15 research outputs found
The thermal conductivity of silicon nitride membranes is not sensitive to stress
We have measured the thermal properties of suspended membranes from 10 K to
300 K for two amplitudes of internal stress (about 0.1 GPa and 1 GPa) and for
two different thicknesses (50 nm and 100 nm). The use of the original 3 \omega
-Volklein method has allowed the extraction of both the specific heat and the
thermal conductivity of each SiN membrane over a wide temperature range. The
mechanical properties of the same substrates have been measured at helium
temperatures using nanomechanical techniques. Our measurements show that the
thermal transport in freestanding SiN membranes is not affected by the presence
of internal stress. Consistently, mechanical dissipation is also unaffected
even though Qs increase with increasing tensile stress. We thus demonstrate
that the theory developed by Wu and Yu [Phys. Rev. B 84, 174109 (2011)] does
not apply to this amorphous material in this stress range. On the other hand,
our results can be viewed as a natural consequence of the "dissipation
dilution" argument [Y. L. Huang and P. R. Saulson, Rev. Sci. Instrum. 69, 544
(1998)] which has been introduced in the context of mechanical damping.Comment: 15 pages, 6 figures. Submitted to PR
Measuring frequency fluctuations in nonlinear nanomechanical resonators
Advances in nanomechanics within recent years have demonstrated an always
expanding range of devices, from top-down structures to appealing bottom-up
MoS and graphene membranes, used for both sensing and component-oriented
applications. One of the main concerns in all of these devices is frequency
noise, which ultimately limits their applicability. This issue has attracted a
lot of attention recently, and the origin of this noise remains elusive up to
date. In this Letter we present a very simple technique to measure frequency
noise in nonlinear mechanical devices, based on the presence of bistability. It
is illustrated on silicon-nitride high-stress doubly-clamped beams, in a
cryogenic environment. We report on the same dependence of the frequency
noise power spectra as reported in the literature. But we also find unexpected
{\it damping fluctuations}, amplified in the vicinity of the bifurcation
points; this effect is clearly distinct from already reported nonlinear
dephasing, and poses a fundamental limit on the measurement of bifurcation
frequencies. The technique is further applied to the measurement of frequency
noise as a function of mode number, within the same device. The relative
frequency noise for the fundamental flexure lies in the range
ppm (consistent with literature for cryogenic MHz devices), and
decreases with mode number in the range studied. The technique can be applied
to {\it any types} of nano-mechanical structures, enabling progresses towards
the understanding of intrinsic sources of noise in these devices.Comment: Published 7 may 201
Nanomechanical damping via electron-assisted relaxation of two-level systems
We report on measurements of dissipation and frequency noise at millikelvin
temperatures of nanomechanical devices covered with aluminum. A clear excess
damping is observed after switching the metallic layer from superconducting to
the normal state with a magnetic field. Beyond the standard model of internal
tunneling systems coupled to the phonon bath, here we consider the relaxation
to the conduction electrons together with the nature of the mechanical
dispersion laws for stressed/unstressed devices. With these key ingredients, a
model describing the relaxation of two-level systems inside the structure due
to interactions with electrons and phonons with well separated timescales
captures the data. In addition, we measure an excess 1/f-type frequency noise
in the normal state, which further emphasizes the impact of conduction
electrons
Universality of thermal transport in amorphous nanowires at low temperatures
Thermal transport properties of amorphous materials at low temperatures are governed by the interaction between phonons and localized excitations referred to as tunneling two-level systems (TLSs). The temperature variation of the thermal conductivity of these amorphous materials is considered as universal and is characterized by a quadratic power law. This is well described by the phenomenological TLS model even though its microscopic explanation is still elusive. Here, by scaling down to the nanometer-scale amorphous systems much below the bulk phonon-TLS mean free path, we probe the robustness of that model in restricted geometry systems. Using very sensitive thermal conductance measurements, we demonstrate that the temperature dependence of the thermal conductance of silicon nitride nanostructures remains mostly quadratic independently of the nanowire section. It does not follow the cubic power law in temperature as expected in a Casimir-Ziman regime of boundary-limited thermal transport. This shows a thermal transport counterintuitively dominated by phonon-TLS interactions and not by phonon boundary scattering in the nanowires. This could be ascribed to an unexpected high density of TLSs on the surfaces which still dominates the phonon diffusion processes at low temperatures and explains why the universal quadratic temperature dependence of thermal conductance still holds for amorphous nanowires
Warped AdS_3 Black Holes
Three dimensional topologically massive gravity (TMG) with a negative
cosmological constant -\ell^{-2} and positive Newton constant G admits an AdS_3
vacuum solution for any value of the graviton mass \mu. These are all known to
be perturbatively unstable except at the recently explored chiral point
\mu\ell=1. However we show herein that for every value of \mu\ell< 3 there are
two other (potentially stable) vacuum solutions given by SL(2,R)x
U(1)-invariant warped AdS_3 geometries, with a timelike or spacelike U(1)
isometry.
Critical behavior occurs at \mu\ell=3, where the warping transitions from a
stretching to a squashing, and there are a pair of warped solutions with a null
U(1) isometry. For \mu\ell>3, there are known warped black hole solutions which
are asymptotic to warped AdS_3. We show that these black holes are discrete
quotients of warped AdS_3 just as BTZ black holes are discrete quotients of
ordinary AdS_3. Moreover new solutions of this type, relevant to any theory
with warped AdS_3 solutions, are exhibited. Finally we note that the black hole
thermodynamics is consistent with the hypothesis that, for \mu\ell>3, the
warped AdS_3 ground state of TMG is holographically dual to a 2D boundary CFT
with central charges c_R={15(\mu\ell)^2+81\over G\mu((\mu\ell)^2+27)} and
c_L={12 \mu\ell^2\over G((\mu\ell)^2+27)}.Comment: 29 page
Nanomechanical damping via electron-assisted relaxation of two-level systems
We report on measurements of dissipation and frequency noise at millikelvin temperatures of nanomechanical devices covered with aluminum. A clear excess damping is observed after switching the metallic layer from superconducting to the normal state with a magnetic field. Beyond the standard model of internal tunneling systems coupled to the phonon bath, here we consider the relaxation to the conduction electrons together with the nature of the mechanical dispersion laws for stressed/unstressed devices. With these key ingredients, a model describing the relaxation of two-level systems inside the structure due to interactions with electrons and phonons with well separated timescales captures the data. In addition, we measure an excess 1/f-type frequency noise in the normal state, which further emphasizes the impact of conduction electrons