5 research outputs found
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 Mn<sub>70</sub> Single-Molecule Magnets of ∼4 nm Diameter
Two Mn<sub>70</sub> torus-like molecules have been obtained from
the alcoholysis in EtOH and 2-ClC<sub>2</sub>H<sub>4</sub>OH of [Mn<sub>12</sub>O<sub>12</sub>(O<sub>2</sub>CMe)<sub>16</sub>(H<sub>2</sub>O)<sub>4</sub>]·4H<sub>2</sub>O·2MeCO<sub>2</sub>H (<b>1</b>) in the presence of NBu<sup>n</sup><sub>4</sub>MnO<sub>4</sub> and an excess of MeCO<sub>2</sub>H. The reaction in EtOH afforded
[Mn<sub>70</sub>O<sub>60</sub>(O<sub>2</sub>CMe)<sub>70</sub>(OEt)<sub>20</sub>(EtOH)<sub>16</sub>(H<sub>2</sub>O)<sub>22</sub>] (<b>2</b>), whereas the reaction in ClC<sub>2</sub>H<sub>4</sub>OH gave [Mn<sub>70</sub>O<sub>60</sub>(O<sub>2</sub>CMe)<sub>70</sub>(OC<sub>2</sub>H<sub>4</sub>Cl)<sub>20</sub>(ClC<sub>2</sub>H<sub>4</sub>OH)<sub>18</sub>(H<sub>2</sub>O)<sub>22</sub>] (<b>3</b>). The complexes
are nearly isostructural, each possessing a Mn<sub>70</sub> torus
structure consisting of alternating near-linear [Mn<sub>3</sub>(μ<sub>3</sub>-O)<sub>4</sub>] and cubic [Mn<sub>4</sub>(μ<sub>3</sub>-O)<sub>2</sub>(μ<sub>3</sub>-OR)<sub>2</sub>] (R = OEt, <b>2</b>; R = OC<sub>2</sub>H<sub>4</sub>Cl, <b>3</b>) subunits,
linked together via <i>syn,syn</i>-μ-bridging MeCO<sub>2</sub><sup>–</sup> and μ<sub>3</sub>-bridging O<sup>2–</sup> 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><sub>eff</sub>) of 23
and 18 K for <b>2</b> and <b>3</b>, respectively. Because
the Mn<sub>70</sub> 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