Spontaneous spin polarization in nanowires. It has been known for some time that spin polarization of carriers in semiconductor nanostructures can result from strongly enhanced many-body interactions, which arise when the carriers are confined in a quantum wire or a quantum dot. It has been understood for some time, for example that the exchange interaction among electrons in quantum dots can lead to the filling of low-lying states in a manner consistent with Hund’s rule. Of interest est here, however, is other work suggesting that electrons in nonmagnetic quantum wires can spontaneously spin polarize at zero magnetic field, forming a local magnetic moment at electron densities where conduction through the wire is about to onset. In our contribution in this area, we have provided evidence for the electrical detection of this spin polarized state, by studying structures comprised of coupled quantum wires. In this experiment, the formation of a local moment in one of the wires is detected as a resonant peak in the conductance of the other wire (see the figure on the left), and a theoretical interpretation that we have developed ascribes the resonance to an enhancement of the density of states of the detector wire, which is effective when it is coupled to the local moment in the other wire. Our current effort in this area is focused on extending this approach to investigate electrical detection of coupled local magnetic moments.

1. T. Morimoto, Y. Iwase, N. Aoki, T. Sasaki, Y. Ochiai, A. Shailos, J. P. Bird, M. P. Lilly, J. L. Reno, and J. A. Simmons, “Nonlocal resonant interaction between coupled quantum wires”, Appl. Phys. Lett. 82, 3952 – 3954 (2003).
2. J. P. Bird and Y. Ochiai, “Electron spin polarization in nanoscale constrictions”, Science 303, 1621 – 1622 (2004).
3. V. I. Puller, L. G. Mourokh, A. Shailos, and J. P. Bird, “Detection of local-moment formation using the resonant interaction between coupled quantum wires”, Phys. Rev. Lett. 92, 096802 (2004).
4. T. Morimoto, M. Henmi, R. Naito, K. Tsubaki, N. Aoki, J. P. Bird, and Y. Ochiai, “Resonantly enhanced nonlinear conductance in long quantum point contacts near pinch-off”, Phys. Rev. Lett. 97, 096801 (2006).
5. Y. Yoon, L. Mourokh, T. Morimoto, N. Aoki, Y. Ochiai, J. L. Reno, and J. P. Bird, “Probing the microscopic structure of bound states in quantum point contacts”, Phys. Rev. Lett. 99, 136805 (2007).

Students involved: Alexandros Shailos (Arizona State University, PhD, graduated 2004), Youngsoo Yoon (University at Buffalo, PhD program), Myoung-Gu Kang (University at Buffalo, PhD program).

Exploitation of quantum carrier phenomena in nanostructures offers the potential to realize new electronic technologies whose functionality goes well beyond that available in conventional CMOS technology. We are exploring several issues related to the development of such technologies, focusing on studies of hybrid semiconductor/nanomagnetic devices, tunable THz detectors based on quantum point contacts, and the use of coupled quantum wires for qubit representation in quantum computing.

Coupled quantum wires for quantum computing. Among the current interest in solid-state approaches to quantum computing, one scheme that has attracted much attention involves using a pair of quantum wires to implement a scalable qubit (or quantum bit). The functionality of this qubit derives by allowing the wavefunctions of two (otherwise independent) quantum wires to overlap in a short coupling region. In numerical simulations of the quantum transport of this system, it has been predicted that the properties (size and local barrier) of this region may be tuned to switch electrons coherently between the wires. The switching is a result of electron interference in the coupling region, and should be sensitive to the presence of a normal magnetic field. Although the implementation of this scheme should lie well within the scope of existing nanofabrication approaches, a proof-of-concept demonstration has thus far not been forthcoming. In our work, we use the split-gate technique to form pairs of wires with tunable inter-wire coupling (as shown in the figure to the left). We have studied the switching properties of these devices at low temperatures, where electron transport should be influenced by electron interference. When the local barrier in the coupling region lies below the Fermi level, allowing for significant wavefunction overlap between the wires, a nonmonotonic switching of the wire currents is obtained in a magnetic field, suggestive of electron interference in the coupling region. In other work yet, we have demonstrated a novel switching behavior that occurs when the confining potential of the two quantum wires is strongly increased close to pinch-off, causing asymmetry in the two wires. At present, we wish to extend this work to investigate mechanisms for two-qubit entanglement based on the Coulomb interaction between qubits.

1. A. Ramamoorthy, J. P. Bird, and J. L. Reno, “Switching characteristics of coupled quantum wires with tunable coupling strength”, Appl. Phys. Lett. 89, 013118 (2006).
2. A. Ramamoorthy, J. P. Bird, and J. L. Reno, “Quantum asymmetry of switching in laterally-coupled quantum wires with tunable coupling strength”, Appl. Phys. Lett. 89, 153128 (2006).
3. A. Ramamoorthy, R. Akis, and J. P. Bird, “Quantum-switching characteristics of realistic electron-waveguide qubits”, IEEE Trans. Nanotech. 5, 712 – 715 (2006).
4. A. Ramamoorthy, J. P. Bird, and J. L. Reno, “Using split-gate structures to explore the implementation of a coupled-electron-waveguide qubit scheme”, J. Phys.: Condens. Matter 19, 276205 (2007).

Students involved: Arunkumar Ramamoorthy (Arizona State University, PhD, graduated 2006).

Electron transport in nanowires fabricated by focused-beam deposition. A convenient approach for the implementation of nanowires involves the technique of focused-beam induced deposition in which either a focused electron or ion beam is used to stimulate the breakdown of a metalorganic gas. In what is a one-step-lithography process, The breakdown results in the formation of a composite nanowire on the region of substrate where the beam is focused. These wires exhibit a composite structure, comprised of small metallic nanocrystals about 2 nm in diameter that are embedded in an amorphous carbon matrix. We have studied the electrical properties of such nanowires, realized using either a focused electron or ion beam. The electron-beam deposited wires show insulating behavior at low bias voltage that we have ascribed to single-electron tunneling via a random distribution of nanosized grains. Due to the small size of these grains, the charging effects remain observable at room temperature and we have also implemented single-electron transistors based on these structures (shown in the figure to the left). In contrast to this behavior, nanowires implemented by the use of a focused ion beam are found to exhibit metallic conductivity, a characteristic that we attribute to doping of the carbon matrix by gallium from the source beam. These results demonstrate very nicely the flexibility of focused-beam deposition as a nanofabrication technique.

1. J.-F. Lin, J. P. Bird, L. Rotkina, and P. A. Bennett, “Classical & quantum transport in focused-ion-beam deposited Pt nano-interconnects”, Appl. Phys. Lett. 82, 802 – 804 (2003).
2. L. Rotkina, J.-F. Lin, and J. P. Bird, “Non-linear current-voltage characteristics of Pt nanowires & nanowire transistors fabricated by electron-beam deposition”, Appl. Phys. Lett. 83, 4426 – 4428 (2003).
3. J.-F. Lin, J. P. Bird, L. Rotkina, A. Sergeev, and V. Mitin, “Large effects due to electron-phonon-impurity interference in the resistivity of Pt/C-Ga composite nanowires”, Appl. Phys. Lett. 84, 3828 – 3830 (2004).
4. J. Fransson, J.-F. Lin, L. Rotkina, J. P. Bird, and P. A. Bennett, “Signatures of band-like tunneling in granular nanowires”, Phys. Rev. B 72, 113411 (2005).

Students involved: Jie-Feng Lin (Arizona State University, PhD, graduated 2004).