arXiv subjects

Alberto F. Morpurgo

Publications and source records attributed to Alberto F. Morpurgo.

At least 91 records · Page 5Linked to original sources

arXiv2013

A ballistic pn junction in suspended graphene with split bottom gates

We have developed a process to fabricate suspended graphene devices with local bottom gates, and tested it by realizing electrostatically controlled pn junctions on a suspended graphene mono-layer nearly 2 micrometers long. Measurements as a function of gate voltage, magnetic field, bias, and temperature exhibit characteristic Fabry-Perot oscillations in the cavities formed by the pn junction and each of the contacts, with transport occurring in the ballistic regime. Our results demonstrate the possibility to achieve a high degree of control on the local electronic properties of ultra-clean suspended graphene layers, a key aspect for the realization of new graphene nanostructures.

cond-mat.mes-hall↗

arXiv2013

Competition between the Superconducting Proximity Effect and Coulomb Interactions in a Graphene Andreev Interferometer

We have investigated transport through graphene Andreev interferometers exhibiting reentrance of the superconducting proximity effect. We observed a crossover in the Andreev conductance oscillations as a function of gate voltage ($V_{BG}$). At high $V_{BG}$ the energy-dependent oscillation amplitude exhibits a scaling predicted for non-interacting electrons, which breaks down at low $V_{BG}$. The phenomenon is a manifestation of electron-electron interactions, whose main effect is to shorten the single-particle phase coherence time $τ_ϕ$. These results indicate that graphene provides a useful experimental platform to investigate the competition between superconducting proximity effect and interactions.

cond-mat.mes-hall↗

arXiv2013

Progress in organic single-crystal field-effect transistors

Research on organic thin-film transistors tends to focus on improvements in device performance, but very little is understood about the ultimate limits of these devices, the microscopic physical mechanisms responsible for their limitations, and, more generally, the intrinsic transport properties of organic semiconductors. These topics are now being investigated through the study of transport in organic transistors realized using molecular single crystals of unprecedented chemical purity and structural quality. These studies are elucidating detailed microscopic aspects of the physics of organic semiconductors and corresponding devices and have also led to unforeseen high values for carrier mobility in these materials. Here, we discuss developments in this area and present a brief outlook on future goals that have now come into experimental reach.

cond-mat.mtrl-sci↗

arXiv2013

Single-Crystal Organic Charge-Transfer Interfaces probed using Schottky-Gated Heterostructures

Organic semiconductors based on small conjugated molecules generally behave as insulators when undoped, but the hetero-interfaces of two such materials can show electrical conductivity as large as in a metal. Although charge transfer is commonly invoked to explain the phenomenon, the details of the process and the nature of the interfacial charge carriers remain largely unexplored. Here we use Schottky-gated heterostructures to probe the conducting layer at the interface between rubrene and PDIF-CN2 single crystals. Gate-modulated conductivity measurements demonstrate that interfacial transport is due to electrons, whose mobility exhibits band-like behavior from room temperature to ~ 150 K, and remains as high as ~ 1 cm2V-1s-1 at 30 K for the best devices. The electron density decreases linearly with decreasing temperature, an observation that can be explained quantitatively based on the heterostructure band diagram. These results elucidate the electronic structure of rubrene-PDIF-CN2 interfaces and show the potential of Schottky-gated organic heterostructures for the investigation of transport in molecular semiconductors.

cond-mat.mtrl-sci↗

arXiv2013

Quantitative Determination of the Band-Gap of WS2 with Ambipolar Ionic Liquid-Gated Transistors

We realized ambipolar Field-Effect Transistors by coupling exfoliated thin flakes of tungsten disulphide (WS2) with an ionic liquid-dielectric. The devices show ideal electrical characteristics, including very steep sub-threshold slopes for both electrons and holes and extremely low OFF-state currents. Thanks to these ideal characteristics, we determine with high precision the size of the band-gap of WS2 directly from the gate-voltage dependence of the source-drain current. Our results demonstrate how a careful use of ionic liquid dielectrics offers a powerful strategy to study quantitatively the electronic properties of nano-scale materials.

cond-mat.mes-hall↗

arXiv2012

Marginal topological properties of graphene: a comparison with topological insulators

The electronic structures of graphene systems and topological insulators have closely-related features, such as quantized Berry phase and zero-energy edge states. The reason for these analogies is that in both systems there are two relevant orbital bands, which generate the pseudo-spin degree of freedom, and, less obviously, there is a correspondence between the valley degree of freedom in graphene and electron spin in topological insulators. Despite the similarities, there are also several important distinctions, both for the bulk topological properties and for their implications for the edge states -- primarily due to the fundamental difference between valley and spin. In view of their peculiar band structure features, gapped graphene systems should be properly characterized as marginal topological insulators, distinct from either the trivial insulators or the true topological insulators.

cond-mat.mes-hall↗

arXiv2012

Crossover from Coulomb blockade to quantum Hall effect in suspended graphene nanoribbons

Suspended graphene nano-ribbons formed during current annealing of suspended graphene flakes have been investigated experimentally. Transport measurements show the opening of a transport gap around charge neutrality due to the formation of "Coulomb islands", coexisting with quantum Hall conductance plateaus appearing at moderate values of magnetic field $B$. Upon increasing $B$, the transport gap is rapidly suppressed, and is taken over by a much larger energy gap due to electronic correlations. Our observations show that suspended nano-ribbons allow the investigation of phenomena that could not so far be accessed in ribbons on SiO$_2$ substrates.

cond-mat.mes-hall↗

arXiv2012

Majorana fermions in superconducting helical magnets

In a variety of rare-earth based compounds singlet superconductivity coexists with helical magnetism. Here we demonstrate that surfaces of these systems should generically host a finite density of zero-energy Majorana modes. In the limit of vanishing disorder, these modes lead to a divergent contribution to zero-energy density of states and to zero-temperature entropy proportional to the sample surface area. When confined to a wire geometry, a discrete number of Majorana modes can be isolated. The relatively large characteristic energy scales for superconductivity and magnetism, compared to other proposals, as well as the lack of need for fine-tuning, make helical magnetic superconducting compounds favorable for the observation and experimental investigation of Majorana fermions.

cond-mat.mes-hall↗

arXiv2012

Very low bias stress in n-type organic single crystal transistors

Bias stress effects in n-channel organic field-effect transistors (OFETs) are investigated using PDIF-CN2 single-crystal devices with Cytop gate dielectric, both under vacuum and in ambient. We find that the amount of bias stress is very small as compared to all (p-channel) OFETs reported in the literature. Stressing the PDIF-CN2 devices by applying 80 V to the gate for up to a week results in a decrease of the source drain current of only ~1% under vacuum and ~10% in air. This remarkable stability of the devices leads to characteristic time constants, extracted by fitting the data with a stretched exponential - that are τ~ 2\cdot10^9 s in air and τ~ 5\cdot10^9 s in vacuum - approximately two orders of magnitude larger than the best values reported previously for p-channel OFETs.

cond-mat.mtrl-sci↗

arXiv2012

Band-Like Electron Transport in Organic Transistors and Implication of the Molecular Structure for Performance Optimization

Single-crystal organic field-effect transistors (OFETs) based on p-channel molecular semiconductors have led to breakthrough carrier mobilities and to the observation of band-like transport. These results represent the limit in our quest for the ultimate OFET performance. However, band-like transport has not been reported for n-channel OFETs and, for p-channel transistors, it is not understood why it occurs only for certain molecular materials. Here we report band-like electron transport for n-channel OFETs based on PDIF-CN2 single-crystals. Devices with different gate dielectrics - vacuum, Cytop, PMMA - are compared and we find that the performance is suppressed for those with larger dielectric constant. This phenomenon parallels that observed for holes in p-channel OFETs, however, the magnitude of the suppression is smaller, an effect that can be rationalized by the semiconductor molecular structure and crystal packing. A quantitative analysis of our findings, together with results on different high-quality p-channel transistors, indicates the importance of the interplay between the semiconductor molecular polarizability and the structure of the charge transport layers in the crystal, as a key factor enabling band-like transport. Based on these considerations, we suggest unprecedented structure-property relationships useful for performance optimization of high-mobility organic transistors.

cond-mat.mtrl-sci↗

arXiv2010

Marginality of bulk-edge correspondence for single-valley Hamiltonians

We study the correspondence between the non-trivial topological properties associated with the individual valleys of gapped bilayer graphene (BLG), as a prototypical multi-valley system, and the gapless modes at its edges and other interfaces. We find that the exact connection between the valley-specific Hall conductivity and the number of gapless edge modes does not hold in general, but is dependent on the boundary conditions, even in the absence of intervalley coupling. This non-universality is attributed to the absence of a well-defined topological invariant within a given valley of BLG; yet, a more general topological invariant may be defined in certain cases, which explains the distinction between the BLG-vacuum and BLG-BLG interfaces.

cond-mat.mes-hall↗

arXiv2010

Magneto-transport through graphene nano-ribbons

We investigate magneto-transport through graphene nano-ribbons as a function of gate and bias voltage, and temperature. We find that a magnetic field systematically leads to an increase of the conductance on a scale of a few tesla. This phenomenon is accompanied by a decrease in the energy scales associated to charging effects, and to hopping processes probed by temperature-dependent measurements. All the observations can be interpreted consistently in terms of strong-localization effects caused by the large disorder present, and exclude that the insulating state observed in nano-ribbons can be explained solely in terms of a true gap between valence and conduction band.

cond-mat.mes-hall↗

arXiv2010

Two-dimensional Dirac fermions in a topological insulator: transport in the quantum limit

Pulsed magnetic fields of up to 55T are used to investigate the transport properties of the topological insulator Bi_2Se_3 in the extreme quantum limit. For samples with a bulk carrier density of n = 2.9\times10^16cm^-3, the lowest Landau level of the bulk 3D Fermi surface is reached by a field of 4T. For fields well beyond this limit, Shubnikov-de Haas oscillations arising from quantization of the 2D surface state are observed, with the ν=1 Landau level attained by a field of 35T. These measurements reveal the presence of additional oscillations which occur at fields corresponding to simple rational fractions of the integer Landau indices.

cond-mat.mes-hall↗

arXiv2009

Electrostatic confinement of electrons in graphene nano-ribbons

Coulomb blockade is observed in a graphene nanoribbon device with a top gate. When two pn junctions are formed via the back gate and the local top gate, electrons are confined between the pn junctions which act as the barriers. When no pn junctions are induced by the gate voltages, electrons are still confined, as a result of strong disorder, but in a larger area. Measurements on five other devices with different dimensions yield consistent results.

cond-mat.mes-hall↗

arXiv2009

High Electron Mobility in Vacuum and Ambient for PDIF-CN2 Single-Crystal Transistors

We have investigated the electron mobility on field-effect transistors based on PDIF-CN$_{2}$ single crystals. The family of the small molecules PDI8-CN$_{2}$ has been chosen for the promising results obtained for vapour-deposited thin film FETs. We used as gate dielectric a layer of PMMA (spinned on top of the SiO$_{2}$), to reduce the possibility of electron trapping by hydroxyl groups present at surface of the oxide. For these devices we obtained a room temperature mobility of 6 cm$^{2}$/Vs in vacuum and 3 cm$^{2}$/Vs in air. Our measurements demonstrate the possibility to obtain n-type OFETs with performances comparable to those of p-type devices.

cond-mat.mtrl-sci↗

arXiv2007

Gate-induced insulating state in bilayer graphene devices

The potential of graphene-based materials consisting of one or a few layers of graphite for integrated electronics originates from the large room-temperature carrier mobility in these systems (approx. 10,000 cm2/Vs). However, the realization of electronic devices such as field-effect transistors will require controlling and even switching off the electrical conductivity by means of gate electrodes, which is made difficult by the absence of a bandgap in the intrinsic material. Here, we demonstrate the controlled induction of an insulating state - with large suppression of the conductivity - in bilayer graphene, by using a double-gate device configuration that allows an electric field to be applied perpendicular to the plane. The dependence of the resistance on temperature and electric field, and the absence of any effect in a single-layer device, strongly suggest that the gate-induced insulating state originates from the recently predicted opening of a bandgap between valence and conduction bands.

cond-mat.mes-hall↗

arXiv2007

Aharonov-Bohm effect in graphene

We investigate experimentally transport through ring-shaped devices etched in graphene and observe clear Aharonov-Bohm conductance oscillations. The temperature dependence of the oscillation amplitude indicates that below 1 K the phase coherence length is comparable to or larger than the size of the ring. An increase in the amplitude is observed at high magnetic field, when the cyclotron diameter becomes comparable to the width of the arms of the ring. By measuring the dependence on gate voltage, we also observe an unexpected linear dependence of the oscillation amplitude on the ring conductance, which had not been reported earlier in rings made using conventional metals or semiconducting heterostructures.

cond-mat.mes-hall↗

arXiv2006

Bipolar supercurrent in graphene

Graphene -a recently discovered one-atom-thick layer of graphite- constitutes a new model system in condensed matter physics, because it is the first material in which charge carriers behave as massless chiral relativistic particles. The anomalous quantization of the Hall conductance, which is now understood theoretically, is one of the experimental signatures of the peculiar transport properties of relativistic electrons in graphene. Other unusual phenomena, like the finite conductivity of order 4e^2/h at the charge neutrality (or Dirac) point, have come as a surprise and remain to be explained. Here, we study the Josephson effect in graphene. Our experiments rely on mesoscopic superconducting junctions consisting of a graphene layer contacted by two closely spaced superconducting electrodes, where the charge density can be controlled by means of a gate electrode. We observe a supercurrent that, depending on the gate voltage, is carried by either electrons in the conduction band or by holes in the valence band. More importantly, we find that not only the normal state conductance of graphene is finite, but also a finite supercurrent can flow at zero charge density. Our observations shed light on the special role of time reversal symmetry in graphene and constitute the first demonstration of phase coherent electronic transport at the Dirac point.

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