Time-reversal symmetry breaking phase in the Hubbard model: a VCA study

We study the stability of the time-reversal symmetry breaking staggered flux phase of a single band Hubbard model, within the Variational Cluster Approach (VCA). For intermediate and small values of the interaction $U$, we find metastable solutions for the staggered flux phase, with a maximum current per bond at $U\approx 3.2$. However, allowing for antiferromagnetic and superconducting long-range order it turns out that in the region at and close to half filling the antiferromagnetic phase is the most favorable energetically. The effect of nearest-neighbour interaction is also considered. Our results show that a negative nearest-neighbour interaction and finite doping favors the stability of the staggered-flux phase. We also present preliminary results for the three-band Hubbard model obtained with a restricted set of variational parameters. For this case, no spontaneous time-reversal symmetry breaking phase is found in our calculations.Comment: 9 pages,8 figure

Half-metallicity and magnetism in the Co2_2MnAl/CoMnVAl heterostructure

We present a study of the electronic structure and magnetism of Co$_2$MnAl, CoMnVAl and their heterostructure. We employ a combination of density-functional theory and dynamical mean-field theory (DFT+DMFT). We find that Co$_2$MnAl is a half-metallic ferromagnet, whose electronic and magnetic properties are not drastically changed by strong electronic correlations, static or dynamic. Non-quasiparticle states are shown to appear in the minority spin gap without affecting the spin-polarization at the Fermi level predicted by standard DFT. We find that CoMnVAl is a semiconductor or a semi-metal, depending on the employed computational approach. We then focus on the electronic and magnetic properties of the Co$_2$MnAl/CoMnVAl heterostructure, predicted by previous first principle calculations as a possible candidate for spin-injecting devices. We find that two interfaces, Co-Co/V-Al and Co-Mn/Mn-Al, preserve the half-metallic character, with and without including electronic correlations. We also analyse the magnetic exchange interactions in the bulk and at the interfaces. At the Co-Mn/Mn-Al interface, competing magnetic interactions are likely to favor the formation of a non-collinear magnetic order, which is detrimental for the spin-polarization.Comment: 15 pages, 16 figure

Locally self-consistent embedding approach for disordered electronic systems

We present a new embedding scheme for the locally self-consistent method to study disordered electron systems. We test this method in a tight-binding basis and apply it to the single band Anderson model. The local interaction zone is used to efficiently compute the local Green's function of a supercell embeded into a local typical medium. We find a quick convergence as the size of the local interaction zone which reduces the computational costs as expected. This method captures the Anderson localization transition and accurately predicts the critical disorder strength. The present work opens the path towards the development of a typical medium embedding scheme for the $O(N)$ multiple scattering methods.Comment: 7 pages, 5 figure

Superconductivity of Bi-III phase of elemental Bismuth: insights from Muon-Spin Rotation and Density Functional Theory

Using muon-spin rotation the pressure-induced superconductivity in the Bi-III phase of elemental Bismuth (transition temperature $T_{\rm c}\simeq7.05$ K) was investigated. The Ginzburg-Landau parameter $\kappa=\lambda/\xi=30(6)$ ($\lambda$ is the magnetic penetration depth, $\xi$ is the coherence length) was estimated which is the highest among single element superconductors. The temperature dependence of the superconducting energy gap [$\Delta(T)$] reconstructed from $\lambda^{-2}(T)$ deviates from the weak-coupled BCS prediction. The coupling strength $2\Delta/k_{\rm B}T_{\rm c}\simeq 4.34$ was estimated thus implying that Bi-III stays within the strong coupling regime. The Density Functional Theory calculations suggest that superconductivity in Bi-III could be described within the Eliashberg approach with the characteristic phonon frequency $\omega_{\rm ln}\simeq 5.5$ meV. An alternative pairing mechanism to the electron-phonon coupling involves the possibility of Cooper pairing induced by the Fermi surface nesting.Comment: 5 pages, 4 figure

Electronic Correlations in Vanadium Revealed by Electron-Positron Annihilation Measurements

The electronic structure of vanadium measured by Angular Correlation of electron-positron Annihilation Radiation (ACAR) is compared with the predictions of the combined Density Functional and Dynamical Mean-Field Theory (DMFT). Reconstructing the momentum density from five 2D projections we were able to determine the full Fermi surface and found excellent agreement with the DMFT calculations. In particular, we show that the local, dynamic self-energy corrections contribute to the anisotropy of the momentum density and need to be included to explain the experimental results

Electron-electron interaction strength in ferromagnetic nickel determined by spin-polarized positron annihilation

The two-photon momentum distribution of annihilating electron-positron pairs in ferromagnetic nickel (Ni) was determined by measuring the spin-polarized two-dimensional angular correlation of annihilation radiation (ACAR). The spectra were compared with theoretical results obtained within LDA+DMFT, a combination of the local density approximation (LDA) and the many-body dynamical mean-field theory (DMFT). The self-energy describing the electronic correlations in Ni is found to make important anisotropic contributions to the momentum distribution which are not present in LDA. Based on a detailed comparison of the theoretical and experimental results the strength of the local electronic interaction U in ferromagnetic Ni is determined as 2.0 +- 0.1 eV

On the superconducting nature of the Bi-II phase of elemental Bismuth

The superconductivity in the Bi-II phase of elemental Bismuth (transition temperature $T_{\rm c}\simeq3.92$ K at pressure $p\simeq 2.80$ GPa) was studied experimentally by means of the muon-spin rotation as well as theoretically by using the Eliashberg theory in combination with Density Functional Theory calculations. Experiments reveal that Bi-II is a type-I superconductor with a zero temperature value of the thermodynamic critical field $B_{\rm c}(0)\simeq31.97$~mT. The Eliashberg theory approach provides a good agreement with the experimental $T_{\rm c}$ and the temperature evolution of $B_{\rm c}$. The estimated value for the retardation (coupling) parameter $k_{\rm B}T_{\rm c}/\omega_{\rm ln} \approx 0.07$ ($\omega_{\rm ln}$ is the logarithmically averaged phonon frequency) suggests that Bi-II is an intermediately-coupled superconductor.Comment: 6 pages, 2 figure

Systematic quantum cluster typical medium method for the study of localization in strongly disordered electronic systems

Great progress has been made in the last several years towards understanding the properties of disordered electronic systems. In part, this is made possible by recent advances in quantum effective medium methods which enable the study of disorder and electron-electronic interactions on equal footing. They include dynamical mean field theory and the coherent potential approximation, and their cluster extension, the dynamical cluster approximation. Despite their successes, these methods do not enable the first-principles study of the strongly disordered regime, including the effects of electronic localization. The main focus of this review is the recently developed typical medium dynamical cluster approximation for disordered electronic systems. This method has been constructed to capture disorder-induced localization, and is based on a mapping of a lattice onto a quantum cluster embedded in an effective typical medium, which is determined self-consistently. Here we provide an overview of various recent applications of the typical medium dynamical cluster approximation to a variety of models and systems, including single and multi-band Anderson model, and models with local and off-diagonal disorder. We then present the application of the method to realistic systems in the framework of the density functional theory.Comment: 58 pages, 46 figure