We present a theoretical study of the static and dynamical properties of the three-dimensional, hyperhoneycomb Kitaev magnet β-Li2IrO3. We argue that the observed incommensurate order can be understood in terms of a long-wavelength twisting of a nearby commensurate period-3 state, with the same key qualitatively features. The period-3 state shows very different structure when either the Kitaev interaction K or the off-diagonal exchange anisotropy Γ is dominant. A comparison of the associated static spin structure factors with reported scattering expoeriments in zero and finite fields gives strong evidence that β-Li2IrO3 lies in the regime of dominant Kitaev coupling, and that the Heisenberg exchange J is much weaker than both K and Γ. Our predictions for the magnon excitation spectra, the dynamical spin structure factors and their polarization dependence provide additional distinctive fingerprints that can be checked experimentally. arXiv:1801.00874v3 [cond-mat.str-el]

We analyze magnetic order in Fe-chalcogenide Fe1+yTe -the parent compound of hightemperature superconductor Fe1+yTe1−xSex. Experiments show that magnetic order in this material contains components with momentum Q1 = (π/2, π/2) and Q2 = (π/2, −π/2) in Fe-only Brillouin zone. The actual spin order depends on the interplay between these two components. Previous works assumed that the ordered state has a single-Q (either Q1 or Q2). In such a state, spins form double stripes along one of diagonals breaking the rotational C4 symmetry. We show that quantum fluctuations actually select another order -a double Q plaquette state with equal weight of Q1 and Q2 components, which preserves C4 symmetry. We argue that the order in Fe1+yTe is determined by the competition between quantum fluctuations and magnetoelastic coupling.Introduction. The analysis of magnetism in parent compounds of iron-based superconductors (FeSCs) is an integral part of the program to understand the origin of superconductivity in these materials [1][2][3][4][5][6][7][8][9][10][11][12]. Parent compounds of Fe-pnictides are moderately correlated metals, [5,13] whose magnetic order can be reasonably well understood within itinerant scenario [7-9, 14, 16] The locations and the shapes of the Fermi surfaces (FSs) select two possible ordered state with momenta (0, π) and (π, 0)-in the Fe-only Brillouin zone (BZ) [9].In each of these two states spins are ordered in a stripe fashion -ferromagnetically along one direction in 2D Feplane and antiferromagnetically in the other. Such an order breaks C 4 lattice rotational symmetry and causes pre-emptive spin-nematic order [15]. The same magnetic order is selected in the strong coupling approach, based on J 1 − J 2 model of localized spins with nearest and second-nearest neighbor spin exchange [17,18]. The actual coupling in Fe-pnictides is neither truly small nor strong enough to cause Mott insulating behavior [13], which makes it extremely useful that the two descriptions agree.There is one family of FeSCs -11 Fe-chalcogenides Fe 1+y Te 1−x Se x , in which smooth evolution between parent and optimally doped compounds does not hold. Magnetism in these materials changes considerably between x = 0 and x ∼ 0.5, where the T c is the largest. Near optimal doping magnetic fluctuations are peaked at or near (0, π) and (π, 0), as in Fe-pnictides, while magnetic order in a parent compound Fe 1+y Te has very different momenta ±(π/2, ±π/2) [19][20][21][22][23]. Upon doping, the spectral weight at ±(π/2, ±π/2) decreases, and the spectral weight at (0, π) and (π, 0) increases [20]. The transport properties of Fe 1+y Te are also quite different from those of parent compounds of Fe-pnictides: the resistivity, ρ(T ), of Fe 1+y Te does not show a prominent increase with increasing T , but instead remains flat and even shows a small increase as T decreases [24]. ARPES studies of Fe 1+y Te show that low-energy spectra are very broad [25], consistent with the notion that electrons are not propagating. These observations lead several

We examine the influence of the excess of interstitial Fe on the magnetic properties of Fe1+yTe compounds. Because in iron chalcogenides the correlations are stronger than in the iron arsenides, we assume in our model that some of the Fe orbitals give rise to localized magnetic moments. These moments interact with each other via exchange interactions as well as phonon-mediated biquadratic interactions that favor a collinear double-stripe state, corresponding to the ordering vectors (±π/2, ±π/2). The remaining Fe orbitals are assumed to be itinerant, giving rise to the first-principle derived Fermi surface displaying nesting features at momenta (π, 0) / (0, π). Increasing the amount of itinerant electrons due to excess Fe, y, leads to changes in the Fermi surface and to the suppression of its nesting properties. As a result, due to the Hund's coupling between the itinerant and localized moments, increasing y leads to modifications in the local moments' exchange interactions via the multi-orbital generalization of the long-range Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. By numerically computing the RKKY corrections and minimizing the resulting effective exchange Hamiltonian, we find, in general, that the excess electrons introduced in the system change the classical magnetic ground state from a double-stripe state to an incommensurate spiral, consistent with the experimental observations. We show that these results can be understood as a result of the suppression of magnetic spectral weight of the itinerant electrons at momenta (π, 0) / (0, π), combined with the transfer of broad magnetic spectral weight from large to small momenta, promoted by the introduction of excess Fe.

We detail the development of an optical probe technique based on time-resolved Raman-induced Kerr effect polarization spectroscopy (tr-RIKES). This technique, termed fs/ps RIKES, combines an ultrafast pump pulse with a narrowband probe that directly allows spectral resolution of low-frequency (0-600 cm(-1)) modes typically observable via RIKES. The narrowband probe pulse alleviates the need to scan the time delay between pump and probe pulses to observe molecular coherences, thus making this multiplexed technique a convenient probe for studying low-frequency molecular dynamics. An important distinguishing characteristic of this polarization-sensitive technique arises from the fact that the delay between the impulsive pump pulse and the picosecond-duration probe pulse is optimized to maximize suppression of nonresonant background signal. Model systems, including the rotational spectrum of gas-phase hydrogen and the low-frequency vibrational spectrum of neat bromoform, are used to compare fs/ps RIKES with the conventional time-resolved RIKES technique.

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