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Scientific Report 2007-2009
Condensed matter physics and biophysics
C5. Phenomenology of transport properties in matter
Transport properties are among the most relevant
properties for the study and characterization of matter.
They may reveal fundamental informations on the electronic
state in solids (such as bandwidth, mobility, localization
effects) or on the mobility of particles in fluids.
Aimed to the study of transport phenomena under
different conditions, our group has developed several experimental
setups which allow us to measure electrical
transport of different materials under a very wide range
of external parameters: temperature (4.2 to 300 K and
above), magnetic fields (up to 16 T), frequency (dc, rf
and microwaves up to 65 GHz). We also developed ad
hoc setups to study the resistivity tensor in anisotropic
materials and microwave resistivity over a very wide,
continuous spectrum.
During the last three years, our group has been using
the measured transport properties to study the dynamical
behaviour of several materials under different physical
conditions. The research has been mainly devoted
to the study of superconductors (both in the normal and
in the superconducting state), while recently the attention
has been also focused on other materials (conducting
polymers, complex liquids).
Figure 1: c-axis resistivity of HTSC as a function of temperature
at different magnetic fields.
For what concerns superconductors, the research has
been focused on the anomalous transport properties of
High temperature superconductors (HTCS) even in their
normal state (i.e. above T c ). The behaviour of resistivity
as a function of temperature reveal several uncommon
features, which are not fully explained by current
theories. Some years ago we proposed a model for the
description of the conductivity in HTCS in their normal
state based on the role of internal barriers. In the last
years, we exploited and extended this model to include
other aspects of the measured properties of superconductors:
charge confinement above T c [1,2], nonlinearity
(both above and below T c ) [4], to end up with the
common interpretation of superfluid (below T c ) and pair
formation (above T c ) within a single phenomenological
frame. Parallel to this study, we studied the specific
case of MgB 2 and in particular its characteristic double
band. Using the measurements at microwave frequencies,
we have been able to identify the contribution of
each band and the contribution of thermal fluctuations
to the obesrved resistivity below T c [3].
Figure 2: Microwave resistivity of MgB 2 as a function of
magnetic field at T=15K. The difference between H c2 as determined
by dc measurements and microwave measurements
is explained in terms of thermal fluctuations.
Conducting polymers films has been studied both at
low frequency (d.c.) and at microwave frequencies. The
growth of these films is not as reproducible as in the
case of common solids. A reliable study can thus be
made only by measuring large sets of samples, in order
to catch the mean behaviours. To this end, we developed
a fast and reliable method to measure the conductivity
of these samples. The combined structural and
electrical characterization allowed us to understand several
relevant aspects of the growing mechanism and the
relevance of microscopic and mesoscopic scale transport
(article submitted to Appl. Phys. A)
The study of complex liquids is still in a ”work in
progress” phase. We realized a cell for the measurement
of the complex permittivity of liquid samples up to 40
GHz. After a relatively long setup procedure, a relevant
data set has been collected (elaboration is in progress).
References
1. M.Giura et al., Supercond. Sci. Technol. 20, 54 (2007)
2. M.Giura et al., Physica C 460-462, 831 (2007)
3. S.Sarti et al., Jou. Supercond. Novel Mag.. 20, 51 (2007)
4. M.Giura et al., Phys. Rev. B 79, 144504 (2009)
Authors
M.Giura, R.Fastampa, S.Sarti
https://server2.phys.uniroma1.it/doc/sarti/g20-
group.html
Sapienza Università di Roma 58 Dipartimento di Fisica