Evolution of the Hall Coefficient and the Peculiar Electronic Structure of the Cuprate Superconductors

Although the Hall coefficient R_H is an informative transport property of metals and semiconductors, its meaning in the cuprate superconductors has been ambiguous because of its unusual characteristics. Here we show that a systematic study of R_H in La_{2-x}Sr_{x}CuO_{4} single crystals over a wide doping range establishes a qualitative understanding of its peculiar evolution, which turns out to reflect a two-component nature of the electronic structure caused by an unusual development of the Fermi surface recently uncovered by photoemission experiments.Comment: 4 pages, 5 figures, final version appeared in Phys. Rev. Let

The Bacterial Photosynthetic Reaction Center as a Model for Membrane Proteins

Membrane proteins participate in many fundamental cellular processes. Until recently, an understanding of the function and properties of membrane proteins was hampered by an absence of structural information at the atomic level. A landmark achievement toward understanding the structure of membrane proteins was the crystallization (1) and structure determination (2-5) the photosynthetic reaction center (RC) from the purple bacteria Rhodopseudomonas viridis, followed by that of the RC from Rhodobacter sphaeroides (6-17). The RC is an integral membrane protein-pigment complex, which carries out the initial steps of photosynthesis (reviewed in 18). RCs from the purple bacteria Rps. viridis and Rb. sphaeroides are composed of three membrane-associated protein subunits (designated L, M, and H), and the following cofactors: four bacteriochlorophylls (Bchl or B), two bacteriopheophytins (Bphe or [phi]), two quinones, and a nonheme iron. The cofactors are organized into two symmetrical branches that are approximately related by a twofold rotation axis (2, 8). A central feature of the structural organization of the RC is the presence of 11 hydrophobic [alpha]-helixes, approximately 20-30 residues long, which are believed to represent the membrane-spanning portion of the RC (3, 9). Five membrane-spanning helixes are present in both the L and M subunits, while a single helix is in the H subunit. The folding of the L and M subunits is similar, consistent with significant sequence similarity between the two chains (19-25). The L and M subunits are approximately related by the same twofold rotation axis that relates the two cofactor branches. RCs are the first membrane proteins to be described at atomic resolution; consequently they provide an important model for discussing the folding of membrane proteins. The structure demonstrates that [alpha]-helical structures may be adopted by integral membrane proteins, and provides confirmation of the utility of hydropathy plots in identifying nonpolar membrane-spanning regions from sequence data. An important distinction between the folding environments of water-soluble proteins and membrane proteins is the large difference in water concentration surrounding the proteins. As a result, hydrophobic interactions (26) play very different roles in stabilizing the tertiary structures of these two classes of proteins; this has important structural consequences. There is a striking difference in surface polarity of membrane and water-soluble proteins. However, the characteristic atomic packing and surface area appear quite similar. A computational method is described for defining the position of the RC in the membrane (10). After localization of the RC structure in the membrane, surface residues in contact with the lipid bilayer were identified. As has been found for soluble globular proteins, surface residues are less well conserved in homologous membrane proteins than the buried, interior residues. Methods based on the variability of residues between homologous proteins are described (13); they are useful (a) in defining surface helical regions of membrane and water-soluble proteins and (b) in assigning the side of these helixes that are exposed to the solvent. A unifying view of protein structure suggests that water-soluble proteins may be considered as modified membrane proteins with covalently attached polar groups that solubilize the proteins in aqueous solution

Doping dependence of charge-transfer excitations in La_{2-x}Sr_xCuO_4

We report a resonant inelastic x-ray scattering (RIXS) study of the doping dependence of charge-transfer excitations in $\rm La_{2-x}Sr_xCuO_4$. The mome ntum dependence of these charge excitations are studied over the whole Brillouin zone in underdoped (x=0.05) and optimally doped (x=0.17) samples, and compared with that of the undoped (x=0) sample. We observe a large change in the RIXS spectra between the x=0 and x=0.17 sample, while the RIXS spectra of the x=0.05 sample are similar to that of the x=0 sample. The most prominent effect of doped-holes on the charge excitation spectra is the appearance of a continuum of intensity, which exhibits a strong momentum-dependence below 2 eV. For the x=0.17 sample, some of the spectral weight from the lowest-lying charge-transfer excitation of the undoped compound is transferred to the continuum intensity below the gap, in agreement with earlier optical studies. However, the higher energy charge-transfer excitation carries significant spectral weight even for the x=0.17 sample. The doping dependence of the dispersion of this charge-transfer excitation is also discussed and compared with recent theoretical calculations.Comment: 7 pages, 6 figures, to appear in Phys. Rev.

Curie-like paramagnetism due to incomplete Zhang-Rice singlet formation in La2-xSrxCuO4

In an effort to elucidate the origin of the Curie-like paramagnetism that is generic for heavily-overdoped cuprates, we have performed high transverse-field muon spin rotation (TF-muSR) measurements of La2-xSrxCuO4 single crystals over the Sr content range 0.145 < x < 0.33. We show that the x-dependence of the previously observed field-induced broadening of the internal magnetic field distribution above the superconducting transition temperature Tc reflects the presence of two distinct contributions. One of these becomes less pronounced with increasing x and is attributed to diminishing antiferromagnetic correlations. The other grows with increasing x, but decreases above x ~ 0.30, and is associated with the Curie-like term in the bulk magnetic susceptibility. In contrast to the Curie-like term, however, this second contribution to the TF-muSR line width extends back into the underdoped regime. Our findings imply a coexistence of antiferromagnetically correlated and paramagnetic moments, with the latter becoming dominant beyond x ~ 0.185. This suggests that the doped holes do not neutralize all Cu spins via the formation of Zhang-Rice singlets. Moreover, the paramagnetic component of the TF-muSR line width is explained by holes progressively entering the Cu 3d_{x^2-y^2} orbital with doping.Comment: 8 pages, 7 figure

Electron interactions and charge ordering in La2−x_{2-x}Srx_xCuO4_4

We present results of inelastic light scattering experiments on single-crystalline La$_{2-x}$Sr$_{x}$CuO$_4$ in the doping range $0.00 \le x=p \le 0.30$ and Tl$_2$Ba$_2$CuO$_{6+\delta}$ at $p=0.20$ and $p=0.24$. The main emphasis is placed on the response of electronic excitations in the antiferromagnetic phase, in the pseudogap range, in the superconducting state, and in the essentially normal metallic state at $x \ge 0.26$, where no superconductivity could be observed. In most of the cases we compare B$_{1g}$ and B$_{2g}$ spectra which project out electronic properties close to $(\pi,0)$ and $(\pi/2, \pi/2)$, respectively. In the channel of electron-hole excitations we find universal behavior in B$_{2g}$ symmetry as long as the material exhibits superconductivity at low temperature. In contrast, there is a strong doping dependence in B$_{1g}$ symmetry: (i) In the doping range $0.20 \le p \le 0.25$ we observe rapid changes of shape and temperature dependence of the spectra. (ii) In La$_{2-x}$Sr$_{x}$CuO$_4$ new structures appear for $x < 0.13$ which are superposed on the electron-hole continuum. The temperature dependence as well as model calculations support an interpretation in terms of charge-ordering fluctuations. For $x \le 0.05$ the response from fluctuations disappears at B$_{1g}$ and appears at B$_{2g}$ symmetry in full agreement with the orientation change of stripes found by neutron scattering. While, with a grain of salt, the particle-hole continuum is universal for all cuprates the response from fluctuating charge order in the range $0.05 \le p < 0.16$ is so far found only in La$_{2-x}$Sr$_{x}$CuO$_4$. We conclude that La$_{2-x}$Sr$_{x}$CuO$_4$ is close to static charge order and, for this reason, may have a suppressed $T_c$.Comment: 17 pages, 15 figure

Inhomogeneous Magnetic-Field Response of YBa2Cu3Oy and La2-xSrxCuO4 Persisting above the Bulk Superconducting Transition Temperature

We report that in YBa2Cu3Oy and La2-xSrxCuO4 there is a spatially inhomogeneous response to magnetic field for temperatures T extending well above the bulk superconducting transition temperature Tc. An inhomogeneous magnetic response is observed above Tc even in ortho-II YBa2Cu3O6.50, which has highly ordered doping. The degree of the field inhomogeneity above Tc tracks the hole doping dependences of both Tc and the density of the superconducting carriers below Tc, and therefore is apparently coupled to superconductivity.Comment: Modified discussio

Breakdown of the universal Josephson relation in spin ordered cuprate superconductors

We present \emph{c} axis infrared optical data on a number of Ba, Sr and Nd-doped cuprates of the La$_{2}$CuO$_{4}$ (La214) series in which we observe significant deviations from the universal Josephson relation linking the normal state transport (DC conductivity $\sigma_{DC}$ measured at $T_{c}$) with the superfluid density ($\rho_{s}$): $\rho_{s}\propto\sigma_{DC}(T_{c})$. We find the violation of Josephson scaling is associated with striking enhancement of the anisotropy in the superfluid density. The data allows us to link the breakdown of Josephson interlayer physics with the development of magnetic order in the CuO$_2$ planes.Comment: 5 pages, 3 figure

Constituents of the "kink" in high-Tc cuprates

Applying the Kramers-Kronig consistent procedure, developed earlier, we investigate in details the formation of the quasiparticle spectrum along the nodal direction of high-Tc cuprates. The heavily discussed "70 meV kink" on the renormalized dispersion exhibits a strong temperature and doping dependence when purified from structural effects. This dependence is well understood in terms of fermionic and bosonic constituents of the self-energy. The latter follows the evolution of the spin-fluctuation spectrum, emerging below T* and sharpening below Tc, and is the main responsible for the formation of the kink in question.Comment: revte