Search

By using ab initio electronic structure calculations within density functional theory, we study the structural, electronic, and magnetic properties of Si doped with a transition metal impurity. We consider the transition metals of the $3d$ series V, Cr, Mn, Fe, Co, and Ni. To get insight into the level filling mechanism and the magnetization saturation, we first investigate the transition metal-Si alloys in the zinc-blende structure. Next, we investigate the doping of bulk Si with a transition metal atom, in which it occupies the substitutional site, the interstitial site with tetrahedral symmetry, and the interstitial site with hexagonal symmetry. It is found that all of these transition metal impurities prefer an interstitial position in Si. Furthermore, we show that it is possible to interpret the electronic and magnetic properties by using a simple level filling picture and a comparison is made to Ge doped with the same transition metal atoms. In order to get insight into the effect of a strained environment, we calculate the formation energy as a function of an applied homogeneous pressure and we show that an applied pressure can stabilize the substitutional position of transition metal impurities in Si. Finally, the energies of the ferromagnetic states are compared to those of the antiferromagnetic states. It is shown that the interstitial site of the Mn dopant helps us to stabilize the nearest neighbor substitutional site to realize the ferromagnetic state. For doping of Si with Cr, a ferrimagnetic behavior is predicted.

We investigate theoretically the magnetic levels and optical properties of zigzag- and armchair-edged hexagonal graphene quantum dots (GQDs) utilizing the tight-binding method. A bound edge state at zero energy appears for the zigzag GQDs in the absence of a magnetic field. The magnetic levels of GQDs exhibit a Hofstadter-butterfly spectrum and approach the Landau levels of two-dimensional graphene as the magnetic field increases. The optical properties are tuned by the size, the type of the edge, and the external magnetic field.

Atomic-layer-epitaxy (ALE) processes for the growth of diamond are evaluated using ab initio molecular dynamics. It is shown that the Cl-based ALE cycle on diamond (111) is not advantageous, due to strong steric hindrances. New ALE processes are proposed and shown to be energetically favorable, self-limiting, and sterically unhindered. They exploit the large heat of formation and the small size of the HF molecule and are based on alternating exposure of the diamond (110) surface to hydrocarbons and fluorocarbons. Alternatively, H and F gases can be used in parts of the cycle.

Off-axis electron holography was used to investigate the barrier profile of the $\mathrm{Py}∕\mathrm{Al}{\mathrm{O}}_x∕\mathrm{Zr}{\mathrm{O}}_y∕\mathrm{Py}$ magnetic tunnel junctions with different $\mathrm{Zr}{\mathrm{O}}_y$ thicknesses. The tunneling magnetoresistance (TMR) has a strong dependence on bias voltage and the bias voltage for maximum TMR is shifted from zero. This shift increases with $\mathrm{Zr}{\mathrm{O}}_y$ barrier thickness due to the increasing barrier asymmetry in the junctions. The evolution of barrier asymmetry was directly observed by the phase change of the off-axis electron holography, which unambiguously shows the barrier profile changes from triangular to trapezoidal shape as increasing of $\mathrm{Zr}{\mathrm{O}}_y$ thickness.

A search for resonant diboson production using a data sample corresponding to $4.7{\mathrm{fb}}^{-1}$ of integrated luminosity collected by the ATLAS experiment at the Large Hadron Collider in $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ is presented. The search for a narrow resonance in the $WW$ or $WZ$ mass distribution is conducted in a final state with an electron or a muon, missing transverse momentum, and at least two jets. No significant excess is observed and limits are set using three benchmark models: $WW$ resonance masses below 940 and 710 GeV are excluded at 95% confidence level for spin-2 Randall–Sundrum and bulk Randall–Sundrum gravitons, respectively; $WZ$ resonance masses below 950 GeV are excluded at 95% confidence level for a spin-1 extended gauge model $W^{\prime }$ boson.

A generic search is presented for a heavy particle decaying to $WZ\to \ell \nu {\ell }^{\prime }{\ell }^{\prime }$ ($\ell$, ${\ell }^{\prime }=e$, $\mu$) final states. The data were recorded by the ATLAS detector in $\sqrt{s}=7\mathrm{TeV}$ $pp$ collisions at the Large Hadron Collider and correspond to an integrated luminosity of $1.02{\mathrm{fb}}^{-1}$. The transverse mass distribution of the selected $WZ$ candidates is found to be consistent with the standard model expectation. Upper limits on the production cross section times branching ratio are derived using two benchmark models predicting a heavy particle decaying to a $WZ$ pair.

First measurements of the azimuthal anisotropy of neutral pions produced in Pb-Pb collisions at a center-of-mass energy of $\sqrt{s_{NN}}=2.76\mathrm{TeV}$ are presented. The amplitudes of the second Fourier component ($v_2$) of the ${\pi }^0$ azimuthal distributions are extracted using an event-plane technique. The values of $v_2$ are studied as a function of the neutral pion transverse momentum ($p_T$) for different classes of collision centrality in the kinematic range $1.6<p_T<8.0\mathrm{GeV}/c$, within the pseudorapidity interval $|\eta |<0.8$. The CMS measurements of $v_2(p_T)$ are similar to previously reported ${\pi }^0$ azimuthal anisotropy results from $\sqrt{s_{NN}}=200\mathrm{GeV}$ Au-Au collisions at RHIC, despite a factor of $\sim 14$ increase in the center-of-mass energy. In the momentum range $2.5<p_T<5.0\mathrm{GeV}/c$, the neutral pion anisotropies are found to be smaller than those observed by CMS for inclusive charged particles.

This paper presents a search for a new heavy particle produced in association with a top or antitop quark. Two models in which the new heavy particle is a color singlet or a color triplet are considered, decaying, respectively, to $\overline{t}q$ or $tq$, leading to a resonance within the $t\overline{t}+\mathrm{\text{jets}}$ signature. The full 2011 ATLAS $pp$ collision data set from the LHC ($4.7{\mathrm{fb}}^{-1}$) is used to search for $t\overline{t}$ events produced in association with jets, in which one of the $W$ bosons from the top quarks decays leptonically and the other decays hadronically. The data are consistent with the Standard Model expectation, and a new particle with mass below 430 Gev for both $W^{\prime }$ boson and color triplet models is excluded at 95% confidence level, assuming unit right-handed coupling.

A search is performed for heavy particle pairs produced in $\sqrt{s}=7\mathrm{TeV}$ proton-proton collisions with $35{\mathrm{pb}}^{-1}$ of data collected by the CMS experiment at the LHC. The search is sensitive to squarks and gluinos of generic supersymmetry models, provided they are kinematically accessible, with minimal assumptions on properties of the lightest superpartner particle. The kinematic consistency of the selected events is tested against the hypothesis of heavy particle pair production using the dimensionless razor variable $R$, related to the missing transverse energy $E_{\mathrm{T}}^{\mathrm{miss}}$. The new physics signal is characterized by a broad peak in the distribution of $M_R$, an event-by-event indicator of the heavy particle mass scale. This new approach is complementary to $E_{\mathrm{T}}^{\mathrm{miss}}$-based searches. After background modeling based on data, and background rejection based on $R$ and $M_R$, no significant excess of events is found beyond the standard model expectations. The results are interpreted in the context of the constrained minimal supersymmetric standard model as well as two simplified supersymmetry models.

A search is presented for gluinos decaying via the supersymmetric partner of the top quark using events with two same-sign leptons, jets, and missing transverse momentum. The analysis is performed with $2.05{\mathrm{fb}}^{-1}$ of integrated luminosity from $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ collected by the ATLAS detector at the LHC. No excess beyond the standard model expectation is observed, and exclusion limits are derived for simplified models where the gluino decays via the supersymmetric partner of the top quark and in the minimal supergravity and constrained minimal supersymmetric standard model framework. In those scenarios, gluino masses below 550 GeV are excluded at 95% C.L. within the parameter space considered, significantly extending the coverage with respect to existing limits. Depending on the model parameters, gluino masses up to 750 GeV can also be excluded at 95% C.L.

We have measured antiproton production cross sections as functions of centrality in collisions of 14.6 GeV/c per nucleon ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ ions with targets of Al, Cu, and Pb. For all targets, the antiproton yields increase linearly with the number of projectile nucleons that have interacted, and show little target dependence. We discuss the implications of this result on the production and absorption of antiprotons within the nuclear medium.

Inclusive double differential multiplicities ${\mathit{d}}^2$N/dy d${\mathit{p}}_{\mathit{t}}$ and related quantities have been measured for protons and deuterons produced in 14.6A GeV/c Si+Al and Si+Pb collisions using the E814 forward spectrometer at the AGS at BNL. Collision ‘‘centrality’’ is determined by measuring ${\mathit{N}}_{\mathit{c}}$, the total charged particle multiplicity in the pseudorapidity range 0.85<η<3.8. For both systems Si + Al and Si + Pb, an increase in the proton rapidity distribution dN/dy at midrapidity and a corresponding decrease at higher rapidities are observed with increasing ${\mathit{N}}_{\mathit{c}}$. For Si+Pb, Boltzmann slope parameters ${\mathit{T}}_{\mathit{B}}$ increase significantly in the most central collisions. The measured distributions exhibit a centrality dependence even when σ/${\mathrm{\sigma }}_{\mathrm{geo}}$≲10%, where full overlap between the Si and Pb nuclei occurs in a simple geometric picture. The proton rapidity distribution dN/dy is presented for the symmetric system Si+Al over the entire rapididty interval. The total number of protons, which is the integral of this quantity over rapidity, varies with ${\mathit{N}}_{\mathit{c}}$. Results are compared with various model calculations, mostly using the hadronic cascade codes ARC and RQMD. No significant nuclear transparency is observed, indicating that large baryon and energy densities are produced in these collisions.

We report a measurement of the average $B$ hadron lifetime using data collected with the SLD detector at the SLAC Linear Collider in 1993. An inclusive analysis selected three-dimensional vertices with $B$ hadron lifetime information in a sample of 50 × ${10}^3$ $Z^0$ decays. A lifetime of 1.564 ± 0.030(stat) ± 0.036(syst) ps was extracted from the decay length distribution of these vertices using a binned maximum likelihood method.

A search is presented for direct top squark pair production in final states with one isolated electron or muon, jets, and missing transverse momentum in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$. The measurement is based on $4.7{\mathrm{fb}}^{-1}$ of data collected with the ATLAS detector at the LHC. Each top squark is assumed to decay to a top quark and the lightest supersymmetric particle (LSP). The data are found to be consistent with standard model expectations. Top squark masses between 230 GeV and 440 GeV are excluded with 95% confidence for massless LSPs, and top squark masses around 400 GeV are excluded for LSP masses up to 125 GeV.

Scanning tunneling microscopy is used to determine the bonding geometry of the spherosiloxane cluster, $\mathrm{H}{}_8\mathrm{Si}{}_8\mathrm{O}{}_{12}$, on $\mathrm{Si}\left(100\right)-2\times 1$. The images obtained are consistent with monovertex bonding to the $\mathrm{Si}\left(100\right)-2\times 1$ surface via activation of a single Si-H bond. Filled and empty state images show good agreement with calculations of the electron density distribution of the cluster as well as the ${\Psi }^2$ highest occupied molecular orbital and lowest unoccupied molecular orbital surface plots of the cluster.

A search for physics beyond the standard model is performed in events with at least three jets and large missing transverse momentum produced in proton-proton collisions at a center-of-mass energy of $\sqrt{s}=7\mathrm{TeV}$. No significant excess of events above the expected backgrounds is observed in $4.98{\mathrm{fb}}^{-1}$ of data collected with the CMS detector at the Large Hadron Collider. The results are presented in the context of the constrained minimal supersymmetric extension of the standard model and more generically for simplified models. For the simplified models of gluino-gluino and squark-squark production, gluino masses below 1.0 TeV and squark masses below 0.76 TeV are excluded in case the lightest supersymmetric particle mass is below 200 GeV. These results significantly extend previous searches.

Transmission x-ray diffraction experiments have been carried out on solid helium grown in porous Vycor glass. Measurements were made at temperatures near 0.7 K and at pressures up to 162 bars. The crystalline phases of solid helium in Vycor are found to differ significantly from the bulk. At pressures from 70 bars through 98 bars the helium is polycrystalline and displays a single size broadened scattering peak. Above 98 bars the peak splits into three close peaks. No higher order peaks are seen at any pressure, indicating significant reduction in intensity due to disorder. A broad peak is present at all pressures, which may indicate the presence of amorphous solid. We tentatively identify the low-pressure phase as bcc and the high-pressure phase as coexistence between bcc and hcp. Size broadening indicates an average grain size of approximately 5 nm, comparable to the Vycor pore size.

Direct searches for lepton flavor violation in decays of the $Z$ boson with the ATLAS detector at the LHC are presented. Decays of the $Z$ boson into an electron or muon and a hadronically decaying $\tau$ lepton are considered. The searches are based on a data sample of proton-proton collisions collected by the ATLAS detector in 2015 and 2016, corresponding to an integrated luminosity of $36.1{\mathrm{fb}}^{-1}$ at a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$. No statistically significant excess of events above the expected background is observed, and upper limits on the branching ratios of lepton-flavor-violating decays are set at the 95% confidence level: $\mathcal{B}(Z\to e\tau )<5.8\times {10}^{-5}$ and $\mathcal{B}(Z\to \mu \tau )<2.4\times {10}^{-5}$. This is the first limit on $\mathcal{B}(Z\to e\tau )$ with ATLAS data. The upper limit on $\mathcal{B}(Z\to \mu \tau )$ is combined with a previous ATLAS result based on $20.3{\mathrm{fb}}^{-1}$ of proton-proton collision data at a center-of-mass energy of $\sqrt{s}=8\mathrm{TeV}$ and the combined upper limit at 95% confidence level is $\mathcal{B}(Z\to \mu \tau )<1.3\times {10}^{-5}$.

This article reports on a search for dark matter pair production in association with a Higgs boson decaying to a pair of bottom quarks, using data from $20.3{\mathrm{fb}}^{-1}$ of $pp$ collisions at a center-of-mass energy of 8 TeV collected by the ATLAS detector at the LHC. The decay of the Higgs boson is reconstructed as a high-momentum $b\overline{b}$ system with either a pair of small-radius jets, or a single large-radius jet with substructure. The observed data are found to be consistent with the expected Standard Model backgrounds. Model-independent upper limits are placed on the visible cross sections for events with a Higgs boson decaying into $b\overline{b}$ and large missing transverse momentum with thresholds ranging from 150 to 400 GeV. Results are interpreted using a simplified model with a $Z^{\prime }$ gauge boson decaying into different Higgs bosons predicted in a two-Higgs-doublet model, of which the heavy pseudoscalar Higgs decays into a pair of dark matter particles. Exclusion limits are also presented for the mass scales of various effective field theory operators that describe the interaction between dark matter particles and the Higgs boson.

A search for a heavy standard model Higgs boson decaying via $H\to ZZ\to {\ell }^{+}{\ell }^{-}\nu \overline{\nu }$, where $\ell =e$, $\mu$, is presented. It is based on proton-proton collision data at $\sqrt{s}=7\mathrm{TeV}$, collected by the ATLAS experiment at the LHC in the first half of 2011 and corresponding to an integrated luminosity of $1.04{\mathrm{fb}}^{-1}$. The data are compared to the expected standard model backgrounds. The data and the background expectations are found to be in agreement and upper limits are placed on the Higgs boson production cross section over the entire mass window considered; in particular, the production of a standard model Higgs boson is excluded in the region $340<m_H<450\mathrm{GeV}$ at the 95% confidence level.