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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.

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 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 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.

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.

The Neel temperature of heavy fermion material CeIn 3 is at 10 K. La doping to Ce site of CeIn 3 introduces increase of distance between Ce ions and reduction of number of 4f electron. It is implied the reduction of 4f electron number with La doping weakens inter site magnetic interaction. An x – T phase diagram was drawn with Neel temperatures estimated from both electrical resistivity and magnetic susceptibility measurements.

We discuss recent specific heat data in high magnetic fields on URu2Si2 and 4% Rh-doped URu2Si2 as well as previously published de Haas-van Alphen data at lower magnetic fields in pure URu2Si2; both of which are consistent with quasiparticle bands formed from a hybridization between 5f-electron Γ 5 doublets and regular conduction electrons. The system exhibits itinerant electron metamagnetism that gives rise to a putative quantum critical point at ∼34 −37 T (depending on the % of Rh) that is subsequently unstable to field-induced phases.

Using electrical transport measurements, we study a composite antidot lattice consisting of two interpenetrating antidot square arrays with a different antidot size and the same lattice period. We show that placing an additional small antidot in the unit cell of the array a higher number of flux quanta per unit cell can be trapped inside the antidots, compared to a reference antidot film without the additional small antidots. As a consequence, the field range in which an enhanced critical current is observed is considerably expanded.

U(Ru 0.96 Rh 0.04 ) 2 Si 2 is an anisotropic itinerant 5f heavy electron system which exhibits a single field induced magnetic phase concealing a quantum critical point. We show that the boundary of this phase corresponds to a first order transition as revealed by (1) a large and symmetric specific heat anomaly, (2) an asymmetric magnetocaloric effect on entering and exiting this phase, and (3) a clear history and time-dependent specific heat. We explore the thermodynamic similarities with the mixed valence compound YbInCu 4 and show that the out-of-equilibrium properties are analogous to those depicted by the critical state model for hard superconductors.

High magnetic field induces quantum criticality in CeIn 3 with suppressing the Neel temperature. Estimated quantum critical field of CeIn 3 is about 60 T where the Neel temperature is suppressed to reach absolute zero. The magnetic field (60 T) is too high to measure electrical resistivity or specific heat precisely. Sn doping to In site of CeIn 3 reduces the Neel temperature. Reduction of Neel temperature indicates lower critical field to facilitate investigation of electronic states at high magnetic fields. Electrical resistivity and specific heat in CeIn 2.85 Sn 0.25 were measured to map an H–T phase diagram.

We report on the magnetic field evolution of the Neel temperature T N in the heavy electron system CeIn 3 - x Sn x ( x = 0 and 0.3), as determined by specific heat, magnetocaloric effect, and electrical transport measurements. For the stoichiometric compound, T N is fully suppressed at H c ∼ 65 T , whereas for x = 0.3 the critical field extrapolates to H c ∼ 30 T . An increasing Sommerfeld coefficient γ , obtained above T N , as field increases is observed and could be related to the a putative quantum critical point at H c .

Specific heat, magnetocaloric effect and magnetization experiments were performed on the 5f-electron system U(Ru0.96Rh0.04)2Si2. This compound exhibits a non-magnetic ground state with a single magnetic field-induced phase between H = 26 and 38 T . We show that the transition into this phase is first-order as revealed by a large irreversible specific heat anomaly and asymmetric magnetocaloric effect. A comparative analysis with the valence transition compound YbInCu4, where similar irreversibility and asymmetry exists, is presented.

We use AC-susceptibility measurements to study the dynamic response of vortices in a Pb thin film with a periodic array of antidots. In the linear regime, from the field dependence of the AC-penetration depth λ we determine the curvature of the pinning potential well a for different matching fields. For applied fields H below the first matching field H 1 , α reaches its highest value due to the strong vortex localization at the pinning sites. For H 1 < H < H 2 , the metallic-like behavior of interstitial vortices gives rise to a reduction of α and a further decrease of the pinning strength is observed for H 2 < H < H 3 . However, for H 3 < H < H 4 , a reentrance in the pinning strength appears, due to a specific configuration of the flux line lattice which strongly restricts the mobility of vortices.

We present studies of the vortex state magnetization M of a single crystal of the nonmagnetic borocarbide superconductor YNi 2 B 2 C. With magnetic field H applied in the square basal plane of the tetragonal unit cell, M has a four-fold anisotropy. This π/2 periodicity occurs deep in the superconductive mixed state, with an unusual field dependence, for a range of temperatures T T c . The observations are well explained by a generalized London model incorporating non-local electrodynamics, using parameters from complementary experiments.

We measured the angular dependence of the irreversible magnetization of a YBa2Cu3O7 single crystal with columnar defects (CD) inclined with respect to the c-axis. At temperatures T ≥ 40K and high fields we observe a sharp maximum centered at the tracks' direction. At T ≤ 20K the behavior is quite different. At high fields a broad bump at the tracks' direction is still visible, but its height is small and it appears only as a perturbation to the angular dependence due to material anisotropy. However, by performing the usual anisotropic rescaling (Blatter, Phys. Rev. Lett. 68, 875, 1992) we recover the strong unidirectional effects due to CD's.