Resonant Multiple-Phonon Absorption Causes Efficient Anti-Stokes Photoluminescence in CsPbBr Nanocrystals.

Lead halide perovskite nanocrystals, such as CsPbBr, exhibit efficient photoluminescence (PL) up-conversion, also referred to as anti-Stokes photoluminescence (ASPL). This is a phenomenon where irradiating nanocrystals up to 100 meV below gap results in higher energy band edge emission. Most surprising is that ASPL efficiencies approach unity and involve single-photon interactions with multiple phonons.

Critical Nature of the Ni Spin State in Doped NdNiO_{2}.

Superconductivity with T_{c}≈15  K was recently found in doped NdNiO_{2}. The Ni^{1+}O_{2} layers are expected to be Mott insulators, so hole doping should produce Ni^{2+} with S=1, incompatible with robust superconductivity. We show that the NiO_{2} layers fall inside a critical region where the large pd hybridization favors a singlet ^{1}A_{1} hole-doped state like in CuO_{2}.

Classification and characterization of nonequilibrium Higgs modes in unconventional superconductors.

Recent findings of new Higgs modes in unconventional superconductors require a classification and characterization of the modes allowed by nontrivial gap symmetry. Here we develop a theory for a tailored nonequilibrium quantum quench to excite all possible oscillation symmetries of a superconducting condensate. We show that both a finite momentum transfer and quench symmetry allow for an identification of the resulting Higgs oscillations.

Extrapolating Quantum Observables with Machine Learning: Inferring Multiple Phase Transitions from Properties of a Single Phase.

We present a machine-learning method for predicting sharp transitions in a Hamiltonian phase diagram by extrapolating the properties of quantum systems. The method is based on Gaussian process regression with a combination of kernels chosen through an iterative procedure maximizing the predicting power of the kernels. The method is capable of extrapolating across the transition lines.

Light Bipolarons Stabilized by Peierls Electron-Phonon Coupling.

It is widely accepted that phonon-mediated high-temperature superconductivity is impossible at ambient pressure, because of the very large effective masses of polarons or bipolarons at strong electron-phonon coupling. Here we challenge this belief by showing that strongly bound yet very light bipolarons appear for strong Peierls coupling. These bipolarons also exhibit many other unconventional properties; e.