James Valles - Academia.edu (original) (raw)

Papers by James Valles

arXiv (Cornell University), Feb 6, 2002

We have fabricated quasi-two-dimensional arrays of nano-scale Pb grains coupled by an overlayer o... more We have fabricated quasi-two-dimensional arrays of nano-scale Pb grains coupled by an overlayer of Ag grains. Their temperature dependent resistive transitions follow predictions for an array of mesoscopic superconductor-normal-superconductor junctions. The decrease of their transition temperatures with Ag overlayer thickness systematically deviates from the Cooper limit theory of the proximity effect as the Pb grain size decreases. The deviations occur when the estimated number of Cooper pairs per grain is less than or equal to 1 and suggest the approach to a superconductor to metal transition.

Bulletin of the American Physical Society, Mar 2, 2012

swim through the coordinated beating of the 1000's of cilia covering their body. We have measured... more swim through the coordinated beating of the 1000's of cilia covering their body. We have measured the swimming speed of populations of Paramecium Caudatam in solutions of different viscosity, η, to see how their propulsion changes with increased drag. We have found the average instantaneous speed, V to decrease monotonically with increasing η. The product ηv is roughly constant over a factor of 7 change in viscosity suggesting that paramecia swim at constant propulsion force. The distribution of swimming speeds is Gaussian. The width appears proportional to the average speed implying that both fast and slow swimmers exert a constant propulsion. We discuss the possibility that this behavior implies that the body cilia beat at constant force with varying viscosity.

arXiv (Cornell University), Nov 25, 2015

Typically the disorder that alters the interference of particle waves to produce Anderson localiz... more Typically the disorder that alters the interference of particle waves to produce Anderson localization is potential scattering from randomly placed impurities. Here we show that disorder in the form of random gauge fields that act directly on particle phases can also drive localization. We present evidence of a superfluid bose glass to insulator transition at a critical level of this gauge field disorder in a nano-patterned array of amorphous Bi islands. This transition shows signs of metallic transport near the critical point characterized by a resistance ∼ 1 2 h 4e 2 , indicative of a quantum phase transition. The critical disorder also depends on interisland coupling in agreement with recent Quantum Monte Carlo simulations. Finally, these experiments are uniquely connected to theory because they employ a method for controlling a disorder parameter that coincides directly with a term that appears in model Hamiltonians. This correspondence will enable further high fidelity comparisons between theoretical and experimental studies of disorder effects on quantum critical systems.

Bulletin of the American Physical Society, Mar 13, 2006

Bulletin of the American Physical Society, Nov 21, 2011

Bulletin of the American Physical Society, Mar 19, 2013

Physica B-condensed Matter, Feb 1, 1994

Electron tunneling measurements of the Coulomb anomaly in the density of states in strongly disor... more Electron tunneling measurements of the Coulomb anomaly in the density of states in strongly disordered quench condensed granular films are presented. The strength of this anomaly grows with increasing sheet resistance, RN, at low RN but saturates at high RN. We suggest that the granular morphology of these films is responsible for the saturation.

Bulletin of the American Physical Society, Mar 16, 2006

Physica C-superconductivity and Its Applications, Mar 1, 2013

Physica B-condensed Matter, Feb 1, 1994

We present electron transport measurements that demonstrate that Pb films that are on the insulat... more We present electron transport measurements that demonstrate that Pb films that are on the insulating side of the two dimensional superconductor to insulator transition can be made superconducting by depositing a normal metal, Ag, on them.

Physical Review B, Apr 1, 2008

We have observed multiple magnetic field driven superconductor-to-insulator transitions ͑SIT͒ in ... more We have observed multiple magnetic field driven superconductor-to-insulator transitions ͑SIT͒ in amorphous Bi films perforated with a nanohoneycomb ͑NHC͒ array of holes. The period of the magnetoresistance, H = H M = h / 2eS, where S is the area of a unit cell of holes, indicates that the field driven transitions are boson dominated. The field-dependent resistance follows R͑T͒ = R 0 ͑H͒e T 0 ͑H͒/T on both sides of the transition so that the evolution between these states is controlled by the vanishing of T 0 → 0. We compare our results to the thickness driven transition in NHC films and the field driven transitions in unpatterned Bi films, other materials, and Josephson junction arrays. Our results suggest a structural source for similar behavior found in some materials and that despite the clear bosonic nature of the SITs, quasiparticle degrees of freedom likely also play an important part in the evolution of the SIT.

Bulletin of the American Physical Society, 2017

Bulletin of the American Physical Society, 2016

Bulletin of the American Physical Society, 2016

ordered nanoporous anodic aluminum oxide (AAO) provides an easy way to fabricate nano structured ... more ordered nanoporous anodic aluminum oxide (AAO) provides an easy way to fabricate nano structured material, such as nano wires and nano particles. We employ AAO as substrates and focus on the thermally evaporated film growth on the surface of the substrate. With various materials deposited onto the substrate, we find the films show different structures, e,g. ordered array of nano particles for Lead and nanohoneycomb structure for Silver. We relate the differing behaviors to the difference of surface energy and diffusion constant. To verify this, the effect of substrate temperature on the film growth has been explored and the structure of the film has been successfully changed through the process. 1 We are grateful for the support of NSF Grants No. DMR-1307290.

Bulletin of the American Physical Society, 2018

Conductor-Insulator Quantum Phase Transitions, 2012

Physical Review Letters, 1987

Physical Review Letters, Oct 5, 2009

Ultrathin amorphous Bi films, patterned with a nano-honeycomb array of holes, can exhibit an insu... more Ultrathin amorphous Bi films, patterned with a nano-honeycomb array of holes, can exhibit an insulating phase with transport dominated by the incoherent motion of Cooper Pairs (CP) of electrons between localized states. Here we show that the magnetoresistance (MR) of this Cooper Pair Insulator (CPI) phase is positive and grows exponentially with decreasing temperature, T , for T well below the pair formation temperature. It peaks at a field estimated to be sufficient to break the pairs and then decreases monotonically into a regime in which the film resistance assumes the T dependence appropriate for weakly localized single electron transport. We discuss how these results support proposals that the large MR peaks in other unpatterned, ultrathin film systems disclose a CPI phase and provide new insight into the CP localization.

arXiv (Cornell University), Feb 6, 2002

We have fabricated quasi-two-dimensional arrays of nano-scale Pb grains coupled by an overlayer o... more We have fabricated quasi-two-dimensional arrays of nano-scale Pb grains coupled by an overlayer of Ag grains. Their temperature dependent resistive transitions follow predictions for an array of mesoscopic superconductor-normal-superconductor junctions. The decrease of their transition temperatures with Ag overlayer thickness systematically deviates from the Cooper limit theory of the proximity effect as the Pb grain size decreases. The deviations occur when the estimated number of Cooper pairs per grain is less than or equal to 1 and suggest the approach to a superconductor to metal transition.

Bulletin of the American Physical Society, Mar 2, 2012

swim through the coordinated beating of the 1000's of cilia covering their body. We have measured... more swim through the coordinated beating of the 1000's of cilia covering their body. We have measured the swimming speed of populations of Paramecium Caudatam in solutions of different viscosity, η, to see how their propulsion changes with increased drag. We have found the average instantaneous speed, V to decrease monotonically with increasing η. The product ηv is roughly constant over a factor of 7 change in viscosity suggesting that paramecia swim at constant propulsion force. The distribution of swimming speeds is Gaussian. The width appears proportional to the average speed implying that both fast and slow swimmers exert a constant propulsion. We discuss the possibility that this behavior implies that the body cilia beat at constant force with varying viscosity.

arXiv (Cornell University), Nov 25, 2015

Typically the disorder that alters the interference of particle waves to produce Anderson localiz... more Typically the disorder that alters the interference of particle waves to produce Anderson localization is potential scattering from randomly placed impurities. Here we show that disorder in the form of random gauge fields that act directly on particle phases can also drive localization. We present evidence of a superfluid bose glass to insulator transition at a critical level of this gauge field disorder in a nano-patterned array of amorphous Bi islands. This transition shows signs of metallic transport near the critical point characterized by a resistance ∼ 1 2 h 4e 2 , indicative of a quantum phase transition. The critical disorder also depends on interisland coupling in agreement with recent Quantum Monte Carlo simulations. Finally, these experiments are uniquely connected to theory because they employ a method for controlling a disorder parameter that coincides directly with a term that appears in model Hamiltonians. This correspondence will enable further high fidelity comparisons between theoretical and experimental studies of disorder effects on quantum critical systems.

Bulletin of the American Physical Society, Mar 13, 2006

Bulletin of the American Physical Society, Nov 21, 2011

Bulletin of the American Physical Society, Mar 19, 2013

Physica B-condensed Matter, Feb 1, 1994

Electron tunneling measurements of the Coulomb anomaly in the density of states in strongly disor... more Electron tunneling measurements of the Coulomb anomaly in the density of states in strongly disordered quench condensed granular films are presented. The strength of this anomaly grows with increasing sheet resistance, RN, at low RN but saturates at high RN. We suggest that the granular morphology of these films is responsible for the saturation.

Bulletin of the American Physical Society, Mar 16, 2006

Physica C-superconductivity and Its Applications, Mar 1, 2013

Physica B-condensed Matter, Feb 1, 1994

We present electron transport measurements that demonstrate that Pb films that are on the insulat... more We present electron transport measurements that demonstrate that Pb films that are on the insulating side of the two dimensional superconductor to insulator transition can be made superconducting by depositing a normal metal, Ag, on them.

Physical Review B, Apr 1, 2008

We have observed multiple magnetic field driven superconductor-to-insulator transitions ͑SIT͒ in ... more We have observed multiple magnetic field driven superconductor-to-insulator transitions ͑SIT͒ in amorphous Bi films perforated with a nanohoneycomb ͑NHC͒ array of holes. The period of the magnetoresistance, H = H M = h / 2eS, where S is the area of a unit cell of holes, indicates that the field driven transitions are boson dominated. The field-dependent resistance follows R͑T͒ = R 0 ͑H͒e T 0 ͑H͒/T on both sides of the transition so that the evolution between these states is controlled by the vanishing of T 0 → 0. We compare our results to the thickness driven transition in NHC films and the field driven transitions in unpatterned Bi films, other materials, and Josephson junction arrays. Our results suggest a structural source for similar behavior found in some materials and that despite the clear bosonic nature of the SITs, quasiparticle degrees of freedom likely also play an important part in the evolution of the SIT.

Bulletin of the American Physical Society, 2017

Bulletin of the American Physical Society, 2016

Bulletin of the American Physical Society, 2016

ordered nanoporous anodic aluminum oxide (AAO) provides an easy way to fabricate nano structured ... more ordered nanoporous anodic aluminum oxide (AAO) provides an easy way to fabricate nano structured material, such as nano wires and nano particles. We employ AAO as substrates and focus on the thermally evaporated film growth on the surface of the substrate. With various materials deposited onto the substrate, we find the films show different structures, e,g. ordered array of nano particles for Lead and nanohoneycomb structure for Silver. We relate the differing behaviors to the difference of surface energy and diffusion constant. To verify this, the effect of substrate temperature on the film growth has been explored and the structure of the film has been successfully changed through the process. 1 We are grateful for the support of NSF Grants No. DMR-1307290.

Bulletin of the American Physical Society, 2018

Conductor-Insulator Quantum Phase Transitions, 2012

Physical Review Letters, 1987

Physical Review Letters, Oct 5, 2009

Ultrathin amorphous Bi films, patterned with a nano-honeycomb array of holes, can exhibit an insu... more Ultrathin amorphous Bi films, patterned with a nano-honeycomb array of holes, can exhibit an insulating phase with transport dominated by the incoherent motion of Cooper Pairs (CP) of electrons between localized states. Here we show that the magnetoresistance (MR) of this Cooper Pair Insulator (CPI) phase is positive and grows exponentially with decreasing temperature, T , for T well below the pair formation temperature. It peaks at a field estimated to be sufficient to break the pairs and then decreases monotonically into a regime in which the film resistance assumes the T dependence appropriate for weakly localized single electron transport. We discuss how these results support proposals that the large MR peaks in other unpatterned, ultrathin film systems disclose a CPI phase and provide new insight into the CP localization.