Search

Two phases have been identified by convergent-beam electron diffraction in multiphase compounds of ${\mathrm{TlBaCaCu}}_3$${\mathrm{O}}_{6.5}$ with a superconductivity temperature of 114 K. They are subsequently found to be superconducting at 120.3 and 90.0 K. These phases are tetragonal with point group 4/mmm and lattice parameters of a=0.547 nm; c=3.61 nm and a=0.547 nm, c=2.97 nm, respectively. We have found by high-resolution lattice imaging that the 3.61 and 2.97 nm phases form coherent intergrowths. Both ordered and disordered intergrowths have been observed.

Tunneling spectroscopy measurements of single tunnel junctions formed between multiwalled carbon nanotubes (MWNTs) and a normal metal are reported. Intrinsic Coulomb interactions in the MWNTs give rise to a strong zero-bias suppression of a tunneling density of states that can be fitted numerically to the environmental quantum-fluctuation theory. An asymmetric conductance anomaly near zero bias is found at low temperatures and interpreted as Fano resonance in the strong tunneling regime.

A series of samples with nominal compositions ${\mathrm{La}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Ba}}_{\mathrm{x}}$${\mathrm{Cu}}_1$${\mathrm{O}}_{3\mathrm{-}\mathrm{y}}$ was prepared by solid-state reaction. The subsolidus phase relation in the CuO-rich region of the La-Ba-Cu-O system has been determined by an x-ray diffraction method. Two ternary compounds were confirmed to exist in the system. The tetragonal or pseudotetragonal phase (${\mathrm{Ba}}_{2\mathrm{-}\mathit{z}}$${\mathrm{La}}_{\mathit{z}}$)${\mathrm{La}}_1$${\mathrm{Cu}}_3$${\mathrm{O}}_{7+\mathrm{\delta }}$ has a wide homogenous phase range from z=0.095 to z=0.578, and a crystal structure closely related to the structure of ${\mathrm{Ba}}_2$${\mathrm{YCu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. Superconductivity of compounds in this system was studied by resistivity and ac susceptibility measurements. The relation between crystal structure and the superconductivity is discussed.

The elastic free energy of carbon nanotubes grown by iron-catalyzed decomposition of acetylene is introduced to describe the possible tubular shapes. The equilibrium shape equation can be obtained by the first variation of the elastic free energy of the slightly distorted tube. The model is consistent with both stable and metastable shapes observed in our experiments. The results are due to the fluctuation of growth conditions, such as the pressure, temperature, and composition of the vapor. © 1996 The American Physical Society.

We report that the conductance of macroscopic multiwall nanotube (MWNT) bundles under pressure shows power laws in temperature and voltage, as corresponding to a network of bulk-bulk connected Tomonaga-Luttinger liquids (LLs). Contrary to individual MWNTs, where the observed power laws are attributed to Coulomb blockade, the measured ratio for the end and bulk obtained exponents, $\sim 2.4$, can be accounted for only by LL theory. At temperatures characteristic of interband separation, it increases due to thermal population of the conducting sheets unoccupied bands.

Both the magnetoconductance and the nonlinear differential conductance of multiwall carbon nanotube ropes have been studied at low temperture. Suppression in the differential conductance was observed at low bias voltages and at low temperatures, indicating the formation of a Coulomb gap. The magnetoconductance was found to follow a scaling law at both high and low temperatures, but with different mechanisms. The analysis of the data provides a sign of a non-Fermi-liquid-like behavior through magnetotransport measurement.

The heterogeneity of electronic states of transition metal oxides produces a variety of properties, which may be discerned by the measurements of their nanocrystals. The structure and optical spectra of chemically synthesized pure ${\mathrm{Fe}}_3{\mathrm{O}}_4$, $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$, and $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanocrystals of different sizes have been investigated. Three types of electronic transitions: ligand to metal charge-transfer transitions, strong magnetic coupled ${\mathrm{Fe}}^{3+}$ ligand field transitions, and pair excitations, occur distinctly in the optical spectra of $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ and $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanocrystals except ${\mathrm{Fe}}_3{\mathrm{O}}_4$. The quantum size effect (for itinerant carriers) and finite size effect (for local magnetic moments) were observed in our nanocrystals, although their bulks are strong electron correlation systems. The localized to delocalized transformation of electrons occurs with time delay in the femtosecond transient absorption spectra of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanocrystal sol, which indicates that heteroresponse times for different transitions agree well with the size-dependent indication of the steady-state absorption bands of these iron oxides nanocrystals. These results are helpful in understanding the relationship among $d\text{−}d$ transition, magnetic pairing, and charge-transfer in transition metal oxides.

A search for supersymmetry in hadronic final states with highly boosted $W$ bosons and $b$ jets is presented, focusing on compressed scenarios. The search is performed using proton-proton collision data at a center-of-mass energy of 8 TeV, collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of $19.7{\mathrm{fb}}^{-1}$. Events containing candidates for hadronic decays of boosted $W$ bosons are identified using jet substructure techniques, and are analyzed using the razor variables $M_R$ and $R^2$, which characterize a possible signal as a peak on a smoothly falling background. The observed event yields in the signal regions are found to be consistent with the expected contributions from standard model processes, which are predicted using control samples in the data. The results are interpreted in terms of gluino-pair production followed by their exclusive decay into top squarks and top quarks. The analysis excludes gluino masses up to 1.1 TeV for light top squarks decaying solely to a charm quark and a neutralino, and up to 700 GeV for heavier top squarks decaying solely to a top quark and a neutralino.

This paper reports a measurement of the cross section for the pair production of top quarks in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$ at the Fermilab Tevatron. The data were collected from the CDF run II detector in a set of runs with a total integrated luminosity of $1.1{\mathrm{fb}}^{-1}$. The cross section is measured in the dilepton channel, the subset of $t\overline{t}$ events in which both top quarks decay through $t\to Wb\to \ell \nu b$, where $\ell =e$, $\mu$, or $\tau$. The lepton pair is reconstructed as one identified electron or muon and one isolated track. The use of an isolated track to identify the second lepton increases the $t\overline{t}$ acceptance, particularly for the case in which one $W$ decays as $W\to \tau \nu$. The purity of the sample may be further improved at the cost of a reduction in the number of signal events, by requiring an identified $b$ jet. We present the results of measurements performed with and without the request of an identified $b$ jet. The former is the first published CDF result for which a $b$-jet requirement is added to the dilepton selection. In the CDF data there are 129 pretag $\mathrm{\text{lepton}}+\mathrm{\text{track}}$ candidate events, of which 69 are tagged. With the tagging information, the sample is divided into tagged and untagged subsamples, and a combined cross section is calculated by maximizing a likelihood. The result is ${\sigma }_{t\overline{t}}=9.6\pm 1.2(\mathrm{stat}{)}_{-0.5}^{+0.6}(\mathrm{sys})\pm 0.6(\mathrm{lum})\mathrm{pb}$, assuming a branching ratio of $\mathrm{BR}(W\to \ell \nu )=10.8\%$ and a top mass of $m_t=175\mathrm{GeV}/c^2$.

We present a measurement of the $t\overline{t}$ production cross section in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$ using events containing a high transverse momentum electron or muon, three or more jets, and missing transverse energy. Events consistent with $t\overline{t}$ decay are found by identifying jets containing candidate heavy-flavor semileptonic decays to muons. The measurement uses a CDF run II data sample corresponding to $2{\mathrm{fb}}^{-1}$ of integrated luminosity. Based on 248 candidate events with three or more jets and an expected background of $79.5\pm 5.3$ events, we measure a production cross section of $9.1\pm 1.6\mathrm{pb}$.

This Letter presents a search for $tb$ resonances in $1.04{\mathrm{fb}}^{-1}$ of LHC proton-proton collision data collected by the ATLAS detector at a center-of-mass energy of 7 TeV. Events with a lepton, missing transverse momentum, and two jets are selected and the invariant mass of the corresponding final state is reconstructed. The search exploits the shape of the $tb$ invariant mass distribution compared to the expected standard model backgrounds. The model of a right-handed $W_R^{\prime }$ with standard model-like couplings is chosen as the benchmark model for this search. No statistically significant excess of events is observed in the data, and upper limits on the cross section times the branching ratio of $W_R^{\prime }$ resonances at 95% C.L. lie in the range of 6.1–1.0 pb for $W_R^{\prime }$ masses ranging from 0.5 to 2.0 TeV. These limits are translated into a lower bound on the allowed right-handed $W_R^{\prime }$ mass, giving $m_{W_R^{\prime }}>1.13\mathrm{TeV}$ at 95% C.L.

A search for direct pair production of supersymmetric top squarks (${\tilde{t}}_1$) is presented, assuming the ${\tilde{t}}_1$ decays into a top quark and the lightest supersymmetric particle, ${\tilde{\chi }}_1^0$, and that both top quarks decay to purely hadronic final states. A total of 16 (4) events are observed compared to a predicted standard model background of ${13.5}_{-3.6}^{+3.7}({4.4}_{-1.3}^{+1.7})$ events in two signal regions based on $\int \mathcal{L}dt=4.7{\mathrm{fb}}^{-1}$ of $pp$ collision data taken at $\sqrt{s}=7\mathrm{TeV}$ with the ATLAS detector at the LHC. An exclusion region in the ${\tilde{t}}_1$ versus ${\tilde{\chi }}_1^0$ mass plane is evaluated: $370<m_{{\tilde{t}}_1}<465\mathrm{GeV}$ is excluded for $m_{{\tilde{\chi }}_1^0}\sim 0\mathrm{GeV}$ while $m_{{\tilde{t}}_1}=445\mathrm{GeV}$ is excluded for $m_{{\tilde{\chi }}_1^0}\le 50\mathrm{GeV}$.

This Letter reports on a search for narrow high-mass resonances decaying into dilepton final states. The data were recorded by the ATLAS experiment in $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ at the Large Hadron Collider and correspond to a total integrated luminosity of 1.08 $(1.21){\mathrm{fb}}^{-1}$ in the $e^{+}{e}^{-}$ (${\mu }^{+}{\mu }^{-}$) channel. No statistically significant excess above the standard model expectation is observed and upper limits are set at the 95% C.L. on the cross section times branching fraction of $Z^{\prime }$ resonances and Randall-Sundrum gravitons decaying into dileptons as a function of the resonance mass. A lower mass limit of 1.83 TeV on the sequential standard model $Z^{\prime }$ boson is set. A Randall-Sundrum graviton with coupling $k/{\overline{M}}_{\mathrm{Pl}}=0.1$ is excluded at 95% C.L. for masses below 1.63 TeV.

We present a measurement of the top quark mass with $t\overline{t}$ dilepton events produced in $p\overline{p}$ collisions at the Fermilab Tevatron ($\sqrt{s}=1.96\mathrm{TeV}$) and collected by the CDF II detector. A sample of 328 events with a charged electron or muon and an isolated track, corresponding to an integrated luminosity of $2.9{\mathrm{fb}}^{-1}$, are selected as $t\overline{t}$ candidates. To account for the unconstrained event kinematics, we scan over the phase space of the azimuthal angles $({\varphi }_{{\nu }_1},{\varphi }_{{\nu }_2})$ of neutrinos and reconstruct the top quark mass for each ${\varphi }_{{\nu }_1}$, ${\varphi }_{{\nu }_2}$ pair by minimizing a ${\chi }^2$ function in the $t\overline{t}$ dilepton hypothesis. We assign ${\chi }^2$-dependent weights to the solutions in order to build a preferred mass for each event. Preferred mass distributions (templates) are built from simulated $t\overline{t}$ and background events, and parametrized in order to provide continuous probability density functions. A likelihood fit to the mass distribution in data as a weighted sum of signal and background probability density functions gives a top quark mass of ${165.5}_{-3.3}^{+3.4}(\mathrm{stat})\pm 3.1(\mathrm{syst})\mathrm{GeV}/c^2$.

We present a search for associated production of the standard model Higgs boson and a $Z$ boson where the $Z$ boson decays to two leptons and the Higgs decays to a pair of $b$ quarks in $p\overline{p}$ collisions at the Fermilab Tevatron. We use event probabilities based on standard model matrix elements to construct a likelihood function of the Higgs content of the data sample. In a CDF data sample corresponding to an integrated luminosity of $2.7{\mathrm{fb}}^{-1}$ we see no evidence of a Higgs boson with a mass between $100\mathrm{GeV}/c^2$ and $150\mathrm{GeV}/c^2$. We set 95% confidence level upper limits on the cross section for $ZH$ production as a function of the Higgs boson mass $m_H$; the limit is 8.2 times the standard model prediction at $m_H=115\mathrm{GeV}/c^2$.

We report a measurement of the single-top-quark production cross section in $2.2{\mathrm{fb}}^{-1}$ of $p\overline{p}$ collision data collected by the Collider Detector at Fermilab at $\sqrt{s}=1.96\mathrm{TeV}$. Candidate events are classified as signal-like by three parallel analyses which use likelihood, matrix element, and neural network discriminants. These results are combined in order to improve the sensitivity. We observe a signal consistent with the standard model prediction, but inconsistent with the background-only model by 3.7 standard deviations with a median expected sensitivity of 4.9 standard deviations. We measure a cross section of ${2.2}_{-0.6}^{+0.7}(\mathrm{stat}+\mathrm{syst})\mathrm{pb}$, extract the Cabibbo-Kobayashi-Maskawa matrix-element value $|V_{tb}|={0.88}_{-0.12}^{+0.13}(\mathrm{stat}+\mathrm{syst})\pm 0.07(\mathrm{theory})$, and set the limit $|V_{tb}|>0.66$ at the 95% C.L.

A search motivated by supersymmetric models with light top squarks is presented using proton-proton collision data recorded with the CMS detector at a center-of-mass energy of $\sqrt{s}=7\mathrm{TeV}$ during 2011, corresponding to an integrated luminosity of $4.98{\mathrm{fb}}^{-1}$. The analysis is based on final states with a single lepton, $b$-quark jets, and missing transverse energy. Standard model yields are predicted from data using two different approaches. The observed event numbers are found to be compatible with these predictions. Results are interpreted in the context of the constrained minimal supersymmetric standard model and of a simplified model with four top quarks in the final state.

We present a search for high-mass neutral resonances using dimuon data corresponding to an integrated luminosity of $2.3{\mathrm{fb}}^{-1}$ collected in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$ by the CDF II detector at the Fermilab Tevatron. No significant excess above the standard model expectation is observed in the dimuon invariant-mass spectrum. We set 95% confidence level upper limits on $\sigma \mathrm{BR}(p\overline{p}\to X\to \mu \overline{\mu })$, where $X$ is a boson with spin-0, 1, or 2. Using these cross section limits, we determine lower mass limits on sneutrinos in $R$-parity-violating supersymmetric models, $Z^{\prime }$ bosons, and Kaluza-Klein gravitons in the Randall-Sundrum model.

We present the first observation in hadronic collisions of the electroweak production of vector boson pairs ($VV$, $V=W$, $Z$) where one boson decays to a dijet final state. The data correspond to $3.5{\mathrm{fb}}^{-1}$ of integrated luminosity of $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$ collected by the CDF II detector at the Fermilab Tevatron. We observe $1516\pm 239(\mathrm{stat})\pm 144(\mathrm{syst})$ diboson candidate events and measure a cross section $\sigma (p\overline{p}\to VV+X)$ of $18.0\pm 2.8(\mathrm{stat})\pm 2.4(\mathrm{syst})\pm 1.1(\mathrm{lumi})\mathrm{pb}$, in agreement with the expectations of the standard model.

A search for new physics is performed in events with two same-sign isolated leptons, hadronic jets, and missing transverse energy in the final state. The analysis is based on a data sample corresponding to an integrated luminosity of $4.98{\mathrm{fb}}^{-1}$ produced in $pp$ collisions at a center-of-mass energy of 7 TeV collected by the CMS experiment at the LHC. This constitutes a factor of 140 increase in integrated luminosity over previously published results. The observed yields agree with the standard model predictions and thus no evidence for new physics is found. The observations are used to set upper limits on possible new physics contributions and to constrain supersymmetric models. To facilitate the interpretation of the data in a broader range of new physics scenarios, information on the event selection, detector response, and efficiencies is provided.