Theory of charge density wave depinning by electromechanical effect

We discuss the first theory for the depinning of low dimensional, incommensurate, charge density waves (CDWs) in the strong electron-phonon (e-p) regime. Arguing that most real CDWs systems invariably develop a gigantic dielectric constant (GDC) at very low frequencies, we propose an electromechanical mechanism which is based on a local field effect. At zero electric field and large enough e-p coupling the structures are naturally pinned by the lattice due to its discreteness, and develop modulation functions which are characterized by discontinuities. When the electric field is turned on, we show that it exists a finite threshold value for the electric field above which the discontinuities of the modulation functions vanish due to CDW deformation. The CDW is then free to move. The signature of this pinning/depinning transition as a function of the increasing electric field can be directly observed in the phonon spectrum by using inelastic neutrons or X-rays experiments.Comment: 4 figures, submitte

Polaron Dissociation at the Insulator-to-Metal Transition

Considering the long range Coulomb interactions between large polarons in dielectrics, we propose a model for their crystallization when no bipolarons are formed. As the density increases, the melting is examined at $T=OK$. One possibility is the delocalization towards a liquid state of polarons. However, we show that this cannot happen if the electron-phonon coupling is larger than some critical value. The other competing mechanism is the dissociation of the polarons themselves, favored owing to their large mass at strong coupling. Finally, we propose a phase diagram for the insulator-to metal transition as a function of the density and electron-phonon coupling.Comment: 5 pages, 3 figures, to be published Mod. Phys. Lett. B; added 1 figure, references and minor change

Nature of metal-nonmetal transition in metal-ammonia solutions. II. From uniform metallic state to inhomogeneous electronic microstructure

Applying semi-analytical models of nonideal plasma, we evaluate the behavior of the metallic phase in metal-ammonia solutions (MAS). This behavior is mainly controlled by the degenerate electron gas, which remains stable down to 5 MPM due to high solvent polarizability and strong dielectric screening of solvated ions. Comparing the behavior of the metallic state with those of localized solvated electrons, we have estimated the miscibility gap $\Delta n$ for various alkali metals and found $\Delta n$(Na)$> \Delta n($K$)$. It is rather narrow in Rb-NH$_3$ and does not occur in Cs-NH$_3$ solutions, which is in full agreement with the experiments. The case of Li is discussed separately. The difference calculated in the excess free energies of the metallic and nonmetallic phases is in the order of $k_BT$, yielding a thermally fluctuating mixed state at intermediate metal concentrations. It results in a continuous metal-nonmetal (MNM) transition above the consolute point $T_c$ and a phase separation below $T_c$. We propose a criterion for the MNM transition which may be attributed to the line of the maximum of compressibility above $T_c$. This line crosses the spinodal one at the critical temperature. Finally, we assert that a new electronic phase similar to microemulsion should also arise between the spinodal and the binodal lines.Comment: 22 pages, 10 figure

Effect of the heliospheric interface on the distribution of interstellar hydrogen atom inside the heliosphere

This paper deals with the modeling of the interstellar hydrogen atoms (H atoms) distribution in the heliosphere. We study influence of the heliospheric interface, that is the region of the interaction between solar wind and local interstellar medium, on the distribution of the hydrogen atoms in vicinity of the Sun. The distribution of H atoms obtained in the frame of the self-consistent kinetic-gasdynamic model of the heliospheric interface is compared with a simplified model which assumes Maxwellian distribution of H atoms at the termination shock and is called often as 'hot' model. This comparison shows that the distribution of H atoms is significantly affected by the heliospheric interface not only at large heliocentric distances, but also in vicinity of the Sun at 1-5 AU. Hence, for analysis of experimental data connected with direct or undirect measurements of the interstellar atoms one necessarily needs to take into account effects of the heliospheric interface. In this paper we propose a new model that is relatively simple but takes into account all major effects of the heliospheric interface. This model can be applied for analysis of backscattered Ly-alpha radiation data obtained on board of different spacecraft.Comment: published in Astronomy Letter

The solar wind as seen by SOHO/SWAN since 1996: comparison with SOHO/LASCO C2 coronal densities

We update the SOHO/SWAN H Lyman-alpha brightness analysis to cover the 1996-2008 time interval. A forward model applied to the intensity maps provides the latitude and time dependence of the interstellar Hydrogen ionisation rate over more than a full solar cycle. The hydrogen ionisation, being almost entirely due to charge-exchange with solar wind ions, reflects closely the solar wind flux. Our results show that the solar wind latitudinal structure during the present solar minimum is strikingly different from the previous minimum, with a much wider slow solar wind equatorial belt which persists until at least the end of 2008. We compute absolute values of the in-ecliptic H ionisation rates using OMNI solar wind data and use them to calibrate our ionisation rates at all heliographic latitudes. We then compare the resulting fluxes with the synoptic LASCO/C2 electron densities at 6 solar radii. The two time-latitude patterns are strikingly similar over all the cycle. This comparison shows that densities at 6 solar radii can be used to infer the solar wind type close to its source, with high (resp. low) densities tracing the slow (resp. fast) solar wind, simply because the density reflects at which altitude occurs the acceleration. The comparison between the two minima suggests that the fast polar wind acceleration occurs at larger distance during the current minimum compared to the previous one. This difference, potentially linked to the magnetic field decrease or(and) the coronal temperature decrease should be reproduced by solar wind expansion models.Comment: Proceedings of the SOHO-23 conference, Sept 21-25, 200

Mobile Bipolarons in the Adiabatic Holstein-Hubbard Model in 1 and 2 dimensions

The bound states of two electrons in the adiabatic Holstein-Hubbard model are studied numerically in one and two dimensions from the anticontinuous limit. This model involves a competition between a local electron-phonon coupling (with a classical lattice) which tends to form pairs of electrons and the repulsive Hubbard interaction $U \geq 0$ which tends to break them. In 1D, the ground-state always consists in a pair of localized polarons in a singlet state. They are located at the same site for U=0. Increasing U, there is a first order transition at which the bipolaron becomes a spin singlet pair of two polarons bounded by a magnetic interaction. The pinning mode of the bipolaron soften in the vicinity of this transition leading to a higher mobility of the bipolaron which is tested numerically. In 2D, and for any $U$, the electron-phonon coupling needs to be large enough in order to form small polarons or bipolarons instead of extended electrons. We calculate the phase diagram of the bipolaron involving first order transitions lines with a triple point. A pair of polarons can form three types of bipolarons: a) on a single site at small $U$, b) a spin singlet state on two nearest neighbor sites for larger $U$ as in 1D and c) a new intermediate state obtained as the resonant combination of four 2-sites singlet states sharing a central site, called quadrisinglet. The breathing and pinning internal modes of bipolarons in 2D generally only weakly soften and thus, they are practically not mobile. On the opposite, in the vicinity of the triple point involving the quadrisinglet, both modes exhibit a significant softening. However, it was not sufficient for allowing the existence of a classical mobile bipolaron (at least in that model)

The flow direction of interstellar neutral H from SOHO/SWAN

Interstellar neutral hydrogen flows into the heliosphere as a mixture of the primary and secondary populations from two somewhat different directions due to splitting occurring in the magnetized outer heliosheath. The direction of inflow of interstellar neutral H observed in the inner heliosphere, confronted with that of the unperturbed flow of interstellar neutral helium, is important for understanding the geometry of the distortion of the heliosphere from axial symmetry. It is also needed for facilitating remote-sensing studies of the solar wind structure based on observations of the helioglow, such as those presently performed by SOHO/SWAN, and in a near future by IMAP/GLOWS. In the past, the only means to measure the flow direction of interstellar hydrogen were spectroscopic observations of the helioglow. Here, we propose a new method to determine this parameter based on a long series of photometric observations of the helioglow. The method is based on purely geometric considerations and does not depend on any model and absolute calibration of the measurements. We apply this method to sky maps of the helioglow available from the SOHO/SWAN experiment and derive the mean flow longitude of interstellar hydrogen. We obtain 253.1\degr \pm 2.8\degr, which is in perfect agreement with the previously obtained results based on spectroscopic observations.Comment: Accepted for Ap