Two-variable Logic with Counting and a Linear Order

We study the finite satisfiability problem for the two-variable fragment of first-order logic extended with counting quantifiers (C2) and interpreted over linearly ordered structures. We show that the problem is undecidable in the case of two linear orders (in the presence of two other binary symbols). In the case of one linear order it is NEXPTIME-complete, even in the presence of the successor relation. Surprisingly, the complexity of the problem explodes when we add one binary symbol more: C2 with one linear order and in the presence of other binary predicate symbols is equivalent, under elementary reductions, to the emptiness problem for multicounter automata

RecSys Challenge 2016: job recommendations based on preselection of offers and gradient boosting

We present the Mim-Solution's approach to the RecSys Challenge 2016, which ranked 2nd. The goal of the competition was to prepare job recommendations for the users of the website Xing.com. Our two phase algorithm consists of candidate selection followed by the candidate ranking. We ranked the candidates by the predicted probability that the user will positively interact with the job offer. We have used Gradient Boosting Decision Trees as the regression tool.Comment: 6 pages, 1 figure, 2 tables, Description of 2nd place winning solution of RecSys 2016 Challange. To be published in RecSys'16 Challange Proceeding

Hydrodynamics in solid state transport, from microscopic to mesoscopic scales

The thesis is devoted to some aspects of the solid-state electronic transport in the so-called viscous or hydrodynamic regime. Hydrodynamic regime in this context means that due to the large carrier density and non-negligible carrier-carrier interactions, the transport properties follow from collective, rather than single-particle phenomena. To capture the dynamics of such a system one may use description based on the conserved quantities, i.e. momentum, energy or charge. If the interactions between the constituents of the system are strong enough, such a description is provided by the hydrodynamic equations which for conserved momentum and energy are the Navier-Stokes equations or their relativistic counterparts. This thesis focuses on such a situation: when the equations governing transport properties follow from conservation of the momentum or, at most, can be treated as a modification of such equations due to weak momentum relaxation. Presented here are two lines of investigation. The first one focuses on the mesoscopic effects, i.e. on the dependence of the outcome of the transport measurements on the physical parameters of the sample such as size and shape. Here also the effects of the weak momentum relaxation are studied. In the second one, the issue of parity and time reversal symmetry breaking, occurring in a 2 dimensional system due to the presence of an external magnetic field, is investigated. An effective model of a strongly coupled quantum system is introduced and used to compute the odd (Hall) viscosity -- a transport coefficient allowed once the discrete symmetries are broken -- as a function of magnetic field, temperature and chemical potential. The first part of results concerns the behaviour of the electronic fluid in a typical AC measurement -- modeled by an elongated channel in which the fluid is subject to a periodically time dependent electric potential. Assuming standard, no-slip boundary conditions, the spatial distribution of the current density is found to be much different to the one known for Ohmic conduction. For small frequency the current distribution has a parabolic profile across the channel, while for high frequency the current in the bulk of the channel becomes flat (position-independent), while two maxima terminating a so-called boundary layer develop. In these boundary layers large gradients of current can be found, contributing to high local entropy production due to the viscous force. Despite this differences in the local current density profile, when the global conductance is measured as a function of the frequency, the result much resembles the well known Drude curve, with a distinct maximum visible in the imaginary part of the AC conductance. There is, however, a global signature of the boundary layer formation -- the scaling of the conductance with the channel width, that changes from quadratic (for parabolic flow) to asymptotically constant (for a flow with boundary layers). Moreover, in the hydrodynamic regime, the position of the Drude peak is not only determined by microscopic parameter but again by a combination of microscopic (viscosity) and mesoscopic (width) parameters. Since the Drude peak occurs for experimentally feasible values of parameters, the mentioned mesoscopic dependence may be used to measure the value of viscosity coefficient. The results discussed above are obtained assuming, as is traditional for hydrodynamics on everyday length-scales, a no-slip boundary condition which forces the fluid to be immobile at the boundary. This boundary condition was also assumed in most of the previous works on the electronic hydrodynamics. However, this is not the only possibility. There exists a one-parameter family of consistent boundary conditions involving velocity and its derivative on the boundary, parametrized by a coefficient called the slip length. Recent theoretical and experimental publications suggest that it may be dependent on the state parameters of the system (i.e temperature, chemical potential) and its value may be relatively large for some experimental situations. One of the consequences of the slip length being large is that hydrodynamic effects are obscured in the simple AC set-up discussed before. In this work it is shown that by an appropriate micro-structuring of the boundary, the effects of slip can be suppressed. Once the array of defects is introduced on the edges of the sample, the no-slip behavior is restored for all the values of the microscopic slip length. Furthermore, the interplay between the microscopic slip length and the sample geometry is investigated and used to propose a simple device for measuring the dependence of the microscopic slip length on the state parameters such as the temperature or the chemical potential. The final part of this thesis is devoted to a different aspect of the hydrodynamic transport -- a computation of the value of hydrodynamic transport coefficients using a microscopic theory. The physical situation of interest is one in which time reversal and parity invariance of a 2-dimensional system are broken, due to the presence of an external magnetic field. In such a situation an unusual class of transport coefficients is allowed in the hydrodynamic description, so-called odd coefficients. The term comes from the fact that they encode response that is transverse to the applied perturbation. These odd coefficients for 2 dimensions were previously studied mostly at weak coupling, i.e. using descriptions based on quasi-particles. This work, however, presents the way of calculating them for strongly coupled model system. To achieve this a high-energy-physics-inspired framework of holographic duality (AdS/CFT) is used. In that approach, an effective model involving magnetically-sourced parity-breaking interactions is constructed for the system at finite temperature and chemical potential. Performing a linear response analysis around the thermal states in that model allows one to read off the transport coefficients, especially the odd (Hall) viscosity coefficient that is of central interest in this study. The mentioned Hall viscosity is found to be non-zero whenever the magnetic field is present, even for zero chemical potential. This is unusual, as odd viscosity is expected to only be non-zero for non-zero charge density states. The mechanism responsible for the presence of Hall viscosity in the discussed case turns out to be the following: charge density in the model is induced by either the chemical potential or the magnetic field, i.e. for non-zero magnetic field even at zero chemical potential some density of charge is present. This charge contributes to the Hall viscosity in the usual way. The odd viscosity coefficient is found to have different scaling behaviors for weak and strong magnetic field. Interestingly, it turns out that the computations of the Hall (and shear) viscosities are relatively straightforward and analytically tractable in the proposed model. This means that the results could be generalized to the zero-temperature case, which however is yet to be done. It also suggests that the model may capture some universal mechanisms of generating the odd viscosity due to the presence of the magnetic field. That intuition is backed by the fact that some of the effective models of quantum Hall states also predict similar mechanism in which charge density is induced by the presence of the magnetic field. Despite these similarities, further studies are needed to establish a solid connection between these systems. In particular, in the model under consideration no mechanism of quantization of the Hall viscosity is found, while the mentioned models of quantum Hall states predict quantization of that transport coefficient

Conformal defects in supergravity - backreacted Dirac delta sources

We construct numerically gravitational duals of theories deformed by localized Dirac delta sources for scalar operators both at zero and at finite temperature. We find that requiring that the backreacted geometry preserves the original scale invariance of the source uniquely determines the potential for the scalar field to be the one found in a certain Kaluza-Klein compactification of $11D$ supergravity. This result is obtained using an efficient perturbative expansion of the backreacted background at zero temperature and is confirmed by a direct numerical computation. Numerical solutions at finite temperatures are obtained and a detailed discussion of the numerical approach to the treatment of the Dirac delta sources is presented. The physics of defect configurations is illustrated with a calculation of entanglement entropy.Comment: 23 pages, 12 figure

Conductivities from attractors

In the context of applications of the AdS/CFT correspondence to condensed matter physics, we compute conductivities for field theory duals of dyonic planar black holes in 3+1-dimensional Einstein-Maxwell-dilaton theories at zero temperature. We combine the near-horizon data obtained via Sen's entropy function formalism with known expressions for conductivities. In this way we express the conductivities in terms of the extremal black hole charges. We apply our approach to three different examples for dilaton theories for which the background geometry is not known explicitly. For a constant scalar potential, the thermoelectric conductivity explicitly scales as $\alpha_{xy}\sim N^{3/2}$, as expected. For the same model, our approach yields a finite result for the heat conductivity $\kappa/T \propto N^{3/2}$ even for $T \rightarrow 0$.Comment: 29 page

Sub-Sharvin conductance and enhanced shot noise in doped graphene

Ideal Sharvin contact in a multimode regime shows the conductance $G\approx{}G_{\rm Sharvin}=g_0k_F{}W/\pi$ (with $g_0$ the conductance quantum, $k_F$ the Fermi momentum, and $W$ the contact width) accompanied by strongly suppressed shot-noise quantified by small Fano factor $F\approx{}0$. For ballistic graphene away from the charge-neutrality point the sub-Sharvin transport occurs, characterised by suppressed conductance $G\approx{}(\pi/4)\,G_{\rm Sharvin}$ and enhanced shot noise $F\approx{}1/8$. All these results can be derived from a basic model of quantum scattering, involving assumptions of infinite height and perfectly rectangular shape of the potential barrier in the sample. Here we have carried out the numerical analysis of the scattering on a family of smooth barriers of finite height interpolating between parabollic and rectangular shapes. We find that tuning the barrier shape one can modify the asymmetry between electron- and hole-doped systems. For electronic dopings, the system crosses from Sharvin to sub-Sharvin transport regime (indicated by both the conductance and the Fano factor) as the potential becomes closer to the rectangular shape. In contrast, for hole dopings, the conductivity is strongly suppressed when the barrier is parabolic and slowly converges to $G\approx{}(\pi/4)\,G_{\rm Sharvin}$ as the potential evolves towards rectangular shape. In such a case the Sharvin transport regime is inaccessible, shot noise is generically enhanced (with much slower convergence to $F\approx{}1/8$) comparing to the electron-doped case, and aperiodic oscillations of both $G$ and $F$ are prominent due to the formation of quasibound states.Comment: RevTeX, 7 pages, 5 figure

Absolute Frequency Measurement of Rubidium 5S-7S Two-Photon Transitions

We report the absolute frequency measurements of rubidium 5S-7S two-photon transitions with a cw laser digitally locked to an atomic transition and referenced to an optical frequency comb. The narrow, two-photon transition, 5S-7S (760 nm) insensitive to first order in a magnetic field, is a promising candidate for frequency reference. The performed tests yield the transition frequency with accuracy better than reported previously.Comment: This paper was published in Optics Letters and is made available as an electronic reprint with the permission of OSA. The paper can be found at http://dx.doi.org/10.1364/OL.38.004581. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under la

Motywacja osiągnięć, kluczowe autoewaluacje i kompetencje społeczne młodocianych przestępców

Adolescence is characterised by dynamic changes in both physical and mental development. During this period, it is particularly easy to develop antisocial behaviours and contact with people from criminal groups. The article presents the results of a pilot study which aim was to compare the results of the level of social competences, core self-evaluations and achievement motivation between a group of adolescents placed in juvenile detention centres and a group of high school students. According to the author, these variables may be psychological resources that contribute to effective resocialization and psychotherapy of adolescents placed in correctional institutions. The analysis of the results showed significant differences in the level of core self-evaluations, except that the group of juvenile offenders scored significantly higher than the comparison group. The remaining differences were not statistically significant. Additional analyses showed a strong relationship between the level of core self-evaluations and the level of social competence and a moderate correlation between the level of achievement motivation and the level of social competence in the group of juvenile offenders. Appropriate use of the results may contribute to increasing the effectiveness of resocialization and reducing the risk of recidivism.Okres dojrzewania charakteryzuje się dynamicznymi zmianami w rozwoju fizycznym i psychicznym. Szczególnie łatwo wówczas o rozwój antyspołecznych zachowań oraz kontakt z osobami z grup o charakterze przestępczym. W artykule zaprezentowano wyniki badania pilotażowego, którego celem było porównanie wyników poziomu kompetencji społecznych, kluczowych autoewaluacji oraz motywacji osiągnięć między grupą młodocianych umieszczonych w ośrodkach resocjalizacyjnych a grupą młodzieży ze szkół średnich. Wymienione zmienne zdaniem autora mogą być zasobami psychologicznymi, które przyczyniają się do efektywnej resocjalizacji oraz psychoterapii młodzieży umieszczonej w zakładach poprawczych. Analiza wyników wykazała istotne różnice w poziomie kluczowych autoewaluacji, z tym że grupa młodocianych przestępców osiągnęła wyniki istotnie wyższe niż grupa porównawcza. Pozostałe różnice nie są istotne statystycznie. Dodatkowe analizy wykazały silny związek między poziomem kluczowych autoewaluacji a poziomem kompetencji społecznych oraz umiarkowaną korelację między poziomem motywacji osiągnięć a poziomem kompetencji społecznych w grupie młodocianych przestępców. Odpowiednie wykorzystanie wyników może przyczynić się do podniesienia efektywności resocjalizacji i zmniejszenia ryzyka recydywy