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Li Yang

Publications and source records attributed to Li Yang.

At least 307 records · Page 17Linked to original sources
arXiv2015

Quantum McEliece public-key encryption scheme

This paper investigates a quantum version of McEliece public-key encryption (PKE) scheme, and analyzes its security. As is well known, the security of classical McEliece PKE is not stronger than the onewayness of related classical one-way function. We prove the security of quantum McEliece PKE ranks between them. Moreover, we propose the double-encryption technique to improve its security, and the security of the improved scheme is proved to be between the original scheme and the quantum one-time pad.

quant-ph↗
arXiv2015

The Classification of Quantum Symmetric-Key Encryption Protocols

The classification of quantum symmetric-key encryption protocol is presented. According to five elements of a quantum symmetric-key encryption protocol: plaintext, ciphertext, key, encryption algorithm and decryption algorithm, there are 32 different kinds of them. Among them, 5 kinds of protocols have already been constructed and studied, and 21 kinds of them are proved to be impossible to construct, the last 6 kinds of them are not yet presented effectively. That means the research on quantum symmetric-key encryption protocol only needs to consider with 5 kinds of them nowadays.

quant-ph↗
arXiv2015

Topological Protected Dirac Cones in Compressed Bulk Black Phosphorus

Using the k.p theory and first-principles simulations, we report that applying a moderate pressure (> 0.6 GPa) on black phosphorus can diminish its band gap and produce one-dimensional and even two-dimensional (2D) Dirac cones, distinguishing this material for use in novel non-compound topological insulators. Similar to topological insulators, these 2D Dirac cones result from two competing mechanisms: the unique linear band dispersion tends to open a gap via a "pseudo spin-orbit" coupling, while the band symmetry requirements preserve the material's gapless spectrum. Moreover, these unique Dirac cones are bulk states that do not require time-reversal symmetry, thus they are robust even in the presence of surface or magnetic perturbations. Ultimately, we show that our predictions can be detected by the material's unusual Landau levels.

cond-mat.mtrl-sci↗
arXiv2014

Quantum Oscillations in Black Phosphorus Two-dimensional Electron Gas

Two-dimensional electron gases (2DEG) have been an important source of experimental discovery and conceptual development in condensed matter physics for decades. When combined with the unique electronic properties of two-dimensional (2D) crystals, rich new physical phenomena can be probed at the quantum level. Here, we create a new 2DEG in black phosphorus, a recent member of the two-dimensional atomic crystal family, using a gate electric field. We achieve high carrier mobility in black phosphorus 2DEG by placing it on a hexagonal boron nitride substrate (h-BN). This allows us, for the first time, to observe quantum oscillations in this material. The temperature and magnetic field dependence of the oscillations yields crucial information about the system, such as cyclotron mass and lifetime of its charge carriers. Our results, coupled with the fact that black phosphorus possesses anisotropic energy bands with a tunable, direct bandgap, distinguishes black phosphorus 2DEG as a novel system with unique electronic and optoelectronic properties.

cond-mat.mes-hall↗
arXiv2014

Highly Anisotropic and Robust Excitons in Monolayer Black Phosphorus

Semi-metallic graphene and semiconducting monolayer transition metal dichalcogenides (TMDCs) are the two-dimensional (2D) materials most intensively studied in recent years. Recently, black phosphorus emerged as a promising new 2D material due to its widely tunable and direct bandgap, high carrier mobility and remarkable in-plane anisotropic electrical, optical and phonon properties. However, current progress is primarily limited to its thin-film form, and its unique properties at the truly 2D quantum confinement have yet to be demonstrated. Here, we reveal highly anisotropic and tightly bound excitons in monolayer black phosphorus using polarization-resolved photoluminescence measurements at room temperature. We show that regardless of the excitation laser polarization, the emitted light from the monolayer is linearly polarized along the light effective mass direction and centers around 1.3 eV, a clear signature of emission from highly anisotropic bright excitons. In addition, photoluminescence excitation spectroscopy suggests a quasiparticle bandgap of 2.2 eV, from which we estimate an exciton binding energy of around 0.9 eV, consistent with theoretical results based on first-principles. The experimental observation of highly anisotropic, bright excitons with exceedingly large binding energy not only opens avenues for the future explorations of many-electron effects in this unusual 2D material, but also suggests a promising future in optoelectronic devices such as on-chip infrared light sources.

cond-mat.mes-hall↗
arXiv2014

Carrier Plasmon Induced Nonlinear Band Gap Renormalization in Two-Dimensional Semiconductors

In reduced-dimensional semiconductors, doping-induced carrier plasmons can strongly couple with quasiparticle excitations, leading to a significant band gap renormalization. We develop a new plasmon-pole theory that efficiently and accurately capture this coupling. Using monolayer molybdenum disulfide (MoS2) as a prototype two-dimensional (2D) semiconductor, we reveal an enhanced band gap renormalization around 400 meV and an unusual nonlinear evolution of its band gap with doping. This 2D prediction significantly differs from the linear behaviors that are common to one-dimensional structures. Our developed approach allows for a quantitative understanding of many-body interactions in general doped 2D semiconductors and paves the way for novel band gap engineering techniques.

cond-mat.mes-hall↗
arXiv2014

Lattice Vibrational Modes and Raman Scattering Spectra of Strained Phosphorene

Strain is prominent in fabricated samples of two-dimensional semiconductors and it also serves as an exploitable tool for engineering their properties. However, quantifying strain and characterizing its spatially inhomogeneous distribution across a material are challenging tasks. Here, we report the lattice vibrational modes and corresponding Raman spectra of strained monolayer black phosphorus (phosphorene) by first-principles simulations. We show that frequencies of vibrational modes of phosphorene and their Raman scattering peaks exhibit substantial and distinct shifts according to the types and size of strain. Therefore, combined with high spatial-resolution Raman scattering measurements, our calculated results can quantify strain distributions in phosphorene. This information is essential for understanding structures of future large-scale fabrication and strain engineering of phosphorene.

cond-mat.mtrl-sci↗
arXiv2014

Enhanced Thermoelectric Efficiency via Orthogonal Electrical and Thermal Conductances in Phosphorene

Thermoelectric devices that utilize the Seebeck effect convert heat flow into electrical energy and are highly desirable for the development of portable, solid state, passively-powered electronic systems. The conversion efficiencies of such devices are quantified by the dimensionless thermoelectric figure of merit (ZT), which is proportional to the ratio of a device's electrical conductance to its thermal conductance. High ZT (>2) has been achieved in materials via all-scale hierarchical architecturing. This efficiency holds at high temperatures (700K~900K) but quickly diminishes at lower temperatures. In this paper, a recently-fabricated two-dimensional (2D) semiconductor called phosphorene (monolayer black phosphorus) is assessed for its thermoelectric capabilities. First-principles and model calculations reveal that phosphorene possesses spatially-anisotropic electrical and thermal conductances. The prominent electrical and thermal conducting directions are orthogonal to one another, enhancing the ratio of these conductances. As a result, ZT can reach 2.5 (the criterion for commercial deployment) along the armchair direction of phosphorene at T=500K and is greater than 1 even at room temperature given moderate doping (~2 x 10^16 m-2). Ultimately, phosphorene stands out as an environmentally sound thermoelectric material with unprecedented qualities: intrinsically, it is a mechanically flexible material that converts heat energy with high efficiency at low temperatures (~ 300K) - one whose performance does not require any sophisticated engineering techniques.

cond-mat.mtrl-sci↗
arXiv2014

Van Hove Singularities and Excitonic Effects in the Optical Conductivity of Twisted Bilayer Graphene

We report a systematic study of the optical conductivity of twisted bilayer graphene (tBLG) across a large energy range (1.2 eV to 5.6 eV) for various twist angles, combined with first-principles calculations. At previously unexplored high energies, our data show signatures of multiple van Hove singularities (vHSs) in the tBLG bands, as well as the nonlinearity of the single layer graphene bands and their electron-hole asymmetry. Our data also suggest that excitonic effects play a vital role in the optical spectra of tBLG. Including electron-hole interactions in first-principles calculations is essential to reproduce the shape of the conductivity spectra, and we find evidence of coherent interactions between the states associated with the multiple vHSs in tBLG.

cond-mat.mes-hall↗
arXiv2014

Tunable Band Gap and Anisotropic Optical Response in Few-layer Black Phosphorus

We report the quasiparticle band gap, excitons, and highly anisotropic optical responses of few-layer black phosphorous (phosphorene). It is shown that these new materials exhibit unique many-electron effects; the electronic structures are dispersive essentially along one dimension, leading to particularly enhanced self-energy corrections and excitonic effects. Additionally, within a wide energy range, including infrared light and part of visible light, few-layer black phosphorous absorbs light polarized along the structure's armchair direction and is transparent to light polarized along the zigzag direction, making them viable linear polarizers for applications. Finally, the number of phosphorene layers included in the stack controls the material's band gap, optical absorption spectrum, and anisotropic polarization energy-window across a wide range.

cond-mat.mes-hall↗
arXiv2014

Unusual Scaling Laws of the Band Gap and Optical Absorption of Phosphorene Nanoribbons

We report the electronic structure and optical absorption spectra of monolayer black phosphorus (phosphorene) nanoribbons (PNRs) via first-principles simulations. The band gap of PNRs is strongly enhanced by quantum confinement. However, differently orientated PNRs exhibit distinct scaling laws for the band gap vs the ribbon width (w). The band gaps of armchair PNRs scale as 1/(w^2), while zigzag PNRs exhibit a 1/w behavior. These distinct scaling laws reflect a significant implication of the band dispersion of phosphorene: electrons and holes behave as classical particles along the zigzag direction, but resemble relativistic particles along the armchair direction. This unexpected merging of classical and relativistic properties in a single material may produce novel electrical and magnetotransport properties of few-layer black phosphorus and its ribbon structures. Finally, the respective PNRs host electrons and holes with markedly different effective masses and optical responses, which are suitable for a wide range of applications.

cond-mat.mes-hall↗
arXiv2014

Proton cloud and the possibility of direct perceiving of a Hydrogen nucleon

We introduce a concept of proton cloud and calculate the radius of the proton cloud of the Hydrogen atom. Then, we estimate the radius of the proton cloud of a Hydrogen atom on highly excited Rydberg states. Based on the size of proton cloud, the stability of the atom and technical level, we guess that the direct perceiving of the Hydrogen nucleon cloud, or proton cloud, is possible in near future.

physics.gen-ph↗
arXiv2014

Strain-Engineering Anisotropic Electrical Conductance of Phosphorene and Few-Layer Black Phosphorus

Newly fabricated monolayer phosphorene and its few-layer structures are expected to be promising for electronic and optical applications because of their finite direct band gaps and sizable but anisotropic electronic mobility. By first-principles simulations, we show that this unique anisotropic conductance can be controlled by using simple strain conditions. With the appropriate biaxial or uniaxial strain, we can rotate the preferred conducting direction by 90 degrees. This will be of useful for exploring quantum Hall effects, and exotic electronic and mechanical applications based on phosphorene.

cond-mat.mes-hall↗
arXiv2014

New Mechanism for Strongly Bound Excitons in Gapless Two-Dimensional Structures

Common wisdom asserts that bound excitons cannot form in high-dimensional (d>1) metallic structures because of their overwhelming screening and unavoidable resonance with nearby continuous bands. Strikingly, here we illustrate that this prevalent assumption is not quite true. A key ingredient that has been overlooked is that of viable decoherence that thwarts the formation of resonances. As an example of this general mechanism, we focus on an experimentally relevant material and predict bound excitons in twisted bilayer graphene, which is a two-dimensional gapless structure exhibiting metallic screening. The binding energies calculated by first-principles simulations are surprisingly large. The low-energy effective model reveals that these bound states are produced by a unique destructive coherence between two alike subband resonant excitons. In particular, this destructive coherent effect is not sensitive to the screening and dimensionality, and hence may persist as a general mechanism for creating bound excitons in various metallic structures, opening the door for excitonic applications based on metallic structures.

cond-mat.mes-hall↗
arXiv2014

Temperature Renormalization of Optical Spectra of Monolayer MoS2

Newly measured optical absorption and photoluminescence spectra reveal substantial frequency shifts of both exciton and trion peaks as monolayer MoS2 is cooled from 363 K to 4 K. First-principles simulations using the GW-Bethe-Salpeter Equation approach satisfactorily reproduce these frequency shifts by incorporating many-electron interactions and the thermal expansion of the in-plane lattice constant. Studying these temperature effects in monolayer MoS2 is crucial for rectifying the results of room-temperature experiments with the previous predictions of zero-temperature-limit simulations. Moreover, we estimate that the thermal expansion coefficient of monolayer MoS2 is around 25% less than that of bulk counterpart by tracking the frequency shifts of the exciton or trion peaks in optical spectra. This may serve as a convenient way to estimate thermal expansion coefficients of general two-dimensional chalcogenides.

cond-mat.mes-hall↗
arXiv2013

Quasiparticle band-edge energy and band offsets of monolayer of molybdenum and tungsten chalcogenide

We report the quasiparticle energy of monolayer of molybdenum and tungsten dichalcogenides, MX2 (M=Mo, W; X=S, Se, Te). Beyond calculating bandgaps, we have achieved converged absolute band energies relative to the vacuum level. Compared with the results from other approaches, the GW calculation reveals substantially larger bandgaps and different absolute band energies because of enhanced many-electron effects. Interestingly, our fully-converged quasiparticle energies ratify the band-gap-center approximation, making it a convenient way to estimate quasiparticle energy. The absolute quasiparticle energies and band offsets obtained in this work are important for designing heterojunction devices and chemical catalysts based on monolayer dichalcogenides.

cond-mat.mes-hall↗
arXiv2013

Distributed quantum election scheme

In an electronic voting protocol, a distributed scheme can be used for forbidding the malicious acts of the voting administrator and the counter during the election, but it cannot prevent them from collaborating to trace the ballots and destroy their privacy after the election. We present a distributed anonymous quantum key distribution scheme and further construct a distributed quantum election scheme with a voting administrator made up of more than one part. This quantum election scheme can resist the malicious acts of the voting administrator and the counter after the election and can work in a system with lossy and noisy quantum channels.

quant-ph↗