arXiv subjects

Folkert K. de Vries

Publications and source records attributed to Folkert K. de Vries.

At least 19 recordsLinked to original sources

arXiv2024

Hard superconducting gap in InSb nanowires

Topological superconductivity is a state of matter that can host Majorana modes, the building blocks of a topological quantum computer. Many experimental platforms predicted to show such a topological state rely on proximity-induced superconductivity. However, accessing the topological properties requires an induced hard superconducting gap, which is challenging to achieve for most material systems. We have systematically studied how the interface between an InSb semiconductor nanowire and a NbTiN superconductor affects the induced superconducting properties. Step by step, we improve the homogeneity of the interface while ensuring a barrier-free electrical contact to the superconductor, and obtain a hard gap in the InSb nanowire. The magnetic field stability of NbTiN allows the InSb nanowire to maintain a hard gap and a supercurrent in the presence of magnetic fields (~ 0.5 Tesla), a requirement for topological superconductivity in one-dimensional systems. Our study provides a guideline to induce superconductivity in various experimental platforms such as semiconductor nanowires, two dimensional electron gases and topological insulators, and holds relevance for topological superconductivity and quantum computation.

cond-mat.mes-hall↗

arXiv2024

Quasiparticle and superfluid dynamics in Magic-Angle Graphene

Magic-Angle Twisted Bilayer Graphene shows a wide range of correlated phases which are electrostatically tunable. Despite a growing knowledge of the material, there is yet no consensus on the microscopic mechanisms driving its superconducting phase. In particular, elucidating the symmetry and formation mechanism of the superconducting phase may provide key insights for the understanding of unconventional, strongly coupled and topological superconductivity. A major obstacle to progress in this direction is that key thermodynamic properties, such as specific heat, electron-phonon coupling and superfluid stiffness, are extremely challenging to measure due to the 2D nature of the material and its relatively low energy scales. Here, we use a gate-defined, radio frequency-biased, Josephson junction to probe the electronic dynamics of magic-angle twisted bilayer graphene (MATBG). We reveal both the electronic quasiparticle dynamics, driven by their thermalization through phonon scattering, as well as the condensate dynamics, driven by the inertia of Cooper pairs. From these properties we recover the evolution of thermalization rates, and the superfluid stiffness across the phase diagram. Our findings favor an anisotropic or nodal pairing state and allow to estimate the strength of electron-phonon coupling. These results contribute to understanding the underlying mechanisms of superconductivity in MATBG while establishing an easy-to-implement method for characterizing thermal and superfluid properties of superconducting 2D materials.

cond-mat.supr-con↗

arXiv2023

Gate-defined superconducting channel in magic-angle twisted bilayer graphene

Magic-angle twisted bilayer graphene (MATBG) combines in one single material different phases like insulating, metallic and superconducting. These phases and their in-situ tunability make MATBG an important platform for the fabrication of superconducting devices. We realize a split gate-defined geometry which enables us to tune the width of a superconducting channel formed in MATBG. We observe a smooth transition from superconductivity to highly resistive transport by progressively reducing the channel width using the split gates or by reducing the density in the channel. Using the gate-defined constriction, we control the flow of the supercurrent, either guiding it through the constriction or throughout the whole device or even blocking its passage completely. This serves as a foundation for developing quantum constriction devices like superconducting quantum point contacts, quantum dots, and Cooper-pair boxes in MATBG.

cond-mat.mes-hall↗

arXiv2023

Kagomé quantum oscillations in graphene superlattices

Periodic systems feature the Hofstadter butterfly spectrum produced by Brown--Zak minibands of electrons formed when magnetic field flux through the lattice unit cell is commensurate with flux quantum and manifested by magneto-transport oscillations. Quantum oscillations, such as Shubnikov -- de Haas effect and Aharonov--Bohm effect, are also characteristic for electronic systems with closed orbits in real space and reciprocal space. Here we show the intricate relation between these two phenomena by tracing quantum magneto-oscillations to Lifshitz transitions in graphene superlattices, where they persist even at relatively low fields and very much above liquid-helium temperatures. The oscillations originate from Aharonov--Bohm interference on cyclotron trajectories that form a kagomé-shaped network characteristic for Lifshitz transitions. In contrast to Shubnikov - de Haas oscillations, the kagomé oscillations are robust against thermal smearing and they can be detected even when the Hofstadter butterfly spectrum is undermined by electron's scattering. We expect that kagomé quantum oscillations are generic to rotationally-symmetric two-dimensional crystals close to Lifshitz transitions.

cond-mat.mes-hall↗

arXiv2022

Gate-defined electron interferometer in bilayer graphene

We present an electron interferometer defined purely by electrostatic gating in encapsulated bilayer graphene. This minimizes possible sample degradation introduced by conventional etching methods when preparing quantum devices. The device quality is demonstrated by observing Aharonov-Bohm (AB) oscillations with a period of h/e, h/2e, h/3e, and h/4e, witnessing a coherence length of many microns. The AB oscillations as well as the type of carriers (electrons or holes) are seamlessly tunable with gating. The coherence length longer than the ring perimeter and semiclassical trajectory of the carrier are established from the analysis of the temperature and magnetic field dependence of the oscillations. Our gate-defined ring geometry has the potential to evolve into a platform for exploring correlated quantum states such as superconductivity in interferometers in twisted bilayer graphene.

cond-mat.mes-hall↗

arXiv2022

A Tunable Monolithic SQUID in Twisted Bilayer Graphene

Magic-angle twisted bilayer graphene (MATBG) hosts a number of correlated states of matter that can be tuned by electrostatic doping. Superconductivity has drawn considerable attention and the mechanism behind it is a topic of active discussion. MATBG has been experimentally characterized by numerous transport and scanning-probe experiments. The material has also emerged as a versatile platform for superconducting electronics, as proven by the realization of monolithic Josephson junctions. However, even though phase-coherent phenomena have been measured, no control of the superconducting phase has been demonstrated so far. Here, we present a Superconducting Quantum Interference Device (SQUID) in MATBG, where the superconducting phase difference is controlled through the magnetic field. We observe magneto-oscillations of the critical current, demonstrating long-range coherence agreeing with an effective charge of 2e for the superconducting charge carriers. We tune to both asymmetric and symmetric SQUID configurations by electrostatically controlling the critical currents through the junctions. With this tunability, we study the inductances in the device, finding values of up to 2μH. Furthermore, we directly observe the current-phase relation of one of the Josephson junctions of the device. Our results show that superconducting devices in MATBG can be scaled up and used to reveal properties of the material. We expect this to foster a more systematic realization of devices of this type, increasing the accuracy with which microscopic characteristics of the material are extracted. We also envision more complex devices to emerge, such as phase-slip junctions or high kinetic inductance detectors.

cond-mat.mes-hall↗

arXiv2021

Tailoring the band structure of twisted double bilayer graphene with pressure

Twisted two-dimensional structures open new possibilities in band structure engineering. At magic twist angles, flat bands emerge, which give a new drive to the field of strongly correlated physics. In twisted double bilayer graphene dual gating allows changing the Fermi level and hence the electron density and also allows tuning the interlayer potential, giving further control over band gaps. Here, we demonstrate that by applying hydrostatic pressure, an additional control of the band structure becomes possible due to the change of tunnel couplings between the layers. We find that the flat bands and the gaps separating them can be drastically changed by pressures up to 2 GPa, in good agreement with our theoretical simulations. Furthermore, our measurements suggest that in finite magnetic field due to pressure a topologically non-trivial band gap opens at the charge neutrality point at zero displacement field.

cond-mat.mes-hall↗

arXiv2021

Fabry-Pérot cavities and quantum dot formation at gate-defined interfaces in twisted double bilayer graphene

The rich and electrostatically tunable phase diagram exhibited by moiré materials has made them a suitable platform for hosting single material multi-purpose devices. To engineer such devices, understanding electronic transport and localization across electrostatically defined interfaces is of fundamental importance. Little is known, however, about how the interplay between the band structure originating from the moiré lattice and electric potential gradients affects electronic confinement. Here, we electrostatically define a cavity across a twisted double bilayer graphene sample. We observe two kinds of Fabry-Pérot oscillations. The first, independent of charge polarity, stems from confinement of electrons between dispersive-band/flat-band interfaces. The second arises from junctions between regions tuned into different flat bands. When tuning the out-of-plane electric field across the device, we observe Coulomb blockade resonances in transport, an indication of strong electronic confinement. From the gate, magnetic field and source-drain voltage dependence of the resonances, we conclude that quantum dots form at the interfaces of the Fabry-Pérot cavity. Our results constitute a first step towards better understanding interfacial phenomena in single crystal moiré devices.

cond-mat.mes-hall↗

arXiv2021

Scattering between minivalleys in a moiré material

A unique feature of the complex band structures of moiré materials is the presence of minivalleys, their hybridization, and scattering between them. Here we investigate magneto-transport oscillations caused by scattering between minivalleys - a phenomenon analogous to magneto-intersubband oscillations - in a twisted double bilayer graphene sample with a twist angle of 1.94°. We study and discuss the potential scattering mechanisms and find an electron-phonon mechanism and valley conserving scattering to be likely. Finally, we discuss the relevance of our findings for different materials and twist angles.

cond-mat.mes-hall↗

arXiv2021

Kondo effect and spin-orbit coupling in graphene quantum dots

The Kondo effect is a cornerstone in the study of strongly correlated fermions. The coherent exchange coupling of conduction electrons to local magnetic moments gives rise to a Kondo cloud that screens the impurity spin. Whereas complete Kondo screening has been explored widely, realizations of the underscreened scenario - where only some of several Kondo channels participate in the screening - remain rare. Here we report the observation of fully screened and underscreened Kondo effects in quantum dots in bilayer graphene. More generally, we introduce a unique platform for studying Kondo physics. In contrast to carbon nanotubes, whose curved surfaces give rise to strong spin-orbit coupling breaking the SU(4) symmetry of the electronic states relevant for the Kondo effect, we study a nominally flat carbon material with small spin-orbit coupling. Moreover, the unusual two-electron triplet ground state in bilayer graphene dots provides a route to exploring the underscreened spin-1 Kondo effect.

cond-mat.mes-hall↗

arXiv2020

Gate-Defined Josephson Junctions in Magic-Angle Twisted Bilayer Graphene

In the past two years, magic-angle twisted bilayer graphene has emerged as a uniquely versatile experimental platform that combines metallic, superconducting, magnetic and insulating phases in a single crystal. In particular the ability to tune the superconducting state with a gate voltage opened up intriguing prospects for novel device functionality. Here we present the first demonstration of a device based on the interplay between two distinct phases in adjustable regions of a single magic-angle twisted bilayer graphene crystal. We electrostatically define the superconducting and insulating regions of a Josephson junction and observe tunable DC and AC Josephson effects. We show that superconductivity is induced in different electronic bands and describe the junction behaviour in terms of these bands, taking in consideration interface effects as well. Shapiro steps, a hallmark of the AC Josephson effect and therefore the formation of a Josephson junction, are observed. This work is an initial step towards devices where separate gate-defined correlated states are connected in single-crystal nanostructures. We envision applications in superconducting electronics and quantum information technology as well as in studies exploring the nature of the superconducting state in magic-angle twisted bilayer graphene.

cond-mat.mes-hall↗

arXiv2020

Combined minivalley and layer control in twisted double bilayer graphene

Control over minivalley polarization and interlayer coupling is demonstrated in double bilayer graphene twisted with an angle of 2.37$^\circ$. This intermediate angle is small enough for the minibands to form and large enough such that the charge carrier gases in the layers can be tuned independently. Using a dual-gated geometry we identify and control all possible combinations of minivalley polarization via the population of the two bilayers. An applied displacement field opens a band gap in either of the two bilayers, allowing us to even obtain full minivalley polarization. In addition, the wavefunctions of the minivalleys are mixed by tuning through a Lifshitz transition, where the Fermi surface topology changes. The high degree of control makes twisted double bilayer graphene a promising platform for valleytronics devices such as valley valves, filters and logic gates.

cond-mat.mes-hall↗

arXiv2019

Hard superconducting gap and diffusion-induced superconductors in Ge-Si nanowires

We show a hard induced superconducting gap in a Ge-Si nanowire Josephson transistor up to in-plane magnetic fields of $250$ mT, an important step towards creating and detecting Majorana zero modes in this system. A hard induced gap requires a highly homogeneous tunneling heterointerface between the superconducting contacts and the semiconducting nanowire. This is realized by annealing devices at $180$ $^\circ$C during which aluminium inter-diffuses and replaces the germanium in a section of the nanowire. Next to Al, we find a superconductor with lower critical temperature ($T_\mathrm{C}=0.9$ K) and a higher critical field ($B_\mathrm{C}=0.9-1.2$ T). We can therefore selectively switch either superconductor to the normal state by tuning the temperature and the magnetic field and observe that the additional superconductor induces a proximity supercurrent in the semiconducting part of the nanowire even when the Al is in the normal state. In another device where the diffusion of Al rendered the nanowire completely metallic, a superconductor with a much higher critical temperature ($T_\mathrm{C}=2.9$ K) and critical field ($B_\mathrm{C}=3.4$ T) is found. The small size of diffusion-induced superconductors inside nanowires may be of special interest for applications requiring high magnetic fields in arbitrary direction.

cond-mat.mes-hall↗

arXiv2019

Crossed Andreev Reflection in InSb Flake Josephson Junctions

We study superconducting quantum interference in InSb flake Josephson junctions. An even-odd effect in the amplitude and periodicity of the superconducting quantum interference pattern is found. Interestingly, the occurrence of this pattern coincides with enhanced conduction at both edges of the flake, as is deduced from measuring a SQUID pattern at reduced gate voltages. We identify the specific crystal facet of the edge with enhanced conduction, and confirm this by measuring multiple devices. Furthermore, we argue the even-odd effect is due to crossed Andreev reflection, a process where a Cooper pair splits up over the two edges and recombines at the opposite contact. An entirely $h/e$ periodic SQUID pattern, as well as the observation of both even-odd and odd-even effects, corroborates this conclusion. Crossed Andreev reflection could be harnessed for creating a topological state of matter or performing experiments on the non-local spin-entanglement of spatially separated Cooper pairs.

cond-mat.mes-hall↗

arXiv2019

Ballistic superconductivity and tunable $π$-junctions in InSb quantum wells

Two-dimensional electron gases (2DEGs) coupled to superconductors offer the opportunity to explore a variety of quantum phenomena. These include the study of novel Josephson effects, superconducting correlations in quantum (spin) Hall systems, hybrid superconducting qubits and emergent topological states in semiconductors with spin-orbit interaction (SOI). InSb is a well-known example of such a strong SOI semiconductor, however hybrid superconducting devices in InSb quantum wells remain unexplored. Here, we interface InSb 2DEGs with a superconductor (NbTiN) to create Josephson junctions (JJs), thus providing the first evidence of induced superconductivity in high quality InSb quantum wells. The JJs support supercurrent transport over several microns and display clear signatures of ballistic superconductivity. Furthermore, we exploit the large Landé g-factor and gate tunability of the junctions to control the current-phase relation, and drive transitions between the $0$ and $π$-states. This control over the free energy landscape allows us to construct a phase diagram identifying these $0$ and $π$-regions, in agreement with theory. Our results establish InSb quantum wells as a promising new material platform to study the interplay between superconductivity, SOI and magnetism.

cond-mat.mes-hall↗

arXiv2018

Josephson effect in a few-hole quantum dot

We use a Ge-Si core-shell nanowire to realise a Josephson field-effect transistor with highly transparent contacts to superconducting leads. By changing the electric field we gain access to two distinct regimes not combined before in a single device: In the accumulation mode the device is highly transparent and the supercurrent is carried by multiple subbands, while near depletion supercurrent is carried by single-particle levels of a strongly coupled quantum dot operating in the few-hole regime. These results establish Ge-Si nanowires as an important platform for hybrid superconductor-semiconductor physics and Majorana fermions.

cond-mat.mes-hall↗

arXiv2017

$h/e$ superconducting quantum interference through trivial edge states in InAs

Josephson junctions defined in strong spin orbit semiconductors are highly interesting for the search for topological systems. However, next to topological edge states that emerge in a sufficient magnetic field, trivial edge states can also occur. We study the trivial edge states with superconducting quantum interference measurements on non-topological InAs Josephson junctions. We observe a SQUID pattern, an indication of superconducting edge transport. Also, a remarkable $h/e$ SQUID signal is observed that, as we find, stems from crossed Andreev states.

cond-mat.mes-hall↗

arXiv2017

Ballistic superconductivity in semiconductor nanowires

Semiconductor nanowires have opened new research avenues in quantum transport owing to their confined geometry and electrostatic tunability. They have offered an exceptional testbed for superconductivity, leading to the realization of hybrid systems combining the macroscopic quantum properties of superconductors with the possibility to control charges down to a single electron. These advances brought semiconductor nanowires to the forefront of efforts to realize topological superconductivity and Majorana modes. A prime challenge to benefit from the topological properties of Majoranas is to reduce the disorder in hybrid nanowire devices. Here, we show ballistic superconductivity in InSb semiconductor nanowires. Our structural and chemical analyses demonstrate a high-quality interface between the nanowire and a NbTiN superconductor which enables ballistic transport. This is manifested by a quantized conductance for normal carriers, a strongly enhanced conductance for Andreev-reflecting carriers, and an induced hard gap with a significantly reduced density of states. These results pave the way for disorder-free Majorana devices.

cond-mat.mes-hall↗