Image of the Month

January 2017

Multiple charge density wave states at the surface of TbTe3


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.

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

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