Research Highlights Archive
Control of magnetism across metal to insulator transitions
J. de la Venta, Siming Wang, J. G. Ramirez, and Ivan K. Schuller
Magnetic properties (coercivity and magnetization) of ferromagnetic films are strongly affected by the proximity to materials that undergo a metal to insulator transition. The authors show that stress associated with structural changes across the metal-insulator phase transition in VO2 and V2O3 produces a magnetoelastic anisotropy in ferromagnetic films (Co and Ni) deposited on top of the oxides.
Origin of 1/f noise in graphene multilayers: Surface vs. volume
Guanxiong Liu, Sergey Rumyantsev, Michael S. Shur, and Alexander A. Balandin
A long-standing question is whether 1/f noise is generated on the surface of electrical conductors or inside their volumes. Unlike the thickness of metal or semiconductor films, the thickness of graphene multilayers can be continuously and uniformly varied all the way down to a single atomic layer of graphene – the actual surface. The measurements with graphene multilayers demonstrated that 1/f noise is dominated by the volume noise when the thickness exceeds ~7 atomic layers (~2.5 nm). The 1/f noise is the surface phenomenon below this thickness.
Revised structural phase diagram of (Ba0.7Ca0.3TiO3)-(BaZr0.2Ti0.8O3)
Dean S. Keeble, Feres Benabdallah, Pam A. Thomas, Mario Maglione, and Jens Kreisel
The temperature-composition phase diagram of barium calcium titanate zirconate (x(Ba0.7Ca0.3TiO3)-(1 − x)(BaZr0.2Ti0.8O3); BCTZ) has been reinvestigated using high-resolution synchrotron x-ray powder diffraction. Contrary to previous reports of an unusual rhombohedral-tetragonal phase transition in this system, we have observed an intermediate orthorhombic phase, isostructural to that present in the parent phase, BaTiO3, and we identify the previously assigned T-R transition as a T-O transition.
Efficiency limits of rectenna solar cells: Theory of broadband photon-assisted tunneling
Saumil Joshi and Garret Moddel
Because rectifiers can convert a wide range of frequencies to dc it was thought that rectenna solar cells–antennas coupled to ultra-high speed diodes–could efficiently harvest the entire solar spectrum and exceed the Shockley-Queisser limit. We show that there are efficiency limits to broadband optical conversion and provide a quantitative analysis using the theory of photon-assisted tunneling.
Demonstration and characterization of an ambipolar high mobility transistor in an undoped GaAs/AlGaAs quantum well
A. F. Croxall, B. Zheng, F. Sfigakis, K. Das Gupta, I. Farrer, C. A. Nicoll, H. E. Beere, and D. A. Ritchie
We report an ambipolar device fabricated in undoped GaAs/AlGaAs quantum wells (widths 10 and 25 nm) with front and backgates that allow almost two orders of magnitude in density to be accessed in the same device (7×109cm−2 to 5×1011cm−2). By changing the well width, the relative electron and hole mobilities can be tuned, approaching similar velocities.
Origin of the defects-induced ferromagnetism in un-doped ZnO single crystals
Peng Zhan, Zheng Xie, Zhengcao Li, Weipeng Wang, Zhengjun Zhang, Zhuoxin Li, Guodong Cheng, Peng Zhang, Baoyi Wang, and Xingzhong Cao
We clarified, in this Letter, that in un-doped ZnO single crystals after thermal annealing in flowing argon, the defects-induced room-temperature ferromagnetism was originated from the surface defects and specifically, from singly occupied oxygen vacancies denoted as F+, by the optical and electrical properties measurements as well as positron annihilation analysis. In addition, a positive linear relationship was observed between the ferromagnetism and the F+ concentration, which is in support with the above clarification.
Cross-plane thermal properties of transition metal dichalcogenides
C. Muratore, V. Varshney, J. J. Gengler, J. J. Hu, J. E. Bultman, T. M. Smith, P. J. Shamberger, B. Qiu, X. Ruan, A. K. Roy, and A. A. Voevodin
The authors explore the thermal properties of hexagonal transition metal dichalcogenide compounds with different average atomic masses but equivalent microstructures. Thermal conductivity values of sputtered thin films were compared to bulk crystals.
Real-time observation of electrical vortex core switching
Kunihiro Nakano, Kenji Tanabe, Ryo Hiramatsu, Daichi Chiba, Norikazu Ohshima, Shinya Kasai, Tomonori Sato, Yoshinobu Nakatani, Koji Sekiguchi, Kensuke Kobayashi, and Teruo Ono
A single vortex-core switching induced by current injection into a ferromagnetic disk is detected in real time using a three-terminal device with the tunneling magnetoresistance junction. The real time observation reveals that the electrical core switching is completed within a few 100 ps.
Gold ion implantation induced high conductivity and enhanced electron field emission properties in ultrananocrystalline diamond films
K. J. Sankaran, H. C. Chen, B. Sundaravel, C. Y. Lee, N. H. Tai, and I. N. Lin
The authors report high conductivity of 185 (Ω cm)−1 and superior electron field emission (EFE) properties, viz. low turn-on field of 4.88 V/μm with high EFE current density of 6.52 mA/cm2 at an applied field of 8.0 V/μm in ultrananocrystalline diamond (UNCD) films due to gold ion implantation. Transmission electron microscopy examinations reveal the presence of Au nanoparticles in films, which result in the induction of nanographitic phases in grain boundaries, forming conduction channels for electron transport.
Fast switching protocol for ferroelectric random access memory based on poly(vinylidene fluoride-trifluoroethylene) copolymer ultrathin films
Ying Hou, Zhaoyue Lü, Tiansong Pu, Yuan Zhang, Guoqiang Xu, and Haisheng Xu
The polarization switching behaviors with different pulse processes for ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer ultrathin films have been studied. The faster switching speed with imprint time is observed for the films with the certain switching and imprint directions.