Optical Properties of Heavy Fermion Systems with SDW Order

The dynamical conductivity $\sigma (\omega)$, reflectivity $R(\omega)$, and tunneling density of states $N(\omega)$ of strongly correlated systems (like heavy fermions) with a spin-density wave (SDW) magnetic order are studied as a function of impurity scattering rate and temperature. The theory is generalized to include strong coupling effects in the SDW order. The results are discussed in the light of optical experiments on heavy-fermion SDW materials. With some modifications the proposed theory is applicable also to heavy fermions with localized antiferromagnetic (LAF) order.Comment: 9 pages, 10 figure

ARPES kink is a "smoking gun" for the theory of high-Tc superconductors: dominance of the electron-phonon interaction with forward scattering peak

The ARPES spectra in high-Tc superconductors show four distinctive features in the quasiparticle self-energy. All of them can be explained consistently by the theory in which the electron phonon interaction (EPI) with the forward scattering peak dominates over the Coulomb scattering. In particular, this theory explains why there is no shift of the nodal kink at 70 meV in the superconducting state, contrary to the clear shift of the anti-nodal singularity at 40 meV. The theory predicts a ``knee''-like structure of the imaginary part of the self-energy, which is phonon dominated for $\omega \approx \omega^{(70)}_{ph}$, and shows linear behavior for $\omega > \omega^{(70)}_{ph}$ - due to the Coulomb scattering. Recent ARPES spectra give that the EPI coupling constant is much larger than the Coulomb one. The dip-hump structure in the spectral function comes out naturally from the proposed theory.Comment: 5 pages, 3 figure

Teachers’ perspective on emergency remote teaching during COVID-19 at tertiary level

Організація процесу навчальної діяльності завжди є складним завданням навіть за звичайних обставин, але все значно ускладнюється під час введення надзвичайного стану, коли до цього додається використання цифрових технологій. У такий період необхідно пам’ятати, що зміст навчання залишається незмінним, але способи його представлення можуть бути інакшими. Зазвичай ми звикли до синхронного прямого спілкування в традиційному класі, але зараз стикаємося з асинхронними каналами та модульним підходом до навчання за допомогою цифрових інструментів, які (не)відомі нам і під час використання яких, незважаючи на їх очевидну значущість, виникають перешкоди в навчальному процесі з різних причин, як-от: доступ до Інтернету, цифрові навички, відповідне обладнання або економічний та соціальний статус. Незважаючи на розвиток цифрових технологій та експоненційне зростання цифрових додатків та інструментів, які проклали шлях до онлайн освіти в усьому світі за останні роки, вимушене швидке реагування під час пандемії виявило недостатню готовність навчальних закладів до такого типу змін. У статті досліджено сприйняття викладачами університетів впровадження екстреного дистанційного навчання у період з березня по вересень 2020 року через пандемію COVID-19. Тема цього дослідження - вивчення афективного ставлення викладачів Університету Приштини в Косовській Митровіці до екстреного дистанційного навчання, завдання дослідження - сприйняття вчителями нової реальності, шляхи їх пристосування до екстреного дистанційного навчання. До дослідження були залучені студенти трьох основних факультетів університету: філософського, мистецтв і медичного з основними напрямами навчання - природничі, гуманітарні та мистецькі науки. Дані були отримані під час опитування, респонденти відбиралися за допомогою зручної вибіркової техніки. Мета дослідження - представлення аналізу отриманих даних з метою розробки майбутньої програми підготовки викладачів ЗВО, яка враховуватиме принципи, наукові засади та практичний досвід дистанційної освіти для покращення якості реагування вищих навчальних закладів на надзвичайні ситуації в цілому.Revising the way we organize the teaching activities is never an easy task even under normal circumstances, and when the state of emergency and the use of digital technology are added to the mix, the situation becomes even more complicated. During this process, it is easy to forget that although the content of teaching remains the same, the ways of delivering that content will change. Accustomed to synchronous, direct communication in a traditional classroom, we are now faced with asynchronous channels and a modular approach to teaching with the help of digital tools that are (un)known to us and which, despite their apparent value, present barriers to teaching and learning for various reasons such as Internet access, digital skills, adequate equipment, and economic and social status. Despite the development of digital technology and the exponential growth of digital applications and tools that have paved the way for online education worldwide in recent years, our emergency response to the pandemic indicates the lack of readiness of educational institutions for this type of change. Therefore, the paper examines how university teachers perceive the emergency remote teaching imposed on them from March to September 2020 due to the COVID-19 pandemic. The research problem of this study concerns the affective attitudes of the lecturers at the University of Priština in Kosovska Mitrovica towards emergency remote teaching. Namely, the authors are set with the task of investigating how teachers perceive the new reality and how they have managed to acclimate to emergency remote teaching. For the purposes of this study, three major schools at the University were selected: the Faculty of Philosophy, the Faculty of Arts, and the Faculty of Medicine. The three schools represent the bulk of the University academic offer: sciences, humanities, and arts. The data is compiled by means of a survey, while the respondents are selected by the convenience sampling technique. The aim of the study is to present the analysis of the data obtained in order to develop a future training programme for university lecturers in principles and practices of the distance education and to improve the emergency response of the higher education institutions as a whole

Charge fluctuations and electron-phonon interaction in the finite-UU Hubbard model

In this paper we employ a gaussian expansion within the finite-$U$ slave-bosons formalism to investigate the momentum structure of the electron-phonon vertex function in the Hubbard model as function of $U$ and $n$. The suppression of large momentum scattering and the onset a small-${\bf q}$ peak structure, parametrized by a cut-off $q_c$, are shown to be essentially ruled by the band narrowing factor $Z_{\rm MF}$ due to the electronic correlation. A phase diagram of $Z_{\rm MF}$ and $q_c$ in the whole $U$-$n$ space is presented. Our results are in more than qualitative agreement with a recent numerical analysis and permit to understand some anomalous features of the Quantum Monte Carlo data.Comment: 4 pages, eps figures include

Decoupling of superconducting layers in magnetic superconductor RuSr_{2}GdCu_{2}O_{8}

We propose the model for magnetic properties of the magnetic superconductor RuSr$_{2}$GdCu$_{2}$O$_{8}$, which incorporates the theory of the superconducting/ferromagnetic multilayers. The transition line $T_{d}(h)$, on which the Josephson coupled superconducting planes are decoupled, i.e. $j_{c}(T_{d})=0$, is calculated as a function of the exchange energy $h$. As the result of this decoupling a nonmonotonic behavior of magnetic properties, like the lower critical field $H_{c1}$, Josephson plasma frequency, etc. is realized near (or by crossing) the $T_{d}(h)$ line. The obtained results are used in analyzing the newly discovered antiferromagnetic ruthenocuprate RuSr$_{2}$GdCu$_{2}$O$_{8}$ with possible weak ferromagnetic order in the RuO planes.Comment: 12 pages, 3 figs embede

Anisotropic impurities in anisotropic superconductors

Physical properties of anisotropic superconductors like the critical temperature and others depend sensitively on the electron mean free path. The sensitivity to impurity scattering and the resulting anomalies are considered a characteristic feature of strongly anisotropic pairing. These anomalies are usually analyzed in terms of s-wave impurity scattering which leads to universal pair breaking effects depending on only two scattering parameters, the mean free path and the impurity cross section. We investigate here corrections coming from anisotropies in the scattering cross section, and find not only quantitative but also qualitative deviations from universal s-wave isotropic pairbreaking. The properties we study are the transition temperature, the density of states, quasiparticle bound states at impurities, and pinning of flux lines by impurities.Comment: 19 page

Time of life as it is in LiFeAs

The time of life of fermionic quasiparticles, the distribution of which in the momentum-energy space can be measured by angle resolved photoemission (ARPES), is the first quantity to look for fingerprints of interaction responsible for the superconducting pairing. Such an approach has been recently used for superconducting cuprates, but its direct application to pnictides was not possible due to essential three-dimensionality of the electronic band structure and magnetic ordering. Here, we report the investigation of the quasiparticle lifetime in LiFeAs, a non-magnetic stoichiometric superconductor with a well separated two-dimensional band. We have found two energy scales: the lower one contains clear fingerprints of optical phonon modes while the higher scale indicates a presence of strong electron-electron interaction. The result suggests that LiFeAs is a phonon mediated superconductor with strongly enhanced electronic density of states at the Fermi level.Comment: reevaluated electron-phonon coupling strength, added reference

Semi-classical buckling of stiff polymers

A quantitative theory of the buckling of a worm like chain based on a semi-classical approximation of the partition function is presented. The contribution of thermal fluctuations to the force-extension relation that allows to go beyond the classical Euler buckling is derived in the linear and non-linear regime as well. It is shown that the thermal fluctuations in the nonlinear buckling regime increase the end-to-end distance of the semiflexible rod if it is confined to 2 dimensions as opposed to the 3 dimensional case. Our approach allows a complete physical understanding of buckling in D=2 and in D=3 below and above the Euler transition.Comment: Revtex, 17 pages, 4 figure

Social Roles and Baseline Proxemic Preferences for a Domestic Service Robot

© The Author(s) 2014. This article is published with open access at Springerlink.com. This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited. The work described in this paper was conducted within the EU Integrated Projects LIREC (LIving with Robots and intEractive Companions, funded by the European Commission under contract numbers FP7 215554, and partly funded by the ACCOMPANY project, a part of the European Union’s Seventh Framework Programme (FP7/2007–2013) under grant agreement n287624The goal of our research is to develop socially acceptable behavior for domestic robots in a setting where a user and the robot are sharing the same physical space and interact with each other in close proximity. Specifically, our research focuses on approach distances and directions in the context of a robot handing over an object to a userPeer reviewe

Nonadiabatic Superconductivity and Vertex Corrections in Uncorrelated Systems

We investigate the issue of the nonadiabatic superconductivity in uncorrelated systems. A local approximation is employed coherently with the weak dependence on the involved momenta. Our results show that nonadiabatic vertex corrections are never negligible, but lead to a strong suppression of $T_c$ with respect to the conventional theory. This feature is understood in terms of the momentum-frequency dependence of the vertex function. In contrast to strongly correlated systems, where the small ${\bf q}$-selection probes the positive part of vertex function, vertex corrections in uncorrelated systems are essentially negative resulting in an effective reduction of the superconducting pairing. Our analysis shows that vertex corrections in nonadiabatic regime can be never disregarded independently of the degree of electronic correlation in the system.Comment: 4 pages, 3 eps fig