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Register a new userA model for the phase separation controlled by doping and the internal chemical pressure in different cuprate superconductors
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In the framework of a two-band model, we study the phase separation regime of different kinds of strongly correlated charge carriers as a function of the energy splitting between the two sets of bands. The narrow (wide) band simulates the more localized (more delocalized) type of charge carriers. By assuming that the internal chemical pressure on the CuO$_2$ layer due to interlayer mismatch controls the energy splitting between the two sets of states, the theoretical predictions are able to reproduce the regime of phase separation at doping higher than 1/8 in the experimental pressure-doping-$T_c$ phase diagram of cuprates at large microstrain as it appears in overoxygenated La$_2$CuO$_4$.
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Single crystals of Ca(Fe1-xRux)2As2 (0<x<0.065) and Ca1-yLay(Fe0.973Ru0.027)2As2 (0<y<0.2) have been synthesized and studied with respect to their structural, electronic and magnetic properties. The partial substitution of Fe by Ru induces a decrease of the c-axis constant leading for x<0.023 to a suppression of the coupled magnetic and structural (tetragonal to orthorhombic) transitions. At x_cr=0.023 a first order transition to a collapsed tetragonal (CT) phase is found, which behaves like a Fermi liquid and which is stabilized by further increase of x. The absence of superconductivity near x_cr is consistent with truly hydrostatic pressure experiments on undoped CaFe2As2. Starting in the CT regime at x=0.027 we investigate the additional effect of electron doping by partial replacement of Ca by La. Most remarkably, with increasing y the CT phase transition is destabilized and the system is tuned back into a tetragonal ground state at y>0.08. This effect is ascribed to a weakening of interlayer As-As bonds by electron doping. Upon further electron doping filamentary superconductivity with Tc of 41 K at y=0.2 is observed.
During the last decade, translational and rotational symmetry-breaking phases -- density wave order and electronic nematicity -- have been established as generic and distinct features of many correlated electron systems, including pnictide and cuprate superconductors. However, in cuprates, the relationship between these electronic symmetry-breaking phases and the enigmatic pseudogap phase remains unclear. Here, we employ resonant x-ray scattering in a cuprate high-temperature superconductor La$_{1.6-x}$Nd$_{0.4}$Sr$_{x}$CuO$_{4}$ (Nd-LSCO) to navigate the cuprate phase diagram, probing the relationship between electronic nematicity of the Cu 3$d$ orbitals, charge order, and the pseudogap phase as a function of doping. We find evidence for a considerable decrease in electronic nematicity beyond the pseudogap phase, either by raising the temperature through the pseudogap onset temperature $T^{*}$ or increasing doping through the pseudogap critical point, $p^{*}$. These results establish a clear link between electronic nematicity, the pseudogap, and its associated quantum criticality in overdoped cuprates. Our findings anticipate that electronic nematicity may play a larger role in understanding the cuprate phase diagram than previously recognized, possibly having a crucial role in the phenomenology of the pseudogap phase.
We study the doping evolution of the electronic structure in the pseudogap state of high-Tc cuprate superconductors, by means of a cluster extension of the dynamical mean-field theory applied to the two-dimensional Hubbard model. The calculated single-particle excitation spectra in the strongly underdoped regime show a marked electron-hole asymmetry and reveal a s-wave pseudogap, which display a finite amplitude in all the directions in the momentum space but not always at the Fermi level: The energy location of the gap strongly depends on momentum, and in particular in the nodal region, it is above the Fermi level. With increasing hole doping, the pseudogap disappears everywhere in the momentum space. We show that the origin and the s-wave structure of the pseudogap can be ascribed to the emergence of a strong-scattering surface, which appears in the energy-momentum space close to the Mott insulator.
Charge order in cuprate superconductors is a possible source of anomalous electronic properties in the underdoped regime. Intra-unit cell charge ordering tendencies point to electronic nematic order involving oxygen orbitals. In this context we investigate charge instabilities in the Emery model and calculate the charge susceptibility within diagrammatic perturbation theory. In this approach, the onset of charge order is signalled by a divergence of the susceptibility. Our calculations reveal three different kinds of order: a commensurate ($q=0$) nematic order, and two incommensurate nematic phases with modulation wavevectors that are either axial or oriented along the Brillouin zone diagonal. We examine the nematic phase diagram as a function of the filling, the interaction parameters, and the band structure. We also present results for the excitation spectrum near the nematic instability, and show that a soft nematic mode emerges from the particle-hole continuum at the transition. The Fermi surface reconstructions that accompany the modulated nematic phases are discussed with respect to their relevance for magneto-oscillation and photoemission measurements. The modulated nematic phases that emerge from the three-band Emery model are compared to those found previously in one-band models.
We have performed a temperature-dependent angle-integrated photoemission study of lightly-doped to heavily-overdoped La$_{2-x}$Sr$_{x}$CuO$_4$ and oxygen-doped La$_2$CuO$_{4.10}$. We found that both the magnitude $Delta$* of the (small) pseudogap and the temperature textit{T}* at which the pseudogap is opened increases with decreasing hole concentration, consistent with previous studies. On the other hand, the superconducting gap $Delta_{sc}$ was found to remain small for decreasing hole concentration. The results can be explained if the superconducting gap opens only on the Fermi arc around the nodal (0,0)-($pi,pi$) direction while the pseudogap opens around $sim$($pi$, 0).
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