Branko P. Stojkovic

MICROSCOPIC THEORY OF WEAK PSEUDOGAP BEHAVIOR IN THE UNDERDOPED CUPRATE SUPERCONDUCTORS : GENERAL THEORY AND QUASIPARTICLE PROPERTIES

We use a solution of the spin fermion model which is valid in the quasistatic limit {pi}T{gt}{omega}{sub sf}, found in the intermediate (pseudoscaling) regime of the magnetic phase diagram of cuprate superconductors, to obtain results for the temperature and doping dependence of the single particle spectral density, the electron-spin fluctuation vertex function, and the low frequency dynamical spin susceptibility. The resulting strong anisotropy of the spectral density and the vertex function lead to the qualitatively different behavior of {ital hot} [around {bold k}=({pi},0)] and {ital cold} [around {bold k}=({pi}/2,{pi}/2)] quasiparticles seen in ARPES experiments. We find that the broad high energy features found in ARPES measurements of the spectral density of the underdoped cuprate superconductors are determined by strong antiferromagnetic (AF) correlations and incoherent precursor effects of an SDW state, with reduced renormalized effective coupling constant. Due to this transfer of spectral weight to higher energies, the low frequency spectral weight of {ital hot} states is strongly reduced but couples very strongly to the spin excitations of the system. For realistic values of the antiferromagnetic correlation length, their Fermi surface changes its general shape only slightly but the strong scattering of hot states makes the Fermi surface crossing invisible abovemorexa0» a pseudogap temperature T{sub {asterisk}}. The electron spin-fluctuation vertex function, i.e., the effective interaction of low energy quasiparticles and spin degrees of freedom, is found to be strongly anisotropic and enhanced for hot quasiparticles; the corresponding charge-fluctuation vertex is considerably diminished. We thus demonstrate that, once established, strong AF correlations act to reduce substantially the effective electron-phonon coupling constant in cuprate superconductors. {copyright} {ital 1999} {ital The American Physical Society}«xa0less

Microscopic model of carbon monoxide binding to myoglobin

We present a microscopic model of carbon monoxide (CO) binding to myoglobin which reproduces the experimentally observed Arrhenius pre-exponential factor of 109u200as−1 and activation enthalpy distribution centered at 12 kJ/mol. The model is based on extensive ab initio calculations of CO interacting with a model heme-imidazole group which we performed using a fully quantum mechanical Hartree–Fock/density functional theory (HF/DFT) hybrid method. We fit the HF/DFT calculated energies, obtained for over 1000 heme-CO structures with varied CO and iron positions and orientations for both high (S=2) and low (S=0) spin states, to a model potential function which includes a bonding interaction in both of the spin states, electrostatic, and anisotropic Lennard-Jones-type interactions. By combining the x-ray determined protein structure with this potential and protein-CO interactions and internal heme interaction potentials obtained from established molecular dynamics literature, we calculate the energy required for ...

CHARGE ORDERING AND LONG-RANGE INTERACTIONS IN LAYERED TRANSITION METAL OXIDES

We study the competition between long-range and short-range interactions among holes within a continuum formulation of the spin density wave picture of layered transition metal oxides. We focus on the problem of charge ordering and the charge phase diagram. The main interactions are the long-range Coulomb interaction and a magnetic dipolar short-range interaction generated by short-range antiferromagnetic fluctuations. Four different phases depending on the strength of the dipolar interaction and the density of holes exist: Wigner crystal, diagonal stripes, horizontal-vertical stripes (loops). and a glassy-clumped phase. The effect of temperature, disorder, and lattice effects on these phases are discussed. {copyright} {ital 1999} {ital The American Physical Society}

WEAK PSEUDOGAP BEHAVIOR IN THE UNDERDOPED CUPRATE SUPERCONDUCTORS

Abstract We report on an exact solution of the nearly antiferromagnetic Fermi liquid spin fermion model in the limit π T ≫ω sf , which demonstrates that the broad high energy features found in ARPES measurements of the spectral density of the underdoped superconductors are determined by strong antiferromagnetic (AF) correlations and precursor effects of an SDW state. We show that the onset temperature, T cr , of weak pseudogap (pseudoscaling) behavior is determined by the strength, ξ, of the AF correlations, and obtain with ξ( T cr )≈2 the generic changes in low frequency magnetic behavior seen in NMR experiments, confirming the Barzykin and Pines crossover criterion.

Charge ordering and long-range interactions in layered transition metal oxides: A quasiclassical continuum study

The competition between long-range and short-range interactions among holes moving in an antiferromagnet (AF) is studied within a model derived from the spin-density-wave picture of layered transition metal oxides.A novel numerical approach is developed that allows one to solve the problem at finite hole densities in very large systems (of the order of hundreds of lattice spacings), albeit in a quasiclassical limit, and to correctly incorporate the long-range part of the Coulomb interaction. The focus is on the problem of charge ordering and the charge phase diagram: at low temperatures four different phases are found, depending on the strength of the magnetic (dipolar) interaction generated by the spin-wave exchange and the density of holes. The four phases are the Wigner crystal, diagonal stripes, a grid phase (horizontal-vertical stripe loops), and a glassy-clumped phase. In the presence of both in-plane and out-of-plane charged impurities the stripe ordering is suppressed, although finite stripe segments persist. At finite temperatures multiscale (intermittency) dynamics is found, reminiscent of that in glasses. The dynamics of stripe melting and its implications for experiments is discussed. (c) 2000 The American Physical Society.

Universal relationship between giant magnetoresistance and anisotropic magnetoresistance in spin valve multilayers

Abstract We measure the giant magnetoresistance (GMR) with the current both parallel and perpendicular to the direction of the magnetization in the ferromagnetic (FM) layers and thus probe the anisotropy of the effective mean free paths for the spin-up and spin-down electrons, seen in the anisotropic magnetoresistance. We find that the difference of the GMR in the two configurations, when expressed in terms of the sheet conductance, displays a nearly universal behavior as a function of GMR. On interpreting the results within the Boltzmann transport formalism we demonstrate the importance of bulk scattering for GMR.

Materials-Driven Science: From High Tc to Complex Adaptive Matter

The past decade has seen a paradigm shift from the study of solids as a collection of weakly interacting elementary excitations to a materials-driven exploration of systems in chemistry physics and biology that display unexpected emergent behavior because they contain collections of agents (electrons atoms molecules) that couple nonlinearly with each other in an enviroment which both influences agent behavior and is influenced by it. Experimental and theoretical investigations of the high Tc cuprate superconductors and related members of the strongly correlated branch of the hard matter family (heavy electron systems organic superconductors manganites) have raised profound questions in physics. Because these questions have deep intellectual and practical connections with the behavior of soft matter and biological matter the exploration of these connections can open new directions for further progress.