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All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Effect of non-stoichiometry and pressure on superconductivity in topological semimetal PdTe

Research into topological superconductivity has been at the forefront of condensed matter physics due to both fundamental interest and potential applications in quantum computing. PdTe, is such a superconductor with a transition temperature T c ∼ 4.5 K and exhibits a nontrivial topological electronic structure, thus receiving significant attention. We report an experimental and theoretical investigation of the pressure effect on superconductivity by applying chemical non-stoichiometry and hydrostatic pressure. While T c decreases with increasing pressure through electrical resistivity, magnetization, and specific heat measurements, chemical pressure has a distinct impact from hydrostatic pressure, which could increase T c by creating negative pressure via non-stoichiometric Pd x Te with x > 1. Accompanied with this is a sign change of the Hall coefficient from negative at x < 1 to positive at x > 1. This indicates extreme sensitivity of the electronic structure to chemical non-stoichiometry, which occurs as a Pd vacancy for x < 1 and Pd interstitial for x > 1.

nonstoichiometry↗
DOE OSTI2020

Aluminium composites prepared by laser cladding assisted by friction stir processing

Abstract A laser cladding and friction stir processing hybrid method was employed to produce an Al matrix composite layer. The microstructure, phase composition, microhardness and conductivity of the composites were investigated. A laser cladding layer with a thickness of approximately 200 μ m was prepared on a 1060 aluminium plate and it was broken up and distributed on the Al matrix after friction stir processing. The particle/Al interfaces exhibited extremely good interfacial integrity. Microstructural observations revealed that an obvious in situ reaction occurred at the particle/Al interfaces, which effectively improved the bonding between the reinforcement phase and the matrix. TEM analysis and selected area diffraction enabled the identification of the intermetallic compounds and confirmed them to be Al 5 Fe 2 and Al 3 Fe. The average microhardness values of the friction stir processed composites reached approximately 85 HV. The electrical resistivity of the friction stir processed composites is slightly higher than that of the aluminium matrix.

Wei, Yanni (ORCID:0000000244443642)↗
DOE OSTI2025

Isolated spin ladders in 𝐿⁢𝑛 2 ⁢Ti 9 ⁢Sb 11 (𝐿⁢𝑛 :La–Nd) metals

Here we present the discovery and characterization of a series of antimonides 𝐿⁢𝑛 2 ⁢Ti 9⁢ Sb 11 (Ln: La–Nd) that exhibit well-isolated, 𝑛 = 2 rare-earth spin ladders. We discuss the structure of these compounds, with a particular focus on the magnetic Ln spin ladders. Nd 2 ⁢Ti 9 ⁢Sb 11 and Ce 2 ⁢Ti 9 ⁢Sb 11 exhibit antiferromagnetic interactions and a well-defined doublet ground state, whereas Pr 2 ⁡Ti 9⁢ Sb 11 exhibits a weakly magnetic singlet ground state. Nd 2 ⁢Ti 9 ⁢Sb 11 is a poor metal with an electrical resistivity of 0.1m⁢Ω cm at 300 K and weak temperature dependence. The thermal conductivity along the ladder exhibits significant field dependence even at 40 K, considerably higher than the magnetic ordering temperature of 1.1 K. Compared to compounds with transition metal spin ladders, the rare-earth elements impart much lower energy scales, making these compounds highly tunable with external stimuli like magnetic fields. In conclusion, the diverse magnetism of the rare-earth ions and Ruderman-Kittel-Kasuya-Yosida interactions further contribute to the potential for a wide array of rich magnetic ground states, positioning these materials as a rare example of an inorganic square spin-ladder platform.

Crystal growth↗
DOE OSTI2025

Magnetic polaron formation in EuZn2P2

Colossal magnetoresistance (CMR) has been observed across many Eu2+-based materials; however, its origin is not completely understood. Here we investigate the antiferromagnetic insulator EuZn2⁢P2 through single crystal x-ray diffraction, transmission electron microscopy, electrical transport, magnetization, dilatometry, and electron spin resonance measurements complemented by density functional theory calculations. Our electrical resistivity data reveal a large negative magnetoresistance, MR=[𝑅⁡(𝐻)−𝑅⁡(0)]/𝑅⁡(0), that reaches MR=−99.7% at 9 T near the antiferromagnetic ordering temperature 𝑇𝑁=23K. Dilatometry measurements show an accompanying field-induced lattice strain. Additionally, Eu2+ electron spin resonance reveals a strong ferromagnetic exchange interaction between Eu2+ and conduction electrons. Our experimental results in EuZn2⁢P2 are consistent with a magnetic polaron scenario and suggest magnetic polaron formation as a prevailing explanation of CMR in Eu2+-based compounds.

Cook, Matt [ORNL] (ORCID:0000000288777505)↗
DOE OSTI2020

Magnetic, thermal, and electronic-transport properties of EuMg 2 Bi 2 single crystals

The trigonal compound EuMg 2 Bi 2 has recently been discussed in terms of its topological band properties. These are intertwined with its magnetic properties. In this paper detailed studies of the magnetic, thermal, and electronic-transport properties of EuMg 2 Bi 2 single crystals are presented. The Eu + 2 spins 7/2 in EuMg 2 Bi 2 exhibit an antiferromagnetic (AFM) transition at a temperature T N = 6.7 K, as previously reported. By analyzing the anisotropic magnetic susceptibility χ data below T N in terms of molecular-field theory (MFT), the AFM structure is inferred to be a c -axis helix, where the ordered moments in the hexagonal a b -plane layers are aligned ferromagnetically in the a b plane with a turn angle between the moments in adjacent moment planes along the c axis of ≈ 120 ° . An alternate but less likely magnetic structure is a planar structure with nearest-neighbor Eu spins aligned at ≈ 120 ° with respect to each other, where these ordered-moment layers are stacked along the c axis. The magnetic heat capacity exhibits a λ anomaly at T N with evidence of dynamic short-range magnetic fluctuations both above and below T N . The high- T limit of the magnetic entropy is close to the theoretical value for spins 7/2. The in-plane electrical resistivity ρ ( T ) data indicate metallic character with a mild and disorder-sensitive upturn below T min = 23 K. An anomalous rapid drop in ρ ( T ) on cooling below T N as found in zero field is replaced by a two-step decrease in magnetic fields. The ρ ( T ) measurements also reveal an additional transition below T N in applied fields of unknown origin that is not observed in the other measurements and may be associated with an incommensurate to commensurate AFM transition. The dependence of T N on the c -axis magnetic field H ⊥ was derived from the field-dependent χ ( T ) , C p ( T ) , and ρ ( T ) measurements. This T N ( H ⊥ ) was found to be consistent with the prediction of MFT for a c -axis helix with S = 7 / 2 and was used to generate a phase diagram in the H ⊥ – T plane.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Canted Eu magnetic structure in EuMnSb 2

Magnetic ordering breaks the time-reversal symmetry, greatly impacting material topological properties. Here, we report the investigation of the magnetic properties of the layered EuMnSb 2 , which has two sets of magnetic sublattices. Both the magnetization and electrical resistivity reveal two phase transitions with one at T N,Eu ~21K and the other at T N,Mn ~346K. Single crystal neutron diffraction refinement indicates that both transitions are originated from magnetic ordering. Below T N,Mn , the Mn sublattice forms the C-type antiferromagnetic (AFM) structure with moments [(4.5±0.6)μB at 7 K] pointing along the a axis. Below T N,Eu , the Eu sublattice forms the canted A-type AFM structure with moments [(5.9±0.8)μB at 7 K] lying in the ac plane but pointing (41 ± 1)° away from the a axis. Quantitative analysis indicates that the spin-spin correlation length, while anisotropic, has long-range characteristic in all directions for both the Eu and Mn sublattices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Canted antiferromagnetism in the quasi-one-dimensional iron chalcogenide BaFe 2 Se 4

We report the synthesis and physical properties studies of quasi-one-dimensional (quais-1D) iron chalcogenide BaFe 2 Se 4 which shares the FeSe 4 tetrahedra building motif commonly seen in the iron chalcogenide superconductors. A high-quality polycrystalline sample was achieved by solid-state reaction method and characterized by x-ray diffraction, electrical resistivity, magnetic susceptibility, and neutron diffraction measurements. BaFe 2 Se 4 is a narrow gap semiconductor that magnetically orders at ~310 K. Therefore, both neutron powder diffraction results and isothermal M-H loops suggest a canted antiferromagnetic structure where Fe sublattices are antiferromagnetically ordered along the c-axis quasi-1D chain direction, resulting in a net ferromagnetic moment in the perpendicular direction along the a axis with tilted angle of 18.7° towards the b axis.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Magnetic order and fluctuations in the quasi-two-dimensional planar magnet Sr( Co 1-x Ni x ) 2 As 2

We use neutron scattering to investigate spin excitations in Sr ( Co 1 - x Ni x ) 2 As 2 , which has a c -axis incommensurate helical structure of the two-dimensional (2D) in-plane ferromagnetic (FM) ordered layers for 0.013 ≤ x ≤ 0.25 . By comparing the wave vector and energy dependent spin excitations in helical ordered Sr ( Co 0.9 Ni 0.1 ) 2 As 2 and paramagnetic SrCo 2 As 2 , we find that Ni doping, while increasing lattice disorder in Sr ( Co 1 - x Ni x ) 2 As 2 , enhances quasi-2D FM spin fluctuations. However, our band structure calculations within the combined density functional theory and dynamic mean field theory ( DFT + DMFT ) failed to generate a correct incommensurate wave vector for the observed helical order from nested Fermi surfaces. Furthermore, since transport measurements reveal increased in-plane and c -axis electrical resistivity with increasing Ni doping and associated lattice disorder, we conclude that the helical magnetic order in Sr ( Co 1 - x Ni x ) 2 As 2 may arise from a quantum order-by-disorder mechanism through the itinerant electron mediated Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Temperature-induced structural evolution in liquid Ag-Ga alloys

Temperature-dependent atomic structural evolutions of liquid Ag 60 Ga 40 and Ag 70 Ga 30 alloys have been studied by in situ high-energy x-ray-diffraction (HEXRD) experiments combined with ab initio molecular-dynamics simulations. The experimental data show a reversible structural crossover at about 1050 ~ 1100 (± 50) K in both liquid Ag 60 Ga 40 and Ag 70 Ga 30 alloys. Obvious changes of the electrical resistivity, absolute thermoelectric power, and atomic diffusivity around the similar temperature range for both Ag-Ga liquids strongly support the HEXRD results. The origin of the liquid-to-liquid crossover in both Ag-Ga liquids was suggested to link with the rearrangements of Ag and Ga atoms, i.e., Ag and Ga atoms prefer to associate with themselves in the higher temperature range above 1100 K, consistent with the accelerated increase of the strong covalently bonded Ga–Ga dimers in both Ag-Ga liquids. In addition, more studies from the energy aspect are still desirable to understand the rearrangements of Ag and Ga atoms in the higher temperature range in both Ag-Ga liquids.

36 MATERIALS SCIENCE↗
DOE OSTI2021

From weak antilocalization to Kondo scattering in a magnetic complex oxide interface

Quantum corrections to electrical resistance can serve as sensitive probes of the magnetic landscape of a material. For example, interference between time-reversed electron paths gives rise to weak localization effects, which can provide information about the coupling between spins and orbital motion, while the Kondo effect is sensitive to the presence of spin impurities. In this work, we use low-temperature magnetotransport measurements to reveal a gradual transition from weak antilocalization (WAL) to Kondo scattering in the quasi-two-dimensional electron gas formed at the interface between SrTiO 3 and the Mott insulator NdTiO 3 . This transition occurs as the thickness of the NdTiO 3 layer is increased. Analysis of the Kondo scattering and WAL points to the presence of atomic-scale magnetic impurities coexisting with nanoscale magnetic regions that affect transport via a strong magnetic exchange interaction. This leads to distinct magnetoresistance behaviors that can serve as a sensitive probe of magnetic properties in two dimensions.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Crossover from Kondo to Fermi-liquid behavior induced by high magnetic field in 1 T -VTe 2 single crystals

The magnetic and magnetotransport properties of metallic 1T -VTe 2 single crystals were investigated at temperatures from 1.3 to 300 K and in magnetic fields up to 35 T. Upon applying a high magnetic field, it is found that the electrical resistivity displays a crossover from the logarithmic divergence of the single-impurity Kondo effect to the Fermi liquid behavior at low temperatures. The Brillouin scale of the negative magnetoresistivity above the Kondo temperature T K = 12 K indicates that the Kondo features originate from intercalated V ions, with S = 1/2. Both magnetic susceptibility and the Hall effect show an anomaly around T K . By using the modified Hamann expression we successfully describe the temperature-dependent resistivity under various magnetic fields, which shows the characteristic peak below T K due to the splitting of the Kondo resonance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

MgPd 2 Sb : A Mg-based Heusler-type superconductor

Here, we report the synthesis and physical properties of a full Heusler compound, MgPd 2 Sb, which we found to show superconductivity below T c = 2.2 K . MgPd 2 Sb was obtained by a two-step solid-state reaction method and its purity and cubic crystal structure [Fm-3m, a = 6.4523(1) Å ] were confirmed by powder x-ray diffraction. Normal and superconducting states were studied by electrical resistivity, magnetic susceptibility, and heat capacity measurements. The results show that MgPd 2 Sb is a type-II, weak coupling superconductor ( λ e-p = 0.53 ). The observed pressure dependence of T c (ΔT c /p ≈ -0.23 K / GPa ) is one of the strongest reported for a superconducting Heusler compound. The electronic structure, phonons, and electron-phonon coupling in MgPd 2 Sb were theoretically investigated. The obtained results are in agreement with the experiment, confirming the electron-phonon coupling mechanism of superconductivity. We compare the superconducting parameters to those of all reported Heusler-type superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2021

High-pressure study of the low- Z rich superconductor Be 22 Re

With T c ~ 9.6K, Be 22 Re exhibits one of the highest critical temperatures among Be-rich compounds.We have carried out a series of high-pressure electrical resistivity measurements on this compound to 30 GPa. The data show that the critical temperature T c is suppressed gradually at a rate of dT c /dP = –0.05K/GPa. Using density functional theory (DFT) calculations of the electronic and phonon density of states (DOS) and the measured critical temperature, we estimate that the rapid increase in lattice stiffening in Be 22 Re overwhelms a moderate increase in the electron-ion interaction with pressure, resulting in the decrease in T c . Furthermore, high-pressure x-ray diffraction measurements show that the ambient pressure crystal structure of Be 22 Re persists to at least 154 GPa.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

$A$-type antiferromagnetic order and magnetic phase diagram of the trigonal Eu spin-$\frac{7}{2}$ triangular-lattice compound $\mathrm{EuSn_2As_2}$

The trigonal compound EuSn 2 As 2 was recently discovered to host Dirac surface states within the bulk band gap and orders antiferromagnetically below the N´eel temperature T N = 23.5(2) K from neutron-diffraction measurements. Here the magnetic ground state of single-crystal EuSn 2 As 2 and the evolution of its properties versus temperature T and applied magnetic field H are reported. Included are the zero-field single-crystal neutron diffraction measurements versus T, magnetization M(H, T), magnetic susceptibility χ(H, T) = M(H,T)/H, heat capacity C p (H, T), and electrical resistivity ρ(H, T) measurements. The neutron-diffraction and χ(T) measurements both indicate a collinear A-type antiferromagnetic (AFM) structure below T N , where the Eu 2+ spins S = 7/2 in a triangular ab-plane layer (hexagonal unit cell) are aligned ferromagnetically in the ab plane whereas the spins in adjacent Eu planes along the c axis are aligned antiferromagnetically. The χ(H ab , T) and χ(Hc, T) data together indicate a smooth crossover between the collinear AFM alignment and an unknown magnetic structure at H ≈ 0.12 T. Dynamic spin fluctuations up to 60 K are evident in the χ(T), Cp(T) and ρ(H, T) measurements, a temperature that is more than twice T N . The ρ(H, T) is consistent with a low-carrier-density metal with strong magnetic scattering and does not reflect a contribution of the topological state of the material as reported earlier by ARPES measurements. This observation is consistent with previous ones for other topological insulators where the chemical potential is above the Dirac point so that ARPES readily detects the surface states, whereas resistivity measurements do not. Finally, the magnetic phase diagrams for both H ∥ c and H ∥ ab in the H-T plane are constructed from the T N (H), χ(H, T), Cp(H, T), and ρ(H, T) data.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Small-moment antiferromagnetic ordering in single-crystalline La 2 Ni 7

Single crystals of La 2 Ni 7 have been grown out of a binary, La-Ni melt. Temperature dependent, zero magnetic field, specific heat, electrical resistivity, and low field magnetization measurements indicate that there is a series of antiferromagnetic phase transitions at T 1 = 61.0 ± 0.2 K, T 2 = 56.5 ± 0.2 K and T 3 = 42.2 ± 0.2 K. The three specific heat anomalies found at these temperatures qualitatively have very small entropy changes associated with them and the anisotropic M(H) data saturate at ~ 0.12 μ B /Ni; both observations strongly suggesting the AFM order is associated with very small, itinerant, moments. Anisotropic, H ||c and H ⟂c , ρ(H) and M(H) isotherms as well as constant field, ρ(T) and M(T) sweeps manifest signatures of multiple phase lines and result in HT phase diagrams that are clearly anisotropic. Analysis of M(T) and M(H) data allow for the identification of the two lower temperature magnetically ordered states as antiferromagnetically ordered, with the moments aligned along the crystallographic c-axis, and the higher temperature, T 2 < T < T 1 , state as having a finite ferromagnetic component. In addition, the metamagnetic transition at low temperatures, for H applied along the crystallographic c-axis (H ||c ) appears to be a near classic example of a spin-flop transition, resulting in a field stabilized antiferromagnetic state with the moments ordered perpendicular to the c-axis. Finally, although the small moment ordering, and existence of multiple phase transitions in field and temperature, suggesting an energetic proximity of these states, could foretell a degree of pressure sensitivity, our measurements of R(T) for applied pressures up to 2.0 GPa indicate that there is very little pressure dependence of T 1 , T 2 and T 3 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Superconducting properties of the spin Hall candidate Ta 3 Sb with eightfold degeneracy

We report the synthesis and characterization of phase pure Ta 3 Sb, a material predicted to be topological with eightfold degenerate fermionic states [Bradlyn et al., Science 353, aaf5037 (2016)] and to exhibit a large spin Hall effect [Derunova et al., Sci. Adv. 5, eaav8575 (2019)]. We observe superconductivity in Ta 3 Sb with T c ~0.67 K in both electrical resistivity ρ(T) and specific heat c(T) measurements. Field-dependent measurements yield the superconducting phase diagram with an upper critical field of H c2 (0) ~0.95 T, corresponding to a superconducting coherence length of ξ ≈ 18.6 nm. The gap ratio deduced from specific heat anomaly, 2Δ 0 /k B T c is 3.46, a value close to the Bardeen-Cooper-Schrieffer value of 3.53. From a detailed analysis of both the transport and thermodynamic data within the Ginsburg-Landau (GL) framework, a GL parameter of κ ≈ 90 is obtained, identifying Ta 3 Sb as an extreme type-II superconductor. The observation of superconductivity in an eightfold degenerate fermionic compound with topological surface states and predicted large spin Hall conductance positions Ta 3 Sb as an appealing platform to further explore exotic quantum states in multifold degenerate systems.

36 MATERIALS SCIENCE↗
DOE OSTI2022

A-type antiferromagnetic order in semiconducting EuMg 2 Sb 2 single crystals

Eu-based Zintl-phase materials EuA 2 Pn 2 (A = Mg, In, Cd, Zn; Pn = Bi, Sb, As, P) have generated significant recent interest owing to the complex interplay of magnetism and band topology. Here, we investigated the crystallographic, magnetic, and electronic properties of the layered Zintlphase single crystals of EuMg 2 Sb 2 with the trigonal CaAl 2 Si 2 crystal structure (space group $P\bar{3}m1$). Electrical resistivity measurements complemented with angle-resolved photoemission spectroscopy (ARPES) studies and density functional theory (DFT) calculations find an activated behavior with intrinsic conductivity at high temperatures indicating a semiconducting electronic ground state with a narrow energy gap of 370 meV. Magnetic susceptibility and zero-field heat capacity measurements indicate that the compound undergoes antiferromagnetic (AFM) ordering at the Néel temperature T N = 8.0(2) K. Here, zero-field neutron-diffraction measurements reveal that the AFM ordering is A-type where the Eu spins (Eu 2+ , S = 7/2) arranged in ab-plane layers are aligned ferromagnetically in the ab plane and the Eu spins in adjacent layers are aligned antiferromagnetically. Eu-moment reorientation within the ab planes in the trigonal AFM domains associated with a very weak inplane magnetic anisotropy is also evident below T N at low fields < 0.05 T. Although isostructural semimetallic EuMg 2 Bi 2 is reported to host Dirac surface states, the observation of narrow-gap semiconducting behavior in EuMg 2 Sb 2 implies a strong role of spin-orbit coupling (SOC) in tuning the electronic states of these materials. Our DFT studies also suggest, besides the SOC, the more electronegative and smaller Sb than Bi shifts the low-lying conduction bands along the Γ-A direction to higher energy, resulting in an indirect bulk band gap between the Γ and M points for EuMg 2 Sb 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗