arXiv subjects

S. -H. Lee

Publications and source records attributed to S. -H. Lee.

At least 145 records · Page 8Linked to original sources

arXiv2007

Quantum spin liquid states in the two dimensional kagome antiferromagnets, ZnxCu4-x(OD)6Cl2

A three-dimensional system of interacting spins typically develops static long-range order when it is cooled. If the spins are quantum (S = 1/2), however, novel quantum paramagnetic states may appear. The most highly sought state among them is the resonating valence bond (RVB) state in which every pair of neighboring quantum spins form entangled spin singlets (valence bonds) and the singlets are quantum mechanically resonating amongst all the possible highly degenerate pairing states. Here we provide experimental evidence for such quantum paramagnetic states existing in frustrated antiferromagnets, ZnxCu4-x(OD)6Cl2, where the S = 1/2 magnetic Cu2+ moments form layers of a two-dimensional kagome lattice. We find that in Cu4(OD)6Cl2, where distorted kagome planes are weakly coupled to each other, a dispersionless excitation mode appears in the magnetic excitation spectrum below ~ 20 K, whose characteristics resemble those of quantum spin singlets in a solid state, known as a valence bond solid (VBS), that breaks translational symmetry. Doping nonmagnetic Zn2+ ions reduces the distortion of the kagome lattice, and weakens the interplane coupling but also dilutes the magnetic occupancy of the kagome lattice. The VBS state is suppressed and for ZnCu3(OD)6Cl2 where the kagome planes are undistorted and 90% occupied by the Cu2+ ions, the low energy spin fluctuations in the spin liquid phase become featureless.

cond-mat.str-el↗

arXiv2006

Magnetic Excitation Spectrum of the Square Lattice S=1/2 Heisenberg Antiferromagnet K2V3O8

We have explored the magnetic excitation spectrum of the S=1/2 square lattice Heisenberg antiferromagnet, K2V3O8 using both triple-axis and time-of-flight inelastic neutron scattering. The long-wavelength spin waves are consistent with the previously determined Hamiltonian for this material. A small energy gap of 72+/-9 micro-eV is observed at the antiferromagnetic zone center and the near-neighbor exchange constant is determined to be 1.08+/-0.03 meV. A finite ferromagnetic interplanar coupling is observed along the crystallographic c-axis with a magnitude of Jc=-0.0036+/-0.006 meV. However, upon approaching the zone boundary, the observed excitation spectrum deviates significantly from the expectation of linear spin wave theory resulting in split modes at the (pi/2,pi/2) zone boundary point. The effects of magnon-phonon interaction, orbital degrees of freedom, multimagnon scattering, and dilution/site randomness are considered in the context of the mode splitting. Unfortunately, no fully satisfactory explanation of this phenomenon is found and further theoretical and experimental work is needed.

cond-mat.str-el↗

arXiv2006

Dielectric properties and lattice dynamics of Ca-doped K$_{0.95}$Li$_{0.05}$TaO$_{3}$

Relaxor behavior and lattice dynamics have been studied for a single crystal of K$_{1-x}$Li$_x$TaO$_3$ $(x=0.05)$, where a small amount of a Ca impurity ($\sim 15$~ppm) was incorporated. The dielectric measurements revealed Debye-like relaxations with Arrhenius activation energies of 80 and 240 meV that are assigned to Li$^+$ dipoles and the Li$^+$-Li$^+$ dipolar pairs, respectively. In the neutron scattering results, diffuse scattering ridges appear around the nuclear Bragg peaks below $\sim 150$ K and phonon line broadening features start to appear at even higher temperatures suggesting that polar nano-regions (PNR's) start to form at these temperatures. These results are supported by the dielectric data that reveal relaxor behavior starting at $\sim 200$ K on cooling. From analyses of the diffuse intensities, the displacements include a uniform phase shift of all of the atoms in addition to the atomic displacements corresponding to a polarization vector of the transverse optic soft ferroelectric mode, a finding that is analogous to that in the prototypical relaxor material Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$.

cond-mat.mtrl-sci↗

arXiv2006

Orbital ordering and enhanced magnetic frustration of strained BiMnO3 thin films

Epitaxial thin films of multiferroic perovskite BiMnO3 were synthesized on SrTiO3 substrates, and orbital ordering and magnetic properties of the thin films were investigated. The ordering of the Mn^{3+} e_g orbitals at a wave vector (1/4 1/4 1/4) was detected by Mn K-edge resonant x-ray scattering. This peculiar orbital order inherently contains magnetic frustration. While bulk BiMnO3 is known to exhibit simple ferromagnetism, the frustration enhanced by in-plane compressive strains in the films brings about cluster-glass-like properties.

cond-mat.mtrl-sci↗

arXiv2005

Nano-magnetic droplets and implications to orbital ordering in La1-xSrxCoO3

Inelastic cold neutron scattering on LaCoO3 provided evidence for a distinct low energy excitation at 0.6 meV coincident with the thermally induced magnetic transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic (AFM) correlations that are dynamic follow the activation to the excited state, identified as the intermediate S=1 spin triplet. This is indicative of dynamical orbital ordering favoring the observed magnetic interactions. With hole doping as in La1-xSrxCoO3, the FM correlations between Co spins become static and isotropically distributed due to the formation of FM droplets. The correlation length and condensation temperature of these droplets increase rapidly with metallicity due to the double exchange mechanism.

cond-mat.str-el↗

arXiv2005

Statics and dynamics of an incommensurate spin order in a geometrically frustrated antiferromagnet CdCr$_2$O$_4$

Using elastic and inelastic neutron scattering we show that a cubic spinel, CdCr$_2$O$_4$, undergoes an elongation along the c-axis ($c > a = b$) at its spin-Peierls-like phase transition at $T_N$ = 7.8 K. The Néel phase ($T < T_N$) has an incommesurate spin structure with a characteristic wave vector \textbf{Q}$_M$ = (0,$δ$,1) with $δ\sim$ 0.09 and with spins lying on the $ac$-plane. This is in stark contrast to another well-known Cr-based spinel, ZnCr$_2$O$_4$, that undergoes a c-axis contraction and a commensurate spin order. The magnetic excitations of the incommensurate Néel state has a weak anisotropy gap of 0.6 meV and it consists of at least three bands extending up to 5 meV.

cond-mat.str-el↗

arXiv2005

Spin chirality on a two-dimensional frustrated lattice

The collective behavior of interacting magnetic moments can be strongly influenced by the topology of the underlying lattice. In geometrically frustrated spin systems, interesting chiral correlations may develop that are related to the spin arrangement on triangular plaquettes. We report a study of the spin chirality on a two-dimensional geometrically frustrated lattice. Our new chemical synthesis methods allow us to produce large single crystal samples of KFe3(OH)6(SO4)2, an ideal Kagome lattice antiferromagnet. Combined thermodynamic and neutron scattering measurements reveal that the phase transition to the ordered ground-state is unusual. At low temperatures, application of a magnetic field induces a transition between states with different non-trivial spin-textures.

cond-mat.str-el↗

arXiv2005

Quantum Critical Scaling and origin of Non-Fermi-Liquid behavior in Sc$_{1-x}$U$_{x}$Pd$_3$

We use inelastic neutron scattering to study magnetic excitations of Sc$_{1-x}$U$_{x}$Pd$_3$ for U concentrations ($x=0.25$, 0.35) near the spin glass quantum critical point (QCP). The excitations are spatially incoherent, broad in energy ($E=\hbarω$), and follow $ω/T$ scaling at all wave vectors investigated. Since similar $ω/T$ scaling has been observed for UCu$_{5-x}$Pd$_x$ and CeCu$_{6-x}$Au$_{x}$ near the antiferromagnetic (AF) QCP, we argue that the observed non Fermi liquid (NFL) behavior in these $f$-electron materials arises from the critical phenomena near a T=0 K phase transition, irrespective of the nature of the transition.

cond-mat.str-el↗

arXiv2004

Orbital and spin chains in ZnV2O4

Our powder inelastic neutron scattering data indicate that \zvo is a system of spin chains that are three dimensionally tangled in the cubic phase above 50 K due to randomly occupied $t_{2g}$ orbitals of V$^{3+}$ ($3d^2$) ions. Below 50 K in the tetragonal phase, the chains become straight due to antiferro-orbital ordering. This is evidenced by the characteristic wave vector dependence of the magnetic structure factor that changes from symmetric to asymmetric at the cubic-to-tetragonal transition.

cond-mat.str-el↗

arXiv2004

"Spin-Flop" Transition and Anisotropic Magnetoresistance in Pr_{1.3-x}La_{0.7}Ce_{x}CuO_{4}: Unexpectedly Strong Spin-Charge Coupling in Electron-Doped Cuprates

We use transport and neutron-scattering measurements to show that a magnetic-field-induced transition from noncollinear to collinear spin arrangement in adjacent CuO_{2} planes of lightly electron-doped Pr_{1.3-x}La_{0.7}Ce_{x}CuO_{4} (x=0.01) crystals affects significantly both the in-plane and out-of-plane resistivity. In the high-field collinear state, the magnetoresistance (MR) does not saturate, but exhibits an intriguing four-fold-symmetric angular dependence, oscillating from being positive at B//[100] to being negative at B//[110]. The observed MR of more than 30% at low temperatures induced by a modest modification of the spin structure indicates an unexpectedly strong spin-charge coupling in electron-doped cuprates.

cond-mat.supr-con↗

arXiv2004

Cold neutron scattering study on diffuse and phonon excitations in the relaxor PbMg(1/3)Nb(2/3)O3

Cold neutron scattering experiments have been performed to explore the energy, temperature, and wave-vector dependence of the diffuse scattering and the transverse acoustic (TA) phonons in the relaxor PbMg(1/3)Nb(2/3)O3. We have observed a weak, but definitive, diffuse scattering cross section above the Burns temperature T_d ~ 600 K. This cross section, which is most likely caused by chemical short-range order, persists down to 100 K, and coexists with the much stronger diffuse scattering that is attributed to the polar nanoregions. A systematic study of the TA phonon around (1, 1, 0) has also been carried out. The phonon is well defined for small wave vectors q, but broadens markedly around q = (0.1, -0.1, 0).

cond-mat.mtrl-sci↗

arXiv2004

Polarized Neutron Measurement of Magnetic Order in YBa2Cu3O6.45

Orbital current order of the d-density-wave type (DDW) has been postulated to explain the pseudogap in the high temperature superconductors. We have performed neutron scattering experiments to search for this order and show here the results obtained on an YBa2Cu3O6.45 sample using the best neutron spectrometers available. We argue that the data are consistent with a small, largely c-axis-directed moment, found below about 200K.

cond-mat.supr-con↗

arXiv2003

Continuum in the spin excitation spectrum of a Haldane chain (erratum)

There was a slight error in calculating the nonlinear sigma-model prediction for the 2-magnon scattering continuum shown in Fig. 2 of cond-mat/0010049, and Phys. Rev. Lett. {\bf 87}, 017202 (2001). Present erratum explains what was incorrect, and shows a corrected version of the figure. The correction is minor and does not affect main conclusions drawn in the original paper.

cond-mat.str-el↗

arXiv2003

Unconventional spin fluctuations in the hexagonal antiferromagnet YMnO$_3$

We used inelastic neutron scattering to show that well below its Néel temperature, $T_{\rm N}$, the two-dimensional (2D) XY nearly-triangular antiferromagnet YMnO$_{3}$ has a prominent {\it central peak} associated with 2D antiferromagnetic fluctuations with a characteristic life time of 0.55(5) ps, coexisting with the conventional long-lived spin-waves. Existence of the two time scales suggests competition between the Néel phase favored by weak interplane interactions, and the Kosterlitz-Thouless phase intrinsic to the 2D XY spin system.

cond-mat.mtrl-sci↗

arXiv2003

Effect of a magnetic field on long-range magnetic order in stage-4 and stage-6 superconducting La2CuO(4+y)

We have measured the enhancement of the static incommensurate spin-density wave (SDW) order by an applied magnetic field in stage-4 and stage-6 samples of superconducting La2CuO(4+y). We show that the stage-6 La2CuO(4+y) (Tc=32 K) forms static long-range SDW order with the same wave-vector as that in the previously studied stage-4 material. We have measured the field dependence of the SDW magnetic Bragg peaks in both stage-4 and stage-6 materials at fields up to 14.5 T. A recent model of competing SDW order and superconductivity describes these data well.

cond-mat.str-el↗

arXiv2003

Effect of a magnetic field on the spin- and charge-density wave order in La1.45Nd0.4Sr0.15CuO4

The spin-density wave (SDW) and charge-density wave (CDW) order in superconducting La1.45Nd0.4Sr0.15CuO4 were studied under an applied magnetic field using neutron and X-ray diffraction techniques. In zero field, incommensurate (IC) SDW order appears below ~ 40 K, which is characterized by neutron diffraction peaks at (1/2 +/- 0.134, 1/2 +/- 0.134, 0). The intensity of these IC peaks increases rapidly below T_Nd ~ 8 K due to an ordering of the Nd^3+ spins. The application of a 1 T magnetic field parallel to the c-axis markedly diminishes the intensity below T_Nd, while only a slight decrease in intensity is observed at higher temperatures for fields up to 7 T. Our interpretation is that the c-axis field suppresses the parasitic Nd^3+ spin order at the incommensurate wave vector without disturbing the stripe order of Cu^2+ spins. Consistent with this picture, the CDW order, which appears below 60 K, shows no change for magnetic fields up to 4 T. These results stand in contrast to the significant field-induced enhancement of the SDW order observed in superconducting La2-xSrxCuO4 with x ~ 0.12 and stage-4 La2CuO4+y. The differences can be understood in terms of the relative volume fraction exhibiting stripe order in zero field, and the collective results are consistent with the idea that suppression of superconductivity by vortices nucleates local patches of stripe order.

cond-mat.supr-con↗

arXiv2002

Commensurate spin dynamics in the superconducting state of an electron-doped cuprate superconductor

We report neutron scattering studies on single crystals of the electron-doped (n-type) superconducting cuprate Nd2-xCexCuO4 (x=0.15) with Tc = 18 K and 25 K. Unlike the hole-doped (p-type) superconducting cuprates where incommensurate magnetic fluctuations commonly exist, the n-type cuprate shows commensurate magnetic fluctuations at the tetragonal (1/2 1/2 0) reciprocal points both in the superconducting and in the normal state. A spin gap opens up when the n-type cuprate becomes superconducting, as in the optimally doped p-type La2-xSrxCuO4. The gap energy, however, increases gradually up to about 4 meV as T decreases from Tc to 2 K, which contrasts with the spin pseudogap behavior with a T-independent gap energy in the superconducting state of p-type cuprates.

cond-mat.str-el↗

arXiv2002

Emergent excitations in a geometrically frustrated magnet

Frustrated systems are ubiquitous and interesting because their behavior is difficult to predict. Magnetism offers extreme examples in the form of spin lattices where all interactions between spins cannot be simultaneously satisfied. Such geometrical frustration leads to macroscopic degeneracies, and offers the possibility of qualitatively new states of matter whose nature has yet to be fully understood. Here we have discovered how novel composite spin degrees of freedom can emerge from frustrated interactions in the cubic spinel ZnCr2O4. Upon cooling, groups of six spins self-organize into weakly interacting antiferromagnetic loops whose directors, defined as the unique direction along which the spins are aligned parallel or antiparallel, govern all low temperature dynamics. The experimental evidence comes from a measurement of the magnetic form factor by inelastic neutron scattering. While the data bears no resemblance to the atomic form factor for chromium, they are perfectly consistent with the form factor for hexagonal spin loop directors. The hexagon directors are to a first approximation decoupled from each other and hence their reorientations embody the long-sought local zero energy modes for the pyrochlore lattice.

cond-mat.str-el↗