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At least 1,747 records · Page 97
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Temperature Dependence of Renormalized Spin Susceptibility for Interacting 2D Electrons in Silicon

By using Shubnikov-de Haas oscillations in crossed magnetic fields, we measured the temperature dependence of the renormalized spin susceptibility χ{sub i}{sup ∗}(T) for strongly interacting itinerant 2D electrons in silicon. The weak δχ{sub i}{sup ∗}(T) dependence, only a few percent over the range T = (0.1 − 1) K, agrees qualitatively with the predicted interaction corrections. However, in strong in-plane magnetic fields, the χ{sup ∗}(T) dependence does not vanish or weaken as expected for the interaction corrections. We found that the susceptibility variations are correlated with the T-dependence of the density of itinerant electrons extracted from the magnetooscillation period. We conclude therefore that the χ{sub i}{sup ∗}(T) dependence is affected by a T-dependent exchange of electrons between the subsystems of itinerant and localized electrons which are in thermodynamic equilibrium.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Magnetic and Transport Properties of Bi{sub 0.5−x}Pr{sub x}Ca{sub 0.5}MnO{sub 3}(0.0 ≤ x ≤ 0.50) Manganites

In this work, the effect of Pr doping at the Bi site on the critical behavior and magnetocaloric and transport properties of Bi{sub 0.5−x}Pr{sub x}Ca{sub 0.5}MnO{sub 3} (0.0 ≤ x ≤ 0.50) manganite synthesized by sol–gel method is reported. An increase in Curie temperature (T{sub C}) with an increase in Pr content is observed. The samples with x = 0.0–0.40 show first-order phase transition whereas the sample with x = 0.50 shows second-order phase transition near T{sub C}. The estimated values of critical exponents β = 0.75 and γ = 0.82 for the sample with x = 0.50 do not fall into any universality class. The sample with x = 0.50 shows maximum magnetic entropy change (|ΔS{sub m}| = (1.22 J kg{sup −1} K{sup −1}) at T{sub C} in an applied magnetic field of 40 kOe, which is associated with magnetocaloric effect (MCE). All the samples show semiconducting behavior and also follow Mott’s variable range hopping (VRH) model below the charge ordering temperature (T{sub CO}). The changes in magnetic and transport properties are correlated with the changes in the structural parameters of the samples. The results are discussed in view of the effect of Pr doping on the 6s{sup 2} lone pair character of Bi{sup 3+} ions and disorder in the samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Sulfur Hydrides: Phase Diagram and the Transition into the Record-High T{sub c} State

Remarkable feature of the phase diagram of sulfur hydrides, the record-high T{sub c} superconductors, is a sharp increase near P ≈ 150 GPa from T{sub c} ≈ 120 to ≈ 200 K. This increase is a signature of the structural transition. The present study is concerned with the nature of this phase transition. One can demonstrate that the symmetry analysis along with an analysis of the impact of lattice deformations lead to the conclusion that we are dealing with the first-order transition. Such a transition is manifested in an abrupt appearance of small pockets on the Fermi surface and, correspondingly, the two-gap energy spectrum.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Quantum Phase Transition Induced by Magnetic Impurity

The Yu–Shiba–Rusinov bound states can arise around magnetic impurities in conventional superconductors. Competition between screened and unscreened states can lead to the quantum phase transition. We discuss basic properties of this quantum phase transition in the case of the triangular lattice with on-site pairing. We show that the quantum phase transistion results from the change of the ground state of the system.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Phase Transitions in Quasi-One-Dimensional System with Unconventional Superconductivity

The paper is devoted to a study of superconducting properties of population-imbalanced fermionic mixtures in quasi-one-dimensional optical lattices. The system can be effectively described by the attractive Hubbard model with the Zeeman magnetic field term. We investigated the ground-state phase diagram of the model as a function of the chemical potential and the magnetic field. The ground state of the system exhibits the conventional BCS-type superconductivity as well as the unconventional so-called Fulde-Ferrell-Larkin-Ovchinnikov state, in which the total momentum of Cooper pairs is non-zero. We determine the orders of transitions as well as the behavior of order parameters with a change of the model parameters.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Synthesis and Superconductivity of Ba{sub 2}Ti{sub 2}(Fe{sub 1−x}Cr{sub x}){sub 2}As{sub 4}O

Ba{sub 2}Ti{sub 2}(Fe{sub 1−x}Cr{sub x}){sub 2}As{sub 4}O (0 ≤ x ≤ 8%) compounds are synthesized by solid-state reaction and their structure and superconductivity are studied with x-ray diffraction, electric resistivity, and magnetization measurements. It is observed that the Cr dopants in the Fe site of Ba{sub 2}Ti{sub 2}Fe{sub 2}As{sub 4}O expand the lattice constants, but do not cause any structure phase transition. Superconducting transition temperature T{sub c} of the Ba{sub 2}Ti{sub 2}(Fe{sub 1−x}Cr{sub x}){sub 2}As{sub 4}O is found monotonically depressed with Cr doping level at an average depression rate of ΔT{sub c}/Cr − 1% = − 2.6 K. Meanwhile, the superconducting volume fraction is also significantly reduced, and the low-temperature resistivity is enhanced. It is believed that the Cr dopants work as pair-breaking impurities, which take responsibility for the depression of superconductivity in the system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Anisotropic Thermal Expansion of p-Terphenyl: a Self-Assembled Supramolecular Array of Poly-p-phenyl Nanoribbons

The recent discovery of superconductivity in a metallic aromatic hydrocarbon, alkali-doped p-terphenyl, has attracted considerable interest. The critical temperature T{sub c} ranges from few to 123 K, the record for organic superconductors, due to uncontrolled competition of multiple phases and dopants concentrations. In the proposed mechanism of Fano resonance in a superlattice of quantum wires with coexisting polarons and Fermi particles, the lattice properties play a key role. Here, we report a study of the temperature evolution of the parent compound p-terphenyl crystal structure proposed to be made of a self-assembled supramolecular network of nanoscale nanoribbons. Using temperature-dependent synchrotron X-ray diffraction, we report the anisotropic thermal expansion in the ab plane, which supports the presence of a nanoscale network of one-dimensional nanoribbons running in the b-axis direction in the P21/a structure. Below the enantiotropic phase transition at 193 K, the order parameter of the C-1 structure follows a power law behavior with the critical exponent α = 0.34 ± 0.02 and the thermal expansion of the a-axis and b-axis show major changes supporting the formation of a two-dimensional bonds network. The large temperature range of the orientation fluctuations in a double well potential of the central phenyl has been determined.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Phenomenological Modeling of Magnetocaloric Effect for Ni{sub 58}Fe{sub 26}Ga{sub 28} Alloy

The magnetocaloric effect (MCE) of shape memory Ni{sub 58}Fe{sub 26}Ga{sub 28} has been investigated. A phenomenological model is used for simulation of magnetization dependence on temperature variation to investigate magnetocaloric properties such as magnetic entropy change and heat capacity change. The results indicate the potential of Ni{sub 58}Fe{sub 26}Ga{sub 28} alloy to achieve the MCE at temperatures near Curie temperature. This alloy presents a good-looking approach for cooling applications in its prospective candidate for a cooling system in a wide temperature interval including room temperature. The results also confirmed that the phenomenological model is useful for prediction of the MCE for magnetic materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Effects of the Random Crystal Field on the Phase Diagrams and the Magnetic Properties of a Spin-1 Blume-Capel Nanoisland

The phase diagrams and the magnetic properties of a spin-1 Blume-Capel nanoisland are investigated by the use of the effective-field theory based on the probability distribution technique. The influence of the random crystal field and the system parameters on the critical behaviors of the nanoisland are examined. The results show that the system can exhibit a first-order phase transition, a tricritical point, and a re-entrant or even a double re-entrant phenomenon.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Random Crystal Field Effect on the Kinetic Spin-1 and Spin-3/2 Under a Time-Dependent Oscillating Field Phase Transitions

The effect of random crystal field on the stationary states of the kinetic spin-1 and spin-3/2 Blume-Capel model is investigated within the framework of the mean field approach. The Glauber-type stochastic dynamics is used to describe the time evolution of the system which is subjected to a time-dependent oscillating external magnetic field. The model exhibits first- and second-order transitions as well as dynamical tricritical, triple and an isolated critical end points. We found that the system displays reentrant phenomenon for both α = 0 and α > 0. Moreover, the system exhibits in the phase space a fixed points and limit cycles with circle, elliptic and parallelogram shapes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Investigation on the Magnetocaloric Effect of the Pr{sub 7}Pd{sub 3} Compound

Magnetic transitional behaviors and the magnetocaloric effect of the intermetallic compound Pr{sub 7}Pd{sub 3} have been studied. A second-order metamagnetic transition between ferromagnetic and paramagnetic states was observed with a Curie temperature (T{sub C}) of 14.9 K. The paramagnetic Curie temperature (𝜃{sub p}) and the effective magnetic moment (μ{sub eff}) were investigated and determined to be 5.1 K and 3.43 μ{sub B}/Pr, respectively. A reversible magnetocaloric effect was observed in this compound originated from the magnetic phase transition. For a magnetic field change of 0–5 T, the maximum magnetic entropy change − ΔS{sub M} reached 5.5 J kg{sup − 1} K{sup − 1}, and the corresponding value of refrigerant capacity was estimated to be about 65 J kg{sup − 1} with no thermal and field hysteresis loss.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Revival of Metal-Insulator and Ferromagnetic-Paramagnetic Transitions by Ni Substitution at Mn Site in Charge-Ordered Monovalent Doped Nd{sub 0.75}Na{sub 0.25}MnO{sub 3} Manganites

Nd{sub 0.75}Na{sub 0.25}Mn{sub 1−x}Ni{sub x}O{sub 3} (x = 0 − 0.07) were synthesized using a conventional solid-state synthesis method to investigate the effect of Ni substitution on their magnetic and electrical transport properties. XRD analysis using the Rietvield refinement method showed an increase in unit cell volume with increasing Ni content, indicating the possibility of substituting Ni for the Mn site. For x = 0, magnetization measurements showed paramagnetic (PM) to antiferromagnetic (AFM) transition at the transition temperature, T{sub N} ∼ 185 K, which may be related to the presence of charge-ordering (CO) state. For Ni-substituted samples, ferromagnetic (FM) to PM transition was induced with Curie temperature (T{sub C}), increasing from 91 K (x = 0.01) to 115 K (x = 0.03), followed by decreases in T{sub C} to 60 K (x = 0.07). Electrical resistivity measurements showed that samples for x ≤ 0.01 exhibited insulator behavior while both x = 0.03 and 0.05 samples exhibited metal-insulator transition suggesting that the CO state is weakened. For x = 0.07, the sample shows re-entrant of insulating characteristic. Inducement of FM metallic (FMM) state, as well as increased of T{sub C} for x = 0.03 is suggested to be related to FM SE between Ni{sup 2+} and Mn{sup 4+}, which suppressed the CO state, hence inducing double-exchange (DE)-like mechanism. In addition, an increase in resistivity was observed as temperature decreased below the resistivity minimum (R{sub min}) at T{sub min} ∼ 40 K for x = 0.05 is possibly due to the Kondo-like effect mechanism. On one hand, the M(H) curve showed the presence of a hysteresis loop for x = 0 and 0.01 at T= 60 and 80 K, indicating the presence of an unstable phase, which may be related with the CO-AFM phase. For x = 0.03, no hysteresis loop was observed, indicating the suppression of the CO-AFM. Intriguingly, for x = 0.05 and 0.07, small hysteresis loop was observed, indicating the possible presence of a minor AFM component. Further, the observed MR behavior in present studies can be understood as a result of a decreased scattering mechanism in the presence of magnetic field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Formation of As–As Interlayer Bonding in the cT Phase of EuFe{sub 2}As{sub 2} and CeFeAsO Under Pressure from First Principle

The electronic band structure and structural phase stability of EuFe{sub 2}As{sub 2} and CeFeAsO compounds were studied using the full-potential linearized augmented plane wave (FP-LAPW) method implemented using WIEN2k. To calculate the structural stability and phase transition of these compounds, the total energies have been computed as a function of reduced volumes and fitted with the Birch–Murnaghan equation. The calculated lattice parameters are found to be in agreement with the available experimental data. The present results show that EuFe{sub 2}As{sub 2} and CeFeAsO compounds undergo structural phase transition from body-centered tetragonal (BCT) into collapsed tetragonal (cT) and tetragonal (T) into cT phase under pressure. The calculated phase transition pressures are in agreement with recent experimental data. The calculated valence charge density of collapsed tetragonal phase reveals that As–As interactions found to be stronger under pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Numerical Simulation of Phase Transitions in Type-II Annular Superconductor Using Time-dependent Ginzburg-Landau Equations

Time-dependent Ginzburg-Landau equations were solved by finite element method in two-dimensional space for order parameter and energy components of the annular superconducting sample in steady magnetic fields. Vortices preferred to penetrate from the inner surface of the annulus due to lesser energy required at the concave surface. A transition magnetic field strength was observed in spatial averages of carrier concentration and energy components, showing small bumps and abrupt variations, indicating phase transition from a non-vortex to vortex state. These effects were observed to repeat with every subsequent entry of a set of vortices into the sample; transition magnetic field strength was found to depend inversely on the annular width of the sample. The present work gives a better understanding of energy variations during phase transition from non-vortex to vortex state and predicts that vortex state can be avoided by tuning the wire thickness in practical applications, e.g., superconducting electromagnets.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Magnetocaloric Effect in BiFe{sub 1−x}Zn{sub x}O{sub 3} Multiferroics

Ceramic BiFe{sub 1−x}Zn{sub x}O{sub 3} multiferroic samples were prepared by the solid combustion method for x = 0.1, 0.15, and 0.2. Structural, magnetic, and magnetocaloric properties of the multiferroics have been studied. For all samples, an antiferromagnetic phase transition is observed in the region of 630 K. With increase in x, the reduction in magnitude of magnetization and Neel temperature is observed. The magnetocaloric properties, entropy, relative cooling power, and heat capacity have been calculated within the framework of thermodynamic theory. It has been established that the maximum changes of magnetocaloric properties of multiferroics are observed in the region of magnetic phase transition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Investigation of Magnetic, Magnetocaloric, and Critical Properties of La{sub 0.5}Ba{sub 0.5}MnO{sub 3} Manganite

We present an extensive study of the magnetic properties of a novel La{sub 0.5}Ba{sub 0.5}MnO{sub 3} perovskite material prepared by the hydrothermal method. The explored sample was structurally studied by the x-ray diffraction (XRD) method which confirms the formation of a pure cubic phase of a perovskite structure with Pm3m space group. The magnetic properties were probed by employing temperature M (T) and external magnetic field M (μ{sub o}H) dependence of magnetization measurements. A magnetic phase transition from ferromagnetic to paramagnetic phase occurs at 339 K in this sample. The maximum magnetic entropy change (|ΔS{sub M}{sup max}|) took a value of 1.4 J kg{sup − 1} K{sup − 1} at the applied magnetic field of 4.0 T for the explored sample and has also been found to occur at Curie temperature (T{sub C}). This large entropy change might be instigated from the abrupt reduction of magnetization at T{sub C}. The magnetocaloric effect (MCE) is maximum at T{sub C} as represented by M (μ{sub o}H) isotherms. The relative cooling power (RCP) is 243.2 J kg{sup − 1} at μ{sub o}H = 4.0 T. Moreover, the critical properties near T{sub C} have been probed from magnetic data. The critical exponents δ, β, and γ with values 3.82, 0.42, and 1.2 are close to the values predicted by the 3D Ising model. Additionally, the authenticity of the critical exponents has been confirmed by the scaling equation of state and all data fall on two separate branches, one for T < T{sub C} and the other for T > T{sub C}, signifying that the critical exponents obtained in this work are accurate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Phenomenological Modeling of Magnetocaloric Effect in La{sub 0.7}Sr{sub x}MnO{sub 3−δ}

The magnetocaloric effect (MCE) is investigated for La{sub 0.7}Sr{sub x}MnO{sub 3−δ} (0.1 ≤ x ≤ 0.3) system closes to a phase transition from a ferromagnetic to a paramagnetic state. The results recommend that La{sub 0.7}Sr{sub x}MnO{sub 3−δ} (LSMO) samples have potential application as a magnetic refrigerant (MR) in an extended temperature range. Consequently, it is hoped that LSMO samples can be MR materials in low magnetic field shifts.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Phase Transition in a Spin-1/2 and Spin-1 Ising Bilayer Film with Non-magnetic Inter-layers

The phase transition, phase diagrams, and magnetizations of an Ising bilayer film with non-magnetic inter-layers are investigated by the use of the effective field theory with correlations. The system is consisted of two magnetic layers where upper and lower layers are consisted of spin-1/2 atoms and spin-1 atoms with a single-ion anisotropy. It is examined how the system may exhibit the tricritical behavior and some characteristic features of ferrimagnetism. The effects of non-magnetic layers on the magnetic properties are clarified.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗