arXiv subjects

Hae-Young Kee

Publications and source records attributed to Hae-Young Kee.

At least 91 records · Page 5Linked to original sources

arXiv2018

Non-local String Order Parameter in the $S = 1/2$ Kitaev-Heisenberg Ladder

We study the spin-$\frac{1}{2}$ Kitaev-Heisenberg (KJ) model in a two-leg ladder. Without a Heisenberg interaction, the Kitaev phase in the ladder model has Majorana fermions with local Z$_2$ gauge fields, and is usually described as a disordered phase without any order parameter. Here we prove the existence of a non-local string order parameter (SOP) in the Kitaev phase which survives with a finite Heisenberg interaction. The SOP is obtained by relating the Kitaev ladder, through a non-local unitary transformation, to a one-dimensional $XY$ chain with an Ising coupling to a dangling spin at every site. This differentiates the Kitaev phases from other nearby phases including a rung singlet. Two phases with non-zero SOP corresponding to ferromagnetic and antiferromagnetic Kitaev interactions are identified. The full phase diagram of the KJ ladder is determined using exact diagonalization and density matrix renormalization group methods, which shows a striking similarity to the KJ model on a two-dimensional honeycomb lattice.

cond-mat.str-el↗

arXiv2018

Magnetic anisotropy of the alkali iridate Na$_{2}$IrO$_{3}$ at high magnetic fields: evidence for strong ferromagnetic Kitaev correlations

The magnetic field response of the Mott-insulating honeycomb iridate Na$_{2}$IrO$_{3}$ is investigated using torque magnetometry measurements in magnetic fields up to 60 tesla. A peak-dip structure is observed in the torque response at magnetic fields corresponding to an energy scale close to the zigzag ordering ($\approx 15~K$) temperature. Using exact diagonalization calculations, we show that such a distinctive signature in the torque response constrains the effective spin models for these classes of Kitaev materials to ones with dominant ferromagnetic Kitaev interactions, while alternative models with dominant antiferromagnetic Kitaev interactions are excluded. We further show that at high magnetic fields, long range spin correlation functions decay rapidly, signaling a transition to a long-sought-after field-induced quantum spin liquid beyond the peak-dip structure. Kitaev systems are thus revealed to be excellent candidates for field-induced quantum spin liquids, similar physics having been suggested in another Kitaev material $α-$RuCl$_{3}$.

cond-mat.str-el↗

arXiv2018

Anisotropic Purity of Entangled Photons From Cooper Pairs in Heterostructures

It was theoretically proposed that a forward-biased p-n junction with a superconducting layer (P-N-S), would produce pure, polarization entangled photons due to inherent spin-singlet pairing of electrons. However, any heterostructure interface generically induces Rashba spin-orbit coupling, which in turn generates a mixed singlet-triplet superconducting order parameter. Here we study the effect of triplet pairing on the purity of photons produced through Cooper pair recombination. A unique directional dependence of the state purity is found for a triplet superconductor with fixed $\vec{d}$: pure, entangled photons are produced when the photon polarization axis is parallel to $\vec{d}$. Induced triplet pairing in a singlet superconductor is shown to degrade the state purity, while induced singlet pairing in a triplet superconductor is shown to enhance the production of entangled pairs. These considerations may aid the design of functional devices to produce entangled photons.

cond-mat.supr-con↗

arXiv2018

Counter-rotating spiral order in three dimensional iridates: signature of hidden symmetry in Kitaev-$Γ$ model

The unconventional magnetic orderings found in Kitaev spin liquid candidates suggest frustration of spin interactions, and raise a possibility of nearby spin liquid phases. In particular, counter-rotating spiral ordering in three-dimensional (3D) iridates is striking, and understanding the microscopic mechanism of such ordering may provide routes to 3D Kitaev spin liquids. We study a minimal 3D model including Kitaev $K$ and a symmetric off-diagonal bond-dependent $Γ$ interaction on the hyperhoneycomb lattice using exact diagonalization. We first show that a 12-site unitary transformation unveils a Heisenberg model with hidden SU(2) symmetry when $K = Γ$. The magnetic ordering in the transformed basis then generically maps to the counter-rotating noncoplanar spiral order of the original spin. The moment direction depends on perturbations away from the SU(2) point. When $K$ and $Γ$ are negative, a positive Heisenberg interaction favors the (110)-direction reported in the neutron scattering measurement on $β$-Li$_2$IrO$_3$. Our findings offer a relevant set of microscopic parameters, which in turn guides a way to approach possible Kitaev spin liquids.

cond-mat.str-el↗

arXiv2018

Selective Phonon Damping in Topological Semimetals

Topological semimetals are characterized by their intriguing Fermi surfaces (FSs) such as Weyl and Dirac points, or nodal FS, and their associated surface states. Among them, topological crystalline semimetals, in the presence of strong spin-orbit coupling, possess a nodal FS protected by non-symmorphic lattice symmetries. In particular, it was theoretically proposed that $\mathrm{SrIrO}_{3}$ exhibits a bulk nodal ring due to glide symmetries, as well as flat two-dimensional surface states related to chiral and mirror symmetries. However, due to the semimetallic nature of the bulk, direct observation of these surface states is difficult. Here we study the effect of flat-surface states on phonon modes for $\mathrm{SrIrO}_{3}$ side surfaces. We show that particular phonon modes, based on mirror symmetry, have qualitatively different damping mechanisms due to the surface states which could be used to infer their existence. Experimental techniques for such measurements are also discussed.

cond-mat.str-el↗

arXiv2018

Path to stable quantum spin liquids in spin-orbit coupled correlated materials

The spin liquid phase is one of the prominent strongly interacting topological phases of matter whose unambiguous confirmation is yet to be reached despite intensive experimental efforts on numerous candidate materials. Recently, a new family of correlated honeycomb materials, in which strong spin-orbit coupling allows for various bond-dependent spin interactions, have been promising candidates to realize the Kitaev spin liquid. Here we study a model with bond-dependent spin interactions and show numerical evidence for the existence of an extended quantum spin liquid region, which is possibly connected to the Kitaev spin liquid state. These results are used to provide an explanation of the scattering continuum seen in neutron scattering on $α$-RuCl$_3$.

cond-mat.str-el↗

arXiv2017

Type-II Dirac Photons

The Dirac equation for relativistic electron waves is the parent model for Weyl and Majorana fermions as well as topological insulators. Simulation of Dirac physics in three-dimensional photonic crystals, though fundamentally important for topological phenomena at optical frequencies, encounters the challenge of synthesis of both Kramers double degeneracy and parity inversion. Here we show how type-II Dirac points---exotic Dirac relativistic waves yet to be discovered---are robustly realized through the nonsymmorphic screw symmetry. The emergent type-II Dirac points carry nontrivial topology and are the mother states of type-II Weyl points. The proposed all-dielectric architecture enables robust cavity states at photonic-crystal---air interfaces and anomalous refraction, with very low energy dissipation.

physics.optics↗

arXiv2017

Helical Majorana fermions at the interface of Weyl semimetal and d-wave superconductor: Application to Iridates and high-Tc Cuprates

Majorana bound states exist inside a core of half-quantum vortex in spin-triplet superconductors. Despite intense efforts, they have not been discovered in any spin-triplet superconductor candidate materials. After the success of topological insulator, another route to achieve Majorana fermions has been suggested: heterostructures of s-wave superconductors and topological insulator or semimetal with strong spin-orbit coupling provide an effective spinless $p+ip$ pairing which supports Majorana bound states in a single vortex. This theoretical observation has stimulated both experimental and theoretical communities to search for Majorana fermions, and recently a localized Majorana state at the end of one-dimensional wire has been reported. Here we study the two-dimensional interface of Weyl semimetal and d-wave superconductor which promotes a pair of helical Majorana fermions propagating along the edge of the interface. We suggest that Iridium oxide layer IrO$_2$ classified as two-dimensional Weyl semimetal in close proximity to high temperature Cuprates would be a best example to explore these helical Majorana modes.

cond-mat.mes-hall↗

arXiv2017

Realizing Haldane Model in Fe-based Honeycomb Ferromagnetic Insulators

The topological Haldane model (THM) on a honeycomb lattice is a prototype of systems hosting topological phases of matter without external fields. It is the simplest model exhibiting the quantum Hall effect without Landau levels, which motivated theoretical and experimental explorations of topological insulators and superconductors. Despite its simplicity, its realization in condensed matter systems has been elusive due to a seemingly difficult condition of spinless fermions with sublattice-dependent magnetic flux terms. While there have been theoretical proposals including elaborate atomic-scale engineering, identifying candidate THM materials has not been successful, and the first experimental realization was recently made in ultracold atoms. Here we suggest that a series of Fe-based honeycomb ferromagnetic insulators, AFe2(PO4)2 (A=Ba,Cs,K,La) possess Chern bands described by the THM. How to detect the quantum anomalous Hall effect is also discussed.

cond-mat.str-el↗

arXiv2017

Elemental topological Dirac semimetal: α-Sn on InSb(111)

Three-dimensional (3D) topological Dirac semimetals (TDSs) are rare but important as a versatile platform for exploring exotic electronic properties and topological phase transitions. A quintessential feature of TDSs is 3D Dirac fermions associated with bulk electronic states near the Fermi level. Using angle-resolved photoemission spectroscopy (ARPES), we have observed such bulk Dirac cones in epitaxially-grown α-Sn films on InSb(111), the first such TDS system realized in an elemental form. First-principles calculations confirm that epitaxial strain is key to the formation of the TDS phase. A phase diagram is established that connects the 3D TDS phase through a singular point of a zero-gap semimetal phase to a topological insulator (TI) phase. The nature of the Dirac cone crosses over from 3D to 2D as the film thickness is reduced.

cond-mat.mtrl-sci↗

arXiv2016

Revealing frustrated local moment model for pressurized hyperhoneycomb iridate: paving a way toward quantum spin liquid

There have been tremendous experimental and theoretical efforts toward discovery of quantum spin liquid phase in honeycomb-based-lattice materials with strong spin-orbit coupling. Here the bond-dependent Kitaev interaction between local moments provides strong magnetic frustration and if it is the only interaction present in the system, it will lead to an exactly solvable quantum spin liquid ground state. In all of these materials, however, the ground state is in a magnetically ordered phase due to additional interactions between local moments. Recently, it has been reported that the magnetic order in hyperhoneycomb material, $β$-Li$_2$IrO$_3$, is suppressed upon applying hydrostatic pressure and the resulting state becomes a quantum paramagnet or possibly a quantum spin liquid. Using ab-initio computations and strong coupling expansion, we investigate the lattice structure and resulting local moment model in pressurized $β$-Li$_2$IrO$_3$. Remarkably, the dominant interaction under high pressure is not the Kitaev interaction nor further neighbor interactions, but a different kind of bond-dependent interaction. This leads to strong magnetic frustration and may provide a platform for discovery of a new kind of quantum spin liquid ground state.

cond-mat.str-el↗

arXiv2016

Quantum spin ice on the breathing pyrochlore lattice

The Coulombic quantum spin liquid in quantum spin ice is an exotic quantum phase of matter that emerges on the pyrochlore lattice and is currently actively searched for. Motivated by recent experiments on the Yb-based breathing pyrochlore material Ba$_3$Yb$_2$Zn$_5$O$_{11}$, we theoretically study the phase diagram and magnetic properties of the relevant spin model. The latter takes the form of a quantum spin ice Hamiltonian on a breathing pyrochlore lattice, and we analyze the stability of the quantum spin liquid phase in the absence of the inversion symmetry which the lattice breaks explicitly at lattice sites. Using a gauge mean-field approach, we show that the quantum spin liquid occupies a finite region in parameter space. Moreover, there exists a direct quantum phase transition between the quantum spin liquid phase and featureless paramagnets, even though none of theses phases break any symmetry. At nonzero temperature, we show that breathing pyrochlores provide a much broader finite temperature spin liquid regime than their regular counterparts. We discuss the implications of the results for current experiments and make predictions for future experiments on breathing pyrochlores.

cond-mat.str-el↗

arXiv2016

ARPES study of the Kitaev Candidate $α$-RuCl$_3$

$α$-RuCl$_3$ has been hinted as a spin-orbital-assisted Mott insulator in proximity to a Kitaev spin liquid state. Here we present ARPES measurements on single crystal $α$-RuCl$_3$ in both the pristine and electron-doped states, and combine them with LDA+SOC+U calculations performed for the several low-energy competing magnetically ordered states as well as the paramagnetic state. A large Mott gap is found in the measured band structure of the pristine compound that persists to more than 20 times beyond the magnetic ordering temperature, though the paramagnetic calculation shows almost no gap. Upon electron doping, spectral weight is transferred into the gap but the new states still maintain a sizable gap from the Fermi edge. These findings are most consistent with a Mott insulator with a somewhat exotic evolution out of the Mott state with both temperature and doping, likely related to unusually strong spin fluctuations.

cond-mat.str-el↗

arXiv2016

3D $Z_2$ Topological Nodes in Nonsymmorphic Photonic Crystals: Ultrastrong Coupling and Anomalous Refraction

We propose to simulate 3D Dirac points and line-nodes with nontrivial $Z_2$ topology in nonsymmorphic all-dielectric photonic-crystals with space-time reversal symmetry, which can be realized at infrared and microwave frequencies. Double degeneracy of all Bloch states in high symmetry planes is achieved via nonsymmorphic screw symmetries despite the fundamental obstacle of no Kramers degeneracy in photonic crystals. Two orthogonal screw axes lead to 3D $Z_2$ Dirac points on high symmetry Brillouin zone boundary lines. On the other hand, $Z_2$ line-nodes emerge as protected twofold degeneracy of Bloch bands with opposite mirror parities on the $k_z=0$ plane. The lowest frequency line-node is deterministic because of a degenerate-partner switching mechanism guaranteed by the fundamental properties of Maxwell equations and the nonsymmorphic screw symmetry. A pair of Fermi arcs with opposite chirality due to $Z_2$ topological Dirac points emerge below the light-line on (100) and (010) photonic-crystal-air interfaces. These robust surface states offer an unique opportunity to realize an "open cavity" with strong interaction between quantum emitters and engineered vacuum with nontrivial Berry phases --- an important step toward topological states of strongly interacting bosons. Realistic calculation for resonant coupling between cavity-photons and phonons in boron nitride thin film yields ultrastrong coupling with vacuum Rabi splitting reaching to $23\%$ of photon frequency. We also show that type-II Dirac cones have anomalous valley selective refraction: birefringence with both positive and negative refractions for one valley, while no refraction for the other.

physics.optics↗

arXiv2016

Cluster Mott insulators and two Curie-Weiss regimes on an anisotropic Kagome lattice

Motivated by recent experiments on the quantum-spin-liquid candidate material LiZn2Mo3O8, we study a single-band extended Hubbard model on an anisotropic Kagome lattice with the 1/6 electron filling. Due to the partial filling of the lattice, the inter-site repulsive interaction is necessary to generate Mott insulators, where electrons are localized in clusters, rather than at lattice sites. It is shown that these cluster Mott insulators are generally U(1) quantum spin liquids with spinon Fermi surfaces. The nature of charge excitations in cluster Mott insulators can be quite different from conventional Mott insulator and we show that there exists a novel cluster Mott insulator where charge fluctuations around the hexagonal cluster induce a plaquette charge order (PCO). The spinon excitation spectrum in this spin-liquid cluster Mott insulator is reconstructed due to the PCO so that only 1/3 of the total spinon excitations are magnetically active. Based on these results, we propose that the two Curie-Weiss regimes of the spin susceptibility in LiZn2Mo3O8 may be explained by finite-temperature properties of the cluster Mott insulator with the PCO as well as fractionalized spinon excitations. Existing and possible future experiments on LiZn2Mo3O8, and other Mo-based cluster magnets are discussed in light of these theoretical predictions.

cond-mat.str-el↗

arXiv2016

Modeling the band structure of the higher manganese silicides starting from Mn$_4$Si$_7$

The higher manganese silicides (HMS), with the chemical formula MnSi$_x$($x \approx 1.73 - 1.75$), have been attracted a lot of attention due to their potential application as thermoelectric materials. While the electronic band structures of HMS have been previously studied using first principle calculations, the relation between crystal structures of Mn and Si atoms and their band structures is not well understood. Here we study Mn$_4$Si$_7$ using first principle calculations and show that a half cell consisting of five Mn atoms is the essential building block for Mn$_4$Si$_7$. Using this insight, we construct a minimal tight-binding model for Mn$_4$Si$_7$ and other HMS including Mn$_{11}$Si$_{19}$ and Mn$_{15}$Si$_{26}$. The role played by the Si atoms and possible ways to achieve higher figure of merit are also discussed.

cond-mat.mtrl-sci↗

arXiv2016

Topological edge states in single layers of honeycomb materials with strong spin-orbit coupling

We study possible edge states in single layers of honeycomb materials such as $α$-RuCl$_3$ and A$_2$IrO$_3$ (A=Li, Na) with strong spin-orbit coupling (SOC). These two dimensional systems exhibit linearly dispersing one-dimensional (1D) edge states when their 1D boundary forms a zig-zag shape. Using an effective tight-binding model based on first principles band structure calculations including Hubbard U and SOC, we find degenerate edge states at the zone center and zone boundary. The roles of chiral symmetry and time-reversal symmetry are presented. The implications to experimental signatures and the effects of disorder are also discussed.

cond-mat.str-el↗

arXiv2016

Topological crystalline semimetals in non-symmorphic lattices

Numerous efforts have been devoted to reveal exotic semimetallic phases with topologically non-trivial bulk and/or surface states in materials with strong spin-orbit coupling. In particular, semimetals with nodal line Fermi surface (FS) exhibit novel properties, and searching for candidate materials becomes an interesting research direction. Here we provide a generic condition for a four-fold degenerate nodal line FS in non-symmorphic crystals with inversion and time-reversal symmetry (TRS). When there are two glide planes or screw axes perpendicular to each other, a pair of Bloch bands related by non-symmorphic symmetry become degenerate on a Brillouin Zone (BZ) boundary. There are two pairs of such bands, and they disperse in a way that the partners of two pairs are exchanged on other BZ boundaries. This enforces a nodal line FS on a BZ boundary plane protected by non-symmorphic symmetries. When TRS is broken, four-fold degenerate Dirac points or Weyl ring FS could occur depending on a direction of the magnetic field. On a certain surface double helical surface states exist, which become double Ferm arcs as TRS is broken.

cond-mat.str-el↗