High-energy Spin Fluctuations Research Articles

Crossover of high-energy spin fluctuations from collective triplons to localized magnetic excitations in Sr14−xCaxCu24O41 ladders

We studied the magnetic excitations in the quasi-one-dimensional (q-1D) ladder subsystem of Sr14−xCaxCu24O41 (SCCO) using Cu L3-edge resonant inelastic X-ray scattering (RIXS). By comparing momentum-resolved RIXS spectra with high (x = 12.2) and without (x = 0) Ca content, we track the evolution of the magnetic excitations from collective two-triplon (2 T) excitations (x = 0) to weakly-dispersive gapped modes at an energy of 280 meV (x = 12.2). Density matrix renormalization group (DMRG) calculations of the RIXS response in the doped ladders suggest that the flat magnetic dispersion and damped excitation profile observed at x = 12.2 originates from enhanced hole localization. This interpretation is supported by polarization-dependent RIXS measurements, where we disentangle the spin-conserving ΔS = 0 scattering from the predominant ΔS = 1 spin-flip signal in the RIXS spectra. The results show that the low-energy weight in the ΔS = 0 channel is depleted when Sr is replaced by Ca, consistent with a reduced carrier mobility. Our results demonstrate that off-ladder impurities can affect both the low-energy magnetic excitations and superconducting correlations in the CuO4 plaquettes. Finally, our study characterizes the magnetic and charge fluctuations in the phase from which superconductivity emerges in SCCO at elevated pressures.

Disentangling the role of bond lengths and orbital symmetries in controlling Tc of optimally doped YBa2Cu3O7

Optimally doped YBCO (YBa$_{2}$Cu$_{3}$O$_{7}$) has a high critical temperature, at 92 K. It is largely believed that Cooper pairs form in YBCO and other cuprates because of spin fluctuations, the issue and the detailed mechanism is far from settled. In the present work, we employ a state-of-the-art \emph{ab initio} ability to compute both the low and high energy spin fluctuations in optimally doped YBCO. We benchmark our results against recent inelastic neutron scattering and resonant inelastic X-ray scattering measurements. Further, we use strain as an external parameter to modulate the spin fluctuations and superconductivity. We disentangle the roles of Barium-apical Oxygen hybridization, the interlayer coupling and orbital symmetries by applying an idealized strain, and also a strain with a fully relaxed structure. We show that shortening the distance between Cu layers is conducive for enhanced Fermi surface nesting, that increases spin fluctuations and drives up $T_{c}$. However, when the structure is fully relaxed electrons flow to the d$_{z^2}$ orbital as a consequence of a shortened Ba-O bond which is detrimental for superconductivity

From mean-field localized magnetism to itinerant spin fluctuations in the “nonmetallic metal” FeCrAs

FeCrAs displays an unusual electrical response that is neither metallic in character nor divergent at low temperatures, as expected for an insulating response, and therefore it has been termed a "nonmetal-metal". We carried out neutron scattering experiments on powder and single crystal samples to study the magnetic dynamics and critical fluctuations in FeCrAs. Magnetic neutron diffraction measurements find Cr3+ magnetic order setting in at 115 K with the mean-field critical exponent. Neutron spectroscopy, however, observes gapless stiff magnetic fluctuations emanating from magnetic positions with propagation wave vector q_0=(1/3,1/3), which persists up to at least 80 meV. The magnetism in FeCrAs therefore displays a response which resembles that of itinerant magnets at high energy transfers, such as chromium alloys. We suggest that the presence of stiff high-energy spin fluctuations is the origin of the unusual temperature dependence of the resistivity.

Neutron scattering studies on the magnetic interaction in iron-based superconductor BaFe<sub>2</sub><sub>−</sub><sub><italic>x</italic></sub>Ni<sub><italic>x</italic></sub>As<sub>2</sub>

Iron-based superconductivity emerges from the suppression of the long-ranged antiferromagnetic order in the parent compounds. To solve the microscopic mechanism of the superconductivity, the key issue is fully understanding about the evolution of magnetic interaction and its relationship between superconductivity. Here, we take the electron-doped iron-based superconductor BaFe2- x Ni x As2 as atypical example, and summarize the neutron scattering results on the electronic phase diagram, magnetic ordered state, low-energy spin excitations and high-energy spin fluctuations, particularly for their systematic evolution versus electron dopings and the recent progress on the spin nematicity. Firstly, we will introduce about the crystal structure and magnetic structure, detailed procedure of the single crystal growth is also presented. By focusing on the structural transition temperature ( T s) and magnetic transition temperature ( T N) near the optimal doping level, we have found both T s and T N vanish above the superconducting transition temperature ( T c) as a first order manner, due to the lattice distortion and magnetically ordered moment decrease beyond the lower limit of the instrument resolution, giving an avoid quantum critical point at the optimal doping. In addition, the magnetic order becomes incommensurate and short-ranged magnetic cluster and competes with superconducting order, suggesting strong interplay between magnetism and superconductivity. Secondly, we will discuss about the itinerant magnetism and spin resonance mode at low energy for spin dynamics. The spin resonance mode in iron pnictides is explained as the collective quasiparticle excitations from the Fermi surface nesting between the hole pocket at the Γ point and electron pocket at the M point. This requires itinerant magnetism from the electrons near Fermi surfaces, which is further confirmed by neutron scattering by discovering the longitudinal mode and line shape change of low-energy spin excitations upon doping. Thirdly, we will present the doping evolution of spin fluctuations throughout the whole phase diagram. Although the spin waves in the parent compound can be described by an effective Heisenberg mode with anisotropic exchange coupling in FeAs plane, the damping features and unexpected low total fluctuation moments reveal both local moments and itinerant electrons contribute to the magnetism. Upon Ni doping, the high energy spin fluctuations is very robust, but the low energy spin excitations are suppressed quickly to zero when cross the boundary of the overdoped superconducting regime. The analysis on the change of the magnetic coupling energy associated with the superconducting transition gives evidence that the magnetic fluctuations are strong enough to drive the condensation of the Cooper pairs. Fourthly, we will further introduce the recent progress on the spin anisotropy by polarized neutron scattering experiments, and spin nematicity measured on detwinned samples. Anisotropic spin excitations at low energy are discovered by polarized neutron scattering, which is caused by the in-plane orbital ordering and persists away up to the tetragonal paramagnetic state. Further experiments on the detwinned sample under uniaxial pressure give direct evidence of the spin nematicity shown as breaking C4 rotation symmetry spin excitations in the tetragonal phase, similar to the anisotropic resistivity and electronic nematic phase discovered in other probes. Such novel electron state in these materials suggests the iron pnictides are unconventional. Finally, we summarize the results about the magnetic phase transition and spin dynamics and discuss its physical origin. Then a possible picture about the spin driven unconventional superconductivity in these materials is proposed. Further perspectives on the research of magnetism in iron-based superconductivity are also given.

NMR investigation of the iron-based superconductors Ca4(Mg,Ti)3Fe2As2O8−yand Ca5(Sc,Ti)4Fe2As2O11−y

${}^{75}$As and ${}^{45}$Sc NMR measurements unravel the electronic state for Fe-based superconductors with perovskite-type blocking layers Ca${}_{4}$(Mg,Ti)${}_{3}$Fe${}_{2}$As${}_{2}$O${}_{8\ensuremath{-}y}$ (${T}_{c}^{\text{onset}}=47$ K) and Ca${}_{5}$(Sc,Ti)${}_{4}$Fe${}_{2}$As${}_{2}$O${}_{11\ensuremath{-}y}$ (${T}_{c}^{\text{onset}}=41$ K). In Ca${}_{5}$(Sc,Ti)${}_{4}$Fe${}_{2}$As${}_{2}$O${}_{11\ensuremath{-}y}$, the nuclear spin relaxation rate $1/{T}_{1}$ shows pseudogap behavior below $\ensuremath{\sim}$80 K, suggesting that the electronic state is similar to that of the LaFeAs(O,F) system with moderate electron doping. The presence of the pseudogap behavior gives an interpretation that the holelike band (so-called $\ensuremath{\gamma}$ pocket) is located just below the Fermi level from the analogy to the LaFeAs(O,F) system and the disappearance of the $\ensuremath{\gamma}$ pocket yields the suppression of the low-energy spin fluctuations. On the other hand, in Ca${}_{4}$(Mg,Ti)${}_{3}$Fe${}_{2}$As${}_{2}$O${}_{8\ensuremath{-}y}$, satisfying the structural optimal condition for higher ${T}_{c}$ among the perovskite systems, the extrinsic contribution, which presumably originates in the Ti moment, is observed in $1/{T}_{1}T$; however, the moderate temperature dependence of $1/{T}_{1}T$ appears by its suppression under high magnetic field. In both systems, the high ${T}_{c}$ of $\ensuremath{\sim}$40 K is realized in the absence of the strong development of the low-energy spin fluctuations. The present results reveal that the structural optimization does not induce the strong development of the low-energy spin fluctuations. If we consider that superconductivity is mediated by spin fluctuations, the structural optimization is conjectured to provide a benefit to the development of the high-energy spin fluctuations irrespective to the low-energy part.

High-Energy Kink in the Single-Particle Spectra of the Two-Dimensional Hubbard Model

Employing dynamical cluster quantum Monte Carlo calculations we show that the single-particle spectral weight A(k,omega) of the one-band two-dimensional Hubbard model displays a high-energy kink in the quasiparticle dispersion followed by a steep dispersion of a broad peak similar to recent angle-resolved photoemission spectroscopy results reported for the cuprates. Based on the agreement between the Monte Carlo results and a simple calculation which couples the quasiparticle to spin fluctuations, we conclude that the kink and the broad spectral feature in the Hubbard model spectra is due to scattering with damped high-energy spin fluctuations.

Weak coupling instabilities of two-dimensional lattice electrons

We present a consistent and systematic study of a two-dimensional extended Hubbard model in the weak coupling limit. Quite generally, the electron gas is unstable towards a superconducting state even in the absence of phonons, since high-energy spin fluctuations create an effective attraction between the quasi-particles. However in the special case of a half-filled nearest-neighbor tight-binding band, the Fermi surface is nested and the system is at a Van Hove singularity. In this situation, there are six competing instabilities: s- and d-wave superconductivity, spin- and charge-density waves and two phases with circulating charge and spin currents, respectively. The required renormalization group formalism can be presented on a most elementary level, connecting the idea of the “parquet summation” to the more modern concept of Wilson's effective action. As a result, a rich phase diagram is obtained as a function of the model interaction. We also argue that the one-loop approximation is insufficient in a case where the Fermi surface is at a Van Hove singularity but not nested.

Weakly interacting electrons and the renormalization group

We present a general method to study weak-coupling instabilities of a large class of interacting electron models in a controlled and unbiased way. Quite generally, the electron gas is unstable towards a superconducting state even in the absence of phonons, since high-energy spin fluctuations create an effective attraction between the quasi-particles. As an example, we show the occurrence of d-wave pairing in the repulsive Hubbard model in two dimensions. In one dimension or if the Fermi surface is nested, there are several competing instabilities. The required renormalization group formalism for this case is presented to lowest (one-loop) order on a most elementary level, connecting the idea of the “parquet summation” to the more modern concept of Wilson's effective action. The validity and restrictions of the one-loop approximation are discussed in detail. As a result, three different renormalization group approaches known in the literature are shown to be equivalent within the regime of applicability. We also briefly discuss the open problem of a two-dimensional Fermi system at Van Hove filling without nesting.

Spin fluctuations and non-Fermi-liquid behavior ofCeNi2Ge2

Neutron scattering shows that non-Fermi-liquid behavior of the heavy-fermion compound ${\mathrm{CeNi}}_{2}{\mathrm{Ge}}_{2}$ is brought about by the development of low-energy spin fluctuations with an energy scale of 0.6 meV. They appear around the antiferromagnetic wave vectors $(\frac{1}{2}\frac{1}{2}0)$ and $(00\frac{3}{4})$ at low temperatures, and coexist with high-energy spin fluctuations with an energy scale of 4 meV and a modulation vector $(0.23,0.23,\frac{1}{2}).$ This unusual energy dependent structure of $\mathrm{Im}\ensuremath{\chi}(\mathit{Q},E)$ in Q space suggests that quasiparticle bands are important.

On the Superexchange Interaction in the Hubbard Model

The origin of the superexchange interaction is reexamined in order to clarify that it works even in metallic phases. The wellknown kinetic exchange process is nothing but the virtual exchange process of high-energy spin excitations between the lower and upper Hubbard bands. The mechanism for Cooper pairing between Gutzwiller's heavy electrons, whose band lies between the lower and upper Hubbard bands, by the superexchange interaction can be regarded as that by high-energy spin fluctuations. Because it can also be regarded as a kind of an excitonic mechanism using high-energy spin excitations, it is plausible that the superexchange interaction causes high-temperature superconductivity of Gutzwiller's heavy-electron liquids.

Paramagnetic spin fluctuations in the weak itinerant-electron ferromagnet MnSi.

Paramagnetic spin fluctuations in the weak itinerant-electron ferromagnet MnSi have been measured by using both unpolarized and polarized neutron scattering, and the scattering intensity was put on an absolute value by the simultaneous measurements of phonon cross sections. The magnetic excitation spectra over the whole Brillouin zone, for a wide energy range from 0 to 20 meV (\ensuremath{\Elzxh}\ensuremath{\omega}/${\mathrm{kT}}_{c}$=7.9) and up to temperatures (T/${T}_{c}$=10) well above ${T}_{c}$, follow closely the Moriya-Kawabata self-consistent-renormalization theory for the weak itinerant-electron ferromagnet. Based on these experimental results, the origin of the increase in the amplitude of the local magnetic moment 〈${M}_{L}^{2}$〉 above ${T}_{c}$ is attributed to the thermal excitations of the high-energy spin fluctuations in the Stoner continuum.