arXiv subjects

Ziyan Zhu

Publications and source records attributed to Ziyan Zhu.

44 records · Page 3Linked to original sources

arXiv2020

Topological Gaseous Plasmon Polariton in Realistic Plasma

Nontrivial topology in bulk matter has been linked with the existence of topologically protected interfacial states. We show that a gaseous plasmon polariton (GPP), an electromagnetic surface wave existing at the boundary of magnetized plasma and vacuum, has a topological origin that arises from the nontrivial topology of magnetized plasma. Because a gaseous plasma cannot sustain a sharp interface with discontinuous density, one must consider a gradual density falloff with scale length comparable or longer than the wavelength of the wave. We show that the GPP may be found within a gapped spectrum in present-day laboratory devices, suggesting that platforms are currently available for experimental investigation of topological wave physics in plasmas.

physics.plasm-ph↗

arXiv2020

Moiré of Moiré: Modeling Mechanical Relaxation in Incommensurate Trilayer van der Waals Heterostructures

The incommensurate stacking of multi-layered two-dimensional materials is a challenging problem from a theoretical perspective and an intriguing avenue for manipulating their physical properties. Here we present a multi-scale model to obtain the mechanical relaxation pattern of twisted trilayer van der Waals (vdW) heterostructures with two independent twist angles, a generally incommensurate system without a supercell description. We adopt the configuration space as a natural description of such incommensurate layered materials, based on the local environment of atomic positions, bypassing the need for commensurate approximations. To obtain the relaxation pattern, we perform energy minimization with respect to the relaxation displacement vectors. We use a continuum model in combination with the Generalized Stacking Fault energy to describe the interlayer coupling, obtained from first-principles calculations based on Density Functional Theory. We show that the relaxation patterns of twisted trilayer graphene and $\mathrm{WSe_2}$ are "moiré of moiré", as a result of the incommensurate coupling two bilayer moiré patterns. We also show that, in contrast to the symmetry-preserving in-plane relaxation in twisted bilayers, trilayer relaxation can break the two-fold rotational symmetry about the xy-plane when the two twist angles are equal.

cond-mat.mes-hall↗

arXiv2020

Electronic structure calculations of twisted multi-layer graphene superlattices

Quantum confinement endows two-dimensional (2D) layered materials with exceptional physics and novel properties compared to their bulk counterparts. Although certain two- and few-layer configurations of graphene have been realized and studied, a systematic investigation of the properties of arbitrarily layered graphene assemblies is still lacking. We introduce theoretical concepts and methods for the processing of materials information, and as a case study, apply them to investigate the electronic structure of multi-layer graphene-based assemblies in a high-throughput fashion. We provide a critical discussion of patterns and trends in tight binding band structures and we identify specific layered assemblies using low-dispersion electronic bands as indicators of potentially interesting physics like strongly correlated behavior. A combination of data-driven models for visualization and prediction is used to intelligently explore the materials space. This work more generally aims to increase confidence in the combined use of physics-based and data-driven modeling for the systematic refinement of knowledge about 2D layered materials, with implications for the development of novel quantum devices.

cond-mat.mes-hall↗

arXiv2019

Angle-Dependent {\it Ab initio} Low-Energy Hamiltonians for a Relaxed Twisted Bilayer Graphene Heterostructure

We present efficient angle-dependent low-energy Hamiltonians to describe the properties of the twisted bilayer graphene (tBLG) heterostructure, based on {\it ab initio} calculations of mechanical relxation and electronic structure. The angle-dependent relaxed atomic geometry is determined by continuum elasticity theory, which induces both in-plane and out-of-plane deformations in the stacked graphene layers. The electronic properties corresponding to the deformed geometry are derived from a Wannier transformation to local interactions obtained from Density Functional Theory calculations. With these {\it ab initio} tight-binding Hamiltonians of the relaxed heterostructure, the low-energy effective theories are derived from the projections near Dirac cones at K valleys. For twist angles ranging from 0.7$^\circ$ to 4$^\circ$, we extract both the intra-layer pseudo-gauge fields and the inter-layer coupling terms in the low-energy Hamiltonians, which extend the conventional low-energy continuum models. We further include the momentum dependent inter-layer scattering terms which give rise to the particle-hole asymmetric features of the electronic structure. Our model Hamiltonians can serve as a starting point for formulating physically meaningful, accurate interacting electron theories.

cond-mat.mes-hall↗

arXiv2019

Coexistence of ultraheavy and ultrarelativistic Dirac quasiparticles in sandwiched trilayer graphene

Electrons in quantum materials exhibiting coexistence of dispersionless (flat) bands piercing dispersive (steep) bands can give rise to strongly correlated phenomena, and are associated with unconventional superconductivity. It is known that in twisted trilayer graphene steep Dirac cones can coexist with band flattening, but the phenomenon is not stable under layer misalignments. Here we show that such a twisted sandwiched graphene (TSWG) -- a three-layer van der Waals heterostructure with a twisted middle layer -- can have very stable flat bands coexisting with Dirac cones near the Fermi energy when twisted to 1.5 degrees. These flat bands require a specific high-symmetry stacking order, and our atomistic calculations predict that TSWG always relaxes to it. Additionally, with external fields, we can control the relative energy offset between the Dirac cone vertex and the flat bands. Our work establishes twisted sandwiched graphene as a new platform for research into strongly interacting phases, and topological transport beyond Dirac and Weyl semimetals.

cond-mat.str-el↗

arXiv2019

An exact continuum model for low-energy electronic states of twisted bilayer graphene

We introduce a complete physical model for the single-particle electronic structure of twisted bilayer graphene (tBLG), which incorporates the crucial role of lattice relaxation. Our model, based on $k \cdot p$ perturbation theory, combines the accuracy of DFT calculations through effective tight-binding Hamiltonians with the computational efficiency and complete control of the twist angle offered by continuum models. The inclusion of relaxation significantly changes the bandstructure at the first magic-angle twist corresponding to flat bands near the Fermi level (the "low-energy" states), and eliminates the appearance of a second magic-angle twist. We show that minimal models for the low-energy states of tBLG can be easily modified to capture the changes in electronic states as a function of twist angle.

cond-mat.mes-hall↗

arXiv2018

Testing General Relativity with the Black Hole Shadow Size and Asymmetry of Sagittarius A*: Limitations from Interstellar Scattering

The Event Horizon Telescope (EHT), a global 230 GHz VLBI array, achieves an angular resolution of ${\approx} 20\,μ{\rm as}$, sufficient to resolve the supermassive black hole Sagittarius A* (Sgr A*). This resolution may soon enable measurement of the black hole "shadow" size and asymmetry, predicted to be ${\approx}50\,μ$as and ${\lesssim} 3\,μ$as, respectively. Measurements that depart from these values could indicate a violation of the "no-hair theorem." However, refractive scattering by the turbulent ionized interstellar medium distorts the image of Sgr A*, affecting its apparent size and asymmetry. In this paper, we present a general analytic approach to quantify the expected image wander, distortion, and asymmetry from refractive scattering. If the turbulence in the scattering material of Sgr A* is close to Kolmogorov, we estimate the mean refractive image wander, distortion, and asymmetry to be 0.53 $μ$as, 0.72 $μ$as, and 0.52 $μ$as at 230 GHz. However, alternative scattering models with flatter power spectra can yield larger values, up to 2.1 $μ$as, 6.3 $μ$as, and 5.0 $μ$as, respectively. We demonstrate that these effects can be reduced by averaging images over multiple observations. For a small number of observations, the effects of scattering can be comparable to or greater than those from black hole spin, and they determine a fundamental limit for testing general relativity via images of Sgr A*.

astro-ph.IM↗

arXiv2017

Chaotic edge density fluctuations in the Alcator C-Mod tokamak

Analysis of the time series obtained with the O-Mode reflectometer (Rhodes et al 1997 Plasma Phys. and Control. Fusion 40 (1998) 493-510) and the gas puff imaging (Cziegler, I. et al 2010 Phys. of Plasmas 17, No. 5 (2010) 056120) systems on the Alcator C-Mod tokamak reveals that the turbulent edge density fluctuations are chaotic. Supporting evidence for this conclusion includes: the observation of exponential power spectra (which is associated with Lorentzian-shaped pulses in the time series), the population of the corresponding Bandt-Pompe probability distributions (Bandt and Pompe 2002 Phys. Rev. Lett. 88 174102), and the location of the signal on the complexity-entropy plane (C-H plane) (Rosso et al 2007 Phys. Rev. Lett. 99, 154102 (2007)). The classification of edge turbulence as chaotic opens the door for further work to understand the underlying process and the impact on turbulent transport.

physics.plasm-ph↗