S.P. Zhao

Modulating magnetism of nitrogen-doped zigzag graphene nanoribbons

We present a study of electronic properties of zigzag graphene nanoribbons (ZGNRs) substitutionally doped with nitrogen atoms at a single edge by first principle calculations. We find that the two edge states near the Fermi level separate due to the asymmetric nitrogen-doping. The ground states of these systems become ferromagnetic because the local magnetic moments along the undoped edges remain and those along the doped edges are suppressed. By controlling the charge-doping level, the magnetic moments of the whole ribbons are modulated. Proper charge doping leads to interesting half-metallic and single-edge conducting ribbons which would be helpful for designing graphene?nanoribbon-based spintronic devices in the future.

Absolute measurement of penetration depth in a superconducting film by the two-coil technique

A two-coil mutual-inductance apparatus that is optimized to allow for the measurement of the absolute value of penetration depth λ in superconducting films is described. Nb films with thickness d ranging from 20 to 90 nm are used to illustrate the measurement. For a 70-nm-thick Nb film at 4.2 K, with d/λ∼0.6, the uncertainty in the measured λ is about ±2.3%. From the results on the Nb film series, we show that a satisfactory determination of the absolute value of λ is possible for these films with d/λ<0.95.

A noise feedback least-mean-square algorithm of data processing for SQUID-based magnetocardiography

We present a modified normalized least-mean-square algorithm for SQUID-based magnetocardiography data processing with a new error function, in which the instantaneous signal component represented approximately by an average of near past error data has been eliminated. In this way, the rebounds of the weight vector W from its optimal value in parameter space due to the signal component can be well avoided.

Excitonic effects of E11, E22, and E33 in armchair-edged graphene nanoribbons

We explore excitonic effects of E11, E22, and E33, which are excitons formed between the three highest valence subbands and the three lowest conduction ones, in armchair-edged graphene nanoribbons by applying the extended tight-binding model including electron-electron interactions. Our results show that the excitation energies and the binding energies decrease inversely with the ribbon widths and can be classified into three categories based on their width indices. We found the relation between the band structures and the binding energies and explained some recent observations of strong excitonic effects in graphene.

Fabrication and properties of all-refractory Nb/Al–AlOx–Ti junctions for microbolometers and microrefrigerators

Fabrication of all-refractory Nb/Al–AlOx–Ti superconducting tunnel junctions using selective titanium etching process (STEP) is described. Results including anodization properties of Ti, and junction’s I–V characteristics and subgap currents measured in the temperature range of 0.4–9.2 K are presented. The junctions show fairly high quality with respect to their stability and reproducibility. Possible utilization of these junctions as superconductor–insulator–normal metal type devices operating around 100 mK and above for ultrasensitive microbolometer and electronic microrefrigerator applications is discussed.

Controlled couplings for Cooper-pair charge qubits

Abstract Quantum manipulation means controlling the states of single and multiple quantum-bit (qubit) by some methods. In this paper, we discuss a Cooper-pair charge qubit system composed of symmetrical dc SQUID’s or low-capacitance Josephson corner junctions made up of d- and s-wave superconductors. Single- and two-qubit operations can be realized in the system by controlling the gate voltages and external magnetic fields in an easy way.

Electronic transport properties of inner and outer shells in near ohmic-contacted double-walled carbon nanotube transistors

We investigate electronic transport properties of field-effect transistors based on double-walled carbon nanotubes, of which inner shells are metallic and outer shells are semiconducting. When both shells are turned on, electron-phonon scattering is found to be the dominant phenomenon. On the other hand, when outer semiconducting shells are turned off, a zero-bias anomaly emerges in the dependence of differential conductance on the bias voltage, which is characterized according to the Tomonaga-Luttinger liquid model describing tunneling into one-dimensional materials. We attribute these behaviors to different contact conditions for outer and inner shells of the double-walled carbon nanotubes. A simple model combining Luttinger liquid model for inner metallic shells and electron-phonon scattering in outer semiconducting shells is given here to explain our transport data at different temperatures.

Transport Properties of Surface-Modulated Gold Atomic-Chains and Nanofilms: Ab initio Calculations

Transport properties of gold atomic-chains and nanofilms under surface modulation are studied by performing self-consistent first-principle calculations. Quantum conducting channels of gold atomic-chains with absorbing atoms can be partly transparent or even blocked for certain injecting energies. Conductances of gold nanofilms with ridges show great dependence on their structures. We demonstrate that the transport properties of gold atomic-chains and nanofilms can be engineered through surface modulation, which may be helpful for designing low-dimensional nanodevices.

Engineering Double-Walled Carbon Nanotubes by Ar Plasma *

Double-walled carbon nanotubes (DWCNTs) have been found to be promising nano-materials for nano-mechanical and nano-electrical devices due to their double-walled structures. Modifying DWCNTs would be one of the key technologies for device construction. We demonstrate engineering the geometry of DWCNTs by etching with Ar plasma. The characterization by atomic force microscopy indicates that single atomic carbon layers could be removed from DWCNTs by Ar plasma. The etching effect is further investigated by electrical measurements on DWCNT field-effect transistors, which allow us to study the interwall screen effect as well.

Piezo-antiferromagnetic effect of sawtooth-like graphene nanoribbons

A type of sawtooth-like graphene nanoribbon (SGNR) with piezo-antiferromagnetic effect is studied numerically. The ground state of the studied SGNR changes from nonmagnetic state to antiferromagnetic state with uniaxial strain. The changes of the spin-charge distributions during the stretching are investigated. The Hubbard model reveals that the hopping integrals between the π-orbitals of the carbon atoms are responsible to the piezo-antiferromagnetic effect. The study sheds light on the application of graphene-based structures to nanosensors and spintronic devices.