arXiv subjects

Adam Z. Kaczmarek

Publications and source records attributed to Adam Z. Kaczmarek.

At least 19 recordsLinked to original sources

arXiv2026

Nonlocal correlations for bosonic fields in black hole quantum atmosphere

Recent theoretical studies propose that Hawking radiation may not emerge strictly at the event horizon but rather from the spatially extended region surrounding a black hole, commonly referred to as the quantum atmosphere. In this work, we explore how this concept influences nonlocal quantum correlations in a bosonic bipartite system located at certain distance from a Schwarzschild black hole. By employing the measurement-induced nonlocality (MIN), as a quantifier of quantum correlations, we analyze the response of bosonic fields to the thermal and geometric characteristics associated with the Hartle-Hawking vacuum. In this manner, we extend previous studies that primarily focused on the fermionic systems. Our results reveal that, when quantum atmosphere is taken into account, the behavior of MIN departs from its conventional near-horizon profile. In particular, bosonic nonlocal correlations are found to exhibit a pronounced degradation at a finite radial distance from the event horizon and to ultimately vanish as scaled distance increases further. To some extent this behavior contrasts with the previously considered fermionic case, indicating that bosonic fields provide potentially stronger response to the quantum atmosphere.

quant-ph↗

arXiv2026

Entropic signatures of market response under concentrated policy communication

The first 100 days of Donald Trump second presidential term (January 20th - April 30th, 2025) featured policy actions with potential market repercussions, constituting a well-suited case study of a concentrated policy scenario. Here, we provide a first look at this period, rooted in the information theory, by analyzing major stock indices across the Americas, Europe as well as Asia and Oceania. Our approach jointly examines dispersion (standard deviation) and information complexity (entropy), but also employs a sliding window cumulative entropy to localize extreme events. We find a notable decoupling between the first two measures, indicating that entropy is not merely a proxy for amplitude but reflects the diversity of populated outcomes. As such, they allow us to capture both market volatility and narrative constraints, signaling large and coherent moves driven by policy changes. In turn, the cumulative entropy is found to notably increase during regional episodes with high information density, providing effective signatures of such events. We argue that the obtained results indicate short-term globally coupled, yet regionally modulated, market impacts with clear connection to introduced policies. In what follows, the presented entropic framework emerges as an efficient complement to standard methods for characterizing markets under turbulent conditions, with potential to enhance forecasting strategies such as the stochastic modeling.

q-fin.ST↗

arXiv2025

Signatures of extreme events in the cumulative entropic spectrum

In this study, the cumulative effect of the empirical probability distribution of a random variable is identified as a factor that amplifies the occurrence of extreme events in datasets. To quantify this observation, a corresponding information measure is introduced, drawing upon Shannon entropy for joint probabilities. The proposed approach is validated using selected market data as case studies, encompassing various instances of extreme events. In particular, the results indicate that the introduced cumulative measure exhibits distinctive signatures of such events, even when the data is relatively noisy. These findings highlight the potential of the discussed concept for developing a new class of related indicators or classifiers.

physics.data-an↗

arXiv2024

Dynamical reconstruction of the $\Lambda$CDM model in the scalar-tensor representation of $f\left(Q,T\right)$ gravity

Motivated by the growing interest in the nonmetricity-matter couplings, we develop the scalar-tensor formulation of recently introduced $f(Q,T)$ gravity, where $Q$ is the nonmetricity and $T$ is the trace of the energy-momentum tensor. The main properties of the scalar-tensor formalism for the Friedmann-Lema{\^ i}tre-Robertson-Walker (FLRW) Universe are discussed, and we introduce an appropriate set of dynamical variables to analyze the cosmic evolution of the scalar-tensor $f(Q,T)$ cosmology as a dynamical system. By considering two distinct cosmic fluids, namely matter and radiation, we have demonstrated that the cosmological phase space exhibits the typical curvature-dominated, radiation-dominated, matter-dominated, and exponentially accelerated fixed points. Furthermore, under an appropriate set of initial conditions compatible with the current observations from the Planck satellite, our analysis shows that the scalar-tensor $f(Q,T)$ successfully yields models indistinguishable from the $\Lambda$CDM cosmology and compatible with the weak-field solar system dynamics, without the inclusion of a cosmological constant $\Lambda$. Thus, the theory introduced herein may be regarded as a suitable candidate to describe the cosmological dynamics of the Universe.

gr-qc↗

arXiv2024

Thermodynamics of the Van der Waals black hole within nonextensive Kaniadiakis entropy

In this work, we have studied the thermodynamic properties of the Van der Waals black hole in the framework of the nonextensive Kaniadakis entropy. We have shown that the black hole properties, such as the mass and temperature, differ from those obtained by using the the Boltzmann-Gibbs approach. Moreover, the nonextensivity \k{appa}-parameter changes behavior of the Gibbs free energy via introduced thermodynamic instabilities, whereas the emission rate is influenced by \k{appa} only at low frequencies. Nonetheless, the pressure-volume (P(V)) characteristics are found independent of \k{appa} and the entropy form, unlike in other anti-de Sitter (AdS) black hole models. In summary, presented findings partially support previous arguments of Gohar and Salzano that under certain circumstances all entropic models are equivalent and indistinguishable [1].

gr-qc↗

arXiv2024

Coherence of multipartite quantum states in the black hole quantum atmosphere

According to the recently introduced concept of quantum atmosphere, the black hole radiation is suggested to originate from the quantum excitations at the effective distance ($r$) near the event horizon ($r_H$). Here, this concept is explored from the quantum resource perspective by analysing the coherence of multipartite quantum systems located near a black hole. For the Greenberger-Horne-Zeilinger state, it is found that signatures of the atmosphere are apparent. This is to say, the coherence exhibits peak close to the event horizon and next decreases, recovering conventional behavior at $r/r_H \rightarrow\infty$. Interestingly, it is shown that as the quantum state gets more complex and the number of parties increases, the role of quantum atmosphere diminishes and the standard behaviour expected for the $N$-partite coherence quantifiers can be observed. That means, in case of complex setups the quantum atmosphere signatures may not be detectable. Hence, our findings show that care should be taken, regarding size of a system, when quantum atmosphere argument is considered.

gr-qc↗

arXiv2024

Dark matter effects of a black hole with nonsingular Yukawa-modified potential in Einstein-Gauss-Bonnet Gravity

This paper investigates the contribution of the nonsingular Yukawa-modified potential in the context of four-dimensional Einstein-Gauss-Bonnet (EGB) gravity modeling by a static and spherically symmetric black hole solution. These Yukawa-type corrections are essentially described along two parameters, $\beta$ and $\lambda$, affecting Newton's law of gravity at large distances, and a deformation parameter $\ell_0$, which is essential at short distances. Primarily, the strongest effect is encoded in $\beta$, which alters the total mass of the black hole with additional mass proportional to $\beta\mathcal{M}$, imitating the effects of dark matter at large distances from the black hole. In contrast, the effect due to $\lambda$ is small for astrophysical values. On the other hand, the EGB gravity is ruled by the Gauss-Bonnet (GB) coupling constant $\alpha$, a fundamental parameter of the theory. We pay particular attention to thermodynamic stability, critical orbits, geodesics and quasinormal modes. The results demonstrate stability of the black hole solution for a range of values of the GB coupling constant $\alpha$. Furthermore, this study investigates the null geodesic motion, namely the shadow behavior, providing intriguing results in relation to the size of the black hole shadow.

gr-qc↗

arXiv2024

Mimetic-$f(Q)$ gravity: cosmic reconstruction and energy conditions

In this study, we present a novel approach to mimetic gravity incorporating a non-zero nonmetricity tensor with vanishing torsion and curvature, establishing a generalized mimetic-$f(Q)$ gravity framework. Using the Lagrange multiplier method, we have obtained and discussed characteristics of the theory's field equations. In order to study cosmic evolution given by the hybrid scale factor, we implemented the reconstruction method in two different ways. In the first case, we have obtained corresponding Lagrange multiplier $\eta$ and potential $U$ for the specific $f(Q)=f(Q)=Q-6\lambda M^2\big(\frac{Q}{6M^2}\big)^\alpha$ function, while in the second scenario we have recovered $f(Q)$ functional and mimetic potential for the given Lagrange multiplier $\eta_0+\gamma H^2$. Subsequently, we explore the fundamental properties of the $f(Q)=Q-6\lambda M^2\big(\frac{Q}{6M^2}\big)^\alpha$ model and analyse the energy conditions to establish its validity. Our findings indicate that the framework introduced herein allows for the derivation of a wide range of viable cosmological models that satisfy energy constraints that are necessary in description of the accelerated expansion.

gr-qc↗

arXiv2023

The hybrid cosmology in the scalar-tensor representation of $f(\mathcal{G},T)$ gravity

In this work, the $f(\mathcal{G},T)$ theory of gravity is recast in terms of the $\phi$ and $\psi$ fields within the scalar-tensor formulation, where $\mathcal{G}$ is the Gauss-Bonnet term and $T$ denotes the trace of the energy-momentum tensor. The general aspects of the introduced reformulation are discussed and the reconstruction of the cosmological scenarios is presented, focusing on the so-called hybrid evolution. As a result, the scalar-tensor $f(\mathcal{G},T)$ theory is successfully reconstructed for the early and late time approximations with the corresponding potentials. The procedure of recovering the $f(\mathcal{G},T)$ theory in the original formulation is performed for the late time evolution and a specific quadratic potential. The scalar-tensor formulation introduced herein not only facilitates the description of various cosmic phases but also serves as a viable alternative portrayal of the $f(\mathcal{G},T)$ gravity which can be viewed as an extension of the well-established scalar Einstein-Gauss-Bonnet gravity.

gr-qc↗

arXiv2023

Cosmological aspects of the unimodular-mimetic $f(\mathcal{G})$ gravity

In this work we introduce and study the unimodular-mimetic $f(\mathcal{G})$ gravity, where unimodular and mimetic constraints are incorporated through corresponding Lagrange multipliers. We present field equations governing this theory and discuss their main properties. By using the reconstruction scheme, we obtain quadratic unimodular-mimetic $f(\mathcal{G})=A\mathcal{G}^2$ gravity capable of describing hybrid expansion law and the power law evolution. Furthermore, we employ an inverted reconstruction technique in order to derive specific $f(\mathcal{G})$ function that reproduces the Hubble rate of symmetric bounce. The unimodular-mimetic $f(\mathcal{G})=A\mathcal{G}^2$ is also shown to be compatible with the BICEP2/Keck and Planck data. To this end, we incorporate updated constraints on the scalar-to-tensor ratio and spectral index, utilizing a perfect fluid approach to the slow-roll parameters. Through an analysis of that kind, we demonstrate that the theoretical framework presented here can indeed characterize inflation that agrees with the observational findings. Consequently, the introduced extension appears to have potential to describe and encompass a wide spectrum of cosmological models.

gr-qc↗

arXiv2023

Signatures of the black hole quantum atmosphere in nonlocal correlations

Recently, it was suggested that the Hawking radiation may originate not at the event horizon but in the quantum region outside of it, known as the quantum atmosphere. The present study attempts to explore this argument further by assessing its role in shaping quantum correlations near a black hole. Herein, these are conveniently captured within the geometric measure of nonlocality, termed as the measurement-induced nonlocality, and found to exhibit signatures of the atmosphere. In particular, a notable loss of correlations is observed well outside the event horizon, coinciding with the peak of particles radiation in the atmosphere region. Still, the correlations are shown to be always finite therein and to continuously scale with not only the radiation temperature but also with the horizon size. Hence, some characteristics of the atmosphere appears to be detectable at the quantum correlations level, providing novel insight and means to help verify the concept of interest.

gr-qc↗

arXiv2023

Entropy of financial time series due to the shock of war

The concept of entropy is not uniquely relevant to the statistical mechanics but among others it can play pivotal role in the analysis of a time series, particularly the stock market data. In this area sudden events are especially interesting as they describe abrupt data changes which may have long-lasting effects. Here, we investigate the impact of such events on the entropy of financial time series. As a case study we assume data of polish stock market in the context of its main cumulative index. This index is discussed for the finite time periods before and after outbreak of the 2022 Russian invasion of Ukraine, acting as the sudden event. The analysis allows us to validate the entropy-based methodology in assessing market changes as driven by the extreme external factors. We show that qualitative features of market changes can be captured quantitatively in terms of the entropy. In addition to that, the magnitude of the impact is analysed over various time periods in terms of the introduced entropic index. To this end, the present work also attempts to answer whether or not the recent war can be considered as a reason or at least catalyst to the current economic crisis.

q-fin.ST↗

arXiv2022

The critical field and specific heat in the electron- and hole-doped graphene superconductors

Doping is one of the most prominent techniques to alter properties of a given material. Herein, the influence of the electron- and hole-doping on the selected superconducting properties of graphene are considered. In details, the Migdal-Eliashberg formalism is employed to analyze the specific heat and the critical magnetic field in the representative case of graphene doped with nitrogen or boron, respectively. It is found that the electron doping is much more favorable in terms of enhancing the aforementioned properties than its hole counterpart. These findings are appropriately summarized by the means of the dimensionless thermodynamic ratios, familiar in the Bardeen-Cooper-Schrieffer theory. To this end, the perspectives for future research on superconductivity in graphene are drawn.

cond-mat.supr-con↗

arXiv2022

The superconducting energy gap in the hole-doped graphene beyond the Migdal's theory

In this work we analyze impact of non-adiabatic effects on the superconducting energy gap in the hole-doped graphene. By using the Eliashberg formalism beyond the Migdal's theorem, we present that the non-adiabatic effects strongly influence the superconducting energy gap in the exemplary boron-doped graphene. In particular, the non-adiabatic effects, as represented by the first order vertex corrections to the electron-phonon interaction, supplement Coulomb depairing correlations and suppress the superconducting state. In summary, the obtained results confirm previous studies on superconductivity in two-dimensional materials and show that the corresponding superconducting phase may be notably affected by the non-adiabatic effects.

cond-mat.supr-con↗

arXiv2022

Measurement-induced nonlocality for observers near a black hole

We present a systematic and complementary study of quantum correlations near a black hole by considering the measurement-induced nonlocality (MIN). The quantum measure of interest is discussed on the same footing for the fermionic, bosonic and mixed fermion-boson modes in relation to the Hawking radiation. The obtained results show that in the infinite Hawking temperature limit, the physically accessible correlations does not vanish only in the fermionic case. However, the higher frequency modes can sustain correlations for the finite Hawking temperature, with mixed system being more sensitive towards increase of the fermionic frequencies than the bosonic ones. Since the MIN for the latter modes quickly diminishes, the increased frequency may be a way to maintain nonlocal correlations for the scenarios at the finite Hawking temperature.

gr-qc↗

arXiv2020

Strong-coupling character of superconducting phase in compressed selenium hydride

At present, metal hydrides are considered highly promising materials for phonon-mediated superconductors, that exhibit high values of the critical temperature. In the present study, the superconducting properties of the compressed selenium hydride in its simplest form (HSe) are analyzed, toward quantitative characterization of this phase. By using the state-of-art Migdal-Eliashberg formalism, it is shown that the critical temperature in this material is relatively high ($T_{c}$=42.65 K) and surpass the level of magnesium diboride superconductor, assuming that the Coulomb pseudopotential takes value of $0.1$. Moreover, the employed theoretical model allows us to characterize other pivotal thermodynamic properties such as the superconducting band gap, the free energy, the specific heat and the critical magnetic field. In what follows, it is shown that the characteristic thermodynamic ratios for the aforementioned parameters differ from the predictions of the Bardeen-Cooper-Schrieffer theory. As a result, we argue that strong-coupling and retardation effects play important role in the discussed superconducting state, which cannot be described within the weak-coupling regime.

cond-mat.supr-con↗

arXiv2019

Characterization of the superconducting phase in tellurium hydride at high pressure

At present, hydrogen-based compounds constitute one of the most promising classes of materials for applications as a phonon-mediated high-temperature superconductors. Herein, the behavior of the superconducting phase in tellurium hydride (HTe) at high pressure ($p=300$ GPa) is analyzed in details, by using the isotropic Migdal-Eliashberg equations. The chosen pressure conditions are considered here as a case study which corresponds to the highest critical temperature value ($T_{c}$) in the analyzed material, as determined within recent density functional theory simulations. It is found that the Migdal-Eliashberg formalism, which constitutes a strong-coupling generalization of the Bardeen-Cooper-Schrieffer (BCS) theory, predicts that the critical temperature value ($T_{c}=52.73$ K) is higher than previous estimates of the McMillan formula. Further investigations show that the characteristic dimensionless ratios for the the thermodynamic critical field, the specific heat for the superconducting state, and the superconducting band gap exceeds the limits of the BCS theory. In this context, also the effective electron mass is not equal to the bare electron mass as provided by the BCS theory. On the basis of these findings it is predicted that the strong-coupling and retardation effects play pivotal role in the superconducting phase of HTe at 300 GPa, in agreement with similar theoretical estimates for the sibling hydrogen and hydrogen-based compounds. Hence, it is suggested that the superconducting state in HTe cannot be properly described within the mean-field picture of the BCS theory.

cond-mat.supr-con↗

arXiv2018

The phonon-mediated superconductivity in bismuthates by non-adiabatic pairing

In the present paper, the impact of small Fermi energy on the selected parameters of the superconducting state in Ba$_{1-x}$K$_{x}$BiO$_{3}$ (BKBO) is studied at $x \in (0.3, 0.4, 0.5)$. This is done by employing the adiabatic and non-adiabatic Eliashberg equations in context of the available experimental data. It is found that the retardation, strong-coupling and the non-adiabatic effects notably influence superconducting phase in BKBO. In particular, the electron-electron interaction, approximated here by the Coulomb pseudopotential, is argued to be reduced by the non-adiabatic effects that supplement retardation and allow for the phonon-mediated superconductivity. These findings are reinforced by further analysis of the isotope effect showing reduction of the isotope coefficient with respect to the canonical Bardeen-Cooper-Schrieffer (BCS) level, as caused by the interplay of all effects mentioned above. Although physics behind the isotope effect appears to be complex, its resulting behavior comply with the scenario for the conventional superconductors. In summary, obtained results confirm recent theoretical and experimental studies that suggest phonon-mediated mechanism of superconductivity in BKBO. However, they also point out that this phase cannot be properly described with the BCS theory due to the existence of somewhat unusual effects.

cond-mat.supr-con↗