Engineering topics

Kaczorowski, D.

Publications and source records attributed to Kaczorowski, D..

7 records

DOE OSTI2022

Observation of the non-linear Meissner effect

A long-standing theoretical prediction is that in clean, nodal unconventional superconductors the magnetic penetration depth λ, at zero temperature, varies linearly with magnetic field. This non-linear Meissner effect is an equally important manifestation of the nodal state as the well studied linear-in-T dependence of λ, but has never been convincingly experimentally observed. Here we present measurements of the nodal superconductors CeCoIn 5 and LaFePO which clearly show this non-linear Meissner effect. We further show how the effect of a small dc magnetic field on λ(T) can be used to distinguish gap nodes from non-nodal deep gap minima. Our measurements of KFe 2 As 2 suggest that this material has such a non-nodal state.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Electronic structure and 4$f$-electron character in Ce 2 PdIn 8 studied by angle-resolved photoemission spectroscopy

The localized-to-itinerant transition of $f$ electrons lies at the heart of heavy-fermion physics, but has only been directly observed in single-layer Ce-based materials. Here, we report a comprehensive study on the electronic structure and nature of the Ce 4$f$ electrons in the heavy-fermion superconductor Ce 2 PdIn 8 , a typical n = 2 Ce n MmIn 3n+2m compound, using high-resolution and 4$d$–4$f$ resonant photoemission spectroscopies. The electronic structure of this material has been studied over a wide temperature range, and hybridization between f and conduction electrons can be clearly observed to form a Kondo resonance near the Fermi level at low temperatures. Here, the characteristic temperature of the localized-to-itinerant transition is around 120 K, which is much higher than its coherence temperature T coh ~ 30K.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Superconductivity in the superhard boride WB 4.2

Here, we show that the superhard boride WB 4.2 is a superconductor with a T c of 2.05(5) K. Temperature-dependent magnetic susceptibility, electrical resistivity, and specific heat measurements were used to characterize the superconducting transition. The Sommerfeld constant gamma for WB 4.2 is 2.07(3) mJ mol( -1 ) K -2 and the Δ C/γ T c = 1.56, which is somewhat higher than what is expected for weakly coupled Bardeen-Cooper-Schrieffer type superconductors. The H c2 versus T plot is linear over a wide temperature range but does show signs of flattening by the lowest temperatures studied and therefore the zero temperature upper critical field ( μ 0 H c2 (0)) for WB 4.2 lies somewhere between the linear extrapolation of μ 0 H c2 (T) to 0 K and expectations based on the Werthamer-Helfand-Hohenberg model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

Mechanism of the Generation of Donor–Acceptor Pairs in Heavily Doped n-ZrNiSn with the Ga Acceptor Impurity

The nature of the mechanism of the simultaneous generation of donor–acceptor pairs under heavy doping of n-ZrNiSn intermetallic semiconductor with the Ga acceptor impurity is established. Such spatial arrangement in the crystal lattice of ZrNiSn{sub 1–x}Ga{sub x} is found when the rate of movement of the Fermi level εF found from calculations of the density distribution of electron states coincides with that experimentally established from dependences lnρ(1/T). It is shown that when the Ga impurity atom (4s{sup 2}4p{sup 1}) occupies the 4b sites of Sn atoms (5s{sup 2}5p{sup 2}), structural defects of both acceptor nature and donor nature in the form of vacancies in the 4b site are simultaneously generated. The results are discussed in the scope of the Shklovskii–Efros model of a heavily doped and compensated semiconductor.

36 MATERIALS SCIENCE↗

DOE OSTI2017

Features of the band structure and conduction mechanisms of n-HfNiSn heavily doped with Y

The crystalline and electronic structures, energy, kinetic, and magnetic characteristics of n-HfNiSn semiconductor heavily doped with Y acceptor impurity are studied in the ranges: T = 80–400 K, N{sub A}{sup Y} ≈ 1.9 × 10{sup 20}–5.7 × 10{sup 21} cm{sup –3} (x = 0.01–0.30), and H ≤ 10 kG. The nature of the mechanism of structural defect generation is determined, which leads to a change in the band gap and the degree of semiconductor compensation, the essence of which is the simultaneous reduction and elimination of structural donor-type defects as a result of the displacement of ~1% of Ni atoms from the Hf (4a) site, and the generation of structural acceptor-type defects by substituting Hf atoms with Y atoms at the 4a site. The results of calculations of the electronic structure of Hf{sub 1–x}Y{sub x}NiSn are in agreement with the experimental data. The discussion is performed within the Shklovskii–Efros model of a heavily doped and compensated semiconductor.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Features of conductivity mechanisms in heavily doped compensated V{sub 1–x}Ti{sub x}FeSb Semiconductor

The crystal and electronic structure and also the energy and kinetic properties of n-VFeSb semiconductor heavily doped with the Ti acceptor impurity are investigated in the temperature and Ti concentration ranges of T = 4.2–400 K and N{sub A}{sup Ti} ≈ 9.5 × 10{sup 19}–3.6 × 10{sup 21} cm{sup –3} (x = 0.005–0.20), respectively. The complex mechanism of the generation of acceptor and donor structural defects is established. It is demonstrated that the presence of vacancies at Sb atomic sites in n-VFeSb gives rise to donor structural defects (“a priori doping”). Substitution of the Ti dopant for V in VFeSb leads simultaneously to the generation of acceptortype structural defects, a decrease in the number of donor defects, and their removal in the concentration range of 0 ≤ x ≤ 0.03 via the occupation of vacancies by Sb atoms, and the generation of donor defects due to the occurrence of vacancies and an increase in their number. The result obtained underlies the technique for fabricating new n-VFeSb-based thermoelectric materials. The results are discussed in the context of the Shklovsky–Efros model for a heavily doped compensated semiconductor.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Superconductivity in CaBi 2

We observed superconductivity with critical temperature T c = 2.0 K in self-flux-grown single crystals of CaBi 2 . This material adopts the ZrSi 2 structure type with lattice parameters a = 4.696(1) Å, b = 17.081(2) Å and c = 4.611(1) Å. The crystals of CaBi 2 were studied by means of magnetic susceptibility, specific heat and electrical resistivity measurements. The heat capacity jump at T c is ΔC/γT c = 1.41, confirming bulk superconductivity; the Sommerfeld coefficient γ = 4.1 mJ mol -1 K -2 and the Debye temperature Θ D = 157 K. The electron–phonon coupling strength is λ el–ph = 0.59, and the thermodynamic critical field H c is low, between 111 and 124 Oe CaBi 2 is a moderate coupling type-I superconductor. Our results of electronic structure calculations are reported and charge densities, electronic bands, densities of states and Fermi surfaces are discussed, focusing on the effects of spin–orbit coupling and electronic property anisotropy. Furthermore, we find a mixed quasi-2D + 3D character in the electronic structure, which reflects the layered crystal structure of the material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗