Liquid Bi shows a peculiar dispersion of the acoustic mode, which is related to the Peierls distortion in the crystalline state. These results ("Anomalous dispersion of the acoustic mode in liquid Bi") will provide valuable inspiration to researchers developing new materials in the nanotechnology field. 
Fig.1: Momentum dependence of excitation energy. Black dots: Energy of acoustic mode obtained from IXS experiments. Dashed line (blue): The energy of the acoustic mode given by the chain model shown in Fig. 2(a). Solid line (red): The energy of the acoustic mode given by the chain model shown in Fig. 2(b) (Image: M. Inui, Graduate School of Integrated Arts and Sciences, Hiroshima University, et al.) Studies of the atomic dynamics in liquid Bi have been revisited more recently. The previous inelastic neutron scattering (INS) results for liquid Bi showed inconsistency for the inelastic excitation of the acoustic mode. These results were also different from the ab initio molecular dynamics (AIMD) prediction that indicated that the peculiar atomic dynamics arose from an anisotropic interatomic force in this monatomic liquid (J. Souto et al., 81, 134201 (2010)). Therefore, it is important to observe the inelastic excitation of the acoustic mode in liquid Bi using inelastic x-ray scattering (IXS). Professor M. Inui at Hiroshima University and his collaborators at Kumamoto University, Keio University, SPring-8/JASRI, and the RIKEN SPring-8 Center measured the IXS on liquid Bi at SPring-8 (A.Q.R Baron et al., , 61, 461 (2000)). This research group found that the dispersion curve of the excitation energy of the acoustic mode exhibits a flat region as a function of the momentum transfer. 
Fig.2: Analytical model. (a) One-dimensional chain where atoms are connected by springs with the same force constant. (b) One-dimensional chain where n atomic pairs connected by a strong spring are connected by weak springs. (Image: M. Inui, Graduate School of Integrated Arts and Sciences, Hiroshima University, et al.) The experiments conducted by Professor Inui et al. used a single-crystal sapphire cell of the Tamura type that was carefully machined to provide a 0.04-mm sample thickness. It is said that only his research group can make full use of this “world-famous” cell, which was used to stably conduct an x-ray beam experiment under high temperatures. Furthermore, this research group reported that the IXS experimental results for liquid Bi clearly show a distinct inelastic excitation of the acoustic mode. This resolves the previous disagreement in the literature. Those researchers said, “Consistent with ab initio calculations of liquid Bi, the dispersion curve was nearly flat from 7 to 15 nm [to the negative 1 power].” 
Fig.3: Structure of crystalline bismuth. Schematic picture using simple cubic lattice, where bold and broken lines denote short strong bonds and long weak ones, respectively. (Image: M. Inui, Graduate School of Integrated Arts and Sciences, Hiroshima University, et al.) They also mentioned, “A long-range force is needed to reproduce the flatness of the dispersion curve, and the long-range force has to strongly be related to a local structure consisting of shorter and longer bounds in the liquid.” This research group demonstrated a possible mechanism for the unusual dispersion of liquid Bi. Their results will greatly contribute to the development of nanotechnology.
read more "Draw out of the predicted interatomic force"
Fig.1: Momentum dependence of excitation energy. Black dots: Energy of acoustic mode obtained from IXS experiments. Dashed line (blue): The energy of the acoustic mode given by the chain model shown in Fig. 2(a). Solid line (red): The energy of the acoustic mode given by the chain model shown in Fig. 2(b) (Image: M. Inui, Graduate School of Integrated Arts and Sciences, Hiroshima University, et al.) Studies of the atomic dynamics in liquid Bi have been revisited more recently. The previous inelastic neutron scattering (INS) results for liquid Bi showed inconsistency for the inelastic excitation of the acoustic mode. These results were also different from the ab initio molecular dynamics (AIMD) prediction that indicated that the peculiar atomic dynamics arose from an anisotropic interatomic force in this monatomic liquid (J. Souto et al., 81, 134201 (2010)). Therefore, it is important to observe the inelastic excitation of the acoustic mode in liquid Bi using inelastic x-ray scattering (IXS). Professor M. Inui at Hiroshima University and his collaborators at Kumamoto University, Keio University, SPring-8/JASRI, and the RIKEN SPring-8 Center measured the IXS on liquid Bi at SPring-8 (A.Q.R Baron et al., , 61, 461 (2000)). This research group found that the dispersion curve of the excitation energy of the acoustic mode exhibits a flat region as a function of the momentum transfer. Fig.2: Analytical model. (a) One-dimensional chain where atoms are connected by springs with the same force constant. (b) One-dimensional chain where n atomic pairs connected by a strong spring are connected by weak springs. (Image: M. Inui, Graduate School of Integrated Arts and Sciences, Hiroshima University, et al.) The experiments conducted by Professor Inui et al. used a single-crystal sapphire cell of the Tamura type that was carefully machined to provide a 0.04-mm sample thickness. It is said that only his research group can make full use of this “world-famous” cell, which was used to stably conduct an x-ray beam experiment under high temperatures. Furthermore, this research group reported that the IXS experimental results for liquid Bi clearly show a distinct inelastic excitation of the acoustic mode. This resolves the previous disagreement in the literature. Those researchers said, “Consistent with ab initio calculations of liquid Bi, the dispersion curve was nearly flat from 7 to 15 nm [to the negative 1 power].” Fig.3: Structure of crystalline bismuth. Schematic picture using simple cubic lattice, where bold and broken lines denote short strong bonds and long weak ones, respectively. (Image: M. Inui, Graduate School of Integrated Arts and Sciences, Hiroshima University, et al.) They also mentioned, “A long-range force is needed to reproduce the flatness of the dispersion curve, and the long-range force has to strongly be related to a local structure consisting of shorter and longer bounds in the liquid.” This research group demonstrated a possible mechanism for the unusual dispersion of liquid Bi. Their results will greatly contribute to the development of nanotechnology.
Conventional superconductors only achieve their zero-resistance state at close to absolute zero. A better understanding of the physical mechanisms responsible for the binding of electron pairs that gives rise to superconductivity could lead to the discovery of room-temperature superconductors. Superconductors conduct electricity with zero resistance, and hence they could potentially revolutionize electric motors, generators and utility grids. However, scientists have yet to discover a material that becomes superconducting at ambient temperature—all known superconductors operate only at cryogenic temperatures, making them impractical for general applications. Unfortunately, progress toward achieving the goal of room-temperature superconductivity has been hindered by scientists’ limited understanding of the fundamental mechanism responsible for the emergence of this remarkable physical phenomenon. Superconductivity occurs as the result of pairs of electrons binding together in such a way that they act as a single quasiparticle. In conventional superconductors, which include elemental materials that become superconducting at temperatures very close to absolute zero, the binding force is provided by vibrations in the atomic lattice through which the electrons travel. Yet not all superconductors behave this way. In unconventional superconductors that do not fit the conventional model, this binding force develops in a different manner and various mechanisms have been proposed for it. One such mechanism is the magnetic or spin fluctuation of the electrons themselves, which binds electrons in pairs through the entanglement of electron spins. However, recent experiments have shown that this mechanism cannot explain the superconducting state in the quintessential unconventional superconductor CeCu
A polaron (blue) is a particle such as an electron that moves through and interacts with a material. Polarons interact with atomic vibrations (phonons) in a ‘phonon bath’ (gridlines) defined by the crystal structure. (Image: Naoto Nagaosa, RIKEN Center for Emergent Matter Science) The movement of electrons in crystals is one of the defining characteristics of materials and determines their behavior in many practical applications. As electrons move through a crystal, they interact with surrounding atoms via atomic vibrations known as phonons. A particle moving in such a ‘phonon bath’ is known as a polaron (Fig. 1). “Polarons occur in almost every transport phenomenon in solids,” explains Nagaosa. Despite their ubiquity, however, deriving a mathematical description of polarons has proved a challenge that has confounded even some of our most famous physicists. The problem is the difficulty of reducing the complexity of interactions that make up a polaron to a few basic simplifications. Although this strategy works well for many problems in physics, phonon systems are so complex they defy simplification. Consequently, previous approaches have been limited to approximations only. In contrast, Nagaosa and his colleagues used mathematically exact computational methods without approximations to calculate results for specific scenarios. They then mapped the information gained from these calculations onto a two-dimensional ‘phase diagram’ of temperature versus strength of interaction between the electron and the surrounding phonon bath. The phase diagram revealed a strong dependence of polaron transport on temperature and strength of interaction, with several distinct transport regimes that explain many of the observed fundamental properties of materials, such as the electrical conductivity of metals and semiconductors. “The study of polarons and particularly polaron mobility is important for technology because polarons are carriers in many modern electronic devices,” says Mishchenko. Although derived in general terms, the researchers’ calculations and the resultant phase diagram are so far limited to describing polarons in one dimension. Extending the theory to higher dimensions will allow a more realistic description of polaron behavior and could lead to a fundamental model that describes a broad range of important effects in materials, such as magnetism and superconductivity.