Publications

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Visualizing near-coexistence of massless Dirac electrons and ultra-massive saddle point electrons

Abhay Kumar Nayak, Jonathan Reiner, Hengxin Tan, Huixia Fu, Henry Ling, Chandra Shekhar, Claudia Felser, Tamar (Tami) Pereg-Barnea, Binghai Yan, Haim Beidenkopf, Nurit Avraham

SciPost Phys. 15, 178 (2023) · published 25 October 2023 | · pdf

Strong singularities in the electronic density of states amplify correlation effects and play a key role in determining the ordering instabilities in various materials. Recently high order van Hove singularities (VHSs) with diverging power-law scaling have been classified in single-band electron models. We show that the 110 surface of Bismuth exhibits high order VHS with an usually high density of states divergence $\sim (E)^{-0.7}$. Detailed mapping of the surface band structure using scanning tunneling microscopy and spectroscopy combined with first-principles calculations show that this singularity occurs in close proximity to Dirac bands located at the center of the surface Brillouin zone. The enhanced power-law divergence is shown to originate from the anisotropic flattening of the Dirac band just above the Dirac node. Such near-coexistence of massless Dirac electrons and ultra-massive saddle points enables to study the interplay of high order VHS and Dirac fermions.
Superfluid response of two-dimensional filamentary superconductors

Giulia Venditti, Ilaria Maccari, Alexis Jouan, Gyanendra Singh, Ramesh C. Budhani, Cheryl Feuillet-Palma, Jérôme Lesueur, Nicolas Bergeal, Sergio Caprara, Marco Grilli

SciPost Phys. 15, 239 (2023) · published 14 December 2023 | · pdf

Different classes of low-dimensional superconducting systems exhibit an inhomogeneous filamentary superconducting condensate whose macroscopic coherence still needs to be fully investigated and understood. Here, we present a thorough analysis of the superfluid response of a prototypical filamentary superconductor embedded in a two-dimensional metallic matrix. By mapping the system into an exactly solvable random impedance network, we show how the dissipative (reactive) response of the system non-trivially depends on both the macroscopic and microscopic characteristics of the metallic (superconducting) fraction. We compare our calculations with resonant microwave transport measurements performed on LaAlO$_3$/SrTiO$_3$ heterostructures over an extended range of temperatures and carrier densities finding that the filamentary character of superconductivity accounts for unusual peculiar features of the experimental data.
Reconstructing the potential configuration in a high-mobility semiconductor heterostructure with scanning gate microscopy

Gaëtan J. Percebois, Antonio Lacerda-Santos, Boris Brun, Benoit Hackens, Xavier Waintal, Dietmar Weinmann

SciPost Phys. 15, 242 (2023) · published 15 December 2023 | · pdf

The weak disorder potential seen by the electrons of a two-dimensional electron gas in high-mobility semiconductor heterostructures leads to fluctuations in the physical properties and can be an issue for nanodevices. In this paper, we show that a scanning gate microscopy (SGM) image contains information about the disorder potential, and that a machine learning approach based on SGM data can be used to determine the disorder. We reconstruct the electric potential of a sample from its experimental SGM data and validate the result through an estimate of its accuracy.
Vertical and lateral manipulation of single Cs atoms on the semiconductor InAs(111)A

Rian A. M. Ligthart, Cristophe Coinon, Ludovic Desplanque, Xavier Wallart, Ingmar Swart

SciPost Phys. 16, 096 (2024) · published 5 April 2024 | · pdf

The tip of the scanning tunneling microscope can be used to position atoms and molecules on surfaces with atomic scale precision. Here, we report the controlled vertical and lateral manipulation of single Cs atoms on the InAs(111)A surface. The Cs adatom adsorbs on the In-vacancy site of the InAs(111)A—(2x2) surface reconstruction. Lateral manipulation is possible in all directions over the surface, not just along high-symmetry directions. Both pushing and pulling modes were observed in the height profile of the tip. We assembled two artificial structures, demonstrating the reliability of the manipulation procedures. Structures remained intact to a temperature of at least 44 K.
Observation of non-local impedance response in a passive electrical circuit

Xiao Zhang, Boxue Zhang, Weihong Zhao, Ching Hua Lee

SciPost Phys. 16, 002 (2024) · published 4 January 2024 | · pdf

In media with only short-ranged couplings and interactions, it is natural to assume that physical responses must be local. Yet, we discover that this is not necessarily true, even in a system as commonplace as an electric circuit array. This work reports the experimental observation of non-local impedance response in a designed circuit network consisting exclusively of passive elements such as resistors, inductors and capacitors (RLC). Measurements reveal that the removal of boundary connections dramatically affects the two-point impedance between certain distant nodes, even in the absence of any amplification mechanism for the voltage signal. This non-local impedance response is distinct from the reciprocal non-Hermitian skin effect, affecting only selected pairs of nodes even as the circuit Laplacian exhibits universally broken spectral bulk-boundary correspondence. Surprisingly, not only are component parasitic resistances unable to erode the non-local response, but they in fact give rise to novel loss-induced topological modes at sufficiently large system sizes, constituting a new manifestation of the critical non-Hermitian skin effect. Our findings chart a new route towards attaining non-local responses in photonic or electrical metamaterials without involving non-linear, non-local, active or amplificative elements.
Single crystal growth of PrNiGe$_2$ compounds

Kousei Ishiwatari, Kaito Hoshii, Keisuke Ida, Masashi Ohashi

SciPost Phys. Proc. 11, 002 (2023) · published 5 June 2023 | · pdf

We report on the magnetic characterization of a praseodym intermetallic PrNiGe$_2$. Several single crystals are grown by the Czochralski method from Ni- deficient sample as the initial one. X-ray analysis of PrNi$_{0.8}$Ge$_2$ indicated the CeNiSi$_2$-type structure as the only phase. The magnetic measurements clearly indicate that PrNi$_x$Ge$_2$ exhibits a ferromagnetic orderings at 12 K, which is independent with of composition of Ni. A strong anisotropy along the three principal crystallographic directions was observed, reflecting the orthorhombic symmetry of the crystal structure. The $b$-axis was found to be the easy axis of magnetization.
Large second-order Josephson effect in planar superconductor-semiconductor junctions

P. Zhang, A. Zarassi, L. Jarjat, V. Van de Sande, M. Pendharkar, J. S. Lee, C. P. Dempsey, A. P. McFadden, S. D. Harrington, J. T. Dong, H. Wu, A. -H. Chen, M. Hocevar, C. J. Palmstrøm, S. M. Frolov

SciPost Phys. 16, 030 (2024) · published 25 January 2024 | · pdf

We investigate the current-phase relations of Al/InAs-quantum well planar Josephson junctions fabricated using nanowire shadowing technique. Based on several experiments, we conclude that the junctions exhibit an unusually large second-order Josephson harmonic, the $\sin(2\varphi)$ term. First, superconducting quantum interference devices (dc-SQUIDs) show half-periodic oscillations, tunable by gate voltages as well as magnetic flux. Second, Josephson junction devices exhibit kinks near half-flux quantum in supercurrent diffraction patterns. Third, half-integer Shapiro steps are present in the junctions. Similar phenomena are observed in Sn/InAs quantum well devices. We perform data fitting to a numerical model with a two-component current phase relation. Analysis including a loop inductance suggests that the sign of the second harmonic term is negative. The microscopic origins of the observed effect remain to be understood. We consider alternative explanations which can account for some but not all of the evidence.
High field magnetism of the triangular lattice antiferromagnet CsFeCl$_3$ under high pressure

Katsuki Nihongi, Takanori Kida, Yasuo Narumi, Nobuyuki Kurita, Hidekazu Tanaka, Yoshiya Uwatoko, Koichi Kindo, Masayuki Hagiwara

SciPost Phys. Proc. 11, 012 (2023) · published 5 June 2023 | · pdf

Magnetization measurements of the triangular lattice antiferromagnet CsFeCl$_{3}$ at 1.4 K for $H \parallel c$ were performed in pulsed high magnetic fields of up to 51 T under high pressures of up to 0.80 GPa. At ambient pressure, the magnetization below $H_{\rm c1}$ = 4 T is small, and then it changes steeply between $H_{\rm c1}$ and $H_{\rm c2}$ = 12 T, followed by a gradual increase in magnetization up to $H_{\rm m}$ = 33 T where a two-step metamagnetic transition occurs. With increasing pressure, the $H_{\rm c1}$ and $H_{\rm m}$ shifted to the low-field side, while the $H_{\rm c2}$ shifted to the opposite side. The change in $H_{\rm m}$ may depend on the difference in the single-ion anisotropies on pressure in the lowest $J$ = 1 and the excited $J$ = 2 states.
Local investigation of the crystal electric field ground-state in CeCu(Sb,Bi)$_2$ heavy fermions

Davi Zau, G. S. Freitas, P. G. Pagliuso, R. R. Urbano

SciPost Phys. Proc. 11, 017 (2023) · published 6 June 2023 | · pdf

In this work, we performed systematic Nuclear Magnetic Resonance (NMR) and magnetic susceptibility experiments in CeCuSb$_2$ single-crystals. The main findings were compared to previous report for CeCuBi$_2$. The NMR spectra and transferred hyperfine coupling for the $^{63}$Cu nuclei were obtained aiming to observe their correlation with the crystal electric field (CEF) effects on the Ce$^{3+}$ ($J=5/2$) multiplet. Besides, in an attempt to elucidate the magnetic structure through NMR measurements at different magnetic fields orientations, we observed a magnetic transition at $T\approx 8\,$K higher than the Néel temperature $T_N$ measured by magnetic susceptibility indicating the development of short-range magnetic ordering above T$_N$. In addition, the wipe out of the main NMR resonance line and a persistent spin-echo signal throughout the whole frequency-swept range suggest the possibility of an incommensurate magnetic structure in CeCuSb$_2$. Furthermore, the small transferred hyperfine coupling constant found for CeCuSb$_2$ indicates a scenario with more localized Ce$^{3+}$ 4f electrons than for CeMIn$_5$ (M $=$ Co,Rh,Ir) heavy fermions family. Additionally, subtle changes in the hybridization between the $^{63}$Cu with the 4f$^1$ Ce$^{3+}$ electrons in distinct magnetic field orientations allowed us to provide detailed information and map out the 4f CEF orbital ground-state of CeCu(Sb,Bi)$_2$ via NMR measurements.
Pressure-dependent structural and electronic instabilities in LaSb$_2$

Theodore I. Weinberger, Christian K. de Podesta, Jiasheng Chen, Stephen A. Hodgson, F. Malte Grosche

SciPost Phys. Proc. 11, 018 (2023) · published 6 June 2023 | · pdf

LaSb$_2$ exhibits a large, non-saturating, linear magnetoresistance at low temperatures, defying the expectations of Fermi liquid theory. This is thought to be caused by charge density wave order emerging below an abrupt, hysteretic anomaly in the resistivity at $\sim$ 355 K. We find that, under hydrostatic pressure, this anomaly becomes much more pronounced, develops strong hysteresis, and shifts rapidly towards lower temperatures. The anomaly is fully suppressed by only 6 kbar. Moreover, we observe a second transition anomaly at lower temperature, which likewise disappears under pressure. These findings are discussed in the context of a structural transition recently discovered in the sister material CeSb$_2$ and of density functional theory calculations, which indicate that the SmSb$_2$-type structure adopted by LaSb$_2$ at ambient conditions is unstable at moderate applied pressures.

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