To fully deploy the potential of semiconductor nanocrystal films as low-cost electronic materials, a better understanding of the amount of dopants required to make their conductivity metallic is needed. In bulk semiconductors, the critical concentration of electrons at the metal-insulator transition is described by the Mott criterion. Here, we theoretically derive the critical concentration nc for films of heavily doped nanocrystals devoid of ligands at their surface and in direct contact with each other. In the accompanying experiments, we investigate the conduction mechanism in films of phosphorus-doped, ligand-free silicon nanocrystals. At the largest electron concentration achieved in our samples, which is half the predicted nc, we find that the localization length of hopping electrons is close to three times the nanocrystals diameter, indicating that the film approaches the metal-insulator transition.Semiconductor nanocrystals (NCs) have shown great potential in optoelectronics applications such as solar cells [1], light emitting diodes [2], and field-effect transistors [3,4] by virtue of their size-tunable optical and electrical properties [5] and low-cost solution-based processing techniques [6,7]. These applications require conducting NC films and the introduction of extra carriers through doping can enhance the electrical conduction. Several strategies for NC doping have been developed. Remote doping, the use of suitable ligands as donors in the vicinity of NC surface, increased the conductivity of PbSe NC films by 12 orders of magnitude [8]. Electrochemical doping, which tunes the carrier concentration accurately and reversibly, resulted in conducting NC films [9,10]. Lately, stoichiometric control has emerged as a strategy to dope lead chalcogenide NCs [11]. Finally, electronic impurity doping of NCs, originally impeded by synthetic challenges [12], was recently achieved in InAs [13] and CdSe [14] NCs.While many experimental studies have been directed towards increasing the conductivity of NC films, there is still no clear consensus on the fundamental question: what is the condition for the metal-insulator transition (MIT) in NC films [15][16][17]? In a bulk semiconductor, the critical electron concentration n M for the MIT depends on the Bohr radius a B according to the well-known Mott criterion [18] n M a 3 B 0.02, where a B = ε 2 /m * e 2 is the effective Bohr radius (in Gaussian units), ε is the dielectric constant of the semiconductor, and m * is the effective electron mass. It is obvious that a dense film of undoped semiconductor NCs is an insulator, while a film of touching metallic NCs with the same geometry is a conductor. Therefore, the MIT has to occur in semiconductor NC films at some criti-FIG. 1. The origin of the metal-to-insulator transition in semiconductor nanocrystal films. The figure shows the cross section of two nanocrystals in contact through facets with radius ρ. The blue spherical cloud represents an electron wave packet which moves through the contact. Such a compact wave pac...

Generation of functional spermatids from human spermatogonial stem cells (SSCs) in vitro is of utmost importance for uncovering mechanisms underlying human germ cell development and treating infertility. Here we report a three-dimensional-induced (3D-I) system by which human SSCs were efficiently differentiated into functional haploid spermatids. Human SSCs were isolated and identified phenotypically. Meiotic chromatin spreads and DNA content assays revealed that spermatocytes and haploid cells were effectively generated from human SSCs by 3D-I system. Haploid cells derived from human SSCs harbored normal chromosomes and excluded Y chromosome microdeletions. RNA sequencing and bisulfite sequencing analyses reflected similarities in global gene profiles and DNA methylation in human SSCs-derived spermatids and normal round spermatids. Significantly, haploid spermatids generated from human SSCs via 3D-I system were capable of fertilizing mouse oocytes, which subsequently enabled the development of hybrid embryos. This study thus provides invaluable human male gametes for treating male infertility.

A Bose condensate subject to a periodic modulation of the two-body interactions was recently observed to emit matter-wave jets resembling "fireworks" [Nature 551, 356(2017)]. In this paper, combining experiment with numerical simulation, we demonstrate that these "Bose fireworks" represent a late stage in a complex time evolution of the driven condensate. We identify a "density wave" stage which precedes jet emission and results from interference of matterwaves. The density waves self-organize and self-amplify without the breaking of long range translational symmetry. Importantly, this density wave structure deterministically establishes the template for the subsequent patterns of the emitted jets. Our simulations, in good agreement with experiment, also address the apparent asymmetry in the jet pattern and show it is fully consistent with momentum conservation.Time-periodic driving, which allows coherent manipulation of many-body systems, is becoming an exciting tool in the ultracold atomic gases. This provides access to new quantum physics, for example, topological states, synthetic gauge fields and Mott transitions [1][2][3][4][5]. Of particular interest is the rather unique capability these atomic systems afford into understanding non-equilibrium many-body dynamics [6]. Also unique to the ultracold gases is the ability, through the Feshbach resonance, to periodically modulate atomic interactions [7]. Recently, this was implemented by the Chicago group [8,9] and the Rice group [10-12] on Bose-Einstein condensates. In the Chicago experiment, a collective emission of matter-wave jets resembling fireworks occurs above a threshold modulation amplitude.The jets were associated with a form of runaway stimulated inelastic scattering occurring in the driven condensate [8].In this paper we use the time-dependent Gross-Pitaevskii (GP) equation to study the evolution of the modulated BEC and the emission of these jets [8]. An unbiased or random noise term is introduced initially to model the fluctuations that seed the jet emission. We show that the simulations capture well the "fireworks" dynamics seen in experiments. Moreover, in combination with a new set of experiments, we identify a previously unobserved stage of the evolution that precedes and underlies the jet-emission. Immediately after modulation, we observe that density waves emerge and grow rapidly in the condensate with quantized wavenumbers determined by the modulation frequency [13]. The density waves arise from the interference between excited matterwaves and the condensate. The pattern is reminiscent of Faraday waves in nonlinear fluids [14,15] and related to that predicted for driven atomic gases [16][17][18][19] as well as observed in the one-dimensional condensate [20].The amplification of these density waves can be considered the matterwave analog of superradiant 30 µm 0 5 9 14 18 t = 28 ms Time t Experiment Simulation Density n (µm -2 ) 0 30 15 Density n/ n 0 0 0.6 0.3FIG. 1. The real space density distribution n(r) (denoted as n) as a comparison betw...

Spermatogonial stem cells (SSCs) have significant applications in reproductive and regenerative medicine. However, nothing is known about genes in mediating human SSCs. Here we have explored for the first time the function and mechanism of P21-activated kinase 1 (PAK1) in regulating the proliferation and apoptosis of the human SSC line. PAK1 level was upregulated by epidermal growth factor (EGF), but not glial cell line-derived neurotrophic factor (GDNF) or fibroblast growth factor 2 (FGF2). PAK1 promoted proliferation and DNA synthesis of the human SSC line, whereas PAK1 suppressed its apoptosis in vitro and in vivo. RNA sequencing identified that PDK1, ZNF367, and KDR levels were downregulated by PAK1 knockdown. Immunoprecipitation and Western blots demonstrated that PAK1 interacted with PDK1. PDK1 and KDR levels were decreased by ZNF367-small interfering RNAs (siRNAs). The proliferation of the human SSC line was reduced by PDK1-, KDR-, and ZNF367-siRNAs, whereas its apoptosis was enhanced by these siRNAs. The levels of phos-ERK1/2, phos-AKT, and cyclin A were decreased by PAK1-siRNAs. Tissue arrays showed that PAK1 level was low in non-obstructive azoospermia patients. Collectively, PAK1 was identified as the first molecule that controls proliferation and apoptosis of the human SSC line through PDK1/KDR/ZNF367 and the ERK1/2 and AKT pathways. This study provides data on novel gene regulation and networks underlying the fate of human SSCs, and it offers new molecular targets for human SSCs in translational medicine.