the other end lie those with little, perhaps negligi- ble, magnetism: electron-doped ZrNCl and HfNCl, and electron-doped TiNCl. There are several other, lower- T$_{c}$ systems, whose behavior seems different still (hydrated Na$_{x}$CoO$_{2}$, Li$_{1}$$_{−}$$_{x}$NbO$_{2}$, and several transition-metal disulfides and diselenides). A common feature of most of these systems is that they are 2D and have a small, but not tiny, con- centration of charge carriers, often in the range of 0.05–0.15 carriers per unit cell. These materials also have ionic character. NaAlSi differs in that it has sp carriers—the others have carriers in d bands—and is self-doped, a compensated semimetal. We suggest that a useful view of NaAlSi is that it be regarded as arising from an underlying ionic semiconductor, but that it has a small negative gap rather than a true gap. Without the overlap of the valence and conduc- tion bands, it would be a 2D, ionic, and somewhat covalent semiconductor like the aforementioned ni- tridochloride compounds, which superconduct in the 15–25 K range. Comparing the characteristics of these two classes of superconductors should further the understanding of 2D superconductivity. This work was supported by DOE grant DE- FG02-04ER46111, the Strategic Sciences Aca- demic Alliance Program under grant DE-FG03- 03NA00071, and by DOE SciDAC Grant No. DE- FC02-06ER25794. VI. ACKNOWLEDGMENTS 1 Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J. Am Chem. Soc. 130 , 3296 (2008). 2 S. Yamanaka, H. Kawaji, K. Hotehama, and M. Ohashi, Adv. Mater. 8 , 771 (1996). 3 S. Shamoto, T. Kato, Y. Ono, Y. Miyazaki, K. Ohoyama, M. Ohashi, Y. Yamaguchi, and T. Ka- jitani, Physica C 306 , 7 (1998). 4 S. Yamanaka, K. Hotehama, and H. Kawaji, Nature 392 , 580 (1998). 5 S. Shamoto, K. Iizawa, M. Yamada, K. Ohoyama, Y. Yamaguchi, and T. Kajitani, J. Phys. Chem. Solids 60 , 1431 (1999). 6 S. Yamanaka, T. Yasunaga, K. Yamaguchi, and M. Tagawa, J. Mater. Chem. 19 , 2573 (2009). 7 S. Kuroiwa, H. Kawashima, H. Kinoshita, H. Okabe, and J. Akimitsu, Physica C 466 , 11 (2007). 8 E. Bustarret, C. Marcenat, P. Achatz, J. Kaˇcmarˇcik, F. L´evy, A. Huxley, L. Ort´ega, E. Bourgeois, X. Blase, D. D´ebarre, and J. Boulmer, Nature 444 , 465 (2006). 9 L. F. Mattheiss, E. M. Gyorgy, and D. W. Johnson, Jr., Phys. Rev. B 37 , 3745 (1988). 10 S. Jin, T. H. Tiefel, R. C. Sherwood, A. P. Ramirez, E. M. Gyorgy, G. W. Kammlott, and R. A. Fastnacht, Appl. Phys. Lett. 53 , 1116 (1988). 11 E. A. Ekimov, V. A. Sidorov, E. D. Bauer, N. N. Mel’nik, N. J. Curro, J. D. Thompson, and S. M. Stishov, Nature 428 , 542 (2004). 12 H. Rosner, A. Kitaigorodsky, and W. E. Pickett, Phys. Rev. Lett. 88 , 127001 (2002). 13 H. Sagayama, Y. Wakabayashi, H. Sawa, T. Kamiyama, A. Hoshikawa, S. Harjo, K. Uozato, A. K. Ghosh, M. Tokunaga, and T. Tamegai, J. Phys. Soc. Jpn. 75 , 043713 (2006). 14 S. Kuroiwa, H. Sagayama, T. Kakiuchi, H. Sawa, Y. Noda, and J. Akimitsu, Phys. Rev. B 74 , 014517 (2006). 15 I. R. Shein, N. I. Medvedeva, and A. L. Ivanovskii, J. Phys.: Condens. Matter 15 , L541 (2003). 16 G. Q. Huang, L. F. Chen, M. Liu, and D. Y. Xing, 8 Phys. Rev. B 69 , 064509 (2004). 17 I. I. Mazin and D. A. Papaconstantopoulos, Phys. Rev. B 69 , 180512 (2004). 18 M. Giantomassi, L. Boeri, and G. B. Bachelet, Phys. Rev. B 72 , 224512 (2005). 19 R. Heid, K. -P.Bohnen, B. Renker, P. Adelmann, T. Wolf, D. Ernst, and H. Schober, J. Low Temp. Phys. 147 , 375 (2007). 20 S. Kuroiwa, A. Q. R. Baron, T. Muranaka, R. Heid, K. -P. Bohnen, and J. Akimitsu, Phys. Rev. B 77 , 140503(R) (2008). 21 K. Koepernik and H. Eschrig, Phys. Rev. B 59 , 1743 (1999). 22 J. P. Perdew and Y. Wang, Phys. Rev. B 45 , 13244 (1992). 23 W. Westerhaus and H. U. Schuster, Z. Naturforsch. 34b , 352 (1979). 24 K. Momma and F. Izumi, J. Appl. Cryst. 41 , 653 (2008). 25 A. B. Kyker and W. E. Pickett, Phys. REv. B 71 , 224517 (2005). 26 H. Rosner, R. Weht, M. D. Johannes, W. E. Pickett, and E. Tosatti, Phys. Rev. Lett. 88 , 027001 (2002). 27 D. J. Singh, Phys. Rev. B 78 , 094511 (2008). 28 I. I. Mazin, D. J. Singh, M. D. Johannes, and M. H. Du, Phys. Rev. Lett 101 , 057003 (2008). 29 A. N. Yaresko, G.-Q. Liu, V. N. Antonov, and O. K. Andersen, Phys. Rev. B 79 , 144421 (2009). 30 F. S. Khan and P. B. Allen, Phys. Rev. Lett. 29 , 3341 (1984). 31 J. F. Janak, Phys. Rev. B 16 , 255 (1977). 32 H. Rietschel and L. J. Sham, Phys. Rev. B 28 , 5100 (1983). 33 M. Grabowski and L. J. Sham, Phys. Rev. B 29 , 6132 (1984). 34 A. Bill, H. Morawitz, and V. Z. Kresin, Phys. Rev. B 66 , 100501 (2002). 35 A. Bill, H. Morawitz, and V. Z. Kresin, Phys. Rev. B 68 , 144519 (2003).