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Experimental signatures of CDW

The phenomena of CDW ordering is usually confirmed experimentally using following experiments or observations.

Source : Inna Vishik

Transport

In a new material, the first suspicion of a CDW often comes from an anomaly (specifically, resistivity increasing with decreasing temperature in some temperature range) in the resistivity. This Behavior near CDW can be a hint to CDW which can then be well-founded by band structure calculations showing fermiology. The following figure shows the anomalous behavior of normalized resistivity as a signature of CDW in NbSe3. Resistivity starts to increase below the CDW temperature because the transition depletes density of states on part of the Fermi surface, but eventually, metallic resistivity (decreasing with decreasing temperature) resumes because a portion of the Fermi surface stays metallic.

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X-ray or Electronic diffraction

The 'proof' for CDW in a given material usually comes from diffraction experiments which show a superstructure--smaller Bragg peaks away from the nuclear Bragg peaks which appear at the CDW onset temperature. The presence of satellite reflections \(\pm\vec{q}\) indicates that an additional periodicity of wavelength \(2\pi/|\vec{q}|\) is superimposed upon the undistorted cell parallel to the \(\vec{a}\) or \(\vec{c}\) axes.

TEM_measurement.png CDW in related compound SmTe3, as measured by TEM. Structural Bragg peaks are intense and labeled, and CDW satellites are the weaker peaks in between. Image source: E. DiMasi et al., Physical Review B 52 14516 (1995)

Scanning tunneling microscopy (STM)

STM can detect a number of features related to the onset of CDW, including charge modulation, periodic distortion of atomic position, and the opening of a gap in the density of states.

STM_scan.png Topographic STM scan (top) in which different colors indicate magnitude and sign of atomic position distortion. Bottom panel shows a gap in the tunneling conductance. All data at 6K. Image source: A. Fang et al., Physical Review Letters 99, 046401 (2007).

Angle resolved photoemission spectroscopy (ARPES)

ARPES can directly show which portions of the Fermi surface are gapped out by the CDW. This can give guidance whether the CDW is driven by Fermi surface nesting (q-vector connecting two parallel Fermi surface segments \(T>T_{CDW}\) is the same as q-vector of satellite peaks in x-ray or electron diffraction) or by something more exotic.

ARPES_CDW.png Fermi surface of TbTe3 below (top) and near (bottom) the CDW onset temperature. Top image shows segments of Fermi surface which are gapped out by CDW. Image source: F. Schmitt et al, New Journal of Physics 13, 063022 (2011)