# Upcoming events

- Clore Seminar on Soft and Biological Physics – Sundays, 13:00, Drory Auditorium
- Statistical Physics Seminar – Mondays, 14:15, Seminar Room A
- Atomic, Molecular, and Optical Seminar (AMOS) – Tuesdays, 13:15, Weismann Auditorium

### Hyperuniformity of driven suspensions

DateMonday, March 25, 2019

Time14:15

DetailsChemistry, TAU

LecturerHaim Diamant

LocationEdna and K.B. Weissman Building of Physical Sciences

AbstractAn arrangement of particles is said to be "hyperuniform" if its density fluctuations over large distances are strongly suppressed relative to a random configuration. Crystals, for example, are hyperuniform. Recently, several disordered materials have been found to be hyperuniform. Examples are sheared suspensions and emulsions, and, possibly, random close packings of particles. We show that externally driven particles in a liquid suspension (as in sedimentation, for example) self-organize hyperuniformly in certain directions relative to the external force. This dynamic hyperuniformity arises from the long-range coupling, induced by the force and carried by the fluid, between the concentration of particles and their velocity field. We obtain the general requirements, which the coupling should satisfy in order for this phenomenon to occur. Under other conditions (e.g., for certain particle shapes), the coupling can lead to the opposite effect -- enhancement of density fluctuations and instability. We confirm these analytical results in a simple two-dimensional simulation.

### Emergence and stability of a Brownian motor

DateMonday, April 8, 2019

Time14:15

DetailsHebrew University

LecturerAlex Feigel

LocationEdna and K.B. Weissman Building of Physical Sciences

AbstractA Brownian motor rectifies thermal noise and creates useful work. Here we address how this machine can emerge without predefined energy minimum in a system out of thermal equilibrium. Intuitively, Brownian motor as any artificial or biological machine should degrade with time. I will show that on contrary, a system with multiple degrees of freedom out of thermal equilibrium can be stable at a state that generates useful work. It is demonstrated with the help of ab initio analysis of a modified Feynman-Smoluchowski ratchet with two degrees of freedom. Out of thermal equilibrium, an environment imposes effective mechanical forces on nano-fabricated devices as well as on microscopic chemical or biological systems. Thus out of thermal equilibrium environment can enforce a specific steady state on the system by creating effective potentials in otherwise homogeneous configuration space. I present an ab initio path from the elastic scattering of a single gas particle by a mechanical system to the transition rate probability between the states of the system with multiple degrees of freedom, together with the corresponding Masters-Boltzmann equation and the average velocities of the system’s degrees of freedom as functions of the macroscopic parameters of the out-of-equilibrium environment. It results in Onsager relations that include the influence of the different degrees of freedom on each other. An interesting finding is that some of these forces persist even in a single temperature environment if the thermodynamic limit does not hold. In addition, the spatial asymmetry of the system’s stable state, together with the corresponding directed motion, may possess preferred chiral symmetry.

### Introduction to the quantum first detection problem

DateMonday, May 20, 2019

Time13:00

DetailsPhysics, BIU

LecturerEli Barkai

LocationEdna and K.B. Weissman Building of Physical Sciences

AbstractWe consider quantum dynamics on a graph, with repeated strong measurements performed locally at a fixed time interval τ. For example, a particle starting on node x and measurements performed on another node x'. From the basic postulates of quantum mechanics the string of measurements yields a sequence: no, no, no, … and finally in the n-th attempt a yes, i.e. the particle is detected. Statistics of the first detection time nτ are investigated, and compared with the corresponding classical first passage problem. Dark states, Zeno physics, a quantum renewal equation, winding number for the first return problem (work of A. Grunbaum et al.), total detection probability, detection time operators and time wave functions are discussed. References [1] H. Friedman, D. Kessler, and E. Barkai Quantum walks: the first detected passage time problem Phys. Rev. E. 95, 032141 (2017). Editor's suggestion. [2] F. Thiel, E. Barkai, and D. A. Kessler First detected arrival of a quantum walker on an infinite line Phys. Rev. Lett. 120, 040502 (2018).