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Register a new userRevealing ultra-strong magnon-photon coupling in a polar antiferromagnet Fe2Mo3O8 by time domain terahertz spectroscopy
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Strong coupling between magnon and electromagnetic wave can lead to the formation of a coupled spinphoton quasiparticle named as magnon-polariton. The phenomenon is well studied for ferromagnetic systems inside microwave cavities in recent years. However, formation of magnon-polariton is rarely seen for an antiferromagnet (AFM) because the strong coupling condition is not easily fulfilled. Here we present time-domain terahertz measurement on a multiferroic polar antiferromagnet Fe2Mo3O8. We find clearly beating between two modes at frequencies above and below the electric-active magnon frequency below TN, which we assign to the formation of AFM magnon-polariton. An ultra-strong spin-photon coupling effect is derived based on the energy level splitting. However, the AFM magnon-polariton is absent in the frequency domain measurement. Our work reveals that the coherent magnon formation driven by the ultrashort THz pulse provides a new way to detect polariton mode splitting.
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Magnetic excitations are investigated for a hexagonal polar magnet Fe2Mo3O8 by terahertz spectroscopy. We observed magnon modes including an electric-field active magnon, electromagnon, in the collinear antiferromagnetic phase with spins parallel to the c axis. We unravel the nature of these excitations by investigating the correlation between the evolution of the mode profile and the magnetic transition from antiferromagnetic to ferrimagnetic order induced by magnetic field or Zn-doping. We propose that the observed electromagnon mode involves the collective precession of the spins with oscillating in-plane electric polarization through the mechanism of the linear magnetoelectric effect.
The ability to achieve strong-coupling has made cavity-magnon systems an exciting platform for the development of hybrid quantum systems and the investigation of fundamental problems in physics. Unfortunately, current experimental realizations are constrained to operate at a single frequency, defined by the geometry of the microwave cavity. In this article we realize a highly-tunable, cryogenic, microwave cavity strongly coupled to magnetic spins. The cavity can be tuned in situ by up to 1.5 GHz, approximately 15% of its original 10 GHz resonance frequency. Moreover, this system remains within the strong-coupling regime at all frequencies with a cooperativity of approximately 800.
We present both static and time-resolved second harmonic generation (SHG) measurements on polar antiferromagnet Fe$_2$Mo$_3$O$_8$ to monitor the evolution of the electric polarization change and its coupling to magnetic order. We find that only one of the second order tensor elements, $chi_{ccc}^{(2)}$ ,shows a prominent change below the Neel temperature $T_N = 60$ K, indicating a magnetic order induced electric polarization change along the c-axis. Time-resolved SHG measurement reveals an ultrafast recovery of the second order tensor element upon the ultrashort laser excitation with fluence above 0.3 $mJ/cm^2$, yielding evidence for a photoinduced ultrafast phase transition from the AFM ordered state to the paramagnetic state. Our work will help understand the spin induced polarization and the ultrafast optical tuning effect in Fe$_{2}$Mo$_{3}$O$_{8}$.
Synthetic antiferromagnet, comprised of two ferromagnetic layers separated by a non-magnetic layer, possesses two uniform precession resonance modes: in-phase acoustic mode and out-of-phase optic mode. In this work, we theoretically and numerically demonstrated the strong coupling between acoustic and optic magnon modes. The strong coupling is attributed to the symmetry breaking of the system, which can be realized by tilting the bias field or constructing an asymmetrical synthetic antiferromagnet. It is found that the coupling strength can be highly adjusted by tuning the tilting angle of bias field, the magnitude of antiferromagnetic interlayer exchange coupling, and the thicknesses of ferromagnetic layers. Furthermore, the coupling between acoustic and optic magnon modes can even reach the ultrastrong coupling regime. Our findings show high promise for investigating quantum phenomenon with a magnonic platform.
We demonstrate terahertz time-domain spectroscopy (THz-TDS) to be an accurate, rapid and scalable method to probe the interaction-induced Fermi velocity renormalization { u}F^* of charge carriers in graphene. This allows the quantitative extraction of all electrical parameters (DC conductivity {sigma}DC, carrier density n, and carrier mobility {mu}) of large-scale graphene films placed on arbitrary substrates via THz-TDS. Particularly relevant are substrates with low relative permittivity (< 5) such as polymeric films, where notable renormalization effects are observed even at relatively large carrier densities (> 10^12 cm-2, Fermi level > 0.1 eV). From an application point of view, the ability to rapidly and non-destructively quantify and map the electrical ({sigma}DC, n, {mu}) and electronic ({ u}F^* ) properties of large-scale graphene on generic substrates is key to utilize this material in applications such as metrology, flexible electronics as well as to monitor graphene transfers using polymers as handling layers.
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