This is the second session of the third season of the Mixed-Gen on-line series aimed mainly at PhD students and researchers in their first post-doc. Our goal is to continue providing a new venue for these young scientists to share their work, get expert feedback and have an opportunity to strengthen scientific relations within the CECAM community.

To participate

If you are a PhD student or a post-doc:

Please use the Participate Tab on this page to start the application. You will have to login using your CECAM account to access the application form. If you don't have a CECAM account yet, use the register option on the top right corner of the login page...and welcome to CECAM!

If you are a more senior scientist:

Please contact the organisers and we shall process your registration.

Submission of posters

(Please note that - at least for the time being - we shall accept posters only from PhD students or researchers in their first post-doc)

After your application is accepted, you will be able to submit a poster. In the CECAM page for this event, go to “My participation” tab and click on “Add a poster”, providing title and abstract following the recommended format. On the same form you can upload your poster file in png or jpg as soon as it is ready. These formats are strict to enable showing of the poster in the Gather session. If the poster file is not ready at the moment of submitting your abstract, you can upload it later by editing your submission (Go to “My participation” tab and click three vertical dots on “Actions” column on table “My posters”). Please upload your poster as soon as possible to enable a decision from the selection committee - see below.

Please note that posters will be visible in the Gather room associated with this session until the end of the series (June 2023) unless otherwise requested.

DEADLINE FOR SUBMISSION: TEN DAYS BEFORE THE EVENT

Selection of posters

Posters will be selected by the event organisers with the support of our main speaker and experts who will take place in the poster session. 

Selection of the two talks by PhD or first year postdocs

These contributions, to be broadcasted in the Zoom webinar in the first part of the event, will be selected, after a preliminary screening by the organisers, the main speaker and guest experts, from the posters selected for the Gather session. Please tick “No” to the question “Upgrade to talk?” in your application if you DO NOT WANT your poster to be considered for upgrade to a talk. 

THE DECISION ON THE POSTER AND THE OUTCOME OF THE SELECTION OF THE TALKS WILL BE COMMUNICATED ONE WEEK BEFORE THE EVENT

POSTER SUBMISSIONS BEYOND THIS DEADLINE WILL BE ACCEPTED BUT NOT CONSIDERED FOR UPGRADE TO TALK. SUBMISSION WILL BE DEFINITELY CLOSED FOUR DAYS BEFORE THE EVENT.

SESSION 2. Title and abstract of talks

Gauge invariance of transport coefficients: fathoming heat transport from the struggle to simulate it

Stefano Baroni , SISSA, Trieste

The simulation of adiabatic (i.e. non-electronic) heat transport from the Green-Kubo theory of linear-response has long been hampered by two misconceptions. First, the linear-response theory of heat transport was deemed incompatible with modern simulation techniques based on electronic-structure theory, because the quantum mechanical expression of the heat flux that enters the Green-Kubo formula is inherently ill defined. Second, it is commonly thought that the numerical application of this theory would require very long molecular-dynamics simulations, much longer in fact than the typical heat-flux auto-correlation times one is required to evaluate. In this talk I will describe the efforts done at SISSA over the past several years to overcome this state of affairs, which resulted in a methodology allowing one to compute heat transport coefficients from equilibrium molecular dynamics (both ab initio and based on deep neural-network potentials), no less than in a deeper understanding of the theory of hydrodynamic fluctuations and their numerical analysis through the statistical theory of stationary time series.

Quantum thermal transport in solids: coherences, disorder, and viscosities

Michele Simoncelli, University of Cambridge

Starting from the Wigner formulation of quantum mechanics, we derive a microscopic equation that describes thermal transport in very general terms and that covers on the same footing crystalline, disordered, and glassy materials. This Wigner transport equation [1] generalizes the Peierls-Boltzmann equation introduced in 1929, and naturally adds a tunnelling term to the drift and scattering of phonon wavepackets. In fact, we show that tunnelling is essential to describe correctly transport in materials that are engineered to be poor thermal conductors, such as thermoelectrics [1], thermal barrier coatings [2], and glasses [3]. We show the predictive accuracy of this formulation by calculating from first-principles thermal transport in various complex crystals and glasses, highlighting also the agreement with experiments at increasing temperatures, where the tunnelling term dominates and offsets the incorrect 1/T decay of Peierls-Boltzmann. Last, we coarse grain the atomistic equations of motion to obtain a set of mesoscopic viscous heat equations [4], which generalize Fourier’s law accounting for the thermal viscosity, in addition to the thermal conductivity. These mesoscopic equationsexplain the experimental observation of hydrodynamic heat propagation in graphite around 100 K [Huberman et al., Science 364 (2019)], and we exploit them to propose a strategy to amplify signatures of hydrodynamic thermal transport [5].

References

[1] M. Simoncelli, N. Marzari, F. Mauri, Nat. Phys., 15, 809-813 (2019)
[2] M. Simoncelli, N. Marzari, F. Mauri, Phys. Rev. X, 12, 041011 (2022)
[3] M. Simoncelli, F. Mauri, N, Marzari arXiv.2209.11201 (2022)
[4] M. Simoncelli, N. Marzari, A. Cepellotti, Phys. Rev. X, 10, 011019 (2020)
[5] J. Dragašević and M. Simoncelli, Viscous heat backflow and temperature resonances in graphite, in preparation.

Sticky coupling as a control variate for the computation of transport coefficents

Shiva Darshan, Ecole des Ponts, ParisTech

A standard method to compute transport coefficients is to simulate Langevin dynamics perturbed by a small non-equilibrium forcing up to a time $T$ and time-average over the trajectory a desired observable divided by the magnitude of the forcing, $\eta$. Unfortunately, this method suffers from large finite-time sampling bias and variance in the limit of small forcing—on the order of $(T\eta)^{-1}$ and $(T\eta^2)^{-1}$ respectively.

For overdamped Langevin dynamics, we propose a method to reduce the bias and variance of this computation using a version of the reference (unperturbed) dynamics sticky coupled [1] to the perturbed dynamics as a control variate. We will show that when the potential of the dynamics is strongly convex at infinity, this sticky coupling based estimator’s reduces the bias and variance by a factor of $\eta^{-1}$ compared to the standard method. The case of strongly convex at infinity potentials includes commonly used systems such as Lennard-Jones particles confined to box by a quadratic potential.

Reference

[1] A. Eberle, R. Zimmer, Ann. Inst. H. Poincaré Probab. Statist., 55, (2019)

Topology, oxidation states, and charge transport in ionic conductors

Paolo Pegolo, SISSA, Trieste

Recent theoretical advances, based on a combination of concepts from Thouless' topological theory of adiabatic charge transport [1] and a newly introduced gauge-invariance principle for transport coefficients [2], have permitted to connect (and reconcile) Faraday's picture of ionic transport---whereby each atom carries a well-defined integer charge---with a rigorous quantum description of the electronic charge-density distribution, which hardly suggests its partition into well defined atomic contributions [3,5]. By relaxing some general topological conditions, one interestingly finds that charge may be transported in ionic conductors without net ionic displacements [4,5]. This allows a new connection between our topological picture and the well-known Marcus-Hush theory of electron transfer, which can be linked with the topology of adiabatic paths drawn by atomic trajectories [5]. As a significant byproduct, this permits the classification of different regimes of ionic transport according to the topological properties of the electronic structure of the conducting material [5].

References

[1] D. Thouless, Phys. Rev. B, 27, 6083-6087 (1983)
[2] A. Marcolongo, P. Umari, S. Baroni, Nature. Phys., 12, 80-84 (2015)
[3] F. Grasselli, S. Baroni, Nat. Phys., 15, 967-972 (2019)
[4] P. Pegolo, F. Grasselli, S. Baroni, Phys. Rev. X, 10, 041031 (2020)
[5] P. Pegolo, S. Baroni, F. Grasselli, Annalen der Physik, 534, 2200123 (2022)