X-Ray Scattering Measurements of the Transient Structure of a Driven Charge-Density-Wave

We report time-resolved x-ray scattering measurements of the transient structural response of the sliding {\bf Q}$_{1}$ charge-density-wave (CDW) in NbSe$_{3}$ to a reversal of the driving electric field. The observed time scale characterizing this response at 70K varies from $\sim$ 15 msec for driving fields near threshold to $\sim$ 2 msec for fields well above threshold. The position and time-dependent strain of the CDW is analyzed in terms of a phenomenological equation of motion for the phase of the CDW order parameter. The value of the damping constant, $\gamma = (3.2 \pm 0.7) \times 10^{-19}$ eV $\cdot$ seconds $\cdot$ \AA$^{-3}$, is in excellent agreement with the value determined from transport measurements. As the driving field approaches threshold from above, the line shape becomes bimodal, suggesting that the CDW does not depin throughout the entire sample at one well-defined voltage.Comment: revtex 3.0, 7 figure

Interesting magnetic properties of Fe1−x_{1-x}Cox_xSi alloys

Solid solution between nonmagnetic narrow gap semiconductor FeSi and diamagnetic semi-metal CoSi gives rise to interesting metallic alloys with long-range helical magnetic ordering, for a wide range of intermediate concentration. We report various interesting magnetic properties of these alloys, including low temperature re-entrant spin-glass like behaviour and a novel inverted magnetic hysteresis loop. Role of Dzyaloshinski-Moriya interaction in the magnetic response of these non-centrosymmetric alloys is discussed.Comment: 11 pages and 3 figure

Tunable variation of optical properties of polymer capped gold nanoparticles

Optical properties of polymer capped gold nanoparticles of various sizes (diameter 3-6 nm) have been studied. We present a new scheme to extract size dependent variation of total dielectric function of gold nanoparticles from measured UV-Vis absorption data. The new scheme can also be used, in principle, for other related systems as well. We show how quantum effect, surface atomic co - ordination and polymer - nanoparticle interface morphology leads to a systematic variation in inter band part of the dielectric function of gold nanoparticles, obtained from the analysis using our new scheme. Careful analysis enables identification of the possible changes to the electronic band structure in such nanoparticles.Comment: 13 pages,7 figures, 1 tabl

Effects of hydrostatic pressure on the magnetic properties of disordered monosilicide FexCo1 - xSi alloys

Disordered FexCo1 - xSi alloys are known to be ferromagnetic in an iron concentration range starting at x = 0.2. We have measured their magnetization in magnetic fields up to 40 kOe at 4.2 K and under hydrostatic pressures up to 8 kbar, as well as their zero pressure susceptibility at 4.2 K in high magnetic fields of 150 kOe. We have also determined the Curie temperatures of the ferromagnetic samples under hydrostatic pressure up to 8 kbar from the temperature variation of the initial susceptibility. We determined a critical iron concentration of x ≃ 0.95 for ferromagnetism in FeSi rich alloys. From some of our results, the ferromagnetism in the CoSi rich alloys seems to appear in a rather homogeneous way, whereas the FeSi rich alloys in contrast exhibit local effects.Les alliages désordonnés FexCo1 - xSi sont ferromagnétiques pour une concentration en fer x supérieure à 0,2. Nous avons mesuré leur aimantation sous champ magnétique jusqu'à 40 kOe à 4,2 K et sous pression hydrostatique jusqu'à 8 kbar, de même que leur susceptibilité à la pression ordinaire sous champ magnétique de 150 kOe. Nous avons aussi déterminé les températures de Curie d'échantillons ferromagnétiques sous pression hydrostatique jusqu'à 8 kbar à partir de la variation thermique de la susceptibilité initiale. Nous avons déterminé une concentration critique x de l'ordre de 0,95 pour l'apparition du ferromagnétisme dans les alliages riches en FeSi. D'après nos résultats, le ferromagnétisme dans les alliages riches en CoSi semble apparaître de façon assez homogène, alors que les alliages riches en FeSi présentent au contraire des effets locaux