Investigation of the electron and thermal transport in rare-earth nitrides

<p>Gadolinium nitride (GdN) and samarium nitride (SmN) have been widely studied to understand their ferromagnetic ordering and electronic structure, and for their promise in spintronics applications. This thesis presents experimental magnetotransport studies of GdN and SmN films in which experimental results have been compared with the existing band structure calculation. Three GdN films have been prepared in different conditions, among them two films are epitaxial quality and one film is polycrystalline in nature, and two films of SmN were also studied. Their magnetic properties were probed by SQUID magnetometry and they are found to be ferromagnetic. The transition temperature differs from sample to sample and this behaviour has been attributed to the presence of magnetic polarons that nucleate around nitrogen vacancies and give rise to an inhomogeneous ferromagnetic state. The charge transport results have been discussed for all GdN and SmN films. A full set of charge/heat transport results are obtained on only one epitaxial GdN. The difference of resistivity among these samples is noticeable. The Hall effect results show the presence of different carrier concentration with at most only weak temperature dependence. We also have noticed the presence of anomalous Hall effect in the paramagnetic region for a lower-concentration epitaxial GdN. The thermopower in both GdN and SmN was measured to provide further insight into the material’s electronic properties. In this thesis we present the first experimental investigation of the thermopower of epitaxial gadolinium nitride and samarium nitride films, measured using an experimental set-up designed for measuring the temperature dependent thermopower of thin films. Our result shows a negative thermopower for both GdN and SmN films and simple, though strong temperature dependence. At low temperatures we observe a peak near the ferromagnetic transition temperature in GdN. The results are interpreted in terms of the diffusion thermopower. Overall the results suggest that the nitrogen vacancy concentration controls the carrier concentration and plays a significant role towards the transport properties. We conclude that all films are either heavily, moderately or weakly doped semiconductors with a metallic characteristic.</p>

Investigation of the electron and thermal transport in rare-earth nitrides

<p>Gadolinium nitride (GdN) and samarium nitride (SmN) have been widely studied to understand their ferromagnetic ordering and electronic structure, and for their promise in spintronics applications. This thesis presents experimental magnetotransport studies of GdN and SmN films in which experimental results have been compared with the existing band structure calculation. Three GdN films have been prepared in different conditions, among them two films are epitaxial quality and one film is polycrystalline in nature, and two films of SmN were also studied. Their magnetic properties were probed by SQUID magnetometry and they are found to be ferromagnetic. The transition temperature differs from sample to sample and this behaviour has been attributed to the presence of magnetic polarons that nucleate around nitrogen vacancies and give rise to an inhomogeneous ferromagnetic state. The charge transport results have been discussed for all GdN and SmN films. A full set of charge/heat transport results are obtained on only one epitaxial GdN. The difference of resistivity among these samples is noticeable. The Hall effect results show the presence of different carrier concentration with at most only weak temperature dependence. We also have noticed the presence of anomalous Hall effect in the paramagnetic region for a lower-concentration epitaxial GdN. The thermopower in both GdN and SmN was measured to provide further insight into the material’s electronic properties. In this thesis we present the first experimental investigation of the thermopower of epitaxial gadolinium nitride and samarium nitride films, measured using an experimental set-up designed for measuring the temperature dependent thermopower of thin films. Our result shows a negative thermopower for both GdN and SmN films and simple, though strong temperature dependence. At low temperatures we observe a peak near the ferromagnetic transition temperature in GdN. The results are interpreted in terms of the diffusion thermopower. Overall the results suggest that the nitrogen vacancy concentration controls the carrier concentration and plays a significant role towards the transport properties. We conclude that all films are either heavily, moderately or weakly doped semiconductors with a metallic characteristic.</p>

Physico-Chemical Characteristics of Spodumene Concentrate and Its Thermal Transformations

Spodumene concentrate from the Pilbara region in Western Australia was characterized by X-ray diffraction (XRD), Scanning Electron Microscope Energy Dispersive Spectroscopy (SEM-EDS) and Mineral Liberation Analysis (MLA) to identify and quantify major minerals in the concentrate. Particle diameters ranged from 10 to 200 microns and the degree of liberation of major minerals was found to be more than 90%. The thermal behavior of spodumene and the concentration of its polymorphs were studied by heat treatments in the range of 900 to 1050 °C. All three polymorphs of the mineral (α, γ and β) were identified. Full transformation of the α-phase was achieved at 975 °C and 1000 °C after 240 and 60 min treatments, respectively. SEM images of thermally treated concentrate revealed fracturing of spodumene grains, producing minor cracks initially which became more prominent with increasing temperature. Material disintegration, melting and agglomeration with gangue minerals were also observed at higher temperatures. The metastable γ-phase achieved a peak concentration of 23% after 120 min at 975 °C. We suggest 1050 °C to be the threshold temperature for the process where even a short residence time causes appreciable transformation, however, 1000 °C may be the ideal temperature for processing the concentrate due to the degree of material disintegration and α-phase transformation observed. The application of a first-order kinetic model yields kinetic parameters which fit the experimental data well. The resultant apparent activation energies of 655 and 731 kJ mol−1 obtained for α- and γ-decay, respectively, confirm the strong temperature dependence for the spodumene polymorph transformations.

Solar-thermal production of graphitic carbon and hydrogen via methane decomposition

This work reports a process in which concentrated irradiation from a simulated solar source converts methane to high-value graphitic carbon and hydrogen gas. Methane flows within a photo-thermal reactor through the pores of a thin substrate irradiated by several thousand suns at the focal peak. The methane decomposes primarily into hydrogen while depositing highly graphitic carbon that grows conformally over ligaments in the porous substrate. The localized solar heating of the porous substrate serves to capture the solid carbon into a readily extractable and useful form while maintaining active deposition site density with persistent catalytic activity. Results indicate a strong temperature dependence with high decomposition occurring in the central heating zone with concentration factors and temperatures above 1000 suns and 1300 K, respectively. Even with a large flow area through regions of lower irradiation and temperature, methane conversion and hydrogen yields of approx. 70\% are achieved, and 58\% of the inlet carbon is captured in graphitic form.

Solar-thermal production of graphitic carbon and hydrogen via methane decomposition

This work reports a process in which concentrated irradiation from a simulated solar source converts methane to high-value graphitic carbon and hydrogen gas. Methane flows within a photo-thermal reactor through the pores of a thin substrate irradiated by several thousand suns at the focal peak. The methane decomposes primarily into hydrogen while depositing highly graphitic carbon that grows conformally over ligaments in the porous substrate. The localized solar heating of the porous substrate serves to capture the solid carbon into a readily extractable and useful form while maintaining active deposition site density with persistent catalytic activity. Results indicate a strong temperature dependence with high decomposition occurring in the central heating zone with concentration factors and temperatures above 1000 suns and 1300 K, respectively. Even with a large flow area through regions of lower irradiation and temperature, methane conversion and hydrogen yields of approx. 70\% are achieved, and 58\% of the inlet carbon is captured in graphitic form.

Superflow decay in a toroidal Bose gas: The effect of quantum and thermal fluctuations

We theoretically investigate the stochastic decay of persistent currents in a toroidal ultracold atomic superfluid caused by a perturbing barrier. Specifically, we perform detailed three-dimensional simulations to model the experiment of Kumar et al. in [Phys. Rev. A 95 021602 (2017)], which observed a strong temperature dependence in the timescale of superflow decay in an ultracold Bose gas. Our ab initio numerical approach exploits a classical-field framework that includes thermal fluctuations due to interactions between the superfluid and a thermal cloud, as well as the intrinsic quantum fluctuations of the Bose gas. In the low-temperature regime our simulations provide a quantitative description of the experimental decay timescales, improving on previous numerical and analytical approaches. At higher temperatures, our simulations give decay timescales that range over the same orders of magnitude observed in the experiment, however, there are some quantitative discrepancies that are not captured by any of the mechanisms we explore. Our results suggest a need for further experimental and theoretical studies into superflow stability.

Thermally boosted upconversion and downshifting luminescence in Sc2(MoO4)3:Yb/Er with two-dimensional negative thermal expansion

Lanthanide (Ln3+)-doped phosphors generally suffer from thermal quenching, in which their photoluminescence (PL) intensities decrease at the higher temperature. Herein, we report a class of unique two-dimensional negative-thermal-expansion phosphor of Sc2(MoO4)3:Yb/Er. By virtue of the reduced distances between sensitizers and emitters as well as confined energy migration with increasing the temperature, a 45-fold enhancement of green upconversion (UC) luminescence and a 450-fold enhancement of near-infrared downshifting (DS) luminescence of Er3+ are achieved from 25 to 500 ˚C. The thermally boosted UC and DS luminescence mechanism is systematically investigated through in situ temperature-dependent Raman spectroscopy, synchrotron X-ray diffraction and PL dynamics. Moreover, the luminescence lifetime of 4I11/2 of Er3+ in Sc2(MoO4)3:Yb/Er displays a strong temperature dependence, enabling ratiometric thermometry with the highest relative sensitivity of 13.4%/K at 298 K. These findings may gain a vital insight into the design of negative-thermal-expansion Ln3+-doped phosphors for versatile applications.

Linear analysis on the onset of thermal convection of highly compressible fluids with variable viscosity and thermal conductivity in spherical geometry: implications for the mantle convection of super-Earths

In this paper, we carried out a series of linear analyses on the onset of thermal convection of highly compressible fluids whose physical properties strongly vary in space in convecting vessels either of a three-dimensional spherical shell or a two-dimensional spherical annulus geometry. The variations in thermodynamic properties (thermal expansivity and reference density) with depth are taken to be relevant for the super-Earths with ten times the Earth’s mass, while the thermal conductivity and viscosity are assumed to exponentially depend on depth and temperature, respectively. Our analysis showed that, for the cases with strong temperature dependence in viscosity and strong depth dependence in thermal conductivity, the critical Rayleigh number is on the order of 108–109, implying that the mantle convection of massive super-Earths is most likely to fall in the stagnant-lid regime very close to the critical condition, if the properties of their mantle materials are quite similar to the Earth’s. Our analysis also demonstrated that the structures of incipient flows of stagnant-lid convection in the presence of strong adiabatic compression are significantly affected by the depth dependence in thermal conductivity and the geometries of convecting vessels, through the changes in the static stability of thermal stratification of the reference state. When the increase in thermal conductivity with depth is sufficiently large, the thermal stratification can be greatly stabilized at depth, further inducing regions of insignificant fluid motions above the bottom hot boundaries in addition to the stagnant lids along the top cold surfaces. We can therefore speculate that the stagnant-lid convection in the mantles of massive super-Earths is accompanied by another motionless regions at the base of the mantles if the thermal conductivity strongly increases with depth (or pressure), even though their occurrence is hindered by the effects the spherical geometries of convecting vessels.

Demonstration of reduced neoclassical energy transport in Wendelstein 7-X

Research on magnetic confinement of high-temperature plasmas has the ultimate goal of harnessing nuclear fusion for the production of electricity. Although the tokamak1 is the leading toroidal magnetic-confinement concept, it is not without shortcomings and the fusion community has therefore also pursued alternative concepts such as the stellarator. Unlike axisymmetric tokamaks, stellarators possess a three-dimensional (3D) magnetic field geometry. The availability of this additional dimension opens up an extensive configuration space for computational optimization of both the field geometry itself and the current-carrying coils that produce it. Such an optimization was undertaken in designing Wendelstein 7-X (W7-X)2, a large helical-axis advanced stellarator (HELIAS), which began operation in 2015 at Greifswald, Germany. A major drawback of 3D magnetic field geometry, however, is that it introduces a strong temperature dependence into the stellarator’s non-turbulent ‘neoclassical’ energy transport. Indeed, such energy losses will become prohibitive in high-temperature reactor plasmas unless a strong reduction of the geometrical factor associated with this transport can be achieved; such a reduction was therefore a principal goal of the design of W7-X. In spite of the modest heating power currently available, W7-X has already been able to achieve high-temperature plasma conditions during its 2017 and 2018 experimental campaigns, producing record values of the fusion triple product for such stellarator plasmas3,4. The triple product of plasma density, ion temperature and energy confinement time is used in fusion research as a figure of merit, as it must attain a certain threshold value before net-energy-producing operation of a reactor becomes possible1,5. Here we demonstrate that such record values provide evidence for reduced neoclassical energy transport in W7-X, as the plasma profiles that produced these results could not have been obtained in stellarators lacking a comparably high level of neoclassical optimization.

Experimental signatures of nodeless multiband superconductivity in a $$\hbox {2H-Pd}_{0.08} \hbox {TaSe}_2$$ single crystal

In order to understand the superconducting gap nature of a $$\hbox {2H-Pd}_{0.08} \hbox {TaSe}_2$$ 2H-Pd 0.08 TaSe 2 single crystal with $$T_{c} = 3.13 \text { K}$$ T c = 3.13 K , in-plane thermal conductivity $$\kappa $$ κ , in-plane London penetration depth $$\lambda _{\text {L}}$$ λ L , and the upper critical fields $$H_{c2}$$ H c 2 have been investigated. At zero magnetic field, it is found that no residual linear term $$\kappa _{0}/T$$ κ 0 / T exists and $$\lambda _{\text {L}}$$ λ L follows a power-law $$T^n$$ T n (T: temperature) with n = 2.66 at $$T \le \frac{1}{3}T_c$$ T ≤ 1 3 T c , supporting nodeless superconductivity. Moreover, the magnetic-field dependence of $$\kappa _{0}$$ κ 0 /T clearly shows a shoulder-like feature at a low field region. The temperature dependent $$H_{c2}$$ H c 2 curves for both in-plane and out-of-plane field directions exhibit clear upward curvatures near $$T_c$$ T c , consistent with the shape predicted by the two-band theory and the anisotropy ratio between the $$H_{c2}$$ H c 2 (T) curves exhibits strong temperature-dependence. All these results coherently suggest that $$\hbox {2H-Pd}_{0.08} \hbox {TaSe}_2$$ 2H-Pd 0.08 TaSe 2 is a nodeless, multiband superconductor.