Abstract
Geometric frustration, in which competing interactions give rise to degenerate ground states, potentially induces various exotic quantum phenomena in magnetic materials. Minimal models comprising triangular units, such as triangular and Kagome lattices, have been investigated for decades to realize novel quantum phases, such as quantum spin liquid. A pentagon is the second-minimal elementary unit for geometric frustration. The realization of such systems is expected to provide a distinct platform for studying frustrated magnetism. Here, we present a spin-1/2 quantum pentagonal lattice in the new organic radical crystal α-2,6-Cl2-V [=α-3-(2,6-dichlorophenyl)-1,5-diphenylverdazyl]. Its unique molecular arrangement allows the formation of a partially corner-shared pentagonal lattice (PCPL). We find a clear 1/3 magnetization plateau and an anomalous change in magnetization in the vicinity of the saturation field, which originate from frustrated interactions in the PCPL.
Highlights
Nitroxide, the π-electron spin density of verdazyl radicals can be delocalized even in non-planar molecular structures
Ab initio molecular orbitals (MOs) calculations indicate the formation of an S = 1 /2 quantum pentagonal lattice consisting of one FM and two AFM interactions
Two AFM exchange interactions, labeled as J1, J2, and J3, are dominant. They are evaluated as J1/kB = − 1.6 K, J2/kB = 3.9 K, and J3/kB = 3.3 K, which are defined in the Heisenberg spin Hamiltonian given by = Jn∑ i,j Si ⋅ Sj, where Σ i,j denotes the sum over the neighboring spin pairs
Summary
Nitroxide, the π-electron spin density of verdazyl radicals can be delocalized even in non-planar molecular structures. The six spins in the unit cell and the 1/3 magnetization plateau suggest the existence of a nonmagnetic singlet state with an excitation energy gap formed by the AFM interactions J2 and/or J3 between the β sites.
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