15,435 research outputs found
Theory of anisotropic exchange in laterally coupled quantum dots
The effects of spin-orbit coupling on the two-electron spectra in lateral
coupled quantum dots are investigated analytically and numerically. It is
demonstrated that in the absence of magnetic field the exchange interaction is
practically unaffected by spin-orbit coupling, for any interdot coupling,
boosting prospects for spin-based quantum computing. The anisotropic exchange
appears at finite magnetic fields. A numerically accurate effective spin
Hamiltonian for modeling spin-orbit-induced two-electron spin dynamics in the
presence of magnetic field is proposed.Comment: 4 pages, 3 figures; paper rewritte
Self-sustained magnetoelectric oscillations in magnetic resonant tunneling structures
The dynamic interplay of transport, electrostatic, and magnetic effects in
the resonant tunneling through ferromagnetic quantum wells is theoretically
investigated. It is shown that the carrier-mediated magnetic order in the
ferromagnetic region not only induces, but also takes part in intrinsic,
robust, and sustainable high-frequency current oscillations over a large window
of nominally steady bias voltages. This phenomenon could spawn a new class of
quantum electronic devices based on ferromagnetic semiconductors.Comment: 5 pages, 4 figure
The Ebers-Moll model for magnetic bipolar transistors
The equivalent electrical circuit of the Ebers-Moll type is introduced for
magnetic bipolar transistors. In addition to conventional diodes and current
sources, the new circuit comprises two novel elements due to spin-charge
coupling. A classification scheme of the operating modes of magnetic bipolar
transistors in the low bias regime is presented.Comment: 4 pages, 2 figure
State-Dependent Approach to Entropic Measurement-Disturbance Relations
Heisenberg's intuition was that there should be a tradeoff between measuring
a particle's position with greater precision and disturbing its momentum.
Recent formulations of this idea have focused on the question of how well two
complementary observables can be jointly measured. Here, we provide an
alternative approach based on how enhancing the predictability of one
observable necessarily disturbs a complementary one. Our
measurement-disturbance relation refers to a clear operational scenario and is
expressed by entropic quantities with clear statistical meaning. We show that
our relation is perfectly tight for all measurement strengths in an existing
experimental setup involving qubit measurements.Comment: 9 pages, 2 figures. v4: published versio
Non-thermal X-rays, a high abundance ridge and fossil bubbles in the core of the Perseus cluster of galaxies
Using a deep Chandra observation of the Perseus cluster of galaxies, we find
a high-abundance shell 250 arcsec (93 kpc) from the central nucleus. This ridge
lies at the edge of the Perseus radio mini-halo. In addition we identify two
Halpha filaments pointing towards this shell. We hypothesise that this ridge is
the edge of a fossil radio bubble, formed by entrained enriched material lifted
from the core of the cluster. There is a temperature jump outside the shell,
but the pressure is continuous indicating a cold front. A non-thermal component
is mapped over the core of the cluster with a morphology similar to the
mini-halo. Its total luminosity is 4.8x10^43 erg/s, extending in radius to ~75
kpc. Assuming the non-thermal emission is the result of inverse Compton
scattering of the CMB and infrared emission from NGC 1275, we map the magnetic
field over the core of the cluster.Comment: 8 pages, colour, accepted by MNRA
Beating the Generator-Enumeration Bound for -Group Isomorphism
We consider the group isomorphism problem: given two finite groups G and H
specified by their multiplication tables, decide if G cong H. For several
decades, the n^(log_p n + O(1)) generator-enumeration bound (where p is the
smallest prime dividing the order of the group) has been the best worst-case
result for general groups. In this work, we show the first improvement over the
generator-enumeration bound for p-groups, which are believed to be the hard
case of the group isomorphism problem. We start by giving a Turing reduction
from group isomorphism to n^((1 / 2) log_p n + O(1)) instances of p-group
composition-series isomorphism. By showing a Karp reduction from p-group
composition-series isomorphism to testing isomorphism of graphs of degree at
most p + O(1) and applying algorithms for testing isomorphism of graphs of
bounded degree, we obtain an n^(O(p)) time algorithm for p-group
composition-series isomorphism. Combining these two results yields an algorithm
for p-group isomorphism that takes at most n^((1 / 2) log_p n + O(p)) time.
This algorithm is faster than generator-enumeration when p is small and slower
when p is large. Choosing the faster algorithm based on p and n yields an upper
bound of n^((1 / 2 + o(1)) log n) for p-group isomorphism.Comment: 15 pages. This is an updated and improved version of the results for
p-groups in arXiv:1205.0642 and TR11-052 in ECC
Spin-orbit coupling and anisotropic exchange in two-electron double quantum dots
The influence of the spin-orbit interactions on the energy spectrum of
two-electron laterally coupled quantum dots is investigated. The effective
Hamiltonian for a spin qubit pair proposed in F. Baruffa et al., Phys. Rev.
Lett. 104, 126401 (2010) is confronted with exact numerical results in single
and double quantum dots in zero and finite magnetic field. The anisotropic
exchange Hamiltonian is found quantitatively reliable in double dots in
general. There are two findings of particular practical importance: i) The
model stays valid even for maximal possible interdot coupling (a single dot),
due to the absence of a coupling to the nearest excited level, a fact following
from the dot symmetry. ii) In a weak coupling regime, the Heitler-London
approximation gives quantitatively correct anisotropic exchange parameters even
in a finite magnetic field, although this method is known to fail for the
isotropic exchange. The small discrepancy between the analytical model (which
employes the linear Dresselhaus and Bychkov-Rashba spin-orbit terms) and the
numerical data for GaAs quantum dots is found to be mostly due to the cubic
Dresselhaus term.Comment: 15 pages, 11 figure
Band-Structure Effects in the Spin Relaxation of Conduction Electrons
Spin relaxation of conduction electrons in metals is significantly influenced
by the Fermi surface topology. Electrons near Brillouin zone boundaries,
special symmetry points, or accidental degeneracy lines have spin flip rates
much higher than an average electron. A realistic calculation and analytical
estimates show that these regions dominate the spin relaxation, explaining why
polyvalent metals have much higher spin relaxation rates (up to three orders of
magnitude) than similar monovalent metals. This suggests that spin relaxation
in metals can be tailored by band-structure modifications like doping,
alloying, reducing the dimensionality, etc.Comment: 10 pages, 2 figures; to appear in the 43rd MMM Conference Proceedings
published in the JA
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