Local observables for quantum phase transitions in strongly correlated systems
Chapter in Understanding Quantum Phase Transitions, (2010) 393-418
Abstract:
This chapter is a progress report on the challenging yet promising frontier of quantum phase transitions (QPTs) in strongly correlated systems from the perspective of modern local probes and recent theoretical developments. The focus will be on our latest developments at this frontier. An outlook based on opportunities and questions emerging from these latest developments concludes the discussion.Nematic electronic structure in the "parent" state of the iron-based superconductor Ca(Fe(1-x)Co(x))2As2.
Science (New York, N.Y.) 327:5962 (2010) 181-184
Abstract:
The mechanism of high-temperature superconductivity in the newly discovered iron-based superconductors is unresolved. We use spectroscopic imaging-scanning tunneling microscopy to study the electronic structure of a representative compound CaFe1.94Co0.06As2 in the "parent" state from which this superconductivity emerges. Static, unidirectional electronic nanostructures of dimension eight times the inter-iron-atom distance a(Fe-Fe) and aligned along the crystal a axis are observed. In contrast, the delocalized electronic states detectable by quasiparticle interference imaging are dispersive along the b axis only and are consistent with a nematic alpha2 band with an apparent band folding having wave vector q vector congruent with +/-2pi/8a(Fe-Fe) along the a axis. All these effects rotate through 90 degrees at orthorhombic twin boundaries, indicating that they are bulk properties. As none of these phenomena are expected merely due to crystal symmetry, underdoped ferropnictides may exhibit a more complex electronic nematic state than originally expected.Heavy d-electron quasiparticle interference and real-space electronic structure of Sr3Ru2O7
Nature Physics Springer Nature 5:11 (2009) 800-804
Spectroscopic fingerprint of phase-incoherent superconductivity in the cuprate pseudogap state [corrected].
Science (New York, N.Y.) 325:5944 (2009) 1099-1103
Abstract:
A possible explanation for the existence of the cuprate "pseudogap" state is that it is a d-wave superconductor without quantum phase rigidity. Transport and thermodynamic studies provide compelling evidence that supports this proposal, but few spectroscopic explorations of it have been made. One spectroscopic signature of d-wave superconductivity is the particle-hole symmetric "octet" of dispersive Bogoliubov quasiparticle interference modulations. Here we report on this octet's evolution from low temperatures to well into the underdoped pseudogap regime. No pronounced changes occur in the octet phenomenology at the superconductor's critical temperature Tc, and it survives up to at least temperature T approximately 1.5 Tc. In this pseudogap regime, we observe the detailed phenomenology that was theoretically predicted for quasiparticle interference in a phase-incoherent d-wave superconductor. Thus, our results not only provide spectroscopic evidence to confirm and extend the transport and thermodynamics studies, but they also open the way for spectroscopic explorations of phase fluctuation rates, their effects on the Fermi arc, and the fundamental source of the phase fluctuations that suppress superconductivity in underdoped cuprates.Evidence for a superglass state in solid 4He.
Science (New York, N.Y.) 324:5927 (2009) 632-636