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Song Sun

Publications and source records attributed to Song Sun.

At least 37 records · Page 2Linked to original sources
arXiv2022

Unprotected quadratic band crossing points and quantum anomalous Hall effect in FeB2 monolayer

Quadratic band crossing points (QBCPs) and quantum anomalous Hall effect (QAHE) have attracted the attention of both theoretical and experimental researchers in recent years. Based on first-principle calculations, we find that the FeB$_2$ monolayer is a nonmagnetic semimetal with QBCPs at $K$. Through symmetry analysis and $\mathbf{k}\cdot\mathbf{p}$ invariant theory, we find that the QBCP is not protected by rotation symmetry and consists of two Dirac points with same chirality (Berry phase of $2π$). Once introducing Coulomb interactions, we find that there is a spontaneous-time-reversal-breaking instability of the spinful QBCPs, which gives rise to a $C=2$ QAH insulator with orbital moment ordering.

cond-mat.mes-hall↗
arXiv2022

Quantized and half-quantized Anomalous Hall effect induced by in-plane magnetic field

In this paper we propose that, quantized and nearly half-quantized intrinsic anomalous Hall effect can be induced by in-plane external magnetic field through the Zeeman coupling in non-magnetic 2D systems with sizeable spin-orbital coupling but without two-fold rotational symmetry. An analytical result is derived for 2D electron gas model with $C_{3v}$ symmetry. Based on the $\boldsymbol{k\cdot p}$ Hamiltonian derived from first principle calculations, we find that quantized and nearly half-quantized conductance can be observed in $\mathrm{Sb_2Te_3}$ thin film in the clean limit with strong in-plane magnetic field $B>20\ \mathrm{T}$ and low temperature $T<100\ \mathrm{mK}$.

cond-mat.mes-hall↗
arXiv2022

Strain-tuned topological phase transition and unconventional Zeeman effect in ZrTe5 microcrystals

The geometric phase (Berry phase) of an electronic wave function is the fundamental basis of the topological properties in solids. Modulating band structure provides a tuning knob for the Berry phase, and in the extreme case drives a topological phase transition. Despite the significant developments in topological materials study, it remains a challenge to tune between different topological phases while tracing the impact of the Berry phase on quantum charge transport, in the same material. Here we report both in a magnetotransport study of ZrTe5. By tuning the band structure with uniaxial strain, we directly map a weak- to strong- topological phase transition through a gapless Dirac semimetal phase via quantum oscillations. Moreover, we demonstrate the impact of the strain-tunable spin-dependent Berry phase on the Zeeman effect through the amplitude of the quantum oscillations. We show that such a spin-dependent Berry phase, largely neglected in solid-state systems, is critical in modeling quantum oscillations in Dirac bands in topological materials.

cond-mat.mtrl-sci↗
arXiv2021

Authentication of optical physical unclonable functions based on single-pixel detection

Physical unclonable function (PUF) has been proposed as a promising and trustworthy solution to a variety of cryptographic applications. Here we propose a non-imaging based authentication scheme for optical PUFs materialized by random scattering media, in which the characteristic fingerprints of optical PUFs are extracted from stochastical fluctuations of the scattered light intensity with respect to laser challenges which are detected by a single-pixel detector. The randomness, uniqueness, unpredictability, and robustness of the extracted fingerprints are validated to be qualified for real authentication applications. By increasing the key length and improving the signal to noise ratio, the false accept rate of a fake PUF can be dramatically lowered to the order of 10^-28. In comparison to the conventional laser-speckle-imaging based authentication with unique identity information obtained from textures of laser speckle patterns, this non-imaging scheme can be implemented at small speckle size bellowing the Nyquist--Shannon sampling criterion of the commonly used CCD or CMOS cameras, offering benefits in system miniaturization and immunity against reverse engineering attacks simultaneously.

cs.CR↗
arXiv2021

Bionic Optical Physical Unclonable Functions for Authentication and Encryption

Information security is of great importance for modern society with all things connected. Physical unclonable function (PUF) as a promising hardware primitive has been intensively studied for information security. However, the widely investigated silicon PUF with low entropy is vulnerable to various attacks. Herein, we introduce a concept of bionic optical PUFs inspired from unique biological architectures, and fabricate four types of bionic PUFs by molding the surface micro-nano structures of natural plant tissues with a simple, low-cost, green and environmentally friendly manufacturing process. The laser speckle responses of all bionic PUFs are statistically demonstrated to be random, unique, unpredictable and robust enough for cryptographic applications, indicating the broad applicability of bionic PUFs. On this ground, the feasibility of implementing bionic PUFs as cryptographic primitives in entity authentication and encrypted communication is experimentally validated, which shows its promising potential in the application of future information security.

cs.CR↗
arXiv2021

Excess-noise suppression for a squeezed state propagating through random amplifying media via wave-front shaping

After propagating through a random amplifying medium, a squeezed state commonly shows excess noise above the shot-noise level. Since large noise can significantly reduce the signal-to-noise ratio, it is detrimental for precision measurement. To circumvent this problem, we propose a noise-reduction scheme using wavefront shaping. It is demonstrated that the average output quantum noise can be effectively suppressed even beyond the shot-noise limit. Both the decrease on amplification strength and the increase on input squeezing strength can give rise to a decrease in the suppressed average quantum noise. Our results not only show the feasibility of manipulating the output quantum noise of random amplifying media, but also indicate potential applications in quantum information processing in complex environments, such as, quantum imaging, quantum communication, and quantum key distribution.

quant-ph↗
arXiv2021

A Liouville theorem on asymptotically Calabi spaces

In this paper, we will study harmonic functions on the complete and incomplete spaces with nonnegative Ricci curvature which exhibit inhomogeneous collapsing behaviors at infinity. The main result states that any nonconstant harmonic function on such spaces yields a definite exponential growth rate which depends explicitly on the geometric data at infinity.

math.DG↗
arXiv2020

Singularities of Hermitian-Yang-Mills connections and Harder-Narasimhan-Seshadri filtrations

This is the first of a series of papers where we relate tangent cones of Hermitian-Yang-Mills connections at an isolated singularity to the complex algebraic geometry of the underlying reflexive sheaf, when the sheaf is locally modelled on the pull-back of a holomorphic vector bundle from the projective space. In this paper we shall impose an extra assumption that the graded sheaf determined by the Harder-Narasimhan-Seshadri filtrations of the vector bundle is reflexive. In general we conjecture that the tangent cone is uniquely determined by the double dual of the associated graded object of a Harder-Narasimhan-Seshadri filtration of an algebraic tangent cone, which is a certain torsion-free sheaf on the projective space. In this paper we also prove this conjecture when there is an algebraic tangent cone which is locally free and stable.

math.DG↗
arXiv2020

Analytic tangent cones of admissible Hermitian-Yang-Mills connections

In this paper we study the analytic tangent cones of admissible Hermitian-Yang-Mills connections near a homogeneous singularity of a reflexive sheaf, and relate it to the Harder-Narasimhan-Seshadri filtration. We also give an algebro-geometric characterization of the bubbling set. This strengthens our previous result.

math.DG↗
arXiv2020

The critical role of shell in enhanced fluorescence of metal-dielectric core-shell nanoparticles

Large scale simulations are performed by means of the transfer-matrix method to reveal optimal conditions for metal-dielectric core-shell particles to induce the largest fluorescence on their surfaces. With commonly used plasmonic cores (Au and Ag) and dielectric shells (SiO2, Al2O3, ZnO), optimal core and shell radii are determined to reach maximum fluorescence enhancement for each wavelength within 550~850 nm (Au core) and 390~500 nm (Ag core) bands, in both air and aqueous hosts. The peak value of the maximum achievable fluorescence enhancement factors of core-shell nanoparticles, taken over entire wavelength interval, increases with the shell refractive index and can reach values up to 9 and 70 for Au and Ag cores, within 600~700 nm and 400~450 nm wavelength ranges, respectively, which is much larger than that for corresponding homogeneous metal nanoparticles. Replacing air by an aqueous host has a dramatic effect of nearly halving the sizes of optimal core-shell configurations at the peak value of the maximum achievable fluorescence. In the case of Au cores,the fluorescence enhancements for wavelengths within the first near-infrared biological window (NIR-I) between 700 and 900 nm can be improved twofold compared to homogeneous Au particle when the shell refractive index ns > 2. As a rule of thumb, the wavelength region of optimal fluorescence (maximal nonradiative decay) turns out to be red-shifted (blue-shifted) by as much as 50 nm relative to the localized surface plasmon resonance wavelength of corresponding optimized core-shell particle. Our results provide important design rules and general guidelines for enabling versatile platforms for imaging, light source, and biological applications.

physics.optics↗
arXiv2019

Topological metals induced by Zeeman effect

In the present paper, we propose a new way to classify centrosymmetric metals by studying the Zeeman effect caused by an external magnetic field described by the momentum dependent g-factor tensor on the Fermi surfaces. Nontrivial U(1) Berry's phase and curvature can be generated once the otherwise degenerate Fermi surfaces are splitted by the Zeeman effect, which will be determined by both the intrinsic band structure and the structure of g-factor tensor on the manifold of the Fermi surfaces. Such Zeeman effect generated Berry's phase and curvature can lead to three important experimental effects, modification of spin-zero effect, Zeeman effect induced Fermi surface Chern number and the in-plane anomalous Hall effect. By first principle calculations, we study all these effects on two typical material, ZrTe$_5$ and TaAs$_2$ and the results are in good agreement with the existing experiments.

cond-mat.mes-hall↗
arXiv2019

First principle calculation of the effective Zeeman's couplings in topological materials

In this paper, we propose a first principle calculation method for the effective Zeeman's coupling based on the second perturbation theory and apply it to a few topological materials. For Bi and Bi$_2$Se$_3$, our numerical results are in good accord with the experimental data; for Na$_3$Bi, TaN, and ZrTe$_5$, the structure of the multi-bands Zeeman's couplings are discussed. Especially, we discuss the impact of Zeeman's coupling on the Fermi surface's topology in Na$_3$Bi in detail.

cond-mat.mtrl-sci↗
arXiv2019

Classification results for expanding and shrinking gradient Kähler-Ricci solitons

We first show that a Kähler cone appears as the tangent cone of a complete expanding gradient Kähler-Ricci soliton with quadratic curvature decay with derivatives if and only if it has a smooth canonical model (on which the soliton lives). This allows us to classify two-dimensional complete expanding gradient Kähler-Ricci solitons with quadratic curvature decay with derivatives. We then show that any two-dimensional complete shrinking gradient Kähler-Ricci soliton whose scalar curvature tends to zero at infinity is, up to pullback by an element of $GL(2,\,\mathbb{C})$, either the flat Gaussian shrinking soliton on $\mathbb{C}^{2}$ or the $U(2)$-invariant shrinking gradient Kähler-Ricci soliton of Feldman-Ilmanen-Knopf on the blowup of $\mathbb{C}^{2}$ at one point. Finally, we show that up to pullback by an element of $GL(n,\,\mathbb{C})$, the only complete shrinking gradient Kähler-Ricci soliton with bounded Ricci curvature on $\mathbb{C}^{n}$ is the flat Gaussian shrinking soliton and on the total space of $\mathcal{O}(-k)\to\mathbb{P}^{n-1}$ for $0<k<n$ is the $U(n)$-invariant example of Feldman-Ilmanen-Knopf. In the course of the proof, we establish the uniqueness of the soliton vector field of a complete shrinking gradient Kähler-Ricci soliton with bounded Ricci curvature in the Lie algebra of a torus. A key tool used to achieve this result is the Duistermaat-Heckman theorem from symplectic geometry. This provides the first step towards understanding the relationship between complete shrinking gradient Kähler-Ricci solitons and algebraic geometry.

math.DG↗
arXiv2018

Algebraic tangent cones of reflexive sheaves

We study the notion of algebraic tangent cones at singularities of reflexive sheaves. These correspond to extensions of reflexive sheaves across a negative divisor. We show the existence of optimal extensions in a constructive manner, and we prove the uniqueness in a suitable sense. The results here are an algebro-geometric counterpart of our previous study on singularities of Hermitian-Yang-Mills connections.

math.DG↗
arXiv2018

Nilpotent structures and collapsing Ricci-flat metrics on K3 surfaces

We exhibit families of Ricci-flat Kahler metrics on K3 surfaces which collapse to an interval, with Tian-Yau and Taub-NUT metrics occurring as bubbles. There is a corresponding continuous surjective map from the K3 surface to the interval, with regular fibers diffeomorphic to either 3-tori or Heisenberg nilmanifolds.

math.DG↗
arXiv2017

Projective embedding of log Riemann surfaces and K-stability

Given a smooth polarized Riemann surface (X, L) endowed with a hyperbolic metric $ω$ with cusp singularities along a divisor D, we show the L^2 projective embedding of (X, D) defined by L^k is asymptotically almost balanced in a weighted sense. The proof depends on sufficiently precise understanding of the behavior of the Bergman kernel in three regions, with the most crucial one being the neck region around D. This is the first step towards understanding the algebro-geometric stability of extremal Kähler metrics with singularities.

math.DG↗
arXiv2017

Explicit Gromov-Hausdorff compactifications of moduli spaces of Kähler-Einstein Fano manifolds

We exhibit the first non-trivial concrete examples of Gromov-Hausdorff compactifications of moduli spaces of Kähler-Einstein Fano manifolds in all complex dimensions bigger than two (Fano K-moduli spaces). We also discuss potential applications to explicit study of moduli spaces of K-stable Fano manifolds with large anti-canonical volume. Our arguments are based on recent progress about the geometry of metric tangent cones and on related ideas about the algebro-geometric study of singularities of K-stable Fano varieties.

math.DG↗