arXiv subjects

Jeil Jung

Publications and source records attributed to Jeil Jung.

At least 19 recordsLinked to original sources

arXiv2026

Layer-Number-Controlled Symmetry Breaking and Surface-State Transport in Rhombohedral Graphene Multilayers

Rhombohedral multilayer graphene hosts layer-polarized flat bands, providing an intriguing platform for correlated and topological electronic states; however, the role of layer number in governing symmetry breaking and surface screening remains elusive. Here we prepare rhombohedral graphene multilayers and systematically conduct electrical transport measurements. We uncover an unconventional layer dependence of phase transitions: the critical displacement field (D$_{c}$) for the layer-antiferromagnetic (LAF)-to-semimetal transitions remains constant across tetralayer to hexalayer graphene, whereas the D$_{c}$ for semimetal-to-layer-polarized-insulator (LPI) transition increases with layer number, defying unscreened Coulomb interaction models. In hexalayer graphene, surface-state-dominated transport emerges, with Landau levels (LLs) and resistive peaks selectively controlled by adjacent gates, a signature of strong interlayer screening absent in thinner stacks. High magnetic fields reveal valley-layer-locked LLs and dissipative states possibly from interlayer backscattering, highlighting the presence of decoupled surface states. Our findings establish layer number as a key tuning knob for engineering correlated and topological phases in rhombohedral graphene multilayers.

cond-mat.mes-hall↗

arXiv2026

Commensuration torques in double-moir\'e twisted trilayer hexagonal boron nitride and graphene heterostructures

We study commensuration-driven torques and angle locking in double-moir\'e trilayer hexagonal boron nitride (hBN) and graphene heterostructures using large-scale atomistic relaxations. In twisted trilayer hBN (t3BN) homostructures, double-moir\'e commensuration ($\theta_{12} = -\theta_{23}$) give rise to local energy minima accompanied by torque sign reversals, signaling a restoring tendency toward the commensurate configuration. The corresponding binding energies are $\sim$0.2-0.3 meV/atom, originating from enhanced overlap of low-energy stacking domains, although the system is globally stable at zero twist. In contrast, in graphene/hBN heterolayers systems the global energy minimum can coincide with the double-moir\'e commensuration angle, particularly near $\sim$0.6$^{\circ}$, reflecting competition between lattice mismatch and interfacial relaxation. Incommensurate atomic structures have reduced stabilization due to suppressed overlap of low-energy stacking and have enhanced superlubricity due to spatial averaging of interfacial energies. These results establish double-moir\'e commensuration as a general, system-dependent mechanism for twist-angle stabilization, whose angular stability is characterized by the torque magnitude and binding energy. Coulomb electrostatic interactions further enhance the stabilization energy without changing the underlying physics.

cond-mat.mes-hall↗

arXiv2026

Supermoir\'{e} domain-resolved effective Hamiltonians and valley topology in helical multilayer graphene

Extending moir\'{e} graphene beyond twisted bilayers, helical trilayer graphene has shown topological bands and correlated states with reshaped moir\'{e} periodicity. Here we develop a theoretical framework for helical multilayer graphene to investigate its supermoir\'{e} relaxation and low-energy electronic structure. Using real-space lattice calculations, we find that relaxation reconstructs the system into locally periodic single-moir\'{e} domains, which provide the basis for a continuum description. Within each reconstructed domain, downfolding the first-shell model yields effective Hamiltonians near the Dirac points that reveal how the low-energy spectrum decomposes into folded Dirac sectors. We further evaluate the valley Chern numbers encoded in these effective Hamiltonians, obtaining domain-dependent and gate-tunable topological responses consistent with the lattice calculations. Our results establish a domain-resolved organizing principle for thicker helical graphene stacks, in which folded Dirac sectors partition the low-energy spectrum, while local stacking families determine the corresponding band character and topological response.

cond-mat.mes-hall↗

arXiv2026

Atomistic substrate relaxation effects in the band gaps of graphene on hexagonal boron nitride

We assess the impact of atomistic substrate lattice relaxation schemes in the primary band gap at charge neutrality and the secondary valence band gap of graphene on hexagonal boron nitride (G/h-BN) as a function of twist angle. For zero twist angle, the primary gap decreases from $\sim 30$~meV in fully relaxed suspended G/h-BN bilayers, to $\sim 9$~meV when the remote h-BN substrate layer is kept rigid, and down to $\sim 3$~meV in completely rigid structures. In the presence of relaxations, the primary gap shows a maximum near $\sim 0.6^{\circ}$ coinciding with energetic stabilization due to alignment between the moir\'e pattern and the graphene lattice vectors, while the secondary valence band gap drops from $\sim 12$~meV down to zero beyond twist angles of $\sim 1^{\circ}$. A small but finite primary gap on the order of $\sim 1$~meV, with a mass sign favoring electronic occupation of carbon atop boron, persists across twist angles from $0^{\circ}$ to $30^{\circ}$ for all sliding configurations, and switches sign for twist angles between $30^{\circ}$ and $60^{\circ}$.

cond-mat.mes-hall↗

arXiv2025

Geometric control of the moire twist angle in heterobilayer flakes

We demonstrate a finite twist-angle stabilization mechanism in lattice-mismatched 2D heterobilayers, which results from the geometric alignment between the flake edges and its moire pattern. Using atomistic simulations of graphene on hexagonal boron nitride flakes with diameters of up to $\sim 2500${\AA}, we identify robust metastable angles at $\sim 0.61^\circ$ for armchair and $\sim1.89^\circ$ for zigzag-edged flakes, tunable via in-plane heterostrain. This locking mechanism, which relies on energy barriers that are an order of magnitude larger than those of nearby metastable twist angles, provides a geometric route to precision twist-angle control of two-dimensional heterostructures and to understand the self-orientation of macroscopic flakes.

cond-mat.mes-hall↗

arXiv2025

Near room temperature magnetoelectric response and tunable magnetic anisotropy in the two-dimensional magnet 1T-CrTe2

Magnets with controllable magnetization and high critical temperature are essential for practical spintronics devices, among which the two-dimensional 1T-CrTe2 stands out because of its high experimental critical temperature up to about 300K down to the single layer limit. By using ab initio density functional theory, we investigate the magnetic properties of monolayer and bilayer 1T-CrTe2 and demonstrate that the magnetic properties, such as the magnetocrystalline anisotropy, critical Curie temperature and magnetizations, can be influenced by strain or electric fields.

cond-mat.mtrl-sci↗

arXiv2025

Correlated interlayer quantum Hall state in large-angle twisted trilayer graphene

Trilayer graphene allows systematic control of its electronic structure through stacking sequence and twist geometry, providing a versatile platform for correlated states. Here we report magnetotransport in alternating twisted trilayer graphene with a twist angle of about 5$^{\circ}$. The data reveal an electron-hole asymmetry that can be captured by introducing layer-dependent potential shifts. At charge neutrality ($\nu_{\mathrm{tot}}=0$), three low-resistance states appear, which Hartree-Fock mean-field analysis attributes to emerging spin-resolved helical edge modes similar to those of quantum spin Hall insulators. At $\nu_{\mathrm{tot}}=-1$, we also observe suppressed resistance when the middle and bottom layers are each half filled while the top layer remains inert at $\nu=-2$, consistent with an interlayer excitonic quantum Hall state. These results demonstrate correlated interlayer quantum Hall phases in alternating twisted trilayer graphene, including spin-resolved edge transport and excitonic order.

cond-mat.mes-hall↗

arXiv2025

Optical control of the moiré twist angle

In this theoretical work, we propose an all-optical method for fast, precise manipulation of two-dimensional multilayers by transferring orbital angular momentum from phase-structured light (e.g. vortex beams) to a 2D material flake. We model the light-matter interaction, analyze the twist dynamics, and develop a phase diagram for optical twists by mapping the system onto an impulsively forced nonlinear pendulum. Our findings reveal rich dynamical responses spanning single- and multi-pulse twist angle control to (quasi)stable dynamical trajectories, and suggest a pathway for all-optical measurement of the twist potential energy. Aided by classical potential estimates for the interlayer energy and numerical simulation, we demonstrate the feasibility of this approach with hexagonal boron nitride bilayers and extend the results to dichalcogenides with first-principles calculations. These results can be generalized to other 2D multilayers, paving the way for scalable and customizable moiré electronics and photonics.

cond-mat.mtrl-sci↗

arXiv2024

Sliding-dependent electronic structures of alternating-twist tetralayer graphene

We study the electronic structure of alternating-twist tetralayer graphene, especially near its magic angle $θ= 1.75^\circ$, for different AA, AB, and SP sliding geometries at their middle interface that divides two twisted bilayer graphenes. This sliding dependence is shown for the bandwidths, band gaps, and $K$-valley Chern numbers of the lowest-energy valence and conduction bands as a function of twist angle and interlayer potential difference. Our analysis reveals that the AA sliding is most favorable for narrow bands and gaps, and the AB sliding is most prone to developing finite valley Chern numbers. We further analyze the linear longitudinal optical absorptions as a function of photon energy and the absorption map in the moiré Brillouin zone for specific transition energies. A self-consistent Hartree calculation reveals that the AA system's electronic structure is the most sensitive to variations in carrier density.

cond-mat.mes-hall↗

arXiv2024

Direct view of gate-tunable miniband dispersion in graphene superlattices near the magic twist angle

Superlattices from twisted graphene mono- and bi-layer systems give rise to on-demand many-body states such as Mott insulators and unconventional superconductors. These phenomena are ascribed to a combination of flat bands and strong Coulomb interactions. However, a comprehensive understanding is lacking because the low-energy band structure strongly changes when the electron filling is varied. Here, we gain direct access to the filling-dependent low energy bands of twisted bilayer graphene (TBG) and twisted double bilayer graphene (TDBG) by applying micro-focused angle-resolved photoemission spectroscopy to in situ gated devices. Our findings for the two systems are in stark contrast: The doping dependent dispersion for TBG can be described in a simple model, combining a filling-dependent rigid band shift with a many-body related bandwidth change. In TDBG, on the other hand, we find a complex behaviour of the low-energy bands, combining non-monotonous bandwidth changes and tuneable gap openings. Our work establishes the extent of electric field tunability of the low energy electronic states in twisted graphene superlattices and can serve to underpin the theoretical understanding of the resulting phenomena.

cond-mat.mes-hall↗

arXiv2024

Moiré exciton polaron engineering via twisted hBN

Twisted hexagonal boron nitride (thBN) exhibits emergent ferroelectricity due to the formation of moiré superlattices with alternating AB and BA domains. These domains possess electric dipoles, leading to a periodic electrostatic potential that can be imprinted onto other 2D materials placed in its proximity. Here we demonstrate the remote imprinting of moiré patterns from twisted hexagonal boron nitride (thBN) onto monolayer MoSe2 and investigate the resulting changes in the exciton properties. We confirm the imprinting of moiré patterns on monolayer MoSe2 via proximity using Kelvin probe force microscopy (KPFM) and hyperspectral photoluminescence (PL) mapping. By developing a technique to create large ferroelectric domain sizes ranging from 1 μm to 8.7 μm, we achieve unprecedented potential modulation of 387 +- 52 meV. We observe the formation of exciton polarons due to charge redistribution caused by the antiferroelectric moiré domains and investigate the optical property changes induced by the moiré pattern in monolayer MoSe2 by varying the moiré pattern size down to 110 nm. Our findings highlight the potential of twisted hBN as a platform for controlling the optical and electronic properties of 2D materials for optoelectronic and valleytronic applications.

cond-mat.mtrl-sci↗

arXiv2024

Ab initio tight-binding Models for Mono- and Bilayer Hexagonal Boron Nitride (h-BN)

Hexagonal boron nitride ($\it h$-BN) exhibits dominant $π$-bands near the Fermi level, similar to graphene. However, unlike graphene, where tight-binding (TB) models accurately reproduce band edges near the $K$ and $K^{\prime}$ points in the Brillouin zone, a wider bandgap in $\it h$-BN necessitates capturing the band edges at both the $K$ and $M$ points for precise bandgap calculations. We present effective TB models derived from $\it ab initio$ calculations using maximally localized Wannier functions (MLWFs) centered on boron and nitrogen sites. These models consider hopping terms of up to four distant neighbors and achieve excellent agreement with $\it ab initio$ results near the $K$ and $M$ points. Furthermore, we compare the band structures from our simplified models with those obtained from $\it ab initio$ calculations and the full tight-binding model to assess their accuracy. To account for the effects of strains, we introduce fitting parametrizations that relate the hopping parameters of the effective TB model to the lattice constant and interlayer distance. Additionally, we utilize the two-center approximation to calculate the interlayer hopping energies based on the relative distances between sublattices to generalize the interlayer hopping parameters across different stacking configurations. We demonstrate the effectiveness of this method by comparing the electronic structure of zero-twist and twisted $\it h$-BN systems with $\it ab initio$ calculations.

cond-mat.mtrl-sci↗

arXiv2024

Interplay of valley, layer and band topology towards interacting quantum phases in moiré bilayer graphene

In Bernal-stacked bilayer graphene (BBG), the Landau levels give rise to an intimate connection between valley and layer degrees of freedom. Adding a moiré superlattice potential enriches the BBG physics with the formation of topological minibands - potentially leading to tunable exotic quantum transport. Here, we present magnetotransport measurements of a high-quality bilayer graphene-hexagonal boron nitride (hBN) heterostructure. The zero-degree alignment generates a strong moiré superlattice potential for the electrons in BBG and the resulting Landau fan diagram of longitudinal and Hall resistance displays a Hofstadter butterfly pattern with a high level of detail. We demonstrate that the intricate relationship between valley and layer degrees of freedom controls the topology of moiré-induced bands, significantly influencing the energetics of interacting quantum phases in the BBG superlattice. We further observe signatures of field-induced correlated insulators, helical edge states and clear quantizations of interaction-driven topological quantum phases, such as symmetry broken Chern insulators.

cond-mat.mes-hall↗

arXiv2024

Moiré flat bands and antiferroelectric domains in lattice relaxed twisted bilayer hexagonal boron nitride under perpendicular electric fields

Local interlayer charge polarization of twisted bilayer hexagonal boron nitride (t2BN) is calculated and parametrized as a function of twist angle and perpendicular electric fields through tight-binding calculations on lattice relaxed geometries Lattice relaxations tend to increase the bandwidth of the nearly flat bands, where widths smaller than 1 meV are expected for angle less than 1.08 degree for parallel BN/BN alignment, and for angle less than 1.5 degree for the antiparallel BN/NB alignment. Local interlayer charge polarization maxima of 2.6 pC/m corresponding are expected at the AB and BA stacking sites of BN/BN aligned t2BN in the long moire period limit for angle less than 1 degree, and evolves non-monotonically with a maximum of 3.5 pC/m at angle equal to 1.6 degree before reaching 2 pC/m for angle equal to 6 degree. The electrostatic potential maxima due to the t2BN are overall enhanced by 20 percentage with respect to the rigid system assuming potential modulation depths of up to 300 mV near its surface. In BN/BN aligned bilayers the relative areas of the AB or BA local stacking regions can be expanded or reduced through a vertical electric field depending on its sign.

cond-mat.mes-hall↗

arXiv2024

Observation of dichotomic field-tunable electronic structure in twisted monolayer-bilayer graphene

Twisted bilayer graphene (tBLG) provides a fascinating platform for engineering flat bands and inducing correlated phenomena. By designing the stacking architecture of graphene layers, twisted multilayer graphene can exhibit different symmetries with rich tunability. For example, in twisted monolayer-bilayer graphene (tMBG) which breaks the C2z symmetry, transport measurements reveal an asymmetric phase diagram under an out-of-plane electric field, exhibiting correlated insulating state and ferromagnetic state respectively when reversing the field direction. Revealing how the electronic structure evolves with electric field is critical for providing a better understanding of such asymmetric field-tunable properties. Here we report the experimental observation of field-tunable dichotomic electronic structure of tMBG by nanospot angle-resolved photoemission spectroscopy (NanoARPES) with operando gating. Interestingly, selective enhancement of the relative spectral weight contributions from monolayer and bilayer graphene is observed when switching the polarity of the bias voltage. Combining experimental results with theoretical calculations, the origin of such field-tunable electronic structure, resembling either tBLG or twisted double-bilayer graphene (tDBG), is attributed to the selectively enhanced contribution from different stacking graphene layers with a strong electron-hole asymmetry. Our work provides electronic structure insights for understanding the rich field-tunable physics of tMBG.

cond-mat.mes-hall↗

arXiv2024

Self-consistent tight-binding calculations with extended Hubbard interactions in rhombohedral multilayer graphene

We study the mean-field broken symmetry phases of charge neutral multilayer rhombohedral graphene within tight-binding approximations including self-consistent extended Hubbard interactions. We used on-site and inter-site Hubbard interactions obtained from a newly developed first-principles calculation method. Our calculations for systems up to eight layers give rise to electron-hole asymmetries, band flatness, band gaps, and layer anti-ferromagnetic ground states in keeping with available experiments. By including the intersite Hubbard interactions up to the next-nearest neighboring sites, the band gaps are shown to open when the number of layers is larger than three, while the trilayer system maintains its metallic nature with two low energy density of state peaks near the Fermi energy whose separation increases with the range of inter-site Hubbard parameters. Within our framework, the calculated band gaps reflect mean-field ground states with extended Hubbard interactions, in closer agreement with experimental estimates. The tight-binding formulation further enables efficient treatment of large rhombohedral chiral systems, including twisted multilayer graphene.

cond-mat.mes-hall↗

arXiv2023

Open-orbit induced low field extremely large magnetoresistance in graphene/h-BN superlattices

We report intriguing and hitherto overlooked low-field room temperature extremely large magnetoresistance (XMR) patterns in graphene/hexagonal boron nitride (h-BN) superlattices that emerge due to the existence of open orbits within each miniband. This finding is set against the backdrop of the experimental discovery of the Hofstadter butterfly in moir superlattices, which has sparked considerable interest in the fractal quantum Hall regime. To cope with the challenge of deciphering the low magnetic field dynamics of moir minibands, we utilize a novel semi-classical calculation method, grounded in zero-field Fermi contours, to predict the nontrivial behavior of the Landau-level spectrum. This is compared with fully quantum simulations, enabling an in-depth and contrasted analysis of transport measurements in high-quality graphene-hBN superlattices. Our results not only highlight the primary observation of the open-orbit induced XMR in this system but also shed new light on other intricate phenomena. These include the nuances of single miniband dynamics, evident through Lifshitz transitions, and the complex interplay of semiclassical and quantum effects between these minibands. Specifically, we document transport anomalies linked to trigonal warping, a semiclassical deviation from the expected linear characteristics of Landau levels, and magnetic breakdown phenomena indicative of quantum tunneling, all effects jointly contributing to the intricacies of a rich electronic landscape uncovered at low magnetic fields.

cond-mat.mes-hall↗

arXiv2023

Tuning electronic properties in transition metal dichalcogenides MX$_2$ (M= Mo/W, X= S/Se) heterobilayers with strain and twist angle

We explore the direct to indirect band gap transitions in MX$_2$ (M= Mo/W, X= S/Se) transition metal dichalcogenides heterobilayers for different system compositions, strains, and twist angles based on first principles density functional theory calculations within the G$_0$W$_0$ approximation. The obtained band gaps that typically range between 1.4$-$2.0 eV are direct/indirect for different/same chalcogen atom systems and can often be induced through expansive/compressive biaxial strains of a few percent. A direct to indirect gap transition is verified for heterobilayers upon application of a finite 16$^{\circ}$ twist that weakens interlayer coupling. The large inter-layer exciton binding energies of the order of $\sim$~250~meV estimated by solving the Bethe-Salpeter equation suggest these systems are amenable to be studied through infrared and Raman spectroscopy.

cond-mat.mtrl-sci↗