E. V. Kuz’min’s research while affiliated with Russian Academy of Sciences and other places

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Publications (22)


Description of a Heisenberg ferromagnet above the Curie point as a spin liquid
  • Article

June 2005

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4 Reads

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2 Citations

E. V. Kuz’min

A Heisenberg ferromagnet (F) with spin S=1/2, found in a spin-liquid (SL) state at temperatures above the Curie point ΤC, is considered. In this spin-liquid state there is no long-range magnetic order but the short-range order is preserved, and the spin correlation functions are isotropic. The spin liquid is described in the framework of a second-order theory by the method of temperature Green functions. The main thermodynamic characteristics of the spin liquid are found as the result of a self-consistent numerical solution of a system of three integral equations. The Curie point ΤC+, at which the dc magnetic susceptibility at wave vector q=0 diverges, is determined. A comparison of the thermodynamic characteristics of the system in the F state (Τ⩽ΤC, spin-wave theory) and in the SL state (Τ⩾ΤC+) is made. It is shown that ΤC+>ΤC, and a modification of spin-wave theory in which ΤC reaches the value ΤC+ is indicated. At the point of the F-SL phase transition the spin correlation functions suffer a finite discontinuity, and with increasing temperature they fall off ∝ 1/Τ. The heat capacity of the ferromagnet at Τ↠ΤC goes to infinity, while in the SL state the heat capacity remains finite at the point ΤC+ and falls off for Τ≫ΤC+ in proportion to 1/Τ2. The susceptibility obeys the Curie-Weiss law.


Dynamic magnetic susceptibility in the t–J model

February 2005

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4 Reads

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2 Citations

The t–J model with a prohibition of “doubles” (doubly occupied sites) in the lower Hubbard band and holes in the upper band is considered in the regime of strong electron correlations. The specific features of the electronic states are described. An expression is obtained for the transverse dynamic susceptibility χ as a function of wave vector q and frequency ω in the random phase approximation (RPA) in the metallic state of the system, and the static susceptibility (ω=0) is analyzed in the limits q=0 and q=Q=(π,π) in a square lattice as a function of the electron density n. The insulating limit (n↠1) is described as a spin liquid (SL) state. An expression for the magnetic susceptibility is proposed which unifies the RPA and the SL concept in the presence of site percolation. It is shown that in a certain concentration region the static susceptibility is negative (i.e., a diamagnetic effect), which indicates the possibility of a superconducting state with an exchange mechanism of pairing.


Quantum spin liquid and antiferromagnetism

July 2003

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4 Reads

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5 Citations

A spin liquid concept for the Heisenberg Hamiltonian (spin s = 1/2) with antiferromagnetic exchange interactions between nearest neighbors is developed. The spin liquid is described by the Green’s function method in the framework of a second-order theory. Equations are presented for the self-consistent calculation of the parameters of the system and its thermodynamic properties at all temperatures. A description of the spin system in the sc and bcc lattices is proposed wherein it is treated as a spatially homogeneous spin liquid with a condensate and with a singlet ground state. It is shown that the modulus of the “staggered” magnetization is expressed uniquely in terms of a condensate at a boundary point of the Brillouin zone and is the long-range order parameter. The existence region in temprature of the ordered state of the spin liquid with a condensate (T<T0) is wider than the existence region of the two-sublattice antiferromagnetism (T0<TN, where TN is the Néel temperature), while the energy is lower. For T>T0 the system passes into an ordinary spin liquid state.


Effect of frustrations on magnetism in the Ru double perovskite Sr2YRuO6

July 2003

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38 Reads

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27 Citations

Physical Review B

Localized Ru5+ spins in Sr2YRuO6 form a fcc lattice with an antiferromagnetic (AFM) nearest-neighbor (NN) coupling J&ap;25 meV and rather low Néel temperature TN=26 K. Analysis of the electronic structure of Sr2RuO4 results in the effective Heisenberg model. We have studied the effect of frustrations on the AFM type-I structure of Sr2YRuO6 in the spin-wave approximation. In the model with only NN coupling the AFM state is unstable due to frustrations, and TN=0. Stabilization of the AFM state occurs due to the next-nearest-neighbor coupling I or due to the magnetic anisotropy D. Very small values D/J˜I/J


Frustrated antiferromagnetism in the Sr 2 YRuO 6 double perovskite

June 2003

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38 Reads

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14 Citations

Journal of Experimental and Theoretical Physics

The spins of Ru5+ ions in Sr2YRuO6 form a face-centered cubic lattice with antiferromagnetic nearest neighbor interaction J≈25 meV. The antiferromagnetic structure of the first type experimentally observed below the Nel temperature T N =26 K corresponds to four frustrated spins of 12 nearest neighbors. In the Heisenberg model in the spin-wave approximation, the frustrations already cause instability of the antiferromagnetic state at T=0 K. This state is stabilized by weak anisotropy D or exchange interaction I with the next-nearest neighbors. Low D/J∼I/J∼10−3 values correspond to the experimental T N and sublattice magnetic moment values.


Quantum spin liquid in the FCC lattice

January 2003

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26 Reads

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13 Citations

Journal of Experimental and Theoretical Physics

The properties of the spin system in the FCC lattice described by the Heisenberg model (s=1/2) with antiferromagnetic interactions between the nearest neighbors were studied. It was shown within the framework of spin-wave theory that long-range antiferromagnetic order was absent because of frustration of exchange coupling and transverse quantum spin fluctuations. The system was in the quantum spin liquid state. A method for describing it within linear second-order theory with self-consistently calculated parameters was suggested. It was proved that the ground spin liquid state was singlet. The thermodynamic properties of the spin liquid in the whole temperature range and the character of spatial spin correlations, which had alternating signs and a finite correlation length, were determined. The theory was constructed based on the method of two-time Green temperature functions.



Two-dimensional Heisenberg model with spin s=1/2 and antiferromagnetic exchange treated as a spin liquid

June 2002

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9 Reads

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8 Citations

Physics of the Solid State

The spin system of the Heisenberg model (s=1/2) on a square lattice with antiferromagnetic (AFM) exchange between nearest neighbors (in which there is no long-range magnetic order at any T≠0) is treated as a spatially homogeneous isotropic spin liquid. The double-time temperature Green’s function method is used in the framework of a second-step decoupling scheme. It is shown that, as T → 0, the spin liquid goes over (without any change in symmetry) to a singlet state with energy (per bond) ɛ0=−0.352 and the correlation length diverges as ξ ∝ T −1 exp(T 0/T). The spatial spin correlators oscillate in sign with distance, as in the AFM state. The theory allows one to calculate the main characteristics of the system in all temperature ranges.


Electronic structure and magnetic mechanism of pairing in HTSC transition metal oxides

January 2002

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9 Reads

The Physics of Metals and Metallography

S.G. Ovchinnikov

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I.O. Baklanov

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A.A. Borisov

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O. Erikson

Starting from the multiband p-d model of strongly correlated electrons, we derive the effective Hamiltonian in the form of the generalized singlet-triplet t-t′-J model which transforms to the usual t-t′-J model in the low-energy limit. Both n- and p-type copper oxides are considered in the mean field approximation for Gorkov type Green functions in the X-operator representation. Different signs of the t′/t ratio result in different normal state properties and Tc values conserving the dx2-y2 gap symmetry both for n- and p-oxides. Comparison with ruthenates and the coexistence of superconductivity and magnetism in RuSr2GdCu2O8 are studied in the extended t-J-I model with ferromagnetic coupling between Ru ions.


Effects of magnetism on the electronic structure and pairing mechanism in oxide superconductors

January 2001

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6 Reads

International Journal of Applied Electromagnetics and Mechanics

Generalized tight-binding method is developed to strongly correlated electron systems. Evolution of the band structure of CuO2 layer with hole doping from the undoped charge transfer antiferromagnetic insulator to the optimally doped paramagnetic metal is obtained. Effective low energy Hamiltonian is given by a t-J-I model that allows to compared magnetic mechanism and symmetry of pairing in cuprates and ruthenates on equal footing.


Citations (4)


... For each of the three lattices, we start with the nearest neighbor case and then add terms up to third neighbors. We emphasize that it is actually unlikely that a mean-field model with only nearest neighbor terms can describe a quantum spin ηX ηT gT ηP gP ηI gI ηΠ xy gΠ xy liquid on the sc and bcc lattices [42][43][44]. This is because on a mean-field level, the range of spinon hopping/pairing amplitudes is directly tied to the range of spin interactions J 1 , J 2 , . . . ...

Reference:

Projective symmetry group classifications of quantum spin liquids on the simple cubic, body centered cubic, and face centered cubic lattices
Quantum spin liquid and antiferromagnetism
  • Citing Article
  • July 2003

... An FCC lattice with nearest-neighbor (NN) antiferromagnetic interactions is the simplest three-dimensional structure exhibiting frustrated magnetism [15]. The magnetic order of this system is theoretically predicted to be unstable down to the lowest temperature when next-nearest neighbor (NNN) interactions and magnetic anisotropy are negligible [15,16]. In this context, Sr 2 YRuO 6 has gained renewed interest as a model system for studying magnetic frustration in FCC-type lattices after two magnetic phase transitions were observed at 30 and 26 K [17]. ...

Frustrated antiferromagnetism in the Sr 2 YRuO 6 double perovskite
  • Citing Article
  • June 2003

Journal of Experimental and Theoretical Physics

... Concerning the physically realizable case of n = 3 (Heisenberg) spins, after much debate [14][15][16][17], there now appears to be a consensus that the model undergoes a first-order phase transition into a collinear AF state [18]. For Heisenberg spins away from the classical limit, i.e., when 1/S = 0, only few studies have addressed the role of quantum fluctuations in the large-S [19] or small-S limits [20], and nearly seven decades after being first attended to in Ref. [21], the determination of the nature of the ground state of the quantum Heisenberg antiferromagnet on the fcc lattice remains a critically outstanding problem. ...

Quantum spin liquid in the FCC lattice
  • Citing Article
  • January 2003

Journal of Experimental and Theoretical Physics