86,410 research outputs found
Boundary Field Theory Approach to the Renormalization of SQUID Devices
We show that the quantum properties of some Josephson SQUID devices are
described by a boundary sine Gordon model. Our approach naturally describes
multi-junction SQUID devices and, when applied to a single junction SQUID (the
rf-SQUID), it reproduces the known results of Glazman and Hekking. We provide a
detailed analysis of the regimes accessible to an rf-SQUID and to a
two-Josephson junction SQUID device (the dc-SQUID). We then compute the normal
component of the current-response of a SQUID device to an externally applied
voltage and show that the equation describing the current-voltage
characteristic function reduces to well-known results when the infrared cutoff
is suitably chosen. Our approach helps in establishing new and interesting
connections between superconducting devices, quantum brownian motion, fermionic
quantum wires and, more generally, quantum impurity problems.Comment: 54 pages, 1 figure, 1 section added, new figur
Metastability in a nano-bridge based hysteretic DC-SQUID embedded in superconducting microwave resonator
We study the metastable response of a highly hysteretic DC-SQUID made of a
Niobium loop interrupted by two nano-bridges. We excite the SQUID with an
alternating current and with direct magnetic flux, and find different stability
zones forming diamond-like structures in the measured voltage across the SQUID.
When such a SQUID is embedded in a transmission line resonator similar diamond
structures are observed in the reflection pattern of the resonator. We have
calculated the DC-SQUID stability diagram in the plane of the exciting control
parameters, both analytically and numerically. In addition, we have obtained
numerical simulations of the SQUID equations of motion, taking into account
temperature variations and non-sinusoidal current-phase relation of the
nano-bridges. Good agreement is found between experimental and theoretical
results
Stacked Josephson junction SQUID
Operation of a Superconducting Quantum Interference Device (SQUID) made of
stacked Josephson junctions is analyzed numerically for a variety of junction
parameters. Due to a magnetic coupling of junctions in the stack, such a SQUID
has certain advantages as compared to an uncoupled multi-junction SQUID.
Namely, metastability of current-flux modulation can be reduced and a
voltage-flux modulation can be improved if junctions in the stack are
phase-locked. Optimum operation of the SQUID is expected for moderately long,
strongly coupled stacked Josephson junctions. A possibility of making a stacked
Josephson junction SQUID based on intrinsic Josephson junctions in high-Tc
superconductor is discussed.Comment: 4 pages, 3 figures, presented at SQUID-2001 (Stenungsbaden September
2001
Superconducting microstrip amplifiers with sub-Kelvin noise temperature near 4 GHz
We present measurements of an amplifier operating at 3.8 GHz with 150 MHz of
bandwidth based on the microstrip input-coil resonance of a dc superconducting
quantum interference device (SQUID) with submicron Josephson junctions. The
noise temperature is measured using two methods: comparing the signal-to-noise
ratio of the system with and without the SQUID in the amplifier chain, and
using a modified Y-factor technique where calibrated narrowband noise is mixed
up to the SQUID amplifier operating frequency. With the SQUID cooled to 0.35 K
we observe a minimum system noise temperature of 0.55 K, dominated
by the contribution from the SQUID amplifier
Dissipative breathers in rf SQUID metamaterials
The existence and stability of dissipative breathers in rf SQUID
(Superconducting Quantum Interference Device) arrays is investigated
numerically. In such arrays, the nonlinearity which is intrinsic to each SQUID,
along with the weak magnetic coupling of each SQUID to its nearest neighbors,
result in the formation of discrete breathers. We analyze several discrete
breather excitations in rf SQUID arrays driven by alternating flux sources in
the presence of losses. The delicate balance between internal power losses and
input power, results in the formation of dissipative discrete breather (DDB)
structures up to relatively large coupling parameters. It is shown that DDBs
may locally alter the magnetic response of an rf SQUID array from paramagnetic
to diamagnetic or vice versa.Comment: 5 pages, 4 figure
Quantum analysis of a nonlinear microwave cavity-embedded dc SQUID displacement detector
We carry out a quantum analysis of a dc SQUID mechanical displacement
detector, comprising a SQUID with mechanically compliant loop segment, which is
embedded in a microwave transmission line resonator. The SQUID is approximated
as a nonlinear, current dependent inductance, inducing an external flux
tunable, nonlinear Duffing self-interaction term in the microwave resonator
mode equation. Motion of the compliant SQUID loop segment is transduced
inductively through changes in the external flux threading SQUID loop, giving a
ponderomotive, radiation pressure type coupling between the microwave and
mechanical resonator modes. Expressions are derived for the detector signal
response and noise, and it is found that a soft-spring Duffing self-interaction
enables a closer approach to the displacement detection standard quantum limit,
as well as cooling closer to the ground state
Time-division SQUID multiplexers with reduced sensitivity to external magnetic fields
Time-division SQUID multiplexers are used in many applications that require
exquisite control of systematic error. One potential source of systematic error
is the pickup of external magnetic fields in the multiplexer. We present
measurements of the field sensitivity figure of merit, effective area, for both
the first stage and second stage SQUID amplifiers in three NIST SQUID
multiplexer designs. These designs include a new variety with improved
gradiometry that significantly reduces the effective area of both the first and
second stage SQUID amplifiers.Comment: 4 pages, 7 figures. Submitted for publication in the IEEE
Transactions on Applied Superconductivity, August 201
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