TOPOLOGICAL defects formed during a rapid symmetry-breaking phase transition in the early Universe1,2 could be responsible for seeding large-scale structure, for the anisotropy of the microwave background radiation, and for the predominance of matter over antimatter3,4. The theory describing this cosmological phase transition is formally analogous to that describing the transition to the superfluid state in liquid 3He, so that in principle the process of cosmological defect formation can be modelled in the laboratory. Here we report the results of an experiment in which the 'primordial fireball' is mimicked using a neutron-induced nuclear reaction (n + 3He → p + 3He + 0.76 MeV) to heat small regions of superfluid 3He above the superfluid transition temperature. These bubbles of normal liquid cool extremely rapidly, and we find that their transition back to the superfluid state is accompanied by the formation of a random network of vortices (the superfluid analogue of cosmic strings). We monitor the evolution of this defect state by rotating the superfluid sample, allowing vortices to escape from the network and thus be probed individually. Our results provide clear confirmation of the idea that topological defects form at a rapid second-order phase transition, and give quantitative support to the Kibble–Zurek mechanism5,6 of cosmological defect formation.

Vortex formation in neutron-irradiated superfluid 3He as an analogue of cosmological defect formation

Quantum dots (QDs) with fluorescence in the second near-infrared window (NIR-II, 1000-1400 nm) are ideal fluorophores for in vivo imaging of deep tissue with high signal-to-noise ratios. Ag₂Se (bulk band gap 0.15 eV) is a promising candidate for preparing NIR-II QDs. By using 1-octanethiol as ligand to effectively balance the nucleation and growth, tuning the fluorescence of Ag₂Se QDs was successfully realized in the NIR-II window ranged from 1080 to 1330 nm. The prepared Ag₂Se QDs can be conveniently transferred to the aqueous phase by ligand exchange, showing great potential for multicolor NIR-II fluorescence imaging in vivo.

Ag2Se Quantum Dots with Tunable Emission in the Second Near-Infrared Window

FeS2 thin films were prepared by thermal sulfidation of iron layers evaporated on glass substrates. Experiments were carried out to optimize the sulfidation parameters. For this purpose, morphological, compositional, crystallographic, optical, and electrical properties of the produced films and their relation with the parameters involved in the sulfidation process (temperature, sulfur pressure) are presented. From x‐ray spectra and energy dispersive analyses of x ray it is concluded that FeS2 starts to be formed at sulfidation temperatures about 200 °C. X‐ray patterns show that crystallinity and grain size of the films improve with the sulfidation temperature with a ‘‘critical value’’ which varies with the sulfur pressure. On the other hand, both the film resistivity and the optical absorption coefficient change abruptly at temperatures higher than 350 °C. Above 500 °C polycrystalline aggregates known as ‘‘framboidal pyrite’’ are observed.

Characterization of FeS2 thin films prepared by thermal sulfidation of flash evaporated iron

We report for the first time the electrical transport in solid-state electrolyte nanowires. Single nanowire transport and in situ transmission electron microscopy studies show that Ag2Se nanowires can be high conducting orthorhombic β-Ag2Se or low conducting cubic α-Ag2Se. It is also the first time that α-Ag2Se is found to be stable at room temperature. A threshold switching phenomenon exists in the low conducting α-Ag2Se with an on−off ratio up to 7 order magnitude. These results provide useful new information for exploring solid-state electrolyte nanowires as resistive switching memory devices.

Electrical switching and phase transformation in silver selenide nanowires.

Three types of amorphous‐titania films prepared by ion‐ and electron‐beam techniques have been annealed thermally. An amorphous‐crystalline transformation is found in each of these film types at around 350 °C. Its resulting microcrystalline structure and the exact transition temperature appear to be dictated by the rutile microcrystalline seed present in the as‐deposited films under different deposition conditions. An amorphous film with a weak rutile seed crystallizes at a lower temperature into the anatase structure, while a film with a relatively strong rutile base crystallizes into the rutile structure at a somewhat higher temperature. It is demonstrated that Raman spectroscopy is a simple and effective tool for characterization of these submicron‐thick amorphous films and for the dynamical study of such a phase transformation. Accompanying this amorphous‐crystalline transformation, a two‐order increase in elastic light scattering is noted implying optical degradation associated with microcrystalline ...

Thermally induced crystallization of amorphous‐titania films

Thin films of silver selenide (Ag2Se) between thicknesses of about 700 and 2200 A have been prepared on glass substrates at room temperature in a vacuum of 5×10−5 Torr. After vacuum annealing the films (at about 373 K for 3 h) electrical resistivity measurements on these films have been carried out in vacuum. From the increase in the rate of decrease of resistance with temperature, the phase transition temperatures (orthorhombic to body‐centered cubic) of the different films have been located. It is found that the phase transition temperature of the thin films is a function of thickness increasing with a decrease in the thickness. This observation has been explained by a recently developed theory [V. Damodara Das and D. Karunakaran, J. Phys. Chem Solids 46, 551 (1985)] of phase transitions in thin films modified further. Also, an order‐of‐magnitude value of the difference in the function of specific surface and interfacial energies of the two phases has also been determined using the theory.

Thickness dependence of the phase transition temperature in Ag2Se thin films

Thermoelectric power of annealed β‐Ag2Se thin films of different thicknesses has been measured both while heating and cooling by the integral method. It is found that it remains practically constant (in β‐Ag2Se phase) during heating while it is a function of temperature while cooling. The thermoelectric power in both heating and cooling cycles is a function of inverse thickness of the films. The difference in behavior between Ag2Se films during heating and cooling is attributed to the possible transformation to monoclinic phase during cooling from the original orthorhombic phase during heating. The inverse thickness dependence has been explained by the size effect theories. Important material parameters like carrier concentration, Fermi energy, effective mass of carriers, and energy dependence of the mean free path have been evaluated for the β‐Ag2Se (orthorhombic) phase.

Thermoelectric power of annealed β-Ag2Se alloy thin films: temperature and size effects ― possibility of a new (β2) phase at low temperatures

Thin films of Ag2Te of various thicknesses in the range 500–1500 A have been prepared by thermal evaporation of the compound under vacuum on clean glass substrates held at room temperature. The electrical resistance of the films has been measured as a function of temperature during heating, which was carried out immediately after the film formation. The observed exponential decrease of resistance with temperature up to the transition point points to the semiconducting nature of the low temperature polymorph of Ag2Te. The band gap of the low temperature phase is calculated for various thicknesses of the films and it is found that the band gap is a function of film thickness, increasing with decreasing thickness. The increase in the band gap, which was found to be inversely proportional to the square of the film thickness, is attributed to quantization of electron momentum component normal to film plane.

Semiconducting behavior of Ag2Te thin films and the dependence of band gap on thickness

${\mathrm{Ag}}_{2}$Te thin films with thicknesses in the range 600---1400 \AA{} have been prepared by vacuum deposition at a pressure of 5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}5}$ Torr on clean glass substrates held at room temperature. The thermoelectromotive force of these films has been measured in the temperature range 300---415 K, that is, below the phase-transition temperature. It is found that the thermoelectric power of ${\mathrm{Ag}}_{2}$Te thin films in the above temperature range exhibits degenerate semiconductor behavior, this is, a linear increase in the thermoelectric power with rising temperature. It is also found that the thermoelectric power obeys the inverse thickness dependence predicted by classical size-effect theories.

D. Karunakaran

Papers

Thickness dependence of the phase transition temperature in Ag2Se thin films

Thermoelectric power of annealed β-Ag2Se alloy thin films: temperature and size effects ― possibility of a new (β2) phase at low temperatures

Semiconducting behavior of Ag2Te thin films and the dependence of band gap on thickness

Thermoelectric studies on semiconducting Ag 2 Te thin films: Temperature and dimensional effects

Thickness dependence of the phase transition temperature in Ag2Te thin films