Singlet Polaron Theory of Low-Energy Optical Excitations in NiPS_{3}.

We develop a theory that explains the low-energy optical excitations near 1.5 eV observed by optical experiments in NiPS_{3}. Using ab initio methods, we construct a two-band Hubbard model for two effective Ni orbitals.

Iron phthalocyanine on Au(111) is a "non-Landau" Fermi liquid.

The paradigm of Landau's Fermi liquid theory has been challenged with the finding of a strongly interacting Fermi liquid that cannot be adiabatically connected to a non-interacting system. A spin-1 two-channel Kondo impurity with anisotropy D has a quantum phase transition between two topologically different Fermi liquids with a peak (dip) in the Fermi level for D < D (D > D). Extending this theory to general multi-orbital problems with finite magnetic field, we reinterpret in a unified and consistent fashion several experimental studies of iron phthalocyanine molecules on Au(111) that were previously described in disconnected and conflicting ways.

Theory of Differential Conductance of Co on Cu(111) Including Co s and d Orbitals, and Surface and Bulk Cu States.

We revisit the theory of the Kondo effect observed by a scanning-tunneling microscope (STM) for transition-metal atoms (TMAs) on noble-metal surfaces, including d and s orbitals of the TMA, surface and bulk conduction states of the metal, and their hopping to the tip of the STM. Fitting the experimentally observed STM differential conductance for Co on Cu(111) including both the Kondo feature near the Fermi energy and the resonance below the surface band, we conclude that the STM senses mainly the Co s orbital and that the Kondo antiresonance is due to interference between states with electrons in the s orbital and a localized d orbital mediated by the conduction states.

Tomography of Zero-Energy End Modes in Topological Superconducting Wires.

We describe the Majorana zero modes in topological hybrid superconductor-semiconductor wires with spin-orbit coupling and magnetic field, in terms of generalized Bloch coordinates φ,θ,δ. When the spin-orbit coupling and the magnetic field are perpendicular, φ and δ are universal in an appropriate coordinate system. We show how to extract the angle θ from the behavior of the Josephson current-phase relation, which enables tomography of the Majorana modes.

Destructive quantum interference in transport through molecules with electron-electron and electron-vibration interactions.

We study the transport through a molecular junction exhibiting interference effects. We show that these effects can still be observed in the presence of molecular vibrations if Coulomb repulsion is taken into account. In the Kondo regime, the conductance of the junction can be changed by several orders of magnitude by tuning the levels of the molecule, or displacing a contact between two atoms, from nearly perfect destructive interference to values of the order of 2e /h expected in Kondo systems.

Ginzburg-Landau theory for the magnetic and structural transitions in La (Ca Sr ) MnO.

We present a phenomenological theory for the ferromagnetic transition temperature, the magnetic susceptibility at high temperatures, and the structural distortion in the La[Formula: see text](Ca[Formula: see text]Sr[Formula: see text])[Formula: see text]MnO[Formula: see text] system. We construct a Ginzburg-Landau free energy that describes the magnetic and the structural transitions, and a competition between them. The parameters of the magnetic part of the free energy are derived from a mean-field solution of the magnetic interaction for arbitrary angular momentum.

Two-stage three-channel Kondo physics for an FePc molecule on the Au(1 1 1) surface.

We study an impurity Anderson model to describe an iron phthalocyanine (FePc) molecule on Au(1 1 1), motivated by previous results of scanning tunneling spectroscopy (STS) and theoretical studies. The model hybridizes a spin doublet consisting in one hole at the [Formula: see text] orbital of iron and two degenerate doublets corresponding to one hole either in the 3d or in the 3d orbital (called π orbitals) with two degenerate Hund-rule triplets with one hole in the 3d orbital and another one in a π orbital. We solve the model using a slave-boson mean-field approximation (SBMFA).

Generalized One-Band Model Based on Zhang-Rice Singlets for Tetragonal CuO.

Tetragonal CuO (T-CuO) has attracted attention because of its structure similar to that of the cuprates. It has been recently proposed as a compound whose study can give an end to the long debate about the proper microscopic modeling for cuprates. In this work, we rigorously derive an effective one-band generalized t-J model for T-CuO, based on orthogonalized Zhang-Rice singlets, and make an estimative calculation of its parameters, based on previous ab initio calculations.

Leading temperature dependence of the conductance in Kondo-correlated quantum dots.

Using renormalized perturbation theory in the Coulomb repulsion, we derive an analytical expression for the leading term in the temperature dependence of the conductance through a quantum dot described by the impurity Anderson model, in terms of the renormalized parameters of the model. Taking these parameters from the literature, we compare the results with published ones calculated using the numerical renormalization group obtaining a very good agreement. The approach is superior to alternative perturbative treatments.

Fractional Spin and Josephson Effect in Time-Reversal-Invariant Topological Superconductors.

Time-reversal-invariant topological superconducting (TRITOPS) wires are known to host a fractional spin ℏ/4 at their ends. We investigate how this fractional spin affects the Josephson current in a TRITOPS-quantum dot-TRITOPS Josephson junction, describing the wire in a model that can be tuned between a topological and a nontopological phase. We compute the equilibrium Josephson current of the full model by continuous-time Monte Carlo simulations and interpret the results within an effective low-energy theory.

Singlet Orbital Ordering in Bilayer Sr_{3}Cr_{2}O_{7}.

We perform an extensive study of Sr_{3}Cr_{2}O_{7}, the n=2 member of the Ruddlesden-Popper Sr_{n+1}Cr_{n}O_{3n+1} system. An antiferromagnetic ordering is clearly visible in the magnetization and the specific heat, which yields a huge transition entropy, Rln(6). By neutron diffraction as a function of temperature we have determined the antiferromagnetic structure that coincides with the one obtained from density functional theory calculations.

Restoring the SU(4) Kondo regime in a double quantum dot system.

We calculate the spectral density and occupations of a system of two capacitively coupled quantum dots, each one connected to its own pair of conducting leads, in a regime of parameters in which the total couplings to the leads for each dot Γ(i) are different. The system has been used recently to perform pseudospin spectroscopy by controlling independently the voltages of the four leads. For an odd number of electrons in the system, equal coupling to the leads Γ1 = Γ2, equal dot levels E1 = E2 and sufficiently large interdot repulsion U12 the system lies in the SU(4) symmetric point of spin and pseudospin degeneracy in the Kondo regime.

Interpretation of experimental results on Kondo systems with crystal field.

We present a simple approach to calculate the thermodynamic properties of single Kondo impurities including orbital degeneracy and crystal field effects (CFE) by extending a previous proposal by Schotte and Schotte (1975 Phys. Lett. 55A 38).

Non-Fermi-liquid behavior in transport through Co-doped Au chains.

We calculate the conductance as a function of temperature G(T) through Au monatomic chains containing one Co atom as a magnetic impurity, and connected to two conducting leads with a fourfold symmetry axis. Using the information derived from ab initio calculations, we construct an effective model Ĥ(eff) that hybridizes a 3d(7) quadruplet at the Co site with two 3d(8) triplets through the hopping of 5d(xz) and 5d(yz) electrons of Au. The quadruplet is split by spin anisotropy due to spin-orbit coupling.

Non-equilibrium conductance through a benzene molecule in the Kondo regime.

Starting from exact eigenstates for a symmetric ring, we derive a low-energy effective generalized Anderson Hamiltonian which contains two spin doublets with opposite momenta and a singlet for the neutral molecule. For benzene, the singlet (doublets) represent the ground state of the neutral (singly charged) molecule. We calculate the non-equilibrium conductance through a benzene molecule, doped with one electron or a hole (i.

Nonequilibrium conductance of a nanodevice for small bias voltage.

Using nonequilibrium renormalized perturbation theory, we calculate the retarded and lesser self-energies, the spectral density ρ(ω) near the Fermi energy, and the conductance G through a quantum dot as a function of a small bias voltage V, in the general case of electron-hole asymmetry and intermediate valence. The linear terms in ω and V are given exactly in terms of thermodynamic quantities. When the energies necessary to add the first electron (Ed) and the second one (Ed + U) to the quantum dot are not symmetrically placed around the Fermi level, G has a term linear in V if, in addition, either the voltage drop or the coupling to the leads is not symmetric.

Universal transport signatures in two-electron molecular quantum dots: gate-tunable Hund's rule, underscreened Kondo effect and quantum phase transitions.

We review here some universal aspects of the physics of two-electron molecular transistors in the absence of strong spin-orbit effects. Several recent quantum dot experiments have shown that an electrostatic backgate could be used to control the energy dispersion of magnetic levels. We discuss how the generally asymmetric coupling of the metallic contacts to two different molecular orbitals can indeed lead to a gate-tunable Hund's rule in the presence of singlet and triplet states in the quantum dot.

Spin-spin indirect interaction at low-energy excitation in zero-dimensional cavities.

We solve the low-energy part of the spectrum of a model that describes a circularly polarized cavity mode strongly coupled to two exciton modes, each of which is coupled to a localized spin of arbitrary magnitude. In the regime in which the excitons and the cavity modes are strongly coupled, forming polaritons, the low-energy part of the spectrum can be described by an effective spin model, which contains a magnetic field, an axial anisotropy, and an Ising interaction between the localized spins. For detunings such that the low-energy states are dominated by nearly degenerate excitonic modes, the description of the low-energy states by a simple effective Hamiltonian ceases to be valid and the effective interaction tends to vanish.

Mechanical control of spin states in spin-1 molecules and the underscreened Kondo effect.

The ability to make electrical contact to single molecules creates opportunities to examine fundamental processes governing electron flow on the smallest possible length scales. We report experiments in which we controllably stretched individual cobalt complexes having spin S = 1, while simultaneously measuring current flow through the molecule. The molecule's spin states and magnetic anisotropy were manipulated in the absence of a magnetic field by modification of the molecular symmetry.

Quantum interference in coherent molecular conductance.

Coherent electronic transport through individual molecules is crucially sensitive to quantum interference. We investigate the zero-bias and zero-temperature conductance through pi-conjugated annulene molecules weakly coupled to two leads for different source-drain configurations, finding an important reduction for certain transmission channels and for particular geometries as a consequence of destructive quantum interference between states with definite momenta. When translational symmetry is broken by an external perturbation we find an abrupt increase of the conductance through those channels.

Nonequilibrium dynamics of a singlet-triplet Anderson impurity near the quantum phase transition.

We study the singlet-triplet Anderson model (STAM) in which a configuration with a doublet is hybridized with another containing a singlet and a triplet, as a minimal model to describe two-level quantum dots coupled to two metallic leads in effectively a one-channel fashion. The model has a quantum phase transition which separates regions of a doublet and a singlet ground state. The limits of integer valence of the STAM (which include a model similar to the underscreened spin-1 Kondo model) are derived and used to predict the behavior of the conductance through the system on both sides of the transition, where it jumps abruptly.

Photoluminescence of a quantum dot hybridized with a continuum of extended states.

We calculate the intensity of photon emission from a trion in a single quantum dot, as a function of energy and gate voltage, using the impurity Anderson model and variational wave functions. Assuming a flat density of conduction states and constant hybridization energy, the results agree with the main features observed in recent experiments: nonmonotonic dependence of the energy on gate voltage, non-Lorentzian line shapes, and a linewidth that increases near the regions of instability of the single electron final state to occupations zero or two.

Dynamical mean field theory of an effective three-band model for NaxCoO2.

We derive an effective Hamiltonian for highly correlated t_{2g} states centered at the Co sites of NaxCoO2. The essential ingredients of the model are an O mediated hopping, a trigonal crystal-field splitting, and on-site effective interactions derived from the exact solution of a multiorbital model in a CoO6 cluster, with parameters determined previously. The effective model is solved by dynamical mean field theory.

Ferrotoroidic moment as a quantum geometric phase.

We present a geometric characterization of the ferrotoroidic moment tau in terms of a set of Abelian Berry phases. We also introduce a fundamental complex quantity z munu, which provides an alternative way to calculate tau and its moments and is derived from the tensor T munu=2 under summation operator jrj muSj nu. This geometric framework defines a natural computational approach for density functional and many-body theories.