arXiv subjects

Claudia Felser

Publications and source records attributed to Claudia Felser.

At least 127 records · Page 7Linked to original sources

arXiv2023

Helicity-dependent Ultrafast Photocurrents in Weyl Magnet Mn$_3$Sn

We present an optical pump-THz emission study on non-collinear antiferromagnet Mn$_3$Sn. We show that Mn$_3$Sn acts as a source of THz radiation when irradiated by femtosecond laser pulses. The polarity and amplitude of the emitted THz fields can be fully controlled by the polarisation of optical excitation. We explain the THz emission with the photocurrents generated via the photon drag effect by combining various experimental measurements as a function of pump polarisation, magnetic field, and sample orientation with thorough symmetry analysis of response tensors.

cond-mat.mes-hall↗

arXiv2022

Large anomalous Hall, Nernst effect and topological phases in the 3d-4d/5d based oxide double perovskites

Magnetic topological quantum materials are attracting considerable attention owing to their potential technological applications. However, only a small number of these materials have been experimentally realized, thereby giving rise to the need for new stable magnetic topological quantum materials. Magnetism and spin-orbit coupling, two essential ingredients of the oxide materials, lead to various topological transport phenomena such as the anomalous Hall and anomalous Nernst effects, which can be significantly enhanced by designing an electronic structure with a large Berry curvature. In that respect, double perovskites with the general formula A$_2$BB'O$_6$ with an alternating ordered arrangement of two transition metal sites, B(3d) and B'(4d/5d), present attractive possibilities as they are robustly stable against oxidation under ambient conditions and versatile. These double perovskites also offer a high energy scale for magnetism as well as strong spin-orbit coupling with a high magnetic ordering temperature. Here, using first-principles density functional theory calculations, we present a comprehensive study of the intrinsic anomalous transport for 3d-4d/5d based cubic and tetragonal stable double perovskite (DP) compounds. A few of the DPs exhibit a very large anomalous Hall effect with a distinct topological band crossing in the vicinity of the Fermi energy. Our results show the importance of symmetries, particularly the mirror planes, as well as the clean topological band crossing near the Fermi energy, which is primarily contributed by the 5d-t$_{2g}$ for large anomalous Hall and Nernst effects.

cond-mat.mtrl-sci↗

arXiv2022

Anisotropic resistance with a 90-degree twist in a ferromagnetic Weyl semimetal, Co2MnGa

Co$_2$MnGa is a ferromagnetic semimetal with Weyl nodal lines identified by ARPES. We studied electrical transport in thin Co$_2$MnGa lamellae (10 $\times$ 10 $\times$ 0.4-5 microns) cut from single-crystals using a focused ion beam. These crystals exhibit an unexpected and highly unusual planar resistance anisotropy ($\sim$10 times) with principal axes that rotate by 90 degrees between the upper and lower faces. Using symmetry arguments and simulations, we find that the observed resistance anisotropy resembles that of an isotropic conductor with anisotropic surface states that are impeded from hybridization with bulk states. The origin of these states awaits further experiments that can correlate the surface bands with the observed 90$^\circ$-twist geometry.

cond-mat.mes-hall↗

arXiv2022

Band structures of (NbSe$_4$)$_3$I and (TaSe$_4$)$_3$I: Reconciling transport, optics and ARPES

Among the quasi one-dimensional transition metal tetrachalcogenides (MSe$_4$)$_n$I (M=Nb,Ta), the $n=3$ compounds are the only ones not displaying charge density waves. Instead, they show structural transitions with puzzling transport behavior. They are semiconductors at the lowest temperatures, but their transport gaps are significantly smaller than those inferred from ARPES and optical conductivity. Recently, a metallic polytype of (TaSe$_4$)$_3$I has been found with ferromagnetism and superconductivity coexisting at low temperature, in contrast to previous reports. In this work we present detailed ab-initio and tight binding band structure calculations for the different (MSe$_4$)$_n$I reported structures. We obtain good agreement with the observed transport gaps, and explain how ARPES and optics experiments effectively probe a gap between different bands due to an approximate translation symmetry, solving the controversy. Finally, we show how small extrinsic hole doping can tune the Fermi level through a Van Hove singularity in (TaSe$_4$)$_3$I and discuss the implications for magnetism and superconductivity.

cond-mat.supr-con↗

arXiv2022

Doping as a tuning mechanism for magneto-thermoelectric effects to improve zT in polycrystalline NbP

Weyl semimetals combine topological and semimetallic effects, making them candidates for interesting and effective thermoelectric transport properties. Here, we present experimental results on polycrystalline NbP, demonstrating the simultaneous existence of a large Nernst effect and a large magneto-Seebeck effect, which is typically not observed in a single material at the same temperature. We compare transport results from two polycrystalline samples of NbP with previously published work, observing a shift in the temperature at which the maximum Nernst and magneto-Seebeck thermopowers occur, while still maintaining thermopowers of similar magnitude. Theoretical modeling shows how doping strongly alters both the Seebeck and Nernst magneto-thermopowers by shifting the temperature-dependent chemical potential, and the corresponding calculations provide a consistent interpretation of our results. Thus, we offer doping as a tuning mechanism for shifting magneto-thermoelectric effects to temperatures appropriate for device applications, improving zT at desirable operating temperature. Furthermore, the simultaneous presence of both a large Nernst and magneto-Seebeck thermopower is uncommon and offers unique device advantages if the thermopowers are used additively. Here, we also propose a unique thermoelectric device which would collectively harness the large Nernst and magneto-Seebeck thermopowers to greatly enhance the output and zT of conventional thermoelectric devices.

cond-mat.mtrl-sci↗

arXiv2022

Molecular Beam Epitaxy of a Half-Heusler Topological Superconductor Candidate YPtBi

The search for topological superconductivity has motivated investigations into materials that combine topological and superconducting properties. The half-Heusler compound YPtBi appears to be such a material, however experiments have thus far been limited to bulk single crystals, drastically limiting the scope of available experiments. This has made it impossible to investigate the potential topological nature of the superconductivity in this material. Experiments to access details about the superconducting state require sophisticated lithographic structures, typically based on thin films. Here we report on the establishment of high crystalline quality epitaxial thin films of YPtBi(111), grown using molecular beam epitaxy on Al2O3(0001) substrates. A robust superconducting state is observed, with both critical temperature and critical field consistent with that previously reported for bulk crystals. Moreover we find that AlOx capping sufficiently protects the sample surface from degradation to allow for proper lithography. Our results pave a path towards the development of advanced lithographic structures, that will allow the exploration of the potentially topological nature of superconductivity in YPtBi.

cond-mat.mtrl-sci↗

arXiv2022

Pressured-induced superconductivity extending across the topological phase transition in thallium-based topological materials TlBi(S1-xSex)2

The coexistence of superconductivity and topology holds the potential to realize exotic quantum states of matter. Here we report that superconductivity induced by high pressure in three thallium-based materials, covering the phase transition from a normal insulator (TlBiS2) to a topological insulator (TlBiSe2) through a Dirac semimetal (TlBiSeS). By increasing the pressure up to 60 GPa, we observe superconductivity phase diagrams with maximal Tc values at 6.0-8.1 K. Our density-functional theory calculations reveal topological surface states in superconductivity phases for all three compounds. Our study paves the path to explore topological superconductivity and topological phase transitions.

cond-mat.supr-con↗

arXiv2022

Observation of an unexpected negative magnetoresistance in magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$

Time-reversal symmetry breaking allows for a rich set of magneto-transport properties related to electronic topology. Focusing on the magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$, we prepared micro-ribbons and investigated their transverse and longitudinal transport properties from 100 K to 180 K in magnetic fields $μ_0 H$ up to 2T. We establish the presence of a magnetoresistance (MR) up to 1 % with a strong anisotropy depending the projection of $H$ on the easy-axis magnetization, which exceeds all other magnetoresistive effects. Based on detailed phenomenological modeling, we attribute the observed results with unexpected form of anisotropy to magnon MR resulting from magnon-electron coupling. Moreover, a similar angular dependence is also found in the transverse resistivity which we show to originate from the combination of ordinary Hall and anomalous Hall effects. Thus the interplay of magnetic and topological properties governs the magnetotransport features of this magnetic Weyl system.

cond-mat.mes-hall↗

arXiv2022

Au4Mn, a localized ferromagnet with strong spin-orbit coupling, long-range ferromagnetic exchange and high Curie temperature

Metallic Mn-based alloys with a nearest-neighbor Mn-Mn distance greater than 0.4 nm exhibit large, well-localized magnetic moments. Here we investigate the magnetism of tetragonal Au4Mn with a Curie temperature of 385 K, where manganese has a spin moment of 4.1 muB and its orbital moment is quenched. Since 80% of the atoms are gold, the spin orbit interaction is strong and Au4Mn exhibits uniaxial magnetocrystalline anisotropy with surface maze domains at room temperature. The magnetic hardness parameter of 1.0 is sufficient to maintain the magnetization along the c-axis for a sample of any shape. Au also reduces the spin moment of Mn through 5d-3d orbital hybridization. An induced moment of 0.05 muB was found on Au under a pulsed field of 40 T. Density functional theory calculations indicate that the Mn-Mn exchange is mediated by spin-polarized gold 5d and 6p electrons. The distance-dependence shows that it is ferromagnetic or zero for the first ten shells of Mn neighbors out to 1.041 nm (64 atoms), and very weak and oscillatory thereafter.

cond-mat.mtrl-sci↗

arXiv2022

Switchable chiral transport in charge-ordered Kagome metal CsV$_3$Sb$_5$

When electric conductors differ from their mirror image, unusual chiral transport coefficients appear that are forbidden in achiral metals, such as a non-linear electric response known as electronic magneto-chiral anisotropy (eMChA). While chiral transport signatures are by symmetry allowed in many conductors without a center of inversion, it reaches appreciable levels only in rare cases when an exceptionally strong chiral coupling to the itinerant electrons is present. So far, observations of chiral transport have been limited to materials in which the atomic positions strongly break mirror symmetries. Here, we report chiral transport in the centro-symmetric layered Kagome metal CsV$_3$Sb$_5$, observed via second harmonic generation under in-plane magnetic field. The eMChA signal becomes significant only at temperatures below $T'\sim$ 35 K, deep within the charge-ordered state of CsV$_3$Sb$_5$ ($T_{\mathrm{CDW}}\sim$ 94 K). This temperature dependence reveals a direct correspondence between electronic chirality, unidirectional charge order, and spontaneous time-reversal-symmetry breaking due to putative orbital loop currents. We show that the chirality is set by the out-of-plane field component and that a transition from left- to right-handed transport can be induced by changing the field sign. CsV$_3$Sb$_5$ is the first material in which strong chiral transport can be controlled and switched by small magnetic-field changes, in stark contrast to structurally chiral materials -- a prerequisite for their applications in chiral electronics.

cond-mat.str-el↗

arXiv2022

Parallel spin-momentum locking in a chiral topological semimetal

Spin-momentum locking in solids describes a directional relationship between the electron's spin angular momentum and its linear momentum over the entire Fermi surface. While orthogonal spin-momentum locking, such as Rashba spin-orbit coupling, has been studied for decades and inspired a vast number of applications, its natural counterpart, the purely parallel spin-momentum locking, has remained elusive in experiments. Recently, chiral topological semimetals that host single- and multifold band crossings have been predicted to realize such parallel locking. Here, we use spin- and angle-resolved photoelectron spectroscopy to probe spin-momentum locking of a multifold fermion in the chiral topological semimetal PtGa via the spin-texture of its topological Fermi-arc surface states. We find that the electron spin of the Fermi-arcs points orthogonal to their Fermi surface contour for momenta close to the projection of the bulk multifold fermion, which is consistent with parallel spin-momentum locking of the latter. We anticipate that our discovery of parallel spin-momentum locking of multifold fermions will lead to the integration of chiral topological semimetals in novel spintronic devices, and the search for spin-dependent superconducting and magnetic instabilities in these materials.

cond-mat.mes-hall↗

arXiv2022

3D Fermi surfaces from charge order in layered CsV$_3$Sb$_5$

The cascade of electronic phases in CsV$_3$Sb$_5$ raises the prospect to disentangle their mutual interactions in a clean, strongly interacting Kagome lattice. When the Kagome planes are stacked into a crystal, its electronic dimensionality encodes how much of the Kagome physics and its topological aspects survive. The layered structure of CsV$_3$Sb$_5$ reflects in Brillouin-zone-sized quasi-2D Fermi surfaces and a significant transport anisotropy. Yet here we demonstrate that CsV$_3$Sb$_5$ is a three-dimensional metal within the charge-density-wave (CDW) state. Small 3D pockets play a crucial role in its low-temperature magneto- and quantum transport. Their emergence at $T_{CDW}\sim 93$ K results in an anomalous sudden increase of the in-plane magnetoresistance by 4 orders of magnitude. The presence of these 3D pockets is further confirmed by quantum oscillations under in-plane magnetic fields - demonstrating their closed nature. These results emphasize the impact of interlayer coupling on the Kagome physics in 3D materials.

cond-mat.str-el↗

arXiv2022

Observation of an Anomalous Hall Effect in Single-crystal Mn3Pt

The Mn3X family of compounds was the first in which a large anomalous Hall effect (AHE) was predicted to arise from a purely antiferromagnetic structure, due to the Berry curvature in momentum space. Nearly simultaneously with this prediction, a large AHE was observed experimentally in one of the hexagonal members of this family, Mn3Sn. Aligning antiferromagnetic domains, a necessary step for observation of the AHE, is more challenging for the cubic members of the Mn3X family, due to a combination of a smaller spontaneous ferromagnetic moment and much stronger magnetic anisotropy. Here, we use a combination of uniaxial stress and applied magnetic field to align domains of bulk single-crystal Mn3Pt, and demonstrate for the first time a substantial AHE in a bulk sample of a cubic member of the Mn3X family. The AHE remains locked in with essentially no quantitative variation when the stress is ramped back to zero, which shows that it is not a consequence of any stress-induced ferromagnetic moment.

cond-mat.mtrl-sci↗

arXiv2022

Hierarchy of quasi-symmetries and degeneracies in chiral crystal materials CoSi

In materials, certain approximated symmetry operations can exist in a lower-order approximation of the effective model but are good enough to influence the physical responses of the system, and these approximated symmetries were recently dubbed "quasi-symmetries" \cite{guo_arxiv_2021}. In this work, we reveal a hierarchy structure of the quasi-symmetries and the corresponding nodal structures that they enforce via two different approaches of the perturbation expansions for the effective model in the chiral crystal material CoSi. In the first approach, we treat the spin-independent linear momentum (k) term as the zero-order Hamiltonian. Its energy bands are four-fold degenerate due to an SU(2)$\times$SU(2) quasi-symmetry. We next consider both the k-independent spin-orbit coupling (SOC) and full quadratic-k terms as the perturbation terms and find that the first-order perturbation leads to a model described by a self-commuting "stabilizer code" Hamiltonian with a U(1) quasi-symmetry that can protect nodal planes. In the second approach, we treat the SOC-free linear-k term and k-independent SOC term as the zero-order. They exhibit an SU(2) quasi-symmetry, which can be reduced to U(1) quasi-symmetry by a choice of quadratic terms. Correspondingly, a two-fold degeneracy for all the bands due to the SU(2) quasi-symmetry is reduced to two-fold nodal planes that are protected by the U(1) quasi-symmetry. For both approaches, including higher-order perturbation will break the U(1) quasi-symmetry and induce a small gap $\sim$ 1 meV for the nodal planes. These quasi-symmetry protected near degeneracies play an essential role in understanding recent quantum oscillation experiments in CoSi.

cond-mat.mes-hall↗

arXiv2022

Magnetic phase crossover in strongly correlated EuMn2P2

Strong electron correlations underlie a plethora of electronic and magnetic components and devices and are often used to identify and probe novel ground states in quantum materials. Herein we report a magnetic phase crossover in EuMn2P2, an insulator which shows Eu antiferromagnetism at TN=17K, but no phase transition attributed to Mn magnetism. The absence of a Mn magnetic phase transition contrasts with the formation of long-range Mn order at T=130K in isoelectronic EuMn2Sb2 and EuMn2As2. Temperature-dependent specific heat and 31P NMR measurements provide evidence for the development of Mn magnetic correlations from T=250-100 K. Density functional theory calculations demonstrate an unusual sensitivity of the band structure to the details of the imposed Mn and Eu magnetic order, with antiferromagnetic Mn order required to recapitulate an insulating state. Our results imply a picture in which long range Mn magnetic order is suppressed by chemical pressure, but that magnetic correlations persist, narrowing bands and producing an insulating state.

cond-mat.str-el↗

arXiv2022

Berry curvature induced anomalous Hall conductivity in magnetic topological oxide double perovskite Sr2FeMoO6

Oxide materials exhibit several novel structural, magnetic, and electronic properties. Their stability under ambient conditions, easy synthesis, and high transition temperatures provide such systems with an ideal ground for realizing topological properties and real-life technological applications. However, experimental evidence of topological states in oxide materials is rare. In this study, we have synthesized single crystals of oxide double perovskite Sr2FeMoO6 and revealed its topological nature by investigating its structural, magnetic, and electronic properties. We observed that the system crystallized in the cubic space group Fm-3m, which is a half-metallic ferromagnet. Transport measurements show an anomalous Hall effect, and it is evident that the Hall contribution originates from the Berry curvature. Assuming a shift of the Fermi energy towards the conduction band, the contribution of the anomalous Hall effect is enhanced owing to the presence of a gaped nodal line. This study can be used to explore and realize the topological properties of bulk oxide systems.

cond-mat.mtrl-sci↗

arXiv2022

Scaling of Berry-curvature monopole dominated large linear positive magnetoresistance

The linear positive magnetoresistance (LPMR) is a widely observed phenomenon in topological materials, which is promising for potential applications on topological spintronics. However, its mechanism remains ambiguous yet and the effect is thus uncontrollable. Here, we report a quantitative scaling model that correlates the LPMR with the Berry curvature, based on a ferromagnetic Weyl semimetal CoS2 that bears the largest LPMR of over 500% at 2 Kelvin and 9 Tesla, among known magnetic topological semimetals. In this system, masses of Weyl nodes existing near the Fermi level, revealed by theoretical calculations, serve as Berry-curvature monopoles and low-effective-mass carriers. Based on the Weyl picture, we propose a relation \[\text{MR}=\frac{e}{\hbar }B{{Ω}_{\text{F}}}\], with B being the applied magnetic field and \[{{Ω}_{\text{F}}}\] the average Berry curvature near the Fermi surface, and further introduce temperature factor to both MR/B slope (MR per unit field) and anomalous Hall conductivity, which establishes the connection between the model and experimental measurements. A clear picture of the linearly slowing down of carriers, i.e., the LPMR effect, is demonstrated under the cooperation of the k-space Berry curvature and real-space magnetic field. Our study not only provides an experimental evidence of Berry curvature induced LPMR for the first time, but also promotes the common understanding and functional designing of the large Berry-curvature MR in topological Dirac/Weyl systems for magnetic sensing or information storage.

cond-mat.mtrl-sci↗

arXiv2022

Topological Metal MoP Nanowire for Interconnect

The increasing resistance of Cu interconnects for decreasing dimensions is a major challenge in continued downscaling of integrated circuits beyond the 7-nm technology node as it leads to unacceptable signal delays and power consumption in computing. The resistivity of Cu increases due to electron scattering at surfaces and grain boundaries of the interconnects at the nanoscale. Topological semimetals, owing to their topologically protected surface states and suppressed electron backscattering, are promising material candidates to potentially replace current Cu interconnects as low-resistance interconnects. Here, we report the attractive resistivity scaling of topological metal MoP nanowires and show that the resistivity values are comparable to those of Cu interconnects below 500 nm$^2$ cross-section areas. More importantly, we demonstrate that the dimensional scaling of MoP nanowires, in terms of line resistance versus total cross-sectional area, is superior to those of effective Cu and barrier-less Ru interconnects, suggesting MoP is an attractive solution to the current scaling challenge of Cu interconnects.

cond-mat.mtrl-sci↗