arXiv subjects

Milind N. Kunchur

Publications and source records attributed to Milind N. Kunchur.

13 recordsLinked to original sources

arXiv2023

The Human Auditory System and Audio

This work reviews the human auditory system, elucidating some of the specialized mechanisms and non-linear pathways along the chain of events between physical sound and its perception. Customary relationships between frequency, time, and phase--such as the uncertainty principle--that hold for linear systems, do not apply straightforwardly to the hearing process. Auditory temporal resolution for certain processes can be a hundredth of the period of the signal, and can extend down to the microseconds time scale. The astonishingly large number of variations that correspond to the neural excitation pattern of 30000 auditory nerve fibers, originating from 3500 inner hair cells, explicates the vast capacity of the auditory system for the resolution of sonic detail. And the ear is sensitive enough to detect a basilar-membrane amplitude at the level of a picometer, or about a hundred times smaller than an atom. This article surveys and provides new insights into some of the impressive capabilities of the human auditory system and explores their relationship to fidelity in reproduced sound.

q-bio.NC↗

arXiv2019

Dissipative-regime measurements as a tool for confirming and characterizing near-room-temperature superconductivity

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

cond-mat.supr-con↗

arXiv2019

Evaluating Superconductors through Current Induced Depairing

The phenomenon of superconductivity occurs in the phase space of three principal parameters: temperature T, magnetic field B, and current density Jd . The critical temperature Tc is one of the first parameters that is measured and in a certain way defines the superconductor. From the practical applications point of view, of equal importance is the upper critical magnetic field Bc2 and conventional critical current density Jc (above which the system begins to show resistance without entering the normal state). However, a seldom-measured parameter, the depairing current density Jd , holds the same fundamental importance as Tc and Bc2, in that it defines a boundary between the superconducting and normal states. A study of Jd sheds unique light on other important characteristics of the superconducting state such as the superfluid density and the nature of the normal state below Tc, information that can play a key role in better understanding newly-discovered superconducting materials. From a measurement perspective, the extremely high values of Jd make it difficult to measure, which is the reason why it is seldom measured. Here, we will review the fundamentals of current-induced depairing and the fast-pulsed current technique that facilitates its measurement and discuss the results of its application to the topological-insulator/chalcogenide interfacial superconducting system. Keywords: pairbreaking, pair-breaking, vortex, vortices, theory, tutorial, RTS, room-temperature supeconductivity

cond-mat.supr-con↗

arXiv2015

Thermally-activated dynamics of spontaneous perpendicular vortices tuned by parallel magnetic fields in thin superconducting films

We report magneto-transport 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 $T>T_v(B)$. Here $T_v$ appears to match the vortex core explosion condition $d\approx 4.4ξ(T_v)$, where $ξ$ is the coherence length. For $T>T_v$, we observed that a nonlinear current-voltage ($IV$) response (Ohmic at low currents and the power law $V\propto I^β$ at higher $I$) is exponentially dependent on $B^2$. We propose a model in which the resistive state at $T>T_v$ is due to thermally-activated hopping of spontaneous perpendicular vortices tuned by the pairbreaking effect of the parallel $B$. keywords: vortex, vortices, fluxon, flux lattice, mixed state, lower critical field

cond-mat.supr-con↗

arXiv2015

Anomalous oscillatory magnetoresistance in superconducting transitions

We have discovered an oscillatory magnetoresistance phenomenon in a wide range of superconducting systems, with a periodicity that is essentially independent of temperature, transport current, magnetic field, and even material parameters. The nearly universal period points to a possible fundamental mechanism deeper than superconductivity itself, and may result from intrinsic pair-breaking mechanisms at sub-atomic length scales.

cond-mat.supr-con↗

arXiv2010

Vortex instability in molybdenum-germanium superconducting films

We studied the high driving force regime of the current-voltage transport response in the mixed state of amorphous molybdenum-germanium superconducting films to the point where the flux flow becomes unstable. The observed nonlinear response conforms with the classic Larkin-Ovchinikov (LO) picture with a quasiparticle-energy-relaxation rate dominated by the quasiparticle recombination process. The measured energy relaxation rate was found to have a magnitude and temperature dependence in agreement with theory. PACS: 74.40.Gh, 74.25.Uv, 72.15.Lh, 73.50.Gr, 73.50.Fq Keywords: fluxon, vortices, TDGL, FFF, negative differential conductivity, NDC, non-monotonic IV curve

cond-mat.supr-con↗

arXiv2010

Evaluating free flux flow in low-pinning molybdenum-germanium superconducting films

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

cond-mat.supr-con↗

arXiv2010

The ballistic acceleration of a supercurrent in a superconductor

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.

cond-mat.supr-con↗

arXiv2006

Energy Relaxation at a Hot-Electron Vortex Instability

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.

cond-mat.supr-con↗

arXiv2004

Current-Induced Pair Breaking in Magnesium Diboride

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.

cond-mat.supr-con↗

arXiv2002

The pair-breaking critical current density of magnesium diboride

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 ΔT_{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

cond-mat.supr-con↗

arXiv2002

Unstable flux flow due to heated electrons in YBaCuO films

A flux instability occurs in superconductors at low temperatures, where ee scattering is more rapid than ep, 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 non-monotonic 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 YBaCuO agree quantitatively with the distinct predictions of this mechanism.

cond-mat.supr-con↗

arXiv2002

Steps in the Negative-Differential-Conductivity Regime of a Superconductor

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.

cond-mat.supr-con↗