Chung-Yu Mou

Ground-state properties of nanographite systems with zigzag edges

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Graphene-based modulation-doped superlattice structures

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Ferromagnetism in armchair graphene nanoribbons

-electron network in nanographite systems with zigzag edges exhibits strongly localized edge states, which are expected to have peculiar properties. We study effects of electron-electron interactions on ground-state properties of zigzag nanographite ribbons and open-ended zigzag nanotubes by means of the weak-coupling renormalization group and the density-matrix renormalization-group method. It is shown that the ground state is a spin-singlet Mott insulator with finite charge and spin gaps. We also find that the edge states are robust against the electronic correlations, resulting in edge-effective spins that can flip almost freely. The schematic picture for the low-energy physics of the systems is discussed.

Josephson effect in graphene SNS junction with a single localized defect

The electronic transport properties of graphene-based superlattice structures are investigated. A graphene-based modulation-doped superlattice structure geometry is proposed consisting of periodically arranged alternate layers: InAs/graphene/GaAs/graphene/GaSb. The undoped graphene/GaAs/graphene structure displays a relatively high conductance and enhanced mobilities at increased temperatures unlike the modulation-doped superlattice structure, which is more steady and less sensitive to temperature and the robust electrical tunable control on the screening length scale. The thermionic current density exhibits enhanced behavior due to the presence of metallic (graphene) monolayers in the superlattice structure. The proposed superlattice structure might be of great use for new types of wide-band energy gap quantum devices.

Induced decoherence and entanglement by interacting quantum spin baths

We study the electronic correlation effects in armchair nanoribbon and nanotube using weak-coupling approach and non-Abelian density-matrix renormalization-group method. We show that upon appropriate doping, the system exhibits a new type of flat-band ferromagnetism, different from the well-known Milke-Tasaki one. The strongly correlated ground state consists of intrinsic magnetic moments of flat-band states and itinerant carriers of dispersive bands, and the exchange coupling between them yields a ferromagnetism. The resultant ferromagnetic state with metallic conductivity has a potential in spintronics applications at nanoscale.Due to the weak spin-orbit interaction and the peculiar relativistic dispersion in graphene, there are exciting proposals to build spin qubits in graphene nanoribbons with armchair boundaries. However, the mutual interactions between electrons are neglected in most studies so far and thus motivate us to investigate the role of electronic correlations in armchair graphene nanoribbon by both analytical and numerical methods. Here we show that the inclusion of mutual repulsions leads to drastic changes and the ground state turns ferromagnetic in a range of carrier concentrations. Our findings highlight the crucial importance of the electron-electron interaction and its subtle interplay with boundary topology in graphene nanoribbons. Furthermore, since the ferromagnetic properties sensitively depend on the carrier concentration, it can be manipulated at ease by electric gates. The resultant ferromagnetic state with metallic conductivity is not only surprising from an academic viewpoint, but also has potential applications in spintronics at nanoscale.

Transport in quantum wells in the presence of interface roughness

Abstract Imperfections change essentially the electronic transport properties of graphene. Motivated by a recent experiment reporting on the possible application of graphene as junctions, we study transport properties in graphene-based junctions with single localized defect. We solve the Dirac–Bogoliubov–de-Gennes equation with a single localized defect superconductor–normal(graphene)–superconductor (SNS) junction. We consider the properties of tunneling conductance and Josephson current through an undoped strip of graphene with heavily doped s-wave superconducting electrodes in the limit l def ⪡ L ⪡ ξ . We find that spectrum of Andreev bound states are modified in the presence of single localized defect in the bulk and the minimum tunneling conductance remains the same. The Josephson junction exhibits sign oscillations.

Effective Potentials for Folding Proteins

The reduced dynamics of a single qubit or two qubits coupled to an interacting quantum spin bath modeled by an XXZ spin chain is investigated. By using the method of a time-dependent density matrix renormalization group t-DMRG, we go beyond the uniform coupling central spin model and nonperturbatively evaluate the induced decoherence and entanglement. It is shown that both the decoherence and the entanglement strongly depend on the phase of the underlying spin bath. We show that in general, spin baths can induce entanglement for an initially disentangled pair of qubits. Furthermore, when the spin bath is in the ferromagnetic phase because the qubits directly couple to the order parameter, the reduced dynamics shows an oscillatory type behavior. On the other hand, only for the paramagnetic and the antiferromagnetic phases do the initially entangled states suffer from an entanglement sudden death. By calculating the concurrence, the finite disentanglement time is mapped out for all of the phases in the phase diagram of the spin bath.

Enhanced surface mobility and quantum oscillations in topological insulator Bi1.5Sb0.5Te1.7Se1.3 nanoflakes

The effective Hamiltonian for two-dimensional quantum wells with rough interfaces is formally derived. Two terms are generated. The first term is identified with local energy-level fluctuations, and was introduced phenomenologically in the literature for interface roughness scattering, however, is now shown to be valid only for an infinite potential well or Hamiltonians with one single length scale. The other term is shown to modulate the wave function and cause fluctuations in the charge density. This will further reduce the electron mobility to a magnitude that is close to the experimental result.

Tunneling conductance of the graphene SNS junction with a single localized defect

A coarse-grained off-lattice model that is not biased in any way to the native state is proposed to fold proteins. To predict the native structure in a reasonable time, the model has included the essential effects of water in an effective potential. Two new ingredients, the dipole-dipole interaction and the local hydrophobic interaction, are introduced and are shown to be as crucial as the hydrogen bonding. The model allows successful folding of the wild-type sequence of protein G and may have provided important hints to the study of protein folding.

Static and dynamical anisotropy effects in the mixed state of d -wave superconductors

In this study, a series of Bi1.5Sb0.5Te1.7Se1.3 (BSTS) flakes 80-nm to 140-μm in thickness was fabricated to investigate their metallic surface states. We report the observation of surface-dominated transport in these topological insulator BSTS nanoflakes. The achievement of surface-dominated transport can be attributed to high surface mobility (∼3000 cm2/V s) and low bulk mobility (12 cm2/V s). Up to 90% of the total conductance, the surface channel was estimated based on the thickness dependence of electrical conductance and the result of the Shubnikov-de Hass oscillations in a 200-nm BSTS. The nature of nontrivial Dirac surface states was also confirmed by the weak anti-localization effect.