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The phonon renormalization across the semiconductor-to-metal crossover in FeSi is investigated by inelastic neutron scattering combined with \textit{ab-initio} lattice dynamical calculations. A significant part of reciprocal space with a particular focus on the 110$-$001 scattering plane is mapped by the time-of-flight inelastic neutron scattering data taken below and above the crossover. Individual momentum values are investigated in more detail as a function of temperature. The data reveal that the anomalous phonon softening upon metallization is not exclusive to the high symmetry $R$ and $\Gamma$ points. Several other phonon modes around the $R$-point as well as the phonon modes at the $M$ and $X$ points of the Brillouin zone exhibit anomalous phonon softening with magnitudes comparable to that observed at the $R$-point. The momentum dependence of the phonon softening is reproduced by the lattice dynamical calculation based on the density functional perturbation theory. We discuss our findings with respect to the nature of the semiconductor-to-metal crossover in FeSi, for which different microscopic origins have been proposed, i.e., lattice thermal disorder and electronic correlation effects., Comment: 11 pages, 9 figures

The origin of the effect of ``colossal magnetoresistance'' (CMR) remains still unexplained. In this work we revisit the spin dynamics of the pseudocubic ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ along the Mn-O-Mn bond direction at four $x$ doping values ($x\ensuremath{\le}0.5$) and various temperatures and report a lattice dynamics study at ${x}_{0}=0.2$, representative of optimal doping for CMR. We propose an interpretation of the spin dynamics in terms of orbital polarons. This picture is supported by the observation of a discrete magnetic energy spectrum ${E}_{n}^{\mathrm{mag}}(q)$ with $n$ levels, characteristic of the internal excitations of ``orbital polarons'' defined by ${\mathrm{Mn}}^{3+}$ neighbors surrounding a central ${\mathrm{Mn}}^{4+}$ hole. Because of its hopping, the hole mixes dynamically all the possible orbital configurations of its surrounding ${\mathrm{Mn}}^{3+}$ with degenerate energies. The ${E}_{n}^{\mathrm{mag}}$ values indicate a lifting of orbital degeneracy by phonon excitations. The $n$ value and the $q$ range are used to characterize these orbital polarons in direct space. At $x=0.125$ and $x=0.3$ the spectrum reveals two-dimensional polarons coupled by exchange and three-dimensional ``free'' polarons, respectively, with sizes $\ensuremath{\ell}=1.67a\ensuremath{\le}2a$ in all bond directions. At ${x}_{0}=0.2$, the spin and the lattice dynamics provide evidence for chains of orbital polarons of size $\ensuremath{\ell}=2a$ with a periodic distribution over $\ensuremath{\approx}3a$ and an interaction energy $\ensuremath{\approx}3$ meV. At $T\ensuremath{\le}{T}_{c}$ the charges propagate together with the longitudinal acoustic phonons along the chains enhancing their ferromagnetic character. The phase separation between metallic and ferromagnetic chains in a nonmetallic matrix may be crucial for CMR.

According to standard theory the magnetoresistance magnitude in ferromagnetic manganites crucially depends on the electron-phonon coupling strength. We showed that in La0.7Sr0.3MnO3 the phonon renormalization is strong, despite its relatively small magnetoresistance. Here, we report results of a similar inelastic neutron-scattering investigation of a closely related compound, La0.8Sr0.2MnO3, where the magnetoresistance is enhanced. We find similar phonon renormalization and dynamic CE-type polaron correlations as in La0.7Sr0.3MnO3. However, quantitative comparison of the results for the two samples shows that only polaron lifetime is well correlated with the strength of the colossal magnetoresistance.

Well defined peaks with energies of about 18 meV have been observed in a variety of inelastic neutron scattering experiments on both single crystals and powders, using either the triple-axis or the time-of-flight technique. They can easily be mistaken for signatures of real excitations. It has been found that they are due to multiple scattering events involving primarily the walls of the sample environment. Hence, they are particularly troublesome in experiments using very small samples, as have been used with recently developed high-intensity neutron spectrometers. Measures required to reduce the unwanted scattering to a minimum are also discussed.

Among colossal magnetoresistive manganites the prototypical ferromagnetic manganite $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ has a relatively small magnetoresistance, and has been long assumed to have only weak electron-lattice coupling. Here we report that $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ has strong electron-phonon coupling: Our neutron and x-ray scattering experiments show strong softening and broadening of transverse acoustic phonons on heating through the Curie temperature ${T}_{C}=350\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Simultaneously, we observe two phases where metallic resistivity and polarons coexist. The ferromagnetic polaronic metal phase between $200\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{C}$ is characterized by quasielastic scattering from dynamic CE-type polarons with the relatively short lifetime of $\ensuremath{\tau}\ensuremath{\approx}1\phantom{\rule{0.16em}{0ex}}\mathrm{ps}$. This scattering is greatly enhanced above ${T}_{C}$ in the paramagnetic polaronic metal phase. Our results suggest that the strength of magnetoresistance in manganites scales with the inverse of the polaron lifetime, not the strength of electron-phonon coupling.

The proton conducting perovskite MZr1−xLnxO3−δHz ceramics are promising electrolytic membranes for fuel cell and water steam electrolyser applications. Simultaneous elastic/quasielastic and diffraction neutron studies were performed in a wide temperature range (25–1150 °C) on protonated Yb-modified BaZrO3 ceramics: dense (97% of theoretical density) and ultradense (99%) using the triple axis spectrometers. The results allowed us to determine: (i) the real content of bulk protonic species ∼1–5 10−3 mol/mol, (ii) the structural modifications caused by the proton doping, and (iii) the bulk proton dynamics. The quasielastic neutron scattering (QNS) results are discussed in the light of neutron diffraction, conductivity, Raman, thermogravimetric, and thermal expansion measurements. The highest bulk proton motion appears in the temperature range where the structural modifications and the energy activation changes are detected. This allows defining the optimum temperature range for the proton dynamics between 400 and 560 °C.

Quasi-elastic neutron scattering, neutron diffraction as well as in situ and ex situ Raman studies were performed on the proton conducting (Ba/Sr)Zr 1- x Ln x O 3-δ dense ceramics. The results confirm the homogeneous distribution of the bulk protonic species, their variable content as a function of Ln content and sample processing as well as successive re-orientation local motion between 400 and 600 °C. The analysis of the elastic incoherent structure factor suggests the complex motion of proton between different sites and with different length jumps. The proton dynamics are accompanied by the presence of structural phase transitions near 350/450 and 550 °C as well as by the changes of conductivity activation energy.

Inelastic x-ray scattering (IXS) was used to study the Cu-O bond-stretching vibrations in the static stripe phase compound La1.48Nd0.4Sr0.12CuO4. It was found that the intrinsic width in Q-space of the previously reported huge anomalous phonon softening and broadening is approximately 0.08r.l.u HWHM. A detailed comparison was also made to inelastic neutron scattering (INS) studies, which indicate a two-peak lineshape (with superimposed broad and narrow peaks) in the vicinity of the anomaly. The high resolution IXS data show that the narrow peak is mostly an artifact of the poor transverse Q-resolution of INS. Otherwise the agreement between the INS and IXS was excellent., Comment: 8 pages 3 figures SNS2007 conference proceedings; version 2: minor changes throughout the text

H+-containing lanthanide-doped perovskites A(Ba, Sr etc.)B(Zr, Ce, Ti etc.)O3 are potential ceramic membranes for fuel cell and medium temperature water electrolysis (300–800 °C). The comparison studies of the hydrated and non-hydrated Yb-doped BaZrO3 and SrZrO3 were performed by thermal expansion, medium–high temperature neutron and room-temperature high-pressure Raman scattering. Neutron diffraction and elastic/quasi-elastic studies carried out for BaZrO3 ceramic show the presence of the protons, their successive diffusion above ∼600 °C, and then their departure above 750 °C (under vacuum). Phase transitions and their modification by proton insertion are discussed. A high-pressure Raman study of SrZrO3 performed at room temperature in the diamond anvil cell reveals the presence of two pressure-induced phase transitions at about 5 and 22 GPa and confirms that proton insertion modifies the phase transition sequences.

Parent compounds of Fe-based superconductors undergo a structural phase transition from a tetragonal to an orthorhombic structure. We investigated the temperature dependence of the frequencies of TA phonons that extrapolate to the shear vibrational mode at the zone center, which corresponds to the orthorhombic deformation of the crystal structure at low temperatures in ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ and ${\mathrm{SrFe}}_{2}{\mathrm{As}}_{2}$. We found that acoustic phonons at small wave vectors soften gradually towards the transition from high temperatures, tracking the increase of the size of slowly fluctuating magnetic domains. On cooling below the transition to base temperature the phonons harden, following the square of the magnetic moment (which we find is proportional to the anisotropy gap). Our results provide evidence for close correlation between magnetic and phonon properties in Fe-based superconductors.

The single-layer perovskite cobaltates have attracted enormous attention due to the recent observation of hour-glass shaped magnetic excitation spectra which resemble the ones of the famous high-temperature superconducting cuprates. Here, we present an overview of our most recent studies of the spin and charge correlations in floating-zone grown cobaltate single crystals. We find that frustration and a novel kind of electronic and magnetic nano phase separation are intimately connected to the appearance of the hour-glass shaped spin excitation spectra. We also point out the difference between nano phase separation and conventional phase separation., * plenary talk SUPERSTRIPES conference 2015

Motivated by model calculations for the heavy fermion superconductor CePt3Si predicting phonon anomalies because of anti-symmetric spin-orbit coupling we performed a detailed experimental study of the lattice dynamical properties of CePt3Si. In particular, we investigated the dispersion of transverse acoustic and low energy optic phonon branches along the [110] direction using inelastic neutron scattering. In these branches, we found deviations from our ab-initio lattice dynamical calculations, which overall give a good description of the phonon dispersion in CePt3Si. However, the agreement for the [110] transverse modes can be improved if we neglect the Ce 4f states, done in an additional calculation. We conclude that the lattice dynamics of CePt3Si are conventional and that the observed deviations are not related to effects of anti-symmetric spin-orbit-coupling. More likely, ab-initio calculations overestimate the exchange between different phonon branches, particularly in the presence of 4f electron states. Our results imply that the ASOC plays less a role in non-centrosymmetric superconductors than commonly believed., Comment: accepted by PRB

We report neutron scattering measurements of the structural correlations associated with the apparent relaxor transition in ${\mathrm{K}}_{1\ensuremath{-}x}{\mathrm{Li}}_{x}{\mathrm{TaO}}_{3}$ for $x=0.02$ [KLT(0.02)]. This compound displays a broad and frequency-dependent peak in the dielectric permittivity, which is the accepted hallmark of all relaxors. However, no evidence of elastic diffuse scattering or any soft-mode anomaly is observed in KLT(0.02) [J. Wen et al., Phys. Rev. B 78, 144202 (2008)], a situation that diverges from that in other relaxors such as ${\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_{3}$. We resolve this dichotomy by showing that the structural correlations associated with the transition in KLT(0.02) are purely dynamic at all temperatures, having a time scale on the order of $\ensuremath{\sim}$ THz. These fluctuations are overdamped, nonpropagating, and spatially uncorrelated. Identical measurements made on pure ${\mathrm{KTaO}}_{3}$ show that they are absent (within experimental error) in the undoped parent material. They exhibit a temperature dependence that correlates well with the dielectric response, which suggests that they are associated with local ferroelectric regions induced by the ${\mathrm{Li}}^{+}$ doping. The ferroelectric transition that is induced by the introduction of ${\mathrm{Li}}^{+}$ cations is therefore characterized by quasistatic fluctuations, which represents a stark contrast to the soft-harmonic-mode-driven transition observed in conventional perovskite ferroelectrics such as ${\mathrm{PbTiO}}_{3}$. The dynamic, glasslike structural correlations in KLT(0.02) are much faster than those measured in random-field-based lead-based relaxors, which exhibit a frequency scale of order $\ensuremath{\sim}$ GHz and are comparatively better correlated spatially. Our results support the view that static random fields give rise to the relaxor phenomena, and that the glasslike dynamics observed here characterize a nascent response.

Inelastic neutron scattering was used to search for an influence of superconductivity on the phonons in optimally doped and in slightly overdoped Ba(Fe1−xCox)2As2, x = 0.06 and x = 0.10. The study focused on phonons with energies close to the superconducting gap energy 2Δ because it is well known that such phonons will respond most strongly to the opening of the gap. We were able to obtain high quality data but nevertheless, we could not detect any influence of superconductivity on the phonons, neither on the linewidths nor on the frequencies. Our results imply that any coupling of low energy phonons to the electrons has to be very small, much smaller than observed in conventional superconductors with a high Tc. Our results are in line with the low coupling strength predicted by density functional theory for the investigated phonon branches.

We present an inelastic neutron scattering study on single-crystalline LiFeAs devoted to the characterization of the incommensurate antiferromagnetic fluctuations at $\mathbf{Q}=(0.5\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta},0.5\ensuremath{\mp}\ensuremath{\delta},{q}_{l})$. Time-of-flight measurements show the presence of these magnetic fluctuations up to an energy transfer of 60 meV, while polarized neutrons in combination with a longitudinal polarization analysis on a triple-axis spectrometer prove the pure magnetic origin of this signal. The normalization of the magnetic scattering to an absolute scale yields that magnetic fluctuations in LiFeAs are by a factor of 8 weaker than the resonance signal in nearly optimally Co-doped ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$, although a factor of 2 is recovered due to the split peaks owing to the incommensurability. The longitudinal polarization analysis indicates weak spin-space anisotropy with a slightly stronger out-of-plane component between 6 and 12 meV. Furthermore, our data suggest a fine structure of the magnetic signal most likely arising from superposing nesting vectors.

Nature Communications 5: Article number: 3467 (2014); Published: 17 March 2014; Updated: 3 July 2014. One of the affiliations for D. Lamago was inadvertently omitted during the preparation of this Article, and is given below: Institute of Solid State Physics, Karlsruhe Institute of Technology, D-76021 Karlsruhe, Germany.

The magnetic excitations in the cuprate superconductors might be essential for an understanding of high-temperature superconductivity. In these cuprate superconductors the magnetic excitation spectrum resembles an hour-glass and certain resonant magnetic excitations within are believed to be connected to the pairing mechanism, which is corroborated by the observation of a universal linear scaling of superconducting gap and magnetic resonance energy. So far, charge stripes are widely believed to be involved in the physics of hour-glass spectra. Here we study an isostructural cobaltate that also exhibits an hour-glass magnetic spectrum. Instead of the expected charge stripe order we observe nano phase separation and unravel a microscopically split origin of hour-glass spectra on the nano scale pointing to a connection between the magnetic resonance peak and the spin gap originating in islands of the antiferromagnetic parent insulator. Our findings open new ways to theories of magnetic excitations and superconductivity in cuprate superconductors.

In superconducting copper oxides, some Cu-O bond-stretching phonons around 70 meV show anomalous giant softening and broadening of electronic origin, and electronic dispersions have large renormalization kinks near the same energy. These observations suggest that phonon broadening originates from quasiparticle excitations across the Fermi surface and the electronic dispersion kinks originate from coupling to anomalous phonons. We measured the phonon anomaly in underdoped ($x=0.05$) and overdoped ($x=0.20$ and 0.25) La${}_{2\ensuremath{-}x}$Sr${}_{x}$CuO${}_{4}$ by inelastic neutron and x-ray scattering with high resolution. Combining these and previously published data, we found that doping dependence of the magnitude of the giant phonon anomaly is very different from that of the ARPES kink, i.e., the two phenomena are not connected. We show that these results provide indirect evidence that the phonon anomaly originates from novel collective charge excitations as opposed to interactions with electron-hole pairs. Their amplitude follows the superconducting dome so these charge modes may be important for superconductivity.

We have determined the boundary of the magnetic A-phase of MnSi from distinct changes of slope in the AC susceptibility for magnetic field B applied parallel to the crystallographic 〈 1 0 0 〉 , 〈 1 1 0 〉 and 〈 1 1 1 〉 directions. The specific heat displays an anomaly of a few percent at the border of the A-phase characteristic of a well-defined phase transition. In addition to the previously reported change of orientation of the helical modulation in the A-phase, we observe a gradual appearance of a ring of neutron small-angle scattering intensity that extends over the field and temperature regime of the A-phase. The estimated integrated intensity of this ring is a sizable fraction of the intensity in the helically ordered state, thus indicating that the change of the magnetic structure near the A-phase cannot be interpreted in terms of an abrupt change of orientation of a simple helical modulation alone.

The crystallographic structure, magnetization, specific heat, and electrical resistivity of polycrystalline CeTi${}_{1\ensuremath{-}x}$V${}_{x}$Ge${}_{3}$ ($0\ensuremath{\le}x\ensuremath{\le}0.42$ and $x$ $=$ 1) have been studied. CeTiGe${}_{3}$ has been reported as one of the rare ferromagnetic Kondo systems with a Curie temperature ${T}_{c}$ $=$ 14 K. We present a detailed study of the low-temperature properties on this compound, including magnetization, specific heat, and electrical resistivity in magnetic fields up to 5 T. Elastic neutron-scattering experiments confirm ferromagnetic and exclude ferrimagnetic order of CeTiGe${}_{3}$. Analysis of the specific heat gives evidence of a strong magnetic anisotropy in this system as evidenced by a gap. Replacing Ti by V reduces ${T}_{c}$, with ${T}_{c}\ensuremath{\rightarrow}0$ for $x\ensuremath{\approx}0.35$, suggesting a possible ferromagnetic quantum critical point.

The insulator-to-metal transition continues to be a challenging subject, especially when electronic correlations are strong. In layered compounds, such as La2-xSrxNiO4 and La2-xBaxCuO4, the doped charge carriers can segregate into periodically-spaced charge stripes separating narrow domains of antiferromagnetic order. Although there have been theoretical proposals of dynamically fluctuating stripes, direct spectroscopic evidence of charge-stripe fluctuations has been lacking. Here we report the detection of critical lattice fluctuations, driven by charge-stripe correlations, in La2-xSrxNiO4 using inelastic neutron scattering. This scattering is detected at large momentum transfers where the magnetic form factor suppresses the spin fluctuation signal. The lattice fluctuations associated with the dynamic charge stripes are narrow in q and broad in energy. They are strongest near the charge stripe melting temperature. Our results open the way towards the quantitative theory of dynamic stripes and for directly detecting dynamical charge stripes in other strongly-correlated systems, including high-temperature superconductors such as La2-xSrxCuO4., Nature Communications, in press, 9 pages 10 figures, completely new version

The incommensurate stripelike magnetic ordering in two single-layered manganites, Nd0.33Sr1.67MnO4 and Pr0.33Ca1.67MnO4, is found to exhibit an hourglasslike excitation spectrum very similar to that seen in various cuprates superconductors, but only for sufficiently short correlation lengths. Several characteristic features of an hourglass dispersion can be identified: enhancement of intensity at the merging of the incommensurate branches, rotation of the intensity maxima with higher energy transfer, and suppression of the outward-dispersing branches at low energy. The correlation length of the magnetic ordering and the large ratio of intra- to interstripe couplings are identified as the decisive parameters causing the hourglass shape of the spectrum.

Magnetic correlations in superconducting LiFeAs were studied by elastic and by inelastic neutron-scattering experiments. There is no indication for static magnetic ordering, but inelastic correlations appear at the incommensurate wave vector ($0.5\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta},0.5\ensuremath{\mp}\ensuremath{\delta},0$) with $\ensuremath{\delta}\ensuremath{\sim}0.07$ slightly shifted from the commensurate ordering observed in other FeAs-based compounds. The incommensurate magnetic excitations respond to the opening of the superconducting gap by a transfer of spectral weight.

Purely local experimental probes of many copper oxide superconductors show that their electronic states are inhomogeneous in real space. For example, scanning tunneling spectroscopic imaging shows strong variations in real space, and according to nuclear quadrupole resonance (NQR) studies, the charge distribution in the bulk varies on the nanoscale. However, the analysis of the experimental results utilizing spatially averaged probes often ignores this fact. We have performed a detailed investigation of the doping dependence of the energy and linewidth of the zone-boundary Cu-O bond-stretching vibration in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ by inelastic neutron scattering. Both our results as well as previously reported angle-dependent momentum widths of the electronic spectral function detected by angle-resolved photoemission can be reproduced by including the same distribution of local environments extracted from the NQR analysis.

The Fe${}_{1+y}$Te${}_{1\ensuremath{-}x}$Se${}_{x}$ compounds belong to the family of iron-based high-temperature superconductors, in which superconductivity often appears upon doping antiferromagnetic parent compounds. Unlike other Fe-based superconductors (in which the antiferromagnetic order is at the Fermi-surface nesting wave vector $[\frac{1}{2},\frac{1}{2},1]$), Fe${}_{1+y}$Te orders at a different wave vector, $[\frac{1}{2},0,\frac{1}{2}]$. Furthermore, the ordering wave vector depends on $y$, the occupation of interstitial sites with excess iron; the origin of this behavior is controversial. Using inelastic neutron scattering on Fe${}_{1.08}$Te, we find incommensurate magnetic fluctuations above the N\'eel temperature, even though the ordered state is bicollinear and commensurate with gapped spin waves. This behavior can be understood in terms of a competition between commensurate and incommensurate order, which we explain as a lock-in transition caused by the magnetic anisotropy.

Motivated by predictions of a substantial contribution of the ``buckling'' vibration of the ${\mathrm{CuO}}_{2}$ layers to $d$-wave superconductivity in the cuprates, we have performed an inelastic neutron scattering study of this phonon in an array of untwinned crystals of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$. The data reveal a pronounced softening of the phonon at the in-plane wave vector $\mathbf{q}=(0,0.3)$ upon cooling below $\ensuremath{\sim}105\text{ }\text{ }\mathrm{K}$, but no corresponding anomaly at $\mathbf{q}=(0.3,0)$. Based on the observed in-plane anisotropy, we argue that the electron-phonon interaction responsible for this anomaly supports an electronic instability associated with a uniaxial charge-density modulation and does not mediate $d$-wave superconductivity.

We report inelastic neutron-scattering experiments on single crystals of Nd${}_{1\ensuremath{-}x}$Sr${}_{x}$MnO${}_{3}$ with $x=0.5$ and $x=0.49$. The spin-wave dispersion in the charge-, orbital-, and spin-ordered state in Nd${}_{0.5}$Sr${}_{0.5}$MnO${}_{3}$ exhibits a strongly anisotropic stiffness. The sign of the anisotropy is characteristic for the site-centered model for charge and orbital ordering in half doped manganites. Within this model, linear spin-wave theory yields a perfect description of the experimental dispersion. In the ferromagnetic metallic state of Nd${}_{1\ensuremath{-}x}$Sr${}_{x}$MnO${}_{3}$ with $x=0.49$ and $x=0.50$ magnetic excitations exhibit nearly the same magnon dispersion. High intense signals near the zone boundary over a wide energy level overlap with a sharp spin-wave dispersion, which can be described with a Heisenberg model including nearest-neighbor interactions.

The critical spin fluctuations in doped compounds Mn$_{1-y}$Fe$_y$Si have been studied by means of ac-susceptibility measurements, polarized neutron small angle scattering and spin echo spectroscopy. It is shown that these compounds undergo the transition from the paramagnetic to helimagnetic phase through continuous, yet well distinguishable crossovers: (i) from paramagnetic to partially chiral, (ii) from partially chiral to highly chiral fluctuating state. The crossover points are identified on the basis of combined analysis of the temperature dependence of ac-susceptibility and polarized SANS data. The whole transition is marked by two inflection point of the temperature dependence of ac-susceptibility: the upper one corresponds to the crossover to partially chiral state at $T^*$, where the inverse correlation length $\kappa \approx 2 k$, the lower one corresponds to the transition to the spin helix structure. The intermediate crossover to the highly chiral phase is observed at the inflection point $T_k$ of the first derivative of ac-susceptibility, where $\kappa \approx k$. The temperature crossovers to the highly chiral fluctuating state is associated with the enhancing influence of the Dzyaloshinskii-Moria interaction close to $T_c$.

Chiral nematic liquid crystals sometimes form blue phases characterized by spirals twisting in different directions. By combining model calculations with neutron-scattering experiments, we show that the magnetic analogue of blue phases does form in the chiral itinerant magnet MnSi in a large part of the phase diagram. The properties of this blue phase explain a number of previously reported puzzling features of MnSi such as partial magnetic order and a two-component specific-heat and thermal-expansion anomaly at the magnetic transition.

The phonon dispersion relations in MnSi were measured using inelastic neutron scattering. At the same time, calculations of the phonon spectra of MnSi were carried out in the framework of density functional theory in the local density and the generalized gradient approximations. The calculations match most of the phonon modes in the frequency range up to 40 meV. The phonon density of states obtained on the basis of the phonon dispersion was used to estimate the lattice contribution to the specific heat of MnSi.

The pressure dependence of a large number of phonon modes in ${\text{CaFe}}_{2}{\text{As}}_{2}$ with energies covering the full range of the phonon spectrum has been studied using inelastic x-ray and neutron scatterings. The pressure range was large enough to cover the first-order phase transition into the so-called collapsed phase where the $c$-axis contracts by about 6% whereas $a$ and $b$ axes expand by about 1.5%. Our main result is that pressure-induced phonon frequency shifts are well explained by the changes in relevant bond lengths throughout the pressure range, including those associated with the first-order phase transition. Specifically, the frequencies of phonons polarized in the $ab$ plane as well as the Fe-As bond lengths change little across the phase transition. On the other hand, the transverse-acoustic phonons propagating along the $c$ direction stiffen very significantly in response to the large contraction of the bonds along the $c$ axis. Nonmagnetic density-functional calculations describe the frequencies in both the zero pressure and in the collapsed phase in a satisfactory way if based on the respective experimental crystal structures. This suggests that there is no need to invoke changes in magnetic moments on Fe atoms to explain the pressure-induced frequency shifts.

X-ray diffraction using synchrotron radiation and polarized neutron small-angle diffraction have been used to evaluate the absolute crystallographic structure and the spin helix chirality of ${\text{Mn}}_{1\ensuremath{-}x}{\text{Fe}}_{x}\text{Si}$. Contrary to previous observations we show that left- and right-handed crystals can be found for MnSi and its iron substituted analogs. The structural chirality rigorously determines the magnetic chirality of these compounds: left-(right-)handed crystalline chirality establishes left (right) handedness of the magnetic helix.

We measured phonon frequencies and linewidths in doped and undoped ${\text{BaFe}}_{2}{\text{As}}_{2}$ single crystals by inelastic x-ray scattering and compared our results with density functional theory calculations. In agreement with previous work, the calculated frequencies of some phonons depended on whether the ground state was magnetic or not and, in the former case, whether phonon wave vector was parallel or perpendicular to the magnetic ordering wave vector. The experimental results agreed better with the magnetic calculation than with zero Fe moment calculations, except the peak splitting expected due to magnetic domain twinning was not observed. Furthermore, phonon frequencies were unaffected by the breakdown of the magnetic ground state due to either doping or increased temperature. Based on these results we propose that phonons strongly couple not to the static order, but to high frequency magnetic fluctuations.

We measured phonon dispersions of CaFe2As2 using inelastic neutron scattering and compared our results to predictions of density functional theory in the local density approximation. The calculation gives correct frequencies of most phonons if the experimental crystal structure is used, except observed linewidths/frequencies of certain modes were larger/softer than predicted. Strong temperature dependence of some phonons near the structural phase transition near 172 K may indicate strong electron-phonon coupling and/or anharmonicity, which may be important for superconductivity.

The magnetic structure of the noncentrosymmetric cubic magnets with Dzyaloshinskii-Moriya interaction ${\text{Mn}}_{1\ensuremath{-}y}{\text{Fe}}_{y}\text{Si}$ with $y=0.0,0.06,0.08,0.10$ has been studied by means of the small angle neutron diffraction and magnetization measurements. The compounds order in the spin helix structure below ${T}_{c}$ that decreases linearly with the Fe doping and approaches zero at $yg0.13$. We build the $(H\text{\ensuremath{-}}T)$ phase diagrams for each compound and interpret them on the basis of the Bak-Jensen hierarchical model of the principal interactions. Among these interactions are the spin-wave stiffness that decreases linearly with $y$, obviously duplicating the change of ${T}_{c}$, and the Dzyaloshinskii interaction showing the moderate evolution with $y$. It is shown that these compounds undergo the transition from the paramagnetic to helimagnetic phase through the intermediate chiral fluctuating phase. The discussion is given on the comparison of the $(T\text{\ensuremath{-}}P)$ phase diagram in pure MnSi and $(T\text{\ensuremath{-}}y)$ phase diagram of the ${\text{Mn}}_{1\ensuremath{-}y}{\text{Fe}}_{y}\text{Si}$ compounds showing the role of the principal interactions on the critical temperature ${T}_{c}$ of these systems.

High resolution spin waves measurements have been carried out in ferromagnetic (F) La(1-x)(Sr,Ca)xMnO3 with x(Sr)=0.15, 0.175, 0.2, 0.3 and x(Ca)=0.3. In all q-directions, close to the zone boundary, the spin wave spectra consist of several energy levels, with the same values in the metallic and the x\approx 1/8 ranges. Mainly the intensity varies, jumping from the lower energy levels determined in the x\approx 1/8 range to the higher energy ones observed in the metallic state. On the basis of a quantitative agreement found for x(Sr)=0.15 in a model of ordered 2D clusters, the spin wave anomalies of the metallic state can be interpreted in terms of quantized spin waves within the same 2D clusters, embedded in a 3D matrix., 4 pages, 5 figures

Extensive inelastic neutron scattering measurements of phonons on a single crystal of CaFe2As2 allowed us to establish a fairly complete picture of phonon dispersions in the main symmetry directions. The phonon spectra were also calculated by density functional theory (DFT) in the local density approximation (LDA). There are serious discrepancies between calculations done for the optimized structure and experiment, because the optimised structure is not the ambient pressure structure but is very close to the collapsed structure reached at p = 3.5 kbar. However, if the experimental crystal structure is used the calculation gives correct frequencies of most phonons. The most important new result is that linewidths/frequencies of certain modes are larger/softer than predicted by DFT-LDA. We also observed strong temperature dependence of some phonons near the structural phase transition near 172 K. This behavior may indicate anomalously strong electron phonon coupling and/or anharmonicity, which may be important to the mechanism of superconductivity.