Electron Transport in Molecular Transistors

In this thesis I will describe the conductance properties of certain organic molecules. I will first show that two metal electrodes can be fabricated with a nm-scale gap between them by causing electomigration-induced failure in a nanoscale wire. These two electrodes are separated by a few nanometers from a metallic gate electrode. Organic molecules can be incorporated into the gap between the electrodes creating a transistor geometry. Transport measurements on metal-organic complexes at low temperatures show Coulomb blockade and Kondo-assisted tunneling. Using the specially designed molecule C$_{140}$, I have studied the coupling between the vibrational modes of the molecule and electron flow through transistors made from it. I have also been able to make magnetic electrodes to pass spin-polarized current through molecules. Using this, I have studied the coexistence of the Kondo effect and ferromagnetism in the electrodes. I have also modified the technique to have a mechanically adjustable distance between the two electrodes, which is useful for studying the influence of the contact on the conductance of a device

From ballistic transport to tunneling in electromigrated ferromagnetic breakjunctions

We fabricate ferromagnetic nanowires with constrictions whose cross section can be reduced gradually from 100 nm to the atomic scale and eventually to the tunneling regime by means of electromigration. These devices are mechanically stable against magnetostriction and magnetostatic effects. We measure magnetoresistances ~ 0.3% for 100*30 nm^2 constrictions, increasing to a maximum of 80% for atomic-scale widths. These results are consistent with a geometrically-constrained domain wall trapped at the constriction. For the devices in the tunneling regime we observe large fluctuations in MR, between -10 and 85%.Comment: 4 pages, 5 figure

Nanoscale Proximity Effect in the High Temperature Superconductor Bi-2212

High temperature cuprate superconductors exhibit extremely local nanoscale phenomena and strong sensitivity to doping. While other experiments have looked at nanoscale interfaces between layers of different dopings, we focus on the interplay between naturally inhomogeneous nanoscale regions. Using scanning tunneling microscopy to carefully track the same region of the sample as a function of temperature, we show that regions with weak superconductivity can persist to elevated temperatures if bordered by regions of strong superconductivity. This suggests that it may be possible to increase the maximum possible transition temperature by controlling the distribution of dopants.Comment: To appear in Physical Review Letter

Tunable magnetic domains in ferrimagnetic MnSb2_2Te4_4

Highly tunable properties make Mn(Bi,Sb)$_2$Te$_4$ a rich playground for exploring the interplay between band topology and magnetism: On one end, MnBi$_2$Te$_4$ is an antiferromagnetic topological insulator, while the magnetic structure of MnSb$_2$Te$_4$ (MST) can be tuned between antiferromagnetic and ferrimagnetic. Motivated to control electronic properties through real-space magnetic textures, we use magnetic force microscopy (MFM) to image the domains of ferrimagnetic MST. We find that magnetic field tunes between stripe and bubble domain morphologies, raising the possibility of topological spin textures. Moreover, we combine in situ transport with domain manipulation and imaging to both write MST device properties and directly measure the scaling of the Hall response with domain area. This work demonstrates measurement of the local anomalous Hall response using MFM, and opens the door to reconfigurable domain-based devices in the M(B,S)T family