Session V11: Heavy Fermions

Magnetic Field Effects in the Heavy Fermion Ce$_{3}$Co$_{4}$Sn$_{13}$

Ce$_{3}$Co$_{4}$Sn$_{13}$ is a heavy Fermion system with a low temperature specific heat as large as 4 J/mol K$^{2}$. Measurements on single crystal specimens of the elastic neutron scattering response and the heat capacity (C) show that Ce$_{3}$Co$_{4}$Sn$_{13}$ is extremely sensitive to applied magnetic fields. The heat capacity measurements show that the broad peak in C/T at 0.8 K at zero field initially moves downward in temperature before moving upward with a characteristic energy scale of 2 T. Neutron diffraction measurements at 0.15 K show that upon application of very small magnetic fields, the intensity on the (100) peak grows and does not saturate below 6 T. Although reduced in magnitude the effect persists to temperatures at least as high at 2 K.   

Thermal Expansion of the Heavy Fermion Borocarbide YbNi$_{2}$B$_{2}$C

YbNi$_{2}$B$_{2}$C is a heavy fermion compound with a coherence temperature of about 10 K. We have measured its thermal expansion from room temperature to below 1 K. The thermal expansion is anisotropic, negative below about 16 K, and deviates from simple metallic behavior near the coherence temperature. We will present and discuss our results in the context of a Gruneisen analysis. This work was supported by the National Science Foundation under DMR-0305397. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405-ENG-82.~ This work was supported by the Director for Energy Research, Office of Basic Energy Sciences.   

Infrared and magneto-optical studies of heavy fermion skutterudites YbFe$_4$Sb$_{12}$ and CeRu$_4$Sb$_{12}$

We will report infrared and magneto-optical results on two heavy fermion skutterudites YbFe$_4$Sb$_{12}$ and CeRu$_4$Sb$_{12}$. Detailed temperature dependence of infrared spectra will be presented for both compounds. In addition, magneto-transmission measurements on YbFe$_4$Sb$_{12}$ in magnetic field as high as 33 Tesla, and magneto-reflection measurements on CeRu$_4$Sb$_{12} $ in 17 Tesla field will be reported. The results reveal suppression of heavy fermion state with magnetic field, and recovery of a more conventional metallic state. In particular, the effective mass of charge carriers is gradual diminished. On the other hand, hybridization gap is much more insensitive to the application of magnetic field.   

Kondo ground state of 6 related Yb-based intermetallic compounds

Isostructrural YbT$_2$Zn$_{20}$ (T=Fe, Co, Ru, Rh, Os, Ir) have less than 5\% concentration atomic Yb, in which the Yb atoms fully occupy one unique crystallographic site of cubic (-43m) symmetry. This series of compounds offer the opportunity to systematically study Kondo lattice systems approaching the single ion limit. With the thermodynamic and transport measurements, we show that the Kondo ground state has different degeneracies for T=Fe, Ru, Rh, Os and Ir, reflecting the competition between the different Kondo temperatures ($T_K$) and the similar crystal-electric field (CEF) splitting temperatures ($T_{CEF}$). On the other hand, the unusual thermodynamic behavior at low temperature suggests that YbCo$_2 $Zn$_{20}$ has low $T_K$ and seems to be very close to a quantum critical point.   

Spin Dynamics in the f-electron non Fermi Liquid alloy Sc1-xUxPd3

We will discuss our recent inelastic neutron scattering experiments probing the spin dynamics in the non Fermi-liquid alloy, Sc$_{1-x}$U$_{x}$Pd$_{3}$. The increased homogeneity of U- sites in this system allows for an investigation of magnetism without the disordering effects of U-site clustering observed in isostructural Y$_{1-x}$U$_{x}$Pd$_{3}$. Spin dynamics indicative of the influence of a spin glass (SG) quantum critical point (QCP) in Sc$_{1-x}$U$_{x}$Pd$_{3}$ have been observed previously. Our current study probes spin fluctuations now doped away from this SG QCP and into the antiferromagnetic (AF) ordered phase of the system. The evolution of the spin excitations as the system is tuned across its phase diagram away from the SG QCP will be discussed, with particular emphasis given to the evolution of the localized spin fluctuations attributed to the QCP in this system.   

Interplay between Disorder and Quantum and Thermal Fluctuations in Ferromagnetic Alloys -- New Systems

Previously$^{1}$ we addressed the effects of disorder on the ferromagnetic ordering temperature, T$_{C}$, in UCu$_{2}$Si$_{2-x}$Ge$_{x}$. In that work the measured non-monotonic variation of T$_{C}$ with disorder (as measured by the resistivity) could be explained within a model$^{2}$ of localized spins interacting with an electronic bath. This model predicts that, in some cases, T$_{C}$ can be \textit{enhanced} by the interplay between quantum and thermal fluctuations with disorder. We have extended this work in other ferromagnetic alloys, with both significantly larger as well as similar variations of T$_{C}$ with doping compared to the $<$10 {\%} variation of T$_{C}$ observed in UCu$_{2}$Si$_{2-x}$Ge$_{x}$. Resistivity, magnetic susceptibility, and specific heat will be presented, along with a comparison to the theory$^{2}$. $^{1}$M. B. Silva Neto, A. H. Castro Neto, D. Mixson, J. S. Kim, and G. R. Stewart, Phys. Rev. Lett. \underline {91}, 257206 (2003). $^{2}$M. B. Silva Neto and A. H. Castro Neto, Europhys. Lett. \underline {62}, 890 (2003).   

Effects of Disorder in the Heavy Fermion Antiferromagnet CeCu$_{6-x}$Au$_{x}$

Using a quick quenching technique that also produces bars of known geometry for absolute resistivity measurements, we have measured the effects of this fairly rapid ($\sim $ 10$^{4}$ K/s) solidification on the high angle x-ray line width, resistivity, magnetic susceptibility, and specific heat of CeCu$_{6-x}$Au$_{x}$ for several compositions. Gradual variations of the disorder and its effect on the properties were investigated by annealing the quenched samples at 700 $^{\circ}$C over periods of days to weeks. The discussion of the effects of the quenched-in disorder on the measured properties will focus on T$_{Neel}$, the low temperature specific heat $\gamma $, and the non-Fermi liquid behavior.   

Elastic property of a high-field ordered state observed in PrFe$_{4}$P$_{12}$

We present experimental results of elastic constants as a function of temperature and magnetic field for the Pr-based heavy fermion system PrFe$_{4}$P$_{12}$, especially in a high-field (HF) ordered phase discovered. Since the HF phase exists in a narrow temperature range below 0.7 K and for high fields above 7 T in a highly limited angular range around the [111] directions, the elastic constants C$_{L}$ = (C$_{11}$+2C$_{12}$+4C$_{44})$/3 and C$_{T}$ = (C$_{11}$-C$_{12}$+C$_{44})$/3 were measured by the longitudinal (L) one propagating along the [111] direction and by the transverse (T) one along the [111] direction polarized to the [1-11] one, respectively. A clear upturn was observed in both of the elastic constants below the HF ordered phase transition temperature. Furthermore, a remarkable elastic softening toward the transition temperature was observed in the temperature dependence of C$_{L}$, whereas no softening was observed in C$_{T}$. These results indicate that the softening is most likely to be due to the bulk modulus (C$_{11}$+2C$_{12})$/3. Thus, it is expected that the HF ordered phase seems to be accompanied by a strain fluctuation with $\Gamma _{1}$ spatial symmetry, namely a scalar phase transition with a multi-polar moment is a candidate for the HF ordered phase.   

Disorder and quantum size effects on Kondo interactions and magnetic correlation in CePt$_{2}$

Measurements of specific heat C(T) and magnetic susceptibility $\chi $(T) on a series of CePt$_{2}$ nanoparticles with size d= 3.1, 22 and 26 nm are compared to those of bulk CePt$_{2}$ to determine the size effects on Kondo interactions and magnetic correlation therein. Kondo interactions (T$_{K}$=5.6 K) and an antiferromagnetic correlation (T$_{N}$=1.6 K) coexist in CePt$_{2}$ with $\sim $60{\%} and $\sim $40{\%} of Ce magnetic ions involved in respectively. While the antiferromagnetism is diminished by size reduction, Kondo behaviour predominates, with a sommerfield constant $\gamma$ increasing from 460 mJ/f.u. K$^{2 }$ for the bulk to 3000 mJ/f.u. K$^{2 }$ for 3.1 nm. Meanwhile, a decrease of Kondo temperature T$_{K}$ from 5.6 to 0.42 K is observed with the size reduction. The consequences were explained by structural disorders, however for d $<$ 10 nm electronic quantum size effect play a more significant role.   

Probing the Magnetic Order of UPt$_{3}$ by Magnetoresistance

It is believed that magnetic order has a strong influence on the unconventional superconducting state of UPt$_{3}$, both as a symmetry breaking field that splits two degenerate superconducting phases, and as an element of the pairing mechanism. There are conflicting reports of the behavior at 5 K, static antiferromagnetic order versus slow fluctuating magnetism. We present measurements of the temperature dependence of the magnetoresistance in bulk single crystals of UPt$_{3}$. Below 5 K, we observe an onset and subsequent rapid increase of a term linear in the applied field. With the field in the basal plane, this linear contribution is positive with current along the c-axis and negative with the current along the b-axis. We identify this behavior in the magnetoresistance with observation of magnetic Bragg peaks from neutron scattering that appear below 5 K. We discuss preliminary results of similar measurements at temperatures in the millikelvin range.   

Doped Mott insulator physics in the 4f antiferromagnet CeIn3 and implications for pressure-tuned superconductivity

We report recent de Haas-van Alphen experiments performed at low temperatures ($T$ $>$ 30 mK) and high magnetic fields in CeIn$_3$ that motivate us to revise our understanding of strongly coupled antiferromagnetism in $f$-electron systems. In addition to the known light Fermi surface sheets, heavy ellipsoid pockets are observed with a symmetry consistent with them being situated at the $ \pi/2,\pi/2,\pi/2$ point in the paramagnetic Brillouin zone. Their topology is the 3D analog of the 2D hole pockets reported to exist at $\pi/2,\pi/2$ in underdoped cuprate superconductors, indicating some degree of similarity between the the electronic structures in the $d$- and $f$-electron systems. The effective masses of the ellipsoids are sufficient to account for all of the electronic specific heat of CeIn $_3$ at ambient pressure within the antiferromagnetic phase, making them strong candidates for the origin of the coexisting superconductivity under pressure. High magnetic fields cause these pockets to undergo a topological deformation, passing through a Lifshitz transition at $\sim$~40~T.   

The superconducting pairing states in CeCu$_2$Si$_2$

The prototype heavy fermion superconductor CeCu$_2$Si$_2$ exhibits very rich physical properties. It has been shown that two distinct superconducting states, one around an AFM QCP at low pressures and the other one around a weak first-order valence transition at high pressures, uniquely exist in the pressurized CeCu$_2$(Si/Ge)$_2$ [1]. In this talk, I will first briefly review the recent progress on the study of these two superconducting phases. Then I will focus on the superconducting pairing state of CeCu$_2$Si$_2$ at $p=0$. Early measurements, e.g., specific heat and $\mu$ SR, revealed quite controversial behavior due to the limitations of experimental techniques and sample quality. In order to elucidate the nature of superconductivity in CeCu$_2$Si$_2$ and to investigate the interplay of AFM and superconductivity, we performed precise measurements of the magnetic penetration depth $\Delta \lambda (T)$ in high quality single crystals down to $T=80$ mK. A linear temperature dependence of $\lambda(T)$ is observed in both A/S-type and S-type CeCu$_2$Si$_2$ below $T=150$ mK, providing uncontroversial evidence for the existence of line nodes in the superconducting energy gap. [1] H. Q. Yuan et al., Science 302, 2104 (2003); Phys. Rev. Lett. 96, 047008 (2006).