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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. La 1.67 Sr 0.33 NiO 4 (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. [ABSTRACT FROM AUTHOR]

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 La2−2xSr1+2xMn2O7 (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 La2−2xSr1+2xMn2O7 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. [ABSTRACT FROM AUTHOR]

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 Tc (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. [ABSTRACT FROM AUTHOR]

The attempt to understand copper oxide superconductors is complicated by the presence of multiple strong interactions in these systems. Many believe that antiferromagnetism is important for superconductivity, but there has been renewed interest in the possible role of electron–lattice coupling. The conventional superconductor MgB2 has a very strong electron–lattice coupling, involving a particular vibrational mode (phonon) that was predicted by standard theory and confirmed quantitatively by experiment. Here we present inelastic scattering measurements that show a similarly strong anomaly in the Cu–O bond-stretching phonon in the copper oxide superconductors La2-xSrxCuO4 (with x = 0.07, 0.15). Conventional theory does not predict such behaviour. The anomaly is strongest in La1.875Ba0.125CuO4 and La1.48Nd0.4Sr0.12CuO4, compounds that exhibit spatially modulated charge and magnetic order, often called stripe order; it occurs at a wave vector corresponding to the charge order. These results suggest that this giant electron–phonon anomaly, which is absent in undoped and over-doped non-superconductors, is associated with charge inhomogeneity. It follows that electron–phonon coupling may be important to our understanding of superconductivity, although its contribution is likely to be indirect. [ABSTRACT FROM AUTHOR]

Only a few metallic phases have been identified in pure crystalline materials. These include normal, ferromagnetic and antiferromagnetic metals, systems with spin and charge density wave order, and superconductors. Fermi-liquid theory provides a basis for the description of all of these phases. It has been suggested that non-Fermi-liquid phases of metals may exist in some heavy-fermion compounds and oxide materials, but the discovery of a characteristic microscopic signature of such phases presents a major challenge. The transition-metal compound MnSi above a certain pressure (pc = 14.6?kbar) provides what may be the cleanest example of an extended non-Fermi-liquid phase in a three-dimensional metal. The bulk properties of MnSi suggest that long-range magnetic order is suppressed at pc (refs 7-12). Here we report neutron diffraction measurements of MnSi, revealing that sizeable quasi-static magnetic moments survive far into the non-Fermi-liquid phase. These moments are organized in an unusual pattern with partial long-range order. Our observation supports the existence of novel metallic phases with partial ordering of the conduction electrons (reminiscent of liquid crystals), as proposed for the high-temperature superconductors and heavy-fermion compounds. [ABSTRACT FROM AUTHOR]

We report results of inelastic neutron scattering measurements of phonon dispersions on a detwinned sample of YBa[sub 2]Cu[sub 3]O[sub 7] 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. [ABSTRACT FROM AUTHOR]

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Superlattice diffraction peaks have been recently observed in underdoped YBaCuO. 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. [ABSTRACT FROM AUTHOR]

Arising from: F. Giustino, M. L. Cohen & S. G. Louie 452, 975–978 (2008)One of the possible mechanisms of high transition temperature (Tc) superconductivity is Cooper pairing with the help of bosons, which change the slope of the electronic dispersion as observed by photoemission. Giustino et al. calculated that in the high temperature superconductor La1.85Sr0.15CuO4 crystal lattice vibrations (phonons) should have a negligible effect on photoemission spectra and concluded that phonons do not have an important role. Here we show that the calculations used by Giustino et al. do not reproduce the huge influence of electron–phonon coupling on important phonons observed in experiments. Thus, we would similarly expect that these calculations do not explain the role of electron–phonon coupling for the electronic dispersion. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented in response to the article "Written Off," in the October 21, 2006 issue.

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 La2−2xSr1+2xMn2O7 (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 La2−2xSr1+2xMn2O7 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. [ABSTRACT FROM AUTHOR]