550 research outputs found
Finite element schemes for Fermi equation
No full textA priori error estimates are derived for the streamline diffusion (SD) finite element methods for the Fermi pencil-beam equation. Two-dimensional numerical examples confirm our theoretical investigations
Stacking-dependent band gap and quantum transport in trilayer graphene
No full textGraphene is an extraordinary two-dimensional (2D) system with chiral charge carriers and fascinating electronic, mechanical and thermal properties. In multilayer graphene, stacking order provides an important yet rarely explored degree of freedom for tuning its electronic properties. For instance, Bernal-stacked trilayer graphene (B-TLG) is semi-metallic with a tunable band overlap, and rhombohedral-stacked trilayer graphene (r-TLG) is predicted to be semiconducting with a tunable band gap. These multilayer graphenes are also expected to exhibit rich novel phenomena at low charge densities owing to enhanced electronic interactions and competing symmetries. Here we demonstrate the dramatically different transport properties in TLG with different stacking orders, and the unexpected spontaneous gap opening in charge neutral r-TLG. At the Dirac point, B-TLG remains metallic, whereas r-TLG becomes insulating with an intrinsic interaction-driven gap ~6 meV. In magnetic fields, well-developed quantum Hall (QH) plateaux in r-TLG split into three branches at higher fields. Such splitting is a signature of the Lifshitz transition, a topological change in the Fermi surface, that is found only in r-TLG. Our results underscore the rich interaction-induced phenomena in trilayer graphene with different stacking orders, and its potential towards electronic applications
A search for neutrino emission from the Fermi bubbles with the ANTARES telescope
No full textAnalysis of the Fermi-LAT data has revealed two extended structures above and below the Galactic Centre emitting gamma rays with a hard spectrum, the so-called Fermi bubbles. Hadronic models attempting to explain the origin of the Fermi bubbles predict the emission of high-energy neutrinos and gamma rays with similar fluxes. The ANTARES detector, a neutrino telescope located in the Mediterranean Sea, has a good visibility to the Fermi bubble regions. Using data collected from 2008 to 2011 no statistically significant excess of events is observed and therefore upper limits on the neutrino flux in TeV range from the Fermi bubbles are derived for various assumed energy cutoffs of the source. © 2014 The Author(s)
Universal Fermi velocity in highly compressed hydride superconductors
No full textThe Fermi velocity vF is one of the primary characteristics of any conductor, including any superconductor. For conductors at ambient pressure, several experimental techniques have been developed to measure vF, and, for instance, Zhou et al. [Nature 423, 398 (2003)] reported that high-Tc cuprates exhibited a universal nodal Fermi velocity vF,univ=2.7±0.5×105 m/s. However, there have been no measurements of vF in highly compressed near-room-temperature superconductors (NRTS), owing to experimental challenges. Here, to answer the question of the existence of a universal Fermi velocity in NRTS materials, we analyze the full inventory of data on the ground-state upper critical field Bc2(0) for these materials and find that this class of superconductors exhibits a universal Fermi velocity vF,univ=1/1.3×2Δ0/kBTc×105 m/s, where Δ(0) is the ground-state amplitude of the energy gap. The ratio 2Δ0/kBTc varies within a narrow range 3.2≤2Δ0/kBTc≤5, and so vF,univ in NRTS materials lies in the range 2.5 × 105 m/s ≤ vF,univ ≤ 3.8 × 105 m/s, which is similar to the range of values found for the high-Tc cuprate counterparts of these materials. © 2022 Author(s)
3D Quantum Hall Effect of Fermi Arc in Topological Semimetals
No full textThe quantum Hall effect is usually observed in 2D systems. We show that the Fermi arcs can give rise to a distinctive 3D quantum Hall effect in topological semimetals. Because of the topological constraint, the Fermi arc at a single surface has an open Fermi surface, which cannot host the quantum Hall effect. Via a "wormhole" tunneling assisted by theWeyl nodes, the Fermi arcs at opposite surfaces can form a complete Fermi loop and support the quantum Hall effect. The edge states of the Fermi arcs show a unique 3D distribution, giving an example of (d - 2)-dimensional boundary states. This is distinctly different from the surface-state quantum Hall effect from a single surface of topological insulator. As the Fermi energy sweeps through theWeyl nodes, the sheet Hall conductivity evolves from the 1/B dependence to quantized plateaus at the Weyl nodes. This behavior can be realized by tuning gate voltages in a slab of topological semimetal, such as the TaAs family, Cd3As2, or Na3Bi. This work will be instructive not only for searching transport signatures of the Fermi arcs but also for exploring novel electron gases in other topological phases of matter.National Basic Research Program of China (NBRPC) [2015CB921102]; Guangdong Innovative and Entrepreneurial Research Team Program [2016ZT06D348]; National Key R D Program [2016YFA0301700]; National Natural Science Foundation of China [11534001, 11474005, 11574127]; Science, Technology and Innovation Commission of Shenzhen Municipality [ZDSYS20170303165926217]SCI(E)ARTICLE1311
THE CLASSICAL DYNAMICS OF MOLECULE WITH FERMI RESONANCE
No full text(1) M. Joyeux, Chem. Phys. 185, 263 (1994) (2) M. Joyeux, Chem. Phys. In press (3)M. Joyeux, J. Mol, Spectry, in press (4) M. Joyeux, J. Chem. Phys. 102, 2816 (1995) (5) M. Joyeux, Chem, Phys. Letters 247, 454 (1995)Author Institution: Laboratoire de Spectrom\bar{e}trle Physique(CNRS UA08), Universit\'{e} Joseph Fourier-GrenobleDue to the 1:2 Fermi resonance, the vibrational levels of molecule like or cannot be obtained by a simple Dunham exponsion, but require instead the diagonallsation of on Homiltonian matrix: The dynamics of the classical counterpart of this Hamiltonian has been studied recently, leading to analytical expressions for the action integrals and classical frequencies, as well as to an exhaustive desceiption of the geometry of the phase space (1,2). These results have been applied to three problems of interest. First, the observed vibrational levels of up to energy have been assigned by replacing the good quantum number, which is destroyed by the fermi resonance, by the remaining semiclassical quantum number obtained from Einstein-Brillouin-Keller(EBK) quantization conditions(3). In addition. EBK quantization enables one to describe the anticrossing of quantum levels as arising classically from the dynamical tunnelling between quantizing torl(4). Also, the first 44 levels arising from combinaations and overtones of the stretching and bending vibrations of were reproduced up to vibrational energy using Berry and Taboors Trace Formula(5)
Lasing in Bose-Fermi mixtures
No full textA.K. acknowledges the support from the EPSRC Established Career Fellowship. V.K., M.D., V.F.S. and A.K. acknowledge support from the Russian Ministry of Science and Education, contract (contract No. 11.G34.31.0067). P.G.S. acknowledges support from Greek GSRT program Aristeia (grant No. 1978). C.S., M. A. J.F., M.K and S.H. acknowledge support from the state of Bavaria.Light amplification by stimulated emission of radiation, well-known for revolutionising photonic science, has been realised primarily in fermionic systems including widely applied diode lasers. The prerequisite for fermionic lasing is the inversion of electronic population, which governs the lasing threshold. More recently, bosonic lasers have also been developed based on Bose-Einstein condensates of exciton-polaritons in semiconductor microcavities. These electrically neutral bosons coexist with charged electrons and holes. In the presence of magnetic fields, the charged particles are bound to their cyclotron orbits, while the neutral exciton-polaritons move freely. We demonstrate how magnetic fields affect dramatically the phase diagram of mixed Bose-Fermi systems, switching between fermionic lasing, incoherent emission and bosonic lasing regimes in planar and pillar microcavities with optical and electrical pumping. We collected and analyzed the data taken on pillar and planar microcavity structures at continuous wave and pulsed optical excitation as well as injecting electrons and holes electronically. Our results evidence the transition from a Bose gas to a Fermi liquid mediated by magnetic fields and light-matter coupling.Peer reviewe
Bulk Fermi surface and momentum density in heavily doped La2?xSrxCuO4 using high-resolution Compton scattering and positron annihilation spectroscopies
No full textWe have observed the bulk Fermi surface (FS) in an overdoped (x=0.3) single crystal of La2?xSrxCuO4 by using Compton scattering. A two-dimensional (2D) momentum density reconstruction from measured Compton profiles yields a clear FS signature in the third Brillouin zone along [100]. The quantitative agreement between density functional theory (DFT) calculations and momentum density experiment suggests that Fermi-liquid physics is restored in the overdoped regime. In particular the predicted FS topology is found to be in good accord with the corresponding experimental data. We find similar quantitative agreement between the measured 2D angular correlation of positron annihilation radiation (2D-ACAR) spectra and the DFT-based computations. However, 2D-ACAR does not give such a clear signature of the FS in the extended momentum space in either the theory or the experiment.RRR/Radiation, Radionuclides and ReactorsApplied Science
Optical characterization of multilayer coating samples
No full textThis paper aims to describe a measurement process studied and applied by the author at CNR IFN - LUXOR laboratories in Padova. The main purpose was to establish the reflectivity performance of some optical mirrors realized with different multilayer technologies. For such devices applications are foreseen in the Metis coronagraph for the ESA Solar Orbiter mission, and in the Free Electron Laser facility in Trieste.
After having studied the bands in which precise reflectance values were needed for the application, measures were took for each sample in the EUV, VIS and NIR range using the facilities available at LUXOR laboratories. The principle of operation of these devices, the structure of the facilities used and the measurement process are going to be described in this paper, then the results will be presented and discussed.
This work was inserted in a more wide sample characterization project which was developed by the LUXOR team. It included reflectance measurements in different spectra, such as Soft-X, and AFM surface analysis, whose data will be presented for completeness.
The aim of these measurements was not only to establish whether some of the samples satisfy the specifications imposed by their application or not, but also to understand which of the multilayer technologies used should possibly be improved and tested for the application. The results, finally, state a performance baseline for the samples, which are going to be exposed to the conditions of their applications (e.g. solar wind in space) in order to establish their performance loose with aging.
\\Questo scritto intende descrivere un processo di misura studiato ed applicato dall'autore ai laboratori LUXOR del CNR IFN. Lo scopo principale era di stabilire le prestazioni in riflettività di specchi ottici realizzati in differenti tecnologie multilayer, e progettati per l'utilizzo nel coronografo Metis, della missione ESA Solar Orbiter, e nella facility Laser ad Elettroni Liberi a Trieste.
Dopo aver studiato le bande nelle quali erano richiesti dall'applicazione precisi valori di riflettività, sono state effettuate misure per ogni campione nelle bande EUV, VIS e NIR utilizzando le facility a disposizione nei laboratori LUXOR. Saranno quindi presentati i principio di funzionamento dei dispositivi, quello delle facility utilizzate nonché il processo di misura, del quale saranno infine presentati e discussi i risultati.
Il lavoro era inserito in un più ampio progetto di caratterizzazione dei campioni, sviluppato dal team LUXOR. Includeva misure di riflettività in differenti bande spettrali, come i Soft-X, e analisi di superficie AFM, i cui dati saranno presentati per completezza.
Il fine di queste misure non era solo di stabilire se alcuni dei campioni soddisfacessero o meno le specifiche imposte dall'applicazione, ma anche di capire quale delle tecnologie usate dovesse eventualmente essere migliorata e testata per l'applicazione. I risultati, infine, stabiliscono una linea di base delle prestazioni dei campioni, che saranno esposti alle condizioni della loro applicazione (per.es. vento solare nello spazio) allo scopo di stabilire la loro perdita in prestazioni dovuta all'invecchiament
Wave turbulence of a rotating array of quantized vortices in the T → 0 temperature limit
No full textThe dynamics of quantized vortices in the zero temperature limit is currently of great interest, particularly in the case of the Fermi superfluid He-B. Here we study wave turbulence, generated by the librating motion of a rotating cylindrical container filled with He-B, in the limit of vanishing viscous forces at temperatures . The polarization of the quantized vortices with respect to the axis of rotation is measured using non-invasive NMR techniques. We observe a decrease of the polarization when the librating motion is started, and a two-stage relaxation process when the modulation of the rotation velocity is stopped. The first relaxation process is associated with the dissipation of large-scale flow stored in inertial waves and the solid body rotation of the vortex array. From the decay of these energy reservoirs we determine the rate of energy dissipation of large-scale flow. The later second process is related to the relaxation of Kelvin waves on individual vortices. This process is monitored by the recovery of the polarization. The existence of a Kelvin wave cascade at the lowest temperatures is currently a central open question. We supply some evidence for the cascade
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