Interaction effects and quantum phase transitions in topological insulators

We study strong correlation effects in topological insulators via the Lanczos algorithm, which we utilize to calculate the exact many-particle ground-state wave function and its topological properties. We analyze the simple, noninteracting Haldane model on a honeycomb lattice with known topological properties and demonstrate that these properties are already evident in small clusters. Next, we consider interacting fermions by introducing repulsive nearest-neighbor interactions. A first-order quantum phase transition was discovered at finite interaction strength between the topological band insulator and a topologically trivial Mott insulating phase by use of the fidelity metric and the charge-density-wave structure factor. We construct the phase diagram at $T = 0$ as a function of the interaction strength and the complex phase for the next-nearest-neighbor hoppings. Finally, we consider the Haldane model with interacting hard-core bosons, where no evidence for a topological phase is observed. An important general conclusion of our work is that despite the intrinsic nonlocality of topological phases their key topological properties manifest themselves already in small systems and therefore can be studied numerically via exact diagonalization and observed experimentally, e.g., with trapped ions and cold atoms in optical lattices.Comment: 13 pages, 12 figures. Published versio

Strong valence fluctuation in the quantum critical heavy fermion superconductor beta-YbAlB4: A hard x-ray photoemission study

Electronic structures of the quantum critical superconductor beta-YbAlB4 and its polymorph alpha-YbAlB4 are investigated by using bulk-sensitive hard x-ray photoemission spectroscopy. From the Yb 3d core level spectra, the values of the Yb valence are estimated to be ~2.73 and ~2.75 for alpha- and beta-YbAlB4, respectively, thus providing clear evidence for valence fluctuations. The valence band spectra of these compounds also show Yb2+ peaks at the Fermi level. These observations establish an unambiguous case of a strong mixed valence at quantum criticality for the first time among heavy fermion systems, calling for a novel scheme for a quantum critical model beyond the conventional Doniach picture in beta-YbAlB4.Comment: 4 pages, 3 figures, revised version accepted for publication in PR

Temperature dependent Eu 3d-4f X-ray Absorption and Resonant Photoemission Study of the Valence Transition in EuNi2(Si0.2Ge0.8)2EuNi_2(Si_{0.2}Ge_{0.8})_2

We study the mixed valence transition ($T$$_{v}$ $\sim$80 K) in EuNi$_{2}$(Si$_{0.2}$Ge$_{0.8}$)$_{2}$ using Eu 3$d-4f$ X-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (RESPES). The Eu$^{2+}$ and Eu$^{3+}$ main peaks show a giant resonance and the spectral features match very well with atomic multiplet calculations. The spectra show dramatic temperature ($T$)-dependent changes over large energies ($\sim$10 eV) in RESPES and XAS. The observed non-integral mean valencies of $\sim$2.35 $\pm$ 0.03 ($T$ = 120 K) and $\sim$2.70 $\pm$ 0.03 ($T$ = 40 K) indicate homogeneous mixed valence above and below $T$$_{v}$. The redistribution between Eu$^{2+}$$4f^7$+$[spd]^0$ and Eu$^{3+}$$4f^6$+$[spd]^1$ states is attributed to a hybridization change coupled to a Kondo-like volume collapse.Comment: 4 pages, 3 figure

Integrated cross-domain object storage in working memory: Evidence from a verbal-spatial memory task

Working-memory theories often include domain-specific verbal and visual stores (e.g., the phonological and visuospatial buffers of Baddeley, 1986), and some also posit more general stores thought to be capable of holding verbal or visuospatial materials (Baddeley, 2000; Cowan, 2005). However, it is currently unclear which type of store is primarily responsible for maintaining objects that include components from multiple domains. In these studies, a spatial array of letters was followed by a single probe identical to an item in the array or differing systematically in spatial location, letter identity, or their combination. Concurrent verbal rehearsal suppression impaired memory in each of these trial types in a task that required participants to remember verbal-spatial binding, but did not impair memory for spatial locations if the task did not require verbal-spatial binding for a correct response. Thus, spatial information might be stored differently when it must be bound to verbal information. This suggests that a cross-domain store such as the episodic buffer of Baddeley (2000) or the focus of attention of Cowan (2001) might be used for integrated object storage, rather than the maintenance of associations between features stored in separate domain-specific buffers

Lower entropy bounds and particle number fluctuations in a Fermi sea

We demonstrate, in an elementary manner, that given a partition of the single particle Hilbert space into orthogonal subspaces, a Fermi sea may be factored into pairs of entangled modes, similar to a BCS state. We derive expressions for the entropy and for the particle number fluctuations of a subspace of a fermi sea, at zero and finite temperatures, and relate these by a lower bound on the entropy. As an application we investigate analytically and numerically these quantities for electrons in the lowest Landau level of a quantum Hall sample.Comment: shorter version, typos fixe

A list-length constraint on incidental item-to-item associations

We investigated the possibility that item-to-item associations form between items concurrently included in a capacity-limited region of working memory, but not outside of that region. Many studies indicate a central capacity limit of 3 to 5 items (e.g., Cowan, 2001). Participants received lists of 3, 6, or 9 words along with an orienting task, selecting the most interesting word from each list. Consistent with expectations, a subsequent, unexpected test showed that memory of whether two words came from the same list or not was superior for 3-word lists compared to 6- and 9-word lists, which did not differ. This effect occurred even though the separation between the list positions of the two probe words was controlled across list lengths. The study demonstrates a source of implicit learning that depends upon a limited-capacity working memory faculty, a finding that should inspire further research on the function of working memory in long-term learning

Sequential therapy of anti-Nogo-A antibody treatment and treadmill training leads to cumulative improvements after spinal cord injury in rats

Intense training is the most clinically successful treatment modality following incomplete spinal cord injuries (SCIs). With the advent of plasticity enhancing treatments, understanding how treatments might interact when delivered in combination becomes critical. Here, we investigated a rational approach to sequentially combine treadmill locomotor training with antibody mediated suppression of the fiber growth inhibitory protein Nogo-A. Following a large but incomplete thoracic lesion, rats were immediately treated with either anti-Nogo-A or control antibody (2 weeks) and then either left untrained or step-trained starting 3 weeks after injury for 8 weeks. It was found that sequentially combined therapy improved step consistency and reduced toe dragging and climbing errors, as seen with training and anti-Nogo-A individually. Animals with sequential therapy also adopted a more parallel paw position during bipedal walking and showed greater overall quadrupedal locomotor recovery than individual treatments. Histologically, sequential therapy induced the greatest corticospinal tract sprouting caudally into the lumbar region and increased the number of serotonergic synapses onto lumbar motoneurons. Increased primary afferent sprouting and synapse formation onto lumbar motoneurons observed with anti-Nogo-A antibody were reduced by training. Animals with sequential therapy also showed the highest reduction of lumbar interneuronal activity associated with walking (c-fos expression). No treatment effects for thermal nociception, mechanical allodynia, or lesion volume were observed. The results demonstrate that sequential administration of anti-Nogo-A antibody followed in time with intensive locomotor training leads to superior recovery of lost locomotor functions, which is probably mediated by changes in the interaction between descending sprouting and local segmental networks after SCI

How Can eHealth Meet the Hearing and Communication Needs of Adults With Hearing Impairment and their Significant Others? A Group Concept Mapping Study

Objectives: To seek the perspectives of key stakeholders regarding: (1) how eHealth could help meet the hearing and communication needs of adults with hearing impairment and their significant others; and (2) how helpful each aspect of eHealth would be to key stakeholders personally. / Design: Group concept mapping, a mixed-methods participatory research method, was used to seek the perspectives of key stakeholders: adults with hearing impairment (n = 39), significant others (n = 28), and hearing care professionals (n = 56). All participants completed a short online survey before completing one or more of the following activities: brainstorming, sorting, and rating. Brainstorming required participants to generate ideas in response to the focus prompt, “One way I would like to use information and communication technologies to address the hearing and communication needs of adults with hearing loss and their family and friends is to….” The sorting task required participants to sort all statements into groups that made sense to them. Finally, the rating task required participants to rate each of the statements according to “How helpful would this idea be to you?” using a 5-point Likert scale. Hierarchical cluster analysis was applied to the “sorting” data to develop a cluster map using the Concept Systems software. The “rating” data were subsequently analyzed at a cluster level and an individual-item level using descriptive statistics. Differences in cluster ratings between stakeholder groups were examined using Kruskal-Wallis tests. / Results: Overall, 123 statements were generated by participants in response to the focus prompt and were included in subsequent analyses. Based on the “sorting” data and hierarchical cluster analysis, a seven-cluster map was deemed to be the best representation of the data. Three key themes emerged from the data, including using eHealth to (1) Educate and Involve Others; (2) Support Aural Rehabilitation; and (3) Educate About and Demonstrate the Impacts of Hearing Impairment and Benefits of Hearing Rehabilitation. Overall median rating scores for each cluster ranged from 3.97 (educate and involve significant others) to 3.44 (empower adults with hearing impairment to manage their hearing impairment from home). / Conclusions: These research findings demonstrate the broad range of clinical applications of eHealth that have the capacity to support the implementation of patient- and family-centered hearing care, with self-directed educational tools and resources typically being rated as most helpful. Therefore, eHealth appears to be a viable option for enabling a more biopsychosocial approach to hearing healthcare and educating and involving significant others in the hearing rehabilitation process without adding more pressure on clinical time. More research is needed to inform the subsequent development of eHealth interventions, and it is recommended that health behavior change theory be adhered to for such interventions