Aqueous dispersions of 1,2‐dioleoyl‐sn‐glycero‐3‐phosphatidylethanolamine (DOPE) exhibit a well‐known phase transition from a lamellar liquid‐crystalline (Lα) to an inverted hexagonal (HII) phase as the temperature is raised above 13°C. Recently, it has been reported [1, 2] that a bicontinuous cubic phase can be induced in this system, when the lipid is rapidly cycled between these two phases. Their X‐ray diffraction results indicated that the cubic lattice is most consistent with either the Pn3m or Pn3 space group. In studies on similar systems [3, 4], two cubic phases could be detected in the Lα‐HII transition region, however. One aim of the present study was to investigate, if still further bicontinuous cubic phases can be induced in simple DOPE dispersions by changing the experimental conditions. Indeed, we found that two cubic lattices may be induced when the lipid system is cycled between –5 and 15°C more than thousand times. The second goal of the present investigation was to study the temperature dependence of the lattice parameters, the stability and transformations of these cubic phases over a wide temperature range. It has been discussed that the presence of cubic phases may be a general feature of HII‐forming lipids in the region of their lamellar to hexagonal phase transition region and that non‐lamellar lipid structures might play an important role as transient intermediates in biochemical processes.

We present direct experimental evidences of Anderson localization induced by the intrinsic alloy compositional disorder of InGaN/GaN quantum wells. Our approach relies on the measurement of the luminescence spectrum under local injection of electrons from a scanning tunneling microscope tip into a near-surface single quantum well. Fluctuations in the emission line shape are observed on a few-nanometer scale. Narrow emission peaks characteristic of single localized states are resolved. Calculations in the framework of the localization landscape theory provide the effective confining potential map stemming from composition fluctuations. This theory explains well the observed nanometer scale carrier localization and the energies of these Anderson-type localized states. The energy spreading of the emission from localized states is consistent with the usually observed very broad photo-or electro-luminescence spectra of InGaN/GaN quantum well structures.InGaN quantum wells (QWs), the active regions of nitride-based LEDs, display broad photo-or electroluminescence spectra. The emission is assumed to be inhomogeneously broadened due to contributions from material regions with different eigenenergies associated with fluctuations of the In content. Spatial variations of emission intensities and/or energies were previously observed in nitride semiconductor structures by far field [1] and near-field [2, 3] photoluminescence microscopy or cathodoluminescence microscopy [4][5][6]. These observations have evidenced so far emitting domains of either micron or submicron (typically 100 nm) size. While significant energy shifts due to these rather large-scale fluctuations were observed, no linewidths below 100 meV at room temperature were detected. These large-scale luminescence fluctuations are usually associated with growth inhomogeneities seen for instance in AFM. However, in the case of random atom positioning in the alloy crystal [7], modeling predicts that the compositional disorder should induce Anderson localization in regions of a few nm size [8][9][10][11][12][13][14]. Moreover, emission from such single localized states is expected to exhibit a narrow linewidth of a few tens of meV at room temperature, i.e. comparable to what was observed from single GaN quantum dots [15]. Such emission features exhibiting size and spectroscopic signatures characteristic of carrier localization induced by the alloy disorder have not been evidenced up to now.Here we report on the observation of fluctuations in the luminescence spectrum on the scale of a few nm as expected for such localization effects. Our approach is based on scanning tunneling luminescence (STL) microscopy [16] which allows luminescence measurements under local carrier injection with a nm scale resolution. In order to reach the required spatial resolution, we used p-type single InGaN/GaN QW structures with a thin top GaN layer separating the QW from the injecting tip of the scanning tunneling microscope (STM) and we analyzed the spatial variations in the luminescence sp...