A. S. Moskvin

57Fe Mössbauer study of unusual magnetic structure of multiferroic 3R-AgFeO2

We report new results of a 57Fe Mössbauer study of hyperfine magnetic interactions in the layered multiferroic 3R-AgFeO2 demonstrating two magnetic phase transitions at T N1 and T N2. The asymptotic value β *u2009u2009≈u2009u20090.34 for the critical exponent obtained from the temperature dependence of the hyperfine field H hf(T) at 57Fe the nuclei below T N1u2009u2009≈u2009u200914u2009K indicates that 3R-AgFeO2 shows quasi-3D critical behavior. The spectra just above T N1 (T N1u2009u2009<u2009u2009Tu2009u2009<u2009u2009Tu2009 *u2009u2009≈u2009u200941u2009K) demonstrate a relaxation behavior due to critical spin fluctuations which indicates the occurrence of short-range correlations. At the intermediate temperature range, T N2u2009u2009<u2009u2009Tu2009u2009<u2009u2009T N1, the 57Fe Mössbauer spectra are described in terms of collinear spin-density-waves (SDW) with the inclusion of many high-order harmonics, indicating that the real magnetic structure of the ferrite appears to be more complicated than a pure sinusoidally modulated SDW. Below Tu2009u2009<u2009u2009T N2u2009u2009≈u2009u20099u2009K, the hyperfine field H hf reveals a large spatial anisotropy (ΔH anisu2009u2009≈u2009u200930 kOe) which is related with a local intra-cluster (FeO6) spin-dipole term that implies a conventional contribution of the polarized oxygen ions. We proposed a simple two-parametric formula to describe the dependence of H anis on the distortions of the (FeO6) clusters. Analysis of different mechanisms of spin and hyperfine interactions in 3R-AgFeO2 and its structural analogue CuFeO2 points to a specific role played by the topology of the exchange coupling and the oxygen polarization in the delafossite-like structures.

[Interaction of Membrane and Calcium Oscillators in Cardiac Pacemaker Cells: Mathematical Modeling].

An integrative model of the calcium dynamics in cardiac pacemaker cells is developed taking the synergetic effect of the coupling of the membrane and the intracellular calcium oscillators (“membrane and Ca2+ clock”) into account. The main feature of the model is a description of the stochastic dynamics of Ca2+ release units within the electron-conformational dynamics of ryanodine channels (RyR-channels). It is shown that interaction of the cellular oscillators provides stable action-potential generation in cardiac pacemaker cells, even in the case of stochastic Ca2+ dynamics. We studied the effect of RyR sensitivity to the intracellular calcium concentration in the sarcoplasmic reticulum and in the dyadic space on the behavior of the calcium-release system in detail. A parametric analysis of an integrative model of pacemaker cells was performed.

Electronic structure of hole centers in CuO2 planes of cuprates

A theoretical analysis and a large amount of experimental data indicate that the structure of the valence hole states in doped cuprates is more complicated than assumed in the simple Zhang-Rice singlet model. In fact, we are dealing with a competition between a hybrid Cu3d–O2pb1g∝dx2−y2-state and purely oxygen nonbonding states with a2g- and eux,y∝px,y-symmetries. Thus, as a cluster analog of a Cu3+ ion, the ground state of a non-Zhang-Rice CuO45− hole center of this sort should be described by complicated A1g1−B2g1,3−Eu1,3 multiplet with a set of charge, orbital, and spin order parameters, some of which are well known (e.g., spin moment or “ferromagnetic” Ising orbital momentum localized on oxygen ions) while others are unconventional or hidden (e.g., “antiferromagnetic” ordering of Ising orbital momenta localized on four oxygen atoms or a combined spin-orbital-quadrupole ordering). The non-Zhang-Rice CuO45− centers are actually singlet-triplet pseudo-Jahn-Teller centers with strong vibron coupling to th...

Electron-conformational transformations govern the temperature dependence of the cardiac ryanodine receptor gating

Temperature influences many aspects of cardiac excitation-contraction coupling, in particular, hypothermia increases the open probability (Popen) of cardiac sarcoplasmic reticulum (SR) Ca2+-release channels (ryanodine-sensitive RyR channels) rising the SR Ca2+ load in mammalian myocytes. However, to the best of our knowledge, no theoretical models are available for that effect. Traditional Markov chain models do not provide a reasonable molecular mechanistic insight on the origin of the temperature effects. Here in the paper we address a simple physically clear electron-conformational model to describe the RyR gating and argue that a synergetic effect of external thermal fluctuation forces (Gaussian–Markovian noise) and internal friction via the temperature stimulation/suppression of the open–close RyR tunneling probability can be considered as a main contributor to temperature effects on the RyR gating. Results of the computer modeling allowed us to successfully reproduce all the temperature effects observed for an isolated RyR gating in vitro under reducing the temperature: increase in Popen and mean open time without any significant effect on mean closed

Unconventional spin-charge phase separation in a model 2D cuprate

In this work, we address a challenging problem of a competition of charge and spin orders for high-Tc cuprates within a simplified 2D spin-pseudospin model which takes into account both conventional Heisenberg Cu2+−Cu2+ antiferromagnetic spin exchange coupling (J) and the on-site (U) and intersite (V) charge correlations in the CuO2 planes with the on-site Hilbert space reduced to only three effective charge states (nominally Cu1+;2+;3+). We performed classical Monte Carlo calculations for large square lattices implying the mobile doped charges and focusing on a case of a small intersite repulsion V ≪ J. The on-site attraction (U < 0) does suppress the antiferromagnetic ordering and gives rise to a checkerboard charge order with the doped charge distributed randomly over a system in the whole temperature range. However, under the on-site repulsion (U > 0) the homogeneous ground state antiferromagnetic solutions of the doped system found in a mean-field approximation are shown to be unstable with respect to a phase separation with the charge and spin subsystems behaving like immiscible quantum liquids. Puzzlingly, with lowering the temperature one can observe two sequential phase transitions: first, an antiferromagnetic ordering in the spin subsystem diluted by randomly distributed charges, then, a charge condensation in the charge droplets. The effects are illustrated by the Monte Carlo calculations of the specific heat and longitudinal magnetic susceptibility.

The temperature effect on cardiac ryanodine receptor gating and conductance: Mathematical modeling

The temperature effect on cardiac ryanodine receptor (RyR) function has been studied within the electron-conformational (EC) model. It’s shown that a simple EC model with an Arrhenius-like temperature dependence of the “internal” and “external” frictions and a specific thermosensitivity of the tunnelling “open↔closed” transitions can provide both qualitative and quantitative description of the temperature effects for isolated RyRs. The potential of the model was illustrated by explaining the experimental data on the temperature dependence of isolated sheep cardiac RyR gating and conductance (R. Sitsapesan et al., J. Physiol. 434, 469(1991)).

Asymptotics of quasi-classical localized states in 2D system of charged hard-core bosons

Abstract The continuous quasi-classical two-sublattice approximation is constructed for the 2D system of charged hard-core bosons to explore metastable inhomogeneous states analogous to inhomogeneous localized excitations in magnetic systems. The types of localized excitations are determined by asymptotic analysis and compared with numerical results. Depending on the homogeneous ground state, the excitations are the ferro and antiferro type vortices, the skyrmion-like topological excitations or linear domain walls.

Charge order–superfluidity transition in a two-dimensional system of hard-core bosons and emerging domain structures

We have used high-performance parallel computations by NVIDIA graphics cards applying the method of nonlinear conjugate gradients and Monte Carlo method to observe directly the developing ground state configuration of a two-dimensional hard-core boson system with decrease in temperature, and its evolution with deviation from a half-filling. This has allowed us to explore unconventional features of a charge order—superfluidity phase transition, specifically, formation of an irregular domain structure, emergence of a filamentary superfluid structure that condenses within of the charge-ordered phase domain antiphase boundaries, and formation and evolution of various topological structures.

The MFA ground states for the extended Bose-Hubbard model with a three-body constraint

Abstract We address the intensively studied extended bosonic Hubbard model (EBHM) with truncation of the on-site Hilbert space to the three lowest occupation states n = 0 , 1 , 2 in frames of the S = 1 pseudospin formalism. Similar model was recently proposed to describe the charge degree of freedom in a model high-T c cuprate with the on-site Hilbert space reduced to the three effective valence centers, nominally Cu1+;2+;3+. With small corrections the model becomes equivalent to a strongly anisotropic S = 1 quantum magnet in an external magnetic field. We have applied a generalized mean-field approach and quantum Monte-Carlo technique for the model 2D S = 1 system with a two-particle transport to find the ground state phase with its evolution under deviation from half-filling.