17,712 research outputs found
Many-body Theory at Extreme Isospin
The structure of nuclei far off beta-stability is investigated by nuclear
many-body theory. In-medium interactions for asymmetric nuclear matter are
obtained by (Dirac-) Brueckner theory thus establishing the link of nuclear
forces to free space interactions. HFB and RPA theory is used to describe
ground and excited states of nuclei from light to heavy masses. In extreme
dripline systems pairing and core polarization are found to be most important
for the binding, especially of halo nuclei. The calculations show that far off
stability mean-field dynamics is gradually replaced by dynamical correlations,
giving rise to the dissolution of shell structures.Comment: 10 pages, 5 figures, to appear in the proceedings of Nuclear Physics
at the Borderline, NPBL2001, Lipari, Sicily, Italy, May 2001 (World
Scientific
Solution to the problem of the poor cyclic fatigue resistance of bulk metallic glasses
The recent development of metallic glass-matrix composites represents a particular milestone in engineering materials for structural applications owing to their remarkable combination of strength and toughness. However, metallic glasses are highly susceptible to cyclic fatigue damage, and previous attempts to solve this problem have been largely disappointing. Here, we propose and demonstrate a microstructural design strategy to overcome this limitation by matching the microstructural length scales (of the second phase) to mechanical crack-length scales. Specifically, semisolid processing is used to optimize the volume fraction, morphology, and size of second-phase dendrites to confine any initial deformation (shear banding) to the glassy regions separating dendrite arms having length scales of ≈2 μm, i.e., to less than the critical crack size for failure. Confinement of the damage to such interdendritic regions results in enhancement of fatigue lifetimes and increases the fatigue limit by an order of magnitude, making these “designed” composites as resistant to fatigue damage as high-strength steels and aluminum alloys. These design strategies can be universally applied to any other metallic glass systems
Weak magnetic anisotropy in GdRhSi studied by magnetic resonance
The antiferromagnetically (AFM) ordered state of GdRhSi which
consists of AFM-stacked ferromagnetic layers is investigated by magnetic
resonance spectroscopy. The almost isotropic Gd paramagnetic resonance
becomes anisotropic in the AFM ordered region below 107 K. The emerging
internal anisotropic exchange-fields are still small enough to allow an
investigation of their magnetization dynamics by using a standard
microwave-frequency magnetic resonance technique. We could characterize this
anisotropy in detail in the ferromagnetic layers of the excitation at 9 and 34
GHz. We derived a resonance condition for the AFM ordered phase to describe the
weak in-plane anisotropic behaviour in combination with a mean-field analysis.Comment: 7 page
Spiral phases and two-particle bound states from a systematic low-energy effective theory for magnons, electrons, and holes in an antiferromagnet
We have constructed a systematic low-energy effective theory for hole- and
electron-doped antiferromagnets, where holes reside in momentum space pockets
centered at and where electrons live in
pockets centered at or . The effective
theory is used to investigate the magnon-mediated binding between two holes or
two electrons in an otherwise undoped system. We derive the one-magnon exchange
potential from the effective theory and then solve the corresponding
two-quasiparticle Schr\"odinger equation. As a result, we find bound state wave
functions that resemble -like or -like symmetry. We also
study possible ground states of lightly doped antiferromagnets.Comment: 2 Pages; Proc. of SCES'07, Housto
GdRhSi: An exemplary tetragonal system for antiferromagnetic order with weak in-plane anisotropy
The anisotropy of magnetic properties commonly is introduced in textbooks
using the case of an antiferromagnetic system with Ising type anisotropy. This
model presents huge anisotropic magnetization and a pronounced metamagnetic
transition and is well-known and well-documented both, in experiments and
theory. In contrast, the case of an antiferromagnetic - system with weak
in-plane anisotropy is only poorly documented. We studied the anisotropic
magnetization of the compound GdRhSi and found that it is a perfect
model system for such a weak-anisotropy setting because the Gd ions in
GdRhSi have a pure spin moment of S=7/2 which orders in a simple AFM
structure with . We observed experimentally in a
continuous spin-flop transition and domain effects for field applied along the
- and the -direction, respectively. We applied a mean field model
for the free energy to describe our data and combine it with an Ising chain
model to account for domain effects. Our calculations reproduce the
experimental data very well. In addition, we performed magnetic X-ray
scattering and X-ray magnetic circular dichroism measurements, which confirm
the AFM propagation vector to be and indicate the absence of
polarization on the rhodium atoms
Homogeneous versus Spiral Phases of Hole-doped Antiferromagnets: A Systematic Effective Field Theory Investigation
Using the low-energy effective field theory for magnons and holes -- the
condensed matter analog of baryon chiral perturbation theory for pions and
nucleons in QCD -- we study different phases of doped antiferromagnets. We
systematically investigate configurations of the staggered magnetization that
provide a constant background field for doped holes. The most general
configuration of this type is either constant itself or it represents a spiral
in the staggered magnetization. Depending on the values of the low-energy
parameters, a homogeneous phase, a spiral phase, or an inhomogeneous phase is
energetically favored. The reduction of the staggered magnetization upon doping
is also investigated.Comment: 35 pages, 5 figure
On researching a health condition that the researcher has also experienced
This paper aims to explicate the particular aspects of reflexivity that arise in a research project when the researcher has personally experienced the condition that is the topic of the research. These issues are illustrated in the context of one specific study, in which the researcher was conducting qualitative research on an intervention for type 2 diabetes, when she herself had had type 1 diabetes since early childhood. The first set of issues concerns the advantages and disadvantages of the researcher’s experiential knowledge of the topic. These include the possibility for greater empathy with participants, the concomitant problem of false assumed similarity, the need to bracket one’s expectations, and the dilemma of whether or not to disclose one’s condition to participants. The second set concerns the researcher’s emotional reactions to the content of the research: becoming aware of the potential harmful consequences of one’s condition, feeling a sense of connectedness with the participants, learning from their struggles, and potentially benefitting personally from conducting the research. These issues are particularly salient in qualitative research, but may also apply to quantitative approaches. They are important to examine, not only because they may affect the trustworthiness of the findings, but also because of the ethical imperative to evaluate the potential impact of the research on both the participants and the researcher
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