Showing 1–14 of 14 results for author: Xue, J M

Quasiparticle Screening near a Bosonic Superconductor-Insulator Transition Revealed by Magnetic Impurity Doping

Authors: Xue Zhang, James C. Joy, Chunshu Wu, Jin-Ho Kim, J. M. Xu, James M. Valles Jr

Abstract: Experiments show that the Cooper pair transport in the insulator phase that forms at thin film superconductor to insulator transitions (SIT) is simply activated. This activated behavior depends on the microscopic factors that drive the localization of the Cooper pairs. To test proposed models, we investigated how a perturbation that weakens Cooper pair binding, magnetic impurity doping, affects th… ▽ More Experiments show that the Cooper pair transport in the insulator phase that forms at thin film superconductor to insulator transitions (SIT) is simply activated. This activated behavior depends on the microscopic factors that drive the localization of the Cooper pairs. To test proposed models, we investigated how a perturbation that weakens Cooper pair binding, magnetic impurity doping, affects the characteristic activation energy, $T_0$. The data show that $T_0$ decreases monotonically with doping in films tuned farther from the SIT and increases and peaks in films that are closer to the SIT critical point. These observations provide strong evidence that the bosonic SIT in thin films is a Mott transition driven by Coulomb interactions that are screened by virtual quasi-particle excitations. This dependence on underlying fermionic degrees of freedom distinguishes these SITs from those in micro-fabricated Josephson Junction Arrays, cold atom systems, and likely in high temperature superconductors with nodes in their quasiparticle density of states. △ Less

Submitted 27 June, 2018; originally announced June 2018.

Comments: 5 pages, 3 figures

Journal ref: Phys. Rev. Lett. 122, 157002 (2019)

Photoemission Study of the Electronic Structure of Valence Band Convergent SnSe

Authors: C. W. Wang, Y. Y. Y. Xia, Z. Tian, J. Jiang, B. H. Li, S. T. Cui, H. F. Yang, A. J. Liang, X. Y. Zhan, G. H. Hong, S. Liu, C. Chen, M. X. Wang, L. X. Yang, Z. Liu, Q. X. Mi, G. Li, J. M. Xue, Z. K. Liu, Y. L. Chen

Abstract: IV-VI semiconductor SnSe has been known as the material with record high thermoelectric performance.The multiple close-to-degenerate valence bands in the electronic band structure has been one of the key factors contributing to the high power factor and thus figure of merit in the SnSe single crystal. To date, there have been primarily theoretical calculations of this particular electronic band st… ▽ More IV-VI semiconductor SnSe has been known as the material with record high thermoelectric performance.The multiple close-to-degenerate valence bands in the electronic band structure has been one of the key factors contributing to the high power factor and thus figure of merit in the SnSe single crystal. To date, there have been primarily theoretical calculations of this particular electronic band structure. In this paper, however, using angle-resolved photoemission spectroscopy, we perform a systematic investigation of the electronic structure of SnSe. We directly observe three predicted hole bands with small energy differences between their band tops and relatively small in-plane effective masses, in good agreement with the ab initio calculations and critical for the enhancement of the Seebeck coefficient while keeping high electrical conductivity. Our results reveal the complete band structure of SnSe and help to provide a deeper understanding of the electronic origin of the excellent thermoelectric performances in SnSe. △ Less

Submitted 12 April, 2018; originally announced April 2018.

Comments: 14 pages, 5 figures

Journal ref: Physical Review B 96, 165118 (2017)

Superconductor to Insulator Transition Tuned by Random Gauge Fields

Authors: H. Q. Nguyen, S. M. Hollen, J. M. Valles Jr., J. Shainline, J. M. Xu

Abstract: 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 d… ▽ 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 of order 0.5 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. △ Less

Submitted 25 November, 2015; originally announced November 2015.

Comments: 6 pages, 3 figures

Disorder Influences the Quantum Critical Transport at a Superconductor to Insulator Transition

Authors: H. Q. Nguyen, S. M. Hollen, J. Shainline, J. M. Xu, J. M. Valles Jr

Abstract: We isolated flux disorder effects on the transport at the critical point of the quantum magnetic field tuned Superconductor to Insulator transition (BSIT). The experiments employed films patterned into geometrically disordered hexagonal arrays. Spatial variations in the flux per unit cell, which grow in a perpendicular magnetic field, constitute flux disorder. The growth of flux disorder with magn… ▽ More We isolated flux disorder effects on the transport at the critical point of the quantum magnetic field tuned Superconductor to Insulator transition (BSIT). The experiments employed films patterned into geometrically disordered hexagonal arrays. Spatial variations in the flux per unit cell, which grow in a perpendicular magnetic field, constitute flux disorder. The growth of flux disorder with magnetic field limited the number of BSITs exhibited by a single film due to flux matching effects. The critical metallic resistance at successive BSITs grew with flux disorder contrary to predictions of its universality. These results open the door for controlled studies of disorder effects on the universality class of an ubiquitous quantum phase transition. △ Less

Submitted 14 October, 2015; v1 submitted 9 February, 2015; originally announced February 2015.

Journal ref: Phys. Rev. B 92, 140501 (2015)

Cooper Pair Insulator phase induced in amorphous Pb$_0.9$Bi$_0.1$ thin films

Authors: S. M. Hollen, J. Shainline, J. M. Xu, J. M. Valles Jr

Abstract: A Cooper pair insulator (CPI) phase emerges near the superconductor-insulator transitions of a number of strongly-disordered thin film systems. Much recent study has focused on a mechanism driving the underlying Cooper pair localization. We present data showing that a CPI phase develops in amorphous Pb$_{0.9}$Bi$_{0.1}$ films deposited onto nano-porous anodized aluminum oxide surfaces just as it h… ▽ More A Cooper pair insulator (CPI) phase emerges near the superconductor-insulator transitions of a number of strongly-disordered thin film systems. Much recent study has focused on a mechanism driving the underlying Cooper pair localization. We present data showing that a CPI phase develops in amorphous Pb$_{0.9}$Bi$_{0.1}$ films deposited onto nano-porous anodized aluminum oxide surfaces just as it has been shown to develop for a-Bi films. This result confirms the assertion that the CPI phase emerges due to the structure of the substrate. It supports the picture that nanoscale film thickness variations induced by the substrate drive the localization. Moreover, it implies that the CPI phase can be induced in any superconducting material that can be deposited onto this surface. △ Less

Submitted 25 January, 2013; originally announced January 2013.

Collapse of the Cooper pair phase coherence length at a superconductor to insulator transition

Authors: S. M. Hollen, G. E. Fernandes, J. M. Xu, J. M. Valles Jr

Abstract: We present investigations of the superconductor to insulator transition (SIT) of uniform a-Bi films using a technique sensitive to Cooper pair phase coherence. The films are perforated with a nanohoneycomb array of holes to form a multiply connected geometry and subjected to a perpendicular magnetic field. Film magnetoresistances on the superconducting side of the SIT oscillate with a period dicta… ▽ More We present investigations of the superconductor to insulator transition (SIT) of uniform a-Bi films using a technique sensitive to Cooper pair phase coherence. The films are perforated with a nanohoneycomb array of holes to form a multiply connected geometry and subjected to a perpendicular magnetic field. Film magnetoresistances on the superconducting side of the SIT oscillate with a period dictated by the superconducting flux quantum and the areal hole density. The oscillations disappear close to the SIT critical point to leave a monotonically rising magnetoresistance that persists in the insulating phase. These observations indicate that the Cooper pair phase coherence length, which is infinite in the superconducting phase, collapses to a value less than the interhole spacing at this SIT. This behavior is inconsistent with the gradual reduction of the phase coherence length expected for a bosonic, phase fluctuation driven SIT. This result starkly contrasts with previous observations of oscillations persisting in the insulating phase of other films implying that there must be at least two distinct classes of disorder tuned SITs. △ Less

Submitted 4 February, 2013; v1 submitted 25 January, 2013; originally announced January 2013.

Cooper pair insulator in amorphous films induced by nanometer-scale thickness variations

Authors: S. M. Hollen, H. Q. Nguyen, E. Rudisaile, M. D. Stewart Jr., J. Shainline, J. M. Xu, J. M. Valles Jr

Abstract: Unusual transport properties of superconducting (SC) materials, such as the under doped cuprates, low dimensional superconductors in strong magnetic fields, and insulating films near the Insulator Superconductor Transition (IST), have been attributed to the formation of inhomogeneous phases. Difficulty correlating the behaviors with observations of the inhomogeneities make these connections uncert… ▽ More Unusual transport properties of superconducting (SC) materials, such as the under doped cuprates, low dimensional superconductors in strong magnetic fields, and insulating films near the Insulator Superconductor Transition (IST), have been attributed to the formation of inhomogeneous phases. Difficulty correlating the behaviors with observations of the inhomogeneities make these connections uncertain. Of primary interest here are proposals that insulating films near the IST, which show an activated resistance and giant positive magnetoresistance, contain islands of Cooper Pairs (CPs). Here we present evidence that these types of inhomogeneities are essential to such an insulating phase in amorphous Bi (a-Bi) films deposited on substrates patterned with nanometer-sized holes. The patterning induces film thickness variations, and corresponding coupling constant variations, that transform the composition of the insulator from localized electrons to CPs. Analyses near the thickness-tuned ISTs of films on nine different substrates show that weak links between SC islands dominate the transport. In particular, the ISTs all occur when the link resistance approaches the resistance quantum for pairs. These observations lead to a detailed picture of CPs localized by spatial variations of the superconducting coupling constant. △ Less

Submitted 28 January, 2011; originally announced January 2011.

Comments: 4 pages, 3 figures, 1 supplemental page with 1 supplemental figure

Journal ref: Phys. Rev. B 84, 064528 (2011)

Observation of Giant Positive Magnetoresistance in a Cooper Pair Insulator

Authors: H. Q. Nguyen, S. M. Hollen, M. D. Stewart Jr., J. Shainline, Aijun Yin, J. M. Xu, J. M. Valles Jr

Abstract: 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 of electrons between localized states. Here we show that the magnetoresistance of this Cooper pair insulator phase is positive and grows exponentially with decreasing temperature, for temperatures well below the pair forma… ▽ 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 of electrons between localized states. Here we show that the magnetoresistance of this Cooper pair insulator phase is positive and grows exponentially with decreasing temperature, for temperatures 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 temperature 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 Cooper Pair Insulator phase and provide new insight into the Cooper pair localization. △ Less

Submitted 1 October, 2009; v1 submitted 23 July, 2009; originally announced July 2009.

Field Tuned Superconductor to Insulator Transitions in an Amorphous Film with an Imposed Multiply Connected Geometry

Authors: M. D. Stewart Jr, Aijun Yin, J. M. Xu, J. M. Valles Jr

Abstract: We have observed multiple magnetic field driven superconductor to insulator transitions (SIT) in amorphous Bi films perforated with a nano-honeycomb (NHC) array of holes. The period of the magneto-resistance, H=H_M=h/2eS where S is the area of a unit cell of holes, indicates the field driven transitions are boson dominated. The field-dependent resistance follows R(T)=R_0(H)exp(T_0(H)/T) on both… ▽ More We have observed multiple magnetic field driven superconductor to insulator transitions (SIT) in amorphous Bi films perforated with a nano-honeycomb (NHC) array of holes. The period of the magneto-resistance, H=H_M=h/2eS where S is the area of a unit cell of holes, indicates the field driven transitions are boson dominated. The field-dependent resistance follows R(T)=R_0(H)exp(T_0(H)/T) on both sides of the transition so that the evolution between these states is controlled by the vanishing of T_0 to0. 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. △ Less

Submitted 6 December, 2007; originally announced December 2007.

Comments: 4 pages, 4 figures

Hopping Conduction in Disordered Carbon Nanotubes

Authors: D. P. Wang, D. E. Feldman, B. R. Perkins, A. J. Yin, G. H. Wang, J. M. Xu, A. Zaslavsky

Abstract: We report electrical transport measurements on individual disordered carbon nanotubes, grown catalytically in a nanoporous anodic aluminum oxide template. In both as-grown and annealed types of nanotubes, the low-field conductance shows as exp[-(T_{0}/T)^{1/2}] dependence on temperature T, suggesting that hopping conduction is the dominant transport mechanism, albeit with different disorder-rela… ▽ More We report electrical transport measurements on individual disordered carbon nanotubes, grown catalytically in a nanoporous anodic aluminum oxide template. In both as-grown and annealed types of nanotubes, the low-field conductance shows as exp[-(T_{0}/T)^{1/2}] dependence on temperature T, suggesting that hopping conduction is the dominant transport mechanism, albeit with different disorder-related coefficients T_{0}. The field dependence of low-temperature conductance behaves an exp[-(xi_{0}/xi)^{1/2}] with high electric field xi at sufficiently low T. Finally, both annealed and unannealed nanotubes exhibit weak positive magnetoresistance at low T = 1.7 K. Comparison with theory indicates that our data are best explained by Coulomb-gap variable range hopping conduction and permits the extraction of disorder-dependent localization length and dielectric constant. △ Less

Submitted 26 October, 2006; originally announced October 2006.

Comments: 10 pages, 5 figures

Ordered Nano-Crystal Arrays Spontaneously Form in Films Evaporated onto Nanopore Array Substrates

Authors: Niravun Pavenayotin, M. D. Stewart Jr., James M. Valles Jr., Aijun Yin, J. M. Xu

Abstract: We present SEM images of films thermally evaporated onto Anodic Aluminum Oxide substrates that are patterned with a hexagonal array of 34 and 80 nm diameter holes spaced by 100 nm. Over a range of film thicknesses, Pb and Sn films spontaneously self assemble into an array of nano-crystals in registry with the underlying hole lattice. The development of the arrays with thickness indicates that su… ▽ More We present SEM images of films thermally evaporated onto Anodic Aluminum Oxide substrates that are patterned with a hexagonal array of 34 and 80 nm diameter holes spaced by 100 nm. Over a range of film thicknesses, Pb and Sn films spontaneously self assemble into an array of nano-crystals in registry with the underlying hole lattice. The development of the arrays with thickness indicates that surface energies drive coalescing grains to move over the holes. Materials that wet the substrate or whose grains do not coalesce at these substrate temperatures do not form arrays. We discuss some potential applications. △ Less

Submitted 1 June, 2005; originally announced June 2005.

Magnetic Flux Periodic Response of Nano-perforated Ultrathin Superconducting Films

Authors: M. D. Stewart Jr., Zhenyi Long, James M. Valles Jr., Aijun Yin, J. M. Xu

Abstract: We have patterned a hexagonal array of nano-scale holes into a series of ultrathin, superconducting Bi/Sb films with transition temperatures 2.65 K $<T_{co} < $5 K. These regular perforations give the films a phase-sensitive periodic response to an applied magnetic field. By measuring this response in their resistive transitions, $R(T)$, we are able to distinguish regimes in which fluctuations o… ▽ More We have patterned a hexagonal array of nano-scale holes into a series of ultrathin, superconducting Bi/Sb films with transition temperatures 2.65 K $<T_{co} < $5 K. These regular perforations give the films a phase-sensitive periodic response to an applied magnetic field. By measuring this response in their resistive transitions, $R(T)$, we are able to distinguish regimes in which fluctuations of the amplitude, both the amplitude and phase, and the phase of the superconducting order parameter dominate the transport. The portion of $R(T)$ dominated by amplitude fluctuations is larger in lower $T_{co}$ films and thus, grows with proximity to the superconductor to insulator transition. △ Less

Submitted 17 October, 2005; v1 submitted 1 June, 2005; originally announced June 2005.

Comments: Revised title, abstract, text, figures

Ultrafast Optical Nonlinearity in PMMA-TiO2 Nanocomposites

Authors: H. I. Elim, W. Ji, A. H. Yuwono, J. M. Xue, J. Wang

Abstract: With 780-nm, 250-fs laser pulses, ultrafast optical nonlinearity has been observed in a series of thin films containing PMMA-TiO2 nanocomposites, which are synthesized by a simple technique of in-situ sol-gel/polymerization. The best figures of merit are found in one of the films prepared with a 60% weight percentage of titanium isopropoxide. TEM shows the presence of the 5-nm-diameter particles… ▽ More With 780-nm, 250-fs laser pulses, ultrafast optical nonlinearity has been observed in a series of thin films containing PMMA-TiO2 nanocomposites, which are synthesized by a simple technique of in-situ sol-gel/polymerization. The best figures of merit are found in one of the films prepared with a 60% weight percentage of titanium isopropoxide. TEM shows the presence of the 5-nm-diameter particles in the film. The observed optical nonlinearity has a recovery time of ~1.5 ps. These findings suggest the strong potential of PMMA-TiO2 nanocomposites for all-optical switching. △ Less

Submitted 27 February, 2003; v1 submitted 10 January, 2003; originally announced January 2003.

Comments: 12 pages, 3 figures. To appear in Appl. Phys. Lett. (2003)

Journal ref: Applied Physics Letters, Vol. 82, (2003), p. 2691; Virtual Journal of Ultrafast Science, Volume 2, Issue 5, May 2003

Superconducting Transition and Vortex Pinning in Nb Films Patterned with Nano-scale Hole-arrays

Authors: U. Welp, Z. L. Xiao, J. S. Jiang, V. K. Vlasko-Vlasov, S. D. Bader, G. W. Crabtree, J. Liang, H. Chik, J. M. Xu

Abstract: Nb films containing extended arrays of holes with 45-nm diameter and 100-nm spacing have been fabricated using anodized aluminum oxide (AAO) as substrate. Pronounced matching effects in the magnetization and Little-Parks oscillations of the superconducting critical temperature have been observed in fields up to 9 kOe. Flux pinning in the patterned samples is enhanced by two orders of magnitude a… ▽ More Nb films containing extended arrays of holes with 45-nm diameter and 100-nm spacing have been fabricated using anodized aluminum oxide (AAO) as substrate. Pronounced matching effects in the magnetization and Little-Parks oscillations of the superconducting critical temperature have been observed in fields up to 9 kOe. Flux pinning in the patterned samples is enhanced by two orders of magnitude as compared to unpatterned reference samples in applied fields exceeding 5 kOe. Matching effects are a dominant contribution to vortex pinning at temperatures as low as 4.2 K due to the extremely small spacing of the holes. △ Less

Submitted 24 April, 2002; originally announced April 2002.