The low-temperature superstructure of a 0 -NaV 2 O 5 is determined by synchrotron radiation x-ray diffraction. The high-temperature structure is confirmed to be centrosymmetric, but the modulated structure is found to be acentric Fmm2 on the 2a 3 2b 3 4c supercell. An analysis using superspace symmetry shows that there are modulated and nonmodulated chains of vanadium atoms, tentatively assigned to magnetic and nonmagnetic chains. The arrangement of these chains is different from any model previously considered. [S0031-9007 (99)09001-8] PACS numbers: 61.44.Fw, 61.50.Ks, 61.66.Fn, 75.30.FvAn intensified interest in low-dimensional quantum systems was incited by the discovery of the first inorganic spin-Peierls compound CuGeO 3 [1]. The magnetic order corresponded to a superstructure comprising a dimerization of the copper atoms along the chains, with displacements of neighboring oxygen atoms through elastic coupling [2]. This model was in full accordance with a pairing of the spins on the copper chains. Modulations on neighboring chains were out of phase, thus explaining the second component of one-half of the modulation wave vector.More recently, a 0 -NaV 2 O 5 was found to exhibit a spin-Peierls transition at T SP 34 K [3]. Superstructure reflections in x-ray scattering were observed at q ͑ 1 2 ,

We report the observation of a new type of charge-density wave (CDW) in the large magnetic-moment rare-earth intermetallic compound, Er 5 Ir 4 Si 10 , which then orders magnetically at low temperatures. Single crystal x-ray diffraction shows the development of a 1D incommensurate CDW at 155 K, which then locks into a purely commensurate state below 55 K. The well-localized Er 31 moments are antiferromagnetically ordered below 2.8 K. We observe very sharp anomalies in the specific heat at 145 and 2.8 K, signifying the bulk nature of these transitions. Our data suggest the coexistence of strongly coupled CDW with local-moment antiferromagnetism in Er 5 Ir 4 Si 10 . PACS numbers: 71.45.Lr, 75.20.Hr, 71.20.Lp Charge-density-wave (CDW) transitions occur in lowdimensional solids where it is possible to achieve nesting of Fermi surfaces that leads to the appearance of a periodic lattice distortion with an accompanying energy gap D [1-4]. These materials show striking nonlinear and anisotropic properties as well as unusual elastic and dynamic behaviors: This makes them an appealing field for experimental and theoretical studies on Fermi surface gapping and electron-phonon coupling. A number of such systems have been previously studied, including quasi-1D organic salts (e.g., TTF-TCNQ) [5] and inorganic chain compounds (e.g., NbSe 3 ) [2]. Here a Peierls-Fröhlich-type [4] phase transformation is observed at a finite temperature T CDW : Above this temperature the coupled electronphonon system is unstable with respect to a deformation of wave vector q 2k F . Below T CDW , the ground state is characterized by a gap in the single-particle excitation spectrum.Although many of the properties of these quasi-1D compounds could be understood within the framework of this continuous phase transition with weak electronphonon and interchain coupling, the thermodynamic behavior of the CDW transition in compounds such as TaSe 2 [6] and the blue bronzes [3] requires a description beyond the weak-coupling model. McMillan [7], using strong electron-phonon coupling and small interchain correlation lengths, was able to provide a microscopic theory, which has given a semiquantitative explanation for the peculiar behavior of these latter systems, like their specific heat jumps and D͞T CDW ratios. In recent years interest in this area has focused on detailed investigation of these conventional CDW systems. However, in order to probe the theory further and search for novel CDW behaviors, especially those involving the interplay with magnetism, new classes of materials are needed. To the best of our knowledge there does not exist a local-moment magnet (mediated by the RKKY interaction between the f and conduction electrons) exhibiting a CDW.We have begun a quest for new CDW systems in intermetallic rare-earth (RE) compounds and found indications in the literature [8][9][10] for such behavior in a series of RE 5 Ir 4 Si 10 materials. Polycrystalline samples showed anomalies in the resistivity above 20 K, which were tentatively attributed to CD...

No abstract

Collective vibrations of the clean and hydrogen-saturated W(l 10) surface have been investigated with high-resolution inelastic helium-atom scattering. In contrast to the clean W(l 10) surface which exhibits "normal" surface phonon dispersion, the hydrogen-saturated surface shows a pronounced surface phonon anomaly in terms of a sharply defined dip in the surface phonon dispersion at Q *0.9 A "' along the F/7 ([001]) direction. The anomalous behavior extends into the neighborhood of this high-symmetry direction.PACS numbers: 79.20. Rf, Hydrogen adsorption on metal surfaces leads to a great variety of surface structures and adsorbate-induced reconstructions [1], and powerful theoretical techniques have been developed to study the dynamics of these hydrogen phases [2]. As a common rule, chemisorbed hydrogen renders densely packed metal surfaces chemically inert and dynamically stable at saturation coverage. Such a behavior indicates that all broken bonds become saturated upon hydrogen adsorption. In this Letter we report on a spectacular exception to this rule: For the first time a strong surface phonon anomaly has been detected which is brought about by saturating a metal surface with chemisorbed hydrogen. The anomaly consists of a sharply defined soft elementary excitation at an incommensurate wave vector. Extensive studies of clean and hydrogen-covered W(110) surfaces have previously been undertaken [3-6]. In particular, the phase diagram with respect to H adsorption is well known [7]. From electron-energy-loss spectroscopy measurements [8] it was concluded that the hydrogen atom is bonded in the long bridge site above the hourglass-shaped hole between the tungsten atoms in the surface layer. In recent LEED studies [9] a loss of symmetry in the diffraction pattern was observed for hydrogen coverages exceeding half a monolayer. This asymmetry has been interpreted by Chung, Ying, and Estrup[9] as being due to an adsorbate-induced shift of all tungsten atoms in the topmost layer along the (001) direction. Even though such a reconstruction leads to a (lxl) superstructure it changes the environment of the tungsten atoms in the surface and should bear on the surface lattice dynamics.The present measurements were performed in a helium scattering apparatus [10] suited for elastic and inelastic (time-of-flight) measurements.The 5 x 10 x 1 -mm 3 tungsten crystal specimen was prepared using standard techniques [11], kept in ultrahigh vacuum, and characterized with LEED and Auger electron spectroscopy (AES). After cleaning, the carbon AES signal was below the detection limit of the analyzer.Elastic helium scattering from the cleanW(HO) surface along the [001], the [HO], and the [ill] directions in the surface yields sharp reflections with weak diffracted intensities typical for weakly corrugated and 2846