A. K. Savchenko

University of Exeter, Exeter, England, United Kingdom

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Publications (80)228.71 Total impact

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    ABSTRACT: Variable-range hopping ~VRH! conductance fluctuations in the gate-voltage characteristics of mesoscopic gallium arsenide and silicon transistors are analyzed by means of their full distribution functions ~DF’s!. The forms of the DF predicted by the theory of Raikh and Ruzin have been verified under controlled conditions for both the long, narrow wire and the short, wide channel geometries. The variation of the mean square fluctuation size with temperature in wires fabricated from both materials is found to be described quantitatively by Lee’s model of VRH along a one-dimensional chain. Armed with this quantitative validation of the VRH model, the DF method is applied to the problem of magnetoconductance in the insulating regime. Here a nonmonotonic variation of the magnetoconductance is observed in silicon metal-oxide-semiconductor fieldeffect transistors whose sign at low magnetic fields is dependent on the channel geometry. The origin of this effect is discussed within the framework of the interference model of VRH magnetoconductance in terms of a narrowing of the DF in a magnetic field.
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    ABSTRACT: The electrical properties of graphene are known to be modified by chemical species that interact with it. We investigate the effect of doping of graphene-based devices by toluene (C6H5CH3). We show that this effect has a complicated character. Toluene is seen to act as a donor, transferring electrons to the graphene. However, the degree of doping is seen to depend on the magnitude and polarity of an electric field applied between the graphene and a nearby electrode. This can be understood in terms of an electrochemical reaction mediated by the graphene crystal.
    Carbon. 01/2011; 49(12):3829-3834.
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    ABSTRACT: We investigate the nonlinear optical properties of graphene flakes using four-wave mixing. The corresponding third-order optical susceptibility is found to be remarkably large and only weakly dependent on the wavelength in the near-infrared frequency range. The magnitude of the response is in good agreement with our calculations based on the nonlinear quantum response theory.
    Physical Review Letters 08/2010; 105(9):097401. · 7.73 Impact Factor
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    ABSTRACT: Recent studies of the electronic properties of graphite have produced conflicting results regarding the positions of the different carrier types within the Brillouin zone, and the possible presence of Dirac fermions. In this paper we report a comprehensive study of the de Haas-van Alphen, Shubnikov-de Haas and Hall effects in a sample of highly orientated pyrolytic graphite, at temperatures in the range 30 mK to 4 K and magnetic fields up to 12 T. The transport measurements confirm the Brillouin-zone locations of the different carrier types assigned by Schroeder et al.: electrons are at the K-point, and holes are near the H-points. We extract the cyclotron mass and scattering time for both carrier types from the temperature- and magnetic-field-dependences of the magneto-oscillations. Our results indicate that the holes experience stronger scattering and hence have a lower mobility than the electrons. We utilise phase-frequency analysis and intercept analysis of the 1/B positions of magneto-oscillation extrema to identify the nature of the carriers in graphite, whether they are Dirac or normal (Schrodinger) fermions. These analyses indicate normal holes and electrons of indeterminate nature.
    Physical review. B, Condensed matter 07/2010; · 3.77 Impact Factor
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    Freddie Withers, Marc Dubois, Alexander K. Savchenko
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    ABSTRACT: We have fabricated transistor structures using fluorinated single-layer graphene flakes and studied their electronic properties at different temperatures. Compared with pristine graphene, fluorinated graphene has very large and strongly temperature dependent resistance in the electro-neutrality region. We show that fluorination creates a mobility gap in graphene's spectrum where electron transport takes place via localised electron states.
    Physical review. B, Condensed matter 05/2010; · 3.77 Impact Factor
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    ABSTRACT: The effect of electron-electron interaction on the low-temperature conductivity of graphene is investigated experimentally. Unlike in other two-dimensional systems, the electron-electron interaction correction in graphene is sensitive to the details of disorder. A new temperature regime of the interaction correction is observed where quantum interference is suppressed by intra-valley scattering. We determine the value of the interaction parameter, F_0 ~ -0.1, and show that its small value is due to the chiral nature of interacting electrons. Comment: 4 pages, 4 figures, 1 table
    Physical review. B, Condensed matter 04/2010; · 3.77 Impact Factor
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    ABSTRACT: Graphene – a single layer of sp2 bonded carbon atoms arranged in a honeycomb pattern – is an indefinitely large aromatic molecule, of unique interest in the field of transparent organic electronics. This is a transparent material where charge carriers (relativistic Dirac fermions) exhibit mobilities (>10^6 cm^2/Vs) higher than Si at room temperature. However, the energy dispersion of graphene is gap-less, and this would limit its applications in electronic devices. For instance, in a graphene-based transistor the absence of the gap in the band-structure results in a relatively small resistance difference between the electro-neutrality (Dirac) region and a region with large carrier concentration (i.e., between the "on" and "off" states). Due to this significant limitation in the use of graphene in electronics, intensive research is currently underway aimed at the creation of a (tunable) gap in graphene's energy spectrum. The ability to chemically functionalize graphene, for instance with fluorine [1] and hydrogen [2] atoms, paved the way towards band-gap engineering. This type of functionalization transforms the graphene planar crystal structure, with sp2 bonds between the carbon atoms, into a three-dimensional structure with sp3 bonding between them. Theoretical predictions show that a band gap of 3.8 eV and 4.2 eV is expected for hydrogen and fluorine for 100% functionalization, respectively [3, 4]. Here I will review recent results on fluorinated graphene transistors [1] produced by mechanical exfoliation of natural graphite which is fluorinated to 24% and 100% (as measured by mass uptake). Transport measurements over a wide range of temperatures (from 4.2K to 300K) show a very large and strongly temperature dependent resistance in the electro-neutrality region. The strong temperature dependence of fluorinated graphene is due to the opening of a mobility in gap in the energy spectrum of graphene where electron transport takes place via localised electron states. Magneto-transport experiments through fluorinated graphene as a function of gate voltage, bias voltage, and temperature show 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 these results demonstrate that disorder induced sub-gap states originate strong localization effects in the transport of charge carriers for energies below the energy-gap of fluorinated graphene.
    Phys. Rev. B Science Phys. Rev. B J. Phys.: Condens. Matter. 01/2010; 82(21):73403-153401.
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    ABSTRACT: We present the first experimental investigation of nonlinear optical properties of graphene flakes. We find that at near infrared frequencies a graphene monolayer exhibits a remarkably high third-order optical nonlinearity which is practically independent of the wavelengths of incident light. The nonlinear optical response can be utilized for imaging purposes, with image contrasts of graphene which are orders of magnitude higher than those obtained using linear microscopy.
    Physical Review Letters - PHYS REV LETT. 12/2009;
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    A. S. Price, A. K. Savchenko, D. A. Ritchie
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    ABSTRACT: We present the first experimental study of mesoscopic fluctuations of Coulomb drag in a system with two layers of composite fermions, which are seen when either the magnetic field or carrier concentration are varied. These fluctuations cause an alternating sign of the average drag. We study these fluctuations at different temperatures to establish the dominant dephasing mechanism of composite fermions. Comment: 4 pages, 4 figures
    Physical review. B, Condensed matter 12/2009; · 3.77 Impact Factor
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    ABSTRACT: We show that quantum interference in graphene can result in antilocalization of charge carriers--an increase of the conductance, which is detected by a negative magnetoconductance. We demonstrate that depending on experimental conditions one can observe either weak localization or antilocalization of carriers in graphene. A transition from localization to antilocalization occurs when the carrier density is decreased and the temperature is increased. We show that quantum interference in graphene can survive at high temperatures, up to T approximately 200 K, due to weak electron-phonon scattering.
    Physical Review Letters 11/2009; 103(22):226801. · 7.73 Impact Factor
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    ABSTRACT: The wave nature of electrons in low-dimensional structures manifests itself in conventional electrical measurements as a quantum correction to the classical conductance. This correction comes from the interference of scattered electrons which results in electron localisation and therefore a decrease of the conductance. In graphene, where the charge carriers are chiral and have an additional (Berry) phase of \pi, the quantum interference is expected to lead to anti-localisation: an increase of the conductance accompanied by negative magnetoconductance (a decrease of conductance in magnetic field). Here we observe such negative magnetoconductance which is a direct consequence of the chirality of electrons in graphene. We show that graphene is a unique two-dimensional material in that, depending on experimental conditions, it can demonstrate both localisation and anti-localisation effects. We also show that quantum interference in graphene can survive at unusually high temperatures, up to T~200 K.
    04/2009;
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    ABSTRACT: We study fluctuations of the conductance of micron-sized graphene devices as a function of the Fermi energy and magnetic field. The fluctuations are studied in combination with analysis of weak localization which is determined by the same scattering mechanisms. It is shown that the variance of conductance fluctuations depends not only on inelastic scattering that controls dephasing but also on elastic scattering. In particular, contrary to its effect on weak localization, strong intervalley scattering suppresses conductance fluctuations in graphene. The correlation energy, however, is independent of the details of elastic scattering and can be used to determine the electron temperature of graphene structures.
    Solid State Communications 03/2009; · 1.53 Impact Factor
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    ABSTRACT: We propose a method of measuring the electron temperature T_{e} in mesoscopic conductors and demonstrate experimentally its applicability to micron-size graphene devices in the linear-response regime (T_{e} approximately T, the bath temperature). The method can be especially useful in case of overheating, T_{e}>T. It is based on analysis of the correlation function of mesoscopic conductance fluctuations. Although the fluctuation amplitude strongly depends on the details of electron scattering in graphene, we show that T_{e} extracted from the correlation function is insensitive to these details.
    Physical Review Letters 02/2009; 102(6):066801. · 7.73 Impact Factor
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    ABSTRACT: We have fabricated graphene devices with a top gate separated from the graphene layer by an air gap-a design which does not decrease the mobility of charge carriers under the gate. This gate is used to realize p-n-p structures where the conducting properties of chiral carriers are studied. The band profile of the structures is calculated taking into account the specifics of the graphene density of states and is used to find the resistance of the p-n junctions expected for chiral carriers. We show that ballistic p-n junctions have larger resistance than diffusive ones. This is caused by suppressed transmission of chiral carriers at angles away from the normal to the junction.
    Nano Letters 08/2008; 8(7):1995-9. · 13.03 Impact Factor
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    ABSTRACT: We show that the manifestation of quantum interference in graphene is very different from that in conventional two-dimensional systems. Because of the chiral nature of charge carriers, it is not only sensitive to inelastic, phase-breaking scattering, but also to a number of elastic scattering processes. We study weak localization in different samples and at different carrier densities, including the Dirac region, and find the characteristic rates that determine it. We show how the shape and quality of graphene flakes affect the values of the elastic and inelastic rates and discuss their physical origin.
    Physical Review Letters 03/2008; 100(5):056802. · 7.73 Impact Factor
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    ABSTRACT: We demonstrate quantitative experimental evidence for a weak localization correction to the conductivity in monolayer and bilayer graphene systems. We show how inter- and intra-valley elastic scattering control the correction in small magnetic fields in a way which is unique to graphene. A clear difference in the forms of the correction is observed in the two systems, which shows the importance of the interplay between the elastic scattering mechanisms and how they can be distinguished. Our observation of the correction at zero-net carrier concentration in both systems is clear evidence of the inhomogeneity engendered into the graphene layers by disorder.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 02/2008; 366(1863):245-50. · 2.89 Impact Factor
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    ABSTRACT: We experimentally demonstrate that weak localisation (WL) exists in graphene fabricated by mechanical exfoliation at different carrier densities. We show that it is controlled not only by inelastic dephasing mechanisms, but also by elastic intra- and inter-valley scattering. By reducing the width of the graphene sample, inter-valley scattering is found to be limited by the sample boundaries. Intravalley scattering, which suppresses WL in one valley, is found to be much faster than inter-valley scattering. In the Dirac point, where there is a zero-net carrier density, we observe a distinct change in both elastic and inelastic scattering.
    Physica E Low-dimensional Systems and Nanostructures 01/2008; · 1.86 Impact Factor
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    A.S. Price, A.K. Savchenko, G. Allison, D.A. Ritchie
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    ABSTRACT: We have observed reproducible fluctuations of the Coulomb drag resistivity, originating from coherent scattering of electrons in two layers. The fluctuations are observed as functions of both the carrier density of each layer, and perpendicular magnetic field. The magnitude of the fluctuations is much larger than expected from the theory of ‘diffusive’ drag and the temperature dependence is stronger than theoretically predicted. We account for this enhancement by considering the ‘ballistic’ nature of the drag in our system. We also present results on the fluctuations at large magnetic fields, where the lowest Landau level is half filled so that coherent drag occurs between composite fermions. The magnitude of fluctuations is seen to be greatly enhanced compared with the small field case, although fluctuations of composite fermion drag show much better agreement with the theory developed for the ‘diffusive’ drag regime.
    Physica E Low-dimensional Systems and Nanostructures 01/2008; · 1.86 Impact Factor
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    ABSTRACT: A comparative study of Coulomb blockade oscillations in a small quantum ring with effective radius 100 nm in ordinary tunnelling and anomalous open regimes is performed. We demonstrate that even though there is no tunnelling barrier between the ring and 2D electron sea in this ring, gigantic Coulomb blockade-like oscillations can be observed. Gate voltage, DC bias and temperature dependences of the ring conductance were experimentally studied at temperatures 0.24 1.4K and magnetic fields up to 1.5 T. These studies show the principal differences between Coulomb blockade oscillations in the tunnelling regime and in the open state. The fundamental single-electron period of the oscillations is found to be the same in both regimes. The role of backscattering processes is concluded to be the reason for the existence of strong Coulomb blockade effects in the open state of the ring.
    Physica E Low-dimensional Systems and Nanostructures 01/2008; 40:1121-1123. · 1.86 Impact Factor
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    ABSTRACT: We report the first experimental study of the quantum interference correction to the conductivity of bilayer graphene. Low-field, positive magnetoconductivity due to the weak localisation effect is investigated at different carrier densities, including those around the electroneutrality region. Unlike conventional 2D systems, weak localisation in bilayer graphene is affected by elastic scattering processes such as intervalley scattering. Analysis of the dephasing determined from the magnetoconductivity is complemented by a study of the field- and density-dependent fluctuations of the conductance. Good agreement in the value of the coherence length is found between these two studies.
    09/2007;