308 research outputs found
Macroscopic Quantum Tunneling and Dissipation of Domain Wall in Ferromagnetic Metals
The depinning of a domain wall in ferromagentic metal via macroscopic quantum
tunneling is studied based on the Hubbard model. The dynamics of the
magnetization verctor is shown to be governed by an effective action of
Heisenberg model with a term non-local in time that describes the dissipation
due to the conduction electron. Due to the existence of the Fermi surface there
exists Ohmic dissipation even at zero temperature, which is crucially different
from the case of the insulator. Taking into account the effect of pinning and
the external magnetic field the action is rewritten in terms of a collective
coordinate, the position of the wall, . The tunneling rate for is
calculated by use of the instanton method. It is found that the reduction of
the tunneling rate due to the dissipation is very large for a thin domain wall
with thickness of a few times the lattice spacing, but is negligible for a
thick domain wall. Dissipation due to eddy current is shown to be negligible
for a wall of mesoscopic size.Comment: of pages 26, to appear in "Quantum Tunneling of Magnetization, ed. B.
Barbara and L. Gunther (Kluwer Academic Pub.), Figures available by FAX
(81-48-462-4649
Superfluids and Supersolids on Frustrated 2D Lattices
We study the ground state of hard-core bosons with nearest-neighbor hopping
and nearest-neighbor interactions on the triangular and Kagom\'e lattices by
mapping to a system of spins (), which we analyze using spin-wave
theory. We find that the both lattices display superfluid and supersolid (a
coexistence of superfluid and solid) order as the parameters and filling are
varied. Quantum fluctuations seem large enough in the Kagom\'e system to raise
the interesting possibility of a disordered ground state.Comment: Latex format, 24 figures available by email upon request. Submitted
to Physical Review
Magnetic Field Dependence of Macroscopic Quantum Tunneling and Coherence of Ferromagnetic Particle
We calculate the quantum tunneling rate of a ferromagnetic particle of diameter in a magnetic field of arbitrary angle. We consider the
magnetocrystalline anisotropy with the biaxial symmetry and that with the
tetragonal symmetry. Using the spin-coherent-state path integral, we obtain
approximate analytic formulas of the tunneling rates in the small -limit for the magnetic field normal to the easy axis (), for the field opposite to the initial easy axis (),
and for the field at an angle between these two orientations (). In addition, we obtain numerically the tunneling rates for
the biaxial symmetry in the full range of the angle of the magnetic
field (), for the values of \epsilon =0.01 and
0.001.Comment: 25 pages of text (RevTex) and 4 figures (PostScript files), to be
published in Phys. Rev.
Berry's phase and Quantum Dynamics of Ferromagnetic Solitons
We study spin parity effects and the quantum propagation of solitons (Bloch
walls) in quasi-one dimensional ferromagnets. Within a coherent state path
integral approach we derive a quantum field theory for nonuniform spin
configurations. The effective action for the soliton position is shown to
contain a gauge potential due to the Berry phase and a damping term caused by
the interaction between soliton and spin waves. For temperatures below the
anisotropy gap this dissipation reduces to a pure soliton mass renormalization.
The gauge potential strongly affects the quantum dynamics of the soliton in a
periodic lattice or pinning potential. For half-integer spin, destructive
interference between soliton states of opposite chirality suppresses nearest
neighbor hopping. Thus the Brillouin zone is halved, and for small mixing of
the chiralities the dispersion reveals a surprising dynamical correlation: Two
subsequent band minima belong to different chirality states of the soliton. For
integer spin, the Berry phase is inoperative and a simple tight-binding
dispersion is obtained. Finally it is shown that external fields can be used to
interpolate continuously between the Bloch wall dispersions for half-integer
and integer spin.Comment: 20 pages, RevTex 3.0 (twocolumn), to appear in Phys. Rev. B 53, 3237
(1996), 4 PS figures available upon reques
A Standardized Morpho-Functional Classification of the Planetâs Humipedons
It was time to take stock. We modified the humipedon classification key published in 2018 to make it easier and more practical. This morpho-functional taxonomy of the topsoil (humipedon) was only available in English; we also translated it into French and Italian. A standardized morphofunctional classification of humipedons (roughly the top 30â40 cm of soil: organic and organomineral surface horizons) would allow for a better understanding of the functioning of the soil ecosystem. This paper provides the founding principles of the classification of humipedon into humus systems and forms. With the recognition of a few diagnostic horizons, all humus systems can be determined. The humus forms that make up these humus systems are revealed by measuring the thicknesses of the diagnostic horizons. In the final part of the article, several figures represent the screenshots of a mobile phone or tablet application that allows for a fast recall of the diagnostic elements of the classification in the field. The article attempts to promote a standardized classification of humipedons for a global and shared management of soil at planet level
A standardized morpho-functional classification of the planetâs humipedons
It was time to take stock. We modified the humipedon classification key published in 2018 to make it easier and more practical. This morpho-functional taxonomy of the topsoil (humipedon) was only available in English; we also translated it into French and Italian. A standardized morphofunctional classification of humipedons (roughly the top 30â40 cm of soil: organic and organomineral surface horizons) would allow for a better understanding of the functioning of the soil ecosystem. This paper provides the founding principles of the classification of humipedon into humus systems and forms. With the recognition of a few diagnostic horizons, all humus systems can be determined. The humus forms that make up these humus systems are revealed by measuring the thicknesses of the diagnostic horizons. In the final part of the article, several figures represent the screenshots of a mobile phone or tablet application that allows for a fast recall of the diagnostic elements of the classification in the field. The article attempts to promote a standardized classification of humipedons for a global and shared management of soil at planet level
Post-tuberculosis lung health: perspectives from the first International symposium
Tuberculosis, although curable, frequently leaves the individual with chronic physical and psycho-social impairment, yet these consequences have to-date been largely neglected. The 1st International Post-Tuberculosis Symposium was devoted entirely to impairment after tuberculosis, and covered a number of multi-disciplinary topics. Using the Delphi process, consensus was achieved for the terms âpost-tuberculosisâ, âpost-tuberculosis lung disease/s (PTLD)â, and âpost-tuberculosis economic, social and psychological well-beingâ (Post-TB ESP)â, to overcome the historical challenge of varied terminology in the literature. A minimum case-definition was proposed by consensus for PTLD in adults and children. Lack of sufficient evidence hampered definitive recommendations in most domains, including prevention and treatment of PTLD, but highlighted the dire need for research and priorities were identified. The heterogeneity of respiratory outcomes and previously employed research methodologies complicates the accurate estimation of disease burden. However, consensus was reached proposing a toolkit for future PTLD measurement, and on PTLD patterns to be considered. The importance of extra-pulmonary consequences and progressive impairment throughout the life-course was identified, including tuberculosis recurrence and increased mortality. Patient advocates emphasised the need for addressing the psychological and social impacts post tuberculosis, and called for clinical guidance. Increased awareness and more research addressing post-tuberculosis complications is urgently needed
Green function techniques in the treatment of quantum transport at the molecular scale
The theoretical investigation of charge (and spin) transport at nanometer
length scales requires the use of advanced and powerful techniques able to deal
with the dynamical properties of the relevant physical systems, to explicitly
include out-of-equilibrium situations typical for electrical/heat transport as
well as to take into account interaction effects in a systematic way.
Equilibrium Green function techniques and their extension to non-equilibrium
situations via the Keldysh formalism build one of the pillars of current
state-of-the-art approaches to quantum transport which have been implemented in
both model Hamiltonian formulations and first-principle methodologies. We offer
a tutorial overview of the applications of Green functions to deal with some
fundamental aspects of charge transport at the nanoscale, mainly focusing on
applications to model Hamiltonian formulations.Comment: Tutorial review, LaTeX, 129 pages, 41 figures, 300 references,
submitted to Springer series "Lecture Notes in Physics
Thin Polymer Brush Decouples Biomaterial's Micro-/Nano-Topology and Stem Cell Adhesion
Surface morphology and chemistry of polymers used as biomaterials, such as tissue engineering scaffolds, have a strong influence on the adhesion and behavior of human mesenchymal stem cells. Here we studied semicrystalline poly(Δ-caprolactone) (PCL) substrate scaffolds, which exhibited a variation of surface morphologies and roughness originating from different spherulitic superstructures. Different substrates were obtained by varying the parameters of the thermal processing, i.e. crystallization conditions. The cells attached to these polymer substrates adopted different morphologies responding to variations in spherulite density and size. In order to decouple substrate topology effects on the cells, sub-100 nm bio-adhesive polymer brush coatings of oligo(ethylene glycol) methacrylates were grafted from PCL and functionalized with fibronectin. On surfaces featuring different surface textures, dense and sub-100 nm thick brush coatings determined the response of cells, irrespective to the underlying topology. Thus, polymer brushes decouple substrate micro-/nano-topology and the adhesion of stem cells
Pairing in nuclear systems: from neutron stars to finite nuclei
We discuss several pairing-related phenomena in nuclear systems, ranging from
superfluidity in neutron stars to the gradual breaking of pairs in finite
nuclei. We focus on the links between many-body pairing as it evolves from the
underlying nucleon-nucleon interaction and the eventual experimental and
theoretical manifestations of superfluidity in infinite nuclear matter and of
pairing in finite nuclei. We analyse the nature of pair correlations in nuclei
and their potential impact on nuclear structure experiments. We also describe
recent experimental evidence that points to a relation between pairing and
phase transitions (or transformations) in finite nuclear systems. Finally, we
discuss recent investigations of ground-state properties of random two-body
interactions where pairing plays little role although the interactions yield
interesting nuclear properties such as 0+ ground states in even-even nuclei.Comment: 74 pages, 33 figs, uses revtex4. Submitted to Reviews of Modern
Physic
- âŠ