arXiv subjects

Q. Huang

Publications and source records attributed to Q. Huang.

At least 109 records · Page 6Linked to original sources

arXiv2008

Structural and Magnetic Properties of Pyrochlore Solid Solutions (Y,Lu)2Ti2-x(Nb,Ta)xO7+/-y

The synthesis and characterization of the pyrochlore solid solutions, Y2Ti2-xNbxO7-y, Lu2Ti2-xNbxO7-y, Y2Ti2-xTaxO7-y and Lu2TiTaO7-y (-0.4<y<0.5), is described. Synthesis at 1600 C, and 10-5 Torr yields oxygen deficiency in all systems. All compounds are found to be paramagnetic and semiconducting, with the size of the local moments being less, in some cases substantially less, than the expected value for the number of nominally unpaired electrons present. Thermogravimetric analysis (TGA) shows that all compounds can be fully oxidized while retaining the pyrochlore structure, yielding oxygen rich pyrochlores as white powders. Powder neutron diffraction of Y2TiNbO7-based samples was done. Refinement of the data for oxygen deficient Y2TiNbO6.76 indicates the presence of a distribution of oxygen over the 8b and 48f sites. Refinement of the data for oxygen rich Y2TiNbO7.5 shows these sites to be completely filled, with an additional half filling of the 8a site. The magnetic and TGA data strongly suggest a preference for a Ti3+/(Nb,Ta)5+ combination, as opposed to Ti4+/(Nb,Ta)4+, in this pyrochlore family. In addition, the evidence clearly points to Ti3+ as the source of the localized moments, with no evidence for localized Nb4+ moments.

cond-mat.mtrl-sci↗

arXiv2007

New estimation of the spectral index of high-energy cosmic rays as determined by the Compton-Getting anisotropy

The amplitude of the Compton-Getting (CG) anisotropy contains the power-law index of the cosmic-ray energy spectrum. Based on this relation and using the Tibet air-shower array data, we measure the cosmic-ray spectral index to be $-3.03 \pm 0.55_{stat} \pm < 0.62_{syst}$ between 6 TeV and 40 TeV, consistent with $-$2.7 from direct energy spectrum measurements. Potentially, this CG anisotropy analysis can be utilized to confirm the astrophysical origin of the ``knee'' against models for non-standard hadronic interactions in the atmosphere.

astro-ph↗

arXiv2007

Magnetic and orbital ordering in the spinel MnV2O4

Neutron inelastic scattering and diffraction techniques have been used to study the MnV2O4 spinel system. Our measurements show the existence of two transitions to long-range ordered ferrimagnetic states; the first collinear and the second noncollinear. The lower temperature transition, characterized by development of antiferromagnetic components in the basal plane, is accompanied by a tetragonal distortion and the appearance of a gap in the magnetic excitation spectrum. The low-temperature noncollinear magnetic structure has been definitively resolved. Taken together, the crystal and magnetic structures indicate a staggered ordering of the V d orbitals. The anisotropy gap is a consequence of unquenched V orbital angular momentum.

cond-mat.str-el↗

arXiv2007

Magnetism and structure of LixCoO2 and comparison to NaxCoO2

The magnetic properties and structure of LixCoO2 for x between 0.5 and 1.0 are reported. Co4+ is found to be high-spin in LixCoO2 for x between 0.94 and 1.0 and low-spin for x between 0.50 and 0.78. Weak antiferromagnetic coupling is observed, increasing in strength as more Co4+ is introduced. At an x value of about 0.65, the temperature-independent contribution to the magnetic susceptibility and the electronic contribution to the specific heat are largest. Neutron diffraction analysis reveals that the lithium oxide layer expands perpendicular to the basal plane and the Li ions displace from their ideal octahedral sites with decreasing x. A comparison of the structures of the NaxCoO2 and LixCoO2 systems reveals that the CoO2 layer changes substantially with alkali content in the former but is relatively rigid in the latter. Further, the CoO6 octahedra in LixCoO2 are less distorted than those in NaxCoO2. We postulate that these structural differences strongly influence the physical properties in the two systems.

cond-mat.str-el↗

arXiv2007

Future plan for observation of cosmic gamma rays in the 100 TeV energy region with the Tibet air shower array : simulation and sensitivity

The Tibet air shower array, which has an effective area of 37,000 square meters and is located at 4300 m in altitude, has been observing air showers induced by cosmic rays with energies above a few TeV. We have a plan to add a large muon detector array to it for the purpose of increasing its sensitivity to cosmic gamma rays in the 100 TeV energy region by discriminating them from cosmic-ray hadrons. We have deduced the attainable sensitivity of the muon detector array using our Monte Carlo simulation. We report here on the detailed procedure of our Monte Carlo simulation.

astro-ph↗

arXiv2007

Future plan for observation of cosmic gamma rays in the 100 TeV energy region with the Tibet air shower array : physics goal and overview

The Tibet air shower array, which has an effective area of 37,000 square meters and is located at 4300 m in altitude, has been observing air showers induced by cosmic rays with energies above a few TeV. We are planning to add a large muon detector array to it for the purpose of increasing its sensitivity to cosmic gamma rays in the 100 TeV (10 - 1000 TeV) energy region by discriminating them from cosmic-ray hadrons. We report on the possibility of detection of gamma rays in the 100 TeV energy region in our field of view, based on the improved sensitivity of our air shower array deduced from the full Monte Carlo simulation.

astro-ph↗

arXiv2007

Neel to Spin-Glass-like Phase Transition versus Dilution in Geometrically Frustrated ZnCr_{2-2x}Ga_{2x}O_4

ZnCr2O4 undergoes a first order spin-Peierls-like phase transition at 12.5 K from a cubic spin liquid phase to a tetragonal Neel state. Using powder diffraction and single crystal polarized neutron scattering, we determined the complex spin structure of the Neel phase. This phase consisted of several magnetic domains with different characteristic wave vectors. This indicates that the tetragonal phase of ZnCr2O4 is very close to a critical point surrounded by many different Neel states. We have also studied, using elastic and inelastic neutron scattering techniques, the effect of nonmagnetic dilution on magnetic correlations in ZnCr_{2-2x}Ga_{2x}O_4 (x=0.05 and 0.3). For x=0.05, the magnetic correlations do not change qualitatively from those in the pure material, except that the phase transition becomes second order. For x= 0.3, the spin-spin correlations become short range. Interestingly, the spatial correlations of the frozen spins in the x=0.3 material are the same as those of the fluctuating moments in the pure and the weakly diluted materials.

cond-mat.str-el↗

arXiv2007

Incommensurate magnetic structure in the orthorhombic perovskite ErMnO_3

By combining dielectric, specific heat, and magnetization measurements and high-resolution neutron powder diffraction, we have investigated the thermodynamic and magnetic/structural properties of the metastable orthorhombic perovskite ErMnO_3 prepared by high-pressure synthesis. The system becomes antiferromagnetically correlated below 42 K and undergoes a lock-in transition at 28 K with propagation wave vector (0,k_b,0), which remains incommensurate at low temperature. The intercorrelation between the magnetic structure and electric properties and the role of the rare earth moment are discussed.

cond-mat.str-el↗

arXiv2007

Effect of antiferromagnetic spin correlations on lattice distortion and charge ordering in Pr$_{0.5}$Ca$_{1.5}$MnO$_{4}$

We use neutron scattering to study the lattice and magnetic structure of the layered half-doped manganite Pr$_{0.5}$Ca$_{1.5}$MnO$_4$. On cooling from high temperature, the system first becomes charge- and orbital- ordered (CO/OO) near $T_{CO}=300$ K and then develops checkerboard-like antiferromagnetic (AF) order below $T_{N}=130$ K. At temperatures above $T_{N}$ but below $T_{CO}$ ($T_N<T<T_{CO}$), the appearance of short-range AF spin correlations suppresses the CO/OO induced orthorhombic strain, contrasting with other half-doped manganites, where AF order has no observable effect on the lattice distortion. These results suggest that a strong spin-lattice coupling and the competition between AF exchange and CO/OO ordering ultimately determines the low-temperature properties of the system.

cond-mat.str-el↗

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↗

arXiv2007

Structural Disorder and Properties of the Stuffed Pyrochlore Ho2TiO5

We report a structural and thermodynamic study of the "stuffed spin ice" material Ho2TiO5 (i.e., Ho2(Ti1.33Ho0.67)O6.67), comparing samples synthesized through two different routes. Neutron powder diffraction and electron diffraction reveal that the previously reported defect fluorite phase has short-range pyrochlore ordering, in that there are domains in which the Ho and Ho/Ti sublattices are distinct. By contrast, a sample prepared through a floating zone method has long range ordering of these sublattices. Despite the differences in crystal structures, the two versions of Ho2TiO5 display similar magnetic susceptibilities. Field dependent magnetization and measured recovered entropies, however, show a difference between the two forms, suggesting that the magnetic properties of the stuffed pyrochlores depend on the local structure.

cond-mat.mtrl-sci↗

arXiv2007

Magnetic Structure and Properties of the S = 5/2 Triangular Antiferromagnet $α$-NaFeO$_2$

The magnetic properties of $α$-NaFeO$_2$ are studied by neutron diffraction and magnetization measurements. An ordered phase with spins aligned along the b$_hex$ axis exists at low temperatures (T < 4 K). At intermediate temperatures (4 K < T < 11 K), the system passes through an incommensurate ordered phase before transforming into a short range ordered state at higher temperatures that persists up to at least 50 K. Although the short range ordering does not persist to room temperature according to neutron diffraction, the magnetic susceptibility does not follow Curie-Weiss behavior, even up to 320 K. This rich magnetic behavior can be understood qualitatively as a competition between different magnetic exchange interactions that are similar in magnitude. The delicate balance between these interactions makes $α$-NaFeO$_2$ a candidate for more detailed theoretical work to understand magnetic behavior in frustrated magnetic systems.

cond-mat.str-el↗

arXiv2007

Direct transition from a disordered to a multiferroic phase on a triangular lattice

Competing interactions and geometric frustration provide favourable conditions for exotic states of matter. Such competition often causes multiple phase transitions as a function of temperature and can lead to magnetic structures that break inversion symmetry, thereby inducing ferroelectricity [1-4]. Although this phenomenon is understood phenomenologically [3-4], it is of great interest to have a conceptually simpler system in which ferroelectricity appears coincident with a single magnetic phase transition. Here we report the first such direct transition from a paramagnetic and paraelectric phase to an incommensurate multiferroic in the triangular lattice antiferromagnet RbFe(MoO4)2 (RFMO). A magnetic field extinguishes the electric polarization when the symmetry of the magnetic order changes and ferroelectricity is only observed when the magnetic structure has chirality and breaks inversion symmetry. Multiferroic behaviour in RFMO provides a theoretically tractable example of ferroelectricity from competing spin interactions. A Landau expansion of symmetry-allowed terms in the free energy demonstrates that the chiral magnetic order of the triangular lattice antiferromagnet gives rise to a pseudoelectric field, whose temperature dependence agrees with that observed experimentally.

cond-mat.mtrl-sci↗

arXiv2006

Search for Magnetic Order in Superconducting RuSr2Eu1.2Ce0.8Cu2O10

Neutron diffraction, polarized neutron transmission, and small angle neutron scattering have been used to investigate the crystal structure and nature of the magnetic order in a polycrystalline sample of RuSr2Eu1.2Ce0.8Cu2O10. The sample was made with the Eu-153 (98.8%) isotope to reduce the high neutron absorption for the naturally occurring element. Full refinements of the crystal structure, space group I4/mmm, are reported. At low temperatures only a single magnetic peak is clearly observed in a relatively wide angular range. A sharp spin reorientation transition (SRT) is observed around 35 K, close to the superconducting transition temperature (Tc~40 K). Between the spin reorientation temperature and the Neel temperature of 59 K, additional magnetic reflections are observed. However, none of these can be simply indexed on the chemical unit cell, either as commensurate peaks or simple incommensurate magnetism, and the paucity of reflections at low T compels the conclusion that these magnetic Bragg peaks arise from an impurity phase. X-ray and neutron diffraction on the pressed pellet both show that the sample does not appear to contain substantial impurity phases, but it turns out that the magnetic impurity peaks exhibit strong preferred orientation with respect to the pellet orientation, while the primary phase does not. We have been unable to observe any magnetic order that can be identified with the ruthenate-cuprate system.

cond-mat.supr-con↗

arXiv2006

Structure and basic magnetic properties of the honeycomb lattice compounds Na2Co2TeO6 and Na3Co2SbO6

The synthesis, structure, and basic magnetic properties of Na2Co2TeO6 and Na3Co2SbO6 are reported. The crystal structures were determined by neutron powder diffraction. Na2Co2TeO6 has a two-layer hexagonal structure (space group P6322) while Na3Co2SbO6 has a single-layer monoclinic structure (space group C2/m). The Co, Te, and Sb ions are in octahedral coordination, and the edge sharing octahedra form planes interleaved by sodium ions. Both compounds have full ordering of the Co2+ and Te6+/Sb5+ ions in the ab plane such that the Co2+ ions form a honeycomb array; the stacking of the honeycomb arrays differ in the two compounds. Both Na2Co2TeO6 and Na3Co2SbO6 reveal antiferromagnetic ordering at low temperatures, with a metamagnetic transition displayed by Na3Co2SbO6.

cond-mat.mtrl-sci↗

arXiv2006

Complex magnetic order in the kagome staircase compound Co3V2O8

Co3V2O8 (CVO) has a geometrically frustrated magnetic lattice, a Kagome staircase. The crystal structure consists of two inequivalent Co sites, one-dimensional chains of Co(2) spine sites, linked by Co(1) cross-tie sites. Neutron powder diffraction has been used to solve the basic magnetic and crystal structures of this system, while polarized and unpolarized single crystal diffraction measurements have been used to reveal a variety of incommensurate phases, interspersed with lock-in transitions to commensurate phases. CVO initially orders magnetically at 11.3 K into an incommensurate, transversely polarized, spin density wave state, with wave vector k=(0,delta,0) with delta=0.55 and the spin direction along the a axis. Delta is found to decrease monotonically with decreasing temperature, and then it locks into a commensurate antiferromagnetic structure with delta=0.5 for 6.9<T<8.6 K. Below 6.9 K the magnetic structure becomes incommensurate again. Delta continues to decrease with decreasing temperature, and locks-in again at delta=1/3 over a narrow temperature range (6.2<T<6.5 K). The system then undergoes a strongly first order transition to the ferromagnetic ground state (delta=0) at Tc=6.2 K. A dielectric anomaly is observed around the ferromagnetic transition temperature of 6.2 K, demonstrating a significant spin-charge coupling present in CVO. A theory based on group theory analysis and a minimal Ising model with competing exchange interactions can explain the basic features of the magnetic ordering.

cond-mat.str-el↗

arXiv2006

Stoichiometric oxygen content in NaxCoO2

The crystal structure and oxygen stoichiometry in two-layer Na0.74CoO2 and Na0.38CoO2 at room temperature are analyzed by powder neutron diffraction. Two sets of diffraction data for each sample, taken at different incident neutron wavelengths, lambda=1.1968 Angstroms and lambda=1.5403 Angstroms, are analyzed simultaneously by the Rietveld method, allowing for the independent refinement of all structural parameters. The fractional oxygen site occupancies are found to be 1.01(1) for Na0.74CoO2 and 0.99(2) for Na0.38CoO2 respectively. These results indicate that the oxygen content of these phases is stoichiometric to a precision of 1 to 2%, and therefore the formal cobalt oxidation state is determined solely by the sodium content. The analysis also reveals that both types of sodium ions in the structure are in off-center distorted trigonal prismatic geometry.

cond-mat.str-el↗

arXiv2006

Crystal Structure and Elementary Properties of NaxCoO2 (x = 0.32, 0.5, 0.6, 0.75, and 0.92) in the Three-Layer NaCoO2 Family

The crystal structures of the NaxCoO2 phases based on three-layer NaCoO2, with x=0.32, x=0.51, x=0.60, x=0.75 and x=0.92, determined by powder neutron diffraction, are reported. The structures have triangular CoO2 layers interleaved by sodium ions, and evolve with variation in Na content in a more complex way than has been observed in the two-layer NaxCoO2 system. The highest and lowest Na containing phases studied (x=0.92 and x=0.32) are trigonal, with three CoO2 layers per cell and octahedral Na ion coordination. The intermediate compositions have monoclinic structures. The x=0.75 compound has one CoO2 layer per cell, with Na in octahedral coordination and an incommensurate superlattice. The x=0.6 and x=0.5 phases are also single-layer, but the Na is found in trigonal prismatic coordination. The magnetic behavior of the phases is similar to that observed in the two-layer system. Both the susceptibility and the electronic contribution to the specific heat are largest for x=0.6.

cond-mat.str-el↗