Explanation of the discrepancy between the measured and atomistically calculated yield stresses in body-centered cubic metals

We propose a mesoscopic model that explains the factor of two to three discrepancy between experimentally measured yield stresses of BCC metals at low temperatures and typical Peierls stresses determined by atomistic simulations of isolated screw dislocations. The model involves a Frank-Read type source emitting dislocations that become pure screws at a certain distance from the source and, owing to their high Peierls stress, control its operation. However, due to the mutual interaction between emitted dislocations the group consisting of both non-screw and screw dislocations can move at an applied stress that is about a factor of two to three lower than the stress needed for the glide of individual screw dislocations.Comment: 4 pages, 2 figures; RevTex4; submitted to PR

Kondo effect and anti-ferromagnetic correlation in transport through tunneling-coupled double quantum dots

We propose to study the transport through tunneling-coupled double quantum dots (DQDs) connected in series to leads, using the finite-$U$ slave-boson mean field approach developed initially by Kotliar and Ruckenstein [Phys. Rev. Lett. {\bf 57}, 1362 (1986)]. This approach treats the dot-lead coupling and the inter-dot tunnelling $t$ nonperturbatively at arbitrary Coulomb correlation $U$, thus allows the anti-ferromagnetic exchange coupling parameter $J=4t^2/U$ to appear naturally. We find that, with increasing the inter-dot hopping, the DQDs manifest three distinct physical scenarios: the Kondo singlet state of each dot with its adjacent lead, the spin singlet state consisting of local spins on each dot and the doubly occupied bonding orbital of the coupled dots. The three states exhibit remarkably distinct behavior in transmission spectrum, linear and differential conductance and their magnetic-field dependence. Theoretical predictions agree with numerical renormalization group and Lanczos calculations, and some of them have been observed in recent experiments.Comment: 5 pages, 5 figures. Physics Review B (Rapid Communication) (in press

Kondo effect in coupled quantum dots under magnetic fields

The Kondo effect in coupled quantum dots is investigated theoretically under magnetic fields. We show that the magnetoconductance (MC) illustrates peak structures of the Kondo resonant spectra. When the dot-dot tunneling coupling $V_C$ is smaller than the dot-lead coupling $\Delta$ (level broadening), the Kondo resonant levels appear at the Fermi level ($E_F$). The Zeeman splitting of the levels weakens the Kondo effect, which results in a negative MC. When $V_{C}$ is larger than $\Delta$, the Kondo resonances form bonding and anti-bonding levels, located below and above $E_F$, respectively. We observe a positive MC since the Zeeman splitting increases the overlap between the levels at $E_F$. In the presence of the antiferromagnetic spin coupling between the dots, the sign of MC can change as a function of the gate voltage.Comment: 6 pages, 3 figure

I-V characteristics of single electron tunneling from symmetric and asymmetric double-barrier tunneling junctions

Copyright 2007 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters, 90(22), 223112, 2007 and may be found at http://dx.doi.org/10.1063/1.274525

Fabrication of nanoscale gaps using a combination of self-assembled molecular and electron beam lithographic techniques

Copyright 2006 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters, 88(22), 223111, 2006 and may be found at http://dx.doi.org/10.1063/1.220920

Phase diagrams of period-4 spin chains consisting of three kinds of spins

We study a period-4 antiferromagnetic mixed quantum spin chain consisting of three kinds of spins. When the ground state is singlet, the spin magnitudes in a unit cell are arrayed as (s-t, s, s+t, s) with integer or half-odd integer s and t (0 <= t < s). The spin Hamiltonian is mapped onto a nonlinear sigma model (NLSM) in a previously developed method. The resultant NLSM includes only two independent parameters originating from four exchange constants for fixed s and t. The topological angle in the NLSM determines the gapless phase boundaries between disordered phases in the parameter space. The phase diagrams for various s and t shows rich structures. We systematically explain the phases in the singlet-cluster-solid picture.Comment: 8 pages (16 figures included

Electron Transport through T-Shaped Double-Dots System

Correlation effects on electron transport through a system of T-shaped double-dots are investigated, for which only one of the dots is directly connected to the leads. We evaluate the local density of states and the conductance by means of the non-crossing approximation at finite temperatures as well as the slave-boson mean field approximation at zero temperature. It is found that the dot which is not directly connected to the leads considerably influences the conductance, making its behavior quite different from the case of a single-dot system. In particular, we find a novel phenomenon in the Kondo regime with a small inter-dot coupling, i.e. Fano-like suppression of the Kondo-mediated conductance, when two dot levels coincide with each other energetically.Comment: 6 pages,7 figure