Quantum critical elasticity

We discuss elastic instabilities of the atomic crystal lattice at zero temperature. Due to long-range shear forces of the solid, at such transitions the phonon velocities vanish, if at all, only along certain crystallographic directions, and, consequently, the critical phonon fluctuations are suppressed to a lower dimensional manifold and governed by a Gaussian fixed-point. In case of symmetry-breaking elastic transitions, a characteristic critical phonon thermodynamics arises that is found, e.g., to violate Debye's $T^3$-law for the specific heat. We point out that quantum critical elasticity is triggered whenever a critical soft mode couples linearly to the strain tensor. In particular, this is relevant for the electronic Ising-nematic quantum phase transition in a tetragonal crystal as discussed in the context of certain cuprates, ruthenates and iron-based superconductors.Comment: 5 pages, 3 figures; (v2) rewritten introduction, supplement included; (v3) minor changes, published versio

Mott metal-insulator transition on compressible lattices

The critical properties of the finite temperature Mott endpoint are drastically altered by a coupling to crystal elasticity, i.e., whenever it is amenable to pressure tuning. Similar as for critical piezoelectric ferroelectrics, the Ising criticality of the electronic system is preempted by an isostructural instability, and long-range shear forces suppress microscopic fluctuations. As a result, the endpoint is governed by Landau criticality. Its hallmark is thus a breakdown of Hooke's law of elasticity with a non-linear strain-stress relation characterized by a mean-field exponent. Based on a quantitative estimate, we predict critical elasticity to dominate the temperature range DeltaT/Tc ~ 8% close to the Mott endpoint of kappa-(BEDT-TTF)2X.Comment: 4 pages, 6 figure

Mott Transition and Quantum Critical Metamagnetism on Compressible Lattices

Solid state phase transitions, which are amenable to pressure, generically, have an intrinsic coupling of the order parameter to the elastic degrees of freedom. The applied pressure primarily affects the lattice by varying the lattice spacing which, in turn, modifies the coupling constants of the critical degrees of freedom. Therefore, a coupling of the strain to the order parameter can strongly affect the phase transition. In this Thesis we investigate this influence on the finite temperature critical point of the Mott metal-insulator transition and the zero temperature quantum critical metamagnetic endpoint. The universality class of the Mott endpoint is a topic which is still under debate. In this Thesis, we show that the nature of the Mott transition is drastically changed when interacting with a compressible lattice. The expected Ising criticality of the electronic system is preempted by an isostructural instability. Due to long ranged shear forces, in the vicinity of the critical endpoint an elastic Landau regime emerges, where the system shows mean-field behavior. The smoking gun criterion to detect the elastic Landau regime is the breakdown of Hooke's law, i.e. a non-linear stress-strain relation. Furthermore, the specific heat coefficient exhibits a finite mean field jump at the transition. For the family of organic salts Kappa-(BEDT-TTF)_2 X, we determine the extent of the elastic Landau regime as \Delta T^\star \approx 2.5 K and \Delta p^\star \approx 50 bar based on thermal expansion experiments. In the second part, we investigate the quantum critical endpoint of itinerant metamagnets. Recently, it was suggested that quantum critical metamagnetism is a generic feature in itinerant ferromagnets such as UCoAl and UGe_2. Within the framework of spin fluctuation theory, we determine the free energy and its temperature dependence, obtained by fluctuation renormalizations, and deduce the critical thermodynamics. Importantly, the compressibility shows the same behavior as the susceptibility which, by definition, diverges at a metamagnetic transition. Therefore, the metamagnetic quantum critical endpoint is intrinsically unstable towards an isostructural transition. This isostructural transition preempts the metamagnetic quantum critical endpoint and the elastic degrees of freedom crucially alter the critical thermodynamics. Most importantly, at the critical field we obtain for lowest but finite temperatures a regime of critical elasticity which is characterized by unusual power laws of the thermodynamic quantities. Whereas the thermal expansion has a much stronger temperature divergence for fields close to the critical field, the specific heat divergence is cut off upon entering this regime. As a consequence, the Grüneisen parameter diverges with an unusual high power of temperature

Multiscale quantum criticality: Pomeranchuk instability in isotropic metals

As a paradigmatic example of multi-scale quantum criticality, we consider the Pomeranchuk instability of an isotropic Fermi liquid in two spatial dimensions, d=2. The corresponding Ginzburg-Landau theory for the quadrupolar fluctuations of the Fermi surface consists of two coupled modes, critical at the same point, and characterized by different dynamical exponents: one being ballistic with dynamical exponent z=2 and the other one is Landau-damped with z=3, thus giving rise to multiple dynamical scales. We find that at temperature T=0, the ballistic mode governs the low-energy structure of the theory as it possesses the smaller effective dimension d+z. Its self-interaction leads to logarithmic singularities, which we treat with the help of the renormalization group. At finite temperature, the coexistence of two different dynamical scales gives rise to a modified quantum-to-classical crossover. It extends over a parametrically large regime with intricate interactions of quantum and classical fluctuations leading to a universal T-dependence of the correlation length independent of the interaction amplitude. The multiple scales are also reflected in the phase diagram and in the critical thermodynamics. In particular, we find that the latter cannot be interpreted in terms of only a single dynamical exponent: whereas, e.g., the critical specific heat is determined by the z=3 mode, the critical compressibility is found to be dominated by the z=2 fluctuations.Comment: 15 pages, 6 figures; (v2) RG implementation with arbitrary dynamical exponent z, discussion on fixed-points adde

ADAPTING mLEARNING ENVIRONMENTS ON LEARNERS’ COGNITIVE STYLES AND VISUAL WORKING MEMORY SPAN

The research that is described in this paper focuses on incorporating theories of individual differences in information processing within the context of mobile hypertext and hypermedia interactive environments. Based on previous findings of the authors in the field of adaptive eLearning, the main purpose was to enhance the quality of information presentation and users’ interactions in the Web by matching their specific needs and preferences. Our more recent experiments, explore how to improve learning process by adapting course content presentation to student cognitive styles and capabilities in mobile environments such as PDA phones. A framework has been developed to comprehensively model student’s cognitive styles and visual working memory span and present the appropriate subject matter, including the content, format, guidance, etc. to suit an individual student by increasing efficiency during interaction. Main aim is to overcome constraints like small screen size and processing/memory capabilities for navigation enhancements that limit the presentation and guidance of the material. An increase on users’ satisfaction as well as more efficient information processing (both in terms of accuracy and task completion time), has been observed in the personalized condition than the original one. Consequently, it is supported that human factors may be used in order to enhance the design of mobile hypertext (or hypermedia) environments in a measurable and meaningful way

Human Factors As A Parameter For Improving Interface Usability And User Satisfaction

The endeavour to optimize HCI should integrate a wide array of user characteristics that have an effect throughout users’ interactions with a system. Human factors such as cognitive traits and current state, from a psychological point of view, are undoubtedly significant in the shaping of the perceived and objective quality of interactions with a system. The research that is presented in this paper focuses on identifying human factors that relate to users’ performance in Web applications that involve information processing, and a framework of personalization rules that are expected to increase users’ performance is depicted. The empirical results that are presented are derived from environments both learning and commercial; in the case of e-learning personalization was beneficial, while the interaction with a commercial site needs to be further investigated due to the implicit character of information processing in the Web

The effect of laser pan-retinal photocoagulation with or without intravitreal bevacizumab injections on the OCT-measured macular choroidal thickness of eyes with proliferative diabetic retinopathy

OBJECTIVES: To investigate the effect of laser pan-retinal photocoagulation with or without intravitreal bevacizumab injections on macular choroidal thickness parameters in eyes with high-risk proliferative diabetic retinopathy. METHODS: High-risk proliferative diabetic retinopathy patients undergoing laser treatment were prospectively enrolled in this study. One eye was randomly selected for laser treatment combined with bevacizumab injections, study group, whereas the corresponding eye was subjected to laser treatment alone, control group. Spectral-domain optical coherence tomography with enhanced depth imaging was used to measure the macular choroidal thickness prior to and 1 month after treatment. Measurements in both groups were compared. Clinicaltrials.gov: NCT01389505. RESULTS: Nineteen patients (38 eyes) with a mean±standard deviation age of 53.4±9.3 years were evaluated, and choroidal thickness measurements for 15 patients were used for comparison. The greatest measurement before treatment was the subfoveal choroidal thickness (341.68±67.66 μm and 345.79±83.66 μm for the study and control groups, respectively). No significant difference between groups was found in terms of macular choroidal thickness measurements at baseline or after treatment. However, within-group comparisons revealed a significant increase in choroidal thickness parameters in 10 measurements in the study group and in only 5 temporal measurements in the control group when 1-month follow-up measurements were compared to baseline values. CONCLUSIONS: The macular choroidal thickness does not appear to be significantly influenced by laser treatment alone but increases significantly when associated with bevacizumab injections in patients with proliferative diabetic retinopathy and macular edema. Because bevacizumab injections reduce short-term laser pan-retinal photocoagulation-induced macular edema, our findings suggest that the choroid participates in its pathogenesis

Effect of panretinal photocoagulation on the peripapillary retinal nerve fiber layer in diabetic retinopathy patients

OBJECTIVES: To determine the effect of panretinal photocoagulation (PRP) on the peripapillary retinal nerve fiber layer (RNFL) in nonglaucomatous patients with proliferative diabetic retinopathy (PDR). METHODS: This is a prospective, single center, observational study. Thirty-eight eyes of 26 diabetic patients underwent PRP for proliferative diabetic retinopathy. Peripapillary RNFL thickness was measured using scanning laser polarimetry (SLP) with variable corneal compensation (GDx VCC; by Carl Zeiss Meditec, Dublin, CA) and spectral-domain optical coherence tomography (OCT) (Heidelberg Spectralis, Carlsbad, USA) at baseline and 12 months after PRP was performed. RESULTS: Thirty-eight eyes of 26 patients (15 female) with a mean age of 53.7 years (range 26 to 74 years) were recruited. No significant difference was found among all RNFL thickness parameters tested by GDx VCC software (p=0.952, 0.464 and 0.541 for temporal-superior-nasal-inferior-temporal (TSNIT) average, superior average, inferior average, respectively). The nerve fiber indicator (NFI) had a nonsignificant increase (p=0.354). The OCT results showed that the average RNFL thickness (360o measurement) decreased nonsignificantly from 97.2 mm to 96.0 mm at 1 year post-PRP (p=0.469). There was no significant difference when separately analyzing all the peripapillary sectors (nasal superior, temporal superior, temporal, temporal inferior, nasal inferior and nasal thickness). CONCLUSION: Our results suggest that PRP, as performed in our study, does not cause significant changes in peripapillary RNFL in diabetic PDR patients after one year of follow-up