6 research outputs found
Phase control of magnons in the van der Waals antiferromagnet NiPS
We demonstrate phase control of magnons in the van der Waals antiferromagnet
NiPS using optical excitation by polarized light. The sign of the coherent
precession of spin amplitude changes upon (1) reversing the helicity of a
circularly polarized pump beam, or (2) rotating the polarization of a linearly
polarized pump by . Because these two excitation pathways have
comparable generation efficiency, the phase of spin precession can be
continuously tuned from 0 to by controlling the polarization state of
the pump pulse. The ability to excite magnons with a desired phase has
potential applications in the design of a spin-wave phased array and ultrafast
spin information processing
Disentangling superconducting and magnetic orders in NaFe_1-xNi_xAs using muon spin rotation
Muon spin rotation and relaxation studies have been performed on a "111"
family of iron-based superconductors NaFe_1-xNi_xAs. Static magnetic order was
characterized by obtaining the temperature and doping dependences of the local
ordered magnetic moment size and the volume fraction of the magnetically
ordered regions. For x = 0 and 0.4 %, a transition to a nearly-homogeneous long
range magnetically ordered state is observed, while for higher x than 0.4 %
magnetic order becomes more disordered and is completely suppressed for x = 1.5
%. The magnetic volume fraction continuously decreases with increasing x. The
combination of magnetic and superconducting volumes implies that a
spatially-overlapping coexistence of magnetism and superconductivity spans a
large region of the T-x phase diagram for NaFe_1-xNi_xAs . A strong reduction
of both the ordered moment size and the volume fraction is observed below the
superconducting T_C for x = 0.6, 1.0, and 1.3 %, in contrast to other iron
pnictides in which one of these two parameters exhibits a reduction below TC,
but not both. The suppression of magnetic order is further enhanced with
increased Ni doping, leading to a reentrant non-magnetic state below T_C for x
= 1.3 %. The reentrant behavior indicates an interplay between
antiferromagnetism and superconductivity involving competition for the same
electrons. These observations are consistent with the sign-changing s-wave
superconducting state, which is expected to appear on the verge of microscopic
coexistence and phase separation with magnetism. We also present a universal
linear relationship between the local ordered moment size and the
antiferromagnetic ordering temperature TN across a variety of iron-based
superconductors. We argue that this linear relationship is consistent with an
itinerant-electron approach, in which Fermi surface nesting drives
antiferromagnetic ordering.Comment: 20 pages, 14 figures, Correspondence should be addressed to Prof.
Yasutomo Uemura: [email protected]
Two-step Mott transition in Ni(S,Se)_{2}: μSR studies and charge-spin percolation model
A pyrite system NiS_{2−x}Se_{x} exhibits a bandwidth controlled Mott transition via (S,Se) substitutions in a two-step process: the antiferromagnetic insulator (AFI) to antiferromagnetic metal (AFM) transition at x∼0.45 followed by the AFM to paramagnetic metal (PMM) transition at x∼1.0. Among a few other Mott systems which exhibit similar two-step transitions, Ni(S,Se)_{2} is of particular interest because a large intermediate AFM region in the phase diagram would provide unique opportunities to study the interplay between the spin and charge order. Muon spin relaxation (μSR) measurements on NiS_{2−x}Se_{x} have been carried out on seven different Se concentrations from x=0 to 1.0. The results on quantum evolution demonstrate significantly random spin correlations in the AFM region associated with a rapid reduction of the average local static Ni moment size with increasing x, yet without signatures of macroscopic phase separation as confirmed by nearly full volume fraction participating in the static muon relaxation process up to x∼ 0.9 at low temperatures. The observed time spectra in the AFM region indicate Lorentzian distribution of static internal field expected for a spatially dilute spin structure. No signature of dynamic critical behavior was observed in thermal phase transitions. The previous neutron scattering studies found sharp magnetic Bragg peaks with a slower reduction of the average ordered moment size in the AFM region. By comparing and combining the muon and neutron results, here we propose a picture where the spin order is maintained by the percolation of “nonmetallic” localized and dangling Ni moments surrounded by S, while the charge transition from AFI to AFM is caused by the percolation of the conducting paths generated by the Ni-Se-Ni bonds. This model of interpenetrating charge and spin percolation captures the behavior of experimental results on (a) Se concentration for the insulator to metal transition, (b) Se concentration for the AFM to PMM transition, (c) variation of Hall effect in the AFM region due to conducting Ni charges on the backbone of the percolating charge network, (d) evolution of the neutron Bragg intensity, (e) evolution of the muon static local fields, and (f) spatial variation of the local conductance observed by STM