  We use microwave-induced dynamic nuclear polarization (DNP) of the
substitutional nitrogen defects (P1 centers) in diamond to hyperpolarize bulk
$^{13}$C nuclei in both single crystal and powder samples at room temperature
at 3.34 T. The large ($>100$-fold) enhancements demonstrated correspond to a
greater than 10,000 fold improvement in terms of signal averaging of the 1\%
abundant $^{13}$C spins. The DNP was performed using low-power solid state
sources under static (non-spinning) conditions. The DNP spectrum (DNP
enhancement as a function of microwave frequency) of diamond powder shows
features that broadly correlate with the EPR spectrum. A well-defined negative
Overhauser peak and two solid effect peaks are observed for the central
($m_I=0$) manifold of the $^{14}$N spins. Previous low temperature measurements
in diamond had measured a positive Overhauser enhancement in this manifold.
Frequency-chirped millimeter-wave excitation of the electron spins is seen to
significantly improve the enhancements for the two outer nuclear spin manifolds
($m_I = \pm 1$) and to blur some of the sharper features associated with the
central manifolds. The outer lines are best fit using a combination of the
cross effect and a truncated cross effect -- which is known to mimic features
of an Overhauser effect. Similar features are also observed in experiments on
single crystal samples. The observation of all of these mechanisms in a single
material system under the same experimental conditions is likely due to the
significant heterogeneity of the high pressure, high temperature (HPHT) type Ib
diamond samples used. Large room temperature DNP enhancements at fields above a
few Tesla enable spectroscopic studies with better chemical shift resolution
under ambient conditions.
