NMR Evidence for 1D Antiferromagnetic Properties in Cs1C60and R

Abstract

We have performed 13 C NMR measurements on the three phases of A 1 C 60 ͑A K, Rb, Cs͒ and alkali NMR in Cs 1 C 60 ( 133 Cs) and Rb 1 C 60 ( 87 Rb). We show that, for Rb 1 C 60 and Cs 1 C 60 , strong antiferromagnetic (AF) fluctuations persist up to room temperature, suggesting a one-dimensional character. However, K 1 C 60 behaves as an ordinary three-dimensional metal. NMR also reveals a magnetic transition at about 25 K in the Rb and Cs polymers. Comparison of the 133 Cs and 13 C spectra supports the occurrence of a spin-flop AF phase. Simulations of the 133 Cs spectrum due to a distribution of dipolar fields are consistent with moments m Ӎ 0.5m B ordered in AF chains.[S0031-9007(96)00161-5] PACS numbers: 76.60. Es, 61.48.+c, 75.50.Ee It is by now well established that the A 1 C 60 compounds display an orthorhombic ͑o͒-phase below room temperature, where the C 60 balls form polymerized chains in the a direction [1,2]. Yet the consequences of this strongly one-dimensional (1D) structure on the symmetry of the electronic properties of these systems are still debated. Shortly after the synthesis of this new phase, Chauvet et al. argued [1], interpreting their ESR measurements, that ͑o͒-Rb 1 C 60 is a quasi-1D conductor, where spin or charge density wave (SDW or CDW) instabilities lead to an insulating ground state. A transition at about 50 K has actually been confirmed for Rb 1 C 60 and Cs 1 C 60 through various experiments [ESR, conductivity [3], and muon spin resonance (mSR) [4-6]]; such a transition is not detected in K 1 C 60 either by ESR or by conductivity despite the structural similarity [3], although the eventual occurrence of a CDW state is controversial [3,7]. The exact nature of the transition in Rb 1 C 60 and Cs 1 C 60 is still unclear: mSR seems to favor a highly disordered magnetic state. On the other hand, Erwin, Krishna, and Mele claim that from band structure calculations [8] in the polymer phase the electronic density at the bonding sites between C 60 balls is small, and therefore the transfer integral along the chain is too weak to display a 1D character. As a consequence, the electronic properties should remain three dimensional (3D).In this context, local magnetic probe studies are highly desirable. We present here a thorough study of the various nuclear spins, which allows us to extract a 1D antiferromagnetic (AF) component of the spin fluctuations in the Rb and Cs polymers. In Cs 1 C 60 , the comparison between the 133 Cs and 13 C low T spectra leads us to conclude that the broadening is not due to static hyperfine fields and is better explained by dipolar fields in a spinflop AF phase.The 13 C relaxation data shown in Fig. 1 demonstrate immediately that T 1 behaves quite differently in Rb 1 C 60 and Cs 1 C 60 , on one hand, and K 1 C 60 , on the other hand. In K 1 C 60 , both T 1 T 300 sec K and the spin susceptibility [3] are T independent, which is the usual situation for an ordinary 3D metal. On the contrary, ͑T 1 T͒ 21 increases with decreasing T 1 as T 1 is nearly T independent in ...

Evidence of a structural anomaly at 14 K in polymerised CsC 60

“…A metal-insulator transition below 50 K is accompanied by the stabilisation of a magnetic state in RbC 60 and CsC 60 , whereas KC 60 remains metallic to low temperatures [4]. The nature of the magnetic transition has remained controversial, as both quasi-one dimensional electronic instabilities [4,5,6,7,8] and three-dimensional magnetic ordering [9,10] have been proposed. Initial X-ray powder diffraction studies [3] have described the structure of the polymers as orthorhombic (space group P mnn) with the orientation of the C − 60 chains about the short axis a described by µ=45±5…”

“…While many experiments reveal one-dimensional characteristics [7,8], electronic band structure calculations predict threedimensional energy bands [9,17]. In addition, recent 13 C magic angle spinning NMR experiments on CsC 60 have shown that the conduction electron density is concentrated along the equator of the C − 60 ions and away from the C-C bridging bonds [18].…”

Design of Spin-Frustrated Monomer-Type C60•− Mott Insulator

“…When the interfullerene distance of the C 60 solids is small (r < 9.4 Å, which is indicated by blue dotted line in Figure 3a,b), the C 60 •− molecules tend to form dimers (9 and 10 in Figure 2b) [37,38] or polymers (11-15 in Figure 2b) by single or double bonds. Polymer 11 with K + is a 3D metal [39], while those with Rb + and Cs + are one-dimensional (1D) metals and become spin-density-wave insulators at low temperatures [40,100]. Single carbon-carbon bonded polymer 12 in Na 2 CsC 60 seems to be an SC with T c about 3 K lower than that for the starting non-polymeric phase.…”

High-field ESR in the metallic fullerides RbC60 and CsC60

“…In contrast, band-structure calculations and the nuclear magnetic resonance (NMR) evidence for spa hybridization of the carbon atoms involved in the intermolecular bonds suggested that the polymer chains do not increase the anisotropy above the extent expected from the orthorhombic structure. Despite these results, short relaxation times and strong line broadening in alkali NMR [8], which suggested the existence of internal magnetic fields in the insulating ground state of RbC 60 and CsC 60 , were ascribed to the formation of a spin density wave (SDW), a magnetic ordering favored by a 1-D electronic structure. In the early 1980s it has been shown on the Bechgaard salts like (TMTSF) 2 PF6 that spin-density-wave ground states result from an instability of highly anisotropic electronic structures [9].…”