Antiferromagnetic Spin Fluctuations Research Articles

Thermal evolution of spin excitations in honeycomb Ising antiferromagnetic FePSe3

We use elastic and inelastic neutron scattering (INS) to study the antiferromagnetic (AF) phase transitions and spin excitations in the two-dimensional (2D) zig-zag antiferromagnet FePSe3. By determining the magnetic order parameter across the AF phase transition, we conclude that the AF phase transition in FePSe3 is first-order in nature. In addition, our INS measurements reveal that the spin waves in the AF ordered state have a large easy-axis magnetic anisotropy gap, consistent with an Ising Hamiltonian, and possible biquadratic magnetic exchange interactions. On warming across TN, we find that dispersive spin excitations associated with three-fold rotational symmetric AF fluctuations change into FM spin fluctuations above TN. These results suggest that the first-order AF phase transition in FePSe3 may arise from the competition between C3 symmetric AF and C1 symmetric FM spin fluctuations around TN, in place of a conventional second-order AF phase transition.

Low-energy spin excitations in detwinned FeSe

Abstract Antiferromagnetic spin fluctuation is regarded as the leading driving force for electron pairing in high-Tc superconductors. In iron-based superconductors, spin excitations at low energy range, especially the spinresonance mode at ER ~5kBTc , are important for understanding the superconductivity. Here, we use inelastic neutron scattering (INS) to investigate the symmetry and in-plane wave-vector dependence of low-energy spin excitations of uniaxial-strain detwinned FeSe. The low-energy spin excitations (E ＜ 10 meV) appear mainly at Q = (±1; 0) in the superconducting state (T ≲ 9 K) and the nematic state (T ≲ 90 K), confirming the constant C 2 rotational symmetry and ruling out the C 4 mode at E ≈ 3 meV reported in a prior INS study. Moreover, our results reveal an isotropic spin resonance in the superconducting state, which is consistent with the s ± wave pairing symmetry. At slightly higher energy, low-energy spin excitations become highly anisotropic. The full width at half maximum (FWHM) of spin excitations is elongated along the transverse (TR) direction. The Q-space isotropic spin resonance and highly anisotropic low-energy spin excitations could arise from dyz intraorbital selective Fermi surface nesting between the hole pocket around Γ point and the electron pockets centered at M X point.

Magnetic Susceptibility Study of Hole-Doped Organic Metal κ-(ET)<sub>4</sub>Hg<sub>3-δ</sub>Cl<sub>8, </sub>δ=22% and κ-(ET)<sub>4</sub>Hg<sub>3-δ</sub>Br<sub>8, </sub>δ=11%

The Non-Fermi-Liquid (NFL) state has been one of central issues in the strongly correlated electrons systems. This deviates from conventional Fermi Liquid (FL) behavior. In the hole-doped triangular lattice organic metal κ-(ET)4Hg3-δBr8, δ = 11% (κ-HgBr), the NFL state is observed as a linear temperature dependence of the resistivity which changed to the temperature square dependence behavior by pressure. The spin susceptibility dependence of the muon Knight shift, K(c), is not linear in the region below 50 K, unlike in other k-type organic superconductors. Furthermore, 13C-NMR study under pressure concluded that strong antiferromagnetic spin fluctuations (AFSF) contribute to the origin of NFL. To understand the underlying correlation of the enhanced AFSF and NFL state in k-HgBr, the K(c) plot study in the sister compound κ-(ET)4Hg3-δCl8, δ=22% (κ-HgCl) which shows metal to insulator transition at TMI ~ 20 K is desirable. In this study, we report a precise susceptibility measurement in both k-HgBr and k-HgCl. Furthermore, we summarize the c(T) data of k-HgBr and k-HgCl and discuss them from the viewpoint of the triangular and square lattice models.

Antiferromagnetic Spin Fluctuations and Unconventional Superconductivity in Topological Superconductor Candidate YPtBi Revealed by ^{195}Pt-NMR.

We report ^{195}Pt nuclear magnetic resonance (NMR) measurements on topological superconductor candidate YPtBi, which has broken inversion symmetry and topological nontrivial band structures due to the strong spin-orbit coupling. In the normal state, we find that Knight shift K is field- and temperature independent, suggesting that the contribution from the topological bands is very small at low temperatures. However, the spin-lattice relaxation rate 1/T_{1} divided by temperature (T), 1/T_{1}T, increases with decreasing T, implying the existence of antiferromagnetic spin fluctuations. In the superconducting state, no Hebel-Slichter coherence peak is seen below T_{c} and 1/T_{1} follows T^{3} variation, indicating the unconventional superconductivity. The finite spin susceptibility at zero-temperature limit and the anomalous increase of the NMR linewidth below T_{c} point to a mixed state of spin-singlet and spin-triplet (or spin-septet) pairing.

Gapless spin excitations in the superconducting state of a quasi-one-dimensional spin-triplet superconductor

Majorana zero modes form as intrinsic defects in an odd-orbital one-dimensional superconductor, thus motivating the search for such materials in the pursuit of Majorana physics. Here, we present combined experimental results and first-principles calculations which suggest that quasi-one-dimensional ${\mathrm{K}}_{2}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}$ may be such a superconductor. Using inelastic neutron scattering we probe the dynamic spin susceptibilities of ${\mathrm{K}}_{2}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}$ and ${\mathrm{K}}_{2}{\mathrm{Mo}}_{3}{\mathrm{As}}_{3}$ and show the presence of antiferromagnetic spin fluctuations in both compounds. Below the superconducting transition, these fluctuations gap in ${\mathrm{K}}_{2}{\mathrm{Mo}}_{3}{\mathrm{As}}_{3}$ but not in ${\mathrm{K}}_{2}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}$. Using first-principles calculations, we show that these fluctuations likely arise from nesting on one-dimensional features of the Fermi surface. Considering these results we propose that while ${\mathrm{K}}_{2}{\mathrm{Mo}}_{3}{\mathrm{As}}_{3}$ is a conventional superconductor, ${\mathrm{K}}_{2}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}$ is likely a spin triplet, and consequently a topological superconductor.

Interrelationships between nematicity, antiferromagnetic spin fluctuations, and superconductivity: Role of hotspots in FeSe1−xSx revealed by high pressure Se77 NMR study

The sulfur-substituted FeSe, FeSe$_{1-x}$S$_{x} $, is one of the unique systems that provides an independent tunability of nematicity, antiferromagnetism and superconductivity under pressure ($p$). Recently Rana et al. [Phys. Rev. B 101, 180503(R) (2020)] reported, from $^{77}$Se nuclear magnetic resonance (NMR) measurements on FeSe$_{0.91}$S$_{0.09}$ under pressure, that there exists a clear role of nematicity on the relationship between antiferromagnetic (AFM) spin fluctuations and superconducting transition temperature ($T_{\rm c}$) where the AFM spin fluctuations are more effective in enhancing $T_{\rm c}$ in the absence of nematicity than with nematicity. Motivated by the work, we carried out $^{77}$Se NMR measurements on FeSe$_{1-x}$S$_{x}$ with $x$= 0.15 and 0.29 under pressure up to 2.10 GPa to investigate the relationship in a wide range of $x$ in the FeSe$_{1-x}$S$_x$ system. Based on the new results together with the previously reported data for $x$=0 [P. Wiecki et al., Phys. Rev. B 96, 180502(R) (2017)] and 0.09 [K. Rana et al. Phys. Rev. B 101, 180503(R) (2020)], we established a $p$ - $x$ - temperature ($T$) phase diagram exhibiting the evolution of AFM spin fluctuations. From the systematic analysis of the NMR data, we found that the superconducting (SC) state in nematic state arises from a non Fermi liquid state with strong stripe-type AFM spin fluctuations while the SC state without nematicity comes from a Fermi liquid state with mild stripe-type AFM spin fluctuations. Furthermore, we show that the previously reported impact of nematicity on the relationship between AFM fluctuations and superconductivity holds throughout the wide range of $x$ from $x$ = 0 to 0.29 in FeSe$_{1-x}$S$_{x}$ under pressure. We discuss the origin of the role of nematicity in terms of the different numbers of hotspots on Fermi surfaces with and without nematicity.

The link between s and d components of electron boson coupling constants in one band d wave Eliashberg theory for high Tc superconductors

The phenomenology of overdoped high Tc uperconductors can be described by a one band d wave Eliashberg theory where the mechanism of superconducting coupling is mediated by antiferromagnetic spin fluctuations and whose characteristic energy Ω0 scales with Tc according to the empirical law Ω0 = 5.8 kBTc. This model presents universal characteristics that are independent of the critical temperature such as the link between the s and d components of electron boson coupling constants and the invariance of the ratio 2∆/kBTc. This situation arises from the particular structure of Eliashberg's equations which, despite being non-linear equations, present solutions with these simple properties.

Topological states and competing magnetic fluctuations in iron germanides

The correlated multiorbital multiband electronic structures of iron-based superconductors harbor an abundance of exotic electronic states, such as spin density waves, superconductivity, topological surface states, and Majorana zero modes. In this paper, we carry out density functional theory combined with dynamical mean-field theory calculations of the electronic structures, spin dynamics, and topological properties in $\mathrm{Y}{\mathrm{Fe}}_{2}{\mathrm{Ge}}_{2}$ and MgFeGe and compare them with $\mathrm{Ba}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ and LiFeAs. We find a coexistence of ferromagnetic and antiferromagnetic spin fluctuations in $\mathrm{Y}{\mathrm{Fe}}_{2}{\mathrm{Ge}}_{2}$ and MgFeGe, which are attributed to the weakly dispersive bands just below the Fermi level. Moreover, we demonstrate that MgFeGe is in a strong topological phase and has two sets of Dirac-cone-like topological surface states on the (001) surface. The coexistence of superconductivity and nontrivial band topology in these two compounds demonstrates that it is interesting to explore topological superconductivity and Majorana zero modes in iron germanides.

Mathematical and Physical Properties of Three-Band s± Eliashberg Theory for Iron Pnictides

The phenomenology of the iron pnictide superconductor can be described by the three-band s± Eliashberg theory in which the mechanism of superconducting coupling is mediated by antiferromagnetic spin fluctuations and whose characteristic energy Ω0 scales with Tc according to the empirical law Ω0=4.65kBTc. This model presents the universal characteristics that are independent of the critical temperature, such as the link between the two free parameters λ13 and λ23 and the ratio Δi/kBTc.

Nested antiferromagnetic spin fluctuations and non-Fermi-liquid behavior in electron-doped CeCo1−xNixIn5

We investigate the electronic state of Ni-substituted ${\mathrm{CeCo}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}{\mathrm{In}}_{5}$ by nuclear quadrupole and magnetic resonance (NQR/NMR) techniques. The heavy fermion superconductivity below ${T}_{\mathrm{c}}=2.3$ K for $x=0$ is suppressed by Ni substitutions, and ${T}_{\mathrm{c}}$ reaches zero for $x=0.25$. The $^{115}\mathrm{In}$ NQR spectra for $x=0.125$ and 0.25 can be explained by simulating the electrical field gradient that is calculated for a virtual supercell with density functional theory. The spin-lattice relaxation rate $1/{T}_{1}$ indicates that Ni substitution weakens antiferromagnetic correlations that are not localized near the substituent but instead are uniform in space. The temperature ($T$) dependence of ${({T}_{1}T)}^{\ensuremath{-}1}$ for $x=0.25$ shows a maximum around ${T}_{\mathrm{g}}=2$ K and ${({T}_{1}T)}^{\ensuremath{-}1}$ decreases toward almost zero when temperature is further reduced as if a gap might be opening in the magnetic excitation spectrum; however, the magnetic specific heat and the static magnetic susceptibility evolve smoothly through ${T}_{\mathrm{g}}$ with a $\ensuremath{-}lnT$ dependence. The peculiar $T$ dependence of ${({T}_{1}T)}^{\ensuremath{-}1}$ and non-Fermi-liquid specific heat and susceptibility can be interpreted in a unified way by assuming nested antiferromagnetic spin fluctuations in a quasi-two-dimensional electronic system.

Shift of the superconducting dome induced by a conduction band of small occupancy: Enhanced d -wave pairing in the overdoped regime

Motivated by the presence of an extremely small electron pocket in the electronic band structure of infinite-layer nickelate superconductors, we systematically explore the superconducting (SC) properties of a two-dimensional Hubbard model influenced by an additional conduction band with small occupancy (dry Fermi sea). Our dynamic cluster quantum Monte Carlo calculations demonstrated the unusual impact of this additional dry metallic layer, which shifts the SC dome of the correlated Hubbard layer towards the overdoped regime, namely the $d$-wave SC can be enhanced (suppressed) in the overdoped (underdoped) regime. Simultaneously, the pseudogap crossover line shifts to the same higher doping regime as well. Our analysis revealed the interplay of the effective pairing interaction, pair-field susceptibility, and antiferromagnetic spin fluctuation in the underlying nonmonotonic physics. We postulate on the physical picture of the SC dome shift in terms of the Kondo-type electron-hole binding in the case of a dry additional conduction band. Our investigation provided valuable insights on tuning the SC properties via external dry bands.

Predicted High-Temperature Superconductivity in Rare Earth Hydride ErH2 at Moderate Pressure

Hydrides offer an opportunity to study high critical temperature (high-T c) superconductivity at experimentally achievable pressures. However, the pressure needed remains extremely high. Using density functional theory calculations, herein we demonstrate that a new rare earth hydride ErH2 could be superconducting with T c ∼ 80 K at 14.5 GPa, the lowest reported value for compressed hydrides to date. Intriguingly, due to Kondo destruction, superconductivity was prone to exist at 15 GPa. We also reveal an energy gap at 20 GPa on the background of normal metallic states. At 20 GPa, this compressed system could act as a host of superconductor judged from a sharp jump of spontaneous magnetic susceptibility with an evanescent spin density of state at Fermi level. Finally, electron pairing glue for ErH2 at these three typical pressures was attributed to the antiferromagnetic spin fluctuation.

Variation of fundamental features of cobalt surface-layered Bi-2212 superconductor materials with diffusion annealing temperature

The present study appears extensively on the role of diffusion annealing temperature intervals 650–850 °C on electrical conductivity, flux pinning ability, superconducting and crystallinity quality of Cobalt (Co) surface-layered Bi-2212 compounds with experimental tests including dc resistivity, bulk density, X-ray diffraction, critical current density measurements, and theoretical calculations. Experimental findings display that the Co ions may be replaced mostly by bismuth sites in the crystal lattice as a consequence of appropriate cation-vacancy, electron configurations of the outer shell, chemical valence states, and electronegativity of chemical contents in the main composition. The fundamental characteristic features refine considerably with 650 °C annealing temperature due to enhancement of antiferromagnetic spin fluctuations in the clusters of microdomains, re-ordering of Cu–O bonds, stabilization of system, pairing mechanism, modulation of insulating Bi–O double layers, and orbital hybridization mechanisms. Accordingly, bulk Bi-2212 ceramic obtained at optimum annealing temperature exhibits the best conductivity because of a decrease in systematic crystallinity problems and potential grain boundary interaction problems expected in the system. Likewise, the optimum annealing temperature triggers the artificial nucleation regions for 2D discrete pancakelike Abrikosov vortices to decelerate thermal fluxon movements. Moreover, the X-ray diffraction results indicate that optimum Co ions in crystal lattice significantly improve crystal structure quality, grain alignment distributions in c-axis orientation, the extension of high-Tc Bi-2223 superconducting phase, and average crystallite size parameters. Additionally, the nucleation activation energy is noticed to reduce with optimum Co ions due to enhancement in the nucleation stability and crystallization temperature values to higher temperature zones. Namely, optimum Co ions easily diffusing into the lattice points support the formation of surface nucleation. In contrast, after a critical value of 650 °C, the characteristic properties mentioned suppress remarkably. In conclusion, the main characteristic features are extensively improved by the optimum diffusion annealing temperature for usage in novel and feasible market areas.

Ferromagnetic exchange field stabilized antiferromagnetic ordering in a cuprate superconductor

We report experimental evidence of formation of antiferromagnetic clusters in composites made of a cuprate superconductor La1.85Sr0.15CuO4 (LCu) and a ferromagnet La0.6Sr0.4CoO3 (LCo). From linear and non-linear ac-susceptibility measurement on this composite material it is found that the exchange field of LCo suppress the dynamic antiferromagnetic spin fluctuation of LCu and convert it into the short range ordered superparamagnetic type antiferromagnetic (AFM) clusters at the cost of superconducting volume fraction. These shrank superconducting volume fraction shows quantum size effect (QSE) and follows DeGennes-Tinkham theory on finite size effect of superconductor.

Comparative studies on superconductivity in Cr3Ru compounds with bcc and A15 structures

Chromium (Cr) is a transition metal element with 3d orbital electrons. In most compounds containing Cr, due to the correlation effect, twofold features, namely localization and itinerancy are expected. The localization gives rise to a magnetic moment, while the latter exhibits as the effective coherent weight for conductivity. Here we report the physical properties of Cr3Ru compounds with body-centered cubic (bcc) and A15 structures by using multiple experimental tools. The resistivity measurements show sharp superconducting transitions at = 2.77 K and = 3.37 K for the bcc and A15 structures, respectively. A high residual resistivity exists in both phases. Magnetization measurements also show rather narrow superconducting transitions, with a clear hump feature in the intermediate temperature region (about 150 K), which may be ascribed to the remaining antiferromagnetic spin fluctuations. A pronounced second peak effect has been observed in magnetization hysteresis loops in the superconducting state only for samples with bcc structure. The specific heat coefficient reveals a clear jump at critical temperatures (). We find that s-wave gaps can be adopted to fit the low temperature specific heat data of both samples yielding ratios of about 3.6, indicating a moderate pairing strength. Interestingly, the Wilson ratios are 3.81 and 3.62 for the bcc and A15 phases, suggesting a moderate correlation effect of conducting electrons in the normal state. Besides, for samples with A15 structure, another specific heat anomaly occurs at about 0.85 K and is sensitive to magnetic fields. In addition, by applying high pressures, both systems will exhibit an enhancement of with a rate of about 0.019 K GPa−1 and 0.013 K GPa−1 for the bcc and A15 phases, respectively. Our combinatory results point to unusual behavior of both superconducting and normal states in these two Cr based alloys.

Possible coexistence of antiferromagnetic and ferromagnetic spin fluctuations in the spin-triplet superconductor UTe2 revealed by Te125 NMR under pressure

A spin-triplet superconducting state mediated by ferromagnetic (FM) spin fluctuations has been suggested to occur in the newly discovered heavy-fermion superconductor ${\mathrm{UTe}}_{2}$. However, the recent neutron scattering measurements revealed the presence of antiferromagnetic (AFM) spin fluctuations in ${\mathrm{UTe}}_{2}$. Here, we report the $^{125}\mathrm{Te}$ nuclear magnetic resonance studies of a single-crystal ${\mathrm{UTe}}_{2}$, suggesting the coexistence of FM and AFM spin fluctuations in ${\mathrm{UTe}}_{2}$. Owing to the two different Te sites in the compound, we conclude that the FM spin fluctuations are dominant within ladders and the AFM spin fluctuations originate from the interladder magnetic coupling. Although AFM spin fluctuations exist in the system, the FM spin fluctuations in the ladders may play an important role in the appearance of the spin-triplet superconducting state of ${\mathrm{UTe}}_{2}$.

Normal-State Transport Properties of Infinite-Layer Sr1-xLaxCuO2 Electron-Doped Cuprates in Optimal- and Over-Doped Regimes.

Transport properties of electron-doped cuprate SrLaCuO thin films have been investigated as a function of doping. In particular, optimal- and over-doped samples were obtained by tuning the Sr:La stoichiometric ratio. Optimal-doped samples show a non-Fermi liquid behavior characterized by linear dependence of the resistivity from room temperature down to intermediate temperature (about 150–170 K). However, by approaching temperatures in the superconducting transition, a Fermi-liquid behavior-characterized by a -scaling law-was observed. Once established, the transition from a linear-T to a quadratic- behavior was successfully traced back in over-doped samples, even occurring at lower temperatures. In addition, the over-doped samples show a crossover to a linear-T to a logarithmic dependence at high temperatures compatible with anti-ferromagnetic spin fluctuations dominating the normal state properties of electron-doped cuprates.

Spin-triplet superconductivity driven by finite-momentum spin fluctuations

A small number of superconductors are believed to exhibit intrinsic spin-triplet pairing, and they are often discussed in terms of a simple, $^{3}\mathrm{He}$-like picture where ferromagnetic spin fluctuations provide the ``glue.'' However, in some cases in which reliable inelastic neutron scattering measurements are available, spin excitations are found to be peaked at finite momentum $\mathbf{q}$ rather than $\mathbf{q}=\mathbit{0}$. Here we investigate some simple models that exhibit triplet pairing arising from antiferromagnetic spin fluctuations. We show that a strong peak at larger $\mathbf{q}$ in the magnetic susceptibility can drive such states and can give rise to pairing states with nodes in the ${k}_{z}$ plane even in the presence of a pure two-dimensional Fermi surface. In these situations, dominant pair scattering processes occur between Fermi surface segments with like signs of the superconducting order parameter, yet they are consistent with an overall odd-parity state. We examine the applicability of these scenarios to the putative triplet superconductor $\mathrm{U}{\mathrm{Te}}_{2}$.

Planar Cu and O NMR and the Pseudogap of Cuprate Superconductors

Recently, an analysis of all available planar oxygen shift and relaxation data for the cuprate high-temperature superconductors showed that the data can be understood with a simple spin susceptibility from a metallic density of states common to all cuprates. It carries a doping dependent but temperature independent pseudogap at the Fermi surface, which causes the deviations from normal metallic behavior, also in the specific heat. Here, a more coherent, unbiased assessment of all data, including planar Cu, is presented and consequences are discussed, since the planar Cu data were collected and analyzed prior to the O data. The main finding is that the planar Cu shifts for one direction of the external magnetic field largely follow from the same states and pseudogap. This explains the shift suppression stated more recently, which leads to the failure of the Korringa relation in contrast to an enhancement of the relaxation due to antiferromagnetic spin fluctuations originally proposed. However, there is still the need for a second spin component that appears to be associated with the Cu 3d(x2−y2) hole to explain the complex Cu shift anisotropy and family dependence. Furthermore, it is argued that the planar Cu relaxation which was reported recently to be rather ubiquitous for the cuprates, must be related to this universal density of states and the second spin component, while not being affected by the simple pseudogap. Thus, while this universal metallic density of states with a pseudogap is also found in the planar Cu data, there is still need for a more elaborate scenario that eludes planar O.

Pseudogap in hole-doped cuprates: Possible insights from the Kondo effect

The ``states-non-conserving'' fermion density of states (DOS), deduced from the specific heat of hole-doped cuprates, could arise from a Kondo or heavy fermionlike DOS being suppressed by antiferromagnetic spin fluctuations. The large Fermi surface predicted by band theory and observed experimentally is still expected for zero pseudogap but with an effective mass corresponding to a Kondo temperature ${T}_{K}\ensuremath{\simeq}800\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. A finite pseudogap could divide it into Fermi arcs. Theoretical results for the asymmetric Anderson model can account for the experimental Wilson ratio.