Virtually all known fluorophores exhibit mysterious episodes of emission intermittency. A remarkable feature of the phenomenon is a power law distribution of onand off-times observed in colloidal semiconductor quantum dots (QDs), nanorods, nanowires and some organic dyes. For nanoparticles the resulting power law extends over an extraordinarily wide dynamic range: nine orders of magnitude in probability density and five to six orders of magnitude in time. Exponents hover about the ubiquitous value of -3/2. Dark states routinely last for tens of seconds, which are practically forever on quantum mechanical time scales. Despite such infinite states of darkness, the dots miraculously recover and start emitting again. Although the underlying mechanism responsible for this phenomenon remains a mystery and many questions persist, we argue that substantial theoretical progress has been made.

We propose a theoretical description of the superconducting state of under-to overdoped cuprates, based on the short coherence length of these materials and the associated strong pairing fluctuations. The calculated Tc and the zero temperature excitation gap ∆(0), as a function of hole concentration x, are in semi-quantitative agreement with experiment. Although the ratio Tc/∆(0) has a strong x dependence, different from the universal BCS value, and ∆(T ) deviates significantly from the BCS prediction, we obtain, quite remarkably, quasi-universal behavior, for the normalized superfluid density ρs(T )/ρs(0) and the Josephson critical current Ic(T )/Ic (0), as a function of T /Tc. While experiments on ρs(T ) are consistent with these results, future measurements on Ic(T ) are needed to test this prediction.PACS numbers: 74.20.-z, 74.25.-q, 74.62.-c, 74.72.-h cond-mat/9807414

Solution-processed mixed halide perovskites are excellent materials for multijunction solar cells. Unfortunately, light-induced halide phase segregation has prevented their effective integration into working devices. In this study, we rationalize and quantify anion photosegregation in stoichiometric and halide-deficient MAPb­(I1–x Br x )3 thin films through kinetic Monte Carlo simulations and complementary optical measurements. Our study reveals that segregation rates are dictated by halide vacancy hopping barriers and are modulated by vacancy concentrations. The simulations further suggest that near-ubiquitous emission energies, which converge on that for MAPb­(I0.8Br0.2)3 (i.e., x ≈ 0.2) following photosegregation, arise from the existence of kinetically trapped Br– within nucleated I-rich domains. An established photosegregation excitation intensity threshold is independent of the number of vacancies and instead depends critically on parameters such as carrier diffusion length, lifetime, and bandgap tunability. The study thus sheds new light on important parameters that define halide photosegregation and presents opportunities for controlling the phenomenon.

We demonstrate how resonant pair scattering of correlated electrons above Tc can give rise to pseudogap behavior. This resonance in the scattering T-matrix appears for superconducting interactions of intermediate strength, within the framework of a simple fermionic model. It is associated with a splitting of the single peak in the spectral function into a pair of peaks separated by an energy gap. Our physical picture is contrasted with that derived from other T-matrix schemes, with superconducting fluctuation effects, and with preformed pair (boson-fermion) models. Implications for photoemission and tunneling experiments in the cuprates are discussed.PACS numbers: 74.20. Mn, 74.25.Fy, 74.25.Nf, It has become clear in recent years that the presence of a pseudogap above the superconducting transition temperature, T c , is a robust feature of the underdoped cuprates. This phenomenon is manifested in thermodynamic [1], magnetic [2], and angle-resolved photoemission spectroscopy (ARPES) data [3]. These ARPES experiments, which have established the presence of a Luttinger volume Fermi surface, place important constraints on any pseudogap scenario: they indicate that the pseudogap appears directly in the spectral function and its magnitude and symmetry [3] seem to evolve smoothly into that of the superconducting state. Furthermore, the minimum gap points in the pseudogap regime retrace the normal state Fermi surface [4].A variety of theoretical scenarios have been proposed, for the origin of the pseudogap. Quantum Monte Carlo simulation studies have been carried out on both positive and negative U Hubbard models [5]. Alternative models relate the pseudogap to either magnetic pairing of spins [6], RVB-like pairing of chargeless spinons [7], or precursor superconductivity effects [8]. The present paper addresses this last scenario, in part because of constraints from ARPES data and in part, because the cuprates are short coherence length, quasi-two dimensional superconductors, with anomalously low plasma frequencies [8,9]. They are, therefore, expected to exhibit important deviations from an abrupt, BCS-like transition.In our physical picture, we associate an important component of the cuprate pseudogap with resonant scattering between electrons of opposite spin and small total momentum. This resonance arises in the presence of intermediate coupling and a sizeable Fermi surface. A depression in the density of states occurs because states near this Fermi surface are unavailable for electrons in the Fermi sea to scatter into; such states are otherwise occupied by relatively long lived (metastable) electron pairs. The related suppression in the spectral weight differs from that derived from conventional low frequency and long wavelength fluctuation effects [10]. In the present case it is the strength of the attractive interaction, rather than the critical slowing down (in proximity to T c ), which leads to the long-lived pair states. It should be noted that our resonant scattering approach is to be distinguished from previo...