scholarly journals Magnon magic angles and tunable Hall conductivity in 2D twisted ferromagnetic bilayers

2020 ◽  
Vol 10 (1) ◽  
Author(s):  
Doried Ghader
Keyword(s):  
Band Structure ◽  
Magnetic Order ◽  
Condensed Matter ◽  
Twist Angle ◽  
Band Gaps ◽  
Novel Device ◽  
Flat Bands ◽  
Research Fields ◽  
Active Research ◽  
The Continuum

Abstract Twistronics is currently one of the most active research fields in condensed matter physics, following the discovery of correlated insulating and superconducting phases in twisted bilayer graphene (tBLG). Here, we present a magnonic analogue of tBLG. We study magnons in twisted ferromagnetic bilayers (tFBL) with collinear magnetic order, including exchange and weak Dzyaloshinskii-Moriya interactions (DMI). For negligible DMI, tFBL presents discrete magnon magic angles and flat moiré minibands analogous to tBLG. The DMI, however, changes the picture and renders the system much more exotic. The DMI in tFBL induces a rich topological magnon band structure for any twist angle. The twist angle turns to a control knob for the magnon valley Hall and Nernst conductivities. Gapped flat bands appear in a continuum of magic angles in tFBL with DMI. In the lower limit of the continuum, the band structure reconstructs to form several topological flat bands. The luxury of twist-angle control over band gaps, topological properties, number of flat bands, and valley Hall and Nernst conductivities renders tFBL a novel device from fundamental and applied perspectives.

scholarly journals Multiple flat bands and topological Hofstadter butterfly in twisted bilayer graphene close to the second magic angle

2021 ◽  
Vol 118 (30) ◽  
pp. e2100006118
Author(s):  
Xiaobo Lu ◽  
Biao Lian ◽  
Gaurav Chaudhary ◽  
Benjamin A. Piot ◽  
Giulio Romagnoli ◽  
...  
Keyword(s):  
Condensed Matter ◽  
Bilayer Graphene ◽  
Band Gaps ◽  
Magic Angle ◽  
Two Dimensional ◽  
Quantum Phases ◽  
Electron Band ◽  
Flat Bands ◽  
Twisted Bilayer Graphene ◽  
Hofstadter Butterfly

Moiré superlattices in two-dimensional van der Waals heterostructures provide an efficient way to engineer electron band properties. The recent discovery of exotic quantum phases and their interplay in twisted bilayer graphene (tBLG) has made this moiré system one of the most renowned condensed matter platforms. So far studies of tBLG have been mostly focused on the lowest two flat moiré bands at the first magic angle θm1 ∼ 1.1°, leaving high-order moiré bands and magic angles largely unexplored. Here we report an observation of multiple well-isolated flat moiré bands in tBLG close to the second magic angle θm2 ∼ 0.5°, which cannot be explained without considering electron–election interactions. With high magnetic field magnetotransport measurements we further reveal an energetically unbound Hofstadter butterfly spectrum in which continuously extended quantized Landau level gaps cross all trivial band gaps. The connected Hofstadter butterfly strongly evidences the topologically nontrivial textures of the multiple moiré bands. Overall, our work provides a perspective for understanding the quantum phases in tBLG and the fractal Hofstadter spectra of multiple topological bands.

Social Movements in Latin America: Mapping the Literature

2020 ◽  
Author(s):  
Nicolás M. Somma
Keyword(s):  
Political Economy ◽  
Social Movements ◽  
Latin American ◽  
Transnational Activism ◽  
Organizational Fields ◽  
Family Labor ◽  
American Sociology ◽  
Research Fields ◽  
Vast Literature ◽  
Active Research

The study of social movements is currently one of the most active research fields in Latin American sociology. This article maps the vast literature on Latin American social movements (LASMs) from the late 1980s to the present. After briefly discussing how scholars have conceptualized LASMs, it presents seven influential approaches: structuralism, political economy, political context, organizational fields, “new social movements,” frames and emotions, and transnational activism. Then it discusses some works that zero in on the specificity of LASMs. It closes with a brief summary of the five coming chapters, each of which is devoted to a specific social movement “family”: labor, women’s, student, indigenous, and anti-globalization.

scholarly journals Spectroscopy of a tunable moiré system with a correlated and topological flat band

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Xiaomeng Liu ◽  
Cheng-Li Chiu ◽  
Jong Yeon Lee ◽  
Gelareh Farahi ◽  
Kenji Watanabe ◽  
...  
Keyword(s):  
Band Structure ◽  
Twist Angle ◽  
Bilayer Graphene ◽  
Scanning Tunneling Spectroscopy ◽  
Scanning Tunneling ◽  
Flat Band ◽  
Correlated Electron ◽  
Partial Filling ◽  
Valley Polarization ◽  
Spectroscopic Signatures

AbstractMoiré superlattices created by the twisted stacking of two-dimensional crystals can host electronic bands with flat energy dispersion in which enhanced interactions promote correlated electron states. The twisted double bilayer graphene (TDBG), where two Bernal bilayer graphene are stacked with a twist angle, is such a moiré system with tunable flat bands. Here, we use gate-tuned scanning tunneling spectroscopy to directly demonstrate the tunability of the band structure of TDBG with an electric field and to show spectroscopic signatures of electronic correlations and topology for its flat band. Our spectroscopic experiments are in agreement with a continuum model of TDBG band structure and reveal signatures of a correlated insulator gap at partial filling of its isolated flat band. The topological properties of this flat band are probed with the application of a magnetic field, which leads to valley polarization and the splitting of Chern bands with a large effective g-factor.

scholarly journals Accurate Prediction of Band Structure of FeS2: A Hard Quest of Advanced First-Principles Approaches

2021 ◽  
Vol 9 ◽  
Author(s):  
Min-Ye Zhang ◽  
Hong Jiang
Keyword(s):  
Band Structure ◽  
First Principles ◽  
High Energy ◽  
Accurate Prediction ◽  
Band Gaps ◽  
Full Potential ◽  
Band Structures ◽  
Electronic Band ◽  
Fundamental Band ◽  
Hybrid Functionals

The pyrite and marcasite polymorphs of FeS2 have attracted considerable interests for their potential applications in optoelectronic devices because of their appropriate electronic and optical properties. Controversies regarding their fundamental band gaps remain in both experimental and theoretical materials research of FeS2. In this work, we present a systematic theoretical investigation into the electronic band structures of the two polymorphs by using many-body perturbation theory with the GW approximation implemented in the full-potential linearized augmented plane waves (FP-LAPW) framework. By comparing the quasi-particle (QP) band structures computed with the conventional LAPW basis and the one extended by high-energy local orbitals (HLOs), denoted as LAPW + HLOs, we find that one-shot or partially self-consistent GW (G0W0 and GW0, respectively) on top of the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation with a converged LAPW + HLOs basis is able to remedy the artifact reported in the previous GW calculations, and leads to overall good agreement with experiment for the fundamental band gaps of the two polymorphs. Density of states calculated from G0W0@PBE with the converged LAPW + HLOs basis agrees well with the energy distribution curves from photo-electron spectroscopy for pyrite. We have also investigated the performances of several hybrid functionals, which were previously shown to be able to predict band gaps of many insulating systems with accuracy close or comparable to GW. It is shown that the hybrid functionals considered in general fail badly to describe the band structures of FeS2 polymorphs. This work indicates that accurate prediction of electronic band structure of FeS2 poses a stringent test on state-of-the-art first-principles approaches, and the G0W0 method based on semi-local approximation performs well for this difficult system if it is practiced with well-converged numerical accuracy.

Physics of Gauge Fields in Quantum Nanosciences

SPIN ◽  
2020 ◽  
Vol 10 (03) ◽  
pp. 2050018
Author(s):  
Seng Ghee Tan ◽  
Mansoor B. A. Jalil ◽  
Ching-Ray Chang ◽  
Shuichi Murakami
Keyword(s):  
Future Development ◽  
Momentum Space ◽  
Condensed Matter ◽  
Equations Of Motion ◽  
Frame Of Reference ◽  
Gauge Fields ◽  
The Past ◽  
Research Fields ◽  
New Research ◽  
The Impact

We review the formulation of gauge fields in terms of the frame of reference as well as the space in which the frame is defined. We highlighted some recent applications of gauge physics in the momentum space — in the modern fields of the spin Hall effect, the magnon Hall, the optical Magnus and the graphene valley Hall. General procedures of gauge transformation which lead to the construction of the gauge curvature and the equations of motion (EOM) are outlined. Central to this review is our intention to illustrate the impact of gauge physics on the past and future development of many new research fields emerging out of condensed matter physics, particularly in quantum nanosciences and nanoelectronics.

Stacking Faults and 3C Quantum Wells in Hexagonal SiC Polytypes

2006 ◽  
Vol 527-529 ◽  
pp. 351-354 ◽  
Author(s):  
M.S. Miao ◽  
Walter R.L. Lambrecht
Keyword(s):  
Band Structure ◽  
Quantum Wells ◽  
First Principles ◽  
Driving Force ◽  
Stacking Faults ◽  
Band Gaps ◽  
Partial Dislocations ◽  
Band Structure Calculations ◽  
Thermodynamic Driving Force ◽  
Structure Calculations

The electronic driving force for growth of stacking faults (SF) in n-type 4H SiC under annealing and in operating devices is discussed. This involves two separate aspects: an overall thermodynamic driving force due to the capture of electrons in interface states and the barriers that need to be overcome to create dislocation kinks which advance the motion of partial dislocations and hence expansion of SF. The second problem studied in this paper is whether 3C SiC quantum wells in 4H SiC can have band gaps lower than 3C SiC. First-principles band structure calculations show that this is not the case due to the intrinsic screening of the spontaneous polarization fields.

scholarly journals Theory of correlated insulating behaviour and spin-triplet superconductivity in twisted double bilayer graphene

2019 ◽  
Vol 10 (1) ◽  
Author(s):  
Jong Yeon Lee ◽  
Eslam Khalaf ◽  
Shang Liu ◽  
Xiaomeng Liu ◽  
Zeyu Hao ◽  
...  
Keyword(s):  
Displacement Field ◽  
Twist Angle ◽  
Vertical Displacement ◽  
Bilayer Graphene ◽  
Magic Angle ◽  
Hartree Fock ◽  
Flat Bands ◽  
Half Filling ◽  
Spin Triplet ◽  
Experimental Findings

AbstractTwo graphene monolayers twisted by a small magic angle exhibit nearly flat bands, leading to correlated electronic states. Here we study a related but different system with reduced symmetry - twisted double bilayer graphene (TDBG), consisting of two Bernal stacked bilayer graphenes, twisted with respect to one another. Unlike the monolayer case, we show that isolated flat bands only appear on application of a vertical displacement field. We construct a phase diagram as a function of twist angle and displacement field, incorporating interactions via a Hartree-Fock approximation. At half-filling, ferromagnetic insulators are stabilized with valley Chern number $${C}_{{\rm{v}}}=\pm 2$$Cv=±2. Upon doping, ferromagnetic fluctuations are argued to lead to spin-triplet superconductivity from pairing between opposite valleys. We highlight a novel orbital effect arising from in-plane fields plays an important role in interpreting experiments. Combined with recent experimental findings, our results establish TDBG as a tunable platform to realize rare phases in conventional solids.

Band Structures of Carbon Nanotubes with Nanoscale Periodic Pores Depending on their Circumferences

2003 ◽  
Vol 02 (01n02) ◽  
pp. 109-116
Author(s):  
Hiroyuki Takeda ◽  
Katsumi Yoshino
Keyword(s):  
Carbon Nanotubes ◽  
Tight Binding ◽  
Band Gaps ◽  
Band Structures ◽  
Electronic Band ◽  
Tight Binding Approximation ◽  
One Dimensional ◽  
Flat Bands ◽  
Porous Graphite ◽  
Electronic Band Structures

We theoretically evaluate the electronic band structures in carbon nanotubes with nanoscale periodic pores with a tight-binding approximation of π electrons, and demonstrate that band gaps of the carbon nanotubes with nanoscale periodic pores differ significantly from those of conventional carbon nanotubes. The band gaps of the carbon nanotubes with nanoscale periodic pores depend strongly on the size of pores and inter-pore distances. In some carbon nanotubes with nanoscale periodic pores, band gaps are constant as a function of their circumferences. In other ones, band gaps have the exact periodicity of three as a function of their circumferences. Those behaviors can be explained by taking properties of nanoscale periodic porous graphite into consideration. In some carbon nanotubes with relatively large nanoscale periodic pores, flat bands appear, which may cause singular properties about magnetism in one-dimensional porous carbon nanotubes.

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