2,238 research outputs found
Charge-noise-free Lateral Quantum Dot Devices with Undoped Si/SiGe Wafer
We develop quantum dots in a single layered MOS structure using an undoped
Si/SiGe wafer. By applying a positive bias on the surface gates, electrons are
accumulated in the Si channel. Clear Coulomb diamond and double dot charge
stability diagrams are measured. The temporal fluctuation of the current is
traced, to which we apply the Fourier transform analysis. The power spectrum of
the noise signal is inversely proportional to the frequency, and is different
from the inversely quadratic behavior known for quantum dots made in doped
wafers. Our results indicate that the source of charge noise for the doped
wafers is related to the 2DEG dopant.Comment: Proceedings of the 12th Asia Pacific Physics Conferenc
Coherent Manipulation of Individual Electron Spin in a Double Quantum Dot Integrated with a Micro-Magnet
We report the coherent manipulation of electron spins in a double quantum dot
integrated with a micro-magnet. We performed electric dipole spin resonance
experiments in the continuous wave (CW) and pump-and-probe modes. We observed
two resonant CW peaks and two Rabi oscillations of the quantum dot current by
sweeping an external magnetic field at a fixed frequency. Two peaks and
oscillations are measured at different resonant magnetic field, which reflects
the fact that the local magnetic fields at each quantum dot are modulated by
the stray field of a micro-magnet. As predicted with a density matrix approach,
the CW current is quadratic with respect to microwave (MW) voltage while the
Rabi frequency (\nu_Rabi) is linear. The difference between the \nu_Rabi values
of two Rabi oscillations directly reflects the MW electric field across the two
dots. These results show that the spins on each dot can be manipulated
coherently at will by tuning the micro-magnet alignment and MW electric field.Comment: 5 pages, 3 figure
On the Lichnerowicz conjecture for CR manifolds with mixed signature
We construct examples of nondegenerate CR manifolds with Levi form of
signature , , which are compact, not locally CR flat, and
admit essential CR vector fields. We also construct an example of a noncompact
nondegenerate CR manifold with signature which is not locally CR flat
and admits an essential CR vector fields. These provide counterexamples to the
analogue of the Lichnerowicz conjecture for CR manifolds with mixed signature.Comment: 7 page
Spin-Echo Measurements for an Anomalous Quantum Phase of 2D Helium-3
Previous heat-capacity measurements of our group had shown the possible
existence of an anomalous quantum phase containing the zero-point vacancies
(ZPVs) in 2D He. The system is monolayer He adsorbed on graphite
preplated with monolayer He at densities () just below the 4/7
commensurate phase (). We carried out
pulsed-NMR measurements in order to examine the microscopic and dynamical
nature of this phase. The measured decay of spin echo signals shows the
non-exponential behaviour. The decay curve can be fitted with the double
exponential function, but the relative intensity of the component with a longer
time constant is small (5%) and does not depend on density and temperature,
which contradicts the macroscopic fluid and 4/7 phase coexistence model. This
slowdown is likely due to the mosaic angle spread of Grafoil substrate and the
anisotropic spin-spin relaxation time in 2D systems with respect to the
magnetic field direction. The inverse value deduced from the major echo
signal with a shorter time constant, which obeys the single exponential
function, decreases linearly with decreasing density from , supporting the
ZPV model.Comment: 4 pages, 6 figure
Surface tension in an intrinsic curvature model with fixed one-dimensional boundaries
A triangulated fixed connectivity surface model is investigated by using the
Monte Carlo simulation technique. In order to have the macroscopic surface
tension \tau, the vertices on the one-dimensional boundaries are fixed as the
edges (=circles) of the tubular surface in the simulations. The size of the
tubular surface is chosen such that the projected area becomes the regular
square of area A. An intrinsic curvature energy with a microscopic bending
rigidity b is included in the Hamiltonian. We found that the model undergoes a
first-order transition of surface fluctuations at finite b, where the surface
tension \tau discontinuously changes. The gap of \tau remains constant at the
transition point in a certain range of values A/N^\prime at sufficiently large
N^\prime, which is the total number of vertices excluding the fixed vertices on
the boundaries. The value of \tau remains almost zero in the wrinkled phase at
the transition point while \tau remains negative finite in the smooth phase in
that range of A/N^\prime.Comment: 12 pages, 8 figure
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