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2Physics

LIGO Science Collaboration announces detection of Gravitational Waves (100 years after Einstein predicted it through his General Theory of Relativity) -- that originated from two coalescing black holes about a billion years back. Here is 2Physics coverage on Gravitational Waves over the last 10 years:
                                                "Gravitational Waves" Pages:   3,   2,   1

Sunday, February 07, 2016

Material Properties of Fire Ant Aggregations

David Hu

Authors: Michael Tennenbaum1, Zhongyang Liu2, David Hu2, Alberto Fernandez-Nieves1

Affiliation:
1School of Physics, Georgia Institute of Technology, Atlanta, Georgia, USA, 
2School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA.

Link to Hu Laboratory for Biolocomotion >>

Michael Tennenbaum

Fire ants are an impressive species. A remarkable property is that they are able to build rafts to survive flooding. These rafts are only made of ants; they contain no twigs or leaves. The raft is solely linked together with ants. This ability to link together means that we can think of ant aggregations as condensed states of matter, like the common fluid or solid states. In this work [1] we begin to characterize the properties of the ant aggregation as a material. However, in contrast to the particles in conventional materials, ants are alive; they are out of equilibrium and also are self-propelled. As a result, ant aggregations are particular examples of active materials.

Alberto Fernandez-Nieves

In fact, the combination of being alive and being able to link together makes fire ants an excellent system to study active materials. Our work began with two simple experiments. The first is a penny that is dropped through a 2D column of fire ants, as shown in figure 1a-c. This behavior is reminiscent of what happens to a ball bearing falling through a viscous liquid. The second experiment in figure 1 shows what happens when an aggregation of fire ants is compressed by a Petri dish. Once compressed the ants spring back to their original shape. This is the classic behavior of elastic materials. We thus conclude that fire ant aggregations are both viscous and elastic. To more carefully study this viscoelastic behavior we employ rheology. We use a modified Anton Paar MCR 501 stress controlled rheometer to investigate the ants.
Figure 1: (a—c) A penny being dropped through a 2D column of ants. (d-f) Compression of an ant aggregation using Petri dishes. Videos are online at antphysics.gatech.edu .

In our recent paper we present the ant aggregation’s response to oscillations. This is called small amplitude oscillatory shear experiments. The premise is to apply a small sinusoidal strain and measure the stress necessary to maintain this strain. In elastic materials this stress is perfectly in phase with the strain. In contrast, for perfectly viscous materials it is completely out of phase. For viscoelastic materials, the response is in between. We can characterize how much energy is stored per cycle (Storage modulus) by the in phase component and how much energy is dissipated per cycle (Loss modulus) by the out of phase component.

Amazingly for ant aggregations these components are equal for the frequency range probed. This means that in each cycle the ants are dissipating energy and storing energy equally. The response is also power law with an exponent of ~0.5. This power law behavior implies that there is a lack of a unique timescale to relax. Polymer gels when they first percolate through the system show this same behavior [2]. In polymer gels this is attributed to the fractal nature of the gel. However, fire ants are not fractal as far as we have seen. The power law exponent of 0.5 combined with the equality of the Storage and Loss moduli is consistent with the Kramers-Kronig relations [3,4], which apply to systems near equilibrium, in the so-called linear regime. However, fire ant aggregations are far from equilibrium and thus it is not obvious as to why linear response works. This is an interesting open question.
Figure 2: Storage modulus G' and Loss modulus G" as a function of frequency (a) Live ant frequency sweeps, (b) Dead ant frequency sweeps.

We are nevertheless sure that our observations are due to the active nature of the ants. When we performed similar measurements using dead ants the result was completely different. Dead ants are elastic. Their storage modulus is always higher than their loss modulus and are frequency independent.

Interestingly, when we increase the density of the live ant aggregation, the behavior approaches that of dead ants and eventually the system becomes predominantly elastic. Looking closer at this we see that there are two regimes. The first is at lower densities and here the storage modulus increases linearly with the fraction of space taken up by ants or volume fraction. In this regime the ants are crowding and the addition of ants decreases the available possibilities for them to rearrange. After a certain point in volume fraction though, the addition of ants causes them to pull their legs in a little. This progressive compression of the ants characterizes the second regime.

A very different response is obtained when the ant aggregation is forced to flow. In this case there is no difference between live and dead ants, and the aggregation behaves as a shear thinning liquid like ketchup, which manifests a smaller viscosity the harder it is pushed. Remarkably though, the amount of shear thinning in the ants is more pronounced relative to that observed in materials like ketchup. This result can ultimately be related to how the energy input is dissipated in the aggregation. For dead ants, this mainly happens at the ants joints, when these give way due to the imposed flow state of the aggregation [5]. The similar behavior exhibited by live ants indicates that this mechanism is also at play for these aggregations.

In summary, we have found (i) that live ants at relatively “low” volume fractions are equally viscous and elastic, (ii) that they become predominantly elastic and approach dead-ant behavior as the volume fraction increases, and (iii) that when forced to flow, they do so with a viscosity that dramatically decreases as they are forced to flow faster. Our work opens the way to many more interesting studies where ants are used as model active particles. We thus hope to use them to address many more relevant problems.

References:
[1] Michael Tennenbaum, Zhongyang Liu, David Hu, Alberto Fernandez-Nieves, "Mechanics of fire ant aggregations", Nature Materials, 15, 54-59, doi:10.1038/nmat4450 (2016). Abstract.
[2] Horst Henning Winter, Marian Mours, "Rheology of Polymers Near Liquid-Solid Transitions" in 'Neutron spin echo spectroscopy viscoelasticity rheology' (Volume 134 of the series 'Advances in Polymer Science', pp 165-234, Springer, 1997). Abstract.
[3] R. Byron Bird, Robert C. Armstrong, Ole Hassager, "Dynamics of polymeric liquids. Vol. 1: Fluid mechanics" (Wiley, 1987).
[4] Michael Stone, Paul Goldbart, "Mathematics for physics: a guided tour for graduate students" (Cambridge University Press, 2009).
[5] Sasha N. Zill, Sumaiya Chaudhry, Ansgar Büschges, Josef Schmitz, "Directional specificity and encoding of muscle forces and loads by stick insect tibial campaniform sensilla, including receptors with round cuticular caps", Arthropod structure & development, 42, 455-467 (2013). Abstract.

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Sunday, January 31, 2016

Graphene Tuned Terahertz Lasers

Thomas Folland (left) and Subhasish Chakraborty

Authors: Subhasish Chakraborty1, Owen Marshall1,2, Thomas Folland1, Yong-Jin Kim2, Alexander Grigorenko2, Konstantin Novoselov2

Affiliation:
1School of Electrical and Electronic Engineering, University of Manchester, UK.
2School of Physics and Astronomy, University of Manchester, UK.

Terahertz light falls between microwave and the infrared in the electromagnetic spectrum, and has the useful property that it can pass through many non-metallic materials, including packaging and clothing. Furthermore, a wealth of material information can be obtained using terahertz waves. As such practical terahertz sources and detectors could be used for security scanning, quality control and wireless communication [1]. High power terahertz waves can be generated by using quantum cascade lasers. These semiconductor chips contain multiple stacked quantum wells, carefully designed so that the conduction band is split into multiple energy levels in which terahertz light is generated [2].

Controlling quantum cascade lasers so that they produce tuneable, single frequency light is particularly difficult. In our earlier work we achieved all-electrical tuning in a terahertz laser over a discrete set of frequencies [3]. We did this by integrating a specially designed photonic lattice into the gold waveguide of the laser, which we dubbed an ‘aperiodic lattice laser’. Such lasers are compact (~1 cm2), powerful (~10 mW of power), and capable of working at a range of terahertz frequencies. However, to be a practical THz source, these lasers need to show continuous tuning over a wide range of THz frequencies.
Figure 1. Graphene integrated laser, showing (from bottom to top) the QCL (Quantum Cascade Laser) chip, the gold grating, the graphene layer, the polymer electrolyte and the top gate.

Enters graphene, a single atomic layer of carbon atoms. Graphene interacts strongly with terahertz waves [4] forming surface plasmons, coupled oscillations between light and the electrons in a material. In graphene the properties of these surface plasmons can be controlled by applying a gate potential to the surface of the device (similar to field effect transistors). This allows graphene to control the properties of a range of optical devices, such as modulators [5]. In our article [6] we integrate graphene into a terahertz quantum cascade laser, allowing us to use tuneable surface plasmons to control the frequency of laser light. We did this by depositing graphene and a polymer electrolyte gate on top of an aperiodic lattice laser (see figure 1). The aperiodic lattice consists of a series sub-wavelength slits, inside which graphene surface plasmons could be excited. When no gate potential was applied to the graphene, a surface plasmon formed inside the slit to suppress light scattering from the lattice. Conversely when a gate potential was applied to the graphene, the surface plasmon wavelength became larger than the slit width, and scattering was enhanced from the lattice. The effect of this change in scattering strength is to change the laser emission frequency, as defined by the properties of the lattice (see figure 2).
Figure 2. Tuning laser emission using a graphene. The figure shows the simulated electric field in each lattice site, and the laser emission spectrum before and after the introduction of a gate potential.

This result shows that graphene plasmons can be excited and used to tune terahertz lasers. Although currently a proof of concept, it is in principle possible to individually gate each slit, allowing us to engineer the laser output. Such control would make terahertz lasers significantly more appealing for real world applications. Furthermore, although the design approach used in our work is most suitable for terahertz lasers, they could in principle be applied to more conventional lasers in the infrared. This would offer an entirely different way of controlling the properties of laser systems.

References:
[1] A. Giles Davies, Andrew D. Burnett, Wenhui Fan, Edmund H. Linfield, John E. Cunningham, "Terahertz spectroscopy of explosives and drugs", Materials Today, 11, 18–26 (2008). Full PDF.
[2] Rüdeger Köhler, Alessandro Tredicucci, Fabio Beltram, Harvey E. Beere, Edmund H. Linfield, A. Giles Davies, David A. Ritchie, Rita C. Iotti, Fausto Rossi, "Terahertz semiconductor-heterostructure laser", Nature, 417, 156–9 (2002). Abstract.
[3] Subhasish Chakraborty, Owen Marshall, Chen Wei Hsin, Md. Khairuzzaman, Harvey Beere, David Ritchie, "Discrete mode tuning in terahertz quantum cascade lasers", Optics Express, 20, B306–14 (2012). Abstract.
[4] Alessandro Tredicucci, Miriam Serena Vitiello, "Device Concepts for Graphene-Based Terahertz Photonics", IEEE Journal for Selected Topics in Quantum Electronics, 20, 130–138 (2014). Full Article.
[5] A. N. Grigorenko, M. Polini, K. S. Novoselov, "Graphene plasmonics", Nature Photonics, 6, 749–758 (2012). Abstract.
[6] S. Chakraborty, O. P. Marshall, T. G. Folland, Y.-J. Kim, A. N. Grigorenko, K. S. Novoselov, "Gain modulation by graphene plasmons in aperiodic lattice lasers", Science, 351, 246–248 (2016). Abstract.

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Sunday, January 03, 2016

All-Optical Reconstruction of Crystal Band Structure

Giulio Vampa
Paul Corkum

Authors: Giulio Vampa1, Thomas Hammond1, Nicolas Thiré2, Bruno Schmidt2, François Légaré2, Chris McDonald1, Thomas Brabec1, Dennis Klug3,
Paul Corkum1,3


Affiliation:
1Department of Physics, University of Ottawa, Ontario, Canada,
2INRS-EMT, Varennes, Québec, Canada,
3National Research Council of Canada, Ottawa, Ontario, Canada.

The electronic band structure of solids determines their key properties, for example electrical conductivity and optical absorption. The energy bands are conventionally measured with Angle Resolved Photoemission Spectroscopy (ARPES), in which the kinetic energy and the momentum of electrons ionized by highly energetic photons are simultaneously measured [1]. The working principle is sketched in the left of Fig. 1. ARPES has been an incredibly successful method to probe conventional and less conventional materials, such as superconductors [2], graphene [3] and topological insulators [4].

However, electrons are not always accessible. Only high vacuum must oppose their propagation from the surface of the sample to the detector. This prevents measuring the band structure of the bulk, or of matter under extreme pressures, where the sample is enclosed in a diamond anvil cell, or in high magnetic fields, where electrons are deviated. Further, many surface chemical reactions that happen at ambient conditions, such as catalysis, are difficult to study with ARPES.
Figure 1: Comparison between photoemission and high harmonic generation. In photoemission experiments, an incoming photon (purple arrow) causes emission of an electron (black vertical arrow) with kinetic energy (K1 and K2) proportional to the initial binding energy (E1 and E2). In high harmonic generation, an electron first tunnels (marked by circled 1) to the conduction band. The electron-hole pair is then accelerated (circled 2) and recombines (circled 3) emitting a harmonic photon in the process. Electrons and holes are denoted by filled and empty blue circles, respectively. Two trajectories are identified by the wiggled solid black arrows, corresponding to different times of creation and recollision of the electron-hole pair.

In our recent letter [5], we report on an optical method to determine the band structure of solids. Because the technique does not detect electrons, it does not need high vacuum. The technique requires focusing a strong femtosecond laser pulse (1 fs = 10-15 s), with a peak field strength of ~0.1-1 V/Ang – comparable to the force binding electrons to their atoms – with a central wavelength in the mid-infrared spectral region (therefore < 1 eV photon energy) inside the bulk of the solid sample to generate several new frequencies at odd multiples of the mid-infrared one. The process, known as High Harmonic Generation [6], proceeds through three steps that are depicted in the right of Fig. 1. First, when the field is close to a maximum, electrons tunnel from the top of the valence to the bottom of the conduction band, creating an electron-hole pair with zero initial crystal momentum (circled “1” in Fig. 1). Then, the laser field accelerates the electron-hole pair to high crystal momentum (it can reach the edge of the Brillouin Zone!) - marked “2” in Fig. 1 – in a way that depends on the shape of their respective bands. Then, at a specific time in the laser cycle, the electron can recombine with the hole, emitting a high harmonic photon in the process with energy equal to the band gap at the momentum of recombination (marked “3” in Fig. 1).

The process is highly coherent: ionization, propagation and recombination are perfectly synchronized with the laser filed. This allows to link each harmonic photon energy to a specific trajectory of the electron-hole pair, which is ultimately determined by when it was created about the peak of the laser field. Therefore, knowledge of the trajectory is sufficient to determine the momentum at recollision, the only ingredient required to determined the band structure – together with the harmonic photon energy. The technique relies on a second laser pulse, superimposed to the first, at twice the mid-infrared frequency, to perturb – and therefore measure – the trajectory.

In a simulated experiment, we are able to fully reconstruct the momentum-dependent band gap of a target model solid, chosen to approximate the band structure of a ZnO crystal. The technique requires measuring many harmonic orders, a task at the moment beyond the capabilities of our experimental setup. In a real experiment we conducted, we are able to measure only 20% of the Brillouin Zone. Even with this limited experimental data, we determine that the split-off valence band on ZnO is the one mostly contributing to tunnelling of electrons to the conduction band.

In conclusion, rather than using weak incoherent fields of high energy photons – as in photoemission experiments – we use intense coherent light of low energy photons to create, accelerate and probe electron-hole pairs in solids. The information about the energy of the bands in which the electron and the hole move is imprinted on a high harmonic photon upon their recombination. Like other spectroscopy techniques based on high harmonic generation in gases [7], the method has intrinsic sub-laser cycle temporal resolution. This could be used to track band structure modifications following laser excitation [8].

References:
[1] Andrea Damascelli, Zahid Hussain, Zhi-Xun Shen, "Angle-resolved photoemission studies of the cuprate superconductors", Review of Modern Physics, 75, 473 (2003). Abstract.
[2] A. Lanzara, P.V. Bogdanov, X.J. Zhou, S.A. Kellar, D.L. Feng, E.D. Lu, T. Yoshida, H. Eisaki, A. Fujimori, K. Kishio, J.-I. Shimoyama, T. Noda, S. Uchida, Z. Hussain, Z.-X. Shen, "Evidence for ubiquitous strong electron–phonon coupling in high-temperature superconductors", Nature, 412, 510 (2001). Abstract.
[3] Søren Ulstrup, Jens Christian Johannsen, Federico Cilento, Jill A. Miwa, Alberto Crepaldi, Michele Zacchigna, Cephise Cacho, Richard Chapman, Emma Springate, Samir Mammadov, Felix Fromm, Christian Raidel, Thomas Seyller, Fulvio Parmigiani, Marco Grioni, Phil D. C. King, Philip Hofmann, "Ultrafast Dynamics of Massive Dirac Fermions in Bilayer Graphene", Physical Review Letters, 112, 257401 (2014). Abstract.
[4] Y. L. Chen, J. G. Analytis, J.-H. Chu, Z.K. Liu, S.-K. Mo, X.L. Qi, H.J. Zhang, D.H. Lu, X. Dai, Z. Fang, S.C. Zhang, I.R. Fisher, Z. Hussain, Z.-X. Shen, "Experimental Realization of a Three-Dimensional Topological Insulator, Bi2Te3", Science 325, 178 (2009). Abstract.
[5] G. Vampa, T.J. Hammond, N. Thiré, B.E. Schmidt, F. Légaré, C.R. McDonald, T. Brabec, D.D. Klug, P.B. Corkum, "All-Optical Reconstruction of Crystal Band Structure", Physical Review Letters, 115, 193603 (2015). Abstract.
[6] Shambhu Ghimire, Anthony D. DiChiara, Emily Sistrunk, Pierre Agostini, Louis F. DiMauro, David A. Reis, "Observation of high-order harmonic generation in a bulk crystal", Nature Physics, 7, 138 (2011). Abstract.
[7] S. Baker, J.S. Robinson, C.A. Haworth, H. Teng, R.A. Smith, C.C. Chirilă, M. Lein, J.W.G. Tisch, J.P. Marangos, "Probing Proton Dynamics in Molecules on an Attosecond Time Scale", Science, 312, 424 (2006). Abstract.
[8] E. N. Glezer, Y. Siegal, L. Huang, E. Mazur, "Laser-induced band-gap collapse in GaAs", Physical Review B, 51, 6959 (1995). Abstract.

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Sunday, December 20, 2015

Topological Defect Lasers

From left to right: Sebastian Knitter, Seng Fatt Liew, Wen Xiong, Hui Cao

Authors: Sebastian Knitter, Seng Fatt Liew, Wen Xiong, Hui Cao

Affiliation: Department of Applied Physics, Yale University, New Haven, Connecticut, USA.

Topological defects have been widely studied in liquid crystals and colloids. One important application of topological defects to photonics is the manipulation of the orbital angular momentum of optical beams. However, they have not been incorporated into lasers. Inspired by the topological defects existing in nematic liquid crystal, we recently designed and fabricated a new type of laser – a topological defect laser [1].

Past 2Physics articles by this group:
April 03, 2011: "Time-reversed Lasing and Coherent Control of Absorption"
 by A. Douglas Stone, Yidong Chong and Hui Cao.

One essential component of a laser is a cavity that confines light. A microscale cavity can be made within a photonic crystal, e.g., a two-dimensional membrane containing a periodic array of circular air holes, as shown in Fig. 1(a). Light, of wavelength comparable to the lattice constant, may be confined in the central square region as a result of high reflectivity from the surrounding photonic crystal walls. Figure 1(b) shows a spatial field profile of a confined mode, and Fig. 1(c) is the spatial map of its energy flow [2].
Figure 1: Introducing topological defect to a photonic crystal with anisotropic unit cell. (a,c) A square lattice of air holes embedded in a dielectric membrane. In (a), the holes have circular cross-section. The 4×4 holes are removed from the center to from a cavity that confines light by high reflection from the surrounding photonic crystal walls. The lattice constant is a = 220nm, and the hole radius is 65nm. To introduce topological defect in (c), the air holes are deformed to ellipses. The major axis of each ellipse is rotated to an angle φ = + c from the horizontal axis. θ is the polar angle of the center position of the ellipse. k = 1 and c = π/4. (b,d) Calculated spatial distribution of the magnetic field for a confined optical mode in (a,b). The wavelength of the mode is 880nm. The topological defect rotates the mode in (d). (c,e) The spatial map of the Poynting vector for the modes in (b,d). Each arrow points in the direction of local energy flux, and its length is proportional to the amplitude of the flux. In the photonic crystal structure, the energy flows out of the central defect region in all four directions. In the topological defect structure, the optical flux circulates in the central region.

To introduce the topological defect to this micro-cavity, we deform the air hole shape from circle to ellipse, and rotate the orientation of individual ellipses to form a vortex-like topological defect, as seen in Fig. 1(d). Light can still be confined in the central region, but the spatial field profile of confined mode is twisted as shown in Fig. 1(e). Figure 1(f) reveals an even more dramatic change in the energy flow: a tightly confined circulating flux pattern arises at the center, in contrast to the outward energy flow in Fig. 1(c). Such change is attributed to the spatial variation of the ellipse orientation in the topological defect structure, which creates the swirling vortex of light [2].

Experimentally we fabricated such structure in a free-standing gallium-arsenide membrane by molecular beam epitaxy, electron-beam lithography, reactive ion etching and wet chemical etching. Figure 2 is the scanning electron micrograph of a fabricated sample. The 190nm-thick membrane contains three layer of quantum dots made of indium arsenide. When pumped by an external laser, the quantum dots are excited to upper states. They subsequently decay to lower states by emitting photons to the confined modes such as the one shown in Fig. 1. When the amplification rate exceeds the leakage rate through the walls, lasing action starts. The laser light swirls around in a vortex.

Fig. 2: Scanning-electron micrograph (SEM) of a topological defect laser fabricated in a GaAs membrane. (a) Top-view SEM image of a square lattice of 32 × 32 air holes with elliptical shape. The ellipticity is ε = 1.4. The lattice constant is a = 220 nm. The air filling fraction is 0.27. At the array center, 4 × 4 air holes are removed. (b) Magnified SEM of a section in (a), highlighted by the gray rectangle. (c) Tilt-view SEM image showing the free-standing GaAs-membrane. Scale-bars in (b) and (c) represent a length of 500 nm.

The power-flow vortex persists beyond the membrane. Our calculation shows that the evanescent field above the top surface of the membrane possess circulating energy flows. These may transfer angular momentum to particles or molecules in the vicinity of the topological defect structure. Thus the optical vortex of the lasing mode may potentially be used for on-chip nanoparticle manipulation. Further experimental study is required to optimize the topological defect laser and exploit the unique potential for applications in areas such as microfluidics for particle sorting and separation. Finally, this work shows that the spatially inhomogeneous variation of the unit cell orientation adds another degree of freedom to the control of a lasing mode, enabling the manipulation of its field pattern and energy flow landscape.

References:
[1] Sebastian Knitter, Seng Fatt Liew, Wen Xiong, Mikhael I. Guy, Glenn S Solomon, Hui Cao, “Topological defect lasers”, Journal of Optics, 18, 014005 (2015). Full Article.
[2] Seng Fatt Liew, Sebastian Knitter, Wen Xiong, Hui Cao, “Photonic crystals with topological defects”, Physical Review A, 91, 023811 (2015). Abstract.

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Sunday, December 06, 2015

Heavy Dark Matter Ignition of Supernovae

Joseph Bramante

Author: Joseph Bramante

Affiliation: Department of Physics, University of Notre Dame, Indiana, USA.

Dark matter makes up most of the mass we observe in our universe. Dark matter's gravitational pull has been identified in the structure of galaxies, the expansion of the universe, and the bending of light through galactic clusters. However, the mass of each dark matter particle, and dark matter's non-gravitational interactions, are still a mystery.

Type Ia supernovae result when a white dwarf's atomic nuclei rapidly fuse, prompting an explosion that spews forth heavier, decaying atomic nuclei. The light released by these decaying nuclei over a hundred days is collected by astronomers, who have noticed a pattern: the maximum brightness of Type Ia supernova can be used to predict how quickly the supernova's light will fade. This is the sense in which Type Ia supernovae are "standard candles" that have been used to measure the accelerating expansion of our universe.

For decades scientists thought that type Ia supernovae were standard candles, because they originated from "Chandraskhar mass" white dwarfs. The Chandrasekhar mass is the maximum mass a white dwarf can have, before it will collapse under its own weight. Contradicting expectations, recent precision studies of type Ia supernovae have shown that many white dwarfs which produce type Ia supernovae are significantly lighter than Chandrasekhar mass [1]. This is a puzzle, that could be solved by heavy dark matter.

A recent paper [2] shows that if dark matter particles are one million times heavier than protons, then enough dark matter could collect into white dwarfs to initiate a period of dark matter collapse. The collapsing dark matter particles would ricochet against white dwarf nuclei fast enough to fuse them, thereby sparking type Ia supernovae.
Figure 1: A schematic indicating that heavier, denser white dwarves would be ignited by dark matter more quickly than lighter, less dense white dwarves.

One prediction of this scenario is that more massive white dwarfs would explode sooner than less massive white dwarfs. This is because heavier white dwarfs are denser, and so the dark matter would be gravitationally bound into a smaller region inside them. Dark matter collected into a smaller region would collapse sooner (see Figure 1). Spurred by this possibility, the author looked for, and found some evidence for heavier white dwarfs exploding sooner in existing type Ia supernovae data (see Figure 2).

There are some plausible, competing explanations for sub-Chandrasekhar mass type Ia supernovae. For example, merging white dwarf stars may precipitate nuclear fusion [3]. The upcoming dark matter search experiments XENON [4] and Lux-Zeppelin [5] could detect or rule out some models of supernova-igniting dark matter. Finally, there is another dramatic consequence of supernova-igniting dark matter: it would collapse neutron stars into black holes at the center of our galaxy, where dark matter is more abundant [6].
Figure 2: (click on the image to view with higher resolution) There is an apparent correlation between the age of stars surrounding supernovae and the supernovae's initial masses. The thick red crosses indicate collected type Ia supernovae data, taken from reference [7]. The solid and dashed lines indicate the predictions of some heavy dark matter models.

References:
[1] R. Scalzo et al. (The Nearby Supernova Factory), "Type Ia supernova bolometric light curves and ejected mass estimates from the Nearby Supernova Factory", Monthly Notices of the Royal Astronomical Society, 440, 1498 (2014). Abstract.
[2] Joseph Bramante, "Dark matter ignition of type Ia supernovae", Physical Review Letters, 115, 141301 (2015). Abstract.
[3] Dan Maoz, Filippo Mannucci, Gijs Nelemans, "Observational clues to the progenitors of Type-Ia supernovae", Annual Reviews of Astronomy and Astrophysics, 52, 107 (2014). Abstract.
[4] E. Aprile et al. (XENON100 collaboration), "Limits on spin-dependent WIMP-nucleon cross sections from 225 live days of XENON100 data",  Physical Review Letters, 111, 021301 (2013). Abstract.
[5] D.S. Akerib et al. (LZ collaboration), "LUX-ZEPLIN (LZ) Conceptual Design Report", arXiv:1509.02910v2 [physics.ins-det].
[6] Joseph Bramante, Tim Linden, "Detecting dark matter with imploding pulsars in the galactic center",  Physical Review Letters, 113, 191301 (2014). Abstract.
[7] Y.-C. Pan, M. Sullivan, K. Maguire, I.M. Hook, P.E. Nugent, D.A. Howell, I. Arcavi, J. Botyanszki, S.B. Cenko, J. DeRose, H.K. Fakhouri, A. Gal-Yam, E. Hsiao, S.R. Kulkarni, R.R. Laher, C. Lidman, J. Nordin, E.S. Walker, D. Xu, "The Host Galaxies of Type Ia Supernovae Discovered by the Palomar Transient Factory", Monthly Notices of the Royal Astronomical Society, 438, 1391 (2014). Abstract.

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Sunday, November 22, 2015

Using Phase Changes to Store Information in the Magnetic Permeability

Authors: Alan S. Edelstein*, John Timmerwilke, Jonathan R. Petrie 

Affiliation: US Army Research Laboratory, Adelphi, Maryland, USA

*Email: aedelstein@cox.net

Finding better methods for storing information was a serious problem for the earliest computers; considerable effort is still being devoted to decreasing the cost and increasing the density and lifetime of stored information. Initial methods of storage included using acoustic delay lines and pixels on the display of cathode ray tubes [1]. Though the current methods for storing information, which include magnetic hard disks, magnetic tape, and various forms of random access memory (RAM) are far superior to these early methods, they still have significant limitations.

As an example, the information on hard disks or magnetic tape is written by a magnetic field and stored as regions, i.e. bits, having different directions for their remanent magnetization. Thus, this information can be erased by exposure to a magnetic field. Furthermore, to avoid thermal upsets of the spin direction of the bits there is a trade-off between how long the information can be stored without acquiring too many incorrect bits and the information density on hard disks. Maintaining a magnetic hard disk lifetime of about seven years has made it difficult to increase the information density in hard disk drives. Magnetic tape degrades in about 20 years.

To avoid these problems, we have been developing a new approach for storing information that we call Magnetic Phase Change Memory (MAG PCM). Instead of using the direction of magnetic remanance, information is stored in bits of soft ferromagnetic material having different values for their magnetic permeability. The initial magnetic permeability of a soft ferromagnetic material is reversible and dependent only on its atomic structure. Thus, it is independent of the magnetic field. The permeability of iron rich FeGa single crystal alloys [2]  is an interesting example of soft magnetic behavior.

The property we use is that high permeability bits attract magnetic fields and low permeability bits do not. To read the information stored in bits with high or low permeability we measure their effect on a probe field. We have primarily focused on writing and changing the permeability of bits of amorphous ferromagnet, 2826 MB Metglas, films by thermally heating with a laser. The nearest neighbor atoms in amorphous ferromagnets materials do not have long range order. Thus, they do not have much crystalline anisotropy or coercivity and are soft ferromagnets with large values for their permeability. When amorphous materials are heated above their glass temperature, they crystalize and have larger values for their coercivity and smaller values for their permeability. The glass temperature of 2826 MB Metglas is 410oC. At temperatures as high as 200oC, 2826 MB Metglas will retain its high permeability for hundreds of years.

Figure 1: a) Microscope view of three 50 micron wide crystallized lines written into amorphous Metglas. b) Voltage of the MTJ reader as it moved over the three crystalline lines written in the presence of a 32 Oe probe field.

We have read the effect on a probe magnetic field near each bit using both magnetic tunnel junction (MTJ) sensors [3,4], and spin transfer oscillators [5]. Figure 1a shows a microscope image of three crystalline lines in Metglas written by a focused 1.966 micron (Tm fiber) laser. The output voltage of an MTJ sensor is plotted in Figure 1b as it is swept over the crystallized Metglas lines shown in Figure 1a. One sees that the crystallized lines affect a 32 Oe probe field. The crystallized lines do not attract the probe field as much as the amorphous Metglas film does which causes the magnetic sensor to measure a larger field. Smaller nanometer-sized bits were created using e-beam lithography. Figure 2a is a microscope image of two square 0.9 mm arrays of 300 nm bits of amorphous Metglas. Figure 2b shows a scanning electron microscope (SEM) image of the 300 nm diameter bits. Laser heating was used to crystallize all of the bits in the left array. Figure 3 shows the voltage output of the MTJ sensor when it is moved over the two arrays before and after the laser heating. One sees that the bits in the left array no longer attract the magnetic field lines of the probe field.
Figure 2. (a) Microscope image of two square 0.9 mm arrays of 300 nm diameter bits of Metglas; (b) scanning electron microscope image of the 300 nm diameter bits of Metglas.

This new approach for storing information has several advantages. One can write bits with at least three different values [6]  for their permeability. The bits will not be corrupted by a magnetic field or thermal upsets and therefore should last decades. It should be possible to write nm sized bits economically by an adaption of heat assisted magnetic recording (HAMR) [7], a technology that is being developed by hard disk companies such as Seagate and Western Digital to cope with the problem mentioned above of maintaining stability against thermal upsets. In HAMR a laser and a near field transducer is used to heat nm sized regions to 710oC to decrease the coercivity so that they can be written without using as large a magnetic field. What we need to do is simpler, in that we do not need a magnetic field and for archiving we do not need to rewrite.

Figure 3: Magnetic tunnel junctions scans before (red, o) and after (blue, x) the left array of 300nm Metglas bits in Fig. 2 were crystallized by heating with a laser.

MAG PCM has the potential for a combination properties not found in other storage technologies. It should have decades of longevity and the rapid access and high density of future hard disks. We have a clear path for developing MAG PCM into commercial products for long term storage applications such as archiving. Though it is unnecessary for archiving, we have found that we can rewrite our bit [8], i.e., return a crystallized bit to an amorphous state.

This work was done while we were at the US Army Research Laboratory.

References:
[1] D.R. Hartree, M.H.A. Newman, M.V. Wilkens, F.C. Williams, J.H. Wilkinson, A.D. Booth, ” A discussion on computing machines”, Proceedings of the Royal Society of London Series A- Mathematical and Physical Sciences, 195:1042, 265 (1948). Abstract.
[2] Harsh Deep Chopra, Manfred Wittig, “Non-Julian magnetostriction”, Nature, 521, 340-343 (2015). Abstract.
[3] J.R. Petrie, K.A. Wieland, R.A. Burke, G.A. Newburgh, J.E. Burnette, G.A. Fischer, A.S. Edelstein, “ A non-erasable magnetic memory based on the magnetic permeability”, Journal of Magnetism and Magnetic Materials, 361, 262 (2014). Abstract.
[4] John Timmerwilke, J.R. Petrie, K.A. Wieland, Raymond Mencia, Sy-Hwang Liou, C.D. Cress, G.A. Newburgh, A.S. Edelstein, “Using magnetic permeability bits to store information”, Journal of Physics D: Applied Physics, 48, 405002 (2015). Abstract.
[5] J.R. Petrie, S. Urazhdin, K.A. Wieland, A.S. Edelstein, “Using a spin torque nano-oscillator to read memory based on the magnetic permeability”, Journal of Physics D: Applied Physics, 47, 055002 (2014). Abstract.
[6] J.R. Petrie, K.A. Wieland, J.M. Timmerwilke, S.C. Barron, R.A. Burke, G.A. Newburgh, J.E. Burnette, G.A. Fischer, and A.S. Edelstein, “A multi-state magnetic memory dependent on the permeability of Metglas”, Applied Physics Letters, 106, 142403 (2015). Abstract.
[7] M.H. Kryder and Soo Kim Chang, “After hard drives—what comes next?” IEEE Transactions on Magnetics, 45, 3406 (2009). Abstract.
[8] Unpublished data.

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Sunday, November 15, 2015

A New Way To Weigh A Star

Nils Andersson (left) and Wynn Ho 

Authors: Nils Andersson, Wynn Ho

Affiliation: Mathematical Sciences and STAG Research Centre, University of Southampton, UK. 

You probably have a pretty good idea of your own weight. And if you need an exact answer it is relatively easy to find out. Get the bathroom scales out, step up and read off the result. But have you ever asked yourself what was actually involved in that measurement? Have you considered that what actually happened was that gravity’s pull was countered by the electromagnetic interaction of the atoms that make up the surface of the scale, and what you actually measured was how hard the atoms had to work to push back on your feet? Possibly not, and the chances are that you have never really considered how one would weigh a distant star, either.

As it turns out, this question has a fairly straightforward answer, which also involves gravity, electromagnetism… and a bit of luck. If you want to figure out how heavy a particular star is, and you are lucky enough that this star has a close companion, then all you need to do is track the star’s motion. The orbital motion of a double-star system is dictated by gravity and you can figure out how much the stars weigh using the same arguments that we use to figure out the mass of the moon (going around the Earth) and the Earth (circling the Sun). If you want to be a bit more precise you should use Einstein’s curved spacetime theory for gravity rather than Newton’s inverse square-law, but this may be a luxury in this exercise.

Using this technique, astronomers have weighed many stars with great precision and they have also managed to work out the masses of a number of pulsars [1]. This is particularly exciting as these systems stretch our understanding of several aspects of fundamental physics.

A pulsar is a highly magnetised rotating neutron star formed when a massive star runs out of nuclear fuel. At this point it can no longer hold itself up against gravity and it starts falling in on itself. This leads to a spectacular explosion – a supernova – the remains of which may be either a neutron star or a black hole. Neutron stars are nature’s own counterparts to the Large Hadron Collider [2]. In essence, they provide a link between astronomy and laboratory work in both high-energy and low-temperature physics. By weighing these stars we gain insight into physics under extreme conditions [3].

Pulsars are named for their rotating beam of electromagnetic radiation, which is observed by telescopes as it sweeps past the Earth, just like the familiar beam of a lighthouse [4]. They are renowned for their incredibly stable rate of rotation [5], but young pulsars occasionally experience so-called glitches where they are found to speed up for a very brief period of time [6]. The prevailing idea is that these glitches provide evidence of exotic states of matter in the star’s interior [3]. The glitches arise when a rapidly spinning superfluid within the star transfers rotational energy to the star's crust, a solid outer layer like a bowl containing a mysterious soup; the component that is tracked by observations. Imagine the bowl spinning at one speed and the soup spinning faster. Friction between the inside of the bowl and its contents, the soup, can cause the bowl to speed up. Whenever this happens, the more soup there is, the faster the bowl will be made to rotate.

Interestingly, it seems that the superfluid soup provides us with a new way of weighing these stars. This new technique is very different from the usual approach as it is not based on gravity, but nuclear physics, and it can also be used for stars in isolation. The star does not have to have a companion.

In a recent paper in Science Advances [7], we have developed this exciting new idea, which relies on a detailed understanding of neutron star superfluidity and the dynamics of the quantum vortices - a kind of ultra-slim tornadoes - by means of which these systems mimic large-scale rotation. Our results are promising and have important implications for the generation of revolutionary radio telescopes, like the Square Kilometre Array (SKA [8]) and the Low Frequency Array (LOFAR [9]), that are being developed by large international collaborations. The discovery and monitoring of many more pulsars is one of the key scientific goals of these projects. We now have a set of scales that may allow us to figure out how much these stars weigh, as well.

References: 
[1] See, for example, the list maintained at stellarcollapse.org
[2] See Large Hadron Collider
[3] J.M. Lattimer, M. Prakash, "The Physics of Neutron Stars", Science, 304, 536-542 (2004). Abstract
[4] The discovery of pulsars was recognized with the Nobel prize in physics in 1974
[5] Gravity tests using the stability of the timing of pulsars was recognized with the Nobel prize in physics in 1993
[6] A catalog of glitches is maintained by Jodrell Bank Centre for Astrophysics. 
[7] W.C.G. Ho, C.M. Espinoza, D. Antonopoulou, N. Andersson, "Pinning down the superfluid and measuring masses using pulsar glitches," Science Advances, 1, e1500578 (2015). Full Article
[8] See Square Kilometre Array.  
[9] See LOFAR

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Sunday, November 08, 2015

A Continuously Pumped Reservoir of Ultracold Atoms

From Left to Right: (top row) Jan Mahnke, Ilka Kruse, Andreas Hüper, (bottom row) Wolfgang Ertmer, Jan Arlt, Carsten Klempt.

Authors: Jan Mahnke1, Ilka Kruse1, Andreas Hüper1, Stefan Jöllenbeck1, Wolfgang Ertmer1, Jan Arlt2, Carsten Klempt1

Affiliation:
1Institut für Quantenoptik, Gottfried Wilhelm Leibniz Universität Hannover, Germany
2Institut for Fysik og Astronomi, Aarhus Universitet, Aarhus C, Denmark

During the last decades, the quantum regime could be accessed through very different systems, including trapped ions, micromechanical oscillators, superconducting circuits and dilute ultracold gases. Mostly, these systems show the desired quantum-mechanical features at ultra-low temperatures only. The lowest temperatures [1] to date are reached in dilute atomic gases by a combination of laser cooling and evaporative cooling. This approach has two disadvantages: It relies on the internal structure of the atoms due to the laser cooling and it can only cool discrete samples instead of continuous beams due to the evaporative cooling.

However, many applications would greatly benefit from a continuous source of cold atoms, for example sympathetic cooling [2] of molecules. Here, the molecules are brought into contact with a bath of cold atoms to redistribute the thermal energy through collisions. Ideally, such a cold bath is realized in absence of disturbing laser light as the rich internal structure of the molecules results in a broad absorption spectrum and any photon can potentially harm the cooling process. Another application of a continuous beam of cold atoms is continuous matter interferometry. Atom interferometry is already in use for the precise measurement of many observables, including time [3], gravity [4] and rotation [5]. These measurements could greatly benefit from a continuous observation instead of the sequential interrogation of discrete samples. Even though continuous sources are highly desired, no continuous sources without the application of laser light have been demonstrated in the microkelvin regime yet.

One possible realization of an ultracold continuous sample was proposed theoretically [6], where a conservative and static trap is loaded by an atomic beam. In this scheme, pre-cooled atoms are guided towards the entrance barrier of an elongated trap with a finite trap depth (see figure 1). If the atoms pass the entrance barrier, they follow the elongated potential until they are reflected by the end of the trap. The strong confinement in the radial direction ensures that most atoms collide with another atom before they reach the entrance barrier again. These collisions allow for a redistribution of the kinetic energy. Consequently, some atoms acquire a kinetic energy larger than the trap depth and escape the trap. Other atoms lose energy and stay trapped. If the trap parameters are chosen well, an equilibrium condition with a surprisingly large phase-space density may be reached.
Figure 1: 3D plot of the static trapping potential in the x–z-plane through the point of the trap minimum.

In our recent publication [7], we demonstrate the first experimental implementation of such a continuous loading of a conservative trap. Our realization is based on a mesoscopic atom chip (see figure 2 and Ref. [8]), a planar structure of millimeter-sized wires. The mesoscopic chip generates the magnetic fields for a three-dimensional magneto-optical trap, a magnetic waveguide and the static trapping potential described above. The three-dimensional magneto-optical trap is periodically loaded with an ensemble of atoms. These ensembles are launched into the magnetic waveguide, where they overlap and produce a continuous atom beam with varying intensity. This beam traverses an aperture which optically isolates the loading region from the static trapping region. In this trapping region, the atom beam is directed onto the elongated magnetic trap, where the atoms accumulate.
Figure 2: Photograph of the mesoscopic atom chip with millimeter-scale wires. The magneto-optical trap is in the lower left area and the static trap is generated in the top right area. The bend wires create a guide connecting the two regions.

With this loading scheme, we create and maintain an atomic reservoir with a total number of 3.8 × 107 trapped atoms at a temperature of 102 µK, corresponding to a peak phase-space density of 9 × 10-8 h-3. This is the first continuously loaded cloud in the microkelvin regime without the application of laser light. Such a continuously replenished ensemble of ultracold atoms presents a new tool for metrological tasks and for the sympathetic cooling of other atomic species, molecules or nanoscopic solid state systems. The scheme is also very versatile in creating cold samples of atoms and molecules directly, as it does not rely on any internal level structure.
Figure 3 Photograph of the experimental setup. The atom chip is visible outside of the vacuum chamber at the top. In the front is the glass cell and the optics for the two-dimensional magneto-optical trap, which is used to load the three dimensional magneto-optical trap on the chip.

References:
[1] A. E. Leanhardt, T. A. Pasquini, M. Saba, A. Schirotzek, Y. Shin, D. Kielpinski, D. E. Pritchard,  W. Ketterle. “Cooling Bose-Einstein condensates below 500 picokelvin”, Science, 301, 1513 (2003). Abstract.
[2] Wade G. Rellergert, Scott T. Sullivan, Svetlana Kotochigova, Alexander Petrov, Kuang Chen, Steven J. Schowalter, Eric R. Hudson, “Measurement of a large chemical reaction rate between ultracold closed-shell 40Ca atoms and open-shell 174Yb+ ions held in a hybrid atom-ion trap”, Physical Review Letters, 107, 243201 (2011). Abstract.
[3] R. Wynands and S. Weyers. “Atomic fountain clocks”, Metrologia, 42 (3), S64 (2005). Abstract.
[4] A. Louchet-Chauvet, S. Merlet, Q. Bodart, A. Landragin, F. Pereira Dos Santos, H. Baumann, G. D'Agostino, C. Origlia, “Comparison of 3 absolute gravimeters based on different methods for the e-MASS project”, Instrumentation and Measurement, IEEE Transactions on, 60(7), 2527-2532 (2011). Abstract.
[5] J. K. Stockton, K. Takase, and M. A. Kasevich. “Absolute geodetic rotation measurement using atom interferometry”, Physical Review Letters, 107, 133001 (2011). Abstract.
[6] C. F. Roos, P. Cren, D. Guéry-Odelin, and J. Dalibard. “Continuous loading of a non-dissipative atom trap”, Europhysics Letters, 61, 187 (2003). Abstract.
[7] J. Mahnke, I. Kruse, A. Hüper, S. Jöllenbeck, W. Ertmer, J. Arlt, C. Klempt. “A continuously pumped reservoir of ultracold atoms”, Journal of Physics B: Atomic Molecular and Optical Physics, 48, 165301 (2015). Abstract.
[8] S. Jöllenbeck, J. Mahnke, R. Randoll, W. Ertmer, J. Arlt, C. Klempt. “Hexapole-compensated magneto-optical trap on a mesoscopic atom chip”, Physical Review A, 83, 043406 (2011). Abstract.

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