45 research outputs found
Inhomogeneous Reionization Models in Cosmological Hydrodynamical Simulations
In this work we present a new hybrid method to simulate the thermal effects
of the reionization in cosmological hydrodynamical simulations. The method
improves upon the standard approach used in simulations of the intergalactic
medium (IGM) and galaxy formation without a significant increase of the
computational cost allowing for efficient exploration of the parameter space.
The method uses a small set of phenomenological input parameters and combines a
semi-numerical reionization model to solve for the topology of reionization and
an approximate model of how reionization heats the IGM, with the massively
parallel \texttt{Nyx} hydrodynamics code, specifically designed to solve for
the structure of diffuse IGM gas. We have produced several large-scale high
resolution cosmological hydrodynamical simulations (, Mpc/h) with different instantaneous and inhomogeneous HI reionization
models that use this new methodology. We study the IGM thermal properties of
these models and find that large scale temperature fluctuations extend well
beyond the end of reionization. Analyzing the 1D flux power spectrum of these
models, we find up to differences in the large scale properties
(low modes, s/km) of the post-reionization power spectrum due
to the thermal fluctuations. We show that these differences could allow one to
distinguish between different reionization scenarios already with existing
Ly forest measurements. Finally, we explore the differences in the
small-scale cutoff of the power spectrum and we find that, for the same heat
input, models show very good agreement provided that the reionization redshift
of the instantaneous reionization model happens at the midpoint of the
inhomogeneous model.Comment: 24 pages, 16 figures. Accepted by MNRAS. Minor changes to match
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Inhomogeneous reionization models in cosmological hydrodynamical simulations
In this work we present a new hybrid method to simulate the thermal effects of reionization in cosmological hydrodynamical simulations. The method improves upon the standard approach used in simulations of the intergalactic medium (IGM) and galaxy formation without a significant increase in the computational cost, thereby allowing for efficient exploration of the parameter space. The method uses a small set of phenomenological input parameters, and combines a seminumerical reionization model to solve for the topology of reionization with an approximate model of how reionization heats the IGM, using the massively parallel Nyx hydrodynamics code which is specifically designed to solve for the structure of diffuse IGM gas. We have produced several medium-scale, high-resolution simulations (20483, Lbox = 40 Mpc h-1) with various instantaneous and inhomogeneous reionization models that use this new methodology. We study the IGM thermal properties of these models and find that large-scale temperature fluctuations extend well beyond the end of reionization. By analysing the 1D flux power spectrum of these models, we find up to {\sim } 50{{\\rm per\cent}} differences in the large-scale properties (low modes, k ≲0.01 s km-1) of the post-reionization power spectrum as a result of the thermal fluctuations. We show that these differences could allow one to distinguish between different reionization scenarios with existing Lyα forest measurements. Finally, we explore the differences in the small-scale cut-off of the power spectrum, finding that, for the same heat input, models show very good agreement provided that the reionization redshift of the instantaneous reionization model occurs at the midpoint of the inhomogeneous model
Orbital Order and Spontaneous Orthorhombicity in Iron Pnictides
A growing list of experiments show orthorhombic electronic anisotropy in the
iron pnictides, in some cases at temperatures well above the spin density wave
transition. These experiments include neutron scattering, resistivity and
magnetoresistance measurements, and a variety of spectroscopies. We explore the
idea that these anisotropies stem from a common underlying cause: orbital order
manifest in an unequal occupation of and orbitals, arising
from the coupled spin-orbital degrees of freedom. We emphasize the distinction
between the total orbital occupation (the integrated density of states), where
the order parameter may be small, and the orbital polarization near the Fermi
level which can be more pronounced. We also discuss light-polarization studies
of angle-resolved photoemission, and demonstrate how x-ray absorption linear
dichroism may be used as a method to detect an orbital order parameter.Comment: Orig.: 4+ pages; Rev.: 4+ pages with updated content and reference
Resonant Enhancement of Charge Density Wave Diffraction in the Rare-Earth Tritellurides
We performed resonant soft X-ray diffraction on known charge density wave
(CDW) compounds, rare earth tri-tellurides. Near the (3d - 4f) absorption
edge of rare earth ions, an intense diffraction peak is detected at a
wavevector identical to that of CDW state hosted on Te planes, indicating a
CDW-induced modulation on the rare earth ions. Surprisingly, the temperature
dependence of the diffraction peak intensity demonstrates an exponential
increase at low temperatures, vastly different than that of the CDW order
parameter. Assuming 4f multiplet splitting due to the CDW states,we present a
model to calculate X-ray absorption spectrum and resonant profile of the
diffraction peak, agreeing well with experimental observations. Our results
demonstrate a situation where the temperature dependence of resonant X-ray
diffraction peak intensity is not directly related to the intrinsic behavior of
the order parameter associated with the electronic order, but is dominated by
the thermal occupancy of the valence states.Comment: 7 pages, 5 figure
High pressure evolution of FeO electronic structure revealed by X-ray absorption
We report the first high pressure measurement of the Fe K-edge in hematite
(FeO) by X-ray absorption spectroscopy in partial fluorescence yield
geometry. The pressure-induced evolution of the electronic structure as
FeO transforms from a high-spin insulator to a low-spin metal is
reflected in the x-ray absorption pre-edge. The crystal field splitting energy
was found to increase monotonically with pressure up to 48 GPa, above which a
series of phase transitions occur. Atomic multiplet, cluster diagonalization,
and density-functional calculations were performed to simulate the pre-edge
absorption spectra, showing good qualitative agreement with the measurements.
The mechanism for the pressure-induced phase transitions of FeO is
discussed and it is shown that ligand hybridization significantly reduces the
critical high-spin/low-spin pressure.Comment: 5 pages, 4 figures and 1 tabl
The Magic Angle "Mystery" in Electron Energy Loss Spectroscopy: Relativistic and Dielectric Corrections
Recently it has been demonstrated that a careful treatment of both
longitudinal and transverse matrix elements in electron energy loss spectra can
explain the mystery of relativistic effects on the {\it magic angle}. Here we
show that there is an additional correction of order where is
the atomic number and the fine structure constant, which is not
necessarily small for heavy elements. Moreover, we suggest that macroscopic
electrodynamic effects can give further corrections which can break the
sample-independence of the magic angle.Comment: 10 pages (double column), 6 figure
Symmetry breaking orbital anisotropy on detwinned Ba(Fe1-xCox)2As2 above the spin density wave transition
Nematicity, defined as broken rotational symmetry, has recently been observed
in competing phases proximate to the superconducting phase in the cuprate high
temperature superconductors. Similarly, the new iron-based high temperature
superconductors exhibit a tetragonal to orthorhombic structural transition
(i.e. a broken C4 symmetry) that either precedes or is coincident with a
collinear spin density wave (SDW) transition in undoped parent compounds, and
superconductivity arises when both transitions are suppressed via doping.
Evidence for strong in-plane anisotropy in the SDW state in this family of
compounds has been reported by neutron scattering, scanning tunneling
microscopy, and transport measurements. Here we present an angle resolved
photoemission spectroscopy study of detwinned single crystals of a
representative family of electron-doped iron-arsenide superconductors,
Ba(Fe1-xCox)2As2 in the underdoped region. The crystals were detwinned via
application of in-plane uniaxial stress, enabling measurements of single domain
electronic structure in the orthorhombic state. At low temperatures, our
results clearly demonstrate an in-plane electronic anisotropy characterized by
a large energy splitting of two orthogonal bands with dominant dxz and dyz
character, which is consistent with anisotropy observed by other probes. For
compositions x>0, for which the structural transition (TS) precedes the
magnetic transition (TSDW), an anisotropic splitting is observed to develop
above TSDW, indicating that it is specifically associated with TS. For
unstressed crystals, the band splitting is observed close to TS, whereas for
stressed crystals the splitting is observed to considerably higher
temperatures, revealing the presence of a surprisingly large in-plane nematic
susceptibility in the electronic structure.Comment: final version published in PNAS, including supplementary informatio