99 research outputs found
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
Financial reporting quality and the quiet Life by Mihir N. Mehta.
Thesis (Ph. D.)--Massachusetts Institute of Technology, Sloan School of Management, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 61-64).Bertrand and Mullainathan (2003) find that managers shielded from the threat of takeovers exert less effort to maximize firm value, consistent with a 'quiet life' hypothesis. I study whether the governance role of financial reporting can mitigate adverse effects arising from managerial preferences for a quiet life. I hypothesize and find evidence that after changes in the mid 1990's to Delaware's takeover protection regime, Delaware firms with higher financial reporting quality (FRQ) have better operating performance and higher capital investment intensity. Furthermore, the above relation between FRQ and performance is stronger for firms operating in less competitive industries, and firms with staggered boards. Overall, the results suggest that financial reporting can help mitigate adverse effects associated with managerial preferences for a quiet life.Ph.D
Optical Hall conductivity of systems with gapped spectral nodes
We calculate the optical Hall conductivity within the Kubo formalism for
systems with gapped spectral nodes, where the latter have a power-law
dispersion with exponent n. The optical conductivity is proportional to n and
there is a characteristic logarithmic singularity as the frequency approaches
the gap energy. The optical Hall conductivity is almost unaffected by thermal
fluctuations and disorder for n=1, whereas disorder has a stronger effect on
transport properties if n=2
Microscopic Polarization in Bilayer Graphene
Bilayer graphene has drawn significant attention due to the opening of a band
gap in its low energy electronic spectrum, which offers a promising route to
electronic applications. The gap can be either tunable through an external
electric field or spontaneously formed through an interaction-induced symmetry
breaking. Our scanning tunneling measurements reveal the microscopic nature of
the bilayer gap to be very different from what is observed in previous
macroscopic measurements or expected from current theoretical models. The
potential difference between the layers, which is proportional to charge
imbalance and determines the gap value, shows strong dependence on the disorder
potential, varying spatially in both magnitude and sign on a microscopic level.
Furthermore, the gap does not vanish at small charge densities. Additional
interaction-induced effects are observed in a magnetic field with the opening
of a subgap when the zero orbital Landau level is placed at the Fermi energy
The electronic properties of bilayer graphene
We review the electronic properties of bilayer graphene, beginning with a
description of the tight-binding model of bilayer graphene and the derivation
of the effective Hamiltonian describing massive chiral quasiparticles in two
parabolic bands at low energy. We take into account five tight-binding
parameters of the Slonczewski-Weiss-McClure model of bulk graphite plus intra-
and interlayer asymmetry between atomic sites which induce band gaps in the
low-energy spectrum. The Hartree model of screening and band-gap opening due to
interlayer asymmetry in the presence of external gates is presented. The
tight-binding model is used to describe optical and transport properties
including the integer quantum Hall effect, and we also discuss orbital
magnetism, phonons and the influence of strain on electronic properties. We
conclude with an overview of electronic interaction effects.Comment: review, 31 pages, 15 figure
Many-body localization in a quantum simulator with programmable random disorder
When a system thermalizes it loses all local memory of its initial
conditions. This is a general feature of open systems and is well described by
equilibrium statistical mechanics. Even within a closed (or reversible) quantum
system, where unitary time evolution retains all information about its initial
state, subsystems can still thermalize using the rest of the system as an
effective heat bath. Exceptions to quantum thermalization have been predicted
and observed, but typically require inherent symmetries or noninteracting
particles in the presence of static disorder. The prediction of many-body
localization (MBL), in which disordered quantum systems can fail to thermalize
in spite of strong interactions and high excitation energy, was therefore
surprising and has attracted considerable theoretical attention. Here we
experimentally generate MBL states by applying an Ising Hamiltonian with
long-range interactions and programmably random disorder to ten spins
initialized far from equilibrium. We observe the essential signatures of MBL:
memory retention of the initial state, a Poissonian distribution of energy
level spacings, and entanglement growth in the system at long times. Our
platform can be scaled to higher numbers of spins, where detailed modeling of
MBL becomes impossible due to the complexity of representing such entangled
quantum states. Moreover, the high degree of control in our experiment may
guide the use of MBL states as potential quantum memories in naturally
disordered quantum systems.Comment: 9 pages, 9 figure
Transport Spectroscopy of Symmetry-Broken Insulating States in Bilayer Graphene
The flat bands in bilayer graphene(BLG) are sensitive to electric fields
E\bot directed between the layers, and magnify the electron-electron
interaction effects, thus making BLG an attractive platform for new
two-dimensional (2D) electron physics[1-5]. Theories[6-16] have suggested the
possibility of a variety of interesting broken symmetry states, some
characterized by spontaneous mass gaps, when the electron-density is at the
carrier neutrality point (CNP). The theoretically proposed gaps[6,7,10] in
bilayer graphene are analogous[17,18] to the masses generated by broken
symmetries in particle physics and give rise to large momentum-space Berry
curvatures[8,19] accompanied by spontaneous quantum Hall effects[7-9]. Though
recent experiments[20-23] have provided convincing evidence of strong
electronic correlations near the CNP in BLG, the presence of gaps is difficult
to establish because of the lack of direct spectroscopic measurements. Here we
present transport measurements in ultra-clean double-gated BLG, using
source-drain bias as a spectroscopic tool to resolve a gap of ~2 meV at the
CNP. The gap can be closed by an electric field E\bot \sim13 mV/nm but
increases monotonically with a magnetic field B, with an apparent particle-hole
asymmetry above the gap, thus providing the first mapping of the ground states
in BLG.Comment: 4 figure
Non-Fermi-liquid d-wave metal phase of strongly interacting electrons
Developing a theoretical framework for conducting electronic fluids
qualitatively distinct from those described by Landau's Fermi-liquid theory is
of central importance to many outstanding problems in condensed matter physics.
One such problem is that, above the transition temperature and near optimal
doping, high-transition-temperature copper-oxide superconductors exhibit
`strange metal' behaviour that is inconsistent with being a traditional Landau
Fermi liquid. Indeed, a microscopic theory of a strange-metal quantum phase
could shed new light on the interesting low-temperature behaviour in the
pseudogap regime and on the d-wave superconductor itself. Here we present a
theory for a specific example of a strange metal---the 'd-wave metal'. Using
variational wavefunctions, gauge theoretic arguments, and ultimately
large-scale density matrix renormalization group calculations, we show that
this remarkable quantum phase is the ground state of a reasonable microscopic
Hamiltonian---the usual t-J model with electron kinetic energy and two-spin
exchange supplemented with a frustrated electron `ring-exchange' term,
which we here examine extensively on the square lattice two-leg ladder. These
findings constitute an explicit theoretical example of a genuine
non-Fermi-liquid metal existing as the ground state of a realistic model.Comment: 22 pages, 12 figures: 6 pages, 7 figures of main text + 16 pages, 5
figures of Supplementary Information; this is approximately the version
published in Nature, minus various subedits in the main tex
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