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We report quantum phenomena in spin-orbit-coupled single crystals that are synthesized using an innovative technology that “field-alters” crystal structures via application of magnetic field during crystal growth. This study addresses a major challenge facing the research community today: A great deal of theoretical work predicting exotic states for strongly spin-orbit-coupled, correlated materials has thus far met very limited experimental confirmation. These conspicuous discrepancies are due in part to the extreme sensitivity of these materials to structural distortions. The results presented here demonstrate that the field-altered materials not only are much less distorted but also exhibit phenomena absent in their non-altered counterparts. The field-altered materials include an array of 4d and 5d transition metal oxides, and three representative materials presented here are Ba4Ir3O10, Ca2RuO4, and Sr2IrO4. This study provides an approach for discovery of quantum states and materials otherwise unavailable.

Measurement and control of magnetic order and correlations in real time is a rapidly developing scientific area relevant for magnetic memory and spintronics. In these experiments an ultrashort laser pulse (pump) is first absorbed by excitations carrying electric dipole moment. These then give their energy to the magnetic subsystem monitored by a time-resolved probe. A lot of progress has been made in investigations of ferromagnets but antiferromagnets are more challenging. Here, we introduce time-resolved two-magnon Raman scattering as a real time probe of magnetic correlations especially well-suited for antiferromagnets. Its application to the antiferromagnetic charge transfer insulator YBa2Cu3O6.1 revealed rapid demagnetization within 90 fs of photoexcitation. The relaxation back to thermal equilibrium is characterized by much slower timescales. We interpret these results in terms of slow relaxation of the charge sector and rapid equilibration of the magnetic sector to a prethermal state characterized by parameters that change slowly as the charge sector relaxes. The limitations of many experimental techniques make it difficult to obtain a clear picture of magnetic interactions in materials, leaving many important questions open. Yang et al. demonstrate that time-resolved two-magnon Raman scattering can probe the dynamics of antiferromagnetic YBa2Cu3O6.1.

Typical density-functional theory calculations that wrongly predict undoped cuprates to be metallic also predict Cu-O half- and full-breathing phonon energies that are significantly softer than observed, presumably because of weak on-site Coulomb repulsion on the Cu 3d orbitals. We used $\mathrm{DFT}+\mathrm{U}$ calculations with antiferromagnetic supercells of ${\mathrm{La}}_{\text{2}}{\mathrm{CuO}}_{\text{4}}$ to establish correlation between the on-site repulsion strength, tuned via adjusting the value of U, and phonon dispersions. We find that breathing and half-breathing phonons reach experimental values when U is tuned to obtain the correct optical gap and magnetic moments. We demonstrate that using distorted supercells within $\mathrm{DFT}+\mathrm{U}$ is a promising framework to model phonons in undoped cuprates and other perovskite oxides with complex, interrelated structural and electronic degrees of freedom.

New surprises continue to be revealed about ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$, the parent compound of the original cuprate superconductor. Here we present neutron scattering evidence that the structural symmetry is lower than commonly assumed. The static distortion results in anisotropic Cu-O bonds within the ${\mathrm{CuO}}_{2}$ planes; such anisotropy is relevant to pinning charge stripes in hole-doped samples. Associated with the extra structural modulation is a soft phonon mode. If this phonon were to soften completely, the resulting change in ${\mathrm{CuO}}_{6}$ octahedral tilts would lead to weak ferromagnetism. Hence, we suggest that this mode may be the ``chiral'' phonon inferred from recent studies of the thermal Hall effect. We also note the absence of interaction between the antiferromagnetic spin waves and low-energy optical phonons, in contrast to what is observed in hole-doped samples.

Electron-phonon coupling, i.e., the scattering of lattice vibrations by electrons and vice versa, is ubiquitous in solids and can lead to emergent ground states such as superconductivity and charge-density wave order. A broad spectral phonon line shape is often interpreted as a marker of strong electron-phonon coupling associated with Fermi surface nesting, i.e., parallel sections of the Fermi surface connected by the phonon momentum. Alternatively broad phonons are known to arise from strong atomic lattice anharmonicity. Here, we show that strong phonon broadening can occur in the absence of both Fermi surface nesting and lattice anharmonicity, if electron-phonon coupling is strongly enhanced for specific values of electron-momentum, k. We use inelastic neutron scattering, soft x-ray angle-resolved photoemission spectroscopy measurements and ab-initio lattice dynamical and electronic band structure calculations to demonstrate this scenario in the highly anisotropic tetragonal electron-phonon superconductor YNi2B2C. This new scenario likely applies to a wide range of compounds., Phonon anomalies are commonly attributed to Fermi-surface nesting or phonon anharmonicity, but these mechanisms do not apply in all cases. Here, the authors propose a new mechanism based on electron-momentum-dependent electron-phonon coupling in the case of YNi2B2C, that could also apply to other materials.

This article introduces software called Phonon Explorer that implements a data mining workflow for large datasets of the neutron scattering function, S(Q, &omega, ), measured on time-of-flight neutron spectrometers. This systematic approach takes advantage of all useful data contained in the dataset. It includes finding Brillouin zones where specific phonons have the highest scattering intensity, background subtraction, combining statistics in multiple Brillouin zones, and separating closely spaced phonon peaks. Using the software reduces the time needed to determine phonon dispersions, linewidths, and eigenvectors by more than an order of magnitude.

Doped antiferromagnets host a vast array of physical properties and learning how to control them is one of the biggest challenges of condensed matter physics. $$\hbox {La}_{1.67}\hbox {Sr}_{0.33}\hbox {NiO}_4$$La1.67Sr0.33NiO4 (LSNO) is a classic example of such a material. At low temperatures holes introduced via substitution of La by Sr segregate into lines to form boundaries between magnetically ordered domains in the form of stripes. The stripes become dynamic at high temperatures, but LSNO remains insulating presumably because an interplay between magnetic correlations and electron–phonon coupling localizes charge carriers. Magnetic degrees of freedom have been extensively investigated in this system, but phonons are almost completely unexplored. We searched for electron–phonon anomalies in LSNO by inelastic neutron scattering. Giant renormalization of plane Ni–O bond-stretching modes that modulate the volume around Ni appears on entering the dynamic charge stripe phase. Other phonons are a lot less sensitive to stripe melting. Dramatic overdamping of the breathing modes indicates that dynamic stripe phase may host small polarons. We argue that this feature sets electron–phonon coupling in nickelates apart from that in cuprates where breathing phonons are not overdamped and point out remarkable similarities with the colossal magnetoresistance manganites.

The cuprates exhibit a prominent charge-density-wave (CDW) instability with a wave vector along [100], i.e., the Cu-O bond direction. Whereas CDW order is most prominent at moderate doping and low temperature, there exists increasing evidence for dynamic charge correlations throughout a large portion of the temperature-doping phase diagram. In particular, the signatures of incipient charge order have been observed as phonon softening and/or broadening near the CDW wave vector approximately halfway through the Brillouin zone. Most of this work is focused on moderately doped cuprates, for which the CDW order is robust, or on optimally doped samples, for which the superconducting transition temperature $({T}_{c})$ attains its maximum. Here we present a time-of-flight neutron scattering study of phonons in simple-tetragonal ${\text{HgBa}}_{2}{\text{CuO}}_{4+\ensuremath{\delta}}\phantom{\rule{4pt}{0ex}}({T}_{c}=55\phantom{\rule{0.28em}{0ex}}\mathrm{K})$ at a low doping level where prior work showed the CDW order to be weak. We employ and showcase a new software-based technique that mines a large number of measured Brillouin zones for useful data in order to improve accuracy and counting statistics. Density-functional theory has not provided an accurate description of phonons in ${\text{HgBa}}_{2}{\text{CuO}}_{4+\ensuremath{\delta}}$, yet we find the right set of parameters to qualitatively reproduce the data. The notable exception is a dispersion minimum in the longitudinal Cu-O bond-stretching branch along [100]. This discrepancy suggests that, while CDW order is weak, there exist significant dynamic charge correlations in the optic phonon range at low doping, near the edge of the superconducting dome.

Stripe order where electrons self-organize into alternating periodic charge-rich and magnetically-ordered charge-poor parallel lines was proposed as a way of optimizing the kinetic energy of holes in a doped Mott insulator. Static stripes detected as extra peaks in diffraction patterns, appear in a number of oxide perovskites as well as some other systems. The more controversial dynamic stripes, which are not detectable by diffraction, may be universally present in copper oxide superconductors. Thus it is important to learn how to detect dynamic stripes as well as to understand their influence on electronic properties. This review article focuses on lattice vibrations (phonons) that might show signatures of the charge component of dynamic stripes. The first part of the article describes recent progress in learning about how the phonon signatures of different types of electronic charge fluctuations including stripes can be distinguished from purely structural instabilities and from each other. Then I will focus on the evidence for dynamic stripes in the phonon spectra of copper oxide superconductors.

We present a comprehensive investigation of lattice dynamics in the double-helix antiferromagnet FeP by means of high-resolution time-of-flight neutron spectroscopy and ab-initio calculations. Phonons can hybridize with the magnetic excitations in noncollinear magnets to significantly influence their properties. We observed a rich spectrum of phonon excitations, which extends up to $\sim$50 meV. We performed detailed analysis of the observed and calculated spectra for all high-symmetry points and high-symmetry directions of the Brillouin zone. We show that the DFT calculations quantitatively capture the essential features of the observed phonons, including both dispersions and scattering intensities. By making use of the detailed intensity comparison between the theory and the data, we were able to identify displacement vectors for the majority of the observed modes. The overall excellent agreement between the DFT predictions and the experimental results breaks down for the lowest mode at the $Y$-point, whose energy is lower than calculated by $\sim$13%. The present study provides vital information on the lattice dynamics in FeP and demonstrates applicability of the DFT to novel pressure-induced phenomena in related materials, such as MnP and CrAs., Comment: 12 pages, 10 figures

${\mathrm{La}}_{1.67}{\mathrm{Sr}}_{0.33}{\mathrm{NiO}}_{4}$ develops charge and spin stripe orders at temperatures of roughly 200 K, with modulation wave vectors that are temperature independent. Various probes of spin and charge response have provided independent evidence for some sort of change below $\ensuremath{\sim}50$ K. In combination with a new set of neutron scattering measurements, we propose a unified interpretation of all of these observations in terms of a freezing of Ni-centered charges stripes, together with a glassy ordering of the spin stripes that shows up in neutron scattering as a slight rotation of the average spin direction.

Electron-phonon interaction in copper oxide superconductors is still enigmatic. Strong coupling for certain optic phonons is now well established experimentally, but theoretical understanding is challenging. Scattering of electrons near the Fermi surface by the longitudinal acoustic (LA) phonons is expected from basic theory because these phonons modulate electron density. We used inelastic neutron scattering on a large single crystal sample of optimally-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ to show that low-energy LA phonons could couple to electronic density fluctuations only at small phonon wavevectors, which naturally limits any interaction to forward scattering. Such scattering should not be pairbreaking in the case of the d-wave gap. We also found that previously the reported low energy phonon spectral weight half-way to the zone boundary is consistent with conventional lattice dynamics and does not reflect an incipient charge density wave.

Measurement and control of magnetic order and correlations in real time is a rapidly developing scientific area relevant for magnetic memory and spintronics. In these experiments an ultrashort laser pulse (pump) is first absorbed by excitations carrying electric dipole moment. These then give their energy to the magnetic subsystem monitored by a time-resolved probe. A lot of progress has been made in investigations of ferromagnets but antiferromagnets are more challenging. Here, we introduce time-resolved two-magnon Raman scattering as a real time probe of magnetic correlations especially well-suited for antiferromagnets. Its application to the antiferromagnetic charge transfer insulator YBa2Cu3O6.1 revealed rapid demagnetization within 90 fs of photoexcitation. The relaxation back to thermal equilibrium is characterized by much slower timescales. We interpret these results in terms of slow relaxation of the charge sector and rapid equilibration of the magnetic sector to a prethermal state characterized by parameters that change slowly as the charge sector relaxes., The limitations of many experimental techniques make it difficult to obtain a clear picture of magnetic interactions in materials, leaving many important questions open. Yang et al. demonstrate that time-resolved two-magnon Raman scattering can probe the dynamics of antiferromagnetic YBa2Cu3O6.1.

We study experimentally the Raman response of the undoped high-${T}_{c}$ parent compound ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6}$, and give a unified theory of the two-magnon Raman peak and optical conductivity based on the Hubbard-Holstein model with electron-phonon coupling (EPC). The Hubbard model without EPC can qualitatively account for the experimentally observed resonance of the Raman response, but only the Hubbard-Holstein model (i) reproduces the asymmetry of the Raman spectrum, (ii) validates the experimental visibility of the two-magnon peak, and (iii) predicts the correct shape and energy of the lower edge of the charge transfer gap in optical conductivity. A comparison of experiments with the theory gives the EPC strength $\ensuremath{\lambda}=0.6$. This result convincingly indicates the vital role of EPC in high-${T}_{c}$ cuprates, providing a clue to the mechanism of high ${T}_{c}$.

Nematicity is ubiquitous in electronic phases of high-${T}_{\mathrm{c}}$ superconductors, particularly in the Fe-based systems. While several experiments have probed nematic fluctuations, they have been restricted to uniform or momentum-averaged fluctuations. Here, we investigate the behavior of finite-momentum nematic fluctuations by utilizing the anomalous softening of acoustic phonon modes in optimally doped $\mathrm{Ba}{({\mathrm{Fe}}_{0.94}{\mathrm{Co}}_{0.06})}_{2}{\mathrm{As}}_{2}$. We determine the nematic correlation length and find that it sharply changes its $T$ dependence at ${T}_{\mathrm{c}}$, revealing a strong connection between nematicity and superconductivity.

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.

We investigated long-wavelength nematic fluctuations in an Fe-based superconductor LiFeAs near q = (0.05, 0, 0) by measuring temperature-dependent renormalization of acoustic phonons through inelastic neutron scattering. We found that the phonons have a conventional behavior, as would be expected in the absence of electronic nematic fluctuations. This observation implies that either electron-phonon coupling is too weak to see any effect or that nematic fluctuations are not present.

Determining the nature of electronic states in doped Mott insulators remains a challenging task. In the case of tetragonal La_{2-x}Sr_{x}NiO_{4}, the occurrence of diagonal charge and spin stripe order in the ground state is now well established. In contrast, the nature of the high-temperature "disordered" state from which the stripe order develops has long been a subject of controversy, with considerable speculation regarding a polaronic liquid. Following the recent detection of dynamic charge stripes, we use neutron scattering measurements on an x=0.25 crystal to demonstrate that the dispersion of the charge-stripe excitations is anisotropic. This observation provides compelling evidence for the presence of electronic nematic order.

In second-order quantum phase transitions from magnetically ordered to paramagnetic states at T = 0, tuned by pressure or chemical substitution, a quantum critical point is expected to appear with critical behavior manifesting in the slowing down of spin fluctuations in the paramagnetic state and a continuous development of the order parameter in the ordered state. Quantum criticality is discussed widely as a possible driving force for unconventional superconductivity and other exotic phenomena in correlated electron systems. In the real world, however, quantum critical points and quantum criticality are often masked by a preceding first-order transition and/or the development of competing states. Pressure tuning of the itinerant-electron helical magnet MnSi is a well-known example of the suppression of a quantum critical point due to a first-order phase transition and resulting destruction of the ordered state. Utilizing muon spin relaxation experiments, here we report that 15% Fe-substituted (Mn,Fe)Si exhibits completely different behavior with pressure tuning, including the restoration of second-order quantum critical behavior and a quantum critical point at p QPC ~ 21–23 kbar, which coincides with the T = 0 crossing point of the extrapolated phase boundary line of pure MnSi. This result is quantitatively consistent with the recent theory of itinerant-electron ferromagnets by Sang, Belitz, and Kirkpatrick, who argued that disorder would restore a quantum critical point which is otherwise hidden by a first-order transition. An exotic quantum state of matter is identified in a new material by researchers in the USA, Switzerland and Germany. Yasutomo Uemura from Columbia University and co-workers observe quantum criticality in a material in which it was previously unseen just by adding iron. As pressure is increasingly applied to pure manganese silicon at absolute zero temperature, it can suddenly change from an ordered magnetic state to an unordered one. Uemura et al. now show that a slower continuous, or second-order, transition is observed instead when fifteen percent of the manganese atoms are replaced with iron atoms. This is indicative that a quantum critical point exists at a pressure of between 21 and 23 kilobar. They believe that criticality is restored because the iron atoms introduce disorder into the system.

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.

Time dynamics of photoexcited electron-hole pairs is important for a number of technologies, in particular solar cells. We combined ultrafast pump-probe Raman scattering and photoemission to directly follow electron-hole excitations as well as the G-phonon in graphite after an excitation by an intense laser pulse. This phonon is known to couple relatively strongly to electrons. Cross-correlating effective electronic and phonon temperatures places new constraints on model-based fits. The accepted two-temperature model predicts that G-phonon population should start to increase as soon as excited electron-hole pairs are created and that the rate of increase should not depend strongly on the pump fluence. Instead we found that the increase of the G-phonon population occurs with a delay of ~65 fs. This time-delay is also evidenced by the absence of the so-called self-pumping for G phonons. It decreases with increased pump fluence. We show that these observations imply a new relaxation pathway: Instead of hot carriers transferring energy to G-phonons directly, the energy is first transferred to optical phonons near the zone boundary K-points, which then decay into G-phonons via phonon-phonon scattering. Our work demonstrates that phonon-phonon interactions must be included in any calculations of hot carrier relaxation in optical absorbers even when only short timescales are considered.

Since conventional superconductivity is mediated by phonons, their role in the mechanism of high temperature superconductivity has been considered very early after the discovery of the cuprates. The initial consensus was that phonons could not produce transition temperatures near 100 K, and the main direction of research focused on nonphononic mechanisms. Subsequent work last reviewed by L. Pintschovius in 2005 showed that electron-phonon coupling in the cuprates is surprisingly strong for some phonons and its role is controversial. Experiments performed since then identified anomalous behavior of certain Cu–O bond-stretching phonons in cuprates as an important phenomenon that is somehow related to the mechanism of superconductivity. A particularly big advance was made in the study of dopedLa2CuO4. This work is reviewed here.

Charge carriers in low-doped semiconductors may distort the atomic lattice around them and through this interaction form so-called small polarons. High carrier concentrations on the other hand can lead to short-range ordered polarons (large polarons) and even to a long-range charge and orbital order. These ordered systems should be insulating with a large electrical resistivity. However, recently a polaronic pseudogap was found in a metallic phase of La(2-2x)Sr(1+2x)Mn(2)O(7) (ref. 7). This layered manganite is famous for colossal magnetoresistance associated with a phase transition from this low-temperature metallic phase to a high-temperature insulating phase. Broad charge-order peaks due to large polarons in the insulating phase disappear when La(2-2x)Sr(1+2x)Mn(2)O(7) becomes metallic. Investigating how polaronic features survive in the metallic phase, here we report the results of inelastic neutron scattering measurements showing that inside the metallic phase polarons remain as fluctuations that strongly broaden and soften certain phonons near the wavevectors where the charge-order peaks appeared in the insulating phase. Our findings imply that polaronic signatures in metals may generally come from a competing insulating charge-ordered phase. Our findings are highly relevant to cuprate superconductors with both a pseudogap and a similar phonon effect associated with a competing stripe order.

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

Phonons in a metal interact with conduction electrons which give rise to a finite linewidth. In the normal state, this leads to a Lorentzian shape of the phonon line. Density functional theory is able to predict the phonon linewidths as a function of wave vector for each branch of the phonon dispersion. An experimental verification of such predictions is feasible only for compounds with very strong electron-phonon coupling. YN2B2C was chosen as a test example because it is a conventional superconductor with a fairly high T c (15.2 K). Inelastic neutron scattering experiments did largely confirm the theoretical predictions. Moreover, they revealed a strong temperature dependence of the linewidths of some phonons with particularly strong electron-phonon coupling which can as yet only qualitatively be accounted for by theory. For such phonons, marked changes of the phonon frequencies and linewidths were observed from room temperature down to 15 K. Further changes were observed on entering into the superconducting state. These changes can, however, not be described simply by a change of the phonon linewidth.

Inelastic neutron scattering was used to study the Cu-O bond-stretching vibrations in optimally doped La1.85Sr0.15CuO4 (Tc = 35 K) and in two other cuprates showing static stripe order at low temperatures, i.e. La1.48Nd0.4Sr0.12CuO4 and La1.875Ba0.125CuO4. All three compounds exhibit a very similar phonon anomaly, which is not predicted by conventional band theory. It is argued that the phonon anomaly reflects a coupling to charge inhomogeneities in the form of stripes, which remain dynamic in superconducting La1.85Sr0.15CuO4 down to the lowest temperatures. These results show that the phonon effect indicating stripe formation is not restricted to a narrow region of the phase diagram around the so-called 1/8 anomaly but occurs in optimally doped samples as well., to appear in J. Low Temp. Phys

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.

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 investigations on optimally doped YBCO revealed a very pronounced temperature dependence of the Cu–O in-plane bond-stretching vibrations along the (0 1 0)-direction: a downward shift of spectral weight with decreasing temperature by at least 10 meV in a narrow range of wave vectors halfway to the zone boundary. The temperature evolution starts around 200 K, well above the superconducting transition temperature. This phonon anomaly provides strong evidence for large electron–phonon coupling. It also indicates an incipient charge density wave instability within the CuO2 planes reminiscent of dynamical charge stripes. The magnetic fluctuations have been investigated in great detail on the same sample. Incommensurate spin fluctuations have been observed for energies both below and above the energy of the resonance peak at E=41 meV. However, the dispersive nature of these fluctuations as well as their apparent isotropy in the basal plane speak against an interpretation of the spin fluctuation spectrum in the framework of the classical stripe phase picture.

MnSi is an itinerant ferromagnet with a long-wavelength helical modulation of the spin structure. Macroscopic measurements suggest that the ordering temperatureTc is reduced with increasing pressure fromTc = 30 K atp = 0 to zero at the critical pressurepc = 14.6 kbar. Resistivity measurements show that MnSi enters a non-Fermi liquid state atpc, which remains to be understood. Neutron scattering techniques have been used to investigate the magnetic structure at and abovepc, i.e. triple-axis spectrometry and small angle neutron scattering. Surprisingly, sizeable quasi-static moments were found to survive to pressures considerably abovepc. They are, however, organized in a highly unusual way such that the magnetic Bragg reflections are sharp in the longitudinal direction but are very broad in the transverse direction, implying a partial magnetic order that was never seen before.

We report results of inelastic neutron scattering measurements of phonon dispersions on a detwinned sample of YBa2Cu3O7 and compare them with model calculations. Plane oxygen bond stretching phonon branches disperse steeply downwards from the zone center in both the a and the b direction indicating a strong electron-phonon coupling. Half way to the zone boundary, the phonon peaks become ill-defined but we see no need to invoke unit cell doubling or charge stripe formation: lattice dynamical shell model calculations predict such behavior as a result of branch anticrossings. There were no observable superconductivity-related temperature effects on selected plane oxygen bond stretching modes measured on a twinned sample.

Superlattice diffraction peaks have been recently observed in underdoped YBa2Cu3O6.6. They have been interpreted to originate from a charge density wave (CDW) formation. It is believed that strong phonon anomalies previously observed near the same wavevectors are related to the CDW. Competition with superconductivity is a salient feature of the CDW peaks. We investigated if the same is true of anomalous bond-stretching phonons around 60 meV by measuring the spectrum of this phonon at 10 K with and without an applied magnetic field of 10 T. Applying the field had no effect on the phonon within the experimental uncertainty.

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.

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.

We present a combined density functional perturbation theory and inelastic neutron scattering study of the lattice dynamical properties of YNi${}_{2}$B${}_{2}$C. In general, very good agreement was found between theory and experiment for both phonon energies and line widths. Our analysis reveals that the strong coupling of certain low energy modes is linked to the presence of large displacements of the light atoms, i.e., B and C, which is unusual in view of the rather low phonon energies. Specific modes exhibiting a strong coupling to the electronic quasiparticles were investigated as a function of temperature. Their energies and line widths showed marked changes on cooling from room temperature to just above the superconducting transition at ${T}_{c}$ = 15.2 K. Calculations simulating the effects of temperature allow us to model the observed temperature dependence qualitatively.

The ferropnictidesuperconductors exhibit a sensitive interplay between the lattice and magnetic degrees of freedom, including a number of phonon modes that are much softer than predicted by nonmagnetic calculations using density functional theory (DFT). However, it is not known what effect, if any, the long-range magnetic order has on phonon frequencies above 23 meV, where several phonon branches are very closely spaced in energy and it is challenging to isolate them from each other. We measured these phonons using inelastic time-of-flight neutron scattering in $\ensuremath{\approx}40$ Brillouin zones, and developed a technique to determine their frequencies. We find this method capable of determining phonon energies to $\ensuremath{\approx}$0.1 meV accuracy, and that the DFT calculations using the experimental structure yield qualitatively correct energies and eigenvectors. We do not find any effect of the magnetic transition on these phonons.

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.

Colossal magnetoresistance (CMR) is associated with the phase transition from a metallic ferromagnetic to insulating paramagnetic phase, which can be controlled by an applied magnetic field. The insulating phase occurs due to trapping of the charge carriers by polaronic lattice distortions, which raise the resistivity. Theories based on local physics predict that the magnitude of the resistivity jump at T-C is determined by how much, on average, the amplitude of these distortions increases at the phase transition. Using neutron scattering, we measured the average distortion amplitude in La0.7Sr0.3MnO3. Surprisingly, its increase from below to above T-C is just as large as in other manganites, which have a much larger resistivity jump. This result suggests that the strength of CMR is determined not by the size of distortions, but by their cooperative nature, specific to each compound. Existing theories need to be extended to include correlations between different unit cells to explain and predict the strength of CMR. (Less)