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Scientific Report 2007-2009<br />
Condensed matter physics and biophysics<br />
C36. Pressure tuning of charge density wave states.<br />
The study of low-dimensional systems has recently become<br />
one of the priorities in condensed matter physics.<br />
These systems not only experience remarkably strong<br />
quantum and thermal fluctuations, but also admit ordering<br />
phenomena that are difficult to obtain in threedimensional<br />
materials, such as charge- and spin-density<br />
wave (CDW and SDW) states. CDW and SDW are<br />
broken symmetry ground states driven by the electronphonon<br />
and electron-electron interactions, respectively.<br />
The intense theoretical and experimental efforts devoted<br />
to the investigation of the onset of density waves in onedimensional<br />
system have provided a rather well established<br />
interpretation framework, although little is known<br />
about the two-dimensional case. External variables (like<br />
temperature, magnetic field, and chemical and applied<br />
pressure) can affect the dimensionality of the interacting<br />
electron gas, and thus the intrinsic electronic properties,<br />
as well as the interplay among different order parameters,<br />
giving rise to rich phase diagrams. Tuning<br />
the dimensionality by applying pressure can thus play a<br />
key role in developing a comprehensive theory. The diand<br />
tri-chalcogenide RTe n (R rare earth, n=2,3) are the<br />
latest paramount examples of low dimensional systems<br />
exhibiting the formation of an incommensurate CDW<br />
state. These materials are characterized by a layered<br />
structure where corrugated R-Te slabs alternate with<br />
planar Te square lattices (single layer for di- and double<br />
layer for tri-tellurides). Metallic conduction occurs along<br />
the Te sheets and unusually large CDW gap, depending<br />
on the rare earth, are observed also at ambient temperature.<br />
Owing to the 2D character of these compounds, the<br />
gap is not isotropic and shows a wave-vector dependence.<br />
In particular, since the vector q* does not nest the whole<br />
Fermi surface (FS), there are regions not gapped where<br />
free charge carriers lead to highly anisotropic metallic<br />
conduction. The study of these compounds could give an<br />
important insight into the interplay between the metallic<br />
state and the broken-symmetry CDW phase.<br />
Through a close collaboraboration of our group with<br />
the ETH (Zurich, CH) and the Department of Applied<br />
Physics at the Stanford University (USA), a whole set of<br />
high-pressure measurements on RTe 2 and RTe 3 has been<br />
carried out. Using diamond anvil cells to pressurize the<br />
samples, we carried out experiments of Raman scattering<br />
(our lab), Infrared (IR) spectroscopy (SISSI beamline<br />
@ELETTRA, Trieste, IT) and x-ray diffraction (ID09A<br />
beamline @ESRF) [1-4].<br />
Figure 2: Left: Single particle excitation energy vs. lattice<br />
constant for CeTe3 under pressures and for the RTe3 series<br />
[1]. Right: X-ray diffraction pattern of single-crystal LaTe 3<br />
and CeTe 3 single-crystal at different P and T. Red circles<br />
highlight the CDW satellite peaks and the modulation vector<br />
q*[4].<br />
Our experiments clearly point out the CDW state and<br />
demonstrate the possibility of tuning and eventually<br />
suppress the CDW state by lattice compression [1,4].<br />
We were able to establish a close equivalence between<br />
chemical (rare-earth substitution) and applied pressure<br />
in governing the onset of the CDW broken symmetry<br />
ground state and in tuning the gap. The reduction and<br />
the suppression of the CDW gap arises in both cases<br />
from internal changes of the effective dimensionality<br />
of the electronic structure, thus strengthening the link<br />
between CDWs and nesting of the FS [1,2]. We propose<br />
that broadening of the bands upon lattice compression<br />
in layered rare earth tellurides removes the perfect<br />
nesting condition of the FS thus diminishing the impact<br />
of the CDW transition on their electronic properties.<br />
The chemical/applied pressure equivalence is confirmed<br />
by Raman measurements, which, moreover, provides<br />
clear evidence for the tight coupling between the CDW<br />
condensate and the vibrational modes [3].<br />
References<br />
1. A. Sacchetti et al., Phys. Rev. Lett. 98, 026401 (2007).<br />
2. M. Lavagnini et al., Phys. Rev. B 77, 165132 (2008).<br />
3. M. Lavagnini et al., Phys. Rev. B 103, 201101(R) (2008).<br />
4. A. Sacchetti et al., Phys. Rev. B 3, 201101(R) (2009).<br />
Authors<br />
P. Postorino, S. Lupi, E. Arcangeletti, L. Baldassarre, M.<br />
Baldini, D. Di Castro, C. Marini, M. Valentini<br />
http://www.phys.uniroma1.it/gr/HPS/HPS.htm<br />
Figure 1: A picture and a schematic representation of a<br />
screw clamped diamond anvil cell.<br />
<strong>Sapienza</strong> Università di Roma 89 Dipartimento di Fisica