Session UU08: V: Complex Structured Materials, Including Graphene IV

Synergistic interplay between Dirac fermions and long-wavelength orders in graphene-insulator heterostructures

In this work, we study the physical properties of a new type of heterostructure system consisted of graphene placed on top of a band-aligned insulating substrate. By virtue of the band alignment, charge carriers can be transferred between graphene and the surface of the substrate under the control of gate voltages, which may yield a long-wavelength charge order at the surface of the substrate through Wigner-crystallization mechanism. The long-wavelength charge order in turn exerts a superlattice Coulomb potential to the Dirac electrons in graphene, which reduces the non-interacting Fermi velocity such that e-e Coulomb interactions would play an important role. Consequently, the Dirac points are spontaneously gapped out by e-e interactions. Meanwhile, the Fermi velocities around the Dirac points are drastically enhanced due to interaction effects, which can give rise to large Landau-level spacing with robust quantization plateaux of Hall resistivity under weak magnetic fields and at high temperatures. We have further performed high-throughput first principles calculations, and found a number of promising insulating materials as candidate substrates for graphene to demonstrate such effects.   

Synthesis method and correlation between compositional, vibrational and electrical properties in graphene oxide fibers

The synthesis method and correlation between compositional, vibrational, and electrical properties in graphene oxide fibers (GOF), synthesized from rice husk by pyrolysis method, are presented here. The variation of carbonization temperature (T_CA) influences oxides concentration (OC). GOF samples exhibits morphology of fibers, compositional response of oxides tuned by T_CA with presence of hydroxyl and epoxy bridges, vibrational characteristics typical of graphene oxide multilayers, and electrical behavior that scale with OC. The correlation between OC and physical properties suggests that by controlling the OC in GOF, it was possible to modify vibrational and electrical properties of great interest in fabrication of advanced electronics.   

Study of experimental anomalous Hall effect at room temperature in reduced graphene oxide films

In this work, we presented an experimental anomalous Hall effect in graphene oxide films (GO)/Quartz, synthesized via the double thermal decomposition method (DTD), employing bamboo precursor as a rich carbon material. SEM-EDS, AFM, and Raman spectroscopy studies revealed high superficial uniformity, a compositional ratio C/O of 93.5%/6.5%, vibrational behavior of reduced graphene oxide, the high charge carrier mobility of 3.78x10⁴ cm² V^-1s^-1 and p-type semiconductor response. These results suggest that GO films are an excellent candidate for advanced electronics of sensors and devices.

    

Magnon scattering across quantum Hall skyrmion crystals

Skyrmion crystals have a rich collective mode spectrum and are hypothesized to appear in quantum Hall ferromagnets in the lowest Landau level at small doping away from one filled level. We develop a model of a ferromagnet-skyrmion crystal-ferromagnet junction, relevant to recent experiments in monolayer graphene, to study the influence of collective modes of skyrmion crystals on the propagation an incoming ferromagnetic magnon[1] – a setup proposed to uncover the nature of the collective modes [2]. We show, using an appropriate set of generalized theta functions, how to smoothly interpolate between regions of zero (ferromagnetic ends) and spatially modulating finite topological charge density (skyrmion crystal). The collective mode equations for such a configuration, from a suitably defined energy functional, map onto the Bogoliubov-de Gennes equation. Using this mapping, along with a slice-wise recursive transfer matrix approach, we calculate the transmission amplitudes of an incoming ferromagnetic magnon. We also show how changing the collective mode spectrum of the skyrmion crystal, by varying the strength of the topological charge density terms in the functional, affects magnon transmission. Our results present unique signatures of skyrmion crystals due to their characteristic collective mode spectrum, and can be used as evidence for their presence in graphene and possibly in twisted bilayer graphene.

 

[1] N. Chakraborty, R. Moessner, B. Doucot: In prep

[2] H. Zhou et al. Nature Physics (2020)

    

Hydrodynamic and diffusive vortices near obstacles in a two-dimensional electron fluid subject to a strong magnetic field

We study current flow near a radially symmetric density perturbation in graphene in the presence of a magnetic field. In both diffusive and hydrodynamic regimes, for strong fields, the current forms spirals around the perturbation. We show that the scaling of the spiral size as a function of magnetic field is different in the two regimes. We discuss how these phenomena can be probed using scanning imaging techniques, including tunneling, potentiometry, magnetometry, and photocurrent measurements.   

Macroscopic Laser Propulsion of Graphene Vehicle by Radiometric Force in Low Vacuum Condition

Known from the Crookes radiometer, radiometric force which originates from unbalanced strike of hot gas molecules in low vacuum condition, is expected as light thruster for space travel and vactrain instead of optical pressure. In the present work, we reported the universal laser launching of various graphene-based materials in rarefied glass tube. The propulsion dependance on vacuum pressure and laser power were systematically investigated to validate the radiometric force and optimize the actuation. By taking advantage of the low density and high diamagnetism of graphene, a maglev graphene boat was rotated constantly with controlled direction by laser illumination on different sides of the sail. We then demonstrated the horizontal propulsion of a maglev graphene train along the magnets track rapidly and smoothly by laser irradiation on the sail back. This work not only demonstrated the effective propulsion of various macroscopic materials with radiometric force, but also exhibited its application potentail on laser launching rockets, and laser-driving of vacuum tube train.

    

Theoretical Model of a Plasmonically Enhanced Tunable Spectrally Selective Infrared Photodetector Based on Intercalation-Doped Nanopatterned Multilayer Graphene

We showed in past work that nanopatterned monolayer graphene (NPG) can be used for realizing an ultrafast and spectrally selective mid-infrared (mid-IR) photodetector based on the photothermoelectric effect and working in the 8–12 μm regime. The absorption wavelength of NPG can be extended to the 3–8 μm regime by tuning geometric patterning parameters and Fermi level of monolayer graphene . Further extension to shorter wavelengths would require a smaller nanohole size that is not attainable with current technology. Here, we show by means of a theoretical model that nanopatterned multilayer graphene intercalated with FeCl₃ (NPMLG-FeCl₃) overcomes this problem by substantially extending the detection wavelength into the range from λ = 1.3 to 3 μm. We present a proof of concept for a spectrally selective infrared (IR) photodetector based on NPMLG-FeCl₃ that can operate from λ = 1.3 to 12 μm and beyond. The localized surface plasmons (LSPs) on the graphene sheets in NPMLG-FeCl₃ allow for electrostatic tuning of the photodetection wavelength. Most importantly, the LSPs along with an optical cavity increase the absorbance from about N × 2.6% for N-layer graphene-FeCl₃ (without patterning) to nearly 100% for NPMLG-FeCl₃, where the strong absorbance occurs locally inside the graphene sheets only. Our IR detection scheme relies on the photothermoelectric effect induced by asymmetric patterning of the multilayer graphene (MLG) sheets. The LSPs on the nanopatterned side create hot carriers that give rise to the Seebeck effect at room temperature, achieving a responsivity of  V/W, a detectivity of D* = 2.3 × 10⁹ Jones, and an ultrafast response time of the order of 100 ns. Our theoretical results can be used to develop graphene-based photodetection, optical IR communication, IR color displays, and IR spectroscopy over a wide IR range.   

Single-photon detection from gated nanostructures

Single-photon detection with high efficiency and low dark count rates is of high importance for a wide range of optical measurements. In this work, we describe efforts to directly measure blue photons emanating from graphene and graphene nanoribbons that are gated at ~10 nm scales. Graphene nanostructures and nanoribbons exhibit remarkable nonlinear optical behavior when subjected to intense local electric fields [1,2]. The experimental signatures of these nonlinear processes are detected through interferometric methods [3]. The single-photon detection platform will allow quantification of these signatures by detection of sum-frequency-generated light in the far field.

[1] Sheridan, E. et al. Nano Lett. (2020) doi:10.1021/acs.nanolett.0c01379

[2] Sheridan, E. et al. APL Materials 9, 071101 (2021)

[3] L. Chen, et al., Light: Science & Appl. 8, 24 (2019).   

Non local transport in Graphene/Chromia

Robust non-local spin voltage signals have been demonstrated experimentally in monolayer Graphene deposited on the (0001) surface of the antiferromagnetic Chromium Oxide (Cr2O3). The measured non-local voltage shows fluctuations at a wide range of energies reminiscent of universal conductance fluctuations but with larger amplitudes. We take a toy model Hamiltonian to explain the main non-local signatures observed in the experiments. Tuning the strength of spin-orbit coupling, interfacial Zeeman exchange, sublattice asymmetries, and orbital effects of the magnetic field cause the Graphene to demonstrate different behaviors as it switches between the quantum spin Hall effect (QSH), quantum anomalous Hall effect (QAHE), and quantum Hall effect (QHE). We use the Landauer-Büttiker formalism to calculate the non-local resistance in the simulations and use self-consistent Born approximation to calculate the dephasing self energy. By including the dephasing effect in the Landauer-Büttiker formalism, we are able to distinguish the non local conductance fluctuation from the main non local signature. The presence of different types of disorder further introduces scattering and quantum effects associated with it in the presence of phase coherence.

    

Ultrafast electron dynamics of graphene quantum dots: High harmonic generation

We study theoretically nonlinear optical properties of graphene quantum dots placed in a field of a short and strong linearly polarized optical pulse. We address the problem of high harmonic generation in quantum dots and how such a nonlinear effect is affected by dephasing processes in a quantum dot. The dephasing makes the ultrafast electron dynamics more irreversible with a large residual population of the excited quantum dot levels. In relation to the high-harmonic spectrum, with increasing the dephasing time, the intensities of the low-frequency harmonics increase while the cutoff energy decreases. The dependence of the cutoff energy on the amplitude of the optical pulse is also sensitive to the frequency of the pulse. When the frequency of the optical pulse is much less than the quantum dot band gap, this dependence is almost linear, but when the frequency of the pulse is comparable to the band gap, the cutoff energy shows saturation behavior at large field amplitude, >0.4 V/Å.