Instabilities of quadratic band crossing points

Using a functional renormalization group approach, we study interaction-driven instabilities in quadratic band crossing point two-orbital models in two dimensions, extending a previous study of Sun et al. [1]. The wavevector-dependence of the Bloch eigenvectors of the free Hamiltonian causes interesting instabilities toward spin nematic, quantum anomalous Hall and quantum spin Hall states. In contrast with other known examples of interaction-driven topological insulators, in the system studied here, the QSH state occurs at arbitrarily small interaction strength and for rather simple intra- and inter-orbital repulsions.Comment: 9 pages, 6 figures; additional information on the fRG method (section 3) and minor change

Self-energy feedback and frequency-dependent interactions in the functional renormalization group flow for the two-dimensional Hubbard model

We study the impact of including the self-energy feedback and frequency-dependent interactions on functional renormalization group grows for the two-dimensional Hubbard model on the square lattice at weak to moderate coupling strength. Previous studies using the functional renormalization group had ignored these two ingredients to large extent, and the question is how much the flows to strong coupling analyzed by this method depend on these approximations. Here we include the imaginary part of the self-energy on the imaginary axis and the frequency-dependence of the running interactions on a frequency mesh of 10 frequencies on the Matsubara axis. We find that i) the critical scales for the flows to strong coupling are shifted downwards by a factor that is usually of order one but can get larger in specific parameter regions, and ii) that the leading channel in this flow does not depend strongly on whether self-energies and frequency-dependence is included or not. We also discuss the main features of the self-energies developing during the flows.Comment: 10 pages, 9 figure

Instabilities of interacting electrons on the honeycomb bilayer

We investigate the instabilities of interacting electrons on the honeycomb bilayer by means of the functional renormalization group for a range of interactions up to the third-nearest neighbor. Besides a novel instability toward a gapless charge-density wave we find that using interaction parameters as determined by ab-initio calculations for graphene and graphite puts the system close to the boundary between antiferromagnetic and quantum spin Hall instabilities. Importantly, the energy scales for these instabilities are large such that imperfections and deviations from the basic model are expected to play a major role in real bilayer graphene, where interaction effects seem to be seen only at smaller scales. We therefore analyze how reducing the critical scale and small doping of the layers affect the instabilities.Comment: 5 pages, 4 figure

Multiband Effects on Superconducting Instabilities Driven by Electron-Electron Interactions

We explore multiband effects on d-wave superconducting instabilities driven by electron-electron interactions. Our models on the two-dimensional square lattice consist of a main band with an extended Fermi surface and predominant weight from $d_{x^2-y^2}$ orbitals, whose orbital character is influenced by the admixture of other energetically neighbored orbitals. Using a functional renormalization group description of the superconducting instabilities of the system and different levels of approximations, we study how the energy scale for pairing and hence the critical temperature is affected by the band structure. We find that a reduction of orbital admixture as a function of the orbital energies can cause a $T_c$ enhancement although the Fermi surface becomes more curved and hence less favorable for antiferromagnetic spin fluctuations. While our study does not allow a quantitative understanding of the $T_c$ differences in realistic high-$T_c$ cuprate systems, it may reveal an underlying mechanism contributing to the actual material trends.Comment: 12 pages, 8 figures, additional interaction terms in section IV and VI included, amended discussion and plots, additional minor change

Interacting electrons on trilayer honeycomb lattices

Few-layer graphene systems come in various stacking orders. Considering tight-binding models for electrons on stacked honeycomb layers, this gives rise to a variety of low-energy band structures near the charge neutrality point. Depending on the stacking order these band structures enhance or reduce the role of electron-electron interactions. Here, we investigate the instabilities of interacting electrons on honeycomb multilayers with a focus on trilayers with ABA and ABC stackings theoretically by means of the functional renormalization group. We find different types of competing instabilities and identify the leading ordering tendencies in the different regions of the phase diagram for a range of local and non-local short-ranged interactions. The dominant instabilities turn out to be toward an antiferromagnetic spin-density wave (SDW), a charge density wave and toward quantum spin Hall (QSH) order. Ab-initio values for the interaction parameters put the systems at the border between SDW and QSH regimes. Furthermore, we discuss the energy scales for the interaction-induced gaps of this model study and put them into context with the scales for single-layer and Bernal-stacked bilayer honeycomb lattices. This yields a comprehensive picture of the possible interaction-induced ground states of few-layer graphene.Comment: 12 pages, 12 figure

Antiferromagnetism in the Hubbard Model on the Bernal-stacked Honeycomb Bilayer

Using a combination of quantum Monte Carlo simulations, functional renormalization group calculations and mean-field theory, we study the Hubbard model on the Bernal-stacked honeycomb bilayer at half-filling as a model system for bilayer graphene. The free bands consisting of two Fermi points with quadratic dispersions lead to a finite density of states at the Fermi level, which triggers an antiferromagnetic instability that spontaneously breaks sublattice and spin rotational symmetry once local Coulomb repulsions are introduced. Our results reveal an inhomogeneous participation of the spin moments in the ordered ground state, with enhanced moments at the three-fold coordinated sites. Furthermore, we find the antiferromagnetic ground state to be robust with respect to enhanced interlayer couplings and extended Coulomb interactions.Comment: 4+ pages, 4 figures; final versio

Do yoga and meditation moderate the relationship between negative life events and depressive symptoms? Analysis of a national cross-sectional survey of Australian women

ObjectivesThis study aims to examine the role of yoga/meditation in the relationship between negative life events, stress and depression.MethodsThe Australian Longitudinal Study on Women’s Health (ALSWH) surveyed 7,186 women aged 36–43 years (mean age 39.2 years; 57.2% university degree) in 2015. Mediation and moderation analyses were conducted to examine whether yoga/meditation practice moderated those relationships.ResultsYoga/meditation was practiced by 27.5% of participants, 33.2% reported negative life events in the past 12 months, and 24% had clinical depression. Perceived stress partially mediated the association between negative life events and depressive symptoms (B = 6.28; 95%CI 5.65; 6.92). Social support (B = −0.38; 95%CI −0.54; −0.23) and optimism (B = −0.25;95%CI −0.31; −0.18) moderated the association between stress and depressive symptoms. Yoga/meditation practice moderated the direct association between negative life events and depressive symptoms (B = −0.92; 95%CI −1.67; −0.18).ConclusionYoga/meditation use was a significant moderator of the relationship between negative life events and depression. Yoga/mediation use did not act via reducing perceived stress, but instead was found to dampen the influence of negative life events on depression directly. More research on how yoga has an impact on depression is warranted

Yoga for stroke rehabilitation

Background: Stroke is a major health issue and cause of long-term disability and has a major emotional and socioeconomic impact. There is a need to explore options for long-term sustainable interventions that support stroke survivors to engage in meaningful activities to address life challenges after stroke. Rehabilitation focuses on recovery of function and cognition to the maximum level achievable, and may include a wide range of complementary strategies including yoga. Yoga is a mind-body practice that originated in India, and which has become increasingly widespread in the Western world. Recent evidence highlights the positive effects of yoga for people with a range of physical and psychological health conditions. A recent non-Cochrane systematic review concluded that yoga can be used as self-administered practice in stroke rehabilitation. Objectives: To assess the effectiveness of yoga, as a stroke rehabilitation intervention, on recovery of function and quality of life (QoL). Search methods: We searched the Cochrane Stroke Group Trials Register (last searched July 2017), Cochrane Central Register of Controlled Trials (CENTRAL) (last searched July 2017), MEDLINE (to July 2017), Embase (to July 2017), CINAHL (to July 2017), AMED (to July 2017), PsycINFO (to July 2017), LILACS (to July 2017), SciELO (to July 2017), IndMED (to July 2017), OTseeker (to July 2017) and PEDro (to July 2017). We also searched four trials registers, and one conference abstracts database. We screened reference lists of relevant publications and contacted authors for additional information. Selection criteria: We included randomised controlled trials (RCTs) that compared yoga with a waiting-list control or no intervention control in stroke survivors. Data collection and analysis: Two review authors independently extracted data from the included studies. We performed all analyses using Review Manager (RevMan). One review author entered the data into RevMan; another checked the entries. We discussed disagreements with a third review author until consensus was reached. We used the Cochrane 'Risk of bias' tool. Where we considered studies to be sufficiently similar, we conducted a meta-analysis by pooling the appropriate data. For outcomes for which it was inappropriate or impossible to pool quantitatively, we conducted a descriptive analysis and provided a narrative summary. Main results: We included two RCTs involving 72 participants. Sixty-nine participants were included in one meta-analysis (balance). Both trials assessed QoL, along with secondary outcomes measures relating to movement and psychological outcomes; one also measured disability. In one study the Stroke Impact Scale was used to measure QoL across six domains, at baseline and post-intervention. The effect of yoga on five domains (physical, emotion, communication, social participation, stroke recovery) was not significant; however, the effect of yoga on the memory domain was significant (mean difference (MD) 15.30, 95% confidence interval (CI) 1.29 to 29.31, P = 0.03), the evidence for this finding was very low grade. In the second study, QoL was assessed using the Stroke-Specifc QoL Scale; no significant effect was found. Secondary outcomes included movement, strength and endurance, and psychological variables, pain, and disability. Balance was measured in both studies using the Berg Balance Scale; the effect of intervention was not significant (MD 2.38, 95% CI -1.41 to 6.17, P = 0.22). Sensititivy analysis did not alter the direction of effect. One study measured balance self-efficacy, using the Activities-specific Balance Confidence Scale (MD 10.60, 95% CI -7.08,= to 28.28, P = 0.24); the effect of intervention was not significant; the evidence for this finding was very low grade. One study measured gait using the Comfortable Speed Gait Test (MD 1.32, 95% CI -1.35 to 3.99, P = 0.33), and motor function using the Motor Assessment Scale (MD -4.00, 95% CI -12.42 to 4.42, P = 0.35); no significant effect was found based on very low-grade evidence. One study measured disability using the modified Rankin Scale (mRS) but reported only whether participants were independent or dependent. No significant effect was found: (odds ratio (OR) 2.08, 95% CI 0.50 to 8.60, P = 0.31); the evidence for this finding was very low grade. Anxiety and depression were measured in one study. Three measures were used: the Geriatric Depression Scale-Short Form (GCDS15), and two forms of State Trait Anxiety Inventory (STAI, Form Y) to measure state anxiety (i.e. anxiety experienced in response to stressful situations) and trait anxiety (i.e. anxiety associated with chronic psychological disorders). No significant effect was found for depression (GDS15, MD -2.10, 95% CI -4.70 to 0.50, P = 0.11) or for trait anxiety (STAI-Y2, MD -6.70, 95% CI -15.35 to 1.95, P = 0.13), based on very low-grade evidence. However, a significant effect was found for state anxiety: STAI-Y1 (MD -8.40, 95% CI -16.74 to -0.06, P = 0.05); the evidence for this finding was very low grade. No adverse events were reported. Quality of the evidence: We assessed the quality of the evidence using GRADE. Overall, the quality of the evidence was very low, due to the small number of trials included in the review both of which were judged to be at high risk of bias, particularly in relation to incompleteness of data and selective reporting, and especially regarding the representative nature of the sample in one study. Authors' conclusions: Yoga has the potential for being included as part of patient-centred stroke rehabilitation. However, this review has identified insufficient information to confirm or refute the effectiveness or safety of yoga as a stroke rehabilitation treatment. Further large-scale methodologically robust trials are required to establish the effectiveness of yoga as a stroke rehabilitation treatment