arXiv subjects

Malgorzata Wierzbowska

Publications and source records attributed to Malgorzata Wierzbowska.

At least 19 recordsLinked to original sources

arXiv2026

Local magnetic correlations and light-sensitive centers in the Cr2AlC MAX phase

Cr2AlC MAX phase is synthesized by high-pressure solid-state annealing and investigated as a candidate platform for optically responsive magnetism. Structural characterization confirms the formation of the Cr2AlC phase, while magnetic and optical-magnetic properties are examined by superconducting quantum interference device (SQUID) magnetometry, electron spin resonance (ESR), and first principles calculations. SQUID magnetometry identifies Cr 2AlC as a weak, field-linear metallic paramagnet dominated by Pauli-like susceptibility of itinerant Cr-derived states. Its non-monotonic temperature dependence is described by an additional contribution from antiferromagnetically coupled Cr-Cr dimers, whereas the low-temperature Curie-like upturn originates from only a trace population of localized Cr centers. Under red-light illumination, SQUID magnetometry does not reveal an intrinsic macroscopic optomagnetic response. In contrast, ESR at 4 K shows a reversible light-induced reduction of a local magnetic signal, but the optically modified spin population corresponds only to several tens of ppm of the Cr sublattice. Ab initio Bethe-Salpeter equation (ai-BSE) calculations combined with the maximally localized Wannier function analysis suggest that optical excitation can redistribute spin polarization between neighboring Cr sites with the opposite local moments. The combined experiment-theory approach therefore establishes the hierarchy of magnetic contributions in Cr 2AlC and identifies the microscopic origin of its local optical sensitivity. This provides a reference for designing MAX phases and related MXenes in which defects, surface terminations or reduced dimensionality may enhance optically active magnetic states.

cond-mat.mtrl-sci↗

arXiv2017

Breathing bands due to molecular order in CH3NH3PbI3

CH3NH3PbI3 perovskite is nowadays amongst the most promising photovoltaic materials for energy conversion. We have studied by ab-initio calculations, using several levels of approximation - namely density functional theory including spin-orbit coupling and quasi-particle corrections by means of the GW method, as well as pseudopotential self-interaction corrections -, the role of the methylammonium orientation on the electronic structure of this perovskite. We have considered many molecular arrangements within 2x2x2 supercells, showing that the relative orientation of the organic molecules is responsible for a huge band gap variation up to 2 eV. The band gap sizes are related to distortions of the PbI3 cage, which are in turn due to electrostatic interactions between this inorganic frame and the molecules. The strong dependence of the band gap on the mutual molecular orientation is confirmed at all levels of approximations. Our results suggest then that the coupling between the molecular motion and the interactions of the molecules with the inorganic cage could help to explain the widening of the absorption spectrum of CH3NH3PbI3 perovskite, consistent with the observed white spectrum.

cond-mat.mtrl-sci↗

arXiv2016

Cascade donor-acceptor organic ferroelectric layers, between graphene sheets, for solar cell applications

Organic ferroelectric layers sandwiched between the graphene sheets are presented as a model of the solar cell. The investigated systems display many advantageous properties: 1) the cascade energy-levels alignment, 2) simultaneous donor and acceptor character depending on the charge-carrier direction, 3) the charge-transfer excitonic type, 4) the induced polarization of the electrodes, leading to a substantial work-function change of the anode and cathode - around +/-1.5 eV, respectively.

cond-mat.mes-hall↗

arXiv2016

Separate-path electron and hole transport across pi-stacked ferroelectrics for photovoltaic applications

Electron and hole separate-path transport is theoretically found in the pi-stacked organic layers and columns. This effect might be a solution for the charge recombination problem. The building molecules, named 1,3,5-tricyano-2,4,6-tricarboxy-benzene, contain the mesogenic flat aromatic part and the terminal dipole groups which make the system ferroelectric. The diffusion path of the electrons cuts through the aromatic rings, while holes hop between the dipole groups. The transmission function and the charge mobilities, especially for the holes, are very sensitive to the distance between the molecular rings, due to the overlap of the pi-type orbitals. We verified that the separation of the diffusion paths is not destroyed by the application of the graphene leads. These features make the system suitable for the efficient solar cells, with the carrier mobilities higher than these in the organometal halide perovskites.

cond-mat.mtrl-sci↗

arXiv2015

New memory devices based on the proton transfer process

Memory devices operating due to the fast proton transfer (PT) process are proposed by means of the first-principles calculations. Writing an information is performed using the electrostatic potential of the scanning tunneling microscopy (STM). Reading an information is based on the effect of the local magnetization induced at the zigzag graphene nanoribbon (Z-GNR) edge - saturated with oxygen or the hydroxy group - and can be realized with the use of the giant magnetoresistance (GMR), magnetic tunnel junction (MTJ) or spin-transfer torque (STT) devices. The energetic barriers for the hop-forward and -backward processes can be tuned by the distance and potential of the STM tip. Thus, enabling to tailor the non-volatile logic states. The proposed system enables very dense packing of the logic cells and could be used in the random access and flash memory devices.

cond-mat.mes-hall↗

arXiv2015

Ferrimagnetism in 2D networks of porphyrin-X and -XO (X=Sc,...,Zn) with acetylene bridges

Magnetism in 2D networks of the acetylene-bridged transition metal porphyrins M(P)-2(C-C)-2 (denoted P-TM), and oxo-TM-porphyrins OM(P)-2(C-C)-2 (denoted P-TMO), is studied with the density functional theory (DFT) and the self-interaction corrected pseudopotential scheme (pSIC). Addition of oxygen lowers magnetism of P-TMO with respect to the corresponding P-TM for most of the first-half $3d$-row TMs. In contrast, binding O with the second-half $3d$-row TMs or Sc increases the magnetic moments. Ferrimagnetism is found for the porphyrin networks with the TMs from V to Co and also for these cases with oxygen. This is a long-range effect of the delocalized spin-polarization, extended even to the acetylene bridges.

cond-mat.mtrl-sci↗

arXiv2015

Contacts for organic switches with carbon-nanotube leads

Molecular devices, as future electronics, seek low-resistivity contacts for the energy saving. At the same time, the contacts should intensify desired properties of tailored electronic elements. In this work, we focus our attention on two classes of organic switches connected to carbon-nanotube leads and operating due to photo- or field-induced proton transfer (PT) process. By means of the first-principles atomistic simulations of the ballistic conductance, we search for atomic contacts which strengthen diversity of the two swapped I-V characteristics between two tautomers of a given molecular system. We emphasize, that the low-resistive character of the contacts is not necessarily in accordance with the switching properties. Very often, the higher-current flow makes it more difficult to distinguish between the logic states of the molecular device. Instead, the resistive contacts multiply a current gear at the tautomeric transition to a larger extent. The low- and high-bias work regimes set additional conditions, which are fulfilled by different contacts. In some cases, the peroxide contacts or the direct connection to the tube perform better than the popular sulfur contact. Additionally, we find that the switching-bias value is not an inherent property of the conducting molecule, but it strongly depends on the chosen contacts.

cond-mat.mes-hall↗

arXiv2014

Multipeak Negative Differential Resistance from Interplay between Nonlinear Stark Effect and Double-Branch Current Flow

Multipeak negative differential resistance (NDR) molecular devices are designed from first principles. The effect of NDR is associated with the non-linear Stark shifts and the electron localization within the conductive region and contacts. Deep I(V)-curve well is formed when the aromatic molecule, containing intramolecular hydrogen bond, is connected to each lead by the double-branch contacts. This effect occurs at the same voltage where a single-junction case exhibits only a flat step in the current characteristics. The multipeak oscillations arise from the mutual effect of the Stark shifts located at the electron-rich contacts and parts of the molecule - this opens the route for further tailoring the desired properties.

cond-mat.mes-hall↗

arXiv2014

Effect of C-face 4H-SiC(0001) deposition on thermopower of single and multilayer graphene in AA, AB and ABC stacking

The Seebeck coefficient in multilayer graphene is investigated within the density-functional theory, using the semiclassical Boltzmann equations and interpolating the bands in a maximally-localized Wannier functions basis set. We compare various graphene stackings (AA, AB and ABC) both free-standing and deposited on a $4H$-SiC(0001) C-terminated substrate. We find that the presence of the SiC substrate can significantly affect the thermopower properties of graphene layers, depending on the stacking, providing a promising way to tailor efficient graphene-based devices.

cond-mat.mtrl-sci↗

arXiv2014

Spin-dependent Seebeck effect and huge growth of thermoelectric parameters at band edges in H- and F-doped graphene, free-standing and deposited on 4H-SiC(0001) C-face

Graphene halfly doped with H or F possesses local magnetization at the undoped C sites. Thus the Seebeck coefficient is different for each spin channel and its sign also changes depending on the spin polarization. Deposition of doped graphene on the C-face 4H-SiC(0001) with two buffer layers substantially varies the electronic and thermoelectric properties. These properties are efficiently calculated from the semiclassical Boltzmann equations, using the maximally-localized Wannier-functions interpolation of the band structures obtained with the density-functional theory. Our results indicate large growth of the thermopower and the ZT efficiency at the band edges. We show in the model discussion that this phenomenon is more general and applies also to other systems than graphene. It gives prospect for developing new spintronic devices working in the band-edge regime.

cond-mat.mtrl-sci↗

arXiv2014

Hole sp3-character and delocalization in (Ga,Mn)As revised with pSIC and MLWF approaches - newly found spin-unpolarized gap states of s-type below 1% of Mn

The dilute magnetic semiconductor (Ga,Mn)As is ferromagnetic in accordance with the p-d Zener model. Hole density function (HDF) localization has been previously studied by means of the density functional theory (DFT) and non-standard DFT methods; however not for dopings near 1%. We have revised (Ga,Mn)As using the DFT with the pseudopotential self-interaction correction (pSIC) and maximally-localized Wannier functions (MLWFs), which show the sp3 character of a HDF. Nature of HDF is extended - for low dopings and the pSIC, 70% of the HDF is located within the inter-impurities region, and contribution of the 3d-Mn states is 3-5% for 1-3% of Mn with the pSIC, and 11% with the DFT. We found that for dopings below 1%, the spin-unpolarized s-type impurity states segregate from the conduction band to the energy gap - in contrast to earlier publications. This implies that donor co-doped dilute samples would be both insulating and nonmagnetic.

cond-mat.str-el↗

arXiv2013

Contrasting Elastic Properties of Heavily B- and N-doped Graphene, with Random Distributions Including Aggregates

We focused on elastic properties of B- and N-doped graphene in wide range of concentrations up to 20%. The Young's, bulk and shear moduli and Poisson's ratio have been calculated by means of the density functional theory for a representative set of supercells with disordered impurity patterns including aggregates. In contrast to earlier work, it is demonstrated that doping with nitrogen even strengthens the graphene layers, whereas incorporation of boron induces large structural and morphological changes seen in simulated STM images. Young's and shear moduli increase or decrease with the doping strength for nitrogen or boron, respectively, while bulk modulus and Poisson's ratio exhibit opposite trends. Elastic properties of samples for both types of impurities are strongly related to the electronic structures, especially for heavy doping (>12%). Local arrangements of dopants and an agregation or separation of impurities play crucial role in the determination of stiffness in the investigated systems. Interestingly, these findings are opossed for B- and N-contained samples.

cond-mat.mtrl-sci↗

arXiv2013

CVD Formation of Graphene on SiC Surface in Argon Atmosphere

We investigate the microscopic processes leading to graphene growth by the chemical vapor deposition of propane in the argon atmosphere at the SiC surface. Experimentally, it is known that the presence of argon fastens the dehydrogenation processes at the surface, in high temperature of about 2000K. We perform ab-initio calculations, at zero temperature, to check whether chemical reactions can explain this phenomenon. Density functional theory and supporting quantum chemistry methods qualitatively describe formation of the graphene wafers. We find that the 4H-SiC(0001) surface exibits large catalytic effect in the adsorption process of hydrocarbon molecules, this is also supported by preliminary molecular dynamics results. Existence of the ArH+ molecule, and an observation from the Raman spectra that the negative charge transfers into the SiC surface, would suggest that presence of argon atoms leads to a deprotonization on the surface, which is necessary to obtain pure carbon add-layer. But the zero-temperature description shows that the cold environment is insufficient to promote the argon-assisted surface cleaning.

cond-mat.mtrl-sci↗

arXiv2012

Poisoning of magnetism in silicon doped with Re, caused by a charge transfer from interstitials to substitutionals, by means of the self-interaction corrected density-functional approach

The self-interaction corrected density-functional calculations are performed for Re impurities and their pairs in silicon. Rhenium ions form in the host crystal not very tight pairs, with impurities separated by one Si atom or by a distance close to two silicon bonds. Comparison of formation energies for various pairs of substitutionals, interstitials, and mixed-site impurities favours the last type. Electron transfer from the interstitial into the substitutional impurity makes the both Re sites nonmagnetic, but the p-type and the n-type co-doping revives magnetism again, the latter more efficiently.

cond-mat.mtrl-sci↗

arXiv2012

Exchange interactions and Tc in rhenium doped silicon: DFT, DFT+U and Monte Carlo calculations

Interactions between rhenium impurities in silicon are investigated by means of the density functional theory (DFT) and the DFT+U scheme. All couplings between impurities are ferromagnetic except the Re-Re dimers which in the DFT method are nonmagnetic, due to formation of the chemical bond supported by substantial relaxation of the geometry. The critical temperature is calculated by means of classical Monte Carlo (MC) simulations with the Heisenberg hamiltonian. The uniform ferromagnetic phase is obtained with the DFT exchange interactions at room temperature for the impurities concentration of 7%. With the DFT+U exchange interactions, the ferromagnetic clusters form above room temperature in MC samples containing only 3% Re.

cond-mat.mtrl-sci↗

arXiv2011

Forces and atomic relaxations in the pSIC approach with ultrasoft pseudopotentials

We present the scheme that allows for efficient calculations of forces in the framework of pseudopotential self-interaction corrected (pSIC) formulation of the density functional theory. The scheme works with norm conserving and also with ultrasoft pseudopotentials and has been implemented in the plane-wave basis code {\sc quantum espresso}. We have performed tests of the internal consistency of the derived expressions for forces considering ZnO and CeO$_2$ crystals. Further, we have performed calculations of equilibrium geometry for LaTiO$_3$, YTiO$_3$, and LaMnO$_3$ perovskites and also for Re and Mn pairs in silicon. Comparison with standard DFT and DFT+U approaches shows that in the cases where spurious self-interaction matters, the pSIC approach predicts different geometry, very often closer to the experimental data.

cond-mat.str-el↗

arXiv2006

Origins of low- and high-pressure discontinuities of $T_{c}$ in niobium

The discontinuities of $T_{c}$ in Niobium under pressure are examined by means of the pseudopotential plane-wave implementation of the electron-phonon coupling calculated from density-functional perturbation theory. Both low- and high-pressure discontinuities of $T_{c}$ have their origin in the Kohn anomalies and are caused by the low-frequency phonons, but the mechanism leading to the discontinuities is different in the two cases. The low-pressure anomaly is associated with a global decrease of the nesting factor in the whole Brillouin Zone and not to a visible change in the band structure. The high-pressure anomaly is instead connected with a well-pronounced change in the band structure.

cond-mat.supr-con↗

arXiv2005

Effect of spin fluctuations on Tc from density-functional theory for superconductors

The transverse spin fluctuations are introduced to the density functional theory for superconductors (SCDFT). Paramagnons are treated within the random phase approximation and assumed to be the same for the normal and superconducting state. The effect of spin fluctuations on Tc is studied for a few simple metals at ambient pressure and niobium at several pressures up to 80 GPa.

cond-mat.supr-con↗