arXiv subjects

A. Scarfato

Publications and source records attributed to A. Scarfato.

7 recordsLinked to original sources

arXiv2016

Stripe and short range order in the charge density wave of 1T-Cu$_x$TiSe$_2$

We study the impact of Cu intercalation on the charge density wave (CDW) in 1T-Cu$_{\text{x}}$TiSe$_{\text{2}}$ by scanning tunneling microscopy and spectroscopy. Cu atoms, identified through density functional theory modeling, are found to intercalate randomly on the octahedral site in the van der Waals gap and to dope delocalized electrons near the Fermi level. While the CDW modulation period does not depend on Cu content, we observe the formation of charge stripe domains at low Cu content (x$<$0.02) and a breaking up of the commensurate order into 2$\times$2 domains at higher Cu content. The latter shrink with increasing Cu concentration and tend to be phase-shifted. These findings invalidate a proposed excitonic pairing as the primary CDW formation mechanism in this material.

cond-mat.mtrl-sci↗

arXiv2015

STM microscopy of the CDW in 1T-TiSe2 in the presence of single atom defects

We present a detailed low temperature scanning tunneling microscopy study of the commensurate charge density wave (CDW) in 1$T$-TiSe$_2$ in the presence of single atom defects. We find no significant modification of the CDW lattice in single crystals with native defects concentrations where some bulk probes already measure substantial reductions in the CDW phase transition signature. Systematic analysis of STM micrographs combined with density functional theory modelling of atomic defect patterns indicate that the observed CDW modulation lies in the Se surface layer. The defect patterns clearly show there are no 2$H$-polytype inclusions in the CDW phase, as previously found at room temperature [Titov A.N. et al, Phys. Sol. State 53, 1073 (2011). They further provide an alternative explanation for the chiral Friedel oscillations recently reported in this compound [J. Ishioka et al., Phys. Rev. B 84, 245125, (2011)].

cond-mat.mtrl-sci↗

arXiv2014

Doping nature of native defects in 1T-TiSe2

The transition metal dichalcogenide 1T-TiSe2 is a quasi two-dimensional layered material with a charge density wave (CDW) transition temperature of TCDW 200 K. Self-doping effects for crystals grown at different temperatures introduce structural defects, modify the temperature dependent resistivity and strongly perturbate the CDW phase. Here we study the structural and doping nature of such native defects combining scanning tunneling microscopy/spectroscopy and ab initio calculations. The dominant native single atom dopants we identify in our single crystals are intercalated Ti atoms, Se vacancies and Se substitutions by residual iodine and oxygen.

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↗