arXiv subjects

Pablo Esquinazi

Publications and source records attributed to Pablo Esquinazi.

16 recordsLinked to original sources

arXiv2019

Control of Magnetic Order in Spinel ZnFe$_2$O$_4$ Thin Films Through Intrinsic Defect Manipulation

We present a systematic study of the magnetic properties of semiconducting ZnFe$_2$O$_4$ thin films fabricated by pulsed laser deposition at low and high oxygen partial pressure and annealed in oxygen and argon atmosphere, respectively. The magnetic response is enhanced by annealing the films at 250$^{\circ}$C and diminished at annealing temperatures above 300$^{\circ}$C. The initial increase is attributed to the formation of oxygen vacancies after argon treatment, evident by the increase in the low energy absorption at $\sim$ 0.9 eV involving Fe$^{2+}$ cations. The weakened magnetic response is related to a decline in disorder with a cation redistribution toward a normal spinel configuration. The structural renormalization is consistent with the decrease and increase in oscillator strength of respective electronic transitions involving tetrahedrally (at $\sim$ 3.5 eV) and octahedrally (at $\sim$ 5.7 eV) coordinated Fe$^{3+}$ cations.

physics.app-ph↗

arXiv2017

Electrical Transport Properties of Polycrystalline and Amorphous TiO2 Single Nanotubes

The electrical transport properties of anodically grown TiO2 nanotubes was investigated. Amorphous nanotubes were anodically grown on titanium foil and transformed through annealing into the anatase phase. Amorphous and polycrystalline single nanotubes were isolated and contacted for measurements of the electrical resistance. Non-linear current-voltage characteristics were explained using the fluctuation induced tunneling conduction model. A clear enhancement of the conductance was induced in an insulating anatase nanotube through low-energy Ar/H ion irradiation. Confocal Raman spectroscopy shows that the annealed samples were in anatase phase and a blueshift due to phonon confinement was observed.

cond-mat.mes-hall↗

arXiv2015

Nanostructured ZnO films: a study of molecular influence on transport properties by impedance spectroscopy

Nanomaterials based on ZnO have been used to build glucose sensors due to its high isoelectric point, which is important when a protein like glucose oxidase (GOx) is attached to a surface. It also creates a biologically friendly environment to preserve the activity of the enzyme. In this work we study the electrical transport properties of ZnO thin films (TFs) and single crystals (SC) in contact with different solutions by using impedance spectroscopy. We have found that the composition of the liquid, by means of the charge of the ions, produces strong changes in the transport properties of the TF. The enzyme GOx and phosphate buffer solutions have the major effect in the conduction through the films, which can be explained by the entrapment of carriers at the grain boundaries of the TFs. These results can help to design a new concept in glucose biosensing.

cond-mat.mtrl-sci↗

arXiv2015

Detection of defect-induced magnetism in low-dimensional ZnO structures by Magnetophotocurrent

The detection of defect-induced magnetic order in single low-dimensional oxide structures is in general difficult because of the relatively small yield of magnetically ordered regions. In this work we have studied the effect of an external magnetic field on the transient photocurrent measured after light irradiation on different ZnO samples at room temperature. We found that a magnetic field produces a change in the relaxation rate of the transient photocurrent only in magnetically ordered ZnO samples. This rate can decrease or increase with field depending whether the magnetic order region is in the bulk or only at the surface of the ZnO sample. The phenomenon reported here is of importance for the development of magneto-optical low-dimensional oxides devices and provide a new guideline for the detection of magnetic order in low-dimensional magnetic semiconductors.

cond-mat.mtrl-sci↗

arXiv2014

Granular superconductivity below 5~K in SPI-II pyrolytic graphite

We have studied the transport properties of transmission electron microscope (TEM) lamellae obtained from a pyrolytic graphite sample of grade B (SPI-II) with electrical contacts at the edges of the graphene layers. The temperature, magnetic field, input current dependence of the resistance as well as the current-voltage characteristic curves are compatible with the existence of granular superconductivity below 5~K. TEM pictures of the studied lamellae reveal clear differences of the embedded interfaces to those existing in more ordered pyrolytic samples, which appear to be the origin for the relatively lower temperatures at which the granular superconductivity is observed.

cond-mat.supr-con↗

arXiv2014

On the superconductivity of graphite interfaces

We propose an explanation for the appearance of superconductivity at the interfaces of graphite with Bernal stacking order. A network of line defects with flat bands appears at the interfaces between two slightly twisted graphite structures. Due to the flat band the probability to find high temperature superconductivity at these quasi one-dimensional corridors is strongly enhanced. When the network of superconducting lines is dense it becomes effectively two-dimensional. The model provides an explanation for several reports on the observation of superconductivity up to room temperature in different oriented graphite samples, graphite powders as well as graphite-composite samples published in the past.

cond-mat.supr-con↗

arXiv2014

Interface size dependence of the Josephson critical behaviour in pyrolytic graphite

We have studied the transport properties of TEM lamellae obtained from a pyrolytic graphite sample with electrical contacts at the edges of the embedded interfaces. The temperature dependence of the resistance as well as the current-voltage characteristic curves are compatible with the existence of Josephson coupled superconducting regions. The transition temperature at which the Josephson behavior sets in, decreases with the interface width and vanishes for width below 200~nm. This unexpected behavior is apparently due to the influence of weak localization effects on the superconducting critical temperature.

cond-mat.supr-con↗

arXiv2013

Invited review: Graphite and its hidden superconductivity

We review experimental results, from transport to magnetization measurements, on different graphite samples, from bulk oriented graphite, thin graphite films to transmission electron microscope lamellae, that indicate the existence of granular superconductivity at temperatures above 100 K. The accumulated evidence speaks for a localization of the superconducting phase(s) at certain interfaces embedded in semiconducting crystalline regions with Bernal stacking order.

cond-mat.supr-con↗

arXiv2008

Graphene Physics in Graphite

Single layers of carbon dubbed "graphenes", from which graphite is built, have attracted broad interest in the scientific community because of recent exciting experimental results. Graphene is interesting from a fundamental research perspective, as well as for potential technological applications. Here, we provide a brief overview of recent developments in this field, focusing especially on the electronic properties of graphite. Experimental evidence indicates that high-quality graphite is a multi-layer system with nearly decoupled 2D graphene planes. Based on experimental observations, we anticipate that thin graphite samples and not single layers will be the most promising candidates for graphene-based electronics.

cond-mat.mtrl-sci↗

arXiv2004

Decoupling transition of two coherent vortex arrays within the surface superconductivity state

In magnetic fields applied within the angular range of the surface superconductivity state a magnetically anisotropic layered medium is created in structurally isotropic, sufficiently thick niobium films. Surface (Kulik) vortices residing in the superconducting sheaths on both main film surfaces in tilted fields are shown to undergo a decoupling transition from a coherent to an independent behavior, similar to the behavior observed for Giaever transformer. At the transition a feature in pinning properties is measured, which implies different pinning for the lattice of surface vortices coherently coupled through the normal layer and for two decoupled vortex arrays in the superconducting surface sheaths.

cond-mat.supr-con↗

arXiv2004

Magnetism in Carbon Structures

We discuss different magnetic phenomena observed in carbon-based structures, in particular the diamagnetism, paramagnetism and ferromagnetism observed in graphite, disordered carbon, fullerenes and irradiated carbon structures.

cond-mat.str-el↗

arXiv2004

Magnetothermal Transport of Oriented Graphite at Low Temperatures

We have studied the magnetic field dependence of the thermal conductivity $κ(T,B)$ of highly oriented pyrolytic graphite samples at temperatures 0.2K$ \le T < 10$K and fields 0T$ \le B \le 9$T. The samples show clear deviations from the Wiedemann-Franz law with a kink behavior at fields $B \sim 0.1 $T near the metal-insulator transition observed in electrical resistivity measurements. We further show that the oscillations in the thermal conductivity at the quantum limit $B > 1 $T, which are correlated with the Landau quantization observed in Hall measurements, increase in amplitude with temperature following a $\sim T^3$ law at $T > 0.2 $K and show a maximum at $T \sim 6 $K, suggesting that they are phonons mediated.

cond-mat.mes-hall↗

arXiv2004

Acoustic Properties of Amorphous Solids at Very Low Temperatures: The Quest for Interacting Tunneling States

We discuss the strain dependence of the acoustic properties of amorphous metals in both normal and superconducting states, in the temperature range 0.1 mK $ \le T \le 1 $K. A crossover is found when the strain energy is of the order of the effective interaction energy between tunneling systems at the corresponding temperature. Our results provide clear evidence for the interaction between tunneling systems, whose energy is in quantitative agreement with theoretical expectations, and reveal that without the knowledge of the corresponding strain dependences, the measured temperature dependences below $\sim 50 $mK of the acoustic properties of disordered solids are rather meaningless.

cond-mat.dis-nn↗

arXiv2003

Influence of temperature on the nodal properties of the longitudinal thermal conductivity of YBa$_2$Cu$_3$O$_{7-x}$

The angle dependence at different temperatures of the longitudinal thermal conductivity $κ_{xx}(θ)$ in the presence of a planar magnetic field is presented. In order to study the influence of the gap symmetry on the thermal transport angular scans were measured up to a few Kelvin below the critical temperature $T_c$. We found that the four-fold oscillation of $κ_{xx}(θ)$ vanishes at $T > 20 $K and transforms into a one-fold oscillation with maximum conductivity for a field of 8 T applied parallel to the heat current. Nevertheless, the results indicate that the d-wave pairing symmetry is the main pairing symmetry of the order parameter up to $T_c$. Numerical results of the thermal conductivity using an Andreev reflection model for the scattering of quasiparticles by supercurrents under the assumption of d-wave symmetry provide a semiquantitative description of the overall results.

cond-mat.supr-con↗

arXiv2002

Thermal Conductivity Tensor in YBa$_2$Cu$_3$O$_{7-x}$: Effects of a Planar Magnetic Field

We have measured the thermal conductivity tensor of a twinned YBa$_2$Cu$_3$O$_{7-x}$ single crystal as a function of angle $θ$ between the magnetic field applied parallel to the CuO$_2$ planes and the heat current direction, at different magnetic fields and at T=13.8 K. Clear fourfold and twofold variations in the field-angle dependence of $κ_{xx}$ and $κ_{xy}$ were respectively recorded in accordance with the d-wave pairing symmetry of the order parameter. The oscillation amplitude of the transverse thermal conductivity $κ^0_{xy}$ was found to be larger than the longitudinal one $κ^0_{xx}$ in the range of magnetic field studied here ($0 T $$ \le B \le 9 $$T$). From our data we obtain quantities that are free from non-electronic contributions and they allow us a comparison of the experimental results with current models for the quasiparticle transport in the mixed state.

cond-mat.supr-con↗

arXiv2000

The Labusch Parameter of a Driven Flux Line Lattice in YBa$_2$Cu$_3$O$_7$ Superconducting Films

We have investigated the influence of a driving force on the elastic coupling (Labusch parameter) of the field-cooled state of the flux line lattice (FLL) in 400 nm thick YBa$_2$Cu$_3$O$_7$ superconducting films. We found that the FLL of a field-cooled state without driving forces is not in an equilibrium state. Results obtained for magnetic fields applied at $0^\circ$ and 30$^\circ$ relative to CuO$_2$ planes, show an enhancement of the elastic coupling of the films at driving current densities several orders of magnitude smaller than the critical one. Our results indicate that the FLL appears to be in a relatively ordered, metastable state after field cooling without driving forces.

cond-mat.supr-con↗