Stuart A. Wolf

Terahertz-field-induced insulator-to-metal transition in vanadium dioxide metamaterial

Electron–electron interactions can render an otherwise conducting material insulating, with the insulator–metal phase transition in correlated-electron materials being the canonical macroscopic manifestation of the competition between charge-carrier itinerancy and localization. The transition can arise from underlying microscopic interactions among the charge, lattice, orbital and spin degrees of freedom, the complexity of which leads to multiple phase-transition pathways. For example, in many transition metal oxides, the insulator–metal transition has been achieved with external stimuli, including temperature, light, electric field, mechanical strain or magnetic field. Vanadium dioxide is particularly intriguing because both the lattice and on-site Coulomb repulsion contribute to the insulator-to-metal transition at 340 K (ref. 8). Thus, although the precise microscopic origin of the phase transition remains elusive, vanadium dioxide serves as a testbed for correlated-electron phase-transition dynamics. Here we report the observation of an insulator–metal transition in vanadium dioxide induced by a terahertz electric field. This is achieved using metamaterial-enhanced picosecond, high-field terahertz pulses to reduce the Coulomb-induced potential barrier for carrier transport. A nonlinear metamaterial response is observed through the phase transition, demonstrating that high-field terahertz pulses provide alternative pathways to induce collective electronic and structural rearrangements. The metamaterial resonators play a dual role, providing sub-wavelength field enhancement that locally drives the nonlinear response, and global sensitivity to the local changes, thereby enabling macroscopic observation of the dynamics. This methodology provides a powerful platform to investigate low-energy dynamics in condensed matter and, further, demonstrates that integration of metamaterials with complex matter is a viable pathway to realize functional nonlinear electromagnetic composites.

Advances and Future Prospects of Spin-Transfer Torque Random Access Memory

Spin-transfer torque random access memory (STT-RAM) is a potentially revolutionary universal memory technology that combines the capacity and cost benefits of DRAM, the fast read and write performance of SRAM, the non-volatility of Flash, and essentially unlimited endurance. In order to realize a small cell size, high speed and achieve a fully functional STT-RAM chip, the MgO-barrier magnetic tunnel junctions (MTJ) used as the core storage and readout element must meet a set of performance requirements on switching current density, voltage, magneto-resistance ratio (MR), resistance-area product (RA), thermal stability factor (¿) , switching current distribution, read resistance distribution and reliability. In this paper, we report the progress of our work on device design, material improvement, wafer processing, integration with CMOS, and testing for a demonstration STT-RAM test chip, and projections based on modeling of the future characteristics of STT-RAM.

The Promise of Nanomagnetics and Spintronics for Future Logic and Universal Memory

This paper is both a review of some recent developments in the utilization of magnetism for applications to logic and memory and a description of some new innovations in nanomagnetics and spintronics. Nanomagnetics is primarily based on the magnetic interactions, while spintronics is primarily concerned with devices that utilize spin polarized currents. With the end of complementary metal-oxide-semiconductor (CMOS) in sight, nanomagnetics can provide a new paradigm for information process using the principles of magnetic quantum cellular automata (MQCA). This paper will review and describe these principles and then introduce a new nonlithographic method of producing reconfigurable arrays of MQCAs and/or storage bits that can be configured electrically. Furthermore, this paper will provide a brief description of magnetoresistive random access memory (MRAM), the first mainstream spintronic nonvolatile random access memory and project how far its successor spin transfer torque random access memory (STT-RAM) can go to provide a truly universal memory that can in principle replace most, if not all, semiconductor memories in the near future. For completeness, a description of an all-metal logic architecture based on magnetoresistive structures (transpinnor) will be described as well as some approaches to logic using magnetic tunnel junctions (MTJs).

Spintronics: a retrospective and perspective

Spintronics is a rapidly emerging field of science and technology that will most likely have a significant impact on the future of all aspects of electronics as we continue to move into the 21st century. Conventional electronics are based on the charge of the electron. Attempts to use the other fundamental property of an electron, its spin, have given rise to a new, rapidly evolving field, known as spintronics, an acronym for spin transport electronics that was first introduced in 1996 to designate a program of the U.S. Defense Advanced Research Projects Agency (DARPA). Initially, the spintronics program involved overseeing the development of advanced magnetic memory and sensors based on spin transport electronics. It was then expanded to included Spins IN Semiconductors (SPINS), in the hope of developing a new paradigm in semiconductor electronics based on the spin degree of freedom of the electron. Studies of spin-polarized transport in bulk and low-dimensional semiconductor structures show promise for the creation of a hybrid device that would combine magnetic storage with gain-in effect, a spin memory transistor. This paper reviews some of the major developments in this field and provides a perspective of what we think will be the future of this exciting field. It is not meant to be a comprehensive review of the whole field but reflects a bias on the part of the authors toward areas that they believe will lead to significant future technologies.

Optical detection in thin granular films of Y‐Ba‐Cu‐O at temperatures between 4.2 and 100 K

It is demonstrated here that granular films of Y‐Ba‐Cu‐O may serve as optical detectors, operating at wavelengths from the visible to the far infrared, at temperatures well above that of liquid helium. Preliminary measurements using a blackbody source show that an upper bound of the minimum detectable power is 1 μW. The response time as determined by a pulsed far‐infrared source is of the order 20 ns. Methods to improve the sensitivity will be discussed.

Directed Self-Assembly of Epitaxial CoFe2O4-BiFeO3 Multiferroic Nanocomposites

CoFe(2)O(4) (CFO)-BiFeO(3) (BFO) nanocomposites are an intriguing option for future memory and logic technologies due to the magnetoelectric properties of the system. However, these nanocomposites form with CFO pillars randomly located within a BFO matrix, making implementation in devices difficult. To overcome this, we present a technique to produce patterned nanocomposites through self-assembly. CFO islands are patterned on Nb-doped SrTiO(3) to direct the self-assembly of epitaxial CFO-BFO nanocomposites, producing square arrays of CFO pillars.

Growth and characterization of vanadium dioxide thin films prepared by reactive-biased target ion beam deposition

Using a novel growth technique called reactive bias target ion beam deposition, the authors have prepared highly oriented VO2 thin films on Al2O3 (0001) substrates at various growth temperatures ranging from 250to550°C. The influence of the growth parameters on the microstructure and transport properties of VO2 thin films was systematically investigated. A change in electrical conductivity of 103 was measured at 341K associated with the well known metal-insulator transition (MIT). It was observed that the MIT temperature can be tuned to higher temperatures by mixing VO2 and other vanadium oxide phases. In addition, a current/electric-field induced MIT was observed at room temperature with a drop in electrical conductivity by a factor of 8. The current densities required to induce the MIT in VO2 are about 3×104A∕cm2. The switching time of the MIT, as measured by voltage pulsed measurements, was determined to be no more than 10ns.

Spintronics: a new paradigm for electronics for the new millennium

SPIN TRansport electrONICS or SPINTRONICS, in which the spin degree of freedom of the electron will play an important role in addition to or in place of the charge degree of freedom in mainstream electronics will be important as we start the new millennium. The prospects for this new electronics in nonvolatile radiation hard magnetic memory for the Department of Defense (DoD) is described.

Properties of superconducting rf sputtered ultrathin films of Nb

Superconducting niobium films, both cylindrical and planar, have been prepared by rf sputtering in an uhv chamber which is routinely pumped to a residual pressure of 3×10−9 Torr (7.6×10−7 Pa). The superconducting transition temperature Tc of films with thicknesses greater than 2000 A was near 9.2 K, the bulk value, while Tc decreased linearly with inverse thickness for films thinner than 1000 A. Superconductivity was observed for the first time in Nb films thinner than 50 A; the Tc of a 40 A film was 4.4 K while for a 27 A film the onset of the transition was about 2 K. Electrical continuity was maintained down to a nominal film thickness of less than 10 A, but this film was not superconducting above 1.5 K. These results will be compared with a model which predicts the dependence of Tc on thickness.

Inhomogeneous superconductivity and the “pseudogap” state of novel superconductors

Abstract Many novel superconducting compounds such as the high T c oxides are intrinsically inhomogeneous systems by virtue of the superconductivity being closely related to the carrier density which is in turn provided in most cases by doping. An inhomogeneous structure is thus created by the statistical nature of the distribution of dopants. At the same time doping also leads to pair-breaking and, consequently, to a local depression of T c . This is a major factor leading to inhomogeneity. As a result, the critical temperature is spatially dependent: T c ≡ T c ( r ) . The “pseudogap” state is characterized by several energy scales: T * , T c * , and T c . The highest energy scale ( T * ) corresponds to phase separation (at T T * ) into a mixed metallic-insulating structure. Especially interesting is the region T c * > T > T c where the compound contains superconducting “islands” embedded in a normal metallic matrix. As a result, the system is characterized by a normal conductance along with an energy gap structure, anomalous diamagnetism, unusual a.c. properties, an isotope effect, and a “giant” Josephson proximity effect. An energy gap may persist to temperatures above T c * caused by the presence of a charge density wave (CDW) or spin density wave (SDW) in the region T > T c * but less than T * , whereas below T c * superconducting pairing also makes a contribution to the energy gap ( T c * is an “intrinsic” critical temperature). The values of T * , T c * , T c depend on the compound and the doping level. The transition at T c into the dissipationless ( R = 0 ) macroscopically coherent state is of a percolation nature.