Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session P12: Skutterudites and Other Heavy Fermions |
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Sponsoring Units: GFC Chair: John Wei, University of Toronto Room: Morial Convention Center 203 |
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P12.00001: Metal-insulator and metal-metal behavior in filled-skutterudites Ling Yang, Shan-Wen Tsai Filled-skutterudite compounds are of the form $LT_4X_{12}$, where $L$ stands for Lanthanide, $T$ stands for transition metal and X stands for Pnictogen. They show many unusual properties, and are also important due to their thermoelectric properties. Here we focus on the metal-insulator and metal-metal behavior observed in $PrRu_4P_{12}$ and $PrOs_4P_{12}$ compounds. We propose a simple model where motion of the L atom inside the atomic cage made by the other atoms plays a crucial role. In this model, the hopping amplitude of the charge carriers depends on the positions of the L atoms in different minima inside the cage, and a phase transition occurs when the temperature is lowered. The nature of the transition depends on the density of carriers. [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P12.00002: Andreev Spectroscopy Study of Multigap Pairing in PrOs$_{4}$Sb$_{12}$ C.S. Turel, J.Y.T. Wei, B. Djurkovic, J.B. Kycia, W.M. Yuhasz, R. Baumbach, M.B. Maple The recent discovery of superconductivity in the filled skutterudite heavy fermion material PrOs$_{4}$Sb$_{12}$ has generated widespread interest. Different experimental studies have reported various field-vs-temperature phase diagrams, with mixed evidence for nodes in the pairing gap. Some experiments have also indicated the presence of multiple gaps, suggesting that the pairing involves either multiple bands or multiple order parameters [1,2]. We present Andreev spectroscopy data, down to 80 mK and up to 2.5 T, taken using ballistic point contacts made with Pt-Ir tips on single crystals of Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$. We observed distinct spectral evidence for gap nodes. We also observed multiple spectral features arising from Ru-doping. We interpret the evolution of these spectral features within the scenario of multigap pairing. \\ $[1]$ G. Seyfarth et al. Phys. Rev. Lett. \textbf{97}, 236403 (2006). \\ $[2]$ R.W. Hill et al. cond-mat/0709.4265. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P12.00003: Defects and broken time-reversal symmetry in superconducting {\boldmath Pr(Os,Ru)$_{4}$Sb$_{12}$} D.E. MacLaughlin, Lei Shu, A.D. Hillier, O.O. Bernal, Y. Aoki, D. Kikuchi, H. Sato, Y. Tunashima, H. Sugawara, T.A. Sayles, T. Yanagisawa, W.M. Yuhasz, M.B. Maple Muon spin relaxation studies of a spontaneous local field $H_\mu$, previously observed in the superconducting state of PrOs$_4$Sb$_{12}$ and attributed to broken time-reversal symmetry, have been extended to Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ alloys. In flux-grown single crystals $H_\mu$ is strongly suppressed but remains observable for $x \le 0.2$. In powder samples prepared by solid state reaction, however, no field is observed for $x = 0.1$ or 0.2. Muon spin relaxation due to dynamic $^{141}$Pr nuclear spin fluctuations is also reduced in the powders. Both results can both be understood if the density of microscopic defects is smaller in the powders: defects increase muon spin relaxation by $^{141}$Pr spins, and supercurrents associated with defects are theoretically predicted to create spontaneous fields in TRS-breaking superconductors. Our results are strong experimental evidence for this prediction. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P12.00004: Multi-symmetry and Multi-band Superconductivity in Superconducting Filled-skutterudites. Robert W. Hill, Shiyan Li, M.B. Maple, Louis Taillefer Thermal conductivity measurements were performed on single crystal samples of the superconducting filled skutterudite compounds PrOs$_4$Sb$_{12}$ and PrRu$_4$Sb$_{12}$ both as a function of temperature and magnetic field applied perpendicular to the heat current. In zero magnetic field, the low temperature electronic thermal conductivity of PrRu$_4$Sb$_{12}$ is vanishingly small, consistent with a fully-gapped Fermi surface. For PrOs$_4$Sb$_ {12}$, however, we find clear evidence for residual electronic conduction as the temperature tends to zero Kelvin which is consistent with the presence of nodes in the superconducting energy gap. The field dependence of the electronic conductivity for both compounds shows a rapid rise immediately above H$_{c1}$ and significant structure over the entire vortex state. In the fully gapped superconductor PrRu$_4$Sb$_{12}$, this is interpreted in terms of multi-band effects. In PrOs$_4 $Sb$_{12}$, we consider the Doppler shift of nodal quasiparticles at low fields and multiband effects at higher fields. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P12.00005: High-field de Haas-van Alphen investigation of the filled skutterudite compound NdOs$_4$Sb$_{12}$ P.-C. Ho, J. Singleton, M.B. Maple, P. Goddard, T. Yanagisawa The filled skutterudite compound NdOs$_4$Sb$_{12}$ displays mean-field type ferromagnetism with a Curie temperature of 0.9~K. The apparent electronic specific heat coefficient $\gamma \sim$ 0.52~Jmol$^{-1}$K$^{-2}$ is very large. In order to search for possible heavy-fermion behavior, de Haas-van Alphen (dHvA) measurements were performed on NdOs$_4$Sb$_{12}$. Experiments for H//[100] show dHvA frequencies at 695$\pm$10, 950$\pm$10, and 2560$\pm$20 T with effective masses ranging from 1.8 to 2.9 $m_e$. Moreover, the angle dependence of dHvA frequencies is very similar to that of LaOs$_4$Sb$_{12}$. Both results suggests that heavy-fermion behavior is not primary responsible for the large $\gamma$. In this context, a theoretical model by Miyake et al. indicates that the tunneling motion of the rare earth ions between off-center sites may enhance the interaction between local phonons and neighboring conduction electrons in the filled skutterudites, resulting in a large $\gamma$. This may be relevant to the extra mode found at 15~K in ultrasound measurements on NdOs$_4$Sb$_{12}$, implying abnormal behavior of the phonon spectrum in this compound. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P12.00006: Non-Fermi liquid behavior in the filled skutterudite compound CeRu$_{4}$As$_{12}$ Ryan Baumbach, P.-C. Ho, T. Sayles, M.B. Maple, R. Wawryk, T. Cichorek, A. Pietraszko, Z. Henkie The filled skutterudite compounds of the form MT$_4$X$_{12}$ (M = alkali metal, alkaline earth, lanthanide, actinide, T = Fe, Ru, Os and X = P, As, Sb) exhibit a wealth of strongly correlated electron phenomena. The CeT$_4$X$_{12}$ subclass is interesting since it includes the only filled skutterudite known to show non - Fermi liquid (NFL) behavior, CeRu$_4$AS$_{12}$, in addition to various semiconductors where the gap size is correlated with the lattice constant. We present electrical resistivity $\rho$, specific heat C, and magnetic susceptibility $\chi$ measurements for the new compound, CeRu$_4$As$_{12}$, which reveal NFL T - dependences at low T, i.e., $\rho$(T) $\sim$ T$^{1.4}$ and weak power law or logarithmic divergences in C(T)/T and $\chi$(T). Measurements also show that the T - dependence of the thermoelectric power S(T) deviates from that in other Ce systems. The NFL behavior appears to be associated with a nonmagnetic or weakly magnetic ground state, as revealed by magnetization M(H,T) measurements. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P12.00007: Low-temperature thermal and transport properties of single-crystalline Ce$_{4}$Pt$_{12}$Sn$_{25}$ Nobuyuki Kurita, Han-Oh Lee, Yoshifumi Tokiwa, Eric Bauer, Joe Thompson, Zachary Fisk, Pei-chun Ho, M. Brian Maple, Roman Movshovich Low-temperature specific heat $C(T)$ and electrical resistivity \textit{$\rho $}($T)$ measurements have been performed on a flux-grown single-crystalline Ce$_{4}$Pt$_{12}$Sn$_{25 }$which has a body-centered cubic structure. As temperature decreased, $C(T)$ increased and showed a huge jump ($\Delta C$/$T\sim $43J/mole-K$^{2}$-Ce) at $T \quad \sim $ 0.2 K, probably due to a magnetic ordering. The entropy gain connected with the ordering reaches a half of $R$ln2 at the peak position and almost full $R$ln2 at 3 K, corresponding to the degeneracy of the fundamental crystal-field doublet. On the other hand, \textit{$\rho $}($T)$ showed metallic behavior and decreased rapidly around the ordering temperature with a clear kink. We will also discuss the results of external-field effect on this compound. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P12.00008: Magnetodynamics of Heavy Fermion YbCo$_{2}$Zn$_{20}$ and YbFe$_{2}$Zn$_{20}$ A.D. Christianson, E.A. Goremychkin, Michael M. Koza , J.L. Zarestky, C.H. Wang, A.I. Kolesnikov, N. Ni, S. Jia, E.D. Mun, S.L. Bud'ko, P.C. Canfield We have performed inelastic neutron scatting experiments on the recently discovered heavy Fermion systems YbCo$_{2}$Zn$_{20}$ and YbFe$_{2}$Zn$_{20}$. The magnetic excitation spectrum demonstrates that YbCo$_{2}$Zn$_{20}$ exhibits crystal field dynamics with an overall splitting 2.72 meV. Despite this, the splitting between the ground state and first excited state is large enough such that the large low temperature electronic specific heat is not due to crystal field entropy and hence YbCo$_{2}$Zn$_{20}$ is a truly heavy Fermion material. YbFe$_{2}$Zn$_{20}$ exhibits significantly different spin dynamics. At 200 K the scattering is quasielastic and as the temperature is lowered the spectral weight is progressively shifted to finite energy transfers. This temperature dependence is characteristic of an intermediate valence system with an unusually low Kondo temperature of $\sim $50 K. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P12.00009: High field de Haas-van Alphen measurements of RT$_{2}$Zn$_{20}$ (R=Yb and Lu,T=Fe, Co and Rh) N. Ni, S. Jia, N. Harrison , G.D. Samolyuk, S.L. Bud'ko, P.C. Canfield The de Haas--van Alphen (dHvA) effect in heavy fermion compounds YbFe$_{2}$Zn$_{20}$, YbCo$_{2}$Zn$_{20}$ and YbRh$_{2}$Zn$_{20}$ as well as the nonmagnetic compounds LuFe$_{2}$Zn$_{20}$, LuCo$_{2}$Zn$_{20}$ and LuRh$_{2}$Zn$_{20}$ have been observed in pulsed fields up to 55 T directed along [110] and in the temperature range 0.4K to 12K. The cyclotron effective masses of~YbT$_{2}$Zn$_{20}$ (T=Fe,Co and Rh) range from 1.8m$_{0}$ to 2.8m$_{0}$. Self-consistent tight binding linear muffin-tin orbital method in the atomic sphere approximation (TB-LMTO-ASA) has been used to construct Fermi surfaces of LuT$_{2}$Zn$_{20}$(T=Fe, Co and Rh). The calculated dHvA frequencies show good agreement with the experiments. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P12.00010: Induced Phase Separation in GdBaCo$_{2-x}$Fe$_{x}$O$_{5.5-\delta }$ by Fe Doping Yan-kun Tang, C.C. Almasan A detailed magnetic study has been carried out for the Fe doped GdBaCo$_{2-x}$Fe$_{x}$O$_{5.5-\delta }$ (x = 0, 0.02, 0.05, 0.10, and 0.20) cobaltites. Fe doping enhances the high temperature T ferromagnetism present in the Fe-free samples and it induces a second ferromagnetic order, which develops at a lower T. Also, the spin-state transition temperature shifts to higher T with increasing the Fe content. Phase separation in two magnetic phases, namely, Fe-rich and Fe-free regions, provides a phenomenological interpretation of these magnetic behaviors. The Fe-rich regions are responsible for the second ferromagnetic order present at lower T. The shift of the low temperature M(T) peak related to this second ferromagnetic order is due to the decrease in the oxygen content in the Fe-rich regions with increasing Fe doping and the presence of the antiferromagnetic Fe-Co superexchange interaction in these regions. The high temperature M(T) peak (corresponding to the Fe-free regions) and the spin-state transition temperature shift to higher temperatures with Fe doping because of the slight compression of the Fe-free regions with Fe doping. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P12.00011: Quantum Critical Phenomena in Ni$_{3}$Al$_{1-x}$Ga$_{x}$ Alloys M.H. Fang, J.H. Yang, Z.A. Xu, B. Chen, Y. Itoh, K. Yoshimura, Z.Q. Mao Considerable study has been devoted to quantum phase transitions (QPTs), which are believed to be a key concept for understanding the physics of strongly correlated electrons. In this talk we report on observation of quantum critical phenomena in Ni$_{3}$Al$_{1-x}$Ga$_{x}$ alloys. Ni$_{3}$Al is a ferromagnetic metal with $T_{c} =41.5K$. With Ga substitution for Al, $T_{c}$ and the spontaneous magnetic moment are gradually suppressed down to zero near the critical composition of $x_{c} \quad \sim $ 0.4. We found that near the critical composition the magnetization as a function of magnetic field M(H) and the magnetic susceptibility as a function of temperature \textit{$\chi $}($T)$ both obey the scaling laws theoretically expected for QPTs, i.e., $M(H) \quad \propto $ $H^{1/3}$ and \textit{$\chi $}$^{-1}\propto T^{4/3}$. In addition, we observed that near $x_{c}$ the derivative derived from the Arrott plots, i.e., \textit{$\gamma $=d(M}$^{2})/d(H/M)$ value$, $exhibits a remarkable peak at about 2-3 T. This peak enhances with decreasing temperature. In terms of a recent theory, we argue that \textit{$\gamma $} reflects characteristics of spin excitation spectrum near QPTs. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P12.00012: Physical properties of new ternary U compounds U$_{3}$Bi$_{4}$M$_{3}$ (M = Rh, Ni) Tomasz Klimczuk, Han-Oh Lee, Filip Ronning, Eric Bauer, Tomasz Durakiewicz, Heather Volz, Joe Thompson A common belief has been that only Sb and Sn can form the U$_{3}$X$_{4}$M$_{3}$ structure, but here we show that also Bi can stabilize the structure and two new U ternary compounds U$_{3}$Bi$_{4}$Ni$_{3}$ and U$_{3}$Bi$_{4}$Rh$_{3}$ can be grown as single crystals out of Bi flux. Both materials form in the same crystal structure as Ce$_{3}$Pt$_{3}$Bi$_{4}$, the well known Kondo insulator. Resistivity and photoemission spectroscopy measurements on U$_{3}$Bi$_{4}$Ni$_{3}$ indicate the presence of a charge gap, suggesting that it might be a Kondo insulator. However, the nonmagnetic reference compound Th$_{3}$Bi$_{4}$Ni$_{3}$ is a semiconductor, and, consequently, U$_{3}$Bi$_{4}$Ni$_{3}$ is rather a band insulator, similar to U$_{3}$Sb$_{4}$Ni$_{3}$. On the other hand, replacing Ni with Rh, which has one less electron than Ni, to form U$_{3}$Bi$_{4}$Rh$_{3}$ produces a metallic resistivity and modestly large electronic specific heat coefficient ($\gamma $(0.4K)=200 mJ/mol-U K$^{2}))$, characteristic of a Kondo-lattice system. Details of specific heat, magnetic susceptibility and electrical resistivity measurements on these single crystals will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P12.00013: Electronic and magnetic properties of single-crystalline UNi$_{0.5}$Sb$_{2}$. B. K. Davis, M. S. Torikachvili, E. D. Mun, J. C. Frederick, G. J. Miller, S. Thimmaiah, S. L. Bud'ko, P. C. Canfield, G. M. Schmiedeshoff We studied the electronic and magnetic properties of antiferromagnetic UNi$_{0.5}$Sb$_{2}$ (T$_{N} \quad \approx $ 161 K) by means of measurements of magnetic susceptibility, thermal expansion, and electrical resistivity ($\rho )$ at ambient pressure, and $\rho $ under hydrostatic pressures up to 20 kbar, in the temperature range from 1.9 to 300 K. The value of \textit{d$\rho $/dT} changes drastically from positive below $T_{N}$ to negative above it, reflecting the loss of spin-disorder scattering in the ordered phase. Two small features in the \textit{$\rho $ vs T} data centered near 40 and 85 K, which are quite hysteretic in temperature, correlate well in temperature with features in the magnetic susceptibility, and thermal expansion. The effect of pressure is to suppress the amplitude of the small features in \textit{$\rho $ vs T} at lower temperatures, and to raise $T_{N}$ at the rate of $\approx $ 0.76 K/kbar. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P12.00014: Tuning the ground state of CeNiGe$_{3}$ by applied magnetic field E.D. Mun, S.L. Bud'ko, P.C. Canfield We studied the thermal, magnetic and electrical transport properties of the ternary intermetallic system CeNiGe$_{3}$. In zero field, antiferromangeic order is observed below 5.1 K. In addition, at low temperatures, two pronounced metamagnetic transitions are also revealed. Applying magnetic fields along the magnetic easy direction leads to a suppression of the Neel temperature, with an apparent critical field $H_{c} \quad \sim $ 32.5 kOe. The $H-T$ phase diagram of CeNiGe$_{3}$ is consistent with a system that manifests field induced, quantum criticality. In the paramagnetic regime, above $H_{c}$ , the resistivity at low temperatures exhibits an unusual temperature dependence. These results make CeNiGe$_{3}$ appear to be most similar to the isostructural, YbNiSi$_{3}$ [1]. \newline [1] S. L. Bud'ko \textit{et al}, Phys. Rev. B 75, 094433(2007). [Preview Abstract] |
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