Designing Quantum Spin-Orbital Liquids in Artificial Mott Insulators

Quantum spin-orbital liquids are elusive strongly correlated states of matter that emerge from quantum frustration between spin and orbital degrees of freedom. A promising route towards the observation of those states is the creation of artificial Mott insulators where antiferromagnetic correlations between spins and orbitals can be designed. We show that Coulomb impurity lattices on the surface of gapped honeycomb substrates, such as graphene on SiC, can be used to simulate SU(4) symmetric spin-orbital lattice models. We exploit the property that massive Dirac fermions form mid-gap bound states with spin and valley degeneracies in the vicinity of a Coulomb impurity. Due to electronic repulsion, the antiferromagnetic correlations of the impurity lattice are driven by a super-exchange interaction with SU(4) symmetry, which emerges from the bound states degeneracy at quarter filling. We propose that quantum spin-orbital liquids can be engineered in artificially designed solid-state systems at vastly higher temperatures than achievable in optical lattices with cold atoms. We discuss the experimental setup and possible scenarios for candidate quantum spin-liquids in Coulomb impurity lattices of various geometries.Comment: 9 pages + supplementary materials, 4 figures; v2: published version, minor changes, references adde

Energy Levels Of Hydrogen-Like Atomsand Fundamental Constants

The present review includes the description of theoretical methods for the investigations of the spectra of hydrogen-like systems. Various versions of the quasipotential approach and the method of the effective Dirac equation are considered. The new methods, which have been developed in the eighties, are described. These are the method for the investigation of the spectra by means of the quasipotential equation with the relativistic reduced mass and the method for a selection of the logarithmic corrections by means of the renormalization group equation. The special attention is given to the construction of a perturbation theory and the selection of graphs, whereof the contributions of different orders of $\alpha$, the fine structure constant, to the energy of the fine and hyperfine splitting in a positronium, a muonium and a hydrogen atom could be calculated. In the second part of this article the comparison of the experimental results and the theoretical results concerning the wide range of topics is produced. They are the fine and hyperfine splitting in the hydrogenic systems, the Lamb shift and the anomalous magnetic moments of an electron and a muon. Also, the problem of the precision determination of a numerical value of the fine structure constant, connected with the above topics, is discussed.Comment: LaTeX file, 68 pp. (figures are available on request

Decay Rate of a Positronium. Review of Theory and Experiment

The present status of theoretical and experimental investigations of the decay rate of a positronium is considered. The increasing interest to this problem has been caused by the disagreement of the calculated value of $\Gamma_3 (o-Ps)$ and the recent series of precise experiments. The necessity of new calculations on the basis of the quantum field methods in bound state theory is pointed out with taking into account the dependence of the interaction kernel on relative energies.Comment: LaTeX file, 9 pp., Preprint IFUNAM FT-93-01

Relativistic peculiarities at stepped surfaces: surprising energetics and unexpected diffusion patterns

We revive intriguing, yet still unexplained, experimental results of Ehrlich and co-workers [ Phys. Rev. Lett. 77 1334 (1996); Phys. Rev. Lett. 67 2509 (1991)] who have observed, that 5d adatoms distributed on (111) surface islands of 5d metals favor the adsorption at the cluster's edge rather than at the cluster's interior, which lies in contrast with the behavior of 4d and 3d elements. Our state of the art ab initio calculations demonstrate that such behavior is a direct consequence of the relativity of 5d metals.Comment: 5 pages, 5 figure

Spiral Spin Order and Transport Anisotropy in Underdoped Cuprates

We discuss the spiral spin density wave model and its application to explain properties of underdoped La$_{2-x}$Sr$_x$CuO$_4$. We argue that the spiral picture is theoretically well justified in the context of the extended $t-J$ model, and then show that it can explain a number of observed features, such as the location and symmetry of the incommensurate peaks in elastic neutron scattering, as well as the in-plane resistivity anisotropy. A consistent description of the low doping region (below 10% or so) emerges from the spiral formulation, in which the holes show no tendency towards any type of charge order and the physics is purely spin driven.Comment: 6 pages, 3 figures; Proceedings of the International Workshop on Effective Models for Low-Dimensional Strongly Correlated Systems, September 2005, Peyresq, Franc

Theory of Anisotropic Hopping Transport due to Spiral Correlations in the Spin-Glass Phase of Underdoped Cuprates

We study the in-plane resistivity anisotropy in the spin-glass phase of the high-$T_{c}$ cuprates, on the basis of holes moving in a spiral spin background. This picture follows from analysis of the extended $t-J$ model with Coulomb impurities. In the variable-range hopping regime the resistivity anisotropy is found to have a maximum value of around 90%, and it decreases with temperature, in excellent agreement with experiments in La$_{2-x}$Sr$_x$CuO$_4$. In our approach the transport anisotropy is due to the non-collinearity of the spiral spin state, rather than an intrinsic tendency of the charges to self-organize.Comment: 5 pages, 4 figures; expanded versio

Valley Order and Loop Currents in Graphene on Hexagonal Boron Nitride

In this letter, we examine the role of Coulomb interactions in the emergence of macroscopically ordered states in graphene supported on hexagonal boron nitride substrates. Due to incommensuration effects with the substrate, graphene can develop gapped low energy modes that spatially conform into a triangular superlattice of quantum rings. In the presence of these modes, we show that Coulomb interactions lead to spontaneous formation of chiral loop currents in bulk and to macroscopic spin-valley order at zero temperature. We show that this exotic state breaks time reversal symmetry and can be detected with interferometry and polar Kerr measurements.Comment: 4.5 pages + supplementary materials; additional changes and references include

Gate control of spin-polarized conductance in alloyed transition metal nano-contacts

To date, endeavors in nanoscale spintronics are dominated by the use of single-electron or single-spin transistors having at their heart a semiconductor, metallic or molecular quantum dot who's localized states are non-spin-degenerate and can be controlled by an external bias applied via a gate electrode. Adjusting the bias of the gate one can realign those states with respect to the chemical potentials of the leads and thus tailor the spin-polarized transmission properties of the device. Here we show that similar functionality can be achieved in a purely metallic junction comprised of a metallic magnetic chains attached to metallic paramagnetic leads and biased by a gate electrode. Our ab initio calculations of electron transport through mixed Pt-Fe (Fe-Pd and Fe-Rh) atomic chains suspended between Pt (Pd and Rh) electrodes show that spin-polarized confined states of the chain can be shifted by the gate bias causing a change in the relative contributions of majority and minority channels to the nano-contact's conductance. As a result, we observe strong dependence of conductance spin-polarization on the applied gate potential. In some cases the spin-polarization of conductance can even be reversed in sign upon gate potential application, which is a remarkable and promising trait for spintronic applications.Comment: 11 pages, 12 figure