Summer 2017

Pairing from Dynamically Screened Coulomb Repulsion in Bismuth

Speaker(s): 
Jonathan Ruhman
Dates: 
Thursday, July 6, 2017 - 4:00pm to 5:00pm

Recently, Prakash et. al. have discovered bulk superconductivity in single crystals of bismuth, which is a semi metal with extremely low carrier density. At such low density, we argue that conventional electron-phonon coupling is too weak to be responsible for the binding of electrons into Cooper pairs. We study a dynamically screened Coulomb interaction with effective attraction generated on the scale of the collective plasma modes. We model the electronic states in bismuth to include three Dirac pockets with high velocity and one hole pocket with a significantly...

Alex Edelman (Chicago): Polaron-Plasmon Superconductivity in Strontium Titanate

Strontium titanate is a bulk insulator that becomes superconducting at remarkably low carrier densities. Even more enigmatic properties become apparent at the strontium titanate/lanthanum aluminate (STO/LAO) interface and it is important to disentangle the effects of reduced dimensionality from the poorly-understood pairing mechanism. Recent experiments measuring the surface photoemission spectrum[1] and bulk tunneling spectrum[1] have found a cross-over, as a function of carrier density, from a polaronic regime with substantial spectral weight associated with strongly coupled phonons, to a more conventional weakly coupled Fermi liquid. Interestingly, it is only the polaronic state that becomes superconducting at low temperatures, although the properties of the superconducting phase itself appear entirely conventional. We interpret these results in a simple analytical model that extends an Engelsberg-Schrieffer theory of electrons coupled to a single longitudinal optic phonon mode to include the response of the electron liquid, and in particular phonon-plasmon hybridization. We perform a Migdal-Eliashberg calculation within our model to obtain this material's unusual superconducting phase diagram.

[1] Z. Wang et al, Nat. Mater. (2016)
[2] A.G. Swartz et al, arXiv:1608.05621

Leena Aggarwal: Tip-induced Superconductivity

In this talk, I am going to present some of the work that I have done during my PhD. In the first part I will mostly focus on building a low-temperature Andreev reflection spectroscopy probe (which is basically a simpler version of an STM). In the second part, I will briefly talk about the observation of a new phase of matter, tip-induced superconductivity (TISC), that emerges only under mesoscopic metallic point contacts on topologically non-trivial semimetals like a 3-D Dirac semimetal Cd3As2, and a Weyl semimetal, TaAs and comment on the possible mechanism that might lead to the emergence of such a surprising phase of matter. All these experiments were done using our home-built low-temperature probe.
If time permits, I will also talk about some experiments that we did using various scanning probe based microscopic techniques, e.g., piezo-response force microscopy (PFM) and ferroelectric lithography. I will show how certain “artifacts” can limit the application of PFM in the investigation of ferroelectric materials, and how, under certain circumstances, such “artifacts” can actually turn out to be useful.

References:
[1] L. Aggarwal, A. Gaurav, G. S. Thakur, Z. Haque, A. K. Ganguli & G. Sheet, Unconventional Superconductivity at Mesoscopic Point-contacts on the 3-Dimensional Dirac Semi-metal Cd3As2.” Nature Materials 15, 32 (2016).
[2] L. Aggarwal, S. Gayen, S. Das, R. Kumar, V. Sϋß, C. Shekhar, C. Felser & G. Sheet, “Mesoscopic superconductivity and high spin-polarization coexisting at metallic point contacts on Weyl semimetal TaAs.” Nature Communications, 8, 13974 (2017).
[3] J. S. Sekhon, L. Aggarwal & G. Sheet, “Voltage induced local hysteretic phase switching in silicon.” Applied Physics Letters 104, 162908 (2014). 

Summer Lectures: Phenomenology of Condensates and Superconductors

Speaker(s): 
David Snoke
Dates: 
Monday, May 29, 2017 - 2:00pm

David Snoke will be giving six lectures at 2 PM on Mondays, May 29, June 5, June 19, June 26, July 3, and July 10, in G-10 Allen Hall.

The basic phenomenology of condensates and superconductors, such as the Gross-Pitaevskii equation, quantized vorticity, the Meissner effect, Josephson junctions, SQUID’s, and so on will be covered. Students who have taken first-year graduate quantum mechanics should have no trouble with the material.

Polaron-Plasmon Superconductivity in Strontium Titanate

Speaker(s): 
Alex Edelman
Dates: 
Wednesday, June 7, 2017 - 4:00pm to 5:00pm

Strontium titanate is a bulk insulator that becomes superconducting at remarkably low carrier densities. Even more enigmatic properties become apparent at the strontium titanate/lanthanum aluminate (STO/LAO) interface and it is important to disentangle the effects of reduced dimensionality from the poorly-understood pairing mechanism. Recent experiments measuring the surface photoemission spectrum[1] and bulk tunneling spectrum[1] have found a cross-over, as a function of carrier density, from a polaronic regime with substantial spectral weight associated with strongly...

Circularly Polarized High-Harmonics – From Symmetries to Applications

Speaker(s): 
Ofer Kfir
Dates: 
Tuesday, May 23, 2017 - 3:00pm to 4:00pm

Generation of high harmonics of light fields had led to numerous scientific and technological advancements since its discovery 30 years ago, including attosecond spectroscopy of atoms, molecules and condensed matter and tabletop high-resolution imaging of nanostructures. However, the polarization state of high harmonics was essentially limited to linear, which prevented experimental access to the role of photon-spin in harmonic generation, and hindered the use of high harmonic radiation for chiro-optical investigation with magnetic and other chiral matter.
My talk...