Nanophysics seminar - spring, 2018
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The (nominal) schedule of the nanophysics seminar (spring, 2018)
| Week | Date | Lecture |
|---|---|---|
| 2 | 02.09. | Simon Zihlman |
| 3 | 02.25. | |
| 4 | 03.04. | Fülöp Bálint |
| 5 | 03.11. | Magyarkuti András |
Tóvári Endre(talk), Gubicza Ági (JC) 2014. febr. 18.
Tóvári Endre: Ultratiszta grafén: felfüggesztett minták, bór-nitrid szubsztrát
Gubicza Ági: Atomic cloud BEC: quantized momentum, hysteresis
Fülöp Gergő (talk), Fülöp Bálint (JC) 2014. febr. 25.
Fülöp Gergő: Ice-assisted e-beam lithography
Fülöp Bálint: Magnetotransport Subband Spectroscopy in InAs Nanowires
Scherübl Zoltán (talk), Magyarkuti András (JC) 2014. márc. 11.
Scherübl Zoltán: Exotic particles in nanophysics
Magyarkuti András: Comprehensive Physical Model of Dynamic Resistive Switching in an Oxide Memristor
Balogh Zoltán (talk), Tóvári Endre (JC), 2014. márc. 18.
Balogh Zoltán: Beszámoló és ballon
Tóvári Endre: Stacking boundaries and transport in bilayer graphene
Fülöp Bálint (lecture), Scherübl Zoltán (JC), 2014.03.25.
Fülöp Bálint: van der Waals heterostructures
JC: Scheriff!
Halász Gábor(lecture), Fülöp Gergő (JC), 2014. április 8.
Fülöp Gergő: JC
- The effect of spin transport on spin lifetime in nanoscale systems
http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.39.html
- Electron Beam Lithography on Irregular Surfaces Using an Evaporated Resist
http://pubs.acs.org/doi/abs/10.1021/nn4064659
- Ultrashort Single-Wall Carbon Nanotubes Reveal Field-Emission Coulomb Blockade and Highest Electron-Source Brightness
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.126805
Halász Gábor: Spin-triplet supercurrents in superconductor / ferromagnet heterostructures
Márc. 18. - Márc. 31. (2014)
Gergő Fülöp
The effect of spin transport on spin lifetime in nanoscale systems
Jeremy Cardellino, Nicolas Scozzaro, Michael Herman, Andrew J. Berger, Chi Zhang, Kin Chung Fong, Ciriyam Jayaprakash, Denis V. Pelekhov & P. Chris Hammel
http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.39.html
All-water-based electron-beam lithography using silk as a resist
Sunghwan Kim, Benedetto Marelli, Mark A. Brenckle, Alexander N. Mitropoulos, Eun-Seok Gil, Konstantinos Tsioris, Hu Tao, David L. Kaplan & Fiorenzo G. Omenetto
http://www.nature.com/nnano/journal/v9/n4/full/nnano.2014.47.html
Boron Nitride–Graphene Nanocapacitor and the Origins of Anomalous Size-Dependent Increase of Capacitance
Gang Shi, Yuranan Hanlumyuang, Zheng Liu, Yongji Gong, Weilu Gao, Bo Li, Junichiro Kono, Jun Lou, Robert Vajtai, Pradeep Sharma, and Pulickel M. Ajayan
http://pubs.acs.org/doi/abs/10.1021/nl4037824
Electron Beam Lithography on Irregular Surfaces Using an Evaporated Resist
Jian Zhang, Celal Con and Bo Cui
http://pubs.acs.org/doi/abs/10.1021/nn4064659
Carbon Nanotube Circuit Integration up to Sub-20 nm Channel Lengths
Max Marcel Shulaker, Jelle Van Rethy, Tony F. Wu, Luckshitha Suriyasena Liyanage, Hai Wei, Zuanyi Li, Eric Pop, Georges Gielen, H.-S. Philip Wong, and Subhasish Mitra
http://pubs.acs.org/doi/abs/10.1021/nn406301r
Improvement of Spin Transfer Torque in Asymmetric Graphene Devices
Chia-Ching Lin, Yunfei Gao, Ashish Verma Penumatcha, Vinh Quang Diep, Joerg Appenzeller and Zhihong Chen
http://pubs.acs.org/doi/abs/10.1021/nn500533b
Quantum Anomalous Hall Effect in Graphene Proximity Coupled to an Antiferromagnetic Insulator
Zhenhua Qiao, Wei Ren, Hua Chen, L. Bellaiche, Zhenyu Zhang, A.H. MacDonald, and Qian Niu
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.116404
Spin Relaxation Mechanism in Graphene: Resonant Scattering by Magnetic Impurities
Denis Kochan, Martin Gmitra, and Jaroslav Fabian
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.116602
Radio-Wave Oscillations of Molecular-Chain Resonators
Stefan Müllegger, Mohammad Rashidi, Karlheinz Mayr, Michael Fattinger, Andreas Ney, and Reinhold Koch
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.117201
Majorana Bound States in Two-Channel Time-Reversal-Symmetric Nanowire Systems
Erikas Gaidamauskas, Jens Paaske, and Karsten Flensberg
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.126402
Coherent Control to Prepare an InAs Quantum Dot for Spin-Photon Entanglement
L.A. Webster, K. Truex, L.-M. Duan, D.?G. Steel, A. S. Bracker, D. Gammon, and L.J. Sham
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.126801
Ultrashort Single-Wall Carbon Nanotubes Reveal Field-Emission Coulomb Blockade and Highest Electron-Source Brightness
A. Pascale-Hamri, S. Perisanu, A. Derouet, C. Journet, P. Vincent, A. Ayari, and S. T. Purcell
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.126805
Tuning the Influence of Microscopic Decoherence on the Superconducting Proximity Effect in a Graphene Andreev Interferometer
Fabio Deon, Sandra Šopic, and Alberto F. Morpurgo
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.126803
Márc. 11. - Márc. 17. (2014)
by Scheriff
Hole spin coherence in a Ge-Si heterostructure nanowire
'A. P. Higginbotham, T. W. Larsen, J. Yao, H. Yan, C. M. Lieber, C. M. Marcus, F. Kuemmeth'
Realizing qubits that simultaneously provide long coherence times and fast control is a key challenge for quantum information processing. Spins in III/V semiconductor quantum dots can be electrically manipulated, but lose coherence due to interactions with nuclear spins. Silicon provides improved coherence because of its low and controllable concentration of nuclear spins. The Ge-Si core-shell heterostructured nanowire is an example of a predominantly zero-nuclear-spin system that is particularly tunable and scalable. Holes in Ge-Si nanowires exhibit large spin-orbit coupling, a useful resource for fast, all-electrical control of single spins. Here, we measure spin coherence times of gate-confined hole spins in a Ge-Si nanowire double quantum dot using high bandwidth electrical control and read out of the spin state. We find inhomogeneous dephasing times T∗2 up to 0.18μs, twenty times longer than in III/V semiconductors. This timescale is consistent with dephasing due to sparse 73Ge nuclear spins. The observed exponential coherence decay suggests a dephasing source with high-frequency spectral content. These results pave the way towards improved spin-orbit qubits and strong spin-cavity coupling in circuit quantum electrodynamics.
http://arxiv.org/abs/1403.2093
Superconducting proximity effect and zero-bias anomaly in transport through quantum dots weakly attached to ferromagnetic leads
'I. Weymann, P. Trocha'
The Andreev transport through a quantum dot coupled to two external ferromagnetic leads and one superconducting lead is studied theoretically by means of the real-time diagrammatic technique in the sequential and cotunneling regimes. We show that the tunnel magnetoresistance (TMR) of the Andreev current displays a nontrivial dependence on the bias voltage and the level detuning, and can be described by analytical formulas in the zero temperature limit. The cotunneling processes lead to a strong modification of the TMR, which is most visible in the Coulomb blockade regime. We find a zero-bias anomaly of the Andreev differential conductance in the parallel configuration, which is associated with a nonequilibrium spin accumulation in the dot triggered by Andreev processes.
http://arxiv.org/abs/1403.2956
Electrical spin protection and manipulation via gate-locked spin-orbit fields
'F. Dettwiler, J. Fu, S. Mack, P. J. Weigele, J. C. Egues, D. D. Awschalom, D. M. Zumbühl'
The spin-orbit (SO) interaction couples electron spin and momentum via a relativistic, effective magnetic field. While conveniently facilitating coherent spin manipulation in semiconductors, the SO interaction also inherently causes spin relaxation. A unique situation arises when the Rashba and Dresselhaus SO fields are matched, strongly protecting spins from relaxation, as recently demonstrated. Quantum computation and spintronics devices such as the paradigmatic spin transistor could vastly benefit if such spin protection could be expanded from a single point into a broad range accessible with in-situ gate-control, making possible tunable SO rotations under protection from relaxation. Here, we demonstrate broad, independent control of all relevant SO fields in GaAs quantum wells, allowing us to tune the Rashba and Dresselhaus SO fields while keeping both locked to each other using gate voltages. Thus, we can electrically control and simultaneously protect the spin. Our experiments employ quantum interference corrections to electrical conductivity as a sensitive probe of SO coupling. Finally, we combine transport data with numerical SO simulations to precisely quantify all SO terms.
http://arxiv.org/abs/1403.3518
Fibonacci Anyons From Abelian Bilayer Quantum Hall States
'Abolhassan Vaezi, Maissam Barkeshli'
The possibility of realizing non-Abelian statistics and utilizing it for topological quantum computation (TQC) has generated widespread interest. However, the non-Abelian statistics that can be realized in most accessible proposals is not powerful enough for universal TQC. In this paper, we consider a simple bilayer fractional quantum Hall (FQH) system with the 1/3 Laughlin state in each layer, in the presence of interlayer tunneling. We show that interlayer tunneling can drive a continuous phase transition to an exotic non-Abelian state that contains the famous `Fibonacci' anyon, whose non-Abelian statistics is powerful enough for universal TQC. Our analysis rests on startling agreements from a variety of distinct methods, including thin torus limits, effective field theories, and coupled wire constructions. The charge gap remains open at the transitions while the neutral gap closes. This raises the question of whether these exotic phases may have already been realized at ν=2/3 in bilayers, as past experiments may not have definitively ruled them out.
http://arxiv.org/abs/1403.3383
High Resolution Coherent Population Trapping on a Single Hole Spin in a Semiconductor Quantum Dot
'Julien Houel, Jonathan H. Prechtel, Andreas V. Kuhlmann, Daniel Brunner, Christopher E. Kuklewicz, Brian D. Gerardot, Nick G. Stoltz, Pierre M. Petroff, and Richard J. Warburton'
We report high resolution coherent population trapping on a single hole spin in a semiconductor quantum dot. The absorption dip signifying the formation of a dark state exhibits an atomic physicslike dip width of just 10 MHz. We observe fluctuations in the absolute frequency of the absorption dip, evidence of very slow spin dephasing. We identify the cause of this process as charge noise by, first, demonstrating that the hole spin g factor in this configuration (in-plane magnetic field) is strongly dependent on the vertical electric field, and second, by characterizing the charge noise through its effects on the optical transition frequency. An important conclusion is that charge noise is an important hole spin dephasing process.
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.107401
Effects of electron-electron interactions on the electronic Raman scattering of graphite in high magnetic fields
'Y. Ma, Y. Kim, N. G. Kalugin, A. Lombardo, A. C. Ferrari, J. Kono, A. Imambekov, and D. Smirnov'
We report the observation of strongly temperature (T)-dependent spectral lines in electronic Raman-scattering spectra of graphite in a high magnetic field up to 45 T applied along the c axis. The magnetic field quantizes the in-plane motion, while the out-of-plane motion remains free, effectively reducing the system dimension from 3 to 1. Optically created electron-hole pairs interact with, or shake up, the one-dimensional Fermi sea in the lowest Landau subbands. Based on the Tomonaga-Luttinger liquid theory, we show that interaction effects modify the spectral line shape from (ω−Δ)−1/2 to (ω−Δ)2α−1/2 at T = 0. At finite T, we predict a thermal broadening factor that increases linearly with T. Our model reproduces the observed T-dependent line shape, determining the electron-electron interaction parameter α to be ∼0.05 at 40 T.
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.121402
Entanglement witnessing and quantum cryptography with nonideal ferromagnetic detectors
'Waldemar Kłobus, Andrzej Grudka, Andreas Baumgartner, Damian Tomaszewski, Christian Schönenberger, and Jan Martinek'
We investigate theoretically the use of nonideal ferromagnetic contacts as a means to detect quantum entanglement of electron spins in transport experiments. We use a designated entanglement witness and find a minimal spin polarization of η>1/3√≈58% required to demonstrate spin entanglement. This is significantly less stringent than the ubiquitous tests of Bell's inequality with η>1/2√4≈84%. In addition, we discuss the impact of decoherence and noise on entanglement detection and apply the presented framework to a simple quantum cryptography protocol. Our results are directly applicable to a large variety of experiments.
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.125404
Maximal Rabi frequency of an electrically driven spin in a disordered magnetic field
'Gábor Széchenyi and András Pályi'
We present a theoretical study of the spin dynamics of a single electron confined in a quantum dot. Spin dynamics is induced by the interplay of electrical driving and the presence of a spatially disordered magnetic field, the latter being transverse to a homogeneous magnetic field. We focus on the case of strong driving, i.e., when the oscillation amplitude A of the electron's wave packet is comparable to the quantum dot length L. We show that electrically driven spin resonance can be induced in this system by subharmonic driving, i.e., if the excitation frequency is an integer fraction (12,13, etc.) of the Larmor frequency. At strong driving we find that (i) the Rabi frequencies at the subharmonic resonances are comparable to the Rabi frequency at the fundamental resonance, and (ii) at each subharmonic resonance, the Rabi frequency can be maximized by setting the drive strength to an optimal, finite value. In the context of practical quantum information processing, these findings highlight the availability of subharmonic resonances for qubit control with effectivity close to that of the fundamental resonance, and the possibility that increasing the drive strength might lead to a decreasing qubit-flip speed. Our simple model is applied to describe electrical control of a spin-valley qubit in a weakly disordered carbon nanotube.
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.115409
Observation of Even Denominator Fractional Quantum Hall Effect in Suspended Bilayer Graphene
'Dong-Keun Ki †, Vladimir I. Fal’ko ‡, Dmitry A. Abanin §, and Alberto F. Morpurgo *†'
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 resistance or 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 hexagonal boron-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.
http://pubs.acs.org/doi/abs/10.1021/nl5003922
Controlled Synthesis of Phase-Pure InAs Nanowires on Si(111) by Diminishing the Diameter to 10 nm
'Dong Pan †, Mengqi Fu ‡, Xuezhe Yu †, Xiaolei Wang †, Lijun Zhu †, Shuaihua Nie †, Siliang Wang †, Qing Chen ‡, Peng Xiong §, Stephan von Molnár §, and Jianhua Zhao *†'
Here we report the growth of phase-pure InAs nanowires on Si (111) substrates by molecular-beam epitaxy using Ag catalysts. A conventional one-step catalyst annealing process is found to give rise to InAs nanowires with diameters ranging from 4.5 to 81 nm due to the varying sizes of the Ag droplets, which reveal strong diameter dependence of the crystal structure. In contrast, a novel two-step catalyst annealing procedure yields vertical growth of highly uniform InAs nanowires 10 nm in diameter. Significantly, these ultrathin nanowires exhibit a perfect wurtzite crystal structure, free of stacking faults and twin defects. Using these high-quality ultrathin InAs nanowires as the channel material of metal-oxide-semiconductor field-effect transistor, we have obtained a high ION/IOFF ratio of 106, which shows great potential for application in future nanodevices with low power dissipation.
http://pubs.acs.org/doi/abs/10.1021/nl4040847
Tunable Charge Transport in Single-Molecule Junctions via Electrolytic Gating
'Brian Capozzi †, Qishui Chen ‡, Pierre Darancet §, Michele Kotiuga §, Marisa Buzzeo , Jeffrey B. Neaton §, Colin Nuckolls ‡, and Latha Venkataraman *†'
We modulate the conductance of electrochemically inactive molecules in single-molecule junctions using an electrolytic gate to controllably tune the energy level alignment of the system. Molecular junctions that conduct through their highest occupied molecular orbital show a decrease in conductance when applying a positive electrochemical potential, and those that conduct though their lowest unoccupied molecular orbital show the opposite trend. We fit the experimentally measured conductance data as a function of gate voltage with a Lorentzian function and find the fitting parameters to be in quantitative agreement with self-energy corrected density functional theory calculations of transmission probability across single-molecule junctions. This work shows that electrochemical gating can directly modulate the alignment of the conducting orbital relative to the metal Fermi energy, thereby changing the junction transport properties.
http://pubs.acs.org/doi/abs/10.1021/nl404459q
Frequency multiplexing for readout of spin qubits
'J. M. Hornibrook1, J. I. Colless1, A. C. Mahoney1, X. G. Croot1, S. Blanvillain1, H. Lu2, A. C. Gossard2 and D. J. Reilly1,a)'
We demonstrate a low loss, chip-level frequency multiplexing scheme for readout of scaled-up spin qubit devices. By integrating separate bias tees and resonator circuits on-chip for each readout channel, we realise dispersive gate-sensing in combination with charge detection based on two radio frequency quantum point contacts. We apply this approach to perform multiplexed readout of a double quantum dot in the few-electron regime and further demonstrate operation of a 10-channel multiplexing device. Limitations for scaling spin qubit readout to large numbers of multiplexed channels are discussed.
http://scitation.aip.org/content/aip/journal/apl/104/10/10.1063/1.4868107
Márc. 4. - Márc. 10. (2014)
Assorted by: Tóvári Endre
Stacking Boundaries and Transport in Bilayer Graphene
'P. San-Jose, R. V. Gorbachev, A. K. Geim, K. S. Novoselov, and F. Guinea'
Pristine bilayer graphene behaves in some instances as an insulator with a transport gap of a few millielectronvolts. This behavior has been interpreted as the result of an intrinsic electronic instability induced by many-body correlations. Intriguingly, however, some samples of similar mobility exhibit good metallic properties with a minimal conductivity of the order of 2e^2/h. Here, we propose an explanation for this dichotomy, which is unrelated to electron interactions and based instead on the reversible formation of boundaries between stacking domains (“solitons”). We argue, using a numerical analysis, that the hallmark features of the previously inferred manybody insulating state can be explained by scattering on boundaries between domains with different stacking order (AB and BA). We furthermore present experimental evidence, reinforcing our interpretation, of reversible switching between a metallic and an insulating regime in suspended bilayers when subjected to thermal cycling or high current annealing.
Nano Lett., Article ASAP, DOI: 10.1021/nl500230a
http://pubs.acs.org/doi/abs/10.1021/nl500230a
Proximity Effect in Graphene–Topological-Insulator Heterostructures
'Junhua Zhang, C. Triola, and E. Rossi'
We formulate a continuum model to study the low-energy electronic structure of heterostructures formed by graphene on a strong three-dimensional topological insulator (TI) for the cases of both commensurate and incommensurate stacking. The incommensurability can be due to a twist angle between graphene and the TI surface or a lattice mismatch between the two systems. We find that the proximity of the TI induces in graphene a strong enhancement of the spin-orbit coupling that can be tuned via the twist angle.
Phys. Rev. Lett. 112, 096802 (2014) DOI: http://dx.doi.org/10.1103/PhysRevLett.112.096802
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.096802
Tunable Fractional Quantum Hall Phases in Bilayer Graphene
'Patrick Maher, Lei Wang, Yuanda Gao, Carlos Forsythe, Takashi Taniguchi, Kenji Watanabe, Dmitry Abanin, Zlatko Papic, Paul Cadden-Zimansky, James Hone, Philip Kim and Cory R. Dean'
Symmetry breaking in a quantum system often leads to complex emergent behavior. In bilayer graphene (BLG), an electric field applied perpendicular to the basal plane breaks the inversion symmetry of the lattice, opening a band gap at the charge neutrality point. In a quantizing magnetic field electron interactions can cause spontaneous symmetry breaking within the spin and valley degrees of freedom, resulting in quantum Hall states (QHS) with complex order. Here we report fractional quantum Hall states (FQHS) in bilayer graphene which show phase transitions that can be tuned by a transverse electric field. This result provides a model platform to study the role of symmetry breaking in emergent states with distinct topological order.
arXiv:1403.2112v1
http://xxx.lanl.gov/pdf/1403.2112v1.pdf
Temperature- and density-dependent transport regimes in a h-BN/bilayer graphene/h-BN heterostructure
'C. Cobaleda, S. Pezzini, E. Diez, and V. Bellani'
We report on multiterminal electrical transport measurements performed on a bilayer graphene sheet enclosed by two hexagonal boron nitride flakes. We characterize the temperature dependence of electrical resistivity from 300 mK to 50 K, varying the carrier densities with a back gate. The resistivity curves clearly show a temperature-independent crossing point at density n=nc≈2.5×10^11 cm^−2 for both positive and negative carriers, separating two distinct regions with dρ/dT<0 and dρ/dT>0, respectively. Our analysis rules out the possibility of a zero-T quantum phase transition, revealing instead the onset of robust ballistic transport for n>nc, while the T dependence close to the neutrality point is the one expected from the parabolic energy-momentum relation. At low temperature (T≪10 K), the data are compatible with transport via variable range hopping mediated by localized impurity sites, with a characteristic exponent 1/3 that is renormalized to 1/2 by Coulomb interaction in the high-density regime.
Phys. Rev. B 89, 121404(R) (2014) DOI: http://dx.doi.org/10.1103/PhysRevB.89.121404
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.121404
Electron-hole entanglement in a quantum spin Hall insulator
'Koji Sato, Mircea Trif, and Yaroslav Tserkovnyak'
We demonstrate that entangled electron-hole pairs can be produced and detected in a quantum spin Hall insulator with a constriction that allows for a weak interedge tunneling. A violation of a Bell inequality, which can be constructed in terms of low-frequency nonlocal current-current correlations, serves as a detection of the entanglement. We show that the maximum violation of a Bell inequality can be naturally achieved in this setup, without a need to fine tune tunneling parameters. This may provide a viable route to producing orbital entanglement in the absence of any correlations and pairing, which is enabled by the helical edge structure of a quantum spin Hall insulator.
Phys. Rev. B 89, 115404 (2014) DOI: http://dx.doi.org/10.1103/PhysRevB.89.115404
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.115404
Quantifying Molecular Stiffness and Interaction with Lateral Force Microscopy
'Alfred John Weymouth,* Thomas Hofmann, Franz J. Giessibl'
The spatial resolution of atomic force microscopy (AFM) can be drastically increased by terminating the tip with a single carbon monoxide (CO) molecule. However, the CO molecule is not stiff, and lateral forces, such as those around the sides of molecules, distort images. This issue begs a larger question of how AFM can probe structures that are laterally weak. Lateral force microscopy (LFM) can probe lateral stiffnesses that are not accessible to normal-force AFM, resulting in higher spatial resolution. With LFM, we determined the torsional spring constant of a CO-terminated tip molecule to be 0.24 newtons per meter. This value is less than that of a surface molecule and an example of a system whose stiffness is a product not only of bonding partners but also local environment.
Science 343, 1120 (2014)
http://www.sciencemag.org/content/343/6175/1120.full.pdf
Theoretical Analysis of a Dual-Probe Scanning Tunneling Microscope Setup on Graphene
'Mikkel Settnes, Stephen R. Power, Dirch H. Petersen, and Antti-Pekka Jauho'
Experimental advances allow for the inclusion of multiple probes to measure the transport properties of a sample surface. We develop a theory of dual-probe scanning tunneling microscopy using a Green’s function formalism, and apply it to graphene. Sampling the local conduction properties at finite length scales yields real space conductance maps which show anisotropy for pristine graphene systems and quantum interference effects in the presence of isolated impurities. Spectral signatures in the Fourier transforms of real space conductance maps include characteristics that can be related to different scattering processes. We compute the conductance maps of graphene systems with different edge geometries or height fluctuations to determine the effects of nonideal graphene samples on dual-probe measurements.
Phys. Rev. Lett. 112, 096801 (2014) DOI: http://dx.doi.org/10.1103/PhysRevLett.112.096801
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.096801
Creating in-plane pseudomagnetic fields in excess of 1000 T by misoriented stacking in a graphene bilayer
'Wen-Yu He, Ying Su, Mudan Yang, and Lin He'
It is well established that some kinds of lattice deformations in graphene monolayer, which change electron hopping in the sublattice and affect in-plane motion of electrons, may induce out-of-plane pseudomagnetic fields as large as 100 T. Here, we demonstrate that stacking misorientation in graphene bilayers mimics the effect of huge in-plane pseudomagnetic fields greater than 1000 T on the interlayer hopping of electrons. As well as addressing the similarity between the effect of in-plane pseudomagnetic fields and the twisting on the electronic band structure of the Bernal graphene bilayer, we point out that the in-plane magnetic fields (or twisting) could modify the low-energy pseudospin texture of the graphene bilayer (the pseudospin winding number is reduced from 2 to 1), thereby changing the chiralities of quasiparticles from those of spin 1 to those of spin 1/2. Our results illustrate the possibility of controllably manipulating electronic properties of Bernal graphene bilayer by introducing the in-plane magnetic field or twisting.
Phys. Rev. B 89, 125418 (2014) DOI: http://dx.doi.org/10.1103/PhysRevB.89.125418
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.125418
From spin-polarized interfaces to giant magnetoresistance in organic spin valves
'Deniz Çakır, Diana M. Otálvaro, and Geert Brocks'
We calculate the spin-polarized electronic transport through a molecular bilayer spin valve from first principles, and establish the link between the magnetoresistance and the spin-dependent interactions at the metal-molecule interfaces. The magnetoresistance of a Fe|bilayer-C70|Fe spin valve attains a high value of 70% in the linear-response regime, but it drops sharply as a function of the applied bias. The current polarization has a value of 80% in linear response and also decreases as a function of bias. Both these trends can be modeled in terms of prominent spin-dependent Fe|C70 interface states close to the Fermi level, unfolding the potential of spinterface science to control and optimize spin currents.
Phys. Rev. B 89, 115407 (2014) DOI: http://dx.doi.org/10.1103/PhysRevB.89.115407
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.115407
Febr. 22. - Márc. 3. (2014)
Assorted by: Magyarkuti András
Sample Corrugation Affects the Apparent Bond Lengths in Atomic Force Microscopy
'Mark P. Boneschanscher, Sampsa K. Hämäläinen, Peter Liljeroth, and Ingmar Swart'
Frequency modulation atomic force microscopy (AFM) allows the chemical structure of planar molecules to be determined with atomic resolution. Typically, these measurements are carried out in constant-height mode using carbon monoxide (CO) terminated tips. Such tips exhibit considerable flexibility, i.e., the CO molecule can bend laterally due to the tip–sample interaction. Using epitaxial graphene as a model system, we demonstrate experimentally that the apparent atomic positions measured by AFM depend on the sample corrugation. Using molecular mechanics simulations, we explain these observations by the interplay of the CO bending and the nonlinear background signal arising from the neighboring atoms. These effects depend nontrivially on the tip–sample distance and limit the achievable accuracy on the bond length determination based on AFM experiments.
ACS Nano, Article ASAP DOI: 10.1021/nn500317r
http://pubs.acs.org/doi/abs/10.1021/nn500317r
Comprehensive Physical Model of Dynamic Resistive Switching in an Oxide Memristor
'Sungho Kim, ShinHyun Choi, and Wei Lu'
Memristors have been proposed for a number of applications from nonvolatile memory to neuromorphic systems. Unlike conventional devices based solely on electron transport, memristors operate on the principle of resistive switching (RS) based on redistribution of ions. To date, a number of experimental and modeling studies have been reported to probe the RS mechanism; however, a complete physical picture that can quantitatively describe the dynamic RS behavior is still missing. Here, we present a quantitative and accurate dynamic switching model that not only fully accounts for the rich RS behaviors in memristors in a unified framework but also provides critical insight for continued device design, optimization, and applications. The proposed model reveals the roles of electric field, temperature, oxygen vacancy concentration gradient, and different material and device parameters on RS and allows accurate predictions of diverse set/reset, analog switching, and complementary RS behaviors using only material-dependent device parameters.
ACS Nano, Article ASAP DOI: 10.1021/nn405827t
http://pubs.acs.org/doi/abs/10.1021/nn405827t
Probing the Spin States of a Single Acceptor Atom
'Joost van der Heijden, Joe Salfi, Jan A. Mol, Jan Verduijn, Giuseppe C. Tettamanzi, Alex R. Hamilton, Nadine Collaert, and Sven Rogge'
We demonstrate a single-hole transistor using an individual acceptor dopant embedded in a silicon channel. Magneto-transport spectroscopy reveals that the ground state splits as a function of magnetic field into four states, which is unique for a single hole bound to an acceptor in a bulk semiconductor. The two lowest spin states are heavy (|mj| = 3/2) and light (|mj| = 1/2) hole-like, a two-level system that can be electrically driven and is characterized by a magnetic field dependent and long relaxation time, which are properties of interest for qubits. Although the bulklike spin splitting of a boron atom is preserved in our nanotransistor, the measured Landé g-factors, |ghh| = 0.81 ± 0.06 and |glh| = 0.85 ± 0.21 for heavy and light holes respectively, are lower than the bulk value.
Nano Lett., Article ASAP DOI: 10.1021/nl4047015
http://pubs.acs.org/doi/abs/10.1021/nl4047015
Valley Splitting in a Silicon Quantum Device Platform
'Jill A. Miwa †, Oliver Warschkow ‡, Damien J. Carter §, Nigel A. Marks §, Federico Mazzola , Michelle Y. Simmons , and Justin W. Wells'
By suppressing an undesirable surface Umklapp process, it is possible to resolve the two most occupied states (1Γ and 2Γ) in a buried two-dimensional electron gas (2DEG) in silicon. The 2DEG exists because of an atomically sharp profile of phosphorus dopants which have been formed beneath the Si(001) surface (a δ-layer). The energy separation, or valley splitting, of the two most occupied bands has critical implications for the properties of δ-layer derived devices, yet until now, has not been directly measurable. Density functional theory (DFT) allows the 2DEG band structure to be calculated, but without experimental verification the size of the valley splitting has been unclear. Using a combination of direct spectroscopic measurements and DFT we show that the measured band structure is in good qualitative agreement with calculations and reveal a valley splitting of 132 ± 5 meV. We also report the effective mass and occupation of the 2DEG states and compare the dispersions and Fermi surface with DFT.
Nano Lett., Article ASAP DOI: 10.1021/nl404738j
http://pubs.acs.org/doi/abs/10.1021/nl404738j
Controlling Graphene Ultrafast Hot Carrier Response from Metal-like to Semiconductor-like by Electrostatic Gating
'S.-F. Shi, T.-T. Tang, B. Zeng, L. Ju, Q. Zhou, A. Zettl, and F. Wang'
We investigate the ultrafast terahertz response of electrostatically gated graphene upon optical excitation. We observe that the photoinduced terahertz absorption increases in charge neutral graphene but decreases in highly doped graphene. We show that this transition from semiconductor-like to metal-like response is unique for zero bandgap materials such as graphene. In charge neutral graphene photoexcited hot carriers effectively increase electron and hole densities and increase the conductivity. In highly doped graphene, however, photoexcitation does not change net conducting carrier concentration. Instead, it mainly increases electron scattering rate and reduce the conductivity.
Nano Lett., Article ASAP DOI: 10.1021/nl404826r
http://pubs.acs.org/doi/abs/10.1021/nl404826r
Graphene Plasmon Enhanced Vibrational Sensing of Surface-Adsorbed Layers
'Yilei Li, Hugen Yan, Damon B. Farmer, Xiang Meng §, Wenjuan Zhu, Richard M. Osgood, Tony F. Heinz, and Phaedon Avouris'
We characterize the influence of graphene nanoribbon plasmon excitation on the vibrational spectra of surface-absorbed polymers. As the detuning between the graphene plasmon frequency and a vibrational frequency of the polymer decreases, the vibrational peak intensity first increases and is then transformed into a region of narrow optical transparency as the frequencies overlap. Examples of this are provided by the carbonyl vibration in thin films of poly(methyl methacrylate) and polyvinylpyrrolidone. The signal depth of the plasmon-induced transparency is found to be 5 times larger than that of light attenuated by the carbonyl vibration alone. The plasmon-vibrational mode coupling and the resulting fields are analyzed using both a phenomenological model of electromagnetically coupled oscillators and finite-difference time-domain simulations. It is shown that this coupling and the resulting absorption enhancement can be understood in terms of near-field electromagnetic interactions.
Nano Lett., Article ASAP DOI: 10.1021/nl404824w
http://pubs.acs.org/doi/abs/10.1021/nl404824w
An antidamping spin–orbit torque originating from the Berry curvature
'H. Kurebayashi, Jairo Sinova, D. Fang, A. C. Irvine, T. D. Skinner, J. Wunderlich, V. Novák, R. P. Campion, B. L. Gallagher, E. K. Vehstedt, L. P. Zârbo, K. Výborný, A. J. Ferguson & T. Jungwirth'
Magnetization switching at the interface between ferromagnetic and paramagnetic metals, controlled by current-induced torques, could be exploited in magnetic memory technologies. Compelling questions arise regarding the role played in the switching by the spin Hall effect in the paramagnet and by the spin–orbit torque originating from the broken inversion symmetry at the interface. Of particular importance are the antidamping components of these current-induced torques acting against the equilibrium-restoring Gilbert damping of the magnetization dynamics. Here, we report the observation of an antidamping spin–orbit torque that stems from the Berry curvature, in analogy to the origin of the intrinsic spin Hall effect. We chose the ferromagnetic semiconductor (Ga,Mn)As as a material system because its crystal inversion asymmetry allows us to measure bare ferromagnetic films, rather than ferromagnetic–paramagnetic heterostructures, eliminating by design any spin Hall effect contribution. We provide an intuitive picture of the Berry curvature origin of this antidamping spin–orbit torque as well as its microscopic modelling. We expect the Berry curvature spin–orbit torque to be of comparable strength to the spin-Hall-effect-driven antidamping torque in ferromagnets interfaced with paramagnets with strong intrinsic spin Hall effect.
Nature Nanotechnology 9, 211–217 (2014) doi:10.1038/nnano.2014.15
http://www.nature.com/nnano/journal/v9/n3/full/nnano.2014.15.html
Evidence for quantum annealing with more than one hundred qubits
'Sergio Boixo, Troels F. Rønnow, Sergei V. Isakov, Zhihui Wang, David Wecker, Daniel A. Lidar, John M. Martinis & Matthias Troyer'
Quantum technology is maturing to the point where quantum devices, such as quantum communication systems, quantum random number generators and quantum simulators may be built with capabilities exceeding classical computers. A quantum annealer, in particular, solves optimization problems by evolving a known initial configuration at non-zero temperature towards the ground state of a Hamiltonian encoding a given problem. Here, we present results from tests on a 108 qubit D-Wave One device based on superconducting flux qubits. By studying correlations we find that the device performance is inconsistent with classical annealing or that it is governed by classical spin dynamics. In contrast, we find that the device correlates well with simulated quantum annealing. We find further evidence for quantum annealing in the form of small-gap avoided level crossings characterizing the hard problems. To assess the computational power of the device we compare it against optimized classical algorithms.
Nature Physics 10, 218–224 (2014) doi:10.1038/nphys2900
http://www.nature.com/nphys/journal/v10/n3/full/nphys2900.html
Electrons go ballistic
'Juan José Palacios'
A recent experiment shows that graphene nanoribbons can be grown to be perect conductors where electrons travel long distances without coming across a single obstacle.
Nature Physics 10, 182–183 (2014) doi:10.1038/nphys2909
http://www.nature.com/nphys/journal/v10/n3/full/nphys2909.html#access
Quantum droplets of electrons and holes
'A. E. Almand-Hunter, H. Li, S. T. Cundiff, M. Mootz, M. Kira & S. W. Koch'
Interacting many-body systems are characterized by stable configurations of objects—ranging from elementary particles to cosmological formations1, 2, 3—that also act as building blocks for more complicated structures. It is often possible to incorporate interactions in theoretical treatments of crystalline solids by introducing suitable quasiparticles that have an effective mass, spin or charge4, 5 which in turn affects the material’s conductivity, optical response or phase transitions2, 6, 7. Additional quasiparticle interactions may also create strongly correlated configurations yielding new macroscopic phenomena, such as the emergence of a Mott insulator8, superconductivity or the pseudogap phase of high-temperature superconductors9, 10, 11. In semiconductors, a conduction-band electron attracts a valence-band hole (electronic vacancy) to create a bound pair, known as an exciton12, 13, which is yet another quasiparticle. Two excitons may also bind together to give molecules, often referred to as biexcitons14, and even polyexcitons may exist15, 16. In indirect-gap semiconductors such as germanium or silicon, a thermodynamic phase transition may produce electron–hole droplets whose diameter can approach the micrometre range17, 18. In direct-gap semiconductors such as gallium arsenide, the exciton lifetime is too short for such a thermodynamic process. Instead, different quasiparticle configurations are stabilized dominantly by many-body interactions, not by thermalization. The resulting non-equilibrium quantum kinetics is so complicated that stable aggregates containing three or more Coulomb-correlated electron–hole pairs remain mostly unexplored. Here we study such complex aggregates and identify a new stable configuration of charged particles that we call a quantum droplet. This configuration exists in a plasma and exhibits quantization owing to its small size. It is charge neutral and contains a small number of particles with a pair-correlation function that is characteristic of a liquid. We present experimental and theoretical evidence for the existence of quantum droplets in an electron–hole plasma created in a gallium arsenide quantum well by ultrashort optical pulses.
Nature 506, 471–475 (27 February 2014) doi:10.1038/nature12994
http://www.nature.com/nature/journal/v506/n7489/full/nature12994.html
Febr. 1. - Febr. 21. (2014)
Assorted by: Fülöp Bálint
Nanowire section (InAs, InSb)
Magnetotransport Subband Spectroscopy in InAs Nanowires
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.076801
Controlled Synthesis of Phase-Pure InAs Nanowires on Si(111) by Diminishing the Diameter to 10 nm
http://pubs.acs.org/doi/abs/10.1021/nl4040847
Phase Separation in Single InxGa1–xN Nanowires Revealed through a Hard X-ray Synchrotron Nanoprobe
http://pubs.acs.org/doi/abs/10.1021/nl4042752
Mobility Enhancement by Sb-mediated Minimisation of Stacking Fault Density in InAs Nanowires Grown on Silicon
http://pubs.acs.org/doi/abs/10.1021/nl5001554
Strong Terahertz Emission and Its Origin from Catalyst-Free InAs Nanowire Arrays
http://pubs.acs.org/doi/abs/10.1021/nl404737r
Long Term Stability of Nanowire Nanoelectronics in Physiological Environments
http://pubs.acs.org/doi/abs/10.1021/nl500070h
Graphene section
Electron–Phonon Interactions and the Intrinsic Electrical Resistivity of Graphene
http://pubs.acs.org/doi/abs/10.1021/nl402696q
Graphene Oxide-Based Carbon Interconnecting Layer for Polymer Tandem Solar Cells
http://pubs.acs.org/doi/abs/10.1021/nl4046284
Relaxation of optically excited carriers in graphene: Anomalous diffusion and Lévy flights
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.075414
Quantum corrections to thermopower and conductivity in graphene
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.075411
Quantum phase transitions into Kondo states in bilayer graphene
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.081101
Quantum Search on Graphene Lattices
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.070504
High-Contrast Terahertz Wave Modulation by Gated Graphene Enhanced by Extraordinary Transmission through Ring Apertures
http://pubs.acs.org/doi/abs/10.1021/nl4041274
Transport Measurement of Landau Level Gaps in Bilayer Graphene with Layer Polarization Control
http://pubs.acs.org/doi/abs/10.1021/nl4043399
Common Origin of Green Luminescence in Carbon Nanodots and Graphene Quantum Dots
http://pubs.acs.org/doi/abs/10.1021/nn500368m
Self-Organized Platinum Nanoparticles on Freestanding Graphene
http://pubs.acs.org/doi/abs/10.1021/nn406394f
Others (MCBJ, AFM, Majorana, QD, etc.)
Auger-Assisted Electron Transfer from Photoexcited Semiconductor Quantum Dots
http://pubs.acs.org/doi/abs/10.1021/nl4041687
Hot-Electron Photodetection with a Plasmonic Nanostripe Antenna
http://pubs.acs.org/doi/abs/10.1021/nl4044373
Electronic states of wires and slabs of topological insulators: Quantum Hall effects and edge transport
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.085305
Annihilation of Colliding Bogoliubov Quasiparticles Reveals their Majorana Nature
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.070604
Signatures of Majorana fermions in topological insulator Josephson junction devices
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.075106
Electrochemical Charge-Transfer Resistance in Carbon Nanotube Composites
http://pubs.acs.org/doi/abs/10.1021/nl404349g
Flexible Power Fabrics Made of Carbon Nanotubes for Harvesting Thermoelectricity
http://pubs.acs.org/doi/abs/10.1021/nn405893t
Just for fun
Synthetic Antiferromagnetic Nanoparticles as Potential Contrast Agents in MRI
http://pubs.acs.org/doi/abs/10.1021/nn406158h
Fully Solution-Processed Semitransparent Organic Solar Cells with a Silver Nanowire Cathode and a Conducting Polymer Anode
http://pubs.acs.org/doi/abs/10.1021/nn406672n
Using Room Temperature Current Noise To Characterize Single Molecular Spectra
http://pubs.acs.org/doi/abs/10.1021/nn404526w
Laser Damage Helps the Eavesdropper in Quantum Cryptography
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.070503
A New Type of Porous Graphite Foams and Their Integrated Composites with Oxide/Polymer Core/Shell Nanowires for Supercapacitors: Structural Design, Fabrication, and Full Supercapacitor Demonstrations
http://pubs.acs.org/doi/abs/10.1021/nl5001778
Ultrafast Modulation of the Plasma Frequency of Vertically Aligned Indium Tin Oxide Rods
http://pubs.acs.org/doi/abs/10.1021/nl4028044
A bulk graphene mimic
http://www.nature.com/nature/journal/v506/n7488/full/506269d.html
Quantum information: Strength of weak measurementsQuantum information: Strength of weak measurements
The back-action of a weak measurement on the electron spin of a nitrogen–vacancy centre in diamond can be used to steer the associated nuclear spin towards a desired state.
http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2908.html
Not so neutral
Switzerland’s science landscape is under threat after a narrow majority of citizens voted for tighter immigration rules that could restrict the number of foreign scientists who work in the country.
http://www.nature.com/news/not-so-neutral-1.14725
Astronomy: Death of a comet
Before it shattered near the Sun, Comet ISON became a scientific celebrity. Now researchers are trying to piece together its lessons.
http://www.nature.com/news/astronomy-death-of-a-comet-1.14741
