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An-Ping Li

Publications and source records attributed to An-Ping Li.

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

Taming Atomic Defects for Quantum Functions

Single atoms provide an ideal system for utilizing fundamental quantum functions. Their electrons have well-defined energy levels and spin properties. Even more importantly, for a given isotope -- say, $^{12}$C -- all the atoms are identical. This creates a perfect uniformity that is impossible to achieve in macroscopic-size quantum systems. However, herding individual atoms is a very difficult task that requires trapping them with magnetic or optical means and cooling them down to temperatures in the nanokelvin range. On the other hand, the counterpart of single atoms -- the single defects -- may be as good as atom-based quantum systems if not better. These defects, also referred as quantum defects, possess the favorable energy, spin, and uniformity properties of single atoms and remain in their place without the help of precisely tuned lasers. While the number of usable isotopes is set, the combinations of defects and their host material are practically limitless, giving us the flexibility to create precisely designed and controlled quantum systems. Furthermore, as we tame these defects for the quantum world, we bring about transformative opportunities to the classical world in forms such as ultradense electronic devices and precise manufacturing. In this research insight, we introduce some of our recent work on precisely controlled creation and manipulation of individual defects with a scanning tunneling microscope (STM). We also discuss possible pathways for utilizing these capabilities for the development of novel systems for Quantum Information Science (QIS) applications such as quantum information processing and ultrasensitive sensors.

cond-mat.mes-hall↗
arXiv2022

Identifying Majorana vortex modes via non-local transport

The combination of two-dimensional Dirac surface states with s-wave superconductivity is expected to generate localized topological Majorana zero modes in vortex cores. Putative experimental signatures of these modes have been reported for heterostructures of proximitized topological insulators, iron-based superconductors or certain transition metal dichalcogenides. Despite these efforts, the Majorana nature of the observed excitation is still under debate. We propose to identify the presence of Majorana vortex modes using a nonlocal transport measurement protocol originally employed for one-dimensional settings. In the case of an isolated subgap state, the protocol provides a spatial map of the ratio of local charge- and probability-density which offers a clear distinction between Majorana and ordinary fermionic modes. We show that these distinctive features survive in the experimentally relevant case of hybridizing vortex core modes.

cond-mat.mes-hall↗
arXiv2021

Graphene nanoribbons for quantum electronics

Graphene nanoribbons (GNRs) are a family of one-dimensional (1D) materials carved from graphene lattice. GNRs possess high mobility and current carrying capability, sizable bandgap, and versatile electronic properties tailored by the orientations and open edge structures. These unique properties make GNRs promising candidates for prospective electronics applications including nano-sized field-effect transistors (FETs), spintronic devices, and quantum information processing. To fully exploit the potential of GNRs, fundamental understanding of structure-property relationship, precise control of atomic structures and scalable production are the main challenges. In the last several years, significant progress has been made toward atomically precise bottom-up synthesis of GNRs and heterojunctions that provide an ideal platform for functional molecular devices, as well as successful production of semiconducting GNR arrays on insulating substrates potentially useful for large-scale digital circuits. With further development, GNRs can be envisioned as a competitive candidate material in future quantum information sciences (QIS). In this Perspective, we review recent progress in GNR research and identify key challenges and new directions likely to develop in the near future.

cond-mat.mes-hall↗
arXiv2021

Van der Waals Epitaxy on Freestanding Monolayer Graphene Membrane by MBE

Research on two-dimensional materials has expanded over the past two decades to become a central theme in condensed matter research today. Significant advances have been made in the synthesis and subsequent reassembly of these materials using mechanical methods into a vast array of hybrid structures with novel properties and ever-increasing potential applications. The key hurdles in realizing this potential are the challenges in controlling the atomic structure of these layered hybrid materials and the difficulties in harnessing their unique functionality with existing semiconductor nanofabrication techniques. Here we report on high-quality van der Waals epitaxial growth and characterization of a layered topological insulator on freestanding monolayer graphene transferred to different mechanical supports. This templated synthesis approach enables direct interrogation of interfacial atomic structure of these as-grown hybrid structures and opens a route towards creating device structures with more traditional semiconductor nanofabrication techniques.

cond-mat.mtrl-sci↗
arXiv2021

Local manifestations of thickness dependent topology and axion edge state in topological magnet MnBi$_2$Te$_4$

The interplay of non-trivial band topology and magnetism gives rise to a series of exotic quantum phenomena, such as the emergent quantum anomalous Hall (QAH) effect and topological magnetoelectric effect. Many of these quantum phenomena have local manifestations when the global symmetry is broken. Here, we report local signatures of the thickness dependent topology in intrinsic magnetic topological insulator MnBi$_2$Te$_4$(MBT), using scanning tunneling microscopy and spectroscopy on molecular beam epitaxy grown MBT thin films. A thickness-dependent band gap with an oscillatory feature is revealed, which we reproduce with theoretical calculations. Our theoretical results indicate a topological quantum phase transition beyond a film thickness of one monolayer, with alternating QAH and axion insulating states for even and odd layers, respectively. At an even-odd layer step, a localized gapped electronic state is observed, in agreement with an axion insulator edge state that results from a phase transition across the step. The demonstration of thickness-dependent topological properties highlights the role of nanoscale control over novel quantum states, reinforcing the necessity of thin film technology in quantum information science applications.

cond-mat.mes-hall↗
arXiv2020

Interlayer magnetism in Fe3-xGeTe2

Fe$_{3-x}$GeTe$_2$ is a layered van der Waals magnetic material with a relatively high ordering temperature and large anisotropy. While most studies have concluded the interlayer ordering to be ferromagnetic, there have also been reports of interlayer antiferromagnetism in Fe$_{3-x}$GeTe$_2$. Here, we investigate the interlayer magnetic ordering by neutron diffraction experiments, scanning tunneling microscopy (STM) and spin-polarized STM measurements, density functional theory plus U calculations and STM simulations. We conclude that the layers of Fe$_{3-x}$GeTe$_2$ are coupled ferromagnetically and that in order to capture the magnetic and electronic properties of Fe$_{3-x}$GeTe$_2$ within density functional theory, Hubbard U corrections need to be taken into account.

cond-mat.mtrl-sci↗
arXiv2020

Realizing gapped surface states in magnetic topological insulator MnBi$_{2-x}$Sb$_{x}$Te$_{4}$

The interplay between magnetism and non-trivial topology in magnetic topological insulators (MTI) is expected to give rise to a variety of exotic topological quantum phenomena, such as the quantum anomalous Hall (QAH) effect and the topological axion states. A key to assessing these novel properties is to tune the Fermi level in the exchange gap of the Dirac surface band. MnBi$_2$Te$_4$ possesses non-trivial band topology with intrinsic antiferromagnetic (AFM) state that can enable all of these quantum states, however, highly electron-doped nature of the MnBi$_2$Te$_4$ crystals obstructs the exhibition of the gapped topological surface states. Here, we tailor the material through Sb-substitution to reveal the gapped surface states in MnBi$_{2-x}$Sb$_{x}$Te$_{4}$ (MBST). By shifting the Fermi level into the bulk band gap of MBST, we access the surface states and show a band gap of 50 meV at the Dirac point from quasi-particle interference (QPI) measured by scanning tunneling microscopy/spectroscopy (STM/STS). Surface-dominant conduction is confirmed below the Néel temperature through transport spectroscopy measured by multiprobe STM. The surface band gap is robust against out-of-plane magnetic field despite the promotion of field-induced ferromagnetism. The realization of bulk-insulating MTI with the large exchange gap offers a promising platform for exploring emergent topological phenomena.

cond-mat.mtrl-sci↗
arXiv2020

Emerging edge states on the surface of the epitaxial semimetal CuMnAs thin film

Epitaxial thin films of CuMnAs have recently attracted attention due to their potential to host relativistic antiferromagnetic spintronics and exotic topological physics. Here we report on the structural and electronic properties of a tetragonal CuMnAs thin film studied using scanning tunneling microscopy (STM) and density functional theory (DFT). STM reveals a surface terminated by As atoms, with the expected semi-metallic behavior. An unexpected zigzag step edge surface reconstruction is observed with emerging electronic states below the Fermi energy. DFT calculations indicate that the step edge reconstruction can be attributed to an As deficiency that results in changes in the density of states of the remaining As atoms at the step edge. This understanding of the surface structure and step edges on the CuMnAs thin film will enable in-depth studies of its topological properties and magnetism.

cond-mat.mes-hall↗
arXiv2020

Single-defect Memristor in MoS$_2$ Atomic-layer

Non-volatile resistive switching, also known as memristor effect in two terminal devices, has emerged as one of the most important components in the ongoing development of high-density information storage, brain-inspired computing, and reconfigurable systems. Recently, the unexpected discovery of memristor effect in atomic monolayers of transitional metal dichalcogenide sandwich structures has added a new dimension of interest owing to the prospects of size scaling and the associated benefits. However, the origin of the switching mechanism in atomic sheets remains uncertain. Here, using monolayer MoS$_2$ as a model system, atomistic imaging and spectroscopy reveal that metal substitution into sulfur vacancy results in a non-volatile change in resistance. The experimental observations are corroborated by computational studies of defect structures and electronic states. These remarkable findings provide an atomistic understanding on the non-volatile switching mechanism and open a new direction in precision defect engineering, down to a single defect, for achieving optimum performance metrics including memory density, switching energy, speed, and reliability using atomic nanomaterials.

cond-mat.mes-hall↗
arXiv2019

Step edge-mediated assembly of periodic arrays of long graphene nanoribbons on Au(111)

The influence of substrate steps on the bottom-up synthesis of atomically precise graphene nanoribbons (GNRs) on an Au(111) surface is investigated. A straight surface step is found to promote the assembly of long and compact arrays of polymers with enhanced interchain pi-pi stacking interaction, which creates a steric hindrance effect on cyclodehydrogenation to suppress the H passivation of polymer ends. The modified two-stage growth process results in periodic arrays of GNRs with doubled average length near step edges.

physics.chem-ph↗
arXiv2019

Local Superconductivity in Vanadium Iron Arsenide

We investigate the chemical substitution of group 5 into BaFe2As2 (122) iron arsenide, in the effort to understand why Fe-site hole doping of this compound (e.g., using group 5 or 6) does not yield bulk superconductivity. We find an increase in c-lattice parameter of the BaFe2As2 with the substitution of V, Nb, or Ta; the reduction in c predicts the lack of bulk superconductivity [1] that is confirmed here through transport and magnetization results. However, our spectroscopy measurements find a coexistence of antiferromagnetic and local superconducting nanoscale regions in V-122, observed for the first time in a transition-metal hole-doped iron arsenide. In BaFe2As2, there is a complex connection between local parameters such as composition and lattice strain, average lattice details, and the emergence of bulk quantum states such as superconductivity and magnetism. [1] L. M. N. Konzen, and A. S. Sefat, J. Phys.: Condens. Matter 29 (2017), 083001.

cond-mat.supr-con↗
arXiv2019

An Air-Stable and Atomically Thin Graphene/Gallium Superconducting Heterostructure

Two-dimensional layered and atomically thin elemental superconductors may be key ingredients in next-generation quantum technologies, if they can be stabilized and integrated into heterostructured devices under ambient conditions. However, atomically thin elemental superconductors are largely unexplored outside ultra-high vacuum due to rapid oxidation, and even 2D layered superconductors require complex encapsulation strategies to maintain material quality. Here we demonstrate environmentally stable, single-crystal, few-atom-thick superconducting gallium, 2D-Ga, produced by confinement heteroepitaxy (CHet) at the interface of epitaxial graphene (EG) and silicon carbide (SiC). 2D-Ga becomes superconducting at 4 K; this elevation over bulk alpha-Ga (Tc~1 K) is primarily attributed to an increased density of states at the Fermi level as the incipient Ga-Ga dimerization seen in alpha-Ga is suppressed by epitaxy to SiC. We also demonstrate the importance of controlling SiC surface morphology (i.e. step height) and defect-engineering in graphene layers prior to intercalation to achieve large-area uniform 2D-Ga layers with isotropic transport properties. This work demonstrates that unique 2D forms of 3D materials can be stabilized at the EG/SiC interface, which represents a scalable route towards air-stable crystalline 2D superconductors as a potential foundation for next-generation quantum technologies.

cond-mat.supr-con↗
arXiv2019

Confinement Heteroepitaxy: Realizing Atomically Thin, Half-van der Waals Materials

Three-dimensional epitaxial heterostructures are based on covalently-bonded interfaces, whereas those from 2-dimensional (2D) materials exhibit van der Waals interactions. Under the right conditions, however, material structures with mixed interfacial van der Waals and covalent bonding may be realized. Atomically thin layers formed at the epitaxial graphene (EG)/silicon carbide (SiC) interface indicate that EG/SiC interfaces provide this unique environment and enable synthesis of a rich palette of 2D materials not accessible with traditional techniques. Here, we demonstrate a method termed confinement heteroepitaxy (CHet), to realize air-stable, structurally unique, crystalline 2D-Ga, In, and Sn at the EG/SiC interface. The first intercalant layer is covalently-bonded to the SiC, and is accompanied by a vertical bonding gradient that ends with van der Waals interactions. Such structures break out of plane centrosymmetry, thereby introducing atomically thin, non-centrosymmetric 2D allotropes of 3D materials as a foundation for tunable superconductivity, topological states, and plasmonic properties.

cond-mat.mtrl-sci↗
arXiv2019

Atomic-scale manipulation and in situ characterization with scanning tunneling microscopy

Scanning tunneling microscope (STM) has presented a revolutionary methodology to the nanoscience and nanotechnology. It enables imaging the topography of surfaces, mapping the distribution of electronic density of states, and manipulating individual atoms and molecules, all at the atomic resolution. In particular, the atom manipulation capability has evolved from fabricating individual nanostructures towards the scalable production of the atomic-sized devices bottom-up. The combination of precision synthesis and in situ characterization of the atomically precise structures has enabled direct visualization of many quantum phenomena and fast proof-of-principle testing of quantum device functions with real-time feedback to guide the improved synthesis. In this article, several representative examples are reviewed to demonstrate the recent development of atomic scale manipulation. Especially, the review focuses on the progress that address the quantum properties by design through the precise control of the atomic structures in several technologically relevant materials systems. Besides conventional STM manipulations and electronic structure characterization with single-probe STM, integration of multiple atomically precisely controlled probes in a multiprobe STM system vastly extends the capability of in situ characterization to a new dimension where the charge and spin transport behaviors can be examined from mesoscopic to atomic length scale. The automation of the atomic scale manipulation and the integration with the well-established lithographic processes would further push this bottom-up approach to a new level that combines reproducible fabrication, extraordinary programmability, and the ability to produce large-scale arrays of quantum structures.

cond-mat.mes-hall↗
arXiv2019

An Extension of Waring's Problem in $\mathbb{Z}_{p^k}$

In this paper, we will investigate the solvability of the equation $x_1^k + x_2^k + \ldots + x_s^k = n$, $n\in \mathbb{Z}_{p^k}$, $x_1,...,x_s\in \mathcal{A}$, $\mathcal{A}\subseteq \mathbb{Z}_{p^k}$. We will give a upper bound of the number of solutions for $s=2$.

math.CO↗
arXiv2018

Direct writing of heterostructures in single atomically precise graphene nanoribbons

Precision control of interfacial structures and electronic properties is the key to the realization of functional heterostructures. Here, utilizing the scanning tunneling microscope (STM) both as a manipulation and characterization tool, we demonstrate the fabrication of a heterostructure in a single atomically precise graphene nanoribbon (GNR) and report its electronic properties. The heterostructure is made of a seven-carbon-wide armchair GNR and a lower band gap intermediate ribbon synthesized bottom-up from a molecular precursor on an Au substrate. The short GNR segments are directly written in the ribbon with an STM tip to form atomic precision intraribbon heterostructures. Based on STM studies combined with density functional theory calculations, we show that the heterostructure has a type-I band alignment, with manifestations of quantum confinement and orbital hybridization. Our finding demonstrates a feasible strategy to create a double barrier quantum dot structure with atomic precision for novel functionalities, such as negative differential resistance devices in GNR-based nanoelectronics.

cond-mat.mes-hall↗