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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Reciprocity between local moments and collective magnetic excitations in the phase diagram of BaFe 2 (As 1-x P x ) 2

Unconventional superconductivity arises at the border between the strong coupling regime with local magnetic moments and the weak coupling regime with itinerant electrons, and stems from the physics of criticality that dissects the two. Unveiling the nature of the quasiparticles close to quantum criticality is fundamental to understand the phase diagram of quantum materials. Here, using resonant inelastic x-ray scattering (RIXS) and Fe-K β emission spectroscopy (XES), we visualize the coexistence and evolution of local magnetic moments and collective spin excitations across the superconducting dome in isovalently-doped BaFe 2 (As 1-x P x ) 2 (0.00 ≤ x ≤0.52). Collective magnetic excitations resolved by RIXS are gradually hardened, whereas XES reveals a strong suppression of the local magnetic moment upon doping. This relationship is captured by an intermediate coupling theory, explicitly accounting for the partially localized and itinerant nature of the electrons in Fe pnictides. Finally, our work identifies a local-itinerant spin fluctuations channel through which the local moments transfer spin excitations to the particle-hole (paramagnons) continuum across the superconducting dome.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Magnetic excitations in hole-doped Sr 2 IrO 4 : Comparison with electron-doped cuprates

We have studied the evolution of magnetic and orbital excitations as a function of hole doping in single-crystal samples of Sr 2 Ir 1- x Rh x O 4 (0.07≤ x ≤0.42) using high-resolution Ir L 3 -edge resonant inelastic x-ray scattering. Within the antiferromagnetically ordered region of the phase diagram ( x ≤0.17) we observe highly dispersive magnon and spin-orbit exciton modes. Interestingly, both the magnon gap energy and the magnon bandwidth appear to increase as a function of doping, resulting in a hardening of the magnon mode with increasing hole doping. As a result, the observed spin dynamics of hole-doped iridates more closely resemble those of the electron-doped, rather than hole-doped, cuprates. Within the paramagnetic region of the phase diagram (0.17≤ x ≤0.42) the low-lying magnon mode disappears, and we find no evidence of spin fluctuations in this regime. In addition, we observe that the orbital excitations become essentially dispersionless in the paramagnetic phase, indicating that magnetic order plays a crucial role in the propagation of the spin-orbit exciton

Resonant inelastic x-ray scattering↗
DOE OSTI2023

Momentum-independent magnetic excitation continuum in the honeycomb iridate H 3 LiIr 2 O 6

Understanding the interplay between the inherent disorder and the correlated fluctuating-spin ground state is a key element in the search for quantum spin liquids. H 3 LiIr 2 O 6 is considered to be a spin liquid that is proximate to the Kitaev-limit quantum spin liquid. Its ground state shows no magnetic order or spin freezing as expected for the spin liquid state. However, hydrogen zero-point motion and stacking faults are known to be present. The resulting bond disorder has been invoked to explain the existence of unexpected low-energy spin excitations, although data interpretation remains challenging. Here, we use resonant X-ray spectroscopies to map the collective excitations in H 3 LiIr 2 O 6 and characterize its magnetic state. In the low-temperature correlated state, we reveal a broad bandwidth of magnetic excitations. The central energy and the high-energy tail of the continuum are consistent with expectations for dominant ferromagnetic Kitaev interactions between dynamically fluctuating spins. Furthermore, the absence of a momentum dependence to these excitations are consistent with disorder-induced broken translational invariance. Our low-energy data and the energy and width of the crystal field excitations support an interpretation of H 3 LiIr 2 O 6 as a disordered topological spin liquid in close proximity to bond-disordered versions of the Kitaev quantum spin liquid.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Magnetic ground state and magnetic excitations in black dioptase Cu 6 Si 6 O 18

The low-temperature magnetic properties and magnetic structure of fully dehydrated black dioptase Cu 6 Si 6 O 18 have been studied by single-crystal neutron diffraction and magnetization measurements in magnetic fields up to 30 T. Here, the intrachain J c and interchain J a b interactions as well as the anisotropy of the exchange coupling J c have been determined using inelastic neutron-scattering techniques. Zero-field antiferromagnetic order at T N = 6.7 K can be described by a commensurate propagation vector of k = ( 0 , 0 , 3 / 2 ) with respect to the hexagonal R 3 ¯ :H unit cell. The Cu magnetic moments are aligned antiferromagnetically along the c axis with about 12 ° tilt and are coupled ferromagnetically between the chains. The high-field magnetization measurements provide strong evidence for the presence of a spin-flop phase above 8 T. We found that J c in black dioptase is significantly increased, while J a b is much weaker compared to its counterpart, green dioptase Cu 6 [ Si 6 O 18 ] 6 H 2 O . We suggest that black dioptase behaves like a nearly ideal S = 1 / 2 antiferromagnetic Heisenberg spiral chain with enhanced quantum fluctuations and weak spinon confinement J a b / J c ~ 0.02 .

1-dimensional spin chains↗
DOE OSTI2023

Magnetic excitations in the square-lattice iridate Ba 2 ⁢IrO 4

Here, we report a resonant inelastic x-ray scattering investigation of ultrathin epitaxial films of Ba 2 ⁢IrO 4 , and compare their low-energy magnetic and spin-orbit excitations to those of their sister compound Sr 2 ⁢IrO 4 . Due to the 180° Ir-O-Ir bond, the bandwidth of the magnon and spin orbiton is significantly larger in Ba 2⁢ IrO 4 , making it difficult to describe these two types of excitations as separate well-defined quasiparticles. Both types of excitations are found to be quite sensitive to the effect of epitaxial strain. In addition, we find that the f-level inversion observed in Sr 2 ⁢IrO 4 is absent in Ba 2 ⁢IrO 4 , as predicted in recent theoretical studies. Our results illustrate that the magnetic properties of Ba 2 ⁢IrO 4 are substantially different from those of Sr 2 ⁢IrO 4 , suggesting that these materials need to be examined more carefully with electron itinerancy taken into account.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Nature of Magnetic Excitations in the High-Field Phase of α–RuCl 3

We present comprehensive electron spin resonance (ESR) studies of in-plane oriented single crystals of α–RuCl 3 , a quasi-two-dimensional material with honeycomb structure, focusing on its high-field spin dynamics. The measurements were performed in magnetic fields up to 16 T, applied along the [110] and [100] directions. Several ESR modes were detected. Combining our findings with recent inelastic neutron- and Raman-scattering data, we identified most of the observed excitations. Most importantly, we show that the low-temperature ESR response beyond the boundary of the magnetically ordered region is dominated by single- and two-particle processes with magnons as elementary excitations. In conclusion, the peculiarities of the excitation spectrum in the vicinity of the critical field are discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Magnetic excitations, nonclassicality, and quantum wake spin dynamics in the Hubbard chain

Recent work has demonstrated that quantum Fisher information (QFI), a witness of multipartite entanglement, and magnetic Van Hove correlations G(r,t), a probe of local real-space real-time spin dynamics, can be successfully extracted from inelastic neutron scattering on spin systems through accurate measurements of the dynamical spin structure factor S(k,ω). In this work, we apply theoretically these ideas to the half-filled Hubbard chain with nearest-neighbor hopping, away from the strong-coupling limit. This model has nontrivial redistribution of spectral weight in S(k,ω) going from the noninteracting limit (U = 0) to strong coupling (U→∞), where it reduces to the Heisenberg quantum spin chain. We use the density matrix renormalization group to find S(k,ω), from which QFI is then calculated. We find that QFI grows with U. With realistic energy resolution it becomes capable of witnessing bipartite entanglement above U=2.5 (in units of the hopping), where it also changes slope. This point is also proximate to slope changes of the bandwidth W(U) and the half-chain von Neumann entanglement entropy. We compute G(r,t) by Fourier transforming S(k,ω). The results indicate a crossover in the short-time short-distance dynamics at low U characterized by ferromagnetic light-cone wavefronts, to a Heisenberg-type behavior at large U featuring antiferromagnetic light cones and spatially period-doubled antiferromagnetism. We find this crossover has largely been completed by U=3. Our results thus provide evidence that, in several aspects, the strong-coupling limit of the Hubbard chain is reached qualitatively already at a relatively modest interaction strength. We discuss experimental candidates for observing the G(r,t) dynamics found at low U.

1-dimensional systems↗
DOE OSTI2022

Magnetic excitations and interactions in the Kitaev hyperhoneycomb iridate β –Li 2 ⁢IrO 3

Here, we present a thorough experimental study of the three-dimensional hyperhoneycomb Kitaev magnet β –Li 2 ⁢IrO 3 , using a combination of inelastic neutron scattering (INS), time-domain terahertz spectroscopy (TDTS), and heat capacity measurements. The main results include a massive low-temperature reorganization of the INS spectral weight that evolves into a broad peak centered around 12 meV, and a distinctive peak in the terahertz data at 2.8(1) meV. A detailed comparison to powder-averaged spin-wave theory calculations reveals that the positions of these two features are controlled by the anisotropic Γ coupling and the Heisenberg exchange J, respectively. The refined microscopic spin model places β –Li 2 ⁢IrO 3 in close proximity to the Kitaev spin liquid phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Magnetic excitations from the hexagonal spin clusters in the 𝑆 = $\frac{1}{2}$ distorted honeycomb lattice antiferromagnet Cu 2 ⁢(pymca)⁢ 3 ⁢(ClO 4 )

Cu 2 ⁢(pymca) ⁢3 (ClO 4 ) (pymca: pyrimidine-2-carboxylate) consists of a slightly distorted honeycomb lattice of Cu 2+ spins, which shows no long-range magnetic order down to 0.6 K. A magnetization study revealed 1/3 and 2/3 plateau phases [A. Okutani et al., J. Phys. Soc. Jpn. 88, 013703 (2019)], which is not expected for regular honeycomb antiferromagnets. Inelastic neutron scattering experiments were performed using a powder sample to investigate the exchange interactions of this material. The spin excitations from the singlet ground state to the first three triplet states, predicted from the antiferromagnetic hexagonal spin cluster interacting with 3.9 meV, were observed. Using the exact diagonalization methods, the intercluster coupling was estimated from the excitation peak width to be about 20% of the intracluster interaction, which is consistent with the previously reported value. Finally, our exchange path model explains the anisotropic exchange interactions in the distorted honeycomb plane.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Tunable magnetic excitations in the honeycomb antiferromagnet (Co,Ni)⁢TiO3

The solid solution of the honeycomb antiferromagnet (AFM) Co0.5⁢Ni0.5⁢TiO3 (CNTO) was synthesized by mixing a 1:1 molar ratio of CoTiO3 (CTO) and NiTiO3 (NTO). Powder neutron scattering was used to determine the structure and magnetic spectrum, where the nuclear and magnetic structures resemble those of the end members. The Néel temperature (T𝑁 = 27 ± 1 K), the ordered magnetic moment (M = 2.29 ± 0.03 𝜇𝐵 per magnetic ion), and lattice parameters are intermediate between those of the two. From inelastic neutron scattering, it is observed that the magnon density of states (MDOS) extends up to 13 meV, and the interaction can be described by an XXZ-type magnetic Hamiltonian. Above 15 meV, spin-orbit excitons (SOEs) are observed similarly to those in CTO, but with modified spin-orbit crystal-field splitting because of the substitution of Ni at Co sites, and the disorder induced by the Ni substitution modifies the spin-orbit crystal field. Thus, the magnetic dynamics of the CNTO cannot be described as a simple average of the two parent compounds.

Rathnayaka, Srimal [University of Virginia, Charlo↗
DOE OSTI2022

Magnetic excitation linking quasi-one-dimensional Chevrel-type selenide and arsenide superconductors

The quasi-one-dimensional Chevrel phases, A 2 M o6 Se 6 (A=Tl, In, K, Rb, Cs), are of interest due to their atypical electronic properties. The Tl and In analogs undergo a superconducting transition whereas the alkali metal analogs show charge gapping of another, not well understood type. Here we report the results of inelastic neutron scattering on polycrystalline In 2 Mo 6 Se 6 (T c =2.85 K) and Rb 2 Mo 6 Se 6 (nonsuperconducting) samples, which reveal a column of intensity with linear dispersion from [0 0 1/2] to [0 0 1] in both compounds. The observed temperature and |Q| independence together suggest the presence of unconventional carriers with a spin contribution to the excitation. This is contrary to the prevailing model for these materials, which is that they are nonmagnetic. The excitation has similar dispersion and S(Q,E,T) behavior as one observed in the structurally related superconducting compounds A 2 Cr 3 As 3 and A 2 Mo 3 As 3 (A=K, Rb, Cs), which has been interpreted as magnetic in origin and related to Fermi surface nesting. The connection is unexpected because the calculated Fermi surface of the arsenides differs substantially from the A 2 Mo 6 Se 6 compounds, and many consider them distinct classes of materials. This observation suggests a hidden link in the physics between both classes of superconductors, perhaps originating from their quasi-low-dimensional character.

1-dimensional systems↗
DOE OSTI2021

Magnetic excitations of the hybrid multiferroic (ND 4 ) 2 FeCl 5 ·D 2 O

We report a comprehensive inelastic neutron scattering study of the hybrid molecule-based multiferroic compound ( ND 4 ) 2 FeCl 5 · D 2 O in the zero-field incommensurate cycloidal phase and the high-field quasicollinear phase. The spontaneous electric polarization changes its direction concurrently with the field-induced magnetic transition, from mostly aligned with the crystallographic a axis to the c axis. To account for such a change in polarization direction, the underlying multiferroic mechanism was proposed to switch from the spin-current model induced via the inverse Dzyaloshinskii-Moriya interaction to the p - d hybridization model. We perform a detailed analysis of the inelastic neutron data of ( ND 4 ) 2 FeCl 5 · D 2 O using linear spin-wave theory to quantify magnetic interaction strengths and investigate the possible impact of different multiferroic mechanisms on the magnetic couplings. Overall our result reveals that the spin dynamics of both multiferroic phases can be well described by a Heisenberg Hamiltonian with easy-plane anisotropy. We do not find notable differences between the optimal model parameters of the two phases. The hierarchy of exchange couplings and the balance among frustrated interactions remain the same between two phases, suggesting that magnetic interactions in ( ND 4 ) 2 FeCl 5 · D 2 O are much more robust than the electric polarization in response to delicate reorganizations of the electronic degrees of freedom in an applied magnetic field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Evolution of the Magnetic Excitations in Electron-Doped La 2 - x Ce x CuO 4

Here we investigated the high energy spin excitations in electron-doped La 2-x Ce x CuO 4 , a cuprate superconductor, by resonant inelastic x-ray scattering (RIXS) measurements. Efforts were paid to disentangle the paramagnon signal from non-spin-flip spectral weight mixing in the RIXS spectrum at Q ∥ =(0.6π,0) and (0.9π,0) along the (1 0) direction. Our results show that, for doping level x from 0.07 to 0.185, the variation of the paramagnon excitation energy is marginal. We discuss the implication of our results in connection with the evolution of the electron correlation strength in this system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Coupled electronic and magnetic excitations in the cuprates and their role in the superconducting transition

The formation of Cooper pairs, a bound state of two electrons of opposite spin and momenta by exchange of a phonon, is a defining feature of conventional superconductivity. In the cuprate high temperature superconductors, even though the superconducting state also consists of Cooper pairs, the pairing mechanism remains intensely debated. Here, we investigate superconducting pairing in the Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) cuprate by employing spectral functions obtained from angle-resolved photoemission as input to the Bethe-Salpeter equation. Assuming Cooper pairing is driven by spin fluctuations, we construct the spin-fluctuation-mediated pairing interaction and use it to compute the eigenfunctions and eigenvalues of the Bethe-Salpeter equation for multiple Bi2212 samples. The leading d-wave eigenvalue increases as the temperature is decreased toward $T_c$, reaching a value of approximately 1 at the $T_c$ corresponding to each doping value. This suggests that spin fluctuations can approximately account for $T_c$ and mediate pairing in the cuprate superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Anisotropic magnetic excitations of a frustrated bilinear-biquadratic spin model: Implications for spin waves of detwinned iron pnictides

Elucidating the nature of spin excitations is important to understanding the mechanism of superconductivity in the iron pnictides. Motivated by recent inelastic neutron scattering measurements in the nearly 100% detwinned BaFe 2 As 2 , we study the spin dynamics of an $\textit{S}$ = 1 frustrated bilinear-biquadratic Heisenberg model in the antiferromagnetic phase with wave vector (π, 0). The biquadratic interactions are treated in a dynamical way using a flavor-wave theory in an $\textit{SU}$(3) representation. Besides the dipolar spin wave (magnon) excitations, the biquadratic interactions give rise to quadrupolar excitations at high energies. In this work, we find that the quadrupolar wave significantly influences, in an energy dependent way, the anisotropy between the spin excitation spectra along the (π, 0) and (0, π) directions in the wave vector space. Our theoretical results capture the essential behavior of the spin dynamics measured in the antiferromagnetic phase of the detwinned BaFe 2 As 2 . More generally, our results underscore the importance of electron correlation effects for the microscopic physics of the iron pnictides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗