Coherent multiple charge transfer in a superconducting NbN tunnel junction

We measured shot noise and submillimeter-wave response in a superconducting NbN tunnel junction that had a subharmonic gap structure on the current-voltage (I-V) curve. We found that the observed effective charge, defined from the noise-current ratio, tends to a steplike function of voltage. In the presence of submillimeter-wave radiation of frequency v, novel step structures spaced by hv/2e below and above the half-gap voltage clearly appeared on the I-V curve, overlapping the ordinary photon-assisted tunneling steps spaced by hv/e. Observation of these features provides clear evidence that coherent multiple Andreev reflection processes occur in the NbN tunnel junction with low barrier transparency.     

强激光加载下高压液氘材料的电导率

液氘在高压下有丰富的电学光学性质。利用反射率和相对介电函数关系并从广义极化角度出发初步建立了计算低Z材料电导率的简易模型;在神光-Ⅱ装置上利用第九路激光冲击加载液氘材料并测量了其在强激光冲击下的高压状态参数和反射率。结合上述理论模型和实验,研究了高压下液氘的电离度和电导率。结果表明,液氘在约70 GPa时的电导率约为2.87105 (Wm)-1,已呈现出较为明显的金属电导特性。显然,冲击加载下液氘从绝缘分子态开始电离并向金属氘转变发生在更低的压强。     

Stop coannihilation in the CMSSM and SubGUT models

Stop coannihilation may bring the relic density of heavy supersymmetric dark matter particles into the range allowed by cosmology. The efficiency of this process is enhanced by stop-antistop annihilations into the longitudinal (Goldstone) modes of the W and Z bosons, as well as by Sommerfeld enhancement of stop annihilations and the effects of bound states. Since the couplings of the stops to the Goldstone modes are proportional to the trilinear soft supersymmetry-breaking A-terms, these annihilations are enhanced when the A-terms are large. However, the Higgs mass may be reduced below the measured value if the A-terms are too large. Unfortunately, the interpretation of this constraint on the stop coannihilation strip is clouded by differences between the available Higgs mass calculators. For our study, we use as our default calculator FeynHiggs 2.13.0, the most recent publicly available version of this code. Exploring the CMSSM parameter space, we find that along the stop coannihilation strip the masses of the stops are severely split by the large A-terms. This suppresses the Higgs mass drastically for \(\mu \) and \(A_0 > 0\), whilst the extent of the stop coannihilation strip is limited for \(A_0 < 0\) and either sign of \(\mu \). However, in sub-GUT models, reduced renormalization-group running mitigates the effect of the large A-terms, allowing larger LSP masses to be consistent with the Higgs mass calculation. We give examples where the dark matter particle mass may reach \(\gtrsim 8\) TeV.     

An upgraded external beam PIXE setup for multielemental analysis of atmospheric aerosol samples

The external beam particle‐induced X‐ray emission (PIXE) setup has been upgraded by introducing a new silicon drift detector with the aim of obtaining better minimum detection limits (MDLs) at the 2 × 1.7 MV Tandetron accelerator of the Beijing Normal University in China. The upgraded external beam PIXE setup is equipped with two silicon drift detectors for PIXE analysis of low and high Z elements. A surface barrier detector for Rutherford backscattering spectrometry monitors beam and helium flow. Two kinds of aerosol filter samples (quartz fiber filters and Teflon filters) were studied. A 200‐μm thick Mylar absorber in front of the medium‐high energy X‐rays detector was adopted, and it got the best MDLs for atmospheric aerosol samples analysis. Multielemental analysis of quartz fiber filter aerosol samples was achieved. For more accurate and better MDLs of low Z elements in PIXE analysis, it is necessary to keep sufficient helium flow behind the thin samples.     

Sensitivity of DNA Sensors in the Presence of Charged Ligands

Factors that influence both the thermodynamics of hybridization and the stability of the DNA–DNA duplexes are analyzed. The noncompetitive DNA hybridization cases in the presence of mono- and bivalent positively charged ligands have been investigated and the comparison is made with the case of uncharged ligands. It has been shown that the charged ligands enhance the sensitivity of the DNA chips as compared with the uncharged ones.     

Frequency and intensity readouts of micro-wave electric field using Rydberg atoms with Doppler effects

A scheme of electric-field measurement of micro-waves is proposed in Rydberg atoms with Doppler effects. A cascade-type electromagnetically-induced-transparency (EIT) system is disturbed by a perturbative field coupling a metastable transition. The original dark state splits and two EIT window appear with a central absorption peak. When a micro-wave (MW) field couples the Rydberg transition, the central absorption peak is divided into two. The frequency splitting of two central peaks is proportional to the MW field intensity, which can be used to probe MW electric field strength. This frequency-readout method based on a double-dark state system increases the probe sensitivity nearly by a factor of 7, compared with that of the single-dark state case. At room temperature, Doppler effects can enhance the absorption spectrum, moreover, its peak value varies linearly with the MW field strength. This can also serve to measure MW electric field strength. Numerical results show that the latter intensity-readout method after Doppler averaging improves the probe sensitivity by a factor of 10 with respect to the case without Doppler effects.     

Evidence for charge Kondo effect in superconducting Tl-doped PbTe

We report results of low-temperature thermodynamic and transport measurements of Pb1-xTlxTe single crystals for Tl concentrations up to the solubility limit of approximately x=1.5%. For all doped samples, we observe a low-temperature resistivity upturn that scales in magnitude with the Tl concentration. The temperature and field dependence of this upturn are consistent with a charge Kondo effect involving degenerate Tl valence states differing by two electrons, with a characteristic Kondo temperature T(K) approximately 6 K. The observation of such an effect supports an electronic pairing mechanism for superconductivity in this material and may account for the anomalously high T(c) values.     

Interchain-frustration-induced metallic state in quasi-one-dimensional Mott insulators

The mechanism that drives a metal-insulator transition in an undoped quasi-one-dimensional Mott insulator is examined in the framework of the Hubbard model with two different hoppings t(perpendicular1) and t(perpendicular2) between nearest-neighbor chains. By applying an N(perpendicular)-chain renormalization group method at the two-loop level, we show how a metallic state emerges when both t(perpendicular1) and t(perpendicular2) exceed critical values. In the metallic phase, the quasiparticle weight becomes finite and develops a strong momentum dependence. We discuss the temperature dependence of the resistivity and the impact of our theory in the understanding of recent experiments on half-filled molecular conductors.     

Current and strain-induced spin polarization in InGaN/GaN superlattices

The lateral current-induced spin polarization in InGaN/GaN superlattices (SLs) without an applied magnetic field is reported. The fact that the sign of the nonequilibrium spin changes as the current reverses and is opposite for the two edges provides a clear signature for the spin Hall effect. In addition, it is discovered that the spin Hall effect can be strongly manipulated by the internal strains. A theoretical work has also been developed to understand the observed strain-induced spin polarization. Our result paves an alternative way for the generation of spin polarized current.     

Dramatic role of critical current anisotropy on flux avalanches in MgB2 films

Anisotropic penetration of magnetic flux in MgB(2) films grown on vicinal sapphire substrates is investigated using magneto-optical imaging. Regular penetration above 10 K proceeds more easily along the substrate surface steps, the anisotropy of the critical current being 6%. At lower temperatures the penetration occurs via abrupt dendritic avalanches that preferentially propagate perpendicular to the surface steps. This inverse anisotropy in the penetration pattern becomes dramatic very close to 10 K where all flux avalanches propagate in the strongest pinning direction. The observed behavior is fully explained using a thermomagnetic model of the dendritic instability.