Influence of chirping the Raman lasers in an atom gravimeter: phase shifts due to the Raman light shift and to the finite speed of light

We present here an analysis of the influence of the frequency dependence of the Raman laser light shifts on the phase of a Raman-type atom gravimeter. Frequency chirps are applied to the Raman lasers in order to compensate gravity and ensure the resonance of the Raman pulses during the interferometer. We show that the change in the Raman light shift when this chirp is applied only to one of the two Raman lasers is enough to bias the gravity measurement by a fraction of $mu$Gal ($1~mu$Gal~=~$10^{-8}$~m/s$^2$). We also show that this effect is not compensated when averaging over the two directions of the Raman wavevector $k$. This thus constitutes a limit to the rejection efficiency of the $k$-reversal technique. Our analysis allows us to separate this effect from the effect of the finite speed of light, which we find in perfect agreement with expected values. This study highlights the benefit of chirping symmetrically the two Raman lasers.

Coexistence and competition of multiple charge-density-wave orders in rare-earth tri-telluride RTe3

The occurrences of collective quantum states, such as superconductivity (SC) and charge- or spin-densitywaves (CDWs or SDWs), are among the most fascinating phenomena in solids. To date much effort has been made to explore the interplay between different orders, yet little is known about the relationship of multiple orders of the same type. Here we report optical spectroscopy study on CDWs in the rare-earth tri-telluride compounds RTe3 (R = rare earth elements). Besides the prior reported two CDW orders, the study reveals unexpectedly the presence of a third CDW order in the series which evolves systematically with the size of R element. With increased chemical pressure, the first and third CDW orders are both substantially suppressed and compete with the second one by depleting the low energy spectral weight. A complete phase diagram for the multiple CDW orders in this series is established.

Electronic properties of 3d transitional metal pnictides : A comparative study by optical spectroscopy

Single-crystalline KFe2As2 and CaT2As2 (T = Fe, Co, Ni, Cu) are synthesized and investigated by resistivity, susceptibility and optical spectroscopy. It is found that CaCu2As2 exhibits a similar transition to the lattice abrupt collapse transitions discovered in CaFe2(As1-xPx)2 and Ca1-xRexFe2As2 (Re-rare earth element). The resistivity of KFe2As2 and CaT2As2 (T = Fe, Co, Ni, Cu) approximately follows the similar T^2 dependence at low temperature, but the magnetic behaviors vary with different samples. Optical measurement reveals the optical response of CaCu2As2 is not sensitive to the transition at 50 K, with no indication of development of a new energy gap below the transition temperature. Using Drude-Lorentz model, We find that two Drude terms, a coherent one and an incoherent one, can fit the low-energy optical conductivity of KFe2As2 and CaT2As2 (T = Fe, Co, Ni) very well. However, in CaCu2As2, which is a sp-band metal, the low-energy optical conductivity can be well described by a coherent Drude term. Lack of the incoherent Drude term in CaCu2As2 may be attributed to the weaker electronic correlation than KFe2As2 and CaT2As2 (T = Fe, Co, Ni). Spectral weight analysis of these samples indicates that the unconventional spectral weight transfer, which is related to Hunds coupling energy J_H, is only observed in iron pnictides, supporting the viewpoint that J_H may be a key clue to seek the mechanism of magnetism and superconductivity in pnictides.

Two successive field-induced spin-flop transitions in single-crystalline CaCo$_{2}$As$_{2}$

CaCo$_{2}$As$_{2}$, a ThCr$_{2}$Si$_{2}$-structure compound, undergoes an antiferromagnetic transition at emph{T$_{N}$}=76K with the magnetic moments being aligned parallel to the emph{c} axis. Electronic transport measurement reveals that the coupling between conducting carriers and magnetic order in CaCo$_{2}$As$_{2}$ is much weaker comparing to the parent compounds of iron pnictide. Applying magnetic field along emph{c} axis induces two successive spin-flop transitions in its magnetic state. The magnetization saturation behaviors with emph{textbf{H}$parallel$c} and emph{textbf{H}$parallel$ab} at 10K indicate that the antiferromagnetic coupling along emph{c} direction is very weak. The interlayer antiferromagntic coupling constant emph{J$_{c}$} is estimated to be about 2 meV.

Three dimensionality of band structure and a large residual quasiparticle population in Ba$_{0.67}$K$_{0.33}$Fe$_2$As$_2$ as revealed by the c-axis polarized optical measurement

We report on a c-axis polarized optical measurement on a Ba$_{0.67}$K$_{0.33}$Fe$_2$As$_2$ single crystal. We find that the c-axis optical response is significantly different from that of high-T$_c$ cuprates. The experiments reveal an anisotropic three-dimensional optical response with the absence of the Josephson plasma edge in R($omega$) in the superconducting state. Furthermore, different from the ab-plane optical response, a large residual quasiparticle population down to $Tsimfrac{1}{5}T_c$ was observed in the c-axis polarized reflectance measurement. We elaborate that there exist nodes for the superconducting gap in regions of the 3D Fermi surface that contribute dominantly to the c-axis optical conductivity.