E. S. Choi

Chinese Academy of Sciences, Peping, Beijing, China

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Publications (131)299.24 Total impact

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    ABSTRACT: We report the valley-selective interlayer conduction of SrMnBi_{2} under in-plane magnetic fields. The c-axis resistivity of SrMnBi_{2} shows clear angular magnetoresistance oscillations indicating coherent interlayer conduction. Strong fourfold variation of the coherent peak in the c-axis resistivity reveals that the contribution of each Dirac valley is significantly modulated by the in-plane field orientation. This originates from anisotropic Dirac Fermi surfaces with strong disparity in the momentum-dependent interlayer coupling. Furthermore, we found a signature of broken valley symmetry at high magnetic fields. These findings demonstrate that a quasi-two-dimensional anisotropic Dirac system can host a valley-polarized interlayer current through magnetic valley control.
    Physical Review Letters 10/2014; 113(15):156602. · 7.73 Impact Factor
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    ABSTRACT: We have carried out high magnetic field studies of single-crystalline Li$_2$MnO$_3$, a honeycomb lattice antiferromagnet. Its magnetic phase diagram was mapped out using magnetization measurements at applied fields up to 35 T. Our results show that it undergoes two successive meta-magnetic transitions around 9 T fields applied perpendicular to the ab-plane (along the c*-axis). These phase transitions are completely absent in the magnetization measured with field applied along the ab-plane. In order to understand this magnetic phase diagram, we developed a mean-field model starting from the correct Neel-type magnetic structure, consistent with our single crystal neutron diffraction data at zero field. Our model calculations succeeded in explaining the two meta-magnetic transitions that arise when Li$_2$MnO$_3$ enters two different spin-flop phases from the zero field Neel phase.
    Physical Review B 09/2014; 90:104412. · 3.66 Impact Factor
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    ABSTRACT: A flop of electric polarization from $P$$\parallel$$c$ ($P_c$) to $P$$\parallel$$a$ ($P_a$) is observed in MnTiO$_3$ as a spin flop transition is triggered by a $c$-axis magnetic field, $H_{\|c}$=7 T. The critical magnetic field $H_{\|c}$ for $P_a$ is significantly reduced in Mn$_{1-x}$Ni$_x$TiO$_3$ (x=0.33). $P_a$ and $P_c$ have been observed with both $H_{\|c}$ and $H_{\|a}$. Neutron diffraction measurements revealed similar magnetic arrangements for the two compositions where the ordered spins couple antiferromagnetically with their nearest intra- and inter-planar neighbors. In the x=0.33 system, the uniaxial and planar anisotropies of Mn$^{2+}$ and Ni$^{2+}$ compete and give rise to a spin reorientation transition at $T_R$. A magnetic field, $H_{\|c}$, aligns the spins along $c$ for $T_R$$<$$T$$<$$T_N$. The rotation of the collinear spins away from the $c$-axis for $T$$<$$T_R$ alters the magnetic point symmetry and gives rise to a new ME susceptibility tensor form. Such linear ME response provides satisfactory explanation for the behavior of the field-induced electric polarization in both compositions. As the Ni content increases to x=0.5 and 0.68, the ME effect disappears as a new magnetic phase emerges.
    Physical Review B 09/2014; 90:144429. · 3.66 Impact Factor
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    ABSTRACT: Spin dynamics and magnetoelectric properties of the coupled-spin tetrahedral compound Cu2Te2O5Cl2
    Physical Review B 08/2014; 90:054418. · 3.66 Impact Factor
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    ABSTRACT: The origin of the spin liquid state in Tb$_2$Ti$_2$O$_7$ has challenged experimentalists and theorists alike for nearly 20 years. To improve our understanding of the exotic magnetism in Tb$_2$Ti$_2$O$_7$, we have synthesized a chemical pressure analog, Tb$_2$Ge$_2$O$_7$. Germanium substitution results in a lattice contraction and enhanced exchange interactions. We have characterized the magnetic ground state of Tb$_2$Ge$_2$O$_7$ with specific heat, ac and dc magnetic susceptibility, and polarized neutron scattering measurements. Akin to Tb$_2$Ti$_2$O$_7$, there is no long-range order in Tb$_2$Ge$_2$O$_7$ down to 20 mK. The Curie-Weiss temperature of $-19.2(1)$ K, which is more negative than that of Tb$_2$Ti$_2$O$_7$, supports the picture of stronger antiferromagnetic exchange. Polarized neutron scattering of Tb$_2$Ge$_2$O$_7$ reveals that at 3.5 K liquid-like correlations dominate in this system. However, below 1 K, the liquid-like correlations give way to intense short-range ferromagnetic correlations with a length scale related to the Tb-Tb distance. Despite stronger antiferromagnetic exchange, the ground state of Tb$_2$Ge$_2$O$_7$ has ferromagnetic character, in stark contrast to the pressure-induced antiferromagnetic order observed in Tb$_2$Ti$_2$O$_7$.
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    ABSTRACT: Two critical points have been revealed in the normal-state phase diagram of the electron-doped cuprate superconductor Nd$_{2-x}$Ce$_x$CuO$_4$ by exploring the Fermi surface properties of high quality single crystals by high-field magnetotransport. First, the quantitative analysis of the Shubnikov-de Haas effect shows that the weak superlattice potential responsible for the Fermi surface reconstruction in the overdoped regime extrapolates to zero at the doping level $x_c = 0.175$ corresponding to the onset of superconductivity. Second, the high-field Hall coefficient exhibits a sharp drop right below optimal doping $x_{\mathrm{opt}} = 0.145$ where the superconducting transition temperature is maximum. This drop is most likely associated with the onset of long-range antiferromagnetic ordering. Thus, for Nd$_{2-x}$Ce$_x$CuO$_4$ the superconducting dome appears to be pinned by two critical points to the normal state phase diagram.
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    ABSTRACT: We have investigated the magnetic and electric ground states of a quasi-two-dimensional triangular lattice antiferromagnet (TLAF), Ba3CoNb2O9, in which the effective spin of Co2+ is 1/2. At zero field, the system undergoes a two-step transition upon cooling at TN2=1.36 K and TN1=1.10 K and enters a 120∘ ordered state. By applying magnetic fields, a series of spin states with fractions of the saturation magnetization Ms are observed. They are spin states with 1/3, 1/2, 2/3 (or √3 /3) Ms. The ferroelectricity emerges in all spin states, either with collinear or noncollinear spin structure, which makes Ba3CoNb2O9 another unique TLAF exhibiting both a series of magnetic phase transitions and multiferroicity. We discuss the role of quantum fluctuations and magnetic anisotropy in contributing more complex phase diagram compared to its sister multiferroic TLAF compound Ba3NiNb2O9 [J. Hwang et al., Phys. Rev. Lett. 109, 257205 (2012), 10.1103/PhysRevLett.109.257205].
    02/2014; 89(10).
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    ABSTRACT: The linear and nonlinear ac susceptibility measurements of Yb-pyrochlores, Yb2X2O7 (X =Sn, Ti, and Ge), show transitions with a ferromagnetic nature at 0.13 and 0.25 K for Yb2Sn2O7 and Yb2Ti2O7, respectively, and an antiferromagnetic ordering at 0.62 K for Yb2Ge2O7. These systematical results (i) provided information about the nature of the unconventional magnetic ground state in Yb2Ti2O7; (ii) realized a distinct antiferromagnetic ordering state in Yb2Ge2O7; and (iii) demonstrated that the application of chemical pressure through the series of Yb-pyrochlores can efficiently perturb the fragile quantum spin fluctuations of the Yb3+ ions and lead to very different magnetic ground states.
    Phys. Rev. B. 02/2014; 89(6):064401.
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    ABSTRACT: We report the electronic and transport properties of the triangular antiferromagnet PdCrO_{2} at high magnetic fields up to 33 T, using measurements of the de Haas-van Alphen oscillations and the Hall resistivity. The de Haas-van Alphen oscillations below the magnetic ordering temperature T_{N} reveal several two-dimensional Fermi surfaces of smaller size than those found in nonmagnetic PdCoO_{2}, consistent with the band structure calculations. This evidences Fermi surface reconstruction due to the 120° helical ordering of the localized Cr spins, suggesting significant coupling of the itinerant electrons to the underlying spin texture. This induces the nonlinear Hall resistivity at low temperatures via the magnetic breakdown in the reconstructed Fermi surface. Furthermore, such a coupling leads to the unconventional anomalous Hall effects near T_{N} due to the field-induced spin chirality at high magnetic fields.
    Physical Review Letters 10/2013; 111(17):176405. · 7.73 Impact Factor
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    ABSTRACT: We present the Fermi-surface map of the spin-chiral bulk states for the non-centrosymmetric semiconductor BiTeI using de Haas-van Alphen and Shubnikov-de Haas oscillations. We identify two distinct Fermi surfaces with a unique spindle-torus-type topology and the non-trivial Berry phases, confirming the spin chirality with oppositely circulating spin-texture. Near the quantum limit at high magnetic fields, we find a substantial Zeeman effect with an effective g-factor of ~ 60 for the Rashba-split Fermi surfaces. These findings provide clear evidence of strong Rashba and Zeeman coupling in the bulk states of BiTeI, suggesting that BiTeI is a good platform hosting the spin-polarized chiral states.
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    ABSTRACT: We report neutron scattering and ac susceptibility measurements on the pyrochlore Yb2Sn2O7. The appearance of magnetic diffuse scattering, low-energy spin excitations, and a nearly temperature-independent relaxation time are signatures that the system enters a short-range-ordered regime with quantum dynamics beneath 2 K. ac susceptibility data further suggest that Yb2Sn2O7 enters a long-range magnetically ordered state below 0.11 K. These results suggest that Yb2Sn2O7 is a ferromagnet with persistent spin dynamics down to 0.05 K. Referencing the phase diagram for Yb pyrochlores, Yb2Sn2O7 is approaching a quantum phase transition near the ferromagnetic ordered critical point.
    Physical Review B 04/2013; 87(13):134408. · 3.66 Impact Factor
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    ABSTRACT: We report a comparative investigation of the magnetostrictive and magnetoconductivity effects in the low-temperature ferrimagnetic phases of the spinel vanadate structure AV2O4 (A = Fe, Mn, and Co). In FeV2O4, both the Fe and V sites have orbital degrees of freedom, the V-V distance places the compound intermediate between localized and itinerant charge carrier behavior, and there are three structural and two magnetic transitions versus temperature. For MnV2O4 and CoV2O4, only the V sites have orbital degrees of freedom. MnV2O4 has the largest V-V distance and the most localized conductivity, with one structural and one magnetic transition. CoV2O4 has the shortest V-V distance and approaches the itinerant charge carrier threshold. CoV2O4 has one ferrimagnetic transition and no long-range ordered structural transition. Working with single crystals, ac dielectric and resistance measurements versus temperature and magnetic field exhibit distinctive physical properties in each case. These include the magnetocaloric signatures of magnetization reversal (Fe and Co), separable isotropic and anisotropic magnetostrictive effects (Mn), glasslike dynamics (Co), and evidence in the field-dependent dielectric response for the alteration of the ferrimagnetic spin structure with increasing magnetic field (Fe). These findings can guide future dielectric- and magnetoconductance-based spinel studies with a focus on the low-temperature and high magnetic field properties of canted ferrimagnetic spin configurations and orbital-lattice ordering effects.
    Physical review. B, Condensed matter 02/2013; 87(5). · 3.77 Impact Factor
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    ABSTRACT: We report the de Haas-van Alphen (dHvA) oscillations and first-principle calculations for triangular antiferromagnet PdCrO2 showing unconventional anomalous Hall effect (AHE). The dHvA oscillations in PdCrO2 reveal presence of several 2 dimensional Fermi surfaces of smaller size than found in nonmagnetic PdCoO2. This evidences Fermi surface reconstruction due to the non-collinear 120 antiferromagnetic ordering of the localized Cr, consistent with the first principle calculations. The temperature dependence of dHvA oscillations shows no signature of additional modification of Cr spin structure below TN. Considering that the 120 helical ordering of Cr spins has a zero scalar spin chirality, our results suggest that PdCrO2 is a rare example of the metallic triangular antiferromagnets whose unconventional AHE can not be understood in terms of the spin chirality mechanism.
    Physical Review Letters 02/2013; · 7.73 Impact Factor
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    ABSTRACT: We report on the emergence of an electronic Griffiths phase in the doped semiconductor FeSb_{2}, predicted for disordered insulators with random localized moments in the vicinity of a metal-insulator transition. Magnetic, transport, and thermodynamic measurements of Fe(Sb_{1-x}Te_{x})_{2} single crystals show signatures of disorder-induced non-Fermi liquid behavior and a Wilson ratio expected for strong electronic correlations. The electronic Griffiths phase states are found on the metallic boundary between the insulating state (x=0) and a long-range albeit weak magnetic order (x≥0.075).
    Physical Review Letters 12/2012; 109(25):256401. · 7.73 Impact Factor
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    ABSTRACT: We report the magnetic and electric properties of Ba_{3}NiNb_{2}O_{9}, which is a quasi-two-dimensional spin-one triangular-lattice antiferromagnet with trigonal structure. At low T and with increasing magnetic field, the system evolves from a 120 degree magnetic ordering phase (A phase) to an up-up-down (uud) phase (B phase) with a change of slope at 1/3 of the saturation magnetization, and then to an "oblique" phase (C phase). Accordingly, the ferroelectricity switches on at each phase boundary with appearance of spontaneous polarization. Therefore, Ba_{3}NiNb_{2}O_{9} is a unique triangular-lattice antiferromagnet exhibiting both uud phase and multiferroicity.
    Physical Review Letters 12/2012; 109(25):257205. · 7.73 Impact Factor
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    ABSTRACT: We have investigated the upper critical field Bc2 of the stoichiometric Fe-based superconductor LiFeAs, via the magnetic torque measurements in dc-magnetic fields up to 35 T and at temperatures down to 0.3 K. Bc2 at 0.3K is obtained to be 26.4 T and 15.5 T for the applied field Ba || ab and Ba || c, respectively, indicating nearly-isotropic superconductivity. The detailed analyses show that, while Bc2 for Ba || c is limited by the orbital effect, the spin-paramagnetic effect is evident in the temperature dependence of Bc2 for Ba || ab.
    Journal of Physics Conference Series 12/2012; 391(1):2133-.
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    ABSTRACT: The pyrochlore Ho2Ge2O7 is a new highly correlated spin ice material. Physical property measurements, including x-ray diffraction, dc susceptibility, and ac susceptibility, confirm that it shares the distinctive characteristics of other known spin ices. Polarized neutron scattering measurements on a powder sample, combined with reverse Monte Carlo (RMC) refinements, give unique information about the spin ice state in Ho2Ge2O7. RMC refinements are used to fit the powder magnetic diffuse scattering and predict the single-crystal magnetic scattering of Ho2Ge2O7, demonstrating consistency with spin ice behavior.
    Physical Review B 10/2012; 86(13):134431. · 3.66 Impact Factor
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    ABSTRACT: We investigated the magnetic phase diagram of single crystals of SrNdFeO4 by measuring the magnetic properties, the specific heat and the dielectric permittivity. The system has two magnetically active ions, Fe3+ and Nd3+. The Fe3+ spins are antiferromagnetically ordered below 360 K with the moments lying in the ab plane, and undergo a reorientation transition at about 35–37 K to an antiferromagnetic order with the moments along the c axis. A short-range, antiferromagnetic ordering of Nd3+ along the c axis was attributed to the reorientation of Fe3+ followed by a long-range ordering at lower temperature [ Oyama et al. J. Phys.: Condens. Matter. 16 1823 (2004)]. At low temperatures and magnetic fields above 8 T, the Nd3+ moments are completely spin polarized. The dielectric permittivity also shows anomalies associated with spin configuration changes, indicating that this compound has considerable coupling between spin and lattice. A possible magnetic structure is proposed to explain the results.
    Physical review. B, Condensed matter 06/2012; 85(22). · 3.77 Impact Factor
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    ABSTRACT: The angular dependence of the magnetothermopower of a charge transfer organic salt α-(ET)(2)KHg(SCN)(4) below (4 K) and above (9 K) the phase transition temperature, T(p) = 8 K, and under fields of 15 T and 25 T, below and above the 'kinkfield', has been studied. We find that for a longitudinal thermoelectric measurement both an interlayer thermopower (the Seebeck effect), S(zz), and a transverse thermopower (the Nernst effect), S(yz), exist in all three different B-T phases (the CDW (0), CDW (x) and metallic states) with large amplitude. Both thermoelectric effects display a resonant-like behavior without a sign reversal at the angles corresponding to angular magnetoresistance oscillation minima and maxima in this compound. The resonant behavior is most evident in the CDW(0) state, indicating a mechanism involving the Fermi surface nesting. Angular dependences reveal different behaviors of the thermopower and Nernst effect in the high magnetic field (CDW(x)) state.
    Journal of Physics Condensed Matter 06/2012; 24(26):265502. · 2.22 Impact Factor
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    ABSTRACT: A comparison among the two sets of studied pyrochlore spin ices, Ho2Sn2O7, Ho2Ti2O7, and Ho2Ge2O7 with Ho3+ spins and Dy2Sn2O7, Dy2Ti2O7, and Dy2Ge2O7 with Dy3+ spins, shows that the application of chemical pressure through each set drives the system toward the antiferromagnetic phase boundary from the spin ice region, which agrees with the prediction of the “dipolar spin ice” model of den Hertog and Gingras. Among all the studied pyrochlore spin ices, Dy2Ge2O7 has the smallest ratio of Jnn/Dnn=-0.73.
    Physical Review Letters 05/2012; 108(20). · 7.73 Impact Factor

Publication Stats

893 Citations
299.24 Total Impact Points


  • 2014
    • Chinese Academy of Sciences
      • Institute of Physics
      Peping, Beijing, China
  • 2000–2014
    • Florida State University
      • • Department of Physics
      • • National High Magnetic Field Laboratory
      Tallahassee, Florida, United States
  • 2013
    • Pohang University of Science and Technology
      • Department of Physics
      Antō, North Gyeongsang, South Korea
  • 2003–2013
    • National High Magnetic Field Laboratory
      Tallahassee, Florida, United States
  • 2005–2006
    • National Institute for Materials Science
      Tsukuba, Ibaraki, Japan
  • 2004
    • RIKEN
      • Condensed Molecular Materials Laboratory
      Вако, Saitama, Japan
  • 2001–2002
    • Ewha Womans University
      • Department of Physics
      Seoul, Seoul, South Korea
  • 1995–2002
    • Korea Research Institute of Bioscience and Biotechnology KRIBB
      • Laboratory of Microbial and Bioprocess Engineering
      Ansan, Gyeonggi, South Korea
  • 1999–2001
    • Kemerovo Cardiology Centre
      Shcheglovsk, Kemerovo, Russia