Heat Dissipation Mechanisms in Hybrid Superconductor-Semiconductor Devices Revealed by Joule Spectroscopy.

Understanding heating and cooling mechanisms in mesoscopic superconductor-semiconductor devices is crucial for their application in quantum technologies. Owing to their poor thermal conductivity, heating effects can drive superconducting-to-normal transitions even at low bias, observed as sharp conductance dips through the loss of Andreev excess currents. Tracking such dips across magnetic field, cryostat temperature, and applied microwave power allows us to uncover cooling bottlenecks in different parts of a device. By applying this "Joule spectroscopy" technique, we analyze heat dissipation in devices based on InAs-Al nanowires and reveal that cooling of superconducting islands is limited by the rather inefficient electron-phonon coupling, as opposed to grounded superconductors that primarily cool by quasiparticle diffusion. We show that powers as low as 50-150 pW are able to suppress superconductivity on the islands. Applied microwaves lead to similar heating effects but are affected by the interplay of the microwave frequency and the effective electron-phonon relaxation time.

Nanoscale-Confined Terahertz Polaritons in a van der Waals Crystal.

Electromagnetic field confinement is crucial for nanophotonic technologies, since it allows for enhancing light-matter interactions, thus enabling light manipulation in deep sub-wavelength scales. In the terahertz (THz) spectral range, radiation confinement is conventionally achieved with specially designed metallic structures-such as antennas or nanoslits-with large footprints due to the rather long wavelengths of THz radiation. In this context, phonon polaritons-light coupled to lattice vibrations-in van der Waals (vdW) crystals have emerged as a promising solution for controlling light beyond the diffraction limit, as they feature extreme field confinements and low optical losses. However, experimental demonstration of nanoscale-confined phonon polaritons at THz frequencies has so far remained elusive. Here, it is provided by employing scattering-type scanning near-field optical microscopy combined with a free-electron laser to reveal a range of low-loss polaritonic excitations at frequencies from 8 to 12 THz in the vdW semiconductor α-MoO3 . In this study, THz polaritons are visualized with: i) in-plane hyperbolic dispersion, ii) extreme nanoscale field confinement (below λo  /75), and iii) long polariton lifetimes, with a lower limit of >2 ps.

Split-Channel Ballistic Transport in an InSb Nanowire.

We report an experimental study of one-dimensional (1D) electronic transport in an InSb semiconducting nanowire. A total of three bottom gates are used to locally deplete the nanowire, creating a ballistic quantum point contact with only a few conducting channels. In a magnetic field, the Zeeman splitting of the corresponding 1D sub-bands is revealed by the emergence of conductance plateaus at multiples of e2/h, yet we find a quantized conductance pattern largely dependent on the configuration of voltages applied to the bottom gates. In particular, we can make the first plateau disappear, leaving a first conductance step of 2 e2/ h, which is indicative of a remarkable 2-fold sub-band degeneracy that can persist up to several tesla. For certain gate voltage settings, we also observe the presence of discrete resonant states producing conductance features that can resemble those expected from the opening of a helical gap in the sub-band structure. We explain our experimental findings through the formation of two spatially separated 1D conduction channels.

Spin-resolved Andreev levels and parity crossings in hybrid superconductor-semiconductor nanostructures.

The physics and operating principles of hybrid superconductor-semiconductor devices rest ultimately on the magnetic properties of their elementary subgap excitations, usually called Andreev levels. Here we report a direct measurement of the Zeeman effect on the Andreev levels of a semiconductor quantum dot with large electron g-factor, strongly coupled to a conventional superconductor with a large critical magnetic field. This material combination allows spin degeneracy to be lifted without destroying superconductivity. We show that a spin-split Andreev level crossing the Fermi energy results in a quantum phase transition to a spin-polarized state, which implies a change in the fermionic parity of the system. This crossing manifests itself as a zero-bias conductance anomaly at finite magnetic field with properties that resemble those expected for Majorana modes in a topological superconductor. Although this resemblance is understood without evoking topological superconductivity, the observed parity transitions could be regarded as precursors of Majorana modes in the long-wire limit.

Zero-bias anomaly in a nanowire quantum dot coupled to superconductors.

We studied the low-energy states of spin-1/2 quantum dots defined in InAs/InP nanowires and coupled to aluminum superconducting leads. By varying the superconducting gap Δ with a magnetic field B we investigated the transition from strong coupling Δ << T(K) to weak-coupling Δ >> T(K), where T(K) is the Kondo temperature. Below the critical field, we observe a persisting zero-bias Kondo resonance that vanishes only for low B or higher temperatures, leaving the room to more robust subgap structures at bias voltages between Δ and 2Δ. For strong and approximately symmetric tunnel couplings, a Josephson supercurrent is observed in addition to the Kondo peak. We ascribe the coexistence of a Kondo resonance and a superconducting gap to a significant density of intragap quasiparticle states, and the finite-bias subgap structures to tunneling through Shiba states. Our results, supported by numerical calculations, own relevance also in relation to tunnel-spectroscopy experiments aiming at the observation of Majorana fermions in hybrid nanostructures.

Effect of stacking order on the electric-field induced carrier modulation in graphene bilayers.

When planar graphene sheets are stacked on top of each other, the electronic structure of the system varies with the position of the subsequent sublattice atoms. Here, we employ scanning photocurrent microscopy to study the disparity in the behavior of charge carriers for two different stacking configurations. It has been found that deviation from the regular Bernal stacking decouples the sheets from each other, which imparts effective electrostatic screening of the farther layer from the underlying backgate. Electrochemical top-gating is demonstrated as a means to selectively tune the charge carrier density in the decoupled upper layer.

Spatially Resolved Potential Distribution in Carbon Nanotube Cross-Junction Devices.

Crossed-nanotube junctions, the basic constituents of carbon nanotube networks, are investigated by scanning photocurrent microscopy. The location of the predominant electrostatic potential drop, at the electrical contacts or at the junction, is found to be highly dependent on the transport regime. Also, whereas Schottky barriers are formed at M-S (metal-semiconductor) nanotube crossings, isotype heterojunctions are formed at S-S ones (figure).

Contact and edge effects in graphene devices.

Electrical transport studies on graphene have been focused mainly on the linear dispersion region around the Fermi level and, in particular, on the effects associated with the quasiparticles in graphene behaving as relativistic particles known as Dirac fermions. However, some theoretical work has suggested that several features of electron transport in graphene are better described by conventional semiconductor physics. Here we use scanning photocurrent microscopy to explore the impact of electrical contacts and sheet edges on charge transport through graphene devices. The photocurrent distribution reveals the presence of potential steps that act as transport barriers at the metal contacts. Modulations in the electrical potential within the graphene sheets are also observed. Moreover, we find that the transition from the p- to n-type regime induced by electrostatic gating does not occur homogeneously within the sheets. Instead, at low carrier densities we observe the formation of p-type conducting edges surrounding a central n-type channel.

Oriented attachment mechanism in anisotropic nanocrystals: a "polymerization" approach.

The kinetic model of stepwise polymerization is revisited, with some adaptations for its application to the kinetics of oriented attachment of nanoparticles in colloidal suspensions, which results in the formation of anisotropic particles. A comparison with experimental data reported in the literature shows good agreement with the model and supports comparisons with other systems.

A kinetic model to describe nanocrystal growth by the oriented attachment mechanism.

The classical model of particle coagulation on colloids is revisited to evaluate its applicability on the oriented attachment of nanoparticles. The proposed model describes well the growth behavior of dispersed nanoparticles during the initial stages of nanoparticle synthesis and during growth induced by hydrothermal treatments. Moreover, a general model, which combines coarsening (i.e., Ostwald ripening) and oriented attachment effects, is proposed as an alternative to explain deviations between experimental results and existing theoretical models.

Oriented attachment: an effective mechanism in the formation of anisotropic nanocrystals.

In this article, we report the formation of dispersed anisotropic nanostructures by the oriented attachment mechanism, under hydrothermal conditions and in a system that all other growth processes are improbable. The comparison of dilute and agglomerated experimental conditions indicate that oriented attachment is an effective mechanism for the formation of anisotropic nanocrystals, and the conclusions can be extended to other nanometric systems.

A simple and novel method to synthesize doped and undoped SnO2 nanocrystals at room temperature.

This work describes the synthesis, at room temperature, of doped and undoped SnO2 nanocrystals (particle size ranging from 1-3 nm) with no thermal or hydrothermal treatment. To the best of our knowledge, this is the first time that the synthesis, at room temperature, of doped and/or undoped SnO2 nanocrystals has been reported. This new synthesis method is based on the controlled oxidation, hydrolysis and polycondensation of tin ions in an ethanol solution. Another novel aspect of this method is the possibility of using surfactants, which may provide improved control over the particle size.