Browsing by Author "Chen, Chih-Wei"

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    Correlations between magneto-transport properties and crystal structure in transition metal pnictides and chalcogenides
    (2016-07-29) Chen, Chih-Wei; Morosan, Emilia
    The interactions between electrons, phonons, and the lattice can result in one or more lattice, charge, spin, and orbital orders. These orders, and the competition between different states they generate, result in many interesting phenomena, such as magnetism, metal-to-insulator transition, giant magnetoresistance, superconductivity, heavy fermion behavior, etc. Here I report the research on three systems: Co$_2$As$_{1-x}$P$_x$, Fe$_x$TaS$_2$, and Sr$_2$Mn$_3$As$_2$O$_2$, and the corresponding phenomena. In Co$_2$As$_{1-x}$P$_x$, the magnetic properties are strongly correlated with their crystal structure. The P doping induces two structural phase transitions. The itinerant ferromagnetism in Co$_2$As is enhanced by the first structural phase transition ($x \sim$ 0.04) and quenched by the second structural phase transition ($x$ between 0.85 and 0.90). In Fe$_x$TaS$_2$, I studied the correlation between magneto-transport properties and the Fe concentration $x$. When x deviates from the two commensurate values 1/4 and 1/3 where Fe ions form superstructures, both the spin misalignment and the magnetoresistance increase. The largest magnetoresistance that has been observed so far is 140$\%$ in Fe$_{0.297}$TaS$_2$, where the Fe concentration is close to the average of two commensurate values. In Sr$_2$Mn$_3$As$_2$O$_2$, I grew the first Sr$_2$Mn$_3$As$_2$O$_2$ single crystals and performed single crystal neutron scattering, which reveals that the magnetic structure at one of its layers has a quasi two-dimensional antiferromagnetic order. The energy dispersion of this magnetic order has linear dependence with wave momentum at the low energy transfer region, which is consistent with the spin wave of antiferromagnetic order. Additionally, band structure calculations indicate that Sr$_2$Mn$_3$As$_2$O$_2$ is an Mott insulator and the Mott transition is both layer- and orbital-selective, where the $d_{x^2 - y^2}$ orbital in this layer dominates the Mott transition.
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    Correlations of crystallographic defects and anisotropy with magnetotransport properties in FexTaS2 single crystals (0.23≤x≤0.35)
    (American Physical Society, 2016) Chen, Chih-Wei; Chikara, Shalinee; Zapf, Vivien S.; Morosan, E.
    Very large magnetoresistance discovered in single crystals of the ferromagnetic Fe-intercalated transition-metal dichalcogenide Fe0.28TaS2 was attributed to the deviation of the Fe concentration from commensurate values (x=1/4 or 1/3), which caused magnetic moment misalignments. Here we report a study of FexTaS2 crystals with 0.23≤x≤0.35, demonstrating that crystallographic defects lead to spin disorder, which correlates with magnetotransport properties, such as switching magnetic field HS, magnetoresistance (MR), and even zero-field resistivity ρ0 and temperature coefficient A in ρ(T)=ρ0+AT2: The ordering temperature TC and Weiss temperature θW are maximized at the superstructure composition x=1/4, whereas Hs, MR, ρ0, and A are minimum. Conversely, at a composition intermediate between the superstructure compositions x=1/4 and 1/3, the corresponding magnetotransport properties reach local maxima.
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    Impurity-Induced Plasmon Damping in Individual Cobalt-Doped Hollow Au Nanoshells
    (American Chemical Society, 2014) Thibodeaux, Christyn A.; Kulkarni, Vikram; Chang, Wei-Shun; Neumann, Oara; Cao, Yang; Brinson, Bruce; Ayala-Orozco, Ciceron; Chen, Chih-Wei; Morosan, Emilia; Link, Stephan; Nordlander, Peter; Halas, Naomi J.; Laboratory for Nanophotonics; Rice Quantum Institute
    The optical properties of plasmonic nanoparticles in the size range corresponding to the electrostatic, or dipole, limit have the potential to reveal effects otherwise masked by phase retardation. Here we examine the optical properties of individual, sub-50 nm hollow Au nanoshells (Co-HGNS), where Co is the initial sacrificial core nanoparticle, using single particle total internal reflection scattering (TIRS) spectroscopy. The residual Co present in the metallic shell induces a substantial broadening of the homogeneous plasmon resonance line width of the Co-HGNS, where the full width at half-maximum (fwhm) broadens proportionately with increasing Co content. This doping-induced line broadening provides a strategy for controlling plasmon line width independent of nanoparticle size, and has the potential to substantially modify the relative decay channels for localized nanoparticle surface plasmons.
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    Magnetic and transport properties of the layered transition-metal pnictides R3T4As4O2−δ (R = La, Ce, Pr, Nd, and Sm, T = Ni, Cu)
    (American Physical Society, 2014) Wang, Jiakui K.; Marcinkova, A.; Chen, Chih-Wei; He, Hua; Aronson, Meigan; Morosan, E.
    The magnetic and transport properties of the novel R3T 4As4O2−δ (R = La, Ce, Pr, Nd and Sm, T = Ni and Cu) layered materials were studied using structural and physical properties measurements. Varying the rare-earth ion led to the observation of diverse physical properties including superconductivity for R = La and T = Ni, ferromagnetic or antiferromagnetic order for R = Ce, Pr, and Sm, or spin-glass behavior in Nd3Ni4As4O2−δ. These complex magnetic and electronic properties are discussed in light of the crystalline anisotropy in these layered compounds.
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    Possible Mott transition in layered Sr2Mn3As2O2ﾠsingle crystals
    (American Physical Society, 2019) Chen, Chih-Wei; Wang, Weiyi; Loganathan, Vaideesh; Carr, Scott V.; Harriger, Leland W.; Georgen, C.; Nevidomskyy, Andriy H.; Dai, Pengcheng; Huang, C.-L.; Morosan, E.
    Single crystals of Sr2Mn3As2O2 have been grown for the first time, for which we show a possible layer-selective Mott insulator behavior. This compound stands out as a hybrid structure of MnO2 and MnAs layers, analogously to the active CuO2 and FeAs layers, respectively, in the cuprate and iron-based high-temperature superconductors. Electrical transport, neutron diffraction measurements, together with density functional theory calculations on Sr2Mn3As2O2 single crystals converge toward a picture of independent magnetic order at T1∼79 K and T2∼360 K for the two Mn sublattices, with insulating behavior at odds with the metallic behavior predicted by calculations. Furthermore, our inelastic neutron-scattering studies of spin-wave dispersions for the Mn(1) sublattice reveal an effective magnetic exchange coupling of SJ∼3.7 meV. This is much smaller than those for the Mn(2) sublattice.
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    Thermally driven analog of the Barkhausen effect at the metal-insulator transition in vanadium dioxide
    (AIP Publishing LLC., 2014) Huber-Rodriguez, Benjamin; Kwang, Siu Yi; Hardy, Will J.; Ji, Heng; Chen, Chih-Wei; Morosan, Emilia; Natelson, Douglas
    The physics of the metal-insulator transition (MIT) inﾠvanadiumﾠdioxide remains a subject of intense interest. Because of the complicating effects of elastic strain on the phase transition, there is interest in comparatively strain-free means of examining VO2ﾠmaterial properties.ﾠWe report contact-free, low-strain studies of the MIT through an inductive bridge approach sensitive to the magnetic response of VO2ﾠpowder.ﾠRather than observing the expected step-like change inﾠsusceptibilityﾠat the transition, we argue that theﾠmeasuredﾠresponse is dominated by an analog of theﾠBarkhausen effect,ﾠdue to the extremely sharp jump in the magnetic response of each grain as a function of time as theﾠmaterialﾠis cycled across the phase boundary. This effect suggests that futureﾠmeasurementsﾠcould access the dynamics of this and similar phase transitions.
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    Very large magnetoresistance in Fe0.28TaS2 single crystals
    (American Physical Society, 2015) Hardy, Will J.; Chen, Chih-Wei; Marcinkova, A.; Ji, Heng; Sinova, Jairo; Natelson, D.; Morosan, E.
    Magnetic moments intercalated into layered transition metal dichalcogenides are an excellent system for investigating the rich physics associated with magnetic ordering in a strongly anisotropic, strong spin-orbit coupling environment. We examine electronic transport and magnetization in Fe0.28TaS2, a highly anisotropic ferromagnet with a Curie temperature TC∼68.8 K. We find anomalous Hall data confirming a dominance of spin-orbit coupling in the magnetotransport properties of this material, and a remarkably large field-perpendicular-to-plane magnetoresistance (MR) exceeding 60% at 2 K, much larger than the typical MR for bulk metals, and comparable to state-of-the-art giant MR in thin film heterostructures, and smaller only than colossal MR in Mn perovskites or high mobility semiconductors. Even within the FexTaS2 series, for the current x=0.28 single crystals the MR is nearly 100× higher than that found previously in the commensurate compound Fe0.25TaS2. After considering alternatives, we argue that the large MR arises from spin-disorder scattering in the strong spin-orbit coupling environment, and suggest that this can be a design principle for materials with large MR.