Recently in Condensed-matter physics Category

In the absence of an applied voltage, an induced electrical current rapidly decays thanks to the scattering of electrons from defects, phonons, and each other. But in a cold metal ring smaller than the electron’s coherence length, it’s possible to induce a dissipationless current, even if the metal is not superconducting. The trick, theorists predicted in the early 1980s, is to thread the ring with a magnetic field, which breaks time-reversal symmetry. The current is revealed only by its magnetic moment μ. And although researchers confirmed the effect early on, mostly using superconducting quantum interference devices (SQUIDs), complete agreement between theory and experiment, and even among experiments, has remained elusive. Jack Harris and colleagues from Yale University and the Free University of Berlin have now developed an elegantly simple alternative measurement scheme. The team deposited aluminum rings on a cantilever whose vibration frequency can be precisely monitored. In a magnetic field B, each ring’s current produces a torque τ = μ × B, recorded as a shift in the cantilever’s resonance frequency of vibration. From that frequency shift, the researchers deduce the current with a precision two orders of magnitude greater than is possible using SQUIDs. For a magnetic flux threading the ring, the current exhibits an Aharonov–Bohm effect, measurable as oscillations, shown here, whose period corresponds to the addition of one flux quantum h/e through the ring. In experiments taken over a broad range of fields, temperatures, and ring sizes, Harris and coworkers find perfect agreement with a noninteracting electron model. (A. C. Bleszynski-Jayich et al., Science 326, 272, 2009.)—R. Mark Wilson

Colossal magnetoresistance is aptly named. By subjecting a piece of appropriately doped manganite to a strong magnetic field and a moderately low temperature, one can raise its electrical conductivity by 10 000%. Despite its prodigious magnitude, CMR has not led to any commercial devices since its discovery 15 years ago. Still, it continues to fascinate physicists. At its most basic level, CMR arises as a paramagnetic insulating phase yields to a ferromagnetic conducting phase. But evidence suggests that a third, charge-ordered antiferromagnetic phase could play a role too. To elucidate the issue, Jing Tao of Brookhaven National Laboratory and her collaborators developed a new experimental technique. Called scanning electron nanoscale diffraction (SEND), their technique combines electron diffraction's ability to reveal the presence of ordered structures with scanning microscopy's ability to reveal those structures' real-space distribution. The patches in the figure correspond to charge-ordered regions. As the temperature approaches the 253-K CMR transition, the volume occupied by the charge-ordered phase increases. Simulations by Elbio Dagotto of Oak Ridge National Laboratory and his collaborators suggest an explanation: The charge-ordered phase vies with the ferromagnetic phase to become the predominant phase below the transition temperature. Although it loses the battle, the charge-ordered phase nevertheless delays and thereby intensifies the onset of CMR. (J. Tao et al., Phys. Rev. Lett. 103, 097202, 2009.)—Charles Day

Two-dimensional spectroscopy of materials, in its simplest form, measures the change in a sample’s interaction with radiation at one frequency caused by excitation at another. First used in nuclear magnetic resonance and later applied at IR and optical frequencies, the technique can separate overlapping peaks in a complicated spectrum or probe the flow of energy through systems such as the light-harvesting molecules involved in photosynthesis. (See Physics Today, July 2005, page 23.) In 2D Fourier-transform optical (FTOPT) spectroscopy, the sample is excited not with monochromatic light but with a pair of phase-related femtosecond pulses separated by a variable delay t₁; the time-domain data are transformed into a spectrum as a function of frequency ω₁. The second field is likewise replaced by a pair of phase-related pulses, one of which is a so-called local oscillator that allows the radiated field to be recorded as a function of time t₃ or frequency ω₃. Now, Keith Nelson and colleagues at MIT have produced 3D FTOPT spectra, in which the signal is a function of ω₁, ω₃, and also ω₂, corresponding to the time t₂ between the two pulse pairs. Applying their technique to semiconductor quantum wells—a system in which electrons are excited from nondegenerate valence-band states into the same conduction-band states—they found that important pathways whose signals were indistinguishable in a 2D spectrum could often be separated in a 3D spectrum. (D. B. Turner et al., J. Chem. Phys., in press.) —Johanna Miller

At absolute zero, a two-dimensional array of identical particles will crystallize into a hexagonal close-packed lattice. At high temperature, the lattice melts. What happens in between has interested physicists for decades. In the 1970s, theorists predicted that 2D melting would proceed via a so-called hexatic state in which the crystal breaks up into patches of local orientational order. The hexatic state's existence has been inferred from changes in resistance and other sample-averaged quantities. Now, the whole melting process—from a crystal through a hexatic state to a liquid—has been directly imaged. Isabel Guillamón of the Autonomous University of Madrid, Spain, and her collaborators melted a lattice of superconducting vortices that forms in a thin film of tungsten under a magnetic field. For their imager, they used a scanning tunneling microscope, which can distinguish the non-superconducting vortex cores from the vortices themselves. As the movie shows, the vortices start off with hexagonal order. As the temperature increases, pentagonal (gold) and heptagonal (green) defects appear that cause dislocations (solid magenta lines). On further heating, the vortices become mobile. Just above 2 K, they move too quickly for the STM to track; they appear as white stripes, whose ordering resembles a liquid crystal's smectic phase. By 3 K, which is 1 K below the film's T_c, the lattice melts completely and the vortices move freely in a gray, undifferentiated blur. (I. Guillamón et al., Nat. Phys., in press.)—Charles Day

The hallmark of a conventional crystal such as table salt is translational symmetry. Quasicrystals do not have that symmetry and so can exhibit a wider structural variety than their more constrained brethren. But quasicrystals, like crystals, do have long-range correlations and display sharp, structure-revealing diffraction patterns. To date, more than 100 quasicrystals have been synthesized in the lab. Now Luca Bindi of the Natural History Museum of Florence has teamed up with Paul Steinhardt and colleagues from Princeton and Harvard universities to present evidence for a natural version of one of those quasicrystals: icosahedral Al₆₃Cu₂₄Fe₁₃. The material, a 100-μm grain, is from a mineral assemblage (left figure) excavated from the Koryak Mountains in Russia and now housed in the Florence museum; the very complexity of the sample argues for its natural formation. In consultation with his US-based colleagues, Bindi identified the sample as possibly hosting a quasicrystal. The US team then probed a small piece of it with transmission electron microscopy. Diffraction patterns such as shown in the right figure identified quasicrystal regions; the 10-fold symmetry cannot be generated by crystals. Subsequent analysis of x rays scattered off pure quasicrystal grains determined the material’s chemical formula. Geologists and physicists have much to learn about the conditions under which quasicrystals form. The study of natural materials can help address that question and may turn up new, never-before contemplated structures. (L. Bindi et al., Science 324, 1306, 2009.) —Steven K. Blau

Below 68 °C vanadium dioxide is an insulator. Above that temperature it’s a metal. The nature of the transition has long remained elusive, though, because bulk VO₂ has a domain structure that complicates its behavior. David Cobden and colleagues from the University of Washington have found an elegant way to avoid that difficulty and map the effective phase diagram. The team grew rectangular nanobeams that were thinner than the characteristic domain size of a few microns. They then suspended each beam between two electrical contacts. The metallic and insulating phases differ in lattice constant, but the constrained geometry creates a uniform stress field in a VO₂ beam such that the two phases coexist in a range of temperatures between 68 °C and 105 °C. Thanks to the dramatic change in optical properties that accompanies the transition, Cobden’s team could visually track the nucleation and growth of the metallic phase as a function of temperature. The figure here shows five snapshots of a 20-μm-long beam (red indicates the insulating phase). By measuring the electrical resistance in the coexistence regime, the researchers found that the resistivity of the insulating phase is independent of temperature. That remarkable result, they argue, implies that the phase transition occurs at a fixed carrier density in the material and is consistent with a picture in which electron–electron interactions drive the transition. (J. Wei, Z. Wang, W. Chen, D. H. Cobden, Nat. Nanotech., doi:10.1038/nnano.2009.141, in press.) —R. Mark Wilson

Decades ago, theorists predicted that under some circumstances, solids could flow like superfluids. In 2004 Moses Chan and Eun-Seong Kim found evidence of such so-called supersolidity: When a torsion oscillator filled with solid helium was cooled below 200 mK, its resonant frequency increased. Some of the He’s mass appeared to have decoupled from the rest. But subsequent experiments revealed a more complicated picture with many aspects unexplained. For example, the oscillator’s dissipation (related to the damping strength) depended on temperature in a way that the theory hadn’t predicted. Now, Cornell University’s Séamus Davis and colleagues have developed a torsion oscillator, shown in the figure, whose position sensor is a superconducting quantum interference device rather than the usual capacitor. The SQUID allows them to measure the dissipation more accurately and to explore a broader range of frequencies and amplitudes than was previously possible. Among their results is the discovery that when the temperature is abruptly lowered, the oscillator’s resonant frequency and dissipation share the same response time constant, which increases steeply with decreasing temperature—much like the characteristic flow time of cooling molten window glass. Some theorists have postulated that He’s behavior results from an ordinary glass transition, not from supersolidity. The relative magnitudes of the changes in frequency and dissipation rule out that possibility. But the ultraslow low-temperature behavior suggests that a glasslike phase may be involved. (B. Hunt et al., Science 324, 632, 2009.) —Johanna Miller

Late Stone Age metal smiths added a little tin to copper to usher in the eponymous Bronze Age; over the ensuing five millennia, many new combinations and applications of the two metals have appeared. Today, for example, a thin tin coating on a copper substrate often serves to interconnect electronic components of various kinds, such as are found in medical devices and satellite equipment. Unfortunately, micron-sized tin whiskers (see figure) sometimes arise spontaneously and can short out the equipment, with great technological and economic repercussions. After decades of widespread effort, the actual mechanism underlying such whisker growth has only now been elucidated. Led by Eric Mittemeijer, a group from the Max Planck Institute for Metals Research in Stuttgart, Germany, working with the Robert Bosch company, examined growing whiskers and their crystallographic environment. Using Laue diffraction measurements made at the Advanced Photon Source at Argonne National Laboratory in Illinois, the researchers noted that at the Cu–Sn interface, Cu₆Sn₅ develops along the tin grain boundaries and is most pronounced directly beneath a whisker's root. That observation, coupled with residual strain measurements, led the team to propose the following mechanism: Deep penetration of Cu₆Sn₅ into the 3-μm-thick tin layer induces in-plane compressive strains near the Cu–Sn interface and in-plane tensile strains nearer the surface. Out-of-plane and in-plane strain gradients—not the strains themselves—then provide the driving force that leads to whisker growth by transporting Sn atoms to the whisker nucleation site as a strain-relief mechanism. (M. Sobiech et al., Appl. Phys. Lett., in press.) —Stephen G. Benka

Before they form snowflakes and other hexagonal crystals, water molecules nucleate in smaller configurations. Determining the structure of those precursors—even in the outwardly simple case of water on a clean metal surface—is an area of ongoing interest and controversy. For example, at submonolayer coverage on a copper (110) surface, water molecules form chains that can grow to many tens of nanometers in length but are just 1 nm wide. The chains’ structure has been a mystery, since no arrangement of water molecules into hexagonal units entirely explains the experimental data. Now, Andrew Hodgson and colleagues of the University of Liverpool in the UK have collaborated with Angelos Michaelides’ group at University College London to find the structure. Michaelides and postdoc Javier Carrasco ran calculations on some 50 possible chain structures; they found that the most energetically stable one also gave the best fit to the Liverpool group’s high-resolution scanning tunneling microscopy images (as shown in the top panel) and vibrational spectra. That structure (bottom panel) is an arrangement of pentagons, not hexagons. The water molecules shown in red and yellow are perpendicular to the Cu surface—the hydrogen atoms pointing up are responsible for the bright spots in the STM images, and the ones pointing down (not visible in the figure) interact with the Cu atoms. The researchers suggest that nonhexagon arrangements might be involved at other water–metal interfaces where the structure of water is unknown. (J. Carrasco et al., Nat. Mater., doi:10.1038/nmat2403.) — Johanna Miller

Spin–orbit coupling is a double-edged sword to physicists who want to exploit the spin degree of freedom in novel electronic devices. On the one hand, the coupling treats up spins differently from down spins, a necessary feature of any spintronic device. But on the other hand, the coupling nullifies the conservation of spin that prevails in free space. Although the net spin polarization averages to zero in a crystal, it fluctuates randomly and locally. Joseph Orenstein of the University of California, Berkeley, and Lawrence Berkeley National Laboratory and his collaborators have demonstrated a way to restore the conservation of spin in a semiconductor quantum well and extend the lifetime of a coherent spin structure. Two tricks are involved. The first is to build a quantum well in which two types of spin–orbit coupling, Rashba and Dresselhaus, are equal. The second trick is to create a particular kind of coherent signal, a spin helix, and send it through the quantum well with a particular wave vector. Mathematically, the spin helix in its specially tuned well shares the same SU(2) symmetry as an isolated spin. Empirically, the spin helix retains its coherence for 1 ns before it diffuses away. That lifetime may seem fleeting, and it's hardly infinite, but it's an order of magnitude longer than that of a coherent spin signal launched without symmetry's sustaining power. To learn more about the spin helix and two other recently observed spin textures, look for the news story on page 12 of the April issue. (J. D. Koralek et al., Nature, in press.) — Charles Day