Marat Gafurov

Coherence times and Rabi oscillations in CaWO4:Cr5+ crystal

Rabi oscillations in the V(15) single molecule magnet embedded in the surfactant (CH(3))(2)[CH(3)(CH(2))(16)CH(2)](2)N(+) have been studied at different microwave powers. An intense damping peak is observed when the Rabi frequency Ω(R) falls in the vicinity of the Larmor frequency of protons ω(N). The experiments are interpreted by a model showing that the damping (or Rabi) time τ(R) is directly associated with decoherence caused by electron-nuclear cross relaxation in the rotating reference frame. This decoherence induces energy dissipation in the range ω(N) - σ(e) < Ω(R) < ω(N), where σ(e) is the mean superhyperfine field induced by protons at V(15). Weaker decoherence without dissipation takes place outside this window. Specific estimations suggest that this rapid cross relaxation in a resonant microwave field, observed for the first time in V(15), should also take place, e.g., in Fe(8) and Mn(12).The coherence times of dopant pentavalent chromium ions in CaWO₄ single crystal (0.0006at.% Cr(5+)) were investigated both theoretically and experimentally. Temperature dependences of spin-lattice relaxation time T₁ and phase memory time T(M) were measured in the temperature range 6-30 K at high (94 GHz, W band) and low (3.5 GHz, S band) frequencies of electron spin resonance. It follows from T(M) calculations that phase relaxation of Cr(5+) ion arises mainly from magnetic dipole interactions between the chromium ions. Anomalously fast damping of Rabi oscillations is detected in both S- and W-band experiments. It is shown that this phenomenon is caused by microwave field inhomogeneity inside the resonator. Relations between the damping time of Rabi oscillations, Rabi frequency and the crystal sample size are obtained. Lumped-element resonators and smaller sample dimensions are suggested to lower spin dephasing during transient nutations.

Paramagnetic Manganese in the Atherosclerotic Plaque of Carotid Arteries

The search for adequate markers of atherosclerotic plaque (AP) instability in the context of assessment of the ischemic stroke risk in patients with atherosclerosis of the carotid arteries as well as for solid physical and chemical factors that are connected with the AP stability is extremely important. We investigate the inner lining of the carotid artery specimens from the male patients with atherosclerosis (27 patients, 42–64 years old) obtained during carotid endarterectomy by using different analytical tools including ultrasound angiography, X-ray analysis, immunological, histochemical analyses, and high-field (3.4 T) pulse electron paramagnetic resonance (EPR) at 94 GHz. No correlation between the stable and unstable APs in the sense of the calcification is revealed. In all of the investigated samples, the EPR spectra of manganese, namely, Mn2+ ions, are registered. Spectral and relaxation characteristics of Mn2+ ions are close to those obtained for the synthetic (nano) hydroxyapatite species but differ from each other for stable and unstable APs. This demonstrates that AP stability could be specified by the molecular organization of their hydroxyapatite components. The origin of the obtained differences and the possibility of using EPR of Mn2+ as an AP stability marker are discussed.