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Results for “fractional quantum Hall effect”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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137 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Intrinsic axion insulating behavior in antiferromagnetic MnBi 6 Te 10

A striking feature of time-reversal symmetry (TRS) protected topological insulators (TIs) is that they are characterized by a half integer quantum Hall effect on the boundary when the surface states are gapped by time-reversal breaking perturbations. While TRS-protected TIs have become increasingly under control, magnetic analogs are still a largely unexplored territory with novel rich structures. In particular, magnetic topological insulators can also host a quantized axion term in the presence of lattice symmetries. Since these symmetries are naturally broken on the boundary, the surface states can develop a gap without external manipulation. In this work, we combine theoretical analysis, density-functional calculations and experimental evidence to reveal intrinsic axion insulating behavior in MnBi 6 Te 10 . Additionally, the quantized axion term arises from the simplest possible mechanism in the antiferromagnetic regime where it is protected by inversion symmetry and the product of a fractional translation and TRS. The anticipated gapping of the Dirac surface state at the edge is subsequently experimentally established using angle resolved photoemission spectroscopy (ARPES). As a result, this system provides the magnetic analog of the simplest TRS-protected TI with a single, gapped Dirac cone at the surface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

GaN/AlGaN 2DEGs in the quantum regime: Magneto-transport and photoluminescence to 60 tesla

Here, using high magnetic fields up to 60 T, we report magneto-transport and photoluminescence (PL) studies of a two-dimensional electron gas (2DEG) in a GaN/AlGaN heterojunction grown by molecular-beam epitaxy. Transport measurements demonstrate that the quantum limit can be exceeded (Landau level filling factor v <1) and show evidence for the v =2/3 fractional quantum Hall state. Simultaneous optical and transport measurements reveal synchronous quantum oscillations of both the PL intensity and the longitudinal resistivity in the integer quantum Hall regime. PL spectra directly reveal the dispersion of occupied Landau levels in the 2DEG and, therefore, the electron mass. These results demonstrate the utility of high (pulsed) magnetic fields for detailed measurements of quantum phenomena in high-density 2DEGs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

High spatial resolution charge sensing of quantum Hall states

Charge distribution offers a unique fingerprint of important properties of electronic systems, including dielectric response, charge ordering, and charge fractionalization. Here, we develop an architecture for charge sensing in two-dimensional electronic systems in a strong magnetic field. We probe local change of the chemical potential in a proximitized detector layer using scanning tunneling microscopy, allowing us to infer the chemical potential and the charge profile in the sample. Our technique has both high energy (<0.3 meV) and spatial (<10 nm) resolution exceeding that of previous studies by an order of magnitude. We apply our technique to study the chemical potential of quantum Hall liquids in monolayer graphene under high magnetic fields and their responses to charge impurities. The chemical potential measurement provides a local probe of the thermodynamic gap of quantum Hall ferromagnets and fractional quantum Hall states. The screening charge profile reveals spatially oscillatory response of the quantum Hall liquids to charge impurities and is consistent with the composite Fermi liquid picture close to the half-filling. Our technique also paves the way to map moiré potentials, probe Wigner crystals, and investigate fractional charges in quantum Hall and Chern insulators.

charge sensing↗
DOE OSTI2024

Microwave and rf Spectroscopy of 2D Carrier Systems and Bilayers (Final Technical Report)

We performed microwave spectroscopic studies of two dimensional electron systems (2DES) exhibiting the quantum Hall effect (QHE) in GaAs/Al${_x}$Ga$_{1-x}$As heterostructures. A common theme of the proposed work is electron solids, of which the simplest is the Wigner crystal, a triangular lattice of individual carriers. The solids are stabilized by carrier-carrier interaction and occur under conditions when the kinetic energy of the system is "frozen out" by high magnetic field, $B$. One of the main reasons for microwave spectroscopic study of 2DES is that the solid phases have striking microwave or rf spectra. In the low wave vector ($q$) limit these spectra exhibit a resonance that is understood as a pinning mode, a small collective oscillation of pieces of the solid within the disorder potential. Pinning modes can be very sharp, with quality factors up to around 30, and are valuable as signatures of the electron solids, and can reveal a solid even when standard dc transport shows an insulator, an integer QHE, or even a fractional QHE The pinning mode also is sensitive to the solid properties, and can reveal transitions between solids.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Pathways from a chiral superconductor to a composite Fermi liquid

Recent experiments have reported chiral time-reversal broken superconductivity in n-layer rhombohedral graphene for n=4,5,6. Introducing a moiré potential by alignement with a hexagonal boron nitride substrate suppresses the superconductivity but leads instead to various fractional quantum anomalous Hall phenomena. Motivated by these observations, we consider the fate of the phase transition between (a chiral) Landau Fermi liquid (LFL) metal and a Composite Fermi Liquid (CFL) metal in the presence of attractive interactions. These are parent states, respectively, for the superconductor and the fractional quantum Hall states. For weak attractive interactions, the LFL is usually unstable to superconductivity while the CFL is stable. This raises the possibility of a direct continuous phase transition between the chiral superconductor and the CFL. However, we show that generically the LFL close to the transition to the CFL is stable against superconductivity. Furthermore the evolution between the CFL and chiral superconductor goes through an intermediate stable LFL phase for weak attractive interactions. With stronger interactions, the evolution can instead go through a non-Abelian paired quantum Hall state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Magic Angles and Fractional Chern Insulators in Twisted Homobilayer Transition Metal Dichalcogenides

We explain the appearance of magic angles and fractional Chern insulators in twisted K-valley homobilayer transition metal dichalcogenides by mapping their continuum model to a Landau level problem. Here, our approach relies on an adiabatic approximation for the quantum mechanics of valence band holes in a layer-pseudospin field that is valid for sufficiently small twist angles and on a lowest Landau level approximation that is valid for sufficiently large twist angles. It provides a simple qualitative explanation for the nearly ideal quantum geometry of the lowest moir´e miniband at particular twist angles, predicts that topological flat bands occur only when the valley-dependent moir´e potential is sufficiently strong compared to the interlayer tunneling amplitude, and provides a convenient starting point for the study of interactions.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Sample shape and boundary dependence of measured transverse thermal properties

In spite of increased interest in thermal Hall measurements for the analysis of insulating quantum materials, there remains large uncertainty in such measurements due to contact misalignment. In this paper, we propose that sample geometry and uncertain boundary conditions provide an additional source of uncertainty in the measurement of $D_{xy}$ or $κ_{xy}$. By running simple simulations in an open source finite-element solver, we demonstrate that measured $D_{xy}$ can be changed by a non-negligible fraction in samples with similar width and length. This geometric corrective factor depends on the distinction between a uniform heat flow and constant temperature boundary couplings to a bath. Sample geometry and boundary conditions can be accounted for through simulation or by using rectangular samples to make thermal Hall measurements more reliable and reproducible. Finally, we detail a contactless optical method for measuring $D_{xy}$ based on an existing photothermal microscope technique. This method is insensitive to the longitudinal diffusivity pollution caused by contact misalignment.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Photonic circuits for laser stabilization with integrated ultra-high Q and Brillouin laser resonators

The integration of stabilized lasers, sources that generate spectrally pure light, will provide compact, low-cost solutions for applications including quantum information sciences, precision navigation and timing, metrology, and high-capacity fiber communications. We report a significant advancement in this field, demonstrating stabilization of an integrated waveguide Brillouin laser to an integrated waveguide reference cavity, where both resonators are fabricated using the same CMOS-compatible integration platform. We demonstrate reduction of the free running Brillouin laser linewidth to a 292 Hz integral linewidth and carrier stabilization to a 4.9 × 10 –13 fractional frequency at 8 ms reaching the cavity-intrinsic thermorefractive noise limit for frequencies down to 80 Hz. We achieve this level of performance using a pair of 56.4 x 10 6 quality factor Si 3 N 4 waveguide ring-resonators that reduce the high-frequency noise by the nonlinear Brillouin process and the low-frequency noise by Pound–Drever–Hall locking to the ultra-low loss resonator. These results represent an important step toward integrated stabilized lasers with reduced sensitivity to environmental disturbances for atomic, molecular, and optical physics (AMO), quantum information processing and sensing, and other precision scientific, sensing, and communications applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Spectroscopy of the fractal Hofstadter energy spectrum

Hofstadter’s butterfly, the predicted energy spectrum for non-interacting electrons confined to a two-dimensional lattice in a magnetic field, is one of the most remarkable fractal structures in nature. At rational ratios of magnetic flux quanta per lattice unit cell, this spectrum shows self-similar distributions of energy levels that reflect its recursive construction. For most materials, Hofstadter’s butterfly is predicted under experimental conditions that are unachievable using laboratory-scale magnetic fields. More recently, electrical transport studies have provided evidence for Hofstadter’s butterfly in materials engineered to have artificially large lattice constants, such as those with moiré superlattices. Yet, so far, direct spectroscopy of the fractal energy spectrum predicted by Hofstadter nearly 50 years ago has remained out of reach. Here, in this work, we use high-resolution scanning tunnelling microscopy/spectroscopy (STM/STS) to investigate the flat electronic bands in twisted bilayer graphene (TBG) near the predicted second magic angle, an ideal setting for spectroscopic studies of Hofstadter’s spectrum. Our study shows the fractionalization of flat moiré bands into discrete Hofstadter subbands and discerns experimental signatures of self-similarity of this spectrum. Moreover, our measurements uncover a spectrum that evolves dynamically with electron density, showing phenomena beyond that of Hofstadter’s original model owing to the combined effects of strong correlations, Coulomb interactions and the quantum degeneracy of electrons in TBG.

electronic properties and materials↗
DOE OSTI2020

Partial equilibration of anti-Pfaffian edge modes at ν = 5 / 2

The thermal Hall conductance K of the fractional quantum Hall state at filling fraction ν = 5/2 has recently been measured to be K = 2.5π 2 $k$$^{2}_{B}$T/3h [M. Banerjee et al. Nature (London) 559, 205 (2018)]. Here, the half-integer value of this result (in units of π 2 $k$$^{2}_{B}$T/3h) provides strong evidence for the presence of a Majorana edge mode and a corresponding quantum Hall state hosting quasiparticles with non-Abelian statistics. Whether this measurement points to the realization of the PH-Pfaffian or the anti-Pfaffian state has been the subject of debate. Here we consider the implications of this measurement for anti-Pfaffian edge-state transport. We show that in the limit of strong Coulomb interaction and an approximate spin degeneracy in the lowest Landau level, the anti-Pfaffian state admits low-temperature edge phases that are consistent with the Hall conductance measurements. Furthermore, these edge phases can exhibit fully equilibrated electrical transport coexisting with partially equilibrated heat transport over a range of temperatures. Through a study of the kinetic equations describing low-temperature electrical and heat transport of these edge states, we determine regimes of parameter space, controlling the interactions between the different edge modes that agree with experiment.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Topological Hall Effect in Antiferromagnetic Co-Doped Fe 3 GaTe 2

Fe 3 GaTe 2 is van der Waals (vdW) ferromagnet with a Curie temperature T C ranging from 350 K to 380 K, followed upon cooling by a ferrimagnetic transition near room temperature. Substituting Fe with Co was previously reported to induce antiferromagnetism (AFM) at a Co fraction dependent Néel temperature TN. In this work, we confirm the overall phase diagram of the Fe 3-x Co x GaTe 2 series as a function of x and temperature via magnetization and electrical transport measurements. For x ≥ 0.6 the Hall effect is observed to mimic the magnetization as the AF ground state is suppressed by the external magnetic field via a metamagnetic transition, thus displaying an anomalous Hall response. At low temperatures, we also observe a pronounced topological Hall signal peaking at μ 0 H = 4 T, or within the metamagnetic transition region of fields. This observation points to the presence of magnetic field-induced chiral spin textures, such as skyrmions upon approaching magnetization saturation. Remarkably, magnetic force microscopy (MFM) reveals the emergence of nearly circular magnetic domains, with diameters on the order of 100–200 nm, within the antiferromagnetic phase. Here, a detailed analysis of the MFM images indicates that the topological Hall effect is closely linked to the field-induced stabilization of magnetic domain structures, likely exhibiting chiral textures. This observation suggests the possible formation of skyrmions already in the AFM phase, i.e., AFM skyrmions, that evolve into ferromagnetic (FM) ones upon increasing the magnetic field. Consequently, Co-doped Fe 3 GaTe 2 might provide a platform to investigate the transformation of skyrmions, initially coupled antiferromagnetically into ferromagnetic skyrmions, and to explore its impact on the topological and skyrmion Hall effects.

anomalous Hall effect↗