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Sep 23, 2013
09/13

by
Eun-Gook Moon

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We study Skyrmion quantum numbers, charge and statistics, in (2+1) dimension induced by quadratic band toucing(QBT) fermions. It is shown that induced charge of Skyrmions is twice bigger than corresponding Dirac particles' and their statistics are always bosonic. Applying to the Bernal stacking bi-layer graphene, we show that Skyrmions of quantum spin Hall(QSH) are charge 4e bosons, so their condensation realizes charge $4e$ superconductivity(SC). The phase transition could be a second order,...

Source: http://arxiv.org/abs/1202.5389v2

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Jun 27, 2018
06/18

by
Eun-Gook Moon

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A conservation law is one of the most fundamental properties in nature, but a certain class of conservation "laws"' could be spoiled by intrinsic quantum mechanical effects, so-called quantum anomalies. Profound properties of the anomalies have deepened our understanding in quantum many body systems. Here, we investigate quantum anomaly effects in quantum phase transitions between competing orders and striking consequences of their presence. We explicitly calculate topological nature...

Topics: Condensed Matter, Strongly Correlated Electrons

Source: http://arxiv.org/abs/1503.05199

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Sep 23, 2013
09/13

by
Eun Gook Moon; Subir Sachdev

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We study the interplay between nematic order and superconductivity, motivated by a recent experiment on FeSe observing strongly distorted vortex shapes (Song et al., Science 332, 1410 (2011)). We show that the nematic order strongly enhances the anisotropy in the superconducting coherence length, beyond that expected from considerations of the Ginzburg-Landau theory. We obtain universal functions describing the coupling between the nematic order and superconductivity, and discuss connections of...

Source: http://arxiv.org/abs/1112.3973v2

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Sep 21, 2013
09/13

by
Eun-Gook Moon; Cenke Xu

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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...

Source: http://arxiv.org/abs/1204.5486v2

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Sep 19, 2013
09/13

by
Eun Gook Moon; Subir Sachdev

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We model the underdoped cuprates using fermions moving in a background with local antiferromagnetic order. The antiferromagnetic order fluctuates in orientation, but not in magnitude, so that there is no long-range antiferromagnetism, but a 'topological' order survives. The normal state is described as a fractionalized Fermi liquid (FL*), with electron-like quasiparticles coupled to the fractionalized excitations of the fluctuating antiferromagnet. The electronic quasiparticles reside near...

Source: http://arxiv.org/abs/1010.4567v3

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Jul 19, 2013
07/13

by
Eun Gook Moon; Subir Sachdev

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We compare the position of an ordering transition in a metal to that in a superconductor. For the spin density wave (SDW) transition, we find that the quantum critical point shifts by order |Delta|, where Delta is pairing amplitude, so that the region of SDW order is smaller in the superconductor than in the metal. This shift is larger than the ~ |Delta|^2 shift predicted by theories of competing orders which ignore Fermi surface effects. For Ising-nematic order, the shift from Fermi surface...

Source: http://arxiv.org/abs/1005.3312v4

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Sep 21, 2013
09/13

by
Eun Gook Moon; Cenke Xu

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We study the many-body physics in thin film topological band insulator, where the inter-edge Coulomb interaction can lead to an exciton condensation transition. We investigate the universality class of the exciton condensation quantum critical point. With different chemical potentials and interactions, the exciton condensation can belong to z = 2 mean field, or 3d XY, or Yukawa-Higgs universality classes. The interplay between exciton condensate and the time-reversal symmetry breaking is also...

Source: http://arxiv.org/abs/1008.0097v2

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Sep 21, 2013
09/13

by
Eun Gook Moon; Subir Sachdev

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We describe the interplay between d-wave superconductivity and spin density wave (SDW) order in a theory of the hole-doped cuprates at hole densities below optimal doping. The theory assumes local SDW order, and associated electron and hole pocket Fermi surfaces of charge carriers in the normal state. We describe quantum and thermal fluctuations in the orientation of the local SDW order, which lead to d-wave superconductivity: we compute the superconducting critical temperature and magnetic...

Source: http://arxiv.org/abs/0905.2608v3

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Jun 30, 2018
06/18

by
Eun-Gook Moon; Yong Baek Kim

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Quantum criticality, a manifestation of emergent scale invariance in electron wavefunctions arises from intricate many-body quantum entanglement. One of the natural venues for the criticality is clean undoped Dirac semimetals, known as a marginally-renormalized critical phase. The ground state is only slightly modified from the Slater-type product wavefunction because the scatterings from weak disorder and Coulomb interactions are suppressed. Here, using the renormalization group (RG) analysis,...

Topics: Strongly Correlated Electrons, Condensed Matter

Source: http://arxiv.org/abs/1409.0573

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Jun 28, 2018
06/18

by
Gil Young Cho; Eun-Gook Moon

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Topological quantum phase transitions intrinsically intertwine self-similarity and topology of many-electron wave-functions, and divining them is one of the most significant ways to advance understanding in condensed matter physics. Our focus is to investigate an unconventional class of the transitions between insulators and Dirac semimetals whose description is beyond conventional pseudo relativistic Dirac Hamiltonian. At the transition without the long-range Coulomb interaction, the...

Topics: Mesoscale and Nanoscale Physics, Strongly Correlated Electrons, Condensed Matter

Source: http://arxiv.org/abs/1508.03777

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Jul 19, 2013
07/13

by
Eun-Gook Moon; Andrey V. Chubukov

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We analyse the onset temperature T_p for the pairing in cuprate superconductors at small doping, when tendency towards antiferromagnetism is strong. We consider the model of Moon and Sachdev (MS), which assumes that electron and hole pockets survive in a paramagnetic phase. Within this model, the pairing between fermions is mediated by a gauge boson, whose propagator remains massless in a paramagnet. We relate the MS model to a generic \gamma-model of quantum-critical pairing with the pairing...

Source: http://arxiv.org/abs/1005.0356v1

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Sep 18, 2013
09/13

by
Eun Gook Moon; Predrag Nikolic; Subir Sachdev

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We consider spin-1/2 fermions of mass m with interactions near the unitary limit. In an applied periodic potential of amplitude V and period a, and with a density of an even integer number of fermions per unit cell, there is a second-order quantum phase transition between superfluid and insulating ground states at a critical V=Vc. We compute the universal ratio Vc m a^2 / h^2 at N=infinity in a model with Sp(2N) spin symmetry. The insulator interpolates between a band insulator of fermions and...

Source: http://arxiv.org/abs/0707.2383v2

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Jun 30, 2018
06/18

by
Lucile Savary; Eun-Gook Moon; Leon Balents

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Magnetic fluctuations and electrons couple in intriguing ways in the vicinity of zero temperature phase transitions - quantum critical points - in conducting materials. Quantum criticality is implicated in non-Fermi liquid behavior of diverse materials, and in the formation of unconventional superconductors. Here we uncover an entirely new type of quantum critical point describing the onset of antiferromagnetism in a nodal semimetal engendered by the combination of strong spin-orbit coupling...

Topics: Strongly Correlated Electrons, Condensed Matter

Source: http://arxiv.org/abs/1403.5255

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Jun 28, 2018
06/18

by
Yejin Huh; Eun-Gook Moon; Yong Baek Kim

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Recently there have been several proposals of materials predicted to be nodal-ring semimetals, where zero energy excitations are characterized by a nodal ring in the momentum space. This class of materials falls between the Dirac-like semimetals and the more conventional Fermi-surface systems. As a step towards understanding this unconventional system, we explore the effects of the long-range Coulomb interaction. Due to the vanishing density of states at the Fermi level, Coulomb interaction is...

Topics: Strongly Correlated Electrons, Condensed Matter

Source: http://arxiv.org/abs/1506.05105

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Jun 29, 2018
06/18

by
SangEun Han; Gil Young Cho; Eun-Gook Moon

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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...

Topics: Superconductivity, Condensed Matter, Strongly Correlated Electrons

Source: http://arxiv.org/abs/1601.00975

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Jun 30, 2018
06/18

by
Bohm-Jung Yang; Eun-Gook Moon; Hiroki Isobe; Naoto Nagaosa

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Topological phase transitions in condensed matters accompany emerging singularities of the electronic wave function, often manifested by gap-closing points in the momentum space. In conventional topological insulators in three dimensions (3D), the low energy theory near the gap-closing point can be described by relativistic Dirac fermions coupled to the long range Coulomb interaction, hence the quantum critical point of topological phase transitions provides a promising platform to test the...

Topics: Mesoscale and Nanoscale Physics, Strongly Correlated Electrons, Condensed Matter

Source: http://arxiv.org/abs/1406.2766

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Sep 23, 2013
09/13

by
Eun-Gook Moon; Cenke Xu; Yong Baek Kim; Leon Balents

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We argue that a class of strongly spin-orbit coupled materials, including some pyrochlore iridates and the inverted band gap semiconductor HgTe, may be described by a minimal model consisting of the Luttinger Hamiltonian supplemented by Coulomb interactions, a problem studied by Abrikosov and collaborators. It contains two-fold degenerate conduction and valence bands touching quadratically at the zone center. Using modern renormalization group methods, we update and extend Abrikosov's classic...

Source: http://arxiv.org/abs/1212.1168v1

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Sep 18, 2013
09/13

by
Martin Veillette; Eun Gook Moon; Austen Lamacraft; Leo Radzihovsky; Subir Sachdev; D. E. Sheehy

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A sufficiently large species imbalance (polarization) in a two-component Feshbach resonant Fermi gas is known to drive the system into its normal state. We show that the resulting strongly-interacting state is a conventional Fermi liquid, that is, however, strongly renormalized by pairing fluctuations. Using a controlled 1/N expansion, we calculate the properties of this state with a particular emphasis on the atomic spectral function, the momentum distribution functions displaying the Migdal...

Source: http://arxiv.org/abs/0803.2517v2

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Sep 20, 2013
09/13

by
Bohm-Jung Yang; Mohammad Saeed Bahramy; Ryotaro Arita; Hiroki Isobe; Eun-Gook Moon; Naoto Nagaosa

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We have constructed a general theory describing the topological quantum phase transitions in 3D systems with broken inversion symmetry. While the consideration of the system's codimension generally predicts the appearance of a stable metallic phase between the normal and topological insulators, it is shown that a direct topological phase transition between two insulators is also possible when an accidental band crossing (ABC) occurs along directions with high crystalline symmetry. At the...

Source: http://arxiv.org/abs/1207.6151v2