Exchange couplings are calculated for Li2VOSiO4 using LDA. While the sum of in-plane couplings J1 + J2 = 9.5 ± 1.5K and the inter-plane coupling J ⊥ ∼ 0.2-0.3K agree with recent experimental data, the ratio J2/J1 ∼ 12 exceeds the reported value by an order of magnitude. Using geometrical considerations, high temperature expansions and perturbative mean field theory, we show that the LDA derived exchange constants lead to a remarkably accurate description of the properties of these materials including specific heat, susceptibility, Neél temperature and NMR spectra. 71.15.Mb,75.10.Jm,75.30.Et In many recently discovered magnetic materials the determination of exchange constants, without input from electronic structure calculations, has proven very difficult and has often led to wildly incorrect parameter values. The interplay of geometry and quantum chemistry has yielded many surprises which could not have been anticipated without a full calculation. Examples are the recently discovered vanadates CaV 4 O 9 [1] and CaV 3 O 7 [2]. In all these cases the dominant exchange interactions were resolved and a good understanding of the material properties obtained only after analyses of electronic structure calculations were carried out.Frustrated square-lattice spin-half Heisenberg antiferromagnets with nearest neighbor exchange J 1 and second neighbor (diagonal) exchange J 2 have received considerable attention recently. The properties of the model with J 2 =0 (or J 1 =0) are well understood at zero and finite temperature [3]. The large J 2 limit of the model is a classic example of quantum order by disorder [4,5], where at the classical level the two sublattices order antiferromagnetically but remain free to rotate with respect to each other. This degeneracy is lifted by quantum fluctuations leading to collinear magnetic order in a columnar pattern. At intermediate J 2 /J 1 there is strong evidence for a spin-gap phase, though the nature of this phase is not fully resolved yet [6].While there has been tremendous theoretical interest in these models, there were no known experimental realizations for intermediate to large J 2 /J 1 , until the investigation of Li 2 VOSiO 4 by Melzi et al. [7,8] Studying the splitting patterns of the 7 Li NMR spectra, these authors presented strong evidence for columnar order [7]. Combining several experiments they derive [8] exchange couplings (with J 2 /J 1 ∼ 1.1) well into the region where model calculations find columnar order.However, several puzzling pieces in that excellent and detailed study remain: (i) The ratio of exchange constants was not well determined from the susceptibility and specific heat data; we will present electronic structure and many-body calculations to show that their estimate J 2 /J 1 ≈ 1 [8], is off by an order of magnitude.(ii)The estimated T = 0 moment was anomalously small for a system well inside the columnar ordered phase. Taking into account the antiferromagnetic inter-plane coupling, we propose that the NMR derived moment is small due to a cancellation...

We have systematically investigated the X-ray-induced radiophotoluminescence (RPL) bands in a silver-activated phosphate glass (PG:Ag), and we have ascribed these bands to Ag 0 , Ag 2+and Ag 2 + centres, using optical characterisation such as absorption, excitation, emission and lifetime measurements. The absorption spectrum of PG:Ag irradiated with X-rays was decomposed into six Gaussian bands on the basis of its strong analogy with irradiated silver-activated sodium chloride (NaCl:Ag). We confirmed that blue emission peaking at 450 nm was connected with the 270 and 345 nm bands of the excitation spectrum, while orange emission peaking at 560 nm was associated with the 308 nm excitation band. Each band of the excitation spectrum was in turn attributed to an Ag 2 + , Ag 0 or Ag 2+ centre by analogy with the RPL of NaCl:Ag. The excited-state lifetimes for each emissive process were also determined for different X-ray doses. In conclusion, we determined that both blue and orange emissions result from radiation-induced (rather than intrinsic) properties in the range of 1.22 -24.5 Gy.

We report here a facile approach for flexible integration of high efficiency surface enhanced Raman scattering (SERS) monitors in a continuous microfluidic channel. In our work, femtosecond laser direct writing was adopted for highly localizable and controllable fabrication of the SERS monitor through a multi-photon absorption (MPA) induced photoreduction of silver salt solution. The silver substrate could be shaped into designed patterns, and could be precisely located at the desired position of the microchannel bed, giving the feasibility for real-time detection during reactions. SEM and TEM images show that the silver substrates were composed of crystallized silver nanoplates with an average thickness of 50 nm. AFM results reveal that the substrates were about 600 nm in height and the surface was very rough. As representative tests for SERS detection, p-aminothiophenol (p-ATP) and flavin adenine dinucleotide (FAD) were chosen as probing molecules for microfluidic analysis at visible light (514.5 nm) excitation, exhibiting an enhancement factor of ~10(8). In addition, the combination of the SERS substrate with the microfluidic channel allows detection of inactive analytes through in situ microfluidic reactions.

We have fabricated a number of high-quality porous silicon optical microcavities operating in the near infrared that exhibit cavity resonances with subnanometer linewidths. This was achieved through the low temperature anodic oxidation of highly doped p-type silicon wafers. We have investigated the optical properties of these microcavities using reflectivity and photoluminescence measurements and compared our results with theoretical predictions. From our analysis, we conclude that, for the low temperature fabrication process, the refractive index difference between adjacent layers of the multilayered structure is maximized while optical losses in the cavity are minimized. Furthermore, by considering the origin of optical losses in these microcavities, we demonstrate that fluctuations in the position of the resonance wavelength and optical absorption play an important role in the realization of high-quality interferometric structures.