reflecting magnetic flux distribution at the crystal surface were taken when the crystal was zero-field cooled to 10 K. The behavior of the vortices, including penetration into and expulsion from the single crystal with increasing and decreasing external fields, respectively, is discussed. The motion behavior is similar to that observed in high-Tc superconducting cuprates with strong vortex pinning; however, the flux-front is irregular due to randomly distributed defects in the crystal.