[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 spin-orbital 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 · 3.74 Impact Factor
[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 off-mirror-plane 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 c-axis 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 single-crystal Yb2V2O7 using dc and ac susceptibility measurements, elastic and inelastic neutron-scattering measurements, and linear spin-wave theory. The experimental data show a ferromagnetic ordering of V4+ ions at 70 K, a short-range ordering of Yb3+ ions below 40 K, and finally a long-range 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 “three-in one-out” spin structure. By modeling the spin-wave excitations, we extract the Hamiltonian parameters. Our results confirm that although the extra inter-sublattice Yb-V interactions dramatically increase the Yb ordering temperature to 15 K, the intra-sublattice Yb-Yb 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 long-range order in
Tb$_2$Ge$_2$O$_7$ down to 20 mK. The Curie-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 at 3.5 K liquid-like correlations dominate in
this system. However, below 1 K, the liquid-like correlations give way to
intense short-range ferromagnetic correlations with a length scale related to
the Tb-Tb distance. Despite stronger antiferromagnetic exchange, the ground
state of Tb$_2$Ge$_2$O$_7$ has ferromagnetic character, in stark contrast to
the pressure-induced 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 spin-liquid 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 long-range 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 short-range ferromagnetic correlations with a length scale similar to the Tb-Tb 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 pressure-induced antiferromagnetic order observed in Tb_{2}Ti_{2}O_{7}.
[Show abstract][Hide abstract] ABSTRACT: We report a de Haas-van 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 1/5-type and a 1/8-type dimerizations. This
verifies that an intriguing quasi-two-dimensional 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 right-handed massless chiral (or Weyl-) fermions. For a certain
class of gapless semiconductors, for which the low-energy band structure is
described in terms of Weyl-fermions, 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 semi-metal or unambiguous evidence for this field-induced
phenomenon. Here, we report the observation of a very large negative
magnetoresistance in the extremely clean quasi-two-dimensional metal PdCoO$_2$.
Our experimental study provides strong support for a scenario where this
unconventional response results from the axial anomaly of field-induced
quasi-one-dimensional 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 three-dimensional
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
spin-5/2 triangular lattice antiferromagnet (TLAF) with weak easy-axis anisotropy. At zero field, we observed a
narrow two-step 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 out-of-plane canting at low temperatures. With
increasing magnetic field, the 120◦ spin structure evolves into up-up-down (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 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report the valley-selective interlayer conduction of SrMnBi_{2} under in-plane magnetic fields. The c-axis resistivity of SrMnBi_{2} shows clear angular magnetoresistance oscillations indicating coherent interlayer conduction. Strong fourfold variation of the coherent peak in the c-axis resistivity reveals that the contribution of each Dirac valley is significantly modulated by the in-plane field orientation. This originates from anisotropic Dirac Fermi surfaces with strong disparity in the momentum-dependent interlayer coupling. Furthermore, we found a signature of broken valley symmetry at high magnetic fields. These findings demonstrate that a quasi-two-dimensional anisotropic Dirac system can host a valley-polarized interlayer current through magnetic valley control.
[Show abstract][Hide abstract] ABSTRACT: We have carried out high magnetic field studies of single-crystalline
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 meta-magnetic transitions
around 9 T fields applied perpendicular to the ab-plane (along the c*-axis).
These phase transitions are completely absent in the magnetization measured
with field applied along the ab-plane. In order to understand this magnetic
phase diagram, we developed a mean-field model starting from the correct
Neel-type magnetic structure, consistent with our single crystal neutron
diffraction data at zero field. Our model calculations succeeded in explaining
the two meta-magnetic transitions that arise when Li$_2$MnO$_3$ enters two
different spin-flop phases from the zero field Neel phase.
Physical Review B 09/2014; 90(10):104412. DOI:10.1103/PhysRevB.90.104412 · 3.74 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$_{1-x}$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 inter-planar 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 field-induced 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 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report on the spin dynamics and discovery of magnetoelectricity in the coupled-spin tetrahedral compound Cu2Te2O5Cl2. Te-125 NMR measurements show an anomalous resonance frequency shift and a signal wipe-out phenomenon around the Neel temperature T-N = 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 10-100 MHz). The critical exponent of (T1T)(-1) proportional to (T - T-N)(-alpha) is 0.40 +/- 0.03, as compared to 0.5 for a three-dimensional mean-field 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 (P-c) is observed along the c axis for temperatures below T-N under finite magnetic field but not sensitive to the electric poling. P-c increases sharply over zero to 2T and then reaches saturation. Below T-N, P-c 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, spin-driven multiferroicity, and an exchange striction of intertetrahedral exchange paths involving the Te4+ lone-pair 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 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A layered perovskite LaSrVO 4 was studied by neutron diffraction, pair distribution function measurement using synchrotron x-ray, susceptibility, and specific heat measurements, and first-principles 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 spin-orbital liquid state related to t 2g orbitals.
Physical Review B 06/2014; 89(23). DOI:10.1103/PhysRevB.89.235131 · 3.74 Impact Factor
[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 normal-state phase diagram of
the electron-doped cuprate superconductor Nd$_{2-x}$Ce$_x$CuO$_4$ by exploring
the Fermi surface properties of high quality single crystals by high-field
magnetotransport. First, the quantitative analysis of the Shubnikov-de 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 high-field 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
long-range antiferromagnetic ordering. Thus, for Nd$_{2-x}$Ce$_x$CuO$_4$ the
superconducting dome appears to be pinned by two critical points to the normal
state phase diagram.
Physical Review B 03/2014; 92(9). DOI:10.1103/PhysRevB.92.094501 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have investigated the magnetic and electric ground states of a quasi-two-dimensional triangular lattice antiferromagnet (TLAF), Ba3CoNb2O9, in which the effective spin of Co2+ is 1/2. At zero field, the system undergoes a two-step 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 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The linear and nonlinear ac susceptibility measurements of Yb-pyrochlores, 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 Yb-pyrochlores 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 · 3.74 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 Haas-van Alphen oscillations and the Hall resistivity. The de Haas-van Alphen oscillations below the magnetic ordering temperature T_{N} reveal several two-dimensional 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 field-induced spin chirality at high magnetic fields.
[Show abstract][Hide abstract] ABSTRACT: We present the Fermi-surface map of the spin-chiral bulk states for the
non-centrosymmetric semiconductor BiTeI using de Haas-van Alphen and
Shubnikov-de Haas oscillations. We identify two distinct Fermi surfaces with a
unique spindle-torus-type topology and the non-trivial Berry phases, confirming
the spin chirality with oppositely circulating spin-texture. Near the quantum
limit at high magnetic fields, we find a substantial Zeeman effect with an
effective g-factor of ~ 60 for the Rashba-split 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 spin-polarized
chiral states.