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The search for new superconducting materials approaching room temperature benefits from having a variety of testing methodologies to confirm and characterize the presence of superconductivity. Often the first signatures of new superconducting species occur incompletely and in very small volume fractions. These trace amounts may be too weak to produce an observable Meissner effect and the resistance may not go completely to zero if the percolation threshold is not met. Under these conditions, secondary behavior--such as transitions or cross overs in the temperature dependence of magnetoresistance, magnetic irreversibility, or thermopower--are often used as indications for the presence of superconductivity. Our group has developed a rather unique set of fast-timescale and dissipative transport measurements that can provide another tool set for confirming and characterizing suspected superconductivity. Here we provide some background for these methods and elucidate their collaborative value in the search for new superconducting materials. Keywords: pairbreaking, pair-breaking, vortex, vortices, theory, tutorial, RTS, room-temperature superconductivity, superconductor, detection, characterization, 8 pages, 9 figures, presented at the "2nd International Workshop on Towards Room Temperature Superconductivity: Hydrides and More

Among the variety of audio configurations in use—from basic monophonic to complex multi-channel surround-sound systems—two-channel stereophonic (stereo) systems have dominated sound reproduction for the purpose of music. Unbeknownst to most of the consumer public, stereo systems in the high end of audio performance can approach the illusion of a live performance, with realistic instrumental timbres and 3D spatialization. Among the three dimensions of the recreated soundstage, the portrayal of elevation remains controversial. In this work, an audio system was assembled that had sufficient fidelity to achieve 3D spatialization, and it was proven through psychoacoustic testing that elevation differentiation of instrumental images can indeed be perceived. The relationship between auditory mechanisms involved in natural-sound localization and stereo-sound imaging is discussed.

We investigated the dissipative regime of the Bi2Te3/FeTe topological insulator-chalcogenide interface superconductor at temperatures well below the Berezinski-Kosterlitz-Thouless transition. We observe a transition in the current-resistance and temperature-resistance curves that quantitatively agrees with the Likharev vortex-explosion phenomenon. In the limit of low temperatures and high current densities, we were able to demonstrate the regime of complete vortex-antivortex dissociation arising from current driven vortex-antivortex pair breaking.

Absence of macroscopic resistance is the most essential trait of superconductors for their practical applications. Yet several mechanisms lead to dissipation in the superconducting state such as weak links, fluctuations, and the motion of flux vortices in the mixed state of type II materials. Thus it is of practical as well as fundamental interest to explore these mechanisms to the fullest extent and to learn how to control them. Regarding the control of flux motion, most research efforts aim at preventing flux motion by introducing a variety of pinning strategies. In the present work, we discuss the circumstances under which it is possible to exclude vortices in the first place, or to short their motion through intrinsic multi-band effects. Experimental results on two model systems, molybdenum-germanium and magnesium diboride films, are presented to support these strategies.

The Bi2Te3/FeTe heterostructure intersects several phenomena and key classes of materials in condensed matter physics: topological insulators, superconductivity, magnetism, and the physics of interfaces. While neither the topological insulator (Bi2Te3) nor the iron chalcogenide (FeTe) are themselves superconductors, superconductivity forms in a thin 7nm interfacial layer between the two. The restricted dimensionality and the extraordinarily conductive normal state, possibly sourced by the topologically protected surface states, have led to the observation of novel phenomena such as the Likharev vortex explosion and transitions in behavior resulting from the interplay between current induced depairing and the Berezinski-Kosterlitz-Thouless regime. Bi2Te3/FeTe also displays the anomalous oscillatory magnetoresistance phenomenon, which we had previously observed in cuprates.

We report oscillatory magnetoresistance in various superconducting films, with a magnetic-field period $\mathrm{\ensuremath{\Delta}}B\ensuremath{\sim}0.1$ T that is essentially independent of sample dimensions, temperature, transport current, and the magnitude and orientation of the magnetic field, including magnetic fields oriented parallel to the film plane. The characteristics of these oscillations seem hard to reconcile with previously established mechanisms for oscillations in magnetoresistance, suggesting the possibility of another type of physical origin.

This work investigates the temporal resolution of human hearing through its discrimination of the time constant of low-pass filtering applied to a periodic signal. While restricting the bandwidth affects both the amplitude spectrum and temporal definition of the signal, the direct amplitude changes in this experiment fall below their just noticeable differences. The discrimination therefore seems to be sensitive to phase in addition to spectral amplitude differences. An upperbound of T ≈ 5 μs was obtained for the threshold time constant, showing that human temporal resolution extends down to time scales shorter than found in the past.

We investigated current induced depairing in the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}/\mathrm{FeTe}$ topological insulator-chalcogenide interface superconductor. The measured depairing current density provides information on the magnetic penetration depth and superfluid density, which in turn shed light on the nature of the normal state that underlies the interfacial superconductivity.

We report the first measurement of the depairing critical current density in the electron-doped infinite-layer Sr 1− x La x CuO 2 system. Resistance-versus-temperature transition curves measured at high pulsed current densities show the classic proportionality between the transition-temperature shift and the two-thirds power of the applied current. The measurement provides an assessment of the superfluid density in the system, and a secondary route for obtaining the penetration depth purely through a transport measurements.

High flux velocities in a superconductor can distort the quasiparticle distribution function and elevate the electronic temperature. Close to T c , a non-thermal distribution function shrinks the vortex core producing the well-known Larkin-Ovchinnikov flux instability. In the present work we consider the opposite limit of low temperatures, where electron-electron scattering is more rapid than electron-phonon, resulting in an electronic temperature rise with a thermal-like distribution function. This produces a different kind of flux instability, due to a reduction in condensate and expansion of the vortex core. Measurements in YBCO films confirm the distinct predictions ofthis mechanism.

Many interesting phenomena and regimes occur when the magnetic field B is oriented parallel to a superconducting film of thickness d much less than the penetration depth λ. The negligible demagnetization effects, together with the nearly complete field penetration, lead to a very large threshold field B c1 ∥ for vortex entry, which is much larger than the bulk B c1. Furthermore, orbital pair breaking is suppressed in parallel fields so that overlap of vortex cores is not the determining condition for the parallel upper critical field B c2 ∥ , but Pauli paramagnetic pair breaking is. In this case B c2 becomes very large and you have novel possibilities such as the Fulde–Ferrell–Larkin–Ovchinnikov phase (FFLO). Finally, when the cross section facing the magnetic field is reduced below a threshold value, Abrikosov vortex cores cease to exist. These features make the determination of the coherence length in the direction of the film thickness problematic. This work demonstrates transport techniques that allow a determination of this important fundamental length through experimental confirmation of the condition when Abrikosov vortices cease to exist.

The electrical transport properties of c-axis oriented Nd1.85Ce0.15CuO4 ? ? superconducting films have been investigated to analyze the pinning mechanism in this material. The samples were grown on SrTiO3 substrates using the dc sputtering high-pressure technique, whereas a detailed analysis of the structure and local composition of the films has been achieved using high-resolution electron microscopy and x-ray microanalysis. Magneto-resistance and current-voltage measurements, in the temperature range from 1.6 to 300 K and in magnetic field up to 9 T, have been reported. In particular, the anisotropic coefficient defined as the ratio between the parallel upper critical field, ab, and the perpendicular one, c, has been evaluated, pointing out the high anisotropy of this compound. Furthermore, the vortex activation energy as a function of the applied magnetic field, parallel and perpendicular to the CuO2 planes, has been derived and compared with the flux-pinning forces to enlighten the peculiar nature of pinning centers in this material.

The motion of flux vortices in Y 1 Ba 2 Cu 3 O 7−δ has been studied in the low temperature limit ( T ∼ 2% of T c ) at extreme current and dissipation densities ( P/V ∼ 10 9 W/cm 3 ). Beyond the regime where flux motion is driven into free flow, occurs an instability that seems to arise from a suppression of the order parameter. The critical resistivity at the point of instability is roughly twice the free-flux-flow value σ * /σ n ∼ 2σ f ∼ 2 B/H c 2 , allowing an estimation of the illusive normal-state resistivity, σ n , in the zero-temperature limit. The estimated σ n saturates to a finite residual value, like a conventional elemental superconductor, and the vortex dissipation continues to be finite as T → 0.

Flux flow was studied over an entire temperature range down to T{approx}2% of T{sub c} by using intense pulsed current densities to overcome flux-vortex pinning. The resistivity at high vortex velocities is proportional to B and roughly follows {rho}{approx}{rho}{sub n}B/H{sub c2} , with a prefactor of order unity. Contrary to some speculation, {rho}{sub n} saturates to a finite residual value as T{yields}0 , indicating a metallic ({rho}{yields}finite ) rather than insulating ({rho}{yields}{infinity}) normal state, and the vortex dissipation continues to be conventional as T{yields}0 . (c) 2000 The American Physical Society.

When the velocity of vortices in a superconductor in the free flux flow regime exceeds the velocity of sound, each vortex produces Cherenkov radiation of sound waves. The velocity field of the lattice ions is localized on the Cherenkov cone, but the forces acting on the lattice are localized on the vortex positions. When the Cherenkov cone coincides with certain directions in the moving vortex lattice, the dissipation exhibits maxima. Since the direction of the Cherenkov cone is determined by the vortex velocity ~electric field! one can expect a series of resonant peaks in the current-voltage characteristic at particular values of electric field. @S0163-1829~99!02638-7#

The transport critical current Jc in a polycrystalline superconductor is a hysteretic function of applied magnetic field H0 due to flux trapping by grains. This effect has been observed by several groups and attempts have been made to calculate the intergranular field Hi as a function of the applied H0 in terms of an effective geometrical demagnetization factor D. In general a first-principles calculation of D is very difficult, and furthermore, D is not constant but is itself a hysteretic function of H0. We develop a self-consistent scheme to extract the D and Hi directly from the Jc(H0) data itself. Our model exploits the fact that there are two field ranges for which the demagnetizing field is a simple function of H0. At low virgin fields, in the Meissner state, the susceptibility χv≈−1/4π is well defined, leading to a multiplicative correction: Hi=H0/(1+D4πχv). For fields that have returned from high values, a complete critical state is established and M is well defined—although χv is unknown. This le...

Much work has gone into the study of the mixed-state Hall effect in high-temperature superconductors (HTS). One of the intriguing features is the reversal of the sign of the Hall angle upon entry into the superconducting state. A common observation for some of the HTS materials, is an empirical decomposition of the measured Hall angle into two terms, one proportional to field (like a normal metal) and the other weakly dependent on field. This observed decomposition can be explained in terms of the Hall effect within the normal vortex cores, combined with the hydrodynamic Magnus force on the body of the vortex. The problem has been treated in the framework of the time-dependent Ginzburg–Landau (TDGL) theory by Kopnin, Ivlev, and Kalatsky [J. Low Temp. Phys. 90 (1993) 1]. We show that the TDGL approach qualitatively explains the observed decomposition, and upon comparing with our data and several other published data, we find rough quantitative agreement as well.

We report a measurement of the depairing critical current density in the electron-doped Nd 2− x Ce x CuO 4− δ cuprate superconductor. Resistance-versus-temperature transition curves measured at high pulsed current densities show the classic proportionality between the transition-temperature shift and the two-thirds power of the applied current. The measurement provides an alternative method for obtaining the penetration depth purely through transport measurements that is not affected by the large paramagnetic background that arises from the Nd 3+ ions, which affects inductive investigations of this quantity.

In type-II superconductors an applied magnetic field B, between the lower and upper critical values, produces a mixed state containing Abrikosov vortices. These vortices contain a quantum of magnetic flux h/2e and consist of a core with depressed order parameter and a pattern of perpetually circulating supercurrents. When B is applied parallel to a thin film, the circulating supercurrents get squeezed by the film surfaces causing the vortex core to become unstable and explode all the way across the film when the thickness d is below the critical value of dc = 4.4ξ; here ξ is the superconducting coherence length. For temperatures above the explosion condition dc(T) > d, the applied B cannot induce single parallel vortices, however perpendicular vortices can be generated spontaneously by thermal fluctuations. We observe a transition from non-dissipative to dissipative behavior at the explosion condition and find that the dynamics of the spontaneous perpendicular vortices can be tuned by the pairbreaking eff...

As the transport current density J in a superconductor is increased beyond its critical value J c , dissipation sets in resulting in a finite resistivity. The superconducting state itself, however, persists up to the pair-breaking value J d , where the kinetic energy associated with the current overcomes the condensation energy. Within this dissipative regime between J c and J d , the transport behavior displays a series of interesting physical phenomena (free flux flow, pair-breaking effect, etc.) as the Lorentz force and kinetic energy associated with the current sequentially overcome various intrinsic (e.g., pair-condensation and interlayer-coupling) and extrinsic (e.g., flux pinning) interactions relevant to the superconducting state. This review discusses these phenomena in the context of recent experiments where the existence of these effects was demonstrated in high-T c superconductors for the first time. Also described are the general principles of the pulsed-current technique used to extend transport measurements to the required high current densities and power-dissipation levels.

We report magnetotransport measurements on a superconducting molybdenum-germanium (MoGe) film of thickness $d=50$ nm in parallel magnetic fields and show evidence of a transition from a Meissner state to a resistive state of spontaneous perpendicular vortices generated by thermal fluctuations above a certain temperature $Tg{T}_{v}(B)$. Here ${T}_{v}$ appears to match the vortex core explosion condition $d\ensuremath{\approx}4.4\ensuremath{\xi}({T}_{v})$, where $\ensuremath{\xi}$ is the coherence length. For $Tg{T}_{v}$, we observed that a nonlinear current-voltage ($I$-$V$) response (Ohmic at low currents and the power law $V\ensuremath{\propto}{I}^{\ensuremath{\beta}}$ at higher $I$) is exponentially dependent on ${B}^{2}$. We propose a model in which the resistive state at $Tg{T}_{v}$ is due to thermally activated hopping of spontaneous perpendicular vortices tuned by the pairbreaking effect of the parallel $B$.

The Hall effect of an YBa[sub 2]Cu[sub 3]O[sub 7[minus][delta]] epitaxial film was studied using high pulsed current densities [ital J] to suppress flux pinning and reveal intrinsic behavior in the free-flux-flow limit. With increasing [ital J] there is an enhancement of the anomalous sign reversal, inconsistent with mechanisms based on pinning and inhomogeneity. At high [ital H] and [ital J] the Hall angle [alpha] appears to decompose into [alpha]=[alpha][sub [ital M]]+[alpha][sub [ital n]]: [alpha][sub [ital M]]([ital T]) saturates to a negative value independent of [ital H] and [ital J], and [alpha][sub [ital n]] ([proportional to][ital H]) has the normal-state value. The results are consistent with time-dependent Ginzburg-Landau calculations.

The equilibrium resistive transition of YBa[sub 2]Cu[sub 3]O[sub 7[minus][delta]] epitaxial films was measured at high pulsed current densities [ital J], to promote homogeneous current flow and free flux flow. The superconducting transition temperature [ital T][sub [ital c]] was then depressed as a function of [ital J] in fixed fields [ital H]. The shifts in [ital T][sub [ital c]], defined near midtransition, scaled following Ginzburg-Landau behavior: [Delta][ital T][sub [ital c]]([ital H],[ital J])/[ital T][sub [ital c]]([ital H],0)=[[ital J]/[ital J][sub 0]][sup 2/3], where [ital J][sub 0] is a field- and current-indpendent constant.

The total resistivity (\ensuremath{\rho}=E/J not dE/dJ) was measured in epitaxial ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ films up to high dissipation levels using a pulsed current source. In the reversible region, \ensuremath{\rho}(J) has an ssS shape; it is Ohmic at low J (thermally activated free-flux motion), goes through a nonlinear transition region (depinning), and becomes Ohmic again at the highest J (free flux flow, i.e., purely viscous motion). The free-flux-flow resistivity ${\mathrm{\ensuremath{\rho}}}_{\mathrm{fff}}$ obeys ${\mathrm{\ensuremath{\rho}}}_{\mathrm{fff}}$/${\mathrm{\ensuremath{\rho}}}_{\mathit{n}}$\ensuremath{\simeq}H/${\mathit{H}}_{\mathit{c}2}$(T), with ${\mathit{dH}}_{\mathit{c}2}$/dT\ensuremath{\approxeq}2 I/K.

A fundamental property of a superconductor in the Meissner state is its ability to carry a current without dissipation. As a result, upon the first application of an electric field one expects to observe an inductive like transient rise in the current until the latter reaches its critical value and dissipation sets in. The present work provides a quantitative real-time experimental demonstration of this transient phase, which yields information on the inertia of the superfluid and shows a signature of superfluid density suppression at high current densities.

Vortex dynamics in molybdenum-germanium superconducting films were found to well approximate the unpinned free limit even at low driving forces. This provided an opportunity to empirically establish the intrinsic character of free flux flow and to test in detail the validity of theories for this regime beyond the Bardeen-Stephen approximation. Our observations are in good agreement with the mean-field result of time dependent Ginzburg-Landau theory. PACS: 74.25.Sv,74.25.Wx,74.25.Uv,74.25.Op,74.25.F- Keywords: vortices, fluxon, Larkin, Ovchinnikov, upper critical magnetic field, Comment: This is the final revised version of a paper that is currently in press. It is expected to appear in Phys. Rev. B in 2010

The mixed state response of disordered magnesium diboride has a con- spicous absence of the usual flux-density (B) dependent broadening of conductance- temperature and voltage-current curves. This B independence implies a vortex vis- cosity that rises linearly with B. We show that this this anomalous viscosity function can arise from the quasiparticle sea of the quenched weaker band and the unusually long electric field penetration depth that delocalizes the vortex electric fields.

One of the most primitive but elusive current-voltage (I-V) responses of a superconductor is when its supercurrent grows steadily after a voltage is first applied. The present work employed a measurement system that could simultaneously track and correlate I(t) and V(t) with sub-nanosecond timing accuracy, resulting in the first clear time-domain measurement of this transient phase where the quantum system displays a Newtonian like response. The technique opens doors for the controlled investigation of other time dependent transport phenomena in condensed-matter systems., 4 pages, 3 figures

The transport critical current density Jc has been measured in high quality polycrystalline YBa2Cu3O7 samples made from extruded preforms. The hysteretic response of Jc to applied magnetic fields was studied as a function of temperature (77–87 K) and sample morphology for various field sweeps in the 0–5 KOe range. A variety of samples show a basic Jc ∼ H−n behavior where n depends on temperature, but is independent of the other variables. A simple model of the local magnetic field distribution is presented and compared to the data, and the theoretical implications of the power law behavior are considered.

The large nonuniform and field-dependent demagnetizing factors of superconducting disks in transverse magnetic fields complicates the determination of the lower critical field and critical current from magnetization. Correcting the applied field with a constant ellipsoidally approximated demagnetization correction D\ensuremath{'} can result in significant errors. In this study of the magnetization characteristics of lead and Nb-Ti disks with various aspect ratios a, we find an empirical relation D\ensuremath{'}(a) that describes the scaling of the applied-field value ${\mathrm{H}}_{\mathrm{c}}^{\ensuremath{'}}$ (${\mathrm{H}}_{\mathrm{c}1}^{\ensuremath{'}}$), at which flux penetration occurs, with respect to the intrinsic (lower) critical fields ${\mathrm{H}}_{\mathrm{c}}$ (${\mathrm{H}}_{\mathrm{c}1}$). A model is described for determining ${\mathrm{H}}_{\mathrm{c}1}$ and ${\mathrm{J}}_{\mathrm{c}}$ for such a geometry. The results have important implications for various magnetic measurements in high-${\mathrm{T}}_{\mathrm{c}}$ superconductors. The errors that can result in the measured values of ${\mathrm{H}}_{\mathrm{c}1}$ and ${\mathrm{J}}_{\mathrm{c}}$, in the inferred penetration depths, and in the effective-mass anisotropies, are discussed.

In conventional flux flow, vortex dissipation is localized in the vicinity of the vortex core leading to a viscous coefficient that is independent of flux density B and a flux-flow conductance Gf 1 /B. An anomalous behavior arises when a substantial quasiparticle population exists away from the cores and when the electric field and dissipation extend into those regions—a scenario that is realized in a disordered two-band superconductor with slow branch-imbalance relaxation. In this case, rises linearly with B and Gf becomes independent of B, as observed in disordered magnesium diboride.

At high dissipation levels, vortex motion in a superconducting film has been observed to become unstable at a certain critical vortex velocity v*. At substrate temperatures substantially below Tc, the observed behavior can be accounted for by a model in which the electrons reach an elevated temperature relative to the phonons and the substrate. Here we examine the underlying assumptions concerning energy flow and relaxation times in this model. A calculation of the rate of energy transfer from the electron gas to the lattice finds that at the instability, the electronic temperature reaches a very high value close to the critical temperature. Our calculated energy relaxation times are consistent with those deduced from the experiments. We also estimate the phonon mean free path and assess its effect on the flow of energy in the film., 8 pages, 7 figures

The flux dynamics in magnesium diboride films becomes altered by scattering resulting in an unprecedented quenching of flux motion that is beyond conventional vortex pinning. At currents approaching the pair-breaking value and magnetic fields approaching the upper-critical field, the conductance diverges and the current-resistance and temperature–conductance curves become parallel, showing mainly a shift in the transition due to pair-breaking with minimal signs of flux motion.

The mixed-state transport behavior of magnesium diboride films shows a suppression of flux motion that is more severe than is typical for conventional vortex pinning. At currents approaching the pair-breaking value and magnetic fields approaching the upper-critical field, the conductance diverges and the current-resistance and temperature-conductance curves become parallel, showing mainly a shift in the transition due to pairbreaking with minimal signs of flux motion.

The transport of electrical current through a superconductor falls into three broad regimes: non-dissipative, dissipative but superconducting, and normal or non-superconducting. These regimes are demarkated by two definitions of critical current: one is the threshold current above which the superconductor enters a dissipative (resistive) state; the other is the thermodynamic threshold above which the superconductivity itself is destroyed and the superconducting order parameter vanishes. The first threshold defines the conventional critical current density Jc and the second defines the depairing (or pair-breaking) current Jd. Type II superconductors in the mixed state have quantized flux vortices, which tend to move when acted upon by the Lorentz driving force of an applied transport current. In such a mixed state the resistance vanishes only when vortices are pinned in place by defects and the applied current is below the threshold Jc required to overcome pinning and mobilize the vortices. Typically Jd >> Jc and a direct experimental measurement of Jd over the entire temperature range (0 < T < Tc) is prohibited by the enormous power dissipation densities (p ~ 10^10 -- 10^12 W/cm^3) needed to reach the normal state. In this work, intense pulsed signals were used to extend transport measurements to unprecedented power densities (p ~ 10^9 -- 10^10 W/cm^3). This together with MgB2's combination of low normal-state resistivity and high transition temperature have permitted a direct estimation of \jd over the entire temperature range. This review describes our experimental investigation of current-induced depairing in MgB2, and provides an introduction to the phenomenological theories of superconductivity and how the observations fit in their context., Accepted for publication as a Topical Review in the Journal of Physics: Condensed Matter (Fall 2004)

A pulsed‐current technique was used to measure the extended I–V characteristics of a wide variety of prototype high‐temperature‐superconductor (HTS) leads. It was found that the average resistivity rises with J(≳Jc) more gradually than in conventional superconductors—often remaining very small compared to silver, for values of J(≫Jc) that are high enough to be practically useful. This observation, combined with the low thermal conductivity (∼50 times smaller than Ag), should extend the utility of HTS leads to the dissipative regime where J/Jc≫1.

We have investigated the low-temperature (T < Tc=10) mixed-state current-voltage (IV) response of magnesium diboride films beyond the point where the superconductivity is completely destroyed and the system enters the normal state. The resistance-versus-current R(I) curves are extremely steep and featureless, with a critical current density j c, marking the onset of dissipation, that is unusually high (j c>j d/10) with respect to the depairing current density j d. At large flux densities Hc2/10 $\lessim$ B $\lessim$Hc2, the R(I) curve has a functional shape that is largely independent of B, indicating that the rise in resistivity with increasing current occurs mainly due to pair-breaking rather than flux motion. The macroscopic destruction current I*, which drives the system normal, has a B/Hc2: $\sqrt{B/H_{c2}}$ flux-density dependence, suggesting that the vortices mainly reduce the effective cross section over which a current of effective density j ~ j d flows.

We have conducted pulsed transport measurements on c-axis-oriented magnesium diboride films over the entire relevant ranges of magnetic field $0\ensuremath{\lesssim}H\ensuremath{\lesssim}{H}_{c2}$ (where ${H}_{c2}$ is the upper critical field) and current density $0\ensuremath{\lesssim}j\ensuremath{\lesssim}{j}_{d}$ (where ${j}_{d}\ensuremath{\sim}{10}^{7}\mathrm{A}/{\mathrm{cm}}^{2}$ is the depairing current density). The intrinsic disorder of the films combined with the large coherence length and three dimensionality, compared to cuprate superconductors, results in a sixfold enhancement of ${H}_{c2}$ (to $17\ifmmode\pm\else\textpm\fi{}3\mathrm{T})$ and raises the depinning current density ${j}_{c}$ to within an order of magnitude of ${j}_{d}.$ The current-voltage response has an unusually steep nonlinear shape at all fields, resulting from a combination of depinning and pair breaking, and has no trace of an Ohmic free-flux-flow regime.

The pair-breaking critical current density j_{d} of magnesium diboride was determined over its entire temperature range by a pulsed dc transport measurement. At fixed low values of current density j, the resistive transition temperature Tc shifts in the classic \Delta T_{C}(j)/T_{C}(0) \propto -[j/j_{d}(0)]^{2/3} manner, with a projected j_{d}(0) \approx 2 \times 10^{7}$ A/cm^{2}. The directly measured j_{d}(0), from current-voltage ($I$-$V$)curves at different fixed temperatures, has a similar value and the overall temperature dependence j_{d}(T) and magnitude are consistent with Ginzburg-Landau theory. Keywords: pair, breaking, depairing, superconductor, superconductivity, flux, fluxon, vortex, mgb2, Comment: Submitted to Phys. Rev. Lett. Related preprints/reprints by our group can be found on http://www.physics.sc.edu/kunchur

A flux instability occurs in superconductors at low temperatures, where $e\mathrm{\text{\ensuremath{-}}}e$ scattering is more rapid than $e\mathrm{\text{\ensuremath{-}}}\mathrm{p}\mathrm{h}$, whereby the dissipation significantly elevates the electronic temperature while maintaining a thermal-like distribution function. The reduction in condensate and rise in resistivity produce a nonmonotonic current-voltage response. In contrast to the Larkin-Ovchinnikov instability where the vortex shrinks, in this scenario the vortex expands and the quasiparticle population rises. Measurements in ${\mathrm{Y}}_{1}{\mathrm{B}\mathrm{a}}_{2}{\mathrm{C}\mathrm{u}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ agree quantitatively with the distinct predictions of this mechanism.

When free flux flow is pushed beyond its instability, the homogeneous flow becomes spatially distorted leading to a new class of dynamic phases with steps in resistivity. At high-flux densities B, the relatively incompressible vortex matter fragments into domains of constant shear curvature, leading to a horizontal-sawtooth-shaped current-voltage characteristic. Measurements on Y 1 Ba 2 Cu 3 O 7 - Φ films confirm this behavior and are quantitatively consistent with the model, which has no adjustable parameters.

The interaction of sound waves with vortices in type-II superconductors is a field of active study: The attenuation of transverse sound waves through a vortex lattice has been analyzed experimentally in Ref. 1; the generation of ultrasonic waves in the mixed state was observed experimentally; 2 sound attenuation by helicon-vortex modes was considered in Ref. 3; sound-vortex interaction and the acoustic Faraday effect has been studied in Ref. 4; the generation of sound waves by moving vortices was considered theoretically. 5 Since vortex motion is driven by an electric current, any peculiarity of the vortex radiation friction due to emission of acoustic waves influences the current-voltage characteristic, and will imprint a nonlinear feature on the I-V

Current-voltage characteristics were measured in the mixed state of Y1Ba2Cu3O(7-delta) superconducting films in the regime where flux flow becomes unstable and the differential conductivity dj/dE becomes negative. Under conditions where its negative slope is steep, the j(E) curve develops a pronounced staircase like pattern. We attribute the steps in j(E) to the formation of a dynamical phase consisting of the succesive nucleation of quantized distortions in the local vortex velocity and flux distribution within the moving flux matter., 5 pages, 3 figures

A model is presented that considers the magnetic relaxation from thermally activated flux creep in a superconductor, after the application of two consecutive field steps separated by a waiting time {ital t}{sub {ital w}}. The time evolution of the resulting superposition of flux distributions is expected to show a characteristic inflection point at a time {ital t}{sub {ital i}}{sup {prime}}. The behavior is reminiscent of, but fundamentally different from, the memory effect of the superconducting-glass model. Measurements on Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} single crystals show the predicted temperature and field dependence of {ital t}{sub {ital i}}{sup {prime}}.

Experiments were conducted to assess the human discriminability of temporal convolution. The experiments employed either lowpass filtering or delays due to spatial misalignment. By using special ultrahigh-fidelity equipment, both experiments demonstrated discernment at a ~ 5 microsecond timescale, which is much shorter than found previously. While the signal manipulations affect both the spectrum and temporal definition of the signal, the spectral changes fall below the known just noticeable differences. The discrimination may therefore involve mechanisms additional to the auditory system's ability to distinguish spectral amplitude differences. Furthermore the present work shows that typical instrumentation used in psychoacoustic research may, for some purposes, have insufficient temporal speed and bandwidth. Also this work proves that that digital sampling rates used in consumer audio are insufficient for fully preserving transparency.