= Solution
In a low-carrier-density semiconductor use a thin, cold sample in a transmission spectrometer. Direct microwave or far-infrared radiation through it, commonly with propagation parallel to $\mathbf B$ and electric field transverse to it. Sweep $B$ at fixed frequency and detect transmitted power. The active <circular polarization> shows an absorption peak, or transmission dip, near $B_{\rm res}=m^*\omega/e$. Resolving it requires a long enough scattering time, $\omega_c\tau\gtrsim1$.
A metal's large <plasma frequency> usually prevents that radiation from sampling its bulk directly. In <surface cyclotron resonance in a metal>, put $\mathbf B$ approximately parallel to a clean metal surface and excite an rf electric field in the <skin depth>, transverse to $\mathbf B$. Electron orbits extend into the bulk and repeatedly return to that layer. Coherent energy absorption occurs when the rf phase repeats on a return, including harmonics $\omega=n\omega_c$. Measure cavity loss, reflected power, or surface impedance rather than attempting ordinary bulk transmission. The characteristic regime has <cyclotron radius> larger than the <skin depth> and sufficiently specular, long-lived orbits.
For the spherical parabolic band the metal harmonic fields are
$$
\boxed{B_n=\frac{m^*\omega}{ne},\qquad n=1,2,\ldots.}
$$
They are evenly spaced in $1/B$, with amplitudes and widths determined by surface geometry and scattering. A semiconductor generally gives the simple bulk fundamental, whereas the metal can show this harmonic series in a surface-loss signal. For an anisotropic real <Fermi surface> the measured mass is the appropriate <cyclotron effective mass> rather than necessarily every component of a transport effective-mass tensor. These classical resonance harmonics are distinct from the Landau-level <quantum oscillations> in <6/c>.
\Image[/past-exam-of-the-condensed-matter-physics-major-option-course-of-the-university-of-oxford/2015-semiconductor-and-metal-cyclotron-resonance.png]
{title=Bulk transmission and surface cyclotron-resonance measurements}
{description=Original schematic arrangements and illustrative field-scan signals. The semiconductor exhibits a bulk transmission dip; a metal is probed through skin-layer losses with harmonic resonance positions. Signal shapes are schematic rather than experimental data.}
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