Recent Research

Quantum Criticality Revealed in a Cuprate Superconductor
In the high-temperature cuprate superconductors, the pervasiveness
of anomalous electronic transport properties suggests that violation of
conventional Fermi liquid behavior is closely tied to superconductivity.
To investigate this, we have performed an extensive set of low temperature
transport experiments on thin films of electron-doped cuprate superconductor
La2-xCexCuO4. This has allowed us to identify quantum critical phases that
surround the superconducting phase as a function of magnetic field and charge
doping. With divergences in electron-electron scattering, anomalous non-Fermi
liquid power laws and quantum critical magnetic field-temperature scaling laws
observed, we have identified signatures of two distinct flavors of quantum fluctuations
suggesting that quantum criticality plays a significant role in shaping the anomalous
properties of the cuprate phase diagram.
 Linear Resistivity in Copper Oxide Superconductors
By studying the magnetotransport in the electron doped cuprate thin films, we showed
a direct correlation between an anomalous linear-in-temperature variation of the electrical
resistance in normal state of the superconducting films and the superconducting Tc. The
linear-in-temperature resistance was shown to arise from scattering of the electrons from
the spin fluctuations. Experiments by others had previously shown that spin fluctuations
were present in all samples that were superconducting. Therefore, our work convinces that
an electron-spin fluctuation interaction is the cause of high Tc superconductivity.
For details, see our paper
Nature 476, 73 (2011)
 Tc Enhancement in Superlattices
We fabricated superlattices (SL's) composed of underdoped/overdoped La2-xCe2-xCuO4 (LCCO)
layers, also undoped/overdoped Pr2-xCe2-xCuO4 (PCCO). All of the constituents either are
nonsuperconducting or have very low Tc. An anomalous Tc enhancement is observed for all these
SL's. Our results couldn't be explained by the theorys, which predict that the enhancement is
caused by the interplay between the large pairing amplitude in underdoped layer and the phase
stiffness originated from the overdoped layer. Furthermore, we find that the Tc enhancement spreads
over a very large length scale, rather than is just confined to the interfaces, again pointing out a
different origin from the theoretical prediction.
For details, see our paper Phys. Rev. B 83, 060511R (2011)