Momentum-resolved electron-phonon interaction in lead determined by neutron resonance spin-echo spectroscopy

Distinguishing Quasiparticle-Phonon Interactions by Ultrahigh-Resolution Lifetime Measurements

We present a determination of quasiparticle-phonon interaction strengths at surfaces through measurements of phonon spectra with ultrahigh energy resolution. The lifetimes of low energy surface phonons on a pristine Ru(0001) surface were determined over a wide range of temperatures and an analysis of the temperature dependence enables us to attribute separate contributions from electron-phonon interactions, phonon-phonon interactions, and defect-phonon interactions. Strong electron-phonon interactions are evident at all temperatures and we show they dominate over phonon-phonon interactions below 400 K.

The linear phase correction of modulation of intensity emerging from zero effort (MIEZE) with magnetic Wollaston prisms

This study proposes the use of two magnetic Wollaston prisms (MWPs) to correct for the linear Larmor phase aberration of modulation of intensity emerging from zero effort (MIEZE), introduced by the transverse size of the sample. With this approach, the contrast of the intensity modulation can be maximized at any scattering angle of interest such that the same contrast as the direct transmission geometry can be fully recovered. The optimum magnetic fields required for the MWPs depend only on the scattering angle and the frequencies of the radio-frequency flippers, and they are independent of the neutron wavelength and beam divergence, which makes the approach suitable for both pulsed and continuous neutron sources.

Direct evaluation of the isotope effect within the framework of density functional theory for superconductors

Within recent developments of density functional theory, its numerical implementation and of the superconducting density functional theory is nowadays possible to predict the superconducting critical temperature, [Formula: see text], with sufficient accuracy to anticipate the experimental verification. In this paper we present an analytical derivation of the isotope coefficient within the superconducting density functional theory. We calculate the partial derivative of [Formula: see text] with respect to atomic masses. We verified the final expression by means of numerical calculations of isotope coefficient in monatomic superconductors (Pb) as well as polyatomic superconductors (CaC₆). The results confirm the validity of the analytical derivation with respect to the finite difference methods, with considerable improvement in terms of computational time and calculation accuracy. Once the critical temperature is calculated (at the reference mass(es)), various isotope exponents can be simply obtained in the same run. In addition, we provide the expression of interesting quantities like partial derivatives of the deformation potential, phonon frequencies and eigenvectors with respect to atomic masses, which can be useful for other derivations and applications.

Magnetic moments induce strong phonon renormalization in FeSi

The interactions of electronic, spin and lattice degrees of freedom in solids result in complex phase diagrams, new emergent phenomena and technical applications. While electron–phonon coupling is well understood, and interactions between spin and electronic excitations are intensely investigated, only little is known about the dynamic interactions between spin and lattice excitations. Noncentrosymmetric FeSi is known to undergo with increasing temperature a crossover from insulating to metallic behaviour with concomitant magnetic fluctuations, and exhibits strongly temperature-dependent phonon energies. Here we show by detailed inelastic neutron-scattering measurements and ab initio calculations that the phonon renormalization in FeSi is linked to its unconventional magnetic properties. Electronic states mediating conventional electron–phonon coupling are only activated in the presence of strong magnetic fluctuations. Furthermore, phonons entailing strongly varying Fe–Fe distances are damped via dynamic coupling to the temperature-induced magnetic moments, highlighting FeSi as a material with direct spin–phonon coupling and multiple interaction paths.

Whilst emergent phenomena and potential technological applications in materials may rely of the coupling between spin, lattice, and electronic degrees of freedom, little is known about direct coupling mechanism between spins and phonons. Here, the authors evidence such behaviour in noncentrosymmetrc FeSi.

Lifetime of gapped excitations in a collinear quantum antiferromagnet

We demonstrate that local modulations of magnetic couplings have a profound effect on the temperature dependence of the relaxation rate of optical magnons in a wide class of antiferromagnets in which gapped excitations coexist with acoustic spin waves. In a two-dimensional collinear antiferromagnet with an easy-plane anisotropy, the disorder-induced relaxation rate of the gapped mode, Γ(imp)≈Γ(0)+A(TlnT)2, greatly exceeds the magnon-magnon damping, Γ(m-m)≈BT5, negligible at low temperatures. We measure the lifetime of gapped magnons in a prototype XY antiferromagnet BaNi2(PO4)2 using a high-resolution neutron-resonance spin-echo technique and find experimental data in close accord with the theoretical prediction. Similarly strong effects of disorder in the three-dimensional case and in noncollinear antiferromagnets are discussed.

Acoustic phonon lifetimes and thermal transport in free-standing and strained graphene

We use first-principles methods based on density functional perturbation theory to characterize the lifetimes of the acoustic phonon modes and their consequences on the thermal transport properties of graphene. We show that using a standard perturbative approach, the transverse and longitudinal acoustic phonons in free-standing graphene display finite lifetimes in the long-wavelength limit, making them ill-defined as elementary excitations in samples of dimensions larger than ∼1 μm. This behavior is entirely due to the presence of the quadratic dispersions for the out-of-plane phonon (ZA) flexural modes that appear in free-standing low-dimensional systems. Mechanical strain lifts this anomaly, and all phonons remain well-defined at any wavelength. Thermal transport is dominated by ZA modes, and the thermal conductivity is predicted to diverge with system size for any amount of strain. These findings highlight strain and sample size as key parameters in characterizing or engineering heat transport in graphene.

Low-temperature dynamics of magnons in a spin-1/2 ladder compound

We have used a combination of neutron resonant spin-echo and triple-axis spectroscopies to determine the energy, fine structure, and linewidth of the magnon resonance in the model spin-1/2 ladder antiferromagnet IPA-CuCl(3) at temperatures T≪Δ(0)/k(B), where Δ(0) is the spin gap at T=0. In this low-temperature regime we find that the results deviate substantially from the predictions of the nonlinear sigma model proposed as a description of magnon excitations in one-dimensional quantum magnets and attribute these deviations to real-space and spin-space anisotropies in the spin Hamiltonian as well as scattering of magnon excitations from a dilute density of impurities. These effects are generic to experimental realizations of one-dimensional quantum magnets.

Energy gaps and Kohn anomalies in elemental superconductors

The momentum and temperature dependence of the lifetimes of acoustic phonons in the elemental superconductors lead and niobium were determined by resonant spin-echo spectroscopy with neutrons. In both elements, the superconducting energy gap extracted from these measurements was found to converge with sharp anomalies originating from Fermi-surface nesting (Kohn anomalies) at low temperatures. The results indicate electron many-body correlations beyond the standard theoretical framework for conventional superconductivity. A possible mechanism is the interplay between superconductivity and spin- or charge-density-wave fluctuations, which may induce dynamical nesting of the Fermi surface.