Kirill Amelin

Although ${\mathrm{BiFeO}}_3$ is one of the most studied multiferroic materials, recent magnetization and neutron scattering studies have revealed a new magnetic phase in this compound—the transverse conical phase. To study the collective spin excitations of this phase, we performed terahertz spectroscopy in magnetic fields up to 17 T at and above room temperature. We observed five spin-wave branches in the magnetic phase with long-wavelength conical modulation. Using a numerical spin dynamics model, we found two kinds of excitations with magnetic moments oscillating either along or perpendicular to the static fields. Remarkably, we detected strong directional dichroism, an optical manifestation of the magnetoelectric effect, for one spin-wave mode of the conical phase. According to our experiments, the stability of the conical state is sensitive to the magnetic-field history, and it can become (meta)stable at or close to zero magnetic field, which may allow exploiting its magnetoelectric properties at room temperature.

Condensed matter magneto-optical investigations can be a powerful probe of a material's microscopic magnetoelectric properties. This is because subtle interactions between electric and magnetic multipoles on a crystal lattice show up in predictable and testable ways in a material's optical response tensor, which dictates the polarization state and absorption spectrum of propagating electromagnetic waves. Magneto-optical techniques are therefore strong complements to probes such as neutron scattering, particularly when spin-lattice coupling effects are present. Here we perform a magneto-optical investigation of vibronic spin-lattice coupling in the magnetically frustrated pyrochlore ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. Coupling of this nature involving quadrupolar mixing between the ${\mathrm{Tb}}^{3+}$ electronic levels and phonons in ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ has been a topic of debate for some time. This is particularly due to its implication for describing the exotic spin-liquid phase diagram of this highly debated system. A manifestation of this vibronic effect is observed as splitting of the ground and first excited crystal field doublets of the ${\mathrm{Tb}}^{3+}$ electronic levels, providing a fine structure to the absorption spectra in the terahertz (THz) frequency range. In this investigation, we apply a static magnetic field along the cubic [111] direction while probing with linearly polarized THz radiation. Through the Zeeman effect, the magnetic field enhances the splitting within the low-energy crystal field transitions revealing new details in our THz spectra. Complementary magneto-optical quantum calculations including quadrupolar terms show that indeed vibronic effects are required to describe our observations at 3 K. A further prediction of our theoretical model is the presence of a novel magneto-optical birefringence as a result of this vibronic process. Essentially, spin-lattice coupling within ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ may break the optical isotropy of the cubic system, supporting two different electromagnetic wave propagations within the crystal. Together our results reveal the significance of considering quadrupolar spin-lattice effects when describing the spin-liquid ground state of ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. They also highlight the potential for future magneto-optical investigations to probe complex materials where spin-lattice coupling is present and reveal new magneto-optical activity in the THz range.

Close to the quantum critical point of the transverse-field Ising spin-chain model, an exotic dynamic spectrum was predicted to emerge upon a perturbative longitudinal field. The dynamic spectrum consists of eight particles and is governed by the symmetry of the $E_8$ Lie algebra. Here we report on high-resolution terahertz spectroscopy of quantum spin dynamics in the ferromagnetic Ising-chain material ${\mathrm{CoNb}}_2{\mathrm{O}}_6$. At 0.25 K in the magnetically ordered phase we identify characteristics of the first six $E_8$ particles, ${\mathrm{m}}_1$ to ${\mathrm{m}}_6$, and the two-particle $({\mathrm{m}}_1+{\mathrm{m}}_2)$ continuum in an applied transverse magnetic field of $B_c^{\text{1D}}=4.75 \mathrm{T}$, before the three-dimensional magnetic order is suppressed above $B_c^{\text{3D}}\approx 5.3 \mathrm{T}$. The observation of the higher-energy particles $({\mathrm{m}}_3 \mathrm{to} {\mathrm{m}}_6)$ above the low-energy two-particle continua features quantum many-body effects in the exotic dynamic spectrum.

Near the transverse-field-induced quantum critical point of the Ising chain, an exotic dynamic spectrum consisting of exactly eight particles was predicted, which is uniquely described by an emergent quantum integrable field theory with the symmetry of the $E_8$ Lie algebra, but rarely explored experimentally. Here we use high-resolution terahertz spectroscopy to resolve quantum spin dynamics of the quasi-one-dimensional Ising antiferromagnet $\mathrm{BaC}{\mathrm{o}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ in an applied transverse field. By comparing to an analytical calculation of the dynamical spin correlations, we identify $E_8$ particles as well as their two-particle excitations.

We measured absorption of THz radiation in ${\mathrm{YFeO}}_3$ single crystals at a temperature of 3 K in the magnetic field up to 17 T applied in all three crystallographic directions. Two spin-wave modes were observed at the $\mathrm{\Gamma }$ point with energies 1.2 meV (9.8 ${\mathrm{cm}}^{-1}$) and 2.4 meV (19.3 ${\mathrm{cm}}^{-1}$) in zero field. From the magnetic-field dependence of mode energies, we have refined the previously proposed model [S. E. Hahn et al., Phys. Rev. B 89, 014420 (2014)] and quantified the parameters of Dzyaloshinskii-Moriya interactions and single-ion anisotropies.