Disordered Critical Quantum Magnetic Systems

Trustees Of Boston University, Boston

Investigators

Abstract

This award supports fundamental theoretical research and education in the area of quantum magnetism and the role of disorder in quantum magnets. The cuprate and heavy-fermion materials have phase transitions between magnetic and non-magnetic phases. Often, these phase transitions are driven by chemical substitution that also introduces disorder into the system. Depending on its nature and strength, disorder can lead to substantial changes in the critical behavior of magnetic systems. The PI aims to study of the effects of disorder and electronic dissipation on the critical behavior and collective excitations of quantum magnetic systems. Contrary to the classical problem with weak disorder, there is no established criteria for the relevance of disorder in quantum critical systems. The PI will search for the relevant criteria in quantum systems. Although there are an enormous number of systems in which critical behavior is modified by disorder, the PI will focus on the study of two experimentally relevant systems: the metallic f-electron alloy UCu5-xPdx where the so-called "non-Fermi liquid" (NFL) behavior has been observed, and the low-dimensional dilute insulating magnet La2Cu1-z(Zn,Mg)zO4. These systems have common features: (i) they can be tuned from a magnetically ordered phase to a paramagnetic phase through a quantum critical point (QCP) via chemical substitution; (ii) they have an anomalous magnetic behavior close to the QCP; (iii) they are disordered alloys. The PI will study these systems using modern field theoretical methods and compare the results with recent experimental data. These comparisons will serve as a filter for the development of the theory. Success of this work, involving developing criteria for disordered quantum magnets and the classification and elucidation of universal behavior, would have broad and direct impact on the area of quantum magnetism. This work has broader impacts in education at the graduate and postgraduate levels. This work also supports efforts to stimulate interest in condensed matter physics by advanced students belonging to underrepresented groups and outreach to elementary school students to stimulate the interest of the next generation of scientists. %%% This award supports fundamental theoretical research and education in the area of quantum magnetism and the role of disorder in quantum magnets. The PI will focus on chemical substitution and the effect of the disorder that often results on the properties of quantum magnets. Chemical substitution enables tuning through a quantum phase transition. Quantum phase transitions are distinct from classical phase transitions and are of great fundamental interest. They may provide a key to understanding unusual properties exhibited by heavy fermion, high temperature superconductors, and other strongly correlated electron materials. This proposal is concerned with the role of disorder, how it changes the properties of the "clean" materials, and the discovery of a criterion that can indicate when disorder is important. Success of this work would have broad and direct impact on the area of quantum magnetism and on a wide class of intriguing and challenging materials. This work has broader impacts in education at the graduate and postgraduate levels. This work also supports efforts to stimulate interest in condensed matter physics by advanced students belonging to underrepresented groups and outreach to elementary school students to stimulate the interest of the next generation of scientists. ***