arXiv subjects

Ludwig Holleis

Publications and source records attributed to Ludwig Holleis.

13 recordsLinked to original sources

arXiv2026

Cryogenic shock exfoliation for ultrahigh mobility rhombohedral graphite nanoelectronics

Rhombohedral multilayer graphene (RMG) offers a highly tunable platform for correlated electron physics, featuring field-effect control of magnetic, superconducting, and topological phases[1-24]. The promise of these materials has been held back by the limited abundance of rhombohedral stacking in natural graphite, which constrains both sample yield and useful area. Here we introduce 'cryogenic shock exfoliation' to produce large area rhombohedral graphene flakes which, combined with a low-pressure van der Waals assembly technique that preserves stacking order, enable highly uniform devices exceeding 1300 $\mu m^2$ with fabrication yields of 90%. Using scanning nanoSQUID-on-tip imaging, we demonstrate uniform spin magnetism over the full central 10 times 10 $\mu m^2$ area of our devices. Transverse magnetic focusing reveals a disorder mean free path exceeding 200 $\mu m$ at low temperatures. Within the flat surface bands of RMG[20], we observe a size-driven crossover from Poiseuille to porous electron flow in the intermediate-temperature regime of strong electron-electron hydrodynamics[16, 25], providing a further signature of ultrahigh device quality. Our approach overcomes a key materials bottleneck in the fabrication of mesoscopic rhombohedral graphene devices, paving the way for incorporating strongly correlated phases into two-dimensional nanoelectronics.

cond-mat.mes-hall↗

arXiv2026

Imaging flat band electron hydrodynamics in biased bilayer graphene

Hydrodynamic electron transport arises when carrier kinetics are dominated by interelectron collisions rather than the relaxation of momentum out of the electron system. In recent years, signatures of electron hydrodynamics have been reported in graphene devices owing to the low disorder and weak electron-phonon coupling. However, these experiments have been performed in regimes where the carrier mass is light, and the electron-electron collision length--though smaller than corresponding lengths for phonon or impurity scattering--remains large in absolute terms, typically several hundred nanometers. This restricts hydrodynamic transport phenomena to large length scales, limiting miniaturization of devices based on hydrodynamic flow. The advent of dual-gated rhombohedral graphene multilayers introduces a new route toward enhanced hydrodynamic behavior via their large--and tunable--effective mass. Here, we employ a scanning superconducting magnetic sensor to image local current flow in dual-gated bilayer graphene. Exploiting a sample geometry sensitive to both laminar and vortical flow, we identify three distinct transport regimes--ballistic, hydrodynamic, and diffusive--across the full phase space spanned by carrier density and displacement field. The strongest hydrodynamic transport is observed in the flat band regime, where fitting our results to a unified Boltzmann transport model reveals the electron-electron scattering length to be comparable to the Fermi wavelength of ~50 nm. High-current measurements, meanwhile, reveal striking nonlinearities in the flow pattern. Our results pave the way for miniaturized electronic devices based on linear and nonlinear electron hydrodynamics.

cond-mat.mes-hall↗

arXiv2025

Flat band surface state superconductivity in thick rhombohedral graphene

Rhombohedral multilayer graphene has recently emerged as a rich platform for studying correlation driven magnetic, topological and superconducting states. While most experimental efforts have focused on devices with N$\leq 9$ layers, the electronic structure of thick rhombohedral graphene features flat-band surface states even in the infinite layer limit. Here, we use layer resolved capacitance measurements to directly detect these surface states for $N\approx 13$ layer rhombohedral graphene devices. Using electronic transport and local magnetometry, we find that the surface states host a variety of ferromagnetic phases, including both valley imbalanced quarter metals and broad regimes of density in which the system spontaneously spin polarizes. We observe several superconducting states localized to a single surface state. These superconductors appear on the unpolarized side of the density-tuned spin transitions, and show strong violations of the Pauli limit consistent with a dominant attractive interaction in the spin-triplet, valley-singlet pairing channel. In contrast to previous studies of rhombohedral multilayers, however, we find that superconductivity can persist to zero displacement field where the system is inversion symmetric. Energetic considerations suggest that superconductivity in this regime is described by the existence of two independent surface superconductors coupled via tunneling through the insulating single crystal graphite bulk.

cond-mat.supr-con↗

arXiv2025

Resolving Intervalley Gaps and Many-Body Resonances in Moiré Superconductor

Magic-angle twisted multilayer graphene stands out as a highly tunable class of moiré materials that exhibit strong electronic correlations and robust superconductivity. However, understanding the relations between the low-temperature superconducting phase and the preceding correlated phases established at higher temperatures remains a challenge. Here, we employ scanning tunneling microscopy and spectroscopy to track the formation sequence of correlated phases established by the interplay of dynamic correlations, intervalley coherence, and superconductivity in magic-angle twisted trilayer graphene (MATTG). We discover the existence of two well-resolved gaps pinned at the Fermi level within the superconducting doping range. While the outer gap, previously associated with pseudogap phase, persists at high temperatures and magnetic fields, the newly revealed inner gap is more fragile in line with superconductivity MATTG transport experiments. Andreev reflection spectroscopy taken at the same location confirms a clear trend that closely follows the doping behaviour of the inner gap, and not the outer one. Moreover, spectroscopy taken at nanoscale domain boundaries further corroborates the contrasting behavior of the two gaps, with the inner gap remaining resilient to structural variations, as expected from the finite superconducting coherence length. By comparing our findings with recent topological heavy-fermion models, we identify that the outer gap originates from the splitting of the Abrikosov-Suhl-Kondo resonance due to the breaking of the valley symmetry arising from correlation-driven effects. Our results suggest an intricate but tractable hierarchy of correlated phases in twisted multilayer graphene.

cond-mat.supr-con↗

arXiv2025

Nanoscale infrared and microwave imaging of stacking faults in multilayer graphene

Graphite occurs in a range of metastable stacking orders characterized by both the number and direction of shifts between adjacent layers by the length of a single carbon-carbon bond. At the extremes are Bernal (or ``ABAB...'') stacking, where the direction of the interlayer shift alternates with each layer, and rhombohedral (or ``ABCABC...'') stacking order where the shifts are always in the same direction. However, for an N-layer system, there are in principle $N-1$ unique metastable stacking orders of this type. Recently, it has become clear that stacking order has a strong effect on the low energy electronic band structure with single-layer shifts completely altering the electronic properties. Most experimental work has focused on the extremal stacking orders in large part due to the difficulty of isolating and identifying intermediate orders. Motivated by this challenge, here we describe two atomic force microscopy (AFM) based techniques to unambiguously distinguish stacking orders and defects in graphite flakes. Photo-thermal infrared atomic force microscope (AFM-IR) is able to distinguish stacking orders across multiple IR wavelengths and readily provides absolute contrast via IR spectral analysis. Scanning microwave impedance microscopy (sMIM) can distinguish the relative contrast between Bernal, intermediate and rhombohedral domains. We show that both techniques are well suited to characterizing graphite van der Waals devices, providing high contrast determination of stacking order, subsurface imaging of graphene flakes buried under a hexagonal boron nitride (hBN) dielectric layer, and identifying nanoscale domain walls. Our results pave the way for the reliable fabrication of graphene multilayer devices of definite interlayer registry.

cond-mat.mes-hall↗

arXiv2025

Fluctuating magnetism and Pomeranchuk effect in multilayer graphene

Magnetism typically arises from the effect of exchange interactions on highly localized fermionic wave functions in f- and d-atomic orbitals. In rhombohedral multilayer graphene (RMG), in contrast, magnetism-manifesting as spontaneous polarization into one or more spin and valley flavors[1-7]-originates from itinerant electrons near a Van Hove singularity. Here, we show experimentally that the electronic entropy in this system shows signatures typically associated with disordered local magnetic moments, unexpected for electrons in a fully itinerant metal. Specifically, we find a contribution $Δ$ S $\approx$ 1k$_B$/charge carrier that onsets at the Curie temperature and survives over one order of magnitude in temperature. First order phase transitions show an isospin `Pomeranchuk effect' in which the fluctuating moment phase is entropically favored over the nearby symmetric Fermi liquid[8, 9]. Our results imply that despite the itinerant nature of the electron wave functions, the spin- and valley polarization of individual electrons are decoupled, a phenomenon typically associated with localized moments, as happens, for example, in solid 3He[10]. Transport measurements, surprisingly, show a finite temperature resistance minimum within the fluctuating moment regime, which we attribute to the interplay of fluctuating magnetic moments and electron phonon scattering. Our results highlight the universality of soft isospin modes to two-dimensional flat band systems.

cond-mat.str-el↗

arXiv2024

Nematicity and Orbital Depairing in Superconducting Bernal Bilayer Graphene with Strong Spin Orbit Coupling

Superconductivity (SC) is a ubiquitous feature of graphite allotropes, having been observed in Bernal bilayers[1], rhombohedral trilayers[2], and a wide variety of angle-misaligned multilayers[3-6]. Despite significant differences in the electronic structure across these systems, supporting the graphite layer on a WSe$_2$ substrate has been consistently observed to expand the range of SC in carrier density and temperature[7-10]. Here, we report the observation of two distinct superconducting states (denoted SC$_1$ and SC$_2$) in Bernal bilayer graphene with strong proximity-induced Ising spin-orbit coupling. Quantum oscillations show that while the normal state of SC$_1$ is consistent with the single-particle band structure, SC$_2$ emerges from a nematic normal state with broken rotational symmetry. Both superconductors are robust to in-plane magnetic fields, violating the paramagnetic limit; however, neither reach fields expected for spin-valley locked Ising superconductors. We use our knowledge of the Fermi surface geometry of SC$_1$ to argue that superconductivity is limited by orbital depairing arising from the imperfect layer polarization of the electron wavefunctions. Finally, a comparative analysis of transport and thermodynamic compressibility measurements in SC$_2$ shows that the proximity to the observed isospin phase boundaries, observed in other rhombohedral graphene allotropes, is likely coincidental, constraining theories of unconventional superconducting pairing mechanisms in theses systems.

cond-mat.supr-con↗

arXiv2023

Intervalley coherence and intrinsic spin-orbit coupling in rhombohedral trilayer graphene

Rhombohedral graphene multilayers provide a clean and highly reproducible platform to explore the emergence of superconductivity and magnetism in a strongly interacting electron system. Here, we use electronic compressibility and local magnetometry to explore the phase diagram of this material class in unprecedented detail. We focus on rhombohedral trilayer in the quarter metal regime, where the electronic ground state is characterized by the occupation of a single spin and valley isospin flavor. Our measurements reveal a subtle competition between valley imbalanced (VI) orbital ferromagnets and intervalley coherent (IVC) states in which electron wave functions in the two momentum space valleys develop a macroscopically coherent relative phase. Contrasting the in-plane spin susceptibility of the IVC and VI phases reveals the influence of graphene's intrinsic spin-orbit coupling, which drives the emergence of a distinct correlated phase with hybrid VI and IVC character. Spin-orbit also suppresses the in-plane magnetic susceptibility of the VI phase, which allows us to extract the spin-orbit coupling strength of $λ\approx 50μ$eV for our hexagonal boron nitride-encapsulated graphene system. We discuss the implications of finite spin-orbit coupling on the spin-triplet superconductors observed in both rhombohedral and twisted graphene multilayers.

cond-mat.mes-hall↗

arXiv2023

Imaging inter-valley coherent order in magic-angle twisted trilayer graphene

Magic-angle twisted trilayer graphene (MATTG) exhibits a range of strongly correlated electronic phases that spontaneously break its underlying symmetries. The microscopic nature of these phases and their residual symmetries stands as a key outstanding puzzle whose resolution promises to shed light on the origin of superconductivity in twisted materials. Here we investigate correlated phases of MATTG using scanning tunneling microscopy and identify striking signatures of interaction-driven spatial symmetry breaking. In low-strain samples, over a filling range of about 2-3 electrons or holes per moiré unit cell, we observe atomic-scale reconstruction of the graphene lattice that accompanies a correlated gap in the tunneling spectrum. This short-scale restructuring appears as a Kekulé supercell -- implying spontaneous inter-valley coherence between electrons -- and persists in a wide range of magnetic fields and temperatures that coincide with the development of the gap. Large-scale maps covering several moiré unit cells further reveal a slow evolution of the Kekulé pattern, indicating that atomic-scale reconstruction coexists with translation symmetry breaking at the much longer moiré scale. We employ auto-correlation and Fourier analyses to extract the intrinsic periodicity of these phases and find that they are consistent with the theoretically proposed incommensurate Kekulé spiral order. Moreover, we find that the wavelength characterizing moiré-scale modulations monotonically decreases with hole doping away from half-filling of the bands and depends only weakly on the magnetic field. Our results provide essential insights into the nature of MATTG correlated phases in the presence of strain and imply that superconductivity emerges from an inter-valley coherent parent state.

cond-mat.str-el↗

arXiv2021

Isospin magnetism and spin-triplet superconductivity in Bernal bilayer graphene

We report the observation of spin-polarized superconductivity in Bernal bilayer graphene when doped to a saddle-point van Hove singularity generated by large applied perpendicular electric field. We observe a cascade of electrostatic gate-tuned transitions between electronic phases distinguished by their polarization within the isospin space defined by the combination of the spin and momentum-space valley degrees of freedom. While all of these phases are metallic at zero magnetic field, we observe a transition to a superconducting state at finite $B_\parallel\approx 150$mT applied parallel to the two dimensional sheet. Superconductivity occurs near a symmetry breaking transition, and exists exclusively above the $B_\parallel$-limit expected of a paramagnetic superconductor with the observed $T_C\approx 30$mK, implying a spin-triplet order parameter.

cond-mat.mes-hall↗

arXiv2021

Anomalous and Anisotropic Nonlinear Susceptibility in the Proximate Kitaev Magnet $α$-RuCl$_3$

The leading order nonlinear (NL) susceptibility, $χ_3$, in a paramagnet is negative and diverges as $T \rightarrow 0$. This divergence is destroyed when spins correlate and the NL response provides unique insights into magnetic order. Dimensionality, exchange interaction, and preponderance of quantum effects all imprint their signatures in the NL magnetic response. Here, we study the NL susceptibilities in the proximate Kitaev magnet $α$-RuCl$_3$ which differs from the expected antiferromagnetic behavior. For $T< T_c$ = 7.5 K and field $B$ in the ab-plane, we obtain contrasting NL responses in low ($<$ 2 ${T}$) and high field regions. For low fields the NL behavior is dominated by a quadratic response (positive $χ_2$), which shows a rapid rise below $T_c$. This large $χ_2 >0$ implies a broken sublattice symmetry of magnetic order at low temperatures. Classical Monte Carlo (CMC) simulations in the standard ${K-H-Γ}$ model secure such a quadratic ${B}$ dependence of ${M}$, only for ${T}$ $\approx$ ${T}_c$ with $χ_2$ being zero as ${T}$ $\rightarrow$ 0. It is also zero for all temperatures in exact diagonalization calculations. On the other hand, we find an exclusive cubic term ($χ_3$) describes the high field NL behavior well. $χ_3$ is large and positive both below and above ${T}_c$ crossing zero only for ${T}$ $>$ 50 K. In contrast, for $B$~$\parallel$~c-axis, no separate low/high field behaviors is measured and only a much smaller $χ_3$ is apparent.

cond-mat.str-el↗

arXiv2019

Magneto Acoustic Quantum Oscillations in High Fields and the Fermi Surface of UPt$_3$

We report magneto-acoustic quantum oscillations (MAQO) in the heavy fermion system UPt$_3$ in magnetic fields B, upto 33 T. For B in the ab-plane of the hexagonal crystal MAQO in the sound velocity commence at $\approx$ 12 T and grow with field. However, in contrast to typical Lifshitz-Kosevich behaviour the frequency corresponding to the dominant oscillation increases continuously as the metamagnetic transition (MMT) at 20 T is reached. This dominant MAQO arises from the $δ$ orbit of band 1 with a large effective mass of 33 m$_e$, for B $<$ 20 T and disappears after the MMT. Thus, the MMT involves a significant change of the Fermi surface, primarily in band 1. For B $\parallel$ ab-plane and $<$ 20 T we reproduce successfully orbits established through previous de Haas-van Alphen and Shubnikov de Haas measurements. We also observe several new orbits, some that can be identified with existing band theory and others not seen previously with completely new frequencies. For B $\parallel$ c-axis we observe MAQOs which also commence at $\approx$ 12 T and grow gradually but break suddenly into a large amplitude and change in frequency at 24.8 T. These enhanced oscillations get weaker again at 30 T. These abrupt changes at 24.8 T and 30 T are signatures of Lifshitz transitions and coincide with the newly discovered spin density wave states for this orientation reported by us recently.

cond-mat.str-el↗

arXiv2018

Field Angle Tuned Metamagnetism and Lifschitz Transitions in UPt3

Strongly correlated electronic systems can harbor a rich variety of quantum spin states. Understanding and controlling such spin states in quantum materials is of great current interest. Focusing on the simple binary system UPt3 with ultrasound (US) as a probe we identify clear signatures in field sweeps demarkating new high field spin states. Magnetostriction (MS) measurements performed up to 65 T also show signatures at the same fields confirming these state transitions. At the very lowest temperatures (<200 mK) we also observe magneto-acoustic quantum oscillations which for theta = 90° and vicinity abruptly become very strong in the 24.8-30 T range. High resolution magnetization measurements for this same angle reveal a continuous variation of the magnetization implying the subtle nature of the implied transitions. With B rotated away from the c-axis, the US signatures occur at nearly the same field. These state transitions merge with the separate sequence of the well known metamagnetic transition which commences at 20 T for theta = 0° but moves to higher fields as 1/cos(θ). This merge, suggesting a tricritical behavior, occurs at θ~ 51° from the ab-plane. This is an unique off-symmetry angle where the length change is precisely zero due to the anisotropic nature of MS in UPt3 for all magnetic field values.

cond-mat.str-el↗