tokens: capabilities on large-aperture telescopes has revolutionized the study of high-redshift ( z ≳ 5.7) quasars (e.g., Fan et al. 2001 , 2003 , 2004 , 2006a ; Cool et al. 2006 ; Goto 2006 ; McGreer et al. 2006 ; Venemans et al. 2007 , 2015 ; Willott et al. 2007 , 2009 , 2010b ; Jiang et al. 2008 , 2009 , 2015 , 2016 ; Mortlock et al. 2009 , 2011 ; Morganson et al. 2012 ; Bañados et al. 2014 , 2016 ; Matsuoka et al. 2016 , 2018a , 2018b ; Reed et al. 2017 ; Tang et al. 2017 ). These earliest quasars, powered by supermassive black holes (SMBHs), not only constrain the physics of black hole (BH) accretion and host galaxy assembly at cosmic dawn but also provide critical information on the physical conditions of the intergalactic medium in the early universe (e.g., Fan et al. 2006a ). Since the first successful identification of z ≳ 6 quasars (Fan et al. 2001 ), the searches for these high- z quasars have matured and dramatically increased the inventory of these rare objects. There are now more than 250 quasars known at z > 5.6, with the most distant quasar reaching z ∼ 7.5 (Bañados et al. 2018b ). These impressive statistics now enable a transition from individual case studies to ensemble studies of high- z quasars. The vast majority of z ≳ 6 quasars are selected from wide-field optical+IR imaging with the dropout technique and are confirmed with optical spectroscopy (e.g., Fan et al. 2001 ). The confirmation optical spectroscopy provides limited information about these high- z quasars themselves, and to probe the physical properties of these systems (e.g., BH mass, spectral properties, broad and narrow absorption lines, etc.), near-IR spectroscopy is necessary to cover the rest-frame UV from C iv 1549 Å to Mg ii 2798 Å. Some earlier near-IR spectra of z ≳ 6 quasars already provided a glimpse of their physical properties, such as BH masses and Eddington ratios (e.g., Jiang et al. 2007 ; Kurk et al. 2007 ; Willott et al. 2010a ; De Rosa et al. 2014 ; Mazzucchelli et al. 2017 ; Kim et al. 2018 ) and chemical abundances in the broad-line region (e.g., Barth et al. 2003 ; Kurk et al. 2007 ; De Rosa et al. 2011 ). These measurements of BH properties also facilitate the study of the relations between BH growth and host galaxy assembly in the early universe, where the host galaxy properties are mostly inferred from the molecular gas emission in the millimeter regime (e.g., Walter et al. 2003 , 2009 ; Riechers et al. 2009 ; Wang et al. 2013 ; Venemans et al. 2016 ; Decarli et al. 2017 , 2018 ; Willott et al. 2017 ). However, near-IR spectroscopy of z ≳ 6 quasars is expensive, and thus only a small fraction of them have existing near-IR spectra, some of which are also of low quality (e.g., insufficient spectral coverage, signal-to-noise ratio, and spectral resolution). Given the importance of understanding the physical properties and growth of these high- z SMBHs, we have assembled a large sample of high-quality near-IR spectra for these objects, as
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