arXiv subjects

A. S. Alexandrov

Publications and source records attributed to A. S. Alexandrov.

At least 91 records · Page 5Linked to original sources

arXiv2004

Isotope effect on the electron band structure of doped insulators

Applying a continuous-time quantum Monte-Carlo algorithm we calculate the exact coherent band dispersion and the density of states of a two dimensional lattice polaron in the region of parameters where any approximation might fail. We find an isotope effect on the band structure, which is different for different wave-vectors of the Brillouin zone and depends on the radius and strength of the electron-phonon interaction. An isotope effect on the electron spectral function is also discussed.

cond-mat.supr-con↗

arXiv2004

Pairing of bosons in the condensed state of the boson-fermion model

A two component model of negative U centers coupled with the Fermi sea of itinerant fermions is discussed in connection with high-temperature superconductivity of cuprates, and superfluidity of atomic fermions. We examine the phase transition and the condensed state of this boson-fermion model (BFM) beyond the ordinary mean-field approximation in two and three dimensions. No pairing of fermions and no condensation are found in two-dimensions for any symmetry of the order parameter. The expansion in the strength of the order parameter near the transition yields no linear homogeneous term in the Ginzburg-Landau-Gor'kov equation and a zero upper critical field in any-dimensional BFM, which indicates that previous mean-field discussions of the model are flawed. Normal and anomalous Green's functions are obtained diagrammatically and analytically in the condensed state of a simplest version of 3D BFM. A pairing of bosons analogous to the Cooper pairing of fermions is found. There are three coupled condensates in the model, described by the off-diagonal single-particle boson, pair-fermion and pair-boson fields. These results negate the common wisdom that the boson-fermion model is adequately described by the BCS theory at weak coupling.

cond-mat.supr-con↗

arXiv2004

How normal is the "normal" state of superconducting cuprates?

High magnetic field studies of the cuprate superconductors revealed a non-BCS temperature dependence of the upper critical field $H_{c2}(T)$ determined resistively by several groups. These determinations caused some doubts on the grounds of the contrasting effect of the magnetic field on the in-plane, $ρ_{ab}$, and out-of-plane, $ρ_{c}$ resistances reported for large sample of Bi2212. Here we present careful measurements of both $ρ_{ab}(B)$ and $ρ_{c}(B)$ of tiny Bi2212 crystals in magnetic fields up to 50 Tesla. None of our measurements revealed a situation when on field increase $ρ_c$ reaches its maximum while $ρ_{ab}$ remains very small if not zero. The resistive $H_{c2}(T)$ estimated from $ρ_{ab}(B)$ and $ρ_{c}(B)$ are approximately the same. We also present a simple explanation of the unusual Nernst signal in superconducting cuprates as a normal state phenomenon. Our results support any theory of cuprates, which describes the state above the resistive phase transition as perfectly 'normal' with a zero off-diagonal order parameter.

cond-mat.supr-con↗

arXiv2004

Nernst effect in poor conductors and the cuprate superconductors

We calculate the Nernst signal in disordered conductors with the chemical potential near the mobility edge. The Nernst effect originates from interference of itinerant and localised-carrier contributions to the thermomagnetic transport. It reveals a strong temperature and magnetic field dependence, which describes quantitatively the anomalous Nernst signal in high-Tc cuprates.

cond-mat.supr-con↗

arXiv2004

Comment on 'Effect of on-site Coulomb repulsion on superconductivity in the boson-fermion model' (T. Domanski, Phys. Rev. B v.66, 134512 (2002))

The two-dimensional boson-fermion model of high-temperature superconductors, numerically studied by Domanski (Phys. Rev. B v.66, 134512 (2002)) is not a superconductor. The critical temperature of the model is zero for any symmetry of the order parameter. The opposite conclusion advocated by Domanski and some other authors stems from an erroneous mean-field approximation neglecting the boson self-energy. The approximation is qualitatively incorrect even in three dimensions. Taking into account the boson self-energy changes the phase transition in 3D BFM, which is not a mean-field second-order phase transition. There are real bosons in the model at and below Tc of 3D BFM, but not only fermions and their Cooper pairs.

cond-mat.supr-con↗

arXiv2004

Condensed phase of converting boson-fermion mixtures

Theory of a condensed state of hybridised bosons and fermions is developed. Normal and anomalous Green's functions are obtained diagrammatically and analytically using the Hamiltonian of the boson-fermion model (BFM). A pairing of bosons analogous to the Cooper pairing of fermions is found. There are three coupled condensates in the model, described by the off-diagonal single-particle boson, pair-fermion and pair-boson fields. The Gor'kov expansion in the strength of the order parameter near the transition yields no linear homogeneous term in the Ginzburg-Landau equation, and the infinite Levanyuk-Ginzburg parameter, Gi, which indicates that previous mean-field discussions of BFM are flawed.

cond-mat.supr-con↗

arXiv2003

Small adiabatic polaron with a long-range electron-phonon interaction

Two-site single electron system interacting with many vibrating ions of a lattice via a long-range (Fröhlich) electron-phonon interaction is studied in the adiabatic regime. The renormalised hopping integral of small adiabatic Fröhlich polarons is calculated and compared with the hopping integral of small adiabatic Holstein polarons.

cond-mat↗

arXiv2003

Comment on `Origin of combination frequencies in quantum magnetic oscillations of two-dimensional multiband metals' by T. Champel [Phys. Rev. B 65, 153403 (2002); cond-mat/0110154]

We analyze the applicability of our analytical theory of combination harmonics in canonical low-dimensional multi-band Fermi liquids, which was recently criticized by Champel (Phys. Rev. B 65, 153403 (2002)). It is shown that his claim that our analytical theory does not apply at low temperatures and in clean samples, is incorrect. We demonstrate that the analytical theory of combination harmonics is in excellent agreement with the exact numerical results even at zero temperature and for clean systems, which are the most challenging for an analytical description.

cond-mat.mes-hall↗

arXiv2003

Polarons in Complex Oxides and Molecular Nanowires

There is a growing understanding that transport properties of complex oxides and individual molecules are dominated by polaron physics. In superconducting oxides the long-range Froehlich and short-range Jahn-Teller electron-phonon interactions bind carriers into real space pairs - small bipolarons with surprisingly low mass but sufficient binding energy, while the long-range Coulomb repulsion keeps bipolarons apart preventing their clustering. The bipolaron theory numerically explains high Tc values without any fitting parameters and describes other key features of the cuprates. The same approach provides a new insite into the theory of transport through molecular nanowires and quantum dots (MQD). Attractive polaron-polaron correlations lead to a "switching" phenomenon in the current-voltage characteristics of MQD. The degenerate MQD with strong electron-vibron coupling has two stable current states (a volatile memory), which might be useful in molecular electronics.

cond-mat.mes-hall↗

arXiv2003

Vortex and critical fields in charged Bose liquids and unconventional superconductors

A single vortex in the charged Bose gas (CBG) has a charged core and its profile different from the vortex in neutral and BCS superfluids. Lower and upper critical fields of CBG are discussed. The unusual resistive upper critical field, Hc2(T), of many cuprates and a few other unconventional superconductors is described as the Bose-Einstein condensation field of preformed bosons-bipolarons. Its nonlinear temperature dependence follows from the scaling arguments. Exceeding the Pauli paramagnetic limit is explained. Controversy in the determination of Hc2(T) of cuprates from kinetic and thermodynamic measurements is addressed in the framework of the bipolaron theory.

cond-mat.supr-con↗

arXiv2003

'Giant' normal state magnetoresistances of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$

Magnetoresistance (MR) of Bi-2212 single crystals with T$_{c}$ $\approx 87-92 K$ is studied in pulsed magnetic fields up to 50T along the c-axis in a wide temperature range. The negative out-of-plane and the positive in-plane MRs are measured in the normal state. Both MRs have similar magnitudes, exceeding any orbital contribution by two orders in magnitude. These are explained as a result of the magnetic pair-breaking of preformed pairs. Resistive upper critical fields H$_{c2}$(T) determined from the in-- and out-of-plane MRs are about the same. They show non-BCS temperature dependences compatible with the Bose-Einstein condensation field of preformed charged bosons.

cond-mat.supr-con↗

arXiv2003

Superlight bipolarons and a checkerboard d-wave condensate in cuprates

The seminal work by Bardeen, Cooper and Schrieffer taken further by Eliashberg to the intermediate coupling solved the problem of conventional superconductors about half a century ago. The Froehlich and Jahn-Teller electron-phonon interactions were identified as an essential piece of physics in all novel superconductors. The BCS theory provides a qualitatively correct description of some of them like magnesium diborade and doped fullerenes (if the polaron formation is taken into account). However, cuprates remain a problem. Here I show that the bipolaron extension of the BCS theory to the strong-coupling regime could be a solution. Low-energy physics in this regime is that of small 'superlight' bipolarons, which are real-space mobile bosonic pairs dressed by phonons. The symmetry and space modulations of the order parameter are explained in the framework of the bipolaron theory. A d-wave Bose-Einstein condensate of bipolarons reveals itself as a checkerboard modulation of the hole density and of the gap below Tc.

cond-mat.supr-con↗

arXiv2003

Hall effect and resistivity in underdoped cuprates

The behaviour of the Hall ratio $R_{H}(T)$ as a function of temperature is one of the most intriguing normal state properties of cuprate superconductors. One feature of all the data is a maximum of $R_{H}(T)$ in the normal state that broadens and shifts to temperatures well above $T_c$ with decreasing doping. We show that a model of preformed pairs-bipolarons provides a selfconsistent quantitative description of $R_{H}(T)$ together with in-plane resistivity and uniform magnetic susceptibility for a wide range of doping.

cond-mat.supr-con↗

arXiv2003

Breakdown of the Wiedemann-Franz law in strongly-coupled electron-phonon system, application to the cuprates

With the superconducting cuprates in mind, a set of unitary transformations was used to decouple electrons and phonons in the strong-coupling limit. While phonons remain almost unrenormalised, electrons are transformed into itinerent singlet and triplet bipolarons and thermally excited polarons. The triplet/singlet exchange energy and the binding energy of the bipolarons are thought to account for the spin and charge pseudogaps in the cuprates, respectively. We calculated the Hall Lorenz number of the system to show that the Wiedemann-Franz law breaks down due to the interference of the polaron and bipolaron contributions to heat flow. The model provides a quantitative fit to magnetotransport data in the cuprates. Furthermore we are able to extract the phonon component of the thermal conductivity with the use of experimental data and the model. Our results further validate the use of a charged Bose gas model to describe normal and superconducting properties of unconventional superconductors.

cond-mat.supr-con↗

arXiv2003

Superlight bipolarons and criterion of BCS-BEC crossover in cuprates

Most of the proposed models of high-temperature superconductivity (HTSC) are based on the short-range electron-electron correlations or/and on a short-range electron-phonon interaction. However, in the cuprates the screening is poor due to a low carrier density, layered crystal structure, and high ionicity of the lattice. We develop further the bipolaron model of HTSC, which explicitly takes into account the long-range origin of all interactions. The long-range electron-phonon (Froehlich) interaction binds carriers into real space pairs (small bipolarons) with surprisingly low mass but sufficient binding energy, while the long-range Coulomb repulsion keeps them from forming larger clusters. The model has explained many key features of cuprates. Here it is shown that real-space pairing takes place in many cuprates at variance with some (incorrect) criteria of the BCS-BEC crossover.

cond-mat.supr-con↗

arXiv2002

Bi-stable tunneling current through a molecular quantum dot

An exact solution is presented for tunneling through a negative-U d-fold degenerate molecular quantum dot weakly coupled to electrical leads. The tunnel current exhibits hysteresis if the level degeneracy of the negative-U dot is larger than two (d>2). Switching occurs in the voltage range V1 < V < V2 as a result of attractive electron correlations in the molecule, which open up a new conducting channel when the voltage is above the threshold bias voltage V2. Once this current has been established, the extra channel remains open as the voltage is reduced down to the lower threshold voltage V1. Possible realizations of the bi-stable molecular quantum dots are fullerenes, especially C60, and mixed-valence compounds.

cond-mat.mes-hall↗

arXiv2002

Current hysteresis and memory effect in a molecular quantum dot with strong electron-vibron interaction

Theory of current hysteresis for tunneling through a molecular quantum dot (MQD) with strong electron-vibron interactions and attractive electron-electron correlations is developed. The dot is modeled as a d-fold degenerate energy level weakly coupled to the leads. The effective attractive interaction between polarons in the dot results in a "switching" phenomenon in the current-voltage characteristics when d>2, in agreement with the results for the phenomenological negative-U model. The degenerate MQD with strong electron-vibron coupling has two stable current states in certain interval of the bias voltage below some critical temperature.

cond-mat.mes-hall↗

arXiv2002

Lorenz number in high Tc superconductors: evidence for bipolarons

Strong electron-phonon interaction in cuprates has gathered support over the last decade in a number of experiments. While phonons remain almost unrenormalised, electrons are transformed into itinerent bipolarons and thermally excited polarons when the electron-phonon interaction is strong. We calculate the Lorenz number of the system to show that the Wiedemann-Franz law breaks down because of the interference of polaron and bipolaron contributions in the heat flow. The model fits numerically the experimental Hall Lorenz number, which provides a direct evidence for bipolarons in the cuprates.

cond-mat.supr-con↗