Effects of Localized Spins in Quasi-Two Dimensional Organic Conductors

Hotta, Chisa; Fukuyama, Hidetoshi

doi:10.1143/jpsj.69.2577

Cited by 79 publications

(45 citation statements)

References 16 publications

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“…Chemical pressure can change this ratio driving the system from a metal (or superconductor) to an insulator [8]. Replacing nonmagnetic Ga 3+ by magnetic Fe 3+ , the electronic parameters change even less [10]. Even though this could also, in principle, drive the system from a metal to an insulator, this could not explain why the metallic phase is restored under magnetic field in a first-order transition.…”

Section: +mentioning

confidence: 99%

Magnetic-field-induced superconductivity in layered organic molecular crystals with localized magnetic moments

2002

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The synthetic organic compound λ-(BETS)2FeCl4 undergoes successive transitions from an antiferromagnetic insulator to a metal and then to a superconductor as a magnetic field is increased. We use a Hubbard-Kondo model to clarify the role of the Fe 3+ magnetic ions in these phase transitions. In the high-field regime, the magnetic field acting on the electron spins is compensated by the exchange field He due to the magnetic ions. This suggests that the field-induced superconducting state is the same as the zero-field superconducting state which occurs under pressure or when the Fe 3+ions are replaced by non-magnetic Ga 3+ ions. We show how He can be extracted from the observed splitting of the Shubnikov-de Haas frequencies. Furthermore, we use this method of extracting He to predict the field range for field-induced superconductivity in other materials.The discovery of magnetic-field-induced superconductivity [1] in the two-dimensional compound λ-(BETS) 2 FeCl 4 [where BETS is bis(ethylenedithio)-tetraselenafulvalene] is a new example of the rich phase diagrams of organic molecular crystals [2]. Whereas previously pressure or chemical substitution has been used to change the electronic properties of these organic materials, it is remarkable that this compound undergoes successive electronic phase transitions as the magnetic field is increased. Below a temperature of 8 K, λ-(BETS) 2 FeCl 4 is an antiferromagnetic (AF) insulator [3]. As a magnetic field is applied, it undergoes a firstorder transition to a metal at 11 T. Close to this field, the magnetic moments associated with the spin 5/2 of the Fe 3+ ions undergo a transition to a polarized paramagnet. If the magnetic field is parallel to the layers, there is a transition to a superconductor at 20 T [1], which is then destroyed above 42 T [4]. The magnetic ions are essential to this behavior, since the compound with non-magnetic ions, λ-(BETS) 2 GaCl 4 , is, in contrast, a superconductor at zero field [5], despite very similar crystal structures [6].In this Letter we focus on three questions: (i) Why does the inclusion of magnetic ions change the ground state from a superconductor to an insulator? (ii) Is the magnetic-field-induced superconductivity due to the the Jaccarino-Peter effect [7] (as has recently been proposed [4]), where the external field is compensated by an internal exchange field due to the magnetic ions? and (iii) Does the Jaccarino-Peter picture survive if one takes into account the spin fluctuations associated with the magnetic ions?Recently, Ziman introduced a two-dimensional Hubbard-Kondo model in order to understand question (i) [3]. The model takes into account the four conduction bands associated with layers of BETS molecules (four HOMO orbitals per unit cell), a Kondo coupling between the localized S = 5/2 spins and the conduction electrons, and the Coulomb repulsion between two electrons on the same BETS molecule. Ziman found that for small electron-electron repulsion the periodic potential due to the magnetic ordering (found self-consi...

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Section: +mentioning

confidence: 99%

Magnetic-field-induced superconductivity in layered organic molecular crystals with localized magnetic moments

2002

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“…In this case, π-d interaction is well described by mean field approximation where conduction electrons feel exchange fields from local d moments [4,6,7].…”

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confidence: 99%

Suppression of electronic susceptibility in metal–Mott-insulator alternating material(Me−3,5−DIP)[Ni(dmit)2
Fujiyama
¹
,
Shitade
²
,
Kanoda
³

et al. 2008
Phys. Rev. B
10
1
4
0
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Frequency shifts and nuclear relaxations of 13 C NMR of the metal-insulator alternating material, [Ni(dmit)2]2, are presented. The NMR absorption lines originating from metallic and insulating layers are well resolved, which evidences the coexistence of localized spins (π loc ) and conduction π-electrons. The insulating layer is newly found to undergo antiferromagnetic long range order at about 2.5 K, suggesting emergence of S = 1/2 Mott insulator. In the metallic layer, we found significant suppressions of static and dynamical susceptibilities of conduction electrons below 35 K, where antiferromagnetic correlation in the insulating layer evolves. We propose a dynamical effect through strong π-π loc coupling between the metallic and insulating layers as an origin of the reduction of the density of states.PACS numbers: 71.20. Rv, Correlations between conduction electrons and localized spins attract much attention in condensed matter physics. Competition between magnetic ordering due to the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction and the formation of a Kondo singlet accompanied by mass enhancement of conduction electrons is a constitutive problem [1]. In the last decades, several organic charge-transfer-salts with anions containing magnetic d ions have been synthesized [2,3]. It is widely recognized that physical properties of molecular conductors are determined by a single band formed by the frontier orbital consisting of hybridized π-electons on molecules, and simple tight binding approximation works extremely well for the description of the band. Such simplicity is a significant advantage in the study of π-d correlation between conduction π-electrons and local d moments.Since hitherto known organic charge-transfer-salts with π-d interaction contain transition metals as localized spins, the wave functions of conduction electrons and localized spins in the anions are weakly hybridized because of large energy gap between the HOMO and the d levels up to 1 eV [4,5]. In this case, π-d interaction is well described by mean field approximation where conduction electrons feel exchange fields from local d moments [4,6,7].A newly synthesized organic conductor, (Me-3,5-DIP)[Ni(dmit) 2 ] 2 , is a candidate material to study the correlation between conduction π-electrons and localized π-spins. This material consists of two dimensional quarter-filled Ni(dmit) 2 anion layers and nested (Me-3,5-DIP) cations as shown in Fig. 1. There are two inequivalently aligned Ni(dmit) 2 layers alternating in a unit cell. An extended Hückel calculation expects an oval Fermi surface originating from one Ni(dmit) 2 layer. In the other Ni(dmit) 2 layer, strong dimerization is expected originating from 30 times as large intradimer transfer integral as interdimer one, which leads to a half-filled band [8]. Experimentally, the results of macroscopic measurements support alternation of metallic and Mott insulating layers [8]. We plot in Fig. 2 the anisotropy of resistivity. The ratio of inter-and intra-layer resistivities, ρ c /ρ b , which...

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“…Another model of the AFI phase was introduced by Hotta and Fukuyama [9]. From a mean-field calculation including the Hubbard model and the π-d interaction, they categorized the BETS system into a unified phase diagram, and proposed that λ-(BETS) 2 FeCl 4 and λ-(BETS) 2 GaCl 4 are very close to the MI boundary of the Mott insulator, and its ground state is easily influenced by the π-d interaction.…”

Section: Theoretical Studiesmentioning

confidence: 99%

“…On the other hand, some theoretical works claim that the Mott transition of the π-electrons is an origin of the MI transition, and then, the π-d coupling forces the Fe moments to be antiferromagnetically ordered [9,10] paramagnetic below T N [11]. This 'chicken or egg' problem of the antiferromagnetic insulating (AFI) phase (which orders first, the d-electrons or the π-electrons), coupled with the ground state of the Fe spins is still open question under strong debate.…”

Section: Introductionmentioning

confidence: 99%

Antiferromagnetic Insulating Ground State of Molecular π-d System Λ-(BETS)2FeCl4 (BETS = Bis(ethylenedithio)tetraselenafulvalene): A Theoretical and Experimental Review

2017

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Abstract:The π-d molecular conductor λ-(BETS) 2 FeCl 4 , where BETS is bis(ethylenedithio) tetraselenafulvalene, has attracted considerable interest for the discovery of its field induced superconducting state. A mystery of this system is its antiferromagnetic insulating ground state. The point still under strong debate is whether the d spins in Fe 3+ are ordered or not. Here, we review experimental and theoretical studies on the antiferromagnetic insulating phase in λ-(BETS) 2 FeCl 4 and mention our perspective based on our ESR measurements for λ-(BETS) 2 Fe x Ga 1−x Cl 4 . Our ESR results indicate that the π-d interaction in the system is very strong and there is no sign of paramagnetic Fe spins in the antiferromagnetic ground state.

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Effects of Localized Spins in Quasi-Two Dimensional Organic Conductors

Cited by 79 publications

References 16 publications

Magnetic-field-induced superconductivity in layered organic molecular crystals with localized magnetic moments

Magnetic-field-induced superconductivity in layered organic molecular crystals with localized magnetic moments

Suppression of electronic susceptibility in metal–Mott-insulator alternating material(Me−3,5−DIP)[Ni(dmit)2
Fujiyama
¹
,
Shitade
²
,
Kanoda
³

et al. 2008
Phys. Rev. B
10
1
4
0
View full text Add to dashboard Cite

Antiferromagnetic Insulating Ground State of Molecular π-d System Λ-(BETS)2FeCl4 (BETS = Bis(ethylenedithio)tetraselenafulvalene): A Theoretical and Experimental Review

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Effects of Localized Spins in Quasi-Two Dimensional Organic Conductors

Cited by 79 publications

References 16 publications

Magnetic-field-induced superconductivity in layered organic molecular crystals with localized magnetic moments

Magnetic-field-induced superconductivity in layered organic molecular crystals with localized magnetic moments

Suppression of electronic susceptibility in metal–Mott-insulator alternating material(Me−3,5−DIP)[Ni(dmit)2Fujiyama1, Shitade2, Kanoda3 et al. 2008Phys. Rev. B10140View full textAdd to dashboardCite

Antiferromagnetic Insulating Ground State of Molecular π-d System Λ-(BETS)2FeCl4 (BETS = Bis(ethylenedithio)tetraselenafulvalene): A Theoretical and Experimental Review

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Product

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Suppression of electronic susceptibility in metal–Mott-insulator alternating material(Me−3,5−DIP)[Ni(dmit)2
Fujiyama
¹
,
Shitade
²
,
Kanoda
³

et al. 2008
Phys. Rev. B
10
1
4
0
View full text Add to dashboard Cite