arXiv subjects
Publications and source records attributed to Qijin Chen.
We explore a BCS Bose Einstein crossover scenario for $0 \leq T \leq T_c$ and its implications for the superfluid density and specific heat. The low lying excitations consist of nodal (fermionic) quasi-particles as well as excited (bosonic) pair states. Semi-quantitative comparison with cuprate data is reasonable, with no compelling indications for Landau parameter effects.
In this paper we study the BCS Bose Einstein condensation (BEC) crossover scenario within the superconducting state, using a T-matrix approach which yields the ground state proposed by Leggett. Here we extend this ground state analysis to finite temperatures T and interpret the resulting physics. We find two types of bosonic-like excitations of the system: long lived, incoherent pair excitations and collective modes of the superconducting order parameter, which have different dynamics. Using a gauge invariant formalism, this paper addresses their contrasting behavior as a function of T and superconducting coupling constant g. At a more physical level, our paper emphasizes how, at finite T, BCS-BEC approaches introduce an important parameter Δ^2_{pg} = Δ^2 - Δ_{sc}^2 into the description of superconductivity. This parameter is governed by the pair excitations and is associated with particle-hole asymmetry effects which are important for sufficiently large g. In the fermionic regime, Δ_{pg}^2 represents the difference between the square of the excitation gap Δ^2 and that of the superconducting order parameter Δ_{sc}^2. The parameter Δ_{pg}^2, which is necessarily zero in the BCS (mean field) limit increases monotonically with the strength of the attractive interaction g. It follows that there is a significant physical distinction between this BCS-BEC crossover approach (in which g is the essential variable which determines Δ_{pg}) and the widely discussed (Coulomb-modulated) phase fluctuation scenario in which the plasma frequency is the tuning parameter.
We investigate the behavior of the superconducting transition temperature within a previously developed BCS-Bose Einstein crossover picture. This picture, based on a decoupling scheme of Kadanoff and Martin, further extended by Patton, can be used to derive a simple form for the superconducting transition temperature in the presence of a pseudogap. We extend previous work which addressed the case of s-wave pairing in jellium, to explore the solutions for T_c as a function of variable coupling in more physically relevant situations. We thereby ascertain the effects of reduced dimensionality, periodic lattices and a d-wave pairing interaction. Implications for the cuprate superconductors are discussed.
We propose a theoretical description of the superconducting state of under- to overdoped cuprates, based on the short coherence length of these materials and the associated strong pairing fluctuations. The calculated $T_c$ and the zero temperature excitation gap $Δ(0)$, as a function of hole concentration $x$, are in semi-quantitative agreement with experiment. Although the ratio $T_c/Δ(0)$ has a strong $x$ dependence, different from the universal BCS value, and $Δ(T)$ deviates significantly from the BCS prediction, we obtain, quite remarkably, quasi-universal behavior, for the normalized superfluid density $ρ_s(T)/ρ_s(0)$ and the Josephson critical current $I_c(T)/I_c(0)$, as a function of $T/T_c$. While experiments on $ρ_s(T)$ are consistent with these results, future measurements on $I_c(T)$ are needed to test this prediction.
The existence of a normal state spectral gap in underdoped cuprates raises important questions about the associated superconducting phase. For example, how does this pseudogap evolve into its below Tc counterpart? In this paper we characterize this unusual superconductor by investigating the nature of the ``residual'' pseudogap below Tc and, find that it leads to an important distinction between the superconducting excitation gap and order parameter. Our approach is based on a conserving diagrammatic BCS Bose-Einstein crossover theory which yields the precise BCS result in weak coupling at any T<Tc and reproduces Leggett's results in the T=0 limit. We explore the resulting experimental implications.
Nucleation and its mechanism of diamond on titanium substrates under very low pressure was studied using hot-filament chemical vapor deposition. Very high nucleation rates and densities (10^8-10^{10} cm^{-2}) were obtained under 1 torr, which were 1-3 orders of magnitude higher than the counterpart (10^7 cm^{-2}) under conventionally low pressure (tens of torr). The effects of substrate temperature and methane concentration under very low pressure were also investigated, revealing that, overly high substrate temperature leads to a relatively low nucleation density, and that higher CH_4 concentration gives rise to a higher density and a higher rate. The nucleation mechanism is discussed in detail. While a large amount of atomic hydrogen creates nucleating sites, sufficient supersaturation of carbon and/or hydrocarbon species on/near the substrate surface is the key factor for nucleation, in competition against the rapid formation of carbide. Very low pressure leads to long mean free path and other benefiting effects, and hence, is critical for rapid, high-density nucleation. Effects of substrate temperature and CH_4 concentration are also important. This further implies that C_2H_x (x<6) and CH_4 also contribute to nucleation, but CH_{1-3} dominates under very low pressure. The very-low-pressure method seems to be the only candidate to make diamond deposition on titanium films applicable. It also sheds light on how to increase the diamond growth rate.
Diamond nucleation under very low pressure (0.1-1.0 torr) was obtained at very high nucleation densities and very rapid rates using hot-filament chemical vapor deposition (HFCVD). The density on mirror-polished silicon was as high as 10^10 - 10^11 cm^{-2}, equivalent to the highest density in a microwave-plasma CVD system. That on scratched silicon substrates was up to 10^9 cm^{-2}, 1-2 orders of magnitude higher than that obtained under conventionally low pressure (tens of torr, 10^7 - 10^8 cm^{-2}). Also, the density on scratched titanium substrates was as high as 10^10 cm^{-2}. The samples were characterized using scanning electron microscopy (SEM) and Raman spectroscopy. The mechanism is investigated in detail, revealing that, under very low pressure, very long mean free path of the gas species, strong electron emission from the hot filament, and high efficiency of decomposition of hydrocarbon species by the filament greatly increase the concentration of reactive hydrocarbon radicals and atomic hydrogen on the substrate surface, and therefore, dramatically enhance the nucleation eventually. This work has great practical applications and theoretical significance.
45-degree rotated epitaxial diamond nucleation on both (001) and {111} planes of a silicon substrate was observed using chemical vapor deposition with nucleation enhancement by electron emission. The epitaxial relationship between diamond and Si is D(001)//Si(001) and D<100>//Si<110> in both cases. While the case of diamond nucleation on Si(001) may be explained in Verwoerd's model, the other case has no theoretical model as yet. The as-grown diamond was characterized by scanning electron microscopy and Raman spectroscopy. It is speculated that a suitable combination of low temperature and low CH_4 concentration leading to low nucleation rate favors the 45-degree rotated epitaxy over those in parallel registry. This observation sheds light on the mechanism of epitaxy of diamond on Si substrates, and has great significance in parameter control in achieving single-crystal epitaxial diamond films.