Sumiran Pujari

Quasiparticle echoes in scanning tunneling microscopy

It is shown that the local density of states (LDOS), measured in an Scanning Tunneling Microscopy (STM) experiment, at a single tip position contains oscillations as a function of energy, due to quasiparticle interference, which is related to the positions of nearby scatterers. We pr opose a method of STM data analysis based on this idea, which can be used to l cate the scatterers. In the case of a superconductor, the method c an potentially distinguish the nature of the scattering by a particular imp urity.

Boson features in STM spectra of cuprate superconductors: Weak-coupling phenomenology

We derive the shape of the high-energy features due to a weakly coupled boson in cuprate superconductors, as seen experimentally in Bi_2 Sr_2 Ca_1 Cu_2 O_8+x (BSCCO) by Lee et al. [Nature (London) 442, 546 (2006)]. A simplified model is used of d-wave Bogoliubov quasiparticles coupled to Einstein oscillators with a momentum-independent electron-boson coupling and an analytic fitting form is derived, which allows us (a) to extract the boson modes frequency and (b) to estimate the electron-boson coupling strength. We further calculate the maximum possible superconducting gap due to an Einstein oscillator with the extracted electron-boson coupling strength, which is found to be less than 0.2 times of the observed gap indicating at the observed bosons non-dominant role in the superconductivitys mechanism. The extracted momentum-independent electron-boson coupling parameter (that we show a posteriori to indeed be in the weak-coupling regime) is then to be interpreted as an (band-structure detail dependent weighted) average over the Brillouin zone of the actual momentum-dependent electron-boson coupling in BSCCO.

Effect of Dirac spinons on angle-resolved photoemission signatures of herbertsmithite

The spinon continues to be an elusive elementary excitation of frustrated antiferromagnets. To solidify evidence for its existence, we address the question of what will be the angle resolved photoemission spectroscopy (ARPES) signatures of single-crystal samples of herbertsmithite assuming it is described by the Dirac spin liquid state. In particular, we show that the electron spectral function will be linear in energy dependence near specific wave vectors and that this dependence is expected even after fluctuations of the mean-field values are taken into account. Observation of this unique signature in ARPES will provide very strong evidence of the existence of spinons in more than one dimensions.