Joule heating in nanowires [journal]
Authors:
Hans Fangohr, Dmitri S. Chernyshenko, Matteo Franchin, Thomas Fischbacher, Guido Meier
Journal: Accepted for publication by Physical Review B
Remark: As many people are talking about moving magnetic domain
walls in nanowires via large electrical current densities, a
technically important question is how much this will heat the
wire. This article studies that question in some detail.
Micromagnetic studies of three-dimensional pyramidal shell structures
[journal]
Authors: A Knittel, M Franchin, T Fischbacher, F Nasirpouri,
S J Bending, H Fangohr
Journal: New Journal of Physics 12, 113048 (23 pages) (2010)
Remark: A study of the micromagnetic behaviour of a particular
magnetic-shell-over-nonmagnetic-core structure that recently has
become experimentally accessible
Micromagnetic simulations of magnetoelectric materials [journal]
Authors: Thomas Fischbacher, Matteo Franchin, Hans Fangohr
Journal: Journal of Applied Physics 109, 07D352 (2011)
Remark: To the best of our knowledge, this is the first article
ever on the simulation of magnetoelectric behaviour using
computational micromagnetism.
arXiv:0907.1587 - Continuum multi-physics modeling with scripting languages: the Nsim simulation compiler prototype for classical field theory
Authors: Thomas Fischbacher, Hans Fangohr
Comments: 50 pages, 5 figures
Journal: [Under Review]
Subjects: Computational Physics (physics.comp-ph);
Mesoscale and Nanoscale Physics (cond-mat.mes-hall);
Distributed, Parallel, and Cluster Computing (cs.DC)
Remark: This work describes a viable approach for
automatically mapping ("compiling") symbolically specified physical
equations to finite element simulation code, and the systems this
technique is applicable to. Two examples (one complex, one simple) are
discussed in detail - the "nmag" micromagnetic simulator that gave the
original incentive for this work, as well as Turing's
reaction-diffusion model for biological pattern formation.
Fabrication and simulation of nanostructures for domain wall magnetoresistance studies on nickel [journal]
Authors: D. Claudio-Gonzalez, M.K. Husaina, C.H. de Groot, G. Bordignon, T. Fischbacher, H. Fangohr
Journal: Journal of Magnetism and Magnetic Materials (2009)
Remark: This work includes both experimental findings as well as
simulation results on the domain wall magnetoresistive effect in
nanobridge geometries. We used our simulation framework "nmag"
to compute the contribution of anisotropic magnetoresistance (AMR)
in complex geometries.
Compression of Boundary Element Matrix in Micromagnetic Simulations [journal]
Authors: A. Knittel, M. Franchin, G. Bordignon, T. Fischbacher, S. Bending, H. Fangohr
Journal: Journal of Applied Physics 105, 07D542 (2009)
Remark:
A general problem of finite element based micromagnetics is that
the long-range interaction of magnetic dipoles couples every piece of
the material not only to its immediate neighbourhood, but every other
piece of material. There are different strategies to deal with this
issue in such a way that computation time does not scale quadratically
with problem size. At the time of this article, the method used in
"nmag"/"nsim" is a hybrid finite element/boundary element approach,
which unfortunately has problems with thin film geometries. This work
is about lossy hierarchic matrix approximation techniques that make
thin film simulations manageable.
Parallel execution and scriptability in micromagnetic simulations [journal]
Authors: Thomas Fischbacher, Matteo Franchin, Giuliano Bordignon, Andreas Knittel, Hans Fangohr
Journal: Journal of Applied Physics 105, 07D527 (2009)
Remark: The "nmag" micromagnetic simulator is designed as an
extension library to the Python scripting language. This makes
higher-level tasks such as combining micromagnetic simulations
with visualization, optimization, data analysis, etc., very
convenient. A fundamental problem here is, however, that
micromagnetic simulations greatly benefit from parallelism,
but parallel programming is a somewhat advanced topic.
With "nmag", the user can benefit from parallel computing
facilities without having to learn parallel programming.
This work gives details.
A new approach to (quasi) periodic boundary conditions in micromagnetics: the macro geometry [journal]
Authors: Hans Fangohr, Giuliano Bordignon, Matteo Franchin, Andreas Knittel, Peter A. J. de Groot, Thomas Fischbacher
Journal: Journal of Applied Physics 105, 07D529 (2009)
Remark: Perhaps of interest to everyone who tries to do
micromagnetic simulations of repetitive structures. There are some
general subtle mathematical issues that arise when "periodic boundary
conditions" are used for systems with long range interactions (such as
in micromagnetism). This work explains the problem and demonstrates a
viable solution.
Current-driven dynamics of domain walls constrained in ferromagnetic nanopillars [journal]
Authors: Matteo Franchin, Thomas Fischbacher, Giuliano Bordignon, Peter de Groot, Hans Fangohr
Journal: Phys Rev B 78, 054447 (2008)
Remark: This work analyzes a "dc-current tunable micromagnetic
microwave generator" that operates by using the torque generated by
spin-polarized electrons to rotate a magnetic domain wall.
The beauty of this system is that the magnetic structure that gives
rise to the domain wall at the same time serves to spin-polarize the
conduction electrons.
Spin-polarized currents in exchange spring systems [journal]
Authors: Matteo Franchin, Giuliano Bordignon, Peter A. J. de Groot, Thomas Fischbacher, Jurgen P. Zimmermann, Guido Meier, Hans Fangohr
Journal: Journal of Applied Physics 103, 07A504 (2008)
Remark: A precursor to "Current-driven dynamics of domain walls
constrained in ferromagnetic nanopillars" - as our team has done quite
some work on multilayer "exchange spring systems" before, it was
natural to study these in the context of spin-polarized currents.
Numerical studies of demagnetizing effects in triangular ring arrays [journal]
Authors: Giuliano Bordignon, Thomas Fischbacher, Matteo Franchin, Jurgen P. Zimmermann, Peter A. J. de Groot, Hans Fangohr
Journal: Journal of Applied Physics 103, 07D932 (2008)
Remark: Arrays of ring-shaped elements have been suggested for
high-density magnetic data storage applications, since magnetic flux
closure reduces the problem of one element influencing the
magnetisation of another. Triangular rings are interesting as they
break the inversion symmetry - unlike squares and circles - and
hence allow much better control of magnetization patterns by
external fields. This work was the first to use the
"macro-geometry approach" for simulations of repetitive structures.
Micromagnetic Modelling of the Dynamics of Exchange Springs in Multi-Layer Systems [journal]
Authors: Matteo Franchin, J.P. Zimmermann, Thomas Fischbacher, Giuliano Bordignon, Peter de Groot, Hans Fangohr
Journal: IEEE Transactions on Magnetics 43, 6, 2887-2889 (2007)
Remark: This work studies the behaviour of a multi-layer system
with different ferromagnetic species (DyFe2/YFe2). Such "exchange springs" show
fairly rich physical behaviour. Here, we use 1-dimensional meshing to model
the system. The "nsim" simulator on top of which "nmag" is implemented can
actually handle simplicial meshes of arbitrary dimension - in principle,
it could e.g. also do 4d finite element simulations.
Analysis of Magnetoresistance in Arrays of Connected Nano-Rings [journal]
Authors: Giuliano Bordignon, Thomas Fischbacher, Matteo Franchin, Jurgen P. Zimmermann, Alexander A. Zhukov, Vitali V. Metlushko, Peter A. J. de Groot, Hans Fangohr
Journal: IEEE Transactions on Magnetics 43, 6, 2881-2883 (2007)
Remark: Our first work on numerical AMR (anisotropic
magnetoresistance) simulations. This clearly shows the importance of
taking the influence of the geometry on the distribution of the
electrical current into account. This uses the multi-physics capabilities
of the "nsim" simulator to combine electric and magnetic physics.
A Systematic Approach to Multiphysics Extensions of Finite-Element-Based Micromagnetic Simulations: Nmag [journal]
Authors: Thomas Fischbacher, Matteo Franchin, Giuliano Bordignon,, Hans Fangohr
Journal: IEEE Transactions on Magnetics 43, 6, 2896-2898 (2007)
Remark: This article briefly describes "nmag". This is the
publication we ask other "nmag" users to cite in their work when they
reference "nmag".
Micromagnetic simulation of the magnetic exchange spring system DyFe2/YFe2 [journal]
Authors: Jurgen P. Zimmermann, Giuliano Bordignon, Richard P. Boardman, Thomas Fischbacher, Hans Fangohr, Kevin Martin, Graham J. Bowden, Alexander A. Zhukov,, Peter A. J. de Groot
Journal: Journal of Applied Physics 99, 08B904 (2006)
Remark: An article on the DyFe2/YFe2 multi-layer system that pre-dates our "nmag" simulator.
I occasionally still do some work on aspects of supersymmetric field theories related to string theory which I personally find aesthetically attractive.
arXiv:1010.4910 - New Supersymmetric and Stable, Non-Supersymmetric Phases in Supergravity and Holographic Field Theory
Authors: Thomas Fischbacher, Krzysztof Pilch, Nicholas P. Warner
Comments: 36 pages, 2 figures
Journal: [to be submitted]
Subjects: High Energy Physics - Theory (hep-th)
Remark: This article establishes that (a) the long known
SO(3)xSO(3) symmetric non-SUSY critical point of SO(8)-gauged N=8
D=4 Supergravity indeed is BF-stable, (b) the new U(1)xU(1) N=1
solution for which numerical evidence was found in arXiv:0912.1636
really exists, and studies stability and associated
renormalization group flows.
arXiv:1007.0600 - Numerical tools to validate stationary points of SO(8)-gauged N=8 D=4 supergravity
Authors: Thomas Fischbacher
Comments: 9 pages
Journal: [to be submitted]
Subjects: High Energy Physics - Theory (hep-th)
Remark: The discovery of 14 new critical points of the
scalar potential of the SO(8)-gauged de Wit-Nicolai N=8
supergravity raised the question how these calculations could be
verified independently. This article provides reference code to
check all claims concerning the properties of known and new
solutions as well as future discoveries.
arXiv:1006.1823 - The two dimensional N=(2,2)
Wess-Zumino Model in the Functional Renormalization Group
Approach
Authors: Franziska Synatschke-Czerwonka, Thomas Fischbacher, Georg Bergner
Comments: 12 pages, 4 figures
Journal: Phys.Rev.D82:085003,2010
Subjects: High Energy Physics - Theory (hep-th)
Remark: This work is the first application of a parallel
numerical tool which we developed to study functional
renormalization group flow equations.
arXiv:0912.1636 - Fourteen new stationary points in the scalar potential of SO(8)-gauged N=8, D=4 supergravity
Authors: Thomas Fischbacher
Comments: 22 pages
Journal: Journal of High Energy Physics JHEP 1009:068,2010
Subjects: High Energy Physics - Theory (hep-th)
Remark: An application of computational techniques whose utility in supergravity
was first demonstrated by the author a year earlier to the perhaps most interesting case.
This managed to solve a long-standing problem that has not seen any progress for 25 years.
arXiv:0908.4182 - On discrete Minimal Flavour Violation
Authors: Roman Zwicky, Thomas Fischbacher
Comments: 25 pages (15 pages main text) and 3 figures
Journal: Phys. Rev. D 80, 076009 (2009) [13 pages]
Subjects: High Energy Physics - Phenomenology (hep-ph)
Remark: Sort-of a premiere. One of the very few works where
I did not write massive amounts of code to do the investigation. Also,
my first (joint) phenomenology paper. This got started over coffee
with an after-talk discussion about group theory when I was visiting
the phenomenology group over in the Physics building...
arXiv:0811.1915 - The many vacua of gauged extended supergravities
Authors: Thomas Fischbacher
Comments: 66 pages, code to reproduce results can be obtained
by downloading paper's source, as described in the text
Journal-ref: Gen.Rel.Grav.41:315-411,2009
Subjects: High Energy Physics - Theory (hep-th)
Remark: Also one of the papers I am rather proud of, for
the new techniques presented here now allow us to run circles around
the problems I studied during my PhD - so I managed to mostly obsolete
the technology underlying my own PhD work through better algorithms.
arXiv:hep-th/0504230 - The structure of E10 at higher A9 levels - a first algorithmic approach
Authors: Thomas Fischbacher
Comments: 26 pages, 1 figure
Journal-ref: JHEP 0508 (2005) 012
Subjects: High Energy Physics - Theory (hep-th)
Remark: Probably the most frustrating paper I ever worked
on. Trying to develop group-theoretic code of about 2000 lines with
initially about one sign error per every 50 lines and no good
strategies to debug it in sight is not fun.
arXiv:hep-th/0412331 - Planar plane-wave matrix theory at the four loop order: Integrability without BMN scaling
Authors: Thomas Fischbacher, Thomas Klose, Jan Plefka
Comments: 59 pages, 6 figures. v2: reference added, minor additions
Journal-ref: JHEP 0502 (2005) 039
Subjects: High Energy Physics - Theory (hep-th)
Remark: This is the one paper I am most proud of. Doing
this work took almost an entire year, and was an enormously risky
project - for the algorithm and code that had to be developed to do
the calculation had to use a broad range of tricks - graph topology,
lazy evaluation, continuation coding, etc. And still we did not know
till the very end whether the calculation would be "too big to
handle". This explains the "gemstone" algorithm and code that indeed
managed to give massive improvements for Feynman diagram type
calculations in the "large N" (i.e. planar graphs only) limit.
arXiv:cs/0406002 - A novel approach to symbolic algebra
Authors: Thomas Fischbacher
Comments: 15 pages
Subjects: Symbolic Computation (cs.SC)
Remark: Another contribution for the "Heinz Billing award"
- essentially, just a quick idea and proof of concept for something
that may turn out useful (hence not submitted to a journal): using
continuations in Scheme to implement a very compact
pattern-matching-based "non-deterministic choice" interactive term
transformation utility with Fierzing calculations as a potential
application in mind... (If anyone is interested in this,
this might be an idea to do a bit of work on!)
arXiv:hep-th/0312262 - Bulk Witten Indices from D=10 Yang Mills Integrals
Authors: Thomas Fischbacher
Comments: 13 pages, source package contains code to redo calculations.
Results added that were missing in the first version due to
a (now fixed) compiler bug
Journal-ref: Nucl.Phys. B694 (2004) 525-535
Subjects: High Energy Physics - Theory (hep-th)
Remark: Matthias Staudacher once mentioned a problem to me
he had with computing certain nested complex integrals. When I looked
into it, I soon noticed how to massively improve the calculations -
this produced new results, but as the size of the computations grows
quite fiercly with the size of the group, I only managed to disprove
some conjectures
arXiv:hep-th/0306276 - Non-semisimple and Complex Gaugings of N=16 Supergravity
Authors: T. Fischbacher, H. Nicolai, H. Samtleben
Comments: LaTeX2e, 30 pages; v2: minor corrections, version published in CMP
Journal-ref: Commun.Math.Phys. 249 (2004) 475-496
Subjects: High Energy Physics - Theory (hep-th)
Remark: This work just scratches the surface on all the
possibilities one has to construct supersymmetric 3-dimensional
Chern-Simons supergravity models with 32 supercharges, e.g. by
performing "Wigner-Inönü" like reductions of semi-simple gauge groups.
Originally, we started out to investigate this by looking at "infinite boosts"
along non-compact directions in E8(8), but one of the amazing things we found
this way is that the famous SO(8)xSO(8) subgroup structure of E8(8) is best
regarded in such a way that we read this as SO(8,C+), where C+ are the
split-complex numbers with i^2=1 - there also are (different!) SO(8,C) and
SO(8,C0) subgroups of E8(8), which correspond to the complex and dual numbers.
arXiv:hep-th/0305176 - Mapping the vacuum structure of gauged maximal supergravities: an application of high-performance symbolic algebra
Authors: Thomas Fischbacher
Comments: PhD thesis, 140 pages, 11 figures
Subjects: High Energy Physics - Theory (hep-th); Symbolic Computation (cs.SC)
Remark: My PhD thesis - contains far more extensive data on the symmetry breaking
structure of low-dimensional models of supergravity with 32 supercharges, as well as an explanation
of computational methods.
arXiv:hep-th/0301017 - Low Level Representations for E10 and E11
Authors: Hermann Nicolai, Thomas Fischbacher
Comments: 37 pages, 2 figures, 2 tables.
Contribution to the Proceedings of the Ramanujan International
Symposium on Kac-Moody Algebras and Applications,
ISKMAA-2002, Jan. 28--31, Chennai,India.
Version published in Contemporary Mathematics, minor corrections.
Source version contains also A2 level decomposition for
Feingold-Frenkel algebra AE3 up to level 56, in addition to
tables for E10 and E11 that contain more data than presented
in the text
Subjects: High Energy Physics - Theory (hep-th); Quantum Algebra (math.QA)
Remark: While most physicists (and mathematicians) know about the Cartan-Dybkin classification of finite-dimensional simple Lie groups, the properties (or even the existence) of infinite-dimensional groups such as E10 are not so widely known. There are some indications that E10 or E11 may be related to the fundamental symmetries of M-theory, but answering the corresponding physically motivated questions turns out to be algebraically extremely challenging. This work contains tables that give extensive data on the structure of particularly interesting finite-dimensional truncations of E10 and E11. (At some point, I should re-do the computations with the LISP code used back then on modern hardware that does not suffer from the 4 GB memory limit and produce more data.)
arXiv:hep-th/0208218 - Introducing LambdaTensor1.0 - A package for explicit symbolic and numeric Lie algebra and Lie group calculations
Authors: Thomas Fischbacher
Comments: 10 pages; to be published in
"Forschung und wissenschaftliches Rechnen - Beitraege
zum Heinz-Billing-Preis 2002"; arXiv version now reflects the
corresponding release of version 1.1, which is described
briefly in an addendum
Subjects: High Energy Physics - Theory (hep-th);
Mathematical Software (cs.MS);
Mathematical Physics (math-ph)
Remark: My contribution to the Max Planck Societies'
"Heinz Billing Award for the Advancement of Scientific computing" in
2002. Received a nomination. Essentially, this describes the analogy
between explicit sparse higher rank tensor arithmetics and relational
database operations and shows how these are implemented in the
LambdaTensor LISP package. Does not mention other parts of
LambdaTensor that e.g. deal with Cartan-Dynkin theory, such as an
implementation of the Fast Freudenthal algorithm to work out weight
multiplicities.
arXiv:hep-th/0207206 - Vacua of Maximal Gauged D=3 Supergravities
Authors: T. Fischbacher, H. Nicolai, H. Samtleben
Comments: LaTeX2e, 46 pages
Journal-ref: Class.Quant.Grav. 19 (2002) 5297-5334
Subjects: High Energy Physics - Theory (hep-th)
Remark: A follow-up to hep-th/0201030 that also looks at
particle mass spectra and perturbative (Breitenlohner-Freedman)
stability of Anti-deSitter vacuum solutions obtained from "the Higgs effect"
in 3-d gauged (Chern-Simons) supergravity models with 32 supercharges.
Note that in three dimensions, a number of fairly exotic (noncompact(!))
gauge groups are possible - such as different real forms of G2xF4.
We study the breaking patterns of a number of those.
arXiv:hep-th/0201030 - Some stationary points of gauged N=16 D=3 supergravity
Authors: Thomas Fischbacher
Comments: 16 pages, 1 figure
Journal-ref: Nucl.Phys. B638 (2002) 207-219
Subjects: High Energy Physics - Theory (hep-th)
Remark: My first journal publication - on "Higgs effect
symmetry breaking" in a supersymmetric field theory that somewhat
resembles the topological models used by Witten to establish the
connection between Gauge Theory and Knot Invariants (such as the Jones
Polynomial), i.e. Chern-Simons theory. The technical challenge was to
work with Euler angle parametrizations of sub-manifolds of the
248-dimensional Lie group E8.
arXiv:0811.2186 - Algebraic mechanics as an accessible toy model demonstrating entropy generation from reversible microscopic dynamics
Authors: Thomas Fischbacher
Comments: 18 pages, 2 figures
Subjects: Mathematical Physics (math-ph)
Remark: A didactic article to help to clarify some of the
confusion associated with the concepts of "entropy", "the arrow of
time", and also "wave function collapse". The original incentive for
this work came from a general audience talk I gave in 2007.