Magnetic effects in sulfur-decorated graphene.

The interaction between two different materials can present novel phenomena that are quite different from the physical properties observed when each material stands alone. Strong electronic correlations, such as magnetism and superconductivity, can be produced as the result of enhanced Coulomb interactions between electrons. Two-dimensional materials are powerful candidates to search for the novel phenomena because of the easiness of arranging them and modifying their properties accordingly. In this work, we report magnetic effects in graphene, a prototypical non-magnetic two-dimensional semi-metal, in the proximity with sulfur, a diamagnetic insulator. In contrast to the well-defined metallic behaviour of clean graphene, an energy gap develops at the Fermi energy for the graphene/sulfur compound with decreasing temperature. This is accompanied by a steep increase of the resistance, a sign change of the slope in the magneto-resistance between high and low fields, and magnetic hysteresis. A possible origin of the observed electronic and magnetic responses is discussed in terms of the onset of low-temperature magnetic ordering. These results provide intriguing insights on the search for novel quantum phases in graphene-based compounds.

Electron Spin Coherence of Shallow Donors in Natural and Isotopically Enriched Germanium.

Germanium is a widely used material for electronic and optoelectronic devices and recently it has become an important material for spintronics and quantum computing applications. Donor spins in silicon have been shown to support very long coherence times (T_{2}) when the host material is isotopically enriched to remove any magnetic nuclei. Germanium also has nonmagnetic isotopes so it is expected to support long T_{2}'s while offering some new properties. Compared to Si, Ge has a strong spin-orbit coupling, large electron wave function, high mobility, and highly anisotropic conduction band valleys which will all give rise to new physics. In this Letter, the first pulsed electron spin resonance measurements of T_{2} and the spin-lattice relaxation (T_{1}) times for ^{75}As and ^{31}P donors in natural and isotopically enriched germanium are presented. We compare samples with various levels of isotopic enrichment and find that spectral diffusion due to ^{73}Ge nuclear spins limits the coherence in samples with significant amounts of ^{73}Ge. For the most highly enriched samples, we find that T_{1} limits T_{2} to T_{2}=2T_{1}. We report an anisotropy in T_{1} and the ensemble linewidths for magnetic fields oriented along different crystal axes but do not resolve any angular dependence to the spectral-diffusion-limited T_{2} in samples with ^{73}Ge.

Embedded binary eutectic alloy nanostructures: a new class of phase change materials.

Phase change materials are essential to a number of technologies ranging from optical data storage to energy storage and transport applications. This widespread interest has given rise to a substantial effort to develop bulk phase change materials well suited for desired applications. Here, we suggest a novel and complementary approach, the use of binary eutectic alloy nanoparticles embedded within a matrix. Using GeSn nanoparticles embedded in silica as an example, we establish that the presence of a nanoparticle/matrix interface enables one to stabilize both nanobicrystal and homogeneous alloy morphologies. Further, the kinetics of switching between the two morphologies can be tuned simply by altering the composition.

Spin susceptibility of noncentrosymmetric heavy-fermion superconductor CeIrSi3 under pressure: 29Si Knight-shift study on single crystal.

We report 29Si NMR study on a single crystal of the heavy-fermion superconductor CeIrSi3 without an inversion symmetry along the c axis. The 29Si Knight-shift measurements under pressure have revealed that the spin susceptibility for the ab plane decreases slightly below T(c), whereas along the c axis it does not change at all. The result can be accounted for by the spin susceptibility in the superconducting state being dominated by the strong antisymmetric (Rashba-type) spin-orbit interaction that originates from the absence of an inversion center along the c axis and it being much larger than superconducting condensation energy. This is the first observation which exhibits an anisotropy of the spin susceptibility below T(c) in the noncentrosymmetric superconductor dominated by strong Rashba-type spin-orbit interaction.

Black Ge based on crystalline/amorphous core/shell nanoneedle arrays.

Direct growth of black Ge on low-temperature substrates, including plastics and rubber is reported. The material is based on highly dense, crystalline/amorphous core/shell Ge nanoneedle arrays with ultrasharp tips ( approximately 4 nm) enabled by the Ni catalyzed vapor-solid-solid growth process. Ge nanoneedle arrays exhibit remarkable optical properties. Specifically, minimal optical reflectance (<1%) is observed, even for high angles of incidence ( approximately 75 degrees ) and for relatively short nanoneedle lengths ( approximately 1 mum). Furthermore, the material exhibits high optical absorption efficiency with an effective band gap of approximately 1 eV. The reported black Ge could potentially have important practical implications for efficient photovoltaic and photodetector applications on nonconventional substrates.

Nanoscale structural engineering via phase segregation: Au-Ge system.

A tunable structural engineering of nanowires based on template-assisted alloying and phase segregation processes is demonstrated. The Au-Ge system, which has a low eutectic temperature and negligible solid solubility (<10(-3) atom %) of Au in Ge at low temperatures, is utilized. Depending on the Au concentration of the initial nanowires, final structures ranging from nearly periodic nanodisk patterns to core/shell and fully alloyed nanowires are produced. The formation mechanisms are discussed in detail and characterized by in situ transmission electron microscopy and energy-dispersive spectrometry analyses. Electrical measurements illustrate the metallic and semiconducting characteristics of the fully alloyed and alternating Au/Ge nanodisk structures, respectively.

Theory of nanocluster size distributions from ion beam synthesis.

Ion beam synthesis of nanoclusters is studied via both kinetic Monte Carlo simulations and the self-consistent mean-field solution to a set of coupled rate equations. Both approaches predict the existence of a steady-state shape for the cluster-size distribution that depends only on a characteristic length determined by the effective diffusion coefficient, the ion solubility, and the volumetric ion flux. The average cluster size in the steady-state regime is determined by the implanted species or matrix interface energy.

Interstitial-mediated diffusion in germanium under proton irradiation.

We report experiments on the impact of 2.5 MeV proton irradiation on self-diffusion and dopant diffusion in germanium (Ge). Self-diffusion under irradiation reveals an unusual depth independent broadening of the Ge isotope multilayer structure. This behavior and the observed enhanced diffusion of B and retarded diffusion of P demonstrates that an interstitial-mediated diffusion process dominates in Ge under irradiation. This fundamental finding opens up unique ways to suppress vacancy-mediated diffusion in Ge and to solve the donor deactivation problem that hinders the fabrication of Ge-based nanoelectronic devices.

Large melting-point hysteresis of Ge nanocrystals embedded in SiO2.

The melting behavior of Ge nanocrystals embedded within SiO2 is evaluated using in situ transmission electron microscopy. The observed melting-point hysteresis is large (+/-17%) and nearly symmetric about the bulk melting point. This hysteresis is modeled successfully using classical nucleation theory without the need to invoke epitaxy.

Optical detection and ionization of donors in specific electronic and nuclear spin States.

We resolve the remarkably sharp bound exciton transitions of highly enriched 28Si using a single-frequency laser and photoluminescence excitation spectroscopy, as well as photocurrent spectroscopy. Well-resolved doublets in the spectrum of the 31P donor reflect the hyperfine coupling of the electronic and nuclear donor spins. The optical detection of the nuclear spin state, and selective pumping and ionization of donors in specific electronic and nuclear spin states, suggests a number of new possibilities which could be useful for the realization of silicon-based quantum computers.

Evidence for p-type doping of InN.

The first evidence of successful p-type doping of InN is presented. It is shown that InN:Mg films consist of a p-type bulk region with a thin n-type inversion layer at the surface that prevents electrical contact to the bulk. Capacitance-voltage measurements indicate a net concentration of ionized acceptors below the -type surface. Irradiation with 2 MeV He+ ions is used to convert the bulk of InN:Mg from p to n-type, at which point photoluminescence is recovered. The conversion is well explained by a model assuming two parallel conducting layers (the surface and the bulk) in the films.

New limit on the neutrinoless betabeta decay of 130Te.

We report the present results of CUORICINO, a search for neutrinoless double-beta (0nu betabeta) decay of 130Te. The detector is an array of 62 TeO2 bolometers with a total active mass of 40.7 kg. The array is cooled by a dilution refrigerator shielded from environmental radioactivity and energetic neutrons, operated at approximately 8 mK in the Gran Sasso Underground Laboratory. No evidence for (0nu betabeta) decay was found and a new lower limit, T(1/2)(0nu) > or = 1.8 x 10(24) yr (90% C.L.) is set, corresponding to [m(nu)] < or = 0.2 to 1.1 eV, depending on the theoretical nuclear matrix elements used in the analysis.

Radiation enhanced silicon self-diffusion and the silicon vacancy at high temperatures.

We report proton radiation enhanced self-diffusion (RESD) studies on Si-isotope heterostructures. Self-diffusion experiments under irradiation were performed at temperatures between 780 degrees C and 872 degrees C for various times and proton fluxes. Detailed modeling of RESD provides direct evidence that vacancies at high temperatures diffuse with a migration enthalpy of H(m)(V)=(1.8+/-0.5) eV significantly more slowly than expected from their diffusion at low temperatures, which is described by H(m)(V)<0.5 eV. We conclude that this diffusion behavior is a consequence of the microscopic configuration of the vacancy whose entropy and enthalpy of migration increase with increasing temperature.

Analysis of broadband x-ray spectra of highly charged krypton from a microcalorimeter detector of an electron-beam ion trap.

Spectra of highly charged Kr ions, produced in an electron-beam ion trap (EBIT), have been recorded in a broad x-ray energy band (0.3 keV to 4 keV) with a microcalorimeter detector. Most of the spectral lines have been identified as transitions of B- to Al-like Kr. The transition energies have been determined with 0.2% uncertainty. A semi-empirical EBIT plasma model has been created to calculate a synthetic spectrum of highly charged Kr and to determine a charge state distribution of Kr ions inside the EBIT.

Large disparity between gallium and antimony self-diffusion in gallium antimonide.

The most fundamental mass transport process in solids is self-diffusion. The motion of host-lattice ('self-') atoms in solids is mediated by point defects such as vacancies or interstitial atoms, whose formation and migration enthalpies determine the kinetics of this thermally activated process. Self-diffusion studies also contribute to the understanding of the diffusion of impurities, and a quantitative understanding of self- and foreign-atom diffusion in semiconductors is central to the development of advanced electronic devices. In the past few years, self-diffusion studies have been performed successfully with isotopically controlled semiconductor heterostructures of germanium, silicon, gallium arsenide and gallium phosphide. Self-diffusion studies with isotopically controlled GaAs and GaP have been restricted to Ga self-diffusion, as only Ga has two stable isotopes, 69Ga and 71Ga. Here we report self-diffusion studies with an isotopically controlled multilayer structure of crystalline GaSb. Two stable isotopes exist for both Ga and Sb, allowing the simultaneous study of diffusion on both sublattices. Our experiments show that near the melting temperature, Ga diffuses more rapidly than Sb by over three orders of magnitude. This surprisingly large difference in atomic mobility requires a physical explanation going beyond standard diffusion models. Combining our data for Ga and Sb diffusion with related results for foreign-atom diffusion in GaSb (refs 8, 9), we conclude that the unusually slow Sb diffusion in GaSb is a consequence of reactions between defects on the Ga and Sb sublattices, which suppress the defects that are required for Sb diffusion.

Exclusion limits on the WIMP-nucleon cross section from the cryogenic dark matter search.

The Cryogenic Dark Matter Search (CDMS) employs Ge and Si detectors to search for weakly interacting massive particles (WIMPs) via their elastic-scattering interactions with nuclei while discriminating against interactions of background particles. CDMS data, accounting for the neutron background, give limits on the spin-independent WIMP-nucleon elastic-scattering cross section that exclude unexplored parameter space above 10 GeV/c2 WIMP mass and, at >75% C.L., the entire 3sigma allowed region for the WIMP signal reported by the DAMA experiment.

Germanium:gallium photoconductors for far infrared heterodyne detection.

Highly compensated Ge:Ga photoconductors have been fabricated and evaluated for high bandwidth heterodyne detection. Bandwidths up to 60 MHz have been obtained with corresponding current responsivity of 0.01 A/W.

Stressed photoconductive detector for far-infrared space applications.

An optimized leaf-spring apparatus for applying uniaxial stress to a Ge:Ga far-infrared photoconductor has been designed and tested. This design has significant advantages for space applications which require high quantum efficiency and stable operation over long periods of time. The important features include adequate spring deflection with relatively small overall size, torque-free stress, easy measurement of applied stress, and a detector configuration with high responsivity. One-dimensional arrays of stressed photoconductors can be constructed using this design. A peak responsivity of 38 A/W is achieved in a detector with a cutoff wavelength of 200 microm, which was operated at a temperature of 2.0 K and a bias voltage equal to one half of the breakdown voltage.