5,242 research outputs found
The Dependence Structure of Macroeconomic Variables in the US
A central role for economic policy involves reducing the incidence of systemic downturns, when key economic variables experience joint extreme events. In this paper, we empirically analyze such dependence using two approaches, correlations and copulas. We document four findings. First, linear correlations and copulas disagree substantially about the nation’s dependence structure, indicating correlation complexity in the US economy. Second, GDP exhibits linear dependence with interest rates and prices, but no extreme dependence with the latter. This is consistent with the existence of liquidity traps. Third, GDP exhibits asymmetric extreme dependence with employment, consumption and investment, with relatively greater dependence during downturns. Fourth, money is neutral, especially during extreme economic conditions.Asymmetric dependence; Copula; Correlation Complexity; Extreme Event; Economic Policy; Money Neutrality; Systemic Downturn
Electronic structures of [111]-oriented free-standing InAs and InP nanowires
We report on a theoretical study of the electronic structures of the
[111]-oriented, free-standing, zincblende InAs and InP nanowires with hexagonal
cross sections by means of an atomistic , spin-orbit interaction
included, nearest-neighbor, tight-binding method. The band structures and the
band state wave functions of these nanowires are calculated and the symmetry
properties of the bands and band states are analyzed based on the
double point group. It is shown that all bands of these nanowires are doubly
degenerate at the -point and some of these bands will split into
non-degenerate bands when the wave vector moves away from the
-point as a manifestation of spin-splitting due to spin-orbit
interaction. It is also shown that the lower conduction bands of these
nanowires all show simple parabolic dispersion relations, while the top valence
bands show complex dispersion relations and band crossings. The band state wave
functions are presented by the spatial probability distributions and it is
found that all the band states show -rotation symmetric probability
distributions. The effects of quantum confinement on the band structures of the
[111]-oriented InAs and InP nanowires are also examined and an empirical
formula for the description of quantization energies of the lowest conduction
band and the highest valence band is presented. The formula can simply be used
to estimate the enhancement of the band gaps of the nanowires at different
sizes as a result of quantum confinement.Comment: 9 pages, 8 figures. arXiv admin note: substantial text overlap with
arXiv:1502.0756
Topological energy gaps in the [111]-oriented InAs/GaSb and GaSb/InAs core-shell nanowires
The [111]-oriented InAs/GaSb and GaSb/InAs core-shell nanowires have been
studied by the Luttinger-Kohn Hamiltonian to
search for non-vanishing fundamental gaps between inverted electron and hole
bands. We focus on the variations of the topologically nontrivial fundamental
gap, the hybridization gap, and the effective gap with the core radius and
shell thickness of the nanowires. The evolutions of all the energy gaps with
the structural parameters are shown to be dominantly governed by quantum size
effects. With a fixed core radius, a topologically nontrivial fundamental gap
exists only at intermediate shell thicknesses. The maximum gap is
meV for GaSb/InAs and meV for InAs/GaSb core-shell nanowires, and
for the GaSb/InAs core-shell nanowires the gap persists over a wider range of
geometrical parameters. The intrinsic reason for these differences between the
two types of nanowires is that in the shell the electron-like states of InAs is
more delocalized than the hole-like state of GaSb, while in the core the
hole-like state of GaSb is more delocalized than the electron-like state of
InAs, and both features favor stronger electron-hole hybridization. Since
similar features of the electron- and hole-like states have been found in
nanowires of other materials, it could serve as a common rule to put the
hole-like state in the core while the electron-like state in the shell of a
core-shell nanowire to achieve better topological properties.Comment: 10 pages, 10 figure
Ground-state fidelity of Luttinger liquids: A wave functional approach
We use a wave functional approach to calculate the fidelity of ground states
in the Luttinger liquid universality class of one-dimensional gapless quantum
many-body systems. The ground-state wave functionals are discussed using both
the Schrodinger (functional differential equation) formulation and a path
integral formulation. The fidelity between Luttinger liquids with Luttinger
parameters K and K' is found to decay exponentially with system size, and to
obey the symmetry F(K,K')=F(1/K,1/K') as a consequence of a duality in the
bosonization description of Luttinger liquids.Comment: 13 pages, IOP single-column format. Sec. 3 expanded with discussion
of short-distance cut-off. Some typos corrected. Ref. 44 in v2 is now
footnote 2 (moved by copy editor). Published versio
Non-damping oscillations at flaring loops
Context. QPPs are usually detected as spatial displacements of coronal loops
in imaging observations or as periodic shifts of line properties in
spectroscopic observations. They are often applied for remote diagnostics of
magnetic fields and plasma properties on the Sun. Aims. We combine imaging and
spectroscopic measurements of available space missions, and investigate the
properties of non-damping oscillations at flaring loops. Methods. We used the
IRIS to measure the spectrum over a narrow slit. The double-component Gaussian
fitting method was used to extract the line profile of Fe XXI 1354.08 A at "O
I" window. The quasi-periodicity of loop oscillations were identified in the
Fourier and wavelet spectra. Results. A periodicity at about 40 s is detected
in the line properties of Fe XXI, HXR emissions in GOES 1-8 A derivative, and
Fermi 26-50 keV. The Doppler velocity and line width oscillate in phase, while
a phase shift of about Pi/2 is detected between the Doppler velocity and peak
intensity. The amplitudes of Doppler velocity and line width oscillation are
about 2.2 km/s and 1.9 km/s, respectively, while peak intensity oscillate with
amplitude at about 3.6% of the background emission. Meanwhile, a quasi-period
of about 155 s is identified in the Doppler velocity and peak intensity of Fe
XXI, and AIA 131 A intensity. Conclusions. The oscillations at about 40 s are
not damped significantly during the observation, it might be linked to the
global kink modes of flaring loops. The periodicity at about 155 s is most
likely a signature of recurring downflows after chromospheric evaporation along
flaring loops. The magnetic field strengths of the flaring loops are estimated
to be about 120-170 G using the MHD seismology diagnostics, which are
consistent with the magnetic field modeling results using the flux rope
insertion method.Comment: 9 pages, 9 figures, 1 table, accepted by A&
NMR Search for the Spin Nematic State in LaFeAsO Single Crystal
We report a 75-As single crystal NMR investigation of LaFeAsO, the parent
phase of a pnictide high Tc superconductor. We demonstrate that spin dynamics
develop a strong two-fold anisotropy within each orthorhombic domain below the
tetragonal-orthorhombic structural phase transition at T[TO]~156 K. This
intermediate state with a dynamical breaking of the rotational symmetry freezes
progressively into a spin density wave (SDW) below T[SDW]~142 K. Our findings
are consistent with the presence of a spin nematic state below T[TO] with an
incipient magnetic order.Comment: Revised manuscript accepted for publication in Phys. Rev. Let
Phase diagram of a Bose-Fermi mixture in a one-dimensional optical lattice in terms of fidelity and entanglement
We study the ground-state phase diagram of a Bose-Fermi mixture loaded in a
one-dimensional optical lattice by computing the ground-state fidelity and
quantum entanglement. We find that the fidelity is able to signal quantum phase
transitions between the Luttinger liquid phase, the density-wave phase, and the
phase separation state of the system; and the concurrence can be used to signal
the transition between the density-wave phase and the Ising phase.Comment: 4 pages 3 figure
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