208 research outputs found
Spin excitations in a 4f-3d heterodimer on MgO
We report on the magnetic properties of HoCo dimers as a model system for the
smallest intermetallic transition metal-lanthanide compound. The dimers are
adsorbed on ultrathin MgO(100) films grown on Ag(100). New for elements,
we detect inelastic excitations with scanning tunneling microscopy and prove by
their behaviour in applied magnetic field that they are spin-excitations. In
combination with density functional theory and spin Hamiltonian analysis we
determine the magnetic level distribution, as well as sign and magnitude of the
exchange interaction between the two atoms. In contrast to typical bulk
compounds, we find ferromagnetic coupling in the dimer
Spontaneous exciton condensation in 1T-TiSe2: a BCS-like approach
Recently strong evidence has been found in favor of a BCS-like condensation
of excitons in 1\textit{T}-TiSe. Theoretical photoemission intensity maps
have been generated by the spectral function calculated within the excitonic
condensate phase model and set against experimental angle-resolved
photoemission spectroscopy data. Here, the calculations in the framework of
this model are presented in detail. They represent an extension of the original
excitonic insulator phase model of J\'erome \textit{et al.} [Phys. Rev. {\bf
158}, 462 (1967)] to three dimensional and anisotropic band dispersions. A
detailed analysis of its properties and further comparison with experiment are
also discussedComment: Submitted to PRB, 11 pages, 7 figure
Quantifying residual hydrogen adsorption in low-temperature STMs
We report on low-temperature scanning tunneling microscopy observations demonstrating that individual Ti atoms on hexagonal boron nitride dissociate and adsorb hydrogen without measurable reaction barrier. The clean and hydrogenated states of the adatoms are clearly discerned by their apparent height and their differential conductance revealing the Kondo effect upon hydrogenation. Measurements at 50 K and 5 × 10−11 mbar indicate a sizable hydrogenation within only 1 h originating from the residual gas pressure, whereas measurements at 4.7 K can be carried out for days without H2 contamination problems. However, heating up a low-T STMto operate it at variable temperature results in very sudden hydrogenation at around 17 K that correlates with a sharp peak in the total chamber pressure. From a quantitative analysis we derive the desorption energies of H2 on the cryostat walls. We find evidence for hydrogen contamination also during Ti evaporation and propose a strategy on how to dose transition metal atoms in the cleanliest fashion. The present contribution raises awareness of hydrogenation under seemingly ideal ultra-high vacuum conditions, it quantifies the H2 uptake by isolated transition metal atoms and its thermal desorption from the gold plated cryostat walls
Atomically precise lateral modulation of a two-dimensional electron liquid in anatase TiO2 thin films
Engineering the electronic band structure of two-dimensional electron liquids
(2DELs) confined at the surface or interface of transition metal oxides is key
to unlocking their full potential. Here we describe a new approach to tailoring
the electronic structure of an oxide surface 2DEL demonstrating the lateral
modulation of electronic states with atomic scale precision on an unprecedented
length scale comparable to the Fermi wavelength. To this end, we use pulsed
laser deposition to grow anatase TiO2 films terminated by a (1 x 4) in-plane
surface reconstruction. Employing photo-stimulated chemical surface doping we
induce 2DELs with tunable carrier densities that are confined within a few TiO2
layers below the surface. Subsequent in-situ angle resolved photoemission
experiments demonstrate that the (1 x 4) surface reconstruction provides a
periodic lateral perturbation of the electron liquid. This causes strong
backfolding of the electronic bands, opening of unidirectional gaps and a
saddle point singularity in the density of states near the chemical potential
Fermi Surface and Quasiparticle Excitations of overdoped Tl2Ba2CuO6+d by ARPES
The electronic structure of the high-T_c superconductor Tl2Ba2CuO6+d is
studied by ARPES. For a very overdoped Tc=30K sample, the Fermi surface
consists of a single large hole pocket centered at (pi,pi) and is approaching a
topological transition. Although a superconducting gap with d_x^2-y^2 symmetry
is tentatively identified, the quasiparticle evolution with momentum and
binding energy exhibits a marked departure from the behavior observed in under
and optimally doped cuprates. The relevance of these findings to scattering,
many-body, and quantum-critical phenomena is discussed.Comment: Revised manuscript, in press on PRL. A high-resolution version can be
found at
http://www.physics.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/Tl2201_LE.pdf
and related material at
http://www.physics.ubc.ca/~quantmat/ARPES/PUBLICATIONS/articles.htm
First observation of spin-helical Dirac fermions and topological phases in undoped and doped Bi2Te3 demonstrated by spin-ARPES spectroscopy
Electron systems that possess light-like dispersion relations or the conical
Dirac spectrum, such as graphene and bismuth, have recently been shown to
harbor unusual collective states in high magnetic fields. Such states are
possible because their light-like electrons come in spin pairs that are
chiral,which means that their direction of propagation is tied to a quantity
called pseudospin that describes their location in the crystal lattice. An
emerging direction in quantum materials research is the manipulation of atomic
spin-orbit coupling to simulate the effect of a spin dependent magnetic
field,in attempt to realize novel spin phases of matter. This effect has been
proposed to realize systems consisting of unpaired Dirac cones that are
helical, meaning their direction of propagation is tied to the electron spin
itself, which are forbidden to exist in graphene or bismuth. The experimental
existence of topological order can not be determined without spin-resolved
measurements. Here we report a spin-and angle-resolved photoemission study of
the hexagonal surface of the Bi2Te3 and Bi{2-x}MnxTe3 series, which is found to
exhibit a single helical Dirac cone that is fully spin-polarized. Our
observations of a gap in the bulk spin-degenerate band and a spin-resolved
surface Dirac node close to the chemical potential show that the low energy
dynamics of Bi2Te3 is dominated by the unpaired spin-helical Dirac modes. Our
spin-texture measurements prove the existence of a rare topological phase in
this materials class for the first time, and suggest its suitability for novel
2D Dirac spin device applications beyond the chiral variety or traditional
graphene.Comment: 13 pages, 4 figure
Temperature dependence of the Kondo resonance and its satellites in CeCu_2Si_2
We present high-resolution photoemission spectroscopy studies on the Kondo
resonance of the strongly-correlated Ce system CeCuSi. Exploiting the
thermal broadening of the Fermi edge we analyze position, spectral weight, and
temperature dependence of the low-energy 4f spectral features, whose major
weight lies above the Fermi level . We also present theoretical
predictions based on the single-impurity Anderson model using an extended
non-crossing approximation (NCA), including all spin-orbit and crystal field
splittings of the 4f states. The excellent agreement between theory and
experiment provides strong evidence that the spectral properties of
CeCuSi can be described by single-impurity Kondo physics down to K.Comment: 4 pages, 3 figure
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