arXiv subjects

T. Q. Duong

Publications and source records attributed to T. Q. Duong.

11 recordsLinked to original sources

arXiv2024

Finite-frequency noise, Fano factor, $ΔT$-noise and cross-correlations in double quantum dots

A theoretical study on electrical current fluctuations in a double quantum dot connected to electronic reservoirs is presented, with the aim of deriving the finite-frequency noise, the Fano factor and the $ΔT$-noise. We establish a general expression for the noise in terms of Green functions in the double quantum dot and self-energies in the reservoirs. This result is then applied to model double quantum dots in various situations. For a non-interacting double quantum dot, we highlight several interesting features in the physical properties of this system. In particular, we demonstrate the possibility of obtaining a significant reduction in zero-frequency noise and Fano factor either when the system is placed in a given operating regime, or when a temperature gradient is applied between the two reservoirs, resulting in a negative $ΔT$-noise being generated. In addition, in the vicinity of honeycomb vertices, a sign change is observed in the finite-frequency cross-correlator between the two reservoirs, in contrast to what is obtained for the zero-frequency cross-correlator, which remains negative throughout the $(\varepsilon_1,\varepsilon_2)$-plane, $\varepsilon_{1,2}$ being the level energies in each of the two dots. By using an approximate first-level numerical approach, we finally study how the finite-frequency noise in a double quantum dot evolves under the influence of Coulomb interactions.

cond-mat.mes-hall↗

arXiv2020

Level anticrossing effect in single-level or multilevel double quantum dots: Electrical conductance, zero-frequency charge susceptibility and Seebeck coefficient

We study electrical and thermoelectrical properties for a double quantum dot system. We consider the cases of both single-level and multilevel quantum dots whatever the way they are coupled, either in a series or in a parallel arrangement. The calculations are performed by using the nonequilibrium Green function theory. In the case of a single-level double quantum dot, the problem is exactly solvable whereas for a multilevel double quantum dot, an analytical solution is obtained in the limit of energy-independent hopping integrals. { We present a detailed discussion about} the dependences of electrical conductance, zero-frequency charge susceptibility and Seebeck coefficient on the gate voltages applied to the dots, allowing us to derive the charge stability diagram. The findings are in agreement with the experimental observations notably with the occurrence of successive sign changes of the Seebeck coefficient when varying the gate voltages. We interpret the results in terms of the bonding and antibonding states produced by the level anticrossing effect which occurs in the presence of a finite interdot coupling. We show that at equilibrium the boundary lines between the domains with different dot occupancies in the charge stability diagram, take place when the bonding and antibonding state levels are aligned with the chemical potentials in the leads. Finally the total dot occupancy is found to be considerably reduced in the case in parallel compared with the case in series, { whenever} the level energies in each dot are equal. We interpret this dip as a direct manifestation of the interference effects occurring in the presence of the two electronic transmission paths provided by each dot.

cond-mat.mes-hall↗

arXiv2019

Full-Duplex MIMO-OFDM Communication with Self-Energy Recycling

This paper focuses on energy recycling in full-duplex (FD) relaying multiple-input-multiple-output orthogonal frequency division multiplexing (OFDM) communication. The loop self-interference (SI) due to full-duplexing is seen as an opportunity for the energy-constrained relay node to replenish its energy requirement through wireless power transfer. In forwarding the source information to the destination, the FD relay can simultaneously harvest energy from the source wireless transmission and also through energy recycling from its own transmission. The objective is to maximize the overall spectral efficiency by designing the optimal power allocation over OFDM sub-carriers and transmit antennas. Due to a large number of sub-carriers, this design problem poses a large-scale nonconvex optimization problem involving a few thousand variables of power allocation, which is very computationally challenging. A new path-following algorithm is proposed, which converges to an optimal solution. This algorithm is very efficient since it is based on \textit{closed-form} calculations. Numerical results for a practical simulation setting show promising results by achieving high spectral efficiency.

cs.IT↗

arXiv2018

Bang-Bang Charging of Electrical Vehicles by Smart Grid Technology

The success of the transportation electricification in this century particularly requires the penentration of the internet of plug-in electric vehicles (PEVs) into the smart power grid. Beside the function of serving the traditional residential power demand, next-generation power grids also aim to support the internet of PEVs at the same time. The distinct difference between the traditional power demand and PEVs' power demand is that while the statistics of the former is rich enough for treating it as inelastic/known before hand, the latter is unknown until random PEVs' arrivals. Massive penentration of PEVs certainly causes the grid unpredictable fluctuation. The present paper considers the joint PEVs charging coordination and grid power generation to minimizing both of the negative impact of PEVs' integration and the cost of power generation while meeting the grid operating constraints and all parties' demand. The bang-bang PEVs charging strategy is adopted to exploit its simple implementation. By using a recently developed model predictive control (MPC) model for this problem, the online compuation is based on a predictive mixed integer nonlinear programming (MINP). A new solution computation for this optimization problem is developed. Its capacity of achieving the globally optimal solution is shown by numerical comparison between its performance and that by an off-line optimal solution.

eess.SY↗

arXiv2018

Charge susceptibility and conductances of a double quantum dot

We calculate the charge susceptibility and the linear and differential conductances of a double quantum dot coupled to two metallic reservoirs both at equilibrium and when the system is driven away from equilibrium. This work is motivated by recent progress in the realization of solid state spin qubits. The calculations are performed by using the Keldysh nonequilibrium Green function technique. In the noninteracting case, we give the analytical expression for the electrical current and deduce from there the linear conductance as a function of the gate voltages applied to the dots, leading to a characteristic charge stability diagram. We determine the charge susceptibility which also exhibits peaks as a function of gate voltages. We show how the study can be extended to the case of an interacting quantum dot.

cond-mat.mes-hall↗

arXiv2018

Non-symmetrized quantum noise in a Kondo quantum dot

The fluctuations of electrical current provide information on the dynamics of electrons in quantum devices. Understanding the nature of these fluctuations in a quantum dot is thus a crucial step insofar as this system is the elementary brick of quantum circuits. In this context, we develop a theory for calculating the quantum noise at finite frequency in a quantum dot connected to two reservoirs in the presence of interactions and for any symmetry of the couplings to the reservoirs. This theory is developed in the framework of the Keldysh non-equilibrium Green function technique. We establish an analytical expression for the quantum noise in terms of the various transmission amplitudes between the reservoirs and of some effective transmission coefficient which we define. We then study the noise as a function of the dot energy level and the bias voltage. The effects of both Coulomb interactions in the dot and asymmetric couplings with the reservoirs are characterized.

cond-mat.mes-hall↗

arXiv2018

NOMA for throughput and EE maximization in Energy Harvesting Enabled Networks

Wireless power transfer via radio-frequency (RF) radiation is regarded as a potential solution to energize energy-constrained users, who are deployed close to the base stations (near-by users). However, energy transfer requires much more transmit power than normal information transfer, which makes it very challenging to provide the quality of service in terms of throughput for all near-by users and cell-edge users. Thus, it is of practical interest to employ non-orthogonal multiple access (NOMA) to improve the throughput of all network users, while fulfilling the energy harvesting requirements of the near-by users. To realize both energy harvesting and information decoding, we consider a transmit time-switching (transmit-TS) protocol. We formulate two important beamfoming problems of users' max-min throughput optimization and energy efficiency maximization under power constraint and energy harvesting thresholds at the nearly-located users. For these problems, the optimization objective and energy harvesting are non-convex in beamforming vectors. Thus, we develop efficient path-following algorithms to solve them. In addition, we also consider conventional power splitting (PS)-based energy harvesting receiver. Our numerical results confirm that the proposed transmit-TS based algorithms clearly outperform PS-based algorithms in terms of both, throughput and energy efficiency.

cs.IT↗

arXiv2018

PMU Placement Optimization for Smart Grid Obvervability and State Estimation

In this paper, phasor measurement unit (PMU) placement for power grid state estimation under different degrees of observability is studied. Observability degree is the depth of the buses' reachability by the placed PMUs and thus constitutes an important characteristic for PMU placement. However, the sole observability as addressed in many works still does not guarantee a good estimate for the grid state. Some existing works also considered the PMU placement for minimizing the mean squared error or maximizing the mutual information between the measurement output and grid state. However, they ignore the observability requirements for computational tractability and thus potentially lead to artificial results such as acceptance of the estimate for an unobserved state component as its unconditional mean. In this work, the PMU placement optimization problem is considered by minimizing the mean squared error or maximizing the mutual information between the measurement output and grid state, under grid observability constraints. The provided solution is free from the mentioned fundamental drawbacks in the existing PMU placement designs. The problems are posed as binary nonlinear optimization problems, for which this paper develops efficient algorithms for computational solutions. The performance of the proposed algorithms is analyzed in detail through numerical examples on large-scale IEEE power networks.

eess.SY↗

arXiv2018

Emission Noise in an Interacting Quantum Dot: Role of Inelastic Scattering and Asymmetric Coupling to the Reservoirs

A theory is developed for the emission noise at frequency $ν$ in a quantum dot in the presence of Coulomb interactions and asymmetric couplings to the reservoirs. We give an analytical expression for the noise in terms of the various transmission amplitudes. Including inelastic scattering contribution, it can be seen as the analog of the Meir-Wingreen formula for the current. A physical interpretation is given on the basis of the transmission of one electron-hole pair to the concerned reservoir where it emits an energy after recombination. We then treat the interactions by solving the self-consistent equations of motion for the Green functions. The results for the noise derivative versus $eV$ show a zero value until $eV = hν$, followed by a Kondo peak in the Kondo regime, in good agreement with recent measurements in carbon nanotube quantum dots.

cond-mat.mes-hall↗

arXiv2017

Model Predictive Control for Smart Grids with Multiple Electric-Vehicle Charging Stations

Next-generation power grids will likely enable concurrent service for residences and plug-in electric vehicles (PEVs). While the residence power demand profile is known and thus can be considered inelastic, the PEVs' power demand is only known after random PEVs' arrivals. PEV charging scheduling aims at minimizing the potential impact of the massive integration of PEVs into power grids to save service costs to customers while power control aims at minimizing the cost of power generation subject to operating constraints and meeting demand. The present paper develops a model predictive control (MPC)- based approach to address the joint PEV charging scheduling and power control to minimize both PEV charging cost and energy generation cost in meeting both residence and PEV power demands. Unlike in related works, no assumptions are made about the probability distribution of PEVs' arrivals, the known PEVs' future demand, or the unlimited charging capacity of PEVs. The proposed approach is shown to achieve a globally optimal solution. Numerical results for IEEE benchmark power grids serving Tesla Model S PEVs show the merit of this approach.

eess.SY↗

arXiv2017

Energy Efficiency in Cell-Free Massive MIMO with Zero-Forcing Precoding Design

We consider the downlink of a cell-free massive multiple-input multiple-output (MIMO) network where numerous distributed access points (APs) serve a smaller number of users under time division duplex operation. An important issue in deploying cell-free networks is high power consumption, which is proportional to the number of APs. This issue has raised the question as to their suitability for green communications in terms of the total energy efficiency (bits/Joule). To tackle this, we develop a novel low-complexity power control technique with zero-forcing precoding design to maximize the energy efficiency of cell-free massive MIMO taking into account the backhaul power consumption and the imperfect channel state information.

cs.IT↗