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Electron Spin Resonance (ESR) experiments of diluted Nd$^{3+}$ ions in the claimed topological insulator (TI) YBiPt are reported. Powdered samples with grain size from $\approx$ 100 $\mu$m to $\approx$ 2,000 $\mu$m were investigated. At low temperatures, 1.6 K $\lesssim$ \emph{T} $\lesssim$ 20 K, the X-band ($9.4$ GHz) ESR spectra show a \emph{g}-value of 2.66(4) and a Dysonian resonance lineshape which shows a remarkably unusual temperature, concentration, microwave power and particle size dependence. These results indicate that metallic and insulating behavior coexist within a skin depth of $\delta \approx$ 15 $\mu$m. Furthermore, the Nd$^{3+}$ spin dynamics in YBiPt are consistent with the existence of a \emph{phonon-bottleneck process} which allows the energy absorbed by the Nd$^{3+}$ ions at resonance to reach the thermal bath via the conduction electrons in the metallic surface states of YBiPt. These results are discussed in terms of the claimed topological semi-metal properties of YBiPt., Comment: 8 pages, 10 figures

We report electron spin resonance (ESR) measurements in the Gd3+ doped semiconducting filled skutterudite compound Ce1-xGdxFe4P12 (x = 0.001). As the temperature T varies from T = 150 K to T = 165 K, the Gd3+ ESR fine and hyperfine structures coalesce into a broad inhomogeneous single resonance. At T = 200 K the line narrows and as T increases further, the resonance becomes homogeneous with a thermal broadening of 1.1(2) Oe/K. These results suggest that the origin of these features may be associated to a subtle interdependence of thermally activated mechanisms that combine: i) an increase with T of the density of activated conduction-carriers across the T-dependent semiconducting pseudogap; ii) the Gd3+ Korringa relaxation process due to an exchange interaction, J_{fd}S.s, between the Gd3+ localized magnetic moments and the thermally activated conduction-carriers and; iii) a relatively weak confining potential of the rare-earth ions inside the oversized (Fe2P3)4 cage, which allows the rare-earths to become rattler Einstein oscillators above T = 148 K. We argue that the rattling of the Gd3+ ions, via a motional narrowing mechanism, also contributes to the coalescence of the ESR fine and hyperfine structure., Comment: 7 pages, 9 figures, accepted for publication in Phys Rev B

We report evidence for a close relation between the thermal activation of the rattling motion of the filler guest atoms, and inhomogeneous spin dynamics of the Eu2+ spins. The spin dynamics is probed directly by means of Eu2+ electron spin resonance (ESR), performed in both X-band (9.4 GHz) and Q-band (34 GHz) frequencies in the temperature interval 4.2 < T < 300 K. A comparative study with ESR measurements on the Beta-Eu8Ga16Ge30 clathrate compound is presented. Our results point to a correlation between the rattling motion and the spin dynamics which may be relevant for the general understanding of the dynamics of cage systems., Comment: 6 pages, 4 figures, accepted for publication in Phys. Rev. B

Electron Spin Resonance (ESR) measurements performed on the filled skutterudite system Ce1-x$YbxFe4P12 (x< 0.003) unequivocally reveal the coexistence of two Yb3+ resonances, associated with sites of considerably different occupations and temperature behaviors. Detailed analysis of the ESR data suggests a scenario where the fraction of oversized (Fe2P3)4 cages that host Yb ions are filled with a low occupation of on-center Yb3+ sites and a highly occupied T-dependent distribution of off-center Yb3+ sites. Analysis of the 171Yb3+ (I=1/2) isotope hyperfine splittings reveal that these two sites are associated with a low (~ 1 GHz) and a high (> 15 GHz) rattling frequency, respectively. Our findings introduce Yb3+ in Th symmetry systems and uses the Yb3+ ESR as a sensitive microscopic probe to investigate the Yb3+ ions dynamics., Comment: 5 pages, 3 figures, final version publisehd in PRB

In this work we report electron spin resonance (ESR) measurements in the semiconducting Ce1-xGdxFe4P12 (x ~ 0.001) filled skutterudite compounds. Investigation of the temperature (T) dependence of the ESR spectra and relaxation process suggests, that in the T-interval of 140-160 K, the onset of a "weak" metal-insulator (M-I) transition takes place due to the increasing density of thermally activated carriers across the semiconducting gap of ~ 1500 K. In addition, the observed low-T fine and hyperfine structures start to collapse at ~ 140 K and is completely absent for > 160 K. We claim that the increasing carrier density is able to trigger the rattling of the Gd3+ ions which in turn is responsible, via a motional narrowing mechanism, for the collapse of the ESR spectra., Comment: 3 pages, 3 figures, presented at QCNP2009, to appear in pssb

Electron Spin Resonance (ESR) can microscopically probe both conduction electrons (ce) and local moment (LM) spin systems in different materials. A ce spin resonance (CESR) is observed in metallic systems based on light elements or with enhanced Pauli susceptibility. LM ESR is frequently seen in compounds with paramagnetic ions and localized d or f electrons. Here we report a remarkable and unprecedented ESR signal in the heavy fermion (HF) superconductor beta-YbAlB4[1] which behaves as a CESR at high temperatures and acquires characteristics of the Yb3+ LM ESR at low temperature. This dual behavior in same ESR spectra strikes as an in situ unique observation of the Kondo quasiparticles giving rise to a new ESR response called Kondo coupled resonant mode (KCRM). The proximity to a quantum critical point (QCP) may favor the observation of a KCRM and its dual character in beta-YbAlB4 may unveil the 4f-electrons nature at the QCP.

Despite extensive research on the skutterudites for the last decade, their electric crystalline field ground state is still a matter of controversy. We show that Electron Spin Resonance (ESR) measurements can determine the full set of crystal field parameters (CFPs) for the Th cubic symmetry (Im3) of the Ce$_{1-x}$R$_{x}$Fe$_{4}$P$_{12}$ (R = Dy, Er, Yb, $x\lesssim 0.003$) skutterudite compounds. From the analysis of the ESR data the three CFPs, B4c, B6c and B6t were determined for each of these rare-earths at the Ce$^{3+}$ site. The field and temperature dependence of the measured magnetization for the doped crystals are in excellent agreement with the one predicted by the CFPs Bnm derived from ESR., Comment: 7 pages, 5 figures, to appear in PRB

In this work we present a systematic Raman Scattering study in the Ca$_{1-x}$% Eu$_{x}$B$_{6}$ series ($0.00\leqslant x\leqslant 1.00$). Our results are the first experimental evidence for the occurrence of a local symmetry break of the crystalline structure in this system. The local symmetry of some Boron octahedra is tetragonal instead of cubic. This result may explain the appearance of ferromagnetism in Eu-hexaborades since magnetic ordering is forbidden in the $O_{h}$ space group of this system., Comment: Submitted to Phys. Rev. Lett

The local environment of Eu$^{2+}$ ($4f^{7}$, $S=7/2$) in Ca$_{1-x}$Eu$_{x}$B$_{6}$ ($0.003\leqslant x\leqslant 1.00$) is investigated by means of electron spin resonance (ESR). For $x\lesssim 0.003$ the spectra show resolved \textit{fine} and \textit{hyperfine} structures due to the cubic crystal \textit{electric} field and nuclear \textit{hyperfine} field, respectively. The resonances have Lorentzian line shape, indicating an \textit{insulating} environment for the Eu$^{2+}$ ions. For $0.003\lesssim x\lesssim 0.07$, as $x$ increases, the ESR lines broaden due to local distortions caused by the Eu/Ca ions substitution. For $0.07\lesssim x\lesssim 0.30$, the lines broaden further and the spectra gradually change from Lorentzian to Dysonian resonances, suggesting a coexistence of both \textit{insulating} and \textit{metallic} environments for the Eu$^{2+}$ ions. In contrast to Ca$_{1-x}$Gd$_{x}$B$_{6}$, the \textit{fine} structure is still observable up to $x\approx 0.15$. For $x\gtrsim 0.30 $ the \textit{fine} and \textit{hyperfine} structures are no longer observed, the line width increases, and the line shape is purely Dysonian anticipating the \textit{semimetallic} character of EuB$_{6}$. This broadening is attributed to a spin-flip scattering relaxation process due to the exchange interaction between conduction and Eu$^{2+}$ $4f$ electrons. High field ESR measurements for $x\gtrsim 0.15$ reveal smaller and anisotropic line widths, which are attributed to magnetic polarons and Fermi surface effects, respectively., Comment: Submitted to PRB

We have studied the low-temperature behavior of the magnetization and the specific heat of the bilayered perovskite system $% La_{2-2x}Sr_{1+2x}Mn_{2}O_{7}$, for $0.30\leq x\leq 0.50$. Our analysis reveals that below 30 K the temperature dependence of the magnetization of the ferromagnetic samples, $x=0.30$, 0.32, and 0.36, in a field of 1T can be interpreted in terms of the thermal excitations of a two-dimensional gas of ferromagnetic magnons. The specific heat in zero field for these samples as for the $x=0.50$ antiferromagnetic one, is linear with temperature between the range of 1.8K $\leq $ T $\leq $ 10K. This behavior can be also explained by the magnon gas model. By comparing specific heat measurements in zero field with those taken in a field of 9T we are able to extract the lattice and electronic contributions and determine the in-plane exchange interactions. That are found to be in reasonable agreement with the values inferred from the analysis of the magnetization data and also with the values reported by inelastic neutron scattering studies. In addition, we found that the electronic density of states obtained for the $x=0.50$ sample is in agreement with previous band structure calculations., Comment: Resubmited to Phys. Rev. B. Changes: correction of some grammatical errors, revision of figures 1,4 and 9, correction of J^{prime} definition, revision of all Tables, new estimaiton of errors, and 3 new references

Neutron-diffraction measurements in LaCrSb{3} show a coexistence of ferromagnetic and antiferromagnetic sublattices below Tc=126 K, with ordered moments of 1.65(4) and 0.49(4) Bohr magnetons per formula unit, respectively (T=10 K), and a spin reorientation transition at ~95 K. No clear peak or step was observed in the specific heat at Tc. Coexisting localized and itinerant spins are suggested., Comment: PRL, in press

The local environment of Gd3+ (4f7 S=7/2) ions in single crystals of Ca1-xGdxB6 (0.0001 < x < 0.01) is studied by means of Electron Spin Resonance (ESR). The spectra for low concentration samples (x < 0.001) show a split spectrum due to cubic crystal field effects(CFE). The lineshape of each fine structure line is lorentzian, indicating an insulating environment for the Gd3+ ions. For higher concentrations (0.003 < x < 0.01), the spectra show a single resonance (g=1.992(4), DH1/2 ~ 30-60 Oe) with no CFE and dysonian lineshape indicating metallic environment for the Gd3+ ions. For intermediate concentrations, a coexistence of spectra corresponding to insulating and metallic regions is observed. Most of the measured samples show the weak ferromagnetism (WF) as reported for Ca1-xLaxB6 (x ~ 0.005), but, surprisingly, this WF has no effect in our ESR spectra either for metallic or insulating environments. This result suggests that the ferromagnetism in these systems might be isolated in clusters (defect-rich regions) and its relationship with metallicity is nontrivial., Comment: 5 pages, 3 figures, 27 references

The $T$-dependence (2- 400 K) of the electron paramagnetic resonance (EPR), magnetic susceptibility, $\chi (T)$, and specific heat, $C_{v}(T)$, of the $normal$ antiferromagnetic (AFM) spinel ZnCr$_{2}$O$_{4}$ and the spin-glass (SG) Zn$_{1-x}$Cd$_{x}$Cr$_{2}$O$_{4}$ ($x=0.05,0.10$) is reported. These systems behave as a strongly frustrated AFM and SG with $% T_{N}$ $ \approx T_{G}\approx 12$ K and -400 K $\gtrsim \Theta_{CW}\gtrsim -500$ K. At high-$T$ the EPR intensity follows the $\chi (T)$ and the $g$-value is $T$-independent. The linewidth broadens as the temperature is lowered, suggesting the existence of short range AFM correlations in the paramagnetic phase. For ZnCr$_{2}$O$_{4}$ the EPR intensity and $\chi (T)$ decreases below 90 K and 50 K, respectively. These results are discussed in terms of nearest-neighbor Cr$^{3+}$ (S $=3/2$%) spin-coupled pairs with an exchange coupling of $| J/k| \approx $ 50 K. The appearance of small resonance modes for $T\lesssim 17$ K, the observation of a sharp drop in $\chi (T)$ and a strong peak in $C_{v}(T)$ at $T_{N}=12$ K confirms, as previously reported, the existence of long range AFM correlations in the low-$T$ phase. A comparison with recent neutron diffraction experiments that found a near dispersionless excitation at 4.5 meV for $T\lesssim T_{N}$ and a continuous gapless spectrum for $T\gtrsim T_{N}$, is also given., Comment: 17 pages, 8 figures, 1 Table. Submitted to Physical Review B