arXiv subjects

E. Silva

Publications and source records attributed to E. Silva.

At least 37 records · Page 2Linked to original sources

arXiv2008

Reduction of the field-dependent microwave surface resistance in YBa_2Cu_3O_7 with sub-micrometric BaZrO_3 inclusions as a function of BaZrO_3 concentration

In order to study the vortex pinning determined by artificially introduced pinning centers in the small-vortex displacement regime, we measured the microwave surface impedance at 47.7 GHz in the mixed state of YBa$_{2}$Cu$_{3}$O$_{7-δ}$ thin films, where sub-micrometric BaZrO$_3$ particles have been incorporated. As a function of the BaZrO$_3$ content, we observe that the absolute losses slightly decrease up to a BaZrO$_3$ content of 5%, and then increase. We found that the magnetic-field-induced losses behave differently, in that they are not monotonic with increasing BaZrO$_3$ concentration: at small concentration (2.5%) the field-induced losses increase, but large reduction of the losses themselves, by factors up to 3, is observed upon further increasing the BaZrO$_3$ concentration in the target up to 7%. Using measurements of both surface resistance and surface reactance we estimate vortex pinning-related parameters. We find that BaZrO$_3$ inclusions introduce deep and steep pinning wells. In particular, the minimum height of the energy barrier for single vortices is raised. At larger BaZrO$_3$ content (5% and 7%) the phenomenon is at its maximum, but it is unclear whether it shows a saturation or not, thus leaving room for further improvements.

cond-mat.supr-con↗

arXiv2008

Reliable determination of vortex parameters from measurements of the microwave complex resistivity

We discuss and propose a complete data treatment, in close contact to typical microwave experimental data, in order to derive vortex parameters, such as pinning constant and viscous drag coefficient (also referred to as ``vortex viscosity''), in a way as model-independent as possible. We show that many of the accepted models for the complex resistivity can be described by a single, very general analytical expression. Using typical measurements of real and imaginary resistivity as a function of the applied field, we show that, even for single-frequency measurements, it is always possible to obtain (a) estimates of viscous drag coefficient and pinning constant with well-defined upper and lower bounds and (b) quantitative information about thermal creep. It turns out that neglecting thermal creep, in particular and counterintuitively at low temperatures, might result in a severe overestimation of the viscous drag coefficient. We also discuss the impact of thermal creep on the determination of the pinning constant. The present results might lead to a reconsideration of several estimates of the vortex parameters.

cond-mat.supr-con↗

arXiv2008

Nonlinear c-axis transport in Bi_2Sr_2CaCu_2O_(8+d) from two-barrier tunneling

Motivated by the peculiar features observed through intrinsic tunneling spectroscopy of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ mesas in the normal state, we have extended the normal state two-barrier model for the c-axis transport [M. Giura et al., Phys. Rev. B {\bf 68}, 134505 (2003)] to the analysis of $dI/dV$ curves. We have found that the purely normal-state model reproduces all the following experimental features: (a) the parabolic $V$-dependence of $dI/dV$ in the high-$T$ region (above the conventional pseudogap temperature), (b) the emergence and the nearly voltage-independent position of the "humps" from this parabolic behavior lowering the temperature, and (c) the crossing of the absolute $dI/dV$ curves at a characteristic voltage $V^\times$. Our findings indicate that conventional tunneling can be at the origin of most of the uncommon features of the c axis transport in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. We have compared our calculations to experimental data taken in severely underdoped and slightly underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ small mesas. We have found good agreement between the data and the calculations, without any shift of the calculated dI/dV on the vertical scale. In particular, in the normal state (above $T^\ast$) simple tunneling reproduces the experimental dI/dV quantitatively. Below $T^\ast$ quantitative discrepancies are limited to a simple rescaling of the voltage in the theoretical curves by a factor $\sim$2. The need for such modifications remains an open question, that might be connected to a change of the charge of a fraction of the carriers across the pseudogap opening.

cond-mat.supr-con↗

arXiv2008

Microwave properties of DyBa_2Cu_3O_(7-x) monodomains and related compounds in magnetic fields

We present a microwave characterization of a DyBa$_{2}$Cu$_{3}$O$_{7-x}$ single domain, grown by the top-seeded melt-textured technique. We report the (a,b) plane field-induced surface resistance, $ΔR_s(H)$, at 48.3 GHz, measured by means of a cylindrical metal cavity in the end-wall-replacement configuration. Changes in the cavity quality factor Q against the applied magnetic field yield $ΔR_s(H)$ at fixed temperatures. The temperature range [70 K ; T_c] was explored. The magnetic field $μ_0 H <$ 0.8 T was applied along the c axis. The field dependence of $ΔR_s(H)$ does not exhibit the steep, step-like increase at low fields typical of weak-links. This result indicates the single-domain character of the sample under investigation. $ΔR_s(H)$ exhibits a nearly square-root dependence on H, as expected for fluxon motion. From the analysis of the data in terms of motion of Abrikosov vortices we estimate the temperature dependences of the London penetration depth $λ$ and the vortex viscosity $η$, and their zero-temperature values $λ(0)=$165 nm and $η(0)=$ 3 10$^{-7}$ Nsm$^{-2}$, which are found in excellent agreement with reported data in YBa$_{2}$Cu$_{3}$O$_{7-x}$ single crystals. Comparison of microwave properties with those of related samples indicate the need for reporting data as a function of T/T_c in order to obtain universal laws.

cond-mat.supr-con↗

arXiv2007

Change of strength of vortex pinning in YBCO due to BaZrO_3 inclusions

We probe the short-range pinning properties with the application of microwave currents at very high driving frequencies (47.7 GHz) on YBa$_2$Cu$_3$O$_{7-δ}$ films with and without sub-micrometer BaZrO$_3$ inclusions. We explore the temperature and field ranges 60 K$<T<T_c$ and 0$<μ_0H<$0.8 T, with the field applied along the c-axis. The magnetic field induces a much smaller increase of the microwave resistivity, $Δρ_1(H)+\mathrm{i}Δρ_2(H)$, in YBa$_2$Cu$_3$O$_{7-δ}$/BaZrO$_3$ with respect to pure YBa$_2$Cu$_3$O$_{7-δ}$. $Δρ_1(H)$ is slightly superlinear in pure YBa$_2$Cu$_3$O$_{7-δ}$ (suggesting a possible contribution of thermal activation), but linear or sublinear in YBa$_2$Cu$_3$O$_{7-δ}$/BaZrO$_3$ (suggesting a possible suppression of thermal activation as a consequence of BaZrO$_3$ inclusions). These features persist up to close to $T_c$. We discuss our data in terms of the ratio $r=ΔX_s'(H)/ΔR_s'(H)$ in the framework of the models for the microwave surface impedance in the mixed state. Large $r$ are found in YBa$_2$Cu$_3$O$_{7-δ}$/BaZrO$_3$, with little field dependence. By contrast, smaller values and stronger field dependences are found in pure YBa$_2$Cu$_3$O$_{7-δ}$. We discuss the different field dependence of the pinning constant.

cond-mat.supr-con↗

arXiv2007

Microwave vortex dynamics in Tl-2212 thin films

We present measurements of the effective surface impedance changes due to a static magnetic field, $ΔZ(H,T)=ΔR(H,T)+\rmi ΔX(H,T)$, in a Tl-2212 thin film with $T_c>$ 103 K, grown on a CeO$_2$ buffered sapphire substrate. Measurements were performed through a dielectric resonator operating at 47.7 GHz, for temperatures 60 K$\leq T ΔR(H)$ in almost the whole $(H,T)$ range explored. A careful analysis of the data allows for an interpretation of these results as dominated by vortex dynamics. In the intermediate-high field range we extract the main vortex parameters by resorting to standard high frequency model and by taking into proper account the creep contribution. The pinning constant shows a marked decrease with the field which can be interpreted in terms of flux lines softening associated to an incipient layer decoupling. Small vortex viscosity, by an order of magnitude lower than in Y-123 are found. Some speculations about these findings are provided.

cond-mat.supr-con↗

arXiv2007

Strong Reduction of the Field-Dependent Microwave Surface Resistance in YBCO with BaZrO_3 Inclusions

We present measurements of the magnetic field dependent microwave surface resistance in laser-ablated YBa$_2$Cu$_3$O$_{7-δ}$ films on SrTiO$_3$ substrates. BaZrO$_3$ crystallites were included in the films using composite targets containing BaZrO$_3$ inclusions with mean grain size smaller than 1 $μ$m. X-ray diffraction showed single epitaxial relationship between BaZrO$_3$ and YBa$_2$Cu$_3$O$_{7-δ}$. The effective surface resistance was measured at 47.7 GHz for 60$< T <$90 K and 0$< μ_0H <$0.8 T. The magnetic field had a very different effect on pristine YBa$_2$Cu$_3$O$_{7-δ}$ and YBa$_2$Cu$_3$O$_{7-δ}$/BaZrO$_3$, while for $μ_0H=$0 only a reduction of $T_c$ in the YBa$_2$Cu$_3$O$_{7-δ}$/BaZrO$_3$ film was observed, consistent with dc measurements. At low enough $T$, in moderate fields YBa$_2$Cu$_3$O$_{7-δ}$/BaZrO$_3$ exhibited an intrinsic thin film resistance lower than the pure film. The results clearly indicate that BaZrO$_3$ inclusions determine a strong reduction of the field-dependent surface resistance. From the analysis of the data in the framework of simple models for the microwave surface impedance in the mixed state we argue that BaZrO$_3$ inclusions determine very steep pinning potentials.

cond-mat.supr-con↗

arXiv2007

Strong reduction of field-dependent microwave surface resistance in YBa$_{2}$Cu$_{3}$O$_{7-δ}$ with sub-micrometric BaZrO$_3$ inclusions

We observe a strong reduction of the field induced thin film surface resistance measured at high microwave frequency ($ν=$47.7 GHz) in YBa$_{2}$Cu$_{3}$O$_{7-δ}$ thin films grown on SrTiO$_3$ substrates, as a consequence of the introduction of sub-micrometric BaZrO$_3$ particles. The field increase of the surface resistance is smaller by a factor of $\sim$3 in the film with BaZrO$_3$ inclusions, while the zero-field properties are not much affected. Combining surface resistance and surface reactance data we conclude (a) that BaZrO$_3$ inclusions determine very deep and steep pinning wells and (b) that the pinning changes nature with respect to the pure film.

cond-mat.supr-con↗

arXiv2006

Microwave properties of YBa$_2$Cu$_3$O$_{7-δ}$ films with BaZrO$_3$ nanoinclusions

We present measurements of the microwave complex surface impedance at 47.7 GHz in YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) films deposited by pulsed laser deposition with the explicit goal to introduce BaZrO$_3$ (BZO) nanoinclusions. Composite targets obtained by addition of BZO powder in molar percents ranging from 2.5 to 7 mol.% have been prepared and characterized. Measurements of the microwave surface impedance indicate a broadened transition in zero field, however compensated by a very large pinning frequency, indicating that while intergrain properties are still to be optimized the effect of nanometric inclusions substantially enhances the intragrain vortex pinning.

cond-mat.supr-con↗

arXiv2006

Tunnel and thermal c-axis transport in BSCCO in the normal and pseudogap state

We consider the problem of c-axis transport in double-layered cuprates, in particular with reference to Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ compounds. We exploit the effect of the two barriers on the thermal and tunnel transport. The resulting model is able to describe accurately the normal state c-axis resistivity in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$, from the underdoped side up to the strongly overdoped. We extend the model, without introducing additional parameters, in order to allow for the decrease of the barrier when an external voltage bias is applied. The extended model is found to describe properly the c-axis resistivity for small voltage bias above the pseudogap temperature $T^{*}$, the c-axis resistivity for large voltage bias even below $T_c$, and the differential $dI/dV$ curves taken in mesa structures.

cond-mat.supr-con↗

arXiv2006

Scaling of the microwave magneto-impedance in Tl$_2$Ba$_2$CaCu$_2$O$_{8+δ}$ thin films

We present measurements of the magnetic field-induced microwave complex resistivity changes at 47 GHz in Tl$_2$Ba$_2$CaCu$_2$O$_{8+δ}$ (TBCCO) thin films, in the ranges 58 K$<T<T_{c}$ and 0$<μ_{0}H<$0.8 T. The large imaginary part $Δρ_{2}(H)$ points to strong elastic response, but the data are not easily reconciled with a rigid vortex model. We find that, over a wide range of temperatures, all the pairs of curves $Δρ_{1}(H)$ and $Δρ_{2}(H)$ can be collapsed on a pair of scaling curves $Δρ_{1}[H/H^{*}(T)]$, $Δρ_{2}[H/H^{*}(T)]$, with the same scaling field $H^{*}(T)$. We argue that $H^{*}(T)$ is related to the loss of vortex rigidity due to a vortex transformation.

cond-mat.supr-con↗

arXiv2006

Vortex state microwave response in superconducting cuprates and MgB$_2$

We investigate the physics of the microwave response in YBa$_{2}$Cu$_{3}$O$_{7-δ}$, SmBa$_{2}$Cu$_{3}$O$_{7-δ}$ and MgB$_{2}$ in the vortex state. We first recall the theoretical basics of vortex-state microwave response in the London limit. We then present a wide set of measurements of the field, temperature, and frequency dependences of the vortex state microwave complex resistivity in superconducting thin films, measured by a resonant cavity and by swept-frequency Corbino disk. The combination of these techniques allows for a comprehensive description of the microwave response in the vortex state in these innovative superconductors. In all materials investigated we show that flux motion alone cannot take into account all the observed experimental features, neither in the frequency nor in the field dependence. The discrepancy can be resolved by considering the (usually neglected) contribution of quasiparticles to the response in the vortex state. The peculiar, albeit different, physics of the superconducting materials here considered, namely two-band superconductivity in MgB$_{2}$ and superconducting gap with lines of nodes in cuprates, give rise to a substantially increased contribution of quasiparticles to the field-dependent microwave response. With careful combined analysis of the data it is possible to extract or infer many interesting quantities related to the vortex state, such as the temperature-dependent characteristic vortex frequency and vortex viscosity, the field dependence of the quasiparticle density, the temperature dependence of the $σ$-band superfluid density in MgB$_{2}$

cond-mat.supr-con↗

arXiv2006

Vortex state microwave resistivity in Tl-2212 thin films

We present measurements of the field induced changes in the 47 GHz complex resistivity, $Δ\tilde ρ(H,T)$, in Tl$_{2}$Ba$_{2}$CaCu$_{2}$O$_{8+x}$ (TBCCO) thin films with $T_{c}\simeq$ 105 K, prepared on CeO$_{2}$ buffered sapphire substrates. At low fields ($μ_{0}H<$10 mT) a very small irreversible feature is present, suggesting a little role of intergranular phenomena. Above that level $Δ\tilde ρ(H,T)$ exhibits a superlinear dependence with the field, as opposed to the expected (at high frequencies) quasilinear behaviour. We observe a crossover between predominantly imaginary to predominantly real (dissipative) response with increasing temperature and/or field. In addition, we find the clear scaling property $Δ\tilde ρ(H,T)=Δ\tilde ρ[H/H^{*}(T)]$, where the scaling field $H^{*}(T)$ maps closely the melting field measured in single crystals. We discuss our microwave results in terms of loss of flux lines rigidity.

cond-mat.supr-con↗

arXiv2006

Mixed-state microwave response in superconducting cuprates

We report measurements of the magnetic-field induced microwave complex resistivity in REBa$_{2}$Cu$_{3}$O$_{7-δ}$ thin films, with RE = Y, Sm. Measurements are performed at 48 GHz by means of a resonant cavity in the end-wall-replacement configuration. The magnetic field dependence is investigated by applying a moderate (0.8 T) magnetic field along the c-axis. The measured vortex state complex resistivity in YBa$_{2}$Cu$_{3}$O$_{7-δ}$ and SmBa$_{2}$Cu$_{3}$O$_{7-δ}$ is analyzed within the well-known models for vortex dynamics. It is shown that attributing the observed response to vortex motion alone leads to inconsistencies in the as-determined vortex parameters (such as the vortex viscosity and the pinning constant). By contrast, attributing the entire response to field-induced pair breaking leads to a nearly quantitative description of the data.

cond-mat.supr-con↗

arXiv2006

Near orders and codes

Hoholdt, van Lint and Pellikaan used order functions to construct codes by means of Linear Algebra and Semigroup Theory only. However, Geometric Goppa codes that can be represented by this method are mainly those based on just one point. In this paper we introduce the concept of near order function with the aim of generalize this approach in such a way that a of wider family of Geometric Goppa codes can be studied on a more elementary setting.

cs.IT↗

arXiv2005

Mixed state microwave resistivity of cuprate superconductors

We present a compared experimental investigation of the (a,b) plane vortex-state complex resistivity at 48 GHz in YBaCuO, SmBaCuO and BiSrCaCuO (2212). In YBaCuO and SmBaCuO the field dependence of the response can be consistently described by a combination of flux flow and strong pair breaking due to the presence of lines of nodes in the gap. In BiSrCaCuO (2212), by contrast, the data might be described by the pair breaking alone.

cond-mat.supr-con↗

arXiv2004

c-axis transport and phenomenology of the pseudo-gap state in $Bi_2Sr_2CaCu_2O_{8+δ}$

We measure and analyze the resistivity of $Bi_2Sr_2CaCu_2O_{8+δ}$ crystals for different doping $δ$. We obtain the fraction of carrier $η(T,δ) = n_g/n_{TOT}$ that do not participate to the c-axis conductivity. All the curves $η(T,δ)$ collapse onto a universal curve when plotted against a reduced temperature $x=[T-Θ(δ)]/Δ^{*}(δ)$. We find that at the superconducting transition $n_g$ is doping independent. We also show that a magnetic field up to 14 T does not affect the degree of localization in the (a,b) planes but widens the temperature range of the x-scaling by suppressing the superconducting phase coherence.

cond-mat.supr-con↗

arXiv2004

Measurements of surface impedance of superconductors as a function of frequency in microwave range

We report measurements of the complex resistivity in YBCO and MgB$_2$ thin films over a continuous frequency spectrum in the microwave range, making use of a Corbino disk geometry. The paper mainly focuses on the extraction of the resistivity from raw data, displaying data analysis procedure and its limits of validity. We obtain and show resistivity curves as a function of frequency and temperature denoting a frequency dependent widening of the superconducting transition.

cond-mat.supr-con↗