Nuclear magnetism and superconductivity : investigations on lithium and rhodium
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Doctoral thesis (article-based)
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AbstractThis thesis describes low temperature experiments on lithium. The experiments concentrate on investigating low temperature phase transitions of two subsystems in this metal: its nuclear spin and electronic systems. These phenomena were investigated in two separate sets of samples, each designed to meet the experimental requirements for the phenomena under investigation. The electronic ground state of lithium at ambient pressures is expected to be superconducting, but earlier experiments show that the metal remains normal at least down to 4 mK. In this thesis research, a lithium sample was cooled down to 0.1 mK, B < 10 nT, but no superconductivity was observed. A new upper limit for the superconducting transition temperature was established, over an order of magnitude lower than the earlier upper limit. Many theoretical estimates predict superconductivity already at higher temperatures, and its absence remains a puzzle. The superconductivity in lithium has been under considerable interest recently, since the superconducting phase was observed in the highly compressed state. The nuclear spin system of lithium was investigated as well. The large magnetic moment of the lithium-7 nuclei made lithium an interesting target for studies of nuclear magnetism. The spin system was cooled down by a two stage adiabatic nuclear demagnetization cryostat. The nuclear magnetic resonance (NMR) spectra, the spin temperature, and the static susceptibility were measured. The NMR spectra revealed the appearance of an anomalous peak at low frequencies. Also, the spin system was found to behave in an irreversible manner at low fields and high nuclear polarizations. These effects give evidence for the existence of a magnetically ordered state of the lithium nuclei. The type of the order was not resolved unambiguously, but some facts point to a ferromagnetic ordering. A tentative phase diagram was constructed for the system. Previously, nuclear magnetism and superconductivity in rhodium metal have been investigated. This thesis also includes some numerical and analytical modeling on the nuclear spin system of rhodium, aimed at the interpretation of the experimental data.
lithium, rhodium, ultralow temperatures, nuclear magnetic ordering, nuclear magnetism, superconductivity
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