Magnetic field enhancement of heat transport in the 2D Heisenberg antiferromagnet K2V3O8

The thermal conductivity and heat capacity of single crystals of the spin 1/2 quasi-2D Heisenberg antiferromagnet K(2)V(3)O(8) have been measured from 1.9 to 300 K in magnetic fields from 0 to 8 T. The zero field thermal conductivity data are consistent with resonant scattering of phonons by magnons near the zone boundary. Application of a magnetic field greater than 1 T, however, produces a new magnetic ground state with substantial heat transport by long wavelength magnons.     

In-plane thermal conductivity of Nd2CuO4: evidence for magnon heat transport

We report the temperature and magnetic field dependence of the in-plane thermal conductivity (kappa(ab)) of high-quality monocrystalline Nd2CuO4. Isothermal measurements of the field dependence of kappa(ab) at low temperatures (2 K相似文献   

Brillouin scattering from ferromagnetic metallic glasses

We report the first measurements of thermally excited surface acoustic phonons, and bulk and surface magnons from metallic glasses using Brillouin scattering. In particular we present data on the compositions Fe₄₀Ni₄₀P₁₄B₆ and Fe₈₀B₂₀. The velocity of the surface phonons was found to agree within 1.5% with the value calculated from elastic continuum theory using known bulk elastic constants. The observed field dependence of the bulk magnon frequency can be fit within experimental error to the classical relation for the k = 0 mode by including alldemagnetizing corrections. The values for the magnetization obtained from both the surface and bulk magnons agree with the bulk values determined previously with a magnetometer. The measured field dependences of the surface magnon frequency allow us to determine the g-factors, yielding 2.08±0.02 for FeB and 2.03±0.03 for FeNiPB.     

Light scattering from thermal acoustic magnons in yttrium iron garnet

Brillouin scattering measurements on yttrium iron garnet (YIG) have revealed the scattering due to thermal acoustic magnons. The intensity and frequency shift of the magnon peaks as a function of applied magnetic field and incident light wavelength have been investigated and are discussed here within the framework of a simple light scattering theory.     

Exact and model operators for magnon-phonon interactions in antiferromagnets

A theoretical study is given of magnon-phonon interactions in antiferromagnetic materials. The roles of magnons and phonons as heat carriers and as sources of thermal resistance have been taken into consideration. The exact collision operator which represents the magnon-phonon interactions involved in the transport Boltzmann equations has been replaced by a model operator which possesses the same important properties. The effect of other scattering processes that either phonons or magnons are involved has also been investigated. A new expression for the thermal conductivity has been derived. It includes terms which represent both Normal and Umklapp magnon-phonon processes. The results obtained by using the new expression agree quantitatively with the experimental measurements on Fe Cl₂     

Magnon heat transport in doped La2CuO4

We present results of the thermal conductivity of La2CuO4 and La(1.8)Eu(0.2)CuO4 single crystals which represent model systems for the two-dimensional spin-1/2 Heisenberg antiferromagnet on a square lattice. We find large anisotropies of the thermal conductivity which are explained in terms of two-dimensional heat conduction by magnons within the CuO2 planes. Nonmagnetic Zn substituted for Cu gradually suppresses this magnon thermal conductivity kappa(mag). A semiclassical analysis of kappa(mag) is shown to yield a magnon mean free path which scales linearly with the reciprocal concentration of Zn ions.     

布里渊散射深入研究Y_(3-x)Bi_xFe_5O_(12)晶体中的磁振子和声学声子(英文)

我们选用x =0 ,0 1 4 ,0 36 ,0 5 4 ,和 0 92的Y3 xBixFe5O12 系列单晶 ,开展了比早期实验更为深入的布里渊散射研究。实验发现对于Bi YIG晶体 ,照射在样品上小于 5mW的激光功率为最佳激发功率。布里渊散射结果表明Y3 xBixFe5O12 晶体中的磁性能和机械性质与Bi含量密切有关。此实验观测到了两种体波性质的磁振子 ,一个是热激发的自旋波 ,另一个被确认为静磁模。实验还发现声学声子的散射强度和声速随Bi含量的增加而减少。     

Raman scattering studies of hexagonal rare‐earth RMnO3 (R = Tb,Dy, Ho,Er) thin films

We have studied the rare‐earth (R) dependence of the phonon and magnon scattering in hexagonal RMnO₃ (R = Tb, Dy, Ho, Er) thin films using Raman scattering spectroscopy. We found, as the ionic radius of R decreases from Tb to Er, the phonons shift to higher energies. Our results indicate that both the lattice constants a and c of hexagonal RMnO₃ would decrease when the ionic radius of R decreases, and the lattice constant c would have a weaker R dependence. The magnons also shift to higher energies when the radius of the R ion decreases, and they show faster upshift than the phonons. In addition, the Néel temperature also shows a systematic increasing behavior when the radius of the R ion decreases. The dependence of the rare‐earth R on the magnons and the Néel temperature can be explained by the rapid increase of the spin‐exchange integral when the Mn–Mn distance decreases. Copyright © 2011 John Wiley & Sons, Ltd.     

Magnon-hole scattering and charge order in Sr14-xCaxCu24O41

The magnon thermal conductivity kappa(mag) of the hole-doped spin ladders in Sr14-xCaxCu24O41 has been investigated at low doping levels x. The analysis of kappa(mag) reveals a strong doping and temperature dependence of the magnon mean free path l(mag), which is a local probe for the interaction of magnons with the doped holes in the ladders. In particular, this novel approach to studying charge degrees of freedom via spin excitations shows that charge ordering of the holes in the ladders leads to a freezing out of magnon-hole scattering processes.     

Effect of dispersion on the phonon focusing and anisotropy of thermal conductivity of silicon single crystals in the boundary scattering regime

The effect of dispersion on the focusing of thermal phonons and on the thermal conductivity of silicon single crystals in the boundary scattering regime has been investigated. Analysis of the spectra of acoustic modes obtained for silicon single crystals from inelastic neutron scattering data has demonstrated that, upon transition from long-wavelength phonons to short-wavelength phonons, the directions of their focusing change. With an increase in temperature, this leads to a change in the anisotropy of thermal conductivity of phonons with different polarizations and, consequently, to a change in the anisotropy of the total thermal conductivity. Analysis of the temperature dependence of the thermal conductivity has revealed that the presence of extended flattened sections in the spectrum of short-wavelength transverse phonons indicates anomalously low values of the group velocity and, accordingly, a significant decrease in the contribution from these phonons to the thermal conductivity with increasing temperature. The contribution from longitudinal phonons to the thermal conductivity also significantly increases even at temperatures higher than 110 K and becomes dominant.     

Second sound and heat conduction of magnons in the ferromagnetic Eu-chalcogenides

The various relaxation times of the thermal magnons in the ferromagnetic Eu-chalcogenides are calculated for temperatures well below the Curie point, and the conditions for the existence of magnon second sound and magnon heat conduction are examined. It is found that for EuS the “frequency window” for magnon second sound exists around 2 MHz at temperatures of about 1 K. In the same material the magnon contribution to the static thermal conductivity should be measurable at temperatures between 1 K and 2 K. Supported in part by the Fonds National Suisse de la Recherche Scientifique.     

多空穴错位分布对石墨纳米带中热输运的影响

利用非平衡格林函数方法研究了石墨纳米带中三空穴错位分布对热输运性质的影响.研究结果发现:三空穴竖直并排结构对低频声子的散射较小,导致低温区域三空穴竖直并排时热导最大,而在高频区域,三空穴竖直并排结构对高频声子的散射较大,导致较高温度区域三空穴竖直并排时热导最小;三空穴的相对错位分布仅能较大幅度地调节面内声学模高频声子的透射概率,而三空穴的相对错位分布能较大幅度地调节垂直振动膜高频声子和低频声子的透射概率,导致三空穴的相对错位分布不仅能大幅调节面内声学模和垂直振动模的高温热导,也能大幅调节垂直振动模的低温热导.研究结果阐明了空穴位置不同的石墨纳米带的热导特性,为设计基于石墨纳米带的热输运量子器件提供了有效的理论依据.     

Thermal conductivity of polycrystalline zinc selenide

The thermal conductivity of optically transparent zinc selenide polycrystals fabricated by vapor deposition was experimentally studied in the temperature range 80–400 K in the as-deposited state and after deformation along the crystal growth direction followed by recrystallization. In the low-temperature range, textured ZnSe samples exhibit anisotropy of the thermal conductivity, which also persisted after their deformation and recrystallization. The anisotropy of the thermal conductivity is caused by phonon scattering by dislocations oriented along the crystal growth direction. The thermal conductivity of ZnSe at T>270 K is shown to be limited by the scattering of acoustic phonons by optical phonons.     

Spin phonon coupling in hexagonal multiferroic YMnO3

Inelastic neutron scattering measurements have been performed on YMnO3, aiming to study the interplay between spin and lattice degrees of freedom in this hexagonal multiferroic material. Our study provides evidence for a strong coupling between magnons and phonons, evidenced by the opening of a gap below T(N) in the dispersion of the transverse acoustic phonon mode polarized along the ferroelectric axis. The resulting upper phonon branch is found to lock on one of the spin-wave modes. These findings are discussed in terms of a possible hybridization between the two types of elementary excitations.     

Possible observation of cycloidal electromagnons in BiFeO3

We unravel the magnon spectra of BiFeO3 by means of low-energy inelastic light scattering. We show the existence of two species of magnons corresponding to spin wave excitations in and out of the cycloidal plane. These excitations might be interpreted as electromagnon modes. The present observations present an unique opportunity to study the competition between ferroelectric and magnetic orders.     

Observation of spontaneous coherence in Bose-Einstein condensate of magnons

The room-temperature dynamics of a magnon gas driven by short microwave pumping pulses is studied. An overpopulation of the lowest energy level of the system following the pumping is observed. Using the sensitivity of the Brillouin light scattering technique to the coherence degree of the scattering magnons we demonstrate the spontaneous emergence of coherence of the magnons at the lowest level, if their density exceeds a critical value. This finding is clear proof of the quantum nature of the observed phenomenon and direct evidence of Bose-Einstein condensation of magnons at room temperature.     

Amplification of non-Cerenkovian magnons by electrons in a laser radiation field

The rate of change of the magnon population due to scattering by conduction electrons in the presence of an electromagnetic wave is calculated. It is found that non-Cerenkovian high-frequency magnons may be amplified over a very narrow band of magnon wavenumbers.     

Solitons and magnons in Sine-Gordon like magnetic chains

We present theoretical results on the dynamic structure factors of both the classical Sine-Gordon chain and thexy-like ferromagnetic chain in a symmetry breaking magnetic field. We investigate the lowest order corrections to the noninteracting soliton/magnon picture and show that interference effects between solitons and magnons considerably reduce the intensity of the soliton induced central peak. We discuss the additional contribution of two magnon processes to the central peak and find that the combined strength is in agreement with numerical results. We calculate magnon intensities including quantum effects and find that the intensity depends strongly on temperature and wavevector. Quantitative results are given for the one-dimensional magnet CsNiF₃ and compared to neutron scattering data. The soliton induced line-width of the long wavelength magnon is also given.     

Magnon scattering by a symmetric atomic well in free standing very thin magnetic films

A theoretical model is presented for the study of the scattering of magnons at an extended symmetric atomic well in very thin magnetic films. The thin film consists of three cubic atomic planes with ordered spins coupled by Heisenberg exchange, and the system is supported on a non-magnetic substrate, and considered otherwise free from magnetic interactions. The coherent transmission and reflection scattering coefficients are derived as elements of a Landauer type scattering matrix. Transmission and reflection scattering cross sections are hence calculated specifically, as a function of the varying local magnetic exchange on the inhomogeneous boundary. Detailed numerical results for the individual incident film magnons, and for the calculated overall magnon conductance, show characteristic transmission properties, with associated Fano resonances, depending on the magnetic boundary conditions and on the magnon incidence.     

二维Mo2C的晶格热导率的第一性原理研究

Mo2C是构建Mxene基器件的重要材料之一,对Mo2C二维材料声子输运的理解非常必要。文章结合第一性原理方法和声子玻尔兹曼输运方程,研究了二维Mo2C材料的晶格热导率。研究表明,室温下二维Mo2C导热系数非常低,其锯齿方向和扶手椅方向的晶格热导率分别为7.20 和 5.04 W/mK。计算了声学振动和光学振动模式对晶格热导率的贡献,揭示总热导率主要由面内声学横波的振动模式所贡献。还进一步计算了声子群速度、声子弛豫时间、三声子散射空间和模式格林艾森参数,发现二维Mo2C中的声子群速度和声子弛豫时间对晶格传输有重要的影响。