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Super-Andreev reflection and longitudinal shift of pseudospin-1 fermions

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Physical Review B 2020年第23期101卷 235417-235417页

作者： Xiaolong Feng Ying Liu Zhi-Ming Yu Zhongshui Ma L. K. Ang Yee Sin Ang Shengyuan A. Yang Research Laboratory for Quantum Materials Singapore University of Technology and Design Singapore 487372 Singapore School of Materials Science and Engineering Hebei University of Technology Tianjin 300130 China Key Lab of Advanced Optoelectronic Quantum Architecture and Measurement (MOE) Beijing Key Lab of Nanophotonics & Ultrafine Optoelectronic Systems and School of Physics Beijing Institute of Technology Beijing 100081 China School of Physics Peking University Beijing 100871 China Science and Math Singapore University of Technology and Design Singapore 487372 Singapore

Novel fermions with a pseudospin-1 structure can be realized as emergent quasiparticles in condensed matter systems. Here, we investigate its unusual properties during the Andreev reflection at a normal-metal/ superconductor (NS) interface. We show that distinct from the previously studied pseudospin-1/2 and two-dimensional electron gas models, the pseudospin-1 fermions exhibit a strongly enhanced Andreev reflection probability, and remarkably, can be further tuned to approach perfect Andreev reflection with unit efficiency for all incident angles, exhibiting a previously unknown super-Andreev reflection effect. The super-Andreev reflection leads to perfect transparency of the NS interface that strongly promotes charge injection into the superconductor and directly manifests as a differential conductance peak which can be readily probed in experiment. Additionally, we find that sizable longitudinal shifts exist in the normal and Andreev reflections of pseudospin-1 fermions. Distinct from the pseudospin-1/2 case, the shift is always in the forward direction in the subgap regime, regardless of whether the reflection is of retro- or specular type.

关键词： Andreev reflection Transport phenomena 2-dimensional systems Superconductors Bogoliubov-de Gennes equations

Thermalization with detailed-balanced two-site Lindblad dissipators

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Physical Review E 2019年第2期100卷 022111-022111页

作者： Mikel Palmero Xiansong Xu Chu Guo Dario Poletti Science and Math Cluster Singapore University of Technology and Design 8 Somapah Road 487372 Singapore Zhengzhou Information Science and Technology Institute Zhengzhou 450004 China Engineering Product Development Singapore University of Technology and Design 8 Somapah Road 487372 Singapore

The use of two-site Lindblad dissipators to generate thermal states and study heat transport was raised to prominence by Prosen and Žnidarič [J. Stat. Mech. (2009) P02035]. Here we propose a variant of this method based on detailed balance of internal levels of the two-site Hamiltonian and characterize its performance. We study the thermalization profile in the chain, the effective temperatures achieved by different single- and two-site observables, and we also investigate the decay of two-time correlations. We find that at a large enough temperature, the steady state approaches closely a thermal state, with a relative error below 1% for the inverse temperature estimated from different observables.

关键词： Matrix product states Open quantum systems & decoherence Quantum master equation Quantum thermodynamics Quantum spin chains

Efficient generation of extreme terahertz harmonics in 3D Dirac Semimetals

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arXiv 2020年

作者： Lim, Jeremy Ang, Y.S. de Abajo, F.J. García Kaminer, I. Ang, L.K. Wong, L.J. Science Math and Technology Singapore University of Technology and Design Singapore ICFO-Institut de Ciencies Fotoniques The Barcelona Institute of Science and Technology Barcelona Spain ICREA-Institució Catalana de Recerca i Estudis Avancats Barcelona Spain Department of Electrical Engineering Technion Haifa Israel School of Electrical and Electronic Engineering Nanyang Technological University Singapore

Frequency multiplication of terahertz signals on a solid state platform is highly sought-after for the next generation of high-speed electronics and the creation of frequency combs. Solutions to efficiently generate extreme harmonics (up to the 31st harmonic and beyond) of a terahertz signal with modest input intensities, however, remain elusive. Using fully nonperturbative simulations and complementary analytical theory, we show that 3D Dirac semimetals (DSMs) have enormous potential as compact sources of extreme terahertz harmonics, achieving energy conversion efficiencies beyond 10−5 at the 31st harmonic with input intensities on the order of 10 MW/cm2, over 105 times lower than in conventional THz high harmonic generation systems. Our theory also reveals a fundamental feature in the nonlinear optics of 3D DSMs: a distinctive regime where higher-order optical nonlinearity vanishes, arising as a direct result of the extra dimensionality in 3D DSMs compared to 2D DSMs. Our findings should pave the way to the development of efficient platforms for high-frequency terahertz light sources and optoelectronics based on 3D DSMs. Copyright © 2020, The Authors. All rights reserved.

关键词： Harmonic analysis

Anomalous Suppression of Higher-Order Nonlinearities in 3D Dirac Semimetals

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Anomalous Suppression of Higher-Order Nonlinearities in 3D D...

IEEE International Conference on Plasma Science (ICOPS)

作者： Jeremy Lim Yee Sin Ang Ricky Ang Francisco J. Garcia de Abajo Ido Kaminer Liang Jie Wong Science and Math Singapore University of Technology and Design Singapore Singapore ICFO The Barcelona Institute of Science and Technology Barcelona Spain ICREA-Institucio Catalana de Recerca i Estudis Avancats Barcelona Spain Dept. of Electrical Engineering TechnionHaifaIsrael3200003 School of Electrical and Electronic Engineering Nanyang Technological University Singapore Singapore

ISBN: (纸本)9781728153087

Three dimensional Dirac semimetals are often thought to share the same essential physics as 2D Dirac materials. Here, we present a counter-intuitive feature of 3D Dirac semimetals that sets them apart from their 2D counterparts: the absence of intraband nonlinearities beyond the third order at near-zero temperatures at or close to a critical field strength. Using complementary analytical theory and rigorous, nonperturbative time-domain simulations, we show that this effect is robust against changes in incident field polarization and remains significant even at finite temperatures. Additionally, we identify regimes where 3D Dirac semimetals effectively function as bulk versions of 2D Dirac semimetals with the potential for superior performance in terms of high-harmonic generation. Our work 1 fills a vital gap in understanding the nonlinear terahertz response of Dirac electrons, and paves the way for the development of chip-integrable, nanophotonic devices and optoelectronics based on Dirac materials.

关键词： Performance evaluation Electric potential Analytical models Three-dimensional displays Conferences Nanoscale devices Plasmas

Revealing Electron Spill-Out in Plasmonic Nanostructures Using Particle Simulation

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Revealing Electron Spill-Out in Plasmonic Nanostructures Usi...

IEEE International Conference on Plasma Science (ICOPS)

作者： Hue Thi Bich Do Wen Jun Ding Xiao Xiong Ching Eng Png Lin Wu Jeremy Zhen Jie Lim Ricky Ang Zackaria Mahfoud Michel Bosman NUS Graduate School of Integrative Sciences and Engineering National University of Singapore Singapore Singapore National University of Singapore Singapore Singapore Institute of High Performance Computing Agency for Science Technology and Research (A*STAR)SingaporeSingapore138632 Institute of High Performance Computing Agency for Science Technology and Research (A*STAR) Singapore Singapore SUTD-MIT International Design Center & Science and Math Singapore University of Technology and Design Singapore Singapore Institute of Materials Research and Engineering Agency for Science Technology and Research (A*STAR) Singapore Singapore

ISBN: (纸本)9781728153087

Classical macroscopic electromagnetic descriptions fail to accurately predict the optical response of plasmonic nanostructures approaching electronic length scales due to the omission of nonlocality and free electron spill-out. Alternatively, microscopic first-principles approaches are accurate, yet too computationally intensive for multiscale problems. Starting from a microscopic viewpoint but more computationally efficient, particle-in-cell (PIC) simulation is probably an appropriate mesoscopic treatment for such problems. Here, we leverage on the inherent soft boundary conditions of PIC to study the spill-out effects of surface electrons in mesoscopic-scale metal nanostructures, specifically Na nanowires [1]. The relationship between the surface plasmon resonance frequency and the radius of the nanowire is investigated, exhibiting a 1/R redshift, which agrees well with the self-consistent hydrodynamic method [2] that includes spill-out of electron density. The developed methodology can be extended to model noble metals by including inter-band transitions, di-metallic nanoplasmonic structures, electron spill-out and charge transport between nearly touching nanoparticles.

关键词： Nanoparticles Integrated optics Optical microscopy Metals Resonant frequency Nanowires Plasmas

Efficient generation of extreme terahertz harmonics in three-dimensional Dirac semimetals

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Physical Review Research 2020年第4期2卷 043252-043252页

作者： Jeremy Lim Yee Sin Ang F. Javier García de Abajo Ido Kaminer Lay Kee Ang Liang Jie Wong Science Math and Technology Singapore University of Technology and Design 8 Somapah Road Singapore Singapore 487372 ICFO-Institut de Ciencies Fotoniques Barcelona Institute of Science and Technology 08860 Castelldefels (Barcelona) Spain ICREA-Institució Catalana de Recerca i Estudis Avançats Passeig Lluís Companys 23 08010 Barcelona Spain Department of Electrical Engineering Technion Haifa 3200003 Israel School of Electrical and Electronic Engineering Nanyang Technological University 50 Nanyang Avenue Singapore Singapore 639798

We show that three-dimensional (3D) Dirac semimetals (DSMs) can achieve highly efficient terahertz high-order harmonic generation (HHG) up to the 31st harmonic with input intensities ≈10MW/cm2—over 105 times lower than required in conventional terahertz HHG systems. Our theory reveals that this extreme nonlinearity is made possible by the existence of an operation regime that differs from previous demonstrations of lower order harmonic generation. We also reveal an unexpected regime in which emitted harmonics abruptly become negligible beyond the third order. This unprecedented vanishing of higher order nonlinearity has a geometrical origin related to the combination of conical dispersion and extra dimensionality in 3D DSMs, breaking the common notion that 3D DSMs share the essential physics of two-dimensional DSMs. Our findings pave the way to unlocking the full potential of 3D DSMs as efficient platforms for terahertz light sources and optoelectronics at moderate intensities.

关键词： High-order harmonic generation Nonlinear optics Ultrafast optics Dirac semimetal

Thermalization with detailed-balanced two-site Lindblad dissipators

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arXiv 2019年

作者： Palmero, Mikel Xu, Xiansong Guo, Chu Poletti, Dario Science and Math Cluster Singapore University of Technology and Design 8 Somapah Road Singapore487372 Singapore Zhengzhou Information Science and Technology Institute Zhengzhou450004 China Engineering Product Development Singapore University of Technology and Design 8 Somapah Road Singapore487372 Singapore

The use of two-site Lindblad dissipator to generate thermal states and study heat transport raised to prominence since [J. Stat. Mech. (2009) P02035] by Prosen and Žnidarič. Here we propose a variant of this method based on detailed balance of internal levels of the two site Hamiltonian and characterize its performance. We study the thermalization profile in the chain, the effective temperatures achieved by different single and two-site observables, and we also investigate the decay of two-time correlations. We find that at a large enough temperature the steady state approaches closely a thermal state, with a relative error below 1% for the inverse temperature estimated from different observables. Copyright © 2019, The Authors. All rights reserved.

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Towards generation of cat states in trapped ions set-ups via FAQUAD protocols and dynamical decoupling

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arXiv 2019年

作者： Palmero, Mikel Simón, Miguel Ángel Poletti, Dario Science and Math Cluster Singapore University of Technology and Design 8 Somapah Road Singapore487372 Singapore Departamento de Química-Física Universidad del País Vasco UPV- EHU Bilbao Spain Engineering Product Development Singapore University of Technology and Design 8 Somapah Road Singapore487372 Singapore

The high fidelity generation of strongly entangled states of many particles, such as cat states, is a particularly demanding challenge. One approach is to drive the system, within a certain final time, as adiabatically as possible, in order to avoid the generation of unwanted excitations. However, excitations can be generated also by the presence of dissipative effects such as dephasing. Here we compare the effectiveness of Local Adiabatic and the FAst QUasi ADiabatic protocols in achieving a high fidelity for a target superposition state both with and without dephasing. In particular we consider trapped ions set-ups in which each spin interacts with all the others with the uniform coupling strength or with a power-law coupling. In order to mitigate the effects of dephasing, we complement the adiabatic protocols with dynamical decoupling and we test its effectiveness. The protocols we study could be readily implemented with state-of-the-art techniques. Copyright © 2019, The Authors. All rights reserved.

关键词： Trapped ions

User association and path planning for UAV-aided mobile edge computing with energy restriction

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arXiv 2019年

作者： Qian, Yuwen Wang, Feifei Li, Jun Shi, Long Cai, Kui Shu, Feng School of Electronic and Optical Engineering Nanjing University of Science and Technology Nanjing China School of Computer Science and Robotics National Research Tomsk Polytechnic University Tomsk634050 Russia Science and Math Cluster Singapore University of Technology and Design Singapore

Mobile edge computing (MEC) provides computational services at the edge of networks by offloading tasks from user equipments (UEs). This letter employs an unmanned aerial vehicle (UAV) as the edge computing server to execute offloaded tasks from the ground UEs. We jointly optimize user association, UAV trajectory, and uploading power of each UE to maximize sum bits offloaded from all UEs to the UAV, subject to energy constraint of the UAV and quality of service (QoS) of each UE. To address the non-convex optimization problem, we first decompose it into three subproblems that are solved with integer programming and successive convex optimization methods respectively. Then, we tackle the overall problem by the multi-variable iterative optimization algorithm. Simulations show that the proposed algorithm can achieve a better performance than other baseline schemes. Copyright © 2019, The Authors. All rights reserved.

关键词： Unmanned aerial vehicles (UAV)