675 research outputs found

    Phonon Bloch oscillations in acoustic-cavity structures

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    We describe a semiconductor multilayer structure based in acoustic phonon cavities and achievable with MBE technology, designed to display acoustic phonon Bloch oscillations. We show that forward and backscattering Raman spectra give a direct measure of the created phononic Wannier-Stark ladder. We also discuss the use of femtosecond laser impulsions for the generation and direct probe of the induced phonon Bloch oscillations. We propose a gedanken experiment based in an integrated phonon source-structure-detector device, and we present calculations of pump and probe time dependent optical reflectivity that evidence temporal beatings in agreement with the Wannier-Stark ladder energy splitting.Comment: PDF file including 4 figure

    Atom interferometry with trapped Fermi gases

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    We realize an interferometer with an atomic Fermi gas trapped in an optical lattice under the influence of gravity. The single-particle interference between the eigenstates of the lattice results in macroscopic Bloch oscillations of the sample. The absence of interactions between fermions allows a time-resolved study of many periods of the oscillations, leading to a sensitive determination of the acceleration of gravity. The experiment proves the superiorness of non interacting fermions with respect to bosons for precision interferometry, and offers a way for the measurement of forces with microscopic spatial resolution.Comment: 4 pages, 4 figure

    Conversion of an Atomic Fermi Gas to a Long-Lived Molecular Bose Gas

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    We have converted an ultracold Fermi gas of 6^6Li atoms into an ultracold gas of 6^6Li2_2 molecules by adiabatic passage through a Feshbach resonance. Approximately 1.5×1051.5 \times 10^5 molecules in the least-bound, v=38v = 38, vibrational level of the X1Σg+^1 \Sigma ^+_g singlet state are produced with an efficiency of 50%. The molecules remain confined in an optical trap for times of up to 1 s before we dissociate them by a reverse adiabatic sweep.Comment: Accepted for publication in Phys. Rev. Letter

    Dynamics of a Quantum Phase Transition

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    We present two approaches to the dynamics of a quench-induced phase transition in quantum Ising model. The first one retraces steps of the standard approach to thermodynamic second order phase transitions in the quantum setting. The second one is purely quantum, based on the Landau-Zener formula for transition probabilities in avoided level crossings. We show that the two approaches yield compatible results for the scaling of the defect density with the quench rate. We exhibit similarities between them, and comment on the insights they give into dynamics of quantum phase transitions.Comment: 4 pages, 3 figures. Replaced by revised versio

    Temperature-dependent magnetization in diluted magnetic semiconductors

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    We calculate magnetization in magnetically doped semiconductors assuming a local exchange model of carrier-mediated ferromagnetic mechanism and using a number of complementary theoretical approaches. In general, we find that the results of our mean-field calculations, particularly the dynamical mean field theory results, give excellent qualitative agreement with the experimentally observed magnetization in systems with itinerant charge carriers, such as Ga_{1-x}Mn_xAs with 0.03 < x < 0.07, whereas our percolation-theory-based calculations agree well with the existing data in strongly insulating materials, such as Ge_{1-x}Mn_x. We comment on the issue of non-mean-field like magnetization curves and on the observed incomplete saturation magnetization values in diluted magnetic semiconductors from our theoretical perspective. In agreement with experimental observations, we find the carrier density to be the crucial parameter determining the magnetization behavior. Our calculated dependence of magnetization on external magnetic field is also in excellent agreement with the existing experimental data.Comment: 17 pages, 15 figure

    Spin Polarization at Semiconductor Point Contacts in Absence of Magnetic Field

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    Semiconductor point contacts can be a useful tool for producing spin-polarized currents in the presence of spin-orbit (SO) interaction. Neither magnetic fields nor magnetic materials are required. By numerical studies, we show that (i) the conductance is quantized in units of 2e^2/h unless the SO interaction is too strong, (ii) the current is spin-polarized in the transverse direction, and (iii) a spin polarization of more than 50% can be realized with experimentally accessible values of the SO interaction strength. The spin-polarization ratio is determined by the adiabaticity of the transition between subbands of different spins during the transport through the point contacts.Comment: 4 pages, 4 figures; minor changes, published in J. Phys. Soc. Jp

    Improved Thermoelectric Cooling Based on the Thomson Effect

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    Traditional thermoelectric Peltier coolers exhibit a cooling limit which is primarily determined by the figure of merit, zT. Rather than a fundamental thermodynamic limit, this bound can be traced to the difficulty of maintaining thermoelectric compatibility. Self-compatibility locally maximizes the cooler's coefficient of performance for a given zT and can be achieved by adjusting the relative ratio of the thermoelectric transport properties that make up zT. In this study, we investigate the theoretical performance of thermoelectric coolers that maintain self-compatibility across the device. We find such a device behaves very differently from a Peltier cooler, and term self-compatible coolers "Thomson coolers" when the Fourier heat divergence is dominated by the Thomson, as opposed to the Joule, term. A Thomson cooler requires an exponentially rising Seebeck coefficient with increasing temperature, while traditional Peltier coolers, such as those used commercially, have comparatively minimal change in Seebeck coefficient with temperature. When reasonable material property bounds are placed on the thermoelectric leg, the Thomson cooler is predicted to achieve approximately twice the maximum temperature drop of a traditional Peltier cooler with equivalent figure of merit (zT). We anticipate the development of Thomson coolers will ultimately lead to solid state cooling to cryogenic temperatures.Comment: The Manuscript has been revised for publication in PR

    Counting statistics in multiple path geometries and the fluctuations of the integrated current in a quantum stirring device

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    The amount QQ of particles that are transported via a path of motion is characterized by its expectation value and by its variance Var(Q)Var(Q). We analyze what happens if a particle has two optional paths available to get from one site to another site, and in particular what is Var(Q)Var(Q) for the current which is induced in a quantum stirring device. It turns out that coherent splitting and the stirring effect are intimately related and cannot be understood within the framework of the prevailing probabilistic theory.Comment: 11 pages, 2 figures, published version, Latex Eq# correcte

    Evidence of strong antiferromagnetic coupling between localized and itinerant electrons in ferromagnetic Sr2FeMoO6

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    Magnetic dc susceptibility (χ\chi) and electron spin resonance (ESR) measurements in the paramagnetic regime, are presented. We found a Curie-Weiss (CW) behavior for χ\chi(T) with a ferromagnetic Θ=446(5)\Theta = 446(5) K and μeff=4.72(9)μB/f.u.\mu_{eff} = 4.72(9) \mu_{B}/f.u., this being lower than that expected for either Fe3+(5.9μB)Fe^{3+}(5.9\mu_{B}) or Fe2+(4.9μB)Fe^{2+}(4.9\mu_{B}) ions. The ESR g-factor g=2.01(2)g = 2.01(2), is associated with Fe3+Fe^{3+}. We obtained an excellent description of the experiments in terms of two interacting sublattices: the localized Fe3+Fe^{3+} (3d53d^{5}) cores and the delocalized electrons. The coupled equations were solved in a mean-field approximation, assuming for the itinerant electrons a bare susceptibility independent on TT. We obtained χe0=3.7\chi_{e}^{0} = 3.7 10−410^{-4} emu/mol. We show that the reduction of μeff\mu_{eff} for Fe3+Fe^{3+} arises from the strong antiferromagnetic (AFM) interaction between the two sublattices. At variance with classical ferrimagnets, we found that Θ\Theta is ferromagnetic. Within the same model, we show that the ESR spectrum can be described by Bloch-Hasegawa type equations. Bottleneck is evidenced by the absence of a gg-shift. Surprisingly, as observed in CMR manganites, no narrowing effects of the ESR linewidth is detected in spite of the presence of the strong magnetic coupling. These results provide evidence that the magnetic order in Sr2FeMoO6Sr_{2}FeMoO_{6} does not originates in superexchange interactions, but from a novel mechanism recently proposed for double perovskites

    Robustness of adiabatic passage trough a quantum phase transition

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    We analyze the crossing of a quantum critical point based on exact results for the transverse XY model. In dependence of the change rate of the driving field, the evolution of the ground state is studied while the transverse magnetic field is tuned through the critical point with a linear ramping. The excitation probability is obtained exactly and is compared to previous studies and to the Landau-Zener formula, a long time solution for non-adiabatic transitions in two-level systems. The exact time dependence of the excitations density in the system allows to identify the adiabatic and diabatic regions during the sweep and to study the mesoscopic fluctuations of the excitations. The effect of white noise is investigated, where the critical point transmutes into a non-hermitian ``degenerate region''. Besides an overall increase of the excitations during and at the end of the sweep, the most destructive effect of the noise is the decay of the state purity that is enhanced by the passage through the degenerate region.Comment: 16 pages, 15 figure
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