Condensed Matter Seminar

Week of Monday, October 16th 2017

Monday, October 16th 2017
Speaker: Prof Oded Agam, Hebrew University Jerusalem,
Subject: Saturation of strong electron-electron umklapp scattering at high temperature.
Please note change of time, room and day for the seminar, this week only.

It is usually believed that in clean metals, at finite temperature, the electron-electron umklapp scattering is proportional to the inelastic scattering rate, 1/\tau_{ee}. In this talk it will be shown that, for three dimensional systems, when \hbar/\tau_{ee} is of order or larger than the band splitting energy, the umklapp scattering rate saturates to a value which is independent of both \tau_{ee} and the temperature, T. This phenomenon sheds new light on the old problem of the resistivity saturation at high temperature.

Faculty Host: Alex Kamenev

Wednesday, October 18th 2017
Speaker: Joe Orenstein, UC Berkeley
Subject: Is there a bound on the nonlinear optical response of crystals?

The second-order nonlinear response, defined by the relation, J^((2)) (ω±ω)=σ^((2) ) (ω)E_ω^2, is allowed only in media without a center of inversion. A new class of inversion breaking materials, “Weyl semimetals,” have been synthesized and are currently under intense investigation. Recently we reported that the first of this class of materials to be discovered, the transition metal monopnictides such as TaAs, and NbAs, exhibit the largest σ^((2) ) (ω)E_ω^2 of any known crystal. Large values of σ^((2) ) (ω) are of interest for applications involving frequency generation and conversion of light to electrical current. The observation of a “giant” response raises two related questions for which I will attempt to provide partial answers: what is special about TaAs, and is there an upper bound on σ^((2) ) (ω) of inversion-breaking crystal?

Faculty Host: Martin Greven

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