Image of the Month

November 2016

Universality of pseudogap and emergent order in lightly doped Mott insulators

Figure 1: Phase-separated Mott/pseudogap electronic structure at 5.5% doping. a, Different spectra in the phase-separated region: Mott like spectrum (blue), with the chemical potential pinned to the UHB, and mixed Mott/pseudogap spectrum (red). The spectra are the average of 180 spectra inside the white circles in c. b, Phenomenological fit function to simultaneously extract both the Mott and pseudogap size. It consists of a density of states (dot-dashed) multiplied by the Mott gap (dashed) plus states inside the Mott gap with a V-shaped pseudogap (dotted). c, The Mott parameter as defined in the text identifies pseudogap puddles (red) and pure Mott regions (blue). Green circles indicate La dopant locations. The triangle on the colour bar indicates the value of the black contour. Inset, definition of the Mott parameter: the integrated DOS inside Mott gap (red) normalized by the one outside the gap (blue). d, Local density of states spectra along the white line in c (each corresponding to a single measurement). The separation is sharp in the sense that a Mott spectrum becomes a pseudogap spectrum within roughly a nanometre. e, ΔPG map extracted from the fitting procedure. The square indicates the region displayed in Fig. 4. Inset, the correlation between ΔMott and ΔPG.

It is widely believed that high-temperature superconductivity in the cuprates emerges from doped Mott insulators. When extra carriers are inserted into the parent state, the electrons become mobile but the strong correlations from the Mott state are thought to survive—inhomogeneous electronic order, a mysterious pseudogap and, eventually, superconductivity appear. How the insertion of dopant atoms drives this evolution is not known, nor is whether these phenomena are mere distractions specific to hole-doped cuprates or represent genuine physics of doped Mott insulators. Here we visualize the evolution of the electronic states of (Sr1-x Lax)2IrO4, which is an effective spin-1/2 Mott insulator like the cuprates, but is chemically radically di_erent2,3. Using spectroscopic-imaging scanning tunneling microscopy (SI-STM), we find that for a doping concentration of x ≈5%, an inhomogeneous, phase separated state emerges, with the nucleation of pseudogap puddles around clusters of dopant atoms. Within these puddles, we observe the same iconic electronic order that is seen in underdoped cuprates.  We investigate the genesis of this state and find evidence at low doping for deeply trapped carriers, leading to fully gapped spectra, which abruptly collapse at a threshold of x≈4%.Our results clarify the melting of the Mott state, and establish phase separation and electronic order as generic features of doped Mott insulators. (Nature Physics Letters. 19 SEPTEMBER 2016 | DOI: 10.1038/NPHYS3894 )

Credits:
I. Battisti1, K. M. Bastiaans1, V. Fedoseev1, A. de la Torre2,3, N. Iliopoulos1, A. Tamai2, E. C. Hunter4, R. S. Perry5, J. Zaanen1, F. Baumberger2,6 and M. P. Allan1*

1Leiden Institute of Physics, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands.

2Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland.

3Department of Physics, California Institute of Technology, Pasadena, California 91125, USA.

4School of Physics and Astronomy, The University of Edinburgh, James Clerk Maxwell Building, Mayfield Road, Edinburgh EH9 2TT, UK. 5London Centre for Nanotechnology and UCL Centre for Materials Discovery, University College London, LondonWC1E 6BT, UK.

6Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland.

Images and data graciously provided by Milan Allan, Leiden Institute of Physics, Leiden University, Leiden, The Netherlands.

Microscope:
Custom Modified Unisoku STM Microscope

Control System:
RHK R9 Control System