Doped CuO 2 Planes in High T c Cuprates: 2D Or Not 2D ?

The key problem in physics of underdoped cuprates is whether the doping is inhomogeneous and holes are expelled into the 1D domains (stripes). Direct comparison of the resistivity vs T curves of the recently discovered 1D spin-ladder cuprates and underdoped cuprates below the pseudogap temperature T* shows a remarkable similarity which implies that the main scattering mechanism is the same for these two classes of cuprates and therefore in the pseudogap regime the transport in underdoped cuprates is essentially 1D. All the resistivity curves in underdoped cuprates can then be fitted by the model of the 1D quantum transport with the inelastic length in charge stripes being fully controlled by the magnetic spin correlation length in even-chain spin-ladders formed in the Cu-O plane as a result of inhomogeneous doping. Knight shift data are also successfully interpreted by taking the same spin-gap value as the one found from resistivity data.     

Isotope Effect at the Fano Resonance in Superconducting Gaps for Multiband Superconductors at a 2.5 Lifshitz Transition

The doping dependent isotope effect in cuprates is explained in the framework of shape resonances in the superconducting gaps (belonging to the class of Fano resonances) in multicondensate superconductors. This new paradigm for high temperature superconductivity is based on the recent Fermiology scenario emerging from dHvA and quantum oscillation data showing a 2.5 Lifshitz topological transition due to the appearance of new small Fermi surface in the underdoped regime. The isotope effect is calculated for an electronic system near a band edge for a superlattice of stripes. The model reproduces the doping dependence of the isotope exponent behavior in cuprates and allows to identify the relative role of the intraband Cooper pairing and the configuration interaction between pairing channels from experimental data.     

Lattice Effects on Charge Localization in Cuprates

Recent experimental results on cuprates and manganites, including those of elastic and inelastic neutron scattering measurements, suggest that charges are not homogeneously distributed even in the metallic state in these compounds. Charge inhomogeneity results from spin/lattice charge constriction. In cuprates the LO phonons strongly reflect the temperature- and composition-dependent charge inhomogeneity and may possibly be involved in causing it. Unlike the static stripes that compete against superconductivity, the charge inhomogeneity seen by the LO phonons is markedly increased in the superconducting phase. A new mechanism of high-temperature superconductivity involving lattice/spin charge constriction is proposed.     

Isotope Effect of Underdoped Cuprates in the Yang-Rice-Zhang Model

The underdoped region of the cuprates phase diagram displays many novel electronic phenomena both in the normal and the superconducting state. Many of these anomalous properties have found a natural explanation within the resonating valence bond spin liquid phenomenological model of Yang-Rice-Zhang (YRZ) which includes the rise of a pseudogap. This leads to Fermi surface reconstruction and profoundly changes the electronic structure. Here, we extend the previous work to consider the shift in critical temperature on ¹⁶O to ¹⁸O substitution. The isotope effect has been found experimentally to be very small at optimal doping yet to rapidly increase to very large values with underdoping. The YRZ model provides a natural explanation of this behavior and supports the idea of a pairing mechanism which is mainly spin fluctuations with a subdominant (～10%) phonon contribution.     

Dimensionality of Spin Modulations in 1/8-Doped Lanthanum Cuprates from the Perspective of NQR and μSR Experiments

We investigate the dimensionality of inhomogeneous spin modulation patterns in the cuprate family of high-temperature superconductors with particular focus on 1/8-doped lanthanum cuprates. We compare one-dimensional stripe modulation pattern with two-dimensional checkerboard of spin vortices in the context of nuclear quadrupole resonance (NQR) and muon spin rotation (μSR) experiments. In addition, we also consider the third pattern, a two-dimensional superposition of spin spirals. Overall, we have found that none of the above patterns leads to a consistent interpretation of the two types of experiments considered. This, in particular, implies that the spin vortex checkerboard cannot be ruled out on the basis of available NQR/μSR experimental results.     

Rotation of orbital stripes and the consequent charge-polarized state in bilayer manganites

Nanoscale self-organization of electrons is ubiquitously observed in correlated electron systems such as complex oxides of transition metals. The phenomenon of charge ordering (CO) or the formation of charge stripes, as observed for layered-structure cuprates and nickelates, is one such example. Among them, CO in manganites is closely tied to the orbital degree of freedom of 3d electrons, leading to staggered orbital ordering or the formation of orbital stripes in the layered structure. Here, we describe the phenomena of thermally induced rotation of the orbital stripes by 90( composite function) for bilayered manganite crystals with half hole doping, that is, a 1:1 ratio of Mn3+/Mn4+. The rotation of orbital stripes and the consequent CO coupled with the underlying lattice distortion were found to produce the charge-polarized state, as also shown by its optical second-harmonic generation activity.     

Effect of Cu-site Spin Polarization on Zhang–Rice State

The stability of Zhang–Rice singlet state is an over-looked issue in derivation of the single-band effective Hamiltonian from the two-band model for high T _c cuprates. By applying the diagonalization method to the two-band model, we show the decoupling of Zhang–Rice singlet by arbitrarily weak spin polarization on the Cu-site. Zhang–Rice state is inadequate for describing the electronic structure of high T _c cuprates in the low doping limit where the effect of local spin polarization is remarkable.     

ARPES Spectra from Bond-Centered Stripe Phase of Moderately Doped Cuprates

Using a combination of the bond formalism and the spin polaron approach we calculate the single particle spectral function for the system of stripes which take the form of two-leg ladder-like domain walls (DWs) between antiferromagnetic (AF) domains with the changing across each DW by π phase of the sublattice magnetization. We assume that the ordered bonds which are in the singlet state, form in the DW a pattern which resembles two layers of a brick wall. We concentrate on the doping level 1/8 and on levels slightly above this value when, as suggested by experiments, the distance between axes of nearest stripes is four lattice spacings and the linear filling of stripes is about 1/2. By comparing the calculated and measured spectra we demonstrate that among three simplest choices of bond order in the DW, singlets formed on rungs, singlets formed on nearest legs on both sides of the stripe axis and the brick wall structure, a most obvious resemblance to the ARPES spectra at the Fermi energy from the doped La₂CuO₄ in the stripe phase shows the single particle spectral weight of the stripe system formed in the underlying spin background with bond order on the nearest leg on both sides of the stripe axis. This conclusion is consistent with the result of the stability analysis reported in a separate paper.     

Implications of Spin Vortex Scenario for 1/8-Doped Lanthanum Cuprates

A superlattice of spin vortices has been proposed in an earlier article as an alternative to the stripe interpretation of spin modulations in 1/8-doped lanthanum cuprates. The present article addresses several additional characteristics of the spin vortex lattice, namely: (i) the nature of extended charge states; (ii) the position of neutron spin peaks as a function of doping; and (iii) the absence of higher-order spin harmonics. All these characteristics afford straightforward connection to the experimental results produced by angle-resolved photoemission spectroscopy, resistivity measurements, and neutron scattering.     

Charge Stripes in Underdoped Cuprates: Ginzburg-Landau Description

There is experimental evidence that the existence of dynamical charge stripes is a universal property of underdoped cuprates. We use a Ginzburg-Landau approach to study the interplay of the three order parameters describing anti-ferromagnetic, charge, and superconducting order. In this context we discuss the relevance of stripes for superconductivity and the effect of magnetically dressed holes on the antiferromagnetic transition.     

Influence of Lattice Parameter Scaling on Local Electronic and Magnetic Properties in La2CuO4

A large amount of static and dynamic NMR experiments have been carried out to determine the spin properties in high temperature superconductors cuprates (HTSC). In the majority of cuprates the magnetic shift at the planar copper nucleus is independent of the temperature when the field is applied perpendicular to the CuO₂ planes and does not exhibit the expected reduction of the spin susceptibility in the superconducting state. This has been explained by a coincidental cancellation of on-site and transferred hyperfine fields at the copper nucleus, which would lead to a vanishing spin shift. To examine this explanation we determine the hyperfine fields by ab initio cluster calculations for La₂CuO₄ within the framework of spin-polarized density functional theory. In particular we investigate the sensitivity of the hyperfine fields on the lattice constants.     

Influence of Lattice Parameter Scaling on Local Electronic and Magnetic Properties in La2CuO4

A large amount of static and dynamic NMR experiments have been carried out to determine the spin properties in high temperature superconductors cuprates (HTSC). In the majority of cuprates the magnetic shift at the planar copper nucleus is independent of the temperature when the field is applied perpendicular to the CuO₂ planes and does not exhibit the expected reduction of the spin susceptibility in the superconducting state. This has been explained by a coincidental cancellation of on-site and transferred hyperfine fields at the copper nucleus, which would lead to a vanishing spin shift. To examine this explanation we determine the hyperfine fields by ab initio cluster calculations for La₂CuO₄ within the framework of spin-polarized density functional theory. In particular we investigate the sensitivity of the hyperfine fields on the lattice constants.     

Spin dynamics and implications for superconductivity: some problems with thed-wave scenario

We review the spin dynamics of the normal state of the cuprates with special emphasis on neutron data in both the YBa₂Cu₃O_7?δ and La_2?x Sr _x CuO₄ systems. When realistic models of the Fermi surface shapes are incorporated, along with a moderate degree of spin fluctuations, we find good semiquantitative agreement with experiment for both cuprates. Building on the success of this Fermi-liquid-based scheme, we explore the implications ford-wave pairing from a number of vantage points. We conclude that our present experimental and theoretical understanding is inadequate to confirm or refute thed-wave scenario.     

A Link between Spin Fluctuation and Fermi Surface in the High T c Cuprates — A Description within the Single-band Hubbard Model

A link between the spin structure and the shape of the Fermi surface is sys-tematically explored for the single-band repulsive Hubbard model with the fluctuation exchange (FLEX) approximation. We show that (i) the ex-perimentally observed band filling dependence of the peak position of the spin structure in the high T _C cuprates can be understood for both the hole-doped (YBCO, Bi2212) and electron-doped (NCCO) regimes. (ii) For La _2?xSr_xCuO₄, the spin-structure incommensurability and the ARPES re-sult for the Fermi surface can be understood simultaneously if the second nearest neighbor hopping decreases with the hole doping.     

Electronic Checkerboard Pattern in Striped Racetrack Domains: A Consistent Picture of Recent Neutron and STM Experiments

We discuss recent elastic neutron scattering and scanning tunneling experiments on high-T _c cuprates exposed to an applied magnetic field. In particular we show that a physical picture consisting of antiferromagnetic vortex cores operating as pinning centers for surrounding stripes is qualitatively consistent with the neutron data provided the stripes have the usual antiphase modulation. Further, we calculate the electronic structure in such a region using a T-matrix method, and find a checkerboard interference pattern consistent with recent scanning tunneling experiments.     

Spin Structures of the Hubbard Clusters and the Pseudogap

The spin and charge structures formed in the Hubbard model for a finite two-dimensional cluster have been studied in the mean field approximation. The self-consistent iterative procedure reduces an uncorrelated initial spin distribution into stable structures with spectral properties typical of stripes. It is shown that the density of states of the system for any doping has a sharp minimum at the Fermi level, the pseudogap. The pinning of the gap at the Fermi level is characteristic not only of a superconducting state, but also typical of a normal state of spin glasses. Our results support conception of the PG state as a state with frozen locally nematic spin structures of antiferromagnetic domains.     

Polaron Dynamics and Bipolarons in Cuprates

The Fröhlich electron–phonon interaction in cuprates, manganites, and other charge-transfer ionic Mott insulators is much stronger than any magnetic interaction. The polaron shift due to the Fröhlich interaction, which is about 1 eV, suggests that carriers in those systems are small (bi)polarons at all temperatures and doping levels. We show both analytically and numerically that (bi)polarons exist in the itinerant Bloch states at temperatures below the characteristic phonon frequency no matter which values the parameters of the system take. The small Fröhlich polaron has spectral features compatible with the single-particle tunneling and photoemission in cuprates. Whereas the band energy dispersion of intersite bipolarons is responsible for the d-wave symmetry of the condensate wave-function, the single-particle excitation spectrum is s-like in agreement with the tunneling data. Two different energy scales in Giaver tunneling and Andreev reflection experiments in cuprates can be understood in the framework of the bipolaron theory as well.     

Unrestricted Hartree-Fock approximation of the extended emery model

We use an unrestricted self-consistent Hartree-Fock approach to calculate the nature of doping states in the three-band Hubbard model. It turns out that for physically relevant parameter values one hole is localized within a small spin-polarized region where five Cu spins are aligned in the same direction. The spin polarization and binding energy between these spinpolaronic states are investigated as a function of different parameters including a Holstein-type electron-phonon coupling on the Cu sites. At higher doping concentration we observe the occurrence of afmon states where the holes are localized in a ring-shaped area. Inside this ring the antiferromagnetic order parameter has inverse sign with respect to the residual antifer-romagnetically ordered plane.     

Electronic Structure in Underdoped Cuprates Due to the Emergence of a Pseudogap

The phenomenological Green’s function developed in the works of Yang, Rice, and Zhang has been very successful in understanding many of the anomalous superconducting properties of the deeply underdoped cuprates. It is based on considerations of the resonating valence bond spin liquid approximation and is designed to describe the underdoped regime of the cuprates. Here, we emphasize the region of doping, x, just below the quantum critical point at which the pseudogap develops. In addition to Luttinger hole pockets centered around the nodal direction, there are electron pockets near the antinodes which are connected to the hole pockets by gapped bridging contours. We determine the contours of nearest approach as would be measured in angular resolved photoemission experiments and emphasize signatures of the Fermi surface reconstruction from the large Fermi contour of Fermi liquid theory (which contains 1+x hole states) to the Luttinger pocket (which contains x hole states). We find that the quasiparticle effective mass renormalization increases strongly toward the edge of the Luttinger pockets beyond which it diverges.     

Spin-Cast Deposition of CdSe-CdS Core-Shell Colloidal Quantum Dots on Doped GaAs Substrates: Structural and Optical Characterization

The detailed study of the effects of spin recipe and GaAs substrate doping (i.e., semi-insulating, n-type, or p-type) on the structural and optical properties of spin-cast CdSe-CdS core-shell CQDs provides insight into the surface adsorption and charge transfer mechanisms that will influence any potential optoelectronic device. The hypotheses of this study are: i) it is possible to establish spin-casting recipes that yield a thin film of CQDs with large surface density and uniform size, and ii) it is possible to control the optical response of CQDs by varying the GaAs substrate doping to influence charge transfer processes. As a result of these measurements, we have been able to demonstrate a strong dependence of spin-cast CQD structural properties on the doping type of the GaAs substrate, as well as evidence from measured optical properties to support the idea that hot carriers photoexcited in the GaAs substrate are transferred either to the CQD surface states through organic surface ligands or directly to confined states within the CQD.