Integrated Calendar

The seminar comprises of methodological developments in solid state Nuclear Magnetic Resonance (ssNMR) and its subsequent application on oriented samples like liquid crystals or single crystals. Two methodologies that have been developed during my graduate studies may be discussed.

The first part describes a new heteronuclear polarization transfer scheme that is devoid of some of the disadvantages of Hartmann- Hahn Cross Polarization (HH-CP) technique. Instead of HH-match, we have employed a homonuclear decoupling sequence (P0) on the I spins followed by a phase shifted version (P90) of the same sequence. A 90o pulse of suitable phase is applied to the S spin in such a way that it is sandwiched between Po and P90. This pulse sequence has been found to transfer polarization similar to HH-CP. Subsequently this sequence, named DAPT (Dipolar Assisted Polarization Transfer), has been extended to different contexts. We have explored its performance in various contexts like, measuring heteronuclear dipolar couplings between spin-1/2’s (1H -13C/15N ), quadrupolar couplings in a fully deuterated liquid crystal), to study second order quadrupolar transition in 14N nucleus etc.

The second part of the talk explains a simple and useful modification of the well-known Separated Local Field (SLF) sequence in solid state known as PISEMA (Polarization Spin Exchange at the Magic Angle). PISEMA is a popular technique for measuring I-S couplings in oriented biological membranes and in liquid crystals. While it has several advantages such as a large dipolar scaling factor, narrow line-widths in the dipolar dimension and ease of implementation it is highly sensitive to the proton off-sets which affect the measured dipolar couplings. The origin of this problem has been analyzed in detail and a solution has been proposed. The modified sequence named as 24-SEMA is found to be highly insensitive to proton offsets and rf in-homogeneity when compared with PISEMA.

The statistical properties of the largest eigenvalue of a
random matrix are of interest in diverse fields ranging from
disordered systems to statistical data analysis and even in string theory.
In this talk I'll
discuss some recent developments in the theory of extremely
rare fluctuations (large deviations) of the largest eigenvalue
using a Coulomb gas method. Such rare fluctuations have also
been measured in recent experiments in coupled laser systems.
I'll also discuss recent applications of this Coulomb
gas method in three different problems: entanglement
in a bipartite system, conductance fluctuation through
a mesoscopic cavity and the vicious random walkers problem.

I will present our recent results on the modelling the mechanisms of adsorption and diffusion or functionalized organic molecules at ionic surfaces, such as TiO2 (110), KBr and NaCl in relation to recent high resolution AFM experiments [1,2]. I will focus on the AFM observations that depositing organic adsorbates onto stable ionic surfaces can induce structural changes of surface features, such as steps and corners [1]. This suggests that by using appropriately chosen/designed molecules it should be possible to tailor the nano-scale structure of polar surfaces for particular applications. This, however, requires a detailed understanding of the mechanisms and driving forces behind such induced reconstructions. I will discuss the multi-scale modelling performed to understand the mechanism of the experimentally observed shape changes of nanoscale islands and pits on the nano-patterned KBr (001) surface upon the adsorption of truxene molecules functionalised with polar organic groups. I will demonstrate how the adsorption of molecules at the edges of surface features lowers activation energies for key transitions for the shape change, enabling them to become thermally accessible. The interplay between molecular diffusion and surface reconstruction is also investigated. These studies offer insights into designing molecules to modify and control surface features on crystal surfaces.

[1] B. Such, T. Trevethan, T. Glatzel, et al. ACS Nano 4, 2429 (2010)
[2] K. Lammle, T. Trevethan, A. Schwarz, et al. Nano Letters 10, 2965 (2010)