Персона: Алексеев, Павел Александрович
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Институт лазерных и плазменных технологий
Стратегическая цель Института ЛаПлаз – стать ведущей научной школой и ядром развития инноваций по лазерным, плазменным, радиационным и ускорительным технологиям, с уникальными образовательными программами, востребованными на российском и мировом рынке образовательных услуг.
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Павел Александрович
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- ПубликацияТолько метаданныеEffect of Nd and Rh substitution on the spin dynamics of the Kondo-insulator CeFe2Al10(2020) Alekseev, P. A.; Mignot, J. M.; Adroja, D. T.; Lazukov, V. N.; Алексеев, Павел АлександровичThe dynamic magnetic properties of the Kondo-insulator state in CeFe2Al10 (spin gap, resonance mode) have been investigated using polarized neutrons on a single crystal of pure CeFe2Al10 . The results indicate that the magnetic excitations are polarized mainly along the orthorhombic a axis and their dispersion along the orthorhombic c direction could be determined. Polycrystalline samples of Nd- and Rh-doped CeFe2Al10 were also studied by the time-of-flight technique, with the aim of finding out how the low-energy magnetic excitation spectra change upon isoelectronic substitution of a rare-earth element (Nd) on the magnetic Ce site or electron doping (Rh) on the transition-element Fe sublattice. The introduction of magnetic Nd impurities strongly modifies the spin gap in the Ce dynamic magnetic response and causes the appearance of a quasielastic signal. The crystal-field excitations of Nd, studied in both LaFe2Al10 and CeFe2Al10, also reveal a significant influence of f -electron hybridization (largest in the case of Ce) on the crystal-field potential. As a function of the Rh concentration, a gradual change is observed from a Kondo-insulator to a metallic Kondo-lattice response, likely reflecting the decrease in the hybridization energy.
- ПубликацияТолько метаданныеDramatic impact of intermediate-valence impurity on induced magnetism in singlet ground state system PrNi(2020) Clementyev, E. S.; Lazukov, V. N.; Savchenkov, P. S.; Alekseev, P. A.; Савченков, Павел Сергеевич; Алексеев, Павел Александрович© 2020 Elsevier B.V.We studied the influence of La (nonmagnetic) and Ce (intermediate valence) impurities on the ferromagnetic ordering temperature Tc of compounds RE1-x(La,Ce)xNi (RE = Pr, Gd). The ordering temperature Tc was measured in a wide range of impurity concentration. Crystal electric field (CEF) splitting for the PrNi and Pr0.25Ce0.75Ni was defined by neutron spectroscopy measurements. The strong difference in the influence of La and Ce on Tc of PrNi and GdNi based systems has been obtained. The introduction of a spin-fluctuating intermediate valence impurity in PrNi leads to a dramatic increase in the area of the ordered state on the phase diagram, due to an increase in Tc, compared to the impact of La-impurity over the entire concentration range. The dependence of Tc on the Ce concentration for the Pr1-xCexNi was calculated within the framework of the model of microscopic states. The essential modification of the CEF splitting by the Ce impurity appears to be the decisive factor affecting Tc wherein the Ce ion carries zero static magnetic moments similar to the La case. The latter explains the identical dependencies of the ordering temperature on the impurity concentration observed in systems with a long-range order of local magnetic moments Gd1-xLaxNi and Gd1-xCexNi.
- ПубликацияТолько метаданныеMagnetic susceptibility of pyrochlores R2Ti2O7: R = Gd, Dy, Tb(2020) Kuznetsov, A. V.; Churkin, O. A.; Ivanov, A. A.; Popov, V. V.; Alekseev, P. A.; Menushenkov, A. P.; Кузнецов, Алексей Владимирович; Чуркин, Олег Алексеевич; Иванов, Андрей Анатольевич; Попов, Виктор Владимирович; Алексеев, Павел Александрович; Менушенков, Алексей Павлович© 2019We present the study of magnetic susceptibility of pyrochlores Gd2Ti2O7, Dy2Ti2O7 and Tb2Ti2O7 which belong to frustrated magnets. The temperature dependence of the susceptibility χ(T) of powder samples was measured in the temperature range from 4.2 to 300 K and analyzed in the framework of mean field approximation taking into account both crystal field effects and anisotropy of the exchange interaction. We developed a simple approach allowing us to estimate interaction energies of magnetic ions from χ(T) dependences and data on wave functions of the crystal-field states of the ions using a minimum number of fitting parameters. We revealed a strong anisotropy of the exchange interaction in the paramagnetic state of Gd2Ti2O7 and obtained nearest-neighbor effective bond energies of the magnetic ions in Tb2Ti2O7 and Dy2Ti2O7 which are in good agreement with the temperatures of formation of the spin-ice state in these compounds.
- ПубликацияТолько метаданныеMagnetization of Crystalline and Amorphous Phases of R(2)Ti(2)O(7)and R2Zr2O7(R = Gd, Dy, Tb)(2020) Kuznetsov, A. V.; Churkin, O. A.; Popov, V. V.; Shchetinin, I. V.; Ivanov, A. A.; Yaroslavtsev, A. A.; Gaynanov, B. R.; Yastrebtsev, A. A.; Chernysheva, O. V.; Alekseev, P. A.; Menushenkov, A. P.; Кузнецов, Алексей Владимирович; Чуркин, Олег Алексеевич; Попов, Виктор Владимирович; Иванов, Андрей Анатольевич; Гайнанов, Булат Радикович; Ястребцев, Алексей Алексеевич; Чернышева, Ольга Викторовна; Алексеев, Павел Александрович; Менушенков, Алексей ПавловичWe present the study of frustrated magnets R2M2O7, where R = Gd, Dy, Tb and M = Ti, Zr. Magnetic susceptibility chi and low-temperature magnetization of perfectly ordered pyrochlores R2Ti2O7, partially disordered fluorites R2Zr2O7, and their completely disordered amorphous phases were measured and analyzed. We have found that structural disorder weakens the exchange interaction of the rare-earth ions. Magnetic moments of dysprosium and terbium, obtained from nearly saturated magnetization, are slightly affected by the disorder while the gadolinium moment is completely independent of it. The local structure of the compounds, including oxygen environment of the rare-earth ions, is discussed in terms of the influence of the crystal electric field on the magnetic properties of R2M2O7.
- ПубликацияТолько метаданныеInduced magnetism and “magnetic hole” in singlet ground state system PrNi(2019) Clementyev, E. S.; Lazukov, V. N.; Savchenkov, P. S.; Alekseev, P. A.; Савченков, Павел Сергеевич; Алексеев, Павел Александрович© 2019 Elsevier B.V.The phenomenon of “induced magnetic ordering” for the local magnetic moments with singlet ground state in a periodical lattice is considered for the prototype intermetallic rare-earth compound PrNi ordered ferromagnetically at temperature TC = 21 K. Within the framework of a realistic model the energy dispersion relations are obtained for the magnetic excitations above Tc along the high symmetry directions. The “soft mode” behaviour observed for the acoustic magnetic excitation along (q 0 0) direction is reproduced. The influence of the substitution of isovalent nonmagnetic La for Pr1−xLaxNi on the temperature of ferromagnetic ordering is investigated. The linear decrease of the ordering temperature TC down to zero value in increasing of the La concentration up to x = 0.5 is observed and described by the model.
- ПубликацияТолько метаданныеBinary and Pseudobinary Invar Materials based on Intermediate Valence Compounds(2019) Serebrennikov, D. A.; Clementyev, E. S.; Alekseev, P. A.; Алексеев, Павел Александрович© 2019, Pleiades Publishing, Ltd.Abstract—: A new type of binary and pseudobinary invar materials based on intermediate valence samarium compounds Sm0.8B6, Sm1 – xLaxB6 (x = 0, 0.1, 0.22, and 0.5), Sm0.67Y0.33S, Sm0.55Y0.45S, and Sm2.75C60 are developed. The Weiss two-level invar model is used to describe the anomalous contribution to the coefficient of thermal expansion (CTE) for Sm0.8B6 and Sm1 – xLaxB6 (x = 0, 0.1, 0.22, and 0.5). The minimum position of the CTE of samarium hexaboride as a function of the degree of lanthanum doping is determined. The obtained dependence allows one to predict the CTE minimum position for Sm1 – xLaxB6 compounds in the temperature range from 40 to 180 K and to optimize the developed composite invar materials, where the positive CTE of the functional material is compensated by the negative CTE of the valence-unstable system.
- ПубликацияТолько метаданныеEvidence of Homogeneous Intermediate Valence Coexisting with Long-range Magnetic Order in EuCu2(Si,Ge)2(2021) Savchenkov, P. S.; Sidorov, V. A.; Guo, J.; Sun, L.; Alekseev, P. A.; Алексеев, Павел Александрович© 2021, Pleiades Publishing, Inc.The phase diagram of the intermediate valence system EuCu2Ge2 is investigated by measuring the temperature dependencies of the heat capacity and the electrical resistance as a function of pressure up to 15 GPa. Transition to the magnetic ordered state is observed at pressures up to 9 GPa. It is established that for EuCu2Ge2 system the effects of the hydrostatic and chemical (substitution of Ge for Si) pressure on Eu-valence are equivalent from the point of view of the transition to the antiferromagnetic ordered ground state. The coexistence of the homogeneous intermediate valence state and long-range magnetic order in EuCu2(Si,Ge)2 has been confidently established. The magnetic phase diagram is presented as a function of Eu average valence. The probable background for the formation of the magnetically ordered intermediate valence state of Eu is considered. There is an indication that it develops on the basis of competition of the two magnetic configurations of Eu, in some analogy with the case of TmSe intermediate valence system.
- ПубликацияТолько метаданныеCooperative and Local Features of the Spin Gap Formation in the Kondo Insulators YbB12 and CeFe2Al10(2022) Lazukov, V. N.; Savchenkov, P. S.; Alekseev, P. A.; Алексеев, Павел Александрович© 2022, Pleiades Publishing, Ltd.Abstract: The formation of the excitation spectra of the ground state for Kondo insulators like YbB12, CeFe2Al10 has been studied and analyzed in terms of correspondence between cooperative and local effects. Experimental results concerning different types of substitutions for the rare earth and d-metal sublattices are discussed on the basis of model calculations. It is shown how the transformation from Kondo insulator to heavy fermions occurs in the f-electron excitation spectra of CeFe2Al10.
- ПубликацияТолько метаданныеNeutron Spectroscopy: Principles and Equipment(2022) Ivanov, A. S.; Alekseev, P. A.; Алексеев, Павел Александрович© 2022, Pleiades Publishing, Inc.Abstract: The physical and methodical principles of inelastic thermal-neutron scattering as applied for studying condensed matter have been briefly described. The basic information essential for understanding the experimental method is presented in a concise form, accessible for non-specialists. This information concerns the crystal and time-of-flight experimental techniques, the use of coherent and incoherent neutron scattering, the main approaches to the study of the atomic and magnetic dynamics of condensed matter, etc. The actual types of neutron sources and the ways to deliver neutron flux to sample are presented. The main principles of operating neutron spectrometers are briefly described. The available and promising equipment designed to create and change physical conditions on a sample (e.g., temperature, pressure, magnetic field) is also considered.
- ПубликацияОткрытый доступAnalysis of anomalous negative magnetic contribution to thermal expansion in Sm0.80B6 and pseudobinary compounds Sm1-xLaxB6 (x = 0, 0.10, 0.22, 0.50)(2019) Serebrennikov, D. A.; Clementyev, E. S.; Alekseev, P. A.; Алексеев, Павел Александрович© 2017 Elsevier B.V. This study is the first attempt to apply the two-level Weiss model for the invar alloy to intermediate-valence samarium systems. This model is instrumental in giving a quantitative characterisation of the anomalous magnetic contribution to the coefficient of thermal expansion (CTE) of Sm0.80B6 and Sm1-xLaxB6 (x = 0, 0.10, 0.22, 0.50). A good agreement between the experimental and calculated magnetic contributions to the CTE is achieved for Sm0.78La0.22B6 over the temperature range from 0 K to 210 K. In other systems, a good fit is obtained over shorter temperature ranges. The model parameters and the dependence of the CTE minimum on the level of lanthanum doping in SmB6 are presented.