Showing papers by "National Institute of Standards and Technology published in 2008"
TL;DR: In this paper, the physics of spin transfer torque in magnetic devices are discussed and an elementary discussion of the mechanism and experimental progress in this field is provided, along with a review of theoretical and experimental results.
1,688 citations
TL;DR: An ultracold dense gas of potassium-rubidium (40K87Rb) polar molecules is created using a single step of STIRAP with two-frequency laser irradiation to coherently transfer extremely weakly bound KRb molecules to the rovibrational ground state of either the triplet or the singlet electronic ground molecular potential.
Abstract: A quantum gas of ultracold polar molecules, with long-range and anisotropic interactions, not only would enable explorations of a large class of many-body physics phenomena but also could be used for quantum information processing We report on the creation of an ultracold dense gas of potassium-rubidium (40K87Rb) polar molecules Using a single step of STIRAP (stimulated Raman adiabatic passage) with two-frequency laser irradiation, we coherently transfer extremely weakly bound KRb molecules to the rovibrational ground state of either the triplet or the singlet electronic ground molecular potential The polar molecular gas has a peak density of 1012 per cubic centimeter and an expansion-determined translational temperature of 350 nanokelvin The polar molecules have a permanent electric dipole moment, which we measure with Stark spectroscopy to be 0052(2) Debye (1 Debye = 3336 × 10–30 coulomb-meters) for the triplet rovibrational ground state and 0566(17) Debye for the singlet rovibrational ground state
1,466 citations
TL;DR: The role of magnetism in the superconductivity that occurs when mobile 'electrons' or 'holes' are doped into the antiferromagnetic parent compounds of rare-earth iron-based oxide systems was investigated in this paper.
Abstract: Following the discovery of long-range antiferromagnetic order in the parent compounds of high-transition-temperature (high-T(c)) copper oxides, there have been efforts to understand the role of magnetism in the superconductivity that occurs when mobile 'electrons' or 'holes' are doped into the antiferromagnetic parent compounds. Superconductivity in the newly discovered rare-earth iron-based oxide systems ROFeAs (R, rare-earth metal) also arises from either electron or hole doping of their non-superconducting parent compounds. The parent material LaOFeAs is metallic but shows anomalies near 150 K in both resistivity and d.c. magnetic susceptibility. Although optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping with electrons to induce superconductivity, there has been no direct evidence of SDW order. Here we report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below 155 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then, at approximately 137 K, develops long-range SDW-type antiferromagnetic order with a small moment but simple magnetic structure. Doping the system with fluorine suppresses both the magnetic order and the structural distortion in favour of superconductivity. Therefore, like high-T(c) copper oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range-ordered antiferromagnetic ground state.
1,441 citations
TL;DR: Repeated measurements during the past year yield a preliminary constraint on the temporal variation of the fine-structure constant α of α ofbatchmode, a regime of operation for atomic clocks based on optical transitions, promising even higher performance.
Abstract: Time has always had a special status in physics because of its fundamental role in specifying the regularities of nature and because of the extraordinary precision with which it can be measured. This precision enables tests of fundamental physics and cosmology, as well as practical applications such as satellite navigation. Recently, a regime of operation for atomic clocks based on optical transitions has become possible, promising even higher performance. We report the frequency ratio of two optical atomic clocks with a fractional uncertainty of 5.2 × 10–17. The ratio of aluminum and mercury single-ion optical clock frequencies νAl+/νHg+ is 1.052871833148990438(55), where the uncertainty comprises a statistical measurement uncertainty of 4.3 × 10–17, and systematic uncertainties of 1.9 × 10–17 and 2.3 × 10–17 in the mercury and aluminum frequency standards, respectively. Repeated measurements during the past year yield a preliminary constraint on the temporal variation of the fine-structure constant α of ![Formula][1] .
[1]: /embed/tex-math-1.gif
1,335 citations
TL;DR: Experiments show that just a few entangled trapped ions can be used to improve the precision of measurements, and if the entanglement in such systems can be scaled up to larger numbers of ions, simulations that are intractable on a classical computer might become possible.
Abstract: To process information using quantum-mechanical principles, the states of individual particles need to be entangled and manipulated. One way to do this is to use trapped, laser-cooled atomic ions. Attaining a general-purpose quantum computer is, however, a distant goal, but recent experiments show that just a few entangled trapped ions can be used to improve the precision of measurements. If the entanglement in such systems can be scaled up to larger numbers of ions, simulations that are intractable on a classical computer might become possible.
1,111 citations
TL;DR: A microporous metal-organic framework, PCN-14, based on an anthracene derivative, 5,5'-(9,10-anthracenediyl)di-isophthalate (H4adip), was synthesized under solvothermal reaction conditions and revealed that it consists of nanoscopic cages suitable for gas storage.
Abstract: A microporous metal-organic framework, PCN-14, based on an anthracene derivative, 5,5‘-(9,10-anthracenediyl)di-isophthalate (H4adip), was synthesized under solvothermal reaction conditions. X-ray single crystal analysis revealed that PCN-14 consists of nanoscopic cages suitable for gas storage. N2-adsorption studies of PCN-14 at 77 K reveal a Langmuir surface area of 2176 m2/g and a pore volume of 0.87 cm3/g. Methane adsorption studies at 290 K and 35 bar show that PCN-14 exhibits an absolute methane-adsorption capacity of 230 v/v, 28% higher than the DOE target (180 v/v) for methane storage.
795 citations
TL;DR: The fabrication and evaluation of a fiber-coupled, photon-number-resolving TES detector optimized for absorption at 1550 and 1310 nm wavelengths is described, which to the authors' knowledge is the highest system detection efficiency reported for a near-infrared single-photon detector.
Abstract: Single-photon detectors operating at visible and near-infrared wavelengths with high detection efficiency and low noise are a requirement for many quantum-information applications. Superconducting transition-edge sensors (TESs) are capable of detecting visible and near-infrared light at the single-photon level and are capable of discriminating between one-and two-photon absorption events; however these capabilities place stringent design requirements on the TES heat capacity, thermometry, and optical detection efficiency. We describe the fabrication and evaluation of a fiber-coupled, photon-number-resolving TES detector optimized for absorption at 1550 and 1310 nm wavelengths. The measured system detection efficiency at 1556 nm is 95 %±2 %, which to our knowledge is the highest system detection efficiency reported for a near-infrared single-photon detector.Work of US government: not subject to US copyright
757 citations
TL;DR: The broadband, coherent nature of narrow-linewidth fiber frequency combs is exploited to measure the full complex spectrum of a molecular gas through multiheterodyne spectroscopy, and the measured phase spectrum agrees with the Kramers-Kronig transformation of the absorption spectrum.
Abstract: The broadband, coherent nature of narrow-linewidth fiber frequency combs is exploited to measure the full complex spectrum of a molecular gas through multiheterodyne spectroscopy. We measure the absorption and phase shift experienced by each of 155 000 individual frequency-comb lines, spaced by 100 MHz and spanning from 1495 to 1620 nm, after passing through hydrogen cyanide gas. The measured phase spectrum agrees with the Kramers-Kronig transformation of the absorption spectrum. This technique can provide a full complex spectrum rapidly, over wide bandwidths, and with hertz-level accuracy.
744 citations
TL;DR: In this article, the authors describe a randomized benchmarking method that yields estimates of the computationally relevant errors without relying on accurate state preparation and measurement, since it involves long sequences of randomly chosen gates, and also verifies that error behavior is stable when used in long computations.
Abstract: A key requirement for scalable quantum computing is that elementary quantum gates can be implemented with sufficiently low error. One method for determining the error behavior of a gate implementation is to perform process tomography. However, standard process tomography is limited by errors in state preparation, measurement and one-qubit gates. It suffers from inefficient scaling with number of qubits and does not detect adverse error-compounding when gates are composed in long sequences. An additional problem is due to the fact that desirable error probabilities for scalable quantum computing are of the order of 0.0001 or lower. Experimentally proving such low errors is challenging. We describe a randomized benchmarking method that yields estimates of the computationally relevant errors without relying on accurate state preparation and measurement. Since it involves long sequences of randomly chosen gates, it also verifies that error behavior is stable when used in long computations. We implemented randomized benchmarking on trapped atomic ion qubits, establishing a one-qubit error probability per randomized $\ensuremath{\pi}/2$ pulse of 0.00482(17) in a particular experiment. We expect this error probability to be readily improved with straightforward technical modifications.
730 citations
TL;DR: An array of 488 Josephson junctions that amplifies and squeezes noise beyond conventional quantum limits is proposed in this article for superconducting qubits and other quantum devices.
Abstract: An array of 488 Josephson junctions that amplifies and squeezes noise beyond conventional quantum limits should prove useful in the study and development of superconducting qubits and other quantum devices.
654 citations
TL;DR: In this paper, a tetragonal-to-orthorhombic distortion associated with the onset of the antiferromagnetic order in the FeAs-based materials was found.
Abstract: The recent discovery of superconductivity in $(\mathrm{Ba},\mathrm{K}){\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$, which crystallizes in the ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$ (122) structure as compared with the LnFeAsO (Ln is lanthanide) systems that possess the ZrCuSiAs (1111) structure, demonstrates the exciting potential of the FeAs-based materials for high-${T}_{C}$ superconductivity. Here we report neutron diffraction studies that show a tetragonal-to-orthorhombic distortion associated with the onset of $\mathbf{q}=(101)$ antiferromagnetic order in ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$, with a saturation moment $0.87(3){\ensuremath{\mu}}_{B}$ per Fe that is orientated along the longer $a$ axis of the $ab$ planes. The simultaneous first-order structural and magnetic transition is in contrast with the separated transitions previously reported in the 1111-type materials. The orientational relation between magnetic alignment and lattice distortion supports a multiorbital nature for the magnetic order.
TL;DR: Comparison of the structural evolution of CeFeAsO(1-x)F(x) with other Fe-based superconductors suggests that the structural perfection of the Fe-As tetrahedron is important for the high-Tc superconductivity in these Fe pnictides.
Abstract: According to a neutron-scattering study of the structural and magnetic properties of the pnictide CeFeAsO1−xFx, the phase diagram of this material shows considerable similarities with the high-Tc cuprate superconductors. These results are an important addition to the effort to find out where superconductivity in these iron–arsenic alloys arises.
TL;DR: A spatially multimode amplifier based on four-wave mixing in a hot vapor is used to produce twin images that exhibit localized entanglement, demonstrating that the system is an ideal source for parallel continuous-variable quantum information protocols.
Abstract: Two beams of light can be quantum mechanically entangled through correlations of their phase and intensity fluctuations. For a pair of spatially extended image-carrying light fields, the concept of entanglement can be applied not only to the entire images but also to their smaller details. We used a spatially multimode amplifier based on four-wave mixing in a hot vapor to produce twin images that exhibit localized entanglement. The images can be bright fields that display position-dependent quantum noise reduction in their intensity difference or vacuum twin beams that are strongly entangled when projected onto a large range of different spatial modes. The high degree of spatial entanglement demonstrates that the system is an ideal source for parallel continuous-variable quantum information protocols.
TL;DR: An improved method for determining lipid areas helps to reconcile long-standing differences in the values of lipid areas obtained from stand-alone x-ray and neutron scattering experiments and poses new challenges for molecular dynamics simulations.
TL;DR: A systematic study of the H2 adsorption on a series of isostructural MOFs, M2(dhtp) (M = Mg, Mn, Co, Ni, Zn), finding a strong correlation between the metal ion radius, the M-H2 distance, and the H1 binding strength, which provides a viable, empirical method to predict the relative H2 binding strength of different open metals.
Abstract: Metal−organic frameworks (MOFs) with open metal sites exhibit a much stronger H2 binding strength than classical MOFs, due to the direct interaction between H2 and the coordinately unsaturated metal ions. Here we report a systematic study of the H2 adsorption on a series of isostructural MOFs, M2(dhtp) (M = Mg, Mn, Co, Ni, Zn). The experimental, initial isosteric heats of adsorption for H2 (Qst) of these MOFs range from 8.5 to 12.9 kJ/mol, with increasing Qst in the following order: Zn, Mn, Mg, Co, and Ni. The H2 binding energies derived from first-principles calculation follow the same trend as the experimental observation on Qst, confirming the electrostatic Coulomb attraction between the H2 and the open metals being the major interaction. We also found a strong correlation between the metal ion radius, the M−H2 distance, and the H2 binding strength, which provides a viable, empirical method to predict the relative H2 binding strength of different open metals.
TL;DR: In this paper, the position of a nanoscale beam using a microwave cavity detector represents a promising step towards being able to measure displacements at the quantum limit, which is the goal of this paper.
Abstract: Measurements of the position of a nanoscale beam using a microwave cavity detector represents a promising step towards being able to measure displacements at the quantum limit.
TL;DR: The aim of this study was to understand the relationship between the redox state of iron-based nanoparticles and their cytotoxicity toward a Gram-negative bacterium, Escherichia coli.
Abstract: Iron-based nanoparticles have been proposed for an increasing number of biomedical or environmental applications although in vitro toxicity has been observed. The aim of this study was to understand the relationship between the redox state of iron-based nanoparticles and their cytotoxicity toward a Gram-negative bacterium, Escherichia coli. While chemically stable nanoparticles (γFe2O3) have no apparent cytotoxicity, nanoparticles containing ferrous and, particularly, zerovalent iron are cytotoxic. The cytotoxic effects appear to be associated principally with an oxidative stress as demonstrated using a mutant strain of E. coli completely devoid of superoxide dismutase activity. This stress can result from the generation of reactive oxygen species with the interplay of oxygen with reduced iron species (FeII and/or Fe0) or from the disturbance of the electronic and/or ionic transport chains due to the strong affinity of the nanoparticles for the cell membrane.
TL;DR: A high-performance remote comparison of optical clocks over kilometer-scale urban distances is realized and the uncertainty of a strontium (Sr) optical lattice clock at the 1 × 10–16 fractional level is evaluated, surpassing the current best evaluations of cesium (Cs) primary standards.
Abstract: Optical atomic clocks promise timekeeping at the highest precision and accuracy, owing to their high operating frequencies. Rigorous evaluations of these clocks require direct comparisons between them. We have realized a high-performance remote comparison of optical clocks over kilometer-scale urban distances, a key step for development, dissemination, and application of these optical standards. Through this remote comparison and a proper design of lattice-confined neutral atoms for clock operation, we evaluate the uncertainty of a strontium (Sr) optical lattice clock at the 1 x 10(-16) fractional level, surpassing the current best evaluations of cesium (Cs) primary standards. We also report on the observation of density-dependent effects in the spin-polarized fermionic sample and discuss the current limiting effect of blackbody radiation-induced frequency shifts.
TL;DR: By connecting device fabrication to molecular design, it is demonstrated that rapid film processing under ambient room conditions and high performance are not mutually exclusive.
Abstract: The use of organic materials presents a tremendous opportunity to significantly impact the functionality and pervasiveness of large-area electronics. Commercialization of this technology requires reduction in manufacturing costs by exploiting inexpensive low-temperature deposition and patterning techniques, which typically lead to lower device performance. We report a low-cost approach to control the microstructure of solution-cast acene-based organic thin films through modification of interfacial chemistry. Chemically and selectively tailoring the source/drain contact interface is a novel route to initiating the crystallization of soluble organic semiconductors, leading to the growth on opposing contacts of crystalline films that extend into the transistor channel. This selective crystallization enables us to fabricate high-performance organic thin-film transistors and circuits, and to deterministically study the influence of the microstructure on the device characteristics. By connecting device fabrication to molecular design, we demonstrate that rapid film processing under ambient room conditions and high performance are not mutually exclusive.
TL;DR: From all-electron fixed-spin-moment calculations, the presence of competing strong antiferromagnetic exchange interactions suggests that magnetism and superconductivity in doped LaFeAsO may be strongly coupled, much like in the high-T(c) cuprates.
Abstract: From all-electron fixed-spin-moment calculations we show that ferromagnetic and checkerboard antiferromagnetic ordering in LaFeAsO are not stable and the stripe antiferromagnetic configuration with is the only stable ground state. The main exchange interactions between Fe ions are large, antiferromagnetic, and frustrated. The magnetic stripe phase breaks the tetragonal symmetry, removes the frustration, and causes a structural distortion. These results successfully explain the magnetic and structural phase transitions in LaFeAsO recently observed by neutron scattering. The presence of competing strong antiferromagnetic exchange interactions suggests that magnetism and superconductivity in doped LaFeAsO may be strongly coupled, much like in the high- cuprates.
13 Jul 2008
TL;DR: This paper proposes an attack graph-based probabilistic metric for network security and studies its efficient computation, and defines and proposes heuristics to improve the efficiency of such computation.
Abstract: To protect critical resources in today's networked environments, it is desirable to quantify the likelihood of potential multi-step attacks that combine multiple vulnerabilities. This now becomes feasible due to a model of causal relationships between vulnerabilities, namely, attack graph. This paper proposes an attack graph-based probabilistic metric for network security and studies its efficient computation. We first define the basic metric and provide an intuitive and meaningful interpretation to the metric. We then study the definition in more complex attack graphs with cycles and extend the definition accordingly. We show that computing the metric directly from its definition is not efficient in many cases and propose heuristics to improve the efficiency of such computation.
TL;DR: In this paper, a photoemission spectroscopy technique was used to study the elementary excitations and energy dispersion in a strongly interacting Fermi gas of 40K atoms.
Abstract: Fermionic superfluidity requires the formation of particle pairs, the size of which varies depending on the system. Many properties of the superfluid depend on the pair size relative to the inter-particle spacing. For example, conventional superconductors comprise a superfluid of loosely bound, large Cooper pairs of electrons, while Bose-Einstein condensates contain tightly bound molecules. The microscopic properties of the fermion pairs can be probed with radio-frequency spectroscopy. However, previous results have been difficult to interpret due to strong final-state interactions that were not well understood. Schunck et al. realize a superfluid spin mixture in an ultracold gas of lithium atoms in which such interactions have negligible influence. They find that the spectroscopic pair size is smaller than the inter-particle spacing. These are the smallest pairs yet observed for fermionic superfluids. In a related experiment, Jin et al use a technique called photoemission spectroscopy to study the excitations in a strongly interacting gas of ultracold potassium atoms. Such studies are of interest because the physics is related to that of the high transition-temperature superconductors, which are not fully understood. Recent experiments using ultracold Fermi gases have demonstrated a phase transition to a superfluid state with strong inter-particle interactions, but these interactions make it difficult to study the behaviour of the atoms in the gas. A technique called photoemission spectroscopy is used to enable a study of the pairing between the atoms. This is of interest because the physics is related to that of the high transition-temperature superconductors. Ultracold atomic gases provide model systems in which to study many-body quantum physics. Recent experiments using Fermi gases have demonstrated a phase transition to a superfluid state with strong interparticle interactions1,2,3,4,5,6. This system provides a realization of the ‘BCS–BEC crossover’7 connecting the physics of Bardeen–Cooper–Schrieffer (BCS) superconductivity with that of Bose–Einstein condensates (BECs). Although many aspects of this system have been investigated, it has not yet been possible to measure the single-particle excitation spectrum (a fundamental property directly predicted by many-body theories). Here we use photoemission spectroscopy to directly probe the elementary excitations and energy dispersion in a strongly interacting Fermi gas of 40K atoms. In the experiments, a radio-frequency photon ejects an atom from the strongly interacting system by means of a spin-flip transition to a weakly interacting state. We measure the occupied density of single-particle states at the cusp of the BCS–BEC crossover and on the BEC side of the crossover, and compare these results to that for a nearly ideal Fermi gas. We show that, near the critical temperature, the single-particle spectral function is dramatically altered in a way that is consistent with a large pairing gap. Our results probe the many-body physics in a way that could be compared to data for the high-transition-temperature superconductors. As in photoemission spectroscopy for electronic materials, our measurement technique for ultracold atomic gases directly probes low-energy excitations and thus can reveal excitation gaps and/or pseudogaps. Furthermore, this technique can provide an analogue of angle-resolved photoemission spectroscopy for probing anisotropic systems, such as atoms in optical lattice potentials.
TL;DR: It is found that this scaling does not apply except in the extreme limit of very small, spherical nanoparticles, and a general, practical map of the resonances is provided for use in locating the desired response for gold nanoantennas.
Abstract: We study the light scattering and surface plasmon resonances of Au nanorods that are commonly used as optical nanoantennas in analogy to dipole radio antennas for chemical and biodetection field-enhanced spectroscopies and scanned-probe microscopies. With the use of the boundary element method, we calculate the nanorod near-field and far-field response to show how the nanorod shape and dimensions determine its optical response. A full mapping of the size (length and radius) dependence for Au nanorods is obtained. The dipolar plasmon resonance wavelength I shows a nearly linear dependence on total rod length L out to the largest lengths that we study. However, L is always substantially less than I/2, indicating the difference between optical nanoantennas and long-wavelength traditional I/2 antennas. Although it is often assumed that the plasmon wavelength scales with the nanorod aspect ratio, we find that this scaling does not apply except in the extreme limit of very small, spherical nanoparticles. The plasmon response depends critically on both the rod length and radius. Large (500 nm) differences in resonance wavelength are found for structures with different sizes but with the same aspect ratio. In addition, the plasmon resonance deduced from the near-field enhancement can be significantly red-shifted due to retardation from the resonance in far-field scattering. Large differences in near-field and far-field response, together with the breakdown of the simple scaling law must be accounted for in the choice and design of metallic I/2 nanoantennas. We provide a general, practical map of the resonances for use in locating the desired response for gold nanoantennas.
TL;DR: A number of recent experiments are reviewed that use laser-cooled neutral atoms localized in a deeply confining optical potential to investigate precision quantum metrology for optical atomic clocks and coherent control of optical interactions of single atoms and photons within the context of cavity quantum electrodynamics.
Abstract: Precision metrology and quantum measurement often demand that matter be prepared in well-defined quantum states for both internal and external degrees of freedom. Laser-cooled neutral atoms localized in a deeply confining optical potential satisfy this requirement. With an appropriate choice of wavelength and polarization for the optical trap, two electronic states of an atom can experience the same trapping potential, permitting coherent control of electronic transitions independent of the atomic center-of-mass motion. Here, we review a number of recent experiments that use this approach to investigate precision quantum metrology for optical atomic clocks and coherent control of optical interactions of single atoms and photons within the context of cavity quantum electrodynamics. We also provide a brief survey of promising prospects for future work.
TL;DR: In this paper, the authors investigate the application of embedded atom method (EAM) interatomic potentials in the study of crystallization kinetics from deeply undercooled melts, focusing on the fcc metals Al and Cu.
Abstract: We investigate the application of embedded atom method (EAM) interatomic potentials in the study of crystallization kinetics from deeply undercooled melts, focusing on the fcc metals Al and Cu. For this application, it is important that the EAM potential accurately reproduces melting properties and liquid structure, in addition to the crystalline properties most commonly fit in its development. To test the accuracy of previously published EAM potentials and to guide the development of new potential in this work, first-principles calculations have been performed and new experimental measurements of the Al and Cu liquid structure factors have been undertaken by X-ray diffraction. We demonstrate that the previously published EAM potentials predict a liquid structure that is too strongly ordered relative to measured diffraction data. We develop new EAM potentials for Al and Cu to improve the agreement with the first-principles and measured liquid diffraction data. Furthermore, we calculate liquid-phase diffus...
TL;DR: The observation of controllable phase separation in a dual-species Bose-Einstein condensate with 85Rb and 87Rb is reported on, which can clearly observe immiscible behavior via a dramatic spatial separation of the two species.
Abstract: We report on the observation of controllable phase separation in a dual-species Bose-Einstein condensate with 85Rb and 87Rb. Interatomic interactions between the different components determine the miscibility of the two quantum fluids. In our experiments, we can clearly observe immiscible behavior via a dramatic spatial separation of the two species. Furthermore, a magnetic-field Feshbach resonance is used to change them between miscible and immiscible by tuning the 85Rb scattering length. The spatial density pattern of the immiscible quantum fluids exhibits complex alternating-domain structures that are uncharacteristic of its stationary ground state.
TL;DR: In this paper, the Ising spin chain magnet with an alternating charge order was found to have collinear-magnetism-driven ferroelectricity, which was attributed to the inversion symmetry breaking in the spin chain.
Abstract: We report discovery of collinear-magnetism-driven ferroelectricity in the Ising chain magnet ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{2\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{O}}_{6}$ ($x\ensuremath{\approx}0.96$). Neutron diffraction shows that ${\mathrm{Co}}^{2+}$ and ${\mathrm{Mn}}^{4+}$ ions alternating along the chains exhibit an up-up-down-down ($\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}$) magnetic order. The ferroelectricity results from the inversion symmetry breaking in the $\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}$ spin chain with an alternating charge order. Unlike in spiral magnetoelectrics where antisymmetric exchange coupling is active, the symmetry breaking in ${\mathrm{Ca}}_{3}(\mathrm{Co},\mathrm{Mn}{)}_{2}{\mathrm{O}}_{6}$ occurs through exchange striction associated with symmetric superexchange.
TL;DR: In this paper, theoretical predictions and experimental measurements for the achievable phase noise, timing jitter, and frequency stability in the coherent transport of an optical frequency over a fiber-optic link are discussed.
Abstract: We present theoretical predictions and experimental measurements for the achievable phase noise, timing jitter, and frequency stability in the coherent transport of an optical frequency over a fiber-optic link. Both technical and fundamental limitations to the coherent transfer are discussed. Measurements of the coherent transfer of an optical carrier over links ranging from 38 to 251 km demonstrate good agreement with theory. With appropriate experimental design and bidirectional transfer on a single optical fiber, the frequency instability at short times can reach the fundamental limit imposed by delay-unsuppressed phase noise from the fiber link, yielding a frequency instability that scales as link length to the 3/2 power. For two-way transfer on separate outgoing and return fibers, the instability is severely limited by differential fiber noise.
TL;DR: Yildirim et al. as discussed by the authors investigated the hydrogen uptake by light transition-metal atoms decorating various carbon-based nanostructures in different types of geometry and dimensionality, such as carbon linear chain, graphene and nanotubes.
Abstract: In a recent letter [T. Yildirim and S. Ciraci, Phys. Rev. Lett. 94, 175501 (2005)], the unusual hydrogen storage capacity of Ti decorated carbon nanotubes has been revealed. The present paper extends this study further to investigate the hydrogen uptake by light transition-metal atoms decorating various carbon-based nanostructures in different types of geometry and dimensionality, such as carbon linear chain, graphene, and nanotubes. Using first-principles plane-wave method we show that not only outer but also inner surface of a large carbon nanotube can be utilized to bind more transition-metal atoms and hence to increase the storage capacity. We also found that scandium and vanadium atoms adsorbed on a carbon nanotube can bind up to five hydrogen molecules. Similarly, light transition-metal atoms can be adsorbed on both sides of graphene and each adsorbate can hold up to four hydrogen molecules yielding again a high-storage capacity. Interestingly, our results suggest that graphene can be considered as a potential high-capacity ${\mathrm{H}}_{2}$ storage medium. We also performed transition state analysis on the possible dimerization of Ti atoms adsorbed on the graphene and single-wall carbon nanotube.
TL;DR: The results show that high harmonic generation and strong-field ionization in polyatomic molecules undergoing bonding or configurational changes involve the participation of multiple molecular orbitals.
Abstract: The attosecond time-scale electron-recollision process that underlies high harmonic generation has uncovered extremely rapid electronic dynamics in atoms and diatomics. We showed that high harmonic generation can reveal coupled electronic and nuclear dynamics in polyatomic molecules. By exciting large amplitude vibrations in dinitrogen tetraoxide, we showed that tunnel ionization accesses the ground state of the ion at the outer turning point of the vibration but populates the first excited state at the inner turning point. This state-switching mechanism is manifested as bursts of high harmonic light that is emitted mostly at the outer turning point. Theoretical calculations attribute the large modulation to suppressed emission from the first excited state of the ion. More broadly, these results show that high harmonic generation and strong-field ionization in polyatomic molecules undergoing bonding or configurational changes involve the participation of multiple molecular orbitals.