49 research outputs found
Toward a Marxist Environmental Ethic: Restoration and Preservation in Focus
Restoration seeks to heal the environment and make amends for damages done by human interference. Preservationists, however, claim that restoration is anthropocentric, hubristic, and ultimately misguided. I defend restoration against these criticisms, and examine narratives from Karl Marx and Lynn White, Jr. to explain human alienation from nature. I use a synthesis of lessons from Marx and White to favor a restoration paradigm over a preservationist model
Bond order wave instabilities in doped frustrated antiferromagnets: "Valence bond solids" at fractional filling
We explore both analytically and numerically the properties of doped t-J
models on a class of highly frustrated lattices, such as the kagome and the
pyrochlore lattice. Focussing on a particular sign of the hopping integral and
antiferromagnetic exchange, we find a generic symmetry breaking instability
towards a twofold degenerate ground state at a fractional filling below half
filling. These states show modulated bond strengths and only break lattice
symmetries. They can be seen as a generalization of the well-known valence bond
solid states to fractional filling.Comment: slightly shortened and reorganized versio
A phase-field approach to studying the temperature-dependent ferroelectric response of bulk polycrystalline PZT
Ferroelectric ceramics are of interest for engineering applications because of their electro-mechanical coupling and the unique ability to permanently alter their atomic-level dipole structure (i.e., their polarization) and to induce large-strain actuation through applied electric fields. Although the underlying multiscale coupling mechanisms have been investigated by modeling strategies reaching from the atomic level across the polycrystalline mesoscale to the macroscopic device level, most prior work has neglected the important influence of temperature on the ferroelectric behavior. Here, we present a phase-field (diffuse-interface) constitutive model for ferroelectric ceramics, which is extended to account for the effects of finite temperature by considering thermal lattice vibrations based on statistical mechanics and by modifying the underlying Landau-Devonshire potential to depend on temperature. Results indicate that the chosen interpolation of the Landau energy coefficients is a suitable approach for predicting the temperature-dependent spontaneous polarization accurately over a broad temperature range. Lowering the energy barrier at finite temperature by the aforementioned methods also leads to better agreement with measurements of the bipolar hysteresis. Based on a numerical implementation via FFT spectral homogenization, we present simulation results of single- and polycrystals, which highlight the effect of temperature on the ferroelectric switching kinetics. We observe that thermal fluctuations (at the phase-field level realized by a thermalized stochastic noise term in the Allen-Cahn evolution equation) promote the nucleation of needle-like domains in regions of high heterogeneity or stress concentration such as grain boundaries. This, in turn, leads to a faster polarization reversal at low electric fields and a simulated domain pattern evolution comparable to experimental observations, stemming from the competition between nucleation and growth of domains. We discuss the development, implementation, validation, and application of the temperature-dependent phase-field framework for ferroelectric ceramics with a focus on tetragonal lead zirconate titanate (PZT), which we demonstrate to admit reasonable model predictions and comparison with experiments
Plaquette bond order wave in the quarter-filled extended Hubbard model on the checkerboard lattice
An extended Hubbard model (including nearest-neighbor repulsion and
antiferromagnetic spin exchange) is investigated on the frustrated checkerboard
lattice, a two-dimensional analog of the pyrochlore lattice. Combining
Gutzwiller renormalized mean-field (MF) calculations, exact diagonalization
(ED) techniques, and a weak-coupling renormalization group (RG) analysis we
provide strong evidence for a crystalline valence bond plaquette phase at
quarter-filling. The ground state is twofold degenerate and breaks translation
symmetry. The bond energies show a staggering while the charge distribution
remains uniform.Comment: 8 pages, 6 figures, published versio
Konservative, physiotherapeutische Behandlungsmethoden beim Karpaltunnelsyndrom
Darstellung des Themas: Das Karpaltunnelsyndrom (KTS) gehört zu den häufigsten Neuropathien der oberen Extremität, doch bisher herrscht Uneinigkeit über evidenzbasierte, konservative Behandlungsmethoden.
Ziel: Das Ziel dieser Arbeit besteht darin, konservative, physiotherapeutische Interventionen zur Behandlung des Karpaltunnelsyndroms aufzuzeigen und die Effektivität zu erläutern.
Methode: Auf medizinischen Datenbanken wurde eine Literaturrecherche durchgeführt. Am Ende verblieben fünf Studien, welche die Einschlusskriterien erfüllten, davon drei RCTs. Sie wurden anschliessend analysiert und auf ihre methodologische Qualität geprüft.
Relevante Ergebnisse: Die Behandlung durch konservative, physiotherapeutische Interventionen (Handgelenksschiene, TENS, Ultraschall, Iontophorese, Neuromobilisation) führte bei milden bis mässigen Beschwerden zu signifikanten Symptomreduktionen. Ebenso wurden die Symptome durch operative Massnahmen signifikant reduziert.
Schlussfolgerung: Konservative, physiotherapeutische Behandlungsmethoden beim KTS erweisen sich als effektiv bei milden bis mässigen Beschwerden. Operative Behandlungen bestätigen ebenfalls signifikante Ergebnisse, dabei spielt der Zeitpunkt des Eingriffs eine wichtige Rolle. Eine Aussage über die Wirksamkeit einzelner konservativer Interventionen kann aufgrund der Heterogenität der Studien nicht gemacht werden. Es braucht weitere Forschung, um die Fragestellung komplett beantworten zu können
Slave-boson mean-field theory of the Mott transition in the two-band Hubbard model
Abstract.: We apply the slave-boson approach of Kotliar and Ruckenstein to the two-band Hubbard model with an Ising like Hund's rule coupling and bands of different widths. On the mean-field level of this approach we investigate the Mott transition and observe both separate and joint transitions of the two bands depending on the choice of the inter- and intra-orbital Coulomb interaction parameters. The mean-field calculations allow for a simple physical interpretation and can confirm several aspects of previous work. Beside the case of two individually half-filled bands we also examine what happens if the original metallic bands possess fractional filling either due to finite doping or due to a crystal field which relatively shifts the atomic energy levels of the two orbitals. For appropriate values of the interaction and of the crystal field we can observe a band insulating state and a ferromagnetic meta
NaxCoO2: Enhanced low-energy excitations of electrons on a 2D triangular lattice
To elucidate the low-energy excitation spectrum of correlated electrons on a
2D triangular lattice, we have studied the electrical resistance and specific
heat down to 0.5 K and in magnetic fields up to 14 T, in NaxCoO2 samples with a
Na content ranging from x \approx 0.5 to 0.82. Two distinct regimes are
observed: for x from about 0.6 to x \approx 0.75 the specific heat is strongly
enhanced, with a pronounced upturn of C/T below about 10 K, reaching 47 mJ/(mol
K^2). This enhancement is suppressed in a magnetic field indicative of strong
low-energy spin fluctuations. At higher Na content the fluctuations are reduced
and mu-SR data confirm the SDW ground state below 22 K and the much reduced
heat capacity is field independent.Comment: Accepted in Physica
A phase-field approach to studying the temperature-dependent ferroelectric response of bulk polycrystalline PZT
Ferroelectric ceramics are of interest for engineering applications because of their electro-mechanical coupling and the unique ability to permanently alter their atomic-level dipole structure (i.e., their polarization) and to induce large-strain actuation through applied electric fields. Although the underlying multiscale coupling mechanisms have been investigated by modeling strategies reaching from the atomic level across the polycrystalline mesoscale to the macroscopic device level, most prior work has neglected the important influence of temperature on the ferroelectric behavior. Here, we present a phase-field (diffuse-interface) constitutive model for ferroelectric ceramics, which is extended to account for the effects of finite temperature by considering thermal lattice vibrations based on statistical mechanics and by modifying the underlying Landau-Devonshire potential to depend on temperature. Results indicate that the chosen interpolation of the Landau energy coefficients is a suitable approach for predicting the temperature-dependent spontaneous polarization accurately over a broad temperature range. Lowering the energy barrier at finite temperature by the aforementioned methods also leads to better agreement with measurements of the bipolar hysteresis. Based on a numerical implementation via FFT spectral homogenization, we present simulation results of single- and polycrystals, which highlight the effect of temperature on the ferroelectric switching kinetics. We observe that thermal fluctuations (at the phase-field level realized by a thermalized stochastic noise term in the Allen-Cahn evolution equation) promote the nucleation of needle-like domains in regions of high heterogeneity or stress concentration such as grain boundaries. This, in turn, leads to a faster polarization reversal at low electric fields and a simulated domain pattern evolution comparable to experimental observations, stemming from the competition between nucleation and growth of domains. We discuss the development, implementation, validation, and application of the temperature-dependent phase-field framework for ferroelectric ceramics with a focus on tetragonal lead zirconate titanate (PZT), which we demonstrate to admit reasonable model predictions and comparison with experiments
Polarization dependence of x-ray absorption spectra in Na_xCoO_2
In order to shed light on the electronic structure of Na_xCoO_2, and
motivated by recent Co L-edge X-ray absorption spectra (XAS) experiments with
polarized light, we calculate the electronic spectrum of a CoO_6 cluster
including all interactions between 3d orbitals. We obtain the ground state for
two electronic occupations in the cluster that correspond nominally to all O in
the O^{-2} oxidation state, and Co^{+3} or Co^{+4}. Then, all excited states
obtained by promotion of a Co 2p electron to a 3d electron, and the
corresponding matrix elements are calculated. A fit of the observed
experimental spectra is good and points out a large Co-O covalency and cubic
crystal field effects, that result in low spin Co 3d configurations. Our
results indicate that the effective hopping between different Co atoms plays a
major role in determining the symmetry of the ground state in the lattice.
Remaining quantitative discrepancies with the XAS experiments are expected to
come from composition effects of itineracy in the ground and excited states.Comment: 10 pages, 4 figure