Showing papers by "Robert J. Birgeneau published in 2005"
TL;DR: In this article, the authors investigated the spin-fluctuations at energy transfers up to the resonance energy of the superconductor in the ortho-II superconductors.
Abstract: We have investigated the spin-fluctuations at energy transfers up to $\ensuremath{\sim}110\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, well above the resonance energy $(33\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$ in the $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.5}$ ortho-II superconductor using neutron time-of-flight and triple-axis techniques. The spectrum at high energies differs from the low-energy incommensurate modulations previously reported where the incommensurate wave vector is largely independent of energy. Well above the resonance the peak of the spin response lies at wave vectors that increase with energy. Within error the excitations at all energies above the resonance are best described by a ring around the $(\ensuremath{\pi},\ensuremath{\pi})$ position. The isotropic wave-vector pattern differs from a recently reported square pattern in different but related systems. The spin spectral weight at high energies is similar to that in the insulator but the characteristic velocity is $\ensuremath{\sim}40%$ lower. We introduce a method of extracting the acoustic and optic weights at all energies from time-of-flight data. We find that the optic spectral weight extends to surprisingly low energies of $\ensuremath{\sim}25\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, and infer that the bilayer spin correlations weaken with increase in hole doping. When the low-energy optic excitations are taken into account we measure the total integrated weight around $(\ensuremath{\pi},\ensuremath{\pi})$, for energies below $120\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, to agree with that expected from the insulator. As a qualitative guide, we compare spin-wave calculations based on models with ordered and disordered stripes and describe the inadequacy of these and other stripe models for the high-energy fluctuations.
156 citations
TL;DR: In this article, a magnetic field-induced transition between magnetically disordered and ordered phases in slightly under-doped La(2-x)SrxCuO4 with x = 0.144 was observed.
Abstract: We report the observation of a magnetic-field-induced transition between magnetically disordered and ordered phases in slightly under-doped La(2-x)SrxCuO4 with x=0.144. Static incommensurate spin-density-wave order is induced above a critical field of about 3 T, as measured by elastic neutron scattering. Our results allow us to constrain the location of a quantum critical point on the phase diagram.
60 citations
TL;DR: In this paper, a detailed study of the phonon lineshape and a comparison of the structure factors to SrTiO 3 was conducted, and it was concluded that the coupling between the acoustic and optic modes is weak.
Abstract: The temperature and zone dependence of the lattice dynamics in Pb(Mg 1/3 Nb 2/3 )O 3 is characterized using neutron inelastic scattering. Through a detailed study of the phonon lineshape and a comparison of the structure factors to SrTiO 3 , we conclude that the coupling between the acoustic and optic modes is weak. However, based on a direct comparison between the (110) Brillouin zone where the diffuse scattering is strong to the (220) zone where the diffuse scattering is weak, a strong coupling between the diffuse component and low-energy acoustic phonon mode is observed. We conclude that the coupling to the diffuse component is the reason for several recent conflicting interpretations of the lattice dynamics based on data from zones with a strong diffuse component.
41 citations
TL;DR: The magnetic properties of the Zn-substituted overdoped high-T$ superconductor have been studied by magnetization measurements and neutron scattering as mentioned in this paper, and the induced Curie constant corresponds to a moment of $0.5\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$ per additional high T$ ion that exceeds $x=0.22$.
Abstract: The magnetic properties of the Zn-substituted overdoped high-${T}_{c}$ superconductor ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Cu}}_{1\ensuremath{-}y}{\mathrm{Zn}}_{y}{\mathrm{O}}_{4}$ have been studied by magnetization measurements and neutron scattering. Magnetization measurements reveal that for Zn-free samples with $x\ensuremath{\geqslant}0.22$ a Curie term is induced in the temperature dependence of the magnetic susceptibility implying the existence of local paramagnetic moments. The induced Curie constant corresponds to a moment of $0.5\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$ per additional ${\mathrm{Sr}}^{2+}$ ion that exceeds $x=0.22$. Zn substitution in the overdoped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ also induces a Curie term that corresponds to $1.2\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$ per ${\mathrm{Zn}}^{2+}$ ion, simultaneously suppressing ${T}_{c}$. The relationship between ${T}_{c}$ and the magnitude of the Curie term for Zn-free ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ with $x\ensuremath{\geqslant}0.22$ and for Zn-substituted ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ with $x=0.22$ are closely similar. This signifies a general competitive relationship between the superconductivity and the induced paramagnetic moment. Neutron scattering measurements show that Zn substitution in overdoped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ anomalously enhances the inelastic magnetic scattering spectra around the $(\ensuremath{\pi},\ensuremath{\pi})$ position, peaking at $\ensuremath{\omega}\ensuremath{\sim}7\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. These facts are discussed on the basis of a ``swiss-cheese'' model of Zn-substituted systems as well as a microscopic phase separation scenario in the overdoped region indicated by muon-spin-relaxation measurements.
17 citations
TL;DR: The robust nature of the N-SmA transition in this system contrasts with cyanobiphenyl-aerosil systems and may be due to the mesogens being nonpolar and having a long nematic range.
Abstract: A high-resolution calorimetric study has been carried out on nanocolloidal dispersions of aerosils in the liquid crystal 4-n-pentylphenylthiol-4'-n-octyloxybenzoate (8S5) as a function of aerosil concentration and temperature spanning the smectic-C to nematic phases. Over this temperature range, this liquid crystal possesses two continuous XY phase transitions: a fluctuation-dominated nematic to smectic-A transition with alpha approximately alphaXY=-0.013 and a mean-field smectic-A to smectic-C transition. The effective critical character of the N-SmA transition remains unchanged over the entire range of the introduced quenched random disorder while the peak height and enthalpy can be well described by considering a cutoff length scale to the quasicritical fluctuations. The robust nature of the N-SmA transition in this system contrasts with cyanobiphenyl-aerosil systems and may be due to the mesogens being nonpolar and having a long nematic range. The character of the SmA-SmC transition changes gradually with increasing disorder but remains mean field like. The heat capacity maximum at the SmA-SmC transition scales as rho with an apparent evolution from tricritical to a simple mean-field step behavior. These results may be generally understood as a stiffening of the liquid crystal (both the nematic elasticity as well as the smectic layer compression modulus B) with silica density.
15 citations
TL;DR: In this article, the authors have observed the charge transfer and molecular-orbital excitations, which exhibit resonances at incident energies of 8.992$ and $8.30$ respectively.
Abstract: Resonant inelastic x-ray scattering (RIXS) at the copper K absorption edge has been performed for heavily overdoped samples of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ with $x=0.25$ and 0.30. We have observed the charge-transfer and molecular-orbital excitations, which exhibit resonances at incident energies of ${E}_{i}=8.992$ and $8.998\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$, respectively. From a comparison with previous results on undoped and optimally doped samples, we determine that the charge-transfer excitation energy increases monotonically as doping increases. In addition, the ${E}_{i}$ dependences of the RIXS spectral weight and absorption spectrum exhibit no clear peak at ${E}_{i}=8.998\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ in contrast to results in the underdoped samples. The low-energy $(\ensuremath{\leqslant}3\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ continuum excitation intensity has been studied utilizing the high-energy resolution of $0.13\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ (full width at half maximum). A comparison of the RIXS profiles at $(\ensuremath{\pi}\phantom{\rule{0.3em}{0ex}}0)$ and $(\ensuremath{\pi}\phantom{\rule{0.3em}{0ex}}\ensuremath{\pi})$ indicates that the continuum intensity exists even at $(\ensuremath{\pi}\phantom{\rule{0.3em}{0ex}}\ensuremath{\pi})$ in the overdoped samples, whereas it has been reported only at (0 0) and $(\ensuremath{\pi}\phantom{\rule{0.3em}{0ex}}0)$ for the $x=0.17$ sample. Furthermore, we also found an additional excitation on top of the continuum intensity at the $(\ensuremath{\pi}\phantom{\rule{0.3em}{0ex}}\ensuremath{\pi})$ and $(\ensuremath{\pi}\phantom{\rule{0.3em}{0ex}}0)$ positions.
8 citations
TL;DR: In this article, the authors present neutron elastic scattering results at high magnetic fields for Mg-doped CuGeO 3, where a spin-Peierls phase is present, and they find an enhancement of the long range antiferromagnetic order in the structural incommensurate phase induced by a high magnetic field.
Abstract: We present neutron elastic scattering results at high magnetic fields for Mg-doped CuGeO 3 For low magnesium concentrations, where a spin-Peierls phase is present, we find an enhancement of the long range antiferromagnetic order in the structural incommensurate phase induced by a high magnetic field In the incommensurate phase, the Neel temperature increases with field leading to a gain in the magnetic Bragg peak intensity at low temperatures We also find that the magnetic Bragg peaks remain commensurate even though the structure is incommensurate at high magnetic fields due to the formation of solitons We interpret these results in terms of the solitons formed at high-magnetic fields effectively acting as doped impurities Our results point to a strongly disordered ground state with the antiferromagnetic order localized around impurity sites
5 citations