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Copenhagen, Denmark

The Nordic Institute for Theoretical Physics, or NORDITA fysik), is an international organisation for research in theoretical physics. It was established in 1957 by Niels Bohr and the Swedish minister Torsten Gustafsson. NORDITA was originally located in Copenhagen , but moved to Stockholm during autumn 2006. The institute is now located at the AlbaNova University Centre. The main research areas at NORDITA are astrophysics, biophysics, condensed matter and particle physics. Wikipedia.

We show that models with deformations of special relativity that have an energy-dependent speed of light have nonlocal effects. The requirement that the arising nonlocality is not in conflict with known particle physics allows us to derive strong bounds on deformations of special relativity and rule out a modification to first order in energy over the Planck mass. © 2010 The American Physical Society.

Wehling T.O.,University of Bremen | Black-Schaffer A.M.,Uppsala University | Balatsky A.V.,NORDITA | Balatsky A.V.,Los Alamos National Laboratory
Advances in Physics | Year: 2014

A wide range of materials, like d-wave superconductors, graphene, and topological insulators, share a fundamental similarity: their low-energy fermionic excitations behave as massless Dirac particles rather than fermions obeying the usual Schrödinger Hamiltonian. This emergent behavior of Dirac fermions in condensed matter systems defines the unifying framework for a class of materials we call "Dirac materials." In order to establish this class of materials, we illustrate how Dirac fermions emerge in multiple entirely different condensed matter systems and we discuss how Dirac fermions have been identified experimentally using electron spectroscopy techniques (angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy). As a consequence of their common low-energy excitations, this diverse set of materials shares a significant number of universal properties in the low-energy (infrared) limit. We review these common properties including nodal points in the excitation spectrum, density of states, specific heat, transport, thermodynamic properties, impurity resonances, and magnetic field responses, as well as discuss many-body interaction effects. We further review how the emergence of Dirac excitations is controlled by specific symmetries of the material, such as time-reversal, gauge, and spin-orbit symmetries, and how by breaking these symmetries a finite Dirac mass is generated. We give examples of how the interaction of Dirac fermions with their distinct real material background leads to rich novel physics with common fingerprints such as the suppression of back scattering and impurity-induced resonant states. © 2014 Taylor & Francis.

Lahtinen V.,NORDITA
New Journal of Physics | Year: 2011

We study the collective states of interacting non-Abelian anyons that emerge in Kitaev's honeycomb lattice model. Vortex-vortex interactions are shown to lead to the lifting of topological degeneracy and the energy is found to exhibit oscillations that are consistent with Majorana fermions being localized at vortex cores. We show how to construct states corresponding to the fusion channel degrees of freedom and obtain the energy gaps characterizing the stability of the topological low-energy spectrum. To study the collective behavior of many vortices, we introduce an effective lattice model of Majorana fermions. We find the necessary conditions for the model to approximate the spectrum of the honeycomb lattice model, and show that bi-partite interactions are responsible for the lifting of degeneracy also in many-vortex systems. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Semenoff G.W.,University of British Columbia | Zarembo K.,NORDITA
Physical Review Letters | Year: 2011

We study tunneling pair creation of W bosons by an external electric field on the Coulomb branch of N=4 supersymmetric Yang-Mills theory. We use AdS/CFT holography to find a generalization of Schwinger's formula for the pair production rate to the strong coupling, planar limit which includes the exchange of virtual massless particles to all orders. We find that the pair creation formula has an upper critical electric field beyond which the process is no longer exponentially suppressed. The value of the critical field is identical to that which occurs in the Born-Infeld action of probe D3-branes in the AdS 5×S5 background, where AdS5 and S5 are 5-dimensional anti-de Sitter space and the 5-sphere, respectively. © 2011 American Physical Society.

Hossenfelder S.,NORDITA
Classical and Quantum Gravity | Year: 2012

It was recently claimed that the Planck length is not a limit to the precision by which we can measure distances, but that instead it is merely the Planck volume that limits the precision by which we can measure volumes. Here, we investigate this claim and show that the argument does not support the conclusion. © 2012 IOP Publishing Ltd.

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