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1.
An in situ x-ray diffraction study of Cs2ZnI4 crystals performed in the 4.2–300 K temperature range is reported. The lattice parameter measurements have revealed three
anomalies corresponding to phase transitions. The thermal expansion coefficient along the c axis was found to vanish in the region of incommensurate and commensurate modulated phases, 120–96 K (the invar effect).
A possible crystallographic model relating modulated atomic displacements to the invar effect is discussed.
Fiz. Tverd. Tela (St. Petersburg) 41, 137–142 (January 1999) 相似文献
2.
L. E. Svistov T. Fujita H. Yamaguchi S. Kimura K. Omura A. Prokofiev A. I. Smirnov Z. Honda M. Hagiwara 《JETP Letters》2011,93(1):21-25
Magnetization of the frustrated S = 1/2 chain compound LiCuVO4, focusing on high magnetic field phases, is reported. Besides a spin-flop transition and the transition from a planar spiral
to a spin modulated structure observed recently, an additional transition was observed just below the saturation field. This
newly observed magnetic phase is considered as a spin nematic phase, which was predicted theoretically but was not observed
experimentally. The critical fields of this phase and its dM/dH curve are in good agreement with calculations performed in a microscopic model (M.E. Zhitomirsky and H. Tsunetsugu, Europhys.
Lett. 92, 37001 (2010)). 相似文献
3.
M. Quilichini J.M. Perez-Mato I. Aramburu O. Hernandez 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,25(4):431-438
Three-axes elastic neutron scattering measurements demonstrate that the five-fold modulated phase (phase 1/5) of BCCD exhibits
under electric field a phase transition without change of superlattice periodicity. Through the monitoring of high-order satellite
diffraction peaks as a function of electric field and temperature, the competition between this phase and neighboring polar
phases with other periods has been characterized. At a threshold electric field of about 20 kV/cm, a rather abrupt redistribution
of the satellite intensities of phase 1/5 is observed, without change of the corresponding primary modulation wave vector
( ⅕). A quantitative analysis of these intensity variations confirms the earlier conjecture based on dielectric experiments that
the modulation essentially changes from a non-polar sequence 5up5down ( <5>) of polarized z-perpendicular layers of basic semicells, to a polar sequence 6up4down ( <64>). The transition is caused by the flip of the average polarization of one of the interface layers, and can then be described as a bounded discrete motion of the
wall separating positive and negative microdomains within the five-fold unit cell. This type of polarization-flip phase transition
had been detected and characterized in one-dimensional theoretical models as generalized Frenkel-Kontorova models or spin
chains with elastic couplings, but had not been anticipated in theoretical analyses of BCCD, for which other phenomenological
or microscopic models (as the ANNNI model) have been considered adequate. Only recently and in view of the experimental results
reported here, we demonstrated, using a general phenomenological displacive model, the possibility of this type of transition
in systems as BCCD [Phys. Rev. B 62, 11418 (2000)]. Phase diagrams with spin-flip phase transitions yield very peculiar phase diagrams with a checkerboard topological
structure and self-similar features. In particular, they may present special critical points as the so-called upsilon points [J. Statistical Phys. 62, 45 (1991)]. BCCD may be then the first experimental system where they could be observed.
Received 20 September 2001 相似文献
4.
M. P. Trubitsyn 《Physics of the Solid State》1999,41(9):1528-1533
The EPR spectra of Mn2+ ions in Rb2ZnCl4 crystals is investigated in the vicinity of the transition from the paraelectric phase to an incommensurate modulated phase.
When these crystals are cooled below the transition temperature T
i
=304 K, a splitting of the resonance lines is observed in the singular spectrum. A one-harmonic model is used to discuss the
contributions that fluctuations in the amplitude and phase of the incommensurate displacement wave make to the local width
of the singular spectra. It is shown that anomalies in the local width of the low-temperature singular peaks observed in the
vicinity of T
i
are caused by amplitude fluctuations.
Fiz. Tverd. Tela (St. Petersburg) 41, 1668–1674 (September 1999) 相似文献
5.
In this paper, discussed is the evolution of two co-propagating optical beams in parallel in nonlocal Kerr media, governed by the nonlocal nonlinear Schrödinger equation (NNLSE). A simplified model to the NNLSE is presented when the media is strongly nonlocal, which is a bridge between the Snyder–Mitchell model (Snyder and Mitchell Science 276 1538, 1997) and the strongly-nonlocal model (Guo, Luo, Yi, Chi, and Xie Phys. Rev. E. 69 016602, 2004). It is found that when one of the soliton beams is much stronger than the other, the weaker (probe beam) can experience $\pi $ nonlinear phase shift, which can be modulated by the stronger (pump beam), within a rather short propagation distance (about 40-m). The comparisons of analytical solutions of the model with numerical simulations of the NNLSE show that the model is of excellent accuracy in the case of strong nonlocality. 相似文献
6.
The phase transitions in boracites are analysed by using the group-theoretical formulation of the Landau theory of phase transitions. It is shown that the orthorhombic, monoclinic and trigonal phase transitions could be induced by the same irreducible representation of the space groupT
d
5
with the star determined by the wave vectork=1/2(b
1+b
2). The corresponding free energy function is constructed and the symmetry of normal modes is discussed.The authors thank Dr. V. Janovec of the Institute of Physics for valuable remarks to this paper. 相似文献
7.
E.I. Efremova A.A. Shiryaev Z.A. Kydryashova L.A. Nosikova S.A. Syrbue V.V. Chernyshev 《Phase Transitions》2015,88(5):503-512
The system of hydrogen-bonded liquid crystals formed from binary mixtures p-n-heptyloxybenzoic (I) acid and p-n-undecloxybenzoic (II) acid has been investigated by polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction, and scattering and dielectric measurements. The T–X phase diagram was obtained for this system. All mixtures show enantiotropic smectic and nematic phases. The crystalline phase represents a (α, β) solid solution. At the 2:1 ratio of I to II, an intermediate phase – co-crystal γ – is formed. The co-crystal γ possesses a much wider mesophase range than the corresponding initial components. In contrast to mixtures based on the solid solutions having a positive anisotropy, the co-crystal shows a negative dielectric anisotropy in the mesophase. 相似文献
8.
James A. Harrington 《Fiber and Integrated Optics》2013,32(3):211-227
Abstract An analysis and experimental data of the time-dependent response of an all-fiber ring resonator in which the ring is illuminated by a phase modulated input beam are presented. The resonator is operated as a high-resolution optical-scanning spectrometer. Since the resolution limit is a function of the finesse and cavity-loop length, we will show that it is not possible to design a high-resolution ring resonator that will not produce a distorted output signal when the phase modulation rate is comparable to the response time of the ring. 相似文献
9.
A study is reported of the anomalous broadening of Mn2+ EPR lines on the high-temperature side of the paraelectric-incommensurate phase transition in Rb2ZnCl4 crystals. It is shown that the resonant-line broadening is inhomogeneous and due to the contribution of low-frequency fluctuations
corresponding to the central peak in the elementary-excitation spectrum. The data obtained have permitted us to obtain the
critical correlation-length exponent ν=0.64±0.02 corresponding to the 3d XY model of Heisenberg.
Fiz. Tverd. Tela (St. Petersburg) 41, 122–125 (January 1999) 相似文献
10.
We define modulated replica symmetry breaking (RSB) schemes which combine tree- and wave-like structures. A modulated scheme and unmodulated RSB are evaluated at 1-step level for a semiconductor model with antiferromagnetic Korenblit-Shender interaction. By comparison of the free energies we find evidence that a T = 0 phase transition in the ferrimagnetic phase leads to a transition between the different RSB-schemes. An embedding factor of Parisi block matrices with sublattice-asymmetrical size is employed as a new variational parameter in the modulated scheme.Received: 11 December 2003, Published online: 15 March 2004PACS:
68.35.Rh Phase transitions and critical phenomena - 75.10.Nr Spin glass and other random models 相似文献
11.
A. I. Belousov S. A. Verzakov Yu. E. Lozovik 《Journal of Experimental and Theoretical Physics》1998,86(1):146-155
The boson lattice Hubbard model is used to study the role of quantum fluctuations of the phase and local density of the superfluid
component in establishing a global superconducting state for a system of mesoscopic Josephson junctions or grains. The quantum
Monte Carlo method is used to calculate the density of the superfluid component and fluctuations in the number of particles
at sites of the two-dimensional lattice for various average site occupation numbers n
0 (i.e., number of Cooper pairs per grain). For a system of strongly interacting bosons, the phase boundary of the ordered
superconducting state lies above the corresponding boundary for its quasiclassical limit—the quantum XY-model—and approaches the latter as n
0 increases. When the boson interaction is weak in the boson Hubbard model (i.e., the quantum fluctuations of the phase are
small), the relative fluctuations of the order parameter modulus are significant when n
0<10, while quantum fluctuations in the phase are significant when n
0<8; this determines the region of mesoscopic behavior of the system. Comparison of the results of numerical modeling with
theoretical calculations show that mean-field theory yields a qualitatively correct estimate of the difference between the
phase diagrams of the quantum XY-model and the Hubbard model. For a quantitative estimate of this difference the free energy and thermodynamic averages of
the Hubbard model are expanded in powers of 1/n
0 using the method of functional integration.
Zh. éksp. Teor. Fiz. 113, 261–277 (January 1998) 相似文献
12.
Helical phases characterized by a helix wave vectorQ which does not point along a major crystallographic axis are a recent theoretical discovery. Such configurations belong to wedge shaped regions in the parameter space and their existence in the tetragonal and hexagonal classical Heisenberg model was proved in the smallQ limit. Here we study this phase for allQ in the tetragonal lattice with competitive interactions up to fifth neighbours. We find that theswinging helix exists for any fifth neighbour interactionJ
5<0, so that the existence of this phase is assured in a region of the parameter space wider than one expects on the basis of the smallQ expansion. The zero temperature phase diagram for the model we consider is given. 相似文献
13.
D.M. Mastrogiuseppe M.E. Torio C.J. Gazza A.O. Dobry 《Physica B: Condensed Matter》2009,404(18):2805-2808
Going beyond a recently proposed microscopic model [D. Mastrogiuseppe, A. Dobry, arXiv:0810.3018v1] for the incommensurate transition in the spin-Peierls TiOX (X=Cl, Br) compounds, in the present work we start by studying the thermodynamics of the model with XY spins and adiabatic phonons. We find that the system enters an incommensurate phase by a first order transition at a low temperature Tc1. At a higher temperature Tc2 a continuous transition to a uniform phase is found. Furthermore, we study the magnetism in the incommensurate phase by density matrix renormalization group (DMRG) calculations on a one-dimensional Heisenberg model where the exchange is modulated by the incommensurate atomic position pattern. When the wave vector q of the modulation is near π, we find local magnetized zones (LMZ) in which spins abandon their singlets as a result of the domain walls induced by the modulated distortion. When q moves far away enough from π, the LMZ disappear and the system develops incommensurate magnetic correlations induced by the structure. We discuss the relevance of this result regarding previous and future experiments in TiOCl. 相似文献
14.
M. Büttiker 《Journal of statistical physics》1989,54(5-6):1425-1425
Overdamped particles subject to a drift in a force field with sinusoidal space dependence and also a sinusoidally modulated space-dependent diffusion, with the same period as the drift, experience a net driving force. The resulting current depends on the amplitude of the modulation of the diffusion and is a periodic function of the phase difference between the sinusoidal drift and the sinusoidal modulation of the diffusion. For small modulation amplitudes a particle subject to state-dependent noise behaves the same way as a particle subject to thermal noise but with a drift which, in addition to the sinusoidal term, contains a net force term [M. Büttiker,Z. Phys. B
68:161 (1987)]. A specific example of this behavior [N. G. van Kampen,IBM J. Res. Dev.
32:107 (1988); R. Landauer,J. Stat. Phys.
53:233 (1988).] is the motion of overdamped particles in a ring subject to a nonuniform temperature field. When the drift and the temperature, which are periodic with a period equal to the ring circumference, are not in phase a noise-induced circulating current results.This paper will appear in a forthcoming issue of theJournal of Statistical Physics. 相似文献
15.
《Journal of Nonlinear Mathematical Physics》2013,20(4):494-526
Abstract We study the q → ∞ limit of the q-deformation of the WZW model on a compact simple and simply connected target Lie group. We show that the commutation relations of the q → ∞ current algebra are underlied by certain affine Poisson structure on the group of holomorphic maps from the disc into the complexification of the target group. The Lie algebroid corresponding to this affine Poisson structure can be integrated to a global symplectic groupoid which turns out to be nothing but the phase space of the q → ∞ limit of the q-WZW model. We also show that this symplectic grupoid admits a chiral decomposition compatible with its (anomalous) Poisson-Lie symmetries. Finally, we dualize the chiral theory in a remarkable way and we evaluate the exchange relations for the q → ∞ chiral WZW fields in both the original and the dual pictures. 相似文献
16.
Effects of signal modulation and coloured cross-correlation of coloured noises on the diffusion of a harmonic oscillator
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The diffusion in a harmonic oscillator driven by coloured noises
$\xi(t)$ and $\eta(t)$ with coloured cross-correlation in which
one of the noises is modulated by a biased periodic signal is
investigated. The exact expression of diffusion coefficient d as a
function of noise parameter, signal parameter, and oscillator
frequency is derived. The findings in this paper are as follows. 1)
The curves of d versus noise intensity D and d versus noises
cross-correlation time $\tau_3$ exist as two different phases. The
transition between the two phases arises from the change of the
cross-correlation coefficient $\lambda$ of the two
Ornstein--Uhlenbeck (O-U) noises. 2) Changing the value of $\tau_3$, the curves of d versus Q, the intensity of colored noise
that is modulated by the signal, can transform from a phase having a
minimum to a monotonic phase. 3) Changing the value of signal
amplitude A, d versus Q curves can transform from a phase
having a minimum to a monotonic phase. The above-mentioned results
demonstrate that a like noise-induced transition appears in the
model. 相似文献
17.
A vibronic charge-transfer exciton, which is a pair of Jahn-Teller electron and hole polarons, is considered as a possible
cause of the appearance of the Müller phase in the virtual ferroelectric SrTiO3 and the “green” luminescence in the virtual ferroelectric KTaO3. The two “green” luminescence bands can be associated with emission from two states of a typical intrinsic defect, viz.,
a vibronic charge-transfer exciton trapped by an oxygen vacancy and an isolated vibronic charge-transfer exciton. In both
cases the “green” luminescence corresponds to the recombination of the electron and the hole in the vibronic charge-transfer
exciton, which is accompanied by the emission of light. The properties of the Müller phase can be attributed to mixing of
the normal state and states of the vibronic charge-transfer exciton phase when they interact with polarization in the soft
SrTiO3 matrix under the conditions of a pseudo-Jahn-Teller (pseudo-JT) effect on a soft TO mode of the displacement type. In this
case the vibronic charge-transfer exciton phase forming the low-lying excited states has “order-disorder” degrees of freedom
and exists at temperatures significantly below the point of the order-disorder ferroelectric transition in SrTiO3 at T=T
Q≈37 K. The corresponding lowering of the symmetry of the vibronic charge-transfer exciton phase to polar symmetry leads to
the possibility of a long-period incommensurate phase in such excited states, which arises as a result of the appearance of
a Lifshitz invariant. The valence-band state making the largest contribution of the pseudo-JT effect corresponds to a wave
vector equal to the critical wave vector of the incommensurate vibronic charge-transfer exciton phase. When the temperature
is lowered, the pseudo-JT distortion increases down to ∼T
Q and subsequently saturates in accordance with the saturation of the dielectric constant. The basic assumption in the model
is that the temperature T=T
Q corresponds to the narrow temperature range for the transition from an intermediate to a strong pseudo-JT effect under the
conditions for the realization of polarization tunneling states. The appearance of a significant admixture of states of the
modulated ferroelectric vibronic charge-transfer exciton phase to the ground state under the conditions for the realization
of polarization tunneling states at low temperatures provides an explanation for the principal properties of the Müller phase.
Fiz. Tverd. Tela (St. Petersburg) 40, 907–909 (May 1998) 相似文献
18.
Abstract High resolution electron microscopy has shown that a modulated structure occurs in the Bi[sbnd]Sr[sbnd]Ca[sbnd]Cu[sbnd]O system. Electron diffraction and x-ray diffraction observations show that this modulation produces extra reflections. Diffraction patterns of the Pb-free material have superstructure reflections with wave vectors [0, 1/4.7, 1] and [0, ?1/4.7, 1]. These two waves vectors can generate all of the superstructure relections associated with each fundamental reflection. When PbO is added to the BSCCO system a new wave vector [0. 1/8.5, 0] is introduced. Observations show that within a single grain the two wave vectors coexist for a Pb and Bi ratio of about 0.25. This produces a non-uniform modulated structure which may be due to a superposition of the two wave vectors. The intensity of the [0, 1/4.7, 1] wave vector is substantially reduced in the Pb-modified material. The b' component of both wave vectors in incommensurate with the lattice period. The same superstructures occurs in both the low-Tc phase and the high-Tc phase. 相似文献
19.
E. A. Shapoval 《JETP Letters》1998,67(5):376-381
A model of strongly coupled electrons on a square lattice with attraction of the electrons to nearest-neighbor and next-nearest
neighbor lattice sites is studied. For this model, the phase diagrams containing d
x
2−y
2, d
xy, and (d
x
2−y
2+id
xy) states are constructed in the variables temperature versus chemical potential for different ratios of the corresponding
potentials.
Pis’ma Zh. éksp. Teor. Fiz. 67, No. 5, 356–360 (10 March 1998) 相似文献
20.
The approach based on an order-disorder transition model combined with the coulomb electrostatic interaction method allows investigating of the order-disorder phase transition in A(B′B′)O3 complex perovskite type ceramics. An analytical analysis is given on how the second nearest neighbour interactions are connected to the quantities characterising the phase transition. While, the criterion proposed by Zhang et al. (Zhang, X.W., Wang, Q. and Gu, B.L. (1991 ). Study of the order-disorder transition in A(B′B′)O3 perovskite type ceramics. J Am Ceram Soc., 74(11), 2846-2850.) in order to predict the phase transition and the behaviour of the curve which describes the quantity ?′ F ′0 T corresponding to order parameter in function of the ionic radius variation and ionic radius ratio are discussed. 相似文献