We theoretically study magnon-phonon hybrid excitations in a square lattice ferromagnet subjected to a magnetic field by varying the field direction. The bulk bands of hybrid excitations, which are referred to as magnon-polarons, are investigated by considering all three phonon modes: vertical phonon, transverse phonon, and longitudinal phonon. We show that the topological proprieties of three hybridizations are different in terms of the Berry curvature and the Chern numbers. We also find that the topological properties of the bands can be controlled by changing the direction of the magnetic field, exhibiting one or more topological phase transitions. The dependence of thermal Hall conductivity as a function of magnetic field direction is proposed as an experiment probe of our theoretical results.
We theoretically study the interaction of magnons, quanta of spin waves, and a domain wall in a one-dimensional easy-axis antiferromagnet in the presence of an external magnetic field applied along the easy axis. To this end, we begin by obtaining the exact solution for spin waves in the background of a domain wall magnetized by an external field. The finite magnetization inside the domain wall is shown to give rise to reflection of magnons scattering off the domain wall, deviating from the well-known result of reflection-free magnons in the absence of a magnetic field. For practical applications of the predicted reflection of magnons, we show that the magnon reflection contributes to the thermally driven domain-wall motion. Our work leads us to envision that inducing a finite magnetization in antiferromagnetic solitons such as vortices and skyrmions can be used to engender phenomena that do not occur in the absence of magnetization.
By performing a series of thermodynamic measurements in an applied magnetic field Hext, we investigated the effects of Eu substitution on the Pr sites in filled skutterudite compound Pr1−xEuxPt4Ge12 (0 ≤ x ≤ 1). A heat capacity Schottky anomaly is present over the whole doping range. For the samples with x > 0.5, the temperature of the maximum Tmax shifts to lower temperature with increasing Hext. We argue that this behavior reflects the antiferromagnetic (AFM) ordering of the Eu moments, as the AFM transition is suppressed by Hext. The Schottky anomaly in the samples with x ≤ 0.5 shift to higher temperatures with increasing magnetic field, signaling the presence of an internal magnetic field due to short-range AFM correlations induced by magnetic moments of neighboring Eu sites. In low Hext, the Schottky gaps show a non-linear relationship with Hext as the magnetic moments become weakly magnetized. In high Hext, the magnetic moments of Eu sites become completely aligned with Hext. Thus, increasing Hext does not further increase the magnetization, hence the Schottky gaps increase linearly with Hext.
Motivated by the recent observation of the time-reversal symmetry broken state in K-doped BaFe2As2 superconducting alloys, we theoretically study the collective modes and the short time dynamics of the superconducting state with s + is-wave order parameter using an effective four-band model with two hole and two electron pockets. The superconducting s + is state emerges for incipient electron bands as a result of hole doping and appears as an intermediate state between s ± (high number of holes) and s ++ (low number of holes). The amplitude and phase modes are coupled giving rise to a variety of collective modes. In the s ± state, we find the Higgs mode at frequencies similar to a two-band model with an absent Leggett mode, while in the s + is and s ++ state, we uncover a new coupled collective soft mode. Finally we compare our results with the s + id solution and find similar behaviour of the collective modes.
Recent experimental advances in creating stable dipolar bosonic systems, including polar molecules with large electric dipole moments, have led to vigorous theoretical activities. Recent reporting of observation of roton feature in dipolar erbium has provided added impetus to theoretical and experimental work. Here we discuss our mean-field theory work on 2D and quasi-2D dipolar bosons with dipoles oriented at an angle to the direction perpendicular to the confining 2D plane, i.e. for non-zero tilt angles. Using Bogoliubov-de Gennes equations, we present results on a number of T=0 properties of both 2D and quasi-2D systems, such as excitation spectra, structure functions, sound velocities, quantum depletion, etc. We explore instabilities at varying tilt angle, density and dipolar coupling. We map out phase diagrams as a function of tilt angle, dipole strength and density. We find the development of maxon-roton behavior leading to roton instabilities at large densities for small tilt angles, and at low densities for large tilt angles. The behavior is anisotropic in k-space; accordingly the roton instabilities occur first in the ky direction, suggestive of inhomogeneity and stripe phase, with density mode becoming soft in the y-direction. Beyond a critical tilt angle, at any density, the dipolar system collapses owing to a phonon instability. We discuss similarities and differences between the properties of 2D and quasi-2D dipolar systems at non-zero tilt angles.
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