Low-energy models for correlated materials: bandwidth renormalization from Coulombic screening

We provide a prescription for constructing Hamiltonians representing the low energy physics of correlated electron materials with dynamically screened Coulomb interactions. The key feature is a renormalization of the hopping and hybridization parameters by the processes that lead to the dynamical screening. The renormalization is shown to be non-negligible for various classes of correlated electron materials. The bandwidth reduction effect is necessary for connecting models to materials behavior and for making quantitative predictions for low-energy properties of solids.Comment: 4 pages, 2 figure

Spectral density of an interacting dot coupled indirectly to conducting leads

We study the spectral density of electrons rho in an interacting quantum dot (QD) with a hybridization lambda to a non-interacting QD, which in turn is coupled to a non-interacting conduction band. The system corresponds to an impurity Anderson model in which the conduction band has a Lorentzian density of states of width Delta2. We solved the model using perturbation theory in the Coulomb repulsion U (PTU) up to second order and a slave-boson mean-field approximation (SBMFA). The PTU works surprisingly well near the exactly solvable limit Delta2 -> 0. For fixed U and large enough lambda or small enough Delta2, the Kondo peak in rho(omega) splits into two peaks. This splitting can be understood in terms of weakly interacting quasiparticles. Before the splitting takes place the universal properties of the model in the Kondo regime are lost. Using the SBMFA, simple analytical expressions for the occurrence of split peaks are obtained. For small or moderate Delta2, the side bands of rho(omega) have the form of narrow resonances, that were missed in previous studies using the numerical renormalization group. This technique also has shortcomings for describing properly the split Kondo peaks. As the temperature is increased, the intensity of the split Kondo peaks decreases, but it is not completely suppressed at high temperatures.Comment: 13 pages, 13 figures, accepted in Phys. Rev.

Hubbard U and Hund's Exchange J in Transition Metal Oxides: Screening vs. Localization Trends from Constrained Random Phase Approximation

In this work, we address the question of calculating the local effective Coulomb interaction matrix in materials with strong electronic Coulomb interactions from first principles. To this purpose, we implement the constrained random phase approximation (cRPA) into a density functional code within the linearized augmented plane wave (LAPW) framework. We apply our approach to the 3d and 4d early transition metal oxides SrMO3 (M=V, Cr, Mn) and (M=Nb, Mo, Tc) in their paramagnetic phases. For these systems, we explicitly assess the differences between two physically motivated low-energy Hamiltonians: The first is the three-orbital model comprising the t2g states only, that is often used for early transition metal oxides. The second choice is a model where both, metal d- and oxygen p-states are retained in the construction of Wannier functions, but the Hubbard interactions are applied to the d-states only ("d-dp Hamiltonian"). Interestingly, since -- for a given compound -- both U and J depend on the choice of the model, so do their trends within a family of these compounds. In the 3d perovskite series SrMO3 the effective Coulomb interactions in the t2g Hamiltonian decrease along the series, due to the more efficient screening. The inverse -- generally expected -- trend, increasing interactions with increasing atomic number, is however recovered within the more localized "d-dp Hamiltonian". Similar conclusions are established in the layered 4d perovskites series Sr2MO4 (M=Mo, Tc, Ru, Rh). Compared to their isoelectronic and isostructural 3d analogues, the 4d 113 perovskite oxides SrMO3 (M=Nb, Mo, Tc) exhibit weaker screening effects. Interestingly, this leads to an effectively larger U on 4d shells than on 3d when a t2g model is constructed.Comment: 21 pages, 7 figure

Long-term effectiveness of a cognitive behavioural therapy (CBT) in the management of fatigue in patients with relapsing remitting multiple sclerosis (RRMS): A multicentre, randomised, open-label, controlled trial versus standard care

Background: Fatigue is a disabling symptom of multiple sclerosis (MS). The lack of effective therapeutics has promoted the development of cognitive behavioural therapy (CBT)-based fatigue management programmes. However, their efficacy does not sustain over time. We proposed to test the long-term effectiveness of a 6-week fatigue programme supplemented with four booster sessions ( € FACETS+') in patients with relapsing remitting MS (RRMS) and fatigue. Methods: This multicentre, randomised, controlled, open-label, parallel-group trial versus standard care enrolled patients with RRMS and fatigue. Participants were randomised to either FACETS+ plus standard care or standard care alone. The primary outcome measure was fatigue impact (Modified Fatigue Impact Scale (MFIS) at 12 months) based on intention-to-treat analyses. Results: From May 2017 to September 2020, 162 patients were screened; 105 were randomly assigned to FACETS+ (n=57) or standard care (n=48) and 88 completed the primary outcome assessment for the MFIS. At month 12, participants showed improved MFIS compared with baseline in the intervention group (mean difference (MD)=14.0 points; (95% CI 6.45 to 21.5)) and the control group (MD=6.1 points; (95% CI -0.30 to 12.5)) with a significant between-group difference in favour of the intervention group (adjusted MD=7.89 points; (95% CI 1.26 to 14.52), standardised effect size=0.52, p=0.021). No trial-related serious adverse events were reported. Conclusions: A 6-week CBT-based programme with four booster sessions is superior to standard care alone to treat MS-related fatigue in the long term (12 months follow-up). The results support the use of the FACETS+ programme for the treatment of MS-related fatigue. Trial registration number: NCT03758820