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Register a new userAnomalous Anisotropic Magnetoresistance in Heavy-Fermion PrFe4P12
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We have investigated the anisotropy of the magnetoresistance in the Pr-based HF compound PrFe4P12. The large anisotropy of effective mass and its strong field dependence have been confirmed by resistivity measurements. Particularly for H||[111], where the effective mass is most strongly enhanced, the non-Fermi liquid behavior has been observed. Also, we have found the angular dependence of the magnetoresistance sharply enhanced at H||[111], which is evidently correlated with both the non-Fermi liquid behavior and the high-field ordered state (B-phase).
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Specific heat and magnetization measurements have been performed on high-quality single crystals of filled-skutterudite PrFe_4P_{12} in order to study the high-field heavy-fermion state (HFS) and low-field ordered state (ODS). From a broad hump observed in C/T vs T in HFS for magnetic fields applied along the <100> direction, the Kondo temperature of ~ 9 K and the existence of ferromagnetic Pr-Pr interactions are deduced. The {141}-Pr nuclear Schottky contribution, which works as a highly-sensitive on-site probe for the Pr magnetic moment, sets an upper bound for the ordered moment as ~ 0.03 mu_B/Pr-ion. This fact strongly indicates that the primary order parameter in the ODS is nonmagnetic and most probably of quadrupolar origin, combined with other experimental facts. Significantly suppressed heavy-fermion behavior in the ODS suggests a possibility that the quadrupolar degrees of freedom is essential for the heavy quasiparticle band formation in the HFS. Possible crystalline-electric-field level schemes estimated from the anisotropy in the magnetization are consistent with this conjecture.
We report the discovery of giant and anisotropic magnetoresistance due to the orbital rearrangement in a non-magnetic correlated metal. In particular, we measured the magnetoresistance under fields up to 31.4 T in the cubic Pr-based heavy fermion superconductor PrV$_2$Al$_{20}$ with a non-magnetic $Gamma _3$ doublet ground state, exhibiting antiferro-quadrupole ordering below 0.7 K. For the [100] direction, we find that the high-field phase appears between 12 T and 25 T, accompanied by a large jump at 12 T in the magnetoresistance ($Delta MR sim $ 100 $% $) and in the anisotropic magnetoresistivity (AMR) ratio by $sim $ 20 $% $. These observations indicate that the strong hybridization between the conduction electrons and anisotropic quadrupole moments leads to the Fermi surface reconstruction upon crossing the field-induced antiferro-quadrupole (orbital) rearrangement.
We grew single crystals of the recently discovered heavy fermion superconductor UTe2, and measured the resistivity, specific heat and magnetoresistance. Superconductivity (SC) was clearly detected at Tsc=1.65K as sharp drop of the resistivity in a high quality sample of RRR=35. The specific heat shows a large jump at Tsc indicating strong coupling. The large Sommerfeld coefficient, 117mJ K-2mol-1 extrapolated in the normal state and the temperature dependence of C/T below Tsc are the signature of unconventional SC. The discrepancy in the entropy balance at Tsc between SC and normal states points out that hidden features must occur. Surprisingly, a large residual value of the Sommerfeld coefficient seems quite robust (gamma_0/gamma ~ 0.5). The large upper critical field Hc2 along the three principal axes favors spin-triplet SC. For H // b-axis, our experiments do not reproduce the huge upturn of Hc2 reported previously. This discrepancy may reflect that Hc2 is very sensitive to the sample quality. A new perspective in UTe2 is the proximity of a Kondo semiconducting phase predicted by the LDA band structure calculations.
The resistance of a metal in a magnetic field can be very illuminating about its ground state. Some famous examples include the integer and fractional quantum Hall effectscite{Klitzing-QHE,Tsui-FQHE}, Shubnikov-de Haas oscillationscite{SdH}, and weak localizationcite{Lee-WL} emph{et al}. In non-interacting metals the resistance typically increases upon the application of a magnetic fieldcite{Pippard-MR}. In contrast, in some special circumstances metals, with anisotropic Fermi surfacescite{Kikugawa-PdCoO2LMR} or a so-called Weyl semimetal for instancecite{Nielsen-ABJ,Son-ChirAnom}, may have negative magnetoresistance. Here we show that semimetallic TaAs$_2$ possesses a gigantic negative magnetoresistance ($-$98% in a field of 3 T at low temperatures), with an unknown mechanism. Density functional calculations illustrate that TaAs$_2$ is a new topological semimetal [$mathbb{Z}_2$ invariant (0;111)] without a Dirac dispersion. This demonstrates that the presence of negative magnetoresistance in non-magnetic semimetals cannot be uniquely attributed to the Adler-Bell-Jackiw anomaly of bulk Dirac/Weyl fermions. Our results also imply that the OsGe$_2$-type monoclinic dipnictides are likely a material basis where unconventional topological semimetals may be found.
The superconducting order parameter of the first heavy-fermion superconductor CeCu2Si2 is currently under debate. A key ingredient to understand its superconductivity and physical properties is the quasiparticle dispersion and Fermi surface, which remains elusive experimentally. Here we present measurements from angle-resolved photoemission spectroscopy. Our results emphasize the key role played by the Ce 4f electrons for the low-temperature Fermi surface, highlighting a band-dependent conduction-f electron hybridization. In particular, we find a very heavy quasi-two-dimensional electron band near the bulk X point and moderately heavy three-dimensional hole pockets near the Z point. Comparison with theoretical calculations reveals the strong local correlation in this compound, calling for further theoretical studies. Our results provide the electronic basis to understand the heavy fermion behavior and superconductivity; implications for the enigmatic superconductivity of this compound are also discussed.
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