arXiv subjects

A. V. Nikulov

Publications and source records attributed to A. V. Nikulov.

At least 73 records · Page 4Linked to original sources

arXiv2006

Quantum detector of noise based on a system of asymmetric Al superconducting rings

The quantum rectification effect observed on asymmetric superconducting loops is proposed to use as a basic of a noise detector with maximum sensitivity. The measurements show that a critical amplitude of noise or ac current decreases down to zero near superconducting transition, Tc, just as superconducting critical current. Therefore any how weak noise, right down to the equilibrium one, can induce the quantum oscillations of the dc voltage near Tc. The transformation of the power of random noise into the dc power observed on asymmetric superconducting loops gives very important advantage: even very weak noise can give enough high output power in a system with large number of loops.

cond-mat.supr-con↗

arXiv2006

Quantum oscillations of the rectified voltage and the critical current of asymmetric mesoscopic superconducting loops

The current-voltage curves and magnetic dependence of the critical current of asymmetric superconducting loops are measured. It was found that sign and value of the asymmetry of the current-voltage curves changes with value of magnetic field, periodically for single loop and system of identical loops. The obtained results allow to explain the quantum oscillation of the dc voltage, observed below superconducting transition in the previous works, as rectification of ac current or noise.

cond-mat.supr-con↗

arXiv2005

Quantum Oscillations of the Critical Current of Asymmetric Aluminum Loops in Magnetic Field

The periodical dependencies in magnetic field of the asymmetry of the current-voltage curves of asymmetric aluminum loop are investigated experimentally at different temperatures below the transition into the superconducting state T < Tc. The obtained periodical dependencies of the critical current on magnetic field allow to explain the quantum oscillations of the dc voltage as consequence of the rectification of the external ac current and to calculate the persistent current at different values of magnetic flux inside the loop and temperatures.

cond-mat.supr-con↗

arXiv2005

How Many Long-Range Orders Are in the Abrikosov State

It is argued that only a single spontaneous long-range order, namely the phase coherence exists in the Abrikosov state, and the prediction of the crystalline long-range order of vortex lattice does not correspond to the facts.

cond-mat.supr-con↗

arXiv2004

Chain of Superconducting Loops as a Possible Quantum Register

The idea of the quantum computation is based on paradoxical principles of quantum physics, superposition and entanglement of quantum states. This idea looks well-founded on the microscopic level in spite of the absence of an universally recognized interpretation of these paradoxical principles since they were corroborated over and over again by reliable experiments on the microscopic level. But the technology can not be able in the near future to work on the microscopic level. Therefore macroscopic quantum phenomenon - superconductivity is very attractive for the realization of the idea of quantum computer. It is shown in the present paper that a chain of superconducting loops can be only possible quantum register. The proposals by some authors to provide the EPR correlation with help of a classical interaction witness the misunderstanding of the entanglement essence. The problem of the possibility of superposition of macroscopically distinct states is considered.

cond-mat.supr-con↗

arXiv2003

Why Vortex Lattice Melting Theory is Science Fiction

Five years ago the talk "The vortex lattice melting theory as example of science fiction" cond-mat/9811051 was presented. Nevertheless this theory predominates up to now and very many people devote oneself to it. It is explained in the present paper why the only first order phase transition observed on the way from the Abrikosov state in the normal state should be interpreted as phase coherence disappearance. The false concept of vortex lattice melting appeared because of some causes main of them are erroneous interpretation of direct observation of the Abrikosov state and the use by theorists the habitual determination of phase coherence invalid for multi-connected superconducting state. The distinguished work by A.A. Abrikosov awarded of the Nobel Prize in Physics for 2003 predicted a periodic lattice structure with crystalline long-rang order only because that it is impossible to obtain any other result for the case of homogeneous, symmetric, infinite space. It is strange that most scientists have lightly admitted that this prediction corresponds to the facts. The long-rang order of superconducting state is phase coherence. Therefore if the Abrikosov state is also the vortex lattice then it have two long-rang orders and two phase transition may be expected. But only phase transition is assumed and observed always. History of this contradiction is considered in the present paper and it is analysed why some delusions about the Abrikosov state became popular. It is emphasized that taking into account thermal fluctuations changes in essence the habitual notion about the mixed state of type II superconductor built because of the Abrikosov result. First of all it shows that the Abrikosov solution is not valid just in the ideal case for which it was obtained.

cond-mat.supr-con↗

arXiv2003

Quantum Power Source. Putting in Order of a Brownian Motion without Maxwell's Demon

The problem of possible violation of the second law of thermodynamics is discussed. It is noted that the task of the well known challenge to the second law called Maxwell's demon is put in order a chaotic perpetual motion and if any ordered (non-chaotic) Brownian motion exists then the second law can be broken without this hypothetical intelligent entity. The postulate of absolute randomness of any Brownian motion saved the second law in the beginning of the 20th century when it was realized as perpetual motion. This postulate can be proved in the limits of classical mechanics but is not correct according to quantum mechanics. Moreover some enough known quantum phenomena, such as the persistent current at non-zero resistance, are an experimental evidence of the non-chaotic Brownian motion with non-zero average velocity. An experimental observation of a dc quantum power source is interpreted as evidence of violation of the second law.

cond-mat.stat-mech↗

arXiv2003

Quantum voltage oscillations observed on segments of an inhomogeneous superconducting loop

A theoretical prediction published in Phys.Rev. B 64, 012505 (2001) is corroborated experimentally. This corroboration of the theoretical result published in Physical Review B was rejected for publication in the same journal. Such inconsistency is connected with a contradiction to the second law of thermodynamics. According to the theory Phys.Rev. B 64, 012505 (2001) the energy of a noise can be transformed to a dc power in superconducting loop and therefore a dc voltage oscillating with magnetic field can be observed on segments of an inhomogeneous loop in the temperature region close to the superconducting transition. Such quantum voltage oscillations observed in the present work can be induced both by an external electrical noise and by internal equilibrium noise (thermal fluctuations). Violation of the second law takes place in the second case. First of all the theoretical result published by Phys.Rev.B is challenge to the second law but about this contradiction was openly written only in the first version (see physics/0105059) of the experimental paper rejected by Referees of Phys.Rev.B. The transformation of an external electrical noise to the dc voltage was confirmed recently in the paper published in JETP Letters 77, 371 (2003) (see also cond-mat/0303538).

cond-mat.supr-con↗

arXiv2003

Reply to Comment on "Quantum Force in a Superconductor"

A reply is made to Comment by D.P.Sheehan in which he takes notice that the quantum force introduced in my earlier paper A.V.Nikulov, Phys. Rev. B 64, 012505 (2001) challenges to the absolute status of the second law of thermodynamics. It is shown that the introduction of the quantum force explains only an experimental challenge to the second law. The observation of the persistent current at non-zero resistance R > 0 is experimental evidence of violation of the postulate saving the second law in the beginning of the 20 century. According to this postulate any Brownian motion is absolutely random whereas the persistent current at R > 0 is a regulated Brownian motion.

cond-mat.supr-con↗

arXiv2003

Induction of dc voltage, proportional to the persistent current, by external ac current on system of inhomogeneous superconducting loops

A dc voltage induced by an external ac current is observed in system of asymmetric mesoscopic superconducting loops. The value and sign of this dc voltage, like the one of the persistent current, depend in a periodical way on a magnetic field with period corresponded to the flux quantum within the loop. The amplitude of the oscillations does not depend on the frequency of the external ac current (in the investigated region 100 Hz - 1 MHz) and depends on its amplitude. The latter dependence is not monotonous. The observed phenomenon of rectification is interpreted as a consequence of a dynamic resistive state induced by superposition of the external current and the persistent current. It is shown that the dc voltage can be added in system of loops connected in series: the dc voltage oscillations with amplitude up to 0.00001 V were observed in single loop, up to 0.00004 V in a system of 3 loops and up to 0.0003 V in a system of 20 loops.

cond-mat.supr-con↗

arXiv2002

Absence of the Transition into Abrikosov Vortex State of Two-Dimensional Type-II Superconductor with Weak Pinning

The resistive properties of thin amorphous NbO_{x} films with weak pinning were investigated experimentally above and below the second critical field H_{c2}. As opposed to bulk type II superconductors with weak pinning where a sharp change of resistive properties at the transition into the Abrikosov state is observed at H_{c4}, some percent below H_{c2} (V.A.Marchenko and A.V.Nikulov, 1981), no qualitative change of resistive properties is observed down to a very low magnetic field, H_{c4} < 0.006 H_{c2}, in thin films with weak pinning. The smooth dependencies of the resistivity observed in these films can be described by paraconductivity theory both above and below H_{c2}. This means that the fluctuation superconducting state without phase coherence remains appreciably below H_{c2} in the two-dimensional superconductor with weak pinning. The difference the H_{c4}/H_{c2} values, i.e. position of the transition into the Abrikosov state, in three- and two-dimensional superconductors conforms to the Maki-Takayama result 1971 year according to which the Abrikosov solution 1957 year is valid only for a superconductor with finite dimensions. Because of the fluctuation this solution obtained in the mean field approximation is not valid in a relatively narrow region below H_{c2} for bulk superconductors with real dimensions and much below H_{c2} for thin films with real dimensions. The superconducting state without phase coherence should not be identified with the mythical vortex liquid because the vortex, as a singularity in superconducting state with phase coherence, can not exist without phase coherence.

cond-mat.supr-con↗

arXiv2002

Existence of the Abrikosov vortex state in two-dimensional type-II superconductors without pinning

Theory alternative to the vortex lattice melting theories is advertised. The vortex lattice melting theories are science fiction cond-mat/9811051 because the Abrikosov state is not the vortex lattice with crystalline long-range order. Since the fluctuation correction to the Abrikosov solution is infinite in the thermodynamic limit (K.Maki and H.Takayama, 1972) any fluctuation theory of the mixed state should consider a superconductor with finite sizes. Such nonperturbative theory for the easiest case of two-dimensional superconductor in the lowest Landau level approximation is presented in this work. The thermodynamic averages of the spatial average order parameter and of the Abrikosov parameter $β_{a}$ are calculated. It is shown that the position H_{c4} of the transition into the Abrikosov state (i.e. in the mixed state with long-range phase coherence) depends strongly on sizes of two-dimensional superconductor. Fluctuations eliminate the Abrikosov vortex state in a wide region of the mixed state of thin films with real sizes and without pinning disorders, i.e. H_{c4} << H_{c2}. The latter has experimental corroboration in Phys.Rev.Lett. 75, 2586 (1995).

cond-mat.supr-con↗

arXiv2002

Fluctuation Effects in Mixed State of Type II Superconductor. Comparison of Experimental Results with Theoretical Predictions

Results of the investigation of fluctuation phenomena in bulk type-II superconductors obtained in 1980 - 1984 years are compared with the analogous results obtained at the investigation of high-Tc superconductors (HTSC). It is shown the latter repeat in the main the results obtained before the HTSC discovery. It was discovered in 1981 year that a sharp change of resistive properties is observed below the second critical field, at H_{c4} < H_{c2}, in high quality samples of type-II superconductors. The same was repeated at investigation of high quality HTSC samples and was interpreted as vortex lattice melting. The difference of the (H_{c2} - H_{c4}) values for low- and high-Tc superconductors corresponds with the scaling law obtained in the lowest Landau level approximation. The linear approximation region above H_{c2}, the critical region and the Abrikosov state are considered. It is emphasized that the rigorous Ami-Maki theory should be used for description of paraconductivity in HTSC and "dirty" conventional superconductors. The approximation dependencies obtained from popular Aslamasov-Larkin, Hikami-Larkin and Lawrence-Doniac works do not have a validity region in these superconductors with wide critical region.

cond-mat.supr-con↗

arXiv2002

What is the Vortex Lattice Melting, Reality or Fiction?

This work fights against the widespread delusion cond-mat/0111052 on the vortex lattice melting. In spite of the outward appearances the Abrikosov state is not the vortex lattice with crystalline long-range order because pinning disorders destroy this order in real superconductors (A.I.Larkin, 1970) and the Abrikosov's solution 1957 year is not valid in the ideal case (K.Maki and H.Takayama, 1972). Only long-range order - long-range phase coherence exists in the Abrikosov state. Therefore the vortex lattice melting theories are science fiction cond-mat/9811051. The phase transition interpreted erroneously as the vortex lattice melting is in reality a disappearance of long-range phase coherence. In spite of the widespread opinion these two transitions are different transitions when a suitable definition of phase coherence is used. The superconducting state without phase coherence differs in essence from the mythical vortex liquid. The vortex, as a singularity in superconducting state with phase coherence, can not exist without phase coherence.

physics.gen-ph↗

arXiv2002

Destruction of the phase coherence by the magnetic field in the fluctuation region of thin superconducting film

It was shown as long ago as 1998 cond-mat/9811051 that the vortex lattice melting theories are science fiction because the Abrikosov state is not the vortex lattice with crystalline long-range order. Nevertheless this false conception is used up to now in many works including the lecture notes cond-mat/0111052 providing a pedagogical introduction. This false conception has resulted from visual but incorrect ideas. In spite of outward appearances only long-range order - long-range phase coherence exists in the Abrikosov state. The phase transition interpreted erroneously as the vortex lattice melting is in reality a disappearance of long-range phase coherence. The investigation of the nonlocal resistivity made in this work is experimental evidence that a state without phase coherence, differed qualitatively from mythical vortex liquid, is observed above this transition.

cond-mat.supr-con↗

arXiv2001

One of Possible Applications of High-Tc Superconductors

Possible application of a result contradicting to the second law of thermodynamics is announced. According to this result the chaotic energy of thermal fluctuation can be transformed in electric energy of direct current in an inhomogeneous superconducting ring. Although the power of this transformation is very weak this effect has optimistic perspective of wide application because useful energy can be obtained from heat energy in the equilibrium state. It is explain why the second law can be broken in some quantum systems. In the last years some authors have concluded almost at once on violation of the second law in different quantum systems: A.E.Allahverdyan and Th.M.Nieuwenhuizen, PRL 85, 1799 (2000); cond-mat/0011389; V.Capec and J.Bok Czech. J. of Phys. 49, 1645 (1999); cond-mat/0012056; Physica A 290, 379 (2001); D. Sheehan, Phys. Plasmas 2 (6), 1893 (1995); Phys. Rev.E 61, 4662 (2000); P. Weiss, Science News, 158, 234 (2000).

physics.gen-ph↗

arXiv2001

Observation of dc voltage on segments of an inhomogeneous superconducting loop

In order to verify a possibility of a dc voltage predicted on segments of an inhomogeneous superconducting loop the Little-Parks oscillations are investigated on symmetrical and asymmetric Al loops. The amplitude of the voltage oscillations $ΔV$ measured on segments of symmetrical loop increases with the measuring current $I_{m}$ and $ΔV = 0$ at $I_{m}= 0$ in accordance with the classical Little-Parks experiment. Whereas the $ΔV$ measured on segments of asymmetric loop has a maximum value at $I_{m}= 0$. The observation of the dc voltage at $I_{m}= 0$ means that one of the loop segments is a dc power source and others is a load. The dc power can be induced by both thermal fluctuation and a external electric noise.

physics.gen-ph↗

arXiv2000

Quantum Force in Superconductor

Transitions between states with continuous (called as classical state) and discrete (called as quantum state) spectrum of permitted momentum values is considered. The persistent current can exist along the ring circumference in the quantum state in contrast to the classical state. Therefore the average momentum can changes at the considered transitions. In order to describe the reiterated switching into and out the quantum state an additional term is introduced in the classical Boltzmann transport equation. The force inducing the momentum change at the appearance of the persistent current is called as quantum force. It is shown that dc potential difference is induced on ring segments by the reiterated switching if the dissipation force is not homogeneous along the ring circumference. The closing of the superconducting state in the ring is considered as real example of the transition from classical to quantum state

cond-mat.supr-con↗