Near-surface tricritical behavior of ${\mathrm{V}}_2\mathrm{H}\left(010\right)\mathrm{}$ at the ${\beta }_1$-${\beta }_2$ phase transition

We present results of an x-ray grazing-incidence diffraction experiment on a V${}_2$H(010) surface. The temperature dependence of the in-plane $(h/2\mathrm{}$ 0 -$h/2)$ superlattice reflections discloses the surface critical behavior across the ${\beta }_1$-${\beta }_2$ phase boundary. The influence of a nearby bulk tricritical point on the expected values for the critical exponents is briefly discussed. Using the depth sensitivity of evanescent scattering and by a proper subtraction of critical diffuse intensity, we determined the surface critical exponent ${\beta }_{\mathrm{surf}}.$ A remnant Bragg contribution above the critical temperature, however, complicates the evaluation of other surface exponents associated with the critical diffuse scattering. We speculate that the origin of this effect is associated with the surface oxide layer necessarily present on the (hydrogen-loaded) metallic crystal.