arXiv subjects

SangEun Han

Publications and source records attributed to SangEun Han.

16 recordsLinked to original sources

arXiv2026

Optical Magnetic Switching in Odd-Parity Magnets with Spin-Orbit Coupling

$p$-wave magnets exhibit odd-parity spin polarization in momentum space, with spin splitting that reverses under $\vec{k}\rightarrow -\vec{k}$, while preserving zero net magnetization. Here we show that, in odd-parity magnets with spin-orbit coupling, elliptically polarized light generates a momentum-independent spin-dependent term that dynamically switches a zero-net-magnetization $p$-wave state into a finite spin-polarized state. The Floquet-engineered bands also acquire a nonzero Chern number whose sign is controlled by the light polarization. For $f$-wave magnets, circularly polarized light induces a net out-of-plane magnetization, offering a direct experimental signature. Our results establish light as an efficient means of controlling magnetic states, with potential applications in spintronics and quantum information.

cond-mat.str-el↗

arXiv2025

Beyond one-loop calculation: Higher-order effects on Gross-Neveu-Yukawa tensorial criticality

We study the Gross-Neveu-Yukawa field theory for the SO($N$) symmetric traceless rank-two tensor order parameter coupled to Majorana fermions using the $\epsilon$-expansion around upper critical dimensions of $3+1$ to two loops. Previously we established in the one-loop calculation that the theory does not exhibit a critical fixed point for $N \geq 4$, but that nevertheless the stable fixed point inevitably emerges at a large number of fermion flavors $N_f$. For $N_f < N_{f,c1} \approx N/2$, no critical fixed point exists; for $N_{f,c1} < N_f < N_{f,c2}$, a real critical fixed point emerges from the complex plane but fails to satisfy the additional stability conditions necessary for a continuous phase transition; and finally only for $N_f > N_{f,c2} \approx N$, the fixed point satisfies the stability conditions as well. In the present work we compute the $O(\epsilon)$ (two-loop) corrections to the critical flavour numbers $N_{f,c1} $ and $N_{f,c2}$. Most importantly, we observe a sharp decrease in $N_{f,c2}$ from its one-loop value, which brings it closer to the point $N_f =1$ relevant to the standard Gross-Neveu model. Some three-loop results are also presented and discussed.

cond-mat.str-el↗

arXiv2024

Gross-Neveu-Yukawa SO(2) and SO(3) tensorial criticality

We investigate the relativistic SO(2)- and SO(3)-invariant Gross-Neveu-Yukawa field theories for real, rank-two, symmetric, traceless tensor order parameters coupled to $N_{\text{f}}$ flavors of two-component Dirac fermions. These field theories arise as an effective description of fractionalized spin-orbital liquids. The two theories are the simplest and special cases of the more general class of field theories with SO($N$) symmetric tensor order parameter coupled to Dirac fermions, in which the symmetry is low enough to allow only one, and not the usual two quartic self-interaction terms. Using two-loop renormalization group near the upper critical dimension, we demonstrate that the theory exhibits a new critical fixed point for $N=3$ and the concomitant continuous phase transition for any value of $N_{\text{f}}$. For $N=2$ the theory is equivalent to the chiral XY model. We discuss the crucial role of the symmetry-allowed sextic self-interactions in the selection of the ground state configuration in the case of SO(3). The universal quantities such as the the anomalous dimensions of order parameters and fermions, the correlation length exponent, and the mass gap ratio between order parameter and fermion masses are computed up to $\epsilon^{2}$ order.

cond-mat.str-el↗

arXiv2024

Gross-Neveu-Yukawa theory of $\text{SO}(2N)\rightarrow \text{SO}(N) \times \text{SO}(N)$ spontaneous symmetry breaking

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 $N_{f,c1} \approx N$ 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 $N$ and $N_{f}$ in the theory. One-loop critical exponents for the universality class at $N_{f, c2}< N_f $ are computed and the flow diagram in various regimes is discussed.

cond-mat.str-el↗

arXiv2024

Spontaneous breaking of the $\text{SO}(2N)$ symmetry in the Gross-Neveu model

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 different 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 $g (\barψ ψ)^2$ 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 the 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.

hep-th↗

arXiv2023

Quantum impurity model for two-stage multipolar ordering and Fermi surface reconstruction

Classification and understanding of quantum phase transitions and critical phenomena in itinerant electron systems are outstanding questions in quantum materials research. Recent experiments on heavy fermion systems with higher-rank multipolar local moments provide a new platform to study such questions. In particular, experiments on $\text{Ce}_{3}\text{Pd}_{20}\text{(Si,Ge)}_{6}$ show novel quantum critical behaviors via two consecutive magnetic field-driven quantum phase transitions. At each transition, the derivative of the Hall resistivity jumps discontinuously, which was attributed to sequential Fermi surface reconstructions. Motivated by this discovery, we consider an effective quantum impurity model of itinerant electrons coupled to local dipolar, quadrupolar, and octupolar moments arising from $\text{Ce}^{3+}$ ions. Using renormalization group analyses, we demonstrate that two-stage multipolar ordering and Fermi surface reconstruction arise depending on which multipolar moments participate in the Fermi surface and which other moments are decoupled via Kondo destruction.

cond-mat.str-el↗

arXiv2023

Complex fixed points of the non-Hermitian Kondo model in a Luttinger liquid

Non-Hermitian physics in open quantum many-body systems provides novel opportunities for the discovery of exotic quantum phenomena unexpected in Hermitian systems. A previous study of the non-Hermitian Kondo problem in ultracold 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 the 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.

cond-mat.str-el↗

arXiv2023

Fermi Surface Bosonization for Non-Fermi Liquids

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 [Delacretaz, Du, Mehta, and Son, Physical Review Research, 4, 033131 (2022)], we present a procedure for bosonizing Fermi surfaces that does not rely on the existence of sharp excitation and is thus directly applicable to non-Fermi liquids. 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 be used for any dimension and Fermi surface of interest. Even at the quadratic order, this action reproduces non-trivial results obtainable only through involved analysis with alternative means. By offering an alternative method directly applicable to studying the low-energy physics of Fermi and non-Fermi liquids, our work potentially stands as an important building block in advancing the comprehension of strange metals and associated phenomena.

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arXiv2022

Non-Fermi liquid behavior and quantum criticality in cubic heavy fermion systems with non-Kramers multipolar local moments

Notable non-Fermi liquid and quantum critical behaviors are observed in rare-earth metallic systems with non-Kramers local moments supporting a number of different multipolar moments. A prominent example is $\text{Pr(Ti,V)}_{2}\text{Al}_{20}$, where the non-Kramers doublet of the $\text{Pr}^{3+}$ ion 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 novel 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.

cond-mat.str-el↗

arXiv2022

Non-Fermi liquid induced by Bose metal with protected subsystem symmetries

Understanding non-Fermi liquids in dimensions higher than one, has been a subject of great interest. Such phases may serve as parent states for other unconventional phases of quantum matter, in a similar manner that conventional broken symmetry states can be understood as instabilities of the Fermi liquid. In this work, we investigate the emergence of a novel non-Fermi liquid in two dimensions, where the fermions with quadratic band-touching dispersion interact with the Bose metal. The bosonic excitations in the 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 (IR) singularity of the Bose metal leads to a line of interacting fixed points of novel 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.

cond-mat.str-el↗

arXiv2022

Realization of fractonic quantum phases in the breathing pyrochlore lattice

Fractonic phases of matter are novel quantum ground states supporting sub-dimensional emergent excitations with mobility restrictions. Due to a sub-extensive ground state degeneracy that is dependent on the geometry of the underlying lattice, fractonic phases are considered as models for quantum memory or quantum glass. While there exist a number of exactly solvable models with interactions between multiple particles/spins, the realization of such models in real materials is extremely challenging. In this work, 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 sub-dimensional space. Using the membrane operators, we demonstrate the existence of a sub-extensive ground state degeneracy explicitly depending on the lattice geometry, which is a useful resource for novel quantum memory.

cond-mat.str-el↗

arXiv2019

Lattice vibration as a knob for novel quantum criticality: Emergence of supersymmetry from spin-lattice coupling

Control of quantum coherence in many-body system is one of the key issues in modern condensed matter. Conventional wisdom is that lattice vibration is an innate source of decoherence, and amounts of research have been conducted to eliminate lattice effects. Challenging this wisdom, here 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 method, and theoretically discover a stable $\mathcal{N}=1$ 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 a new perspective that lattice vibration may be a knob for exotic quantum many-body states.

cond-mat.str-el↗

arXiv2019

Emergent Anisotropic Non-Fermi Liquid at a Topological Phase Transition in Three Dimensions

Understanding correlation effects in topological phases and their transitions is a cutting-edge area of research in recent condensed matter physics. 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 $N_f$ theory and the $ε= 4-d$ method with fermion flavor number $N_f$ and spatial dimension $d$, we obtain the anomalous dimensions of electrons ($η_f=0.366/N_f $) in large $N_f$ theory and the associated anisotropic scaling relations of various physical observables. Our results may be observed in candidate materials for DWSMs such as HgCr$_2$Se$_4$ or SrSi$_2$ when the system undergoes a TQPT.

cond-mat.str-el↗

arXiv2018

Quantum Criticalities with Infinite Anisotropy in Topological Phase Transitions between Dirac and Weyl Semi-metals

We study quantum phase transitions (QPTs) associated with splitting nodal Fermi points, motivated by topological phase transitions between Dirac and Weyl semi-metals. A Dirac point in Dirac semi-metals 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 QPTs 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 {\it infinite anisotropy} of physical quantities, which cannot appear in a Lifshitz transition. By using the standard renormalization group (RG) method, two types of infinitely anisotropic quantum criticalities are found in three spatial dimensions varying with the number of the Dirac points ($N_f$). For $N_f = 1$, the ratio of the fermion velocity to the velocity of order parameter excitations becomes universal ($\sqrt{2}-1$) along the Dirac point splitting direction . For $N_f >1$, we find that fermions are parametrically faster than order parameter excitations in all directions. Our RG 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.

cond-mat.str-el↗

arXiv2018

Long-range Coulomb Interaction effects on Topological Phase Transitions between Semi-metals 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$_4$, protected by a mirror symmetry and DSM in Na$_3$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.

cond-mat.str-el↗

arXiv2016

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.

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