1,059 research outputs found
Bulk Witten indices from D=10 Yang-Mills integrals
No full textValues 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 negativ
Some stationary points of gauged N=16 D=3 supergravity
No full textFive nontrivial stationary points are found for maximal gauged N=16 supergravity in three dimensions with gauge group SO(8)xSO(8) by restricting the potential to a submanifold of the space of SU(3)C(SO(8)xSO(8) diag singlets. The construction presented here uses the embedding of E_{7(+7)CE8(+8) to lift the analysis of N=8, D=4 supergravity performed by N. Warner to N=16, D=3, and hence, these stationary points correspond to some of the known extrema of gauged N=8, D=4 supergravity
The many vacua of gauged extended supergravities
No full textA novel method is presented which employs advanced numerical techniquesused in the engineering sciences to find and study the properties of nontrivialvacua of gauged extended supergravity models. While this method only producesapproximate numerical data rather than analytic results, it overcomes the previouslimitation of only being able to find vacua with large residual unbroken gauge symmetrygroups. The effectiveness of this method is demonstrated by applying it to thetechnically most challenging D ? 3 scalar potential—that of SO(8) × SO(8) gaugedN = 16 Chern–Simons Supergravity in D = 3. Extensive data on the properties of99 different vacua (92 of them new) of this model are given. Furthermore, techniquesare briefly discussed which should allow using this numerical information as an inputto the construction of semi-automatic stringent analytic proofs on the locations andproperties of vacua. It hence is argued that these combined techniques presumably arepowerful enough to systematically map all the nontrivial vacua of every supergravitymodel
A systematic approach to multiphysics extensions of finite-element-based micromagnetic simulations: Nmag
No full textExtensions of the basic micromagnetic model that include effects such as spin-current interaction, diffusion of thermal energy or anisotropic magnetoresistance are often studied by performing simulations that use case-specific ad-hoc extensions of widely used software packages such as OOMMF or Magpar. We present the novel software framework 'Nmag' that handles specifications of micromagnetic systems at a sufficiently abstract level to enable users with little programming experience to automatically translate a description of a large class of dynamical multifield equations plus a description of the system's geometry into a working simulation. Conceptually, this is a step towards a higher-level abstract notation for classical multifield mutliphysics simulations, similar to the change from assembly language to a higher level human-and-machine-readable formula notation for mathematical terms (FORTRAN) half a century ago. We demonstrate the capability of this approach through two examples, showing 1) a reduced dimensionality model coupling two copies of the micromagnetic sector and 2) the computation of a spatial current density distribution for anisotropic magnetoresistance (AMR). For cross-wise validation purposes, we also show how Nmag compares to the OOMMF and Magpar packages on a selected micromagnetic toy system. We furthermore, briefly discuss the limiations of our framework and related conceptual questions
Mapping the vacuum structure of gauged maximal supergravities: an application of high performance symbolic algebra
No full textThe 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 , , 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 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 analyse
Nonsemisimple and complex gaugings of N=16 supergravity
No full textMaximal and non-maximal supergravities in three dimensions allow for a large variety of semisimple (Chern-Simons) gauge groups. In this paper, we analyze non-semisimple and complex gauge groups that satisfy the pertinent consistency relations for a maximal (N=16) gauged supergravity to exist. We give a general procedure how to generate non-semisimple gauge groups from known admissible semisimple gauge groups by a singular boost within E_{8(8)}. Examples include the theories with gauge group SO(8) x T_{28} that describe the reduction of IIA/IIB supergravity on the seven-sphere. In addition, we exhibit two 'strange embeddings' of the complex gauge group SO(8,C) into (real) E_{8(8)} and prove that both can be consistently gauged. We discuss the structure of the associated scalar potentials as well as their relation to those of D>3 gauged supergravitie
Joule heating in nanowires
No full textWe 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. [APL89, 222513 (2006)]. We provide an analytical expression for the time tc that marks the upper applicability limit of the You model. After tc, 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. We provide an analytical expression, valid for all pulse durations, that allows one to accurately compute this temperature increase in the nanowire on thin substrate
Vacua of Maximal Gauged D=3 Supergravities
No full textWe analyze the scalar potentials of maximal gauged three-dimensional supergravities which reveal a surprisingly rich structure. In contrast to maximal supergravities in dimensions D>3, all these theories admit a maximally supersymmetric (N=16) ground state with negative cosmological constant Lambda<0, except for the gauge group SO(4,4)^2, for which Lambda=0. We compute the mass spectra of bosonic and fermionic fluctuations around these vacua and identify the unitary irreducible representations of the relevant background (super)isometry groups to which they belong. In addition, we find several stationary points which are not maximally supersymmetric, and determine their complete mass spectra as well. In particular, we show that there are analogs of all stationary points found in higher dimensions, among them de Sitter vacua in the theories with noncompact gauge groups SO(5,3)^2 and SO(4,4)^2, as well as anti-de Sitter vacua in the compact gauged theory preserving 1/4 and 1/8 of the supersymmetries. All the dS vacua have tachyonic instabilities, whereas there do exist non-supersymmetric AdS vacua which are stable, again in contrast to the D>3 theories
Micromagnetic modelling of the dynamics of exchange springs in multi-layer systems
No full textExchange springs are formed in multilayers of alternating hard and soft ferromagnetic materials which are exchange coupled at their interfaces. These systems are rich of interesting physical properties, which can be tuned by selecting suitable geometries and compositions. In this paper, we present a computational study of the dynamics of a tri-layer DyFe2/YFe2/DyFe2 exchange spring system near the bending field (the field required to twist the magnetization of the soft YFe2 layer out of the aligned state). The dynamical reaction of the system to small variations of the applied field is studied and its oscillatory nature is analyzed numerically. The behaviors of the decay times, the frequencies, and amplitudes reveal enhanced responses of the system near the bending field
Discrete minimal flavour violation
No full textWe investigate the consequences of replacing the global flavor symmetry of minimal flavor violation (MFV) SU(3)Q×SU(3)U×SU(3)D×[centered ellipsis] by a discrete [script D]Q×[script D]U×[script D]D×[centered ellipsis] symmetry. Goldstone bosons resulting from the breaking of the flavor symmetry generically lead to bounds on new flavor structure many orders of magnitude above the TeV scale. The absence of Goldstone bosons for discrete symmetries constitute the primary motivation of our work. Less symmetry implies further invariants and renders the mass-flavor 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 DeltaF=2 operators. If though the DeltaF=2 transitions are generated by two subsequent DeltaF=1 processes, as, for example, in the standard model, then the four crystal-like groups Sigma(168)[approximate]PSL(2,[openface F]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 (nonfaithful) representation corresponding to an A4 symmetry. Moreover we argue that the, apparently often omitted, (D) groups are subgroups of an appropriate Delta(6g2). We would like to stress that we do not provide an actual model that realizes the MFV scenario nor any other theory of flavor
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