We investigate the stability of a three spin state mixture of ultracold fermionic 6 Li atoms over a range of magnetic fields encompassing three Feshbach resonances. For most field values, we attribute decay of the atomic population to three-body processes involving one atom from each spin state and find that the three-body loss coefficient varies by over four orders of magnitude. We observe high stability when at least two of the three scattering lengths are small, rapid loss near the Feshbach resonances, and two unexpected resonant loss features. At our highest fields, where all pairwise scattering lengths are approaching at = −2140a0, we measure a three-body loss coefficient L3 ≃ 5 × 10 −22 cm 6 /s and a trend toward lower decay rates for higher fields indicating that future studies of color superfluidity and trion formation in a SU(3) symmetric Fermi gas may be feasible.
We observe enhanced three-body recombination in a three-component ;{6}Li Fermi gas attributable to an excited Efimov trimer state intersecting the three-atom scattering threshold near 895 G. From measurements of the recombination rate we determine the Efimov parameters kappa_{*} and eta_{*} for the universal region above 600 G which includes three overlapping Feshbach resonances. The value of kappa_{*} also predicts the locations of loss features previously observed near 130 and 500 G [T. B. Ottenstein, Phys. Rev. Lett. 101, 203202 (2008)10.1103/PhysRevLett.101.203202; J. H. Huckans, Phys. Rev. Lett. 102, 165302 (2009)10.1103/PhysRevLett.102.165302] suggesting they are associated with a ground-state Efimov trimer near threshold. We also report on the realization of a degenerate three-component Fermi gas with approximate SU(3) symmetry.
We have measured the interaction energy and three-body recombination rate for a two-component Fermi gas near a narrow Feshbach resonance and found both to be strongly energy dependent. Even for de Broglie wavelengths greatly exceeding the van der Waals length scale, the behavior of the interaction energy as a function of temperature cannot be described by atoms interacting via a contact potential. Rather, energy-dependent corrections beyond the scattering length approximation are required, indicating a resonance with an anomalously large effective range. For fields where the molecular state is above threshold, the rate of three-body recombination is enhanced by a sharp, two-body resonance arising from the closed-channel molecular state which can be magnetically tuned through the continuum. This narrow resonance can be used to study strongly correlated Fermi gases that simultaneously have a sizable effective range and a large scattering length.
For the first time thick orientation-patterned GaP (OPGaP) was repeatedly grown heteroepitaxially on OPGaAs templates as a quasi-phase matched medium for frequency conversion in the mid and longwave IR, and THz regions. The OP templates were fabricated by wafer-bonding and in a MBE-assisted polarity inversion process. Standard low-pressure hydride vapor phase epitaxy (LP-HVPE) was used for one-step growth of up to 400 µm thick device quality OPGaP with excellent domain fidelity. The presented results can be viewed as the missing link between a welldeveloped technique for preparation of OP templates, using one robust nonlinear optical material (GaAs), and the subsequent thick epitaxial growth on them of another material (GaP). The reason for these efforts is that the second material has some indisputable advantages in point of view of thermal and optical properties but the preparation of native templates encounters challenges, which makes it difficult to obtain high quality homoepitaxial growth at an affordable price. Successful heteroepitaxial growth at such a relatively high lattice mismatch (-3.6%) in a close to equilibrium growth process such as HVPE is noteworthy, especially when previously reported attempts, for example, growth of OPZnSe on OPGaAs templates at about 10 times smaller lattice mismatch (+ 0.3%) have produced only limited results. Combining the advantages of the two most promising nonlinear materials, GaAs and GaP, is a solution that will accelerate the development of high power, tunable laser sources for the IR and THz region, which are in great demand on the market.
We report an s-wave collisional frequency shift of an atomic clock based on fermions. In contrast to bosons, the fermion clock shift is insensitive to the population difference of the clock states, set by the first pulse area in Ramsey spectroscopy, θ1. The fermion shift instead depends strongly on the second pulse area θ2. It allows the shift to be canceled, nominally at θ2 = π/2, but correlations shift the null to slightly larger θ2. The shift applies to optical lattice clocks and increases with the spatial inhomogeneity of the clock excitation field, naturally large at optical frequencies. PACS numbers: 06.30.Ft, 34.50.Cx, 37.10.Jk At ultracold temperatures, atom-atom interactions can only occur through s-wave collisions. While s-wave collisions are allowed for bosons and are the most important limitation to the the accuracy of clocks that establish international atomic time [1,2], they are forbidden for identical fermions by the Pauli exclusion principle. Even when dephasing made fermions distinguishable, collision shifts were absent [3]. Thus, ultracold fermions were thought to be immune to swave collisional frequency shifts (sCFS's) making them ideally suited for precision metrology [4][5][6][7][8] and quantum memories [9][10][11][12]. However, recent theoretical work predicted that fermions can have an sCFS because generally occurring inhomogeneities of excitation fields makes particles distinguishable [13][14][15].Here we experimentally observe an s-wave collisional frequency shift of an atomic clock based on a thermal gas of ultracold fermions. Ramsey spectroscopy clearly distinguishes the novel behaviors of the sCFS, via specific dependences on the first and second Ramsey pulse areas, θ 1 and θ 2 . We demonstrate that the shift is insensitive to θ 1 and thereby the difference of the spin populations [13], in stark contrast with the shifts for bosons and the often-used mean-field expression. Instead, the fermion sCFS depends strongly on θ 2 , which reads out the interaction induced phase shifts of each atom. The shift is canceled if the atoms' phases are detected, on average, with equal sensitivity [13]. Interestingly, we show that correlations in the sample perturb the null of the sCFS to θ 2 slightly greater than π/2. We explicitly see that the sCFS increases as expected with the inhomogeneity of the clock field, which we characterize independently. The fermion sCFS we observe in the resolved sideband regime is exactly analogous to those of optical lattice clocks [6], for which the spatial field inhomogeneity is naturally large at optical frequencies. Recently, the fermion sCFS was simulated using an 87 Rb Bose gas [16]. They worked, in contrast, with unresolved trap sidebands to directly excite a spin-wave, and observed the predicted dependence on θ 2 , but an unexpected and unexplained dependence on θ 1 . They elegantly showed a direct link between spin-waves and the fermion sCFS. Here we observe these predicted spin-waves in the resolved sideband regime, demonstrating the dependence of the sCFS ...
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