Measurements of the distances to type Ia supernovae have produced strong evidence that the expansion of the Universe is accelerating, implying the existence of a nearly uniform component of dark energy with negative pressure. We show that constraints to this mysterious component based upon large-scale structure nicely complement the SN Ia data, and that together they require ${\Omega }_X={\rho }_X/{\rho }_{\mathrm{crit}}\in (0.6,0.7)$ and ${w=p}_X/{\rho }_X<-0.6\mathrm{}$ ( $95\%$ C.L.), for the favored flat Universe. Other cosmological data support this conclusion. The simplest explanation, a cosmological constant, is consistent with this, while some of the other possibilities are not.

Dynamic force spectroscopy was applied to single specific bonds between immunoglobulins of type G and protein A, a staphylococcal receptor for IgG. The resulting spectra of yield forces indicated the crossover from force induced to spontaneous bond dissociation. Moreover, failure of unloaded bonds was observed directly. Extrapolation to vanishing loading rate and direct observation yielded coinciding results.

We consider two different ways to encode quantum information, by parallel or antiparallel pairs of spins. We find that there is more information in the antiparallel ones. This purely quantum mechanical effect is due to entanglement, not of the states but occurring in the course of the measuring process. We also introduce a range of quantum information processing machines, such as spin flip and anticloning.

We have compared the decay rates of $K_L$ and $K_S$ to ${\pi }^{+}{\pi }^{-}$ and ${\pi }^0{\pi }^0$ final states using a subset of the data from the KTeV experiment (E832) at Fermilab. We find that the direct-CP-violation parameter $\mathrm{Re}\left({\epsilon }^{\prime }/\epsilon \right)$ is equal to $[28.0\pm 3.0(\mathrm{stat})\pm 2.8(\mathrm{syst}\left)\right]\times {10}^{-4\mathrm{}}$. This result definitively establishes the existence of CP violation in a decay process.

The ${}^{17}\mathrm{F}(p,\gamma {)}^{18}\mathrm{Ne}$ reaction is important in stellar explosions, but its rate has been uncertain because of an expected $3^{+}$ state in ${}^{18}\mathrm{Ne}$ that has never been conclusively observed. This state would provide a strong $\ell =0\mathrm{}$ resonance and, depending on its excitation energy, could dominate the stellar reaction rate. We have observed this missing $3^{+}$ state by measuring the ${}^1\mathrm{H}{(}^{17}\mathrm{F},p{)}^{17}\mathrm{F}$ excitation function with a radioactive ${}^{17}\mathrm{F}$ beam at the ORNL Holifield Radioactive Ion Beam Facility. We find that the state lies at a center-of-mass energy of $E_r=599.8\mathrm{}\pm 1{.5}_{\mathrm{stat}}\pm {2.0}_{\mathrm{sys}}\mathrm{keV}$ ( $E_x=4523.7\mathrm{}\pm 2.9\mathrm{keV}$) and has a width of $\Gamma =18\mathrm{}\pm 2_{\mathrm{stat}}\pm 1_{\mathrm{sys}}\mathrm{keV}$.

We report the observation of small group velocities of order 90 m/s and large group delays of greater than 0.26 ms, in an optically dense hot rubidium gas ( $\approx 360\mathrm{K}$). Media of this kind yield strong nonlinear interactions between very weak optical fields and very sharp spectral features. The result is in agreement with previous studies on nonlinear spectroscopy of dense coherent media.

The electronic and phase-coherent transport properties of doped carbon nanotube junctions are studied. It is shown that there are regions of negative differential resistance in the $I-V$ characteristic of small radius metallic tubes, which are not seen for semiconducting tubes. Their origin, as discussed, is different from that of traditional Esaki diodes and resonant tunneling structures. Semiconducting tube characteristics, however, show a region of zero current for nonzero voltages which is asymmetric with respect to the applied bias if there is different doping on each side of the junction.

In the high-κ cubic superconductor ${\mathrm{V}}_3\mathrm{Si}$, phase transitions of the flux lattice structure occur as a function of applied field and temperature. With the field parallel to the fourfold [001] axis, the flux lattice transforms from triangular to square symmetry at approximately 1 T. With the field parallel to the twofold [110] axis, the lattice, which is a nearly perfect hexagonal array at the lowest fields, distorts as the field is increased; the rate of increase in this distortion changes abruptly at 1.3 T. As $T_c$ is approached, the system tends towards a more isotropic hexagonal array of the flux lines. These transitions are largely but not completely in agreement with a recent theory of the effects on the flux line arrangements of nonlocal electrodynamics in the London limit.

A new technique for determining electron impact metastable excitation cross sections which exploits angular deflection of the target is presented. The deflected targets, further excited by laser radiation which is resonant between the metastable state and a high- $n$ Rydberg state, are detected using field ionization. The target deflection angle and flight time determines the associated electron scattering angle, whereas the laser probes individual states of the electron excited manifold. New information over a full range of scattering angles from forward to backward scattering is then obtained.

The local atomic environment of Co atoms in a CoO crystal was imaged by hard x-ray holography. The complete three dimensional picture of the first two cobalt neighbor shells with nearly isotropic resolution was obtained, using a combination of normal and inverse, synchrotron and laboratory experiments. No a priori knowledge of the structure was used.

We report on both small angle neutron scattering and magnetization measurements in the cubic $(\mathrm{K},\mathrm{Ba}){\mathrm{BiO}}_3$ superconductor. We show that the flux lines are ordered into a hexagonal lattice at low field ( $B<\mathrm{}{B}_e\sim 0.6\mathrm{T}$) and that the diffracted intensity continuously drops to zero as the magnetic field is increased towards $B_e$. This intensity fall is attributed to a static Debye-Waller factor associated with disorder induced longitudinal fluctuations of the vortex lines. We show that $B_e$ lies close to the onset of the anomalous “peak effect” in magnetization measurements, suggesting that this peak is related to the change in the structure of the vortex solid.

We describe the operation of a laser cooled cesium fountain clock in the quantum limited regime. An ultrastable cryogenic sapphire oscillator is used to measure the short-term frequency stability of the fountain as a function of the number of detected atoms $N_{\mathrm{at}}$. For $N_{\mathrm{at}}$ varying from $4\times {10}^4$ to $6\times {10}^5$ the Allan standard deviation of the frequency fluctuations is in excellent agreement with the $N_{\mathrm{at}}^{-1/2\mathrm{}}$ law of atomic projection noise. With $6\times {10}^5$ atoms, the relative frequency stability is $4\times {10}^{-14\mathrm{}}{\tau }^{-1/2\mathrm{}}$, where τ is the integration time in seconds. This is the best short-term stability ever reported for primary frequency standards, a factor of 5 improvement over previous results.

We report on a novel approach to measure the phase relaxation length and femtosecond lifetime of hot quasiparticles on metal surfaces. A 4 K scanning tunneling microscope has been used to study the spatial decay of interference patterns in the local density of states for surface state electrons on Ag(111) and Cu(111). This decay is governed by inelastic electron-electron scattering. We find a $({E-E}_F{)}^{-2\mathrm{}}$ energy dependence of the lifetimes for both Ag and Cu, and our values are comparable to the corresponding bulk electron lifetimes. This indicates that electron-electron interaction of hot surface state electrons with the Fermi sea is dominated by the underlying bulk electrons.

We suggest that the transition to superconductivity in a single crystal of ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{BiO}}_3$ with $T_c=32\mathrm{}\mathrm{K}$, and having critical fields with anomalous temperature dependencies and vanishing discontinuities in specific heat and magnetic susceptibility, may well be an example of a fourth order (in Ehrenfest's sense) phase transition. We have derived a free energy functional for a fourth order transition and calculated (for the temperature range $T_c/2<T\simeq T_c$) the temperature dependence of the critical fields. We find $H_{c1\mathrm{}}\left(T\right)\propto (1-T/T_c{)}^3$, $H_0\left(T\right)\propto (1-T/T_c{)}^2$, and $H_{c2\mathrm{}}\left(T\right)\propto (1-T/T_c{)}^1$ in general agreement with experiments.

We demonstrate an ultrastable ${\mathrm{CO}}_2$ laser trap that provides tight confinement of neutral atoms with negligible optical scattering and minimal laser-noise-induced heating. Using this method, fermionic ${}^6\mathrm{Li}$ atoms are stored in a 0.4 mK deep well with a $1/e$ trap lifetime of 300 sec, consistent with a background pressure of ${10}^{-11\mathrm{}}\mathrm{Torr}$. To our knowledge, this is the longest storage time ever achieved with an all-optical trap, comparable to the best reported magnetic traps.

The general theory of surface NMR imaging of large electromagnetically active systems is considered, motivated by geophysical applications. A general imaging equation is derived for the NMR voltage response, valid for arbitrary transmitter and receiver loop geometry and arbitrary conductivity structure of the sample. When the conductivity grows to the point where the electromagnetic skin depth becomes comparable to the sample size, significant diffusive retardation effects occur that strongly affect the signal. Accounting for these now allows more accurate imaging than previously possible. It is shown that the time constant $T_1$ may in principle be inferred directly from the diffusive tail of the signal.

A multiphoton microscopy based on coherent anti-Stokes Raman scattering is accomplished with near-infrared ultrashort laser pulses. We demonstrate vibrational imaging of chemical and biological samples with high sensitivity, high spatial resolution, noninvasiveness, and three-dimensional sectioning capability.

We have measured the velocity of a running crack in brittle single crystal silicon as a function of energy flow to the crack tip. The experiments are designed to permit direct comparison with molecular dynamics simulations; therefore the experiments provide an indirect but sensitive test of interatomic potentials. Performing molecular dynamics simulations of brittle crack motion at the atomic scale we find that experiments and simulations disagree showing that interatomic potentials are not yet well understood.

Ultrashort pulse laser techniques are applied to study optically induced modulation in exchange biased ferromagnetic/antiferromagnetic (FM/AF) thin bilayer films (NiFe/NiO). Photoexcitation of the FM/AF interface with subpicosecond laser pulses induces large modulation in the unidirectional exchange bias field ( $H_{\mathrm{ex}}$) on an ultrashort time scale. The “unpinning” of the exchange bias leads to coherent magnetization rotation in the permalloy film which is time resolved by the experiment and corresponds to a large modulation in the magnetization component ( $\Delta M_Z{/M}_S\sim 0.5$), on a time scale of 100 psec.

Strong infrared fields can be used for controlled spinning of molecules to very high angular momentum states. The angular momentum acquired can be sufficient to break molecular bonds. The approach is suitable for all anisotropic molecules, and we illustrate it by dissociating a homonuclear diatomic ${\mathrm{Cl}}_2$, with optical centrifuge efficiently separating ${\mathrm{Cl}}^{35}$ and ${\mathrm{Cl}}^{37}$ isotopes and thus demonstrating high sensitivity to the moment of inertia.