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Zheng Han

Publications and source records attributed to Zheng Han.

34 records · Page 2Linked to original sources
arXiv2017

High-Yield Proximity-Induced Chemical Vapor Deposition of Graphene Over Millimeter-Sized Hexagonal Boron Nitride

We present a transfer-free preparation method for graphene on hexagonal boron nitride (h-BN) crystals by chemical vapor deposition of graphene via a catalytic proximity effect, i.e. activated by a Cu catalyst close-by . We demonstrate the full coverage by monolayer graphene of half-millimeter-sized hexagonal boron nitride crystals exfoliated on a copper foil prior to growth. We demonstrate that the proximity of the copper catalyst ensures high yield with the growth rate estimated between of 2μm/min to 5μm/min . Optical and electron microscopies together with confocal micro-Raman mapping confirm that graphene covers the top surface of h-BN crystals that we attribute to be a lateral growth from the supporting catalytic copper substrate. Structural and electron transport characterization of the in-situ grown graphene present an electronic mobility of about 20, 000cm2/(V.s). Comparison with graphene/h-BN stacks obtained by manual transferring of similar CVD graphene onto h-BN, confirms the better neutrality reached by the self-assembled structures.

cond-mat.mtrl-sci↗
arXiv2016

Unconventional Correlation between Quantum Hall Transport Quantization and Bulk State Filling in Gated Graphene Devices

We report simultaneous transport and scanning microwave impedance microscopy to examine the correlation between transport quantization and filling of the bulk Landau levels in the quantum Hall regime in gated graphene devices. Surprisingly, a comparison of these measurements reveals that quantized transport typically occurs below the complete filling of bulk Landau levels, when the bulk is still conductive. This result points to a revised understanding of transport quantization when carriers are accumulated by gating. We discuss the implications on transport study of the quantum Hall effect in graphene and related topological states in other two-dimensional electron systems.

cond-mat.mes-hall↗
arXiv2016

Primal-Dual Active-Set Methods for Isotonic Regression and Trend Filtering

Isotonic regression (IR) is a non-parametric calibration method used in supervised learning. For performing large-scale IR, we propose a primal-dual active-set (PDAS) algorithm which, in contrast to the state-of-the-art Pool Adjacent Violators (PAV) algorithm, can be parallized and is easily warm-started thus well-suited in the online settings. We prove that, like the PAV algorithm, our PDAS algorithm for IR is convergent and has a work complexity of O(n), though our numerical experiments suggest that our PDAS algorithm is often faster than PAV. In addition, we propose PDAS variants (with safeguarding to ensure convergence) for solving related trend filtering (TF) problems, providing the results of experiments to illustrate their effectiveness.

math.OC↗
arXiv2016

Electron optics with ballistic graphene junctions

Electrons transmitted across a ballistic semiconductor junction undergo refraction, analogous to light rays across an optical boundary. A pn junction theoretically provides the equivalent of a negative index medium, enabling novel electron optics such as negative refraction and perfect (Veselago) lensing. In graphene, the linear dispersion and zero-gap bandstructure admit highly transparent pn junctions by simple electrostatic gating, which cannot be achieved in conventional semiconductors. Moreover ballistic transport over micron length scales at ambient temperature has been realized, providing an ideal platform to realize a new generation of device based on electron lensing. Robust demonstration of these effects, however, has not been forthcoming. Here we employ transverse magnetic focusing to probe propagation across an electrostatically defined graphene junction. We find perfect agreement with the predicted Snells law for electrons, including observation of both positive and negative refraction. Resonant transmission across the pn junction provides a direct measurement of the angle dependent transmission coefficient, and we demonstrate good agreement with theory. Comparing experimental data with simulation reveals the crucial role played by the effective junction width, providing guidance for future device design. Our results pave the way for realizing novel electron optics based on graphene pn junctions.

cond-mat.mes-hall↗
arXiv2015

Phonon-Induced Transparency in Functionalized Single Layer Graphene

Herein, intervalley scattering is exploited to account for anomalous antiresonances in the infrared spectra of doped and disordered single layer graphene. We present infrared spectroscopy measurements of graphene grafted with iodophenyl moieties in both reflection microscopy and transmission configurations. Asymmetric transparency windows at energies corresponding to phonon modes near the Γ and K points are observed, in contrast to the featureless spectrum of pristine graphene. These asymmetric antiresonances are demonstrated to vary as a function of the chemical potential. We propose a model which involves coherent intraband scattering with defects and phonons, thus relaxing the optical selection rule forbidding access to ${\bf q} \neq$ Γ phonons. This interpretation of the new phenomenon is supported by our numerical simulations that reproduce the experimental features.

cond-mat.mes-hall↗
arXiv2015

High-performance and power-efficient 2${\times}$2 optical switch on Silicon-on-Insulator

A compact (15μm${\times}$μm) and highly-optimized 2${\times}$2 optical switch is demonstrated on a CMOS-compatible photonic crystal technology. On-chip insertion loss are below 1dB, static and dynamic contrast are 40dB and >20dB respectively. Owing to efficient thermo-optic design, the power consumption is below 3 mW while the switching time is 1 μs.

physics.optics↗
arXiv2015

Fractional fractal quantum Hall effect in graphene superlattices

The Hofstadter energy spectrum provides a uniquely tunable system to study emergent topological order in the regime of strong interactions. Previous experiments, however, have been limited to the trivial case of low Bloch band filling where only the Landau level index plays a significant role. Here we report measurement of high mobility graphene superlattices where the complete unit cell of the Hofstadter spectrum is accessible. We observe coexistence of conventional fractional quantum Hall effect (QHE) states together with the integer QHE states associated with the fractal Hofstadter spectrum. At large magnetic field, a new series of states appear at fractional Bloch filling index. These fractional Bloch band QHE states are not anticipated by existing theoretical pictures and point towards a new type of many-body state.

cond-mat.mes-hall↗
arXiv2014

Quantum confinement of zero-dimensional hybrid organic-inorganic polaritons at room temperature

We report on the quantum confinement of zero-dimensional polaritons in perovskite-based microcavity at room temperature. Photoluminescence of discrete polaritonic states are observed for polariton localized in symmetric sphere-like defects which are spontaneously nucleated on the top dielectric Bragg mirror. The linewidth of these confined states are found much sharper (almost one order of magnitude) than that of photonic modes in the perovskite planar microcavity. Our results show the possibility to study organic-inorganic cavity polariton in confined microstructure and suggest a fabrication method to realize integrated polaritonic devices operating at room temperature.

cond-mat.mes-hall↗
arXiv2014

Strain superlattices and macroscale suspension of Graphene induced by corrugated substrates

We investigate the organized formation of strain, ripples and suspended features in macroscopic CVD-prepared graphene sheets transferred onto a corrugated substrate made of an ordered arrays of silica pillars of variable geometries. Depending on the aspect ratio and sharpness of the corrugated array, graphene can conformally coat the surface, partially collapse, or lay, fakir-like, fully suspended between pillars over tens of micrometers. Upon increase of pillar density, ripples in collapsed films display a transition from random oriented pleats emerging from pillars to ripples linking nearest neighboring pillars organized in domains of given orientation. Spatially-resolved Raman spectroscopy, atomic force microscopy and electronic microscopy reveal uniaxial strain domains in the transferred graphene, which are induced and controlled by the geometry. We propose a simple theoretical model to explain the transition between suspended and collapsed graphene. For the arrays with high aspect ratio pillars, graphene membranes stays suspended over macroscopic distances with minimal interaction with pillars tip apex. It offers a platform to tailor stress in graphene layers and open perspectives for electron transport and nanomechanical applications.

cond-mat.mes-hall↗
arXiv2014

Collapse of superconductivity in a hybrid tin-graphene Josephson junction array

When a Josephson junction array is built with hybrid superconductor/metal/superconductor junctions, a quantum phase transition from a superconducting to a two-dimensional (2D) metallic ground state is predicted to happen upon increasing the junction normal state resistance. Owing to its surface-exposed 2D electron gas and its gate-tunable charge carrier density, graphene coupled to superconductors is the ideal platform to study the above-mentioned transition between ground states. Here we show that decorating graphene with a sparse and regular array of superconducting nanodisks enables to continuously gate-tune the quantum superconductor-to-metal transition of the Josephson junction array into a zero-temperature metallic state. The suppression of proximity-induced superconductivity is a direct consequence of the emergence of quantum fluctuations of the superconducting phase of the disks. Under perpendicular magnetic field, the competition between quantum fluctuations and disorder is responsible for the resilience at the lowest temperatures of a superconducting glassy state that persists above the upper critical field. Our results provide the entire phase diagram of the disorder and magnetic field-tuned transition and unveil the fundamental impact of quantum phase fluctuations in 2D superconducting systems.

cond-mat.supr-con↗
arXiv2012

Suppression of Multilayer Graphene Patches during CVD Graphene growth on Copper

By limiting the carbon segregation at the copper surface defects, a pulsed chemical vapor deposition method for single layer graphene growth is shown to inhibit the formation of few-layer regions, leading to a fully single-layered graphene homogeneous at the centimeter scale. Graphene field-effect devices obtained after transfer of pulsed grown graphene on oxidized silicon exhibit mobilities above 5000 cm^2.V^-1.s^-1.

cond-mat.mes-hall↗
arXiv2012

Electrical Control of the Superconducting-to-Insulating Transition in Graphene/Metal Hybrids

Graphene is a sturdy and chemically inert material exhibiting an exposed two-dimensional electron gas of high mobility. These combined properties enable the design of graphene composites either based on covalent or non- covalent coupling of adsorbates, or on multilayered structures. These systems have exhibited novel tunable electronic properties such as bandgap- engineering, reversible metal/insulating transition or supramolecular spintronics. Tunable superconductivity is expected as well, but experimental realization is lacking. Here, we show experiments based on me tal/graphene hybrid composites, enabling the control of proximity coupling of an array of superconducting nanoparticles onto a macroscopic graphene sheet. This material allows at low temperature to tune the superconductivity down to a strongly insulating state. It results from the combination of a proximity-induced superconductivity generated by the metallic nanoparticles array with the two-dimensional and tunable metallicity of graphene. The resulting hybrid material behaves, as a whole, like a granular 2D superconductor showing universal transition threshold and localization of Cooper pairs in the insulating phase. This experiment sheds light on the emergence of superconductivity in inhomogeneous superconduc- tors, and more generally it demonstrates the potential of graphene as a versatile building-block for the realization of novel superconducting materials.

cond-mat.mes-hall↗
arXiv2012

On the unconditional uniqueness for NLS in $H^s$

In this article, we study the unconditional uniqueness of $\dot H^s$, $0<s< 1$, solutions for the nonlinear Schrödinger equation $i\partial_t u +Δu+ c |u|^αu=0$ in ${\mathbb R}^n$. We give a unified proof of the previously known results in the subcritical cases and critical cases, and we also extend these results to some previously unsettled cases. Our proof uses in particular negative order Sobolev spaces (or Besov spaces), general Strichartz estimates, and the improved regularity property for the difference of two solutions.

math.AP↗
arXiv2010

Continuous dependence for NLS in fractional order spaces

We consider the Cauchy problem for the nonlinear Schrödinger equation $iu_t+ Δu+ λ|u|^αu=0$ in $\R^N $, in the $H^s$-subcritical and critical cases $0<α\le 4/(N-2s)$, where $0<s<N/2$. Local existence of solutions in $H^s$ is well known. However, even though the solution is constructed by a fixed-point technique, continuous dependence in $H^s$ does not follow from the contraction mapping argument. In this paper, assuming furthermore $s<1$, we show that the solution depends continuously on the initial value in the sense that the local flow is continuous $H^s \to H^s$. If, in addition, $α\ge 1$ then the flow is Lipschitz. This completes previously known results concerning the cases $s=0,1,2$.

math.AP↗
arXiv2008

The nonlinear Schrödinger equations with combined nonlinearities of power-type and Hartree

This paper is devoted to a comprehensive study of the nonlinear Schrödinger equations with combined nonlinearities of the power-type and Hartree-type in any dimension n\ge3. With some structural conditions, a nearly whole picture of the interactions of these nonlinearities in the energy space is given. The method is based on the Morawetz estimates and perturbation principles.

math.AP↗