J. W. Lee

Nanoporous Si as an efficient thermoelectric material.

Room-temperature thermoelectric properties of n-type crystalline Si with periodically arranged nanometer-sized pores are computed using a combination of classical molecular dynamics for lattice thermal conductivity and ab initio density functional theory for electrical conductivity, Seebeck coefficient, and electronic contribution to the thermal conductivity. The electrical conductivity is found to decrease by a factor of 2-4, depending on doping levels, compared to that of bulk due to confinement. The Seebeck coefficient S yields a 2-fold increase for carrier concentrations less than 2 x 10(19) cm(-3), above which S remains closer to the bulk value. Combining these results with our calculations of lattice thermal conductivity, we predict the figure of merit ZT to increase by 2 orders of magnitude over that of bulk. This enhancement is due to the combination of the nanometer size of pores which greatly reduces the thermal conductivity and the ordered arrangement of pores which allows for only a moderate reduction in the power factor. We find that while alignment of pores is necessary to preserve power factor values comparable to those of bulk Si, a symmetric arrangement is not required. These findings indicate that nanoporous semiconductors with aligned pores may be highly attractive materials for thermoelectric applications.

Centrality dependence of charged-hadron transverse-momentum spectra in d+Au collisions at sqrt[s(NN)]=200 GeV.

We have measured transverse momentum distributions of charged hadrons produced in d+Au collisions at sqrt[s(NN)]=200 GeV. The spectra were obtained for transverse momenta 0.25<p(T)<6.0 GeV/c, in a pseudorapidity range of 0.2<eta<1.4 in the deuteron direction. The evolution of the spectra with collision centrality is presented in comparison to p+pmacr; collisions at the same collision energy. With increasing centrality, the yield at high transverse momenta increases more rapidly than the overall particle density, leading to a strong modification of the spectral shape. This change in spectral shape is qualitatively different from observations in Au+Au collisions at the same energy. The results provide important information for discriminating between different models for the suppression of high-p(T) hadrons observed in Au+Au collisions.

Thermal Transport in Nanoporous Silicon: Interplay between Disorder at Mesoscopic and Atomic Scales

We present molecular and lattice dynamics calculations of the thermal conductivity of nanoporous silicon, and we show that it may attain values 10-20 times smaller than in bulk Si for porosities and surface-to-volume ratios similar to those obtained in recently fabricated nanomeshes. Further reduction of almost an order of magnitude is obtained in thin films with thickness of 20 nm, in agreement with experiment. We show that the presence of pores has two main effects on heat carriers: appearance of non-propagating, diffusive modes and reduction of the group velocity of propagating modes. The former effect is enhanced by the presence of disorder at the pore surfaces, while the latter is enhanced by decreasing film thickness.

Pseudorapidity distribution of charged particles in d+Au collisions at sqrt[sNN]=200 GeV.

The measured pseudorapidity distribution of primary charged particles in minimum-bias d+Au collisions at sqrt[s(NN)]=200 GeV is presented for the first time. This distribution falls off less rapidly in the gold direction as compared to the deuteron direction. The average value of the charged particle pseudorapidity density at midrapidity is |eta|< or =0.6)=9.4+/-0.7(syst) and the integrated primary charged particle multiplicity in the measured region is 82+/-6(syst). Estimates of the total charged particle production, based on extrapolations outside the measured pseudorapidity region, are also presented. The pseudorapidity distribution, normalized to the number of participants in d+Au collisions, is compared to those of Au+Au and p+(-)p systems at the same energy. The d+Au distribution is also compared to the predictions of the parton saturation model, as well as microscopic models.

Charged hadron transverse momentum distributions in Au+Au collisions at sNN=200 GeV

We present transverse momentum distributions of charged hadrons produced in Au+Au collisions at sqrt(s_NN) = 200 GeV. The spectra were measured for transverse momenta p_T from 0.25 to 4.5 GeV/c in a rapidity range of 0.2 < y_pi < 1.4. The evolution of the spectra is studied as a function of collision centrality, from 65 to 344 participating nucleons. The results are compared to data from proton-antiproton collisions and Au+Au collisions at lower RHIC energies. We find a significant change of the spectral shape between proton-antiproton and peripheral Au+Au collisions. Comparing peripheral to central Au+Au collisions, we find that the yields at high p_T exhibit approximate scaling with the number of participating nucleons, rather than scaling with the number of binary collisions.

Thermoelectric properties of nanoporous Ge

We computed thermoelectric properties of nanoporous Ge (np-Ge) with aligned pores along the [001] direction through a combined classical molecular dynamics and first-principles electronic structure approach. A significant reduction in the lattice thermal conductivity of np-Ge leads to a 30-fold increase in the thermoelectric figure-of-merit (ZT) compared to that of bulk. Detailed comparisons with the recently proposed np-Si show that although the maximum ZT (ZTmax) of Ge is nine times larger than that of Si in the bulk phase, ZTmax of np-Ge is twice as large as that of np-Si due to the similarity in lattice thermal conductivity of the two np systems. Moreover, ZTmax is found to occur at a carrier concentration two orders of magnitude lower than that for with np-Si due to the dissimilarities in their electronic structure.

Magnetic properties in graphene-graphane superlattices

National Science Foundation (U.S.) (Network for Computational Nanotechnology Grant EEC-0634750)

Pseudorapidity dependence of charged hadron transverse momentum spectra in d+Au collisions at sqrt[SNN]=200 GeV

We have measured the transverse momentum distributions of charged hadrons in d+Au collisions at sqrt sNN = 200 GeV in the range of 0.5<p_T<4.0 GeV/c. The total range of pseudorapidity, eta, is 0.2<eta<1.4, where positive eta is in the deuteron direction. The data has been divided into three regions of pseudorapidity, covering 0.2<eta<0.6, 0.6<eta<1.0, and 1.0<eta<1.4 and has been compared to charged hadron spectra from p+pbar collisions at the same energy. There is a significant change in the spectral shape as a function of pseudorapidity. As eta increases we see a decrease in the nuclear modification factor RdAu.

Charged hadron transverse momentum distributions in Au+Au collisions at GeV

We present transverse momentum distributions of charged hadrons produced in Au+Au collisions at sqrt(s_NN) = 200 GeV. The spectra were measured for transverse momenta p_T from 0.25 to 4.5 GeV/c in a rapidity range of 0.2 < y_pi < 1.4. The evolution of the spectra is studied as a function of collision centrality, from 65 to 344 participating nucleons. The results are compared to data from proton-antiproton collisions and Au+Au collisions at lower RHIC energies. We find a significant change of the spectral shape between proton-antiproton and peripheral Au+Au collisions. Comparing peripheral to central Au+Au collisions, we find that the yields at high p_T exhibit approximate scaling with the number of participating nucleons, rather than scaling with the number of binary collisions.

Pseudorapidity dependence of charged hadron transverse momentum spectra in d+Au collisions at s(NN)**(1/2) = 200 GeV

We have measured the transverse momentum distributions of charged hadrons in d+Au collisions at sqrt sNN = 200 GeV in the range of 0.5<p_T<4.0 GeV/c. The total range of pseudorapidity, eta, is 0.2<eta<1.4, where positive eta is in the deuteron direction. The data has been divided into three regions of pseudorapidity, covering 0.2<eta<0.6, 0.6<eta<1.0, and 1.0<eta<1.4 and has been compared to charged hadron spectra from p+pbar collisions at the same energy. There is a significant change in the spectral shape as a function of pseudorapidity. As eta increases we see a decrease in the nuclear modification factor RdAu.