arXiv subjects

Michał J. Winiarski

Publications and source records attributed to Michał J. Winiarski.

At least 19 recordsLinked to original sources

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

Band-Selective Tunneling and Anisotropic Multiband Superconductivity in V$_2$Ga$_5$

Multiband superconductors with structural anisotropy offer a fertile ground for exploring unconventional quantum states, yet disentangling their directional pairing characteristics remains a formidable challenge. Here, we present a comprehensive thermodynamic and spectroscopic study of the tetragonal intermetallic superconductor $\text{V}_2\text{Ga}_5$ ($T_{\rm c} \approx 3.5$~K), combining first-principles electronic structure calculations with highly sensitive AC calorimetry and directional low-temperature scanning tunneling spectroscopy. By constructing a self-consistent, anisotropic multiband singlet $s$-wave pairing model within the fully symmetric $A_{1g}$ representation, we successfully reconcile the experimental specific heat and upper critical field anomalies. Crucially, we reveal that the apparent reversal of bulk gap hierarchies in directional tunneling experiments is a direct consequence of band-selective tunneling. This effect is governed by an elegant interplay between localized Fermi velocity 'hot spots' and specific Fermi surface topologies, rather than raw thermodynamic gap magnitudes alone. Our findings provide a clear microscopic picture of direction-dependent, band-selective tunneling in a highly uniaxial anisotropic superconductor, demonstrating how orientation-dependent transport constraints shape the observable signatures of multiband quantum condensates.

cond-mat.supr-con↗

arXiv2026

Heat Capacity-A Powerful Tool for Studying Exotic States of Matter

Heat capacity measurements are a powerful tool that researchers rely on when studying the relationship between microscopic degrees of freedom and macroscopic behavior in condensed matter. This uniqueness stems from heat capacity capturing contributions from lattice, electronic, and magnetic components, as well as energy-level populations, enabling an effective approach to studying phase transitions and excitations across different classes of materials. However, analyzing heat capacity data presents a common, appreciable challenge for new researchers. Although comprehensive theoretical aspects of heat capacity are presented in several elegant textbooks, practical application remains a daunting task. To overcome this challenge, this tutorial guides researchers in collecting, analyzing, and interpreting heat capacity data in contemporary quantum materials. We outline the connections between thermodynamics, heat capacity, and entropy, as well as measurement methodology and data analysis for representative examples, including phonon dynamics, spin waves, superconductors, magnetic skyrmions, proximate quantum spin liquids, and heavy-fermion materials. Our goal is to provide a concise, accessible guide that enables new researchers to utilize heat capacity as a quantitative lens for understanding exotic states of matter.

cond-mat.str-el↗

arXiv2026

Cluster glass behavior and magnetocaloric effect in the hexagonal polymorph of disordered Ce$_2$PdGe$_3$

In this work, we study the hexagonal variant of the $\text{Ce}_2\text{PdGe}_3$ system that crystallizes in the $\text{AlB}_2$-type structure (space group $P6/mmm$, $hP3$) and exhibits cluster spin glass type behavior. The physical properties were studied by magnetization, heat capacity and electric resistivity, which showed that $\text{AlB}_2$-type $\text{Ce}_2\text{PdGe}_3$ ($h\text{-Ce}_2\text{PdGe}_3$) can be classified as a cluster glass material with the freezing temperature $T_f = 3.44 \text{ K}$ in contrast to the behavior of the previously described tetragonal variant, which shows a double antiferromagnetic transition at $T_{N1} = 11 \text{ K}$ and $T_{N2} = 2.3 \text{ K}$. The X-ray photoelectron spectroscopy measurements reveal that the $\text{Ce } 4f$ states are well localized. In addition, we examine the magnetocaloric effect in this compound. The maximum values of magnetocaloric parameters appear in the vicinity of $7\text{--}9 \text{ K}$. For a magnetic field change of $50 \text{ kOe}$, the value of the change in magnetic entropy is $2.6(1) \text{ J kg}^{-1} \text{ K}^{-1}$ and the adiabatic temperature change is $\sim 8 \text{ K}$.

cond-mat.mtrl-sci↗

arXiv2026

Effect of Pressure and Oxygen-Isotope Substitution on Density-Wave Transitions in La$_4$Ni$_3$O$_{10}$

Understanding the interplay between magnetism and superconductivity in nickelate systems is a key objective in condensed matter physics. Here, we present a systematic muon-spin rotation/relaxation ($μ$SR) and resistivity study of the trilayer Ruddlesden-Popper nickelate La$_4$Ni$_3$O$_{10}$ under ambient and applied pressure, combined with oxygen-isotope substitution. At ambient pressure, two incommensurate spin-density-wave (SDW) transitions are identified at $T_{SDW}\simeq132$ K and $T^\ast\simeq80-90$ K. Comparison of the internal magnetic fields with dipole-field calculations reveals a magnetic structure consistent with antiferromagnetically coupled SDW order on the outer two Ni layers, with smaller moments on the inner layer. Above $T^\ast$, the moments lie mainly in the $ab$ plane, whereas below this temperature they develop a $c$-axis component. The internal fields at the muon stopping sites appear abruptly at $T_{SDW}$, suggesting a first-order-like SDW transition closely linked to the charge-density-wave (CDW) order occurring at the same temperature ($T_{SDW}=T_{CDW}$). Under pressure, all transition temperatures -- $T_{SDW}$, $T^\ast$, and $T_{CDW}$ -- are suppressed at a nearly uniform rate of $\simeq-13$ K/GPa. This contrasts with bilayer La$_3$Ni$_2$O$_7$, where pressure enhances the separation between the SDW and CDW transitions. Oxygen-isotope substitution ($^{16}$O $\rightarrow$ $^{18}$O) shifts $T_{CDW}$ to higher values. The isotope effect on $T_{SDW}$ and $T^\ast$ differs markedly: when CDW and SDW are intertwined, a notable isotope effect is observed on $T_{SDW}$, yielding nearly identical isotope shifts for $T_{CDW}$ and $T_{SDW}$, whereas no isotope effect is detected at $T^\ast$, where the SDW transition occurs independently of the CDW.

cond-mat.supr-con↗

arXiv2026

Possible Proximity to Ferromagnetism in the V$_2$Ga$_5$ Superconductor

Superconductivity and ferromagnetism are generally competing ground states in $d$-electron systems, making their interplay of fundamental interest. We report a comprehensive study of high-quality single- and polycrystalline V$_2$Ga$_5$, a bulk type-II superconductor ($T_c = 3.54 \ K$) with a quasi-one-dimensional crystal structure, supplemented with density functional theory (DFT) calculations, suggesting possible proximity to ferromagnetic order. Below $T \approx 10 \ K$, magnetic susceptibility shows ZFC/FC splitting, along with saturation and hysteresis in $M(H)$. Moreover, electrical transport measurements reveal a magnetic-field-dependent resistivity upturn, while specific heat is enhanced in magnetic fields. DFT calculations show that the Fermi level in V$_2$Ga$_5$ is located at a peak in the density of states, with a small magnetic moment per unit cell comparable to the experimental value. Together, these results indicate the possibility that ferromagnetic correlations develop below $T \approx 10 \ K$, well above $T_c$, with long-range ferromagnetic order suppressed by the superconducting transition.

cond-mat.supr-con↗

arXiv2025

YRu$_3$B$_2$ -- a kagome lattice superconductor

We report the synthesis and physical properties of a polycrystalline, hexagonal boride YRu$_3$B$_2$. Our resistivity and heat capacity measurements indicate that YRu$_3$B$_2$ is a weakly coupled superconductor, with critical temperature $T_c$ = 0.63 K and upper critical field $μ_0 H_{c2}$ (0)=0.11 T. Density functional theory calculations, together with chemical-bonding analysis, reveal that the electronic states at and near the Fermi energy level are dominated by the Ru kagome sublattice.

cond-mat.supr-con↗

arXiv2025

Quantum oscillations and transport properties of layered single-crystal SrCu$_4$As$_2$

We report a systematic investigation of the physical properties and Fermi-surface topology of layered single-crystal \ce{SrCu4As2} using electrical transport, magnetotransport, and quantum-oscillation experiments plus band-structure calculations. The temperature-dependent electrical resistivity reveals a hysteretic phase transition at $T_P$ = 59 K, most likely associated with a structural change. Hall resistivity data suggest a marked change in the average hole density resulting from the latter phase transition near $T_P$. A large, linear, and nonsaturating magnetoresistance is observed at low temperatures in \ce{SrCu4As2}, likely attributable to the multipocket Fermi surface. Quantum-oscillation data measured in magnetic fields of up to 60 T show several oscillation frequencies exhibiting low effective masses, indicating the presence of Dirac-like band dispersion in \ce{SrCu4As2}, as suggested by the band structure calculations.

cond-mat.str-el↗

arXiv2025

Antibonding and Electronic Instabilities in GdRu2X2 (X = Si, Ge, Sn): A New Pathway Toward Developing Centrosymmetric Skyrmion Materials

Chemical bonding is key to unlocking the potential of magnetic materials for future information technology. Magnetic skyrmions are topologically protected nano-sized spin textures that can enable high-density low-power spin-based electronics. Despite increasing interest in the discovery of new skyrmion hosts and their characterization, the electronic origins of the skyrmion formation remain unknown. Here, we study GdRu2X2 (X = Si, Ge, Sn) as a model system to study the connection among chemical bonding, electronic instability, and the critical temperature and magnetic field at which skyrmions evolve. The nature of the electronic structure of GdRu2X2 is characterized by chemical bonding, Fermi surface analysis, and density of energy function. As X-p orbitals become more extended from Si-3p to Ge-4p and Sn-5p, improved interactions between the Gd spins and the [Ru2X2] conduction layer and increased destabilizing energy contributions are obtained. GdRu2Si2 possesses a Fermi surface nesting (FSN) vector [Q = (q, 0, 0)], whereas GdRu2Ge2 displays two inequivalent FSN vectors [Q = (q, 0, 0); QA = (q, q, 0)] and GdRu2Sn2 features multiple Q vectors. In addition, competing ferromagnetic and antiferromagnetic exchange interactions in the Gd plane become more pronounced as a function of X. These results reveal some correlation among the electronic instability, the competing interaction strength, and the temperature and magnetic field conditions at which the skyrmions emerge. This work demonstrates how chemical bonding and electronic structure enable a new framework for understanding and developing skyrmions under desired conditions that would otherwise be impossible.

cond-mat.str-el↗

arXiv2025

Kondo-like behavior in a mixed valent oxypnictide $\mathrm{La_{3}Cu_{4}P_{4}O_{2}}$

We have synthesized and characterized the physical properties of a layered, mixed valent oxypnictide $\mathrm{La_{3}Cu_{4}P_{4}O_{2}}$ via magnetization, electrical resistivity, and specific heat measurements. Although $\mathrm{La_{3}Cu_{4}P_{4}O_{2}}$ does not exhibit superconductivity down to T = 0.5 K, it demonstrates an intriguing resistivity minimum observed at $\mathrm{T_{min}}$ = 13.7 K. Disappearance of the resistivity minimum under an applied magnetic field of $\mathrm{μ_{0}H}$ = 9 T together with the negative magnetoresistance at low and positive at high temperatures are observed, which are typical for both Kondo-like spin-dependent scattering and 3D weak localization. We argue that the Kondo scattering is a more plausible explanation due to the low-temperature deviation from a Curie-Weiss law observed in the magnetic susceptibility, consistent with the presence of magnetic interactions between paramagnetic $\mathrm{Cu^{2+}}$ ions and Kondo screening of these $\mathrm{Cu^{2+}}$ moments. We supplemented the experimental characterization with a detailed description of chemical bonding, employing density functional theory (DFT) calculations and crystal orbital Hamilton population (COHP) analysis for $\mathrm{La_{3}Cu_{4}P_{4}O_{2}}$ and isostructural $\mathrm{La_{3}Ni_{4}P_{4}O_{2}}$, which is a superconductor with $\mathrm{T_c = 2.2}$ K. Based on the calculations performed, we present the difference between $\mathrm{La_{3}Cu_{4}P_{4}O_{2}}$ and $\mathrm{La_{3}Ni_{4}P_{4}O_{2}}$ in the character of electronic states at the Fermi level. This discrepancy impacts structural stability and may cause a lack of superconductivity in $\mathrm{La_{3}Cu_{4}P_{4}O_{2}}$ down to T = 0.5 K.

cond-mat.str-el↗

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

Noncentrosymmetric Triangular Magnet CaMnTeO$_6$: Strong Quantum Fluctuations and Role of s0 vs. s2 Electronic States in Competing Exchange Interactions

Noncentrosymmetric triangular magnets offer a unique platform for realizing strong quantum fluctuations. However, designing these quantum materials remains an open challenge attributable to a knowledge gap in the tunability of competing exchange interactions at the atomic level. Here, we create a new noncentrosymmetric triangular S = 3/2 magnet CaMnTeO$_6$ based on careful chemical and physical considerations. The model material displays competing magnetic interactions and features nonlinear optical responses with the capability of generating coherent photons. The incommensurate magnetic ground state of CaMnTeO$_6$ with an unusually large spin rotation angle of 127 deg.(1) indicates that the anisotropic interlayer exchange is strong and competing with the isotropic interlayer Heisenberg interaction. The moment of 1.39(1) $μ$B, extracted from low-temperature heat capacity and neutron diffraction measurements, is only 46% of the expected value of the static moment 3 $μ$B. This reduction indicates the presence of strong quantum fluctuations in the half-integer spin S = 3/2 CaMnTeO$_6$ magnet, which is rare. By comparing the spin-polarized band structure, chemical bonding, and physical properties of AMnTeO$_6$ (A = Ca, Sr, Pb), we demonstrate how quantum-chemical interpretation can illuminate insights into the fundamentals of magnetic exchange interactions, providing a powerful tool for modulating spin dynamics with atomically precise control.

cond-mat.str-el↗

arXiv2023

The electronic structure of intertwined kagome, honeycomb, and triangular sublattices of the intermetallics MCo$_2$Al$_9$

Intermetallics are an important playground to stabilize a large variety of physical phenomena, arising from their complex crystal structure. The ease of their chemical tuneabilty makes them suitable platforms to realize targeted electronic properties starting from the symmetries hidden in their unit cell. Here, we investigate the family of the recently discovered intermetallics MCo$_2$Al$_9$ (M: Sr, Ba) and we unveil their electronic structure for the first time. By using angle-resolved photoelectron spectroscopy and density functional theory calculations, we discover the existence of Dirac-like dispersions as ubiquitous features in this family, coming from the hidden kagome and honeycomb symmetries embedded in the unit cell. Finally, from calculations, we expect that the spin-orbit coupling is responsible for opening energy gaps in the electronic structure spectrum, which also affects the majority of the observed Dirac-like states. Our study constitutes the first experimental observation of the electronic structure of MCo$_2$Al$_9$ and proposes these systems as hosts of Dirac-like physics with intrinsic spin-orbit coupling. The latter effect suggests MCo$_2$Al$_9$ as a future platform for investigating the emergence of non-trivial topology.

cond-mat.str-el↗

arXiv2019

Iridium 5$\textit{d}$-electron driven superconductivity in ThIr$_3$

Polycrystalline sample of superconducting ThIr$_{3}$ was obtained by arc-melting Th and Ir metals. Powder x-ray diffraction revealed that the compound crystalizes in a rhombohedral crystal structure (R-3m, s.g. no. 166) with the lattice parameters: a = 5.3394(1) $Å$ and c = 26.4228(8) $Å$. Normal and superconducting states were studied by magnetic susceptibility, electrical resistivity and heat capacity measurements. The results showed that ThIr$_{3}$ is a type II superconductor (Ginzburg-Landau parameter $κ$ = 38) with the critical temperature T$_{c}$ = 4.41 K. The heat capacity data yielded the Sommerfeld coefficient $γ$ = 17.6 mJ mol$^{-1}$ K$^{-2}$ and the Debye temperature $Θ_{D}$ = 169 K. The ratio $Δ$C / ($γ$ T$_{c}$) = 1.6, where $Δ$C stands for the specific heat jump at T$_{c}$, and the electron-phonon coupling constant $λ_{e-p}$ = 0.74 suggest that ThIr$_{3}$ is a moderate-strength superconductor. The experimental studies were supplemented by band structure calculations, which indicated that the superconductivity in ThIr$_{3}$ is governed mainly by 5d states of iridium. The significantly smaller band-structure value of Sommerfeld coefficient as well as the experimentally observed quadratic temperature dependence of resistivity and enhanced magnetic susceptibility suggest presence of electronic interactions in the system, which compete with superconductivity.

cond-mat.supr-con↗

arXiv2017

Magnetism and charge density waves in RNiC$_2$ (R = Ce, Pr, Nd)

We have compared the magnetic, transport, galvanomagnetic and specific heat properties of CeNiC$_2$, PrNiC$_2$ and NdNiC$_2$ to study the interplay between charge density waves and magnetism in these compounds. The negative magnetoresistance in NdNiC$_2$ is discussed in terms of the partial destruction of charge density waves and an irreversible phase transition stabilized by the field induced ferromagnetic transformation is reported. For PrNiC$_2$ we demonstrate that the magnetic field initially weakens the CDW state, due to the Zeeman splitting of conduction bands. However, the Fermi surface nesting is enhanced at a temperature related to the magnetic anomaly.

cond-mat.str-el↗

arXiv2016

Field induced suppression of charge density wave in GdNiC$_2$

We report the specific heat, magnetic, magnetotransport and galvanomagnetic properties of polycrystalline GdNiC$_2$. In the intermediate temperature region above $T_N$ = 20 K, we observe large negative magnetoresistance due to Zeeman splitting of the electronic bands and partial destruction of a charge density wave ground state. Our magnetoresistance and Hall measurements show that at low temperatures a magnetic field induced transformation from antiferromagnetic order to a metamagnetic phase results in the partial suppression of the CDW.

cond-mat.str-el↗

arXiv2016

Crystal structure and low-energy Einstein mode in ErV$_2$Al$_{20}$ intermetallic cage compound

Single crystals of a new ternary aluminide ErV$_2$Al$_{20}$ were grown using a self-flux method. The crystal structure was determined by powder X-ray diffraction measurements and Rietveld refinement, and physical properties were studied by means of electrical resistivity, magnetic susceptibility and specific heat measurements. These measurements reveal that ErV$_2$Al$_{20}$ is a Curie-Weiss paramagnet down to 1.95 K with an effective magnetic moment $μ_{eff}$ = 9.27(1) $μ_B$ and Curie-Weiss temperature $Θ_{CW}$ = -0.55(4) K. The heat capacity measurements show a broad anomaly at low temperatures that is attributed to the presence of a low-energy Einstein mode with characteristic temperature $Θ_E$ = 44 K, approximately twice as high as in the isostructural 'Einstein solid' VAl$_{10.1}$.

cond-mat.mtrl-sci↗