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Fisk, Zachary

Publications and source records attributed to Fisk, Zachary.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Effect of Pressure on Crystal Structure and Phonon Density of States of FeSi

The strongly correlated material FeSi displays several unusual thermal, magnetic, and structural properties under varying pressure-temperature (P-T) conditions. It is a potential thermoelectric alloy and a material with several geochemical implications as a possible constituent at the Earth's core-mantle boundary (CMB). Previous theoretical studies predicted a pressure-induced B20-B2 transition at ambient temperature below 40 GPa; however, experimentally, the structural transition is observed only at high P-T conditions. In this study, we have performed high-pressure powder X-ray diffraction (XRD) up to 90 GPa and Nuclear Resonant Inelastic X-ray Scattering (NRIXS) measurements up to 120 GPa to understand the phase stability and lattice dynamics. Our study provides evidence for a non-hydrostatic stress-induced B20-B2 transition in FeSi at around 36 GPa. We deduced the Fe partial phonon density of states (PDOS) and thermal parameters from NRIXS measurements up to 120 GPa and compared them with density functional theory (DFT) calculations. Furthermore, the computations show pressure-induced metallization and the band gap closing around 12 GPa.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

40 Years of SCES at Los Alamos

Reports of unconventional superconductivity in UBe 13 in 1983 and soon thereafter of the possible coexistence of bulk superconductivity and spin fluctuations in UPt 3 marked the beginning of a 40-year adventure in the study of strongly correlated quantum materials and phenomena at Los Alamos. The subsequent discovery and exploration of heavy-fermion magnetism, cuprates, Kondo insulators, Ce- and Pu-115 superconductors and, more broadly, quantum states of narrow-band systems provided challenges for the next 30 years. Progress was not made in a vacuum but benfitted from significant advances in the Americas, Asia and Europe as well as from essential collaborations, visitors and Los Alamos students and postdocs, many subsequently setting their own course in SCES. As often the case, serendipity played a role in shaping this history.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Probing FeSi, a d -electron topological Kondo insulator candidate, with magnetic field, pressure, and microwaves

Recently, evidence for a conducting surface state (CSS) below 19 K was reported for the correlated d -electron small gap semiconductor FeSi. In the work reported herein, the CSS and the bulk phase of FeSi were probed via electrical resistivity ρ measurements as a function of temperature T , magnetic field B to 60 T, and pressure P to 7.6 GPa, and by means of a magnetic field-modulated microwave spectroscopy (MFMMS) technique. The properties of FeSi were also compared with those of the Kondo insulator SmB 6 to address the question of whether FeSi is a d -electron analogue of an f -electron Kondo insulator and, in addition, a “topological Kondo insulator” (TKI). The overall behavior of the magnetoresistance of FeSi at temperatures above and below the onset temperature T S = 19 K of the CSS is similar to that of SmB 6 . The two energy gaps, inferred from the ρ( T ) data in the semiconducting regime, increase with pressure up to about 7 GPa, followed by a drop which coincides with a sharp suppression of T S . Several studies of ρ( T ) under pressure on SmB 6 reveal behavior similar to that of FeSi in which the two energy gaps vanish at a critical pressure near the pressure at which T S vanishes, although the energy gaps in SmB 6 initially decrease with pressure, whereas in FeSi they increase with pressure. The MFMMS measurements showed a sharp feature at T S ≈ 19 K for FeSi, which could be due to ferromagnetic ordering of the CSS. However, no such feature was observed at T S ≈ 4.5 K for SmB 6 .

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Nodeless superconductivity in the noncentrosymmetric compound ThIrSi

The superconductor ThIrSi, with Tc=6.5K, is expected to show unusual features in view of its noncentrosymmetric structure and the presence of heavy elements featuring a sizable spin-orbit coupling. Here, we report a comprehensive study of its electronic properties by means of magnetization, muon-spin rotation and relaxation (μ⁢SR) and nuclear magnetic resonance (NMR) measurements. Both the superfluid density ρ sc ⁡(T) (determined via transverse-field μ⁢SR) and the spin-lattice relaxation rate T–1 1⁡(T) (determined via NMR) suggest a nodeless superconductivity. Furthermore, the absence of spontaneous magnetic fields below T c , as evinced from zero-field μ⁢SR measurements, indicates a preserved time-reversal symmetry in the superconducting state of ThIrSi. Finally, temperature-dependent upper critical fields as well as field-dependent superconducting muon-spin relaxations suggest the presence of multiple superconducting gaps in ThIrSi.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Bulk transport paths through defects in floating zone and Al flux grown SmB 6

We investigate the roles of disorder on low-temperature transport in SmB 6 crystals grown by both the Al flux and floating zone methods. We used the inverted resistance method with Corbino geometry to investigate whether low-temperature variations in the standard resistance plateau arise from a surface or a bulk channel in floating zone samples. The results show significant sample-dependent residual bulk conduction, in contrast to smaller amounts of residual bulk conduction previously observed in Al flux grown samples with Sm vacancies. In this work, we consider hopping in an activated impurity band as a possible source for the observed bulk conduction, but it is unlikely that the large residual bulk conduction seen in floating zone samples is solely due to Sm vacancies. We therefore propose that one-dimensional defects, or dislocations, contribute as well. Using chemical etching, we find evidence for dislocations in both flux and floating zone samples, with higher dislocation density in floating zone samples than in Al flux grown samples. In addition to the possibility of transport through one-dimensional dislocations, we also discuss our results in the context of recent theoretical models of SmB 6 .

36 MATERIALS SCIENCE↗
DOE OSTI2020

Hall-coefficient diagnostics of the surface state in pressurized SmB 6

In this study, we report results of the high-pressure Hall coefficient (R H ) measurements in the putative topological Kondo insulator SmB 6 up to 37 GPa. Below 10 GPa, our data reveal that R H (T) exhibits a prominent peak upon cooling below 20 K. Remarkably, the temperature at which surface conduction dominates coincides with the temperature of the peak in R H (T). The temperature dependent resistance and Hall coefficient can be well fitted by a two-channel model with contributions from the metallic surface and the thermally activated bulk states. When the bulk of SmB 6 becomes metallic and magnetic at ~10 GPa, both the R H (T) peak and the resistance plateau disappear simultaneously. Here, our results indicate that the R H (T) peak is a fingerprint to diagnose the presence of a metallic surface state in SmB 6 . The high-pressure magnetic state of SmB 6 is robust to 180 GPa, and no evidence of superconductivity is observed in the metallic phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Exploring itinerant states in divalent hexaborides using rare-earth L edge resonant inelastic x-ray scattering

We present a study of resonant inelastic x-ray scattering (RIXS) spectra collected at the rare-earth L edges of divalent hexaborides YbB 6 and EuB 6 . In both systems, RIXS-active features are observed at two distinct resonances separated by ~10 eV in incident energy, with angle-dependence suggestive of distinct photon scattering processes. RIXS spectra collected at the divalent absorption peak resemble the unoccupied 5d density of states calculated using density functional theory. We discuss possible origins of this correspondence including a scenario which changes the 4f valence. In addition, anomalous resonant scattering is observed at higher incident energy, where no corresponding absorption feature is present. Our results demonstrate the potential for L-edge RIXS to assess the itinerant-state properties of -electron materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Putative hybridization gap in CaMn 2 Bi 2 under applied pressure

Here, we study electrical transport measurements on CaMn 2 Bi 2 single crystals under applied pressure. At ambient pressure and high temperatures, CaMn 2 Bi 2 behaves as a single-band semimetal hosting Néel order at T N = 150 K. At low temperatures, multiband behavior emerges along with an activated behavior typical of degenerate semiconductors. The activation gap is estimated to be Δ~ 20 K. Applied pressure not only favors the antiferromagnetic order at a rate of 0.40(2) K/kbar, but it also enhances the activation gap at 20 kbar by about 70%. This gap enhancement is usual of correlated narrow-gap semiconductors such as FeSi and Ce 3 Bi 4 Pt 3 , and it places CaMn 2 Bi 2 as a Mn-based hybridization-gap semiconductor candidate. Ab initio calculations based on density functional theory are demonstrated to be insufficient to describe the ground state of CaMn 2 Bi 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Transport gap in SmB 6 protected against disorder

Significance The realization of modern electrical and optoelectrical devices is possible because of the successful control of point defects in semiconductors or narrow band-gapped insulators. Recently, a new subclass of these semiconductors has been discovered, so-called topological insulators (TIs). These 3D TIs harbor a unique 2D conduction layer on the surfaces that can be potentially applicable for novel quantum devices. The access of these surface states, however, is easily hindered by bulk conduction from unintentional defects. Surprisingly, by using a unique resistance measurement in a strongly correlated TI S m B 6 , we find that typical defect signatures of a semiconductor in electrical transport are absent in the S m B 6 bulk, including samples prepared with large samarium-to-boron off-stoichiometry.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Quantum Oscillations in Flux-Grown SmB 6 with Embedded Aluminum

SmB 6 is a candidate topological Kondo insulator that displays surface conduction at low temperatures. In this study, we perform torque magnetization measurements as a means to detect de Haas–van Alphen (dHvA) oscillations in SmB 6 crystals grown by aluminum flux. We find that dHvA oscillations occur in single crystals containing embedded aluminum, originating from the flux used to synthesize SmB 6 . Measurements on a sample with multiple, unconnected aluminum inclusions show that aluminum crystallizes in a preferred orientation within the SmB 6 cubic lattice. The presence of aluminum is confirmed through bulk susceptibility measurements, but does not show a signature in transport measurements. Lastly, we discuss the ramifications of our results.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Physical properties of Sm x B 6 single crystals

We study the magnetic susceptibility, electrical resistivity, Hall effect, and heat capacity of single crystals of Sm x B 6 for x=1,0.94,0.8, and 0.75. Remarkably, the overall properties of the crystals do not qualitatively depend on the density of vacancies. The topological surface states are seen at low temperatures in the electrical transport properties as the bulk conductivity freezes out. Even a large number of Sm vacancies does not close the hybridization gap. The linear term in the specific heat γ, of unclear origin, remains large but decreases gradually with x. The shoulder in the density of states, brought by the hybridized 4f states and observed in the susceptibility and the specific heat, does not move but decreases its height with decreasing x. The specific heat also displays a weak Schottky anomaly at about 1.5 K for all x, except for the x = 0.75 sample. This Schottky anomaly is only slightly field dependent and of unknown origin.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Localized magnetic moments in metallic SrB 6 single crystals

We report the specific heat C p of metallic SrB 6 single crystals shows an anomalous behavior for T ≲ 5 K which varies strongly with an applied magnetic field. This is consistent with a two-level Schottky system. We ascribe the excess of C p in this temperature range to localized magnetic moments. In addition, features that are attributable to a partial ferromagnetic polarization of a conduction electron gas are observed. Finally, these results are supported by magnetization measurements and are compatible with the transport properties reported previously (Stankiewicz 2016 Phys. Rev. B 94 125141).

36 MATERIALS SCIENCE↗
DOE OSTI2018

Magnetic and defect probes of the SmB 6 surface state

The impact of nonmagnetic and magnetic impurities on topological insulators is a central focus concerning their fundamental physics and possible spintronics and quantum computing applications. Combining scanning tunneling spectroscopy with transport measurements, we investigate, both locally and globally, the effect of nonmagnetic and magnetic substituents in SmB 6 , a predicted topological Kondo insulator. Around the so-introduced substitutents and in accord with theoretical predictions, the surface states are locally suppressed with different length scales depending on the substituent’s magnetic properties. For sufficiently high substituent concentrations, these states are globally destroyed. Similarly, using a magnetic tip in tunneling spectroscopy also resulted in largely suppressed surface states. Hence, a destruction of the surface states is always observed close to atoms with substantial magnetic moment. This points to the topological nature of the surface states in SmB 6 and illustrates how magnetic impurities destroy the surface states from microscopic to macroscopic length scales.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Structural differences between single crystal and polycrystalline UBe 13

Here, we report on observations of structural and chemical differences between samples of UBe 13 that were synthesised using two different methods. Unexplained discrepancies in properties between samples with differing synthesis had previously been found in this heavy fermion superconductor. A polycrystalline UBe 13 sample was made by arc-melting the constituents. Single crystals were grown using an aluminium flux and had a consistently slightly larger lattice parameter than the polycrystals, which merited further study. Neutron diffraction data were collected at the Lujan Center at LANSCE on the HIPPO diffractometer. Aluminium was detected by inductively coupled plasma mass spectrometry (ICP-MS) in the flux-grown single crystal (0.803 wt%), and small amounts (~0.2 wt%) of thorium were detected in the UBe 13 polycrystalline sample. In order to probe the implications of the presence of Al, calculations by spin-polarised DFT-GGA+U show that the incorporation of Al onto the 96i site (the lowest symmetry site in the structure) is energetically more favourable than on other sites. In general, the trends calculated by DFT for bond lengths and lattice parameter increases are consistent with bond lengths experimentally observed by neutron diffraction, but specific percentage changes with aluminium incorporation may be obscured by the unexpected thorium in the polycrystalline sample. The aggregate of our initial observations suggests that incorporation of aluminium from the flux into single crystal UBe 13 is significant.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Evolution of ground-state wave function in CeCoIn 5 upon Cd or Sn doping

We present linear polarization-dependent soft-x-ray absorption spectroscopy data at the Ce M 4,5 edges of Cd- and Sn-doped CeCoIn 5 . The 4f ground-state wave functions have been determined for their superconducting, antiferromagnetic, and paramagnetic ground states. The absence of changes in the wave functions in CeCo (In 1- x Cd x ) 5 suggests that the 4f-conduction-electron (c f) hybridization is not affected by global Cd doping, thus supporting the interpretation of magnetic droplets nucleating long-range magnetic order. This is contrasted by changes in the wave function due to Sn substitution. Increasing Sn in CeCo (In 1 - y Sn y ) 5 compresses the 4f orbitals into the tetragonal plane of these materials, suggesting enhanced c f hybridization with the in-plane In(1) atoms and a homogeneous altering of the electronic structure. As these experiments show, the 4 f wave functions are a very sensitive probe of small changes in the hybridization of 4f and conduction electrons, even conveying information about direction dependencies.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Dual roles of f electrons in mixing Al 3 p character into d -orbital conduction bands for lanthanide and actinide dialuminides

Correlated electron phenomena in lanthanide and actinide materials are driven by a complex interplay between the f and d orbitals. Here in this study, aluminum K-edge x-ray absorption spectroscopy and density functional theory calculations are used to evaluate the electronic structure of the dialuminides, MAl 2 (M = Ce, Sm, Eu, Yb, Lu, U, and Pu). The results show how the energy and occupancy of the 4f or 5f orbitals impacts mixing of Al 3p character into the 5d or 6d conduction bands, which has implications for understanding the magnetic and structural properties of correlated electron systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗