123 research outputs found
Lifting restrictions on coherence loss when characterizing non-transparent hypersonic phononic crystals
Abstract Hypersonic phononic bandgap structures confine acoustic vibrations whose wavelength is commensurate with that of light, and have been studied using either time- or frequency-domain optical spectroscopy. Pulsed pump-probe lasers are the preferred instruments for characterizing periodic multilayer stacks from common vacuum deposition techniques, but the detection mechanism requires the injected sound wave to maintain coherence during propagation. Beyond acoustic Bragg mirrors, frequency-domain studies using a tandem Fabry–Perot interferometer (TFPI) find dispersions of two- and three-dimensional phononic crystals (PnCs) even for highly disordered samples, but with the caveat that PnCs must be transparent. Here, we demonstrate a hybrid technique for overcoming the limitations that time- and frequency-domain approaches exhibit separately. Accordingly, we inject coherent phonons into a non-transparent PnC using a pulsed laser and acquire the acoustic transmission spectrum on a TFPI, where pumped appear alongside spontaneously excited (i.e. incoherent) phonons. Choosing a metallic Bragg mirror for illustration, we determine the bandgap and compare with conventional time-domain spectroscopy, finding resolution of the hybrid approach to match that of a state-of-the-art asynchronous optical sampling setup. Thus, the hybrid pump–probe technique retains key performance features of the established one and going forward will likely be preferred for disordered samples
Mode Spectroscopy and Level Coupling in Ballistic Electron Waveguides
A tunable quantum point contact with modes occupied in both transverse
directions is studied by magnetotransport experiments. We use conductance
quantization of the one-dimensional subbands as a tool to determine the mode
spectrum. A magnetic field applied along the direction of the current flow
couples the modes. This can be described by an extension of the Darwin-Fock
model. Anticrossings are observed as a function of the magnetic field, but not
for zero field or perpendicular field directions, indicating coupling of the
subbands due to nonparabolicity in the electrical confinement.Comment: 4 pages, 3 figure
Cytokines and regulation of glucose and lipid metabolism in the obesity
The article presents data of the influence of cytokines of different directions of glucose and lipid metabolism in obesity. A change of the basic paradigm regarding adipose tissue has contributed to a number of recent discoveries. This concerns such basic concepts as healthy and diseased adipocytes, and, as a consequence, changes of their metabolism under the influence of cytokins. Distinguishing the concept of organokines demonstrates that despite the common features of cytokine regulation, each organ has its own specifics features of cytokine regulation, each organ has its own specific an important section of this concept is the idea of the heterogeneity of adipose tissue. Knowledge of the function of adipose tissue localized in different compartments of the body is expanding. There are date about the possibility of transition of one type of adipose tissue to another. A possible mechanism linking adipose tissue inflammation and the formation of insulin resistance (IR) is presented in this paper. The mechanism of IR development is closely connected with to proinflammatory cytokins disordering the insulin signal, accompanied by a decrease of the work of glucose transporters. A decrease of the income of glucose into cells leads to a change of glycolysis level to an increase of the fatty acids oxidation. Cytokins are able to participate in the process of the collaboration of some cells with others, that occurs both during physiological and pathological process
Voltage dependent conductance and shot noise in quantum microconstriction with single defects
The influence of the interference of electron waves, which are scattered by
single impurities and by a barrier on nonlinear conductance and shot noise of
metallic microconstriction is studied theoretically. It is shown that the these
characteristics are nonmonotonic functions on the applied bias.Comment: 18 pages,5 figure
Electrical detection of picosecond acoustic pulses in vertical transport devices with nanowires
Picosecond acoustic pulses, generated in a thin aluminum transducer, are injected into semiconductor vertical transport devices consisting of core-shell GaAsP nanowires. The acoustic pulses induce current pulses in the device with amplitude ∼1 μA. The spectrum of the electrical response is sensitive to the elastic properties of the device and has a frequency cutoff at ∼10 GHz. This work shows the potential of the technique for studies the elastic properties of complex semiconductor nanodevices.Peer reviewe
Synchronous Spatial Oscillation of Electron- and Mn-Spin Polarizations in Dilute-Magnetic-Semiconductor Quantum Wells under Spin-Orbit Effective Magnetic Fields
In semiconductors, spin-orbit effective magnetic fields, i.e., the Rashba and
Dresselhaus fields, are used to control electron-spin polarization. This
operation, however, destroys the electron-spin coherence, and the spin
polarization is limited to the vicinity of a ferromagnetic source electrode. In
this paper, we propose the use of dilute magnetic semiconductors to improve the
coherence of spatially oscillating electron-spin polarization. In dilute
magnetic semiconductors, the electron-spin polarization near the source
electrode dynamically induces the local spin polarization of magnetic
impurities through s-d spin-flip scattering. This impurity-spin polarization
improves, in turn, the coherence of the electron-spin polarization, and this
improved electron-spin polarization induces impurity-spin polarization farther
in the adjacent region. Because of this positive feedback, the coherent and
synchronized spatial oscillations of electron- and impurity-spin polarizations
grow cooperatively. A numerical calculation for a CdMnTe quantum well
demonstrates the validity of this mechanism.Comment: 30 pages, 6 figures, 1 tabl
Quantum Conductance in Semimetallic Bismuth Nanocontacts
Electronic transport properties of bismuth nanocontacts are analyzed by means
of a low temperature scanning tunneling microscope. The subquantum steps
observed in the conductance versus elongation curves give evidence of atomic
rearrangements in the contact. The underlying quantum nature of the conductance
reveals itself through peaks in the conductance histograms. The shape of the
conductance curves at 77 K is well described by a simple gliding mechanism for
the contact evolution during elongation. The strikingly different behaviour at
4 K suggests a charge carrier transition from light to heavy ones as the
contact cross section becomes sufficiently small.Comment: 5 pages including 4 figures. Accepted for publication in Phys. Rev.
Let
Femtosecond nonlinear ultrasonics in gold probed with ultrashort surface plasmons
Fundamental interactions induced by lattice vibrations on ultrafast time
scales become increasingly important for modern nanoscience and technology.
Experimental access to the physical properties of acoustic phonons in the THz
frequency range and over the entire Brillouin zone is crucial for understanding
electric and thermal transport in solids and their compounds. Here, we report
on the generation and nonlinear propagation of giant (1 percent) acoustic
strain pulses in hybrid gold/cobalt bilayer structures probed with ultrafast
surface plasmon interferometry. This new technique allows for unambiguous
characterization of arbitrary ultrafast acoustic transients. The giant acoustic
pulses experience substantial nonlinear reshaping already after a propagation
distance of 100 nm in a crystalline gold layer. Excellent agreement with the
Korteveg-de Vries model points to future quantitative nonlinear femtosecond
THz-ultrasonics at the nano-scale in metals at room temperature
- …