1,710 research outputs found
Keck Deep Fields. II. The UV Galaxy Luminosity Function at z~4, 3, and 2
We use very deep UGRI multi-field imaging obtained at the Keck telescope to
study the evolution of the rest-frame 1700A galaxy luminosity function as the
Universe doubles its age from z~4 to z~2. The depth of our imaging allows us to
constrain the faint end of the luminosity function reaching M_1700A ~ -18.5 at
z~3 (equivalent to ~1M_sun/yr) accounting for both N^1/2 uncertainty in the
number of galaxies and for cosmic variance. We carefully examine many potential
sources of systematic bias in our LF measurements before drawing the following
conclusions. We find that the luminosity function of Lyman Break Galaxies
evolves with time and that this evolution is likely differential with
luminosity. The result is best constrained between the epochs at z~4 and z~3,
where we find that the number density of sub-L* galaxies increases with time by
at least a factor of 2.3 (11sigma statistical confidence); while the faint end
of the LF evolves, the bright end appears to remain virtually unchanged,
indicating that there may be differential, luminosity-dependent evolution
significant at the 97% level. Potential systematic biases restric our ability
to draw strong conclusions about continued evolution of the luminosity function
to lower redshifts, z~2.2 and z~1.7, but, nevertheless, it appears certain that
the number density of z~2.2 galaxies at all luminosities we studied,
-22<M_1700A<-18, is at least as high as that of their counterparts at z~3.
While it is not yet clear what mechanism underlies the observed evolution, the
fact that this evolution is differential with luminosity opens up new avenues
of improving our understanding of how galaxies form and evolve at high
redshift.Comment: Accepted for publication in ApJ. Updated preprint to reflect this
final versio
Strained bilayer graphene: Band structure topology and Landau level spectrum
We show that topology of the low-energy band structure in bilayer graphene
critically depends on mechanical deformations of the crystal which may easily
develop in suspended graphene flakes. We describe the Lifshitz transition that
takes place in strained bilayers upon splitting the parabollic bands at
intermediate energies into several Dirac cones at the energy scale of few meV.
Then, we show how this affects the electron Landau level spectra and the
quantum Hall effect.Comment: slightly over 4 pages, 3 figures, updated discussion and references;
almost identical to the published versio
Keck Deep Fields. III. Luminosity-dependent Evolution of the Ultraviolet Luminosity and Star Formation Rate Densities at z~4, 3, and 2
We use the Keck Deep Fields UGRI catalog of z~4, 3, and 2 UV-selected
galaxies to study the evolution of the rest-frame 1700A luminosity density at
high redshift. The ability to reliably constrain the contribution of faint
galaxies is critical and our data do so as they reach to M*+2 even at z~4 and
deeper still at lower redshifts. We find that the luminosity density at high
redshift is dominated by the hitherto poorly studied galaxies fainter than L*,
and, indeed, the the bulk of the UV light in the high-z Universe comes from
galaxies in the luminosity range L=0.1-1L*. It is these faint galaxies that
govern the behavior of the total UV luminosity density. Overall, there is a
gradual rise in luminosity density starting at z~4 or earlier, followed by a
shallow peak or a plateau within z~3--1, and then followed by the well-know
plunge at lower redshifts. Within this total picture, luminosity density in
sub-L* galaxies evolves more rapidly at high redshift, z>~2, than that in more
luminous objects. However, this is reversed at lower redshifts, z<~1, a
reversal that is reminiscent of galaxy downsizing. Within the context of the
models commonly used in the observational literature, there seemingly aren't
enough faint or bright LBGs to maintain ionization of intergalactic gas even as
late as z~4. This is particularly true at earlier epochs and even more so if
the faint-end evolutionary trends we observe at z~3 and 4 continue to higher
redshifts. Apparently the Universe must be easier to reionize than some recent
studies have assumed. Nevertheless, sub-L* galaxies do dominate the total UV
luminosity density at z>~2 and this dominance further highlights the need for
follow-up studies that will teach us more about these very numerous but thus
far largely unexplored systems.Comment: Accepted for publication in the Astrophysical Journal. Abstract
abridge
Strain-induced modifications of transport in gated graphene nanoribbons
We investigate the effects of homogeneous and inhomogeneous deformations and
edge disorder on the conductance of gated graphene nanoribbons. Under
increasing homogeneous strain the conductance of such devices initially
decreases before it acquires a resonance structure, and finally becomes
completely suppressed at larger strain. Edge disorder induces mode mixing in
the contact regions, which can restore the conductance to its ballistic value.
The valley-antisymmetric pseudo-magnetic field induced by inhomogeneous
deformations leads to the formation of additional resonance states, which
either originate from the coupling into Fabry-Perot states that extend through
the system, or from the formation of states that are localized near the
contacts, where the pseudo-magnetic field is largest. In particular, the n=0
pseudo-Landau level manifests itself via two groups of conductance resonances
close to the charge neutrality point.Comment: 10 pages, 6 figure
Spin-rotationally symmetric domain flux phases in underdoped cuprates
We propose a new form of inhomogeneous phases consisting of out-of-phase
staggered flux domains separated by diagonal charged domain walls centered on
bonds or on sites. Remarkably, such domain flux phases are spin-rotationally
symmetric and exhibit cone-like quasiparticle dispersion as well as
incommensurate order of orbital currents. Such features are consistent with the
pseudogap behavior and the diagonal stripes observed experimentally in lightly
doped cuprates. A renormalized mean field theory shows that such solutions are
competitive candidates within the -- model.Comment: 6 pages, 3 figure
Spectral features due to inter-Landau-level transitions in the Raman spectrum of bilayer graphene
We investigate the contribution of the low-energy electronic excitations
towards the Raman spectrum of bilayer graphene for the incoming photon energy
Omega >> 1eV. Starting with the four-band tight-binding model, we derive an
effective scattering amplitude that can be incorporated into the commonly used
two-band approximation. Due to the influence of the high-energy bands, this
effective scattering amplitude is different from the contact interaction
amplitude obtained within the two-band model alone. We then calculate the
spectral density of the inelastic light scattering accompanied by the
excitation of electron-hole pairs in bilayer graphene. In the absence of a
magnetic field, due to the parabolic dispersion of the low-energy bands in a
bilayer crystal, this contribution is constant and in doped structures has a
threshold at twice the Fermi energy. In an external magnetic field, the
dominant Raman-active modes are the n_{-} to n_{+} inter-Landau-level
transitions with crossed polarisation of in/out photons. We estimate the
quantum efficiency of a single n_{-} to n_{+} transition in the magnetic field
of 10T as I_{n_{-} to n_{+}}~10^{-12}.Comment: 7 pages, 3 figures, expanded version published in PR
Unidirectional d-wave superconducting domains in the two-dimensional t-J model
Motivated by the recently observed pattern of unidirectional domains in
high-T_c superconductors [Y. Kohsaka et al., Science 315, 1380 (2007)], we
investigate the emergence of spontaneous modulations in the d-wave
superconducting resonating valence bond phase using the t-J model at x=1/8
doping. Half-filled charge domains separated by four lattice spacings are found
to form along one of the crystal axis leading to modulated superconductivity
with out-of-phase d-wave order parameters in neighboring domains. Both
renormalized mean-field theory and variational Monte Carlo calculations yield
that the energies of modulated and uniform phases are very close to each other.Comment: 4 pages, 5 figures, minor revisions, and added reference
N-particle nonclassicality without N-particle correlations
Most of known multipartite Bell inequalities involve correlation functions
for all subsystems. They are useless for entangled states without such
correlations. We give a method of derivation of families of Bell inequalities
for N parties, which involve, e.g., only (N-1)-partite correlations, but still
are able to detect proper N-partite entanglement. We present an inequality
which reveals five-partite entanglement despite only four-partite correlations.
Classes of inequalities introduced here can be put into a handy form of a
single non-linear inequality. An example is given of an N qubit state, which
strongly violates such an inequality, despite having no N-qubit correlations.
This surprising property might be of potential value for quantum information
tasks.Comment: 5 page
An algorithm for solving the pulsar equation
We present an algorithm of finding numerical solutions of pulsar equation.
The problem of finding the solutions was reduced to finding expansion
coefficients of the source term of the equation in a base of orthogo- nal
functions defined on the unit interval by minimizing a multi-variable mismatch
function defined on the light cylinder. We applied the algorithm to Scharlemann
& Wagoner boundary conditions by which a smooth solu- tion is reconstructed
that by construction passes success- fully the Gruzinov's test of the source
function exponent.Comment: 4 pages, 4 figures, accepted for publication in ApSS (a shortened
version of the previous one
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