download report - Sapienza

Scientific Report 2007-2009
Condensed matter physics and biophysics
C3. Charge inhomogeneities and criticality in cuprate
superconductors
Strong correlations have the marked tendency to
destabilize the metallic state. The formation of a Mott
insulator is a ”classical” case, but the breakdown of the
metallic phase may also lead to superconductivity, competing
phases, and inhomogeneities. Near these instabilities
the metallic state can acquire an anomalous behavior
and violate the standard Landau paradigm of Fermi
liquids. Also superconductivity can be realized in unusual
forms violating the standard BCS scheme. It is
therefore of great interest to study strongly correlated
systems in the proximity of their instabilities. This is the
main framework of our investigation of new paradigms of
the normal and superconducting states. This research regards
fundamental concepts in solid-state physics, but it
is also relevant for the understanding of physical systems
of applicative interest like magnetic materials, spintronics,
superconductivity, nano- and mesoscopic systems.
Since many years our group realized that strongly
correlated systems are often prone to phase separation,
although this may be prevented by Coulombic
interactions. This is the so-called frustrated phase
separation (FPS) giving rise to the general phenomenon
of mesoscopic-, micro-, or nano-phase separation, which
is by now a general chapter of condensed matter physics
as it occurrs in many systems ranging from charged
colloidal to two-dimensional electron gas, to ruthenates,
manganites thin films, and high temperature superconductors
(HTSC). In this last case FPS provides a
mechanism of how attraction for pair formation can
be generated by repulsive correlations. Recently we
classified the transition to frustrated states in two
universality classes [1] corresponding to the anomalies
often found in a variety of strongly correlated electronic
models: short range compressibility negative in a finite
interval of density or delta like divergent due to the
free energy crossing of two homogeneous phases. In this
last case in 2D the system always breaks into domains
in a narrow range of densities, no matter how big the
Coulombic frustration is. For the case of negative
compressibility, shown by our group to be relevant for
the cuprates, we have provided [2] the phase diagram in
three dimension in the density- frustration plane with
transitions from the homogeneous phases to the different
morphologies of clusters (from a bcc crystal of droplets,
to a triangular lattice of rods, to a layered structure)
(see Fig. 1). Inclusion of a strong anisotropy allows
for second- and first-order transition lines joined by a
tricritical point and for the discussion of the evolution
from a sinusoidal charge density wave modulation to
anharmonic stripes. These topics are strictly related to
the physics of HTSC, according to our proposal that
these systems are on the verge of a charge-ordering
(CO) instability due to FPS. Although CO may not be
fully realized because of low dimensionality, disorder,
and Cooper-pair formation, there is a tendency to
perform a second-order transition to a CO state with
a transition line T CO ending around optimal doping
Figure 1: Schematic phase diagram of FPS in 3D isotropic
systems (after Ref.2).
(for which the superconducting T c is the highest) into a
quantum critical point (QCP) at zero temperature (see
Fig.2). Near this QCP the collective charge fluctuations
Figure 2: Schematic phase diagram of the HTSC focusing
on the different CO regions and the CO-QCP
have an intrisically dynamic character and with their
low energetic cost they provide an effective scattering
mechanism for the metal quasiparticles possibly also
leading to superconducting pairing. This ”uncomplete
criticality” makes it available low-energy quasi-critical
fluctuations over extended regions of the phase diagram,
whose effect on optical conductivity and Raman
scattering[3], angle-resolved photoemission spectroscopy
(ARPES) and STM [4] have been investigated. This
analysis of spectroscopic signatures of the low-energy
quasi-critical charge fluctuations has allowed to interpret
several peculiar features of the spectra in HTSC and to
identify the specific momentum and energy dependence
of the collective excitations in these systems. More
recently the dynamical character of the CO fluctuations
has been exploited to account for the rather elusive
character of CO in these materials: dynamic CO can
substantially affect the ARPES and STM spectra at
finite energy showing the 1D stripe self-organization,
while leaving untouched the states near the Fermi level.
References
1. C. Ortix et al., Physica B 404, 499 (2009)
2. C. Ortix et al., Phys. Rev. Lett. 100, 246402 (2008)
3. M. Grilli, et al., Physica B.404, 3070 (2009)
4. G. Seibold, et al. Phys. Rev. Lett. 103,217005 (2009)
Authors
S. Caprara, C. Di Castro, M. Grilli, J. Lorenzana 3
http://theprestige.phys.uniroma1.it/clc/
Sapienza Università di Roma 56 Dipartimento di Fisica