Chiu Fan Bowen "Leo" Lo

I am an incoming physics PhD student at the Condensed Matter Theory group at Harvard University, working with Professor Ashvin Vishwanath. I am broadly interested in topological phases of matters, superconductivity, and strongly correlated electron materials. Click here for my resume.

When I am not doing physics (which is becoming increasingly rare!), I like playing the flute, coming up with random melodies, and biking. And here is a random rock in our solar system.

news

Apr 21, 2022	I was elected to Phi Beta Kappa!
Apr 4, 2022	I was awarded the NSF Graduate Research Fellowship!
Mar 1, 2022	Our paper on inherited topological superconductivity in 2D Dirac semimetals is published!
Mar 26, 2021	I was named a Goldwater Scholar!

selected publications

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.
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.
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.
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.