Vortex Pinning Research Articles

Tuning of anisotropy in Bi-based high temperature superconducting thin films

• Highly anisotropic Bi-based superconducting thin films. • Development of planar defects in thin films enhance vortex pinning. • Pb substitution strengthen 2D pancake arrangement of vortices in CuO planes. • Origin of anisotropy influence the axial dependence of pinning potential. The origin of the anisotropy behavior in the thin films of Bi 1.75 Pb 0.25 Sr 2 Ca 2 Cu 3 O 10±δ grown using pulsed laser deposition technique on three single crystalline substrates was investigated using magnetotransport measurements. Based on the evolution of the activation energy deduced from the magnetotransport data, it is surmised that Pb substitution in the present system strengthens the two-dimensional pancake arrangement of the vortices in the superconducting slab, which enhances the pinning of the vortices to the defect sites. The development of these planar defects with the decrease in the dimensionality of the sample was found to be the reason for the enhanced value of the pinning potential obtained in the thin film samples compared to the bulk. The interplay of Pb-assisted cation chemistry was found to have an influence on the anisotropy behavior in these films leading to an axial dependence of the pinning potential. Film grown on SrTiO 3 substrate with an anisotropy factor of ∼ 26 and higher superconducting parameters is the most anisotropic among the series of samples investigated.

Intrinsic vortex pinning in superconducting quasicrystals

We numerically show that a vortex pinning occurs in superconducting quasicrystal without impurities and defects. This vortex pinning is intrinsic since the superconducting order parameter in quasicrystal is always inhomogeneous due to the lack of the translational symmetry. We propose that experiments influenced by vortex pinning effects can detect the atomic-scale inhomogeneous superconducting order parameter in quasicrystals. We develop a numerical method to solve the Bogoliubov-de Gennes equations and gap equations in large systems, which is based on the localized-Krylov subspace and a sparse modeling technique. Two two-dimensional quasicrystals, the Penrose and Amman-Beenker tiling, are considered.

Critical current density and vortex phase diagram in the superconductor Sn0.55In0.45Te

Critical current density and vortex pinning dynamics have been studied in the superconductor ${\mathrm{Sn}}_{0.55}{\mathrm{In}}_{0.45}\mathrm{Te}$. Analysis of the temperature-dependent lower critical field shows that it has a weakly anisotropic single energy gap. The critical current density ${J}_{c}(0)$ and pinning potential ${U}_{0}(H)$ values reach as high as $2.56\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.16em}{0ex}}\mathrm{A}/{\mathrm{cm}}^{2}$ at 1.8 K and $2.1\ifmmode\times\else\texttimes\fi{}{10}^{3}$ K at ${\ensuremath{\mu}}_{0}H=0.01\phantom{\rule{3.33333pt}{0ex}}\mathrm{T}$, respectively. Based on the collective pinning model, we demonstrate the coexistence of vortex pinning regimes in ${\mathrm{Sn}}_{0.55}{\mathrm{In}}_{0.45}\mathrm{Te}$. One is a $\ensuremath{\delta}{T}_{\mathrm{c}}$ pinning regime induced by the spatial fluctuations of the transition temperature in a low field. The other is a dominantly $\ensuremath{\delta}l$ pinning regime associated with the spatial variations of the charge-carrier mean free path in a higher field. This causes a nonconstant exponent of the power-law behavior ${J}_{c}(T)\ensuremath{\propto}{H}^{n}$. A very weak vortex fluctuation is unveiled by a narrow separation between the irreversibility field ${\ensuremath{\mu}}_{0}{H}_{\mathrm{irr}}(T)$ and upper critical field ${\ensuremath{\mu}}_{0}{H}_{\mathrm{c}2}(T)$ in the vortex phase diagram. We discuss the potential application in superconducting electronics like the single-photon detector in thin film form.

Three-Dimensional Molecular Dynamics Method for Vortex pinning by Splayed Columnar Defects

We have developed three-dimensional molecular dynamics method, in which anisotropy of a superconductor and vortex core energy are included. We have applied it to a superconductor with splayed columnar defects. We have found that under a driving force due to an external current, vortices show wandering structures, which comes from the pinning force from splayed columnar defects. Especially, for weak core energy, vortices become soft or flexible. In an isotropic superconductor, these wandering structures disappear.

High Critical Current Density of Nanostructured MgB2 Bulk Superconductor Densified by Spark Plasma Sintering.

In situ MgB2 superconducting samples were prepared by using the spark plasma sintering method. The density of the obtained bulks was up to 95% of the theoretical value predicted for the material. The structural and microstructural characterizations of the samples were investigated using X-ray diffraction and SEM and correlated to their superconducting properties, in particular their critical current densities, Jc, which was measured at 20 K. Extremely high critical current densities of up to 6.75 × 105 A/cm2 in the self-field and above 104 A/cm2 at 4 T were measured at 20 K, indicating that vortex pinning is very strong. This property is mainly attributed to the sample density and MgB2 nanograins in connection to the presence of MgO precipitates and areas rich in boron.

Transition of vortex pinning behaviour induced by an artificial microstructure design in Ba(Fe0.94Co0.06)2As2 pnictide superconductor

Among various systems of iron-based superconductors, the doped Ba122 polycrystals have shown the largest application value because of their brilliant properties and chemical stability. Since there have been few reports on the vortex dynamics in Ba122 polycrystal, we firstly conduct a comparative investigation on the differences of vortex dynamics in Ba(Fe0.94Co0.06)2As2 single crystal, grain oriented polycrystal and random oriented powder in this paper. Herein, the polycrystal, which has c-axis grain texture, is prepared by the compaction of designed single crystal powder. The second magnetization peak (SMP) effects are observed in the hysteresis loops of single crystal and totally disappear in those of polycrystal and powder. Conventional magnetization relaxation and Maley's method are applied to analyse the vortex dynamics. In single crystal, the SMP effect is primarily related to the crossover of elastic-plastic vortex creep. The weak pinning in single vortex regime results in large magnetization relaxation rates (S) in low magnetic field regime. In polycrystal for H//AD (axial direction), the introduction of strong pins is effective to single vortex pinning, which brings about much lower S values in low magnetic field regime. Furthermore, the results of single crystal for H//ab, polycrystal for H//RD (radial direction) and random oriented powder are analyzed. Synergistic effects of strong pins and microscopic anisotropy on the vortex dynamics are discussed. These results will provide new knowledge for the design of high-performance Ba122 superconductors.

Optimizing vortex pinning in YBa2Cu3O7-x superconducting films up to high magnetic fields

The magnetic flux pinning capabilities of YBa2Cu3O7-x (YBCO) coated conductors vary strongly across different regions of the magnetic field–temperature phase diagram and with the orientation of the magnetic field θ. Here, we determine the optimal pinning landscape for a given region of the phase diagram by investigating the critical current density Jc(H,θ,T) in the 5–77 K temperature range, from self-field to high magnetic fields of 35 T. Our systematic analysis reveals promising routes for artificially engineering YBCO coated conductors in any region of interest of the phase diagram. In solution-derived nanocomposites, we identify the relevance of coexisting high amounts of short stacking faults, Cu-O vacancy clusters, and segmentation of twin boundaries, in combination with nanoparticles, for enhanced pinning performance at high magnetic fields and low temperatures. Moreover, we demonstrate that twin boundaries preserve a high pinning energy in thick YBCO films, which is beneficial for the pinning performance at high magnetic fields and high temperatures.

Effects of magnetic and non-magnetic doping on the vortex lattice in MgB2

Small-angle neutron scattering has been used to study the vortex lattice in superconducting MgB2 doped with either manganese or carbon to achieve a similar suppression of the critical temperature. Measurements were performed with the magnetic field applied along the c axis, where the vortex lattice in pure MgB2 is known to undergo a field- and temperature-driven 30° rotation transition. For Mn doping, the vortex lattice phase diagram remains qualitatively similar to that of pure MgB2, indicating only a modest effect on the vortex–vortex interaction. In contrast, the vortex lattice rotation transition is completely suppressed in the C-doped case, probably due to a change in the electronic structure which affects the two-band/two-gap nature of superconductivity in MgB2. The vortex lattice longitudinal correlation length shows the opposite behavior, remaining roughly unchanged between pure and C-doped MgB2 while it is significantly reduced in the Mn-doped case. However, the extensive vortex lattice metastability and related activated behavior, observed in conjunction with the vortex lattice transition in pure MgB2, are also seen in the Mn-doped sample. This shows that the vortex lattice disordering is not associated with a substantially increased vortex pinning.

Superconducting and structural properties of the noncentrosymmetric Re6Hf superconductor under high pressure

We report the effect of high pressure on the superconducting, vortex pinning, and structural properties of a polycrystalline non-centrosymmetric superconductor Re6Hf. The superconducting transition temperature, Tc, reveals a modest decrease as pressure P increases with a slope -0.046 K/GPa (-0.065 K/GPa) estimated from resistivity measurements up to 8 GPa (magnetization measurement ~ 1.1 GPa). Structural analysis up to ~18 GPa reveals monotonic decreases of lattice constant without undergoing any structural transition and a high value of bulk modulus B0= 333.63 GPa, indicating the stability of the structure. Furthermore, the upper critical field and lower critical field at absolute temperature (Hc2(0) & Hc1(0)) decreases slightly from the ambient pressure value as pressure increases up to 2.5 GPa. In addition, up to P ~ 2.5 GPa using thermally activated flux flow of vortices revealed a double linearity field dependence of activation energy of vortices, confirming the coexistence of single and collective pinning vortex states. Moreover, analysis of critical current density using the collective pinning theory showed the transformation of {\delta}Tc to {\delta}l pinning as pressure increases, possibly due to migration of grain boundaries. Besides, the band structure calculations using density functional theory show that density of states decreases modestly with pressure, which may be a possible reason for such a small decrease in Tc by pressure.

Analysis of local burnout in a sub-scale test coil for the 32 T magnet after spontaneous quenches during fast ramping

Industrial production of REBa2Cu3O7-δ (REBCO) coated conductors has made it possible to construct the 32 T magnet, the first successful all-superconducting user magnet to exceed 30 T, which now serves users as SCM4 (superconducting magnet) at the NHMFL. Here, we present an analysis of the damage that occurred in late-stage proof testing of the 32 T prototype coil after many essential facets of the design had been proven through more than 100 intentionally triggered quenches at fields up to 24 T. This prototype coil was then subjected to accelerated charge–discharge cycles at a rate 44 times faster than its design ramp rate in an attempt to address its fatigue tolerance. The extra hysteresis loss of the fast ramps led to heating of the end pancakes, which was induced after 55 fatigue cycles, three spontaneous quenches at progressively lower currents. Recognizing that the coil was damaged, the pancakes were then unwound and their REBCO tapes run through our continuous in-field transport I c and remnant-field magnetization monitoring device, YateStar, which revealed three highly localized zones of low I c in the end pancake that induced quench. Careful examination of these zones, especially the most intensely damaged one, revealed that the worst hot spot reached at least 779 °C during the quenches. Magneto-optical imaging showed that this damaged zone was about 5 mm in diameter and indeed the perpendicular damage length induced in neighboring turns by this localized quench heating was almost as great. Although there is much present concern about fatigue crack propagation from edge defects, we actually attribute this damage not to fatigue but to fluctuations in vortex pinning density due to imperfect BaZrO3 (BZO) nanorod growth that locally reduced the critical current I c. These localized low-I c regions then had to shed their excess current into the copper stabilizer, producing intense heating. We provide transmission and scanning electron microscopy evidence for local fluctuations of the BZO pinning structure and relate it to recent work that shows significant variations of 4 K, high field I c values due to apparent production fluctuations of the growth conditions of the Zr-doped metal-organic chemical vapor deposition REBCO used for this test magnet.

Self-Organized Nanocomposite Structure Controlled by Elemental Site Occupancy to Improve Vortex Pinning in YBa2Cu3O7 Superconducting Films

Self-Organized Nanocomposite Structure Controlled by Elemental Site Occupancy to Improve Vortex Pinning in YBa₂Cu₃O₇ Superconducting Films

Improving critical current density of Nb3Sn by optimizing pinning potential of grain boundary and grain size

Nb3Sn superconductor is of significant interest for applications in constructing high-field magnets beyond the limit of NbTi. However, its critical current density decreases rapidly at high magnetic fields (12 T) and the state-of-the-art level of Nb3Sn superconductors still cannot meet the requirements of the planned future accelerator magnets. The primary flux pinning centers for Nb3Sn wire mainly arise from the grain boundaries (GBs). In the present paper, we theoretically investigate, through time-dependent Ginzburg-Landau theory and with graphics-processing unit parallel technique, the vortex pinning and the critical current density in large-scale polycrystalline Nb3Sn superconductor by varying the pinning potential of GB and grain size at various magnetic fields. Unlike the conventional dot-like pinning systems, it is found that the critical current is not a monotonous function by suppressing the superconductivity of the GBs. The optimal pinning potential of GB for maximum critical current density strongly depends on magnetic fields. Furthermore, we find that the critical current density can be significantly enhanced by reducing grain size at low magnetic fields, while increase of critical current density cannot always be observed at high magnetic fields. Actually, critical current density even decreases by reducing grain size, which depends on the superconductivity of GBs. The findings in the paper provide theoretical foundations to achieve further improvement of Nb3Sn with optimizing the flux pinning.

Interface Engineering for Enhanced Magnetic Vortex Pinning by 1D-BZO APCs in a Wide Angular Range

Microstructural analysis of the BaZrO3 (BZO)/YBa2Cu3O7 (YBCO) interface has revealed a highly defective and oxygen deficient 2-3 nm thick YBCO column surrounding the BZO one-dimensional artificial pinning centers (1D-APCs). The resulting semi-coherent interface is the consequence of the ∼7.7% BZO/YBCO lattice mismatch and is responsible for the low pinning efficiency of BZO 1D-APCs. Herein, we report an interface engineering approach of dynamic Ca/Cu replacement on YBCO lattice to reduce/eliminate the BZO/YBCO lattice mismatch for improved pinning at a wide angular range of the magnetic field orientation. The Ca/Cu replacement induces a local elongation of the YBCO c-lattice near the BZO/YBCO interface, thereby ensuring a reduction in the BZO/YBCO lattice mismatch to ∼1.4% and a coherent BZO/YBCO interface. This has resulted in enhanced pinning at B//c-axis and a broad angular range of B-field orientation. For example, the 6 vol.% BZO/YBCO film with interface engineering exhibits F p ∼158 GN/m3 at 65 K and B//c-axis, which is 440% higher than the ∼36.1 GN/m3 for the reference 6% BZO/YBCO sample, and enhanced Jc and F p in a wide angular range up to ∼ 80°. This result illustrates a facile scheme for engineering 1D-APC/YBCO interface to resume the pristine pinning efficiency of the 1D-APCs.

No interface energy barrier and increased surface pinning in low temperature baked niobium

Superconducting Radio-Frequency cavities are currently made out of niobium. Niobium cavities are limited by the magnetic field on the cavity walls due to the entry of vortices at the field of first vortex penetration, H_{vp}. Low temperature baking in vacuum or low pressure gas atmosphere removes the strong decrease of the quality factor with accelerating gradient (high field Q-slope). Some cavities reach surface magnetic field above the lower critical field H_{c1}. One hypothesis for this performance increase is that the outer layer affected by the treatments acts as a barrier for vortex penetration (effective bilayer). Using a vibrating sample magnetometer the field of first flux penetration (H_{vp}) was measured for Nb ellipsoids with various low temperature treatments. All H_{vp} values were found to be consistent with the lower critical field, H_{c1}, as predicted for clean niobium. This led to the conclusion that a metastable flux free state above H_{c1} cannot be observed in DC magnetometry for low temperature baked niobium unlike for bilayers consisting of two superconductors as previously published. The effect of flux pinning differed significantly between treatments, suggesting that the high field Q-slope mitigation might be related to vortex pinning in the surface of the cavities.

Peak effect in a superconductor/normal-metal strip in a vortex-free state

We theoretically predict that the critical current ${I}_{c}$ and magnetization $M$ of a hybrid superconductor/normal-metal (SN) strip may have nonmonotonous dependence on a perpendicular magnetic field---so-called peak effect. In contrast to familiar peak effect, which is connected either with vortex entry to the superconductor or with peculiarities of vortex pinning, the found phenomenon exists at low fields, in the vortex-free (Meissner) phase. We argue that the effect appears at specific parameters of the studied hybrid structure when its in-plane current-supervelocity relation has two maxima. We expect that the same peak effect may exist in two-band superconductors (like ${\mathrm{MgB}}_{2}$) where similar current-supervelocity dependence was predicted at low temperatures.

Rising Speed Limits for Fluxons via Edge-Quality Improvement in Wide MoSi Thin Films

Ultra-fast vortex motion has recently become a subject of extensive investigations, triggered by the fundamental question regarding the ultimate speed limits for magnetic flux quanta and enhancements of single-photon detectors. In this regard, the current-biased quench of a dynamic flux-flow regime - flux-flow instability (FFI) - has turned into a widely used method for the extraction of information about the relaxation of quasiparticles (unpaired electrons) in the superconductor. However, the large relaxation times $\tau_\epsilon$ deduced from FFI for many superconductors are often inconsistent with the fast relaxation processes implied by their single-photon counting capability. Here, we investigate FFI in $15$ nm-thick $182$ $\mu$m-wide MoSi strips with rough and smooth edges produced by laser etching and milling by a focused ion beam. For the strip with smooth edges we deduce, from the current-voltage ($I$-$V$) curve measurements, a factor of 3 larger critical currents $I_\mathrm{c}$, a factor of 20 higher maximal vortex velocities of 20 km/s, and a factor of 40 shorter $\tau_\epsilon$. We argue that for the deduction of the intrinsic $\tau_\epsilon$ of the material from the $I$-$V$ curves, utmost care should be taken regarding the edge and sample quality and such a deduction is justified only if the field dependence of $I_\mathrm{c}$ points to the dominating edge pinning of vortices.

Critical current density in granular REBa2Cu3O7-δ superconductors with different interlayer magnetic coupling

Current-voltage (IV) characteristics of REBa2Cu3O7-δ have been used to extract critical current density (JcG and Jc) using an electric field (E) criterion below the onset critical temperature (Tc). By changing the intrinsic magnetic moment via four different rare earth (RE) atoms the interlayer coupling between superconducting planes is modified strongly. Variations of granular critical current density (JcG (T)) and total critical current density (Jc) are used to extract an exponent which we propose to be important to understand the nature of the vortex pinning by distributed magnetic moments. Also the effect of the grain boundary angles on pinning exponents extracted from both Jc (T) and JcG (T) has been discussed.

Magneto-optical studies of flux pinning in high-temperature superconductors

Abstract Quantitative magneto-optical imaging of magnetic flux distributions has developed in a powerful tool for the analysis of the local transport properties of superconductors. It allows a model-independent determination of the current density distribution of thin films and, thus, the local current density through individual defects. Also, local metastable properties are detectable, such as the local electric field distribution E with a high sensitivity down to 10–12 V/m caused by thermally activated flux creep. Based on these tools, in this paper we present a systematic comparison of vortex pinning, vortex movement and current transfer of two kinds of planar defects which are typically present in high-temperature superconducting thin films: low-angle grain boundaries and antiphase boundaries. Special attention is drawn to the local magnetic field dependence of the critical current density and to the spatial distribution of E, giving insight into the collective behavior of vortices at planar defects.

Mechanical frequency control in inductively coupled electromechanical systems

Nano-electromechanical systems implement the opto-mechanical interaction combining electromagnetic circuits and mechanical elements. We investigate an inductively coupled nano-electromechanical system, where a superconducting quantum interference device (SQUID) realizes the coupling. We show that the resonance frequency of the mechanically compliant string embedded into the SQUID loop can be controlled in two different ways: (1) the bias magnetic flux applied perpendicular to the SQUID loop, (2) the magnitude of the in-plane bias magnetic field contributing to the nano-electromechanical coupling. These findings are quantitatively explained by the inductive interaction contributing to the effective spring constant of the mechanical resonator. In addition, we observe a residual field dependent shift of the mechanical resonance frequency, which we attribute to the finite flux pinning of vortices trapped in the magnetic field biased nanostring.

Dynamical Mechanisms of Vortex Pinning in Superfluid Thin Films.

We characterize the mechanisms of vortex pinning in a superfluid thin film described by the two-dimensional Gross-Pitaevskii equation. We consider a vortex "scattering experiment" whereby a single vortex in a superfluid flow interacts with a circular, uniform pinning potential. By an analogy with linear dielectrics, we develop an analytical hydrodynamic approximation that predicts vortex trajectories, the vortex fixed point and the unpinning velocity. We then solve the Gross-Pitaevskii equation to validate this model, and build a phase portrait of vortex pinning. We identify two different dynamical pinning mechanisms marked by distinctive phonon emission signatures: one enabled by acoustic radiation and another mediated by vortex dipoles nucleated within the pin. Relative to obstacle size, we find that pinning potentials on the order of the healing length are more effective for vortex capture. Our results could be useful in mitigating the negative effects of drag due to vortices in superfluid channels, in analogy to maximizing supercurrents in type-II superconductors.