Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session L5: Competing Ground-States and Novel Excitations in Strongly Correlated Metals |
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Sponsoring Units: DCMP Chair: Andrea Bianchi, Université de Montréal Room: 401/402 |
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L5.00001: Quantum criticality in a cubic heavy fermion cage compound Invited Speaker: Matter at the absolute zero in temperature may reach a highly exotic state: Where two distinctly different ground states are separated by a second order phase transition the system is far from being frozen; it is undecided in which state to be and therefore undergoes strong collective quantum fluctuations. Quantum criticality describes these fluctuations, their extension to finite temperatures, and the resulting unconventional physical properties. Heavy fermion compounds have been much investigated in the past few years as model systems. An important recent finding is that in the tetragonal compound YbRh$_2$Si$_2$ [1] a new energy scale vanishes at the quantum critical point and is in addition to the second-order phase transition scale that governs the behavior of conventional quantum critical points [2,3]. New theoretical scenarios can account for this finding if 2-dimensional spin fluctuations are assumed [4]. Here similar behaviour of the new heavy fermion compound Ce$_3$Pd$_{20}$Si$_6$ [5] is discussed in which the cubic crystal structure and the highly symmetric local environment of the Ce atoms in molecular ``cages'' makes 2-dimensional spin fluctuations rather unlikely.\\[4pt] [1] For a review see Gegenwart et al., Nat. Phys. 4, 186 (2008).\\[0pt] [2] Paschen et al., Nature 432, 881 (2004).\\[0pt] [3] Gegenwart et al., Science 316, 90 (2007).\\[0pt] [4] Si et al., Nature 413, 804 (2001).\\[0pt] [5] Paschen et al., J. Magn. Magn. Mater 316, 90 (2007) and Refs. herein. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:42PM |
L5.00002: A New Route to Quantum Criticality in Yb$_{3}$Pt$_{4}$ Invited Speaker: The vanishing of magnetic order at a quantum critical point (QCP) is a central feature of virtually all classes of correlated electron systems, and may be accompanied by unconventional ordered states such as superconductivity, and by anomalous critical scattering. Some of the most detailed studies have focused on f-electron heavy electron compounds, and here the picture has emerged that magnetic order requires the formation of moments, provided by the divergence of the quasiparticle mass at the QCP. We combine specific heat, magnetization, and electrical resistivity measurements on the new compound Yb$_{3}$Pt$_{4}$ to argue that alternative routes to quantum criticality are also possible. The weakly first order antiferromagnetic transition in Yb$_{3}$Pt$_{4}$ can be tuned by field to a critical end point, which is extended to a quantum critical point at 1.62 T. Both the ordered and paramagnetic phases are Fermi liquids at low temperatures, but the quasiparticle mass does not diverge at the QCP. Instead, a divergence of the zero temperature susceptibility and the quasiparticle scattering is observed, controlled by a zero field fixed point and not the nearby QCP. We argue that Yb$_{3}$Pt$_{4} $ is the first example of a heavy electron systems where magnetic order occurs at the QCP due to increasingly strong quasiparticle interactions, much as is found in $^{3}$He, Stoner ferromagnets, and spin density wave systems. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 4:18PM |
L5.00003: Magnetism and the Fermi surface in heavy fermion metals Invited Speaker: With a plethora of different phases and quantum critical points, heavy fermion materials should reign supreme as the prototype for competing order, a major contemporary theme in condensed matter physics. One key feature that differentiates the types of magnetic phases/critical points is the presence or absence of Kondo screening. This singlet formation is dramatically manifested in the Fermi surface, which provides important experimental insight into the problem. The size of the Fermi surface therefore becomes an important issue. To provide a theoretical basis for the different types of magnetism, we have recently carried out asymptotically exact studies of the Kondo lattice model inside both the antiferromagnetic [1] and ferromagnetic [2] phases. A fundamental aspect of the approach is to map the magnetic Hamiltonian for the local f-moments onto a quantum nonlinear sigma model (QNLsM). The Kondo interaction results in an effective coupling between the QNLsM fields and the conduction electrons. Renormalization group analyses show that the Fermi surface in the corresponding ordered states is small (not incorporating the f-moments) for both the ferromagnetic and antiferromagnetic cases. These results are of relevance to a number of materials, including YbRh2Si2 and CeRu2Ge2, where experimental measurements of magnetotransport and de Haas van Alphen effects [3,4] have provided evidence for the small Fermi surface phases. The implications of our results for the heavy fermion quantum critical points will also be discussed.\\[4pt] [1] S. J. Yamamoto and Q. Si, PRL 99, 016401 (2007); Physica B 403, 1414 (2008); \\[0pt] [2] S. J. Yamamoto and Q. Si, to be published;\\[0pt] [3] S. Paschen et al, Nature 432, 881 (2004); H. Shishido et al, JPSJ 74, 1103 (2005);\\[0pt] [4] C. A. King and G. G. Lonzarich, Physica B 171, 161 (1991). [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:54PM |
L5.00004: How Do Heavy Fermions Get Polarized And Die? Invited Speaker: In paramagnetic heavy fermion systems the f-spins dissolve into Kondo singlets and reappear within the Fermi volume, producing a ``large" Fermi surface populated by heavy quasiparticles. According to theory, when a very large magnetic field is applied to such a system the Kondo singlets are broken and the fully polarized bare f-spins vanish from the Fermi volume, leaving behind a ``small" Fermi surface populated by light quasiparticles. How the system passes from the low-field to the high-field limit is not clear. This talk will discuss recent transport and de Haas van Alphen studies of the archetypal heavy fermion systems $\rm CeRu_2Si_2$ [1] and $\rm YbRh_2Si_2$ [2], which are interpreted as showing that the f-electron disappears from the Fermi volume via two successive Lifshitz transitions: in the first transition a majority spin band sinks below the Fermi level, while in the second a new minority spin band appears at the Fermi level. While this interpretation is in accord with recent theoretical work of Kusminskiy et al. [3], it could be criticized on the grounds that only the first of the two postulated Lifshitz transitions have so far been observed.\\[4pt] References:\\[0pt] [1] R. Daou, C. Bergemann and S.R. Julian, ``Continuous evolution of the Fermi surface of $\rm CeRu_2Si_2$ across the metamagnetic transition,'' Physical Review Letters {\bf 96} (2006) 026401.\\[0pt] [2] P.M.C. Rourke, A. McCollam, G. Lapertot, G. Knebel, J. Flouquet and S.R. Julian, ``Magnetic field dependence of the $\rm YbRh_2Si_2$ Fermi surface,'' arXiv:0807.3970; accepted, Physical Review Letters.\\[0pt] [3] S. Viola-Kusminskiy, K.S.D. Beach, A.H. Castro Neto and D.K. Campbell, ``Mean-field study of the heavy fermion metamagnetic transition,'' Physical Review B {\bf 77} (2008) 094419. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:30PM |
L5.00005: Electron spin resonance in Kondo systems Invited Speaker: Well-defined electron spin resonance (ESR) lines have been detected recently in several heavy fermion compounds, in which ferromagnetic correlations appear to be present [1]. We first discuss [2] the theory of ESR for the Kondo impurity system at temperatures T$<<$T$_{K}$ (Kondo temperature), where the local spin ESR line has a width of order T$_{K}$ and is therefore unobservably broad. By contrast, in the Anderson lattice system in the Kondo regime the ESR linewidth is narrow, and gets broadened by spin lattice relaxation and quasiparticle interaction processes. We show [2] that the spin lattice induced ESR linewidth is greatly reduced by an effective mass factor. The quasiparticle induced linewidth is small in the Fermi liquid regime, proportional to max(T$^{2}$,B$^{2})$ (T=temperature, B=Zeeman energy). The total ESR linewidth is reduced by exchange narrowing induced by a ferromagnetic exchange interaction. This explains the available ESR data. \\[4pt] [1] C. Krellner et al., Phys. Rev. Lett. \textbf{100}, 066401 (2008) \\[0pt] [2] E. Abrahams and P. Woelfle, Phys. Rev. B\textbf{78}, 104423 (2008) [Preview Abstract] |
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