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 Å. 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

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

Coupled Replicator Equations for the Dynamics of Learning in Multiagent Systems

Starting with a group of reinforcement-learning agents we derive coupled replicator equations that describe the dynamics of collective learning in multiagent systems. We show that, although agents model their environment in a self-interested way without sharing knowledge, a game dynamics emerges naturally through environment-mediated interactions. An application to rock-scissors-paper game interactions shows that the collective learning dynamics exhibits a diversity of competitive and cooperative behaviors. These include quasiperiodicity, stable limit cycles, intermittency, and deterministic chaos--behaviors that should be expected in heterogeneous multiagent systems described by the general replicator equations we derive.Comment: 4 pages, 3 figures, http://www.santafe.edu/projects/CompMech/papers/credlmas.html; updated references, corrected typos, changed conten

An Evaluation of Body-grip Trap Trigger Configurations for Reducing River Otter Take Incidental to Beaver Trapping

River otter (Lontra canadensis) populations in North America have been the focus of significant restoration efforts. Wildlife management agencies, concerned about the unintentional take of river otters incidental to beaver (Castor canadensis) trapping, may recommend techniques to avoid capturing river otters. River otter avoidance techniques that are ineffective or diminish trap performance for beavers are undesirable. We conducted a field evaluation in 2015 and 2016 in Wisconsin to assess how two trigger configurations (offset and center) on body-grip traps would affect the incidental capture rate of river otters during beaver trapping. We also evaluated effects of each configuration on beaver capture rates, body lengths, and anatomical locations of trap-jaw strikes. We used size 330 body-grip traps equipped with identical triggers and alternated between trigger configurations during beaver damage management activities. We captured 8 river otters with each trap trigger configuration. Trap-jaw strikes on beavers differed between trigger configurations, with offset triggers resulting in more abdomen strikes and center triggers causing more cervical vertebrae strikes. We found that an offset trigger configuration did not reduce incidental take of otters and was less effective for trapping beavers

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

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

Random Field and Random Anisotropy Effects in Defect-Free Three-Dimensional XY Models

Monte Carlo simulations have been used to study a vortex-free XY ferromagnet with a random field or a random anisotropy on simple cubic lattices. In the random field case, which can be related to a charge-density wave pinned by random point defects, it is found that long-range order is destroyed even for weak randomness. In the random anisotropy case, which can be related to a randomly pinned spin-density wave, the long-range order is not destroyed and the correlation length is finite. In both cases there are many local minima of the free energy separated by high entropy barriers. Our results for the random field case are consistent with the existence of a Bragg glass phase of the type discussed by Emig, Bogner and Nattermann.Comment: 10 pages, including 2 figures, extensively revise