EFFECT OF MAGNETIC FIELD ON THE THERMAL CONDUCTIVITY OF SINGLE CRYSTAL OF YBa2Cu3O7-

ABSTRACT

This research investigated the effect of magnetic field on the thermal conductivity of high temperature type II superconductors. The result suggested that the thermal conductivity of high temperature type II superconductor YBa₂Cu₃O_7- decreases as the applied magnetic field increases at a given temperature. We also found out that the superconducting energy gap of YBa₂Cu₃O_7- decreases in response to increasing temperature and applied magnetic field. At a critical temperature of about 100K, we noted a sharp decrease in the energy gap of the substance. This implies that, the superconducting energy gap decreases in response to increase in temperature until at a critical temperature of about 100K,the material transits to normal state, thus resulting to increase in superconducting energy gap again. Our finding also revealed that specific heat of YBa₂Cu₃O_7- is proportional to electron density.

CHAPTER ONE

General Introduction

• Introduction and Discovery of Superconductivity

             The phenomenon of superconductivity was first observed by Kamerlingh Onnes in Leiden in 1911[1], three years after he liquefied helium gas. He then measured the electrical resistivity of metals such as gold, platinium and mercury. He found that the electrical resistivity of mercury vanished almost completely below 4.2K. The phenomenon by which a material loses all its electrical resistivity below a certain temperature is called superconductivity [2]. The temperature at which this occurs is known as the critical or transition temperature and it is normally denoted by T_c. At temperatures below the critical temperature, the superconducting electrons are ordered and therefore, do not carry heat. Thus, the ordered nature of superconducting electrons reduce the thermal conductivity of superconductors since there is no exchange of heat energy due to non- interactive nature of the super- conducting electrons with the lattice [3].

Superconductivity occurs in many metallic elements of the periodic alloys, and inter-metallic compounds at either low or high temperature. The search for new superconductors is an ongoing process by material scientists with superconducting transition temperature (T_c) above 30K in a mixture of lanthanum and barium-copper oxide [4] La_2-x­Ba_xCuO_x . High temperature superconductors, otherwise known as high-T_c superconductors were first discovered by Bednorz and Müller in 1986 [4].  Attempts to substitute yittrium (Y) for lanthanum (La) resulted in a polyphase mixture containing a new superconductor with T_c ≈ 90K [5]. Several other copper oxide superconductors were discovered, some with   T_c  above  120K [6]. Magnesium dibromide MgB₂ was found to be superconducting with T_c of 39K. [7]

Anderson identified three essential features of the new superconductors [8]. First the materials are quasi–two dimension (2D); the key structural units seem to be the presence of CuO₂ plane and the interplane coupling is very weak. Second, high–T_c superconductivity is created by doping a “Mott” insulator. A Mott insulator is a material in which the conductivity vanishes as temperature tends to zero, even though band theory would predict it to be metallic [9]. Third, Anderson proposed that the combination of proximity to a Mott insulating phase and low dimensionality would cause the doped material to exhibit fundamentally new behaviour, not explicable in terms of conventional metal physics.

Generally, superconductors can be categorised into type I and type II superconductors. In type I superconductors, the transition from superconducting state to normal state in the presence of applied magnetic field is very sharp while in type II superconductors, the transition from super-conducting state to normal state in the presence of applied magnetic field takes place after going through a mixed state region. Table 1.1 gives the transition temperature (  T_c  ) and the year of the discovery of some novel superconductors.

Table 1.1 High Temperature Superconductors, T_c and Year of Discovery     

Most of the cuprate superconductors were derived from YBa₂Cu₃O_7- by replacing yittrium with either bismuth (Bi), thalium (Tl )or mercury (Hg) or barium (Ba) with lead (Pb), for example. It is therefore necessary to examine the crystal structure of YBa₂Cu₃O_7- in some detail.

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