The classical nature of nuclear spin noise near clock transitions of Bi donors in silicon

Whether a quantum bath can be approximated as classical noise is a fundamental issue in central spin decoherence and also of practical importance in designing noise-resilient quantum control. Spin qubits based on bismuth donors in silicon have tunable interactions with nuclear spin baths and are first-order insensitive to magnetic noise at so-called clock-transitions (CTs). This system is therefore ideal for studying the quantum/classical nature of nuclear spin baths since the qubit-bath interaction strength determines the back-action on the baths and hence the adequacy of a classical noise model. We develop a Gaussian noise model with noise correlations determined by quantum calculations and compare the classical noise approximation to the full quantum bath theory. We experimentally test our model through dynamical decoupling sequence of up to 128 pulses, finding good agreement with simulations and measuring electron spin coherence times approaching one second - notably using natural silicon. Our theoretical and experimental study demonstrates that the noise from a nuclear spin bath is analogous to classical Gaussian noise if the back-action of the qubit on the bath is small compared to the internal bath dynamics, as is the case close to CTs. However, far from the CTs, the back-action of the central spin on the bath is such that the quantum model is required to accurately model spin decoherence.Comment: 5 pages, 3 figure

Uncovering many-body correlations in nanoscale nuclear spin baths by central spin decoherence

Many-body correlations can yield key insights into the nature of interacting systems; however, detecting them is often very challenging in many-particle physics, especially in nanoscale systems. Here, taking a phosphorus donor electron spin in a natural-abundance 29Si nuclear spin bath as our model system, we discover both theoretically and experimentally that many-body correlations in nanoscale nuclear spin baths produce identifiable signatures in the decoherence of the central spin under multiple-pulse dynamical decoupling control. We find that when the number of decoupling -pulses is odd, central spin decoherence is primarily driven by second-order nuclear spin correlations (pairwise flip-flop processes). In contrast, when the number of -pulses is even, fourth-order nuclear spin correlations (diagonal interaction renormalized pairwise flip-flop processes) are principally responsible for the central spin decoherence. Many-body correlations of different orders can thus be selectively detected by central spin decoherence under different dynamical decoupling controls, providing a useful approach to probing many-body processes in nanoscale nuclear spin baths

Construction of a polarization insensitive lens from a quasi-isotropic metamaterial slab

We propose to employ the quasiisotropic metamaterial (QIMM) slab to construct a polarization insensitive lens, in which both E- and H-polarized waves exhibit the same refocusing effect. For shallow incident angles, the QIMM slab will provide some degree of refocusing in the same manner as an isotropic negative index material. The refocusing effect allows us to introduce the ideas of paraxial beam focusing and phase compensation by the QIMM slab. On the basis of angular spectrum representation, a formalism describing paraxial beams propagating through a QIMM slab is presented. Because of the negative phase velocity in the QIMM slab, the inverse Gouy phase shift and the negative Rayleigh length of paraxial Gaussian beam are proposed. We find that the phase difference caused by the Gouy phase shift in vacuum can be compensated by that caused by the inverse Gouy phase shift in the QIMM slab. If certain matching conditions are satisfied, the intensity and phase distributions at object plane can be completely reconstructed at image plane. Our simulation results show that the superlensing effect with subwavelength image resolution could be achieved in the form of a QIMM slab.Comment: 25 pages, 8 figure

Pure spin current in a two-dimensional topological insulator

We predict a mechanism to generate a pure spin current in a two-dimensional topological insulator. As the magnetic impurities exist on one of edges of the two-dimensional topological insulator, a gap is opened in the corresponding gapless edge states but another pair of gapless edge states with opposite spin are still protected by the time-reversal symmetry. So the conductance plateaus with the half-integer values $e^2/h$ can be obtained in the gap induced by magnetic impurities, which means that the pure spin current can be induced in the sample. We also find that the pure spin current is insensitive to weak disorder. The mechanism to generate pure spin currents is generalized for two-dimensional topological insulators.Comment: 5 pages, 6 figure

Plastic properties and microstructure evolution of 20CrMoA steel during warm deformation

The plastic properties and microstructure evolution of 20CrMoA steel was analyzed at 600-750 °C and strain rate of 0,01-10 s-1.The result reveals that the deformation behavior is hardening followed by softening at low strain rates(0,01 s-1 and 0,1 s-1), but hardening is dominant in the whole deformation process at high strain rates(1 s-1and 10 s-1) and low temperature(600 °C and 650 °C). The strain rate sensitivity exponent increases with the increasing deformation temperature except for 650 °C and high strain rate. The spheroidization mechanism of cementite is the mechanical fracture and the dissolution of cementite particles. At 700 °C, spheroidized particles are finer and their distribution is more uniform than that at 750 °C

Study on cold spinning deformation behavior and load mechanism of conical thin-walled aluminum alloy parts

Aiming at the problem that the wall thickness uniformity of conical thin-walled aluminum alloy parts formed by single-pass spinning is difficult to control, this paper systematically studies the influence of different spinning parameters on the material evolution law and load mechanism during the forming process of 1070 thin-walled aluminum alloy. The effects of slab thickness, roller gap and feed ratio on the macroscopic forming quality and mechanical load of 1070 thin-walled aluminum alloy conical parts were simulated by Simufact.forming software. Finally, the optimal spinning parameters of 1070 thin-walled aluminum alloy conical parts were obtained. The final results show that increasing the diameter-thickness ratio of the blank can prevent wrinkling, and reasonable spinning parameters can improve the uniformity of the wall thickness of the product

Study on cold spinning deformation behavior and load mechanism of conical thin-walled aluminum alloy parts

Aiming at the problem that the wall thickness uniformity of conical thin-walled aluminum alloy parts formed by single-pass spinning is difficult to control, this paper systematically studies the influence of different spinning parameters on the material evolution law and load mechanism during the forming process of 1070 thin-walled aluminum alloy. The effects of slab thickness, roller gap and feed ratio on the macroscopic forming quality and mechanical load of 1070 thin-walled aluminum alloy conical parts were simulated by Simufact.forming software. Finally, the optimal spinning parameters of 1070 thin-walled aluminum alloy conical parts were obtained. The final results show that increasing the diameter-thickness ratio of the blank can prevent wrinkling, and reasonable spinning parameters can improve the uniformity of the wall thickness of the product

Large-deviation analysis for counting statistics in mesoscopic transports

We present an efficient approach, based on a number-conditioned master equation, for large-deviation analysis in mesoscopic transports. Beyond the conventional full-counting-statistics study, the large-deviation approach encodes complete information of both the typical trajectories and the rare ones, in terms of revealing a continuous change of the dynamical phase in trajectory space. The approach is illustrated with two examples: (i) transport through a single quantum dot, where we reveal the inhomogeneous distribution of trajectories in general case and find a particular scale invariance point in trajectory statistics; and (ii) transport through a double dots, where we find a dynamical phase transition between two distinct phases induced by the Coulomb correlation and quantum interference.Comment: 8 pages, 3 figure

The Trophic Life Cycle Stage of the Opportunistic Fungal Pathogen \u3cem\u3ePneumocystis murina\u3c/em\u3e Hinders the Ability of Dendritic Cells to Stimulate CD4\u3csup\u3e+\u3c/sup\u3e T Cell Responses

The life cycle of the opportunistic fungal pathogen Pneumocystis murina consists of a trophic stage and an ascus-like cystic stage. Infection with the cyst stage induces proinflammatory immune responses, while trophic forms suppress the cytokine response to multiple pathogen-associated molecular patterns (PAMPs), including β-glucan. A targeted gene expression assay was used to evaluate the dendritic cell response following stimulation with trophic forms alone, with a normal mixture of trophic forms and cysts, or with β-glucan. We demonstrate that stimulation with trophic forms downregulated the expression of multiple genes normally associated with the response to infection, including genes encoding transcription factors. Trophic forms also suppressed the expression of genes related to antigen processing and presentation, including the gene encoding the major histocompatibility complex (MHC) class II transactivator, CIITA. Stimulation of dendritic cells with trophic forms, but not a mixture of trophic forms and cysts, reduced the expression of MHC class II and the costimulatory molecule CD40 on the surface of the cells. These defects in the expression of MHC class II and costimulatory molecules corresponded with a reduced capacity for trophic form-loaded dendritic cells to stimulate CD4+ T cell proliferation and polarization. These data are consistent with the delayed innate and adaptive responses previously observed in immunocompetent mice inoculated with trophic forms compared to responses in mice inoculated with a mixture of trophic forms and cysts. We propose that trophic forms broadly inhibit the ability of dendritic cells to fulfill their role as antigen-presenting cells

The Final SDSS High-Redshift Quasar Sample of 52 Quasars at z>5.7

We present the discovery of nine quasars at $z\sim6$ identified in the Sloan Digital Sky Survey (SDSS) imaging data. This completes our survey of $z\sim6$ quasars in the SDSS footprint. Our final sample consists of 52 quasars at $5.7<z\le6.4$, including 29 quasars with $z_{\rm AB}\le20$ mag selected from 11,240 deg$^2$ of the SDSS single-epoch imaging survey (the main survey), 10 quasars with $20\le z_{\rm AB}\le20.5$ selected from 4223 deg$^2$ of the SDSS overlap regions (regions with two or more imaging scans), and 13 quasars down to $z_{\rm AB}\approx22$ mag from the 277 deg$^2$ in Stripe 82. They span a wide luminosity range of $-29.0\le M_{1450}\le-24.5$. This well-defined sample is used to derive the quasar luminosity function (QLF) at $z\sim6$. After combining our SDSS sample with two faint ($M_{1450}\ge-23$ mag) quasars from the literature, we obtain the parameters for a double power-law fit to the QLF. The bright-end slope $\beta$ of the QLF is well constrained to be $\beta=-2.8\pm0.2$. Due to the small number of low-luminosity quasars, the faint-end slope $\alpha$ and the characteristic magnitude $M_{1450}^{\ast}$ are less well constrained, with $\alpha=-1.90_{-0.44}^{+0.58}$ and $M^{\ast}=-25.2_{-3.8}^{+1.2}$ mag. The spatial density of luminous quasars, parametrized as $\rho(M_{1450}<-26,z)=\rho(z=6)\,10^{k(z-6)}$, drops rapidly from $z\sim5$ to 6, with $k=-0.72\pm0.11$. Based on our fitted QLF and assuming an IGM clumping factor of $C=3$, we find that the observed quasar population cannot provide enough photons to ionize the $z\sim6$ IGM at $\sim90$\% confidence. Quasars may still provide a significant fraction of the required photons, although much larger samples of faint quasars are needed for more stringent constraints on the quasar contribution to reionization.Comment: 20 pages, 12 figures, Accepted for publication in The Astrophysical Journa