Digital Thesis - LTPL

We show in this page the digital version of the PhD Thesis published by the graduate students of the laboratory.
Versiones digitales de las Tesis Doctorales publicadas por los graduados del laboratorio.

2007
Alejandro Moreno - Doctoral Thesis
"Caracterización y respuesta dinámica de la red de vórtices en monocristales de YBCO con distinto contenido de oxígeno."
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2005
Guillermo Jorge - Doctoral Thesis
"Magnetic excitations in high temperature superconductors and other strongly correlated systems."
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2005
Miguel Monteverde - Doctoral Thesis
"Electric conduction in carbon nanotubes under pressure."
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2005
Gastón Garbarino - Doctoral Thesis
"Efectos de la presión en el transporte eléctrico y en la transición ferromagnética en manganitas y materiales afines."
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2001
Sergio Valenzuela - Doctoral Thesis
"Dinámica oscilatoria de la red de vórtices en superconductores de alta temperatura crítica."
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Alejandro Moreno

Doctoral Thesis

2007

Alejandro Moreno

Caracterización y respuesta dinámica de la red de vórtices en monocristales de YBCO con distinto contenido de oxígeno.

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Keywords: High temperature superconductors, YBa2Cu3O7-δ , vortices, ac susceptibility, dc magnetization, pinning mechanisms, surface barriers, mobility, order.

Abstract

The static and dynamic properties of the vortex lattice (VL) in type II superconductors conform an active field of study, not only from the point of view of technological applications, but as of the study of basic physics as well. The structure of the VL is very rich due to competing interactions between vortices, between these and the disorder of the superconducting material and thermal fluctuations. In several high critical temperature superconducting compounds the effect of doping, i.e. the modification of the number of available carriers, is of interest because these materials present a very rich phase diagram with different magnetic behaviors. In the superconducting phase the modification of the number of carriers by means of changes in the oxygen content allows the study of the interaction of vortex matter with defects generated by this procedure, and provides information about the relevance of the different mechanisms of pinning at different temperatures and magnetic fields. It is of particular interest the study of vortex dynamics when the excitation is of oscillatory character.

The objective of this thesis was to measure the response of VL in twinned YBCO single crystals with different oxygen contents, to study the effect of oxygen doping in the interactions between vortices and in the diverse pinning mechanisms of the VL. In this thesis several techniques were used: a high resolution ac susceptibility technique, dc magnetization and remnant magnetization measured with a on-conventional ac procedure. These techniques involve different characteristic measurement times (from some hundredth of millisecond to some minutes) to explore how the pinning strength changes with the degree of oxygen content in different time windows. Additionally we studied how VL dynamics changes as a result of different thermomagnetic histories of the sample. In particular we studied the effect of applying ac fields of large amplitude to the VL to study its order and mobility.

The dynamic measurements of ac susceptibility were modeled within the framework of a phenomenological model that includes the effect of pinning centers and relaxation by thermal activation. We obtained the parameters of the model like the critical current density at T = 0, the typical scale of the pinning potentials and the average vortex propagation speed. The regime of pinning for the oxygenated and deoxygenated samples was also determined. The measurements of the remnant magnetization with a non-conventional technique were presented within the framework of a critical state model that includes relaxation by thermal activation, in order to determine the effective critical current densities. Measurements of dc magnetization at different fields and temperatures were made to study the different pinning mechanisms measuring them in a different temporary scale.

We have observed that the deoxygenated samples present less pinning strength than the oxygenated ones and that in crystals with low oxygen content surface barriers hinder the penetration of magnetic flux in a wide range of temperatures. The pinning strength is stronger in the oxygenated samples, possibly because the VL is in the individual vortex pinning regime in a wide range of temperatures and fields, whereas the removal of oxygen makes the collective pinning regime the predominant one. At temperatures near the critical temperature the surface barriers are made more effective independently of the oxygen content.

We have observed that ac magnetic fields of high amplitude can not order a previously disordered VL and that they disorder a previously ordered VL. The observed memory effects in oxygenated crystals are absent in deoxygenated crystals for the same range of fields and temperatures.

   

Guillermo Jorge

Doctoral Thesis

2005

Guillermo Jorge

Magnetic excitations in high temperature superconductors and other strongly correlated systems

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Keywords: Strongly correlated electrons, specific heat, superconductivity, organic conductors, quantum magnets.

Abstract

In this work, we studied three strongly correlated systems by means of  specific heat and magnetization measurements at high magnetic fields. The specific heat measurements were performed using thin amorphous silicon nitride membrane calorimeters, allowing the determination of the heat capacity of small crystals (less than one milligram of mass). We used also a silicon platform calorimeter for larger samples. The method used to measure the specific heat was the thermal relaxation time technique.

We studied the specific heat and magnetization in fields up to 15 T at the simultaneous onset of superconductivity and incommensurate magnetic order (Tc = Tm = 42 K) in a high temperature superconductor, La2CuO4.11. The peak in the specific heat due to the superconducting transition was observed, and another broad anomaly centered at 50 K was seen after the subtraction of the specific heat of a deoxygenated sample. We attributed this to the fluctuations of the spin system enhanced by the interplay between charge and spin degrees of freedom. In the magnetization measurement we found evidence of strong vortex fluctuations.

We also investigated the specific heat in fields up to 15 T at the dimerization transition of butyl and ethyl neutral radical organic conductors. The transition temperatures were 350 K and 150 K respectively. For the first compound the transition temperature shifted slightly to lower temperatures in applied fields and had hysteresis, whereas for the second compound the transition temperature did not move with applied field and hysteresis was not observed. The associated entropies were different in both cases. A thermally induced molecular flopping mechanism was proposed to explain those differences.

Finally, we studied the compound SrCu2(BO3)2, a two-dimensional spin system with a orthogonal dimer network. We performed specific heat measurements in fields up to 33 T, magnetic susceptibility measurements at 4 T, and magnetization measurements in pulsed fields of 45 T. We compare our data with numerical simulations, obtaining a good fitting with the insert of an anysotropic term in the Hamiltonian, which suggests a broken symmetry at low temperatures induced by a strong lattice distortion.

   

Miguel Monteverde

Doctoral Thesis

2005

 

Electric conduction in carbon nanotubes under pressure

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Keywords: nanotubes, SWNT, MWNT, pressure, electric conduction, transport properties, Luttinger liquid, nanoscopic system, tunnelling.

Abstract:

An extensive and detailed work on electric transport properties on networks of single wall (SWNT) and multiwall (MWNT) nanotubes under pressure was realised. Although we present measurements of networks consisting on great number of nanotubes, its electric conduction was interpreted as a network of tunnel junctions so it is in close relation with the conduction of a single nanotube. These results were interpreted using a one dimensional system model, Luttinger liquid theory, obtaining power law behaviour in conductance vs. temperature and in conductance vs. voltage, obtaining a good agreement with the predicted scaling law.

The effect of pressure is focalized in changing the amount of conducting channels of nanotubes at both sides of each tunnel junction. This fact allows to test expressions for transition probabilities of a tunnel junction as function of the number of channels.

We observe and explain an anomalous behaviour of the electric transport at high temperatures and at all pressures in MWNT with a simple model. This model takes into account the thermal excitations of electrons or holes that begin to fill (or leave) bands that otherwise, at low temperatures, should be totally empty or filled so they dont collaborate to transport. We also observe evidence of SWNT collapse under high pressure.

   

Gastón Garbarino

Doctoral Thesis

2005

Efectos de la presión en el transporte eléctrico y en la transición ferromagnética en manganitas y materiales afines.

PDF file

   

Sergio Valenzuela

Doctoral Thesis

2001

Dinámica oscilatoria de la red de vórtices en superconductores de alta temperatura crítica.

PDF file

 

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