Band-Structure Trend in Hole-Doped Cuprates and Correlation withTcmax

Abstract

By calculation and analysis of the bare conduction bands in a large number of hole-doped high-temperature superconductors, we have identified the energy of the so-called axial-orbital as the essential, material-dependent parameter. It is uniquely related to the range of the intra-layer hopping. It controls the Cu 4s-character, influences the perpendicular hopping, and correlates with the observed Tc at optimal doping. We explain its dependence on chemical composition and structure, and present a generic tight-binding model. PACS numbers: 74.25.Jb, 74.62.Bf, 74.62.Fj, The mechanism of high-temperature superconductivity (HTSC) in the hole-doped cuprates remains a puzzle [1]. Many families with CuO 2 -layers have been synthesized and all exhibit a phase diagram with T c going through a maximum as a function of doping. The prevailing explanation is that at low doping, superconductivity is destroyed with rising temperature by the loss of phase coherence, and at high doping by pair-breaking [2]. For the materials-dependence of T c at optimal doping, T c max , the only known, but not understood, systematics is that for materials with multiple CuO 2 -layers, such as HgBa 2 Ca n−1 Cu n O 2n+2 , T c max increases with the number of layers, n, until n ∼3. There is little clue as to why for n fixed, T c max depends strongly on the family, e.g. why for n=1, T c max is 40 K for La 2 CuO 4 and 85 K for Tl 2 Ba 2 CuO 6 , although the Neel temperatures are fairly similar. A wealth of structural data has been obtained, and correlations between structure and T c have often been looked for as functions of doping, pressure, uniaxial strain, and family. However, the large number of structural and compositional parameters makes it difficult to find what besides doping controls the superconductivity. Insight was recently provided by Seo et al. [3] who grew ultrathin epitaxial La 1.9 Sr 0.1 CuO 4 films with varying degrees of strain and measured all relevant structural parameters and physical properties. For this single-layer material it was concluded that the distance between the charge reservoir and the CuO 2 -plane is the key structural parameter determining the normal state and superconducting properties.Most theories of HTSC are based on a Hubbard model with one Cu d x 2 −y 2 -like orbital per CuO 2 unit. The oneelectron part of this model is, in the k-representation:with t, t ′ , t ′′ , ... denoting the hopping integrals (≥ 0) on the square lattice (Fig. 1) Relation between the one-orbital model (t, t ′ , t ′′ , ...) and the nearest-neighbor four-orbital model [4] (ε d − εp ∼ 1 eV, t pd ∼ 1.5 eV, εs − εp ∼ 16 − 4 eV, tsp ∼ 2 eV) .The LDA band structure of the best known, and only stoichiometric optimally doped HTSC, YBa 2 Cu 3 O 7 , is more complicated than what can be described with the t-t ′ model. Nevertheless, careful analysis has shown [4] that the low-energy, layer-related features, which are the only generic ones, can be described by a nearest-neighbor, tight-binding model with four orbitals per layer (Fig. 1), Cu d x 2 −...

Spin-fluctuation mechanism of superconductivity in cuprates

“…These facts give a renewed support to the scenario that the spin fluctuations in cuprates represent the lowest bosonic mode relevant for the mechanism of the d-wave SC pairing. One of the novel results of our approach is the importance of the NNN hopping t ′ , consistent with the emerging evidence for the variation of T c among different families of cuprates [15].We consider the t-J modelincluding both the NN hopping t ij = t and the NNN hopping t ij = t ′ . The projection in fermionic operators,c is = (1 − n i,−s )c is leads to a nontrivial EQM, which can be in the k basis represented as where c h is the hole concentration and m kq is the effective spin-fermion coupling m kq = 2Jγ q + ǫ 0 k−q , while ǫ 0 k =…”

Damped spin excitations in a doped cuprate superconductor with orbital hybridization

“…Moreover, it is known from band-structure calculations and experiments that the nextnearest-neighbor (diagonal) hopping integral t constitutes a non-negligible fraction of t [10]. Empirically [11], the superconducting transition scales with the ratio t /t whereas Hubbard-type models predict the opposite trend [12,13]. As a resolution, a two-orbital model-in which hybridization of d z 2 and d x 2 −y 2 states suppresses T c and enhances t -has been put forward [14].…”

Nanoscale Interplay of Strain and Doping in a High-Temperature Superconductor

“…Theoretical models have argued for the primacy of charge, 8 strain, 9 or a carefully tuned combination of both. 10 A set of strain theories have specifically explored the relationship between the apical oxygen height and the superconducting pairing strength, predicting both enhancement [11][12][13][14] and reduction 15 of pairing strength with increasing apical oxygen height. However, microscopic theoretical understanding of cause and effect has been stalled by uncertainty about the precise dopant locations.…”