Session H27: Correlated Electrons: Heavy Fermions and Exotic Magnets

Magnetism and metallicity in FeSb2-xTex

Single crystals of FeSb2-xTex (0$\le $x$\le $1) were grown by molten metallic flux technique. A rich variety of electronic and magnetic ground states will be presented.   

Phase transitions in $R_{5}$NiPb$_{3}$ ($R$=Ce,Nd,Gd)

We report magnetic and thermodynamic measurements for recently-synthesized $R_{5}$NiPb$_{3}$ ($R$=Ce,Nd,Gd) (hexagonal Hf$_{5}$CuSn$_{3}$-type structure), as well as non-magnetic La-based analogs. High-temperature Curie-Weiss fits yield effective moments of 2.43, 3.70 and 9 $\mu_{B}$ for Ce$_{5}$NiPb$_{3}$, Nd$_{5}$NiPb$_{3}$ and Gd$_{5}$NiPb$_{3}$ respectively. These are close to the $R^{3+}$ ionic moments, showing that Ni is nonmagnetic in all cases. For Ce$_{5}$NiPb$_{3}$ a peak seen in both the magnetization and specific heat at 48 K indicates an apparent ferromagnetic transition at that temperature, which is also confirmed by field dependent heat capacity and a positive Curie-Weiss temperature. Nd$_{5}$NiPb$_{3}$ exhibits two magnetic transitions, an antiferromagnetic transition at 42 K and an apparently weak ferromagnetic canting transition at 8 K. Ce$_{5}$NiPb$_{3}$ shows a kink in both the magnetization and specific heat at 68 K indicates a ferro- or ferrimagnetic transition at that temperature, which is also confirmed by a positive Curie-Weiss temperature. For this material, ZFC and FC measurements show irreversibility at transition temperature. For Ce and Nd samples $M$-$H$ curves show metamagnetism at low temperatures. We will compare the results with the non magnetic analog La$_{5}$NiPb$_{3}$.   

Universal heat transport in the heavy-fermion superconductor CeIrIn5

In superconductors with nodes in the gap, zero-energy quasiparticles give rise to a residual linear term in the thermal conductivity at T=0. When the quasiparticle density of states is linear in energy, this term is universal, in the sense that it does not depend on impurity concentration. Such universal heat transport has been observed in cuprates and ruthenates but never in heavy-fermion superconductors. Here we show that heat transport in the heavy-fermion superconductor CeIrIn5 is unchanged by the doping of La impurities. This universal transport confirms the presence of a line node in the gap [1]. \newline [1] H. Shakeripour et al., Phys. Rev. Lett. 99, 187004 (2007).   

Fermi Surface Investigation of Cd doped CeCoIn5

CeCoIn5 is a 2.3K superconductor member of the 115 family of heavy fermion compounds that have attracted much attention due to the competing magnetism and superconductivity. The superconductivity in CeCoIn5 emerges from a metallic state that exhibits strong deviations from the Fermi Liquid theory, with the presence of a field-tuned antiferromagnetic quantum critical point near the upper critical field. Recently it was shown that Cd substitution can tune the ground state from superconducting to antiferromagnetic. One important question is how the shape of the Fermi surface influences the stability of the ground state. We will present de-Haas-van-Alphen results in Cd doped LnCoIn5 (Ln=La,Ce) and discuss the origin of magnetic order in this system.   

Two fluid analysis of La and Cd doped CeIrIn$_{5}$.

The heavy fermion superconductor CeIrIn$_{5}$ has been shown to have a ground state dependence on Cd doping which induces antiferromagnetism while suppressing superconductivity. This system provides an interesting regime in which to study the onset of the heavy fermion superconducting and antiferromagnetic ground states as well as the interplay between the different electronic interactions at low temperatures. The Two Fluid Model has quantified the energy scales in the related La doped CeCoIn5. We attempted a two fluid analysis on specific heat and susceptibility measurements of La and Cd doped CeIrIn$_{5}$ single crystals.   

Anisotropic effect of Cd-doping on superconducting phase in CeCoIn$_{5}$ at high fields

Unconventional superconductor CeCoIn$_{5}$ at high magnetic field displays first order superconducting (SC) transition, and an additional high field-low temperature SC phase (previously proposed to be an inhomogeneous superconducting FFLO state). We have studied Cd-doping effect on the FFLO state and the first order SC transition by measuring specific heat $C$(T) of CeCo(In$_{1-x}$Cd$_{x})_{5}$ (x=0.01, 0.02 and 0.03) at low temperatures and high fields. Our data show that the FFLO state is already destroyed by 1{\%} Cd-doping. The effect of Cd doping on the first order SC transition is anisotropic. The cross-over temperature $T_{0}$, where the superconducting transition changes its nature from first to second order, decreases rapidly with increasing doping for $H$//[100] and disappears already at x=0.02, while it remains rather temperature-independent for $H$//[001] up to x=0.03.   

Angular Dependent Magnetic Properties of CeCoIn$_{5}$ at Low Temperatures

The heavy-fermion compound CeCoIn$_{5}$ exhibits a superconducting transition at 2.3 K. As an unconventional superconductor, many unusual physical properties of the compound have been actively studied. In particular, evidence of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state in this compound was reported first by Radovan and co-workers.$^{\dag }$ At the lowest temperature ($\sim $ 20 mK), the FFLO state of CeCoIn$_{5}$ was observed in fields between 10 and 11.7 T when the \textit{ab}-plane of the compound was placed parallel to the external magnetic field. In addition, at these low temperatures, the angular dependent peak effect was observed and interpreted as a crossover between Abrikosov and Josephson vortex lattices.$^{\ddag }$ Further experimental studies of the low temperature ($>$ 12 mK) magnetic properties of CeCoIn$_{5}$, performed in various sample orientations with respect to magnetic field will be presented. $^{\dag }$H.A. Radovan, et al., Nature \textbf{425} (2003) 51. $^{\ddag }$H.A. Radovan, et al., Philosophical Magazine \textbf{86} (2006) 3569.   

Evolution of the FFLO state of CeCo(In$_{1-x}$Hg$_{x})_{5}$

Specific heat C(T) of the unconventional superconductor CeCoIn$_{5 }$displays an anomaly within the superconducting (SC) state, indicating an additional SC phase in the high field and low temperature corner of the SC state of the H-T phase diagram. This SC state was previously proposed to be an inhomogeneous superconducting FFLO state. We have performed specific heat measurements of very low Hg-doped samples with x=0.001, 0.002 and 0.003, in order to study the evolution of the proposed FFLO state in CeCoIn$_{5}$. In a sample with x=0.001, C(T) still shows the additional transition inside the SC state, but it is broadened. The transition temperature is slightly higher than that of pure compound. C(T) of a higher Hg-doped sample with x=0.003 displays a very shallow and broad hump around the same temperature. Given that only 20{\%} of the nominal Hg concentration goes into the sample, the FFLO state appears to be extremely sensitive to the presence of impurities.   

Supression of superconductivity in CeRh$_{1-x}$Co$_{x}$In$_5$ by incommensurate antiferromagnetic order

CeCoIn$_5$ shows superconductivity (SC) below 2.3~K, while the SC is highly suppressed in a related system CeRhIn$_5$, which exhibits the antiferromagnetic (AF) order below 3.8~K\@. Then the mixed compound CeRh$_{1-x}$Co$_{x}$In$_5$ is expected to have a complex $x$-$T$ phase diagram where the SC and magnetism coexist. In order to understand the relationship between the SC and the antiferromagnetism in CeRh$_{1-x}$Co$_{x}$In$_5$, we have performed neutron diffraction measurements on this system for various $x$\@. A commensurate (C) AF order with a propagating vector {\boldmath $q$}$_c=(1/2, 1/2, 1/2)$ is observed for $0.3 \le x \le 0.6$, coexisting with the SC\@. However, they are strongly suppressed at $x \sim 0.3$, and an incommensurate (IC) AF order with {\boldmath $q$}$_I=(1/2, 1/2, 0.298)$ simultaneously appears. We interpret that there is no intrinsic coexistence between the IC- and C-AF orders and that the SC competes with the IC-AF order but coexists with the C-AF one. These results imply that particular areas on the Fermi surface nested by {\boldmath $q$}$_I$ play an active role in forming the superconducting state in CeCoIn$_5$.   

Chiral d-wave superconductivity in the heavy fermion compound $CeIrIn_5$

A recent experiment by Shakeripour et al indicates that the superconductivity in heavy fermion compound $CeIrIn_5$ is very different from the one in $CeCoIn_5$. We have re-examined their data and concluded that it belongs to the chiral d-wave SC or $\Delta(\vec{k})\sim e^{\pm i\phi}\cos(ck_z)$.   

Fabrication of artificial heavy fermion superlattices by the molecular beam epitaxy

We have grown artificial superlattices of CeIn$_3$ ($m$) / LaIn$_3$ ($n$), in which $m$-layers of heavy-fermion antiferromagnet CeIn$_3$ and $n$-layers of a non-magnetic isostructual compound LaIn$_3$ are stacked alternately, by a molecular beam epitaxy. Growth process were monitored by reflection high energy electron diffraction (RHEED). Sharp streak pattern of RHEED indicates the epitaxial growth of thin films. Satellite peaks observed in a X-ray diffraction pattern also indicates the superlattice structure. By reducing the thickness of CeIn$_3$, we observe a suppression of antiferromagnetic order and an enhancement of effective mass inferred from the resistivity coefficient, which both imply new `dimensional tuning' towards a quantum critical point.   

Antiferromagnetism in the Kondo lattice system Ce(Ni$_{0.25}$In$_{1.75})$.

The pseudobinary compound Ce(Ni$_{0.25}$In$_{1.75})$, which crystallizes in a hexagonal AlB$_{2}$ type structure with space group P6/mmm, exhibits antiferromagnetic ordering below the temperature 3.9 K, as revealed in the magnetic susceptibility, electrical resistivity and low-temperature specific-heat data. A ln(T) dependence is seen in the high temperature region for the magnetic contribution to the resistivity $\rho _{m}$, which is one of the characteristic features of dense Kondo systems. The magnetic entropy S$_{m}$(T$_{N})$ associated with the magnetic structure of Ce(Ni$_{0.25}$In$_{1.75)})$ is found to be only 62{\%} of Rln(2), corresponding to a reduction of 38{\%} of the cerium moment. This large magnetic reduction may be able to be attributed to Kondo effect. The heat capacity C(T) of Ce(Ni$_{0.25}$In$_{1.75})$, in the paramagnetic state at temperatures between 8 and 20 K, can be fitted to the expression C$_{n}$ = $\gamma $T + $\beta $T$^{3}$ by a least squares analysis, which yields the value $\gamma $ = 123 mJ/mol K$^{2}$ and $\beta $ = 1.2 mJ/mol K$^{4}$, the latter value corresponding to the Debye temperature $\Theta _{D}$ = 169 K.   

Angle dependent quasiparticle weights in correlated metals

The variation in the quasiparticle weight $Z$ on moving around the fermi surface in correlated metals is studied theoretically. Our primary example is a heavy Fermi liquid treated within the standard hybridization mean field theory. The most dramatic variation in the quasiparticle weight happens in situations where the hybridization vanishes along certain directions in momentum space. Such a ``hybridization node" is demonstrated for a simplified model of a Cerium-based cubic heavy electron metal. We show that the quasiparticle weight varies from almost unity in some directions, to values approaching zero in others. This is accompanied by a similar variation in the quasiparticle effective mass. Some consequences of such hybridization nodes and the associated angle dependence are explored. Comparisons to somewhat similar phenomena in the normal metallic state of the cuprate materials are discussed. A phenomenological picture of the pseudogap state in the cuprates with a large Fermi surface with a severely anisotropic spectral weight is explored.   

Magnetic properties of RFe$_2$Zn$_{20}$ (R = Y, Gd - Lu): $4f$ moments embedded in strongly correlated electron host

The RFe$_2$Zn$_{20}$ series of compounds manifest varied magnetic properties, from near ferromagnetism (R = Y, Lu), to enhanced ferromagnetic (FM) ordering of local moments (R = Gd to Tm), to heavy Fermi ground state (R = Yb). Thermodynamic and transport measurement results reveal that these varied magnetic states can be understood in the framework of $4f$ moments embedded in a nearly ferromagnetic Fermi liquid. In such a highly polarizable electronic host, the different type of 4f moments, null moments for Y$^{3+}$ and Lu$^{3+}$, pure spin- contributed Gd$^{3+}$, spin-orbital-coupling contributed Tb$^ {3+}$ to Tm$^{3+}$, and the hybridized Yb ions, correlated with the itinerant electrons, lead to this magnetic versatility. For local moment members (R = Gd to Tm), the Curie temperatures roughly scale with the de Gennes parameter, indicating negligible crystal electric field (CEF) effect on the magnetic ordering, although the CEF on the $4f$ local moments leads to anisotropic FM ground state for R = Tb to Tm. For hybridized moment member (R = Yb), the Kondo temperature seems to be enhanced, associated with the electronic host.   

Low-Energy Magnetic Excitations in the Itinerant Quantum Ferromagnet ZrZn$_2$

High resolution de Haas-van Alphen measurements offer a powerful method to probe the low energy magnetic excitations in ferromagnets. Here we report measurements of the temperature dependence of the exchange splitting $\Delta(T)$ in the weak itinerant ferromagnet ZrZn$_2$ (Curie temperature $T_c=28$\,K) using the de Haas-van Alphen (dHvA) effect. Quantitative comparison with the magnetic moment $M(T)$ shows that longitudinal or `Stoner' excitations dominate, in strong contrast with observations on metallic ferromagnets like Fe and Ni, where spin-waves dominate at low $T$. We ascribe the difference to the proximity of a quantum critical point (QCP) in ZrZn$_2$. The results are discussed in terms of a phenomenological Ginzburg-Landau model which includes effects of thermal fluctuations and enhanced fluctuations due to the nearby quantum critical point. We find that the model accurately predicts both the rate of collapse of $M(T)$ with $T$ and the relative reduction of $\Delta(T)$ and $M(T)$. We suggest a picture for the evolving nature of magnetic excitations within the ferromagnetic state as the QCP is approached.