  M dwarfs are prominent targets of planet search projects, and their chemical
composition is crucial to understanding the formation process or interior of
orbiting exoplanets. However, measurements of elemental abundances of M dwarfs
have been limited due to difficulties in the analysis of their optical spectra.
We conducted a detailed chemical analysis of five M dwarfs (T_eff ~3200--3800
K), which form binary systems with G/K-type stars, by performing a line-by-line
analysis based on high-resolution (R ~80,000) near-infrared (960--1710 nm)
spectra obtained with CARMENES. We determined the chemical abundances of eight
elements (Na, Mg, K, Ca, Ti, Cr, Mn, and Fe), which are in agreement with those
of the primary stars within measurement errors (~0.2 dex). Through the analysis
process, we investigated the unique behavior of atomic lines in a cool
atmosphere. Most atomic lines are sensitive to changes in abundance not only of
the corresponding elements but also of other elements, especially dominant
electron donors such as Na and Ca. The Ti I lines show a negative correlation
with the overall metallicity at T_eff < 3400 K due to the consumption of
neutral titanium by the formation of TiO molecules. These findings indicate
that to correctly estimate the overall metallicity or the abundance of any
element, we need to determine the abundances of other individual elements
consistently.
