Calculations : The electronic structure of monolayer VSe2 was calculated within the density functional theory ( DFT ) . The DFT calculations were performed using the VASP package , utilizing the projector augmented phase wave ( PAW ) method , [ 49 ] and the Perdew Burke and Ernzerhof ( PBE ) exchange - correlation functional . [ 50 ] An on - site Coulomb interaction was included within the GGA+U approach using the Dudarev method , [ 51 ] as implemented in VASP . We chose a value − = 3 eV for the V 3d electrons . A separation of 15 Å between V layers in our supercell was found to be sufficient to represent an isolated monolayer . We   Hz , 50 mV ) shows a differential conductance dip around the Fermi level with a size of ~26 meV , due to a CDW order . Its inset shows a zoom - in of the gap feature .    Figure 2 . Synchrotron - PES data of pristine and ambient - exposed monolayer VSe2 . a , b ) V 2p and Se 3d core - levels of monolayer VSe2 . The V 2p3/2 and 2p1/2 peaks are located at 513.4 eV and 520.9 eV , respectively , thus indicating a spin - orbit splitting of 7.5 eV. The Se 3d5/2 and 3d3/2 peaks are positioned at 53.4 eV and 54.2 eV , respectively . c ) The valence band of the monolayer shows a sharp Fermi edge originating from the V 3dz 2 band . The spectral features between 1 and 5 eV are contributed by the Se 4p derived bands , while the broad bump beyond 6 eV is a substrate peak . d ) Work function of monolayer VSe2 extracted from the secondary electron cutoff , using a photon energy of 60 eV. The upper panel ( red spectra ) shows the effects of air - exposure . In particular , additional peaks , which are located at the higher BE side of the main peaks , are seen to develop at the V 2p and Se 3d core - levels . The sharp Fermi edge of the initial valence band has been reduced substantially , accompanied with a work function decrease of 0.25 eV. Figure 3 . V 3d 1 electronic configuration of 1T - VSe2 , and element - specific XAS and XMCD measurements of monolayer VSe2 . a ) Schematic diagram of the 3d electronic states of 1T - VSe2 in an octahedral crystal - field . The 3d - orbitals of the V 4 + ion are split into two sets of eg and t2 g orbitals , separated by an energy ∆oct = 10 Dq . b ) TEY detection geometry for the XAS and XMCD data shown in ( c ) . c ) Upper panel shows the XAS spectra of the monolayer measured at 30 K. Spectra highlighted in red and green are , respectively , the XAS obtained with opposite magnetic field ( ~±300 Oe ) directions , and their sum gives the total XAS marked in purple . Atomic multiplet structure , identified as a - e , provides direct proof of the d 1 configuration . Lower panel in ( c ) shows the XMCD signal that is within the experimental error , indicating no signs of a ferromagnetic coupling in the monolayer . This result is supported by measurements taken at 16 K and with a 1 T field ( Figure S5 , Supporting Information ) . VSe2 . An overlap between the field - cooling ( FC ) and zero - field - cooling ( ZFC ) curves down to 2 K and in a 7 T field , together with a broad maximum for the measured χ , a characteristic feature of low - dimensional magnetic systems with a short - range antiferromagnetic ( AF ) interaction , indicates the presence of spin frustration in monolayer VSe2 . Lower panel , a negative Weiss constant obtained from the Curie - Weiss ( CW ) fit to the high - temperature region of 1 / χ , suggesting an antiferromagnetic ( AF ) origin of the exchange interaction in monolayer VSe2 . d ) A triangular spin - lattice with AF coupling . In this geometry , three neighboring spins can not be mutually antialigned , thus leading to spin fluctuation and suppressed AF correlation .   Figure 5 . ARPES data of monolayer VSe2 . a ) Intensity map as a function of the surface momentum component k// as acquired at 300 K. b ) Zoom - in of ( a ) in the Fermi level BE range . c ) Experimental band dispersion extracted from ( b ) using a second derivative filter . b - f ) Same as ( a - c ) , with data acquired at 11 K. Zero energy represents the Fermi level position . Upon cooling , the V 3d and Se 4p bands are observed to shift towards higher BE . g ) These bands appear as a double - peak structure in the EDCs , which is not resolved at 300 K. The measured intensities of the EDCs have been normalized by the FD function , in order to highlight the effect of temperature on the spectral broadening . A CDW gap opening is evidenced by a leading - edge midpoint shift toward higher BE when cooling down to 11 K from 300 K.