E. S. Choi

Pohang University of Science and Technology, Antō, North Gyeongsang, South Korea

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Publications (123)211.61 Total impact

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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.
    03/2014;
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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. · 7.94 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.
    06/2013;
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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. · 3.77 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).
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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; · 7.94 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. · 7.94 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. · 7.94 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. · 3.77 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).
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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. · 2.36 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). · 7.94 Impact Factor
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    ABSTRACT: Polycrystalline Pb3TeCo3V2O14, a structural analogue of the multiferroic Ba3NbFe3Si2O14, was synthesized and characterized using X-ray diffraction, magnetic susceptibility, specific heat, dielectric constant, and neutron diffraction. Magnetic susceptibility, specific heat capacity measurements, and bond valence analysis confirmed that the V5+ ion is nonmagnetic, while Co2+ is in its high spin state (S = 3/2). Two magnetic transitions were seen at TN1 = 8.6 K and TN2 = 6.0 K where the spins first adopt a magnetic structure with propagation vector k = (0.752,0,1/2) and reorder into a commensurate structure with propagation vector k = (5/6,5/6,1/2). Changes in the dielectric constant at both magnetic phase transitions suggest that magnetoelectric coupling exists in Pb3TeCo3V2O14.
    Chemistry of Materials 01/2012; 24(4):664-670. · 8.24 Impact Factor
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    ABSTRACT: We present the upper critical fields μâH{sub c2}(T) and Hall effect in β-FeSe single crystals. The μâH{sub c2}(T) increases as the temperature is lowered for fields applied parallel and perpendicular to (101), the natural growth facet of the crystal. The μâH{sub c2}(T) for both field directions and the anisotropy at low temperature increase under pressure. Hole carriers are dominant at high magnetic fields. However, the contribution of electron-type carriers is significant at low fields and low temperature. Our results show that multiband effects dominate μâH{sub c2}(T) and electronic transport in the normal state.
    Physical review. B, Condensed matter 01/2012; 85(9).
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    ABSTRACT: The magnetic field and angle dependences of the thermopower and Nernst effect for the quasi-two dimensional (q2D) organic conductor α-(ET)2KHg(SCN)4 are measured at temperatures below (4 K) and above (9 K) the transition temperature in fields of up to 31 T. In addition, a theoretical model involving a magnetic breakdown effect between the q1D and q2D bands is proposed in order to simulate the data. Analysis of the background components of the thermopower and Nernst effect imply that at low temperatures, in the CDW state, the properties of α-(ET)2KHg(SCN)4 are determined mostly by the orbits on new, open Fermi sheets. The quantum oscillations observed in both thermoelectric effects at fields above 8 T are caused only by the α orbit.
    Low Temperature Physics 10/2011; 37(10). · 0.82 Impact Factor
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    ABSTRACT: We have measured the dielectric constant and the pyroelectric current of orthorhombic (space group $Pbam$) NdCrTiO$_5$ polycrystalline samples. The dielectric constant and the pyroelectric current show features associated with ferroelectric transitions at the antiferromagnetic transition temperature ($T_{\text{N}}$ = 21 K). The effect of magnetic fields is to enhance the features almost linearly up to the maximum measured field (7 T) with a spontaneous polarization value of $\sim 3.5 \mu$C/m$^2$. Two possible scenarios, the linear magnetoelectric effect and multiferroicity (antiferromagnetism + ferroelectricity), are discussed as possible explanations for the observations.
    Physical review. B, Condensed matter 08/2011;
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    ABSTRACT: The magnetic field and angular dependencies of the thermopower and Nernst effect of the quasi-two-dimensional organic conductor alpha-(ET)2KHg(SCN)4 are experimentally measured at temperatures below (4 K) and above (9 K) the transition temperature to fields of In addition, a theoretical model which involves a magnetic breakdown effect between the q1D and q2D bands is proposed in order to simulate the data. Analysis of the background components of the thermopower and Nernst effect imply that at low temperatures, in the CDW state, the properties of alpha-(ET)2KHg(SCN)4 are determined mostly by the orbits on the new open Fermi sheets. Quantum oscillations observed in the both thermoelectric effects, at fields above 8 T, originate only from the alpha orbit.
    07/2011;
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    ABSTRACT: The synthesis and characterization of Ba3CuSb2O9, which has a layered array of Cu2+ spins in a triangular lattice, are reported. The magnetic susceptibility and neutron scattering experiments of this material show no magnetic ordering down to 0.2 K with a θ(CW) = -55  K. The magnetic specific heat reveals a T-linear dependence with a γ = 43.4  mJ  K(-2)  mol(-1) below 1.4 K. These observations suggest that Ba3CuSb2O9 is a new quantum spin liquid candidate with a S = 1/2 triangular lattice.
    Physical Review Letters 04/2011; 106(14):147204. · 7.94 Impact Factor
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    ABSTRACT: We report on the upper critical field B_c2 of a superconducting LiFeAs single crystal with T_c~16 K, determined from magnetic torque measurements in dc-magnetic fields up to 35 T and at temperatures down to 0.3 K. B_c2 at 0.3 K is obtained to be 26.4 T and 15.5 T for the applied field B_a||ab and B_a||c, respectively. The anisotropy parameter $\Gamma$=$B_c2^ab / B_c2^c$ is ~ 3 at T_c and decreases to 1.7 as $T \rightarrow 0$, showing rather isotropic superconductivity. While B_c2 is orbitally-limited for B_a||c, the spin-paramagnetic effect is evident in the temperature dependence of B_c2 for B_a||ab Comment: 4 pages, 4 figures, revised version to be published in J. Phys. Soc. Jpn. as a letter article
    Journal of the Physical Society of Japan 11/2010; · 2.09 Impact Factor

Publication Stats

467 Citations
211.61 Total Impact Points

Institutions

  • 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
  • 2000–2013
    • Florida State University
      • • Department of Physics
      • • National High Magnetic Field Laboratory
      Tallahassee, Florida, United States
  • 2005–2006
    • National Institute for Materials Science
      Tsukuba, Ibaraki, 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