205 research outputs found
Graphene: Kinks, Superlattices, Landau levels, and Magnetotransport
We review recent work on superlattices in monolayer and bilayer graphene. We
highlight the role of the quasiparticle chirality in generating new Dirac
fermion modes with tunable anisotropic velocities in one dimensional (1D)
superlattices in both monolayer and bilayer graphene. We discuss the structure
of the Landau levels and magnetotransport in such superlattices over a wide
range of perpendicular (orbital) magnetic fields. In monolayer graphene, we
show that an orbital magnetic field can reverse the anisotropy of the transport
imposed by the superlattice potential, suggesting possible switching-type
device applications. We also consider topological modes localized at a kink in
an electric field applied perpendicular to bilayer graphene, and show how
interactions convert these modes into a two-band Luttinger liquid with tunable
Luttinger parameters. The band structures of electric field superlattices in
bilayer graphene (with or without a magnetic field) are shown to arise
naturally from a coupled array of such topological modes. We briefly review
some bandstructure results for 2D superlattices. We conclude with a discussion
of recent tunneling and transport experiments and point out open issues.Comment: Invited Review Article for Special Issue on Graphene, References
added, Typos correcte
Valley-Selective Landau-Zener Oscillations in Semi-Dirac p-n Junctions
We study transport across p-n junctions of gapped two-dimensional semi-Dirac
materials: nodal semimetals whose energy bands disperse quadratically and
linearly along distinct crystal axes. The resulting electronic properties ---
relevant to materials such as TiO/VO multilayers and
-(BEDT-TTF)I salts --- continuously interpolate between those
of mono- and bi-layer graphene as a function of propagation angle. We
demonstrate that tunneling across the junction depends on the orientation of
the tunnel barrier relative to the crystalline axes, leading to strongly
non-monotonic current-voltage characteristics, including negative differential
conductance in some regimes. In multi-valley systems these features provide a
natural route to engineering valley-selective transport.Comment: 7 pages, 7 figures, appendice
Localized systems coupled to small baths: from A to Z
We investigate what happens if an Anderson localized system is coupled to a
small bath, with a discrete spectrum, when the coupling between system and bath
is specially chosen so as to never localize the bath. We find that the effect
of the bath on localization in the system is a non-monotonic function of the
coupling between system and bath. At weak couplings, the bath facilitates
transport by allowing the system to 'borrow' energy from the bath. But above a
certain coupling the bath produces localization, because of an orthogonality
catastrophe, whereby the bath 'dresses' the system and hence suppresses the
hopping matrix element. We call this last regime the regime of
"Zeno-localization", since the physics of this regime is akin to the quantum
Zeno effect, where frequent measurements of the position of a particle impede
its motion. We confirm our results by numerical exact diagonalization
Remnants of Anderson localization in prethermalization induced by white noise
We study the nonequilibrium evolution of a one-dimensional quantum Ising chain with spatially disordered, time-dependent, transverse fields characterized by white noise correlation dynamics. We establish prethermalization in this model, showing that the quench dynamics of the on-site transverse magnetization first approaches a metastable state unaffected by noise fluctuations, and then relaxes exponentially fast toward an infinite temperature state as a result of the noise. We also consider energy transport in the model, starting from an inhomogeneous state with two domain walls which separate regions characterized by spins with opposite transverse magnetization. We observe at intermediate timescales a phenomenology akin to Anderson localization: energy remains localized within the two domain walls, until the Markovian noise destroys coherence and, accordingly, disorder-induced localization, allowing the system to relax toward the late stages of its nonequilibrium dynamics. We compare our results with the simpler case of a noisy quantum Ising chain without disorder, and we find that the prethermal plateau is a generic property of spin chains with weak noise, while the phenomenon of prethermal Anderson localization is a specific feature arising from the competition of noise and disorder in the real-time transport properties of the system
Localization-protected quantum order
Closed quantum systems with quenched randomness exhibit many-body localized regimes wherein they do not equilibrate, even though prepared with macroscopic amounts of energy above their ground states. We show that such localized systems can order, in that individual many-body eigenstates can break symmetries or display topological order in the infinite-volume limit. Indeed, isolated localized quantum systems can order even at energy densities where the corresponding thermally equilibrated system is disordered, i.e., localization protects order. In addition, localized systems can move between ordered and disordered localized phases via nonthermodynamic transitions in the properties of the many-body eigenstates. We give evidence that such transitions may proceed via localized critical points. We note that localization provides protection against decoherence that may allow experimental manipulation of macroscopic quantum states. We also identify a “spectral transition” involving a sharp change in the spectral statistics of the many-body Hamiltonian
Common path interference in Zener tunneling is a universal phenomenon
We show that the probability of electric field induced interband tunneling in
solid state systems is generically a non-monotonic (oscillatory) function of
the applied field. This unexpected behavior can be understood as arising due to
a common path interference between two distinct tunneling solutions. The
phenomenon is insensitive to magnetic field, and arises whenever the low energy
dispersion relation contains higher order terms in addition to the usual
term. Such higher order terms are generically present, albeit with small
co-efficient, so that the oscillatory Zener tunneling is a universal
phenomenon. However, the first `Zener oscillation' occurs at a transmission
probability which is exponentially small when the co-efficient of the higher
order terms is small. This explains why this oscillatory aspect of Zener
tunneling has been hitherto overlooked, despite its universality. The common
path interference is also destroyed by the presence of odd powers of in the
low energy dispersion relation. Since odd powers of are strictly absent
only when the tunneling barrier lies along an axis of mirror symmetry, it
follows that the robustness of the oscillatory behavior depends on the
orientation of the tunneling barrier. Bilayer graphene is identified as a
particularly good material for observation of common path interference, due to
its unusual nearly isotropic dispersion relation, where the term makes
the leading contribution
Orbital Diamagnetism of Weak-doped Bilayer Graphene in Magnetic Field
We investigate the orbital diamagnetism of a weak-doped bilayer graphene
(BLG) in spatially smoothly varying magnetic field and obtain the general
analytic expression of the orbital susceptibility of BLG, with finite wave
number and Fermi energy, at zero temperature. We find that the magnetic field
screening factor of BLG is dependent with the wave number, which results in a
more complicated screening behavior compared with that of monolayer graphene
(MLG). We also study the induced magnetization, electric current in BLG, under
nonuniform magnetic field, and find that they are qualitatively different from
that in MLG and two-dimensional electron gas (2DEG). However, similar to the
MLG, the magnetic object placed above BLG is repelled by a diamagnetic force
from BLG, approximately equivalent to a force produced by its mirror image on
the other side of BLG with a reduced amplitude dependent with the typical
length of the systems. BLG shows crossover behaviors in the responses to the
external magnetic field as the intermediate between MLG and 2DEG.Comment: 8 pages, 2 figure
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