51 research outputs found
Assessing N competition between outplanted conifer seedlings and early successional plants using ion-exchange membranes
Non-Peer ReviewedDuring the early establishment phase, outplanted white spruce (Picea glauca (Moench) Voss.)
and jack pine (Pinus banksiana Lamb.) seedlings are vulnerable to lethargic growth or mortality
because of interspecific competition for soil nutrients, particularly nitrogen (N). Accurately
quantifying the degree of N competition is essential for supporting effective vegetation
management decisions. This study evaluated N competition at four boreal forest sites, three
years following outplanting, using two-week in situ burials of ion-exchange membrane (IEM) in
plots with and without vegetation management (VM). The effect of noncrop N uptake on soil N
availability also was assessed using conventional 2N KCl extractions. Vegetation management
continued to support increased conifer seedling growth, with no effect on survival compared to
control plots. Although the N supply rate measured using IEM (Plant Root Simulator™-probes)
were not correlated (P >0.05) with 2N KCl-extracted N concentration, there was a correlation (R2
= 0.68 to 0.76, P <0.01) between N supply rate and seedling growth. Ammonium-N supply rate
was better correlated than NO3--N with conifer seedling growth, which is in agreement with
preferential NH4+-N uptake by conifer species. The results of this study support the use of in situ
IEM burials for monitoring soil N bioavailability during the early establishment phase
The optical conductivity of the quasi one-dimensional conductors: the role of forward scattering by impurities
We calculate the average conductivity sigma (omega) of interacting electrons
in one dimension in the presence of a long-range random potential (forward
scattering disorder). Taking the curvature of the energy dispersion into
account, we show that weak disorder leads to a transport scattering rate that
vanishes as omega^2 for small frequency omega. This implies that the real part
of the conductivity remains finite for omega -> 0, while the imaginary part
diverges. These effects are lost within the usual bosonization approach, which
relies on the linearization of the energy dispersion. We discuss our result in
the light of a recent experiment.Comment: 5 RevTex pages; more careful comparison with experiments and
discussion of interchain hopping added; some references added; to appear in
Phys. Rev.
The Nature of Heavy Quasiparticles in Magnetically Ordered Heavy Fermions
The optical conductivity of the heavy fermions UPd2Al3 and UPt3 has been
measured in the frequency range from 10 GHz to 1.2 THz (0.04 meV to 5 meV) at
temperatures 1 K < T < 300 K. In both compounds a well pronounced pseudogap of
less than a meV develops in the optical response at low temperatures; we relate
this to the antiferromagnetic ordering. From the energy dependence of the
effective electronic mass and scattering rate we derive the energies essential
for the heavy quasiparticle. We find that the enhancement of the mass mainly
occurs below the energy which is related to magnetic correlations between the
local magnetic moments and the itinerant electrons. This implies that the
magnetic order in these compounds is the pre-requisite to the formation of the
heavy quasiparticle and eventually of superconductivity.Comment: RevTeX, 4 pages, 3 figures, email:
[email protected]
Resonant Two-Magnon Raman Scattering in Cuprate Antiferromagnetic Insulators
We present results of low-temperature two-magnon resonance Raman excitation
profile measurements for single layer Sr_2CuO_2Cl_2 and bilayer YBa_2Cu_3O_{6 +
\delta} antiferromagnets over the excitation region from 1.65 to 3.05 eV. These
data reveal composite structure of the two-magnon line shape and strong
nonmonotic dependence of the scattering intensity on excitation energy. We
analyze these data using the triple resonance theory of Chubukov and Frenkel
(Phys. Rev. Lett., 74, 3057 (1995)) and deduce information about magnetic
interaction and band parameters in these materials.Comment: REVTeX, 4 pages + 2 PostScript (compressed) figure
Tc for heavy Fermion superconductors linked with other physical properties at zero and applied pressure
The superconducting transition temperature Tc has earlier been correlated
with coherence length and effective mass for a series of heavy Fermion (HF)
materials at atmospheric pressure. Here, a further physical property, the dc
electrical conductivity sigma(Tc), is one focal point, another being the
pressure dependence of both Tc and sigma(Tc) for several HF materials. The
relaxation time tau(Tc) is also studied in relation to an Uncertainty Principle
limit, involving only the thermal energy kB Tc and Planck's constant.Comment: Supercond. Sci. Tech., to appea
Non-Fermi liquid behavior of SrRuO_3 -- evidence from infrared conductivity
The reflectivity of the itinerant ferromagnet SrRuO_3 has been measured
between 50 and 25,000 cm-1 at temperatures ranging from 40 to 300 K, and used
to obtain conductivity, scattering rate, and effective mass as a function of
frequency and temperature. We find that at low temperatures the conductivity
falls unusually slowly as a function of frequency (proportional to
\omega^{-1/2}), and at high temperatures it even appears to increase as a
function of frequency in the far-infrared limit. The data suggest that the
charge dynamics of SrRuO_3 are substantially different from those of
Fermi-liquid metals.Comment: 4 pages, 3 postscript figure
Electronic Structure and Charge Dynamics of Huesler Alloy Fe2TiSn Probed by Infrared and Optical Spectroscopy
We report on the electrodynamics of a Heusler alloy Fe2TiSn probed over four
decades in energy: from the far infrared to the ultraviolet. Our results do not
support the suggestion of Kondo-lattice behavior inferred from specific heat
measurements. Instead, we find a conventional Drude-like response of free
carriers, with two additional absorption bands centered at around 0.1 and 0.87
eV. The latter feature can be interpreted as excitations across a pseudogap, in
accord with band structure calculations.Comment: 3 pages, 4 figure
Two--magnon scattering and the spin--phonon interaction beyond the adiabatic approximation
We consider a model of Raman scattering for a two--dimensional
Heisenberg Anti-Ferromagnet which includes a {\it dynamical} spin--phonon
interaction. We observe a broadening of the line shape due to increased
coupling with excited high--energy spin states. Our results are close to a
model of random static exchange interactions, first introduced in this context
by Haas {\it et al.} [J. Appl. Phys. {\bf 75}, 6340, (1994)], which, when
extended to large numbers of spins, explains experiments in the parent
insulating compounds of high- superconductors.Comment: 14 pages (revtex format), 8 postscript figure
The Antiferromagnetic Band Structure of La2CuO4 Revisited
Using the Becke-3-LYP functional, we have performed band structure
calculations on the high temperature superconductor parent compound, La2CuO4.
Under the restricted spin formalism (rho(alpha) equal to rho(beta)), the
R-B3LYP band structure agrees well with the standard LDA band structure. It is
metallic with a single Cu x2-y2/O p(sigma) band crossing the Fermi level. Under
the unrestricted spin formalism (rho(alpha) not equal to rho(beta)), the UB3LYP
band structure has a spin polarized antiferromagnetic solution with a band gap
of 2.0 eV, agreeing well with experiment. This state is 1.0 eV (per formula
unit) lower than that calculated from the R-B3LYP. The apparent high energy of
the spin restricted state is attributed to an overestimate of on-site Coulomb
repulsion which is corrected in the unrestricted spin calculations. The
stabilization of the total energy with spin polarization arises primarily from
the stabilization of the x2-y2 band, such that the character of the eigenstates
at the top of the valence band in the antiferromagnetic state becomes a strong
mixture of Cu x2-y2/O p(sigma) and Cu z2/O' p(z). Since the Hohenberg-Kohn
theorem requires the spin restricted and spin unrestricted calculations give
exactly the same ground state energy and total density for the exact
functionals, this large disparity in energy reflects the inadequacy of current
functionals for describing the cuprates. This calls into question the use of
band structures based on current restricted spin density functionals (including
LDA) as a basis for single band theories of superconductivity in these
materials.Comment: 13 pages, 8 figures, to appear in Phys. Rev. B, for more information
see http://www.firstprinciples.co
Proposal of an extended t-J Hamiltonian for high-Tc cuprates from ab initio calculations on embedded clusters
A series of accurate ab initio calculations on Cu_pO-q finite clusters,
properly embedded on the Madelung potential of the infinite lattice, have been
performed in order to determine the local effective interactions in the CuO_2
planes of La_{2-x}Sr_xCuO_4 compounds. The values of the first-neighbor
interactions, magnetic coupling (J_{NN}=125 meV) and hopping integral
(t_{NN}=-555 meV), have been confirmed. Important additional effects are
evidenced, concerning essentially the second-neighbor hopping integral
t_{NNN}=+110meV, the displacement of a singlet toward an adjacent colinear
hole, h_{SD}^{abc}=-80 meV, a non-negligible hole-hole repulsion
V_{NN}-V_{NNN}=0.8 eV and a strong anisotropic effect of the presence of an
adjacent hole on the values of the first-neighbor interactions. The dependence
of J_{NN} and t_{NN} on the position of neighbor hole(s) has been rationalized
from the two-band model and checked from a series of additional ab initio
calculations. An extended t-J model Hamiltonian has been proposed on the basis
of these results. It is argued that the here-proposed three-body effects may
play a role in the charge/spin separation observed in these compounds, that is,
in the formation and dynamic of stripes.Comment: 24 pages, 4 figures, submitted to Phys. Rev.
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