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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,783 records · Page 99
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DOE OSTI2019

Space-Modulated Structures in BiFeO{sub 3} Films

In this work, we explore the properties of antiferromagnetic cycloid and the phase transitions between commensurate and incommensurate magnetic states in epitaxial BiFeO{sub 3} film. Additional magnetic anisotropy induced by strain effects in the films allocates cycloids with the definite directions of spin rotation. Peculiar feature of the cycloids propagating in the films whose symmetry is different from the single crystals is the orientation of spin rotational plane that does not contain electric polarization in contrast with the bulk materials. We construct a diagram of phase transitions induced by magnetic field applied along normal to the surface and show considerable decrease of the strength of magnetic field destroying cycloid in films compared with the bulk.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Topological Structures in Unconventional Scenario for 2D Cuprates

Numerous experimental data point to cuprates as d-d charge transfer unstable systems whose description implies the inclusion of the three many-electron valence states CuO{sub 4}{sup 7−,6−,5−} (nominally Cu{sup 1+,2+,3+}) on an equal footing as a well-defined charge triplet. We introduce a minimal model to describe the charge degree of freedom in cuprates with the on-site Hilbert space reduced to only the three states and make use of the S = 1 pseudospin formalism. The formalism constitutes a powerful method to study complex phenomena in interacting quantum systems characterized by the coexistence and competition of various ordered states. Overall, such a framework provides a simple and systematic methodology to predict and discover new kinds of orders. In particular, the pseudospin formalism provides the most effective way to describe different topological structures, in particular, due to a possibility of a geometrical two-vector description of the on-site states. We introduce and analyze effective pseudospin Hamiltonian with on-site and inter-site charge correlations, two types of a correlated one-particle transfer and two-particle, or the composite boson transfer. The latter is of a principal importance for the HTSC perspectives. The 2D S = 1 pseudospin system is prone to a creation of different topological structures, which form topologically protected inhomogeneous distributions of the eight local S = 1 pseudospin order parameters. We present a short overview of localized topological structures, typical for S = 1 (pseudo)spin systems, focusing on unexpected antiphase domain walls in parent cuprates and so-called quadrupole skyrmion, which are believed to be candidates for a topological charge excitation in parent or underdoped cuprates. Puzzlingly, these unconventional structures can be characterized by an uniform distribution of the mean on-site charge, that makes these invisible for X-rays. Quasi-classical approximation and computer simulation are applied to analyze localized topological defects and evolution of the domain structures in “negative-U” model under charge order-superfluid phase transition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Effect of CuO{sub 2} Lattice Strain on the Electronic Structure and Properties of High-T{sub c} Cuprate Family

Doping and strain dependences of the electronic structure of the CuO{sub 6}-octahedra layer within LDA+GTB method in the frameworks of six-band p-d model are calculated. Band structure and Fermi surface of the quasiparticle excitations in the effective Hubbard model are characterized by inhomogeneous distribution of the k-dependent spectral weight. Doping results in reconstruction of the band structure, redistribution of the spectral weight over dispersion surface, and reconstruction of Fermi surface from small hole pockets in the underdoped region to large hole contour in the overdoped region through two quantum phase transitions (QPT). Strain increasing leads to displacement of the valence and conductivity bands, bandwidths decreasing, and shift of the concentrations corresponding to QPTs. Strain dependences of the antiferromagnetic exchange coupling and DOS at the Fermi level determining superconducting temperature T{sub c} are obtained. Effective exchange coupling in the equation for T{sub c} demonstrates monotonic strain dependence.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

The Superconductor-Superinsulator Transition: S-duality and the QCD on the Desktop

We show that the nature of quantum phases around the superconductor-insulator transition (SIT) is controlled by charge-vortex topological interactions, and does not depend on the details of material parameters and disorder. We find three distinct phases, superconductor, superinsulator, and bosonic topological insulator. The superinsulator is a state of matter with infinite resistance in a finite temperature range, which is the S-dual of the superconductor and in which charge transport is prevented by electric strings binding charges of opposite sign. The electric strings ensuring linear confinement of charges are generated by instantons and are dual to superconducting Abrikosov vortices. Material parameters and disorder enter the London penetration depth of the superconductor, the string tension of the superinsulator and the quantum fluctuation parameter driving the transition between them. They are entirely encoded in four phenomenological parameters of a topological gauge theory of the SIT. Finally, we point out that, in the context of strong coupling gauge theories, the many-body localization phenomenon that is often referred to as an underlying mechanism for superinsulation is a mere transcription of the well-known phenomenon of confinement into solid-state physics language and is entirely driven by endogenous disorder embodied by instantons with no need of exogenous disorder.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Structural Analysis, Magnetocaloric Effect, and Critical Exponents for La{sub 0.6}Sr{sub 0.2}Na{sub 0.2}MnO{sub 3} Manganite

This work focuses on the structural, magnetocaloric effect, and critical exponent investigations of La{sub 0.6}Sr{sub 0.2}Na{sub 0.2}MnO{sub 3} manganite prepared via sol–gel method. XRD analysis confirms the rhombohedral structure with R 3-bar c symmetry for this sample, and Rietveld method was used to estimate its different structural parameters. The sample presents FM-PM phase transition of second-order type at T{sub C} = 309 K. The maximum of magnetic entropy change (−ΔS{sub M}{sup max}) and relative cooling power (RCP) reach values of about 3.57 J kg{sup −1} K{sup −1} and 214.46 J kg{sup −1}, respectively (at μ{sub 0}H = 5 T). These values indicate that La{sub 0.6}Sr{sub 0.2}Na{sub 0.2}MnO{sub 3} sample can be used as active magnetic refrigerator. Critical exponents (β, γ, and δ) of the sample have studied by analyzing the M(μ{sub 0}H, T) isotherms using different techniques. The estimated values of these critical exponents are found to be different to those predicted for standard theoretical models.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Studies of the Magnetocaloric and Electrical Properties of La{sub 0.7}Er{sub 0.05}Sr{sub 0.15}Ca{sub 0.1}Mn{sub 1−x}In{sub x}O{sub 3} (0 ≤ x ≤ 0.30)

We report on the study of In substitution for Mn on the magnetic and transport properties of layered manganese oxides La{sub 0.7}Er{sub 0.05}Sr{sub 0.15}Ca{sub 0.1}Mn{sub 1−x}In{sub x}O{sub 3} (0 ≤ x ≤ 0.30) synthesized via sol-gel method. For all the samples, all peaks are satisfactorily indexed in the rhombohedra structure with R 3-bar c space group. Magnetic data, under a magnetic field of 0.05 T, revealed that our compounds exhibit a continuous (second-order) transition from a ferromagnetic (FM) to a paramagnetic (PM) state upon decreasing the temperature. Also, magnetic entropy change and relative cooling power for magnetic field variation were predicted. Then, we have fitted the experimental data of resistivity using non-linear curve fitting, a typical mathematical function (Gauss function). This function can describe the carrier transport behavior of manganites as a function of temperature around the electrical phase transition. The obtained results were discussed. These predicted parameters agree with the experimental data.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Hexanuclear and Heptanuclear Nickel(II) Complexes of with a Non-Schiff-Base Tetradentate Ligand: an Example of Slow Motion Ferromagnetic Phase Transition at Very Low Temperature

Hexanuclear and heptanuclear nickel(II) complexes with N,N′-bis(2-hydroxy-3,5-dimethylbenzyl-1,2-ethanediamine (H{sub 2}hdde), [Ni{sub 6}(hdde){sub 3}(OH){sub 4}(NO{sub 3}){sub 2}(H{sub 2}O){sub 2}]·2dmf (dmf = N,N-dimethylformamide) and [Ni{sub 7}(hdde){sub 3}(OH){sub 6}(dmac){sub 3}](ClO{sub 4}){sub 2} (dmac = N,N-dimethylacetamide), were investigated. The latter complex was newly synthesized and the single-crystal X-ray structure analysis revealed a wheel-shaped heptanuclear cluster. These complexes showed a ferromagnetic interaction in the temperature dependence of magnetic susceptibilities. The micro-SQUID data of the former complex did not show SMM property, whereas the AC susceptibility measurements suggested SMM-like behavior and the zero-field-cooled and field-cooled studies showed a ferromagnetic transition near 240 mK for the latter complex.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Superconductivity in SrTiO{sub 3}: Dielectric Function Method for Non-Parabolic Bands

The dielectric function method for superconductivity has been applied to SrTiO{sub 3} accounting for the non-parabolic dispersion of charge carriers in the conduction band and for the dispersion of optical phonons based on density functional theory calculations. The obtained critical temperatures of the superconducting phase transition in SrTiO{sub 3} are in agreement with experiments in the density range n ∼ 5 × 10{sup 18} to 5 × 10{sup 20}cm{sup − 3}. The dielectric function method predicts also the sign of the anomalous isotope effect in strontium titanate, in line with recent observations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Study of Critical Behavior and Magnetocaloric Effect in Nd{sub 1− x} Sr{sub x}MnO{sub 3} Compounds

In this paper, we have investigated the influence of substitution of Nd by Sr on the structural, magnetic, and magnetocaloric properties of Nd{sub 1 − x}Sr{sub x}MnO{sub 3} (x = 0.3 and x = 0.4). These compounds were prepared using the solid state method. The structural, morphological, and magnetic properties of our system were characterized by XRD, MEB techniques, and a magnetometer for magnetic characterization. The Rietveld refinement has revealed the coexistence of both Pnma orthorhombic and R-3c rhombohedral phases. The magnetic data indicate that the compounds exhibit a continuous paramagnetic (PM) to ferromagnetic (FM) phase transition. In addition, the critical behavior in both compounds associated with the magnetic phase transition has been studied by the magnetization isotherms. The critical exponents are deduced using various techniques such as the modified Arrott plot, Kouvel–Fisher plot, and critical isotherm technique. Finally, the results obtained of the magnetocaloric effect are in agreement with the reported values of previous researches. These results show that our samples can be used as a magnet refrigerant at room temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Multiple Magnetic Phase Transitions in Mn-deficient Co{sub 0.4}Ni{sub 0.6}Mn{sub 1−x}Ge Compounds

The structural and magnetic transition in Mn-deficient Co{sub 0.4}Ni{sub 0.6}Mn{sub 1−x}Ge was investigated by DC magnetization and temperature-dependent X-ray diffraction measurements. By introducing Mn vacancies in the stoichiometric Co{sub 0.4}Ni{sub 0.6}MnGe compound, multiple magnetic phase transitions and tunable martensitic transitions were observed. A paramagnetic martensite (Ni{sub 2}In-type structure) to ferromagnetic austenite (TiNiSi-type structure) transition was realized in Co{sub 0.4}Ni{sub 0.6}Mn{sub 0.98}Ge around 275 K. A large volume expansion of 2.6% accompanied this transition. In addition, a martensitic transition induced by the magnetic field was also observed near room temperature. Furthermore, a significant magnetic entropy variation of − 21.3 J/kg K at 263 K (∆B = 5 T) was obtained in the Co{sub 0.4}Ni{sub 0.6}Mn{sub 0.98}Ge compound.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

The Effect of Electron Flow on Coercivity and Remanence of FePt Nanocomposites Under Heat Treatment

Phase transition to L1{sub 0} for the FePt crystal requires a temperature approximately 600 °C; however, in this temperature, their size control is exceptionally imperative. Two methods of particle size control with unique advantages have been combined. On the SiO{sub 2} wafers, a thin layer of FePT/carbon nanotubes (CNTs) has been coated and post-annealed at 700 °C under electron flux. Results show the Hc and Mr/Ms of FePt/CNT nanocomposites is raised to 0.75 T and 0.9, respectively, and the average size of the FePt nanoparticles (NPs) is about 5 nm.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Critical Properties of Perovskite Manganese Oxides Pr{sub 1−x}Ba{sub x}MnO{sub 3}

The critical properties of polycrystalline Pr{sub 1−x}Ba{sub x}MnO{sub 3} (x = 0.13 and 0.23) samples prepared by using a standard ceramic fabrication method were investigated. Pr{sub 1−x}Ba{sub x}MnO{sub 3} exhibited a second-order magnetic transition. As per the modified Arrott plot method and double logarithmic M-μ{sub 0}H curve, the critical exponents (CE) were calculated as β = 0.459 ± 0.03, γ = 1.346 ± 0.02, δ = 4.32 ± 0.04, and T{sub C} ≈ 130 K, for x = 0.13, and β = 0.424 ± 0.03, γ = 1.362 ± 0.02, δ = 4.19. ± 0.03, and T{sub C} ≈ 163 K, for x = 0.23. To confirm the reliability of such CE values, the M (H) data of Pr{sub 1−x}Ba{sub x}MnO{sub 3} were checked by the scaling equation M(H, t) = |t|{sup β}f{sub ±}(H/|t|{sup β + γ}), where t=(T/(T{sub C}))−1 refers to the reduced temperature, f{sub ±} is the scaling function above (+)/below (−) T{sub C}. The β values for Pr{sub 1−x}Ba{sub x}MnO{sub 3} (x = 0.13 and 0.23) are between β = 0.365 (3D Heisenberg exponent) and β = 0.5 (mean-field exponent), whereas γ values are fairly close to γ = 1.386. This reveals long- and short-range order coexisting in Pr{sub 1−x}Ba{sub x}MnO{sub 3}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Structure and Magnetic Properties of the Ti-Doped Pyrochlore Molybdate Y{sub 2}Mo{sub 2(1-x)}Ti{sub 2x}O{sub 7}

The structural, heat capacity, and magnetic properties of Y{sub 2}Mo{sub 2(1-x)}Ti{sub 2x}O{sub 7} were investigated in this work. Y{sub 2}Mo{sub 2(1-x)}Ti{sub 2x}O{sub 7} maintains the pyrochlore structure, but lattice constants are observed to decrease continuously with the Ti{sup 4+} doping. The magnetic measurements indicate a magnetic phase transition at low temperature for all Ti-doped samples, and the Ti{sup 4+} doping suppresses the transition temperature. This is ascribed to the diluted effect of nonmagnetic Ti{sup 4+} substitution of magnetic Mo{sup 4+}. The heat capacity data confirm that the low-temperature magnetic phase is spin glass in the Ti{sup 4+}-doped Y{sub 2}Mo{sub 2(1-x)}Ti{sub 2x}O{sub 7} system. The M(H) data reveals the coexistence of antiferromagnetic and ferromagnetic interactions at low temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Impact of Titanium Doping on Structural, Magnetic, and Magnetocaloric Properties and Order of Transition in La{sub 0.5}Pr{sub 0.3}Ba{sub 0.2}Mn{sub 1-x}Ti{sub x}O{sub 3} (x = 0.0 and 0.1) Manganite

In this paper, we study the structural, magnetic, and magnetocaloric properties of the polycrystalline peroviskite manganites La{sub 0.5}Pr{sub 0.3}Ba{sub 0.2}Mn{sub 1-x}Ti{sub x}O{sub 3} with x = 0 and 0.1. Our samples were prepared by solid-state reaction method. The XRD study has been analyzed by Rietveld refinement technique. Both samples crystallize in the rhombohedral symmetry with R 3-bar c space group. The second order of magnetic phase transition in all samples is confirmed by the Arrott analysis and the behavior of the master curve. Moreover, the magnetic entropy change ∆S{sub M} has been determined by the Maxwell relation, using phenomenological model and Landau theory. The relative cooling power (RCP) value under a magnetic field of 5 T is 236.29 J kg{sup −1} for La{sub 0.5}Pr{sub 0.3}Ba{sub 0.2}Mn{sub 0.9}Ti{sub 0.1}O{sub 3} sample which will be an interesting compound for application materials. Also, an agreement was found between the magnetic entropy change values calculated by Landau theory and those obtained using Maxwell relation above T{sub C} and a difference below it, for a magnetic field of 2 T.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

The Curie–Weiss model with Complex Temperature: Phase Transitions

We study the partition function of the Curie–Weiss model with complex temperature, and partially describe its phase transitions. As a consequence, we obtain information on the locations of zeros of the partition function (the Fisher zeros).

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Non-robust Phase Transitions in the Generalized Clock Model on Trees

Pemantle and Steif provided a sharp threshold for the existence of a robust phase transition (RPT) for the continuous rotator model and the Potts model in terms of the branching number and the second eigenvalue of the transfer matrix whose kernel describes the nearest neighbor interaction along the edges of the tree. Here a RPT is said to occur if an arbitrarily weak coupling with symmetry-breaking boundary conditions suffices to induce symmetry breaking in the bulk. They further showed that for the Potts model RPT occurs at a different threshold than PT (phase transition in the sense of multiple Gibbs measures), and conjectured that RPT and PT should occur at the same threshold in the continuous rotator model. We consider the class of four- and five-state rotation-invariant spin models with reflection symmetry on general trees which contains the Potts model and the clock model with scalarproduct-interaction as limiting cases. The clock model can be viewed as a particular discretization which is obtained from the classical rotator model with state space S{sup 1}. We analyze the transition between PT=RPT and PT≠RPT, in terms of the eigenvalues of the transfer matrix of the model at the critical threshold value for the existence of RPT. The transition between the two regimes depends sensitively on the third largest eigenvalue.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

An Investigation of Chaotic Diffusion in a Family of Hamiltonian Mappings Whose Angles Diverge in the Limit of Vanishingly Action

The chaotic diffusion for a family of Hamiltonian mappings whose angles diverge in the limit of vanishingly action is investigated by using the solution of the diffusion equation. The system is described by a two-dimensional mapping for the variables action, I, and angle, θ and controlled by two control parameters: (i) ϵ, controlling the nonlinearity of the system, particularly a transition from integrable for ϵ=0 to non-integrable for ϵ≠0 and; (ii) γ denoting the power of the action in the equation defining the angle. For ϵ≠0 the phase space is mixed and chaos is present in the system leading to a finite diffusion in the action characterized by the solution of the diffusion equation. The analytical solution is then compared to the numerical simulations showing a remarkable agreement between the two procedures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Stochastic Spatial Models in Ecology: A Statistical Physics Approach

Ecosystems display a complex spatial organization. Ecologists have long tried to characterize them by looking at how different measures of biodiversity change across spatial scales. Ecological neutral theory has provided simple predictions accounting for general empirical patterns in communities of competing species. However, while neutral theory in well-mixed ecosystems is mathematically well understood, spatial models still present several open problems, limiting the quantitative understanding of spatial biodiversity. In this review, we discuss the state of the art in spatial neutral theory. We emphasize the connection between spatial ecological models and the physics of non-equilibrium phase transitions and how concepts developed in statistical physics translate in population dynamics, and vice versa. We focus on non-trivial scaling laws arising at the critical dimension D=2 of spatial neutral models, and their relevance for biological populations inhabiting two-dimensional environments. We conclude by discussing models incorporating non-neutral effects in the form of spatial and temporal disorder, and analyze how their predictions deviate from those of purely neutral theories.

54 ENVIRONMENTAL SCIENCES↗