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At least 1,117 records · Page 62
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Size effects of 109° domain walls in rhombohedral barium titanate single crystals—A molecular statics analysis

Ferroelectric functional materials are of great interest in science and technology due to their electromechanically coupled material properties. Therefore, ferroelectrics, such as barium titanate, are modeled and simulated at the continuum scale as well as at the atomistic scale. Due to recent advancements in related manufacturing technologies the modeling and simulation of smart materials at the nanometer length scale is getting more important not only to predict but also fundamentally understand the complex material behavior of such materials. In this study, we analyze the size effects of 109° nanodomain walls in ferroelectric barium titanate single crystals in the rhombohedral phase using a recently proposed extended molecular statics algorithm. We study the impact of domain thicknesses on the spontaneous polarization, the coercive field, and the lattice constants. Moreover, we discuss how the electromechanical coupling of an applied electric field and the introduced strain in the converse piezoelectric effect is affected by the thickness of nanodomains.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

High-mobility BaSnO{sub 3} grown by oxide molecular beam epitaxy

High-mobility perovskite BaSnO{sub 3} films are of significant interest as new wide bandgap semiconductors for power electronics, transparent conductors, and as high mobility channels for epitaxial integration with functional perovskites. Despite promising results for single crystals, high-mobility BaSnO{sub 3} films have been challenging to grow. Here, we demonstrate a modified oxide molecular beam epitaxy (MBE) approach, which supplies pre-oxidized SnO{sub x}. This technique addresses issues in the MBE of ternary stannates related to volatile SnO formation and enables growth of epitaxial, stoichiometric BaSnO{sub 3}. We demonstrate room temperature electron mobilities of 150 cm{sup 2} V{sup −1} s{sup −1} in films grown on PrScO{sub 3}. The results open up a wide range of opportunities for future electronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Spin reorientation transition in Sm{sub 0.5}Tb{sub 0.5}FeO{sub 3} orthoferrite single crystal

Single crystal of Sm{sub 0.5}Tb{sub 0.5}FeO{sub 3} was grown by an optical floating zone method. The spin reorientation transition behavior (Γ{sub 2} → Γ{sub 24} → Γ{sub 4}) is observed in the temperature range from 150 to 250 K. The dynamics of spin reorientation transition is studied by terahertz time-domain spectroscopy (THz-TDS). FM mode was only discovered when temperature is below 55 K. And the FM mode resonant frequency increases from 0.216 THz to 0.268 THz with temperature deceasing from 55 K to 39 K. The AFM mode resonant frequency is fixed at 0.542 THz except in the very low temperature range. Our results demonstrate that THz-TDS is an effective means to study the dynamical iron ions SRT in RFeO{sub 3}.

36 MATERIALS SCIENCE↗
DOE OSTI2016

All-in-all-out magnetic domain size in pyrochlore iridate thin films as probed by local magnetotransport

Pyrochlore iridates have attracted growing attention because of a theoretical prediction of a possible topological semimetal phase originating from all-in-all-out spin ordering. Related to the topological band structure, recent findings of the magnetic domain wall conduction have stimulated investigations of magnetic domain distribution in this system. Here, we investigate the size of magnetic domains in Eu{sub 2}Ir{sub 2}O{sub 7} single crystalline thin films by magnetoresistance (MR) using microscale Hall bars. Two distinct magnetic domains of the all-in-all-out spin structure are known to exhibit linear MR but with opposite signs, which enables us to estimate the ratio of the two domains in the patterned channel. The linear MR for 80 × 60 μm{sup 2} channel is nearly zero after zero-field cooling, suggesting random distribution of domains smaller than the channel size. In contrast, the wide distribution of the value of the linear MR is detected in 2 × 2 μm{sup 2} channel, reflecting the detectable domain size depending on each cooling-cycle. Compared to simulation results, we estimate the average size of a single all-in-all-out magnetic domain as 1–2 μm.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Improved neutron-gamma discrimination for a {sup 6}Li-glass neutron detector using digital signal analysis methods

A {sup 6}Li-glass scintillator (GS20) based neutron Anger camera was developed for time-of-flight single-crystal diffraction instruments at Spallation Neutron Source. Traditional Pulse-Height Analysis (PHA) for Neutron-Gamma Discrimination (NGD) resulted in the neutron-gamma efficiency ratio (defined as NGD ratio) on the order of 10{sup 4}. The NGD ratios of Anger cameras need to be improved for broader applications including neutron reflectometers. For this purpose, six digital signal analysis methods of individual waveforms acquired from photomultiplier tubes were proposed using (i) charge integration, (ii) pulse-amplitude histograms, (iii) power spectrum analysis combined with the maximum pulse-amplitude, (iv) two event parameters (a{sub 1}, b{sub 0}) obtained from a Wiener filter, (v) an effective amplitude (m) obtained from an adaptive least-mean-square filter, and (vi) a cross-correlation coefficient between individual and reference waveforms. The NGD ratios are about 70 times those from the traditional PHA method. Our results indicate the NGD capabilities of neutron Anger cameras based on GS20 scintillators can be significantly improved with digital signal analysis methods.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Strain profiles in ion implanted ceramic polycrystals: An approach based on reciprocal-space crystal selection

The determination of the state of strain in implanted materials is a key issue in the study of their mechanical stability. Whereas this question is nowadays relatively easily solved in the case of single crystals, it remains a challenging task in the case of polycrystalline materials. In this paper, we take benefit of the intense and parallel beams provided by third generation synchrotron sources combined with a two-dimensional detection system to analyze individual grains in polycrystals, hence obtaining “single crystal-like” data. The feasibility of the approach is demonstrated with implanted UO{sub 2} polycrystals where the in-depth strain profile is extracted for individual grains using numerical simulations of the diffracted signal. The influence of the implantation dose is precisely analyzed for several diffracting planes and grains. This work suggests that, at low fluences, the development of strain is mainly due to ballistic effects with little effect from He ions, independently from the crystallographic orientation. At higher fluences, the evolution of the strain profiles suggests a partial and anisotropic plastic relaxation. With the present approach, robust and reliable structural information can be obtained, even from complex polycrystalline ceramic materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Spin coherence in a Mn{sub 3} single-molecule magnet

Spin coherence in single crystals of the spin S = 6 single-molecule magnet (SMM) [Mn{sub 3}O(O{sub 2}CEt){sub 3}(mpko){sub 3}]{sup +} (abbreviated Mn{sub 3}) has been investigated using 230 GHz electron paramagnetic resonance spectroscopy. Coherence in Mn{sub 3} was uncovered by significantly suppressing dipolar contribution to the decoherence with complete spin polarization of Mn{sub 3} SMMs. The temperature dependence of spin decoherence time (T{sub 2}) revealed that the dipolar decoherence is the dominant source of decoherence in Mn{sub 3} and T{sub 2} can be extended up to 267 ns by quenching the dipolar decoherence.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Advanced fabrication of single-crystalline silver nanopillar on SiO{sub 2} substrate

Nanoscale crystallographic textures have received very little attention in research on surface plasmons using metallic nanostructures. A single-crystalline metallic nanostructure with a controlled crystallographic texture is expected to reduce optical losses. We elucidated the grain growth mechanism in silver thin films deposited on a highly transparent SiO{sub 2} substrate by electron backscatter diffraction methods with nanoscale resolution. At higher substrate temperatures, the grain growth was facilitated but the preferred orientation was not achieved. Moreover, we fabricated a single-crystalline silver nanopillar in a (111)-oriented large growing grain, which was controlled by varying the substrate temperature during film deposition by focused ion-beam milling. Furthermore, the light intensity of the scattering spectrum was measured for a single-crystalline silver nanopillar (undersurface diameter: 200 nm) for which surface plasmon resonance was observed. The single-crystalline silver nanopillar exhibits a stronger and sharper spectrum than the polycrystalline silver nanopillar. These results can be applied to the direct fabrication of a single-crystalline silver nanopillar using only physical processing.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

X-ray topography using the forward transmitted beam under multiple-beam diffraction conditions

X-ray topographs are taken for a sapphire wafer with the [0001] surface normal, as an example, by forward transmitted synchrotron x-ray beams combined with two-dimensional electronic arrays in the x-ray detector having a spatial resolution of 1 μm. They exhibit no shape deformation and no position shift of the dislocation lines on the topographs. Since the topography is performed under multiple-beam diffraction conditions, the topographic images of a single diffraction (two-wave approximation condition) or plural diffractions (six-wave approximation condition) can be recorded without large specimen position changes. As usual Lang topographs, it is possible to determine the Burgers vector of each dislocation line. Because of high parallelism of the incoming x-rays and linear sensitivity of the electronic arrays to the incident x-rays, the present technique can be used to visualize individual dislocations in single crystals of the dislocation density as high as 1 × 10{sup 5} cm{sup −2}.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Giant semiclassical magnetoresistance in high mobility TaAs{sub 2} semimetal

We report the observation of colossal positive magnetoresistance (MR) in single crystalline, high mobility TaAs{sub 2} semimetal. The excellent fit of MR by a single quadratic function of the magnetic field B over a wide temperature range (T = 2–300 K) suggests the semiclassical nature of the MR. The measurements of Hall effect and Shubnikov-de Haas oscillations, as well as band structure calculations, suggest that the giant MR originates from the nearly perfectly compensated electrons and holes in TaAs{sub 2}. The quadratic MR can even exceed 1 200 000% at B = 9 T and T = 2 K, which is one of the largest values among those of all known semi-metallic compounds, including the very recently discovered WTe{sub 2} and NbSb{sub 2}. The giant positive magnetoresistance in TaAs{sub 2} not only has a fundamentally different origin from the negative colossal MR observed in magnetic systems but also provides a nice complemental system that will be beneficial for applications in magnetoelectronic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

A diamond 14 MeV neutron energy spectrometer with high energy resolution

A self-standing single-crystal chemical vapor deposited diamond was obtained using lift-off method. It was fabricated into a radiation detector and response function measurements for 14 MeV neutrons were taken at the fusion neutronics source. 1.5% of high energy resolution was obtained by using the {sup 12}C(n, α){sup 9}Be reaction at an angle of 100° with the deuteron beam line. The intrinsic energy resolution, excluding energy spreading caused by neutron scattering, slowing in the target and circuit noises was 0.79%, which was also the best resolution of the diamond detector ever reported.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Spin dynamics, electronic, and thermal transport properties of two-dimensional CrPS{sub 4} single crystal

2-Dimensional (2D) CrPS{sub 4} single crystals have been grown by the chemical vapor transport method. The crystallographic, magnetic, electronic, and thermal transport properties of the single crystals were investigated by the room-temperature X-ray diffraction, electrical resistivity ρ(T), specific heat C{sub P}(T), and the electronic spin response (ESR) measurements. CrPS{sub 4} crystals crystallize into a monoclinic structure. The electrical resistivity ρ(T) shows a semiconducting behavior with an energy gap E{sub a} = 0.166 eV. The antiferromagnetic transition temperature is about T{sub N} = 36 K. The spin flipping induced by the applied magnetic field is observed along the c axis. The magnetic phase diagram of CrPS{sub 4} single crystal has been discussed. The extracted magnetic entropy at T{sub N} is about 10.8 J/mol K, which is consistent with the theoretical value R ln(2S + 1) for S = 3/2 of the Cr{sup 3+} ion. Based on the mean-field theory, the magnetic exchange constants J{sub 1} and J{sub c} corresponding to the interactions of the intralayer and between layers are about 0.143 meV and −0.955 meV are obtained based on the fitting of the susceptibility above T{sub N}, which agree with the results obtained from the ESR measurements. With the help of the strain for tuning the magnetic properties, monolayer CrPS{sub 4} may be a promising candidate to explore 2D magnetic semiconductors.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Giant low-field magnetocaloric effect in single-crystalline EuTi{sub 0.85}Nb{sub 0.15}O{sub 3}

The magnetocaloric effect in ferromagnetic single crystal EuTi{sub 0.85}Nb{sub 0.15}O{sub 3} has been investigated using magnetization and heat capacity measurements. EuTi{sub 0.85}Nb{sub 0.15}O{sub 3} undergoes a continuous ferromagnetic phase transition at T{sub C} = 9.5 K due to the long range ordering of magnetic moments of Eu{sup 2+} (4f{sup 7}). With the application of magnetic field, the spin entropy is strongly suppressed and a giant magnetic entropy change is observed near T{sub C}. The values of entropy change ΔS{sub m} and adiabatic temperature change ΔT{sub ad} are as high as 51.3 J kg{sup −1} K{sup −1} and 22 K, respectively, for a field change of 0–9 T. The corresponding magnetic heating/cooling capacity is 700 J kg{sup −1}. This compound also shows large magnetocaloric effect even at low magnetic fields. In particular, the values of ΔS{sub m} reach 14.7 and 23.8 J kg{sup −1} K{sup −1} for field changes of 0–1 T and 0–2 T, respectively. The low-field giant magnetocaloric effect, together with the absence of thermal and field hysteresis makes EuTi{sub 0.85}Nb{sub 0.15}O{sub 3} a very promising candidate for low temperature magnetic refrigeration.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Optical properties of single crystalline SrMoO{sub 3} thin films

The optical properties of pulsed laser deposited highly crystalline SrMoO{sub 3} thin films were investigated. Due to their low resistivity below 30 μΩ cm, thin films of SrMoO{sub 3} are candidates for transparent conductor applications. The transparency of SrMoO{sub 3} extends into the ultraviolet range to about 300 nm. In this range, SrMoO{sub 3} has a higher transparency at similar sheet resistance as compared to alternative oxide or metallic materials. Density functional theory shows that electron-electron correlation effects are small in SrMoO{sub 3} as compared to other low-resistivity transition metal oxides and predicts the optical properties in good agreement with experiment.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Short range ferromagnetic, magneto-electric, and magneto-dielectric effect in ceramic Co{sub 3}TeO{sub 6}

We report observation of magneto-electric and magneto-dielectric couplings along with short range ferromagnetic order in ceramic Cobalt Tellurate (Co{sub 3}TeO{sub 6}, CTO) using magnetic, structural, dielectric, pyroelectric, and polarization studies. DC magnetization along with dielectric constant measurements indicate a coupling between magnetic order and electrical polarization. A strong anomaly in the dielectric constant at ∼17.4 K in zero magnetic field indicates spontaneous electric polarization, consistent with a recent neutron diffraction study. Observation of weak short range ferromagnetic order at lower temperatures is attributed to the Griffiths-like ferromagnetism. Furthermore, magnetic field dependence of the ferroelectric transition follows earlier theoretical predictions, applicable to single crystal CTO. Finally, combined dielectric, pyroelectric, and polarization measurements suggest that the ground state of CTO may possess spontaneous symmetry breaking in the absence of magnetic field.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Electronic spectrum of non-tetrahedral acceptors in CdTe:Cl and CdTe:Bi,Cl single crystals

The electronic spectra of complex acceptors in compensated CdTe:Cl, CdTe:Ag,Cl, and CdTe:Bi,Cl single crystals are studied using low-temperature photoluminescence (PL) measurements under both nonresonant and resonant excitation of distant donor–acceptor pairs (DAP). The wavelength modulation of the excitation source combined with the analysis of the differential PL signal is used to enhance narrow spectral features obscured because of inhomogeneous line broadening and/or excitation transfer for selectively excited DAPs. For the well-known tetrahedral (T{sub D}) Ag{sub Cd} acceptor, the energies of four excited states are measured, and the values obtained are shown to be in perfect agreement with the previous data. Moreover, splitting between the 2P{sub 3/2} (Γ{sub 8}) and 2S{sub 3/2} (Γ{sub 8}) states is clearly observed for Ag{sub Cd} centers located at a short distance (5–7 nm) from a hydrogen-like donor (Cl{sub Te}). This splitting results from the reduction of the T{sub D} symmetry taking place when the acceptor is a member of a donor–acceptor pair. For the Cl-related complex acceptor with an activation energy of ∼121 meV (A-center), the energies of eight excited states are measured. It is shown that this defect produces low-symmetry central-cell correction responsible for the strong splitting of S-like T{sub D} shells. The energy spectrum of the Bi-related shallow acceptor with an activation energy of ∼36 meV is measured as well. The spectrum obtained differs drastically from the hydrogen-like set of levels, which indicates the existence of repulsive low-symmetry perturbation of the hydrogen-like Coulomb potential. It is also shown that the spectra of selectively excited PL recorded for a macroscopic ensemble of distant donor–acceptor pairs allow one to detect the low symmetry of acceptors of a given type caused by their complex nature or by the Jahn–Teller distortion. This method does not require any additional (external) field and is applicable to acceptors in diverse zinc-blende compound semiconductors.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Traceable atomic force microscopy of high-quality solvent-free crystals of [6,6]-phenyl-C{sub 61}-butyric acid methyl ester

We report high-resolution, traceable atomic force microscopy measurements of high-quality, solvent-free single crystals of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). These were grown by drop-casting PCBM solutions onto the spectrosil substrates and by removing the residual solvent in a vacuum. A home-built atomic force microscope featuring a plane mirror differential optical interferometer, fiber-fed from a frequency-stabilized laser (emitting at 632.8 nm), was used to measure the crystals' height. The optical interferometer together with the stabilized laser provides traceability (via the laser wavelength) of the vertical measurements made with the atomic force microscope. We find that the crystals can conform to the surface topography, thanks to their height being significantly smaller compared to their lateral dimensions (namely, heights between about 50 nm and 140 nm, for the crystals analysed, vs. several tens of microns lateral dimensions). The vast majority of the crystals are flat, but an isolated, non-flat crystal provides insights into the growth mechanism and allows identification of “molecular terraces” whose height corresponds to one of the lattice constants of the single PCBM crystal (1.4 nm) as measured with X-ray diffraction.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗