publications

2022

1. Phys. Rev. B

   Inherited topological superconductivity in two-dimensional Dirac semimetals

   Lo, Chiu Fan Bowen, Po, Hoi Chun, and Nevidomskyy, Andriy H.

   Physical Review B Mar 2022

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   Under what conditions does a superconductor inherit topologically protected nodes from its parent normal state? In the context of inter-Fermi-surface pairing in three-dimensional Weyl semimetals with broken time-reversal symmetry, the pairing order parameter is classified by monopole harmonics and is necessarily nodal [Li and Haldane, Phys. Rev. Lett. 120, 067003 (2018)]. Here, we show that a similar conclusion could also be drawn for 2D Dirac semimetals, although the conditions for the existence of nodes are more complex, depending on the pairing matrix structure in the valley and sublattice space. We analytically and numerically analyze the Bogoliubov-de Gennes quasiparticle spectra for Dirac systems based on the monolayer as well as twisted bilayer graphene. We find that in the cases of intravalley intrasublattice and intervalley intersublattice pairings, the point nodes in the BdG spectra (which are inherited from the Dirac cone in the normal state) are protected by a 1D winding number. The nodal structure of the superconductivity is confirmed numerically using tight-binding models of monolayer and twisted bilayer graphene. Notably, the BdG spectrum is nodal even with a momentum-independent “bare” pairing, which, however, acquires a momentum dependence and point nodes upon projection to the Bloch states on the topologically nontrivial Fermi surface, similar in spirit to the Li-Haldane monopole superconductor and the Fu-Kane proximity-induced superconductor on the surface of a topological insulator.

2021

1. Nat. Commun.

   Terahertz response of monolayer and few-layer WTe2 at the nanoscale

   Jing, Ran, Shao, Yinming, Fei, Zaiyao, Lo, Chiu Fan Bowen, Vitalone, Rocco A., Ruta, Francesco L., Staunton, John, Zheng, William J.-C., Mcleod, Alexander S., Sun, Zhiyuan, Jiang, Bor-yuan, Chen, Xinzhong, Fogler, Michael M., Millis, Andrew J., Liu, Mengkun, Cobden, David H., Xu, Xiaodong, and Basov, D. N.

   Nature Communications 2021

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   Tungsten ditelluride (WTe2) is an atomically layered transition metal dichalcogenide whose physical properties change systematically from monolayer to bilayer and few-layer versions. In this report, we use apertureless scattering-type near-field optical microscopy operating at Terahertz (THz) frequencies and cryogenic temperatures to study the distinct THz range electromagnetic responses of mono-, bi- and trilayer WTe2 in the same multi-terraced micro-crystal. THz nano-images of monolayer terraces uncovered weakly insulating behavior that is consistent with transport measurements. The near-field signal on bilayer regions shows moderate metallicity with negligible temperature dependence. Subdiffractional THz imaging data together with theoretical calculations involving thermally activated carriers favor the semimetal scenario with Δ≈−10meV over the semiconductor scenario for bilayer WTe2. Also, we observed clear metallic behavior of the near-field signal on trilayer regions. Our data are consistent with the existence of surface plasmon polaritons in the THz range confined to trilayer terraces in our specimens. Finally, data for microcrystals up to 12 layers thick reveal how the response of a few-layer WTe2 asymptotically approaches the bulk limit.

2. Nano Lett.

   Long-lived phonon polaritons in hyperbolic materials

   Ni, Guangxin, McLeod, Alexander S., Sun, Zhiyuan, Matson, Joseph R., Lo, Chiu Fan Bowen, Rhodes, Daniel A., Ruta, Francesco L., Moore, Samuel L., Vitalone, Rocco A., Cusco, Ramon, Artús, Luis, Xiong, Lin, Dean, Cory R., Hone, James C., Millis, Andrew J., Fogler, Michael M., Edgar, James H., Caldwell, Joshua D., and Basov, D. N.

   Nano Letters 2021

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   Natural hyperbolic materials with dielectric permittivities of opposite signs along different principal axes can confine long-wavelength electromagnetic waves down to the nanoscale, well below the diffraction limit. Confined electromagnetic waves coupled to phonons in hyperbolic dielectrics including hexagonal boron nitride (hBN) and α-MoO_3 are referred to as hyperbolic phonon polaritons (HPPs). HPP dissipation at ambient conditions is substantial, and its fundamental limits remain unexplored. Here, we exploit cryogenic nanoinfrared imaging to investigate propagating HPPs in isotopically pure hBN and naturally abundant α-MoO3 crystals. Close to liquid-nitrogen temperatures, losses for HPPs in isotopic hBN drop significantly, resulting in propagation lengths in excess of 8 μm, with lifetimes exceeding 5 ps, thereby surpassing prior reports on such highly confined polaritonic modes. Our nanoscale, temperature-dependent imaging reveals the relevance of acoustic phonons in HPP damping and will be instrumental in mitigating such losses for miniaturized mid-infrared technologies operating at liquid-nitrogen temperatures.

3. Nat. Commun.

   Hyperbolic enhancement of photocurrent patterns in minimally twisted bilayer graphene

   Sunku, Sai S., Halbertal, Dorri, Stauber, Tobias, Chen, Shaowen, McLeod, Alexander S., Rikhter, Andrey, Berkowitz, Michael E., Lo, Chiu Fan Bowen, Gonzalez-Acevedo, Derick E., Hone, James C., Dean, C. R., Fogler, M. M., and Basov, D. N.

   Nature Communications 2021

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   Quasi-periodic moiré patterns and their effect on electronic properties of twisted bilayer graphene have been intensely studied. At small twist angle θ, due to atomic reconstruction, the moiré superlattice morphs into a network of narrow domain walls separating micron-scale AB and BA stacking regions. We use scanning probe photocurrent imaging to resolve nanoscale variations of the Seebeck coefficient occurring at these domain walls. The observed features become enhanced in a range of mid-infrared frequencies where the hexagonal boron nitride substrate is optically hyperbolic. Our results illustrate the capabilities of the nano-photocurrent technique for probing nanoscale electronic inhomogeneities in two-dimensional materials.

4. Nano Lett.

   Dual-gated graphene devices for near-field nano-imaging

   Sunku, Sai S., Halbertal, Dorri, Engelke, Rebecca, Yoo, Hyobin, Finney, Nathan R., Curreli, Nicola, Ni, Guangxin, Tan, Cheng, McLeod, Alexander S., Lo, Chiu Fan Bowen, Dean, Cory R., Hone, James C., Kim, Philip, and Basov, D. N.

   Nano Letters 2021

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   Graphene-based heterostructures display a variety of phenomena that are strongly tunable by electrostatic local gates. Monolayer graphene (MLG) exhibits tunable surface plasmon polaritons, as revealed by scanning nano-infrared experiments. In bilayer graphene (BLG), an electronic gap is induced by a perpendicular displacement field. Gapped BLG is predicted to display unusual effects such as plasmon amplification and domain wall plasmons with significantly larger lifetime than MLG. Furthermore, a variety of correlated electronic phases highly sensitive to displacement fields have been observed in twisted graphene structures. However, applying perpendicular displacement fields in nano-infrared experiments has only recently become possible [Li, H.; Nano Lett. 2020, 20, 3106−3112]. In this work, we fully characterize two approaches to realizing nano-optics compatible top gates: bilayer MoS2 and MLG. We perform nano-infrared imaging on both types of structures and evaluate their strengths and weaknesses. Our work paves the way for comprehensive near-field experiments of correlated phenomena and plasmonic effects in graphene-based heterostructures.

2020

1. Nano Lett.

   Hyperbolic cooper-pair polaritons in planar graphene/cuprate plasmonic cavities

   Berkowitz, Michael E., Kim, Brian SY, Ni, Guangxin, McLeod, Alexander S., Lo, Chiu Fan Bowen, Sun, Zhiyuan, Gu, Genda, Watanabe, Kenji, Taniguchi, Takashi, Millis, Andrew J., Hone, James C., Fogler, Michael M., Averitta, Richard D., and Basov, D. N.

   Nano Letters 2020

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   Hyperbolic Cooper-pair polaritons (HCP) in cuprate superconductors are of fundamental interest due to their potential for providing insights into the nature of unconventional superconductivity. Here, we critically assess an experimental approach using near-field imaging to probe HCP in Bi2Sr2CaCu2O8+x (Bi-2212) in the presence of graphene surface plasmon polaritons (SPP). Our simulations show that inherently weak HCP features in the near-field can be strongly enhanced when coupled to graphene SPP in layered graphene/hexagonal boron nitride (hBN)/Bi-2212 heterostructures. This enhancement arises from our multilayered structures effectively acting as plasmonic cavities capable of altering collective modes of a layered superconductor by modifying its electromagnetic environment. The degree of enhancement can be selectively controlled by tuning the insulating spacer thickness with atomic precision. Finally, we verify the expected renormalization of room-temperature graphene SPP using near-field infrared imaging. Our modeling, augmented with data, attests to the validity of our approach for probing HCP modes in cuprate superconductors.

2. Nat. Mater.

   Excitons in strain-induced one-dimensional moiré potentials at transition metal dichalcogenide heterojunctions

   Bai, Yusong, Zhou, Lin, Wang, Jue, Wu, Wenjing, McGilly, Leo J., Halbertal, Dorri, Lo, Chiu Fan Bowen, Liu, Fang, Ardelean, Jenny, Rivera, Pasqual, Finney, Nathan R., Yang, Xu-Chen, Basov, D. N., Yao, Wang, Xu, Xiaodong, Hone, James, Pasupathy, Abhay N., and Zhu, X.-Y.

   Nature Materials 2020

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   The possibility of confining interlayer excitons in interfacial moiré patterns has recently gained attention as a strategy to form ordered arrays of zero-dimensional quantum emitters and topological superlattices in transition metal dichalcogenide heterostructures. Strain is expected to play an important role in the modulation of the moiré potential landscape, tuning the array of quantum dot-like zero-dimensional traps into parallel stripes of one-dimensional quantum wires. Here, we present real-space imaging of unstrained zero-dimensional and strain-induced one-dimensional moiré patterns along with photoluminescence measurements of the corresponding excitonic emission from WSe2/MoSe2 heterobilayers. Whereas excitons in zero-dimensional moiré traps display quantum emitter-like sharp photoluminescence peaks with circular polarization, the photoluminescence emission from excitons in one-dimensional moiré potentials shows linear polarization and two orders of magnitude higher intensity. These results establish strain engineering as an effective method to tailor moiré potentials and their optoelectronic response on demand.

2017

1. Appl. Phys. Lett.

   Rigorous numerical modeling of scattering-type scanning near-field optical microscopy and spectroscopy

   Chen, Xinzhong, Lo, Chiu Fan Bowen, Zheng, William, Hu, Hai, Dai, Qing, and Liu, Mengkun

   Applied Physics Letters 2017

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   Over the last decade, scattering-type scanning near-field optical microscopy and spectroscopy have been widely used in nano-photonics and material research due to their fine spatial resolution and broad spectral range. A number of simplified analytical models have been proposed to quantitatively understand the tip-scattered near-field signal. However, a rigorous interpretation of the experimental results is still lacking at this stage. Numerical modelings, on the other hand, are mostly done by simulating the local electric field slightly above the sample surface, which only qualitatively represents the near-field signal rendered by the tip-sample interaction. In this work, we performed a more comprehensive numerical simulation which is based on realistic experimental parameters and signal extraction procedures. By directly comparing to the experiments as well as other simulation efforts, our methods offer a more accurate quantitative description of the near-field signal, paving the way for future studies of complex systems at the nanoscale.