arXiv subjects

Fangyuan Yang

Publications and source records attributed to Fangyuan Yang.

13 recordsLinked to original sources

arXiv2025

Entropy of strongly correlated electrons in a partially filled Landau level

We use high-resolution chemical potential measurements to extract the entropy of monolayer and bilayer graphene in the quantum Hall regime via the Maxwell relation $\left.\frac{dμ}{dT}\right|_N = -\left.\frac{dS}{dN}\right|_T$. Measuring the entropy from $T=300$K down to $T=200$mK, we identify the sequential emergence of quantum Hall ferromagnetism, fractional quantum Hall states (FQH), and various charge orders by comparing the measured entropy in different temperature regimes with theoretical models. At the lowest temperature of $T \approx 200$mK we perform a detailed study of the entropy near even-denominator fractional quantum Hall states in bilayer graphene, and comment on the possible topological origin of the observed excess entropy.

cond-mat.mes-hall↗

arXiv2023

Cascade of multi-electron bubble phases in monolayer graphene at high Landau level filling

The phase diagram of an interacting two-dimensional electron system in a high magnetic field is enriched by the varying form of the effective Coulomb interaction, which depends strongly on the Landau level index. While the fractional quantum Hall states that dominate in the lower energy Landau levels have been explored experimentally in a variety of two-dimensional systems, much less work has been done to explore electron solids owing to their subtle transport signatures and extreme sensitivity to disorder. Here we use chemical potential measurements to map the phase diagram of electron solid states in $N=2$, $N=3$, and $N=4$ Landau levels in monolayer graphene. Direct comparison between our data and theoretical calculations reveals a cascade of density-tuned phase transitions between electron bubble phases up to two, three or four electrons per bubble in the N=2, 3 and 4 Landau levels respectively. Finite temperature measurements are consistent with melting of the solids for T$\approx$1K.

cond-mat.mes-hall↗

arXiv2023

Energy gap of the even-denominator fractional quantum Hall state in bilayer graphene

Bernal bilayer graphene hosts even denominator fractional quantum Hall states thought to be described by a Pfaffian wave function with nonabelian quasiparticle excitations. Here we report the quantitative determination of fractional quantum Hall energy gaps in bilayer graphene using both thermally activated transport and by direct measurement of the chemical potential. We find a transport activation gap of 5.1K at B = 12T for a half-filled N=1 Landau level, consistent with density matrix renormalization group calculations for the Pfaffian state. However, the measured thermodynamic gap of 11.6K is smaller than theoretical expectations for the clean limit by approximately a factor of two. We analyze the chemical potential data near fractional filling within a simplified model of a Wigner crystal of fractional quasiparticles with long-wavelength disorder, explaining this discrepancy. Our results quantitatively establish bilayer graphene as a robust platform for probing the non-Abelian anyons expected to arise as the elementary excitations of the even-denominator state.

cond-mat.mes-hall↗

arXiv2023

DataChat: Prototyping a Conversational Agent for Dataset Search and Visualization

Data users need relevant context and research expertise to effectively search for and identify relevant datasets. Leading data providers, such as the Inter-university Consortium for Political and Social Research (ICPSR), offer standardized metadata and search tools to support data search. Metadata standards emphasize the machine-readability of data and its documentation. There are opportunities to enhance dataset search by improving users' ability to learn about, and make sense of, information about data. Prior research has shown that context and expertise are two main barriers users face in effectively searching for, evaluating, and deciding whether to reuse data. In this paper, we propose a novel chatbot-based search system, DataChat, that leverages a graph database and a large language model to provide novel ways for users to interact with and search for research data. DataChat complements data archives' and institutional repositories' ongoing efforts to curate, preserve, and share research data for reuse by making it easier for users to explore and learn about available research data.

cs.IR↗

arXiv2021

Isospin magnetism and spin-triplet superconductivity in Bernal bilayer graphene

We report the observation of spin-polarized superconductivity in Bernal bilayer graphene when doped to a saddle-point van Hove singularity generated by large applied perpendicular electric field. We observe a cascade of electrostatic gate-tuned transitions between electronic phases distinguished by their polarization within the isospin space defined by the combination of the spin and momentum-space valley degrees of freedom. While all of these phases are metallic at zero magnetic field, we observe a transition to a superconducting state at finite $B_\parallel\approx 150$mT applied parallel to the two dimensional sheet. Superconductivity occurs near a symmetry breaking transition, and exists exclusively above the $B_\parallel$-limit expected of a paramagnetic superconductor with the observed $T_C\approx 30$mK, implying a spin-triplet order parameter.

cond-mat.mes-hall↗

arXiv2020

Electrical switching of magnetic order in an orbital Chern insulator

Magnetism typically arises from the joint effect of Fermi statistics and repulsive Coulomb interactions, which favors ground states with non-zero electron spin. As a result, controlling spin magnetism with electric fields---a longstanding technological goal in spintronics and multiferroics---can be achieved only indirectly. Here, we experimentally demonstrate direct electric field control of magnetic states in an orbital Chern insulator, a magnetic system in which non-trivial band topology favors long range order of orbital angular momentum but the spins are thought to remain disordered. We use van der Waals heterostructures consisting of a graphene monolayer rotationally faulted with respect to a Bernal-stacked bilayer to realize narrow and topologically nontrivial valley-projected moiré minibands. At fillings of one and three electrons per moiré unit cell within these bands, we observe quantized anomalous Hall effects with transverse resistance approximately equal to $h/2e^2$, which is indicative of spontaneous polarization of the system into a single-valley-projected band with a Chern number equal to two. At a filling of three electrons per moiré unit cell, we find that the sign of the quantum anomalous Hall effect can be reversed via field-effect control of the chemical potential; moreover, this transition is hysteretic, which we use to demonstrate nonvolatile electric field induced reversal of the magnetic state. A theoretical analysis indicates that the effect arises from the topological edge states, which drive a change in sign of the magnetization and thus a reversal in the favored magnetic state. Voltage control of magnetic states can be used to electrically pattern nonvolatile magnetic domain structures hosting chiral edge states, with applications ranging from reconfigurable microwave circuit elements to ultralow power magnetic memory.

cond-mat.str-el↗

arXiv2020

Isospin Pomeranchuk effect and the entropy of collective excitations in twisted bilayer graphene

In condensed matter systems, higher temperatures typically disfavors ordered phases leading to an upper critical temperature for magnetism, superconductivity, and other phenomena. A notable exception is the Pomeranchuk effect in 3He, in which the liquid ground state freezes upon increasing the temperature due to the large entropy of the paramagnetic solid phase. Here we show that a similar mechanism describes the finite temperature dynamics of spin and valley-isospins in magic-angle twisted bilayer graphene. Most strikingly a resistivity peak appears at high temperatures near superlattice filling factor nu = -1, despite no signs of a commensurate correlated phase appearing in the low-temperature limit. Tilted field magnetotransport and thermodynamic measurements of the inplane magnetic moment show that the resistivity peak is adiabatically connected to a finite-field magnetic phase transition at which the system develops finite isospin polarization. These data are suggestive of a Pomeranchuk-type mechanism, in which the entropy of disordered isospin moments in the ferromagnetic phase stabilizes it relative to an isospin unpolarized Fermi liquid phase at elevated temperatures. Measurements of the entropy, S/kB indeed find it to be of order unity per unit cell area, with a measurable fraction that is suppressed by an in-plane magnetic field consistent with a contribution from disordered physical spins. In contrast to 3He, however, no discontinuities are observed in the thermodynamic quantities across this transition. Our findings imply a small isospin stiffness, with implications for the nature of finite temperature transport as well as the mechanisms underlying isospin ordering and superconductivity in twisted bilayer graphene and related systems.

cond-mat.mes-hall↗

arXiv2020

Experimental determination of the energy per particle in partially filled Landau levels

We describe an experimental technique to measure the chemical potential, $μ$, in atomically thin layered materials with high sensitivity and in the static limit. We apply the technique to a high quality graphene monolayer to map out the evolution of $μ$ with carrier density throughout the N=0 and N=1 Landau levels at high magnetic field. By integrating $μ$ over filling factor, $ν$, we obtain the ground state energy per particle, which can be directly compared with numerical calculations. In the N=0 Landau level, our data show exceptional agreement with numerical calculations over the whole Landau level without adjustable parameters, as long as the screening of the Coulomb interaction by the filled Landau levels is accounted for. In the N=1 Landau level, comparison between experimental and numerical data reveals the importance of valley anisotropic interactions and the presence of valley-textured electron solids near odd filling.

cond-mat.mes-hall↗

arXiv2018

Quantum Hall Effect in Electron-Doped Black Phosphorus Field-Effect Transistors

The advent of black phosphorus field-effect transistors (FETs) has brought new possibilities in the study of two-dimensional (2D) electron systems. In a black phosphorus FET, the gate induces highly anisotropic 2D electron and hole gases. Although the 2D hole gas in black phosphorus has reached high carrier mobilities that led to the observation of the integer quantum Hall effect, the improvement in the sample quality of the 2D electron gas (2DEG) has however been only moderate; quantum Hall effect remained elusive. Here, we obtain high quality black phosphorus 2DEG by defining the 2DEG region with a prepatterned graphite local gate. The graphite local gate screens the impurity potential in the 2DEG. More importantly, it electrostatically defines the edge of the 2DEG, which facilitates the formation of well-defined edge channels in the quantum Hall regime. The improvements enable us to observe precisely quantized Hall plateaus in electron-doped black phosphorus FET. Magneto-transport measurements under high magnetic fields further revealed a large effective mass and an enhanced Landé g-factor, which points to strong electron-electron interaction in black phosphorus 2DEG. Such strong interaction may lead to exotic many-body quantum states in the fractional quantum Hall regime.

cond-mat.mes-hall↗

arXiv2017

Strain-modulated Bandgap and Piezo-resistive Effect in Black Phosphorus Field-effect Transistors

Energy bandgap largely determines the optical and electronic properties of a semiconductor. Variable bandgap therefore makes versatile functionality possible in a single material. In layered material black phosphorus, the bandgap can be modulated by the number of layers; as a result, few-layer black phosphorus has discrete bandgap values that are relevant for opto-electronic applications in the spectral range from red, in monolayer, to mid-infrared in the bulk limit. Here, we further demonstrate continuous bandgap modulation by mechanical strain applied through flexible substrates. The strain-modulated bandgap significantly alters the charge transport in black phosphorus at room temperature; we for the first time observe a large piezo-resistive effect in black phosphorus field-effect transistors (FETs). The effect opens up opportunities for future development of electro-mechanical transducers based on black phosphorus, and we demonstrate strain gauges constructed from black phosphorus thin crystals.

cond-mat.mtrl-sci↗

arXiv2016

Direct Observation of Layer-Dependent Electronic Structure in Phosphorene

Phosphorene, a single atomic layer of black phosphorus, has recently emerged as a new twodimensional (2D) material that holds promise for electronic and photonic technology. Here we experimentally demonstrate that the electronic structure of few-layer phosphorene varies significantly with the number of layers, in good agreement with theoretical predictions. The interband optical transitions cover a wide, technologically important spectrum range from visible to mid-infrared. In addition, we observe strong photoluminescence in few-layer phosphorene at energies that match well with the absorption edge, indicating they are direct bandgap semiconductors. The strongly layer-dependent electronic structure of phosphorene, in combination with its high electrical mobility, gives it distinct advantages over other twodimensional materials in electronic and opto-electronic applications.

cond-mat.mtrl-sci↗

arXiv2015

Quantum Hall Effect in Black Phosphorus Two-dimensional Electron Gas

Development of new, high quality functional materials has been at the forefront of condensed matter research. The recent advent of two-dimensional black phosphorus has greatly enriched the material base of two-dimensional electron systems. Significant progress has been made to achieve high mobility black phosphorus two-dimensional electron gas (2DEG) since the development of the first black phosphorus field-effect transistors (FETs)$^{1-4}$. Here, we reach a milestone in developing high quality black phosphorus 2DEG - the observation of integer quantum Hall (QH) effect. We achieve high quality by embedding the black phosphorus 2DEG in a van der Waals heterostructure close to a graphite back gate; the graphite gate screens the impurity potential in the 2DEG, and brings the carrier Hall mobility up to 6000 $cm^{2}V^{-1}s^{-1}$. The exceptional mobility enabled us, for the first time, to observe QH effect, and to gain important information on the energetics of the spin-split Landau levels in black phosphorus. Our results set the stage for further study on quantum transport and device application in the ultrahigh mobility regime.

cond-mat.mtrl-sci↗

arXiv2014

Gate-tunable Phase Transitions in 1T-TaS$_2$

The ability to tune material properties using gate electric field is at the heart of modern electronic technology. It is also a driving force behind recent advances in two-dimensional systems, such as gate-electric-field induced superconductivity and metal-insulator transition. Here we describe an ionic field-effect transistor (termed "iFET"), which uses gate-controlled lithium ion intercalation to modulate the material property of layered atomic crystal 1T-TaS$_2$. The extreme charge doping induced by the tunable ion intercalation alters the energetics of various charge-ordered states in 1T-TaS$_2$, and produces a series of phase transitions in thin-flake samples with reduced dimensionality. We find that the charge-density-wave states in 1T-TaS$_2$ are three-dimensional in nature, and completely collapse in the two-dimensional limit defined by their critical thicknesses. Meanwhile the ionic gating induces multiple phase transitions from Mott-insulator to metal in 1T-TaS$_2$ thin flakes at low temperatures, with 5 orders of magnitude modulation in their resistance. Superconductivity emerges in a textured charge-density-wave state induced by ionic gating. Our method of gate-controlled intercalation of 2D atomic crystals in the bulk limit opens up new possibilities in searching for novel states of matter in the extreme charge-carrier-concentration limit.

cond-mat.mtrl-sci↗