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Last update: 06.2014

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Welcome to the QUANTUM ELECTRONICS GROUP website


We use nano-electronic devices to investigate microscopic electronic processes in different classes of materials, with the aim to push further our fundamental understanding of their electronic properties, to discover new physical phenomena, and to contribute to the development of new practical electronic applications.

Look at some of our recent papers:

 Mono- and Bi-layer WS2 Light-Emitting Transistors

 We have realized ambipolar ionic liquid gated field-effect
 transistors based on WS2 mono- and bilayers, and
 investigated their opto-electronic response. A thorough
 characterization of the transport properties
 demonstrates the high quality of these devices for both
 electron and hole accumulation, which enables
 the quantitative determination of the band gap (Δ1L = 2.14 eV for monolayers and
 Δ2L = 1.82 eV for bilayers). It also enables the operation of the transistors in
 the ambipolar injection regime with electrons and holes injected simultaneously at
 the two opposite contacts of the devices in which we observe light emission from
 the FET channel. A quantitative analysis of the spectral properties of the emitted
 light, together with a comparison with the band gap values obtained from
 transport, show the internal consistency of our results and allow a quantitative
 estimate of the excitonic binding energies to be made. Our results demonstrate
 the power of ionic liquid gating in combination with nanoelectronic systems,
 as well as the compatibility of this technique with optical measurements
 on semiconducting transition metal dichalcogenides. These findings further open
 the way to the investigation of the optical properties of these systems in
 a carrier density range much broader than that explored until now.

 Even denominator fractional quantum Hall effect
 in suspended multiterminal bilayer graphene

 We investigate low-temperature magneto-transport
 in recently developed, high-quality multiterminal
 suspended bilayer graphene devices, enabling the
 independent measurement of the longitudinal and
 transverse resistance. We observe clear signatures of the fractional quantum Hall
 effect with different states that are either fully developed, and exhibit a clear
 plateau in the transverse resistance with a concomitant dip in longitudinal
 resistanceor incipient, and exhibit only a longitudinal resistance minimum.
 All observed states scale as a function of filling factor ν, as expected.
 An unprecedented even-denominator fractional state is observed at ν = −1/2 on
 the hole side, exhibiting a clear plateau in Rxy quantized at the expected value of
 2h/e2 with a precision of 0.5%. Many of our observations, together with a recent
 electronic compressibility measurement performed in graphene bilayers
 on hexagonalboron-nitride (hBN) substrates, are consistent with a recent
 theory that accounts for the effect of the degeneracy between the N = 0
 and N = 1 Landau levels in the fractional quantum Hall effect and predicts
 the occurrence of a Moore-Read type ν = −1/2 state. Owing to the experimental
 flexibility of bilayer graphene, which has a gate-dependent band structure, can be
 easily accessed by scanning probes, and can be contacted with materials such
 as superconductors, our findings offer new possibilities to explore the microscopic
 nature of even-denominator fractional quantum Hall effect.

 Tuning the influence of microscopic decoherence
 on superconducting proximity effect in a graphene
 Andreev interferometer devices

 We discuss transport measurements through graphene
 Andreev interferometers exhibiting reentrance of
 the superconducting proximity effect. We observe that
 at high gate voltage (VBG) the energy dependence of
 the Andreev conductance oscillations exhibits a scaling in agreement with
 theoretical expectations, which breaks down at low VBG, when the Fermi energy
 approaches the charge neutrality point. The phenomenon is a manifestation
 of single particle dephasing that increasingly limits the propagation of
 superconducting correlations away from the superconductor-graphene interface.
 Our work addresses the interplay between microscopic decoherence and
 superconductivity, and shows that graphene provides a useful experimental
 platform to investigate unexplored regimes and phenomena in the superconducting
 proximity effect.

 Tailoring the molecular structure to suppress
 extrinsic disorder in organic transistors

 In organic field-effect transistors, the structure
 of the constituent molecules can be tailored to minimize
 the disorder experienced by charge carriers.
 Experiments on two perylene derivatives show that
 disorder can be suppressed by attaching longer core
 substituents – thereby reducing potential fluctuations in the transistor channel and
 increasing the mobility in the activated regime – without altering the intrinsic
 transport properties.

 High-quality multi-terminal suspended graphene

 We introduce a new scheme to realize suspended,
 multiterminal graphene structures that can be current
 annealed successfully to obtain uniform, very high
 quality devices. A key aspect is that the bulky metallic
 contacts are not connected directly to the part of
 graphene probed by transport measurements, but only through etched
 constriction, which prevents the contacts from acting invasively. The device high
 quality and uniformity is demonstrated by a reproducibly narrow (δn 109 cm–2)
 resistance peak around charge neutrality, by carrier mobility values exceeding
 106 cm2 V–1 s–1, by the observation of integer quantum Hall plateaus starting
 at 30 mT and of symmetry broken states at about 200 mT, and by the occurrence
 of a negative multiterminal resistance directly proving the occurrence of ballistic
 transport. As these multiterminal devices enable measurements that cannot be
 done in a simpler two-terminal configuration, we anticipate that their use in future
 studies of graphene-based systems will be particularly relevant.

 More papers...


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