International Conference on Condensed Matter Physics

For a long time, researchers assumed that phonons could not contribute to the thermal Hall effect because of their lack of charge and spin. New work challenges this assumption. Figure 1:   Only scattering processes that involve at least two (virtual) collisions with collective fluctuations can contribute to a Hall effect. Now researchers show that phonon scattering by a fluctuating quantum field meets this condition and leads to a phonon Hall effect. The results resolve the “Achilles’ heel” of thermal conductivity measurements. How heat flows in interacting quantum many-body systems is one of the most interesting open problems in condensed-matter physics. Understanding thermal transport is particularly challenging in systems where charge-carrier contributions to energy transport are strongly suppressed, such as in insulators and superconductors. In such systems, heat transport cannot therefore be understood in terms of electronic carriers alone. In insulators, acoustic phonons are...

  By firing laser pulses of two different colors at a nanosized metal tip, researchers create an interference effect that turns electron emission on and off with femtosecond timing. Figure 1: In a two-color scheme for controlling electron emission, a strong laser pulse at the fundamental frequency (ω)(𝜔) is combined with a much weaker pulse at the second harmonic (2ω)(2𝜔). When the pulses strike a nanotip, they can excite electrons above the emission energy threshold (solid yellow line on right). Two photoemission pathways exist along the intermediate states (dashed white lines). Interference between these pathways can switch the emission on or off, depending on the relative phase between the fundamental and second harmonic pulses. The ability to control electron pulses on the femtosecond time scale is at the heart of several ambitious research endeavors, ranging from ultrafast electron microscopes [1, 2] to tabletop particle accelerators and intense x-ray sources [3]. A first ap...

A special microscope has visualized changes of electron current distribution that clearly indicate a transition from ohmic to viscous electron flow in graphene. Sketch of the experimental setup used by Jenkins and colleagues [1]. A nitrogen-vacancy-center probe scans the area around a constriction etched in graphene, imaging the current flowing through the constriction. Imagine a breeze of moist air condensing into water drops and dripping down on a cold glass. Electrons can undergo a transition that resembles this gas-to-fluid condensation: the transition is controlled by temperature and produces a fluid-like state in which electrons display remarkably different dynamics than in the gas-like state. Unlike the condensation of water vapor, however, the electron transition cannot be directly imaged with a camera. One reason for this difficulty is that the pattern of this electron fluid varies at submicron scales that can’t be clearly resolved by visible light. Another reason is that elec...

Single electrons stay stationary in superconductors with “flat-band” electronic structures, which could lead to low-energy-consumption devices made from such materials. In 2018, researchers discovered that two layers of graphene, stacked and twisted at a specific angle, could exhibit superconductivity. Theorists have determined that the electronic structure of such a twisted material approximately resembles a “flat band,” which means that the energy of the materials’ free electrons remains constant regardless of the electrons’ momenta. This phenomenon inspired a flurry of work on systems that exhibit flat-band superconductivity. However, most of the research has focused on how such systems behave under equilibrium conditions. Now Päivi Törmä of Aalto University in Finland and her colleagues have probed the behavior of superconducting flat-band systems under nonequilibrium conditions . The findings could help in the design of superconducting devices with low energy consumption. Törmä a...

Researchers have demonstrated that a solid can exhibit an enhanced nonlinear optical phenomenon usually seen only in cold atomic gases. A light wave propagating in a vacuum (top) has its wavelength shortened when it propagates in a high-refractive-index medium such as Cu2O (middle and bottom). Because of the Kerr effect, the refractive index of the material depends on the intensity of the light. A medium-intensity beam (middle) generates a certain density of Rydberg excitons (orange circles) and induces a change in the refractive index. A high-intensity beam (bottom) generates a greater density of Rydberg excitons and produces a higher modification in the refractive index. Dotted lines show the exaggerated phase shift between medium- and high-intensity light. Among the benefits brought about by the invention of the laser in the 1960s is the ability to generate light at an intensity great enough to produce nonlinear optical effects. Such nonlinear effects have entered daily use in app...

  A new observation of heat transport by surface waves involving electrons could lead to improved cooling strategies for microscale electronic components. Carried away. When a laser (dark green) creates a hot spot in a disk-shaped titanium thin film sitting on a slab of silicon dioxide, surface electromagnetic excitations called surface plasmon polaritons (light green) can help to dissipate the heat. The effect is larger in circular films with radii of 2 cm or more. As electronic devices get ever smaller, keeping them cool is a major challenge. Even tiny electric currents can produce enough heating to damage very small components. New experiments show that surface electron waves called surface plasmon polaritons (SPPs) could help to dissipate heat in microscale devices [ 1 ]. The researchers measured the heat carried by these surface excitations in thin metal films and found that the SPP contribution could boost the total heat dissipation by as much as a quarter. Further engineerin...

  Germanene undergoes a topological phase transition and then becomes a normal insulator when the strength of an applied electric field is dialed up. Graphene’s valence and conduction bands meet at a point, making the single-layer crystal a semimetal. Researchers have predicted that spin-orbit coupling of carbon’s outer electrons opens a narrow gap between these bands—but only for the crystal’s bulk. Along the edges, spin-dependent states bridge the band gap, allowing the resistance-free flow of electrons: a quantum spin Hall effect. The weakness of carbon’s spin-orbit coupling means that this quantum spin Hall effect is too fragile to observe, however. Now Pantelis Bampoulis of the University of Twente in the Netherlands and his collaborators have seen the quantum spin Hall effect in graphene’s germanium (Ge) analog, germanene [ 1 ]. Furthermore, they show that germanene’s structure—a honeycomb like graphene’s, but lightly buckled—allows the quantum spin Hall effect to be turned o...

  Soft-matter physics, is a young sub-field of condensed matter physics. This field is generally described as materials oriented with a strong focus on understanding macromolecular assemblies. These meso-scale or medium sized constituents often self-assemble or organize into macro-scale materials and demonstrate many novel and unexpected phenomenon. Many of these materials are extremely familiar from everyday life and have a vast number of technologically important applications. For example we utilize personal care and food products that are comprised of materials such as liquid crystals, gels, foams, polymers, granular matter and emulsions just to name a few. Many current theoretical descriptions of soft matter are derived through classical physics, and are often described using the tools of equilibrium and non-equilibrium statistical mechanics, symmetry breaking and many body physics. At Georgetown, we are interested in developing tools and methods to better describe these comple...

  Ceramic solid electrolyte cells (vertical rectangular shapes) with dendrites (the lightning-like dark structures inside the rectangles) growing inside them from the bottom to the top. These solid electrolytes are floating on a liquid (blue puddle) which represents a liquid electrolyte. The reflections of the solid electrolytes in the blue liquid, particularly the dark dendrites, show the similarity in the dendrite initiation process in both the liquid and solid. (Photo: Rajeev Gopal/Bai lab) “Our results reveal surprising similarities between the liquid and the solid electrolytes, and that allows us to borrow some ideas from the successful liquid electrolytes to help our design of the solid electrolytes,” said Peng Bai, an assistant professor of energy, environmental and chemical engineering. “Before our work, solid electrolytes, at least the ceramic ones we studied here, are considered distinctly different from their liquid counterparts.” Batteries power so much of our lives, so...

A team of researchers based in Manchester, the Netherlands, Singapore, Spain, Switzerland and the USA has published a new review on a field of computer device development known as spintronics, which could use graphene as a building block for next-generation electronics. Recent theoretical and experimental advances and phenomena in studies of electronic spin transport in graphene and related two-dimensional (2D) materials have emerged as a fascinating area of research and development. Spintronics is the combination of electronics and magnetism at nanoscale and could allow electronic development at speeds exceeding Moore’s law, which observes that computer processing power roughly doubles every two years, while the price halves. Spintronic devices may offer higher energy efficiency and lower dissipation as compared to conventional electronics, which rely on charge currents. In principle, we could have phones and tablets operating with spin-based transistors and memories, greatly improvi...