Disorder induced Coulomb gaps in graphene constrictions with different aspect ratios

We present electron transport measurements on lithographically defined and etched graphene nanoconstrictions with different aspect ratios including different lengths (L) and widths (W). A roughly length-independent disorder induced effective energy gap can be observed around the charge neutrality point. This energy gap scales inversely with the width even in regimes where the length of the constriction is smaller than its width (L<W). In very short constrictions, we observe both resonances due to localized states or charged islands and an elevated overall conductance level (0.1-1e2/h), which is strongly length-dependent in the gap region. This makes very short graphene constrictions interesting for highly transparent graphene tunneling barriers.Comment: 4 pages, 4 figure

From diffusive to ballistic transport in etched graphene constrictions and nanoribbons

Graphene nanoribbons and constrictions are envisaged as fundamental components of future carbon-based nanoelectronic and spintronic devices. At nanoscale, electronic effects in these devices depend heavily on the dimensions of the active channel and the nature of edges. Hence, controlling both these parameters is crucial to understand the physics in such systems. This review is about the recent progress in the fabrication of graphene nanoribbons and constrictions in terms of low temperature quantum transport. In particular, recent advancements using encapsulated graphene allowing for quantized conductance and future experiments towards exploring spin effects in these devices are presented. The influence of charge carrier inhomogeneity and the important length scales which play a crucial role for transport in high quality samples are also discussed.Comment: 32 pages, 6 figures. Will appear in Annalen der Physi

Size quantization of Dirac fermions in graphene constrictions

Quantum point contacts (QPCs) are cornerstones of mesoscopic physics and central building blocks for quantum electronics. Although the Fermi wave-length in high-quality bulk graphene can be tuned up to hundreds of nanometers, the observation of quantum confinement of Dirac electrons in nanostructured graphene systems has proven surprisingly challenging. Here we show ballistic transport and quantized conductance of size-confined Dirac fermions in lithographically-defined graphene constrictions. At high charge carrier densities, the observed conductance agrees excellently with the Landauer theory of ballistic transport without any adjustable parameter. Experimental data and simulations for the evolution of the conductance with magnetic field unambiguously confirm the identification of size quantization in the constriction. Close to the charge neutrality point, bias voltage spectroscopy reveals a renormalized Fermi velocity ($v_F \approx 1.5 \times 10^6 m/s$) in our graphene constrictions. Moreover, at low carrier density transport measurements allow probing the density of localized states at edges, thus offering a unique handle on edge physics in graphene devices.Comment: 24 pages including 20 figures and 1 table. Corrected typos. To appear in Nature Communication

Probing relaxation times in graphene quantum dots

Graphene quantum dots are attractive candidates for solid-state quantum bits. In fact, the predicted weak spin-orbit and hyperfine interaction promise spin qubits with long coherence times. Graphene quantum dot devices have been extensively investigated with respect to their excitation spectrum, spin-filling sequence, and electron-hole crossover. However their relaxation dynamics remain largely unexplored. This is mainly due to challenges in device fabrication, in particular regarding the control of carrier confinement and the tunability of the tunnelling barriers, both crucial to experimentally investigate decoherence times. Here, we report on pulsed-gate transient spectroscopy and relaxation time measurements of excited states in graphene quantum dots. This is achieved by an advanced device design, allowing to tune the tunnelling barriers individually down to the low MHz regime and to monitor their asymmetry with integrated charge sensors. Measuring the transient currents through electronic excited states, we estimate lower limit of charge relaxation times on the order of 60-100 ns.Comment: To be published in Nature Communications. The first two authors contributed equally to this work. Main article: 10 pages, 4 figures. Supplementary information: 4 pages, 4 figure

Graphene-based charge sensors

We discuss graphene nanoribbon-based charge sensors and focus on their functionality in the presence of external magnetic fields and high frequency pulses applied to a nearby gate electrode. The charge detectors work well with in-plane magnetic fields of up to 7 T and pulse frequencies of up to 20 MHz. By analyzing the step height in the charge detector's current at individual charging events in a nearby quantum dot, we determine the ideal operation conditions with respect to the applied charge detector bias. Average charge sensitivities of 1.3*10^-3 e/sqrt{Hz} can be achieved. Additionally, we investigate the back action of the charge detector current on the quantum transport through a nearby quantum dot. By setting the charge detector bias from 0 to 4.5 mV, we can increase the Coulomb peak currents measured at the quantum dot by a factor of around 400. Furthermore, we can completely lift the Coulomb blockade in the quantum dot.Comment: 7 pages, 7 figure

The Electrical Transport Study Of Graphene Nanoribbons And 2d Materials Beyond Graphene

The electrical transport measurements on a suspended ultra-low-disorder graphene nanoribbon (GNR) with nearly atomically smooth edges that reveal a high mobility exceeding 3000 cm2 V-1 s-1 and an intrinsic bandgap was reported in this study. The experimentally derived bandgap is in quantitative agreement with the results of our electronic-structure calculations on chiral GNRs with comparable width taking into account the electron-electron interactions, indicating that the origin of the bandgap in non-armchair GNRs is partially due to the magnetic zigzag edges. In addition, electrical transport measurements show that current-annealing effectively removes the impurities on the suspended graphene nanoribbons, uncovering the intrinsic ambipolar transfer characteristic of graphene. Further increasing the annealing current creates a narrow constriction in the ribbon, leading to the formation of a large band-gap and subsequent high on/off ratio (which can exceed 104). This work shows for the first time that ambipolar field effect characteristics and high on/off ratios at room temperature can be achieved in relatively wide graphene nanoribbon (15 nm ~50 nm) by controlled current annealing. Moreover, a simple one-stage solution-based method was developed to produce graphene nanoribbons by sonicating graphite powder in organic solutions with polymer surfactant. Single-layer and few-layer graphene nanoribbons with a width ranging from sub-10 nm to tens of nm and length ranging from hundreds of nm to 1 ìm were routinely observed. Electrical transport properties of individual graphene nanoribbons were measured in both the back-gate and polymer-electrolyte top-gate configurations. The mobility of the graphene nanoribbons was found to be over an order of magnitude higher when measured in the latter than in the former configuration (without the polymer electrolyte), which can be attributed to the screening of the charged impurities by the counter-ions in the polymer electrolyte. This finding suggests that the charge transport in these solution-produced graphene nanoribbons is largely limited by charged impurity scattering. We also report electrical characterization of monolayer molybdenum disulfide (MoS2) devices using a thin layer of polymer electrolyte consisting of poly (ethylene oxide) (PEO) and lithium perchlorate (LiClO4) as both a contact-barrier reducer and channel mobility booster. We find that bare MoS2 devices (without polymer electrolyte) fabricated on Si/SiO2 have low channel mobility and large contact resistance, both of which severely limit the field-effect mobility of the devices. A thin layer of PEO/ LiClO4 deposited on top of the devices not only substantially reduces the contact resistance but also boost the channel mobility, leading up to three-orders-of-magnitude enhancement of the field-effect mobility of the device. When the polymer electrolyte is used as a gate medium, the MoS2 field-effect transistors exhibit excellent device characteristics such as a near ideal subthreshold swing and an on/off ratio of 106 as a result of the strong gate-channel coupling. In addition, the ambipolar field-effect transistors of atomically thin MoS2 with an ionic liquid gate were realized in this study. A record high On-Off current ratio greater than 106 is achieved for hole transport in a bilayer MoS2 transistor, while that for electron transport exceeds 107. The scaled transconductance of the device reaches 11.8 µS/µm at a drain-source voltage of 1V, which is an order of magnitude large than that observed in MoS2 transistors with a high-ê top-gate dielectric. A near ideal subthreshold swing of 47mV/dec at 230 K is also achieved in the bilayer MoS2 device

Spins, disorder and interactions in GaAs and graphene

This thesis describes experiments on semiconductor spin physics under the influence of diverse disorder and carrier-carrier interaction. Motivated by recent observations of GaAs spin qubit coherence limited by hyperfine coupling to nuclear-spin en- semble fluctuations, we started out to find ways to study the electron-spin nuclear-spin coupling or to avoid the nuclear spin bath altogether. This can be done in several different ways and here we pursued two fairly different approaches. One is the investigation of the dynamics of nuclear spin polarization in GaAs and the other aims at spin-related effects in graphene na- nostructures which possibly have negligible nuclear spin contributions due to the natural abundance (about 99 %) of zero- spin isotopes. The experiments on GaAs are performed using a non-local spin injection device with Fe ferromagnetic contacts on a degene- rately n-doped epilayer. At low temperatures, where the injected spin polarization allows dynamic polarization of the nuclear spins via hyperfine interaction, distinct spin signals are used to study the dynamics of the nuclear spin system both in presen- ce and absence of net electron spin polarization. The nuclear spin-lattice relaxation in an unpolarized environment reveals an unexpected breakdown of the Korringa law of nuclear spin relaxation otherwise valid for metallic systems. This is manifested in the observed deviation from a linear tem- perature dependence of the nuclear T_1 time and is interpreted as a result of hyperfine coupling to conduction electrons which are influenced by the interplay of disorder and carrier-carrier interaction. This finding therefore gives important insight into the strong influence of intimate coupling between the electron and nuclear spin sub-systems. Transport experiments on lithographically defined graphene quantum dots are performed at low temperatures. Three graphe- ne quantum dots of different nanometer sizes fabricated on a single graphene flake allow a detailed investigation of the size dependence of the Coulomb interaction, the energy spectra, and the influence of disorder within the nanostructures. The onset of Landau quantization in perpendicular magnetic fields reveals signatures of the electron-hole crossover reflecting the bandstructure symmetry of graphene. Suppression of orbital effects by applying external magnetic fields parallel to the sam- ple plane allows to address spin effects of the charge transitions in the quantum dots. The observed field dependence of Cou- lomb blockade peak splittings is not inconsistent with the Zeeman splitting proportional to an expected g-factor of 2. The transport data evidence strong influence of disorder supposably induced by both charged impurities in the close vicinity of the quantum dots and by edge disorder as a result of the fabrication process lacking precise control of the edge structures