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The cross section for the neutron-induced fission of 238U nuclei has been measured using the time-of-flight spectrometer of the GNEIS neutron complex at the Petersburg Nuclear Physics Institute, National Research Center Kurchatov Institute, in the neutron energy range of 0.3–500 MeV. Fission fragments have been detected by low-pressure position-sensitive multiwire proportional counters. The cross section for 238U(n, f) fission has been measured with respect to the cross section for 235U(n, f) fission, which is an accepted international standard. Data on the energy dependence of the anisotropy of the angular distribution of fragments of neutron-induced 238U nuclei are also presented. The data obtained have been compared to previous experiments carried out using both similar and significantly different methods.

The measured angular distributions of fission fragments of 240Pu nuclei have been presented and theoretically analyzed. Measurements at energies above 10 MeV have been performed for the first time. A model description of the energy dependence of the angular anisotropy of fission fragments in the entire studied range of neutron energies has been proposed.

The experimental data on the angular distributions of fragments from the fission of 237Np nuclei induced by 1–200 MeV neutrons have been presented. Such measurements for neutron energies above 16 MeV have been performed for the first time. These distributions have been analyzed and the angular anisotropy in the center-of-mass of fragments has been determined in the entire energy range under study. A method involvng the complex dynamics of the formation and decay of highly excited nuclei and based on the TALYS program has been proposed to calculate the angular distributions of fission fragments in a wide energy range for the collision of nuclei with neutrons. It has been shown that the developed model describes well the main features of the energy dependence of the angular anisotropy for the 237Np target nuclei and can be used to extract new information on the reaction and fission process.

The specific heat and susceptibilities for the two- and one-dimensional spin--orbital models are calculated in the framework of a spherically symmetric self-consistent approach at different temperatures and relations between the parameters of the system. It is shown that even in the absence of the long-range spin and orbital order, the system exhibits the features in the behavior of thermodynamic characteristics, which are typical of those manifesting themselves at phase transitions. Such features are attributed to the quantum entanglement of the coupled spin and orbital degrees of freedom., Comment: 7 pages, 9 figures, submitted to JETP Letters

The angular distributions of fragments from the neutron-induced fission of natPb and 239Pu nuclei have been measured in the energy of range 1–200 MeV using the neutron time-of-flight spectrometer GNEIS. Fission fragments have been detected by position sensitive multiwire proportional counters. The results for the anisotropy of fission fragments deduced from the measured angular distributions have been presented. The results have been compared with the experimental data of other authors.

In the framework of the spin-fermion model, to which the Emery model is reduced in the limit of strong electron correlations, it is shown that the fermion quasiparticles in cuprate high-Tc superconductors (HTSCs) arise under a strong effect of exchange coupling between oxygen holes and spins of copper ions. This underlies the spin-polaron nature of fermion quasiparticles in cuprate HTSCs. The Cooper instability with respect to the d-wave symmetry of the order parameter is revealed for an ensemble of such quasiparticles. For the normal phase, the spin-polaron concept allows us to reproduce the fine details in the evolution of the Fermi surface with the changes in the doping level x observed in experiment for La2-xSrxCuO4. The calculated T–x phase diagram correlates well with the available experimental data for cuprate HTSCs.

New results of the neutron-induced fission experiments carried out at the neutron time-of-flight spectrometer GNEIS of the PNPI are given. Angular distributions of fission fragments from the neutron-induced fission of 233U and 209Bi nuclei have been measured in the energy range 1–200 MeV using position sensitive multiwire proportional counters as fission fragment detector. The recent improvements of the measurement and data processing procedures are described. The data on anisotropy of fission fragments deduced from the measured angular distributions are presented in comparison with the experimental data of other authors.

It has been shown that, because of the two-orbital character of the subsystem of holes located at oxygen sites and the spatial separation of this subsystem from that of spins at copper ions, the superconducting phase in high-T c superconductors is stable with respect to the strong Coulomb repulsion of holes located at nearestneighbor oxygen sites if the order parameter has the $${d_{{x^2} - {y^2}}}$$ symmetry. This effect is due to the symmetry characteristics of the Coulomb potential, owing to which the equation determining the Cooper pairing in the $${d_{{x^2} - {y^2}}}$$ channel does not include this potential.

A matrix projection formalism is developed for the description of a frustrated two-dimensional antiferromagnet. This approach keeps safe all the advantages of the spherically symmetric self-consistent treatment (it strictly meets the spin constraint and the Mermin—Wagner theorem) but does not have its disadvantages. In particular, even in its simplest implementation, this approach provides a good agreement with the numerical simulations for two best-studied reference points: the zero frustration point and the point of a transition from the antiferromagnetic ordered phase to the disordered phase.

Angular distributions of fission fragments from the neutron-induced fission of 232Th, 235U, and 238U have been measured in the energy range 1–200 MeV at the neutron time-of-flight (TOF) spectrometer GNEIS using position sensitive multiwire proportional counters as fission fragment detector. A short description of the experimental equipment and measurement procedure is given. The anisotropy of fission fragments deduced from the data on measured angular distributions is presented in comparison with experimental data of other authors.

For a two-dimensional optimally doped antiferromagnet with spin S =1/2, we study the temperature dependence of the electrical resistivity ρ(T), Hall coefficient RH(T), and thermopower Q(T). The temperature dependence corresponding to optimally doped layered high-Tc superconducting cuprates has been obtained simultaneously for all three transport coefficients within the unified spin-polaron approach for the Kondo lattice. The key features of our study are the usage of the multimoment method for solving the kinetic equations (seven moments for the nonequilibrium distribution function ensure a good convergence) and the form of ac spin susceptibility χ(q, ω) for localized spins. For χ(q, ω), we choose a self-consistent expression which takes into account the “crossover” between the spin susceptibility determined by the self-consistent spherically symmetric Green’s function method and the semiphenomenological spin susceptibility corresponding to the critical decay of magnons.

For the spin-fermion model, it has been shown that the concept of spin polarons makes it possible to reproduce fine details of the Fermi surface evolution in the nodal direction of La2 − xSrxCuO4 occurring with changes in the doping level x. The physics here is determined by the spin-correlated hopping of charge carriers and by the doping dependence of the inverse magnetic correlation length.

The ground state of the J 1-J 2 Heisenberg model with arbitrary signs of exchange is studied for spin S = 1/2 in the case of the two-dimensional (2D) square lattice. The states with different types of spin long-range order (antiferromagnetic checkerboard, stripe, collinear ferromagnetic) as well as the disordered spin liquid states are described in the framework of one analytical approach. In particular, it is shown that the phase transition between the ferromagnetic spin liquid and the ferromagnet with long-range order is of the second order. In the vicinity of such transition, we have found the ferromagnetic state with a rapidly varying condensate function.

An orthonormalized basis describing the single-hole states of the CuO2 plane has been constructed exactly taking into account the p-d exchange coupling between the spins of a hole and the two nearest copper ions. The introduced basis makes it possible to include the effects caused by the dependence of the Heisenberg interaction between the nearest copper ions on the occupation of the intermediate oxygen ion by the hole in the calculation of the spectrum of the collective excitations of nonlocal three-spin polarons. The inclusion of this correlation, as well as the nonlocality of the p-d exchange coupling, is the main difference of the developed approach to the description of the energy structure from the calculations with the use of the Zhang-Rice singlet.

The phase diagram of the frustrated J 1-J 2-J 3 S = 1/2 Heisenberg model on a square lattice is studied in the range of parameters corresponding to the spin-liquid state of the system. The study is performed using the self-consistent two-time retarded spin-spin Green’s functions, which do not break both translational and SU(2) symmetries. The inclusion of the damping of spin fluctuations allows us to obtain a good agreement with the cluster calculations. Using a consistent analytical approach, we have found continuous transitions via the spin-liquid state between the phases with three types of the long-range order: checkerboard, stripe, and (k, k) helical. In addition, there are indications confirming the existence of the (k, π) helical phase.

It has been shown that two-and three-center interactions arise in the strong-coupling regime for the 2D Kondo lattice; these interactions both induce and suppress the Cooper instability. It is important that, in contrast to the t-J* model, the three-center interactions promote the Cooper pairing and ensure the appearance of the superconducting phase with a high critical temperature Tc. The calculated concentration dependences of Tc agree well with the experimental data for the cuprate superconductors.

In the Kondo model of the two-dimensional lattice with a strong spin-hole antiferromagnetic exchange, the pseudogap behavior of the carrier spectral function A(k, ω) is considered in the optimal and almost dielectric doping limits. A distinctive feature of the analysis is the introduction of the spin polaron even in the mean-field approximation that leads to the formation of two bands (the analogs of the upper and lower Hubbard bands) and makes it possible to immediately take into account the main rearrangement of A(k, ω). The inclusion of the scattering of the mean-field polaron (within the irreducible Green’s functions) describes the further rearrangement of A(k, ω), in particular, the unusual appearance of the pseudogap near the points N = (±π/2, ±π/2).

Experimental data on the non-spin-wave behavior of the cuprate magnetic susceptibility χ(q, ω) in the superconducting state are reconstructed on the basis of a spherically symmetric self-consistent approach within the framework of the frustrated Heisenberg model. For the Green’s function of the spin-wave excitations, a polarization operator resonant with the superconducting state is introduced phenomenologically. This gives rise to an additional (compared to the normal state) branch of the incommensurate peaks and to a pronounced non-spin-wave behavior of the susceptibility χ(q, ω) in the frequency range near 80 meV (the so-called “dark region”). The renormalization of the real part of the polarization operator plays a crucial role in the theory.

The problem of constructing the kinetic equation with the description of motion of a hole in systems with strong spin-hole interaction (such as high-temperature superconductors) in terms of the spin polaron has been considered in the framework of the regular antiferromagnetic s-d model. It has been shown by the example of electrical resistivity that the kinetics is determined by the properties of the bands of the spin polariton (rather than “bar hole”) and their quasiparticle residues Z k. The cases of low and optimal doping of the CuO2 plane have been considered. It has been shown that the rearrangement of the spectrum of the lower polaron band, as well as the strong doping dependence of the quasiparticle residues Z k, is decisive in the unified consideration of these cases.

A spherically symmetric approach is developed for the two-plane frustrated Heisenberg model with the inclusion of finite damping in the dynamical spin susceptibility χ(q, ω) at finite temperatures. For yttrium cuprates, the theory allows the description of the neutron experimental results for the acoustic, χac (q, ω), and optical, χopt(q, ω), branches of the susceptibility in a wide doping interval. The problem of the reconstruction of the exchange constants, the stripe scenario for forming the 40-meV peak in the q integrated susceptibility χ2D″ (ω), strong renormalization of the spin-excitation spectrum (due to finite damping), and differences of the approach from the linear-spin-wave method are discussed.