Subject Hall coefficient

All the cuprates described till now are hole superconductors. The nature of holes has been subject of considerable discussion (Chakraverty et al. 1988 Rao et al. 19896 Sarma Rao 1989). There has been no experimental evidence for the presence of Cum type species in the doped cuprates. Instead, there is considerable evidence from electron and X-ray spectroscopies for the presence of hybridized oxygen holes which can be represented as O-. The detailed description of the holes in terms of the d and p characters has been investigated (Bianconi 1990). Essentially, the mobile holes in the cuprates are present in the in-plane n band which has 0-2p character. The concentration of holes (in all but the T1 cuprates) are easily determined by iodometry or Fen-Fem titrations (Rao el al. 1991a Shafer Penney 1990). Since the Hall coefficients are temperature dependent, the chemical titration method becomes invaluable. 

Details of the method employed for measuring Seebeck coefficients have been described by Heller and Danielson (5). The Hall coefficient of Li0.37W03 was measured by a d.c. method and is therefore subject to error from the Ettings-hausen effect. This error is not expected to exceed 10%. 

Hall effect - The development of a transverse potential difference Uin a conducting material when subjected to a magnetic field H perpendicular to the direction of the current. The potential difference is given by U = Rjj BJt, where B is the magnetic induction,/the current density, fthe thickness of the specimen in the direction of the potential difference, and is called the Hall coefficient. 

There are, however, also some drawbacks to these devices. Because of their poor sensitivity, silicon Hall elements provide only a very small output voltage, which must be highly amplified. Their inevitable offset voltage is subject to significant fluctuations because of amplitude and temperature coefficients, and the strong temperature-dependent sensitivity requires adequate compensation. Thus, similarly to the resistive current principle, the amplifying circuit often defines the quality of the sensor. 

A surface heat transfer coefficient h can be defined as the quantity of heat flowing per unit time normal to the surface across unit area of the interface with unit temperature difference across the interface. When there is no resistance to heat flow across the interface, h is infinite. The heat transfer coefficient can be compared with the conductivity the conductivity relates the heat flux to the temperature gradient the surface heat transfer coefficient relates the heat flux to a temperature difference across an unknowm distance. Some theoretical work has been done on this subject [8], but since it is rarely possible to achieve in practice the boundary conditions assumed in the mathematical formulation, it is better to regard it as an empirical factor to be determined experimentally. Some typical values are given in Table 2. Cuthbert [9] has suggested that values greater than about 6000 W/m K can be regarded as infinite. The spread of values in the Table is caused by mold pressure and by different fluid velocities. Heat loss by natural convection also depends on whether the sample is vertical or horizontal. Hall et al. [10] have discussed the effect of a finite heat transfer coefficient on thermal conductivity measurement. 

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