  We present new observations of the faint "Sirius-like" companion discovered
to orbit HD 114174. Previous attempts to image HD 114174 B at mid-infrared
wavelengths using NIRC2 at Keck have resulted in a non-detection. Our new
L'-band observations taken with the Large Binocular Telescope and LMIRCam
recover the companion ($\Delta L$ = 10.15 $\pm$ 0.15 mag, $\rho$ = 0.675''
$\pm$ 0.016'') with a high signal-to-noise ratio (10 $\sigma$). This
measurement represents the deepest L' high-contrast imaging detection at
sub-arcsecond separations to date, including extrasolar planets. We confirm
that HD 114174 B has near-infrared colors consistent with the interpretation of
a cool white dwarf ($J-L'$ = 0.76 $\pm$ 0.19 mag, $K-L'$ = 0.64 $\pm$ 0.20).
New model fits to the object's spectral energy distribution indicate a
temperature $T_{\rm eff}$ = 4260 $\pm$ 360 K, surface gravity log g = 7.94
$\pm$ 0.03, a cooling age t$_{c} \approx$ 7.8 Gyr, and mass $M$ = 0.54 $\pm$
0.01 $M_{\odot}$. We find that the cooling age given by theoretical atmospheric
models do not agree with the age of HD 114174 A derived from both
isochronological and gyrochronological analyses. We speculate on possible
scenarios to explain the apparent age discrepancy between the primary and
secondary. HD 114174 B is a nearby benchmark white dwarf that will ultimately
enable a dynamical mass estimate through continued Doppler and astrometric
monitoring. Efforts to characterize its physical properties in detail will test
theoretical atmospheric models and improve our understanding of white dwarf
evolution, cooling, and progenitor masses.
