Recent interesting articles

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''Válogatta: Scherübl Zoltán''
''Válogatta: Scherübl Zoltán''
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'''In situ tunable g factor for a single electron confined inside an InAs quantum dot'''
 +
W. Liu1, S. Sanwlani1, R. Hazbun2, J. Kolodzey2, A. S. Bracker3, D. Gammon3, and M. F. Doty1
 +
 +
Tailoring the properties of single spins confined in self-assembled quantum dots (QDs) is critical to the development of new optoelectronic logic devices. However, the range of heterostructure engineering techniques that can be used to control spin properties is severely limited by the requirements of QD self-assembly. We demonstrate a new strategy for rationally engineering the spin properties of single confined electrons or holes by adjusting the composition of the barrier between a stacked pair of InAs QDs coupled by coherent tunneling to form a quantum dot molecule (QDM). We demonstrate this strategy by designing, fabricating, and characterizing a QDM in which the g-factor for a single confined electron can be tuned in situ by over 50% with a minimal change in applied voltage.
 +
 +
http://prb.aps.org/abstract/PRB/v84/i12/e121304
 +
 +
----
 +
'''Hole-spin initialization and relaxation times in InAs/GaAs quantum dots'''
 +
 +
F. Fras, B. Eble, P. Desfonds, F. Bernardot, C. Testelin, and M. Chamarro
 +
 +
We study, at low temperature and zero magnetic field, the hole-spin dynamics in InAs/GaAs quantum dots. We measure the hole-spin relaxation time at a time scale longer than the dephasing time (about ten nanoseconds), imposed by the hole-nuclear hyperfine coupling. We use a pump-probe configuration and compare two experimental techniques based on differential absorption. The first one works in the time domain, and the second one is a new experimental method, the dark-bright time-scanning spectroscopy (DTS), working in the frequency domain. The measured hole-spin relaxation times, using these two techniques, are very similar, in the order of TNh≈1 μs. It is mainly imposed by the inhomogeneous hole hyperfine coupling in the hole localization volume. The DTS technique allows us also to measure the hole-spin initialization time τi. The hole spin is initialized by a periodic train of circularly polarized pulses at 76 MHz; we have observed that τi decreases as the power density increases, and we have measured a minimum value of τi≈100 ns in good agreement with a simple model [see B. Eble, P. Desfonds, F. Fras, F. Bernardot, C. Testelin, M. Chamarro, A. Miard and A. Lemaître Phys. Rev. B 81 045322 (2010)].
 +
 +
http://prb.aps.org/abstract/PRB/v84/i12/e125431
 +
 +
----
 +
'''Efficient terahertz emission from InAs nanowires'''
 +
 +
Denis V. Seletskiy1,4,*, Michael P. Hasselbeck1, Jeffrey G. Cederberg2, Aaron Katzenmeyer3, Maria E. Toimil-Molares3, François Léonard3, A. Alec Talin3,†, and Mansoor Sheik-Bahae1
 +
 +
We observe intense pulses of far-infrared electromagnetic radiation emitted from arrays of InAs nanowires. The terahertz radiation power efficiency of these structures is ∼15 times higher than a planar InAs substrate. This is explained by the preferential orientation of coherent plasma motion to the wire surface, which overcomes radiation trapping by total-internal reflection. We present evidence that this radiation originates from a low-energy acoustic surface plasmon mode of the nanowire. This is supported by independent measurements of electronic transport on individual nanowires, ultrafast terahertz spectroscopy, and theoretical analysis. Our combined experiments and analysis further indicate that these plasmon modes are specific to high aspect ratio geometries.
 +
 +
http://prb.aps.org/abstract/PRB/v84/i11/e115421
 +
 +
----
 +
'''Magnetic Proximity Effect as a Pathway to Spintronic Applications of Topological Insulators'''
 +
 +
Ivana Vobornik*†, Unnikrishnan Manju‡, Jun Fujii†, Francesco Borgatti§, Piero Torelli†, Damjan Krizmancic†, Yew San Hor, Robert J. Cava, and Giancarlo Panaccione†
 +
 +
We complete our recently introduced theoretical framework treating the double-quantum-dot system with a generalized form of Hubbard model. The effects of all quantum parameters involved in our model on the charge-stability diagram are discussed in detail. A general formulation of the microscopic theory is presented and, truncating at one orbital per site, we study the implication of different choices of the model confinement potential on the Hubbard parameters as well as the charge-stability diagram. We calculate the charge-stability diagram keeping three orbitals per site and find that the effect of additional higher-lying orbitals on the subspace with lowest-energy orbitals only can be regarded as a small renormalization of Hubbard parameters, thereby justifying our practice of keeping only the lowest orbital in all other calculations. The role of the harmonic-oscillator frequency in the implementation of the Gaussian model potential is discussed, and the effect of an external magnetic field is identified to be similar to choosing a more localized electron wave function in microscopic calculations. The full matrix form of the Hamiltonian, including all possible exchange terms and several peculiar charge-stability diagrams due to unphysical parameters, is presented in the Appendices, thus emphasizing the critical importance of a reliable microscopic model in obtaining the system parameters defining the Hamiltonian.
 +
 +
http://prb.aps.org/abstract/PRB/v84/i11/e115301
 +
 +
----
 +
'''Direct measurement of quantum phases in graphene via photoemission spectroscopy'''
 +
 +
Choongyu Hwang1, Cheol-Hwan Park2, David A. Siegel1,2, Alexei V. Fedorov3, Steven G. Louie1,2,*, and Alessandra Lanzara1,2,
 +
 +
Quantum phases provide us with important information for understanding the fundamental properties of a system. However, the observation of quantum phases, such as Berry's phase and the sign of the matrix element of the Hamiltonian between two nonequivalent localized orbitals in a tight-binding formalism, has been challenged by the presence of other factors, e.g. , dynamic phases and spin or valley degeneracy, and the absence of methodology. Here, we report a way to directly access these quantum phases, through polarization-dependent angle-resolved photoemission spectroscopy (ARPES), using graphene as a prototypical two-dimensional material. We show that the momentum- and polarization-dependent spectral intensity provides direct measurements of (i) the phase of the band wavefunction and (ii) the sign of matrix elements for nonequivalent orbitals. Upon rotating light polarization by π/2, we found that graphene with a Berry's phase of nπ (n=1 for single- and n=2 for double-layer graphene for Bloch wavefunction in the commonly used form) exhibits the rotation of ARPES intensity by π/n, and that ARPES signals reveal the signs of the matrix elements in both single- and double-layer graphene. The method provides a technique to directly extract fundamental quantum electronic information on a variety of materials.
 +
 +
http://prb.aps.org/abstract/PRB/v84/i12/e125422
 +
 +
----
 +
'''Current correlations in the interacting Cooper-pair beam-splitter'''
 +
 +
J. Rech, D. Chevallier, T. Jonckheere, T. Martin
 +
 +
Using a conserving many-body treatment, we propose an approach allowing the computation of currents and their correlations in interacting multi-terminal mesoscopic systems involving quantum dots coupled to normal and/or superconducting leads. We illustrate our method with the Cooper-pair beam-splitter setup recently proposed, which we model as a double quantum dot with weak interactions, connected to a superconducting lead and two normal ones. Our results suggest that even a weak Coulomb repulsion tends to favor positive current cross-correlations.
 +
 +
http://xxx.lanl.gov/abs/1109.2476
 +
 +
----
 +
'''Quantum Hall effect and semimetallic behavior in dual-gated ABA trilayer graphene'''
 +
 +
E. A. Henriksen, D. Nandi, J. P. Eisenstein
 +
 +
The electronic structure of multilayer graphenes depends strongly on the number of layers as well as the stacking order. Here we explore the electronic transport of purely ABA-stacked trilayer graphenes in a dual-gated field effect device configuration. We find both the quantum Hall effect (QHE) and low-field transport to be distinctly different from the mono- and bilayer graphenes, showing electron-hole asymmetries that are strongly suggestive of a semimetallic band overlap. When subject to an electric field perpendicular to the sheet, Landau level splittings due to breaking of the lattice mirror symmetry are clearly observed.
 +
 +
http://xxx.lanl.gov/abs/1109.2385
 +
 +
----
 +
'''Measuring the complex admittance of a carbon nanotube double quantum dot
 +
 +
S.J. Chorley, J. Wabnig, Z.V. Penfold-Fitch, K.D. Petersson, J. Frake, C.G. Smith, M.R. Buitelaar
 +
 +
We investigate radio-frequency (rf) reflectometry in a tunable carbon nanotube double quantum dot coupled to a resonant circuit. By measuring the in-phase and quadrature components of the reflected rf signal, we are able to determine the complex admittance of the double quantum dot as a function of the energies of the single-electron states. The measurements are found to be in good agreement with a theoretical model of the device in the incoherent limit. Besides being of fundamental interest, our results present an important step forward towards non-invasive charge and spin state readout in carbon nanotube quantum dots.
 +
 +
http://xxx.lanl.gov/abs/1109.1827
 +
 +
----
 +
'''Imaging the lateral shift of a quantum-point contact using scanning-gate microscopy'''
 +
 +
S. Schnez, C. Rössler, T. Ihn, K. Ensslin, C. Reichl, W. Wegscheider
 +
 +
We perform scanning-gate microscopy on a quantum-point contact. It is defined in a high-mobility two-dimensional electron gas of an AlGaAs/GaAs heterostructure, giving rise to a weak disorder potential. The lever arm of the scanning tip is significantly smaller than that of the split gates defining the conducting channel of the quantum-point contact. We are able to observe that the conducting channel is shifted in real space when asymmetric gate voltages are applied. The observed shifts are consistent with transport data and numerical estimations.
 +
 +
http://xxx.lanl.gov/abs/1109.1544
 +
 +
----
 +
'''Carbon tips as electrodes for single-molecule junctions'''
 +
 +
Andres Castellanos-Gomez, Stefan Bilan, Linda A. Zotti, Carlos R. Arroyo, Nicolas Agrait, Juan Carlos Cuevas, Gabino Rubio-Bollinger
 +
 +
We study electron transport through single-molecule junctions formed by an octanethiol molecule bonded with the thiol anchoring group to a gold electrode and the opposing methyl endgroup to a carbon tip. Using the scanning tunneling microscope based break junction technique, we measure the electrical conductance of such molecular junctions. We observe the presence of well-defined conductance plateaus during the stretching of the molecular bridge, which is the signature of the formation of a molecular junction.
 +
 +
http://xxx.lanl.gov/abs/1109.2089
== Szept. 01. - Szept. 07. (2011) ==
== Szept. 01. - Szept. 07. (2011) ==

Revision as of 22:18, 18 September 2011

Science this week issue

Nature last week

Nature Physics last week

Nature Nanotechnology last week

Nano Letters ASAP

ACS Nano ASAP

Cond-mat Mesoscale and Nanoscale Physics - recent papers Note: all the papers in a certain month can be listed as e.g. http://xxx.lanl.gov/list/cond-mat.mes-hall/1104 , where 11 stands for 2011 and 04 for April.

Physical Review Letters

Physical Review B

Szept. 08. - Szept. 14. (2011)

Válogatta: Scherübl Zoltán

In situ tunable g factor for a single electron confined inside an InAs quantum dot

W. Liu1, S. Sanwlani1, R. Hazbun2, J. Kolodzey2, A. S. Bracker3, D. Gammon3, and M. F. Doty1

Tailoring the properties of single spins confined in self-assembled quantum dots (QDs) is critical to the development of new optoelectronic logic devices. However, the range of heterostructure engineering techniques that can be used to control spin properties is severely limited by the requirements of QD self-assembly. We demonstrate a new strategy for rationally engineering the spin properties of single confined electrons or holes by adjusting the composition of the barrier between a stacked pair of InAs QDs coupled by coherent tunneling to form a quantum dot molecule (QDM). We demonstrate this strategy by designing, fabricating, and characterizing a QDM in which the g-factor for a single confined electron can be tuned in situ by over 50% with a minimal change in applied voltage.

http://prb.aps.org/abstract/PRB/v84/i12/e121304

Hole-spin initialization and relaxation times in InAs/GaAs quantum dots

F. Fras, B. Eble, P. Desfonds, F. Bernardot, C. Testelin, and M. Chamarro

We study, at low temperature and zero magnetic field, the hole-spin dynamics in InAs/GaAs quantum dots. We measure the hole-spin relaxation time at a time scale longer than the dephasing time (about ten nanoseconds), imposed by the hole-nuclear hyperfine coupling. We use a pump-probe configuration and compare two experimental techniques based on differential absorption. The first one works in the time domain, and the second one is a new experimental method, the dark-bright time-scanning spectroscopy (DTS), working in the frequency domain. The measured hole-spin relaxation times, using these two techniques, are very similar, in the order of TNh≈1 μs. It is mainly imposed by the inhomogeneous hole hyperfine coupling in the hole localization volume. The DTS technique allows us also to measure the hole-spin initialization time τi. The hole spin is initialized by a periodic train of circularly polarized pulses at 76 MHz; we have observed that τi decreases as the power density increases, and we have measured a minimum value of τi≈100 ns in good agreement with a simple model [see B. Eble, P. Desfonds, F. Fras, F. Bernardot, C. Testelin, M. Chamarro, A. Miard and A. Lemaître Phys. Rev. B 81 045322 (2010)].

http://prb.aps.org/abstract/PRB/v84/i12/e125431

Efficient terahertz emission from InAs nanowires

Denis V. Seletskiy1,4,*, Michael P. Hasselbeck1, Jeffrey G. Cederberg2, Aaron Katzenmeyer3, Maria E. Toimil-Molares3, François Léonard3, A. Alec Talin3,†, and Mansoor Sheik-Bahae1

We observe intense pulses of far-infrared electromagnetic radiation emitted from arrays of InAs nanowires. The terahertz radiation power efficiency of these structures is ∼15 times higher than a planar InAs substrate. This is explained by the preferential orientation of coherent plasma motion to the wire surface, which overcomes radiation trapping by total-internal reflection. We present evidence that this radiation originates from a low-energy acoustic surface plasmon mode of the nanowire. This is supported by independent measurements of electronic transport on individual nanowires, ultrafast terahertz spectroscopy, and theoretical analysis. Our combined experiments and analysis further indicate that these plasmon modes are specific to high aspect ratio geometries.

http://prb.aps.org/abstract/PRB/v84/i11/e115421

Magnetic Proximity Effect as a Pathway to Spintronic Applications of Topological Insulators

Ivana Vobornik*†, Unnikrishnan Manju‡, Jun Fujii†, Francesco Borgatti§, Piero Torelli†, Damjan Krizmancic†, Yew San Hor, Robert J. Cava, and Giancarlo Panaccione†

We complete our recently introduced theoretical framework treating the double-quantum-dot system with a generalized form of Hubbard model. The effects of all quantum parameters involved in our model on the charge-stability diagram are discussed in detail. A general formulation of the microscopic theory is presented and, truncating at one orbital per site, we study the implication of different choices of the model confinement potential on the Hubbard parameters as well as the charge-stability diagram. We calculate the charge-stability diagram keeping three orbitals per site and find that the effect of additional higher-lying orbitals on the subspace with lowest-energy orbitals only can be regarded as a small renormalization of Hubbard parameters, thereby justifying our practice of keeping only the lowest orbital in all other calculations. The role of the harmonic-oscillator frequency in the implementation of the Gaussian model potential is discussed, and the effect of an external magnetic field is identified to be similar to choosing a more localized electron wave function in microscopic calculations. The full matrix form of the Hamiltonian, including all possible exchange terms and several peculiar charge-stability diagrams due to unphysical parameters, is presented in the Appendices, thus emphasizing the critical importance of a reliable microscopic model in obtaining the system parameters defining the Hamiltonian.

http://prb.aps.org/abstract/PRB/v84/i11/e115301

Direct measurement of quantum phases in graphene via photoemission spectroscopy

Choongyu Hwang1, Cheol-Hwan Park2, David A. Siegel1,2, Alexei V. Fedorov3, Steven G. Louie1,2,*, and Alessandra Lanzara1,2,

Quantum phases provide us with important information for understanding the fundamental properties of a system. However, the observation of quantum phases, such as Berry's phase and the sign of the matrix element of the Hamiltonian between two nonequivalent localized orbitals in a tight-binding formalism, has been challenged by the presence of other factors, e.g. , dynamic phases and spin or valley degeneracy, and the absence of methodology. Here, we report a way to directly access these quantum phases, through polarization-dependent angle-resolved photoemission spectroscopy (ARPES), using graphene as a prototypical two-dimensional material. We show that the momentum- and polarization-dependent spectral intensity provides direct measurements of (i) the phase of the band wavefunction and (ii) the sign of matrix elements for nonequivalent orbitals. Upon rotating light polarization by π/2, we found that graphene with a Berry's phase of nπ (n=1 for single- and n=2 for double-layer graphene for Bloch wavefunction in the commonly used form) exhibits the rotation of ARPES intensity by π/n, and that ARPES signals reveal the signs of the matrix elements in both single- and double-layer graphene. The method provides a technique to directly extract fundamental quantum electronic information on a variety of materials.

http://prb.aps.org/abstract/PRB/v84/i12/e125422

Current correlations in the interacting Cooper-pair beam-splitter

J. Rech, D. Chevallier, T. Jonckheere, T. Martin

Using a conserving many-body treatment, we propose an approach allowing the computation of currents and their correlations in interacting multi-terminal mesoscopic systems involving quantum dots coupled to normal and/or superconducting leads. We illustrate our method with the Cooper-pair beam-splitter setup recently proposed, which we model as a double quantum dot with weak interactions, connected to a superconducting lead and two normal ones. Our results suggest that even a weak Coulomb repulsion tends to favor positive current cross-correlations.

http://xxx.lanl.gov/abs/1109.2476

Quantum Hall effect and semimetallic behavior in dual-gated ABA trilayer graphene

E. A. Henriksen, D. Nandi, J. P. Eisenstein

The electronic structure of multilayer graphenes depends strongly on the number of layers as well as the stacking order. Here we explore the electronic transport of purely ABA-stacked trilayer graphenes in a dual-gated field effect device configuration. We find both the quantum Hall effect (QHE) and low-field transport to be distinctly different from the mono- and bilayer graphenes, showing electron-hole asymmetries that are strongly suggestive of a semimetallic band overlap. When subject to an electric field perpendicular to the sheet, Landau level splittings due to breaking of the lattice mirror symmetry are clearly observed.

http://xxx.lanl.gov/abs/1109.2385

Measuring the complex admittance of a carbon nanotube double quantum dot

S.J. Chorley, J. Wabnig, Z.V. Penfold-Fitch, K.D. Petersson, J. Frake, C.G. Smith, M.R. Buitelaar

We investigate radio-frequency (rf) reflectometry in a tunable carbon nanotube double quantum dot coupled to a resonant circuit. By measuring the in-phase and quadrature components of the reflected rf signal, we are able to determine the complex admittance of the double quantum dot as a function of the energies of the single-electron states. The measurements are found to be in good agreement with a theoretical model of the device in the incoherent limit. Besides being of fundamental interest, our results present an important step forward towards non-invasive charge and spin state readout in carbon nanotube quantum dots.

http://xxx.lanl.gov/abs/1109.1827

Imaging the lateral shift of a quantum-point contact using scanning-gate microscopy

S. Schnez, C. Rössler, T. Ihn, K. Ensslin, C. Reichl, W. Wegscheider

We perform scanning-gate microscopy on a quantum-point contact. It is defined in a high-mobility two-dimensional electron gas of an AlGaAs/GaAs heterostructure, giving rise to a weak disorder potential. The lever arm of the scanning tip is significantly smaller than that of the split gates defining the conducting channel of the quantum-point contact. We are able to observe that the conducting channel is shifted in real space when asymmetric gate voltages are applied. The observed shifts are consistent with transport data and numerical estimations.

http://xxx.lanl.gov/abs/1109.1544

Carbon tips as electrodes for single-molecule junctions

Andres Castellanos-Gomez, Stefan Bilan, Linda A. Zotti, Carlos R. Arroyo, Nicolas Agrait, Juan Carlos Cuevas, Gabino Rubio-Bollinger

We study electron transport through single-molecule junctions formed by an octanethiol molecule bonded with the thiol anchoring group to a gold electrode and the opposing methyl endgroup to a carbon tip. Using the scanning tunneling microscope based break junction technique, we measure the electrical conductance of such molecular junctions. We observe the presence of well-defined conductance plateaus during the stretching of the molecular bridge, which is the signature of the formation of a molecular junction.

http://xxx.lanl.gov/abs/1109.2089

Szept. 01. - Szept. 07. (2011)

Válogatta: Pósa László

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Authors: ???

Abstract

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