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The recent discovery of superconductivity in La_{3}Ni_{2}O_{7} with T_{c}≃80 K under high pressure opens up a new route to high-T_{c} superconductivity. This material realizes a bilayer square lattice model featuring a strong interlayer hybridization unlike many unconventional superconductors. A key question in this regard concerns how electronic correlations driven by the interlayer hybridization affect the low-energy electronic structure and the concomitant superconductivity. Here, we demonstrate using a cluster dynamical mean-field theory that the interlayer electronic correlations (IECs) induce a Lifshitz transition resulting in a change of Fermi surface topology. By solving an appropriate gap equation, we further show that the leading pairing instability, s± wave, is enhanced by the IECs. The underlying mechanism is the quenching of a strong ferromagnetic channel, resulting from the Lifshitz transition driven by the IECs. Based on this picture, we provide a possible reason of why superconductivity emerges only under high pressure.
RESUMEN
Mott-Hubbard and Hund electron correlations have been realized thus far in separate classes of materials. Here, we show that a single moiré homobilayer encompasses both kinds of physics in a controllable manner. We develop a microscopic multiband model that we solve by dynamical mean-field theory to nonperturbatively address the local many-body correlations. We demonstrate how tuning with twist angle, dielectric screening, and hole density allows us to switch between Mott-Hubbard and Hund correlated states in a twisted WSe2 bilayer. The underlying mechanism is based on controlling Coulomb-interaction-driven orbital polarization and the energetics of concomitant local singlet and triplet spin configurations. From a comparison to recent experimental transport data, we find signatures of a filling-controlled transition from a triplet charge-transfer insulator to a Hund-Mott metal. Our finding establishes twisted transition-metal dichalcogenides as a tunable platform for exotic phases of quantum matter emerging from large local spin moments.
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Motivated by the recent discovery of superconductivity in infinite-layer nickelates RE_{1-δ}Sr_{δ}NiO_{2} (RE=Nd, Pr), we study the role of Hund coupling J in a quarter-filled two-orbital Hubbard model, which has been on the periphery of the attention. A region of negative effective Coulomb interaction of this model is revealed to be differentiated from three- and five-orbital models in their typical Hund metal active fillings. We identify distinctive regimes including four different correlated metals, one of which stems from the proximity to a Mott insulator, while the other three, which we call "intermediate" metal, weak Hund metal, and valence-skipping metal, from the effect of J being away from Mottness. Defining criteria characterizing these metals is suggested, establishing the existence of Hund metallicity in two-orbital systems.
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Thermoelectrics directly converts waste heat into electricity and is considered a promising means of sustainable energy generation. While most of the recent advances in the enhancement of the thermoelectric figure of merit (ZT) resulted from a decrease in lattice thermal conductivity by nanostructuring, there have been very few attempts to enhance electrical transport properties, i.e., the power factor. Here we use nanochemistry to stabilize bulk bismuth telluride (Bi2Te3) that violates phase equilibrium, namely, phase-pure n-type K0.06Bi2Te3.18. Incorporated potassium and tellurium in Bi2Te3 far exceed their solubility limit, inducing simultaneous increase in the electrical conductivity and the Seebeck coefficient along with decrease in the thermal conductivity. Consequently, a high power factor of â¼43 µW cm-1 K-2 and a high ZT > 1.1 at 323 K are achieved. Our current synthetic method can be used to produce a new family of materials with novel physical and chemical characteristics for various applications.
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Recent experimental findings of two dimensional ferromagnetism in Fe3GeTe2, whose critical temperature can reach room temperature by gating, has attracted great research interest. Here we performed elaborate ab initio studies using density functional theory, dynamical mean-field theory and magnetic force response theory. In contrast to the conventional wisdom, it is unambiguously shown that Fe3GeTe2 is not ferromagnetic but is antiferromagnetic, carrying zero net moment in its stoichiometric phase. Fe defect and hole doping are the keys to make this material ferromagnetic as supported by previously disregarded experiments. Furthermore, we found that electron doping also induces the antiferro- to ferro-magnetic transition. It is crucial to understand the notable recent experiments on gate-controlled ferromagnetism. Our results not only reveal the origin of ferromagnetism of this material but also show how it can be manipulated with defects and doping.
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We performed comparative analysis for DFT+U functionals to better understand their applicability to non-collinear magnetism. Taking LiNiPO4 and Sr2IrO4 as examples, we investigated the results out of two formalisms based on charge-only density and spin density functional plus U calculations. Our results show that the ground state spin order in terms of tilting angle is strongly dependent on Hund J. In particular, the opposite behavior of canting angles as a function of J is found for LiNiPO4. The dependence on the other physical parameters such as Hubbard U and Slater parameterization [Formula: see text] is investigated. We also discuss the formal aspects of these functional dependences as well as parameter dependences. The current study provides useful information and important intuition for the first-principles calculation of non-collinear magnetic materials.
RESUMEN
A systematic comparative study has been performed to better understand DFT+U (density functional theory + U) method. We examine the effect of choosing different double counting and exchange-correlation functionals. The calculated energy distribution and the Hund-J dependence of potential profile for representative configurations clearly show the different behaviors of each DFT+U formalism. In particular, adopting spin-dependent exchange-correlation functionals likely leads to undesirable magnetic solution. Our analyses are further highlighted by real material examples ranging from insulating oxides (MnO and NiO) to metallic magnetic systems (SrRuO3 and BaFe2As2). The current work sheds new light on understanding DFT+U and provides a guideline to use the related methods.
RESUMEN
A systematic first-principles study has been performed to understand the magnetism of thin film SrRuO3 which lots of research efforts have been devoted to but no clear consensus has been reached about its ground state properties. The relative t 2g level difference, lattice distortion as well as the layer thickness play together in determining the spin order. In particular, it is important to understand the difference between two standard approximations, namely LDA and GGA, in describing this metallic magnetism. Landau free energy analysis and the magnetization-energy-ratio plot clearly show the different tendency of favoring the magnetic moment formation, and it is magnified when applied to the thin film limit where the experimental information is severely limited. As a result, LDA gives a qualitatively different prediction from GGA in the experimentally relevant region of strain whereas both approximations give reasonable results for the bulk phase. We discuss the origin of this difference and the applicability of standard methods to the correlated oxide and the metallic magnetic systems.