arXiv subjects

Songxue Chi

Publications and source records attributed to Songxue Chi.

At least 37 records · Page 2Linked to original sources

arXiv2020

Parallel Spin Stripes and Their Coexistance with Superconducting Ground States at Optimal and High Doping in La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$

Quasi-two dimensional quantum magnetism is clearly highly correlated with superconducting ground states in cuprate-based High T$_c$ superconductivity. Three dimensional, commensurate long range magnetic order in La$_2$CuO$_4$ quickly evolves to quasi-two dimensional, incommensurate correlations on doping with mobile holes, and superconducting ground states follow for x as small as 0.05 in the La$_{2-x}$Sr$_x$/Ba$_x$CuO$_4$ family of superconductors. It has long been known that the onset of superconducting ground states in these systems is coincident with a remarkable rotation of the incommensurate spin order from "diagonal stripes" below x = 0.05, to "parallel stripes" above. However, little is known about the spin correlations at optimal and high doping levels, where the dome of superconductivity draws to an end. Here we present new elastic and inelastic neutron scattering measurements on single crystals of La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$ with x = 0.125, 0.19, 0.24 and 0.26, and show that two dimensional, quasi-static, parallel spin stripes are observed to onset at temperatures such that the parallel spin stripe phase envelopes all superconducting ground states in this system. Parallel spin stripes stretch across 0.05 < < 0.26, with rapidly decreasing moment size and onset temperatures for x > 0.125. We also show that the low energy, parallel spin stripe fluctuations for optimally doped x = 0.19 display dynamic spectral weight which grows with decreasing temperature and saturates below its superconducting T$_c$. The elastic order parameter for x = 0.19 also shows plateau behavior coincident with the onset of superconductivity. This set of observations assert the foundational role played by two dimensional parallel spin stripe order and fluctuations in High T$_c$ cuprate superconductivity.

cond-mat.supr-con↗

arXiv2020

Competition of three-dimensional magnetic phases in Ca$_2$Ru$_{1-x}$Fe$_x$O$_4$: A structural perspective

The crystalline and magnetic structures of Ca$_2$Ru$_{1-x}$Fe$_x$O$_4$ (x=0.02, 0.05, 0.08 and 0.12) have been studied using neutron and X-ray diffraction. The Fe-doping reduces the Ru-O bond length in both apical and planar directions. The smaller Ru(Fe)O$_6$ octahedron leads to its reduced distortion. The $Pbca$ space group is maintained in all the Fe-dopings, so is the octahedral flattening. Warming has a similar effect on the lattice to that of the Fe-doping in releasing the distorted octahedra but precipitates an abrupt octahedral elongation near the N$\acute{e}$el temperature. Two competing antiferromagnetic orders, $A$- and $B$-centered phases have been observed. The Fe-doping-relaxed crystal structure prefers the latter to the former. As the doping increases, the $B$-centered phase continuously grows at the cost of the $A$-centered one and eventually replaces it at x=0.12. The absence of the two-dimensional antiferromagnetic critical fluctuations above the magnetic transition temperature and the three-dimensional magnetic correlation below the transition, together with the anomalous lattice response, point to an important role of orbital degree of freedom in driving the magnetic phase competition.

cond-mat.str-el↗

arXiv2020

Topological Singularity Induced Chiral Kohn Anomaly in a Weyl Semimetal

The electron-phonon interaction (EPI) is instrumental in a wide variety of phenomena in solid-state physics, such as electrical resistivity in metals, carrier mobility, optical transition and polaron effects in semiconductors, lifetime of hot carriers, transition temperature in BCS superconductors, and even spin relaxation in diamond nitrogen-vacancy centers for quantum information processing. However, due to the weak EPI strength, most phenomena have focused on electronic properties rather than on phonon properties. One prominent exception is the Kohn anomaly, where phonon softening can emerge when the phonon wavevector nests the Fermi surface of metals. Here we report a new class of Kohn anomaly in a topological Weyl semimetal (WSM), predicted by field-theoretical calculations, and experimentally observed through inelastic x-ray and neutron scattering on WSM tantalum phosphide (TaP). Compared to the conventional Kohn anomaly, the Fermi surface in a WSM exhibits multiple topological singularities of Weyl nodes, leading to a distinct nesting condition with chiral selection, a power-law divergence, and non-negligible dynamical effects. Our work brings the concept of Kohn anomaly into WSMs and sheds light on elucidating the EPI mechanism in emergent topological materials.

cond-mat.mtrl-sci↗

arXiv2020

Anisotropic effect of a magnetic field on the neutron spin resonance in FeSe

We use inelastic neutron scattering to study the effect of a magnetic field on the neutron spin resonance (Er = 3.6 meV) of superconducting FeSe (Tc = 9 K). While a field aligned along the in-plane direction broadens and suppresses the resonance, a c-axis aligned field does so much more efficiently, consistent with the anisotropic field-induced suppression of the superfluid density from the heat capacity measurements. These results suggest that the resonance in FeSe is associated with the superconducting electrons arising from orbital selective quasi-particle excitations between the hole and electron Fermi surfaces.

cond-mat.str-el↗

arXiv2020

Noncollinear magnetic structure and magnetoelectric coupling in buckled honeycomb Co$_4$Nb$_2$O$_9$:A single crystal neutron diffraction study

Through analysis of single crystal neutron diffraction data, we present the magnetic structures of magnetoelectric Co4Nb2O9 under various magnetic fields. In zero-field, neutron diffraction experiments below TN=27 K reveal that the Co2+ moments order primarily along the a* direction without any spin canting along the c axis, manifested by the magnetic symmetry C2/c'. The moments of nearest neighbor Co atoms order ferromagnetically with a small cant away from the next nearest neighbor Co moments along the c axis. In the applied magnetic field H//a, three magnetic domains were aligned with their major magnetic moments perpendicular to the magnetic field with no indication of magnetic phase transitions. The influences of magnetic fields on the magnetic structures associated with the observed magnetoelectric coupling are discussed.

cond-mat.str-el↗

arXiv2019

Flat band magnetism and helical magnetic order in Ni-doped SrCo$_2$As$_2$

A series of Sr(Co$_{1-x}$Ni$_x$)$_2$As$_2$ single crystals was synthesized allowing a comprehensive phase diagram with respect to field, temperature, and chemical substitution to be established. Our neutron diffraction experiments revealed a helimagnetic order with magnetic moments ferromagnetically (FM) aligned in the $ab$ plane and a helimagnetic wavevector of $q=(0,0,0.56)$ for $x$ = 0.1. The combination of neutron diffraction and angle-resolved photoemission spectroscopy (ARPES) measurements show that the tuning of a flat band with $d_{x^2-y^2}$ orbital character drives the helimagnetism and indicates the possibility of a quantum order-by-disorder mechanism.

cond-mat.str-el↗

arXiv2019

Near-room-temperature giant topological Hall effect in antiferromagnetic kagome metal YMn6Sn6

The kagome lattice, consisting of interconnected triangles and hexagons uniquely, is an excellent model system for study frustrated magnetism, electronic correlation and topological electronic structure. After an intensive investigation on frustrated magnetism in insulating magnetic kagome lattices, the interplay between charge and spin degrees of freedom via spin-orbital coupling in metallic systems (kagome metals) has become an attractive topic recently. Here, we study centrosymmetric YMn6Sn6 with Mn kagome lattice. We discover that it exhibits giant topological Hall effect near room temperature, ascribed to the field-induced non-collinear spin texture, possibly a skyrmion lattice (SkL) state. Combined with the large intrinsic anomalous Hall effect, YMn6Sn6 shows a synergic effect of real- and momentum-space Berry phase on physical properties. Since the features of tunable magnetic interaction and flexible structure in this large homologous series, it provides a novel platform for understanding the influence of electronic correlations on topological quantum states in both real and momentum spaces.

cond-mat.str-el↗

arXiv2019

Anomalous Magnetic Behavior in Ba2CoO4 with Isolated CoO4 Tetrahedra

The dimensionality of the electronic and magnetic structure of a given material is generally predetermined by its crystal structure. Here, using elastic and inelastic neutron scattering combined with magnetization measurements, we find unusual magnetic behavior in three-dimensional (3D) Ba2CoO4. In spite of isolated CoO4 tetrahedra, the system exhibits a 3D noncollinear antiferromagnetic order in the ground state with an anomalously large Curie-Weiss temperature of 110 K compared to TN = 26 K. More unexpectedly, spin dynamics displays quasi-2D spin wave dispersion with an unusually large spin gap, and 1D magnetoelastic coupling. Our results indicate that Ba2CoO4 is a unique system for exploring the interplay between isolated polyhedra, low-dimensional magnetism, and novel spin states in oxides.

cond-mat.str-el↗

arXiv2018

ω/T scaling and magnetic quantum criticality in BaFe2(As0.7P0.3)2

We used transport and inelastic neutron scattering to study the optimally phosphorus-doped BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$ superconductor ($T_c = 30$ K). In the normal state, we find that the previously reported linear temperature dependence of the resistivity below room temperature extends to $\sim$ 500 K. Our analysis of the temperature and energy ($E=\hbarω$) dependence of spin dynamical susceptibility at the antiferromagnetic (AF) ordering wave vector $χ^{\prime\prime}({\bf Q}_{\rm AF},ω)$ reveal an $ω/ T$ scaling within $1.1<E/k_BT<110$. These results suggest that the linear temperature dependence of the resistivity is due to the presence of a magnetic quantum critical point in the cleanest iron pnictides near optimal superconductivity. Moreover, the results reconcile the strange-metal temperature dependences with the weakly first-order nature of the quantum transition out of the AF and nematic orders.

cond-mat.supr-con↗

arXiv2018

Neutron spin resonance as a probe of Fermi surface nesting and superconducting gap symmetry in Ba$_{0.67}$K$_{0.33}$(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$

We use inelastic neutron scattering to study energy and wave vector dependence of the superconductivity-induced resonance in hole-doped Ba$_{0.67}$K$_{0.33}$(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ ($x=0,0.08$ with $T_c\approx 37, 28$ K, respectively). In previous work on electron-doped Ba(Fe$_{0.963}$Ni$_{0.037}$)$_2$As$_2$ ($T_N=26$ K and $T_c=17$ K), the resonance is found to peak sharply at the antiferromagnetic (AF) ordering wave vector ${\bf Q}_{\rm AF}$ along the longitudinal direction, but disperses upwards away from ${\bf Q}_{\rm AF}$ along the transverse direction. For hole doped $x=0, 0.08$ without AF order, we find that the resonance displays ring-like upward dispersion away from ${\bf Q}_{\rm AF}$ along both the longitudinal and transverse directions. By comparing these results with calculations using the random phase approximation, we conclude that the dispersive resonance is a direct signature of isotropic superconducting gaps arising from nested hole-electron Fermi surfaces.

cond-mat.supr-con↗

arXiv2018

Local orthorhombic lattice distortions in the paramagnetic tetragonal phase of superconducting NaFe$_{1-x}$Ni$_x$As

Understanding the interplay between nematicity, magnetism and superconductivity is pivotal for elucidating the physics of iron-based superconductors. Here we use neutron scattering to probe magnetic and nematic orders throughout the phase diagram of NaFe$_{1-x}$Ni$_x$As, finding that while both static antiferromagnetic and nematic orders compete with superconductivity, the onset temperatures for these two orders remain well-separated approaching the putative quantum critical points. We uncover local orthorhombic distortions that persist well above the tetragonal-to-orthorhombic structural transition temperature $T_{\rm s}$ in underdoped samples and extend well into the overdoped regime that exhibits neither magnetic nor structural phase transitions. These unexpected local orthorhombic distortions display Curie-Weiss temperature dependence and become suppressed below the superconducting transition temperature $T_{\rm c}$, suggesting they result from a large nematic susceptibility near optimal superconductivity. Our results account for observations of rotational symmetry-breaking above $T_{\rm s}$, and attest to the presence of significant nematic fluctuations near optimal superconductivity.

cond-mat.supr-con↗

arXiv2018

Dynamic spin-lattice coupling and nematic fluctuations in NaFeAs

We use inelastic neutron scattering to study acoustic phonons and spin excitations in single crystals of NaFeAs, a parent compound of iron pnictide superconductors. NaFeAs exhibits a tetragonal-to-orthorhombic structural transition at $T_s\approx 58$ K and a collinear antiferromagnetic (AF) order at $T_N\approx 45$ K. While longitudinal and out-of-plane transverse acoustic phonons behave as expected, the in-plane transverse acoustic phonons reveal considerable softening on cooling to $T_s$, and then harden on approaching $T_N$ before saturating below $T_N$. In addition, we find that spin-spin correlation lengths of low-energy magnetic excitations within the FeAs layer and along the $c$-axis increase dramatically below $T_s$, and show weak anomaly across $T_N$. These results suggest that the electronic nematic phase present in the paramagnetic tetragonal phase is closely associated with dynamic spin-lattice coupling, possibly arising from the one-phonon-two-magnon mechanism.

cond-mat.str-el↗

arXiv2018

Coexistence of superconductivity and short-range double-stripe spin correlations in Te-vapor annealed FeTe$_{1-x}$Se$_{x}$ with $x\le0.2$

In as-grown bulk crystals of Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ with $x\lesssim0.3$, excess Fe ($y>0$) is inevitable and correlates with a suppression of superconductivity. At the same time, there remains the question as to whether the character of the antiferromagnetic correlations associated with the enhanced anion height above the Fe planes in Te-rich samples is compatible with superconductivity. To test this, we have annealed as-grown crystals with $x=0.1$ and 0.2 in Te vapor, effectively reducing the excess Fe and inducing bulk superconductivity. Inelastic neutron scattering measurements reveal low-energy magnetic excitations consistent with short-range correlations of the double-stripe type; nevertheless, cooling into the superconducting state results in a spin gap and a spin resonance, with the extra signal in the resonance being short-range with a mixed single-stripe/double-stripe character, which is different than other iron-based superconductors. The mixed magnetic character of these superconducting samples does not appear to be trivially explainable by inhomogeneity.

cond-mat.supr-con↗

arXiv2017

Suppression of the antiferromagnetic order when approaching the superconducting state in a phase-separated crystal of K$_x$Fe$_{2-y}$Se$_2$

We have combined elastic and inelastic neutron scattering techniques, magnetic susceptibility and resistivity measurements to study single-crystal samples of K$_x$Fe$_{2-y}$Se$_2$, which contain the superconducting phase that has a transition temperature of $\sim$31 K. In the inelastic neutron scattering measurements, we observe both the spin-wave excitations resulting from the block antiferromagnetic ordered phase and the resonance that is associated with the superconductivity in the superconducting phase, demonstrating the coexistence of these two orders. From the temperature dependence of the intensity of the magnetic Bragg peaks, we find that well before entering the superconducting state, the development of the magnetic order is interrupted, at $\sim$42 K. We consider this result to be evidence for the physical separation of the antiferromagnetic and superconducting phases; the suppression is possibly due to the proximity effect of the superconducting fluctuations on the antiferromagnetic order.

cond-mat.supr-con↗

arXiv2017

Mn-induced magnetic symmetry breaking and its correlation with the metal-insulator transition in bilayered Sr3(Ru1-xMnx)2O7

Bilayered Sr3Ru2O7 is an unusual metamagnetic metal with inherently antiferromagnetic (AFM) and ferromagnetic (FM) fluctuations. Partial substitution of Ru by Mn results in the establishment of metal-insulator transition (MIT) at TMIT and AFM ordering at TM in Sr3(Ru1-xMnx)2O7. Using elastic neutron scattering we determined the effect of Mn doping on the magnetic structure and in-plane magnetic correlation lengths in Sr3(Ru1-xMnx)2O7 (x = 0.06 and 0.12). With increasing Mn doping (x) from 0.06 to 0.12 or decreasing temperatures for x=0.12, an evolution from an in-plane short-range to long-range double-stripe AFM ground state occurs. For both compounds, the onset of magnetic correlation with an anisotropic behavior coincides with the sharp rise of the electrical resistivity and the specific heat. Since it does not induce measurable lattice distortion, the double-stripe magnetic order with anisotropic spin texture breaks the symmetry from C4v crystal lattice to C2v magnetic sublattice. These observations shed new light on an age-old question of Slater versus Mott-type MIT.

cond-mat.str-el↗

arXiv2017

Phase diagram and neutron spin resonance of superconducting NaFe$_{1-x}$Cu$_x$As

We use transport and neutron scattering to study the electronic phase diagram and spin excitations of NaFe$_{1-x}$Cu$_x$As single crystals. Similar to Co- and Ni-doped NaFeAs, a bulk superconducting phase appears near $x\approx2\%$ with the suppression of stripe-type magnetic order in NaFeAs. Upon further increasing Cu concentration the system becomes insulating, culminating in an antiferromagnetically ordered insulating phase near $x\approx 50\%$. Using transport measurements, we demonstrate that the resistivity in NaFe$_{1-x}$Cu$_x$As exhibits non-Fermi-liquid behavior near $x\approx1.8\%$. Our inelastic neutron scattering experiments reveal a single neutron spin resonance mode exhibiting weak dispersion along $c$-axis in NaFe$_{0.98}$Cu$_{0.02}$As. The resonance is high in energy relative to the superconducting transition temperature $T_{\rm c}$ but weak in intensity, likely resulting from impurity effects. These results are similar to other iron pnictides superconductors despite the superconducting phase in NaFe$_{1-x}$Cu$_x$As is continuously connected to an antiferromagnetically ordered insulating phase near $x\approx 50\%$ with significant electronic correlations. Therefore, electron correlations is an important ingredient of superconductivity in NaFe$_{1-x}$Cu$_x$As and other iron pnictides.

cond-mat.supr-con↗

arXiv2016

A Mott insulator continuously connected to iron pnictide superconductors

Iron-based superconductivity develops near an antiferromagnetic order and out of a bad metal normal state, which has been interpreted as originating from a proximate Mott transition. Whether an actual Mott insulator can be realized in the phase diagram of the iron pnictides remains an open question. Here we use transport, transmission electron microscopy, X-ray absorption spectroscopy, and neutron scattering to demonstrate that NaFe$_{1-x}$Cu$_x$As near $x\approx 0.5$ exhibits real space Fe and Cu ordering, and are antiferromagnetic insulators with the insulating behavior persisting above the Néel temperature, indicative of a Mott insulator. Upon decreasing $x$ from $0.5$, the antiferromagnetic ordered moment continuously decreases, yielding to superconductivity around $x=0.05$. Our discovery of a Mott insulating state in NaFe$_{1-x}$Cu$_x$As thus makes it the only known Fe-based material in which superconductivity can be smoothly connected to the Mott insulating state, highlighting the important role of electron correlations in the high-$T_{\rm c}$ superconductivity.

cond-mat.supr-con↗

arXiv2016

The magnetic precursor of the pressure-induced superconductivity in Fe-ladder compound

The pressure effects on the antiferromagentic orders in iron-based ladder compounds CsFe$_2$Se$_3$ and BaFe$_2$S$_3$ have been studied using neutron diffraction. With identical crystal structure and similar magnetic structures, the two compounds exhibit highly contrasting magnetic behaviors under moderate external pressures. In CsFe$_2$Se$_3$ the ladders are brought much closer to each other by pressure, but the stripe-type magnetic order shows no observable change. In contrast, the stripe order in BaFe$_2$S$_3$, undergoes a quantum phase transition where an abrupt increase of N$\acute{e}$el temperature by more than 50$\%$ occurs at about 1 GPa, accompanied by a jump in the ordered moment. With its spin structure unchanged, BaFe$_2$S$_3$ enters an enhanced magnetic phase that bears the characteristics of an orbital selective Mott phase, which is the true neighbor of superconductivity emerging at higher pressures.

cond-mat.supr-con↗