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Manipulating the superconducting states of high transition temperature (high-Tc) cuprate superconductors in an efficient and reliable way is of great importance for their applications in next-generation electronics. Here, employing ionic liquid gating, a selective control of volatile and non-volatile superconductivity is achieved in pristine insulating Pr2CuO4±δ (PCO) films, based on two distinct mechanisms. Firstly, with positive electric fields, the film can be reversibly switched between superconducting and non-superconducting states, attributed to the carrier doping effect. Secondly, the film becomes more resistive by applying negative bias voltage up to − 4 V, but strikingly, a non-volatile superconductivity is achieved once the gate voltage is removed. Such phenomenon represents a distinctive route of manipulating superconductivity in PCO, resulting from the doping healing of oxygen vacancies in copper-oxygen planes as unravelled by high-resolution scanning transmission electron microscope and in situ X-ray diffraction experiments. The effective manipulation of volatile/non-volatile superconductivity in the same parent cuprate brings more functionalities to superconducting electronics, as well as supplies flexible samples for investigating the nature of quantum phase transitions in high-Tc superconductors.

Superconductors are well known for their ability to screen out magnetic fields. In type-II superconductors, as the magnetic field pressure is progressively increased, magnetic flux accumulates at the periphery of the sample, very much like charges accumulate in a capacitor when voltage is increased. As for capacitors, exceeding certain threshold field causes the blocked magnetic flux to abruptly penetrate into the sample. This phenomenon, triggered by a thermomagnetic instability, is somewhat analogous to the dielectric breakdown of the capacitor and leaves behind a similar Lichtenberg imprinting. Even though electrical breakdown threshold has been extensively studied in dielectrics, little information is known about the statistical distribution of the thermomagnetic breakdown in superconductors. In this work, we address this problem by performing magneto-optical imaging experiments on a Nb film where nanometric heating elements are used to rapidly erase the magnetic history of the sample. We demonstrate that the size and shape distributions of avalanches permits to unambiguously identify the transition between two regimes where either thermal diffusivity or magnetic diffusivity dominates. Clear criteria for discriminating athermal dynamic avalanches from thermally driven avalanches are introduced. This allows us to provide the first precise determination of the threshold field of the thermomagnetic breakdown and unveil the details of the transition from finger-like magnetic burst to dendritic branching morphology. These findings open a new avenue in the interdisciplinary exploration of catastrophic avalanches through non destructive repeatable experiments.

Despite the ubiquity of Raman spectroscopy, fluorescence, poor signal strength and photobleaching pose a significant challenge to researchers in the biomedical field. Here, we demonstrate a 17-fold signal enhancement in Raman spectra of crystal violet via surface-enhanced Raman scattering (SERS). The SERS substrate was fabricated by electron beam lithography (EBL); the nanostructured surface was an array of G-shaped elements made of Au on SiO2/Si. In addition to the SERS spectra, finite-difference time-domain simulations were performed to illustrate the distribution of electric-field hot-spots on the SERS substrate. The electric-field hot-spots were prominent at the vertices and edges of the nanostructured G-shaped motifs. The results presented here demonstrate that EBL is a high-end choice for SERS substrate fabrication that opens the way for more complex Raman spectroscopies, for instance involving nonlinear optics or chiral analytes.

In this work, we show that targeted and controlled modifications of the Josephson-junction properties of a bridge-type $\mathrm{Nb}$ nanoSQUID can be achieved by an electroannealing process allowing us to tune and tailor the response of a single device. The electroannealing consists in substantial Joule heating produced by large current densities followed by a rapid temperature quench. We report on a highly nontrivial evolution of the material properties when performing subsequent electroannealing steps. As the current density is increased, an initial stage characterized by a modest improvement of the superconducting critical temperature and normal-state conductivity of the bridges, is observed. This is followed by a rapid deterioration of the junction properties, i.e., decrease of critical temperature and conductivity. Strikingly, further electroannealing leads to a noteworthy recovery before irreversible damage is produced. Within the electroannealing regime where this remarkable resurrection of the superconducting properties are observed, the nanoSQUID can be operated in nonhysteretic mode in the whole temperature range and without compromising the critical temperature of the device. The proposed postprocessing is particularly appealing in view of its simplicity and robustness.

A graded distribution of pinning centers (antidots) in superconducting MoGe thin films has been investigated by magnetization and magneto-optical imaging. The pinning landscape has maximum density at the border, decreasing progressively towards the center. At high temperatures and low fields, where this landscape mimics the vortex distribution predicted by the Bean model, an increase of the critical current is observed. At low temperatures and fields, the superconducting performance of the non-uniform sample is also improved due to suppression of thermomagnetic avalanches. These findings emphasize the relevance of non-uniform pinning landscapes, so far experimentally unexplored, on the enhancement of pinning efficiency.

Quantitative magneto-optical imaging of a type-II superconductor thin film cooled under zero, homogeneous, and inhomogeneous applied magnetic fields, indicates that the latter procedure leads to an enhancement of the screening capacity. Such an observation is corroborated by both B-independent and B-dependent critical state model analyses. Furthermore, repulsive (attractive) vortex-(anti)vortex interactions were found to have a decisive role in the shielding ability, with initial states prepared with vortices resulting in a shorter magnetic flux front penetration depth than those prepared with antivortices. The proposed strategy could be implemented to boost the performance of thin superconducting devices., Comment: 6 pages and 5 figures

Considering a noncentrosymmetric pinning texture composed of a square array of triangular holes, the magnetic flux penetration and expulsion are investigated experimentally and theoretically. A direct visualization of the magnetic landscape obtained using a magneto-optical technique on a Nb film is complemented by a multiscale numerical modeling. This combined approach allows the magnetic flux dynamics to be identified from the single flux quantum limit up to the macroscopic electromagnetic response. Within the theoretical framework provided by time-dependent Ginzburg-Landau simulations, an estimation of the in-plane current anisotropy is obtained and its dependence with the radius of the curvature of hole vertices is addressed. These simulations show that current crowding plays an important role in channeling the flux motion, favoring hole-to-hole flux hopping rather than promoting interstitial flux displacement in between the holes. The resulting anisotropy of the critical current density gives rise to a distinct pattern of discontinuity lines for increasing and decreasing applied magnetic fields, in sharp contrast to the invariable patterns reported for centrosymmetric pinning potentials. This observation is partially accounted for by the rectification effect, as demonstrated by finite-element modeling. At low temperatures, where magnetic field penetration is dominated by thermomagnetic instabilities, highly directional magnetic flux avalanches with a fingerlike shape are observed to propagate along the easy axis of the pinning potential. This morphology is reproduced by numerical simulations. Our findings demonstrate that anisotropic pinning landscapes and, in particular, ratchet potentials produce subtle modifications to the critical state field profile that are reflected in the distribution of discontinuity lines.

The past years have witnessed major advancements in all-electrical doping control on cuprates. In the vast majority of cases, the tuning of charge carrier density has been achieved via electric field effect by means of either a ferroelectric polarization or using a dielectric or electrolyte gating. Unfortunately, these approaches are constrained to rather thin superconducting layers and require large electric fields in order to ensure sizable carrier modulations. In this work, we focus on the investigation of oxygen doping in an extended region through current-stimulated oxygen migration in YBa2Cu3O7−δ superconducting bridges. The underlying methodology is rather simple and avoids sophisticated nanofabrication process steps and complex electronics. A patterned multiterminal transport bridge configuration allows us to electrically assess the directional counterflow of oxygen atoms and vacancies. Importantly, the emerging propagating front of current-dependent doping δ is probed in situ by optical microscopy and scanning electron microscopy. The resulting imaging techniques, together with photoinduced conductivity and Raman scattering investigations, reveal an inhomogeneous oxygen vacancy distribution with a controllable propagation speed permitting us to estimate the oxygen diffusivity. These findings provide direct evidence that the microscopic mechanism at play in electrical doping of cuprates involves diffusion of oxygen atoms with the applied current. The resulting fine control of the oxygen content would permit a systematic study of complex phase diagrams and the design of electrically addressable devices., The authors acknowledge the financial support from the Fonds de la Recherche ScientifiqueFNRS, Spanish Ministry of Economy and Competitiveness through the Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015- 0496), SUMATE project (RTI2018-095853-B-C21), cofinanced by the European Regional Development Fund, the COST programme NanoCoHybri (CA 16218), and from the Catalan Government with 2017 SGR 1519. The work of A. Fernańdez-Rodriǵuez has been done in the framework of the doctorate in Materials Science of the Autonomous University of Barcelona (UAB) and is supported by the Spanish Ministry of Economy (BES-2016-077310). The work of A. V. Silhanek is partially supported by the grant CDR J.0151.19 of the F.R.S.- FNRS. S. Blanco Alvarez, S. Melinte, and A. V. Silhanek acknowledge financial support through the grant PDR T.0106.16 of the F.R.S.-FNRS. S. Marinković acknowledges support from F.R.S.-FNRS (Research Fellowship ASP). B. Maiorov was supported by the U.S. DOE, Office of Science, BES, Materials Sciences and Engineering Division. The authors thank the ULiege Microscopy facility CAREM for the SEM investigations. G. Rius acknowledges the Ramon y Cajal grant RYC-2016-21412.

We numerically investigate the effect of an edge indentation on the threshold field of thermomagnetic instabilities in superconducting films subjected to a ramping magnetic field, applied perpendicular to the plane of the film. In particular, we are able to address the question on whether edge indentations promote magnetic flux avalanches. For the magnetic field-independent critical current density model, the triggering of the first magnetic flux avalanche systematically occurs at the edge indentation. In contrast to that, for the more realistic field-dependent critical current density model the first flux avalanche can take place either at or away from the indentation. This selective triggering of magnetic flux avalanches is shown to result from two effects. Namely, (i) the variation of the threshold magnetic field for the first flux avalanche triggered at the indentation and (ii) the reduction of the critical current density by large local magnetic fields at the tip of the indentation which translates in a lower power density dissipated near the tip. We demonstrate that this interplay can be tuned by varying the indentation size, ramp rate of applied field ${\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{H}}_{a}$, and working temperature ${T}_{0}$. We build up a phase diagram in the ${\ensuremath{\mu}}_{0}{\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{H}}_{a}\ensuremath{-}{T}_{0}$ plane with well-defined boundaries separating three distinct regimes of thermomagnetic instability.

Efficient conversion of a spin signal into an electric voltage in mainstream semiconductors is one of the grand challenges of spintronics. This process is commonly achieved via a ferromagnetic tunnel barrier where non-linear electric transport occurs. In this work, we demonstrate that non-linearity may lead to a spin-to-charge conversion efficiency larger than 10 times the spin polarization of the tunnel barrier when the latter is under bias of a few mV. We identify the underlying mechanisms responsible for this remarkably efficient spin detection as the tunnel barrier deformation and the conduction band shift resulting from a change of applied voltage. In addition, we derive an approximate analytical expression for the detector spin sensitivity $P_{\textrm{det}}(V)$. Calculations performed for different barrier shapes show that this enhancement is present in oxide barriers as well as in Schottky tunnel barriers even if the dominant mechanisms differs with the barrier type. Moreover, although the spin signal is reduced at high temperatures, it remains superior to the value predicted by the linear model. Our findings shed light into the interpretation and understanding of electrical spin detection experiments and open new paths to optimize the performance of spin transport devices., 11 pages, 9 figures

Superconductors are well known for their ability to screen out magnetic fields. In type-II superconductors, as the magnetic field pressure is progressively increased, magnetic flux accumulates at the periphery of the sample, very much like charges accumulate in a capacitor when voltage is increased. As for capacitors, exceeding certain threshold field causes the blocked magnetic flux to abruptly penetrate into the sample. This phenomenon, triggered by a thermomagnetic instability, is somewhat analogous to the dielectric breakdown of the capacitor and leaves behind a similar Lichtenberg imprinting. Even though electrical breakdown threshold has been extensively studied in dielectrics, little information is known about the statistical distribution of the thermomagnetic breakdown in superconductors. In this work, we address this problem by performing magneto-optical imaging experiments on a Nb film where nanometric heating elements are used to rapidly erase the magnetic history of the sample. We demonstrate that the size and shape distributions of avalanches permits to unambiguously identify the transition between two regimes where either thermal diffusivity or magnetic diffusivity dominates. Clear criteria for discriminating athermal dynamic avalanches from thermally driven avalanches are introduced. This allows us to provide the first precise determination of the threshold field of the thermomagnetic breakdown and unveil the details of the transition from finger-like magnetic burst to dendritic branching morphology. These findings open a new avenue in the interdisciplinary exploration of catastrophic avalanches through non destructive repeatable experiments.

Local polarization of magnetic materials has become a well-known and widely used method for storing binary information. Numerous applications in our daily life such as credit cards, computer hard drives, and the popular magnetic drawing board toy, rely on this principle. In this work, we review the recent advances on the magnetic recording of inhomogeneous magnetic landscapes produced by superconducting films. We summarize the current compelling experimental evidence showing that magnetic recording can be applied for imprinting in a soft magnetic layer the flux trajectory taking place in a superconducting layer at cryogenic temperatures. This approach enables the ex situ observation at room temperature of the imprinted magnetic flux landscape obtained below the critical temperature of the superconducting state. The undeniable appeal of the proposed technique lies in its simplicity and the potential to improve the spatial resolution, possibly down to the scale of a few vortices.

Bacterial infections are one of the leading causes of disease worldwide. Conventional antibiotics are becoming less efficient, due to antibiotic-resistant bacterial strains. Therefore, the development of novel antibacterial materials and advanced treatment strategies are becoming increasingly important. In the present work, we developed a simple and efficient strategy for effective bacterial capture and their subsequent eradication through photothermal killing. The developed device consists of a flexible nanoheater, comprising a Kapton/Au nanoholes substrate, coated with reduced graphene oxide–polyethyleneimine (K/Au NH/rGO–PEI) thin films. The Au NH plasmonic structure was tailored to feature strong absorption in the near-infrared (NIR) region, where most biological matter has limited absorption, while PEI was investigated for its strong binding with bacteria through electrostatic interactions. The K/Au NH/rGO–PEI device was demonstrated to capture and eliminate effectively both planktonic Gram-positive Staphilococcus aureus (S. aureus) and Gram-negative Escherichia coli (E. coli) bacteria after 10 min of NIR (980 nm) irradiation and, to destroy and eradicate Staphilococcus epidermidis (S. epidermidis) biofilms after 30 min irradiation. The technique developed herein is simple and universal with potential applications for eradication of different micro-organisms.

We report on metal assisted chemical etching of Si for the synthesis of mechanically-stable, hybrid crystallographic orientation Si superstructures with high aspect ratio, above 200. This one-pot-type method sustains high etching rates and facilitates reproducible results. The protocol enables the control of the number, angle and location of kinks via successive etch-quench sequences. We analysed relevant Au mask catalyst features to systematically assess their impact on a wide spectrum of etched morphologies that can be easily attained and customized by fine tuning of the critical etching parameters. For instance, the designed kinked Si nanowires can be internalized in biological cells, without affecting their viability. An accessible numerical model is provided to explain the etch profiles and the physico-chemical events at the Si-Au-electrolyte interface and offers guidelines for the development of finite-element modeling of metal assisted Si chemical etching.

© 2018 American Physical Society. Theoretical proposals for spin-ice analogs based on nanostructured superconductors have suggested larger flexibility for probing the effects of fluctuations and disorder than in the magnetic systems. In this paper, we unveil the particularities of a vortex ice system by direct observation of the vortex distribution in a kagome lattice of paired antidots using scanning Hall probe microscopy. The theoretically suggested vortex ice distribution, lacking long-range order, is observed at half matching field (H1/2). Moreover, the vortex ice state formed by the pinned vortices is still preserved at 2H1/3. This unexpected result is attributed to the introduction of interstitial vortices at these magnetic-field values. Although the interstitial vortices increase the number of possible vortex configurations, it is clearly shown that the vortex ice state observed at 2H1/3 is less prone to defects than at H1/2. In addition, the nonmonotonic variations of the vortex ice quality on the lattice spacing indicates that a highly ordered vortex ice state cannot be attained by simply reducing the lattice spacing. The optimal design to observe defect-free vortex ice is discussed based on the experimental statistics. The direct observations of a tunable vortex ice state provides new opportunities to explore the order-disorder transition in artificial ice systems. ispartof: PHYSICAL REVIEW B vol:97 issue:13 status: published

Electromigration has evolved from an important cause of failure in electronic devices to an appealing method, capable of modifying the material properties and geometry of nanodevices. Although this technique has been successfully used by researchers to investigate low dimensional systems and nanoscale objects, its low controllability remains a serious limitation. This is in part due to the inherent stochastic nature of the process, but also due to the inappropriate identification of the relevant control parameters. In this study, we identify a suitable process variable and propose a novel control algorithm that enhances the controllability and, at the same time, minimizes the intervention of an operator. As a consequence, the algorithm facilitates the application of electromigration to systems that require exceptional control of, for example, the width of a narrow junction. It is demonstrated that the electromigration rate can be stabilized on pre-set values, which eventually defines the final geometry of the electromigrated structures. ispartof: REVIEW OF SCIENTIFIC INSTRUMENTS vol:89 issue:4 ispartof: location:United States status: published

We report on magneto-optical imaging and the temperature dependency of the upper critical fields ${H}_{c2}^{c}(T)$ parallel to the $c$ axis and ${H}_{c2}^{ab}(T)$ parallel to the $ab$ plane in ${\mathrm{Ba}}_{2}{\mathrm{Ti}}_{2}{\mathrm{Fe}}_{2}{\mathrm{As}}_{4}\mathrm{O}$ single crystals. These data were inferred from the measurements of the temperature-dependent resistance in static magnetic fields up to 14 T and magnetoresistance in pulsed fields up to 60 T. ${H}_{c2}$ values are found to be 52 and 50 T for $H\ensuremath{\parallel}ab$ and $H\ensuremath{\parallel}c$, respectively. These values are 1.2--1.35 times larger than the weak-coupling Pauli paramagnetic limit $({H}_{p}\ensuremath{\sim}1.84{T}_{c})$, indicating that enhanced paramagnetic limiting is essential and this superconductor is unconventional. Our observations of strong bending in the ${H}_{c2}^{ab}(T)$ curves and a nearly isotropic maximum upper critical field ${H}_{c2}^{ab}(0)\phantom{\rule{0.16em}{0ex}}\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}{H}_{c2}^{c}(0)$ support the presence of a strong Pauli paramagnetic effect. We show that the Werthamer-Helfand-Hohenberg (WHH) formula that includes the spin-orbit scattering can effectively describe the ${H}_{c2}^{ab}(T)$ curve, whereas ${H}_{c2}$ deviates from the conventional WHH theoretical model without considering the spin paramagnetic effect for the $H\ensuremath{\parallel}c$ and $H\ensuremath{\parallel}ab$ directions. For $H\ensuremath{\parallel}c$, a two-band model is required to fully reproduce the behavior of ${H}_{c2}$, while for $H\ensuremath{\parallel}ab$ the spin paramagnetic effect is responsible for the behavior of ${H}_{c2}$. The anisotropy of ${H}_{c2}$ is close to 3 near ${T}_{c}$ and decreases rapidly at lower temperatures.

We present a detailed quantitative magneto-optical imaging study of several superconductor/ferromagnet hybrid structures, including Nb deposited on top of thermomagnetically patterned NdFeB, and permalloy/niobium with erasable and tailored magnetic landscapes imprinted in the permalloy layer. The magneto-optical imaging data is complemented with and compared to scanning Hall probe microscopy measurements. Comprehensive protocols have been developed for calibrating, testing, and converting Faraday rotation data to magnetic field maps. Applied to the acquired data, they reveal the comparatively weaker magnetic response of the superconductor from the background of larger fields and field gradients generated by the magnetic layer., Comment: 21 pages, including 2 pages of supplementary material

We demonstrate the in situ engineering of superconducting nanocircuitry by targeted modulation of material properties through high applied current densities. We show that the sequential repetition of such customized electro-annealing in a niobium (Nb) nanoconstriction can broadly tune the superconducting critical temperature T-c and the normal-state resistance R-n in the targeted area. Once a sizable R-n is reached, clear magneto-resistance oscillations are detected along with a Fraunhofer-like field dependence of the critical current, indicating the formation of a weak link but with further adjustable characteristics. Advanced Ginzburg-Landau simulations fully corroborate this picture, employing the detailed parametrization from the electrical characterization and high resolution electron microscope images of the region within the constriction where the material has undergone amorphization by electro-annealing.

We investigate the degeneracy of the superconducting vortex matter ground state by directly visualizing the vortex configurations in a kagome lattice of elongated antidots via scanning Hall probe microscopy. The observed vortex patterns, at specific applied magnetic fields, are in good agreement with the configurations obtained using time-dependent Ginzburg-Landau simulations. Both results indicate that the long-range interaction in this nanostructured superconductor is unable to lift the degeneracy between different vortex states and the pattern formation is mainly ruled by the nearest-neighbor interaction. This simplification makes it possible to identify a set of simple rules characterizing the vortex configurations.We demonstrate that these rules can explain both the observed vortex distributions and the magnetic-field-dependent degree of degeneracy. ispartof: Physical Review B vol:96 issue:2 pages:1-6 status: published

International audience; The electromigration process has the potential capability to move atoms one by onewhen properly controlled. It is therefore an appealing tool to tune the cross sectionof monoatomic compounds with ultimate resolution or, in the case of polyatomiccompounds, to change the stoichiometry with the same atomic precision. Asdemonstrated here, a combination of electromigration and anti-electromigration canbe used to reversibly displace atoms with a high degree of control. This enables afine adjustment of the superconducting properties of Al weak links, whereas in Nbthe diffusion of atoms leads to a more irreversible process. In a superconductor witha complex unit cell (La2−xCexCuO4), the electromigration process acts selectively onthe oxygen atoms with no apparent modification of the structure. This allows to adjustthe doping of this compound and switch from a superconducting to an insulatingstate in a nearly reversible fashion. In addition, the conditions needed to replacefeedback controlled electromigration by a simpler technique of electropulsing arediscussed. These findings have a direct practical application as a method to explorethe dependence of the characteristic parameters on the exact oxygen content and pavethe way for a reversible control of local properties of nanowires.

International audience; We investigate the magnetic field and current distributions in structures made of superposed supercon- ducting films. A recent experimental study of a planar bilayer structure (Tamegai 2017) has revealed unusual pat- terns in the critical state, with an unexpected discontinuity line arising in the region where the layers overlap. We use a finite element model to investigate the critical states in such 3D structures with several layers and different overlapping configurations. We show that the experimentally observed discontinuity line arises as a result of a magnetic field dependence of the critical current density. Additionally, we show that the network of discontinuity lines not only depends on the geometry of the system but also evolves with the applied magnetic field as the structures are magnetized in a zero-field-cooled mode. The strength of the magnetic coupling is shown to depend on the number of layers, their overlap and separation, with a direct effect on the length and the shape of the discontinuity lines.

We theoretically investigate, through Ginzburg–Landau simulations, the possibility to induce an open circuit voltage in absence of applied current, by dragging superconducting vortices with a dynamic pinning array as for instance that created by a nearby sliding vortex lattice or moving laser spots. Different dynamic regimes, such as synchronous vortex motion or dynamic vortex chains consisting of laggard vortices, can be observed by varying the velocity of the sliding pinning potential and the applied magnetic field. Additionally, due to the edge barrier, significantly different induced voltage is found depending on whether the vortices are dragged along the superconducting strip or perpendicular to the lateral edges. The output voltage in the proposed mesoscopic superconducting dynamo can be tuned by varying size, density and directions of the sliding pinning potential.

We present direct experimental mapping of the lateral magnetic near-field distribution in plasmonic nanoantennas using aperture scanning near-field optical microscopy (SNOM). By means of full-field simulations it is demonstrated how the coupling of the hollow-pyramid aperture probe to the nanoantenna induces an effective magnetic dipole which efficiently excites surface plasmon resonances only at lateral magnetic field maxima. This excitation in turn affects the detected light intensity enabling the visualization of the lateral magnetic near-field distribution of multiple odd and even order plasmon modes with subwavelength spatial resolution.

Quantized magnetic vortices driven by electric current determine key electromagnetic properties of superconductors. While the dynamic behavior of slow vortices has been thoroughly investigated, the physics of ultrafast vortices under strong currents remains largely unexplored. Here, we use a nanoscale scanning superconducting quantum interference device to image vortices penetrating into a superconducting Pb film at rates of tens of GHz and moving with velocities of up to tens of km/s, which are not only much larger than the speed of sound but also exceed the pair-breaking speed limit of superconducting condensate. These experiments reveal formation of mesoscopic vortex channels which undergo cascades of bifurcations as the current and magnetic field increase. Our numerical simulations predict metamorphosis of fast Abrikosov vortices into mixed Abrikosov-Josephson vortices at even higher velocities. This work offers an insight into the fundamental physics of dynamic vortex states of superconductors at high current densities, crucial for many applications., Ultrafast vortex dynamics driven by strong currents define eletromagnetic properties of superconductors, but it remains unexplored. Here, Embon et al. use a unique scanning microscopy technique to image steady-state penetration of super-fast vortices into a superconducting Pb film at rates of tens of GHz and velocities up to tens of km/s.

The main dissipation mechanism in superconducting nanowires arises from phase slips. Thus far, most of the studies focus on long nanowires where coexisting events appear randomly along the nanowire. In the present work we investigate highly confined phase slips at the contact point of two superconducting leads. Profiting from the high current crowding at this spot, we are able to shrink in-situ the nanoconstriction. This procedure allows us to investigate, in the very same sample, thermally activated phase slips and the probability density function of the switching current Isw needed to trigger an avalanche of events. Furthermore, for an applied current larger than Isw, we unveil the existence of two distinct thermal regimes. One corresponding to efficient heat removal where the constriction and bath temperatures remain close to each other, and another one in which the constriction temperature can be substantially larger than the bath temperature leading to the formation of a hot spot. Considering that the switching current distribution depends on the exact thermal properties of the sample, the identification of different thermal regimes is of utmost importance for properly interpreting the dissipation mechanisms in narrow point contacts.

International audience; The influence of a conducting layer on the magnetic flux penetration in a superconducting Nb film is studied by magneto-optical imaging. The metallic layer partially covering the superconductor provides an additional velocity-dependent damping mechanism for the flux motion that helps to protect the superconducting state when thermomagnetic instabilities develop. If the flux advances with a velocity slower than w = 2/μ 0 σ t, where σ is the cap layer conductivity and t is its thickness, the flux penetration remains unaffected, whereas for incoming flux moving faster than w, the metallic layer becomes an active screening shield. When the metallic layer is replaced by a perfect conductor, it is expected that the flux braking effect will occur for all flux velocities. We investigate this effect by studying Nb samples with a thickness step. Some of the observed features, namely the deflection of the flux trajectories at the border of the thick center, as well as the favored flux penetration at the indentation, are reproduced by time-dependent Ginzburg-Landau simulations.

We investigate by scanning susceptibility microscopy the response of a thin Pb strip, with a square array of submicron antidots, to a low-frequency ac magnetic field applied perpendicularly to the film plane. By mapping the local permeability of the sample within the field range where vortices trapped by the antidots and interstitial vortices coexist, we observed two distinct dynamical regimes occurring at different temperatures. At a temperature just belowthe superconducting transition, T/Tc = 0.96, the sample response is essentially dominated by the motion of highly mobile interstitial vortices. However, at a slightly lower temperature, T/Tc = 0.93, the interstitial vortices freeze up leading to a strong reduction of the ac screening length.We propose a simple model for the vortex response in this system which fits well to the experimental data. Our analysis suggests that the observed switching to the high mobility regime stems from a resonant effect, where the period of the ac excitation is just large enough to allow interstitial vortices to thermally hop through the weak pinning landscape produced by random material defects. This argument is further supported by the observation of a pronounced enhancement of the out-of-phase response at the crossover between both dynamical regimes. ispartof: Physical Review B vol:94 issue:2 pages:1-12 status: published

Local polarization of a magnetic layer, a well-known method for storing information, has found its place in numerous applications such as the popular magnetic drawing board toy or the widespread credit cards and computer hard drives. Here we experimentally show that a similar principle can be applied for imprinting the trajectory of quantum units of flux (vortices), travelling in a superconducting film (Nb), into a soft magnetic layer of permalloy (Py). In full analogy with the magnetic drawing board, vortices act as tiny magnetic scribers leaving a wake of polarized magnetic media in the Py board. The mutual interaction between superconducting vortices and ferromagnetic domains has been investigated by the magneto-optical imaging technique. For thick Py layers, the stripe magnetic domain pattern guides both the smooth magnetic flux penetration as well as the abrupt vortex avalanches in the Nb film. It is however in thin Py layers without stripe domains where superconducting vortices leave the clearest imprints of locally polarized magnetic moment along their paths. In all cases, we observe that the flux is delayed at the border of the magnetic layer. Our findings open the quest for optimizing magnetic recording of superconducting vortex trajectories.

On the basis of the relationship between charge dimensionality and singular field behavior, it is proven that in a volumetric description of a volume current carrying topology, half rooftops of different binary hierarchical level are allowed without introducing numerical difficulties. This opens the possibility to use a very efficient multilevel hierarchical meshing scheme in a volumetric method-of-moments (V-MoM) algorithm. The new meshing scheme is validated by numerical calculations and experiments. It paves the way toward a much more efficient use of MoM in the description of arbitrarily shaped nanostructures at infrared and optical frequencies.

The current stimulated atomic diffusion in YBa2Cu3O7–d superconducting bridges is investigated. A superconductor to insulator transition can be induced by the current controlled electromigration process, whereas the partial recovery of the superconducting state can be achieved by inverting the polarity of the bias. Interestingly, the temperature dependence of the current density JEM(T), above which atomic migration takes place, intersects the critical current density Jc(T) at certain temperature T*. Therefore, for T < T*, the current-induced dissipative state cannot be accessed without leading to irreversible modifications of the material properties. This phenomenon could also lead to the local deterioration of high critical temperature superconducting films abruptly penetrated by thermomagnetic instabilities., The authors acknowledge the - nancial support form the Fonds de la Recherche Sci- enti que - FNRS, Spanish Ministry of Economy and Competitiveness through the Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496), CONSOLIDER Excellence Network (MAT2015-68994- REDC), COACHSUPENERGY project (MAT2014- 51778-C2-1-R), co- nanced by the European Regional Development Fund, the Research Foundation - Flanders (FWO-Vlaanderen), the COST programme NanoCoHy- bri (CA 16218), and from the Catalan Government with 2017 SGR 1519. A.F.-R. thank Spanish Ministry of Econ- omy for his FPI Spanish grant (BES-2016-077310).

We present a thorough investigation by magneto-optical imaging of the magnetic flux penetration in Nb thin films with lithographically defined border indentations. We demonstrate that discontinuity lines ($d$ lines), caused by the abrupt bending of current streamlines around the indentations, depart from the expected parabolic trend close to the defect and depend on the shape and size of the indentation as well as on the temperature. These findings are backed up and compared with theoretical results obtained by numerical simulations and analytical calculations highlighting the key role played by demagnetization effects and the creep exponent $n$. In addition, we show that the presence of nearby indentations and submicrometer random roughness of the sample border can severely modify the flux front topology and dynamics. Strikingly, in contrast to what has been repeatedly predicted in the literature, we do not observe that indentations act as nucleation spots for flux avalanches, but they instead help to release the flux pressure and avoid thermomagnetic instabilities.

Superconducting nanowires currently attract great interest due to their application in single-photon detectors and quantum-computing circuits. In this context, it is of fundamental importance to understand the detrimental fluctuations of the superconducting order parameter as the wire width shrinks. In this paper, we use controlled electromigration to narrow down aluminium nanoconstrictions. We demonstrate that a transition from thermally assisted phase slips to quantum phase slips takes place when the cross section becomes less than ∼150 nm2. In the regime dominated by quantum phase slips the nanowire loses its capacity to carry current without dissipation, even at the lowest possible temperature. We also show that the constrictions exhibit a negative magnetoresistance at low-magnetic fields, which can be attributed to the suppression of superconductivity in the contact leads. These findings reveal perspectives of the proposed fabrication method for exploring various fascinating superconducting phenomena in atomic-size contacts., Nanostructured superconductors allow dissipationless electrical transport to be exploited in technologically relevant devices. Here, the authors follow how detrimental fluctuations of the superconducting order parameter evolve in Al atomic contacts as their width is controlled by electromigration.

An experimental determination of the mean vortex velocity in superconductors mostly relies on the measurement of flux-flow resistance with magnetic field, temperature, or driving current. In the present work we introduce a method combining conventional transport measurements and a frequency-tuned flashing pinning potential to obtain reliable estimates of the vortex velocity. The proposed device is characterized using the time-dependent Ginzburg-Landau formalism, where the velocimetry method exploits the resonances in mean vortex dissipation when temporal commensuration occurs between the vortex crossings and the flashing potential. We discuss the sensitivity of the proposed technique on applied current, temperature and heat diffusion, as well as the vortex core deformations during fast motion.

A comprehensive study of the doping dependence of the phase diagram of FeSe-based superconductors is still required due to the lack of a clean and systematic means of doping control. Here, we report on the magneto-optical imaging, thermodynamic and transport properties, as well as in situ angle-resolved photoemission spectroscopy (ARPES) studies of impurity scattering in stoichiometric FeSe single crystals. Co doping at the Fe site is found to decrease the superconducting transition temperature $({T}_{c})$. The upper critical field and specific heat all indicate a possible multiband superconductivity with strong coupling in the Co-doped system. A remarkable feature in FeSe is that its temperature dependent resistivity exhibits a wide hump at high temperatures, a signature of a crossover from a semiconductinglike behavior to metallic behavior. A structural tetragonal-to-orthorhombic phase transition $({T}_{s})$ (a consequence of the electronic nematicity) is suppressed by either physical or chemical pressures. Due to the reconstruction of the Fermi surface at ${T}_{s}$, specific heat anomalies at ${T}_{s}$ present $\mathrm{\ensuremath{\Delta}}{C}_{p}/{T}_{s}\ensuremath{\approx}{\ensuremath{\gamma}}_{n}$, being ${\ensuremath{\gamma}}_{n}$ the Sommerfield coefficient at low temperature. This reflect an additional electronic instability in the ${\mathrm{FeSe}}_{1\ensuremath{-}x}{\mathrm{S}}_{x}$ system. ARPES data between 180 and 282 K indicates the existence of a chemical potential shift with increasing thermal excitations, resulting in a change of the Fermi-surface topology and exhibiting a semimetal behavior. We found that the temperature-induced Lifshitz transition is much higher than the temperature for the nematic order.

We present a comprehensive experimental and theoretical study of the electronic and magnetic properties of two quasi-two-dimensional (2D) honeycomb lattice monoclinic compounds ${A}_{3}\mathrm{N}{\mathrm{i}}_{2}\mathrm{Sb}{\mathrm{O}}_{6}\phantom{\rule{0.16em}{0ex}}(A=\mathrm{Li},\phantom{\rule{0.16em}{0ex}}\mathrm{Na})$. Magnetic susceptibility and specific heat data are consistent with the onset of antiferromagnetic long-range order with N\'eel temperatures of \ensuremath{\sim}14 and 16 K for $\mathrm{L}{\mathrm{i}}_{3}\mathrm{N}{\mathrm{i}}_{2}\mathrm{Sb}{\mathrm{O}}_{6}$ and $\mathrm{N}{\mathrm{a}}_{3}\mathrm{N}{\mathrm{i}}_{2}\mathrm{Sb}{\mathrm{O}}_{6}$, respectively. The effective magnetic moments of $4.3\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.\phantom{\rule{0.16em}{0ex}}(\mathrm{L}{\mathrm{i}}_{3}\mathrm{N}{\mathrm{i}}_{2}\mathrm{Sb}{\mathrm{O}}_{6})$ and $4.4\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.\phantom{\rule{0.16em}{0ex}}(\mathrm{N}{\mathrm{a}}_{3}\mathrm{N}{\mathrm{i}}_{2}\mathrm{Sb}{\mathrm{O}}_{6})$, where f.u. is formula units, indicate that $\mathrm{N}{\mathrm{i}}^{2+}$ is in a high-spin configuration ($S=1$). The temperature dependence of the inverse magnetic susceptibility follows the Curie-Weiss law in the high-temperature region and shows positive values of the Weiss temperature, $\ensuremath{\sim}8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ ($\mathrm{L}{\mathrm{i}}_{3}\mathrm{N}{\mathrm{i}}_{2}\mathrm{Sb}{\mathrm{O}}_{6}$) and \ensuremath{\sim}12 K ($\mathrm{N}{\mathrm{a}}_{3}\mathrm{N}{\mathrm{i}}_{2}\mathrm{Sb}{\mathrm{O}}_{6}$), pointing to the presence of nonnegligible ferromagnetic interactions, although the system orders antiferromagnetically at low temperatures. In addition, the magnetization curves reveal a field-induced (spin-flop type) transition below ${T}_{\mathrm{N}}$ that can be related to the magnetocrystalline anisotropy in these systems. These observations are in agreement with density functional theory calculations, which show that both antiferromagnetic and ferromagnetic intralayer spin exchange couplings between $\mathrm{N}{\mathrm{i}}^{2+}$ ions are present in the honeycomb planes, supporting a zigzag antiferromagnetic ground state. Based on our experimental measurements and theoretical calculations, we propose magnetic phase diagrams for the two compounds.

We study the instability of the superconducting state in a mesoscopic geometry for the low pinning material Mo$_3$Ge characterized by a large Ginzburg-Landau parameter. We observe that in the current driven switching to the normal state from a nonlinear region of the Abrikosov flux flow, the mean critical vortex velocity reaches a limiting maximum velocity as a function of the applied magnetic field. Based on time dependent Ginzburg-Landau simulations we argue that the observed behavior is due to the high velocity vortex dynamics confined on a mesoscopic scale. We build up a general phase diagram which includes all possible dynamic configurations of Abrikosov lattice in a mesoscopic superconductor., Comment: 7 pages, 6 figures

Flux avalanches are known to occur as a consequence of thermomagnetic instabilities. Some of their fingerprints are jumps in magnetization curves, or a paramagnetic reentrance in AC susceptibility measurements. In this work we have studied flux avalanches triggered by an AC field cycle by means of AC susceptibility and residual magnetization after an applied AC field measured as a function of an AC excitation field (h). These measurements allow comparing both results with magneto-optical imaging carried out in similar conditions. The results show a correspondence for the onset of the avalanche activity, as well as between the residual magnetic moment and the mean gray value calculated from the magneto-optical images in the remanent state.

Many self-propelled microorganisms are attracted to surfaces. This makes their dynamics in restricted geometries very different from that observed in the bulk. Swimming along walls is beneficial for directing and sorting cells, but may be detrimental if homogeneous populations are desired, such as in counting microchambers. In this work, we characterize the motion of human sperm cells $60 \mu m$ long, strongly confined to $25 \mu m$ shallow chambers. We investigate the nature of the cell trajectories between the confining surfaces and their accumulation near the borders. Observed cell trajectories are composed of a succession of quasi-circular and quasi-linear segments. This suggests that the cells follow a path of intermittent trappings near the top and bottom surfaces separated by stretches of quasi-free motion in between the two surfaces, as confirmed by depth resolved confocal microscopy studies. We show that the introduction of artificial petal-shaped corrugation in the lateral boundaries removes the tendency of cells to accumulate near the borders, an effect which we hypothesize may be valuable for microfluidic applications in biomedicine., Comment: 9 pages, latex. In accepted version on April 14, v2: abstract modified, information added to Sec. II.A and experiments added to Sec. III.A and Fig.3. Sec. III.C was deleted. Requested references added

In 1976, Larkin and Ovchinnikov [Zh. Eksp. Teor. Fiz. 68, 1915 (1975) [Sov. Phys.\char21{}JETP 41, 960 (1976)]] predicted that vortex matter in superconductors driven by an electrical current can undergo an abrupt dynamic transition from a flux-flow regime to a more dissipative state at sufficiently high vortex velocities. Typically, this transition manifests itself as a large voltage jump at a particular current density, so-called instability current density ${J}^{*}$, which is smaller than the depairing current. By tuning the effective pinning strength in Al films, using an artificial periodic pinning array of triangular holes, we show that a unique and well-defined instability current density exists if the pinning is strong, whereas a series of multiple voltage transitions appear in the relatively weaker pinning regime. This behavior is consistent with time-dependent Ginzburg-Landau simulations, where the multiple-step transition can be unambiguously attributed to the progressive development of vortex chains and subsequently phase-slip lines. In addition, we explore experimentally the magnetic braking effects, caused by a thick Cu layer deposited on top of the superconductor, on the instabilities and the vortex ratchet effect.

Introducing artificial pinning centers is a well established strategy to trap quantum vortices and increase the maximal magnetic field and applied electric current that a superconductor can sustain without dissipation. In case of spatially periodic pinning, a clear enhancement of the superconducting critical current arises when commensurability between the vortex configurations and the pinning landscape occurs. With recent achievements in (ultrafast) optics and nanoengineered plasmonics it has become possible to exploit the interaction of light with superconductivity, and create not only spatially periodic imprints on the superconducting condensate, but also temporally periodic ones. Here we show that in the latter case, temporal matching phenomena develop, caused by stroboscopic commensurability between the characteristic frequency of the vortex motion under applied current and the frequency of the dynamic pinning. The matching resonances persist in a broad parameter space, including magnetic field, driving current, or material purity, giving rise to unusual features such as externally variable resistance/impedance and Shapiro steps in currentvoltage characteristics. All features are tunable by the frequency of the dynamic pinning landscape. These findings open further exploration avenues for using flashing, spatially engineered, and/or mobile excitations on superconductors, permitting us to achieve advanced functionalities. ispartof: Scientific Reports vol:5 issue:1 ispartof: location:England status: published

The response of superconducting devices to electromagnetic radiation is a core concept implemented in diverse applications, ranging from the currently used voltage standard to single photon detectors in astronomy. Suprisingly, a sufficiently high power subgap radiation may stimulate superconductivity itself. The possibility of stimulating type II superconductors, in which the radiation may interact also with vortex cores, remains however unclear. Here we report on superconductivity enhanced by GHz radiation in type II superconducting Pb films in the presence of vortices. The stimulation effect is more clearly observed in the upper critical field and less pronounced in the critical temperature. The magnetic field dependence of the vortex related microwave losses in a film with periodic pinning reveals a reduced dissipation of mobile vortices in the stimulated regime due to a reduction of the core size. Results of numerical simulations support the validy of this conclusion. Our findings may have intriguing connections with holographic superconductors in which the possibility of stimulation is under current debate, This work has been supported in parts by Spanish MINECO (MAT2012-32743), and Comunidad de Madrid (NANOFRONTMAG-CM S2013/MIT-2850) and NANO-SC COST-Action MP-1201. A. Lara thanks UAM for FPI-UAM fellowship. The work of A.V.S. was partially supported by Mandat ‘‘d’Impulsion Scientifique’’ of the F.R.S.-FNRS

Using scanning susceptibility microscopy, we shed light on the dynamics of individual superconducting vortices and examine the hypotheses of the phenomenological models traditionally used to explain the macroscopic ac electromagnetic properties of superconductors. The measurements, carried out on a 2H-NbSe2 single crystal at relatively high temperature (T = 6.8 K), show a linear amplitude dependence of the global ac susceptibility for excitation amplitudes between 0.3 and 2.6 Oe. We observe that the low amplitude response, typically attributed to the oscillation of vortices in a potential well defined by a single, relaxing, Labusch constant, actually corresponds to strongly nonuniform vortex shaking. This is particularly pronounced in the field-cooled disordered phase, which undergoes a dynamic reorganization above 0.8 Oe as evidenced by the healing of lattice defects and a more uniform oscillation of vortices. These observations are corroborated by molecular dynamics simulations when choosing the microscopic input parameters from the experiments. The theoretical simulations allow us to reconstruct the vortex trajectories, providing deeper insight into the thermally induced hopping dynamics and the vortex lattice reordering. ispartof: Physical Review B, Condensed Matter and Materials Physics vol:90 issue:13 pages:134508-1 status: published

Using two experimental techniques, we studied single crystals of the 122-FeAs family with almost the same critical temperature, Tc. We investigated the temperature dependence of the lower critical field of a single crystal under static magnetic fields parallel to the axis. The temperature dependence of the London penetration depth can be described equally well either by a single anisotropic -wave-like gap or by a two-gap model, while a d-wave approach cannot be used to fit the London penetration depth data. Intrinsic multiple Andreev reflection effect spectroscopy was used to detect bulk gap values in single crystals of the intimate compound, with the same Tc. We estimated the range of the large gap value 6-8 meV (depending on small variation of and its a space anisotropy of about 30%, and the small gap 1.7 meV. This clearly indicates that the gap structure of our investigated systems more likely corresponds to a nodeless s-wave two gaps., 9 pages, 6 figures

The guidance of human sperm cells under confinement in quasi-2D microchambers is investigated using a purely physical method to control their distribution. Transport property measurements and simulations are performed with diluted sperm populations, for which effects of geometrical guidance and concentration are studied in detail. In particular, a trapping transition at convex angular wall features is identified and analyzed. We also show that highly efficient microratchets can be fabricated by using curved asymmetric obstacles to take advantage of the spermatozoa specific swimming strategy. Fil: Guidobaldi, Héctor Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones Biológicas y Tecnológicas. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto de Investigaciones Biológicas y Tecnológicas; Argentina Fil: Jeyaram, Y.. Katholikie Universiteit Leuven; Bélgica Fil: Berdakin, Ivan. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina Fil: Moshchalkov, V.V.. Katholikie Universiteit Leuven; Bélgica Fil: Condat, Carlos. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina Fil: Marconi, Veronica Iris. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina Fil: Giojalas, Laura Cecilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones Biológicas y Tecnológicas. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto de Investigaciones Biológicas y Tecnológicas; Argentina Fil: Silhanek, A. V.. Université de Liège; Bélgica

We study the influence of artificial pinning centers on the vortex critical velocity in Al thin films deposited on top of a periodic array of Permalloy (FeNi) square rings. We demonstrate that the field dependence of the flux flow velocity strongly depends on the particular magnetic state of the rings. In particular, we find that, even when the rings are in a flux closure state, i.e. with little stray field, the vortex critical velocity shows a non-monotonic magnetic field dependence. This behaviour is in sharp contrast with the results obtained in a reference plain film, with no rings underneath. A comparison with the intrinsic strong pinning Nb films previously studied, suggests an interpretation in terms of a channel-like motion of vortices, here induced by the artificial pinning structure. (C) 2010 Elsevier B.V. All rights reserved. ispartof: Physica c-superconductivity and its applications vol:470 issue:19 pages:904-906 ispartof: location:GREECE, Rhodes status: published

We present the second harmonic generation - circular dichroism response from G-shaped gold nanostructures for P-polarized incoming light. By eliminating the contributions of anisotropy to the signal, an absolute measure of the second harmonic generation - circular dichroism is presented. The system of planar nanostructures is an excellent model for studying the correlation between optical properties and chiral morphology at the nanoscale. Samples with both enantiomorphs of the chiral pattern were prepared and, demonstrating the unambiguous presence of chirality, the second harmonic generation - circular dichroism response from two such samples was fitted simultaneously with a single set of fitting parameters.

Studies of single crystal YNi{sub 2}B {sub 2}C have revealed a fourfold anisotropy of the equilibrium magnetization in the square crystallographic basal plane. This {pi}/2 periodicity occurs deep in the superconductive mixed state. In this crystal symmetry, an ordinary superconductive mass anisotropy (as in usual London theory) allows only a constant, isotropic response. In contrast, the experimental results are well described by generalized London theory incorporating nonlocal electrodynamics, as needed for this clean, intermediate-{kappa} superconductor. {copyright} {ital 1999} {ital The American Physical Society }