CAREER: A Multifaceted Approach for Manipulation and Investigation of Quantum Phases and Phase Transitions in Prototypical 2-D Metallic Systems

University Of Virginia Main Campus, Charlottesville VA

Investigators

Abstract

Non-Technical Explanation: Commonly, materials undergo phase transitions due to change in temperature. A typical example is the transformation of water into ice at zero degrees Celsius. There are also examples of phase transitions, which are controlled solely by non-thermal parameters such as pressure, magnetic field, and compositional disorder. A complete characterization of the key principles behind quantum phase transitions (QPTs) is yet to be accomplished in two-dimensional (2-D) metallic systems, which is believed to be crucial to the realization of intriguing behaviors in various materials of fundamental interest and potential technological import such as cuprate and pnictide high temperature superconductors, ruthenates and heavy fermions. This research project aims at fundamental studies of QPTs in model systems by incorporating a set of complementary experimental techniques. In accordance to its strong commitment to the development of world-class science, technology, engineering and mathematics workforce in the country, this project, in addition to supporting the PhD studies of a graduate student, provides hands-on, research-based, education to undergraduate students at an early stage and local high school students through summer internship. The synergy fostered by this project's integration of education into research is also anticipated to accomplish an important task: preparing the next generation of scientists in the field of synchrotron-based research in the United States. Further education and outreach endeavors of this project comprise development of web based educational materials pertaining to advanced materials characterization techniques for non-experts and broader dissemination of the research activities of the PI's group through lecture-demonstrations, targeted at local non-scientific audiences. Technical Description: Comprehensive experimental investigations of quantum phase transitions (QPTs) in the phase diagrams of two-dimensional (2-D) metallic systems, in general, are rather intricate. This is mostly because the subtle interplay of competing interactions in these systems often shrouds the relevant quantum critical points stymieing direct experimental access to their quantum criticality. In order to address this issue, this research project adopts a straightforward approach: (i) finds a simple model system with a well defined ordered state, (ii) continuously tunes critical temperature of this state to zero by changing certain non-thermal parameter, and (iii) directly measures changes in various physical properties along with the pertinent order parameter through the quantum critical point. The systems of interest for the current studies are a set of layered transition metal dichalcogenides, namely 2H-NbSe2, 2H-TaS2 and 2H-TaSe2, which possess simple crystal and electronic structures. Each of these compounds exhibits a well defined phase transition via the formation of waves in space with alternating regions of higher and lower density of charges, known as a charge density wave (CDW). Moreover, their CDW transition temperatures can continuously be tuned to zero through various non-thermal processes. In the first avenue of research, CDW orders of these compounds are to be melted quantum mechanically via methodical introduction of electronic and structural disorders in single crystal samples of these materials. In the second line of research, the thickness of single crystal samples are to be gradually diminished transforming them eventually into few- to single-layer thick crystals. Using a combination of Angle Resolved Photoemission Spectroscopy, X-ray diffraction and electrical resistivity measurements, critical insights into quantum criticality and quantum phase transitions in 2-D systems are to be attained by (a) unveiling evolution of structural, electrical and electronic properties of these materials along with their CDW order parameters through quantum critical points in temperature-disorder phase diagram, and (b) investigating the impact of quantum confinement on CDW instability and phase transition in few- to single-layer thick samples. In this context, exact theoretical treatment of quantum criticality in metals is highly challenging because one needs to take into account both single particle excitations and order parameter fluctuations in the same footing. The proposed integration of structural, spectroscopic and transport probes in this project provides a unique opportunity to do so experimentally. Furthermore, this project provides a well-defined methodology to investigate the role of complex disorders on electronic and structural properties of a physical system. This is at the heart of developing materials, which exhibit strongly enhanced responses to external stimuli, useful in energy as well as in device applications.