arXiv subjects

Markus Kriener

Publications and source records attributed to Markus Kriener.

At least 19 recordsLinked to original sources

arXiv2026

EuIn2Sb2: epitaxially stabilized axion insulator candidate with strong spin-orbit coupling

Eu triangular lattice layer compounds described by the general formula EuA2X2 have attracted growing attention due to a wide range of layered crystal structures and corresponding magnetic topological phases, arising from the coexistence of large Eu magnetic moments and energetically inverted A and X bands. Among the EuA2X2 family, EuIn2Sb2 has been predicted to host robust axion insulator and higher-order topological insulator states by stronger spin-orbit coupling but has not been experimentally realized, including the structural identification. Here we report the epitaxial stabilization of EuIn2Sb2 by adopting the molecular beam epitaxy technique. Structural characterization reveals that EuIn2Sb2 films are based on a unique In-on-In stacking, forming a new trilayer structure distinct from previously predicted structures in the EuA2X2 family. In addition to the in-plane antiferromagnetic ordering at TN=10.5K, the stronger spin-orbit coupling and enhanced In-In hybridization in EuIn2Sb2 make it an ideal candidate of axion insulator and higher-order topological insulator phases. These results establish EuIn2Sb2 as a new member of the EuA2X2 family and offer an advanced platform for exploring these magnetic topological phases.

cond-mat.mtrl-sci↗

arXiv2026

Observation of in-plane anomalous Nernst effect

The Nernst effect, which enables the conversion of a heat current into a transverse voltage under magnetic field or spin magnetization, holds significant promise for energy harvesting and thermal management in future electronics. However, the conventional Nernst effect is fundamentally constrained by the orthogonality requirement that the applied field or spontaneous magnetization must be perpendicular to the plane defined by the temperature gradient and the induced voltage. Here we report that symmetry-tailored ultrathin films of a prototypical ferromagnetic oxide exhibit anomalous Nernst effect arising from intrinsic coupling to spontaneous in-plane spin magnetization. Systematic magnetothermoelectric measurements under spherical rotations of the magnetic field reveal that a pronounced Nernst signal, comparable in magnitude to the out-of-plane response, emerges robustly associated with out-of-plane orbital magnetization. Our findings demonstrate that the anomalous Nernst effect is no longer limited by the orthogonality condition, opening new opportunities for more flexible designs of magnetothermoelectric materials and devices.

cond-mat.str-el↗

arXiv2026

Single-crystal structural phase diagram of stoichiometric bilayer nickelate La3Ni2O7 under hydrostatic pressure

The bilayer nickelate La3Ni2O7 has attracted intense interest following the discovery of high-temperature superconductivity under pressure, representing the first nickelate superconductor realized in bulk form. However, the crystal structure of the superconducting phase remains under active discussion, complicating efforts to establish its microscopic origin. Here we resolve these structural controversies by establishing a definitive pressure-temperature phase diagram, including the superconducting region of stoichiometric La3Ni2O7 single crystals under hydrostatic conditions using helium as the pressure-transmitting medium. At ambient pressure, La3Ni2O7 adopts a polar orthorhombic Am2m structure characterized by charge order between inequivalent Ni sites and NiO6 octahedral tilting. Upon compression, the system undergoes a direct transition from the charge-ordered Am2m phase to the tetragonal I4/mmm phase near 10 GPa, coinciding with the onset of bulk superconductivity. These results establish the intrinsic structural evolution of La3Ni2O7 and provide a structural framework for microscopic theories of nickelate superconductivity.

cond-mat.supr-con↗

arXiv2026

Density-wave phases, anisotropic transport, and Planckian dissipation in single crystals of the superconductor La3Ni2O7

Pressure-induced superconductivity in bilayer nickelates provides a platform for investigating intertwined roles of charge/spin orders and electric transport in unconventional superconductivity. However, important quantitative information on the transport, such as the absolute value of the resistivity, the anisotropy, and the scattering rate of carriers, remains insufficient due to the lack of accurate measurements using large single crystals. Here we establish a high-precision pressure-temperature phase diagram of high-quality La3Ni2O7 single crystals, by measuring the in-plane and out-of-plane resistivities. We resolve two distinct anomalies associated with density-wave formation with contrasting pressure dependences. The pressure-induced structural transition enhances not only the resistivity values for both directions, but also its anisotropy at low temperatures, demonstrating a pronounced effect of density-wave order on the charge dynamics. Superconductivity with zero-resistance emerges near the boundary where the density-wave phases are fully suppressed, and above Tc, the resistivity exhibits a temperature-linear dependence over a wide temperature range while the scattering rate falls within a regime of the Planckian limit. Our results show that pressure dramatically changes the anisotropic charge transport via modifying density-wave orders, and eventually produces a pronounced strange-metal state with strong scatterings, from which superconductivity develops. This establishes robust density-wave correlations and Planckian dissipation as remarkable features of La3Ni2O7.

cond-mat.supr-con↗

arXiv2025

Bulk superconductivity in the kagome metal YRu3B2

Materials with a kagome sublattice have been heavily studied recently for their exotic electronic band structure, structural frustration, high-temperature charge order transitions, and unconventional electron-phonon coupling. In LaRu3Si2, it was proposed that electronic flat bands conspire with the characteristic phonon spectrum of the kagome lattice to drive enhanced superconductivity at Tc = 7 K. Here, we report bulk superconductivity in the structural analogue YRu3B2, which hosts a structurally pristine kagome lattice. We observe a superconducting transition at Tc = 0.7 K through magnetization, resistivity, and heat-capacity measurements in this novel kagome metal.

cond-mat.supr-con↗

arXiv2025

Successive orthorhombic distortions in kagome metals by molecular orbital formation

The kagome lattice, with its inherent frustration, hosts a plethora of exotic phenomena, including the emergence of $3\mathbf{q}$ charge density wave order. The high rotational symmetry, required to realize such an unconventional charge order, is broken in many kagome materials by orthorhombic distortions at high temperature, the origin of which is much less discussed despite their ubiquity. In this study, synchrotron X-ray diffraction reveals a structural phase transition from a parent hexagonal phase to an orthorhombic ground state, mediated by a critical regime of diffuse scattering in the prototypical kagome metals $R$Ru$_3$Si$_2$ ($R$=rare-earth). Structural analysis uncovers an interlayer dimerization of kagome atoms in the low-temperature phase. Accordingly, a dimer model with one-dimensional disorder on kagome layers successfully reproduces the diffuse scattering. The observations point to molecular orbital formation between kagome $4d_{z^2}$ orbitals as the driving force behind the transition, consistent with \textit{ab initio} calculations. A framework based on electronegativity and atomic radii is proposed to evaluate the stability of the hexagonal phase in kagome metals, guiding the design of highly symmetric materials.

cond-mat.str-el↗

arXiv2025

Chemical enhancement of superconductivity in LaRu3Si2 with mode-selective coupling between kagome phonons and flat bands

In kagome metals, flat electronic bands induced by frustrated hopping are a platform for strong electron correlations. In particular, a selective coupling of flat band states to certain kagome phonon modes is proposed as a universal origin of superconductivity in this material class. Here, we investigate the superconductivity in the kagome system LaRu$_3$(Si$_{1-x}$Ge$_x$)$_2$ by chemical pressure tuning while preserving the Ru-$4d$ states that constitute the kagome flat bands. We observe a sizable enhancement in the density of states up to $x = 0.07$, as determined by the specific heat, with a concomitant increase in the superconducting transition temperature $T_\mathrm{c}$. Ge-dopants induce a uniaxial lattice expansion along the $c$-axis. Our first-principles calculations suggest that this mitigates the detrimental effect of hybridization between kagome layers and reduces the dispersion of the Ru-$4d_{x^2-y^2}$ flat band. The calculated chemical potential moves closer to the maximum in the energy-dependent density of states. Our result is consistent with a theoretical prediction of tunable flat band superconductivity in LaRu$_3$Si$_2$ by mode-selective coupling between specific kagome phonons and the Ru-$d_{x^2-y^2}$ orbitals.

cond-mat.supr-con↗

arXiv2025

Stark difference in the in-plane anomalous Hall response in Zintl compounds EuA2Sb2 (A = Zn, Cd) thin films

Recent observation of the in-plane anomalous Hall effect in magnetic Weyl semimetal EuCd2Sb2 has drawn attention to out-of-plane orbital magnetization induced by an in-plane field component. Here we study EuZn2Sb2, a sister compound of EuCd2Sb2, to demonstrate sensitive changes of the in-plane anomalous Hall effect on the band modulation. The Hall resistivity measured with rotating the magnetic field within the (001) principal plane of EuZn2Sb2 films exhibits a clear three-fold component corresponding to the in-plane anomalous Hall effect, which is distinct from the two-fold component of the planar Hall effect. The in-plane anomalous Hall effect of EuZn2Sb2 is highly contrasting to EuCd2Sb2, especially in terms of its opposite sign and field dependence, which can be explained by model calculations with different band inversion parameters. Our results pave the way for systematically controlling the in-plane anomalous Hall effect and orbital magnetization through elaborate band engineering.

cond-mat.mtrl-sci↗

arXiv2025

Anomalous Hall effect in Dirac semimetal probed by in-plane magnetic field

Intrinsic anomalous Hall effect (AHE) formulated by geometric properties of Bloch wavefunctions is a ubiquitous transport phenomenon not limited to magnetic systems but also allowed in non-magnetic ones under an external field breaking time-reversal symmetry. On the other hand, detection of field-induced AHE is practically challenging because the band modulation through the Zeeman and spin-orbit couplings is typically small compared to other contributions as induced by the Lorentz force. Here, we demonstrate on Dirac semimetal Cd$_3$As$_2$ films that the field-induced AHE in non-magnetic systems can be quantitatively probed by applying and rotating the magnetic field within the Hall deflection plane. Measurements on the Cd$_3$As$_2$ (112) plane reveal that AHE emerges as a clear three-fold symmetric component for the in-plane field rotation. This intrinsic response becomes more pronounced in ultralow-electron-density films where significant variations in the geometric properties are expected under the magnetic field. Our findings open new opportunities in the research of Hall responses manifested as orbital magnetization in non-magnetic systems.

cond-mat.mtrl-sci↗

arXiv2025

Probing Mixed Valence States by Nuclear Spin-Spin Relaxation Time Measurements

Several elements in the periodic table exhibit an interesting and often overlooked feature: They skip certain valence states which is discussed in the field of superconductivity to be in favor of fostering higher transition temperatures $T_c$. However, from the experimental point of view, it is often deemed difficult to probe changes in the valence state. Here we demonstrate that the latter are accessible by the spin-spin relaxation rate $1/T_2$ in nuclear magnetic resonance. As target material, we chose the solid solution Ge$_{1-x}$In$_x$Te, where valence-skipping In induces superconductivity and changes its valence state as a function of $x$. We observe a strong enhancement in $1/T_2(x)$ and, most importantly, find that $1/T_2$ and $T_c$ exhibit a strikingly similar $x$ dependence. These results underline the importance of valence physics for the evolution of superconductivity in Ge$_{1-x}$In$_x$Te. A model based on a Ruderman-Kittel-Kasuya-Yosida type of interaction among the In nuclei is proposed which fully accounts for the experimental results.

cond-mat.supr-con↗

arXiv2024

Enhancement of the Thermoelectric Figure of Merit in the Dirac Semimetal Cd$_{3}$As$_{2}$ by Band-Structure and -Filling Control

Topological materials attract a considerable research interest because of their characteristic band structure giving rise to various new phenomena in quantum physics. Beside this, they are tempting from a functional materials point of view: Topological materials bear potential for an enhanced thermoelectric efficiency because they possess the required ingredients, such as intermediate carrier concentrations, large mobilities, heavy elements etc. Against this background, this work reports an enhanced thermoelectric performance of the topological Dirac semimetal Cd$_{3}$As$_{2}$ upon alloying the trivial semiconductor Zn$_{3}$As$_{2}$. This allows to gain fine-tuned control over both the band filling and the band topology in Cd$_{3-x}$Zn$_{x}$As$_{2}$. As a result, the thermoelectric figure of merit exceeds 0.5 around $x = 0.6$ and $x = 1.2$ at elevated temperatures. The former is due to an enhancement of the power factor, while the latter is a consequence of a strong suppression of the thermal conductivity. In addition, in terms of first-principle band structure calculations, the thermopower in this system is theoretically evaluated, which suggests that the topological aspects of the band structure change when traversing $x = 1.2$.

cond-mat.mtrl-sci↗

arXiv2024

Distinct topological Hall responses in CeCu$_2$-type EuZn$_2$ and EuCd$_2$ films

Rare earth intermetallic compounds crystallized in AlB$_2$-type and its low-symmetry derivative CeCu$_2$-type structures potentially host diverse frustrated magnetic structures and rich magnetotransport phenomena. We report the film growth of CeCu$_2$-type EuZn$_2$ by molecular beam epitaxy and the observation of topological Hall responses highly contrastive to isostructural EuCd$_2$. While their magnetization curves are rather similar, the topological Hall effect observed in EuZn$_2$ is simpler, with the only one component enhanced at the magnetic transition field. EuZn$_2$ may be a unique system for studying the magnetic domain boundary effect on topological Hall responses among the CeCu$_2$-type rare-earth intermetallic compounds.

cond-mat.mtrl-sci↗

arXiv2024

Unconventional two-dimensional quantum oscillations in three-dimensional thick SrRuO$_3$ films

SrRuO$_3$ is a prototypical transition metal oxide which hosts rich physical properties including itinerant ferromagnetism, high conductivity, and intrinsic Hall effect originating in the Weyl points. Recently, high-quality SrRuO$_3$ films with residual resistivity ratios of more than 50 have been reported to exhibit quantum oscillations at low temperatures in spite of its strong electron correlation. While the origin of the oscillations has been discussed in relation to Weyl orbits based on the Weyl semimetal band structure, so far experimentally reported results are neither consistent with each other nor with theoretically expected behavior, leaving the origin of the oscillations in SrRuO$_3$ films still elusive. In this report, we have carefully evaluated the quantum oscillations observed in three-dimensional thick SrRuO$_3$ films with a high residual resistivity ratio of RRR = 82. We reveal the coexistence of two oscillation components both derived from two-dimensional electronic states and with slightly different masses, suggesting the involvement of the surface Fermi arc states formed between different Weyl point pairs.

cond-mat.str-el↗

arXiv2024

Observation of in-plane anomalous Hall effect associated with orbital magnetization

For over a century, the Hall effect, a transverse effect under out-of-plane magnetic field or magnetization, has been a cornerstone for magnetotransport studies and applications. Modern theoretical formulation based on the Berry curvature has revealed the potential that even in-plane magnetic field can induce anomalous Hall effect, but its experimental demonstration has remained difficult due to its potentially small magnitude and strict symmetry requirements. Here we report observation of the in-plane anomalous Hall effect by measuring low-carrier density films of magnetic Weyl semimetal EuCd$_2$Sb$_2$. Anomalous Hall resistance exhibits distinct three-fold rotational symmetry for changes in the in-plane field component, and this can be understood in terms of out-of-plane Weyl points splitting or orbital magnetization induced by in-plane field, as also confirmed by model calculation. Our findings demonstrate the importance of in-plane field to control the Hall effect, accelerating materials development and further exploration of various in-plane field induced phenomena.

cond-mat.mes-hall↗

arXiv2024

Ferromagnetic state with large magnetic moments realized in epitaxially strained Sr3Ru2O7 films

Technical advancement of oxide molecular beam epitaxy (MBE) has opened new avenues for studying various quantum transport phenomena in correlated transition-metal oxides, as exemplified by the exotic superconductivity of Sr$_2$RuO$_4$ and quantum oscillations of SrRuO$_3$. On the other hand, film research of another Ruddlesden-Popper strontium ruthenate Sr$_3$Ru$_2$O$_7$ which exhibits a unique quantum phase related to metamagnetism in bulk systems did not progress well. Here we report the fabrication of high-quality Sr$_3$Ru$_2$O$_7$ thin films by oxide MBE and the observation of a strain-induced ferromagnetic ground state. The change in magnetic exchange coupling evaluated by first-principles calculations indicates a systematic relation between the compression of the $c$-axis length and induced ferromagnetism. Giant epitaxial strain in high-quality films will be a key to a comprehensive understanding of the magnetism in Ruddlesden-Popper strontium ruthenates Sr$_{n+1}$Ru$_n$O$_{3n+1}$, which sensitively depends on the ratio of in-plane to out-of-plane Ru-Ru distances.

cond-mat.str-el↗

arXiv2024

Berry curvature derived negative magnetoconductivity observed in type-II magnetic Weyl semimetal films

Here we study nonmonotonic features which appear both in magnetoresistivity and anomalous Hall resistivity during the simple magnetization process, by systematically measuring type-II magnetic Weyl semimetal EuCd$_2$Sb$_2$ films over a wide carrier density range. We find that a positive magnetoresistivity hump can be explained as manifestation of a field-linear term in the generalized magnetoconductivity formula including the Berry curvature. As also confirmed by model calculation, the term can be negative and pronounced near the Weyl point energy in the case that the Weyl cones are heavily tilted. Our findings demonstrate extensive effects of the Berry curvature on various magnetotransport in magnetic Weyl semimetals beyond the anomalous Hall effect.

cond-mat.mtrl-sci↗

arXiv2024

Striped electronic phases in an incommensurately modulated van der Waals superlattice

Electronic properties of crystals can be manipulated using spatially periodic modulations. Long-wavelength, incommensurate modulations are of particular interest, exemplified recently by moiré patterned van der Waals (vdW) heterostructures. Bulk vdW superlattices hosting interfaces between clean 2D layers represent scalable bulk analogs of vdW heterostructures and present a complementary venue to explore incommensurately modulated 2D states. Here we report the bulk vdW superlattice SrTa$_2$S$_5$ realizing an incommensurate 1D modulation of 2D transition metal dichalcogenide (TMD) $H$-TaS$_2$ layers. High-quality electronic transport in the $H$-TaS$_2$ layers, evidenced by quantum oscillations, is made anisotropic by the modulation and shows commensurability oscillations akin to lithographically modulated 2D systems. We also find unconventional, clean-limit superconductivity (SC) in SrTa$_2$S$_5$ with a pronounced suppression of interlayer coherence relative to intralayer coherence. Such a hierarchy can arise from pair-density wave (PDW) SC with mismatched spatial arrangement in adjacent superconducting layers. Examining the in-plane magnetic field $H_{ab}$ dependence of interlayer critical current density $J_c$, we find anisotropy with respect to $H_{ab}$ orientation: $J_c$ is maximized (minimized) when $H_{ab}$ is perpendicular (parallel) to the stripes, consistent with 1D PDW SC. From diffraction we find the structural modulation is shifted between adjacent $H$-TaS$_2$ layers, suggesting mismatched 1D PDW is seeded by the striped structure. With a high-mobility Fermi liquid in a coherently modulated structure, SrTa$_2$S$_5$ is a promising host for novel phenomena anticipated in clean, striped metals and superconductors. More broadly, SrTa$_2$S$_5$ establishes bulk vdW superlattices as macroscopic platforms to address long-standing predictions for modulated electronic phases.

cond-mat.mtrl-sci↗

arXiv2024

Intrinsic insulating transport characteristics in low-carrier density EuCd2As2 films

Searching for an ideal magnetic Weyl semimetal hosting only a single pair of Weyl points has been a focal point for systematic clarification of its unique magnetotransport derived from the interplay between topology and magnetization. Among the candidates, triangular-lattice antiferromagnet EuCd$_2$As$_2$ has been attracting special attention due to the prediction of the ideal Weyl semimetal phase in the ferromagnetic state, however, transport properties of low-carrier density samples have remained elusive. Here we report molecular beam epitaxy growth of EuCd$_2$As$_2$ films, achieving low-hole density in the range of $10^{15}$-$10^{16}$ cm$^{-3}$ at low temperature. Transport measurements of such low-carrier density films reveal an insulating behavior with an activation gap of about 200 meV, which persists even in the field-induced ferromagnetic state. Our work provides an important experimental clue that EuCd$_2$As$_2$ is intrinsically insulating, contrary to the previous prediction.

cond-mat.mtrl-sci↗