Seminar

Statistical mechanics of the transition to turbulence

Speaker(s): 
Dr. Nigel Goldenfeld
Dates: 
Wednesday, September 26, 2018 - 10:00am

How do fluids become turbulent as their flow velocity is increased? In recent years, careful experiments in pipes and Taylor-Couette systems have revealed that the lifetime of transient turbulent regions in a fluid appears to diverge with flow velocity just before the onset of turbulence, faster than any power law or exponential function. I show how this superexponential scaling of the turbulent lifetime in pipe flow is related to extreme value statistics, which I show is a manifestation of a mapping between transitional turbulence and the statistical mechanics model of directed percolation.  This mapping itself arises from a further surprising and remarkable connection: laminar and turbulent regions in a fluid behave as a predator-prey ecosystem. Such ecosystems are governed by individual fluctuations in the population and being naturally quantized, are solvable by path integral techniques from field theory. I explain the evidence for this mapping, and propose how a unified picture of the transition to turbulence emerges in systems ranging from turbulent convection to magnetohydrodynamics. 

The Physics of Brain Science: Quasicriticality, An Organizing Principle?

Speaker(s): 
Dr. Rashid Williams-Garcia
Dates: 
Thursday, November 8, 2018 - 4:00pm

Empirical evidence suggests that living neural networks operate near a continuous phase transition, conjectured to be an optimal point for information storage and processing. Applying theoretical approaches, however, is challenging since vital features of neural networks present numerous obstacles to the applicability of traditional statistical physics tools, many of which have not yet been adapted to neuroscience. I will describe a simple cellular automaton model which allows for the characterization of the out-of-equilibrium transition and demonstrates an explicit symmetry breaking due...

Using Interfacial Electric Fields at Domain Walls to Stabilize Novel Ground States

Speaker(s): 
Dr. Julia Mundy
Dates: 
Thursday, September 27, 2018 - 4:00pm

Interfaces between two distinct complex oxide materials can display ground states which diverge greatly from the parent compounds, making them a playground to establish emergent phenomena. Particularly intriguing are the so-called polar interfaces where a diverging electrostatic potential leads to charge transfer. The canonical polar interface between two insulating oxides, LaAlO3/SrTiO3, forms a two-dimensional electron liquid which superconductors at low-temperature and where the conductivity can be manipulated by changing the film surface. Here, I will demonstrate novel functionality at...

Reciprocal and nonreciprocal amplification at the quantum level

Speaker(s): 
Dr. Anja Metelmann
Dates: 
Wednesday, November 28, 2018 - 2:00pm

Preserving the quantum coherence of signals is of paramount importance for components utilized in quantum information processing, quantum computation and quantum measurement setups. In recent years a tremendous progress has been made in the development of quantum-limited components, such as reciprocal and nonreciprocal amplifiers, circulators and isolators. A promising way to design these devices is based on parametric modulation of coupled modes, where the required mode-mixing processes are realized by utilizing Josphson junction-based tunable couplers or via coupling to mechanical...

Status of the Search for Majorana Fermions in Semiconductor Nanowires

Speaker(s): 
Dr. Sergey Frolov
Dates: 
Monday, August 27, 2018 - 4:00pm

Majorana fermions are non-trivial quantum excitations that have remarkable topological properties and can be used to protect quantum information against decoherence. Tunneling spectroscopy measurements on one-dimensional superconducting hybrid materials have revealed signatures of Majorana fermions which are the edge states of a bulk topological superconducting phase. We couple strong spin-orbit semiconductor InSb nanowires to conventional superconductors (NbTiN, Al) to obtain additional signatures of Majorana fermions and to explore the  topological phase transition. A potent alternative explanation for many of the recent experimental Majorana reports is that a non-topological Andreev state localizes near the end of a nanowire. We compare Andreev and Majorana modes and investigate ways to clearly distinguish the two phenomena. We are also exploring how Andreev states can be chained together along the nanowire to realize the one-dimensional Kitaev model, a discrete way of generating Majorana modes.

Pages