8,491 research outputs found
Quasiparticle interference and the interplay between superconductivity and density wave order in the cuprates
Scanning tunneling spectroscopy (STS) is a useful probe for studying the
cuprates in the superconducting and pseudogap states. Here we present a
theoretical study of the Z-map, defined as the ratio of the local density of
states at positive and negative bias energies, which frequently is used to
analyze STS data. We show how the evolution of the quasiparticle interference
peaks in the Fourier transform Z-map can be understood by considering different
types of impurity scatterers, as well as particle-hole asymmetry in the
underlying bandstructure. We also explore the effects of density wave orders,
and show that the Fourier transform Z-map may be used to both detect and
distinguish between them.Comment: final version published in Phys. Rev.
Time-resolved photoemission of correlated electrons driven out of equilibrium
We describe the temporal evolution of the time-resolved photoemission
response of the spinless Falicov-Kimball model driven out of equilibrium by
strong applied fields. The model is one of the few possessing a metal-insulator
transition and admitting an exact solution in the time domain. The
nonequilibrium dynamics, evaluated using an extension of dynamical mean-field
theory, show how the driven system differs from two common viewpoints - a
quasiequilibrium system at an elevated effective temperature (the "hot"
electron model) or a rapid interaction quench ("melting" of the Mott gap) - due
to the rearrangement of electronic states and redistribution of spectral
weight. The results demonstrate the inherent trade-off between energy and time
resolution accompanying the finite width probe pulses, characteristic of those
employed in pump-probe time-domain experiments, which can be used to focus
attention on different aspects of the dynamics near the transition.Comment: Original: 5 pages, 3 figures; Replaced: updated text and figures, 5
pages, 4 figure
Towards Axion Monodromy Inflation with Warped KK-Modes
We present a particularly simple model of axion monodromy: Our axion is the
lowest-lying KK-mode of the RR-2-form-potential in the standard
Klebanov-Strassler throat. One can think of this inflaton candidate as being
defined by the integral of over the cycle of the throat. It obtains
an exponentially small mass from the IR-region in which the shrinks to
zero size both with respect to the Planck scale and the mass scale of local
modes of the throat. Crucially, the cycle has to be shared between two
throats, such that the second locus where the shrinks is also in a warped
region. Well-known problems like the potentially dangerous back-reaction of
brane/antibrane pairs and explicit supersymmetry breaking are not present in
our scenario. However, the inflaton back-reaction starts to deform the geometry
strongly once the field excursion approaches the Planck scale. We derive the
system of differential equations required to treat this effect quantitatively.
Numerical work is required to decide whether back-reaction makes the model
suitable for realistic inflation. While we have to leave this crucial issue to
future studies, we find it interesting that such a simple and explicit stringy
monodromy model allows an originally sub-Planckian axion to go through many
periods with full quantitative control before back-reaction becomes strong.
Also, the mere existence of our ultra-light throat mode (with double
exponentially suppressed mass) is noteworthy.Comment: 28 pages, 3 figures; v2: references added; v3: Corrected an
underestimate of supergravity back-reaction in Eq. (36); results changed
accordingly; added section 6 which develops the methodology for the 10d
non-linear back-reaction; added reference
Controlled quantum stirring of Bose-Einstein condensates
By cyclic adiabatic change of two control parameters of an optical trap one
can induce a circulating current of condensed bosons. The amount of particles
that are transported per period depends on the "radius" of the cycle, and this
dependence can be utilized in order to probe the interatomic interactions. For
strong repulsive interaction the current can be regarded as arising from a
sequence of Landau-Zener crossings. For weaker interaction one observes either
gradual or coherent mega crossings, while for attractive interaction the
particles are glued together and behave like a classical ball. For the analysis
we use the Kubo approach to quantum pumping with the associated Dirac monopoles
picture of parameter space.Comment: 12 pages, 8 figure
Decorrelation of neural-network activity by inhibitory feedback
Correlations in spike-train ensembles can seriously impair the encoding of
information by their spatio-temporal structure. An inevitable source of
correlation in finite neural networks is common presynaptic input to pairs of
neurons. Recent theoretical and experimental studies demonstrate that spike
correlations in recurrent neural networks are considerably smaller than
expected based on the amount of shared presynaptic input. By means of a linear
network model and simulations of networks of leaky integrate-and-fire neurons,
we show that shared-input correlations are efficiently suppressed by inhibitory
feedback. To elucidate the effect of feedback, we compare the responses of the
intact recurrent network and systems where the statistics of the feedback
channel is perturbed. The suppression of spike-train correlations and
population-rate fluctuations by inhibitory feedback can be observed both in
purely inhibitory and in excitatory-inhibitory networks. The effect is fully
understood by a linear theory and becomes already apparent at the macroscopic
level of the population averaged activity. At the microscopic level,
shared-input correlations are suppressed by spike-train correlations: In purely
inhibitory networks, they are canceled by negative spike-train correlations. In
excitatory-inhibitory networks, spike-train correlations are typically
positive. Here, the suppression of input correlations is not a result of the
mere existence of correlations between excitatory (E) and inhibitory (I)
neurons, but a consequence of a particular structure of correlations among the
three possible pairings (EE, EI, II)
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