Magnetic Excitations and Continuum of a Possibly Field-Induced Quantum Spin Liquid in α-RuCl_{3}

We report on terahertz spectroscopy of quantum spin dynamics in α-RuCl_{3}, a system proximate to the Kitaev honeycomb model, as a function of temperature and magnetic field. We follow the evolution of an extended magnetic continuum below the structural phase transition at T_{s2}=62  K. With the onset of a long-range magnetic order at T_{N}=6.5  K, spectral weight is transferred to a well-defined magnetic excitation at ℏω_{1}=2.48  meV, which is accompanied by a higher-energy band at ℏω_{2}=6.48  meV. Both excitations soften in a magnetic field, signaling a quantum phase transition close to B_{c}=7  T, where a broad continuum dominates the dynamical response. Above B_{c}, the long-range order is suppressed, and on top of the continuum, emergent magnetic excitations evolve. These excitations follow clear selection rules and exhibit distinct field dependencies, characterizing the dynamical properties of a possibly field-induced quantum spin liquid.

Formation of the S = 1 paramagnetic centers in the bond-diluted spin-gap magnet

Electron spin resonance experiment reveals that non-magnetic bond doping of the spin-gap magnet (C4H12N2)Cu2Cl6 (abbreviated PHCC) results in the formation of S  =  1 paramagnetic centers that dominate low-temperature ESR response. We have followed evolution of this signal with doping impurity content and have found that the concentraion of these centers is quadratic over the impurity content. We also observe coexistence of the ESR responses from these local centers and from delocalized triplet excitations over a certain temperature range.

Formation of gapless triplets in the bond-doped spin-gap antiferromagnet (C4H12N2)(Cu2Cl6)

We report results of an electron spin resonance (ESR) study of a spin-gap antiferromagnet (C(4)H(12)N(2))(Cu(2(Cl(6)) (nicknamed PHCC) with chlorine ions partially substituted by bromine. We found that up to 10% of nominal doping the contribution of the random defects to the absorption spectra remains at about 0.1% per copper ion, which is almost the same as in the pure system. Instead, a particular kind of ESR absorption corresponding to gapless S = 1 triplets is observed at low temperatures in samples with high nominal bromine content x ≥ 5%. Increase of bromine concentration also leads to the systematic broadening of ESR absorption line, indicating reduction of the quasi-particles lifetime.

Infrared spectroscopy of small-molecule endofullerenes

Hydrogen is one of the few molecules that has been incarcerated in the molecular cage of C₆₀ to form the endohedral supramolecular complex H₂@C₆₀. In this confinement, hydrogen acquires new properties. Its translation motion, within the C₆₀ cavity, becomes quantized, is correlated with its rotation and breaks inversion symmetry that induces infrared (IR) activity of H₂. We apply IR spectroscopy to study the dynamics of hydrogen isotopologues H₂, D₂ and HD incarcerated in C₆₀. The translation and rotation modes appear as side bands to the hydrogen vibration mode in the mid-IR part of the absorption spectrum. Because of the large mass difference of hydrogen and C₆₀ and the high symmetry of C₆₀ the problem is almost identical to a vibrating rotor moving in a three-dimensional spherical potential. We derive potential, rotation, vibration and dipole moment parameters from the analysis of the IR absorption spectra. Our results were used to derive the parameters of a pairwise additive five-dimensional potential energy surface for H₂@C₆₀. The same parameters were used to predict H₂ energies inside C₇₀. We compare the predicted energies and the low-temperature IR absorption spectra of H₂@C₇₀.

Infrared spectroscopy of endohedral HD and D2 in C60

We report on the dynamics of two hydrogen isotopomers, D(2) and HD, trapped in the molecular cages of a fullerene C(60) molecule. We measured the infrared spectra and analyzed them using a spherical potential for a vibrating rotor. The potential, vibration-rotation Hamiltonian, and dipole moment parameters are compared with previously studied H(2)@C(60) parameters [M. Ge, U. Nagel, D. Hüvonen, T. Rõõm, S. Mamone, M. H. Levitt, M. Carravetta, Y. Murata, K. Komatsu, J. Y.-C. Chen, and N. J. Turro, J. Chem. Phys. 134, 054507 (2011)]. The isotropic part of the potential is similar for all three isotopomers. In HD@C(60), we observe mixing of the rotational states and an interference effect of the dipole moment terms due to the displacement of the HD rotation center from the fullerene cage center.

Optical spectroscopy of superconducting Ba0.55K0.45Fe2As2: evidence for strong coupling to low-energy bosons

Normal state optical spectroscopy on single crystals of the new iron arsenide superconductor Ba0.55K0.45Fe2As2 shows that the infrared spectrum consists of two major components: a strong metallic Drude band and a well-separated midinfrared absorption centered at 0.7 eV. It is difficult to separate the two components unambiguously but several fits using Lorentzian peaks suggest a model with a Drude peak having a plasma frequency of 1.6 to 2.1 eV and a midinfrared peak with a plasma frequency of 2.5 eV. Detailed analysis of the frequency dependent scattering rate shows that the charge carriers interact with a broad bosonic spectrum extending beyond 100 meV with a very large coupling constant lambda=3.4 at low temperature. As the temperature increases this coupling weakens to lambda=0.78 at ambient temperature. This suggests a bosonic spectrum that is similar to what is seen in the lower Tc cuprates.