arXiv subjects

Ming-Hao Liu

Publications and source records attributed to Ming-Hao Liu.

At least 19 recordsLinked to original sources

arXiv2026

Quantum transport across normal-superlattice-normal graphene junctions: Fabry-Pérot interference, Hofstadter butterfly, and supersnake states

Electrostatic modulation of graphene provides a tunable route to engineering miniband structures. We perform quantum transport simulations on a gate-defined graphene superlattice junction, formed by confining a two-dimensional superlattice graphene (SGr) region between two normal graphene (NGr) regions. In the low-field regime at low carrier densities, robust Fabry-Pérot interference fringes emerge even in the unipolar regime due to Fermi-velocity renormalization in the SGr region. At stronger magnetic fields but only up to 3 T, the conductance map clearly reveals the Hofstadter butterfly spectrum. At intermediate fields, our finite-width transport simulations reveal a new type of snake state, the supersnake state, composed of alternating anomalous cyclotron arcs on the SGr side and conventional semicircular arcs on the NGr side, forming a weaving trajectory along the junction. The resulting conductance oscillations agree well with geometrical conditions derived from semiclassical cyclotron orbits. Our results demonstrate that gate-defined NGr-SGr-NGr junctions provide a versatile platform hosting multiple transport regimes within a single device architecture and can be generalized to other types of superlattices not restricted to graphene.

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arXiv2026

Electronic Reconstruction at the Quasicrystal-Moiré Crossover in Twisted Bilayer Graphene

Large twist angles in twisted bilayer graphene are widely expected to be electronically trivial, with negligible interlayer coupling and no electronic reconstruction, in contrast to the rich moiré-driven band reconstruction and correlated physics that emerge at small twist angles. Here, we show that this paradigm breaks down near a twist angle of 29°, where the system crosses over between quasicrystalline and commensurate order. Atomic-resolution transmission electron microscopy directly reveals the coexistence of near-dodecagonal quasicrystalline symmetry and emerging moiré periodicity, indicating an intermediate, nonperiodic structural regime. Magnetotransport measurements uncover strong interlayer hybridization mediated by Umklapp scattering, manifested by magneto-intersubband oscillations and a highly unconventional Landau-level spectrum. Remarkably, the Landau-level degeneracy evolves from 4- to 12-fold with increasing temperature, a behavior incompatible with two decoupled graphene monolayers. These findings establish large-angle twisted bilayer graphene as a platform where quasiperiodic symmetry fundamentally reshapes low-energy electronic states beyond the conventional moiré framework.

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arXiv2026

Dimensional and doping stability of Peierls charge density waves in arrays of coupled one-dimensional chains

The Peierls instability, the spontaneous dimerization of a one-dimensional metallic chain at half filling, is a paradigmatic mechanism for charge-density-wave (CDW) formation. Here we test its robustness under finite doping and interchain hybridization in finite-thickness arrays of identical chains. We find that the stacking geometry plays a decisive role in stabilizing CDW order away from half filling. In particular, parallel-coupled chains exhibit a bistable regime where the normal and dimerized states coexist as local minima of the total energy, while skew-coupled chains display reentrant CDW order upon doping. Our results demonstrate that even minimal models of coupled atomic chains host rich phase diagrams controlled by doping, lattice rigidity, and interchain coupling geometry.

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arXiv2026

Probing the Potential Profile of Twisted Bilayer Graphene via Fabry-Pérot Interference

We use Fabry-Pérot interference to probe the internal potential profile of large-angle twisted bilayer graphene. We trace anomalous resistance oscillations in the nominally unipolar regime to a hidden cavity formed by unintentional local doping in our device. By analyzing zero-field interference patterns, we determine the location and size of this inhomogeneity. Magnetotransport measurements support the model, distinguishing local cavity modes from global resonances through their different magnetic dependence. Simulations using our extracted profile reproduce the experimental features. Our results highlight interference spectroscopy as a simple, non-invasive probe for identifying local defects and internal potential barriers in ballistic devices.

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arXiv2026

Efficient Magnetic Spin-Filtering and Persistent Spin-Currents in Lifshitz-Transitioned Altermagnets: A Route to Open-Orbit Spintronics

Altermagnets offer a unique venue for spin transport due to their vanishing net magnetization and momentum-dependent spin splitting. We demonstrate that a homotopic Lifshitz transition in two-dimensional altermagnets creates a regime where carriers are confined to geometrically protected, spin-selective open channels. These channels originate from non-contractible Fermi contours and act as metallic analogues of topological edge modes: they are sharply directional, spin-pure, and protected by Fermi-surface winding rather than an energy gap or boundary confinement. We predict three striking magneto-transport signatures of such topologically reconfigured altermagnets: open-orbit focusing with perfect lensing and retroreflection, high-efficiency magnetic spin filtering, and chirality-tunable spin persistent currents in altermagnetic nanotubes. Our results establish altermagnets as a platform where Fermi-surface winding directly engineers spin transport, bypassing the requirements for ferromagnetism or strong spin-orbit coupling. These findings identify Lifshitz-transitioned altermagnets as a route to topology-enabled spintronics that transcends the limitations of conventional edge-state paradigms.

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arXiv2026

Scalable tight-binding model for strained graphene

We generalize the scalable tight-binding model for graphene, which allows for efficient quantum transport simulations in the Dirac regime, to account for elastic strain. We show that the original scalable model with scaling factor $s$ is readily applicable to strained graphene, provided that the displacement fields corresponding to the deformed graphene lattice are properly scaled. In particular, we show that the long-wavelength theory remains invariant when the strain tensor is scaled by $s$. This is achieved in practice by scaling the in-plane displacement fields by $s$ while the out-of-plane displacements have to be scaled by $\sqrt{s}$. We confirm these scaling laws by extensive numerical simulations, starting with the pseudomagnetic field and the local density of states for different scaled lattices. The latter allows us to study pseudo-Landau levels as well as hybrid Landau levels in the presence of an external magnetic field. Finally, we consider quantum transport simulations motivated by a recent experiment, where a uniaxial strain barrier is engineered in monolayer graphene by vertically misaligned gates. Our work generalizes the scalable tight-binding model to allow for efficient modeling of quantum transport in large-scale strained graphene devices.

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arXiv2026

Gate-Tunable Resonances and 1D Channel in a Graphene Nanoslide

We present a theory of the graphene nanoslide, a fundamental device for graphene straintronics that realizes a single pseudogauge barrier. We solve the scattering problem in closed form and demonstrate that the nanoslide gives rise to a hybrid pseudogauge and electrostatic cavity in the bipolar regime, and hosts one-dimensional transverse channels. The latter can be tuned using a bottom gate between valley-chiral or counterpropagating modes, as well as one-dimensional flatbands. Hence, the local density of states near the barrier depends strongly on the gate voltage with a tunable sublattice and electron-hole asymmetry. In the presence of electron-electron interactions, the nanoslide allows for \textit{in-situ} tuning between a chiral and ordinary Tomonaga-Luttinger liquid.

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arXiv2025

Anisotropic transport in ballistic bilayer graphene cavities

Closing the gap between ray tracing simulations and experimentally observed electron jetting in bilayer graphene (BLG), we study all-electronic, gate-defined BLG cavities using tight-binding simulations and semiclassical equations of motion. Such cavities offer a rich playground to investigate anisotropic electron transport due to the trigonally warped Fermi surfaces. In this work, we achieve two things: First, we verify the existence of triangular modes (as predicted by classical ray tracing calculations) in the quantum solutions of closed circular BLG cavities. Then, we explore signatures of said triangular modes in transport through open BLG cavities connected to leads. We show that the triangular symmetry translates into anisotropic transport and present an optimal setup for experimental detection of the triangular modes as well as for controlled modulation of transport in preferred directions.

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arXiv2025

Pseudomagnetotransport in Strained Graphene

In graphene, long-wavelength deformations that result in elastic shear strain couple to the low-energy Dirac electrons as pseudogauge fields. Using a scalable tight-binding model, we consider analogs to magnetotransport in mesoscopic strained graphene devices with nearly uniform pseudomagnetic fields. In particular, we consider transverse pseudomagnetic focusing in a bent graphene ribbon and show that a focused valley-polarized current can be generated with characteristic conductance oscillations. Importantly, our scaling method allows for quantum transport calculations with realistic device geometries, and leaves the Dirac physics and pseudogauge fields invariant as long as the atomic displacements vary slowly with respect to the scaled lattice. Our results show that pseudomagnetotransport is a promising new route for graphene straintronics, and our scaling method provides a new framework for the modeling, design, and interpretation of straintronics experiments and applications.

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arXiv2024

Magnetic Field-Induced Polar Order in Monolayer Molybdenum Disulfide Transistors

In semiconducting monolayer transition metal dichalcogenides (ML-TMDs), broken inversion symmetry and strong spin-orbit coupling result in spin-valley lock-in effects so that the valley degeneracy may be lifted by external magnetic fields, potentially leading to real-space structural transformation. Here, we report magnetic field (B)-induced giant electric hysteretic responses to back-gate voltages in ML-MoS2 field-effect transistors (FETs) on SiO2/Si at temperatures < 20 K. The observed hysteresis increases with |B| up to 12 T and is tunable by varying the temperature. Raman spectroscopic and scanning tunneling microscopic studies reveal significant lattice expansion with increasing |B| at 4.2 K, and this lattice expansion becomes asymmetric in ML-MoS2 FETs on rigid SiO2/Si substrates, leading to out-of-plane mirror symmetry breaking and the emergence of a tunable out-of-plane ferroelectric-like polar order. This broken symmetry-induced polarization in ML-MoS2 shows typical ferroelectric butterfly hysteresis in piezo-response force microscopy, adding ML-MoS2 to the single-layer material family that exhibit out-of-plane polar order-induced ferroelectricity, which is promising for such technological applications as cryo-temperature ultracompact non-volatile memories, memtransistors, and ultrasensitive magnetic field sensors. Moreover, the polar effect induced by asymmetric lattice expansion may be further generalized to other ML-TMDs and achieved by nanoscale strain engineering of the substrate without magnetic fields.

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arXiv2024

Fractal Quantum Transport on MoS2 Superlattices: a System with Tunable Symmetry

Electron doping is an excellent tuning knob to explore different phases of matter in two-dimensional (2D) materials. For example, tuning the Fermi level at a van Hove singularity in twisted bilayer graphene can enhance electron-electron interactions and induce a diverse range of correlated phases. Here, using a single-particle picture, we study the electronic reconstruction of the band edges of a 2D semiconductor, monolayer MoS2, on a hexagonal moire potential induced by another MoS2 monolayer. We find that such system transitions from a honeycomb to a hexagonal symmetry when the Fermi level is tuned from the conduction to the valence side. We also study the system under magnetic fields, and construct the Hofstadter's butterfly in the electron- and hole-doped side. Our findings are confirmed by simulating the conductance across a large-scale two-terminal device. We conclude that this duality is a general property that MoS2 and other transition-metal-dichalcogenides exhibit under non-symmetric superlattice potentials.

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arXiv2024

Four-band effective square lattice model for Bernal-stacked bilayer graphene

Bernal-stacked bilayer graphene (BLG) provides an ideal basis for gate-controlled, and free of etching, electronic devices. Theoretical modeling of realistic devices is an essential part of research, however, simulations of large-scale BLG devices continue to be extremely challenging. Micrometer-sized systems are predominantly beyond the reach of the commonly used atomistic tight-binding method, while other numerical approaches based on the two dimensional Dirac equation are not straightforward to conduct due to the fermion doubling problem. Here we present an approach based on the continuum model, unharmed by the fermion doubling. The discretization of the BLG continuum Hamiltonian leads to an effective four-band model, with both valleys built-in. We demonstrate its performance with realistic, large-scale systems, and obtain results consistent with experiments and with the tight-binding model, over a broad range of magnetic field.

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arXiv2024

Magnetotransport Signatures of the Radial Rashba Spin-Orbit Coupling in Proximitized Graphene

Graphene-based van der Waals heterostructures take advantage of tailoring spin-orbit coupling (SOC) in the graphene layer by proximity effect. At long-wavelength -- saddled by the electronic states near the Dirac points -- the proximitized features can be effectively modelled by the Hamiltonian involving novel SOC terms and allow for an admixture of the tangential and radial spin textures -- by the so-called Rashba angle $θ_{\text{R}}$. Taking such effective models we perform realistic large-scale magneto-transport calculations -- transverse magnetic focusing and Dyakonov-Perel spin relaxation -- and show that there are unique qualitative and quantitative features allowing for an unbiased experimental disentanglement of the conventional Rashba SOC from its novel radial counterpart, called here the radial Rashba SOC. Along with that, we propose a scheme for a direct estimation of the Rashba angle by exploring the magneto-response symmetries when swapping an in-plane magnetic field. To complete the story, we analyze the magneto-transport signatures in the presence of an emergent Dresselhaus SOC and also provide some generic ramifications about possible scenarios of the radial superconducting diode effect.

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arXiv2024

Terahertz ratchet in graphene 2D metamaterial formed by a patterned gate with an antidot arrayd

We report the observation of the terahertz-induced ratchet effect in graphene-based two-dimensional (2D) metamaterials. The metamaterial consists of a graphite gate patterned with an array of triangular antidots placed under a graphene monolayer. We show that the ratchet current appears due to the noncentrosymmetry of the periodic structure unit cell. The ratchet current is generated owing to the combined action of a spatially periodic in-plane electrostatic potential and a periodically modulated radiation electric field caused by near-field diffraction. The magnitude and direction of the ratchet current are shown to be controlled by voltages applied to both back and patterned gates, which change the lateral asymmetry, carrier type and density. The phenomenological and microscopic theories of ratchet effects in graphene-based 2D metamaterials are developed. The experimental data are discussed in the light of the theory based on the solution of the Boltzmann kinetic equation and the calculated electrostatic potential profile. The theory describes well all the experimental results and shows that the observed ratchet current consists of the Seebeck thermoratchet contribution as well as the linear contribution, which is sensitive to the orientation of the radiation electric field vector with respect to the triangles.

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arXiv2024

Electron quantum optics in graphene

In the last decade, graphene has become an exciting platform for electron optical experiments, in many aspects superior to conventional two-dimensional electron gases (2DEGs). A major advantage, besides the ultra-large mobilities, is the fine control over the electrostatics, which gives the possibility of realising gap-less and compact p-n interfaces with high precision. The latter host non-trivial states, \eg, snake states in moderate magnetic fields, and serve as building blocks of complex electron interferometers. Thanks to the Dirac spectrum and its non-trivial Berry phase, the internal (valley and sublattice) degrees of freedom, and the possibility to tailor the band structure using proximity effects, such interferometers open up a completely new playground based on novel device architectures. In this review, we introduce the theoretical background of graphene electron optics, fabrication methods used to realise electron-optical devices, and techniques for corresponding numerical simulations. Based on this, we give a comprehensive review of ballistic transport experiments and simple building blocks of electron optical devices both in single and bilayer graphene, highlighting the novel physics that is brought in compared to conventional 2DEGs. After describing the different magnetic field regimes in graphene p-n junctions and nanostructures, we conclude by discussing the state of the art in graphene-based Mach-Zender and Fabry-Perot interferometers.

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arXiv2023

Probing miniband structure and Hofstadter butterfly in gated graphene superlattices via magnetotransport

The presence of periodic modulation in graphene leads to a reconstruction of the band structure and formation of minibands. In an external uniform magnetic field, a fractal energy spectrum called Hofstadter butterfly is formed. Particularly interesting in this regard are superlattices with tunable modulation strength, such as electrostatically induced ones in graphene. We perform quantum transport modeling in gate-induced square two-dimensional superlattice in graphene and investigate the relation to the details of the band structure. At low magnetic field the dynamics of carriers reflects the semi-classical orbits which depend on the mini band structure. We theoretically model transverse magnetic focusing, a ballistic transport technique by means of which we investigate the minibands, their extent and carrier type. We find a good agreement between the focusing spectra and the mini band structures obtained from the continuum model, proving usefulness of this technique. %positions of van Hove singularities at high magnetic field the calculated four-probe resistance fit the Hofstadter butterfly spectrum obtained for our superlattice. Our quantum transport modeling provides an insight into the mini band structures, and can be applied to other superlattice geometries.

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arXiv2023

Ballistic transport spectroscopy of spin-orbit-coupled bands in monolayer graphene on WSe$_2$

Van der Waals interactions with transition metal dichalcogenides was shown to induce strong spin-orbit coupling (SOC) in graphene, offering great promises to combine large experimental flexibility of graphene with unique tuning capabilities of the SOC that can rotate spin by moving electrons or vice versa. Here, we probe SOC-driven band splitting and electron dynamics in graphene on WSe$_2$ by measuring ballistic transverse magnetic focusing. We found a clear splitting in the first focusing peak whose evolution in charge density and magnetic field is well reproduced by calculations using SOC strength of ~13 meV and no splitting in the second peak that indicates stronger Rashba SOC. A possible suppression of electron-electron scatterings was also found in temperature dependence measurement. Further, we found that Shubnikov-de Haas oscillations exhibit SOC strength of ~3.4 meV, suggesting that it probes different electron dynamics, calling for new theory. Our study demonstrates an interesting possibility to exploit ballistic electron motion pronounced in graphene for emerging spin-orbitronics.

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arXiv2023

Quantum capacitive coupling between large-angle twisted graphene layers

Large-angle twisted bilayer graphene (tBLG) is known to be electronically decoupled due to the spatial separation of the Dirac cones corresponding to individual graphene layers in the reciprocal space. The close spacing between the layers causes strong capacitive coupling, opening possibilities for applications in atomically thin devices. Here, we present a self-consistent quantum capacitance model for the electrostatics of decoupled graphene layers, and further generalize it to deal with decoupled tBLG at finite magnetic field and large-angle twisted double bilayer graphene at zero magnetic field. We probe the capacitive coupling through the conductance, showing good agreement between simulations and experiments for all the systems considered. We also propose a new experiment utilizing the decoupling effect to induce a huge and tunable bandgap in bilayer graphene by applying a moderately low bias. Our model can be extended to systems composed of decoupled graphene multilayers as well as non-graphene systems, opening a new realm of quantum-capacitively coupled materials.

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