arXiv subjects

Jung Hoon Han

Publications and source records attributed to Jung Hoon Han.

At least 91 records · Page 5Linked to original sources

arXiv2012

Emergence of Chiral Magnetism in Spinor Bose-Einstein Condensate with Rashba Coupling

Hydrodynamic theory of the spinor BEC condensate with Rashba spin-orbit coupling is presented. A close mathematical analogy of the Rashba-BEC model to the recently developed theory of chiral magnetism is found. Hydrodynamic equations for mass density, superfluid velocity, and the local magnetization are derived. The mass current is shown to contain an extra term proportional to the magnetization direction, as a result of the Rashba coupling. Elementary excitations around the two known ground states of the Rashba-BEC Hamiltonian, the plane-wave and the stripe states, are worked out in the hydrodynamic framework, highlighting the cross-coupling of spin and superflow velocity excitations due to the Rashba term.

cond-mat.quant-gas↗

arXiv2012

Skyrmion Generation by Current

Skyrmions, once a hypothesized field-theoretical object believed to describe the nature of elementary particles, became common sightings in recent years among several non-centrosymmetric metallic ferromagnets. For more practical applications of Skyrmionic matter as carriers of information, thus realizing the prospect of "Skyrmionics", it is necessary to have the means to create and manipulate Skyrmions individually. We show through extensive simulation of the Landau-Lifshitz-Gilbert equation that a circulating current imparted to the metallic chiral ferromagnetic system can create isolated Skyrmionic spin texture without the aid of external magnetic field.

cond-mat.str-el↗

arXiv2011

Observation of warping effects in the band and angular momentum structures of topological insulator Bi2Te3

We performed angle resolved photoemission (ARPES) experiments on Bi2Te3 with circularly polarized light. ARPES data show very strong circular dichroism, indicating existence of orbital angular momentum (OAM). Moreover, the alignment of OAM is found to have a strong binding energy dependence. Such energy dependence comes from a relatively strong band warping effect in Bi2Te3 compared to Bi2Se3. OAM close to Dirac point has an ideal chiral structure (sin ?) without out-of-plane component. Warping effect comes in as the binding energy decreases and circular dichroism along a constant energy contour can no longer be explained by a simple sin? function but requires a sin3? term. When the warping effect becomes even stronger near the Fermi energy, circular dichroism gains an additional sin6? term. Such behavior is found to be compatible with the theoretically predicted OAM structure.

cond-mat.mtrl-sci↗

arXiv2011

Emergence of Orbital Angular Momentum by Inversion Symmetry Breaking and Its Detection by ARPES

Rashba-split surface band is characterized by a one-to-one correspondence between the electron's momentum k and its spin orientation. Here we show that a similar correspondence between momentum and orbital angular momentum (OAM) must exist on surface bands once the inversion symmetry is broken. The correspondence is valid even when there is no spin-orbit interaction. Tight-binding and first-principles calculations are presented to support our claim. As a method to detect such OAM-momentum correspondence, we propose the circular dichroism (CD) experiment using the angle-resolved photoemission (ARPES) setup. CD-ARPES experiment performed on Cu surface confirms the existence of chiral OAM. A new concept of "orbital Galvanic effect" is proposed.

cond-mat.mtrl-sci↗

arXiv2011

Detecting Chiral Orbital Angular Momentum by Circular Dichroism ARPES

We show, by way of tight-binding and first-principles calculations, that a one-to-one correspondence between electron's crystal momentum k and non-zero orbital angular momentum (OAM) is a generic feature of surface bands. The OAM forms a chiral structure in momentum space much as its spin counterpart in Rashba model does, as a consequence of the inherent inversion symmetry breaking at the surface but not of spin-orbit interaction. Circular dichroism (CD) angle-resolved photoemission (ARPES) experiment is an efficient way to detect this new order, and we derive formulas explicitly relating the CD-ARPES signal to the existence of OAM in the band structure. The cases of degenerate p- and d-orbital bands are considered.

cond-mat.mtrl-sci↗

arXiv2011

Orbital-Angular-Momentum Based Origin of Rashba-Type Surface Band Splitting

We propose that existence of local orbital angular momentum (OAM) on the surfaces of high-Z materials play a crucial role in the formation of Rashba-type surface band splitting. Local OAM state in a Bloch state produces asymmetric charge distribution (electric dipole). Presence of surface electric field aligns the electric dipole and results in chiral OAM states and the relevant Rashba- type splitting. Therefore, the band splitting originates from electric dipole interaction, not from the Zeeman splitting as proposed in the original Rashba picture. The characteristic spin chiral structure of Rashba states is formed through the spin-orbit coupling and thus is a secondary effect to the chiral OAM. Results from first principles calculations on a single Bi layer under an external electric field verify the key predictions of the new model, including the direction of the spin textures which is predicted to be in opposite direction in the Rashba picture.

cond-mat.mtrl-sci↗

arXiv2011

Chiral orbital-angular-momentum in the surface states of Bi2Se3

Locking of the spin of a quasi-particle to its momentum in split bands of on the surfaces of metals and topological insulators (TIs) is understood in terms of Rashba effect where a free electron in the surface states feels an effective magnetic field. On the other hand, the orbital part of the angular momentum (OAM) is usually neglected. We performed angle resolved photoemission experiments with circularly polarized lights and first principles density functional calculation with spin-orbit coupling on a TI, Bi2Se3, to study the local OAM of the surface states. We show from the results that OAM in the surface states of Bi2Se3 is significant and locked to the electron momentum in opposite direction to the spin, forming chiral OAM states. Our finding opens a new possibility to have strong light-induced spin-polarized current in the surface states.

cond-mat.mtrl-sci↗

arXiv2011

Spin-Orbit Coupled Bose-Einstein Condensate under Rotation

We examine the combined effects of Rashba spin-orbit (SO) coupling and rotation on trapped spinor Bose-Einstein condensates (BECs). Nature of single particle states is thoroughly examined in the Landau level basis and is shown to support the formation of half-quantum vortex. In the presence of weak s-wave interactions, the ground state at strong SO coupling develops ring-like structures with domains whose number shows step behavior with increasing rotation. For fast rotation case, the vortex pattern favors triangular lattice, accompanied by the density depletion in the central region and weakened Skyrmionic character as the SO coupling is enhanced. Giant vortex formation is facilitated when SO coupling and rotation are both strong.

cond-mat.quant-gas↗

arXiv2011

Dynamics of Skyrmion Crystals in Metallic Thin Films

We study the collective dynamics of the Skyrmion crystal (SkX) in thin films of ferromagnetic metals resulting from the nontrivial Skyrmion topology. We show that the current-driven motion of the crystal reduces the topological Hall effect and the Skyrmion trajectories bend away from the direction of the electric current (the Skyrmion Hall effect). We find a new dissipation mechanism in non-collinear spin textures that can lead to a much faster spin relaxation than Gilbert damping, calculate the dispersion of phonons in the SkX, and discuss effects of impurity pinning of Skyrmions.

cond-mat.mes-hall↗

arXiv2011

Landau level states on a topological insulator thin film

We analyze the four-dimensional Hamiltonian proposed to describe the band structure of the single-Dirac-cone family of topological insulators in the presence of a uniform perpendicular magnetic field. Surface Landau level(LL) states appear, decoupled from the bulk levels and following the quantized energy dispersion of a purely two-dimensional surface Dirac Hamiltonian. A small hybridization gap splits the degeneracy of the central n=0 LL with dependence on the film thickness and the field strength that can be obtained analytically. Explicit calculation of the spin and charge densities show that surface LL states are localized within approximately one quintuple layer from the surface termination. Some new surface-bound LLs are shown to exist at a higher Landau level index.

cond-mat.mes-hall↗

arXiv2010

Zero-temperature phases for chiral magnets in three dimensions

We calculate the energies of various multiple-spiral spin configurations for the three-dimensional model of chiral magnets. The ground-state phase diagram is obtained in the space of anisotropy, magnetic field, and interaction parameters. Regimes with multiple-spiral, or spin crystal ground states of two- and three-dimensional nature are identified. The Skyrmion number responsible for anomalous Hall effect is examined for each spin crystal configuration. A geometric interpretation is given for the three-dimensional spin crystal lattice structures as the periodic arrangement of hedgehog and anti-hedgehog singularities.

cond-mat.str-el↗

arXiv2010

Skyrmion Lattice in Two-Dimensional Chiral Magnet

We develop a theory of the magnetic field-induced formation of Skyrmion crystal state in chiral magnets in two spatial dimensions, motivated by the recent discovery of the Skyrmionic phase of magnetization in thin film of Fe$_{0.5}$Co$_{0.5}$Si and in the A-phase of MnSi. Ginzburg-Landau functional of the chiral magnet re-written in the CP$^1$ representation is shown to be a convenient framework for the analysis of the Skyrmion states. Phase diagram of the model at zero temperature gives a sequence of ground states, helical spin $\rightarrow$ Skyrme crystal $\rightarrow$ ferromagnet, as the external field $B$ increases, in good accord with the thin-film experiment. In close analogy with Abrikosov's derivation of the vortex lattice solution in type-II superconductor, the CP$^1$ mean-field equation is solved and shown to reproduce the Skyrmion crystal state.

cond-mat.other↗

arXiv2009

Duality Argument for the Chiral-Nematic Phase of Planar Spins

A duality argument for the recently discovered chiral-nematic phase of the XY model in a triangular lattice is presented. We show that a new Ising variable naturally emerges in mapping the antiferromagnetic J1-J2 classical XY spin Hamiltonian onto an appropriate Villain model on a triangular lattice. The new variable is the chirality degree of freedom, which exists in addition to the usual vortex variables, in the dual picture. Elementary excitations and the associated phase transition of the Ising degrees of freedom are discussed in some detail.

cond-mat.stat-mech↗

arXiv2009

Hierarchy of Spin and Valley Symmetry Breaking in Quantum Hall Single Layer Graphene

We explore several microscopic mechanisms for breaking the $n=0$ fourfold Landau level degeneracy in a single-layer graphene. Valley-scattering random potential, Zeeman interaction, and electron-phonon coupling are considered in the presence of SU(4)-symmetric Coulomb exchange interaction. Among all the mechanisms considered, it is the electron-phonon coupling combined with the Zeeman interaction which leads to the full splitting of the $n=0$ Landau levels. A recent controversy of "valley-first" or "spin-first" breaking of SU(4) symmetry of the $n=0$ graphene Landau level is examined in light of our results. Existence of midgap states between Landau levels of opposite valley polarity are demonstrated.

cond-mat.mes-hall↗

arXiv2009

Magnetic response of the J1-J2 spin Hamiltonian from classical Monte Carlo and Schwinger boson mean field theory

Magnetic susceptibilities at several potential ordering wave vectors (0,0), (π,0), and (π,π) are analyzed for the antiferromagnetic J1-J2 spin Hamiltonian by classical Monte Carlo and Schwinger boson mean field theories over the parameter range 0 \le 2J_2/J_1 \le 2. We find a nearly linear-T behavior of the uniform susceptibility that extends up to the temperature scale of J_1 within both calculation schemes when 2J_2/J_1 is sufficiently removed from the critical point 2J_2/J_1=1. The window of linear temperature dependence diminishes as the critical point is approached.

cond-mat.str-el↗

arXiv2009

Skyrmions and Anomalous Hall Effect in a Dzyloshinskii-Moriya Spiral Magnet

Monte Carlo simulation study of a classical spin model with Dzylosinskii-Moriya interaction and the spin anisotropy under the magnetic field is presented. We found a rich phase diagram containing the multiple spin spiral (or skyrme crystal) phases of square, rectangular, and hexagonal symmetries in addition to the spiral spin state. The Hall conductivity $σ_{xy}$ is calculated within the $sd$ model for each of the phases. While $σ_{xy}$ is zero in the absence of external magnetic field, applying a field strength $H$ larger than a threshold value $H_c$ leads to the simultaneous onset of nonzero chirality and Hall conductivity. We find $H_c = 0$ for the multiple spin spiral states, but $H_c > 0$ for a single spin spiral state regardless of the field orientation. Relevance of the present results to MnSi is discussed.

cond-mat.str-el↗

arXiv2008

Chiral spin states in the pyrochlore Heisenberg magnet: Fermionic mean-field theory and variational Monte Carlo calculations

Fermionic mean-field theory and variational Monte Carlo calculations are employed to shed light on the possible uniform ground states of the Heisenberg model on the pyrochlore lattice. Among the various flux configurations, we find the chiral spin states carrying \pm pi/2 flux through each triangular face to be the most stable both within the mean-field theory and the projected wave-function studies. Properties of the spin-spin correlation function and the chirality order parameter are calculated for the projected wave functions. Mean-field band structures are examined.

cond-mat.str-el↗

arXiv2008

Theory of magnetic field-induced metaelectric critical end point in BiMn$_2$O$_5$

A recent experiment on the multiferroic BiMn$_2$O$_5$ compound under a strong applied magnetic field revealed a rich phase diagram driven by the coupling of magnetic and charge (dipolar) degrees of freedom. Based on the exchange-striction mechanism, we propose here a theoretical model with the intent to capture the interplay of the spin and dipolar moments in the presence of a magnetic field in BiMn$_2$O$_5$. Experimentally observed behavior of the dielectric constants, magnetic susceptibility, and the polarization is, for the most part, reproduced by our model. The critical behavior observed near the polarization reversal $(P=0)$ point in the phase diagram is interpreted as arising from the proximity to the critical end point.

cond-mat.mtrl-sci↗