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