arXiv subjects

Piotr Witkowski

Publications and source records attributed to Piotr Witkowski.

16 recordsLinked to original sources

arXiv2024

Sub-Sharvin conductance and incoherent shot-noise in graphene disks at magnetic field

Highly-doped graphene samples show the conductance reduced and the shot-noise power enhanced compared to standard ballistic systems in two-dimensional electron gas. These features can be understood within a model assuming incoherent scattering of Dirac electrons between two interfaces separating the sample and the leads. Here we find, by adopting the above-mentioned model for the edge-free (Corbino) geometry and by means of the computer simulation of quantum transport, that another graphene-specific feature should be observable when the current flow through a doped disk is blocked by high magnetic field. In case the conductance drops to zero, the Fano factor approaches the value of $F\approx{}0.56$, with a very weak dependence on the disk radii ratio. The role of finite source-drain voltages and the system behavior upon tuning the electrostatic potential barrier from a rectangular to parabolic shape are also discussed.

cond-mat.mes-hall↗

arXiv2023

Thermoelectric properties of the Corbino disk in graphene

Thermopower and the Lorentz number for an edge-free (Corbino) graphene disk in the quantum Hall regime is calculated within the Landauer-B\"{u}ttiker formalism. We find, by varying the electrochemical potential, that amplitude of the Seebeck coefficient follows a modified Goldsmid-Sharp relation, in which energy gap is identified with the interval between zero-th and first Landau level in bulk graphene. Analogous relation for the Lorentz number is also determined. Therefore, these thermoelectric properties are solely defined by the magnetic field, temperature, the Fermi velocity in graphene, and fundamental constants including the electrons charge, the Planck and Boltzmann constants, being independent on the system geometric dimensions. This suggests that the Corbino disk in graphene may operate as a thermoelectric thermometer, allowing to determine small temperature difference between two reservoirs, if mean temperature and magnetic field are known.

cond-mat.mes-hall↗

arXiv2022

Theory of the sub-Sharvin charge transport in graphene disks

Ballistic graphene samples in a multimode regime show the sub-Sharvin charge transport, characterized by the conductance reduced by a factor of $\pi/4$ comparing to standard Sharvin contacts in two-dimensional electron gas, and the shot-noise power enhanced up to $F\approx{}1/8$ (with $F$ the Fano factor) [Phys. Rev. B 104, 165413 (2021)]. Here we consider the disk-shaped (Corbino) setup in graphene, with inner radius $r_1$ and outer radius $r_2$, finding that the multimode conductance is slightly enhanced for any $r_1<r_2$, reaching $(4\!-\!\pi)\approx{}0.8684$ of the Sharvin value for $r_1\ll{}r_2$. At the same limit, the Fano factor is reduced, approaching $(9\pi-28)/(12-3\pi)\approx{}0.1065<1/8$. Closed-form approximating expressions for any $r_1/r_2$ ratio are derived supposing incoherent scattering of Dirac fermions on asymmetric double barrier and compared with exact numerical results following from the mode-matching method. Sub-Sharvin values are restored in the narrow-disk limit $r_1/r_2\rightarrow{}1$. For experimentally-accessible radii ratios $0.5\leqslant{}r_1/r_2\leqslant{}0.8$ both the conductance and the Fano factor are noticeably closer to the values predicted for the $r_1\ll{}r_2$ limit, yet still differ from standard Sharvin transport characteristics. The system behavior upon tuning the electrostatic potential barrier from a rectangular to parabolic shape is studied numerically, and the crossover from the sub-Sharvin to standard Sharvin transport regime is demonstrated. Implications for a finite section of the disk are also discussed.

cond-mat.mes-hall↗

arXiv2021

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.

cond-mat.mes-hall↗

arXiv2019

Boundary condition and geometry engineering in electronic hydrodynamics

We analyze the role of boundary geometry in viscous electronic hydrodynamics. We address the twin questions of how boundary geometry impacts flow profiles, and how one can engineer boundary conditions -- in particular the effective slip parameter -- to manipulate the flow in a controlled way. We first propose a micropatterned geometry involving finned barriers, for which we show by an explicit solution that one can obtain effectively no-slip boundary conditions regardless of the detailed microscopic nature of the channel surface. Next we analyse the role of mesoscopic boundary curvature on the effective slip length, in particular its impact on the Gurzhi effect. Finally we investigate a hydrodynamic flow through a circular junction, providing a solution, which suggests an experimental set-up for determining the slip parameter. We find that its transport properties differ qualitatively from the case of ballistic conduction, and thus presents a promising setting for distinguishing the two.

cond-mat.mes-hall↗

arXiv2019

Hall Viscosity in a Strongly Coupled Magnetized Plasma

We show how a Hall viscosity induced by a magnetic field can be generated in strongly coupled theories with a holographic dual. This is achieved by considering parity-breaking higher derivative terms in the gravity dual. These terms couple the Riemann curvature tensor to the field strength of a gauge field dual to the charge current, and have an analog in the field theory side as a coupling between the "Euler current" and the electromagnetic field. As a concrete example, we study the effect of the new terms in the thermodynamic and transport properties of a strongly coupled magnetized plasma dual to a dyonic black hole in $AdS_4$. As a new property of the holographic model, we find that for a state that is initially neutral at zero magnetic field, a charge density and non-dissipative Hall transport are present when the magnetic field is turned on. Remarkably, we also observe that the results from the holographic model are consistent with hydrodynamics even at magnetic fields much larger than temperature.

hep-th↗

arXiv2018

A note on symmetries in the path integral formulation of the Langevin dynamics

We study a dissipative Langevin dynamics in the path integral formulation using the Martin-Siggia-Rose formalism. The effective action is supersymmetric and we identify the supercharges. In addition we study the transformations generated by superderivatives, which were recently included in the cohomological structure emerging in the dissipative systems. We find that these transformations do not generate Ward identities, which are explicitly broken, however, they lead to universal identities, which we derive from Schwinger-Dyson equations. We confirm that the above identities hold in an explicit example of Ornstein-Uhlenbeck process.

hep-th↗

arXiv2018

Completing the Picture: Complexity of Graded Modal Logics with Converse

A complete classification of the complexity of the local and global satisfiability problems for graded modal language over traditional classes of frames have already been established. By "traditional" classes of frames, we mean those characterized by any positive combination of reflexivity, seriality, symmetry, transitivity, and the Euclidean property. In this paper, we fill the gaps remaining in an analogous classification of the graded modal language with graded converse modalities. In particular, we show its NExpTime-completeness over the class of Euclidean frames, demonstrating this way that over this class the considered language is harder than the language without graded modalities or without converse modalities. We also consider its variation disallowing graded converse modalities, but still admitting basic converse modalities. Our most important result for this variation is confirming an earlier conjecture that it is decidable over transitive frames. This contrasts with the undecidability of the language with graded converse modalities.

cs.LO↗

arXiv2018

Pulsating flow and boundary layers in viscous electronic hydrodynamics

Motivated by experiments on a hydrodynamic regime in electron transport, we study the effect of an oscillating electric field in such a setting. We consider a long two-dimensional channel of width $L$, whose geometrical simplicity allows an analytical study as well as hopefully permitting experimental realisation. The response depends on viscosity $ν$, driving frequency, $ω$ and ohmic heating coefficient $γ$ via the dimensionless complex variable $\frac{L^2}ν(iω+γ)=iΩ+Σ$. While at small $Ω$, we recover the static solution, a new regime appears at large $Ω$ with the emergence of a boundary layer. This includes a splitting of the location of maximal flow velocity from the centre towards the edges of the boundary layer, an an increasingly reactive nature of the response, with the phase shift of the response varying across the channel. The scaling of the total optical conductance with $L$ differs between the two regimes, while its frequency dependence resembles a Drude form throughout, even in the complete absence of ohmic heating, against which, at the same time, our results are stable. Current estimates for transport coefficients in graphene and delafossites suggest that the boundary layer regime should be experimentally accessible.

cond-mat.str-el↗

arXiv2017

Time dependence of entanglement for steady state formation in AdS$_3$/CFT$_2$

We consider a holographic model of two 1+1-dimensional heat baths at different temperatures joined at time $t=0$, such that a steady state heat-current region forms and expands in space for times $t>0$. After commenting on the causal structure of the dual 2+1-dimensional spacetime, we present how to calculate the time-dependent entanglement entropy of the boundary system holographically. We observe that the increase rate of the entanglement entropy satisfies certain bounds known from the literature on entanglement tsunamis. Furthermore, we check the validity of several non-trivial entanglement inequalities in this dynamic system.

hep-th↗

arXiv2017

Time evolution of entanglement for holographic steady state formation

Within gauge/gravity duality, we consider the local quench-like time evolution obtained by joining two 1+1-dimensional heat baths at different temperatures at time t=0. A steady state forms and expands in space. For the 2+1-dimensional gravity dual, we find that the shockwaves expanding the steady-state region are of spacelike nature in the bulk despite being null at the boundary. However, they do not transport information. Moreover, by adapting the time-dependent Hubeny-Rangamani-Takayanagi prescription, we holographically calculate the entanglement entropy and also the mutual information for different entangling regions. For general temperatures, we find that the entanglement entropy increase rate satisfies the same bound as in the "entanglement tsunami" setups. For small temperatures of the two baths, we derive an analytical formula for the time dependence of the entanglement entropy. This replaces the entanglement tsunami-like behaviour seen for high temperatures. Finally, we check that strong subadditivity holds in this time-dependent system, as well as further more general entanglement inequalities for five or more regions recently derived for the static case.

hep-th↗

arXiv2017

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 $α_{xy}\sim N^{3/2}$, as expected. For the same model, our approach yields a finite result for the heat conductivity $κ/T \propto N^{3/2}$ even for $T \rightarrow 0$.

hep-th↗

arXiv2016

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.

cs.LO↗

arXiv2016

Bounded Model Checking of Pointer Programs Revisited

Bounded model checking of pointer programs is a debugging technique for programs that manipulate dynamically allocated pointer structures on the heap. It is based on the following four observations. First, error conditions like dereference of a dangling pointer, are expressible in a~fragment of first-order logic with two-variables. Second, the fragment is closed under weakest preconditions wrt. finite paths. Third, data structures like trees, lists etc. are expressible by inductive predicates defined in a fragment of Datalog. Finally, the combination of the two fragments of the two-variable logic and Datalog is decidable. In this paper we improve this technique by extending the expressivity of the underlying logics. In a~sequence of examples we demonstrate that the new logic is capable of modeling more sophisticated data structures with more complex dependencies on heaps and more complex analyses.

cs.LO↗

arXiv2015

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.

hep-th↗

arXiv2012

Satisfiability vs. Finite Satisfiability in Elementary Modal Logics

We study elementary modal logics, i.e. modal logic considered over first-order definable classes of frames. The classical semantics of modal logic allows infinite structures, but often practical applications require to restrict our attention to finite structures. Many decidability and undecidability results for the elementary modal logics were proved separately for general satisfiability and for finite satisfiability [11, 12, 16, 17]. In this paper, we show that there is a reason why we must deal with both kinds of satisfiability separately -- we prove that there is a universal first-order formula that defines an elementary modal logic with decidable (global) satisfiability problem, but undecidable finite satisfiability problem, and, the other way round, that there is a universal formula that defines an elementary modal logic with decidable finite satisfiability problem, but undecidable general satisfiability problem.

cs.LO↗