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Register a new userElectronic structure and two-band superconductivity in unconventional high-$T_c$ cuprates Ba$_2$CuO$_{3+delta}$
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The recently discovered cuprate superconductor Ba$_2$CuO$_{3+delta}$ exhibits a high $T_csimeq73$K at $deltasimeq0.2$. The polycrystal grown under high pressure has a structure similar to La$_2$CuO$_4$, but with dramatically different lattice parameters due to the CuO$_6$ octahedron compression. The crystal field in the compressed Ba$_2$CuO$_4$ leads to an inverted Cu $3d$ $e_g$ complex with the $d_{x^2-y^2}$ orbital sitting below the $d_{3z^2-r^2}$ and an electronic structure highly unusual compared to the conventional cuprates. We construct a two-orbital Hubbard model for the Cu $d^9$ state at hole doping $x=2delta$ and study the orbital-dependent strong correlation and superconductivity. For the undoped case at $x=0$, we found that strong correlation drives an orbital-polarized Mott insulating state with the spin-$1/2$ moment of the localized $d_{3z^2-r^2}$ orbital. In contrast to the single-band cuprates where superconductivity is suppressed in the overdoped regime, hole doping the two-orbital Mott insulator leads to orbital-dependent correlations and the robust spin and orbital exchange interactions produce a high-$T_c$ antiphase $d$-wave superconductor even in the heavily doped regime at $x=0.4$. We conjecture that Ba$_2$CuO$_{3+delta}$ realizes mixtures of such heavily hole-doped superconducting Ba$_2$CuO$_4$ and disordered Ba$_2$CuO$_{3}$ chains in a single-layer or predominately separated bilayer structure. Our findings suggest that unconventional cuprates with liberated orbitals as doped two-band Mott insulators can be a direction for realizing high-T$_c$ superconductivity with enhanced transition temperature $T_c$.
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Angle-dependent magnetoresistance measurements are used to determine the isotropic and anisotropic components of the transport scattering rate in overdoped Tl$_2$Ba$_2$CuO$_{6+delta}$ for a range of $T_c$ values between 15K and 35K. The size of the anisotropic scattering term is found to scale linearly with $T_c$, establishing a link between the superconducting and normal state physics. Comparison with results from angle resolved photoemission spectroscopy indicates that the transport and quasiparticle lifetimes are distinct.
First principles investigations of the high temperature superconducting system Ba$_2$CuO$_{3+delta}$, recently discovered at $deltaapprox0.2$ at $T_c=70$ K, are applied to demonstrate the effects of oxygen ordering on the electronic and magnetic properties. The observed `highly over-doped superconducting phase displays stretched Cu-planar oxygen O$_{rm P}$ distances and anomalously shortened Cu-apical O$_{rm A}$ separations compared with other cuprates. The stoichiometric system $delta=0$, with its strongly one-dimensional (1D) Cu-O$_{rm P}$ chain structure, when nonmagnetic shows 1D Fermi surfaces that lead, within density functional theory, to antiferromagnetic Cu-O$_{rm P}$ chains (a spin-Peierls instability). Accounting for 1D fluctuations and small interchain coupling according to the theory of Schulz indicates this system, like Sr$_2$CuO$_3$, is near the 1D Luttinger-liquid quantum critical phase. The unusual Cu-O bond lengths per se have limited effects on other properties for $delta$=0. We find that a `doubled bilayer structure of alternating Cu-O$_{rm P}$ chains and wide rung Cu$_3$O$_4$ ladders is the energetically preferred one of three possibilities where the additional oxygen ions bridge Cu-O$_{rm P}$ chains in the superconducting phase $delta=1/4$. Nominal formal valences of the three Cu sites are discussed. The six-fold (octahedral) site is the most highly oxidized, accepting somewhat more holes in the $d_{z^2}$ orbital than in the $d_{x^2-y^2}$ orbital. The implication is that two-band physics is involved in the pairing mechanism and the superconducting carriers. The Fermi surfaces of this metallic bilayer structure show both 1D and 2D strong (incipient) nesting instabilities, possibly accounting for the lack of clean single-phase samples based on this structure and suggesting importance for the pairing mechanism.
We have performed angle-resolved photoemission and core-level x-ray photoemission studies of the single-layer cuprate Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+delta}$ (Bi2201) and revealed the doping evolution of the electronic structure from the lightly-doped to optimally-doped regions. We have observed the formation of the dispersive quasi-particle band, evolution of the Fermi ``arc into the Fermi surface and the shift of the chemical potential with hole doping as in other cuprates. The doping evolution in Bi2201 is similar to that in Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_2$ (Na-CCOC), where a rapid chemical potential shift toward the lower Hubbard band of the parent insulator has been observed, but is quite different from that in La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO), where the chemical potential does not shift, yet the dispersive band and the Fermi arc/surface are formed around the Fermi level already in the lightly-doped region. The (underlying) Fermi surface shape and band dispersions are quantitatively analyzed using tight-binding fit, and the deduced next-nearest-neighbor hopping integral $t$ also confirm the similarity to Na-CCOC and the difference from LSCO.
We report on the phase diagram of antiferromagnetism (AFM) and superconductivity (SC) in three-layered Ba_2Ca_2Cu_3O_6(F,O)_2 by means of Cu-NMR measurements. It is demonstrated that AFM and SC uniformly coexist in three-layered compounds as well as in four- and five-layered ones. The critical hole density p_c for the long range AFM order is determined as p_c ~ 0.075, which is larger than p_c ~ 0.02 and 0.055 in single- and bi-layered compounds, and smaller than p_c ~ 0.08-0.09 and 0.10-0.11 in four- and five-layered compounds, respectively. This variation of p_c is attributed to the magnetic interlayer coupling which becomes stronger as the stacking number of CuO_2 layers increases; that is, the uniform coexistence of AFM and SC is a universal phenomenon in underdoped regions when a magnetic interlayer coupling is strong enough to stabilize an AFM ordering. In addition, we highlight an unusual pseudogap behavior in three-layered compounds -- the gap behavior in low-energy magnetic excitations collapses in an underdoped region where the ground state is the AFM-SC mixed phase.
This article describes new polar angle-dependent magnetoresistance (ADMR) measurements in the overdoped cuprate Tl$_2$Ba$_2$CuO$_{6+delta}$ over an expanded range of temperatures and azimuthal angles. These detailed measurements re-affirm the analysis of earlier data taken over a more restricted temperature range and at a single azimuthal orientation, in particular the delineation of the intraplane scattering rate into isotropic and anisotropic components. These new measurements also reveal additional features in the temperature and momentum dependence of the scattering rate, including anisotropy in the $T^2$ component and the preservation of both the $T$-linear and $T^2$ components up to 100 K. The resultant form of the scattering rate places firm constraints on the development of any forthcoming theoretical framework for the normal state charge response of high temperature superconducting cuprates.
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