Localization and dephasing driven by magnetic fluctuations in low carrier density colossal magnetoresistance materials

Localization and dephasing of conduction electrons in a low carrier density ferromagnet due to scattering on magnetic fluctuations is considered. We claim the existence of the “mobility edge”, which separates the states with fast diffusion and the states with slow diffusion; the latter is determined by the dephasing time. When the “mobility edge” crosses the Fermi energy a large and sharp change of conductivity is observed. The theory provides an explanation for the observed temperature dependence of conductivity in ferromagnetic semiconductors and manganite pyrochlores. Received 17 January 1999 and Received in final form 12 March 1999     

Single-electron coherence: Finite temperature versus pure dephasing

We analyze a coherent injection of single electrons on top of the Fermi sea in two situations, at finite-temperature and in the presence of pure dephasing. Both finite-temperature and pure dephasing change the property of the injected quantum states from pure to mixed. However, we show that the temperature-induced mixedness does not alter the coherence properties of these single-electron states. In particular two such mixed states exhibit perfect antibunching while colliding at an electronic wave splitter. This is in striking difference with the dephasing-induced mixedness which suppresses antibunching. On the contrary, a single-particle shot noise is suppressed at finite temperatures but is not affected by pure dephasing. This work therefore extends the investigation of the coherence properties of single-electron states to the case of mixed states and clarifies the difference between different types of mixedness.     

Reprint of : Single-electron coherence: Finite temperature versus pure dephasing

We analyze a coherent injection of single electrons on top of the Fermi sea in two situations, at finite-temperature and in the presence of pure dephasing. Both finite-temperature and pure dephasing change the property of the injected quantum states from pure to mixed. However, we show that the temperature-induced mixedness does not alter the coherence properties of these single-electron states. In particular two such mixed states exhibit perfect antibunching while colliding at an electronic wave splitter. This is in striking difference with the dephasing-induced mixedness which suppresses antibunching. On the contrary, a single-particle shot noise is suppressed at finite temperatures but is not affected by pure dephasing. This work therefore extends the investigation of the coherence properties of single-electron states to the case of mixed states and clarifies the difference between different types of mixedness.     

The effect of stochastic dephasing on the entanglement and coherence of qutrits

By using negativity, we study the effect on the quantum entanglement of 2-qutrit states of stochastic dephasing which obeys the stationary Gauss-Markov process. The linear entropy used to measure coherence loss due to the stochastic dephasing is discussed. With analysis, we find that the time evolution of entanglement and the linear entropy depends not only on the structure of states of concern, but also on the strength of the fluctuations of the random variable in the stochastic process.     

Anomalous dephasing of bosonic excitons interacting with phonons in the vicinity of the Bose-Einstein condensation

The dephasing and relaxation kinetics of bosonic excitons interacting with a thermal bath of acoustic phonons is studied after coherent pulse excitation. The kinetics of the induced excitonic polarization is calculated within Markovian equations both for subcritical and supercritical excitation with respect to a Bose-Einstein condensation (BEC). For excited densities n below the critical density , an exponential polarization decay is obtained, which is characterized by a dephasing rate . This dephasing rate due to phonon scattering shows a pronounced exciton-density dependence in the vicinity of the phase transition. It is well described by the power law that can be understood by linearization of the equations around the equilibrium solution. Above the critical density we get a non-exponential relaxation to the final condensate value p⁰ with that holds for all densities. Furthermore we include the full self-consistent Hartree-Fock-Bogoliubov (HFB) terms due to the exciton-exciton interaction and the kinetics of the anomalous functions . The collision terms are analyzed and an approximation is used which is consistent with the existence of BEC. The inclusion of the coherent exciton-exciton interaction does not change the dephasing laws. The anomalous function F_k exhibits a clear threshold behaviour at the critical density. Received 13 December 1999     

Statistical model for the effects of dephasing on transport properties of large samples

We present a statistical model for the effects of dephasing on the transport properties of large devices. The physical picture is different from earlier models which assume that dephasing happens continuously throughout the sample, whereas we model the dephasing in a statistical sense, assuming a distribution of completely phase randomizing regions between which the transport is coherent and described by the nonequilibrium Green’s function method. Thus the sample is effectively divided into smaller parts making the numerical treatment more efficient. As a first application the conductances of ordered and disordered linear tight-binding chains are calculated and compared to the results of other phenomenological models in the literature.     

Background fluctuations in the GPS-microlab-1 ionospheric radio sounding experiment

We analyze the spectra of the phase and amplitude fluctuations of radio signals recorded in the course of four ionospheric radio-sounding sessions at altitudes from 70 to 120 km. Our study is aimed at determining the sources of these fluctuations. Comparing the statistical properties of fluctuations measured at two wavelengths with theoretical calculations, we conclude that only the low-frequency part of background fluctuations is of ionospheric origin, while the high-frequency fluctuations are caused by noise of the measuring system. The amplitude fluctuations are more informative in the high-frequency region, since they are mainly due to ionospheric irregularities. We discuss the possibility of separation of the components of amplitude fluctuations caused by ionospheric irregularities and the irregularities of the neutral atmosphere in the case of sounding at the altitudes at which those components have comparable values. It is shown that this problem cannot be solved by dispersion and extrapolation methods, which are used for separation of the regular ionospheric and atmospheric components of the amplitude and phase variations of radio signals. A. N. Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, Moscow, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 6, pp. 511–523, June 1999.     

Evolution of Berry's phase in a graded-index medium

Evolution of the polarization vector in an inhomogeneous isotropic medium is investigated using the quantum-mechanical formalism of coherent states. The relation between the fluctuations of Berry's phase of wave nature caused by the “spin-orbit” interaction of a photon and depolarization of an incident pure state of polarization is clarified.     

Microcavity polariton scattering probed by coherent control experiments

We present coherent control experiments which simultaneously probe both the coherence and the population dynamics of the exciton–photon polariton states in a semiconductor microcavity. The coherent manipulation of either the spin orientation or the density of polaritons is demonstrated leading to the measurement of the optical dephasing time. The polariton scattering by acoustical phonons or by mutual collision processes are investigated by a simultaneous measurement of both the optical dephasing time T₂and the decay time T₁of the radiant states. These results clearly evidence a quenching of the different scattering processes at resonance.     

Holstein model and Datta model of dephasing in one-dimensional chains

We consider the recently proposed “momentum conserving” dephasing model of Golizadeh-Mojarad and Datta [R. Golizadeh-Mojarad, S. Datta, Phys. Rev. B 75 (2007) 081301(R)] versus the “momentum relaxing” Holstein dephasing model. For both models, a detailed analysis of the coherent and incoherent components of the transmission coefficient is presented. The comparison between the two models reveals significant differences in the scaling properties of the coherent and incoherent contributions as functions of the dephasing strength and the length of the region where it acts. We also provide an analytic model that describes the peculiar behavior of the coherent component of the transmission coefficient. Our simulations are based on the Keldysh Green’s function method in the tight-binding framework.     

Role of phase fluctuation and dephasing in the enhancing continuous variable entanglement of a two-photon coherent beat laser

A steady state analysis of the nonclassical features and statistical properties of the cavity radiation of a two-photon coherent beat laser is presented. Results show that the degree of two-mode squeezing, detectable entanglement and intensity of the cavity radiation can increase with the deviation of the phase fluctuations of the laser employed in preparing the atoms, but decrease with the increasing rate at which the induced coherence superposition decays. Although it is found that varying the phase fluctuations and dephasing can lead to modification in the quantum features and statistical properties of the radiation, it does not alter the similarity in the nature of the degree of entanglement detectable by the criteria following from Duan-Giedke-Cirac-Zoller and logarithmic negativity in a perceivable manner. Since the intensity and quantum features can be readily enhanced, this system is expected to be a viable source of a strong robust entangled (squeezed) light under various conditions. Moreover, comparison of the mean number of photon pairs with intensity difference shows that the chance of inciting a two-photon process can be enhanced by changing the rate of dephasing and phase fluctuations.     

Quantum phase properties of the field in a two-photon micromaser

In this paper, we study the quantum phase properties of the field in a two-photon micromaser, including the effects of the finite detuning of the intermediate level. For initial coherent state of the cavity field and atoms initially in their excited state multipeak phase structure appears which eventually leads to the randomization of the cavity field phase. However, the approach towards the randomization depends upon the detuning. If the atoms are injected in a coherent superposition of their upper and lower atomic states then the phase distribution evolves into two-peak structure. For initial thermal state and atoms in polarized state, cavity field acquires some phase. We also consider the effect of finite Q of the cavity, random injection of the atoms and fluctuations in the interaction time.     

奇偶相干态中测量相位算符的涨落及其压缩

利用Ｂａｒｎｅｔｔ和Ｐｅｇｇ提出的测量相位算符讨论了奇偶相干态中的相位涨落及其高阶涨落，在测不准关系和高阶测不准关系基础上给出了测量相位压缩和高阶压缩的二类定义，并用这二类定义研究了奇偶相干态中测量相位的二阶和高阶压缩情况。     

Dephasing effects in topological insulators

Topological insulators, a class of typical topological materials in both two dimensions and three dimensions,are insulating in bulk and metallic at surface. The spin-momentum locked surface states and peculiar transport properties exhibit promising potential applications on quantum devices, which generate extensive interest in the last decade. Dephasing is the process of the loss of phase coherence, which inevitably exists in a realistic sample. In this review, we focus on recent progress in dephasing effects on the topological insulators. In general, there are two types of dephasing processes: normal dephasing and spin dephasing. In two-dimensional topological insulators, the phenomenologically numerical investigation shows that the longitudinal resistance plateaus is robust against normal dephasing but fragile with spin dephasing. Several microscopic mechanisms of spin dephasing are then discussed. In three-dimensional topological insulators, the helical surface states exhibit a helical spin texture due to the spin-momentum locking mechanism. Thus, normal dephasing has close connection to spin dephasing in this case, and gives rise to anomalous “gap-like” feature. Dephasing effects on properties of helical surface states are investigated.     

热噪声的相干态和压缩态中的高阶涨落

利用热场动力学的方法研究了具有热噪声的相干态和压缩态中场正交分量的高阶涨落和高阶压缩.利用测量相位算符讨论了这些热化态中相位的高阶涨落.从而得到在有限温度下这些态的低阶及高阶涨落与温度的关系和压缩特性与温度的关系.由于实际的场态总是在一有限的温度下，所以讨论温度对涨落和压缩的影响是很有意义的. 关键词：     

Self-Consistent Consideration of Fluctuations in Singlet Superconducting Phases with s and d Symmetry

We analyze the behavior of thermal fluctuations of the superconducting order parameter with extended s-wave and $${{d}_{{{{x}^{2}} - {{y}^{2}}}}}$$-wave symmetry. For this purpose, we develop a method of self-consistent consideration of the order parameter fluctuations and charge carrier scatterers by fluctuations of coupled electron pairs using the theory of functional integration. The study is performed based on the quasi-two-dimensional one-band model with attraction between electrons located at neighboring sites. We obtain the distribution functions of the phase fluctuation probabilities depending on temperature, charge carrier concentrations, and model parameters. It is shown that the phase of the order parameter in the superconducting region is coherent, and the density of states has a dip at the Fermi level. In approaching the incoherent region of the phase diagram, the dip in the density of states disappears simultaneously with the loss of phase coherence. At the same time, the order parameter amplitude averaged over fluctuations remains finite at any temperature and concentration of charge carriers. Our results show that the pseudogap state cannot be explained in the frames of this scenario.     

Controlled dephasing of a quantum dot in the Kondo regime

Kondo correlation in a spin polarized quantum dot (QD) results from the dynamical formation of a spin singlet between the dot's net spin and a Kondo cloud of electrons in the leads, leading to enhanced coherent transport through the QD. We demonstrate here significant dephasing of such transport by coupling the QD and its leads to potential fluctuations in a nearby "potential detector." The qualitative dephasing is similar to that of a QD in the Coulomb blockade regime in spite of the fact that the mechanism of transport is quite different. A much stronger than expected suppression of coherent transport is measured, suggesting that dephasing is induced mostly in the "Kondo cloud" of electrons within the leads and not in the QD.     

Squeezed states of light pulses in the presence of a self-effect in an inertial nonlinear medium

A systematic theory of the formation of squeezed states during the propagation of coherent light pulses in a medium with an inertial Kerr nonlinearity is developed. It is established that the region of the spectrum where the quadrature fluctuations are weaker than the shot noise depends on both the relaxation time of the nonlinearity and the magnitude of the nonlinear phase shift. It is also shown that the frequency at which suppression of the fluctuations is greatest can be controlled by adjusting the phase of the pulse. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 7, 481–485 (10 April 1999)     

Formation of correlated states and tunneling for a low energy and controlled pulsed action on particles

We consider a method for optimizing the tunnel effect for low-energy particles by using coherent correlated states formed under controllable pulsed action on these particles. Typical examples of such actions are the effect of a pulsed magnetic field on charged particles in a gas or plasma. Coherent correlated states are characterized most comprehensively by the correlation coefficient r(t); an increase of this factor elevates the probability of particle tunneling through a high potential barrier by several orders of magnitude without an appreciable increase in their energy. It is shown for the first time that the formation of coherent correlated states, as well as maximal |r(t)|_max and time-averaged 〈|r(t)|〉 amplitudes of the correlation coefficient and the corresponding tunneling probability are characterized by a nonmonotonic (oscillating) dependence on the forming pulse duration and amplitude. This result makes it possible to optimize experiments on the realization of low-energy nuclear fusion and demonstrates the incorrectness of the intuitive idea that the tunneling probability always increases with the amplitude of an external action on a particle. Our conclusions can be used, in particular, for explaining random (unpredictable and low-repeatability) experimental results on optimization of energy release from nuclear reactions occurring under a pulsed action with fluctuations of the amplitude and duration. We also consider physical premises for the observed dependences and obtain optimal relations between the aforementioned parameters, which ensure the formation of an optimal coherent correlated state and optimal low-energy tunneling in various physical systems with allowance for the dephasing action of a random force. The results of theoretical analysis are compared with the data of successful experiments on the generation of neutrons and alpha particles in an electric discharge in air and gaseous deuterium.     

Phase properties of superposed squeezed states

The phase properties of the superposed squeezed states are studied. The superposed squeezed states are obtained by inserting two squeezers in a Mach-Zehnder interferometer one for each arm. The visibility of the states as a measure of the coherence of the generated photons in the nonlinear interferometer is analyzed. And the mean-square phase fluctuations are also considered to analyze the phase properties of the superposed squeezed states. The quantities are obtained as a function of gain and the input phase, when the amplified input is a coherent state, a number state, a thermal state, a displaced number state, or a coherent thermal state.