Antiferromagnetic 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.

Nonlinear phenomena in charge transport properties of a hole-doped organic spin-liquid compound

The nonlinear electric conductivity of κ-(BEDT-TTF)4Hg2.89Br8, which is known as a hole-doped spin liquid, has been observed by single crystal transport measurements using dc and ac excitations. This compound is known as a charge transfer complex with strong magnetic fluctuations accompanied by electron mass enhancement at ambient pressure. We discuss the nonlinear dc conductivity drastically emerging below 100 K. We also performed ac impedance measurements of this compound and compared the results with those of a typical dimer-Mott compound of deuterated κ-(d8-BEDT-TTF)2Cu[N(CN)2]Br, located just near the Mott boundary. The analyses of the Nyquist plots of κ-(d8-BEDT-TTF)2Cu[N(CN)2]Br and κ-(BEDT-TTF)4Hg2.89Br8 reveal qualitatively different features. The former shows a behavior pertinent to the inhomogeneous distribution of domains due to a mixing of metal and insulating phases, taking into account the influence of the proximity effect, while the doped spin liquid has a distinct semicircle-type frequency dependence in its Nyquist plot in the whole temperature range studied. We conclude that the nonlinear conductivity is intrinsically peculiar to the doped dimer Mott system, where the charge degrees of freedom dominate the itinerancy. We attribute the anomalous features, such as non-Fermi liquid behavior, heat capacity enhancement, and strong antiferromagnetic fluctuations in κ-(BEDT-TTF)4Hg2.89Br8, to a kind of charge confinement effect retained in the hole-doped spin-liquid state.

Unconventional anomalous Hall effect in Zintl thermoelectric Eu[formula omitted]ZnSb[formula omitted

The outstanding performance of thermoelectric materials is intimately related to complicated electronic band structures that may comprise topological linear dispersion bands. In this work, the exotic magnetism and magneto-transport properties of thermoelectric Eu2ZnSb2 single crystal are investigated. Temperature- and field-dependent magnetization reveal an antiferromagnetic (AFM) ground state and highly localized magnetic moments of Eu2+ ions. The longitudinal resistivity [ρxx(T)] and magnetoresistivity (MR) unveil the coexistence of underlying ferromagnetic (FM) and AFM fluctuations. Furthermore, an unconventional anomalous Hall effect (UAHE) for both above and below the AFM phase transition temperature (TN) is observed, which is suggested to be caused by the vacancy-induced Weyl bands and strong magnetic fluctuations. Our work is benefit for understanding the exotic band structures and thermoelectric properties in Eu2ZnSb2.

Electronic spin resonance study of room-temperature ferromagnet cr[formula omitted]te[formula omitted

Chromium telluride Cr3Te4 with intrinsic room-temperature ferromagnetism has substantially practical application in spintronics. In this work, the microwave response of Cr3Te4 single crystal is investigated by the electronic spin resonance (ESR) with the magnetic field applied along various directions. Two resonance lines are observed for both H//ab and H//c, which are suggested to result from ferromagnetic (FM) and antiferromagnetic (AFM) fluctuations respectively. The angle-dependent ESR spectra show considerable anisotropy, with a strong microwave response for H//ab and a weak microwave response for H//c. The analysis of the ESR spectra reveals the coexistence of FM and AFM fluctuations in this system. Moreover, this anisotropic microwave response in single crystal Cr3Te4 could benefit the microwave-based spintronic device.

Spin-fluctuation glue disfavors high-critical temperature of superconductivity?

Antiferromagnetic fluctuations are believed to be a promising glue to drive high-temperature superconductivity especially in cuprates. Here, we perform a close inspection of the superconducting mechanism from spin fluctuations in the Eliashberg framework by employing a typical one-band model on a square lattice. While spin fluctuations can eventually drive superconductivity as is well established, we find that the superconducting tendency is suppressed substantially by a seemingly negligible contribution from a small momentum transfer far away from . This suppression comes from phase frustration of the pairing gap and is expected to be a general feature due to the repulsive pairing interaction of spin fluctuations. Furthermore, we find that the momentum dependence of the pairing gap largely deviates from the functional form of , although this form is well established in cuprate superconductors. We argue that an instantaneous magnetic interaction plays the important role to understand high-critical temperature of superconductivity as well as the momentum dependence of the pairing gap.

Spin fluctuations in Sr1.8La0.2RuO4

We use inelastic neutron scattering to study spin fluctuations in Sr$_{1.8}$La$_{0.2}$RuO$_4$, where Lanthanum doping triggers a Lifshitz transition by pushing the van Hove singularity in the $\gamma$ band to the Fermi energy. Strong spin fluctuations emerge at an incommensurate wave vector $\mathbf{Q}_{ic} = (0.3,0.3)$, corresponding to the nesting vector between $\alpha$ and $\beta$ Fermi sheets. The incommensurate antiferromagnetic fluctuations shift toward $(0.25,0.25)$ with increasing energy up to ${\sim}110$ meV. By contrast, scatterings near the ferromagnetic wave vectors $\mathbf{Q} = (1,0)$ and $(1,1)$ remain featureless at all energies. This contradicts the weak-coupling perspective that suggests a sharp enhancement of ferromagnetic susceptibility due to the divergence of density of states in the associated $\gamma$ band. Our findings imply that ferromagnetic fluctuations in Sr$_2$RuO$_4$ and related materials do not fit into the weak-coupling paradigm, but instead are quasi-local fluctuations induced by Hund's coupling. This imposes significant constraints for the pairing mechanism involving spin fluctuations.

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.

Local pairs in high-temperature superconductors: The concept of pseudogap

The origin of the high-temperature superconductivity of cuprates remains a subject of debate after almost four decades of investigation. One of the main questions — what is the physics behind the mechanism of superconducting pairing, which makes it possible to obtain real Cooper pairs at temperatures much higher than 100 K, remains open. It is believed that the answer can be obtained by the studying the properties of cuprates in the normal state above Tc, where the pseudogap (PG) opens at T* >> Tc. The number of papers devoted to the study of PG is extraordinarily large, but its physics also remains in question. In cuprates, the question arises whether the pseudogap phase arises exclusively due to antiferromagnetic fluctuations, spin or charge density waves, or whether it can be explained by the formation below T* of specific paired fermions, the so-called local pairs. We review studies on both approaches to PG that should clarify this issue. In addition to theoretical considerations, we analyze and discuss various experimental results of fluctuation conductivity and PG measurements, as well as magnetic measurements, in an attempt to prove the decisive role of local pairs in the formation of the PG state. Accordingly, various types of supposed magnetic interactions can be considered as possible mechanisms of superconducting pairing in high-temperature superconductors.

Magnetic phase diagram and multiple field-induced states in the intermetallic triangular-lattice antiferromagnet NdAuAl4Ge2 with Ising-like spins

Geometrical frustration and the enhancement of strong quantum fluctuations in two-dimensional triangular antiferromagnets can lead to various intriguing phenomena. Here, we studied the spin-1/2 triangular lattice antiferromagnet NdAuAl$_4$Ge$_2$. Thermodynamic and transport properties, such as magnetization and specific heat together with the resistivity measurements were performed. In zero field, two successive phase transitions were observed at $T_{\rm N1}=1.75\pm 0.02$ K and $T_{\rm N2}=0.49\pm 0.02$ K, respectively. Under magnetic field, $\rm XXZ$-type anisotropy was revealed, with the moments pointing along the easy $c$ axis. For $B\parallel c$, multiple field-induced states were observed, and the magnetic phase diagram was established based on the specific heat and magnetization data. The temperature-dependent resistivity measurements indicate that NdAuAl$_4$Ge$_2$ is a good metal. It is very likely that both the long-range RKKY interactions and the geometrical frustration play an important roles in this case.

Ubiquitous spin freezing in the superconducting state of UTe2

In most superconductors electrons form Cooper pairs in a spin-singlet state mediated by either phonons or by long-range interactions such as spin fluctuations. The superconductor UTe2 is a rare material wherein electrons are believed to form pairs in a unique spin-triplet state with potential topological properties. While spin-triplet pairing may be mediated by ferromagnetic or antiferromagnetic fluctuations, experimentally, the magnetic properties of UTe2 are unclear. By way of muon spin rotation/relaxation (μSR) measurements on independently grown UTe2 single crystals we demonstrate the existence of magnetic clusters that gradually freeze into a disordered spin frozen state at low temperatures. Our findings suggest that inhomogeneous freezing of magnetic clusters is linked to the ubiquitous residual linear term in the temperature dependence of the specific heat (C) and the low-temperature upturn in C/T versus T. The omnipresent magnetic inhomogeneity has potential implications for experiments aimed at establishing the intrinsic low-temperature properties of UTe2.

Enhanced magnetism in Ru‐doped hybrid improper perovskite Ca3Mn2O7 via experimental and first‐principles study

AbstractThe quasi‐2D Ruddlesden–Popper layered perovskites with single‐phase multiferroicity have attracted much attention in recent magnetoelectric memory applications. We have systematically investigated the optical and magnetic properties of Ru‐doped hybrid improper perovskite Ca3Mn2O7 both experimentally and using first‐principles calculations. The Ca3RuxMn2−xO7 (x = 0, 0.02, 0.06, 0.10) powders were successfully synthesized using the sol–gel method. Ru doping enhanced the ferromagnetism of Ca3Mn2O7. The quasi‐2D antiferromagnetic (AFM) fluctuation effect was observed in Ca3Mn2O7 and in certain doped samples. We also found that the optical bandgaps of the doped samples were reduced after doping, agreeing with the results of the first‐principles calculations. Infrared absorption spectra indicated the distortion of the Mn–O bonds was possibly due to the Jahn–Teller effect. To analyze the electronic structures and to understand the detailed atomic contributions of magnetic moments, the Crystal Orbital Hamilton Population (COHP) and Integrated COHP of the (Mn,Ru)O6 octahedra were also calculated. The Ru‐doped materials with the coexistence of ferromagnetic and AFM orderings are expected to be excellent candidates for magnetoelectric devices.

Fluctuating Ru trimer precursor to a two-stage electronic transition in RuP

Superconductivity in binary ruthenium pnictides occurs proximal to and upon suppression of a nonmagnetic ground state, preceded by a pseudogap phase associated with Fermi surface instability, and its critical temperature, ${T}_{c}$, is maximized around the pseudogap quantum critical point. By analogy with isoelectronic iron-based counterparts, antiferromagnetic fluctuations became ``usual suspects'' as putative mediators of superconducting pairing. Here we report on a high-temperature local symmetry breaking in RuP, the nonsuperconducting parent of the maximum-${T}_{c}$ branch of these novel superconductors, revealed by combined nanostructure-sensitive powder and single-crystal x-ray total scattering analyses. Large local distortions of Ru chains associated with orbital-charge trimerization, further assembled into hexamers, exist above the two-stage electronic transition in RuP. In the pseudogap regime the precursors order with distortions retaining their strength, whereas they acquire spin-singlet characteristics which dramatically enhances the distortion as the nonmagnetic ground state establishes. The precursors enable the nonmagnetic ground state and presumed complex oligomerization, and the relevance of pseudogap fluctuations for superconductivity emerges as a distinct prospect. As a transition metal system in which partial d-manifold filling combined with high crystal symmetry promotes electronic instabilities, this represents a further example of local electronic precursors underpinning the macroscopic collective behavior of quantum materials.

Antiferromagnetic fluctuations and orbital-selective Mott transition in the van der Waals ferromagnet Fe3−xGeTe2

Fe3-xGeTe2 is a layered magnetic van der Waals material of interest for both fundamental and applied research. Despite the observation of intriguing physical properties, open questions exist even on the basic features related to magnetism: is it a simple ferromagnet or are there antiferromagnetic regimes and are the moments local or itinerant. Here, we demonstrate that antiferromagnetic spin fluctuations coexist with the ferromagnetism through comprehensive elastic and inelastic neutron scattering and thermodynamic measurements. Our realistic dynamical mean-field theory calculations reveal that the competing magnetic fluctuations are driven by an orbital selective Mott transition, where only the plane-perpendicular a1g orbital of the Fe(3d) manifold remains itinerant. Our results highlight the multi-orbital character in Fe3-xGeTe2 that supports a rare coexistence of local and itinerant physics within this material.

Antiferromagnetic fluctuations and dominant dxy -wave pairing symmetry in nickelate-based superconductors

Motivated by recent experimental studies on superconductivity found in nickelate-based materials, we study the temperature dependence of the spin correlation and the superconducting pairing interaction within an effective two-band Hubbard model by the quantum Monte Carlo method. Based on parameters extracted from first-principles calculations, our intensive numerical results reveal that the pairing with a $d_{xy}$-wave symmetry firmly dominates over other pairings at low temperature, which is mainly determined by the Ni 3$d$ orbital. It is also found that the effective pairing interaction is enhanced as the on-site interaction increases, demonstrating that the superconductivity is driven by strong electron-electron correlation. Even though the $(\pi,\pi)$ antiferromagnetic correlation could be enhanced by electronic interaction, there is no evidence for long-range antiferromagnetic order exhibited in nickelate-based superconductors. Moreover, our results offer possible evidence that the pure electron correlation may not account for the charge density wave state observed in nickelates.

Large antiferromagnetic fluctuation enhancement of the thermopower at a critical doping in magnetic semimetal Cr1+δTe2

Cr1+dTe2 is a self-intercalated transition metal dichalcogenide that hosts tunable electronic filling and magnetism in its semimetallic band structure. Recent angle-resolved photoemission spectroscopy (ARPES) studies have unveiled a systematic shift in this semimetallic band structure relative to the chemical potential with increased Cr doping. This report presents the temperature and magnetic field dependence of the longitudinal thermopower Sxx for different Cr1+dTe2 compositions. We show that as doping increases, the sign of Sxx changes from positive to negative at the critical doping level of d ~ 0.5. This observed doping-dependent trend in the thermopower is consistent with the evolution of the semimetallic band structure from ARPES. Importantly, an anomalous enhancement of the thermoelectric response is also observed around d~0.5. Combining information from magnetometry and ARPES measurements, existence of the critical nature of the doping level dc (~0.5) is unveiled in magnetic semimetal Cr1+dTe2, where antiferromagnetic fluctuation and near-Fermi-energy pseudogap formation play a potential vital role in enhancing thermoelectric energy conversion.

Superconductivity in the crystallogenide LaFeSiO1−δ with squeezed FeSi layers

Pnictogens and chalcogens are both viable anions for promoting Fe-based superconductivity, and intense research activity in the related families has established a systematic correlation between the Fe-anion height and the superconducting critical temperature Tc, with an optimum Fe-anion height of ~1.38 Å. Here, we report the discovery of superconductivity in the compound LaFeSiO1−δ that incorporates a crystallogen element, Si, and challenges the above picture: considering the strongly squeezed Fe–Si height of 0.94 Å, the superconducting transition at Tc = 10 K is unusually high. In the normal state, the resistivity displays non-Fermi-liquid behavior while NMR experiments evidence weak antiferromagnetic fluctuations. According to first-principles calculations, the Fermi surface of this material is dominated by hole pockets without nesting properties, which explains the strongly suppressed tendency toward magnetic order and suggests that the emergence of superconductivity materializes in a distinct set-up, as compared to the standard s±- and d-wave electron-pocket-based situations. These properties and its simple-to-implement synthesis make LaFeSiO1−δ a particularly promising platform to study the interplay between structure, electron correlations, and superconductivity.

Multipole-fluctuation pairing mechanism of dx2−y2 + ig superconductivity in Sr2RuO4

Despite of many experimental and theoretical efforts, the pairing symmetry of superconductivity in Sr$_2$RuO$_4$ remains undecided. The accidentally degenerate $d_{x^2-y^2}+ig$ is consistent with most current experiments and seems to be one of the most probable candidates, but we still lack a satisfactory theoretical mechanism for its appearance. Here we construct a phenomenological model combining realistic electronic band structures and all symmetry-allowed multipole fluctuations as potential pairing glues, and make a systematic survey of major pairing states within the Eliashberg framework. Our calculations show that $d_{x^2-y^2}+ig$ can arise naturally from the interplay of antiferromagnetic, ferromagnetic, and electric multipole fluctuations whose coexistence is manifested in previous experiments and calculations. Our work provides a physically reasonable basis supporting the possibility of $d_{x^2-y^2}+ig$ pairing in superconducting Sr$_2$RuO$_4$.

Quantifying the role of antiferromagnetic fluctuations in the superconductivity of the doped Hubbard model

We study the contribution of the electron-spin fluctuation coupling to the superconducting state of the two dimensional Hubbard model within dynamical cluster approximation (DCA) using a numerical exact continuous time Monte Carlo solver. By analyzing the frequency dependence of the self energy, we show that only about half of the superconductivity can be attributed to a "pairing glue" arising from treating spin fluctuations as a pairing boson in the standard one-loop theory.

Competing magnetic and nonmagnetic states in monolayer VSe2 with charge density wave

The field of two-dimensional ferromagnets has been reinvigorated by the discovery of ${\mathrm{VSe}}_{2}$ monolayer grown on van der Waals substrates, which is reported to be ferromagnetic with a Curie point higher than 330 K. However, the ferromagnetic and nonmagnetic states of pristine monolayer ${\mathrm{VSe}}_{2}$ are highly debated. Here, employing density functional theory, Wannier function calculations, and the band unfolding method, we explore the electronic structure of monolayer ${\mathrm{VSe}}_{2}$ with a $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{7}$ charge density wave (CDW). Certain qualitative aspects of the calculated unfolded band dispersion and unfolded Fermi surface of monolayer ${\mathrm{VSe}}_{2}$ with the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{7}$ CDW in the nonmagnetic state agree well with previous angle-resolved photoemission spectroscopy results, albeit with uncertainty about whether these experiments probed single or multiple domains. Specifically, we find that an isolated CDW domain naturally induces a strong breaking of the threefold symmetry of the electronic structure. In addition we find that, relative to the undistorted structure, the CDW structure shows a strong competition between nonmagnetic and various magnetic states, with an energy difference less than 5 meV/f.u. For the CDW structure in the antiferromagnetic state, the band dispersions and Fermi surface are similar to those in the nonmagnetic state, while the unfolded bands of the ferromagnetic CDW state display a sizable exchange splitting. These results indicate the possibility of various antiferromagnetic fluctuations in ${\mathrm{VSe}}_{2}$ to coexist and compete with ferromagnetic order and the experimentally reported CDW order. Our calculations build insights for exploring the interplay between magnetism and CDW behaviors more generally in transition metal dichalcogenides.

Mechanism of superconductivity in the Hubbard model at intermediate interaction strength

We study the fluctuations responsible for pairing in the d-wave superconducting state of the two-dimensional Hubbard model at intermediate coupling within a cluster dynamical mean-field theory with a numerically exact quantum impurity solver. By analyzing how momentum- and frequency-dependent fluctuations generate the d-wave superconducting state in different representations, we identify antiferromagnetic fluctuations as the pairing glue of superconductivity in both the underdoped and the overdoped regime. Nevertheless, in the intermediate coupling regime, the predominant magnetic fluctuations may differ significantly from those described by conventional spin fluctuation theory.