scholarly journals The theory of the magneto-resistance effects in metals

1947 ◽  
Vol 190 (1023) ◽  
pp. 435-455 ◽  
Keyword(s):  
Magnetic Field ◽  
Electrical Resistance ◽  
Low Temperatures ◽  
Lorenz Number ◽  
Strong Magnetic Fields ◽  
Pass Through ◽  
Set Up ◽  
Magneto Resistance ◽  
The Magnetic Field ◽  
Overlapping Bands

General formulae are obtained for the effect of a magnetic field on the electrical and thermal conductivities of a metal in which there are two overlapping bands of normal form. Simple formulae are set up which, though not strictly valid for all temperatures and fields, reduce to the correct expressions in the three limiting cases of high temperatures, low temperatures and very strong magnetic fields. The behaviour of the electrical resistance at low temperatures is discussed, and it is shown that in certain cases the resistance may pass through a minimum as the temperature is increased provided the magnetic field is large enough. It is also shown that in general the Lorenz number is increased by the presence of a magnetic field, but that the thermal conductivity of the lattice is unaffected by a magnetic field.

scholarly journals The change of resistance of gold crystals at very low temperatures in a magnetic field and supra-conductivity

1930 ◽  
Vol 126 (803) ◽  
pp. 683-695 ◽  
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Low Temperatures ◽  
Crystalline State ◽  
Specific Resistance ◽  
Systematic Research ◽  
Continuous Curve ◽  
Strong Magnetic Fields ◽  
Relative Change ◽  
The Magnetic Field

In a paper published last year the author described a systematic research on the change of resistance which occurs in a number of metals in strong magnetic fields. As a result of these investigations the following formulæ expressing the relative change of resistance ∆R/R o with the field H were found to hold :— ∆R/R o = β' H 2 /3H k H ≼ H k , (1) and ∆R/R o = β' ( H-H k +H k 2 /3H) H ≽ H k , (2) where β' and H k are constant for a given sample of a metal and at a given temperature. These two expressions form a continuous curve, and it is evident that the formula (1) which holds for the weaker fields, shows that the resistance increases as the square of H, and formula (2) indicates that the change of resistance in strong fields approaches a linear law. These two formulæ have been obtained mathematically on the following assumption. It is known that in a metal which is not in a perfect crystalline state, and which contains even small traces of impurities, there exists a disturbance which increases its specific resistance. My hypothesis was that a magnetic field increases the specific resistance in a similar way to these imperfections, so that they are equivalent to an internal magnetic field H k , orientated at random. Then, if the metal is brought under the influence of an outside magnetic field H, the increase of resistance is such as would be produced by a combination of the two fields. Further, I assumed that the increase of resistance is proportional to the magnetic field, and this led to formulæ (1) and (2) which appear to fit all my experimental results very well. Several important consequences follow from this hypothesis.

scholarly journals The magneto-resistance effect in cadmium at low temperatures

1937 ◽  
Vol 160 (901) ◽  
pp. 207-229 ◽  
Keyword(s):  
Magnetic Field ◽  
Field Strength ◽  
Electrical Resistance ◽  
Low Temperatures ◽  
Linear Part ◽  
Experimental Curve ◽  
Linear Variation ◽  
Experimental Range ◽  
Maximum Field ◽  
Magneto Resistance

The experiments of Kapitza (1929) showed that the increase of electrical resistance produced in a metal by a magnetic field H is not proportional to H 2 , as was previously supposed. In the new experimental range made available by his method (Kapitza 1927) of producing very strong fields up to 300 kilogauss, Kapitza found that the increase of resistance tended towards a linear variation with the field strength. The result may be expressed in the formula ΔR / R 0 = b ( H - H k ), for H ≫ H k , where R 0 is the resistance at 0° C. This gives the asymptote to the experimental curve: but if experiments are made at field strengths up to a maximum H m , and H m ≫ H k , then over a large part of the experimental range the curve obtained is practically identical with the asymptote. If the linear part of the curve is then extrapolated back to meet the axis of H , its intercept on that axis gives the parameter H k , and the slope of the line gives the parameter b . If, however, the maximum field used is only of the order of H k , the linear variation is only reached outside the experimental range; and some formula must be employed, in effect, to extrapolate to the region where the linear law holds, before the position of the asymptote and the values of the parameters can be derived. It is obvious that the values so obtained will vary according to the particular formula adopted.

The theory of the magneto-resistance effects in polar semi-conductors

1955 ◽  
Vol 227 (1169) ◽  
pp. 241-251 ◽  
Keyword(s):  
Magnetic Field ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Hall Coefficient ◽  
Low Temperatures ◽  
Magneto Resistance

General expressions are obtained for the Hall coefficient and transverse magneto-resistance effect in polar semi-conductors, and the variation of these effects with temperature, magnetic field strength and degeneracy of the electrons is discussed. At low temperatures the magneto-resistance effect may become very large, contrary to the prediction of the freepath theory.

scholarly journals The hall effect in liquid electrolytes

1913 ◽  
Vol 88 (606) ◽  
pp. 588-604 ◽  
Keyword(s):  
Magnetic Field ◽  
Hall Effect ◽  
Electric Current ◽  
Metal Plate ◽  
Satisfactory Explanation ◽  
Thermo Electric ◽  
Field Independent ◽  
Set Up ◽  
The Magnetic Field ◽  
Thin Metal Plate

The distortion of the lines of flow of an electric current in a thin metal plate by the action of a magnetic field was discovered in 1879. Hall attributed this to the action of the magnetic field on the molecular currents in the metal film, which were deflected to one side or the other and accompanied by a corresponding twist of the equipotential lines. This explanation did not pass without criticism, and another theory of the effect found by Hall was published in 1884. In that paper the author seeks to explain the effect by assuming a combination of certain mechanical strains and Peltier effects, a thermo-electric current being set up between the strained and the unstrained portions. The effect of such strain was to produce a reverse effect in some metals, and these were precisely the metals for which the Hall effect was found to reverse. Aluminium was the only exception. In other respects, however, as shown by Hall in a later paper, Bidwell's theory did not stand the test of experiment, and the results lend no support to his theory, while they are in complete accordance withe the explanation that the molecular currents are disturbed by the action of the magnetic field. On the electron theory of metallic conduction, the mechanism of the Hall effect is more obvious, but at present no satisfactory explanation of the reversal found in some metals is known. Further experiments have made it clear that there is a real deflection of the elementary currents, due to the application of the magnetic field, independent of any effect due to strain.

scholarly journals Photons in a cold axion background and strong magnetic fields: Polarimetric consequences

2015 ◽  
Vol 30 (17) ◽  
pp. 1550099 ◽  
Author(s):  
Domènec Espriu ◽  
Albert Renau
Keyword(s):  
Magnetic Field ◽  
Dark Matter ◽  
Galactic Halo ◽  
Local Density ◽  
Polarization Plane ◽  
Strong Magnetic Fields ◽  
Two Photon ◽  
Main Effect ◽  
Optical Table ◽  
The Magnetic Field

In this work, we analyze the propagation of photons in an environment where a strong magnetic field (perpendicular to the photon momenta) coexists with an oscillating cold axion background with the characteristics expected from dark matter in the galactic halo. Qualitatively, the main effect of the combined background is to produce a three-way mixing among the two photon polarizations and the axion. It is interesting to note that in spite of the extremely weak interaction of photons with the cold axion background, its effects compete with those coming from the magnetic field in some regions of the parameter space. We determine (with one plausible simplification) the proper frequencies and eigenvectors as well as the corresponding photon ellipticity and induced rotation of the polarization plane that depend both on the magnetic field and the local density of axions. We also comment on the possibility that some of the predicted effects could be measured in optical table-top experiments.

The Shape of Analytical Curves in Zeeman Atomic Absorption Spectroscopy. II. Theoretical Analysis and Experimental Evidence for Absorption Maximum in the Analytical Curve

1980 ◽  
Vol 34 (4) ◽  
pp. 464-472 ◽  
Author(s):  
M. T. C. De Loos-Vollebregt ◽  
L. de Galan
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Atomic Absorption ◽  
Absorption Spectroscopy ◽  
Atomic Absorption Spectroscopy ◽  
Stray Light ◽  
Strong Nonlinearity ◽  
Analytical Curve ◽  
Pass Through ◽  
The Magnetic Field

The analysis of the shape of analytical curves in Zeeman atomic absorption spectroscopy has been extended toward higher concentrations. Nonlinearity in the conventional atomic absorption signal due to stray light or nonlinear electronic response causes both theoretically calculated and experimental analytical curves in Zeeman atomic absorption to go through a maximum at a certain concentration. The height and the position of the maximum depend on the magnetic system used, the strength of the magnetic field and the amount of nonlinearity. In all magnetic systems the maximum attainable absorbance is enhanced by increasing the magnetic field strength and decreasing the amount of nonlinearity. Over the normal concentration range a maximum in the Zeeman atomic absorption analytical curve only occurs under the extreme conditions of a very weak magnetic field and strong nonlinearity. Ultimately, however, all Zeeman atomic absorption analytical curves pass through a maximum unless the optics and electronics are perfect. For practical systems strong ac modulated magnetic fields are to be preferred over dc magnetic fields.

scholarly journals Oscillatory convection in strong magnetic fields and origin of active regions

1968 ◽  
Vol 35 ◽  
pp. 127-130 ◽  
Author(s):  
S. I. Syrovatsky ◽  
Y. D. Zhugzhda
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Active Regions ◽  
Oscillatory Convection ◽  
Strong Magnetic Fields ◽  
Various Boundary Conditions ◽  
Convective Cells ◽  
The Magnetic Field ◽  
Polytropic Atmosphere

The convection in a compressible inhomogeneous conducting fluid in the presence of a vertical uniform magnetic field has been studied. It is shown that a new mode of oscillatory convection occurs, which exists in arbitrarily strong magnetic fields. The convective cells are stretched along the magnetic field, their horizontal dimensions are determined by radiative cooling. Criteria for convective instability in a polytropic atmosphere are obtained for various boundary conditions in the case when the Alfvén velocity is higher compared with the velocity of sound.The role of oscillatory convection in the origin of sunspots and active regions is discussed.

scholarly journals The study of the specific resistance of bismuth crystals and its change in strong magnetic fields and some allied problems

1928 ◽  
Vol 119 (782) ◽  
pp. 358-443 ◽  
Keyword(s):  
Magnetic Field ◽  
Specific Resistance ◽  
Cleavage Plane ◽  
Time Lag ◽  
General View ◽  
Time Lags ◽  
Strong Magnetic Fields ◽  
First Moment ◽  
The Magnetic Field ◽  
A Current

It is well known that in a magnetic field bismuth shows a greater change of resistance than any other substance, and it is also known that in the case of a crystal this phenomenon varies very much with the orientation of the crystal. A great deal of literature exists on this subject. The general view of the phenomenon is that the increase of resistance is largest when the cleavage plane of the crystal is parallel to the magnetic field, and when the current is flowing perpendicular to it. It is also known that the resistance in a magnetic field increases very rapidly with decreasing temperature. A complication in all these phenomena arises through certain time lags. When a current is passed through bismuth placed in a magnetic field, the resistance at the first moment is large, and then gradually decreases to its final value. This time lag accounts for the fact, first discovered by Lenard, that bismuth has a larger resistance for alternating currents than for direct currents. This phenomenon also depends on the crystal state of the bismuth.

The Effect of a Magnetic Field on Individual Convective Cells

1974 ◽  
Vol 2 (5) ◽  
pp. 267-269
Author(s):  
J. O. Murphy
Keyword(s):  
Magnetic Field ◽  
Close Association ◽  
Cell Structure ◽  
Longitudinal Field ◽  
Strong Magnetic Fields ◽  
Convective Processes ◽  
Granulation Pattern ◽  
Convective Cells ◽  
The Magnetic Field

The darkness of sunspots has been attributed by many authors (Biermann 1941; Danielson 1961) to the inhibition of the normal solar convective processes by the presence of strong magnetic fields. Observations of the solar photospheric granulation pattern have also shown that a weak longitudinal field exists outside the activity regions. Although these observations have not revealed any close association between the magnetic field and individual granules, nor the exact reasons for the darker cell boundaries, it must be accepted that, overall, the role of the magnetic field must be such as to influence the cell structure and reduce the normal heat transfer by convection.

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