Non-contact friction at miscroscopic scale

Friction is a daily phenomena and everybody has been used to it and learned how to use it by instinct. And the macroscopic law is usually stunningly simple and universal and relative motion is a necessary condition. On the other hand, how it comes about microscopically has been one among the mootest topics that arouse curiousities. Here is a review on a recent work in this field:

To unravel this tangle of vdW, electrostatic and phononic friction, Kisiel et al. incorporated a second experimental approach that aims to distinguish between phononic and electronic dissipation. In both vdW and electrostatic friction, dissipation ultimately takes place through forced motion of charge in a resistive medium. To separate this effect from phononic dissipation, one can vary the electrical resistivity of one of the two mating materials. This can be done most elegantly in a temperature-dependent experiment in which resistivity is switched on and off by going through a superconductivity transition. An experiment of this type was performed in 1998 by Dayo and colleagues⁵, who used a quartz microbalance to observe increased slip times of nitrogen adhered to a metal surface. Their work triggered significant debate, which continues to this day, because the observed behaviour did not show the predicted temperature dependence around the critical temperature.

In their experiments, Kisiel et al. used a nanoscale cantilevered tip vibrating in close proximity to the surface of a conductor. An atomic force microscope allowed both precise positioning and friction measurements. By positioning a silicon tip close to a niobium surface at temperatures above niobium's superconductivity transition temperature, T_c, they measured friction coefficients down to ~10⁻¹² kg s⁻¹. Moreover, they collected evidence of an electromagnetic origin of friction by verifying the dependence on the distance and applied voltage as predicted by Volokitin and Persson². Going through T_c to lower temperatures, the friction dropped to one third of the initial value. Below T_c, they argue, electronic dissipation is excluded and phononic interaction should govern the friction. Again, the dependence on the distance and applied voltage fitted the prediction, and even the temperature dependence they found is somewhat more gradual than that in the experiments of Dayo et al. [http://www.nature.com/nmat/journal/v10/n2/full/nmat2947.html]

2DEG at the surface of STO

Reduced dimension electron systems boast of very interesting physics from both the fundamental and practical point of view. For example, 2DEG houses quantum hall effects, which even today constitutes a rich arena of study [Nature, 469:185–188]. Such 2DEG exists in a diversity of systems, such as semiconductor or transition metal oxide hetero structures. More recently, it defines topological insulators. Now a work in Nature shows that, 2DEG exists also in simple surfaces of strontium titanate, and more interestingly, this 2DEG has similar characteristics with those found at the interfaces of STO with the second compound [doi:10.1038/nature09720]. This offers an easy way to fabricate 2DEG systems. More work is needed in elucidating the mechanism.

As silicon is the basis of conventional electronics, so strontium titanate (SrTiO3) is the foundation of the emerging field of oxide electronics1,2. SrTiO3 is the preferred template for the creation of exotic, two-dimensional (2D) phases of electronmatter at oxide interfaces3–5 that havemetal–insulator transitions6,7, superconductivity8,9 or large negative magnetoresistance10. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs), which is crucial to understanding their remarkable properties11,12, remains elusive. Here we show, using angle-resolved photoemission spectroscopy, that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO3 (including the non-doped insulating material) independently of bulk carrier densities over more than seven decades. This 2DEG is confined within a region of about five unit cells and has a sheet carrier density of 0.33 electrons per square lattice parameter. The electronic structure consists of multiple subbands of heavy and light electrons. The similarity of this 2DEG to those reported in SrTiO3-based heterostructures6,8,13 and field-effect transistors9,14 suggests that different forms of electron confinement at the surface of SrTiO3 lead to essentially the same 2DEG. Our discovery provides a model system for the study of the electronic structure of 2DEGs in SrTiO3-based devices and a novel means of generating 2DEGs at the surfaces of transition-metal oxides.

Cloaks work with a mirror

Cloaks are such inspiring objects that intrigue both physical and atheistic innovative can only be quested by physicists to the end. Now a work from Singapore reported a cloak working with a mirrow: the thing of a 2mm to be hided is placed on a mirror and covered by a cloak made of calcite. One only perceives the mirror but not thing under carpet [PRL 106, 033901 (2011)].

Invisibility cloaks, a subject that usually occurs in science fiction and myths, have attracted wide interest recently because of their possible realization. The biggest challenge to true invisibility is known to be the cloaking of a macroscopic object in the broad range of wavelengths visible to the human eye. Here we experimentally solve this problem by incorporating the principle of transformation optics into a
conventional optical lens fabrication with low-cost materials and simple manufacturing techniques. A transparent cloak made of two pieces of calcite is created. This cloak is able to conceal a macroscopic object with a maximum height of 2 mm, larger than 3500 free-space-wavelength, inside a transparent liquid environment. Its working bandwidth encompassing red, green, and blue light is also demonstrated.

Trions detected in CNTs

An exciton is a bound pair of one electron and one hole, analogous to a hydrogen atom, but with much smaller binding energy. It can be excited in many semiconductors by light. An electron in valence band can be lifted by a photon to the conduction band and in the meanwhile a hole-quasiparticle- will be left behind. This electron shall attract with the hole due to electrostatic interaction and make an exciton. A trion is like an ionized hydrogen molecule, containing one electron and two holes. Such objects are of course less stable than an exciton, but have been observed. Now some scientists from Kyoto University observed these trions in carbon nanotubes. They seem much stable with binding energy of a tenth of an electron volt [Phys. Rev. Lett. 106, 037404 (2011) ]. A view point can be found here [Physics 4, 5 (2011)] :

The smoking gun of the trions in the experiment of Matsunaga et al. is the appearance of an additional peak in the optical spectra on the low-energy end of the exciton peak. Matsunaga et al. work hard to show that this peak indeed originates from trions and not, for example, from defects introduced through doping. They first investigate the influence of different dopants and doping concentrations and find that new peaks associated with trions in nanotubes appear at the same energy, regardless of the dopant species. They also become stronger with increasing doping concentration, along with a reduction of the exciton peak. Moreover, they find that excitons in nanotubes with different diameters and twist angles all come along with a corresponding trion partner and show clear “family patterns,” similar to those known for excitons in nanotubes. All this provides strong evidence for trions.

How does black holes sette in galaxies

Every galaxy is believed to harbor a compact massive body-black hole- at its center. How it gets in? A natural way is through galaxy evolution based on the Big Bang model. However, there is the other way around, which is said being enigmatic [Nature,469:305].

Bulges and their black holes seem to be a natural consequence of structure formation in the hot Big Bang theory of the expanding Universe. According to this theory, galaxies grew by gravitational assembly of matter into clumps that gathered into larger clumps, and so on to galaxies. In galaxies with bulges, including ellipticals, which have bulges and no disks, the mass of the central black hole correlates not only with the mass of the bulge, but also, as Kormendy, Bender and Cornell¹ note ( page 374), with the average spread of velocities of the bulge stars (see Fig. 2a on page 375). The plausible explanation is that part of the gas out of which bulge stars formed settled instead near to the black hole, in part increasing its mass and in part fuelling explosions that blew the gas away and suppressed bulge-star formation. That is, the growth of bulge and black hole may have controlled each other. The timing looks right. Bulge stars are old: they formed when the expanding Universe was roughly a third of its present size (redshift about 2). This is when the rate of star formation per unit of matter was near its maximum (more than 10 times the present rate³). It is also when quasars — explosions powered by the central black holes — were most abundant (100 times more common than now⁴), probably an explosive result of overfeeding of the black holes as the early generations of stars were forming.

........

In theory, galaxies both with and without bulges were growing by the gravitational collection of clumps of matter when the star-formation rate was near its peak. That would suggest that the clumps contained stars; a recent discussion puts roughly comparable masses in stars and gas⁶. So where are these early generations of stars? Not in disks, because there is nothing that would slow the motion of a star to allow it to settle onto a disk. Bulges contain old stars, and it has been suggested that this is where the early stars ended up. But we now see that this is not plausible: why would these old stars have avoided our bulgeless Galaxy and settled instead in the bulge of our neighbour M31? Maybe the old stars are in diffuse stellar haloes. If so, it seems curious that the stellar halo of our Galaxy is much less prominent than that of M31. But more studies of other nearby galaxies will be required to check for inventories of stars that are old enough and abundant enough to account for stars that formed before disks.

Relativity starts your automobile

I have already mentioned this interesting work. But it is so funny as to deserve a further click from Nature [469:269]:

Cars are started using lead–acid batteries, which generate energy using electrochemical reactions between lead compounds and sulphuric acid. Rajeev Ahuja of Uppsala University in Sweden and his colleagues modelled the reactions and found that as electrons move at high speed around a lead nucleus, their energy levels change owing to relativity. The authors conclude that the change accounts for 1.7–1.8 volts of a standard 2.13-volt lead–acid cell.

Reality Check at the LHC

A synopsis of what has been achieved in the beginning run of LHC:
http://physicsworld.com/cws/article/indepth/44805

Is this energy change possible ?

Mpemba Effect

Water is just mundane and seems well-understood in many respects. However, there are still quite a lot of things that motivate people to find more. For example, how water molecules arrange themselves when they adsorbed on an adsorbate. Another instance is, I think more associated with the thermodynamics of water: it has been claimed that, hot water cools faster than cold water when they are placed in the same chamber. This was named after its discoverer, a middle school student Mpemba. There came a latest study on this [http://arxiv.org/ftp/arxiv/papers/1101/1101.2684.pdf]:

In this paper we have presented data confirming that water initially at higher temperature cools at a faster rate than water initially at a lower temperature and that this trend continues past the point at which the two samples reach the same temperature: the crossover temperature. Furthermore, our data indicates that the starting temperature affects the crossover temperature in a reproducible manner. We have confirmed that warmer water indeed cools faster than colder water and that, surprisingly, this trend continues past the point where the temperatures of the two samples are the same. Our results show that when using optimal initial temperature conditions, the crossover temperature is found to be 2.7 oC whereas our other set of initial conditions gave a crossover temperature of -0.07 oC. These data taken together provide a definite quantitative evidence of the Mpemba effect.

Light-Induced Superconductivity in a Stripe-Ordered Cuprate

This is a funny study, which shows how light mobilizes carriers in gapped states to become superconducting temporarily. [http://www.sciencemag.org/content/331/6014/189.full.pdf]. What does all these imply for the microscopic theory of cuprates, anyway?

One of the most intriguing features of some high-temperature cuprate superconductors is the interplay between one-dimensional “striped” spin order and charge order, and superconductivity. We used mid-infrared femtosecond pulses to transform one such stripe-ordered compound, nonsuperconducting La1.675Eu0.2Sr0.125CuO4, into a transient three-dimensional superconductor.
The emergence of coherent interlayer transport was evidenced by the prompt appearance of a Josephson plasma resonance in the c-axis optical properties. An upper limit for the time scale needed to form the superconducting phase is estimated to be 1 to 2 picoseconds, which is significantly faster than expected. This places stringent new constraints on our understanding of stripe order and its relation to superconductivity.

Vacuum friction

Can the vacuum drag a spinning objects ? Considering the famous Casimir effect, the answer is a definite yes. This recent study calculated this dragging force in a full quantum way and shows that it is experimentally detectable. Let's see if new experiments will come out soon !

We study the stopping of spinning particles in vacuum. A torque is produced by fluctuations of the vacuum electromagnetic field and the particle polarization. Expressions for the frictional torque and the power radiated by the particle are obtained as a function of rotation velocity and the temperatures of the particle and the surrounding vacuum. We solve this problem following two different approaches: (i) a semiclassical calculation based upon the fluctuation-dissipation theorem (FDT), and (ii) a fully quantum-mechanical theory within the framework of quantum electrodynamics, assuming that the response of the particle is governed by bosonic excitations such as phonons and plasmons. Both calculations lead to identical final expressions, thus confirming the suitability of the FDT to deal with problems that are apparently out of equilibrium, and also providing comprehensive insight into the physical processes underlying thermal and vacuum friction.We adapt the quantum-mechanical theory to describe particles whose electromagnetic response is produced by fermionic excitations. Furthermore, we extend our FDT formalism to fully account for magnetic polarization, which dominates friction when the particle is a good conductor. Finally, we present numerically calculated torques and stopping times for the relevant cases of graphite and gold nanoparticles. [PHYSICAL REVIEW A 82, 063827 (2010)]

Cloak for sounds

The idea of cloaks that make objects disappear is really capturing. It has been realized at least in the lab by a theory called "transformation optics", which relates to the transformation properties of Maxwell's equations. However, this idea not only blossoms in optics but also in acoustics, where similar equations exist in 2D. A new paper in PRL designs a cloak for ultrasonic waves[Phys. Rev. Lett. 106, 024301 (2011) – Published January 10, 2011]. The difficulty lies in varying the density of materials in a desired way[Physics 4, 2 (2011)].

Invisibility devices based on coordinate transformation have opened up a new field of considerable interest. We present here the first practical realization of a low-loss and broadband acoustic cloak for underwater ultrasound. This metamaterial cloak is constructed with a network of acoustic circuit elements, namely, serial inductors and shunt capacitors. Our experiment clearly shows that the acoustic cloak can effectively bend the ultrasound waves around the hidden object, with reduced scattering and
shadow. Because of the nonresonant nature of the building elements, this low-loss ( 6 dB=m) cylindrical cloak exhibits invisibility over a broad frequency range from 52 to 64 kHz. Furthermore, our experimental study indicates that this design approach should be scalable to different acoustic frequencies and offers the possibility for a variety of devices based on coordinate transformation.

A varying J is not neccessary

In the spin-fermion model, which is explicitly two-band, proposed by the blog owner for cuprate superconductors, three terms are considered relevant: the kinetic energy of doped carriers, the on-site AFM spin coupling between the carrier and the local Cu spin, and the AFM coupling (with energy J) between nearest local Cu spins. In this entry, I note that, the J might not be the same for every pair of Cu spins, in the presence of carriers. The reason is clear: the J actually stems from the virtual hopping of Cu electrons and such virtual hopping occurs through the shared O site. When carriers are there on the O site, the virtual hopping shall be affected and hence the J be affected. Nonetheless, it is still realistic to maintain a uniform J for these considerations: (1) the J, as non-uniform as it may be, is always positive, thus no special thing such as frustration could take place; (2) the non-uniformity must be very small, of the order of kintic energy /16, because each carrier in Zhang-Rice orbital becomes diluted by a factor of 1/4; (3) the overall effect may be just to enlarge J a bit (on average). So, this model with uniform J is still very physically reasonable.

Physics and Physicists: High-Tc Superconductors Are Very Kinky - Update 10

Physics and Physicists: High-Tc Superconductors Are Very Kinky - Update 10

Relativity and the Lead-Acid Battery

This relation may seem at first glance quite unusual. For most, relativity needs be concerned only when the considered speed becomes comparable to that of light. The Lead -acid battery seems having no attachment to this speed. However, if you look into the electrons that are busy inside the substance, you may change your mind. Lead is a heavy element, and relativity effects, as you can derive from Dirac's equation, scale as the quartic power of the atomic numbers. That is how relativity plays a role, which proves crucial for your cars to start, in Lead-Acid Battery [http://prl.aps.org/pdf/PRL/v106/i1/e018301].

The energies of the solid reactants in the lead-acid battery are calculated ab initio using two different basis sets at nonrelativistic, scalar-relativistic, and fully relativistic levels, and using several exchange correlation potentials. The average calculated standard voltage is 2.13 V, compared with the experimental value of 2.11 V. All calculations agree in that 1.7–1.8 V of this standard voltage arise from relativistic effects, mainly from PbO2 but also from PbSO4.

Kinks cannot be ascribed to phonons

In cuprate superconductors, the dispersion of energy with wave vectors features some kinks. As regards the cause of such kinks, suppliers with phonons has appeared. However, this paper [http://www.fkf.mpg.de/metzner/manske/Pub/PRL100_137001.pdf] disfavors such choice.

Using the local density approximation and a realistic phonon spectrum we determine the momentum and frequency dependence of 2F k;! in YBa2Cu3O7 for the bonding, antibonding, and chain band. The resulting self-energy is rather small near the Fermi surface. For instance, for the antibonding band the maximum of Re as a function of frequency is about 7 meVat the nodal point in the normal state and the ratio of bare and renormalized Fermi velocities is 1.18. These values are a factor of 3–5 too small compared to the experiment showing that only a small part of can be attributed to phonons. Furthermore, the frequency dependence of the renormalization factor Z k;! is smooth and has no anomalies at the observed kink frequencies which means that phonons cannot produce well-pronounced kinks in stoichiometric YBa2Cu3O7, at least, within the local density approximation.

Physics and Physicists: More Evidence The Public Can't Tell The Difference Between Scientific and Anecdotal Evidence

Physics and Physicists: More Evidence The Public Can't Tell The Difference Between Scientific and Anecdotal Evidence

Ferromagetism demises SC in overdoped LSCO ?

The phase diagram of cuprate superconductors is as perplexing as one can imagine. It is like winning a holy grail to theoretically explicate this diagram. One feature in it is the SC dome: the SC phase survives only a segment between x1 and x2. Much attention has been paid to the regime around x1, while little to that around x2. A basic question is, what destroys SC in the course of overdoping ? Experiments actually have found no traces of what can be seen in the vicinity of x1, the Mottness behaviors showing significant AFM fluctuations. Some claimed that, the compound becomes a normal Fermi liquid beyond x2, which is however questioned for several reasons [http://www.pnas.org/content/104/15/6123.full.pdf]. A novel proposal surmised that, there is a new order, a ferromagnetic order, that competes with SC [http://www.pnas.org/content/104/15/6123.full.pdf]. Their proposal finds advocate from an upturn in the uniform magnetic susceptibility that is usually associated with FM correlations. More interestingly, a recent experiment claimed a direct observation of this FM phase [http://www.pnas.org/content/107/40/17131.full.pdf]. This makes the x2-region very fashionable.

Cloak enough to cover visible objects

A team in UK recently fabricated a cloak that works under green light to make optical delusions [http://arxiv.org/ftp/arxiv/papers/1012/1012.2783.pdf]. This cloak is large and able to cover an object as large as 10mm^3. The physics is governed by Maxwell's equations. The substance is calcite, which has anisotropic optical properties.

Varma's Current

The short review [http://arxiv.org/PS_cache/arxiv/pdf/1012/1012.5461v1.pdf] of the history of High Tc superconductivity by J.Zannen has mentioned the work by C. M.Varma, who proposed the so-called circulating current phase as a candidate explanation of the observed pseudogap phase [http://prl.aps.org/pdf/PRL/v83/i17/p3538_1]. As beautiful as it seems, one should take caution with this: such current is incompatible with the formation of Zhang-Rice singlets. The only claimed experimental support of this phase comes from Neutron scattering and optical experiments which detected a time-reversal symmetry breaking state. However, it might be too hasty to identify this phase with the Varma's phase. Let's wait and see how the holy grail will be won !