Electron-doped Side Research Articles

Odd-parity triplet superconducting phase in multiorbital materials with a strong spin-orbit coupling: application to doped Sr₂IrO₄.

We explore possible superconducting states in t(2g) multiorbital correlated electron systems with strong spin-orbit coupling (SOC). In order to study such systems in a controlled manner, we employ large-scale dynamical mean-field theory (DMFT) simulations with the hybridization expansion continuous-time quantum Monte Carlo (CTQMC) impurity solver. To determine the pairing symmetry, we go beyond the local DMFT formalism using parquet equations to introduce the momentum dependence in the two-particle vertex and correlation functions. In the strong SOC limit, a singlet, d-wave pairing state in the electron-doped side of the phase diagram is observed at weak Hund's coupling, which is triggered by antiferromagnetic fluctuations. When the Hund's coupling is comparable to SOC, a twofold degenerate, triplet p-wave pairing state with relatively high transition temperature emerges in the hole-doped side of the phase diagram, which is associated with enhanced charge fluctuations. Experimental implications to doped Sr2IrO4 are discussed.

Competing phases of the Hubbard model on a triangular lattice: Insights from the entropy

In this Rapid Communication, we present a comprehensive study of the Hubbard model on the isotropic triangular lattice by using the recently developed ladder dual-fermion approach. This method is a nonlocal extension of the dynamical mean-field theory and is free of finite-size effect. In addition to confirming the much-discussed phase diagram at half-filling, our work provides insights into both hole- and electron-doped regimes and, in particular, the finite-temperature phase diagrams. We find the triangular system to be short-range correlated with an associated magnetic phase diagram, which is asymmetric with respect to hole and electron doping. In contrast to the unfrustrated lattice, it can adiabatically be cooled by increasing the interactions. Strikingly, at the electron-doped side, the entropy displays a maximum. This latter example, as well as other results of our work, may provide insights for a variety of correlated triangular materials, such as the water-intercalated sodium-doped cobaltates.

Doping dependence of spin fluctuations and electron correlations in iron pnictides

Doping dependence of the spin fluctuations and the electron correlations in the effective five-band Hubbard model for iron pnictides is investigated using the fluctuation-exchange approximation. For a moderate hole doping, we find a dominant low-energy spin excitation at Q=(\pi,0), which becomes critical at low temperature. The low-energy spin excitations in the heavily hole-doped region are characterized by weak Q dependence. The electron doping leads to an appearance of a pseudogap in spin excitation spectrum. Correspondingly, the NMR-1/T1 relaxation rate is strongly enhanced on the hole-doped side and suppressed on the electron-doped side of the phase diagram. This behavior can be to large extent understood by systematic changes of the Fermi-surface topology.

Electron doping and superconductivity in the two-dimensional Hubbard model

An elaborate variational wave function is used for studying superconductivity in the repulsive twodimensional Hubbard model, including both nearest- and next-nearest-neighbor hoppings. A marked asymmetry is found between the “localized” hole-doped region and the more itinerant electron-doped region. Superconductivity with d-wave symmetry turns out to be restricted to densities where the Fermi surface crosses the magnetic zone boundary. A concomitant peak in the magnetic structure factor at , clearly points to a magnetic mechanism. One of the central issues in the field of high-temperature superconductors has been—and for some researchers still is—the question whether pairing in the cuprates arises from purely repulsive interactions, as proposed by Anderson two decades ago. 1 This question has been studied extensively in the framework of the two-dimensional 2Drepulsive Hubbard model and the related t-J model. Recent progress, both in dynamical mean-field theory 2,3 and in variational calculations, 4 has strengthened the case for the existence of a superconducting phase in the Hubbard model, with a d-wave gap parameter reasonably close to the experimental values for intermediate interaction strengths U of the order of the bandwidth. This conclusion has been challenged on the basis of Monte Carlo simulations, 5 which we believe to be not conclusive, as discussed below. Most previous studies of the Hubbard model have been restricted to nearest-neighbor hopping, where electron doping does not differ from hole doping. Here we show that the addition of second-neighbor hopping which breaks the electron-hole symmetry changes this behavior substantially. While the hole-doped side is “localized” and shows kineticenergy-driven superconductivity with a large condensation energy, the electron-doped side is itinerant with a potentialenergy-driven superconductivity and a small condensation energy.

Structural and electronic response upon hole doping of rare-earth iron oxyarsenides Nd1−xSrxFeAsO (0 < x≤ 0.2)

Hole doping of NdFeAsO via partial replacement of Nd3+ by Sr2+ is a successful route to obtaining superconducting phases (T(c) = 13.5 K for a Sr2+ content of 20%); however, the structural and electronic response with doping is different from and non-symmetric to that in the electron-doped side of the phase diagram.

Doping Dependence of Superconductivity and Lattice Constants in Hole Doped La1-xSrxFeAsO

By using solid state reaction method we have fabricated the hole doped $La_{1-x}Sr_xFeAsO$ superconductors with Sr content up to 0.13. It is found that the sharp anomaly at about 150 K and the low temperature upturn of resistivity are suppressed by doping holes into the parent phase. Interestingly both the superconducting transition temperature $T_c$ and the lattice constants (a-axis and c-axis) increase monotonously with hole concentration, in sharp contrast with the electron doped side where the $T_c$ increases with a continuing shrinkage of the lattice constants either by dope more fluorine or oxygen vacancies into the system. Our data clearly illustrate that the superconductivity can be induced by doping holes via substituting the trivalent La with divalent Sr in the LaFeAsO system with single FeAs layer, and the $T_c$ in the present system exhibits a symmetric behavior at the electron and hole doped sides, as we reported previously.

Local tunneling spectroscopy of the electron-doped cuprate superconductorSm1.85Ce0.15CuO4

We present local tunneling spectroscopy in the optimally electron-doped cuprate ${\mathrm{Sm}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$, $x=0.15$. A clear signature of the superconducting gap is observed with an amplitude ranging from place to place and from sample to sample ($\ensuremath{\Delta}\ensuremath{\sim}3.5--6\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$. Another spectroscopic feature is simultaneously observed at high energy with an amplitude ranging from $\ifmmode\pm\else\textpm\fi{}60$ to $\ifmmode\pm\else\textpm\fi{}80\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. Its energy scale and temperature evolution are found to be compatible with previous photoemission and optical experiments. If interpreted as the signature of antiferromagnetic order in the samples, these results could suggest the coexistence on the local scale of antiferromagnetism and superconductivity on the electron-doped side of cuprate superconductors.

Doping of Ce inT−La2CuO4: Rigorous test for electron-hole symmetry for high-Tcsuperconductivity

We report that Ce doping was achieved in ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ with the ${\mathrm{K}}_{2}{\mathrm{NiF}}_{4}\phantom{\rule{0.3em}{0ex}}(T)$ structure for the first time by molecular beam epitaxy. A synthesis temperature of as low as $\ensuremath{\sim}$ $630\phantom{\rule{0.3em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and an appropriate substrate choice, $i.e.$, $(001){\mathrm{LaSrGaO}}_{4}\phantom{\rule{0.3em}{0ex}}({a}_{s}=3.843\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$, enabled us to incorporate Ce into the ${\mathrm{K}}_{2}{\mathrm{NiF}}_{4}$ lattice and to obtain Ce-doped $T\text{\ensuremath{-}}{\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}$ up to $x\ensuremath{\sim}0.06$. The doping of Ce makes $T\text{\ensuremath{-}}{\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ more insulating, which is in sharp contrast to Sr (or Ba) doping in $T\text{\ensuremath{-}}{\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$, which makes the compound metallic and superconducting. The observed smooth increase in resistivity from the hole-doped side $(T\text{\ensuremath{-}}{\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4})$ to the electron-doped side $(T\text{\ensuremath{-}}{\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4})$ indicates that the electron-hole symmetry is broken in the $T$-phase materials.

Ce doping in T-La 2CuO 4 films: Broken electron–hole symmetry for high- Tc superconductivity

We attempted Ce doping in La 2CuO 4 with the K 2NiF 4 ( T) structure by molecular beam epitaxy. At low growth temperature and with an appropriate substrate choice, we found that Ce can be incorporated into the K 2NiF 4 lattice up to x ∼ 0.06, which had not yet been realized in bulk synthesis. The doping of Ce makes T-La 2− x Ce x CuO 4 more insulating, which is in sharp contrast to Ce doping in La 2CuO 4 with the Nd 2CuO 4 structure, which makes the compounds superconducting. The observed smooth increase in resistivity from hole-doped side ( T-La 2− x Sr x CuO 4) to electron-doped side ( T-La 2− x Ce x CuO 4) indicates that electron–hole symmetry is broken in the T-phase materials.

Phase separation over an extended compositional range: Studies of theCa1−xBixMnO3(x<~0.25)phase diagram

Phase transitions on the electron-doped side of the ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}{\mathrm{MnO}}_{3}$ system $(x<~0.25)$ have been investigated using high-resolution synchrotron x-ray and neutron powder-diffraction techniques, electrical transport and magnetic susceptibility measurements. At room temperature all samples investigated were single phase, paramagnetic conductors $(\ensuremath{\rho}<0.1\ensuremath{\Omega}\mathrm{cm}),$ isostructural with ${\mathrm{GdFeO}}_{3}$ (space group Pnma). The Mn-O-Mn angles remain nearly constant from $x=0$ to $x=0.25,$ while the Mn-O distances steadily increase with the ${\mathrm{Mn}}^{3+}$ content. Three distinct phases are observed at 25 K. The first one, observed from $0.15>~x>~0.03,$ is characterized by the absence of charge and orbital ordering, a canted G-type antiferromagnetic spin structure, and delocalized electron transport. The second phase, observed from $0.25>~x>~0.12$ (single phase at $x=0.18),$ is characterized by pronounced orbital ordering, a C-type antiferromagnetic spin structure, and insulating behavior. The third low-temperature phase, observed for $x>~0.20,$ is characterized by orbital and magnetic ordering similar to the Wigner crystal structure previously observed for ${\mathrm{Ca}}_{0.67}{\mathrm{La}}_{0.33}{\mathrm{MnO}}_{3},$ but with a $4a\ifmmode\times\else\texttimes\fi{}b\ifmmode\times\else\texttimes\fi{}2c$ unit cell. The most striking feature of the phase diagram is the wide compositional range over which low-temperature phase separation is observed. Only those samples with $x<0.12$ and $x=0.18$ did not undergo phase separation upon cooling. We show that this behavior cannot be attributed to compositional variations, and therefore, propose that anisotropic strain interactions between crystallites may be partially responsible for this behavior.