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Chi-Hang Lam

Publications and source records attributed to Chi-Hang Lam.

At least 55 records · Page 3Linked to original sources
arXiv2012

Probing the quantum behaviors of a nanomechanical resonator coupled to a double quantum dot

We propose a current correlation spectrum approach to probe the quantum behaviors of a nanome-chanical resonator (NAMR). The NAMR is coupled to a double quantum dot (DQD), which acts as a quantum transducer and is further coupled to a quantum-point contact (QPC). By measuring the current correlation spectrum of the QPC, shifts in the DQD energy levels, which depend on the phonon occupation in the NAMR, are determined. Quantum behaviors of the NAMR could, thus, be observed. In particular, the cooling of the NAMR into the quantum regime could be examined. In addition, the effects of the coupling strength between the DQD and the NAMR on these energy shifts are studied. We also investigate the impacts on the current correlation spectrum of the QPC due to the backaction from the charge detector on the DQD.

cond-mat.mes-hall↗
arXiv2012

Cooling a nanomechanical resonator by a triple quantum dot

We propose an approach for achieving ground-state cooling of a nanomechanical resonator (NAMR) capacitively coupled to a triple quantum dot (TQD). This TQD is an electronic analog of a three-level atom in $Λ$ configuration which allows an electron to enter it via lower-energy states and to exit only from a higher-energy state. By tuning the degeneracy of the two lower-energy states in the TQD, an electron can be trapped in a dark state caused by destructive quantum interference between the two tunneling pathways to the higher-energy state. Therefore, ground-state cooling of an NAMR can be achieved when electrons absorb readily and repeatedly energy quanta from the NAMR for excitations.

cond-mat.mes-hall↗
arXiv2010

Kinetic Monte Carlo simulation of shape transition in strained quantum dots

The pyramid-to-dome transition in Ge$_{x}$Si$_{1-x}$ on Si(100) initiated by step bunching on pyramidal quantum dots is atomistically simulated using a novel multi-state lattice model incorporating effective surface reconstructions. Results are explained by a simple theory based on a shallow island approximation. Under given deposition conditions in $d$ dimensions, the shape transition is shown to occur at island size $n_c$ following $n_c^{1/d} \propto x^{-ζ}$ independent of temperature and deposition rate, where $ζ\alt 2$ and $x$ is the actual Ge concentration in the island. The transition has an energy barrier dominated by the facet interface energy. Fast deposition however can out-run and delay the transition to larger island sizes.

cond-mat.mtrl-sci↗
arXiv2010

Backaction of a charge detector on a double quantum dot

We develop a master equation approach to study the backaction of quantum point contact (QPC) on a double quantum dot (DQD) at zero bias voltage. We reveal why electrons can pass through the zero-bias DQD only when the bias voltage across the QPC exceeds a threshold value determined by the eigenstate energy difference of the DQD. This derived excitation condition agrees well with experiments on QPC-induced inelastic electron tunneling through a DQD [S. Gustavsson et al., Phys. Rev. Lett. 99, 206804(2007)]. Moreover, we propose a new scheme to generate a pure spin current by the QPC in the absence of a charge current.

cond-mat.mes-hall↗
arXiv2010

Cooling a mechanical resonator by quantum interference in a triple quantum dot

We propose an approach to cool a mechanical resonator (MR) via quantum interference in a triple quantum dot (TQD) capacitively coupled to the MR. The TQD connected to three electrodes is an electronic analog of a three-level atom in $Λ$ configuration. The electrons can tunnel from the left electrode into one of the two dots with lower-energy states, but can only tunnel out from the higher-energy state at the third dot to the right electrode. When the two lower-energy states are tuned to be degenerate, an electron in the TQD can be trapped in a superposition of the degenerate states called the dark state. This effect is caused by the destructive quantum interference between tunneling from the two lower-energy states to the higher-energy state. Under this condition, an electron in the dark state readily absorbs an energy quantum from the MR. Repeating this process, the MR can be cooled to its ground state. Moreover, we propose a scheme for verifying the cooling result by measuring the current spectrum of a charge detector adjacent to a double quantum dot coupled to the MR.

cond-mat.mes-hall↗
arXiv2009

Kinetic Monte Carlo simulation of faceted islands in heteroepitaxy using multi-state lattice model

A solid-on-solid model is generalized to study the formation of Ge pyramid islands bounded by (105) facets on Si(100) substrates in two dimensions. Each atomic column is not only characterized by the local surface height but also by two deformation state variables dictating the local surface tilt and vertical extension. These deformations phenomenologically model surface reconstructions in (105) facets and enable the formation of islands which better resemble faceted pyramids. We demonstrate the model by application to a kinetic limited growth regime. We observe significantly reduced growth rates after faceting and a continuous nucleation of new islands until overcrowding occurs.

cond-mat.mes-hall↗
arXiv2009

Shot noise in electron transport through a double quantum dot: A master equation approach

We study shot noise in tunneling current through a double quantum dot connected to two electric leads. We derive two master equations in the occupation-state basis and the eigenstate basis to describe the electron dynamics. The approach based on the occupation-state basis, despite widely used in many previous studies, is valid only when the interdot coupling strength is much smaller than the energy difference between the two dots. In contrast, the calculations using the eigenstate basis are valid for an arbitrary interdot coupling. We show that the master equation in the occupation-state basis includes only the low-order terms with respect to the interdot coupling compared with the more accurate master equation in the eigenstate basis. Using realistic model parameters, we demonstrate that the predicted currents and shot-noise properties from the two approaches are significantly different when the interdot coupling is not small. Furthermore, properties of the shot noise predicted using the eigenstate basis successfully reproduce qualitative features found in a recent experiment.

cond-mat.mes-hall↗
arXiv2008

Readout of a single electron spin in a double quantum dot using a quantum point contact

We study the dynamics of a single electron spin in a double quantum dot (DQD) and its readout via a quantum point contact (QPC). We model the system microscopically and derive rate equations for the reduced electron density matrix of the DQD. Two cases with one and two electrons in the DQD are studied. In the one-electron case, with different Zeeman splittings in the two dots, the electron spin states are distinctly characterized by a constant and an oscillatory current through the QPC. In the two-electron case, the readout of the spin state of the electron in one of the dots called the qubit dot is essentially similar after considering hyperfine interactions between the electrons and the nuclear spins of the host materials and a uniform magnetic field applied to the DQD. Moreover, to ensure that an electron is properly injected into the qubit dot, we propose to determine the success of the electron injection from the variations of the QPC current after applying an oscillating magnetic field to the qubit dot.

cond-mat.mes-hall↗
arXiv2008

Shot noise in a quantum dot coupled to non-magnetic leads: Effects of Coulomb interaction

We study electron transport through a quantum dot, connected to non-magnetic leads, in a magnetic field. A super-Poissonian electron noise due to the effects of both interacting localized states and dynamic channel blockade is found when the Coulomb blockade is partially lifted. This is sharp contrast to the sub-Poissonian shot noise found in the previous studies for a large bias voltage, where the Coulomb blockade is completely lifted. Moreover, we show that the super-Poissonian shot noise can be suppressed by applying an electron spin resonance (ESR) driving field. For a sufficiently strong ESR driving field strength, the super-Poissonian shot noise will change to be sub-Poissonian.

cond-mat.mes-hall↗
arXiv2008

Fast kinetic Monte Carlo simulation of strained heteroepitaxy in three dimensions

Accelerated algorithms for simulating the morphological evolution of strained heteroeptiaxy based on a ball and spring lattice model in three dimensions are explained. We derive exact Green's function formalisms for boundary values in the associated lattice elasticity problems. The computational efficiency is further enhanced by using a superparticle surface coarsening approximation. Atomic hoppings simulating surface diffusion are sampled using a multi-step acceptance-rejection algorithm. It utilizes quick estimates of the atomic elastic energies from extensively tabulated values modulated by the local strain. A parameter controls the compromise between accuracy and efficiency of the acceptance-rejection algorithm.

cond-mat.stat-mech↗
arXiv2007

Leakage-induced decoherence during single electron spin manipulation in a double quantum dot

Coherent single electron spin oscillation in a double quantum dot system driven by a magnetic electron spin resonance field is studied theoretically using a Bloch-type rate equation approach. The oscillation frequency and relaxation time obtained using typical model parameters are consistent with experiment findings. The dominant decoherence mechanism is identified to be a leakage current through a Coulomb blockade barrier at a quantum dot during the spin manipulation. Nuclear field fluctuations which induce a long relaxation time are found to contribute only negligibly to the decoherence despite an earlier suggestion.

cond-mat.mes-hall↗
arXiv2006

Dissipative dynamics of coupled quantum dots under quantum measurement

We have studied the dissipative dynamics of a solid-state qubit with an extra electron confined to either one of two coupled quantum dots. Previous theoretical work based on Bloch-type rate equations gave an unphysical uniform occupation probability of the electron in the quantum dots even for non-identical dots. We show that this is due to neglecting higher order interactions in the analysis. By including higher order terms, we obtain expected asymmetric occupation probabilities for non-identical dots. Our work demonstrates that the high order interaction terms can lead to important qualitative impacts on the operation of the qubit.

cond-mat.mes-hall↗
arXiv2005

Exact scalings in competitive growth models

A competitive growth model (CGM) describes aggregation of a single type of particle under two distinct growth rules with occurrence probabilities $p$ and $1-p$. We explain the origin of scaling behaviors of the resulting surface roughness with respect to $p$ for two CGMs which describe random deposition (RD) competing with ballistic deposition (BD) and RD competing with the Edward Wilkinson (EW) growth rule. Exact scaling exponents are derived and are in agreement with previously conjectured values. Using this analytical result we are able to derive theoretically the scaling behaviors of the coefficients of the continuous equations that describe their universality classes. We also suggest that, in some CGM, the $p-$dependence on the coefficients of the continuous equation that represents the universality class can be non trivial. In some cases the process cannot be represented by a unique universality class. In order to show this we introduce a CGM describing RD competing with a constrained EW (CEW) model. This CGM show a transition in the scaling exponents from RD to a Kardar-Parisi-Zhang behavior when $p \to 0$ and to a Edward Wilkinson one when $p \to 1$. Our simulation results are in excellent agreement with the analytic predictions.

cond-mat.stat-mech↗
arXiv2005

Island, pit and groove formation in strained heteroepitaxy

We study the morphological evolution of strained heteroepitaxial films using a kinetic Monte Carlo method in three dimensions. The elastic part of the problem uses a Green's function method. Isolated islands are observed under deposition conditions for deposition rates slow compared with intrinsic surface roughening rates. They are semi-spherical and truncated conical for high and low temperature cases respectively. Annealing of films at high temperature leads to the formation of closely packed islands consistent with an instability theory. At low temperature, pits form via a layer-by-layer nucleation mechanism and subsequently develop into grooves.

cond-mat.stat-mech↗
arXiv2004

Fluid Invasion in Porous Media: Viscous Gradient Percolation

We suggest that the dynamics of stable viscous invasion fronts in porous media depends on the volume capacitance of the media. At high volume capacitance, our network simulations provide numerical evidence of a scaling relation between the front width and its velocity. In the low volume capacitance regime, we derive a new effective scaling supported by network simulations and is in agreement with previous experiments on imbibition in paper and collections of glass beads.

cond-mat.stat-mech↗
arXiv2002

Competing roughening mechanisms in strained heteroepitaxy: a fast kinetic Monte Carlo study

We study the morphological evolution of strained heteroepitaxial films using kinetic Monte Carlo simulations in two dimensions. A novel Green's function approach, analogous to boundary integral methods, is used to calculate elastic energies efficiently. We observe island formation at low lattice misfit and high temperature that is consistent with the Asaro-Tiller-Grinfeld instability theory. At high misfit and low temperature, islands or pits form according to the nucleation theory of Tersoff and LeGoues.

cond-mat.mtrl-sci↗
arXiv1997

Anomaly in Numerical Integrations of the KPZ Equation and Improved Discretization

We demonstrate and explain that conventional finite difference schemes for direct numerical integration do not approximate the continuum Kardar-Parisi-Zhang (KPZ) equation due to microscopic roughness. The effective diffusion coefficient is found to be inconsistent with the nominal one. We propose a novel discretization in 1+1 dimensions which does not suffer from this deficiency and elucidates the reliability and limitations of direct integration approaches.

cond-mat.stat-mech↗