Dark Monopoles in Grand Unified Theories

We consider a Yang-Mills-Higgs theory with gauge group $G=SU(n)$ broken to $G_{v} = [SU(p)\times SU(n-p)\times U(1)]/Z$ by a Higgs field in the adjoint representation. We obtain monopole solutions whose magnetic field is not in the Cartan Subalgebra. Since their magnetic field vanishes in the direction of the generator of the electromagnetic group $U(1)_{em}$, we call them Dark Monopoles. These Dark Monopoles must exist in some Grand Unified Theories (GUTs) without the need to introduce a dark sector. We analyze the particular case of $SU(5)$ GUT, where we obtain that their mass is $M = 4\pi v \widetilde{E}(\lambda/e^{2})/e$, where $\widetilde{E}(\lambda/e^{2})$ is a monotonically increasing function of $\lambda/e^{2}$ with $\widetilde{E}(0)=1.294$ and $\widetilde{E}(\infty)=3.262.$ We also give a geometrical interpretation to their non-abelian magnetic charge.Comment: 22 pages; added some comments on possible cosmological implications of Dark Monopoles in the last section and added some references. Published Versio

Ab initio study of charge transport through single oxygen molecules in atomic aluminum contacts

We present ab initio calculations of transport properties of atomic-sized aluminum contacts in the presence of oxygen. The experimental situation is modeled by considering a single oxygen atom (O) or one of the molecules O2 and O3 bridging the gap between electrodes forming ideal, atomically sharp pyramids. The transport characteristics are computed for these geometries with increasing distances between the leads, simulating the opening of a break junction. To facilitate comparison with experiments further, the vibrational modes of the oxygen connected to the electrodes are studied. It is found that in the contact regime the change of transport properties due to the presence of oxygen is strong and should be detectable in experiments. All three types of oxygen exhibit a comparable behavior in their vibrational frequencies and conductances, which are well below the conductance of pure aluminum atomic contacts. The conductance decreases for an increasing number of oxygen atoms. In the tunneling regime the conductance decays exponentially with distance and the decay length depends on whether or not oxygen is present in the junction. This fact may provide a way to identify the presence of a gas molecule in metallic atomic contacts.Comment: 8 pages, 9 figures; added appendi