Image of the Month

Image of the Month
posted January 1, 2017

FIG. 1. (a) At left, the crystal structure for TbTe3 with a black rectangle outlining the unit cell. The dotted line indicates the a-c cleave plane between the double Te layers. At right, the square lattice of the Te layer which is exposed by cleaving as well as the locations of the closest Tb ions in the rare-earth block layer directly below. The unit cell is again shown in the structures at right for reference. The crystal structures were constructed using Vesta software. (b) Topographic image taken over a 90 A ̊ square region at I = 65 pA, VSample = −350 mV. The Te square lattice of the exposed surface can be clearly seen as well as superimposed “stripes” associated with a unidirectional CDW state along the a1 crystal axis.

We studied TbTe3 using scanning tunneling microscopy (STM) in the temperature range of 298–355 K. Our measurements detect a unidirectional charge density wave (CDW) state in the surface Te layer with a wave vector consistent with that of the bulk qCDW = 0.30 ± 0.01c∗. However, unlike previous STM measurements, and differing from measurements probing the bulk, we detect two perpendicular orientations for the unidirectional CDW with no directional preference for the in-plane crystal axes (a or c axis) and no noticeable difference in wave vector magnitude. In addition, we find regions in which the bidirectional CDW states coexist. We propose that observation of two unidirectional CDW states indicates a decoupling of the surface Te layer from the rare-earth block layer below, and that strain variations in the Te surface layer drive the local CDW direction to the specific unidirectional or, in rare occurrences, bidirectional CDW orders observed. This indicates that similar driving mechanisms for CDW formation in the bulk, where anisotropic lattice strain energy is important, are at play at the surface. Furthermore, the wave vectors for the bidirectional order we observe differ from those theoretically predicted for checkerboard order competing with stripe order in a Fermi-surface nesting scenario, suggesting that factors beyond Fermi-surface nesting drive CDW order in TbTe3. Finally, our temperature-dependent measurements provide evidence for localized CDW formation above the bulk transition temperature TCDW.

Credits:
Ling Fu,1 Aaron M. Kraft,1 Bishnu Sharma,1 Manoj Singh,1 Philip Walmsley,2,3 Ian R. Fisher,2,3 and Michael C. Boyer1,*
1Department of Physics, Clark University, Worcester, Massachusetts 01610, USA
2Geballe Laboratory for Advanced Materials and Department of Applied Physics, Stanford University, Stanford, California 94305-4045, USA
3Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA

Microscope:
PanScan UHV

Control System:
RHK R9 Control System

+Show More
Image of the Month
posted November 1, 2016

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

+Show More
Image of the Month
posted October 1, 2016

Figure 1: STM images of Ag(111) after exposure to AO (atomic oxygen) at Tdep = 490 K. Exposure duration is labeled in the upper left corner of each image, and the scale bar is in the lower right corner. Panels (A) and (B) show that the p(4 X 5√3) domain was predominant after brief exposures, as were areas of clean Ag(111) with isolated O adatoms that were observed as black depressions, as shown in panel (B). Panels (C)−(F) show that, with increasing AO exposure, several domains coexisted until the surface became uniformly covered in the striped pattern after 300 and 600 s exposures. Imaging conditions for each image were as follows: (A) i = 280 pA, V = 1.0 V; (B) i = 300 pA, V = 800 mV; (C) I = 260 pA, V = 0.400 mV; (D) i = 200 pA, V = 800 mV; (E) i = 300 pA, V = 900 mV; and (F) i = 260 pA, V = 0.970 mV. (ACS Catal. 2016, 6, 4640−4646)

A long-standing challenge in the study of heterogeneously catalyzed reactions on silver surfaces has been the determination of what oxygen species are of greatest chemical importance. This is due to the coexistence of several different surface reconstructions on oxidized silver surfaces. A further complication is subsurface oxygen (Osub ). Osub  are O atoms absorbed into the near surface region of a metal, and are expected to alter the surface in terms of chemistry and structure; however, these effects have yet to be well characterized. We studied oxidized Ag(111) surfaces after exposure to gas-phase O atoms to determine how Osub  is formed and how its presence alters the surface structure. Using a combination of surface science techniques to quantify Osub  formation and the resultant surface structure, we observed that once 0.1 ML of Osub  formed, the surface was dramatically, and uniformly, reconstructed to striped structures at the expense of all other surface structures. Furthermore, Osub  formation was hindered at temperatures above 500 K. The thermal dependence for Osub  formation suggests that, under the industrial catalytic conditions of 475− 500 K for the epoxidation of ethylene to ethylene oxide, Osub  would be present and is a factor in the subsequent reactivity of the catalysts. These findings point to the need for the incorporation of Osub  into catalytic models, as well as further theoretical investigation of the resultant structure observed in the presence of Osub. (ACS Catal. 2016, 6, 4640−4646)

Credits:
Jonathan Derouin,† Rachael G. Farber,† Marie E. Turano,† Erin V. Iski,‡ and Daniel R. Killelea*,†

(ACS Catal. 2016, 6, 4640−4646)

†Department of Chemistry & Biochemistry, Loyola University Chicago, 1068 W. Sheridan Rd., Chicago, Illinois 60660, United States

‡Department of Chemistry and Biochemistry, The University of Tulsa, 800 S. Tucker Dr., Tulsa, Oklahoma 74104, United States

Images and data graciously provided by Dan Killelea, Loyola University Chicago, Chicago, Illinois.

Microscope:
RHK PanScan Freedom Microscope

Control System:
RHK R9 Control System

+Show More
Image of the Month
posted September 1, 2016

Figure 1:  The sample is graphene grown on SiC by Joshua Robinson’s group at Penn State. The image is a large scale (50nm) high resolution (2048px) simultaneously collected dI/dV and topo image. Taken at 200mV and 0.1nA. dI/dV setting is 10mV excitation at 1kHz. The Moire pattern is clear in the main image, while the zoom shows atomic resolution.

Graphene grown on SiC is one of the most promising routes for producing large-scale graphene devices and heterostructures. Here we studied topography and variations in local density of states of graphene grown on SiC by Joshua Robinson’s group at Penn State.  This characterization of the base graphene growth is in preparation for developing work that aims to understand the local electronic states across graphene heterostructures. The figure shows a large scale image (50nm) taken at a high resolution (2048px) and a simultaneously collected dI/dV map (at 200mV and 0.1nA) from Shawna Hollen’s group at University of New Hampshire. The large scale image shows a superposition of a Moire pattern and electronic variations, while the zoom shows details down to atomic resolution.

Shruti Subramanian in Joshua Robinson’s group at Penn State grew the graphene on SiC samples. Jake Riffle in Shawna Hollen’s group at University of New Hampshire took the STM/STS data.

Credits:
Jake Riffle2, Shruti Subramanian1, Joshua Robinson1 and Shawna Hollen2

Department of Materials Science and Engineering and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, Pennsylvania 16802, USA

University of New Hampshire, Department of Physics, 9 Library Way, Durham, NH 03824

Images and data graciously provided by Professor Shawna Hollen, University of New Hampshire, Durham, New Hampshire.

Microscope:
RHK PanScan Freedom STM/AFM

Control System:
RHK R9 Control System

+Show More