arXiv subjects

S. V. Dubonos

Publications and source records attributed to S. V. Dubonos.

32 records · Page 2Linked to original sources

arXiv2005

Direct Observation of Vortex Shells and Magic Numbers in Mesoscopic Superconducting Disks

We have visualised and studied vortex configurations in mesoscopic superconducting disks using the Bitter decoration technique. For a broad range of vorticities L the circular geometry is found to result in the formation of concentric shells of vortices. From images obtained on disks of different sizes in a range of magnetic fields we were able to trace the evolution of vortex states and identify stable and metastable configurations. Furthermore, quantitative analysis of shell filling with increasing L allowed us to identify 'magic' numbers corresponding to the appearance of consecutive new shells.

cond-mat.supr-con↗

arXiv2005

Two Dimensional Electron and Hole Gases at the Surface of Graphite

We report high-quality two-dimensional (2D) electron and hole gases induced at the surface of graphite by the electric field effect. The 2D carriers reside within a few near-surface atomic layers and exhibit mobilities up to 15,000 and 60,000 cm2/Vs at room and liquid-helium temperatures, respectively. The mobilities imply ballistic transport at micron scale. Pronounced Shubnikov-de Haas oscillations reveal the existence of two types of carries in both 2D electron and hole gases.

cond-mat.mes-hall↗

arXiv2004

Room-temperature electric field effect and carrier-type inversion in graphene films

The ability to control electronic properties of a material by externally applied voltage is at the heart of modern electronics. In many cases, it is the so-called electric field effect that allows one to vary the carrier concentration in a semiconductor device and, consequently, change an electric current through it. As the semiconductor industry is nearing the limits of performance improvements for the current technologies dominated by silicon, there is a constant search for new, non-traditional materials whose properties can be controlled by electric field. Most notable examples of such materials developed recently are organic conductors [1], oxides near a superconducting or magnetic phase transition [2] and carbon nanotubes [3-5]. Here, we describe another system of this kind - thin monocrystalline films of graphite - which exhibits a pronounced electric field effect, such that carriers in the conductive channel can be turned into either electrons or holes. The films remain metallic, continuous and of high quality down to a few atomic layers in thickness. The demonstrated ease of preparing such films of nearly macroscopic sizes and of their processing by standard microfabrication techniques, combined with submicron-scale ballistic transport even at room temperature, offer a new two-dimensional system controllable by electric-field doping and provide a realistic promise of device applications.

cond-mat.mes-hall↗

arXiv2004

Electric Field Effect in Atomically Thin Carbon Films

We report a naturally-occurring two-dimensional material (graphene that can be viewed as a gigantic flat fullerene molecule, describe its electronic properties and demonstrate all-metallic field-effect transistor, which uniquely exhibits ballistic transport at submicron distances even at room temperature.

cond-mat.mtrl-sci↗

arXiv2004

Long-range nonlocal flow of vortices in narrow superconducting channels

We report a new nonlocal effect in vortex matter, where an electric current confined to a small region of a long and sufficiently narrow superconducting wire causes vortex flow at distances hundreds of inter-vortex separations away. The observed remote traffic of vortices is attributed to a very efficient transfer of a local strain through the one-dimensional vortex lattice, even in the presence of disorder. We also observe mesoscopic fluctuations in the nonlocal vortex flow, which arise due to "traffic jams" when vortex arrangements do not match a local geometry of a superconducting channel.

cond-mat.supr-con↗

arXiv2003

Subatomic movements of a domain wall in the Peierls potential

Movements of individual domain walls in a ferromagnetic garnet were studied with angstrom resolution. The measurements reveal that domain walls can be locked between adjacent crystallographic planes and propagate by distinct steps matching the lattice periodicity. Domain walls are found to be weakly mobile within valleys of the atomic washboard but become unexpectedly flexible on Peierls ridges, where they can be kept in a bi-stable state by ac magnetic field. We describe the latter observation in terms of a single magnetic kink propagating along a domain wall.

cond-mat.mes-hall↗

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

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↗

arXiv2003

Proximity Effect and Spontaneous Vortex Phase in Planar SF-Structures

The proximity effect in SF structures was examined. It is shown that, due to the oscillations of the induced superconducting order parameter in a ferromagnet, the critical temperature of an SF-bilayer becomes minimal when the thickness of the ferromagnetic layer is close to a quarter of the period of spatial oscillations. It is found that the spontaneous vortex state arisen in the superconductor due to the proximity of the magnetic domain structure of a ferromagnet brings about noticeable magnetoresistive effects.

cond-mat.supr-con↗

arXiv2002

Multiple flux jumps and irreversible behavior of thin Al superconducting rings

An experimental and theoretical investigation was made of flux jumps and irreversible magnetization curves of mesoscopic Al superconducting rings. In the small magnetic field region the change of vorticity with magnetic field can be larger than unity. This behavior is connected with the existence of several metastable states of different vorticity. The intentional introduction of a defect in the ring has a large effect on the size of the flux jumps. Calculations based on the time-dependent Ginzburg-Landau model allows us to explain the experimental results semi-quantitatively.

cond-mat.supr-con↗

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

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

Non-Quantized Penetration of Magnetic Field in the Vortex State of Superconductors

As first pointed out by Bardeen and Ginzburg in the early sixties, the amount of magnetic flux carried by vortices depends on their distance to the sample edge and can be smaller than one flux quantum, f0 = h/2e. In bulk superconductors, this reduction of flux becomes negligible already at submicron distances from the edge but, in thin films, the effect may survive at much larger distances. In the absence of any experimental observation, such flux reduction is perceived to be an exotic or unimportant effect, and it is often assumed that magnetic field enters type-II superconductors in units of f0. Here we report the measurements of the amount of flux associated with the entrance of individual vortices in a superconducting film and show that the flux that they bring in, always differs substantially from f0. We have observed vortices that carry as little as 0.001 f0 as well as seemingly "negative vortices" whose penetration leads to the expulsion of magnetic field. We distinguish two phenomena responsible for non-quantized flux penetration: the finite-size effect and a non-linear screening of magnetic field in the presence of a surface barrier. The latter effect has previously not been considered but is likely to cause non-quantized penetration in many cases.

cond-mat.supr-con↗

arXiv2000

Fine Structure in Magnetization of Individual Fluxoid States

Each time a vortex enters or exits a small superconductor, a different fluxoid state develops. We have observed splitting and sharp kinks on magnetization curves of such individual states. The features are the manifestation of first and second order transitions, respectively, and reveal the existence of distinct vortex phases within a superconducting state with a fixed number of fluxoids. We show that the kinks indicate the merger of individual vortices into a single giant vortex while the splitting is attributed to transitions between different arrays of the same number of vortices.

cond-mat.supr-con↗