210 research outputs found
Imbibition in mesoporous silica: rheological concepts and experiments on water and a liquid crystal
We present, along with some fundamental concepts regarding imbibition of
liquids in porous hosts, an experimental, gravimetric study on the
capillarity-driven invasion dynamics of water and of the rod-like liquid
crystal octyloxycyanobiphenyl (8OCB) in networks of pores a few nanometers
across in monolithic silica glass (Vycor). We observe, in agreement with
theoretical predictions, square root of time invasion dynamics and a sticky
velocity boundary condition for both liquids investigated.
Temperature-dependent spontaneous imbibition experiments on 8OCB reveal the
existence of a paranematic phase due to the molecular alignment induced by the
pore walls even at temperatures well beyond the clearing point. The ever
present velocity gradient in the pores is likely to further enhance this
ordering phenomenon and prevent any layering in molecular stacks, eventually
resulting in a suppression of the smectic phase in favor of the nematic phase.Comment: 18 pages, 8 figure
Thermal Analyzer for Planetary Soil (TAPS): an in Situ Instrument for Mineral and Volatile-element Measurements
Thermal Analyzer for Planetary Soil (TAPS) offers a specific implementation for the generic thermal analyzer/evolved-gas analyzer (TA/EGA) function included in the Mars Environmental Survey (MESUR) strawman payload; applications to asteroids and comets are also possible. The baseline TAPS is a single-sample differential scanning calorimeter (DSC), backed by a capacitive-polymer humidity sensor, with an integrated sampling mechanism. After placement on a planetary surface, TAPS acquires 10-50 mg of soil or sediment and heats the sample from ambient temperature to 1000-1300 K. During heating, DSC data are taken for the solid and evolved gases are swept past the water sensor. Through ground based data analysis, multicomponent DSC data are deconvolved and correlated with the water release profile to quantitatively determine the types and relative proportions of volatile-bearing minerals such as clays and other hydrates, carbonates, and nitrates. The rapid-response humidity sensors also achieve quantitative analysis of total water. After conclusion of soil-analysis operations, the humidity sensors become available for meteorology. The baseline design fits within a circular-cylindrical volume less than 1000 cm(sup 3), occupies 1.2 kg mass, and consumes about 2 Whr of power per analysis. Enhanced designs would acquire and analyze multiple samples and employ additional microchemical sensors for analysis of CO2, SO2, NO(x), and other gaseous species. Atmospheric pumps are also being considered as alternatives to pressurized purge gas
Bostonia: The Boston University Alumni Magazine. Volume 33
Founded in 1900, Bostonia magazine is Boston University's main alumni publication, which covers alumni and student life, as well as university activities, events, and programs
Recurrence of hepatitis C virus during leucocytopenia and spontaneous clearance after recovery from cytopenia: a case report
<p>Abstract</p> <p>Introduction</p> <p>There is little information about the risk of HCV recurrence in immunosuppressed patients. Although the presence of antibodies to HCV and the absence of HCV-RNA is usually considered to indicate viral elimination, the virus may not be completely eliminated but may be under control of an effective immune response.</p> <p>Case presentation</p> <p>A 69 year old man presented with jaundice, elevated ALT, AST, lipase and concomitant abdominal pain. He was found to be positive for HCV-RNA (genotype 3a) and was diagnosed with acute hepatitis C. Six weeks later no HCV-RNA was detected, and the patient was diagnosed with hyperthyreosis and started on propylthiouracil. After 4 weeks of propylthiouracil treatment, the patient developed leucocytopenia, followed by liver function deterioration and reappearance of HCV-RNA. Propylthiouracil was discontinued and his leukocyte counts increased. Twenty-eight weeks after onset of acute hepatitis C, no HCV-RNA was detected.</p> <p>Conclusion</p> <p>This case history shows the risk of recurrence of HCV during leucocytopenia. These findings indicate that patients who are anti-HCV positive but HCV-RNA negative may be at risk of cytopenia-induced HCV reactivation.</p
Pseudogap and charge density waves in two dimensions
An interaction between electrons and lattice vibrations (phonons) results in
two fundamental quantum phenomena in solids: in three dimensions it can turn a
metal into a superconductor whereas in one dimension it can turn a metal into
an insulator. In two dimensions (2D) both superconductivity and charge-density
waves (CDW) are believed to be anomalous. In superconducting cuprates, critical
transition temperatures are unusually high and the energy gap may stay unclosed
even above these temperatures (pseudogap). In CDW-bearing dichalcogenides the
resistivity below the transition can decrease with temperature even faster than
in the normal phase and a basic prerequisite for the CDW, the favourable
nesting conditions (when some sections of the Fermi surface appear shifted by
the same vector), seems to be absent. Notwithstanding the existence of
alternatives to conventional theories, both phenomena in 2D still remain the
most fascinating puzzles in condensed matter physics. Using the latest
developments in high-resolution angle-resolved photoemission spectroscopy
(ARPES) here we show that the normal-state pseudogap also exists in one of the
most studied 2D examples, dichalcogenide 2H-TaSe2, and the formation of CDW is
driven by a conventional nesting instability, which is masked by the pseudogap.
Our findings reconcile and explain a number of unusual, as previously believed,
experimental responses as well as disprove many alternative theoretical
approaches. The magnitude, character and anisotropy of the 2D-CDW pseudogap are
intriguingly similar to those seen in superconducting cuprates.Comment: 14 pages including figures and supplementary informatio
Fermi surface nesting in several transition metal dichalcogenides
By means of high-resolution angle resolved photoelectron spectroscopy (ARPES)
we have studied the fermiology of 2H transition metal dichalcogenide polytypes
TaSe2, NbSe2, and Cu0.2NbS2. The tight-binding model of the electronic
structure, extracted from ARPES spectra for all three compounds, was used to
calculate the Lindhard function (bare spin susceptibility), which reflects the
propensity to charge density wave (CDW) instabilities observed in TaSe2 and
NbSe2. We show that though the Fermi surfaces of all three compounds possess an
incommensurate nesting vector in the close vicinity of the CDW wave vector, the
nesting and ordering wave vectors do not exactly coincide, and there is no
direct relationship between the magnitude of the susceptibility at the nesting
vector and the CDW transition temperature. The nesting vector persists across
the incommensurate CDW transition in TaSe2 as a function of temperature despite
the observable variations of the Fermi surface geometry in this temperature
range. In Cu0.2NbS2 the nesting vector is present despite different doping
level, which lets us expect a possible enhancement of the CDW instability with
Cu-intercalation in the CuxNbS2 family of materials.Comment: Accepted to New J. Phy
1.2-km Timing-Stabilized, Polarization-Maintaining Fiber Link with Sub-Femtosecond Residual Timing Jitter
A 1.2-km timing-stabilized, polarization-maintaining fiber link based on balanced optical cross-correlationwas demonstrated with ~0.9 fs RMS timing jitter over 16 days and ~0.2 fs RMS timing jitter over 3 days
Electrical tuning of elastic wave propagation in nanomechanical lattices at MHz frequencies
Nanoelectromechanical systems (NEMS) that operate in the megahertz (MHz) regime allow energy transducibility between different physical domains. For example, they convert optical or electrical signals into mechanical motions and vice versa. This coupling of different physical quantities leads to frequency-tunable NEMS resonators via electromechanical non-linearities. NEMS platforms with single- or low-degrees of freedom have been employed to demonstrate quantum-like effects, such as mode cooling, mechanically induced transparency, Rabi oscillation, two-mode squeezing and phonon lasing. Periodic arrays of NEMS resonators with architected unit cells enable fundamental studies of lattice-based solid-state phenomena, such as bandgaps, energy transport, non-linear dynamics and localization, and topological properties, directly transferrable to on-chip devices. Here we describe one-dimensional, non-linear, nanoelectromechanical lattices (NEML) with active control of the frequency band dispersion in the radio-frequency domain (10–30 MHz). The design of our systems is inspired by NEMS-based phonon waveguides and includes the voltage-induced frequency tuning of the individual resonators. Our NEMLs consist of a periodic arrangement of mechanically coupled, free-standing nanomembranes with circular clamped boundaries. This design forms a flexural phononic crystal with a well-defined bandgap, 1.8 MHz wide. The application of a d.c. gate voltage creates voltage-dependent on-site potentials, which can significantly shift the frequency bands of the device. Additionally, a dynamic modulation of the voltage triggers non-linear effects, which induce the formation of a phononic bandgap in the acoustic branch, analogous to Peierls transition in condensed matter. The gating approach employed here makes the devices more compact than recently proposed systems, whose tunability mostly relies on materials’ compliance and mechanical non-linearities
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