Alexey Bezryadin - Profile on Academia.edu (original) (raw)

Papers by Alexey Bezryadin

Bulletin of the American Physical Society, Mar 24, 2011

We report on observations of nonequilibrium pulsing states in microwave (i.e., GHz) coplanar wave... more We report on observations of nonequilibrium pulsing states in microwave (i.e., GHz) coplanar waveguide(CPW) resonators consisting of superconducting MoGe strips interrupted by a trench and connected by one or more suspended superconducting nanowires. The Lorentzian resonance peak shows a "crater" when driven past the critical current of the nanowire, leading to a "pulsing" state. In the pulsing state, the supercurrent grows until it reaches the critical current, at which point all stored energy quickly dissipates through Joule heating. We develop a phenomenological model of resonator-nanowire systems, which explains the experimental data quantitatively. For the case of resonators comprising two parallel nanowires and subject to an external magnetic field, we find field-driven oscillations of the onset power for crater formation, as well as the occurrence of a new state, in which the periodic pulsing effect is such that only the weaker wire participates in the dissipation process.

Bulletin of the American Physical Society, Mar 25, 2011

W.Meissner [Z. Physik. 86, 787 (1933)], who observed zero resistance in SNS pressed contacts, man... more W.Meissner [Z. Physik. 86, 787 (1933)], who observed zero resistance in SNS pressed contacts, many manifestations of the superconducting proximity effect have been reported. Recently it was shown that when closely spaced superconducting leads are placed on graphene, the proximity effect is induced and a supercurrent can flow between the electrodes. Here we fabricate graphene proximityeffect junctions (GPJ) and compare them to Josephson junctions (JJ). As the bias current is increased to near the critical current, a thermal escape from the washboard potential can occur driving the junction into the runaway voltage state. The standard deviation of the switching current is measured as a function of temperature and compared to the thermal and quantum escape models for JJs. We find that the temperature dependence of the standard deviation of switching currents of graphene proximity junctions is qualitatively different from the well-studied behavior of the insulator-based JJs. Possible reasons will be discussed.

Physical review, Oct 14, 2016

We present a type of transmon split-junction qubit which can be tuned by Meissner screening curre... more We present a type of transmon split-junction qubit which can be tuned by Meissner screening currents flowing in the adjacent superconducting film electrodes. The best detected relaxation time (T 1) was of the order of 50 μs and the dephasing time (T 2) about 40 μs. The achieved period of oscillation with magnetic field was much smaller than in the usual SQUID-based transmon qubits; thus a strong effective field amplification has been realized. This Meissner qubit allows a strong mixing of the current flowing in the qubit junctions and the currents generated by the Abrikosov vortices. We present a quantitative analysis of the radiation-free relaxation in qubits coupled to the Abrikosov vortices. The observation of coherent quantum oscillations provides strong evidence that the position of the vortex as well as its velocity do not have to accept exact values but can be smeared in the quantum mechanical sense. The eventual relaxation of such states contributes to an increased relaxation rate of the qubit coupled to vortices. Such relaxation is described using basic notions of the Caldeira-Leggett quantum dissipation theory.

Physical Review X, Jun 10, 2015

Macroscopic quantum tunneling is a fundamental phenomenon of quantum mechanics related to the act... more Macroscopic quantum tunneling is a fundamental phenomenon of quantum mechanics related to the actively debated topic of quantum-to-classical transition. The ability to realize macroscopic quantum tunneling affects implementation of qubit-based quantum computing schemes and their protection against decoherence. Decoherence in qubits can be reduced by means of topological protection, e.g., by exploiting various parity effects. In particular, paired phase slips can provide such protection for superconducting qubits. Here, we report on the direct observation of quantum paired phase slips in thin-wire superconducting loops. We show that in addition to conventional single phase slips that change the superconducting order parameter phase by 2π, there are quantum transitions that change the phase by 4π. Quantum paired phase slips represent a synchronized occurrence of two macroscopic quantum tunneling events, i.e., cotunneling. We demonstrate the existence of a remarkable regime in which paired phase slips are exponentially more probable than single ones.

Physical Review B, Jul 9, 2012

We measure quantum and thermal phase-slip rates using the standard deviation of the switching cur... more We measure quantum and thermal phase-slip rates using the standard deviation of the switching current in superconducting nanowires. Our rigorous quantitative analysis provides firm evidence for the presence of quantum phase slips (QPS) in homogeneous nanowires at high bias currents. We observe that as temperature is lowered, thermal fluctuations freeze at a characteristic crossover temperature Tq, below which the dispersion of the switching current saturates to a constant value, indicating the presence of QPS. The scaling of the crossover temperature Tq with the critical temperature Tc is linear, Tq ∝ Tc, which is consistent with the theory of macroscopic quantum tunneling. We can convert the wires from the initial amorphous phase to a single crystal phase, in situ, by applying calibrated voltage pulses. This technique allows us to probe directly the effects of the wire resistance, critical temperature and morphology on thermal and quantum phase slips.

Physical review, Sep 11, 2017

We study nanostructures based on two ultrathin superconducting nanowires connected in parallel to... more We study nanostructures based on two ultrathin superconducting nanowires connected in parallel to form a superconducting quantum interference device (SQUID). The measured function of the critical current versus magnetic field, IC (B), is multivalued, asymmetric and its maxima and minima are shifted from the usual integer and half integer flux quantum points. We also propose a lowtemperature-limit model which generates accurate fits to the IC (B) functions and provides verifiable predictions. The key assumption of our model is that each wire is characterized by a sample-specific critical phase, φC , defined as the phase difference at which the supercurrent in the wire is the maximum. For our nanowires φC is much greater than the usual π/2, which makes a qualitative difference in the behavior of the SQUID. The nanowire current-phase relation is assumed linear, since the wires are much longer than the coherence length. The model explains single-valuedness regions where only one vorticity value, nv, is stable. Also, it predicts regions where multiple vorticity values are stable because the Little-Parks (LP) diamonds, which describe the region of stability for each winding number nv in the current-field diagram, can overlap. We also observe and explain regions in which the standard deviation of the switching current is independent of the magnetic field. We develop a technique that allows a reliable detection of hidden phase slips and use it to determine the boundaries of the LP diamonds even at low currents where IC (B) is not directly measurable.

Applied Physics Letters, Jun 13, 2011

A thin-film Fabry-Perot superconducting resonator is used to reveal the Little and Parks ͑LP͒ eff... more A thin-film Fabry-Perot superconducting resonator is used to reveal the Little and Parks ͑LP͒ effect ͓Phys. Rev. Lett. 9, 9 ͑1962͔͒, even at temperatures much lower than the critical temperature. A pair of parallel nanowires is incorporated into the resonator at the point of the supercurrent antinode. As the magnetic field is ramped, Meissner currents develop, changing the resonance frequency of the resonator. The LP oscillation is revealed as a periodic set of distorted parabolas observed in the transmission of the resonator and corresponds to the states of the wire loop having different vorticities. We also report a direct observation of single and double phase slip events.

Physical Review Letters, Feb 28, 2012

We study the stochastic nature of switching current in hysteretic current-voltage characteristics... more We study the stochastic nature of switching current in hysteretic current-voltage characteristics of superconductor-graphene-superconductor (SGS) junctions. We find that the dispersion of the switching current distribution scales with temperature as σI ∝ T α G with αG as low as 1/3. This observation is in sharp contrast with the known Josephson junction behavior where σI ∝ T α J with αJ = 2/3. We propose an explanation using a generalized version of Kurkijärvi's theory for the flux stability in rf-SQUID and attribute this anomalous effect to the temperature dependence of the critical current which persists down to low temperatures.

Nanotechnology, Sep 5, 2011

We present a method for in situ tuning of the critical current (or switching current) and critica... more We present a method for in situ tuning of the critical current (or switching current) and critical temperature of a superconducting MoGe nanowire using high bias voltage pulses. Our main finding is that as the pulse voltage is increased, the nanowire demonstrates a reduction, a minimum and then an enhancement of the switching current and critical temperature. Using controlled pulsing, the switching current of a superconducting nanowire can be set exactly to a desired value. These results correlate with in situ transmission electron microscope imaging where an initially amorphous nanowire transforms into a single crystal nanowire by high bias voltage pulses. We compare our transport measurements to a thermally activated model of Little's phase slips in nanowires.

Physical Review B, Oct 13, 2010

We report microwave transmission measurements of superconducting Fabry-Perot resonators (SFPR), h... more We report microwave transmission measurements of superconducting Fabry-Perot resonators (SFPR), having a superconducting nanowire placed at a supercurrent antinode. As the plasma oscillation is excited, the supercurrent is forced to flow through the nanowire. The microwave transmission of the resonator-nanowire device shows a nonlinear resonance behavior, significantly dependent on the amplitude of the supercurrent oscillation. We show that such amplitude-dependent response is due to the nonlinearity of the current-phase relationship (CPR) of the nanowire. The results are explained within a nonlinear oscillator model of the Duffing oscillator, in which the nanowire acts as a purely inductive element, in the limit of low temperatures and low amplitudes. The low quality factor sample exhibits a "crater" at the resonance peak at higher driving power, which is due to dissipation. We observe a hysteretic bifurcation behavior of the transmission response to frequency sweep in a sample with a higher quality factor. The Duffing model is used to explain the Duffing bistability diagram. We also propose a concept of a nanowire-based qubit that relies on the current dependence of the kinetic inductance of a superconducting nanowire.

New Journal of Physics, Jun 13, 2017

The demand for low-dissipation nanoscale memory devices is as strong as ever. As Moore's law is s... more The demand for low-dissipation nanoscale memory devices is as strong as ever. As Moore's law is staggering, and the demand for a low-power-consuming supercomputer is high, the goal of making information processing circuits out of superconductors is one of the central goals of modern technology and physics. So far, digital superconducting circuits could not demonstrate their immense potential. One important reason for this is that a dense superconducting memory technology is not yet available. Miniaturization of traditional superconducting quantum interference devices is difficult below a few micrometers because their operation relies on the geometric inductance of the superconducting loop. Magnetic memories do allow nanometer-scale miniaturization, but they are not purely superconducting (Baek et al 2014 Nat. Commun. 5 3888). Our approach is to make nanometer scale memory cells based on the kinetic inductance (and not geometric inductance) of superconducting nanowire loops, which have already shown many fascinating properties (Aprili 2006 Nat. Nanotechnol. 1 15; Hopkins et al 2005 Science 308 1762). This allows much smaller devices and naturally eliminates magnetic-field cross-talk. We demonstrate that the vorticity, i.e., the winding number of the order parameter, of a closed superconducting loop can be used for realizing a nanoscale nonvolatile memory device. We demonstrate how to alter the vorticity in a controlled fashion by applying calibrated current pulses. A reliable read-out of the memory is also demonstrated. We present arguments that such memory can be developed to operate without energy dissipation. Power management and cooling demands of high performance processors have become one of the main obstacles to further progress of the computing devices. Thus development of the superconductor-based cryogenic computers which appear particularly suitable for overcoming these problems attracts much attention [4]. Nanoscale lowdissipation memory that could be integrated naturally with superconducting circuits remains one of the most essential elements that still needs to be demonstrated. Typical 'single-flux quanta' (SFQ) digital superconductor devices are based on manipulation of individual quanta of magnetic flux in circuits composed of Josephson junctions and inductive loops [5], and recently made much progress towards the large-scale practical logic circuits-see, e.g. [6-9]. However, direct applications of the SFQ principles to memory devices (see, e.g., [10, 11]) remain not quite competitive with other approaches because of the relatively large size, in the micrometer range, of the memory cells, determined by both the size of the Josephson junctions and geometric inductances. This motivates a search for hybrid memory based either on direct incorporation of semiconducting memory elements into superconductor circuits [12] or on the development of Josephson junctions with ferromagnetic barriers-see, e.g., [1, 13-16]. While promising in several respects, hybrid structures face many problems related to conversion between different forms of information representation and fabrication difficulties, and still did not reach the level of completely satisfactory practical circuits. The goal of this work is to suggest and demonstrate the main operating principles of all-superconducting memory cells which can be scaled down in size into the range of few tens of nanometers, and which do not suffer from the aforementioned problems. The memory is based on the loops made of superconducting nanowires [3], in which

arXiv (Cornell University), Mar 11, 2013

We perform measurements of phase-slip-induced switching current events on different types of supe... more We perform measurements of phase-slip-induced switching current events on different types of superconducting weak links and systematically study statistical properties of the switching current distributions. We employ two types of devices in which a weak link is formed either by a superconducting nanowire or by a graphene flake subject to proximity effect. We demonstrate that, independently on the nature of the weak link, higher moments of the distribution take universal values. In particular, the third moment (skewness) of the distribution is close to −1 both in thermal and quantum regimes. The fourth moment (kurtosis) also takes a universal value close to 5. The discovered universality of skewness and kurtosis is confirmed by an analytical model. Our numerical analysis shows that introduction of extraneous noise into the system leads to significant deviations from the universal values. We suggest to use the discovered universality of higher moments as a robust tool for checking against undesirable effects on noise in various types of measurements.

arXiv (Cornell University), Feb 1, 2012

We measure quantum and thermal phase-slip rates using the standard deviation of the switching cur... more We measure quantum and thermal phase-slip rates using the standard deviation of the switching current in superconducting nanowires. Our rigorous quantitative analysis provides firm evidence for the presence of quantum phase slips (QPSs) in homogeneous nanowires at high bias currents. We observe that as temperature is lowered, thermal fluctuations freeze at a characteristic crossover temperature Tq, below which the dispersion of the switching current saturates to a constant value, indicating the presence of QPSs. The scaling of the crossover temperature Tq with the critical temperature Tc is linear, Tq ∝ Tc, which is consistent with the theory of macroscopic quantum tunneling. We can convert the wires from the initial amorphous phase to a single-crystal phase, in situ, by applying calibrated voltage pulses. This technique allows us to probe directly the effects of the wire resistance, critical temperature, and morphology on thermal and quantum phase slips.

Anomalous supercurrent switching in graphene under proximity effect

arXiv (Cornell University), Sep 28, 2011

ABSTRACT We report a study of hysteretic current-voltage characteristics in superconductor-graphe... more ABSTRACT We report a study of hysteretic current-voltage characteristics in superconductor-graphene-superconductor (SGS) junctions. The stochastic nature of the phase slips is characterized by measuring the distribution of the switching currents. We find that in SGS junctions the dispersion of the switching current scales with temperature as σIT^αG with αG 1/3. This observation is in sharp contrast with the known Josephson junction behavior where σIT^αJ with αJ=2/3. We propose an explanation using a modified version of Kurkijarvi's theory for the flux stability in rf-SQUID and attribute this anomalous effect to the temperature dependence of the critical current which persists down to low temperatures.

Counting statistics of phase slips in superconducting interferometers

Bulletin of the American Physical Society, Mar 20, 2013

ABSTRACT In the superconducting proximity circuits, stochastic switching from the super-current c... more ABSTRACT In the superconducting proximity circuits, stochastic switching from the super-current carrying state to dissipative normal state is triggered by the topological fluctuations of the order parameter - phase slips. We study theoretically switching current statistics in a double-nanowire quantum interferometer as a function of the applied magnetic field perpendicular to the plane of the device. This system is a prototype of the double-slit experiment in optics which allows to probe macroscopic coherence of superconducting condensates. Magnetic field induces Meissner currents in the leads that lock superconducting phases across the wires. As a results phase slips that occur in the wires are not independent. We calculate dispersion of the switching current distribution as well as higher moment and find that they oscillate as the function of the field.

Bulletin of the American Physical Society, Mar 21, 2005

CORPORATION COLLABORATION-We demonstrated room-temperature quasi-ballistic electron conduction in... more CORPORATION COLLABORATION-We demonstrated room-temperature quasi-ballistic electron conduction in doublewall carbon nanotubes (DWNTs) produced using a modified arc-discharge method [1]. Conductance dependence on the length of DWNT was measured by submerging the sample into liquid mercury. The conductance versus length plots show plateaus, indicating weak dependence of the electrical resistance of the DWNTs on the length of the nanotubes segment connecting electrodes. We infer a mean free path between 0.6-10 micron meter for 80% of the tubes, which is in good agreement with the results of calculations based on the electron scattering by acoustic-phonons and by disorder. [1] H. Kajiura et al. Chem Phys Lett 398(2004)476-9.

Bulletin of the American Physical Society, Mar 6, 2007

Submitted for the MAR07 Meeting of The American Physical Society Protein translocating as unfolde... more Submitted for the MAR07 Meeting of The American Physical Society Protein translocating as unfolded chains through solid-state nanopores THOMAS AREF, ALEXEY BEZRYADIN, UIUC-We have detected translocation of the protein shrimp alkaline phosphatase (SAP) through a solidstate nanopore. The nanopores were fabricated in a silicon nitride membrane using a highly focused electron beam in a transmission electron microscope. Once formed, the nanopore was wet with an electrolytic solution and current was driven through it by application of an electric potential. When introduced to the negative side of the nanopore, the negatively charged SAP produced current blockages as the protein molecules were driven through the pore by the electric field. No current blockages occurred when protein had not been added to the electrolytic solution nor when polarity of the applied electric field was reversed. Furthermore, this globular protein does not appear to translocate as a sphere as might be expected, but rather goes through as an unfolded chain. Our current blockage events are similar to signals produced by lambda DNA translocating through a nanopore significantly larger than the DNA's diameter. This has implications for future experiments using nanopores to probe proteins.

Chemical Physics Letters, Nov 1, 2004

Room-temperature quasi-ballistic electron transport in double-wall carbon nanotubes (DWNT) is dem... more Room-temperature quasi-ballistic electron transport in double-wall carbon nanotubes (DWNT) is demonstrated. Conductance dependence on the length was measured by submerging DWNTs into liquid mercury. The conductance plots show plateaus, indicating weak dependence of the electrode-tube-electrode electrical resistance on the length of the connecting nanotube. We infer a mean free path between 0.6 and 10 µm for ~80% of the DWNTs, which is in good agreement with calculations based on the electron scattering by acoustic phonons and by disorder.

Entropy, Apr 18, 2016

The maximum entropy production principle (MEPP) is a type of entropy optimization which demands t... more The maximum entropy production principle (MEPP) is a type of entropy optimization which demands that complex non-equilibrium systems should organize such that the rate of the entropy production is maximized. Our take on this principle is that to prove or disprove the validity of the MEPP and to test the scope of its applicability, it is necessary to conduct experiments in which the entropy produced per unit time is measured with a high precision. Thus we study electric-field-induced self-assembly in suspensions of carbon nanotubes and realize precise measurements of the entropy production rate (EPR). As a strong voltage is applied the suspended nanotubes merge together into a conducting cloud which produces Joule heat and, correspondingly, produces entropy. We introduce two types of EPR, which have qualitatively different significance: global EPR (g-EPR) and the entropy production rate of the dissipative cloud itself (DC-EPR). The following results are obtained: (1) As the system reaches the maximum of the DC-EPR, it becomes stable because the applied voltage acts as a stabilizing thermodynamic potential; (2) We discover metastable states characterized by high, near-maximum values of the DC-EPR. Under certain conditions, such efficient entropy-producing regimes can only be achieved if the system is allowed to initially evolve under mildly non-equilibrium conditions, namely at a reduced voltage; (3) Without such a "training" period the system typically is not able to reach the allowed maximum of the DC-EPR if the bias is high; (4) We observe that the DC-EPR maximum is achieved within a time, T e , the evolution time, which scales as a power-law function of the applied voltage; (5) Finally, we present a clear example in which the g-EPR theoretical maximum can never be achieved. Yet, under a wide range of conditions, the system can self-organize and achieve a dissipative regime in which the DC-EPR equals its theoretical maximum.

Bulletin of the American Physical Society, Mar 19, 2013

We study statistical properties of the switching current in superconductor-graphene-superconducto... more We study statistical properties of the switching current in superconductor-graphene-superconductor proximity junctions and superconductor-nanowire-superconductor devices. The fluctuations of the switching current are related to Little's phase slips, generated by thermal and quantum fluctuations of the superconducting order parameter. The study focuses on higher moments of the statistical probability distributions of the switching current. Namely we study the skewness, which defines the asymmetry of the distribution, and kurtosis, which is a measure of the "peakedness." The skewness is defined as sk= m 3 /m 3/2 2 where m 2 is the second moment of the distribution, called the variance, and m 3 is the third moment. Kurtosis is defined as kur= m 4 /m 2 2 , where m 4 is the fourth moment of the distribution. It is known that for Gaussian distributions sk=0 and kur=3. On our devices we find, in most cases, sk ∼-1 and kur ∼ 5. These results are in agreement with numerical simulations as well as an analytic model. Finally we present preliminary experimental results for a two-nanowire device. We have found that the standard deviation, skewness and kurtosis of the switching current distributions in these devices vary periodically with magnetic field.

Bulletin of the American Physical Society, Mar 24, 2011

We report on observations of nonequilibrium pulsing states in microwave (i.e., GHz) coplanar wave... more We report on observations of nonequilibrium pulsing states in microwave (i.e., GHz) coplanar waveguide(CPW) resonators consisting of superconducting MoGe strips interrupted by a trench and connected by one or more suspended superconducting nanowires. The Lorentzian resonance peak shows a "crater" when driven past the critical current of the nanowire, leading to a "pulsing" state. In the pulsing state, the supercurrent grows until it reaches the critical current, at which point all stored energy quickly dissipates through Joule heating. We develop a phenomenological model of resonator-nanowire systems, which explains the experimental data quantitatively. For the case of resonators comprising two parallel nanowires and subject to an external magnetic field, we find field-driven oscillations of the onset power for crater formation, as well as the occurrence of a new state, in which the periodic pulsing effect is such that only the weaker wire participates in the dissipation process.

Bulletin of the American Physical Society, Mar 25, 2011

W.Meissner [Z. Physik. 86, 787 (1933)], who observed zero resistance in SNS pressed contacts, man... more W.Meissner [Z. Physik. 86, 787 (1933)], who observed zero resistance in SNS pressed contacts, many manifestations of the superconducting proximity effect have been reported. Recently it was shown that when closely spaced superconducting leads are placed on graphene, the proximity effect is induced and a supercurrent can flow between the electrodes. Here we fabricate graphene proximityeffect junctions (GPJ) and compare them to Josephson junctions (JJ). As the bias current is increased to near the critical current, a thermal escape from the washboard potential can occur driving the junction into the runaway voltage state. The standard deviation of the switching current is measured as a function of temperature and compared to the thermal and quantum escape models for JJs. We find that the temperature dependence of the standard deviation of switching currents of graphene proximity junctions is qualitatively different from the well-studied behavior of the insulator-based JJs. Possible reasons will be discussed.

Physical review, Oct 14, 2016

We present a type of transmon split-junction qubit which can be tuned by Meissner screening curre... more We present a type of transmon split-junction qubit which can be tuned by Meissner screening currents flowing in the adjacent superconducting film electrodes. The best detected relaxation time (T 1) was of the order of 50 μs and the dephasing time (T 2) about 40 μs. The achieved period of oscillation with magnetic field was much smaller than in the usual SQUID-based transmon qubits; thus a strong effective field amplification has been realized. This Meissner qubit allows a strong mixing of the current flowing in the qubit junctions and the currents generated by the Abrikosov vortices. We present a quantitative analysis of the radiation-free relaxation in qubits coupled to the Abrikosov vortices. The observation of coherent quantum oscillations provides strong evidence that the position of the vortex as well as its velocity do not have to accept exact values but can be smeared in the quantum mechanical sense. The eventual relaxation of such states contributes to an increased relaxation rate of the qubit coupled to vortices. Such relaxation is described using basic notions of the Caldeira-Leggett quantum dissipation theory.

Physical Review X, Jun 10, 2015

Macroscopic quantum tunneling is a fundamental phenomenon of quantum mechanics related to the act... more Macroscopic quantum tunneling is a fundamental phenomenon of quantum mechanics related to the actively debated topic of quantum-to-classical transition. The ability to realize macroscopic quantum tunneling affects implementation of qubit-based quantum computing schemes and their protection against decoherence. Decoherence in qubits can be reduced by means of topological protection, e.g., by exploiting various parity effects. In particular, paired phase slips can provide such protection for superconducting qubits. Here, we report on the direct observation of quantum paired phase slips in thin-wire superconducting loops. We show that in addition to conventional single phase slips that change the superconducting order parameter phase by 2π, there are quantum transitions that change the phase by 4π. Quantum paired phase slips represent a synchronized occurrence of two macroscopic quantum tunneling events, i.e., cotunneling. We demonstrate the existence of a remarkable regime in which paired phase slips are exponentially more probable than single ones.

Physical Review B, Jul 9, 2012

We measure quantum and thermal phase-slip rates using the standard deviation of the switching cur... more We measure quantum and thermal phase-slip rates using the standard deviation of the switching current in superconducting nanowires. Our rigorous quantitative analysis provides firm evidence for the presence of quantum phase slips (QPS) in homogeneous nanowires at high bias currents. We observe that as temperature is lowered, thermal fluctuations freeze at a characteristic crossover temperature Tq, below which the dispersion of the switching current saturates to a constant value, indicating the presence of QPS. The scaling of the crossover temperature Tq with the critical temperature Tc is linear, Tq ∝ Tc, which is consistent with the theory of macroscopic quantum tunneling. We can convert the wires from the initial amorphous phase to a single crystal phase, in situ, by applying calibrated voltage pulses. This technique allows us to probe directly the effects of the wire resistance, critical temperature and morphology on thermal and quantum phase slips.

Physical review, Sep 11, 2017

We study nanostructures based on two ultrathin superconducting nanowires connected in parallel to... more We study nanostructures based on two ultrathin superconducting nanowires connected in parallel to form a superconducting quantum interference device (SQUID). The measured function of the critical current versus magnetic field, IC (B), is multivalued, asymmetric and its maxima and minima are shifted from the usual integer and half integer flux quantum points. We also propose a lowtemperature-limit model which generates accurate fits to the IC (B) functions and provides verifiable predictions. The key assumption of our model is that each wire is characterized by a sample-specific critical phase, φC , defined as the phase difference at which the supercurrent in the wire is the maximum. For our nanowires φC is much greater than the usual π/2, which makes a qualitative difference in the behavior of the SQUID. The nanowire current-phase relation is assumed linear, since the wires are much longer than the coherence length. The model explains single-valuedness regions where only one vorticity value, nv, is stable. Also, it predicts regions where multiple vorticity values are stable because the Little-Parks (LP) diamonds, which describe the region of stability for each winding number nv in the current-field diagram, can overlap. We also observe and explain regions in which the standard deviation of the switching current is independent of the magnetic field. We develop a technique that allows a reliable detection of hidden phase slips and use it to determine the boundaries of the LP diamonds even at low currents where IC (B) is not directly measurable.

Applied Physics Letters, Jun 13, 2011

A thin-film Fabry-Perot superconducting resonator is used to reveal the Little and Parks ͑LP͒ eff... more A thin-film Fabry-Perot superconducting resonator is used to reveal the Little and Parks ͑LP͒ effect ͓Phys. Rev. Lett. 9, 9 ͑1962͔͒, even at temperatures much lower than the critical temperature. A pair of parallel nanowires is incorporated into the resonator at the point of the supercurrent antinode. As the magnetic field is ramped, Meissner currents develop, changing the resonance frequency of the resonator. The LP oscillation is revealed as a periodic set of distorted parabolas observed in the transmission of the resonator and corresponds to the states of the wire loop having different vorticities. We also report a direct observation of single and double phase slip events.

Physical Review Letters, Feb 28, 2012

We study the stochastic nature of switching current in hysteretic current-voltage characteristics... more We study the stochastic nature of switching current in hysteretic current-voltage characteristics of superconductor-graphene-superconductor (SGS) junctions. We find that the dispersion of the switching current distribution scales with temperature as σI ∝ T α G with αG as low as 1/3. This observation is in sharp contrast with the known Josephson junction behavior where σI ∝ T α J with αJ = 2/3. We propose an explanation using a generalized version of Kurkijärvi's theory for the flux stability in rf-SQUID and attribute this anomalous effect to the temperature dependence of the critical current which persists down to low temperatures.

Nanotechnology, Sep 5, 2011

We present a method for in situ tuning of the critical current (or switching current) and critica... more We present a method for in situ tuning of the critical current (or switching current) and critical temperature of a superconducting MoGe nanowire using high bias voltage pulses. Our main finding is that as the pulse voltage is increased, the nanowire demonstrates a reduction, a minimum and then an enhancement of the switching current and critical temperature. Using controlled pulsing, the switching current of a superconducting nanowire can be set exactly to a desired value. These results correlate with in situ transmission electron microscope imaging where an initially amorphous nanowire transforms into a single crystal nanowire by high bias voltage pulses. We compare our transport measurements to a thermally activated model of Little's phase slips in nanowires.

Physical Review B, Oct 13, 2010

We report microwave transmission measurements of superconducting Fabry-Perot resonators (SFPR), h... more We report microwave transmission measurements of superconducting Fabry-Perot resonators (SFPR), having a superconducting nanowire placed at a supercurrent antinode. As the plasma oscillation is excited, the supercurrent is forced to flow through the nanowire. The microwave transmission of the resonator-nanowire device shows a nonlinear resonance behavior, significantly dependent on the amplitude of the supercurrent oscillation. We show that such amplitude-dependent response is due to the nonlinearity of the current-phase relationship (CPR) of the nanowire. The results are explained within a nonlinear oscillator model of the Duffing oscillator, in which the nanowire acts as a purely inductive element, in the limit of low temperatures and low amplitudes. The low quality factor sample exhibits a "crater" at the resonance peak at higher driving power, which is due to dissipation. We observe a hysteretic bifurcation behavior of the transmission response to frequency sweep in a sample with a higher quality factor. The Duffing model is used to explain the Duffing bistability diagram. We also propose a concept of a nanowire-based qubit that relies on the current dependence of the kinetic inductance of a superconducting nanowire.

New Journal of Physics, Jun 13, 2017

The demand for low-dissipation nanoscale memory devices is as strong as ever. As Moore's law is s... more The demand for low-dissipation nanoscale memory devices is as strong as ever. As Moore's law is staggering, and the demand for a low-power-consuming supercomputer is high, the goal of making information processing circuits out of superconductors is one of the central goals of modern technology and physics. So far, digital superconducting circuits could not demonstrate their immense potential. One important reason for this is that a dense superconducting memory technology is not yet available. Miniaturization of traditional superconducting quantum interference devices is difficult below a few micrometers because their operation relies on the geometric inductance of the superconducting loop. Magnetic memories do allow nanometer-scale miniaturization, but they are not purely superconducting (Baek et al 2014 Nat. Commun. 5 3888). Our approach is to make nanometer scale memory cells based on the kinetic inductance (and not geometric inductance) of superconducting nanowire loops, which have already shown many fascinating properties (Aprili 2006 Nat. Nanotechnol. 1 15; Hopkins et al 2005 Science 308 1762). This allows much smaller devices and naturally eliminates magnetic-field cross-talk. We demonstrate that the vorticity, i.e., the winding number of the order parameter, of a closed superconducting loop can be used for realizing a nanoscale nonvolatile memory device. We demonstrate how to alter the vorticity in a controlled fashion by applying calibrated current pulses. A reliable read-out of the memory is also demonstrated. We present arguments that such memory can be developed to operate without energy dissipation. Power management and cooling demands of high performance processors have become one of the main obstacles to further progress of the computing devices. Thus development of the superconductor-based cryogenic computers which appear particularly suitable for overcoming these problems attracts much attention [4]. Nanoscale lowdissipation memory that could be integrated naturally with superconducting circuits remains one of the most essential elements that still needs to be demonstrated. Typical 'single-flux quanta' (SFQ) digital superconductor devices are based on manipulation of individual quanta of magnetic flux in circuits composed of Josephson junctions and inductive loops [5], and recently made much progress towards the large-scale practical logic circuits-see, e.g. [6-9]. However, direct applications of the SFQ principles to memory devices (see, e.g., [10, 11]) remain not quite competitive with other approaches because of the relatively large size, in the micrometer range, of the memory cells, determined by both the size of the Josephson junctions and geometric inductances. This motivates a search for hybrid memory based either on direct incorporation of semiconducting memory elements into superconductor circuits [12] or on the development of Josephson junctions with ferromagnetic barriers-see, e.g., [1, 13-16]. While promising in several respects, hybrid structures face many problems related to conversion between different forms of information representation and fabrication difficulties, and still did not reach the level of completely satisfactory practical circuits. The goal of this work is to suggest and demonstrate the main operating principles of all-superconducting memory cells which can be scaled down in size into the range of few tens of nanometers, and which do not suffer from the aforementioned problems. The memory is based on the loops made of superconducting nanowires [3], in which

arXiv (Cornell University), Mar 11, 2013

We perform measurements of phase-slip-induced switching current events on different types of supe... more We perform measurements of phase-slip-induced switching current events on different types of superconducting weak links and systematically study statistical properties of the switching current distributions. We employ two types of devices in which a weak link is formed either by a superconducting nanowire or by a graphene flake subject to proximity effect. We demonstrate that, independently on the nature of the weak link, higher moments of the distribution take universal values. In particular, the third moment (skewness) of the distribution is close to −1 both in thermal and quantum regimes. The fourth moment (kurtosis) also takes a universal value close to 5. The discovered universality of skewness and kurtosis is confirmed by an analytical model. Our numerical analysis shows that introduction of extraneous noise into the system leads to significant deviations from the universal values. We suggest to use the discovered universality of higher moments as a robust tool for checking against undesirable effects on noise in various types of measurements.

arXiv (Cornell University), Feb 1, 2012

We measure quantum and thermal phase-slip rates using the standard deviation of the switching cur... more We measure quantum and thermal phase-slip rates using the standard deviation of the switching current in superconducting nanowires. Our rigorous quantitative analysis provides firm evidence for the presence of quantum phase slips (QPSs) in homogeneous nanowires at high bias currents. We observe that as temperature is lowered, thermal fluctuations freeze at a characteristic crossover temperature Tq, below which the dispersion of the switching current saturates to a constant value, indicating the presence of QPSs. The scaling of the crossover temperature Tq with the critical temperature Tc is linear, Tq ∝ Tc, which is consistent with the theory of macroscopic quantum tunneling. We can convert the wires from the initial amorphous phase to a single-crystal phase, in situ, by applying calibrated voltage pulses. This technique allows us to probe directly the effects of the wire resistance, critical temperature, and morphology on thermal and quantum phase slips.

Anomalous supercurrent switching in graphene under proximity effect

arXiv (Cornell University), Sep 28, 2011

ABSTRACT We report a study of hysteretic current-voltage characteristics in superconductor-graphe... more ABSTRACT We report a study of hysteretic current-voltage characteristics in superconductor-graphene-superconductor (SGS) junctions. The stochastic nature of the phase slips is characterized by measuring the distribution of the switching currents. We find that in SGS junctions the dispersion of the switching current scales with temperature as σIT^αG with αG 1/3. This observation is in sharp contrast with the known Josephson junction behavior where σIT^αJ with αJ=2/3. We propose an explanation using a modified version of Kurkijarvi's theory for the flux stability in rf-SQUID and attribute this anomalous effect to the temperature dependence of the critical current which persists down to low temperatures.

Counting statistics of phase slips in superconducting interferometers

Bulletin of the American Physical Society, Mar 20, 2013

ABSTRACT In the superconducting proximity circuits, stochastic switching from the super-current c... more ABSTRACT In the superconducting proximity circuits, stochastic switching from the super-current carrying state to dissipative normal state is triggered by the topological fluctuations of the order parameter - phase slips. We study theoretically switching current statistics in a double-nanowire quantum interferometer as a function of the applied magnetic field perpendicular to the plane of the device. This system is a prototype of the double-slit experiment in optics which allows to probe macroscopic coherence of superconducting condensates. Magnetic field induces Meissner currents in the leads that lock superconducting phases across the wires. As a results phase slips that occur in the wires are not independent. We calculate dispersion of the switching current distribution as well as higher moment and find that they oscillate as the function of the field.

Bulletin of the American Physical Society, Mar 21, 2005

CORPORATION COLLABORATION-We demonstrated room-temperature quasi-ballistic electron conduction in... more CORPORATION COLLABORATION-We demonstrated room-temperature quasi-ballistic electron conduction in doublewall carbon nanotubes (DWNTs) produced using a modified arc-discharge method [1]. Conductance dependence on the length of DWNT was measured by submerging the sample into liquid mercury. The conductance versus length plots show plateaus, indicating weak dependence of the electrical resistance of the DWNTs on the length of the nanotubes segment connecting electrodes. We infer a mean free path between 0.6-10 micron meter for 80% of the tubes, which is in good agreement with the results of calculations based on the electron scattering by acoustic-phonons and by disorder. [1] H. Kajiura et al. Chem Phys Lett 398(2004)476-9.

Bulletin of the American Physical Society, Mar 6, 2007

Submitted for the MAR07 Meeting of The American Physical Society Protein translocating as unfolde... more Submitted for the MAR07 Meeting of The American Physical Society Protein translocating as unfolded chains through solid-state nanopores THOMAS AREF, ALEXEY BEZRYADIN, UIUC-We have detected translocation of the protein shrimp alkaline phosphatase (SAP) through a solidstate nanopore. The nanopores were fabricated in a silicon nitride membrane using a highly focused electron beam in a transmission electron microscope. Once formed, the nanopore was wet with an electrolytic solution and current was driven through it by application of an electric potential. When introduced to the negative side of the nanopore, the negatively charged SAP produced current blockages as the protein molecules were driven through the pore by the electric field. No current blockages occurred when protein had not been added to the electrolytic solution nor when polarity of the applied electric field was reversed. Furthermore, this globular protein does not appear to translocate as a sphere as might be expected, but rather goes through as an unfolded chain. Our current blockage events are similar to signals produced by lambda DNA translocating through a nanopore significantly larger than the DNA's diameter. This has implications for future experiments using nanopores to probe proteins.

Chemical Physics Letters, Nov 1, 2004

Room-temperature quasi-ballistic electron transport in double-wall carbon nanotubes (DWNT) is dem... more Room-temperature quasi-ballistic electron transport in double-wall carbon nanotubes (DWNT) is demonstrated. Conductance dependence on the length was measured by submerging DWNTs into liquid mercury. The conductance plots show plateaus, indicating weak dependence of the electrode-tube-electrode electrical resistance on the length of the connecting nanotube. We infer a mean free path between 0.6 and 10 µm for ~80% of the DWNTs, which is in good agreement with calculations based on the electron scattering by acoustic phonons and by disorder.

Entropy, Apr 18, 2016

The maximum entropy production principle (MEPP) is a type of entropy optimization which demands t... more The maximum entropy production principle (MEPP) is a type of entropy optimization which demands that complex non-equilibrium systems should organize such that the rate of the entropy production is maximized. Our take on this principle is that to prove or disprove the validity of the MEPP and to test the scope of its applicability, it is necessary to conduct experiments in which the entropy produced per unit time is measured with a high precision. Thus we study electric-field-induced self-assembly in suspensions of carbon nanotubes and realize precise measurements of the entropy production rate (EPR). As a strong voltage is applied the suspended nanotubes merge together into a conducting cloud which produces Joule heat and, correspondingly, produces entropy. We introduce two types of EPR, which have qualitatively different significance: global EPR (g-EPR) and the entropy production rate of the dissipative cloud itself (DC-EPR). The following results are obtained: (1) As the system reaches the maximum of the DC-EPR, it becomes stable because the applied voltage acts as a stabilizing thermodynamic potential; (2) We discover metastable states characterized by high, near-maximum values of the DC-EPR. Under certain conditions, such efficient entropy-producing regimes can only be achieved if the system is allowed to initially evolve under mildly non-equilibrium conditions, namely at a reduced voltage; (3) Without such a "training" period the system typically is not able to reach the allowed maximum of the DC-EPR if the bias is high; (4) We observe that the DC-EPR maximum is achieved within a time, T e , the evolution time, which scales as a power-law function of the applied voltage; (5) Finally, we present a clear example in which the g-EPR theoretical maximum can never be achieved. Yet, under a wide range of conditions, the system can self-organize and achieve a dissipative regime in which the DC-EPR equals its theoretical maximum.

Bulletin of the American Physical Society, Mar 19, 2013

We study statistical properties of the switching current in superconductor-graphene-superconducto... more We study statistical properties of the switching current in superconductor-graphene-superconductor proximity junctions and superconductor-nanowire-superconductor devices. The fluctuations of the switching current are related to Little's phase slips, generated by thermal and quantum fluctuations of the superconducting order parameter. The study focuses on higher moments of the statistical probability distributions of the switching current. Namely we study the skewness, which defines the asymmetry of the distribution, and kurtosis, which is a measure of the "peakedness." The skewness is defined as sk= m 3 /m 3/2 2 where m 2 is the second moment of the distribution, called the variance, and m 3 is the third moment. Kurtosis is defined as kur= m 4 /m 2 2 , where m 4 is the fourth moment of the distribution. It is known that for Gaussian distributions sk=0 and kur=3. On our devices we find, in most cases, sk ∼-1 and kur ∼ 5. These results are in agreement with numerical simulations as well as an analytic model. Finally we present preliminary experimental results for a two-nanowire device. We have found that the standard deviation, skewness and kurtosis of the switching current distributions in these devices vary periodically with magnetic field.