Dynamic exchange coupling in magnetic bilayers

A long-range dynamic interaction between ferromagnetic films separated by normal-metal spacers is reported, which is communicated by nonequilibrium spin currents. It is measured by ferromagnetic resonance and explained by an adiabatic spin-pump theory. In such a resonance the spin-pump mechanism of spatially separated magnetic moments leads to an appreciable increase in the resonant linewidth when the resonance fields are well apart, and results in a dramatic linewidth narrowing when the resonant fields approach each other.     

Current detection of superradiance and induced entanglement of double quantum dot excitons

We propose to measure the superradiance effect by observing the current through a semiconductor double-dot system. An electron and a hole are injected separately into one of the quantum dots to form an exciton and then recombine radiatively. We find that the stationary current shows oscillatory behavior as one varies the interdot distance. The amplitude of oscillation can be increased by incorporating the system into a microcavity. Furthermore, the current is suppressed if the dot distance is small compared to the wavelength of the emitted photon. This photon trapping phenomenon generates the entangled state and may be used to control the emission of single photons at predetermined times.     

Quantum chaos triggered by precursors of a quantum phase transition: the dicke model

We consider the Dicke Hamiltonian, a simple quantum-optical model which exhibits a zero-temperature quantum phase transition. We present numerical results demonstrating that at this transition the system changes from being quasi-integrable to quantum chaotic. By deriving an exact solution in the thermodynamic limit we relate this phenomenon to a localization-delocalization transition in which a macroscopic superposition is generated. We also describe the classical analogs of this behavior.     

Background gradient suppression in pulsed gradient stimulated echo measurements

Pulsed gradient spin echo (PGSE) experiments can be used to measure the probability distribution of molecular displacements. In homogeneous samples this reports on the molecular diffusion coefficient, and in heterogeneous samples, such as porous media and biological tissue, such measurements provide information about the sample's morphology. In heterogeneous samples however background gradients are also present and prevent an accurate measurement of molecular displacements. The interference of time independent background gradients with the applied magnetic field gradients can be removed through the use of bipolar gradient pulses. However, when the background gradients are spatially non-uniform molecular diffusion introduces a temporal modulation of the background gradients. This defeats simple bipolar gradient suppression of background gradients in diffusion related measurements. Here we introduce a new method that requires the background gradients to be constant over coding intervals only. Since the coding intervals are typically at least an order of magnitude shorter than the storage time, this new method succeeds in suppressing cross-terms for a much wider range of heterogeneous samples.     

Raman modes of index-identified freestanding single-walled carbon nanotubes

Using electron diffraction on freestanding single-walled carbon nanotubes, we have determined the structural indices (n,m) of tubes in the diameter range from 1.4 to 3 nm. On the same freestanding tubes, we have recorded Raman spectra of the tangential modes and the radial breathing mode. For the smaller diameters (1.4-1.7 nm), these measurements confirm previously established radial breathing mode frequency versus diameter relations and would be consistent with the theoretically predicted proportionality to the inverse diameter. However, for extending the relation to larger diameters, either a yet unexplained environmental constant has to be assumed, or the linear relation has to be abandoned.     

Dynamics of a large spin with strong dissipation

We study the generalization of the spin-boson model to spins greater than one-half in the strong-coupling regime. This model applies to dissipative large spins as well as to ensembles of identical two-state systems coupled to a common environment. Using a combination of polaron transformations and master equations, we find nonexponential spin relaxation towards one of two possible equilibrium states. For Ohmic dissipation the relaxation is approximately logarithmic in time.     

Determination of water compartments in rat myocardium using combined D-T1 and T1-T2 experiments

We have used combined D-T1 and T1-T2 correlation experiments to explore water compartments in rat heart tissue (myocardium). The results show that two main compartments can be identified, which we assign to extracellular (ec) and intracellular (ic) water. The exchange rate of water across the cell membrane was found to be on the order of 0.1 Hz. In addition, the T1-T2 correlation measurements indicate that the ic compartment contain two T2 populations.