A. Rakhmanov - Academia.edu (original) (raw)
Papers by A. Rakhmanov
Advances in Superconductivity XII, 2000
Physical Review B, 2001
ABSTRACT
The WWR-SM research reactor in Uzbekistan has operated at 10 MW since 1979, using Russian-supplie... more The WWR-SM research reactor in Uzbekistan has operated at 10 MW since 1979, using Russian-supplied IRT-3M fuel assemblies containing 90% enriched uranium. Burnup tests of three full-sized IRT-3M FA with 36% enrichment were successfully completed to a burn up of about â¼50% in 1987-1989. In August 1998, four IRT-3M FA with 36% enriched uranium were loaded into the core to initiate conversion of the entire core to 36% enriched fuel. This paper presents the results of equilibrium fuel cycle comparisons of the reactor using HEU (90%) and HEU (36%) IRT-3M fuel and compares results with the performance of IRT-4M FA containing LEU (19.75%). The results show that an LEU (19.75%) density of 3.8 g/cm³ is required to match the cycle length of the HEU (90%) core and an LEU density 3.9 g/cm³ is needed to match the cycle length of the HEU (36%) core.
Reviews of Modern Physics, 1981
This review is devoted to the problem of critical state stability in hard superconductors and sup... more This review is devoted to the problem of critical state stability in hard superconductors and superconducting normal composites. An introduction is given to the properties of hard and composite superconductors, and to the qualitative nature of the physical processes that occur in these materials in the critical state. The dynamics of the development of instabilities of various kinds are treated
Physical Review Letters, 2005
The phase diagram for doped manganites and related compounds is analyzed in terms of the Kondo-la... more The phase diagram for doped manganites and related compounds is analyzed in terms of the Kondo-lattice model taking into account an interplay between electrons localized due to lattice distortions and those in the band states. It is shown that the number of itinerant charge carriers can be significantly lower than that implied by the doping level. The competition between the homogeneous (ferromagnetic or antiferromagnetic) and phase-separated states is discussed and a strong tendency to the phase separation was revealed for a wide doping range.
Physical Review B, 2009
Spin-state transitions, observed in many transition metal compounds containing Co 3+ and Fe 2+ , ... more Spin-state transitions, observed in many transition metal compounds containing Co 3+ and Fe 2+ , may occur with the change of temperature, pressure, but also with doping, in which case the competition of single-site effects and kinetic energy of doped carriers can favor a change in the spin state. We consider this situation in a simple model, formally resembling that used for manganites in Ref. 1. Based on such a model, we predict the possibility of a jump-like change in the number of Co 3+ ions undergoing spin-state transition caused by hole doping. A tendency to the electronic phase separation within a wide doping range is demonstrated. Phase diagrams with the regions of phase separation are constructed at different values of the characteristic parameters of the model.
Physical Review B, 2006
ABSTRACT The structure of bound magnetic polarons in an antiferromagnetic matrix is studied in th... more ABSTRACT The structure of bound magnetic polarons in an antiferromagnetic matrix is studied in the framework of a two-dimensional (2D) and three-dimensional (3D) Kondo-lattice models in the double exchange limit (JH⪢t). The conduction electron is bound by a nonmagnetic donor impurity and forms a ferromagnetic core of the size about the electron localization length (bound magnetic polaron). We find that the magnetic polaron produces rather long-range extended spin distortions of the antiferromagnetic background around the core. In a wide range of distances, these distortions decay as 1∕r2 and 1∕r4 in 2D and 3D cases, respectively. In addition, the magnetization of the core is smaller than its saturation value. Such a magnetic polaron state is favorable in energy in comparison to the usually considered one (saturated core without extended distortions).
Physical Review B, 2004
The structure of magnetic polarons (ferrons) is studied for an 1D antiferromagnetic chain doped b... more The structure of magnetic polarons (ferrons) is studied for an 1D antiferromagnetic chain doped by non-magnetic donor impurities. The conduction electrons are assumed to be bound by the impurities. Such a chain can be described as a set of ferrons at the antiferromagnetic background. We found that two types of ferrons can exist in the system. The ground state of the chain corresponds to the ferrons with the sizes of the order of the localization length of the electron near the impurity. The ferrons of the second type produce a more extended distortion of spins in the chain. They are stable within a finite domain of the system parameters and can be treated as excitations above the ground state. The ferrons in the excited states can appear in pairs only. The energy of the excited states decreases with the growth in density of impurities. This can be interpreted as a manifestation of an attractive interaction between ferrons.
Physical Review B, 2013
We investigate the doping of AA-stacked graphene bilayers. Applying a mean field theory at zero t... more We investigate the doping of AA-stacked graphene bilayers. Applying a mean field theory at zero temperature we find that, at half-filling, the bilayer is an antiferromagnetic insulator. Upon doping, the homogeneous phase becomes unstable with respect to phase separation. The separated phases are undoped antiferromagnetic insulator and metal with a non-zero concentration of charge carriers. At sufficiently high doping, the insulating areas shrink and disappear, and the system becomes a homogeneous metal. The conductivity changes drastically upon doping, so the bilayer may be used as a switch in electronic devices. The effects of finite temperature are also discussed.
Physical Review B, 2008
A possible mechanism of electronic phase separation in the systems with orbital ordering is analy... more A possible mechanism of electronic phase separation in the systems with orbital ordering is analyzed. We suggest a simple model taking into account an interplay between the delocalization of charge carriers introduced by doping and the cooperative ordering of local lattice distortions. The proposed mechanism is quite similar to the double exchange usually invoked for interpretation of phase separation in doped magnetic oxides like manganites, but can be efficient even in the absence of any magnetic ordering. It is demonstrated that the delocalized charge carriers favor the formation of nanoscale inhomogeneities with the orbital structure different from that in the undoped material. The directional character of orbitals leads to inhomogeneities of different shapes and sizes.
Physical Review B, 2006
A "minimal model" of the Kondo-lattice type is used to describe a competition between the localiz... more A "minimal model" of the Kondo-lattice type is used to describe a competition between the localization and metallicity in doped manganites and related magnetic oxides with Jahn-Teller ions. It is shown that the number of itinerant charge carriers can be significantly lower than that implied by the doping level x. A strong tendency to the phase separation is demonstrated for a wide range of intermediate doping concentrations vanishing at low and high doping. The phase diagram of the model in the x − T plane is constructed. At low temperatures, the system is in a state with a long-range magnetic order: antiferromagnetic (AF), ferromagnetic (FM), or AF-FM phase separated (PS) state. At high temperatures, there can exist two types of the paramagnetic (PM) state with zero and nonzero density of the itinerant electrons. In the intermediate temperature range, the phase diagram includes different kinds of the PS states: AF-FM, FM-PM, and PM with different content of itinerant electrons. The applied magnetic field changes the phase diagram favoring the FM ordering. It is shown that the variation of temperature or magnetic field can induce the metal-insulator transition in a certain range of doping levels.
Physical Review B, 2005
The thermal behavior of a one-dimensional antiferromagnetic chain doped by donor impurities was a... more The thermal behavior of a one-dimensional antiferromagnetic chain doped by donor impurities was analyzed. The ground state of such a chain corresponds to the formation of a set of ferromagnetically correlated regions localized near impurities (bound magnetic polarons). At finite temperatures, the magnetic structure of the chain was calculated simultaneously with the wave function of a conduction electron bound by an impurity. The calculations were performed using an approximate variational method and a Monte Carlo simulation. Both these methods give similar results. The analysis of the temperature dependence of correlation functions for neighboring local spins demonstrated that the ferromagnetic correlations inside a magnetic polaron remain significant even above the Néel temperature T N implying rather high stability of the magnetic polaron state. In the case when the electron-impurity coupling energy V is not too high (for V lower that the electron hopping integral t), the magnetic polaron could be depinned from impurity retaining its magnetic structure. Such a depinning occurs at temperatures of the order of T N . At even higher temperatures (T ∼ t) magnetic polarons disappear and the chain becomes completely disordered.
Physical Review B, 2013
We analyze the phase diagram for a system of weakly-coupled electrons having an electron-and a ho... more We analyze the phase diagram for a system of weakly-coupled electrons having an electron-and a hole-band with imperfect nesting. Namely, both bands have spherical Fermi surfaces, but their radii are slightly different, with a mismatch proportional to the doping. Such a model is used to describe: the antiferromagnetism of chromium and its alloys, pnictides, AA-stacked graphene bilayers, as well as other systems. Here we show that the uniform ground state of this model is unstable with respect to electronic phase separation in a wide range of model parameters. Physically, this instability occurs due to the competition between commensurate and incommensurate antiferromagnetic states and could be of importance for other models with imperfect nesting.
Physical Review B, 2007
The two-band Hubbard model is used to analyze a possibility of a nonuniform charge distribution i... more The two-band Hubbard model is used to analyze a possibility of a nonuniform charge distribution in a strongly correlated electron system with two types of charge carriers. It is demonstrated that in the limit of strong on-site Coulomb repulsion, such a system has a tendency to phase separate into the regions with different charge densities even in the absence of magnetic or any other ordering. This tendency is especially pronounced if the ratio of the bandwidths is large enough. The characteristic size of inhomogeneities is estimated, accounting for the surface energy and the electrostatic energy related to the charge imbalance.
Journal of Magnetism and Magnetic Materials, 2006
The behavior of a one-dimensional antiferromagnetic (AF) chain doped by non-magnetic donor impuri... more The behavior of a one-dimensional antiferromagnetic (AF) chain doped by non-magnetic donor impurities is analyzed, in the limit of low impurity density n. The doping leads to the formation of ferromagnetically correlated regions localized near impurities (bound magnetic polarons or ferrons). The temperature evolution of the chain is calculated using an approximate variational method, and a Monte Carlo simulation. Both these methods give the similar results. The analysis of correlation functions for neighboring local spins demonstrates that the ferromagnetic correlations inside a ferron are significant even at high temperatures. The AF correlations in the rest part of the chain decay much faster with temperature. So, the ferron is a stable object that does not disappear even above the Ne´el temperature T N . At rather small values of the electron-impurity coupling energy V (for V lower then the electron hopping integral t), the bound ferron depins from impurity retaining its magnetic structure. Such a depinning occurs at T$V .
IEEE Transactions on Magnetics, 1977
Advances in Superconductivity XII, 2000
Physical Review B, 2001
ABSTRACT
The WWR-SM research reactor in Uzbekistan has operated at 10 MW since 1979, using Russian-supplie... more The WWR-SM research reactor in Uzbekistan has operated at 10 MW since 1979, using Russian-supplied IRT-3M fuel assemblies containing 90% enriched uranium. Burnup tests of three full-sized IRT-3M FA with 36% enrichment were successfully completed to a burn up of about â¼50% in 1987-1989. In August 1998, four IRT-3M FA with 36% enriched uranium were loaded into the core to initiate conversion of the entire core to 36% enriched fuel. This paper presents the results of equilibrium fuel cycle comparisons of the reactor using HEU (90%) and HEU (36%) IRT-3M fuel and compares results with the performance of IRT-4M FA containing LEU (19.75%). The results show that an LEU (19.75%) density of 3.8 g/cm³ is required to match the cycle length of the HEU (90%) core and an LEU density 3.9 g/cm³ is needed to match the cycle length of the HEU (36%) core.
Reviews of Modern Physics, 1981
This review is devoted to the problem of critical state stability in hard superconductors and sup... more This review is devoted to the problem of critical state stability in hard superconductors and superconducting normal composites. An introduction is given to the properties of hard and composite superconductors, and to the qualitative nature of the physical processes that occur in these materials in the critical state. The dynamics of the development of instabilities of various kinds are treated
Physical Review Letters, 2005
The phase diagram for doped manganites and related compounds is analyzed in terms of the Kondo-la... more The phase diagram for doped manganites and related compounds is analyzed in terms of the Kondo-lattice model taking into account an interplay between electrons localized due to lattice distortions and those in the band states. It is shown that the number of itinerant charge carriers can be significantly lower than that implied by the doping level. The competition between the homogeneous (ferromagnetic or antiferromagnetic) and phase-separated states is discussed and a strong tendency to the phase separation was revealed for a wide doping range.
Physical Review B, 2009
Spin-state transitions, observed in many transition metal compounds containing Co 3+ and Fe 2+ , ... more Spin-state transitions, observed in many transition metal compounds containing Co 3+ and Fe 2+ , may occur with the change of temperature, pressure, but also with doping, in which case the competition of single-site effects and kinetic energy of doped carriers can favor a change in the spin state. We consider this situation in a simple model, formally resembling that used for manganites in Ref. 1. Based on such a model, we predict the possibility of a jump-like change in the number of Co 3+ ions undergoing spin-state transition caused by hole doping. A tendency to the electronic phase separation within a wide doping range is demonstrated. Phase diagrams with the regions of phase separation are constructed at different values of the characteristic parameters of the model.
Physical Review B, 2006
ABSTRACT The structure of bound magnetic polarons in an antiferromagnetic matrix is studied in th... more ABSTRACT The structure of bound magnetic polarons in an antiferromagnetic matrix is studied in the framework of a two-dimensional (2D) and three-dimensional (3D) Kondo-lattice models in the double exchange limit (JH⪢t). The conduction electron is bound by a nonmagnetic donor impurity and forms a ferromagnetic core of the size about the electron localization length (bound magnetic polaron). We find that the magnetic polaron produces rather long-range extended spin distortions of the antiferromagnetic background around the core. In a wide range of distances, these distortions decay as 1∕r2 and 1∕r4 in 2D and 3D cases, respectively. In addition, the magnetization of the core is smaller than its saturation value. Such a magnetic polaron state is favorable in energy in comparison to the usually considered one (saturated core without extended distortions).
Physical Review B, 2004
The structure of magnetic polarons (ferrons) is studied for an 1D antiferromagnetic chain doped b... more The structure of magnetic polarons (ferrons) is studied for an 1D antiferromagnetic chain doped by non-magnetic donor impurities. The conduction electrons are assumed to be bound by the impurities. Such a chain can be described as a set of ferrons at the antiferromagnetic background. We found that two types of ferrons can exist in the system. The ground state of the chain corresponds to the ferrons with the sizes of the order of the localization length of the electron near the impurity. The ferrons of the second type produce a more extended distortion of spins in the chain. They are stable within a finite domain of the system parameters and can be treated as excitations above the ground state. The ferrons in the excited states can appear in pairs only. The energy of the excited states decreases with the growth in density of impurities. This can be interpreted as a manifestation of an attractive interaction between ferrons.
Physical Review B, 2013
We investigate the doping of AA-stacked graphene bilayers. Applying a mean field theory at zero t... more We investigate the doping of AA-stacked graphene bilayers. Applying a mean field theory at zero temperature we find that, at half-filling, the bilayer is an antiferromagnetic insulator. Upon doping, the homogeneous phase becomes unstable with respect to phase separation. The separated phases are undoped antiferromagnetic insulator and metal with a non-zero concentration of charge carriers. At sufficiently high doping, the insulating areas shrink and disappear, and the system becomes a homogeneous metal. The conductivity changes drastically upon doping, so the bilayer may be used as a switch in electronic devices. The effects of finite temperature are also discussed.
Physical Review B, 2008
A possible mechanism of electronic phase separation in the systems with orbital ordering is analy... more A possible mechanism of electronic phase separation in the systems with orbital ordering is analyzed. We suggest a simple model taking into account an interplay between the delocalization of charge carriers introduced by doping and the cooperative ordering of local lattice distortions. The proposed mechanism is quite similar to the double exchange usually invoked for interpretation of phase separation in doped magnetic oxides like manganites, but can be efficient even in the absence of any magnetic ordering. It is demonstrated that the delocalized charge carriers favor the formation of nanoscale inhomogeneities with the orbital structure different from that in the undoped material. The directional character of orbitals leads to inhomogeneities of different shapes and sizes.
Physical Review B, 2006
A "minimal model" of the Kondo-lattice type is used to describe a competition between the localiz... more A "minimal model" of the Kondo-lattice type is used to describe a competition between the localization and metallicity in doped manganites and related magnetic oxides with Jahn-Teller ions. It is shown that the number of itinerant charge carriers can be significantly lower than that implied by the doping level x. A strong tendency to the phase separation is demonstrated for a wide range of intermediate doping concentrations vanishing at low and high doping. The phase diagram of the model in the x − T plane is constructed. At low temperatures, the system is in a state with a long-range magnetic order: antiferromagnetic (AF), ferromagnetic (FM), or AF-FM phase separated (PS) state. At high temperatures, there can exist two types of the paramagnetic (PM) state with zero and nonzero density of the itinerant electrons. In the intermediate temperature range, the phase diagram includes different kinds of the PS states: AF-FM, FM-PM, and PM with different content of itinerant electrons. The applied magnetic field changes the phase diagram favoring the FM ordering. It is shown that the variation of temperature or magnetic field can induce the metal-insulator transition in a certain range of doping levels.
Physical Review B, 2005
The thermal behavior of a one-dimensional antiferromagnetic chain doped by donor impurities was a... more The thermal behavior of a one-dimensional antiferromagnetic chain doped by donor impurities was analyzed. The ground state of such a chain corresponds to the formation of a set of ferromagnetically correlated regions localized near impurities (bound magnetic polarons). At finite temperatures, the magnetic structure of the chain was calculated simultaneously with the wave function of a conduction electron bound by an impurity. The calculations were performed using an approximate variational method and a Monte Carlo simulation. Both these methods give similar results. The analysis of the temperature dependence of correlation functions for neighboring local spins demonstrated that the ferromagnetic correlations inside a magnetic polaron remain significant even above the Néel temperature T N implying rather high stability of the magnetic polaron state. In the case when the electron-impurity coupling energy V is not too high (for V lower that the electron hopping integral t), the magnetic polaron could be depinned from impurity retaining its magnetic structure. Such a depinning occurs at temperatures of the order of T N . At even higher temperatures (T ∼ t) magnetic polarons disappear and the chain becomes completely disordered.
Physical Review B, 2013
We analyze the phase diagram for a system of weakly-coupled electrons having an electron-and a ho... more We analyze the phase diagram for a system of weakly-coupled electrons having an electron-and a hole-band with imperfect nesting. Namely, both bands have spherical Fermi surfaces, but their radii are slightly different, with a mismatch proportional to the doping. Such a model is used to describe: the antiferromagnetism of chromium and its alloys, pnictides, AA-stacked graphene bilayers, as well as other systems. Here we show that the uniform ground state of this model is unstable with respect to electronic phase separation in a wide range of model parameters. Physically, this instability occurs due to the competition between commensurate and incommensurate antiferromagnetic states and could be of importance for other models with imperfect nesting.
Physical Review B, 2007
The two-band Hubbard model is used to analyze a possibility of a nonuniform charge distribution i... more The two-band Hubbard model is used to analyze a possibility of a nonuniform charge distribution in a strongly correlated electron system with two types of charge carriers. It is demonstrated that in the limit of strong on-site Coulomb repulsion, such a system has a tendency to phase separate into the regions with different charge densities even in the absence of magnetic or any other ordering. This tendency is especially pronounced if the ratio of the bandwidths is large enough. The characteristic size of inhomogeneities is estimated, accounting for the surface energy and the electrostatic energy related to the charge imbalance.
Journal of Magnetism and Magnetic Materials, 2006
The behavior of a one-dimensional antiferromagnetic (AF) chain doped by non-magnetic donor impuri... more The behavior of a one-dimensional antiferromagnetic (AF) chain doped by non-magnetic donor impurities is analyzed, in the limit of low impurity density n. The doping leads to the formation of ferromagnetically correlated regions localized near impurities (bound magnetic polarons or ferrons). The temperature evolution of the chain is calculated using an approximate variational method, and a Monte Carlo simulation. Both these methods give the similar results. The analysis of correlation functions for neighboring local spins demonstrates that the ferromagnetic correlations inside a ferron are significant even at high temperatures. The AF correlations in the rest part of the chain decay much faster with temperature. So, the ferron is a stable object that does not disappear even above the Ne´el temperature T N . At rather small values of the electron-impurity coupling energy V (for V lower then the electron hopping integral t), the bound ferron depins from impurity retaining its magnetic structure. Such a depinning occurs at T$V .
IEEE Transactions on Magnetics, 1977