Alexander V. Balatsky

Spin Current and Magnetoelectric Effect in Noncollinear Magnets

A new mechanism of the magnetoelectric effect based on the spin supercurrent is theoretically presented in terms of a microscopic electronic model for noncollinear magnets. The electric polarization P(ij) produced between the two magnetic moments S(i) and S(j) is given by P proportional e(ij) X (S(i) X S(j)) with e(ij) being the unit vector connecting the sites i and j. Applications to the spiral spin structure and the gauge theoretical interpretation are discussed.

Mott Transition in VO2 Revealed by Infrared Spectroscopy and Nano-Imaging

Electrons in correlated insulators are prevented from conducting by Coulomb repulsion between them. When an insulator-to-metal transition is induced in a correlated insulator by doping or heating, the resulting conducting state can be radically different from that characterized by free electrons in conventional metals. We report on the electronic properties of a prototypical correlated insulator vanadium dioxide in which the metallic state can be induced by increasing temperature. Scanning near-field infrared microscopy allows us to directly image nanoscale metallic puddles that appear at the onset of the insulator-to-metal transition. In combination with far-field infrared spectroscopy, the data reveal the Mott transition with divergent quasi-particle mass in the metallic puddles. The experimental approach used sets the stage for investigations of charge dynamics on the nanoscale in other inhomogeneous correlated electron systems.

Interplay of electron-lattice interactions and superconductivity in Bi2Sr2CaCu2O8+δ

Formation of electron pairs is essential to superconductivity. For conventional superconductors, tunnelling spectroscopy has established that pairing is mediated by bosonic modes (phonons); a peak in the second derivative of tunnel current d2I/dV2 corresponds to each phonon mode. For high-transition-temperature (high-Tc) superconductivity, however, no boson mediating electron pairing has been identified. One explanation could be that electron pair formation and related electron–boson interactions are heterogeneous at the atomic scale and therefore challenging to characterize. However, with the latest advances in d2I/dV2 spectroscopy using scanning tunnelling microscopy, it has become possible to study bosonic modes directly at the atomic scale. Here we report d2I/dV2 imaging studies of the high-Tc superconductor Bi2Sr2CaCu2O8+δ. We find intense disorder of electron–boson interaction energies at the nanometre scale, along with the expected modulations in d2I/dV2 (refs 9, 10). Changing the density of holes has minimal effects on both the average mode energies and the modulations, indicating that the bosonic modes are unrelated to electronic or magnetic structure. Instead, the modes appear to be local lattice vibrations, as substitution of 18O for 16O throughout the material reduces the average mode energy by approximately 6 per cent—the expected effect of this isotope substitution on lattice vibration frequencies. Significantly, the mode energies are always spatially anticorrelated with the superconducting pairing-gap energies, suggesting an interplay between these lattice vibration modes and the superconductivity.

Superconductivity and quantum criticality in CeCoIn5

Electrical resistivity measurements on a single crystal of the heavy-fermion superconductor CeCoIn5 at pressures to 4.2 GPa reveal a strong crossover in transport properties near P(*) approximately 1.6 GPa, where T(c) is a maximum. The temperature-pressure phase diagram constructed from these data provides a natural connection to cuprate physics, including the possible existence of a pseudogap.

Scanning tunneling microscopy of DNA-wrapped carbon nanotubes.

We employ scanning tunneling microscopy (STM) to reveal the structure of DNA-carbon nanotube complexes with unprecedented spatial resolution and compare our experimental results to molecular dynamics simulations. STM images show strands of DNA wrapping around (6,5) nanotubes at approximately 63 degrees angle with a coiling period of 3.3 nm, in agreement with the theoretical predictions. In addition, we observe width modulations along the DNA molecule itself with characteristic lengths of 1.9 and 2.5 nm, which remain unexplained. In our modeling we use a helical coordinate system, which naturally accounts for tube chirality along with an orbital charge density distribution and allows us to simulate this hybrid system with the optimal pi-interaction between DNA bases and the nanotube. Our results provide novel insight into the self-assembling mechanisms of nanotube-DNA hybrids and can be used to guide the development of novel DNA-based nanotube separation and self-assembly methods, as well as drug delivery and cancer therapy techniques.

IMPURITY-INDUCED VIRTUAL BOUND STATES IN D-WAVE SUPERCONDUCTORS

It is shown that a single, strongly scattering impurity produces a bound or a virtual bound quasiparticle state inside the gap in a

Imaging the Fano lattice to ‘hidden order’ transition in URu2Si2

d

Dynamical magnetoelectric coupling in helical magnets.

-wave superconductor. The explicit form of the bound state wave function is found to decay exponentially with angle-dependent range. These states provide a natural explanation of the second Cu NMR rate arising from the sites close to Zn impurities in the cuprates. Finally, for finite concentration of impurities in a

Theory of Scanning Tunneling Microscopy Probe of Impurity States in a D-Wave Superconductor.

d

Infrared spectroscopy and nano-imaging of the insulator-to-metal transition in vanadium dioxide

-wave superconductor, we reexamine the growth of these states into an impurity band, and discuss the Mott criterion for this band.