Quantum information processing (QIP) could revolutionize areas ranging from chemical modeling to cryptography. One key figure of merit for the smallest unit for QIP, the qubit, is the coherence time (T2), which establishes the lifetime for the qubit. Transition metal complexes offer tremendous potential as tunable qubits, yet their development is hampered by the absence of synthetic design principles to achieve a long T2. We harnessed molecular design to create a series of qubits, (Ph4P)2[V(C8S8)3] (1), (Ph4P)2[V(β-C3S5)3] (2), (Ph4P)2[V(α-C3S5)3] (3), and (Ph4P)2[V(C3S4O)3] (4), with T2s of 1–4 μs at 80 K in protiated and deuterated environments. Crucially, through chemical tuning of nuclear spin content in the vanadium(IV) environment we realized a T2 of ∼1 ms for the species (d20-Ph4P)2[V(C8S8)3] (1′) in CS2, a value that surpasses the coordination complex record by an order of magnitude. This value even eclipses some prominent solid-state qubits. Electrochemical and continuous wave electron paramagnetic resonance (EPR) data reveal variation in the electronic influence of the ligands on the metal ion across 1–4. However, pulsed measurements indicate that the most important influence on decoherence is nuclear spins in the protiated and deuterated solvents utilized herein. Our results illuminate a path forward in synthetic design principles, which should unite CS2 solubility with nuclear spin free ligand fields to develop a new generation of molecular qubits.

There is great interest in finding materials possessing quasiparticles with topological properties. Such materials may have novel excitations that exist on their boundaries which are protected against disorder. We report experimental evidence that magnons in an insulating kagome ferromagnet can have a topological band structure. Our neutron scattering measurements further reveal that one of the bands is flat due to the unique geometry of the kagome lattice. Spin wave calculations show that the measured band structure follows from a simple Heisenberg Hamiltonian with a Dzyaloshinkii-Moriya interaction. This serves as the first realization of an effectively two-dimensional topological magnon insulator-a new class of magnetic material that should display both a magnon Hall effect and protected chiral edge modes. DOI: 10.1103/PhysRevLett.115.147201 PACS numbers: 75.30.Ds When quantum particles are confined to move in reduced dimensions, such as in planes, unexpectedly rich physics can emerge as a result of the geometry and interactions. The quantum Hall effect is a famous example, which results from placing a two-dimensional (2D) gas of electrons or quasiparticles in a large magnetic field [1]. Pioneering theoretical work by Haldane showed that some systems may inherently possess topological bands that allow them to exhibit quantum Hall physics without applied magnetic fields [2]. The discovery of materials in which strong spin-orbit coupling leads to topological bands, such as topological insulators, has led to a flurry of activity in condensed matter physics research [3,4]. Recently, theoretical studies have focused on 2D topological band structures that include flat bands due to the possibility of achieving fractional quantum hall physics in the absence of magnetic fields [5]. Flat bands (bands that are dispersionless in energy) hold unique interest because the interaction energy between particles may dominate the kinetic energy, leading to novel correlated electron states. A number of theoretical models for the fractional quantum Hall effect have been proposed based on flat topological bands [6][7][8]; however, these invariably require tuning of parameters, which is difficult to control in real materials.Topological band structures are not unique to systems with electronlike quasiparticles. It has been demonstrated that topological photon modes can be realized in experimental systems [9][10][11]. Possible realizations of topological bosonic systems that include flat bands have been proposed using dipolar molecules trapped in an optical lattice [12], and using photonic lattices [13] based on the interaction between photons and arrays of superconducting circuits [14], although experimental confirmation has yet to be demonstrated. In this Letter, we show that topological bands exist for another class of quasiparticles: magnons in an insulating ferromagnet. Our material serves as the first realization of an effectively 2D topological magnon insulator [15], an electrically insulating state in which the spin degre...

Slow magnetic relaxation is observed for [(tpa(Mes))Fe](-), a trigonal pyramidal complex of high-spin iron(II), providing the first example of a mononuclear transition metal complex that behaves as a single-molecule magnet. Dc magnetic susceptibility and magnetization measurements reveal a strong uniaxial magnetic anisotropy (D = -39.6 cm(-1)) acting on the S = 2 ground state of the molecule. Ac magnetic susceptibility measurements indicate the absence of slow relaxation under zero applied dc field as a result of quantum tunneling of the magnetization. Application of a 1500 Oe dc field initiates slow magnetic relaxation, which follows a thermally activated tunneling mechanism at high temperature to give an effective spin-reversal barrier of U(eff) = 42 cm(-1) and follows a temperature-independent tunneling mechanism at low temperature. In addition, the magnetic relaxation time shows a pronounced dc-field dependence, with a maximum occurring at approximately 1500 Oe.