arXiv subjects

Jurgen H. Smet

Publications and source records attributed to Jurgen H. Smet.

At least 19 recordsLinked to original sources

arXiv2026

Magnetic field-driven phase switching in the antiferromagnetic Mott insulator Ca$_3$(Ru$_{0.99}$Ti$_{0.01}$)$_2$O$_7$

A bandwidth-controlled antiferromagnetic Mott-insulating phase in Ca$_3$(Ru$_{1-x}$Ti$_x$)$_2$O$_7$ is realized through isovalent substitution at the Ru site. For a dilute substitution with only 1% Ti, the Mott insulator ground state remains nearly degenerate with the ground state of pristine Ca$_3$Ru$_2$O$_7$, where the Ru moments are ferromagnetically aligned within the metallic RuO$_2$ bilayers stacked in an antiferromagnetic fashion. The exceptionally shallow free energy landscape of this doped compound arises from intertwined electron-electron and electron-lattice interactions. This makes its magnetic and transport properties highly sensitive to external perturbations. We systematically investigated magnetic-field-induced phase switching in Ca$_3$(Ru$_{0.99}$Ti$_{0.01}$)$_2$O$_7$ to explore its magnetic $H$-$T$ phase diagram. With the field applied along the easy $b$-axis, parallel to the antiferromagnetic moments, the magnetization exhibits a first-order spin-flop transition at $\approx $ 6 T, indicating reorientation of the Ru moments perpendicular to the field. The transition is accompanied by a decrease in the electrical resistance, but the spin-flop phase remains insulating. Above 10.5 T, all Ru moments align with the $b$-axis, resulting in a forced ferromagnetic metallic phase. In contrast, neither spin-flop nor forced-ferromagnetic phases are observed up to 14 T, when the field is applied along the $a$-axis. While the electronic kinetic energy and the electron-lattice coupling contribute to the free-energy balance of this system, the resulting $H$-$T$ phase diagram is remarkably simple and closely resembles that of a canonical anisotropic antiferromagnet, albeit with substantially renormalized critical fields.

cond-mat.str-el↗

arXiv2025

Magic continuum in multi-moiré twisted trilayer graphene

Moiré lattices provide a highly tunable platform for exploring the interplay between electronic correlations and band topology. Introducing a second moiré pattern extends this paradigm: interference between the two moiré patterns produces a supermoiré modulation, opening a route to further tailor electronic properties. Twisted trilayer graphene generally exemplifies such a system: two distinct moiré patterns arise from the relative twists between adjacent graphene layers. Here, we report the observation of correlated phenomena across a wide range of twisted trilayer graphene devices whose twist angles lie along two continuous lines in the twist-angle parameter space. Depending on the degree of lattice relaxation, twisted trilayer graphene falls into two classes: moiré polycrystals, composed of periodic domains with locally commensurate moiré order, and moiré quasicrystals, characterized by smoothly varying local moiré configurations. In helically twisted moiré polycrystals, we observe an anomalous Hall effect, consistent with topological bands arising from domains with broken $xy$-inversion symmetry. In contrast, superconductivity appears generically in our moiré quasicrystals. A subset of these systems exhibits signatures of spatially modulated superconductivity, which we attribute to the supermoiré structure. Our findings uncover the organizing principles of the observed correlated phases in twisted trilayer graphene, highlight the critical roles of the supermoiré modulation and lattice relaxation, and suggest a broader framework in which magic conditions arise not as isolated points but as extended manifolds within the multi-dimensional twist-angle space of complex moiré materials.

cond-mat.mes-hall↗

arXiv2024

Observation of 1/3 fractional quantum Hall physics in balanced large angle twisted bilayer graphene

Magnetotransport of conventional semiconductor based double layer systems with barrier suppressed interlayer tunneling has been a rewarding subject due to the emergence of an interlayer coherent state that behaves as an excitonic superfluid. Large angle twisted bilayer graphene offers unprecedented strong interlayer Coulomb interaction, since both layer thickness and layer spacing are of atomic scale and a barrier is no more needed as the twist induced momentum mismatch suppresses tunneling. The extra valley degree of freedom also adds richness. Here we report the observation of fractional quantum Hall physics at 1/3 total filling for balanced layer population in this system. Monte Carlo simulations support that the ground state is also an excitonic superfluid but the excitons are composed of fractional rather than elementary charges. The observed phase transitions with an applied displacement field at this and other fractional fillings are also addressed with simulations. They reveal ground states with different topology and symmetry properties.

cond-mat.mes-hall↗

arXiv2023

Non-linear Landau fan diagram for graphene electrons exposed to a moiré potential

Due to Landau quantization, the conductance of two-dimensional electrons exposed to a perpendicular magnetic field exhibits oscillations that generate a fan of linear trajectories when plotted in the parameter space spanned by density and magnetic field. This fan looks identical irrespective of the electron dispersion details that determines the field dependence of the Landau level energy. This is no surprise, since the position of conductance minima solely depends on the level degeneracy which is linear in flux. The fractal energy spectrum that emerges within each Landau band when electrons are also exposed to a two-dimensional superlattice potential produces numerous additional oscillations, but they too create just linear fans for the same reason. Here, we report on conductance oscillations of graphene electrons exposed to a moiré potential that defy this general rule of flux linearity and attribute the anomalous behavior to the simultaneous occupation of multiple minibands and magnetic breakdown.

cond-mat.mes-hall↗

arXiv2023

Resonant Inelastic X-ray Scattering from Electronic Excitations in $α$-RuCl$_3$ Nanolayers

We present Ru $L_3$-edge resonant inelastic x-ray scattering (RIXS) measurements of spin-orbit and d-d excitations in exfoliated nanolayers of the Kitaev spin-liquid candidate RuCl$_3$. Whereas the spin-orbit excitations are independent of thickness, we observe a pronounced red-shift and broadening of the d-d excitations in layers with thickness below $\sim$7 nm. Aided by model calculations, we attribute these effects to distortions of the RuCl$_6$ octahedra near the surface. Our study paves the way towards RIXS investigations of electronic excitations in various other 2D materials and heterostructures.

cond-mat.str-el↗

arXiv2023

Gate-tuned ambipolar superconductivity with strong pairing interaction in intrinsic gapped monolayer 1T'-MoTe2

Gate tunable two-dimensional (2D) superconductors offer significant advantages when studying superconducting phase transitions. Here, we address superconductivity in exfoliated 1T'-MoTe2 monolayers with an intrinsic band gap of ~7.3 meV using electrostatic doping. Despite large differences in the dispersion of the conduction and the valence bands, superconductivity can be achieved easily for both electrons and holes. The onset of superconductivity occurs near 7-8K for both charge carrier types. This temperature is much higher than in bulk samples. Also the in-plane upper critical field is strongly enhanced and exceeds the BCS Pauli limit in both cases. Gap information is extracted using point-contact spectroscopy. The gap ratio exceeds multiple times the value expected for BCS weak-coupling. All these observations suggest a strong enhancement of the pairing interaction.

cond-mat.supr-con↗

arXiv2021

Odd integer quantum Hall states with interlayer coherence in twisted bilayer graphene

We report on the quantum Hall effect in two stacked graphene layers rotated by 2 degree. The tunneling strength among the layers can be varied from very weak to strong via the mechanism of magnetic breakdown when tuning the density. Odd-integer quantum Hall physics is not anticipated in the regime of suppressed tunneling for balanced layer densities, yet it is observed. We interpret this as a signature of Coulomb interaction induced interlayer coherence and Bose Einstein condensation of excitons that form at half filling of each layer. A density imbalance gives rise to reentrant behavior due to a phase transition from the interlayer coherent state to incompressible behavior caused by simultaneous condensation of both layers in different quantum Hall states. With increasing overall density, magnetic breakdown gains the upper hand. As a consequence of the enhanced interlayer tunneling, the interlayer coherent state and the phase transition vanish.

cond-mat.mes-hall↗

arXiv2020

Rectification by hydrodynamic flow in an encapsulated graphene Tesla valve

Systems in which interparticle interactions prevail can be described by hydrodynamics. This regime is typically difficult to access in the solid state for electrons. However, the high purity of encapsulated graphene combined with its advantageous phonon properties make it possible, and hydrodynamic corrections to the conductivity of graphene have been observed. Examples include electron whirlpools, enhanced flow through constrictions as well as a Poiseuille flow profile. An electronic device relying specifically on viscous behaviour and acting as a viscometer has however been lacking. Here, we implement the analogue of the Tesla valve. It exhibits nonreciprocal transport and can be regarded as an electronic viscous diode. Rectification occurs at carrier densities and temperatures consistent with the hydrodynamic regime, and disappears both in the ballistic and diffusive transport regimes. In a device in which the electrons are exposed to a Moiré superlattice, the Lifshitz transition when crossing the Van Hove singularity is observed in the rectifying behaviour.

cond-mat.mes-hall↗

arXiv2019

Reliable postprocessing improvement of van der Waals heterostructures

The successful assembly of heterostructures consisting of several layers of different 2D materials in arbitrary order by exploiting van der Waals forces has truly been a game changer in the field of low dimensional physics. For instance, the encapsulation of graphene or MoS2 between atomically flat hexagonal boron nitride (hBN) layers with strong affinity and graphitic gates that screen charge impurity disorder provided access to a plethora of interesting physical phenomena by drastically boosting the device quality. The encapsulation is accompanied by a self-cleansing effect at the interfaces. The otherwise predominant charged impurity disorder is minimized and random strain fluctuations ultimately constitute the main source of residual disorder. Despite these advances, the fabricated heterostructures still vary notably in their performance. While some achieve record mobilities, others only possess mediocre quality. Here, we report a reliable method to improve fully completed van der Waals heterostructure devices with a straightforward post-processing surface treatment based on thermal annealing and contact mode AFM. The impact is demonstrated by comparing magnetotransport measurements before and after the AFM treatment on one and the same device as well as on a larger set of treated and untreated devices to collect device statistics. Both the low temperature properties as well as the room temperature electrical characteristics, as relevant for applications, improve on average substantially. We surmise that the main beneficial effect arises from reducing nanometer scale corrugations at the interfaces, i.e. the detrimental impact of random strain fluctuations.

cond-mat.mtrl-sci↗

arXiv2019

Even denominator fractional quantum Hall states in higher Landau levels of graphene

An important development in the field of the fractional quantum Hall effect has been the proposal that the 5/2 state observed in the Landau level with orbital index $n = 1$ of two dimensional electrons in a GaAs quantum well originates from a chiral $p$-wave paired state of composite fermions which are topological bound states of electrons and quantized vortices. This state is theoretically described by a "Pfaffian" wave function or its hole partner called the anti-Pfaffian, whose excitations are neither fermions nor bosons but Majorana quasiparticles obeying non-Abelian braid statistics. This has inspired ideas on fault-tolerant topological quantum computation and has also instigated a search for other states with exotic quasiparticles. Here we report experiments on monolayer graphene that show clear evidence for unexpected even-denominator fractional quantum Hall physics in the $n=3$ Landau level. We numerically investigate the known candidate states for the even-denominator fractional quantum Hall effect, including the Pfaffian, the particle-hole symmetric Pfaffian, and the 221-parton states, and conclude that, among these, the 221-parton appears a potentially suitable candidate to describe the experimentally observed state. Like the Pfaffian, this state is believed to harbour quasi-particles with non-Abelian braid statistics

cond-mat.mes-hall↗

arXiv2019

Acousto-electric study of microwave-induced current domains

Surface Acoustic Waves (SAW) have been utilized to investigate the properties of a two-dimensional electron system, subjected to a perpendicular magnetic field and monochromatic microwave radiation, in the regime where the so-called microwave-induced zero-resistance states form. Contrary to conventional magneto-transport in Hall bar and van der Pauw geometries, the collimated SAW beam probes only the bulk of the electronic system exposed to this wave. Clear signatures appear in the SAW propagation velocity corroborating that neither contacts, nor sample edges are a root source for their emergence. By virtue of the directional nature of this probing method and with the assistance of theoretical modelling, we were also able to demonstrate that the SAW response depends on the angle between its propagation vector and the orientation of domains which spontaneously form when zero-resistance is observed in transport. This confirms in unprecedented manner the formation of an inhomogeneous phase under these non-equilibrium conditions.

cond-mat.mes-hall↗

arXiv2019

Anomalously large spin-current voltages on the surface of SmB$_6$

The spin-polarized surface states of topological insulators have attracted interest both from a fundamental and applied point of view. A recent proposal describes a method of probing these surface states with ferromagnetic contacts, which was subsequently applied to a variety of materials. In this study, we use this method on the potential topological insulator SmB$_6$ with a new design based on the Corbino geometry. Though the signal behaves as predicted for all orientations of current and magnetic field, its magnitude is much larger than expected. Possible parasitic effects such as stray field-induced Hall voltages are excluded, leaving the origin of the observations uncertain. This corroborates the need for careful analysis when interpreting results of similar experiments.

cond-mat.mes-hall↗

arXiv2019

Spin-split band hybridization in graphene proximitized with $α$-RuCl$_3$ nanosheets

Proximity effects induced in the 2D Dirac material graphene potentially open access to novel and intriguing physical phenomena. Thus far, the coupling between graphene and ferromagnetic insulators has been experimentally established. However, only very little is known about graphene's interaction with antiferromagnetic insulators. Here, we report a low temperature study of the electronic properties of high quality van der Waals heterostructures composed of a single graphene layer proximitized with $α$-RuCl$_3$. The latter is known to become antiferromagnetically ordered below 10 K. Shubnikov de Haas oscillations in the longitudinal resistance together with Hall resistance measurements provide clear evidence for a band realignment that is accompanied by a transfer of electrons originally occupying the graphene's spin degenerate Dirac cones into $α$-RuCl$_3$ band states with in-plane spin polarization. Left behind are holes in two separate Fermi pockets, only the dispersion of one of which is distorted near the Fermi energy due to spin selective hybridization with these spin polarized $α$-RuCl$_3$ band states. This interpretation is supported by our DFT calculations. An unexpected damping of the quantum oscillations as well as a zero field resistance upturn close to the N$é$el temperature of $α$-RuCl$_3$ suggests the onset of additional spin scattering due to spin fluctuations in the $α$-RuCl$_3$.

cond-mat.mtrl-sci↗

arXiv2019

Single spin resonance in a van der Waals embedded paramagnetic defect

Spins constitute a group of quantum objects forming a key resource in modern quantum technology. Two-dimensional (2D) van der Waals materials are of fundamental interest for studying nanoscale magnetic phenomena. However, isolating singular paramagnetic spins in 2D systems is challenging. We report here on a quantum emitting source embedded within hexgonal boron nitride (h-BN) exhibiting optical magnetic resonance (ODMR). We extract an isotropic $g$ factor close to 2 and derive an upper bound for a zero field splitting (ZFS) ($\leq$ 4 MHz). Photoluminescence (PL) behavior under temperature cycling using different excitations is presented, assigning probable zero phonon lines (ZPLs) / phonon side band (PSBs) to emission peaks, compatible with h-BN's phonon density of states, indicating their intrinsic nature. Narrow and inhomogeneous broadened ODMR lines differ significantly from monoatomic vacancy defect lines known in literature. We derive a hyperfine coupling of around 10 MHz. Its angular dependence indicates an unpaired electron in an out-of-plane $π$-orbital, probably originating from an additional substitutional carbon impurity or other low mass atom. We determine the spin relaxation time $T_1$ to be around 17 $μ$s.

cond-mat.mes-hall↗

arXiv2019

Type-II Ising Pairing in Few-Layer Stanene

Spin-orbit coupling has proven indispensable in realizing topological materials and more recently Ising pairing in two-dimensional superconductors. This pairing mechanism relies on inversion symmetry breaking and sustains anomalously large in-plane polarizing magnetic fields whose upper limit is expected to diverge at low temperatures, although experimental demonstration of this has remained elusive due to the required fields. In this work, the recently discovered superconductor few-layer stanene, i.e. epitaxially strained $α$-Sn, is shown to exhibit a new type of Ising pairing between carriers residing in bands with different orbital indices near the $Γ$-point. The bands are split as a result of spin-orbit locking without the participation of inversion symmetry breaking. The in-plane upper critical field is strongly enhanced at ultra-low temperature and reveals the sought for upturn.

cond-mat.supr-con↗

arXiv2017

Quantum light in curved low dimensional hexagonal boron nitride systems

Low-dimensional wide bandgap semiconductors open a new playing field in quantum optics using sub-bandgap excitation. In this field, hexagonal boron nitride (h-BN) has been reported to host single quantum emitters (QEs), linking QE density to perimeters. Furthermore, curvature/perimeters in transition metal dichalcogenides (TMDCs) have demonstrated a key role in QE formation. We investigate a curvature-abundant BN system - quasi one-dimensional BN nanotubes (BNNTs) fabricated via a catalyst-free method. We find that non-treated BNNT is an abundant source of stable QEs and analyze their emission features down to single nanotubes, comparing dispersed/suspended material. Combining high spatial resolution of a scanning electron microscope, we categorize and pin-point emission origin to a scale of less than 20 nm, giving us a one-to-one validation of emission source with dimensions smaller than the laser excitation wavelength, elucidating nano-antenna effects. Two emission origins emerge: hybrid/entwined BNNT. By artificially curving h-BN flakes, similar QE spectral features are observed. The impact on emission of solvents used in commercial products and curved regions is also demonstrated. The 'out of the box' availability of QEs in BNNT, lacking processing contamination, is a milestone for unraveling their atomic features. These findings open possibilities for precision engineering of QEs, puts h-BN under a similar 'umbrella' of TMDC's QEs and provides a model explaining QEs spatial localization/formation using electron/ion irradiation and chemical etching.

cond-mat.mes-hall↗

arXiv2016

Structural attributes and photo-dynamics of visible spectrum quantum emitters in hexagonal boron nitride

Newly discovered van der Waals materials like MoS$_2$, WSe$_2$, hexagonal boron nitride (h-BN), and recently $\mathrm{C}_2\mathrm{N}$ have sparked intensive research to unveil the quantum behavior associated with their 2D structure. Of great interest are 2D materials that host single quantum emitters. h-BN, with a band gap of 5.95 eV, has been shown to host single quantum emitters which are stable at room temperature in the UV and visible spectral range. In this paper we investigate correlations between h-BN structural features and emitter location from bulk down to the monolayer at room temperature. We demonstrate that chemical etching and ion irradiation can generate emitters in h-BN. We analyze the emitters' spectral features and show that they are dominated by the interaction of their electronic transition with a single Raman active mode of h-BN. Photodynamics analysis reveals diverse rates between the electronic states of the emitter. The emitters show excellent photo stability even under ambient conditions and in monolayers. Comparing the excitation polarization between different emitters unveils a connection between defect orientation and the h-BN hexagonal structure. The sharp spectral features, color diversity, room-temperature stability, long-lived metastable states, ease of fabrication, proximity of the emitters to the environment, outstanding chemical stability, and biocompatibility of h-BN provide a completely new class of systems that can be used for sensing and quantum photonics applications.

cond-mat.mes-hall↗

arXiv2016

Negative permittivity attests to local attractive interactions in bubble and stripe phases

The physics of itinerant electrons in condensed matter is by and large governed by repulsive Coulomb forces. However, rare cases exist where local attractive interactions emerge and prevail in determining the ground state of the system despite the dominant Coulomb repulsion. The most notable example is no doubt electron pairing, which leads to superconductivity and is mediated by electron phonon coupling or more intricate mechanisms such as antiferromagnetic spin order in high-temperature superconductors. The interplay of attractive and repulsive interaction components may also instigate spontaneous symmetry lowering and clustering of charges in geometric patterns such as bubbles and stripes, provided these interactions act on different length scales. In high-temperature superconductors, for instance, fluctuating stripe or nematic ordering is intertwined with superconductivity itself. Both types of attractive interaction based physics, pairing and charge ordering, are also at play in high-quality two-dimensional electron systems exposed to a quantizing perpendicular magnetic field for electrons describing larger orbits. This has been concluded indirectly from the transport behaviour when such phases form. An unambiguous manifestation of an underlying attractive interaction between the electrons has however remained elusive. Here we report the observation of a negative permittivity as an immediate consequence of local attractive interaction rather than repulsion among the electrons when bubble and stripe phases form. The implemented technique based on surface acoustic waves offers directionality and confirms the stripe phase to be a strongly anisotropic medium.

cond-mat.mes-hall↗