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 $(p,q)$, $2\leq p\leq q$, 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 $(1,n-1)$ 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 $^{3}$He. The system is monolayer $^{3}$He adsorbed on graphite preplated with monolayer $^{4}$He at densities ($\rho$) just below the 4/7 commensurate phase ($0.8\leq \rho /\rho_{4/7}\leq 1$). 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 $T_{2}$ in 2D systems with respect to the magnetic field direction. The inverse $T_2$ value deduced from the major echo signal with a shorter time constant, which obeys the single exponential function, decreases linearly with decreasing density from $n=1$, 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