Session M22: Heavy Fermions: Ce-115 and Yb-based Compounds: Experiment

An Angle Resolved Photoemission Survey of the Band Structure of the Heavy Fermion Superconductor, CeCoIn$_5$

With the highest T$_c$ of the non-radioactive heavy fermion materials, CeCoIn$_5$ has been extensively studied by a host of techniques. However direct measurements of the band structure via angle resolved photo-emission spectroscopy has been limited to just a few experiments. We will present our studies of the momentum, temperature, photon energy and polarization dependence of the band structure of CeCoIn$_5$. We will compare our results with theory and other experimental results.   

Landau Renormalizations of Superfluid Density in the Heavy-Fermion Superconductor CeCoIn5

The formation of heavy fermion (HF) bands can occur by means of the conversion of a periodic array of local moments into itinerant electrons via the Kondo effect and the huge consequent Fermi-liquid(FL) renormalizations. Leggett predicted for liquid $^3$He that FL renormalizations change in the superconducting state, leading to a temperature(T) dependence of the London penetration depth~$\Lambda$ quite different from that in the BCS theory. Using Leggett's theory, as modified for HF, it is possible to extract from the measured T dependence of $\Lambda$ in high quality samples both Landau parameters $F_0^s$ and $F_1^s$; this has never been accomplished before. A modification of the T dependence of the specific heat $C_{\mathrm{el}}$, related to that of $\Lambda$, is also expected. We have carefully determined the magnitude and T dependence of $\Lambda$ in CeCoIn$_5$ by muon spin relaxation rate measurements to obtain $F_0^s = 36 \pm 1$ and $F_1^s = 1.2 \pm 0.3$, and find a consistent change in the T dependence of electronic specific heat $C_{\mathrm{el}}$. This, the first determination of $F_1^s$ with a value~$\ll F_0^s$ in a HF compound, tests the basic assumption of the theory of HF,that the frequency dependence of the self-energy is much more important than its momentum dependence.   

Time Reversal Symmetry Breaking in Polar Kerr Measurements of the Heavy-Fermion Superconductor CeCoIn$_5$

The heavy-fermion superconductor CeCoIn$_5$ is of great interest, as it shares many features with high-$T_C$ d-wave superconductors, with unconventional pairing and competition between superconducting and magnetic phases. Understanding the mechanisms of superconductivity in this material can elucidate the physics of high-$T_C$ and of unconventional superconductivity in general. We present measurements of the polar Kerr effect in CeCoIn$_5$ using a zero-area Sagnac interferometer. We observe an onset of Kerr rotation near the superconducting transition temperature, indicating an order parameter which breaks time reversal symmetry. We discuss the relation of this symmetry breaking to the superconducting state, and place our measurement in context with other experiments on this material.   

Ultrafast dynamics in CeCoIn$_{5}$

We present ultrafast pump-probe and transient grating spectroscopy studies of the heavy Fermion superconductor CeCoIn$_{5}$. In pump-probe experiments, a 100-femtosecond 800nm-wavelength pulse creates transient electronic excitations whose decay is probed by studying transient changes in reflectivity as a function of time. We observe changes in pump-probe decay dynamics across the Kondo coherence temperature. In transient grating spectroscopy, two pump-pulses are interfered to produce a spatially periodic excitation, and the system's response to this periodic perturbation is studied through a diffracted probe beam. The temporal evolution of this signal indicates a non-trivial motion of the excitation grating in the heavy electron regime.   

Field induced density wave in the heavy fermion compound CeRhIn5

We will present evidence for a magnetic field induced phase-transition to a state akin to a density-wave (DW) in the heavy fermion superconductor CeRhIn$_5$. The DW state is signaled by a hysteretic anomaly in the in-plane resistivity accompanied by the appearance of non-linear electrical transport at high magnetic fields ($>$27T), which are distinctive characteristics of density-wave states. The differential resistance dV/dI is strongly suppressed by currents in excess of an critical electric field E$_c$ 15mV/cm, which would be a typical value for depinning thresholds in incommensurate density waves such as NbSe$_3$ or TaS$_3$. Intriguingly, the out-of-plane resistivity as well as the magnetic torque remain unaffected by the transition. The untypically large hysteresis enables us to directly investigate the Fermi surface of a supercooled electronic system and to clearly associate a Fermi surface reconstruction with the transition: additional frequencies suddenly appear at the transition in Shubnikov-de Haas measurements. Key to our observation is the fabrication of single crystal microstructures with $\mu m^2$ cross-sections, which are found to be highly sensitive to phase transitions involving small portions of the Fermi surface.   

Observation of field-induced Fermi surface reconstruction in CeRhIn5

CeRhIn5 provides a prototype compound for studying quantum criticality and its interplay with superconductivity. Application of pressure suppresses the antiferromagnetic (AF) order and gives rise to superconductivity [1]. A sharp change of Fermi surface was observed just at the pressure-tuning AF quantum critical point (QCP) [2], which was argued to support the scenario of local quantum criticality [3]. By measuring the dHvA oscillations and specific heat in a pulsed magnetic field, we have demonstrated the existence of a field-induced AF QCP around Bc0$=$50T in this compound [4]. In this presentation, we will report the measurements of dHvA effect and Hall resistivity of CeRhIn5 performed by using the 45T hybrid magnet and the pulsed field magnet at NHMFL. Field-induced changes of the dHvA frequencies and Hall coefficient are observed around B*$=$31T. Detailed analyses suggest that the Fermi surface reconstruction at B* corresponds to a localized-itinerant transition of Ce 4f-electrons attributed to the Kondo effect. Our results indicate that multiple quantum phase transitions may exist in CeRhIn5 which can be classified by the measurements of Fermi surface topology. [1] T. Park et. al., Nature 440, 65 (2006). [2] H. Shishido et. al., J Phys Soc Jpn 74,1103 (2005). [3] Q. Si, F. Steglich, Science 329,1161 (2010). [4] L. Jiao et al., arXiv:1308.0294.   

The magnitude of the magnetic exchange interaction in the heavy fermion antiferromagnet CeRhIn$_{5}$

The family of heavy fermion compounds Ce$T$In$_{5}$ ($T$ = Co, Rh, Ir) has been a fertile ground to explore and understand the interplay between magnetism, unconventional superconductivity and quantum criticality due to their tunability by pressure, substitution and magnetic field. CeRhIn$_{5}$ is a heavy fermion antiferromagent which can be tuned to quantum criticality under pressure. The strength of the magnetic exchange interaction, which is a key parameter to understand its complex properties, however remained unknown. We have used high-resolution neutron spectroscopy to determine the complete spin wave spectrum in CeRhIn$_{5}$. The spin wave dispersion can be quantitatively reproduced with a simple frustrated $J_1-J_2$ model that also naturally explains the magnetic spin-spiral ground state of CeRhIn$_{5}$ and yields a dominant in-plane nearest-neighbor magnetic exchange constant $J_0 = 0.74(3)$ meV. Our results pave the way to a quantitative understanding of the rich low-temperature phase diagram of the prominent Ce$T$In$_{5}$ class of heavy fermion materials.   

Fine tuning of the quantum criticality in the heavy fermion superlattices CeRhIn$_{5}$/YbRhIn$_{5}$

Bulk CeRhIn$_{5}$ shows an antiferromagnetic order at $T_{\rm N}$ = 3.8 K. Using molecular beam epitaxy, we fabricate artificial superlattices CeRhIn$_{5}(m)$/YbRhIn$_{5}$(7) ($m$ = 2, 3, 4, 5, 9) containing $m$ layers of CeRhIn$_{\rm 5}$ alternating with seven layers of the nonmagnetic metal YbRhIn$_{5}$. With decreasing $m$, $T_{\rm N}$ is seriously reduced and nearly vanishes at the $m$ = 3, indicating the dimensional tuning of the quantum criticality. When the magnetic field is applied to $m$ = 3 superlattice perpendicular to the plane, $T$-linear resistivity, a hallmark of non-Fermi liquid, persists down to 50 mK, demonstrating the fine tuning of the quantum critical point   

Pressure studies of the quantum critical alloy Ce$_{0.93}$Yb$_{0.07}$CoIn$_{5}$

We performed experimental and theoretical studies of the effect of pressure on the heavy fermion quantum critical alloy Ce$_{0.93}$Yb$_{0.07}$CoIn$_{5}$. As observed in resistivity measurements, the Ce$_{\mathrm{1-x}}$Yb$_{\mathrm{x}}$CoIn$_{5}$ system exhibits non-Fermi liquid behavior with two distinct contributions to resistivity (linear-in-T and square-root-in-T). Our measurements suggest that linear in T resistivity is governed by heavy/large Fermi surface and is suppressed with pressure, together with the suppression of the quantum fluctuations with pressure in Ce$_{0.93}$Yb$_{0.07}$CoIn$_{5}$. The square-root-in-T dependence originates from two different physics: (i) the $\surd $T dependence just above T$_{\mathrm{c}}$ is suppressed with the application of pressure, and is a result of superconducting fluctuations; (ii) the higher temperature $\surd $T contribution to resistivity remains insensitive to pressure, indicating that the scattering processes in this T range are governed by the scattering of light electrons from the small Fermi surface. We demonstrate that the growth of the coherence temperature with pressure, as well as the decrease of the residual resistivity, can be accurately described by employing the coherent potential approximation for a disordered Kondo lattice.   

Chemical substitution study on magnetism and superconductivity in Ce$_{1-x}$Sm$_x$CoIn$_5$

We have investigated the system Ce$_{1-x}$Sm$_x$CoIn$_5$ (0 $<$ $x$ $<$ 1) by means of x-ray diffraction, electrical resistivity, specific heat, and magnetization measurements. We observe a crossover from a coherent Kondo lattice exhibiting superconductivity to a single-ion impurity Kondo effect coexisting with magnetic order on the Sm-rich side of the phase diagram. The superconducting transition temperature, $T_c$, and Kondo lattice coherence temperature, $T_{coh}$, are suppressed near $x$ $\sim$ 0.2 and $x$ $\sim$ 0.5, respectively, which is consistent with the effect of substitution with other rare-earth (RE) ions on CeCoIn$_5$. After $T_{coh}$ is suppressed to 0 K, a single-ion impurity Kondo effect is observed for 0.5 $<$ $x$ $\leq$ 0.85. The compound SmCoIn$_5$ exhibits three distinct magnetic phase transitions at roughly 8, 10, and 12 K, which are presumably associated with magnetic order; similar features are observed in the related compound SmIn$_3$. These transition temperatures are gradually suppressed by Ce substitution and completely vanish near $x$ $\sim$ 0.2. We establish the phase diagram of the system Ce$_{1-x}$Sm$_x$CoIn$_5$ and compare our results with those obtained from chemical substitution studies of CeCoIn$_5$ involving other RE ions.   

Tuning the Kondo effect in YbFe$_{1-x}$Co$_{x}$Zn$_{20}$

YbCo$_{2}$Zn$_{20}$ is a heavy fermion compound with a Sommerfeld coefficient, $\gamma$ value, of about 8000 mJ/mol-K$^{2}$ with an estimated single ion Kondo temperature, T$_{K}$, of about 1.5 K. One the other hand, YbFe$_{2}$Zn$_{20}$ is less heavy with $\gamma \sim$ 500 mJ/mol-K$^{2}$ and T$_{K} \sim$ 30 K. From a generalized Kadowaki-Woods picture, degeneracies that relate to their Kondo phenomena are large while different: 8 for YbFe$_{2}$Zn$_{20}$ and 4 for YbCo$_{2}$Zn$_{20}$ [1]. In order to understand the effects of Fe-Co substitution on the Kondo effect, a family of YbFe$_{1-x}$Co$_{x}$Zn$_{20}$ were studied. We performed zero-field resistivity and specific heat measurements on single crystals of YbFe$_{1-x}$Co$_{x}$Zn$_{20}$ that were synthesized using a high-temperature solution growth technique [2]. The Kondo characteristic temperatures do not change monotonically in between pure YbFe$_{2}$Zn$_{20}$ and YbCo$_{2}$Zn$_{20}$. Data and a summarize phase diagram of characteristic temperatures as a function of Co doping will be presented and discussed. \\[4pt] [1] M. S. Torikachvili et al. Proc. Natl. Acad. Sci. USA 104, 9960 (2007) \\[0pt] [2] S. Jia et al. Nat. Phys. 3, 334 (2007)   

Thermal expansion and quantum criticality of the heavy fermion antiferromagnet YbBiPt

YbBiPt is a stoichiometric heavy fermion compound with an enormous Sommerfeld coefficient and an antiferromagnetic ground state that is suppressed by magnetic fields of about 0.4 T. Non-Fermi liquid behavior, and other signatures of field-induced quantum criticality have been observed. In this talk we will present measurements of the thermal expansion of YbBiPt along the [111] axis from 30K to below 400 mK and in magnetic fields as high as 9 T. We will discuss the implications of our measurements on the quantum criticality of YbBiPt and we will discuss an unusual feature in the data near 5K. Work at Ames Laboratory was supported by the Department of Energy, Basic Energy Sciences under Contract No. DE-AC02-07CH11358. Work at Occidental College was supported by the National Science Foundation under DMR-1408598.   

Hybridization and coherence in the intermediate valence compound YbAl$_{3}$ via quasiparticle scattering spectroscopy (QPS)$^{\ast }$

Band renormalization and hybridization in Anderson lattices has been a subject of continued interest [1]. The intermediate valence compound YbAl$_{3}$, which does not order magnetically nor superconducts, is a good model system for the study of the hybridization process. A microscopic understanding is still lacking although some characteristic temperature and energy scales have been identified. As shown by our previous works [1,2], QPS is a powerful tool to investigate the hybridization process via measurement of the hybridization gap. Here we report our recent QPS results on YbAl$_{3}$ [3]. Conductance spectra over a wide temperature range indicate that the hybridization process begins around 110 K, a new temperature scale found in this study, well before the full coherence is achieved ($\sim$ 34 K). Our observations agree with the slow crossover scenario discussed recently in the literature [4]. The hybridization gap opens concomitantly with the emergence of a coherent Fermi liquid, suggesting that its measurement can be a more rigorous way to define the coherence temperature. $^{\ast}$The work at UIUC is supported by the NSF DMR 12-06766 and the work done at Ames Lab. was supported under Contract No. DE-AC02-07CH11358.\\[4pt] [1] W. K. Park \textit{et al}., PRL \textbf{108}, 246403 (2012); [2] W. K. Park \textit{et al}., Philos. Mag. (2014), DOI:10.1080/14786435.2014.909613; [3] W. K. Park \textit{et al}., to be submitted; [4] S. Burdin and V. Zlati\'{c}, PRB \textbf{79}, 115139 (2009).   

Visualizing Heavy Fermions in Thin Films by in situ ARPES

Heavy Fermions are an important class of materials, which has attracted a lot of interest as they seemingly host a number of exotic ground states viz. unconventional superconductivity, Quantum Critical Fermi Liquid, FFLO states etc. Stabilizing these materials in a thin film form and extraction of their spectral function via ARPES opens up new possibilities of dimensional and strain tunability and in understanding and designing materials with exotic emergent properties. I will present our recent efforts in stabilizing thin films of Yb based heavy fermion compound YbAl$_{3}$ and the conventional metal analog LuAl$_{3}$ on MgO substrates. With the aid of an Al buffer layer crystalline, phase pure and fully-oriented epitaxial thin films can be grown with sub-nm surface roughness. Using \textit{insitu} angle resolved photoemission we, for the first time have been able to directly map out their electronic bandstructure and Fermi Surface. Measurements on LuAl$_{3}$ were found to be in good agreement with ab-initio calculations that provided us with an excellent reference to identify the signatures of heavy fermion formation in YbAl$_{3}$.