2,374 research outputs found
Gamow shell-model calculations of drip-line oxygen isotopes
We employ the Gamow shell model (GSM) to describe low-lying states of the
oxygen isotopes 24O and 25O. The many-body Schrodinger equation is solved
starting from a two-body Hamiltonian defined by a renormalized low-momentum
nucleon-nucleon (NN) interaction, and a spherical Berggren basis. The Berggren
basis treats bound, resonant, and continuum states on an equal footing, and is
therefore an appropriate representation of loosely bound and unbound nuclear
states near threshold. We show that such a basis is necessary in order to
obtain a detailed and correct description of the low-lying 1+ and 2+ excited
states in 24O. On the other hand, we find that a correct description of binding
energy systematics of the ground states is driven by proper treatment and
inclusion of many-body correlation effects. This is supported by the fact that
we get 25O unstable with respect to 24O in both oscillator and Berggren
representations starting from a 22O core. Furthermore, we show that the
structure of these loosely bound or unbound isotopes are strongly influenced by
the 1S0 component of the NN interaction. This has important consequences for
our understanding of nuclear stability.Comment: 5 pages, 3 figure
Origin of intrinsic dark count in superconducting nanowire single-photon detectors
The origin of the decoherence in superconducting nanowire single-photon
detectors, the so-called dark count, was investigated. We measured the
direct-current characteristics and bias-current dependencies of the dark count
rate in a wide range of temperatures from 0.5 K to 4 K, and analyzed the
results by theoretical models of thermal fluctuations of vortices. Our results
indicate that the current-assisted unbinding of vortex-antivortex pairs is the
dominant origin of the dark count.Comment: 10 pages, 2 figure
High resolution chronology of late Cretaceous-early Tertiary events determined from 21,000 yr orbital-climatic cycles in marine sediments
A number of South Atlantic sites cored by the Deep Sea Drilling Project (DSDP) recovered late Cretaceous and early Tertiary sediments with alternating light-dark, high-low carbonate content. The sedimentary oscillations were turned into time series by digitizing color photographs of core segments at a resolution of about 5 points/cm. Spectral analysis of these records indicates prominent periodicity at 25 to 35 cm in the Cretaceous intervals, and about 15 cm in the early Tertiary sediments. The absolute period of the cycles that is determined from paleomagnetic calibration at two sites is 20,000 to 25,000 yr, and almost certainly corresponds to the period of the earth's precessional cycle. These sequences therefore contain an internal chronometer to measure events across the K/T extinction boundary at this scale of resolution. The orbital metronome was used to address several related questions: the position of the K/T boundary within magnetic chron 29R, the fluxes of biogenic and detrital material to the deep sea immediately before and after the K/T event, the duration of the Sr anomaly, and the level of background climatic variability in the latest Cretaceous time. The carbonate/color cycles that were analyzed contain primary records of ocean carbonate productivity and chemistry, as evidenced by bioturbational mixing of adjacent beds and the weak lithification of the rhythmic sequences. It was concluded that sedimentary sequences that contain orbital cyclicity are capable of providing resolution of dramatic events in earth history with much greater precision than obtainable through radiometric methods. The data show no evidence for a gradual climatic deterioration prior to the K/T extinction event, and argue for a geologically rapid revolution at this horizon
Impact of time-ordered measurements of the two states in a niobium superconducting qubit structure
Measurements of thermal activation are made in a superconducting, niobium
Persistent-Current (PC) qubit structure, which has two stable classical states
of equal and opposite circulating current. The magnetization signal is read out
by ramping the bias current of a DC SQUID. This ramping causes time-ordered
measurements of the two states, where measurement of one state occurs before
the other. This time-ordering results in an effective measurement time, which
can be used to probe the thermal activation rate between the two states.
Fitting the magnetization signal as a function of temperature and ramp time
allows one to estimate a quality factor of 10^6 for our devices, a value
favorable for the observation of long quantum coherence times at lower
temperatures.Comment: 14 pages, 4 figure
Role of low- component in deformed wave functions near the continuum threshold
The structure of deformed single-particle wave functions in the vicinity of
zero energy limit is studied using a schematic model with a quadrupole deformed
finite square-well potential. For this purpose, we expand the single-particle
wave functions in multipoles and seek for the bound state and the Gamow
resonance solutions. We find that, for the states, where is
the -component of the orbital angular momentum, the probability of each
multipole components in the deformed wave function is connected between the
negative energy and the positive energy regions asymptotically, although it has
a discontinuity around the threshold. This implies that the
resonant level exists physically unless the component is inherently large
when extrapolated to the well bound region. The dependence of the multipole
components on deformation is also discussed
Ultrathin MgB2 films fabricated on Al2O3 substrate by hybrid physical-chemical vapor deposition with high Tc and Jc
Ultrathin MgB2 superconducting films with a thickness down to 7.5 nm are
epitaxially grown on (0001) Al2O3 substrate by hybrid physical-chemical vapor
deposition method. The films are phase-pure, oxidation-free and continuous. The
7.5 nm thin film shows a Tc(0) of 34 K, which is so far the highest Tc(0)
reported in MgB2 with the same thickness. The critical current density of
ultrathin MgB2 films below 10 nm is demonstrated for the first time as Jc ~
10^6 A cm^{-2} for the above 7.5 nm sample at 16 K. Our results reveal the
excellent superconducting properties of ultrathin MgB2 films with thicknesses
between 7.5 and 40 nm on Al2O3 substrate.Comment: 7 pages, 4 figures, 2 table
Role of low- component in deformed wave functions near the continuum threshold
The structure of deformed single-particle wave functions in the vicinity of
zero energy limit is studied using a schematic model with a quadrupole deformed
finite square-well potential. For this purpose, we expand the single-particle
wave functions in multipoles and seek for the bound state and the Gamow
resonance solutions. We find that, for the states, where is
the -component of the orbital angular momentum, the probability of each
multipole components in the deformed wave function is connected between the
negative energy and the positive energy regions asymptotically, although it has
a discontinuity around the threshold. This implies that the
resonant level exists physically unless the component is inherently large
when extrapolated to the well bound region. The dependence of the multipole
components on deformation is also discussed
Measurement of spontaneous emission from a two-dimensional photonic band gap defined microcavity at near-infrared wavelengths
An active, photonic band gap-based microcavity emitter in the near infrared is demonstrated. We present direct measurement of the spontaneous emission power and spectrum from a microcavity formed using a two-dimensional photonic band gap structure in a half wavelength thick slab waveguide. The appearance of cavity resonance peaks in the spectrum correspond to the photonic band gap energy. For detuned band gaps, no resonances are observed. For devices with correctly tuned band gaps, a two-time enhancement of the extraction efficiency was demonstrated compared to detuned band gaps and unpatterned material
Electrochemical development of hydrogen silsesquioxane by applying an electrical potential
We present a new method for developing hydrogen silsesquioxane (HSQ) by using electrical potentials and deionized water. Nested-L test structures with a pitch as small as 9 nm were developed using this electrochemical technique in saline solution without adding hydroxyl ions. Furthermore, we showed that high-resolution structures can be electrochemically developed in deionized water alone. Electrochemical development is controlled by the applied voltage and may overcome several of the limitations discussed for alkaline developers, such as poor hydroxyl anion diffusion and charge repulsion effects in small trenches.National Science Foundation (U.S.)Massachusetts Institute of Technology. Materials Processing CenterMassachusetts Institute of Technology. Center for Materials Science and Engineerin
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