arXiv subjects

Youngju Park

Publications and source records attributed to Youngju Park.

15 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.

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arXiv2026

Commensuration torques in double-moiré twisted trilayer hexagonal boron nitride and graphene heterostructures

We study commensuration-driven torques and angle locking in double-moiré trilayer hexagonal boron nitride (hBN) and graphene heterostructures using large-scale atomistic relaxations. In twisted trilayer hBN (t3BN) homostructures, double-moiré commensuration ($θ_{12} = -θ_{23}$) 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-moiré 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-moiré 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.

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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 moiré 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 $60^{\circ}$.

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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.

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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.

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arXiv2023

Chirality and correlations in the spontaneous spin-valley polarization of rhombohedral multilayer graphene

We investigate the total energies of spontaneous spin-valley polarized states in bi-, tri-, and tetralayer rhombohedral graphene where the long-range Coulomb correlations are accounted for within the random phase approximation. Our analysis of the phase diagrams for varying carrier doping and perpendicular electric fields shows that the exchange interaction between chiral electrons is the main driver of spin-valley polarization, while the presence of Coulomb correlations brings the flavor polarization phase boundaries to carrier densities close to the complete filling of the Mexican hat shape top at the Dirac points. We find that the tendency towards spontaneous spin-valley polarization is enhanced with the chirality of the bands and therefore with increasing number of layers.

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arXiv2023

Interaction-driven spontaneous broken-symmetry insulator and metals in ABCA tetralayer graphene

Interactions among charge carriers in graphene can lead to the spontaneous breaking of multiple degeneracies. When increasing the number of graphene layers following rhombohedral stacking, the dominant role of Coulomb interactions becomes pronounced due to the significant reduction in kinetic energy. In this study, we employ phonon-polariton assisted near-field infrared imaging to determine the stacking orders of tetralayer graphene devices. Through quantum transport measurements, we observe a range of spontaneous broken-symmetry states and their transitions, which can be finely tuned by carrier density n and electric displacement field D. Specifically, we observe a layer antiferromagnetic insulator at n = D = 0 with a gap of approximately 15 meV. Increasing D allows for a continuous phase transition from a layer antiferromagnetic insulator to a layer polarized insulator. By simultaneously tuning n and D, we observe isospin polarized metals, including spin-valley-polarized and spin-polarized metals. These transitions are associated with changes in Fermi surface topology and are consistent with the Stoner criteria. Our findings highlight the efficient fabrication of specially stacked multilayer graphene devices and demonstrate that crystalline multilayer graphene is an ideal platform for investigating a wide range of broken symmetries driven by Coulomb interactions.

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arXiv2023

Topological flat bands in rhombohedral tetralayer and multilayer graphene on hexagonal boron nitride moire superlattices

We show that rhombohedral four-layer graphene (4LG) nearly aligned with a hexagonal boron nitride (hBN) substrate often develops nearly flat isolated low energy bands with non-zero valley Chern numbers. The bandwidths of the isolated flatbands are controllable through an electric field and twist angle, becoming as narrow as $\sim10~$meV for interlayer potential differences between top and bottom layers of $|Δ|\approx 10\sim15~$meV and $θ\sim 0.5^{\circ}$ at the graphene and boron nitride interface. The local density of states (LDOS) analysis shows that the nearly flat band states are associated to the non-dimer low energy sublattice sites at the top or bottom graphene layers and their degree of localization in the moire superlattice is strongly gate tunable, exhibiting at times large delocalization despite of the narrow bandwidth. We verified that the first valence bands' valley Chern numbers are $C^{ν=\pm1}_{V1} = \pm n$, proportional to layer number for $n$LG/BN systems up to $n = 8$ rhombohedral multilayers.

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arXiv2023

Commensuration torques and lubricity in double moire systems

We study the commensuration torques and layer sliding energetics of alternating twist trilayer graphene (t3G) and twisted bilayer graphene on hexagonal boron nitride (t2G/BN) that have two superposed moire interfaces. Lattice relaxations for typical graphene twist angles of $\sim 1^{\circ}$ in t3G or t2G/BN are found to break the out-of-plane layer mirror symmetry, give rise to layer rotation energy local minima dips of the order of $\sim 10^{-1}$ meV/atom at double moire alignment angles, and have sliding energy landscape minima between top-bottom layers of comparable magnitude. Moire superlubricity is restored for twist angles as small as $\sim 0.03^\circ$ away from alignment resulting in suppression of sliding energies by several orders of magnitude of typically $\sim 10^{-4}$ meV/atom, hence indicating the precedence of rotation over sliding in the double moire commensuration process.

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arXiv2022

Spin-orbit coupling-enhanced valley ordering of malleable bands in twisted bilayer graphene on WSe2

New phases of matter can be stabilized by a combination of diverging electronic density of states, strong interactions, and spin-orbit coupling. Recent experiments in magic-angle twisted bilayer graphene (TBG) have uncovered a wealth of novel phases as a result of interaction-driven spin-valley flavour polarization. In this work, we explore correlated phases appearing due to the combined effect of spin-orbit coupling-enhanced valley polarization and large density of states below half filling ($ν\lesssim 2$) of the moiré band in a TBG coupled to tungsten diselenide. We observe anomalous Hall effect, accompanied by a series of Lifshitz transitions, that are highly tunable with carrier density and magnetic field. Strikingly, the magnetization shows an abrupt sign change in the vicinity of half-filling, confirming its orbital nature. The coercive fields reported are about an order of magnitude higher than previous studies in graphene-based moiré systems, presumably aided by a Stoner instability favoured by the van Hove singularities in the malleable bands. While the Hall resistance is not quantized at zero magnetic fields, indicative of a ground state with partial valley polarization, perfect quantization and complete valley polarization are observed at finite fields. Our findings illustrate that singularities in the flat bands in the presence of spin-orbit coupling can stabilize ordered phases even at non-integer moiré band fillings.

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arXiv2022

Electronic structure of lattice relaxed alternating twist tNG-multilayer graphene: from few layers to bulk AT-graphite

We calculate the electronic structure of AA'AA'...-stacked alternating twist N-layer (tNG) graphene for N = 3, 4, 5, 6, 8, 10, 20 layers and bulk alternating twist (AT) graphite systems where the lattice relaxations are modeled by means of molecular dynamics simulations. We show that the symmetric AA'AA'... stacking is energetically preferred among all interlayer sliding geometries for progressively added layers up to N=6. Lattice relaxations enhance electron-hole asymmetry, and reduce the magic angles with respect to calculations with fixed tunneling strengths that we quantify from few layers to bulk AT-graphite. Without a perpendicular electric field, the largest magic angle flat-band states locate around the middle following the largest eigenvalue eigenstate in a 1D-chain model of layers, while the density redistributes to outer layers for smaller magic twist angles corresponding to higher order effective bilayers in the 1D chain. A perpendicular electric field decouples the electronic structure into $N$ Dirac bands with renormalized Fermi velocities with distinct even-odd band splitting behaviors, showing a gap for N=4 while for odd layers a Dirac cone remains between the flat band gaps. The magic angle error tolerance estimated from density of states maxima expand progressively from $0.05^{\circ}$ in t2G to up to $0.2^{\circ}$ in AT-graphite, hence allowing a greater flexibility in multilayers. Decoupling of tNG into t2G with different interlayer tunneling proportional to the eigenvalues of a 1D layers chain allows to map tNG-multilayers bands onto those of periodic bulk AT-graphite's at different $k_z$ values. We also obtain the Landau level density of states in the quantum Hall regime for magnetic fields of up to 50~T and confirm the presence of nearly flat bands around which we can develop suppressed density of states gap regions by applying an electric field in N > 3 systems.

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arXiv2021

Topological phases in N-layer ABC-graphene boron-nitride moire superlattices

Rhombohedral $N = 3$ trilayer graphene on hexagonal boron nitride (TLG/BN) hosts gate-tunable, valley-contrasting, nearly flat topological bands that can trigger spontaneous quantum Hall phases under appropriate conditions of the valley and spin polarization. Recent experiments have shown signatures of C = 2 valley Chern bands at 1/4 hole filling, in contrast to the predicted value of C = 3. We discuss the low-energy model for rhombohedral N-layer graphene (N = 1, 2, 3) aligned with hexagonal boron nitride (hBN) subject to off-diagonal moire vector potential terms that can alter the valley Chern numbers. Our analysis suggests that topological phase transitions of the flat bands can be triggered by pseudomagnetic vector field potentials associated to moire strain patterns, and that a nematic order with broken rotational symmetry can lead to valley Chern numbers that are in agreement with recent Hall conductivity observations.

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arXiv2020

Electron-hole asymmetry and band gaps of commensurate double moire patterns in twisted bilayer graphene on hexagonal boron nitride

Spontaneous orbital magnetism observed in twisted bilayer graphene (tBG) on nearly aligned hexagonal boron nitride (BN) substrate builds on top of the electronic structure resulting from combined G/G and G/BN double moire interfaces. Here we show that tBG/BN commensurate double moire patterns can be classified into two types, each favoring the narrowing of either the conduction or valence bands on average, and obtain the evolution of the bands as a function of the interlayer sliding vectors and electric fields. Finite valley Chern numbers $\pm 1$ are found in a wide range of parameter space when the moire bands are isolated through gaps, while the local density of states associated to the flat bands are weakly affected by the BN substrate invariably concentrating around the AA-stacked regions of tBG. We illustrate the impact of the BN substrate for a particularly pronounced electron-hole asymmetric band structure by calculating the optical conductivities of twisted bilayer graphene near the magic angle as a function of carrier density. The band structures corresponding to other $N$-multiple commensurate moire period ratios indicate it is possible to achieve narrow width $W \lesssim 30$ meV isolated folded band bundles for tBG angles $θ\lesssim 1^{\circ}$.

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arXiv2020

Gate tunable topological flat bands in twisted monolayer-bilayer graphene

We investigate the band structure of twisted monolayer-bilayer graphene (tMBG), or twisted graphene on bilayer graphene (tGBG), as a function of twist angles and perpendicular electric fields in search of optimum conditions for achieving isolated nearly flat bands. Narrow bandwidths comparable or smaller than the effective Coulomb energies satisfying $U_{\textrm{eff}} /W \gtrsim 1$ are expected for twist angles in the range of $0.3^{\circ} \sim 1.5^{\circ}$, more specifically in islands around $θ\sim 0.5^{\circ}, \, 0.85^{\circ}, \,1.3^{\circ}$ for appropriate perpendicular electric field magnitudes and directions. The valley Chern numbers of the electron-hole asymmetric bands depend intrinsically on the details of the hopping terms in the bilayer graphene, and extrinsically on factors like electric fields or average staggered potentials in the graphene layer aligned with the contacting hexagonal boron nitride substrate. This tunability of the band isolation, bandwidth, and valley Chern numbers makes of tMBG a more versatile system than twisted bilayer graphene for finding nearly flat bands prone to strong correlations.

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arXiv2017

Electronic and magnetic properties of single-layer MPX$_3$ metal phosphorous trichalcogenides

We systematically investigate the electronic structure and magnetic properties of two dimensional (2D) MPX$_3$ (M= V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and X = S, Se, Te) transition metal chacogenophosphates to examine their potential role as single-layer van der Waals materials that exhibit magnetic order. Our {\em ab initio} calculations predict that most of these single-layer materials are antiferromagnetic semiconductors. The band gaps of the antiferromagnetic states decrease as the atomic number of the chalcogen atom increases (from S to Se, Te), leading in some cases to half-metallic ferromagnetic states or to non-magnetic metallic states. We find that the phase transition from antiferromagnetic semiconductor to ferromagnetic half-metal can be controlled by gating or by strain engineering. The sensitive interdependence we find between magnetic, structural, and electronic properties establishes the potential of this 2D materials class for applications in spintronics.

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