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Search Results: 1 - 10 of 144967 matches for " Manuel V Ramallo "
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An effective-charge model for the trapping of impurities of fluids in channels with nanostructured walls
Manuel V Ramallo
Nanoscale Research Letters , 2013, DOI: 10.1186/1556-276X-8-19
Abstract: We present model equations for the trapping and accumulation of particles in a 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. The effective charge is gradually reduced as the flow runs through the channel and the trapped impurities cover the internal walls. Based on the proposed equations, the position and time dependence of the areal density of trapped impurities, and the filtration performance, may be calculated. It is proposed that experimentally testing these results may help to understand the enhanced trapping capability observed in many diverse nanotextured channel structures.
On the energy saved by interlayer interactions in the superconducting state of cuprates
Manuel V. Ramallo
Physics , 2003, DOI: 10.1209/epl/i2003-10075-5
Abstract: A Ginzburg-Landau-like functional is proposed reproducing the main low-energy features of various possible high-Tc superconducting mechanisms involving energy savings due to interlayer interactions. The functional may be used to relate these savings to experimental quantities. Two examples are given, involving the mean-field specific heat jump at Tc and the superconducting fluctuations above Tc. Comparison with existing data suggests, e.g., that the increase of Tc due to the so-called interlayer tunneling (ILT) mechanism of interlayer kinetic-energy savings is negligible in optimally-doped Bi-2212.
Effects of critical temperature inhomogeneities on the voltage-current characteristics of a planar superconductor near the Berezinskii-Kosterlitz-Thouless transition
Noelia Coton,Manuel V. Ramallo,Felix Vidal
Physics , 2011, DOI: 10.1088/0953-2048/24/8/085013
Abstract: We analyze numerically how the voltage-current (V-I) characteristics near the so-called Berezinskii-Kosterlitz-Thouless (BKT) transition of 2D superconductors are affected by a random spatial Gaussian distribution of critical temperature inhomogeneities with long characteristic lengths (much larger than the in-plane superconducting coherence length amplitude). Our simulations allow to quantify the broadening around the average BKT transition temperature of both the exponent alpha in V I^alpha and of the resistance V/I. These calculations reveal that strong spatial redistributions of the local current will occur around the transition as either I or the temperature T are varied. Our results also support that the condition alpha=3 provides a good estimate for the location of the average BKT transition temperature, and that extrapolating to alpha->1 the alpha(T) behaviour well below the transition provides a good estimate for the average mean-field critical temperature.
Comment on "Phase fluctuation superconductivity in overdoped La2-xSrxCuO4"
Jesús Mosqueira,Manuel V. Ramallo,Félix Vidal
Physics , 2011,
Abstract: By measuring the in-plane resistivity as a function of temperature and magnetic field in overdoped La2-xSrxCuO4 (LSCO) superconductors, in a recent Letter Rourke et al. [Nature Physics 7, 455 (2011)] have addressed the long standing but still open issue of the interplay between pseudogap and superconducting fluctuations in cuprates. Unfortunately, these authors have analyzed their interesting measurements without taking into account the unavoidable inhomogeneities of the zero-field critical temperature Tc0 associated with the random distribution of the Sr ions, in spite of the different works on this issue in the same compounds. Here we will show that such a shortcoming invalidates most of the conclusions of Rourke et al.
Measurements of the doping effects on the in-plane paraconductivity in cuprate superconductors
Felix Vidal,Manuel V. Ramallo,Gonzalo Ferro,Jose Antonio Veira
Physics , 2006,
Abstract: We will summarize here some of our measurements of the superconducting fluctuations effects on the in-plane electrical resistivity (the so-called in-plane paraconductivity) in La_{2-x}Sr_xCuO_4 thin films with different Sr content. Our results suggest that these superconducting fluctuations effects are not related to the opening of a pseudogap in the normal-state of underdoped compounds.
Breakdown by a magnetic field of the superconducting fluctuations in the normal state in Pb_{1-x}In_x alloys
Felix Soto,Carlos Carballeira,Jesus Mosqueira,Manuel V. Ramallo,Mauricio Ruibal,Jose A. Veira,Felix Vidal
Physics , 2003,
Abstract: The effects induced on the magnetization by coherent fluctuating Cooper pairs in the normal state have been measured in Pb_{1-x}In_x alloys up to high magnetic fields, of amplitudes above Hc2(0), the upper critical field extrapolated to T=0K. Our results show that in dirty alloys these superconducting fluctuation effects are, in the entire H-T phase diagram above Hc2(T), independent of the amount of impurities and that they vanish when H~1.1Hc2(0). These striking results are consistent with a phenomenological estimate that takes into account the limits imposed by the uncertainty principle to the shrinkage, when H increases, of the superconducting wave function.
Comment on "High Field Studies of Superconducting Fluctuations in High-Tc Cuprates. Evidence for a Small Gap distinct from the Large Pseudogap"
Manuel V. Ramallo,Carlos Carballeira,Ramon I. Rey,Jesus Mosqueira,Felix Vidal
Physics , 2011, DOI: 10.1103/PhysRevB.85.106501
Abstract: By using high magnetic field data to estimate the background conductivity, Rullier-Albenque and coworkers have recently published [Phys.Rev.B 84, 014522 (2011)] experimental evidence that the in-plane paraconductivity in cuprates is almost independent of doping. In this Comment we also show that, in contrast with their claims, these useful data may be explained at a quantitative level in terms of the Gaussian-Ginzburg-Landau approach for layered superconductors, extended by Carballeira and coworkers to high reduced-temperatures by introducing a total-energy cutoff [Phys.Rev.B 63, 144515 (2001)]. When combined, these two conclusions further suggest that the paraconductivity in cuprates is conventional, i.e., associated with fluctuating superconducting pairs above the mean-field critical temperature.
Observation of enhanced fluctuation diamagnetism in lanthanum superconductors with dilute magnetic impurities
Felix Soto,Lucia Cabo,Jesus Mosqueira,Manuel V. Ramallo,Jose A. Veira,Felix Vidal
Physics , 2005, DOI: 10.1209/epl/i2005-10423-5
Abstract: The fluctuation-induced diamagnetism (FD), associated with the presence of precursor Cooper pairs in the normal state, has been measured in lanthanum with dilute magnetic (Pr) and nonmagnetic (Lu) impurities. It is found that while for pure La and La-Lu alloys the FD agrees, as expected, with the theoretical predictions, it is much larger for La-Pr alloys (around a factor 5 for La-2at.%Pr). These results suggest the existence of an indirect contribution to the FD arising from the interaction between fluctuating Cooper pairs and magnetic impurities.
Breakdown by a magnetic field of the superconducting fluctuations in the normal state: A simple phenomenological explanation
Manuel V. Ramallo,Jesus Mosqueira,Carlos Carballeira,Felix Soto,Felix Vidal
Physics , 2006, DOI: 10.1016/j.jpcs.2005.10.028
Abstract: We first summarize our recent observations, through magnetization measurements in different low-Tc superconductors, of a rather sharp disappearance of the superconducting fluctuations in the normal state when the magnetic field approaches Hc2(0), the upper critical field extrapolated to T=0K. We propose that a crude phenomenological description of the observed effects may be obtained if the quantum limits associated with the uncertainty principle are introduced in the Gaussian-Ginzburg-Landau description of the fluctuation-induced magnetization.
On the consequences of the uncertainty principle on the superconducting fluctuations well inside the normal state
Felix Vidal,Carlos Carballeira,Severiano R. Curras,Jesus Mosqueira,Manuel V. Ramallo,Jose Antonio Veira,Jose Vina
Physics , 2001, DOI: 10.1209/epl/i2002-00190-3
Abstract: We first argue that the collective behaviour of the Cooper pairs created by thermal fluctuations well above the superconducting transition temperature, Tc, is dominated by the uncertainty principle which, in particular, leads to a well-defined temperature, T^C, above which the superconducting coherence vanishes. On the grounds of the BCS approach, the corresponding reduced-temperature, ln(T^C/Tc), is estimated to be around 0.55, i.e., above T^C \approx 1.7Tc coherent Cooper pairs cannot exist. The implications of these proposals on the superfluid density are then examined using the Gaussian-Ginzburg-Landau approximation. Then we present new measurements of the thermal fluctuation effects on the electrical conductivity and on the magnetization in different low- and high-Tc superconductors with different dopings which are in excellent agreement with these proposals and that demonstrate the universality of ln(T^C/Tc).
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