3,595 research outputs found
The Mass-Metallicity and Luminosity-Metallicity Relation from DEEP2 at z ~ 0.8
We present the mass-metallicity (MZ) and luminosity-metallicity (LZ)
relations at z ~ 0.8 from ~1350 galaxies in the Deep Extragalactic Evolutionary
Probe 2 (DEEP2) survey. We determine stellar masses by fitting the spectral
energy distribution inferred from photometry with current stellar population
synthesis models. This work raises the number of galaxies with metallicities at
z ~ 0.8 by more than an order of magnitude. We investigate the evolution in the
MZ and LZ relations in comparison with local MZ and LZ relations determined in
a consistent manner using ~21,000 galaxies in the Sloan Digital Sky Survey. We
show that high stellar mass galaxies (log(M/M_solar)~10.6) at z ~ 0.8 have
attained the chemical enrichment seen in the local universe, while lower
stellar mass galaxies (log(M/M_solar)~9.2) at z ~ 0.8 have lower metallicities
(Delta log(O/H)~0.15 dex) than galaxies at the same stellar mass in the local
universe. We find that the LZ relation evolves in both metallicity and B-band
luminosity between z ~ 0.8 and z~ 0, with the B-band luminosity evolving as a
function of stellar mass. We emphasize that the B-band luminosity should not be
used as a proxy for stellar mass in chemical evolution studies of star-forming
galaxies. Our study shows that both the metallicity evolution and the B-band
luminosity evolution for emission-line galaxies between the epochs are a
function of stellar mass, consistent with the cosmic downsizing scenario of
galaxy evolution.Comment: Accepted Version: 18 pages, 13 figure
Multiplet resonance lifetimes in resonant inelastic X-ray scattering involving shallow core levels
Resonant inelastic X-ray scattering (RIXS) spectra of model copper- and
nickel-based transition metal oxides are measured over a wide range of energies
near the M-edge (h=60-80eV) to better understand the properties of
resonant scattering involving shallow core levels. Standard multiplet RIXS
calculations are found to deviate significantly from the observed spectra.
However, by incorporating the self consistently calculated decay lifetime for
each intermediate resonance state within a given resonance edge, we obtain
dramatically improved agreement between data and theory. Our results suggest
that these textured lifetime corrections can enable a quantitative
correspondence between first principles predictions and RIXS data on model
multiplet systems. This accurate model is also used to analyze resonant elastic
scattering, which displays the elastic Fano effect and provides a rough upper
bound for the core hole shake-up response time.Comment: 6 pages, 3 figure
\u3cem\u3eColloquium\u3c/em\u3e: Topological Insulators
Topological insulators are electronic materials that have a bulk band gap like an ordinary insulator but have protected conducted states on their edge or surface. These states are possible due to the combination of spin-orbit interactions and time-reversal symmetry. The two-dimensional (2D) topological insulator is a quantum spin Hall insulator, which is a close cousin of the integer quantum Hall state. A three-dimensional (3D) topological insulator supports novel spin-polarized 2D Dirac fermions on its surface. In this Colloquium the theoretical foundation for topological insulators and superconductors is reviewed and recent experiments are described in which the signatures of topological insulators have been observed. Transport experiments on HgTe/CdTe quantum wells are described that demonstrate the existence of the edge states predicted for teh quantum spin hall insulator. Experiments on Bi1-xSbx, Bi\u3c2Se3, Bi2Te3 and Sb2Te3 are then discussed that establish these materials as 3D topological insulators and directly probe the topology of their surface states. Exotic states are described that can occur at the surface of a 3D topological insulator due to an induced energy gap. A magnetic gap leads to a novel quantum Hall state that gives rise to a topological magnetoelectric effect. A superconducting energy gap leads to a state that supports Majorana fermions and may provide a new venue for realizing proposals for topological quantum computation. Prospects for observing these exotic states are also discussed, as well as other potential device applications of topological insulators
Quantum master equation scheme of time-dependent density functional theory to time-dependent transport in nano-electronic devices
In this work a practical scheme is developed for the first-principles study
of time-dependent quantum transport. The basic idea is to combine the transport
master-equation with the well-known time-dependent density functional theory.
The key ingredients of this paper include: (i) the partitioning-free initial
condition and the consideration of the time-dependent bias voltages which base
our treatment on the Runge-Gross existence theorem; (ii) the non-Markovian
master equation for the reduced (many-body) central system (i.e. the device);
and (iii) the construction of Kohn-Sham master equation for the reduced
single-particle density matrix, where a number of auxiliary functions are
introduced and their equations of motion (EOM) are established based on the
technique of spectral decomposition. As a result, starting with a well-defined
initial state, the time-dependent transport current can be calculated
simultaneously along the propagation of the Kohn-Sham master equation and the
EOM of the auxiliary functions.Comment: 9 pages, no figure
Direct tunneling through high- amorphous HfO: effects of chemical modification
We report first principles modeling of quantum tunneling through amorphous
HfO dielectric layer of metal-oxide-semiconductor (MOS) nanostructures in
the form of n-Si/HfO/Al. In particular we predict that chemically modifying
the amorphous HfO barrier by doping N and Al atoms in the middle region -
far from the two interfaces of the MOS structure, can reduce the
gate-to-channel tunnel leakage by more than one order of magnitude. Several
other types of modification are found to enhance tunneling or induce
substantial band bending in the Si, both are not desired from leakage point of
view. By analyzing transmission coefficients and projected density of states,
the microscopic physics of electron traversing the tunnel barrier with or
without impurity atoms in the high- dielectric is revealed.Comment: 5 pages, 5 figure
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