527 research outputs found
Electron backscattering in a cavity: ballistic and coherent effects
Numerous experimental and theoretical studies have focused on low-dimensional
systems locally perturbed by the biased tip of a scanning force microscope. In
all cases either open or closed weakly gate-tunable nanostructures have been
investigated, such as quantum point contacts, open or closed quantum dots, etc.
We study the behaviour of the conductance of a quantum point contact with a
gradually forming adjacent cavity in series under the influence of a scanning
gate. Here, an initially open quantum point contact system gradually turns into
a closed cavity system. We observe branches and interference fringes known from
quantum point contacts coexisting with irregular conductance fluctuations.
Unlike the branches, the fluctuations cover the entire area of the cavity. In
contrast to previous studies, we observe and investigate branches under the
influence of the confining stadium potential, which is gradually built up. We
find that the branches exist only in the area surrounded by cavity top gates.
As the stadium shrinks, regular fringes originate from tip-induced
constrictions leading to quantized conduction. In addition, we observe arc-like
areas reminiscent of classical electron trajectories in a chaotic cavity. We
also argue that electrons emanating from the quantum point contact spread out
like a fan leaving branch-like regions of enhanced backscattering.Comment: 7 pages, 4 figure
Dynamic photoconductive gain effect in shallow-etched AlGaAs/GaAs quantum wires
We report on a dynamic photoconductive gain effect in quantum wires which are
lithographically fabricated in an AlGaAs/GaAs quantum well via a shallow-etch
technique. The effect allows resolving the one-dimensional subbands of the
quantum wires as maxima in the photoresponse across the quantum wires. We
interpret the results by optically induced holes in the valence band of the
quantum well which shift the chemical potential of the quantum wire. The
non-linear current-voltage characteristics of the quantum wires also allow
detecting the photoresponse effect of excess charge carriers in the conduction
band of the quantum well. The dynamics of the photoconductive gain are limited
by the recombination time of both electrons and holes
Nematic state of the FeSe superconductor
We study the crystal structure of the tetragonal iron selenide FeSe and its nematic phase transition to the low-temperature orthorhombic structure using synchrotron x-ray and neutron scattering analyzed in both real space and reciprocal space. We show that in the local structure the orthorhombic distortion associated with the electronically driven nematic order is more pronounced at short length scales. It also survives to temperatures above 90 K, where reciprocal-space analysis suggests tetragonal symmetry. Additionally, the real-space pair distribution function analysis of the synchrotron x-ray diffraction data reveals a tiny broadening of the peaks corresponding to the nearest Fe-Fe, nearest Fe-Se, and next-nearest Fe-Se bond distances as well as the tetrahedral torsion angles at a short length scale of 20 Ă…. This broadening appears below 20 K and is attributed to a pseudogap. However, we did not observe any further reduction in local symmetry below orthorhombic down to 3 K. Our results suggest that the superconducting gap anisotropy in FeSe is not associated with any symmetry-lowering short-range structural correlations
Energy scales in 4f1 delafossite magnets: crystal-field splittings larger than the strength of spin-orbit coupling in KCeO2
Ytterbium-based delafossites with effective S=1/2 moments are investigated
intensively as candidates for quantum spin-liquid ground states. While the
synthesis of related cerium compounds has also been reported,many important
details concerning their crystal, electronic, and magnetic structures are
unclear. Here we analyze the S=1/2 system KCeO2, combining complementary
theoretical methods. The lattice geometry was optimized and the band structure
investigated using density functional theory extended to the level of a GGA+U
calculation in order to reproduce the correct insulating behavior. The Ce 4f1
states were then analyzed in more detail with the help of ab initio
wave-function-based computations. Unusually large effective crystal-field
splittings of up to 320 meV are predicted, which puts KCeO2 in the strong field
coupling regime. Our results reveal a subtle interplay between ligand-cage
electrostatics and the trigonal field generated by the extended crystalline
surroundings, relevant in the context of recent studies on tuning the nature of
the ground-state wave function in 4f triangular-lattice and pyrochlore
compounds. It also makes KCeO2 an interesting model system in relation to the
effect of large crystal-field splittings on the anisotropy of intersite
exchange in spin-orbit coupled quantum magnets.Comment: 6 pages, 2 figures, and 3 table
The phase plane of moving discrete breathers
We study anharmonic localization in a periodic five atom chain with
quadratic-quartic spring potential. We use discrete symmetries to eliminate the
degeneracies of the harmonic chain and easily find periodic orbits. We apply
linear stability analysis to measure the frequency of phonon-like disturbances
in the presence of breathers and to analyze the instabilities of breathers. We
visualize the phase plane of breather motion directly and develop a technique
for exciting pinned and moving breathers. We observe long-lived breathers that
move chaotically and a global transition to chaos that prevents forming moving
breathers at high energies.Comment: 8 pages text, 4 figures, submitted to Physical Review Letters. See
http://www.msc.cornell.edu/~houle/localization
Optically induced transport properties of freely suspended semiconductor submicron channels
We report on optically induced transport phenomena in freely suspended
channels containing a two-dimensional electron gas (2DEG). The submicron
devices are fabricated in AlGaAs/GaAs heterostructures by etching techniques.
The photoresponse of the devices can be understood in terms of the combination
of photogating and a photodoping effect. The hereby enhanced electronic
conductance exhibits a time constant in the range of one to ten milliseconds
The electronic structure of the doped one-dimensional transition metal oxide Y1-xCaxBaNiO5 studied using x-ray absorption
A strong anisotropic distribution of the holes in Ni 3d and O 2p orbitals is
observed in the polarization dependent O1s and Ni2p3/2 x-ray absorption
spectroscopy of the linear-chain nickelate Y1-xCaxBaNiO5 (x = 0, 0.05, 0.1,
0.2), which demonstrates the one-dimensional nature of the electronic state in
these compounds. Furthermore, the additional holes introduced by Ca-doping
occupy both O 2p and Ni 3d orbitals along the NiO5 chains. By comparing the
experimental Ni 2p3/2 absorption spectra of Y1-xCaxBaNiO5 to those from charge
transfer multiplet calculations we can derive the orbital character of the
additional holes to be of ca. 60% O2p and ca. 40% Ni 3d.Comment: pdf only. Submitted to PR
Fractional Generalization of Gradient Systems
We consider a fractional generalization of gradient systems. We use
differential forms and exterior derivatives of fractional orders. Examples of
fractional gradient systems are considered. We describe the stationary states
of these systems.Comment: 11 pages, LaTe
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