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Register a new userDiffusive superconductors beyond Usadel approximation: Electron-hole asymmetry and large photoelectric effect
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We extend the quasiclassical formalism for diffusive superconductors by deriving anisotropic (gradient) corrections to the Usadel equation. We demonstrate that in a number of physical situations such corrections may play a crucial role being responsible for the effects which cannot be recovered within the standard Usadel approximation. One of them is the so-called photoelectric effect in superconductors and superconducting-normal (SN) hybrid structures. Provided a superconducting part of the system is irradiated by an external ac electromagnetic field the charge imbalance develops and a non-vanishing dc voltage is induced across the SN interface. In the presence of magnetic impurities in a superconductor the magnitude of this effect becomes large and can easily be detected in modern experiments.
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High-temperature superconductivity (HTSC) mysteriously emerges upon doping holes or electrons into insulating copper oxides with antiferromagnetic (AFM) order. It has been thought that the large energy scale of magnetic excitations, compared to phonon energies for example, lies at the heart of an electronically-driven superconducting phase at high temperatures. However, despite extensive studies, little information is available for comparison of high-energy magnetic excitations of hole- and electron-doped superconductors to assess a possible correlation with the respective superconducting transition temperatures. Here, we use resonant inelastic x-ray scattering (RIXS) at the Cu L3-edge to reveal high-energy collective excitations in the archetype electron-doped cuprate Nd2-xCexCuO4 (NCCO). Surprisingly, despite the fact that the spin stiffness is zero and the AFM correlations are short-ranged, magnetic excitations harden significantly across the AFM-HTSC phase boundary, in stark contrast with the hole-doped cuprates. Furthermore, we find an unexpected and highly dispersive mode in superconducting NCCO that is undetected in the hole-doped compounds, which emanates from the zone center with a characteristic energy comparable to the pseudogap, and may signal a quantum phase distinct from superconductivity. The uncovered asymmetry in the high-energy collective excitations with respect to hole and electron doping provides additional constraints for modeling the HTSC cuprates.
We present here a microscopic two-band model based on the structure of energetic levels of holes in $mathrm{CuO}_{2}$ conducting layers of cuprates. We prove that two energetically near-lying interacting bands can explain the electron-hole asymmetry. Indeed, we rigorously analyze the phase diagram of the model and show that the critical temperatures for fermion densities below half-filling can manifest a very different behavior as compared to the case of densities above half-filling. This fact results from the inter-band interaction and intra-band Coulomb repulsion in interplay with thermal fluctuations between two energetic levels. So, if the energy difference between bands is too big then the asymmetry disappears. Moreover, the critical temperature turns out to be a non-monotonic function of the fermion density and the phase diagram of our model shows superconducting domes as in high-$T_{c}$ cuprate superconductors. This explains why the maximal critical temperature is attained at donor densities away from the maximal one. Outside the superconducting phase and for fermion densities near half-filling the thermodynamics governed by our Hamiltonian corresponds, as in real high-$T_c$ materials, to a Mott-insulating phase. The nature of the inter-band interaction can be electrostatic (screened Coulomb interaction), magnetic (for instance some Heisenberg-type one-site spin-spin interaction), or a mixture of both. If the inter-band interaction is predominately magnetic then - additionally to the electron-hole asymmetry - we observe a reentering behavior meaning that the superconducting phase can only occur in a finite interval of temperatures. This phenomenon is rather rare, but has also been observed in the so-called magnetic superconductors.
We show, from first-principles calculations, that the hole-doped side of FeAs-based compounds is different from its electron-doped counterparts. The electron side is characterized as Fermi surface nesting, and SDW-to-NM quantum critical point (QCP) is realized by doping. For the hole-doped side, on the other hand, orbital-selective partial orbital ordering develops together with checkboard antiferromagnetic (AF) ordering without lattice distortion. A unique SDW-to-AF QCP is achieved, and $J_2$=$J_1/2$ criteria (in the approximate $J_1&J_2$ model) is satisfied. The observed superconductivity is located in the vicinity of QCP for both sides.
Unconventional superconductivity in molecular conductors is observed at the border of metal-insulator transitions in correlated electrons under the influence of geometrical frustration. The symmetry as well as the mechanism of the superconductivity (SC) is highly controversial. To address this issue, we theoretically explore the electronic properties of carrier-doped molecular Mott system $kappa$-(BEDT-TTF)$_2$X. We find significant electron-hole doping asymmetry in the phase diagram where antiferromagnetic (AF) spin order, different patterns of charge order, and SC compete with each other. Hole-doping stabilizes AF phase and promotes SC with $d_{xy}$-wave symmetry, which has similarities with high-$T_c$ cuprates. In contrast, in the electron-doped side, geometrical frustration destabilizes the AF phase and the enhanced charge correlation induces another SC with extended-$s$+$d_{x^2-y^2}$-wave symmetry. Our results disclose the mechanism of each phase appearing in filling-control molecular Mott systems, and elucidate how physics of different strongly-correlated electrons are connected, namely, molecular conductors and high-$T_c$ cuprates.
Electron-hole asymmetry is a fundamental property in solids that can determine the nature of quantum phase transitions and the regime of operation for devices. The observation of electron-hole asymmetry in graphene and recently in the phase diagram of bilayer graphene has spurred interest into whether it stems from disorder or from fundamental interactions such as correlations. Here, we report an effective new way to access electron-hole asymmetry in 2D materials by directly measuring the quasiparticle self-energy in graphene/Boron Nitride field effect devices. As the chemical potential moves from the hole to the electron doped side, we see an increased strength of electronic correlations manifested by an increase in the band velocity and inverse quasiparticle lifetime. These results suggest that electronic correlations play an intrinsic role in driving electron hole asymmetry in graphene and provide a new insight for asymmetries in more strongly correlated materials.
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