Porphyrins and other tetrapyrrole macrocycles possess an impressive variety of functional properties that have been exploited in natural and artificial systems. Different metal centres incorporated within the tetradentate ligand are key for achieving and regulating vital processes, including reversible axial ligation of adducts, electron transfer, light-harvesting and catalytic transformations. Tailored substituents optimize their performance, dictating their arrangement in specific environments and mediating the assembly of molecular nanoarchitectures. Here we review the current understanding of these species at well-defined interfaces, disclosing exquisite insights into their structural and chemical properties, and also discussing methods by which to manipulate their intramolecular and organizational features. The distinct characteristics arising from the interfacial confinement offer intriguing prospects for molecular science and advanced materials. We assess the role of surface interactions with respect to electronic and physicochemical characteristics, and describe in situ metallation pathways, molecular magnetism, rotation and switching. The engineering of nanostructures, organized layers, interfacial hybrid and bio-inspired systems is also addressed.

Porphyrins at interfaces

https://poggiolab.unibas.ch/full/2012PootPR.pdf

Mechanical systems in the quantum regime

Mechanical systems are ideal candidates for studying quantumbehavior of macroscopic objects. To this end, a mechanical resonator has to be cooled to its ground state and its position has to be measured with great accuracy. Currently, various routes to reach these goals are being explored. In this review, we discuss different techniques for sensitive position detection and we give an overview of the cooling techniques that are being employed. The latter include sideband cooling and active feedback cooling. The basic concepts that are important when measuring on mechanical systems with high accuracy and/or at very low temperatures, such as thermal and quantum noise, linear response theory, and backaction, are explained. From this, the quantum limit on linear position detection is obtained and the sensitivities that have been achieved in recent opto and nanoelectromechanical experiments are compared to this limit. The mechanical resonators that are used in the experiments range from meter-sized gravitational wave detectors to nanomechanical systems that can only be read out using mesoscopic devices such as single-electron transistors or superconducting quantum interference devices. A special class of nanomechanical systems are bottom-up fabricated carbon-based devices, which have very high frequencies and yet a large zero-point motion, making them ideal for reaching the quantum regime. The mechanics of some of the different mechanical systems at the nanoscale is studied. We conclude this review with an outlook of how state-of-the-art mechanical resonators can be improved to study quantum {\it mechanics}.

/pdf/mechanical-systems-in-the-quantum-regime-2g3p0kkzaq.pdf

As the first in a large family of 2D van der Waals (vdW) materials, graphene has attracted enormous attention owing to its remarkable properties. The recent development of simple experimental techniques for combining graphene with other atomically thin vdW crystals to form heterostructures has enabled the exploration of the properties of these so-called vdW heterostructures. Hexagonal boron nitride is the second most popular vdW material after graphene, owing to the new physics and device properties of vdW heterostructures combining the two. Hexagonal boron nitride can act as a featureless dielectric substrate for graphene, enabling devices with ultralow disorder that allow access to the intrinsic physics of graphene, such as the integer and fractional quantum Hall effects. Additionally, under certain circumstances, hexagonal boron nitride can modify the optical and electronic properties of graphene in new ways, inducing the appearance of secondary Dirac points or driving new plasmonic states. Integrating other vdW materials into these heterostructures and tuning their new degrees of freedom, such as the relative rotation between crystals and their interlayer spacing, provide a path for engineering and manipulating nearly limitless new physics and device properties. This is an overview of the new physics that emerges in van der Waals heterostructures consisting of graphene and hexagonal boron nitride, including the integer and fractional quantum Hall effects, novel plasmonic states and the effects of emergent moire superlattices.

van der Waals heterostructures combining graphene and hexagonal boron nitride

The properties of a quantum bit coupled to both a microwave cavity and a phonon mode in a micromechanical resonator suggest that such systems may allow for storage of quantum information in long-lived phonon states and read-out via microwave photons, with applications in quantum information control. In the emerging field of quantum information technologies the next advances are expected to involve the combination of different types of quantum systems to harness various degrees of freedom. In this spirit, this paper describes the construction of a solid-state system combining a memory element, which has long-lived quantum states, with a quantum interface that offers easy read-out. This is achieved by coupling an artificial two-level atom in the form of a superconducting transmon qubit, to two different resonant cavities — a microwave resonator and a nanomechanical resonator. In the resulting hybrid device the low-frequency phonon cavity stores the quantum information from the qubit, and the electrical microwave resonator communicates with the outside world. Hybrid quantum systems with inherently distinct degrees of freedom have a key role in many physical phenomena. Well-known examples include cavity quantum electrodynamics1, trapped ions2, and electrons and phonons in the solid state. In those systems, strong coupling makes the constituents lose their individual character and form dressed states, which represent a collective form of dynamics. As well as having fundamental importance, hybrid systems also have practical applications, notably in the emerging field of quantum information control. A promising approach is to combine long-lived atomic states2,3 with the accessible electrical degrees of freedom in superconducting cavities and quantum bits4,5 (qubits). Here we integrate circuit cavity quantum electrodynamics6,7 with phonons. Apart from coupling to a microwave cavity, our superconducting transmon qubit8, consisting of tunnel junctions and a capacitor, interacts with a phonon mode in a micromechanical resonator, and thus acts like an atom coupled to two different cavities. We measure the phonon Stark shift, as well as the splitting of the qubit spectral line into motional sidebands, which feature transitions between the dressed electromechanical states. In the time domain, we observe coherent conversion of qubit excitation to phonons as sideband Rabi oscillations. This is a model system with potential for a quantum interface, which may allow for storage of quantum information in long-lived phonon states, coupling to optical photons or for investigations of strongly coupled quantum systems near the classical limit.

Hybrid circuit cavity quantum electrodynamics with a micromechanical resonator

One of the suggested ways of controlling the electronic properties of graphene is to establish a periodic potential modulation on it, which could be achieved by self-assembly of ordered molecular lattices. We have studied the self-assembly of cobalt phthalocyanines (CoPc) on chemical vapor deposition (CVD) grown graphene transferred onto silicon dioxide (SiO2) and hexagonal boron nitride (h-BN) substrates. Our scanning tunneling microscopy (STM) experiments show that, on both substrates, CoPc forms a square lattice. However, on SiO2, the domain size is limited by the corrugation of graphene, whereas on h-BN, single domain extends over entire terraces of the underlying h-BN. Additionally, scanning tunneling spectroscopy (STS) measurements suggest that CoPc molecules are doped by the substrate and that the level of doping varies from molecule to molecule. This variation is larger on graphene on SiO2 than on h-BN. These results suggest that graphene on h-BN is an ideal substrate for the study of molecular se...

Molecular self-assembly on graphene on SiO2 and h-BN substrates

The detection of mechanical vibrations near the quantum limit is a formidable challenge since the displacement becomes vanishingly small when the number of phonon quanta tends toward zero. An interesting setup for on-chip nanomechanical resonators is that of coupling them to electrical microwave cavities for detection and manipulation. Here we show how to achieve a large cavity coupling energy of up to (2π) 1 MHz/nm for metallic beam resonators at tens of megahertz. We used focused ion beam (FIB) cutting to produce uniform slits down to 10 nm, separating patterned resonators from their gate electrodes, in suspended aluminum films. We measured the thermomechanical vibrations down to a temperature of 25 mK, and we obtained a low number of about 20 phonons at the equilibrium bath temperature. The mechanical properties of Al were excellent after FIB cutting, and we recorded a quality factor of Q ∼ 3 × 10(5) for a 67 MHz resonator at a temperature of 25 mK. Between 0.2 and 2 K we find that the dissipation is linearly proportional to the temperature.

/pdf/strong-gate-coupling-of-high-q-nanomechanical-resonators-274fjebz1k.pdf

Strong Gate Coupling of High-Q Nanomechanical Resonators

We study inelastic energy relaxation in graphene for low energies to find out how electrons scatter with acoustic phonons and other electrons. By coupling the graphene to superconductors, we create a strong dependence of the measured signal, i.e., critical Josephson current, on the electron population on different energy states. Since the relative population of high- and low-energy states is determined by the inelastic scattering processes, the critical current becomes an effective probe for their strength. We argue that the electron-electron interaction is the dominant relaxation method and we estimate a scattering time ${\ensuremath{\tau}}_{e\ensuremath{-}e}=0.1\dots{}1$ ps at $T=500$ mK, 1-2 orders of magnitude smaller than predicted for normal two-dimensional diffusive systems.

/pdf/energy-relaxation-in-graphene-and-its-measurement-with-35gppy6xkk.pdf

Energy relaxation in graphene and its measurement with supercurrent

Single electron transistors (SETs) fabricated from single-walled carbon nanotubes (SWNTs) can be operated as highly sensitive charge detectors reaching sensitivity levels comparable to metallic radio frequency SETs (rf-SETs). Here, we demonstrate how the charge sensitivity of the device can be improved by using the mechanical oscillations of a single-walled carbon nanotube quantum dot. To optimize the charge sensitivity dQ, we drive the mechanical resonator far into the nonlinear regime and bias it to an operating point where the mechanical third order nonlinearity is canceled out. This way we enhance dQ, from 6 mu e/(Hz)(1/2) for the static case to 0.97 mu e/(Hz)(1/2) at a probe frequency of similar to 1.3 kHz.

Charge Sensitivity Enhancement via Mechanical Oscillation in Suspended Carbon Nanotube Devices

We have investigated the microwave response of nanotube Josephson junctions at 600-900 MHz at microwave powers corresponding to currents from 0 to $2\times I_{\mathrm C}$ in the junction. Compared with theoretical modeling, the response of the junctions correspond well to the lumped element model of resistively and capacitively shunted junction. We demonstrate the operation of these superconducting FETs as charge detectors at high frequencies without any matching circuits. Gate-voltage-induced charge $Q_{\rm G}$ modifies the critical current $I_{\rm C}$, which changes the effective impedance of the junction under microwave irradiation. This change, dependent on the transfer characteristics $dI_{\mathrm C}/dQ_{\rm G}$, modifies the reflected signal and it can be used for wide band electrometry. We measure a sensitivity of $3.1\times10^{-5}$ $e/\sqrt{\mathrm{Hz}}$ from a sample which has a maximum switching current of 2.6 nA.

/pdf/wideband-superconducting-nanotube-electrometer-3k7kelmzhp.pdf

Pasi Häkkinen

Papers

Molecular self-assembly on graphene on SiO2 and h-BN substrates

Strong Gate Coupling of High-Q Nanomechanical Resonators

Energy relaxation in graphene and its measurement with supercurrent

Charge Sensitivity Enhancement via Mechanical Oscillation in Suspended Carbon Nanotube Devices

Wideband superconducting nanotube electrometer