Publications (124)345.23 Total impact
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ABSTRACT: Results from detailed investigations of the magnetic properties of a powder sample of βCoPc for the temperatures T=0.4 K to 300 K and in magnetic fields H up to 90 kOe are reported. Xray diffraction confirmed the βphase and scanning electron microscopy showed platelike morphology of the sample. For T>3 K, the data of magnetic susceptibility χ vs. T fit the CurieWeiss (CW) law yielding θ= 2.5 K, µ=2.16 µB per Co2+ and g=2.49 for spin S=1/2 of the low spinstate of Co2+. However for T<3 K, the χ vs. T data deviates from the CW law yielding a peak in χ at Tmax=1.9 K. It is shown that the χ vs. T data from 0.4 K to 300 K fits well with the predictions of the BonnerFisher (BF) model for S=1/2 Heisenberg linear chain antiferromagnet with the Co2+Co2+ exchange J/kB=−1.5 K (Ĥ=2J Σ Si•Si+1). The data of magnetization M vs. H at T=1 K agrees with the predictions of the BF model with J/kB=−1.5 K, yielding saturation magnetization MS=12.16 emu/g above 60 kOe corresponding to complete alignment of the spins.  [Show abstract] [Hide abstract]
ABSTRACT: Polycrystalline samples of the quarterdoped manganites $R_{0.75}$Ca$_{0.25}$MnO$_3$ ($R$ = Y, Tb, Dy, Ho, and Er) were studied by Xray diffraction and AC/DC susceptibility measurements. All five samples are orthorhombic and exhibit similar magnetic properties: enhanced ferromagnetism below $T_1$ ($\sim80$ K) and a spin glass (SG) state below $T_{SG}$ ($\sim30$ K). With increasing $R^{3+}$ ionic size, both $T_1$ and $T_{SG}$ generally increase. The single crystal neutron diffraction results on Tb$_{0.75}$Ca$_{0.25}$MnO$_3$ revealed that the SG state is mainly composed of a shortrange ordered version of a novel canted (i.e. noncollinear) antiferromagnetic spin state. Furthermore, calculations based on the double exchange model for quarterdoped manganites reveal that this new magnetic phase provides a transition state between the ferromagnetic state and the theoretically predicted spinorthogonal stripe phase.  [Show abstract] [Hide abstract]
ABSTRACT: We report the pressureinduced topological quantum phase transition of BiTeI single crystals using Shubnikovde Haas oscillations of bulk Fermi surfaces. The sizes of the inner and the outer FSs of the Rashbasplit bands exhibit opposite pressure dependence up to P = 3.35 GPa, indicating pressuretunable Rashba effect. Above a critical pressure P ∼ 2 GPa, the Shubnikovde Haas frequency for the inner Fermi surface increases unusually with pressure, and the Shubnikovde Haas oscillations for the outer Fermi surface shows an abrupt phase shift. In comparison with band structure calculations, we find that these unusual behaviors originate from the Fermi surface shape change due to pressureinduced band inversion. These results clearly demonstrate that the topological quantum phase transition is intimately tied to the shape of bulk Fermi surfaces enclosing the timereversal invariant momenta with band inversion.  [Show abstract] [Hide abstract]
ABSTRACT: Elastic neutron scattering, ac susceptibility, and specific heat experiments on the pyrochlores Er$_{2}$Ge$_{2}$O$_{7}$ and Yb$_{2}$Ge$_{2}$O$_{7}$ show that both systems are antiferromagnetically ordered in the $\Gamma_5$ manifold. The ground state is a $\psi_{3}$ phase for the Er sample and a $\psi_{2}$ or $\psi_{3}$ phase for the Yb sample, which suggests "Order by Disorder"(ObD) physics. Furthermore, we unify the various magnetic ground states of all known R$_{2}$B$_{2}$O$_{7}$ (R = Er, Yb, B = Sn, Ti, Ge) compounds through the enlarged XY type exchange interaction $J_{\pm}$ under chemical pressure. The mechanism for this evolution is discussed in terms of the phase diagram proposed in the theoretical study [Wong et al., Phys. Rev. B 88, 144402, (2013)].  [Show abstract] [Hide abstract]
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.  [Show abstract] [Hide abstract]
ABSTRACT: High quality single crystals of BaFe12O19 were grown using the floating zone technique in 100 atm of flowing oxygen. Single crystal neutron diffraction was used to determine the nuclear and magnetic structures of BaFe12O19 at 4 K and 295 K. At both temperatures, there exist local electric dipoles formed by the offmirrorplane displacements of magnetic Fe3+ 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 1.90 to 300 K. The inverse dielectric permittivity, 1/ε, 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/ε. These features resemble those of classic quantum paraelectrics such as SrTiO3. The presence of the upturn in 1/ε indicates that BaFe12O19 is a critical quantum paraelectric system with Fe3+ 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.  [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$.  [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}.  [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.  [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.  [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.  [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.  [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. 
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.  [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.  [Show abstract] [Hide abstract]
ABSTRACT: The hysteresis loss (Qhyst) of a conductor is a very important parameter for magnet design. Due to the anisotropy of the REBCO conductor, its Qhyst strongly depends on magnetic field orientation. During a field ramp of a REBCO magnet, at locations where radial field component is significant, Qhyst may be large. This may significantly reduce the design temperature margin and increase the liquid helium consumption. Therefore, it is very important to measure the field and field angular dependence of Qhyst of the REBCO conductor. In this paper, we measured Qhyst of a small REBCO sample by vibrating sample magnetometry. The measurements were performed in a resistive magnet at the NHMFL with magnetic fields up to 35 T. The magnetization of a 1×1 mm2 sample was measured in different fields, field orientations, and temperatures. The implication of our experimental results on the REBCO magnet design is discussed.  [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].
Publication Stats
1k  Citations  
345.23  Total Impact Points  
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Institutions

20002016

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


20032014

National High Magnetic Field Laboratory
Tallahassee, Florida, United States


20012002

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