Phonon Thermal Transport in UO_{2} via Self-Consistent Perturbation Theory.

Computing thermal transport from first-principles in UO_{2} is complicated due to the challenges associated with Mott physics. Here, we use irreducible derivative approaches to compute the cubic and quartic phonon interactions in UO_{2} from first principles, and we perform enhanced thermal transport computations by evaluating the phonon Green's function via self-consistent diagrammatic perturbation theory. Our predicted phonon lifetimes at T=600 K agree well with our inelastic neutron scattering measurements across the entire Brillouin zone, and our thermal conductivity predictions agree well with previous measurements. Both the changes due to thermal expansion and self-consistent contributions are nontrivial at high temperatures, though the effects tend to cancel, and interband transitions yield a substantial contribution.

Resilience of the Aurivillius structure upon La and Cr doping in a Bi5Ti3FeO15 multiferroic.

Combining the experimental techniques of high-resolution X-ray diffraction, magnetometry, specific heat measurement, and X-ray photoelectron, Raman and dielectric spectroscopy techniques, we have studied the influence of La and Cr doping on the crystal structure and magnetism of the room temperature Aurivillius multiferroic Bi5Ti3FeO15 by investigating the physical properties of (Bi4La)Ti3FeO15 and Bi5Ti3 (Fe0.5Cr0.5)O15. The parent (Bi5Ti3FeO15) and the doped ((Bi4La)Ti3FeO15 and Bi5Ti3(Fe0.5Cr0.5)O15) compounds crystallize in the A21am space group, which is confirmed through our analysis of high-resolution synchrotron X-ray diffraction data obtained on phase-pure polycrystalline powders. We determined the oxidation states of the metal atoms in the studied compounds as Fe3+, Ti4+, Cr3+, and La3+ through the analysis of X-ray photoelectron spectroscopy data. The magnetic susceptibilities of the three compounds are marked by the absence of a long-range ordered ground state, but dominated by superparamagnetic clusters with dominant antiferromagnetic interactions. This signature of short-range magnetism is also seen in specific heat as a low temperature enhancement which is suppressed upon the application of external magnetic fields up to 8 T. Our dielectric spectroscopy experiments showed that the three studied compounds display similar features in the dielectric constant measured as a function of frequency. However, upon doping La at the Bi site, the width of the ferroelectric hysteresis loop increases for (Bi4La)Ti3FeO15 compared to that of the parent compound Bi5Ti3FeO15, and with Cr doping, Bi5Ti3(Fe0.5Cr0.5)O15 becomes a leaky dielectric. The resilience of the Aurivillius crystal structure towards doping of La at the Bi site and Cr at the Fe site is clearly seen in the bulk properties of magnetic susceptibility, specific heat and the average crystal structure. The relevance of changes in the local structure is evident from our Raman spectroscopy and X-ray pair distribution function studies.

Crystal structure and magnetism of actinide oxides: a review.

In actinide systems, the 5felectrons experience a uniquely delicate balance of effects and interactions having similar energy scales, which are often difficult to properly disentangle. This interplay of factors such as the dual nature of 5f-states, strong electronic correlations, and strong spin-orbit coupling results in electronically unusual and intriguing behavior such as multi-k antiferromagnetic ordering, multipolar ordering, Mott-physics, mixed valence configurations, and more. Despite the inherent allure of their exotic properties, the exploratory science of even the more basic, binary systems like the actinide oxides has been limited due to their toxicity, radioactivity, and reactivity. In this article, we provide an overview of the available synthesis techniques for selected binary actinide oxides, including the actinide dioxides, sesquioxides, and a selection of higher oxides. For these oxides, we also review and evaluate the current state of knowledge of their crystal structures and magnetic properties. In many aspects, substantial knowledge gaps exist in the current body of research on actinide oxides related to understanding their electronic ground states. Bridging these gaps is vital for improving not only a fundamental understanding of these systems but also of future nuclear technologies. To this end, we note the experimental techniques and necessary future investigations which may aid in better elucidating the nature of these fascinating systems.

Evidence of Long-Range and Short-Range Magnetic Ordering in the Honeycomb Na3Mn2SbO6 Oxide.

We present a comprehensive study of the synthesis, structure, and magnetic properties of the honeycomb oxide Na3Mn2SbO6 supported by neutron diffraction, heat capacity, and magnetization measurements. The refinements of the neutron diffraction patterns (150, 50, and 45 K) using the Rietveld method confirm the monoclinic (S. G. C2/m) structure. Temperature-dependent magnetic susceptibilities measured at varying fields along with the heat capacity measurements demonstrate the coexistence of long-range ordering (∼42 K) and short-range ordering (∼65 K). The field-dependent isothermal magnetization measurements at 5 K indicate a spin-flop transition around 5 T. Rietveld refinements of the low-temperature (below 45 K) neutron diffraction data further confirm the long-range magnetic ordering. In addition, the temperature variation of the lattice parameters obtained from the neutron powder diffraction analysis exhibited a distinct anomaly near the antiferromagnetic transition temperature. The appearance of the concomitant broadened backgrounds in the neutron powder diffraction data collected at 80, 50, and 45 K supports the short-range ordering. The resultant magnetic structure consists of spins that are aligned antiparallel with the nearest neighbors and also with the spins of the adjacent honeycomb layers. The occurrence of a fully ordered magnetic ground state (Neel antiferromagnetic (AFM)) in Na3Mn2SbO6 consolidates the significance of fabricating new honeycomb oxides.

Magnetic structures and excitations in sawtooth olivine chalcogenides Mn2SiX4 (X = S, Se).

The Mn lattice in olivine chalcogenide Mn2SiX4 (X = S, Se) compounds forms a sawtooth, which is of special interest in magnetism owing to the possibility of realizing flat bands in magnon spectra, a key component in magnonics. In this work, we investigate the Mn2SiX4 olivines using magnetic susceptibility, and X-ray and neutron diffraction. We have determined the average and local crystal structures of Mn2SiS4 and Mn2SiSe4 using synchrotron X-ray, neutron diffraction, and X-ray total scattering data followed by Rietveld and pair distribution function analyses. It is found from the pair distribution function analysis that the Mn triangle that constitutes the sawtooth is isosceles in Mn2SiS4 and Mn2SiSe4. The temperature evolution of magnetic susceptibility of Mn2SiS4 and Mn2SiSe4 shows anomalies below 83 K and 70 K, respectively, associated with magnetic ordering. From the neutron powder diffraction measurements the magnetic space groups of Mn2SiS4 and Mn2SiSe4 are found to be Pnma and Pnm'a', respectively. We find that the Mn spins adopt a ferromagnetic alignment on the sawtooth in both Mn2SiS4 and Mn2SiSe4 but along different crystallographic directions for the S and the Se compounds. From the temperature evolution of Mn magnetic moments obtained from refining neutron diffraction data, the transition temperatures are accurately determined as TN(S) = 83(2) K and TN(Se) = 70.0(5) K. Broad diffuse magnetic peaks are observed in both the compounds, and are prominently seen close to TN, suggesting the presence of a short-range magnetic order. The magnetic excitations studied using inelastic neutron scattering reveal a magnon excitation with an energy corresponding to approximately 4.5 meV in both S and Se compounds. Spin correlations are observed to persist up to 125 K much above the ordering temperature and we suggest the possibility of short-range spin correlations responsible for this.

Advances in actinide thin films: synthesis, properties, and future directions.

Actinide-based compounds exhibit unique physics due to the presence of 5f electrons, and serve in many cases as important technological materials. Targeted thin film synthesis of actinide materials has been successful in generating high-purity specimens in which to study individual physical phenomena. These films have enabled the study of the unique electron configuration, strong mass renormalization, and nuclear decay in actinide metals and compounds. The growth of these films, as well as their thermophysical, magnetic, and topological properties, have been studied in a range of chemistries, albeit far fewer than most classes of thin film systems. This relative scarcity is the result of limited source material availability and safety constraints associated with the handling of radioactive materials. Here, we review recent work on the synthesis and characterization of actinide-based thin films in detail, describing both synthesis methods and modeling techniques for these materials. We review reports on pyrometallurgical, solution-based, and vapor deposition methods. We highlight the current state-of-the-art in order to construct a path forward to higher quality actinide thin films and heterostructure devices.

Physical Properties of Candidate X-Ray Detector Material Rb4Ag2BiBr9.

Recently, metal halide perovskites have emerged as promising semiconductor candidates for sensitive X-ray photon detection due to their suitable bandgap energies, excellent charge transport properties, and low material cost afforded by their low-temperature solution-processing preparation. Here, we report an improved methodology for single crystal (SC) growth, thermal and electrical properties of a two-dimensional (2D) layered halide material Rb4Ag2BiBr9, which has been identified as a potential candidate for X-ray radiation detection applications. The measured heat capacity for Rb4Ag2BiBr9 implies that there are no structural phase transitions upon cooling. Temperature dependence of thermal transport measurements further suggest remarkably low thermal conductivities of Rb4Ag2BiBr9 that are comparable to the lowest reported in literature. The bulk crystal resistivity is determined to be 2.59×109 Ω·cm from the current-voltage (I-V) curve. Density of trap states are estimated to be ~1010 cm-3 using the space-charge-limited-current (SCLC) measurements. The fabricated Rb4Ag2BiBr9-based X-ray detector shows good operational stability with no apparent current drift, which may be ascribed to the 2D crystal structure of Rb4Ag2BiBr9. Finally, by varying the X-ray tube current to change the corresponding dose rate, the Rb4Ag2BiBr9 X-ray detector sensitivity is determined to be 222.03 uCGy-1cm-2 (at an electric field of E = 24 V/mm).

Thermal Energy Transport in Oxide Nuclear Fuel.

To efficiently capture the energy of the nuclear bond, advanced nuclear reactor concepts seek solid fuels that must withstand unprecedented temperature and radiation extremes. In these advanced fuels, thermal energy transport under irradiation is directly related to reactor performance as well as reactor safety. The science of thermal transport in nuclear fuel is a grand challenge as a result of both computational and experimental complexities. Here we provide a comprehensive review of thermal transport research on two actinide oxides: one currently in use in commercial nuclear reactors, uranium dioxide (UO2), and one advanced fuel candidate material, thorium dioxide (ThO2). In both materials, heat is carried by lattice waves or phonons. Crystalline defects caused by fission events effectively scatter phonons and lead to a degradation in fuel performance over time. Bolstered by new computational and experimental tools, researchers are now developing the foundational work necessary to accurately model and ultimately control thermal transport in advanced nuclear fuels. We begin by reviewing research aimed at understanding thermal transport in perfect single crystals. The absence of defects enables studies that focus on the fundamental aspects of phonon transport. Next, we review research that targets defect generation and evolution. Here the focus is on ion irradiation studies used as surrogates for damage caused by fission products. We end this review with a discussion of modeling and experimental efforts directed at predicting and validating mesoscale thermal transport in the presence of irradiation defects. While efforts in these research areas have been robust, challenging work remains in developing holistic tools to capture and predict thermal energy transport across widely varying environmental conditions.

Magnetoelastic standing waves induced in UO2 by microsecond magnetic field pulses.

Magnetoelastic dilatometry of the piezomagnetic antiferromagnet UO2 was performed via the fiber Bragg grating method in magnetic fields up to 150 T generated by a single-turn coil setup. We show that in microsecond timescales, pulsed-magnetic fields excite mechanical resonances at temperatures ranging from 10 to 300 K, in the paramagnetic as well as within the robust antiferromagnetic state of the material. These resonances, which are barely attenuated within the 100-µs observation window, are attributed to the strong magnetoelastic coupling in UO2 combined with the high crystalline quality of the single crystal samples. They compare well with mechanical resonances obtained by a resonant ultrasound technique and superimpose on the known nonmonotonic magnetostriction background. A clear phase shift of π in the lattice oscillations is observed in the antiferromagnetic state when the magnetic field overcomes the piezomagnetic switch field H[Formula: see text] T. We present a theoretical argument that explains this unexpected behavior as a result of the reversal of the antiferromagnetic order parameter at Hc.

Magnetic structure, excitations and short-range order in honeycomb Na2Ni2TeO6.

Na2Ni2TeO6has a layered hexagonal structure with a honeycomb lattice constituted by Ni2+and a chiral charge distribution of Na+that resides between the Ni layers. In the present work, the antiferromagnetic (AFM) transition temperature of Na2Ni2TeO6is confirmed atTN≈ 27 K, and further, it is found to be robust up to 8 T magnetic field and 1.2 GPa external pressure; and, without any frequency-dependence. Slight deviations from nominal Na-content (up to 5%) does not seem to influence the magnetic transition temperature,TN. Isothermal magnetization curves remain almost linear up to 13 T. Our analysis of neutron diffraction data shows that the magnetic structure of Na2Ni2TeO6is faithfully described by a model consisting of two phases described by the commensurate wave vectorsk→c,0.500and0.500.5, with an additional short-range order component incorporated in to the latter phase. Consequently, a zig-zag long-range ordered magnetic phase of Ni2+results in the compound, mixed with a short-range ordered phase, which is supported by our specific heat data. Theoretical computations based on density functional theory predict predominantly in-plane magnetic exchange interactions that conform to aJ1-J2-J3model with a strongJ3term. The computationally predicted parameters lead to a reliable estimate forTNand the experimentally observed zig-zag magnetic structure. A spin wave excitation in Na2Ni2TeO6atE≈ 5 meV atT= 5 K is mapped out through inelastic neutron scattering experiments, which is reproduced by linear spin wave theory calculations using theJvalues from our computations. Our specific heat data and inelastic neutron scattering data strongly indicate the presence of short-range spin correlations, atT>TN, stemming from incipient AFM clusters.

Absence of long-range magnetic order in lithium-containing honeycombs in the Li-Cr-Sb(Te)-O phases.

Li3((LiCr)(Te/Sb))O6compounds where Cr atoms along with Li and Te or Sb are part of a honeycomb and are studied using magnetic susceptibility, specific heat, x-ray photoelectron spectroscopy and neutron diffraction. The oxides stoichiometries as determined from the neutron diffraction studies are Li4.47Cr0.53TeO6and Li3.88Cr1.12SbO6with a stable oxidation state of +3 for Cr. Both the compounds crystallize in space groupC2/mwith intermixing of cations at the 4gsites leaving the 2asites preferentially for Te or Sb. Again, the Li+ions alone predominantly occur in the interlayer sites. Both the compounds show a broad anomaly in specific heat at 8 K, which is robust against 8 T. A corresponding anomaly is absent in the magnetic susceptibility but recovers from its derivative, dχ(T)/dT. We ascertain the magnetic anomaly temperatures (Ta) of Li4.47Cr0.53TeO6and Li3.88Cr1.12SbO6as 5.9 K and 6.7 K respectively from specific heat. Although the physical properties indicated a low temperature anomaly, neutron diffraction data did not reveal a magnetic signal or a structural anomaly down to 1.5 K. This rules out a conventional long-range ordered magnetic ground state in either compounds. Combining the results from specific heat, neutron diffraction and electron paramagnetic resonance, we put forth a scenario of depleted honeycomb lattice of Cr3+with predominant short-range magnetic correlations as the magnetic ground states of the title compounds.

Magnetocaloric Effect in a Frustrated Gd-Garnet with No Long-Range Magnetic Order.

In this paper, the hyperkagome lattice of Gd spins in a garnet compound, Gd3CrGa4O12, is studied using bulk measurements and density functional computations, and the observation of large magnetocaloric effect corresponding to an entropy change, ΔSm = 45 J kg-1K-1 (≈ 45 J mol-1K-1) at 2 K, 8 T is reported. Though the compound defies long-range magnetic order down to 0.4 K, a broad feature below 10 K is observed in the specific heat with two low temperature anomalies at T* ≈ 0.7 K and TS ≈ 2.45 K. The anomaly at T* is reminiscent of one in Gd3Ga5O12, where it is related to the development of a complex magnetic phase, whereas the TS-peak is accounted for by a multilevel Schottky-like model. The spin-lattice relaxation times studied by nuclear magnetic resonance experiments show that the relaxation is dominated by the magnetic fluctuations in Cr which has a longer relaxation time compared to that of the garnet, Lu3CrGa4O12 containing a nonmagnetic rare earth. Our first-principles density functional theory calculations agree well with the experimental results and support short-range magnetic order in the Gd-sublattice and antiferromagnetism in the Cr-sublattice. The importance of spin fluctuations and short-range order in the rare earth and transition metal lattices in garnets resulting in large magnetocaloric effect is brought out through this work.

Observation of gapped state in rare-earth monopnictide HoSb.

The rare-earth monopnictide family is attracting an intense current interest driven by its unusual extreme magnetoresistance (XMR) property and the potential presence of topologically non-trivial surface states. The experimental observation of non-trivial surface states in this family of materials are not ubiquitous. Here, using high-resolution angle-resolved photoemission spectroscopy, magnetotransport, and parallel first-principles modeling, we examine the nature of electronic states in HoSb. Although we find the presence of bulk band gaps at the [Formula: see text] and X-symmetry points of the Brillouin zone, we do not find these gaps to exhibit band inversion so that HoSb does not host a Dirac semimetal state. Our magnetotransport measurements indicate that HoSb can be characterized as a correlated nearly-complete electron-hole-compensated semimetal. Our analysis reveals that the nearly perfect electron-hole compensation could drive the appearance of non-saturating XMR effect in HoSb.

Experimental observation of drumhead surface states in SrAs3.

The topological nodal-line semimetal (TNS) is a unique class of materials with a one dimensional line node accompanied by a nearly dispersionless two-dimensional surface state. However, a direct observation of the so called drumhead surface state within current nodal-line materials is still elusive. Here, using high-resolution angle-resolved photoemission spectroscopy (ARPES) along with first-principles calculations, we report the observation of a topological nodal-loop (TNL) in SrAs3, whereas CaAs3 exhibits a topologically trivial state. Our data reveal that surface projections of the bulk nodal-points are connected by clear drumhead surface states in SrAs3. Furthermore, our magneto-transport and magnetization data clearly suggest the presence (absence) of surface states in SrAs3 (CaAs3). Notably, the observed topological states in SrAs3 are well separated from other bands in the vicinity of the Fermi level. RAs3 where R = Ca, Sr, thus, offers a unique opportunity to realize an archetype nodal-loop semimetal and establish a platform for obtaining a deeper understanding of the quantum phase transitions.

Local inhomogeneity and filamentary superconductivity in Pr-doped CaFe2As2.

We use multiscale techniques to determine the extent of local inhomogeneity and superconductivity in Ca0.86Pr0.14Fe2As2 single crystal. The inhomogeneity is manifested as a spatial variation of the praseodymium concentration, local density of states, and superconducting order parameter. We show that the high-Tc superconductivity emerges from cloverlike defects associated with Pr dopants. The highest Tc is observed in both the tetragonal and collapsed tetragonal phases, and its filamentary nature is a consequence of nonuniform Pr distribution that develops localized, isolated superconducting regions within the crystals.

Imaging the three-dimensional Fermi-surface pairing near the hidden-order transition in URu2Si2 using angle-resolved photoemission spectroscopy.

We report angle-resolved photoemission spectroscopy experiments probing deep into the hidden-order state of URu(2)Si(2), utilizing tunable photon energies with sufficient energy and momentum resolution to detect the near Fermi-surface (FS) behavior. Our results reveal (i) the full itinerancy of the 5f electrons, (ii) the crucial three-dimensional k-space nature of the FS and its critical nesting vectors, in good comparison with density-functional theory calculations, and (iii) the existence of hot-spot lines and pairing of states at the FS, leading to FS gapping in the hidden-order phase.

Aligned carbon nanotubes sandwiched in epitaxial NbC film for enhanced superconductivity.

Highly aligned carbon nanotube (CNT) ribbons were sandwiched in epitaxial superconducting NbC films by a chemical solution deposition method. The incorporation of aligned long CNTs into NbC film enhances the normal-state conductivity and improves the superconducting properties of the assembly.