Unifying the Phase Diagrams of the Magnetic and Transport Properties of La_(2-x)Sr_xCuO_4, 0 < x < 0.05

An extensive experimental and theoretical effort has led to a largely complete mapping of the magnetic phase diagram of La_(2-x)Sr_xCuO_4, and a microscopic model of the spin textures produced in the x < 0.05 regime has been shown to be in agreement with this phase diagram. Here we use this same model to derive a theory of the impurity-dominated, low temperature transport. Then, we present an analysis of previously published data for two samples: x = 0.002 data from Chen et. al., and x = 0.04 data from Keimer et. al. We show that the transport mechanisms in the two systems are the same, even though they are on opposite sides of the observed insulator-to-metal transition. Our model of impurity effects on the impurity band conduction, variable-range hopping conduction, and coulomb gap conduction, is similar to that used to describe doped semiconductors. However, for La_(2-x)Sr_xCuO_4 we find that in addition to impurity-generated disorder effects, strong correlations are important and must be treated on a equal level with disorder. On the basis of this work we propose a phase diagram that is consistent with available magnetic and transport experiments, and which connects the undoped parent compound with the lowest x value for which La_(2-x)Sr_xCuO_4 is found to be superconducting, x about 0.06.Comment: 7 pages revtex with one .ps figur

On the contrasting spin dynamics of La2−xSrxCuO4La_{2-x}Sr_xCuO_4, Nd2−xCexCuO4Nd_{2-x}Ce_xCuO_4 and YBa2Cu3O6+xYBa_2Cu_3O_{6+x} near half filling

We present simple calculations which show that incommensurability upon doping and the width of the magnetically ordered phase in Mott-Hubbard insulators depend strongly on the location of the hole/electron pockets in the Brillouin zone. For $LaSrCuO$ systems, we found the pockets at $(\pm \pi/2,\pm \pi/2)$, in which case the corrections to the antiferromagnetic spin stiffness grow with doping and destroy the commensurate antiferromagnetic ordering already at a very small doping. On the other hand, in $NdCeCuO$, the hole pockets are located at $(\pi,0)$ and the symmetry related points, in which case the corrections to the stiffness scale linearly with the density of carriers and do not destroy commensurate spin ordering. For $YBCuO$, systems the situation is less certain, but our results favor hole pockets at $(\pi/2,\pi/2)$. We also discuss briefly the tendency towards phase separation.Comment: 18 pages, LaTe

Sr impurity effects on the magnetic correlations of LaSrCuO

We examine the low-temperature magnetic properties of moderately doped LaSrCuO paying particular attention to the spin-glass (SG) phase and the C-IC transition as they are affected by Sr impurity disorder. New measurements of the low-temperature susceptibility in the SG phase show an increase of an anomalously small Curie constant with doping. This behaviour is explained in terms of our theoretical work that finds small clusters of AFM correlated regions separated by disordered domain walls. The domain walls lead to a percolating sequence of paths connecting the impurities. We predict that for this spin morphology the Curie constant should scale as $1/(2 \xi(x,T=0)^2)$, a result that is quantitatively in agreement with experiment. Also, we find that the magnetic correlations in the ground states in the SG phase are commensurate, and that this behaviour should persist at higher temperatures where the holes should move along the domain walls. However, our results show that incommensurate correlations develop continuously around 5 % doping, consistent with recent measurements by Yamada.Comment: 30 pages, revtex, 8 .ps format figures (2 meant to be in colour), to be published in Physical Review B

Charge pairing, superconducting transition and supersymmetry in high-temperature cuprate superconductors

We propose a model for high-T$_{c}$ superconductors, valid for $0\leq\delta\leq\delta_{SC}$, that includes both the spin fluctuations of the Cu$^{++}$ magnetic ions and of the O$^{--}$ doped holes. Spin-charge separation is taken into account with the charge of the doped holes being associated to quantum skyrmion excitations (holons) of the Cu$^{++}$ spin background. The holon effective interaction potential is evaluated as a function of doping, indicating that Cooper pair formation is determined by the competition between the spin fluctuations of the Cu$^{++}$ background and of spins of the O$^{--}$ doped holes (spinons). The superconducting transition occurs when the spinon fluctuations dominate, thereby reversing the sign of the interaction. At this point ($\delta = \delta_{SC}$), the theory is supersymmetric at short distances and, as a consequence, the leading order results are not modified by radiative corrections. The critical doping parameter for the onset of superconductivity at T=0 is obtained and found to be a universal constant determined by the shape of the Fermi surface. Our theoretical values for $\delta_{SC}$ are in good agreement with the experiment for both LSCO and YBCO.Comment: RevTex, 4 pages, no figure

Validity of the rigid band picture for the t-J model

We present an exact diagonalization study of the doping dependence of the single particle Green's function in 16, 18 and 20 site clusters of t-J model. We find evidence for rigid-band behaviour starting from the half-filled case: upon doping, the topmost states of the quasiparticle band observed in the photoemisson spectrum at half-filling cross the chemical potential and reappear as the lowermost states of the inverse photoemission spectrum. Features in the inverse photoemission spectra which are inconsistent with rigid-band behaviour are shown to originate from the nontrivial point group symmetry of the ground state with two holes, which enforces different selection rules than at half-filling. Deviations from rigid band behaviour which lead to the formation of the `large Fermi surface' in the momentum distribution occur only at energies far from the chemical potential. A Luttinger Fermi surface and a nearest neighbor hopping band do not exist.Comment: Remarks: Revtex file + 7 figures attached as compressed postscript files Figures can also be obtained by ordinary mail on reques

d-Wave Superconductivity Induced by Chern-Simons Term in High-TcT_c Cuprates

We show that a Chern-Simons term for a gauge field describing a fluctuation of spins is induced by integrating out hole fields in the presence of spin-orbit coupling which originates from a buckling of the CuO$_2$ plane. Through the Chern-Simons term, holes behave like skyrmion excitations in a spin system and become a superconducting state with $d_{x^2-y^2}$ symmetry after the antiferromagnetic long-range order is destroyed.Comment: 4 pages, 1 figure, the condition of taking continuum limit included, references added, typos corrected, to be published in Journal of Physical Society of Japa

Associations of Blood Pressure and&nbsp;Cholesterol Levels During Young&nbsp;Adulthood With Later&nbsp;Cardiovascular Events.

BackgroundBlood pressure (BP) and cholesterol are major modifiable risk factors for cardiovascular disease (CVD), but effects of exposures during young adulthood on later life CVD risk have not been well quantified.ObjectiveThe authors sought to evaluate the independent associations between young adult exposures to risk factors and later life CVD risk, accounting for later life exposures.MethodsThe authors pooled data from 6 U.S. cohorts with observations spanning the life course from young adulthood to later life, and imputed risk factor trajectories for low-density lipoprotein (LDL) and high-density lipoprotein cholesterols, systolic and diastolic BP starting from age 18 years for every participant. Time-weighted average exposures to each risk factor during young (age 18 to 39 years) and later adulthood (age ≥40 years) were calculated and linked to subsequent risks of coronary heart disease (CHD), heart failure (HF), or stroke.ResultsA total of 36,030 participants were included. During a median follow-up of 17 years, there were 4,570 CHD, 5,119 HF, and 2,862 stroke events. When young and later adult risk factors were considered jointly in the model, young adult LDL&nbsp;≥100&nbsp;mg/dl (compared with&nbsp;&lt;100&nbsp;mg/dl) was associated with a 64% increased risk for CHD, independent of later adult exposures. Similarly, young adult SBP&nbsp;≥130&nbsp;mm&nbsp;Hg (compared with&nbsp;&lt;120&nbsp;mm&nbsp;Hg) was associated with a 37% increased risk for HF, and young adult DBP&nbsp;≥80&nbsp;mm&nbsp;Hg (compared with&nbsp;&lt;80&nbsp;mm&nbsp;Hg) was associated with a 21% increased risk.ConclusionsCumulative young adult exposures to elevated systolic BP, diastolic BP and LDL were associated with&nbsp;increased CVD risks in later life, independent of later adult exposures

Quantum skyrmions and the destruction of long-range antiferromagnetic order in the high-Tc superconductors La(2-x)Sr(x)CuO(4) and YBa(2)Cu(3)O(6+x)

We study the destruction of the antiferromagnetic order in the high-Tc superconductors La(2-x)Sr(x)CuO(4) and YBa(2)Cu(3)O(6+x) in the framework of the CP1-nonlinear sigma model formulation of the 2D quantum Heisenberg antiferromagnet. The dopants are introduced as independent fermions with appropriate dispersion relations determined by the shape of the Fermi surface. The energy of skyrmion topological defects, which are shown to be introduced by doping, is used as an order parameter for antiferromagnetic order. We obtain analytic expressions for this as a function of doping which allow us to plot the curves T_N(x_c)\times x_c and M(x)\times x, for both YBCO and LSCO, in good quantitative agreement with the experimental data.Comment: 4 pages, revtex, 5 embeeded figure

Doping dependence of the Neel temperature in Mott-Hubbard antiferromagnets: Effect of vortices

The rapid destruction of long-range antiferromagnetic order upon doping of Mott-Hubbard antiferromagnetic insulators is studied within a generalized Berezinskii-Kosterlitz-Thouless renormalization group theory in accordance with recent calculations suggesting that holes dress with vortices. We calculate the doping-dependent Neel temperature in good agreement with experiments for high-Tc cuprates. Interestingly, the critical doping where long-range order vanishes at zero temperature is predicted to be xc ~ 0.02, independently of any energy scales of the system.Comment: 4 pages with 3 figures included, minor revisions, to be published in PR