arXiv subjects

D. B. Haviland

Publications and source records attributed to D. B. Haviland.

16 recordsLinked to original sources

arXiv2020

IQ Mixer Calibration for Superconducting Circuits

An important device for modulation and frequency translation in the field of circuit quantum electrodynamics is the IQ mixer, an analog component for which calibration is necessary to achieve optimal performance. In this paper, we introduce techniques originally developed for wireless communication applications to calibrate upconversion and downconversion mixers. A Kalman filter together with a controllable carrier frequency offset calibrates both mixers without removing them from the embedding measurement infrastructure. These techniques can be embedded into room temperature control electronics and they will find widespread use as circuit QED devices continue to grow in size and complexity.

quant-ph↗

arXiv2010

Reconstructing nonlinearities with intermodulation spectroscopy

We describe a method of analysis which allows for reconstructing the nonlinear disturbance of a high Q harmonic oscillator. When the oscillator is driven with two or more frequencies, the nonlinearity causes intermodulation of the drives, resulting in a complicated spectral response. Analysis of this spectrum allows one to approximate the nonlinearity. The method, which is generally applicable to measurements based on resonant detection, increases the information content of the measurement without requiring large detection bandwidth, and optimally uses the enhanced sensitivity near resonance to extract information and minimize error due to detector noise.

cond-mat.mes-hall↗

arXiv2007

Spin injection and relaxation in a mesoscopic superconductor

We study spin accumulation and spin relaxation in a superconducting nanowire. Spins are injected and detected by using a set of magnetic tunnel contact electrodes, closely spaced along the nanowire. We observe a giant enhancement of the spin accumulation of up to five orders of magnitude on transition into the superconducting state, consistent with the expected changes in the density of states. The spin relaxation length decreases by an order of magnitude from its value in the normal state. These measurements combined with our theoretical model, allow us to distinguish the individual spin flip mechanisms present in the transport channel. Our conclusion is that magnetic impurities rather than spin-orbit coupling dominate spin-flip scattering in the superconducting state.

cond-mat.mes-hall↗

arXiv2006

Phase-Charge Duality of a Josephson junction in a fluctuating electromagnetic environment

We have measured the current-voltage characteristics of a single Josephson junction placed in a high impedance environment. The transfer of Cooper pairs through the junction is governed by overdamped quasicharge dynamics, leading to Coulomb blockade and Bloch oscillations. Exact duality exists to the standard overdamped phase dynamics of a Josephson junction, resulting in a dual shape of the current-voltage characteristic, with current and voltage changing roles. We demonstrate this duality with experiments which allow for a quantitative comparison with a theory that includes the effect of fluctuations due to finite temperature of the electromagnetic environment.

cond-mat.mes-hall↗

arXiv2006

Coulomb blockade of Cooper pair tunneling and parity effects in the Cooper pair transistor

We have measured the Cooper Pair Transistor (CPT) in a tunable electromagnetic environment consisting of four one-dimensional SQUID arrays. The transport properties of the CPT in the high impedance limit, Z_env>>R_Q=6.45~kΩ, are studied for different ratios of the Josephson coupling energy to the charging energy. As the impedance of the environment is increased, the current-voltage characteristic (IVC) of the CPT develops a Coulomb blockade of Cooper pair tunneling and the measured IVCs agree qualitatively with a theory based on quasicharge dynamics for a CPT. Increasing the impedance of the environment induces a transition in the modulation of the IVC with the gate charge from e-periodic to 2e-periodic.

cond-mat.mes-hall↗

arXiv2006

Diode Effect in Asymmetric Double Tunnel Barriers with Single Metal Nanoclusters

Asymmetric double tunnel barriers with the center electrode being a metal cluster in the quantum regime are studied. The zero dimensionality of the clusters used and the associated quantized energy spectra are manifest in well-defined steps in the current voltage characteristic (IVC). Record high current rectification ratios of 10000 for tunneling through such clusters are demonstrated at room temperature. We are able to account for all of the experimentally observed features by modeling our double barrier structures using a combination of discrete states and charging effects for tunneling through quantum dots.

cond-mat.other↗

arXiv2006

Direct demonstration of decoupling of spin and charge currents in nanostructures

The notion of decoupling of spin and charge currents is one of the basic principles underlying the rapidly expanding feld of Spintronics. However, no direct demonstration of the phenomenon exists. We report a novel measurement, in which a non-equilibrium spin population is created by a point-like injection of current from a ferromagnet across a tunnel barrier into a one dimensional spin channel, and detected differentially by a pair of ferromagnetic electrodes placed symmetrically about the injection point. We demonstrate that the spin current is strictly isotropic about the injection point and, therefore, completely decoupled from the uni-directional charge current.

cond-mat.mes-hall↗

arXiv2006

Enhanced spin accumulation in a superconductor

A lateral array of ferromagnetic tunnel junctions is used to inject and detect non-equilibrium quasi-particle spin distribution in a superconducting strip made of Al. The strip width and thickness is kept below the quasi particle spin diffusion length in Al. Non-local measurements in multiple parallel and antiparallel magnetic states of the detectors are used to in-situ determine the quasi-particle spin diffusion length. A very large increase in the spin accumulation in the superconducting state compared to that in the normal state is observed and is attributed to a diminishing of the quasi-particle population by opening of the gap below the transition temperature.

cond-mat.mes-hall↗

arXiv2006

Spin flip scattering at Al surfaces

Non-local measurements are performed on a multi terminal device to $in-situ$ determine the spin diffusion length and in combination with resistivity measurements also the spin relaxation time in Al films. By varying the thickness of Al we determine the contribution to spin relaxation from surface scattering. From the temperature dependence of the spin diffusion length it is established that the spin relaxation is impurity dominated at low temperature. A comparison of the spin and momentum relaxation lengths for different thicknesses reveals that the spin flip scattering at the surfaces is weak compared to that within the bulk of the Al films.

cond-mat.mes-hall↗

arXiv2004

Fast switching current detection at low critical currents

A pulse-and-hold technique is used to measure the switching of small critical current Josephson junctions. This technique allows one to achieve a good binary detection and therefore measure switching probabilities. The technique overcomes limitations on simple square pulses and allows for the measurement of junctions with critical currents of the order of 10nA with bias pulses of the order of 100ns. A correlation analysis of the switching events is performed to show how the switching probability depends on the wait time between repeated bias pulses.

cond-mat.supr-con↗

arXiv2004

Giant fluctuations of superconducting order parameter in Ferromagnet/superconductor single electron transistors

Spin dependent transport in a ferromagnet/superconductor/ferromagnet single electron transistor is studied theoretically with spin accumulation, spin relaxation, gap suppression, and charging effects taken into account. A strong dependence of the gap on the magnetic state of the outer electrodes is found, which gives rise to a negative magneto-resistance of up to 100 %. We predict that fluctuations of the spin accumulation due to tunneling of quasi-particles can play such an important role as to cause the island to fluctuate between the superconducting

cond-mat.mes-hall↗

arXiv2002

Escape from a zero current state in a one dimensional array of Josephson junctions

A long one dimensional array of small Josephson junctions exhibits Coulomb blockade of Cooper pair tunneling. This zero current state exists up to a switching voltage, Vsw, where there is a sudden onset of current. In this paper we present histograms showing how Vsw changes with temperature for a long array and calculations of the corresponding escape rates. Our analysis of the problem is based on the existence of a voltage dependent energy barrier and we do not make any assumptions about its shape. The data divides up into two temperature regimes, the higher of which can be explained with Kramers thermal escape model. At low temperatures the escape becomes independent of temperature.

cond-mat.mes-hall↗

arXiv2000

Random background charges and Coulomb blockade in one-dimensional tunnel junction arrays

We have numerically studied the behavior of one dimensional tunnel junction arrays when random background charges are included using the ``orthodox'' theory of single electron tunneling. Random background charge distributions are verified in both amplitude and density. The use of a uniform array as a transistor is discussed both with and without random background charges. An analytic expression for the gain near zero gate voltage in a uniform array with no background charges is derived. The gate modulation with background charges present is simulated.

cond-mat.mes-hall↗

arXiv2000

Superconducting and Insulating Behavior in One-Dimensional Josephson Junction Arrays

Experiments on one-dimensional small capacitance Josephson Junction arrays are described. The arrays have a junction capacitance that is much larger than the stray capacitance of the electrodes, which we argue is important for observation of Coulomb blockade. The Josephson energy can be tuned in situ and an evolution from Josephson-like to Coulomb blockade behavior is observed. This evolution can be described as a superconducting to insulating, quantum phase transition. In the Coulomb blockade state, hysteretic current-voltage characteristics are described by a dynamic model which is dual to the resistively shunted junction model of classical Josephson junctions.

cond-mat.mes-hall↗

arXiv1998

Two-dimensional arrays of low capacitance tunnel junctions: general properties, phase transitions and Hall effect

We describe transport properties of two-dimensional arrays of low capacitance tunnel junctions, such as the current voltage characteristic and its dependence on external magnetic field and temperature. We discuss several experiments in which the small capacitance of the junctions plays an important role. In arrays where the junctions have a relatively large charging energy, (i.e. when they have a low capacitance) and a high normal state resistance, the low bias resistance increases with decreasing temperature and eventually at very low temperature the array becomes insulating even though the electrodes in the array are superconducting. This transition to the insulating state can be described by thermal activation. In an intermediate region where the junction resistance is of the order of the quantum resistance and the charging energy is of the order of the Josephson coupling energy, the arrays can be tuned between a superconducting and an insulating state with a magnetic field. We describe measurements of this magnetic-field-tuned superconductor insulator transition, and we show that the resistance data can be scaled over several orders of magnitude. Four arrays follow the same universal function. At the transition the transverse (Hall) resistance is found to be very small in comparison with the longitudinal resistance. However, for magnetic field values larger than the critical value.we observe a substantial Hall resistance. The Hall resistance of these arrays oscillates with the applied magnetic field. features in the magnetic field dependence of the Hall resistance can qualitatively be correlated to features in the derivative of the longitudinal resistance, similar to what is found in the quantum Hall effect.

cond-mat.mes-hall↗

arXiv1997

Coulomb blockade effects in anodised niobium nanostructures

Niobium thin film wires were fabricated using electron beam lithography with a four layer liftoff mask system, and subsequently thinned by anodisation. The resistance along the wire was monitored in situ and trimmed by controlling the anodisation voltage. Depending on the room temperature sheet resistance, samples showed either superconducting or insulating behaviour at low temperatures. A Coulomb blockade was observed for samples exceeding 6 kOhm per square. Samples were also made in a single electron transistor-like geometry with two weak links made by combined angular evaporation and anodisation. Their current-voltage characteristics could be modulated by a voltage applied to an overlapping gate.

cond-mat.mes-hall↗