arXiv subjects

Kyusung Hwang

Publications and source records attributed to Kyusung Hwang.

24 records · Page 2Linked to original sources

arXiv2014

Signatures of spin-triplet excitations in optical conductivity of valence bond solids

We show that the optical responses below the Mott gap can be used to probe the spin-triplet excitations in valence bond solid (VBS) phases in Mott insulators. The optical conductivity in this regime arises due to the electronic polarization mechanism via virtual electron hopping processes. We apply this mechanism to the Hubbard model with spin-orbit couplings and/or the corresponding spin model with significant Dzyaloshinskii-Moriya (DM) interactions, and compute the optical conductivity of VBS states on both ideal and deformed Kagome lattices. In case of the deformed Kagome lattice, we study the antiferromagnet, Rb$_2$Cu$_3$SnF$_{12}$ with the pinwheel VBS state. In case of the ideal Kagome lattice, we explore the optical conductivity signatures of the spin-triplet excitations for three VBS states with (1) a 12-site unit cell, (2) a 36-site unit cell with six-fold rotation symmetry, and (3) a 36-site unit cell with three-fold rotation symmetry, respectively. We find that increasing the DM interactions generally leads to broad and smooth features in the optical conductivity with interesting experimental consequences. The optical conductivity reflects the features of the spin-triplet excitations that can be measured in future experiments.

cond-mat.str-el↗

arXiv2014

Emergent quantum phases in a frustrated J1-J2 Heisenberg model on hyperhoneycomb lattice

We investigate possible quantum ground states as well as the classical limit of a frustrated J1-J2 Heisenberg model on the three-dimensional (3D) hyperhoneycomb lattice. Our study is inspired by the recent discovery of beta-Li2IrO3, where Ir^{4+} ions form a 3D network with each lattice site being connected to three nearest neighbors. We focus on the influence of magnetic frustration caused by the second-nearest neighbor spin interactions. Such interactions are likely to be significant due to large extent of 5d orbitals in iridates or other 5d transition metal oxides. In the classical limit, the ground state manifold is given by line degeneracies of the spiral magnetic-order wavevectors when J2/J1\gtrsim 0.17 while the collinear stripy order is included in the degenerate manifold when J2/J1 = 0.5. Quantum order-by-disorder effects are studied using both the semi-classical $1/S$ expansion in the spin wave theory and Schwinger boson approach. In general, certain coplanar spiral orders are chosen from the classical degenerate manifold for a large fraction of the phase diagram. Nonetheless quantum fluctuations favor the collinear stripy order over the spiral orders in an extended parameter region around J2/J1 = 0.5, despite the spin-rotation invariance of the underlying Hamiltonian. This is in contrast to the emergence of stripy order in the Heisenberg-Kitaev model studied earlier on the same lattice, where the Kitaev-type Ising interactions are important for stabilizing the stripy order. As quantum fluctuations become stronger, U(1) and Z2 quantum spin liquid phases are shown to arise via quantum disordering of the Neel, stripy and spiral magnetically-ordered phases. The effects of magnetic anisotropy and their relevance to future experiments are also discussed.

cond-mat.str-el↗

arXiv2014

Topological and magnetic phases with strong spin-orbit coupling on the hyperhoneycomb lattice

We study the general phase diagram of correlated electrons for iridium-based (Ir) compounds on the hyperhoneycomb lattice---a crystal structure where the Ir$^{4+}$ ions form a three-dimensional network with three-fold coordination recently realized in the $β$-Li${}_{2}$IrO${}_{3}$ compound. Using a combination of microscopic derivations, symmetry analysis, and density functional calculations, we determine the general model for the electrons occupying the $j_{\text{eff}}=1/2$ orbitals at the Ir$^{4+}$ sites. In the non-interacting limit, we find that this model allows for both topological and trivial electronic band insulators along with metallic states. The effect of Hubbard-type electron-electron repulsion on the above electronic structure in stabilizing $\mathbf{q}=\mathbf{0}$ magnetic order reveals a phase diagram with continuous phase transition between a topological band insulator and a Neel ordered magnetic insulator.

cond-mat.str-el↗

arXiv2013

Three-dimensional nematic spin liquid in the stacked triangular lattice 6H-B structure

Recently, a number of experiments indicate the possible presence of spin liquid phases in quantum magnets with spin-1/2 and spin-1 moments sitting on triangular-lattice-based structures in Ba3CuSb2O9 and Ba3NiSb2O9 respectively. In relation to these experiments, several theoretical proposals have been made for spin liquid phases and spin-liquid-like behaviours on the stacked triangular lattice. While the crystal structures of these materials are currently under debate, it is nonetheless interesting to understand possible spin liquid phases on such frustrated lattices. In this work, we apply Schwinger boson mean-field theory and projective symmetry group (PSG) analysis to investigate spin liquid phases on the fully three-dimensional 6H-B structure, in contrast to previous works that considered two-dimensional systems. We find that a nematic Z2 spin liquid phase, where the lattice-rotational symmetry is spontaneously broken, is the most promising spin liquid phase that is consistent with spiral magnetic ordering in the classical limit. We discuss the implications of our results to future theoretical and experimental works.

cond-mat.str-el↗

arXiv2012

Influence of Dzyaloshinskii-Moriya interactions on magnetic structure of a spin-1/2 deformed kagome lattice antiferromagnet

Motivated by the recent neutron scattering experiment on Rb2Cu3SnF12 [Nat. Phys. 6, 865 (2010)], we investigate the effect of Dzyaloshinskii-Moriya interactions in a theoretical model for the magnetic structure of this material. Considering the valence bond solid ground state, which has a 12-site unit cell, we develop the bond operator mean-field theory. It is shown that the Dzyaloshinskii-Moriya interactions significantly modify the triplon dispersions around the Gamma point and cause a shift of the spin gap (the minimum triplon gap) position from the K to Gamma point in the first Brilloin zone. The spin gap is also evaluated in exact diagonalization studies on a 24-site cluster. We discuss a magnetic transition induced by the Dzyaloshinskii-Moriya interactions in the bond operator framework. Moreover, the magnetization process under external magnetic fields is studied within the exact diagonalization and strong coupling expansion approaches. We find that the results of all above approaches are consistent with the experimental findings.

cond-mat.str-el↗

arXiv2011

Spin cluster operator theory for the Kagome lattice antiferromagnet

The spin-1/2 quantum antiferromagnet on the Kagome lattice provides a quintessential example in the strongly correlated electron physics where both effects of geometric frustration and quantum fluctuation are pushed to their limit. Among possible non-magnetic ground states, the valence bond solid (VBS) with a 36-site unit cell is one of the most promising candidates. A natural theoretical framework for the analysis of such VBS order is to consider quantum states on a bond connecting the nearest-neighboring sites as fundamental quantum modes of the system and treat them as effectively independent "bond particles." While correctly describing the VBS order in the ground state, this approach, known as the bond operator theory, significantly overestimates the lowest spin excitation energy. To overcome this problem, we take a next logical step in this paper to improve the bond operator theory and consider extended spin clusters as fundamental building blocks of the system. Depending on two possible configurations of the VBS order, various spin clusters are considered: (i) in the VBS order with staggered hexagonal resonance, we consider one spin cluster for a David star and two spin clusters with each composed of a perfect hexagon and three attached dimers, and (ii) in the VBS order with uniform hexagonal resonance, one spin cluster composed of a David star and three attached dimers. It is shown that the majority of low-energy spin excitations are nearly or perfectly flat in energy. With most of its weight coming from the David star, the lowest spin excitation has a gap much lower than the previous value obtained by the bond operator theory, narrowing the difference against exact diagonalization results.

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