Session P13: Focus Session: High-Tc Cuprates and Nickelates

Enhanced polaron formation, suppression of superconducting T$_c$, and the isotope effect in the Hubbard model with phonons

Using a dynamical cluster quantum Monte Carlo approximation we investigate the effect of dynamical Holstein, buckling and breathing phonons on the physics of the 2D Hubbard model at small doping. For all three phonon modes the interplay of electronic correlations and the electron-phonon interaction produces two competing effects, an enhancement of the effective $d$-wave pairing interaction and a strong suppression of the single-particle quasiparticle weight. Due to the later effect we find that Holstein, buckling and breathing phonons suppress superconductivity in the region of parameter space relevant for cuprate superconductors. The renormalization of the single- particle propagator, associated with polaron formation, is significantly enhanced by the presence of antiferromagnetic correlations. Moreover, as a complementray effect, the electron-phonon scattering strongly enhances the spin correlations at finite doping, showing a synergistic interplay between the electron-phonon coupling and antiferromagnetic correlations. The suppression of superconductivity due to polaron formation can explain the isotope effect observed in cuprates. We find a positive and large isotope exponent in the underdoped region where the antiferromagnetic correlations are strong and a small positive isotope exponent in the optimally doped region, in agreement with experiment.   

The effect of strong impurity scattering on superconductivity in the 2D Hubbard model

We study the effect of strong impurity scattering in the two-dimensional Hubbard model to model the effect of Zn substitution in the cuprates, using the dynamical cluster quantum Monte Carlo framework. The superconducting Tc is strongly suppressed by impurity doping, while the spin susceptibility indicates moment formation. We will discuss the dependence of Tc on the strength of the impurity scattering potential, and by investigating the properties of sites neighboring the impurity, the relevance to the experimental STM image of Zn impurity in cuprates.   

Exact thermodynamics of pairing and charge-spin separation in Hubbard nanoclusters

An exact thermal studies of charge-spin separation, pairing fluctuations and pseudogaps are carried out by exact diagonalization of 4-site, frustrated (three dimensional) tetrahedral Hubbard and planar (2x4) clusters. Our exact results for 4-site cluster strongly suggest the existence of a quantum critical points in small Hubbard clusters for particle-particle/hole pair binding, antiferromagnetism, unsaturated and saturated ferromagnetism. Exact studies of larger planar and three dimensional Hubbard clusters yield more intriguing insight supporting the analytical results obtained for the 4-site clusters. Our microscopic theory reproduces electron pairing correlations, phase separation and magnetism in clusters, small nanoparticles, and, surprisingly, in transition metal oxides and high T$_c$ doped cuprates. Theory describes also the effect of pressure on the superconducting transition temperature, the presence of a dormant magnetic state in a narrow region of doping and variation of spin pseudogap with doping level, etc.   

Field Induced Suppression of the Resonance Mode in N-type High-T$_{c}$ Cuprate Pr$_{.88}$LaCe$_{.12}$CuO$_{4-\delta }$ (T$_{c}$=24K)

We discuss the results of our recent inelastic neutron scattering experiments probing the magnetic field dependence of the resonance mode in an electron-doped high-T$_{c}$ cuprate. The resonance mode in the high-T$_{c}$ superconductors is a magnetic excitation widely believed to be fundamentally connected to the superconducting mechanism. The mode itself appears only below T$_{c}$ in optimally-doped cuprates, and its characteristic energy follows the universal relation E$_{Resonance}$=5.8k$_{B}$T$_{c}$ in all classes of cuprate systems. Using a c-axis aligned magnetic field, superconductivity in the electron-doped cuprate, Pr$_{.88}$LaCe$_{.12}$CuO$_{4-\delta }$ (PLCCO), can be completely suppressed with an experimentally realizable field of $\sim $9 T at 2 K. This fact combined with the recent discovery of the resonance mode in this PLCCO system, allows, for the first time, an experimental observation of the evolution of the resonance mode as a cuprate system is driven into its field-suppressed ground state. We will present such a study in a nearly optimally-doped sample of PLCCO (T$_{c}$=24K). The simultaneous emergence under field of static antiferromagnetic (AF) order at the commensurate AF ordering wavevector will also be discussed along with the influence of a c-axis field on low energy excitations in this system. Changes in magnetism coupled to the suppression of the superconducting phase in this PLCCO system will be given particular focus.   

Local Electronic Structure of Bi2Sr2CaCu2O8 near Oxygen Dopants: A Window on the High-Tc Pairing Mechanism

The cuprate material Bi2Sr2CaCu2O8(BSCCO-2212) is believed to be doped by a combination of cation switching and excess oxygen. The interstitial oxygen dopants are of particular interest because scanning tunneling microscopy (STM) experiments have shown that they are positively correlated with the local value of the superconducting gap, and calculations suggest that the fundamental attraction between electrons is modulated locally. In this work, we use density functional theory to try to ascertain which locations in the crystal are energetically most favorable for the O dopant atoms, and how the surrounding cage of atoms deforms. Our results provide support for the identification of STM resonances at -1.0 eV with dopant interstitial O atoms, and show how the local electronic structure is modified nearby.   

Metal to insulator transition and ground state electronic structure of La$_{2-x}$Sr$_x$CuO$_4$

We use the self-interaction corrected local spin-density (SIC-LSD) method to study the ground state electronic structure of La$_{2-x}$Sr$_x$CuO$_4$ as a function of Sr doping $x$. SIC-LSD is a parameter free method based on Density Functional Theory that has proven reliable for the study of strongly correlated electron systems. Sr is introduced via the virtual crystal approximation by linearly mixing the La and Sr potentials. In our calculations, we find that the nature of Cu-$d_{x^2-y^2}$ orbital changes character with varying Sr concentration. In the under-doped regime, one of the Cu-$d_{x^2-y^2}$ orbitals per atom is fully occupied and localized on the Cu site, leading to the formation of magnetic moments on Cu aligned antiferromagnetically in the CuO2 plane. Sr doping introduces holes mainly into the O-p bands and the system is a doped charge transfer insulator. In the over-doped regime, the Cu-$d_{x^2-y^2}$ orbitals are band-like and not spin-split. The moments on the Cu atoms vanish and the system is a nonmagnetic metal. In the orthorhombic structure, the transition from localized to band-like Cu-$d_{x^2-y^2}$ states occurs at about18\% Sr doping, i.e. within the region of optimal doping for superconductivity. We find a similar behavior if the calculations are performed with the idealized tetragonal structure.   

A pressure induced insulator-to-metal critical point in the copper-oxides

The presence of a quantum critical point inside the superconducting dome is a novel ideal unifying high-Tc superconductivity in the copper-oxides with that of other unconventional superconductors in strongly correlated materials. Experimental progress, however, has been difficult since superconductivity protects it from most direct measurements. Yet, the tuning parameter of all efforts to date has been chemical doping, which varies crystal fields, electron-phonon, and electron-electron interactions with potentially very different physical metrics. We report pressure tuned Raman and x-ray scattering data revealing an insulator-to-metal critical point near 20GPa with anomalies in six physical quantities: electronic Raman background, phonon lineshape and temperature dependence, density dependent behaviour of phonon and magnon frequencies, and a subtle structural change in the c-axis. We also suggest why this critical point may be near optimal doping in the high-Tc phase diagram.   

Suppression of charge stripes in highly strained, epitaxial La$_{5/3}$Sr$_{1/3}$NiO$_{4}$ films

We have successfully grown epitaxial La$_{5/3}$Sr$_{1/3}$NiO$_{4}$ films with a small crystalline mosaic using pulsed laser deposition. Using synchrotron radiation, the x-ray diffraction peaks associated with charge stripes have been successfully observed for relatively thick films with little strain. Anomalies due to the charge-ordering transition have been examined using four-point probe resistivity measurement. We also have produced highly strained films with the same total thickness through the use of multilayers of La$_{5/3}$Sr$_{1/3}$NiO$_{4}$ alternating with SrTiO$_{3}$. These films remain under in-plane tension. A thorough search for the charge stripe peaks in the strained multilayers has been negative; the stripes appear to be suppressed under these conditions. This suggests that electron-lattice interactions are critical for the formation of stripe phases. This work is supported through NSF DMR-0239667. Some data was taken at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences.   

Study of the antiferromagnetism in electron-doped cuprate superconductors with disorder

On the basis of the Hubbard model, we study the antiferromagnetic (AF) properties in electron-doped cuprates using the fluctuation-exchange approach. Taking into account the spin fluctuations in combination with the impurity scattering effect due to the randomly distributed dopant-atoms, we formulate the theory of antiferromagnetism in the system. By self-consistently solving the integral-equations for the Green's function, the Neel's temperature is determined by the condition that the Goldstone mode from the transverse spin susceptiblility first appears as the temperature is lowered. Our numerical calculation shows that the Goldstone mode always is pinned at ($pi$,$pi$), insensitive to the doping level. We also calculate the onset temperature of the pseudogap formation which is due to the antiferomagnetic fluctuations, the single particle spectral density, the Fermi surface evolution with doping concentration, and the staggered magnetization. It is shown that the results obtained by the present approach are in very good agreement with the experiments. In the present approach, the density of states (DOS) of the antiferromagnetic phase exhibits a zero-energy peak in the under-doped region.