Browsing by Author "Chen, Jian"
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Item Adiabatic cooling of atoms by an intense standing wave(American Physical Society, 1992) Chen, Jian; Story, J.G.; Tollett, J.J.; Hulet, Randall G.; Rice Quantum InstituteLithium atoms channeled in the nodes of an intense standing-wave radiation field are cooled to near the recoil limit by adibatically reducing the radiation intensity. The final momentum distribution has a narrow component with a root-mean-squared momentum of 2ħk in one dimension, where ħk is the momentum of a radiation-field photon. The data are compared with the results of a Monte Carlo simulation using a two-level atom model. This process may be useful for cooling and increasing the phase-space density of atoms confined in a magnetic trap.Item Evolution of atomic motion in an intense standing wave(American Physical Society, 1993) Chen, Jian; Story, J.G.; Hulet, Randall G.; Rice Quantum InstituteWe have investigated the effect of the dipole force and its fluctuation on the motion of Li atoms in an intense, one-dimensional, near-resonant standing light wave. The duration of the interaction of the atoms with the standing wave was varied from several tens of spontaneous-emission lifetimes to several hundreds. For a standing-wave frequency blue detuned from resonance, diffusive heating can dominate the time-averaged dissipative dipole force so that there is no steady-state momentum distribution. However, for sufficiently large blue detunings, the rate of diffusion is so slow that the resulting distribution approaches a quasisteady state. For red detunings, the diffusion is balanced with the force and a true steady state is achieved. We apply a Monte Carlo method based on the density-matrix equations in the dressed-state representation to simulate the atomic motion. The dynamics of atom channeling is discussed.Item Heavy fermion quantum criticality at dilute carrier limit in CeNi2−δ(As1−xPx)2(Springer Nature, 2019) Chen, Jian; Wang, Zhen; Li, Yupeng; Feng, Chunmu; Dai, Jianhui; Xu, Zhu’an; Si, QimiaoWe study the quantum phase transitions in the nickel pnctides, CeNi2−δ(As1−xPx)2 (δ ≈ 0.07–0.22) polycrystalline samples. This series displays the distinct heavy fermion behavior in the rarely studied parameter regime of dilute carrier limit. We systematically investigate the magnetization, specific heat and electrical transport down to low temperatures. Upon increasing the P-content, the antiferromagnetic order of the Ce-4f moment is suppressed continuously and vanishes at xc ~ 0.55. At this doping, the temperature dependences of the specific heat and longitudinal resistivity display non-Fermi liquid behavior. Both the residual resistivity ρ0 and the Sommerfeld coefficient γ0 are sharply peaked around xc. When the P-content reaches close to 100%, we observe a clear low-temperature crossover into the Fermi liquid regime. In contrast to what happens in the parent compound x = 0.0 as a function of pressure, we find a surprising result that the non-Fermi liquid behavior persists over a nonzero range of doping concentration, xc < x < 0.9. In this doping range, at the lowest measured temperatures, the temperature dependence of the specific-heat coefficient is logarithmically divergent and that of the electrical resistivity is linear. We discuss the properties of CeNi2−δ(As1−xPx)2 in comparison with those of its 1111 counterpart, CeNi(As1−xPx)O. Our results indicate a non-Fermi liquid phase in the global phase diagram of heavy fermion metals.Item Investigation of atomic motion in an intense standing light wave and application to adiabatic laser cooling of atoms(1993) Chen, Jian; Hulet, Randall G.The effect of the dipole force and its fluctuation on the motion of Li atoms in an intense, one-dimensional, near-resonant standing light wave has been investigated, both theoretically and experimentally. The duration of the interaction of the atoms with the standing wave was varied from several tens of spontaneous emission lifetimes to several hundred. For a standing wave frequency blue-detuned from resonance, diffusive heating can dominate the time-averaged dissipative dipole force so that there is no steady-state momentum distribution. However, for sufficiently large blue detunings the rate of diffusion is so slow that the resulting distribution approaches a quasi-steady state. For red detunings the diffusion is balanced with the force and a true steady-state is achieved. A Monte Carlo method based on the density matrix equations in the dressed-state representation is applied to simulate the atomic motion. The dynamics of atom channeling is studied. One of the important applications of this investigation is adiabatic laser cooling of atoms. Lithium atoms channeled in the nodes of an intense standing wave radiation field are cooled to near the recoil limit, a single photon's momentum $\hbar{k}$, by adiabatically reducing the radiation intensity. The final momentum distribution has a narrow component with a root mean squared momentum of 2 $\hbar{k}$ in one-dimension. The data are compared with the results obtained by the Monte Carlo method. This process may be useful for cooling and increasing the phase-space density of atoms confined in a magnetic trap.Item Spectroscopic and kinetic studies of ionized helium excited by a laser-produced plasma soft x-ray source(1988) Chen, Jian; Sauerbrey, Roland A.The work of this thesis describes the experimental set-up and measurements for spectroscopic and kinetic studies of ionized helium gas. An injection-controlled KrF 248nm laser beam focussed to an intensity of 3 $\times$ 10$\sp{11}$ Wcm$\sp{-2}$ was used to generate highly ionized tantalum plasma. Helium gas was photoionized by the broadband soft x-ray flux from the laser-produced Ta plasma. The possibilities for realizing a short wavelength laser (below 2000A) have been investigated. The spectra of He$\sp+$ transitions 4 $\to$ 2 and 3 $\to$ 2 were experimentally measured by an optical multichannel analyzer, and the time evolution of the fluorescences were observed by a fast response photomultiplier. The effects of helium pressure on the fluorescence were also studied. In addition to the experiments, a kinetic model was developed. The properties of ionized helium predicted by this model are in relatively good agreement with the experimental results.