11,862 research outputs found
Phonons in a Nanoparticle Mechanically Coupled to a Substrate
The discrete nature of the vibrational modes of an isolated nanometer-scale
solid dramatically modifies its low-energy electron and phonon dynamics from
that of a bulk crystal. However, nanocrystals are usually coupled--even if only
weakly--to an environment consisting of other nanocrystals, a support matrix,
or a solid substrate, and this environmental interaction will modify the
vibrational properties at low frequencies. In this paper we investigate the
modification of the vibrational modes of an insulating spherical nanoparticle
caused by a weak {\it mechanical} coupling to a semi-infinite substrate. The
phonons of the bulk substrate act as a bath of harmonic oscillators, and the
coupling to this reservoir shifts and broadens the nanoparticle's modes. The
vibrational density of states in the nanoparticle is obtained by solving the
Dyson equation for the phonon propagator, and we show that environmental
interaction is especially important at low frequencies. As a probe of the
modified phonon spectrum, we consider nonradiative energy relaxation of a
localized electronic impurity state in the nanoparticle, for which good
agreement with experiment is found.Comment: 10 pages, Revte
Dissipation due to tunneling two-level systems in gold nanomechanical resonators
We present measurements of the dissipation and frequency shift in
nanomechanical gold resonators at temperatures down to 10 mK. The resonators
were fabricated as doubly-clamped beams above a GaAs substrate and actuated
magnetomotively. Measurements on beams with frequencies 7.95 MHz and 3.87 MHz
revealed that from 30 mK to 500 mK the dissipation increases with temperature
as , with saturation occurring at higher temperatures. The relative
frequency shift of the resonators increases logarithmically with temperature up
to at least 400 mK. Similarities with the behavior of bulk amorphous solids
suggest that the dissipation in our resonators is dominated by two-level
systems
Cavity-enhanced room-temperature magnetometry using absorption by nitrogen-vacancy centers in diamond
We demonstrate a cavity-enhanced room-temperature magnetic field sensor based
on nitrogen-vacancy centers in diamond. Magnetic resonance is detected using
absorption of light resonant with the 1042 nm spin-singlet transition. The
diamond is placed in an external optical cavity to enhance the absorption, and
significant absorption is observed even at room temperature. We demonstrate a
magnetic field sensitivity of 2.5 nT/sqrt(Hz), and project a photon
shot-noise-limited sensitivity of 70 pT/sqrt(Hz) for a few mW of infrared
light, and a quantum projection-noise-limited sensitivity of 250 fT/sqrt(Hz)
for the sensing volume of 90 um x 90 um 200 um.Comment: main text 5 pages, supplementary material 3 page
Stimulated Neutrino Transformation with Sinusoidal Density Profiles
Large amplitude oscillations between the states of a quantum system can be
stimulated by sinusoidal external potentials with frequencies that are similar
to the energy level splitting of the states or a fraction thereof. Situations
when the applied frequency is equal to an integer fraction of the energy level
splittings are known as parametric resonances. We investigate this effect for
neutrinos both analytically and numerically for the case of arbitrary numbers
of neutrino flavors. We look for environments where the effect may be observed
and find that supernova are the one realistic possibility due to the necessity
of both large densities and large amplitude fluctuations. The comparison of
numerical and analytic results of neutrino propagation through a model
supernova reveals it is possible to predict the locations and strengths of the
stimulated transitions that occur.Comment: 14 pages, 6 figure
Nonlinear modal coupling in a high-stress doubly-clamped nanomechanical resonator
We present results from a study of the nonlinear intermodal coupling between
different flexural vibrational modes of a single high-stress, doubly-clamped
silicon nitride nanomechanical beam. The measurements were carried out at 100
mK and the beam was actuated using the magnetomotive technique. We observed the
nonlinear behavior of the modes individually and also measured the coupling
between them by driving the beam at multiple frequencies. We demonstrate that
the different modes of the resonator are coupled to each other by the
displacement induced tension in the beam, which also leads to the well known
Duffing nonlinearity in doubly-clamped beams.Comment: 15 pages, 7 figure
Metastability in spin polarised Fermi gases and quasiparticle decays
We investigate the metastability associated with the first order transition from normal to superfluid phases in the phase diagram of two-component polarised Fermi gases.We begin by detailing the dominant decay processes of single quasiparticles.Having determined the momentum thresholds of each process and calculated their rates, we apply this understanding to a Fermi sea of polarons by linking its metastability to the stability of individual polarons, and predicting a region of metastability for the normal partially polarised phase. In the limit of a single impurity, this region extends from the interaction strength at which a polarised phase of molecules becomes the groundstate, to the one at which the single quasiparticle groundstate changes character from polaronic to molecular. Our argument in terms of a Fermi sea of polarons naturally suggests their use as an experimental probe. We propose experiments to observe the threshold of the predicted region of metastability, the interaction strength at which the quasiparticle groundstate changes character, and the decay rate of polarons
Graduate dress code: How undergraduates are planning to use hair, clothes and make-up to smooth their transition to the workplace
This article explores the relationship between students’ identities, their ideas about professional appearance and their anticipated transition to the world of work. It is based on a series of semi-structured interviews with 13 students from a vocationally-focused university in England. It was found that participants viewed clothing and appearance as an important aspect of their transition to the workplace. They believed that, if carefully handled, their appearance could help them to fit in and satisfy the expectations of employers, although some participants anticipated that this process of fitting in might compromise their identity and values. The article addresses students’ anticipated means of handling the tension between adapting to a new environment and ‘being themselves’. It is argued that the way this process is handled is intertwined with wider facets of identity – most notably those associated with gender.The article is based on research funded by the University of Derby. © 2015 IP Publishing Ltd. ((http://www.ippublishing.com). Reproduced by permission
Analyzing the success of T-matrix diagrammatic theories in representing a modified Hubbard model
We present a systematic study of various forms of renormalization that can be
applied in the calculation of the self-energy of the Hubbard model within the
T-matrix approximation. We compare the exact solutions of the attractive and
repulsive Hubbard models, for linear chains of lengths up to eight sites, with
all possible taxonomies of the T-matrix approximation. For the attractive
Hubbard model, the success of a minimally self-consistent theory found earlier
in the atomic limit (Phys. Rev. B 71, 155111 (2005)) is not maintained for
finite clusters unless one is in the very strong correlation limit. For the
repulsive model, in the weak correlation limit at low electronic densities --
that is, where one would expect a self-consistent T-matrix theory to be
adequate -- we find the fully renormalized theory to be most successful. In our
studies we employ a modified Hubbard interaction that eliminates all Hartree
diagrams, an idea which was proposed earlier (Phys. Rev. B 63, 035104 (2000)).Comment: Includes modified discussion of 1st-order phase transition. Accepted
for publication in J. Phys.: Condensed Matte
Germline mutations in the oncogene EZH2 cause Weaver syndrome and increased human height.
The biological processes controlling human growth are diverse, complex and poorly understood. Genetic factors are important and human height has been shown to be a highly polygenic trait to which common and rare genetic variation contributes. Weaver syndrome is a human overgrowth condition characterised by tall stature, dysmorphic facial features, learning disability and variable additional features. We performed exome sequencing in four individuals with Weaver syndrome, identifying a mutation in the histone methyltransferase, EZH2, in each case. Sequencing of EZH2 in additional individuals with overgrowth identified a further 15 mutations. The EZH2 mutation spectrum in Weaver syndrome shows considerable overlap with the inactivating somatic EZH2 mutations recently reported in myeloid malignancies. Our data establish EZH2 mutations as the cause of Weaver syndrome and provide further links between histone modifications and regulation of human growth
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Conceptualising quality of life for older people with aphasia
Background: There is an increasing need in speech and language therapy for clinicians to provide intervention in the context of the broader life quality issues for people with aphasia. However, there is no descriptive research that is explicitly focused on quality of life (QoL) from the perspectives of older people with aphasia.
Aims: The current study explores how older people with chronic aphasia who are living in the community describe their QoL in terms of what contributes to and detracts from the quality in their current and future lives. The study is descriptive in nature, and the purpose is to conceptualize the factors that influence QoL.
Methods & Procedures: Thirty older participants (16 women, 14 men) with mild to moderate aphasic impairment took part. All participants had adequate communication skills to participate: demonstrating reliable yes/no response and moderate auditory comprehension ability. Participants were interviewed in their own homes using six brief unprompted open questions about QoL, in a structured interview. The first five questions were drawn from previous gerontological research (Farquhar, 1995), and a sixth question specifically targeting communication was added. Content analysis was used, identifying discrete units of data and then coding these into concepts and factors. Additional demographic information was collected, and participants’ mood on day of interviewing was assessed using the Geriatric Depression Scale (Sheikh & Yesavage, 1986).
Outcomes & Results: Activities, verbal communication, people, and body functioning were the core factors in QoL for these participants, and they described how these factors both contributed quality in life as well as detracted from life quality. Other factors that influenced QoL included stroke, mobility, positive personal outlook, in/dependence, home and health. Whilst the findings are limited by the lack of probing of participants’ responses, the study does present preliminary evidence for what is important in QoL to older people with aphasia.
Conclusions: Quality of life for older people with predominantly mild to moderate chronic aphasia who are living in the community is multifactorial in nature. Some factors lie within the remit of speech and language therapy, some lie beyond the professional role, but all are relevant for consideration in rehabilitation and community practice. Further qualitative research is implicated to better understand QoL with aphasia, using in-depth interviewing with a broader range of people with aphasia
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