SangEun Han

pdf version of CV: here (updated at 2024/06/15)
TA class materials: here

Publications on Google ScholarGoogle Scholar
Publications on arXiv
Publications on INSPIREINSPIRE
Publications on ORCIDORCID
Publications on ResearchGateResearchGate
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Publications on Web of ScienceWeb of Science
Publications on Scopus

Articles related to my works:
                    KAIST Compass 2019 Fall (Newsletter of College of Natural Science)
                    Physics & High Technology 2019 09 (KPS Webzine, in Korean)
                    KAIST Department of Physics Newsletter Fall 2018, Vol. 07 (in Korean)
                    KAIST Department of Physics Newsletter Autumn 2017, Vol. 05 (in Korean)

  • SangEun Han and Igor F. Herbut
    We construct and study the relativistic Gross-Neveu-Yukawa field theory for the $\text{SO}(2N)$ real symmetric second-rank tensor order parameter coupled to $N_f$ flavors of $4N$-component Majorana fermions in 2+1 dimensions. Such a tensor order parameter unifies all Lorentz-invariant mass-gap orders for $N$ two-component Dirac fermions in two dimensions except for the $\text{SO}(2N)$-singlet anomalous quantum Hall state. The value $N_f=1$ corresponds to the canonical Gross-Neveu model. Within the leading-order $\epsilon$-expansion around the upper critical dimension of $3+1$ the field theory exhibits a critical fixed point in its renormalization group flow which describes spontaneous symmetry breaking to $\text{SO}(N)\times \text{SO}(N)$ for the number of flavors of Majorana fermions higher than a critical value $N_{f,c2}\approx 2N$. For $N_{f, c1}< N_f < N_{f,c2}$ , with $N_{f,c1} \approx 1.080 N$ the critical fixed point resides in the unstable region of the theory where the effective potential is unbounded from below, whereas for $N_f < N_{f,c1}$ there is no real critical fixed point, and the flow runs away. In either case, for $N_f < N_{f,c2}$ the transition should become fluctuation-induced first-order, and we discuss the dependence of its size on the parameters in the theory. One-loop critical exponents for the new universality class at $N_{f, c2}< N_f $ are computed and the flow diagram in various regimes is discussed.
  • W
     "Spontaneous breaking of the SO(2N) symmetry in the Gross-Neveu model"
    SangEun Han and Igor F. Herbut
    Phys. Rev. D 109, 096026 (2024) (arXiv:2403.09627)
    The canonical Gross-Neveu model for $N$ two-component Dirac fermions in $2+1$ dimensions suffers a continuous phase transition at a critical interaction $g_{c1} \sim 1/N$ at large $N$, at which its continuous symmetry $\text{SO}(2N)$ is preserved and a discrete (Ising) symmetry becomes spontaneously broken. A recent mean-field calculation, however, points to an additional transition at a diffferent critical $g_{c2}\sim -N g_{c1}$, at which $\text{SO}(2N) \rightarrow \text{SO}(N) \times \text{SO}(N)$. To study the latter phase transition we rewrite the Gross-Neveu interaction $g (\bar{\psi} \psi)^2$ in terms of three different quartic terms for the single ($L=1$) $4N$-component real (Majorana) fermion, and then extend the theory to $L>1$. This allows us to track the evolution of the fixed points of the renormalization group transformation starting from $L\gg 1$, where one can discern three distinct critical points which correspond to continuous phase transitions into (1) $\text{SO}(2N)$-singlet mass-order-parameter, (2) $\text{SO}(2N)$-symmetric-tensor mass-order-parameters, and (3) $\text{SO}(2N)$-adjoint nematic-order-parameters, down to $L=1$ value that is relevant to standard Gross-Neveu model. Below the critical value of $L_c (N)\approx 0.35 N$ for $N\gg1$ only the Gross-Neveu critical point (1) still implies a diverging susceptibility for its corresponding ($\text{SO}(2N)$-singlet) order parameter, whereas the two new critical points that existed at large $L$ ultimately become equivalent to the Gaussian fixed point at $L=1$. We interpret this metamorphosis of the $\text{SO}(2N)$-symmetric-tensor fixed point from critical to spurious as an indication that the transition at $g_{c2}$ in the original Gross-Neveu model is turned first-order by fluctuations.
  • SangEun Han, Félix Desrochers, and Yong Baek Kim
    Understanding non-Fermi liquids in dimensions higher than one remains one of the most formidable challenges in modern condensed matter physics. These systems, characterized by an abundance of gapless degrees of freedom and the absence of well-defined quasiparticles, defy conventional analytical frameworks. Inspired by recent work on the bosonization of Fermi surfaces [Delacretaz, Du, Mehta, and Son, Physical Review Research, 4, 033131 (2022)], we present a procedure for bosonizing non-Fermi liquids, which provides a holistic approach capable of addressing their intricate physics. Our method involves parameterizing the generalized fermionic distribution function through a bosonic field that describes frequency-dependent local variations of the chemical potential in momentum space. We propose an effective action that produces the collisionless quantum Boltzmann equation as its equation of motion and can straightforwardly be used for any dimension and Fermi surface of interest. Even at the quadratic order, this action reproduces highly non-trivial results obtainable only through involved analysis with alternative means. By offering a comprehensive description of the physics of non-Fermi liquids, our work stands as an important building block in advancing the comprehension of strange metals and associated phenomena.
  • SangEun Han, Daniel J. Schultz, and Yong Baek Kim
    Phys. Rev. B 108, L060401 (2023) (arXiv:2207.07661)
    Recent experiments on heavy fermion systems with higher-rank multipolar local moments provide a new platform to study such questions. In particular, experiments on Ce3Pd20(Si,Ge)6 show novel quantum critical behaviors via two consecutive magnetic field-driven quantum phase transitions. At each transition, the derivative of the Hall conductivity jumps discontinuously, which was attributed to sequential Fermi surface reconstructions. Motivated by this discovery, we consider a microscopic model of itinerant electrons coupled to local dipolar, quadrupolar, and octupolar moments arising from Ce3+ ions. Using renormalization group analyses, we demonstrate that numerous transitions can occur depending on which multipolar moments participate in the Fermi surface and which other moments are decoupled via Kondo destruction, and identify order parameters consistent with experiments. Our work offers a new theoretical framework for understanding multipolar quantum materials.
  • SangEun Han, Daniel J. Schultz, and Yong Baek Kim
    Phys. Rev. B 107, 235153 (2023) (arXiv:2302.07883)
    Non-Hermitian physics in open quantum many-body systems provides novel opportunities for discovery of exotic quantum phenomena unexpected in Hermitian systems. A previous study of the non-Hermitian Kondo problem in ultra-cold atoms reports reversion of renormalization group flows which violates the g-theorem and produces an unusual quantum phase transition. In this work, we study the effect of electron-electron interactions by considering the non-Hermitian Kondo problem in a Luttinger liquid. By performing a perturbative renormalization group analysis to two-loop order, we find that the interplay between non-Hermitian Kondo couplings and electron-electron interactions can produce a pair of complex fixed points. Complex fixed points have often been discussed in an attempt to understand extremely long correlation length of Hermitian systems with weakly first-order transitions. Here, we show that complex fixed points arise naturally and can be physically realized in open quantum systems. We discuss consequences of the complex fixed points and future directions.
  • SangEun Han, Daniel J. Schultz, and Yong Baek Kim
    Phys. Rev. B 106, 155155 (2022) (arXiv:2206.02808)
    The non-Kramers doublet of the Pr3+ ion in Pr(Ti,V)2Al20 allows quadrupolar and octupolar moments, but lacks a dipolar moment. Previous theoretical studies show that a single impurity Kondo problem with such an unusual local moment leads to exotic non-Fermi liquid states. In this work, we investigate possible quantum critical behaviors arising from the competition between non-Fermi liquid states and multipolar-ordered phases induced by the RKKY interaction. We consider a local version of the corresponding Kondo lattice model, namely the Bose-Fermi Kondo model. Here, the multipolar local moments are coupled to fermionic and bosonic bath degrees of freedom representing the multipolar Kondo effect and RKKY interactions. Using a perturbative renormalization group (RG) study up to two loop order, we find critical points between non-Fermi liquid Kondo fixed points and a quadrupolar ordered fixed point. The critical points describe quantum critical behaviors at the corresponding phase transitions and can be distinguished by higher order corrections in the octupolar susceptibility that can be measured by ultrasound experiments. Our results imply the existence of a rich expansion of the phases and quantum critical behaviors in multipolar heavy fermion systems.
  • SangEun Han and Yong Baek Kim
    Phys. Rev. B 106, L081106 (2022) (arXiv:2102.05052)
    We investigate the emergence of an exotic non-Fermi liquid in two dimensions, where the fermions with quadratic band-touching dispersion interact with Bose metal. The bosonic excitations in Bose metal possess an extended nodal-line spectrum in momentum space, which arises due to the subsystem symmetry or the restricted motion of bosons. Using renormalization group analysis and direct computations, we show that the extended infrared singularity of Bose metal leads to a line of interacting fixed points of an exotic non-Fermi liquids, where the anomalous dimension of the fermions varies continuously, akin to the Luttinger liquid in one dimension. Further, the generalization of the model with multiple low-energy excitations is used to explore other unusual features of the resulting ground state.
  • SangEun Han, Adarsh S. Patri, and Yong Baek Kim
    Phys. Rev. B 105, 235120 (2022) (arXiv:2109.03835)
    We provide a realistic quantum model of quadratic spin interactions on the breathing pyrochlore lattice of existing materials. We show that the emergent “cluster charge” excitations arise as vacuum fluctuations residing on the boundary of membrane objects, and move in a subdimensional space. Using the membrane operators, we demonstrate the existence of a subextensive ground state degeneracy explicitly depending on the lattice geometry, which is a useful resource for novel quantum memory.
  • SangEun Han, Junhyun Lee, Eun-Gook Moon
    Phys. Rev. B 103, 014435 (2021) (arXiv:1911.01435)
    We show that lattice vibration may not be a decoherence source but an impetus of a novel coherent quantum many-body state. We demonstrate the possibility by studying the transverse-field Ising model on a chain with renormalization group and density-matrix renormalization group methods and theoretically discover a stable N=1 supersymmetric quantum criticality with central charge c=3/2. Thus, we propose an Ising spin chain with strong spin-lattice coupling as a candidate to observe supersymmetry. Generic precursor conditions of novel quantum criticality are obtained by generalizing the Larkin-Pikin criterion of thermal transitions. Our work provides the perspective that lattice vibration may be a knob for exotic quantum many-body states.
  • We study topological quantum phase transitions (TQPTs) between double-Weyl semimetals (DWSMs) and insulators, and argue that a novel class of quantum criticality appears at the TQPT characterized by emergent anisotropic non-Fermi-liquid behaviors, in which the interplay between the Coulomb interaction and electronic critical modes induces not only anisotropic renormalization of the Coulomb interaction but also strongly correlated electronic excitation in three spatial dimensions. Using the standard renormalization group methods, large Nf theory, and the ε=4−d method with a fermion flavor number Nf and spatial dimension d, we obtain the anomalous dimensions of electrons (ηf=0.366/Nf) in large Nf theory and the associated anisotropic scaling relations of various physical observables. Our results may be observed in candidate materials for DWSMs such as HgCr2Se4 or SrSi2 when the system undergoes a TQPT.
  • We study quantum phase transitions associated with splitting nodal Fermi points, motivated by topological phase transitions between Dirac and Weyl semimetals. A Dirac point in Dirac semimetals may be split into two Weyl points by breaking a lattice symmetry or time-reversal symmetry, and the Lifshitz transition is commonly used to describe the phase transitions. Here, we show that the Lifshitz description is fundamentally incorrect in quantum phase transitions with splitting nodal Fermi points. We argue that correlations between fermions, order parameter, and the long-range Coulomb interaction must be incorporated from the beginning. One of the most striking correlation effects we find is infinite anisotropy of physical quantities, which cannot appear in a Lifshitz transition. By using the standard renormalization group method, two types of infinitely anisotropic quantum criticalities are found in three spatial dimensions, varying with the number of the Dirac points N. Our renormalization group analysis is fully controlled by the fact that order parameter and fermion fluctuations are at the upper critical dimension, and thus our stable fixed points demonstrate the presence of weakly coupled quantum criticalities with infinite anisotropy.
  • SangEun Han and Eun-Gook Moon
    Phys. Rev. B 97, 241101(R) (2018) (arXiv:1802.05727)
    Chiral symmetry is one of the most fundamental symmetries in nature, which prohibits mass generation of fermions. Remarkable advances in topological matter reveal the chiral symmetry may be realized as lattice symmetries of topological state. A topological phase transition is intrinsically tied to breaking a chiral symmetry. The topological transition, however, is lack of the Lorentz symmetry in contrast to particle physics. In particular, the Coulomb interaction is instantaneous, and it is imperative to understand its effects on chiral symmetry breaking transitions. We show that a topological transition associated with chiral symmetry is stable under the presence of a Coulomb interaction and the electron velocity always becomes faster than the one of a chiral symmetry order parameter. Thus, the transition must not be relativistic, which implies that supersymmetry is intrinsically forbidden by the long-range Coulomb interaction. Asymptotically exact universal ratios of physical quantities such as the energy gap ratio are obtained, and connections with experiments and recent theoretical proposals are also discussed.
  • We show intriguing phenomena of interplay between symmetry and topology in three-dimensional topological phase transitions associated with line-nodal superconductors. More specifically, we discover an exotic universality class out of topological line-nodal superconductors. The order parameter of broken symmetries is strongly correlated with underlying line-nodal fermions, and this gives rise to a large anomalous dimension in sharp contrast to that of the Landau-Ginzburg theory. Remarkably, hyperscaling violation and emergent relativistic scaling appear in spite of the presence of nonrelativistic fermionic excitation. We also propose characteristic experimental signatures around the phase transitions, for example, a linear phase boundary in a temperature-tuning parameter phase diagram, and discuss the implication of recent experiments in pnictides and heavy-fermion systems.
  • "Explaining the Lepton Non-universality at the LHCb and CMS from an Unified Framework"
    Sanjoy Biswas, Debtosh Chowdhury, SangEun Han, and Seung J. Lee
    JHEP 02, 142 (2015) (arXiv:1409.0882)
Academic Affiliations
  • Department of Physics, Simon Fraser University, Burnaby, Canada September 2023 - Present
  • Postdoctoral Fellow
  • Department of Physics, University of Toronto, Toronto, Canada November 2020 - August 2023
  • Postdoctoral Fellow
  • School of Computational Sciences, KIAS, Seoul, Korea August 2020 - October 2020
  • Visiting Scholar
  • Department of Physics, KAIST, Daejeon, Korea March 2013 - August 2020
  • Candidate of Integrated Master's and Ph.D Program

  • Korea Advanced Institute of Science and Technology, Daejeon, Korea March 2013 - August 2020
    Doctor of Philosophy in Physics
    Thesis Adviosr: Prof. Eun-Gook Moon
  • Korea Advanced Institute of Science and Technology, Daejeon, Korea Feburary 2010 - Feburary 2013
    Bachelor of Science, Magna Cum Laude, Feburary, 2013
    Double major in Physics and Mathematical Sciences
  • Hankuk University of Foreign Studies, Seoul, Korea March 2006 - January 2008
    in Department of Physics

  • Scholarship of Center for Theoretical Physics of the Universe, IBS March 2014 - June 2015
  • Academic Scholarship in Hankuk University of Foreign Studies 2006 Fall - 2008 Spring

  • 2018 Workshop on Spin-orbit Coupled Topological states Outstanding Poster Award October 2018
  • 2018 Pre-doctoral Fellow of Physics at KAIST August 2018
  • 2014 Spring Outstanding Teaching Assistant Awards September 2014
  • 2011 Collegiate Engineering Mathematics Competition Bronze December 2011
  • 2011 Fall Semester's FDC Award, Presidential Design Award Feburary 2012

  • Reviewer August 2022 - Present
    of Nature Communications
  • Referee January 2020 - Present
    of Phys. Rev. Research
  • Referee April 2019 - Present
    of Phys. Rev. Lett.
  • Referee September 2018 - Present
    of Phys. Rev. B
  • Military service Feburary 2008 - January 2010
    at Republic of Korea Army, the 30th Division, Republic of Korea

Research Interests
  • Theoretical Physics, Condensed matter physics, Strongly correlated systems, Quantum phase transitions, Theoretical particle physics, Quantum field theory, Mathematical physics, Statistical physics (equilibrium or non-equilibrium), Quantum spin liquids, Quantum chaos, SYK physics, Non-Fermi liquids, Non-Hermitian systems, Flat bands, Deconfined Quantum Critical Points, Fractons, etc.

Teaching Experiences
  • Teaching Assistant for the Physics Department, KAIST September 2017 - December 2017
    Course: Graduate Quantum Mechanics 2
  • Teaching Assistant for the Physics Department, KAIST March 2017 - June 2017
    Course: Graduate Quantum Mechanics 1
  • Teaching Assistant for the Physics Department, KAIST September 2016 - December 2016
    Course: Colloquium & Seminar
  • Teaching Assistant for the Physics Department, KAIST March 2016 - June 2016
    Course: Graduate Quantum Mechanics 1
  • Teaching Assistant for the Physics Department, KAIST September 2015 - December 2015
    Course: Undergraduate Quantum Mechanics 2
  • Teaching Assistant for the Physics Department, KAIST March 2015 - June 2015
    Course: Undergraduate Quantum Mechanics 1
  • Teaching Assistant for the Physics Department, KAIST March 2014 - June 2014
    Course: Quantum Field Theory 2
  • Teaching Assistant for the Physics Department, KAIST September 2013 - December 2013
    Course: General Physics 2
  • Teaching Assistant for the Physics Department, KAIST March 2013 - June 2013
    Course: General Physics 1
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2024: 2024/01 / 2024/02 / 2024/03 / 2024/04 / 2024/05 / 2024/06 / 2024/07 /