Physical Review Letters

Popov, Igor ; Seifert, Gotthard ; Tománek, David

doi: 10.1103/PhysRevLett.108.156802pmid: 22587274

Studying the reason why single-layer molybdenum disulfide ( MoS 2 ) appears to fall short of its promising potential in flexible nanoelectronics, we find that the nature of contacts plays a more important role than the semiconductor itself. In order to understand the nature of MoS 2 / metal contacts, we perform ab initio density functional theory calculations for the geometry, bonding, and electronic structure of the contact region. We find that the most common contact metal (Au) is rather inefficient for electron injection into single-layer MoS 2 and propose Ti as a representative example of suitable alternative electrode materials.

Horn, Rolf ; Abolghasem, Payam ; Bijlani, Bhavin J; Kang, Dongpeng J; Helmy, A. S; Weihs, Gregor S

doi: 10.1103/PhysRevLett.108.153605pmid: 22587254

The creation of monolithically integratable sources of single and entangled photons is a top research priority with formidable challenges: The production, manipulation, and measurement of the photons should all occur in the same material platform, thereby fostering stability and scalability. Here we demonstrate efficient photon pair production in a semiconductor platform, gallium arsenide. Our results show type-I spontaneous parametric down-conversion of laser light from a 2.2 mm long Bragg-reflection waveguide, and we estimate its internal pair production efficiency to be 2.0 × 10 - 8 (pairs/pump photon). This is the first time that significant pair production has been demonstrated in a structure that can be electrically self-pumped and which can form the basis for passive optical circuitry, bringing us markedly closer to complete integration of quantum optical technologies.

Aad, G; Abbott, B; Abdallah, J; Abdelalim, A. A; Abdesselam, A. A; Abdinov, O. A; Abi, B. A; Abolins, M. A; AbouZeid, O. S; Abramowicz, H. S; Abreu, H. S; Acerbi, E. S; Acharya, B. S; Adamczyk, L. S; Adams, D. L; Addy, T. N;

Taylor, Edward ; Kallin, Catherine

doi: 10.1103/PhysRevLett.108.157001pmid: 22587276

We identify an intrinsic Hall effect in multiband chiral superconductors in the absence of a magnetic field (i.e., an anomalous Hall effect). This effect arises from interband transitions involving time-reversal symmetry-breaking chiral Cooper pairs. We discuss the implications of this effect for the putative chiral p -wave superconductor, Sr 2 RuO 4 , and show that it can contribute significantly to Kerr rotation experiments. Since the magnitude of the effect depends on the structure of the order parameter across the bands, this result may be used to distinguish between different models proposed for the superconducting state of Sr 2 RuO 4 .

Ovadyahu, Z

doi: 10.1103/PhysRevLett.108.156602pmid: 22587271

We report on measurements of absorption from applied ac fields in Anderson-localized indium-oxide films. The absorption shows a roll-off at a frequency that is much smaller than the electron-electron scattering rate measured at the same temperature in diffusive samples of this material. These results are interpreted as evidence for discreteness of the energy spectrum.

Mosca Conte, A; Pulci, O; Knapik, A; Bagniuk, J; Del Sole, R; Lojewska, J; Missori, M

doi: 10.1103/PhysRevLett.108.158301pmid: 22587292

The yellowing of paper on aging causes major aesthetic damages of cultural heritage. It is due to cellulose oxidation, a complex process with many possible products still to be clarified. By comparing ultraviolet-visible reflectance spectra of ancient and artificially aged modern papers with ab initio time-dependent density functional theory calculations, we identify and estimate the abundance of oxidized functional groups acting as chromophores and responsible of paper yellowing. This knowledge can be used to set up strategies and selective chemical treatments preventing paper yellowing.

Tsai, Shirley C; Lin, Shih K; Mao, Rong W; Tsai, Chen S

doi: 10.1103/PhysRevLett.108.154501pmid: 22587258

This Letter reports the first observation and theoretical analysis of a new phenomenon: one large spherical water drop ejecting simultaneously a very large number of monodisperse microdroplets. An ultrasonic nozzle with multiple-Fourier horns in resonance enables controlled excitation of megahertz Faraday waves on the free water surface. The temporal instability of such waves leads to the ejection of 3.5 – 4.4 μ m monodisperse droplets at a high rate ( <?format ?> > 4.0 × 10 7 droplets / sec ). This is in stark contrast to the Rayleigh-Plateau instability, which ejects one droplet at a time.

Gamba, Andrea ; Nicodemi, Mario ; Soriano, Jordi ; Ott, Albrecht

doi: 10.1103/PhysRevLett.108.158103pmid: 22587289

The formation of a hollow cellular sphere is often one of the first steps of multicellular embryonic development. In the case of Hydra , the sphere breaks its initial symmetry to form a foot-head axis. During this process a gene, k s 1 , is increasingly expressed in localized cell domains whose size distribution becomes scale-free at the axis-locking moment. We show that a physical model based solely on the production and exchange of k s 1 -promoting factors among neighboring cells robustly reproduces the scaling behavior as well as the experimentally observed spontaneous and temperature-directed symmetry breaking.

King, John T; Ross, Matthew R; Kubarych, Kevin J

doi: 10.1103/PhysRevLett.108.157401pmid: 22587280

Ultrafast two-dimensional infrared (2D-IR) spectroscopy is used to study the picosecond dynamics of a vibrational probe molecule dissolved in a fragile glass former. The spectral dynamics are observed as the system is cooled to within a few degrees of the glass transition temperature ( T g ). We observe nonexponential relaxation of the frequency-frequency correlation function, similar to what has been reported for other dynamical correlation functions. In addition, we see evidence for α -like relaxation, typically associated with long-time, cooperative molecular motion, on the ultrafast time scale. The data suggests that the spectral dynamics are sensitive to cooperative motion occurring on time scales that are necessarily longer than the observation time.

Bulgac, Aurel ; Luo, Yuan-Lung ; Roche, Kenneth J

doi: 10.1103/PhysRevLett.108.150401pmid: 22587233

We show that in the collision of two superfluid fermionic atomic clouds one observes the formation of quantum shock waves as discontinuities in the number density and collective flow velocity. Domain walls, which are topological excitations of the superfluid order parameter, are also generated and exhibit abrupt phase changes by π and slower motion than the shock waves. The domain walls are distinct from the gray soliton train or number density ripples formed in the wake of the shock waves and observed in the collisions of superfluid bosonic atomic clouds. Domain walls with opposite phase jumps appear to collide elastically.