Defect-related optical absorption bands in CdSiP2 crystals

When used as optical parametric oscillators, CdSiP2 crystals generate tunable output in the mid-infrared. Their performance, however, is often limited by unwanted optical absorption bands that overlap the pump wavelengths. A broad defect-related optical absorption band peaking near 800 nm, with a shoulder near 1 µm, can be photoinduced at room temperature in many CdSiP2 crystals. This absorption band is efficiently produced with 633 nm laser light and decays with a lifetime of ∼0.5 s after removal of the excitation light. The 800 nm band is accompanied by a less intense absorption band peaking near 1.90 µm. Data from eight CdSiP2 crystals grown at different times show that the singly ionized silicon vacancy (VSi− role= presentation style= box-sizing: border-box; display: inline; font-size: 12.880000114440918px; line-height: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative; \u3e−) is responsible for the photoinduced absorption bands. Electron paramagnetic resonance (EPR) is used to identify and directly monitor these silicon vacancies

Variable-range hopping in quasi-one-dimensional electron crystals

We study the effect of impurities on the ground state and the low-temperature dc transport in a 1D chain and quasi-1D systems of many parallel chains. We assume that strong interactions impose a short-range periodicicity of the electron positions. The long-range order of such an electron crystal (or equivalently, a $4 k_F$ charge-density wave) is destroyed by impurities. The 3D array of chains behaves differently at large and at small impurity concentrations $N$. At large $N$, impurities divide the chains into metallic rods. The low-temperature conductivity is due to the variable-range hopping of electrons between the rods. It obeys the Efros-Shklovskii (ES) law and increases exponentially as $N$ decreases. When $N$ is small, the metallic-rod picture of the ground state survives only in the form of rare clusters of atypically short rods. They are the source of low-energy charge excitations. In the bulk the charge excitations are gapped and the electron crystal is pinned collectively. A strongly anisotropic screening of the Coulomb potential produces an unconventional linear in energy Coulomb gap and a new law of the variable-range hopping $-\ln\sigma \sim (T_1 / T)^{2/5}$. $T_1$ remains constant over a finite range of impurity concentrations. At smaller $N$ the 2/5-law is replaced by the Mott law, where the conductivity gets suppressed as $N$ goes down. Thus, the overall dependence of $\sigma$ on $N$ is nonmonotonic. In 1D, the granular-rod picture and the ES apply at all $N$. The conductivity decreases exponentially with $N$. Our theory provides a qualitative explanation for the transport in organic charge-density wave compounds.Comment: 20 pages, 7 figures. (v1) The abstract is abridged to 24 lines. For the full abstract, see the manuscript (v2) several changes in presentation per referee's comments. No change in result

Scaling slowly rotating asteroids with stellar occultations

Context. As evidenced by recent survey results, the majority of asteroids are slow rotators (spin periods longer than 12 h), but lack spin and shape models because of selection bias. This bias is skewing our overall understanding of the spins, shapes, and sizes of asteroids, as well as of their other properties. Also, diameter determinations for large (>60 km) and medium-sized asteroids (between 30 and 60 km) often vary by over 30% for multiple reasons. Aims. Our long-term project is focused on a few tens of slow rotators with periods of up to 60 h. We aim to obtain their full light curves and reconstruct their spins and shapes. We also precisely scale the models, typically with an accuracy of a few percent. Methods. We used wide sets of dense light curves for spin and shape reconstructions via light-curve inversion. Precisely scaling them with thermal data was not possible here because of poor infrared datasets: large bodies tend to saturate in WISE mission detectors. Therefore, we recently also launched a special campaign among stellar occultation observers, both in order to scale these models and to verify the shape solutions, often allowing us to break the mirror pole ambiguity. Results. The presented scheme resulted in shape models for 16 slow rotators, most of them for the first time. Fitting them to chords from stellar occultation timings resolved previous inconsistencies in size determinations. For around half of the targets, this fitting also allowed us to identify a clearly preferred pole solution from the pair of two mirror pole solutions, thus removing the ambiguity inherent to light-curve inversion. We also address the influence of the uncertainty of the shape models on the derived diameters. Conclusions. Overall, our project has already provided reliable models for around 50 slow rotators. Such well-determined and scaled asteroid shapes will, for example, constitute a solid basis for precise density determinations when coupled with mass information. Spin and shape models in general continue to fill the gaps caused by various biases

Vogel-Fulcher like dielectric response and variable range hopping behaviour in molybdenum bronze

International audienceWe report on transport and dielectric properties of molybdenum bronze. Dielectric constant measurements in blue bronze samples show a low temperature anomaly exhibiting two distinct relaxational processes. One of these peaks follows a Vogel- Fulcher law and is discussed in comparison with ferroelectric relaxors. The temperature dependence of the conductivity in the ohmic regime as well as the non-linear transport properties for ET , E , EpT can be well described by a variable range hopping mechanism in both Blue and Red Bronze. We show that transport and dielectric properties are both very sensitive to the heat treatment applied to the sample

A multiple origin for non-linear transport properties in

We report on non-linear transport properties in \ab{K_{0.3}MoO_3} molybdenum blue bronze. We show that the conductivity sharply increases above a characteristic threshold field E_{\ab T}^* in three perpendicular crystallographic directions. This threshold field is visible up to room temperature in the direction perpendicular to the ($\bar201$) plane. Data suggests that this singularity is probably not related to the motion of the Charge Density Wave (CDW). Along the CDW axis, the electric field dependence of the conductivity for E < E_{\ab T}^* can be very well described by the scaling theory of the CDW creep in the presence of randomly distributed pinning centers

Determination of the magnetocaloric entropy change in the presence of phase separation and metastability: The case of Eu0.58Sr0.42MnO3

International audienceThe magnetocaloric effect (MCE) in the manganite Eu0.58Sr0.42MnO3 was derived by different methods, in a field range very sensitive to the phenomenon of phase separation. It turns out that a strong scatter in the MCE features was observed. When the applied field is less than the field required to complete the transition, it is found that the MCE can be strongly overestimated by “standard” indirect measurements. A way to properly estimate the MCE around a first order transition in the presence of phase separation and metastability is proposed

Determination of the magnetocaloric entropy change in the presence of phase separation and metastability: The case of Eu0.58Sr0.42MnO3

International audienc

Observation of a metallic resistivity behavior below 60 K in annealed molybdenum blue bronzes

When annealing molybdenum blue bronzes in the presence of a metal (Ag or Cu) a metallic resistivity behavior is observed below T ≈ 60 K in the three crystal directions. Nevertheless, DC conductivity and dielectric constant measurements still clearly exhibit the presence of a charge density wave (CDW). Electronic Paramagnetic Resonance (EPR) studies of annealed samples revealed a high concentration of Mo5+ sites with respect to untreated samples suggesting that due to some chemical reactions between the metal and the blue bronze some Mo6+ sites have been change into Mo5+ sites. We have discussed possible interpretations of such anomaly