21,494 research outputs found
Tunable Exchange Interaction in Quantum Dot Devices
We theoretically discuss the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction
between semiconductor quantum dots (QDs). When each QD having a local spin is
coupled to the conduction electrons in semiconductors, an indirect exchange
interaction, i.e., the RKKY interaction, is induced between two local spins.
The RKKY interaction between QDs, which is mediated by the Fermi sea in
semiconductors, is modulated by changing the Fermi energy, and the magnitude or
even the sign of the exchange interaction can be tuned, which leads to a
tunable magnetic transition in QD devices. We estimate the magnitude of the
RKKY interaction in QDs as a function of the electron density and the inter-dot
distance
A Canonical Ensemble Approach to the Fermion/Boson Random Point Processes and its Applications
We introduce the boson and the fermion point processes from the elementary
quantum mechanical point of view. That is, we consider quantum statistical
mechanics of canonical ensemble for a fixed number of particles which obey
Bose-Einstein, Fermi-Dirac statistics, respectively, in a finite volume.
Focusing on the distribution of positions of the particles, we have point
processes of the fixed number of points in a bounded domain. By taking the
thermodynamic limit such that the particle density converges to a finite value,
the boson/fermion processes are obtained. This argument is a realization of the
equivalence of ensembles, since resulting processes are considered to describe
a grand canonical ensemble of points. Random point processes corresponding to
para-particles of order two are discussed as an application of the formulation.
A statistics of a system of composite particles at zero temperature are also
considered as a model of determinantal random point processes.Comment: 26pages, Some typos are corrected, to be published in Commun. Math.
Phy
A Random Point Field related to Bose-Einstein Condensation
The random point field which describes the position distribution of the
system of ideal boson gas in a state of Bose-Einstein condensation is obtained
through the thermodynamic limit. The resulting point field is given by
convolution of two independent point fields: the so called boson process whose
generating functional is represented by inverse of the Fredholm determinant for
an operator related to the heat operator and the point field whose generating
functional is represented by a resolvent of the operator. The construction of
the latter point field in an abstract formulation is also given.Comment: 21 page
Double- Order in a Frustrated Random Spin System
We use the three-dimensional Heisenberg model with site randomness as an
effective model of the compound Sr(FeMn)O. The model consists
of two types of ions that correspond to Fe and Mn ions. The nearest-neighbor
interactions in the ab-plane are antiferromagnetic. The nearest-neighbor
interactions along the c-axis between Fe ions are assumed to be
antiferromagnetic, whereas other interactions are assumed to be ferromagnetic.
From Monte Carlo simulations, we confirm the existence of the
double- ordered phase characterized by two wave numbers,
and . We also identify the spin ordering pattern in
the double- ordered phase.Comment: 5pages, 3figure
Fano-Kondo interplay in a side-coupled double quantum dot
We investigate low-temperature transport characteristics of a side-coupled
double quantum dot where only one of the dots is directly connected to the
leads. We observe Fano resonances, which arise from interference between
discrete levels in one dot and the Kondo effect, or cotunneling in general, in
the other dot, playing the role of a continuum. The Kondo resonance is
partially suppressed by destructive Fano interference, reflecting novel
Fano-Kondo competition. We also present a theoretical calculation based on the
tight-binding model with slave boson mean field approximation, which
qualitatively reproduces the experimental findings.Comment: 4 pages, 4 figure
Multiobjective Decision Making - Utility Theoretic Approach
One of the difficult problems in decision analysis relates to the situation, when the decision must be undertaken by a committee. There exist several formalizations of decision making process based on the utility function approach. This approach is however very difficult to apply in the group decision case, since the number of coefficients characterizing the utility function is very high and it is practically impossible to directly identify such utility function. Therefore, reduction of dimensionality of the parameter space is necessary.
In this paper a concept of convex dependence between two conflicting decision makers is presented. This concept was effectively used by the author to develop a decomposition principle of the group utility function as well as to formulate the conditions necessary to perform such a decomposition. The concept was successfully applied for a practical example
Observational Test of Environmental Effects on The Local Group Dwarf Spheroidal Galaxies
In this paper, we examine whether tidal forces exerted by the Galaxy or M31
have an influence on the Local Group dwarf spheroidal galaxies (dSphs) which
are their companions. We focus on the surface brightness profiles of the dSphs,
especially their core radii because it is suggested based on the numerical
simulations that tidal disturbance can make core radii extended. We examine the
correlation for the dSphs between the distances from their parent galaxy (the
Galaxy or M31) and the compactnesses of their surface brightness profiles by
using a parameter ``C'' defined newly in this paper. Consequently, we find no
significant correlation. We make some remarks on the origin of this result by
considering three possible scenarios; tidal picture, dark matter picture, and
heterogeneity of the group of dSphs, each of which has been often discussed to
understand fundamental properties and formation processes of dSphs.Comment: 14 pages LaTeX, 2 PostScript figures, to appear in ApJ Letter
Singlet-triplet splitting, correlation and entanglement of two electrons in quantum dot molecules
Starting with an accurate pseudopotential description of the single-particle
states, and following by configuration-interaction treatment of correlated
electrons in vertically coupled, self-assembled InAs/GaAs quantum
dot-molecules, we show how simpler, popularly-practiced approximations, depict
the basic physical characteristics including the singlet-triplet splitting,
degree of entanglement (DOE) and correlation. The mean-field-like
single-configuration approaches such as Hartree-Fock and local spin density,
lacking correlation, incorrectly identify the ground state symmetry and give
inaccurate values for the singlet-triplet splitting and the DOE. The Hubbard
model gives qualitatively correct results for the ground state symmetry and
singlet-triplet splitting, but produces significant errors in the DOE because
it ignores the fact that the strain is asymmetric even if the dots within a
molecule are identical. Finally, the Heisenberg model gives qualitatively
correct ground state symmetry and singlet-triplet splitting only for rather
large inter-dot separations, but it greatly overestimates the DOE as a
consequence of ignoring the electron double occupancy effect.Comment: 13 pages, 9 figures. To appear in Phys. Rev.
Magnetic field effects on two-dimensional Kagome lattices
Magnetic field effects on single-particle energy bands (Hofstadter
butterfly), Hall conductance, flat-band ferromagnetism, and magnetoresistance
of two-dimensional Kagome lattices are studied. The flat-band ferromagnetism is
shown to be broken as the flat-band has finite dispersion in the magnetic
field. A metal-insulator transition induced by the magnetic field (giant
negative magnetoresistance) is predicted. In the half-filled flat band, the
ferromagnetic-paramagnetic transition and the metal-insulator one occur
simultaneously at a magnetic field for strongly interacting electrons. All of
the important magnetic fields effects should be observable in mesoscopic
systems such as quantum dot superlattices.Comment: 10 pages, 4 figures, and 1 tabl
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