3,695 research outputs found
Seniority in quantum many-body systems. I. Identical particles in a single shell
A discussion of the seniority quantum number in many-body systems is
presented. The analysis is carried out for bosons and fermions simultaneously
but is restricted to identical particles occupying a single shell. The emphasis
of the paper is on the possibility of {\em partial} conservation of seniority
which turns out to be a peculiar property of spin-9/2 fermions but prevalent in
systems of interacting bosons of any spin. Partial conservation of seniority is
at the basis of the existence of seniority isomers, frequently observed in
semi-magic nuclei, and also gives rise to peculiar selection rules in
one-nucleon transfer reactions.Comment: 41 pages, 7 figures, 1 tables, submitted to Annals of Physic
Role of tip size, orientation, and structural relaxations in first-principles studies of magnetic exchange force microscopy and spin-polarized scanning tunneling microscopy
Using first-principles calculations based on density functional theory (DFT),
we investigate the exchange interaction between a magnetic tip and a magnetic
sample which is detected in magnetic exchange force microscopy (MExFM) and also
occurs in spin-polarized scanning tunneling microscopy (SP-STM) experiments. As
a model tip-sample system, we choose Fe tips and one monolayer Fe on W(001)
which exhibits a checkerboard antiferromagnetic structure and has been
previously studied with both SP-STM and MExFM. We calculate the exchange forces
and energies as a function of tip-sample distance using different tip models
ranging from single Fe atoms to Fe pyramids consisting of up to 14 atoms. We
find that modelling the tip by a single Fe atom leads to qualitatively
different tip-sample interactions than using clusters consisting of several
atoms. Increasing the cluster size changes the calculated forces quantitatively
enhancing the detectable exchange forces. Rotating the tip with respect to the
surface unit cell has only a small influence on the tip-sample forces.
Interestingly, the exchange forces on the tip atoms in the nearest and
next-nearest layers from the apex atom are non-negligible and can be opposite
to that on the apex atom for a small tip. In addition, the apex atom interacts
not only with the surface atoms underneath but also with nearest-neighbors in
the surface. We find that structural relaxations of tip and sample due to their
interaction depend sensitively on the magnetic alignment of the two systems. As
a result the onset of significant exchange forces is shifted towards larger
tip-sample separations which facilitates their measurement in MExFM. At small
tip-sample separations, structural relaxations of tip apex and surface atoms
can either enhance or reduce the magnetic contrast measured in SP-STMComment: 14 pages, 13 figure
Drain Voltage Scaling in Carbon Nanotube Transistors
While decreasing the oxide thickness in carbon nanotube field-effect
transistors (CNFETs) improves the turn-on behavior, we demonstrate that this
also requires scaling the range of the drain voltage. This scaling is needed to
avoid an exponential increase in Off-current with drain voltage, due to
modulation of the Schottky barriers at both the source and drain contact. We
illustrate this with results for bottom-gated ambipolar CNFETs with oxides of 2
and 5 nm, and give an explicit scaling rule for the drain voltage. Above the
drain voltage limit, the Off-current becomes large and has equal electron and
hole contributions. This allows the recently reported light emission from
appropriately biased CNFETs.Comment: 4 pages, 4 EPS figure, to appear in Appl. Phys. Lett. (issue of 15
Sept 2003
Unexpected Scaling of the Performance of Carbon Nanotube Transistors
We show that carbon nanotube transistors exhibit scaling that is
qualitatively different than conventional transistors. The performance depends
in an unexpected way on both the thickness and the dielectric constant of the
gate oxide. Experimental measurements and theoretical calculations provide a
consistent understanding of the scaling, which reflects the very different
device physics of a Schottky barrier transistor with a quasi-one-dimensional
channel contacting a sharp edge. A simple analytic model gives explicit scaling
expressions for key device parameters such as subthreshold slope, turn-on
voltage, and transconductance.Comment: 4 pages, 4 figure
Moderately Discontinuous Algebraic Topology for Metric Subanalytic Germs
We have developed both a homology theory and a homotopy theory in the context of metric subanalytic germs (see Definition 2.1). The former is called MD homology and is covered in Chapter 2, which contains a paper that is joined work with my PhD advisors Javier Fernández de Bobadilla and María Pe Pereira and with Edson Sam- paio. The latter is called MD homotopy and is covered in Chapter 3. Both theories are functors from a category of germs of metric subanalytic spaces (resp. germs of metric subanalytic spaces that are punctured in a way that will be defined) to a cat- egory of commutative diagrams of groups. For the concrete definition of the domain categories see Definition 2.10 and Definition 3.47 respectively; for the target categories see Definition 2.42 and Definition 3.52 respectively. Similarly to classical homology and homotopy theories, the groups appearing in the target category are abelian in the homology theory for any degree and in the homotopy theory for degree n > 1
Structurally-driven magnetic state transition of biatomic Fe chains on Ir(001)
Using first-principles calculations, we demonstrate that the magnetic
exchange interaction and the magnetocrystalline anisotropy of biatomic Fe
chains grown in the trenches of the 5x1 reconstructed Ir(001) surface depend
sensitively on the atomic arrangement of the Fe atoms. Two structural
configurations have been considered which are suggested from recent
experiments. They differ by the local symmetry and the spacing between the two
strands of the biatomic Fe chain. Since both configurations are very close in
total energy they may coexist in experiment. We have investigated collinear
ferro- and antiferromagnetic solutions as well as a collinear state with two
moments in one direction and one in the opposite direction (up-down-up-state).
For the structure with a small interchain spacing, there is a strong exchange
interaction between the strands and the ferromagnetic state is energetically
favorable. In the structure with larger spacing, the two strands are
magnetically nearly decoupled and exhibit antiferromagnetic order along the
chain. In both cases, due to hybridization with the Ir substrate the exchange
interaction along the chain axis is relatively small compared to freestanding
biatomic iron chains. The easy magnetization axis of the Fe chains also
switches with the structural configuration and is out-of-plane for the
ferromagnetic chains with small spacing and along the chain axis for the
antiferromagnetic chains with large spacing between the two strands. Calculated
scanning tunneling microscopy images and spectra suggest the possibility to
experimentally distinguish between the two structural and magnetic
configurations.Comment: Accepted for publication in Physical Review
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