Y. J. Uemura

Columbia University, New York City, New York, United States

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Publications (391)1111.79 Total impact

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    ABSTRACT: A new diluted ferromagnetic semiconductor (Sr,Na)(Zn,Mn)2As2 is reported, in which charge and spin doping are decoupled via Sr/Na and Zn/Mn substitutions, respectively, being distinguished from classic (Ga,Mn)As where charge & spin doping are simultaneously integrated. Different from the recently reported ferromagnetic (Ba,K)(Zn,Mn)2As2, this material crystallizes into the hexagonal CaAl2Si2-type structure. Ferromagnetism with a Curie temperature up to 20 K has been observed from magnetization. The muon spin relaxation measurements suggest that the exchange interaction between Mn moments of this new system could be different to the earlier DMS systems. This system provides an important means for studying ferromagnetism in diluted magnetic semiconductors.
    Physical Review B 11/2014; 90(15). DOI:10.1103/PhysRevB.90.155202 · 3.66 Impact Factor
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    ABSTRACT: In order to realize significant benefits from assembling solid-state materials from molecular cluster superatomic building blocks, several criteria must be met. Reproducible syntheses must reliably produce macroscopic amounts of pure material; the cluster-assembled solids must show properties that are more than simply averages of those of the constituent subunits; and rational changes to the chemical structures of the subunits must result in predictable changes in the collective properties of the solid. In this report we show that we can meet these requirements. Using a combination of magnetometry and muon spin relaxation measurements, we demonstrate that crystallographically defined superatomic solids assembled from molecular nickel telluride clusters and fullerenes are paramagnetic at high temperature, but they order into ferromagnets at low temperature. Moreover we show that when we modify the constituent superatoms, the cooperative magnetic properties change in predictable ways.
    Journal of the American Chemical Society 11/2014; DOI:10.1021/ja5098622 · 11.44 Impact Factor
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    ABSTRACT: Here we report the successful synthesis of a spin- & charge-decoupled diluted magnetic semiconductor (Ca,Na)(Zn,Mn)2As2, crystallizing into the hexagonal CaAl2Si2 structure. The compound shows a ferromagnetic transition with a Curie temperature up to 33 K with 10% Na doping, which gives rise to carrier density of np~10^20 cm^-3. The new DMS is a soft magnetic material with HC<400 Oe. The anomalous Hall effect is observed below the ferromagnetic ordering temperature. With increasing Mn doping, ferromagnetic order is accompanied by an interaction between the local spin and mobile charge, giving rise to a minimum in resistivity at low temperatures and localizing the conduction electrons. The system provides an ideal platform for studying the interaction of the local spins and conduction electrons.
    Journal of Applied Physics 10/2014; 116(16). DOI:10.1063/1.4899190 · 2.19 Impact Factor
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    ABSTRACT: A quantum critical point (QCP) occurs upon chemical doping of the weak itinerant ferromagnet Sc_{3.1}In. Remarkable for a system with no local moments, the QCP is accompanied by non-Fermi liquid (NFL) behavior, manifested in the logarithmic divergence of the specific heat both in the ferro- and the paramagnetic states. Sc_{3.1}In displays critical scaling and NFL behavior in the ferromagnetic state, akin to what had been observed only in f-electron, local moment systems. With doping, critical scaling is observed close to the QCP, as the critical exponents, and delta, gamma and beta have weak composition dependence, with delta nearly twice, and beta almost half of their respective mean-field values. The unusually large paramagnetic moment mu_PM~1.3 mu_B/F.U. is nearly composition-independent. Evidence for strong spin fluctuations, accompanying the QCP at x_c = 0.035 +- 0.005, may be ascribed to the reduced dimensionality of Sc_{3.1}In, associated with the nearly one-dimensional Sc-In chains.
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    ABSTRACT: The electronic and magnetic properties of a new diluted magnetic semiconductor (DMS) Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$, which is isostructural to so-called 122-type Fe-based superconductors, are investigated by x-ray absorption spectroscopy (XAS) and resonance photoemission spectroscopy (RPES). Mn $L_{2,3}$-edge XAS indicates that the doped Mn atoms have the valence 2+ and strongly hybridize with the $4p$ orbitals of the tetrahedrally coordinating As ligands. The Mn $3d$ partial density of states (PDOS) obtained by RPES shows a peak around 4 eV and relatively high between 0-2 eV below the Fermi level ($E_{F}$) with little contribution at $E_{F}$, similar to that of the archetypal DMS Ga$_{1-x}$Mn$_{x}$As. This energy level creates $d^{5}$ electron configuration with $S=5/2$ local magnetic moments at the Mn atoms. Hole carriers induced by K substitution for Ba atoms go into the top of the As $4p$ valence band and are weakly bound to the Mn local spins. The ferromagnetic correlation between the local spins mediated by the hole carriers induces ferromagnetism in Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$
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    ABSTRACT: Itinerant and local moment magnetism have substantively different origins, and require distinct theoretical treatment. A unified theory of magnetism has long been sought after, and remains elusive, mainly due to the limited number of known itinerant magnetic systems. In the case of the two such examples discovered several decades ago, the itinerant ferromagnets ZrZn_2 and Sc_3In, the understanding of their magnetic ground states draws on the existence of 3d electrons subject to strong spin fluctuations. Similarly, in Cr, an elemental itinerant antiferromagnet (IAFM) with a spin density wave (SDW) ground state, its 3d character has been deemed crucial to it being magnetic. Here we report the discovery of the first IAFM compound with no magnetic constituents, TiAu. Antiferromagnetic order occurs below a Neel temperature T_N ~ 36 K, about an order of magnitude smaller than in Cr, rendering the spin fluctuations in TiAu more important at low temperatures. This new IAFM challenges the currently limited understanding of weak itinerant antiferromagnetism, while providing long sought-after insights into the effects of spin fluctuations in itinerant electron systems.
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    ABSTRACT: We report muon spin relaxation/rotation (uSR) measurements of single crystal Ba(Fe1-xCox)2As2 with x=0.038 and 0.047. Zero field (ZF)-uSR and Transverse field (TF)-iSR measurements of these underdoped samples find the presence of magnetism and superconductivity. We find internal fields along the c-axis whose magnitude decreases with increasing doping. We find evidence for a low-temperature volume fraction that is only weakly magnetic, where that volume fraction increases with increasing Co doping of the sample. TF-uSR measurements show slight changes in the spectra that indicate magnetic inhomogeneities due to the loss of Fe moments in the system, the effect of which is larger in the higher Co doping. We discuss the existence of superconductivity in these samples in close proximity to strong magnetic order.
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    ABSTRACT: We use muon spin relaxation (muSR) to investigate the magnetic properties of a bulk form diluted ferromagnetic semiconductor (DFS) Li1.15(Zn0.9Mn0.1)P with T_C ~ 22 K. MuSR results confirm the gradual development of ferromagnetic ordering below T_C with a nearly 100% magnetic ordered volume. Despite its low carrier density, the relation between static internal field and Curie temperature observed for Li(Zn,Mn)P is consistent with the trend found in (Ga,Mn)As and other bulk DFSs, indicating these systems share a common mechanism for the ferromagnetic exchange interaction. Li1+y(Zn1-xMnx)P has the advantage of decoupled carrier and spin doping, where Mn2+ substitution for Zn2+ introduces spins and Li+ off-stoichiometry provides carriers. This advantage enables us to investigate the influence of overdoped Li on the ferromagnetic ordered state. Overdoping Li suppresses both T_C and saturation moments for a certain amount of spins, which indicates that more carriers are detrimental to the ferromagnetic exchange interaction, and that a delicate balance between charge and spin densities is required to achieve highest T_C.
    Physical Review B 06/2014; 90(8). DOI:10.1103/PhysRevB.90.085123 · 3.66 Impact Factor
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    ABSTRACT: We have studied the magnetism in superconducting single crystals of EuFe2As1.4P0.6 by using the local probe techniques of zero-field muon spin rotation/relaxation and 151Eu/57Fe Mössbauer spectroscopy. All of these measurements reveal magnetic hyperfine fields below the magnetic ordering temperature TM=18K of the Eu2+ moments. The analysis of the data shows that there is a coexistence of antiferromagnetism, resulting from Eu2+ moments ordered along the crystallographic c-axis, and superconductivity below TSC≈TSC≈ 10 K. We find indications for a change in the dynamics of the small Fe magnetic moments (~0.07 μBμB) at T⁎≃15K that may be triggering the onset of superconductivity: below T⁎ the Fe magnetic moments seem to be “frozen” within the ab-plane.
    Solid State Communications 06/2014; 187:18–22. DOI:10.1016/j.ssc.2014.02.001 · 1.70 Impact Factor
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    ABSTRACT: Understanding the role played by broken symmetry states such as charge, spin, and orbital orders in the mechanism of emergent properties such as high-temperature superconductivity (HTSC) is a major current topic in materials research. That the order may be within one unit cell, such as nematic, was only recently considered theoretically, but its observation in the iron-pnictide and doped cuprate superconductors places it at the forefront of current research. Here we show that the recently discovered BaTi$_2$Sb$_2$O superconductor and its "parent" compound BaTi$_2$As$_2$O form a symmetry-breaking nematic ground state that can be naturally explained as an intra-unit-cell charge order with $d$-wave symmetry, pointing to the ubiquity of the phenomenon. These findings, together with the key structural features in these materials being intermediate between the cuprate and iron-pnictide HTSC materials, render the titanium oxypnictides an important new material system to understand the nature of nematic order and its relationship to superconductivity.
    Nature Communications 05/2014; 5. DOI:10.1038/ncomms6761 · 10.74 Impact Factor
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    ABSTRACT: Understanding the role played by broken symmetry states such as charge, spin, and orbital orders in the mechanism of emergent properties such as high-temperature superconductivity (HTSC) is a major current topic in materials research. That the order may be within one unit cell, such as nematic, was only recently considered theoretically, but its observation in the iron-pnictide and doped cuprate superconductors places it at the forefront of current research. Here we show that the recently discovered BaTi$_2$Sb$_2$O superconductor and its "parent" compound BaTi$_2$As$_2$O form a symmetry-breaking nematic ground state that can be naturally explained as an intra-unit-cell charge order with $d$-wave symmetry, pointing to the ubiquity of the phenomenon. These findings, together with the key structural features in these materials being intermediate between the cuprate and iron-pnictide HTSC materials, render the titanium oxypnictides an important new material system to understand the nature of nematic order and its relationship to superconductivity.
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    ABSTRACT: The B-site ordered double perovskite Ba2CaOsO6 was studied by d.c. magnetic susceptibility, powder neutron diffraction and muon spin relaxation methods. The lattice parameter is a = 8.3619(6) A at 280K and cubic symmetry (Fm-3m) is retained to 3.5K with a = 8.3426(5) A. Curie-Weiss susceptibility behavior is observed for T > 100K and the derived constants are C = 0.3361(1)emu-K/mole and T_CW = -156.2(3) K, in excellent agreement with literature values. This Curie constant is much smaller than the spin-only value of 1.00 emu-K/mole for a 5d^2 Os6+ configuration, indicating a major influence of spin-orbit coupling. Previous studies had detected both susceptibility and heat capacity anomalies near 50K but no definitive conclusion was drawn concerning the nature of the ground state. While no ordered Os moment could be detected by powder neutron diffraction, muon spin relaxation (muSR) data show clear long-lived oscillations indicative of long-range magnetic order below T_C = 50K. An estimate of the ordered moment on Os6+ is ~0.2 mu_B, based upon a comparison with muSR data for Ba2YRuO6 with a known ordered moment of 2.2 mu_B. These results are compared with those for isostructural Ba2YReO6 which contains Re5+, also 5d^2, and has a nearly identical unit cell constant, a = 8.3628(2) A. In contrast, Ba2YReO6 shows T_CW = -616 K, and a complex spin-disordered and, ultimately, spin-frozen ground state below 50 K, indicating a much higher level of geometric frustration than in Ba2CaOsO6. A comparison is made to recent theory on d^2 double perovskites.
    Journal of Physics Condensed Matter 12/2013; 26(30). DOI:10.1088/0953-8984/26/30/306003 · 2.22 Impact Factor
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    ABSTRACT: We present the results of muon spin relaxation/rotation, transmission electron microscopy, and neutron diffraction measurements performed on several specimens of BaTi2(As1-xSbx)2O, which is known to have either charge density or spin density wave ordering at TDW for all x, and superconductivity below Tc ≈ 1 K for x = 1. Zero-field muon spin relaxation measurements show no significant increase in relaxation rate at the density wave ordering temperature for any composition, indicating that the density wave is of the charge rather than spin type. The absence of any superstructure peaks in selected area electron and high-resolution neutron diffraction measurements below TDW suggests that the charge density wave does not involve modulation of atomic arrangement. Transverse field muon spin rotation measurements reveal a robust superconducting state below Tc ≈ 1 K for x = 1.
    Physical Review B 12/2013; DOI:10.1103/PhysRevB.88.214506 · 3.66 Impact Factor
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    ABSTRACT: The doping effect of Sr and transition metals Mn, Fe, Co into the direct-gap semiconductor LaZnAsO has been investigated. Our results indicate that the single phase ZrCuSiAs-type tetragonal crystal structure is preserved in (La1-xSrx)(Zn1-xTMx)AsO (TM = Mn, Fe, Co) with the doping level up to x = 0.1. While the system remains semiconducting, doping with Sr and Mn results in ferromagnetic order with TC ~ 30K, and doping with Sr and Fe results in a spin glass like state below ~6K with a saturation moment of ~0.02 muB/Fe, an order of magnitude smaller than the ~0.4 muB/Mn of Sr and Mn doped samples. The same type of magnetic state is observed neither for (Zn,Fe) substitution without carrier doping, nor for Sr and Co doped specimens.
    EPL (Europhysics Letters) 11/2013; 103(6). DOI:10.1209/0295-5075/103/67011 · 2.27 Impact Factor
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    ABSTRACT: We report low-temperature specific heat and positive muon spin relaxation/rotation (μSR) measurements on both polycrystalline and single-crystal samples of the pyrochlore magnet Yb_{2}Ti_{2}O_{7}. This material is believed to possess a spin Hamiltonian able to support a quantum spin ice (QSI) ground state. Yb_{2}Ti_{2}O_{7} displays sample variation in its low-temperature heat capacity and, while our two samples exhibit extremes of this variation, our μSR measurements indicate a similar disordered low-temperature state down to 16 mK in both. We report little temperature dependence to the muon spin relaxation and no evidence for ferromagnetic order, in contrast to reports by Chang et al. [Nat. Comm. 3, 992 (2012)] and Yasui et al. [J. Phys. Soc. Japan. 72, 11 (2003)]. Transverse field (TF) μSR measurements show changes in the temperature dependence of the muon Knight shift that coincide with heat capacity anomalies, which, incidentally, prove that the implanted muons are not diffusing in Yb_{2}Ti_{2}O_{7}. From these results, we are led to propose that Yb_{2}Ti_{2}O_{7} enters an unconventional ground state below T_{c}∼265 mK. As found for all the current leading experimental candidates for a quantum spin liquid state, the precise nature of the state below T_{c} in Yb_{2}Ti_{2}O_{7} remains unknown and, at this time, defined by what is not as opposed to what it is: lacking simple periodic long-range order or a frozen spin glass state.
    Physical Review B 10/2013; 88(13). DOI:10.1103/PhysRevB.88.134428 · 3.66 Impact Factor
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    ABSTRACT: We report the discovery of a new diluted magnetic semiconductor, Li(Zn,Mn)P, in which charge and spin are introduced independently via lithium off-stoichiometry and the isovalent substitution of Mn2+ for Zn2+, respectively. Isostructural to (Ga,Mn)As, Li(Zn,Mn)P was found to be a p-type ferromagnetic semiconductor with excess Lithium providing charge doping. First principles calculations indicate that excess Li is favored to partially occupy the Zn site, leading to hole doping. Ferromagnetism is mediated in semiconducting samples of relative low mobile carriers with a small coercive force, indicating an easy spin flip.
    Physical Review B 08/2013; 88(8). DOI:10.1103/PhysRevB.88.081203 · 3.66 Impact Factor
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    ABSTRACT: We report the synthesis and characterization of a bulk diluted magnetic semiconductor (La1-xBax)(Zn1-xMnx)AsO (0 <= x <= 0.2) with a layered crystal structure identical to that of the "1111" FeAs superconductors. No ferromagnetic order occurs for (Zn,Mn) substitution in the parent compound LaZnAsO without charge doping. Together with carrier doping via (La,Ba) sub- stitution, a small amount of Mn substituting for Zn results in ferromagnetic order with TC up to ~40 K, although the system remains semiconducting. Muon spin relaxation measurements confirm the development of ferromagnetic order in the entire volume, with the relationship between the internal field and TC consistent with the trend found in (Ga,Mn)As, the "111" Li(Zn,Mn)As, and the "122" (Ba,K)(Zn,Mn)2As2 systems.
    Physical Review B 06/2013; 88(4). DOI:10.1103/PhysRevB.88.041102 · 3.66 Impact Factor
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    ABSTRACT: Diluted magnetic semiconductors (DMS) have received much attention due to its potential applications to spintronics devices. A prototypical system (Ga,Mn)As has been widely studied since 1990s. The simultaneous spin and charge doping via hetero-valence (Ga3+,Mn2+) substitution, however, resulted in severely limited solubility without availability of bulk specimens. Previously we synthesized a new diluted ferromagnetic semiconductor of bulk Li(Zn,Mn)As with Tc up to 50K, where isovalent (Zn,Mn) spin doping was separated from charge control via Li concentrations. Here we report the synthesis of a new diluted ferromagnetic semiconductor (Ba1-xKx)(Zn1-yMny)2As2, isostructural to iron 122 system, where holes are doped via (Ba2+, K1+), while spins via (Zn2+,Mn2+) substitutions. Bulk samples with x=0.1-0.3 and y=0.05-0.15 exhibit ferromagnetic order with TC up to 180K, comparable to that of record high Tc for Ga(MnAs), significantly enhanced than Li(Zn,Mn)As. Moreover the (Ba,K)(Zn,Mn)2As2 shares the same 122 crystal structure with semiconducting BaZn2As2, antiferromagnetic BaMn2As2, and superconducting (Ba,K)Fe2As2, which makes them promising to the development of multilayer functional devices.
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    ABSTRACT: Diluted magnetic semiconductors have received much attention due to their potential applications for spintronics devices. A prototypical system (Ga,Mn)As has been widely studied since the 1990s. The simultaneous spin and charge doping via hetero-valent (Ga(3+),Mn(2+)) substitution, however, resulted in severely limited solubility without availability of bulk specimens. Here we report the synthesis of a new diluted magnetic semiconductor (Ba(1-x)K(x))(Zn(1-y)Mn(y))(2)As(2), which is isostructural to the 122 iron-based superconductors with the tetragonal ThCr(2)Si(2) (122) structure. Holes are doped via (Ba(2+), K(1+)) replacements, while spins via isovalent (Zn(2+),Mn(2+)) substitutions. Bulk samples with x=0.1-0.3 and y=0.05-0.15 exhibit ferromagnetic order with T(C) up to 180 K, which is comparable to the highest T(C) for (Ga,Mn)As and significantly enhanced from T(C) up to 50 K of the '111'-based Li(Zn,Mn)As. Moreover, ferromagnetic (Ba,K)(Zn,Mn)(2)As(2) shares the same 122 crystal structure with semiconducting BaZn(2)As(2), antiferromagnetic BaMn(2)As(2) and superconducting (Ba,K)Fe(2)As(2), which makes them promising for the development of multilayer functional devices.
    Nature Communications 02/2013; 4:1442. DOI:10.1038/ncomms2447 · 10.74 Impact Factor
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    ABSTRACT: Diluted magnetic semiconductor (DMS) exhibits unique magnetic and transport properties. The well known DMS is (Ga,Mn)As that is in thin film form so far only. However Mn dopant brings both localized spins and carriers in Ga1-xMnxAs, result in the lack of independent control of local moment and carrier densities. It is therefore difficult to study or control charge and spin variable quantitatively. Here we report that an individual control of carrier and spin can be realized in a new type of DMS for Mn-doped I-II-V type semiconductor Li1+yZn1-xMnxAs in bulk. The Li(Zn,Mn)As polycrystalline samples showed ferromagnetic transition temperature (TC) up to 50K and spontaneous magnetization up to 2.9 μB per Mn. The other advantage of new DMS over III-V DMSs is the availability of bulk specimens so far for these type of DMS Moreover the new DMS shows a soft magnetic behavior with the coercive force < 100Oe which would be promising for prospective application. Muon spin relaxtion (μSR) probed static magnetic order of full volume in the ferromagnetic region. Electrical transport properties showed Li1+y(Zn,Mn)As (y = 0.05-0.1) compounds have p-type carriers since excess Li substitutes Zn sites presumably[1].
    Journal of Physics Conference Series 12/2012; 400(3):2033-. DOI:10.1088/1742-6596/400/3/032033

Publication Stats

9k Citations
1,111.79 Total Impact Points

Institutions

  • 1989–2014
    • Columbia University
      • Department of Physics
      New York City, New York, United States
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, Maryland, United States
  • 2013
    • Canadian Institute For Advanced Research
      Toronto, Ontario, Canada
  • 2010
    • McMaster University
      • Department of Physics and Astronomy
      Hamilton, Ontario, Canada
  • 1979–2010
    • TRIUMF
      Vancouver, British Columbia, Canada
  • 1997–2007
    • CUNY Graduate Center
      New York City, New York, United States
  • 2000
    • Technion - Israel Institute of Technology
      • Department of Physics
      H̱efa, Haifa, Israel
  • 1998
    • Institut Laue-Langevin
      Grenoble, Rhône-Alpes, France
  • 1995
    • University of Tsukuba
      Tsukuba, Ibaraki, Japan
  • 1993
    • Fukui University
      Hukui, Fukui, Japan
  • 1978–1991
    • The University of Tokyo
      • Department of Physics
      Tōkyō, Japan
    • University of British Columbia - Vancouver
      Vancouver, British Columbia, Canada
  • 1983–1989
    • Brookhaven National Laboratory
      • Physics Department
      New York City, New York, United States
  • 1988
    • College of William and Mary
      Williamsburg, Virginia, United States
  • 1985
    • AT&T Labs
      Austin, Texas, United States