Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session X4: Quantum Order in Chiral Magnets |
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Sponsoring Units: DCMP Chair: Chandra Varma, University of California, Riverside Room: Colorado Convention Center Korbel 2B-3B |
Friday, March 9, 2007 8:00AM - 8:36AM |
X4.00001: Quantum order in chiral magnets: 3D Non-Fermi Liquid Phase and Blue Quantum Fog in MnSi Invited Speaker: The discovery of a distinct change from Fermi liquid to non-Fermi liquid resistivity and the observation of partial magnetic order in MnSi under high pressure [1,2] has generated great scientific interest in the properties of itinerant-electron systems with weak chiral spin-orbit interactions. Recent theoretical predictions include the spontaneous formation of a skyrmion phase at the boundary of conventional helical order [3] and the existence of a new type of Goldstone-like excitation, so called helimagnons [4]. New experimental work using sophisticated neutron scattering techniques and bulk properties exploring the question of skyrmion textures and helimagnon excitations, as well as studies of the thermal expansion under pressure using a newly developed ultra-high resolution neutron spin-resonance technique (Larmor diffraction) will be reviewed. \newline \newline [1] C. Pfleiderer, S. R. Julian, G. G. Lonzarich, Nature {\bf 414}, 427 (2001). \newline [2] C. Pfleiderer, et al., Nature {\bf 427}, 227 (2004). \newline [3] U. R{\"o}{\ss}ler, A. B. Bogdanov, C. Pfleiderer, Nature {\bf 442}, 797 (2006). \newline [4] D. Belitz, T. R. Kirkpatrick, A. Rosch, Phys. Rev. B {\bf 73}, 054431 (2006). [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 9:12AM |
X4.00002: Real Space Observation of Helical Spin Order Invited Speaker: When a symmetry gets spontaneously broken in a phase transition, topological defects are routinely formed. There are numerous examples of topological defects in condensed matter systems, such as, vortices in superconductors, vortices in superfluid helium, monopoles and strings in liquid crystals, etc. A similar picture would emerge in helimagnets. It is therefore interesting to deepen our understanding of how, what kind of, and why magnetic defects form and how they evolve after formation in helimagnets. In recent years, there have been significant advances in the experiment [1] and in the theories [2] of phases and textures in helimagnets. This will have a significant impact on our understanding of not only the puzzling behavior of the helimagnet MnSi with non-Fermi-liquid transport properties [3], but also phase transitions and phase diagrams in different condensed matter systems. \newpage In this paper, we describe the current status of our experiments. To see the helical spin order and magnetic defects in metal silicides such as (Fe, Co)Si and FeGe in real space, we used Lorentz electron microscopy, combined with the transport of intensity equation (TIE) analysis or holographic interference microscopy. This method has allowed us to find the topological defect similar to atomic dislocations in the crystal lattice. Furthermore, by applying magnetic fields, we directly observed the deformation processes of the helical spin order, accompanied by nucleation, movement, and annihilation of the magnetic defects. \newline \newline [1] M. Uchida \textit{et al.}, Science \textbf{311}, 359 (2006). \newline [2] U. K. R\"{o}{\ss}ler, A. N. Bogdanov, and C. Pfleiderer, Nature \textbf{442}, 797 (2006); B. Binz, A. Vishwanath, and V. Aji, Phys. Rev. Lett$.,$ \textbf{96}, 207202 (2006); S. Tewari, D. Belitz, and T. R. Kirkpatric, Phys. Rev. Lett., \textbf{96}, 47207 (2006). \newline [3] C. Pfleiderer \textit{et al}., Nature \textbf{427}, 227 (2004). [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:48AM |
X4.00003: Theory of the helical spin crystals Invited Speaker: Recent experiments in the ``partial order'' regime at high pressure in MnSi quite intriguingly suggest diffuse spin correlations and slow dynamics in a pure crystalline metal. As a starting point for a theoretical description of this phase, we are investigating the nature of its dominant spin correlations. Particularly, the observed location of maximal neutron scattering intensity around $\langle 110\rangle$ is difficult to explain in terms of fluctuating helical spin-density waves alone. We therefore investigate helical spin crystals. These are magnetic structures obtained by superimposing distinct spin spirals, via a process reminiscent of crystallization. Based on a phenomenological Landau description, we identify which spin crystal structures may be energetically stabilized and study their properties. One of these states, a bcc spin crystal, is compatible with existing data on MnSi from neutron scattering and magnetic field studies. It also shows new and interesting phenomena, such as symmetry stabilized topological textures, missing higher order Bragg reflections and an octupolar order parameter. Possible routes towards ``partial order,'' which requires the destruction of long-range order by some mechanism, will be briefly discussed. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:24AM |
X4.00004: Investigation of the Metallic State in Cubic FeGe beyond its Quantum Phase Transition Invited Speaker: FeGe and MnSi are prominent examples where the Dzyaloshinskii-Moriya interaction causes a modulation of the ferromagnetic structure as a consequence of the lack of inversion symmetry in the $B20$ structure (space group $P2_13 $). In FeGe, helimagnetism sets in through a first order phase transition at $T_{\rm C}=280\,$K with a saturated moment of $m=1 \mu_B$ per Fe atom. The helical modulation has a period of about $700\,$\AA\ and propagates along the spiral propagation vector ${\bf k} \parallel [1 0 0]$. It alters its direction to ${\bf k}\parallel [1 1 1]$ at $T_2\approx 211-245\,$K without a change in the period. In MnSi, however, the helical order occurs below $T_{\rm C}=29\,$K. The modulation has a wavelength of $175\,$\AA\ and the ordered moments of about $m=0.4\,\mu_B$ per Mn atom are perpendicular to ${\bf k}\parallel [1 1 1]$. It is well established that the second order phase transition is driven first order for a sufficiently weak magnetic interaction close to the critical pressure, $p_{\rm c}=1.46\,$GPa. In light of these structural and magnetic similarities between FeGe and MnSi, a volume compression in FeGe could tune its $T_C$ to zero temperature with the chance to reveal peculiar electronic ground state properties at the verge of the magnetic order. Indeed, the electrical resistivity measurements, $\rho(T) $, show a suppression of the helical order at $p_c\approx 19 \,$GPa. The strong deviations from a Fermi-liquid behavior in a wide pressure range above $p_c$ suggest that the suppression of $T_C$ disagrees with the standard notion of a quantum critical phase transition. Our band-structure calculations suggest that disorder due to zero-point motion is strong enough to close the narrow gap expected for compressed FeGe, stabilizing a new magnetic ground state above $p_c$. An anomaly observed at $T_X$ in the $\rho(T)$ curves recorded above $p_c$ might be related to this magnetic phase. The isothermal structural data at low temperature revealed a discontinuous change in the pressure dependence of the shortest Fe-Ge interatomic distance close to the $T_C(p)$ phase line. The $(T,V)$ phase diagram will be discussed and the connection with MnSi and the semiconducting properties of FeSi will be addressed. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 11:00AM |
X4.00005: Quantum PhaseTransitions and Exotic Phases in Metallic Helimagnets Invited Speaker: I will review some of the current theoretical understanding of the exotic properties of chiral magnets, in particular the metallic helimagnet MnSi. In the ordered phase, a helical Goldstone mode leads to corrections to Fermi-liquid behavior, and to a non-Fermi liquid single-particle relaxation rate [1]. On the phase boundary, a tricritical point pushes the quantum critical point to a nonzero external magnetic field, where the quantum critical behavior has been determined exactly [2]. In the disordered phase, an analogy with chiral liquid crystals suggests a first-order transition from a chiral liquid to a chiral gas as an explanation for neutron scattering data [3]. The observed non-Fermi-liquid transport behavior in the disordered phase [4] remains an open problem. \medskip\par\noindent [1] D. Belitz, T.R. Kirkpatrick, and A. Rosch, Phys. Rev. B {\bf 73}, 054431 (2006); Phys. Rev. B {\bf 74}, 024409 (2006). \smallskip\par\noindent [2] D. Belitz, T.R. Kirkpatrick, and J. Rollb{\"u}hler, Phys. Rev. Lett. {\bf 94}, 027205 (2005). \smallskip\par\noindent [3] S. Tewari, D. Belitz, and T.R. Kirkpatrick, Phys. Rev. Lett. {\bf 96}, 047207 (2006). \smallskip\par\noindent [4] C. Pfleiderer, S.R. Julian, and G.G. Lonzarich, Nature {\bf 414}, 427 (2004). [Preview Abstract] |
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