arXiv subjects

Claudia Felser

Publications and source records attributed to Claudia Felser.

At least 235 records · Page 13Linked to original sources

arXiv2020

Observation of giant spin-split Fermi-arc with maximal Chern number in the chiral topological semimetal PtGa

Non-symmorphic chiral topological crystals host exotic multifold fermions, and their associated Fermi arcs helically wrap around and expand throughout the Brillouin zone between the high-symmetry center and surface-corner momenta. However, Fermi-arc splitting and realization of the theoretically proposed maximal Chern number rely heavily on the spin-orbit coupling (SOC) strength. In the present work, we investigate the topological states of a new chiral crystal, PtGa, which has the strongest SOC among all chiral crystals reported to date. With a comprehensive investigation using high-resolution angle-resolved photoemission spectroscopy, quantum-oscillation measurements, and state-of-the-art ab initio calculations, we report a giant SOC-induced splitting of both Fermi arcs and bulk states. Consequently, this study experimentally confirms the realization of a maximal Chern number equal to |4| for the first time in multifold fermionic systems, thereby providing a platform to observe large-quantized photogalvanic currents in optical experiments.

cond-mat.mtrl-sci↗

arXiv2020

Generalized Electron Hydrodynamics, Vorticity Coupling, and Hall Viscosity in Crystals

Theoretical and experimental studies have revealed that electrons in condensed matter can behave hydrodynamically, exhibiting fluid phenomena such as Stokes flow and vortices. Unlike classical fluids, preferred directions inside crystals lift isotropic restrictions, necessitating a generalized treatment of electron hydrodynamics. We explore electron fluid behaviors arising from the most general viscosity tensors in two and three dimensions, constrained only by thermodynamics and crystal symmetries. Hexagonal 2D materials such as graphene support flows indistinguishable from those of an isotropic fluid. By contrast 3D materials including Weyl semimetals, exhibit significant deviations from isotropy. Breaking time-reversal symmetry, for example in magnetic topological materials, introduces a non-dissipative Hall component to the viscosity tensor. While this vanishes by isotropy in 3D, anisotropic materials can exhibit nonzero Hall viscosity components. We show that in 3D anisotropic materials the electronic fluid stress can couple to the vorticity without breaking time-reversal symmetry. Our work demonstrates the anomalous landscape for electron hydrodynamics in systems beyond graphene, and presents experimental geometries to quantify the effects of electronic viscosity.

cond-mat.mtrl-sci↗

arXiv2020

A combined laser-based ARPES and 2PPES study of Td-WTe$_2$

Laser-based angle-resolved photoemission spectroscopy (ARPES) and two-photon photoemission spectroscopy (2PPES) are employed to study the valence electronic structure of the Weyl semimetal candidate Td-WTe$_2$ along two high symmetry directions and for binding energies between $\approx$ -1 eV and 5 eV. The experimental data show a good agreement with band structure calculations. Polarization dependent measurements provide furthermore information on initial and intermediate state symmetry properties with respect to the mirror plane of the Td structure of WTe$_2$.

cond-mat.mtrl-sci↗

arXiv2020

Intrinsic anomalous Hall effect in Ni-substituted magnetic Weyl semimetal Co3Sn2S2

Topological materials have recently attracted considerable attention among materials scientists as their properties are predicted to be protected against perturbations such as lattice distortion and chemical substitution. However, any experimental proof of such robustness is still lacking. In this study, we experimentally demonstrate that the topological properties of the ferromagnetic kagome compound Co3Sn2S2 are preserved upon Ni substitution. We systematically vary the Ni content in Co3Sn2S2 single crystals and study their magnetic and anomalous transport properties. For the intermediate Ni substitution, we observe a remarkable increase in the coercive field while still maintaining significant anomalous Hall conductivity. The large anomalous Hall conductivity of these compounds is intrinsic, consistent with first-principle calculations, which proves its topological origin. Our results can guide further studies on the chemical tuning of topological materials for better understanding.

cond-mat.mtrl-sci↗

arXiv2020

Controlling Dzyaloshinskii-Moriya interactions in the skyrmion host candidates FePd$_{1-x}$Pt$_x$Mo$_3$N

Ferromagnets crystallizing in structures described by chiral cubic space groups, including compounds with the B20 or $β$-Mn structures, are known to host long-period chiral spin textures such as skyrmion lattices. These spin textures are stabilized by a competition between ferromagnetic exchange and antisymmetric Dyzaloshinskii-Moriya (DM) exchange, which is enhanced by the spin-orbit coupling associated with high-atomic-number elements. For real-world application, it is desirable to find materials that can host compact skyrmion lattices at readily accessible temperatures. Here, we report on the crystal chemistry and magnetic phase diagrams of a family of compounds with the filled $β$-Mn structure, FePd$_{1-x}$Pt$_x$Mo$_3$N with $T_C$ ranging from 175 K to 240 K. DC and AC magnetization measurements reveal magnetic phase diagrams consistent with the formation of a skyrmion pocket just below $T_C$. The magnitudes of ferromagnetic and DM exchanges are determined from the phase diagrams, demonstrating that the introduction of increasing amounts of Pt can be used to increase spin-orbit coupling in order to control the expected skyrmion lattice parameter between 140 nm and 65 nm while simultaneously increasing $T_C$.

cond-mat.mes-hall↗

arXiv2020

Identification of chirality of chiral multifold fermions in anti-crystals

The chirality of chiral multifold fermions in reciprocal space is related to the chirality of the crystal lattice structure in real space. In this work, we propose a strategy to detect and identify opposite-chirality multifold fermions in nonmagnetic systems by means of second-order optical transports. The chiral crystals are related by an inversion operation and cannot overlap with each other by any experimental operation, and the chiral multifold fermions in the crystals host opposite chiralities for a given k-point. A change of chirality is indicated by a sign change of the second-order charge current dominated by chiral fermions. This strategy is effective to study the relationship between chiralities in reciprocal and real spaces by utilizing bulk transport.

cond-mat.mtrl-sci↗

arXiv2019

Strong bulk photovoltaic effect in chiral crystal in the visible spectrum

Structurally chiral materials hosting multifold fermions with large topological number have attracted considerable attention because of their naturally long surface Fermi arcs and bulk quantized circular photogalvanic effect (CPGE). Multifold fermions only appear in metallic states, and therefore, most studies so far have only focused on the semimetals in compounds with chiral crystal structures. In this work, we show that the structurally chiral topological trivial insulators are also exotic states, which is interesting from the application point of view, owing to their natural advantage to host a large bulk photovoltaic effect in the visible wavelength region. In the last decades, the shift current in the visible wavelength region was limited to be 10 uA/V2 . By scanning the insulators with chiral structure, we found a class of compounds with photoconductivity ranging from 20 to 80 uA/V2 , which is approximately one order of magnitude larger than that reported in other real materials. This work illustrates that the compounds with chiral structure can host both quantum CPGE and a strong shift current in the second order optical response. Moreover, this work offers a good platform for the study of the shift current and its future application by putting the focus on insulator with chiral lattices, so far overlooked in photovoltaic technologies.

cond-mat.mtrl-sci↗

arXiv2019

Anomalous and topological Hall effects in epitaxial thin films of the noncollinear antiferromagnet Mn$_{3}$Sn

Noncollinear antiferromagnets with a D0$_{19}$ (space group = 194, P6$_{3}$/mmc) hexagonal structure have garnered much attention for their potential applications in topological spintronics. Here, we report the deposition of continuous epitaxial thin films of such a material, Mn$_{3}$Sn, and characterize their crystal structure using a combination of x-ray diffraction and transmission electron microscopy. Growth of Mn$_{3}$Sn films with both (0001) c-axis orientation and (40$\bar{4}$3) texture is achieved. In the latter case, the thin films exhibit a small uncompensated Mn moment in the basal plane, quantified via magnetometry and x-ray magnetic circular dichroism experiments. This cannot account for the large anomalous Hall effect simultaneously observed in these films, even at room temperature, with magnitude $σ_{\mathrm{xy}}$ ($μ_{0}H$ = 0 T) = 21 $\mathrmΩ^{-1}\mathrm{cm}^{-1}$ and coercive field $μ_{0}H_{\mathrm{C}}$ = 1.3 T. We attribute the origin of this anomalous Hall effect to momentum-space Berry curvature arising from the symmetry-breaking inverse triangular spin structure of Mn$_{3}$Sn. Upon cooling through the transition to a glassy ferromagnetic state at around 50 K, a peak in the Hall resistivity close to the coercive field indicates the onset of a topological Hall effect contribution, due to the emergence of a scalar spin chirality generating a real-space Berry phase. We demonstrate that the polarity of this topological Hall effect, and hence the chiral-nature of the noncoplanar magnetic structure driving it, can be controlled using different field cooling conditions.

cond-mat.mes-hall↗

arXiv2019

Observation of Topological Photocurrents in the Chiral Weyl Semimetal RhSi

Weyl semimetals are crystals in which electron bands cross at isolated points in momentum space. Associated with each crossing point (or Weyl node) is an integer topological invariant known as the Berry monopole charge. The discovery of new classes of Weyl materials is driving the search for novel properties that derive directly from the Berry charge. The circular photogalvanic effect (CPGE), whereby circular polarized light generates a current whose direction depends on the helicity of the absorbed photons, is a striking example of a macroscopic property that emerges from Weyl topology. Recently, it was predicted that the rate of current generation associated with optical transitions near a Weyl node is proportional to its monopole charge and independent of material-specific parameters. In Weyl semimetals that retain mirror symmetry this universal photogalvanic current is strongly suppressed by opposing contributions from energy equivalent nodes of opposite charge. However, when all mirror symmetries are broken, as in chiral Weyl systems, nodes with opposite topological charge are no longer degenerate, opening a window of photon energies where the topological CPGE can emerge. In this work we test this theory through measurement of the photon-energy dependence of the CPGE in the chiral Weyl semimetal RhSi. The spectrum is fully consistent with a topological CPGE, as it reveals a response in a low-energy window that closes at 0.65 eV, in quantitative agreement with the theoretically-derived bandstucture.

cond-mat.mes-hall↗

arXiv2019

Giant anomalous Hall effect in a ferromagnetic Kagome-lattice semimetal

Magnetic Weyl semimetals with broken time-reversal symmetry are expected to generate strong intrinsic anomalous Hall effects, due to their large Berry curvature. Here, we report a magnetic Weyl semimetal candidate Co3Sn2S2 with a quasi-two-dimensional crystal structure consisting of stacked Kagome lattices. This lattice provides an excellent platform for hosting exotic quantum topological states. We observe a negative magnetoresistance that is consistent with the chiral anomaly expected from the presence of Weyl fermions close to the Fermi level. The anomalous Hall conductivity is robust against both increased temperature and charge conductivity, which corroborates the intrinsic Berry-curvature mechanism in momentum space. Owing to the low carrier density in this material and the significantly enhanced Berry curvature from its band structure, the anomalous Hall conductivity and the anomalous Hall angle simultaneously reach 1130 S cm-1 and 20%, respectively, an order of magnitude larger than typical magnetic systems. Combining the Kagome-lattice structure and the long-range out-of-plane ferromagnetic order of Co3Sn2S2, we expect that this material is an excellent candidate for observation of the quantum anomalous Hall state in the two-dimensional limit.

cond-mat.mtrl-sci↗

arXiv2019

Large Resistivity Reduction in Mixed-Valent CsAuBr$_3$ Under Pressure

We report on high-pressure $p \leq 45$ GPa resistivity measurements on the perovskite-related mixed-valent compound CsAuBr$_3$. The compounds high-pressure resistivity can be classified into three regions: For low pressures ($p < 10$ GPa) an insulator to metal transition is observed; between $p= 10$ GPa and 14 GPa the room temperature resistivity goes through a minimum and increases again; above $p = 14$ GPa a semiconducting state is observed. From this pressure up to the highest pressure of $p = 45$ GPa reached in this experiment, the room-temperature resistivity remains nearly constant. We find an extremely large resistivity reduction between ambient pressure and 10 GPa by more than 6 orders of magnitude. This decrease is among the largest reported changes in the resistivity for this narrow pressure regime. We show - by an analysis of the electronic band structure evolution of this material - that the large change in resistivity under pressure in not caused by a crossing of the bands at the Fermi level. We find that it instead stems from two bands that are pinned at the Fermi level and that are moving towards one another as a consequence of the mixed-valent to single-valent transition. This mechanism appears to be especially effective for the rapid buildup of the density of states at the Fermi level.

cond-mat.mtrl-sci↗

arXiv2019

Magneto-Optics of the Weyl Semimetal TaAs in the THz and IR Regions

The magnetic-field dependence of optical reflectivity [$R(ω)$] and optical conductivity [$σ(ω)$] spectra of the ideal type-I Weyl semimetal TaAs has been investigated at the temperature of 10 K in the terahertz (THz) and infrared (IR) regions. The obtained $σ(ω)$ spectrum in the THz region of $\hbarω\leq15$ meV is strongly affected by the applied magnetic field ($B$): The Drude spectral weight is rapidly suppressed and an energy gap originating from the optical transition in the lowest Landau levels appears with a gap size that increases in proportion to $\sqrt{B}$, which suggests linear band dispersions. The obtained THz $σ(ω)$ spectra could be scaled not only in the energy scale by $\sqrt{B}$ but also in the intensity by $1/\sqrt{B}$ as predicted theoretically. In the IR region for $\hbarω\geq17$ meV, on the other hand, the observed $R(ω)$ peaks originating from the optical transitions in higher Landau levels are proportional to linear-$B$ suggesting parabolic bands. The different band dispersions originate from the crossover from the Dirac to the free-electron bands.

cond-mat.str-el↗

arXiv2019

Signatures for half-metallicity and nontrivial surface states in a Kagome-lattice magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$

Weyl semimetals with time reversal symmetry breaking are expected to show various fascinating physical behaviors, such as intrinsic giant anomalous Hall effect, chiral anomaly effect in the bulks, and Fermi arcs on the surfaces. Here we report a scanning tunneling microscopy study on the magnetic Weyl semimetal candidate Co$_3$Sn$_2$S$_2$. According to the morphology and local density of states of the surface, we provide assignments to different surface terminations. The measured local density of states reveals a semimetal gap of ~300 mV, which is further verified as the gap in spin-minority bands using spin-resolved tunneling spectra. Additionally, signature for the nontrivial surface states around 50 mV is proposed. This is further confirmed by the observations of standing waves around a step-edge of the sample. Our observations and their comparison with band structure calculations provide direct yet timely evidence for the bulk and surface band structures of the magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$.

cond-mat.mtrl-sci↗

arXiv2019

Surface states in bulk single crystal of topological semimetal Co$_3$Sn$_2$S$_2$ towards water oxidation

The band inversion in topological phase matters bring exotic physical properties such as the emergence of a topologically protected surface states. They strongly influence the surface electronic structures of the investigated materials and could serve as a good platform to gain insight into the catalytic mechanism of surface reactions. Here we synthesized high-quality bulk single crystals of the topological semimetal Co$_3$Sn$_2$S$_2$. We found that at room temperature, Co$_3$Sn$_2$S$_2$ naturally hosts the band structure of a topological semimetal. This guarantees the existence of robust surface states from the Co atoms. Bulk single crystal of Co$_3$Sn$_2$S$_2$ exposes their Kagome lattice that constructed by Co atoms and have high electrical conductivity. They serves as catalytic centers for oxygen evolution process (OER), making bonding and electron transfer more efficient due to the partially filled $e_g$ orbital. The bulk single crystal exhibits outstanding OER catalytic performance, although the surface area is much smaller than that of Co-based nanostructured catalysts. Our findings emphasize the importance of tailoring topological non-trivial surface states for the rational design of high-activity electrocatalysts.

cond-mat.mtrl-sci↗

arXiv2019

Magnetically Switchable Light-Matter Interaction in the Two-Dimensional Magnet CrI3

Coupling different physical properties is a fascinating subject of physics. Already well-known are the multiferroics which show properties of ferroelectrics and magnets. But ferroelectricity by itself also entails the bulk photovoltaic effect, a light-matter interaction which generates dc currents. Here we propose a magnetic photogalvanic effect that couples the magnetism to the light-matter interaction. This phenomenon emerges from the $\mathbf{k}$ to $\mathbf{-k}$ symmetry-breaking in the band structure and does not require a static polarization. It is distinct from other known bulk photovoltaic mechanisms such as the shift current. We demonstrate such phenomena in a newly discovered layered magnetic insulator CrI$_3$. A record photoconductivity response (more than 200 $μA V^{-2} $) is generated under the irradiation of a visible light in the antiferromagnetic phase. The current can be reversed and switched by controllable magnetic phase transitions. Our work paves a new route for photovoltaic and optoelectronic devices and provides a sensitive probe for the magnetic transition.

cond-mat.mes-hall↗

arXiv2019

Thickness dependence of the anomalous Hall effect in thin films of the topological semimetal Co$_2$MnGa

Topological magnetic semimetals promise large Berry curvature through the distribution of the topological Weyl nodes or nodal lines and further novel physics with exotic transport phenomena. We present a systematic study of the structural and magnetotransport properties of Co$_2$MnGa films from thin (20 nm) to bulk like behavior (80 nm), in order to understand the underlying mechanisms and the role on the topology. The magnetron sputtered Co$_2$MnGa films are $L$$2_{\mathrm {1}}$-ordered showing very good heteroepitaxy and a strain-induced tetragonal distortion. The anomalous Hall conductivity was found to be maximum at a value of 1138 S/cm, with a corresponding anomalous Hall angle of 13 %, which is comparatively larger than topologically trivial metals. There is a good agreement between the theoretical calculations and the Hall conductivity observed for the 80 nm film, which suggest that the effect is intrinsic. Thus, the Co$_2$MnGa compound manifests as a promising material towards topologically-driven spintronic applications.

cond-mat.mtrl-sci↗

arXiv2019

Berry curvature unravelled by the Nernst effect in Mn$_3$Ge

The discovery of topological quantum materials represents a striking innovation in modern condensed matter physics with remarkable fundamental and technological implications. Their classification has been recently extended to topological Weyl semimetals, i.e., solid state systems which exhibit the elusive Weyl fermions as low-energy excitations. Here we show that the Nernst effect can be exploited as a sensitive probe for determining key parameters of the Weyl physics, applying it to the non-collinear antiferromagnet Mn$_3$Ge. This compound exhibits anomalous thermoelectric transport due to enhanced Berry curvature from Weyl points located extremely close to the Fermi level. We establish from our data a direct measure of the Berry curvature at the Fermi level and, using a minimal model of a Weyl semimetal, extract for the first time the Weyl point energy and their distance in momentum-space.

cond-mat.str-el↗

arXiv2019

Extremely high conductivity observed in the triple point topological metal MoP

Weyl and Dirac fermions have created much attention in condensed matter physics and materials science. Recently, several additional distinct types of fermions have been predicted. Here, we report ultra-high electrical conductivity in MoP at low temperature, which has recently been established as a triple point Fermion material. Here we show that the electrical resistivity is 6 n-ohm cm at 2 K with a large mean free path of 11 microns. de Haas-van Alphen oscillations reveal spin splitting of the Fermi surfaces. In contrast to noble metals with similar conductivity and number of carriers, the magnetoresistance in MoP does not saturate up to 9 T at 2 K. Interestingly, the momentum relaxing time of the electrons is found to be more than 15 times larger than the quantum coherence time. This difference between the scattering scales shows that momentum conserving scattering dominates in MoP at low temperatures.

cond-mat.mtrl-sci↗