Landau-Zener transitions in qubits controlled by electromagnetic fields

We investigate the influence of a dipole interaction with a classical radiation field on a qubit during a continuous change of a control parameter. In particular, we explore the non-adiabatic transitions that occur when the qubit is swept with linear speed through resonances with the time-dependent interaction. Two classical problems come together in this model: the Landau-Zener and the Rabi problem. The probability of Landau-Zener transitions now depends sensitively on the amplitude, the frequency and the phase of the Rabi interaction. The influence of the static phase turns out to be particularly strong, since this parameter controls the time-reversal symmetry of the Hamiltonian. In the limits of large and small frequencies, analytical results obtained within a rotating-wave approximation compare favourably with a numerically exact solution. Some physical realizations of the model are discussed, both in microwave optics and in magnetic systems.Comment: 12 pages, 5 figure

Competing magnetostructural phases in a semiclassical system

The interplay between charge, structure, and magnetism gives rise to rich phase diagrams in complex materials with exotic properties emerging when phases compete. Molecule-based materials are particularly advantageous in this regard due to their low energy scales, flexible lattices, and chemical tunability. Here, we bring together high pressure Raman scattering, modeling, and first principles calculations to reveal the pressure-temperature-magnetic field phase diagram of Mn[N(CN)2]2. We uncover how hidden soft modes involving octahedral rotations drive two pressure-induced transitions triggering the low ??? high magnetic anisotropy crossover and a unique reorientation of exchange planes. These magnetostructural transitions and their mechanisms highlight the importance of spin-lattice interactions in establishing phases with novel magnetic properties in Mn(II)-containing systems

Molecules at the Quantum–Classical Nanoparticle Interface: Giant Mn₇₀ Single-Molecule Magnets of ∼4 nm Diameter

Two Mn₇₀ torus-like molecules have been obtained from the alcoholysis in EtOH and 2-ClC₂H₄OH of [Mn₁₂O₁₂(O₂CMe)₁₆(H₂O)₄]·4H₂O·2MeCO₂H (<b>1</b>) in the presence of NBuⁿ₄MnO₄ and an excess of MeCO₂H. The reaction in EtOH afforded [Mn₇₀O₆₀­(O₂CMe)₇₀­(OEt)₂₀­(EtOH)₁₆­(H₂O)₂₂] (<b>2</b>), whereas the reaction in ClC₂H₄­OH gave [Mn₇₀­O₆₀­(O₂CMe)₇₀­(OC₂H₄Cl)₂₀­(ClC₂H₄OH)₁₈­(H₂O)₂₂] (<b>3</b>). The complexes are nearly isostructural, each possessing a Mn₇₀ torus structure consisting of alternating near-linear [Mn₃(μ₃-O)₄] and cubic [Mn₄(μ₃-O)₂(μ₃-OR)₂] (R = OEt, <b>2</b>; R = OC₂H₄Cl, <b>3</b>) subunits, linked together via <i>syn,syn</i>-μ-bridging MeCO₂^– and μ₃-bridging O^2– groups. <b>2</b> and <b>3</b> have an overall diameter of ∼4 nm and crystallize as highly ordered supramolecular nanotubes. Alternating current (ac) magnetic susceptibility measurements, performed on microcrystalline samples in the 1.8–10 K range and a 3.5 G ac field with oscillation frequencies in the 5–1500 Hz range, revealed frequency-dependent out-of-phase signals below ∼2.4 K for both molecules indicative of the slow magnetization relaxation of single-molecule magnets (SMMs). Single-crystal, magnetization vs field studies on both complexes revealed hysteresis loops below 1.5 K, thus confirming <b>2</b> and <b>3</b> to be new SMMs. The hysteresis loops do not show the steps that are characteristic of quantum tunneling of magnetization (QTM). However, low-temperature studies revealed temperature-independent relaxation rates below ∼0.2 K for both compounds, the signature of ground state QTM. Fitting of relaxation data to the Arrhenius equation gave effective barriers for magnetization reversal (<i>U</i>_eff) of 23 and 18 K for <b>2</b> and <b>3</b>, respectively. Because the Mn₇₀ molecule is close to the classical limit, it was also studied using a method based on the Néel–Brown model of thermally activated magnetization reversal in a classical single-domain magnetic nanoparticle. The field and sweep-rate dependence of the coercive field was investigated and yielded the energy barrier, the spin, the Arrhenius pre-exponential, and the cross-over temperature from the classical to the quantum regime. The validity of this approach emphasizes that large SMMs can be considered as being at or near the quantum–classical nanoparticle interface

Two Frustrated, Bitetrahedral Single-Molecule Magnets

Two unusual mixed-valent {Mn III 6MnII} bitetrahedra display frustrated magnetic exchange, leading to S = 13/2 ± 1 and 11/2 ± 1 ground states and slow magnetization relaxation