The Special Issue contains theoretical and experimental works that report on studies of impurities in quantum gases, fundamental properties and universal aspects of quasiparticles and other related ...many-body phenomena. Particular focus is placed on the Fermi and Bose polarons. The Special Issue contains ten research articles and two reviews. M. G. Skou et al. report on the experimental observation of time dynamics of Bose polarons. Theoretical studies by H. Tajima et al., L. A. Ardila, and G. Panochko and V. Pastukhov touch upon the physics of multiple impurities, in particular, the induced impurity–impurity interactions in different spatial dimensions and the formation of multi-polaron states. G. M. Koutentakis et al. elaborate on the phenomenon of temporal orthogonality catastrophe in low dimensions. Polaritons in an electron gas are discussed by M. A. Bastarrachea-Magnani et al. M. Brooks et al. describe the emergence of anyons originating from angulons. F. Scazza et al. provide an overview of our current understanding of repulsive Bose and Fermi polarons. C. D’Errico and M. G. Tarallo explicate the effects of disorder in bosonic systems. The Special Issue also includes studies of correlated atom pairs in bosonic mixtures by O. Alon, the behavior of the three-body decay rate coefficients into shallow dimers in mass-imbalanced three-atom systems by P. Giannakeas and C. H. Greene, population and angular momentum transfer in Raman-coupled Bose–Einstein condensates by K. Mukherjee et al.
We explore repulsive Fermi polarons in one-dimensional harmonically trapped few-body mixtures of ultracold atoms using as a case example a 6Li-40K mixture. A characterization of these ...quasiparticle-like states, whose appearance is signaled in the impurity's radiofrequency spectrum, is achieved by extracting their lifetime and residua. Increasing the number of 40K impurities leads to the occurrence of both single and multiple polarons that are entangled with their environment. An interaction-dependent broadening of the spectral lines is observed suggesting the presence of induced interactions. We propose the relative distance between the impurities as an adequate measure to detect induced interactions independently of the specifics of the atomic mixture, a result that we showcase by considering also a 6Li-173Yb system. This distance is further shown to be indicative of the generation of entanglement independently of the size of the bath (6Li) and the atomic species of the impurity. The generation of entanglement and the importance of induced interactions are revealed with an emphasis on the regime of intermediate interaction strengths.
We study the ground state properties and non-equilibrium dynamics of two spinor bosonic impurities immersed in a one-dimensional bosonic gas upon applying an interspecies interaction quench. For the ...ground state of two non-interacting impurities we reveal signatures of attractive induced interactions in both cases of attractive or repulsive interspecies interactions, while a weak impurity-impurity repulsion forces the impurities to stay apart. Turning to the quench dynamics we inspect the time-evolution of the contrast unveiling the existence, dynamical deformation and the orthogonality catastrophe of Bose polarons. We find that for an increasing postquench repulsion the impurities reside in a superposition of two distinct two-body configurations while at strong repulsions their corresponding two-body correlation patterns show a spatially delocalized behavior evincing the involvement of higher excited states. For attractive interspecies couplings, the impurities exhibit a tendency to localize at the origin and remarkably for strong attractions they experience a mutual attraction on the two-body level that is imprinted as a density hump on the bosonic bath.
The small‐polaron hopping model has been used for six decades to rationalize electronic charge transport in oxides. The model was developed for binary oxides, and, despite its significance, its ...accuracy has not been rigorously tested for higher‐order oxides. Here, the small‐polaron transport model is tested by using a spinel system with mixed cation oxidation states (MnxFe3−xO4). Using molecular‐beam epitaxy (MBE), a series of single crystal MnxFe3−xO4 thin films with controlled stoichiometry, 0 ≤ x ≤ 2.3, and lattice strain are grown, and the cation site‐occupation is determined through X‐ray emission spectroscopy (XES). Density functional theory + U analysis shows that charge transport occurs only between like‐cations (Fe/Fe or Mn/Mn). The site‐occupation data and percolation models show that there are limited stoichiometric ranges for transport along Fe and Mn pathways. Furthermore, due to asymmetric hopping barriers and formation energies, the MnOh2+ polaron is energetically preferred to the FeOh2+ polaron, resulting in an asymmetric contribution of Mn/Mn pathways. All of these findings are not contained in the conventional small‐polaron hopping model, highlighting its inadequacy. To correct the model, new parameters in the nearest‐neighbor hopping equation are introduced to account for percolation, cross‐hopping, and polaron‐distribution, and it is found that a near‐perfect correlation can be made between experiment and theory for the electronic conductivity.
The small‐polaron hopping model was developed six decades ago to rationalize electronic charge transport in oxides. The limitations of the old model are highlighted in this work and a new model that includes percolation, cross‐hopping, and polaron distribution parameters in the nearest‐neighbor hopping equation is presented. The new accurate model opens new avenues for designing high‐performance devices such as batteries and supercapacitors.
We report transition metal oxide materials have attracted much attention for photoelectrochemical water splitting, but problems remain, e.g. the sluggish transport of excess charge carriers in these ...materials, which is not well understood. Here we use periodic, spin-constrained and gap-optimised hybrid density functional theory to uncover the nature and transport mechanism of holes and excess electrons in a widely used water splitting material, bulk-hematite (α-Fe2O3). We find that upon ionisation the hole relaxes from a delocalized band state to a polaron localised on a single iron atom with localisation induced by tetragonal distortion of the 6 surrounding iron-oxygen bonds. This distortion is responsible for sluggish hopping transport in the Fe-bilayer, characterised by an activation energy of 70 meV and a hole mobility of 0.031 cm2/Vs. By contrast, the excess electron induces a smaller distortion of the iron-oxygen bonds resulting in delocalisation over two neighbouring Fe units. We find that 2-site delocalisation is advantageous for charge transport due to the larger spatial displacements per transfer step. As a result, the electron mobility is predicted to be a factor of 3 higher than the hole mobility, 0.098 cm2/Vs, in qualitative agreement with experimental observations. Our study demonstrates that constrained DFT is a very powerful tool for the prediction of charge transfer rates and mobilities in application-relevant oxide materials.
Density functional theory calculations and electrochemical impedance spectroscopy have been combined to derive a defect model for ZrV2O7 to rationalize its electrical conductivity behavior. ZrV2O7 ...shows slight vanadium over-stoichiometry, yielding the predominant defect pair: V5+ on Zr sites (VZr•) charge-compensated by electron polarons as V4+ on V sites (VV′). Small polaron hopping is the dominating conduction mechanism with a calculated polaron self-trapping energy of −0.22 eV. The polarons can, however, be strongly bound as V4+ substituted on Zr sites (VZr×), with a calculated binding energy of −0.97 eV with respect to free polarons. The temperature dependency of the electrical conductivity exhibits a crossover between two regimes at 550 °C, and the apparent activation energy increases with increasing temperature from 0.3 to 0.86 eV. According to the defect model, which includes the concentration ratio of free and bound polarons, free polaron hopping predominates the electrical conductivity at lower temperatures while the thermally activated transition from bound to free polarons predominates at higher temperatures.
•DFT calculations and electrochemical impedance spectroscopy have been combined to derive a defect model for ZrV2O7.•The temperature dependency of the electrical conductivity exhibits a crossover between two different regimes at 550 °C.•Free polaron hopping predominates the electrical conductivity below 550 °C.•The thermally activated transition from bound to free polarons predominates the electrical conductivity above 550 °C.
The dynamics of a mobile quantum impurity in a degenerate Fermi system is a fundamental problem in many-body physics. The interest in this field has been renewed due to recent ground-breaking ...experiments with ultracold Fermi gases1, 2, 3, 4, 5. Optical creation of an exciton or a polariton in a two-dimensional electron system embedded in a microcavity constitutes a new frontier for this field due to an interplay between cavity coupling favouring ultralow-mass polariton formation6 and exciton-electron interactions leading to polaron or trion formation7, 8. Here, we present cavity spectroscopy of gate-tunable monolayer MoSe2 (ref. 9) exhibiting strongly bound trion and polaron resonances, as well as non-perturbative coupling to a single microcavity mode10, 11. As the electron density is increased, the oscillator strength determined from the polariton splitting is gradually transferred from the higher-energy repulsive exciton-polaron resonance to the lower-energy attractive exciton-polaron state. Simultaneous observation of polariton formation in both attractive and repulsive branches indicates a new regime of polaron physics where the polariton impurity mass can be much smaller than that of the electrons. Our findings shed new light on optical response of semiconductors in the presence of free carriers by identifying the Fermi polaron nature of excitonic resonances and constitute a first step in investigation of a new class of degenerate Bose-Fermi mixtures12, 13.
Understanding “efficiency roll‐off” (i.e., the drop in emission efficiency with increasing current) is critical if efficient and bright emissive technologies are to be rationally designed. Emerging ...light‐emitting electrochemical cells (LECs) can be cost‐ and energy‐efficiently fabricated by ambient‐air printing by virtue of the in situ formation of a p‐n junction doping structure. However, this in situ doping transformation renders a meaningful efficiency analysis challenging. Herein, a method for separation and quantification of major LEC loss factors, notably the outcoupling efficiency and exciton quenching, is presented. Specifically, the position of the emissive p‐n junction in common singlet‐exciton emitting LECs is measured to shift markedly with increasing current, and the influence of this shift on the outcoupling efficiency is quantified. It is further verified that the LEC‐characteristic high electrochemical‐doping concentration renders singlet‐polaron quenching (SPQ) significant already at low drive current density, but also that SPQ increases super‐linearly with increasing current, because of increasing polaron density in the p‐n junction region. This results in that SPQ dominates singlet‐singlet quenching for relevant current densities, and significantly contributes to the efficiency roll‐off. This method for deciphering the LEC efficiency roll‐off can contribute to a rational realization of all‐printed LEC devices that are efficient at highluminance.
The dynamic doping operation of light‐emitting electrochemical cells (LECs) renders the understanding of the efficiency roll‐off challenging. Herein, a method for quantification of major LEC loss factors, notably the outcoupling efficiency and exciton quenching, is presented. It reveals that singlet‐polaron quenching strongly affects the efficiency roll‐off in singlet‐emitting LECs because of increasing polaron concentration in the p‐n junction with increasing current .
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