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@ARTICLE{Foroughi:861560,
      author       = {Foroughi, F. and Mol, J.-M. and Müller, T. and Kirtley, J.
                      R. and Moler, K. A. and Bluhm, Hendrik},
      title        = {{A} micro-{SQUID} with dispersive readout for magnetic
                      scanning microscopy},
      journal      = {Applied physics reviews},
      volume       = {112},
      number       = {25},
      issn         = {1077-3118},
      address      = {New York, NY},
      publisher    = {AIP74335},
      reportid     = {FZJ-2019-02011},
      pages        = {252601 -},
      year         = {2018},
      abstract     = {We have designed and characterized a micro-SQUID with
                      dispersive readout for use in low temperature scanning probe
                      microscopy systems. The design features a capacitively
                      shunted RF SQUID with a tunable resonance frequency from 5
                      to 12 GHz, micrometer spatial resolution, and integrated
                      superconducting coils for local application of magnetic
                      fields. The SQUID is operated as a nonlinear oscillator with
                      a flux- and power-dependent resonance frequency.
                      Measurements for device characterization and noise
                      benchmarking were carried out at 4 K. The measured flux
                      noise above 10 kHz at 4 K is 80 nΦ0 Hz−1∕2 at a
                      bandwidth of 200 MHz. Estimations suggest that one can
                      benefit from parametric gain based on inherent nonlinearity
                      of the Josephson junction and reduce the flux noise to 30
                      nΦ0Hz–1∕2 at 100 mK, which corresponds to 10.6
                      μBHz–1∕2 for a magnetic moment located at the center of
                      the pickup loop.Measuring the magnetic response of
                      mesoscopic samples or mapping it vs. position for extended
                      systems is effective methods for revealing the fundamental
                      quantum properties of condensed matter systems. Over the
                      past few decades, many advanced magnetic imaging schemes
                      have been developed, including magnetic force microscopy,1
                      scanning Hall probe microscopy,2 superconducting
                      interference devices (SQUIDs),3,4 and NV center based
                      magnetometry.5 The high sensitivity, low back action, and
                      low power dissipation of SQUIDs make them attractive for
                      many types of low temperature experiments. One classic
                      example is the central role of scanning SQUID microscopy in
                      tests of pairing symmetry of high-Tc cuprate
                      superconductors.6 Other prominent applications are the
                      thermodynamic characteristics of persistent currents in
                      normal metal rings7 and proof of edge states in topological
                      insulators in the quantum spin Hall regime.8A key
                      performance for a SQUID is its flux sensitivity. Parametric
                      amplification has been harnessed in nano-SQUIDs based on
                      aluminum junctions9,10 in order to reduce the flux noise
                      down to 30 nΦ0Hz–1∕2 with a bandwidth exceeding
                      60 MHz.10 Recent studies on parametric amplifiers and
                      dispersive readout of superconducting qubits also harness
                      the nonlinearity of the Josephson junction to boost
                      sensitivity.11–13In scanning SQUID microscopy, signal
                      sources often behave dipole-like, such as superconducting
                      vortices in mesoscopic dots or currents in mesoscopic rings.
                      Smaller SQUIDs provide better coupling to smaller samples
                      which leads to higher spin sensitivity3,14 as the magnetic
                      field of a dipole decreases with 1/r3. Various approaches
                      have been pursued to reduce the size. The smallest
                      nano-SQUID to date has been fabricated by evaporating Nb or
                      Pb15 onto the apex of a sharp quartz tip reaching a loop
                      diameter of below nm and a spin sensitivity of 0.38
                      μBHz−1/2 at tens of kHz bandwidth. Compared to such
                      size-optimized devices, micro-SQUIDs fabricated using a
                      standard Nb technique have the advantage of on-chip field
                      coils and modulation coils,14 and can be operated at 4 K
                      allowing for a wide range of applications. However, these
                      come at the cost of larger pick-up loops of a few microns
                      and a resulting poorer spin sensitivity of 200 μBHz−1/2
                      for a magnetic moment located at the center of the pickup
                      loop.The limitation of large pickup loops was previously
                      only overcome by devices with pickup loops defined by the
                      focused-ion-beam,16 but has recently also been achieved by
                      improvements in lithography and shown to offer better
                      spatial resolution as well.17 Here, we present a scanning
                      micro-SQUID which is based on the same fabrication
                      technology incorporating another major advancement. Our
                      design exploits the parametric amplification based on
                      nonlinearities of the SQUID to boost sensitivity. The
                      combination of a smaller pickup loop and parametric
                      amplification of the signal leads to better spin sensitivity
                      (10.6 μBHz−1/2) and higher bandwidth (200 MHz) compared
                      to traditional DC micro-SQUIDs in which the flux sensitivity
                      is mainly limited by internal dissipation.Our devices were
                      fabricated at Hypres Inc. using planarized Nb/AlOx/Nb
                      trilayer Josephson junction technology, including two
                      planarized Nb layers with approximately 0.8 μm minimum
                      feature size. The SQUID consists of a superconducting ring
                      shunted by one Josephson junction with a designed critical
                      current of 20 μA [Fig. 1(a)]. With this geometry, the SQUID
                      is gradiometric, i.e., the effect of any homogeneous
                      background magnetic field approximately cancels out. The
                      smallest pickup loops implemented have an inner (outer)
                      diameter of 1 μm (2 μm). The superconducting loop is
                      shunted by an on-chip parallel-plate capacitor with
                      amorphous silicon dioxide as dielectric which was designed
                      to have a capacitance of 80 pF. The superconducting loop
                      and the capacitor form a resonator with a flux dependent
                      resonance frequency from 5 to 12 GHz. The pickup loops are
                      placed on opposite corners of the parallel-plate capacitor
                      at a distance of about 450 μm so that one loop can be
                      located in close proximity to the sample while the other is
                      kept far away from it. External fields can be applied to the
                      sample by local single-turn field coils situated around each
                      pickup loop. The field coils are fully integrated into the
                      chip layout and can also be used to bias the SQUID at its
                      most sensitive point. Compared to using additional
                      modulation coils,14 this approach has the advantage of
                      reducing the total device inductance, which leads to a
                      higher sensitivity.18 In a scanning microscope, the field
                      coil at the rear end (away from the sample) can be used to
                      bias the SQUID or operate it in flux feedback such that
                      resulting stray fields will not act on the sample, apart
                      from a weak screening current through the pickup loop. The
                      effective diameter of each coil is 7.5 μm, resulting in a
                      mutual inductance of about 0.34 Φ0/mA (for the smallest
                      pickup loop). The SQUID resonator is connected by an on-chip
                      Z0 = 50 Ω coplanar waveguide (CPW) transmission line to
                      the bonding pads. The two field coils are connected by two
                      smaller CPWs to the bottom edge bonding pads [Fig. 1(c)],
                      ending in two parallel-stripline waveguides running
                      alongside the edges of the capacitor towards the respective
                      field coils. Based on estimations, it should thus be
                      possible to apply high frequency signals of at least several
                      GHz to SQUID and sample.},
      cin          = {PGI-11 / JARA-FIT},
      ddc          = {530},
      cid          = {I:(DE-Juel1)PGI-11-20170113 / $I:(DE-82)080009_20140620$},
      pnm          = {144 - Controlling Collective States (POF3-144)},
      pid          = {G:(DE-HGF)POF3-144},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000435987400035},
      doi          = {10.1063/1.5030489},
      url          = {https://juser.fz-juelich.de/record/861560},
}