Electronic Correlations Stabilize the Antiferromagnetic Mott State inCs3C60

Abstract

Cs3C60 in the A15 structure is an antiferromagnet at ambient pressure in contrast with other superconducting trivalent fullerides. Superconductivity is recovered under pressure and reaches the highest critical temperature of the family. Comparing density-functional calculations with generalized gradient approximation to the hybrid functional HSE, which includes a suitable component of exchange, we establish that the antiferromagnetic state of Cs3C60 is not due to a Slater mechanism, and it is stabilized by electron correlation. HSE also reproduces the pressure-driven metalization. Our findings corroborate previous analyses suggesting that the properties of this compound can be understood as the result of the interplay between electron correlations and Jahn-Teller electronphonon interaction. [5]. The unavoidable question about the "pairing glue" in the fullerides seems to have an answer at least for the standard members of the family, like K 3 C 60 or Rb 3 C 60 . For these compounds there is convincing evidence of an electronphonon driven superconductivity, in which Jahn-Tellercoupled intramolecular vibrations play the main role [6]. Nonetheless, the anomalous properties of "expanded" fullerides with large intermolecular distance, like ammoniated compounds [7] and most notably Cs 3 C 60 suggest that labeling these materials as standard superconductors described by the Bardeen-Cooper-Schrieffer (BCS) theory is at least hazardous.The anomalies are particularly clear in Cs 3 C 60 , in which the large Cs ions increase the lattice spacing in comparison with K-and Rb-doping. This compound is synthesized in two different crystal structure, the fcc which is common to most doped fullerides and a A15 structure, with a bipartite lattice [8]. A15 Cs3C 60 at ambient pressure is an antiferromagnetic (AFM) insulator with T N = 46K and a spin-1/2 moment at each molecule [9], as opposed to K 3 C 60 and Rb 3 C 60 , which are superconductors in the same conditions. A superconducting state is recovered under external pressure of around 4Kbar. Even more interestingly, the critical temperature has a bell-shaped behavior as a function of pressure, with a maximum of 38K at P = 7KBar. The proximity between superconductivity and antiferromagnetism and the existence of a maximum remind of the phase diagram of several exotic superconductors, ranging from the high-temperature copper-oxides to heavy fermion materials, even if in the present case the bell-shaped curve is a function of lattice spacing instead of doping. However, since the phase diagrams of these latter materials are believed to be dominated by strong electron-electron correlations and by the proximity to a Mott insulator, this similarity suggests that also the physics of Cs 3 C 60 can be understood in the same framework.The role of electron-electron correlations has been underlined well before Cs 3 C 60 was known on the basis of estimates of the Coulomb interaction and the bandwidth [10,11] In this light, the synthesis of Cs 3 C 60 provides a system in which the correlation ef...

Antiferromagnetic Slater Insulator Phase of Na2IrO3

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Nonempirical Calculation of Superconducting Transition Temperatures in Light‐Element Superconductors

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Modelling the electronic structure and magnetic properties of LiFeAs and FeSe using hybrid-exchange density functional theory

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“…The hybrid functional such as B3LYP [20] and PBE0 [21] can balance the tendencies of delocalising and localising wave functions and partially eliminates the self-interaction error by mixing exact exchange. For example, the band gap in various compounds can be predicted more accurately by hybrid functional than LDA or GGA [22] and the effect of electron correlation in CsC 60 can be assessed using hybrid exchange functional [19]. A satisfactory theoretical description of electronic structure of FeSCs which can take electron correlation into account properly within DFT formalism is still rare [10,16,23,24,25,26,27,28].…”