E.J. Romans

Three-dimensional nanoscale superconducting quantum interference device pickup loops

Nanoscale superconducting quantum interference devices (SQUIDs) have sensitivities approaching that required for single-spin detection, but they only measure fields perpendicular to their plane and can be difficult to tightly couple to magnetic sources on the same chip. To remove these limitations we used focused-ion-beam-induced chemical vapor deposition to directly write a SQUID structure with three-dimensional, freestanding pickup loops using superconducting tungsten nanowires. By applying a localized field, we investigated the pickup loop response, and found that it exhibits Meissner screening corresponding to a penetration depth λ(T) consistent with BCS theory in the dirty limit and λ(0)=330 nm.

Pulsed laser deposition of YBa2Cu3O7-δ and NdBa2Cu3O7-δ thin films : A comparative study

Abstract Thin films of YBa2Cu3O7−δ and NdBa2Cu3O7−δ were grown by pulsed laser deposition. The statistical methods of experimental design and regression analysis were used to correlate the electrical and morphological properties of the films to the growth conditions. For optimised transition temperatures the deposition parameter settings of target–substrate distance, oxygen pressure and laser energy density were found to differ significantly for YBa2Cu3O7−δ and NdBa2Cu3O7−δ. While the maximum transition temperatures obtained were similar (∼90 K) for the two materials, all NdBa2Cu3O7−δ films had a roughness comparable to the c-axis unit cell dimension whereas the YBa2Cu3O7−δ films had a surface roughness which varied between 5–17 nm depending on the growth conditions. In addition, there were differences in the average size and density of non-stoichiometric outgrowths on the two types of film. These differences we relate to a difference in growth mode. Our atomic force microscope and scanning tunnelling microscope studies suggest a 3D screw dislocation mediated growth for YBa2Cu3O7−δ and a 2D layer-by-layer process for NdBa2Cu3O7−δ.

Highly balanced long-baseline single-layer high-Tc superconducting quantum interference device gradiometer

We describe a direct-current superconducting quantum interference device (SQUID) first-order gradiometer fabricated from a single layer of YBa2Cu3O7 on a 30×10 mm2 bicrystal substrate. The device has a baseline of 13 mm and an intrinsic balance of ∼10−3. The gradient sensitivity at 77 K and 1 kHz is 50 fT/(cmHz) in magnetic shielding and 260 fT/(cmHz) when operated unshielded in our laboratory. An antiparallel two-SQUID coupling scheme is employed to optimize the device’s balance to at least 3×10−5.

Electromagnetic nondestructive evaluation: moving HTS SQUIDs, inducing field nulling and dual frequency measurements

We have previously shown that simple, single layer HTS SQUIDs can be used effectively in electromagnetic nondestructive evaluation (NDE) using eddy current techniques in a magnetically unshielded environment. HTS SQUID systems for NDE applications are expected to be small and portable allowing non-stationary measurements to be carried out in the Earths field above a stationary sample. Here we present application-oriented results showing the ability of our HTS electronic gradiometer to cope with the movement of the sensors above a series of simulated flaws in aircraft grade aluminum samples. To permit the detection of fine surface and subsurface structures we have applied active field nulling to the two SQUIDs to increase the effective signal to noise ratio. The excitation signal is applied via a non-superconducting coil to provide a lower field environment for each device. We also present results using a dual frequency eddy current technique to allow depth profiling of flaws in multilayer structures.

Focused Ion Beam NanoSQUIDs as Novel NEMS Resonator Readouts

Nano electromechanical systems (NEMS) represent an important new class of device with wide ranging applications. In this paper we report proposals and calculations for novel methods for excitation and readout of cantilever-style NEMS resonators which are applicable over a wide temperature range. We suggest a Lorentz force-based excitation method and discuss an ultrasensitive readout provided by a nanoSQUID, where the cantilever vibration modulates the inductance of the nanoSQUID loop, allowing potentially sub-picometer amplitude sensitivity to be achieved.

Coupled NanoSQUIDs and Nano-Electromechanical Systems (NEMS) Resonators

Recent developments in cryogenic nano-electromechanical (NEMS) resonators have shown that they can address some fundamental physics, for example in achieving the ground state of a mechanical resonator and entanglement with an electromagnetic resonator. In contrast, little work has been reported on using what is arguably the most sensitive measuring device, a SQUID, to directly interact with, and thus interrogate, a NEMS resonator. We report here our initial experimental results aimed towards forming an optimized coupled micro/nano-mechanical resonator and a focused ion beam patterned Nb SQUID, possessing exceptionally low noise (~200nΦ0/Hz1/2 above 1 kHz), and operating above 4.2 K. We describe our first results from a paddle-shaped mechanical resonator with a diameter of 15 μm coupled to a Nb SQUID loop. Finally, we describe the construction of our first true nanoscale-coupled, double-clamped cantilever and nanoSQUID (rectangular loop area 100 nm × 900 nm).

Proximity effect bilayer nano superconducting quantum interference devices for millikelvin magnetometry

Superconducting quantum interference devices (SQUIDs) incorporating thin film nanobridges as weak links have sensitivities approaching that required for single spin detection at 4.2 K. However, due to thermal hysteresis they are difficult to operate at much lower temperatures which hinder their application to many quantum measurements. To overcome this, we have developed nanoscale SQUIDs made from titanium-gold proximity bilayers. We show that their electrical properties are consistent with a theoretical model developed for heat flow in bilayers and demonstrate that they enable magnetic measurements to be made on a sample at system temperatures down to 60 mK.

High-Tc gradiometric superconducting quantum interference device and its incorporation into a single-layer gradiometer

We describe a first-order gradiometric dc superconducting quantum interference device (SQUID) and its incorporation into a first-order directly coupled single-layer gradiometer. The gradiometric SQUIDs were fabricated from a single layer of YBa2Cu3O7, with a silicon dioxide insulating layer and a gold crossover structure. For several gradiometric SQUIDs, with estimated inductances of order 67 pH, we measured parasitic effective areas in the range 1–2 μm2, approximately two orders of magnitude lower than for conventional narrow linewidth SQUIDs of similar inductance. For a single-layer gradiometer incorporating a gradiometric SQUID, we measured a parasitic effective area of 95 μm2. We demonstrate that for this device, the SQUID itself makes a negligible contribution to the overall parasitic effective area. We show that the improved balance leads to better performance in an unshielded environment.

Planar SQUID gradiometers fabricated on 24/spl deg/ and 30/spl deg/ SrTiO/sub 3/ bicrystals

HTS dc SQUID gradiometers have been fabricated on 24/spl deg/ and 30/spl deg/ SrTiO/sub 3/ bicrystal substrates with slots and flux dams in the SQUID washers. The spatial response of such devices has been measured experimentally and compared with modelled results. Single layer devices, in terms of gradient sensitivity, have characteristics that deviate only slightly from idealized first order gradiometers. The low frequency flux noise of these devices is discussed with particular emphasis on both the unshielded properties and the effect of various cooldown procedures on the noise.

Noise Performance of Niobium Nano-SQUIDs in Applied Magnetic Fields

We have fabricated two different designs of niobium dc nano-SQUIDs using focussed ion beam (FIB) lithography with loops and track widths down to 70 nm. We report on the voltage-flux and noise performance of the devices in zero field and with magnetic fields of up to 1 T applied either in-plane or perpendicular to the plane at temperatures between 5 K and 9 K. We compare the measurements with modeled estimates of the inductance of the structures and the conventional theory of noise in dc SQUIDs.