217 research outputs found
Interaction of Phonons and Dirac Fermions on the Surface of Bi2Se3: A Strong Kohn Anomaly
We report the first measurements of phonon dispersion curves on the (001)
surface of the strong three-dimensional topological insulator Bi2Se3. The
surface phonon measurements were carried out with the aid of coherent helium
beam surface scattering techniques. The results reveal a prominent signature of
the exotic metallic Dirac fermion quasi-particles, including a strong Kohn
anomaly. The signature is manifest in a low energy isotropic convex dispersive
surface phonon branch with a frequency maximum of 1.8 THz, and having a
V-shaped minimum at approximately 2kF that defines the Kohn anomaly.
Theoretical analysis attributes this dispersive profile to the renormalization
of the surface phonon excitations by the surface Dirac fermions. The
contribution of the Dirac fermions to this renormalization is derived in terms
of a Coulomb-type perturbation model
Temperature dependence of surface magnetization in local-moment systems
We present a theory to study the temperature-dependent behavior of surface
states in a ferromagnetic semi-infinite crystal. Our approach is based on the
single-site approximation for the \emph{s-f} model. The effect of the
semi-infinite nature of the crystal is taken into account by a localized
perturbation method. Using the mean-field theory for the layer-dependent
magnetization, the local density of states and the electron-spin polarization
are investigated at different temperatures for ordinary and surface transition
cases. The results show that the surface magnetic properties may differ
strongly from those in the bulk and the coupling constant of atoms plays a
decisive role in the degree of spin polarization. In particular, for the case
in which the exchange coupling constant on the surface and between atoms in the
first and second layer is higher than the corresponding in the bulk, an
enhancement of surface Curie temperature and hence the spin polarization can be
obtained.Comment: 9 pages,8 figure
Temperature-Dependent Anomalies in the Structure of the (001) Surface of LiCu2O2
Surface corrugation functions, derived from elastic helium atom scattering
(HAS) diffraction patterns at different temperatures, reveal that the Cu2+ rows
in the (001) surface of LiCu2O2 undergo an outward displacement of about 0.15
{\AA} as the surface was cooled down to 140 K. This is probably the first time
that isolated one-dimensional magnetic ion arrays were realized, which
qualifies the Li1+Cu2+O2-2 surface as a candidate to study one-dimensional
magnetism. The rising Cu2+ rows induce a surface incommensurate structural
transition along the a-direction. Surface equilibrium analysis showed that the
surface Cu2+ ions at bulk-like positions experience a net outward force along
the surface normal which is relieved by the displacement. Temperature-dependent
changes of the surface phonon dispersions obtained with the aid of inelastic
HAS measurements combined with surface lattice dynamical calculations are also
reported.Comment: 4 pages, 7 figure
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