Entropic Stabilization of Tunable Planar Modulated Superstructures

Self-assembling novel ordered structures with nanoparticles has recently received much attention. Here we use computer simulations to study a two-dimensional model system characterized by a simple isotropic interaction that could be realized with building blocks on the nanoscale. We find that the particles arrange themselves into hexagonal superstructures of twin boundaries whose superlattice vector can be tuned reversibly by changing the temperature. Thermodynamic stability is confirmed by calculating the free energy with a combination of thermodynamic integration and the Frenkel-Ladd method. Different contributions to the free energy difference are discussed.Comment: 4 pages, 5 figures plus 7 pages of supplementary figure

Idealistische wiskundedocenten ontwikkelen een eigen methode

Voor docenten die op zoek zijn naar wiskundeonderwijs waarin geen genoegen wordt genomen met het aanleren van manieren, is de Wageningse Methode een uitkomst. De opgaven zijn geen stapelopgaven (elkaar opvolgende onderdelen), maar doeopdrachten met een wiskundig puzzelkarakter. De Wageningse Methode bestrijkt, in tegenstelling tot de meest gebruikt methode Getal en Ruimte, de iets minder gebruikte methode ModerneWiskunde en de nog minder gebruikte methode Netwerk, een minimaal deel van de wiskundeonderwijsmarkt. De auteurs zijn niet uit op winstbejag. Verschillendematerialen zijn vrij toegankelijk op het internet. Het is de moeite waard deze idealistische benadering meer bekendheid te geven. Daan van Smaalen, een jonge auteur, licht een tipje van de sluier op. Als docent maakte hij kennis met de Wageningse Methode. Het was liefde op het eerste gezicht. Sinds 2005 is Daan van Smaalen actief betrokken bij Stichting de Wageningse Methode

Raman signatures of charge ordering in K0.3WO3

We present polarization- and temperature-dependent Raman spectroscopic study of hexagonal tungsten bronze, K0.3WO3. The observed asymmetry in phonon line shapes indicate the presence of strong lattice anharmonicity arising due to the nonstoichiometry of the material. We observed a broad multipeak Raman feature at low frequency due to the local modes of K atoms known as local structural excitations. The observed vibrational features indicate a second-order phase transition around T=200 K accompanied by a frequency softening of low-frequency phonon modes. The observed phonon anomalies hint a physical picture involving a continuous symmetry change toward a charge-ordered state below 200 K. These observations indicate that K0.3WO3 may exhibit a weak charge-density-wave ground state at low temperatures.

Incommensurate interactions and non-conventional spin-Peierls transition in TiOBr

Temperature-dependent x-ray diffraction of the low-dimensional spin 1/2 quantum magnet TiOBr shows that the phase transition at T_{c2} = 47.1 (4) K corresponds to the development of an incommensurate superstructure. Below T_{c1} = 26.8 \pm 0.3 K the incommensurate modulation locks in into a two-fold superstructure similar to the low-temperature spin-Peierls state of TiOCl. Frustration between intra- and interchain interations within the spin-Peierls scenario, and competition between two-dimensional magnetic order and one-dimensional spin-Peierls order are discussed as possible sources of the incommensurability.Comment: 5 pages including 3 figures and 1 tabl

Inverse pressure-induced Mott transition in TiPO4_4

TiPO$_4$ shows interesting structural and magnetic properties as temperature and pressure are varied, such as a spin-Peierls phase transition and the development of incommensurate modulations of the lattice. Recently, high pressure experiments for TiPO$_4$ reported two new structural phases appearing at high pressures, the so-called phases IV and V [M. Bykov et al., Angew. Chem. Int. Ed. 55, 15053]. The latter was shown to include the first example of 5-fold O-coordinated P-atoms in an inorganic phosphate compound. In this work we characterize the electronic structure and other physical properties of these new phases by means of ab-initio calculations, and investigate the structural transition. We find that the appearance of phases IV and V coincides with a collapse of the Mott insulating gap and quenching of magnetism in phase III as pressure is applied. Remarkably, our calculations show that in the high pressure phase V, these features reappear, leading to an antiferromagnetic Mott insulating phase, with robust local moments