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Chen, Yiyao

Publications and source records attributed to Chen, Yiyao.

15 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Revealing aggregation of non-fullerene acceptors in intermixed phase by ultraviolet-visible absorption spectroscopy

Non-fullerene acceptor (NFA) aggregation is crucial in determining bulk-heterojunction (BHJ) organic photovoltaic (OPV) performance. However, it is still a big challenge to characterize the nanostructure of NFAs in the disordered donor-acceptor intermixed phase. Here, we demonstrate a method to characterize NFA aggregation and composition in the intermixed phase by measuring NFA concentration-dependent ultraviolet-visible (UV-vis) absorption spectroscopy of BHJ films. In various polymer:NFA films, an absorption shift as a function of increasing molecular concentration (ASIMC) phenomenon is observed, and different NFA aggregation behaviors can be distinguished. The ASIMC method was then applied to study the influence of processing conditions on the NFA concentration in the intermixed phase of devices to establish a correlation with device efficiency. The current work provides a feasible tool to study the nanostructure of NFAs in the complex polymer matrix and to understand the variations in the NFA concentration in the intermixed phase under non-equilibrium conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Quantum Magnetic Properties and Metal‐to‐Insulator Transition in Chemically Doped Calcium Ruthenate Perovskite

Ruthenates provide a comprehensive platform to study a plethora of novel properties, such as quantum magnetism, superconductivity, and magnetic fluctuation mediated metal–insulator transition (MIT). Herein, an overview of quantum mechanical phenomenology in calcium ruthenium oxide with varying compositions is provided. While the stoichiometric composition of exhibits non‐fermi liquid (FL) behavior with quasi‐criticality, chemically doped compounds depict prominent signatures of quantum magnetic fluctuations at low temperature that in some cases are argued to mediate in metal–insulator transition. In the case of cobalt‐doped , an unusual continuum fluctuation is found to persist deep inside the glassy phase of the material. These observations reflect the richness of the ruthenate research platform in the study of quantum magnetic phenomena of fundamental importance.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Magnetic charge and geometry confluence for ultra-low forward voltage diode in artificial honeycomb lattice

We report spin diode is important prerequisite to practical manifestation of spin electronics. Yet, a functioning magnetic diode at room temperature is still illusive. Here, we reveal diode-type phenomena due to magnetic charge mediated conduction in artificial honeycomb geometry, made of concave shape single domain permalloy element. We find that honeycomb lattice defies symmetry by populating vertices with low and high multiplicity magnetic charges, causing asymmetric magnetization, in applied current of opposite polarity. High multiplicity units create highly resistive network, thereby inhibiting magnetic charge dynamics propelled electrical conduction. However, practical realization of this effect requires modest demagnetization factor in constituting element. Concave structure fulfills the condition. Subsequently, magnetic diode behavior emerges across broad thermal range of T = 40 K–300 K. The finding is a departure from the prevailing notion of spin-charge interaction as the sole guiding principle behind spintronics. Consequently, a new vista, mediated by magnetic charge interaction, is envisaged for spintronic research.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Local Spin Ice Order Induced Planar Hall Effect in Nd–Sn Artificial Honeycomb Lattice

Geometrically frustrated materials, such as spin ice or kagome lattice, are known to exhibit exotic Hall effect phenomena due to spin chirality. For the first time, Hall effect mechanism is explored in an artificial honeycomb spin ice of Nd–Sn element using Hall probe and polarized neutron reflectivity measurements. In an interesting observation, a strong enhancement in Hall signal at relatively higher temperature of T ~20 K is detected. The effect is attributed to the planar Hall effect due to magnetic moment configuration in spin ice state in low field application. In the antiferromagnetic state of neodymium at low temperature, applied field induced coupling between atomic Nd moments and conduction electrons in underlying lattice causes distinct increment in Hall resistivity at very modest field of H ~ 0.015 T. The experimental findings suggest the development of a new research vista to study the planar and the field induced Hall effects in artificial spin ice.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Quantum Disordered State of Magnetic Charges in Nanoengineered Honeycomb Lattice

Abstract A quantum magnetic state due to magnetic charges is never observed, even though they are treated as quantum mechanical variables in theoretical calculations. Here, the occurrence of a novel quantum disordered state of magnetic charges in a nanoengineered magnetic honeycomb lattice of ultra‐small connecting elements is demonstrated. The experimental research, performed using spin resolved neutron scattering, reveals a massively degenerate ground state, comprised of low integer and energetically forbidden high integer magnetic charges, that manifests cooperative paramagnetism at low temperature. The system tends to preserve the degenerate configuration even under large magnetic field application. It exemplifies the robustness of disordered correlation of magnetic charges in a 2D honeycomb lattice. The realization of quantum disordered ground state elucidates the dominance of exchange energy, which is enabled due to the nanoscopic magnetic element size in nanoengineered honeycomb. Consequently, an archetypal platform is envisaged to study quantum mechanical phenomena due to emergent magnetic charges.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Microscopic Nature of Magnetic Ground State in CeAuSb 2

Here, the synergistic investigation of ground-state magnetic correlation in the single-crystal heavy fermion compound CeAuSb 2 using detailed neutron scattering measurements and density functional calculations is reported. Unlike the previous reports of single antiferromagnetic transition at T N ≈ 6 K, three successive transitions with distinct critical exponents at T N = 5.3, 4.46, and 3.76 K, respectively, are detected in CeAuSb. The low-temperature ground-state magnetic correlation is described by the spin density wave order in the basal plane with propagation wave vector $τ$ = (0.135, 0.135, 0.5) rlu. The spin density wave order arises due to the nesting of hole pockets in the Fermi surface, with parallel surfaces being separated by the experimentally found propagation vector τ. The comprehensive investigation of magnetic ground-state properties is expected to provide new insights in understanding the emerging quantum magnetism in this system, including the debated quantum critical state and magnetic field-induced metamagnetic transitions at low temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Field and Current Control of the Electrical Conductivity of an Artificial 2D Honeycomb Lattice

The conductivity of a neodymium-based arti ficial honeycomb lattice undergoes dramatic changes upon application of magnetic fi elds and currents. We attribute these changes to a redistribution of magnetic charges that are formed at the vertices of the honeycomb due to the nonvanishing net flux of magnetization from adjacent magnetic elements. We suggest that the application of a large magnetic fi eld or a current causes a transition from a disordered state, in which magnetic charges are distributed at random, to an ordered state, in which they are regularly arranged on the sites of two interpenetrating triangular Wigner crystals. The field and current tuning of electrical properties are greatly desirable functionalities for spintronics applications. As a result, a new spintronics research platform can be envisaged in arti cial magnetic honeycomb lattice.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Spin correlation in trigonal EuMn 2 As 2

The trigonal structure of EuMn 2 As 2 is an anomaly in the tetragonal 122-type pnictide family. Here, we report a detailed investigation of the underlying magnetic correlations in single crystal EuMn 2 As 2 using high-resolution elastic neutron-scattering measurements. The system undergoes two successive antiferromagnetic transitions at T = 135 K and 14.4 K, respectively. Numerical modeling of the experimental data reveals the long-range antiferromagnetic correlation of Mn-ions in the a - b plane below T N 1 = 135 K . Mn spins are aligned closer to the diagonal axis of the unit cell. The lower temperature transition, below T N 2 = 14.4 K , is discovered to arise due to the long-range antiferromagnetic correlation of Eu spins that are rotated by θ = 55 ° from the c axis of the unit cell.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Quantum Magnetic Properties in Perovskite with Anderson Localized Artificial Spin‐1/2

Abstract Quantum magnetic properties in a geometrically frustrated lattice of spin‐1/2 magnet, such as quantum spin liquid or solid and the associated spin fractionalization, are considered key in developing a new phase of matter. The feasibility of observing the quantum magnetic properties, usually found in geometrically frustrated lattice of spin‐1/2 magnet, in a perovskite material with controlled disorder is demonstrated. It is found that the controlled chemical disorder, due to the chemical substitution of Ru ions by Co‐ions, in a simple perovskite CaRuO 3 creates a random prototype configuration of artificial spin‐1/2 that forms dimer pairs between the nearest and further away ions. The localization of the Co impurity in the Ru matrix is analyzed using the Anderson localization formulation. The dimers of artificial spin‐1/2, due to the localization of Co impurities, exhibit singlet‐to‐triplet excitation at low temperature without any ordered spin correlation. The localized gapped excitation evolves into a gapless quasi‐continuum as dimer pairs break and create freely fluctuating fractionalized spins at high temperature. Together, these properties hint at a new quantum magnetic state with strong resemblance to the resonance valence bond system.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Nonequilibrium behavior of the Pb wetting layer on Si(111)7×7

The growth and stability of the Pb8 x 8 wetting layer on Si(111)7 x 7, which provides a foundation for growing quantum-size-effect nanocrystals, was investigated by in situ x-ray scattering. Our experimental results reveal that the wetting-layer structure evolves temporally over a remarkably broad range of temperatures and that there are two distinct temperature regimes of nonequilibrium behavior. When grown at lower temperature (below 170 C), it was discovered that the wetting-layer structure changes with time, indicating that its disordered structure is not static; annealing in this regime improves the order of the wetting layer. Growth at higher temperature (170 C < T < 250 C), however, leads to a time-dependent degradation of the 8 x 8 structure due to the deterioration of the underlying Si(111)7 x 7. Thermal measurements determined an activation energy of 0.4 eV in the low-temperature regime, whereas in the high-temperature regime, a two-step process is observed, which has activation energies of approximately 1.3 and 1.9 eV. The results provide important considerations for understanding the anomalous kinetic behavior of quantum-size-effect Pb nanocrystals on Si(111)7 x 7, which is facilitated by the wetting layer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗