1. Quantum Criticality in Magnetic Single-Electron Transistors T p Physics of non-Fermi-liquid Metals Qimiao Si, Rice University, DMR-0424125 Quantum criticality influences the physical properties of a wide range of quantum materials. Traditional theory describes it in terms of the spatial and temporal fluctuations of a classical variable – Landau’s order parameter. There are recent indications that quantum entanglement effect may also play an important role. Here, we propose to probe this entanglement effect using a special type of magnetic transistor. The entanglement effect has a clear signature in the electrical conductance of the device, through its dependence on temperature and frequency. At the quantum critical point, the Kondo entanglement effect is critical, in a way similar to the theory of local quantum criticality proposed earlier in Q. Si et al., Nature 413, 804 (2001). (a) The setup. The arrows denote the direction of the magnetization of the two ferromagnetic metallic leads; (b) The temperature dependence of the electrical conductance. Quantum criticality is reached through a variation of its gate voltage.

2. Hall-Effect probe of Heavy Fermion Quantum Critical Point T p Physics of non-Fermi-liquid Metals Qimiao Si, Rice University, DMR-0424125 Hall coefficient showing a rapid crossover as a function of the control parameter (magnetic field B2) across the quantum magnetic phase transition in the heavy fermion compound YbRh2Si2. By extrapolation to zero temperature, the crossover turns into a sharp jump at the quantum critical point. In collaboration with the group of F. Steglich and S. Paschen from Max- Planck Institute, and P. Coleman of Rutgers U. One of the prototype systems in which quantum critical points have been observed is magnetic heavy fermions. Here quantum criticality has been known to be responsible for non-Fermi liquid behavior and also for the emergence of novel electronic states such as unconventional superconductors. Our earlier theoretical work, including those described in Q. Si et al., Nature 413, 804 (2001), have suggested a class of local quantum criticality with not only critical magnetic fluctuations but also a destruction of Kondo entanglement. In an experiment-theory collaboration, we report the observation of a rapid crossover of the Hall coefficient across a heavy fermion quantum critical point. The result provides evidence for a Fermi surface jump at zero temperature at the quantum critical point, which is a salient feature of the local quantum criticality.

3. S. Paschen et al., Nature 432, 881-885 (2004) [in collaboration with F. Steglich, S. Paschen et al of Max-Planck Institute for Chemical Physics, Dresden, Germany and P. Coleman of Rutgers University] S. Kirchner, L. Zhu, Q. Si, & D. Natelson, Proc. Natl. Acad. Sci. USA 102, 18824- 18829 (2005). Work in this area has involved two graduate students (L. Zhu and S. Yamamoto), two postdocs (S. Kirchner and E. Pivovarov) and one undergraduate student (Misha Teplitskiy), and previously three other undergraduates for their thesis research. The PI organized a workshop on Quantum Criticality, at the Lorentz Center, Netherlands in 2006 (co-organizers M. Fisher, K. Schoutens, and M. Vojta). The PI has also served on the advisory committees for one international conference and one journal. The PI gave many talks on these research at Conferences/Workshops and Universities, including some pedagogical ones aimed for graduate students. Publication on magnetic transistor as a probe of quantum criticality Publication on Hall effect across a heavy fermion quantum critical point Education and Outreach Physics of non-Fermi-liquid Metals Qimiao Si, Rice University, DMR-0424125