Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session K45: Heavy Fermions: Experiment |
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Sponsoring Units: DCMP Chair: M. Aronson, University of Michigan Room: Baltimore Convention Center 348 |
Tuesday, March 14, 2006 2:30PM - 2:42PM |
K45.00001: Thermal and Magnetic Properties of Ferromagnetic YbPt$_{2}$ Moo Sung Kim, Marcus Bennett, Meigan Aronson, Jasmine Millican, Julia Chan We present the results of magnetization and heat capacity measurements carried out on needle-like single crystals of YbPt$_{2}$. Above 250 K, the magnetic susceptibility is well described by a Curie-Weiss expression with a moment of 3.35 $\mu_{B}$ per Yb and a Weiss temperature of 73 K, indicating ferromagnetic interactions among almost localized moments. At the lowest temperatures, the magnetization is increasingly nonlinear in field, approaching a saturation value of 3.5 $\mu_{B}$/Yb at 1.8 K. However, an Arrott plot analysis indicates that ferromagnetic order occurs at a much lower temperature, approximately 0.85 K. Above $\sim$10 K,we find that the heat capacity C=$\gamma$T+$\beta$T$^{3}$, with $\gamma$=0.358 J/mol-K$^{2}$, while between 10 K and 3 K, C/T drops by a factor of four, perhaps due to the opening of a gap at the Fermi surface. A large anomaly is found in the heat capacity at 0.83 K, suggesting that the transition to the ferromagnetic state is weakly first order. Our results imply that YbPt$_{2}$ is a rare example of a heavy fermion ferromagnet. [Preview Abstract] |
Tuesday, March 14, 2006 2:42PM - 2:54PM |
K45.00002: Magnetic Field Suppression of Ferromagnetism in Yb$_2$Pt$_3$ Marcus Bennett, Dmitry Sokolov, William Gannon, Meigan Aronson, Carl Henderson, Jasmine Millican, Julia chan We have measured the heat capacity, resistivity and magnetization of single crystals of Yb$_2$Pt$_3$ grown from Pb flux. The magnetization is anisotropic and strongly non linear with field, with an Arrott plot analysis indicating ferromagnetic order. The zero field AC susceptibility is peaked at 2.4 K, where the zero field heat capacity shows a first order paramagnetic to ferromagnetic transition. This transition is suppressed by magnetic field, disappearing at a critical point, 1.75 T and 1.3 K. The zero field resistivity shows a metallic temperature dependence, and for all fields we find $\rho(T,H) = \rho_0(H) + A(H)T^2$ at sufficiently low temperature. The coefficient, A(H), has a strong maximum for H=1.75 T. It is remarkable that the application of a magnetic field decreases T$_C$ in Yb$_2$Pt$_3$, contradicting conventional expectations. [Preview Abstract] |
Tuesday, March 14, 2006 2:54PM - 3:06PM |
K45.00003: Complete Elastic Constants of the Heavy Fermion YbAgGe I. Mihut, J. Betts, S. Harrington, A. Migliori, S. L. Bud'ko, P. C. Canfield We measured the elastic constants of the heavy fermion YbAgGe using resonant ultrasound spectroscopy at temperatures from 300K to 0.5K and magnetic fields to 7T. As a function of magnetic field and below 7K, the elastic constants show strong non-monotonic behavior, suggesting a field-induced quantum critical point. Only at the lowest magnetic fields is there a strong temperature dependence. [Preview Abstract] |
Tuesday, March 14, 2006 3:06PM - 3:18PM |
K45.00004: Studies of the Non-Fermi Liquid Kondo System $Lu_{1-x} Yb_x Rh_2 Si_2 $. Samuel MaQuilon, Long Pham, Han-oh Lee, Peter Klavins, Zach Fisk Single Crystalline Platelets of the $YbRh_2 Si_2 $ system show strong magnetic anisotropy. Specific Heat, Magnetic Susceptibility and Electrical Resistivity data are presented for the alloy system$Lu_{1-x} Yb_x Rh_2 Si_2 $across the entire composition range. The data suggests that the effective Kondo scale for $YbRh_2 Si_2 $is smaller than that for the dilute alloys of $Yb$ in $LuRh_2 Si_2 $. This work was supported by NSF DMR-0433560. [Preview Abstract] |
Tuesday, March 14, 2006 3:18PM - 3:30PM |
K45.00005: Quantum critical behaviour of heavy Fermion CeNi2Ge2 Bilal Zoghbi, Almut Schroeder, Collin Broholms Neutron scattering data of CeNi2Ge2 collected at SPINS at NIST will be presented to characterize the magnetic correlations and dynamics close to an antiferromagnetic quantum critical point (AF QCP). The dynamical susceptibility $\chi $''(q,E) has been measured in a temperature range 0.1K$<$T$<$30K. While the q-independent fluctuations remain unchanged for T$<$30K, the enhanced susceptibility $\Delta \chi $'' close to the wavevector q=(0.5,0.5,0), shows a relaxation rate following the absolute temperature kBT down to 5K but then remains finite towards the lowest T = 0.1 K. The Lorentzian linewidth is reduced to about 0.4 meV, a factor of ten smaller than the rate observed in the q- independent spectrum. The energy, q, and T dependence of $\Delta \chi $'' shows characteristics of the fluctuations expected close to an AF QCP in 3 dimensions, stating that CeNi2Ge2 lies beside the AF QCP. [Preview Abstract] |
Tuesday, March 14, 2006 3:30PM - 3:42PM |
K45.00006: Time-resolved optical spectroscopy of the itinerant antiferromagnets UMGa$_{5}$ (M=Ni, Pt) Ee Min Elbert Chia, Hae Ja Lee, Eric Bauer, Namjung Hur, Richard Averitt, Antoinette Taylor, John Sarrao We present time-resolved optical conductivity measurements of the itinerant antiferromagnets UNiGa$_{5}$ ($T_{N}$=85K) and UPtGa$_{5}$ ($T_{N}$=25K), as well as the parent material UGa$_{3}$, using a pump-probe technique. The relaxation time $\tau$ diverges near $T_{N}$, which we attribute to the opening of a spin gap. $\tau$ also diverges at the lowest temperatures, which is similar to that shown by the heavy fermion YbAgCu$_{4}$, but with no blocking of electron-phonon scattering within the DOS peak. The transient amplitude exhibits a sign change at $T_{N}$, whose temperature dependence is also consistent with the appearance of a spin gap. We will also attempt to analyze our data using the Rothwarf-Taylor model. [Preview Abstract] |
Tuesday, March 14, 2006 3:42PM - 3:54PM |
K45.00007: Ferromagnetism and Crystalline Electric Field Effects in Cubic UX$_2$Zn$_{20}$ (X=Co, Rh, Ir) E.D. Bauer, F. Ronning, A. Silhanek, N. Harrison, J.D. Thompson, J.L. Sarrao, R. Movshovich, M.F. Hundley, M. Jaime, E. Daniel, C.H. Booth The properties of a new class of cubic UX$_2$Zn$_{20}$ (X=Co, Rh, Ir) heavy fermion compounds have been investigated by means of magnetic susceptibility, specific heat, electrical resistivity, and x-ray absorption spectroscopy. Both UCo$_2$Zn$_{20}$ and URh$_2$Zn$_{20}$ show peaks in $C(T)$ and $\chi(T)$ at $\sim$5-10 K suggesting the presence of crystalline electric field (CEF) effects in these materials, i.e., a localized $5f^2 $ configuration of uranium. In addition, measurements in high magnetic fields up to 40 T are consistent with a CEF model of a nonmagnetic ground state and a magnetic first excited state separated by $\sim$ 20 K. In contrast, UIr$_2$Zn$_{20}$ exhibits a first-order ferromagnetic transition at T$_c$=2.75 K with a saturation moment $\mu_{sat}$=0.5 $\mu_B$ in the ferromagnetic state. All compounds in this series are heavy fermion materials with enhanced electronic specific heat coefficients $\gamma \sim$ 150-300 mJ/molK$^2$. The physical properties of UX$_2$Zn$_{20}$ (X=Co, Rh, Ir) will be discussed. [Preview Abstract] |
Tuesday, March 14, 2006 3:54PM - 4:06PM |
K45.00008: NMR Probes Two Kinds of HO Effects in URu$_2$Si$_2$ Single Crystals O.O. Bernal, J. Seunghun, A.P. Reyes, P.L. Kuhns, D.E. MacLaughlin, J.A. Mydosh, T.J. Gortenmulder We performed $^{29}$Si NMR experiments in two single crystals
of URu$_2$Si$_2$ for 4~$ |
Tuesday, March 14, 2006 4:06PM - 4:18PM |
K45.00009: Ordered states of URu$_2$Si$_2$ under hydrostatic pressure J. R. Jeffries, N. P. Butch, J. Paglione, M. B. Maple Oriented single crystalline samples of URu$_2$Si$_2$ have been synthesized and investigated under nearly hydrostatic pressure via electrical resistivity and ac susceptibility measurements. The heavy fermion compound URu$_2$Si$_2$ exhibits three distinct ordered states as a function of temperature and pressure: ``hidden order'' (HO), in which the order parameter has yet to be determined; antiferromagnetism (AFM), which seemingly develops out of the HO state at $P \leq 15$ kbar; and superconductivity (SC), which exists at ambient pressure. Careful measurements of the HO transition, occurring at $T_{HO} \sim 16.5$ K at ambient pressure, have been performed at several different pressures, yielding a $P$-$T$ phase diagram for the HO state as a function of pressure. In addition, measurements of the superconducting critical temperature, $T_c \sim 1.4$ K at ambient pressure, and the upper critical field $H_{c2}$ have been performed at low temperatures using a $^3$He-$^4$He dilution refrigerator, revealing the superconducting $H$-$T$-$P$ phase diagram. Possible correlations between the HO, AFM, and SC states will be discussed. This research was supported by the National Nuclear Security Administration under the Stewardship Science Academic Alliances program through DOE Research Grant No. DE-FG52-03NA00068. [Preview Abstract] |
Tuesday, March 14, 2006 4:18PM - 4:30PM |
K45.00010: Hall Effect Evolution Across a Field-Induced Quantum Critical Point in U$($Ru$_{0.96}$Rh$_{0.04}$$)$$_{2}$Si$_{2}$ Yoon Seok Oh, Kee Hoon Kim, N. Harrison, P. A. Sharma, H. Amitsuka, J. A. Mydosh The heavy fermion compound U$($Ru$_{0.96}$Rh$_{0.04}$$)$$_{2} $Si$_{2}$ has been recently identified as a unique system in which a magnetic field-induced quantum critical point $($QCP$)$ is avoided by the creation of a single magnetic phase [1]. To further understand the nature of phase formation across the putative QCP, we have measured the Hall effect of U$($Ru$_{0.96}$Rh$_{0.04}$$)$$_{2}$Si$_{2}$ between 0.6 and 30K using a 50T mid-pulse magnet. We find that the Hall coefficient $R_{\rm H}$ abruptly increases inside the new phase formed between 28 and 38T, around $B_{\rm QCP}$ $\sim$ 34T, evidencing formation of a gap in the Fermi surface. Furthermore, low temperature $R_{\rm H}$ at $B$ $>$ 38T is much smaller than that at $B$ $<$ 28T, indicating the different Fermi surface area in each Fermi liquid states. While the field-induced phase disappeared above 9.5K, $R_{\rm H}$ still exhibits a broad maximum near $B_{\rm QCP}$, signaling a finite temperature crossover of the Fermi surface change. All of above findings are consistent with an electronic structure evolution from a heavy fermion to spin-polarized Fermi liquids across $B_{\rm QCP}$. [1] K. H. Kim ${\it et~al.} $, Phys. Rev. Lett. ${\bf 93}$, 206402 $($2004$)$ [Preview Abstract] |
Tuesday, March 14, 2006 4:30PM - 4:42PM |
K45.00011: Irreversible dynamics of the phase boundary in U(Ru$_{1-x}$Rh$_x$)$_2$Si$_2$ Victor Fanelli, A.V. Silhanek, M. Jaime, N. Harrison, C.D. Batista, H. Amitsuka, L. Balicas, J.L. Sarrao, J.A. Mydosh Specific heat and magneto-caloric effect measuments on the systems: U(Ru$_{1-x}$Rh$_x$)$_2$Si$_2$ and Yb$_{1-x}$Y$_x$In Cu$_4$ were carried out at high magnetic fields. Thermodynamic similarities are found at the phase boundary into the magnetic field-induced phase (phase II) of the former and at the valence transition of the latter. We show that the irreversible properties of the transition into the phase II of U(Ru$_{1-x} $Rh$_x$)$_2$Si$_2$ are very similar to that associated with the valence transition of Yb$_{1-x}$Y$_x$In Cu$_4$. To explain these results, we propose a bootstrap mechanism by which lattice degrees of freedom within phase II become coupled to the 5f-electron hybridization, giving raise to a valence change at the transition. [Preview Abstract] |
Tuesday, March 14, 2006 4:42PM - 4:54PM |
K45.00012: Neutron scattering studies of itinerancy and ``hidden order'' in URu$_{2}$Si$_{2}$ C.R. Wiebe, J.A. Janik, G.J. MacDougall, G.M. Luke, J.D. Garrett, Y. Qiu, J.R.D. Copley, C. Broholm, W.J.L. Buyers The heavy fermion superconductor URu$_{2}$Si$_{2}$ has been of interest for the last two decades but the nature of the ``hidden order'' in the phase below T$_{0}$ = 17.5 K has not been identified. The large specific heat jump at this temperature cannot be accounted for by the weak ordered moment which is only a fraction of a Bohr magneton on the uranium site (0.03 $\mu _{B})$. In recent neutron scattering experiments we set new limits for the maximum size of an ordered moment which could be observed in the ``hidden order'' phase, and we investigated the possibility of orbital currents. Furthermore, our experiments have unveiled a new feature in the inelastic spectrum -- the precursor phase above 17.5 K exhibits incommensurate fluctuations that are highly correlated in Q and extend to high energies. This suggests that (1) the spins are itinerant, rather than localized and (2) these excitations must play a role in the heat capacity anomaly at the phase transition, as they become gapped at incommensurate wave-vectors below T$_{0}$ and cover a large region of phase space. Implications of these new measurements will be discussed. [Preview Abstract] |
Tuesday, March 14, 2006 4:54PM - 5:06PM |
K45.00013: Region of Quantum Critical Behavior in Ce(Ru$_{1-x}$Rh$_{x})_{2}$Si$_{2}$ G.R. Stewart, J.S. Kim, D.J. Mixson, D. Burnette The magnetic phase diagram of Ce(Ru$_{1-x}$Rh$_{x})_{2}$Si$_{2}$ is quite rich, with a spin density ordering temperature approaching T=0 around x$_{QCP }$= 0.4 and with - from the Rh rich side where T$_{N}$ of CeRh$_{2}$Si$_{2}$ is 36 K - a long range, local moment ordering temperature approaching T=0 at approximately x$_{QCP}$ = 0.6. Specific heat data down to 0.04 K for a number of compositions indicate a \textit{region} of non-Fermi liquid behavior between x=0.4 and 0.6 associated with the two quantum critical points. The nature of the nFl temperature dependence in the specific heat changes from Moriya-like weak spin fluctuation behavior at the Ru rich side to strong fluctuation, C/T $\propto $logT behavior at the Rh rich end. How this smooth progression of behavior with increasing x provides insights into our understanding of non-Fermi liquid behavior in general will be discussed. The disagreement in the literature over whether C/T is Moriya-like or follows logT at x=0.5 is resolved in favor of the former temperature dependence. [Preview Abstract] |
Tuesday, March 14, 2006 5:06PM - 5:18PM |
K45.00014: Magnetic ordering and valence instability in Kondo system CeMn$_{2-x}$Cu$_{x}$Si$_{2}$ Gan Liang, Shelley Keith, Jesse Vernon, F. Yen The transition from a 3d-antiferromagnetically ordered mixed valence system to a Kondo lattice system has been studied for the CeMn$_{2-x}$Cu$_{x}$Si$_{2 }$(0 $\le \quad x \quad \le $ 1) series. The Ce L$_{3}$-edge x-ray absorption result shows that the series evolves from a Ce mixed valence system at $x$ = 0 to a nearly trivalent system at $x$ = 2. The resistivity results show that in the low Ce valence region ($x$ greater than 1.0), the system exhibits a crystalline-field modified Kondo lattice behavior. In the high valence region, however, the system displays a behavior of the prototype mixed valence compound CePd$_{3}$. Magnetization (M) data were taken in both zero-field-cooled (ZFC) and field cooled (FC) processes. It is found that for 0 $\le \quad x \quad <$ 0.4, Neel temperature decreases rapidly with the increase of the Cu concentration $x$. In the range of $x$ from 0.4 $\le $ x $<$ 0.8, ferromagnetic phase is observed below 150 K, and both the ordering temperature $T_{c}$ and Qurie-Weiss temperature $\theta $ decrease with the increase of $x$. For $x \quad \ge $ 0.8, the $M$ (T) curves are reversible and display paramagnetic behavior. Thus, the system is non-magnetic as it approaches heavy-fermion compound CeCu$_{2}$Si$_{2}$. Finally, a magnetic phase diagram is proposed for this compound series. [Preview Abstract] |
Tuesday, March 14, 2006 5:18PM - 5:30PM |
K45.00015: Deviations from Drude response in CaRuO$_3$ Saeid Kamal, Steve Dodge, Dong-Min Kim, Chang-Beom Eom We report on the low frequency optical conductivity measurements of epitaxial thin films of CaRuO$_3$, obtained with time-domain terahertz spectroscopy. We find that the conductivity does not follow a simple Drude form, even at low temperatures and frequencies; alternative power-law forms also fail to describe it adequately. To achieve a good fit to the experimental data, we find it necessary to use a multi-component model consisting of multiple {\em Drude} terms and a finite frequency Lorentzian. This parameterization allows us to set an upper bound on the free-carrier spectral weight, from which we can set a lower bound on the renormalized mass of $m^*/m\cong 10$. If the scattering is assumed to be isotropic, our results imply a frequency dependence in the mass enhancement and scattering rate that extends well below terahertz frequencies. However, our results are also consistent with frequency-independent carrier dynamics that vary strongly over the Fermi surface. [Preview Abstract] |
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