Nuclear spin relaxation in strongly disordered superconductors

The influence of nonmagnetic impurity and spin-orbit scattering on the nuclear spin-lattice relaxation rate in strongly disordered superconductors is presented. Using Anderson's exact-eigenstate formalism, it is shown that there exist two effects of disorder onT ₁ ^–1 . Firstly, nonmagnetic impurity and spin-orbit scattering enhances the magnitude of the relaxation rate in the same manner as in the normal dirty metal due to the diffusive nature of quasiparticle motion. Secondly, the Hebel-Slichter peak becomes suppressed due to the disorder enhancement of the quasiparticle inelastic scattering rate due to phonon, Coulomb, and/or spin-fluctuation interactions. Comparison with the available experimental data is made.     

Anomalous magnetotransport in chemically doped carbon nanotubes

We report on anomalous magnetotransport features in chemically doped, weakly disordered carbon nanotubes. Under the application of a magnetic field parallel to the tube axis, hole conduction is shown to be strongly affected by impurity scattering with short mean free path and negative magnetoresistance, strongly different from electron conduction with much longer mean free path and positive magnetoresistance behavior.     

The electrical property dependence of disordered copper oxide on oxygen content

We have deposited disordered copper oxide films on glass substrates, with varying oxygen to copper ratios, by the reactive sputtering method. The variation in the ratio is enough to take the system from having disordered metallic like conduction at low oxygen content to hopping like behaviour at higher oxygen content. The hopping like behaviour is described by an unusual band structure, where Mott-variable range hopping is seen at lower temperatures and electron-electron variable range hopping at higher temperatures in some samples. The metallic region shows the expected effects of quantum corrections to the conductivity, with the often seen influence of spin-orbit scattering in copper. The transition between the hopping and metallic state is of the percolation type, where the conduction path between Cu islands is broken as the oxygen content increases.     

Spin relaxation in the presence of electron-electron interactions

The D'yakonov-Perel' spin relaxation induced by the spin-orbit interaction is examined in disordered two-dimensional electron gas. It is shown that, because of the electron-electron interactions, substantially different spin relaxation rates may be observed depending on the technique used to extract them. It is demonstrated that the relaxation rate of a spin population is proportional to the spin-diffusion constant D(s), while the spin-orbit scattering rate controlling the weak-localization corrections is proportional to the diffusion constant D, i.e., the conductivity. The two diffusion constants get strongly renormalized by the electron-electron interactions, but in different ways. As a result, the corresponding relaxation rates are different, with the difference between the two being especially strong near a magnetic instability or near the metal-insulator transition.     

Dephasing by extremely dilute magnetic impurities revealed by Aharonov-Bohm oscillations

We have probed the magnetic field dependence of the electron phase coherence time tau(phi) by measuring the Aharonov-Bohm conductance oscillations of mesoscopic Cu rings. Whereas tau(phi) determined from the low-field magnetoresistance saturates below 1 K, the amplitude of Aharonov-Bohm h/e oscillations increases strongly on a magnetic field scale proportional to the temperature. This provides strong evidence that a likely explanation for the frequently observed saturation of tau(phi) at low temperature in weakly disordered metallic thin films is the presence of extremely dilute magnetic impurities.     

Numerical studies on the Anderson localization problem

The localization properties of three different gauge invariant disordered electronic systems are studied by numerical methods with the purpose to clarify their localization properties and to evaluate the dc-resistivity atT=0 K as far as possible. The three different models, two of which involve also spin-dependent scattering processes, represent three different universality classes, corresponding to orthogonal, unitary, and symplectic matrix ensembles, respectively, in a field-theoretic representation. For the symplectic case, which corresponds to a situation with dominating spin-orbit scattering, we find hints for an unconventional transition, separating weakly antilocalized from exponentially localized states, whereas in the other two cases instead of a sharp transition only a drastic crossover between weak localization and exponential localization seems to happen. For the symplectic model also the magnetoresistivity is calculated; we find a negative magnetoresistivity if the Zeeman splitting is neglected, whereas by inclusion of Zeeman splitting the magnetoresistivity is positive.     

Spin—Dependent Scattering Effects and Dimensional Crossover in a Quasi—Two—Dimensional Disordered Electron System

Two kinds of spin-dependent scattering effects (magnetic-impurity and spin-orbit scatterings) are investigated theoretically in a quasi-tow-dimensional (quasi-2D) disordered electron system.By making use of the diagrammatic techniques in perturbation theory,we have calculated the dc conductivity and magnetoresistance due to weak-localization effects,the analytical expressions of them are obtained as functions of the interlayer hopping energy and the characteristic times:elastic,inelastic,magnetic and spin-orbit scattering times.The relevant dimensional crossover behavior from 3D to 2D with decreasing the interlayer coupling is discussed,and the condition for the crossover is shown to be dependent on the aforementioned scattering times.At low temperature there exists a spin-dependent-scattering-induced dimensional crossover in this system.     

金纳米粒子的二维组装与表面增强拉曼散射研究

金纳米粒子的二维组装与表面增强拉曼散射研究朱涛王健符小艺张续刘忠范（北京大学化学与分子工程学院智能材料研究中心北京１００８７１）Ｔｗｏ┐ＤｉｍｅｎｓｉｏｎａｌＧｏｌｄＮａｎｏｐａｒｔｉｃｌｅＡｓｅｍｂｌｉｅｓａｓＡｃｔｉｖｅＳｕｂｓｔｒａｔｅｓｆｏ...     

Two-dimensional electrons in the presence of spin-orbit scattering by impurities

A two-dimensional disordered system of interacting electrons when the principal source of spin relaxation is the spin-orbit scattering by impurities, is studied. It is shown that in this system conductivity has a very complicated behaviour since there is a focus on the phase plane of the renormalization group equations.     

Inelastic electron backscattering in a generic helical edge channel

We evaluate the low-temperature conductance of a weakly interacting one-dimensional helical liquid without axial spin symmetry. The lack of that symmetry allows for inelastic backscattering of a single electron, accompanied by forward scattering of another. This joint effect of weak interactions and potential scattering off impurities results in a temperature-dependent deviation from the quantized conductance, δG ∝ T4. In addition, δG is sensitive to the position of the Fermi level. We determine numerically the parameters entering our generic model for the Bernevig-Hughes-Zhang Hamiltonian of a HgTe/CdTe quantum well in the presence of Rashba spin-orbit coupling.     

Metal to insulator transition in films of molecularly linked gold nanoparticles

We report a metal to insulator transition (MIT) in disordered films of molecularly linked gold nanoparticles (NPs). As the number of carbons (n) of alkanedithiol linker molecules (C(n)S2) is varied, resistance (R) at low temperature (T = 2 K) and at 200 K, as well as trends in R vs T data at intermediate temperatures, all point to an MIT occurring at n = 5. We describe these results in a context of a Mott-Hubbard MIT. We find that all insulating samples (n > or = 5) exhibit a universal scaling behavior R approximately exp[(T0/T)nu] with nu = 0.65, and all metallic samples (n < or = 5) exhibit weaker R-T dependencies than bulk gold. We discuss these observations in terms of competitive thermally activated processes and strong, T-independent elastic scattering, respectively.     

Effect of spin-orbit scattering on the magnetic and superconducting properties of nearly ferromagnetic metals: application to granular Pt

We calculate the effect of scattering on the static, exchange enhanced, spin susceptibility and show that, in particular, spin-orbit scattering leads to a reduction of the giant moments and spin glass freezing temperature due to dilute magnetic impurities. The harmful spin fluctuation contribution to the intragrain pairing interaction is strongly reduced opening the way for BCS superconductivity. We are thus able to explain the superconducting and magnetic properties recently observed in granular Pt as being due to scattering effects in single small grains.     

Tunneling anisotropic magnetoresistance driven by resonant surface states: first-principles calculations on an Fe(001) surface

Fully relativistic first-principles calculations of the Fe(001) surface demonstrate that resonant surface (interface) states may produce sizable tunneling anisotropic magnetoresistance in magnetic tunnel junctions with a single magnetic electrode. The effect is driven by the spin-orbit coupling. It shifts the resonant surface band via the Rashba effect when the magnetization direction changes. We find that spin-flip scattering at the interface is controlled not only by the strength of the spin-orbit coupling, but depends strongly on the intrinsic width of the resonant surface states.     

Magnetoconductance of Au-SiO2 films

Measurements of the magnetoconductance in Au-SiO₂ thin films show that spin-orbit coupling is dominant at low temperatures. By fitting the data with the theory of Maekawa and Fukuyama the temperature dependence of the inelastic scattering time has been determined to be approximately T>^?1.5. The spin-orbit scattering times turn out to be significantly smaller than those used by Bergmann.     

Origin of orbital ferromagnetism and giant magnetic anisotropy at the nanoscale

The origin of orbital magnetism recently observed in different nanostructured films and particles is discussed as a consequence of spin-orbit coupling. It is shown that contact potentials induced at the thin film surface by broken symmetries, as domain boundaries in self-assembled monolayers, lead to orbital states that in some cases are of large radius. The component of the angular momentum normal to the surface can reach very high values that decrease the total energy by decreasing spin-orbit interaction energy. Intraorbital ferromagnetic spin correlations induce orbital momenta alignment. The estimated values of the magnetic moments per atom are in good agreement with the experimental observations in thiol capped gold films and nanoparticles.     

Particle dynamics in polymer-metal nanocomposite thin films on nanometer-length scales

X-ray photon correlation spectroscopy was used in conjunction with resonance-enhanced grazing-incidence small-angle x-ray scattering to probe slow particle dynamics and kinetics in gold/polystyrene nanocomposite thin films. Such enhanced coherent scattering enables, for the first time, measurement of the particle dynamics at wave vectors up to approximately 1 nm(-1) (or a few nanometers spatially) in a disordered system, well in the regime where entanglement, confinement, and particle interaction dominate the dynamics and kinetics. Measurements of the intermediate structure factor f(q,t) indicate that the particle dynamics differ from Stokes-Einstein Brownian motion and are explained in terms of viscoelastic effects and interparticle interactions.     

Upper critical field of a superconductor with arbitrary spin-orbit scattering: CeCu2Si2 and UBe13

The upper critical field is determined for an even-parity singlet pairing state in the presence of arbitrary spin-orbit scattering. Comparison with critical field experiments suggests that superconductivity in CeCu₂Si₂ is a singlet pairing state, and in UBe₁₃ is either a triplet pairing state or is a singlet state with restrictive conditions that the pair orbital be nearly isotropic and that strong spin-orbit scattering increase strongly as the field increases.     

Instabilities of waves in nonlinear disordered media

We develop a self-consistent theory of temporal fluctuations of a speckle pattern resulting from the multiple scattering of a coherent wave in a weakly nonlinear disordered medium. The speckle pattern is shown to become unstable if the nonlinearity exceeds a threshold value. The instability is due to a feedback provided by the multiple scattering and manifests itself in spontaneous fluctuations of the scattered intensity. The development of instability is independent of the sign of nonlinearity.     

Amplified extended modes in random lasers

We report on a new random laser phenomenon that gives rise to narrow emission modes without requiring optical cavities. Sharp emission peaks are observed experimentally over a broad range of scattering strengths and analyzed in numerical calculations. We find that the introduction of exponential gain in a multiple light scattering process strongly increases the importance of very long light paths. Such long paths are rare and often neglected in passive disordered materials but we show that they can dominate the emission spectrum from an amplifying disordered system.     

Scattering of surface plasmon-polaritons and volume waves by thin gold films

The specific features of elastic scattering of volume waves and surface plasmon polaritons by polycrystalline gold films have been investigated. An analysis of the relative scattered energy, power spectral density of surface roughness, and integral and angular dependences of scattering of waves of different nature indicates a strong nonradiative multiple scattering of surface plasmon polaritons in gold films. When roughness increases, this scattering leads to an increase in scattering isotropy and to a partial loss of structural information about gold films. The analysis of the scattered energy of surface plasmon polaritons with application of the data on multifractal dimension of gold surface indicates also that the radiative scattering of surface plasmon polaritons depends on both the rms surface roughness and the surface wave propagation length.