  The topology of the Fermi surface significantly influences the transport
properties of a material. Firstly measured through quantum oscillation
experiments, the Fermi surfaces of crystals are now commonly characterized
using angle-resolved photoemission spectroscopy (ARPES), given the larger
information volume it provides. In the case of Weyl semimetals, ARPES has
proven remarkably successful in verifying the existence of the Weyl points and
the Fermi arcs, which define a Weyl Fermi surface. However, ARPES is limited in
resolution, leading to significant uncertainty when measuring relevant features
such as the distance between the Weyl points. While quantum oscillation
measurements offer higher resolution, they do not reveal insights into the
cross-sectional shape of a Fermi surface. Moreover, both techniques lack
critical information about transport, like the carriers mean free path. Here,
we report measurements unveiling the distinctive peanut-shaped cross-section of
the Fermi surface of Weyl fermions and accurately determine the separation
between Weyl points in the Weyl semimetal NbP. To surpass the resolution of
ARPES, we combine quantum oscillation measurements with transverse electron
focusing (TEF) experiments, conducted on microstructured single-crystals. The
TEF spectrum relates to the Fermi surface shape, while the frequency of the
quantum oscillations to its area. Together, these techniques offer
complementary information, enabling the reconstruction of the distinctive Weyl
Fermi surface geometry. Concurrently, we extract the electrical transport
properties of the bulk Weyl fermions. Our work showcases the integration of
quantum oscillations and transverse electron focusing in a singular experiment,
allowing for the measurements of complex Fermi surface geometries in
high-mobility quantum materials.
