Luis Carlos Pardo
I am Luis Carlos Pardo, and I am currently at the "Grup de Caracterització de Materials" GCM as associate professor. In my research I am mainly concerned with disordered systems such as liquids and Plastic Phases, and the glasses they can form when their movements are frozen. I am also interested in liquid water and its interaction with molecules of bilogical interest. However if you want to know more about me and my research, I recommend you to surf a little bit in my web pages. You can also have a look at my publications here.
- Physics of disordered systems: liquids, plastic crystal and its glassy phases
Disordered systems are all around us, beginning with the bottle of water I am now looking at, and finishing, at a much greater scale with my office table. My research interest is however, only focused in the first kind of systems (up to now).
Concerning microscopic disordered systems, I am mainly interested in molecular liquids and Plastic Phases. The first kind of systems is widely known, although not completely understood, being the molecules in such a state of matter rotating and translating almost randomly, and exactly this "almost" is what I am interested in. The second kind of disordered systems, although not so widely known, are simpler than the liquids because the molecules are ordered in a lattice, but they can rotate around the equilibrium positions "almost" freely, and therefore there are less "almost" to have into account, what makes this system much affordable to study. Also, I am interested on liquid water, and how liquid water is ordering around molecules of biological interest.
If the aforementioned systems are interesting , it is even more interesting what happens when some of these systems are cooled down (or its pressure is increased) at high enough rates: some, or all movements of the molecules, can be frozen, giving rise to the so called "glassy state". The most known glass is the material used in the glass containing the water that I have just now poored in: a glass (made of glass, of course). The silica glass, as that used in my glass, is in fact a liquid, but flowing so slow that it seems solid (if you can read spanish, and want to know more about glasses, CD's and frogs, click here ).
Due to the difficulties in understanding the glasses formed when a liquid is cooled down because molecules are disordered in orientation and position, I try to understand a simpler system (which is not simple at all), that is, glasses formed from plastic crystals. In this case, only the rotation of molecules must be had in to account, and therefore that simplifies the original problem... in principle!. If after this introduction, you want to know something more specific you can read the following sections, otherwise, thank you to arrive here without falling asleep.
I am interested in the study of the structure of disordered systems such as liquids and plastic phases. For this reason the ANGULA software has been recently developed to extract the orientational order of the molecules in a liquid, given a configuration (obtained from whatever the method: RMC, MD, EPSR...). If you want to know more about that, you can have a look at this poster, or you can read:
Comparison of short-range-order in liquid- and rotator-phase states of a simple molecular liquid: A reverse Monte Carlo and molecular dynamics analysis of neutron diffraction data
Pardo L.C., Tamarit J.Ll., Veglio N., Bermejo F.J., and Cuello G.J.
Phys. Rev. B 76, 134203 (2007)
Although it seems that movements taking place in the liquid are understood (using for example mode coupling theory), the dynamics when this systems enter the glassy phase are far to be understood. Specially puzzling is the fast movement of molecules (JG-beta relaxation), that take place simultaneously in the substance as the slow movements associated with the macroscopic viscosity of the system (overall tumbling of molecules). If this is caused by movements of every molecule, or different molecules are moving with different velocities is also a matter of controversy. I am interested in such problems, and also in others related to plastic phases. If you want to know more, you can read, for example :
Dielectric spectroscopy in benzophenone beta relaxation and its relation to the mode-coupling cole-cole peak
Pardo L.C., Lunkenheimer P., Loidl A.
Phys. Rev. E 76, 030502 (2007)
X-Ray and neutron diffraction
Using X-ray and neutron diffraction you can access the microscopic structure of materials. Moreover, using different wavelengths,and different kind of detectors you can look at very different length scales. Starting from the smallest length, the molecule, you can use small wavelengths to study the intramolecular and short range order of disordered systems. With bigger wavelengths you can obtain information about lattice sizes and geometries, and using detectors placed far away form the sample you can have a look at even bigger structures as clusters. If you want to know more, read, for example:
Direct experimental assessment of the strength of orientational correlations in polar liquids
Veglio N., Bermejo F.J., Pardo L.C., Tamarit J.Ll., Cuello G.J.
Phys. Rev. E 72 (3), 031502 (2005)
I have learned Dielectric spectroscopy during my post-doc in the laboratory of Augsburg: EPV, mainly with Dr. P. Lunkenheimer. In order to analyze dielectric spectra you can use the FIDEWA algorithm inserted in the more general software FABADA, to fit dielectric spectra using the multiplicative ansatz, following the ideas of Williams and Watts.
Inelastic-Quasielastic Neutron scattering
I have worked as Post-doc in the backscattering spectrometer SPHERES, in the new Neutron Source from Munich FRMII. Currently I am performing experiments in different expectrometers, and I am mainly interested inthe dynamics of liquid and plastic phases, and their glassy phases. I am also interested in the process of data fitting using a Bayesian ansatz. You can find the FABADA software developed to do that here.
Here you can also find some tips to work with the data analysis with FRIDA1
High pressure devices
High pressure devices have been developed the last years in our laboratory GCM. In my case I learned these techniques from Dr. A. Würflinger from Ruhr Universität (Bochum). We have currently on work a high pression densitometer, and a high pressure thermal analysis device. This allows us to control whether to decrease mobility of molecules via thermal vibrations by cooling down the sample, or by decreasing the accessible volume by increasing pressure, which helps us to better understand the behaviour of glassy phases. In addition, it also allows to find new phases for materials, see the disorder mixtures section, hidden at normal pressure. To know more you can have a look here:
Multiple crossed isopolymorphism: Two-component systems CCl4+CBr2Cl2 and CBrCl3+CBr2Cl2; inference of a metastable rhombohedral phase of CBr2Cl2
Barrio M., Negrier P.,Tamarit, J. Ll.,Pardo, L. C., Mondieig D.
J. Phys. Chem. B 111 (30), 8899-8909 (2007)