65 research outputs found

    Pair Partitioning in time reversal acoustics

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    Time reversal of acoustic waves can be achieved efficiently by the persistent control of excitations in a finite region of the system. The procedure, called Time Reversal Mirror, is stable against the inhomogeneities of the medium and it has numerous applications in medical physics, oceanography and communications. As a first step in the study of this robustness, we apply the Perfect Inverse Filter procedure that accounts for the memory effects of the system. In the numerical evaluation of such procedures we developed the Pair Partitioning method for a system of coupled oscillators. The algorithm, inspired in the Trotter strategy for quantum dynamics, obtains the dynamic for a chain of coupled harmonic oscillators by the separation of the system in pairs and applying a stroboscopic sequence that alternates the evolution of each pair. We analyze here the formal basis of the method and discuss his extension for including energy dissipation inside the medium.Comment: 6 pages, 4 figure

    Decoherent time-dependent transport beyond the Landauer-B\"uttiker formulation: a quantum-drift alternative to quantum jumps

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    We present a model for decoherence in time-dependent transport. It boils down into a form of wave function that undergoes a smooth stochastic drift of the phase in a local basis, the Quantum Drift (QD) model. This drift is nothing else but a local energy fluctuation. Unlike Quantum Jumps (QJ) models, no jumps are present in the density as the evolution is unitary. As a first application, we address the transport through a resonant state ∣0⟩\left\vert 0\right\rangle that undergoes decoherence. We show the equivalence with the decoherent steady state transport in presence of a B\"{u}ttiker's voltage probe. In order to test the dynamics, we consider two many-spin systems whith a local energy fluctuation. A two-spin system is reduced to a two level system (TLS) that oscillates among ∣0⟩\left\vert 0\right\rangle ≡\equiv ∣↑↓⟩ \left\vert \uparrow \downarrow \right\rangle and ∣1⟩≡\left\vert 1\right\rangle \equiv ∣↓↑⟩\left\vert \downarrow \uparrow \right\rangle . We show that QD model recovers not only the exponential damping of the oscillations in the low perturbation regime, but also the non-trivial bifurcation of the damping rates at a critical point, i.e. the quantum dynamical phase transition. We also address the spin-wave like dynamics of local polarization in a spin chain. The QD average solution has about half the dispersion respect to the mean dynamics than QJ. By evaluating the Loschmidt Echo (LE), we find that the pure states ∣0⟩\left\vert 0\right\rangle and ∣1⟩\left\vert 1\right \rangle are quite robust against the local decoherence. In contrast, the LE, and hence coherence, decays faster when the system is in a superposition state. Because its simple implementation, the method is well suited to assess decoherent transport problems as well as to include decoherence in both one-body and many-body dynamics.Comment: 10 pages, 5 figure

    Gaussian to Exponential Crossover in the Attenuation of Polarization Echoes in NMR

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    An ingenious pulse sequence devised by S. Zhang, B. H. Meier, and R. R. Ernst (Phys. Rev. Lett. {\bf 69}, 2149 (1992)) reverses the time evolution (``spin diffusion'') of the local polarization in a dipolar coupled 1^{1}H spin system. This refocusing originates a Polarization Echo whose amplitude attenuates by increasing the time tRt_R elapsed until the dynamics is reversed. Different functional attenuations are found for a set of dipolar coupled systems: ferrocene, (C5_5H5_5)2_2Fe, cymantrene, (C5_5H5_5)Mn(CO)3_3, and cobaltocene, (C5_5H5_5)2_2Co. To control a relevant variable involved in this attenuation a pulse sequence has been devised to progressively reduce the dipolar dynamics. Since it reduces the evolution of the polarization echo it is referred as REPE sequence. Two extreme behaviors were found while characterizing the materials: In systems with a strong source of relaxation and slow dynamics, the attenuation follows an exponential law (cymantrene). In systems with a strong dipolar dynamics the attenuation is mainly Gaussian. By the application of the REPE sequence the characteristic time of the Gaussian decay is increased until the presence of an underlying dissipative mechanism is revealed (cobaltocene). For ferrocene, however, the attenuation remains Gaussian within the experimental time scale. These two behaviors suggest that the many body quantum dynamics presents an extreme intrinsic instability which, in the presence of small perturbations, leads to the onset of irreversibility. This experimental conclusion is consistent with the tendencies displayed by the numerical solutions of model systems.Comment: 7 pages + 7 Postscript figure

    Tuning laser-induced bandgaps in graphene

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    Could a laser field lead to the much sought-after tunable bandgaps in graphene? By using Floquet theory combined with Green's functions techniques, we predict that a laser field in the mid-infrared range can produce observable bandgaps in the electronic structure of graphene. Furthermore, we show how they can be tuned by using the laser polarization. Our results could serve as a guidance to design opto-electronic nano-devices.Comment: 4 pages, 3 figures, to appear in Applied Physics Letter

    Spin projection chromatography

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    We formulate the many-body spin dynamics at high temperature within the non-equilibrium Keldysh formalism. For the simplest XY interaction, analytical expressions in terms of the one particle solutions are obtained for linear and ring configurations. For small rings of even spin number, the group velocities of excitations depend on the parity of the total spin projection. This should enable a dynamical filtering of spin projections with a given parity i.e. a Spin projection chromatography.Comment: 13 pages, 3 figure

    Quantum Interference Phenomena in the Local Polarization Dynamics of Mesoscopic Systems: An NMR Observation

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    It was predicted that local spin polarization in a ring of five dipolar coupled spins should present a particular fingerprint of quantum interferences reflecting both the discrete and finite nature of the system [Phys. Rev. Lett. 75 (1995) 4310]. We report its observation for the proton system of a (C5_5H5_5)2_2Fe molecule using a rare 13^{13}C as {\it local probe}. Novel high frequency (≃60k\simeq 60kHz) polarization oscillations appear because incomplete 13^{13}C-1^1H cross-polarization transfer {\it splits} the polarization state, in a portion that wanders in the proton system and one that remains in the 13^{13} C. They interfere with each other after rejoining.Comment: 12 pages, RevTex, 4 Figures available upon request, to appear in Chemical Physics Letter
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