Phase tuning in transition metal pnictide CrAs

In unconventional superconductors, electronic and structural degrees of freedom are strongly intertwined and play a key role in stabilizing ordered phases outside the superconducting state, with also direct implications for superconductivity. A detailed understanding of these interactions is therefore crucial for clarifying the underlying mechanism of unconventional superconductivity.
In this thesis, the unconventional superconductor chromium arsenide (CrAs) was chosen as a model system due to its strong coupling between magnetic order and lattice degrees of freedom. CrAs exhibits an anomalous magnetoelastic effect across its first-order magnetostructural transition, which is progressively suppressed under applied pressure as magnetic order disappears and superconductivity emerges. Suppression of magnetic order in CrAs by applied pressure is accompanied by signatures associated with quantum critical behavior. When the same magnetic instability is suppressed through phosphorus substitution in CrAs$_{1-x}$P$_x$, similar quantum critical signatures are observed, even though superconductivity remains absent. In this context, previous work on the interplay between competing orders in CrAs has primarily addressed the coexistence between magnetic and superconducting phases at low temperatures. ... mehrBy contrast, phase coexistence at higher temperatures - between magnetically ordered and non-magnetic phases - has received significantly less attention in both pure and phosphorus-substituted CrAs, despite the potential influence of proximity to a quantum critical regime. Investigating phase coexistence within the normal state is therefore highly relevant for probing how the interplay between magnetic, structural, and electronic degrees of freedom may influence the emergence of superconductivity. On this basis, this thesis focused on the study of phase coexistence through the structural and lattice response, examining its relation to the magnetostructural transition and its evolution under external control parameters such as hydrostatic pressure and chemical substitution, using Raman spectroscopy and Transmission Electron Microscopy (TEM).

In this thesis, high quality single crystals of CrAs and phosphorus-substituted CrAs$_{1-x}$P$_x$, were grown and characterized by powder X-ray diffraction and magnetometry measurements. Raman spectroscopy performed on single crystals revealed that phase coexistence in CrAs-based systems is intrinsically local. Phase coexistence was directly tracked through the simultaneous observation of two distinct $A_g$ phonon modes, associated with the high temperature non-magnetic and low temperature magnetically ordered phases. Raman mapping further revealed strongly position dependent spectra, consistent with a temperature dependent domain reconfiguration dynamics, with some regions retaining signatures of phase coexistence down to the lowest temperatures. Under hydrostatic pressure on pure CrAs, Raman measurements showed a progressive suppression of previously reported linewidth anomalies, confirming their magnetoelastic nature. In addition, Raman measurements showed evidence for a previously reported low pressure transition around 0.2-0.3 GPa. In phosphorus-substituted CrAs$_{1-x}$P$_x$, the magnetostructural transition was similarly suppressed, while enhanced phonon linewidths pointed to the important role of disorder, indicating a clear deviation from the analogy between chemical and hydrostatic pressure. TEM provided direct real-space evidence that phase coexistence occurs on nanometric length scales and, when combined with Raman spectroscopy, offered a complementary perspective on how the coexistence evolves across different tuning parameters. In phosphorus-substituted samples, the coexistence can organize into stripe-like configurations, pointing to a more complex coupling between magnetic, structural, and electronic degrees of freedom.