56 research outputs found
Strain controlled valley filtering in multi-terminal graphene structures
Valley-polarized currents can be generated by local straining of
multi-terminal graphene devices. The pseudo-magnetic field created by the
deformation allows electrons from only one valley to transmit and a current of
electrons from a single valley is generated at the opposite side of the locally
strained region. We show that valley filtering is most effective with bumps of
a certain height and width. Despite the fact that the highest contribution to
the polarized current comes from electrons from the lowest sub-band,
contributions of other sub-bands are not negligible and can significantly
enhance the output current.Comment: 4 pages, 4 figure
Strong plasmon-phonon splitting and hybridization in 2D materials revealed through a self-energy approach
We reveal new aspects of the interaction between plasmons and phonons in 2D
materials that go beyond a mere shift and increase in plasmon width due to
coupling to either intrinsic vibrational modes of the material or phonons in a
supporting substrate. More precisely, we predict strong plasmon splitting due
to this coupling, resulting in a characteristic avoided crossing scheme. We
base our results on a computationally efficient approach consisting in
including many-body interactions through the electron self-energy. We specify
this formalism for a description of plasmons based upon a tight-binding
electron Hamiltonian combined with the random-phase approximation. This
approach is accurate provided vertex corrections can be neglected, as is is the
case in conventional plasmon-supporting metals and Dirac-fermion systems. We
illustrate our method by evaluating plasmonic spectra of doped graphene
nanotriangles with varied size, where we predict remarkable peak splittings and
other radical modifications in the spectra due to plasmons interactions with
intrinsic optical phonons. Our method is equally applicable to other 2D
materials and provides a simple approach for investigating coupling of plasmons
to phonons, excitons, and other excitations in hybrid thin nanostructures
Strained graphene structures: from valleytronics to pressure sensing
Due to its strong bonds graphene can stretch up to 25% of its original size
without breaking. Furthermore, mechanical deformations lead to the generation
of pseudo-magnetic fields (PMF) that can exceed 300 T. The generated PMF has
opposite direction for electrons originating from different valleys. We show
that valley-polarized currents can be generated by local straining of
multi-terminal graphene devices. The pseudo-magnetic field created by a
Gaussian-like deformation allows electrons from only one valley to transmit and
a current of electrons from a single valley is generated at the opposite side
of the locally strained region. Furthermore, applying a pressure difference
between the two sides of a graphene membrane causes it to bend/bulge resulting
in a resistance change. We find that the resistance changes linearly with
pressure for bubbles of small radius while the response becomes non-linear for
bubbles that stretch almost to the edges of the sample. This is explained as
due to the strong interference of propagating electronic modes inside the
bubble. Our calculations show that high gauge factors can be obtained in this
way which makes graphene a good candidate for pressure sensing.Comment: to appear in proceedings of the NATO Advanced Research Worksho
A comparison between experiment and theory on few-quantum-dot nanolasing in a photonic-crystal cavity
Detection of Pneumocystis DNA in samples from patients suspected of bacterial pneumonia- a case-control study
BACKGROUND: Pneumocystis jiroveci (formerly known as P. carinii f.sp. hominis) is an opportunistic fungus that causes Pneumocystis pneumonia (PCP) in immunocompromised individuals. Pneumocystis jiroveci can be detected by polymerase chain reaction (PCR). To investigate the clinical importance of a positive Pneumocystis-PCR among HIV-uninfected patients suspected of bacterial pneumonia, a retrospective matched case-control study was conducted. METHODS: Respiratory samples from 367 patients suspected of bacterial pneumonia were analysed by PCR amplification of Pneumocystis jiroveci. To compare clinical factors associated with carriage of P. jiroveci, a case-control study was done. For each PCR-positive case, four PCR-negative controls, randomly chosen from the PCR-negative patients, were matched on sex and date of birth. RESULTS: Pneumocystis-DNA was detected in 16 (4.4%) of patients. The median age for PCR-positive patients was higher than PCR-negative patients (74 vs. 62 years, p = 0.011). PCR-positive cases had a higher rate of chronic or severe concomitant illness (15 (94%)) than controls (32 (50%)) (p = 0.004). Twelve (75%) of the 16 PCR positive patients had received corticosteroids, compared to 8 (13%) of the 64 PCR-negative controls (p < 0.001). Detection of Pneumocystis-DNA was associated with a worse prognosis: seven (44%) of patients with positive PCR died within one month compared to nine (14%) of the controls (p = 0.01). None of the nine PCR-positive patients who survived had developed PCP at one year of follow-up. CONCLUSIONS: Our data suggest that carriage of Pneumocystis jiroveci is associated with old age, concurrent disease and steroid treatment. PCR detection of P. jiroveci has low specificity for diagnosing PCP among patients without established immunodeficiency. Whether overt infection is involved in the poorer prognosis or merely reflects sub-clinical carriage is not clear. Further studies of P. jiroveci in patients receiving systemic treatment with corticosteroids are warranted
Many-body calculations of plasmon and phonon satellites in angle-resolved photoelectron spectra using the cumulant expansion approach
The interaction of electrons with crystal lattice vibrations (phonons) and
collective charge-density fluctuations (plasmons) influences profoundly the
spectral properties of solids revealed by photoemission spectroscopy
experiments. Photoemission satellites, for instance, are a prototypical example
of quantum emergent behavior that may result from the strong coupling of
electronic states to plasmons and phonons. The existence of these spectral
features has been verified over energy scales spanning several orders of
magnitude (from 50 meV to 15-20 eV) and for a broad class of compounds such as
simple metals, semiconductors, and highly-doped oxides. During the past few
years the cumulant expansion approach, alongside with the GW approximation and
the theory of electron-phonon and electron-plasmon coupling in solids, has
evolved into a predictive and quantitatively accurate approach for the
description of the spectral signatures of electron-boson coupling entirely from
first principles, and it has thus become the state-of-the-art theoretical tool
for the description of these phenomena. In this chapter we introduce the
fundamental concepts needed to interpret plasmon and phonon satellites in
photoelectron spectra, and we review recent progress on first-principles
calculations of these features using the cumulant expansion method
Standing waves for acoustic levitation
Standing waves are the most popular method to achieve acoustic trapping. Particles with greater acoustic impedance than the propagation medium will be trapped at the pressure nodes of a standing wave. Acoustic trapping can be used to hold particles of various materials and sizes, without the need of a close-loop controlling system. Acoustic levitation is a helpful and versatile tool for biomaterials and chemistry, with applications in spectroscopy and lab-on-a-droplet procedures. In this chapter, multiple methods are presented to simulate the acoustic field generated by one or multiple emitters. From the acoustic field, models such as the Gor'kov potential or the Flux Integral are applied to calculate the force exerted on the levitated particles. The position and angle of the acoustic emitters play a fundamental role, thus we analyse commonly used configurations such as emitter and reflector, two opposed emitters, or arrangements using phased arrays
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