Liheng An - Academia.edu (original) (raw)

Papers by Liheng An

arXiv (Cornell University), Jun 9, 2020

The recently discovered nonlinear Hall effect (NHE) in a few non-interacting systems provides a n... more The recently discovered nonlinear Hall effect (NHE) in a few non-interacting systems provides a novel mechanism to generate second harmonic electrical Hall signals under timereversal-symmetric conditions. Here, we introduce a new approach to engineering NHE by using twisted moiré structures. We find that the twisted WSe2 bilayer exhibits a NHE when tuning the Fermi level to the moiré flat bands. Near half-filling of the first moiré band, the nonlinear Hall signal shows a sharp peak with the generation efficiency at least two orders of magnitude larger than those in previous experiments. We propose that the giant NHE and diverging generation efficiency originate from a mass-diverging type continuous Mott transition, which is evidenced by resistivity measurements. This work demonstrates not only how interaction effects can couple to Berry curvature dipoles to produce novel quantum phenomena, but also what NHE measurements can provide for developing a new tool to study the quantum criticality.

The recently discovered nonlinear Hall effect (NHE) in a few non-interacting systems provides a n... more The recently discovered nonlinear Hall effect (NHE) in a few non-interacting systems provides a novel mechanism to generate second harmonic electrical Hall signals under timereversal-symmetric conditions. Here, we introduce a new approach to engineering NHE by using twisted moiré structures. We find that the twisted WSe2 bilayer exhibits a NHE when tuning the Fermi level to the moiré flat bands. Near half-filling of the first moiré band, the nonlinear Hall signal shows a sharp peak with the generation efficiency at least two orders of magnitude larger than those in previous experiments. We propose that the giant NHE and diverging generation efficiency originate from a mass-diverging type continuous Mott transition, which is evidenced by resistivity measurements. This work demonstrates not only how interaction effects can couple to Berry curvature dipoles to produce novel quantum phenomena, but also what NHE measurements can provide for developing a new tool to study the quantum crit...

Cornell University - arXiv, May 27, 2022

Twisted van der Waals heterostructures have recently been proposed as a condensedmatter platform ... more Twisted van der Waals heterostructures have recently been proposed as a condensedmatter platform for realizing controllable quantum models due to the low-energy moiré bands with specific charge distributions in moiré superlattices. Here, combining angleresolved photoemission spectroscopy with sub-micron spatial resolution (μ-ARPES) and scanning tunneling microscopy (STM), we performed a systematic investigation on the electronic structure of 5.1° twisted bilayer WSe2 that hosts correlated insulating and zeroresistance states. Interestingly, contrary to one's expectation, moiré bands were observed only at-valley but not-valley in μ-ARPES measurements; and correspondingly, our STM measurements clearly identified the real-space honeycomb-and Kagome-shaped charge distributions at the moiré length scale associated with the-valley moiré bands. These results not only reveal the unsual valley dependent moiré-modified electronic structure in twisted transition metal dichalcogenides, but also highlight the-valley moiré bands as a promising platform for exploring strongly correlated physics in emergent honeycomb and Kagome lattices at different energy scales.

Nanoscale, 2021

Layer-dependent interface reconstruction and associated strain effects in marginally twisted WSe2... more Layer-dependent interface reconstruction and associated strain effects in marginally twisted WSe2 are revealed by quantitative electron diffraction analysis, atomic-resolution imaging and electrical transport detection of multiple ultra-flat minibands.

Physical Review B, 2018

We investigated the superconductivity in a few-layer NbSe 2-graphene heterojunction by differenti... more We investigated the superconductivity in a few-layer NbSe 2-graphene heterojunction by differential conductance spectroscopy. Because of the gate-tunable Fermi level of the few-layer graphene, used here as a tunneling electrode in a nano-point-contact spectroscopy setup, the differential conductance of the heterojunction showed highly sensitive dependence on the gate voltage, which allowed us to probe the nature of the superconducting gap functions with unprecedented detail by continuously tuning the transparency of the junction between the spectroscopic tunneling and the Andreev reflection limits. Characteristic features associated with a two-gap superconductivity in NbSe 2 were reproducibly observed in both limits and between, e.g., in the form of a central conductance dip with two sets of coherence peaks when the Fermi level was close to the charge neutrality point of graphene. From fits with the Blonder-Tinkham-Klapwijk model, two gaps with their temperature dependence were extracted. The two gaps associated with the two-band superconductivity in NbSe 2 followed the expected temperature behavior in the limit of weak interband scattering, with a gap to T c ratio suggesting a weak to moderately strong coupling in few-layer systems.

Nanotechnology, Jan 20, 2017

Atomically-thin black phosphorus (BP) field effect transistors show strong-weak localization tran... more Atomically-thin black phosphorus (BP) field effect transistors show strong-weak localization transition, which is tunable through gate voltages. Hopping transports through charge impurity-induced localized states are observed at low carrier density regime. Variable-range hopping model is applied to simulate scattering behaviors of charge carriers. In the high carrier concentration regime, a negative magnetoresistance indicates weak localization effects. The extracted phase coherence length is power-law temperature-dependent (~T^-0.48±0.03) and demonstrates inelastic electron-electron interactions and the 2D transport features in few-layer BP field effect devices. The competition between localization and phase coherence lengths is investigated and analyzed based on observed gate-tunable strong-weak localization transition in few-layer BP.

Nanoscale Horizons, 2020

Twisted bilayer graphene provides a new two-dimensional platform for studying electron interactio... more Twisted bilayer graphene provides a new two-dimensional platform for studying electron interaction phenomena and flat band properties such as correlated insulator transition, superconductivity and ferromagnetism at certain magic angles.

Nature Communications, 2021

Moiré superlattices in van der Waals heterostructures provide a tunable platform to study emergen... more Moiré superlattices in van der Waals heterostructures provide a tunable platform to study emergent properties that are absent in the natural crystal form. Twisted bilayer transition metal dichalcogenides (TB-TMDs) can host moiré flat bands over a wide range of twist angles. For twist angle close to 60°, it was predicted that TB-TMDs undergo a lattice reconstruction which causes the formation of ultra-flat bands. Here, by using scanning tunneling microscopy and spectroscopy, we show the emergence of multiple ultra-flat bands in twisted bilayer WSe2 when the twist angle is within 3° of 60°. The ultra-flat bands are manifested as narrow tunneling conductance peaks with estimated bandwidth less than 10 meV, which is only a fraction of the estimated on-site Coulomb repulsion energy. The number of these ultra-flat bands and spatial distribution of the wavefunctions match well with the theoretical predictions, strongly evidencing that the observed ultra-flat bands are induced by lattice re...

arXiv (Cornell University), Jun 9, 2020

The recently discovered nonlinear Hall effect (NHE) in a few non-interacting systems provides a n... more The recently discovered nonlinear Hall effect (NHE) in a few non-interacting systems provides a novel mechanism to generate second harmonic electrical Hall signals under timereversal-symmetric conditions. Here, we introduce a new approach to engineering NHE by using twisted moiré structures. We find that the twisted WSe2 bilayer exhibits a NHE when tuning the Fermi level to the moiré flat bands. Near half-filling of the first moiré band, the nonlinear Hall signal shows a sharp peak with the generation efficiency at least two orders of magnitude larger than those in previous experiments. We propose that the giant NHE and diverging generation efficiency originate from a mass-diverging type continuous Mott transition, which is evidenced by resistivity measurements. This work demonstrates not only how interaction effects can couple to Berry curvature dipoles to produce novel quantum phenomena, but also what NHE measurements can provide for developing a new tool to study the quantum criticality.

The recently discovered nonlinear Hall effect (NHE) in a few non-interacting systems provides a n... more The recently discovered nonlinear Hall effect (NHE) in a few non-interacting systems provides a novel mechanism to generate second harmonic electrical Hall signals under timereversal-symmetric conditions. Here, we introduce a new approach to engineering NHE by using twisted moiré structures. We find that the twisted WSe2 bilayer exhibits a NHE when tuning the Fermi level to the moiré flat bands. Near half-filling of the first moiré band, the nonlinear Hall signal shows a sharp peak with the generation efficiency at least two orders of magnitude larger than those in previous experiments. We propose that the giant NHE and diverging generation efficiency originate from a mass-diverging type continuous Mott transition, which is evidenced by resistivity measurements. This work demonstrates not only how interaction effects can couple to Berry curvature dipoles to produce novel quantum phenomena, but also what NHE measurements can provide for developing a new tool to study the quantum crit...

Cornell University - arXiv, May 27, 2022

Twisted van der Waals heterostructures have recently been proposed as a condensedmatter platform ... more Twisted van der Waals heterostructures have recently been proposed as a condensedmatter platform for realizing controllable quantum models due to the low-energy moiré bands with specific charge distributions in moiré superlattices. Here, combining angleresolved photoemission spectroscopy with sub-micron spatial resolution (μ-ARPES) and scanning tunneling microscopy (STM), we performed a systematic investigation on the electronic structure of 5.1° twisted bilayer WSe2 that hosts correlated insulating and zeroresistance states. Interestingly, contrary to one's expectation, moiré bands were observed only at-valley but not-valley in μ-ARPES measurements; and correspondingly, our STM measurements clearly identified the real-space honeycomb-and Kagome-shaped charge distributions at the moiré length scale associated with the-valley moiré bands. These results not only reveal the unsual valley dependent moiré-modified electronic structure in twisted transition metal dichalcogenides, but also highlight the-valley moiré bands as a promising platform for exploring strongly correlated physics in emergent honeycomb and Kagome lattices at different energy scales.

Nanoscale, 2021

Layer-dependent interface reconstruction and associated strain effects in marginally twisted WSe2... more Layer-dependent interface reconstruction and associated strain effects in marginally twisted WSe2 are revealed by quantitative electron diffraction analysis, atomic-resolution imaging and electrical transport detection of multiple ultra-flat minibands.

Physical Review B, 2018

We investigated the superconductivity in a few-layer NbSe 2-graphene heterojunction by differenti... more We investigated the superconductivity in a few-layer NbSe 2-graphene heterojunction by differential conductance spectroscopy. Because of the gate-tunable Fermi level of the few-layer graphene, used here as a tunneling electrode in a nano-point-contact spectroscopy setup, the differential conductance of the heterojunction showed highly sensitive dependence on the gate voltage, which allowed us to probe the nature of the superconducting gap functions with unprecedented detail by continuously tuning the transparency of the junction between the spectroscopic tunneling and the Andreev reflection limits. Characteristic features associated with a two-gap superconductivity in NbSe 2 were reproducibly observed in both limits and between, e.g., in the form of a central conductance dip with two sets of coherence peaks when the Fermi level was close to the charge neutrality point of graphene. From fits with the Blonder-Tinkham-Klapwijk model, two gaps with their temperature dependence were extracted. The two gaps associated with the two-band superconductivity in NbSe 2 followed the expected temperature behavior in the limit of weak interband scattering, with a gap to T c ratio suggesting a weak to moderately strong coupling in few-layer systems.

Nanotechnology, Jan 20, 2017

Atomically-thin black phosphorus (BP) field effect transistors show strong-weak localization tran... more Atomically-thin black phosphorus (BP) field effect transistors show strong-weak localization transition, which is tunable through gate voltages. Hopping transports through charge impurity-induced localized states are observed at low carrier density regime. Variable-range hopping model is applied to simulate scattering behaviors of charge carriers. In the high carrier concentration regime, a negative magnetoresistance indicates weak localization effects. The extracted phase coherence length is power-law temperature-dependent (~T^-0.48±0.03) and demonstrates inelastic electron-electron interactions and the 2D transport features in few-layer BP field effect devices. The competition between localization and phase coherence lengths is investigated and analyzed based on observed gate-tunable strong-weak localization transition in few-layer BP.

Nanoscale Horizons, 2020

Twisted bilayer graphene provides a new two-dimensional platform for studying electron interactio... more Twisted bilayer graphene provides a new two-dimensional platform for studying electron interaction phenomena and flat band properties such as correlated insulator transition, superconductivity and ferromagnetism at certain magic angles.

Nature Communications, 2021

Moiré superlattices in van der Waals heterostructures provide a tunable platform to study emergen... more Moiré superlattices in van der Waals heterostructures provide a tunable platform to study emergent properties that are absent in the natural crystal form. Twisted bilayer transition metal dichalcogenides (TB-TMDs) can host moiré flat bands over a wide range of twist angles. For twist angle close to 60°, it was predicted that TB-TMDs undergo a lattice reconstruction which causes the formation of ultra-flat bands. Here, by using scanning tunneling microscopy and spectroscopy, we show the emergence of multiple ultra-flat bands in twisted bilayer WSe2 when the twist angle is within 3° of 60°. The ultra-flat bands are manifested as narrow tunneling conductance peaks with estimated bandwidth less than 10 meV, which is only a fraction of the estimated on-site Coulomb repulsion energy. The number of these ultra-flat bands and spatial distribution of the wavefunctions match well with the theoretical predictions, strongly evidencing that the observed ultra-flat bands are induced by lattice re...