Jae-Seung Jeong

Curvature-enhanced spin-orbit coupling in a carbon nanotube

Structure of the spin-orbit coupling varies from material to material and thus finding the correct spin-orbit coupling structure is an important step toward advanced spintronic applications. We show theoretically that the curvature in a carbon nanotube generates two types of the spin-orbit coupling, one of which was not recognized before. In addition to the topological phase-related contribution of the spin-orbit coupling, which appears in the off-diagonal part of the effective Dirac Hamiltonian of carbon nanotubes, there is another contribution that appears in the diagonal part. The existence of the diagonal term can modify spin-orbit coupling effects qualitatively, an example of which is the electron-hole asymmetric spin splitting observed recently, and generate four qualitatively different behavior of energy-level dependence on parallel magnetic field. It is demonstrated that the diagonal term applies to a curved graphene as well. This result should be valuable for spintronic applications of graphitic materials.

Curvature-induced spin-orbit coupling and spin relaxation in a chemically clean single-layer graphene

The study of spin-related phenomena in materials requires knowledge on the precise form of effective spin-orbit coupling of conducting carriers in the solid-states systems. We demonstrate theoretically that curvature induced by corrugations or periodic ripples in single-layer graphenes generates two types of effective spin-orbit coupling. In addition to the spin-orbit coupling reported previously that couples with sublattice pseudospin and corresponds to the Rashba-type spin-orbit coupling in a corrugated single-layer graphene, there is an additional spin-orbit coupling that does not couple with the pseudospin, which can not be obtained from the extension of the curvature-induced spin-orbit coupling of carbon nanotubes. Via numerical calculation we show that both types of the curvature-induced spin-orbit coupling make the same order of contribution to spin relaxation in chemically clean single-layer graphene with nanoscale corrugation. The spin relaxation dependence on the corrugation roughness is also studied.

Ballistic spin field-effect transistors: Multichannel effects

We study a ballistic spin field-effect transistor (SFET) with special attention to the issue of multichannel effects. The conductance modulation of the SFET as a function of the Rashba spin-orbit coupling strength is numerically examined for the number of channels, ranging from a few to close to 100. Even with the ideal spin injector and collector, the conductance modulation ratio, defined as the ratio between the maximum and minimum conductances, decays rapidly and approaches 1 with the increase of the channel number. It turns out that the decay is considerably faster when the Rashba spin-orbit coupling is larger. Effects of the electronic coherence are also examined in the multichannel regime, and it is found that the coherent Fabry-Perot-like interference in the multichannel regime gives rise to a nested peak structure. For a nonideal spin injector and/or collector structure, which consists of a conventional metallic ferromagnet-thin insulator two-dimensional electron-gas heterostructure, the Rashba-coupling-induced conductance modulation is strongly affected by large resonance peaks that arise from the electron confinement effect of the insulators. Finally, scattering effects are briefly addressed and it is found that in the weakly diffusive regime, the positions of the resonance peaks fluctuate, making the conductance modulation signal sample dependent.

Nonlocal spin transport in single-walled carbon nanotube networks

Spin transport in carbon-based materials has stimulated much interest due to their ballistic conductance and a long phase coherence length. While much research has been conducted on individual carbon nanotubes, current growth and placement techniques are incompatible with large-scale fabrication. Here we report nonlocal spin injection and detection in single wall carbon nanotube networks. We observe spin transport over a distance of 1 um, and extract a spin diffusion length of 1.6 - 2.4 um with an injected spin polarization from CoFe into nanotube network of 18 - 41%. Our observations demonstrate that spin transport is possible in carbon nanotube networks due to the formation of natural tunnel barriers between nanotubes and metallic contacts.

Emergent quantum phases in a frustratedJ1−J2Heisenberg model on the 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

Bilayer mapping of the paired quantum Hall state: Instability toward anisotropic pairing

1/S

Mediating Effect of Internet Addiction on the Association between Resilience and Depression among Korean University Students: A Structural Equation Modeling Approach

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.

Competing states for the fractional quantum Hall effect in the 1/3-filled second Landau level

One of the most dominant candidates for the paired quantum Hall (QH) state at filling factor

Suppression of Spin Dephasing in a Two-Dimensional Electron Gas with a Quantum Point Contact

\nu=5/2

Rotational symmetry breaking and topological phase transition in the exciton-polariton condensate of gapped 2D Dirac material

is the Moore-Read (MR) Pfaffian state. A salient problem, however, is that it does not occur exactly at the Coulomb interaction, but rather at a modified interaction, which favors particle-hole symmetry breaking. In an effort to find a better state, in this work, we investigate the possible connection between the paired QH state and the antisymmetrized bilayer ground state, which is inspired by the intriguing identity that the MR Pfaffian state is entirely equivalent to the antisymmetrized projection of the bilayer QH state called the Halperin (331) state, which is valid at interlayer distance,