High-frequency nanotube mechanical resonators

We report on a simple method to fabricate high-frequency nanotube mechanical resonators reproducibly. We measure resonance frequencies as high as 4.2 GHz for the fundamental eigenmode and 11 GHz for higher order eigenmodes. The high-frequency resonances are achieved using short suspended nanotubes and by introducing tensile stress in the nanotube. These devices allow us to determine the coefficient of the thermal expansion of an individual nanotube, which is negative and is about -0.7E-5 1/K at room temperature. High-frequency resonators made of nanotubes hold promise for mass sensing and experiments in the quantum limit

Symmetry breaking in a mechanical resonator made from a carbon nanotube

Nanotubes behave as semi-flexible polymers in that they can bend by a sizeable amount. When integrating a nanotube in a mechanical resonator, the bending is expected to break the symmetry of the restoring potential. Here we report on a new detection method that allows us to demonstrate such symmetry breaking. The method probes the motion of the nanotube resonator at nearly zero-frequency; this motion is the low-frequency counterpart of the second overtone of resonantly excited vibrations. We find that symmetry breaking leads to the spectral broadening of mechanical resonances, and to an apparent quality factor that drops below 100 at room temperature. The low quality factor at room temperature is a striking feature of nanotube resonators whose origin has remained elusive for many years. Our results shed light on the role played by symmetry breaking in the mechanics of nanotube resonators.Comment: manuscript and supplementary material, 7 figure

Sensitivity of mixing layers to three-dimensional forcing

It is well known that turbulent mixing layers are dominated by large scale, fairly coherent structures, and that these structures are related to the stability characteristics of the flow. These facts have led researchers to attempt controlling such flows by selectively forcing certain unstable modes, which can in addition have the effect of suppressing other modes. Much of the work on controlling the mixing layer has relied on forcing 2-D instabilities. The results of forcing 3-D instabilities are addressed. The objectives of the work are twofold: to understand how a mixing layer responds to 3-D perturbations, and to test the validity of an amplitude expansion in predicting the mixing layer development. The amplitude expansion could be very useful in understanding and predicting the 3-D response of the flow to a variety of initial conditions

From spin-Peierls to superconductivity: (TMTTF)_2PF_6 under high pressure

The nature of the attractive electron-electron interaction, leading to the formation of Cooper-pairs in unconventional superconductors has still to be fully understood and is subject to intensive research. Here we show that the sequence spin-Peierls, antiferromagnetism, superconductivity observed in (TMTTF)_2PF_6 under pressure makes the (TM)_2X phase diagram universal. We argue that the suppression of the spin-Peierls transition under pressure, the close vicinity of antiferromagnetic and superconducting phases at high pressure as well as the existence of critical antiferromagnetic fluctuations above T_c strongly support the intriguing possibility that the interchain exchange of antiferromagnetic fluctuations provides the pairing mechanism required for bound charge carriers.Comment: 4 pages, revtex, 4 figures (jpeg,eps,png

Spin chain from membrane and the Neumann-Rosochatius integrable system

We find membrane configurations in AdS_4 x S^7, which correspond to the continuous limit of the SU(2) integrable spin chain, considered as a limit of the SU(3) spin chain, arising in N=4 SYM in four dimensions, dual to strings in AdS_5 x S^5. We also discuss the relationship with the Neumann-Rosochatius integrable system at the level of Lagrangians, comparing the string and membrane cases.Comment: LaTeX, 16 pages, no figures; v2: 17 pages, title changed, explanations and references added; v3: more explanations added; v4: typos fixed, to appear in Phys. Rev.

Modification and Assessment of the Bedside Pediatric Early Warning Score in the Pediatric Allogeneic Hematopoietic Cell Transplant Population

OBJECTIVES: To determine the validity of the Bedside Pediatric Early Warning Score system in the hematopoietic cell transplant population, and to determine if the addition of weight gain further strengthens the association with need for PICU admission. DESIGN: Retrospective cohort study of pediatric allogeneic hematopoietic cell transplant patients from 2009 to 2016. Daily Pediatric Early Warning Score and weights were collected during hospitalization. Logistic regression was used to identify associations between maximum Pediatric Early Warning Score or Pediatric Early Warning Score plus weight gain and the need for PICU intervention. The primary outcome was need for PICU intervention; secondary outcomes included mortality and intubation. SETTING: A large quaternary free-standing children's hospital. PATIENTS: One-hundred two pediatric allogeneic hematopoietic cell transplant recipients. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Of the 102 hematopoietic cell transplant patients included in the study, 29 were admitted to the PICU. The median peak Pediatric Early Warning Score was 11 (interquartile range, 8-13) in the PICU admission cohort, compared with 4 (interquartile range, 3-5) in the cohort without a PICU admission (p < 0.0001). Pediatric Early Warning Score greater than or equal to 8 had a sensitivity of 76% and a specificity of 90%. The area under the receiver operating characteristics curve was 0.83. There was a high negative predictive value at this Pediatric Early Warning Score of 90%. When Pediatric Early Warning Score greater than or equal to 8 and weight gain greater than or equal to 7% were compared together, the area under the receiver operating characteristic curve increased to 0.88. CONCLUSIONS: In this study, a Pediatric Early Warning Score greater than or equal to 8 was associated with PICU admission, having a moderately high sensitivity and high specificity. This study adds to literature supporting Pediatric Early Warning Score monitoring for hematopoietic cell transplant patients. Combining weight gain with Pediatric Early Warning Score improved the discriminative ability of the model to predict the need for critical care, suggesting that incorporation of weight gain into Pediatric Early Warning Score may be beneficial for monitoring of hematopoietic cell transplant patients