arXiv subjects

Songxue Chi

Publications and source records attributed to Songxue Chi.

At least 73 records · Page 4Linked to original sources

arXiv2013

In-plane spin excitation anisotropy in the paramagnetic phase of NaFeAs

We use unpolarized and polarized inelastic neutron scattering to study low-energy spin excitations in NaFeAs, which exhibits a tetragonal-to-orthorhombic lattice distortion at $T_s\approx 58$ K followed by a collinear antiferromagnetic (AF) order below $T_N\approx 45$ K. In the AF ordered state ($T T_s$), spin excitations become more isotropic. Since similar magnetic anisotropy is also observed in the paramagnetic tetragonal phase of superconducting BaFe$_{1.904}$Ni$_{0.096}$As$_2$, our results suggest that the spin excitation anisotropy in superconducting iron pnictides originates from similar anisotropy already present in their parent compounds.

cond-mat.supr-con↗

arXiv2013

The magnetic and crystal structures of Sr2IrO4: A neutron diffraction study

We report a single-crystal neutron diffraction study of the layered $\rm Sr_2IrO_4$. This work unambiguously determines the magnetic structure of the system and reveals that the spin orientation rigidly tracks the staggered rotation of the $\rm IrO_6$ octahedra in $\rm Sr_2IrO_4$. The long-range antiferromagnetic order has a canted spin configuration with an ordered moment of 0.208(3) $μ_B$/Ir site within the basal plane; a detailed examination of the spin canting yields 0.202(3) and 0.049(2) $μ_B$/site for the a axis and the b axis, respectively. It is intriguing that forbidden nuclear reflections of space group $I4_1/acd$ are also observed in a wide temperature range from 4 K to 600 K, which suggests a reduced crystal structure symmetry. This neutron-scattering work provides a direct, well-refined experimental characterization of the magnetic and crystal structures that are crucial to the understanding of the unconventional magnetism exhibited in this unusual magnetic insulator.

cond-mat.str-el↗

arXiv2013

Uniaxial pressure effect on structural and magnetic phase transitions in NaFeAs and its comparison with as-grown and annealed BaFe$_2$As$_2$

We use neutron scattering to study the effect of uniaxial pressure on the tetragonal-to-orthorhombic structural ($T_s$) and paramagnetic-to-antiferromagnetic ($T_N$) phase transitions in NaFeAs and compare the outcome with similar measurements on as-grown and annealed BaFe$_2$As$_2$. In previous work on as-grown BaFe$_2$As$_2$, uniaxial pressure necessary to detwin the sample was found to induce a significant increase in zero pressure $T_N$ and $T_s$. However, we find that similar uniaxial pressure used to detwin NaFeAs and annealed BaFe$_2$As$_2$ has a very small effect on their $T_N$ and $T_s$.Since transport measurements on these samples still reveal resistivity anisotropy above $T_N$ and $T_s$, we conclude that such anisotropy cannot be due to uniaxial strain induced $T_N$ and $T_s$ shifts, but must arise from intrinsic electronic anisotropy in these materials.

cond-mat.supr-con↗

arXiv2013

Neutron Scattering Study on Spin Dynamics in Superconducting (Tl,Rb)2Fe4Se5

We observed in superconducting (Tl,Rb)2Fe4Se5 spin-wave branches that span an energy range from 6.5 to 209 meV. Spin dynamics are successfully described by a Heisenberg localized spin model whose dominant in-plane interactions include only the nearest (J1 and J1') and next nearest neighbor (J2 and J2') exchange terms within and between the tetramer spin blocks, respectively. These experimentally determined exchange constants would crucially constrain the theoretical viewpoints on magnetism and superconductivity in the Fe-based materials.

cond-mat.supr-con↗

arXiv2012

Magnetic orders and spin-flop transitions in the cobalt doped multiferroic $\rm Mn_{1-x}Co_{x}WO_4$

We present a comprehensive single crystal neutron diffraction investigation of the $\rm Mn_{1-x}Co_{x}WO_4$ with $0.02\leq x \leq0.30$. At lower concentration $x \leq 0.05$, the system is quickly driven into the multiferroic phase with spin structure forming an elliptical spiral order similar to the parent compound. The reduction of electric polarization is ascribed to the tilting of the spiral plane. For $x\sim 0.075$, the magnetic structure undergoes a spin flop transition that is characterized by a sudden rotation of the spin helix envelope into the $ac$ plane. This spin structure persists for concentration up to $x=0.15$, where additional competing magnetic orders appear at low temperature. For $0.17 \leq x \leq 0.30$, the system experiences another spin flop transition and recovers the low-$x$ spiral spin configuration. A simple commensurate spin structure with $\vec{q}=(0.5,0,0)$ is found to coexist with the incommensurate spiral order. The complex evolution of magnetic structure in Co doped $\rm MnWO_4$ contrasts sharply with other transition metal ion doped $\rm Mn_{1-x}A_xWO_4$ (A=Zn, Mg, Fe) where the chemical substitutions stabilize only one type of magnetic structure. The rich phase diagram of $\rm Mn_{1-x}Co_{x}WO_4 $ results from the interplay between magnetic frustration and spin anisotropy of the Co ions.

cond-mat.str-el↗

arXiv2012

Direct evidence of a zigzag spin chain structure in the honeycomb lattice: A neutron and x-ray diffraction investigation on single crystal $\rm Na_2IrO_3$

We have combined single crystal neutron and x-ray diffractions to investigate the magnetic and crystal structures of the honeycomb lattice $\rm Na_2IrO_3$. The system orders magnetically below $18.1(2)$ K with Ir$^{4+}$ ions forming zigzag spin chains within the layered honeycomb network with ordered moment of $\rm 0.22(1) μ_B$/Ir site. Such a configuration sharply contrasts the N{é}el or stripe states proposed in the Kitaev-Heisenberg model. The structure refinement reveals that the Ir atoms form nearly ideal 2D honeycomb lattice while the $\rm IrO_6$ octahedra experience a trigonal distortion that is critical to the ground state. The results of this study provide much-needed experimental insights into the magnetic and crystal structure crucial to the understanding of the exotic magnetic order and possible topological characteristics in the 5$d$-electron based honeycomb lattice.

cond-mat.str-el↗

arXiv2012

Block magnetism coupled with local distortion in the iron-based spin-ladder compound BaFe2Se3

Magnetism in the insulating BaFe$_2$Se$_3$ was examined through susceptibility, specific heat, resistivity and neutron diffraction measurements. After formation of a short-range magnetic correlation, a long-range ordering was observed below $T_{\rm N}\sim 255$ K. The transition is obscured by bulk properties. Magnetic moments ($\parallel a$) are arranged to form a Fe$_4$ ferromagnetic unit, and each Fe$_4$ stacks antiferromagnetically. This block magnetism is of the third type among magnetic structures of ferrous materials. The magnetic ordering drives unusually large distortion via magnetoelastic coupling.

cond-mat.str-el↗

arXiv2011

Response of acoustic phonons to charge and orbital order in the 50\% doped bilayer manganite LaSr$_2$Mn$_2$O$_7$

We report an inelastic neutron scattering study of acoustic phonons in the charge and orbitally ordered bilayer manganite LaSr$_2$Mn$_2$O$_7$. For excitation energies less than $15\,\rm{meV}$, we observe an abrupt increase (decrease) of the phonon energies (linewidths) of a transverse acoustic phonon branch at $\mathbf{q} = (h, h, 0)$, $h\le0.3$, upon entering the low temperature charge and orbital ordered state ($T_{COO} = 225\,\rm{K}$). This indicates a reduced electron-phonon coupling due to a decrease of electronic states at the Fermi level leading to a partial removal of the Fermi surface below $T_{COO}$ and provides direct experimental evidence for a link between electron-phonon coupling and charge order in manganites.

cond-mat.str-el↗

arXiv2011

Neutron Scattering Studies of spin excitations in hole-doped Ba0.67K0.33Fe2As2 superconductor

We report inelastic neutron scattering experiments on single crystals of superconducting Ba0.67K0.33Fe2As2 (Tc = 38 K). In addition to confirming the resonance previously found in powder samples, we find that spin excitations in the normal state form longitudinally elongated ellipses along the QAFM direction in momentum space, consistent with density functional theory predictions. On cooling below Tc, while the resonance preserves its momentum anisotropy as expected, spin excitations at energies below the resonance become essentially isotropic in the in-plane momentum space and dramatically increase their correlation length. These results suggest that the superconducting gap structures in Ba0.67Ka0.33Fe2As2 are more complicated than those suggested from angle resolved photoemission experiments.

cond-mat.supr-con↗

arXiv2011

Spin Glass and Semiconducting Behavior in 1D BaFe2-δSe3 Crystals

We investigate the physical properties and electronic structure of BaFe2-δSe3 crystals, which were grown out of tellurium flux. The crystal structure of the compound, an iron-deficient derivative of the ThCr2Si2-type, is built upon edge-shared FeSe4 tetrahedra fused into double chains. The semiconducting BaFe2-δSe3 with δ \approx 0.2 (ρ295K = 0.18 Ω\cdotcm and Eg = 0.30 eV) does not order magnetically, however there is evidence for short-range magnetic correlations of spin glass type (Tf \approx 50 K) in magnetization, heat capacity and neutron diffraction results. A one-third substitution of selenium with sulfur leads to a slightly higher electrical conductivity (ρ295K = 0.11 Ω\cdotcm and Eg = 0.22 eV) and a lower spin glass freezing temperature (Tf \approx 15 K), corroborating with higher electrical conductivity reported for BaFe2S3. According to the electronic structure calculations, BaFe2Se3 can be considered as a one-dimensional ladder structure with a weak interchain coupling.

cond-mat.supr-con↗

arXiv2011

Common origin of the two types of magnetic fluctuations in iron chalcogenides

We use inelastic neutron scattering to study the low energy spin excitations in moderately doped non-superconducting Fe$_{1.01}$Te$_{0.72}$Se$_{0.28}$. The spin excitations in this system contain components near (0.5,0,0) and (0.5,0.5,0) in a-b plane reciprocal lattice units using tetragonal unit cell notation (a=b=3.772 Åand c=6.061 Å). At low energies the scattering is centered around (0.5,0,0). With increasing energy, the spectral weight of low energy spin excitations centered around (0.5,0,0) abruptly shifts around 3 meV to the incommensurate spin excitations centered around (0.5,0.5,0). However both types of spin fluctuations exhibit the identical temperature dependence. These results indicate that the (0.5,0,0) type spin excitations and the incommensurate excitations around the (0.5,0.5,0) position have a common origin and both must be taken into account to understand the nature of magnetism and superconducting pairing in the iron chalcogenides.

cond-mat.str-el↗

arXiv2011

Magnetic field tuning of antiferromagnetic Yb$_{3}$Pt$_{4}$

We present measurements of the specific heat, magnetization, magnetocaloric effect and magnetic neutron diffraction carried out on single crystals of antiferromagnetic Yb$_{3}$Pt$_{4}$, where highly localized Yb moments order at $T_{\rm N}=2.4$ K in zero field. The antiferromagnetic order was suppressed to $T_{\rm N}\rightarrow 0$ by applying a field of 1.85 T in the $ab$ plane. Magnetocaloric effect measurements show that the antiferromagnetic phase transition is always continuous for $T_{\rm N}>0$, although a pronounced step in the magnetization is observed at the critical field in both neutron diffraction and magnetization measurements. These steps sharpen with decreasing temperature, but the related divergences in the magnetic susceptibility are cut off at the lowest temperatures, where the phase line itself becomes vertical in the field-temperature plane. As $T_{\rm N}\rightarrow0$, the antiferromagnetic transition is increasingly influenced by a quantum critical endpoint, where $T_{\rm N}$ ultimately vanishes in a first order phase transition.

cond-mat.str-el↗

arXiv2011

Nitrogen Contamination in Elastic Neutron Scattering

Nitrogen gas accidentally sealed in a sample container produces various spurious effects in elastic neutron scattering measurements. These effects are systematically investigated and the details of the spurious scattering are presented.

cond-mat.mtrl-sci↗

arXiv2010

Evidence for local moment magnetism in superconducting FeTe0.35Se0.65

The nature of the magnetic correlations in Fe-based superconductors remains a matter of controversy. To address this issue, we use inelastic neutron scattering to characterize the strength and temperature dependence of low-energy spin fluctuations in FeTe$_{0.35}$Se$_{0.65}$ ($T_c \sim 14$ K). Integrating magnetic spectral weight for energies up to 12 meV, we find a substantial moment ($\agt 0.26 μ_B/$Fe) that shows little change with temperature, from below T$_c$ to 300 K. Such behavior cannot be explained by the response of conduction electrons alone; states much farther from the Fermi energy must have an instantaneous local spin polarization. It raises interesting questions regarding the formation of the spin gap and resonance peak in the superconducting state.

cond-mat.supr-con↗

arXiv2010

Antiferromagnetic Critical Fluctuations in BaFe$_2$As$_2$

Magnetic correlations near the magneto-structural phase transition in the bilayer iron pnictide parent compound, BaFe$_2$As$_2$, are measured. In close proximity to the antiferromagnetic phase transition in BaFe$_2$As$_2$, a crossover to three dimensional critical behavior is anticipated and has been preliminarily observed. Here we report complementary measurements of two-dimensional magnetic fluctuations over a broad temperature range about T$_N$. The potential role of two-dimensional critical fluctuations in the magnetic phase behavior of BaFe$_2$As$_2$ and their evolution near the anticipated crossover to three dimensional critical behavior and long-range order are discussed.

cond-mat.supr-con↗

arXiv2010

Antiferromagnetic critical pressure in URu2Si2 under hydrostatic conditions

The onset of antiferromagnetic order in URu2Si2 has been studied via neutron diffraction in a helium pressure medium, which most closely approximates hydrostatic conditions. The antiferromagnetic critical pressure is 0.80 GPa, considerably higher than values previously reported. Complementary electrical resistivity measurements imply that the hidden order-antiferromagnetic bicritical point far exceeds 1.02 GPa. Moreover, the redefined pressure-temperature phase diagram suggests that the superconducting and antiferromagnetic phase boundaries actually meet at a common critical pressure at zero temperature.

cond-mat.str-el↗

arXiv2010

Doping Dependence of Spin Dynamics in Electron-Doped Ba(Fe1-xCox)2As2

The spin dynamics in single crystal, electron-doped Ba(Fe1-xCox)2As2 has been investigated by inelastic neutron scattering over the full range from undoped to the overdoped regime. We observe damped magnetic fluctuations in the normal state of the optimally doped compound (x=0.06) that share a remarkable similarity with those in the paramagnetic state of the parent compound (x=0). In the overdoped superconducting compound (x=0.14), magnetic excitations show a gap-like behavior, possibly related to a topological change in the hole Fermi surface (Lifshitz transition), while the imaginary part of the spin susceptibility prominently resembles that of the overdoped cuprates. For the heavily overdoped, non-superconducting compound (x=0.24) the magnetic scattering disappears, which could be attributed to the absence of a hole Fermi-surface pocket observed by photoemission.

cond-mat.supr-con↗

arXiv2010

Disappearance of static magnetic order and evolution of spin fluctuations in Fe$_{1+δ}$Se$_{x}$Te$_{1-x}$

We report neutron scattering studies on static magnetic orders and spin excitations in the Fe-based chalcogenide system Fe$_{1+δ}$Se$_{x}$Te$_{1-x}$ with different Fe and Se compositions. Short-range static magnetic order with the "bicollinear" spin configuration is found in all non-superconducting samples, with strong low-energy magnetic excitations near the $(0.5,0)$ in-plane wave-vector (using the two-Fe unit cell) for Se doping up to 45%. When the static order disappears and bulk superconductivity emerges, the spectral weight of the magnetic excitations shifts to the region of reciprocal space near the in-plane wave-vector $(0.5,0.5)$, corresponding to the "collinear" spin configuration. Our results suggest that spin fluctuations associated with the collinear magnetic structure appear to be universal in all Fe-based superconductors, and there is a strong correlation between superconductivity and the character of the magnetic order/fluctuations in

cond-mat.supr-con↗