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We show that the upper critical field in nanometer-sized Nb particles isgoverned by the changes in the effective Ginzburg-Landau coherence lengthoccurring due to two competing factors: (i) the decrease in the grain size andconsequent increase of disorder, and (ii) the effective decrease in the densityof states at the Fermi level due to the formation of a Kubo gap. As a result,the upper critical field (HC2) and irreversibility fields (Hirr) innanostructured Nb show non-monotonic grain size dependences. Between 60nm to20nm, HC2 is found to increase by 2.5times while there is no appreciabledecrease in the superconducting transition temperature (TC) from its bulk valueof 9.4K. This can be ascribed to a decrease in the coherence length due to areduction in the mean free path with decreasing size. Below 20 nm, however, HC2decreases with decreasing size. In this size range (<20 nm), there also occursa decrease in the TC as well as the superconducting energy gap. The decrease inHC2 in this regime can be ascribed to the decrease in the density of states atthe Fermi level due to a quantization in the electronic energy levels

Upper critical field in nanostructured Nb: Competing effects of the reduction in density of states and the mean free path