Influence of the Magnetic Tip on Heterodimers in Electron Spin Resonance Combined with Scanning Tunneling Microscopy.

Investigating the quantum properties of individual spins adsorbed on surfaces by electron spin resonance combined with scanning tunneling microscopy (ESR-STM) has shown great potential for the development of quantum information technology on the atomic scale. A magnetic tip exhibiting high spin polarization is critical for performing an ESR-STM experiment. While the tip has been conventionally treated as providing a static magnetic field in ESR-STM, it was found that the tip can exhibit bistability, influencing ESR spectra. Ideally, the ESR splitting caused by the magnetic interaction between two spins on a surface should be independent of the tip. However, we found that the measured ESR splitting of a metal atom-molecule heterodimer can be tip-dependent. Detailed theoretical analysis reveals that this tip-dependent ESR splitting is caused by a different interaction energy between the tip and each spin of the heterodimer. Our work provides a comprehensive reference for characterizing tip features in ESR-STM experiments and highlights the importance of employing a proper physical model when describing the ESR tip, in particular, for heterospin systems.

Electron spin resonance of single iron phthalocyanine molecules and role of their non-localized spins in magnetic interactions.

Electron spin resonance (ESR) spectroscopy is a crucial tool, through spin labelling, in investigations of the chemical structure of materials and of the electronic structure of materials associated with unpaired spins. ESR spectra measured in molecular systems, however, are established on large ensembles of spins and usually require a complicated structural analysis. Recently, the combination of scanning tunnelling microscopy with ESR has proved to be a powerful tool to image and coherently control individual atomic spins on surfaces. Here we extend this technique to single coordination complexes-iron phthalocyanines (FePc)-and investigate the magnetic interactions between their molecular spin with either another molecular spin (in FePc-FePc dimers) or an atomic spin (in FePc-Ti pairs). We show that the molecular spin density of FePc is both localized at the central Fe atom and also distributed to the ligands (Pc), which yields a strongly molecular-geometry-dependent exchange coupling.

Coherent Spin Control of Single Molecules on a Surface.

Control of single electron spins constitutes one of the most promising platforms for spintronics, quantum sensing, and quantum information processing. Utilizing single molecular magnets as their hosts establishes an interesting framework since their molecular structure is highly flexible and chemistry-based large-scale synthesis directly provides a way toward scalability. Here, we demonstrate coherent spin manipulation of single molecules on a surface, which we control individually using a scanning tunneling microscope in combination with electron spin resonance. We previously found that iron phthalocyanine (FePc) molecules form a spin-1/2 system when placed on an insulating thin film of magnesium oxide (MgO). Performing Rabi oscillation and Hahn echo measurements, we show that the FePc spin can be coherently manipulated with a phase coherence time T2Echo of several hundreds of nanoseconds. Tunneling current-dependent measurements demonstrate that interaction with the tunneling electrons is the dominating source of decoherence. In addition, we perform Hahn echo measurements on small self-assembled arrays of FePc molecules. We show that, despite additional intermolecular magnetic coupling, spin resonance and T2Echo are much less perturbed by T1 spin flip events of neighboring spins than by the tunneling current. This will potentially allow for individual addressable molecular spins in self-assemblies and with application for quantum information processing.

Bright CdSe/CdS Quantum Dot Light-Emitting Diodes with Modulated Carrier Dynamics via the Local Kirchhoff Law.

Addressing the interactions between optical antennas and ensembles of emitters is particularly challenging. Charge transfer and Coulomb interactions complicate the understanding of the carrier dynamics coupled by antennas. Here, we show how Au antennas enhance the luminescence of CdSe/CdS quantum dot assemblies through carrier dynamics control within the framework of the local Kirchhoff law. The Au antennas inject hot electrons into quantum dot assemblies via plasmon-induced hot electron transfer that increases the carrier concentration. Also, the localized surface plasmon resonances of Au antennas favorably tilt the balance between nonradiative Auger processes and radiative recombination in the CdSe core. Eventually, a high bright (125,091.6 cd/m2) deep-red quantum dot light-emitting diode is obtained by combining with Au antennas. Our findings suggest a new understanding of light emission of assembled emitters coupled by antennas, which is of essential interest for the description of light-matter interaction in advanced optoelectronics.

Wide Critical Fluctuations of the Field-Induced Phase Transition in Graphite.

In the immediate vicinity of the critical temperature (T_{c}) of a phase transition, there are fluctuations of the order parameter that reside beyond the mean-field approximation. Such critical fluctuations usually occur in a very narrow temperature window in contrast to Gaussian fluctuations. Here, we report on a study of specific heat in graphite subject to a high magnetic field when all carriers are confined in the lowest Landau levels. The observation of a BCS-like specific heat jump in both temperature and field sweeps establishes that the phase transition discovered decades ago in graphite is of the second order. The jump is preceded by a steady field-induced enhancement of the electronic specific heat. A modest (20%) reduction in the amplitude of the magnetic field (from 33 to 27 T) leads to a threefold decrease of T_{c} and a drastic widening of the specific heat anomaly, which acquires a tail spreading to two times T_{c}. We argue that the steady departure from the mean-field BCS behavior is the consequence of an exceptionally large Ginzburg number in this dilute metal, which grows steadily as the field lowers. Our fit of the critical fluctuations indicates that they belong to the 3DXY universality class as in the case of the ^{4}He superfluid transition.

Field-Effect Transistor and Photo-Transistor of Narrow-Band-Gap Nanocrystal Arrays Using Ionic Glasses.

The gating of nanocrystal films is currently driven by two approaches: either the use of a dielectric such as SiO2 or the use of electrolyte. SiO2 allows fast bias sweeping over a broad range of temperatures but requires a large operating bias. Electrolytes, thanks to large capacitances, lead to the significant reduction of operating bias but are limited to slow and quasi-room-temperature operation. None of these operating conditions are optimal for narrow-band-gap nanocrystal-based phototransistors, for which the necessary large-capacitance gate has to be combined with low-temperature operation. Here, we explore the use of a LaF3 ionic glass as a high-capacitance gating alternative. We demonstrate for the first time the use of such ionic glasses to gate thin films made of HgTe and PbS nanocrystals. This gating strategy allows operation in the 180 to 300 K range of temperatures with capacitance as high as 1 µF·cm-2. We unveil the unique property of ionic glass gate to enable the unprecedented tunability of both magnitude and dynamics of the photocurrent thanks to high charge-doping capability within an operating temperature window relevant for infrared photodetection. We demonstrate that by carefully choosing the operating gate bias, the signal-to-noise ratio can be improved by a factor of 100 and the time response accelerated by a factor of 6. Moreover, the good transparency of LaF3 substrate allows back-side illumination in the infrared range, which is highly valuable for the design of phototransistors.

Impact of dimensionality and confinement on the electronic properties of mercury chalcogenide nanocrystals.

We demonstrate the growth of 2D nanoplatelets (NPLs) made of a HgTe/CdS heterostructure, with an optical absorption reaching the shortwave infrared range. The material is an interesting platform to investigate the effect of dimensionality (0D vs. 2D) and confinement on the electronic spectrum and carrier dynamics in colloidal materials. We bring consistent evidence for the p-type nature of this material from transport and photoemission measurements. The majority carrier dynamics probed using pump-probe photoemission is found to be mostly dependent on the presence of a confinement barrier at the surface rather than on the material dimensionality. The minority carrier, on the other hand, is strongly affected by the material shape showing a longer lived minority carrier in 2D NPLs compared to their 0D equivalent with a similar band gap. Finally, we test the potential of this material for photodetection in the short-wave infrared range (SWIR) and show that fast photoresponse and detectivity reaching 109 Jones at room temperature can be achieved.

Spin-Orbit induced phase-shift in Bi2Se3 Josephson junctions.

The transmission of Cooper pairs between two weakly coupled superconductors produces a superfluid current and a phase difference; the celebrated Josephson effect. Because of time-reversal and parity symmetries, there is no Josephson current without a phase difference between two superconductors. Reciprocally, when those two symmetries are broken, an anomalous supercurrent can exist in the absence of phase bias or, equivalently, an anomalous phase shift φ0 can exist in the absence of a superfluid current. We report on the observation of an anomalous phase shift φ0 in hybrid Josephson junctions fabricated with the topological insulator Bi2Se3 submitted to an in-plane magnetic field. This anomalous phase shift φ0 is observed directly through measurements of the current-phase relationship in a Josephson interferometer. This result provides a direct measurement of the spin-orbit coupling strength and open new possibilities for phase-controlled Josephson devices made from materials with strong spin-orbit coupling.

HgSe Self-Doped Nanocrystals as a Platform to Investigate the Effects of Vanishing Confinement.

Self-doped colloidal quantum dots (CQDs) attract a strong interest for the design of a new generation of low-cost infrared (IR) optoelectronic devices because of their tunable intraband absorption feature in the mid-IR region. However, very little remains known about their electronic structure which combines confinement and an inverted band structure, complicating the design of optimized devices. We use a combination of IR spectroscopy and photoemission to determine the absolute energy levels of HgSe CQDs with various sizes and surface chemistries. We demonstrate that the filling of the CQD states ranges from 2 electrons per CQD at small sizes (<5 nm) to more than 18 electrons per CQD at large sizes (≈20 nm). HgSe CQDs are also an interesting platform to observe vanishing confinement in colloidal nanoparticles. We present lines of evidence for a semiconductor-to-metal transition at the CQD level, through temperature-dependent absorption and transport measurements. In contrast with bulk systems, the transition is the result of the vanishing confinement rather than the increase of the doping level.

Shiba Bound States across the Mobility Edge in Doped InAs Nanowires.

We present a study of Andreev quantum dots fabricated with small-diameter (30 nm) Si-doped InAs nanowires where the Fermi level can be tuned across a mobility edge separating localized states from delocalized states. The transition to the insulating phase is identified by a drop in the amplitude and width of the excited levels and is found to have remarkable consequences on the spectrum of superconducting subgap resonances. While at deeply localized levels only quasiparticle cotunneling is observed, for slightly delocalized levels Shiba bound states form and a parity-changing quantum phase transition is identified by a crossing of the bound states at zero energy. Finally, in the metallic regime, single Andreev resonances are observed.

Electronic properties of (Sb;Bi)2Te3 colloidal heterostructured nanoplates down to the single particle level.

We investigate the potential use of colloidal nanoplates of Sb2Te3 by conducting transport on single particle with in mind their potential use as 3D topological insulator material. We develop a synthetic procedure for the growth of plates with large lateral extension and probe their infrared optical and transport properties. These two properties are used as probe for the determination of the bulk carrier density and agree on a value in the 2-3 × 1019 cm-3 range. Such value is compatible with the metallic side of the Mott criterion which is also confirmed by the weak thermal dependence of the conductance. By investigating the transport at the single particle level we demonstrate that the hole mobility in this system is around 40 cm2V-1s-1. For the bulk material mixing n-type Bi2Te3 with the p-type Sb2Te3 has been a successful way to control the carrier density. Here we apply this approach to the case of colloidally obtained nanoplates by growing a core-shell heterostructure of Sb2Te3/Bi2Te3 and demonstrates a reduction of the carrier density by a factor 2.5.

Charge Dynamics and Optolectronic Properties in HgTe Colloidal Quantum Wells.

We investigate the electronic and transport properties of HgTe 2D colloidal quantum wells. We demonstrate that the material can be made p- or n-type depending on the capping ligands. In addition to the control of majority carrier type, the surface chemistry also strongly affects the photoconductivity of the material. These transport measurements are correlated with the electronic structure determined by high resolution X-ray photoemission. We attribute the change of majority carriers to the strong hybridization of an n-doped HgS layer resulting from capping the HgTe nanoplatelets by S2- ions. We further investigate the gate and temperature dependence of the photoresponse and its dynamics. We show that the photocurrent rise and fall times can be tuned from 100 µs to 1 ms using the gate bias. Finally, we use time-resolved photoemission spectroscopy as a probe of the transport relaxation to determine if the observed dynamics are limited by a fundamental process such as trapping. These pump probe surface photovoltage measurements show an even faster relaxation in the 100-500 ns range, which suggests that the current performances are rather limited by geometrical factors.

Superconducting parity effect across the Anderson limit.

How small can superconductors be? For isolated nanoparticles subject to quantum size effects, P.W. Anderson in 1959 conjectured that superconductivity could only exist when the electronic level spacing δ is smaller than the superconducting gap energy Δ. Here we report a scanning tunnelling spectroscopy study of superconducting lead (Pb) nanocrystals grown on the (110) surface of InAs. We find that for nanocrystals of lateral size smaller than the Fermi wavelength of the 2D electron gas at the surface of InAs, the electronic transmission of the interface is weak; this leads to Coulomb blockade and enables the extraction of electron addition energy of the nanocrystals. For large nanocrystals, the addition energy displays superconducting parity effect, a direct consequence of Cooper pairing. Studying this parity effect as a function of nanocrystal volume, we find the suppression of Cooper pairing when the mean electronic level spacing overcomes the superconducting gap energy, thus demonstrating unambiguously the validity of the Anderson criterion.

Transport in a Single Self-Doped Nanocrystal.

Addressing the optical properties of a single nanoparticle in the infrared is particularly challenging, thus alternative methods for characterizing the conductance spectrum of nanoparticles in this spectral range need to be developed. Here we describe an efficient method of fabricating single nanoparticle tunnel junctions on a chip circuit. We apply this method to narrow band gap nanoparticles of HgSe, which band structure combines the inverted character of the bulk semimetal with quantum confinement and self-doping. Upon tuning the gate bias, measurement reveals the presence of two energy gaps in the spectrum. The wider gap results from the interband gap, while the narrower gap results from intraband transitions. The observation of the latter near zero gate voltage confirms the doped character of the nanoparticle at the single particle level, which is in full agreement with the ensemble optical and transport measurements. Finally we probe the phototransport within a single quantum dot and demonstrate a large photogain mechanism resulting from photogating.

Nernst effect in metals and superconductors: a review of concepts and experiments.

The Nernst effect is the transverse electric field produced by a longitudinal thermal gradient in the presence of a magnetic field. At the beginning of this century, Nernst experiments on cuprates were analyzed assuming that: (i) the contribution of quasi-particles to the Nernst signal is negligible; and (ii) Gaussian superconducting fluctuations cannot produce a Nernst signal well above the critical temperature. Both these assumptions were contradicted by subsequent experiments. This paper reviews experiments documenting multiple sources of a Nernst signal, which, according to the Bridgman relation, measures the flow of transverse entropy caused by a longitudinal particle flow. Along the lines of Landauer's approach to transport phenomena, the magnitude of the transverse magneto-thermoelectric response is linked to the quantum of thermoelectric conductance and a number of material-dependent length scales: the mean free path, the Fermi wavelength, the de Broglie thermal wavelength and the superconducting coherence length. Extremely mobile quasi-particles in dilute metals generate a widely-documented Nernst signal. Fluctuating Cooper pairs in the normal state of superconductors have been found to produce a detectable Nernst signal with an amplitude conforming to the Gaussian theory, first conceived by Ussishkin, Sondhi and Huse. In addition to these microscopic sources, mobile Abrikosov vortices, mesoscopic objects simultaneously carrying entropy and magnetic flux, can produce a sizeable Nernst response. Finally, in metals subject to a magnetic field strong enough to truncate the Fermi surface to a few Landau tubes, each exiting tube generates a peak in the Nernst response. The survey of these well-established sources of the Nernst signal is a helpful guide to identify the origin of the Nernst signal in other controversial cases.

Infrared Photodetection Based on Colloidal Quantum-Dot Films with High Mobility and Optical Absorption up to THz.

Infrared thermal imaging devices rely on narrow band gap semiconductors grown by physical methods such as molecular beam epitaxy and chemical vapor deposition. These technologies are expensive, and infrared detectors remain limited to defense and scientific applications. Colloidal quantum dots (QDs) offer a low cost alternative to infrared detector by combining inexpensive synthesis and an ease of processing, but their performances are so far limited, in terms of both wavelength and sensitivity. Herein we propose a new generation of colloidal QD-based photodetectors, which demonstrate detectivity improved by 2 orders of magnitude, and optical absorption that can be continuously tuned between 3 and 20 µm. These photodetectors are based on the novel synthesis of n-doped HgSe colloidal QDs whose size can be tuned continuously between 5 and 40 nm, and on their assembly into solid nanocrystal films with mobilities that can reach up to 100 cm(2) V(-1) s(-1). These devices can be operated at room temperature with the same level of performance as the previous generation of devices when operated at liquid nitrogen temperature. HgSe QDs can be synthesized in large scale (>10 g per batch), and we show that HgSe films can be processed to form a large scale array of pixels. Taken together, these results pave the way for the development of the next generation mid- and far-infrared low-cost detectors and camera.

Electrolyte-gated colloidal nanoplatelets-based phototransistor and its use for bicolor detection.

Colloidal nanocrystals are appealing candidates for low cost optoelectronic applications because they can combine the advantages of both organic materials, such as their easy processing, and the excellent performance of inorganic systems. Here, we report the use of two-dimensional colloidal nanoplatelets for photodetection. We show that the nanoplatelets photoresponse can be enhanced by two to three orders of magnitude when they are incorporated in an all solid electrolyte-gated phototransistor. We extend this technique to build the first colloidal quantum dot-based bicolor detector with a response switchable between the visible and near-IR.

In-vacuum projection of nanoparticles for on-chip tunneling spectroscopy.

Starting with a discussion of the percolation problem applied to the trapping of conducting nanoparticles between nanometer-spaced electrodes, we show that a good strategy to trap a single nanoparticle between the electrodes is to prepare chips with low coverage of nanoparticles to avoid percolating current paths. To increase the probability of trapping a single nanoparticle, we developed a new method where nanoparticles are projected in-vacuum on the chip, followed by a measure of the tunnel current, in a cycle that is repeated up to a few thousand times until a preset threshold value is reached. A plot of the tunneling current as a function of time allows discriminating between the two possible current paths, i.e., a single nanoparticle trapped between the electrodes or a percolating path across many nanoparticles. We applied the method to prepare chip circuits with single gold nanoparticles, as demonstrated by the observation of Coulomb blockade. Furthermore, we applied the method to trap single magnetite nanoparticles for the study of electric-field-induced switching from insulator to metal in single nanoparticles.