Cr 1Àx Al x exhibits semiconducting behavior for x ¼ 0:15-0:26. This Letter uses hard x-ray photoemission spectroscopy and density functional theory to further understand the semiconducting behavior. Photoemission measurements of an epitaxial Cr 0:80 Al 0:20 thin film show several features in the valence band region, including a gap at the Fermi energy (E F ) for which the valence band edge is 95 AE 14 meV below E F . Theory agrees well with the valence band measurements, and shows an incomplete gap at E F due to the hole band at M shifting almost below E F . DOI: 10.1103/PhysRevLett.105.236404 PACS numbers: 71.20.Lp, 71.23.Àk, 71.28.+d, 79.60.Bm Intermetallic compounds containing transition metals and sp elements often form a gap at the Fermi energy (E F ) due to hybridization. This gap can be exploited for applications, making these compounds the subject of intense study. For example, intermetallic semiconductors are attractive for thermoelectric devices due to their typically small gaps and large Seebeck coefficients (e.g., ZrNiSn) [1]. In magnetic compounds, the gap is asymmetric with spin; if a gap occurs at E F for one spin but not the other, the result is a half-metal (e.g., Co 2 MnAl) [2]. Half-metals are important for spintronics circuits such as spin transistors and nonvolatile logic.Compounds of the form A 2 BD or A 3 D, where A and B are transition metals and D is an sp element, typically crystallize in the ternary (L2 1 ) or binary (D0 3 ) full-Heusler structures. These compounds are usually ferro-or ferrimagnetic, with the magnetic moment well predicted by a Slater-Pauling counting scheme: M ¼ Z À 24, where Z is the total number of valence electrons in the unit cell [3]. For Z ¼ 24, there is no net magnetization and a gap in both the majority and minority spin density of states (DOS), resulting in a semiconducting gap (e.g., pseudogap in Fe 2 VAl) [4]. This tunability and predictability of parameters with Z makes Heusler compounds an attractive class of materials to work with.Cr 3 AlðCr 1Àx Al x with x % 0:25Þ is an exception to this scheme. According to the Slater-Pauling counting scheme Z ¼ 21 so it should be a metal with M ¼ À3. Instead, Cr 1Àx Al x is found to be antiferromagnetic for x ¼ 0-0:50. Cr is an antiferromagnet with a spin-density wave (SDW) incommensurate with the lattice. The addition of Al causes the SDW to become commensurate, i.e., a simple antiferromagnetic structure, for x ! 0:03. For x ¼ 0:15-0:26, the Cr magnetic moment reaches 1 B with a high Néel temperature of about 800 K [5].In the same range of x, Cr 1Àx Al x displays semiconducting behavior not yet adequately explained by theory [6]. The gap has been estimated to be between 6 and 60 meV, making Cr 1Àx Al x a narrow-gap semiconductor [7][8][9].In this Letter, we probe the electronic structure of Cr 0:80 Al 0:20 through hard x-ray photoemission spectroscopy (HXPS) and density functional theory (DFT). Hard x rays are advantageous over soft x rays due to enhanced bulk sensitivity and relatively enhanced photoionization ...
Antiferromagnetism and chemical ordering have both been previously suggested as causes of the observed semiconductorlike behavior in Cr 3 Al. Two films of Cr 3 Al(001)/MgO(001) were grown under different conditions to achieve different types of chemical ordering and electronic properties: one X-phase structure (semiconducting) and one C11 b structure (metallic). The films were investigated by x-ray and neutron diffraction. Both films show commensurate antiferromagnetic order, with a high Néel temperature greater than 578 K, showing that the antiferromagnetism in Cr 3 Al is quite robust. Density-functional theory calculations were performed and it was shown that the well-known antiferromagnetic pseudogap in the density of states occurs for all types of chemical ordering considered. The conclusion of these studies is that the antiferromagnetism causes a pseudogap in the density of states, which is a necessary condition for the semiconductorlike transport behavior; however, that antiferromagnetism is seen in both metallic and semiconducting Cr 3 Al samples shows that antiferromagnetism is not a sufficient condition for semiconducting behavior. Chemical ordering is equally important.
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