arXiv subjects

A. S. Alexandrov

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

At least 73 records · Page 4Linked to original sources

arXiv2006

Unconventional pairing in bipolaronic theories

Various mechanisms have been put forward for cuprate superconductivity, which fit largely into two camps: spin-fluctuation and electron-phonon (el-ph) mechanisms. However, in spite of a large effort, electron-phonon interactions are not fully understood away from clearly defined limits. To this end, we use a numerically exact algorithm to simulate the binding of bipolarons. We present the results of a continuous-time quantum Monte-Carlo (CTQMC) algorithm on a tight-binding lattice, for bipolarons with arbitrary interaction range in the presence of strong coulomb repulsion. The algorithm is sufficiently efficient that we can discuss properties of bipolarons with various pairing symmetries. We investigate the effective mass and binding energies of singlet and triplet real-space bipolarons for the first time, and discuss the extensions necessary to investigate $d$-symmetric pairs.

cond-mat.supr-con↗

arXiv2006

Superlight small bipolarons in the presence of strong Coulomb repulsion

We study a lattice bipolaron on a staggered triangular ladder and triangular and hexagonal lattices with both long-range electron-phonon interaction and strong Coulomb repulsion using a novel continuous-time quantum Monte-Carlo (CTQMC) algorithm extended to the Coulomb-Frohlich model with two particles. The algorithm is preceded by an exact integration over phonon degrees of freedom, and as such is extremely efficient. The bipolaron effective mass and bipolaron radius are computed. Lattice bipolarons on such lattices have a novel crablike motion, and are small but very light in a wide range of parameters, which leads to a high Bose-Einstein condensation temperature. We discuss the relevance of our results with current experiments on cuprate high-temperature superconductors and propose a route to room temperature superconductivity.

cond-mat.supr-con↗

arXiv2006

Comment on 'Hysteresis, Switching, and Negative Differential Resistance in Molecular Junctions: a Polaron Model', by M. Galperin, M.A. Ratner, and A. Nitzan, Nano Lett. 5, 125 (2005)

It is shown that the ``hysteresis'' in a polaron model of electron transport through the molecule found by M.Galperin et al. [Nano Lett. 5, 125 (2005)] is an artefact of their ``mean-field'' approximation. The reason is trivial: after illegitimate replacement $\hat{n}^{2}=\hat{n}n_{0},$ where \hat{n} is the electron number operator, n_{0} the average molecular level occupation, Galperin et al. obtained non-physical dependence of a renormalized molecular energy level on the non-integer mean occupation number n_{0} (i.e. the electron self-interaction) and the resulting non-linearity of current. The exact theory of correlated polaronic transport through molecular quantum dots (MQDs) that we proposed earlier [Phys. Rev. B67, 235312 (2003)] proved that there is no hysteresis or switching in current-voltage characteristics of non-degenerate, d=1, or double degenerate, d=2, molecular bridges, contrary to the mean-field result. Switching could only appear in multiply degenerate MQDs with d>2 due to electron correlations. Most of the molecular quantum dots are in the regime of weak coupling to the electrodes addressed in our formalism.

cond-mat.mtrl-sci↗

arXiv2006

Phase coexistence and resistivity near the ferromagnetic transition of manganites

Pairing of oxygen holes into heavy bipolarons in the paramagnetic phase and their magnetic pair-breaking in the ferromagnetic phase [the so-called current-carrier density collapse (CCDC)] has accounted for the first-order ferromagnetic phase transition, colossal magnetoresistance (CMR), isotope effect, and pseudogap in doped manganites. Here we propose an explanation of the phase coexistence and describe the magnetization and resistivity of manganites near the ferromagnetic transition in the framework of CCDC. The present quantitative description of resistivity is obtained without any fitting parameters by using the experimental resistivities far away from the transition and the experimental magnetization, and essentially model independent.

cond-mat.str-el↗

arXiv2006

Hall-Lorenz number paradox in cuprate superconductors

Significantly different normal state Lorenz numbers have been found in two independent direct measurements based on the Righi-Leduc effect, one about 6 times smaller and the other one about 2 times larger than the Sommerfeld value in single cuprate crystals of the same chemical composition. The controversy is resolved in the model where charge carriers are mobile lattice bipolarons and thermally activated nondegenerate polarons. The model numerically fits several longitudinal and transverse kinetic coefficients providing a unique explanation of a sharp maximum in the temperature dependence of the normal state Hall number in underdoped cuprates.

cond-mat.supr-con↗

arXiv2005

Normal state Nernst effect, semiconducting-like resistivity and diamagnetism of underdoped cuprates

Semiconducting-like low-temperature in-plane resistivity indicates that there are no remnants of superconductivity above the resistive phase transition at T > Tc in underdoped cuprates. The model with the chemical potential pinned near the mobility edge inside the charge-transfer optical gap describes quantitatively the Nernst effect, thermopower, diamagnetism and the unusual low-temperature resistivity of underdoped cuprates as normal state phenomena above Tc.

cond-mat.supr-con↗

arXiv2005

Strong-Coupling Theory of High Temperature Superconductivity

High-temperature superconductivity (HTS) of cuprates represents a challenge to the conventional theory. Here I review a multi-polaron approach to the problem based on our extension of the BCS theory to the strong-coupling regime. Since there is almost no retardation (i.e. no Tolmachev-Morel-Anderson logarithm) reducing the Coulomb repulsion, e-ph interactions should be relatively strong to overcome the direct Coulomb repulsion, so carriers must be polaronic to form pairs in novel superconductors. I identify the Froehlich electron-phonon interaction as the most essential for pairing in superconducting cuprates, and discuss the "Froehlich-Coulomb" model of HTS, low-energy structures, and the phase diagram of cuprates. "Individual" versus Cooper pairing, normal state properties, in particular in-plane resistivity, the Hall effect, magnetic susceptibility, the Lorenz number, the Nernst effect, diamagnetism, spin and charge pseudogaps, and c-axis transport are also discussed. I present a parameter-free evaluation of Tc, an explanation of isotope effects, specific heat anomalies, upper critical fields, symmetries and space modulations of the order parameter. Finally I introduce a model of overdoped cuprates as mixtures of mobile bipolarons and degenerate lattice polarons.

cond-mat.supr-con↗

arXiv2005

Strong-coupling theory of high-temperature superconductivity and colossal magnetoresistance

We argue that the extension of the BCS theory to the strong-coupling regime describes the high-temperature superconductivity of cuprates and the colossal magnetoresistance (CMR) of ferromagnetic oxides if the phonon dressing of carriers and strong attractive correlations are taken into account. The long-range Froehlich electron-phonon interaction has been identified as the most essential in cuprates providing "superlight" lattice polarons and bipolarons. Here some kinetic, magnetic, and more recent thermomagnetic normal state measurements are interpreted in the framework of the strong-coupling theory, including the Nernst effect and normal state diamagnetism. Remarkably, a similar strong-coupling approach offers a simple explanation of CMR in ferromagnetic oxides. The pairing of oxygen holes into heavy bipolarons in the paramagnetic phase and their magnetic pair-breaking in the ferromagnetic phase account for the first-order ferromagnetic phase transition, CMR, isotope effects, and pseudogaps in doped manganites. Here we propose an explanation of the phase coexistence and describe the shape of resistivity of manganites near the transition in the framework of the strong-coupling approach.

cond-mat.supr-con↗

arXiv2005

Effects of lattice geometry and interaction range on polaron dynamics

We study the effects of lattice type on polaron dynamics using a continuous-time quantum Monte-Carlo approach. Holstein and screened Froehlich polarons are simulated on a number of different Bravais lattices. The effective mass, isotope coefficients, ground state energy and energy spectra, phonon numbers, and density of states are calculated. In addition, the results are compared with weak and strong coupling perturbation theory. For the Holstein polaron, it is found that the crossover between weak and strong coupling results becomes sharper as the coordination number is increased. In higher dimensions, polarons are much less mobile at strong coupling, with more phonons contributing to the polaron. The total energy decreases monotonically with coupling. Spectral properties of the polaron depend on the lattice type considered, with the dimensionality contributing to the shape and the coordination number to the bandwidth. As the range of the electron-phonon interaction is increased, the coordination number becomes less important, with the dimensionality taking the leading role.

cond-mat.str-el↗

arXiv2005

Normal state diamagnetism of charged bosons in cuprate superconductors

Normal state orbital diamagnetism of charged bosons quantitatively accounts for recent high-resolution magnetometery results near and above the resistive critical temperature Tc of superconducting cuprates. Our parameter-free descriptions of normal state diamagnetism, Tc, upper critical fields and specific heat anomalies unambiguously support the 3D Bose-Einstein condensation at Tc of preformed real-space pairs with zero off-diagonal order parameter above Tc, at variance with phase fluctuation (or vortex) scenarios of the "normal" state of cuprates.

cond-mat.supr-con↗

arXiv2005

Reply to the comment by C. Capan and K. Behnia on "Nernst effect in poor conductors and in the cuprate superconductors" (cond-mat/0501288)

The comment criticisms (cond-mat/0501288) are completely out of line with the context of the commented theory (Phys. Rev. Lett. v.93, 217002 (2004)). The comment neglected essential parts of the theory, which actually addressed all relevant experimental observations. I argue that the coexistence of the large Nernst signal and the insulating-like in-plane resistivity in underdoped cuprates rules out the vortex scenario, but agrees remarkably well with our theory.

cond-mat.supr-con↗

arXiv2005

Magnetic quantum oscillations in nanowires

Analytical expressions for the magnetization and the longitudinal conductivity of nanowires are derived in a magnetic field, B. We show that the interplay between size and magnetic field energy-level quantizations manifests itself through novel magnetic quantum oscillations in metallic nanowires. There are three characteristic frequencies of de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) oscillations, F=F_0,F_1, and F_2 in contrast with a single frequency F'_0 in simple bulk metals. The amplitude of oscillations is strongly enhanced in some "magic" magnetic fields. The wire cross-section S can be measured along with the Fermi surface cross-section, S_F.

cond-mat.mes-hall↗

arXiv2005

Effect of electron-phonon interaction range on lattice polaron dynamics: a continuous-time quantum Monte Carlo study

We present the numerically exact ground state energy, effective mass, and isotope exponents of a one-dimensional lattice polaron, valid for any range of electron-phonon interaction, applying a new continuous-time Quantum Monte Carlo (QMC) technique in a wide range of coupling strength and adiabatic ratio. The QMC method is free from any systematic finite-size and finite-time-step errors. We compare our numerically exact results with analytical weak-coupling theory and with the strong-coupling $1/λ$ expansion. We show that the exact results agree well with the canonical Fröhlich and Holstein-Lang-Firsov theories in the weak and strong coupling limits, respectively, for any range of interaction. We find a strong dependence of the polaron dynamics on the range of interaction. An increased range of interaction has a similar effect to an increased (less adiabatic) phonon frequency: specifically, a reduction in the effective mass.

cond-mat.str-el↗

arXiv2005

High temperature superconductivity due to the long-range electron-phonon interaction, application to isotope effects, thermomagnetic transport and nanoscale heterogeneity in cuprates

Strong electron-phonon interactions in cuprates and other high-temperature superconductors have gathered support over the last decade in a large number of experiments. Here I briefly introduce the Froehlich-Coulomb multi-polaron model of high-temperature superconductivity, which includes strong on-site repulsive correlations and the long-range Coulomb and electron-phonon (e-ph) interactions. Our extension of the BCS theory to the strong-coupling regime with the long-range \emph{unscreened} electron-phonon interaction could naturally explain the isotope effects, unconventional thermomagnetic transport, and checkerboard modulations of the tunnelling density of states in cuprates.

cond-mat.supr-con↗

arXiv2004

Vortex matter in the charged Bose liquid at absolute zero

The Gross-Pitaevskii-type equation is solved for the charge Bose liquid in the external magnetic field at zero temperature. There is a vortex lattice with locally broken charge neutrality. The boson density is modulated in real space and each vortex is charged. Remarkably, there is no upper critical field at zero temperature, so the density of single flux-quantum vortices monotonously increases with the magnetic field up to B=infinity and no indication of a phase transition. The size of each vortex core decreases as about 1/sqrt(B) keeping the system globally charge neutral. If bosons are composed of two fermions, a phase transition to a spin-polarized Fermi liquid at some magnetic field larger than the pair-breaking field is predicted.

cond-mat.supr-con↗

arXiv2004

Unconventional critical behavior of fermions hybridized with bosons

A phase transition into the condensed state of fermions hybridized with immobile bosons is examined beyond the ordinary mean-field approximation (MFA) in two and three dimensions. The hybridization interaction does not provide the Cooper pairing of fermions and the Bose-condensation in two-dimensions. In the three-dimensional (3D) boson-fermion model (BFM) an expansion in the strength of the order parameter near the transition yields no linear homogeneous term in the Ginzburg-Landau-Gor'kov equation. This indicates that previous mean-field discussions of the model are flawed in any dimensions. In particular, the conventional (MFA) upper critical field is zero in any-dimensional BFM.

cond-mat.str-el↗

arXiv2004

The "normal" state of superconducting cuprates might really be normal after all

High magnetic field studies of 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 both the contrasting effect of the magnetic field on the in-plane and out-of-plane resistances reported for large Bi2212 sample and the large Nernst signal \emph{well above} $T_{c}$. Here we present 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 the field increase $ρ_c$ reaches its maximum while $ρ_{ab}$ remains very small if not zero. The resistive %upper critical fields estimated from the in-plane and out-of-plane $H_{c2}(T)$ estimated from $ρ_{ab}(B)$ and $ρ_{c}(B)$ are approximately the same. Our results support any theory of cuprates that describes the state above the resistive phase transition as perfectly normal with a zero off-diagonal order parameter. In particular, the anomalous Nernst effect above the resistive phase transition in high-$T_{c}$ cuprates can be described quantitatively as a normal state phenomenon in a model with itinerant and localised fermions and/or charged bosons.

cond-mat.supr-con↗

arXiv2004

Checkerboard density of states in strong-coupling superconductors

The Bogoliubov-de Gennes (BdG) equations are solved in the strong-coupling limit, where real-space (preformed) pairs bose-condense with finite center-of-mass momenta. There are two energy scales in this regime, a temperature independent incoherent gap $Δ_p$ and a temperature dependent coherent gap $Δ_c (T)$, modulated in real space. The single-particle density of states (DOS) reveals checkerboard modulations similar to the tunnelling DOS in cuprates.

cond-mat.supr-con↗