Engineering topics

Thomas, Sean Michael

Publications and source records attributed to Thomas, Sean Michael.

At least 19 records

DOE OSTI2024

Nanometric modulations of the magnetic structure of the element Nd

The rare earth neodymium arguably exhibits the most complex magnetic ordering and series of magnetic phase transitions of the elements. Here we report the results of small-angle neutron scattering (SANS) measurements as a function of temperature and applied magnetic field to study magnetic correlations on nanometer length scales in Nd. We support these results with additional neutron diffraction, thermodynamic and synchrotron x-ray measurements. The SANS measurements reveal the presence of modulation vectors characterizing the ordered spin configuration which exhibit changes in magnitude and direction that are phase dependent. Between 5.9 and 7.6 K an additional modulation vector is observed with magnitude Q = 0.12 Å -1 . This scattering appears to originate from order of the Nd layers which contain a center of inversion. Tracking this modulation vector as a function of magnetic field indicates a phase boundary at ≈1 T. The modulation vectors observed in this study establish the presence of nanometer length scale spin textures which are likely stabilized by frustrated Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Probing quantum criticality in ferromagnetic CeRh 6 Ge 4

CeRh 6 Ge 4 is unusual in that its ferromagnetic transition can be suppressed continuously to zero temperature, i.e., to a ferromagnetic quantum-critical point (QCP), through the application of modest hydrostatic pressure. This discovery has raised the possibility that the ferromagnetic QCP may be of the Kondo-breakdown type characterized by a jump in Fermi volume, to which thermopower S measurements should be sensitive. Further, though S/T changes both sign and magnitude around the critical pressure P c ≈ 0.8 GPa, these changes are not abrupt but extend over a pressure interval from within the ferromagnetic state up to P c . Together with temperature and pressure variations in electrical resistivity and previously reported heat capacity, thermopower results point to the near coincidence of two sequential effects near P c , delocalization of 4f degrees of freedom through orbital-selective hybridization followed by quantum criticality of itinerant ferromagnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2024

Two-Stage Cryogenic HEMT-Based Amplifier for Low-Temperature Detectors

To search for dark matter candidates with masses below $\mathcal{O}$ (MeV), the SPLENDOR (Search for Particles of Light dark mattEr with Narrow-gap semiconDuctORs) experiment is developing novel narrow-bandgap semiconductors with electronic bandgaps on the order of 1–100 meV. In order to detect the charge signal produced by scattering or absorption events, SPLENDOR has designed a two-stage cryogenic HEMT-based amplifier with an estimated charge resolution approaching the single-electron level. A low-capacitance (~ 1.6 pF) HEMT is used as a buffer stage at T = 10 mK to mitigate effects of stray capacitance at the input. The buffered signal is then amplified by a higher-capacitance (~ 200 pF) HEMT amplifier stage at T = 4 K. Importantly, the design of this amplifier makes it usable with any insulating material—allowing for rapid prototyping of a variety of novel detector materials. Here, we present the two-stage cryogenic amplifier design, preliminary voltage noise performance, and estimated charge resolution of 7.2 electrons.

47 OTHER INSTRUMENTATION↗

DOE OSTI2023

Localized 𝑓-electron magnetism in the semimetal Ce 3 ⁢ Bi 4 ⁢ Au 3

Ce 3 ⁢Bi 4 ⁢ Au 3 crystallizes in the same noncentrosymmetric cubic structure as the prototypical Kondo insulator Ce 3 ⁢ Bi 4 ⁢Pt 3 . Here we report the physical properties of Ce 3 ⁢Bi 4 ⁢ Au 3 single crystals using magnetization, thermodynamic, and electrical-transport measurements. Magnetic-susceptibility and heat-capacity data reveal antiferromagnetic order below 𝑇 𝑁 =3.2K. The magnetic entropy 𝑆 mag reaches 𝑅⁢ ln⁡ 2 slightly above 𝑇 𝑁 , which suggests localized 4⁢𝑓 moments in a doublet ground state. Multiple field-induced magnetic transitions are observed at temperatures below 𝑇 𝑁 , which indicate a complex spin structure with competing interactions. Ce 3 ⁢Bi 4 ⁢ Au 3 shows semimetallic behavior in electrical resistivity in contrast to the majority of reported cerium-based 343 compounds which are semiconducting. Electrical-resistivity measurements under hydrostatic pressure reveal a slight enhancement of 𝑇 𝑁 under pressures up to 2.3 GPa, which supports a scenario wherein Ce 3 ⁢Bi 4 ⁢Au 3 belongs to the far left of the Doniach phase diagram dominated by Ruderman-Kittel-Kasuya-Yosida interactions. Using realistic many-body simulations, we confirm the semimetallic electronic structure of Ce 3 ⁢Bi 4 ⁢ Au 3 and quantitatively reproduce its local moment behavior in the paramagnetic state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

$μ^+$ Knight shift in UTe 2 : Evidence for relocalization in a Kondo lattice

The local magnetic susceptibility of the spin-triplet superconductor UTe 2 has been investigated by positive muon (μ + ) Knight shift measurements in the normal state. Three distinct μ + Knight shift components are observed for a magnetic field applied parallel to the c axis. Two of these exhibit a breakdown in the linear relationship with the bulk magnetic susceptibility (χ) below a temperature T*~30K, which points to a gradual emergence of a correlated Kondo liquid. Below T r ~12K linearity is gradually restored, indicating relocalization of the Kondo liquid quasiparticles. The third Knight shift component is two orders of magnitude larger, and despite the c-axis alignment of the external field, scales with the a axis χ above T r ~12K. We conjecture that this component is associated with magnetic clusters and the change in the temperature dependence of all three Knight shift components below T r is associated with a change in magnetic correlations. Finally, our findings indicate that prior to the onset of superconductivity the development of the itinerant heavy-electron fluid is halted by a gradual development of local U 5⁢ƒ-moment fluctuations.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Optimizing Conductivity and Hardness in Cu–Nb Nanolamellar Composites Fabricated Through Accumulative Roll Bonding Without Intermittent Heat Treatments

High strength/high conductivity Cu–XNb nanolamellar bimetallic composites (where X < 50 pct) were successfully processed via accumulative roll bonding. No intermittent heat treatments were used which increases industrial applicability. Estimates of conductivity from published models were validated at length scales of ~ 150 nm but were shown to be four times too low for composites with layer thicknesses < 40 nm. Nanoscale composites with conductivity of ~ 47 pct IACS and 99 Rockwell B hardness were fabricated.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Differences in the resistive and thermodynamic properties of the single crystalline chiral superconductor candidate SrPtAs

The locally noncentrosymmetric superconductor SrPtAs is proposed to host a topological chiral d-wave state, but experimental reports have been limited to polycrystalline samples. Here we report the synthesis of single crystalline SrPtAs grown from Pb flux. SrPtAs crystallizes in the hexagonal space group P⁢6 3 /mmc with lattice parameters a=4.2445⁢(4) Å and c=8.9513⁢(18) Å. Additionally, magnetic susceptibility and electrical resistivity measurements reveal a superconducting transition at T c ~2.2 K, in agreement with previous reports on polycrystalline samples. Surprisingly, heat capacity data show only a small bulk transition at 0.7 K. We discuss the possible origins of the discrepancy between the various measurements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

Direct growth of crystalline SiGe nanowires on superconducting NbTiN thin films

Novel heterostructures created by coupling one-dimensional semiconductor nanowires with a superconducting thin film show great potential toward next-generation quantum computing. Here, by growing high-crystalline SiGe nanowires on a NbTiN thin film, the resulting heterostructure exhibits Ohmic characteristics as well as a shift of the superconducting transition temperature (T c ). The structure was characterized at atomic resolution showing a sharp SiGe/NbTiN interface without atomic interdiffusion. Lattice spacing, as calculated from large-area x-ray diffraction experiments, suggests a potential preferred d-spacing matching between (200) NbTiN and (110) SiGe grains. The observed out-of-plane compressive strain within the NbTiN films coupled with SiGe nanowires explains the downward shift of the superconductivity behavior. The presented results post scientific insights toward functional heterostructures by coupling multi-dimensional materials, which could enable tunable superconductivity that benefits the quantum science applications.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2023

Ubiquitous spin freezing in the superconducting state of UTe 2

In most superconductors electrons form Cooper pairs in a spin-singlet state mediated by either phonons or by long-range interactions such as spin fluctuations. The superconductor UTe 2 is a rare material wherein electrons are believed to form pairs in a unique spin-triplet state with potential topological properties. While spin-triplet pairing may be mediated by ferromagnetic or antiferromagnetic fluctuations, experimentally, the magnetic properties of UTe 2 are unclear. By way of muon spin rotation/relaxation (μSR) measurements on independently grown UTe 2 single crystals we demonstrate the existence of magnetic clusters that gradually freeze into a disordered spin frozen state at low temperatures. Our findings suggest that inhomogeneous freezing of magnetic clusters is linked to the ubiquitous residual linear term in the temperature dependence of the specific heat (C) and the low-temperature upturn in C/T versus T. The omnipresent magnetic inhomogeneity has potential implications for experiments aimed at establishing the intrinsic low-temperature properties of UTe 2 .

36 MATERIALS SCIENCE↗

DOE OSTI2022

Interwoven atypical quantum states in CeLiBi 2

Here we report the discovery of CeLiBi 2 , the first example of a material in the tetragonal CeTX 2 ( T = transition metal; X = pnictogen) family wherein an alkali cation replaces the typical transition metal. Magnetic susceptibility and neutron powder diffraction measurements are consistent with a crystal-field Γ 6 ground-state Kramers doublet that orders antiferromagnetically below T N = 3.4 K with an incommensurate propagation wave vector k = ( 0, 0.0724(4), 0.5) that generates a nanometric modulation of the magnetic structure. The best model of the ordered state is an elliptical cycloid with Ce moments primarily residing in the ab plane. This is highly unusual, as all other Γ 6 CeTX 2 members order ferromagnetically. Further, we observe an atypical hard-axis metamagnetic transition at 2 T in magnetostriction, magnetization, and resistivity measurements. CeLiBi 2 is a rare example of a highly conductive material with dominant skew scattering leading to a large anomalous Hall effect. Quantum oscillations with five frequencies arise in magnetostriction and magnetic susceptibility data to T = 30 K and μ 0 H = 55 T, which indicate small Fermi pockets of light carriers with effective masses as low as 0.07 m e . Density functional theory calculations indicate that square-net Dirac-like Bi-p bands are responsible for these ultralight carriers. Together, our results show that CeLiBi 2 enables multiple atypical magnetic and electronic properties in a single clean material.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Metastable phase of UTe 2 formed under high pressure above 5 GPa

Uranium ditelluride (UTe 2 ) has attracted recent interest due to its unique superconducting properties, which include the potential for a topological odd-parity superconducting state. Recently, ac-calorimetry measurements under pressure indicate a change in the ground state of UTe 2 from superconducting to antiferromagnetic at 1.4 GPa. In this work, we investigate the effect of pressure on the crystal structure of UTe 2 up to 25 GPa at room temperature using x-ray diffraction. We find that UTe 2 , which at ambient conditions has an orthorhombic (Immm) structure, transforms to a body-centered tetragonal (I4/mmm) structure at 5 GPa in a quasihydrostatic neon (Ne) pressure-transmitting medium. In the absence of a pressure-transmitting medium, this transformation occurs between 5 and 8 GPa. The data were fit with a third-order Birch-Murnaghan equation of state resulting in values of B 0 = 46.0 ± 0.6 GPa , B' = 9.3 ± 0.5 (no pressure medium), and B 0 = 42.5 ± 2.0 GPa , B' = 9.3 (fixed) (neon pressure medium) for the Immm phase. For the I4/mmm phase, B 0 = 78.9 ± 0.5 GPa GPa and B' = 4.2 ± 0.1 (no pressure-transmitting medium), and B 0 = 70.0 ± 1.1 GPa and B' = 4.1 ± 0.2 (neon pressure medium). The high-pressure tetragonal phase is retained after decompression to ambient pressure, with approximately 30% remaining after 2 days. We argue that the observed phase transition into a higher-symmetry structure at P ~ 5 GPa (orthorhombic to tetragonal) is accompanied by an increase in the shortest distance between uranium atoms from 3.6 Å (orthorhombic) to 3.9 Å (tetragonal), which suggests localization of the 5ƒ electrons, albeit with a 10.7% decrease in volume.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Ground state of Ce 3 Bi 4 Pd 3 unraveled by hydrostatic pressure

Noncentrosymmetric Ce 3 Bi 4 Pd 3 has attracted a lot of attention as a candidate for strongly correlated topological material, yet its experimental ground state remains a matter of contention. Two conflicting scenarios have emerged from a comparison to the prototypical Kondo insulator Ce 3 Bi 4 Pd 3 : Either Ce 3 Bi 4 Pd 3 is a spin-orbit-driven topological semimetal or a Kondo insulator with smaller Kondo coupling than its Pt counterpart. Here, we determine the ground state of Ce 3 Bi 4 Pd 3 via electrical resistivity measurements under hydrostatic pressure, which is a clean symmetry-preserving tuning parameter that increases hybridization but virtually preserves spin-orbit coupling. Ce 3 Bi 4 Pd 3 becomes more insulating under pressures up to 2.3 GPa, which is a signature of Ce-based Kondo insulating materials in the considered pressure range. Its small zero-pressure gap increases quadratically with pressure, similar to the behavior observed in the series Ce 3 Bi 4 (Pt 1 - x Pd x ) 3 , which indicates that Pt substitution and applied pressure have a similar effect. Finally, our result not only demonstrates that Kondo coupling, rather than spin-orbit coupling, is the main tuning parameter in this class of materials, but it also establishes that Ce 3 Bi 4 Pd 3 has a narrow-gap Kondo insulating ground state.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Anisotropic magnetotransport properties of the heavy-fermion superconductor CeRh 2 As 2

Here, we report anisotropic resistivity measurements of the heavy-fermion superconductor CeRh 2 As 2 in magnetic fields up to 16 T and temperatures down to 0.35 K. The measured CeRh 2 As 2 resistivity shows a signature corresponding to the suggested quadrupole-density-wave order state at T 0 ~0.5 K for both measured directions. For a magnetic field applied along the tetragonal a axis, T 0 is enhanced with magnetic field reaching ~1.75 K at 16 T. Further, a magnetic field-induced transition occurs at μ 0 H m ~ 8.1 T corresponding to a change to a new broken symmetry state. For a magnetic field applied along the c axis, T 0 is suppressed below our base temperature ~0.35 K by μ 0 H ~ 4.5 T, a field close to the previously reported field-induced transition within the superconducting state suggested to be from an even-parity to an odd-parity state. Our results indicate that the multiple superconducting phases in CeRh 2 As 2 are intimately tied to the suppression of the proposed quadrupole-density-wave phase at T 0 .

36 MATERIALS SCIENCE↗

DOE OSTI2022

Physical properties of the layered $\mathcal{f}$-electron van der Waals magnet Ce 2 Te 5

Here. we report a detailed study of the magnetic, transport, and thermodynamic properties of Ce 2 Te 5 single crystals, a layered $\mathcal{f}$-electron van der Waals magnet. Four consecutive transitions at ~5.2, 2.1, 0.9, and 0.4 K were observed in the ac-plane electrical resistivity $\rho(T)$, which were further confirmed in specific heat $C_\text{p}(T)$ measurements. Analysis of the magnetic susceptibility $\chi(T)$, the magnetic-field variation of $\rho(T)$, and the increase of the first transition temperature ($T_c$ ~ 5.2 K) with applied magnetic field indicates ferromagnetic order, while the decrease of the other transitions with field suggests different states with dominant antiferromagnetic interactions below $T_2$ ~ 2.1 K, $T_3$ ~ 0.9 K, and $T_4$ = 0.4 K. Critical behavior analysis around Tc that gives critical exponents $\beta$ = 0.31(2), $\gamma$ = 0.99(2), $\delta$ = 4.46(1), and $T_c$ = 5.32(1) K indicates that Ce 2 Te 5 shows a three-dimensional magnetic critical behavior. Moreover, the Hall resistivity ρxy indicates that Ce2Te5 is a multiband system with a relatively high electron mobility ~2900 cm 2 V –1 s –1 near $T_c$, providing further opportunities for future device applications.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Thermodynamic and electrical transport properties of UTe 2 under uniaxial stress

Despite intense experimental efforts, the nature of the unconventional superconducting order parameter of UTe 2 remains elusive. This puzzle stems from reports of either a single or a double superconducting transition at ambient pressure as well as a complex pressure-temperature phase diagram. To address this issue, we measured the heat capacity and electrical resistivity of UTe 2 under compressive uniaxial stress σ applied along different crystallographic directions. We find that the critical temperature T c of the single observed bulk superconducting transition decreases with σ along [100] and [110] but increases with σ along [001]. Aside from its effect on T c , c-axis stress leads to a significant piezoresistivity. Importantly, an in-plane shear stress σ xy does not induce any observable splitting of the superconducting transition over a stress range of σ xy ≈ 0.17GPa. Furthermore, this result suggests that the superconducting order parameter of UTe 2 may be single component at ambient pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Predicting electrical conductivity in Cu/Nb composites: A combined model-experiment study

The generation of high magnetic fields requires materials with high electric conductivity and good strength properties. Cu/Nb composites are considered to be good candidates for this purpose. In this work, we aim to predict, from theory, the dependence of electric conductivity on the microstructure, most notably on the layer thickness and grain sizes. We also conducted experiments to calibrate and validate our simulations. Bimetal interfaces and grain boundaries are confirmed to have the largest impact on conductivity in this composite material. In this approach, a distribution of the layer thickness is accounted for in order to better model the experimentally observed microstructure. Because layer thicknesses below the mean free path of Cu significantly degrade the conductivity, an average layer thickness larger than expected may be needed to meet conductivity requirements in order to minimize these smaller layers in the distribution. We also investigate the effect of variations in volume fraction of Nb and temperature on the material’s conductivity.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Colossal piezoresistance in narrow-gap Eu 5 In 2 Sb 6

Piezoresistance, the change of the electrical resistance (R) of a material in response to an applied mechanical stress (σ), is the driving principle of electromechanical devices such as strain gauges, accelerometers, and cantilever force sensors. Enhanced piezoresistance has been traditionally observed in two classes of uncorrelated materials: nonmagnetic semiconductors and composite structures. In this study, we report the discovery of a remarkably large piezoresistance in Eu 5 In 2 Sb 6 single crystals, wherein anisotropic metallic clusters naturally form within a semiconducting matrix due to electronic interactions. Eu 5 In 2 Sb 6 shows a highly anisotropic piezoresistance, and uniaxial pressure along [001] of only 0.4 GPa leads to a resistivity drop of >99.95%, which results in a colossal piezoresistance factor of 5000×10 –11 Pa –1 . Our result not only reveals the role of interactions and phase separation in the realization of colossal piezoresistance, but it also highlights a route to multifunctional devices with large responses to both pressure and magnetic field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Single thermodynamic transition at 2 K in superconducting UTe 2 single crystals

UTe 2 is a newly-discovered unconventional superconductor wherein multicomponent topological superconductivity is anticipated based on the presence of two superconducting transitions and time-reversal symmetry breaking in the superconducting state. The observation of two superconducting transitions, however, remains controversial. Here we demonstrate that UTe 2 single crystals displaying an optimal superconducting transition temperature at 2 K exhibit a single transition and remarkably high quality supported by their large residual resistance ratio and small residual heat capacity in the superconducting state. Our results shed light on the intrinsic superconducting properties of UTe 2 and bring into question whether UTe 2 is a multicomponent superconductor at ambient pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗