Phonon-Modulated Magnetic Interactions and Spin Tomonaga-Luttinger Liquid in thep-Orbital AntiferromagnetCsO2

Abstract

The magnetic response of antiferromagnetic CsO2, coming from the p-orbital S = 1/2 spins of anionic O − 2 molecules, is followed by 133 Cs nuclear magnetic resonance across the structural phase transition occuring at Ts1 = 61 K on cooling. Above Ts1, where spins form a square magnetic lattice, we observe a huge, nonmonotonic temperature dependence of the exchange coupling originating from thermal librations of O − 2 molecules. Below Ts1, where antiferromagnetic spin chains are formed as a result of p-orbital ordering, we observe a spin Tomonaga-Luttinger-liquid behavior of spin dynamics. These two interesting phenomena, which provide rare simple manifestations of the coupling between spin, lattice and orbital degrees of freedom, establish CsO2 as a model system for molecular solids.PACS numbers: 75.10. Pq, 75.30.Et, 75.25.Dk, In many magnetic insulators, spins are well decoupled from other degrees of freedom, which implies simple Hamiltonians completely defined by the short-range magnetic exchange interactions. Model systems of this kind provide an excellent playground for the understanding of collective quantum phenomena, including TomonagaLuttinger liquid (TLL) in one-dimensional (1D) antiferromagnets [1], Bose-Einstein condensation of magnons in dimer spin systems [2], quantum criticality in gapped antiferromagnets [3][4][5][6] and spin-liquid behavior in frustrated spin systems [7].In molecular solids, a class of magnetic insulators containing molecules as structural and magnetic units, spins cannot be decoupled from lattice and orbital degrees of freedom. This is particularly pronounced in systems based on small and light anionic O − 2 molecules: alkali superoxides, AO 2 (A = Na, K, Rb, Cs) [8][9][10][11], and alkali sesquioxides, A 4 O 6 (A = Rb, Cs) [12][13][14][15]. Here, the O − 2 anion carries an S = 1/2 spin in a pair of p-derived degenerate π * orbitals [16]. A strong coupling between spin, lattice and orbital degrees of freedom leads to complex physics [12][13][14][15][16][17][18][19][20][21][22], which is nevertheless based on two relatively simple mechanisms characteristic of molecular solids: (i) the O − 2 "dumbbells" can easily reorient, which modulates the overlaps of π * orbitals and thus the exchange coupling between the neighboring spins [11]; (ii) the degeneracy of the π * orbitals is lifted by a structural phase transition involving the tilting of O − 2 dumbbells, which is reminiscent of the Jahn-Teller effect [16]. Calorimetric and magnetic studies indeed revealed several structural phase transitions in AO 2 systems back in the 1970s [9-11], but their origin remained largely unexplained. These interesting observations were systematically revisited only in recent studies [16][17][18][19][20][21][22]. Among them, an important X-ray and Raman scattering study of CsO 2 clearly demonstrated the ordering of p orbitals below the structural phase transition at T s ≈ 70 K, which leads to the formation of 1D antiferromagnetic spin-1/2 chains in an otherwise 2D magnetic lattice [8], as sketched in F...

π-electron S = ½ quantum spin-liquid state in an ionic polyaromatic hydrocarbon

“…These systems crystallize in architectures of varied dimensionality that range from 1D to 3D and encompass molecular metals and superconductors, such as organic charge-transfer salts 9 and the alkali fullerides [10][11][12] , which provide the highest T c (38 K) (ref. 13) together with molecular magnets 14 , including QSLs 6-8 and quantum-spin chains and ladders [15][16][17] . In particular, the interest in all-carbon π-electron systems was recently revived by the report that alkali intercalation of a polyaromatic hydrocarbon (PAH), the five-membered-ring picene C 22 H 14 , led to a superconductor with T c = 18 K, but of unknown composition and structure 18 .…”

Spin-dimer ground state driven by consecutive charge and orbital ordering transitions in the anionic mixed-valence compound Rb4O6

“…4(b) and Ref. [17]]. In CsO 2 , the increase of 1/T 1 with decreasing temperature between 220 and 80 K was ascribed to the dynamic modulation of the antiferromagnetic exchange coupling between nearest-neighbouring O − 2 spins due to their thermal librations [17].…”

“…(b) The temperature dependencies of 5/T1 (solid circles) and 1/T2 (open circles) relaxation rates. In the inset, the 133 Cs 1/T1 data of CsO2 is shown for comparison (data taken from [17]). (c) Solid circles show 1/T1 in the reciprocal 1/T scale.…”

Magnetic phase transitions and magnetoelastic coupling in S=12 Heisenberg antiferromagnets

“…These subtle modifications act to stabilize the low temperature long-range ordered state although there is no spin-lattice coupling across T N within our sensitivity. Getting back to the overall temperature trend, we point out that modulation of the exchange integral by lattice vibrations may lead to temperature dependence of exchange constants [14]. By contrast, CuF 2 (H 2 O) 2 (3-Clpy) shows no evidence of structural changes or local lattice distortions down to 4.2 K [38].…”

“…While high field magnetization and microscopic modeling demonstrate that magnetic interactions dominate this process [11][12][13], magnetically-driven transitions are not always spinonly processes. There is, for instance, plenty of evidence suggesting that spin and lattice can be inextricably linked [14][15][16][17][18][19] -at least under certain circumstances -although a test to determine the requirements for a lattice contribution has, so far, been missing. Two S=1/2 Heisenberg antiferromagnets, [Cu(pyz) 2 (HF 2 )]PF 6 and CuF 2 (H 2 O) 2 (3-Clpy), provide an opportunity to unravel this puzzle.…”