arXiv subjects

P. Esquinazi

Publications and source records attributed to P. Esquinazi.

At least 73 records · Page 4Linked to original sources

arXiv2005

Magnetic properties of carbon phases synthesized using high pressure-high temperature treatment

Two sets of samples were synthesized at 3.5 GPa near the point of C60 cage collapse at different annealing times. A clear structural transformation from mixture of C60 polymeric phases to graphite-like hard carbon phase was confirmed by X-ray diffraction and Raman spectroscopy. Magnetic force microscopy and superconducting quantum interference device were used to characterize the magnetic properties of the synthesized samples. We found that the sample preparation conditions used in this study are not suitable to produce bulk magnetic carbon.

cond-mat.mtrl-sci↗

arXiv2005

Induced Magnetic Order by Ion Irradiation of Carbon-Based Structures

In this chapter we review the magnetic effects produced by proton irradiation on graphite, disordered carbon and fullerene films. The advantage of proton irradiation is twofold: it enables us to make an impurity analysis simultaneously to the implantation of hydrogen. In this chapter we will review the main effects obtained after proton irradiation in different carbon-based structures. This chapter is organized as follows. In section 2 we provide the main characteristics of our irradiation facility. In this section we show an example of element analysis obtained in one of the graphite samples used for the irradiation studies. The irradiation effects are reviewed in section 3. This section is divided in two main subsections that describe the effects in oriented graphite and carbon-based thin films. In section 4 we discuss some of the effects observed after annealing the sample at high temperatures in vacuum or after leaving it at room temperature for a long period of time.

cond-mat.mtrl-sci↗

arXiv2005

Higher harmonics of ac voltage response in narrow strips of YBa2Cu3O7 thin films: Evidence for strong thermal fluctuations

We report on measurements of higher harmonics of the ac voltage response in strips of YBa2Cu3O7 thin films as a function of temperature, frequency and ac current amplitude. The third (fifth) harmonic of the local voltage is found to exhibit a negative (positive) peak at the superconducting transition temperature and their amplitudes are closely related to the slope (derivative) of the first (Ohmic) harmonic. The peaks practically do not depend on frequency and no even (second or fourth) harmonics are detected. The observed data can be interpreted in terms of ac current induced thermal modulation of the sample temperature added to strong thermally activated fluctuations in the transition region.

cond-mat.supr-con↗

arXiv2004

Searching for a magnetic proximity effect in magnetite-carbon structures

In order to study a possible magnetic proximity effect in magnetite-carbon structures, we have performed magnetization measurements of graphite-magnetite composites with different mass ratios as well as the measurement of the magnetoresistance of one of them and of the magnetization of a magnetite-carbon bilayer. The overall results do not indicate the induction of bulk ferromagnetism in graphite and disordered carbon structures through their contact with magnetite.

cond-mat.mtrl-sci↗

arXiv2003

Magnetothermal Conductivity of Highly Oriented Pyrolytic Graphite in the Quantum Limit

We report on the magnetic field (0T$ \le B \le 9$T) dependence of the longitudinal thermal conductivity $κ(T,B)$ of highly oriented pyrolytic graphite in the temperature range 5 K $\le T\le$ 20 K for fields parallel to the $c-$axis. We show that $κ(T,B)$ shows large oscillations in the high-field region (B > 2 T) where clear signs of the Quantum-Hall effect are observed in the Hall resistance. With the measured longitudinal electrical resistivity we show that the Wiedemann-Franz law is violated in the high-field regime.

cond-mat.str-el↗

arXiv2003

Induced Magnetic Ordering by Proton Irradiation in Graphite

We provide evidence that proton irradiation of energy 2.25 MeV on highly-oriented pyrolytic graphite samples triggers ferro- or ferrimagnetism. Measurements performed with a superconducting quantum interferometer device (SQUID) and magnetic force microscopy (MFM) reveal that the magnetic ordering is stable at room temperature.

cond-mat.str-el↗

arXiv2003

The origin of paramagnetic magnetization in field-cooled YBa2Cu3O7 films

Temperature dependences of the magnetic moment have been measured in YBa_2Cu_3O_{7-δ} thin films over a wide magnetic field range (5 <= H <= 10^4 Oe). In these films a paramagnetic signal known as the paramagnetic Meissner effect has been observed. The experimental data in the films, which have strong pinning and high critical current densities (J_c ~ 2 \times 10^6 A/cm^2 at 77 K), are quantitatively shown to be highly consistent with the theoretical model proposed by Koshelev and Larkin [Phys. Rev. B 52, 13559 (1995)]. This finding indicates that the origin of the paramagnetic effect is ultimately associated with nucleation and inhomogeneous spatial redistribution of magnetic vortices in a sample which is cooled down in a magnetic field. It is also shown that the distribution of vortices is extremely sensitive to the interplay of film properties and the real experimental conditions of the measurements.

cond-mat.supr-con↗

arXiv2003

Reentrant Metallic Behavior of Graphite in the Quantum Limit

Magnetotransport measurements performed on several well-characterized highly oriented pyrolitic graphite and single crystalline Kish graphite samples reveal a reentrant metallic behavior in the basal-plane resistance at high magnetic fields, when only the lowest Landau levels are occupied. The results suggest that the quantum Hall effect and Landau-level-quantization-induced superconducting correlations are relevant to understand the metallic-like state(s) in graphite in the quantum limit.

cond-mat.str-el↗

arXiv2003

A Field-Effect-Transistor from Graphite: No Effect of Low Gate Fields

Inspired by the striking similarities between the metal-insulator transitions in graphite and Si-MOSFET's and the recent attention to charge doping in carbon-based materials, we have made attempts to fabricate a field-effect transistor based on graphite. A relatively thick layer of boron nitride turned out to be able to serve as a gate dielectric. This, however, limits the achievable electric gate field, which might be the reason for our observation of no charge-doping effect.

cond-mat.str-el↗

arXiv2002

Absence of Metal-Insulator-Transition and Coherent Interlayer Transport in oriented graphite in parallel magnetic fields

Measurements of the magnetoresistivity of graphite with a high degree of control of the angle between the sample and magnetic field indicate that the metal-insulator transition (MIT), shown to be induced by a magnetic field applied perpendicular to the layers, does not appear in parallel field orientation. Furthermore, we show that interlayer transport is coherent in less ordered samples and high magnetic fields, whereas appears to be incoherent in less disordered samples. Our results demonstrate the two-dimensionality of the electron system in ideal graphite samples.

cond-mat.str-el↗

arXiv2002

Metal-Insulator-Metal Transitions, Superconductivity and Magnetism in Graphite

This article reviews recent results of magnetotransport and magnetization measurements performed on highly oriented pyrolitic graphite (HOPG) and single crystalline Kish graphite samples. Both metal-insulator and insulator-metal transitions driven by magnetic field applied perpendicular to the basal planes of graphite were found and discussed in the light of relevant theories. The results provide evidence for the existence of localized superconducting domains in HOPG even at room temperature, as well as an interplay between superconducting and ferromagnetic correlations. We also present experimental evidence for the superconductivity occurrence in graphite-sulfur composites.

cond-mat.str-el↗

arXiv2002

Field-induced metal-insulator Field-induced metal-insulator transition in the c-axis resistivity of graphite

We show that the resistivity perpendicular $ρ_c$ and parallel $ρ_a$ to the basal planes of different graphite samples show similar magnetic-field-driven metal-insulator-transitions at a field $B_c \sim 0.1 T$ applied parallel to the c-axis. Our results demonstrate the universality of the recently found scaling in $ρ_a$ of graphite and indicate that the metallic-like temperature dependence of $ρ_c$ is directly correlated to that of $ρ_a$. The similar magnetoresistance found for both resistivities, the violation of Kohler's rule and the field-induced transition indicate that the semiclassical transport theory is inadecuate to understand the transport properties of graphite.

cond-mat.str-el↗

arXiv2002

Ferromagnetism in Oriented Graphite Samples

We have studied the magnetization of various, well characterized samples of highly oriented pyrolitic graphite (HOPG), Kish graphite and natural graphite to investigate the recently reported ferromagnetic-like signal and its possible relation to ferromagnetic impurities. The magnetization results obtained for HOPG samples for applied fields parallel to the graphene layers - to minimize the diamagnetic background - show no correlation with the magnetic impurity concentration. Our overall results suggest an intrinsic origin for the ferromagnetism found in graphite. We discuss possible origins of the ferromagnetic signal.

cond-mat.str-el↗

arXiv2002

Strain dependence of the acoustic properties of amorphous metals below 1K: Evidence for the interaction between tunneling states

We have conducted a thorough study of the acoustic properties between 10^-4 and 1 Kelvin for the amorphous metal Zr_x Cu_1-x (x=0.3 and x=0.4), by measuring the relative change of sound velocity dv/v and internal friction Q^-1 as a function of temperature and also of the applied strain, in both superconducting and normal state. We have found that when plotted versus the ratio of strain energy to thermal energy, all measurements display the same behavior: a crossover from a linear regime of ``independent'' tunneling systems at very low strains and/or high enough temperatures to a nonlinear regime where dv/v and Q^-1 depend on applied strain and the tunneling systems cannot be considered as independent.

cond-mat.dis-nn↗

arXiv2001

Comment on "Quantum Melting of the Quasi-Two-Dimensional Vortex Lattice in $κ-(ET)_2Cu(NCS)_2$"

In a recent Letter Mola et al. \cite{mola} reported magnetization measurements $M(H,θ)$ performed on the organic superconductor $κ-$(ET)$_2$Cu(NCS)$_2 (T_c = 9.1$ K) as a function of the magnetic field $H$ applied at different angles $θ$ with respect to the $a$-axis direction. The results \cite{mola} demonstrate: (a) the occurrence of pronounced irreversible magnetization $M_{\rm irr}(H)$ jumps and (b) their sudden cessation for $H \ge H_m(T,θ)$. The boundary line $H_m(T)$ has been interpreted by Mola et al. as the Q2D vortex lattice (VL) quantum melting phase transition line \cite{mola}. The purpose of this comment is to show that the results can be understood in a simple way without invoking "quantum melting phase transition".

cond-mat.supr-con↗

arXiv2001

Comment on ``New Vortex-Matter Size Effect Observed in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$"

The Letter of Wang et al. \cite{wan} is related to the recently discovered vanishing of the second magnetization peak (SMP) in high-$T_c$ superconductors reducing the sample size \cite{kope1,kope2,gal}. This phenomenon has been interpreted \cite{kope1,kope2,gal} in terms of thermomagnetic instability (TMI) effects. The TMI approach has also been adopted by Wang et al. in their previous work \cite{wan2} on the critical sample size temperature dependence $R_{cr}(T)$ measured in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi2212) single crystals. In contrast, in the recently published Letter \cite{wan} the authors claimed that the $R_{cr}(T)$ obtained for Bi2212 crystals is at odds with the TMI model and proposed an alternative explanation of the results assuming that the SMP vanishes as the sample width (or diameter) becomes smaller than the in-plane vortex correlation length $R_c^{2D}(T) \sim J_c(T)^{-1/2}$, where $J_c(T)$ is the critical current density. We demonstrate below that the authors conclusion is misleading.

cond-mat.supr-con↗

arXiv2001

Angle Dependence of the Transverse Thermal Conductivity in YBa$_2$Cu$_3$O$_7$ single crystals: Doppler Effect vs. Andreev scattering

We have measured the transverse thermal conductivity $κ_{xy}$ of twinned and untwinned YBa$_2$Cu$_3$O$_7$ single crystals 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 temperatures. For both crystals we observed a clear twofold variation in the field-angle dependence of $κ_{xy}(θ) = - κ^0_{xy}(T,B) \sin(2θ)$. We have found that the oscillation amplitude $κ^0_{xy}$ depends on temperature and magnetic field. Our results show that $κ^0_{xy} = a B \ln(1/(bB))$ with the temperature- and sample-dependent parameters $a$ and $b$. We discuss our results in terms of Andreev scattering of quasiparticles by vortices and a recently proposed theory based on the Doppler shift in the quasiparticle spectrum.

cond-mat.supr-con↗

arXiv2001

Nonmonotonic Temperature Dependence of the Thermal Hall Angle of a YBa$_{2}$Cu$_{3}$O$_{6.95}$ Single Crystal

We have performed high-resolution measurements of the magnetic field (0~T~$ \le B \le 9~$T) and temperature (10 K~$\le T < 140~$K) dependence of the longitudinal and transverse Hall thermal conductivity of a twinned YBa$_{2}$Cu$_{3}$O$_{6.95}$ single crystal. We have used and compared two recently published methods to extract the thermal Hall angle $θ_H(T,B)$. Our results indicate that $\cot(θ_H)$ varies quite accurately as $T^4$ in the intermediate temperature range $\sim 0.3 < T/T_c$. It shows a well defined minimum at $T_m \simeq 20~$K which resembles that observed in the $c$-axis microwave conductivity. The electronic part of the longitudinal and the transverse thermal conductivity show the scaling behavior for transport properties predicted for d-wave superconductors in the temperature range $\sim 18~$K~$\le T \le 30~$K.

cond-mat.supr-con↗