Archive for the ‘ATOMIC PHYSICS’ Category

Giant Milky Way bubbles blown by black hole merger

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Did an insurgent dwarf galaxy create these colossal, high-energy bubbles (shown in false colour)? (Image: NASA/GSFC)

Did an insurgent dwarf galaxy create these colossal, high-energy bubbles (shown in false colour)? (Image: NASA/GSFC)

A tiny galaxy that collided with the Milky Way spawned two huge bubbles of high-energy particles that now tower over the centre of our galaxy. This new model for the birth of the mysterious bubbles also explains discrepancies in the ages of stars at the galactic middle.

In 2010, sky maps made by NASA’s Fermi Gamma-ray Space Telescope revealed two lobes of particles billowing out from the heart of the Milky Way, each one stretching 25,000 light years beyond the galactic plane.

Astronomers suspected the bubbles were inflated by a period of violence in the galactic centre about 10 million years ago, but no one could say what had triggered the outburst.

Earlier this year, Kelly Holley-Bockelmann from Vanderbilt University in Nashville, Tennessee, was discussing the problem with Tamara Bogdanović from the Georgia Institute of Technology in Atlanta.

“We pieced together all the evidence and realised they could be explained by a single catastrophic event – the collision between two black holes,” recalls Holley-Bockelmann.

Tango and crash

We know that a supermassive black hole weighing as much as 4 million suns lurks at the core of the Milky Way. We also have an array of dwarf galaxies orbiting our much larger spiral galaxy, as well as hints that past satellite dwarfs have collided with us.

According to the new theory, a small galaxy with its own central black hole dove into the Milky Way and began spiralling through our galaxy. After billions of years, the stripped-down dwarf’s black hole made it to the galactic centre.

The two black holes then performed a tight gravitational tango before finally merging. This final act produced violent forces that flung out many of the stars that were born in the Milky Way’s middle, explaining why astronomers now find far fewer old stars there than they have every right to expect.

The whirling black holes also disrupted giant clouds of gas, some of which got squeezed so much that they collapsed to form clusters of bright new stars. Much of the rest of the gas swirled into the merged black holes, getting so hot from compression that it radiated huge amounts of energy.

“We think it’s both the energy from this ‘burp’ near the black hole and the winds of gas from the starburst that inflated the Fermi bubbles,” says Holley-Bockelmann.

Round up the runaways

“This hypothesis is probably worth considering,” says Mark Morris, an authority on the galactic centre at the University of California, Los Angeles. But he cautions against making it a leading explanation without more evidence.

Holley-Bockelmann and colleagues think that the colliding dwarf galaxy was formed early in the history of the universe and consisted mainly of dark matter and the central black hole, without many stars. That would account for why we see no tell-tale trail of stars left behind as the dwarf galaxy fragmented.

Instead, the team proposes another test of their model: hunting for the old stars catapulted outwards during the black hole merger.

“It should have carved nearly 1000 stars out of the galactic centre,” says Bogdanović. “These stars should still be racing through space, about 10,000 light years from their original orbits.” The team is now searching for these runaway stars in data amassed by the Sloan Digital Sky Survey, which has observed the properties of hundreds of millions of stars in our galaxy.

Journal reference: Monthly Notices of the Royal Astronomical Society,

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March 8, 2013 at 1:34 pm

Transforming noise into mechanical energy at nanometric level

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Driving a Macroscopic Oscillator with the Stochastic Motion of a Hydrogen Molecule
Christian Lotze, Martina Corso, Katharina J. Franke, Felix von Oppen, Jose Ignacio Pascual

Energy harvesting from noise is a paradigm proposed by the theory of stochastic resonances. We demonstrate that the random switching of a hydrogen (H2) molecule can drive the oscillation of a macroscopic mechanical resonator. The H2 motion was activated by tunneling electrons and caused fluctuations of the forces sensed by the tip of a noncontact atomic force microscope. The stochastic molecular noise and the periodic oscillation of the tip were coupled in a concerted dynamic that drives the system into self-oscillation. This phenomenon could be a way for enhancing the transfer of energy from incoherent sources into coherent dynamics of a molecular engine.
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Read also: Vibrating molecule drives a motor

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November 12, 2012 at 4:18 pm

Posted in ATOMIC PHYSICS, Thermodynamics

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Nov. 8, 1895: Roentgen Stumbles Upon X-Rays

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1895: German physicist Wilhelm Roentgen is working in his laboratory in Würzburg when he accidentally discovers the X-ray.

Roentgen was conducting experiments with a Crookes tube — basically a glass gas bulb that gives off fluorescent light when a high-voltage current is passed through it — when he noticed that the beam turned a screen 9 feet away a greenish fluorescent color, despite the tube being shielded by heavy black cardboard.

Roentgen concluded, correctly, that he was dealing with a new kind of ray, one that cast the shadow of a solid object when passed through an opaque covering from its point of origin. Not knowing what kind of ray he was dealing with, exactly, led him to call it an X-ray. The name stuck.

To test his discovery, Roentgen made an X-ray image of his wife Bertha’s hand, clearly showing the bones of her hand and a pretty hefty wedding ring.

In the next couple of months, Roentgen published a paper about his discovery: “On a New Kind of Rays.” He made a presentation before the Würzburg Medical Society and X-rayed the hand of a prominent anatomist, who proposed naming the new ray after Roentgen.

You don’t hear them called Roentgen rays much these days, but the term roentgenology is still current, and the roentgen is a radiological unit of measure.

X-rays are no longer a mystery, but a major tool of medical diagnosis.

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Written by physicsgg

November 8, 2012 at 6:59 pm


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Building the Smallest Possible Ice Crystal

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It may sound like a Zen koan, but it’s a serious scientific question: How many molecules of water does it take to make the smallest possible ice crystal? Because crystals are defined by a repeated, three-dimensional arrangement of molecules, you can’t necessarily take any small group of bonded-together molecules and call them a crystal. That’s especially true for water: When it freezes, the weak hydrogen bonds that loosely bind the water molecules together pull the disordered clusters of molecules (left) into a more open—but also more rigid—cagelike arrangement (cross section of cluster at right). This roomy lattice is also why ice is less dense than water (and therefore floats). So to calculate the minimum number of molecules needed to make an ice lattice, a team of researchers shone infrared lasers on clusters of water molecules containing between 80 and 500 molecules. The team paid particular attention to how much energy the clusters absorbed from the lasers between the wavelengths of 2.63 micrometers and 3.57 micrometers—the range in which the oxygen-hydrogen bonds in water continually stretch and shrink. A particular peak of energy absorption occurred at a wavelength of about 3.125 micrometers—denoting the spectral characteristic of ice—and only appeared for clusters containing more than 275 water molecules, the researchers report online today in Science. That number of molecules yields a tiny ice cluster between 1 nanometer and 3 nanometers across—the ultimate in crushed ice.

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Read also: How many water molecules does it take to make ice?

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September 21, 2012 at 12:39 pm

Ultracold fermions simulate spin–orbit coupling

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Artist’s impression of the spin diode created by researchers at MIT. Atoms with clockwise spin can only move in one direction, while atoms with anticlockwise spin move in the opposite direction. (Courtesy: Christine Daniloff)

Two independent groups of physicists are the first to use ultracold fermionic atoms to simulate “spin–orbit coupling” – an interaction that plays an important role in the electronic properties of solid materials. Both experiments were done by firing laser beams at the atoms, which caused their momentum to change by an amount that depends on their intrinsic spin. Because the interactions between atoms in such simulations can be adjusted with great precision, the breakthrough could shed further light on a range of physical phenomena, including magnetism, topological insulators and Majorana fermions.

Spin–orbit coupling describes the interaction between the intrinsic spin of an electron in a material and the magnetic field induced by the electron’s movement relative to its surrounding ions. As well as playing a key role in the magnetic properties of materials, spin–orbit coupling also influences the performance of “spintronic” devices – those that exploit the spin, rather than the charge, of electrons and that could one day lead to faster and more energy-efficient computers.

Quantum simulators

Because of its fundamental nature, physicists are therefore very keen to use clouds of ultracold atoms to simulate spin–orbit coupling. Such “quantum simulations” are carried out by subjecting the gas to laser light and magnetic fields, which lets researchers create interactions between atoms that are similar to those experienced by electrons in a solid. The advantage of these simulations is that – unlike in a solid – the strength of these interactions can be easily adjusted, allowing physicists to test theories of condensed-matter physics.

In 2011 Ian Spielman and colleagues at the National Institute of Standards and Technology (NIST) in Maryland were the first to simulate spin–orbit coupling in an ultracold gas of bosonic atoms. Now, two independent groups – one in China led by Hui Zhai of Tsinghua University and Jing Zhang of Shanxi University, and the other in the US headed by Martin Zwierlein and Lawrence Cheuk at the Massachusetts Institute of Technology (MIT) – have extended Spielman’s technique to fermions. As electrons are fermions and not bosons, the new work is much more relevant to electron physics.

Using potassium-40…

The Chinese team began with about two million potassium-40 atoms that are held in an optical trap and cooled to well below the ensemble’s Fermi temperature. This means that nearly all the atoms in the gas are in the lowest possible energy state, like the conduction electrons in a metal. The team focused on two closely spaced magnetic energy states, which are used to simulate the spin of the electron – one state corresponding to spin up and the other to spin down.

The team then fired two laser beams into the gas from opposing directions. The laser light is set to resonate with a transition between the two spin states – a process that involves the atoms continuously absorbing and emitting photons. As these photons carry momentum, if an atom absorbs a photon moving in one direction and then re-emits it in the same direction, the atom’s momentum will not change. However, an atom can also be stimulated by the opposing beam to emit the photon in the opposite direction – thus changing the atom’s momentum. Such an interaction involves a change in the direction of the atom’s spin and is therefore analogous to spin–orbit coupling – albeit in 1D.

The Chinese team used its system to study several aspects of spin–orbit coupling. In one experiment, the researchers began with a state in which all the spins are initially pointing in the same direction. They then turned on the spin–orbit interaction by pulsing the lasers for a very short time – just a few hundred microseconds. They found that the spins began to point in different directions in a process known as “dephasing”. This is expected from fermions because atoms with the same spin cannot have the same momentum and therefore each atom will be affected differently by the spin–orbit interaction.

Understanding dephasing is important because it has a detrimental effect on technological applications of spin such as spintronics and quantum computing. The team also looked at several other effects related to spin–orbit coupling, including its effect on the momentum distribution of the atoms.

…and lithium-6

The MIT physicists, meanwhile, used a gas of lithium-6 atoms, which meant that their realization of spin–orbit coupling was more difficult than for the Chinese team. The problem is that lighter atoms such as lithium are more prone to heating via the resonant absorption of light. So to get round this problem, the MIT team kept most of its atoms in “reservoir states” in which they do not interact with the light and stayed cool – using radio waves to drive a small number of atoms into the spin–orbit coupling states.

The MIT team focused on showing that ultracold atoms can be used to simulate a “spin diode” – a device that is likely to play a key role in the development of spintronic circuits. It allows spin-up atoms to flow forwards but not backwards, and spin-down atoms backwards but not forward. “The gas acts as a quantum diode, a device that regulates the flow of spin currents,” says Cheuk.

Simulating band structure

By applying radio-frequency radiation to the gas, the MIT physicists were also able to simulate a periodic potential similar to that found in a 1D lattice. As expected for real materials, the periodic potential led to the existence of spin-dependent energy bands. According to the team, the ability to create spin-dependent band structures in this way could lead to the simulation of topological insulators.

The spin–orbit coupling simulated by both teams occurs only in 1D and therefore cannot be used to simulate the 2D and 3D systems found in most real-life electronic devices. However, there are several interesting scenarios that can be investigated in a 1D system. For example, it could be used to simulate the behaviour of electrons in semiconductor/superconductor nanowires. Such systems are believed to harbour quasiparticles that resemble Majorana fermions – long sought-after particles that are also their own antiparticle.

Both experiments are described in Physical Review Letters.
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September 5, 2012 at 8:32 am

Quantum dynamics of the avian compass

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(Color online) Decay of coherence in the Bloch sphere picture. For initially pure states, trajectories begin on the unit sphere before undergoing rapid decay of the z component and slow decay of the x and y components

Zachary B. Walters
The ability of migratory birds to orient relative to the Earth’s magnetic field is believed to involve a coherent superposition of two spin states of a radical electron pair. However, the mechanism by which this coherence can be maintained in the face of strong interactions with the cellular environment has remained unclear. This Letter addresses the problem of decoherence between two electron spins due to hyperfine interaction with a bath of spin 1/2 nuclei. Conditions necessary for long lived coherence are identified, and a simple yet robust model for sensing magnetic field orientation is presented.
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Read also: Eye bath’ to thank for quantum vision in birds

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August 22, 2012 at 12:57 pm

A Matterwave Transistor Oscillator

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The atomtronic transistor

Seth C. Caliga, Cameron J. E. Straatsma, Alex A. Zozulya, Dana Z. Anderson
A triple-well atomtronic transistor combined with forced RF evaporation is used to realize a driven matterwave oscillator circuit. The transistor is implemented using a metalized compound glass and silicon substrate. On-chip and external currents produce a cigar-shaped magnetic trap, which is divided into transistor source, gate, and drain regions by a pair of blue-detuned optical barriers projected onto the magnetic trap through a chip window. A resonant laser beam illuminating the drain portion of the atomtronic transistor couples atoms emitted by the gate to the vacuum. The circuit operates by loading the source with cold atoms and utilizing forced evaporation as a power supply that produces a positive chemical potential in the source, which subsequently drives oscillation. High-resolution in-trap absorption imagery reveals gate atoms that have tunneled from the source and establishes that the circuit emits a nominally mono-energetic matterwave with a frequency of 23.5(1.0) kHz by tunneling from the gate, corresponding to a vacuum energy of 2.67 MHz x h, where h is Planck’s constant, and a vacuum wavelength of 29 nm. Time-of-flight measurements indicate that the transistor exhibits ohmic cooling, i.e. negative resistance, and therefore has gain. Time-of-flight measurements are also used to determine an upper bound of the atomtronic transresistance, r<0.01 Hz/Hz x h/m^2 where m is the mass of rubidium 87, and that the closed-loop circuit energy gain varies between 3.1 and 3.6.
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Read also: First Atomtronic Radio Broadcasts Matter Waves

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August 20, 2012 at 3:20 pm


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Magic Angle

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Diagram shows measurement of the magic angle, which is useful for a range of scientific experiments. (Licensed under the Creative Commons Attribution-ShareAlike 3.0 License)

by Glenn Roberts Jr.
The magic angle is all about mathematics, not hocus pocus, but it can work wonders in gathering useful data in scientific experiments and in medical imaging, too.

John Bozek, an instrument scientist at SLAC’s Linac Coherent Light Source, said the angle, which measures about 54.7 degrees, is important for a range of experiments with the X-ray laser, particularly those involving gases.

In photoelectron spectroscopy, a common research technique at LCLS, X-ray laser pulses strike a sample and eject electrons from it. Instruments called spectrometers measure the energy range, or spectrum, of the ejected electrons.

To get the most accurate measurements of the energy spectrum of those electrons, a detector can be placed at the magic angle relative to the path of the laser beam through the sample.

A detector placed at other angles can provide conflicting data, though it is possible to get an accurate measurement by comparing observations from multiple angles.

“If you measure the spectrum at 90 degrees, then measure it at zero degrees, some of the peaks in the spectra completely disappear; the intensity of the peaks will depend on the angle,” Bozek said. “So either you measure at a few angles, or you measure at the magic angle.”

One of the spectrometers at the LCLS’s Atomic, Molecular and Optical Science instrument is typically aligned at the magic angle, Bozek noted, to help cancel out such effects.

The magic angle is also useful in magnetic resonance imaging, among other applications.

The magic angle can be formed by drawing a line from a bottom corner in the inside of a cube diagonally to its opposite corner at the top of the cube, and measuring the angle between that diagonal line and the vertical line that rises from that bottom corner.

Written by physicsgg

July 27, 2012 at 6:02 pm