J. Fedor

Tunable transport in magnetically coupled MoGe/Permalloy hybrids

We demonstrate controlled magnetotransport anisotropy of magnetically coupled superconductor-ferromagnet MoGe/Permalloy hybrids. The rotatable anisotropy Permalloy ferromagnet with stripe domain structure induces in-plane anisotropy in superconducting order parameter. We show that near the superconductor-normal state phase boundary the superconductivity in MoGe is localized in narrow mesoscopic channels just above the magnetic domain walls. Changing the in-plane direction of magnetic stripe domains it is possible to reconfigure the direction of the superconducting channels and controllably rotate the direction of the in-plane anisotropy axis in the superconductor.

Coexistence and coupling of two distinct charge density waves in Sm2Te5.

Sm2Te5 is an example of compound that has two separate charge density wave (CDW) distortions of its square nets of tellurium. The two CDWs originate from electronic instabilities in two distinct parts of the structure but create unique structural patterns due to mutual interference.

Visualizing domain wall and reverse domain superconductivity

In magnetically coupled, planar ferromagnet-superconductor (F/S) hybrid structures, magnetic domain walls can be used to spatially confine the superconductivity. In contrast to a superconductor in a uniform applied magnetic field, the nucleation of the superconducting order parameter in F/S structures is governed by the inhomogeneous magnetic field distribution. The interplay between the superconductivity localized at the domain walls and far from the walls leads to effects such as re-entrant superconductivity and reverse domain superconductivity with the critical temperature depending upon the location. Here we use scanning tunnelling spectroscopy to directly image the nucleation of superconductivity at the domain wall in F/S structures realized with Co-Pd multilayers and Pb thin films. 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 nucleus by applying an external magnetic field, potentially useful to guide vortices for computing application.

The local effect of magnetic impurities on superconductivity in CoxNbSe2 and MnxNbSe2 single crystals

We investigate the effect of individual atomic impurities on the superconducting state that they are embedded in. Using low temperature scanning tunneling microscopy and spectroscopy we could identify Co and Mn atoms in the Co(x)NbSe(2) and Mn(x)NbSe(2) single crystals and observe the influence on the local electronic density of states (LDOS) at 0.4 K. We find that Co is in the weak scattering limit. In this case the LDOS is quite homogeneous on the sample surface, despite the number of defects, and retains sharp coherent superconducting peaks. This is in strong contrast to the effects of Mn impurities, which locally destroy superconductivity. In this case the LDOS shows a strong enhancement of spectral weight inside the superconducting gap even far from the Mn atoms. Moreover, two impurity bound states are found within the superconducting gap at E/Δ(0) = 0.18 and 0.36 at locations close to defects.

Low-temperature scanning tunneling microscopy and spectroscopy measurements of ultrathin Pb films

We have investigated the electronic properties of ultrathin Pb films by low temperature scanning tunneling microscopy and spectroscopy. Our results show that 30 nm thick Pb(111) films grown on atomically flat highly oriented pyrolytic graphite (HOPG) and on amorphous SiO2 are both in the strong-coupling limit with transition temperature and energy gap close to the bulk value. Conductance maps and spectroscopy in the vortex state reveal a bound state at the center of the vortices, which suggest that the films are in the clean limit. Measurements of 3 nm Pb films grown on HOPG show a clear crossover to the weak-coupling regime and dirty limit.

Adjustable Superconducting Anisotropy in Superconductor-Ferromagnet Bilayers

We demonstrate novel magneto-transport properties of magnetically coupled superconductor-ferromagnet (MoGe/Permalloy) bilayers. Ferromagnetic Permalloy film exhibiting stripe domains with rotatable anisotropy induces modulation of the order parameter in the superconducting thin film. Domain-wall superconductivity is localized above the magnetic domain walls forming mesoscopic superfluid channels. We can achieve non-volatile re-configuration of the in-plane direction of the superconducting channels by changing the orientation of the stripe domains in the Permalloy. We discuss the influence of magnetic domain structure on Abrikosov vortex dynamics in this system.

Switching of magnetic domains in Permalloy microstructures using two-dimensional electron gas

The authors demonstrate the ability to monitor and alter the magnetization state of microscopic Permalloy element deposited on the active area of a two-dimensional electron gas (2DEG) Hall probe using the current through the 2DEG. Magnetic force microscopy imaging shows the exact magnetization state of the Permalloy thin film ellipse at different applied magnetic fields. Recorded Hall voltage signal provides information on local magnetization of the ferromagnetic element at the same time. Application of short, but intense current pulses through the Hall probe changes the magnetization state of the Permalloy ellipse.