Search

We report the details of the recent x-ray back diffraction experiments, in which interference fringes due to x-ray cavity resonance are unambiguously observed. The Fabry-Pérot type cavities, the tested crystal devices of reflectivity $R\simeq 0.5$ and finesse $F\simeq 2.3$, consist of monolithic two-plate and eight-plate silicon crystals. They were prepared by using x-ray lithographic techniques. The thicknesses of the crystal plates and the gaps between the two adjacent plates are a few tens to hundreds $\mu \mathrm{m}$. The (12 4 0) back reflection and synchrotron x-radiation of energy resolution $\Delta E=0.36\mathrm{meV}$ at $14.4388\mathrm{keV}$ are employed. Interference fringes in angle- and photon-energy scans for two-plate and eight-plate cavities are shown. Considerations on the temporal and spatial coherence for observable resonance interference fringes using synchrotron x-rays are presented. The details about the accompanied simultaneous 24-beam diffraction in relation to x-ray photon energy are also described.

Ferroelectric perovskite oxides possess a large electrocaloric (EC) effect, but usually at high temperatures near the ferroelectric/paraelectric phase transition temperature, which limits their potential application as next-generation solid-state cooling devices. We use classical molecular dynamics to study the electric-field-induced phase transitions and EC effect in PMN-PT $(\mathrm{PbM}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}{\mathrm{O}}_3-\mathrm{PbTi}{\mathrm{O}}_3)$. We find that the maximum EC strength of PMN-PT occurs within the morphotropic phase boundary (MPB) region at 300 K. The large adiabatic temperature change is caused by easy rotation of polarization within the MPB region.

Oxygen interstitial site energies and migration barriers in 15 hexagonal close-packed (hcp) metals have been calculated with first-principles density functional theory. Multiple hcp systems show a preference for the hexahedral site over the tetrahedral site, as well as a stable crowdion site. More surprisingly, in more than half of the hcp systems, the oxygen does not choose the large octahedral interstitial as its ground state. We explain this result based on the effective valence of the metal from crystal-field splitting and the $c/a$ ratio. Diffusion constants for oxygen in all 15 hcp systems are calculated from analytically derived diffusion equations and match available experimental data.

A method for direct determination of resonance phase shifts in a (001) CdTe/InSb thin-film system is developed using soft x-ray three-wave resonance diffraction. At the (002) Bragg peaks of CdTe and InSb, two inversion-symmetry related three-wave diffractions are systematically identified according to crystal symmetry and the resonance phase shifts versus photon energies are measured without turning the thin film upside down. The momentum-transfer selectivity at (002) reflections facilitates the quantitative determination of the phase shifts near the $\text{Cd}{L}_3$, $\text{Te}{L}_3$, and $\text{Sb}{L}_2$ edges.

X-ray diffraction with photon energies near the Ru $L_2$-absorption edge was used to detect resonant reflections characteristic of a $G$-type superstructure in ${\mathrm{RuSr}}_2{\mathrm{GdCu}}_2{\mathrm{O}}_8$ single crystals. A polarization analysis confirms that these reflections are due to magnetic order of Ru moments, and the azimuthal-angle dependence of the scattering amplitude reveals that the moments lie along a low-symmetry axis with substantial components parallel and perpendicular to the ${\mathrm{RuO}}_2$ layers. Complemented by susceptibility data and a symmetry analysis of the magnetic structure, these results reconcile many of the apparently contradictory findings reported in the literature.

Angle-resolved photoemission extended fine structure (ARPEFS) was used to determine the structure of c(2×2)P/Fe(100). Photoemission data were collected normal to the (100) surface and 45° off-normal along the [011] direction at room temperature. A close analysis of the autoregressive linear-prediction-based Fourier transform indicates that the P atoms adsorb in the high-coordination fourfold hollow sites. Curved-wave multiple-scattering calculations confirmed the fourfold hollow adsorption site. The P atoms were determined to bond 1.02 Å above the first layer of Fe atoms and the Fe-P-Fe bond angle is 140.6°. Additionally, it was determined that there was no expansion of the Fe surface. Self-consistent-field Xα scattered-wave calculations were performed for the c(2×2)P/Fe(100) and the c(2×2)S/Fe(100) systems. These independent results are in excellent agreement with this P/Fe structure and the S/Fe structure previously published, confirming the ARPEFS determination that the ${\mathrm{Fe}}_1$ -${\mathrm{Fe}}_2$ interlayer spacing is contracted from the bulk value for S/Fe but not for P/Fe. Finally, this structure is compared to structures from the literature of atomic nitrogen, atomic oxygen, and sulfur adsorbed on the Fe(100) surface.

The resistivity ρ and Hall coefficient ${\mathit{R}}_{\mathit{H}}$ have been measured for samples of the two-plane superconductor ${\mathrm{HgBa}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{6+\mathrm{\delta }}$ (${\mathit{T}}_{\mathit{c}}$∼124 K) with different annealing histories. The observed temperature dependence ${\mathit{R}}_{\mathit{H}}$∼1/T (up to 470 K) matches the steep dependence previously seen in 90-K ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ and indicates low-impurity scattering. We find T-linear behavior for ρ in the lowest-resistivity samples and confirm that cot${\mathrm{\theta }}_{\mathit{H}}$∼${\mathit{T}}^2$. These findings add support to Anderson’s proposal of distinct longitudinal and transverse scattering times.

Semiperipheral collisions in the ${}^{124}\mathrm{Sn}+{}^{64}\mathrm{Ni}$ reaction at 35 MeV/nucleon were studied using the forward part of the Charged Heavy Ion Mass and Energy Resolving Array. Nearly completely determined ternary events involving projectilelike fragments (PLF), targetlike fragments (TLF), and intermediate mass fragments (IMF) were selected. A new method of studying the reaction mechanism, focusing on the analysis of the correlations between relative velocities in the IMF+PLF and IMF+TLF subsystems, is proposed. The relative velocity correlations provide information on the time sequence and time scale of the neck fragmentation processes leading to production of IMFs. It is shown that the majority of light IMFs are produced within 40–80 fm/c after the system starts to reseparate. Heavy IMFs are formed at times of about 120 fm/c or later and can be viewed as resulting from two-step (sequential) neck rupture processes.

Flexible control of magnetization switching by electrical means is crucial for applications of spin orbitronics. Besides a switching current that is parallel to an applied field, a bias current that is normal to the switching current is introduced to tune the magnitude of effective damping-like and field-like torques and further electrically control magnetization switching. Symmetrical and asymmetrical control over the critical switching current by the bias current with opposite polarities is realized in both Pt/Co/MgO and $\alpha$-Ta/CoFeB/MgO systems, respectively. This research not only identifies the influences of field-like and damping-like torques on the switching process, it also demonstrates an electrical method to control it.

A precision measurement by the Alpha Magnetic Spectrometer on the International Space Station of the positron fraction in primary cosmic rays in the energy range from 0.5 to 350 GeV based on $6.8\times {10}^6$ positron and electron events is presented. The very accurate data show that the positron fraction is steadily increasing from 10 to $\sim 250\mathrm{GeV}$, but, from 20 to 250 GeV, the slope decreases by an order of magnitude. The positron fraction spectrum shows no fine structure, and the positron to electron ratio shows no observable anisotropy. Together, these features show the existence of new physical phenomena.

We present high-statistics, precision measurements of the detailed time and energy dependence of the primary cosmic-ray electron flux and positron flux over 79 Bartels rotations from May 2011 to May 2017 in the energy range from 1 to 50 GeV. For the first time, the charge-sign dependent modulation during solar maximum has been investigated in detail by leptons alone. Based on $23.5\times {10}^6$ events, we report the observation of short-term structures on the timescale of months coincident in both the electron flux and the positron flux. These structures are not visible in the $e^{+}/e^{-}$ flux ratio. The precision measurements across the solar polarity reversal show that the ratio exhibits a smooth transition over $830\pm 30$ days from one value to another. The midpoint of the transition shows an energy dependent delay relative to the reversal and changes by $260\pm 30$ days from 1 to 6 GeV.

A precision measurement of the nitrogen flux with rigidity (momentum per unit charge) from 2.2 GV to 3.3 TV based on $2.2\times {10}^6$ events is presented. The detailed rigidity dependence of the nitrogen flux spectral index is presented for the first time. The spectral index rapidly hardens at high rigidities and becomes identical to the spectral indices of primary He, C, and O cosmic rays above $\sim 700\mathrm{GV}$. We observed that the nitrogen flux ${\mathrm{\Phi }}_{\mathrm{N}}$ can be presented as the sum of its primary component ${\mathrm{\Phi }}_{\mathrm{N}}^P$ and secondary component ${\mathrm{\Phi }}_{\mathrm{N}}^S$, ${\mathrm{\Phi }}_{\mathrm{N}}={\mathrm{\Phi }}_{\mathrm{N}}^P+{\mathrm{\Phi }}_{\mathrm{N}}^S$, and we found ${\mathrm{\Phi }}_{\mathrm{N}}$ is well described by the weighted sum of the oxygen flux ${\mathrm{\Phi }}_{\mathrm{O}}$ (primary cosmic rays) and the boron flux ${\mathrm{\Phi }}_{\mathrm{B}}$ (secondary cosmic rays), with ${\mathrm{\Phi }}_{\mathrm{N}}^P=(0.090\pm 0.002)\times {\mathrm{\Phi }}_{\mathrm{O}}$ and ${\mathrm{\Phi }}_{\mathrm{N}}^S=(0.62\pm 0.02)\times {\mathrm{\Phi }}_{\mathrm{B}}$ over the entire rigidity range. This corresponds to a change of the contribution of the secondary cosmic ray component in the nitrogen flux from 70% at a few GV to $<30\%$ above 1 TV.

We report the magnetic and electronic properties of ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$, which has a structure based on a triangular bilayer of Mn, rather than the ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$ structure commonly encountered for 122 compounds in intermetallic systems. ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$ has an antiferromagnetic ground state, with a ${\mathrm{T}}_N$ of 150 K, and for a 250 K temperature range above ${\mathrm{T}}_N$ does not exhibit Curie-Weiss behavior, indicating the presence of strong magnetic correlations at high temperatures. Resistivity measurements show that ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$ exhibits semiconducting properties at low temperatures, with an energy gap of only 62 meV, indicating it to be a very narrow band gap semiconductor. The electronic structure of ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$, examined via ab-initio electronic structure calculations, indicates that Mn 3d orbital hybridization is essential for the formation of the band gap, suggesting that ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$ may be a hybridization-gap semiconductor.

We report the observation of a new structure in the ${\mathrm{\Lambda }}_b^0{\pi }^{+}{\pi }^{-}$ spectrum using the full LHCb data set of $pp$ collisions, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$, collected at $\sqrt{s}=7$, 8, and 13 TeV. A study of the structure suggests its interpretation as a superposition of two almost degenerate narrow states. The masses and widths of these states are measured to be $m_{{\mathrm{\Lambda }}_b{(6146)}^0}=6146.17\pm 0.33\pm 0.22\pm 0.16\mathrm{MeV},m_{{\mathrm{\Lambda }}_b{(6152)}^0}=6152.51\pm 0.26\pm 0.22\pm 0.16\mathrm{MeV},{\mathrm{\Gamma }}_{{\mathrm{\Lambda }}_b{(6146)}^0}=2.9\pm 1.3\pm 0.3\mathrm{MeV},{\mathrm{\Gamma }}_{{\mathrm{\Lambda }}_b{(6152)}^0}=2.1\pm 0.8\pm 0.3\mathrm{MeV},$with a mass splitting of $\mathrm{\Delta }m=6.34\pm 0.32\pm 0.02\mathrm{MeV}$, where the first uncertainty is statistical, the second systematic. The third uncertainty for the mass measurements derives from the knowledge of the mass of the ${\mathrm{\Lambda }}_b^0$ baryon. The measured masses and widths of these new excited states suggest their possible interpretation as a doublet of ${\mathrm{\Lambda }}_b{(1D)}^0$ states.

The four decays, ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Sigma }}_c^{(*)++}{D}^{(*)-}{K}^{-}$, are observed for the first time using proton-proton collision data collected with the LHCb detector at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of $6{\mathrm{fb}}^{-1}$. By considering the ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\overline{D}}^0{K}^{-}$ decay as reference channel, the following branching fraction ratios are measured to be $\frac{\mathcal{B}({\mathrm{\Lambda }}_b^0\to {\mathrm{\Sigma }}_c^{++}{D}^{-}{K}^{-})}{\mathcal{B}({\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\overline{D}}^0{K}^{-})}=0.282\pm 0.016\pm 0.016\pm 0.005$, $\frac{\mathcal{B}({\mathrm{\Lambda }}_b^0\to {\mathrm{\Sigma }}_c^{*++}{D}^{-}{K}^{-})}{\mathcal{B}({\mathrm{\Lambda }}_b^0\to {\mathrm{\Sigma }}_c^{++}{D}^{-}{K}^{-})}=0.460\pm 0.052\pm 0.028$, $\frac{\mathcal{B}({\mathrm{\Lambda }}_b^0\to {\mathrm{\Sigma }}_c^{++}{D}^{*-}{K}^{-})}{\mathcal{B}({\mathrm{\Lambda }}_b^0\to {\mathrm{\Sigma }}_c^{++}{D}^{-}{K}^{-})}=2.261\pm 0.202\pm 0.129\pm 0.046$, $\frac{\mathcal{B}({\mathrm{\Lambda }}_b^0\to {\mathrm{\Sigma }}_c^{*++}{D}^{*-}{K}^{-})}{\mathcal{B}({\mathrm{\Lambda }}_b^0\to {\mathrm{\Sigma }}_c^{++}{D}^{-}{K}^{-})}=0.896\pm 0.137\pm 0.066\pm 0.018$, where the first uncertainties are statistical, the second are systematic, and the third are due to uncertainties in the branching fractions of intermediate particle decays. These initial observations mark the beginning of pentaquark searches in these modes, with more datasets to become available following the LHCb upgrade.

The fraction of ${\chi }_{c1}$ and ${\chi }_{c2}$ decays in the prompt $J/\psi$ yield, $F_{{\chi }_c\to J/\psi }={\sigma }_{{\chi }_c\to J/\psi }/{\sigma }_{J/\psi }$, is measured by the LHCb detector in $p\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}}=8.16\mathrm{TeV}$. The study covers the forward ($1.5<y^{*}<4.0$) and backward ($-5.0<y^{*}<-2.5$) rapidity regions, where $y^{*}$ is the $J/\psi$ rapidity in the nucleon-nucleon center-of-mass system. Forward and backward rapidity samples correspond to integrated luminosities of $13.6\pm 0.3$ and $20.8\pm 0.5{\mathrm{nb}}^{-1}$, respectively. The result is presented as a function of the $J/\psi$ transverse momentum $p_{\mathrm{T},J/\psi }$ in the range $1<p_{\mathrm{T},J/\psi }<20\mathrm{GeV}/c$. The $F_{{\chi }_c\to J/\psi }$ fraction at forward rapidity is compatible with the LHCb measurement performed in $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$, whereas the result at backward rapidity is $2.4\sigma$ larger than in the forward region for $1<p_{\mathrm{T},J/\psi }<3\mathrm{GeV}/c$. The increase of $F_{{\chi }_c\to J/\psi }$ at low $p_{\mathrm{T},J/\psi }$ at backward rapidity is compatible with the suppression of the $\psi (2S)$ contribution to the prompt $J/\psi$ yield. The lack of in-medium dissociation of ${\chi }_c$ states observed in this study sets an upper limit of 180 MeV on the free energy available in these $p\mathrm{Pb}$ collisions to dissociate or inhibit charmonium state formation.

We present the first results from an all-sky all-frequency (ASAF) search for an anisotropic stochastic gravitational-wave background using the data from the first three observing runs of the Advanced LIGO and Advanced Virgo detectors. Upper limit maps on broadband anisotropies of a persistent stochastic background were published for all observing runs of the LIGO-Virgo detectors. However, a broadband analysis is likely to miss narrowband signals as the signal-to-noise ratio of a narrowband signal can be significantly reduced when combined with detector output from other frequencies. Data folding and the computationally efficient analysis pipeline, PyStoch, enable us to perform the radiometer map-making at every frequency bin. We perform the search at 3072 HEALPix equal area pixels uniformly tiling the sky and in every frequency bin of width $1/32\mathrm{Hz}$ in the range 20–1726 Hz, except for bins that are likely to contain instrumental artefacts and hence are notched. We do not find any statistically significant evidence for the existence of narrowband gravitational-wave signals in the analyzed frequency bins. Therefore, we place 95% confidence upper limits on the gravitational-wave strain for each pixel-frequency pair, the limits are in the range $(0.030-9.6)\times {10}^{-24}$. In addition, we outline a method to identify candidate pixel-frequency pairs that could be followed up by a more sensitive (and potentially computationally expensive) search, e.g., a matched-filtering-based analysis, to look for fainter nearly monochromatic coherent signals. The ASAF analysis is inherently independent of models describing any spectral or spatial distribution of power. We demonstrate that the ASAF results can be appropriately combined over frequencies and sky directions to successfully recover the broadband directional and isotropic results.

A flavor-tagged time-dependent angular analysis of the decay $B_s^0\to \varphi \varphi$ is performed using $pp$ collision data collected by the LHCb experiment at the center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of $6{\mathrm{fb}}^{-1}$. The $CP$-violating phase and direct $CP$-violation parameter are measured to be ${\varphi }_s^{s\overline{s}s}=-0.042\pm 0.075\pm 0.009\mathrm{rad}$ and $|\lambda |=1.004\pm 0.030\pm 0.009$, respectively, assuming the same values for all polarization states of the $\varphi \varphi$ system. In these results, the first uncertainties are statistical and the second systematic. These parameters are also determined separately for each polarization state, showing no evidence for polarization dependence. The results are combined with previous LHCb measurements using $pp$ collisions at center-of-mass energies of 7 and 8 TeV, yielding ${\varphi }_s^{s\overline{s}s}=-0.074\pm 0.069\mathrm{rad}$ and $|\lambda |=1.009\pm 0.030$. This is the most precise study of time-dependent $CP$ violation in a penguin-dominated $B$ meson decay. The results are consistent with $CP$ symmetry and with the standard model predictions.

An amplitude analysis of $B^0\to J/\psi \varphi K_{\text{S}}^0$ decays is performed using proton-proton collision data, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$, collected with the LHCb detector at center-of-mass energies of 7, 8, and 13 TeV. Evidence with a significance of 4.0 standard deviations of a structure in the $J/\psi K_{\text{S}}^0$ system, named $T_{\psi s1}^{\theta }(4000{)}^0$, is seen, with its mass and width measured to be ${3991}_{-10}^{+12}{}_{-17}^{+9}\mathrm{MeV}/{\mathrm{c}}^2$ and ${105}_{-25}^{+29}{}_{-23}^{+17}\mathrm{MeV}$, respectively, where the first uncertainty is statistical and the second systematic. The $T_{\psi s1}^{\theta }(4000{)}^0$ state is likely to be the isospin partner of the $T_{\psi s1}^{\theta }(4000{)}^{+}$ state, previously observed in the $J/\psi K^{+}$ system of the $B^{+}\to J/\psi \varphi K^{+}$ decay. When isospin symmetry for the charged and neutral $T_{\psi s1}^{\theta }(4000)$ states is assumed, the signal significance increases to 5.4 standard deviations.

A simultaneous analysis of the $B^{+}\to K^{+}{\ell }^{+}{\ell }^{-}$ and $B^0\to K^{*0}{\ell }^{+}{\ell }^{-}$ decays is performed to test muon-electron universality in two ranges of the square of the dilepton invariant mass, $q^2$. The measurement uses a sample of beauty meson decays produced in proton-proton collisions collected with the LHCb detector between 2011 and 2018, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$. A sequence of multivariate selections and strict particle identification requirements produce a higher signal purity and a better statistical sensitivity per unit luminosity than previous LHCb lepton universality tests using the same decay modes. Residual backgrounds due to misidentified hadronic decays are studied using data and included in the fit model. Each of the four lepton universality measurements reported is either the first in the given $q^2$ interval or supersedes previous LHCb measurements. The results are compatible with the predictions of the Standard Model.