Crystal growth, magnetic and thermal properties of quasi two dimensional quantum magnets

Abstract

Quantum materials is a broad and evolving term with no universally agreed-upon definition. Generally, materials are considered quantum in nature when their specific properties cannot be fully explained using classical paradigms and are intrinsically governed by quantum mechanical principles. Initially, the term ”quantum materials” was associated with strongly correlated electron systems, such as heavy fermion systems and high-temperature superconductors. Over time, it has expanded to encompass a wide range of systems, including low-dimensional quantum magnets, two-dimensional van der Waals materials, frustrated spin systems (e.g., spin-1/2 or spin-1), semimetals (Dirac, Weyl, and nodal-line semimetals), two-dimensional and three-dimensional topological insulators, and more. On a microscopic scale, these materials involve the complex interplay of huge number of electrons interacting with each other or with ions in the materials in ways that give rise to exotic phases of matter. These phenomena often challenge conventional understanding and remain a frontier of exploration. Examples include quantum spin liquids with fractionalized excitations, quantum spin ices, the up-up-down (uud) field-induced 1/3 magnetization plateau in triangular lattice antiferromagnets, exotic multiband superconductivity in iron pnictides, large and non-saturating positive magnetoresistance in topological semimetals, the chiral anomaly in Weyl semimetals, and remarkable effects like the giant anomalous Nernst and Hall effects in topological materials. These discoveries illustrate the rich and diverse physics enabled by the quantum nature of these systems. A notable special case within quantum materials is the realm of two-dimensional (2D) magnets. These materials consist of atomically thin layers, often just a few atoms thick, where quantum effects are significantly enhanced due to their reduced dimensionality. In traditional three-dimensional materials, magnetic order is typically governed by long-range interactions that diminish in lower dimensions. However, 2D magnets defy these classical rules and maintain magnetic order down to a single atomic layer in some systems, which provides a unique opportunity to explore quantum magnetism in reduced dimensions. More over, 2D magnets are highly tunable, with their magnetic properties easily controlled by external factors such as electric fields or strain, making them ideal for next-generation tech nologies. Their integration with other 2D materials, like graphene, makes them promising candidates for applications in spintronics, quantum information processing, and advanced magnetic memory devices. Hence, exploring new 2D magnets or reinvestigating the huge pool of existing 2D magnets is essential for the discovery of new exotic quantum phases , which not only deepen our understanding of fundamental physics but also pave the way for future technological applications in areas like quantum computing, spintronics, and low-power electronic devices. This thesis focuses on the high-quality single crystal growth of novel low-dimensional Honeycomb quantum magnets where the magnetic spins are decorated on the honeycomb or pseudo-honeycomb lattice. By employing various crystal growth techniques and optimizing the thermodynamic conditions, we have grown defect/impurity-free large, high-quality single crystals of these materials. The absence of defects and impurities is essential for accurately probing the intrinsic physical properties of these materials, which can otherwise be obscured or altered. The structural quality of the grown single crystals is rigorously characterized by a range of X-ray diffraction methods to ensure the crystals are of high quality. Following this, detailed investigations of magnetic and thermal properties are carried out in order to reveal the nature of their magnetic ground states, as well as the effect of external perturbations, such as doping and applied magnetic fields.