Rapid (nanosecond-scale) electrical pulsing is used to study drift-velocity saturation in graphene field-effect devices. In these experiments, high-field pulses are utilized to drive graphene's carriers on time scales much faster than that on which energy loss to the underlying substrate can occur, thereby allowing the observation of the highest saturation velocities reported to date. In a dramatic departure from the behavior exhibited by conventional metals and semiconductors, as the electron or hole density is reduced toward the charge-neutrality point, the drift velocity is found to reach values comparable to the Fermi velocity itself. Corresponding current densities are as large as 10(9) A/cm(2), similar to the values reported for carbon nanotubes and for graphene-on-diamond transistors. In essence, our approach of rapid pulsing allows us to "free" graphene from the deleterious influence of its substrate, revealing a pathway to achieve the superior electrical performance promised by this material. The usefulness of this approach is not merely limited to graphene but should extend also to a broad variety of two-dimensional semiconductors.

We use pulsed electrical studies to investigate the various processes that limit the current carrying capacity of graphene high frequency transistors. By investigating the transient response of these devices over a time scale that spans some twelve orders of magnitude, we identify the presence of four distinct processes that degrade the current: (1) charge injection into deep traps within the interior of the oxide; (2) Joule heating of the transistor substrate by hot carriers in the graphene channel; (3) equilibration of interfacial-state filling in response to voltage transients, and; (4) leakage of captured charge from the deep traps, once the pulsed voltage is removed. The time scale associated with these processes ranges from nanoseconds to hours, with process (1) being the fastest and process (4) the slowest. By pulsing the transistors on time intervals as short as a few nanoseconds, we therefore demonstrate how it is possible to obtain output characteristics from them that are essentially free from the influence of these different mechanisms. Under such conditions, the hot-carrier drift velocity is shown to saturate at the large values expected for intrinsic graphene. Beyond graphene, this approach of pulsed characterization of transistor performance should be broadly applicable to studies of other twodimensional semiconductors, including transition-metal dichalcogenides, black phosphorous, silicene, and topological insulators.

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Elastocaloric cooling has been identified as a promising alternative to high global warming potential vapor compression cooling. Two key bottlenecks to adoption are the need for bulky/expensive actuators to provide sufficient uniaxial stress and inadequate elastocaloric material fatigue life. This paper defines the physics that govern performance of axisymmetric flexural bending for use as an emerging low-force and low-fatigue elastocaloric heating and cooling mechanism and further demonstrates a continuous rotary-driven cooling prototype using polycrcrystalline Ni50.7Ti48.9. Elastocaloric material performance is determined using infrared thermography during uniaxial-tension and four-point bending thermomechanical testing. A systematic study reveals the effects of strain rate (from 0.001 to 0.025 s -1 ), maximum strain (from 2 to 8%), and strain mode on the temperature evolution, mechanical response, and coefficient of performance. Four-point bending experiments demonstrate a temperature reduction up to 11.3°C, material coefficients of performance between 2.31 and 21.71, and a 6.09-to 7.75-fold reduction in required actuation force compared to uniaxial tension. The absence of Lüders bands and reduced mechanical dissipation during flexure represent reduced microstructure degradation and improved fatigue life. The rotary-based elastocaloric cooling prototype is shown to provide similar thermomechanical performance with the added benefit of discrete hot and cold zones, continuous cooling, inexpensive rotary actuation, and scalability, which represents a significant advancement for compact, long lifetime, and inexpensive elastocaloric cooling. MAIN TEXTImportant climate change legislation has been proposed in the United States, as well as Canada, Mexico, and the European Union, to phase out high global warming potential (GWP) Hydrofluorocarbon (HFC) refrigerants used in vapor-compression (VC) cooling [1]. Solid-state,

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