  We present results from NuSTAR and XMM-Newton observations of the new black
hole X-ray binary MAXI J1820+070 at low accretion rates (below 1% of the
Eddington luminosity). We detect a narrow Fe K$\alpha$ emission line, in
contrast to the broad and asymmetric Fe K$\alpha$ line profiles commonly
present in black hole binaries at high accretion rates. The narrow line, with
weak relativistic broadening, indicates that the Fe K$\alpha$ line is produced
at a large disk radius. Fitting with disk reflection models assuming standard
disk emissivity finds a large disk truncation radius (a few tens to a few
hundreds of gravitational radii, depending on the disk inclination). In
addition, we detect a quasi-periodic oscillation (QPO) varying in frequency
between $11.6\pm0.2$~mHz and $2.8\pm0.1$~mHz. The very low QPO frequencies
suggest a large size for the optically-thin Comptonization region according to
the Lense-Thirring precession model, supporting that the accretion disk recedes
from the ISCO and is replaced by advection-dominated accretion flow at low
accretion rates. We also discuss the possibility of an alternative accretion
geometry that the narrow Fe K$\alpha$ line is produced by a lamppost corona
with a large height illuminating the disk.
