Joshua Paramanandam , Matthew Bell, and Michael Gershenson

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## Joshua Paramanandam , Matthew Bell, and Michael Gershenson

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**Magnetic-Field-Driven**Phase Transitions in Unconventional Josephson Arrays Joshua Paramanandam, Matthew Bell, and Michael Gershenson Department of Physics and Astronomy, Rutgers University, New Jersey, USA Theoretical encouragement: Lev Ioffe (Rutgers) and MishaFeigelman (Landau Inst.) “Strongly Disordered Superconductors and Electronic Segregation” Lorentz Center, Leiden, 26 Aug. 2011**Several long-standing (~20 years) issues:**- magnetic-field-induced “metallicity” in Josephson arrays; - dissipation mechanisms; - transport in the insulating regime. Our weapon of choice: Josephson arrays with a large number of nearest-neighbor islands. “S-I” transition at EJ/Ec~ 1, the “critical” resistance varies by three orders of magnitude depending on screening. “Metallicity”: several alternating “S” and “I” phases (commensurability) with very small (T) characteristic energies. Insulating regime (no traces of emergent inhomogeneity…): - “Arrhenius” activation energy correlates with the “offset” voltage across the whole array ??? - the power threshold of quasiparticle generation is “universal” and scales with the array area ??? Outline:**Bosonic Model of SIT (preformed Cooper pairs)**R Insulator RQ superconductor T Efetov et al., ‘80 Ma, Lee ‘85 Kapitulnik, Kotliar ‘85 Fisher ‘90 Wen and Zee ‘90 Only phase fluctuations Josephson energy The SIT is driven by the competition between Cooper pair hopping and Coulomb repulsion: Charging energy Charge-vortex duality (M. Fisher, ’90) van der Zant et al, ‘96 B=0**Magnetic-field-driven SIT in Josephson Arrays**T (K) At odds with the “dirty boson” model,a T-independent (“metallic”) resistivity was observed over a wide range of R. f = /0 f=0.27 f=0 Chen et al., (’94) • Potential complications: • Random charges in the environment (static and fluctuating) • Flux noise • Random scatter of Josephson energies and its fluctuations Static and dynamic disorder ? disorder + B-induced frustrations emergent inhomogeneity, glassines, etc.**Characteristic energies**per island (no gate electrode, CJ>>Cg ): JJ arrays with large number of nearest-neighbor islands J Potential advantages of large N: • better averaging of the fluctuations of the parameters of individual JJs. • the effect of magnetic field is expected to be stronger (NEJEJNin B>0/A); • exploration of a much wider range of the JJ parameters • (e.g., junctions with RN >>RQ). The characteristic energies are 2-3 times smaller than that for the conventional arrays (still exceed the temperature of the quasiparticle “freeze-out”, ~0.2K).**Array Fabrication**• Experimental realization: • “Manhattan pattern” nanolithography • Multi-angle deposition of Al Typical normal-state R of individual junctions: no ground plane: 30-200 k with ground plane: up to1 M • Aarray~ 100100m2 B0/Aarray N=10 array IC(nA) B (G) - in line with numerical simulations (Sadovskyy)**Array A**R(2K)=15.2 k RJ =133 k EC = 1.8K EJ = 0.06 K N2(EJ/EC) = 3.3 Array B R(2K)= 5.0 k RJ = 43 k EC = 1.2 K EJ = 0.18 K N2(EJ/EC) = 15 Arrays without ground plane Arrays: 8x8 “supercells” (100×100 m2) C(per island) ~ 5 fF, EC (per island) ~ 0.2 K C/Cg ~ 100 Incoherent transport of Cooper pairs Quasiparticle freeze-out A • The “critical” R ~ 3-20 k for the arrays without a ground plane. R (k) NEJ Mag. field B T (K)**Arrays with conducting ground plane**Al2O3 3 nm Al 20 nm 3 The “S-I” transition at NEJ/Ecisland~1. NEJ 2 resistances at 2K 1 The “critical” R~1M for this array with a ground plane.**Probably, the first experiment which shows that**(EJ/EC)island is the only relevant parameter, the critical resistance Rcr can vary a great deal depending on the capacitance matrix.**Arrays without ground plane: more detailed look at the SIT**B A R (k) R (k) f =/0 – normalized flux per 10 unit cells f f Multiple SITs(commensurate structure) at different R ~ 3-20 k. R (k) R (k) alternating “S” and “I” phases f f van der Zant et al, ‘96**Finite-Bias Transport**Rarray(4K)= 18.9 k RJ = 160 k EC ~ 2K, EJ ~ 0.05K N2(EJ/EC) ~ 2.5 Color-coded differential resistance dV/dI(I,B) f I (nA)**Direct “S” “I ” Transitions**Array B “insulator”: 20 R (k) “superconductor”: T (K) -20 Low Rcr(< 10 k): direct “S” – “I” transitions.**Lack of Duality at High Rcr**Array A A R (k) T (K) f I (nA) High Rcr(>10 k): Lack of “duality”.**“Metallicity”:**T (K) At least partially due to alternating S and I phases (commensurability) with very small activation energies. The phase transitions observed at low “critical” R < 10kfollow the “dirty boson” scenario (direct SIT). However, the duality is lacking for the transitions observed at larger R> 10k. f=0.27 f = /0 f=0 Chen et al., (’94)**Array I (8x8 supercells)**R(2K)= 16.63 k Array II (4x4 supercells) R(2K)= 16.47 k RJ = 156 k EC = 2.5 K EJ = 0.05 K N2(EJ/EC) = 2 “Insulating” Regime Sub-pA bias is required in the “insulating” regime. B V* V (V) V* is the voltage drop across the whole array I R (k) B I (nA) 500 Lines: 1/T (1/K) R(T) ~ exp[2eV*/kBT] I II II 2eV*(B)/kB (mK) 250 R (k) B 0 0 0.5 1.0 1.5 B (G) 1/T (1/K)**N = 4 array**Rarray(300K)= 37.5 k EC ~ 1.2K, EJ ~ 0.23K EJ/EC~ 0.2 N2(EJ/EC) ~ 3 Insulating Regime in N = 4 Array 2eV*(B) ~ kBT0(B) f = /0 Arrhenius: R(T)=R0exp(T0/T) T0= T0(B) R0 104 **Possible Explanations?**2eV*(B)~kBT0(B) could be signatures of a collective process. Emergent inhomogeneity? Cooper pair hopping along the chain of islands with an effective charge close to (2n+1)e (costs no energy to add/subtract a Cooper pair). The “bottleneck” is the island with a larger deviation of its q from (2n+1)e. - The voltage drops across the most resistive link with the largest localT0. 2eV*(B)=kBT0(B) However, the same values of the resistance observed for two halves of the array seem to rule out the latter option.**Macroscopic Homogeneity in the “Insulating” Regime**Solid curves: total array Dashed curves: one half No significant difference in the resistance and T0 for two halves of the array was observed.**System-size dependence of T0and VT in thin films**T0~ lnL VT, mV 2eVT(L) ~ (10100) kBT0 (L)**Threshold of Quasiparticle Generation**The “threshold” power does not depend on the zero-bias resistance. For all studied arrays Pth 10-14 -10-13 W.**Threshold Power V *I ***N = 11 array Rarray(4K)= 15.4 k RJJ ~ 150 k EC ~ 0.7K, EJ ~ 0.06K EJ/EC ~ 0.08 N2(EJ/EC) ~ 10 Pth is T-independent below ~ 0.2K, whereas R(I=0) and Ithstill depend on T.**Scaling with Array Area**Two arrays on the same chip: The “threshold” power is proportional to the array’s area (the total number of junctions)**Unconventional Josephson arrays with a large number of**nearest-neighbor islands have been fabricated. Multiple “S-I” transitions (due to commensurate effects) over a wide range of critical resistances R ~ 3-20 k were observed. “Metallisity” – due to alternating “S” and “I” phases with very low (typically < 100 mK) characteristic energies. The phase transitions observed for these arrays resemble the “dirty boson” SIT at low “critical” Rcr ~ few k, however the duality is lacking for the transitions observed at larger Rcr . On the “insulating” side of the SIT, the R(T) dependences can be fitted with the Arrhenius law R(T)~exp(T0/T), where kBT0 is close to the “Coulomb” gap 2eV* (V* is the offset voltage across the whole array). The threshold for quasiparticle generation at high bias currents is surprisingly universal for samples with vastly different zero-bias resistances. This power scales with the array area. Summary: