View source for Cyclotron resonance

{{Short description|Motion of charged particles}}
'''Cyclotron resonance''' describes the interaction of external forces with charged particles experiencing a [[magnetic field]], thus already moving on a circular path. It is named after the [[cyclotron]], a cyclic [[particle accelerator]] that utilizes an oscillating electric field tuned to this resonance to add kinetic energy to charged particles.

==Cyclotron resonance frequency==
The '''cyclotron frequency''' or '''gyrofrequency''' is the frequency of a [[charged particle]] moving perpendicular to the direction of a uniform magnetic field ''B'' (constant magnitude and direction). Since that motion is always circular,<ref name="Physics">Physics by M. Alonso & E. Finn, Addison Wesley 1996.</ref> the cyclotron frequency is given by equality of [[centripetal force]] and magnetic [[Lorentz force]]

:<math>\frac{mv^2}{r} = qBv</math>

with the particle mass ''m'', its charge ''q'', velocity ''v'', and the circular path radius ''r'', also called [[gyroradius]].

The [[angular speed]] is then:
:<math>\omega = \frac{v}{r} = \frac{qB}{m}</math>.

Giving the [[rotational frequency]] (being the cyclotron frequency) as:
:<math>f = \frac{\omega}{2\pi} = \frac{qB}{2\pi m}</math>,

It is notable that the cyclotron frequency is independent of the radius and velocity and therefore independent of the particle's kinetic energy; all particles with the same charge-to-mass ratio rotate around magnetic field lines with the same frequency. This is only true in the non-relativistic limit, and underpins the principle of operation of the [[cyclotron]].

The cyclotron frequency is also useful in non-uniform magnetic fields, in which (assuming slow variation of magnitude of the magnetic field) the movement is approximately helical - in the direction parallel to the magnetic field, the motion is uniform, whereas in the plane perpendicular to the magnetic field the movement is, as previously circular. The sum of these two motions gives a trajectory in the shape of a [[helix]].

When the charged particle begins to approach relativistic speeds, the centripetal force should be multiplied by the [[Lorentz factor]], yielding a corresponding factor in the angular frequency:
:<math>\omega=\frac{qB}{\gamma m}</math>.

===Gaussian units===
The above is for [[SI units]]. In some cases, the cyclotron frequency is given in [[Gaussian units]].<ref>Kittel, Charles. [[Introduction to Solid State Physics]], 8th edition. pp. 153</ref> In Gaussian units, the Lorentz force differs by a factor of 1/''c'', the speed of light, which leads to:
:<math>\omega = \frac{v}{r} = \frac{qB}{mc}</math>.

For materials with little or no magnetism (i.e. <math>\mu \approx 1</math>) <math>H \approx B</math>, so we can use the easily measured [[Magnetic field#The H-field|magnetic field intensity ''H'']]  instead of ''B'':<ref>Ashcroft and Mermin. Solid State Physics. pp12</ref>

:<math>\omega = \frac{qH}{mc}</math>.

Note that converting this expression to SI units introduces a factor of the [[vacuum permeability]].

===Effective mass===
{{See also|Effective mass (solid-state physics)#Cyclotron effective mass}}
For some materials, the motion of electrons follows loops that depend on the applied magnetic field, but not exactly the same way. For these materials, we define a cyclotron effective mass, <math>m^*</math> so that:

:<math>\omega = \frac{qB}{m^*}</math>.

== See also ==
* [[Ion cyclotron resonance]]
* [[Electron cyclotron resonance]]

== References ==
{{reflist}}

==External links==
* [http://www.wolframalpha.com/input/?i=cyclotron+frequency Calculate Cyclotron frequency with Wolfram Alpha]
[[Category:Condensed matter physics]]
[[Category:Electric and magnetic fields in matter]]
[[Category:Accelerator physics]]
[[Category:Scientific techniques]]
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