Polarons in the 21st Century
- Cesare Franchini (University of Vienna, Austria)
- Georg Kresse (University of Vienna, Austria)
- Jacques Tempere (University Antwerpen, Belgium)
- Jozef Devreese (University Antwerpen, Belgium)
Objective: Gathering together an interdisciplinary group of researchers working on polaron physics from different perspectives, including: first-principles calculations, quantum-field Hamiltonian approaches, theoretical/mathematical models, and experiments, representative of the condensed-matter and ultracold atoms communities. The workshop will offer the opportunity to report the status of the art, discuss new ideas, developments and establish possible interactions among the different communities.
Scientific aim of the workshop
The polaron concept was proposed by Landau in 1933  to describe an electron moving in a dielectric crystal where the atoms are displaced from their equilibrium positions to screen the charge of the electron. Depending on the spatial extent of the polaron wavefunction and associated structural distortions two general types of polarons can be identified: small-polaron and large-polaron. Historically , the polaron problem has been studied using quantum field theory effective Hamiltonians: the large-polaron Fröhlich Hamiltonian  within the ‘continuum approximation’ and the small-polarons models of Yamasita-Kurosawa  and Holstein  describing the ‘hopping’ motion of small polarons. Feynman developed a superior description for the Fröhlich polaron, using path integrals , which represented a corner-stone in the theory of polarons, but these models Hamiltonians are often too complex to be solved analytically (for all electron-phonon coupling strength), and approximations are often needed.
Researchers have also tried to study polarons using analog systems consisting of gases of ultracold atoms. The results of these simulations provide insights into the underlying physics, as well as offer verification for the approximations made in polaron models [7,8]. More recently, first principles methods based on suitable extension of the density functional theory (DFT) turned out to provide an accurate microscopic description of both large and small polarons and predicted material-specific polaron properties [9,10,11,12]. Using DFT-based approaches the Vienna co-organizers (Franchini & Kresse) have
The field of polarons has not only been a testing ground for the development of novel analytical, semi-analytical, and numerical techniques. In fact, polarons are central to the often exotic behavior of oxides and polar semiconductors and are of key importance in photonic, photovoltaic, and photochemistry applications. The physical properties of the polaron (mobility, effective mass, optical characteristics, etc.) can be very different from those of the bare electron, leading to strong modifications of the electrical, thermal transport and even catalytic properties of the material. From the experimental point of view, there are different type of measurements that can be conducted which can provide useful insights on the properties of polarons both in real materials (Electron paramagnetic resonance, EPR , Angle resolved photoemission spectroscopy, ARPES , Scanning tunneling microscopy , UV/IR spectroscopy  and time-resolved optical Kerr, effect TR-OKE ) and in ultracold atoms [16, 17].
In this workshop we plan to provide an overview of the state of the art of polaron physics, actual challenges and future directions of research, by congregating international renowned experts coming from interdisciplinary sub-fields of physics, representative of the variegated scientific communities working on the polaron physics. Specifically:
A) Theory of polarons using effective quantum-field Hamiltonians. Following the tradition based on the seminal works of Landau, Fröhlich, Holstein, Feynman and Bogoliubov, the polaron problem stimulated a lot research aiming to solve the polaron Hamiltonians using different type of techniques, such as path integrals, strong-coupling perturbation expansions, advanced variational and exact diagonalization, Diagrammatic Monte Carlo, Dynamical Mean Field Theory.
B) Ultracold atoms. Gases of ultracold atoms, which can be artificially realized using very advanced methods such as laser cooling and evaporative cooling, represent an ideal playground for studying quantum phase transition including Bose-Einstein condensation (BEC) but also Bose and Fermi polarons. The great experimental control available in ultracold atoms provides valuable insights into the properties of interacting quantum systems and provide a unique platform to verify advanced numerical schemes.
C) First-principles quantum-mechanical modeling. First-principles approaches based on density functional theory represent a powerful theoretical and computational tool to model polarons in real materials and acquire informations on material-dependent properties. In the past 10 years there have been an upsurge of interest in the realistic modeling of polarons using DFT and a few new approaches have been proposed to improve the treatment of electron localization and electron-phonon interactions.
D) Experiments on polaronic materials. Since the first experimental observation of polaron in UO2 in 1963 several new measurements have been conducted to disclose fingerprints of polaronic behavior in materials. Not many experimental probes can be employed to inspect the nature of polarons in materials, in particular EPR, STM and ARPES, but also XPS,TR-OKE. In most cases the support of first principles calculation in necessary to provide a sound interpretation of the observations.
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