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Kłopotowski Ł, Backes C, Mitioglu AA, Vega-Mayoral V, Hanlon D, Coleman JN, Ivanov VY, Maude DK, Plochocka P. Revealing the nature of excitons in liquid exfoliated monolayer tungsten disulphide. NANOTECHNOLOGY 2016; 27:425701. [PMID: 27606691 DOI: 10.1088/0957-4484/27/42/425701] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
Transition metal dichalcogenides (TMD) hold promise for applications in novel optoelectronic devices. There is therefore a need for materials that can be obtained in large quantities and with well understood optical properties. In this report, we present thorough photoluminescence (PL) investigations of monolayer tungsten disulphide obtained via liquid phase exfoliation. As shown by microscopy studies, the exfoliated nanosheets have dimensions of tens of nanometers and thickness of 2.5 monolayers on average. The monolayer content is about 20%. Our studies show that at low temperature the PL is dominated by excitons localized on nanosheet edges. As a consequence, the PL is strongly sensitive to the environment and exhibits an enhanced splitting in magnetic field. As the temperature is increased, the excitons are thermally excited out of the defect states and the dominant transition is that of the negatively charged exciton. Furthermore, upon excitation with a circularly polarized light, the PL retains a degree of polarization reaching 50% and inherited from the valley polarized photoexcited excitons. The studies of PL dynamics reveal that the PL lifetime is on the order of 10 ps, which is probably limited by non-radiative processes. Our results underline the potential of liquid exfoliated TMD monolayers in large scale optoelectronic devices.
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Alexander-Webber JA, Huang J, Maude DK, Janssen TJBM, Tzalenchuk A, Antonov V, Yager T, Lara-Avila S, Kubatkin S, Yakimova R, Nicholas RJ. Giant quantum Hall plateaus generated by charge transfer in epitaxial graphene. Sci Rep 2016; 6:30296. [PMID: 27456765 PMCID: PMC4960615 DOI: 10.1038/srep30296] [Citation(s) in RCA: 26] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2016] [Accepted: 06/24/2016] [Indexed: 11/09/2022] Open
Abstract
Epitaxial graphene has proven itself to be the best candidate for quantum electrical resistance standards due to its wide quantum Hall plateaus with exceptionally high breakdown currents. However one key underlying mechanism, a magnetic field dependent charge transfer process, is yet to be fully understood. Here we report measurements of the quantum Hall effect in epitaxial graphene showing the widest quantum Hall plateau observed to date extending over 50 T, attributed to an almost linear increase in carrier density with magnetic field. This behaviour is strong evidence for field dependent charge transfer from charge reservoirs with exceptionally high densities of states in close proximity to the graphene. Using a realistic framework of broadened Landau levels we model the densities of donor states and predict the field dependence of charge transfer in excellent agreement with experimental results, thus providing a guide towards engineering epitaxial graphene for applications such as quantum metrology.
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Piot BA, Desrat W, Maude DK, Kazazis D, Cavanna A, Gennser U. Disorder-Induced Stabilization of the Quantum Hall Ferromagnet. PHYSICAL REVIEW LETTERS 2016; 116:106801. [PMID: 27015501 DOI: 10.1103/physrevlett.116.106801] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/13/2015] [Indexed: 06/05/2023]
Abstract
We report on an absolute measurement of the electronic spin polarization of the ν=1 integer quantum Hall state. The spin polarization is extracted in the vicinity of ν=1 (including at exactly ν=1) via resistive NMR experiments performed at different magnetic fields (electron densities) and Zeeman energy configurations. At the lowest magnetic fields, the polarization is found to be complete in a narrow region around ν=1. Increasing the magnetic field (electron density) induces a significant depolarization of the system, which we attribute to a transition between the quantum Hall ferromagnet and the Skyrmion glass phase theoretically expected as the ratio between Coulomb interactions and disorder is increased. These observations account for the fragility of the polarization previously observed in high mobility 2D electron gas and experimentally demonstrate the existence of an optimal amount of disorder to stabilize the ferromagnetic state.
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Desrat W, Piot BA, Maude DK, Wasilewski ZR, Henini M, Airey R. W line shape in the resistively detected nuclear magnetic resonance. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2015; 27:275801. [PMID: 26086605 DOI: 10.1088/0953-8984/27/27/275801] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
The resistively detected nuclear magnetic resonance (RDNMR) performed on a two-dimensional electron gas is known to exhibit a peculiar 'dispersive' line shape at some filling factors, especially around ν = 1. Here, we study in detail the inversion of the dispersive line shape as a function of the filling factor from ν = 1 to 2/3. The RDNMR spectra show a new characteristic W line shape in the longitudinal resistance, whereas dispersive lines detected in the Hall resistance remain unchanged. This W resonance, like the dispersive line, can be fitted correctly by a model of two independent response functions, which are the signatures of polarized and unpolarized electronic sub-systems.
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Mitioglu AA, Plochocka P, Granados del Aguila Á, Christianen PCM, Deligeorgis G, Anghel S, Kulyuk L, Maude DK. Optical Investigation of Monolayer and Bulk Tungsten Diselenide (WSe₂) in High Magnetic Fields. NANO LETTERS 2015; 15:4387-4392. [PMID: 26065723 DOI: 10.1021/acs.nanolett.5b00626] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Optical spectroscopy in high magnetic fields B ≤ 65 T is used to reveal the very different nature of carriers in monolayer and bulk transition metal dichalcogenides. In monolayer WSe2, the exciton emission shifts linearly with the magnetic field and exhibits a splitting that originates from the magnetic field induced valley splitting. The monolayer data can be described using a single particle picture with a Dirac-like Hamiltonian for massive Dirac Fermions, with an additional term to phenomenologically include the valley splitting. In contrast, in bulk WSe2 where the inversion symmetry is restored, transmission measurements show a distinctly excitonic behavior with absorption to the 1s and 2s states. Magnetic field induces a spin splitting together with a small diamagnetic shift and cyclotron like behavior at high fields, which is best described within the hydrogen model.
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Vorobyova JS, Vorob'ev AB, Prinz VY, Toropov AI, Maude DK. Magnetotransport in two-dimensional electron gas in helical nanomembranes. NANO LETTERS 2015; 15:1673-1678. [PMID: 25650698 DOI: 10.1021/nl504305s] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Heterostructures containing high-mobility two-dimensional electron gas were rolled into freestanding helically shaped contacted Hall bars. Magnetotransport measurements in these structures at high magnetic fields revealed minima in the longitudinal magnetoresistance corresponding to integer and fractional filling factors. A strong asymmetry of the longitudinal magnetoresistance with respect to the external magnetic field direction was observed. For this new type of structures, an edge state picture was considered, and calculations based on the Landauer-Büttiker formalism are performed.
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Jadczak J, Plochocka P, Mitioglu A, Breslavetz I, Royo M, Bertoni A, Goldoni G, Smolenski T, Kossacki P, Kretinin A, Shtrikman H, Maude DK. Unintentional high-density p-type modulation doping of a GaAs/AlAs core-multishell nanowire. NANO LETTERS 2014; 14:2807-2814. [PMID: 24745828 DOI: 10.1021/nl500818k] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
Achieving significant doping in GaAs/AlAs core/shell nanowires (NWs) is of considerable technological importance but remains a challenge due to the amphoteric behavior of the dopant atoms. Here we show that placing a narrow GaAs quantum well in the AlAs shell effectively getters residual carbon acceptors leading to an unintentional p-type doping. Magneto-optical studies of such a GaAs/AlAs core-multishell NW reveal quantum confined emission. Theoretical calculations of NW electronic structure confirm quantum confinement of carriers at the core/shell interface due to the presence of ionized carbon acceptors in the 1 nm GaAs layer in the shell. Microphotoluminescence in high magnetic field shows a clear signature of avoided crossings of the n = 0 Landau level emission line with the n = 2 Landau level TO phonon replica. The coupling is caused by the resonant hole-phonon interaction, which points to a large two-dimensional hole density in the structure.
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Alexander-Webber JA, Baker AMR, Janssen TJBM, Tzalenchuk A, Lara-Avila S, Kubatkin S, Yakimova R, Piot BA, Maude DK, Nicholas RJ. Phase space for the breakdown of the quantum Hall effect in epitaxial graphene. PHYSICAL REVIEW LETTERS 2013; 111:096601. [PMID: 24033057 DOI: 10.1103/physrevlett.111.096601] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/17/2013] [Indexed: 06/02/2023]
Abstract
We report the phase space defined by the quantum Hall effect breakdown in polymer gated epitaxial graphene on SiC (SiC/G) as a function of temperature, current, carrier density, and magnetic fields up to 30 T. At 2 K, breakdown currents (I(c)) almost 2 orders of magnitude greater than in GaAs devices are observed. The phase boundary of the dissipationless state (ρ(xx)=0) shows a [1-(T/T(c))2] dependence and persists up to T(c)>45 K at 29 T. With magnetic field I(c) was found to increase ∝B(3/2) and T(c)∝B2. As the Fermi energy pproaches the Dirac point, the ν=2 quantized Hall plateau appears continuously from fields as low as 1 T up to at least 19 T due to a strong magnetic field dependence of the carrier density.
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Plochocka P, Mitioglu AA, Maude DK, Rikken GLJA, del Águila AG, Christianen PCM, Kacman P, Shtrikman H. High magnetic field reveals the nature of excitons in a single GaAs/AlAs core/shell nanowire. NANO LETTERS 2013; 13:2442-2447. [PMID: 23634970 DOI: 10.1021/nl400417x] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/02/2023]
Abstract
Magneto-photoluminescence measurements of individual zinc-blende GaAs/AlAs core/shell nanowires are reported. At low temperature, a strong emission line at 1.507 eV is observed under low power (nW) excitation. Measurements performed in high magnetic field allowed us to detect in this emission several lines associated with excitons bound to defect pairs. Such lines were observed before in epitaxial GaAs of very high quality, as reported by Kunzel and Ploog. This demonstrates that the optical quality of our GaAs/AlAs core/shell nanowires is comparable to the best GaAs layers grown by molecular beam epitaxy. Moreover, strong free exciton emission is observed even at room temperature. The bright optical emission of our nanowires in room temperature should open the way for numerous optoelectronic device applications.
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Amado M, Diez E, Rossella F, Bellani V, López-Romero D, Maude DK. Magneto-transport of graphene and quantum phase transitions in the quantum Hall regime. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2012; 24:305302. [PMID: 22771627 DOI: 10.1088/0953-8984/24/30/305302] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
In this paper we show the electronic transport and the quantum phase transitions that characterize the quantum Hall regime in graphene placed on SiO(2) substrates at magnetic fields up to 28 T and temperatures down to 4 K. The analysis of the temperature dependence of the Hall and longitudinal resistivity reveals intriguing non-universalities of the critical exponents of the plateau-insulator transition. These exponents depend on the type of disorder that governs the electrical transport and its characterization is important for the design and fabrication of novel graphene nano-devices.
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Schneider JM, Piot BA, Sheikin I, Maude DK. Using the de Haas-van Alphen effect to map out the closed three-dimensional Fermi surface of natural graphite. PHYSICAL REVIEW LETTERS 2012; 108:117401. [PMID: 22540506 DOI: 10.1103/physrevlett.108.117401] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/10/2011] [Indexed: 05/31/2023]
Abstract
The Fermi surface of graphite has been mapped out using de Haas-van Alphen (dHvA) measurements at low temperature with in-situ rotation. For tilt angles θ>60° between the magnetic field and the c axis, the majority electron and hole dHvA periods no longer follow a cos(θ) behavior demonstrating that graphite has a three-dimensional closed Fermi surface. The Fermi surface of graphite is accurately described by highly elongated ellipsoids. A comparison with the calculated Fermi surface suggests that the Slonczewski-Weiss-McClure trigonal warping parameter γ(3) is significantly larger than previously thought.
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Stern M, Piot BA, Vardi Y, Umansky V, Plochocka P, Maude DK, Bar-Joseph I. NMR probing of the spin polarization of the ν=5/2 quantum Hall state. PHYSICAL REVIEW LETTERS 2012; 108:066810. [PMID: 22401109 DOI: 10.1103/physrevlett.108.066810] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/27/2011] [Indexed: 05/31/2023]
Abstract
Resistively detected nuclear magnetic resonance is used to measure the Knight shift of the 75As nuclei and determine the electron spin polarization of the fractional quantum Hall states of the second Landau level. We show that the 5/2 state is fully polarized within experimental error, thus confirming a fundamental assumption of the Moore-Read theory. We measure the electron heating under radio frequency excitation and show that we are able to detect NMR at electron temperatures down to 30 mK.
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dos Santos LF, Gobato YG, Teodoro MD, Lopez-Richard V, Marques GE, Brasil MJSP, Orlita M, Kunc J, Maude DK, Henini M, Airey RJ. Circular polarization in a non-magnetic resonant tunneling device. NANOSCALE RESEARCH LETTERS 2011; 6:101. [PMID: 21711613 PMCID: PMC3211145 DOI: 10.1186/1556-276x-6-101] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 09/13/2010] [Accepted: 01/25/2011] [Indexed: 05/31/2023]
Abstract
We have investigated the polarization-resolved photoluminescence (PL) in an asymmetric n-type GaAs/AlAs/GaAlAs resonant tunneling diode under magnetic field parallel to the tunnel current. The quantum well (QW) PL presents strong circular polarization (values up to -70% at 19 T). The optical emission from GaAs contact layers shows evidence of highly spin-polarized two-dimensional electron and hole gases which affects the spin polarization of carriers in the QW. However, the circular polarization degree in the QW also depends on various other parameters, including the g-factors of the different layers, the density of carriers along the structure, and the Zeeman and Rashba effects.
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39
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Patanè A, Mori N, Makarovsky O, Eaves L, Zambrano ML, Arce JC, Dickinson L, Maude DK. Manipulating and imaging the shape of an electronic wave function by magnetotunneling spectroscopy. PHYSICAL REVIEW LETTERS 2010; 105:236804. [PMID: 21231494 DOI: 10.1103/physrevlett.105.236804] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/02/2010] [Indexed: 05/30/2023]
Abstract
We measure the current due to electrons tunneling through the ground state of hydrogenic Si donors placed in a GaAs quantum well in the presence of a magnetic field tilted at an angle to the plane of the well. The component of B parallel to the direction of current compresses the donor wave function. By measuring the current as a function of the perpendicular component of B, we probe how the magnetocompression affects the spatial form of the wave function and observe directly the transition from Coulombic to magnetic confinement at high fields.
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40
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Stern M, Plochocka P, Umansky V, Maude DK, Potemski M, Bar-Joseph I. Optical probing of the spin polarization of the ν=5/2 quantum Hall state. PHYSICAL REVIEW LETTERS 2010; 105:096801. [PMID: 20868182 DOI: 10.1103/physrevlett.105.096801] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/17/2010] [Indexed: 05/29/2023]
Abstract
We apply polarization resolved photoluminescence spectroscopy to measure the spin polarization of a two dimensional electron gas in perpendicular magnetic field. We find that the splitting between the σ+ and σ- polarizations exhibits a sharp drop at ν=5/2 and is equal to the bare Zeeman energy, which resembles the behavior at even filling factors. We show that this behavior is consistent with filling factor ν=5/2 being unpolarized.
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Orlita M, Faugeras C, Schneider JM, Martinez G, Maude DK, Potemski M. Graphite from the viewpoint of Landau level spectroscopy: an effective graphene bilayer and monolayer. PHYSICAL REVIEW LETTERS 2009; 102:166401. [PMID: 19518730 DOI: 10.1103/physrevlett.102.166401] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/26/2009] [Indexed: 05/27/2023]
Abstract
We describe an infrared transmission study of a thin layer of bulk graphite in magnetic fields up to B=34 T. Two series of absorption lines whose energy scales as sqrt[B] and B are present in the spectra and identified as contributions of massless holes at the H point and massive electrons in the vicinity of the K point, respectively. We find that the optical response of the K point electrons corresponds, over a wide range of energy and magnetic field, to a graphene bilayer with an effective interlayer coupling 2gamma_{1}, twice the value for a real graphene bilayer, which reflects the crystal ordering of bulk graphite along the c axis. The K point electrons thus behave as massive Dirac fermions with a mass enhanced twice in comparison to a true graphene bilayer.
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Schneider JM, Orlita M, Potemski M, Maude DK. Consistent interpretation of the low-temperature magnetotransport in graphite using the Slonczewski-Weiss-McClure 3D band-structure calculations. PHYSICAL REVIEW LETTERS 2009; 102:166403. [PMID: 19518732 DOI: 10.1103/physrevlett.102.166403] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/11/2009] [Indexed: 05/27/2023]
Abstract
Magnetotransport of natural graphite and highly oriented pyrolytic graphite has been measured at mK temperatures. Quantum oscillations for both electron and hole carriers are observed with an orbital angular momentum quantum number up to N approximately 90. A remarkable agreement is obtained when comparing the data and the predictions of the Slonczewski-Weiss-McClure tight binding model for massive fermions. No evidence for Dirac fermions is observed in the transport data which are dominated by the crossing of the Landau bands at the Fermi level, corresponding to dE/dk_{z}=0, which occurs away from the H point where Dirac fermions are expected.
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Plochocka P, Schneider JM, Maude DK, Potemski M, Rappaport M, Umansky V, Bar-Joseph I, Groshaus JG, Gallais Y, Pinczuk A. Optical absorption to probe the quantum Hall ferromagnet at filling factor nu=1. PHYSICAL REVIEW LETTERS 2009; 102:126806. [PMID: 19392309 DOI: 10.1103/physrevlett.102.126806] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/23/2009] [Indexed: 05/27/2023]
Abstract
Optical absorption measurements are used to probe the spin polarization in the integer and fractional quantum Hall effect regimes. The system is fully spin polarized only at filling factor nu=1 and at very low temperatures ( approximately 40 mK). A small change in filling factor (deltanu approximately +/-0.01) leads to a significant depolarization. This suggests that the itinerant quantum Hall ferromagnet at nu=1 is surprisingly fragile against increasing temperature, or against small changes in filling factor.
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Orlita M, Faugeras C, Plochocka P, Neugebauer P, Martinez G, Maude DK, Barra AL, Sprinkle M, Berger C, de Heer WA, Potemski M. Approaching the dirac point in high-mobility multilayer epitaxial graphene. PHYSICAL REVIEW LETTERS 2008; 101:267601. [PMID: 19437673 DOI: 10.1103/physrevlett.101.267601] [Citation(s) in RCA: 81] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/08/2023]
Abstract
Multilayer epitaxial graphene is investigated using far infrared transmission experiments in the different limits of low magnetic fields and high temperatures. The cyclotron-resonance-like absorption is observed at low temperature in magnetic fields below 50 mT, probing the nearest vicinity of the Dirac point. The carrier mobility is found to exceed 250,000 cm2/(V x s). In the limit of high temperatures, the well-defined Landau level quantization is observed up to room temperature at magnetic fields below 1 T, a phenomenon unusual in solid state systems. A negligible increase in the width of the cyclotron resonance lines with increasing temperature indicates that no important scattering mechanism is thermally activated.
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45
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Makarovsky O, Thomas O, Balanov AG, Eaves L, Patanè A, Campion RP, Foxon CT, Vdovin EE, Maude DK, Kiesslich G, Airey RJ. Fock-Darwin-like quantum dot states formed by charged Mn interstitial ions. PHYSICAL REVIEW LETTERS 2008; 101:226807. [PMID: 19113508 DOI: 10.1103/physrevlett.101.226807] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/08/2008] [Indexed: 05/27/2023]
Abstract
We report a method of creating electrostatically induced quantum dots by thermal diffusion of interstitial Mn ions out of a p-type (GaMn)As layer into the vicinity of a GaAs quantum well. This approach creates deep, approximately circular, and strongly confined dotlike potential minima in a large (200 microm) mesa diode structure without need for advanced lithography or electrostatic gating. Magnetotunneling spectroscopy of an individual dot reveals the symmetry of its electronic eigenfunctions and a rich energy level spectrum of Fock-Darwin-like states with an orbital angular momentum component |lz| from 0 to 11.
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46
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Orlita M, Faugeras C, Martinez G, Maude DK, Sadowski ML, Potemski M. Dirac fermions at the H point of graphite: magnetotransmission studies. PHYSICAL REVIEW LETTERS 2008; 100:136403. [PMID: 18517976 DOI: 10.1103/physrevlett.100.136403] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/17/2007] [Indexed: 05/26/2023]
Abstract
We report on far infrared magnetotransmission measurements on a thin graphite sample prepared by exfoliation of highly oriented pyrolytic graphite. In a magnetic field, absorption lines exhibiting a blueshift proportional to sqrt[B] are observed. This is a fingerprint for massless Dirac holes at the H point in bulk graphite. The Fermi velocity is found to be c[over ] =( 1.02+/-0.02) x 10(6) m/s and the pseudogap |Delta| at the H point is estimated to be below 10 meV. Although the holes behave to a first approximation as a strictly 2D gas of Dirac fermions, the full 3D band structure has to be taken into account to explain all the observed spectral features.
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47
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Takashina K, Brun M, Ota T, Maude DK, Fujiwara A, Ono Y, Takahashi Y, Hirayama Y. Anomalous resistance ridges along filling factor nu=4i. PHYSICAL REVIEW LETTERS 2007; 99:036803. [PMID: 17678309 DOI: 10.1103/physrevlett.99.036803] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/15/2006] [Indexed: 05/16/2023]
Abstract
We report anomalous structure in the magnetoresistance of SiO(2)/Si(100)/SiO(2) quantum wells. When Landau levels of opposite valleys are driven through coincidence at the Fermi level, the longitudinal resistance displays elevations at filling factors that are integer multiples of 4 (nu=4i) accompanied by suppression on either side of nu=4i. This persists when either the magnetic field or the valley splitting is swept, leading to resistance ridges running along nu=4i. The range of field over which they are observed points to the role of spin degeneracy, which is directly confirmed by their disappearance with the addition of an in-plane magnetic field. The data suggest a new type of many-body effect arising from the combined degeneracy due to the valley and the spin degrees of freedom.
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Allison G, Mori N, Patanè A, Endicott J, Eaves L, Maude DK, Hopkinson M. Strong effect of resonant impurities on Landau-level quantization. PHYSICAL REVIEW LETTERS 2006; 96:236802. [PMID: 16803389 DOI: 10.1103/physrevlett.96.236802] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/06/2006] [Indexed: 05/10/2023]
Abstract
We investigate experimentally the effect of a random distribution of nitrogen (N) impurities on the Landau-level spectrum of a GaAs quantum well. Our magnetotunneling study reveals complex and nonequally spaced Landau levels and a quenching of the Landau states at a well-defined bias and electron energy which is resonant with that of the N atoms. Analysis of the magnetic field dependence of the tunnel current into the Landau levels of the well also provides quantitative information about the nonresonant component of the N-related scattering potential.
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Könemann J, Haug RJ, Maude DK, Fal'ko VI, Altshuler BL. Spin-orbit coupling and anisotropy of spin splitting in quantum dots. PHYSICAL REVIEW LETTERS 2005; 94:226404. [PMID: 16090418 DOI: 10.1103/physrevlett.94.226404] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/03/2004] [Indexed: 05/03/2023]
Abstract
In lateral quantum dots, the combined effect of both Dresselhaus and Bychkov-Rashba spin-orbit coupling is equivalent to an effective magnetic field +/- B(SO) which has the opposite sign for s(z)= +/- 1/2 spin electrons. When the external magnetic field is perpendicular to the planar structure, the field B(SO) generates an additional splitting for electron states as compared to the spin splitting in the in-plane field orientation. The anisotropy of spin splitting has been measured and then analyzed in terms of spin-orbit coupling in several AlGaAs/GaAs quantum dots by means of resonant tunneling spectroscopy. From the measured values and sign of the anisotropy we are able to determine the dominating spin-orbit coupling mechanism.
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50
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Patanè A, Mori N, Fowler D, Eaves L, Henini M, Maude DK, Hamaguchi C, Airey R. Magnetic-field-induced suppression of electronic conduction in a superlattice. PHYSICAL REVIEW LETTERS 2004; 93:146801. [PMID: 15524824 DOI: 10.1103/physrevlett.93.146801] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/04/2004] [Indexed: 05/24/2023]
Abstract
We use a magnetic field applied along the axis of a semiconductor superlattice (SL) as a controllable means of creating a one-dimensional band structure. We demonstrate that the current flow through the SL is strongly suppressed when the electron motion perpendicular to the SL axis is strongly confined by the quantizing magnetic field. By modeling this behavior using semiclassical and nonequilibrium Green's function methods, we show that the observed quenching arises from a qualitative change in electron dynamics caused by increasing quantum confinement.
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