arXiv subjects

Kyoung-Min Kim

Publications and source records attributed to Kyoung-Min Kim.

26 records · Page 2Linked to original sources

arXiv2023

Controllable magnetic domains in twisted trilayer magnets

The use of moiré patterns to manipulate two-dimensional materials has facilitated new possibilities for controlling material properties. The moiré patterns in the two-dimensional magnets can cause peculiar spin texture, as shown by previous studies focused on twisted bilayer systems. In our study, we develop a theoretical model to investigate the magnetic structure of twisted trilayer magnets. Unlike the twisted bilayer, the twisted trilayer magnet has four different local stacking structures distinguished by the interlayer couplings between the three layers. Our results show that the complex interlayer coupling effects in the moiré superlattice can lead to the stabilization of rich magnetic domain structures; these structures can be significantly manipulated by adjusting the twist angle. Additionally, external magnetic fields can easily manipulate these domain structures, indicating potential applications in spintronics devices.

cond-mat.mes-hall↗

arXiv2023

Anderson transition of in-gap quasiparticles in a quasi-two-dimensional disordered superconductor

The Anderson transition of Bogoliubov-de Gennes (BdG) quasiparticles in superconducting state has been studied theoretically for last three decades. However, its experimental proof is lacking. In particular, the relationship of the superconducting order-parameter fluctuations and the Anderson transition of BdG quasiparticles have not been well understood. Our study, based on scanning tunneling microscopy measurements, investigates how BdG quasiparticles become Anderson-localized and delocalized as a function of energy and applied magnetic field in a quasi-two-dimensional Fe-based superconductor with sufficient zero-bias BdG quasiparticles. The anomalous multifractal spectra based on the spatial distributions of the pairing gaps and the coherent peak heights suggest that superconducting fluctuations play a key role in the delocalization of in-gap BdG quasiparticles. Our real-space Hartree-Fock-BCS-Anderson simulations and renormalization group analysis with pairing fluctuations support quasiparticle localization and suggest that enhanced pairing fluctuations lead to delocalization of BdG quasiparticles and "weak localization" of phase-fluctuating Cooper pairs in quasi-two-dimensional disordered superconductors. The present study proposes that the 10-fold way classification scheme has to be generalized to take order-parameter fluctuations in actual quantum matter. Also, it shed light on how ac energy loss due to quasiparticles at Fermi level can be controlled in a quasi-2d superconductor with sufficient pairing fluctuation.

cond-mat.str-el↗

arXiv2022

Theory of Moire Magnets and Topological Magnons: Applications to Twisted Bilayer CrI3

We develop a comprehensive theory of twisted bilayer magnetism. Starting from the first-principles calculations of two-dimensional honeycomb magnet CrI3, we construct the generic spin models that represent a broad class of twisted bilayer magnetic systems. Using Monte-Carlo method, we discover a variety of non-collinear magnetic orders and topological magnons that have been overlooked in the previous theoretical and experimental studies. As a function of the twist angle, the collinear magnetic order undergoes the phase transitions to the non-collinear order and the magnetic domain phase. In the magnetic domain phase, we find that the spatially varying interlayer coupling produces the magnetic skyrmions even in the absence of the Dzyaloshinskii-Moriya interactions. In addition, we describe the critical phenomena of the magnetic phase transitions by constructing the field theoretical model of the moire magnet. Our continuum model well-explains the nature of the phase transitions observed in the numerical simulations. Finally, we classify the topological properties of the magnon excitations. The magnons in each phases are characterized by the distinct mass gaps with different physical origins. In the collinear ferromagnetic order, the higher-order topological magnonic insulator phase occurs. It serves as a unique example of the higher-order topological phase in magnonic system, since it does not require non-collinear order or asymmetric form of the interactions. In the magnetic domain phases, the magnons are localized along the domain wall and form one-dimensional topological edge mode. As the closed domain walls deform to a open network, the confined edge mode extends to form a network model of the topological magnons.

cond-mat.str-el↗

arXiv2021

Disorder-driven Phase Transitions of Second-order Non-Hermitian Skin Effects

Non-Hermitian skin effect exhibits the collapse of the extended bulk modes into the extensive number of localized boundary states in open boundary conditions. Here we demonstrate the disorder-driven phase transition of the trivial non-Hermitian system to the higher-order non-Hermitian skin effect phase. In contrast to the clean systems, the disorder-induced boundary modes form an arc in the complex energy plane, which is the manifestation of the disorder-driven dynamical phase transition. At the phase transition, the localized corner modes and bulk modes characterized by trivial Hamiltonian coexist within the single-band but are separated in the complex energy plane. This behavior is analogous to the mobility edge phenomena in the disordered Hermitian systems. Using effective medium theory and numerical diagonalizations, we provide a systematic characterization of the disorder-driven phase transitions.

cond-mat.mes-hall↗

arXiv2018

Emergence of Weyl metals driven by doped magnetic impurities in spin-orbit coupled semiconductors

Constructing an effective field theory in terms of doped magnetic impurities (described by an O(3) vector model with a random mass term), itinerant electrons of spin-orbit coupled semiconductors (given by a Dirac theory with a relatively large mass term), and effective interactions between doped magnetic ions and itinerant electrons (assumed by an effective Zeeman coupling term), we perform the perturbative renormalization group analysis in the one-loop level based on the dimensional regularization technique. As a result, we find that the mass renormalization in dynamics of itinerant electrons acquires negative feedback effects due to quantum fluctuations involved with the Zeeman coupling term, in contrast with that of the conventional problem of quantum electrodynamics, where such interaction effects enhance the fermion mass more rapidly. Recalling that the applied magnetic field decreases the band gap in the presence of spin-orbit coupling, this renormalization group analysis shows that the external magnetic field overcomes the renormalized band gap, allowed by doped magnetic impurities even without ferromagnetic ordering. In other words, the Weyl metal physics can be controlled by doping magnetic impurities into spin-orbit coupled semiconductors, even if the external magnetic field alone cannot realize the Weyl metal phase due to relatively large band gaps of semiconductors. Furthermore, we emphasize that quasiparticles do not exist in this emergent disordered Weyl metal phase due to correlations with strong magnetic fluctuations. This non-Fermi liquid type Weyl metal state may be regarded to be a novel metallic phase in the respect that a topologically nontrivial band structure appears in the vicinity of quantum criticality.

cond-mat.str-el↗

arXiv2016

Two-parameter scaling theory of the longitudinal magnetoconductivity in a Weyl metal phase: Chiral anomaly, weak disorder, and finite temperature

It is at the heart of modern condensed matter physics to investigate the role of a topological structure in anomalous transport phenomena. In particular, chiral anomaly turns out to be the underlying mechanism for the negative longitudinal magnetoresistivity in a Weyl metal phase. Existence of a dissipationless current channel causes enhancement of electric currents along the direction of a pair of Weyl points or applied magnetic fields ($B$). However, temperature ($T$) dependence of the negative longitudinal magnetoresistivity has not been understood yet in the presence of disorder scattering since it is not clear at all how to introduce effects of disorder scattering into the "topological-in-origin" transport coefficient at finite temperatures. The calculation based on the Kubo formula of the current-current correlation function is simply not known for this anomalous transport coefficient. Combining the renormalization group analysis with the Boltzmann transport theory to encode the chiral anomaly, we reveal how disorder scattering renormalizes the distance between a pair of Weyl points and such a renormalization effect modifies the topological-in-origin transport coefficient at finite temperatures. As a result, we find breakdown of $B/T$ scaling, given by $B/T^{1 + η}$ with $0 < η< 1$. This breakdown may be regarded to be a fingerprint of the interplay between disorder scattering and topological structure in a Weyl metal phase.

cond-mat.dis-nn↗

arXiv2015

Interplay between disorder and inversion symmetry: Extreme enhancement of the mobility near the Weyl point in BiTeI

We show experimental and theoretical evidence that BiTeI hosts a novel disordered metallic state named diffusive helical Fermi liquid (DHFL), characterized by a pair of concentric spin-chiral Fermi surfaces with negligible inter-valley scattering. Key experimental observations are extreme disparity of the mobility between inner and outer helical Fermi surfaces near the Weyl point and existence of the so called universal scaling behavior for the Hall resistivity. Although the extreme enhancement of the inner-Fermi-surface mobility near the Weyl point is quantitatively explained within the self-consistent Born approximation, the existence of universal scaling in the Hall resistivity shows its breakdown, implying necessity of mass renormalization in the inner Fermi-surface beyond the independent electron picture.

cond-mat.str-el↗

arXiv2014

Dilute magnetic topological semiconductors: What's new beyond the physics of dilute magnetic semiconductors?

Role of localized magnetic moments in metal-insulator transitions lies at the heart of modern condensed matter physics, for example, the mechanism of high T$_{c}$ superconductivity, the nature of non-Fermi liquid physics near heavy fermion quantum criticality, the problem of metal-insulator transitions in doped semiconductors, and etc. Dilute magnetic semiconductors have been studied for more than twenty years, achieving spin polarized electric currents in spite of low Curie temperatures. Replacing semiconductors with topological insulators, we propose the problem of dilute magnetic topological semiconductors. Increasing disorder strength which corresponds to the size distribution of ferromagnetic clusters, we suggest a novel disordered metallic state, where Weyl metallic islands appear to form inhomogeneous mixtures with topological insulating phases. Performing the renormalization group analysis combined with experimental results, we propose a phase diagram in $(λ_{so},Γ,T)$, where the spin-orbit coupling $λ_{so}$ controls a topological phase transition from a topological semiconductor to a semiconductor with temperature $T$ and the distribution for ferromagnetic clusters $Γ$ gives rise to a novel insulator-metal transition from either a topological insulating or band insulating phase to an inhomogeneously distributed Weyl metallic state with such insulating islands. Since electromagnetic properties in Weyl metal are described by axion electrodynamics, the role of random axion electrodynamics in transport phenomena casts an interesting problem beyond the physics of percolation in conventional disorder-driven metal-insulator transitions. We also discuss how to verify such inhomogeneous mixtures based on atomic force microscopy.

cond-mat.dis-nn↗