Showing papers in "Bulletin of the American Physical Society in 2012"

Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection

Abstract: Modern computing technology is based on writing, storing and retrieving information encoded as magnetic bits. Although the giant magnetoresistance effect has improved the electrical read out of memory elements, magnetic writing remains the object of major research efforts. Despite several reports of methods to reverse the polarity of nanosized magnets by means of local electric fields and currents, the simple reversal of a high-coercivity, single-layer ferromagnet remains a challenge. Materials with large coercivity and perpendicular magnetic anisotropy represent the mainstay of data storage media, owing to their ability to retain a stable magnetization state over long periods of time and their amenability to miniaturization. However, the same anisotropy properties that make a material attractive for storage also make it hard to write to. Here we demonstrate switching of a perpendicularly magnetized cobalt dot driven by in-plane current injection at room temperature. Our device is composed of a thin cobalt layer with strong perpendicular anisotropy and Rashba interaction induced by asymmetric platinum and AlOx interface layers. The effective switching field is orthogonal to the direction of the magnetization and to the Rashba field. The symmetry of the switching field is consistent with the spin accumulation induced by the Rashba interaction and the spin-dependent mobility observed in non-magnetic semiconductors, as well as with the torque induced by the spin Hall effect in the platinum layer. Our measurements indicate that the switching efficiency increases with the magnetic anisotropy of the cobalt layer and the oxidation of the aluminium layer, which is uppermost, suggesting that the Rashba interaction has a key role in the reversal mechanism. To prove the potential of in-plane current switching for spintronic applications, we construct a reprogrammable magnetic switch that can be integrated into non-volatile memory and logic architectures. This device is simple, scalable and compatible with present-day magnetic recording technology.

Two types of luminescence blinking revealed by spectroelectrochemistry of single quantum dots

Abstract: Photoluminescence blinking—random switching between states of high (ON) and low (OFF) emissivities—is a universal property of molecular emitters found in dyes, polymers, biological molecules and artificial nanostructures such as nanocrystal quantum dots, carbon nanotubes and nanowires. For the past 15 years, colloidal nanocrystals have been used as a model system to study this phenomenon. The occurrence of OFF periods in nanocrystal emission has been commonly attributed to the presence of an additional charge, which leads to photoluminescence quenching by non-radiative recombination (the Auger mechanism). However, this ‘charging’ model was recently challenged in several reports. Here we report time-resolved photoluminescence studies of individual nanocrystal quantum dots performed while electrochemically controlling the degree of their charging, with the goal of clarifying the role of charging in blinking. We find that two distinct types of blinking are possible: conventional (A-type) blinking due to charging and discharging of the nanocrystal core, in which lower photoluminescence intensities correlate with shorter photoluminescence lifetimes; and a second sort (B-type), in which large changes in the emission intensity are not accompanied by significant changes in emission dynamics. We attribute B-type blinking to charge fluctuations in the electron-accepting surface sites. When unoccupied, these sites intercept ‘hot’ electrons before they relax into emitting core states. Both blinking mechanisms can be electrochemically controlled and completely suppressed by application of an appropriate potential.

Room temperature sub-diffraction-limited plasmon laser by total internal reflection

Abstract: Plasmon lasers can operate at dimensions well below the diffraction limit. Their small size promises uses in nanophotonic circuits and for other size-critical applications. The demonstration of a sub-diffraction-limited plasmon laser with low losses, which enables its room-temperature operation, takes a significant step towards realizing the potential of these lasers.

Theory of double-resonant Raman spectra in graphene: intensity and line shape of defect-induced and two-phonon bands

Abstract: We calculate the double resonant (DR) Raman spectrum of graphene, and determine the lines associated to both phonon-defect processes, and two-phonons ones. Phonon and electronic dispersions reproduce calculations based on density functional theory corrected with GW. Electron-light, -phonon, and -defect scattering matrix elements and the electronic linewidth are explicitly calculated. Defect-induced processes are simulated by considering different kind of idealized defects. For an excitation energy of $\epsilon_L=2.4$ eV, the agreement with measurements is very good and calculations reproduce: the relative intensities among phonon-defect or among two-phonon lines; the measured small widths of the D, $D'$, 2D and $2D'$ lines; the line shapes; the presence of small intensity lines in the 1800, 2000 cm$^{-1}$ range. We determine how the spectra depend on the excitation energy, on the light polarization, on the electronic linewidth, on the kind of defects and on their concentration. According to the present findings, the intensity ratio between the $2D'$ and 2D lines can be used to determine experimentally the electronic linewidth. The intensity ratio between the $D$ and $D'$ lines depends on the kind of model defect, suggesting that this ratio could possibly be used to identify the kind of defects present in actual samples. Charged impurities outside the graphene plane provide an almost undetectable contribution to the Raman signal.

Anatomy of a Nanoscale Conduction Channel Reveals the Mechanism of a High-Performance Memristor

Abstract: By employing a precise method for locating and directly imaging the active switching region in a resistive random access memory (RRAM) device, a nanoscale conducting channel consisting of an amorphous Ta(O) solid solution surrounded by nearly stoichiometric Ta(2) O(5) is observed. Structural and chemical analysis of the channel combined with temperature-dependent transport measurements indicate a unique resistance switching mechanism.

Finding density functionals with machine learning

Abstract: Machine learning is used to approximate density functionals. For the model problem of the kinetic energy of noninteracting fermions in 1D, mean absolute errors below 1 kcal/mol on test densities similar to the training set are reached with fewer than 100 training densities. A predictor identifies if a test density is within the interpolation region. Via principal component analysis, a projected functional derivative finds highly accurate self-consistent densities. The challenges for application of our method to real electronic structure problems are discussed.

Robustness of interdependent networks under targeted attack

Abstract: When an initial failure of nodes occurs in interdependent networks, a cascade of failure between the networks occurs. Earlier studies focused on random initial failures. Here we study the robustness of interdependent networks under targeted attack on high or low degree nodes. We introduce a general technique which maps the targeted-attack problem in interdependent networks to the random-attack problem in a transformed pair of interdependent networks. We find that when the highly connected nodes are protected and have lower probability to fail, in contrast to single scale-free (SF) networks where the percolation threshold pc = 0, coupled SF networks are significantly more vulnerable with pc significantly larger than zero. The result implies that interdependent networks are difficult to defend by strategies such as protecting the high degree nodes that have been found useful to significantly improve robustness of single networks.

Revealing the Superfluid Lambda Transition in the Universal Thermodynamics of a Unitary Fermi Gas

Abstract: Fermi gases, collections of fermions such as neutrons and electrons, are found throughout nature, from solids to neutron stars. Interacting Fermi gases can form a superfluid or, for charged fermions, a superconductor. We have observed the superfluid phase transition in a strongly interacting Fermi gas by high-precision measurements of the local compressibility, density, and pressure. Our data completely determine the universal thermodynamics of these gases without any fit or external thermometer. The onset of superfluidity is observed in the compressibility, the chemical potential, the entropy, and the heat capacity, which displays a characteristic lambda-like feature at the critical temperature T(c)/T(F) = 0.167(13). The ground-state energy is 3/5ξN E(F) with ξ = 0.376(4). Our measurements provide a benchmark for many-body theories of strongly interacting fermions.

Chiral superconductivity from repulsive interactions in doped graphene

Abstract: Chiral superconducting states are expected to support a variety of exotic and potentially useful phenomena. Theoretical analysis suggests that just such a state could emerge in a doped graphene monolayer.

The Dark Energy Survey

Abstract: Dark Energy is the dominant constituent of the universe and we have little understanding of it. We describe a new project aimed at measuring the dark energy equation of state parameter, w, to a statistical precision of ~5% with four separate techniques. The survey will image 5000 deg2 in the southern sky and collect 300 million galaxies, 30,000 galaxy clusters, and 2000 Type Ia supernovae. The survey will be carried out using a new 3 deg2 mosaic camera mounted at the prime focus of the 4m Blanco telescope at CTIO.

Cold Atmosphere Plasma in Cancer Therapy

Abstract: Recent progress in atmospheric plasmas has led to the creation of cold plasmas with ion temperature close to room temperature. This paper outlines recent progress in understanding of cold plasma physics as well as application of cold atmospheric plasma (CAP) in cancer therapy. Varieties of novel plasma diagnostic techniques were developed recently in a quest to understand physics of CAP. It was established that the streamer head charge is about 108 electrons, the electrical field in the head vicinity is about 107 V/m, and the electron density of the streamer column is about 1019 m−3. Both in-vitro and in-vivo studies of CAP action on cancer were performed. It was shown that the cold plasma application selectively eradicates cancer cells in-vitro without damaging normal cells and significantly reduces tumor size in-vivo. Studies indicate that the mechanism of action of cold plasma on cancer cells is related to generation of reactive oxygen species with possible induction of the apoptosis pathway. It is also shown that the cancer cells are more susceptible to the effects of CAP because a greater percentage of cells are in the S phase of the cell cycle.

Multiferroicity due to Charge Ordering

Abstract: In this contribution to the special issue on multiferroics we focus on multiferroicity driven by different forms of charge ordering. We will present the generic mechanisms by which charge ordering can induce ferroelectricity in magnetic systems. There is a number of specific classes of materials for which this is relevant. We will discuss in some detail (i) perovskite manganites of the type (PrCa)MnO3, (ii) the complex and interesting situation in magnetite Fe3O4, (iii) strongly ferroelectric frustrated LuFe2O4 and (iv) an example of a quasi-one-dimensional organic system. All these are ‘type-I’ multiferroics, in which ferroelectricity and magnetism have different origins and occur at different temperatures. In the second part of this article we discuss ‘type-II’ multiferroics, in which ferroelectricity is completely due to magnetism, but with charge ordering playing an important role, such as (v) the newly discovered multiferroic Ca3CoMnO6, (vi) possible ferroelectricity in rare earth perovskite nickelates of the type RNiO3, (vii) multiferroic properties of manganites of the type RMn2O5, (viii) perovskite manganites with magnetic E-type ordering and (ix) bilayer manganites. (Some figures in this article are in colour only in the electronic version)

Giant Flexoelectric Effect in Ferroelectric Epitaxial Thin Films

Abstract: We report on nanoscale strain gradients in ferroelectric HoMnO3 epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane x-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.

Two-dimensional Fourier transform electronic spectroscopy

Abstract: Two-dimensional Fourier transform electronic spectra of the cyanine dye IR144 in methanol are used to explore new aspects of optical 2D spectroscopy on a femtosecond timescale. The experiments reported here are pulse sequence and coherence pathway analogs of the two-dimensional magnetic resonance techniques known as COSY (correlated spectroscopy) and NOESY (nuclear Overhauser effect spectroscopy). Noncollinear three pulse scattering allows selection of electronic coherence pathways by choice of phase matching geometry, temporal pulse order, and Fourier transform variables. Signal fields and delays between excitation pulses are measured by spectral interferometry. Separate real (absorptive) and imaginary (dispersive) 2D spectra are generated by measuring the signal field at the sample exit, performing a 2D scan that equally weights rephasing and nonrephasing coherence pathways, and phasing the 2D spectra against spectrally resolved pump–probe signals. A 3D signal propagation function is used to correct the...

Stabilizing Rabi oscillations in a superconducting qubit using quantum feedback

Abstract: Real-time quantum feedback based on weak measurement of the quantum state is used to stabilize the oscillation phase of a driven quantum bit. By performing weak measurements of a quantum state, it is possible to slow the rate of collapse of its wavefunction, so that information about the quantum state can be gradually acquired. Such information can be used to continuously track and steer the quantum state using feedback. This paper reports quantum feedback control of a superconducting quantum bit (qubit) coupled to a microwave cavity. The qubit undergoes coherent oscillations that can be made to speed up, slow down or persist indefinitely. This ability to actively suppress decoherence could find many applications in quantum error correction, quantum-state stabilization and purification, entanglement generation and adaptive measurements. The act of measurement bridges the quantum and classical worlds by projecting a superposition of possible states into a single (probabilistic) outcome. The timescale of this ‘instantaneous’ process can be stretched using weak measurements1,2, such that it takes the form of a gradual random walk towards a final state. Remarkably, the interim measurement record is sufficient to continuously track and steer the quantum state using feedback3,4,5,6,7,8. Here we implement quantum feedback control in a solid-state system, namely a superconducting quantum bit (qubit) coupled to a microwave cavity9. A weak measurement of the qubit is implemented by probing the cavity with microwave photons, maintaining its average occupation at less than one photon. These photons are then directed to a high-bandwidth, quantum-noise-limited amplifier10,11, which allows real-time monitoring of the state of the cavity (and, hence, that of the qubit) with high fidelity. We demonstrate quantum feedback control by inhibiting the decay of Rabi oscillations, allowing them to persist indefinitely12. Such an ability permits the active suppression of decoherence and enables a method of quantum error correction based on weak continuous measurements13,14. Other applications include quantum state stabilization4,7,15, entanglement generation using measurement16, state purification17 and adaptive measurements18,19.

Mechanical Control of Spin States in Spin-1 Molecules and the Underscreened Kondo Effect

Abstract: Spin Control Through Molecular Stretching Molecules with high symmetry, such as metal complexes with several equivalent ligands, can, in principle, have this symmetry broken by stresses that lengthen bonds in one direction. Parks et al. (p. 1370; see the Perspective by Jarillo-Herrero) placed cobalt complexes in a break-junction contact and then applied a mechanical force to slowly open the contact. Low-temperature measurement of differential conductance revealed a splitting of the Kondo peak at zero-applied voltage into two features, which occurred by breaking the degeneracy of S = 1 triplet states. This assignment of the spin state was confirmed by the evolution of splitting with magnetic field and by comparison to theory for a case where the conduction electrons only partially screen the spin states. Controlled stretching of individual transition-metal complexes enables direct manipulation of the molecule’s spin states. The ability to make electrical contact to single molecules creates opportunities to examine fundamental processes governing electron flow on the smallest possible length scales. We report experiments in which we controllably stretched individual cobalt complexes having spin S = 1, while simultaneously measuring current flow through the molecule. The molecule’s spin states and magnetic anisotropy were manipulated in the absence of a magnetic field by modification of the molecular symmetry. This control enabled quantitative studies of the underscreened Kondo effect, in which conduction electrons only partially compensate the molecular spin. Our findings demonstrate a mechanism of spin control in single-molecule devices and establish that they can serve as model systems for making precision tests of correlated-electron theories.

Conduction at domain walls in insulating Pb(Zr$_{0.2}$Ti$_{0.8}$)O$_3$ thin films

Abstract: Domain wall conduction in insulating Pb(Zr(0.2) Ti(0.8))O(3) thin films is demonstrated. The observed electrical conduction currents can be clearly differentiated from displacement currents associated with ferroelectric polarization switching. The domain wall conduction, nonlinear and highly asymmetric due to the specific local probe measurement geometry, shows thermal activation at high temperatures, and high stability over time.

Graphyne- and Graphdiyne-based Nanoribbons: Density Functional Theory Calculations of Electronic Structures

Abstract: We report on the configurations and electronic properties of graphyne and graphdiyne nanoribbons with armchair and zigzag edges investigated with first principles calculations. Our results show that all the nanoribbons are semiconductors with suitable band gaps similar to silicon. And their band gaps decrease as widths of nanoribbons increase. We also find that the band gap is at the Gamma point for all graphdiyne ribbons and it is at the X point for all graphyne ribbons. Of particular interest, the band gap of zigzag graphyne nanoribbons show a unique step effect as the width increases. This property is good for tuning of the energy band gap, as in a certain range of the ribbon width, the energy gap remains constant and in reality the edge cannot be as neat as that in a theoretic model.