El premio Wheatley de la American Physical Society: una aventura patagónica

A principios de 1990, la American Physical Society (APS) aprobó la creación de un premio en reconocimiento a la dedicación de físicos norteamericanos que hubieran realizado, contribuciones al desarrollo Física en países del Tercer Mundo. El premio fue llamado "The Wheatley Award", en reconocimiento a John Whealtey, un descollante físico norteamericano que dedicó dieciocho meses a la formación de un Laboratorio de Bajas temperaturas en Bariloche

Critical current and topology of the supercooled vortex state in NbSe2

We study the behavior of the critical current, Ic(H,T), of pure and Fe doped NbSe2 crystals in the denominated disordered vortex region, limited by the critical field Hc2(T) and the field Hp(T) at which the peak effect in Ic(H,T) is detected. The critical current follows an individual pinning response as demonstrated by its field independent universal function of the superfluid density. Transport measurements combined with Bitter decorations show no evidence of the existence of an amorphous phase in the high temperature region.Comment: 7 pages, figures included. Submitted to Phys. Rev.

Low-noise high-voltage DC power supply for nanopositioning applications

Nanopositioning techniques currently applied to characterize physical properties of materials interesting for applications at the microscopic scale rely on high-voltage electronic control circuits that should have the lowest possible noise level. Here we introduce a simple, flexible, and custom-built power supply circuit that can provide +375\,V with a noise level below 10\,ppm. The flexibility of the circuit comes from its topology based on discrete MOSFET components that can be suitable replaced in order to change the polarity as well as the output voltage and current.Comment: 3 pages, 2 figure

Thermodynamic, structural and magnetic properties of vortex nanocrystals

Mediante este proyecto planeamos estudiar experimentalmente las propiedades termodinámicas, estructurales y magnéticas de nanocristales de vórtices. Aplicaremos técnicas de medición a bajas temperaturas demagnetometría Hall local y decoración magnética, y de microfabricación de muestras en la sala limpia del Centro Atómico Bariloche. Se fabricarán muestras superconductoras micrométricas en las que se nuclean nanocristales de vórtices extremadamente anisotrópicos. Se planea además incursionar en la técnica de nanocalorimetría diferencial para realizar mediciones con una técnica alternativa del salto de entropía en la transición de fusión de la materia de vórtices nanocristalina.This project aims to experimentally study the thermodynamic, structural and magnetic properties of vortex nanocrystals. We plan to apply low-temperature measurement techniques as local Hall magnetometry, magnetic decoration, as well as sample microfabrication at the clean room of the Centro Atómico Bariloche. We will produce superconducting samples with micrometric size and use them to nucleate extremely anisotropic vortex nanocrystals. We also plan to start implementing the differential nanocalorimetry technique as to quantify the entropy-jump at the first order vortex melting transition in nanocrystals with an alternative technique

Detection of discretized single-shell penetration in mesoscopic vortex matter

We investigated configurational changes in mesoscopic vortex matter with less than thousand vortices during flux penetration in freestanding 50 $\mu$m diameter disks of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8 +\delta}$. High-resolution AC and DC local magnetometry data reveal oscillations in the transmittivity echoed in peaks in the third-harmonics magnetic signal fainting on increasing vortex density. By means of extra experimental evidence and a simple geometrical analysis we show that these features fingerprint the discretized entrance of single-shells of vortices having a shape that mimics the sample edge

Fingerprint of dynamical charge/spin correlations in the tunneling spectra of colossal magnetoresistive manganites

We present temperature-dependent scanning tunneling spectroscopy measurements on $La_{1-x}Ca_{x}MO_{3}$ ($x\sim0.33$) films with different degrees of biaxial strain. A depletion in normalized conductance around the Fermi level is observed both above and below the insulator-to-metal transition temperature $T_{MI}$, for weakly as well as highly-strained films. This pseudogap-like depletion globally narrows on cooling. The zero-bias conductance decreases on cooling in the insulating phase, reaches a minimum close to $T_{MI}$ and increases on cooling in the metallic phase, following the trend of macroscopic conductivity. These results support a recently proposed scenario in which dynamical short-range antiferromagnetic/charge order correlations play a preeminent role in the transport properties of colossal magnetoresistive manganites [R. Yu \textit{et al}., Phys. Rev. B \textbf{77}, 214434 (2008)].Comment: 9 pages, 4 figure

Hyperuniformity in Type-II Superconductors with Point and Planar Defects

We use vortex matter in type-II superconductors as a playground to study how different types of disorder affect the long wavelength density fluctuations of the system. We find that irrespective of the vortex-vortex interaction, in the case of samples with weak and dense point defects the system presents the hidden order of hyperuniformity characterized by an algebraic suppression of density fluctuations when increasing the system size. We also reveal that, on the contrary, for samples with planar defects hyperuniformity is suppressed since density fluctuations have a tendency to unboundedness on increasing the system size. Although some of these results were known from previous works, this paper makes the fundamental discovery that the ability of planar disorder to suppress hyperuniformity grows on increasing the softness of the structure for more diluted systems.Comment: 5 pages, 2 figures, Conference LT29, Journal of the Physical Society of Japan: Conference Proceedings accepted 202

Disorder, critical currents, and vortex pinning energies in isovalently substituted BaFe2_{2}(As1−x_{1-x}Px_{x})2_{2}

We present a comprehensive overview of vortex pinning in single crystals of the isovalently substituted iron-based superconductor BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$, a material that qualifies as an archetypical clean superconductor, containing only sparse strong point-like pins [in the sense of C.J. van der Beek {\em et al.}, Phys. Rev. B {\bf 66}, 024523 (2002)]. Widely varying critical current values for nominally similar compositions show that flux pinning is of extrinsic origin. Vortex configurations, imaged using the Bitter decoration method, show less density fluctuations than those previously observed in charge-doped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals. Analysis reveals that the pinning force and -energy distributions depend on the P-content $x$. However, they are always much narrower than in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$, a result that is attributed to the weaker temperature dependence of the superfluid density on approaching $T_{c}$ in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$. Critical current density measurements and pinning force distributions independently yield a mean distance between effective pinning centers $\bar{\mathcal L} \sim 90$ nm, increasing with increasing P-content $x$. This evolution can be understood as being the consequence of the P-dependence of the London penetration depth. Further salient features are a wide vortex free "Meissner belt", observed at the edge of overdoped crystals, and characteristic chain-like vortex arrangements, observed at all levels of P-substitution.Comment: 11 page

Multifractal scaling of flux penetration in the Iron-based Superconductor Ba(Fe0.925_{0.925}Co0.075_{0.075})2_{2}As2_2

International audienceThe penetration of magnetic flux fronts in the optimally doped iron based superconductor Ba(Fe$_{0.925}$Co$_{0.075}$)$_{2}$As$_2$ is studied by means of high resolution magneto-optic imaging. The analysis of roughening and growth of the magnetic flux front reveals anomalous scaling properties. While higher-order spatial correlation functions reveal multifractal behavior for the roughening, the usual Kardar-Parisi-Zhang growth exponent is found. Both exponents are found to independent of temperature. We propose a scenario for vortex penetration based on 2D percolation and cluster aggregation in an inhomogeneously disordered superconductor

Disordered hyperuniform vortex matter with rhombic distortions in FeSe at low fields

In the current quest to synthesize hyperuniform materials with interesting applications, addressing the coupling of the objects composing the system to the physical properties of the host medium is crucial. With this aim we study a model system: vortices in the FeSe superconductor subject to a considerable magneto-elastic coupling with the host sample. We reveal that the low-field FeSe vortex structure is of the weakest hyperuniform type possibly due to the relevance of the anisotropic and long-ranged interaction term introduced by the magneto-elastic coupling. This work indicates that it is possible to tailor the hyperuniformity class of material systems by tuning the coupling of interacting objects with elastic properties of the host medium.Comment: 16 figure