Publications (144)382.48 Total impact
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ABSTRACT: The specific heat, the susceptibility under pressure, and the dielectric constant were measured for single crystals ${\mathrm{Y}}_{1$${}x}{\mathrm{La}}_{x}{\mathrm{TiO}}_{3}$. The observed ${T}^{2}$dependent specific heat at low temperatures for $0.17$\le${}x$\le${}0.3$ samples shows a spinorbital liquid state between the ferromagnetic/orbital ordering $(x<0.17)$ and antiferromagnetic/possible orbital liquid phase $(x>0.3)$. The nonmonotonous pressure dependence of ${T}_{\text{C}}$ and the glassy behavior of the dielectric loss for the $x=0.23$ sample suggest that it is approaching a possible quantum critical point. All these properties result from the coupling between the strong spin and orbital fluctuations while approaching the phase boundary.Physical Review B 04/2015; 91(16):161106. DOI:10.1103/PhysRevB.91.161106 · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: High quality single crystals of BaFe$_{12}$O$_{19}$ were grown using the floating zone technique in flowing oxygen pressurized to 100 atm. Single crystal neutron diffraction was used to determine the nuclear and magnetic structure of BaFe$_{12}$O$_{19}$ at 4 K and 295 K. At both temperatures, there exist local electric dipoles formed by the offmirrorplane displacements of magnetic Fe$^{3+}$ ions at the bipyramidal sites. The displacement at 4 K is about half of that at room temperature. The temperature dependence of the specific heat shows no anomaly associated with long range polar ordering in the temperature range from 1.90300 K. The inverse dielectric permittivity, $1/\varepsilon$, along the caxis shows a $T^2$ temperature dependence between 10 K and 20 K, with a significantly reduced temperature dependence displayed below 10 K. Moreover, as the sample is cooled below 1.4 K there is an anomalous sharp upturn in $1/\varepsilon$. These features resemble those of classic quantum paraelectrics such as SrTiO$_3$. The presence of the upturn in $1/\varepsilon$ indicates that BaFe$_{12}$O$_{19}$ is a critical quantum paraelectric system with Fe$^{3+}$ ions involved in both magnetic and electric dipole formation.  [Show abstract] [Hide abstract]
ABSTRACT: We studied the magnetic properties of singlecrystal Yb2V2O7 using dc and ac susceptibility measurements, elastic and inelastic neutronscattering measurements, and linear spinwave theory. The experimental data show a ferromagnetic ordering of V4+ ions at 70 K, a shortrange ordering of Yb3+ ions below 40 K, and finally a longrange noncollinear ordering of Yb3+ ions below 15 K. With external magnetic field oriented along the [111] axis, the Yb sublattice experiences a spin flop transition related to the “threein oneout” spin structure. By modeling the spinwave excitations, we extract the Hamiltonian parameters. Our results confirm that although the extra intersublattice YbV interactions dramatically increase the Yb ordering temperature to 15 K, the intrasublattice YbYb interactions, based on the pyrochlore lattice, still stabilize the Yb ions' noncollinear spin structure and spin flop transition.Physical review. B, Condensed matter 02/2015; 91(6):064425. DOI:10.1103/PhysRevB.91.064425 · 3.66 Impact Factor  [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$.Physical Review Letters 12/2014; 113:267205. DOI:10.1103/PhysRevLett.113.267205 · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: After nearly 20 years of study, the origin of the spinliquid state in Tb_{2}Ti_{2}O_{7} remains a challenge for experimentalists and theorists alike. To improve our understanding of the exotic magnetism in Tb_{2}Ti_{2}O_{7}, we synthesize a chemical pressure analog: Tb_{2}Ge_{2}O_{7}. Substitution of titanium by germanium results in a lattice contraction and enhanced exchange interactions. We characterize 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 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 liquidlike correlations dominate in this system at 3.5 K. However, below 1 K, the liquidlike correlations give way to intense shortrange ferromagnetic correlations with a length scale similar to the TbTb nearest neighbor 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}.Physical Review Letters 12/2014; 113(26):267205. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report a de Haasvan Alphen (dHvA) oscillation study on IrTe2 single crystals showing complex dimer formations. By comparing the angle dependence of dHvA oscillations with band structure calculations, we show distinct Fermi surface reconstruction induced by a 1/5type and a 1/8type dimerizations. This verifies that an intriguing quasitwodimensional conducting plane across the layers is induced by dimerization in both cases. A phase transition to the 1/8 phase with higher dimer density reveals that local instabilities associated with intra and interdimer couplings are the main driving force for complex dimer formations in IrTe2.Physical Review Letters 12/2014; 113(26). DOI:10.1103/PhysRevLett.113.266406 · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The axial anomaly leads to the violation of separate number conservation laws for left and righthanded massless chiral (or Weyl) fermions. For a certain class of gapless semiconductors, for which the lowenergy band structure is described in terms of Weylfermions, the application of a magnetic field parallel to the electrical current is predicted to induce a large suppression of the electrical resistivity. To date, there is no concrete experimental realization of a Weyl semimetal or unambiguous evidence for this fieldinduced phenomenon. Here, we report the observation of a very large negative magnetoresistance in the extremely clean quasitwodimensional metal PdCoO$_2$. Our experimental study provides strong support for a scenario where this unconventional response results from the axial anomaly of fieldinduced quasionedimensional conduction channels. The observation of this effect in PdCoO$_2$ demonstrates that the axial anomaly is a general feature of the longitudinal magnetotransport of clean and weakly correlated threedimensional metals.  [Show abstract] [Hide abstract]
ABSTRACT: We have performed magnetic, electric, thermal, and neutron powder diffraction (NPD) experiments as well as density functional theory (DFT) calculations on Ba3MnNb2O9. All results suggest that Ba3MnNb2O9 is a spin5/2 triangular lattice antiferromagnet (TLAF) with weak easyaxis anisotropy. At zero field, we observed a narrow twostep transition at TN1 = 3.4 K and TN2 = 3.0 K. The neutron diffraction measurement and the DFT calculation indicate a 120◦ spin structure in the ab plane with outofplane canting at low temperatures. With increasing magnetic field, the 120◦ spin structure evolves into upupdown (uud) and oblique phases showing successive magnetic phase transitions, which fits well to the theoretical prediction for the 2D Heisenberg TLAF with classical spins. Multiferroicity is observed when the spins are not collinear but suppressed in the uud and oblique phases.Physical Review B 12/2014; 90(22):224402. DOI:10.1103/PhysRevB.90.224402 · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report the valleyselective interlayer conduction of SrMnBi_{2} under inplane magnetic fields. The caxis resistivity of SrMnBi_{2} shows clear angular magnetoresistance oscillations indicating coherent interlayer conduction. Strong fourfold variation of the coherent peak in the caxis resistivity reveals that the contribution of each Dirac valley is significantly modulated by the inplane field orientation. This originates from anisotropic Dirac Fermi surfaces with strong disparity in the momentumdependent interlayer coupling. Furthermore, we found a signature of broken valley symmetry at high magnetic fields. These findings demonstrate that a quasitwodimensional anisotropic Dirac system can host a valleypolarized interlayer current through magnetic valley control.Physical Review Letters 10/2014; 113(15):156602. DOI:10.1103/PhysRevLett.113.156602 · 7.73 Impact Factor  [Show abstract] [Hide abstract]
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.Physical Review B 09/2014; 90(10):104412. DOI:10.1103/PhysRevB.90.104412 · 3.66 Impact Factor  [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.Physical Review B 09/2014; 90:144429. DOI:10.1103/PhysRevB.90.144429 · 3.66 Impact Factor 
Article: Spin dynamics and magnetoelectric properties of the coupledspin tetrahedral compound Cu2Te2O5Cl2
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ABSTRACT: We report on the spin dynamics and discovery of magnetoelectricity in the coupledspin tetrahedral compound Cu2Te2O5Cl2. Te125 NMR measurements show an anomalous resonance frequency shift and a signal wipeout phenomenon around the Neel temperature TN = 18.2 K, which could be attributed to the anomalous critical slowing down of the Cu spin fluctuations on the NMR time scale (similar to 10100 MHz). The critical exponent of (T1T)(1) proportional to (T  TN)(alpha) is 0.40 +/ 0.03, as compared to 0.5 for a threedimensional meanfield model. This is in contrast to the Br compound [S.H. Baek et al., Phys. Rev. B 86, 180405 (2012)], which exhibits pronounced singlet dynamics with a large spin gap. Electric polarization (Pc) is observed along the c axis for temperatures below TN under finite magnetic field but not sensitive to the electric poling. Pc increases sharply over zero to 2T and then reaches saturation. Below TN, Pc changes its sign depending on the applied magnetic field direction, positive for the H perpendicular to c axis and negative for H parallel to c axis. We discuss possible explanations for the observed magnetoelectric (ME) behavior in terms of linear ME effect, spindriven multiferroicity, and an exchange striction of intertetrahedral exchange paths involving the Te4+ lonepair ions. Our results suggest that Cu2Te2O5Cl2 is a type of ME material whose properties are tuned by intertetrahedral exchange interactions involving polarizable Te4+ ions.Physical Review B 08/2014; 90(5):054418. DOI:10.1103/PhysRevB.90.054418 · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A layered perovskite LaSrVO 4 was studied by neutron diffraction, pair distribution function measurement using synchrotron xray, susceptibility, and specific heat measurements, and firstprinciples calculation. The results show (i) a weak structural distortion around 100 K with the existence of orbital fluctuations both above and below it; (ii) the absence of the long range magnetic ordering down to 0.35 K but the appearance of a short range magnetic ordering around 11 K with a T 2 behavior of the specific heat below it. Meanwhile, the calculation based on the density functional theory predicts a magnetic ordered ground state. All facts indicate a melting of the magnetic ordering due to the orbital fluctuations in LaSrVO 4 , which makes it a rare candidate for the spinorbital liquid state related to t 2g orbitals.Physical Review B 06/2014; 89(23). DOI:10.1103/PhysRevB.89.235131 · 3.66 Impact Factor  [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.  [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].Physical Review B 02/2014; 89(10). DOI:10.1103/PhysRevB.89.104420 · 3.66 Impact Factor  [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.Physical Review B 02/2014; 89(6):064401. DOI:10.1103/PhysRevB.89.064401 · 3.66 Impact Factor  [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. DOI:10.1103/PhysRevLett.111.176405 · 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.  [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. DOI:10.1103/PhysRevB.87.134408 · 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). DOI:10.1103/PhysRevB.87.054432 · 3.66 Impact Factor
Publication Stats
1k  Citations  
382.48  Total Impact Points  
Top Journals
Institutions

2000–2015

Florida State University
 • Department of Physics
 • National High Magnetic Field Laboratory
Tallahassee, Florida, United States


2003–2014

National High Magnetic Field Laboratory
Tallahassee, Florida, United States


2005

Okayama University
Okayama, Okayama, Japan


1995–2004

Korea Research Institute of Bioscience and Biotechnology KRIBB
 Laboratory of Microbial and Bioprocess Engineering
Anzan, Gyeonggi Province, South Korea 
Korea Institute of Science and Technology
Sŏul, Seoul, South Korea 
Institute for Genetic Engineering
Maddham Gram, West Bengal, India


2001–2002

Ewha Womans University
 Department of Physics
Seoul, Seoul, South Korea
