B. Freelon

University of California, Berkeley, Berkeley, California, United States

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Publications (22)48.98 Total impact

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    ABSTRACT: We perform a combined experimental-theoretical study of the Fe-oxychalcogenides (FeO$\emph{Ch}$) series La$_{2}$O$_{2}$Fe$_{2}$O\emph{M}$_{2}$ (\emph{M}=S, Se), which is the latest among the Fe-based materials with the potential \ to show unconventional high-T$_{c}$ superconductivity (HTSC). A combination of incoherent Hubbard features in X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS) spectra, as well as resitivity data, reveal that the parent FeO$\emph{Ch}$ are correlation-driven insulators. To uncover microscopics underlying these findings, we perform local density approximation-plus-dynamical mean field theory (LDA+DMFT) calculations that unravel a Mott-Kondo insulating state. Based upon good agreement between theory and a range of data, we propose that FeO$\emph{Ch}$ may constitute a new, ideal testing ground to explore HTSC arising from a strange metal proximate to a novel selective-Mott quantum criticality.
    Full-text · Article · Oct 2015 · Physical Review B
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    Dataset: changSM

    Full-text · Dataset · Oct 2013
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    ABSTRACT: We report the momentum-resolved measurement of a two-dimensional electron gas at the LaTiO_{3}/SrTiO_{3} interface by angle-resolved photoemission spectroscopy (ARPES). Thanks to an advanced sample preparation technique, the orbital character of the conduction electrons and the electronic correlations can be accessed quantitatively as each unit cell layer is added. We find that all of these quantities change dramatically with distance from the interface. These findings open the way to analogous studies on other heterostructures, which are traditionally a forbidden field for ARPES.
    Full-text · Article · Sep 2013 · Physical Review Letters
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    ABSTRACT: This article is about resettled Afghan Hazaras in Australia, many of whom are currently undergoing a complex process of transition (from transience into a more stable position) for the first time in their lives. Despite their permanent residency status, we show how resettlement can be a challenging transitional experience. For these new migrants, we argue that developing a sense of belonging during the transition period is a critical rite of passage in the context of their political and cultural identity. A study of forced migrants such as these, moving out of one transient experience into another transitional period (albeit one that holds greater promise and permanence) poses a unique intellectual challenge. New understandings about the ongoing, unpredictable consequences of ‘transience’ for refugee communities is crucial as we discover what might be necessary, as social support structures, to facilitate the process of transition into a distinctly new environment. The article is based on a doctoral ethnographic study of 30 resettled Afghan Hazara living in the region of Dandenong in Melbourne, Australia. Here, we include four of these participants’ reflections of transition during different phases of their resettlement. These reflections were particularly revealing of the ways in which some migrants deal with change and acquire a sense of belonging to the community. Taking a historical view, and drawing on Bourdieu’s notion of symbolic social capital to highlight themes in individual experiences of belonging, we show how some new migrants adjust and learn to ‘embody’ their place in the new country. Symbolic social capital illuminates how people access and use resources such as social networks as tools of empowerment, reflecting how Hazara post-arrival experiences are tied to complex power relations in their everyday social interactions and in their life trajectories as people in transition. We learned that such tools can facilitate the formation of Hazara migrant identities and are closely tied to their civic community participation, English language development, and orientation in, as well as comprehension of local cultural knowledge and place. This kind of theorization allows refugee, post-refugee and recent migrant narratives to be viewed not merely as static expressions of loss, trauma or damage, but rather as individual experiences of survival, adaptation and upward mobility.
    No preview · Article · Jan 2012 · Physical Review B
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    ABSTRACT: Spin fluctuations and the local spin susceptibility in isovalently Zn-substituted La2−xSrxCu1−yZnyO4 (x=0.25, y≈0.01) are measured via inelastic neutron-scattering techniques. As Zn2+ is substituted onto the Cu2+-sites, an anomalous enhancement of the local spin susceptibility χ′′(ω) appears due to the emergence of a commensurate antiferromagnetic excitation centered at wave vector Q =(π,π,0) that coexists with the known incommensurate spin-density wave excitations at QHK=(π±δ,π),(π,π±δ). Our results support a picture of Zn-induced antiferromagnetic (AF) fluctuations appearing through a local staggered polarization of Cu2+ spins, and the simultaneous suppression of Tc as AF fluctuations are slowed in proximity to Zn impurities suggests the continued importance of high-energy AF fluctuations at the far overdoped edge of superconductivity in the cuprates.
    Full-text · Article · Dec 2011
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    ABSTRACT: Soft X-ray absorption and emission spectroscopy was used to investigate the electronic structure of BaFe2As2 and oxygen-vacated PrFeAsO iron pnictides. The effect of carrier doping on the conduction band was investigated. Iron–arsenic hybridized spectral features reveal that Fe–As bonding is involved in the process of electron addition near the Fermi-level for PrFeAsO. X-ray spectra reveals an increase in the Fe density of states at the PrFeAsO Fermi-level as the doping decreases. This doping dependent electronic behavior indicates the possibility of a magnetic instability in undoped PrFeAsO.
    No preview · Article · Sep 2011 · Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment
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    ABSTRACT: High-resolution X-ray scattering studies of the nonpolar thermotropic liquid crystal 4-n-pentylphenylthiol-4'-n-octyloxybenzoate (\bar{8}S5) in aerosil gel nano-networks reveal that the aerosil-induced disorder significantly alters both the nematic to Smectic-A and Smectic-A to Smectic-C phase transitions. The limiting Smectic-A correlation length follows a power-law dependence on the aerosil density in quantitative agreement with the limiting lengths measured previously in other Smectic-A liquid crystal gels. The Smectic-A to Smectic-C liquid crystalline phase transition is altered fundamentally by the presence of the aerosil gel. The onset of the Smectic-C phase remains relatively sharp but there is an extended coexistence region where Smectic-A and Smectic-C domains can exist.
    Preview · Article · Sep 2011 · Physical Review E
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    ABSTRACT: Heat-capacity measurements on exceptionally high-quality samples of Ba1-xKxFe2As2 give the values of a number of parameters relevant to the electronic structure, the structural/magnetic transition in the undoped parent compound, and the superconducting transition in the near optimally doped, x = 0.41 sample. In BaFe2As2 the changes in the lattice and magnetic structure appear as a single, sharp first-order transition at 140 K; the Sommerfeld coefficient is γ = 5.1 mJ K−2 mol−1. For the superconducting, x = 0.41, sample, the residual density of electron states in the superconducting states is essentially zero. The specific-heat anomaly at Tc suggests extreme strong coupling, with Δ(0)/kBTc ≈ 2.6, and a normal-state Sommerfeld coefficient γn ≈ 45 mJ K−2 mol−1.
    Full-text · Article · Feb 2011 · Journal of Physics Conference Series
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    ABSTRACT: Soft X-ray absorption and emission spectroscopy was used to determine the electronic density of states of oxygen vacated PrFeAsO. The measured oxygen and iron spectra are in good agreement with mean field theory and local density approximations. Iron-arsenic hybridized spectral features reveal that Fe-As bonding is involved in the process of electron addition near the Fermi-level. X-ray emission and absorption spectra shows an increase in the Fe density of states at the Fermi-level as the doping decreases. This doping dependent electronic behavior indicates the possibility of a magnetic instability in the undoped compound.
    No preview · Article · Jan 2011 · Journal of Physics Conference Series
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    ABSTRACT: We performed in situ angle-resolved photoemission spectroscopy measurements on La 2/3 Sr 1/3 MnO 3 thin and ultrathin films deposited onto SrTiO 3 substrates in order to determine their electronic band structure. We directly identified, in ultrathin films, an enhancement of the occupation of the out-of-plane Mn 3d e g 3z 2 − r 2 band at the expense of the in-plane 3d e g x 2 − y 2 band. Such a disproportionate change in orbital occupation is interpreted as the origin of the strong depression of the magnetotransport properties at the interface between manganite films and substrates.
    Full-text · Article · Dec 2010 · Physical Review B
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    ABSTRACT: Magnetic correlations near the magneto-structural phase transition in the bilayer iron pnictide parent compound, BaFe$_2$As$_2$, are measured. In close proximity to the antiferromagnetic phase transition in BaFe$_2$As$_2$, a crossover to three dimensional critical behavior is anticipated and has been preliminarily observed. Here we report complementary measurements of two-dimensional magnetic fluctuations over a broad temperature range about T$_N$. The potential role of two-dimensional critical fluctuations in the magnetic phase behavior of BaFe$_2$As$_2$ and their evolution near the anticipated crossover to three dimensional critical behavior and long-range order are discussed.
    Full-text · Article · Sep 2010 · Physical review. B, Condensed matter
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    ABSTRACT: Heat capacity, x-ray diffraction, and resistivity measurements on a high-quality BaFe2As2 sample show an evolution of the magnetostructural transition with successive annealing periods. After a 30-day anneal the resistivity in the (ab) plane decreases by more than an order of magnitude, to 12 μΩ cm, with a residual resistance ratio ∼36; the heat capacity anomaly at the transition sharpens, to an overall width of less than K, and shifts from 135.4 to 140.2 K. The heat capacity anomaly in both the as-grown sample and after the 30-day anneal shows a hysteresis of ∼0.15 K, and is unchanged in a magnetic field μ0H=14 T. The x-ray and heat capacity data combined suggest that there is a first-order jump in the structural order parameter. The entropy of the transition is reported.
    Full-text · Article · Aug 2010 · Physical review. B, Condensed matter
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    ABSTRACT: We report the results of Oxygen K-edge soft X-ray absorption and emission spectroscopy that was performed on an infinite-layer insulating cuprate thin film CaCuO2. Experimentally obtained spectra are consistent with local density approximation calculations. X-ray absorption spectra show a close resemblance to spectra obtained from homologous single crystal cuprates. In addition to d–d excitations, X-ray emission spectra reveal the presence of Zhang-Rice singlet states in the infinite-layer CuO2 planes. The question of whether the Zhang-Rice singlet features are masked by the O 2p main-band is addressed: it is possible to quantify the position of the Zhang-Rice singlet using emission intensity profiles. X-ray emission is demonstrated as a tool for understanding CuO2 planar electronic correlation in the prototypical infinite-layer. The energy difference, 2.0 eV, between the oxygen main-band and the Zhang-Rice singlet band is found to match values obtained theoretically using established planar electronic correlation parameters.
    No preview · Article · Feb 2010 · Physica C Superconductivity
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    ABSTRACT: Commonalities among the order parameters of the ubiquitous antiferromagnetism present in the parent compounds of the iron arsenide high temperature superconductors are explored. Additionally, comparison is made between the well established two-dimensional Heisenberg-Ising magnet, K$_2$NiF$_4$ and iron arsenide systems residing at a critical point whose structural and magnetic phase transitions coincide. In particular, analysis is presented regarding two distinct classes of phase transition behavior reflected in the development of antiferromagnetic and structural order in the three main classes of iron arsenide superconductors. Two distinct universality classes are mirrored in their magnetic phase transitions which empirically are determined by the proximity of the coupled structural and magnetic phase transitions in these materials. Comment: 6 pages, 4 figures
    Full-text · Article · Oct 2009 · Physical review. B, Condensed matter
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    ABSTRACT: The electronic phase diagram of PrFeAsO1-xFx (0≤x≤0.225) has been determined using synchrotron x-ray powder-diffraction, magnetization, and resistivity measurements. The structural transition temperature is suppressed from 154 to ≈120K and the magnetic phase transitions of both iron and praseodymium ions are completely suppressed by x≈0.08 fluorine doping, coinciding with the emergence of superconductivity. The optimal doping is x≈0.15 when TC=47K while the maximum solubility of fluorine in PrFeAsO1-xFx is reached around x=0.22 . The structural, magnetic, and superconducting phase diagram is presented.
    Full-text · Article · Oct 2009 · Physical review. B, Condensed matter
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    ABSTRACT: We present the results of an investigation of both the magnetic and structural phase transitions in a high quality single crystalline sample of the undoped iron pnictide compound BaFe2As2. Both phase transitions are characterized via neutron diffraction measurements which reveal simultaneous, continuous magnetic and structural orderings with no evidence of hysteresis, consistent with a single second-order phase transition. The onset of long-range antiferromagnetic order can be described by a simple power-law dependence ϕ(T)2∝(1−T/TN)2β with β=0.103±0.018; a value near the β=0.125 expected for a two-dimensional Ising system. Biquadratic coupling between the structural and magnetic order parameters is also inferred along with evidence of three-dimensional critical scattering in this system.
    Full-text · Article · May 2009 · Physical review. B, Condensed matter
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    ABSTRACT: We present the results of an investigation of both the magnetic and structural phase transitions in a high quality single crystalline sample of the undoped, iron pnictide compound BaFe$_2$As$_2$. Both phase transitions are characterized via neutron diffraction measurements which reveal simultaneous, continuous magnetic and structural orderings with no evidence of hysteresis, consistent with a single second order phase transition. The onset of long-range antiferromagnetic order can be described by a simple power law dependence $\phi(T)^2\propto(1-\frac{T}{T_N})^{2\beta}$ with $\beta=0.103\pm0.018$; a value near the $\beta=0.125$ expected for a two-dimensional Ising system. Biquadratic coupling between the structural and magnetic order parameters is also inferred along with evidence of three-dimensional critical scattering in this system. Comment: New figure and discussion added. Length: 11 pages, 7 figures
    Full-text · Article · Mar 2009
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    ABSTRACT: We applied soft X-ray absorption spectroscopy to study the Ti L-edge in ferroelectric capacitors using a modified total electron yield method. The inner photo currents and the X-ray absorption spectra were polarization state dependent. The results are explained on the basis of photo electric effects and the inner potential in the ferroelectric capacitors as a result of back-to-back Schottky barriers superimposed by the potential due to the depolarization field. In general, the presented method offers the opportunity to investigate the electronic structure of buried metal-insulator and metal-semiconductor interfaces in thin film devices. Corresponding author: h.h.kohlstedt@fz-juelich.de
    Full-text · Article · Nov 2008
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    ABSTRACT: We report the first observation of soft-x-ray scattering from spin fluctuations in a high-Tc parent compound. An antiferromagnetic charge transfer insulator, CaCuO2, was irradiated by Cu M-edge soft x-rays. Ultra-high resolution measurements of scattered intensity revealed magnon-magnon excitations, due to spin exchange scattering, as low-energy loss features. The process is analogous to optical Raman scattering. The spectra provide the first measurement of the two-magnon excitation energy and the antiferromagnetic exchange parameter in infinite-layer CaCuO2. The results reveal resonant inelastic soft x-ray scattering as a novel probe of the spin dynamics in cuprates.
    Full-text · Article · Jul 2008
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    ABSTRACT: The struggle to understand high-temperature superconductivity (HTSC) has taken place in the field of condensed matter physics for a relatively long time. The difficulty in discovering a HTSC mechanism seems to originate from the confluence of two very complicated problems; electron correlation and quantum magnetism. Novel techniques that specifically probe the electronic or magnetic behavior of these materials are highly sought. We report the direct observation of spin-flip (SF) excitations using resonant inelastic soft x-ray scattering (RIXS). The antiferromganetic (AFM) charge transfer insulator, CaCuO2, was irradiated by soft x-rays tuned to the Cu M-edge (˜ 75 eV). Magnon-magnon (2 spin-flips) excitations were revealed as low-energy loss (0.39 eV) features in photon-in/photon-out experiments. The process is analogous to 2-magnon laser Raman scattering. Collected RIXS spectra provide a measurement of the 2 spin-flip excitation energy and the extraction of the AFM exchange of CaCuO2. These results mark the first report of two-spin-flip magneto-optical excitations revealed through Raman scattering using x-rays. This work reveals the utility of RIXS as a new probe of the spin structure in strongly correlated electron materials.
    No preview · Article · Mar 2006