Evidence for reduced magnetic braking in polars from binary population models

We present the first population synthesis of synchronous magnetic cataclysmic variables, called polars, taking into account the effect of the white dwarf (WD) magnetic field on angular momentum loss. We implemented the reduced magnetic braking (MB) model proposed by Li, Wu & Wickramasinghe into the Binary Stellar Evolution (BSE) code recently calibrated for cataclysmic variable (CV) evolution. We then compared separately our predictions for polars and non-magnetic CVs with a large and homogeneous sample of observed CVs from the Sloan Digital Sky Survey. We found that the predicted orbital period distributions and space densities agree with the observations if period bouncers are excluded. For polars, we also find agreement between predicted and observed mass transfer rates, while the mass transfer rates of non-magnetic CVs with periods ≳3 h drastically disagree with those derived from observations. Our results provide strong evidence that the reduced MB model for the evolution of highly magnetized accreting WDs can explain the observed properties of polars. The remaining main issues in our understanding of CV evolution are the origin of the large number of highly magnetic WDs, the large scatter of the observed mass transfer rates for non-magnetic systems with periods ≳3 h, and the absence of period bouncers in observed samples

Scanning Gate Spectroscopy of transport across a Quantum Hall Nano-Island

We explore transport across an ultra-small Quantum Hall Island (QHI) formed by closed quan- tum Hall edge states and connected to propagating edge channels through tunnel barriers. Scanning gate microscopy and scanning gate spectroscopy are used to first localize and then study a single QHI near a quantum point contact. The presence of Coulomb diamonds in the spectroscopy con- firms that Coulomb blockade governs transport across the QHI. Varying the microscope tip bias as well as current bias across the device, we uncover the QHI discrete energy spectrum arising from electronic confinement and we extract estimates of the gradient of the confining potential and of the edge state velocity.Comment: 13 pages, 3 figure

Gas and liquid-fuelled HVOF spraying of Ni50Cr coating: microstructure and high temperature oxidation

Ni50Cr thermally sprayed coatings are widely used for high temperature oxidation and corrosion in thermal power plants. In this study, a commercially available gas atomised Ni50Cr powder was sprayed onto a power plant alloy (ASME P92) using both gas and liquid fuelled high velocity oxy-fuel (HVOF) thermal spray. Microstructures of the two coatings were examined using SEM-EDX, XRD, oxygen content analysis and mercury intrusion porosimeter. The gas fuelled coating had higher levels of oxygen content and porosity. Shorter term air oxidation tests (4 h) of the free-standing deposits in a thermogravimetric analyser (TGA) and longer term air oxidation tests (100 h) of the coated substrates were performed at 700 °C. The kinetics of oxidation and the oxidation products were characterized in detail in SEM/EDX and XRD. In both samples, oxides of various morphologies developed on top of the Ni50Cr coatings. Cr2O3 was the main oxidation product on the surface of the coatings along with a small amount of NiO and NiCr2O4. Rietveld analysis was performed on the XRD data to quantify the phase composition of the oxides on both Ni50Cr coatings and their evolution with exposure time

Catching VY Sculptoris in a low state

Context. In the context of a large campaign to determine the system parameters of high mass transfer cataclysmic variables, we found VY Scl in a low state in 2008. Aims. Making use of this low state, we study the stellar components of the binary with little influence of the normally dominating accretion disc. Methods. Time-resolved spectroscopy and photometry of VY Scl taken during the low state are presented. We analysed the light-curve and radial velocity curve and use time-resolved spectroscopy to calculate Doppler maps of the dominant emission lines. Results. The spectra show narrow emission lines of Halpha, Hbeta, HeI, NaID, and FeII, as well as faint TiO absorption bands that trace the motion of the irradiated secondary star, and Halpha and HeI emission line wings that trace the motion of the white dwarf. From these radial velocities, we find an orbital period of 3.84 h, and put constraints on binary parameters such as the mass ratio M2/M1 of 0.43 and the inclination of 15 deg. With a secondary's mass between 0.3 and 0.35 Msol, we derive the mass for the white dwarf as M1 = 0.6-0.1 Msol.Comment: 8 pages, 9 figures, accepted for publication in A&

Statistical model of dephasing in mesoscopic devices introduced in the scattering matrix formalism

We propose a phenomenological model of dephasing in mesoscopic transport, based on the introduction of random phase fluctuations in the computation of the scattering matrix of the system. A Monte Carlo averaging procedure allows us to extract electrical and microscopic device properties. We show that, in this picture, scattering matrix properties enforced by current conservation and time reversal invariance still hold. In order to assess the validity of the proposed approach, we present simulations of conductance and magnetoconductance of Aharonov-Bohm rings that reproduce the behavior observed in experiments, in particular as far as aspects related to decoherence are concerned.Comment: 6 pages, 6 figure

Formation of quantum dots in the potential fluctuations of InGaAs heterostructures probed by scanning gate microscopy

The disordered potential landscape in an InGaAs/InAlAs two-dimensional electron gas patterned into narrow wires is investigated by means of scanning gate microscopy. It is found that scanning a negatively charged tip above particular sites of the wires produces conductance oscillations that are periodic in the tip voltage. These oscillations take the shape of concentric circles whose number and diameter increase for more negative tip voltages until full depletion occurs in the probed region. These observations cannot be explained by charging events in material traps, but are consistent with Coulomb blockade in quantum dots forming when the potential fluctuations are raised locally at the Fermi level by the gating action of the tip. This interpretation is supported by simple electrostatic simulations in the case of a disorder potential induced by ionized dopants. This work represents a local investigation of the mechanisms responsible for the disorder-induced metal-to-insulator transition observed in macroscopic two-dimensional electron systems at low enough density

Scanning-gate microscopy of semiconductor nanostructures: an overview

This paper presents an overview of scanning-gate microscopy applied to the imaging of electron transport through buried semiconductor nanostructures. After a brief description of the technique and of its possible artifacts, we give a summary of some of its most instructive achievements found in the literature and we present an updated review of our own research. It focuses on the imaging of GaInAs-based quantum rings both in the low magnetic field Aharonov-Bohm regime and in the high-field quantum Hall regime. In all of the given examples, we emphasize how a local-probe approach is able to shed new, or complementary, light on transport phenomena which are usually studied by means of macroscopic conductance measurements.Comment: Invited talk by SH at 39th "Jaszowiec" International School and Conference on the Physics of Semiconductors, Krynica-Zdroj, Poland, June 201

Imaging Electron Wave Functions Inside Open Quantum Rings

Combining Scanning Gate Microscopy (SGM) experiments and simulations, we demonstrate low temperature imaging of electron probability density $|\Psi|^{2}(x,y)$ in embedded mesoscopic quantum rings (QRs). The tip-induced conductance modulations share the same temperature dependence as the Aharonov-Bohm effect, indicating that they originate from electron wavefunction interferences. Simulations of both $|\Psi|^{2}(x,y)$ and SGM conductance maps reproduce the main experimental observations and link fringes in SGM images to $|\Psi|^{2}(x,y)$.Comment: new titl

Local Density of States in Mesoscopic Samples from Scanning Gate Microscopy

We study the relationship between the local density of states (LDOS) and the conductance variation $\Delta G$ in scanning-gate-microscopy experiments on mesoscopic structures as a charged tip scans above the sample surface. We present an analytical model showing that in the linear-response regime the conductance shift $\Delta G$ is proportional to the Hilbert transform of the LDOS and hence a generalized Kramers-Kronig relation holds between LDOS and $\Delta G$. We analyze the physical conditions for the validity of this relationship both for one-dimensional and two-dimensional systems when several channels contribute to the transport. We focus on realistic Aharonov-Bohm rings including a random distribution of impurities and analyze the LDOS-$\Delta G$ correspondence by means of exact numerical simulations, when localized states or semi-classical orbits characterize the wavefunction of the system.Comment: 8 pages, 8 figure

Effects of nicotine administration in rats on MMP2 and VEGF levels in periodontal membrane

Background: Nicotine is associated with increased incidence of periodontal disease and poor response to therapy. This article aimed at identifying the expression of matrix metalloproteinases 2 (MMPs2) and vascular endothelial growth factor (VEGF) proteins on extracellular matrix, fibrous distribution and angiogenetic development in periodontitis caused by nicotine effects on periodontal membrane.Materials and methods: In this experimental study, rats were divided into nicotine and control groups. While the rats in the nicotine group (n = 6) were administered 2 mg/kg nicotine sulphate for 28 days, the animals in the control group (n = 6) were only administered 1.5 mL physiologic saline solution subcutaneously for 28 days.Results: Histological sections were prepared and immunohistochemically stained for MMP2 and VEGF. The sections stained with Trichrome-Masson were observed under light microscope. VEGF and MMP2 immunoreactivity of periodontal gingiva and dentin was assessed by immunohistochemical staining.Conclusions: Nicotine reduces MMP production, disrupts collagen synthesis and causes periodontitis. We observed that nicotine increases periodontitis by disrupting periodontal membrane and prevents tooth to anchor in dental alveoli by disrupting epithelial structure