461 research outputs found
Luttinger liquid versus charge density wave behaviour in the one-dimensional spinless fermion Holstein model
We discuss the nature of the different ground states of the half-filled
Holstein model of spinless fermions in 1D. In the metallic regime we determine
the renormalised effective coupling constant and the velocity of the charge
excitations by a density-matrix renormalisation group (DMRG) finite-size
scaling approach. At low (high) phonon frequencies the Luttinger liquid is
characterised by an attractive (repulsive) effective interaction. In the
charge-density wave Peierls-distorted state the charge structure factor scales
to a finite value indicating long-range order.Comment: 2 pages, 3 figures, submitted to SCES'0
Spin polarons in triangular antiferromagnets
The motion of a single hole in a 2D triangular antiferromagnet is
investigated using the t-J model. The one-hole states are described by strings
of spin deviations around the hole. Using projection technique the one-hole
spectral function is calculated. For large J/t we find low-lying
quasiparticle-like bands which are well separated from an incoherent background
by a gap of order J. However, for small J/t this gap vanishes and the spectrum
becomes broad over an energy range of several t. The results are compared with
SCBA calculations and numerical data.Comment: 4 pages, 6 figs, to be publish in PR
Crystal Field, Magnetic Anisotropy and Excitations in Rare-Earth Hexaborides
We clarify the role of crystalline electric field (CEF) induced magnetic
anisotropy in the ground state and spin-wave spectrum of cubic rare-earth
materials with dominating isotropic magnetic exchange interactions. In
particular we study the hexaboride NdB_6 which is shown to exhibit strong
spin-quadrupolar coupling. The CEF scheme is analyzed and a non-collinear
magnetization response is found. The spin orientation in the
antiferromagnetically ordered ground-state is identified. Moreover, the spin
excitations are evaluated and in agreement with inelastic neutron scattering a
suppression of one of the two magnetic modes in the strong-coupling regime is
predicted.Comment: 4 pages, 1 eps-figur
Destruction of long-range antiferromagnetic order by hole doping
We study the renormalization of the staggered magnetization of a
two-dimensional antiferromagnet as a function of hole doping, in the framework
of the t-J model. It is shown that the motion of holes generates decay of spin
waves into ''particle-hole'' pairs, which causes the destruction of the
long-range magnetic order at a small hole concentration. This effect is mainly
determined by the coherent motion of holes. The value obtained for the critical
hole concentration, of a few percent, is consistent with experimental data for
the doped copper oxide high-Tc superconductors.Comment: 12 pages, 2 figure
Linear response within the projection-based renormalization method: Many-body corrections beyond the random phase approximation
The explicit evaluation of linear response coefficients for interacting
many-particle systems still poses a considerable challenge to theoreticians. In
this work we use a novel many-particle renormalization technique, the so-called
projector-based renormalization method, to show how such coefficients can
systematically be evaluated. To demonstrate the prospects and power of our
approach we consider the dynamical wave-vector dependent spin susceptibility of
the two-dimensional Hubbard model and also determine the subsequent magnetic
phase diagram close to half-filling. We show that the superior treatment of
(Coulomb) correlation and fluctuation effects within the projector-based
renormalization method significantly improves the standard random phase
approximation results.Comment: 17 pages, 7 figures, revised versio
PREDICTING THE SUMMER TEMPERATURE OF SMALL STREAMS IN SOUTHWESTERN WISCONSIN 1
One of the biggest challenges in managing cold water streams in the Midwest is understanding how stream temperature is controlled by the complex interactions among meteorologic processes, channel geometry, and ground water inflow. Inflow of cold ground water, shade provided by riparian vegetation, and channel width are the most important factors controlling summer stream temperatures. A simple screening model was used to quantitatively evaluate the importance of these factors and guide management decisions. The model uses an analytical solution to the heat transport equation to predict steady-state temperature throughout a stream reach. The model matches field data from four streams in southwestern Wisconsin quite well (typically within 1°C) and helps explain the observed warming and cooling trends along each stream reach. The distribution of ground water inflow throughout a stream reach has an important influence on stream temperature, and springs are especially effective at providing thermal refuge for fish. Although simple, this model provides insight into the importance of ground water and the impact different management strategies, such as planting trees to increase shade, may have on summer stream temperature.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74032/1/j.1752-1688.2005.tb03714.x.pd
Spin polaron damping in the spin-fermion model for cuprate superconductors
A self-consistent, spin rotational invariant Green's function procedure has
been developed to calculate the spectral function of carrier excitations in the
spin-fermion model for the CuO2 plane. We start from the mean field description
of a spin polaron in the Mori-Zwanzig projection method. In order to determine
the spin polaron lifetime in the self-consistent Born approximation, the
self-energy is expressed by an irreducible Green's function. Both, spin polaron
and bare hole spectral functions are calculated. The numerical results show a
well pronounced quasiparticle peak near the bottom of the dispersion at
(pi/2,pi/2), the absence of the quasiparticle at the Gamma-point, a rather
large damping away from the minimum and an asymmetry of the spectral function
with respect to the antiferromagnetic Brillouin zone. These findings are in
qualitative agreement with photoemission data for undoped cuprates. The direct
oxygen-oxygen hopping is responsible for a more isotropic minimum at
(pi/2,pi/2).Comment: 18 pages, 13 figure
4f-spin dynamics in La(2-x-y)Sr(x)Nd(y)CuO(4)
We have performed inelastic magnetic neutron scattering experiments on
La(2-x-y)Sr(x)Nd(y)CuO(4) in order to study the Nd 4f-spin dynamics at low
energies. In all samples we find at high temperatures a quasielastic line
(Lorentzian) with a line width which decreases on lowering the temperature. The
temperature dependence of the quasielastic line width Gamma/2(T) can be
explained with an Orbach-process, i.e. a relaxation via the coupling between
crystal field excitations and phonons. At low temperatures the Nd-4f magnetic
response S(Q,omega) correlates with the electronic properties of the
CuO(2)-layers. In the insulator La(2-y)Nd(y)CuO(4) the quasielastic line
vanishes below 80 K and an inelastic excitation occurs. This directly indicates
the splitting of the Nd3+ ground state Kramers doublet due to the static
antiferromagnetic order of the Cu moments. In La(1.7-x)Sr(x)Nd(0.3)CuO(4) with
x = 0.12, 0.15 and La(1.4-x)Sr(x)Nd(0.6)CuO(4) with x = 0.1, 0.12, 0.15, 0.18
superconductivity is strongly suppressed. In these compounds we observe a
temperature independent broad quasielastic line of Gaussian shape below T about
30 K. This suggests a distribution of various internal fields on different Nd
sites and is interpreted in the frame of the stripe model. In
La(1.8-y)Sr(0.2)Nd(y)CuO(4) (y = 0.3, 0.6) such a quasielastic broadening is
not observed even at lowest temperature.Comment: 8 pages, 10 figures included, to appear in Phys. Rev.
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