11,715 research outputs found
Measurement of a Sign-Changing Two-Gap Superconducting Phase in Electron-Doped Ba(Fe_{1-x}Co_x)_2As_2 Single Crystals using Scanning Tunneling Spectroscopy
Scanning tunneling spectroscopic studies of (x =
0.06, 0.12) single crystals reveal direct evidence for predominantly two-gap
superconductivity. These gaps decrease with increasing temperature and vanish
above the superconducting transition . The two-gap nature and the slightly
doping- and energy-dependent quasiparticle scattering interferences near the
wave-vectors and are consistent with
sign-changing -wave superconductivity. The excess zero-bias conductance and
the large gap-to- ratios suggest dominant unitary impurity scattering.Comment: 4 pages, 4 figures. Paper accepted for publication in Physical Review
Letters. Contact author: Nai-Chang Yeh ([email protected]
Pattern Competition in the Photorefractive Semiconductors
We analytically study the photorefractive Gunn effect in n-GaAs subjected to
two external laser beams which form a moving interference pattern (MIP) in the
semiconductor. When the intensity of the spatially independent part of the MIP,
denoted by , is small, the system has a periodic domain train (PDT),
consistent with the results of linear stability analysis. When is large,
the space-charge field induced by the MIP will compete with the PDT and result
in complex dynamics, including driven chaos via quasiperiodic route
Scattering-free plasmonic optics with anisotropic metamaterials
We develop an approach to utilize anisotropic metamaterials to solve one of
the fundamental problems of modern plasmonics -- parasitic scattering of
surface waves into free-space modes, opening the road to truly two-dimensional
plasmonic optics. We illustrate the developed formalism on examples of
plasmonic refractor and plasmonic crystal, and discuss limitations of the
developed technique and its possible applications for sensing and imaging
structures, high-performance mode couplers, optical cloaking structures, and
dynamically reconfigurable electro-plasmonic circuits
Scanning Tunneling Spectroscopic Studies of the Low-Energy Quasiparticle Excitations in Cuprate Superconductors
We report scanning tunneling spectroscopic (STS) studies of the low-energy quasiparticle excitations of cuprate superconductors as a function of magnetic field and doping level. Our studies suggest that the origin of the pseudogap (PG) is associated with competing orders (COs), and that the occurrence (absence) of PG above the superconducting (SC) transition T_c is associated with a CO energy Î_(CO) larger (smaller) than the SC gap Î_(SC). Moreover, the spatial homogeneity of Î_(SC) and Î_(CO) depends on the type of disorder in different cuprates: For optimally and under-doped YBa_2Cu_3O_(7âÎŽ) (Y-123), we find that Î_(SC) < Î_(CO) and that both Î_(SC) and Î(CO) exhibit long-range spatial homogeneity, in contrast to the highly inhomogeneous STS in Bi_2Sr_2CaCu_2O_(8+x) (Bi-2212). We attribute this contrast to the stoichiometric cations and ordered apical oxygen in Y-123, which differs from the non-stoichiometric Bi-to-Sr ratio in Bi-2212 with disordered Sr and apical oxygen in the SrO planes. For Ca-doped Y-123, the substitution of Y by Ca contributes to excess holes and disorder in the CuO_2 planes, giving rise to increasing inhomogeneity, decreasing Î_(SC) and Î_(CO), and a suppressed vortex-solid phase. For electron-type cuprate Sr_(0.9)La_(0.1)CuO_2 (La-112), the homogeneous Î_(SC) and Î_(CO) distributions may be attributed to stoichiometric cations and the absence of apical oxygen, with Î_(CO) < Î_(SC) revealed only inside the vortex cores. Finally, the vortex-core radius (Ο_(halo)) in electron-type cuprates is comparable to the SC coherence length Ο_(SC), whereas Ο_(halo) ⌠10Ο_(SC) in hole-type cuprates, suggesting that Ο_(halo) may be correlated with the CO strength. The vortex-state irreversibility line in the magnetic field versus temperature phase diagram also reveals doping dependence, indicating the relevance of competing orders to vortex pinning
Scattering of slow-light gap solitons with charges in a two-level medium
The Maxwell-Bloch system describes a quantum two-level medium interacting
with a classical electromagnetic field by mediation of the the population
density. This population density variation is a purely quantum effect which is
actually at the very origin of nonlinearity. The resulting nonlinear coupling
possesses particularly interesting consequences at the resonance (when the
frequency of the excitation is close to the transition frequency of the
two-level medium) as e.g. slow-light gap solitons that result from the
nonlinear instability of the evanescent wave at the boundary. As nonlinearity
couples the different polarizations of the electromagnetic field, the
slow-light gap soliton is shown to experience effective scattering whith
charges in the medium, allowing it for instance to be trapped or reflected.
This scattering process is understood qualitatively as being governed by a
nonlinear Schroedinger model in an external potential related to the charges
(the electrostatic permanent background component of the field).Comment: RevTex, 14 pages with 5 figures, to appear in J. Phys. A: Math. Theo
Scanning Tunnelling Spectroscopic Studies of Dirac Fermions in Graphene and Topological Insulators
We report novel properties derived from scanning tunnelling spectroscopic (STS) studies of Dirac fermions in graphene and the surface state (SS) of a strong topological insulator (STI), Bi_2Se_3. For mono-layer graphene grown on Cu by chemical vapour deposition (CVD), strain-induced scalar and gauge potentials are manifested by the charging effects and the tunnelling conductance peaks at quantized energies, respectively. Additionally, spontaneous time-reversal symmetry breaking is evidenced by the alternating anti-localization and localization spectra associated with the zero-mode of two sublattices while global time-reversal symmetry is preserved under the presence of pseudo-magnetic fields. For Bi_2Se_3 epitaxial films grown on Si(111) by molecular beam epitaxy (MBE), spatially localized unitary impurity resonances with sensitive dependence on the energy difference between the Fermi level and the Dirac point are observed for samples thicker than 6 quintuple layers (QL). These findings are characteristic of the SS of a STI and are direct manifestation of strong topological protection against impurities. For samples thinner than 6-QL, STS studies reveal the openup of an energy gap in the SS due to overlaps of wave functions between the surface and interface layers. Additionally, spin-preserving quasiparticle interference wave-vectors are observed, which are consistent with the Rashba-like spin-orbit splitting
mixing and the next-to-leading-order power correction
The next-to-leading-order power correction for and
form factors are evaluated and employed to explore the
mixing. The parameters of the two mixing angle scheme are
extracted from the data for form factors, two photon decay widths and radiative
decays. The analysis gives the result:
, where
and are the decay constants and the mixing
angles for the singlet (octet) state. In addition, we arrive at a stringent
range for MeV MeV.Comment: 23 pages, 9 figures, To be publshied in Phys. Rev.
Spherical harmonic decomposition applied to spatial-temporal analysis of human high-density EEG
We demonstrate an application of spherical harmonic decomposition to analysis
of the human electroencephalogram (EEG). We implement two methods and discuss
issues specific to analysis of hemispherical, irregularly sampled data.
Performance of the methods and spatial sampling requirements are quantified
using simulated data. The analysis is applied to experimental EEG data,
confirming earlier reports of an approximate frequency-wavenumber relationship
in some bands.Comment: 12 pages, 8 figures, submitted to Phys. Rev. E, uses APS RevTeX
style
Superconducting and normal-state interlayer-exchange-coupling in LaSrMnO-YBaCuO_{0.67}_{0.33}{3}$ epitaxial trilayers
The issue of interlayer exchange coupling in magnetic multilayers with
superconducting (SC) spacer is addressed in LaSrMnO
(LSMO) - YBaCuO (YBCO) - LaSrMnO
(LSMO) epitaxial trilayers through resistivity, ac-susceptibility and
magnetization measurements. The ferromagnetic (FM) LSMO layers possessing
in-plane magnetization suppress the critical temperature (T of the
c-axis oriented YBCO thin film spacer. The superconducting order, however,
survives even in very thin layers (thickness d 50 {\AA}, 4
unit cells) at T 25 K. A predominantly antiferromagnetic (AF) exchange
coupling between the moments of the LSMO layers at fields 200 Oe is seen in
the normal as well as the superconducting states of the YBCO spacer. The
exchange energy J ( 0.08 erg/cm at 150 K for d = 75
{\AA}) grows on cooling down to T, followed by truncation of this growth
on entering the superconducting state. The coupling energy J at a fixed
temperature drops exponentially with the thickness of the YBCO layer. The
temperature and d dependencies of this primarily non-oscillatory J
are consistent with the coupling theories for systems in which transport is
controlled by tunneling. The truncation of the monotonic T dependence of
J below T suggests inhibition of single electron tunneling across
the CuO planes as the in-plane gap parameter acquires a non-zero value.Comment: Accepted for publication in Phys. Rev.
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