arXiv subjects

Guangyu Zhang

Publications and source records attributed to Guangyu Zhang.

At least 37 records · Page 2Linked to original sources

arXiv2024

Probing band topology in ABAB and ABBA stacked twisted double bilayer graphene

Twisted graphene moire superlattice has been demonstrated as an exotic platform for investigating correlated states and nontrivial topology. Among the moire family, twisted double bilayer graphene (TDBG) is a tunable flat band system expected to show stacking-dependent topological properties. However, electron correlations and the band topology are usually intertwined in the flat band limit, rendering the unique topological property due to stacking still elusive. Focusing on a large-angle TDBG with weak electron correlations, here we probe the Landau level (LL) spectra in two differently stacked TDBG, i.e. ABBA- and ABAB-TDBG, to unveil their distinct topological properties. For ABBA-TDBG, we observe non-trivial topology at zero electric displacement filed, evident from both the emergence of Chern bands from half fillings and the closure of gap at CNP above a critical magnetic field. For ABAB-TDBG, by contrast, we find that the moire band is topologically trivial, supported by the absence of LLs from half fillings and the persistence of the gap at CNP above the critical magnetic fields. In addition, we also observe an evolution of the trivial-to-nontrivial topological transition at finite D fields, confirmed by the emerged Landau fans originating from quarter filling v = 1. Our result demonstrates, for the first time, the unique stacking-dependent topology in TDBG, offering a promising avenue for future investigations on topological states in correlated systems.

cond-mat.mes-hall↗

arXiv2024

Observation of electrical high-harmonic generation

High-harmonic generation (HHG), an extreme nonlinear effect, introduces an unprecedented paradigm to detect emergent quantum phases and electron dynamics inconceivable in the framework of linear and low-order nonlinear processes. As an important manifestation, the optical HHG (o-HHG) enables extraordinary opportunities to underpin attosecond physics. In addition to nonlinear optics, emerging nonlinear electric transport has been demonstrated recently and opens new paradigms to probe quantum phase transition, symmetry breaking, band geometrical and topological properties. Thus far, only electrical second-/third-harmonic generation in perturbative regime has been elucidated, while the electrical HHG (e-HHG) that can advance to extreme non-perturbative physics remains elusive. Here we report the observation of e-HHG up to 300th-order. Remarkably, the e-HHG shows a clear non-perturbative character and exhibits periodic oscillations with the reciprocal of driving current. Further, theoretical simulations corroborate the experiments, suggesting the contribution of singular distribution of Berry curvature near band edges. Our results demonstrate e-HHG in extreme nonlinear regime and may shed light on a plethora of exotic physics and applications, such as extreme non-equilibrium quantum phenomena, ultra-fast and coherent electrical signal generations and detections.

cond-mat.mes-hall↗

arXiv2024

Correlated Charge Density Wave Insulators in Chirally Twisted Triple Bilayer Graphene

Electrons residing in flat-band system can play a vital role in triggering spectacular phenomenology due to relatively large interactions and spontaneous breaking of different degeneracies. In this work we demonstrate chirally twisted triple bilayer graphene, a new moiré structure formed by three pieces of helically stacked Bernal bilayer graphene, as a highly tunable flat-band system. In addition to the correlated insulators showing at integer moiré fillings, commonly attributed to interaction induced symmetry broken isospin flavors in graphene, we observe abundant insulating states at half-integer moiré fillings, suggesting a longer-range interaction and the formation of charge density wave insulators which spontaneously break the moiré translation symmetry. With weak out-of-plane magnetic field applied, as observed half-integer filling states are enhanced and more quarter-integer filling states appear, pointing towards further quadrupling moiré unit cells. The insulating states at fractional fillings combined with Hartree-Fock calculations demonstrate the observation of a new type of correlated charge density wave insulators in graphene and points to a new accessible twist manner engineering correlated moiré electronics.

cond-mat.mes-hall↗

arXiv2024

Sliding ferroelectric memories and synapses

Ferroelectric materials with switchable electric polarization hold great promise for a plethora of emergent applications, such as post-Moore's law nanoelectronics, beyond-Boltzmann transistors, non-volatile memories, and above-bandgap photovoltaic devices. Recent advances have uncovered an exotic sliding ferroelectric mechanism, which endows to design atomically thin ferroelectrics from non-ferroelectric parent monolayers. Although notable progress has been witnessed in understanding its fundamental properties, functional devices based on sliding ferroelectrics, the key touchstone toward applications, remain elusive. Here, we demonstrate the rewritable, non-volatile memory devices at room-temperature utilizing a two-dimensional (2D) sliding ferroelectric semiconductor of rhombohedral-stacked bilayer molybdenum disulfide. The 2D sliding ferroelectric memories (SFeMs) show superior performances with a large memory window of >8V, a high conductance ratio of above 106, a long retention time of >10 years, and a programming endurance greater than 104 cycles. Remarkably, flexible SFeMs are achieved with state-of-the-art performances competitive to their rigid counterparts and maintain their performances post bending over 103 cycles. Furthermore, synapse-specific Hebbian forms of plasticity and image recognition with a high accuracy of 97.81% are demonstrated based on flexible SFeMs. Our work demonstrates the sliding ferroelectric memories and synaptic plasticity on both rigid and flexible substrates, highlighting the great potential of sliding ferroelectrics for emerging technological applications in brain-inspired in-memory computing, edge intelligence and energy-efficient wearable electronics.

cond-mat.mes-hall↗

arXiv2024

Interplay of Landau quantization and interminivalley scatterings in a weakly coupled moiré superlattice

Double layer quantum systems are promising platforms for realizing novel quantum phases. Here, we report a study of quantum oscillations (QOs) in a weakly coupled double layer system, composed of a large angle twisted double bilayer graphene (TDBG). We observe two different QOs at low temperature, one with a periodicity in carrier density (n), i.e. Shubnikov de Haas oscillation (SdHO) due to Landau quantization, and the other one in displacement field (D), resulting a grid pattern. We quantify the interlayer coupling strength by measuring the interlayer capacitance from the grid pattern with a capacitance model, revealing an electron hole asymmetry. At high temperature when SdHO are thermal smeared, we observe resistance peaks when LLs from two minivalleys in the moiré Brillion zone are aligned, regardless of carrier density; eventually, it results in a two fold increase of oscillating frequency in D, serving as a smoking gun evidence of the magneto intersubband oscillations (MISO) in a double layer system. The temperature dependence of MISO suggests electron-electron interaction between two minivalleys play a crucial rule in the scattering, and the scattering times obtained from MISO thermal damping are found to be correlated with the interlayer coupling strength. Our study reveals an intriguing interplay among Landau quantization, moiré band structure, and scatterings.

cond-mat.mes-hall↗

arXiv2024

Robust Control of An Aerial Manipulator Based on A Variable Inertia Parameters Model

Aerial manipulator, which is composed of an UAV (Unmanned Aerial Vehicle) and a multi-link manipulator and can perform aerial manipulation, has shown great potential of applications. However, dynamic coupling between the UAV and the manipulator makes it difficult to control the aerial manipulator with high performance. In this paper, system modeling and control problem of the aerial manipulator are studied. Firstly, an UAV dynamic model is proposed with consideration of the dynamic coupling from an attached manipulator, which is treated as disturbance for the UAV. In the dynamic model, the disturbance is affected by the variable inertia parameters of the aerial manipulator system. Then, based on the proposed dynamic model, a disturbance compensation robust $H_{\infty}$ controller is designed to stabilize flight of the UAV while the manipulator is in operation. Finally, experiments are conducted and the experimental results demonstrate the feasibility and validity of the proposed control scheme.

cs.RO↗

arXiv2024

Guided Time-optimal Model Predictive Control of a Multi-rotor

Time-optimal control of a multi-rotor remains an open problem due to the under-actuation and nonlinearity of its dynamics, which make it difficult to solve this problem directly. In this paper, the time-optimal control problem of the multi-rotor is studied. Firstly, a thrust limit optimal decomposition method is proposed, which can reasonably decompose the limited thrust into three directions according to the current state and the target state. As a result, the thrust limit constraint is decomposed as a linear constraint. With the linear constraint and decoupled dynamics, a time-optimal guidance trajectory can be obtained. Then, a cost function is defined based on the time-optimal guidance trajectory, which has a quadratic form and can be used to evaluate the time-optimal performance of the system outputs. Finally, based on the cost function, the time-optimal control problem is reformulated as an MPC (Model Predictive Control) problem. The experimental results demonstrate the feasibility and validity of the proposed methods.

cs.RO↗

arXiv2024

An Aerial Manipulator for Robot-to-robot Torch Relay Task: System Design and Control Scheme

Torch relay is an important tradition of the Olympics and heralds the start of the Games. Robots applied in the torch relay activity can not only demonstrate the technological capability of humans to the world but also provide a sight of human lives with robots in the future. This article presents an aerial manipulator designed for the robot-to-robot torch relay task of the Beijing 2022 Winter Olympics. This aerial manipulator system is composed of a quadrotor, a 3 DoF (Degree of Freedom) manipulator, and a monocular camera. This article primarily describes the system design and system control scheme of the aerial manipulator. The experimental results demonstrate that it can complete robot-to-robot torch relay task under the guidance of vision in the ice and snow field.

cs.RO↗

arXiv2023

Room-temperature correlated states in twisted bilayer MoS$_2$

Moiré superlattices have emerged as an exciting condensed-matter quantum simulator for exploring the exotic physics of strong electronic correlations. Notable progress has been witnessed, but such correlated states are achievable usually at low temperatures. Here, we report the transport evidences of room-temperature correlated electronic states and layer-hybridized SU(4) Hubbard model simulator in AB-stacked MoS$_2$ homo-bilayer moiré superlattices. Correlated insulating states at moiré band filling factors v = 1, 2, 3 are unambiguously established in twisted bilayer MoS$_2$. Remarkably, the correlated electronic states can persist up to a record-high critical temperature of over 285 K. The realization of room-temperature correlated states in twisted bilayer MoS$_2$ can be understood as the cooperation effects of the stacking-specific atomic reconstruction and the resonantly enhanced interlayer hybridization, which largely amplify the moiré superlattice effects on electronic correlations. Furthermore, extreme large non-linear Hall responses up to room-temperature are uncovered near correlated insulating states, demonstrating the quantum geometry of moiré flat conduction band.

cond-mat.mtrl-sci↗

arXiv2023

Persistence of Monoclinic Crystal Structure in Three-Dimensional Second-Order Topological Insulator Candidate 1T'-MoTe2 Thin Flake without Structural Phase transition

A van der Waals material, MoTe2 with a monoclinic 1T' crystal structure is a candidate for three-dimensional (3D) second-order topological insulators (SOTIs) hosting gapless hinge states and insulating surface states. However, due to the temperature-induced structural phase transition, the monoclinic 1T' structure of MoTe2 would be transformed into the orthorhombic Td structure as the temperature is lowered, which hinders the experimental verification and the electronic applications of the predicted SOTI state at low temperatures. Here, we present systematic Raman spectroscopy studies of the exfoliated MoTe2 thin flakes with variable thicknesses at different temperatures. As a spectroscopic signature of the orthorhombic Td structure of MoTe2, the out-of-plane vibration mode D at ~ 125 cm-1 is always visible below a certain temperature in the multilayer flakes thicker than ~ 27.7 nm, but vanishes in the temperature range from 80 K to 320 K when the flake thickness becomes lower than ~ 19.5 nm. The absence of the out-of-plane vibration mode D in the Raman spectra here demonstrates not only the disappearance of the monoclinic-to-orthorhombic phase transition but also the persistence of the monoclinic 1T' structure in the MoTe2 thin flakes thinner than ~ 19.5 nm at low temperatures down to 80 K, which may be caused by the high enough density of the holes introduced during the gold-enhanced exfoliation process and exposure to air. The MoTe2 thin flakes with the low-temperature monoclinic 1T' structure provide a material platform for realizing SOTI states in van der Waals materials at low temperatures, which paves the way for developing a new generation of electronic devices based on SOTIs.

cond-mat.mtrl-sci↗

arXiv2023

Weyl phonons in chiral crystals

Chirality is an indispensable concept that pervades fundamental science and nature, manifesting itself in diverse forms such as chiral quasiparticles and chiral structures. Of particular interest are Weyl phonons carrying specific Chern numbers and chiral phonons doing circular motions in crystals. Up to now, Weyl and chiral phonons have been studied independently and the interpretations of chirality seem to be different in these two concepts, impeding our understanding. Here, we demonstrate that Weyl and chiral phonons are entangled in chiral crystals. Employing a typical chiral crystal of elementary tellurium (Te) as a case study, we expound on the intrinsic relationship between Chern number of Weyl phonons and pseudo-angular momentum (PAM) of chiral phonons. In light of the mutual coupling, we propose Raman scattering as a new technique to demonstrate the existence of Weyl phonons in Te, by detecting the chirality-induced energy splitting between the two constituent chiral phonon branches for Weyl phonons. By using the same experimental approach, we also observe the obstructed phonon surface states for the first time.

cond-mat.mtrl-sci↗

arXiv2023

Observation of first-order quantum phase transitions and ferromagnetism in twisted double bilayer graphene

Twisted graphene multilayers are highly tunable flatband systems for developing new phases of matter. Thus far, while orbital ferromagnetism has been observed in valley polarized phases, the long-range orders of other correlated phases as well as the quantum phase transitions between different orders mostly remain unknown. Here, we report an observation of Coulomb interaction driven first-order quantum phase transitions and ferromagnetism in twisted double bilayer graphene (TDBG). At zero magnetic field, the transitions are revealed in a series of step-like abrupt resistance jumps with prominent hysteresis loop when either the displacement field (D) or the carrier density (n) is tuned across symmetry-breaking boundary near half filling, indicating a formation of ordered domains. It is worth noting that the good turnability and switching of these states gives a rise to a memory performance with a large on/off ratio. Moreover, when both spin and valley play the roles at finite magnetic field, we observe abundant first-order quantum phase transitions among normal metallic states from charge neutral point, orbital ferromagnetic states from quarter filling, and spin-polarized states from half filling. We interpret these first-order phase transitions in the picture of phase separations and spin domain percolations driven by multi-field tunable Coulomb interactions, in agreement with Lifshitz transition from Hartree-Fock calculations. The observed multi-filed tunable domain structure and its hysteresis resembles the characteristics of multiferroics, revealing intriguing magnetoelectric properties. Our result enriches the correlated phase diagram in TDBG for discovering novel exotic phases and quantum phase transitions, and it would benefit other twisted moiré systems as well.

cond-mat.mes-hall↗

arXiv2023

Quantum oscillations in field-induced correlated insulators of a moiré superlattice

We report an observation of quantum oscillations (QOs) in the correlated insulators with valley anisotropy of twisted double bilayer graphene (TDBG). The anomalous QOs are best captured in the magneto resistivity oscillations of the insulators at v = -2, with a period of 1/B and an oscillation amplitude as high as 150 kΩ. The QOs can survive up to ~10 K, and above 12 K, the insulating behaviors are dominant. The QOs of the insulator are strongly D dependent: the carrier density extracted from the 1/B periodicity decreases almost linearly with D from -0.7 to -1.1 V/nm, suggesting a reduced Fermi surface; the effective mass from Lifshitz-Kosevich analysis depends nonlinearly on D, reaching a minimal value of 0.1 me at D = ~ -1.0 V/nm. Similar observations of QOs are also found at v = 2, as well as in other devices without graphite gate. We interpret the D sensitive QOs of the correlated insulators in the picture of band inversion. By reconstructing an inverted band model with the measured effective mass and Fermi surface, the density of state at the gap, calculated from thermal broadened Landau levels, agrees qualitatively with the observed QOs in the insulators. While more theoretical understandings are needed in the future to fully account for the anomalous QOs in this moiré system, our study suggests that TDBG is an excellent platform to discover exotic phases where correlation and topology are at play.

cond-mat.mes-hall↗

arXiv2023

Electron-infrared phonon coupling in ABC trilayer graphene

Stacking order plays a crucial role in determining the crystal symmetry and has significant impacts on electronic, optical, magnetic, and topological properties. Electron-phonon coupling, which is central to a wide range of intriguing quantum phenomena, is expected to be intricately connected with stacking order. Understanding the stacking order-dependent electron-phonon coupling is essential for understanding peculiar physical phenomena associated with electron-phonon coupling, such as superconductivity and charge density waves. In this study, we investigate the effect of stacking order on electron-infrared phonon coupling in graphene trilayers. By using gate-tunable Raman spectroscopy and excitation frequency-dependent near-field infrared nanoscopy, we show that rhombohedral ABC-stacked trilayer graphene has a significantly stronger electron-infrared phonon coupling strength than the Bernal ABA-stacked trilayer graphene. Our findings provide novel insights into the superconductivity and other fundamental physical properties of rhombohedral ABC-stacked trilayer graphene, and can enable nondestructive and high-throughput imaging of trilayer graphene stacking order using Raman scattering.

physics.optics↗

arXiv2022

Numerical investigation of azimuthal thermoacoustic instability in a gas turbine model combustor

Self-excited spinning mode azimuthal instability in an annular combustor with non-swirling flow is investigated using large eddy simulation (LES). Compressible Navier-Stoke equations are solved with a flamelet combustion model to describe the subgrid chemistry$-$turbulence interactions. Two flamelet models, with and without heat loss effects, are compared to elucidate the non-adiabatic wall effects on the thermoacoustic instability. The azimuthal modes are captured well by both models with only marginal differences in the computed frequencies and amplitudes. By comparing with the experimental measurements, the frequencies given by the LES are approximately 10\% higher and the amplitudes are well predicted. Further analysis of the experimental and LES data shows a similar dominant anti-clockwise spinning mode, under which a good agreement is observed for the phase-averaged heat release rate fluctuations. Dynamic mode decomposition (DMD) is applied to shed more light on this spinning mode. The LES and experimental DMD modes reconstructed for their azimuthal mode frequencies agree very well for the heat release fluctuations. The DMD mode structure for the acoustic pressure from the LES shows a considerable non-zero profile at the combustor outlet, which could be essential for azimuthal modes to establish in this annular combustor. Finally, a low-order modelling study was conducted using an acoustic network combined with the flame transfer function extracted from LES. The results show that the dominant mode is associated with the plenum showing a first longitudinal and azimuthal mixed mode structure. By tuning the plenum length to match the effective volume, the predicted frequency becomes very close to the measured value.

physics.flu-dyn↗

arXiv2022

Temperature-linear Resistivity in Twisted Double Bilayer Graphene

We report an experimental study of carrier density (n), displacement field (D) and twist angle (θ) dependence of temperature (T)-linear resistivity in twisted double bilayer graphene (TDBG). For a large twist angle (θ>1.5°) where correlated insulating states are absent, we observe a T-linear resistivity (with the slope of the order ~10Ω/K) over a wide range of carrier density and its slope decreases with increasing of n, in agreement with acoustic phonon scattering model semi-quantitatively. The slope of T-linear resistivity is non-monotonically dependent on the displacement field with a single peak structure. For device with θ~1.23° at which correlated states emerge, the slope of T-linear resistivity is found maximum (~100Ω/K) at the boundary of the halo structure where phase transition occurs, with signatures of continuous phase transition, Planckian dissipation, and the diverging effective mass; these observations are in line with quantum critical behaviors, which might be due to the symmetry-breaking instability at the critical points. Our results shed new light on correlated physics in TDBG and other twisted moiré systems.

cond-mat.mes-hall↗

arXiv2022

Hofstadter Butterfly in Graphene

This is a short review of the recent progresses on Hofstadter butterfly in graphene, organized in the following. We first briefly introduce the Hofstadter butterfly, including the theoretic toy model of Bloch electrons in the magnetic field, the resulting Harper's equation and its solution, and realizations in non-graphene systems. Then, we discuss the ways to realize the fractal Hofstadter spectra in graphene, and introduce three types of graphene superlattice structure, including graphene-hBN, twisted graphene layers, nanofabricated graphene superlattice. In particular, details about the fractal Hofstadter spectrum in graphene will be focused and discussed in the graphene-hBN superlattice and the twisted bilayer graphene.

cond-mat.mes-hall↗

arXiv2022

Layer-by-Layer Epitaxy of Multilayer MoS2 Wafers

Two-dimensional (2D) semiconductor of MoS2 has great potential for advanced electronics technologies beyond silicon1-9. So far, high-quality monolayer MoS2 wafers10-12 are already available and various demonstrations from individual transistors to integrated circuits have also been shown13-15. In addition to the monolayer, multilayers have narrower band gaps but improved carrier mobilities and current capacities over the monolayer5,16-18. However, achieving high-quality multilayer MoS2 wafers remains a challenge. Here we report the growth of high quality multilayer MoS2 4-inch wafers via the layer-by-layer epitaxy process. The epitaxy leads to well-defined stacking orders between adjacent epitaxial layers and offers a delicate control of layer numbers up to 6. Systematic evaluations on the atomic structures and electronic properties were carried out for achieved wafers with different layer numbers. Significant improvements on device performances were found in thicker-layer field effect transistors (FETs), as expected. For example, the average field-effect mobility (μFE) at room temperature (RT) can increase from ~80 cm2V-1s-1 for monolayer to ~110/145 cm2V-1s-1 for bilayer/trilayer devices. The highest RT μFE=234.7 cm2V-1s-1 and a record-high on-current densities of 1.704 mAμm-1 at Vds=2 V were also achieved in trilayer MoS2 FETs with a high on/off ratio exceeding 107. Our work hence moves a step closer to practical applications of 2D MoS2 in electronics.

physics.app-ph↗