arXiv subjects

V. Hasse

Publications and source records attributed to V. Hasse.

3 recordsLinked to original sources

arXiv2024

Interlayer charge transfer induced by electronic instabilities in the natural van der Waals hetrostructure 4H$_b$-TaS$_2$

The natural van der Waals heterostructure 4H$_b$-TaS$_2$ composed of alternating 1T- and 1H-TaS$_2$ layers serves as a platform for investigating the electronic correlations and layer-dependent properties of novel quantum materials. The temperature evolution of the conductivity spectra $\sigma(\omega)$ obtained through infrared spectroscopy elucidates the influence of band modifications associated with the charge-density-wave (CDW) superlattice on the 1T layer, resulting in a room-temperature energy gap, $\Delta_{\rm CDW}\approx$ 0.35 eV. However, there is no gap associated to the 1H layer. Supported by density functional theory calculations, we attribute the behavior of interband transitions to the convergence of the layers, which amplifies the charge transfer from the 1T to the 1H layers, progressing as the temperature decreases. This phenomenon leads to an enhanced low-energy spectral weight and carrier density. The presence of an energy gap and the temperature-tunable charge transfer within the bulk of 4H$_b$-TaS$_2$ driven by layer-dependent CDW states contribute to a more comprehensive understanding of other complex compounds of transition-metal dichalcogenides.

cond-mat.mes-hall↗

arXiv2024

Intrinsic negative magnetoresistance from the chiral anomaly of multifold fermions

The chiral anomaly, a hallmark of chiral spin-1/2 Weyl fermions, is an imbalance between left- and right-moving particles that underpins both high and low energy phenomena, including particle decay and negative longitudinal magnetoresistance in Weyl semimetals. The discovery that chiral crystals can host higher-spin generalizations of Weyl quasiparticles without high-energy counterparts, known as multifold fermions, raises the fundamental question of whether the chiral anomaly is a more general phenomenon. Answering this question requires materials with chiral quasiparticles within a sizable energy window around the Fermi level, that are unaffected by trivial extrinsic effects such as current jetting. Here we report the chiral anomaly of multifold fermions in CoSi, which features multifold bands within about 0.85 eV around the Fermi level. By excluding current jetting through the squeezing test, we measure an intrinsic, longitudinal negative magnetoresistance. We develop the semiclassical theory of magnetotransport of multifold fermions that shows that the negative magnetoresistance originates in their chiral anomaly, despite a sizable and detrimental orbital magnetic moment contribution, previously unaccounted for. A concomitant nonlinear Hall effect supports the multifold-fermion origin of magnetotransport. Our work confirms the chiral anomaly of higher-spin generalizations of Weyl fermions, currently inaccessible outside the solid-state.

cond-mat.mes-hall↗

arXiv2024

Probing the shape of the Weyl Fermi surface of NbP using transverse electron focusing

The topology of the Fermi surface significantly influences the transport properties of a material. Firstly measured through quantum oscillation experiments, the Fermi surfaces of crystals are now commonly characterized using angle-resolved photoemission spectroscopy (ARPES), given the larger information volume it provides. In the case of Weyl semimetals, ARPES has proven remarkably successful in verifying the existence of the Weyl points and the Fermi arcs, which define a Weyl Fermi surface. However, ARPES is limited in resolution, leading to significant uncertainty when measuring relevant features such as the distance between the Weyl points. While quantum oscillation measurements offer higher resolution, they do not reveal insights into the cross-sectional shape of a Fermi surface. Moreover, both techniques lack critical information about transport, like the carriers mean free path. Here, we report measurements unveiling the distinctive peanut-shaped cross-section of the Fermi surface of Weyl fermions and accurately determine the separation between Weyl points in the Weyl semimetal NbP. To surpass the resolution of ARPES, we combine quantum oscillation measurements with transverse electron focusing (TEF) experiments, conducted on microstructured single-crystals. The TEF spectrum relates to the Fermi surface shape, while the frequency of the quantum oscillations to its area. Together, these techniques offer complementary information, enabling the reconstruction of the distinctive Weyl Fermi surface geometry. Concurrently, we extract the electrical transport properties of the bulk Weyl fermions. Our work showcases the integration of quantum oscillations and transverse electron focusing in a singular experiment, allowing for the measurements of complex Fermi surface geometries in high-mobility quantum materials.

cond-mat.mtrl-sci↗