230 research outputs found
Selective Dynamic Nuclear Spin Polarization in Spin-Blocked Double-Dot
We study the mechanism of dynamical nuclear spin polarization by hyperfine
interaction in spin-blocked double quantum dot system. We calculate the
hyperfine transition rates and solve the master equations for the nuclear
spins. Specifically, we incorporate the effects of the nuclear quadrupole
coupling due to the doping-induced local lattice distortion and strain. Our
results show that nuclear quadrupole coupling induced by the 5% indium
substitution can be used to explain the recent experimental observation of
missing arsenic NMR signal in the spin-blocked double dots.Comment: 4 pages, 3 figure
Many Body Effects on Electron Tunneling through Quantum Dots in an Aharonov-Bohm Circuit
Tunneling conductance of an Aharonov-Bohm circuit including two quantum dots
is calculated based on the general expression of the conductance in the linear
response regime of the bias voltage. The calculation is performed in a wide
temperature range by using numerical renormalization group method. Various
types of AB oscillations appear depending on the temperature and the potential
depth of the dots. Especially, AB oscillations have strong higher harmonics
components as a function of the magnetic flux when the potential of the dots is
deep. This is related to the crossover of the spin state due to the Kondo
effect on quantum dots. When the temperature rises up, the amplitude of the AB
oscillations becomes smaller reflecting the breaking of the coherency.Comment: 21 pages, 11 PostScript figures, LaTeX, uses jpsj.sty epsbox.st
Kondo Effect in Single Quantum Dot Systems --- Study with Numerical Renormalization Group Method ---
The tunneling conductance is calculated as a function of the gate voltage in
wide temperature range for the single quantum dot systems with Coulomb
interaction. We assume that two orbitals are active for the tunneling process.
We show that the Kondo temperature for each orbital channel can be largely
different. The tunneling through the Kondo resonance almost fully develops in
the region T \lsim 0.1 T_{K}^{*} \sim 0.2 T_{K}^{*}, where is the
lowest Kondo temperature when the gate voltage is varied. At high temperatures
the conductance changes to the usual Coulomb oscillations type. In the
intermediate temperature region, the degree of the coherency of each orbital
channel is different, so strange behaviors of the conductance can appear. For
example, the conductance once increases and then decreases with temperature
decreasing when it is suppressed at T=0 by the interference cancellation
between different channels. The interaction effects in the quantum dot systems
lead the sensitivities of the conductance to the temperature and to the gate
voltage.Comment: 22 pages, 18 figures, LaTeX, to be published in J. Phys. Soc. Jpn.
Vol. 67 No. 7 (1998
Singlet-triplet transition in a single-electron transistor at zero magnetic field
We report sharp peaks in the differential conductance of a single-electron
transistor (SET) at low temperature, for gate voltages at which charge
fluctuations are suppressed. For odd numbers of electrons we observe the
expected Kondo peak at zero bias. For even numbers of electrons we generally
observe Kondo-like features corresponding to excited states. For the latter,
the excitation energy often decreases with gate voltage until a new zero-bias
Kondo peak results. We ascribe this behavior to a singlet-triplet transition in
zero magnetic field driven by the change of shape of the potential that
confines the electrons in the SET.Comment: 4 p., 4 fig., 5 new ref. Rewrote 1st paragr. on p. 4. Revised author
list. More detailed fit results on page 3. A plotting error in the horizontal
axis of Fig. 1b and 3 was corrected, and so were the numbers in the text read
from those fig. Fig. 4 was modified with a better temperature calibration
(changes are a few percent). The inset of this fig. was removed as it is
unnecessary here. Added remarks in the conclusion. Typos are correcte
Electron-phonon interaction in ultrasmall-radius carbon nanotubes
We perform analysis of the band structure, phonon dispersion, and
electron-phonon interactions in three types of small-radius carbon nanotubes.
We find that the (5,5) can be described well by the zone-folding method and the
electron-phonon interaction is too small to support either a charge-density
wave or superconductivity at realistic temperatures. For ultra-small (5,0) and
(6,0) nanotubes we find that the large curvature makes these tubes metallic
with a large density of states at the Fermi energy and leads to unusual
electron-phonon interactions, with the dominant coupling coming from the
out-of-plane phonon modes. By combining the frozen-phonon approximation with
the RPA analysis of the giant Kohn anomaly in 1d we find parameters of the
effective Fr\"{o}lich Hamiltonian for the conduction electrons. Neglecting
Coulomb interactions, we find that the (5,5) CNT remains stable to
instabilities of the Fermi surface down to very low temperatures while for the
(5,0) and (6,0) CNTs a CDW instability will occur. When we include a realistic
model of Coulomb interaction we find that the charge-density wave remains
dominant in the (6,0) CNT with around 5 K while the
charge-density wave instability is suppressed to very low temperatures in the
(5,0) CNT, making superconductivity dominant with transition temperature around
one Kelvin.Comment: 20 pages. Updated 7/23/0
Modified Perturbation Theory Applied to Kondo-Type Transport through a Quantum Dot under a Magnetic Field
Linear conductance through a quantum dot is calculated under a finite
magnetic field using the modified perturbation theory. The method is based on
the second-order perturbation theory with respect to the Coulomb repulsion, but
the self-energy is modified to reproduce the correct atomic limit and to
fulfill the Friedel sum rule exactly. Although this method is applicable only
to zero temperature in a strict sense, it is approximately extended to finite
temperatures. It is found that the conductance near electron-hole symmetry is
suppressed by the application of the magnetic field at low temperatures.
Positive magnetoconductance is observed in the case of large electron-hole
asymmetry.Comment: 4pages, 5 figure
Ground-Laboratory to In-Space Atomic Oxygen Correlation for the Polymer Erosion and Contamination Experiment (PEACE) Polymers
The Materials International Space Station Experiment 2 (MISSE 2) Polymer Erosion and Contamination Experiment (PEACE) polymers were exposed to the environment of low Earth orbit (LEO) for 3.95 years from 2001 to 2005. There were 41 different PEACE polymers, which were flown on the exterior of the International Space Station (ISS) in order to determine their atomic oxygen erosion yields. In LEO, atomic oxygen is an environmental durability threat, particularly for long duration mission exposures. Although spaceflight experiments, such as the MISSE 2 PEACE experiment, are ideal for determining LEO environmental durability of spacecraft materials, ground-laboratory testing is often relied upon for durability evaluation and prediction. Unfortunately, significant differences exist between LEO atomic oxygen exposure and atomic oxygen exposure in ground-laboratory facilities. These differences include variations in species, energies, thermal exposures and radiation exposures, all of which may result in different reactions and erosion rates. In an effort to improve the accuracy of ground-based durability testing, ground-laboratory to in-space atomic oxygen correlation experiments have been conducted. In these tests, the atomic oxygen erosion yields of the PEACE polymers were determined relative to Kapton H using a radio-frequency (RF) plasma asher (operated on air). The asher erosion yields were compared to the MISSE 2 PEACE erosion yields to determine the correlation between erosion rates in the two environments. This paper provides a summary of the MISSE 2 PEACE experiment; it reviews the specific polymers tested as well as the techniques used to determine erosion yield in the asher, and it provides a correlation between the space and ground laboratory erosion yield values. Using the PEACE polymers asher to in-space erosion yield ratios will allow more accurate in-space materials performance predictions to be made based on plasma asher durability evaluation
Tunable Kondo effect in a single donor atom
The Kondo effect has been observed in a single gate-tunable atom. The
measurement device consists of a single As dopant incorporated in a Silicon
nanostructure. The atomic orbitals of the dopant are tunable by the gate
electric field. When they are tuned such that the ground state of the atomic
system becomes a (nearly) degenerate superposition of two of the Silicon
valleys, an exotic and hitherto unobserved valley Kondo effect appears.
Together with the regular spin Kondo, the tunable valley Kondo effect allows
for reversible electrical control over the symmetry of the Kondo ground state
from an SU(2)- to an SU(4) -configuration.Comment: 10 pages, 8 figure
From the Kondo Regime to the Mixed-Valence Regime in a Single-Electron Transistor
We demonstrate that the conductance through a single-electron transistor at
low temperature is in quantitative agreement with predictions of the
equilibrium Anderson model. When an unpaired electron is localized within the
transistor, the Kondo effect is observed. Tuning the unpaired electron's energy
toward the Fermi level in nearby leads produces a cross-over between the Kondo
and mixed-valence regimes of the Anderson model.Comment: 3 pages plus one 2 page postscript file of 5 figures. Submitted to
PR
Excess Kondo resonance in a quantum dot device with normal and superconducting leads: the physics of Andreev-normal co-tunneling
We report on a novel Kondo phenomenon of interacting quantum dots coupled
asymmetrically to a normal and a superconducting lead. The effects of intradot
Coulomb interaction and Andreev tunneling give rise to Andreev bound
resonances. As a result, a new type of co-tunneling process which we term
Andreev-normal co-tunneling, is predicted. At low temperatures, coherent
superposition of these co-tunneling processes induces a Kondo effect in which
Cooper pairs directly participate formation of a spin singlet, leading to four
Kondo resonance peaks in the local density of states, and enhancing the
tunneling current.Comment: 4 pages, 2 figures, Late
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