678 research outputs found
Magneto-electrical subbands of freely suspended quantum point contacts
We present a versatile design of freely suspended quantum point contacts with
particular large one-dimensional subband quantization energies of up to 10meV.
The nanoscale bridges embedding a two-dimensional electron system are
fabricated from AlGaAs/GaAs heterostructures by electron-beam lithography and
etching techniques. Narrow constrictions define quantum point contacts that are
capacitively controlled via local in-plane side gates. Employing transport
spectroscopy, we investigate the transition from electrostatic subbands to
Landau-quantization in a perpendicular magnetic field. The large subband
quantization energies allow us to utilize a wide magnetic field range and
thereby observe a large exchange splitted spin-gap of the two lowest
Landau-levels
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
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
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
Laterally defined freely suspended quantum dots in GaAs/AlGaAs heterostructures
Free standing beams containing a two-dimensional electron system are shaped
from a GaAs/AlGaAs heterostructure. Quantum point contacts and (double) quantum
dots are laterally defined using metal top gates. We investigate the electronic
properties of these nanostructures by transport spectroscopy. Tunable localized
electron states in freely suspended nanostructures are a promising tool to
investigate the electron-phonon-interaction
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
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 electron-hole liquid in a polar semiconductor: Cubic SiC
The binding energy EB = (17 +/- 3) meV and density n = (9.2 +/- 1.7) x 1018 cm-3 of the EHL in cubic SiC are determined from excitation-dependent spectra. Comparing these values with ground state properties calculated with and without electron-phonon-interaction using newly determined valence band parameters evidence for the importance of e.p.i. in SiC is found.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23743/1/0000715.pd
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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