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Bang Y.,Chonnam National University | Bang Y.,Asia Pacific Center for Theoretical Physics
Physical Review Letters | Year: 2010

We studied the field dependencies of specific heat coefficient γ(H)=limT→0C(T,H)/T and thermal conductivity coefficient limT→0κ(T,H)/T of the ±s-wave state in the mixed state. It is a generic feature of the two band s-wave state with the unequal sizes of gaps, small ΔS and large ΔL, that Doppler shift of the quasiparticle excitations (Volovik effect) creates a finite density of states, on the extended states outside of vortex cores, proportional to H in contrast to the √H dependence of the d-wave state. The impurity scattering effect makes this generic H-linear dependence sublinear approaching to the √H behavior. Our calculations of limT→0κ(T,H)/T successfully fit the experimental data of Ba(Fe1-xCox)2 As2 with different Co-doping x by systematically varying the gap size ratio R=|ΔS|/|ΔL|. We also resolve the dilemma of a substantial value of γ(H→0) but almost zero value of limT→0κ(T,H→0)/T. © 2010 The American Physical Society.

Reunchan P.,Asia Pacific Center for Theoretical Physics | Jhi S.-H.,Pohang University of Science and Technology
Applied Physics Letters | Year: 2011

First-principles calculations are carried out to study the role of various metal atoms on porous graphene for molecular hydrogen (H2) adsorption. The binding sites of each metal atom on porous graphene are investigated and the binding energies are determined. It is shown that H 2 exhibits different adsorption characteristics onto alkaline, alkaline-earth, or transition metals in porous graphene. In particular, Ca-decorated porous graphene is investigated and found to be feasible for high-capacity hydrogen storage. Our results provide a general picture on the interactions of H2 with porous graphene decorated with various metals. © 2011 American Institute of Physics.

Choi K.-Y.,Asia Pacific Center for Theoretical Physics | Choi K.-Y.,Pohang University of Science and Technology | Seto O.,Hokkai Gakuen University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

We show that a Dirac right-handed scalar neutrino can be dark matter (DM) as a weakly interacting massive particle in the neutrinophilic Higgs model. When the additional Higgs fields couple only to the leptonic sector through neutrino Yukawa couplings, the right number of relic density of DM can be obtained from thermal freeze-out of the DM annihilation into charged leptons and neutrinos. At present epoch, this tree-level annihilation into fermions is suppressed by the velocity of DM, and the one-loop annihilation cross section into γγ can be dominant because relevant coupling constants are different. Hence, the recently observed (tentative) gamma-ray line signal in the Fermi-Large Area Telescope can be naturally explained by the annihilation of right-handed sneutrino DM. © 2012 American Physical Society.

Sahlmann H.,Asia Pacific Center for Theoretical Physics | Sahlmann H.,Pohang University of Science and Technology
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

In general relativity, the fields on a black hole horizon are obtained from those in the bulk by pullback and restriction. Similarly, in quantum gravity, the quantized horizon degrees of freedom should result from restricting, or pulling back, the quantized bulk degrees of freedom. This is not yet fully realized in the-otherwise very successful-quantization of isolated horizons in loop quantum gravity. In this work we outline a setting in which the quantum horizon degrees of freedom are simply components of the quantized bulk degrees of freedom. There is no need to quantize them separately. We present evidence that for a horizon of sphere topology, the resulting horizon theory is remarkably similar to what has been found before. © 2011 American Physical Society.

Higaki T.,Keio University | Jeong K.S.,Pusan National University | Kitajima N.,Asia Pacific Center for Theoretical Physics | Takahashi F.,Tohoku University | Takahashi F.,University of Tokyo
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2016

We argue that the QCD axion can arise from many aligned axions with decay constants much smaller than the conventional axion window. If the typical decay constant is of O(100) GeV to 1 TeV, one or more of the axions or saxions may account for the recently found diphoton excess at ~750 GeV. Our scenario predicts many axions and saxions coupled to gluons with decay constants of order the weak scale, and therefore many collider signatures by heavy axions and saxions will show up at different energy scales. In particular, if the inferred broad decay width is due to multiple axions or saxions, a non-trivial peak structure may become evident when more data is collected. We also discuss cosmological implications of the aligned QCD axion scenario. In the Appendix we give a possible UV completion and argue that the high quality of the Peccei-Quinn symmetry is naturally explained in our scenario. © 2016 Published by Elsevier B.V.

Bourgine J.-E.,Asia Pacific Center for Theoretical Physics
International Journal of Modern Physics A | Year: 2014

In this paper, we study a class of grand-canonical partition functions with a kernel depending on a small parameter ε. This class is directly relevant to Nekrasov partition functions of N = 2 SUSY gauge theories on the 4d Ω-background, for which ε is identified with one of the equivariant deformation parameter. In the Nekrasov-Shatashvili limit ε→0, we show that the free energy is given by an on-shell effective action. The equations of motion take the form of a TBA equation. The free energy is identified with the Yang-Yang functional of the corresponding system of Bethe roots. We further study the associated canonical model that takes the form of a generalized matrix model. Confinement of the eigenvalues by the short-range potential is observed. In the limit where this confining potential becomes weak, the collective field theory formulation is recovered. Finally, we discuss the connection with the alternative expression of instanton partition functions as sums over Young tableaux. © World Scientific Publishing Company.

Bourgine J.-E.,Asia Pacific Center for Theoretical Physics
Nuclear Physics B | Year: 2014

Mayer cluster expansion is an important tool in statistical physics to evaluate grand canonical partition functions. It has recently been applied to the Nekrasov instanton partition function of N=2 4d gauge theories. The associated canonical model involves coupled integrations that take the form of a generalized matrix model. It can be studied with the standard techniques of matrix models, in particular collective field theory and loop equations. In the first part of these notes, we explain how the results of collective field theory can be derived from the cluster expansion. The equalities between free energies at first orders is explained by the discrete Laplace transform relating canonical and grand canonical models. In a second part, we study the canonical loop equations and associate them with similar relations on the grand canonical side. It leads to relate the multi-point densities, fundamental objects of the matrix model, to the generating functions of multi-rooted clusters. Finally, a method is proposed to derive loop equations directly on the grand canonical model. © 2014 The Author.

Wan M.-B.,Asia Pacific Center for Theoretical Physics
Classical and Quantum Gravity | Year: 2011

We study the neutron star axisymmetric critical solution previously found in the numerical studies of neutron star mergers. Using neutron star-like initial data and performing similar merger simulations, we demonstrate that the solution is indeed a semi-attractor on the threshold plane separating the basin of a neutron star and the basin of a black hole in the solution space of the Einstein equations. In order to explore the extent of the attraction basin of the neutron star semi-attractor, we construct initial data phase spaces for these neutron star-like initial data. From these phase spaces, we also observe several interesting dynamical scenarios where the merged object is supported from prompt collapse. The properties of the critical index of the solution, in particular, its dependence on conserved quantities, are then studied. From the study, it is found that a family of neutron star semi-attractors exist that can be classified by both their rest masses and ADM masses. © 2011 IOP Publishing Ltd.

Bourgine J.-E.,Asia Pacific Center for Theoretical Physics
Journal of High Energy Physics | Year: 2015

Abstract: The Spherical Hecke central (SHc) algebra has been shown to act on the Nekrasov instanton partition functions of N=2 gauge theories. Its presence accounts for both integrability and AGT correspondence. On the other hand, a specific limit of the Omega background, introduced by Nekrasov and Shatashvili (NS), leads to the appearance of TBA and Bethe like equations. To unify these two points of view, we study the NS limit of the SHc algebra. We provide an expression of the instanton partition function in terms of Bethe roots, and define a set of operators that generates infinitesimal variations of the roots. These operators obey the commutation relations defining the SHc algebra at first order in the equivariant parameter ϵ2. Furthermore, their action on the bifundamental contributions reproduces the Kanno-Matsuo-Zhang transformation. We also discuss the connections with the Mayer cluster expansion approach that leads to TBA-like equations. © 2015, The Author(s).

Appleby S.,Asia Pacific Center for Theoretical Physics
Journal of Cosmology and Astroparticle Physics | Year: 2015

We search for self tuning solutions to the Einstein-scalar field equations for the simplest class of 'Fab-Four' models with constant potentials. We first review the conditions under which self tuning occurs in a cosmological spacetime, and by introducing a small modification to the original theory - introducing the second and third Galileon terms - show how one can obtain de Sitter states where the expansion rate is independent of the vacuum energy. We then consider whether the same self tuning mechanism can persist in a spherically symmetric inhomogeneous spacetime. We show that there are no asymptotically flat solutions to the field equations in which the vacuum energy is screened, other than the trivial one (Minkowski space). We then consider the possibility of constructing Schwarzschild de Sitter spacetimes for the modified Fab Four plus Galileon theory. We argue that the only model that can successfully screen the vacuum energy in both an FLRW and Schwarzschild de Sitter spacetime is one containing 'John' ∼ Gμνμφνφ and a canonical kinetic term ∼ αφ αφ. This behaviour was first observed in [1]. The screening mechanism, which requires redundancy of the scalar field equation in the 'vacuum', fails for the 'Paul' term in an inhomogeneous spacetime. © 2015 IOP Publishing Ltd and Sissa Medialab srl.

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