Showing 1–9 of 9 results for author: Hasse, V

Detecting the full photoemission cone from laser-based ARPES experiments by leveraging deflector technology

Authors: Nicolas Gauthier, Benson Kwaku Frimpong, Dario Armanno, Akib Jabed, Francesco Goto, Vicky Hasse, Claudia Felser, Genda Gu, Heide Ibrahim, Francois Légaré, Fabio Boschini

Abstract: Angle-resolved photoemission spectroscopy (ARPES) provides a direct access to the electronic band structure of solid and molecular systems. The momentum range accessible by this technique depends directly on the photon energy used, and low-photon-energy sources are insufficient to photoemit electrons over the full Brillouin zone of most quantum materials. In addition, while electrons are emitted o… ▽ More Angle-resolved photoemission spectroscopy (ARPES) provides a direct access to the electronic band structure of solid and molecular systems. The momentum range accessible by this technique depends directly on the photon energy used, and low-photon-energy sources are insufficient to photoemit electrons over the full Brillouin zone of most quantum materials. In addition, while electrons are emitted over a 2$π$ solid angle, conventional hemispherical analyzers only collect a small subset of those electrons. A previous work [RSI 92, 123907 (2021)] demonstrated that electrons emitted over a larger field-of-view can be acquired in one fixed configuration by accelerating them towards the analyzer with a bias voltage. Here, we extend this work by leveraging the deflector technology of novel ARPES hemispherical analyzers. We demonstrate the ability to detect all $2π$ photoemitted electrons in a fixed configuration for various materials such as gold, cuprates and transition-metal dichalcogenides. This approach is especially advantageous for time-resolved ARPES, as electron dynamics over a large momentum range can be accessed with identical measurement conditions. △ Less

Submitted 23 January, 2026; originally announced January 2026.

Orientation Dependent Resistivity Scaling in Mesoscopic NbP Crystals

Authors: Gianluca Mariani, Federico Balduini, Nathan Drucker, Lorenzo Rocchino, Vicky Hasse, Claudia Felser, Heinz Schmid, Cezar Zota, Bernd Gotsmann

Abstract: The scaling of Si transistor technology has resulted in a remarkable improvement in the performance of integrated circuits over the last decades. However, scaled transistors also require reduced electrical interconnect dimensions, which lead to greater losses and power dissipation at circuit level. This is mainly caused by enhanced surface scattering of charge carriers in copper interconnect wires… ▽ More The scaling of Si transistor technology has resulted in a remarkable improvement in the performance of integrated circuits over the last decades. However, scaled transistors also require reduced electrical interconnect dimensions, which lead to greater losses and power dissipation at circuit level. This is mainly caused by enhanced surface scattering of charge carriers in copper interconnect wires at dimensions below 30 nm. A promising approach to mitigate this issue is to use directional conductors, i.e. materials with anisotropic Fermi surface, where proper alignment of crystalline orientation and transport direction can minimize surface scattering. In this work, we perform a resistivity scaling study of the anisotropic semimetal NbP as a function of crystalline orientation. We use here focused ion beam to pattern and scale down NbP crystallites to dimensions comparable to the electron scattering length at cryogenic temperatures. The experimental transport properties are correlated with the Fermi surface characteristics through a theoretical model, thus identifying the physical mechanisms that influence the resistivity scaling of anisotropic conductors. Our methodology provides an effective approach for early evaluation of anisotropic materials as future ultra-scalable interconnects, even when they are unavailable as epitaxial films. △ Less

Submitted 18 February, 2025; originally announced February 2025.

Comments: 18 pages, 3 figures

Spin-forbidden excitations in the magneto-optical spectra of CrI$_3$ tuned by covalency

Authors: Connor A. Occhialini, Luca Nessi, Luiz G. P. Martins, Ahmet Kemal Demir, Qian Song, Vicky Hasse, Chandra Shekhar, Claudia Felser, Kenji Watanabe, Takashi Taniguchi, Valentina Bisogni, Jonathan Pelliciari, Riccardo Comin

Abstract: Spin-forbidden ($ΔS \neq 0$) multiplet excitations and their coupling to magnetic properties are of increasing importance for magneto-optical studies of correlated materials. Nonetheless, the mechanisms for optically brightening these transitions and their generality remain poorly understood. Here, we report magnetic circular dichroism (MCD) spectroscopy on the van der Waals (vdW) ferromagnet (FM)… ▽ More Spin-forbidden ($ΔS \neq 0$) multiplet excitations and their coupling to magnetic properties are of increasing importance for magneto-optical studies of correlated materials. Nonetheless, the mechanisms for optically brightening these transitions and their generality remain poorly understood. Here, we report magnetic circular dichroism (MCD) spectroscopy on the van der Waals (vdW) ferromagnet (FM) CrI$_3$. Previously unreported spin-forbidden ($ΔS = 1$) ${}^4A_{2\mathrm{g}} \to{}^2E_\mathrm{g}/{}^2T_{1\mathrm{g}}$ Cr${}^{3+}$ $dd$ excitations are observed near the ligand-to-metal charge transfer (LMCT) excitation threshold. The assignment of these excitations and their Cr$^{3+}$ multiplet character is established through complementary Cr $L_3$-edge resonant inelastic X-ray scattering (RIXS) measurements along with charge transfer multiplet (CTM) calculations and chemical trends in the chromium trihalide series (CrX$_3$, X = Cl, Br, I). We utilize the high sensitivity of MCD spectroscopy to study the thickness dependent optical response. The spin-forbidden excitations remain robust down to the monolayer limit and exhibit a significant magnetic field tunability across the antiferromagnetic to FM transition in few-layer samples. This behavior is associated to changes in the metal-ligand covalency with magnetic state, as supported by our CTM analysis. Our results clarify the magneto-optical response of CrI$_3$ and identify covalency as a central mechanism for the brightening and field-tunability of spin-forbidden multiplet excitations. △ Less

Submitted 29 January, 2025; originally announced January 2025.

Engineering Magnetotransport Through Hierarchical Symmetry in Weyl Semimetal Superlattices

Authors: Nathan C. Drucker, Federico Balduini, Jules Schadt, Lorenzo Rocchino, Tathagata Paul, Vicky Hasse, Claudia Felser, Heinz Schmid, Cezar B. Zota, Bernd Gotsmann

Abstract: Superlattice engineering is a powerful way to tune the transport properties of a material. In this work we show that magnetotransport can be modified by superlattices in 3D materials based on the relative symmetry between the Fermi-surface and superlattice. We demonstrate commensuration oscillations in the ballistic transport regime of a nanostructured 3D material with the Weyl semimetal NbP, a si… ▽ More Superlattice engineering is a powerful way to tune the transport properties of a material. In this work we show that magnetotransport can be modified by superlattices in 3D materials based on the relative symmetry between the Fermi-surface and superlattice. We demonstrate commensuration oscillations in the ballistic transport regime of a nanostructured 3D material with the Weyl semimetal NbP, a signature typically limited to superlattices in 2D materials. The behavior of the oscillations encodes information about the shared properties between the quasiparticles at the Fermi-surface--including their momentum, charge, mass, and rotational symmetry--and the structure of the superlattice. The magnetic field and temperature dependence of the commensuration oscillations enables us to extract the Fermi-momenta and quasiparticle mass at an order of magnitude lower magnetic field and higher temperature than Shubnikov-de Haas quantum oscillations. Furthermore, we use a chiral superlattice to engineer asymmetric longitudinal magnetoresistance based on the charge of the quasiparticles and superlattice enantiomer. These results demonstrate nanopatterned superlattices as an effective method for fermiology, and also point towards new ways of engineering quantum transport in these systems based on the mutual properties of the superlattice and Fermi-surface. △ Less

Submitted 2 May, 2025; v1 submitted 27 January, 2025; originally announced January 2025.

Comments: 25 Pages, 4 main figures, 4 supplementary figures

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

Authors: R. Mathew Roy, X. Feng, M. Wenzel, V. Hasse, C. Shekhar, M. G. Vergniory, C. Felser, A. V. Pronin, M. Dressel

Abstract: 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 $σ(ω)$ obtained through infrared spectroscopy elucidates the influence of band modifications associated with the… ▽ More 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 $σ(ω)$ 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, $Δ_{\rm CDW}\approx$ 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. △ Less

Submitted 12 November, 2024; originally announced November 2024.

Comments: 11 pages including SM

Report number: Phys. Rev. Lett. 135, 116503

Momentum-Resolved Fingerprint of Mottness in Layer-Dimerized Nb$_3$Br$_8$

Authors: Mihir Date, Francesco Petocchi, Yun Yen, Jonas A. Krieger, Banabir Pal, Vicky Hasse, Emily C. McFarlane, Chris Körner, Jiho Yoon, Matthew D. Watson, Vladimir N. Strocov, Yuanfeng Xu, Ilya Kostanovski, Mazhar N. Ali, Sailong Ju, Nicholas C. Plumb, Michael A. Sentef, Georg Woltersdorf, Michael Schüler, Philipp Werner, Claudia Felser, Stuart S. P. Parkin, Niels B. M. Schröter

Abstract: In a well-ordered crystalline solid, insulating behaviour can arise from two mechanisms: electrons can either scatter off a periodic potential, thus forming band gaps that can lead to a band insulator, or they localize due to strong interactions, resulting in a Mott insulator. For an even number of electrons per unit cell, either band- or Mott-insulators can theoretically occur. However, unambiguo… ▽ More In a well-ordered crystalline solid, insulating behaviour can arise from two mechanisms: electrons can either scatter off a periodic potential, thus forming band gaps that can lead to a band insulator, or they localize due to strong interactions, resulting in a Mott insulator. For an even number of electrons per unit cell, either band- or Mott-insulators can theoretically occur. However, unambiguously identifying an unconventional Mott-insulator with an even number of electrons experimentally has remained a longstanding challenge due to the lack of a momentum-resolved fingerprint. This challenge has recently become pressing for the layer dimerized van der Waals compound Nb$_3$Br$_8$, which exhibits a puzzling magnetic field-free diode effect when used as a weak link in Josephson junctions, but has previously been considered to be a band-insulator. In this work, we present a unique momentum-resolved signature of a Mott-insulating phase in the spectral function of Nb$_3$Br$_8$: the top of the highest occupied band along the out-of-plane dimerization direction $k_z$ has a momentum space separation of $Δk_z=2π/d$, whereas the valence band maximum of a band insulator would be separated by less than $Δk_z=π/d$, where $d$ is the average spacing between the layers. As the strong electron correlations inherent in Mott insulators can lead to unconventional superconductivity, identifying Nb$_3$Br$_8$ as an unconventional Mott-insulator is crucial for understanding its apparent time-reversal symmetry breaking Josephson diode effect. Moreover, the momentum-resolved signature employed here could be used to detect quantum phase transition between band- and Mott-insulating phases in van der Waals heterostructures, where interlayer interactions and correlations can be easily tuned to drive such transition. △ Less

Submitted 21 October, 2024; originally announced October 2024.

Comments: 9 pages, 3 figures

Intrinsic negative magnetoresistance from the chiral anomaly of multifold fermions

Authors: F. Balduini, A. Molinari, L. Rocchino, V. Hasse, C. Felser, M. Sousa, C. Zota, H. Schmid, A. G. Grushin, B. Gotsmann

Abstract: 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… ▽ More 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. △ Less

Submitted 30 April, 2024; originally announced April 2024.

Journal ref: Nat Commun 15, 6526 (2024)

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

Authors: F. Balduini, L. Rocchino, A. Molinari, T. Paul, G. Mariani, V. Hasse, C. Felser, C. Zota, H. Schmid, B. Gotsmann

Abstract: 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 v… ▽ More 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. △ Less

Submitted 19 April, 2024; v1 submitted 15 January, 2024; originally announced January 2024.

Topological Metal MoP Nanowire for Interconnect

Authors: Hyeuk Jin Han, Sushant Kumar, Xiaoyang Ji, James L. Hart, Gangtae Jin, David J. Hynek, Quynh P. Sam, Vicky Hasse, Claudia Felser, David G. Cahill, Ravishankar Sundararaman, Judy J. Cha

Abstract: 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 semime… ▽ More 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. △ Less

Submitted 4 August, 2022; originally announced August 2022.

Comments: 4 figures