If the fuel cell is to become the modern steam engine, basic research must provide breakthroughs in understanding, materials, and design to make a hydrogen-based energy system a vibrant and competitive force

If solar energy is to become a practical alternative to fossil fuels, we must have efficient ways to convert photons into electricity, fuel, and heat. The need for better conversion technologies is a driving force behind many recent developments in biology, materials, and especially nanoscience.

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The study of the anisotropic superconductor MgB2 using a combination of scanning tunneling microscopy and spectroscopy reveals two distinct energy gaps at ∆1=2.3 meV and ∆2=7.1 meV. Different spectral weights of the partial superconducting density of states (PDOS) are a reflection of different tunneling directions in this multi-band system. Our experimental observations are consistent with the existence of two-band superconductivity in the presence of interband superconducting pair interaction and quasiparticle scattering. Temperature evolution of the tunneling spectra follows the BCS scenario [1] with both gaps vanishing at the bulk Tc. Indeed, the study of tunneling junctions exhibiting only the small gap (c-axis tunneling) clearly and reproducibly show that this gap persists up to the bulk Tc. The data confirm the importance of Fermi-surface sheet dependent superconductivity in MgB2 proposed in the multigap model by Liu et al. [2] .The discovery of superconductivity in MgB 2 [3] at 39K sparked great interest in the fundamental physics and practical applications of this material. There has already been rapid progress in understanding the physical properties of this superconductor. Specific heat measurements [4,5] show that MgB 2 is an s-wave superconductor and the presence of the isotope effect [6,7] points towards phonon-mediated pairing. Tunneling and photoemission spectroscopy directly measures the superconducting energy gap and can provide further understanding of the origin of the superconductivity in this material. Earlier tunneling spectroscopy measurements show a large spread in the gap values [8][9][10] each consistent with the BCS form. More recent experiments, including STM tunneling spectroscopy [11], point-contact spectroscopy [12,13], specific heat measurements [4,5], and Raman spectroscopy [14] point towards the existence of two distinct gaps. This scenario has been predicted theoretically by Liu et al. [2]. First principle calculations show that the Fermi surface of MgB 2 consists of 2D cylindrical sheets arising from σ antibonding states of B p xy orbitals, and 3D tubular networks arising from π bonding and antibonding states of B p z orbitals. In this theoretical framework [2] two different energy gaps exist, the smaller one being an induced gap associated with the 3D bands and the larger one associated with the superconducting 2D bands. Furthermore both superconducting gaps should vanish at the bulk critical temperature T c . Due to this highly anisotropic band structure the superconducting gaps should be momentum-dependent reflecting the strength of the electron-phonon coupling of the carriers in the different bands. Up to now there has been no direct experimental evidence of the orientation dependence of the order parameter in this material. Moreover, the temperature dependence of the two gaps would give further insights into the nature of superconductivity in MgB 2 . Scanning tunneling spectroscopy is a unique technique that allows direct measure of the DOS near the Fermi energy with high...

Resistive-type palladium structures for hydrogen sensing remains as a research focus for their simplicity in device construction. We demonstrate that a siloxane self-assembled monolayer placed between a substrate and an evaporated ultrathin Pd film promotes the formation of small Pd nanoclusters and reduces the stiction between the palladium and the substrate. The resulting Pd nanocluster film can detect 2 % H 2 with a rapid response time of ϳ70 ms and is sensitive to 25 ppm hydrogen, detectable by a 2% increase in conductance due to the hydrogen-induced palladium lattice expansion.