download report - Sapienza

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