arXiv subjects

F. C. Chou

Publications and source records attributed to F. C. Chou.

At least 19 recordsLinked to original sources

arXiv2020

Magnetic properties and magnetic structure of the frustrated quasi-one-dimensional antiferromagnet SrCuTe$_2$O$_6$

Magnetization measurements on single-crystal cubic SrCuTe$_2$O$_6$ with an applied magnetic field of along three inequivalent high symmetry directions $[100]$, $[110]$, and $[111]$ reveal weak magnetic anisotropy. The fits of the magnetic susceptibility to the result from a quantum Monte Carlo simulation on the Heisenberg spin-chain model, where the chain is formed via the dominant third-nearest-neighbor exchange interaction $J_3$, yield the intra-chain interaction $(J_3/k_B)$ between 50.12(7) K for the applied field along $[110]$ and 52.5(2) K along $[100]$ with about the same $g$-factor of 2.2. Single-crystal neutron diffraction unveils the transition to the magnetic ordered state as evidenced by the onset of the magnetic Bragg intensity at $T_\textrm{N1}=5.25(9)$ K with no anomaly of the second transition at $T_\textrm{N2}$ reported previously. Based on irreducible representation theory and magnetic space group analysis of powder and single-crystal neutron diffraction data, the magnetic structure in the Shubnikov space group $P4_132$, where the Cu$^{2+}$~$S=1/2$ spins antiferromagnetically align in the direction perpendicular to the spin chain, is proposed. The measured ordered moment of $0.52(6)~μ_B$, which represents 48% reduction from the expected value of $1~μ_B$, suggests the remaining influence of frustration resulting from the $J_1$ and $J_2$ bonds.

cond-mat.str-el↗

arXiv2020

Spin-Orbit Exciton in a Honeycomb Lattice Magnet CoTiO$_3$: Revealing Link Between Rare Earth and Transition Metal Magnetism

We carried out inelastic neutron scattering to study the spin-orbital (SO) exciton in a single crystal sample of CoTiO$_3$ as a function of temperature. CoTiO$_3$ is a honeycomb magnet with dominant XY-type magnetic interaction and an A-type antiferromagnetic order below $\mathrm{T_N} \approx 38$~K. We found that the SO exciton becomes softer, but acquires a larger bandwidth in the paramagnetic phase, compared to that in the magnetically ordered phase. Moreover, an additional mode is only observed in the intermediate temperature range, as the sample is warmed up above the lowest accessible temperature below $\mathrm{T_N}$. Such an unusual temperature dependence observed in this material suggests that its ground states (an $S_{\mathrm{eff}}=\frac{1}{2}$ doublet) and excited states multiplets are strongly coupled, and therefore cannot be treated independently, as often done in a pseudo-spin model. Our observations can be explained by a multi-level theory within random phase approximation that explicitly takes into account both the ground and excited multiplets. The success of our theory, which is originally developed to explain temperature dependence of magnetic excitations in the rare-earth magnets, highlight the similarity between the magnetic excitations in rare-earth systems and those in transition metal systems with strong spin orbit coupling.

cond-mat.str-el↗

arXiv2020

Magnetic and orbital correlations in multiferroic CaMn$_7$O$_{12}$ probed by x-ray resonant elastic scattering

The quadruple perovskite CaMn$_7$O$_{12}$ is a topical multiferroic, in which the hierarchy of electronic correlations driving structural distortions, modulated magnetism, and orbital order is not well known and may vary with temperature. x-ray resonant elastic scattering (XRES) provides a momentum-resolved tool to study these phenomena, even in very small single crystals, with valuable information encoded in its polarization- and energy-dependence. We present an application of this technique to CaMn$_7$O$_{12}$. By polarization analysis, it is possible to distinguish superstructure reflections associated with magnetic order and orbital order. Given the high momentum resolution, we resolve a previously unknown splitting of an orbital order superstructure peak, associated with a distinct \textit{locked-in} phase at low temperatures. A second set of orbital order superstructure peaks can then be interpreted as a second-harmonic orbital signal. Surprisingly, the intensities of the first- and second-harmonic orbital signal show disparate temperature and polarization dependence. This orbital re-ordering may be driven by an exchange mechanism, that becomes dominant over the Jahn-Teller instability at low temperature.

cond-mat.mtrl-sci↗

arXiv2019

Dirac magnons in a honeycomb lattice quantum XY magnet CoTiO3

The discovery of massless Dirac electrons in graphene and topological Dirac-Weyl materials has prompted a broad search for bosonic analogues of such Dirac particles. Recent experiments have found evidence for Dirac magnons above an Ising-like ferromagnetic ground state in a two-dimensional (2D) kagome lattice magnet and in the van der Waals layered honeycomb crystal CrI$_3$, and in a 3D Heisenberg magnet Cu$_3$TeO$_6$. Here we report on our inelastic neutron scattering investigation on large single crystals of a stacked honeycomb lattice magnet CoTiO$_3$, which is part of a broad family of ilmenite materials. The magnetically ordered ground state of CoTiO$_3$ features ferromagnetic layers of Co$^{2+}$, stacked antiferromagnetically along the $c$-axis. We discover that the magnon dispersion relation exhibits strong easy-plane exchange anisotropy and hosts a clear gapless Dirac cone along the edge of the 3D Brillouin zone. Our results establish CoTiO$_3$ as a model pseudospin-$1/2$ material to study interacting Dirac bosons in a 3D quantum XY magnet.

cond-mat.str-el↗

arXiv2019

Fundamental insights to topological quantum materials: A real-space view of 13 cases by supersymmetry of valence bonds approach

We present a real-space view of one-dimensional (1D) to three-dimensional (3D) topological materials with 13 representative samples selected from each class, including 1D trans-polyacetylene, two-dimensional (2D) graphene, and 3D topological insulators, Dirac semimetals, Weyl semimetals, and nodal-line semimetals. This review is not intended to present a complete up-to-date list of publications on topological materials, nor to provide a progress report on the theoretical concepts and experimental advances, but rather to focus on an analysis based on the valence-bond model to help the readers gain a more balanced view of the real-space bonding electron characteristics at the molecular level versus the reciprocal-space band picture of topological materials. Starting from a brief review of low-dimensional magnetism with `toy models' for a 1D Heisenberg antiferromagnetic (HAF) chain, 1D trans-polyacetylene and 2D graphene are found to have similar conjugated (π)-bond systems, and the Dirac cone is correlated to their unconventional 1D and 2D conduction mechanisms. Strain-driven and symmetry-protected topological insulators are introduced from the perspective of material preparation and valence-electron sharing in the valence-bond model analysis. The valence-bond models for the newly developed Dirac semimetals, Weyl semimetals, and nodal line semimetals are examined with more emphasis on the bond length and electron sharing, which is found consistent with the band picture.

cond-mat.str-el↗

arXiv2018

Dynamic surface electronic reconstruction as symmetry-protected topological orders in topological insulator Bi2Se3

Layered narrow band gap semiconductor Bi2Se3 is composed of heavy elements with strong spin-orbital coupling (SOC), which has been identified both as a good candidate of thermoelectric material of high thermoelectric figure-of-merit (ZT) and a topological insulator of Z2-type with a gapless surface band in Dirac cone shape. The existence of a conjugated pi-bond system on the surface of each Bi2Se3 quintuple layer is proposed based on an extended valence bond model having valence electrons distributed in the hybridized orbitals. Supporting experimental evidences of a 2D conjugated pi-bond system on each quintuple layer of Bi2Se3 are provided by electron energy-loss spectroscopy (EELS) and electron density (ED) mapping through inverse Fourier transform of X-ray diffraction data. Quantum chemistry calculations support the pi-bond existence between partially filled 4pz orbitals of Se via side-to-side orbital overlap positively. The conjugated pi-bond system on the surface of each quintuple Bi2Se3 layer is proposed being similar to that found in graphite (graphene) and responsible for the unique 2D conduction mechanism. The van der Waals (vdW) attractive force between quintuple layers is interpreted being coming from the anti-ferroelectrically ordered effective electric dipoles which are constructed with pi-bond trimer pairs on Se-layers across the vdW gap of minimized Coulomb repulsion.

cond-mat.mtrl-sci↗

arXiv2018

Resistivity Anomaly in Weyl Semimetal candidate Molybdenum Telluride

The Weyl semi-metal candidate MoTe$_{2}$ is expected to exhibit a range of exotic electronic transport properties. It exhibits a structural phase transition near room temperature that is evident in the thermal hysteresis in resistivity and thermopower (Seebeck coefficient) as well as large spin-orbit interaction. Here, we also document a resistivity anomaly of up to 13% in the temperature window between 25 and 50 K, which is found to be strongly anisotropic. Based on the experimental data in conjunction with density functional theory calculations, we conjecture that the anomaly can be related to the presence of defects in the system. These findings open opportunities for further investigations and understanding of the transport behavior in these newly discovered semi-metallic layered systems.

cond-mat.mes-hall↗

arXiv2017

On Nematicity and Charge Order in Superoxygenated La$_{2-x}$Sr$_x$CuO$_{4+y}$

A nematic order in stripe-ordered cuprates was recently identified with (001) reflection at resonant energies associated with the in-plane states. However, whether this resonant reflection is ubiquitous among all 214 cuprates is still unknown. Here we report a Resonant soft X-ray Scattering (RXS) measurement on two La$_{2-x}$Sr$_x$CuO$_{4+y}$ crystals. Charge order was found in La$_2$CuO$_{4+y}$ with a total hole concentration near 0.125/Cu but no measurable (001) peak at any resonance, while in a La$_{1.94}$Sr$_{0.06}$CuO$_{4+y}$ sample near 0.16/Cu a (001) peak resonant was identified to be consistent with the presence of LTT tilting. The lack of such a (001) peak in a compound with stripe-like charge order raises questions about nematicity and the origin of the scattering feature.

cond-mat.str-el↗

arXiv2017

Topological Phase Transition Under Pressure in the Topological Nodal Line Superconductor PbTaSe$_2$

A first-order-like resistivity hysteresis is induced by a subtle structural transition under hydrostatic pressure in the topological nodal-line superconductor PbTaSe$_2$. This structure transition is quickly suppressed to zero at pressure $\sim$0.25 GPa. As a result, superconductivity shows a marked suppression, accompanied with fundamental changes in the magnetoresistance and Hall resistivity, suggesting a Lifshitz transition around $\sim$0.25 GPa. The first principles calculations show that the spin-orbit interactions partially gap out the Dirac nodal line around $K$ point in the Brillouin zone upon applying a small pressure, whilst the Dirac states around $H$ point are completely destroyed. The calculations further reveal a second structural phase transition under a pressure as high as $\sim$30 GPa, through which a transition from a topologically nontrivial phase to a trivial phase is uncovered, with a superconducting dome emerging under this high-pressure phase.

cond-mat.supr-con↗

arXiv2017

Bose-Einstein condensation of triplons in the S=1 tetramer antiferromagnet K2Ni2(MoO4)3: A compound close to quantum critical point

The structure of K2Ni2(MoO4)3 consists of S=1 tetramers formed by Ni^{2+} ions. The magnetic susceptibility chi(T) and specific heat Cp(T) data on a single crystal show a broad maximum due to the low-dimensionality of the system with short-range spin correlations. A sharp peak is seen in chi(T) and Cp(T) at about 1.13 K, well below the broad maximum. This is an indication of magnetic long-range order i.e., the absence of spin-gap in the ground state. Interestingly, the application of a small magnetic field (H>0.1 T) induces magnetic behavior akin to Bose-Einstein condensation (BEC) of triplon excitations observed in some spin-gap materials. Our results demonstrate that the temperature-field (T-H) phase boundary follows a power-law (T-T_{N})propotional to H^(1/alpha) with the exponent 1/alpha close to 2/3, as predicted for BEC scenario. The observation of BEC of triplon excitations in small H infers that K2Ni2(MoO4)3 is located in the proximity of a quantum critical point, which separates the magnetically ordered and spin-gap regions of the phase diagram.

cond-mat.str-el↗

arXiv2017

$μ$SR study of noncentrosymmetric superconductor PbTaSe$_2$

We present muon spin rotation ($μ$SR) measurements on the noncentrosymmetric superconductor PbTaSe$_2$. From measurements in an applied transverse field between $H_{c1}$ and $H_{c2}$, we extract the superfluid density as a function of temperature in the vortex state. This data can be fit with a fully gapped two-band model, consistent with previous evidence from ARPES, thermal conductivity, and resistivity. Furthermore, zero field measurements show no evidence for a time reversal symmetry breaking field greater than 0.05~G in the superconducting state. This makes exotic fully gapped spin-triplet states unlikely, and hence we contend that PbTaSe$_2$ is characterized by conventional BCS s-wave superconductivity in multiple bands.

cond-mat.supr-con↗

arXiv2017

Optical phonon dynamics and electronic fluctuations in the Dirac semimetal Cd3As2

Raman scattering in the three-dimensional Dirac semimetal Cd3As2 shows an intricate interplay of electronic and phonon degrees of freedom. We observe resonant phonon scattering due to interband transitions, an anomalous anharmonicity of phonon frequency and intensity, as well as quasielastic (E~0) electronic scattering. The latter two effects are governed by a characteristic temperature scale T* ~ 100 K that is related to mutual fluctuations of lattice and electronic degrees of freedom. A refined analysis shows that this characteristic temperature corresponds to the energy of optical phonons which couple to interband transitions in the Dirac states of Cd3As2. As electron-phonon coupling in a topological semimetal is primarily related to phonons with finite momenta, the back action on the optical phonons in only observed as anharmonicities via multi-phonon processes involving a broad range of momenta. The resulting energy density fluctuations of the coupled system have previously only been observed in low dimensional or frustrated spin systems with suppressed long range ordering.

cond-mat.mtrl-sci↗

arXiv2017

Sodium layer chiral distribution and spin structure of Na$_2$Ni$_2$TeO$_6$ with a honeycomb network

The nature of Na ion distribution, diffusion path, and the spin structure of $P2$-type Na$_2$Ni$_2$TeO$_6$ with a Ni honeycomb network has been explored. The nuclear density distribution of Na ions reveals a 2D chiral pattern within Na layers without breaking the original 3D crystal symmetry, which has been achieved uniquely via an inverse Fourier transform (iFT)-assisted neutron diffraction technique. The Na diffusion pathway described by the calculated iso-surface of Na ion bond valence sum (BVS) map is found consistent to a chiral diffusion mechanism. The Na site occupancy and Ni$^{2+}$ spin ordering were examined in detail with the electron density mapping, neutron diffraction, magnetic susceptibility, specific heat, thermal conductivity and transport measurements. Signatures of both strong incommensurate (ICM) and weak commensurate (CM) antiferromagnetic (AFM) spin ordering were identified in the polycrystalline sample studied, and the CM-AFM spin ordering was confirmed by using a single crystal sample through the $k$-scan in the momentum space corresponding to the AFM peak of ($\frac{1}{2}$, 0, 1).

cond-mat.mtrl-sci↗

arXiv2017

Anisotropy in the thermal hysteresis of resistivity and charge density wave nature of single crystal SrFeO3-delta: X-ray absorption and photoemission studies

The local electronic and atomic structures of the high-quality single crystal of SrFeO3-{delta} ({delta}~0.19) were studied using temperature-dependent x-ray absorption and valence-band photoemission spectroscopy (VB-PES) to investigate the origin of anisotropic resistivity in the ab-plane and along the c-axis close to the region of thermal hysteresis (near temperature for susceptibility maximum, Tm~78 K). All experiments herein were conducted during warming and cooling processes. The Fe L3,2-edge X-ray linear dichrois results show that during cooling from room temperature to below the transition temperature, the unoccupied Fe 3d eg states remain in persistently out-of-plane 3d3z2-r2 orbitals. In contrast, in the warming process below the transition temperature, they change from 3d3z2-r2 to in-plane 3dx2-y2 orbitals. The nearest-neighbor (NN) Fe-O bond lengths also exhibit anisotropic behavior in the ab-plane and along the c-axis below Tm. The anisotropic NN Fe-O bond lengths and Debye-Waller factors stabilize the in-plane Fe 3dx2-y2 and out-of-plane 3d3z2-r2 orbitals during warming and cooling, respectively. Additionally, a VB-PES study further confirms that a relative band gap opens at low temperature in both the ab-plane and along the c-axis, providing the clear evidence of the charge-density-wave nature of SrFeO3-{delta} ({delta}~ 0.19) single crystal.

cond-mat.str-el↗

arXiv2016

Topological quantum phase transition and superconductivity induced by pressure in the bismuth tellurohalide BiTeI

A pressure-induced topological quantum phase transition has been theoretically predicted for the semiconductor BiTeI with giant Rashba spin splitting. In this work, the evolution of the electrical transport properties in BiTeI and BiTeBr is investigated under high pressure. The pressure-dependent resistivity in a wide temperature range passes through a minimum at around 3 GPa, indicating the predicted transition in BiTeI. Superconductivity is observed in both BiTeI and BiTeBr while the resistivity at higher temperatures still exhibits semiconducting behavior. Theoretical calculations suggest that the superconductivity may develop from the multi-valley semiconductor phase. The superconducting transition temperature Tc increases with applied pressure and reaches a maximum value of 5.2 K at 23.5 GPa for BiTeI (4.8 K at 31.7 GPa for BiTeBr), followed by a slow decrease. Our results demonstrate that BiTeX (X = I, Br) compounds with non-trivial topology of electronic states display new ground states upon compression.

cond-mat.supr-con↗

arXiv2016

Large transverse Hall-like signal in topological Dirac semimetal Cd3As2

Cadmium arsenide ($\rm Cd_3As_2$) is known for its inverted band structure and ultra-high electron mobility. It has been theoretically predicted and also confirmed by ARPES experiments to exhibit a 3D Dirac semimetal phase containing degenerate Weyl nodes. From magneto-transport measurements in high quality single crystals of $\rm Cd_3As_2$, a small effective mass $m^* \approx$ 0.05 $m_e$ is determined from the Shubnikov-de Haas (SdH) oscillations. In certain field orientations, we find a splitting of the SdH oscillation frequency in the FFT spectrum suggesting a possible lifting of the double degeneracy in accord with the helical spin texture at outer and inner Fermi surfaces with opposite chirality predicted by our \textit{ab initio} calculations. Strikingly, a large antisymmetric magnetoresistance with respect to the applied magnetic fields is uncovered over a wide temperature range in needle crystal of $\rm Cd_3As_2$ with its long axis along [112] crystal direction. It reveals a possible contribution of intrinsic anomalous velocity term in the transport equation resulting from a unique 3D Rashba-like spin splitted bands that can be obtained from band calculations with the inclusion of Cd antisite defects.

cond-mat.mes-hall↗

arXiv2016

Ferrimagnetic ordering and spin entropy of field-dependent intermediate spins in Na0.82CoO2

The peculiar field-dependent magnetism of Na0.82CoO2 has been investigated through an analysis of its DC and AC spin susceptibilities. To account for the easily activated narrow b2g-a1g gap of the crystal field for Co in the cobalt oxide layer, the spin-state transition of Co3+ (3d6) between the low spin (LS) state b2g^2-a1g^0 of S=0 and the intermediate spin (IS) state b2g^1-a1g^1 of S=1 is thus seen as thermally activated and exhibits a Boltzmann distribution. The IS state of Co3+ within each sqrt(13a) hexagonal superlattice formed by the S=1/2 state of the Co4+ ions appears randomly within each supercell and shows significant temperature and field dependence. The magnetic field is found to assist in pinning down the thermally activated state of Co3+ and swings the Boltzmann distribution weight toward a higher fraction of the IS state. The field dependence of the in-plane magnetic moment from the added number of S=1 spins is used to explain the origin of A-type antiferromagnetic (AF) ordering, particularly that the ferromagnetic (FM)-like behavior below TN at low field is actually a ferrimagnetic IS spin ordering of Co3+.

cond-mat.str-el↗

arXiv2016

Tellurium-bridged two-leg spin ladder in Ba$_2$CuTeO$_6$

We present single-crystal growth and magnetic property studies of tellurium-bridged copper spin-1/2 system Ba$_2$CuTeO$_6$. The spin-exchange interaction among copper spins via Cu-O-Te-O-Cu super-superexchange route leads to a novel two-leg spin ladder system. Spin susceptibility $χ$(T) data indicates that the triclinic Ba$_2$CuTeO$_6$ undergoes a stepwise crossover for exchange couplings revealed by a broad maximum near T$_{max}$$\sim$75~K and an anisotropic cusp in $\frac{dχ}{dT}$(T) to signify a three dimensional (3D) antiferromagnetic long-range ordering (LRO). The 3D LRO has been suggested from the anisotropic behavior of $χ$(T) with strong c-axis spin anisotropy and the signature of spin flop transition from the isothermal magnetization below $T_N$. Analysis of magnetic heat capacity (Cm) at $T_N$~15 K indicates that most of the spin entropy (~92% ) has already been released above $T_N$, which supports the picture of consecutive spin entropy reduction upon cooling with Te-bridged two-leg spin ladder system with strong intraladder and interladder couplings. Theoretical DFT+U calculations have been performed to search for the ground state magnetic configuration and also to evaluate exchange coupling constants that support the magnetic model deduced from the combined spin susceptibility and crystal structure symmetry analysis.

cond-mat.str-el↗