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>

Optical Properties of Rare Earth Nitrides

<p>This experimental thesis uncovers the fundamental optical features of rare earth nitride compounds and relates them to their electronic structure. Experimental observations for the optical energy gaps for thin films of GdN, DyN, SmN and EuN are made for the first time. Thin films are grown by thermal evaporation in ultra high vacuum environment and are passivated by MgF₂ layers. Initial characterizations indicate the polycrystalline thin films of RENs are strongly textured along [111] direction. Optical characterization techniques, Fourier transform infrared and conventional UV/Vis spectrometers are used in conjunction with SQUID magnetometer and DC electrical resistivity. Transmission and reflection spectra for rare earth nitride thin films were obtained in the photon energy range 0.5 – 5.5 eV in their paramagnetic and ferromagnetic phases. Paramagnetic GdN has a direct energy gap of 1.30±0.05 eV which coincides well with theoretically predicted energy gap. A red-shift in the fundamental absorption edge of ferromagnetic GdN is observed along with onset of absorption at higher energy attributable to the exchange splitting of conduction and valence bands of GdN. The spin split joint density of states is in remarkable agreement with theoretically calculated spin polarized band structure of GdN. Similarly for DyN a consensus is found between theory and experiment on the energy gap of 1.20±0.05 eV at room temperature. However, in the case of SmN, an energy gap of 1.30±0.1 eV is underestimated by theory to 0.81 eV. For EuN, the experimentally determined value of energy gap is 0.97±0.05 eV. This value is used to tune the band structure calculation by QSGW theory which returns a ferromagnetic semiconducting solution for EuN.</p>

Optical Properties of Rare Earth Nitrides

<p>This experimental thesis uncovers the fundamental optical features of rare earth nitride compounds and relates them to their electronic structure. Experimental observations for the optical energy gaps for thin films of GdN, DyN, SmN and EuN are made for the first time. Thin films are grown by thermal evaporation in ultra high vacuum environment and are passivated by MgF₂ layers. Initial characterizations indicate the polycrystalline thin films of RENs are strongly textured along [111] direction. Optical characterization techniques, Fourier transform infrared and conventional UV/Vis spectrometers are used in conjunction with SQUID magnetometer and DC electrical resistivity. Transmission and reflection spectra for rare earth nitride thin films were obtained in the photon energy range 0.5 – 5.5 eV in their paramagnetic and ferromagnetic phases. Paramagnetic GdN has a direct energy gap of 1.30±0.05 eV which coincides well with theoretically predicted energy gap. A red-shift in the fundamental absorption edge of ferromagnetic GdN is observed along with onset of absorption at higher energy attributable to the exchange splitting of conduction and valence bands of GdN. The spin split joint density of states is in remarkable agreement with theoretically calculated spin polarized band structure of GdN. Similarly for DyN a consensus is found between theory and experiment on the energy gap of 1.20±0.05 eV at room temperature. However, in the case of SmN, an energy gap of 1.30±0.1 eV is underestimated by theory to 0.81 eV. For EuN, the experimentally determined value of energy gap is 0.97±0.05 eV. This value is used to tune the band structure calculation by QSGW theory which returns a ferromagnetic semiconducting solution for EuN.</p>

Chiral singlet superconductivity in the weakly correlated metal LaPt3P

Chiral superconductors are novel topological materials with finite angular momentum Cooper pairs circulating around a unique chiral axis, thereby spontaneously breaking time-reversal symmetry. They are rather scarce and usually feature triplet pairing: a canonical example is the chiral p-wave state realized in the A-phase of superfluid He3. Chiral triplet superconductors are, however, topologically fragile with the corresponding gapless boundary modes only weakly protected against symmetry-preserving perturbations in contrast to their singlet counterparts. Using muon spin relaxation measurements, here we report that the weakly correlated pnictide compound LaPt3P has the two key features of a chiral superconductor: spontaneous magnetic fields inside the superconducting state indicating broken time-reversal symmetry and low temperature linear behaviour in the superfluid density indicating line nodes in the order parameter. Using symmetry analysis, first principles band structure calculation and mean-field theory, we unambiguously establish that the superconducting ground state of LaPt3P is a chiral d-wave singlet.

Chiral singlet superconductivity in the weakly correlated metal LaPt3P

Topological superconductors (SCs) are novel phases of matter with nontrivial bulk topology. They host at their boundaries and vortex cores zero-energy Majorana bound states, potentially useful in fault-tolerant quantum computation [1]. Chiral SCs [2] are particular examples of topological SCs with finite angular momentum Cooper pairs circulating around a unique chiral axis, thus spontaneously breaking time-reversal symmetry (TRS). They are rather scarce and usually feature triplet pairing: best studied examples in bulk materials are UPt<3> and Sr<2>RuO<4> proposed to be f-wave and p-wave SCs respectively, although many open questions still remain [2]. Chiral triplet SCs are, however, topologically fragile with the gapless Majorana modes weakly protected against symmetry preserving perturbations in contrast to chiral singlet SCs [3, 4]. Using muon spin relaxation (μSR) measurements, here we report that the weakly correlated pnictide compound LaPt<3>P has the two key features of a chiral SC: spontaneous magnetic fields inside the superconducting state indicating broken TRS and low-temperature linear behaviour in the superfluid density indicating line nodes in the order parameter. Using symmetry analysis, first principles band structure calculation and mean-field theory, we unambiguously establish that the superconducting ground state of LaPt<3>P is chiral d-wave singlet.