Charge density waves in individual nanoribbons of orthorhombic-TaS3.

Orthorhombic-TaS3 is a quasi-1D material that undergoes a Peierls' transition to become a charge density wave conductor at low temperatures. Electrical transport measurements of individual single-crystalline TaS3 nanoribbons prepared by a novel bottom-up method from elemental precursors indicate a depression of the Peierls' ordering temperature to 205 K, broadening of the electric-field-induced depinning of the charge density wave below the Peierls' transition temperature, and a greatly increased threshold voltage for nucleation of charge density wave dislocations posited to be a result of surface confinement and finite size effects. Single-nanoribbon measurements of broad-band noise indicate discrete phase slip events near the depinning threshold. Three distinct regimes are identified with the normalized noise spectrum showing a distinctive maxima near the threshold voltage for depinning of the charge density wave, corresponding to sampling of different metastable states that balance ordered and sliding charge density waves.

Scalable hydrothermal synthesis of free-standing VO2 nanowires in the M1 phase.

VO2 nanostructures derived from solution-phase methods are often plagued by broadened and relatively diminished metal-insulator transitions and adventitious doping due to imperfect control of stoichiometry. Here, we demonstrate a stepwise scalable hydrothermal and annealing route for obtaining VO2 nanowires exhibiting almost 4 orders of magnitude abrupt (within 1 °C) metal-insulator transitions. The prepared nanowires have been characterized across their structural and electronic phase transitions using single-nanowire Raman microprobe analysis, ensemble differential scanning calorimetry, and single-nanowire electrical transport measurements. The electrical band gap is determined to be 600 meV and is consistent with the optical band gap of VO2, and the narrowness of differential scanning calorimetry profiles indicates homogeneity of stoichiometry. The preparation of high-quality free-standing nanowires exhibiting pronounced metal-insulator transitions by a solution-phase process allows for scalability, further solution-phase processing, incorporation within nanocomposites, and integration onto arbitrary substrates.

Intermediate metallic phase in VO2 observed with scanning tunneling spectroscopy.

This investigation focuses on the formation of nanoscale puddles of an intermediate metallic phase (IMP) in the metal-insulator transition (MIT) temperature regime of single-crystalline vanadium dioxide (VO2) nanowires. The electronic structure of VO2 nanowires was examined with scanning tunneling spectroscopy. The evolution of the local density of states of individual nanowires throughout the MIT regime is presented with differential tunneling conductance spectra and images measured as the temperature was increased. Our results show that the formation of an IMP plays an important role in the MIT of intrinsic VO2.

Microwave spectroscopy evidence of superconducting pairing in the magnetic-field-induced metallic state of InO(x) films at zero temperature.

We investigate the field-tuned quantum phase transition in a 2D low-disorder amorphous InO(x) film in the frequency range of 0.05 to 16 GHz employing microwave spectroscopy. In the zero-temperature limit, the ac data are consistent with a scenario where this transition is from a superconductor to a metal instead of a direct transition to an insulator. The intervening metallic phase is unusual with a small but finite resistance that is much smaller than the normal state sheet resistance at the lowest measured temperatures. Moreover, it exhibits a superconducting response on short length and time scales while global superconductivity is destroyed. We present evidence that the true quantum critical point of this 2D superconductor metal transition is located at a field B(sm) far below the conventionally defined critical field B(cross) where different isotherms of magnetoresistance cross each other. The superfluid stiffness in the low-frequency limit and the superconducting fluctuation frequency from opposite sides of the transition both vanish at B≈B(sm). The lack of evidence for finite-frequency superfluid stiffness surviving B(cross) signifies that B(cross) is a crossover above which superconducting fluctuations make a vanishing contribution to dc and ac measurements.

Measurements of the magnetic-field-tuned conductivity of disordered two-dimensional Mo43Ge57 and InOx superconducting films: evidence for a universal minimum superfluid response.

Our measurements of the low frequency ac conductivity in strongly disordered two-dimensional films near the magnetic-field-tuned superconductor-to-insulator transition show a sudden drop in the phase stiffness of superconducting order with either increased temperature or magnetic field. Surprisingly, for two different material systems, the abrupt drop in the superfluid density in a magnetic field has the same universal value as that expected for a Berezinskii-Kosterlitz-Thouless transition in a zero magnetic field. The characteristic temperature at which phase stiffness is suddenly lost can be tuned to zero at a critical magnetic field, following a power-law behavior with a critical exponent consistent with that obtained in previous dc transport studies on the dissipative side of the transition.

Single-nanowire raman microprobe studies of doping-, temperature-, and voltage-induced metal-insulator transitions of W(x)V(1-x)O2 nanowires.

Considerable recent research interest has focused on mapping the structural phase diagrams of anisotropic VO(2) nanobeams as model systems for elucidating single-domain behavior within strongly correlated electronic materials, to examine in particular the coupling of lattice and orbital degrees of freedom. Nevertheless, the role of substitutional doping in altering the phase stabilities of competing ground states of VO(2) remains underexplored. In this study, we use individual nanowire Raman microprobe mapping to examine the structural phase progressions underlying the metal-insulator transitions of solution-grown W(x)V(1-x)O(2) nanowires. The structural phase progressions have been monitored for three distinctive modes of inducing the electronic metal-insulator phase transition: as a function of (a) W doping at constant temperature, (b) varying temperature for specific W dopant concentrations, and (c) varying applied voltage for specific W dopant concentrations. Our results suggest the establishment of a coexistence regime within individual nanowires wherein M1 and R phases simultaneously exist before the percolation threshold is reached and the nanowire becomes entirely metallic. Such a coexistence regime has been found to exist during both temperature- and voltage-induced transitions. No evidence of an M2 phase is observed upon inducing the electronic phase transition by any of the three distinctive methods (temperature, doping, and applied voltage), suggesting that substitutional tungsten doping stabilizes the M1 phase over its M2 counterpart and further corroborating that the latter phase is not required to mediate M1→R transformations.

Colossal above-room-temperature metal-insulator switching of a Wadsley-type tunnel bronze.

We show evidence of the manifestation of enormous metal-insulator switching ranging up to almost six orders of magnitude in individual nanowires of ß'-Cu(x)V(2)O(5). The magnitude and temperature of the phase transition is in strong contrast to data reported over several decades for other Wadsley-type mixed valence tunnel bronze structures.

Pinning-mode resonance of a Skyrme crystal near Landau-level filling factor ν=1.

Microwave pinning-mode resonances found around integer quantum Hall effects, are a signature of crystallized quasiparticles or holes. Application of in-plane magnetic field to these crystals, increasing the Zeeman energy, has negligible effect on the resonances just below Landau-level filling ν=2, but increases the pinning frequencies near ν=1, particularly for smaller quasiparticle or hole densities. The charge dynamics near ν=1, characteristic of a crystal order, are affected by spin, in a manner consistent with a Skyrme crystal.

Synthesis, spectroscopic characterization, and observation of massive metal-insulator transitions in nanowires of a nonstoichiometric vanadium oxide bronze.

Metal-insulator transitions in strongly correlated materials, induced by varying either temperature or dopant concentration, remain a topic of enduring interest in solid-state chemistry and physics owing to their fundamental importance in answering longstanding questions regarding correlation effects. We note here the unprecedented observation of a four-orders-of-magnitude metal-insulator transition in single nanowires of delta-K(x)V(2)O(5), when temperature is varied, which thus represents a rare new addition to the pantheon of materials exhibiting pronounced metal-insulator transitions in proximity to room temperature.

Pinning mode resonances of 2D electron stripe phases: effect of an in-plane magnetic field.

We study the anisotropic pinning-mode resonances in the rf conductivity spectra of the stripe phase of 2D electron systems around a Landau level filling of 9/2, in the presence of an in-plane magnetic field B(ip). The polarization along which the resonance is observed switches as B(ip) is applied, consistent with the reorientation of the stripes. The resonance frequency, a measure of the pinning interaction between the 2D electron systems and disorder, increases with B(ip). The magnitude of this increase indicates that disorder interaction is playing an important role in determining the stripe orientation.

Observation of pinning mode of stripe phases of 2D systems in high Landau levels.

We study the radio-frequency diagonal conductivities of the anisotropic stripe phases of higher Landau levels near half-integer fillings. In the hard direction, in which larger dc resistivity occurs, the spectrum exhibits a striking resonance, while in the orthogonal, easy direction, no resonance is discernible. The resonance is interpreted as a pinning mode of the stripe phase.

Nanowire acting as a superconducting quantum interference device.

We present the results from an experimental study of the magnetotransport of superconducting wires of amorphous indium-oxide having widths in the range 40-120 nm. We find that, below the superconducting transition temperature, the wires exhibit clear, reproducible, oscillations in their resistance as a function of magnetic field. The oscillations are reminiscent of those that underlie the operation of a superconducting quantum interference device.

Experimental evidence for a collective insulating state in two-dimensional superconductors.

We present the results of an experimental study of the current-voltage characteristics in a strong magnetic field (B) of disordered, superconducting, thin films of amorphous indium oxide. As the B strength is increased superconductivity degrades, until a critical field (B(c)) where the system is forced into an insulating state. We show that the differential conductance measured in the insulating phase vanishes abruptly below a well-defined temperature, resulting in a clear threshold for conduction. Our results indicate that a new collective state emerges in two-dimensional superconductors at high B.

Superconductivity-related insulating behavior.

We present the results of an experimental study of superconducting, disordered, thin films of amorphous indium oxide. These films can be driven from the superconducting phase to a reentrant insulating state by the application of a perpendicular magnetic field (B). We find that the high-B insulator exhibits activated transport with a characteristic temperature, TI. TI has a maximum value (TpI) that is close to the superconducting transition temperature (Tc) at B=0, suggesting a possible relation between the conduction mechanisms in the superconducting and insulating phases. Tp(I) and Tc display opposite dependences on the disorder strength.