46 research outputs found
Energy-dependent spatial texturing of the charge order in -CuTiSe
We report a detailed study of the microscopic effects of Cu intercalation on
the charge density wave (CDW) in 1\textit{T}-CuTiSe. Scanning tunneling
microscopy and spectroscopy (STM/STS) reveal a unique, Cu driven spatial
texturing of the charge ordered phase, with the appearance of energy dependent
CDW patches and sharp -phase shift domain walls (DWs). The energy and
doping dependencies of the patchwork are directly linked to the inhomogeneous
potential landscape due to the Cu intercalants. They imply a CDW gap with
unusual features, including a large amplitude, the opening below the Fermi
level and a shift to higher binding energy with electron doping. Unlike the
patchwork, the DWs occur independently of the intercalated Cu
distribution. They remain atomically sharp throughout the investigated phase
diagram and occur both in superconducting and non-superconducting specimen.
These results provide unique atomic-scale insight on the CDW ground state,
questioning the existence of incommensurate CDW domain walls and contributing
to understand its formation mechanism and interplay with superconductivity
Dimensional cross-over of the charge density wave order parameter in thin exfoliated 1T-VSe
The capability to isolate one to few unit-cell thin layers from the bulk
matrix of layered compounds opens fascinating prospects to engineer novel
electronic phases. However, a comprehensive study of the thickness dependence
and of potential extrinsic effects are paramount to harness the electronic
properties of such atomic foils. One striking example is the charge density
wave (CDW) transition temperature in layered dichalcogenides whose thickness
dependence remains unclear in the ultrathin limit. Here we present a detailed
study of the thickness and temperature dependences of the CDW in VSe by
scanning tunnelling microscopy (STM). We show that mapping the real-space CDW
periodicity over a broad thickness range unique to STM provides essential
insight. We introduce a robust derivation of the local order parameter and
transition temperature based on the real space charge modulation amplitude.
Both quantities exhibit a striking non-monotonic thickness dependence that we
explain in terms of a 3D to 2D dimensional crossover in the FS topology. This
finding highlights thickness as a true tuning parameter of the electronic
ground state and reconciles seemingly contradicting thickness dependencies
determined in independent transport studies
Holographic imaging of the complex charge density wave order parameter
The charge density wave (CDW) in solids is a collective ground state
combining lattice distortions and charge ordering. It is defined by a complex
order parameter with an amplitude and a phase. The amplitude and wavelength of
the charge modulation are readily accessible to experiment. However, accurate
measurements of the corresponding phase are significantly more challenging.
Here we combine reciprocal and real space information to map the full complex
order parameter based on topographic scanning tunneling microscopy (STM)
images. Our technique overcomes limitations of earlier Fourier space based
techniques to achieve distinct amplitude and phase images with high spatial
resolution. Applying this analysis to transition metal dichalcogenides provides
striking evidence that their CDWs consist of three individual charge
modulations whose ordering vectors are connected by the fundamental rotational
symmetry of the crystalline lattice. Spatial variations in the relative phases
of these three modulations account for the different contrasts often observed
in STM topographic images. Phase images further reveal topological defects and
discommensurations, a singularity predicted by theory for a nearly commensurate
CDW. Such precise real space mapping of the complex order parameter provides a
powerful tool for a deeper understanding of the CDW ground state whose
formation mechanisms remain largely unclear
Subharmonic gap structures and Josephson effect in MgB2/Nb micro-constrictions
Superconducting micro-constrictions between Nb tips and high quality
MgB pellets have been realized by means of a point-contact inset, driven
by a micrometric screw. Measurements of the current-voltage characteristics and
of the dynamical conductance versus bias have been performed in the temperature
range between 4.2 K and 500 K. Above the Nb critical temperature T,
the conductance of the MgB/normal-metal constrictions behaves as predicted
by the BTK model for low resistance contacts while high resistance junctions
show quasiparticle tunneling characteristics. Consistently, from the whole set
of data we infer the value meV for the
three-dimensional gap of MgB. Below T, low resistance contacts
show Josephson current and subharmonic gap structures (SGS), due to multiple
Andreev reflections. Simultaneous observations of both features, unambiguously
indicate coupling of the 3D band of MgB with the Nb superconducting order
parameter. We found that the temperature dependence of the Josephson critical
current follows the classical Ambegaokar-Baratoff behavior with a value
meV at low temperatures.Comment: 8 pages, 5 figures. Replaced with published versio
Point Contact Spectra on YBaCuO/LaCaMnO bilayers
We present conductance characteristics of point contact junctions realized
between a normal Pt-Ir tip and
YBaCuO/LaCaMnO (YBCO/LCMO) bilayers. The
point contact characteristics show a zero bias conductance peak, as a
consequence of the formation of Andreev bound states at the YBCO Fermi level.
The temperature evolution of the spectra reveals a depressed zero bias peak and
a reduced superconducting energy gap, both explainable in terms of spin
polarization effects due to the LCMO layer.Comment: 4 pages, 4 EPS figures. Proceedings of EUCAS 2005. Accepted in
Journal of Physics: Conference Serie
Towards surface diffusion potential mapping on atomic length scale
The surface diffusion potential landscape plays an essential role in a number
of physical and chemical processes such as self-assembly and catalysis.
Diffusion energy barriers can be calculated theoretically for simple systems,
but there is currently no experimental technique to systematically measure them
on the relevant atomic length scale. Here, we introduce an atomic force
microscopy based method to semiquantitatively map the surface diffusion
potential on an atomic length scale. In this proof of concept experiment, we
show that the atomic force microscope damping signal at constant
frequency-shift can be linked to nonconservative processes associated with the
lowering of energy barriers and compared with calculated single-atom diffusion
energy barriers.Comment: 8 pages and 3 figure
Adsorption Behavior of Asymmetric Pd Pincer Complexes on a Cu(111) Surface
We address the adsorption of asymmetric Pd pincer complexes on a Cu(111) surface by scanning tunneling microscopy. The structural asymmetry is manifested in the observation of two chiral enantiomers. To enable an unambiguous identification of individual constituents, three closely related complexes with small modifications are investigated in parallel. Thereby, methyl substituents determine attractive molecule-molecule interaction. Depending on their distribution, dimerization and tetramerization can be observed
Imaging the spontaneous formation of vortex-antivortex pairs in planar superconductor/ferromagnet hybrid structures
Low-temperature magnetic force microscopy has been used to visualize spontaneous formation of vortex-antivortex pairs in hybrid ferromagnet/superconductor systems. Vortex-antivortex pairs are induced by the periodic stray field of the ferromagnet. We find general equilibrium conditions for which spontaneous vortex-antivortex pairs are formed during zero-field cooling of the hybrid ferromagnet/superconductor bilayers. Vortices can be generated by the ferromagnet domains in the absence of an external field and they are thermodynamically stable for values of the stray field and the period of the stripe magnetic domains that exceed a certain threshold