Multiferroicity induced by dislocated spin-density waves

We uncover a new pathway towards multiferroicity, showing how magnetism can drive ferroelectricity without relying on inversion symmetry breaking of the magnetic ordering. Our free-energy analysis demonstrates that any commensurate spin-density-wave ordering with a phase dislocation, even if it is collinear, gives rise to an electric polarization. Because of the dislocation, the electronic and magnetic inversion centers do not coincide, which turns out to be a sufficient condition for multiferroic coupling. The novel mechanism explains the formation of multiferroic phases at the magnetic commensurability transitions, such as the ones observed in YMn(2)O(5) and related compounds. We predict that in these multiferroics an oscillating electrical polarization is concomitant with the uniform polarization. On the basis of our theory, we put forward new types of magnetic materials that are potentially ferroelectric.     

Imperfect nesting and Peierls instability for a two-dimensional tight-binding model

Based on a half-filled two-dimensional tight-binding model with nearest-neighbour and next nearest-neighbour hopping the effect of imperfect Fermi surface nesting on the Peierls instability is studied at zero temperature. Two dimerization patterns corresponding to a phonon vector (π,π) are considered. It is found that the Peierls instability will be suppressed with an increase of next nearest-neighbour hopping which characterizes the nesting deviation. First and second order transitions to a homogeneous state are possible. The competition between the two dimerized states is discussed. Received 22 December 2000     

Ferroelectricity induced by acentric spin-density waves in YMn2O5

The commensurate and incommensurate magnetic structures of the magnetoelectric system YMn2O5, as determined from neutron diffraction, were found to be spin-density waves lacking a global center of symmetry. We propose a model, based on a simple magnetoelastic coupling to the lattice, which enables us to predict the polarization based entirely on the observed magnetic structure. Our data accurately reproduce the temperature dependence of the spontaneous polarization, particularly its sign reversal at the commensurate-incommensurate transition.     

The phase diagram of charge- and spin-density waves in polymeric C60

\chem{(C_{60})_{\chemindex{x}}} at interfullerene distances larger than . At smaller ball separation, a special charge-density-wave ground state occurs, which exhibits polarized spheres. At around experimental bond distances, both CDW and SDW are present with rather small amplitudes. Received: 13 September 1996/Accepted: 11 November 1996     

Oscillatory exchange bias due to an antiferromagnet with incommensurate spin-density waves

Oscillatory exchange bias in both magnitude and in sign has been observed in epitaxial (100)Cr/Ni(81)Fe19 bilayers due to the incommensurate spin-density waves in antiferromagnetic (100)Cr layers. Salient effects due to the spin-flip transition between longitudinal and transverse spin-density waves as well as that of expanding wavelength have been observed.     

Tunnel characteristics of partially gapped non-superconducting metals with charge- or spin-density waves

Current-voltage characteristics for symmetrical and non-symmetrical tunnel junctions involving non-superconducting partially dielectrized metals are calculated. Root singularities at eV = ±Σ and jumps at eV = ±2Σ, where V is the bias voltage, Σ is the dielectric gap, and e is the elementary charge, are obtained for symmetrical junctions. For non-symmetrical ones current-voltage characteristics are non-symmetrical and depend on the sign of Σ. The results obtained agree qualitatively with tunnel and point-contact measurements for layered dichalcogenides, NbSe₃, and URu₂Si₂.     

First-principles study of spin-density waves in Fe/Cr and V/Cr multilayers

A first-principles electronic structure calculation for Fe/Cr and V/Cr multilayers is performed, and spin-density wave order in the Cr layer is investigated. It is found that for a sufficiently large spacer thickness of the Cr layer the spin-density wave order grows spontaneously and becomes similar to that in bulk Cr in the vicinity of the middle of the Cr layer.     

Triangular lattices of charge-and spin-density waves in chemisorbed metal monolayers on the surface of diamond structure semiconductors

The electronic spectrum of a planar triangular lattice is analyzed, and the possible occurrence of charge (CDWs) and spin density waves (SDWs) in the lattice is discussed. Commensurate CDW and SDW structures of two types are considered in the weak and strong electron-electron interaction approximations. The CDW and SDW models are applied to the specific case of chemisorbed metal monolayers on the (111) surface of diamond structure semiconductors with a coverage close to 1/3.     

Unusual spectral behavior of charge-density waves with imperfect nesting in a quasi-one-dimensional metal

Low-temperature electronic properties of the charge-density-wave system NbSe3 are reported from angle-resolved photoemission at 15 K. The effect of two instabilities q(1) and q(2) on the k-resolved spectral function is observed for the first time. With a pseudogap background, the gap spectra exhibit maxima at Delta*(1) approximately 110 meV and Delta*(2) approximately 45 meV. Imperfectly nested sections of the Fermi surface lack a Fermi-Dirac edge, and show the signature of a dispersion that is modified by self-energy effects. The energy scale is of the order of the effective gap 2 Delta*(2). The effect disappears above T2, suggesting a correlation with the charge-density-wave state.