Influence of random roughness on cantilever curvature sensitivity

In this work we explore the influence of random surface roughness on the cantilever sensitivity to respond to curvature changes induced by changes in surface stress. The roughness is characterized by the out-of-plane roughness amplitude w, the lateral correlation length x, and the roughness or Hurst exponent H (0<H<1). The cantilever sensitivity is found to decrease with increasing roughness (decreasing H and/or increasing ratio w/x) or equivalently increasing local surface slope. Finally, analytic expressions of the cantilever sensitivity as a function of the parameters w, x, and H are derived in order to allow direct implementation in sensing systems.Comment: 10 pages, 3 figure

Stabilized hot electron bolometer heterodyne receiver at 2.5 THz

We report on a method to stabilize a hot electron bolometer (HEB) mixer at 2.5 THz. The technique utilizes feedback control of the local oscillator (LO) laser power by means of a swing-arm actuator placed in the optical beam path. We demonstrate that this technique yields a factor of 50 improvement in the spectroscopic Allan variance time which is shown to be over 30 s in a 12 MHz noise fluctuation bandwidth. Furthermore, broadband signal direct detection effects may be minimized by this technique. The technique is versatile and can be applied to practically any local oscillator at any frequency

Heterodyne Receiver Development at the Caltech Submillimeter Observatory

The Caltech Submillimeter Observatory (CSO) operates at the summit of Mauna Kea, Hawaii, at an elevation of 4200 m. The site was chosen for its very dry climate and stable atmosphere, enabling submillimeter observations in the astrophysically important 1.3 mm to 300 μm atmospheric windows. Ever since its inception, the CSO has proven itself to be a productive test-bed for new detector technologies. In this paper we review the heterodyne (coherent) receiver development at the CSO, and highlight some of the ways it has helped to shape the field of submillimeter and terahertz high spectral resolution far-infrared astronomy

Cluster evolution in a cold Ising ferromagnet:Disappearance of magic numbers with temperature rise

Decay and growth of clusters at low and intermediate temperatures based on the two-dimensional square-lattice Ising model has been studied with Monte Carlo simulations employing Glauber (or Metropolis) dynamics, exploiting a procedure where the starting configuration is a cluster (that tend to grow in the applied magnetic field) on a relatively small lattice. The behavior of such a cluster is stochastic and only when typical several thousands of identical clusters are analyzed will the underlying deterministic behavior become apparent. At 0.4Tc, the time-dependent cluster size distribution is relatively broad, but smooth, i.e., Gaussian, and the decay and growth behaviors of various relative compact clusters are compared. At lower temperatures, modulations in the size distribution occur with minima at magic sizes corresponding to n=m×m+1 and n=m×(m+1)+1 with m integer values. A quantitative analysis of the amplitude of the modulations as a function of temperature is performed. Also the relation between the distributions of size and of the number of internal cluster bonds (or cluster perimeter) is scrutinized.

Resolving hydrogen atoms at metal-metal hydride interfaces

Hydrogen as a fuel can be stored safely with high volumetric density in metals. It can, however, also be detrimental to metals causing embrittlement. Understanding fundamental behavior of hydrogen at atomic scale is key to improve the properties of metal-metal hydride systems. However, currently, there is no robust technique capable of visualizing hydrogen atoms. Here, we demonstrate that hydrogen atoms can be imaged unprecedentedly with integrated differential phase contrast, a recently developed technique performed in a scanning transmission electron microscope. Images of the titanium-titanium monohydride interface reveal remarkable stability of the hydride phase, originating from the interplay between compressive stress and interfacial coherence. We also uncovered, thirty years after three models were proposed, which one describes the position of the hydrogen atoms with respect to the interface. Our work enables novel research on hydrides and is extendable to all materials containing light and heavy elements, including oxides, nitrides, carbides and borides

IF impedance and mixer gain of NbN hot electron bolometers

The intermediate frequency (IF) characteristics, the frequency dependent IF impedance, and the mixer conversion gain of a small area hot electron bolometer (HEB) have been measured and modeled. The device used is a twin slot antenna coupled NbN HEB mixer with a bridge area of 1×0.15 µm^2, and a critical temperature of 8.3 K. In the experiment the local oscillator frequency was 1.300 THz, and the (IF) 0.05–10 GHz. We find that the measured data can be described in a self-consistent manner with a thin film model presented by Nebosis et al. [Proceedings of the Seventh International Symposium on Space Terahertz Technology, Charlottesville, VA, 1996 (unpublished), pp. 601–613], that is based on the two temperature electron-phonon heat balance equations of Perrin-Vanneste [J. Phys. (Paris) 48, 1311 (1987)]. From these results the thermal time constant, governing the gain bandwidth of HEB mixers, is observed to be a function of the electron-phonon scattering time, phonon escape time, and the electron temperature. From the developed theory the maximum predicted gain bandwidth for a NbN HEB is found to be 5.5–6 GHz. In contrast, the gain bandwidth of the device under discussion was measured to be ~2.3 GHz which, consistent with the outlined theory, is attributed to a somewhat low critical temperature and nonoptimal film thickness (6 nm)

Chalcogenides by Design:Functionality through Metavalent Bonding and Confinement

A unified picture of different application areas for incipient metals is presented. This unconventional material class includes several main‐group chalcogenides, such as GeTe, PbTe, Sb2Te3, Bi2Se3, AgSbTe2 and Ge2Sb2Te5. These compounds and related materials show a unique portfolio of physical properties. A novel map is discussed, which helps to explain these properties and separates the different fundamental bonding mechanisms (e.g., ionic, metallic, and covalent). The map also provides evidence for an unconventional, new bonding mechanism, coined metavalent bonding (MVB). Incipient metals, employing this bonding mechanism, also show a special bond breaking mechanism. MVB differs considerably from resonant bonding encountered in benzene or graphite. The concept of MVB is employed to explain the unique properties of materials utilizing it. Then, the link is made from fundamental insights to application‐relevant properties, crucial for the use of these materials as thermoelectrics, phase change materials, topological insulators or as active photonic components. The close relationship of the materials' properties and their application potential provides optimization schemes for different applications. Finally, evidence will be presented that for metavalently bonded materials interesting effects arise in reduced dimensions. In particular, the consequences for the crystallization kinetics of thin films and nanoparticles will be discussed in detail

High Resolution Imaging of Chalcogenide Superlattices for Data Storage Applications:Progress and Prospects

Phase-change materials (PCMs) based on Ge–Sb–Te alloys are a strong contender for next-generation memory technology. Recently, PCMs in the form of GeTe–Sb 2 Te 3 superlattices (CSLs) have shown superior performance compared to ordinary PCM memory, which relies on switching between amorphous and crystalline phases. Although detailed atomic structure switching models have been developed with the help of ab-initio simulations, there is still fierce scientific debate concerning the experimental verification of the actual crystal structures pertaining to the two CSL memory states. One of the strongest techniques to provide this information is (scanning) transmission electron microscopy ((S)TEM). The present article reviews the analyses of CSLs using TEM-based techniques published during the last seven years since the seminal 2011 Nature Nanotechnology paper of Simpson et al., showing the superior performance of the CSL memory. It is critically reviewed what relevant information can be extracted from the (S)TEM results, also showing the impressive progress that has been achieved in a relatively short time frame. Finally, an outlook is given including several open questions. Although debate on actual switching mechanism in CSL memory is clearly not settled, still there is consensus in this field that CSL research has a bright future