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Register a new userElectromagnon in the Z-type hexaferrite $({rm Ba}_{x}{rm Sr}_{1-x})_3rm Co_2Fe_{24}O_{41}$
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We studied experimentally the high-temperature magnetoelectric $({rm Ba}_{x}{rm Sr}_{1-x})_3rm Co_2Fe_{24}O_{41}$ prepared as ceramics (x = 0, 0.2) and a single crystal (x = 0.5) using inelastic neutron scattering, THz time-domain, Raman and far-infrared spectroscopies. The spectra, measured with varying temperature and magnetic field, reveal rich information about the collective spin and lattice excitations. In the ceramics, we observed an infrared-active magnon which is absent in $E^{omega}perp z$ polarized THz spectra of the crystal, and we assume that it is an electromagnon active in $E^{omega} | z$ polarized spectra. On heating from 7 to 250 K, the frequency of this electromagnon drops from 36 to 25 cm$^{-1}$ and its damping gradually increases, so it becomes overdamped at room temperature. Applying external magnetic field has a similar effect on the damping and frequency of the electromagnon, and the mode is no more observable in the THz spectra above 2 T, as the transverse-conical magnetic structure transforms into a collinear one. Raman spectra reveal another spin excitation with a slightly different frequency and much higher damping. Upon applying magnetic field higher than 3 T, in the low-frequency part of the THz spectra, a narrow excitation appears whose frequency linearly increases with magnetic field. We interpret this feature as the ferromagnetic resonance.
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We investigated static and dynamic magnetoelectric properties of single crystalline BaSrCoZnFe$_{11}$AlO$_{22}$ which is a room-temperature multiferroic with Y-type hexaferrite crystal structure. Below $300,rm K$, a purely electric-dipole-active electromagnon at $approx 1.2,rm THz$ with the electric polarization oscillating along the hexagonal axis was observed by THz and Raman spectroscopies. We investigated the behavior of the electromagnon with applied DC magnetic field and linked its properties to static measurements of the magnetic structure. Our analytical calculations determined selection rules for electromagnons activated by the magnetostriction mechanism in various magnetic structures of Y-type hexaferrite. Comparison with our experiment supports that the electromagnon is indeed activated by the magnetostriction mechanism involving spin vibrations along the hexagonal axis.
We study the effect of Hunds splitting of repulsive interactions on electronic phase transitions in the multiorbital topological crystalline insulator Pb$_{1-x}$Sn$_{x}$Te, when the chemical potential is tuned to the vicinity of low-lying Type-II Van Hove singularities. Nontrivial Berry phases associated with the Bloch states impart momentum-dependence to electron interactions in the relevant band. We use a multipatch parquet renormalization group (RG) analysis for studying the competition of different electronic phases, and find that if the dominant fixed-point interactions correspond to antiparallel spin configurations, then a chiral $p$-wave Fulde-Ferrell-Larkin-Ovchinnikov(FFLO) state is favored, otherwise, none of the commonly encountered electronic instabilities occur within the one-loop parquet RG approach.
We report an electron spin resonance (ESR) study of single crystals of the spin-chain spin-ladder compound (Sr,La,Ca)_{14}Cu_{24}O_{41}. The data suggest that in intrinsically hole doped Sr_{14-x}Ca_xCu_{24}O_{41} only a small amount of holes is transferred from the chains to the ladders with increasing x, resulting in a crossover from spin dimerized to uniform spin chains. In the samples of La_{14-x}Ca_xCu_{24}O_{41} with reduced hole content a very broad signal is observed in the paramagnetic state, indicative of a surprisingly strong anisotropy of the nearest neighbor exchange in the chains.
The valence and spin state evolution of Mn and Co on TbMn$_{rm 1-x}$Co$_{rm x}$O$_3$ series is precisely determined by means of soft and hard x-ray absorption spectroscopy (XAS) and K$beta$ x-ray emission spectroscopy (XES). Our results show the change from Mn$^{3+}$ to Mn$^{4+}$ both high-spin (HS) together with the evolution from Co$^{2+}$ HS to Co$^{3+}$ low-spin (LS) with increasing $rm x$. In addition, high energy resolution XAS spectra on the K pre-edge region are interpreted in terms of the strong charge transfer and hybridization effects along the series. These results correlate well with the spin values of Mn and Co atoms obtained from the K$beta$ XES data. From this study, we determine that Co enters into the transition metal sublattice of TbMnO$_3$ as a divalent ion in HS state, destabilizing the Mn long range magnetic order since very low doping compositions (${rm x} le 0.1$). Samples in the intermediate composition range ($0.4 le {rm x} le 0.6$) adopt the crystal structure of a double perovskite with long range ferromagnetic ordering which is due to Mn$^{4+}$-O-Co$^{2+}$ superexchange interactions with both cations in HS configuration. Ferromagnetism vanishes for ${rm x} ge 0.7$ due to the structural disorder that collapses the double perovskite structure. The spectroscopic techniques reveal the occurrence of Mn$^{4+}$ HS and a fluctuating valence state Co$^{2+}$ HS/Co$^{3+}$ LS in this composition range. Disorder and competitive interactions lead to a magnetic glassy behaviour in these samples.
The interplay between the nematic order and magnetism in FeSe is not yet well understood. There is a controversy concerning the relationship between orbital and spin degrees of freedom in FeSe and their relevance for superconductivity. Here we investigate the effect of S substitution on the nematic transition temperature ($T_{rm n}$) and the low-energy spin fluctuations (SF) in FeSe single crystals. We show that the low-energy SF emerge below the nematic transition. The difference between the onset temperature for the critical SF ($T_{rm SF}$) and $T_{rm n}$ is small for FeSe but significantly increases with S substitution. Below $T_{rm SF}$ the Korringa relation is violated and the effective muon hyperfine coupling constant changes a sign. Our results exclude a direct coupling of the low-energy SF to the electronic nematic order indicating a presence of multiple spin degrees of freedom in FeSe$_{rm 1-x}$S$_{rm x}$.
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