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Register a new userMagnetic Excitations in the Spin Gap System TlCuCl$_3$
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Single-crystal neutron inelastic scattering was performed in order to investigate the magnetic excitations in the spin gap system TlCuCl$_3$. The constant-${bf Q}$ energy scan profiles were collected in the $a^*-c^*$ plane. Three excitations are observed for $E{leq}15$ meV. One of the excitations is identified to be magnetic excitation. The lowest magnetic excitation with $Esim 0.5$ meV occurs at ${bf Q}=(1, 0, 1)$, as observed in KCuCl$_3$. The dispersion relation of the magnetic excitation can be fitted to the dispersion formula derived from the weakly coupled dimer model. The intradimer interaction is evaluated as $J=5.23$ meV, which coincides with the value estimated from the susceptibility data. However, one of the interdimer interactions obtained is so large that the weakly coupled dimer model is broken down.
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We present high-resolution measurements of the thermal expansion and the magnetostriction of TlCuCl$_{3}$ which shows field-induced antiferromagnetic order. We find pronounced anomalies in the field and temperature dependence of different directions of the lattice signaling a large magnetoelastic coupling. The phase boundary is extremely sensitive to pressure, e.g. the transition field would change by about +/- 185$%/GPa under uniaxial pressure applied along certain directions. This drastic effect can unambiguously be traced back to changes of the intradimer coupling under uniaxial pressure. The interdimer couplings remain essentially unchanged under pressure, but strongly change when Tl is replaced by K.
Neutron elastic scattering experiments have been performed on the spin gap system TlCuCl$_3$ in magnetic fields parallel to the $b$-axis. The magnetic Bragg peaks which indicate the field-induced N{e}el ordering were observed for magnetic field higher than the gap field $H_{rm g}approx 5.5$ T at $Q=(h, 0, l)$ with odd $l$ in the $a^*-c^*$ plane. The spin structure in the ordered phase was determined. The temperature and field dependence of the Bragg peak intensities and the phase boundary obtained were discussed in connection with a recent theory which describes the field-induced N{e}el ordering as a Bose-Einstein condensation of magnons.
Time-of-flight inelastic neutron scattering measurements on Sr2IrO4 single crystals were performed to access the spin Hamiltonian in this canonical Jeff=1/2 spin-orbital Mott insulator. The momentum of magnetic scattering at all inelastic energies that were measured is revealed to be $L$-independent, indicative of idealized two-dimensional in-plane correlations. By probing the in-plane energy and momentum dependence up to ~80 meV we model the magnetic excitations and define a spin-gap of 0.6(1) meV. Collectively the results indicate that despite the strong spin-orbit entangled isospins an isotropic two-dimensional S=1/2 Heisenberg model Hamiltonian accurately describes the magnetic interactions, pointing to a robust analogy with unconventional superconducting cuprates.
What is so unique in TlCuCl3 which drives so many unique magnetic features in this compound? To study these properties, here we employ a combination of ab-initio band structure, tight-binding model, and an effective quantum field theory. Within a density-functional theory (DFT) calculation, we find an unexpected bulk Dirac cone without spin-orbit coupling (SOC). Tracing back to its origin, we identify, for the first time, the presence of a Su-Schrieffer-Heeger (SSH) like dimerized Cu chain lying in the 3D crystal structure. The SSH chain, combined with SOC, stipulates an anisotropic 3D Dirac cone where chiral and helical states are intertwined. As a Heisenberg interaction is introduced, we show that the dimerized Cu sublattices of the SSH chain condensate into spin-singlet, dimerized magnets. In the magnetic ground state, we also find a topological phase, distinguished by the axion angle. Finally, to study how the topological axion term couples to magnetic excitations, we derive a Chern-Simons-Ginzburg-Landau action from the 3D SSH Hamiltonian. We find that axion term provides an additional mass term to the Higgs mode, and a lifetime to paramagnons, which are independent of the quantum critical physics. The axion-Higgs interplay can be probed with electric and magnetic field applied parallel or anti-parallel to each other.
We calculate spectra of magnetic excitations in the spin-spiral state of perovskite manganates. The spectra consist of several branches corresponding to different polarizations and different ways of diffraction from the static magnetic order. Goldstone modes and opening of gaps at zero and non-zero energies due to the crystal field and the Dzyaloshinski-Moriya anisotropies are discussed. Comparing results of the calculation with available experimental data we determine values of effective exchange parameters and anisotropies. To simplify the spin-wave calculation and to get a more clear physical insight in the structure of excitations we use the {sigma}-model-like effective field theory to analyze the Heisenberg Hamiltonian and to derive the spectra.
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