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Fisk, Zachary

Publications and source records attributed to Fisk, Zachary.

28 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

An effect of Sm vacancies on the hybridization gap in topological Kondo insulator candidate SmB 6

A necessary element for the predicted topological state in Kondo insulator SmB 6 is the hybridization gap which opens in this compound at low temperatures. Here, in this work, we present a comparative study of the in-gap density of states due to Sm vacancies by Raman scattering spectroscopy and heat capacity for samples where the number of Sm vacancies is equal to or below 1%. We demonstrate that hybridization gap is very sensitive to the presence of Sm vacancies. Lastly, at the amount of vacancies above 1% the gap fills in with impurity states and low temperature heat capacity is enhanced.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Quantum phase transition and destruction of Kondo effect in pressurized SmB 6

SmB 6 has been a well-known Kondo insulator for decades, but recently attracts extensive new attention as a candidate topological system. Studying SmB 6 under pressure provides an opportunity to acquire the much-needed understanding about the effect of electron correlations on both the metallic surface state and bulk insulating state. Here we do so by studying the evolution of two transport gaps (low temperature gap E l and high temperature gap E h ) associated with the Kondo effect by measuring the electrical resistivity under high pressure and low temperature (0.3 K) conditions. We associate the gaps with the bulk Kondo hybridization, and from their evolution with pressure we demonstrate an insulator-to-metal transition at ~4 GPa. At the transition pressure, a large change in the Hall number and a divergence tendency of the electron-electron scattering coefficient provide evidence for a destruction of the Kondo entanglement in the ground state. In conclusion, our results raise the new prospect for studying topological electronic states in quantum critical materials settings.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Effects of spin excitons on the surface states of SmB 6 : A photoemission study

We present the results of a high-resolution valence-band photoemission spectroscopic study of SmB 6 which shows evidence for a V-shaped density of states of surface origin within the bulk gap. The spectroscopy data are interpreted in terms of the existence of heavy 4 f surface states, which may be useful in resolving the controversy concerning the disparate surface Fermi-surface velocities observed in experiments. Most importantly, we find that the temperature dependence of the valence-band spectrum indicates that a small feature appears at a binding energy of about - 9 meV at low temperatures. We also attribute this feature to a resonance caused by the spin-exciton scattering in SmB 6 which destroys the protection of surface states due to time-reversal invariance and spin-momentum locking. Thus, the existence of a low-energy spin exciton may be responsible for the scattering, which suppresses the formation of coherent surface quasiparticles and the appearance of the saturation of the resistivity to temperatures much lower than the coherence temperature associated with the opening of the bulk gap.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Quasi-particle Interference of Heavy Fermions in Resonant X-ray Scattering

Resonant x-ray scattering (RXS) has recently become an increasingly important tool for the study of ordering phenomena in correlated electron systems. Yet, the interpretation of RXS experiments remains theoretically challenging because of the complexity of the RXS cross section. Central to this debate is the recent proposal that impurity-induced Friedel oscillations, akin to quasi-particle interference signals observed with a scanning tunneling microscope (STM), can lead to scattering peaks in RXS experiments. The possibility that quasi-particle properties can be probed in RXS measurements opens up a new avenue to study the bulk band structure of materials with the orbital and element selectivity provided by RXS. We test these ideas by combining RXS and STM measurements of the heavy fermion compound CeMIn 5 (M = Co, Rh). Temperature- and doping-dependent RXS measurements at the Ce-M 4 edge show a broad scattering enhancement that correlates with the appearance of heavy f-electron bands in these compounds. The scattering enhancement is consistent with the measured quasi-particle interference signal in the STM measurements, indicating that the quasi-particle interference can be probed through the momentum distribution of RXS signals. Overall, our experiments demonstrate new opportunities for studies of correlated electronic systems using the RXS technique.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Electrical transport properties of single-crystal CaB 6 , SrB 6 , and BaB 6

We measure the electrical resistivity and Hall effect of alkaline-earth-metal hexaboride single crystals as a function of temperature, hydrostatic pressure, and magnetic field. The transport properties vary weakly with the external parameters and are modeled in terms of intrinsic variable-valence defects. These defects can stay either in (1) delocalized shallow levels or in (2) localized levels resonant with the conduction band, which can be neutral or negatively charged. Satisfactory agreement is obtained for electronic transport properties in a broad temperature and pressure range, though fitting the magnetoresistance is less straightforward and a combination of various mechanisms is needed to explain the field and temperature dependences.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Unusual Kondo-hole effect and crystal-field frustration in Nd-doped CeRhIn 5

In this research, we investigate single crystals of Ce 1₋x Nd x RhIn 5 by means of x-ray-diffraction, microprobe, magnetic susceptibility, heat capacity, and electrical resistivity measurements. Our data reveal that the antiferromagnetic transition of CeRhIn 5 , at T$Ce\atop{N}$=3.8 K, is linearly suppressed with x Nd . We associate this effect with the presence of a “Kondo hole” created by Nd substitution. The extrapolation of T$Ce\atop{N}$ to zero temperature, however, occurs at x c ~0.3, which is below the two-dimensional percolation limit found in Ce 1₋x La x RhIn 5 . This result strongly suggests the presence of a crystal-field induced magnetic frustration. Near x Nd ~0.2, the Ising antiferromagnetic order from Nd 3+ ions is stabilized and T$Nd\atop{N}$ increases up to 11 K in NdRhIn 5 . Finally, our results shed light on the effects of magnetic doping in heavy-fermion antiferromagnets and stimulate the study of such systems under applied pressure.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Fermi surface topology and hot spot distribution in the Kondo lattice system CeB 6

Rare-earth hexaborides have attracted considerable attention recently in connection to a variety of correlated phenomena including heavy fermions, superconductivity, and low-temperature magnetic phases. Here, we present high-resolution angle-resolved photoemission spectroscopy studies of trivalent CeB 6 and divalent BaB 6 rare-earth hexaborides. Here we find that the Fermi surface electronic structure of CeB 6 consists of large oval-shaped pockets around the X points of the Brillouin zone, whereas the states around the zone center Γ point are strongly renormalized. Our first-principles calculations agree with our experimental results around the X points but not around the Γ point, indicating areas of strong renormalization located near Γ. The Ce quasiparticle states participate in the formation of hot spots at the Fermi surface, whereas the incoherent f states hybridize and lead to the emergence of dispersive features absent in the non-$f$ counterpart BaB 6 . Lastly, our results provide an understanding of the electronic structure in rare-earth hexaborides, which will be useful in elucidating the nature of the exotic low-temperature phases in these materials.

36 MATERIALS SCIENCE↗