L. Shu

University of California, San Diego, San Diego, California, United States

Are you L. Shu?

Claim your profile

Publications (68)110.95 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Measurements of physical properties show that Yb enters the single crystals systematically and in registry with the nominal Yb concentration x of the starting material dissolved in the molten indium flux.
    07/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Muon spin rotation and relaxation ($\mu$SR) experiments have been carried out to characterize magnetic and superconducting ground states in the Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ alloy series. In the ferromagnetic end compound NdOs$_4$Sb$_{12}$ the spontaneous local field at positive-muon ($\mu^+$) sites below the ordering temperature $T_C$ is greater than expected from dipolar coupling to ferromagnetically aligned Nd$^{3+}$ moments, indicating an additional indirect RKKY-like transferred hyperfine mechanism. For 0.45 $\le x \le$ 0.75, $\mu^+$ spin relaxation rates in zero and weak longitudinal applied fields indicate that static fields at $\mu^+$ sites below $T_C$ are reduced and strongly disordered. We argue this is unlikely to be due to reduction of Nd$^{3+}$ moments, and speculate that the Nd$^{3+}$-$\mu^+$ interaction is suppressed and disordered by Pr doping. In an $x$ = 0.25 sample, which is superconducting below $T_c$ = 1.3 K, there is no sign of "spin freezing" (static Nd$^{3+}$ magnetism), ordered or disordered, down to 25 mK. Dynamic $\mu^+$ spin relaxation is strong, indicating significant Nd-moment fluctuations. The $\mu^+$ diamagnetic frequency shift and spin relaxation in the superconducting vortex-lattice phase decrease slowly below $T_c$, suggesting pair breaking and/or possible modification of Fermi-liquid renormalization by Nd spin fluctuations. For 0.25 $\le x \le$ 0.75, the $\mu$SR data provide evidence against phase separation; superconductivity and Nd$^{3+}$ magnetism coexist on the atomic scale.
    04/2014;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report measurements of electrical resistivity, magnetic susceptibility, specific heat, and thermoelectric power on the system Pr1-xCexPt4Ge12. Superconductivity is suppressed with increasing Ce concentration up to x = 0.5, above which there is no evidence for superconductivity down to 1.1 K. The Sommerfeld coefficient {\gamma} increases with increasing x from 48 mJ/mol K^2 up to 120 mJ/mol K^2 at x = 0.5, indicating an increase in strength of electronic correlations. The temperature dependence of the specific heat at low temperatures evolves from roughly T^3 for x = 0 to e^(-\Delta /T) behavior for x = 0.05 and above, suggesting a crossover from a nodal to a nodeless superconducting energy gap or a transition from multiband to single-band superconductivity. Fermi-liquid behavior is observed throughout the series in low-temperature magnetization, specific heat, and electrical resistivity measurements.
    03/2014; 89(3).
  • [Show abstract] [Hide abstract]
    ABSTRACT: The electronic structure of Ce_{1−x}Yb_{x}CoIn_{5} has been studied by a combination of photoemission, x-ray absorption, and bulk property measurements. Previous findings of a Ce valence near 3+ for all x and of an Yb valence near 2.3+ for x⩾0.3 were confirmed. One new result of this study is that the Yb valence for x⩽0.2 increases rapidly with decreasing x from 2.3 toward 3+, which correlates well with de Haas van Alphen results showing a change of Fermi surface around x=0.2. Another new result is the direct observation by angle resolved photoemission Fermi surface maps of ≈50% cross-sectional area reductions of the α and β sheets for x=1 compared to x=0, and a smaller, essentially proportionate, size change of the α sheet for x=0.2. These changes are found to be in good general agreement with expectations from simple electron counting. The implications of these results for the unusual robustness of superconductivity and Kondo coherence with increasing x in this alloy system are discussed.
    Physical Review B. 10/2013; 88(16).
  • [Show abstract] [Hide abstract]
    ABSTRACT: One of the greatest challenges to Landau's Fermi liquid theory-the standard theory of metals-is presented by complex materials with strong electronic correlations. In these materials, non-Fermi liquid transport and thermodynamic properties are often explained by the presence of a continuous quantum phase transition that happens at a quantum critical point (QCP). A QCP can be revealed by applying pressure, magnetic field, or changing the chemical composition. In the heavy-fermion compound CeCoIn5, the QCP is assumed to play a decisive role in defining the microscopic structure of both normal and superconducting states. However, the question of whether a QCP must be present in the material's phase diagram to induce non-Fermi liquid behavior and trigger superconductivity remains open. Here, we show that the full suppression of the field-induced QCP in CeCoIn5 by doping with Yb has surprisingly little impact on both unconventional superconductivity and non-Fermi liquid behavior. This implies that the non-Fermi liquid metallic behavior could be a new state of matter in its own right rather than a consequence of the underlying quantum phase transition.
    Proceedings of the National Academy of Sciences 04/2013; · 9.74 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The electronic structure of (Ce,Yb)CoIn5 has been studied by a combination of photoemission, x-ray absorption and bulk property measurements. Previous findings of a Ce valence near 3+ for all x and of an Yb valence near 2.3+ for x>0.3 were confirmed. One new result of this study is that the Yb valence for x<0.2 increases rapidly with decreasing x from 2.3+ toward 3+, which correlates well with de Haas van Alphen results showing a change of Fermi surface around x=0.2. Another new result is the direct observation by angle resolved photoemission Fermi surface maps of about 50% cross sectional area reductions of the \alpha- and \beta-sheets for x=1 compared to x=0, and a smaller, essentially proportionate, size change of the \alpha-sheet for x=0.2. These changes are found to be in good general agreement with expectations from simple electron counting. The implications of these results for the unusual robustness of superconductivity and Kondo coherence with increasing x in this alloy system are discussed.
    Physical Review B 03/2013; · 3.66 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Recent x-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements on the superconducting heavy fermion system Ce1-xYbxCoIn5 reveal that the correlated electron state is stabilized throughout the range 0 < x < 0.8, apparently due to cooperative behavior of Ce and Yb ions involving their unstable valences. Phase separation occurs for x > 0.8. Interestingly, the superconducting critical temperature decreases linearly with x from 2.3 K at x = 0 towards 0 K at x = 1. Transverse-field muon spin rotation experiments have been performed on Ce1-xYbxCoIn5 alloys. Based on these measurements, we report the absolute value of magnetic penetration depth as a function of x and discuss whether Tc is controlled by the superfluid density of superconducting carriers. The results are compared to a recently proposed theory for the superconductivity in Ce1-xYbxCoIn5
    03/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: The pseudo ternary system Pr1-xNdxOs4Sb12 has been used as a model system to investigate the effect of ferromagnetism (FM) on the unconventional superconductivity (SC) and quantum critical behavior of PrOs4Sb12 [1]. SC in this system disappears near a critical concentration xcr,1˜0.58 and FM appears above xcr,2˜0.33 [1,2]. The new μSR measurements have been performed on samples with x = 0.25, 0.75, and 1. For x = 1 and 0.75, the estimated frozen moments agree with the Nd^3+ CEF ground state moment. For x = 0.25, neither time reversal symmetry breaking nor evidence of freezing of Nd^3+ spins was observed in zero-field μSR measurements, the behavior of which is very different than what is observed for x = 0.45-0.55 [2]. In the SC state, an unexpected linear T dependence of the Gaussian relaxation rate was also found in the transverse field μSR data for x = 0.25, which is different than the plateau in PrOs4Sb12 below 1.3K [3]. [1] Ho, et al., PRB 83, 024511 (2011).[2] Ho, et al., 2010 APS March Meeting, A38.00005 (2010). [3] MacLaughlin et al., PRL 105, 019701 (2010).
    03/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: The temperature-dependent ac susceptibility of the filled skutterudite superconductor PrPt4Ge12 has been measured under hydrostatic He-gas pressure to 0.58 GPa. The superconducting transition temperature Tc decreases linearly with pressure P from 7.91 K at ambient pressure to 7.83 K at 0.58 GPa, giving the rate dTc/dP = −0.19 ± 0.03 K/GPa. Evidence is presented that suggests that the value of Tc in this compound is slightly reduced due to magnetic pair-breaking effects from the Pr3+ cations.
    Physica C Superconductivity 02/2013; 485:160–162. · 0.72 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report measurements of the thermoelectric power Sand electrical resistivity ρ for correlated electron systems Ln 3Pt4Ge6and LnPt4Ge12(Ln= Ce, Pr) and X 2 T 12P7(X= Yb, Hf and T= Fe, Co). The thermoelectric power factor S 2/ ρ is utilized as a means to assess the potential viability of these materials for thermoelectric applications. Sis observed to be sensitive to Lnin Ln 3Pt4Ge6and LnPt4Ge12with Ce-based compounds providing a much larger Sand S 2/ ρ than Pr-based materials. The character of Sfor the Ce-based compounds is consistent with an intermediate Ce valence. In the case of X 2 T 12P7compounds reported herein, it appears that it is possible to tune the magnitude of Smore effectively by varying Trather than X; the magnitude of Sis significantly larger with T= Fe than when T= Co.
    01/2013;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: One of the greatest challenges to Landau's Fermi liquid theory - the standard theory of metals - is presented by complex materials with strong electronic correlations. In these materials, non-Fermi liquid transport and thermodynamic properties are often explained by the presence of a continuous quantum phase transition which happens at a quantum critical point (QCP). A QCP can be revealed by applying pressure, magnetic field, or changing the chemical composition. In the heavy-fermion compound CeCoIn$_5$, the QCP is assumed to play a decisive role in defining the microscopic structure of both normal and superconducting states. However, the question of whether QCP must be present in the material's phase diagram to induce non-Fermi liquid behavior and trigger superconductivity remains open. Here we show that the full suppression of the field-induced QCP in CeCoIn$_5$ by doping with Yb has surprisingly little impact on both unconventional superconductivity and non-Fermi liquid behavior. This implies that the non-Fermi liquid metallic behavior could be a new state of matter in its own right rather then a consequence of the underlying quantum phase transition.
    08/2012;
  • [Show abstract] [Hide abstract]
    ABSTRACT: The pseudo ternary system Pr1-xNdxOs4Sb12 has been used as a model system to investigate the effect of ferromagnetism (FM) on the unconventional superconductivity (SC), the high field ordered phase (HFOP), and quantum critical behavior [1], that was observed in PrOs4Sb12. SC in this system disappears near the Nd concentration x ˜ 0.58. Between x ˜ 0.33 and ˜ 0.58, weak FM, confirmed by the μSR experiments [2], was found to coexist with SC. In order to further inspect the possible quantum critical behavior, a power-law analysis of the temperature dependence of the electrical resistivity data was performed. Upon suppression of SC, for samples in the range 0.33 < x < 0.58, the power-law exponent decreases from ˜ 1.8 toward 1 in the temperature region below 2.5,, resembling non-Fermi liquid behavior. Detailed T-x, H-x, and H-T phase diagrams for various x will be discussed. [4pt] [1] Ho, et. al., PRB 83, 024511 (2011).[0pt] [2] Ho, et. al., 2010 APS March Meeting, A38.00005 (2010).
    02/2012;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Generally, rare-earth substitution for Ce in the heavy fermion superconductor CeCoIn5 suppresses superconductivity rapidly. However, it was recently reported that the correlated electron ground state of Ce1-xYbxCoIn5 is stabilized over an anomalously large range in x, perhaps because of cooperative valence fluctuations of the Ce and Yb ions. Motivated by this possibility, we studied the effect of applied pressure on the superconducting critical (Tc) and Kondo-lattice coherence (T^*) temperatures of Ce1-xRxCoIn5 with R = Yb, Y, and Gd in order to compare the effect of Yb substitution with other magnetic and non-magnetic rare-earth ion substitutions. We performed electrical resistivity measurements under pressures up to a maximum of ˜2.3 GPa in a piston-cylinder clamped high pressure cell using a 50:50 mixture of n-pentane and isoamyl alcohol for the pressure transmitting medium. It was found that the variations of Tc and T^* in Ce1-xRxCoIn5 under pressure were approximately independent of R. This result implies that the effect of pressure is independent of the magnetic configuration of the rare-earth ion being introduced.
    02/2012;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We performed magnetoresistance and Hall effect measurements on Yb-doped CeCoIn5. The longitudinal resistivity data measured in 14 T show that the onset of coherence in the dilute Kondo lattice remains robust with respect to Yb concentration. In addition, we find that the superconducting transition temperature is weakly suppressed with doping (x<=0.2). Our analysis of the magnetoresistance data allowed us to identify the magnetic field induced quantum critical point and its evolution upon doping. At high Yb concentrations, our Hall effect data point to a possible valence transition of Yb ions. At small doping, our results provide an insight into the nature of the interplay between quantum criticality, magnetism, and unconventional superconductivity, while the behavior of this system at high doping can be characterized by a subtle interplay between Kondo screening on Ce sites and strong valence fluctuations on Yb sites.
    02/2012;
  • [Show abstract] [Hide abstract]
    ABSTRACT: X-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements on Ce(1-x)Yb(x)CoIn5 (0≤x≤1) reveal that many of the characteristic features of the x=0 correlated electron state are stable for x≤0.775 and that phase separation occurs for x>0.775. The stability of the correlated electron state is apparently due to cooperative behavior of the Ce and Yb ions, involving their unstable valences. Low-temperature non-Fermi liquid behavior is observed and varies with x, even though there is no readily identifiable quantum critical point. The superconducting critical temperature T(c) decreases linearly with x towards 0 K as x→1, in contrast with other HF superconductors where T(c) scales with T(coh).
    Physical Review Letters 04/2011; 106(15):156403. · 7.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Zero-field muon spin relaxation experiments have been carried out in the Pr(Os1-xRux)4Sb12 and Pr1-yLayOs4Sb12 alloy systems to investigate broken time-reversal symmetry (TRS) in the superconducting state, signaled by the onset of a spontaneous static local magnetic field Bs. In both alloy series Bs initially decreases linearly with solute concentration. Ru doping is considerably more efficient than La doping, with a ~50% faster initial decrease. The data suggest that broken TRS is suppressed for Ru concentration x≳0.6 but persists for essentially all La concentrations. Our data support a crystal-field excitonic Cooper pairing mechanism for TRS-breaking superconductivity.
    Physical review. B, Condensed matter 03/2011; 83(10). · 3.77 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have investigated the chemical substitution series Ce1-xYbxCoIn5 (0
    03/2011;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Ce1-xYbxCoIn5 (YbCe115) is a new 115 alloy series with long range magnetic order suppressed in the whole substitution range. Measurements reveal a rich phase diagram in which the Kondo lattice is robust against Yb substitution, superconductivity persists to high values of x, and the non-Fermi-liquid behavior is enhanced by Yb substitution [1].We have characterized the electronic structure of this new alloy by x-ray and angle resolved photoemission spectroscopy (XPS and ARPES) performed at the Advanced Light Source (ALS). Yb 4f XPS spectra vary with increasing x from dominantly Yb^3+ to a mix of Yb^3+ and Yb^2+, in agreement with inferences from the magnetic susceptibility [1]. We will present ARPES data to show the x-dependence of the Fermi surface and discuss the relation of the electronic structure to the transport properties. The effect of surface contributions to the XPS and ARPES data will be assessed. [4pt] [1] Lei Shu et al, to be published
    03/2011;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Zero-field muon spin relaxation (muSR) experiments have been carried out in the Pr(Os1-xRux)4Sb12 and Pr1-yLayOs4Sb12 alloy systems to investigate broken time-reversal symmetry (TRS) in the superconducting state, signaled by the onset of a spontaneous static local magnetic field Bs. In both alloy series Bs initially decreases linearly with solute concentration. Ru doping is considerably more efficient than La doping ,with a ˜50% faster initial decrease. The data suggest that broken TRS is suppressed for Ru concentration larger than 0.6, but persists for essentially all La concentrations. Our data support the theory of TRS-breaking superconductivity via crystal-field excitonic Cooper pairing of Koga, Matsumoto, and Shiba.
    03/2011;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Zero-field muon spin relaxation experiments have been carried out in the Pr(Os_{1-x}Ru_x)_4Sb_12 and Pr_{1-y}La_yOs_4Sb_12 alloy systems to investigate broken time-reversal symmetry (TRS) in the superconducting state, signaled by the onset of a spontaneous static local magnetic field B_s. In both alloy series B_s initially decreases linearly with solute concentration. Ru doping is considerably more efficient than La doping, with a ~50% faster initial decrease. The data suggest that broken TRS is suppressed for Ru concentration x >~ 0.6, but persists for essentially all La concentrations. Our data support a crystal-field excitonic Cooper pairing mechanism for TRS-breaking superconductivity.
    02/2011;

Publication Stats

137 Citations
110.95 Total Impact Points

Institutions

  • 2009–2013
    • University of California, San Diego
      • Department of Physics
      San Diego, California, United States
  • 2005–2011
    • University of California, Riverside
      • Department of Physics and Astronomy
      Riverside, CA, United States
    • TRIUMF
      Vancouver, British Columbia, Canada
  • 2008
    • Applied Physical Sciences
      Groton, Connecticut, United States