arXiv subjects

Marcin Kurpas

Publications and source records attributed to Marcin Kurpas.

At least 19 recordsLinked to original sources

arXiv2025

Ferroelectric switching control of spin current in graphene proximitized by In$_2$Se$_3$

By utilizing the proximity effect, we introduce a platform that exploits ferroelectric switching to modulate spin currents in graphene proximitized by ferroelectric In$_2$Se$_3$ monolayer. Through first-principles calculations and tight-binding modeling, we studied the electronic structure of graphene/In$_2$Se$_3$ heterostructure for twist angles of 0$^{\circ}$ and 17.5$^{\circ}$, considering both ferroelectric polarizations. We discover that switching the ferroelectric polarization reverses the sign of the charge-to-spin conversion coefficients, acting as a chirality switch of the in-plane spin texture in graphene. For the twisted heterostructure, we observed emergence of unconventional radial Rashba field for one ferroelectric polarization direction. Additionally, we demonstrated that the Rashba phase can be directly extracted from the ratio of conversion efficiency coefficients, providing a straightforward approach to characterize the in-plane spin texture in graphene. All the unique features of the studied graphene/In$_2$Se$_3$ heterostructure can be experimentally detected, offering a promising approach for developing advanced spintronic devices with enhanced performance and efficiency.

cond-mat.mtrl-sci↗

arXiv2025

Optimizing optical properties of bilayer PtSe$_2$: the role of twist angle and hydrostatic pressure

Two-dimensional van der Waals materials offer exceptional tunability in their electronic properties. In this paper, we explore how twisting and hydrostatic pressure can be leveraged to engineer the electronic and optical characteristics of bilayer PtSe$_2$. Using state-of-the-art first-principles density functional methods, we calculate the electronic band structure and the imaginary part of the dielectric function across multiple twist angles and pressure values. We find, that at the twist angle $\theta=13.17^\circ$, bilayer PtSe$_2$, which is intrinsically an indirect semiconductor, transforms into a direct-gap semiconductor. Moreover, we demonstrate that hydrostatic out-of-plane pressure boosts near-infrared optical activity, further expanding the functional potential of PtSe$_2$ bilayers. The demonstrated high tunability of electronic and optical properties by twisting and pressure opens new application directions of PtSe$_2$ in optoelectronics.

cond-mat.mtrl-sci↗

arXiv2025

Strain-tuning of spin anisotropy in single-layer phosphorene: insights from Elliott-Yafet and Dyakonov-Perel spin relaxation rates

Materials and systems that exhibit persistent spin texture provide a platform for creating robust spin states that can be used in quantum computing, memory storage, and other advanced technological applications. In this paper we show that persistent spin-texture in single-layer phosphorene electrons close to the $Γ$ point, subjected to the finite perpendicular electric field, can be achieved by appropriately tuning the extrinsic spin-orbit coupling strength using the tensile strain of about $1.2\%$ in the zigzag direction. This is confirmed by detailed numerical investigations of the effects of strain on the intrinsic and extrinsic spin-orbit coupling, and by the effective spin-orbit Hamiltonian of phosphorene electrons and holes around the $Γ$ point, assuming the presence of the perpendicular electric field. Furthermore, the calculated spin relaxation rates due to the Dyakonov-Perel mechanism indicate a giant anisotropy of the in-plane spin, up to $10^5$, which is directly related to the discovered persistent spin texture of phosphorene electrons close to the $Γ$ point. %%%%%%%% We also show, that strain can reverse the anisotropy of spin mixing parameter $b^2$ connected to the Elliott-Yafet spin relaxation mechanism which dominates spin relaxation in phosphorene. We find the conditions under which Elliott-Yafet spin lifetime anisotropy can be largely enhanced due to synergy of spin mixing and g-factor anisotropy. %%%%%%%%%% Our results suggest that spin texture in phosphorene can be modulated by strain, enabling its potential usage in the field of spintronics.

cond-mat.mes-hall↗

arXiv2025

Hexatic Phase in Covalent Two-Dimensional Silver Iodide

According to the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory, the transition from a solid to liquid in two dimensions proceeds through an orientationally ordered liquid-like hexatic phase. However, alternative mixed melting scenarios, in which melting proceeds through the hexatic phase with both continuous and discontinuous transitions, have also been observed in some two-dimensional systems. In this study, we imaged silver iodide embedded in multilayer graphene using time- and temperature-resolved in situ atomic-resolution scanning transmission electron microscopy and nanobeam electron diffraction. We observed the hexatic phase and provide evidence supporting a mixed melting scenario.

cond-mat.mtrl-sci↗

arXiv2024

Friedel oscillations and chiral superconductivity in monolayer NbSe$_2$

In 1965 Kohn and Luttinger proposed a genuine electronic mechanism for superconductivity. Despite the bare electrostatic interaction between two electrons being repulsive, in a metal electron-hole fluctuations can give rise to Friedel oscillations of the screened Coulomb potential. Cooper pairing among the electrons then emerges when taking advantage of the attractive regions. The nature of the leading pairing mechanism in some two-dimensional transition metal dichalcogenides is still debated. Focusing on NbSe$_2$, we show that superconductivity can be induced by the Coulomb interaction when accounting for screening effects on the trigonal lattice with multiple orbitals. Using ab initio-based tight-binding parametrizations for the relevant low-energy d-bands, we evaluate the screened interaction microscopically, in a scheme including Bloch overlaps. In the direct space, we find long-range Friedel oscillations alternating in sign, a key to the Kohn-Luttinger mechanism. The momentum-resolved gap equations predict two degenerate solutions at the critical temperature Tc, signaling the unconventional nature of the pairing. Their complex linear combination, i.e., a chiral gap with p-like symmetry, provides the ground state of the system. Our prediction of a fully gapped chiral phase well below Tc is in excellent agreement with the spectral function extracted from tunneling spectroscopy measurements of single-layer NbSe$_2$.

cond-mat.supr-con↗

arXiv2024

Armchair carbon nanotube on Pt and hBN/Pt: from strong metallic contact to coherent spin transport regime

We study spin-orbit proximity effects in an armchair (4,4) carbon nanotube on the Pt(111) surface. By employing first-principles calculations, we show that the Dirac cone of the metallic nanotube is altered due to strong hybridization with the Pt substrate. Inserting a monolayer hexagonal boron nitride (hBN) between the nanotube and the substrate limits the hybridization effects leading to recovering the Dirac cone. The Dirac bands display asymmetric spin splitting, 0.7\,meV for the right movers and 1.7\,meV for the left movers at the K valley, due to the proximity to the Pt substrate. We find that the Dirac states exhibit almost perfect spin polarization, transverse to the nanotube axis and to the stacking direction, forming a proper condition for charge-to-spin conversion with coherent spin transport in the nanotube. We propose an effective Hamiltonian describing the proximity-induced effects on the Dirac electrons and their spin texture.

cond-mat.mes-hall↗

arXiv2024

Hydrostatic pressure control of the spin-orbit proximity effect, spin relaxation, and thermoelectricity in a phosphorene-WSe$_2$ heterostructure

Effective control of interlayer interactions is a key element in modifying the properties of van der Waals heterostructures and the next step toward their practical applications. Focusing on the phosphorene-WSe$_2$ heterostructure, we demonstrate, using first-principles calculations, proximity-induced amplification of the spin-orbit coupling in phosphorene by applying vertical pressure. We simulate external pressure by changing the interlayer distance between bilayer constituents and show that it is possible to tune the spin-orbit field of phosphorene holes in a controllable way. By fitting effective electronic states of the proposed Hamiltonian to the first principles data, we reveal that the spin-orbit coupling in phosphorene hole bands is enhanced more than two times for experimentally accessible pressures up to 17 kbar. Correspondingly, we find that the pressure-enhanced spin-orbit coupling boosts the Dyakonov-Perel spin relaxation mechanism, reducing the spin lifetime of phosphorene holes by factor 4. We further explore the role of the lateral shift on the spin-orbit field and reveal that the spin-orbit strength of phosphorene holes can be sizably modulated when strong pressure is applied. We also found that the thermopower is governed mainly by the phosphorene and pressure reduces the overall thermoelectric efficiency of the heterostructure.

cond-mat.mes-hall↗

arXiv2024

Giant asymmetric proximity-induced spin-orbit coupling in twisted graphene/SnTe heterostructure

We analyze the spin-orbit coupling effects in a three-degree twisted bilayer heterostructure made of graphene and an in-plane ferroelectric SnTe, with the goal of transferring the spin-orbit coupling from SnTe to graphene, via the proximity effect. Our results indicate that the point-symmetry breaking due to the incompatible mutual symmetry of the twisted monolayers and a strong hybridization has a massive impact on the spin splitting in graphene close to the Dirac point, with the spin splitting values greater than 20 meV. The band structure and spin expectation values of graphene close to the Dirac point can be described using a symmetry-free model, triggering different types of interaction with respect to the threefold symmetric graphene/transition-metal dichalcogenide heterostructure. We show that the strong hybridization of the Dirac cone's right movers with the SnTe band gives rise to a large asymmetric spin splitting in the momentum space. Furthermore, we discover that the ferroelectricity-induced Rashba spin-orbit coupling in graphene is the dominant contribution to the overall Rashba field, with the effective in-plane electric field that is almost aligned with the (in-plane) ferroelectricity direction of the SnTe monolayer. We also predict an anisotropy of the in-plane spin relaxation rates. Our results demonstrate that the group-IV monochalcogenides MX (M=Sn, Ge; X=S, Se, Te) are a viable alternative to transition-metal dichalcogenides for inducing strong spin-orbit coupling in graphene.

cond-mat.mes-hall↗

arXiv2024

Probing type-II Ising pairing using the spin-mixing parameter

The immunity of Ising superconductors to external magnetic fields originates from a spin locking of the paired electrons to an intrinsic Zeeman-like field. The spin-momentum locking in non-centrosymmetric crystalline materials leads to type-I Ising pairing in which the direction of the intrinsic field can be deduced from the spin expectation values. Conversely, in centrosymmetric crystals the electron spins locked to the orbitals can form Ising type-II pairs consisting of spin-orbit split doublets. Due to time-reversal symmetry, the doublets are spin degenerate, making it difficult to read the spin polarization of bands and the direction of spin-orbit fields. Here we present an efficient approach to determine the direction of the intrinsic field using the spin-mixing parameter $b^2$. Using first principles calculations based on the density functional theory, we study monolayer transition metal dichalcogenide superconductors PdTe$_2$, NbTe$_2$, and TiSe$_2$ with the 1T structure. We calculate $b^2$ for individual Fermi pockets and provide a general picture of possible Ising type-II pairing within the full Brillouin zone. In order to complement our first principles results, we use group theory to provide a detailed picture of spin-orbit coupling and spin mixing in the relevant bands forming Fermi pockets. We demonstrate that contrary to the anticipated effects of spin-orbit locking, not every spin-orbit split spin doublet actively participates in Ising pairing. Finally, by connecting the spin-mixing parameter $b^2$ with the intrinsic out-of-plane Zeeman field we estimate the upper in-plane critical magnetic field.

cond-mat.mes-hall↗

arXiv2024

Proximity-enabled control of spin-orbit coupling in phosphorene symmetrically and asymmetrically encapsulated by WSe$_2$ monolayers

We analyze, using first-principles calculations and the method of invariants, the spin-orbit proximity effects in trilayer heterostructures comprising phosphorene and encapsulating WSe$_2$ monolayers. We focus on four different configurations, in which the top/bottom WSe$_2$ monolayer is twisted by 0 or 60 degrees with respect to phosphorene, and analyze the spin splitting of phosphorene hole bands around the $Γ$ point. Our results show that the spin texture of phosphorene hole bands can be dramatically modified by different encapsulations of phosphorene monolayer. For a symmetrically encapsulated phosphorene, the momentum-dependent spin-orbit field has the out-of-plane component only, simulating the spin texture of phosphorene-like group-IV monochalcogenide ferroelectrics. Furthermore, we reveal that the direction of the out-of-plane spin-orbit field can be controlled by switching the twist angle from 0 to 60 degrees. Finally, we show that the spin texture in asymmetrically encapsulated phosphorene has the dominant in-plane component of the spin-orbit field, comparable to the Rashba effect in phosphorene with an applied sizable external electric field. Our results confirm that the significant modification and control of the spin texture is possible in low common-symmetry heterostructures, paving the way for using different substrates to modify spin properties in materials important for spintronics.

cond-mat.mes-hall↗

arXiv2023

Proximity-induced spin-orbit coupling in phosphorene on a WSe$_2$ monolayer

We investigate, using first-principles methods and effective-model simulations, the spin-orbit coupling proximity effects in a bilayer heterostructure comprising phosphorene and WSe$_2$ monolayers. We specifically analyze holes in phosphorene around the $Γ$ point, at which we find a significant increase of the spin-orbit coupling that can be attributed to the strong hybridization of phosphorene with the WSe$_2$ bands. We also propose an effective spin-orbit model based on the ${\bf C}_{1{\rm v}}$ symmetry of the studied heterostructure. The corresponding spin-orbit field can be divided into two parts: the in-plane field, present due to the broken nonsymmorphic horizontal glide mirror plane symmetry, and the dominant out-of-plane field triggered by breaking the out-of-plane rotational symmetry of the phosphorene monolayer. Furthermore, we also demonstrate that a heterostructure with 60$^\circ$ twist angle exhibits an opposite out-of-plane spin-orbit field, indicating that the coupling can effectively be tuned by twisting. The studied phosphorene/WSe$_2$ bilayer is a prototypical low common-symmetry heterostructure in which the proximity effect can be used to engineer the spin texture of the desired material.

cond-mat.mes-hall↗

arXiv2023

A learning by confusion approach to characterize phase transitions

Recently, the learning by confusion (LBC) approach has been proposed as a machine learning tool to determine the critical temperature Tc of phase transitions without any prior knowledge of its even approximate value. However, the effectiveness of the method has been demonstrated only for continuous phase transitions, where confusion can result only from a deliberate incorrect labeling of the data and not from the coexistence of different phases. To verify whether the confusion scheme can also be used for discontinuous phase transitions, in this work, we apply the LBC method to three microscopic models, the Blume-Capel, the q-state Potts, and the Falicov-Kimball models, which undergo continuous or discontinuous phase transitions depending on model parameters. With the help of a simple model, we predict that the phase coexistence present in discontinuous phase transitions can make the neural network more confused and thus decrease its performance. However, numerical calculations performed for the models mentioned above indicate that other aspects of this kind of phase transition are more important and can render the LBC method less effective. Nevertheless, we demonstrate that in some cases the same aspects allow us to use the LBC method to identify the order of a phase transition

cond-mat.dis-nn↗

arXiv2023

Proximity spin-orbit coupling in an armchair carbon nanotube on monolayer bismuthene

We study spin-orbit proximity effects in a hybrid heterostructure build of a one-dimensional (1D) armchair carbon nanotube and two-dimensional (2D) buckled monolayer bismuthene. We show, by performing first-principles calculations, that Dirac electrons in the nanotube exhibit large spin-orbit coupling due to a close vicinity of bismuthene. The calculated low-energy band structures of the proximized nanotube display a strong dependence on the position of the nanotube on the substrate, similar to twist-angle dependence found in 2D heterostructures. Based on the first-principles results, we formulate an effective low-energy Hamiltonian of the nanotube and identify key interactions governing the proximity spin-orbit coupling. The proximity-induced spin splitting of Dirac cone bands is in meV range, confirming an efficient transfer of spin-orbit coupling from bismuthene to the nanotube.

cond-mat.mes-hall↗

arXiv2020

Intrinsic and extrinsic spin-orbit coupling and spin relaxation in monolayer PtSe$_2$

Monolayer PtSe$_2$ is a semiconducting transition metal dichalcogenide characterized by an indirect band gap, space inversion symmetry, and high carrier mobility. Strong intrinsic spin-orbit coupling and the possibility to induce extrinsic spin-orbit fields by gating make PtSe$_2$ attractive for fundamental spin transport studies as well as for potential spintronics applications. We perform a systematic theoretical study of the spin-orbit coupling and spin relaxation in this material. Specifically, we employ first principles methods to obtain the basic orbital and spin-orbital properties of PtSe$_2$, also in the presence of an external transverse electric field. We calculate the spin mixing parameters $b^2$ and the spin-orbit fields $Ω$ for the Bloch states of electrons and holes. This information allows us to predict the spin lifetimes due to the Elliott-Yafet and D'yakonov-Perel mechanisms. We find that $b^2$ is rather large, on the order of $10^{-2}$ and $10^{-1}$, while $Ω$ varies strongly with doping, being about $10^{3} - 10^{4}$\,ns$^{-1}$ for %typical Fermi levels in the interval $(10-100)$ meV, carrier density in the interval $10^{13}-10^{14}$\,cm$^{-2}$ at the electric field of 1 V/nm. We estimate the spin lifetimes to be on the picosecond level.

cond-mat.mes-hall↗

arXiv2019

$k \cdot p$ theory for phosphorene: Effective g-factors, Landau levels, and excitons

Phosphorene, a single layer of black phosphorus, is a direct-band gap two-dimensional semiconductor with promising charge and spin transport properties. The electronic band structure of phosphorene is strongly affected by the structural anisotropy of the underlying crystal lattice. We describe the relevant conduction and valence bands close to the $Γ$ point by four- and six-band (with spin) $k \cdot p$ models, including the previously overlooked interband spin-orbit coupling which is essential for studying anisotropic crystals. All the $k \cdot p$ parameters are obtained by a robust fit to {\it ab initio} data, by taking into account the nominal band structure and the $k$-dependence of the effective mass close to $Γ$-point. The inclusion of interband spin-orbit coupling allows us to determine dipole transitions along both armchair and zigzag directions. The interband coupling is also key to determine the effective g-factors and Zeeman splittings of the Landau levels. We predict the electron and hole g-factor correction of $\approx 0.03$ due to the intrinsic contributions in phosphorene, which lies within the existing range of experimental data. Furthermore, we investigate excitonic effects using the $k \cdot p$ models and find exciton binding energy (0.81 eV) and exciton diameters consistent with experiments and {\it ab initio} based calculations. The proposed $k \cdot p$ Hamiltonians should be useful for investigating magnetic, spin, transport, optical properties and many-body effects in phosphorene.

cond-mat.mes-hall↗

arXiv2019

Spin-orbit coupling in elemental two-dimensional materials

The fundamental spin-orbit coupling and spin mixing in graphene and rippled honeycomb lattice materials silicene, germanene, stanene, blue phosphorene, arsenene, antimonene, and bismuthene is investigated from first principles. The intrinsic spin-orbit coupling in graphene is revisited using multi-band $k\cdot p$ theory, showing the presence of non-zero spin mixing in graphene despite the mirror symmetry. However, the spin mixing itself does not lead to the the Elliott-Yafet spin relaxation mechanism, unless the mirror symmetry is broken by external factors. For other aforementioned elemental materials we present the spin-orbit splittings at relevant symmetry points, as well as the spin admixture $b^2$ as a function of energy close to the band extrema or Fermi levels. We find that spin-orbit coupling scales as the square of the atomic number Z, as expected for valence electrons in atoms. For isolated bands, it is found that $b^2\sim Z^4$. The spin-mixing parameter also exhibits giant anisotropy which, to a large extent, can be controlled by tuning the Fermi level. Our results for $b^2$ can be directly transferred to spin relaxation time due to the Elliott-Yafet mechanism, and therefore provide an estimate of the upper limit for spin lifetimes in materials with space inversion center.

cond-mat.mes-hall↗

arXiv2019

The Electronic Thickness of Graphene

The van-der-Waals stacking technique enables the fabrication of heterostructures, where two conducting layers are atomically close. In this case, the finite layer thickness matters for the interlayer electrostatic coupling. Here we investigate the electrostatic coupling of two graphene layers, twisted by 22 degrees such that the layers are decoupled by the huge momentum mismatch between the K and K' points of the two layers. We observe a splitting of the zero-density lines of the two layers with increasing interlayer energy difference. This splitting is given by the ratio of single-layer quantum capacitance over interlayer capacitance C and is therefore suited to extract C. We explain the large observed value of C by considering the finite dielectric thickness d of each graphene layer and determine d=2.6 Angstrom. In a second experiment we map out the entire density range with a Fabry-Pérot resonator. We can precisely measure the Fermi-wavelength in each layer, showing that the layers are decoupled. We find that the Fermi wavelength exceeds 600nm at the lowest densities and can differ by an order of magnitude between the upper and lower layer. These findings are reproduced using tight-binding calculations.

cond-mat.mes-hall↗

arXiv2019

Spin-orbit coupling and spin relaxation in phosphorene: Intrinsic versus extrinsic effects

First-principles calculations of the essential spin-orbit and spin relaxation properties of phosphorene are performed. Intrinsic spin-orbit coupling induces spin mixing with the probability of $b^2 \approx 10^{-4}$, exhibiting a large anisotropy, following the anisotropic crystalline structure of phosphorene. For realistic values of the momentum relaxation times, the intrinsic (Elliott--Yafet) spin relaxation times are hundreds of picoseconds to nanoseconds. Applying a transverse electric field (simulating gating and substrates) generates extrinsic $C_{2v}$ symmetric spin-orbit fields in phosphorene, which activate the D'yakonov--Perel' mechanism for spin relaxation. It is shown that this extrinsic spin relaxation also has a strong anisotropy, and can dominate over the Elliott-Yafet one for strong enough electric fields. Phosphorene on substrates can thus exhibit an interesting interplay of both spin relaxation mechanisms, whose individual roles could be deciphered using our results.

cond-mat.mes-hall↗