Hyejin Ryu

Stony Brook University, Stony Brook, New York, United States

Are you Hyejin Ryu?

Claim your profile

Publications (16)36.41 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We use angle-resolved photoemission spectroscopy to study heavy fermion superconductor Ce2RhIn8. The Fermi surface is rather complicated and consists of several hole and electron pock- ets. We do not observe kz dispersion of Fermi sheets, which is consistent with 2D character of the electronic structure. Comparison of the ARPES data and band structure calculations points to a localized picture of f electrons. Our findings pave the way for understanding the transport and thermodynamical properties of this material.
    11/2014;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The normal-state in-plane resistivity below the zero-field superconducting transition temperature $T_c$ and the upper critical field Hc2 were measured by suppressing superconductivity in pulsed magnetic fields for K0.70Fe1.46Se1.85Te0.15. The normal-state resistivity $\rho_{ab}$ is found to increase logarithmically with decrasing temperature as $\frac{T}{T_c}\rightarrow 0$. Similar to granular metals, our results suggest that a superconductor - insulator transition below zero-field T$_{c}$ may be induced in high magnetic fields. This is related to the intrinsic real-space phase-separated states common to all inhomogeneous superconductors.
    08/2014;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Spin-1/2 Heisenberg antiferromagnets Cs2CuCl4 and Cs2CuBr4 with distorted triangular-lattice structures are studied by means of electron spin resonance spectroscopy in magnetic fields up to the saturation field and above. In the magnetically saturated phase, quantum fluctuations are fully suppressed, and the spin dynamics is defined by ordinary magnons. This allows us to accurately describe the magnetic excitation spectra in both materials and, using the harmonic spin-wave theory, to determine their exchange parameters. The viability of the proposed method was proven by applying it to Cs2CuCl4, yielding J/kB=4.7(2) K, J'/kB=1.42(7) K, [J'/J≃0.30] and revealing good agreement with inelastic neutron-scattering results. For the isostructural Cs2CuBr4, we obtain J/kB=14.9(7) K, J'/kB=6.1(3) K, [J'/J≃0.41], providing exact and conclusive information on the exchange couplings in this frustrated spin system.
    Physical Review Letters 02/2014; 112(7):077206. · 7.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Spin-1/2 Heisenberg antiferromagnets Cs$_2$CuCl$_4$ and Cs$_2$CuBr$_4$ with distorted triangular-lattice structures are studied by means of electron spin resonance spectroscopy in magnetic fields up to the saturation field and above. In the magnetically saturated phase, quantum fluctuations are fully suppressed, and the spin dynamics is defined by ordinary magnons. This allows us to accurately describe the magnetic excitation spectra in both materials and, using the harmonic spin-wave theory, to determine their exchange parameters. The viability of the proposed method was proven by applying it to Cs$_2$CuCl$_4$, yielding $J/k_B=4.7(2)$ K, $J'/k_B=1.42(7)$ K [$J'/J\simeq 0.30$] and revealing good agreement with inelastic neutron-scattering results. For the isostructural Cs$_2$CuBr$_4$, we obtain $J/k_B=14.9(7)$ K, $J'/k_B=6.1(3)$ K, [$J'/J\simeq 0.41$], providing exact and conclusive information on the exchange couplings in this frustrated spin system.
    01/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: ZeTe3 belongs to the ample class of chainlike chalcogenide charge-density-wave (CDW) materials. Its peculiarity consists in the formation of the CDW condensate at TCDW along the crystallographic directions perpendicular to the b-axis, Zr-Zr chains. Ni and Cu intercalation of ZrTe3 leads to the onset of bulk superconductivity at Tc
    12/2013; 89(3).
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have synthesized K0.95(1)Ni1.86(2)Se2 single crystals. The single crystals contain K and Ni deficiencies not observed in KNi2Se2 polycrystals. Unlike KNi2Se2 polycrystals, the superconductivity is absent in single crystals. The detailed physical property study indicates that the K0.95Ni1.86Se2 single crystals exhibit heavy-fermion-like characteristics. The transition to a heavy fermion state below T ∼ 30 K results in an enhancement of the electron-like carrier density whereas the magnetic susceptibility shows little anisotropy and suggests the presence of both itinerant and localized Ni orbitals.
    Journal of Physics Condensed Matter 11/2013; 26(1):015701. · 2.22 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Magnetic excitations in Cs$_2$CuBr$_4$, a spin-1/2 antiferromagnet with a distorted triangular lattice, are probed by means of high-field electron spin resonance (ESR) spectroscopy. We show that the high-energy excitation spectrum of this material is not appreciably affected by the 3D ordering, indicating the presence of strong quantum fluctuations competing with the magnetic order down to well below $T_N$. Such a behavior is consistent with the quantum spin-liquid scenario with the spin dynamics determined by short-range-order correlations (presumably of 1D nature, albeit being strongly influenced by frustrated interchain interactions). The observation of three ESR branches is in a prefect agreement with a general phenomenological macroscopic theory, predicting the presence of three Goldstone modes (two gapped and one gapless) as a consequence of the complete breaking of the rotational SO(3) symmetry in anisotropic spin systems with a noncollinear ground state.
    06/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report physical properties and ground state phase diagram of Kx(Fe,Ni)2-ySe2 single crystal alloy series. The ground state evolves from a heavy-Fermion-like metal KxNi2-ySe2 (I4/mmm) to a phase separated superconducting KxFe2-ySe2 (I4/m and I4/mmm space groups). Intermediate alloys show rich variety of ground states including semiconducting magnetic spin glass as Ni is replaced by Fe. We will address magnetic, thermodynamic, electronic and thermal transport properties and their connection to relevant structural parameters. 1Work at Brookhaven is supported by the U.S. DOE under Contract No. DE-AC02-98CH10886 and in part by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the U.S. DOE, Office for Basic Energy Science (H. L. and C. P). Work at the National High Magnetic Field Laboratory is supported by the DOE NNSA DEFG52-10NA29659 (D.G.), by the NSF Cooperative Agreement No. DMR-0654118, and by the state of Florida.
    03/2013;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Iron chalcogenide superconductors have become one of the most investigated superconducting materials in recent years due to high upper critical fields, competing interactions and complex electronic and magnetic phase diagrams. The structural complexity, defects and atomic site occupancies significantly affect the normal and superconducting states in these compounds. In this work we review the vortex behavior, critical current density and high magnetic field pair-breaking mechanism in iron chalcogenide superconductors. We also point to relevant structural features and normal-state properties.
    Science and Technology of Advanced Materials 12/2012; 13(5). · 3.75 Impact Factor
  • [Show abstract] [Hide abstract]
    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
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have successfully synthesized a new layered iron oxychalcogenide BaFe2OSe2 single crystal. This compound is built up of Ba and Fe-Se(O) layers alternatively stacked along the c-axis. The Fe-Se(O) layers contain double chains of edge-shared Fe-Se(O) tetrahedra that propagate along the b-axis and are bridged by oxygen along the a-axis. Physical property measurements indicate that BaFe2OSe2 is a semiconductor without the Curie-Weiss behavior up to 350 K. There is a possible long range antiferromagnetic (AFM) transition at 240 K, corresponding to the peak in specific heat measurement and two glassy transitions at 115 K and 43 K. The magnetic entropy up to 300 K is much smaller than the expected value for Fe2+ in tetrahedral crystal fields and Mosbauer spectrum indicates that long range magnetic order is unlikely at 294 K. Both results suggest that a short range magnetic correlations exist above the room temperature.
    Physical Review B 06/2012; 86(19). · 3.66 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report low-temperature electron spin resonance (ESR) studies of single-crystalline samples of Cs2CuBr4, a spin-1/2 antiferromagnet with a triangular spin-lattice structure. A remarkable angular dependence of the resonance field, including the splitting of the ESR line for some orientations of the magnetic field, and the presence of a gap in the ESR excitation spectrum at temperatures above TN˜ 1.3 K have been revealed. Our observations suggest that uniform Dzyaloshinskii-Moriya interaction affects the low-energy excitation spectrum in this frustrated compound. The results are compared with that obtained recently for the isostructural material Cs2CuCl4 [Povarov et al., Phys. Rev. Lett. 107, 037204 (2011)].
    02/2012;
  • Source
    [Show abstract] [Hide abstract]
    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). · 3.77 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report significantly enhanced magnetic moment in K0.69(2)Fe1.45(1)Se2.00(1) single crystals with sharp Tc and bulk superconductivity obtained by postannealing and quenching process. There are two Fe sites in the K0.69(2)Fe1.45(1)Se2.00(1) unit cell: Fe1, which has higher symmetry with longer average Fe-Se bond length, and Fe2, which has lower symmetry with shorter average Fe-Se bond length. Temperature-dependent x-ray absorption fine-structure (XAFS) analysis results on quenched and as-grown K0.69(2)Fe1.45(1)Se2.00(1) crystals show that quenched K0.69(2)Fe1.45(1)Se2.00(1) have increased average Fe-Se bond length and decreased static disorder. Our results indicate that nonzero population of Fe1 sites is the key structural parameter that governs the bulk superconductivity. We also show clear evidence that Fe1 sites carry higher magnetic moment than Fe2 sites.
    Physical review. B, Condensed matter 11/2011; 85(22). · 3.77 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We studied the anisotropy in physical properties of Ba1.00(4)Fe1.9(1)Se3.1(1) single crystals. BaFe2Se3 is a semiconductor below 300 K. Magnetization measurements show that there is a crossover from short-range antiferromagnetic (AFM) correlations at room temperature to a long-range AFM order with TN = 255 K. The anisotropy of magnetization is consistent with the previous neutron results. This crossover is supported by the heat-capacity measurement, where the phase-transition peak is absent at 255 K. The superconducting transition at about 10 K is likely due to the small amount of beta-FeSe impurities.
    Physical review. B, Condensed matter 10/2011; 84. · 3.77 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report significantly enhanced magnetic moment in K0.69(2)Fe1.45(1)Se2.00(1) single crystals with sharp Tc and bulk superconductivity obtained by post-annealing and quenching process. There are two Fe sites in K0.69(2)Fe1.45(1)Se2.00(1) unit cell: Fe1 which has higher symmetry with longer average Fe-Se bond length, and Fe2 which has lower symmetry with shorter average Fe-Se bond length. Temperature dependent X-ray absorption fine structure (XAFS) analysis results on quenched and as grown K0.69(2)Fe1.45(1)Se2.00(1) crystals show that quenched K0.69(2)Fe1.45(1)Se2.00(1) have increased average Fe-Se bond length and decreased static disorder. Our results indicate that nonzero population of Fe1 sites is the key structural parameter that governs the bulk superconductivity. We also show clear evidence that Fe1 sites carry higher magnetic moment than Fe2 sites.