Quantum Interference of Dual-Channel Excited Magnons in Spin-1 Bose-Einstein Condensates Atomic Chain

We investigate the quantum interference of spin wave excitations of a spin-1 atomic Bose condensate confined in an optical lattice. Single-channel and dual-channel interactions are employed in our system, and their induced excitations are compared. Also we consider the interplay of magneto-optical excitations, which leads to a constructive or destructive effect for the creation of magnons based on background excitations. The population distributions of excited magnons can be well controlled by steering the long-range dipole-dipole interactions. Such a scheme can be used to demonstrate conventional quantum-optical phenomena like dynamical Casimir effect at finite temperatures.     

Elementary spin excitations in ultrathin itinerant magnets

Elementary spin excitations (magnons) play a fundamental role in condensed matter physics, since many phenomena e.g. magnetic ordering, electrical (as well as heat) transport properties, ultrafast magnetization processes, and most importantly electron/spin dynamics can only be understood when these quasi-particles are taken into consideration. In addition to their fundamental importance, magnons may also be used for information processing in modern spintronics.     

强关联材料霍尔热导率实验测量综

在材料中输入热流并在垂直于热流的方向上施加磁场时,热载流子将可能被磁场偏转,获 得横向速度,从而导致材料在横向出现一个温度梯度。这种效应被称为热霍尔效应 (THE)。与电 霍尔效应类似,热霍尔效应被预言将在一些拥有非平庸贝利曲率的材料中出现,因此它可以揭示 材料的拓扑性质。然而,热霍尔效应并不像电霍尔一样,只局限于载流子带电的体系;相反,任 何种类的准粒子都可以导热。因此,热霍尔效应也可以用来探索强关联电子体系材料 (尤其是绝缘 体) 的奇异性质。因此,热霍尔效应更具有普适性,并日益成为探测电中性激发,如声子和磁振子 的强有力手段。不仅如此,有如手性声子这样超越一般非平庸贝利曲率图像的因素仍可导致热霍 尔效应;探查其中的热霍尔效应将为理解材料中复杂的微观机理指明方向。但是,热信号比电信 号要微弱得多。尤其是测量热霍尔效应,往往要在较大背景噪音中提取微弱的有效信号,这使霍 尔热导的测量极具挑战性。但是得益于科研工作者大量的努力,该领域在近几年发展迅速,得到 了许多十分有趣的结果。在本文中,我们将简要总结现有的一些令人兴奋的在霍尔热导率测量方 面的成果,指出尚未解决的问题,并提出未来可能的方向。      

Self-trapping of magnon Bose-Einstein condensates in the ground state and on excited levels: from harmonic to box confinement

Long-lived coherent spin precession of (3)He-B at low temperatures around 0.2T(c) is a manifestation of Bose-Einstein condensation of spin-wave excitations or magnons in a magnetic trap which is formed by the order-parameter texture and can be manipulated experimentally. When the number of magnons increases, the orbital texture reorients under the influence of the spin-orbit interaction and the profile of the trap gradually changes from harmonic to a square well, with walls almost impenetrable to magnons. This is the first experimental example of Bose condensation in a box. By selective rf pumping the trap can be populated with a ground-state condensate or one at any of the excited energy levels. In the latter case the ground state is simultaneously populated by relaxation from the exited level, forming a system of two coexisting condensates.     

Direct measurement of the spin Hamiltonian and observation of condensation of magnons in the 2D frustrated quantum magnet Cs2CuCl4

We propose a method for measuring spin Hamiltonians and apply it to the spin- 1/2 Heisenberg antiferromagnet Cs2CuCl4, which shows a 2D fractionalized resonating valence bond state at low fields. By applying strong fields we fully align the spin moment of Cs2CuCl4, transforming it into an effective ferromagnet. In this phase the excitations are conventional magnons and their dispersion relation measured using neutron scattering give the exchange couplings directly, which are found to form an anisotropic triangular lattice with small Dzyaloshinskii-Moriya terms. Using the field to control the excitations we observe Bose condensation of magnons into an ordered ground state.     

Effect of photons on the magnon splitting

The self energy of a uniform magnon due to splitting into two magnons of oppositely directed momenta is calculated in the presence of a magnon-photon interaction, and it is found that the magnons and the photons form excitations of mixed character. Thus there is a net correction to the magnon splitting due to an interference.     

Dynamical theory of photon superradiative emission by nanoscale system of Bose-condensed magnons

We have shown the possibility of non-Dicke superradiance for non-ideal magnon Bose-Einstein condensate (BEC) in a broadband frequency bath. Here, it is found that all the stored energy in the system of Bose-condensed magnons can be irradiated into a short pulse with a time delay caused by the strong frequency modulation of magnons due to direct inter-particle interactions in the Bose-condensed state. The last mechanism radically distinguishes this effect from the well-known Dicke superradiance of two-level atomic ensemble where the delay is connected with enhancement of the inter-atomic correlations due to exchange by virtual photons. In our case, the superradiance is the consequence of Bose-condensation in the coherent state where the particles are coupled by direct interaction. We have discussed the conditions for observation of this effect for Bose-condensed magnons in a solid-state sample with a spatial size smaller comparing with the wavelength of the emitted field. In general, we had shown that this kind of superradiance can proceed in samples with ferromagnetic type interaction. As for the antiferromagnetic ones, the effect of magnon superradiance takes place without delay.     

Phase-coherent current-driven magnons in magnetic multilayers

We report the detection of electromagnetic waves radiated by current-driven magnons in a Co/Cu magnetic multilayer. The magnons were excited by means of a high current density ≈10⁸ A/cm² injected into the multilayer through a point contact. The point contact itself was used as a high frequency mixer to mix electromagnetic waves radiated by the current-driven magnons with externally generated microwave radiation. Here the external microwaves are used as a direct probe of the high-frequency behavior and partial phase coherence of the current-induced excitations. When the external frequency equaled the frequency of the magnons generated in the multilayer a DC voltage was found to develop across the contact. Investigation of how this voltage varies with exciting current, magnetic field, and microwave frequency provides detailed information on the spectrum of the current-driven magnons. Our observations support the feasibility of a spin-wave maser, or spin-wave amplification by stimulated emission of radiation.     

Elementary excitations at magnetic surfaces and their spin dependence

The elementary surface excitations are studied by spin-polarized electron energy loss spectroscopy on a prototype oxide surface [an oxygen passivated Fe(001)-p(1×1) surface], where the various excitations coexist. For the first time, the surface phonons and magnons are measured simultaneously and are distinguished based on their different spin nature. The dispersion relation of all excitations is probed over the entire Brillouin zone. The different phonon modes observed in our experiment are described by means of ab initio calculations.     

Stripe-like topological excitations in a two-dimensional Heisenberg ferromagnet

For a two-dimensional Heisenberg ferromagnet, a class of steady-state nonlinear excitations above the ground state is considered. The excitations have the form of stripes and exhibit quasiparticle properties. The effect of an external magnetic field on the basic characteristics of these nonlinear topological excitations is investigated. The magnetic field is found to destroy the instanton-type solutions (kinks) and to generate new particles with the properties of vortex-antivortex pairs: each particle has a zero topological charge and an energy close to the double skyrmion energy 8πJS². The dispersion of the quasiparticles and the dependences of their energy and momentum on the number of magnons localized by one excitation are discussed.     

Beliaev theory of spinor Bose–Einstein condensates

By generalizing the Green’s function approach developed by Beliaev [S.T. Beliaev, Sov. Phys. JETP 7 (1958) 299; S.T. Beliaev, Sov. Phys. JETP 7 (1958) 289], we study effects of quantum fluctuations on the energy spectra of spin-1 spinor Bose–Einstein condensates, in particular, of a ⁸⁷Rb condensate in the presence of an external magnetic field. We find that due to quantum fluctuations, the effective mass of magnons, which characterizes the quadratic dispersion relation of spin-wave excitations, increases compared with its mean-field value. The enhancement factor turns out to be the same for two distinct quantum phases: the ferromagnetic and polar phases, and it is a function of only the gas parameter. The lifetime of magnons in a spin-1 ⁸⁷Rb spinor condensate is shown to be much longer than that of phonons due to the difference in their dispersion relations. We propose a scheme to measure the effective mass of magnons in a spinor Bose gas by utilizing the effect of magnons’ nonlinear dispersion relation on the time evolution of the distribution of transverse magnetization. This type of measurement can be applied, for example, to precision magnetometry.     

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.     

Critical dynamics and relaxation of elementary excitations of the nematic phase of a non-heisenberg magnet with spin <Emphasis Type="Italic">S</Emphasis> = 1

The relaxation of elementary excitations (magnons) in the nematic phase of a magnet with spin S = 1 at the critical point of the nematic-to-ferromagnetic phase transition has been studied. Magnons in a three-dimensional nematic at the critical point have all of the properties of Goldstone excitations; in particular, their damping decrement tends to zero faster than the excitation frequency as the wave vector tends to zero. In the two-dimensional case, the ratio of the damping decrement to the frequency is small in the long-wavelength limit. The similarity between the behaviors of magnons in the spin nematic and in the isotropic Heisenberg ferromagnet has been underlined.     

利用光晶格自旋链中磁振子的激发模拟有限温度下光子的动力学 Casimir 效应

研究了在静磁场诱导的磁偶极-偶极相互作用和外部激光场诱导产生的偶极-偶极相互作用下光晶格自旋链中磁振子激发的动力学特征. 文中选取了蓝失谐光晶格, 提出了等效温度的概念, 并将系统中磁振子的激发过程与光学振动腔中光子的激发过程进行了类比. 研究表明, 通过选取适当的系统参数, 可以在磁振子系统中重现有限温度下光子的动力学Casimir效应. 关键词： 光晶格 偶极－偶极相互作用 磁振子 Casimir效应     

Crystal field and low energy excitations measured by high resolution RIXS at the L3 edge of Cu, Ni and Mn

Resonant inelastic x-ray scattering in the soft x-ray regime has been profiting much from technical advances that have lowered considerably the instru- mental linewidth. At the ADRESS beam line of the Swiss Light Source the SAXES spectrometer can be used to measure RIXS spectra at the L edges of the 3d transition metals with unprecedented energy resolution, of the order of 100 meV for Mn, Ni and Cu. We present here some preliminary spectra on CuO, malachite, NiO, NiCl₂, MnO and LaMnO₃. The dd excitations are very well resolved allowing accurate experimental evaluation of 3d state energy splitting. The low energy scale becomes accessible opening the way to the study of collective excitations in strongly correlated electron systems, like magnons and orbitons.     

Spin pumping by parametrically excited exchange magnons

We experimentally show that exchange magnons can be detected by using a combination of spin pumping and the inverse spin-Hall effect proving its wavelength integrating capability down to the submicrometer scale. The magnons were injected in a ferrite yttrium iron garnet film by parametric pumping and the inverse spin-Hall effect voltage was detected in an attached Pt layer. The role of the density, wavelength, and spatial localization of the magnons for the spin pumping efficiency is revealed.     

Plasmon-magnon interaction in magnetic semiconductors

In appreciably doped semiconductors (e.g. EuO, CdCr₂S₄, etc.) plasmon and magnon energies are comparable. Therefore, there will be resonant interaction between these modes of excitations. On the basis of a new microscopic theory formulated for plasmon-magnon interaction, the effect of this interaction on the energies and lifetimes of plasmons and magnons has been calculated using the double-time Green’s function. The energy shifts are very small and the lifetimes of plasmons,τ ^p , and magnons,τ ^m , are of the order of 10⁻² and 10⁻³ sec respectively.     

Effective spin spin interactions in the damped ferromagnets

Simanek's theory is extended by taking into account the nuclear spin excitations due to the motion of the electronic system. Damping of the magnons is found to be an extra criteria for having a finite interaction range for the nuclear spin system. The resonance excitation of Simanek's theory is due to the neglect of the second order in the hyperfine interaction.     

Magnon–magnon interactions in O(3) ferromagnets and equations of motion for spin operators

The method of equations of motion for spin operators in the case of O(3) Heisenberg ferromagnet is systematically analyzed starting from the effective Lagrangian. It is shown that the random phase approximation and the Callen approximation can be understood in terms of perturbation theory for type B magnons. Also, the second order approximation of Kondo and Yamaji for one dimensional ferromagnet is reduced to the perturbation theory for type A magnons. An emphasis is put on the physical picture, i.e. on magnon–magnon interactions and symmetries of the Heisenberg model. Calculations demonstrate that all three approximations differ in manner in which the magnon–magnon interactions arising from the Wess–Zumino term are treated, from where specific features and limitations of each of them can be deduced.