arXiv subjects

A. V. Syromyatnikov

Publications and source records attributed to A. V. Syromyatnikov.

50 records · Page 3Linked to original sources

arXiv2010

Spectrum of short-wavelength magnons in two-dimensional quantum Heisenberg antiferromagnet on a square lattice: third order expansion in 1/S

The spectrum of short-wavelength magnons in two-dimensional quantum Heisenberg antiferromagnet on a square lattice is calculated in the third order in $1/S$ expansion. It is shown that $1/S$ series for $S=1/2$ converges fast in the whole Brillouin zone except for the neighborhood of the point ${\bf k}=(π,0)$, at which absolute values of the third and the second order $1/S$-corrections are approximately equal to each other. It is shown that the third order corrections make deeper the roton-like local minimum at ${\bf k}=(π,0)$ improving the agreement with the recent experiments and numerical results in the neighborhood of this point. It is suggested that $1/S$ series converges slowly near ${\bf k}=(π,0)$ also for $S=1$ although the spectrum renormalization would be small in this case due to very small values of high-order $1/S$ corrections.

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arXiv2009

Instability of the collinear phase in two-dimensional ferromagnet in strong in-plane magnetic field

It is well-known that in thin ferromagnetic film with a net magnetization perpendicular to the film the collinear arrangement of spins is unstable in an in-plane field $H$ smaller than its saturation value $H_c$. Existence of a stripe phase was proposed with elongated domains of alternating direction of magnetization component perpendicular to the film. We consider in the present paper the strong-field regime $H<H_c$ and discuss the minimal microscopic model describing this phenomenon, two-dimensional Heisenberg ferromagnet with strong easy-axis anisotropy and dipolar forces. The noncollinear (stripe) phase is discussed using the technique of the Bose-Einstein condensation of magnons. Some previously unknown results are observed concerning the stripe phase. Evolution is established of the spin arrangement in the noncollinear phase upon the field rotation within the plane. We find a rapid decreasing of the period of the stripe structure as the field decreases. We demonstrate that spins components perpendicular to the film form a sinusoid in the noncollinear phase at $H\approx H_c$ that transforms to a step-like profile upon the field decreasing so that the domain wall density decreases from unity to a value much smaller than unity. The spin-wave spectrum in the noncollinear phase is discussed.

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arXiv2009

Collective excitations in two-dimensional antiferromagnet in strong magnetic field

We discuss spin-$\frac12$ two-dimensional (2D) Heisenberg antiferromagnet (AF) on a square lattice at T=0 in strong magnetic field H near its saturation value $H_c$. A perturbation approach is proposed to obtain spectrum of magnons with momenta not very close to AF vector in the leading order in small parameter $(H_c-H)/H_c$. We find that magnons are well-defined quasi-particles at $H>0.9H_c$ although the damping is quite large near the zone boundary. A characteristic rotonlike local minimum in the spectrum is observed at ${\bf k}=(π,0)$ accompanied by decrease of the damping near $(π,0)$. The suggested approach can be used in discussion of short-wavelength excitations in other 2D Bose gases of particles or quasi-particles.

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arXiv2008

Spin-wave interaction in two-dimensional ferromagnets with dipolar forces

We discuss the spin-wave interaction in two-dimensional (2D) Heisenberg ferromagnet (FM) with dipolar forces at $T_C\gg T\ge0$ using 1/S expansion. A comprehensive analysis is carried out of the first 1/S corrections to the spin-wave spectrum. In particular, similar to 3D FM discussed in our previous paper A.V. Syromyatnikov, PRB {\bf 74}, 014435 (2006), we obtain that the spin-wave interaction leads to the {\it gap} in the spectrum $ε_{\bf k}$ renormalizing greatly the bare gapless spectrum at small momenta $k$. Expressions for the spin-wave damping $Γ_{\bf k}$ are derived self-consistently and it is concluded that magnons are well-defined quasi-particles in both quantum and classical 2D FMs at small $T$. We observe thermal enhancement of both $Γ_{\bf k}$ and $Γ_{\bf k}/ε_{\bf k}$ at small momenta. In particular, a peak appears in $Γ_{\bf k}$ and $Γ_{\bf k}/ε_{\bf k}$ at small $k$ and at any given direction of $\bf k$. If $S\sim1$ the height of the peak in $Γ_{\bf k}/ε_{\bf k}$ is not larger than a value proportional to $T/D\ll1$, where $D$ is the spin-wave stiffness. In the case of large spins $S\gg1$ the peak in $Γ_{\bf k}/ε_{\bf k}$ cannot be greater than that of the classical 2D FM found at $k=0$ which height is small only {\it numerically}: $Γ_{\bf 0}/ε_{\bf 0}\approx0.16$ for the simple square lattice. Frustrating next-nearest-neighbor exchange coupling increases $Γ_{\bf 0}/ε_{\bf 0}$ in classical 2D FM only slightly. We find expressions for spin Green's functions and the magnetization. The latter differs from the well-known result by S.V. Maleev, Sov. Phys. JETP {\bf 43}, 1240 (1976). The effect of the exchange anisotropy is also discussed briefly.

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arXiv2007

Nonfrustrated magnetoelectric with incommensurate magnetic order in magnetic field

We discuss a model nonfrustrated magnetoelectric in which strong enough magnetoelectric coupling produces incommensurate magnetic order leading to ferroelectricity. Properties of the magnetoelectric in magnetic field directed perpendicular to wave vector describing the spin helix are considered in detail. Analysis of classical energy shows that in contrast to naive expectation the onset of ferroelectricity takes place at a field $H_{c1}$ that is lower than the saturation field $H_{c2}$. One has $H_{c1}=H_{c2}$ at strong enough magnetoelectric coupling. We show that at H=0 the ferroelectricity appears at $T=T_{FE}<T_N$. Qualitative discussion of phase diagram in $H-T$ plane is presented within mean field approach.

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arXiv2007

Bose-Einstein condensation of magnons in magnets with predominant ferromagnetic interaction

We discuss Bose-Einstein condensation of magnons (BEC) in magnets with predominant ferromagnetic (FM) interaction in magnetic field $H$ near saturation ($H_c$). Because $H_c$ is independent of FM couplings, magnetic materials of this type can have small $H_c$ that makes them promising candidates for experimental investigation of BEC. Ferromagnets with easy-plane anisotropy and antiferromagnets (AFs) containing weakly coupled FM planes or chains are discussed in detail. We observe small effective interaction between magnons near the QCP in such magnets, in contrast to AFs with strong AF coupling previously discussed. In particular, this smallness allows us to find crossovers in the critical temperature $T_c(H)\propto (H_c-H)^{1/ϕ}$ from $ϕ=3/2$ to $ϕ=1$ in quasi-1D magnets, and from $ϕ=3/2$ to $ϕ\approx1$ ($T_c\ln T_c\propto H_c-H$) in quasi-2D ones.

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arXiv2006

Frustrated impurity spins in ordered two-dimensional quantum antiferromagnets

Dynamical properties of an impurity spin coupled symmetrically to sublattices of ordered 2D Heisenberg quantum antiferromagnet (i.e., frustrated impurity spin) are discussed at $T\ge0$ (existence of a small interaction stabilizing the long range order at $T\ne0$ is implied). We continue our study on this subject started in Phys.Rev.B 72, 174419 (2005), where spin-1/2 defect is discussed and the host spins fluctuations are considered within the spin-wave approximation (SWA). In the present paper we i) go beyond SWA and ii) study impurities with spins $S\ge1/2$. It is demonstrated that in contrast to defects coupled to sublattices asymmetrically longitudinal host spins fluctuations play important role in the frustrated impurity dynamics. The spectral function, that is proportional to $ω^2$ within SWA, acquires new terms proportional to $ω^2$ and $ωT^2$. It is observed that the spin-1/2 impurity susceptibility has the same structure as that obtained within SWA: the Lorenz peak and the non-resonant term. The difference is that the width of the peak becomes larger being proportional to $f^2(T/J)^3$ rather than $f^4(T/J)^3$, where $f$ is the dimensionless coupling parameter. We show that transverse static susceptibility acquires a new negative logarithmic contribution. In accordance with previous works we find that host spins fluctuations lead to an effective one-ion anisotropy on the impurity site. Then defects with $S>1/2$ appears to be split. We observe strong reduction of the value of the splitting due to longitudinal host spins fluctuations. We demonstrate that the dynamical impurity susceptibility contains $2S$ Lorenz peaks corresponding to transitions between the levels, and the non-resonant term.

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arXiv2006

Magnon Bose condensation in symmetry breaking magnetic field

Magnon Bose condensation (BC)in the symmetry breaking magnetic field is a result of unusual form of the Zeeman energy, which has terms linear in the spin-wave operators and terms mixing excitations differ in the Wave-vector of the magnetic structure. The following examples are considered: simple easy-plane tetragonal antiferromagnets (AF), frustrated AF family$R_2Cu O_4$ where $R=Pr,Nd$ etc. and cubic magnets with the Dzyaloshinskii-Moriya interaction ($Mn Si$ etc.). In all cases the BC becomes important when the magnetic field becomes comparable with the spin-wave gap. The theory is illustrated by existing experimental results.

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arXiv2006

Renormalization of the spin-wave spectrum in three-dimentional ferromagnets with dipolar interaction

Renormalization of the spin-wave spectrum is discussed in a cubic ferromagnet with dipolar forces at $T_C\gg T\ge0$. First 1/S-corrections are considered in detail to the bare spectrum $ε_{\bf k} = \sqrt{Dk^2 (Dk^2 + Sω_0\sin^2θ_{\bf k})}$, where $D$ is the spin-wave stiffness, $θ_{\bf k}$ is the angle between $\bf k$ and the magnetization and $ω_0$ is the characteristic dipolar energy. In accordance with previous results we obtain the thermal renormalization of constants $D$ and $ω_0$ in the expression for the bare spectrum. Besides, a number of previously unknown features are revealed. We observe terms which depend on azimuthal angle of the momentum $\bf k$. It is obtained an isotropic term proportional to $k$ which makes the spectrum linear rather than quadratic when $\sinθ_{\bf k}=0$ and $k \ll ω_0/D$. In particular a spin-wave gap proportional to $\sinθ_{\bf k}$ is observed. Essentially, thermal contribution from the Hartree-Fock diagram to the isotropic correction as well as to the spin-wave gap are proportional to the demagnetizing factor in the direction of domain magnetization. This nontrivial behavior is attributed to the long-range nature of the dipolar interaction. It is shown that the gap screens infrared singularities of the first 1/S-corrections to the spin-wave stiffness and longitudinal dynamical spin susceptibility (LDSS) obtained before. We demonstrate that higher order 1/S-corrections to these quantities are small at $T\llω_0$. However the analysis of the entire perturbation series is still required to derive the spectrum and LDSS when $T\ggω_0$.

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arXiv2005

Frustrated two-level impurities in two-dimensional antiferromagnets

Dynamical properties of the impurity spin-$\frac12$ in 2D and quasi-2D Heisenberg antiferromagnets (AFs) at $T\ge0$ are discussed. The specific case of an impurity coupled symmetrically to two neighboring host spins is considered. The specific feature of this problem is that the defect is degenerate (frustrated) being located in zero molecular field. It is shown that this problem can be described by spin-boson model without tunneling term and with a more complex interaction. We demonstrate that the effect of the host system on the defect is completely described by the spectral function. It is found within the spin-wave approximation that for not too small $ω$ the spectral function is proportional to $ω^2/J^3$, where $J$ is the exchange constant between the host spins. The defect dynamical susceptibility is derived using Abrikosov's pseudofermion technique and diagrammatic expansion. The calculations are performed within the fourth order of the dimensionless coupling parameter $f$. It is found that transverse impurity susceptibility $χ_\perp(ω)$ has a Lorenz peak with the width proportional to $f^4J(T/J)^3$ which disappears at T=0, and a non-resonant term. The later term diverges logarithmically as $ω,T \to 0$. The static susceptibility $χ(0)$ has the free-spin-like contribution $1/(4T)$, and a logarithmic correction proportional to $f^2\ln(J/T)$. The influence of finite concentration of the defects $n$ on the low-temperature properties of AF is also investigated. A logarithmic correction to spin-wave velocity of the form $nf^4\ln|J/ω|$ and an anomalous damping of spin waves proportional to $nf^4|ω|$ are obtained.

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arXiv2005

Chiral fluctuations in triangular antiferromagnets at $T \ll T_N$

Chiral fluctuations in triangular antiferromagnets (TAFs) at $T \ll T_N$ are studied theoretically. The case of a ferromagnetic interaction along c axis (which is directed perpendicular to the plane of the lattice), Dzyaloshinskii-Moriya interaction ${\bf D}\| c$ and a weak magnetic field ${\bf H}\| c$ is considered in detail. Previously, this model has been proposed to describe quantum TAF CsCuCl$_3$. Expressions for dynamical chirality (DC) are derived within the linear spin-wave approximation. In contrast to non-frustrated antiferromagnets, DC is found to be nonzero even at $D,H=0$ in a one-domain sample. We argue that this unusual behavior stems from the fact that a ground state of XY and Heisenberg TAFs is characterized by an axial vector along which DC is directed.

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arXiv2003

Low-energy singlet dynamics of spin-$\frac12$ kagome Heisenberg antiferromagnets

We give a physical picture of the low-energy sector of the spin-$\frac12$ kagome Heisenberg antiferromagnets (KAFs). It is shown that kagome lattice can be represented as a set of blocks containing 12 spins, having form of stars and arranged in a triangular lattice. Each of these stars has two degenerate singlet ground states which can be considered in terms of pseudospin-$\frac12$. It is shown using symmetry consideration that the KAF lower singlet band is made by inter-star interaction from these degenerate states. We demonstrate that this band is described by effective Hamiltonian of a magnet in the external magnetic field. The general form of this Hamiltonian is established. The Hamiltonian parameters are calculated up to the third order of perturbation theory. The ground state energy calculated in the considered model is lower than that evaluated numerically in the previous finite clusters studies. A way of experimental verification of this picture using neutron scattering is discussed. It is shown that the approach presented can not be expanded directly on KAFs with larger spin values.

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arXiv2003

Double-peak specific heat feature in frustrated antiferromagnetic clusters

We study the nature of the double-peak specific heat structure in kagome clusters. That containing 12 spins is considered thoroughly by numerical diagonalization. Simple models are proposed revealing the low-$T$ peak nature at $T_l<Δ$ ($Δ$ is the spin gap) in this case and in those of larger clusters studied so far. We show that the rapid increase in density of states just above the spin gap gives rise to this peak. These models establish the reason for the weak magnetic field sensitivity of the low-$T$ peak. Our discussion could be appropriate for other frustrated antiferromagnetic systems too.

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arXiv2002

Hidden long range order in Heisenberg Kagome antiferromagnets

We give a physical picture of the low-energy sector of the spin 1/2 Heisenberg Kagome antiferromagnet (KAF). It is shown that Kagome lattice can be presented as a set of stars which are arranged in a triangular lattice and contain 12 spins. Each of these stars has two degenerate singlet ground states which can be considered in terms of pseudospin. As a result of interaction between stars we get Hamiltonian of the Ising ferromagnet in magnetic field. So in contrast to the common view there is a long range order in KAF consisting of definite singlet states of the stars.

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