[Nano Electromechanical System(NEMS)]
Nat. Mat. 7 459
Nanlelectromechanical system (NEMS), scaled-down system of microelectromechanical system(MEMS), is an integrated system of electrical and mechanical functionality on the nanoscale and is widely applied as actuators, mass/force sensors, accelerometer and bio/chemical sensors.
Typical NEMS have a mechanical degree of freedom. And NEMS are fabricated as doubly clamped beam resonator structure at nanoscale. Small mass density and high aspect ratio of material based NEMS are very sensitive to loading mass and external force, and recently yoctogram (1yg=1e-24g) resolution NEMS mass sensors are reported.
Furthermore, superior mechanical properties of carbon nanotube, graphene, nanowire based NEMS show the ultra high frequency mechanical resonators. Ultra high frequency mechanical resonators can open the opportunity to observe the quantum behavior at mesoscopic scale. To approach the quantum regime, mechanical energy must be larger than thermal excitation energy. Ultra high frequency response mechanical resonator can show quantum behavior at ~mK range which are approchable temperature of commercial dilution refrigerator.
Some research groups try to the measure the quantum behavior of mechanical resonators by coupling the superconductor qubit and optical cooling method. As described, NEMS are widely used from ultra sensitive application sensor to the fundamental research tools.
Generally, NEMS are freely suspended structures and NEMS based materials are isolated from substrate and environment interactions. Thus NEMS can be optimized characterization tools for electrical, thermal and mechanical properties of materials with minimized perturbation. For example, thermal properties such as thermal conductivity are precisely measured by reducing the heat path through the substrate.
Electrical properties characterization using a NEMS reduces the substrate induced perturbation and scattering. Characterizations of mechanical properties at nanoscale are very challenging, due to mechanical characterization instruments are larger than materials and have limited detection resolution.
[Diluted Magnetic Semiconductor with MBE (DMS)]
The field of semiconductor spintronics (spin electronics) was initiated by the proposal of Datta and Das, which has been described as spin field effect transistor. The idea is to utilize the spin-degree of freedom in addition to the charge of the carrier in doped semiconductors. But this idea cannot be easily realized. In realizing the spin-transistor, there are several independent tasks to overcome, such as spin injection and detection of spin polarized current.
One of the most important issue of spin injection is known as conductivity mismatch problem. So, to overcome this problem, the diluted magnetic semiconductor (DMS) has been regarded as a good candidate for spintronics devices.
In the past decades, semiconductor spintronic device research has progressed and has achieved remarkable success. Among them, III-Mn-V ferromagnetic semiconductors are the most intensively studied and best understood materials due to its advantage of Mn acting dual role as local magnetic source and acceptor.
In this DMS system, hole carriers participate in the magnetic ordering. A particularly strong manifestation of valence band spin orbit coupling occurs in the antisymmetric off diagonal element of the resistivity tensor. Thus, the anomalous Hall effect has become one of the key tools used to detect the paramagnetic-ferromagnetic transition.
AHE has allowed for indirect characterization of magnetic properties as well as demonstrating the novelty of carrier mediated ferromagnetic ordering in diluted magnetic semiconductors. Although the AHE measurement is widely used in the research of ferromagnetic materials, its exact origin is a still controversial issue in both theory and experiment. To further its application, it is important to understand the exact mechanism of AHE in ferromagnetic semiconductor.
