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Wen Yang

Publications and source records attributed to Wen Yang.

At least 217 records · Page 12Linked to original sources
arXiv2017

Enhanced circular photogalvanic effect in HgTe quantum wells in the heavily inverted regime

Thanks to the strong spin-orbit interaction (SOI), HgTe-based quantum wells (QWs) exhibit very rich spin-related properties. But the full descriptions of them are beyond the simple parabolic band models and conventional Rashba and Dresselhaus SOI terms, as a result of the strong interband coupling of the narrow gap band structures. Here, we develop a theoretical method to calculate the circular photogalvanic effect (CPGE) in Hg$_{0.3}$Cd$_{0.7}$Te/HgTe/Hg$_{0.3}$Cd$_{0.7}$Te quantum wells (HgTe QWs) based on the realistic eight-band $\mathbf{k}\cdot\mathbf{p}$ model with density matrix formalism. Our method could take account of the unusual band structures and SOIs of HgTe QWs, therefore can be used to calculate the CPGE currents in HgTe QWs with non-parabolic, Dirac-like and inverted energy dispersions. The microscopic origin of CPGE and the interplay effect of structure inversion asymmetry (SIA) and bulk inversion asymmetry (BIA) is also investigated. In addition, this method is extended to study the pure spin currents (PSCs) in HgTe QWs injected by linearly polarized light at normal incidence. Our calculation results support the following findings: (i) In the inverted phase regime, the energy dispersion of heavily inverted HgTe QWs could be strongly distorted, lead to a significant enhancement of CPGE at a certain range of energy spectrum. (ii) The interplay of SIA and BIA could lead to the CPGE currents anisotropically dependent on the azimuth angle of oblique incident light. (iii) The PSC $j_{y'}^{x'}$ ($x'\parallel[110]$ and $y'\parallel[\bar{1}10]$) produced by [110]-linearly-polarized light could change sign with HgTe QW transformed from normal phase to inverted phase. These findings might be utilized in developing the HgTe-based infrared/terahertz optoelectronic and spintronic devices.

cond-mat.mes-hall↗
arXiv2017

Quantum many-body theory for electron spin decoherence in nanoscale nuclear spin baths

Decoherence of electron spins in nanoscale systems is important to quantum technologies such as quantum information processing and magnetometry. It is also an ideal model problem for studying the crossover between quantum and classical phenomena. At low temperatures or in light-element materials where the spin-orbit coupling is weak, the phonon scattering in nanostructures is less important and the fluctuations of nuclear spins become the dominant decoherence mechanism for electron spins. Since 1950s, semiclassical noise theories have been developed for understanding electron spin decoherence. In spin-based solid-state quantum technologies, the relevant systems are in the nanometer scale and the nuclear spin baths are quantum objects which require a quantum description. Recently, quantum pictures have been established to understand the decoherence and quantum many-body theories have been developed to quantitatively describe this phenomenon. Anomalous quantum effects have been predicted and some have been experimentally confirmed. A systematically truncated cluster correlation expansion theory has been developed to account for the many-body correlations in nanoscale nuclear spin baths that are built up during the electron spin decoherence. The theory has successfully predicted and explained a number of experimental results in a wide range of physical systems. In this review, we will cover these recent progresses. The limitations of the present quantum many-body theories and possible directions for future development will also be discussed.

cond-mat.mes-hall↗
arXiv2017

Sharp estimates for solutions of mean field equation with collapsing singularity

The pioneering work of Brezis-Merle [7], Li-Shafrir [27], Li [26] and Bartolucci-Tarantello [4] showed that any sequence of blow up solutions for (singular) mean field equations of Liouville type must exhibit a "mass concentration" property. A typical situation of blow-up occurs when we let the singular (vortex) points involved in the equation (see (1.1) below) collapse together. However in this case Lin-Tarantello in [30] pointed out that the phenomenon: "bubbling implies mass concentration" might not occur and new scenarios open for investigation. In this paper, we present two explicit examples which illustrate (with mathematical rigor) how a "non-concentration" situation does happen and its new features. Among other facts, we show that in certain situations, the collapsing rate of the singularities can be used as blow up parameter to describe the bubbling properties of the solution-sequence. In this way we are able to establish accurate estimates around the blow-up points which we hope to use towards a degree counting formula for the shadow system (1.34) below.

math.AP↗
arXiv2017

General Green's function formalism for layered systems: Wave function approach

The single-particle Green's function (GF) of mesoscopic structures plays a central role in mesoscopic quantum transport. The recursive GF technique is a standard tool to compute this quantity numerically, but it lacks physical transparency and is limited to relatively small systems. Here we present a numerically efficient and physically transparent GF formalism for a general layered structure. In contrast to the recursive GF that directly calculates the GF through the Dyson equations, our approach converts the calculation of the GF to the generation and subsequent propagation of a scattering wave function emanating from a local excitation. This viewpoint not only allows us to reproduce existing results in a concise and physically intuitive manner, but also provides analytical expressions of the GF in terms of a generalized scattering matrix. This identifies the contributions from each individual scattering channel to the GF and hence allows this information to be extracted quantitatively from dual-probe STM experiments. The simplicity and physical transparency of the formalism further allows us to treat the multiple reflection analytically and derive an analytical rule to construct the GF of a general layered system. This could significantly reduce the computational time and enable quantum transport calculations for large samples. We apply this formalism to perform both analytical analysis and numerical simulation for the two-dimensional conductance map of a realistic graphene p-n junction. The results demonstrate the possibility of observing the spatially-resolved interference pattern caused by negative refraction and further reveal a few interesting features, such as the distance-independent conductance and its quadratic dependence on the carrier concentration, as opposed to the linear dependence in uniform graphene.

cond-mat.mes-hall↗
arXiv2016

On Rank Two Toda System with Arbitrary Singularities: Local Mass and New Estimates

For all rank two Toda systems with an arbitrary singular source, we use a unified approach to prove: (i) The pair of local masses $(σ_1,σ_2)$ at each blowup point has the expression $$σ_i=2(N_{i1}μ_1+N_{i2}μ_2+N_{i3}),$$ where $N_{ij}\in\mathbb{Z},~i=1,2,~j=1,2,3.$ (ii) Suppose at each vortex point $p_t$, $(α_1^t,α_2^t)$ are integers and $ρ_i\notin 4π\mathbb{N}$, then all the solutions of Toda systems are uniformly bounded. (iii) If the blow up point $q$ is not a vortex point, then $$u^k(x)+2\log|x-x^k|\leq C,$$ where $x^k$ is the local maximum point of $u^k$ near $q$. (iv) If the blow up point $q$ is a vortex point $p_t$ and $α_t^1,α_t^2$ and $1$ are linearly independent over $Q$, then $$u^k(x)+2\log|x-p_t|\leq C.$$ The Harnack type inequalities of (iii) or (iv) is important for studying the bubbling behaves near each blow up point.

math.AP↗
arXiv2016

Quantum Zeno effect in a nitrogen-vacancy center embedded in a spin bath

We study the longitudinal relaxation of a nitrogen-vacancy (NV) center surrounded by a $^{13}$C nuclear spin bath in diamond. By means of cluster-correlation expansion (CCE), we numerically demonstrate the decay process of electronic state induced by cross relaxation at low temperature. It is shown that the CCE method is not only capable of describing pure-dephasing effect at large-detuning regime, but it can also simulate the quantum dynamics of populations in the nearly resonant regime. We present a proposal to slow down the decay of NV center via implementing quantum Zeno effect (QZE). The numerical result shows that QZE can effectively inhibit the decay of NV center.

quant-ph↗
arXiv2016

A mean field equation involving positively supported probability measures: blow-up phenomena and variational aspects

We are concerned with an elliptic problem which describes a mean field equation of the equilibrium turbulence of vortices with variable intensities. In the first part of the paper we describe the blow-up phenomenon and highlight the differences from the standard mean field equation. In the second part we discuss the Moser-Trudinger inequality in terms of the blow-up masses and get the existence of solutions in a non-coercive regime by means of a variational argument, which is based on some improved Moser-Trudinger inequalities.

math.AP↗
arXiv2016

Analytic aspects of the Tzitzéica equation: blow-up analysis and existence results

We are concerned with the following class of equations with exponential nonlinearities: $$ Δu+h_1e^u-h_2e^{-2u}=0 \qquad \mbox{in } B_1\subset\mathbb{R}^2, $$ which is related to the Tzitzéica equation. Here $h_1, h_2$ are two smooth positive functions. The purpose of the paper is to initiate the analytical study of the above equation and to give a quite complete picture both for what concerns the blow-up phenomena and the existence issue. In the first part of the paper we provide a quantization of local blow-up masses associated to a blowing-up sequence of solutions. Next we exclude the presence of blow-up points on the boundary under the Dirichlet boundary conditions. In the second part of the paper we consider the Tzitzéica equation on compact surfaces: we start by proving a sharp Moser-Trudinger inequality related to this problem. Finally, we give a general existence result.

math.AP↗
arXiv2016

Hidden quantum mirage by negative refraction in semiconductor P-N junctions

We predict a novel quantum interference based on the negative refraction across a semiconductor P-N junction: with a local pump on one side of the junction, the response of a local probe on the other side behaves as if the disturbance emanates not from the pump but instead from its mirror image about the junction. This phenomenon is guaranteed by translational invariance of the system and matching of Fermi surfaces of the constituent materials, thus it is robust against other details of the junction (e.g., junction width, potential profile, and even disorder). The recently fabricated P-N junctions in 2D semiconductors provide ideal platforms to explore this phenomenon and its applications to dramatically enhance charge and spin transport as well as carrier-mediated long-range correlation.

cond-mat.mtrl-sci↗
arXiv2016

On the Topological degree of the Mean field equation with two parameters

We consider the following class of equations with exponential nonlinearities on a compact surface $M$: $$ - Δu = ρ_1 \left( \frac{h_1 \,e^{u}}{\int_M h_1 \,e^{u} } - \frac{1}{|M|} \right) - ρ_2 \left( \frac{h_2 \,e^{-u}}{\int_M h_2 \,e^{-u} } - \frac{1}{|M|} \right), $$ which is associated to the mean field equation of the equilibrium turbulence with arbitrarily signed vortices. Here $h_1, h_2$ are smooth positive functions and $ρ_1, ρ_2$ are two positive parameters. We start by proving a concentration phenomena for the above equation, which leads to a-priori bound for the solutions of this problem provided $ρ_i\notin 8π\mathbb{N}, \, i=1,2$. Then we study the blow up behavior when $ρ_1$ crosses $8π$ and $ρ_2 \notin 8π\mathbb{N}$. By performing a suitable decomposition of the above equation and using the shadow system that was introduced for the $SU(3)$ Toda system, we can compute the Leray-Schauder topological degree for $ρ_1 \in (0,8π) \cup (8π,16π)$ and $ρ_2 \notin 8π\mathbb{N}$. As a byproduct our argument, we give new existence results when the underlying manifold is a sphere and a new proof for some known existence result.

math.AP↗
arXiv2016

Classification of blow-up limits for the sinh-Gordon equation

The aim of this paper is to use a selection process and a careful study of the interaction of bubbling solutions to show a classification result for the blow-up values of the elliptic sinh-Gordon equation $$Δu+h_1e^u-h_2e^{-u}=0 \quad \mathrm{in}~B_1\subset\mathbb{R}^2.$$ In particular we get that the blow-up values are multiple of $8π.$ It generalizes the result of Jost, Wang, Ye and Zhou \cite{jwyz} where the extra assumption $h_1 = h_2$ is crucially used.

math.AP↗
arXiv2016

High fidelity transfer and storage of photon states in a single nuclear spin

Building a quantum repeater network for long distance quantum communication requires photons and quantum registers that comprise qubits for interaction with light, good memory capabilities and processing qubits for storage and manipulation of photons. Here we demonstrate a key step, the coherent transfer of a photon in a single solid-state nuclear spin qubit with an average fidelity of 98% and storage over 10 seconds. The storage process is achieved by coherently transferring a photon to an entangled electron-nuclear spin state of a nitrogen vacancy centre in diamond, confirmed by heralding through high fidelity single-shot readout of the electronic spin states. Stored photon states are robust against repetitive optical writing operations, required for repeater nodes. The photon-electron spin interface and the nuclear spin memory demonstrated here constitutes a major step towards practical quantum networks, and surprisingly also paves the way towards a novel entangled photon source for photonic quantum computing.

cond-mat.mes-hall↗
arXiv2015

Manipulation of a single magnetic atom using polarized single electron transport in a double quantum dot

We consider theoretically a magnetic impurity spin driven by polarized electrons tunneling through a double quantum dot system. Spin blockade effect and spin conservation in the system make the magnetic impurity sufficiently interact with each transferring electron. As a results, a single collected electron carries information about spin change of the magnetic impurity. The scheme may develop all electrical manipulation of magnetic atoms by means of single electrons, which is significant for the implementation of scalable logical gates in information processing systems.

cond-mat.mes-hall↗
arXiv2015

Strongly polarizing weakly coupled $^{13}$C nuclear spins with optically pumped nitrogen-vacancy center

Enhancing the polarization of nuclear spins surrounding the nitrogen-vacancy (NV) center in diamond has attracted widespread attention recently due to its various applications. Here we present an analytical theory and comprehensive understanding on how to optimize the dynamic nuclear polarization by an optically pumped NV center near the ground state level anticrossing. Our results not only provide a parameter-free explanation and a clearly physics picture for the recently observed polarization dependence on the magnetic field for strongly coupled $^{13}$C nuclei [H. J. Wang \textit{et al}., Nat. Commun. 4, 1 (2013)], but also demonstrate the possibility to strongly polarize weakly coupled $^{13}$C nuclei under weak optical pumping and suitably chosen magnetic field. This allows sensitive magnetic control of the $^{13}$C nuclear spin polarization for NMR applications and significant suppression of the $^{13}$C nuclear spin noise to prolong the NV spin coherence time.

quant-ph↗
arXiv2015

Theory of nuclear spin dephasing and relaxation by optically illuminated nitrogen-vancy center

Dephasing and relaxation of the nuclear spins coupled to the nitrogen-vacancy (NV) center during optical initialization and readout is an important issue for various applications of this hybrid quantum register. Here we present both an analytical description and a numerical simulation for this process, which agree reasonably with the experimental measurements. For the NV center under cyclic optical transition, our analytical formula not only provide a clear physics picture, but also allows controlling the nuclear spin dissipation by tuning an external magnetic field. For more general optical pumping, our analytical formula reveals significant contribution to the nuclear spin dissipation due to electron random hopping into/out of the $m=0$ (or $m=\pm1$) subspace. This contribution is not suppressed even under saturated optical pumping and/or vanishing magnetic field, thus providing a possible solution to the puzzling observation of nuclear spin dephasing in zero perpendicular magnetic field [M. V. G. Dutt \textit{et al}., Science \textbf{316}, 1312 (2007)]. It also implies that enhancing the degree of spin polarization of the nitrogen-vacancy center can reduce the effect of optical induced nuclear spin dissipation.

quant-ph↗
arXiv2015

Heralded entanglement of two distant quantum dot spins via optical interference

We present a proposal for heralded entanglement between two quantum dots via Hong--Ou--Mandel effect. Each of the quantum dots, drived off-resonance by two lasers, can be entangled with the coherent cavity mode. The output photons from the two coherent cavity modes interfering by a beamsplitter, we could entangle the two QDs with nearly unit success probability. Our scheme requires neither direct coupling between qubits nor the detection of single photons. Moreover the quantum dots do not need to have the same frequencies and coupling constants.

quant-ph↗
arXiv2015

All-optical controlled phase gate in quantum dot molecules

We propose a two-qubit optically controlled phase gate in quantum dot molecules via adiabatic passage and hole tunneling. Our proposal combines the merits of the current generation of vertically stacked self-assembled InAs quantum dots and adiabatic passage. The simulation shows an implementation of the gate with a fidelity exceeding 0.98.

quant-ph↗