10,549 research outputs found
Investigating the Physical Origin of Unconventional Low-Energy Excitations and Pseudogap Phenomena in Cuprate Superconductors
We investigate the physical origin of unconventional low-energy excitations
in cuprate superconductors by considering the effect of coexisting competing
orders (CO) and superconductivity (SC) and of quantum fluctuations and other
bosonic modes on the low-energy charge excitation spectra. By incorporating
both SC and CO in the bare Green's function and quantum phase fluctuations in
the self-energy, we can consistently account for various empirical findings in
both the hole- and electron-type cuprates, including the excess subgap
quasiparticle density of states, ``dichotomy'' in the fluctuation-renormalized
quasiparticle spectral density in momentum space, and the occurrence and
magnitude of a low-energy pseudogap being dependent on the relative gap
strength of CO and SC. Comparing these calculated results with experiments of
ours and others, we suggest that there are two energy scales associated with
the pseudogap phenomena, with the high-energy pseudogap probably of magnetic
origin and the low-energy pseudogap associated with competing orders.Comment: 10 pages, 5 figures. Invited paper for the 2006 Taiwan International
Conference on Superconductivity. Correspondence author: Nai-Chang Yeh
(e-mail: [email protected]
Frozen light in periodic stacks of anisotropic layers
We consider a plane electromagnetic wave incident on a periodic stack of
dielectric layers. One of the alternating layers has an anisotropic refractive
index with an oblique orientation of the principal axis relative to the normal
to the layers. It was shown recently (A. Figotin and I. Vitebskiy, Phys. Rev.
E68, 036609 2003) that an obliquely incident light, upon entering such a
periodic stack, can be converted into an abnormal axially frozen mode with
drastically enhanced amplitude and zero normal component of the group velocity.
The stack reflectivity at this point can be very low, implying nearly total
conversion of the incident light into the frozen mode with huge energy density,
compared to that of the incident light. Supposedly, the frozen mode regime
requires strong birefringence in the anisotropic layers - by an order of
magnitude stronger than that available in common anisotropic dielectric
materials. In this paper we show how to overcome the above problem by
exploiting higher frequency bands of the photonic spectrum. We prove that a
robust frozen mode regime at optical wavelengths can be realized in stacks
composed of common anisotropic materials, such as YVO₄, LiNb,
CaCO₃, and the like.Comment: to be submitted to Phys. Rev.
Observation of vortices and hidden pseudogap from scanning tunneling spectroscopic studies of electron-doped cuprate superconductor
We present the first demonstration of vortices in an electron-type cuprate
superconductor, the highest (= 43 K) electron-type cuprate
. Our spatially resolved quasiparticle tunneling spectra
reveal a hidden low-energy pseudogap inside the vortex core and unconventional
spectral evolution with temperature and magnetic field. These results cannot be
easily explained by the scenario of pure superconductivity in the ground state
of high- superconductivity.Comment: 6 pages, 4 figures. Two new graphs have been added into Figure 2.
Accepted for publication in Europhysics Letters. Corresponding author:
Nai-Chang Yeh (E-mail: [email protected]
Heterostructure solar cells
The performance of gallium arsenide solar cells grown on Ge substrates is discussed. In some cases the substrate was thinned to reduce overall cell weight with good ruggedness. The conversion efficiency of 2 by 2 cm cells under AMO reached 17.1 percent with a cell thickness of 6 mils. The work described forms the basis for future cascade cell structures, where similar interconnecting problems between the top cell and the bottom cell must be solved. Applications of the GaAs/Ge solar cell in space and the expected payoffs are discussed
Experimental investigation of the asymmetric spectroscopic characteristics of electron- and hole-doped cuprates
Quasiparticle tunneling spectroscopic studies of electron- (n-type) and hole-doped (p-type) cuprates reveal that the pairing symmetry, pseudogap phenomenon and spatial homogeneity of the superconducting order parameter are all non-universal. We compare our studies of p-type YBa2Cu3O7-delta and n-type infinite-layer Sr(0.9)Ln(0.1)CuO(2) (Ln = La, Gd) systems with results from p-type Bi2Sr2CaCu2Ox and n-type one-layer Nd1.85Ce0.15CuO4 cuprates, and attribute various non-universal behavior to different competing orders in p-type and n-type cuprates
Discrete solitons and nonlinear surface modes in semi-infinite waveguide arrays
We discuss the formation of self-trapped localized states near the edge of a
semi-infinite array of nonlinear waveguides. We study a crossover from
nonlinear surface states to discrete solitons by analyzing the families of odd
and even modes centered at different distances from the surface, and reveal the
physical mechanism of the nonlinearity-induced stabilization of surface modes.Comment: 4 double-column pages, 5 figures, submitted to Optics Letter
High Resolution Crystal Structures of the Wild Type and Cys-55 right-arrow Ser and Cys-59 right-arrow Ser Variants of the Thioredoxin-like [2Fe-2S] Ferredoxin from Aquifex aeolicus
The [2Fe-2S] ferredoxin (Fd4) from Aquifex aeolicus adopts a thioredoxin-like polypeptide fold that is distinct from other [2Fe-2S] ferredoxins. Crystal structures of the Cys-55 right-arrow Ser (C55S) and Cys-59 right-arrow Ser (C59S) variants of this protein have been determined to 1.25 Ă… and 1.05 Ă… resolution, respectively, whereas the resolution of the wild type (WT) has been extended to 1.5 Ă…. The improved WT structure provides a detailed description of the [2Fe-2S] cluster, including two features that have not been noted previously in any [2Fe-2S] cluster-containing protein, namely, pronounced distortions in the cysteine coordination to the cluster and a Calpha -H-Sgamma hydrogen bond between cluster ligands Cys-55 and Cys-9. These features may contribute to the unusual electronic and magnetic properties of the [2Fe-2S] clusters in WT and variants of this ferredoxin. The structures of the two variants of Fd4, in which single cysteine ligands to the [2Fe-2S] cluster are replaced by serine, establish the metric details of serine-ligated Fe-S active sites with unprecedented accuracy. Both the cluster and its surrounding protein matrix change in subtle ways to accommodate this ligand substitution, particularly in terms of distortions of the Fe2S2 inorganic core from planarity and displacements of the polypeptide chain. These high resolution structures illustrate how the interactions between polypeptide chains and Fe-S active sites reflect combinations of flexibility and rigidity on the part of both partners; these themes are also evident in more complex systems, as exemplified by changes associated with serine ligation of the nitrogenase P cluster
Quasiparticle spectroscopy and high-field phase diagrams of cuprate superconductors -- An investigation of competing orders and quantum criticality
We present scanning tunneling spectroscopic and high-field thermodynamic
studies of hole- and electron-doped (p- and n-type) cuprate superconductors.
Our experimental results are consistent with the notion that the ground state
of cuprates is in proximity to a quantum critical point (QCP) that separates a
pure superconducting (SC) phase from a phase comprised of coexisting SC and a
competing order, and the competing order is likely a spin-density wave (SDW).
The effect of applied magnetic field, tunneling current, and disorder on the
revelation of competing orders and on the low-energy excitations of the
cuprates is discussed.Comment: 10 pages, 5 figures. Accepted for publication in the International
Journal of Modern Physics B. (Correspondence author: Nai-Chang Yeh, e-mail:
[email protected]
Macroscopic evidence for quantum criticality and field-induced quantum fluctuations in cuprate superconductors
We present macroscopic experimental evidence for field-induced microscopic
quantum fluctuations in different hole- and electron-type cuprate
superconductors with varying doping levels and numbers of CuO layers per
unit cell. The significant suppression of the zero-temperature in-plane
magnetic irreversibility field relative to the paramagnetic field in all
cuprate superconductors suggests strong quantum fluctuations due to the
proximity of the cuprates to quantum criticality.Comment: 3 figures. To appear in Phys. Rev. B, Rapid Communications (2007).
For correspondence, contact: Nai-Chang Yeh (e-mail: [email protected]
Discrete surface solitons in semi-infinite binary waveguide arrays
We analyze discrete surface modes in semi-infinite binary waveguide arrays,
which can support simultaneously two types of discrete solitons. We demonstrate
that the analysis of linear surface states in such arrays provides important
information about the existence of nonlinear surface modes and their
properties. We find numerically the families of both discrete surface solitons
and nonlinear Tamm (gap) states and study their stability properties.Comment: 3 pages, 4 figures, submitted to Opt. Let
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