Disorder suppression and precise conductance quantization in constrictions of PbTe quantum wells

Conductance quantization was measured in submicron constrictions of PbTe, patterned into narrow,12 nm wide quantum wells deposited between Pb$_{0.92}$Eu$_{0.08}$Te barriers. Because the quantum confinement imposed by the barriers is much stronger than the lateral one, the one-dimensional electron energy level structure is very similar to that usually met in constrictions of AlGaAs/GaAs heterostructures. However, in contrast to any other system studied so far, we observe precise conductance quantization in $2e^2/h$ units, {\it despite of significant amount of charged defects in the vicinity of the constriction}. We show that such extraordinary results is a consequence of the paraelectric properties of PbTe, namely, the suppression of long-range tails of the Coulomb potentials due to the huge dielectric constant.Comment: 7 pages, 6 figures, submitted to Phys. Rev.

Heat Capacity of PbS: Isotope Effects

In recent years, the availability of highly pure stable isotopes has made possible the investigation of the dependence of the physical properties of crystals, in particular semiconductors, on their isotopic composition. Following the investigation of the specific heat ($C_p$, $C_v$) of monatomic crystals such as diamond, silicon, and germanium, similar investigations have been undertaken for the tetrahedral diatomic systems ZnO and GaN (wurtzite structure), for which the effect of the mass of the cation differs from that of the anion. In this article we present measurements for a semiconductor with rock salt structure, namely lead sulfide. Because of the large difference in the atomic mass of both constituents ($M_{\rm Pb}$= 207.21 and ($M_{\rm S}$=32.06 a.m.u., for the natural isotopic abundance) the effects of varying the cation and that of the anion mass are very different for this canonical semiconductor. We compare the measured temperature dependence of $C_p \approx C_v$, and the corresponding derivatives with respect to ($M_{\rm Pb}$ and $M_{\rm S}$), with \textit{\textit{ab initio}} calculations based on the lattice dynamics obtained from the local density approximation (LDA) electronic band structure. Quantitative deviations between theory and experiment are attributed to the absence of spin-orbit interaction in the ABINIT program used for the electronic band structure calculations.Comment: 17 pages including 10 Fig

A Neptune Orbiter Concept Using Drag Modulated Aerocaptue (DMA) and the Adaptable, Deployable Entry and Placement Technology (ADEPT)

Conceptual Neptune orbiter was designed for the purpose of assessing mission feasibilityBuilt off of the 2017 Pre-Decadal Study, but adapted for drag modulation aerocapture.Science payload includes: Narrow Angle camera, Doppler Imager, Magnetometer, Atmospheric Probe (w/ ASI, Nephelometer, Mass Spectrometer). Baseline concept of operations releases probe prior to orbit insertion, but investigations are ongoing to assess the feasibility of bringing the probe to orbit before release

Microscopic mechanism of low thermal conductivity in lead-telluride

The microscopic physics behind low lattice thermal conductivity of single crystal rocksalt lead telluride (PbTe) is investigated. Mode-dependent phonon (normal and umklapp) scattering rates and their impact on thermal conductivity were quantified by the first-principles-based anharmonic lattice dynamics calculations that accurately reproduce thermal conductivity in a wide temperature range. The low thermal conductivity of PbTe is attributed to the scattering of longitudinal acoustic phonons by transverse optical phonons with large anharmonicity, and small group velocity of the soft transverse acoustic phonons. This results in enhancing the relative contribution of optical phonons, which are usually minor heat carrier in bulk materials.Comment: 18 pages, 4 figures, accepted for publication in Phys. Rev.

Materials Science and Engineering A xxx (2006) xxx--xxx

The mechanical behavior and microstructure of pure iron subjected to dominant shear loading has been characterized over a wide range of strain rates. Pure iron is found to be highly strain-rate sensitive. Iron exhibits marked strain softening at # 850 MPa that is unexpected for the annealed material, as characterized by TEM, but is identical to that of iron preshocked at 40 GPa [G.M. Weston, J., Mater. Sc. Lett. 11 (1992) 1361]. The microstructure is found to undergo significant refinement with increasing strain rate, from large initial grains (50 #m), through dislocation cells and large twinning, and finally micro-twins and dynamically recrystallized 200 nm grains at the higher strain rates. In situ temperature measurements indicate the release of an external heat source, other that the thermomechanical conversion of plastic work, which is identified as dynamic recrystallization. The present results suggest the operation of the # (BCC) # (HCP) phase transition that is known to occur during hydrostatic or shock loading at 13 GPa. The combination of the high strain-rate sensitivity and dominant shear loading conditions seem to trigger this phase transition, thus supporting recent work [K.J. Caspersen, A. Lew, M. Ortiz, M., E.A. Carter, Phys. Rev. Lett. 10 (2004) 115501] emphasizing the role of shear

Chinese Mirror from the Late Sarmatian Site Cherny Yar

This paper provides the description and results of interdisciplinary research into the Chinese mirror from the elite Late Sarmatian burial mound Cherny Yar, located in the south of the Orenburg region (oblast) in the southern Urals, and dated by the 2nd – 3rd cc. AD. The mirror was found in a wooden case made of wooden twigs rolled in a spiral. It is a circular disc with a diameter of 17 cm with a loop-shaped handle in the center, and its outer surface is decorated with a complex ornament and symbols. We’ve conducted X-ray fluorescence analysis, optical and scanning electron microscopy with an energydispersive spectrometer, as well as linguistic analysis. The metallographic examination shows that the mirror was produced by casting in a low-heat-conducting clay mould or a stone mould, while its microstructure was characteristic for cast high-tin bronzes with lead. The object was cast and, most likely, quickly cooled. There are no traces of silver or tin coating on the mirror. The addition of lead was necessary to reduce porosity and improve fluidity, as well as to obtain the desired pattern from the mould. Both methods of studying microstructure (metallography and electron microscopy) confirm that, when it comes to mirror manufacturing technology, this sample could not be produced in the Sarmatian culture environment, but had analogies in other territories, such as China, in particular. This is also confirmed by a characteristic ornament and the presence of symbols on the mirror. The inscriptions on the mirror refer to the style of Zhuangshu, to its simplified version – Xiaozhuan. Symbols on the mirror show 12 signs, the so-called earthly branches, which are associated with 12 animals of the annual cycle, some of which are depicted side by side