arXiv subjects

A. M. Cucolo

Publications and source records attributed to A. M. Cucolo.

12 recordsLinked to original sources

arXiv2011

Field emission properties of as-grown multiwalled carbon nanotube films

Multiwalled carbon nanotubes have been produced by ethylene catalytic chemical vapor deposition and used to fabricate thick and dense freestanding films ("buckypapers") by membrane filtering. Field emission properties of buckypapers have been locally studied by means of high vacuum atomic force microscopy with a standard metallic cantilever used as anode to collect electrons emitted from the sample. Buckypapers showed an interesting linear dependence in the Fowler-Nordheim plots demonstrating their suitability as emitters. By precisely tuning the tip-sample distance in the submicron region we found out that the field enhancement factor is not affected by distance variations up to 2um. Finally, the study of current stability showed that the field emission current with intensity of about 3,3*10-5A remains remarkably stable (within 5% fluctuations) for several hours.

cond-mat.mes-hall↗

arXiv2008

Field emission from single multi-wall carbon nanotubes

Electron field emission characteristics of individual multiwalled carbon nanotubes have been investigated by a piezoelectric nanomanipulation system operating inside a scanning electron microscopy chamber. The experimental setup ensures a high control capability on the geometric parameters of the field emission system (CNT length, diameter and anode-cathode distance). For several multiwalled carbon nanotubes, reproducible and quite stable emission current behaviour has been obtained with a dependence on the applied voltage well described by a series resistance modified Fowler-Nordheim model. A turn-on field of about 30 V/um and a field enhancement factor of around 100 at a cathode-anode distance of the order of 1 um have been evaluated. Finally, the effect of selective electron beam irradiation on the nanotube field emission capabilities has been extensively investigated.

cond-mat.mtrl-sci↗

arXiv2008

Local probing of the field emission stability of vertically aligned multiwalled carbon nanotubes

Metallic cantilever in high vacuum atomic force microscope has been used as anode for field emission experiments from densely packed vertically aligned multi-walled carbon nanotubes. The high spatial resolution provided by the scanning probe technique allowed precise setting of the tip-sample distance in the submicron region. The dimension of the probe (curvature radius below 50nm) allowed to measure current contribution from sample areas smaller than 1um^2. The study of long-term stability evidenced that on these small areas the field emission current remains stable (within 10% fluctuations) several hours (at least up to 72 hours) at current intensities between 10-5A and 10-8A. Improvement of the current stability has been observed after performing long-time Joule heating conditioning to completely remove possible adsorbates on the nanotubes.

cond-mat.mtrl-sci↗

arXiv2007

Evidence of s-Wave Subdominant Order Parameter in YBCO from Break Junction Tunneling Spectra

The tunneling spectra of YBa2Cu3O7 break-junctions have been investigated for the tunneling direction close to the node one. The zero-bias conductance peak (ZBCP) and Josephson current have been studied with temperature and magnetic field. The observed deep splitting of ZBCP which starts at Ts<20-30K is in agreement with the theory for the dx2-y2+-is order parameter [Y. Tanuma, Y. Tanaka, and S. Kashiwaya, Phys. Rev. B 64, 214519 (2001)]. The low (0.04T) magnetic field depresses significantly such splitting. The 1/T temperature dependence of maximum Josephson current goes to saturation at T<Ts also confirming the mixed order parameter formation.

cond-mat.supr-con↗

arXiv2007

A local field emission study of partially aligned carbon-nanotubes by AFM probe

We report on the application of Atomic Force Microscopy (AFM) for studying the Field Emission (FE) properties of a dense array of long and vertically quasi-aligned multi-walled carbon nanotubes grown by catalytic Chemical Vapor Deposition on a silicon substrate. The use of nanometric probes enables local field emission measurements allowing investigation of effects non detectable with a conventional parallel plate setup, where the emission current is averaged on a large sample area. The micrometric inter-electrode distance let achieve high electric fields with a modest voltage source. Those features allowed us to characterize field emission for macroscopic electric fields up to 250 V/$μ$m and attain current densities larger than 10$^5$ A/cm$^2$. FE behaviour is analyzed in the framework of the Fowler-Nordheim theory. A field enhancement factor $γ\approx$ 40-50 and a turn-on field $E_{turn-on} \sim$15 V/$μ$m at an inter-electrode distance of 1 $μ$m are estimated. Current saturation observed at high voltages in the I-V characteristics is explained in terms of a series resistance of the order of M$Ω$. Additional effects as electrical conditioning, CNT degradation, response to laser irradiation and time stability are investigated and discussed.

cond-mat.mtrl-sci↗

arXiv2006

Local Tunneling Study of Three-Dimensional Order Parameter in the $π$-band of Al-doped MgB$_2$ Single Crystals

We have performed local tunneling spectroscopy on high quality Mg$_{1-x}$Al$_x$B$_2$ single crystals by means of Variable Temperature Scanning Tunneling Spectroscopy (STS) in magnetic field up to 3 Tesla. Single gap conductance spectra due to c-axis tunneling were extensively measured, probing different amplitudes of the three-dimensional $Δ_π$ as a function of Al content. Temperature and magnetic field dependences of the conductance spectra were studied in S-I-N configuration: the effect of the doping resulted in a monotonous reduction of the locally measured $T_C$ down to 24K for x=0.2. On the other hand, we have found that the gap amplitude shows a maximum value $Δ_π= 2.3$ meV for x=0.1, while the $Δ_π/ T_C$ ratio increases monotonously with doping. The locally measured upper critical field was found to be strongly related to the gap amplitude, showing the maximum value $H_{c2}\simeq3T$ for x=0.1 substituted samples. For this Al concentration the data revealed some spatial inhomogeneity in the distribution of $Δ_π$ on nanometer scale.

cond-mat.supr-con↗

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↗

arXiv2005

Superconducting vortex profile from fixed point measurements The Lazy Fisherman tunnelling microscopy method

We introduce a mode of operation for studying the vortex phase in superconductors using scanning tunnelling microscopy (STM). While in the conventional STM method, the tip is scanned over a sample in which a fixed vortex pattern is prepared, in our "Lazy Fisherman" method the STM tip is kept fixed at a selected location while the vortices are being moved by varying the applied magnetic field. By continuously acquiring the local tunnelling conductance spectra, dI/dV(V), we detect the changes in the local density of states under the tip due to the vortex motion. With no need for scanning, the method permits one to extend the study of vortices to samples in which scanning is difficult or even impossible due to surface nonuniformity and allows one to study vortex dynamics. Using a statistical analysis of the spectra, we reconstruct the single vortex zero bias conductance profile. We apply the method to the c-axis face of an MgB$_2$ single crystal sample and obtain a vortex profile with a coherence length, xi of 57+-2 nm.

cond-mat.supr-con↗

arXiv2004

Field dependence of the microwave resistivity in SmBaCuO thin films

We report measurements of the microwave complex resistivity at 48 GHz in SmBaCuO thin films. Measurements are performed with a moderate magnetic field, $μ_{0}H<$ 0.8T, applied along the c-axis. We find that the complex resistivity presents clear sublinear field dependences, and that the imaginary part is remarkably sensitive to the moderate magnetic field. Interpretation considering an unusually strong pinning leads to very anomalous field dependences of the single-vortex viscosity and of the pinning constant. By contrast, allowing for a significant effect of the magnetic field on the depletion of the condensate, the data are quantitatively described by simple free-flux flow model, supplemented with two-fluid conductivity. In this frame, we obtain the vortex viscosity from the data. We compare vortex viscosity in SmBaCuO and in YBaCuO.

cond-mat.supr-con↗

arXiv2004

Two regimes in the magnetic field response of superconducting MgB$_2$

Using Scanning Tunneling Microscope at low temperature we explore the superconducting phase diagram in the $π$-band of the two-band superconductor MgB$_2$. In this band the peculiar shape of the local tunneling spectra and their dynamics in the magnetic field reveal the complex character of the quasiparticle density of states (DOS). The gap in the DOS is first rapidly filled with states in raising the magnetic field up to 0.5 T and then slowly approaches the normal state value : The gap is observed up to 2 T. Such a change in the DOS dynamics suggests the existence of two terms in the DOS of the $π$-band: a first one, reflecting an intrinsic superconductivity in the band and a second one, originating from an inter-band coupling to the $σ$-band. Our findings allow a deeper understanding of the unique phase diagram of MgB$_2$.

cond-mat.supr-con↗

arXiv2004

Microwave fluctuational conductivity in YBa$_{2}$Cu$_{3}$O$_{7-δ}$

We present measurements of the microwave complex conductivity at 23.9 and 48.2 GHz in YBa$_{2}$Cu$_{3}$O$_{7-δ}$ films, in the fluctuational region above $T_{c}$. With increasing temperature, the fluctuational excess conductivity drops much faster than the well-known calculations within the time-dependent Ginzburg-Landau theory [H. Schmidt {\em Z. Phys.} {\bf 216}, 336]. Approaching the transition temperature, slowing down of the fluctuations is observed. We discuss the results in terms of a modified gaussian theory for finite-frequency fluctuational conductivity, where renormalization is introduced in order to account for the $T\to T_{c}$ regime, and a spectral cutoff is inserted in order to discard high-momentum modes. The data are in excellent agreement with the modified theory, when formulated for three-dimensional, anisotropic superconductors, in the whole experimentally accessible temperature range, and for both frequencies.

cond-mat.supr-con↗

arXiv1998

Role of Inter-Electron Interaction in the Pseudo-Gap Opening in High T $_c$ Tunneling Experiments

The analysis of tunneling experiments showing the pseudogap type behavior is carried out based on the idea of the renormalization of density of states due to the inter-electron interaction in the Cooper channel (superconducting fluctuations contribution in tunneling current). It is demonstrated that the observed kink of the zero-bias conductance $G(0,T)$ of $YBaCuO/Pb$ junctions in the vicinity of $T_c$ can be explained in terms of fluctuation theory in a quite wide range of temperature above $T_c$, using the values of microscopic parameters of the $YBaCuO$ electron spectrum taken from independent experiments. The approach proposed also permits to explain qualitatively the shape of the tunneling anomalies in $G(V,T)$ and gives a correct estimate for the pseudogap position and amplitude observed in the experiments on $BiSrCaCuO$ junctions.

cond-mat.supr-con↗