Dimensionality and Criticality in Strongly-Correlated Materials

The present doctoral work aims at describing the behavior of two classes of novelmaterials in extreme conditions, in particular very high pressure. My doctoral thesis developed around the construction and testing of hybrid piston-clamped anvil pressure cells for nuclearmag- netic resonance andmagnetizationmeasurements. In the first part, after spotting some basic concepts related to the investigatedmaterials (Chapter 1) and introducing the employed methods (Chapter 2), the detailed project, conceptual design, and testing of a new pressure cell is discussed (Chapter 3). The second part of the thesis (Chapters 4 and 5) is dedicated to two investigations on quantum magnets under pressure, namely Cu(pz)2(ClO4)2, a 2D spin-1/2 quantum Heisenberg antiferromagnet, and Na2IrO3, a honeycomb iridate, claimed to offer a potential realization of the quantum-spin-liquid state. The third and last part deals with several experiments on novel superconductors, including the investigation of the noncentrosymmetric superconductor SrAuSi3 (Chapter 6), the study of the doping-induced superconductivity in the transition-metal diborides ZrB2 and HfB2 (Chapter 7), the gap symmetry in the series of iron-based superconductors LaOFeAs1 x Px (Chapter 8), and the pressure effects on the electronic properties of ThFeAsN, a recently synthesized iron-based superconductor (Chapter 9). These studies suggest that hydrostatic pressure is a power- ful and reversible parameter to tune the structural, electronic, and magnetic properties ofmaterials and that it offers the possibility to assess the competition and cooperation of interactions in strongly-correlatedmaterials, like quantummagnets and unconventional superconductors