Anomalous Pinning Fields in Helimagnets Dietrich Belitz, University of Oregon Eugene, DMR 0529966

1. Anomalous Pinning Fields in HelimagnetsDietrich Belitz, University of Oregon Eugene, DMR 0529966 • In helical magnets, the magnetization follows a helical or spiral pattern. Examples: MnSi, FeGe. This leads to collective excitations called helimagnons. • Problem: • An external magnetic field B aligns the helix in the direction of B at a critical field H c1, and destroys the helix at a second critical field H c2. • Experimentally, the ratio • Δ = H c1/ H c2≈ 0.2 • in MnSi (see figure). • Theoretically, a standard Landau theory yields • Δ theoretical ≈ 0.01 • Resolution: • Exchange of helimagnons leads to an attractive interaction between electrons. Observed phase diagram of MnSi (after Mühlbauer et al 2009) • Screening of this interaction renormalizes the coefficients of the Landau theory • This affects H c1, but not H c2 • Δ derived from the renormalized theory agrees with the experimental value. Conclusion: Many-body renormalizations of the coefficients in a Landau theory are very important for a quantitative understanding of electronic systems.

2. Anomalous Pinning Fields in HelimagnetsDietrich Belitz, University of Oregon Eugene, DMR 0529966 Community Service: Education, and Development of Human Resources: • Qi Li, graduate student (PhD 2008) • Kuan-Yuet Ho, current graduate student • Ronojoy Saha, postdoc • Yan Sang, current graduate student • Lubo Zhou, graduate student (PhD 2006) Our collaborative research project supported by the two grants fosters close contact between our respective groups, with interactions between, and exchange of, students, postdocs, and visitors. Personnel supported, in full or in part, by the grants include • PI Kirkpatrick serves as Divisional Associate Editor for Physical Review Letters, and as an editor for the electronic journal, JSTAT. • PI Belitz serves as Associate Editor for Condensed Matter Theory for Reviews of Modern Physics.