arXiv subjects

Konstantin Semeniuk

Publications and source records attributed to Konstantin Semeniuk.

15 recordsLinked to original sources

arXiv2026

Unconventional superconductivity in locally non-centrosymmetric CeNi$_2$As$_2$

Recent years have seen intense research on heavy fermion superconductivity, inspired by the discovery of multiple superconducting phases in UTe$_\text{2}$ and CeRh$_\text{2}$As$_\text{2}$. In the latter material, two superconducting phases observed under applied magnetic field oriented along the crystallographic $\textit{c}$ direction are associated with local inversion symmetry breaking. To advance understanding of these phenomena, it is highly desirable to find further examples of heavy fermion superconductivity in materials with locally non-centrosymmetric structure. We have succeeded in this quest, by growing single crystals of the CaBe$_\text{2}$Ge$_\text{2}$ isomorph of CeNi$_\text{2}$As$_\text{2}$. The resulting superconductivity brings more than we had foreseen. In addition to providing the opportunity to compare and contrast with that of CeRh$_\text{2}$As$_\text{2}$, the condensation occurs from an incoherent normal state quantitatively similar to that of UBe$_\text{13}$. Our findings therefore raise profound questions not just about superconductivity in the locally non-centrosymmetric structures, but about unconventional superconductivity itself.

cond-mat.supr-con↗

arXiv2026

Crystallographic imperfections and exotic superconductivity of UBe$_{13}$

The symmetry of the superconducting gap is related to the symmetry of the crystal structure. In unconventional superconductors, big changes of the critical temperature, critical field or even the gap structure can happen even for small perturbations of the lattice. In this letter, we use microstructuring to study aluminium-free crystals of UBe$_{13}$ which are expected to be closer to "perfect" material than previously studied aluminium-grown single crystals. We compare the effect of minuscule imperfections on the value of the critical temperature and critical field, which, in the case of UBe$_{13}$, has drastic effects, supporting its unconventional nature. We conjecture that this likely arises from the oxidation state of uranium, which is sensitive to its crystal environment. Our findings suggests that uranium-based materials provide not only an excellent reservoir of new unconventional phenomena, but also a way to identify the gap symmetry of unconventional superconductors.

cond-mat.supr-con↗

arXiv2026

Emergent Surface Kondo Flat Band Driven by Competing Interactions in a Topological Ferromagnet

A central goal of modern condensed matter physics is to uncover new quantum states of matter arising from the intertwined effects of strong electron correlations, magnetism, and band topology. Heavy-fermion phases, generated by Kondo interactions, represent one of the most remarkable manifestations of electronic correlations, and topological heavy-fermion states have been identified in several non-magnetic materials. Yet, the consequences of their competition with magnetic order have remained largely unexplored. Here, we reveal a new phenomenon: the spontaneous spatial separation of correlated quantum phases. By showing that magnetism can drive distinct strongly correlated electronic states to coexist in different regions of a single material, our work establishes a previously unknown mechanism for organizing quantum matter and opens a new direction in the study of correlated topological systems. Using \emph{bulk-sensitive} probes, we show that UAsS crystals are, in the bulk, metallic ferromagnets with only moderate correlation-driven band renormalizations. First-principles calculations reveal a topological electronic structure hosting both nodal lines and Weyl points, pointing to a rich underlying topology. Angle-resolved photoemission spectroscopy (ARPES) measurements are consistent with these predictions, resolving the nodal lines and Weyl crossings. In striking contrast, \emph{surface-sensitive} ARPES and scanning tunneling microscopy/spectroscopy (STM/STS) measurements reveal a pronounced flat band pinned at the Fermi level, accompanied by a sharp resonance -- hallmarks of an emergent, strongly correlated Kondo state not captured by first-principles calculations.

cond-mat.str-el↗

arXiv2026

Interlayer interactions reshape charge-density wave through electronic elasticity in 4H$_{\mathrm{b}}$-TaS$_2$

Incommensurate charge-density waves (CDWs) in layered quantum materials frequently exhibit widely varying ordering wave vectors, even among nominally identical samples, obscuring their intrinsic electronic properties. Here we identify an inherent origin of this variability through the electronic elasticity of an incommensurate CDW using the model heterostructure 4H$_{\mathrm{b}}$-TaS$_2$, composed of alternating commensurate CDW on 1T and incommensurate CDW on 1H layers. Low-temperature scanning tunneling microscopy, combined with Fourier and quasiparticle-interference analysis, exploits the lattice-pinned 1T CDW as an internal reference to resolve discrete compressive ($-2.3\%$) and tensile ($+3.2\%$) elastic states of the neighboring 1H CDW selected by the interlayer registry of the adjacent layers. Corresponding few-meV shifts of a flat band demonstrate that weak interlayer interactions reshape the low-energy electronic structure through the intrinsic elasticity of the incommensurate CDW. These findings establish electronic elasticity as a mechanism by which subtle interlayer interactions control correlated electronic states in van der Waals heterostructures.

cond-mat.str-el↗

arXiv2026

Basal-plane anisotropy of field-induced multipolar order in tetragonal CeRh$_2$As$_2$

Unconventional superconductivity in Ce-based Kondo-lattice materials emerges almost exclusively in the vicinity of weak dipolar magnetic orders, while higher multipolar orders are only known to occur in a few Pr-based unconventional superconductors and possibly URu$_2$Si$_2$. The multiphase superconductor CeRh$_2$As$_2$ appears to be a notable exception from this trend. Showing clear signatures of magnetism, this tetragonal system is suspected to host a concomitant quadrupolar order, which could be causing the strong enhancement of the ordering temperature when a magnetic field is applied perpendicular to the fourfold ($c$) axis of the lattice. In this work, we show that the field-temperature phase diagram of CeRh$_2$As$_2$ has a remarkable basal-plane anisotropy. This finding supports the scenario of coupled magnetic and multipolar ordering, which may have implications for the pairing mechanism of the superconductivity, and guides the development of the next iteration of theoretical models.

cond-mat.str-el↗

arXiv2024

Exposing the odd-parity superconductivity in CeRh$_2$As$_2$ with hydrostatic pressure

Odd-parity superconductivity is a fundamentally interesting but rare state of matter with a potential for applications in topological quantum computing. Crystals with staggered locally noncentrosymmetric structures have been proposed as platforms where a magnetic field can induce a transition between even- and odd-parity superconducting (SC) states. The strongly correlated superconductor CeRh$_2$As$_2$ with the critical temperature $T_{\mathrm{c}}\approx0.4\,\mathrm{K}$ is likely the first example material showing such a phase transition, which occurs at the magnetic field $μ_{0}H^{*}=4\,\mathrm{T}$ applied along the crystallographic $c$ axis. CeRh$_2$As$_2$ also undergoes a phase transition of an unknown origin at $T_{0}=0.5\,\mathrm{K}$. By subjecting CeRh$_2$As$_2$ to hydrostatic pressure and mapping the resultant changes to the SC phase diagrams we investigated how the lattice compression and changes to the electronic correlations affect the stability and relative balance of the two SC states. The abnormally high in-plane upper critical field becomes even higher close to a quantum critical point of the $T_{0}$ order. Remarkably, the SC phase-switching field $H^{*}$ is drastically reduced under pressure, dropping to $0.3\,\mathrm{T}$ at $2.7\,\mathrm{GPa}$. This result signals an apparent strengthening of the local noncentrosymmetricity and forecasts a possible stabilization of the putative odd-parity state down to zero field, hitherto not considered by theoretical models.

cond-mat.supr-con↗

arXiv2024

Pressure-tuned quantum criticality in the locally non-centrosymmetric superconductor CeRh$_2$As$_2$

The unconventional superconductor CeRh$_2$As$_2$ (critical temperature $T_{\mathrm{c}}\approx0.4\,\mathrm{K}$) displays an exceptionally rare magnetic-field-induced transition between two distinct superconducting (SC) phases, proposed to be states of even and odd parity of the SC order parameter, which are enabled by a locally noncentrosymmetric structure. The superconductivity is preceded by a phase transition of unknown origin at $T_{0}\approx0.5\,\mathrm{K}$. Electronic low-temperature properties of CeRh$_2$As$_2$ show pronounced non-Fermi-liquid behavior, indicative of a proximity to a quantum critical point (QCP). The role of quantum fluctuations and normal state orders for the superconductivity in a system with staggered Rashba interaction is currently an open question, pertinent to explaining the occurrence of two-phase superconductivity. In this work, using measurements of resistivity and specific heat under hydrostatic pressure, we show that the $T_{0}$ order vanishes completely at a modest pressure of $P_{0}=0.5\,\mathrm{GPa}$, revealing a QCP. In line with the quantum criticality picture, the linear temperature dependence of the resistivity at $P_{0}$ evolves into a Fermi-liquid quadratic dependence as quantum critical fluctuations are suppressed by increasing pressure. Furthermore, the domelike behavior of $T_{\mathrm{c}}$ around $P_{0}$ implies that the fluctuations of the $T_{0}$ order are involved in the SC pairing mechanism.

cond-mat.str-el↗

arXiv2024

Decoupling multi-phase superconductivity from normal state ordering in CeRh$_2$As$_2$

CeRh$_2$As$_2$ is a multi-phase superconductor with $T_{\textrm{c}}=0.26\,\textrm{K}$. The two superconducting (SC) phases, SC1 and SC2, observed for a magnetic field $H$ parallel to the $c$ axis of the tetragonal unit cell, have been interpreted as even- and odd-parity SC states, separated by a phase boundary at $μ_{\textrm{0}}H^{*}=4\,\textrm{T}$. Such parity switching is possible due to a strong Rashba spin-orbit coupling at the Ce sites located in locally non-centrosymmetric environments of the globally centrosymmetric lattice. Existence of another ordered state (Phase I) below a temperature $T_{\textrm{0}}\approx0.4\,\textrm{K}$ suggests an alternative interpretation of the $H^{*}$ transition: It separates a mixed SC+I (SC1) and a pure SC (SC2) state. Here, we present a detailed study of higher quality single crystals of CeRh$_2$As$_2$, showing much sharper signatures at $T_{\textrm{c}}=0.31\,\textrm{K}$ and $T_{\textrm{0}}=0.48\,\textrm{K}$. We refine the $T$-$H$ phase diagram of CeRh$_2$As$_2$ and demonstrate that $T_{0}(H)$ and $T_{\textrm{c}}(H)$ lines meet at $μ_{\textrm{0}}H\approx6\,\textrm{T}$, well above $H^{*}$, implying no influence of Phase I on the SC phase switching. A basic analysis with the Ginzburg-Landau theory indicates a weak competition between the two orders.

cond-mat.supr-con↗

arXiv2022

Truncated mass divergence in a Mott metal

Metal-insulator transitions in clean, crystalline solids can be driven by two distinct mechanisms. In a conventional insulator, the charge carrier concentration vanishes, when an energy gap separates filled and unfilled electronic states. In the established picture of a Mott insulator, by contrast, electronic interactions cause coherent charge carriers to slow down and eventually stop in electronic grid-lock without materially affecting the carrier concentration itself. This description has so far escaped experimental verification by quantum oscillation measurements, which directly probe the velocity distribution of the coherent charge carriers. By extending this technique to high pressure we were able to examine the evolution of carrier concentration and velocity in the strongly correlated metallic state of the clean, crystalline material NiS$_2$, while tuning the system towards the Mott insulating phase. Our results confirm that pronounced electronic slowing down indeed governs the approach to the insulating state. However, the critical point itself, at which the carrier velocity would reach zero and the effective carrier mass diverge, is concealed by the insulating sector of the phase diagram. In the resulting, more nuanced view of Mott localisation, the inaccessibility of the low temperature Mott critical point resembles that of the threshold of magnetic order in clean metallic systems, where criticality is almost universally interrupted by first order transitions, tricritical behaviour or novel emergent phases such as unconventional superconductivity.

cond-mat.str-el↗

arXiv2021

Synthesis of Murunskite Single Crystals: A Bridge Between Cuprates and Pnictides

Numerous contemporary investigations in condensed matter physics are devoted to high temperature (high-$T_c$ ) cuprate superconductors. Despite its unique effulgence among research subjects, the enigma of the high-$T_c$ mechanism still persists. One way to advance its understanding is to discover and study new analogous systems. Here we begin a novel exploration of the natural mineral murunskite, K$_2$FeCu$_3$S$_4$, as an interpolation compound between cuprates and ferropnictides, the only known high-$T_c$ superconductors at ambient pressure. Because in-depth studies can be carried out only on single crystals, we have mastered the synthesis and growth of high quality specimens. Similar to the cuprate parent compounds, these show semiconducting behavior in resistivity and optical transmittance, and an antiferromagnetic ordering at 100 K. Spectroscopy (XPS) and calculations (DFT) concur that the sulfur 3$p$ orbitals are partially open, making them accessible for charge manipulation, which is a prerequisite for superconductivity in analogous layered structures. DFT indicates that the valence band is more cuprate-like, while the conduction band is more pnictide-like. With appropriate doping strategies, this parent compound promises exciting future developments.

cond-mat.mtrl-sci↗

arXiv2021

Unidirectional Kondo scattering in layered NbS2

Crystalline defects can modify quantum interactions in solids, causing unintuitive, even favourable, properties such as quantum Hall effect or superconducting vortex pinning. Here we present another example of this notion - an unexpected unidirectional Kondo scattering in single crystals of 2H-NbS2. This manifests as a pronounced low-temperature enhancement in the out-of-plane resistivity and thermopower below 40 K, hidden for the in-plane charge transport. The anomaly can be suppressed by the c-axis-oriented magnetic field, but is unaffected by field applied along the planes. The magnetic moments originate from layers of 1T-NbS2, which inevitably form during the growth, undergoing a charge-density-wave reconstruction with each superlattice cell (David-star-shaped cluster of Nb atoms) hosting a localised spin. Our results demonstrate the unique and highly anisotropic response of a spontaneously formed Kondo lattice heterostructure, intercalated in a layered conductor.

cond-mat.str-el↗

arXiv2020

Structural phase transition and bandgap control through mechanical deformation in layered semiconductors 1T-ZrX2 (X = S, Se)

Applying elastic deformation can tune a material physical properties locally and reversibly. Spatially modulated lattice deformation can create a bandgap gradient, favouring photo-generated charge separation and collection in optoelectronic devices. These advantages are hindered by the maximum elastic strain that a material can withstand before breaking. Nanomaterials derived by exfoliating transition metal dichalcogenides TMDs are an ideal playground for elastic deformation, as they can sustain large elastic strains, up to a few percent. However, exfoliable TMDs with highly strain-tunable properties have proven challenging for researchers to identify. We investigated 1T-ZrS2 and 1T-ZrSe2, exfoliable semiconductors with large bandgaps. Under compressive deformation, both TMDs dramatically change their physical properties. 1T-ZrSe2 undergoes a reversible transformation into an exotic three-dimensional lattice, with a semiconductor-to-metal transition. In ZrS2, the irreversible transformation between two different layered structures is accompanied by a sudden 14 % bandgap reduction. These results establish that Zr-based TMDs are an optimal strain-tunable platform for spatially textured bandgaps, with a strong potential for novel optoelectronic devices and light harvesting.

cond-mat.mtrl-sci↗

arXiv2020

Preferential out-of-plane conduction and quasi-one-dimensional electronic states in layered 1T-TaS2

Layered transition metal dichalcogenides (TMDs) are commonly classified as quasi-two-dimensional materials, meaning that their electronic structure closely resembles that of an individual layer, which results in resistivity anisotropies reaching thousands. Here, we show that this rule does not hold for 1T-TaS2 - a compound with the richest phase diagram among TMDs. While the onset of charge density wave order makes the in-plane conduction non-metallic, we reveal that the out-of-plane charge transport is metallic and the resistivity anisotropy is close to one. We support our findings with ab-initio calculations predicting a pronounced quasi-one-dimensional character of the electronic structure. Consequently, we interpret the highly debated metal-insulator transition in 1T-TaS2 as a quasi-one-dimensional instability, contrary to the long-standing Mott localisation picture. In a broader context, these findings are relevant for the newly born field of van der Waals heterostructures, where tuning interlayer interactions (e.g. by twist, strain, intercalation, etc.) leads to new emergent phenomena.

cond-mat.mtrl-sci↗

arXiv2018

Composition dependence of bulk superconductivity in YFe2Ge2

In the layered iron-based superconductor YFe2Ge2, a high Sommerfeld ratio of ~100 mJ/molK^2 and a T^(3/2) temperature dependence of the electrical resistivity at low temperature T indicate strong electronic correlations and point towards an unconventional pairing state. We have investigated the role of composition and annealing conditions in optimizing the growth of high-quality YFe2Ge2. Our findings confirm that bulk superconductivity is observed in samples with disorder scattering rates less than 2 k_B T_c/hbar. Fe deficiency on the Fe site is identified as the dominant source of disorder, which can be minimised by precipitating from a slightly iron-rich melt, following by annealing.

cond-mat.str-el↗

arXiv2015

Unconventional superconductivity in the layered iron germanide YFe$_2$Ge$_2$

Since the discovery of superconductivity in LaFePO in 2006, numerous iron-based superconductors have been identified within diverse structure families, all of which combine iron with a group-V (pnictogen) or group-VI (chalco- gen) element. Unconventional superconductivity is extremely rare among transition metal compounds outside these layered iron systems and the cuprates, and it is almost universally associated with highly anisotropic electronic properties and nearly 2D Fermi surface geometries. The iron-based intermetallic YFe$_2$Ge$_2$ features a 3D Fermi surface and a strongly enhanced low temperature heat capacity, which signals strong electronic correlations. We present data from a new generation of high quality samples of YFe$_2$Ge$_2$, which show superconducting transition anomalies below 1.8 K in thermodynamic as well as transport measurements, establishing that superconductivity is intrinsic in this layered iron compound outside the known superconducting iron pnictide or chalcogenide families. The Fermi surface geometry of YFe$_2$Ge$_2$ resembles that of KFe$_2$As$_2$ in the high pressure collapsed tetragonal phase, in which superconductivity at temperatures as high as 10 K has recently been reported, suggesting an underlying connection between the two systems.

cond-mat.str-el↗