PhD Seminar C2N - 23rd october

"Spin and orbital current conversion using topological insulator Bi1-xSbx alloy" by Mudit Jain.

Officient conversion of spin and orbital angular momentum currents into charge currents is central to advancing spintronic and orbitronic memory and logic technologies. Conventional ferromagnet/heavy-metal heterostructures rely on the spin Hall and inverse spin Hall effects, whereas topological insulators (TIs) such as Bi₁₋ₓSbₓ offer an alternative route with potentially superior spin- and orbital-to-charge conversion (SOCC) due to their spin–momentum–locked surface states. In BiSb, strong spin–orbit coupling gives rise to both spin and orbital textures at the surface, enabling the generation of both spin and possibly orbital angular momentum currents. However, these topological surface states are highly sensitive to magnetic proximity effects from adjacent ferromagnets (FMs), and disentangling spin versus orbital channels is complicated by interfacial hybridization, exchange coupling, and bulk contributions. 
In this work, we investigate Bi₁₋ₓSbₓ/FM heterostructures using two complementary probes: terahertz time-domain spectroscopy (THz-TDS) emission, which detects ultrafast charge currents generated by femtosecond laser excitation (spin-to-charge conversion), and 2nd harmonic hall voltage technique to look into charge-to-spin conversion. And we grow these magnetic heterostructures using MBE and Sputtering.  

"Topology-Driven Quantum Interference in Multi-Bridged π-Conjugated Molecular Junctions for Thermoelectric Applications" by Melih Aktas. 

One dimensional topological π-conjugated materials, such as single-bridged polyacene polymers [1-3], can show electronic topological states without the need of spin-orbit interactions. These systems can be described with the Su-Schrieffer-Heeger (SSH) model and exhibit exceptional molecular transport properties via quantum interference (QI) effects between their topological edge states [4].

While single-bridge polyacene polymers have been widely studied, the impact of the number of connections between monomers on transport properties is missing. More precisely, double-bridged polyacene polymers can be described with a double-stacked SSH model [5] and effectively host twice as much topological states with respect to the single-bridged configurations. Therefore, investigating these stacked topological structures as molecular junctions (MJs) can reveal novel topological transport effects.

In this work, using state of the art ab initio techniques and non-equilibrium Green's function calculations, we studied coherent molecular electronic transport and linear-response thermoelectric properties of both single- and double-bridged polyacene MJs. Our results revealed that, in contrast with single-bridged systems, the QI between the frontier topological states of double-bridged polyacene MJs can intrinsically generate a Fano resonance in the electronic transmission spectrum, without the need of an interfering pendant group. This Fano resonance can strongly increase the thermoelectric figure of merit ZT. Furthermore, in both systems, we also observed that the sign of the Seebeck coefficient can be reversed with a topological phase transition.

Our work demonstrates an alternative way to generate Fano resonances in the absence of a pendant group in π-conjugated organic materials by modulating the topological phase of stacked-SSH-type organic MJs, leading to enhanced thermoelectric properties via QI effects.

"A silicon-based multilayer integration platform for photonics" by Gervasio Adriano D’anzieri.