Observation of extremely long spin relaxation times in an organic nanowire spin valve

Organic semiconductors that are pi-conjugated are emerging as an important platform for 'spintronics', which purports to harness the spin degree of freedom of a charge carrier to store, process and/or communicate information. Here, we report the study of an organic nanowire spin valve device, 50 nm in diameter, consisting of a trilayer of ferromagnetic cobalt, an organic, Alq3, and ferromagnetic nickel. The measured spin relaxation time in the organic is found to be exceptionally long-between a few milliseconds and a second-and it is relatively temperature independent up to 100 K. Our experimental observations strongly suggest that the primary spin relaxation mechanism in the organic is the Elliott-Yafet mode, in which the spin relaxes whenever a carrier scatters and its velocity changes.     

Resonant spikes of the nuclear spin qubits relaxation rate in quantum-Hall systems with magnetic impurities

The magnetic field dependence of the local phononassisted nuclear spin qubit relaxation rate in 2DES with magnetic impurities is studied theoretically. For weak and strong scattering limits under a strong magnetic field, we show that the magnetic field dependence of T/sub 1//sup -1/ exhibits giant spikes, due to the energy matching of the electron Zeeman splitting with the energy spacing between the vibrational mode energies. The localized mode is created by the lattice distortion around the impurity. This resonance phenomenon could be used as a local probe for studying the localized vibrational modes and their coupling to electrons and for the resonant manipulation of the nuclear spin qubits.     

Growth of Ge-Si nanowire heterostructures via chemical vapor deposition

The growth of Ge-Si and Ge-Si nanowire (NW) heterostructures was demonstrated via chemical vapor deposition. Due to the influence of interface energy, differing topographies of the heterostructures were observed. On initially grown Ge NWs, numerous Si NW branches were grown near the tip due to Au migration. However, on initially grown Si NWs, high-density Ge nanodots were observed.     

Direct detection of hole gas in Ge-Si core-shell nanowires by enhanced Raman scattering

We report the direct detection of hole accumulation in the core of Ge-Si core-shell nanowire heterostructures by a Fano resonance between free holes and the F2g mode in Raman spectra. Raman enhancements of 10-10 000 with respect to bulk were observed and explained using finite difference time domain simulations of the electric fields concentrated in the nanowire. Numerical modeling of the radial carrier concentration revealed that the asymmetric line-shape is strongly influenced by inhomogeneous broadening.     

Spatial carrier confinement in core-shell and multishell nanowire heterostructures

We have derived an analytical effective-mass model and employed first-principles density functional theory to study the spatial confinement of carriers in core-shell and multishell structured semiconductor nanowires. The band offset effect is analyzed based on the subband charge density distributions, which is strongly dependent upon the strain relaxation. First-principles calculation results for spatially confined Si/Ge and GaN/GaP nanowires indicate accumulation of a Ge-core hole gas and a GaN-core electron gas, respectively, in agreement with experimental observations.     

Vertically aligned ZnO/amorphous-Si core-shell heterostructured nanowire arrays

We report the synthesis of vertically aligned ZnO/a-Si core-shell nanowire arrays (ZnO nanowires coated with amorphous silicon) through chemical vapor deposition. The core-shell heterostructured nanowires possessed uniform morphology and the thickness of the amorphous silicon shells could be controlled easily by tuning the deposition duration and temperature. The core-shell heterostructured nanowires exhibited enhanced antireflection and absorption performance as well as tunable PL properties. Because the individual ZnO/a-Si nanowires showed p-type characteristics and the ZnO cores were n-type semiconductors, the core-shell nanowires formed p-n junctions naturally.     

Focused ion beam fabrication of novel core-shell nanowire structures

A novel method of indirect deposition by means of a focused ion beam (FIB) is utilized to develop metal/insulator/semiconductor nanowire core-shell structures. This method is based upon depositing an annular pattern centered on a nanowire, with secondary deposition then coating the wire. Typical cross-sectional deposition area increments as a function of ion doses are 1.3 × 10(-2)?μm(2)?nC(-1) for Pt and 3.5 × 10(-2)?μm(2)?nC(-1) for SiO(2). The structures are examined with a transmission electron microscope (TEM) using a new nanowire TEM sample preparation method that allows direct examinations of individually selected core-shell nanowires fabricated under different indirect FIB deposition conditions. Elemental analyses by means of energy dispersive x-ray spectroscopy and electron energy filtered TEM imaging verify the deposition of SiO(2) and Pt layers. Relatively uniform Pt and SiO(2) coatings on individual GaP nanowires can be achieved with overall thickness deviation of about 10% for deposition up to 25-30?nm thick Pt or SiO(2) shells. It should be possible to extend this approach to any nanowire/nanotube system, and to a wide range of coatings in any desired layer sequences.     

Slow relaxation in spin glasses

The remarkable remanence effects observed in spin glasses are discussed. Some theoretical approaches and results are reviewed. Invited talk given at the Indian Academy of Sciences, Bangalore, Academy discussion meeting on phase transitions, June 21st, 1978.     

Enhancing the radiative rate in III-V semiconductor plasmonic core-shell nanowire resonators

We investigate the radiative properties of plasmonic core-shell nanowire resonators and, using boundary element method calculations, demonstrate enhanced radiative decay rate by up to 3500 times in nanoscale compound semiconductor/metal cavities. Calculation of the local density of optical states enables identification of new types of modes in cavities with mode volumes on the order of 10(-4)(λ/n)(3). These modes dramatically enhance the radiative decay rate and significantly modify the polarization of far-field emission.     

Role of confinement on carrier transport in Ge-Si(x)Ge(1-x) core-shell nanowires

We examine the impact of shell content and the associated hole confinement on carrier transport in Ge-Si(x)Ge(1-x) core-shell nanowires (NWs). Using NWs with different Si(x)Ge(1-x) shell compositions (x = 0.5 and 0.7), we fabricate NW field-effect transistors (FETs) with highly doped source/drain and examine their characteristics dependence on shell content. The results demonstrate a 2-fold higher mobility at room temperature, and a 3-fold higher mobility at 77K in the NW FETs with higher (x = 0.7) Si shell content by comparison to those with lower (x = 0.5) Si shell content. Moreover, the carrier mobility shows a stronger temperature dependence in Ge-Si(x)Ge(1-x) core-shell NWs with high Si content, indicating a reduced charge impurity scattering. The results establish that carrier confinement plays a key role in realizing high mobility core-shell NW FETs.     

Synthesis and strain relaxation of Ge-core/Si-shell nanowire arrays

Analogous to planar heteroepitaxy, misfit dislocation formation and stress-driven surface roughening can relax coherency strains in misfitting core-shell nanowires. The effects of coaxial dimensions on strain relaxation in aligned arrays of Ge-core/Si-shell nanowires are analyzed quantitatively by transmission electron microscopy and synchrotron X-ray diffraction. Relating these results to reported continuum elasticity models for coaxial nanowire heterostructures provides valuable insights into the observed interplay of roughening and dislocation-mediated strain relaxation.     

Nuclear spin lattice relaxation rate in the excitonic state

The nuclear spin lattice relaxation rate in the excitonic state is studied. Use is made of the two-band model, where the Fermi radii of the electron band and the hole band are assumed to be of the same size. It is important to distinguish two cases; the spin singlet case (i.e., the nonmagnetic excitonic state) and the spin triplet case (i.e., the antiferromagnetic state). In the case of the spin singlet excitonic state the nuclear spin lattice relaxation rate first increases in the excitonic state just below the transition temperature and then decreases rapidly as the temperature decreases. In the case of the spin triplet excitonic state the relaxation rate depends on whether the nuclear spin is polarized parallel or perpendicular to the spin-density wave describing the excitonic state. In the parallel case the relaxation rate decreases monotonically in the excitonic state as the temperature decreases, while in the transverse case it has a small peak just below the transition temperature.     

Ion-induced surface relaxation: controlled bending and alignment of nanowire arrays

It is a well-known fact that a sphere offers less surface area, and thus less surface energy, than any other arrangement of the same volume. From this perspective, all other shapes are metastable objects. In this paper, we present and discuss a manifestation of this metastability: the spontaneous alignment of free-standing amorphous nanowires towards, and ultimately parallel to, a flux of directional ion irradiation. The behavior expected from surface energy reduction is the opposite of that predicted by both theory and experiment regarding defect generation in crystalline nanowires, but is consistent with other observations on non-crystalline materials. We verify our expectations by bending and aligning finely stranded amorphous silica nanowires, noting that such nanostructures are particularly susceptible to bending through ion-induced surface energy reduction. We offer support for this mechanism through bending rate studies, thermal annealing experiments and mathematical modeling. Experimentally, we also demonstrate selective reorientation of nanowires in patterned areas, as well as conformal coating of reoriented arrays with functional materials. These capabilities offer the prospect of exploiting engineered surface anisotropies in optical, fluidic and micromechanical applications.     

Anomalous nuclear spin relaxation of adsorbed helium-3

Within the framework of a general theory of two-dimensional NMR we are able to elucidate certain anomalous features of spin relaxation in adsorbed and porous systems. We explain the linear dependence of relaxation time on applied magnetic field, and this is demonstrated to be related to the relative insensitivity ofT ₁to temperature. We also show thatT ₁remains field dependent on the “fast” side of theT ₁minimum.     

The growth and characterization of ZnO/ZnTe core-shell nanowires and the electrical properties of ZnO/ZnTe core-shell nanowire field effect transistor

Vertically aligned ZnO/ZnTe core-shell nanowires were grown on a-plane sapphire substrate by using chemical vapor deposition with gold as catalyst for the growth of ZnO core and then followed by growing ZnTe shell using metal-organic chemical vapor deposition (MOCVD). Transmission electron microscope (TEM) and Raman scattering indicate that the core-shell nanostructures have good crystalline quality. Three-dimensional fluorescence images obtained by using laser scanning confocal microscope demonstrate that the nanowires have good optical properties. The core-shell nanowire was then fabricated into single nanowire field effect transistor by standard e-beam photolithography. Electrical measurements reveals that the p-type ZnO/ZnTe FET device has a turn on voltage of -1.65 V and the hole mobility is 13.3 cm2/V s.     

Spin relaxation in the channel of a spin field-effect transistor

We examine the major spin relaxation mechanism (D'yakonov-Perel') in the channel of a spin field-effect transistor (SPINFET) and show analytically that it can be completely eliminated if the channel is a quantum wire and transport is strictly single moded (only the lowest subband is occupied). Single-moded transport also produces the largest "on" to "off" conductance ratio and the largest transconductance of the transistor.     

Retardation effects in the longitudinal spin relaxation in metals. II

The memory kernel of the longitudinal dynamical impurity susceptibility is calculated as a function of frequency, magnetic field, and temperature up to third order in the exchange coupling of thes-d model. It is shown that the strong retardation effects for the spin relaxation due to freezing out of the spin-flip scattering are modified by Kondo anomalies only slightly.Supported by the Deutscher Akademischer Austauschdienst.     

Nanocrystal shape and the mechanism of exciton spin relaxation

The rate of exciton spin relaxation (flips) between the bright exciton states (F = +/-1) of CdSe nanocrystals is reported as a function of shape, for dots and nanorods. The spin relaxation is measured using an ultrafast transient grating method with a crossed linearly polarization sequence. It is found that the spin relaxation rate depends on the radius, not length, of the nanocrystals. That observation is explained by deriving an expression for the electronic coupling matrix element that mixes the bright exciton states.     

Thermal variation of the spin relaxation rate in nearly magnetic metals

Considering either the nuclear spin resonance on the nucleus of a nearly magnetic atom or the electron spin resonance of a magnetic impurity in a nearly magnetic host, it is shown that taking into account an explicit temperature dependence of the Stoner susceptibility of the interacting electrons yields an important thermal decrease of 1/T ₁ T around the spin-fluctuation temperature. The physical case of the nuclear spin relaxation rate of platinum is discussed.