We investigate the effects of transition metals ͑TM͒ on the electronic doping and scattering in graphene using molecular-beam epitaxy combined with in situ transport measurements. The room-temperature deposition of TM onto graphene produces clusters that dope n type for all TM investigated ͑Ti, Fe, and Pt͒. We also find that the scattering by TM clusters exhibits different behavior compared to 1 / r Coulomb scattering. At high coverage, Pt films are able to produce doping that is either n type or weakly p type, which provides experimental evidence for a strong interfacial dipole favoring n-type doping as predicted theoretically.

When high quality bismuth or graphite crystals are placed in a magnetic field directed along the c axis (trigonal axis for bismuth) and the temperature is lowered, the resistance increases as it does in an insulator but then saturates. We show that the combination of unusual features specific to semimetals, i.e., low carrier density, small effective mass, high purity, and an equal number of electrons and holes (compensation), gives rise to a unique ordering and spacing of three characteristic energy scales, which not only is specific to semimetals but which concomitantly provides a wide window for the observation of apparent field-induced metal-insulator behavior. Using magnetotransport and Hall measurements, the details of this unusual behavior are captured with a conventional multiband model, thus confirming the occupation by semimetals of a unique niche between conventional metals and semiconductors.

We present a gauge-invariant effective action for the Abelian Higgs model (scalar electrodynamics) with a chemical potential µ on a 1+1 dimensional lattice. This formulation provides an expansion in the hopping parameter κ which we test with Monte Carlo simulations for a broad range of the inverse gauge coupling β pl and small values of the scalar self-coupling λ. In the opposite limit of infinitely large λ, the partition function can be written as a traced product of local tensors which allows us to write exact blocking formulas. Their numerical implementation requires truncations but there is no sign problem for arbitrary values of µ. We show that the time continuum limit of the blocked transfer matrix can be obtained numerically and, in the limit of infinite β pl and with a spin-1 truncation, the small volume energy spectrum is identical to the low energy spectrum of a two-species Bose-Hubbard model in the limit of large onsite repulsion. We extend this procedure for finite β pl and derive a spin-1 approximation of the Hamiltonian. It involves new terms corresponding to transitions among the two species in the Bose-Hubbard model. We propose an optical lattice implementation involving a ladder structure.

We examine theoretically the signatures of magnetic adatoms in graphene probed by scanning tunneling spectroscopy (STS). When the adatom hybridizes equally with the two graphene sublattices, the broadening of the local adatom level is anomalous and can scale with the cube of the energy. In contrast to ordinary metal surfaces, the adatom local moment can be suppressed by the proximity of the probing scanning tip. We propose that the dependence of the tunneling conductance on the distance between the tip and the adatom can provide a clear signature for the presence of local magnetic moments. We also show that tunneling conductance can distinguish whether the adatom is located on top of a carbon atom or in the center of a honeycomb hexagon.

We connect explicitly the classical O(2) model in 1+1 dimensions, a model sharing important features with U (1) lattice gauge theory, to physical models potentially implementable on optical lattices and evolving at physical time. Using the tensor renormalization group formulation, we take the time continuum limit and check that finite dimensional projections used in recent proposals for quantum simulators provide controllable approximations of the original model. We propose two-species Bose-Hubbard models corresponding to these finite dimensional projections at strong coupling and discuss their possible implementations on optical lattices using a 87 Rb and 41 K Bose-Bose mixture.