G. Blumberg

Constant effective mass across the phase diagram of high-Tc cuprates

We investigate the hole dynamics in two prototypical high temperature superconducting systems: La2�xSrxCuO4 and YBa2Cu3Oy, using a combination of dc transport and infrared spectroscopy. By exploring the effective spectral weight obtained with optics in conjunction with dc Hall results, we find that the transition to the Mott insulating state in these systems is of the “vanishing carrier number” type since we observe no substantial enhancement of the mass as one proceeds to undoped phases. Further, the effective mass remains constant across the entire underdoped regime of the phase diagram. We discuss the implications of these results for the understanding of both transport phenomena and pairing mechanism in high-Tc systems.

Inhomogeneous CuO6 tilt distribution and charge-spin correlations in La2-x-yNdySrxCuO4 around the commensurate hole concentration

Phononic and magnetic Raman scattering are studied in La2-x-yNdySrxCuO4 with three doping concentrations: x1/8, y = 0; x1/8, y = 0.4; and x = 0.01, y = 0. We observe strong disorder in the tilt pattern of the CuO6 octahedra in both the orthorhombic and tetragonal phases that persist down to 10 K and are coupled to bond disorder in the cation layers around 1/8 doping independent of Nd concentration. The weak magnitude of existing charge-spin modulations in the Nd-doped structure does not allow us to detect the specific Raman signatures of lattice dynamics or two-magnon scattering around 2200 cm–1. ©2003 The American Physical Society

Resonant two-magnon Raman scattering in cuprate antiferromagnetic insulators

We present results of low-temperature two-magnon resonance Raman excitation profile measurements for single-layer Sr{sub 2}CuO{sub 2}Cl{sub 2} and bilayer YBa{sub 2}Cu{sub 3}O{sub 6+{delta}} antiferromagnets over the excitation region from 1.65 to 3.05 eV. These data reveal composite structure of the two-magnon line shape and strong nonmonotic dependence of the scattering intensity on excitation energy. We analyze these data using the {ital triple} {ital resonance} theory of Chubukov and Frenkel [Phys. Rev. Lett. {bold 74}, 3057 (1995)] and deduce information about magnetic interaction and band parameters in these materials. {copyright} {ital 1996 The American Physical Society.}

Compact nanomechanical plasmonic phase modulators

Researchers exploit the strong dependence of gap-plasmon phase velocity on gap width to make a compact phase-modulator. An electromechanically variable gap size enables a 23-μm-long non-resonant modulator with moderate losses.

CHARGE AND SPIN DYNAMICS OF AN ORDERED STRIPE PHASE IN LA12/3SR1/3NIO4 INVESTIGATED BY RAMAN SPECTROSCOPY

For La_(1 2/3)Sr_(1/3)NiO_4 -- a commensurately doped Mott-Hubbard system -- charge- and spin-ordering in a stripe phase has been investigated by phononic and magnetic Raman scattering. Formation of a superlattice and an opening of a pseudo-gap in the electron-hole excitation spectra as well as two types of double-spin excitations -- within the antiferromagnetic domain and across the domain wall -- are observed below the charge-ordering transition. The temperature dependence suggests that the spin ordering is driven by charge ordering and that fluctuating stripes persist above the ordering transition.

Evolution of superconductivity in electron-doped cuprates: Magneto-Raman spectroscopy

The electron-doped cuprates Pr_{2-x}Ce_xCuO_4 and Nd_{2-x}Ce_xCuO_4 have been studied by electronic Raman spectroscopy across the entire region of the superconducting (SC) phase diagram. The SC pairing strength is found to be consistent with a weak-coupling regime except in the under-doped region where we observe an in-gap collective mode at 4.5 k_{B}T_c while the maximum amplitude of the SC gap is ~8 k_{B}T_{c}. In the normal state, doped carriers divide into coherent quasi-particles (QPs) and carriers that remain incoherent. The coherent QPs mainly reside in the vicinity of (\pi/2, \pi/2) regions of the Brillouin zone (BZ). We find that only coherent QPs contribute to the superfluid density in the B_{2g} channel. The persistence of SC coherence peaks in the B_{2g} channel for all dopings implies that superconductivity is mainly governed by interactions between the hole-like coherent QPs in the vicinity of (\pi/2, \pi/2) regions of the BZ. We establish that superconductivity in the electron-doped cuprates occurs primarily due to pairing and condensation of hole-like carriers. We have also studied the excitations across the SC gap by Raman spectroscopy as a function of temperature (T) and magnetic field (H) for several different cerium dopings (x). Effective upper critical field lines H*_{c2}(T, x) at which the superfluid stiffness vanishes and H^{2\Delta}_{c2}(T, x) at which the SC gap amplitude is suppressed by field have been determined; H^{2\Delta}_{c2}(T, x) is larger than H*_{c2}(T, x) for all doping concentrations. The difference between the two quantities suggests the presence of phase fluctuations that increase for x< 0.15. It is found that the magnetic field suppresses the magnitude of the SC gap linearly at surprisingly small fields.

Critical quadrupole fluctuations and collective modes in iron pnictide superconductors

The multiband nature of iron pnictides gives rise to a rich temperature-doping phase diagram of competing orders and a plethora of collective phenomena. At low dopings, the tetragonal-to-orthorhombic structural transition is closely followed by a spin-density-wave transition both being in close proximity to the superconducting phase. A key question is the nature of high-

Crystal structure and high-field magnetism of La2CuO4

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C-axis electronic raman scattering in Bi_2Sr_2CaCu_2O_(8+d)

superconductivity and its relation to orbital ordering and magnetism. Here we study the

Evolution of magnetic and superconducting fluctuations with doping of high-Tc superconductors: An electronic Raman scattering study

{\mathrm{NaFe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{As}