Judy Cherian

Spin Exchange Interaction in Substituted Copper Phthalocyanine Crystalline Thin Films.

The origins of spin exchange in crystalline thin films of Copper Octabutoxy Phthalocyanine (Cu-OBPc) are investigated using Magnetic Circular Dichroism (MCD) spectroscopy. These studies are made possible by a solution deposition technique which produces highly ordered films with macroscopic grain sizes suitable for optical studies. For temperatures lower than 2 K, the contribution of a specific state in the valence band manifold originating from the hybridized lone pair in nitrogen orbitals of the Phthalocyanine ring, bears the Brillouin-like signature of an exchange interaction with the localized d-shell Cu spins. A comprehensive MCD spectral analysis coupled with a molecular field model of a σπ − d exchange analogous to sp-d interactions in Diluted Magnetic Semiconductors (DMS) renders an enhanced Zeeman splitting and a modified g-factor of −4 for the electrons that mediate the interaction. These studies define an experimental tool for identifying electronic states involved in spin-dependent exchange interactions in organic materials.

Ultrafast pump–probe spectroscopy in gallium arsenide at 25 T

We have conducted pump-probe spectroscopy of bulk GaAs in the Split Florida-Helix at 15 K and 25 T. We observe an electronic rise followed by a slower decay with a superimposed oscillatory response.

Ultrafast pump-probe spectroscopy in gallium arsenide at 25 tesla

We have conducted pump-probe spectroscopy of bulk GaAs in the Split Florida-Helix at 15 K and 25 T. We observe an electronic rise followed by a slower decay with a superimposed oscillatory response.

Organic analogues of diluted magnetic semiconductors: bridging quantum chemistry to condensed matter physics

The selective coupling between polarized photons and electronic states in materials enables polarization-resolved spectroscopy studies of exchange interactions, spin dynamics, and collective magnetic behavior of conduction electrons in semiconductors. Here we report on Magnetic Circular Dichroism (MCD) studies of magnetic properties of electrons in crystalline thin films of small molecule organic semiconductors. Specifically, the focus was on the magnetic exchange interaction properties of d-shell ions (Cu2+, Co2+ and Mn2+) metal phthalocyanine (Pc) thin films that one may think of as organic analogues of diluted magnetic semiconductors (DMS). These films were deposited in-house using a recently developed pen-writing method that results in crystalline films with macroscopic long range ordering and improved electronic properties, ideally suited for spectroscopy techniques. Our experiments reveal that, in analogy to DMS, the extended π-orbitals of the Pc molecule mediate the spin exchange between highly localized d-like unpaired spins. We established that exchange mechanisms involve different electronic states in each species and/or hybridization between d-like orbitals and certain delocalized π-orbitals. Unprecedented 25T MCD and PL conducted in the unique 25T Split Florida HELIX magnet at the National High Magnetic Field Laboratory (NHMFL) will prove useful in probing these exchange interactions.

Tuning exchange interactions in organometallic semiconductors

Organic semiconductors are emerging as a leading area of research as they are expected to overcome limitations of inorganic semiconductor devices for certain applications where low cost manufacturing, device transparency in the visible range or mechanical flexibility are more important than fast switching times. Solution processing methods produce thin films with millimeter sized crystalline grains at very low cost manufacturing prices, ideally suited for optical spectroscopy investigations of long range many-body effects in organic systems. To this end, we synthesized an entire family of organosoluble 3-d transition metal Pc’s and successfully employed a novel solution-based pen-writing deposition technique to fabricate long range ordered thin films of mixtures of metal-free (H2Pc) molecule and organometallic phthalocyanines (MPcs). Our previous studies on the parent MPc crystalline thin films identified different electronic states mediating exchange interactions in these materials. This understanding of spin-dependent exchange interaction between delocalized π-electrons with unpaired d spins enabled the further tuning of these interactions by mixing CoPc and H2Pc in different ratios ranging from 1:1 to 1000:1 H2Pc:MPc. The magnitude of the exchange is also tunable as a function of the average distance between unpaired spins in these materials. Furthermore, high magnetic field (B < 25T) MCD and magneto-photoluminescence show evidence of spin-polarized band-edge excitons in the same materials.