arXiv subjects

Chenglin Zhang

Publications and source records attributed to Chenglin Zhang.

43 records · Page 3Linked to original sources

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

In-plane magnetic field effect on the neutron spin resonance in optimally doped FeSe$_{0.4}$Te$_{0.6}$ and BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ superconductors

We use inelastic neutron scattering to study the effect of an in-plane magnetic field on the magnetic resonance in optimally doped superconductors FeSe$_{0.4}$Te$_{0.6}$ ($T_c=14$ K) and BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ ($T_c=20$ K). While the magnetic field up to 14.5 Tesla does not change the energy of the resonance, it particially suppresses $T_c$ and the corresponding superconductivity-induced intensity gain of the mode. However, we find no direct evidence for the field-induced spin-1 Zeeman splitting of the resonance. Therefore, it is still unclear if the resonance is the long-sought singlet-triplet excitation directly coupled to the superconducting electron Cooper pairs.

cond-mat.supr-con↗

arXiv2011

Spin Waves and magnetic exchange interactions in insulating Rb$_{0.89}$Fe$_{1.58}$Se$_2$

The discovery of alkaline iron selenide $A$Fe$_{1.6+x}$Se$_2$ ($A=$ K, Rb, Cs) superconductors has generated considerable excitement in the condensed matter physics community because superconductivity in these materials may have a different origin from the sign reversed s-wave electron pairing mechanism, a leading candidate proposed for all other Fe-based superconductors. Although $A$Fe$_{1.6+x}$Se$_2$ are isostructural with the metallic antiferromagnetic (AF) iron pnictides such as (Ba,Ca,Sr)Fe$_2$As$_2$, they are insulators near $x=0$ and form a $\sqrt{5}\times\sqrt{5}$ blocked AF structure (Fig. 1a) completely different from the iron pnictides. If magnetism is responsible for superconductivity of all iron-based materials, it is important to determine their common magnetic features. Here we use neutron scattering to map out spin waves in the AF insulating Rb$_{0.89}$Fe$_{1.58}$Se$_2$. We find that although Rb$_{0.89}$Fe$_{1.58}$Se$_2$ has a N$\rm \acute{e}$el temperature ($T_N=475$ K) much higher than that of the iron pnictides ($T_N\leq 220$ K), spin waves for both classes of materials have similar zone boundary energies. A comparison of the fitted effective exchange couplings using a local moment Heisenberg Hamiltonian in Rb$_{0.89}$Fe$_{1.58}$Se$_2$, (Ba,Ca,Sr)Fe$_2$As$_2$, and iron chalcogenide Fe$_{1.05}$Te reveals that their next nearest neighbor (NNN) exchange couplings are similar. Therefore, superconductivity in all Fe-based materials may have a common magnetic origin that is intimately associated with the NNN magnetic exchange interactions, even though they have metallic or insulating ground states, different AF orders and electronic band structures.

cond-mat.supr-con↗

arXiv2010

Normal-State Hourglass Dispersion of the Spin Excitations in FeSe$_{x}$Te$_{1-x}$

We use cold neutron spectroscopy to study the low-energy spin excitations of superconducting (SC) FeSe$_{0.4}$Te$_{0.6}$ and essentially non-superconducting (NSC) FeSe$_{0.45}$Te$_{0.55}$. In contrast to BaFe$_{2-x}$(Co,Ni)$_{x}$As$_2$, where the low-energy spin excitations are commensurate both in the SC and normal state, the normal-state spin excitations in SC FeSe$_{0.4}$Te$_{0.6}$ are incommensurate and show an hourglass dispersion near the resonance energy. Since similar hourglass dispersion is also found in the NSC FeSe$_{0.45}$Te$_{0.55}$, we argue that the observed incommensurate spin excitations in FeSe$_{1-x}$Te$_{x}$ are not directly associated with superconductivity. Instead, the results can be understood within a picture of Fermi surface nesting assuming extremely low Fermi velocities and spin-orbital coupling.

cond-mat.supr-con↗

arXiv2010

Anisotropic Neutron Spin Resonance in Superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_2$

We use polarized inelastic neutron scattering to show that the neutron spin resonance below $T_c$ in superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_2$ ($T_c=20$ K) is purely magnetic in origin. Our analysis further reveals that the resonance peak near 7~meV only occurs for the planar response. This challenges the common perception that the spin resonance in the pnictides is an isotropic triplet excited state of the singlet Cooper pairs, as our results imply that only the $S_{001}=\pm1$ components of the triplet are involved.

cond-mat.supr-con↗

arXiv2010

Electron-doping evolution of the low-energy spin excitations in the iron arsenide BaFe$_{2-x}$Ni$_{x}$As$_{2}$ superconductors

We use elastic and inelastic neutron scattering to systematically investigate the evolution of the low-energy spin excitations of the iron arsenide superconductor BaFe2-xNixAs2 as a function of nickel doping x. In the undoped state, BaFe2As2 exhibits a tetragonal-to-orthorhombic structural phase transition and simultaneously develops a collinear antiferromagnetic (AF) order below TN = 143 K. Upon electron-doping of x = 0.075 to induce bulk superconductivity with Tc = 12.3 K, the AF ordering temperature reduces to TN = 58 K.We show that the appearance of bulk superconductivity in BaFe1.925Ni0.075As2 coincides with a dispersive neutron spin resonance in the spin excitation spectra, and a reduction in the static ordered moment. For optimally doped BaFe1.9Ni0.1As2 (Tc = 20 K) and overdoped BaFe1.85Ni0.15As2 (Tc = 15 K) superconductors, the static AF long-range order is completely suppressed and the spin excitation spectra are dominated by a resonance and spin-gap at lower energies. We determine the electron-doping dependence of the neutron spin resonance and spin gap energies, and demonstrate that the three-dimensional nature of the resonance survives into the overdoped regime. If spin excitations are important for superconductivity, these results would suggest that the three-dimensional character of the electronic superconducting gaps are prevalent throughout the phase diagram, and may be critical for superconductivity in these materials.

cond-mat.supr-con↗

arXiv2009

Resonance as a probe of the electron superconducting gap in BaFe1.9Ni0.1As2

The discovery of high-transition temperature (high-Tc) superconductivity near antiferromagnetism in iron arsenides raised the possibility of an unconventional superconducting mechansim1-8. The observation of clear Fermi surfaces and nodeless superconducting gaps by angle resolved photoemission9-12 suggests that electron pairing in these materials may be mediated by quasiparticle excitations between sign reversed hole and electron Fermi pockets5-8. Although the presence of a 'resonance' in the spin excitation spectrum found by inelastic neutron scattering13-17 is consistent with this picture18-20, there has been no direct evidence connecting the resonance to the superconducting gap energy. Here we show that for the optimally electron doped BaFe1.9Ni0.1As2 (Tc =20 K, Fig. 1c) iron arsenide superconductor, application of a magnetic field that suppresses the superconductivity and superconducting gap energy also reduces the intensity and energy of the resonance. These results suggest that the energy of the resonance is proportional to the electron pairing energy, and thus indicate that spin fluctuations are intimately related to the mechanism of superconductivity in iron arsenides.

cond-mat.supr-con↗