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Register a new userNickelate superconductors $--$ Multiorbital nature and spin freezing
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Superconductivity with a remarkably high $T_c$ has recently been found in Sr-doped NdNiO$_2$ thin films. While this system bears strong similarities to the cuprates, some differences, such as a weaker antiferromagnetic exchange coupling and possible high-spin moments on the doped Ni sites have been pointed out. Here, we investigate the effect of Hund coupling and crystal field splitting in a simple model system and argue that a multiorbital description of nickelate superconductors is warranted, especially in the strongly hole-doped regime. We then look at this system from the viewpoint of the spin-freezing theory of unconventional superconductivity, which provides a unified understanding of unconventional superconductivity in a broad range of compounds. Sr$_{0.2}$Nd$_{0.8}$NiO$_2$ falls into a parameter regime influenced by two spin-freezing crossovers, one related to the emergent multi-orbital nature in the strongly doped regime and the other related to the single-band character and square lattice geometry in the weakly doped regime.
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We investigate the phase diagram of the spin-orbit-coupled three orbital Hubbard model at arbitrary filling by means of dynamical mean-field theory combined with continuous-time quantum Monte Carlo. We find that the spin-freezing crossover occurring in the metallic phase of the non-relativistic multiorbital Hubbard model can be generalized to a $mathbf{J}$-freezing crossover, with $mathbf{J}=mathbf{L}+mathbf{S}$, in the spin-orbit-coupled case. In the $mathbf{J}$-frozen regime the correlated electrons exhibit a non-trivial flavor selectivity and energy dependence. Furthermore, in the regions near $n=2$ and $n=4$ the metallic states are qualitatively different from each other, which reflects the atomic Hunds third rule. Finally, we explore the appearance of magnetic order from exciton condensation at $n=4$ and discuss the relevance of our results for real materials.
We analyze the nature of Mott metal-insulator transition in multiorbital systems using dynamical mean-field theory (DMFT). The auxiliary multiorbital quantum impurity problem is solved using continuous time quantum Monte Carlo (CTQMC) and the rotationally invariant slave-boson (RISB) mean field approximation. We focus our analysis on the Kanamori Hamiltonian and find that there are two markedly different regimes determined by the nature of the lowest energy excitations of the atomic Hamiltonian. The RISB results at $Tto0$ suggest the following rule of thumb for the order of the transition at zero temperature: a second order transition is to be expected if the lowest lying excitations of the atomic Hamiltonian are charge excitations, while the transition tends to be first order if the lowest lying excitations are in the same charge sector as the atomic ground state. At finite temperatures the transition is first order and its strength, as measured e.g. by the jump in the quasiparticle weight at the transition, is stronger in the parameter regime where the RISB method predicts a first order transition at zero temperature. Interestingly, these results seem to apply to a wide variety of models and parameter regimes.
Superconducting nickelates appear to be difficult to synthesize. Since the chemical reduction of ABO3 (A: rare earth; B transition metal) with CaH2 may result in both, ABO2 and ABO2H, we calculate the topotactic H binding energy by density functional theory (DFT). We find intercalating H is energetically favorable for LaNiO2 but not for Sr-doped NdNiO2. This has dramatic consequences for the electronic structure as determined by DFT+dynamical mean field theory: that of 3d9 LaNiO2 is similar to (doped) cuprates, 3d8 LaNiO2H is a two-orbital Mott insulator. Topotactical H might hence explain why some nickelates are superconducting and others are not.
Spin textures in k-space arising from spin-orbit coupling in non-centrosymmetric crystals find numerous applications in spintronics. We present a mechanism that leads to appearance of k-space spin texture due to spontaneous symmetry breaking driven by electronic correlations. Using dynamical mean-field theory we show that doping a spin-triplet excitonic insulator provides a means of creating new thermodynamic phases with unique properties. The numerical results are interpreted using analytic calculations within a generalized double-exchange framework.
We study the interplay of crystal field splitting and Hund coupling in a two-orbital model which captures the essential physics of systems with two electrons or holes in the e_g shell. We use single site dynamical mean field theory with a recently de
veloped impurity solver which is able to access strong couplings and low temperatures. The fillings of the orbitals and the location of phase boundaries are computed as a function of Coulomb repulsion, exchange coupling and crystal field splitting. We find that the Hund coupling can drive the system into a novel Mott insulating phase with vanishing orbital susceptibility. Away from half-filling, the crystal field splitting can induce an orbital selective Mott state.
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