280 likes | 426 Views
Bosonic IST’s in Nano-HoneyComb (NHC) Films. M. D. Stewart, Jr. (Stew) J. M. Xu, Aijun Yin, and Jim Valles Brown University, Providence, RI USA. amorphous-Bi films Haviland, Liu and Goldman. Insulators in 2D. Insulator to Superconductor Transition (IST) thickness or H-tuned
E N D
Bosonic IST’s in Nano-HoneyComb (NHC) Films M. D. Stewart, Jr. (Stew) J. M. Xu, Aijun Yin, and Jim Valles Brown University, Providence, RI USA
amorphous-Bi films Haviland, Liu and Goldman Insulators in 2D Insulator to Superconductor Transition (IST) thickness or H-tuned What’s the insulating phase? localized Cooper pairs (bosons) or localized electrons (fermions) or novel mixture
H amorphous-Indium Oxide (Hebard and Paalanen) Insulators in 2D Insulator to Superconductor Transition (IST) H-tuned What’s the insulating phase? Scaling Superinsulator behavior Magnetoresistance peak …probably localized bosons
Boson to Boson IST’s in NHC Films • Amorphous Nano-HoneyComb films • Patterning allows Cooper Pair detection • Thickness tuned IST • Patterning transforms IST • Direct observation of localized Cooper pairs • Vanishing energy scale at critical point • Magnetic Field tuned IST • Vanishing energy scale at critical point • Comparison to Indium Oxide films
Nano-honeycomb Films o 1 mm
Cooper Pair Detection Cooper pair phase obeys: S H flux periodic properties period Only need, Lφ > S1/2 frustration
Nano-honeycomb Sheet Resistance (Ω/□) Temperature (K) Features: Reentrance, stronger insulator, broad superconducting transition Thickness Tuned IST’s Unpatterned Sheet Resistance (Ω/□) Temperature (K)
Insulator is exponentially localized NHC films R=R0 exp(-To/T)
R□ (kΩ) ∆G (1/kΩ) -1 0 1 Log [T(K)] Insulating Unpatterned Films IST of a-Bi/Sb films PRB 59, 11209 (1999) Insulator’s conductance: weakly localized fermions
Insulator exhibits flux oscillations! 1)Period: HM = h/2eS 2e charge carriers 2) Activation energy oscillates
1e vs. 2e oscillations • Patterned PbTe Films (1e) • Oscillation period h/2eS • Negative magnetoresistance • 1e hopping • Poyarkov et al., JETP Lett. 44, 373 (1987) • NHC Films (2e) • Oscillation period h/2eS • Positive Magnetoresistance • 2e transport
Energy scales in the insulator Oscillations Unpatterned film To (f=1/2) To (f=0) IST where To 0
Charge 2e Carriers • Cooper pairs in the insulator • Oscillations appear where unpatterned films can support pairing • Cooper pair transport in the insulator • Differs from quasi-particle transport in structurally granular films
grain JJ What governs the NHC film IST? ? = Josephson Junction Array physics? localizing charging energies vs. delocalizing Josephson energies Ec > Ej gives an insulator Ej oscillates in magnetic field
L JJA picture Estimate : For L = 50 nm, Ec = 104K/ Too large to be relevant! Effective islands encompass many holes? --- Need 1000’s of holes
Thickness tuned IST in NHC Films • Direct observation of strongly localized Cooper pairs • Patterning enhances boson localization • Vanishing energy scale at the critical point • JJA model requires large islands
H a-Indium Oxide Films Hebard and Paalanen (1990) Field tuned SIT • Reentrance • Metal at H = Hc • Dirty Boson scaling
Field tuned SIT – NHC films • Reentrant • Exponential insulator - R=Roexp(To(H)/T) • Metallic at the critical point?
To vs. H/HM H controls a single energy scale
Magneto-Oscillations ContinueMultiple field tuned IST’s oscillations bosons Weak superconductors at matching fields
Effects of Nano-Honeycomb Geometry AAO Substrate • Multiply connected • Surface undulations Exp(To/T) R(H) peak
Magnetoresistance Peaks Indium Oxide Film PRL 107005 (2004) NHC Bi/Sb Film
R(T)’s of Indium Oxide Films Reentrance Hebard and Paalanen, PRL 65, 927 (1990) Activated Sambandamurthy et al., PRL 107005 (2004)
Boson to Boson IST’s in NHC Films • Method to “see” Cooper pairs in non-superconducting system • Created boson dominated SIT’s • How holes tend to localize Cooper pairs is unclear • Similarities to Indium Oxide films