arXiv subjects

Klaus Koepernik

Publications and source records attributed to Klaus Koepernik.

At least 19 recordsLinked to original sources

arXiv2026

Fermi surface and topology of multiband superconductor BeAu

The chiral material BeAu was recently identified as a multiband type-I superconductor with a critical temperature of 3.2 K. As a member of the B20 crystal family (space group $P2_13$), its band structure hosts multifold fermions at high-symmetry points, unpaired Weyl points and even nodal surfaces. This renders BeAu an appealing system to investigate the interplay between superconductivity and topology. Here we present a comprehensive first-principles analysis of BeAu's electronic structure focusing on its Fermi surface's topology and the implications for superconductivity. Together with the presence of four- and six-fold fermions at high-symmetry points, we identify several additional isolated Weyl points near the Fermi level. We also determine the associated topological edge states -- the surface Fermi arcs. Computing the Chern number associated to different Fermi surface sheets, we show that BeAu harbors a $ν= 4$ topological superconducting phase in the presence of $s$-wave pairing of alternating sign ($s_\pm$ pairing). Notably, we also identify a Fermi surface with a Chern number of +6; the highest value reported to date. Finally, our analysis reveals strong inhomogeneity in the orbital character of electronic states at the Fermi level, suggesting a link to the observed multigap superconductivity.

cond-mat.supr-con↗

arXiv2025

Room temperature Planar Hall effect in nanostructures of trigonal-PtBi2

Trigonal-PtBi2 has recently garnered significant interest as it exhibits unique superconducting topological surface states due to electron pairing on Fermi arcs connecting bulk Weyl nodes. Furthermore, topological nodal lines have been predicted in trigonal-PtBi2, and their signature was measured in magnetotransport as a dissipationless, i.e. odd under a magnetic field reversal, anomalous planar Hall effect. Understanding the topological superconducting surface state in trigonal-PtBi2 requires unravelling the intrinsic geometric properties of the normal state electronic wavefunctions and further studies of their hallmarks in charge transport characteristics are needed. In this work, we reveal the presence of a strong dissipative, i.e. even under a magnetic field reversal, planar Hall effect in PtBi2 at low magnetic fields and up to room temperature. This robust response can be attributed to the presence of Weyl nodes close to the Fermi energy. While this effect generally follows the theoretical prediction for a planar Hall effect in a Weyl semimetal, we show that it deviates from theoretical expectations at both low fields and high temperatures. We also discuss the origin of the PHE in our material, and the contributions of both the topological features in PtBi2 and its possible trivial origin. Our results strengthen the topological nature of PtBi2 and the strong influence of quantum geometric effects on the electronic transport properties of the low energy normal state.

cond-mat.mes-hall↗

arXiv2025

Topological nodal $i$-wave superconductivity in PtBi$_2$

Most superconducting materials are well-understood and conventional in the sense that the pairs of electrons that cause the superconductivity by their condensation have the highest possible symmetry. Famous exceptions are the enigmatic high-$T_c$ cuprate superconductors. Nodes in their superconducting gap are the fingerprint of their unconventional character and imply superconducting pairing of $d$-wave symmetry. Here, using angle-resolved photoemission spectroscopy, we observe that the Weyl semimetal PtBi$_2$ harbors nodes in its superconducting gap, implying unconventional $i$-wave pairing symmetry. At temperatures below $10\,\mathrm{K}$, the superconductivity in PtBi$_2$ gaps out its topological surface states, the Fermi arcs, while its bulk states remain normal. The nodes in the superconducting gap that we observe are located exactly at the center of the Fermi arcs, and imply the presence of topologically protected Majorana cones around this locus in momentum space. From this, we infer theoretically that robust zero-energy Majorana flat bands emerge at surface step edges. This not only establishes PtBi$_2$ surfaces as unconventional, topological $i$-wave superconductors but also as a promising material platform in the ongoing effort to generate and manipulate Majorana bound states.

cond-mat.supr-con↗

arXiv2025

Large anomalous Hall conductivity in Weyl ferrimagnet Cs$_{2}$Co$_{3}$S$_4$ predicted by density-functional calculations

The identification of topological Weyl semimetals has recently gained considerable attention. Here, we report the results of density-functional theory calculations regarding the magnetic properties, the electronic structure, and the intrinsic anomalous Hall conductivity of the title compound, which was synthesized already 50 years ago but received little attention, hitherto. We found Cs$_{2}$Co$_{3}$S$_4$ to be a ferrimagnetic half-metal with a total spin magnetic moment of about 3 $μ_B$ per formula unit. It shows energy band gap of 0.36 eV in the majority-spin channel and a pseudo-gap at the Fermi level in the minority-spin channel. We identified several sets of low-energy Weyl points and traced their dependence on the direction of magnetization. The intrinsic anomalous Hall conductivity is predicted to reach a magnitude up to 500 $Ω^{-1}$cm$^{-1}$, which is comparable to values obtained in other celebrated Weyl semimetals.

cond-mat.mtrl-sci↗

arXiv2024

Dissipationless transport signature of topological nodal lines

Topological materials, such as topological insulators or semimetals, usually not only reveal the nontrivial properties of their electronic wavefunctions through the appearance of stable boundary modes, but also through very specific electromagnetic responses. The anisotropic longitudinal magnetoresistance of Weyl semimetals, for instance, carries the signature of the chiral anomaly of Weyl fermions. However for topological nodal line semimetals -- materials where the valence and conduction bands cross each other on one-dimensional curves in the three-dimensional Brillouin zone -- such a characteristic has been lacking. Here we report the discovery of a peculiar charge transport effect generated by topological nodal lines: a dissipationless transverse signal in the presence of coplanar electric and magnetic fields, which originates from a Zeeman-induced conversion of topological nodal lines into Weyl nodes under infinitesimally small magnetic fields. We evidence this dissipationless topological response in trigonal \ce{PtBi2} persisting up to room temperature, and unveil the extensive topological nodal lines in the band structure of this non-magnetic material. These findings provide a new pathway to engineer Weyl nodes by arbitrary small magnetic fields and reveal that bulk topological nodal lines can exhibit non-dissipative transport properties.

cond-mat.mes-hall↗

arXiv2024

Electronic structure of the surface superconducting Weyl semimetal PtBi$_2$

Trigonal PtBi$_2$ is a layered semimetal without inversion symmetry, featuring 12 Weyl points in the vicinity of the Fermi energy. Its topological Fermi arcs were recently shown to superconduct at low temperatures where bulk superconductivity is absent. Here, we perform first-principles calculations to investigate in detail the bulk and surface electronic structure of PtBi$_2$, and obtain the spin texture as well as the momentum-dependent localization of the arcs. Motivated by the experimentally observed recovery of inversion symmetry under pressure or upon doping, we interpolate between the two structures and determine the energy and momentum dependence of the Weyl nodes. For deeper insights into the surface superconductivity of PtBi$_2$, we construct a symmetry-adapted effective four-band model that accurately reproduces the Weyl points of PtBi$_2$. We supplement this model with an analysis of the symmetry-allowed pairings between the Fermi arcs, which naturally mix spin-singlet and spin-triplet channels. Moreover, the presence of surface-only superconductivity facilitates an intrinsic superconductor-semimetal-superconductor Josephson junction, with the semimetallic phase sandwiched between the two superconducting surfaces. For a phase difference of $π$, zero-energy Andreev bound states develop between the two terminations.

cond-mat.supr-con↗

arXiv2023

Superconducting Arcs

An essential ingredient for the production of Majorana fermions that can be used for quantum computing is the presence of topological superconductivity. As bulk topological superconductors remain elusive, the most promising approaches exploit proximity-induced superconductivity making systems fragile and difficult to realize. Weyl semimetals due to their intrinsic topology belong to potential candidates too, but search for Majorana fermions has always been connected with the superconductivity in the bulk, leaving the possibility of intrinsic superconductivity of the Fermi surface arcs themselves practically without attention, even from the theory side.Here, by means of angle-resolved photoemission spectroscopy and ab-initio calculations, we unambiguously identify topological Fermi arcs on two opposing surfaces of the non-centrosymmetric Weyl material PtBi2. We show that these states become superconducting at different temperatures around 10K. Remarkably, the corresponding coherencepeaks appear as the strongest and sharpest excitations ever detected by photoemission from solids, suggesting significant technological relevance. Our findings indicate that topological superconductivity in PtBi2 occurs exclusively at the surface, which not only makes it an ideal platform to host Majorana fermions, but may also lead to a unique quantum phase - an intrinsic topological SNS Josephson junction.

cond-mat.supr-con↗

arXiv2021

A Charge-Density-Wave Topological Semimetal

Topological physics and strong electron-electron correlations in quantum materials are typically studied independently. However, there have been rapid recent developments in quantum materials in which topological phase transitions emerge when the single-particle band structure is modified by strong interactions. We here demonstrate that the room-temperature phase of (TaSe$_4$)$_2$I is a Weyl semimetal with 24 pairs of Weyl nodes. Owing to its quasi-1D structure, (TaSe$_4$)$_2$I hosts an established CDW instability just below room temperature. Using X-ray diffraction, angle-resolved photoemission spectroscopy, and first-principles calculations, we find that the CDW in (TaSe$_4$)$_2$I couples the bulk Weyl points and opens a band gap. The correlation-driven topological phase transition in (TaSe$_4$)$_2$I provides a route towards observing condensed-matter realizations of axion electrodynamics in the gapped regime, topological chiral response effects in the semimetallic phase, and represents an avenue for exploring the interplay of correlations and topology in a solid-state material.

cond-mat.mtrl-sci↗

arXiv2020

Spin-orbitronic materials with record spin-charge conversion from high-throughput ab initio calculations

The spin Hall effect (SHE) is an important spintronics phenomenon, which allows transforming a charge current into a spin current and vice versa without the use of magnetic materials or magnetic fields. To gain new insight into the physics of the SHE and to identify materials with a substantial spin Hall conductivities (SHC), we performed high-precision, high-throughput ab initio electronic structure calculations of the intrinsic SHC for over 20,000 non-magnetic crystals. The calculations reveal a strong and unexpected relation of the magnitude of the SHC with the crystalline symmetry, which we show exists because large SHC is typically associated with mirror symmetry protected nodal lines in the band structure. From the new developed database, we identify new promising materials. This includes eleven materials with a SHC comparable or even larger than that the up to now record Pt as well as materials with different types of spin currents, which could allow for new types of spin-obitronics devices.

cond-mat.mtrl-sci↗

arXiv2020

Comprehensive scan for nonmagnetic Weyl semimetals with nonlinear optical response

As the development of topological band theory, comprehensive databases about time reversal and crystalline symmetries protected nonmagnetic topological materials were developed via first-principles calculations recently. However, owing to the low symmetry requirement of Weyl points, the symmetry-based topological indicator cannot be applied to Weyl semimetals (WSMs). Hitherto, the WSMs with Weyl points in arbitrary positions are still absent in the well-known databases. In this work, we develop an efficient algorithm to search for Weyl points automatically and establish a database of nonmagnetic WSMs with Weyl points near Fermi level based on the total experimental noncentrosymmetric crystal structures in the Inorganic Crystal Structure Database (ICSD). Totally 46 Weyl semimetals were discovered to have nearly clean Fermi surface and Weyl points near Fermi level within 300 meV, and 9 of them are chiral structures which may host the quantized circular photogalvanic effect. In addition, the nonlinear optical response is studied and giant shift current is explored in the end. Besides nonmagnetic WSMs, our powerful tools can also be used in the discovery of magnetic topological materials.

cond-mat.mtrl-sci↗

arXiv2019

Dual topology in Jacutingaite Pt$_2$HgSe$_3$

Topological phases of electronic systems often coexist in a material, well-known examples being systems which are both strong and weak topological insulators. More recently, a number of materials have been found to have the topological structure of both a weak topological insulator and a mirror-protected topological crystalline insulator. In this work, we first focus on the naturally occurring mineral called Jacutingaite, Pt$_2$HgSe$_3$, and show based on density-functional calculations that it realizes this dual topological phase and that the same conclusion holds for Pd$_2$HgSe$_3$. Second, we introduce tight-binding models that capture the essential topological properties of this dual topological phase in materials with three-fold rotation symmetry and use these models to describe the main features of the surface spectral density of different materials in the class.

cond-mat.mes-hall↗

arXiv2018

Strongly enhanced Berry dipole at topological phase transitions in BiTeI

Transitions between topologically distinct electronic states have been predicted in different classes of materials and observed in some. A major goal is the identification of measurable properties that directly expose the topological nature of such transitions. Here we focus on the giant-Rashba material bismuth tellurium iodine (BiTeI) which exhibits a pressure-driven phase transition between topological and trivial insulators in three-dimensions. We demonstrate that this transition, which proceeds through an intermediate Weyl semi-metallic state, is accompanied by a giant enhancement of the Berry curvature dipole which can be probed in transport and optoelectronic experiments. From first-principles calculations, we show that the Berrry-dipole --a vector along the polar axis of this material-- has opposite orientations in the trivial and topological insulating phases and peaks at the insulator-to-Weyl critical points, at which the nonlinear Hall conductivity can increase by over two orders of magnitude.

cond-mat.mes-hall↗

arXiv2018

From colossal to zero: Controlling the Anomalous Hall Effect in Magnetic Heusler Compounds via Berry Curvature Design

Since the discovery of the anomalous Hall effect (AHE), the anomalous Hall conductivity (AHC) has been thought to be zero when there is no net magnetization. However, the recently found relation between the intrinsic AHE and the Berry curvature predicts other possibilities, such as a large AHC in non-colinear antiferromagnets with no net magnetization but net Berry curvature. Vice versa, the AHE in principle could be tuned to zero, irrespective of a finite magnetization. Here, we experimentally investigate this possibility and demonstrate that, the symmetry elements of Heusler magnets can be changed such that the Berry curvature and all the associated properties are switched while leaving the magnetization unaffected. This enables us to tune the AHC from 0 Ω-1cm-1 up to 1600 Ω-1cm-1 with an exceptionally high anomalous Hall angle up to 12 %, while keeping the magnetization same. Our study shows that the AHC can be controlled by selectively changing the Berry curvature distribution, independent of the magnetization.

cond-mat.mtrl-sci↗

arXiv2018

Prediction of a magnetic Weyl semimetal without spin-orbit coupling and strong anomalous Hall effect in the Heusler compensated ferrimagnet Ti2MnAl

We predict a magnetic Weyl semimetal in the inverse Heusler Ti2MnAl, a compensated ferrimagnet with a vanishing net magnetic moment and a Curie temperature of over 650 K. Despite the vanishing net magnetic moment, we calculate a large intrinsic anomalous Hall effect (AHE) of about 300 S/cm. It derives from the Berry curvature distribution of the Weyl points, which are only 14 meV away from the Fermi level and isolated from trivial bands. Different from antiferromagnets Mn3X (X= Ge, Sn, Ga, Ir, Rh, and Pt), where the AHE originates from the non-collinear magnetic structure, the AHE in Ti2MnAl stems directly from the Weyl points and is topologically protected. The large anomalous Hall conductivity (AHC) together with a low charge carrier concentration should give rise to a large anomalous Hall angle. In contrast to the Co-based ferromagnetic Heusler compounds, the Weyl nodes in Ti2MnAl do not derive from nodal lines due to the lack of mirror symmetries in the inverse Heusler structure. Since the magnetic structure breaks spin-rotation symmetry, the Weyl nodes are stable without SOC. Moreover, because of the large separation between Weyl points of opposite topological charge, the Fermi arcs extent up to 75% of the reciprocal lattice vectors in length. This makes Ti2MnAl an excellent candidate for the comprehensive study of magnetic Weyl semimetals. It is the first example of a material with Weyl points, large anomalous Hall effect and angle despite a vanishing net magnetic moment.

cond-mat.mtrl-sci↗

arXiv2018

Strong spin-Hall and Nernst effects in a p-band semimetal

Since spin currents can be generated, detected, and manipulated via the spin Hall effect (SHE), the design of strong SHE materials has become a focus in the field of spintronics. Because of the recent experimental progress also the spin Nernst effect (SNE), the thermoelectrical counterpart of the SHE, has attracted much interest. Empirically strong SHEs and SNEs are associated with $d$-band compounds, such as transition metals and their alloys -- the largest spin Hall conductivity (SHC) in a $p$-band material is $\sim 450$ $\left(\hbar/e\right)\left(Ω\cdot cm\right)^{-1}$ for a Bi-Sb alloy, which is only about a fifth of platinum. This raises the question whether either the SHE and SNE are naturally suppressed in $p$-bands compounds, or favourable $p$-band systems were just not identified yet. Here we consider the $p$-band semimetal InBi, and predict it has a record SHC $σ_{xy}^{z}\approx 1100 \ \left(\hbar/e\right)\left(Ω\cdot cm\right)^{-1}$ which is due to the presence of nodal-lines in its band structure. Also the spin-Nernst conductivity $α_{zx}^y\approx 1.2 \ (\hbar/e)(A/m\cdot K)$ is very large, but our analysis shows its origin is different as the maximum appears in a different tensor element. This insight gained on InBi provides guiding principles to obtain a strong SHE and SNE in $p$-band materials and establishes a more comprehensive understanding of the relationship between the SHE and SNE.

cond-mat.mtrl-sci↗

arXiv2017

Generic coexistence of Fermi arcs and Dirac cones on the surface of time-reversal invariant Weyl semimetals

The hallmark of Weyl semimetals is the existence of open constant-energy contours on their surface -- the so-called Fermi arcs -- connecting Weyl points. Here, we show that for time-reversal symmetric realizations of Weyl semimetals these Fermi arcs in many cases coexist with closed Fermi pockets originating from surface Dirac cones pinned to time-reversal invariant momenta. The existence of Fermi pockets is required for certain Fermi-arc connectivities due to additional restrictions imposed by the six $\mathbb{Z}_2$ topological invariants characterizing a generic time-reversal invariant Weyl semimetal. We show that a change of the Fermi-arc connectivity generally leads to a different topology of the surface Fermi surface, and identify the half-Heusler compound LaPtBi under in-plane compressive strain as a material that realizes this surface Lifshitz transition. We also discuss universal features of this coexistence in quasi-particle interference spectra.

cond-mat.mes-hall↗

arXiv2017

Correlation of Fe-based Superconductivity and Electron-Phonon Coupling in an FeAs/Oxide Heterostructure

Interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have received considerable attention due to the possibility of enhancing preexisting superconductivity. Using scanning tunneling spectroscopy, we studied the correlation between superconductivity and e-ph interaction with interfacial-phonons in an iron-based superconductor Sr$_2$VO$_3$FeAs ($T_c \approx$ 33 K) made of alternating FeSC and oxide layers. The quasiparticle interference measurement over regions with systematically different average superconducting gaps due to the e-ph coupling locally modulated by O vacancies in VO$_2$ layer, and supporting self-consistent momentum-dependent Eliashberg calculations provide a unique real-space evidence of the forward-scattering interfacial phonon contribution to the total superconducting pairing.

cond-mat.supr-con↗

arXiv2016

Magnetotransport and de Haas-van Alphen measurements in the type-II Weyl semimetal TaIrTe$_4$

The layered ternary compound TaIrTe$_4$ has been predicted to be a type-II Weyl semimetal with only four Weyl points just above the Fermi energy. Performing magnetotransport measurements on this material we find that the resistivity does not saturate for fields up to 70 T and follows a $ ρ\sim B^{1.5}$ dependence. Angular-dependent de Haas-van Alphen (dHvA) measurements reveal four distinct frequencies. Analyzing these magnetic quantum oscillations by use of density functional theory (DFT) calculations we establish that in TaIrTe$_4$ the Weyl points are located merely $\sim$ 40-50 meV above the chemical potential, suggesting that the chemical potential can be tuned into the four Weyl nodes by moderate chemistry or external pressure, maximizing their chiral effects on electronic and magnetotransport properties.

cond-mat.str-el↗