Spin fluctuations in the 0.7-anomaly in quantum point contacts

It has been argued that the 0.7 anomaly in quantum point contacts (QPCs) is due to an enhanced density of states at the top of the QPC-barrier (van Hove ridge), which strongly enhances the effects of interactions. Here, we analyze their effect on dynamical quantities. We find that they pin the van Hove ridge to the chemical potential when the QPC is subopen; cause a temperature dependence for the linear conductance that qualitatively agrees with experiment; strongly enhance the magnitude of the dynamical spin susceptibility; and significantly lengthen the QPC traversal time. We conclude that electrons traverse the QPC via a slowly fluctuating spin structure of finite spatial extent

Reversible Jump Metropolis Light Transport using Inverse Mappings

We study Markov Chain Monte Carlo (MCMC) methods operating in primary sample space and their interactions with multiple sampling techniques. We observe that incorporating the sampling technique into the state of the Markov Chain, as done in Multiplexed Metropolis Light Transport (MMLT), impedes the ability of the chain to properly explore the path space, as transitions between sampling techniques lead to disruptive alterations of path samples. To address this issue, we reformulate Multiplexed MLT in the Reversible Jump MCMC framework (RJMCMC) and introduce inverse sampling techniques that turn light paths into the random numbers that would produce them. This allows us to formulate a novel perturbation that can locally transition between sampling techniques without changing the geometry of the path, and we derive the correct acceptance probability using RJMCMC. We investigate how to generalize this concept to non-invertible sampling techniques commonly found in practice, and introduce probabilistic inverses that extend our perturbation to cover most sampling methods found in light transport simulations. Our theory reconciles the inverses with RJMCMC yielding an unbiased algorithm, which we call Reversible Jump MLT (RJMLT). We verify the correctness of our implementation in canonical and practical scenarios and demonstrate improved temporal coherence, decrease in structured artifacts, and faster convergence on a wide variety of scenes

Coannihilation without chemical equilibrium

Chemical equilibrium is a commonly made assumption in the freeze-out calculation of coannihilating dark matter. We explore the possible failure of this assumption and find a new conversion-driven freeze-out mechanism. Considering a representative simplified model inspired by supersymmetry with a neutralino- and sbottom-like particle we find regions in parameter space with very small couplings accommodating the measured relic density. In this region freeze-out takes place out of chemical equilibrium and dark matter self-annihilation is thoroughly inefficient. The relic density is governed primarily by the size of the conversion terms in the Boltzmann equations. Due to the small dark matter coupling the parameter region is immune to direct detection but predicts an interesting signature of disappearing tracks or displaced vertices at the LHC. Unlike freeze-in or superWIMP scenarios, conversion-driven freeze-out is not sensitive to the initial conditions at the end of reheating.Comment: 12 pages + references, 10 figures; v2: Discussion of kinetic equilibrium extended, matches published versio

A global fit of the γ\gamma-ray galactic center excess within the scalar singlet Higgs portal model

We analyse the excess in the $\gamma$-ray emission from the center of our galaxy observed by Fermi-LAT in terms of dark matter annihilation within the scalar Higgs portal model. In particular, we include the astrophysical uncertainties from the dark matter distribution and allow for unspecified additional dark matter components. We demonstrate through a detailed numerical fit that the strength and shape of the $\gamma$-ray spectrum can indeed be described by the model in various regions of dark matter masses and couplings. Constraints from invisible Higgs decays, direct dark matter searches, indirect searches in dwarf galaxies and for $\gamma$-ray lines, and constraints from the dark matter relic density reduce the parameter space to dark matter masses near the Higgs resonance. We find two viable regions: one where the Higgs-dark matter coupling is of ${\cal O}(10^{-2})$, and an additional dark matter component beyond the scalar WIMP of our model is preferred, and one region where the Higgs-dark matter coupling may be significantly smaller, but where the scalar WIMP constitutes a significant fraction or even all of dark matter. Both viable regions are hard to probe in future direct detection and collider experiments.Comment: 20 pages + references, 12 figures; v2: minor changes in presentation, references added, improved scan coverage and updated plots in figs. 6, 8, 9, 10 and 12 accordingly, conclusions unchanged, matches journal versio

Yu-Shiba-Rusinov States of Molecules on Pb(100)

(Metal-)Phthalocyanine molecules adsorbed on a superconducting Pb(100) surface are investigated with low-temperature scanning tunneling microscopy (STM). Although the molecules are originally diamagnetic, they acquire a net magnetic moment upon assembly into supramolecules or within self-assembled islands. This effect is revealed by the observation of Yu-Shiba-Rusinov (YSR) resonances on the respective molecules. H2-phthalocyanine (H2Pc) supramolecules are manually assembled on the surface via manipulation with the STM tip. It is shown that the lowest unoccupied molecular orbital (LUMO) of the central molecule shifts toward the Fermi level and thus becomes partially filled, which results in the net magnetic moment responsible for the YSR state. A phenomenological model is developed that explains the underlying molecular interactions. The magnetic moments are furthermore finely tuned by targeted repositioning of the inner hydrogen atoms of neighboring H2Pc molecules. In addition, lead phthalocyanine (PbPc) molecules on Pb(100) are investigated. The influence of different adsorption geometries and neighbor molecules is studied in detail, revealing a possible influence of the electrostatic dipole moments of molecular neighbors. The sharp YSR resonances are utilized for inelastic electron tunneling spectroscopy (IETS) of molecular vibrations of PbPc. A significantly higher sensitivity is obtained compared to earlier experiments, and thus 46 separate vibrational excitations are observed. Good agreement is found in comparison with on-surface DFT calculations allowing an assignment of the observed modes. The high spectral energy resolution is used to study energy shifts of the modes caused by different influences. Supplementary, three other molecules carrying YSR states are investigated. Finally, the construction, test, and redesign of a home-built non-evaporable getter (NEG) pump is presented