Hyejin Ryu

Brookhaven National Laboratory, Брукхейвен, New York, United States

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Publications (23)72.96 Total impact

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    ABSTRACT: We report on high-field electron spin resonance (ESR) studies of magnetic excitations in the spin-1/2 triangular-lattice antiferromagnet Cs2CuBr4. Frequency-field diagrams of ESR excitations are measured for different orientations of magnetic fields up to 25 T. We show that the substantial zero-field energy gap, K, observed in the low-temperature excitation spectrum of Cs2CuBr4, (Zvyagin et al 2014 Phys. Rev. Lett.112 077206) is present well above TN. Noticeably, the transition into the long-range magnetically ordered phase does not significantly affect the size of the gap, suggesting that even below TN the high-energy spin dynamics in Cs2CuBr4 is determined by short-range-order spin correlations. The experimental data are compared with results of model spin-wave-theory calculations for spin-1/2 triangular-lattice antiferromagnet.
    No preview · Article · Nov 2015 · New Journal of Physics
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    ABSTRACT: We present Co substitution effects in KxFe2-y-zCozSe2 (0.06≤z≤1.73) single-crystal alloys. By 3.5% of Co doping superconductivity is suppressed, whereas phase separation of semiconducting K2Fe4Se5 and superconducting/metallic KxFe2Se2 is still present. We show that the arrangement and distribution of the superconducting phase (stripe phase) are connected with the arrangement of K, Fe, and Co atoms. Semiconducting spin glass is found in proximity to the superconducting state, persisting for large Co concentrations. At high Co concentrations a ferromagnetic metallic state emerges above the spin glass. This is coincident with changes of the unit cell and arrangement and connectivity of the stripe conducting phase.
    No preview · Article · Nov 2015 · Physical Review B
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    ABSTRACT: Polarized Raman scattering spectra of superconducting Kx Fe2−y Se2 and non-superconducting K0.8Fe1.8Co0.2Se2 single crystals were measured in the temperature range from 10 K up to 300 K. Two Raman active modes from the I4/mmm phase and seven from the I4/m phase are observed in the frequency range from 150 to 325 cm−1 in both compounds, suggesting that the K0.8Fe1.8Co0.2Se2 single crystal also has a two-phase nature. The temperature dependence of the Raman mode energy is analyzed in terms of lattice thermal expansion and phonon–phonon interaction. The temperature dependence of the Raman mode linewidth is dominated by temperature-induced anharmonic effects. It is shown that the change in Raman mode energy with temperature is dominantly driven by thermal expansion of the crystal lattice. An abrupt change of the A1g mode energy near was observed in Kx Fe2−y Se2, whereas it is absent in non-superconducting K0.8Fe1.8Co0.2Se2. Phonon energy hardening at low temperatures in the superconducting sample is a consequence of superconductivity-induced redistribution of the electronic states below the critical temperature.
    No preview · Article · Nov 2015 · Journal of Physics Condensed Matter
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    ABSTRACT: We report electron doping effects by Ni in ${\text{K}}_{x}{\text{Fe}}_{2$-${}$\delta${}$-${}y}{\text{Ni}}_{y}{\text{Se}}_{2}\phantom{\rule{4pt}{0ex}}(0.06$\le${}y$\le${}1.44)$ single-crystal alloys. A rich ground-state phase diagram is observed. A small amount of Ni ($$\sim${}4$%) suppressed superconductivity below 1.8 K, inducing insulating spin-glass magnetic ground state for higher Ni content. With further Ni substitution, metallic resistivity is restored. For high Ni concentration in the lattice the unit cell symmetry is high symmetry $I4/mmm$ with no phase separation whereas both $I4/m+I4/mmm$ space groups were detected in the phase separated crystals when concentration of Ni $<$ Fe. The absence of superconductivity coincides with the absence of crystalline Fe vacancy order.
    No preview · Article · May 2015 · Physical Review B
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    ABSTRACT: We report discovery of ${\mathrm{KFe}}_{1.05}{\mathrm{Ag}}_{0.88}{\mathrm{Te}}_{2}$ single crystals with semiconducting spin glass ground state. Composition and structure analyses suggest nearly stoichiometric $I4/\mathit{mmm}$ space group but allow for the existence of vacancies, absent in long-range semiconducting antiferromagnet ${\mathrm{KFe}}_{0.85}{\mathrm{Ag}}_{1.15}{\mathrm{Te}}_{2}$. The subtle change in stoichometry in Fe-Ag sublattice changes magnetic ground state but not conductivity, giving further insight into the semiconducting gap mechanism.
    No preview · Article · May 2015 · Physical Review B
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    ABSTRACT: We use angle-resolved photoemission spectroscopy (ARPES) to study the two-dimensional (2D) heavy-fermion superconductor, Ce2RhIn8. The Fermi surface is rather complicated and consists of several hole and electron pockets with one of the sheets displaying strong nesting properties with a q vector of (0.32, 0.32) π/a. We do not observe kz dispersion of the Fermi sheets, which is consistent with the expected 2D character of the electronic structure. Comparison of the ARPES data to band-structure calculations suggests that a localized picture of the f electrons works best. While there is some agreement in the overall band dispersion and location of the Fermi sheets, the model does not reproduce all observed bands and is not completely accurate for those it does. Our data paves the way for improving the band-structure calculations and the general understanding of the transport and thermodynamical properties of this material.
    No preview · Article · Apr 2015 · Physical Review B
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    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.
    Preview · Article · Nov 2014
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    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.
    Preview · Article · Aug 2014 · Physical Review X
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    ABSTRACT: We report evidence for multiband transport and an insulating low-temperature normal state in superconducting K0.50Na0.24Fe1.52Se2 with T-c approximate to 20 K. The temperature-dependent upper critical field H-c2 is well described by a two-band BCS model. The normal-state resistance, accessible at low temperatures only in pulsed magnetic fields, shows an insulating logarithmic temperature dependence as T -> 0 after superconductivity is suppressed. This is similar as for high-T-c copper oxides and granular type-I superconductors, suggesting that the superconductor-insulator transition observed in high magnetic fields is related to intrinsic nanoscale phase separation.
    No preview · Article · Jul 2014 · Physical Review B
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    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.
    Full-text · Article · Feb 2014 · Physical Review Letters
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    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.
    Full-text · Article · Jan 2014
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    ABSTRACT: ZeTe(3) belongs to the ample class of chainlike chalcogenide charge-density-wave (CDW) materials. Its peculiarity consists in the formation of the CDW condensate at T-CDW 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 T-c < T-CDW, raising questions about the possible coexistence as well as competition between both broken-symmetry ground states. Here, we report on the optical investigation of Ni0.05ZrTe3 and Cu0.05ZrTe3 single crystals performed with electromagnetic radiation polarized along the in-plane crystallographic a and b axes, over a broad spectral range and as a function of temperature. The charge dynamics of the CDW state displays a polarization dependence within the ab plane and gives evidence for a partial gapping of the Fermi surface, which affects almost exclusively the crystallographic direction parallel to the a axis. We provide a complementary analysis of the spectral weight distribution, disentangling its reshuffling in the excitation spectrum across the symmetry-breaking CDW transition. While the superconducting energy gap lies outside our measurable spectral range, we observe nonetheless that the CDW-gap feature persists at T < T-c. We propose that CDW and superconductivity coexist along the a axis.
    No preview · Article · Dec 2013 · Physical Review B
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    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.
    Preview · Article · Nov 2013 · Journal of Physics Condensed Matter
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    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.
    Full-text · Article · Jun 2013
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    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.
    No preview · Article · Mar 2013
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    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.
    Full-text · Article · Dec 2012 · Science and Technology of Advanced Materials
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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).
    No preview · Article · Dec 2012 · Physical Review Letters
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    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.
    Full-text · Article · Jun 2012 · Physical Review B
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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.
    Full-text · Article · Mar 2012 · Physical review. B, Condensed matter
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    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)].
    No preview · Article · Feb 2012

Publication Stats

110 Citations
72.96 Total Impact Points

Institutions

  • 2012-2015
    • Brookhaven National Laboratory
      • Condensed Matter Physics & Materials Science Department
      Брукхейвен, New York, United States
    • University of Maryland, College Park
      • Department of Physics
      Maryland, United States
  • 2011-2015
    • Stony Brook University
      • Department of Physics and Astronomy
      스토니브룩, New York, United States