Circularly polarized near-field optical mapping of spin-resolved quantum Hall chiral edge states.

We have successfully developed a circularly polarized near-field scanning optical microscope (NSOM) that enables us to irradiate circularly polarized light with spatial resolution below the diffraction limit. As a demonstration, we perform real-space mapping of the quantum Hall chiral edge states near the edge of a Hall-bar structure by injecting spin polarized electrons optically at low temperature. The obtained real-space mappings show that spin-polarized electrons are injected optically to the two-dimensional electron layer. Our general method to locally inject spins using a circularly polarized NSOM should be broadly applicable to characterize a variety of nanomaterials and nanostructures.

Gate-voltage dependence of zero-bias anomalies in multiwall carbon nanotubes.

Temperature dependence of zero-bias conductance of the vanadium (V)/multiwall carbon nanotube (MWNT)/V structure is studied. As temperature is reduced, the conductance decreases with a functional form consistent with a power law. For the first time, we find that the exponent depends significantly on gate voltage. This exponent dependence cannot be explained by Luttinger-liquid theory for ballistic MWNTs. We interpret the obtained results within the framework of the nonconventional Coulomb blockade theory for strongly disordered MWNTs.

Quantum phase transition in one-dimensional arrays of resistively shunted small Josephson junctions.

We have observed a superconductor-insulator transition in one-dimensional (1D) arrays of small Josephson junctions by changing both the resistance R(S) of normal metal resistors shunting each junction and the ratio of the Josephson coupling energy E(J) to the charging energy E(C). The phase boundary lies at R(S) approximately R(Q) (R(Q) identical with h/4e(2)=6.45 kOmega) when E(J)/E(C) is smaller than about unity. We discuss the obtained phase diagram in terms of theoretical models of the dissipation-driven quantum phase transition, with particular attention to differences from 2D arrays.