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TATEH 275-06 (office)

ayklein @ umn.edu

I am a theoretical physicist, visiting from Israel to do postdoctoral research with Prof. Andrey Chubukov. I like studying the way that the laws of quantum mechanics interplay with the more everyday, so-called "classical" laws of physics, in the behavior of many materials, both solids and fluids. Before coming here, I studied in the Hebrew University in Jerusalem. I also spent some years teaching computer science to high school students.

Our modern understanding of quantum mechanical systems, and of many classical systems as well, is founded on particle-wave duality. In essence, this is the idea that physical systems consist of ‘particle’ excitations with well-defined classical attributes like energy, position, and momentum, but that nevertheless interfere with each other like waves. However, in many condensed-matter systems, interactions between particles are strong enough that even the these ‘wavelike’ particles are no longer well-defined, and their place is taken by exotic collective excitations with quantum properties. My research focuses on understanding the dynamics of these excitations: how they evolve in time, how they can be measured, and especially their interplay with the classical world around, for example when responding to external stimulus.

Some current research subjects:

*Dynamics of itinerant quantum critical fermions at long wavelength* - A long-standing unsolved problem in condensed-matter physics is the behavior of itinerant fermions near a quantum critical phase transition. It is believed that several important physical systems, such as the cuprate- and iron-based superconductors, display such behavior. Near a QCP, low-energy excitations couple strongly to the fermions, leading to a breakdown of the quasiparticle Fermi liquid picture. In recent years I have focused on such systems near long-wavelength transitions, especially nematic transitions – where the fermions spontaneously develop a quadrupole moment over large distances. In nematic systems, the quantum dynamics of the non-Fermi liquid fermions is strongly affected by classical constraints like charge conservation, leading to unique behavior and experimental signatures.

*Nonlinear structure and dynamics of quantum vortices*

Superfluids are an example of a strongly-correlated bosonic system. Superfluids exhibit beautiful excitations of angular momentum – vortices in the fluid with quantized fluid velocity. These objects are very strange – they are neither very localized nor very extended, and as a result, vortices interact strongly with one another, creating a strongly-correlated system within another one! At the same time, their structure is protected by the quantization – they are topological defects. One aspect I have studied is how the nonlinear behavior of the ‘classical’ fluid affects the quantum vortex excitations. For example, the vortex core structure can deform as a result of current flow. These deformations are themselves elementary excitations of the core itself, and are similar to the quantized cyclotron orbits of in the quantum Hall effect.

Other topics I have thought about/am thinking about are the mixing of classical and quantum correlations in disordered systems, integrable models of dynamics of liquids and applications of random matrix theory to disordered systems. Some of this work appears in the publication list below.

To get an idea of my work subjects I suggest the following publications:

A Klein, S Lederer, D Chowdhury, E Berg, and A Chubukov, Phys. Rev. B 98, 041101(R) (2018)

A Klein, S Lederer, D Chowdhury, E Berg, and A Chubukov, Phys. Rev. B 97, 155115 (2017)

A Klein, O Agam, and IL Aleiner, Phys. Rev. Lett. 118, 085303 (2017)

A Klein, O Agam, and B Spivak, Phys. Rev. A 94, 013828 (2016)

A Klein and O Agam, J. Phys. A: Math. Theor. 45 355003 (2012)

Hebrew University in Jerusalem (B.Sc., M.Sc., Ph.D.)