arXiv subjects

M. V. Ramallo

Publications and source records attributed to M. V. Ramallo.

13 recordsLinked to original sources

arXiv2024

Dimensional crossovers in the Gaussian critical fluctuations above $T_c$ of two-layer and three-layer superconductors

By using a Ginzburg-Landau functional in the Gaussian approximation, we calculate the energy of superconducting fluctuations above the transition, at zero external magnetic field, of a system composed by a small number $N$ of parallel two-dimensional superconducting planes, each of them Josephson coupled to its first neighbour, with special focus in the $N=2$ and $3$ cases. This allows us to obtain expressions for the critical contributions to various observables (fluctuation specific heat and magnetic susceptibility and Aslamazov-Larkin paraconductivity). Our results suggest that these systems may display deviations from pure 2D behaviour and interesting crossover effects, with both similitudes and differences to those known to occur in infinite-layers superconductors. Some challenges for future related research are also outlined.

cond-mat.supr-con↗

arXiv2024

Calculations of some doping nanostructurations and patterns improving the functionality of high-temperature superconductors for bolometer device applications

We calculate the effects of doping nanostructuration and the patterning of thin films of high-temperature superconductors (HTS) with the aim of optimizing their functionality as sensing materials for resistive transition-edge bolometer devices (TES). We focus, in particular, on spatial variations of the carrier doping into the CuO$_2$ layers due to oxygen off-stoichiometry, (that induce, in turn, critical temperature variations) and explore following two major cases of such structurations: First, the random nanoscale disorder intrinsically associated to doping levels that do not maximize the superconducting critical temperature; our studies suggest that this first simple structuration already improves some of the bolometric operational parameters with respect to the conventional, nonstructured HTS materials used until now. Secondly, we consider the imposition of regular arrangements of zones with different nominal doping levels (patterning); we find that such regular patterns may improve the bolometer performance even further. We find one design that improves, with respect to nonstructured HTS materials, both the saturation power and the operating temperature width by more than one order of magnitude. It also almost doubles the response of the sensor to radiation.

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arXiv2024

A scenario for the critical fluctuations near the transition of few-bilayer films of high-temperature cuprate superconductors

We study the critical fluctuations near the resistive transition of very thin films of high-temperature cuprate superconductors composed of a number $N$ of only a few unit cells of superconducting bilayers. For that, we solve the fluctuation spectrum of a Gaussian-Ginzburg-Landau model for few-bilayers superconductors considering two alternating Josephson interlayer interaction strengths, and we obtain the corresponding paraconductivity above the transition. Then, we extend these calculations to temperatures below the transition through expressions for the Ginzburg number and Kosterlitz--Thouless-like critical region. When compared with previously available data in YBa$_2$Cu$_3$O$_{7-δ}$ few-bilayers systems, with $N=1$ to $4$, our results seem to provide a plausible scenario for their critical regime.

cond-mat.supr-con↗

arXiv2024

Superconductivity in Nanosystems: A Fruitful Path to New Phenomenology in Quantum Materials

We reason that various recent works by different groups reporting new phenomenologies in superconductors can be understood in a unifying way as instances of the appearance of novel competitions (or synergies in some cases) between the coexisting orders at play in superconducting materials. In particular, we argue that the main common feature of such phenomenologies is to have emerged from the induction, by nanoengineering, of novel characteristic lengths for each order, or of custom regular spatial patterns affecting them. We claim, thus, that a fruitful path to discover new phenomenology is opened by these and future searches of novel nanostructurations of superconducting materials.

cond-mat.supr-con↗

arXiv2024

Design of structured La$_{2-x}$Sr$_{x}$CuO$_{4}$ films as superconducting transition-edge sensors at 4.2K

We calculate the effects of carrier-density structuration and patterning on thin films of the cuprate superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$, in order to optimize its functional characteristics as sensing material for resistive transition-edge bolometers at liquid-He temperature. We perform finite-element computations considering two major contributions to structuration: The intrinsic random nanoscale disorder associated to carrier density nonstoichiometry, plus the imposition of regular arrangements of zones with different nominal carrier densities. Using ad-hoc seek algorithms, we obtain various structuration designs that markedly improve the bolometric performance, mainly the saturation power and dynamic range. Bolometric operation becomes favorable even in the easier-to-implement constant current mode of measurement.

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arXiv2024

Nanostructured micrometric-pore membranes for nanofiltration: Micrometric geometry may optimize performance, energy efficiency and operational lifetime

Media and membranes composed of micrometric-diameter pores are well known in academia and industry to be capable of efficacious nanofiltration of fluids once the pore inner surfaces are coated with nanostructures. Given the large mismatch between the two very different scales of these hybrid systems, it could be expected that trapping of nanoimpurities would almost entirely depend on the characteristics of the nanostructures. However, we show here that the micrometric-scale nominal geometry does have noticeable impact on the nanofiltration performance, on its evolution with time, and on the energy spent per trapped impurity. For that, we apply stochastic calculations customized to combine the cumulative probabilities of wall-impurity attraction and binding, supplemented with continuity equations as the fluid flows; this allows tracking the nanofiltration without a many-particle simulation, prohibitive in such multi-scale system. We focus on the influence of the micrometric nominal geometry over filtration features like logarithmic removal value LRV, operational lifetimes, energy balances, or spatial profiles of the trapped-impurity layer in the pore inner walls. Our results identify some pore geometries (e.g., decreasing-conical and sinusoidal-corrugated) with about 4-fold larger initial nanofiltering performance and operational lifetime than simple cylinders of the same average diameter. The optimal geometry is also shown to depend on the LRV value acceptable for each specific application.

cond-mat.mes-hall↗

arXiv2019

The conductivity and the magnetization around Tc in optimally-doped YBa$_2$Cu$_3$O$_{7-δ}$ revisited: quantitative analysis in terms of fluctuating superconducting pairs

We first present detailed measurements of the rounding behavior around the superconducting transition temperature, $T_c$, of the in-plane electrical conductivity, magnetoconductivity and magnetization, including the low and moderate magnetic field regimes, in a high-quality single crystal and a thin film of the prototypical optimally-doped YBa$_2$Cu$_3$O$_{7-δ}$ (OPT Y-123), in which the inhomogeneity effects are minimized. Then, it is presented a comparison of these experimental data with the phenomenological Ginzburg-Landau (GL) approach that takes into account the unavoidable contribution of the fluctuating pairs, the only theoretical scenario allowing at present to analyze these roundings at the quantitative level. These analyses demonstrate that the measured rounding effects around $T_c$ may be explained quantitative and consistently in terms of the GL scenario, even up to the rounding onset temperatures if the quantum localization, associated with the shrinkage of the superconducting wavefunction, is taken into account. The implications of our results on the pseudogap physics of optimally-doped cuprates are also discussed.

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arXiv2015

Magnetic field dependence of the precursor diamagnetism in La superconductors with magnetic Pr impurities

The interplay between fluctuating Cooper pairs and magnetic impurities in conventional BCS low-Tc superconductors has been studied through measurements of the magnetic field dependence of the fluctuation diamagnetism (FD) above Tc in lanthanum with praseodymium impurities. These measurements provide a crucial confirmation of our previous observation [Europhys. Lett. 73, 587 (2006)] that in the dilute impurity regime the FD increases almost linearly with the concentration of magnetic impurities. This striking effect is attributed to a variation due to the presence of the fluctuating Cooper pairs of the coupling between magnetic impurities. To describe these results at a phenomenological level, we propose a Gaussian Ginzburg-Landau model for the FD which includes an indirect contribution proportional to both the impurities concentration and the Cooper pairs density. Our approach is able to explain simultaneously the FD increase due to magnetic impurities and its decrease with the application of large magnetic fields.

cond-mat.supr-con↗

arXiv2013

Critical temperatures for superconducting phase-coherence and condensation in La2-xSrxCuO4

A pivotal ongoing debate about cuprate superconductors (HTS) is the location of the transition temperatures for the superconducting wave function phase coherence and condensation, Tphase and Tcond. This shall elucidate which of two very different interactions dictate the macroscopic superconducting phase diagram of HTS: either those between normal-state carriers or those between preformed vortices and antivortices. Here, we present unambiguous experimental determinations of Tphase and Tcond in the prototypical HTS La2-xSrxCuO4 as a function of the doping level x. Tphase is measured as a sharp change in the exponent alpha of the voltage-current characteristics (V proportional to I^alpha). Tcond is determined from the critical rounding of the ohmic resistivity above Tphase. Our measurements indicate that the transition to macroscopic superconductivity is accompanied by phase coherence due to vortex-antivortex binding and also that, for all x, Tcond lies only a few Kelvin above Tphase, limiting then the shift of the transition due to vortex-antivortex correlations.

cond-mat.supr-con↗

arXiv2013

New results on the anomalous precursor diamagnetism in the underdoped La1.9Sr0.1CuO4 superconductor

The new results summarized here, including a brief comparison with the paraconductivity, further suggest that the anomalous precursor (above Tc) diamagnetism recently observed in the underdoped La1.9Sr0.1CuO4 superconductor could be attributed to the presence, in addition to the conventional superconducting pair fluctuations, of Tc-inhomogeneities with long characteristic lengths associated with chemical disorder.

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arXiv2013

Precursor superconducting diamagnetism in YBa2Cu3O7-d with in-plane or intercalated magnetic impurities

The magnetization around the superconducting transition was measured in YBa2Cu3O7-d with magnetic impurities in the CuO2 layers (Cu substituted by Zn or Ni) or between them (Y substituted by Gd or Pr). While some of these impurities have an important effect on the superconducting transition temperature (Tc), the precursor diamagnetism observed above Tc is not appreciably affected. This result contrasts with recent observations in a conventional BCS superconductor (La), in which the precursor diamagnetism was found to increase several orders of magnitude with the addition of a small amount (a few atomic percent) of Gd or Pr magnetic impurities.

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arXiv2013

Comment on 'Diamagnetism and Cooper pairing above Tc in cuprates'

It is shown that the magnetization rounding measured by L. Li and coworkers above the superconducting transition in optimally doped YBa2Cu3O7-d crystals under magnetic fields up to 14 T [Phys. Rev. B 81, 054510 (2010), arXiv:0906.1823] may be explained at a phenomenological level in terms of the mean field Gaussian-Ginzburg-Landau (GGL) approach for layered superconductors. This result challenges the claims of L. Li and coworkers, who write '[...] we are observing the phase-disordering mechanism, rather than Gaussian mean-field fluctuations', but it is in full agreement with earlier magnetization measurements by different authors in optimally-doped YBa2Cu3O7-d under lower magnetic fields. The adequacy of the mean-field Ginzburg-Landau descriptions is further confirmed when analyzing in terms of the GL scaling in the lowest-Landau-level (GL-LLL) approximation the magnetization data reported below Tc by L. Li and coworkers.

cond-mat.supr-con↗

arXiv2012

An effective-charge model for the trapping of impurities of fluids in channels with nanostructured walls

We present model equations for the trapping and accumulation of particles in a short cylindrical channel with nanostructured inner walls when a fluid passes through, carrying a moderate load of impurities. The basic ingredient of the model is the introduction of a phenomenological "effective-charge density" of the walls, related to the electrical charges exposed in the nanotexture, and which is gradually reduced as the flow runs through the channel and the trapped impurities cover the internal walls. By solving the proposed equations, three regimes are predicted for the channel: a linear or clean-filter regime, a logarithmic or half-saturation regime, and the saturation limit. It is proposed that experimentally testing these regimes may help to understand the enhanced trapping capability observed in many diverse nanotextured channel structures.

cond-mat.mes-hall↗