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By means of the ionic-liquid-gating technique (ILG), we have successfully tuned the critical transition temperatures (Tc) of superconducting FeSe and La1.9Ce0.1CuO4 (LCCO) films, whose thicknesses largely exceed the working depth of electrostatic fields. The magnetic responses of gated samples were measured in situ using the homemade two-coil mutual inductance devices. Through the analysis of the imaginary part of the induced pick-up coil voltage, we conclude that the gating process influences the entire film thickness rather than just a few layers near the surface. This bulk effect suggests that the electrochemical effect rather than electrostatic effect plays a primary role in our experiments. These findings will shed new light on the mechanisms of ILG in tuning superconducting films.

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.

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.

Despite intense efforts on all known quasi-two-dimensional superconductors, the origin and exact boundary of the electronic orderings, particularly charge density waves and superconductivity, are still attractive problems with several open questions. Here, in order to reveal how the superconducting gap evolves, we report on high quality complementary measurements of magneto-optical imaging, specific heat, magnetic susceptibility, resistivity measurements, Andreev spectroscopy, and London penetration depth ${\ensuremath{\lambda}}_{ab}(T)$ measurements supplemented with theoretical calculations for $2\mathrm{H}\text{\ensuremath{-}}\mathrm{Nb}{\mathrm{Se}}_{2}$ and $2\mathrm{H}\text{\ensuremath{-}}\mathrm{Nb}{\mathrm{S}}_{2}$ single crystals. The temperature dependence of ${\ensuremath{\lambda}}_{ab}(T)$ calculated from the lower critical field and Andreev spectroscopy can be well described by using a two-band model with $s$-wave-like gaps. The effect of pressure on the superconducting gap of both systems illustrates that both bands are practically affected. Upon compression, the Fermi surfaces do not change significantly, and the nesting remains almost unaffected compared to that at ambient condition. However, a strong bending in the upper critical fields (${H}_{\mathrm{c}2}$) curves is obtained under pressure and support the presence of a strong Pauli paramagnetic effect. In $\mathrm{Nb}{\mathrm{Se}}_{2}$, using a two-band model with $s$-wave-like gaps, the temperature dependence ${H}_{\mathrm{c}2}(T)$ can be properly described. In contrast to that, the behavior of ${H}_{\mathrm{c}2}$ for $\mathrm{Nb}{\mathrm{S}}_{2}$ is ruled by the spin paramagnetic effect. The estimated values of the penetration depth at $T=0\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ confirm that $\mathrm{Nb}{\mathrm{Se}}_{2}$ and $\mathrm{Nb}{\mathrm{S}}_{2}$ superconductors depart from a Uemura-style relationship between ${T}_{\mathrm{c}}$ with ${\mathbit{\ensuremath{\lambda}}}_{ab}^{\ensuremath{-}2}(T)$, the in-plane superconducting penetration depth.

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.

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

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 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; 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.

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.

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).

Using scanning Hall probe microscopy a direct visualization of the flux distribution in a Pb film covering a fivefold Penrose array of Co dots is obtained. We demonstrate that stable vortex configurations can be found for fields H ∼ 0.8 H 1 , H 1 and 1.6 H 1 , where H 1 corresponds to one flux quantum per pinning site. The vortex pattern at 0.8 H 1 corresponds to one vacancy in one of the vertices of the thin tiles whereas at 1.6 H 1 the vortex structure can be associated with one interstitial vortex inside each thick tile. Strikingly, for H = 1.6 H 1 interstitial and pinned vortices arrange themselves in ring-like structures (“vortex corrals”) which favor the formation of a giant vortex state at their center.

The vortex dynamics in a superconducting thin Al film with a periodic Honeycomb or Kagome array of antidots has been investigated by electrical transport measurements. The large values of the superconducting coherence length and penetration depth of the Al films guarantee a maximum of one flux quantum trapped per pinning site. This allows us to directly compare the experimental results with previous theoretical investigations based on molecular dynamics simulations. For the Kagome lattice, two submatching features not anticipated theoretically at H/H1=1/3 and 2/3, where H1 is the field at which the number of vortices coincides with the number of pinning sites, are observed. Possible corresponding stable vortex patterns are suggested. For the Honeycomb pinning landscape, the commensurability effects are in agreement with the theoretical expectations. A preliminary analysis of the vortex mobility in this lattice shows the presence of a weak vortex guidance.

We demonstrate circular dichroism (CD) in the second harmonic generation (SHG) signal from chiral assemblies of G-shaped nanostructures made of gold. The arrangement of the G shapes is crucial since upon reordering them the SHG-CD effect disappears. Microscopy reveals SHG "hotspots" assemblies, which originate in enantiomerically sensitive plasmon modes, having the novel property of exhibiting a chiral geometry themselves in relation with the handedness of the material. These results open new frontiers in studying chirality.

The results of theoretical and experimental investigations of the peculiarities of the nucleation of superconductivity in the presence of a nonuniform magnetic field induced by ferromagnetic films or dots with out-of-plane magnetic anisotropy are reviewed. It is shown that the phase transition line of superconductor-ferromagnet hybrids in the H-T plane (H is an external magnetic field and T is temperature) is determined significantly by the spatial distribution of nonuniform magnetic field. It allows to control the thermodynamic and transport properties of superconductor-ferromagnet hybrids by tuning the magnetic state of the ferromagnet.

The transport properties of Al superconducting thin films covering arrays of Co/Pt pinning centers of two different sizes are measured and compared. Detailed magnetization characterization of the dots allow us to gain control over the average magnetization of the dots through the application of an external magnetic field. By increasing the magnetization of the dots a gradually increasing shift of the superconducting phase boundary is observed. This shift is explained in terms of vortex–antivortex pair generation. It is shown that the resulting field-induced-superconductivity is strongly dependent on the size of the magnetic dots.

Voltage–current (V(I)) characteristics are investigated in the superconducting state of an equilateral triangle of 2.2 μm2 area. Several steps in the V(I) curves indicate the presence of phase slip centers in the triangle. The field dependence of these steps follows the Little-Parks oscillations of the phase boundary, associated with the change of vorticity. The influence of the contacts manifests itself as an excess voltage and a negative differential resistance. In addition we show that the position of the current contacts modify the superconducting phase boundary measurements.

The effect of a nonuniform magnetic field induced by a ferromagnet on the magnetoresistance of thin-film superconductor/ferromagnet hybrid structures was investigated experimentally. Two different magnetic textures with out-of-plane magnetization were considered: a plain ferromagnetic film with bubble domains and a regular array of ferromagnetic dots. The stray fields of the structures are able to affect the spatial profile of the superconducting condensate, leading to a modification of the dependence of the critical temperature T c on an external magnetic field H. We showed how the standard linear T c ( H ) dependence with a single maximum at H = 0 can be continuously transformed into so-called reentrant phase boundary with two T c peaks. We demonstrated that both domain-wall superconductivity and field-induced superconductivity are different manifestations of the magnetic confinement effect in various magnetic patterns.

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.

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

We investigate the dynamic response of a vortex lattice in nanostructured superconducting Pb films by means of ac-magnetization measurements. We compare the flux pinning properties of superconducting Pb films with arrays of fully perforated holes (antidots) and partially drilled holes (blind holes). Our results show that an array of blind holes, characterized by a thin superconducting bottom layer, gives rise to a less efficient pinning potential than an array of antidots. On top of that, a lower number of flux quanta trapped per pinning site (the saturation number), is observed for the blind hole array.

We report on electro-transport measurements on a Pb thin film with a square array of antidots, for the whole angular range of in-plane current orientations. We demonstrate that the interaction of a moving flux line lattice with a periodic pinning potential, breaks the rotational symmetry and induces a guided vortex motion along the principal symmetry orientations of the pinning array. As a consequence, we found that for orientations different than the symmetry directions, the vortex velocity is not aligned with the Lorentz force. We show that this anisotropic vortex motion is temperature dependent and progressively fades out when approaching the normal state.

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

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 investigate the nucleation of superconductivity in an Al/Al 2 O 3 /Py trilayer system by electrical transport measurements. Magnetic force microscopy images taken at room temperature show that the 0.7 μm thick Py-film form stripes of magnetic domains with alternating out-of-plane stray field. After applying a strong out of plane magnetic field H the superconductor/normal phase boundary becomes asymmetric with respect to H = 0. This lack of field polarity symmetry results from the unbalanced size distribution of domains with opposite polarity.

The transport properties of an Al type-II superconducting thin film covering a Py plain film with a rectangular array of triangular holes are investigated. We show that, although the magnetization of the Py lies in the plane of the structure, both the critical temperature and the critical current are asymmetric with respect to the polarity of the external field, which is applied perpendicularly to the structure. The asymmetric nucleation can be explained in terms of field compensation effects between internal and external magnetic fields, whereas the presence of vortex–antivortex pairs are responsible for the observed features in the critical current.

In this work we present experimental results on the rectification of vortices in a superconductor/ferromagnet system under a high frequency drive. The two-dimensional pinning landscape, induced by the stray fields of the ferromagnetic template, has no intrinsic asymmetry. Nevertheless, an asymmetric potential is artificially induced by an applied dc bias. The experimental results unambiguously show a biased, discrete motion of the vortices in the periodic potential at frequencies above 10 MHz. This synchronized motion is very sensitive to the external applied field. Increasing temperature leads to a reduction of the pinning potential, which in turn results in a lower ac power needed to drive the vortex lattice.