Quasiparticles in Mott Insulators, Strange Metals and Spin liquids probed by Low Temperature Spectroscopic-Imaging Scanning Tunneling Microscopy

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Non-technical Abstract: An exciting area of current condensed matter research is systems where the particles participating in transport bear little resemblance to conventional electrons. One example is fractionalization where electrons split into two separate pieces; one part carrying the charge and the other carrying the spin as is predicted to happen in spin liquids. While such fractional particles have been postulated to exist for many decades, the fact that correlations are usually important in such systems makes theoretical handling difficult. This combined with a lack of experimental data (with the exception of the fractional quantum Hall effect) has slowed progress. Another example of is that `strange metals’ where transport properties show unexplained behavior. In these materials, resistance varies linearly with temperature down to quite low temperatures. So far there is no accepted theory for this linear resistivity. Some theories have suggested that the phenomenon may result from the breakdown of basic quantum mechanical ideas, in a state where the particles become entangled with each other – a phenomenon described as “unparticle physics”. The behavior of particles in the strange metal phase and the spin liquid ground state, thus remain open interesting problems at the frontiers of condensed matter physics. So far, most experimental data on these systems has been obtained by transport and other bulk probes and there is very little nanoscale spectroscopic data on these systems due to multiple reasons including the dearth of suitable samples. In the last few years however, some of the established Mott insulators have been shown to exhibit behavior consistent with strange metals/spin liquids, opening up new possibilities for their exploration. The goal of this project is to use low temperature scanning tunneling microscopy (STM) and spectroscopy (STS) to investigate the physics of such systems. Technical Abstract:The materials being studied include the Sr3(RuxIr1-x)2O7 system and bilayer graphene (BLG) where transport shows linear resistivity up to high temperatures; and 1T-TaS2/1T-NbSe2 monolayer- and few-layer films which are Mott insulators where the spin-liquid phase may be realized even at high temperatures. One of the biggest advantages of STM is that one can probe the real space electronic structure and obtain simultaneous information on momentum-space (k-space) dispersion. By applying powerful technique of quasiparticle interference to these materials classes, the goal is to obtain spectroscopic data that will help distinguish between different theories and provide new insights into the physics of these elusive phases. Recent theory on spin-liquids has provided concrete predictions for signatures of spinons in the charge channel which makes this an exciting time to study such systems with STM. The success of the project will enhance the research experience for women and minorities in physics. The new course being developed will train students in modern ideas and techniques in condensed matter. The PI's integrated outreach and education activities will expose talented high school students to cutting edge science. The students working on these projects will be trained on materials and instruments at the forefront of today’s research.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.