39 research outputs found
Low-temperature thermodynamics of the classical frustrated ferromagnetic chain in magnetic field
Low-temperature magnetization curves of the classical frustrated
ferromagnetic chain in the external magnetic field near the transition point
between the ferromagnetic and the helical phases is studied. It is shown that
the calculation of the partition function in the scaling limit reduces to the
solution of the Schr\"{o}dinger equation of the special form for the quantum
particle. It is proposed that the magnetization of the classical model in the
ferromagnetic part of the phase diagram including the transition point defines
the universal scaling function which is valid for quantum model as well.
Explicit analytical formulae for the magnetization are given in the limiting
cases of low and high magnetic fields. The influence of the easy-axis
anisotropy on the magnetic properties of the model is studied. It is shown that
even small anisotropy essentially changes the behavior of the susceptibility in
the vicinity of the transition point.Comment: 14 pages, 5 figure
Semiclassical dynamics of domain walls in the one-dimensional Ising ferromagnet in a transverse field
We investigate analytically and numerically the dynamics of domain walls in a
spin chain with ferromagnetic Ising interaction and subject to an external
magnetic field perpendicular to the easy magnetization axis (transverse field
Ising model). The analytical results obtained within the continuum
approximation and numerical simulations performed for discrete classical model
are used to analyze the quantum properties of domain walls using the
semiclassical approximation. We show that the domain wall spectrum shows a band
structure consisting of 2 non-intersecting zones.Comment: 15 pages, 9 figure
β-Lactoglobulin Adsorption Layers at the Water/Air Surface: 5. Adsorption Isotherm and Equation of State Revisited, Impact of pH
The theoretical description of the adsorption of proteins at liquid/fluid interfaces suffers
from the inapplicability of classical formalisms, which soundly calls for the development of more
complicated adsorption models. A Frumkin-type thermodynamic 2-D solution model that accounts
for nonidealities of interface enthalpy and entropy was proposed about two decades ago and has been
continuously developed in the course of comparisons with experimental data. In a previous paper
we investigated the adsorption of the globular protein β-lactoglobulin at the water/air interface
and used such a model to analyze the experimental isotherms of the surface pressure, Î (c), and the
frequency-, f-, dependent surface dilational viscoelasticity modulus, E(c)f
, in a wide range of protein
concentrations, c, and at pH 7. However, the best fit between theory and experiment proposed
in that paper appeared incompatible with new data on the surface excess, Γ, obtained from direct
measurements with neutron reflectometry. Therefore, in this work, the same model is simultaneously
applied to a larger set of experimental dependences, e.g., Π(c), Γ(c), E(Π)f
, etc., with E-values
measured strictly in the linear viscoelasticity regime. Despite this ambitious complication, a best
global fit was elaborated using a single set of parameter values, which well describes all experimental
dependencies, thus corroborating the validity of the chosen thermodynamic model. Furthermore, we
applied the model in the same manner to experimental results obtained at pH 3 and pH 5 in order to
explain the well-pronounced effect of pH on the interfacial behavior of β-lactoglobulin. The results
revealed that the propensity of β-lactoglobulin globules to unfold upon adsorption and stretch at the
interface decreases in the order pH 3 > pH 7 > pH 5, i.e., with decreasing protein net charge. Finally,
we discuss advantages and limitations in the current state of the mode
Topological solitons in highly anisotropic two dimensional ferromagnets
e study the solitons, stabilized by spin precession in a classical
two--dimensional lattice model of Heisenberg ferromagnets with non-small
easy--axis anisotropy. The properties of such solitons are treated both
analytically using the continuous model including higher then second powers of
magnetization gradients, and numerically for a discrete set of the spins on a
square lattice. The dependence of the soliton energy on the number of spin
deviations (bound magnons) is calculated. We have shown that the
topological solitons are stable if the number exceeds some critical value
. For and the intermediate values of anisotropy
constant ( is an exchange constant), the soliton
properties are similar to those for continuous model; for example, soliton
energy is increasing and the precession frequency is decreasing
monotonously with growth. For high enough anisotropy we found some fundamentally new soliton features absent for continuous
models incorporating even the higher powers of magnetization gradients. For
high anisotropy, the dependence of soliton energy E(N) on the number of bound
magnons become non-monotonic, with the minima at some "magic" numbers of bound
magnons. Soliton frequency have quite irregular behavior with
step-like jumps and negative values of for some regions of . Near
these regions, stable static soliton states, stabilized by the lattice effects,
exist.Comment: 17 page
Schwinger-boson approach to quantum spin systems: Gaussian fluctuactions in the "natural" gauge
We compute the Gaussian-fluctuation corrections to the saddle-point
Schwinger-boson results using collective coordinate methods. Concrete
application to investigate the frustrated J1-J2 antiferromagnet on the square
lattice shows that, unlike the saddle-point predictions, there is a quantum
nonmagnetic phase for 0.53 < J2/J1 < 0.64. This result is obtained by
considering the corrections to the spin stiffness on large lattices and
extrapolating to the thermodynamic limit, which avoids the infinite-lattice
infrared divergencies associated to Bose condensation. The very good agreement
of our results with exact numerical values on finite clusters lends support to
the calculational scheme employed.Comment: 4 pages, Latex, 3 figures included as eps files,minor correction
Non-linear dynamics and two-dimensional solitons for spin ferromagnets with biquadratic exchange
We develop a consistent semiclassical theory of spin dynamics for an
isotropic ferromagnet with a spin taking into consideration both
bilinear and biquadratic over spin operators exchange interaction. For such
non-Heisenberg magnets, a peculiar class of spin oscillations and waves, for
which the quantum spin expectation value does
not change it direction, but changes in length, is presented. Such
``longitudinal'' excitations do not exist in regular magnets, dynamics of which
are described in terms of the Landau-Lifshitz equation or by means of the spin
Heisenberg Hamiltonian. We demonstrate the presence of non-linear uniform
oscillations and waves, as well as self-localized dynamical excitations
(solitons) with finite energy. A possibility of excitation of such oscillations
by ultrafast laser pulse is discussed.Comment: 11 pages, 7 figures, MikTE
Modified Spin Wave Thoery of the Bilayer Square Lattice Frustrated Quantum Heisenberg Antiferromagnet
The ground state of the square lattice bilayer quantum antiferromagnet with
nearest and next-nearest neighbour intralayer interaction is studied by means
of the modified spin wave method. For weak interlayer coupling, the ground
state is found to be always magnetically ordered while the quantum disordered
phase appear for large enough interlayer coupling. The properties of the
disordered phase vary according to the strength of the frustration. In the
regime of weak frustration, the disordered ground state is an almost
uncorrelated assembly of interlayer dimers, while in the strongly frustrated
regime the quantum spin liquid phase which has considerable N\'eel type short
range order appears. The behavior of the sublattice magnetization and spin-spin
correlation length in each phase is discussed.Comment: 15 pages, revtex, figures upon reques
Quantum Phase Transition in the Frustrated Heisenberg Antiferromagnet
Using the J_1-J_2 model, we present a description of quantum phase transition
from Neel ordered to the spin-liquid state based on the modified spin wave
theory. The general expression for the gap in the spectrum in the spin-liquid
phase is presented.Comment: 8 pages of REVTeX 3.0, one PostScript file appended (Eq. 15
corrected, two recent references added, + some minor changes