Using controlled disorder to probe quantum phase transitions under the dome of superconductivity

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Non-technical abstractModern technologies rely on the unusual properties of novel materials created in labs purposefully targeting needed functionalities. Of particular interest are materials exhibiting coexistence and interplay of seemingly incompatible properties, such as magnetic, electronic, and superconducting ordering of electrons. Sometimes one type of order dominates and quenches the other. The point where it happens at zero temperature is called a quantum critical point. The suppressed order does not give up easily, and the quantum critical regime in the vicinity of this point is full of surprising and largely unexplored properties. It is the goal of this project to identify these features and harness their unusual properties for future technologies, devices, and applications. Specifically, the team is studying complex materials where superconductivity coexists with charge-density wave. In this peculiar state, the electron density is spatially modulated while simultaneously superconducting. The interplay between these two ordering tendencies is tuned by the introduction of artificial scattering centers to study the underlying quantum critical point. The ultimate goal of this research is to learn how to harness the unusual physics of the quantum critical point. The research program provides an excellent hands-on training platform for a diverse group of undergraduate and graduate students in all aspects of experimental research. The underlying concepts and results of the project will be incorporated into an upper-level physics course to highlight the place of quantum criticality in modern condensed matter physics. Technical abstractThis proposal is systematically studying the interplay between charge-density-wave (CDW) and superconductivity (SC), not in a singular compound such as well-studied NbSe2, but in novel 3-4-13 stannides (Ca,Sr)3(Ir1-xRhx)4Sn13. In these compounds, the CDW is tuned continuously to a structural quantum critical point (QCP) by changing the composition. The results are compared with materials where spin-density-wave (SDW) coexists with superconductivity, specifically, (Ba,K)(Fe,T)2(As,P)2 (T=Co, Ni, Rh). QCPs under the dome of superconductivity in some of these systems have already been established. However, there is limited knowledge of their structure and response to disorder. Specifically, it is unknown how robust QCPs are to structural point-like disorder. Theoretical predictions range from the QCP completely disappearing, to being robust and even stabilized by disorder. The research involves structural, thermodynamic and transport measurements. In particular, x-ray scattering, muon spin rotation spectroscopy, electrical and thermal transport, London penetration depth, magnetization, and elastoresistivity. The measurements are performed across temperature-composition phase diagrams, emphasizing the quantum critical behavior near the QCP in the normal and the superconducting phases. Controlled point-like disorder introduced by MeV-range electron irradiation will perturb the studied systems at fixed chemical, electronic, and magnetic configurations. Specific questions being addressed are: (1) does SC weaken or protect the QCP? (2) is the QCP inside the SC state more robust against disorder than in the normal metal? (3) is the universality class of the QCP inside the SC phase the same as in the normal state? (4) are there novel emergent phenomena associated with disorder, such as large, rare regions leading to Griffiths singularities near the QCP?This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.