Millielectronvolt

Millielectronvolt (meV) is a unit of energy equal to one thousandth of an electron volt. By definition, 1 eV is 1.602176634 × 10^-19 joule, so 1 meV equals 1.602176634 × 10^-22 joule. In terms of temperature, using Boltzmann's constant (k_B ≈ 8.617333262 × 10^-5 eV/K), 1 meV corresponds to about 11.6 kelvin; thus 25 meV is roughly room temperature (about 290 K).

The meV scale is used to describe low-energy phenomena in physics. It is common in condensed matter physics, nanoscale systems, and quantum devices, where energy gaps, excitation energies, and thermal energies are on the meV scale. Examples include the superconducting gap in many materials (often a few meV), phonon energies in crystals, magnon and spin-excitation energies in magnets, and level spacings in quantum dots or two-dimensional electron systems. Measurements in this range rely on techniques such as tunneling spectroscopy, infrared and terahertz spectroscopy, and low-temperature transport experiments.

Conversions and notes of context are useful for intuition. A photon with energy 1 meV corresponds to a wavelength of about 1.24 millimeters (in the far-infrared to microwave range). Because meV energies are small compared with typical chemical-bond energies (eV scale) but large on the scale of thermal fluctuations at cryogenic temperatures, the meV unit is especially convenient for describing quantum and thermal phenomena in solid-state systems.