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Register a new userTwo-Magnon Bound States in the Kitaev Model in a $[111]$-Field
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It is now well established that the Kitaev honeycomb model in a magnetic field along the $[111]$-direction harbors an intermediate gapless quantum spin liquid (QSL) phase sandwiched between a gapped non-abelian QSL at low fields $H< H_{c1}$ and a partially polarized phase at high fields $H> H_{c2}$. Here, we analyze the low field and high field phases and phase transitions in terms of single- and two-magnon excitations using exact diagonalization (ED) and density matrix renormalization group (DMRG) methods. We find that the energy to create a bound state of two-magnons $Delta_p$ becomes lower than the energy to create a single spin flip $Delta_s$ near $H_{c2}$. In the entire Kitaev spin liquid $Delta_p<Delta_s$ and both gaps vanish at $H_{c2}$. We make testable predictions for magnon pairing that could be observable in Raman scattering measurements on Kitaev QSL candidate materials.
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The pure Kitaev honeycomb model harbors a quantum spin liquid in zero magnetic fields, while applying finite magnetic fields induces a topological spin liquid with non-Abelian anyonic excitations. This latter phase has been much sought after in Kitaev candidate materials, such as $alpha$-RuCl$_3$. Currently, two competing scenarios exist for the intermediate field phase of this compound ($B=7-10$ T), based on experimental as well as theoretical results: (i) conventional multiparticle magnetic excitations of integer quantum number vs. (ii) Majorana fermionic excitations of possibly non-Abelian nature with a fractional quantum number. To discriminate between these scenarios a detailed investigation of excitations over a wide field-temperature phase diagram is essential. Here we present Raman spectroscopic data revealing low-energy quasiparticles emerging out of a continuum of fractionalized excitations at intermediate fields, which are contrasted by conventional spin-wave excitations. The temperature evolution of these quasiparticles suggests the formation of bound states out of fractionalized excitations.
Phonon-assisted 2-magnon absorption is studied at T=4 K in the spin-1/2 two-leg ladders of Ca_14-x La_x Cu_24 O_41 (x=5 and 4) for polarization of the electrical field parallel to the legs and the rungs, respectively. Two peaks at about 2140 and 2800 1/cm reflect van-Hove singularities in the density of states of the strongly dispersing 2-magnon singlet bound state, and a broad peak at about 4000 1/cm is identified with the 2-magnon continuum. Two different theoretical approaches (Jordan-Wigner fermions and perturbation theory) describe the data very well for J_parallel = 1050 - 1100 1/cm and J_parallel / J_perp = 1 - 1.1. A striking similarity of the high-energy continuum absorption of the ladders and of the undoped high T_c cuprates is observed.
A repulsive Coulomb interaction between electrons in different orbitals in correlated materials can give rise to bound quasiparticle states. We study the non-hybridized two-orbital Hubbard model with intra (inter)-orbital interaction $U$ ($U_{12}$) and different band widths using an improved dynamical mean field theory numerical technique which leads to reliable spectra on the real energy axis directly at zero temperature. We find that a finite density of states at the Fermi energy in one band is correlated with the emergence of well defined quasiparticle states at excited energies $Delta=U-U_{12}$ in the other band. These excitations are inter-band holon-doublon bound states. At the symmetric point $U=U_{12}$, the quasiparticle peaks are located at the Fermi energy, leading to a simultaneous and continuous Mott transition settling a long-standing controversy.
We calculate magnon dispersions and damping in the Kitaev-Heisenberg model with an off-diagonal exchange $Gamma$ and isotropic third-nearest-neighbor interaction $J_3$ on a honeycomb lattice. This model is relevant to a description of the magnetic properties of iridium oxides $alpha$-Li$_2$IrO$_3$ and Na$_2$IrO$_3$, and Ru-based materials such as $alpha$-RuCl$_3$. We use an unconventional parametrization of the spin-wave expansion, in which each Holstein-Primakoff boson is represented by two conjugate hermitian operators. This approach gives us an advantage over the conventional one in identifying parameter regimes where calculations can be performed analytically. Focusing on the parameter regime with the zigzag spin pattern in the ground state that is consistent with experiments, we demonstrate that one such region is $Gamma = K>0$, where $K$ is the Kitaev coupling. Within our approach we are able to obtain explicit analytical expressions for magnon energies and eigenstates and go beyond the standard linear spin-wave theory approximation by calculating magnon damping and demonstrating its role in the dynamical structure factor. We show that the magnon damping effects in both Born and self-consistent approximations are very significant, underscoring the importance of non-linear magnon coupling in interpreting broad features in the neutron-scattering spectra.
We use time-domain terahertz spectroscopy to measure the low energy conductivity and magnons in RuCl$_3$ under external magnetic field. At zero field, an oscillation with a frequency of 0.62 THz is clearly observed in time-domain spectrum below T$_N$, which is identified as a magnon excitation in the magnetic order state. The magnon excitation is not affected by the external magnetic field $textbf{H}_{DC}$ when it is applied along the c-axis, but is clearly suppressed when $textbf{H}_{DC}$ is applied within ab plane. More interestingly, when the magnetic component of THz wave $textbf{h}(t)$ is perpendicular to the applied in-plane magnetic field, we observe another coherent oscillation at slightly higher energy scale at the field above 2 T, which is eventually suppressed for $H_{DC}>$5 T. The measurement seems to indicate that the in-plane magnetic field can lift the degeneracy of two branches of low energy magnons at $Gamma$ point. The low energy optical conductivity calculated from the measured transmission spectrum is dominated by a broad continuum contribution, which is not affected by changing either temperature or external magnetic field. The continuum is likely to be related to the fractional spin excitation due to dominated Kitaev interaction in the material.
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