Copenhagen, Denmark
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.

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Gustavsson K.,Gothenburg University | Mehlig B.,Gothenburg University | Mehlig B.,NORDITA
Advances in Physics | Year: 2016

The dynamics of heavy particles suspended in turbulent flows is of fundamental importance for a wide range of questions in astrophysics, atmospheric physics, oceanography, and technology. Laboratory experiments and numerical simulations have demonstrated that heavy particles respond in intricate ways to turbulent fluctuations of the carrying fluid: non-interacting particles may cluster together and form spatial patterns even though the fluid is incompressible, and the relative speeds of nearby particles can fluctuate strongly. Both phenomena depend sensitively on the parameters of the system. This parameter dependence is difficult to model from first principles since turbulence plays an essential role. Laboratory experiments are also very difficult, precisely since they must refer to a turbulent environment. But in recent years it has become clear that important aspects of the dynamics of heavy particles in turbulence can be understood in terms of statistical models where the turbulent fluctuations are approximated by Gaussian random functions with appropriate correlation functions. In this review, we summarise how such statistical-model calculations have led to a detailed understanding of the factors that determine heavy-particle dynamics in turbulence. We concentrate on spatial clustering of heavy particles in turbulence. This is an important question because spatial clustering affects the collision rate between the particles and thus the long-term fate of the system. © 2016 Informa UK Limited, trading as Taylor & Francis Group.

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.

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.

Eichhorn R.,NORDITA
Physical Review Letters | Year: 2010

Isomeric molecules that only differ in the spatial orientation of their constituents are called stereoisomers. We demonstrate that different stereoisomers are transported typically at different velocities in a fluid streaming through a straight microchannel. As the underlying mechanism, we identify a translation-rotation coupling in the motion of the molecules which is specific for the molecule structure. This effect can be exploited for the separation of stereoisomers, a task of immense importance in biotechnology and pharmaceutics. © 2010 The American Physical Society.

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.

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.

Black-Schaffer A.M.,NORDITA
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We study the carrier-mediated exchange interaction, the so-called Ruderman-Kittel-Kasuya-Yoshida (RKKY) coupling, between two magnetic impurity moments in graphene using exact diagonalization on the honeycomb lattice. By using the tight-binding nearest-neighbor band structure of graphene we also avoid the use of a momentum cutoff which plagues perturbative results in the Dirac continuum model formulation. We extract both the short and long impurity-impurity distance behavior and show on a qualitative agreement with earlier perturbative results in the long-distance limit but also report on a few new findings. In the bulk the RKKY coupling is proportional to 1/ |R| 3 and displays [1+cos (2 kD R)] -type oscillations. A-A sublattice coupling is always ferromagnetic whereas A-B subattice coupling is always antiferromagnetic and three times as large. We also study the effect of edges in zigzag graphene nanoribbons (ZGNRs). We find that for impurities on the edge the RKKY coupling decays exponentially because of the localized zero-energy edge states and we also conclude that a nonperturbative treatment is essential for these edge impurities. For impurities inside a ZGNR the bulk characteristics are quickly regained. © 2010 The American Physical Society.

Black-Schaffer A.M.,NORDITA
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We study self-consistently a microscopic interface between a quantum spin Hall insulator (QSHI) and a superconductor (SC), focusing on properties related to Majorana fermion creation. For an s-wave SC we show that odd-in-momentum, or p-wave, order parameters exist for all doping levels of the QSHI and that they can be related to different spinless Cooper pair amplitudes. Despite this, the induced superconducting gap in the QSHI always retains its s-wave character, validating the commonly used effective model for superconductivity in a topological insulator. For a dxy-wave SC, we show that a Majorana mode is only created at finite doping and that there is no excitation gap protecting this mode. © 2011 American Physical Society.

Hossenfelder S.,NORDITA
Living Reviews in Relativity | Year: 2013

We review the question of whether the fundamental laws of nature limit our ability to probe arbitrarily short distances. First, we examine what insights can be gained from thought experiments for probes of shortest distances, and summarize what can be learned from different approaches to a theory of quantum gravity. Then we discuss some models that have been developed to implement a minimal length scale in quantum mechanics and quantum field theory. These models have entered the literature as the generalized uncertainty principle or the modified dispersion relation, and have allowed the study of the effects of a minimal length scale in quantum mechanics, quantum electrodynamics, thermodynamics, black-hole physics and cosmology. Finally, we touch upon the question of ways to circumvent the manifestation of a minimal length scale in short-distance physics.

Black-Schaffer A.M.,NORDITA
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We show that the carrier-mediated exchange interaction, the so-called Ruderman-Kittel-Kasuya-Yoshida (RKKY) coupling, between two magnetic impurity moments in graphene is significantly modified in the presence of electron-electron (el-el) interactions. Within the mean-field approximation of the Hubbard- U model we show that for increasing el-el interactions the oscillations disappear and the power-law decay becomes more long ranged. In zigzag graphene nanoribbons the effects are even more striking with any finite U rendering the RKKY coupling distance independent. Since the RKKY coupling is directly proportional to the magnetic susceptibility, these results are important for any physical property of graphene related to magnetism. Comparing our mean-field results with first-principles results we also extract a surprisingly large value of U indicating that graphene is very close to an antiferromagnetic instability. © 2010 The American Physical Society.

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