RDS
STM measurements taken at the surface of a F/S bilayer. The top image is the conductance map acquired just below the critical temperature in the presence of an applied magnetic field that compensates one type of magnetic domains of the ferromagnet. The middle image is the conductance map at the same location and in the same applied field at T=1.5 K. The bottom image is the topography of the superconducting layer. The cartoon shows the magnetic domains of the underlaying ferromagnetic layer.
VAVmaps
Vortex Chains of opposite polarities localized above the ferromagnetic domain of the same polarity.
Magnetically coupled planar ferromagnet-superconductor (F/S) hybrid structures offer new avenues for manipulation of the superconductivity at the nanoscale and convenient means to control vortex dynamics. The nonuniform magnetic field produced by the ferromagnet spatially confines superconductivity. When the temperature is decreased below Tc, the superconductivity is expected to nucleate first at the location where the stray field is minimum, i.e. at the domain wall, which is a realization of domain wall superconductivity. Therefore, the superconductivity is confined in tiny channels that can more or less interact depending upon the distance between them. When an external magnetic field is applied the superconducting nuclei will shift to the center of the compensated domain (reverse domain superconductivity). Furthermore, by reversing the polarity of the applied field there will be a switching of a normal region in superconducting and viceversa.

Our results demonstrate that such F/S structures are attractive model systems that offer the possibility to control the strength and the location of the superconducting nuclei by applying an external magnetic field.

These types of S/F systems hold the potential to increase the critical current density of the superconducting layer through the interaction of the vortices with the magnetic moment of the underlying magnetic domain. In this case vortices form chain structures above the magnetic domains with same polarity as the applied field and they are confined to move along the channels determined by the underlying magnetic landscape of the ferromagnet.

flat_Pb_islands
STM topography image of flat hexagonal Pb islands about 14nm in diameter grown on highly oriented pyrolytic graphite.
Our goal is to understand the fundamental features that underlie the behavior of vortices under confinement in mesoscopic superconductors. When the size of a superconductor is small compared to the coherence length or the penetration depth material's property greatly differ from the bulk properties. The superconducting critical density can be enhanced and the vortex configuration can be strongly influenced by the sample geometry. As a result of the confinement, the presence of the sample's boundaries promote the appearance of exotic vortex states (giant vortex states, vortex clusters, shell configurations), otherwise forbidden in bulk samples.

The interplay between different ground states like charge density wave (CDW), superconductivity (SC) and antiferromagnetism (AF) is a major topic in condensed matter physics. Modern research techniques allow us to explore and control emergent behavior that arises from complex correlations using local real space probes (low temperature Scanning Probe Microscopy and Spectroscopy) and correlating them with k-space techniques such as ARPES and X-rays.

We are exploring this topic in dichalcogenides.

Phys. Rev. B 85, 155103 (2012)
CDW_TiSe2
Atomically resolved STM image of a TiSe2 single crystal showing a strong superlattice modulation 2a0x2a0. Line profiles along the unit vectors a1, a2 and a3 in two different regions of the image reveal the presence of chiral CDW domains.
CDW_CuxTiSe2
Atomically revolved STM image of a Cu0.05TiSe2 single crystal and filtered CDW modulations showing the presence of chiral domains also in Cu-doped TiSe2 crystals. Atomically resolved STM image of a TiSe2 single crystal showing a strong superlattice modulation 2a0x2a0. Line profiles along the unit vectors a1, a2 and a3 in two different regions of the image reveal the presence of chiral CDW domains.
0