1,696 research outputs found
Local Compressibility Measurements of Correlated States in Suspended Bilayer Graphene
Bilayer graphene has attracted considerable interest due to the important
role played by many-body effects, particularly at low energies. Here we report
local compressibility measurements of a suspended graphene bilayer. We find
that the energy gaps at filling factors v = 4 do not vanish at low fields, but
instead merge into an incompressible region near the charge neutrality point at
zero electric and magnetic field. These results indicate the existence of a
zero-field ordered state and are consistent with the formation of either an
anomalous quantum Hall state or a nematic phase with broken rotational
symmetry. At higher fields, we measure the intrinsic energy gaps of
broken-symmetry states at v = 0, 1 and 2, and find that they scale linearly
with magnetic field, yet another manifestation of the strong Coulomb
interactions in bilayer graphene.Comment: 9 pages, including 4 figures and supplementary material
Incompressible strips in dissipative Hall bars as origin of quantized Hall plateaus
We study the current and charge distribution in a two dimensional electron
system, under the conditions of the integer quantized Hall effect, on the basis
of a quasi-local transport model, that includes non-linear screening effects on
the conductivity via the self-consistently calculated density profile. The
existence of ``incompressible strips'' with integer Landau level filling factor
is investigated within a Hartree-type approximation, and non-local effects on
the conductivity along those strips are simulated by a suitable averaging
procedure. This allows us to calculate the Hall and the longitudinal resistance
as continuous functions of the magnetic field B, with plateaus of finite widths
and the well-known, exactly quantized values. We emphasize the close relation
between these plateaus and the existence of incompressible strips, and we show
that for B values within these plateaus the potential variation across the Hall
bar is very different from that for B values between adjacent plateaus, in
agreement with recent experiments.Comment: 13 pages, 11 figures, All color onlin
Coulomb-driven broken-symmetry states in doubly gated suspended bilayer graphene
The non-interacting energy spectrum of graphene and its bilayer counterpart
consists of multiple degeneracies owing to the inherent spin, valley and layer
symmetries. Interactions among charge carriers are expected to spontaneously
break these symmetries, leading to gapped ordered states. In the quantum Hall
regime these states are predicted to be ferromagnetic in nature whereby the
system becomes spin polarized, layer polarized or both. In bilayer graphene,
due to its parabolic dispersion, interaction-induced symmetry breaking is
already expected at zero magnetic field. In this work, the underlying order of
the various broken-symmetry states is investigated in bilayer graphene that is
suspended between top and bottom gate electrodes. By controllably breaking the
spin and sublattice symmetries we are able to deduce the order parameter of the
various quantum Hall ferromagnetic states. At small carrier densities, we
identify for the first time three distinct broken symmetry states, one of which
is consistent with either spontaneously broken time-reversal symmetry or
spontaneously broken rotational symmetry
Self-consistent local-equilibrium model for density profile and distribution of dissipative currents in a Hall bar under strong magnetic fields
Recent spatially resolved measurements of the electrostatic-potential
variation across a Hall bar in strong magnetic fields, which revealed a clear
correlation between current-carrying strips and incompressible strips expected
near the edges of the Hall bar, cannot be understood on the basis of existing
equilibrium theories. To explain these experiments, we generalize the
Thomas-Fermi--Poisson approach for the self-consistent calculation of
electrostatic potential and electron density in {\em total} thermal equilibrium
to a {\em local equilibrium} theory that allows to treat finite gradients of
the electrochemical potential as driving forces of currents in the presence of
dissipation. A conventional conductivity model with small values of the
longitudinal conductivity for integer values of the (local) Landau-level
filling factor shows that, in apparent agreement with experiment, the current
density is localized near incompressible strips, whose location and width in
turn depend on the applied current.Comment: 9 pages, 7 figure
Tunneling out of a time-dependent well
Solutions to explicit time-dependent problems in quantum mechanics are rare.
In fact, all known solutions are coupled to specific properties of the
Hamiltonian and may be divided into two categories: One class consists of
time-dependent Hamiltonians which are not higher than quadratic in the position
operator, like i.e the driven harmonic oscillator with time-dependent
frequency. The second class is related to the existence of additional
invariants in the Hamiltonian, which can be used to map the solution of the
time-dependent problem to that of a related time-independent one.
In this article we discuss and develop analytic methods for solving
time-dependent tunneling problems, which cannot be addressed by using quadratic
Hamiltonians. Specifically, we give an analytic solution to the problem of
tunneling from an attractive time-dependent potential which is embedded in a
long-range repulsive potential.
Recent progress in atomic physics makes it possible to observe experimentally
time-dependent phenomena and record the probability distribution over a long
range of time. Of special interest is the observation of macroscopical
quantum-tunneling phenomena in Bose-Einstein condensates with time-dependent
trapping potentials. We apply our model to such a case in the last section.Comment: 11 pages, 3 figure
Microrheology with optical tweezers: data analysis
We present a data analysis procedure that provides the solution to a long-standing issue in microrheology studies, i.e. the evaluation of the fluids' linear viscoelastic properties from the analysis of a finite set of experimental data, describing (for instance) the time-dependent mean-square displacement of suspended probe particles experiencing Brownian fluctuations. We report, for the first time in the literature, the linear viscoelastic response of an optically trapped bead suspended in a Newtonian fluid, over the entire range of experimentally accessible frequencies. The general validity of the proposed method makes it transferable to the majority of microrheology and rheology techniques
Revealing and Controlling Energy Barriers and Valleys at Grain Boundaries in Ultrathin Organic Films
Anomalous spin-splitting of two-dimensional electrons in an AlAs Quantum Well
We measure the effective Lande g-factor of high-mobility two-dimensional
electrons in a modulation-doped AlAs quantum well by tilting the sample in a
magnetic field and monitoring the evolution of the magnetoresistance
oscillations. The data reveal that |g| = 9.0, which is much enhanced with
respect to the reported bulk value of 1.9. Surprisingly, in a large range of
magnetic field and Landau level fillings, the value of the enhanced g-factor
appears to be constant.Comment: 4 pages, 3 figure
Time Resolved Correlation measurements of temporally heterogeneous dynamics
Time Resolved Correlation (TRC) is a recently introduced light scattering
technique that allows to detect and quantify dynamic heterogeneities. The
technique is based on the analysis of the temporal evolution of the speckle
pattern generated by the light scattered by a sample, which is quantified by
, the degree of correlation between speckle images recorded at
time and . Heterogeneous dynamics results in significant
fluctuations of with time . We describe how to optimize TRC
measurements and how to detect and avoid possible artifacts. The statistical
properties of the fluctuations of are analyzed by studying their
variance, probability distribution function, and time autocorrelation function.
We show that these quantities are affected by a noise contribution due to the
finite number of detected speckles. We propose and demonstrate a method to
correct for the noise contribution, based on a extrapolation
scheme. Examples from both homogeneous and heterogeneous dynamics are provided.
Connections with recent numerical and analytical works on heterogeneous glassy
dynamics are briefly discussed.Comment: 19 pages, 15 figures. Submitted to PR
High resolution amplitude and phase gratings in atom optics
An atom-field geometry is chosen in which an atomic beam traverses a field
interaction zone consisting of three fields, one having frequency propagating in the direction and the other two having
frequencies and propagating in the
- direction. For and , where and are positive integers and
is the pulse duration in the atomic rest frame, the atom-field interaction
results in the creation of atom amplitude and phase gratings having period . In this manner, one can use optical fields having
wavelength to produce atom gratings having periodicity much less
than .Comment: 11 pages, 14 figure
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