D. J. Safarik

Superconductivity in the Heusler Family of Intermetallics

Przeprowadzono badania szeregu związkow nadprzewodzących z klasy związkow Heuslera, w szczegolności rodziny (Sc, Y, Lu)Pd2Sn i APd2M (A= Hf, Zr, i M = In, Al). Zwrocono uwage na istotny wplyw sprzezenia elektron - fonon na obserwowane nadprzewodnictwo.

Band Structure of SnTe Studied by Photoemission Spectroscopy

We present an angle-resolved photoemission spectroscopy study of the electronic structure of SnTe and compare the experimental results to ab initio band structure calculations as well as a simplified tight-binding model of the p bands. Our study reveals the conjectured complex Fermi surface structure near the L points showing topological changes in the bands from disconnected pockets, to open tubes, and then to cuboids as the binding energy increases, resolving lingering issues about the electronic structure. The chemical potential at the crystal surface is found to be 0.5 eV below the gap, corresponding to a carrier density of p=1.14 × 10(21)  cm(-3) or 7.2 × 10(-2) holes per unit cell. At a temperature below the cubic-rhombohedral structural transition a small shift in spectral energy of the valance band is found, in agreement with model predictions.

Tin telluride: a weakly co-elastic metal

We report resonant ultrasound spectroscopy (RUS), dilatometry/magnetostriction, magnetotransport, magnetization, specific-heat, and 119Sn Mossbauer spectroscopy measurements on SnTe and Sn0.995Cr0.005Te. Hall measurements at T=77 K indicate that our Bridgman-grown single crystals have a p-type carrier concentration of 3.4×1019 cm−3 and that our Cr-doped crystals have an n-type concentration of 5.8×1022 cm−3. Although our SnTe crystals are diamagnetic over the temperature range 2≤T≤1100 K, the Cr-doped crystals are room-temperature ferromagnets with a Curie temperature of 294 K. For each sample type, three-terminal capacitive dilatometry measurements detect a subtle 0.5 μm distortion at Tc≈85 K. Whereas our RUS measurements on SnTe show elastic hardening near the structural transition, pointing to co-elastic behavior, similar measurements on Sn0.995Cr0.005Te show a pronounced softening, pointing to ferroelastic behavior. Effective Debye temperature, θD, values of SnTe obtained from 119Sn Mossbauer studies show a hardening of phonons in the range 60–115 K (θD=162 K) as compared with the 100–300 K range (θD=150 K). In addition, a precursor softening extending over approximately 100 K anticipates this collapse at the critical temperature and quantitative analysis over three decades of its reduced modulus finds ΔC44/C44=A|(T−T0)/T0|−κ with κ=0.50±0.02, a value indicating a three-dimensional softening of phonon branches at a temperature T0∼75 K, considerably below Tc. We suggest that the differences in these two types of elastic behaviors lie in the absence of elastic domain-wall motion in the one case and their nucleation in the other.

Evidence for direct impact damage in metamict titanite CaTiSiO5

We have measured the dose dependence of the degree of amorphization of titanite, CaTiSiO(5). Titanite is an often metamict mineral which has been considered as a matrix for the encapsulation of radiogenic waste, such as Pu. The amorphous fraction p of geologically irradiated samples (ages between 0.3 and 1 Ga) follows p = 1 - exp(-B(a)D) where D is the total dose and the characteristic amorphization mass is B(a) = 2.7(3) × 10(-19) g. Amorphization follows the direct impact mechanism where each α-decay leads to a recoil of the radiogenic atoms (mostly Th and U), which then, in turn, displaces some 5000 atoms of the titanite matrix. The amorphization behaviour is almost identical with that of zircon, ZrSiO(4), which has a similar molecular mass. While the recrystallization mechanism and elastic behaviour of the two minerals are very different, we do not find significant differences for the amorphization mechanism. Our samples have undergone little reheating over their geological history, since heating over 800 K would lead to rapid recrystallization for which we have found no evidence.

Determination of iron sites and the amount of amorphization in radiation-damaged titanite (CaSiTiO5).

Iron is a ubiquitous impurity in metamict (radiation-damaged and partially amorphized) materials such as titanite (CaSiTiO(5)). Using (57)Fe Mössbauer spectroscopy we find that iron in metamict titanite is partitioned between amorphous and crystalline regions based on valence. Trivalent iron exists in the crystalline titanite matrix whereas divalent iron exists almost exclusively in radiation-amorphized regions. We find that the relative abundances of the oxidation states correlate with the volume fraction of amorphous and crystalline regions. Our data also show that oxidation of iron proceeds along with the recrystallization of the amorphized regions. Recrystallization is confirmed to occur over the range 700 °C < T < 925 °C, and no further structural changes are observed at higher temperatures. It is surprising that our Mössbauer measurements show divalent iron to be surrounded by titanite with a high degree of short-range structural order in the amorphized regions. This observation is fundamentally different from other metamict materials such as zircon (ZrSiO(4)), where amorphized regions show no short-range order.

Elastic softening of metamict titanite CaTiSiO5: Radiation damage and annealing

Abstract We have measured the elastic response of radiation-damaged titanite, CaTiSiO5, as a function of thermal annealing. We estimate the bulk modulus of the damaged samples (-24% amorphous) to be 85 GPa, which is much softer than for undamaged crystalline titanite [131.4 GPa; Angel et al. (1999)]. Conversely, the lowest shear modulus of the radiation-damaged material is 52-58 GPa, which is harder that of the undamaged titanite, 46-52 GPa. The bulk and shear moduli of the radiation-damaged materials are close to those of thermal titanite glass, Bglass ≈ 75 GPa and Gglass ≈ 47 GPa, and are much smaller than expected based on other radiation-damaged materials such as zircon (ZrSiO4). Surprisingly, annealing of the damaged titanite in the range 600 < T < 1000 K leads to additional massive softening of the shear moduli. During annealing the shear modulus of titanite sample 1 softened from 58 to 29 GPa, and sample 2 softened from 52 to 19 GPa. The temperature range for the softening coincides with that found for crystallization of the amorphous regions, as measured previously by diffraction and spectroscopic methods. In contrast to the huge softening of the ultrasonically measured shear modulus, the calorimetrically measured Debye temperature θD increases by -5%, suggesting a small intrinsic hardening of the acoustic shear modes. Additional heating to 1473 K leads, in one titanite sample, to a steep increase of the shear modulus to values much larger than that of the initial, radiation-damaged material. Theoretical models are discussed to rationalize the massive softening due to both radiation damage and subsequent anneal.

Spectral analysis of resonance ultrasonic spectroscopy: Kramers–Kronig analysis, Fano profiles, and the case of precursor softening in SnTe:Cr

The analysis of resonant ultrasound spectroscopy (RUS) spectra is exemplified by the study of elastic softening in single-crystal Sn0.995Cr0.005Te near the ferroelastic phase transition at T≃100 K. Kramers–Kronig analysis of the resonance peaks shows that the elastic response is linear over the entire temperature range. In the paraelastic phase the Cole–Cole plots of the RUS spectra are circles with small gaps that are related to linear damping. In the ferroelastic phase strong coupling with domain boundary movement occurs, and results in distortion of the Cole–Cole circles. The RUS line profiles in the ferroelastic phase are well-described by the sum of a resonance term and a Fano spectrum with a Fano parameter of q=0.46. The general equations and some simple approximations, which can conveniently be used to analyze RUS spectra, are summarized. We expect that this analysis is transportable to a large parameter space and can be applied to most RUS spectra for both ferroic and nonferroic materials.

Order-parameter coupling in the improper ferroelectric lawsonite

Low-temperature specific heat and thermal expansion measurements are used to study the hydrogen-based ferroelectric lawsonite over the temperature range 1.8 K ≤ T ≤ 300 K. The second-order phase transition near 125 K is detected in the experiments, and the low-temperature phase is determined to be improper ferroelectric and co-elastic. In the ferroelectric phase T ≤ 125 K, the spontaneous polarization P(s) is proportional to (1) the volume strain e(s), and (2) the excess entropy ΔS(e). These proportionalities confirm the improper character of the ferroelectric phase transition. We develop a structural model that allows the off-centering of hydrogen positions to generate the spontaneous polarization. In the low-temperature limit we detect a Schottky anomaly (two-level system) with an energy gap of Δ ∼ 0.5 meV.

Superconductivity in the Einstein solid V Al10.1

We used magnetic susceptibility, resistivity and heat capacity measurements to characterize the superconducting state in the Einstein solid VAl(10.1). We find that VAl(10.1) is a weak-coupling, type-II superconductor with T(c) = 1.53 K and an upper critical field of H(c2)(0) = 800 Oe. The heat capacity data in the range 0.07 K < T < 1.53 K are consistent with an isotropic energy gap of Δ(0) = 0.23 meV.

Structure and paramagnetism in weakly correlated Y8Co5

We report the basic physical properties of monoclinic Y8Co5 determined by means of magnetic susceptibility, electrical resistivity, and specific heat measurements. The crystal structure of Y8Co5 is monoclinic (P2(1)/c) with lattice parameters a = 7.0582(6) Å, b = 7.2894(6) Å, c = 24.2234(19) Å, and β = 102.112(6)° as refined by using synchrotron powder x-ray diffraction data. The compound shows temperature independent paramagnetism with χ0 = 2.1 × 10(-3) emu mol(-1) and Sommerfeld parameter γ = 63 mJ mol(-1) K(-2). The calculated Wilson ratio for Y8Co5, R(W) = 1.4, is close to that expected for a free electron gas R(W) = 1. Low temperature resistivity under high pressure does not reveal superconductivity in this compound down to 1.2 K, up to hydrostatic pressures of 5.56 GPa. Band structure calculations (full-potential linearized augmented plane wave, FP-LAPW) derive the Stoner exchange interaction parameter S = 0.24, excluding magnetic behavior for Y8Co5.