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- ItemCoherent Rabi dynamics of a superradiant spin ensemble in a microwave cavity(College Park, Md. : APS, 2017) Rose, B.C.; Tyryshkin, A.M.; Riemann, H.; Abrosimov, N.V.; Becker, P.; Pohl, H.-J.; Thewalt, M.L.W.; Itoh, K.M.; Lyon, S.A.We achieve the strong-coupling regime between an ensemble of phosphorus donor spins in a highly enriched 28Si crystal and a 3D dielectric resonator. Spins are polarized beyond Boltzmann equilibrium using spin-selective optical excitation of the no-phonon bound exciton transition resulting in N=3.6×1013 unpaired spins in the ensemble. We observe a normal mode splitting of the spin-ensemble–cavity polariton resonances of 2g√N=580 kHz (where each spin is coupled with strength g) in a cavity with a quality factor of 75 000 (γ≪κ≈60 kHz, where γ and κ are the spin dephasing and cavity loss rates, respectively). The spin ensemble has a long dephasing time (T∗2=9 μs) providing a wide window for viewing the dynamics of the coupled spin-ensemble–cavity system. The free-induction decay shows up to a dozen collapses and revivals revealing a coherent exchange of excitations between the superradiant state of the spin ensemble and the cavity at the rate g√N. The ensemble is found to evolve as a single large pseudospin according to the Tavis-Cummings model due to minimal inhomogeneous broadening and uniform spin-cavity coupling. We demonstrate independent control of the total spin and the initial Z projection of the psuedospin using optical excitation and microwave manipulation, respectively. We vary the microwave excitation power to rotate the pseudospin on the Bloch sphere and observe a long delay in the onset of the superradiant emission as the pseudospin approaches full inversion. This delay is accompanied by an abrupt π-phase shift in the peusdospin microwave emission. The scaling of this delay with the initial angle and the sudden phase shift are explained by the Tavis-Cummings model.
- ItemModulated martensite: Why it forms and why it deforms easily(Milton Park : Taylor & Francis, 2011) Kaufmann, S.; Niemann, R.; Thersleff, T.; Rößler, U.K.; Heczko, O.; Buschbeck, J.; Holzapfel, B.; Schultz, L.; Fähler, S.Diffusionless phase transitions are at the core of the multifunctionality of (magnetic) shape memory alloys, ferroelectrics and multiferroics. Giant strain effects under external fields are obtained in low symmetric modulated martensitic phases. We outline the origin of modulated phases, their connection with tetragonal martensite and consequences owing to their functional properties by analysing the martensitic microstructure of epitaxial Ni–Mn–Ga films from the atomic to the macroscale. Geometrical constraints at an austenite–martensite phase boundary act down to the atomic scale. Hence, a martensitic microstructure of nanotwinned tetragonal martensite can form. Coarsening of twin variants can reduce twin boundary energy, a process we could observe from the atomic to the millimetre scale. Coarsening is a fractal process, proceeding in discrete steps by doubling twin periodicity. The collective defect energy results in a substantial hysteresis, which allows the retention of modulated martensite as a metastable phase at room temperature. In this metastable state, elastic energy is released by the formation of a 'twins within twins' microstructure that can be observed from the nanometre to the millimetre scale. This hierarchical twinning results in mesoscopic twin boundaries. Our analysis indicates that mesoscopic boundaries are broad and diffuse, in contrast to the common atomically sharp twin boundaries of tetragonal martensite. We suggest that the observed extraordinarily high mobility of such mesoscopic twin boundaries originates from their diffuse nature that renders pinning by atomistic point defects ineffective.