32,491 research outputs found
Spin-Orbit Coupling and Magnetic Anisotropy in Iron-Based Superconductors
We determine theoretically the effect of spin-orbit coupling on the magnetic
excitation spectrum of itinerant multi-orbital systems, with specific
application to iron-based superconductors. Our microscopic model includes a
realistic ten-band kinetic Hamiltonian, atomic spin-orbit coupling, and
multi-orbital Hubbard interactions. Our results highlight the remarkable
variability of the resulting magnetic anisotropy despite constant spin-orbit
coupling. At the same time, the magnetic anisotropy exhibits robust universal
behavior upon changes in the bandstructure corresponding to different materials
of iron-based superconductors. A natural explanation of the observed
universality emerges when considering optimal nesting as a resonance
phenomenon. Our theory is also of relevance to other itinerant system with
spin-orbit coupling and nesting tendencies in the bandstructure.Comment: 15 pages, 9 figure
Collective magnetic excitations of symmetric magnetic states in iron-based superconductors
We study the collective magnetic excitations of the recently discovered
symmetric spin-density wave states of iron-based superconductors with
particular emphasis on their orbital character based on an itinerant
multiorbital approach. This is important since the symmetric
spin-density wave states exist only at moderate interaction strengths where
damping effects from a coupling to the continuum of particle-hole excitations
strongly modifies the shape of the excitation spectra compared to predictions
based on a local moment picture. We uncover a distinct orbital polarization
inherent to magnetic excitations in symmetric states, which provide a
route to identify the different commensurate magnetic states appearing in the
continuously updated phase diagram of the iron-pnictide family.Comment: 5+7 pages, 3+2 figure
The origin of a and e' orderings in NaCoO
It has often been suggested that correlation effects suppress the small e_g'
Fermi surface pockets of NaxCoO_2 that are predicted by LDA, but absent in
ARPES measurements. It appears that within the dynamical mean field theory
(DMFT) the ARPES can be reproduced only if the on-site energy of the eg'
complex is lower than that of the a1g complex at the one-electron level, prior
to the addition of local correlation effects. Current estimates regarding the
order of the two orbital complexes range from -200 meV to 315 meV in therms of
the energy difference. In this work, we perform density functional theory
calculations of this one-electron splitting \Delta= \epsilon_a1g-\epsilon_e_g'
for the full two-layer compound, Na2xCo2O4, accounting for the effects of Na
ordering, interplanar interactions and octahedral distortion. We find that
\epsilon a_1g-\epsilon e_g' is negative for all Na fillings and that this is
primarily due to the strongly positive Coulomb field created by Na+ ions in the
intercalant plane. This field disproportionately affects the a_1g orbital which
protrudes farther upward from the Co plane than the e_g' orbitals. We discuss
also the secondary effects of octahedral compression and multi-orbital filling
on the value of \Delta as a function of Na content. Our results indicate that
if the e_g' pockets are indeed suppressed that can only be due to nonlocal
correlation effects beyond the standard DMFT.Comment: 4 pages, 3 figure
Time-Dependent Random Walks and the Theory of Complex Adaptive Systems
Motivated by novel results in the theory of complex adaptive systems, we
analyze the dynamics of random walks in which the jumping probabilities are
{\it time-dependent}. We determine the survival probability in the presence of
an absorbing boundary. For an unbiased walk the survival probability is
maximized in the case of large temporal oscillations in the jumping
probabilities. On the other hand, a random walker who is drifted towards the
absorbing boundary performs best with a constant jumping probability. We use
the results to reveal the underlying dynamics responsible for the phenomenon of
self-segregation and clustering observed in the evolutionary minority game.Comment: 5 pages, 2 figure
Knight Shift and Leading Superconducting Instability From Spin Fluctuations in Sr2RuO4
Recent nuclear magnetic resonance studies [A. Pustogow {\it et al.},
arXiv:1904.00047] have challenged the prevalent chiral triplet pairing scenario
proposed for SrRuO. To provide guidance from microscopic theory as to
which other pair states might be compatible with the new data, we perform a
detailed theoretical study of spin-fluctuation mediated pairing for this
compound. We map out the phase diagram as a function of spin-orbit coupling,
interaction parameters, and band-structure properties over physically
reasonable ranges, comparing when possible with photoemission and inelastic
neutron scattering data information. We find that even-parity pseudospin
singlet solutions dominate large regions of the phase diagram, but in certain
regimes spin-orbit coupling favors a near-nodal odd-parity triplet
superconducting state, which is either helical or chiral depending on the
proximity of the band to the van Hove points. A surprising
near-degeneracy of the nodal - and -wave solutions leads
to the possibility of a near-nodal time-reversal symmetry broken
pair state. Predictions for the temperature dependence
of the Knight shift for fields in and out of plane are presented for all
states.Comment: 5 pages (3 figures) + supplementary informatio
Role of multiorbital effects in the magnetic phase diagram of iron-pnictides
We elucidate the pivotal role of the bandstructure's orbital content in
deciding the type of commensurate magnetic order stabilized within the
itinerant scenario of iron-pnictides. Recent experimental findings in the
tetragonal magnetic phase attest to the existence of the so-called charge and
spin ordered density wave over the spin-vortex crystal phase, the latter of
which tends to be favored in simplified band models of itinerant magnetism.
Here we show that employing a multiorbital itinerant Landau approach based on
realistic bandstructures can account for the experimentally observed magnetic
phase, and thus shed light on the importance of the orbital content in deciding
the magnetic order. In addition, we remark that the presence of a hole pocket
centered at the Brillouin zone's -point favors a magnetic stripe
rather than a tetragonal magnetic phase. For inferring the symmetry properties
of the different magnetic phases, we formulate our theory in terms of magnetic
order parameters transforming according to irreducible representations of the
ensuing D point group. The latter method not only provides
transparent understanding of the symmetry breaking schemes but also reveals
that the leading instabilities always belong to the subset
of irreducible representations, independent of their C or C nature.Comment: 11 pages, 6 figure
The GRB early optical flashes from internal shocks: application to GRB990123, GRB041219a and GRB060111b
With the successful launch of the Swift Gamma-Ray Burst Explorer, people
expected the prompt optical flash like GRB990123 would be easily detected.
However the fact that early optical flash have not been detected for a number
of GRBs indicates the reverse shock must be suppressed. Here we explore the
possibility that the optical flash may arise from the internal shock. We find
that, under certain circumstance, the optical flash of GRB990123 and GRB060111b
can really be explained by the internal shock. For GRB041219a, the prompt
optical emission was correlated with the gamma-ray emission, we explain this
feature also in the internal shock scenario, the optical emission is the low
energy extension of the gamma-ray emission, and we can restrict its redshift
. As for GRB050904, we have shown in previous paper that the optical
flash was produced by synchrotron radiation and the X-ray flare was produced by
the synchrotron-self-Compton mechanism. Therefore we conclude that the early
optical flash of GRBs can usually come from the internal shock. Meanwhile since
the condition to produce the optical flash is not easily satisfied, so the
optical flash like GRB990123 should not be common in GRBs. In addition, we also
discussed the synchrotron-self-Compton effect in the internal shock model, and
find that for different values of parameters, there would be soft gamma-ray
(100 KeV), hard gamma-ray (10 MeV) and GeV flare accompanying the optical
flash. For GRB like GRB990123, a GeV flare with fluence about 10^{-8} erg
cm^{-2} s^{-1} is expected, which may be detected by the GLAST satellite.Comment: 11 pages, minor revision, accepted by MNRA
The Reionization History and Early Metal Enrichment inferred from the Gamma-Ray Burst Rate
Based on the gamma-ray burst (GRB) event rate at redshifts of , which is assessed by the spectral peak energy-to-luminosity relation
recently found by Yonetoku et al., we observationally derive the star formation
rate (SFR) for Pop III stars in a high redshift universe. As a result, we find
that Pop III stars could form continuously at . Using the
derived Pop III SFR, we attempt to estimate the ultraviolet (UV) photon
emission rate at in which redshift range no observational
information has been hitherto obtained on ionizing radiation intensity. We find
that the UV emissivity at can make a noticeable contribution
to the early reionization. The maximal emissivity is higher than the level
required to keep ionizing the intergalactic matter at .
However, if the escape fraction of ionizing photons from Pop III objects is
smaller than 10%, then the IGM can be neutralized at some redshift, which may
lead to the double reionization. As for the enrichment, the ejection of all
metals synthesized in Pop III objects is marginally consistent with the IGM
metallicity, although the confinement of metals in Pop III objects can reduce
the enrichment significantly.Comment: 12 pages, 2 figures, ApJL accepte
Low latency via redundancy
Low latency is critical for interactive networked applications. But while we
know how to scale systems to increase capacity, reducing latency --- especially
the tail of the latency distribution --- can be much more difficult. In this
paper, we argue that the use of redundancy is an effective way to convert extra
capacity into reduced latency. By initiating redundant operations across
diverse resources and using the first result which completes, redundancy
improves a system's latency even under exceptional conditions. We study the
tradeoff with added system utilization, characterizing the situations in which
replicating all tasks reduces mean latency. We then demonstrate empirically
that replicating all operations can result in significant mean and tail latency
reduction in real-world systems including DNS queries, database servers, and
packet forwarding within networks
- …