We calculate the effects of the spin-lattice coupling on the magnon spectrum of thin ferromagnetic films consisting of the magnetic insulator yttrium-iron garnet. The magnon-phonon hybridisation generates a characteristic minimum in the spin dynamic structure factor which quantitatively agrees with recent Brillouin light scattering experiments. We also show that at room temperature the phonon contribution to the magnon damping exhibits a rather complicated momentum dependence: In the exchange regime the magnon damping is dominated by Cherenkov type scattering processes, while in the long-wavelength dipolar regime these processes are subdominant and the magnon damping is two orders of magnitude smaller. We supplement our calculations by actual measurements of the magnon relaxation in the dipolar regime. Our theory provides a simple explanation of a recent experiment probing the different temperatures of the magnon and phonon gases in yttrium-iron garnet.

We investigate the spin transport properties of a topological magnon insulator, a magnetic insulator characterized by topologically nontrivial bulk magnon bands and protected magnon edge modes located in the bulk band gaps. Employing the Landau-Lifshitz-Gilbert phenomenology, we calculate the spin current driven through a normal metal|topological magnon insulator|normal metal heterostructure by a spin accumulation imbalance between the metals, with and without random lattice defects. We show that bulk and edge transport are characterized by different length scales. This results in a characteristic system size where the magnon transport crosses over from being bulk-dominated for small systems to edge-dominated for larger systems. These findings are generic and relevant for topological transport in systems of nonconserved bosons.

We show that the formation of a magnon condensate in thin ferromagnetic films can be explained within the framework of a classical stochastic non-Markovian Landau-Lifshitz-Gilbert equation where the properties of the random magnetic field and the dissipation are determined by the underlying phonon dynamics. We have numerically solved this equation for a tangentially magnetized yttrium-iron garnet film in the presence of a parallel parametric pumping field. We obtain a complete description of all stages of the nonequilibrium time evolution of the magnon gas which is in excellent agreement with experiments. Our calculation proves that the experimentally observed condensation of magnons in yttrium-iron garnet at room temperature is a purely classical phenomenon which should be called Rayleigh-Jeans rather than Bose-Einstein condensation.PACS numbers: 75.30. Ds, 75.10.Hk, 05.30.Jp In the past decade the nonequilibrium dynamics of parametrically pumped magnons in thin yttrium-iron garnet (YIG) films has been investigated by many experimental studies [1][2][3][4][5][6][7][8][9]. Very rich physics was found, including the overpopulation of the lowest energy state, which was interpreted as Bose-Einstein condensation (BEC) of magnons at room temperature and finite momentum. Using the technique of Brillouin light scattering it is even possible to measure the magnon distribution with momentum and time resolution [10]. This allows an observation of the parametric resonance and of the subsequent thermalization leading to the formation of the condensate in detail [6,7]. Unfortunately, a complete theoretical understanding of this phenomenon is still lacking and there is no theory that can simultaneously describe all stages of the experiment. While the so-called S-theory [11][12][13][14][15][16][17][18] is able to describe the parametric resonance used to populate certain magnon states, it does not properly take magnon-magnon scattering into account and therefore cannot describe the cascade of relaxation processes leading to the formation of a magnon condensate. On the other hand, theories that focus on the condensate usually do not take the pumping dynamics into account and start with some given quasiequilibrium state which can be identified with the ground state of some effective quantum mechanical Hamiltonian [19][20][21]. Phenomenological approaches of the Ginzburg-Landau type also have been used to study the condensation dynamics [22]. Finally, theories dealing with the relaxation processes and kinetics of excited magnons did not include the possibility of magnon condensation [23][24][25][26].Since BEC is a manifestation of quantum mechanics, it seems at the first sight reasonable that quantized magnons obeying Bose statistics are essential to obtain a satisfactory theoretical description of magnon condensation in YIG. However, since the experiments are performed at room temperature, which is large compared with the relevant magnon energies, the equilibrium distribution of the magnons is the Rayleigh-Jeans rather than t...

We develop a microscopic theory of spin-lattice interactions in magnetic insulators, separating rigid-body rotations and the internal angular momentum, or spin, of the phonons, while conserving the total angular momentum. In the low-energy limit, the microscopic couplings are mapped onto experimentally accessible magnetoelastic constants. We show that the transient phonon spin contribution of the excited system can dominate over the magnon spin, leading to nontrivial Einstein-de Haas physics.

We investigate phonon spin transport in an insulating ferromagnet−nonmagnet−ferromagnet heterostructure. We show that the magnetoelastic interaction between the spins and the phonons leads to nonlocal spin transfer between the magnets. This transfer is mediated by a local phonon spin current and accompanied by a phonon spin accumulation. The spin conductance depends nontrivially on the system size, and decays over millimeter length scales for realistic material parameters, far exceeding the decay lengths of magnonic spin currents.

We generalize the spin-wave expansion in powers of the inverse spin to time-dependent quantum spin models describing rotating magnets or magnets in time-dependent external fields. We show that in these cases, the spin operators should be projected onto properly defined rotating reference frames before the spin components are bosonized using the Holstein-Primakoff transformation. As a first application of our approach, we calculate the reorganization of the magnetic state due to Bose-Einstein condensation of magnons in the magnetic insulator yttrium-iron garnet; we predict a characteristic dip in the magnetization which should be measurable in experiments.Comment: 6 pages, 5 figures, final version published in PR

Using a non-perturbative functional renormalization group approach we calculate the renormalized quasi-particle velocity v(k) and the static dielectric function (k) of suspended graphene as functions of an external momentum k. Our numerical result for v(k) can be fitted by v(k)/vF = A+B ln(Λ0/k), where vF is the bare Fermi velocity, Λ0 is an ultraviolet cutoff, and A = 1.37, B = 0.51 for the physically relevant value (e 2 /vF = 2.2) of the coupling constant. In contrast to calculations based on the static random-phase approximation, we find that (k) approaches unity for k → 0. Our result for v(k) agrees very well with a recent measurement by Elias et al. [Nat. Phys. 7, 701 (2011)].PACS numbers: 81.05.ue, 11.10.Hi, At low energies the physical properties of graphene are dominated by the Dirac points where the energy dispersion vanishes linearly. In this regime many-body effects become important and can be measured experimentally [1]. In view of the great interest in graphene both for fundamental research and applied physics, it is important to gain a thorough understanding of correlation effects. Of particular interest is the renormalization of the Fermi velocity at the Dirac points by long-range Coulomb interactions, which has been observed experimentally in suspended graphene using cyclotron resonance [2], in ARPES measurements of quasi-freestanding graphene on SiC [3], and in graphene on hexagonal boron nitride (hBN) [4]. Early one-loop renormalization group (RG) calculations [5] predicted a logarithmic enhancement of the renormalized Fermi velocity,where Λ is the infrared cutoff introduced in the RG procedure, Λ 0 is an ultraviolet cutoff of the order of the inverse lattice spacing, v F = 10 6 m/s is the bare Fermi velocity, and α = e 2 /v F is the relevant dimensionless coupling constant. Because for graphene suspended in vacuum α ≈ 2.2 is rather large, perturbative RG calculations are not expected to be quantitatively accurate.In this work, we use a functional renormalization group (FRG) approach [6,7] to derive non-perturbative RG flow equations for the cutoff-and momentum-dependent velocity v Λ (k) and the static dielectric function Λ (q) of suspended graphene. Since we are interested in the RG flow of momentum-dependent functions, the field theoretical RG is not sufficient, because with this method one can only keep track of a finite set of coupling constants. We show here that this problem can be solved within the FRG formalism [6,7]; specifically, we derive two coupled integro-differential equations for the cutoff-dependent functions v Λ (k) and Λ (q) which are non-perturbative in α and self-consistently describe the interplay between self-energy and screening effects.Our starting point is the following effective Hamiltonian describing the low-energy physics of graphene,where p = ± labels the two Dirac points of the underlying tight-binding model on a honeycomb lattice, v p = pv F is the bare Fermi velocity at Dirac point p, andψ p (k) are two-component fermionic field operators whose components are as...

Dzyaloshinskii-Moriya interaction in magnets, which is usually derived from inversion symmetry breaking at interfaces or in noncentrosymmetric crystals, plays a vital role in chiral spintronics. Here we report that an emergent Dzyaloshinskii-Moriya interaction can be achieved in a centrosymmetric material, La 0.67 Sr 0.33 MnO 3 , by a graded strain. This strain-driven Dzyaloshinskii-Moriya interaction not only exhibits distinctive two coexisting nonreciprocities of spin-wave propagation in one system, but also brings about a robust room-temperature magnetic skyrmion lattice as well as a spiral lattice at zero magnetic field. Our results demonstrate the feasibility of investigating chiral spintronics in a large category of centrosymmetric magnetic materials.