69 research outputs found
Gauge Theory for a Doped Antiferromagnet in a Rotating Reference-Frame
We study a doped antiferromagnet (AF) using a rotating reference-frame.
Whereas in the laboratory reference-frame with a globally fixed
spin-quantization axis (SQA) the long-wavelength, low-energy physics is given
by the O(3) non-linear -model with current-current interactions between
the fermionic degrees of freedom and the order-parameter field for the
spin-background, an alternative description in form of an U(1) gauge theory can
be derived by choosing the SQA defined by the local direction of the
order-parameter field via a SU(2) rotation of the fermionic spinor. Within a
large- expansion of this U(1) gauge theory we obtain the phase diagram for
the doped AF and identify the relevant terms due to doping that lead to a
quantum phase transition at from the antiferromagnetically ordered N\'eel
phase to the quantum-disordered (QD) spin-liquid phase. Furthermore, we
calculate the propagator of the corresponding U(1) gauge field, which mediates
a long-range transverse interaction between the bosonic and fermionic fields.
It is found that the strength of the propagator is proportional to the gap of
the spin-excitations. Therefore, we expect as a consequence of this long-range
interaction the formation of bound states when the spin-gap opens, i.e.\ in the
QD spin-liquid phase. The possible bound states are spin-waves with a (spin-)
gap in the excitation spectrum, spinless fermions and pairs of fermions. Thus,
an alternative picture for charge-spin separation emerges, with composite
charge-separated excitations. Moreover, the present treatment shows an intimate
connection between the opening of the spin-gap and charge-spin separation as
well as pairing.Comment: 13 pages, also available at
http://www.physik.uni-augsburg.de/theo2/Publications
Quantum disordered phase in a doped antiferromagnet
A quantitative description of the transition to a quantum disordered phase in
a doped antiferromagnet is obtained with a U(1) gauge-theory, where the gap in
the spin-wave spectrum determines the strength of the gauge-fields. They
mediate an attractive long-range interaction whose possible bound-states
correspond to charge-spin separation and pairing.Comment: 11 pages, LaTex, chris-preprint-1994-
Searching for zeroes: unconventional superconductors in a magnetic field
We review the results of the microscopic approach to the calculation of the
anisotropy in the specific heat in unconventional superconductors under rotated
field. Treating vortex scattering on equal footing with the energy shift we
find that the electronic specific heat may have minima or maxima when the field
is aligned with the nodes, depending on the temperature and field range. We
discuss the influence of the paramagnetic limiting and Fermi surface shape on
the location of the inversion line.Comment: Proceedings of SCES-0
Thermal conductivity in the vortex state of d-wave superconductors
We present the results of a microscopic calculation of the longitudinal
thermal conductivity of quasiparticles, , in a 2D d-wave
superconductor in the vortex state. Our approach takes into account both
impurity scattering and a contribution to the thermal transport lifetime due to
the scattering of quasiparticles off of vortices. We compare the results with
the experimental measurements on high-T cuprates and organic
superconductors.Comment: 2 pages, submitted to proceedings of M2S-HTSC-VI (Houston
Plant water uptake modelling : added value of cross-disciplinary approaches
Abstract In recent years, research interest in plant water uptake strategies has significantly grown in many disciplines such as hydrology, plant ecology and ecophysiology. Quantitative modelling approaches to estimate plant water uptake and the spatio-temporal dynamics significantly advanced from different disciplines across scales. Despite this progress, major limitations, i.e. to predict plant water uptake under drought or it?s impact at large-scales remain. These are less attributed to limitations in process understanding, but rather to a lack of implementation of cross-disciplinary insights in plant water uptake model structure. The main goal of this review is to highlight how the 4 dominant model approaches, e.g. Feddes approach, hydrodynamic approach, optimality and statistical approaches, can be and have been used to create interdisciplinary hybrid models enabeling a holistic system understanding that e.g. embeds plant water uptake plasticity into a broader conceptual view of soil-plant feedbacks of water, nutrient and carbon cycling or reflects observed drought responses of plant-soil feedbacks and their dynamics under e.g. drought. Specifically, we provide examples of how integration of Bayesian and hydrodynamic approaches might overcome challenges in interpreting plant water uptake related to e.g. different travel and residence times of different plant water sources or trade-offs between root system optimization to forage for water and nutrients during different seasons and phenological stages.In recent years, research interest in plant water uptake strategies has rapidly increased in many disciplines, such as hydrology, plant ecology and ecophysiology. Quantitative modelling approaches to estimate plant water uptake and spatiotemporal dynamics have significantly advanced through different disciplines across scales. Despite this progress, major limitations, for example, predicting plant water uptake under drought or drought impact at large scales, remain. These are less attributed to limitations in process understanding, but rather to a lack of implementation of cross-disciplinary insights into plant water uptake model structure. The main goal of this review is to highlight how the four dominant model approaches, that is, Feddes approach, hydrodynamic approach, optimality and statistical approaches, can be and have been used to create interdisciplinary hybrid models enabling a holistic system understanding that, among other things, embeds plant water uptake plasticity into a broader conceptual view of soil-plant feedbacks of water, nutrient and carbon cycling, or reflects observed drought responses of plant-soil feedbacks and their dynamics under, that is, drought. Specifically, we provide examples of how integration of Bayesian and hydrodynamic approaches might overcome challenges in interpreting plant water uptake related to different travel and residence times of different plant water sources or trade-offs between root system optimization to forage for water and nutrients during different seasons and phenological stages.Peer reviewe
Angle Dependence of the Transverse Thermal Conductivity in YBaCuO single crystals: Doppler Effect vs. Andreev scattering
We have measured the transverse thermal conductivity of twinned
and untwinned YBaCuO single crystals as a function of angle
between the magnetic field applied parallel to the CuO planes and
the heat current direction, at different magnetic fields and temperatures. For
both crystals we observed a clear twofold variation in the field-angle
dependence of . We
have found that the oscillation amplitude depends on
temperature and magnetic field. Our results show that with the temperature- and sample-dependent parameters and .
We discuss our results in terms of Andreev scattering of quasiparticles by
vortices and a recently proposed theory based on the Doppler shift in the
quasiparticle spectrum.Comment: 5 pages, 4 figure
A model for the doped copper oxide compounds
We present a relativistic spin-fermion model for the cuprates, in which both
the charge and spin degrees of freedom are treated dynamically. The spin-charge
coupling parameter is associated with the doping fraction. The model is able to
account for the various phases of the cuprates and their properties, not only
at low and intermediate doping but also for (highly) over-doped compounds. In
particular, we acquire a qualitative understanding of high-T_c
superconductivity through Bose-Einstein condensation of bound charge pairs. The
mechanism that binds these pairs does not require a Fermi sea.Comment: 9 pages, 2 postscript figures. Version accepted for publication in
Europhys. Let
Transport Properties of d-Wave Superconductors in the Vortex State
We calculate the magnetic field dependence of quasiparticle transport
properties in the vortex state of a d-wave superconductor arising solely from
the quasiparticle's Doppler shift in the superflow field surrounding the
vortex. Qualitative features agree well with experiments on cuprate and heavy
fermion superconductors at low fields and temperatures. We derive scaling
relations in the variable valid at sufficiently low temperatures
and fields , but show that these relations depend on the scattering
phase shift, and are in general fulfilled only approximately even in the clean
limit, due to the energy dependence of the quasiparticle relaxation time.Comment: 5 pages, 2 Postscript figure
Anisotropic thermodynamics of d-wave superconductors in the vortex state
We show that the density of states and the thermodynamic properties of a 2D
d-wave superconductor in the vortex state with applied magnetic field
in the plane depend on the angle between and the order parameter nodes.
Within a semiclassical treatment of the extended quasiparticle states, we
obtain fourfold oscillations of the specific heat, measurement of which
provides a simple probe of gap symmetry. The frequency dependence of the
density of states and the temperature dependence of thermodynamic properties
obey different power laws for field in the nodal and anti-nodal direction. The
fourfold pattern is changed to twofold when orthorhombicity is considered.Comment: 5 pages, figures included, minor changes, published versio
Quasiparticle transport in the vortex state of YBa_2Cu_3O_6.9
The effect of vortices on quasiparticle transport in cuprate superconductors
was investigated by measuring the low temperature thermal conductivity of
YBa_2Cu_3O_6.9 in magnetic fields up to 8 T. The residual linear term (as T \to
0) is found to increase with field, directly reflecting the occupation of
extended quasiparticle states. A study for different Zn impurity concentrations
reveals a good agreement with recent calculations for a d-wave superconductor,
thereby shedding light on the nature of scattering by both impurities and
vortices. It also provides a quantitative measure of the gap near the nodes.Comment: 4 pages, 2 included eps figures, significant new analysis wrt other
experiments, to appear in Phys Rev Lett 29 March 199
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