844 research outputs found
Magnetotransport near a quantum critical point in a simple metal
We use geometric considerations to study transport properties, such as the
conductivity and Hall coefficient, near the onset of a nesting-driven spin
density wave in a simple metal. In particular, motivated by recent experiments
on vanadium-doped chromium, we study the variation of transport coefficients
with the onset of magnetism within a mean-field treatment of a model that
contains nearly nested electron and hole Fermi surfaces. We show that most
transport coefficients display a leading dependence that is linear in the
energy gap. The coefficient of the linear term, though, can be small. In
particular, we find that the Hall conductivity is essentially
unchanged, due to electron-hole compensation, as the system goes through the
quantum critical point. This conclusion extends a similar observation we made
earlier for the case of completely flat Fermi surfaces to the immediate
vicinity of the quantum critical point where nesting is present but not
perfect.Comment: 11 pages revtex, 4 figure
Oceanic redox conditions during the terminal Cambrian extinction event
Marine animal diversity during the late Cambrian was reduced by a series of extinctions that have generally been attributed to oceanic anoxic events associated with positive carbon isotope excursions. Here we present carbon and uranium isotope ratios (δ13C and δ238U values) as proxies for global carbon cycle and oceanic redox conditions, respectively, from carbonate rocks of the Wa'ergang section, South China. The dataset spans an interval that includes the last major negative δ13C excursion (TOCE) of the Cambrian Period. The TOCE is a globally documented event, recovery from which corresponds to the terminal Cambrian extinction event. The δ13C and δ238U values covary through the section, shifting initially to lower values, with δ238U falling below the modern open-ocean seawater value from the start to the middle of the profile, followed by a shift to higher values towards the end of the Cambrian. Neither the co-occurrence of δ13C and δ238U negative excursions, nor the association of rising δ238U with extinction have been commonly reported. Here we argue that robust positive coupling of δ13C and δ238U relates to the existence of extensive intermediate reducing settings (from low-O2 suboxia to intermittent anoxia) during the late Cambrian alongside low atmospheric pO2 and a greenhouse climate. Similarly, a stepwise increase in the δ238U baseline in carbonates across the Ediacaran−Cambrian boundary is consistent with the growing importance of an intermediate reducing sink through that interval. We propose further that divergent trends in lower and upper ocean redox conditions could have driven the parallel isotope excursions. An expansion of intermediate reducing conditions, rather than persistent anoxic euxinia, is consistent with the recovery of δ13C and δ238U to higher values, as well as the presence of benthic fauna and shoreward extension of deeper-water fauna that may have had a greater tolerance against hypoxia
Exploring the ‘middle ground’ between state and market: the example of China
Studies of housing systems lying in the ‘middle ground’ between state and market are subject to three important shortcomings. First, the widely used Esping-Andersen (EA) approach assesses only a subset of the key housing outcomes and may be less helpful for describing changes in housing policy regimes. Second, there is too much emphasis on tenure transitions, and an assumed close correspondence between tenure labels and effective system functioning may not be valid. Third, due attention has not been given to the spatial dimensions in which housing systems operate, in particular when housing policies have a significant devolved or localised emphasis. Updating EA’s framework, we suggest a preliminary list of housing system indicators in order to capture the nature of the housing systems being developed and devolved. We verified the applicability of this indicator system with the case of China. This illustrates clearly the need for a more nuanced and systematic basis for categorising differences and changes in welfare and housing policies
Violation of the Wiedemann-Franz Law in a Large-N Solution of the t-J Model
We show that the Wiedemann-Franz law, which holds for Landau Fermi liquids,
breaks down in a large-n treatment of the t-J model. The calculated ratio of
the in-plane thermal and electrical conductivities agrees quantitatively with
experiments on the normal state of the electron-doped Pr_{2-x}Ce_xCuO_4 (x =
0.15) cuprate superconductor. The violation of the Wiedemann-Franz law in the
uniform phase contrasts with other properties of the phase that are Fermi
liquid like.Comment: 4 pages, 2 figures. Typos corrected, one added reference, revised
discussion of experiment on 214 cuprate material (x = 0.06
The self-organization of grid cells in 3D
Do we expect periodic grid cells to emerge in bats, or perhaps dolphins, exploring a three-dimensional environment? How long will it take? Our self-organizing model, based on firing-rate adaptation, points at a complex answer. The mathematical analysis leads to asymptotic states resembling FCC and HCP crystal structures, which are calculated to be very close to each other in terms of cost function. The simulation of the full model, however, shows that the approach to such asymptotic states involves several sub-processes over distinct time scales. The smoothing of the initially irregular multiple fields of individual units and their arrangement into hexagonal grids over certain best planes are observed to occur relatively fast, even in large 3D volumes. The correct mutual orientation of the planes, though, and the coordinated arrangement of different units, take a longer time, with the network showing no sign of convergence towards either a pure FCC or HCP orderin
A Fermi liquid model for the overdoped and optimally doped cuprate superconductors: scattering rate, susceptibility, spin resonance peak and superconducting transition
This paper treats a number of issues of the cuprates, ranging from the spin
resonance peak and the linear one-particle scattering rate to the
superconducting transition, in the frame of a Fermi liquid model. Recent ARPES
expts. by Valla et al., Science vol. 285, 2110 (1999), and e-print
cond-mat/0003407, directly support the linearity of the one-particle scattering
rate everywhere in the Brillouin zone we obtained here. We show that the origin
of this linearity is the strong linear in energy term of the imaginary part of
the carrier susceptibility. This result yields directly a linear in temperature
resistivity and linear in 1/energy optical conductivity. We show that the low
energy dependence of the susceptibility can have a purely fermionic origin. We
introduce an antiferromagnetic ansatz for the susceptibility of the
carriers.Inter alia, this ansatz may explain the appearance of the spin
resonance peak (observed in neutron scattering) in the normal state of the
cuprates. Further, we obtain particularly high transition temperatures
from our Eliashberg scheme by using this ansatz: we have a gap
with K for nearest neighbour hopping .Comment: This version corrects a few misprints of the printed versio
Neutron scattering and superconducting order parameter in YBa2Cu3O7
We discuss the origin of the neutron scattering peak at 41 meV observed in
YBaCuO below . The peak may occur due to spin-flip electron
excitations across the superconducting gap which are enhanced by the
antiferromagnetic interaction between Cu spins. In this picture, the experiment
is most naturally explained if the superconducting order parameter has -wave
symmetry and opposite signs in the bonding and antibonding electron bands
formed within a CuO bilayer.Comment: In this version, only few minor corrections and the update of
references were done in order to make perfect correspondence with the
published version. RevTeX, psfig, 5 pages, and 3 figure
On the multi-orbital band structure and itinerant magnetism of iron-based superconductors
This paper explains the multi-orbital band structures and itinerant magnetism
of the iron-pnictide and chalcogenides. We first describe the generic band
structure of an isolated FeAs layer. Use of its Abelian glide-mirror group
allows us to reduce the primitive cell to one FeAs unit. From
density-functional theory, we generate the set of eight Fe and As
localized Wannier functions for LaOFeAs and their tight-binding (TB)
Hamiltonian, . We discuss the topology of the bands, i.e. allowed and
avoided crossings, the origin of the d6 pseudogap, as well as the role of the
As orbitals and the elongation of the FeAs tetrahedron. We then
couple the layers, mainly via interlayer hopping between As orbitals,
and give the formalism for simple and body-centered tetragonal stackings. This
allows us to explain the material-specific 3D band structures. Due to the high
symmetry, several level inversions take place as functions of or
pressure, resulting in linear band dispersions (Dirac cones). The underlying
symmetry elements are, however, easily broken, so that the Dirac points are not
protected, nor pinned to the Fermi level. From the paramagnetic TB Hamiltonian,
we form the band structures for spin spirals with wavevector by coupling
and . The band structure for stripe order is studied as a
function of the exchange potential, , using Stoner theory. Gapping of
the Fermi surface (FS) for small requires matching of FS dimensions
(nesting) and -orbital characters. The origin of the propeller-shaped FS is
explained. Finally, we express the magnetic energy as the sum over
band-structure energies, which enables us to understand to what extent the
magnetic energies might be described by a Heisenberg Hamiltonian, and the
interplay between the magnetic moment and the elongation of the FeAs4
tetrahedron
Resonance peak in underdoped cuprates
The magnetic susceptibility measured in neutron scattering experiments in
underdoped YBaCuO is interpreted based on the self-consistent
solution of the t-J model of a Cu-O plane. The calculations reproduce correctly
the frequency and momentum dependencies of the susceptibility and its variation
with doping and temperature in the normal and superconducting states. This
allows us to interpret the maximum in the frequency dependence -- the resonance
peak -- as a manifestation of the excitation branch of localized Cu spins and
to relate the frequency of the maximum to the size of the spin gap. The
low-frequency shoulder well resolved in the susceptibility of superconducting
crystals is connected with a pronounced maximum in the damping of the spin
excitations. This maximum is caused by intense quasiparticle peaks in the hole
spectral function for momenta near the Fermi surface and by the nesting.Comment: 9 pages, 6 figure
- …