Search

This paper reports on a cavity haloscope search for dark matter axions in the galactic halo in the mass range $2.81$-$3.31$ ${\mu}eV$. This search excludes the full range of axion-photon coupling values predicted in benchmark models of the invisible axion that solve the strong CP problem of quantum chromodynamics, and marks the first time a haloscope search has been able to search for axions at mode crossings using an alternate cavity configuration. Unprecedented sensitivity in this higher mass range is achieved by deploying an ultra low-noise Josephson parametric amplifier as the first stage signal amplifier.

This Letter reports results from a haloscope search for dark matter axions with masses between 2.66 and 2.81 $\mu$eV. The search excludes the range of axion-photon couplings predicted by plausible models of the invisible axion. This unprecedented sensitivity is achieved by operating a large-volume haloscope at sub-kelvin temperatures, thereby reducing thermal noise as well as the excess noise from the ultra-low-noise SQUID amplifier used for the signal power readout. Ongoing searches will provide nearly definitive tests of the invisible axion model over a wide range of axion masses., Comment: 5 pages, 4 figures

We report a new method to study two level fluctuators (TLFs) by measuring the offset charge induced after applying a sudden step voltage to the gate electrode of a single electron transistor. The offset charge is measured for more than 20 hours for samples made on three different substrates. We find that the offset charge drift follows a logarithmic increase over four orders of magnitude in time and that the logarithmic slope increases linearly with the step voltage. The charge drift is independent of temperature, ruling out thermally activated TLFs and demonstrating that the charge fluctuations involve tunneling. These observations are in agreement with expectations for an ensemble of TLFs driven out of equilibrium. From our model, we extract the density of TLFs assuming either a volume density or a surface density., Comment: 5 pages, 5 figures

We report a new class of $c$-axis Josephson tunneling experiments in which a conventional superconductor (Pb) is deposited across a single twin boundary of a $\mathrm{YBa}{}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) crystal. We measure the critical current as a function of the magnitude and angle of a magnetic field applied in the plane of the junction. In all samples, we observe a clear experimental signature of an order parameter phase shift across the twin boundary. These results provide strong evidence for mixed $d$- and $s$-wave pairing in YBCO, with a reversal in the sign of the $s$-wave component across the twin boundary.

We have fabricated a two-dimensional array of Josephson junctions within 100nm of a two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure. The screening provided by the 2DEG causes the array to show superconducting behavior despite a large junction resistance. Varying the resistance per square of the 2DEG changes the dissipation in the electrodynamic environment of the array independently of any other parameters in the system. As the resistance increases, the current-voltage characteristics of the array change from superconducting to insulating in character. {copyright} {ital 1997} {ital The American Physical Society}

In heavily twinned crystals or films of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ (YBCO), ${d}_{{x}^{2}{\ensuremath{-}y}^{2}}$ pairing symmetry is expected to cause the cancellation of first-order Josephson tunneling through a YBCO-Pb tunnel junction grown on an a-b face; any residual tunneling is thus second order. As a result, microwaves at frequency f are predicted to induce steps on the current-voltage characteristic at voltages that are multiples of $\frac{1}{2}(hf/2e)$. Experimentally, steps are observed only at multiples of $hf/2e$, suggesting that s-wave pairing is present in YBCO; however, the simultaneous presence of d-wave pairing is by no means ruled out.

Phase-sensitive spectroscopy is used to characterize fully the complex resistivity tensor of YBa{sub 2}Cu{sub 3}O{sub {ital x}} films in the mixed state, in the frequency range 100 to 500 GHz and temperature range 10 K to {ital T}{sub {ital c}}. We compare our results with the predictions of a vortex model containing three parameters: viscosity, pinning, and Magnus force. The viscosity and Magnus parameter we measure are in disagreement with theoretically predicted values. We show that the discrepancies can be explained by incorporating the effects of a highly anisotropic or {ital d}-wave gap.

The magnetic flux noise generated by films and crystals of B${\mathrm{i}}_{2}$S${\mathrm{r}}_{2}$CaC${\mathrm{u}}_{2}$ ${\mathrm{O}}_{8+y}$ and YB${\mathrm{a}}_{2}$C${\mathrm{u}}_{3}$ ${\mathrm{O}}_{7\ensuremath{-}x}$, up to 30 \ensuremath{\mu}m thick and cooled in nominally zero magnetic field, has been measured at opposing surfaces by two dc superconducting quantum interference devices. For both materials, the noise sources at the two surfaces were highly correlated at specific temperatures in a given cooldown. This result suggests that the observed vortices moved as rigid rods. At other temperatures, the noise was mostly uncorrelated, suggesting that the relevant vortices were pinned at more than one point along their length.

The spectral density S(Φ)(f) = A(2)/(f/1 Hz)(α) of magnetic flux noise in ten dc superconducting quantum interference devices (SQUIDs) with systematically varied geometries shows that α increases as the temperature is lowered; in so doing, each spectrum pivots about a nearly constant frequency. The mean-square flux noise, inferred by integrating the power spectra, grows rapidly with temperature and at a given temperature is approximately independent of the outer dimension of a given SQUID. These results are incompatible with a model based on the random reversal of independent, surface spins.

Coherent time-domain spectroscopy was used to measure the complex transmission tensor of several $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ thin films in magnetic fields up to 6 T at temperatures from 10 to 200 K. The complex conductivity tensor was determined from transmission measurements in the frequency range 150-800 GHz without the need for Kramers-Kronig analysis. Both the real and imaginary parts of ${\ensuremath{\sigma}}_{\mathrm{xy}}$ were found to peak near 40 K, exceeding their normal state values by more than a factor of 10. This behavior is ascribed to the Hall effect of quasiparticles in the superconducting state.

The observation of magnetic flux noise in superconducting thin-film flux transformers of YBa[sub 2]Cu[sub 3]O[sub 7[minus][ital x]] enables us to measure a very small, frequency-dependent resistance [ital R][sub [ital v]]([ital f]) in the films, caused by vortex motion. In static magnetic fields below 10[sup [minus]6] T and at voltages down to 10[sup [minus]20] V Hz[sup [minus]1/2],[ital R][sub [ital v]] is proportional to the frequency [ital f] at which the measurement is made, is approximately independent of temperature, and decreases with increasing static current in the transformer. This behavior cannot be explained by certain currently accepted models of dissipation arising from vortex motion, but is consistent with a model in which independent vortices hop between two potential wells in a confined region.

Hidden U(1) gauge symmetries are common to many extensions of the standard model proposed to explain dark matter. The hidden gauge vector bosons of such extensions may mix kinetically with standard model photons, providing a means for electromagnetic power to pass through conducting barriers. The axion dark matter experiment detector was used to search for hidden vector bosons originating in an emitter cavity driven with microwave power. We exclude hidden vector bosons with kinetic couplings $\ensuremath{\chi}g3.48\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ for masses less than $3\text{ }\text{ }\ensuremath{\mu}\mathrm{eV}$. This limit represents an improvement of more than 2 orders of magnitude in sensitivity relative to previous cavity experiments.

The magnetic-flux noise generated by a patterned film of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ is reversibly suppressed by the application of a supercurrent. The total noise power decreases by an order of magnitude, in quantitative agreement with a model involving thermally activated vortex hopping between pairs of pinning sites of nearly equal pinning energy. The model yields a force constant of 6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ N/m for a vortex at a typical pinning site, and a hopping distance which increases with temperature from 13 nm at 4.2 K to 64 nm at 60 K. The number density of vortices is at least 0.02 \ensuremath{\mu}${\mathrm{m}}^{\mathrm{\ensuremath{-}}2}$, even in a magnetic field of less than 1 \ensuremath{\mu}T.

The domain structure in the nearly commensurate charge-density-wave phase of 1T-${\mathrm{TaS}}_{2}$ has been experimentally determined. Fourier-transformed scanning-tunneling-microscopy images display fine satellite structure in excellent agreement with that predicted by the amplitude- and phase-domain model of Nakanishi and Shiba. Earlier analyses of scanning-tunneling- and atomic-force-microscopy images of 1T-${\mathrm{TaS}}_{2}$ in the nearly commensurate phase are shown to be inadequate for the unambiguous identification and characterization of domain structure.

The spectral density of the magnetic flux noise measured in high-temperature superconductors in low magnetic fields scales approximately as the inverse of the frequency and increases with temperature. We use the temperature and frequency dependence of the noise to determine the pinning energies of individual flux vortices in thermal equilibrium. The distribution of pinning energies peaks below 0.1 eV in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ and near 0.2 eV in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$. The noise power is proportional to the ambient magnetic field, indicating that the vortex motion is uncorrelated.

We propose a model for 1/f flux noise in superconducting devices (f is frequency). The noise is generated by the magnetic moments of electrons in defect states which they occupy for a wide distribution of times before escaping. A trapped electron occupies one of the two Kramers-degenerate ground states, between which the transition rate is negligible at low temperature. As a result, the magnetic moment orientation is locked. Simulations of the noise produced by randomly oriented defects with a density of 5x10(17) m(-2) yield 1/f noise magnitudes in good agreement with experiments.

We present a theory for a finite-length superconducting nanowire coupled to an environment. We show that in the absence of dissipation quantum phase slips always destroy superconductivity, even at zero temperature. Dissipation stabilizes the superconducting phase. We apply this theory to explain the "anti-proximity effect" recently seen by Tian et. al. in Zinc nanowires., Comment: 4 pages, 3 figures

We demonstrate that protons in a liquid acquire a static polarization perpendicular to the plane of a rotating magnetic field. The rotating field was reduced adiabatically to zero, transforming the static polarization in the rotating frame to the laboratory frame. The application of a small magnetic field perpendicular to the polarization induced a free induction decay (FID) that was detected by a superconducting quantum interference device. The results agree with the predictions of the modified Bloch equations. The FID remained observable in the presence of magnetic material, suggesting that this technique may find practical applications.

We measure the effect of dissipation on the minimum zero-bias conductance, G{sup min}{sub 0} , of a superconducting single electron transistor (sSET) capacitively coupled to a two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure. Depleting the 2DEG with a back gate voltage decreases the dissipation experienced by the sSET in situ. We find that G{sup min}{sub 0} increases as the dissipation is increased or the temperature is reduced; the functional forms of these dependences are compared with the model of Wilhelm etal.in which the leads coupled to the sSET are represented by lossy transmission lines.

We have used a dc superconducting quantum interference device to measure the magnetic flux noise generated by the equilibrium vortex density fluctuations associated with the Kosterlitz-Thouless-Berezinskii (KTB) transition in an overdamped Josephson junction array. At temperatures slightly above the KTB transition temperature, the noise is white for {ital f}{lt}{ital f}{sub {xi}} and scales as 1/{ital f} for {ital f}{approx_gt}{ital f}{sub {xi}}. Here {ital f}{sub {xi}}{proportional_to}{xi}{sup {minus}{ital z}}, where {xi} is the correlation length and {ital z} is the dynamic exponent. Moreover, when all frequencies are scaled by {ital f}{sub {xi}}, data for different temperatures and frequencies collapse onto a single curve. {copyright} {ital 1996 The American Physical Society.}

We have measured the current-voltage ( $I$- $V$) characteristics of one- and two-dimensional arrays of normal metal islands linked by small tunnel junctions. The tunneling resistance is large compared to the resistance quantum, and a ground plane reduces the screening length to much less than the interisland spacing. At temperatures well below the island charging energy, we find a threshold voltage ${V}_{T}$ below which little current flows. For $Vg{V}_{T}$, $I$ scales as $({V/V}_{T}\ensuremath{-}1{)}^{\ensuremath{\zeta}}$ where $\ensuremath{\zeta}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.36\ifmmode\pm\else\textpm\fi{}0.1$ (1D) and $1.80\ifmmode\pm\else\textpm\fi{}0.16$ (2D). We interpret this behavior as a dynamic critical phenomenon.

The current-voltage characteristics of submicron normal-metal tunnel junctions at millikelvin temperatures are observed to exhibit a sharp Coulomb blockade with high-resistance thin-film leads, but to be heavily smeared for low-resistance leads. As the temperature is lowered, the zero-bias differential resistance tends asymptotically to a limit that is greater for junctions with high-resistance leads. Both observations are explained in terms of a model in which quantum fluctuations in the external circuit enhance the low-temperature tunneling rate. The predictions are in reasonable agreement with the data.

A reply to the comment on flux pinning energies in high-{ital T}{sub {ital c}} superconductors is given. The extraction of flux-pinning energies from different types of measurements remains a highly contorversial issues. (AIP)

A pair-quasiparticle potential difference arising from a quasiparticle charge imbalance has been observed in superconducting tin films along which there exist both a supercurrent, $I$, and a temperature gradient, $\ensuremath{\nabla}T$. The voltage is proportional to $I\ensuremath{\nabla}T$ at a given temperature, in agreement with the prediction of Pethick and Smith, and diverges as ${(1\ensuremath{-}\frac{T}{{T}_{c}})}^{\ensuremath{-}1}$ for given values of $I$ and $\ensuremath{\nabla}T$.

We report the first observation of quantized energy levels for a macroscopic variable, namely the phase difference across a current-biased Josephson junction in its zero-voltage state. The position of these energy levels is in quantitative agreement with a quantum mechanical calculation based on parameters of the junction that are measured in the classical regime.

The observed dependence of the $\frac{1}{f}$ noise in metal films on the temperature coefficient of resistance and on sample volume, and the observation of frequency-dependent spatial correlations of the noise, suggest the noise is due to a thermal-diffusion mechanism. The assumption of a spectrum for the intrinsic temperature fluctuations with an explicit $\frac{1}{f}$ region, and the condition $〈{(\ensuremath{\Delta}T)}^{2}〉=\frac{k{T}^{2}}{{C}_{V}}$, yield quantitative predictions of the noise power in excellent agreement with experiment.

The spectral density of the voltage noise has been measured in current-biased resistively shunted Josephson junctions in which quantum corrections to the noise are expected to be important. The experimental data are in excellent agreement with theoretical pretions, demonstrating clearly the contribution of zero-point fluctuations that are generated in the shunt at frequencies near the Josephson frequency and mixed down to the measurement frequency.

Al-Al/sub 2/O/sub 3/-Al tunnel junctions were used to measure large increases in the energy gap of superconducting aluminum films in the presence of 10-GHz microwave radiation. When the microwave power was increased at constant temperature, enchancement occurred only for temperatures at which twice the equilibrium energy only for temperatures at which twice the equilibrium energy gap exceeded the photon energy. When the temperature was increased at constant microwave power, enhancement was observed at higher temperatures.

The charge-imbalance voltage induced in a clean superconductor by a supercurrent density J/sub s/ in the presence of a temperature gradient delT is shown to be V=2..pi..v/sub F/llambda/sup 2/..delta.. (J/sub s/xdelT)/3c/sup 2/k/sub B/T/sup 2/ cosh/sup 2/(..delta../2k/sub B/T) g/sub N/S(1-Z), where v/sub F/, l, and lambda (T) are the Fermi velocity, mean free path, and penetration depth, g/sub N/S is the normalized tunnel conductance of the probe junction, and Z (..delta../T) is a known function. This result agrees well with the experiments of Clarke, Fjordboge, and Lindelof.

It is shown that the low-frequency spectral density of the voltage noise in a current-biased Josephson junction with critical current ${I}_{0}$, shunt resistance $R$, and small capacitance is $\frac{e{{I}_{0}}^{2}{R}^{3}}{\ensuremath{\pi}V}$ in the limit $\mathrm{eV}\ensuremath{\gg}{k}_{\mathrm{B}}T{(\frac{I}{{I}_{0}})}^{2}$ and $Ig{I}_{0}$, where $V$ is the voltage and $I$ is the current. The noise arises from zero-point current fluctuations in the shunt resistor. The rounding of the current-voltage characteristic caused by the quantum fluctuations and the effects of nonzero junction capacitance are calculated.

Precessing nuclear quadrupole moments are observed to induce oscillating electric dipole moments in neighboring atoms. The $^{35}\mathrm{Cl}$ nuclei of a single crystal of NaCl${\mathrm{O}}_{3}$ placed between the plates of a capacitor are excited into precession by a rf pulse. After the pulse, the electric polarization induced by the chlorine nuclear quadrupole moments produces a voltage across the capacitor, which is amplified with a dc SQUID. This technique provides information on the location and polarizability of atoms near nuclear quadrupole moments in noncentrosymmetric crystals.

The lifetime $\ensuremath{\tau}$ of the zero-voltage state of a current-biased Josephson junction in the thermal limit has been measured in the presence of a weak microwave perturbation. When the microwave frequency is close to the plasma frequency of the junction, the junction is "resonantly activated" out of the zero-voltage state, with a corresponding reduction in $\ensuremath{\tau}$. The results are well explained by numerical simulations.

The quasiparticle transport current induced in a superconducting aluminum film by a temperature gradient has been detected by measurements of the spatially decaying charge imbalance generated near the end of the sample where the current is divergent. The results are in good agreement with theory when the nonuniformity of the temperature gradient is taken into account. In particular, the inferred value of the thermopower agrees well with the value obtained when the aluminum is in the normal state.

The power spectra of fluctuations in the mean square of the Johnson-noise voltage across small semiconductor and metal films in thermal equilibrium were measured down to ${10}^{\ensuremath{-}4}$ Hz. The spectra have a $\frac{1}{f}$-like behavior that matches the resistance-fluctuation spectra obtained by passing a current through the samples. These measurements contribute strong evidence that $\frac{1}{f}$ noise is due to equilibrium resistance fluctuations.

The current-voltage characteristics of Josephson tunnel junctions shunted by a resistance with a substantial self-inductance exhibit stable negative-resistance regions. Very large increases in the low-frequency voltage noise with noise temperatures of 10/sup 6/ K or more observed near these regions arise from switching between subharmonic modes. Moderate increases in the noise, with temperatures of about 10/sup 3/ K, arise from chaotic behavior. These results are substantiated by analog and digital simulations.

When 500-keV electron irradiation was used to induce defects in polycrystalline copper films maintained at 90 K, the spectral density of the $\frac{1}{f}$ noise voltage across the films increased by as much as 1 order of magnitude, while the resistivity increased by at most 10%. When the films were annealed at progressively higher temperatures, both the $\frac{1}{f}$ noise and the resistivity were reduced; however, at lower annealing temperatures, the fractional reduction in the added noise was substantially more than that in the added resistivity.

The escape rate of an underdamped ($Q\ensuremath{\approx}30$), current-biased Josephson junction from the zero-voltage state has been measured. The relevant parameters of the junction were determined in situ in the thermal regime from the dependence of the escape rate on bias current and from resonant activation in the presence of microwaves. At low temperatures, the escape rate became independent of temperature with a value that, with no adjustable parameters, was in excellent agreement with the zero-temperature prediction for macroscopic quantum tunneling.

An effect analogous to the fountain effect in superfluid helium has been observed in a superconductor for the first time. A temperature gradient across a Pb-Pb point-contact junction produced a small supercurrent, which was detected by measuring the asymmetry of the critical current.

A theory is given of the observable potential difference between pairs and quasiparticles due to the imbalance in the populations of the electronlike and holelike branches of the excitation spectrum of a superconductor, caused by injection of a quasiparticle current.

Using the ac Josephson effect we have demonstrated experimentally that $\frac{2e}{h}$ is identical in lead, tin, and indium to within 1 part in ${10}^{8}$.

Small, two-dimensional islands of silver and gold formed in situ by evaporation onto graphite cleaved in ultrahigh vacuum are imaged atom by atom with a scanning tunneling microscope. These islands contain ordered regions of roughly 50 atoms in rectangular lattices, incommensurate with the substrate lattice, that are not close packed as in the bulk fcc structure. In one series of images, the shorter lattice spacing remained constant at 2.35 +- 0.15 A whereas the longer decreased from 4.05 +- 0.1 to 3.5 +- 0.1 A over a period of roughly 10 min.

A reply to the comment on macroscopic quantum tunneling out of zero/minus/ voltage state of acurrent/minus/biased josephson junction is given.It is blieved that the authors data strongly disprove the classical theory and are con/minus/ sistent with the quantum theory. (AIP)

A Comment on the Letter by Shechao Feng, Patrick A. Lee, and A. Douglas Stone, Phys. Rev. Lett. 56, 1960 (1986).