arXiv subjects

Bartlomiej Wiendlocha

Publications and source records attributed to Bartlomiej Wiendlocha.

At least 19 recordsLinked to original sources

arXiv2026

On the dual character of Zn impurity in SnTe: Tuning thermoelectric and topological properties

We present a first-principles study of the electronic structure and thermoelectric properties of Zn-doped SnTe using the Korringa-Kohn-Rostoker method within the coherent potential approximation, complemented by pseudopotential calculations. SnTe is a lead-free analogue of PbTe and a candidate thermoelectric material in which Zn doping has been experimentally reported to enhance the performance of $p$-type samples. We show that Zn introduces a resonant-like impurity state, located within the conduction band, which evolves strongly depending on the Zn concentration. This feature leads to a significant enhancement of the thermopower in $n$-type SnTe. In the valence band, Zn doping induces L-$Σ$ band convergence, also resulting in an increased $p$-type Seebeck coefficient over a broad concentration range and delaying the onset of the bipolar effect. We further demonstrate that the Zn-induced band-structure modifications drive a transition from an inverted to a trivial band ordering, indicating a controllable topological phase transition. Our results clarify the microscopic role of Zn in SnTe and identify doping as a mechanism for simultaneously tuning thermoelectric and topological properties.

cond-mat.mtrl-sci↗

arXiv2026

Strong-coupling multigap superconductivity in the Heusler compound ScAu$_2$Al

ScAu$_2$Al is the Heusler superconductor with the highest reported transition temperature, Tc, close to 5 K. Here, we combine high-resolution ac calorimetry and electrical-transport measurements with first-principles-based Eliashberg calculations. The large specific-heat jump, $ΔC/{γ_nT_c} =$ 2.2, well above the weak-coupling BCS value of 1.43, together with an electron-phonon coupling constant $λ = $1.1, establishes ScAu2Al as a strong-coupling superconductor. The electronic specific heat is described better by a two-gap α model, with $2Δ_S/k_BT_c = $ 4.1 and $2Δ_L/k_BT_c = $ 4.8, than by a single-gap model. The thermodynamic upper critical field follows a conventional WHH-like temperature dependence with $B_{c2}(0) = 0.18$ T, whereas the resistively determined critical field reaches values about three times larger and exhibits a pronounced positive curvature. We attribute the enhanced resistive field scale to superconductivity in disordered grain-boundary and interfacial regions. The experimental results are supported by calculations of the heat capacity and upper critical field within the Eliashberg formalism using Fermi-surface and electron-phonon parameters obtained from first-principles calculations

cond-mat.supr-con↗

arXiv2026

Type-I superconductivity in a quasi-2D topologically nontrivial YbBi$_2$

Intrinsic superconductivity in stoichiometric materials with nontrivial electronic topology remains uncommon, limiting opportunities to investigate how these two phenomena coexist within a single electronic system. Here, we report bulk type-I superconductivity below $T_c$ $\sim$ 0.9~K in YbBi$_2$, a layered rare-earth compound with a nonsymmorphic crystal structure and a quasi-two-dimensional Fermi surface. Thermodynamic and transport measurements establish the superconducting ground state, while quantum oscillations reveal exceptionally light carriers, with a cyclotron mass as low as 0.07 $m_e$, and a nonzero Berry phase of approximately 0.82 $π$. The latter closely matches the calculated value of the corresponding Wilson phase 0.99 $π$ for the corresponding orbit near a symmetry-protected band degeneracy. ARPES measurements show good agreement with key features of the calculated electronic structure, providing complementary experimental constraints on the normal-state band structure. The combination of intrinsic type-I superconductivity, light quasi-two-dimensional carriers, and signatures of nontrivial electronic topology identifies YbBi$_2$ as a distinct stoichiometric platform for investigating superconductivity in a topologically nontrivial electronic environment.

cond-mat.supr-con↗

arXiv2026

Multiband superconductivity in the kagome-lattice superconductor Re2Zr

We present a first-principles investigation of the electronic structure, lattice dynamics, electron-phonon coupling, and superconducting properties of the hexagonal kagome-lattice compound Re$_2$Zr, in which time-reversal symmetry breaking and unconventional pairing have recently been proposed. We examine whether superconductivity in Re$_2$Zr can be explained within a conventional phonon-mediated framework by performing fully ab initio calculations within the density functional theory for superconductors. The electronic structure is characterized by a mixture of seven two- and three-dimensional Fermi surface sheets, giving rise to multiband superconductivity with many overlapping superconducting gaps. A spin-orbit-induced van Hove singularity is identified in the density of states near $E_F$. The electron-phonon interaction is moderately strong, with a coupling constant $λ\simeq 0.88$. Superconducting gap calculations reveal significant anisotropy and a broad distribution of gap values across different Fermi surface sheets. Spin fluctuations introduce additional depairing effects and reduce the critical temperature from 7.7 K to 6.4 K, in excellent agreement with the experimental value of 6.65 K. Taken together, our results show that the superconducting energy scale in Re$_2$Zr can be quantitatively reproduced within a conventional phonon-mediated framework, while the resulting state exhibits pronounced multiband and anisotropic gap structure. The origin of the experimentally suggested time-reversal symmetry breaking, however, remains an open question.

cond-mat.supr-con↗

arXiv2025

Thermoelectric performance of Ni-Au metallic alloys determined by resonant scattering

This work presents a theoretical study of the electronic structure and transport properties of Ni-Au alloys, recently identified as excellent thermoelectric metals with a power factor significantly exceeding that of conventional semiconductor thermoelectrics. Using first-principles calculations based on the Korringa-Kohn-Rostoker method combined with the coherent-potential approximation (KKR-CPA) and the Kubo-Greenwood formalism, we demonstrate the key role of resonant scattering in determining the thermoelectric properties of these alloys. This is supported by calculated densities of states, Bloch spectral functions, electrical conductivity, and thermopower. Alloying Ni with Au not only induces resonant scattering but also leads to the formation of a flat band below the Fermi level. The combination of these two features results in high thermopower, arising from a transition between resonant and weak scattering regimes near the Fermi level. Our findings are further compared with analogous calculations for constantan, a Ni-Cu alloy long regarded as a reference thermoelectric metal. We show that differences between the Ni-Au and Ni-Cu systems explain why Ni-Au exhibits nearly twice the thermopower of Ni-Cu. Finally, we simulate the effect of lattice parameter variation on the thermoelectric performance of Ni-Au and suggest that this is a promising pathway for further enhancement, for example through additional alloying or layer deposition.

cond-mat.mtrl-sci↗

arXiv2025

Anisotropic, multiband, and strong-coupling superconductivity of the Pb0.64Bi0.36 alloy

This paper presents theoretical and experimental studies on the superconductivity of Pb${_{0.64}}$Bi$_{0.36}$ alloy, which is a prototype of strongly coupled superconductors and exhibits one of the strongest coupling under ambient pressure among the materials studied so far. The critical temperature, the specific heat in the superconducting state, and the magnetic critical fields are experimentally determined. Deviations from the single-gap s-wave BCS-like behavior are observed. The electronic structure, phonons and electron-phonon interactions are analyzed in relation to the metallic Pb, explaining why the Pb-Bi alloy exhibits such a large value of the electron-phonon coupling parameter $λ\simeq 2$. Superconductivity is studied using the isotropic Eliashberg formalism as well as the anisotropic density functional theory for superconductors. We find that while Pb is a two-gap superconductor with well-defined separate superconducting gaps, in the Pb-Bi alloy an overlapped three-gap-like structure is formed with a strong anisotropy. Furthermore, the chemical disorder, inherent to this alloy, leads to strong electron scattering, which is found to reduce the critical temperature.

cond-mat.supr-con↗

arXiv2024

Superconductivity in ternary Mg$_4$Pd$_7$As$_6$

We report the synthesis and characterization of a new compound Mg$_4$Pd$_7$As$_6$, which was found to be a superconductor with $T_c=5.45$~K. Powder X-ray diffraction confirms the U$_4$Re$_7$Si$_6$ structure (space group $Im$-$3m$, no. 229) with the lattice parameter $a$ = 8.2572(1)~Å. Magnetization, specific heat, and electrical resistivity measurements indicate that it is a moderate-coupling ($λ= 0.72$) type-II superconductor. The electronic and phonon structures are calculated, highlighting the importance of antibonding Pd-As interactions in determining the properties of this material. The calculated electron-phonon copling parameter $λ= 0.76$ agrees very well with the experimental finding, which confirms the conventional pairing mechanism in Mg$_4$Pd$_7$As$_6$.

cond-mat.supr-con↗

arXiv2024

High thermoelectric power factor through topological flat bands

Thermoelectric (TE) materials are useful for applications such as waste heat harvesting or efficient and targeted cooling. While various strategies towards superior thermoelectrics through a reduction of the lattice thermal conductivity have been developed, a path to enhance the power factor is pressing. Here, we report large power factors up to 5 mW m$^{-1}$ K$^{-2}$ at room temperature in the kagome metal Ni$_3$In$_{1-x}$Sn$_x$. This system is predicted to feature almost dispersionless flat bands in conjunction with highly dispersive Dirac-like bands in its electronic structure around the Fermi energy $E_\text{F}$ [L. Ye et al., Nature Physics 1-5 (2024)]. Within this study, we experimentally and theoretically showcase that tuning this flat band precisely below $E_\text{F}$ by chemical doping $x$ boosts the Seebeck coefficient and power factor, as highly mobile charge carriers scatter into the flat-band states. Our work demonstrates the prospect of engineering extremely flat and highly dispersive bands towards the Fermi energy in kagome metals and introduces topological flat bands as a novel tuning knob for thermoelectrics.

cond-mat.str-el↗

arXiv2023

Local distortions of the crystal structure and their influence on the electronic structure and superconductivity of the high-entropy alloy (TaNb)0.67(HfZrTi)0.33

Local distortions of the crystal structure and their influence on the electronic structure, electron-phonon interaction, and superconductivity are theoretically studied in the superconducting high-entropy alloy (HEA) (TaNb)$_{0.67}$(HfZrTi)$_{0.33}$. Distortions of the crystal lattice are caused by the relaxation of atomic positions and are studied in the twelve models of supercells. The largest relative changes in the interatomic distances due to relaxation reach 8\%. On average, local distortions tend to lower the density of states at the Fermi level and significantly reduce the electron-phonon coupling parameter $λ$. As a result, the calculated superconducting critical temperature is reduced to about 50\% of the initial value, which shows the strong impact of structural disorder on superconductivity in this prototype high-entropy alloy. With the reduced value of $λ$, the theoretical $T_c$ is closer to the experiment for typical values of the Columomb pseudopotential parameter $μ^*$. The experimental $T_c$ can be reproduced by taking a slightly enhanced $μ^* = 0.176$, which leaves little room for the possibility of suppression of superconductivity by disorder.

cond-mat.supr-con↗

arXiv2023

Strong-coupling superconductivity of the Heusler-type compound ScAu2Al: Ab-initio studies

The ScAu$_2$Al superconducting Heusler-type compound was recently characterized to have the highest critical temperature of $T_c = 5.12$ K and the strongest electron-phonon coupling among the Heusler family. In this work, the electronic structure, phonons, electron-phonon coupling, and superconductivity of ScAu$_2$Al are studied using \textit{ab initio} calculations. The spin-orbit coupling significantly changes the electronic structure removing the van Hove singularity from the vicinity of the Fermi level. In the phonon spectrum, low frequency acoustic modes, additionally softened by the spin-orbit interaction, strongly couple with electrons, leading to the electron-phonon coupling constant $λ=1.25$, a record high among Heuslers. The density functional theory for superconductors is then used to analyze superconducting {state in this two-band superconductor}. The effect of spin fluctuations (SF) on superconductivity is also analyzed. The calculated critical temperatures of $T_c = 5.16$ K (4.79 K with SF) agree very well with the experiment, confirming the electron-phonon mechanism of superconductivity and showing a weak spin-fluctuations effect. The superconducting gaps formed on two Fermi surface sheets exhibit moderate anisotropy. Their magnitudes confirm the strong coupling regime, as the reduced average values are $2Δ_1/k_BT_c \simeq 4.1$ and $2Δ_2/k_BT_c \simeq 4.3$. Anisotropy of the gaps and large spread in their values significantly affect the calculated quasiparticle density of states.

cond-mat.supr-con↗

arXiv2023

Superconductivity near the Mott-Ioffe-Regel limit in the high-entropy alloy superconductor (ScZrNb)$_{1-x}$(RhPd)$_x$ with a CsCl-type lattice

Theoretical analysis of the electronic structure of the high-entropy-type superconductor (ScZrNb)$_{1-x}$(RhPd)$_x$, $x \in (0.35, 0.45)$ is presented. The studied material is a partially ordered CsCl-type structure, with two sublattices, randomly occupied by Sc, Zr, Nb (first sublattice) and Nb, Rh, and Pd (second sublattice). Calculations were done using the Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA) and take into account the substitutional disorder. Our total energy calculations confirm the preference for the partially ordered structure over the fully random {\it bcc}-type one. Electronic densities of states $N(E)$, dispersion relations, and McMillan-Hopfield parameters $η$ (electronic contribution to electron-phonon coupling) are studied as a function of composition. The computed increasing trends in $N(E_F)$ and $η$ with $x$ are opposite to what we expected based on the experimental results, where the decrease in the critical temperature with increasing $x$ was found. Very strong electron scattering due to disorder is observed, as the electronic dispersion relations are strongly smeared. As a result, the computed electronic lifetimes $τ$ are very short, leading to a small mean-free path of electrons of the order of interatomic distance, which puts (ScZrNb)$_{1-x}$(RhPd)$_x$ near the Mott-Ioffe-Regel limit. The trend in $τ(x)$ is similar to the trend observed experimentally in $T_c(x)$, suggesting that disorder may be the factor that influences $T_c$ in this series of alloys.

cond-mat.supr-con↗

arXiv2022

Pressure effects on the electronic structure, phonons, and superconductivity of noncentrosymmetric ThCoC2

The electronic structure, phonons, electron-phonon coupling, and superconductivity are theoretically studied in noncentrosymmetric superconductor ThCoC$_2$ as a function of pressure, in the pressure range 0 - 20 GPa. We found that the electronic band splitting induced by the spin-orbit coupling is enhanced under pressure. In spite of the overall stiffening of the crystal lattice, the electron-phonon coupling constant $λ$ increases with pressure, from 0.583 at 0 GPa to 0.652 at 20 GPa. If the isotropic Eliashberg electron-phonon coupling theory is used to simulate the effect on the critical temperature $T_c$, such an increase in $λ$ results in a substantial increase of $T_c$ from 2.5~K at 0 GPa to 4~K at 20 GPa. This shows that examining the effect of pressure offers a chance to resolve the pairing mechanism in ThCoC$_2$.

cond-mat.supr-con↗

arXiv2022

Electronic structure of CeIr3 superconductor: DMFT studies

We present the band structure of CeIr$_3$ superconductor calculated within the dynamical mean field theory (DMFT). Standard GGA and GGA+U methods fail to reproduce the experimental electronic specific heat coefficient $γ_{\rm expt.}$ due to the underestimated density of states at the Fermi level $N(E_F)$ followed by an overestimated strength of electron-phonon coupling (EPC) calculated as a renormalization of $γ_{\rm expt.}$. The DMFT study shows a strong hybridization of $4f$ states of Ce with $5d$-states of Ir, which leads to the larger $N(E_F)$ giving the correct $γ_{\rm expt.}$ with a moderate EPC in agreement with the experimental data.

cond-mat.str-el↗

arXiv2021

The effect of Ni doping on the electronic structure and superconductivity in the noncentrosymmetric ThCoC2

ThCoC$_2$ is a non-BCS, noncentrosymmetric superconductor with $T_c \simeq 2.5$~K, in which pairing mechanism was suggested to be either spin fluctuations or the electron-phonon coupling. Earlier experimental work revealed that $T_c$ can be greatly enhanced upon Ni doping in ThCo$_{1-x}$Ni$_x$C$_2$, up to 12~K for $x = 0.4$. In this work, using the Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA), the evolution of the electronic structure upon Ni doping was studied and the increase of the density of states at the Fermi level was found. The electron-phonon coupling constant $λ$ was calculated using the rigid muffin tin approximation (RMTA) and the increase in $λ(x)$ was found. This indirectly confirms the electron-phonon coupling scenario as the trend in $λ(x)$ is in qualitative agreement with the experiment when the electron-phonon pairing is assumed.

cond-mat.supr-con↗

arXiv2021

Electronic structure, electron-phonon coupling and superconductivity in noncentrosymmetric ThCoC2 from ab initio calculations

Superconductors without inversion symmetry in their crystal structure are known to exhibit unconventional properties. Recently, based on the measured temperature dependence of the magnetic field penetration depth, superconductivity in noncentrosymmetric $\mathrm{ThCoC_2}$ was proposed to be a nodal $d$-wave and mediated by the spin fluctuations. Moreover, a non-BCS behavior of the temperature dependence of the electronic specific heat and the magnetic upper critical field were reported. In this work, the electronic structure, phonons and electron-phonon coupling are studied in $\mathrm{ThCoC_2}$ on the basis of \textit{ab initio} computations. The effect of the spin-orbit coupling on the electronic structure and electron-phonon interaction is analyzed, and a large splitting of the electronic band structure is found. The calculated electron-phonon coupling constant $λ= 0.59$ remains in decent agreement with the experimental estimates, suggesting that the electron-phonon interaction is strong enough to explain superconductivity with $T_c \simeq 2.5$~K. Nevertheless, we show that the conventional isotropic Eliashberg formalism is unable to describe the thermodynamic properties of the superconducting state, as calculated temperature dependence of the electronic specific heat and magnetic penetration depth deviate from experiments, which is likely driven by the strong spin-orbit coupling and inversion symmetry breaking. In addition, to shed more light on the pairing mechanism, we propose to measure the carbon isotope effect, as our calculations based on the electron-phonon coupling predict the observation of the isotope effect with an exponent $α\simeq 0.15$.

cond-mat.supr-con↗

arXiv2021

Strong-coupling superconductivity of SrIr2 and SrRh2: Phonon engineering of metallic Ir and Rh

Experimental and theoretical studies on superconductivity in SrIr$_2$ and SrRh$_2$ Laves phases are presented. The measured resistivity, heat capacity, and magnetic susceptibility confirm the superconductivity of these compounds with $T_c$ = 6.07 K and 5.41 K, respectively. Electronic structure calculations show that the Fermi surface is mostly contributed by 5$d$ (4$d$) electrons of Ir (Rh), with Sr atoms playing the role of electron donors. The effect of the spin-orbit coupling is analyzed and found to be important in both materials. Lattice dynamics and electron-phonon coupling (EPC) are studied and the strong electron-phonon interaction is found, contributed mostly by the low-frequency Ir and Rh vibrations. The enhancement of EPC when compared to weakly-coupled metallic Ir and Rh is explained by the strong modifications in the propagation of phonons in the network of Ir (Rh) tetrahedrons, which are the building blocks of the Laves phase, and originate from the metallic fcc structures of elemental iridium and rhodium.

cond-mat.supr-con↗

arXiv2021

NbIr$_2$B$_2$ and TaIr$_2$B$_2$ -- new low symmetry noncentrosymmetric superconductors with strong spin orbit coupling

Superconductivity was first observed more than a century ago, but the search for new superconducting materials remains a challenge. The Cooper pairs in superconductors are ideal embodiments of quantum entanglement. Thus, novel superconductors can be critical for both learning about electronic systems in condensed matter and for possible application in future quantum technologies. Here two previously unreported materials, NbIr$_2$B$_2$ and TaIr$_2$B$_2$, are presented with superconducting transitions at 7.2 and 5.2 K, respectively. They display a unique noncentrosymmetric crystal structure, and for both compounds the magnetic field that destroys the superconductivity at 0 K exceeds one of the fundamental characteristics of conventional superconductors (the Pauli limit), suggesting that the superconductivity may be unconventional. Supporting this experimentally based deduction, first-principle calculations show a spin split Fermi surface due to the presence of strong spin-orbit coupling. These materials may thus provide an excellent platform for the study of non-BCS superconductivity in intermetallic compounds.

cond-mat.supr-con↗

arXiv2018

A Family of Pb-based Superconductors with Variable Cubic to Hexagonal Packing

We describe three previously unreported superconductors, BaPb3, Ba0.89Sr0.11Pb3 and Ba0.5Sr0.5Pb3. These three materials, together with SrPb3, form a distinctive isoelectronic family of intermetallic superconductors based on the stacking of Pb planes, with crystal structures that display a hexagonal to cubic perovskite-like progression, as rarely seen in metals. The superconducting transition temperatures (Tc) are similar for all - 2.2 K for BaPb3, 2.7 K for Ba0.89Sr0.11Pb3 and 2.6 K for Ba0.5Sr0.5Pb3, and the previously reported Tc of SrPb3, ~ 2 K, is confirmed. The materials are moderate coupling superconductors, and calculations show that the electronic densities of states at the Fermi energy are primarily contributed by Pb. The observations suggest that the Pb-stacking variation has only a minor effect on the superconductivity.

cond-mat.supr-con↗