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By single crystal diffraction we characterize the chemo-structural disorder introduced by Zn-Cu site mixing in the kagom\'e spin $S$-1/2 systems herbertsmithite ZnCu$_3$(OH)$_6$Cl$_2$ and YCu$_3$(OH)$_{6}$Br$_{2}$[Br$_x$(OH)$_{1-x}$]. For an untwinned single crystal of herbertsmithite of composition Zn$_{0.95(1)}$Cu$_{2.99(3)}$O$_{5.9(1)}$H$_{5.8(1)}$Cl$_2$ we find substitution by Cu of the Zn atoms in the layers separating the kagom\'e layers as well as substantial Zn substitution for Cu in the kagom\'e layers. In YCu$_3$(OH)$_{6}$Br$_{2}$[Br$_x$(OH)$_{1-x}$] site mixing disorder is present for intermediate $x$. Analogous to the Cl homologous system in crystals with $x = 1/3$ disorder is absent and a low-temperature structural transition emerges driven by strong magneto-phonon coupling as a release of frustration. Apart from this structural anomaly we find the physical properties of these crystals unchanged compared to intermediate $x$ and closely resembling the Cl homologue where long-range magnetic order was observed.

To search for a conceptual picture describing the magnetization plateau phenomenon, we surveyed the crystal structures and the spin lattices of those magnets exhibiting plateaus in their magnetization vs. magnetic field curves by probing the three questions: (a) why only certain magnets exhibit magnetization plateaus, (b) why there occur several different types of magnetization plateaus, and (c) what controls the widths of magnetization plateaus. We show that the answers to these questions lie in how the magnets under field absorb Zeeman energy hence changing their magnetic structures. The magnetic structure of a magnet insulator is commonly described in terms of its spin lattice, which requires the determination of the spin exchanges nonnegligible strengths between the magnetic ions. Our work strongly suggests that a magnet under magnetic field partitions its spin lattice into antiferromagnetic (AFM) or ferrimagnetic fragments by breaking its weak magnetic bonds. Our supposition of the field-induced partitioning of spin lattice into magnetic fragments is supported by the anisotropic magnetization plateaus of Ising magnets and by the highly anisotropic width of the 1/3-magnetization plateau in azurite. The answers to the three questions (a) - (c) emerge naturally by analyzing how these fragments are formed under magnetic field., Comment: A survey of the magnetization plateau phenomenon; 128 pages including the Supporting Information

The perovskite oxide EuTiO3 (ETO) has attracted increased scientific interest due to its potential multiferroic properties and magnetic activity above and below its structural phase transition at TS=282K. Various experiments have indirectly evidenced that this transition is neither a cubic tetragonal nor the only one occurring in ETO. Here, we show new results demonstrating two further instabilities below TS based on lattice dynamics and spin-phonon interactions combined with a Landau free energy model with coupled order parameters. The new transition temperatures perfectly agree with available experimental data where further instabilities have been anticipated., Comment: 14 pages, 5 figures

We have investigated the magnetic ground state properties of the quantum spin trimer compound strontium hydroxy copper chloride SrCu(OH)$_3$Cl using bulk magnetization, specific heat measurements, nuclear magnetic resonance (NMR), and electron spin resonance (ESR) spectroscopy. SrCu(OH)$_3$Cl consists of layers with isolated Cu$^{2+}$ triangles and hence provides an opportunity to understand the magnetic ground state of an isolated system of \textit{S} = 1/2 arranged on an equilateral triangle. Although magnetization measurements do not exhibit a phase transition to a long-range ordered state down to \textit{T} = 2 K, they reveal the characteristic behavior of isolated trimers with an exchange of $J = 154$~K. The Curie-Weiss behavior changes around 50--80~K, as is also seen in the NMR spin-lattice relaxation rate. In zero magnetic field, our specific heat data establish a second-order phase transition to an antiferromagnetic ground state below \textit{T}= 1.2 K. We have drawn a magnetic field-temperature ($H$-$T$) phase diagram based on the specific heat measurements. The ESR data show divergence of the linewidth at lower temperatures, which precedes the phase transition to an antiferromagnetic long-range ordered state with unconventional critical exponents. The temperature variation of the $g$-factor further confirms the antiferromagnetic phase transition and reflects the underlying magneto-crystalline anisotropy of the compound.

Polar and chiral crystal symmetries confer a variety of potentially useful functionalities upon solids by coupling otherwise noninteracting mechanical, electronic, optical, and magnetic degrees of freedom. We describe two unstudied phases of the 3D perovskite, CsSnBr$_3$, which emerge below 85 K due to the formation of Sn(II) lone pairs and their interaction with extant octahedral tilts. Phase II (77 K<$T$<85 K, space group $P2_1/m$) exhibits ferroaxial order driven by a noncollinear pattern of lone pair-driven distortions within the plane normal to the unique octahedral tilt axis, preserving the inversion symmetry observed at higher temperatures. Phase I ($T$<77 K, space group $P2_1$) additionally exhibits ferroelectric order due to distortions along the unique tilt axis, breaking both inversion and mirror symmetries. This polar and chiral phase exhibits second harmonic generation from the bulk and a large, intrinsic polarization$-$electrostriction coefficient along the polar axis ($Q_{22}\approx$1.1 m$^4$ C$^{-2}$), resulting in acute negative thermal expansion ($\alpha_V=-9\times10^{-5}$ K$^{-1}$) through the onset of spontaneous polarization. The unprecedented structures of phases I and II were predicted by recursively following harmonic phonon instabilities to generate a tree of candidate structures and subsequently corroborated by synchrotron X-ray powder diffraction and polarized Raman and $^{81}$Br nuclear quadrupole resonance spectroscopies. Relativistic electronic structure scenarios compatible with reported photoluminescence measurements are discussed. Together, the polar symmetry, small bandgap, large spin-orbit splitting of Sn 5$p$ orbitals, and predicted strain sensitivity of the symmetry-breaking distortions suggest bulk samples and epitaxial films of CsSnBr$_3$ or its neighboring solid solutions as strong candidates for bulk Rashba effects.

We examined what interactions control the sign and strength of the interlayer coupling in van der Waals ferromagnets such as Fe3-xGeTe2, Cr2Ge2Te6, CrI3 and VI3, to find that high-spin orbital interaction across the van der Waals gaps are a key to understanding their ferromagnetism. Interlayer ferromagnetic coupling in Fe3-xGeTe2 Cr2Ge2Te6, and CrI3 is governed by the high-spin two-orbital two-electron destabilization, but that in VI3 by the high-spin four-orbital two-electron stabilization. These interactions explain a number of seemingly puzzling observations in van der Waals fer-romagnets., Comment: 17 pages, 11 figures

Superconducting bismuthates (Ba,K)BiO$_{3}$ (BKBO) constitute an interesting class of superconductors in that superconductivity with a remarkably high $T_\mathrm{c}$ of 30 K arises in proximity to charge density wave (CDW) order. Prior understanding on the driving mechanism of the CDW and superconductivity emphasizes the role of either bismuth (negative $U$ model) or oxygen ions (ligand hole model). While holes in BKBO presumably reside on oxygen owing to their negative charge transfer energy, so far there has been no other comparative material studied. Here, we introduce (Ba,K)SbO$_{3}$ (BKSO) in which the Sb 5$s$ orbital energy is higher than that of the Bi 6$s$ orbitals enabling tuning of the charge transfer energy from negative to slightly positive. The parent compound BaSbO$_{3-\delta}$ shows a larger CDW gap compared to the undoped bismuthate BaBiO$_{3}$. As the CDW order is suppressed via potassium substitution up to 65 %, superconductivity emerges, rising up to $T_\mathrm{c}$ = 15 K. This value is lower than the maximum $T_\mathrm{c}$ of BKBO, but higher by more than a factor of two at comparable potassium concentrations. The discovery of an enhanced CDW gap and superconductivity in BKSO indicates that the sign of the charge transfer energy may not be crucial, but instead strong metal-oxygen covalency plays the essential role in constituting a CDW and high-$T_\mathrm{c}$ superconductivity in the main-group perovskite oxides.

We determined the spin exchanges between the Cu2+ ions in the kagome layers of volborthite, Cu3V2O7(OH)2x2H2O, by performing the energy-mapping analysis based on DFT+U calculations, to find that the kagom\'e layers of Cu2+ ions are hardly spin-frustrated, and the magnetic properties of volborthite below ~75 K should be described by very weakly interacting antiferromagnetic uniform chains made up of effective S=1/2 pseudospin units. This conclusion was verified by synthesizing single crystals of not only Cu3V2O7(OH)2x2H2O but also its deuterated analogue Cu3V2O7(OD)2x2D2O and then by investigating their magnetic susceptibilities and specific heats. Each kagome layer consists of intertwined two-leg spin ladders with rungs of linear spin trimers. With the latter acting as S=1/2 pseudospin units, each two-leg spin ladder behaves as a chain of S=1/2 pseudospins. Adjacent two-leg spin ladders in each kagome layer interact very weakly, so it is required that all nearest-neighbor spin exchange paths of every two-leg spin ladder remain antiferromagnetically coupled in all spin ladder arrangements of a kagome layer. This constraint imposes three sets of entropy spectra with which each kagome layer can exchange energy with the surrounding on lowering the temperature below ~1.5 K and on raising the external magnetic field B. We discovered that the specific heat anomalies of volborthite observed below ~1.5 K at B = 0 are suppressed by raising the magnetic field B to ~4.2 T, that a new specific heat anomaly occurs when B is increased above ~5.5 T, and that the imposed three sets of entropy spectra are responsible for the field-dependence of the specific heat anomalies., Comment: 36 pages incl. Supplementary Information

Using an {\it ab initio} approach, we report a phonon soft mode in the tetragonal structure described by the space group $I4_{1}22$ of the $1$ K $5d$ superconductor Cd$_2$Re$_2$O$_7$. It induces an orthorhombic distortion to a crystal structure described by the space group $F222$ which hosts the superconducting state. This new phase has a lower total energy than the other known crystal structures of Cd$_2$Re$_2$O$_7$. Comprehensive temperature dependent Raman scattering experiments on isotope enriched samples, $^{116}$Cd$_2$Re$_2{^{18}}$O$_7$, not only confirm the already known structural phase transitions but also allow us to identify a new characteristic temperature regime around $\sim 80$ K, below which the Raman spectra undergo remarkable changes with the development of several sharp modes and mode splitting. Together with the results of the \textit{ab initio} phonon calculations we take these observations as strong evidence for another phase transition to a novel low-temperature crystal structure of Cd$_2$Re$_2$O$_7$., Comment: 7 pages, 8 figures

Spin-orbit coupling of as large as a half eV for electrons in 5$d$ orbitals often gives rise to the formation of spin-orbital entangled objects, characterized by the effective total angular momentum $J_{eff}$. Of particular interest are the $J_{eff}$ = 3/2 states realized in 5$d^{1}$ transition metal ions surrounded by an anion octahedron. The pure $J_{eff}$ = 3/2 quartet does not have any magnetic dipolar moment (<$M$> = 0) but hosts hidden pseudo-dipolar moments accompanied by charge quadrupoles and magnetic octupoles. Cs$_2$TaCl$_6$ and Rb$_2$TaCl$_6$ are correlated insulators with 5$d^{1}$ Ta$^{4+}$ ions in a regular Cl octahedron. Here we demonstrate that these Ta chlorides have a substantially suppressed effective magnetic dipolar moment of ~ 0.2 ${\mu}_B$. Two phase transitions are observed at low temperatures that are not pronounced in the magnetization but accompanied with large electronic entropy of $R$ln4. We ascribe the two transitions to the ordering of hidden multipoles.

The mineral linarite, PbCuSO$_4$(OH)$_2$, is a spin 1/2 chain with frustrating nearest neighbor ferromagnetic and next-nearest neighbor antiferromagnetic exchange interactions. Our inelastic neutron scattering experiments performed above the saturation field establish that the ratio between these exchanges is such that linarite is extremely close to the quantum critical point between spin-multipolar phases and the ferromagnetic state. However, the measured complex magnetic phase diagram depends strongly on the magnetic field direction. The field-dependent phase sequence is explained by our classical simulations of a nearly critical model with tiny orthorhombic exchange anisotropy. The simulations also capture qualitatively the measured variations of the wave vector as well as the staggered and the uniform magnetizations in an applied field.

Recent interest in topological semimetals has lead to the proposal of many new topological phases that can be realized in real materials. Next to Dirac and Weyl systems, these include more exotic phases based on manifold band degeneracies in the bulk electronic structure. The exotic states in topological semimetals are usually protected by some sort of crystal symmetry and the introduction of magnetic order can influence these states by breaking time reversal symmetry. Here we show that we can realize a rich variety of different topological semimetal states in a single material, $\rm CeSbTe$. This compound can exhibit different types of magnetic order that can be accessed easily by applying a small field. It allows, therefore, for tuning the electronic structure and can drive it through a manifold of topologically distinct phases, such as the first nonsymmorphic magnetic topological material with an eight-fold band crossing at a high symmetry point. Our experimental results are backed by a full magnetic group theory analysis and ab initio calculations. This discovery introduces a realistic and promising platform for studying the interplay of magnetism and topology.

H2S is converted under ultrahigh pressure (> 110 GPa) to a metallic phase that becomes superconducting with a record Tc of 200 K. It has been proposed that the superconducting phase is body-centered cubic H3S ( Im3m , a = 3.089 {\AA}) resulting from a decomposition reaction 3H2S --> 2H3S + S. The analogy of H2S and H2O leads us to a very different conclusion. The well-known dissociation of water into H3O+ and OH- increases by orders of magnitude under pressure. An equivalent behavior of H2S is anticipated under pressure with the dissociation, 2H2S --> H3S+ + SH- forming a perovskite structure (SH-)(H3S+), which consists of corner-sharing SH6 octahedra with SH- at each A-site (i.e., the center of each S8 cube). Our DFT calculations show that the perovskite (SH-)(H3S+) is thermodynamically more stable than the Im3m structure of H3S, and suggest that the A-site H atoms are most likely fluxional even at Tc., Comment: 17 pages, 6 figures, 5 tables The mistakes for the enthalpy and the free energy changes for Eq. 1 (namely, half the energy of H2 was used instead of the full energy of H2) was corrected. Our corrections of this error plus other discussion were given in a corrigendum instead of fixing the text. We added a corrigendum

The cause for the preferred spin orientation in magnetic systems containing spin-1/2 transition-metal ions was explored by studying the origin of the easy-plane anisotropy of the spin-1/2 Cu2+ ions in CuCl2.2H2O, LiCuVO4, CuCl2 and CuBr2 on the basis of density functional theory and magnetic dipole-dipole energy calculations as well as a perturbation theory treatment of the spin-orbit coupling. We find that the spin orientation observed for these spin-1/2 ions is not caused by their anisotropic spin exchange interactions, nor by their magnetic dipole-dipole interactions, but by the spin-orbit coupling associated with their crystal-field split d-states. Our study also predicts in-plane anisotropy for the Cu2+ ions of Bi2CuO4 and Li2CuO2. The results of our investigations dispel the mistaken belief that magnetic systems with spin-1/2 ions have no magnetic anisotropy induced by spin-orbit coupling., Comment: 30 pages, 5 figures; J. Chem. Phys., in press; Typos corrected

The magnitude and the temperature dependence of the superconducting order parameter {\Delta}(T) of single-crystals of Cd2Re2O7 (Tc = 1.02 K) was measured using point contact spectroscopy. In order to fit the conductance spectra and to extract the order parameter at different temperatures we generalized the Blonder-Tinkham-Klapwijk theory by including the self-energy of the quasiparticles into the Bogoliubov equations. This modification enabled excellent fits of the conductance spectra. {\Delta}(T) increases steeply below the superconducting transition temperature of 1.02 K and levels off below about 0.8 K depending on measurement directions where its value varied from 0.22(1) meV to 0.26(1) meV. Our results indicate the presence of a strong electron-phonon interaction and an enhanced quasiparticle damping and may be related to a possible phase transition within the superconducting region at about 0.8 K., Comment: 14 pages, 9 figures

Low-energy excitations of a single water molecule are studied when confined within a nano-size cage formed by the ionic crystal lattice. Optical spectra are measured of manganese doped beryl single crystal Mn:Be3Al2Si6O18, that contains water molecules individually isolated in 0.51 nm diameter voids within the crystal lattice. Two types of orientation are distinguished: water-I molecules have their dipole moments aligned perpendicular to the c axis, and dipole moments of water-II molecules are parallel to the c-axis. The optical conductivity and permittivity spectra are recorded in terahertz and infrared ranges, at frequencies from several wavenumbers up to 7000cm-1, at temperatures from 5K to 300K and for two polarizations, when the electric vector E of the radiation is parallel and perpendicular to the c-axis. Comparative experiments on as-grown and on dehydrated samples allow to identify the conductivity and permittivity spectra caused exclusively by water molecules. In the infrared range well-known internal modes nu1, nu2 and nu3 of the H2O molecule are observed for both polarizations, indicating the presence of water-I and water-II molecules in the crystal. Spectra recorded below 1000cm-1 reveal a rich set of highly anisotropic features in the low-energy response of H2O molecule in a crystalline nanocage. While for E parallel to c only two absorption peaks are detected, at 90cm-1 and 160cm-1, several absorption bands are discovered for E perpendicular to c, each consisting of narrower resonances. The bands are assigned to librational and translational vibrations of water-I molecule that is weakly, via hydrogen bonds, coupled to the nanocage walls. A model is presented that explains the fine structure of the bands by splitting of the energy levels due to quantum tunneling between the minima in a six-well potential relief felt by a molecule within the cage., Comment: 16 figures, 3 tables

In the s = 1/2 antiferromagnetic spin chain material TiPO4 the formation of a spin gap takes place in a two step process with two characteristic temperatures, T*=111 K and TSP=74 K. We observe an unusual lattice dynamics over a large temperature regime as well as evidence for an orbital instability preceding the spin-Peierls transition. We relate different intrachain exchange interactions of the high temperature compared to the spin-Peierls phase to a modification of the orbital ordering pattern. In particular, our observation of a high energy excitation of mixed electronic and lattice origin suggests an exotic dimerization process different from other spin-Peierls materials.

When water is confined to nanocavities, its quantum-mechanical behavior can be revealed by terahertz spectroscopy. We place H2O molecules in the nanopores of a beryl crystal-lattice and observe a rich and highly anisotropic set of absorption lines in the terahertz spectral range. Two bands can be identified, which originate from translational and librational motions of the water molecule isolated within the cage; they correspond to the analogous broad bands in liquid water and ice. In the present case of well-defined and highly symmetric nanocavities, the observed fine structure can be explained by macroscopic tunneling of the H2O molecules within a six-fold potential caused by the interaction of the molecule with the cavity walls.

The spin reorientation (SR) phenomenon of the square-lattice antiferromagnets RMnAsO (R = Ce, Nd) was investigated by analyzing the spin exchange interactions between the rare-earth and transition-metal ions (R3+ and Mn2+, respectively) on the basis of density functional calculations. It is found that the symmetry and strength of the Dzyaloshinskii-Moriya (DM) interaction are determined primarily by the partially filled 4f states of the R3+ ions, and that the DM and biquadratic (BQ) exchanges between the R3+ and Mn2+ ions are unusually strong and control the observed spin reorientation phenomenon. Below their SR temperature, the Mn2+ and Ce3+ moments are orthogonal in CeMnAsO but are collinear in NdMnAsO, because the DM interaction dominates over the BQ interaction for CeMnAsO while the opposite is the case for NdMnAsO. Experiments designed to test the implications of our findings are proposed., Comment: 33 pages, 9 figures; Inorganic Chemistry 2012

The spin-1/2 Cu2+ ions of LiCuVO4 form one-dimensional chains along the b-direction, and the spin frustration in LiCuVO4 is described in terms of the nearest-neighbor ferromagnetic exchange J1 and the next-nearest-neighbor antiferromagnetic exchange J2 in these chains. Recently, it has become controversial whether or not J1 is stronger in magnitude than J2. To resolve this controversy, we determined the crystal structure of LiCuVO4 at 1.6 K by neutron diffraction, analyzed the magnetic susceptibility of LiCuVO4 to deduce the Curie-Weiss temperature \theta and the J2/J1 ratio, and finally extracted the spin exchange constants of LiCuVO4 on the basis of density functional calculations. Our work shows unambiguously that the Curie-Weiss temperature \theta of LiCuVO4 is negative in the range of -20 K, so that J2 is substantially stronger in magnitude than J1., Comment: 30 pages, 4 tables, 4 figures

The spin lattice appropriate for Azurite Cu3(CO3)2(OH)2 was determined by evaluating its spin exchange interactions on the basis of first principles density functional calculations. It is found that Azurite is not described by an isolated diamond chain with no spin frustration, but by a two-dimensional spin lattice in which diamond chains with spin frustration interact through the interchain spin exchange in the ab-plane. Our analysis indicates that the magnetic properties of Azurite at low temperatures can be approximated by two independent contributions, i.e., an isolated dimer and an effective uniform chain contributions. This prediction was verified by analyzing the magnetic susceptibility and specific heat data of Azurite., Comment: 23 pages, 3 figures, 3 tables. J. Phys.: Condes. Matter, in press

We review the characteristics of some carbon based novel superconductors which emerged in the past two decades since the discovery of superconductivity in the high-T_c oxocuprates. In particular, we summarize the properties of ternary layered carbide halides of the rare rarth metals with composition RE2C2X2 (RE = Y, La; X=Cl, Br, I) and of the rare earth di- and sesquicarbides, YC2, LaC2 and La2C3. Finally, we briefly discuss the properties of the recently discovered Ca and Yb intercalated graphite superconductors, CaC6 and YbC6., Comment: Carbon Based Superconductors

The crystal structure of the copper(II) tellurium(IV) oxochloride Cu$_{4}$Te$_{5}$O$_{12}$Cl$_{4}$ (Cu-45124) is composed of weakly coupled tetrahedral Cu clusters and shows crystallographic similarities with the intensively investigated compound Cu$_{2}$Te$_{2}$O$_{5}$X$_{2}$, with X~=~Cl, Br (Cu-2252). It differs from the latter by a larger separation of the tetrahedra within the crystallographic ab plane, that allows a more direct assignment of important inter-tetrahedra exchange paths and the existence of an inversion center. Magnetic susceptibility and specific heat evidence antiferromagnetic, frustrated correlations of the Cu spin moments and long range ordering with $T_{c}$=13.6 K. The entropy related to the transition is reduced due to quantum fluctuations. In Raman scattering a well structured low energy magnetic excitation is observed at energies of $\approx$50K (35cm$^{-1})$. This energy scale is reduced as compared to Cu-2252., Comment: 11 pages, 9 figures, further information see http://www.peter-lemmens.de

Thermodynamic properties of the multiband superconductor MgB$_{2}$ have often been described using a simple sum of the standard BCS expressions corresponding to $\sigma$- and $\pi$-bands. Although, it is \textit{a priori} not clear if this approach is working always adequately, in particular in cases of strong interband scattering. Here we compare the often used approach of a sum of two independent bands using BCS-like $\alpha$-model expressions for the specific heat, entropy and free energy to the solution of the full Eliashberg equations. The superconducting energy gaps, the free energy, the entropy and the heat capacity for varying interband scattering rates are calculated within the framework of two-band Eliashberg theory. We obtain good agreement between the phenomenological two-band $\alpha$-model with the Eliashberg results, which delivers for the first time the theoretical verification to use the $\alpha$-model as a useful tool for a reliable analysis of heat capacity data. For the thermodynamic potential and the entropy we demonstrate that only the sum over the contributions of the two bands has physical meaning., Comment: 27 pages, 10 figures, 1 table, submitted to Phys. Rev. B

The structural properties under hydrostatic pressure up to 3 GPa of the layered rare earth carbide halide superconductor Y2C2I2 are studied by neutron powder diffraction at room temperature. The compressibilities are anisotropic, such that the compressibility perpendicular to the layers being approximately twice as large as within the layers. The atomic positional parameters determined from the powder diffraction patterns as a function of pressure were used as the basis for highly resolved electronic structure calculations. These reveal the electronic density of states at E_F to increase with pressure. As is shown quantitatively, this effect outweighs the pressure induced lattice stiffening effects and is responsible for the pressure induced increase and saturation of Tc towards 2 GPa., Comment: 12 pages, 6 figures

We argue, based on band structure calculations and Eliashberg theory, that the observed decrease of $T_c$ of Al and C doped MgB$_2$ samples can be understood mainly in terms of a band filling effect due to the electron doping by Al and C. A simple scaling of the electron-phonon coupling constant $\lambda$ by the variation of the density of states as function of electron doping is sufficient to capture the experimentally observed behavior. Further, we also explain the long standing open question of the experimental observation of a nearly constant $\pi$ gap as function of doping by a compensation of the effect of band filling and interband scattering. Both effects together generate a nearly constant $\pi$ gap and shift the merging point of both gaps to higher doping concentrations, resolving the discrepancy between experiment and theoretical predictions based on interband scattering only., Comment: accepted by PRL

High-resolution X-ray diffraction data show that the low-temperature superstructure of alpha-NaV2O5 has an F-centered orthorhombic 2a x 2b x 4c superlattice. A structure model is proposed, that is characterized by layers with zigzag charge order on all ladders and stacking disorder, such that the averaged structure has space group Fmm2. This model is in accordance with both X-ray scattering and NMR data. Variations in the stacking order and disorder offer an explanation for the recently observed devils staircase of the superlattice period along c., Comment: REVTEX, 4 pages including 2 figures, shortened, submitted to PRL