Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L40: Invited Session: New Horizons for Magnetism and Competing Phases in Heavy Fermions |
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Sponsoring Units: DCMP Chair: Paul Canfield, Ames Laboratory Room: Mile High Ballroom 2B-3B |
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L40.00001: Tuning magnetism by Kondo effect and frustration Invited Speaker: Hilbert v.L\"{o}hneysen Heavy-fermion systems are an ideal playground for studying the quantum phase transition (QPT) between paramagnetic and magnetically ordered ground states arising from the competition between Kondo and RKKY interactions [1]. Two different routes have been identified by various experiments, i. e., the more traditional spin-density-wave (SDW) [2] and the Kondo-breakdown [3] approaches. However, up to now an \textit{a-priori} assignment of a given system to these different routes has not been possible. Yet another route to quantum criticality not included in the above approaches might be geometric frustration of magnetic moments, a route well known for insulating magnets with competing interactions [4]. First experiments on metallic systems have recently been conducted. In the canonical partially frustrated antiferromagnetic system CePd$_{\mathrm{1-x}}$Ni$_{\mathrm{x}}$Al, the N\'{e}el temperature $T_{\mathrm{N}}(x)$ decreases, with $T_{\mathrm{N}}\to $ 0 at the critical concentration $x_{c}\approx $ 0.144. The low-temperature specific heat $C(T)$ evolves toward $C$/$T\alpha $ ln($T_{\mathrm{0}}$/$T)$ for $x\to x_{c}$ [5]. The unusual $T$ dependence of $C$/$T$ is compatible with the Hertz-Millis-Moriya (HMM) scenario of quantum criticality [2] if the quantum-critical fluctuations are two-dimensional in nature. Here two-dimensionality might arise from antiferromagnetic planes that are effectively decoupled by the frustrated Ce atoms in between. An exciting possibility is that the planes of frustrated Ce moments form a two-dimensional spin liquid. In the prototypical heavy-fermion system CeCu$_{\mathrm{6-x}}$Au$_{\mathrm{x}}$ the experiments by Schr\"{o}der et al.[6] provided the initial evidence of local quantum criticality. While concentration and pressure tuning of the quantum phase transition (QPT) are described by this scenario, magnetic-field tuning the QPT is in line with the SDW scenario [7]. Elastic neutron scattering experiments on CeCu$_{\mathrm{5.5}}$Au$_{\mathrm{0.5}}$ under hydrostatic pressure $p$ [8] show that at $p=$ 8 kbar, $T_{\mathrm{N}}$ and the magnetic propagation vector attain almost the values of CeCu$_{\mathrm{5.7}}$Au$_{\mathrm{0.3}}$. This $x-p$ analogy away from the QPT is highly remarkable since the ambient-pressure magnetic structures for $x=$ 0.3 and 0.5 are quite different. These results give clues to a general ($x$,$p$,$B)$ phase diagram at $T=$ 0 and might explain the existence of different universality classes. \\[4pt] [1] H. v. L\"{o}hneysen et al., Rev. Mod. Phys. \textbf{79}, 1015 (2007).\\[0pt] [2] J. A. Hertz, Phys. Rev. B \textbf{14}, 1165 (1976); A. J. Millis, Phys. Rev. B \textbf{48}, 7113 (1993); T. Moriya and T. Takamoto, J. Phys. Soc. Jpn. \textbf{64}, 960 (1995).\\[0pt] [3] Q. Si et al., Nature \textbf{413}, 804 (2001).\\[0pt] [4] B. Keimer and S. Sachdev, Physics Today \textbf{64} (2), 29 (2011).\\[0pt] [5] V. Fritsch et al., arXive 1301.6062, submitted for publication (2013).\\[0pt] [6] A. Schr\"{o}der et al., Nature \textbf{407}, 351-355 (2000).\\[0pt] [7] O. Stockert et al., Phys. Rev. Lett. \textbf{99}, 237203 (2007).\\[0pt] [8] A. Hamann et al., Phys. Rev. Lett. \textbf{110}, 096404 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L40.00002: Magnetic field tuned quantum criticality of heavy fermion system YbPtBi Invited Speaker: Eundeok Mun Quantum criticality triggers an emergence of new quantum phase of matters due to the critical behavior of quantum fluctuations. Heavy fermion (HF) compounds have provided the cleanest evidence for the quantum phase transition. The face-centered cubic YbPtBi is one of the few frustrated stoichiometric Yb-based HF compounds. Measurements of magnetic field and temperature dependent resistivity, specific heat, thermal expansion, Hall effect, and thermoelectric power indicate that the antiferromagnetic (AFM) order ($T_{\mathrm{N}}$ $\sim$ 0.4 K) can be suppressed by applied magnetic field of $H_{\mathrm{c}}$ $\sim$ 4 kOe. In the $H$-$T$ phase diagram of YbPtBi, three regimes of its low temperature states emerges: (I) AFM state, characterized by spin density wave (SDW) like feature, which can be suppressed to $T =$ 0 by the relatively small magnetic field of $H_{\mathrm{c}}$ $\sim$ 4 kOe, (II) field induced anomalous state in which the electrical resistivity follows $\rho (T)$ $\sim$ $T^{1.5}$ between $H_{\mathrm{c}}$ and $\sim$ 8 kOe, and (III) Fermi liquid (FL) state in which $\rho (T)$ $\sim$ $T^{2}$ for $H$ \textgreater 8 kOe. Regions I and II are separated at $T =$ 0 by what appears to be a quantum critical point. Whereas region III appears to be a FL associated with the hybridized 4$f$ states of Yb, region II may be a manifestation of a spin liquid state. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L40.00003: Scaling of the magnetic Gr\"{u}neisen ratio near quantum critical point Invited Speaker: Yoshi Tokiwa The magnetic Gr\"{u}neisen ratio $\Gamma_{\mathrm{H}}=$(1/T)dT/dH is the most sensitive probe of quantum criticality. Its divergence signals the underlying instability. We have studied quantum criticality in the frustrated Kondo lattice system YbAgGe and the heavy fermion superconductor CeCoIn$_{\mathrm{5}}$ by high-precision magnetocaloric effect measurements. In the former, NFL behavior appears around a metamagnetic spin-flop transition between two symmetry broken phases. Previously, it was unclear how the two ordered phases are related to the NFL state. Here, we propose a novel quantum bicritical point (QBCP) scenario, which is distinct from either quantum critical end point or ordinary QCPs with single symmetry broken phase. The observed scaling behavior of $\Gamma_{\mathrm{H}}$ and its characteristic asymmetry across the critical field are consistent with a QBCP scenario. We also report a possible violation of Wiedemann-Franz law at the QBCP in YbAgGe. In CeCoIn$_{\mathrm{5}}$ indications of a quantum critical field hidden inside the superconducting (SC) phase have been extensively debated. We show $\Gamma_{\mathrm{H}}$ data and scaling analysis in the normal state, which surprisingly suggests a zero-field QCP. Anomalous behaviors of $\Gamma_{\mathrm{H}}$ and specific heat within the SC state further support this conclusion.\\[4pt] Work done in collaboration with Markus Garst, Institute for Theoretical Physics, University of Cologne; Jinkui Dong, I. Physical Institute, University of Goettingen; Sergey Bud'ko, Ames Laboratory; Eric Bauer; Los Alamos National Laboratory; Paul Canfield, Ames Laboratory; and Philipp Gegenwart, I. Physical Institute, University of Goettingen. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:24AM |
L40.00004: Quantum Critical Behavior in Heavy-Fermion Iron Oxypnictide Ce(Ru$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})$PO Invited Speaker: Kenji Ishida Quantum phase transition in itinerant ferromagnets is one of the major topics in a strongly correlated electron system, since it has been suggested to be always first order when the ferromagnetic (FM) order is suppressed by pressure or chemical doping [1]. In order to obtain universal features of the FM quantum criticality, we have studied the two-dimensional heavy-fermion (HF) system Ce(Ru$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})$PO from microscopic $^{31}$P-NMR measurements [2-4]. A HF ferromagnet CeRuPO turns into a HF paramagnet by an isovalent Fe substitution for Ru. We found that Ce(Ru$_{0.15}$Fe$_{0.85})$PO shows critical fluctuations down to $\sim$ 0.3 K, as well as the continuous suppression of Curie temperature and the ordered moments by the Fe substitution. These experimental results suggest the presence of a FM quantum critical point (QCP) at around x $=$ 0.86, which is a rare example among itinerant ferromagnets. In addition, we point out that the critical behaviors in Ce(Ru$_{0.15}$Fe$_{0.85})$PO share a similarity with those in YbRh$_{2}$Si$_{2}$ [5], where the local criticality of f electrons has been discussed [6]. We reveal that Ce(Ru$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})$PO is a new system to study FM quantum criticality in HF compound. \\[4pt] [1] D. Belitz, T. R. Kirkpatrick, and J. Rollb\"uhler, Phys. Rev. Lett. \textbf{94}, 247205 (2005).\\[4pt] [2] S. Kitagawa\textit{ et al.}, Phys. Rev. Lett. \textbf{107,} 277002 (2011).\\[0pt] [3] S. Kitagawa \textit{et al.}, K. Ishida, T. Nakamura, M. Matoba, and Y. Kamihara, Phys. Rev. Lett. \textbf{109}, 227004 (2012).\\[0pt] [4] S. Kitagawa \textit{et al.}, J. Phys. Soc. Jpn. \textbf{82}, 033704 (2013). \\[0pt] [5] K. Ishida \textit{et al.}, Phys. Rev. Lett. \textbf{89}, 107202 (2002).\\[0pt] [6] Q. Si, S. Rabello, K. Ingersent, and J. L. Smith, Nature (London) \textbf{413}, 804 (2001). [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 11:00AM |
L40.00005: Tricritical point and wing structure in the phase diagram of UGe$_2$ Invited Speaker: Valentin Taufour Among the numerous reports on quantum criticality, studies on ferromagnets are less common than studies on antiferromagnetic compounds. This is surprising since the paramagnetic to ferromagnetic transition is a textbook example of second order transition and there are several examples where the ferromagnetic transition can be tuned to zero temperature by applied pressure, chemical doping or magnetic field. However, it seems that the transition becomes first order at a tricritical point before being fully suppressed, changing the quantum critical point to a first order quantum phase transition. I will present the case of the superconducting ferromagnet UGe$_2$. In this material, we experimentally located the tricritical point in the temperature-pressure phase diagram. By applying magnetic field, the critical end point, which corresponds to the tricritical point at zero field, can be located leading to a wing-structure in the temperature-pressure-magnetic field phase diagram. The suppression of the critical end point to zero temperature leads to a new kind of quantum criticality: a quantum critical end point. The case of UGe$_2$ will be compared with other ferromagnets, in particular LaCr$_{1-x}$V$_x$Ge$_3$. The work on UGe$_2$ was performed at CEA Grenoble, France with D. Aoki, G. Knebel, H. Kotegawa, L. Malone, I. Sheikin and J. Flouquet. The work on other compounds is performed at my present institution Ames Laboratory, Iowa State University, Ames, Iowa, U.S.A. with U. Kaluarachchi, X. Lin, S. K. Kim, S. L Bud'ko and P. C. Canfield supported by AFOSR-MURI grant FA9550-09-1-0603. \\[4pt] [1] Tricritical point and wing structure in the itinerant ferromagnet UGe$_2$ V. Taufour, D. Aoki, G. Knebel, and J. Flouquet Physical Review Letters 105, 217201 (2010) \\[0pt] [2] Evolution toward Quantum Critical End Point in UGe$_2$ H. Kotegawa, V. Taufour, D. Aoki, G. Knebel, and J. Flouquet Journal of the Physical Society of Japan Vol. 80, No. 8, 083703 (2011) \\[0pt] [3] Ferromagnetic Quantum Critical Endpoint in UCoAl D. Aoki, T. Combier, V. Taufour, T. D. Matsuda, G. Knebel, H. Kotegawa, and J. Flouquet Journal of the Physical Society of Japan Vol. 80, No. 9, 094711 (2011) \\[0pt] [4] Suppression of ferromagnetism in the LaV$_x$Cr$_{1-x}$Ge$_3$ system Lin, Xiao and Taufour, Valentin and Bud'ko, Sergey L. and Canfield, Paul C. Phys. Rev. B 88 094405 (2013) [Preview Abstract] |
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