arXiv subjects

Thomas Fischbacher

Publications and source records attributed to Thomas Fischbacher.

At least 37 records · Page 2Linked to original sources

arXiv2011

The Encyclopedic Reference of Critical Points for SO(8)-Gauged N=8 Supergravity Part 1: Cosmological Constants in the Range -Λ/g^2 \in [6;14.7)

This article is part of a collection that strives to collect and provide in an unified form data about all the critical points on the scalar manifold of SO(8)-gauged N=8 supergravity in four dimensions known so far. The vast majority of these were obtained using the enhanced sensitivity backpropagation method introduced by the author in 2008. This part of the collection describes 41 critical points, 7 of which have been known for more than two decades, 8 of which were discovered recently, and 26 are novel. The residual gauge symmetries of these 41 critical points (likely) are SO(8) with N=8 SUSY (1x), SO(7) (2x), SU(4) (1x), G2 with N=1 SUSY (1x), SU(3)xU(1) with N=2 SUSY (1x), SO(3)xSO(3) (2x), SO(3)xU(1)xU(1) (1x), SO(3)xU(1) (3x), SO(3) (3x), U(1)xU(1) with N=1 SUSY (1x), U(1)xU(1) without SUSY (4x), U(1) (11x), and None (10x). Analytic conjectures (not yet proven but overwhelmingly likely correct) are given for the locations and cosmological constants of some critical points.

hep-th↗

arXiv2011

Fourteen new stationary points in the scalar potential of SO(8)-gauged N=8, D=4 supergravity

The list of six previously known nontrivial stationary points in the scalar potential of N=8, D=4 supergravity with gauge group SO(8) is extended by fourteen new entries, whose properties have been obtained numerically using the sensitivity backpropagation technique. Eight of the new solutions break the gauge group completely, while three have a residual symmetry of U(1). Three further ones break the gauge group to U(1)xU(1), and while the approximate numerical data are somewhat inconclusive, there is evidence that one of these may have a residual N=1 supersymmetry, hence correspond to a stable vacuum. It must be pointed out that this list of new solutions most likely is not exhaustive.

hep-th↗

arXiv2011

Numerical tools to validate stationary points of SO(8)-gauged N=8 D=4 supergravity

Until recently, the preferred strategy to identify stationary points in the scalar potential of SO(8)-gauged N=8 supergravity in D=4 has been to consider truncations of the potential to sub-manifolds of E_{7(+7)}/SU(8) that are invariant under some postulated residual gauge group G of SO(8). As powerful alternative strategies have been shown to exist that allow one to go far beyond what this method can achieve -- and in particular have produced numerous solutions that break the SO(8) gauge group to no continuous residual symmetry -- independent verification of results becomes a problem due to both the complexity of the scalar potential and the large number of new solutions. This article introduces a conceptually simple self-contained piece of computer code that allows independent numerical validation of claims on the locations of newly discovered stationary points.

hep-th↗

arXiv2011

Joule heating in nanowires

We study the effect of Joule heating from electric currents flowing through ferromagnetic nanowires on the temperature of the nanowires and on the temperature of the substrate on which the nanowires are grown. The spatial current density distribution, the associated heat generation, and diffusion of heat is simulated within the nanowire and the substrate. We study several different nanowire and constriction geometries as well as different substrates: (thin) silicon nitride membranes, (thick) silicon wafers, and (thick) diamond wafers. The spatially resolved increase in temperature as a function of time is computed. For effectively three-dimensional substrates (where the substrate thickness greatly exceeds the nanowire length), we identify three different regimes of heat propagation through the substrate: regime (i), where the nanowire temperature increases approximately logarithmically as a function of time. In this regime, the nanowire temperature is well-described analytically by You et al. [APL 89, 222513 (2006)]. We provide an analytical expression for the time t_c that marks the upper applicability limit of the You model. After t_c, the heat flow enters regime (ii), where the nanowire temperature stays constant while a hemispherical heat front carries the heat away from the wire and into the substrate. As the heat front reaches the boundary of the substrate, regime (iii) is entered where the nanowire and substrate temperature start to increase rapidly. For effectively two-dimensional substrates (where the nanowire length greatly exceeds the substrate thickness), there is only one regime in which the temperature increases logarithmically with time for large times, before the heat front reaches the substrate boundary. We provide an analytical expression, valid for all pulse durations, that allows one to accurately compute this temperature increase in the nanowire on thin substrates.

cond-mat.mtrl-sci↗

arXiv2011

Enhanced spin transfer torque effect for transverse domain walls in cylindrical nanowires

Recent studies have predicted extraordinary properties for transverse domain walls in cylindrical nanowires: zero depinning current, the absence of the Walker breakdown, and applications as domain wall oscillators. In order to reliably control the domain wall motion, it is important to understand how they interact with energy barriers. In this paper, we study the motion and depinning of transverse domain walls through potential barriers in ferromagnetic cylindrical nanowires. We use magnetic fields and spin-polarized currents to drive the domain walls along the wire. Using magnetic fields, we find that the minimum and the maximum fields required to push the domain wall through the barrier differ by 30 %. On the contrary, using spin-polarized currents, we find variations of a factor 130 between the minimum value of the depinning current density and the maximum value. We study the depinning current density as a function of the height of the energy barrier using numerical and analytical methods. We find that, for a barrier of 40 k_B T, a depinning current density of about 5 uA is sufficient to depin the domain wall. We reveal and explain the mechanism that leads to these unusually low depinning currents. One requirement for this new depinning mechanism is for the domain wall to be able to rotate around its own axis. With the right barrier design, the spin torque transfer term is acting exactly against the damping in the micromagnetic system, and thus the low current density is sufficient to accumulate enough energy quickly. These key insights may be crucial in furthering the development of novel memory technologies, such as the racetrack memory, that can be controlled through low current densities.

cond-mat.mtrl-sci↗

arXiv2011

On discrete Minimal Flavour Violation

We investigate the consequences of replacing the global flavour symmetry of Minimal Flavour Violation (MFV) SU(3)_QxSU(3)_UxSU(3)_Dx..., by a discrete D_QxD_UxD_Dx.. symmetry. Goldstone bosons resulting from the breaking of the flavour symmetry generically lead to bounds on new flavour structure many orders of magnitude above the TeV-scale. The absence of Goldstone bosons for discrete symmetries constitute the \emph{primary} motivation of our work. Less symmetry implies further invariants and renders the mass flavour basis transformation observable in principle and calls for a hierarchy in the Yukawa matrix expansion. We show, through the dimension of the representations, that the (discrete) symmetry in principle does allow for additional Delta F = 2 operators. If though the Delta F = 2 transitions are generated by two subsequent Delta F = 1 processes, as for example in the Standard Model, then the four crystal-like groups Sigma(168) ~ PSL(2,7), Sigma(72phi), Sigma(216phi) and especially Sigma(360phi) do provide enough protection for a TeV-scale discrete MFV scenario. Models where this is not the case have to be investigated case by case. Interestingly Sigma(216phi) has a (non-faithful) representation corresponding to an A4-symmetry. Moreover we argue that the, apparently often omitted, (D)-groups are subgroups of an appropriate Delta(6g^2). We would like to stress that we do not provide an actual model that realizes the MFV scenario nor any other theory of flavour.

hep-ph↗

arXiv2010

The two dimensional N=(2,2) Wess-Zumino Model in the Functional Renormalization Group Approach

We study the supersymmetric N=(2,2) Wess-Zumino model in two dimensions with the functional renormalization group. At leading order in the supercovariant derivative expansion we recover the nonrenormalization theorem which states that the superpotential has no running couplings. Beyond leading order the renormalization of the bare mass is caused by a momentum dependent wave function renormalization. To deal with the partial differential equations we have developed a numerical toolbox called FlowPy. For weak couplings the quantum corrections to the bare mass found in lattice simulations is reproduced with high accuracy. But in the regime with intermediate couplings higher-order-operators that are not constrained by the nonrenormalization theorem yield the dominating contribution to the renormalized mass.

hep-th↗

arXiv2010

New Supersymmetric and Stable, Non-Supersymmetric Phases in Supergravity and Holographic Field Theory

We establish analytically that the potential of N=8 supergravity in four dimensions has a new N=1 supersymmetric critical point with U(1) x U(1) symmetry. We work within a consistent N=1 supersymmetric truncation and obtain the holographic flow to this new point from the N=8 critical point. The operators that drive the flow in the dual field theory are identified and it is suggested the new critical point might represent a new conformal phase in the holographic fermion droplet models with sixteen supersymmetries. The flow also has c_{IR}/c_{UV}} = 1/2. We examine the stability of all twenty known critical points and show that the SO(3) x SO(3) point is a perturbatively stable non-supersymmetric fixed point. We also locate and describe a novel critical point that also has SO(3)\times SO(3) symmetry and is related to the previously known one by triality in a similar manner to the way that the SO(7)^\pm points are related to one another.

hep-th↗

arXiv2009

Continuum multi-physics modeling with scripting languages: the Nsim simulation compiler prototype for classical field theory

We demonstrate that for a broad class of physical systems that can be described using classical field theory, automated runtime translation of the physical equations to parallelized finite-element numerical simulation code is feasible. This allows the implementation of multiphysics extension modules to popular scripting languages (such as Python) that handle the complete specification of the physical system at script level. We discuss two example applications that utilize this framework: the micromagnetic simulation package "Nmag" as well as a short Python script to study morphogenesis in a reaction-diffusion model.

physics.comp-ph↗

arXiv2008

Algebraic mechanics as an accessible toy model demonstrating entropy generation from reversible microscopic dynamics

One observes that a considerable level of confusion remains about some of those aspects of irreversibility, entropy generation and `the arrow of time' which actually are well understood. This demands that great care must be taken in any discussion of irreversibility to use clear-cut notions and precise language in order to be definite about which property follows from which assumption. In this work, a novel toy model of `algebraic mechanics' is presented that elucidates specific key aspects of entropy generation in a system with extremely simple reversible fundamental dynamics. It is argued why insights gained through a detailed quantitative study of this toy model also have to be taken into account for any realistic model of microscopic dynamics, classical or quantum alike. As irreversibility also touches upon the quantum mechanical measurement process (through the `proof' of the `H-Theorem'), a simple way to address the tenacious question `when (and how) the wave function collapses' is offered.

math-ph↗

arXiv2008

The many vacua of gauged extended supergravities

A novel method is presented which employs advanced numerical techniques used in the engineering sciences to find and study the properties of nontrivial vacua of gauged extended supergravity models. While this method only produces approximate numerical data rather than analytic results, it overcomes the previous limitation of only being able to find vacua with large residual unbroken gauge symmetry groups. The effectiveness of this method is demonstrated by applying it to the technically most challenging D>=3 scalar potential -- that of SO(8) X SO(8) gauged N=16 Chern-Simons Supergravity in D=3. Extensive data on the properties of 99 different vacua (92 of them new) of this model are given. Furthermore, techniques are briefly discussed which should allow using this numerical information as an input to the construction of semi-automatic stringent analytic proofs on the locations and properties of vacua. It hence is argued that these combined techniques presumably are powerful enough to systematically map all the nontrivial vacua of every supergravity model.

hep-th↗

arXiv2005

The structure of E10 at higher A9 levels - a first algorithmic approach

The conjecture of a hidden $E_{10}$ symmetry of M-theory is supported by the close connection between the dynamics of D=11 supergravity near a spacelike singularity and a truncation of an one-dimensional $σ$-model with $E_{10}$ symmetry where all representations beyond SL(10) level $\ell=3$ are omitted. If this conjecture is right, higher-level representations should especially capture the dynamics of further M-theory degrees of freedom. Unfortunately, the level by level determination of $E_{10}$ commutators which is necessary to extend the model to higher levels is both an involved and toilsome task that requires computer aid. In this work, some of the relevant problems are exposed and algorithmic methods are developed which simplify key steps in the determination of explicit $E_{10}$ commutators at higher levels. As an application, we compute the commutator of the level-two six-form with itself.

hep-th↗

arXiv2005

Planar plane-wave matrix theory at the four loop order: Integrability without BMN scaling

We study SU(N) plane-wave matrix theory up to fourth perturbative order in its large N planar limit. The effective Hamiltonian in the closed su(2) subsector of the model is explicitly computed through a specially tailored computer program to perform large scale distributed symbolic algebra and generation of planar graphs. The number of graphs here was in the deep billions. The outcome of our computation establishes the four-loop integrability of the planar plane-wave matrix model. To elucidate the integrable structure we apply the recent technology of the perturbative asymptotic Bethe Ansatz to our model. The resulting S-matrix turns out to be structurally similar but nevertheless distinct to the so far considered long-range spin-chain S-matrices of Inozemtsev, Beisert-Dippel-Staudacher and Arutyunov-Frolov-Staudacher in the AdS/CFT context. In particular our result displays a breakdown of BMN scaling at the four-loop order. That is, while there exists an appropriate identification of the matrix theory mass parameter with the coupling constant of the N=4 superconformal Yang-Mills theory which yields an eigth order lattice derivative for well seperated impurities (naively implying BMN scaling) the detailed impurity contact interactions ruin this scaling property at the four-loop order. Moreover we study the issue of ``wrapping'' interactions, which show up for the first time at this loop-order through a Konishi descendant length four operator.

hep-th↗

arXiv2004

A novel approach to symbolic algebra

A prototype for an extensible interactive graphical term manipulation system is presented that combines pattern matching and nondeterministic evaluation to provide a convenient framework for doing tedious algebraic manipulations that so far had to be done manually in a semi-automatic fashion.

cs.SC↗

arXiv2004

Bulk Witten Indices from D=10 Yang Mills Integrals

Values for the bulk Witten indices for D=10 Yang-Mills integrals for some regular simple groups of rank 4 and 5 are calculated by employing the BRST deformation technique by Moore, Nekrasov and Shatashvili. The results cannot be reconciled with the double assumption that the number of normalizable ground states is given by certain simple partition functions given by Kac and Smilga as well as that the corresponding boundary term is always negative.

hep-th↗

arXiv2003

Mapping the vacuum structure of gauged maximal supergravities: an application of high-performance symbolic algebra

The analysis of the extremal structure of the scalar potentials of gauged maximally extended supergravity models in five, four, and three dimensions, and hence the determination of possible vacuum states of these models is a computationally challenging task due to the occurrence of the exceptional Lie groups $E_6$, $E_7$, $E_8$ in the definition of these potentials. At present, the most promising approach to gain information about nontrivial vacua of these models is to perform a truncation of the potential to submanifolds of the $G/H$ coset manifold of scalars which are invariant under a subgroup of the gauge group and of sufficiently low dimension to make an analytic treatment possible. New tools are presented which allow a systematic and highly effective study of these potentials up to a previously unreached level of complexity. Explicit forms of new truncations of the potentials of four- and three-dimensional models are given, and for N=16, D=3 supergravities, which are much more rich in structure than their higher-dimensional cousins, a series of new nontrivial vacua is identified and analysed.

hep-th↗

arXiv2003

Introducing LambdaTensor1.0 - A package for explicit symbolic and numeric Lie algebra and Lie group calculations

Due to the occurrence of large exceptional Lie groups in supergravity, calculations involving explicit Lie algebra and Lie group element manipulations easily become very complicated and hence also error-prone if done by hand. Research on the extremal structure of maximal gauged supergravity theories in various dimensions sparked the development of a library for efficient abstract multilinear algebra calculations involving sparse and non-sparse higher-rank tensors, which is presented here.

hep-th↗

arXiv2003

Low Level Representations for E10 and E11

We work out the decomposition of the indefinite Kac Moody algebras ${E_{10}}$ and ${E_{11}}$ w.r.t. their respective subalgebras $A_9$ and $A_{10}$ at low levels. Tables of the irreducible representations with their outer multiplicities are presented for ${E_{10}}$ up to level $\ell = 18$ and for ${E_{11}}$ up to level $\ell =10$. On the way we confirm and extend existing results for ${E_{10}}$ root multiplicities, and for the first time compute non-trivial root multiplicities of ${E_{11}}$.

hep-th↗