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 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).
6. A. Ramamoorthy, L. Mourokh, J. L. Reno, and J. P. Bird, “Tunneling spectroscopy of a ballistic quantum wire”, Phys. Rev. B 78, 035335 (2008).
7. Y. Yoon, M.-G. Kang, T. Morimoto, L. Mourokh, N. Aoki, J. L. Reno, J. P. Bird, and Y. Ochiai, “Detector backaction on the self-consistent bound state in quantum point contacts”, Phys. Rev. B 79, 121304(R) (2009).
8. Y. Yoon, M.-G. Kang, P. Ivanushkin, L. Mourokh, T. Morimoto, N. Aoki, J. L. Reno, Y. Ochiai, and J. P. Bird, “Non-local bias spectroscopy of the self-consistent bound state in quantum point contacts near pinch-off”, Appl. Phys. Lett. 94, 213103 (2009).

Students involved: Myoung-Gu Kang (University at Buffalo, PhD program), Arunkumar Ramamoorthy (Arizona State University, PhD, graduated 11/06), Alexandros Shailos (Arizona State University, PhD, graduated 2004), Youngsoo Yoon (University at Buffalo, PhD, graduated 2008).

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.

Hybrid semiconductor/nanomagnetic devices. The integration of nanoscale magnetic elements with semiconductor nanostructures, such as quantum wires and dots, offers the potential to realize a new class of electronics, characterized by reduced energy dissipation, increased switching speed, and higher storage density. While there has been enormous progress in recent years in the development of metal-based magnetoelectronics, including devices that utilize the giant- and tunneling-magneto-resistance effects, the development of analogous semiconductor structures is lagging far behind. We are currently exploring the use of hybrid semiconductor nanowire/nanomagnet structures for application as the building components of integrated-circuit technology in which the logic and memory functions are achievable within the same device structure. Investigations of a hybrid "Mag-FET" structure have demonstrated an enhanced tunneling magneto-resistance near pinch-off that could be of interest for future applications that combine logic and non-volatile memory. We have also proposed the operation of novel magnetic field sensors and memory cells that utilize hybrid semiconductor/nanomagnetic structures to achieve their functionality.

1. J.-F. Song, Y. Ochiai, and J. P. Bird, “Fano resonances in open quantum dots and their application as spin filters”, Appl. Phys. Lett. 82, 4561 – 4563 (2003).
2. J.-F. Song, J. P. Bird, and Y. Ochiai, “Manipulating the transmission of a two-dimensional electron gas via spatially-varying magnetic fields”, Appl. Phys. Lett. 86, 062106 (2005).
3. J.-F. Song, J. P. Bird, and Y. Ochiai, “Nanowire magnetic-memory cell based on a periodic magnetic superlattice”, J. Phys.: Condens. Matter 17, 5263 – 5268 (2005).
4. J.-U. Bae, T.-Y. Lin, Y. Yoon, S. J. Kim, J. P. Bird, A. Imre, W. Porod, and J. L. Reno, “Large hysteretic magneto-resistance in high-mobility semiconductor quantum wires bridged by single-domain nanomagnets”, Appl. Phys. Lett. 91, 022105 (2007).
5. J.-U. Bae, T.-Y. Lin, Y. Yoon, S. J. Kim, A. Imre, J. L. Reno, W. Porod, and J. P. Bird, “Large tunneling magneto-resistance in a field-effect transistor with a nanoscale ferromagnetic gate”, Appl. Phys. Lett. 92, 253101 (2008).
6. J.-U. Bae, T.-Y. Lin, J. L. Reno, and J. P. Bird, “Non-linear characteristics of the hysteretic magneto-resistance of a hybrid nanomagnetic field effect transistor”, Appl. Phys. Lett. 93, 143109 (2008).
7. J.-U. Bae, T.-Y. Lin, Y. Yoon, S. J. Kim, J. P. Bird, A. Imre, W. Porod, and J. L. Reno, “Characterization of nanomagnet fringing fields in hybrid semiconductor/ferromagnetic devices”, IEEE Trans. Mag. 44, 4706 (2008).

Students involved: Jong-Uk Bae (University at Buffalo, PhD, graduated 2007), Girish Bohra (University at Buffalo, PhD program), Himanshu Kothari (Arizona State University, graduated 10/07), Teng-Yin Lin (University at Buffalo, PhD program).

(a) Optical image of one of our QPC devices with an integrated bow-tie antenna, showing the position of the gates and antenna, relative to the Hall bar. The Hall bar features eight Ohmic contacts (mottled regions in the image) that allow current to be driven through the QPC. The Hall bar is 300 μm wide and 1 mm long. (b). The left axis shows the QPC conductance measured at 2.5 K, with (line with solid symbols) and without (line with no symbols) THz irradiation. The right axis shows the current change induced in the QPC by THz irradiation (DITHz, line with open symbols). The upper inset is an electron-microscope image of the central portion of the device, with gates indicated by the lighter regions. The lower inset indicates the measurement circuit.

Nanoelectronic devices for THz applications. The terahertz (THz) region of the electromagnetic spectrum is of interest for applications ranging from defense and homeland-security to biomedical engineering. The realization of THz spectral-imaging systems for applications of these types requires frequency-tunable compact sources and detectors, and compatibility of these with conventional semiconductor microfabrication techniques is highly desired. Standard THz detection schemes, on the other hand, are either cryogenically-cooled (< 4.2 K) or low sensitivity.  More importantly, these methods do not provide-frequency resolution for rapid spectroscopic imaging. The strong confinement of carriers in nanostructures results in the formation of discretely (or quasi-discretely) quantized levels with a characteristic energy separation (~meV) that is well matched to the THz range. We are therefore exploring the use of quantum point contacts as nanoscale solid-state terahertz detectors with high sensitivity and frequency resolution. By utilizing in-situ gate control of the confinement energies of these nanostructures, we are interested in developing frequency-selective and tunable detectors, which should should eventually lead to advances in areas such as package monitoring, electronic surveillance, short-link secure wireless communications, and pharmaceutical science. Recent published work has demonstrated a novel rectification of THz radiation in QPCs, and in current effort we are seeking to identyify signatures of THz photon-assisted tunneling in these structures.

1. J. W. Song, N. A. Kabir, Y. Kawano, K. Ishibashi, G. R. Aizin, L. Mourokh, J. L. Reno, A. G. Markelz, and J. P. Bird, “Terahertz response of quantum point contacts”, Appl. Phys. Lett. 92, 223115 (2008).


Student involved: Jungwoo Song (University at Buffalo, PhD program).

(a) Scanning AFM image of epitaxially-formed NiSi NWs. (b) Scanning electron micrograph showing an individually-contacted NiSi NW whose MR characteristics are plotted in the figure. (c) MR of the NiSi NW at several different temperatures (indicated). Open circles were obtained while sweeping B from +8- to −8-T, while red crosses were obtained sweeping from −8- to +8-T. In all cases, the magnetic-field sweep rate was 0.2 T/min.