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The magnetic and superconducting properties of a series of underdoped $Ba_{1-x}Na_{x}Fe_{2}As_{2}$ (BNFA) single crystals with $0.19 \leq x\leq 0.34$ has been investigated with the complementary muon-spin-rotation ($\mu$SR) and infrared spectroscopy techniques. The focus has been on the different antiferromagnetic states in the underdoped regime and their competition with superconductivity, especially for the ones with a tetragonal crystal structure and a so-called double-$Q$ magnetic order. Besides the collinear state with a spatially inhomogeneous spin-charge-density wave (i-SCDW) order at $x=0.24$ and $0.26$, that was previously identified in BNFA, we obtained evidence for an orthomagnetic state with a "hedgehog"-type spin vortex crystal (SVC) structure at $x=0.32$ and $0.34$. Whereas in the former i-SCDW state the infrared spectra show no sign of a superconducting response down to the lowest measured temperature of about 10K, in the SVC state there is a strong superconducting response similar to the one at optimum doping. The magnetic order is strongly suppressed here in the superconducting state and at $x=0.34$ there is even a partial re-entrance into a paramagnetic state at $T<

Fe$^{3+}$ $S = 5/2$ ions form saw-tooth like chains along the $a$ axis of the oxo-selenite Fe$_2$O(SeO$_3$)$_2$ and an onset of long-range magnetic order is observed for temperatures below $T_C = 105$ K. This order leads to distinct fingerprints in phonon mode linewidths and energies as resolved by Raman scattering. In addition, new excitations with small linewidths emerge below $T = 150$ K, and are assigned to two-magnon scattering processes with the participation of flat-band and high energy magnon branches. From this a set of exchange coupling constants is estimated. The specific ratio of the saw-tooth spine-spine and spine-vertex interactions may explain the instability of the dimer quantum ground state against an incommensurate 3D magnetic order.

With muon spin rotation ($ \mu $SR) we studied the transition between the orthorhombic antiferromagnetic (o-AF) and the tetragonal antiferromagnetic (t-AF) states of a weakly underdoped Ba$ _{1-x} $K$ _{x} $Fe$ _{2} $As$ _{2} $ single crystal. We observed some characteristic changes of the magnitude and the orientation of the magnetic field at the muon site which, due to the fairly high point symmetry of the latter, allow us to identify the magnetic structure of the t-AF state. It is the so-called, inhomogeneous double-$\mathbf{Q}$ magnetic structure with $ c $-axis oriented moments which has a vanishing magnetic moment on half of the Fe sites., Comment: 5 pages, 4 figures. Supplementary Material: 8 figures

The low-dimensional s=1/2 compound (NO)[Cu(NO3)3] has recently been suggested to follow the Nersesyan-Tsvelik model of coupled spin chains. Such a system shows unbound spinon excitations and a resonating valence bond ground state due spin frustration. Our Raman scattering study demonstrates phonon anomalies as well as the suppression of a broad magnetic scattering continuum for temperatures below a characteristic temperature, T

The topological insulator Bi2Se3 shows a Raman scattering response related to topologically protected surface states amplified by a resonant interband transition. Most significantly this signal has a characteristic Lorentzian lineshape and spin-helical symmetry due to collision dominated scattering of Dirac states at the Fermi level E_F on bulk valence states. Its resonance energy, temperature and doping dependence points to a high selectivity of this process. Its scattering rate (Gamma=40 cm-1=5 meV) is comparable to earlier observations, e.g. in spin-polaron systems. Although the observation of topological surface states in Raman scattering is limited to resonance conditions, this study represents a quite clean case which is not polluted by symmetry forbidden contributions from the bulk, Comment: 12 pages, 5 figures, 1 table

We report zero field muon spin relaxation (muSR) measurements on RFeAsO with R = La, Ce, Pr, and Sm. We study the interaction of the FeAs and R (rare earth) electronic systems in the non superconducting magnetically ordered parent compounds of RFeAsO{1-x}Fx superconductors via a detailed comparison of the local hyperfine fields at the muon site with available Moessbauer spectroscopy and neutron scattering data. These studies provide microscopic evidence of long range commensurate magnetic Fe order with the Fe moments not varying by more than 15 % within the series RFeAsO with R = La, Ce, Pr, and Sm. At low temperatures, long range R magnetic order is also observed. Different combined Fe and R magnetic structures are proposed for all compounds using the muon site in the crystal structure obtained by electronic potential calculations. Our data point to a strong effect of R order on the iron subsystem in the case of different symmetry of Fe and R order parameters resulting in a Fe spin reorientation in the R ordered phase in PrFeAsO. Our symmetry analysis proves the absence of collinear Fe--R Heisenberg interactions in RFeAsO. A strong Fe--Ce coupling due to non--Heisenberg anisotropic exchange is found in CeFeAsO which results in a large staggered Ce magnetization induced by the magnetically ordered Fe sublattice far above T_N{Ce}. Finally, we argue that the magnetic R--Fe interaction is probably not crucial for the observed enhanced superconductivity in RFeAsO{1-x}Fx with a magnetic R ion., Comment: 21 pages, 12 figures, text modified, citations updated, accepted for publication in Physical Review B

Raman scattering in the spin-crossover system [Fe(pmd)(H2O){Au(CN)2}2]*H2O reveals a complex three-phase spin-state transition in contrast to earlier observations in magnetization measurements. We observe different spin state phases as function of temperature and electromagnetic radiation in the visible spectral range. There exists a fluctuating spin state phase with an unexpected wipeout of the low frequency phonon scattering intensity. Furthermore we observe one phase with reduced symmetry that is attributed to a cooperative Jahn-Teller effect. Pronounced electron-phonon interaction manifests itself as a strong Fano-resonance of phonons related to {FeN6} and {FeN4O2} coordination octahedra. Density functional theory supports this interpretation., Comment: 9 pages, 9 figures, 3 tables

We report magnetic susceptibility, specific heat, and Raman scattering investigations of alpha-TeVO4 containing V-O edge-sharing chains. These chains promote a system of ferromagnetic/antiferromagnetic spin-1/2 Heisenberg alternating exchange chains with pronounced spin frustration. The magnetic susceptibility and Raman scattering evidence a crossover at T* = 85 K with different slopes of the reciprocal susceptibility and a magnetic phase transition into a long-range-ordered state at Tc = 16 K. From Raman scattering data a strong mutual coupling between lattice and magnetic degrees of freedom is deduced. A comparison to model calculations and prior Raman scattering on other chain systems yield a plausible interpretation of the microscopic mechanism for the crossover behavior.

We report muon spin relaxation measurements on the magnetic structures of RBaCo_2O_5.5 with R=Y, Tb, Dy, and Ho. Three different phases, one ferrimagnetic and two antiferromagnetic, are identified below 300 K. They consist of different ordered spin state arrangements of high-, intermediate-, and low-spin Co^3+ of CoO_6 octahedra. Phase separation into well separated regions with different spin state order is observed in the antiferromagnetic phases. The unusual strongly anisotropic magnetoresistance and its onset at the FM-AFM phase boundary is explained., Comment: 4 pages, accepted for publication in Phys. Rev. Lett

Raman scattering experiments on CdCr2S4 single crystals show pronounced anomalies in intensity and frequency of optical phonon modes with an onset temperature T*=130 K that coincides with the regime of giant magnetocapacitive effects. A loss of inversion symmetry and Cr off-centering are deduced from the observation of longitudinal optical and formerly infrared active modes for T

Giant softening by 30 cm$^{-1}$ of the 490- and 620-cm$^{-1}$ modes is observed by Raman scattering measurements below the Curie temperature of single crystalline LaMnO$_{3+\delta}$ ($0.085\leq \delta\leq0.125$). A pseudogap-like electronic continuum and a Fano antiresonance at 144 cm$^{-1}$ appear below the charge ordering temperature. This gives evidence for the presence of an orbital fluctuating state and the formation of a hole stripe, respectively. This is further corroborated by a unstructured broadening and shifting of multiphonon features with increasing doping $\delta$. Our study suggests the significance of double exchange mechanism in the charged ordered insulating state., Comment: 4 pages, 5 figures, 1 table

We report on Raman scattering measurements of single crystalline La$_{1-x}$Sr$_x$MnO$_3$ ($x$=0, 0.06, 0.09 and 0.125), focusing on the high frequency regime. We observe multi-phonon scattering processes up to fourth-order which show distinct features: (i) anomalies in peak energy and its relative intensity and (ii) a pronounced temperature-, polarization-, and doping-dependence. These features suggest a mixed orbiton-phonon nature of the observed multi-phonon Raman spectra., Comment: 6pages, 6figures, submitted to PRB

We report a neutron and Raman scattering study of a single-crystal of La(2)CuO(4.05) prepared by high temperature electrochemical oxidation. Elastic neutron scattering measurements show the presence of two phases, corresponding to the two edges of the first miscibility gap, all the way up to 300 K. An additional oxygen redistribution, driven by electronic energies, is identified at 250 K in Raman scattering (RS) experiments by the simultaneous onset of two-phonon and two-magnon scattering, which are fingerprints of the insulating phase. Elastic neutron scattering measurements show directly an antiferromagnetic ordering below a N\'eel temperature of T_N =210K. The opening of the superconducting gap manifests itself as a redistribution of electronic Raman scattering below the superconducting transition temperature, T_c = 24K. A pronounced temperature-dependent suppression of the intensity of the (100) magnetic Bragg peak has been detected below T_c. We ascribe this phenomenon to a change of relative volume fraction of superconducting and antiferromagnetic phases with decreasing temperature caused by a form of a superconducting proximity effect., Comment: 9 pages, including 9 eps figures, submitted to PRB

The Raman-active phonons in perovskite-like LaMn1-xCoxO3 (x = 0, 0.2, 0.3, 0.4, and 1.0) were studied by measuring Raman spectra at temperatures of 295 and 5 K. The changes in the spectra with Co doping are correlated with the decrease of orthorhombic distortions. Surprisingly more phonon lines than allowed for the rhombohedral LaCoO3 structure were observed in the spectra., Comment: 8 pages, 4 figures, 1 table

We present an inelastic light scattering study of single crystalline (La$_{1-y}$Pr$_y$)$_{1-x}$Sr$_{x}$MnO$_3$ ($0\leq x\leq0.14$,$y=0$ and $x=1/8$,$0\leq y\leq0.5$). A giant softening up to 20 - 30 cm$^{-1}$ of the Mn-O breathing mode has been observed only for the ferromagnetic insulating (FMI) samples ($0.11\leq x \leq 0.14$) upon cooling below the Curie temperature. With increasing Pr-doping the giant softening is gradually suppressed. This is attributed to a coupling of the breathing mode to orbital polarons which are present in the FMI phase., Comment: 4 pages, 5 figures

The NQR on Ba nuclei was studied in four samples of BaBiO3 prepared in different ways and, in addition, in BaPbO3. The spectrum of $^{137}$Ba at T=4.2 K consists of relatively broad line centered near 18 MHz for all BaBiO3 samples and near 13 MHz for BaPbO3. The integrated intensity of $^{137}$Ba resonance in ceramic sample BaBiO3 synthesized at 800 C is approximately two times larger than in ceramic samples and single crystal prepared at 930-1080 C. The decrease of the NQR signal can be attributed to the partial disordering of charge disproportionated Bi ions on the two inequivalent sites. The broadening of the resonance indicates that local distortions of crystal structure exist in both compounds. The point charge model was used to analyse the electric field gradient on the Ba sites, Comment: 6 pages, 5 figures, 2 tables, RevTex, twocolumn

The processes of Coulomb gas ordering in 3D layered system are studied by means of Brownian dynamics approach. It is found that at different densities of the carriers the 3D lattice of charges as well as new specific structures are possible in the system. In particular, at some small density the particles inside the layers can associate into droplets that collectively repulse between neighbouring layers. These droplets possess local stripe structure which orders spontaneously along arbitrary chosen direction. At higher densities a specific ordering of the charges into the tetragonal-like or hexagonal-like structures is observed visually and described numerically. Specific ''pairing'' of the charges from different layers plays an essential role in formation of all above structures. It is essentially many-body effect in 3D system which can be a reason for unusual properties of layered crystals., Comment: 11 pages, 6 figures, manuscript, ReVTeX

We report Raman light-scattering and optical conductivity measurements on a single crystal of La(1.775)Sr(0.225)NiO(4) which exhibits incommensurate charge-stripe order. The extra phonon peaks induced by stripe order can be understood in terms of the energies of phonons that occur at the charge-order wave vector, Q_c. A strong Fano antiresonance for a Ni-O bond-stretching mode provides clear evidence for finite dynamical conductivity within the charge stripes., Comment: 4 pages, 2-col style, including 5 eps figures; Fig.2 replaced with new Raman data plus some small changes to the text