Magnetoresistance of a quantum dot with spin-active interfaces

We study the zero-bias magnetoresistance MR of an interacting quantum dot connected to two ferromagnetic leads and capacitively coupled to a gate voltage source Vg. We investigate the effects of the spin-activity of the contacts between the dot and the leads by introducing an effective exchange field in an Anderson model. This spin-activity makes easier negative MR effects, and can even lead to a giant MR effect with a sign tunable with Vg. Assuming a twofold orbital degeneracy, our approach allows to interpret in an interacting picture the MR(Vg) measured by S. Sahoo et al. [Nature Phys. 2, 99 (2005)] in single wall carbon nanotubes with ferromagnetic contacts. If this experiment is repeated on a larger Vg-range, we expect that the MR(Vg) oscillations are not regular like in the presently available data, due to Coulomb interactions.Comment: 9 pages, 6 figures, to appear in Phys. Rev.

Charge transport and mobility engineering in two-dimensional transition metal chalcogenide semiconductors

Two-dimensional (2D) van der Waals semiconductors represent the thinnest, air stable semiconducting materials known. Their unique optical, electronic and mechanical properties hold great potential for harnessing them as key components in novel applications for electronics and optoelectronics. However, the charge transport behavior in 2D semiconductors is more susceptible to external surroundings (e.g. gaseous adsorbates from air and trapped charges in substrates) and their electronic performance is generally lower than corresponding bulk materials due to the fact that surface and bulk coincide. In this article, we review recent progress on the charge transport properties and carrier mobility engineering of 2D transition metal chalcogenides, with a particular focus on the markedly high dependence of carrier mobility on thickness. We unveil the origin of this unique thickness dependence and elaborate the devised strategies to master it for carrier mobility optimization. Specifically, physical and chemical methods towards the optimization of the major factors influencing the extrinsic transport such as electrode/semiconductor contacts, interfacial Coulomb impurities and atomic defects are discussed. In particular, the use of \textit{ad-hoc} molecules makes it possible to engineer the interface with the dielectric and heal the vacancies in such materials. By casting fresh light onto the theoretical and experimental works, we provide a guide for improving the electronic performance of the 2D semiconductors, with the ultimate goal of achieving technologically viable atomically thin (opto)electronics.Comment: 33 pages, 19 figures and 6 table

Hall effect of quasi-hole gas in organic single-crystal transistors

Hall effect is detected in organic field-effect transistors, using appropriately shaped rubrene (C42H28) single crystals. It turned out that inverse Hall coefficient, having a positive sign, is close to the amount of electric-field induced charge upon the hole accumulation. The presence of the normal Hall effect means that the electromagnetic character of the surface charge is not of hopping carriers but resembles that of a two-dimensional hole-gas system

Mass Estimates of a Giant Planet in a Protoplanetary Disk from the Gap Structures

A giant planet embedded in a protoplanetary disk forms a gap. An analytic relationship among the gap depth, planet mass $M_{p}$, disk aspect ratio $h_p$, and viscosity $\alpha$ has been found recently, and the gap depth can be written in terms of a single parameter $K= (M_{p}/M_{\ast})^2 h_p^{-5} \alpha^{-1}$. We discuss how observed gap features can be used to constrain the disk and/or planet parameters based on the analytic formula for the gap depth. The constraint on the disk aspect ratio is critical in determining the planet mass so the combination of the observations of the temperature and the image can provide a constraint on the planet mass. We apply the formula for the gap depth to observations of HL~Tau and HD~169142. In the case of HL~Tau, we propose that a planet with $\gtrsim 0.3$ is responsible for the observed gap at $30$~AU from the central star based on the estimate that the gap depth is $\lesssim 1/3$. In the case of HD~169142, the planet mass that causes the gap structure recently found by VLA is $\gtrsim 0.4 M_J$. We also argue that the spiral structure, if observed, can be used to estimate the lower limit of the disk aspect ratio and the planet mass.Comment: 16 pages, 5 figures, accepted for publication in The Astrophysical Journal Letter