3,024 research outputs found
Moire superlattice effects in graphene/boron-nitride van der Waals heterostructures
Van der Waals heterostructures of graphene and hexagonal boron nitride
feature a moir\'e superlattice for graphene's Dirac electrons. Here, we review
the effects generated by this superlattice, including a specific miniband
structure featuring gaps and secondary Dirac points, and a fractal spectrum of
magnetic minibands known as Hofstadter's butterfly.Comment: 25 pages, 7 figure
Non-perturbative field theories
SIGLEAvailable from British Library Document Supply Centre- DSC:D85083 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
Symmetry and Mesoscopic Physics
Symmetry is one of the most important notions in natural science; it lies at the heart of fundamental laws of nature and serves as an important tool for understanding the properties of complex systems, both classical and quantum. Another trend, which has in recent years undergone intensive development, is mesoscopic physics. This branch of physics also combines classical and quantum ideas and methods. Two main directions can be distinguished in mesoscopic physics. One is the study of finite quantum systems of mesoscopic sizes. Such systems, which are between the atomic and macroscopic scales, exhibit a variety of novel phenomena and find numerous applications in creating modern electronic and spintronic devices. At the same time, the behavior of large systems can be influenced by mesoscopic effects, which provides another direction within the framework of mesoscopic physics. The aim of the present book is to emphasize the phenomena that lie at the crossroads between the concept of symmetry and mesoscopic physics
The Breakdown of Alfven's Theorem in Ideal Plasma Flows
This paper presents both rigorous results and physical theory on the
breakdown of magnetic flux conservation for ideal plasmas, by nonlinear
effects. Our analysis is based upon an effective equation for
magnetohydrodynamic (MHD) modes at length-scales with smaller scales
eliminated, as in renormalization-group methodology. We prove that
flux-conservation can be violated for an arbitrarily small length-scale
and in the absence of any non-ideality, but only if singular current sheets and
vortex sheets both exist and intersect in sets of large enough dimension. This
result gives analytical support to and rigorous constraints on theories of fast
turbulent reconnection. Mathematically, our theorem is analogous to Onsager's
result on energy dissipation anomaly in hydrodynamic turbulence. As a physical
phenomenon, the breakdown of magnetic-flux conservation in ideal MHD is similar
to the decay of magnetic flux through a narrow superconducting ring, by
phase-slip of quantized flux lines. The effect should be observable both in
numerical MHD simulations and in laboratory plasma experiments at moderately
high magnetic Reynolds numbers.Comment: 38 pages, 1 figur
Cascades and transitions in turbulent flows
Turbulence is characterized by the non-linear cascades of energy and other
inviscid invariants across a huge range of scales, from where they are injected
to where they are dissipated. Recently, new experimental, numerical and
theoretical works have revealed that many turbulent configurations deviate from
the ideal 3D/2D isotropic cases characterized by the presence of a strictly
direct/inverse energy cascade, respectively. We review recent works from a
unified point of view and we present a classification of all known transfer
mechanisms. Beside the classical cases of direct and inverse cascades, the
different scenarios include: split cascades to small and large scales
simultaneously, multiple/dual cascades of different quantities, bi-directional
cascades where direct and inverse transfers of the same invariant coexist in
the same scale-range and finally equilibrium states where no cascades are
present, including the case when a condensate is formed. We classify all
transitions as the control parameters are changed and we analyse when and why
different configurations are observed. Our discussion is based on a set of
paradigmatic applications: helical turbulence, rotating and/or stratified
flows, MHD and passive/active scalars where the transfer properties are altered
as one changes the embedding dimensions, the thickness of the domain or other
relevant control parameters, as the Reynolds, Rossby, Froude, Peclet, or Alfven
numbers. We discuss the presence of anomalous scaling laws in connection with
the intermittent nature of the energy dissipation in configuration space. An
overview is also provided concerning cascades in other applications such as
bounded flows, quantum, relativistic and compressible turbulence, and active
matter, together with implications for turbulent modelling. Finally, we present
a series of open problems and challenges that future work needs to address.Comment: accepted for publication on Physics Reports 201
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