The electronic structure of transition metal oxides featuring correlated electrons can be rationalized within the Zaanen-Sawatzky-Allen framework. Following a brief description of the present ...paradigms of electronic behavior, we focus on the physics of rare-earth nickelates as an archetype of complexity emerging within the charge transfer regime. The intriguing prospect of realizing the physics of high-
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cuprates through heterostructuring resulted in a massive endeavor to epitaxially stabilize these materials in ultrathin form. A plethora of new phenomena unfolded in such artificial structures due to the effect of epitaxial strain, quantum confinement, and interfacial charge transfer. Here we review the present status of artificial rare-earth nickelates in an effort to uncover the interconnection between the electronic and magnetic behavior and the underlying crystal structure. We conclude by discussing future directions to disentangle the puzzle regarding the origin of the metal-insulator transition, the role of oxygen holes, and the true nature of the antiferromagnetic spin configuration in the ultrathin limit.
Recently developed numerical methods have enabled the explicit construction of the superconducting state of the Hubbard model of strongly correlated electrons in parameter regimes where the model ...also exhibits a pseudogap and a Mott insulating phase. d(x(2)-y(2)) symmetry superconductivity is found to occur in proximity to the Mott insulator, but separated from it by a pseudogapped nonsuperconducting phase. The superconducting transition temperature and order parameter amplitude are found to be maximal at the onset of the normal-state pseudogap. The emergence of superconductivity from the normal state pseudogap leads to a decrease in the excitation gap. All of these features are consistent with the observed behavior of the copper-oxide superconductors.
The quench dynamics of a system involving two competing orders is investigated using a Ginzburg-Landau theory with relaxational dynamics. We consider the scenario where a pump rapidly heats the ...system to a high temperature, after which the system cools down to its equilibrium temperature. We study the evolution of the order parameter amplitude and fluctuations in the resulting time-dependent free-energy landscape. Exponentially growing thermal fluctuations dominate the dynamics. The system typically evolves into the phase associated with the faster-relaxing order parameter, even if it is not the global free-energy minimum. This theory offers a natural explanation for the widespread experimental observation that metastable states may be induced by laser-induced collapse of a dominant equilibrium order parameter.
A combination of density functional and dynamical mean field theory calculations are used to show that the remarkable metal-insulator transition in the rare-earth-element nickelate perovskites arises ...from a site-selective Mott phase, in which the d electrons on half of the Ni ions are localized to form a fluctuating moment while the d electrons on other Ni ions form a singlet with holes on the surrounding oxygen ions. The calculation reproduces key features observed in the nickelate materials, including an insulating gap in the paramagnetic state, a strong variation of static magnetic moments among Ni sites and an absence of charge order. A connection between structure and insulating behavior is documented. The site-selective Mott transition may be a more broadly applicable concept in the description of correlated materials.
We present a comprehensive dynamical mean field study of the triangular lattice moiré Hubbard model, which is believed to represent the physics of moiré bilayer transition metal dichalcogenides. In ...these materials, important aspects of the band structure including the bandwidth and the order and location of van Hove singularities can be tuned by varying the interlayer potential. We present a magnetic and metal-insulator phase diagram and a detailed study of the dependence of the resistivity on temperature, band filling, and interlayer potential. We find that transport displays Fermi liquid, strange metal, and quantum critical behaviors in distinct regions of the phase diagram. Specifically, we find that the cube-root van Hove singularity ρ(ε)∼|ε|−1/3 gives a strange metal behavior with aT-linear scattering rate andω/Tscaling. We show how magnetic order affects the resistivity. Our results elucidate the physics of the correlated states and the metal-insulator continuous transition recently observed in twisted homobilayerWSe2and heterobilayerMoTe2/WSe2experiments.
In several families of iron-based superconducting materials, a d-wave pairing instability may compete with the leading s-wave instability. Here, we show that when both states have comparable free ...energies, superconducting and nematic degrees of freedom are strongly coupled. Whereas nematic order causes a sharp nonanalytic increase in T(c), nematic fluctuations can change the character of the s-wave to d-wave transition, favoring an intermediate state that does not break time-reversal symmetry but does break tetragonal symmetry. The coupling between superconductivity and nematicity is also manifested in the strong softening of the shear modulus across the superconducting transition. Our results show that nematicity can be used as a diagnostic tool to search for unconventional pairing states in iron pnictides and chalcogenides.
We show that in excitonic insulators with s-wave electron-hole pairing, an applied electric field (either pulsed or static) can induce a p-wave component to the order parameter, and further drive it ...to rotate in the s+ip plane, realizing a Thouless charge pump. In one dimension, each cycle of rotation pumps exactly two electrons across the sample. Higher dimensional systems can be viewed as a stack of one-dimensional chains in momentum space in which each chain crossing the Fermi surface contributes a channel of charge pumping. Physics beyond the adiabatic limit, including in particular dissipative effects is discussed.
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