Eun-Gook Moon

Topological Phase Transitions in Line-nodal Superconductors

Fathoming interplay between symmetry and topology of many-electron wave-functions has deepened understanding of quantum many body systems, especially after the discovery of topological insulators. Topology of electron wave-functions enforces and protects emergent gapless excitations, and symmetry is intrinsically tied to the topological protection in a certain class. Namely, unless the symmetry is broken, the topological nature is intact. We show novel interplay phenomena between symmetry and topology in topological phase transitions associated with line-nodal superconductors. The interplay may induce an exotic universality class in sharp contrast to that of the phenomenological Landau-Ginzburg theory. Hyper-scaling violation and emergent relativistic scaling are main characteristics, and the interplay even induces unusually large quantum critical region. We propose characteristic experimental signatures around the phase transitions in three spatial dimensions, for example, a linear phase boundary in a temperature-tuning parameter phase-diagram.

Exotic continuous quantum phase transition betweenZ2topological spin liquid and Néel order

Recent numerical simulations with different techniques have all suggested the existence of a continuous quantum phase transition between the Z2 topological spin liquid phase and a conventional Neel order. Motivated by these numerical progresses, we propose a candidate theory for such Z2-Neel transition. We first argue on general grounds that, for a SU(2) invariant system, this transition cannot be interpreted as the condensation of spinons in the Z2 spin liquid phase. Then we propose that such Z2-Neel transition is driven by proliferating the bound state of the bosonic spinon and vison excitation of the Z2 spin liquid, i.e. the so called (e, m)-type excitation. Universal critical exponents associated with this exotic transition are computed using 1/N expansion. This theory predicts that at the Z2-Neel transition, there is an emergent quasi long range power law correlation of columnar valence bond solid order parameter.

Long-range Coulomb interaction effects on the topological phase transitions between semimetals and insulators

Topological states may be protected by a lattice symmetry in a class of topological semi-metals. In three spatial dimensions, the Berry flux around gapless excitations in momentum space defines a chirality concretely, so a protecting symmetry may be referred to as a chiral symmetry. Prime examples include Dirac semi-metal (DSM) in a distorted spinel, BiZnSiO

Ab initio study of dihydrogen binding in metal-decorated polyacetylene for hydrogen storage

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Skyrmions with quadratic band touching fermions: A way to achieve charge4esuperconductivity

, protected by a mirror symmetry and DSM in Na

Vestiges of Topological Phase Transitions in Kitaev Quantum Spin Liquids

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Emergent Anisotropic Non-Fermi Liquid.

Bi, protected by a rotational symmetry. In these states, topology and a chiral symmetry are intrinsically tied. In this work, we investigate characteristics interplay between a chiral symmetry order parameter and instantaneous long-range Coulomb interaction with the standard renormalization group method. We show that a topological transition associated with a chiral symmetry is stable under the presence of the Coulomb interaction and the electron velocity always becomes faster than one of a chiral symmetry order parameter. Thus, the transition {\it must not} be relativistic, which implies a supersymmetry is intrinsically forbidden by the long-range Coulomb interaction. {Asymptotically exact} universal ratios of physical quantities such as energy gap ratio are obtained, and connections with experiments and recent theoretical proposals are also discussed.