background using data from Advanced LIGO’s second observing run'
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*Introduction*— A superposition of gravitational waves from many astrophysical and cosmological sources creates a stochastic gravitational-wave background (SGWB). Sources which may contribute to the  include compact binary coalescences [@2013MNRAS.431..882Z; @PhysRevD.84.124037; @PhysRevD.85.104024; @PhysRevD.84.084004; @2011ApJ...739...86Z; @rosado_stoch; @MarassiEA_2011; @ZhuEA_2013], core collapse supernovae [@Buonanno:2004tp; @Sandick:2006sm; @2009MNRAS.398..293M; @2010MNRAS.409L.132Z; @PhysRevD.72.084001; @PhysRevD.73.104024], neutron stars [@Ferrari:1998jf; @Regimbau:2001kx; @PhysRevD.87.063004; @2012PhRvD..86j4007R; @2011ApJ...729...59Z; @Rosado:2012bk; @2011MNRAS.411.2549M; @2011MNRAS.410.2123H; @WuEA_2013; @2004MNRAS.351.1237H], stellar core collapse [@2015PhRvD..92f3005C; @2017PhRvD..95f3015C], cosmic strings [@2005PhRvD..71f3510D; @1976JPhA....9.1387K; @2002PhLB..536..185S; @2007PhRvL..98k1101S; @O1_cosmic_strings], primordial black holes [@MandicEA_2016; @SasakiEA_2016; @Wang:2016ana], superradiance of axion clouds around black holes [@Brito:2017wnc; @Brito:2017zvb; @Fan:2017cfw; @Tsukada:2018mbp], and gravitational waves produced during inflation [@1994PhRvD..50.1157B; @1979JETPL..30..682S; @2007PhRvL..99v1301E; @2012PhRvD..85b3525B; @2012PhRvD..85b3534C; @Lopez:2013mqa; @1997PhRvD..55..435T; @2006JCAP...04..010E; @axion_inflation]. A particularly promising source is the  from compact binary coalescences, especially in light of the detections of one binary neutron star and ten binary black hole mergers [@gw170814; @gw170608; @gw170104; @gw151226; @gw150914; @o1rates; @gw170817; @gwtc-1] by the Advanced LIGO Detector, installed in the Laser Interferometer Gravitational-wave Observatory (LIGO) [@aLIGO_2015], and by Advanced Virgo [@aVirgo_2015] so far. Measurements of the rate of binary black hole and binary neutron star mergers imply that the  may be large enough to detect with the Advanced LIGO-Virgo detector network [@gw150914_stoch; @gw170817_stoch]. The stochastic background 