Physics

There has long been a discrepancy between the size distributions of Ar + n clusters measured by different groups regarding whether or not magic numbers appear at sizes corresponding to the closure of icosahedral (sub-)shells. We show that the previously observed magic cluster size distributions are likely the result of an unresolved Ar n H + component, that is, from protonated argon clusters. We find that the proton impurity gives cluster geometries that are much closer to those for neutral rare gas clusters, which are known to form icosahedral structures, than the pure cationic clusters, explaining why the mass spectra from protonated argon clusters better matches these structural models. Our results thus show that even small impurities, e.g.\ a single proton, can significantly influence the properties of clusters.

We investigate the role of exchange bridges in molecular magnets. We explore their effects on the distribution of the valence electrons and their contribution to the exchange processes. The present study is focused on a spin-half dimer with nonequivalent exchange bridges. Here, we derive an effective Hamiltonian that allows for an accurate estimation of the observables associated to the magnetic properties of the magnet. Our results are compared to those obtained by means of the conventional Heisenberg model that usually fails.

The magnetic moment of a single impurity atom in a finite free electron gas is studied in a combined x-ray magnetic circular dichroism spectroscopy and density functional theory study of size-selected free chromium-doped gold clusters. The observed size-dependence of the local magnetic moment can essentially be understood in terms of the Anderson impurity model. Electronic shell closure in the host metal minimizes the interaction of localized impurity states with the confined free electron gas and preserves the full magnetic moment of $\unit[5]{\mu_B}$ in CrAu + 2 and CrAu + 6 clusters. Even for open-shell species, large local moments are observed that scale with the energy gap of the gold cluster. This indicates that an energy gap in the free electron gas generally stabilizes the local magnetic moment of the impurity.

We study the magnetic excitations of the trimeric magnetic compounds A 3 Cu 3 (PO 4 ) 4 (A = Ca, Sr, Pb). The spectra are analyzed in terms of the Heisenberg model and a generic spin Hamiltonian that accounts for the changes in valence electrons distribution along the bonds among magnetic ions. The analytical results obtained in the framework of both Hamiltonians are compared to each other and to the available experimental measurements. The results based on our model show better agreement with the experimental data than those obtained with the aid of the Heisenberg model. For all trimers, our analysis reveals the existence of one thin energy band referring to the flatness of observed excitation peaks.

Here we follow, both experimentally and theoretically, the development of magnetism in Tb clusters from the atomic limit, adding one atom at a time. The exchange interaction is, surprisingly, observed to drastically increase compared to that of bulk, and to exhibit irregular oscillations as a function of the interatomic distance. From electronic structure theory we find that the theoretical magnetic moments oscillate with cluster size in exact agreement with experimental data. Unlike the bulk, the oscillation is not caused by the RKKY mechanism. Instead, the inter-atomic exchange is shown to be driven by a competition between wave-function overlap of the 5d shell and the on-site exchange interaction, which leads to a competition between ferromagnetic double-exchange and antiferromagnetic super-exchange. This understanding opens up new ways to tune the magnetic properties of rare-earth based magnets with nano-sized building blocks.

We experimentally observe the action of multiple light pulses on the transverse motion of a continuous beam of fullerenes. The light potential is generated by non-resonant ultra-short laser pulses in perpendicular spatial overlap with the molecule beam. We observe a small but clear enhancement of the number of molecules in the center fraction of the molecular beam. Relatively low light intensity and short laser pulse duration prevent the molecule from fragmentation and ionization. Experimental results are confirmed by Monte Carlo trajectory simulations.

Three C60 fragmentation regimes in fullerite bombarded by Cs+ are identified as a function of its energy. C2 is the major species sputtered at all energies. For E(Cs+) < 1 keV C2 emissions dominate. C2 and C1 have highest intensities between 1 and 3 keV with increasing contributions from C3 and C4. Intensities of all fragments maximize around 2 keV. Above 3 keV, fragments densities stabilize. The roles of and the contributions from direct recoils and collision cascades are determined. Maximum direct recoil energy delivered to the C60 fullerite cage is 210 eV at which only C2 emissions occur is identified and an explanation provided. The three fragmentation regimes under continued Cs+ bombardment eventually lead to complete destruction of the C60 cages transforming fullerite into amorphous carbon

The atom-photon entanglement of dressed atom and its spontaneous emission in a Double-Lambda closed-loop atomic system is studied in multi-photon resonance condition. It is shown that, even in the absence of quantum interference due to the spontaneous emission, the von Neumann entropy is phase-sensitive and it can be controlled by either intensity or relative phase of the applied fields. It is demonstrated that, for the special case of Rabi frequency of the applied fields the system is maximally entangled. Moreover, the open-loop configuration is considered and it is shown that the degree of entanglement measure (DEM) can be controlled by intensity of the applied fields. Furthermore, in electromagnetically induced transparency condition, the system is disentangled. Such a system can be used for quantum information processing via entanglement using optical switching.

Relaxation modes are the collective modes in which all probability deviations from equilibrium states decay with the same relaxation rates. In contrast, a first passage time is the required time for arriving for the first time from one state to another. In this paper, we discuss how and why the slowest relaxation rates of relaxation modes are reconstructed from the first passage times. As an illustrative model, we use a continuous-time Markov state model of vacancy diffusion in KCl nanoclusters. Using this model, we reveal that all characteristics of the relaxations in KCl nanoclusters come from the fact that they are hybrids of two kinetically different regions of the fast surface and slow bulk diffusions. The origin of the different diffusivities turns out to come from the heterogeneity of the activation energies on the potential energy landscapes. We also develop a stationary population method to compute the mean first passage times as mean times required for pair annihilations of particle-hole pairs, which enables us to obtain the symmetric results of relaxation rates under the exchange of the sinks and the sources. With this symmetric method, we finally show why the slowest relaxation times can be reconstructed from the mean first passage times.

We utilize two-photon high-precision microwave spectroscopy of ng→(n+2)g transitions to precisely measure the high-angular-momentum g -series quantum defect of 85 Rb. Samples of cold Rydberg atoms in the ng state are prepared via a three-photon optical excitation combined with controlled electric-field mixing and probed with 40- μ s-long microwave interaction pulses. The leading systematic uncertainty arises from DC Stark shifts, which is addressed by a cancellation of background electric fields in all three dimensions. From our measurements and an analysis of systematic uncertainties from DC and AC Stark shifts, van der Waals interactions, and microwave frequency calibration, we obtain δ 0 =0.0039990(21) and δ 2 =−0.0202(21) . We discuss our results in context with recent work elsewhere, as well as applications towards precision measurement.