We report on the focalization of Bogoliubov-de Gennes excitations of the nonlinear Schrödinger equation in the defocusing regime (Gross-Pitaevskii equation for repulsive Bose-Einstein condensates) with a spatially modulated periodic potential. Exploiting the modification of the dispersion relation induced by the modulation, we demonstrate the existence of localized structures of the Bogoliubov-de Gennes excitations, in both the linear and nonlinear regimes (linear and nonlinear "bullets"). These traveling Bogoliubov-de Gennes bullets, localized both spatially and temporally in the comoving reference frame, are robust and propagate remaining stable, without spreading or filamentation. The phenomena reported in this Letter could be observed in atomic Bose-Einstein condensates in the presence of a spatially periodic potential induced by an optical lattice. DOI: 10.1103/PhysRevLett.118.044103 The nonlinear Schrödinger equation (NLSE) [5], and Bose-Einstein condensates. In the latter system, the NLSE is known as the Gross-Pitaevskii equation [6]. The NLSE, despite its apparent simplicity, supports a variety of fascinating nonlinear structures like solitons, breathers, recurrences, etc., hence being an attractive and rich model both in the description of natural phenomena and in technology-oriented research areas.In particular, the localized solutions of the NLSE and its generalizations have attracted considerable attention, and a huge amount of the scientific literature has been devoted to characterizing their existence and stability in various spatial dimensions.Written in a normalized form, the NLSE, for the temporal evolution of the field amplitude Aðr; tÞ defined in spacer and evolving in time t, readswhere c is the nonlinearity coefficient and ∇ 2 the Laplace operator.It is well known that the one-dimensional NLSE supports solitons in the strict mathematical sense, for focusing nonlinearity (c > 0). Zakharov and Shabat showed that a bright soliton solution exists [7]: In this case, the effects of nonlinear focusing and diffractive spreading are counterbalanced, and spatially localized nonlinear modes are stable. Bright solitons of the NLSE have been observed in nonlinear optics [8]. In fiber optics, solitons provide a powerful tool for optical communication systems [9].If a finite field background is present, coherent structures localized in space and in time, exhibiting periodic oscillations, the so-called breathers, have been predicted analytically [10][11][12] and observed experimentally [13]. Furthermore, new solutions on the finite background can be constructed by using refined mathematical techniques [14] showing the incredible richness of the NLSE.For c < 0, the NLSE is defocusing, and its homogeneous solution (the condensate) is stable. In this case, the NLSE supports the existence of vortices and dark or gray solitons [8] but not the bright soliton. Simply speaking, this is because both the diffraction and the nonlinearity tend to spread (defocus) the excitations over the homogeneous background. Ther...