arXiv subjects

Konrad J. Kapcia

Publications and source records attributed to Konrad J. Kapcia.

18 recordsLinked to original sources

arXiv2026

Phase structure of the one-dimensional $\mathbb{Z}_2$ lattice gauge theory with second nearest-neighbor interactions

We investigate the ground-state phase diagram of a one-dimensional $\mathbb{Z}_2$ lattice gauge theory (LGT) model with hard-core bosons at half-filling, extending previous studies by including second nearest-neighbor (2NN) interactions. Using matrix product state techniques within the density matrix renormalization group, we compute charge gap, static structure factor, pair-pair correlation functions, and entanglement entropy for various interaction strengths and field parameters. We analyze two representative neatest-neighbor interaction strengths ($V_1$) that correspond to the Luttinger liquid (LL) and Mott insulator (MI) phases in the absence of the 2NN interactions. We introduce the 2NN coupling $V_2$ and investigate its impact on the system. Our results reveal very rich behavior. As the 2NN repulsion increases, in the case of small $V_1$, we observe a direct transition from the LL phase to a charge-ordered insulator (COI) phase with four-site ordering pattern, whereas for large $V_1$, we observe a transition from the MI phase with two-site ordering pattern (previously found with only $V_1$ included), going through an intermediate LL region, and finally reaching the COI regime. Additionally, the inclusion of 2NN interactions enhances charge order and suppresses pair coherence, evidenced by sharp peaks in the structure factor and rapid decay in pair-pair correlators. Our work extends the well-studied phase structure of 1D $\mathbb{Z}_2$ LGT models and demonstrates the interplay between gauge fields, confinement, and extended interactions.

cond-mat.str-el↗

arXiv2026

Electronic mechanism of sub-100-fs demagnetization induced by a femtosecond light pulse

A quantitative understanding of the processes that trigger light-induced demagnetization on ultrashort timescales is crucial for achieving an ultrafast, radiation-controlled magnetic response in materials. This milestone is essential for developing next-generation magnetic storage devices and ultrafast magnetic switches. In this theoretical study, we investigated demagnetization triggered in a single magnetic domain by light pulses ranging from a few to a few tens of femtoseconds in duration, with photon energies spanning the optical and X-ray regimes, under strongly non-equilibrium conditions. We predicted a loss of magnetization in the sub-100-fs range in all cases, primarily due to the excitation of the electronic system and the subsequent redistribution of electrons within the magneto-sensitive band. The considered timescales were too short for phonon-mediated processes or inter-site Heisenberg exchange processes to contribute significantly. These findings pave the way for highly accurate, radiation-driven magnetization control in magnetic materials at sub-100-femtosecond timescales with potential practical applications.

cond-mat.mtrl-sci↗

arXiv2023

Interplay of thermal and non-thermal effects in x-ray-induced ultrafast melting

X-ray laser-induced structural changes in silicon undergoing femtosecond melting have been investigated by using an x-ray pump-x-ray probe technique. The experimental results for different initial sample temperatures reveal that the onset time and the speed of the atomic disordering are independent of the initial temperature, suggesting that equilibrium atomic motion in the initial state does not play a pivotal role in the x-ray-induced ultrafast melting. By comparing the observed time-dependence of the atomic disordering and the dedicated theoretical simulations, we interpret that the energy transfer from the excited electrons to ions via electron-ion coupling (thermal effect) as well as a strong modification of the interatomic potential due to electron excitations (non-thermal effect) trigger the ultrafast atomic disordering. Our finding of the interplay of thermal and non-thermal effects in the x-ray-induced melting demonstrates that accurate modeling of intense x-ray interactions with matter is essential to ensure a correct interpretation of experiments using intense x-ray laser pulses.

physics.app-ph↗

arXiv2023

Spin-polarized superconducting phase in semiconducting system with next-nearest-neighbor hopping on the honeycomb lattice

The particles in the honeycomb lattice with on-site $s$-wave pairing exhibit many interesting behaviors, which can be described in the framework of the Hubbard model. Among others, at the half-filling, some critical value $|U_c|$ of pairing interaction $U<0$ exists that, for $U<U_c$, the superconducting phase becomes unstable. Introduction of the nonzero hopping $t'$ between next-nearest-neighbor sites strongly modifies the physical properties of the system. Here, we discuss the behavior of the system for $t' \neq 0$ (at the ground state), where the hopping between next-nearest neighbors leads to change of the order of phase transition between superconducting and normal phases from discontinuous to continuous one in the external magnetic field $h$. We show that this behavior is strongly dependent on $t'$ and associated with the Dirac cones in the non-interacting band structure of the system. For non-zero magnetic field and for some range of $t'$, a spin-polarized superconducting phase occurs in the ground state phase diagram (only at the half-filling and for $h\neq 0$).

cond-mat.supr-con↗

arXiv2023

Femtosecond reduction of atomic scattering factors triggered by intense x-ray pulse

X-ray diffraction of silicon irradiated with tightly focused femtosecond x-ray pulses (photon energy: 11.5 keV, pulse duration: 6 fs) was measured at various x-ray intensities up to $4.6\times10^{19}$ W/cm$^2$. The measurement reveals that the diffraction intensity is highly suppressed when the x-ray intensity reaches of the order of $10^{19}$ W/cm$^2$. With a dedicated simulation, we confirm the observed reduction of the diffraction intensity is attributed to the femtosecond change in individual atomic scattering factors due to the ultrafast creation of highly ionized atoms through photoionization, Auger decay, and subsequent collisional ionization. We anticipate that this ultrafast reduction of atomic scattering factor will be a basis for new x-ray nonlinear techniques, such as pulse shortening and contrast variation x-ray scattering.

physics.atom-ph↗

arXiv2023

Quench dynamics of Fano-like resonances in the presence of the on-dot superconducting pairing

We explore the electron dynamics of a system composed of double quantum dot embedded between metallic and superconducting leads in a T-shape geometry. In nanoscopic systems, where electron transfer between electrodes can be realized via different paths, interference effects play an important role. For double quantum dot system in the chosen geometry, interference of electrons transferred between electrodes via the interfacial quantum dot and electrons scattered on the side dot gives rise to Fano-like interference. If such a system is additionally coupled to a superconducting electrode, together with the well-understood Fano resonance an additional resonance appears on the opposite side of the Fermi level. In the recent work [Sci. Rep. 10, 2881 (2020)], we showed that this resonance occurs solely as a result of the local pairing of non-scattered electrons with scattered ones. In this work, considering the quench dynamics, we explore how much time is required for formation of each of these resonances. In particular, (i) we analyze the charge oscillations between subsystems; (ii) we estimate the time required for each resonance to achieve stable equilibrium upon an abrupt change of interdot connection; (iii) we discuss a typical energy and time scales for experiments on similar architectures.

cond-mat.mes-hall↗

arXiv2023

Electrical polarization switching in bulk single crystal GaFeO$_{3}$

The electrical polarization switching on stoichiometric GaFeO$_{3}$ single crystal was measured, and a new model of atomic displacements responsible for the polarization reverse was proposed. The widely adapted mechanism of polarization switching in GaFeO$_{3}$ can be applied to stoichiometric, perfectly ordered crystals. However, the grown single crystals, as well as thin films of Ga-Fe-O, show pronounced atomic disorder. By piezoresponse force microscopy, the electrical polarization switching on a crystal surface perpendicular to the electrical polarization direction was demonstrated. Atomic disorder in the crystal was measured by X-ray diffraction and Mössbauer spectroscopy. These measurements were supported by ab initio calculations. By analysis of atomic disorder and electronic structure calculations, the energies of defects of cations in foreign cationic sites were estimated. The energies of the polarization switch were estimated, confirming the proposed mechanism of polarization switching in GaFeO$_{3}$ single crystals.

cond-mat.mtrl-sci↗

arXiv2023

Ultrafast demagnetization in bulk nickel induced by X-ray photons tuned to Ni $M_{3}$ and $L_3$ absorption edges

Studies of light-induced demagnetization started with the experiment performed by Beaupaire et al. on nickel. Here, we present theoretical predictions for X-ray induced demagnetization of nickel, with X-ray photon energies tuned to its $M_3$ and $L_3$ absorption edges. We show that the specific feature in the density of states of the d-band of Ni, a sharp peak located just above the Fermi level, strongly influences the change of the predicted magnetic signal, making it stronger than in the previously studied case of cobalt. We believe that this finding will inspire future experiments on magnetic processes in X-ray irradiated nickel.

cond-mat.mtrl-sci↗

arXiv2023

Chiral phonons in binary compounds $A$Bi ($A$ = K, Rb, Cs) with P2$_1$/c structure

Binary compounds $A$Bi ($A$ = K, Rb, Cs) crystallize in P2$_1$/c structure containing both clockwise and anticlockwise chiral chains of Bi atoms. Electronic band structure exhibits the insulating nature of these systems, with the band gap about $0.25$ eV. The presented study of dynamical properties confirm a stability of the system with P2$_1$/c symmetry. The crystal structure contains the quasi-one-dimensional Bi chains, exhibiting four-fold-like rotational ``local'' symmetry. Nevertheless, the system formally posses two-fold rotational symmetry. Independently of the absence of the three-fold (or higher) rotational symmetry axes for the whole crystal, the chiral modes propagate along the Bi atom chains in these systems. We discuss basic properties of these modes in monoatomic chiral chains. We show that the two-fold rotational symmetry axis affects the main properties of the chiral phonons, which are not realized at the high-symmetry points, but along some paths between them in the reciprocal space. In addition, in the doped system, the chiral phonons possess non-zero total angular momentum.

cond-mat.mtrl-sci↗

arXiv2023

Electronic processes occurring during ultrafast demagnetization of cobalt triggered by X-ray photons tuned to Co L$_3$ resonance

Magnetization dynamics triggered with ultrashort laser pulses has been attracting significant attention, with strong focus on the dynamics excited by VIS/NIR pulses. Only recently, strong magnetic response in solid materials induced by intense X-ray pulses from free-electron lasers (FELs) has been observed. The exact mechanisms that trigger the X-ray induced demagnetization are not yet fully understood. They are subject of on-going experimental and theoretical investigations. Here, we present a theoretical analysis of electronic processes occurring during demagnetization of Co multilayer system irradiated by X-ray pulses tuned to L$_3$-absorption edge of cobalt. We show that, similarly as in the case of X-ray induced demagnetization at M-edge of Co, electronic processes play a predominant role in the demagnetization until the pulse fluence does not exceed the structural damage threshold. The impact of electronic processes can reasonably well explain the available experimental data, without a need to introduce the mechanism of stimulated elastic forward scattering.

cond-mat.mtrl-sci↗

arXiv2022

Stress-Tuned Optical Transitions in Layered 1T-MX2 (M= Hf, Zr, Sn; X= S, Se) Crystals

Optical measurements under externally applied stresses allow us to study the materials' electronic structure by comparing the pressure evolution of optical peaks obtained from experiments and theoretical calculations. We examine the stress-induced changes in electronic structure for the thermodynamically stable 1T polytype of selected MX2 compounds (M=Hf, Zr, Sn; X=S, Se), using the density functional theory. We demonstrate that considered 1T-MX2 materials are semiconducting with indirect character of the band gap, irrespective to the employed pressure as predicted using modified Becke-Johnson potential. We determine energies of direct interband transitions between bands extrema and in band-nesting regions close to Fermi level. Generally, the studied transitions are optically active, exhibiting in-plane polarization of light. Finally, we quantify their energy trends under external hydrostatic, uniaxial, and biaxial stresses by determining the linear pressure coefficients. Generally, negative pressure coefficients are obtained implying the narrowing of the band gap. The semiconducting-to-metal transition are predicted under hydrostatic pressure. We discuss these trends in terms of orbital composition of involved electronic bands. In addition, we demonstrate that the measured pressure coefficients of HfS2 and HfSe2 absorption edges are in perfect agreement with our predictions. Comprehensive and easy-to-interpret tables containing the optical features are provided to form the basis for assignation of optical peaks in future measurements.

cond-mat.mtrl-sci↗

arXiv2022

Delayed onset and directionality of x-ray-induced atomic displacements observed on subatomic length scales

Transient structural changes of Al$_2$O$_3$ on subatomic length scales following irradiation with an intense x-ray laser pulse (photon energy: 8.70 keV; pulse duration: 6 fs; fluence: 8$\times$10$^2$ J/cm$^{2}$) have been investigated by using an x-ray pump x-ray probe technique. The measurement reveals that aluminum and oxygen atoms remain in their original positions by $\sim$20 fs after the intensity maximum of the pump pulse, followed by directional atomic displacements at the fixed unit cell parameters. By comparing the experimental results and theoretical simulations, we interpret that electron excitation and relaxation triggered by the pump pulse modifies the potential energy surface and drives the directional atomic displacements. Our results indicate that high-resolution x-ray structural analysis with the accuracy of 0.01 Åis feasible even with intense x-ray pulses by making the pulse duration shorter than the timescale needed to complete electron excitation and relaxation processes, which usually take up to a few tens of femtoseconds.

physics.optics↗

arXiv2021

Thermodynamic and electromagnetic properties of the eta-pairing superconductivity in the Penson-Kolb model

In the paper, we study the thermodynamic and electromagnetic properties of the Penson-Kolb (PK) model, i.e., the tight-binding model for fermionic particles with the pair-hopping interaction $J$. We focus on the case of repulsive $J$ (i.e., $J<0$), which can stabilize the eta-pairing superconductivity with Cooper-pair center-of-mass momentum $\vec{q}=\vec{Q}$, $\vec{Q}=(π/a$,$π/a$,\ldots). Numerical calculations are performed for several $d$-dimensional hypercubic lattices: $d=2$ (the square lattice, SQ), $d=3$ (the simple cubic lattice) and $d=\infty$ hypercubic lattice (for arbitrary particle concentration $0 0$ as a function of $n$ and pairing strength. The analysis of the effects of the Fock term on the ground state phase boundaries and on selected PK model characteristics is performed as well as the influence of the phase fluctuations on the eta-pairing superconductivity is investigated. Within the Kosterlitz-Thouless scenario, the critical temperatures $T_{KT} $ are estimated for $d=2$ SQ lattice and compared with the critical temperature $T_c$ obtained from HFA. We also determine the temperature $T_m$ at which minimal gap between two quasiparticle bands vanishes in the eta-phase. Our results for repulsive $J$ are contrasted with those found earlier for the PK model with attractive $J$ (i.e., with $J>0$).

cond-mat.supr-con↗

arXiv2021

Electronic and dynamical properties of CeRh$_{2}$As$_{2}$: Role of Rh$_{2}$As$_{2}$ layers and expected hidden orbital order

Recently discovered heavy fermion CeRh$_{2}$As$_{2}$ compound crystallizes in the nonsymmorphic $P4/nmm$ symmetry, which enables the occurrence of topological protection. Experimental results show that this material exhibits unusual behavior, which is manifested by the appearance of two superconducting phases. In this work, we uncover and discuss a role of Rh$_{2}$As$_{2}$ layers and their impact on the electronic and dynamical properties of the system. The location of Ce atoms between two non-equivalent layers allows for the realization of hidden orbital order. We point out that the electronic band structure around the Fermi level is associated mostly with Ce $4f$ and Rh $4d$ orbitals and suggest the occurrence of the Lifshitz transition induced by the external magnetic field. We discuss also the role played by the $f$--$d$ orbital hybridization in the electronic band structure.

cond-mat.mtrl-sci↗

arXiv2020

Low-Temperature Phase of the Cd$_2$Re$_2$O$_7$ Superconductor: Ab initio Phonon Calculations and Raman Scattering

Using an {\it ab initio} approach, we report a phonon soft mode in the tetragonal structure described by the space group $I4_{1}22$ of the $1$ K $5d$ superconductor Cd$_2$Re$_2$O$_7$. It induces an orthorhombic distortion to a crystal structure described by the space group $F222$ which hosts the superconducting state. This new phase has a lower total energy than the other known crystal structures of Cd$_2$Re$_2$O$_7$. Comprehensive temperature dependent Raman scattering experiments on isotope enriched samples, $^{116}$Cd$_2$Re$_2{^{18}}$O$_7$, not only confirm the already known structural phase transitions but also allow us to identify a new characteristic temperature regime around $\sim 80$ K, below which the Raman spectra undergo remarkable changes with the development of several sharp modes and mode splitting. Together with the results of the \textit{ab initio} phonon calculations we take these observations as strong evidence for another phase transition to a novel low-temperature crystal structure of Cd$_2$Re$_2$O$_7$.

cond-mat.supr-con↗

arXiv2017

The ab initio study of unconventional superconductivity in CeCoIn$_{5}$ and FeSe

Electronic structure and the shape of the Fermi surface are known to be of fundamental importance for the superconducting instability in real materials. We demonstrate that such an instability may be explored by static Cooper pair susceptibility renormalized by pairing interaction and present an efficient method of its evaluation using Wannier orbitals derived from ab initio calculation. As an example, this approach is used to search for an unconventional superconducting phase of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type in a heavy-fermion compound CeCoIn$_5$ and an iron-based superconductor FeSe. The results suggest that the FFLO superconducting phase occurs at finite magnetic field in both materials.

cond-mat.supr-con↗

arXiv2017

Magnetic Lifshitz transition and its consequences in multi-band iron-based superconductors

In this paper we address Lifshitz transition induced by applied external magnetic field in a case of iron-based superconductors, in which a difference between the Fermi level and the edges of the bands is relatively small. We introduce and investigate a two-band model with intra-band pairing in the relevant parameters regime to address a generic behaviour of a system with hole-like and electron-like bands in external magnetic field. Our results show that two Lifshitz transitions can develop in analysed systems and the first one occurs in the superconducting phase and takes place at approximately constant magnetic field. The chosen sets of the model parameters can describe characteristic band structure of iron-based superconductors and thus the obtained results can explain the experimental observations in FeSe and Co-doped BaFe$_{2}$As$_{2}$ compounds.

cond-mat.supr-con↗

arXiv2017

Critical behaviour in one dimension: unconventional pairing, phase separation, BEC-BCS crossover and magnetic Lifshitz transition

We study the superconducting properties of population-imbalanced ultracold Fermi mixtures in one-dimensional (1D) optical lattices that can be effectively described by the spin-imbalanced attractive Hubbard model (AHM) in the presence of a Zeeman magnetic field. We use the mean-field theory approach to obtain the ground state phase diagrams including some unconventional superconducting phases such as the Fulde--Ferrell--Larkin--Ovchinnikov (FFLO) phase, and the $η$ phase (an extremal case of the FFLO phase), both for the case of a fixed chemical potential and for a fixed number of particles. It allows to determine optimal regimes for the FFLO phase as well as $η$-pairing stability. We also investigate the evolution from the weak coupling (BCS-like limit) to the strong coupling limit of tightly bound local pairs (BEC) with increasing attraction, at $T=0$. Finally, the obtained results show that despite of the occurrence of the Lifshitz transition induced by an external magnetic field, the superconducting state can still exist in the system, at higher magnetic field values.

cond-mat.quant-gas↗