The electrical and magnetic properties of magnetite from 100-300°K have been studied. Specimens were obtained from natural crystals of local origin and fabricated in the form of rectangular bars. When this was not possible, due to excessive cracks in the crystal, the electrical properties of the resulting irregularly shaped disc were measured by use of the Van der Pauw theorem. Observation of the temperature variation of the resistivity showed that the resistivity increased as the temperature was decreased and that it increased by a factor 24 over a 10°K temperature interval when the specimen was cooled below 115°K. This phenomenon is related to an order-disorder transition at this temperature, in which the crystalline symmetry changes from cubic to orthorhombic. One specimen did not show the resistivity transition, probably due to excessive impurity content, which would prevent ordering from occurring. The results are explained qualitatively by the Verwey model, which postulates a conduction mechanism involving the jumping of electrons between octahedral sites of the Fe₃O₄ unit cell, as opposed to the conventional band type of conduction. An attempt was made to measure the Hall mobility of the specimens, but a Hall voltage was not detected within the sensitivity of the apparatus. An upper limit of 1 cm²/volt-sec. for the mobility was established. A negative magnetoresistance effect was observed in both specimens and was measured as a function of temperature for both transverse and longitudinal magnetic fields. A pronounced minimum at the transition temperature was observed for one specimen, while the other showed no change here. The AC permeability was observed over the transition region and again a sharp decrease was observed on cooling through the transition. Normal induction curves were obtained for the specimen at various fixed temperatures from the permeability data, from which it was deduced that the specimen was much harder to saturate below the transition. The permeability on warming was found to be independent of the magnetic state of the sample while cooling through the transition, and it was again observed that one crystal did not undergo the phase transition at 115°K. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Small free radicals trapped in solid argon, krypton and carbon tetrachloride at 4.2°K have been studied using electron spin resonance (ESR). An attempt was made to determine whether the methylene radical, produced by the photolysis of diazomethane and ketene trapped in the solid matrix, has a triplet ground state. No signal definitely attributable to the methylene radical was observed. It is postulated that the zero field splitting due to the spin-spin coupling of the unpaired electrons broadened any ESR signal beyond detectability„ The ESR signal of trapped methyl radicals was observed in some experiments, and it is suggested that they were formed by abstraction of hydrogen atoms from another deposited material by methylene radicals. An experiment in which diazomethane was photolysed in the presence of D₂O in an argon matrix at 4.2°K yielded an ESR signal which may possibly have been due to the CH₂D radical. An investigation has been carried out of the populations of the rotational levels of methyl radicals produced by the photolysis of trapped methyl iodide and dimethyl mercury at 4.2°K. For thermal equilibrium freely rotating radicals should populate only the ground state at this temperature, but it was found that the lowest two levels were both populated. It is suggested that either there was not thermal equilibrium, or, more likely, the methyl radicals were undergoing hindered rotation. Room temperature equilibrium mixtures of N₂F₄ - NF₂ were trapped in the three matrices at 4.2°K, and ESR absorption due to the trapped NF₂ radicals was observed. Three lines were observed at this temperature, with the centre one of greater amplitude and smaller line width than the outer two. During warmup the amplitudes and widths of these lines became approximately equal and two further triplets appeared, symmetrically distributed about the centre line. From the warmup spectra the isotropic hyperfine splitting constants for fluorine and nitrogen have been deduced to be 168 and 48 mc./sec. respectively. It is suggested that the radicals underwent slow isotropic rotation at 4.2°K. The degree of s-character of the molecular orbital containing the unpaired electron is discussed in the light of the isotropic hyperfine splitting constants. An unsuccessful attempt to find hyperfine and rotational structure in the ESR signal of the NF₂ radical in the gas phase was carried out. The photolysis of CF₃I in krypton and carbon tetrachloride matrices at 4.2°K yielded a very complicated ESR spectrum. A phase reversal of some of the lines was observed. A broad single line was observed when CF₃I in carbon tetrachloride was irradiated at 77°K. At the time of writing no definite interpretation of the spectra can be suggested. / Science, Faculty of / Chemistry, Department of / Graduate

Short resumés of the theories of propagation of electromagnetic waves in an anisotropic medium, of the Faraday effect, and of the influence of paramagnetic resonance on the Faraday effect are given. The Poincaré sphere is introduced to describe polarized light. A paramagnetic resonance spectrometer is described, which was modified so as to allow the study of magneto-optical phenomena under the influence of paramagnetic resonance. The spectrometer operated in the X-band using a 2K39 Klystron. The samples were located in a transmission type cavity operating in the TE 101 mode, and immersed in liquid helium. The cavity was provided with holes allowing the passage of light through the sample in a direction parallel to the external magnetic field. An optical system provided plane-polarized monochromatic light ( ג = 5461 Å ) incident on the sample. The light emerging from the sample passed through a Glan-Thompson prism analyzer. The relative intensity of the light passing the analyzer could be measured by a photomultiplier circuit and could be displayed as a function of time on an oscilloscope. Experiments are described in detail in which the spin-lattice relaxation time was measured as a function of temperature and external magnetic field. In these experiments, for a given temperature and magnetic field, the Faraday rotation was reduced by pulses of microwave power of varying length applied to the cavity. The return of the Faraday rotation as a function of time to its equilibrium value after the microwave power was cut off could be inferred from the intensity versus time relationship of the light transmitted by the analyzer. Photographic records of this intensity versus time relationship were obtained and the relaxation time was deduced from these records. Results of the measurement of the spin-lattice relaxation time of neodymium ethylsulfate for fields between 780 and 2540 Oerstedt and temperatures between 1.38°K and 4.22°K are given. The relaxation times measured were of the order of .001 to .1 seconds. The relaxation time appeared to be inversely proportional to the third power of the temperature and showed only small field dependence, except for a large dip at a field corresponding to the resonance field for the microwave frequencies used. Within the accuracy of the experiments no effect of the length of the microwave pulses on the relaxation time could be observed. An experiment on cerium ethylsulfate is described, which showed that the spin-lattice relaxation time must be smaller than 1 millisecond for this salt. No accurate determination of the relaxation time could be made in this case. Mention is made of an "overshoot effect" observed in one particular crystal of neodymium ethylsulfate. A possible explanation for this effect is given, by assuming that the crystal in question was twinned. In this case one may infer that the relaxation time is strongly dependent on the orientation of the optical axis of the crystal with respect to the external magnetic field The results were found to disagree with present-day theories of paramagnetic relaxation. Assumptions of doubtful validity in the theory are discussed as possible reasons for such disagreement. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

In this thesis, I demonstrate Doppler and Sisyphus cooling of a cryogenic buffer-gas beam of CaH molecules. I detail the construction and optimization of the experimental apparatus, including the cryogenic source, laser systems, vacuum systems and detection schemes. I demonstrate that the cryogenic source produces a bright and slow beam of CaH molecules via ablation of a solid chemical target and thermalization with a He buffer gas. The molecular beam exits the ablation cell with an average forward velocity of 250 ±200 m/s and a molecular beam flux per ablation pulse of ≈ 1×1010 per steradian per pulse. I present the spectroscopic determination of the molecular transitions necessary to pursue laser cooling. These include the X2Σ+ → A2Π1/2 and the X2Σ+ → B2Σ+ transitions that each contain two spin-rotation states, J = 1/2 and J = 3/2, and a further pair of hyperfine states, F = 0,1 and F = 1,2, respectively. Finally, I describe the vibrational repumping transitions between the four hyperfine states of the J = 1/2 and J = 3/2 branches of the V = 1 vibrational state back to the ground state via decay from an intermediary state, X2Σ+(V = 1) → B2Σ+(V = 0) → X2Σ+(V = 0). I present measurements of the vibrational decay probabilities from the B2Σ+(V = 0) and A2Π1/2(V = 0) excited states to the V = 0,1 and 2 states of the ground X2Σ+ state. Next, I show that we can achieve a high scattering rate of ≈ 1.6E6 photons/second while cycling on the X2Σ+ → A2Π1/2 transition. Finally, I demonstrate the ability to perform transverse cooling of a beam of CaH molecules through both the Doppler mechanism and magnetically assisted Sisyphus mechanism. With the help of a transverse standing wave of laser light, I show that we are able to lower the molecular beam’s transverse temperature from 12.2±1.2 mK to 5.7±1.1mK. This thesis represents a promising start to laser slowing and magneto-optical trapping of CaH molecules, which could provide trapped ultracold samples of atomic hydrogen upon dissociation of the trapped CaH molecules.

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Solid-state NMR measurement at cryogenic temperature shows significant potential for biological analysis due to its advantages for sample stability and detection sensitivity. However, at low temperature lineshape broadening and low spectral resolution are commonly observed and limit the applications for complex protein systems. Here, we explored the hypotheses for the underlying mechanisms of broad linewidths at low temperatures by studying E. coli Dihydrofolate reductase (DHFR). Our results support the hypothesis that conformational heterogeneity is a major source of linebroadening. We measured the protein backbone torsion angle (Ψ) at 105 K. In a selectively enriched protein sample with only one amide 13 C’- 15 N correlation expected, we identified three different conformations with distinct N chemical shift values accounting for the dramatic broadening observed in low temperature NMR spectra. We presume that backbone torsion angle fluctuates among the conformers on picosecond timescale at room temperature and are ‘frozen out’ giving rise to static heterogeneity at cryogenic temperatures. MD simulations support this hypothesis. QM/MM predicted chemical shifts based on snapshots from a MD simulation show excellent agreement with our data in that the average agrees well with the room temperature shift and the distribution agrees well with the low temperature spectral lineshape. On the other hand, our data suggest that there is no relationship between the μs - ms motions at room temperature and the lineshape broadening at low temperature. Resonance assignments of the apoenzyme in solution and associated liganded states were accomplished to identify the conformational transition in chemical exchange. We analyzed the 15 N relaxation dispersion profile of each residue at room temperature in solution; the rates appear to be organized in functional groups that exchange in a concerted fashion, with shift differences related to ligated-vs-unligated changes. The chemical shift changes associated with μs - ms exchange phenomena (and with ligation) are about an order of magnitude too small to explain the low temperature lineshapes, and also have no correlation with the low temperature lineshapes.

In narrow electron bands in which the Coulomb interaction energy becomes comparable to the bandwidth, interactions can drive new quantum phases. In this dissertation, we achieve narrow bands by twisting two atomically thin layers of the semiconducting van der Waals material WSe₂. The resulting moiré potential from the twist angle modulates the electronic bands, yielding minibands of tens of meV on the valence band. We perform transport measurements at cryogenic temperatures and observe signatures of collective phases over twist angles that range from 4 to 5.1°. At half-band filling, a correlated insulator appeared that is tunable with both twist angle and displacement field. Near the boundary between ordered and disordered quantum phases, several experiments have demonstrated metallic behaviour that defies the Landau Fermi paradigm. We find that the metal-insulator transition as a function of both density and displacement field is continuous. At the metal–insulator boundary, the resistivity displays strange metal behaviour at low temperatures, with dissipation comparable to that at the Planckian limit. Further into the metallic phase, Fermi liquid behaviour is recovered at low temperature, and this evolves into a quantum critical fan at intermediate temperatures, before eventually reaching an anomalous saturated regime near room temperature. An analysis of the residual resistivity indicates the presence of strong quantum fluctuations in the insulating phase. We further show via magnetotransport measurements that new correlated electronic phases can exist independent of moiré commensurability, and are instead driven by weak interactions in twisted WSe₂. The first of these phases is an antiferromagnetic metal that is driven by proximity to the van Hove singularity (vHS), which trails a range of incommensurate dopings. The temperature, magnetic field and density dependence of the Hall effect carry signatures of the reconstructed Fermi surface due to itinerant magnetic ordering. The second is an excitonic metal-insulator phase that exists at high external magnetic field in the vicinity of half-filling of the moiré superlattice. For a 4.2° sample, magnetic field dependence of the longitudinal resistance shows metallic behavior at fields above 5 T, but transitions to an insulating state above ∼ 24 T. A detailed analysis of of the Landau fans and the high field 𝝆_𝜘𝛾 near the gap rules out the possibility of a trivial insulator. We propose an Ising excitonic insulator as the most likely scenario. Moreover, in the electron-imbalanced excitonic metal, a set of correlated Landau levels emerge. The observation of tunable collective phases in a simple band, which hosts only two holes per unit cell at full filling, establishes twisted bilayer transition metal dichalcogenides as an ideal platform to study correlated physics in two dimensions on a triangular lattice.