439 research outputs found
The quantized Hall conductance of a single atomic wire: A proposal based on synthetic dimensions
We propose a method by which the quantization of the Hall conductance can be
directly measured in the transport of a one-dimensional atomic gas. Our
approach builds on two main ingredients: (1) a constriction optical potential,
which generates a mesoscopic channel connected to two reservoirs, and (2) a
time-periodic modulation of the channel, specifically designed to generate
motion along an additional synthetic dimension. This fictitious dimension is
spanned by the harmonic-oscillator modes associated with the tightly-confined
channel, and hence, the corresponding "lattice sites" are intimately related to
the energy of the system. We analyze the quantum transport properties of this
hybrid two-dimensional system, highlighting the appealing features offered by
the synthetic dimension. In particular, we demonstrate how the energetic nature
of the synthetic dimension, combined with the quasi-energy spectrum of the
periodically-driven channel, allows for the direct and unambiguous observation
of the quantized Hall effect in a two-reservoir geometry. Our work illustrates
how topological properties of matter can be accessed in a minimal
one-dimensional setup, with direct and practical experimental consequences.
Conductance quantization and snake states in graphene magnetic waveguides
We consider electron waveguides (quantum wires) in graphene created by
suitable inhomogeneous magnetic fields. The properties of uni-directional snake
states are discussed. For a certain magnetic field profile, two spatially
separated counter-propagating snake states are formed, leading to conductance
quantization insensitive to backscattering by impurities or irregularities of
the magnetic field.Comment: 5 pages, 4 figures, final version accepted as Rapid Comm. in PR
Analyse zur Netto-Lebensmittelproduktion und zum AckerflÀchenbedarf von Rindermastsystemen
Die zukĂŒnftigen Rahmenbedingungen lassen eine steigende Ressourcenkonkurrenz zwischen Nahrungs- und Futtermittelproduktion erwarten. Gerade die Rindermast wird in diesem Zusammenhang sehr oft kritisch betrachtet. Das Ziel der vorliegenden Arbeit war es daher, unterschiedlich intensive Rindermastsysteme hinsichtlich ihres AckerflĂ€chenbedarfs und ihres Beitrags zur Netto- Lebensmittelproduktion zu untersuchen. Die Analysen beruhen auf Daten eines Mastversuchs mit Kalbinnen, Ochsen und Stieren der Rasse Fleckvieh auf Grassilage- bzw. Maissilagebasis. Die Tiere der Grassilagegruppen wurden auf drei unterschiedlichen Kraftfutterniveaus (extensiv, niedrig, hoch) und die Maissilagegruppen auf hohem Kraftfutterniveau gemĂ€stet.
Die Lebensmittelkonversionseffizienz (LKE, humanernĂ€hrungstauglicher Output/ humanernĂ€hrungstauglicher Input) verringert sich bei den Grassilage- FĂŒtterungsgruppen mit steigender Kraftfuttergabe und war fĂŒr die Maissilage- FĂŒtterungsgruppen sowohl auf Basis Bruttoenergie als auch Rohprotein an niedrigsten. Im Mittel ĂŒber alle Versuchsgruppen lag die LKE-Energie mit 0,29 (0,6-0,56) und LKE- Protein mit 0,44 (0,21- 0,87) unter den derzeitigen Bedingungen deutlich unter 1, was einer negativen Netto- Lebensmittelproduktion entspricht. Allerdings muss berĂŒcksichtigt werden, dass die ProteinqualitĂ€t auf der Output- Seite um den Faktor 1,5 bis 1,9 höher war als auf der Input- Seite. Kombiniert man diese qualitativen Unterschiede mit den quantitativen Ănderungen im humanernĂ€hrungstauglichen Protein, so steigern extensiv gefĂŒtterte Kalbinnen und Ochsen die Wertigkeit des Proteins FĂŒr die menschliche ErnĂ€hrung. Mit steigender FĂŒtterungsintensitĂ€t stieg die NĂ€hrstoffversorgung ĂŒber Futtermittel von AckerflĂ€chen in den Grassilagegruppen deutlich an und der höchste AckerflĂ€chenbedarf ergab sich in den Maissilagegruppen
Electron Correlations in a Quantum Dot with Bychkov-Rashba Coupling
We report on a theoretical approach developed to investigate the influence of
Bychkov-Rashba interaction on a few interacting electrons confined in a quantum
dot. We note that the spin-orbit coupling profoundly influences the energy
spectrum of interacting electrons in a quantum dot. Inter-electron interaction
causes level crossings in the ground state and a jump in magnetization. As the
coupling strength is increased, that jump is shifted to lower magnetic fields.
Low-field magnetization will therefore provide a direct probe of the spin-orbit
coupling strength in a quantum dot
Strain Relaxation in Graded InGaAs and InP Buffer Layers on GaAs (001)
We investigate compositionally graded Inxoâ€xâ€0.5Ga1-xAs and InP buffer layers which are prepared by molecular beam epitaxy on (001) GaAs substrate. The initial In content xo is equal to 0, 0.12, 0.18, 0.24, and 0.5 for the different samples. The In composition of the graded buffer increases linearly between xo and 0.5 with a fixed slope of 50% In-content per ÎŒm. The idea was to combine the advantage of surface flatness in homogeneous buffer layers and the reduced density of threading dislocations on the surface for graded buffer layers. The best compromise in terms of photoluminescence intensity and linewidth, electron mobility and crystal quality is achieved for xo = 0.18. For comparison to the InGaAs layers, we investigated also homogenous InP buffer layers on GaAs substrate. A strong photoluminescence peak with a linewidth of 5 meV is observed for 1 ÎŒm InP grown at 450°C applying a GaP decomposition source. The density of threading dislocations in the surface region is lower than in relaxed In0.5Ga0.5As layers but still by far not as low as for the graded buffer layers
Bosonization of strongly interacting electrons
Strong repulsive interactions in a one-dimensional electron system suppress
the exchange coupling J of electron spins to a value much smaller than the
Fermi energy E_F. The conventional theoretical description of such systems
based on the bosonization approach and the concept of Tomonaga-Luttinger liquid
is applicable only at energies below J. In this paper we develop a theoretical
approach valid at all energies below the Fermi energy, including a broad range
of energies between J and E_F. The method involves bosonization of the charge
degrees of freedom, while the spin excitations are treated exactly. We use this
technique to calculate the spectral functions of strongly interacting electron
systems at energies in the range J<<epsilon<< E_F$. We show that in addition to
the expected features at the wavevector k near the Fermi point k_F, the
spectral function has a strong peak centered at k=0. Our theory also provides
analytical description of the spectral function singularities near 3k_F (the
"shadow band" features).Comment: 21 pages, 4 figure
Tomonaga-Luttinger liquid parameters of magnetic waveguides in graphene
Electronic waveguides in graphene formed by counterpropagating snake states in suitable inhomogeneous magnetic fields are shown to constitute a realization of a Tomonaga-Luttinger liquid. Due to the spatial separation of the right- and left-moving snake states, this non-Fermi liquid state induced by electron-electron interactions is essentially unaffected by disorder. We calculate the interaction parameters accounting for the absence of Galilei invariance in this system, and thereby demonstrate that non-Fermi liquid effects are significant and tunable in realistic geometries
Effective charge-spin models for quantum dots
It is shown that at low densities, quantum dots with few electrons may be
mapped onto effective charge-spin models for the low-energy eigenstates. This
is justified by defining a lattice model based on a many-electron pocket-state
basis in which electrons are localised near their classical ground-state
positions. The equivalence to a single-band Hubbard model is then established
leading to a charge-spin () model which for most geometries reduces to a
spin (Heisenberg) model. The method is refined to include processes which
involve cyclic rotations of a ``ring'' of neighboring electrons. This is
achieved by introducing intermediate lattice points and the importance of ring
processes relative to pair-exchange processes is investigated using high-order
degenerate perturbation theory and the WKB approximation. The energy spectra
are computed from the effective models for specific cases and compared with
exact results and other approximation methods.Comment: RevTex, 24 pages, 7 figures submitted as compressed and PostScript
file
Signatures of electron correlations in the transport properties of quantum dots
The transition matrix elements between the correlated and
electron states of a quantum dot are calculated by numerical diagonalization.
They are the central ingredient for the linear and non--linear transport
properties which we compute using a rate equation. The experimentally observed
variations in the heights of the linear conductance peaks can be explained. The
knowledge of the matrix elements as well as the stationary populations of the
states allows to assign the features observed in the non--linear transport
spectroscopy to certain transition and contains valuable information about the
correlated electron states.Comment: 4 pages (revtex,27kB) + 3 figures in one file ziped and uuencoded
(postscript,33kB), to appear in Phys.Rev.B as Rapid Communicatio
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