Flux-flow Resistivity Research Articles

Anomalous upper critical field in the quasicrystal superconductor Ta1.6Te

Superconductivity in quasicrystals poses a new challenge in condensed matter physics. We measured the resistance and ac magnetic susceptibility of a Ta1.6Te dodecagonal quasicrystal, which is superconducting below Tc ~ 1 K. We show that the upper critical field increases linearly with a large slope of − 4.4 T/K with decreasing temperature down to 0.04 K, with no tendency to level off. The extrapolated zero-temperature critical field exceeds the Pauli limit by a factor of 2.3. We also observed flux-flow resistance with thermally activated behavior and an irreversibility field that is distinct from the upper critical field. We discuss these peculiarities in terms of the nonuniform superconducting gap and spin-orbit interaction in quasicrystal structures.

Guided vortex motion in a ferromagnet-superconductor bilayer

Vortex motion in ferromagnetic-superconducting bilayers constituted by thin manganite La0.67Sr0.33MnO3 film deposited on the top of thin YBa2Cu3O7−δ film has been investigated. Both films were deposited on uniformly twinned single-crystalline substrates and patterned into strips directed at different angles to the twin lines pattern. These conditions lead to the guided vortex motion at different angles to the periodic pinning potential. It has been established that critical current and flux flow resistance in such an anisotropic pinning landscape-scale follow the elliptic dependence on the angle of the direction of the strip.

Vortex dynamics in NbTi films at high frequency and high DC magnetic fields

We report on the characterization of NbTi films at sim 11 GHz and in DC magnetic fields up to 4 T, performed by means of the coplanar waveguide resonator technique, providing quantitative information about the penetration depth, the complex impedance, and the vortex-motion-induced complex resistivity. This kind of characterization is essential for the development of radiofrequency cavity technology. To access the vortex-pinning parameters, the complex impedance was analyzed within the formalism of the Campbell penetration depth. Measurements in this frequency range allowed us to determine the complete set of vortex-pinning parameters and the flux flow resistivity, both analyzed and discussed in the framework of high-frequency vortex dynamics models. The analysis also benefits from the comparison with results obtained by a dielectric-loaded resonator technique on similar samples and by other ancillary structural and electromagnetic characterization techniques that provide us with a comprehensive picture of the material. It turns out that the normalized flux flow resistivity follows remarkably well the trend predicted by the time dependent Ginzburg-Landau theory, while the pinning constant exhibits a decreasing trend with the field which points to a collective pinning regime.

Measurements of Surface Impedance in MgB2 in DC Magnetic Fields: Insights in Flux-Flow Resistivity

We present the multifrequency measurements of the surface resistance of spark-plasma-sintered MgB2 performed through a dielectric loaded resonator operating at 16.5 and 26.7 GHz. By normally applying magnetic fields ≤1.2 T to the sample surface, we drove it in the mixed state. By means of data-rooted analysis, we found that the sample vortex dynamics could be fully described within a single-component approach. Pinning phenomena were present and characterized by a depinning frequency smaller than the measurement ones. The multiband nature of the superconductor emerged in the flux-flow resistivity, whose field dependence could be interpreted well within theoretical models. By exploiting them, the upper critical field was extracted in the low-temperature range, which exhibited a consistent temperature trend with the values obtained at the onset of the resistive transition near Tc, and was well in line with literature data on other polycrystalline samples.

A Noninductive Bifilar Coil to Design Compact Flux Pumps for HTS Magnets

High-temperature superconducting (HTS) flux pump is a promising way of energizing superconducting magnets without direct electrical contacts. It can remove the resistive heating and heat leakage from the current leads at room temperature. It is well known that applying the current over the critical current of the superconductor will force the superconducting layer to enter into the flux flow regime. This flux flow resistivity generates a dc voltage across the length of the HTS tape. This phenomenon is well understood; however, it is difficult to generate a high dc charging voltage without a high current. In this article, we demonstrate a novel flux pumping technique by using a noninductive bifilar bridge wound in parallel to the HTS coil. This configuration can generate large dc voltages using relatively small currents. It results in effectively pumping dc currents equal to the critical current for large HTS magnets.

Effect of S-doping on the magnetic and electrical properties of FeSe superconductor

Here we report on the magnetic and magneto-transport behavior of Fe-based superconductor with nominal compositions FeSe1−xSx (x=0.1 and 0.2). The impact of external hydrostatic pressure (P) and chemical pressure on the superconducting state of the FeSe system has been investigated in this work. Superconducting transition temperature (Tc) is found to be shifted towards higher temperatures with 10% S doping at the Se site of the FeSe alloy. On further increase in the doping concentration (for x=0.2), the superconducting Tc decreases. We have also observed a significant effect of external P on the superconducting properties of these S-doped alloys. Different superconducting parameters calculated from our experimental data confirm the type-II superconducting nature of the studied compounds. The signature of flux flow resistance is observed in the isothermal resistivity data as a function of the external magnetic field.

Josephson vortices and intrinsic Josephson junctions in the layered iron-based superconductor Ca10(Pt3As8)((Fe0.9Pt0.1)2As2)5

Modulated electronic state due to the layered crystal structures brings about moderate anisotropy of superconductivity in the iron-based superconductors and thus Abrikosov vortices are expected in the mixed state. However, based on the angular and temperature dependent transport measurements in iron-based superconductor Ca10(Pt3As8)((Fe0.9Pt0.1)2As2)5 with T c ≃ 12 K, we find clear evidences of a crossover from Abrikosov vortices to Josephson vortices at a crossover temperature T* ≃ 7 K, when the applied magnetic field is parallel to the superconducting FeAs layers, i.e., the angle between the magnetic field and the FeAs layers θ = 0°. This crossover to Josephson vortices is demonstrated by an abnormal decrease (increase) of the critical current (flux-flow resistance) below T*, in contrast to the increase (decrease) of the critical current (flux-flow resistance) above T* expected for Abrikosov vortices. Furthermore, when θ is larger than 0.5°, the flux-flow resistance and critical current have no anomalous behaviors across T*. These anomalous behaviors can be understood in terms of the distinct transition from the well-pinned Abrikosov vortices to the weakly-pinned Josephson vortices upon cooling, when the coherent length perpendicular to the FeAs layers ξ ⊥ becomes shorter than half of the interlayer distance d/2. These experimental findings indicate the existence of intrinsic Josephson junctions below T* and thus quasi-two-dimensional superconductivity in Ca10(Pt3As8)((Fe0.9Pt0.1)2As2)5, similar to those in the cuprate superconductors.

Electronic States and Energy Dissipations of Vortex Core in Pure FeSe Single Crystals Investigated by Microwave Surface Impedance Measurements

In order to clarify electronic states and energy dissipations due to a motion of a vortex core in pure FeSe, which is a candidate superconductor possessing a super-clean core, we measured the microwave surface impedance of pure FeSe single crystals under finite magnetic fields. From the magnetic-field dependence of the flux-flow resistivity, we found that a barometer of electronic states inside the vortex core $\omega_{0}\tau_{\rm core}$ is $1\pm0.5$, suggesting that the vortex core of pure FeSe is in the moderately clean regime contrary to the expectation of the super-clean core. We also found that the mean-free path inside the vortex core is suppressed at the distance of the order of the core radius. Based on observed results and previous reports, we discussed possible origins of rather small $\omega_{0}\tau_{\rm core}$ value in terms of the multiple-bands nature of FeSe and additional mechanisms producing extra energy dissipations specific to the vortex core in motion.

Interplay between Vortex Dynamics and Superconducting Gap Structure in Layered Organic Superconductors

Layered organic superconductors motivate intense investigations because they provide various unexpected issues associated with their low dimensionality and the strong electron correlation. Since layered organic superconductors possess simple Fermi surface geometry and they often share similarities to the high temperature oxide superconductors and heavy fermion compounds, research on layered organic superconductors is suitable for understanding the essence and nature of strongly correlated electron systems. In strongly correlated electron systems, one of the central problems concerning the superconducting (SC) state is the symmetry of the SC gap, which is closely related to the paring mechanism. Thus, experimental determination of the SC gap structure is of essential importance. In this review, we present the experimental results for the in-plane angular variation of the flux-flow resistance in layered organic superconductors k-(ET)2Cu(NCS)2, β″-(ET)2SF5CH2CF2SO3, and λ-(BETS)2GaCl4. The interplay between the vortex dynamics and nodal structures is discussed for these superconductors.

Linear and nonlinear flux-flow behaviors in superconducting hot-electron bolometer mixers

We study experimentally the resistive behaviors of a niobium nitride superconducting hot-electron bolometer (HEB) device driven by magnetic field and terahertz radiation. We find that the resistance of the superconducting HEB device emerges due to time-dependent changes of the macroscopic quantum phase by flux-flow events. The flux-flow resistance is found to be increased linearly with magnetic field at low vortex velocities and is enhanced by terahertz radiation. At high vortex velocities, there is an obvious non-linear change, which can be well interpreted by viscous flux-flow instability predicted by the Larkin and Ovchinnikov theory. In addition, we demonstrate a technique taking use of the device resistance dependence on magnetic field to stabilize the superconducting HEB device.

Pinning, Flux Flow Resistivity, and Anisotropy of Fe(Se,Te) Thin Films From Microwave Measurements Through a Bitonal Dielectric Resonator

We report on the anisotropy of the vortex motion surface impedance of a FeSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> thin film grown on a CaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> substrate. The dependence on the magnetic field intensity up to 1.2 T and direction, both parallel and perpendicular to the sample c-axis, was explored at fixed temperature at two distinct frequencies, ~ 16 GHz and ~ 27 GHz, by means of a bitonal dielectric resonator. The free flux flow resistivity ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ff</sub> was obtained by exploiting standard models for the high frequency dynamics, whereas the angle dependence was studied in the framework of the well known and widely used Blatter-Geshkenbein-Larkin (BGL) scaling theory for anistropic superconductors. Excellent agreement with the scaling law prescription by the fluxon flux flow resistivity was obtained. From the scaling analysis, a low-field mass anisotropy ~ 1.8was obtained, well within the value ranges reported in literature. The angular dependence of the pinning constant suggests that pinning is dominated by random, isotropic point pins, consistently with critical current density measurements.

Microwave measurements of the high magnetic field vortex motion pinning parameters in Nb3Sn

The high frequency vortex motion in Nb3Sn was analyzed in this work up to 12 T. We used a dielectric loaded resonator tuned at 15 GHz to evaluate the surface impedance Z of a Nb3Sn bulk sample (24.8 at.%Sn). From the field induced variation of Z, the high frequency vortex parameters (the pinning constant k p , the depinning frequency ν p and the flux flow resistivity ρ ff ) were obtained over a large temperature and field range; their field and temperature dependencies were analyzed. Comparison with other superconducting materials shows that high frequency applications in strong magnetic fields are also feasible with Nb3Sn. In the present work, we report the first measurements about the microwave response in Nb3Sn in strong magnetic fields.

Influence of variable Ca-doping on the critical current density of low-angle grain boundaries in YBa2Cu3O7−d

The rapid decrease in the critical current density Jc with grain boundary (GB) misorientation angle θ strongly limits the current-carrying capability of yttrium–barium–copper-oxide (YBCO), and the residual low-angle GB distribution is the most important current-limiting mechanism in biaxially textured YBCO coated conductors. To deepen the understanding of the Ca doping in low-angle GBs in YBCO, transport characteristics of low-angle GBs in YBa2Cu3O7−d bicrystals with different Ca contents x (Y1−xCaxBa2Cu3O7−d) were examined to verify the influence of the carrier density at the GB due to the Ca doping level. Y1−xCaxBa2Cu3O7−d (x = 0, 0.10, 0.15, and 0.30) epitaxial films were deposited by pulsed laser deposition on single crystal SrTiO3 (STO) substrates and 7° and 9° symmetric [001]-tilt bicrystal STO substrates. We found that 15% Ca doping was the most effective at increasing the Jc behavior across the 7° GB in intermediate fields, completely eliminating the GB dissipation signature in the V–I characteristics. For the 9° GB, 10% Ca doping yielded the highest depairing current Jd across the GB, calculated from the flux flow resistivity, even though a significant GB dissipation signature remained in the V–I curves.

Microwave properties of Fe(Se,Te) thin films in a magnetic field: pinning and flux flow

We present here a microwave (16 GHz) investigation of the complex surface impedance in a dc magnetic field (up to 1 T) of Fe(Se,Te) thin ( 300 nm) films. We derive the vortex parameters: the flux-flow resistivity yields information on the dynamics of the quasiparticles, the Labusch parameter yields a measure of the steepness of the pinning potential wells, and the depinning frequency assesses the frequency range where the material is suitable for high-frequency applications in a dc magnetic field. We compare the results to the data obtained in YBCO, Nb and Nb3Sn.

Superconducting-Gap Anisotropy of Iron Pnictides Investigated via Combinatorial Microwave Measurements

One of the most significant issues for superconductivity is clarifying the momentum-dependent superconducting gap Δ({boldsymbol{k}}), which is closely related to the pairing mechanism. To elucidate the gap structure, it is essential to investigate Δ({boldsymbol{k}}) in as many different physical quantities as possible and to crosscheck the results obtained in different methods with each other. In this paper, we report a combinatorial investigation of the superfluid density and the flux-flow resistivity of iron-pnictide superconductors; LiFeAs and BaFe2(As1−xPx)2 (x = 0.3, 0.45). We evaluated Δ({boldsymbol{k}}) by fitting these two-independent quantities with a two-band model simultaneously. The obtained Δ({boldsymbol{k}}) are consistent with the results observed in angle-resolved photoemission spectroscopy (ARPES) and scanning-tunneling spectroscopy (STS) studies. We believe our approach is a powerful method for investigating Δ({boldsymbol{k}}) because it does not require a sample with clean surface unlike ARPES and STS experiments, or a rotational magnetic-field system for direct measurements of the angular dependence of thermodynamic quantities.

Physics of vortex motion by means of microwave surface impedance measurements (Review article)

In this paper we present a short overview on the results that can be obtained through the study of vortex motion at high frequencies. The phenomenological force balance for isolated-like vortices shaken by microwave currents and subjected to viscous drag, pinning forces and thermal creep is recalled and physically presented. The derived vortex motion resistivity, together with the main vortex parameters (viscosity η and flux flow resistivity ρff, pinning constant kp, creep factor χ), is then commented. Sample measurements are reported to illustrate the main aspects of the involved physical models.

Angular magnetic-field dependence of vortex matching in pinning lattices fabricated by focused or masked helium ion beam irradiation of superconducting YBa2Cu3O7−δ thin films

The angular dependence of magnetic-field commensurability effects in thin films of the cuprate high-critical-temperature superconductor YBa2Cu3O7−δ (YBCO) with an artificial pinning landscape is investigated. Columns of point defects are fabricated by two different methods of ion irradiation — scanning the focused 30 keV ion beam in a helium ion microscope or employing the wide-field 75 keV He+ beam of an ion implanter through a stencil mask. Simulations of the ion-target interactions and the resulting collision cascades reveal that with both methods square arrays of defect columns with sub-μm spacings can be created. They consist of dense point-defect clusters, which act as pinning centers for Abrikosov vortices. This is verified by the measurement of commensurable peaks of the critical current and related minima of the flux-flow resistance vs magnetic field at the matching fields. In oblique magnetic fields, the matching features are exclusively governed by the component of the magnetic field parallel to the axes of the columnar defects, which confirms that the magnetic flux is penetrated along the defect columns. We demonstrate that the latter dominate the pinning landscape despite of the strong intrinsic pinning in thin YBCO films.

Intrinsic anisotropy and pinning anisotropy in nanostructured YBa2Cu3O7−δ from microwave measurements

Anisotropy is an intrinsic factor that dictates the magnetic properties of YBa2Cu3O7−δ and therefore has a great impact on many applications. Artificial pinning centres are often introduced in an attempt to mitigate its effect, resulting in less anisotropic electrical and magnetic properties. However, the nanoengineering of superconductors makes the quantification of the anisotropy itself uncertain: the intrinsic anisotropy due to the layered structure, quantified by the anisotropy factor γ, mixes with the additional anisotropy due to pinning. As a consequence, there is no consensus on the experimental anisotropy factor γ that can result in YBa2Cu3O7−δ when directional (twin planes, nanorods) or isotropic defects are present. We present measurements of the magnetic field and angular dependent surface impedance in very different nanostructured YBa2Cu3O7−δ films, grown by a chemical route and by pulsed laser deposition with different kind of defects (nanorods, twin planes, nanoparticles). We show that the surface impedance measurements are able to disentangle the intrinsic anisotropy from the directional pinning anisotropy, thanks to the possibility of extracting the true anisotropic flux–flow resistivity and by correctly exploiting the angular scaling. We find in all films that the intrinsic anisotropy γ = 5.3 ± 0.7. By contrast, the pinning anisotropy determines a much more complex, feature–rich and nonuniversal, sample–dependent angular landscape.

Collective flux pinning in hexatic vortex fluid in a-MoGe thin film

We investigate the magnetic field variation of the thermally activated flux flow resistivity, ρTAFF and flux flow critical current density, Jc, in a weakly pinned thin film of the amorphous superconductor a-MoGe, where vortices are in a fluid state over a large range of magnetic fields. We show that both quantities can be understood within the framework of collective pinning theory. In particular, our results demonstrate that a ‘peak effect’ can arise at the order–disorder transition of the vortex lattice even when both the ordered and disordered states are vortex fluids, such as the boundary between a hexatic vortex fluid and an isotropic vortex liquid.

Vortex Pinning and Flux Flow Microwave Studies of Coated Conductors

Demanding microwave applications in a magnetic field require the material optimization not only in zero-field but, more important, in the in-field flux motion dominated regime. However, the effect of artificial pinning centers (APC) remains unclear at high frequency. Moreover, in coated conductors the evaluation of the high frequency material properties is difficult due to the complicated electromagnetic problem of a thin superconducting film on a buffered metal substrate. In this paper we present an experimental study at 48 GHz of 150-200 nm YBa$_2$Cu$_3$O$_{7-x}$ coated conductors, with and without APCs, on buffered Ni-5at%W tapes. By properly addressing the electromagnetic problem of the extraction of the superconductor parameters from the measured overall surface impedance $Z$, we are able to extract and to comment on the London penetration depth, the flux flow resistivity and the pinning constant, highlighting the effect of artificial pinning centers in these samples.