Publications (131)281.22 Total impact
 Physical Review B 09/2014; 90:104412. · 3.66 Impact Factor
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ABSTRACT: We have carried out high magnetic field studies of singlecrystalline 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 metamagnetic transitions around 9 T fields applied perpendicular to the abplane (along the c*axis). These phase transitions are completely absent in the magnetization measured with field applied along the abplane. In order to understand this magnetic phase diagram, we developed a meanfield model starting from the correct Neeltype magnetic structure, consistent with our single crystal neutron diffraction data at zero field. Our model calculations succeeded in explaining the two metamagnetic transitions that arise when Li$_2$MnO$_3$ enters two different spinflop phases from the zero field Neel phase.09/2014;  [Show abstract] [Hide abstract]
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 longrange order in Tb$_2$Ge$_2$O$_7$ down to 20 mK. The CurieWeiss 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 liquidlike correlations dominate in this system. However, below 1 K, the liquidlike correlations give way to intense shortrange ferromagnetic correlations with a length scale related to the TbTb distance. Despite stronger antiferromagnetic exchange, the ground state of Tb$_2$Ge$_2$O$_7$ has ferromagnetic character, in stark contrast to the pressureinduced antiferromagnetic order observed in Tb$_2$Ti$_2$O$_7$.04/2014;  [Show abstract] [Hide abstract]
ABSTRACT: Two critical points have been revealed in the normalstate phase diagram of the electrondoped cuprate superconductor Nd$_{2x}$Ce$_x$CuO$_4$ by exploring the Fermi surface properties of high quality single crystals by highfield magnetotransport. First, the quantitative analysis of the Shubnikovde 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 highfield 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 longrange antiferromagnetic ordering. Thus, for Nd$_{2x}$Ce$_x$CuO$_4$ the superconducting dome appears to be pinned by two critical points to the normal state phase diagram.03/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We have investigated the magnetic and electric ground states of a quasitwodimensional triangular lattice antiferromagnet (TLAF), Ba3CoNb2O9, in which the effective spin of Co2+ is 1/2. At zero field, the system undergoes a twostep 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).  [Show abstract] [Hide abstract]
ABSTRACT: The linear and nonlinear ac susceptibility measurements of Ybpyrochlores, 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 Ybpyrochlores 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.  [Show abstract] [Hide abstract]
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$_{1x}$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 interplanar 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 fieldinduced 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.arXiv:1409.1258. 01/2014; 
Article: Spin dynamics and magnetoelectric properties of the coupledspin tetrahedral compound Cu2Te2O5Cl2
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ABSTRACT: Spin dynamics and magnetoelectric properties of the coupledspin tetrahedral compound Cu2Te2O5Cl2PHYSICAL REVIEW B. 01/2014; 90:054418.  [Show abstract] [Hide abstract]
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 Haasvan Alphen oscillations and the Hall resistivity. The de Haasvan Alphen oscillations below the magnetic ordering temperature T_{N} reveal several twodimensional 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 fieldinduced spin chirality at high magnetic fields.Physical Review Letters 10/2013; 111(17):176405. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present the Fermisurface map of the spinchiral bulk states for the noncentrosymmetric semiconductor BiTeI using de Haasvan Alphen and Shubnikovde Haas oscillations. We identify two distinct Fermi surfaces with a unique spindletorustype topology and the nontrivial Berry phases, confirming the spin chirality with oppositely circulating spintexture. Near the quantum limit at high magnetic fields, we find a substantial Zeeman effect with an effective gfactor of ~ 60 for the Rashbasplit 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 spinpolarized chiral states.06/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We report neutron scattering and ac susceptibility measurements on the pyrochlore Yb2Sn2O7. The appearance of magnetic diffuse scattering, lowenergy spin excitations, and a nearly temperatureindependent relaxation time are signatures that the system enters a shortrangeordered regime with quantum dynamics beneath 2 K. ac susceptibility data further suggest that Yb2Sn2O7 enters a longrange 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  [Show abstract] [Hide abstract]
ABSTRACT: We report a comparative investigation of the magnetostrictive and magnetoconductivity effects in the lowtemperature 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 VV 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 VV distance and the most localized conductivity, with one structural and one magnetic transition. CoV2O4 has the shortest VV distance and approaches the itinerant charge carrier threshold. CoV2O4 has one ferrimagnetic transition and no longrange 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 fielddependent dielectric response for the alteration of the ferrimagnetic spin structure with increasing magnetic field (Fe). These findings can guide future dielectric and magnetoconductancebased spinel studies with a focus on the lowtemperature and high magnetic field properties of canted ferrimagnetic spin configurations and orbitallattice ordering effects.Physical review. B, Condensed matter 02/2013; 87(5). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report the de Haasvan Alphen (dHvA) oscillations and firstprinciple 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 noncollinear 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  [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 metalinsulator transition. Magnetic, transport, and thermodynamic measurements of Fe(Sb_{1x}Te_{x})_{2} single crystals show signatures of disorderinduced nonFermi 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 longrange albeit weak magnetic order (x≥0.075).Physical Review Letters 12/2012; 109(25):256401. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report the magnetic and electric properties of Ba_{3}NiNb_{2}O_{9}, which is a quasitwodimensional spinone triangularlattice 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 upupdown (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 triangularlattice antiferromagnet exhibiting both uud phase and multiferroicity.Physical Review Letters 12/2012; 109(25):257205. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We have investigated the upper critical field Bc2 of the stoichiometric Febased superconductor LiFeAs, via the magnetic torque measurements in dcmagnetic 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 nearlyisotropic superconductivity. The detailed analyses show that, while Bc2 for Ba  c is limited by the orbital effect, the spinparamagnetic effect is evident in the temperature dependence of Bc2 for Ba  ab.Journal of Physics Conference Series 12/2012; 391(1):2133.  [Show abstract] [Hide abstract]
ABSTRACT: The pyrochlore Ho2Ge2O7 is a new highly correlated spin ice material. Physical property measurements, including xray 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 singlecrystal magnetic scattering of Ho2Ge2O7, demonstrating consistency with spin ice behavior.Physical Review B 10/2012; 86(13):134431. · 3.66 Impact Factor  [Show abstract] [Hide abstract]
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 shortrange, antiferromagnetic ordering of Nd3+ along the c axis was attributed to the reorientation of Fe3+ followed by a longrange 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  [Show abstract] [Hide abstract]
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 BT phases (the CDW (0), CDW (x) and metallic states) with large amplitude. Both thermoelectric effects display a resonantlike 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  [Show abstract] [Hide abstract]
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
871  Citations  
281.22  Total Impact Points  
Top Journals
Institutions

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


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
