Development of optical magnetometry based on Nitrogen Vacancy centers in diamond to study unconventional superconductors
Date
2021-08
Authors
Joshi, Kamal Raj
Major Professor
Advisor
Prozorov, Ruslan
Tanatar, Makariy A.
Fei, Zhe
Pruski, Marek
Wetstein, Matthew
Committee Member
Journal Title
Journal ISSN
Volume Title
Publisher
Altmetrics
Abstract
Unconventional superconductivity often emerges in close proximity to a magnetic instability. Upon suppressing the magnetic transition down to zero temperature by tuning the carrier concentration, pressure, magnetic field, or disorder, the superconducting transition temperature T$_c$ acquires its maximum value. A major challenge is the elucidation of the relationship between the superconducting phase and the strong quantum fluctuations expected near a quantum phase transition (QPT). While unusual normal state properties, such as non-Fermi liquid behavior of the resistivity, are commonly associated with strong quantum fluctuations, evidence for its presence inside the superconducting dome are much scarcer. Uncovering the existence of QPT hidden inside the superconducting dome may allow to understand the unconventional superconductivity. BaFe$_2$As$_2$ is an ideal system to study the possibility of QPT because there are very clear two phase lines (structural and magnetic phase transition lines) passing through the superconducting dome. \par
We developed the technique to measure lower critical field, H$_{c1}$ and associated London penetration depth (\la) by employing minimally invasive optical magnetometry based on nitrogen-vacancy centers in diamond that allows accurate detection of the vector magnetic field with subgauss sensitivity and submicrometer spatial resolution. We apply this sensitive and precise technique to probe the doping evolution of the T = 0 penetration depth in the electron-doped iron-based superconductor Ba(Fe$_{1-x}$Co$_x$)As$_2$. A non-monotonic evolution with a pronounced peak in the vicinity of the putative magnetic QPT is found. This behavior is reminiscent to that previously seen in isovalently-substituted BaFe$_2$(As$_{1-x}$P$_x$)$_2$ compounds, despite the notable differences between these two systems. Whereas the latter is a very clean system that displays nodal superconductivity and a single simultaneous first-order nematic–magnetic transition, the former is a charge-doped and significantly dirtier system with fully gapped superconductivity and split second-order nematic and magnetic transitions. Thus, our observation of a sharp peak in λ(x) near optimal doping, combined with the theoretical result that a QPT alone does not mandate the appearance of such peak, unveils a puzzling and seemingly universal manifestation of magnetic quantum fluctuations in iron-based superconductors and unusually robust quantum phase transition under the dome of superconductivity.
Series Number
Journal Issue
Is Version Of
Versions
Series
Academic or Administrative Unit
Physics and Astronomy
Type
article