Local distributions of the 1D dilute Ising model

The local distributions of the one-dimensional dilute annealed Ising model with charged impurities are studied. Explicit expressions are obtained for the pair distribution functions and correlation lengths, and their low-temperature asymptotic behavior is explored depending on the concentration of impurities. For a more detailed consideration of the ordering processes, we study local distributions. Based on the Markov property of the dilute Ising chain, we obtain an explicit expression for the probability of any finite sequence and find a geometric probability distribution for the lengths of sequences consisting of repeating blocks. An analysis of distributions shows that the critical behavior of the spin correlation length is defined by ferromagnetic or antiferromagnetic sequences, while the critical behavior of the impurity correlation length is defined by the sequences of impurities or by the charge-ordered sequences. For the dilute Ising chain, there are no other repeating sequences whose mean length diverges at zero temperature. While both the spin correlation and the impurity correlation lengths can diverge only at zero temperature, the ordering processes result in a maximum of the specific heat at finite temperature defined by the maximum rate of change of the impurity-spin pairs concentration. A simple approximate equation is found for this temperature. We show that the non-ordered dilute Ising chains correspond to the regular Markov chains, while various orderings generate the irregular Markov chains of different types. © 2020 Elsevier B.V.This work was supported by Program 211 of the Government of the Russian Federation, Agreement 02.A03.21.0006, and the Ministry of Education and Science of the Russian Federation, project FEUZ-2020-0054

Critical Temperatures of a Model Cuprate

Abstract: The problem of competing orderings in the high-temperature cuprate materials is widely discussed for the last years. We present the mean-field approximation results for the spin-pseudospin model accounting for the on-site and inter-site correlations, the antiferromagnetic exchange coupling, the one- and two-particle transport. The explicit form of the equations for the critical temperatures of the most significant order parameters of the model are given. © 2019, Pleiades Publishing, Ltd.This work was supported by Program 211 of the Government of the Russian Federation (Agreement 02.A03.21.0006), the Ministry of Education and Science of the Russian Federation (projects nos. 2277 and 5719)

Metastable austenitic steel structure and mechanical properties evolution in the process of cold radial forging

The article presents the influence of structure formation on the properties of 321 metastable austenitic stainless steel in the process of cold radial forging (CRF). The steel under study after austenitization was subjected to CRF at room temperature with degrees of true strain (e) 0.26, 0.56, 1.00, 1.71 and 2.14. It has been shown that structure formation of the studied steel during CRF consists of three stages: formation of the lamellar structure of austenite, formation of the trapezoidal structure, and formation of the equiaxial grain structure. The kinetics of the strain-induced α'-martensitic transformation is related to the stages of structure evolution. Hardness, ultimate tensile strength and yield strength uniformly increase in all stages of structure formation with a significant decrease of elongation to fracture during the first stage of structure formation while the value of elongation to fracture remains constant in the subsequent stages of deformation. Impact strength of fatigue cracked specimens (KCT) decreases sharply at the first stage of structure formation and smoothly increases at the second and third stages. However, the impact strength of V-notch specimens (KCV) continuously decreases when deformation degree increases in the overall investigated deformation range. © 2019 by the authors.Ministry of Science and Higher Education of the Russian FederationFunding: The work has been performed under the project No.11.8213.2017/8.9 within the framework of the basic part of the state assignment to universities in the field of scientific activity financed by the Ministry of Science and Higher Education of the Russian Federation

Phase Diagram of Semi-Hard-Core Bosons on a Square Lattice

Abstract: Phase diagrams of charged “semi-hard-core” bosons are studied in the mean field approximation. An increase in the parameter of local correlations is shown to lead to the transformation of the phase diagram of the system from the form characteristic of “hard-core” bosons to the limiting form with a parabolic dependence of the critical temperature of charge ordering on the boson concentration. The evolution between these limiting cases is dependent on the ratio between the model parameters and is accompanied by various effects, such as the change in the phase transition type, the appearance of new order–order transition, and the appearance of new critical points. © 2021, Pleiades Publishing, Ltd.Ministry of Education and Science of the Russian Federation, Minobrnauka: FEUZ-2020-0054; Government Council on Grants, Russian FederationThis work was supported by the program 211 of the Government of the Russian Federation (Agreements no. 02.A03.21.0006) and the Ministry of Education and Science of the Russian Federation (project no. FEUZ-2020-0054

Model of Charge Triplets for High-Tc Cuprates

Starting with a minimal model for the CuO2 planes with the on-site Hilbert space reduced to a charge triplet of the three effective valence centers [CuO4]7−,6−,5− (nominally Cu1+,2+,3+) with different conventional spin, different orbital symmetry, and different local lattice configuration, we develop a unified non-BCS spin–pseudospin model to describe the main phase states of doped cuprates. We argue that antiferromagnetic insulating, charge ordered, superconducting, and Fermi-liquid phases are possible phase states of a model parent cuprate, while typical phase state of a doped cuprate, in particular mysterious pseudogap phase, is a result of a phase separation. Superconductivity of cuprates is not a consequence of pairing of doped holes, but the result of quantum transport of on-site composite hole bosons, whereas main peculiarities of normal state can be related to an electron–hole interplay for unusual Fermi-liquid phase and features of the phase separation. Puzzlingly, but it is the electron–lattice interaction, which in the BCS model determines s-wave pairing, in the model of local composite bosons gives dx2−y2-symmetry of the superconducting order parameter, thus showing once again a substantial involvement of the lattice in the cuprate's HTSC. © 2022 Elsevier B.V.This research was supported by the Ministry of Education and Science of Russian Federation, project No FEUZ-2020-0054

Unconventional phase separation in the model 2D spin-pseudospin system

The competition of charge and spin orderings is a challenging problem for strongly correlated systems, in particular, for high-Tc cuprates. We addressed a simplified static 2D spin-pseudospin model which takes into account both conventional spin exchange coupling and the on-site and inter-site charge correlations. Classical Monte-Carlo calculations for large square lattices show that homogeneous ground state antiferromagnetic solutions found in a mean-field approximation are unstable with respect to phase separation into the charge and spin subsystems behaving like immiscible quantum liquids. In this case, with lowering of a temperature one can observe two sequential phase transitions: first, antiferromagnetic ordering in the spin subsystem diluted by randomly distributed charges, then, the charge condensation in the charge droplets. The inhomogeneous droplet phase reduces the energy of the system and changes the diagram of the ground states. On the other hand, the ground state energy of charge-ordered state in a mean-field approximation exactly matches the numerical Monte-Carlo calculations. The doped charges in this case are distributed randomly over a system in the whole temperature range. Various thermodynamic properties of the 2D spin-pseudospin system are studied by Monte-Carlo simulation. © 2018 The Authors, published by EDP Sciences.The work is supported by Act 211 Government of the Russian Federation, agreement No 02.A03.21.0006 and by the Ministry of Education and Science, projects 2277 and 5719

Singlet-Triplet Pseudo-Jahn-Teller Centers in Copper Oxides

One of the most exciting features of the hole centers CuO45- in doped cuprates is an unusually complicated ground state which is the result of the electronic quasi-degeneracy. An additional hole, doped to the basic CuO46- cluster with the b1g hole can occupy both the same hybrid Cu3d-O2p orbital state resulting in a Zhang-Rice singlet 1A1g and the purely oxygen eu molecular orbital resulting in a singlet or triplet 1,3Eu term with the close energies. We present detailed analysis of the (pseudo)-Jahn-Teller (PJT) effect driven by the near-degeneracy within the (1A1g, 1,3Eu)-manifold.We thank Drs. M.V. Eremin, B.I. Kochelaev, Yu.V.Yablokov, S.Yu. Shashkin, A.E. Nikiforov, A.S. Ovchinnikov, V.Ya. Mitrofanov and A.Ya. Fishman for stimulating discussions

The influence of local correlations on the phase states in the model of semi-hard-core bosons on a square lattice

The work considers a model of charged "semi-hard-core" bosons on a square lattice with a possible filling number at each node, ranging from 0 to 2. Temperature phase diagrams of the model are obtained using numerical Monte Carlo quantum simulation methods, and the influence of local charge correlations is examined. Comparison with results from mean-field methods shows that local charge correlations contribute to an increased role of quantum fluctuations in the formation of phase states.Comment: 5 pages, 2 figure

Singlet-triplet pseudo Jahn-Teller centers in copper oxides

One of the most exciting features of the hole centers CuO45- in doped cuprates is an unusually complicated ground state which is the result of the electronic quasi-degeneracy. An additional hole, doped to the basic CuO46- cluster with the b1g hole can occupy both the same hybrid Cu3d-O2p orbital state resulting in a Zhang-Rice singlet 1A1g and the purely oxygen eu molecular orbital resulting in a singlet or triplet 1,3Eu term with the close energies. We present detailed analysis of the (pseudo)-Jahn-Teller (PJT) effect driven by the near-degeneracy within the (1A1g, 1,3Eu)-manifold.We thank Drs. M.V. Eremin, B.I. Kochelaev, Yu.V.Yablokov, S.Yu. Shashkin, A.E. Nikiforov, A.S. Ovchinnikov, V.Ya. Mitrofanov and A.Ya. Fishman for stimulating discussions