Session F56: Graphene: Bandstructure Engineering, Device Physics

Electronic Transport and Photoemission Fingerprints of Floquet Graphene Antidot Lattices

The Floquet graphene antidot lattice is a periodically hole-patterned graphene sheet driven periodically by electromagnetic radiation [1]. Notably, the equilibrium semiconducting state in such a system can be steered through Floquet Dirac, selectively dynamically localized, or Floquet semi-Dirac electronic phases by irradiation with circularly polarized near-IR radiation of suitable intensity. Experimentally, light-driven materials are probed by electronic transport measurements [2] and time- and angle- resolved photoemission spectroscopy (TR-ARPES) [3]. Thus, in order to connect the predicted Floquet electronic phases to experiment, we set out to model these measurement schemes. For electronic transport, the components of the frequency-dependent electrical conductivity tensor were computed at the level of linear response by means of the Floquet-Kubo formula [4]. Additionally, we utilized subcycle resolved photocurrent theory for TR-ARPES [5]. For the phases under consideration, the combination of the computed conductivity frequency sweeps and photocurrent spectra provides an unambiguous mapping between the quasienergy band structures predicted within the Floquet formalism and experimentally accessible quantities.

 

[1] Floquet Graphene Antidot Lattices, Cupo et al., Phys. Rev. B, 2021

[2] Light-Induced Anomalous Hall Effect in Graphene, McIver et al., Nat. Phys., 2020

[3] Selective Scattering Between Floquet–Bloch and Volkov States in a Topological Insulator, Mahmood et al., Nat. Phys., 2016

[4] Floquet-Drude Conductivity, Wackerl et al., Phys. Rev. B, 2020

[5] Theory of Subcycle Time-Resolved Photoemission: Application to Terahertz Photodressing in Graphene, Schüler and Sentef, arXiv:2103.15900v2, 2021   

Quantum point contacts fabricated in graphene van der Waals heterostructures using AFM anodic oxidation

Quantum Hall interferometers rely on high-quality quantum point contacts (QPCs) to properly function as a tool to observe exchange statistics of quasiparticles. Graphene van der Waals heterostructures offer the advantage to gate-define QPCs using encapsulating graphite layers allowing for high-quality, tunable edge potential profiles. Recent work on such devices has relied on traditional electron-beam lithography to form the required gate structures, and challenges have emerged in reducing disorder resulting from the etching process. Here we report on using an alternative method, anodic oxidation lithography, which utilizes a voltage-driven Atomic Force Microscope (AFM) tip in a controlled environment to create high-quality incisions in graphite gates. In this talk we focus on optimizing this method for fabricating QPCs and characterize the resulting devices using transport measurements. These results pave the way for the observation of anyonic exchange statistics in future generations of graphene quantum Hall interferometers.   

Nanometer-scale lateral junctions in graphene/α-RuCl3 heterostructures

Accessing nano-scale and high-quality lateral p-n junctions in graphene is crucial for studying numerous emergent quantum interfacial phenomena. Here we investigate graphene/α-RuCl₃ heterostructures that are revealed to host sharp p-n interfaces at the boundaries of intrinsic nanobubbles. We utilized scanning tunneling microscopy (STM) and spectroscopy (STS) and scattering-type scanning nearfield optical microscopy (s-SNOM) as multi-messenger local probes to explore both the electronic and plasmonic properties of nanobubble p-n junctions. Both STS and s-SNOM results demonstrate a substantial shift in Dirac point (~0.6 eV) as a result of work function mediated charge transfer between α-RuCl₃ and graphene. Further, we observe the formation of an abrupt junction along nanobubble boundaries with an exceptionally sharp lateral width (<3 nm). This is one order of magnitude smaller than junctions produced via state-of-the-art lithographically defined gated devices. Our results are supported by density functional theory (DFT) calculations which help to elucidate the origins of both in-plane and out-of-plane charge transfer behavior. Our work opens up routes toward device engineering via interfacial charge transfer in graphene and other low-density 2D materials.   

Topological Metallicity in Defective Graphene

Disordered graphene surfaces are presented which have a non-vanishing density of states at the Fermi level driven by topological defects in the sheet structure, i.e. heptagonal and pentagonal rings. We observe a crossover in the relative representation of five and seven-fold rings in the site-resolved density of states upon passing across the Fermi level, with signs that gapping near the Fermi surface is connected to the development of “tails” of reversed ring character that extend into these gaps. The generality of this phenomenon suggests that it may be a universal feature of disordered sp² carbons.   

Thermionic Cascade in Graphene-Boron Nitride Heterostructures

We investigate the inter- and intra-layer photocurrent of a single pixel consisting of two graphene layers separated by an ultrathin hexagonal boron nitride barrier. The basic device response displays an interlayer photocurrent which increases super-linearly with excitation power and has a voltage-tunable time constant (~1 ps) that increases linearly with the inverse interlayer voltage. This is consistent with the thermally driven drift of hot electronic charge carriers across the interlayer barrier. In contrast to this ordinary behavior, we also observed an extraordinarily strong enhancement of the interlayer photoconductance near the Dirac point. This Dirac point photoconductance is sensitive to even a weak intralayer excitation voltage and can be strongly quenched at temperatures near 70K and below. This photoconductance can be further suppressed into negative differential photoconductance as the intralayer excitation voltage increases. We attribute this remarkable behavior to ultrafast thermionic cascade triggered by enhanced electronic temperatures near the Dirac point. Furthermore, its efficient quenching achieved through intralayer excitation voltage indicates non-conventional and efficient cooling pathways that may serve as a hallmark of Dirac fluid in graphene.   

Gate-tunable Veselago Interference in a Bipolar Graphene Microcavity

We present a novel device architecture of a graphene microcavity defined by carefully engineered local strain and electrostatic fields. We create a controlled electron-optic interference process at zero magnetic fields as a result of consecutive Veselago refractions in the microcavity and provide direct experimental evidence through low-temperature electrical transport measurements. The experimentally observed first-, second-, and third-order interference peaks agree quantitatively with the Veselago physics in a microcavity. In addition, we demonstrate decoherence of the interference by an external magnetic field, as the cyclotron radius becomes comparable to the interference length scale. For its application in electron-optics, we use Veselago interference to localize uncollimated electrons and characterize its contribution in further improving collimation efficiency in graphene pn-junctions [1-2]. Our work sheds new light on relativistic single-particle physics and provides important technical improvements toward next-generation quantum devices based on the coherent manipulation of electron momentum and trajectory.

[1] Chen, S. et al. Science 353, 1522–1525 (2016).

[2] Wang, K. et al. PNAS USA 116, 6575–6579 (2019)   

Graphene – Narrow Bandgap Semiconductor Heterostructures for Mid-Infrared Photodetection

Graphene-based semiconductor heterostructures have attracted attention for realizing optoelectronic devices, especially for photodetection from the deep-UV to the mid-infrared region. These photodetectors utilize graphene as a photoconductive channel to circulate photocurrent, fabricated on top of a narrow semiconductor substrate that will use a phenomenon known as photogating to obtain a high photoconductive gain. While a common material used in the substrate is silicon (Si), indium antimonide (InSb) offers a promising substitute, as it has a direct and narrow bandgap semiconductor. These detectors show high photo-response to the mid-infrared incident light at the wavelength of 10 µm. The work provides a method for achieving high-performance optoelectronics operating in the mid-infrared region at room temperature.   

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Graphene has emerged as the novel Van der Waals heterostructure for performing electron transport experiments. The ability to fabricate ultraclean devices with high electron mobility along with the tunability of the carrier density has made it an ideal material for studying ballistic electron transport. At low carrier densities, electrons in graphene obey ohmic transport, which is characterized by dominant bulk scattering and leads to a local relation between the current density and the electric field. This changes at high densities when the transport becomes ballistic and non-local. In this talk, I will present our non-invasive study of this crossover to non-local electron transport, imaged using a scanning single-electron transistor. Deep in the ballistic regime and under small non-quantizing perpendicular magnetic fields, we observe that the transverse electric field Ey shows a curious spatial structure. Specifically, we see that with increasing magnetic field, Ey measured in a rectangular channel develops a central cusp which splits into two peaks. Using electron billiards simulations, we show that these spatial structures reflect the internal structure of the semi-classical skipping orbits at the sample boundaries.   

Quantum straintronics: controlling the valley polarization, quantized flat bands and 2D electronic correlation by nanoscale strain engineering of single-layer graphene

We employed nanoscale strain engineering techniques to induce periodic strain on single-layer graphene (SLG) [1]. By placing strain-free SLG[2] and h-BN on top of e-beam lithography-fabricated periodic nano-pillar/cone arrays on a SiO₂/Si substrate, periodic graphene wrinkles with controlled strain were induced. Besides scanning tunneling spectroscopic evidence of strain-induced giant pseudo-magnetic fields and valley polarization [1], electrical transport studies of valley-Hall transistors built on strained graphene wrinkles were carried out. Nonlocal resistance and Hall resistance taken on multiple devices of different strain in zero external magnetic fields and at 1.8 K confirmed the existence of valley Hall effect (VHE), quantized Landau levels, and varying electron correlation proportional to the strain. Moreover, quantum VHE (QVHE), quantum anomalous Hall effect (QAHE) and spontaneous spin polarization were observed in the ballistic limit for strong correlation. Our approach thus establishes a new paradigm of quantum straintronics for tuning the 2D electron correlation. This work is jointly supported by ARO/MURI (Award #W911NF-16-1-0472), and NSF/IQIM at Caltech (Award #1733907).   

Strain-induced pseudo gauge potential based graphene valley beam splitter

Valley polarization is paramount for leveraging the valley degree of freedom in encoding and computing information. Using scattering theory based quantum transport calculations, his work proposes a graphene-based multi-terminal valley beam splitter for separating valley polarized beams in real space. Our proposed approach leverages the effect of strain combined with the negative refraction property in graphene. The uniaxial smooth strain applied along a transverse axis to the transport direction induces the pseudo gauge potential, which acts differently on K and K’ transverse wavevector components and directs their trajectories in opposite directions along the longitudinal axis. Graphene acts as a meta-surface with negative refraction properties for PN junction. Thus, the angular transmission spectra of each valley are configured to design robust polarization for the finite energy range by modulating strain and doping electrostatically.

Reference:

1. Fujita, T., et. al. Applied Physics Letters 97, no. 4 (2010): 043508

2. McRae, A. C. et.al. Physical Review Applied 11, no. 5 (2019): 054019.   

Thermo-electric response in two-dimensional Dirac systems: the role of particle-hole pairs

Clean two-dimensional Dirac systems have received a lot of attention for being a prime candidate to observe hydrodynamical transport behavior in interacting electronic systems. This is mostly due to recent advances in the preparation of ultrapure samples with sufficiently strong interactions. In this paper, we investigate the role of collective modes in the thermo-electric transport properties of those systems. We find that dynamical particle-hole pairs, plasmons, make a sizeable contribution to the thermal conductivity. While the increase at the Dirac point is moderate, it becomes large towards larger doping. We suspect, that this is a generic feature of ultraclean two-dimensional electronic systems, also applicable to degenerate systems.   

Impurity induced parallel multi-flat bands in graphene

The electronic structure of graphene suppresses the transfer of the single active electron when the adjacent carbon atom is doped. This causes the emergence of a robust and universal flat band in the band structure and subsequently a mid-gap state in the DOS diagram close to the zero of the energy. This effect can be modified by adding extra impurities to the graphene supercells. We found that one can observe multiple flat bands by adding the odd number of epoxy groups around the initial impurity point. In addition, different types of neighbor impurities are compared with each other in their impurity-induced flat band effect. The origin of the multiple flat bands is investigated by a series of analyses with different types of impurities and configurations. Therefore, any periodic arrangement of impurity strongly localizes electrons and produces alternated superlattice bands whose width depends on the parameters of the added impurity. Accessing multiple flat bands can pave the way in the engineering of suitable platforms in studying other phenomena such as superconductivity and fractional quantum Hall effect.   

Connecting edge states in bilayer graphene nanoribbon with SSH ladder

We study edge states in AB-stacked bilayer graphene (BLG) ribbon in the simplest tight-binding model. Topological features of edge states when two layers ended with identical or distinct edge terminations (zigzag, bearded, armchair) are discussed. Existence of edge states near Fermi level are protected by chiral symmetry, whose topology is well captured by Su-Schrieffer-Heeger (SSH) ladder model. These zero-energy edge states can exist in the whole k_y region when two layers are zigzag and bearded, respectively. Winding number calculation shows a topological phase transition between two distinct non-trivial topological phases when crossing Dirac points. Additionally, we illustrate the importance of  stacking configurations of BLG ribbon since they can lead to unexpected edge states without protection from chiral symmetry both near the Fermi level in armchair-armchair case and in the gap within bulk bands that are away from Fermi level in the general case. The origin of these states are explained by lattice struture of  SSH ladder near the decoupling point k_y=π.   

Photon assisted tunneling in bilayer graphene double quantum dots

Spin qubits in semiconductor quantum dots (QDs) are attractive candidates for solid state quantum computation. Singlet-triplet qubits, where the logical qubits are encoded in a two-electron spin system in double quantum dots (DQDs), turned out to be of special interest. In such systems, control over the interdot tunnel coupling and, hence, the exchange interaction is essential. Bilayer graphene (BLG) is an attractive host material for spin qubits due to its small spin-orbit and hyperfine interaction, as well as its gate voltage controllable band gap. Only recently, it has become possible to confine single electrons in BLG QDs and to understand their spin and valley texture [1]. However, microwave manipulation has not been demonstrated, so far.

Here, we perform photon-assisted tunneling (PAT) spectroscopy, which relies on resonant microwave excitation of electrons across the interdot transition [2]. In power-dependent measurements, we explore multi-photon processes up to 10^th order. We use PAT as a probe for the interdot tunnel coupling in BLG DQDs. We can control and measure the interdot tunnel coupling in a range of several GHz which is suitable for qubit operations.

[1] L. Banszerus et al., Nat. Commun. 12,5250 (2021).

[2] T. H. Oosterkamp et al., Nature 395, 873 (1998).