We report x-ray total scattering and pair distribution function (PDF) studies of the structural relaxation around interstitial manganese (Mn$_i$) in ferromagnetic Mn$_{1+delta}$Sb ($0.03 le delta le 0.23$) alloys, guided by density functional theory
(DFT). Refinements to the experimental PDF using a crystallographically constrained structural model indicate an expansion in the equatorial plane of the Mn$_i$Sb$_5$ trigonal bipyramidal site, which introduces significant positional disorder in addition to the nominally-random occupation of interstitial voids. Observation of a weak diffuse signal near the symmetry-forbidden (001) reflection position is indicative of correlated disorder from the clustering of Mn$_i$. Density functional relaxation of supercells approximating the $delta = 0.08$, $0.15,$ and $0.23$ compositions provides improved models that accurately describe the short-range structural distortions captured in the PDFs. Such structural relaxation increases the DFT calculated moment on Mn$_i$, which aligns antiparallel to the primary Mn moments, but leads to insubstantial changes in the average Mn and Sb moments and moments of Mn and Sb proximal to interstitials, thus providing a more accurate description of the observed bulk magnetic properties.
With the increased availability of high intensity time-of-flight neutron and synchrotron X-ray scattering sources that can access wide ranges of momentum transfer, the pair distribution function method has become a standard analysis technique for stu
dying disorder of local coordination spheres and at intermediate atomic separations. In some cases, rational modeling of the total scattering data (Bragg and diffuse) becomes intractable with least-squares approaches and necessitates reverse Monte Carlo (RMC) simulations using large supercells. However, the extraction of meaningful information from the resulting atomistic ensembles is challenging, especially at intermediate length scales. We use representational analysis to describe displacements of atoms in RMC ensembles from an ideal crystallographic structure. Rewriting the displacements in terms of a local basis that is descriptive of the ideal crystallographic symmetry provides a robust approach to characterizing medium-range order (and disorder) and symmetry breaking in complex and disordered crystalline materials. This method enables the extraction of statistically relevant displacement modes (orientation, amplitude, and distribution) of the crystalline disorder and provides directly meaningful information in a symmetry-adapted basis set that is most descriptive of the crystal chemistry and physics.
KNi$_2$Se$_2$ exhibits an increase of symmetry on cooling below $Tle50$ K, as observed by Raman spectroscopy and synchrotron x-ray diffraction. X-ray absorption spectroscopy confirms that the symmetry increase is due to changes in nickel-nickel inter
actions and suppression of charge density wave fluctuations. Density functional theory calculations reveal a zone- boundary lattice instability that provides a model of the room-temperature x-ray pair distribution function data, but fails to describe the higher local symmetry observed for $Tle50$K. Together, these results support many- body correlation effects as drivers for the unusual heavy fermion electronic ground state in KNi$_2$Se$_2$.
The emergence of complex electronic behaviour from simple ingredients has resulted in the discovery of numerous states of matter. Many examples are found in systems exhibiting geometric magnetic frustration, which prevents simultaneous satisfaction o
f all magnetic interactions. This frustration gives rise to complex magnetic properties such as chiral spin structures orbitally-driven magnetism, spin-ice behavior exhibiting Dirac strings with magnetic monopoles, valence bond solids, and spin liquids. Here we report the synthesis and characterization of LiZn2Mo3O8, a geometrically frustrated antiferromagnet in which the magnetic moments are localized on small transition metal clusters rather than individual ions. By doing so, first order Jahn-Teller instabilities and orbital ordering are prevented, allowing the strongly interacting magnetic clusters in LiZn2Mo3O8 to probably give rise to an exotic condensed valence-bond ground state reminiscent of the proposed resonating valence bond state. Our results also link magnetism on clusters to geometric magnetic frustration in extended solids, demonstrating a new approach for unparalleled chemical control and tunability in the search for collective, emergent electronic states of matter.
The nearby red supergiant (RSG) Betelgeuse has a complex circumstellar medium out to at least 0.5 parsecs from its surface, shaped by its mass-loss history within the past 0.1 Myr, its environment, and its motion through the interstellar medium (ISM)
. In principle its mass-loss history can be constrained by comparing hydrodynamic models with observations. Observations and numerical simulations indicate that Betelgeuse has a very young bow shock, hence the star may have only recently become a RSG. To test this possibility we calculated a stellar evolution model for a single star with properties consistent with Betelgeuse. We incorporated the resulting evolving stellar wind into 2D hydrodynamic simulations to model a runaway blue supergiant (BSG) undergoing the transition to a RSG near the end of its life. The collapsing BSG wind bubble induces a bow shock-shaped inner shell which at least superficially resembles Betelgeuses bow shock, and has a similar mass. Surrounding this is the larger-scale retreating bow shock generated by the now defunct BSG winds interaction with the ISM. We investigate whether this outer shell could explain the bar feature located (at least in projection) just in front of Betelgeuses bow shock.
Understanding the complexities of electronic and magnetic ground states in solids is one of the main goals of solid-state physics. Materials with the canonical ThCr$_2$Si$_2$-type structure have proved particularly fruitful in this regards, as they e
xhibit a wide range of technologically advantageous physical properties described by many-body physics, including high-temperature superconductivity and heavy fermion behavior. Here, using high-resolution synchrotron X-ray diffraction and time-of-flight neutron scattering, we show that the isostructural mixed valence compound, KNi$_2$S$_2$, displays a number of highly unusual structural transitions, most notably the presence of charge density wave fluctuations that disappear on cooling. This behavior occurs without magnetic or charge order, in contrast to expectations based on all other known materials. Furthermore, the low-temperature electronic state of KNi$_2$S$_2$ is found to exhibit many characteristics of heavy-fermion behavior, including a heavy electron state ($m^*/m_e sim$ 24), with a negative coefficient of thermal expansion, and superconductivity below $T_c$ = 0.46(2) K. In the potassium nickel sulfide, these behaviors arise in the absence of localized magnetism, and instead appear to originate in proximity to charge order.
A magnetically driven piston pump for xenon gas recirculation is presented. The pump is designed to satisfy extreme purity and containment requirements, as is appropriate for the recirculation of isotopically enriched xenon through the purification s
ystem and large liquid xenon TPC of EXO-200. The pump, using sprung polymer gaskets, is capable of pumping more than 16 standard liters per minute (SLPM) of xenon gas with 750 torr differential pressure.
In many ostensibly crystalline materials, unit-cell-based descriptions do not always capture the complete physics of the system due to disruption in long-range order. In the series of cobalt hydroxides studied here, Co(OH)$_{2-x}$(Cl)$_x$(H$_2$O)$_{n
}$, magnetic Bragg diffraction reveals a fully compensated Neel state, yet the materials show significant and open magnetization loops. A detailed analysis of the local structure defines the aperiodic arrangement of cobalt coordination polyhedra. Representation of the structure as a combination of distinct polyhedral motifs explains the existence of locally uncompensated moments and provides a quantitative agreement with bulk magnetic measurements and magnetic Bragg diffraction.
EXO-200 uses 468 large area avalanche photodiodes (LAAPDs) for detection of scintillation light in an ultra-low-background liquid xenon (LXe) detector. We describe initial measurements of dark noise, gain and response to xenon scintillation light of
LAAPDs at temperatures from room temperature to 169K - the temperature of liquid xenon. We also describe the individual characterization of more than 800 LAAPDs for selective installation in the EXO-200 detector.
