arXiv subjects

E. Lähderanta

Publications and source records attributed to E. Lähderanta.

15 recordsLinked to original sources

arXiv2026

Hydrostatic pressure effect on the pseudogap in Y0.77Pr0.23Ba2Cu3O7-δsingle crystals

We studied the changes of resistivity $ρ(T)$, superconducting transition temperature $T_c$, fluctuation conductivity $σ'(T)$, and pseudogap $Δ^*(T)$ of $Y_{1-x}Pr_xBa_2Cu_3O_{7-δ}$ (YPrBCO) ($x = 0.23$) single crystal under hydrostatic pressure (HP) up to $\sim 1.1$ GPa. Defects created by magnetic inclusions of $PrBa_2Cu_3O_{7-δ}$ (PrBCO) were found to play a significant role in the behavior of the sample. The largest decrease in $ρ(T)$ was found depending on HP at a rate of $d\lnρ(100~K)/dP = -(29 \pm 0.2) \% \cdot GPa^{-1}$. This indicates that the mechanisms of the influence of HP on the $ρ(T)$ and $T_c$ of YPrBCO and $YBa_2Cu_3O_{7-δ}$ single crystals are different. From the analysis of the pseudogap, it was revealed that the mechanism of the interaction of charge carriers with magnetic PrBCO impurities changes three times with increasing HP. Relatively low HP, up to $\sim 0.5$ GPa, promotes the formation of defects caused by PrBCO magnetic inclusions. Starting from 0.6 GPa, the HP neutralizes the influence of magnetic impurities, and the magnetic maximum on $Δ^*(T)$ disappears. Above $\sim 1.0$ GPa, HP aligns the magnetic moments of PrBCO, and the magnetic maximum reappears on $Δ^*(T)$, more pronounced than at $P = 0$.

cond-mat.supr-con↗

arXiv2026

Specific features of the magnetic-field dependences of electrical resistivity in Bi--Mn solid solutions with low Mn content

For the first time, the field dependences of the magnetoresistance of a textured polycrystalline Bi$_{88.08}$Mn$_{11.92}$ sample were investigated in the $H \perp I$ and $H \parallel I$ configurations, and the results were compared with those previously obtained for the solid solution Bi$_{95.69}$Mn$_{3.69}$Fe$_{0.62}$ with a lower manganese concentration. It was demonstrated that the magnetoresistance field dependencies for Bi$_{88.08}$Mn$_{11.92}$ differ significantly from those of Bi$_{95.69}$Mn$_{3.69}$Fe$_{0.62}$ below 100 K, becoming nearly identical as the temperature approaches room temperature. It was established that the maximum relative magnetoresistance values for Bi$_{88.08}$Mn$_{11.92}$ are observed at a magnetic field of 90 kOe and a temperature of 100 K in both the $H \perp I$ and $H \parallel I$ configurations, amounting to 3170\% and 380\%, respectively. These values are significantly lower than those previously reported for the solid solution containing less manganese (Bi$_{95.69}$Mn$_{3.69}$Fe$_{0.62}$), which were 3877\% and 742\%, respectively. Analysis of the results indicates that differences in field dependence among the studied materials are associated with varying amounts of magnetic $α$-BiMn. The anomalous magnetoresistance behavior, as compared with that of pure bismuth, is attributed to the influence of internal magnetism on electrical transport within the bismuth matrix of the studied solid solutions.

cond-mat.mtrl-sci↗

arXiv2025

Modeling adsorption processes on the core-shell-like polymer structures: star and comb topologies

Coagulation-flocculation of pollutants and chelation of heavy metal ions are two widely used techniques in wastewater purification. Despite the differences between their respective mechanisms and inherent length scales, they bear much similarity on a larger scale, and can both be treated as adsorption of obstacles on a polymer structure. In this regime, their adsorbing efficiency is predominantly affected by conformation statistics of involved polymers, and this approach has been used in our previous studies based on lattice polymer model for a linear polymer adsorbent. There is a strong experimental evidence that branched adsorbents are more efficient than their linear counterparts. In this study we focus on two simplest representatives of the core-shell branched architectures: the star-like (with zero-dimensional point-like core) and comb-like polymers (with one-dimensional rigid core) with various number of branches, $f$, branch lengths, $N$, and branches separations, $S$ (for the case of comb-like structure). The polymers are grown on a lattice using the Monte Carlo simulations with the pruned-enriched Rosenbluth algorithm. The quantitative estimates for adsorption capacity in terms of adsorbed obstacles per monomer and the average number of bonds per adsorbed particle (average adsorption strength) have been evaluated in a wide range of parameters $f$, $N$, and $S$. Both the case of implicit diffusion of obstacles (with averaging over different arrangements of immobilized obstacles) and explicit diffusion of obstacles (allowing to study dynamics of adsorption process) have been analyzed. We found that comb-like polymers display the higher adsorption capacity but lower adsorption strength, comparing to the star-like polymers, and these effects are more pronounced with increasing branches separations $S$.

cond-mat.soft↗

arXiv2025

Effect of a magnetic field up to 9 T on the temperature dependence of the pseudogap in YBa$_2$Cu$_3$O$_{7-δ}$ films

The work analyzes the effect of a magnetic field $B$ directed along the $c$ axis ($B \parallel c$) up to 9~T on the resistivity $ρ(T)$, fluctuation conductivity (FLC) $σ'(T)$ and pseudogap $Δ^*(T)$ in thin films of YBa$_2$Cu$_3$O$_{7-δ}$ with a critical temperature of the superconducting transition $T_c = 88.8$~K. In contrast to previous work, where the magnetic field was directed along the $ab$ plane ($B \parallel ab$), the influence of the field on the sample is stronger due to the contribution of both spin--orbit and Zeeman effects. It was found that the BEC--BCS transition temperature, $T_{\mathrm{pair}}$, which corresponds to the maximum of the $Δ^*(T)$ dependence, shifts to the region of lower temperatures with increasing $B$, and the maximum value of $Δ^*(T_{\mathrm{pair}})$ decreases in fields $B > 5$~T. It was found that with increasing field, the low-temperature maximum near $T_0$ is smeared and disappears at $B > 1$~T. In addition, above the Ginzburg temperature $T_G$, for $B > 1$~T, a minimum appears on $Δ^*(T)$ at $T_{\min}$, which becomes very pronounced with a subsequent increase in $B$. As a result, the overall value of $Δ^*(T_G)$ decreases noticeably, most likely due to the pair-breaking effect. At the same time, $ΔT_{\mathrm{fl}}$ and $ξ_c(0)$ increase sharply by approximately 3 times with increasing $B$ above 1~T. Our results confirm the possibility of the formation of a vortex state in YBa$_2$Cu$_3$O$_{7-δ}$ by a magnetic field and its evolution with increasing $B$.

cond-mat.supr-con↗

arXiv2024

Mapping self-avoiding walk on obstacle-ridden lattice onto chelation of heavy metal ions: Monte Carlo study

Self-avoiding walk (SAW) represents linear polymer chain on a large scale, neglecting its chemical details and emphasizing the role of its conformational statistics. The role of the latter is important in formation of agglomerates and complexes involving polymers and organic or inorganic particles, such as polymer-stabilized colloidal suspensions, microemulsions, or micellar solutions. When such particles can be adsorbed on a polymer of considerably larger dimensions than themselves, this setup may represent chelation of heavy metal ions by polymeric chelants. We consider the SAW of the length $N$ on a cubic lattice ridden by randomly distributed obstacles of the concentration $p$ interpreted as ions. The SAW monomers can bind to the obstacles with variable binding energy $\varepsilon$ mimicking formation of the chelation bond. Pruned-enriched Rosenbluth method (PERM) Monte Carlo (MC) algorithm is applied to simulate system behaviour. We focus on several relevant properties related to the chelation efficiency and strength, as functions of the variables set $\{p,N,\varepsilon\}$. The results are interpreted in terms of conformational freedom, excluded volume effects and loop formation for the SAW, and the tendencies being predicted are in agreement with some experimental data.

cond-mat.soft↗

arXiv2024

Coagulation-flocculation process on a lattice: Monte Carlo simulations

Coagulation-flocculation, the physicochemical process widely used for purification a wastewater, is affected both by chemical details of involved polymers and by the statistics of their conformations on a large scale. The latter aspect is covered in this study by employing a coarse-grained modelling approach based on a combination of two paradigms of statistical mechanics. One is the self-avoiding walk (SAW) which generates a range of conformations for a linear polymer of $N_{\rm SAW}$ monomers. Another one is a non-trivial diffusion limited aggregation (DLA) process of $N_{\rm DLA}$ impurities (referred thereafter as "particles") which describes their coagulation occurring with the probability $0< p \leq 1$ ($p=1$ recovers a standard DLA). DLA of diffusive particles is complemented by their irreversible adsorption on the SAW monomers occurring with the probability equal to one, both processes resulting in formation of the DLA-SAW agglomerates. The dynamics of formation of such agglomerates, as well as their fractal dimensions and internal structure are of practical interest. We consider a range of related characteristics, such as: (i) absolute $N_a$ and relative $n_a$ adsorbing efficiencies of SAW; (ii) effective gyration radius $R_{g {\rm DLA-SAW}}$ of the DLA-SAW agglomerates; and (iii) the fractal dimension $D_{\rm DLA-SAW}$ of these aggregates. These are studied within a wide range for each parameter from a set $\{p,N_{\rm DLA},N_{\rm SAW}\}$.

cond-mat.soft↗

arXiv2024

Evolution of the pseudogap temperature dependence in YBa$_2$Cu$_3$O$_{7-δ}$ films under the influence of a magnetic field

The evolution of the temperature dependence of pseudogap $Δ$*(T) in optimally doped (OD) YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) films with $T_{c}$ = 88.7 K under the influence of a magnetic field $B$ up to 8 T has been studied in detail. It has been established that the shape of $Δ$*(T) for various $B$ over the entire range from the pseudogap opening temperature $T$* to $T_{01}$, below which superconducting fluctuations occur, has a wide maximum at the BEC-BCS crossover temperature $T_{pair}$, which is typical for OD films and untwinned YBCO single crystals. $T$* was shown to be independent on $B$, whereas $T_{pair}$ shifts to the low temperature region along with increase of $B$, while the maximum value of $Δ$*($T_{pair}$) remains practically constant regardless of $B$. It was revealed that as the field increases, the low-temperature maximum near the 3D-2D transition temperature $T_{0}$ is blurred and disappears at $B$ > 5 T. Moreover, above the Ginzburg temperature $T_{G}$, which limits superconducting fluctuations from below, at $B$ > 0.5 T, a minimum appears on $Δ$*(T) at $T_{min}$, which becomes very pronounced with a further increase in the field. As a result, the overall value of $Δ$*(T) decreases noticeably most likely due to pair-breaking affect of a magnetic field. A comparison of $Δ$*(T) near $T_{c}$ with the Peters-Bauer theory shows that the density of fluctuating Cooper pairs actually decreases from < > $\approx$ 0.31 at $B$ = 0 to < > $\approx$ 0.28 in the field 8 T. The observed behavior of $Δ$*(T) around $T_{min}$ is assumed to be due to the influence of a two-dimensional vortex lattice created by the magnetic field, which prevents the formation of fluctuating Cooper pairs near $T_{c}$.

cond-mat.supr-con↗

arXiv2021

Prediction of a Heusler alloy with switchable metal-to-half-metal behavior

We propose a ferromagnetic Heusler alloy that can switch between a metal and a half-metal. Thiseffect can provide tunable spintronics properties. Using the density functional theory (DFT) withreliable implementations of the electron correlation effects, we find Mn2ScSi total energy curvesconsisting of distinct branches with a very small energy difference. The phase at low lattice crystalvolume is a low magnetic half-metallic state while the phase at high lattice crystal volume is a highmagnetic metallic state. We suggest that the transition between half-metallic and metallic statescan be triggered by a triaxial contraction/expansion of the crystal lattice or by an external magneticfield if we assume that the lattice is cubic and remains cubic under expansion/contraction. However,the phase at high volume can also undergo an austenite-martensite phase transition because of thepresence of Jahn-Teller active3delectrons on the Mn atoms.

cond-mat.mtrl-sci↗

arXiv2020

Electronic structure beyond the generalized gradient approximation for Ni$_2$MnGa

The stability of the nonmodulated martensitic phase, the austenitic Fermi surface and the phonon dispersion relations for ferromagnetic Ni$_2$MnGa are studied using density functional theory. Exchange-correlation effects are considered with various degrees of precision, starting from the simplest local spin density approximation (LSDA), then adding corrections within the generalized gradient approximation (GGA) and finally, including the meta-GGA corrections within the strongly constrained and appropriately normed (SCAN). We discuss a simple procedure to reduce a possible overestimation of magnetization and underestimation of nesting vector in SCAN by parametrically decreasing self-interaction corrections.

cond-mat.mtrl-sci↗

arXiv2019

Correlation effects on ground-state properties of ternary Heusler alloys: first-principles study

The strongly constrained and appropriately normed (SCAN) semi-local functional for exchange-correlation is deployed to study the ground-state properties of ternary Heusler alloys transforming martensitically. The calculations are performed for ferromagnetic, ferrimagnetic, and antiferromagnetic phases. Comparisons between SCAN and generalized gradient approximation (GGA) are discussed. We find that SCAN yields smaller lattice parameters and higher magnetic moments compared to the GGA corresponding values for both austenite and martensite phases. Furthermore, in the case of ferromagnetic and non-magnetic Heusler compounds, GGA and SCAN display similar trends in the total energy as a function of lattice constant and tetragonal ratio. However, for some ferrimagnetic Mn-rich Heusler compounds, different magnetic ground states are found within GGA and SCAN.

cond-mat.mtrl-sci↗

arXiv2019

Gate-tunable magnetism of C adatoms on graphene

We have performed density functional theory calculations of graphene decorated with carbon adatoms, which bind at the bridge site of a C--C bond. Earlier studies have shown that the C adatoms have magnetic moments and have suggested the possibility of ferromagnetism with high Curie temperature. Here we propose to use a gate voltage to fine tune the magnetic moments from zero to 1$μ_B$ while changing the magnetic coupling from antiferromagnetism to ferromagnetism and again to antiferromagnetism. These results are rationalized within the Stoner and RKKY models. When the SCAN meta-GGA correction is used, the magnetic moments for zero gate voltage are reduced and the Stoner band ferromagnetism is slightly weakened in the ferromagnetic region.

cond-mat.mes-hall↗

arXiv2017

Superconducting spin valves controlled by spiral re-orientation in B20-family magnets

We propose a superconducting spin-triplet valve, which consists of a superconductor and an itinerant magnetic material, with the magnet showing an intrinsic non-collinear order characterized by a wave vector that may be aligned in a few equivalent preferred directions under control of a weak external magnetic field. Re-orienting the spiral direction allows one to controllably modify long-range spin-triplet superconducting correlations, leading to spin-valve switching behavior. Our results indicate that the spin-valve effect may be noticeable. This bilayer may be used as a magnetic memory element for cryogenic nanoelectronics. It has the following advantages in comparison to superconducting spin valves proposed previously: (i) it contains only one magnetic layer, which may be more easily fabricated and controlled, (ii) its ground states are separated by a potential barrier, which solves the "half-select" problem of the addressed switch of memory elements.

cond-mat.supr-con↗

arXiv2017

A nontrivial crossover in topological Hall effect regimes

We propose a new theory of the topological Hall effect (THE) in systems with chiral magnetization vortices such as magnetic skyrmions. We solve the problem of electron scattering on a magnetic skyrmion exactly, for an arbitrary strength of exchange interaction and the skyrmion size. We report the existence of different regimes of THE and resolve the apparent contradiction between the adiabatic Berry phase theoretical approach and the perturbation theory for THE. We traced how the topological charge Hall effect transforms into the spin Hall effect upon varying the exchange interaction strength or the skyrmion size. This transformation has a nontrivial character: it is accompanied by an oscillating behavior of both charge and spin Hall currents. This hallmark of THE allows one to differentiate the chirality driven contribution to Hall response in the experiments.

cond-mat.mes-hall↗

arXiv2015

Resonant 2D-2D tunneling with account for spin-orbit interaction

We present a theory of quantum tunneling between 2D layers with account for Rashba and Dresselhaus spin-orbit interaction (SOI) in the layers. Energy and momentum conservation results in a single resonant peak in the tunnel conductance between two 2D layers as has been experimentally observed for two quantum wells (QW) in GaAs/AlGaAs heterostructures. The account for SOI in the layers leads to a complex pattern in the tunneling characteristic with typical features corresponding to SOI energy. For this manifestation of SOI to be observed experimentally the characteristic energy should exceed the resonant broadening related to the particles quantum lifetime in the layers. We perform an accurate analysis of the known experimental data on electron and hole 2D-2D tunneling in AlGaAs/GaAs heterostructures. It appears that for the electron tunneling the manifestation of SOI is difficult to observe, but for the holes tunneling the parameters of the real structures used in the experiments are very close to those required by the resolution criteria. We also consider a new promising candidate for the effect to be observed, that is p-doped SiGe strained heterostructures. The reported parameters of cubic Rashba SOI and quantum lifetime in strained Ge QWs fabricated up to date already match the criteria for observing SOI in 2D-2D heavy holes tunneling. As supported by our calculations small adjustments of the parameters for AlGaAs/GaAs p-type QWs or simply designing a 2D-2D tunneling experiment for SiGe case are very likely to reveal the SOI features in the 2D-2D tunneling.

cond-mat.mes-hall↗

arXiv2012

Pecularities of Hall effect in GaAs/δ /GaAs/In\timesGa1-\timesAs/GaAs (\times {\approx} 0.2) heterostructures with high Mn content

Transport properties of GaAs/δ /GaAs/In\timesGa1-\timesAs/GaAs structures containing InxGa1-xAs (\times {\approx} 0.2) quantum well (QW) and Mn delta layer (DL) with relatively high, about one Mn monolayer (ML) content, are studied. In these structures DL is separated from QW by GaAs spacer with the thickness ds = 2-5 nm. All structures possess a dielectric character of conductivity and demonstrate a maximum in the resistance temperature dependence Rxx(T) at the temperature {\approx} 46K which is usually associated with the Curie temperature Tc of ferromagnetic (FM) transition in DL. However, it is found that the Hall effect concentration of holes pH in QW does not decrease below TC as one ordinary expects in similar systems. On the contrary, the dependence pH(T) experiences a minimum at T = 80-100 K depending on the spacer thickness, then increases at low temperatures more strongly than ds is smaller and reaches a giant value pH = (1-2)\cdot10^13 cm^(-2). Obtained results are interpreted in the terms of magnetic proximity effect of DL on QW, leading to induce spin polarization of the holes in QW. Strong structural and magnetic disorder in DL and QW, leading to the phase segregation in them is taken into consideration. The high pH value is explained as a result of compensation of the positive sign normal Hall effect component by the negative sign anomalous Hall effect component.

cond-mat.mes-hall↗