654 research outputs found
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.
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
Current distribution inside Py/Cu lateral spin-valve device
We have investigated experimentally the non-local voltage signal (NLVS) in
the lateral permalloy (Py)/Cu/Py spin valve devices with different width of Cu
stripes. We found that NLVS strongly depends on the distribution of the
spin-polarized current inside Cu strip in the vicinity of the Py-detector. To
explain these data we have developed a diffusion model describing spatial (3D)
distribution of the spin-polarized current in the device. The results of our
calculations show that NLVS is decreased by factor of 10 due to spin
flip-scattering occurring at Py/Cu interface. The interface resistivity on
Py/Cu interface is also present, but its contribution to reduction of NLVS is
minor. We also found that most of the spin-polarized current is injected within
the region 30 nm from Py-injector/Cu interface. In the area at Py-detector/Cu
interface, the spin-polarized current is found to flow mainly close on the
injector side, with 1/e exponential decay in the magnitude within the distance
80 nm.Comment: 10 pages, 14 figure
A graphene bilayer channel transistor with a very high Ion/Ioff ratio, graphene single electron devices and graphene nano-mechanical structures for NEMS
First-principles investigation of spin polarized conductance in atomic carbon wire
We analyze spin-dependent energetics and conductance for one dimensional (1D)
atomic carbon wires consisting of terminal magnetic (Co) and interior
nonmagnetic (C) atoms sandwiched between gold electrodes, obtained employing
first-principles gradient corrected density functional theory and Landauer's
formalism for conductance. Wires containing an even number of interior carbon
atoms are found to be acetylenic with sigma-pi bonding patterns, while cumulene
structures are seen in wires containing odd number of interior carbon atoms, as
a result of strong pi-conjugation. Ground states of carbon wires containing up
to 13 C atoms are found to have anti-parallel spin configurations of the two
terminal Co atoms, while the 14 C wire has a parallel Co spin configuration in
the ground state. The stability of the anti-ferromagnetic state in the wires is
ascribed to a super-exchange effect. For the cumulenic wires this effect is
constant for all wire lengths. For the acetylenic wires, the super-exchange
effect diminishes as the wire length increases, going to zero for the atomic
wire containing 14 carbon atoms. Conductance calculations at the zero bias
limit show spin-valve behavior, with the parallel Co spin configuration state
giving higher conductance than the corresponding anti-parallel state, and a
non-monotonic variation of conductance with the length of the wires for both
spin configurations.Comment: revtex, 6 pages, 5 figure
Spin dependent transport of ``nonmagnetic metal/zigzag nanotube encapsulating magnetic atoms/nonmagnetic metal'' junctions
Towards a novel magnetoresistance (MR) device with a carbon nanotube, we
propose ``nonmagnetic metal/zigzag nanotube encapsulating magnetic
atoms/nonmagnetic metal'' junctions. We theoretically investigate how
spin-polarized edges of the nanotube and the encapsulated magnetic atoms
influence on transport. When the on-site Coulomb energy divided by the
magnitude of transfer integral, , is larger than 0.8, large MR effect
due to the direction of spins of magnetic atoms, which has the magnitude of the
MR ratio of about 100%, appears reflecting such spin-polarized edges.Comment: 4 pages, 3 figures, accepted for publication in Synth. Metal
Transport in two dimensional periodic magnetic fields
Ballistic transport properties in a two dimensional electron gas are studied
numerically, where magnetic fields are perpendicular to the plane of two
dimensional electron systemsand periodically modulated both in and
directions. We show that there are three types of trajectories of classical
electron motions in this system; chaotic, pinned and runaway trajectories. It
is found that the runaway trajectories can explain the peaks of
magnetoresistance as a function of external magnetic fields, which is believed
to be related to the commensurability effect between the classical cyclotron
diameter and the period of magnetic modulation. The similarity with and
difference from the results in the antidot lattice are discussed.Comment: 4 pages, 7 figures, to appear in J. Phys. Soc. Jpn., vol. 67 (1998)
Novembe
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