arXiv subjects

Yiran Zhang

Publications and source records attributed to Yiran Zhang.

At least 91 records · Page 5Linked to original sources

arXiv2022

Spin-Orbit Enhanced Superconductivity in Bernal Bilayer Graphene

In the presence of a large perpendicular electric field, Bernal-stacked bilayer graphene (BLG) features several broken-symmetry metallic phases as well as magnetic-field-induced superconductivity. The superconducting state is quite fragile, however, appearing only in a narrow window of density and with a maximum critical temperature $T_c\approx30$~mK. Here, we show that placing monolayer tungsten diselenide (WSe$_{2}$) on BLG promotes Cooper pairing to an extraordinary degree: superconductivity appears at zero magnetic field, exhibits an order of magnitude enhancement in $T_c$, and occurs over a density range that is wider by a factor of eight. By mapping quantum oscillations in BLG-WSe$_2$ as a function of electric field and doping, we establish that superconductivity emerges throughout a region whose normal state is polarized, with two out of four spin-valley flavours predominantly populated. In-plane magnetic field measurements further reveal a striking dependence of the critical field on doping, with the Chandrasekhar-Clogston (Pauli) limit roughly obeyed on one end of the superconducting dome yet sharply violated on the other. Moreover, the superconductivity arises only for perpendicular electric fields that push BLG hole wavefunctions towards WSe$_2$ -- suggesting that proximity-induced (Ising) spin-orbit coupling plays a key role in enhancing the pairing. Our results pave the way for engineering robust, highly tunable, and ultra-clean graphene-based superconductors.

cond-mat.supr-con↗

arXiv2022

Power-law Spectrum of Energetic Particles in Classical Thermal Equilibrium by Pitch-angle Scattering Process

The Boltzmann-Gibbs thermodynamic equilibrium state of charged particles pitch-angle scattered by weak plasma waves is discussed. Degrees of freedom of these waves play a fundamental role in constructing the grand canonical ensemble. Via the gyro-resonance condition, fast particles have an inverse break power-law spectrum for $ \varepsilon -μ\ll T $, where $ \varepsilon $ is the particle energy, $ μ$ is the chemical potential, $ T $ is the temperature. The break energies are the rest energy and $ -μ$. For $ \varepsilon \ll -μ\ll T $, the energy spectral index $ α$ is $ δ/2+1 $ and $ δ+1 $ for non- and ultra-relativistic particles, respectively, with $ δ$ an effective fractal dimension of background magnetic field lines. The spectral index for $ -μ\ll \varepsilon \ll T $ is $ α+1 $. This thermal equilibrium scenario, combined with the leaky-box model and cosmic-ray observations, seems to suggest that the Galactic magnetic field is super-diffusive with $ δ\approx 1.4 $.

astro-ph.HE↗

arXiv2022

A Three-component Model for Cosmic-ray Spectrum and Dipole Anisotropy

Using a three-component, multi-scale diffusion model, we show that the cosmic-ray (CR) proton and helium spectra and the dipole anisotropy can be explained with reasonable parameters. The model includes a nearby source associated with the supernova remnant (SNR) that gave rise to the Geminga pulsar, a source at the Galactic center, and a component associated with the Galactic disk. The CR flux below TeV is dominated by the disk component. The center source with a continuous injection of CRs starting about 18 Myr ago is needed to explain the anisotropy above 100 TeV. With the assumption of universal CR spectra injected by all SNRs, the nearby source can produce a TeV spectral bump observed at Earth via slow diffusion across the interstellar magnetic field, which needs to have an angle $ θ\approx 5^{\circ} $ between the field line and the line of sight toward the source, and have weak magnetic turbulence with the Alfvén Mach number $ M_{\text{A}}\approx 0.1 $. Considering the modulation of the Galactic-scale anisotropy by this magnetic field, in a quasi-local approach the field may be directed at a right ascension about $ -90^{\circ} $ and a declination about $ -7.4^{\circ} $ in the equatorial coordinate system.

astro-ph.HE↗

arXiv2022

Brain-Inspired Modelling and Decision-making for Human-Like Autonomous Driving in Mixed Traffic Environment

In this paper, a human-like driving framework is designed for autonomous vehicles (AVs), which aims to make AVs better integrate into the transportation ecology of human driving and eliminate the misunderstanding and incompatibility of human drivers to autonomous driving. Based on the analysis of the real world INTERACTION dataset, a driving aggressiveness estimation model is established with the fuzzy inference approach. Then, a human-like driving model, which integrates the brain emotional learning circuit model (BELCM) with the two-point preview model, is designed. In the human-like lane-change decision-making algorithm, the cost function is designed comprehensively considering driving safety and travel efficiency. Based on the cost function and multi-constraint, the dynamic game algorithm is applied to modelling the interaction and decision making between AV and human driver. Additionally, to guarantee the lane-change safety of AVs, an artificial potential field model is built for collision risk assessment. Finally, the proposed algorithm is evaluated through human-in-the-loop experiments on a driving simulator, and the results demonstrated the feasibility and effectiveness of the proposed method.

cs.RO↗

arXiv2021

Ascendance of Superconductivity in Magic-Angle Graphene Multilayers

Graphene moire superlattices have emerged as a platform hosting and abundance of correlated insulating, topological, and superconducting phases. While the origins of strong correlations and non-trivial topology are shown to be directly linked to flat moire bands, the nature and mechanism of superconductivity remain enigmatic. In particular, only alternating twisted stacking geometries of bilayer and trilayer graphene are found to exhibit robust superconductivity manifesting as zero resistance and Fraunhofer interference patterns. Here we demonstrate that magic-angle twisted tri-, quadri-, and pentalayers placed on monolayer tungsten diselenide exhibit flavour polarization and superconductivity. We also observe insulating states in the trilayer and quadrilayer arising at finite electric displacement fields, despite the presence of dispersive bands introduced by additional graphene layers. Moreover, the three multilayer geometries allow us to identify universal features in the family of graphene moire structures arising from the intricate relations between superconducting states, symmetry-breaking transitions, and van Hove singularities. Remarkably, as the number of layers increases, superconductivity emerges over a dramatically enhanced filling-factor range. In particular, in twisted pentalayers, superconductivity extends well beyond the filling of four electrons per moire unit cell, demonstrating the non-trivial role of the additional bands. Our results highlight the importance of the interplay between flat and dispersive bands in extending superconducting regions in graphene moire superlattices and open new frontiers for developing graphene-based superconductors.

cond-mat.supr-con↗

arXiv2021

Spectroscopic Signatures of Strong Correlations and Unconventional Superconductivity in Twisted Trilayer Graphene

Magic-angle twisted trilayer graphene (MATTG) has emerged as a novel moiré material that exhibits both strong electronic correlations and unconventional superconductivity. However, spectroscopic studies of its electronic properties are lacking, and the nature of superconductivity and the corresponding order parameter in this system remain elusive. Here we perform high-resolution scanning tunneling microscopy and spectroscopy of MATTG and reveal extensive regions of atomic reconstruction that favor mirror-symmetric stacking. In these regions, we observe a cascade of symmetry-breaking electronic transitions and doping-dependent band structure deformations similar to those realized in magic-angle bilayers, as expected theoretically given the commonality of flat bands. More strikingly, in a density window spanning two to three holes per moire unit cell, spectroscopic signatures of superconductivity are manifest as pronounced dips in the tunneling conductance at the Fermi level accompanied by coherence peaks that become gradually suppressed at elevated temperatures and magnetic fields. The observed evolution of the conductance with doping is consistent with a gate-tunable transition from a gapped to a nodal superconductor, which we show theoretically is compatible with a sharp transition from a Bardeen-Cooper-Schrieffer (BCS) to a Bose-Einstein condensation (BEC) superconductor with a nodal order parameter. Within this doping window, we also detect peak-dip-hump structures suggesting that superconductivity is driven by strong coupling to bosonic modes of MATTG. Our results pave the way for further understanding of superconductivity and correlated states in graphene-based moiré structures beyond twisted bilayers, where unconventional superconductivity and nodal pairing were reported.

cond-mat.mes-hall↗

arXiv2021

New Nonexistence Results on Circulant Weighing Matrices

A circulant weighing matrix $W = (w_{i,j})$ is a square matrix of order $n$ and entries $w_{i,j}$ in $\{0, \pm 1\}$ such that $WW^T=kI_n$. In his thesis, Strassler gave a table of existence results for such matrices with $n \leq 200$ and $k \leq 100$. In the latest version of Strassler's table given by Tan \cite{arXiv:1610.01914} there are 34 open cases remaining. In this paper we give nonexistence proofs for 12 of these cases, report on preliminary searches outside Strassler's table, and characterize the known proper circulant weighing matrices.

math.CO↗

arXiv2021

A Marker-free Head Tracker Using Vision-based Head Pose Estimation with Adaptive Kalman Filter

The immersion and the interaction are the important features of the driving simulator. To improve these characteristics, this paper proposes a low-cost and mark-less driver head tracking framework based on the head pose estimation model, which makes the view of the simulator can automatically align with the driver's head pose. The proposed method only uses the RGB camera without the other hardware or marker. To handle the error of the head pose estimation model, this paper proposes an adaptive Kalman Filter. By analyzing the error distribution of the estimation model and user experience, the proposed Kalman Filter includes the adaptive observation noise coefficient and loop closure module, which can adaptive moderate the smoothness of the curve and keep the curve stable near the initial position. The experiments show that the proposed method is feasible, and it can be used with different head pose estimation models.

eess.IV↗

arXiv2021

Interaction-driven Band Flattening and Correlated Phases in Twisted Bilayer Graphene

Flat electronic bands, characteristic of magic-angle twisted bilayer graphene (TBG), host a wealth of correlated phenomena. Early theoretical considerations suggested that, at the magic angle, the Dirac velocity vanishes and the entire width of the moiré bands becomes extremely narrow. Yet, this scenario contradicts experimental studies that reveal a finite Dirac velocity as well as bandwidths significantly larger than predicted. Here we use spatially resolved spectroscopy in finite and zero magnetic fields to examine the electronic structure of moiré bands and their intricate connection to correlated phases. By following the relative shifts of Landau levels in finite fields, we detect filling-dependent band flattening, that unexpectedly starts already at ~1.3 degrees, well above the magic angle and hence nominally in the weakly correlated regime. We further show that, as the twist angle is reduced, the moiré bands become maximally flat at progressively lower doping levels. Surprisingly, when the twist angles reach values for which the maximal flattening occurs at approximate filling of $-2$, $+1$,$+2$,$+3$ electrons per moiré unit cell, the corresponding zero-field correlated phases start to emerge. Our observations are corroborated by calculations that incorporate an interplay between the Coulomb charging energy and exchange interactions; together these effects produce band flattening and hence a significant density-of-states enhancement that facilitates the observed symmetry-breaking cascade transitions. Besides emerging phases pinned to integer fillings, we also experimentally identify a series of pronounced correlation-driven band deformations and soft gaps in a wider doping range around $\pm 2$ filling where superconductivity is expected. Our results highlight the role of interaction-driven band-flattening in forming robust correlated phases in TBG.

cond-mat.str-el↗

arXiv2020

Tracing out Correlated Chern Insulators in Magic Angle Twisted Bilayer Graphene

Magic-angle twisted bilayer graphene (MATBG) exhibits a range of correlated phenomena that originate from strong electron-electron interactions. These interactions make the Fermi surface highly susceptible to reconstruction when $ \pm 1, \pm 2, \pm 3$ electrons occupy each moir\' e unit cell and lead to the formation of correlated insulating, superconducting and ferromagnetic phases. While some phases have been shown to carry a non-zero Chern number, the local microscopic properties and topological character of many other phases remain elusive. Here we introduce a set of novel techniques hinging on scanning tunneling microscopy (STM) to map out topological phases in MATBG that emerge in finite magnetic field. By following the evolution of the local density of states (LDOS) at the Fermi level with electrostatic doping and magnetic field, we visualize a local Landau fan diagram that enables us to directly assign Chern numbers to all observed phases. We uncover the existence of six topological phases emanating from integer fillings in finite fields and whose origin relates to a cascade of symmetry-breaking transitions driven by correlations. The spatially resolved and electron-density-tuned LDOS maps further reveal that these topological phases can form only in a small range of twist angles around the magic-angle value. Both the microscopic origin and extreme sensitivity to twist angle differentiate these topological phases from the Landau levels observed near charge neutrality. Moreover, we observe that even the charge-neutrality Landau spectrum taken at low fields is considerably modified by interactions and exhibits an unexpected splitting between zero Landau levels that can be as large as ${\sim }\,3-5$ meV. Our results show how strong electronic interactions affect the band structure of MATBG and lead to the formation of correlation-enabled topological phases.

cond-mat.str-el↗

arXiv2020

Superconductivity without insulating states in twisted bilayer graphene stabilized by monolayer WSe$_2$

Magic-angle twisted bilayer graphene (TBG), with rotational misalignment close to 1.1$^\circ$, features isolated flat electronic bands that host a rich phase diagram of correlated insulating, superconducting, ferromagnetic, and topological phases. The origins of the correlated insulators and superconductivity, and the interplay between them, are particularly elusive. Both states have been previously observed only for angles within $\pm0.1^\circ$ from the magic-angle value and occur in adjacent or overlapping electron density ranges; nevertheless, it is still unclear how the two states are related. Beyond the twist angle and strain, the dependence of the TBG phase diagram on the alignment and thickness of insulating hexagonal boron nitride (hBN) used to encapsulate the graphene sheets indicates the importance of the microscopic dielectric environment. Here we show that adding an insulating tungsten-diselenide (WSe$_2$) monolayer between hBN and TBG stabilizes superconductivity at twist angles much smaller than the established magic-angle value. For the smallest angle of $θ$ = 0.79$^\circ$, we still observe clear superconducting signatures, despite the complete absence of the correlated insulating states and vanishing gaps between the dispersive and flat bands. These observations demonstrate that, even though electron correlations may be important, superconductivity in TBG can exist even when TBG exhibits metallic behaviour across the whole range of electron density. Finite-magnetic-field measurements further reveal breaking of the four-fold spin-valley symmetry in the system, consistent with large spin-orbit coupling induced in TBG via proximity to WSe$_2$. Our results highlight the importance of symmetry breaking effects in stabilizing electronic states in TBG and open new avenues for engineering quantum phases in moiré systems.

cond-mat.supr-con↗

arXiv2020

Self-similar Blast Wave for A Two-component Fluid with Variable Adiabatic Index

We propose a self-similar (SS) solution to hydrodynamic non-relativistic flow behind a spherical strong blast wave (BW) passing through a homogeneous plasma with efficient relativistic particle acceleration at the shock front. The flow is described by an ideal two-fluid model with a relativistic component so that the post-shock gas has an effective SS adiabatic index $ γ$ varying from $ 5/3 $ to $ 4/3 $. This solution is calculated numerically and compared with the standard Sedov solution. We find that the BW center in our solution is dominated by the relativistic component with $ γ=4/3 $ for the divergence of expansion there, and the relativistic component dominates the interior for a moderate acceleration efficiency at the shock front. The overall efficiency of relativistic particle acceleration can be enhanced by a factor of $ 2 $ due to the slower adiabatic energy loss rate of the relativistic component during expansion. Tendency of the dominance by the relativistic component may be common in expanding astrophysical two-fluid systems such as supernova remnants, lobes of radio galaxies.

astro-ph.HE↗

arXiv2019

Imaging Electronic Correlations in Twisted Bilayer Graphene near the Magic Angle

Twisted bilayer graphene with a twist angle of around 1.1° features a pair of isolated flat electronic bands and forms a strongly correlated electronic platform. Here, we use scanning tunneling microscopy to probe local properties of highly tunable twisted bilayer graphene devices and show that the flat bands strongly deform when aligned with the Fermi level. At half filling of the bands, we observe the development of gaps originating from correlated insulating states. Near charge neutrality, we find a previously unidentified correlated regime featuring a substantially enhanced flat band splitting that we describe within a microscopic model predicting a strong tendency towards nematic ordering. Our results provide insights into symmetry breaking correlation effects and highlight the importance of electronic interactions for all filling factors in twisted bilayer graphene.

cond-mat.mes-hall↗

arXiv2018

Global Constraints on Diffusive Particle Acceleration by Strong Nonrelativistic Shocks

Estimating the cosmic-ray acceleration efficiency $ ε$ in supernova remnants (SNRs) through observations is a challenging task in general. Based on the Rankine-Hugoniot shock conditions, we find an anticorrelation between $ ε$ and the power-law spectral index $ α$ of relativistic particle distribution produced via diffusive particle acceleration by nonrelativistic shocks, implying more efficient acceleration in older SNRs with harder radio spectra. Then $ ε$ may be estimated from some hard radio spectral index measurements. Assuming the particle distribution in downstream of strong shocks to be a nonrelativistic Maxwellian plus a relativistic power law with a high-energy cutoff, we also find that the injection rate for relativistic particles $ η$ needs to $ \gtrsim 10^{-6} $ for a prominent decrease of the adiabatic index in SNRs, which implies higher compression ratio and lower values of $ α$. This threshold of $ η$ increases with the shock speed $ u_1 $, which may explain the relatively harder radio spectra of older SNRs with lower $ u_1 $. We show that $ η$ and/or the relativistic cutoff momentum $ p_\text{m} $ need to be low for old SNRs, and expect a gradual increase of $ ε$ as SNR evolves with gradually decreasing $ η$ and $ p_\text{m} $.

astro-ph.HE↗

arXiv2018

How Cox models react to a study-specific confounder in a patient-level pooled dataset: Random-effects better cope with an imbalanced covariate across trials unless baseline hazards differ

Combining patient-level data from clinical trials can connect rare phenomena with clinical endpoints, but statistical techniques applied to a single trial may become problematical when trials are pooled. Estimating the hazard of a binary variable unevenly distributed across trials showcases a common pooled database issue. We studied how an unevenly distributed binary variable can compromise the integrity of fixed and random effects Cox proportional hazards models. We compared fixed effect and random effects Cox proportional hazards models on a set of simulated datasets inspired by a 17-trial pooled database of patients presenting with ST-segment elevation myocardial infarction (STEMI) and non-STEMI undergoing percutaneous coronary intervention. An unevenly distributed covariate can bias hazard ratio estimates, inflate standard errors, raise type I error, and reduce power. While uneveness causes problems for all Cox proportional hazards models, random effects suffer least. Compared to fixed effect models, random effects suffer lower bias and trade inflated type I errors for improved power. Contrasting hazard rates between trials prevent accurate estimates from both fixed and random effects models. When modeling a covariate unevenly distributed across pooled trials with similar baseline hazard rates, Cox proportional hazards models with a random trial effect more accurately estimate hazard ratios than fixed effects. Differing between-trial baseline hazard rates bias both random and fixed effect models. With an unevenly-distributed covariate and similar baseline hazard rates across trials, a random effects Cox proportional hazards model outperforms a fixed effect model, but cannot overcome contrasting baseline hazard rates.

stat.AP↗

arXiv2018

Crossing points in survival analysis sensitively depend on system conditions

Crossing survival curves complicate how we interpret results from a clinical trial's primary endpoint. We find the function to determine a crossing point's location depends exponentially on individual survival curves. This exponential relationship between survival curves and the crossing point transforms small survival curve errors into large crossing point errors. In most cases, crossing points are sensitive to individual survival errors and may make accurately locating a crossing point unsuccessful. We argue more complicated analyses for mitigating crossing points should be reserved only after first exploring a crossing point's variability, or hypothesis tests account for crossing point variability.

stat.AP↗

arXiv2018

Assimilated LVEF: A Bayesian technique combining human intuition with machine measurement for sharper estimates of left ventricular ejection fraction and stronger association with outcomes

The cardiologist's main tool for measuring systolic heart failure is left ventricular ejection fraction (LVEF). Trained cardiologist's report both a visual and machine-guided measurement of LVEF, but only use this machine-guided measurement in analysis. We use a Bayesian technique to combine visual and machine-guided estimates from the PARTNER-IIA Trial, a cohort of patients with aortic stenosis at moderate risk treated with bioprosthetic aortic valves, and find our combined estimate reduces measurement errors and improves the association between LVEF and a 1-year composite endpoint.

stat.AP↗

arXiv2018

Band dependence of charge density wave in quasi-one-dimensional Ta2NiSe7 probed by orbital magnetoresistance

Ta2NiSe7 is a quasi-one-dimensional (quasi-1D) transition-metal chalcogenide with Ta and Ni chain structure. An incommensurate charge-density wave (CDW) in this quasi-1D structure was well studied previously using tunnelling spectrum, X-ray and electron diffraction, whereas its transport property and the relation to the underlying electronic states remain to be explored. Here we report our results of magnetoresistance (MR) on Ta2NiSe7. A breakdown of the Kohler's rule is found upon entering the CDW state. Concomitantly, a clear change of curvature in the field dependence of MR is observed. We show that the curvature change is well described by two-band orbital MR, with the hole density being strongly suppressed in the CDW state, indicating that the $p$ orbitals from Se atoms dominate the change in transport through the CDW transition.

cond-mat.mtrl-sci↗