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A. Mourachkine

Publications and source records attributed to A. Mourachkine.

At least 19 recordsLinked to original sources
arXiv2008

Template nanowires for spintronics applications: nanomagnet microwave resonators functioning in zero applied magnetic field

Low-cost spintronic devices functioning in zero applied magnetic field are required for bringing the idea of spin-based electronics into the real-world industrial applications. Here we present first microwave measurements performed on nanomagnet devices fabricated by electrodeposition inside porous membranes. In the paper, we discuss in details a microwave resonator consisting of three nanomagnets, which functions in zero external magnetic field. By applying a microwave signal at a particular frequency, the magnetization of the middle nanomagnet experiences the ferromagnetic resonance (FMR), and the device outputs a measurable direct current (spin-torque diode effect). Alternatively, the nanodevice can be used as a microwave oscillator functioning in zero field. In order to test the resonators at microwave frequencies, we developed a simple measurement set-up.

cond-mat.mes-hall↗
arXiv2008

Comment on Evolution of the electronic excitation spectrum with strongly diminishing hole density in superconducting Bi-2212 (J. W. Alldredge et al., Nature Phys. 4, 319 (2008))

In a recent article, Alldredge et al. reported tunnelling data obtained at T = 4.2 K by STM in Bi-2212 having different doping levels, p, and a new way to fit the measured differential conductances, g(V), which are linearly proportional to the local density of states of quasiparticle excitations at the surface, N(E). The main point of the fitting procedure is to use an energy-dependent inelastic scattering rate, G = aE (a is a constant). They argue that by using an equation of the d-wave BCS density of states (Bardeen-Cooper-Schrieffer) modified by Dynes et al. with G = aE, they were able to fit any conductance obtained in Bi-2212 with a doping level between 0.1 and 0.22. In addition, they discuss a new energy scale Delta_0(p) similar to that obtained in Raman measurements. Unfortunately, the paper is misleading.

cond-mat.supr-con↗
arXiv2008

Comment on Visualization of the interplay between high-temperature superconductivity, the pseudogap and impurity resonances

In a recent letter, Chatterjee et al. (Nature Phys. 4, 108 (2008)) reported tunnelling data obtained below and above the critical temperature (Tc = 15 K) by STM in overdoped Bi2-yPbySr2CuO6+x (Bi2201) and some interpretations of the data1. The main experimental findings of their work are observations of native-impurity resonances which coexist spatially with the superconducting (SC) gap below Tc and survive unchanged on warming through Tc. No doubts, the data in the letter are an important piece of information for assembling the high-Tc SC jigsaw puzzle; however, there are inaccuracies in the discussion. In addition, I propose another interpretation of the data.

cond-mat.supr-con↗
arXiv2007

A possible scenario for the mechanism of high-Tc superconductivity based on experimental data

The issue of the mechanism of high-Tc superconductivity remains open. In this contribution, we propose a new scenario for the mechanism of superconductivity in cuprates based on analysis of experimental data, mainly tunneling, neutron scattering and muon-spin-relaxation data, made earlier (see e.g. Mod. Phys. Lett. B 19 (2005) 743). A specific feature of this scenario is the mechanism of the establishment of long-range phase coherence among Cooper pairs, based on recent experimental data obtained in nonsuperconducting materials.

cond-mat.str-el↗
arXiv2007

Route to Room-Temperature Superconductivity from a Practical Point of View

To synthesize a new superconductor which has a critical temperature, Tc, exceeding the room temperature, one needs to know what chemical components to start with. This chapter presents analysis of experimental data which allow one to draw a conclusion about components and the structure of a potential room-temperature superconductor. The two essential components of a room temperature superconductor are large organic molecules (polymers, tissues) and atoms/molecules which are magnetic in the intercalated state. This conclusion is fully based on experimental facts known today, and does not require any assumptions about the mechanism of room-temperature superconductivity. This, however, does not mean that to synthesize a room-temperature superconductor is an easy task.

cond-mat.supr-con↗
arXiv2007

Origin of dips in tunneling dI/dV characteristics of cuprates

Extensive efforts have been made to understand the electronic properties of high-Tc superconductors. One feature which has been discussed in the literature during the past few years is the dips in tunneling conductances obtained in cuprates. In this contribution, we focus our attention on the origin of these dips. On the basis of experimental data obtained in Bi2Sr2CaCu2O8 and YBa2Cu3O7, we show that the dips appear naturally in tunneling spectra due to a superposition of the peaks and humps and, therefore, have no physical meaning.

cond-mat.supr-con↗
arXiv2006

Room-Temperature Superconductivity

This is the first book on the subject of room-temperature superconductivity. The main purpose of the book is twofold. First, to show that, under suitable conditions, superconductivity can occur above room temperature. Secondly, to present general guidelines on how to synthesize a room temperature superconductor. The book begins with an introduction into the physics of the superconducting state and superconducting materials. The mechanisms of conventional, half-conventional and unconventional superconductivity are discussed in the following chapters. The last three chapters of the book are devoted to room temperature superconductivity. In Chapter 2, an attempt to review the basic properties of the superconducting state independently of any specific mechanism is made for the first time. In addition, four principles of superconductivity valid for any type of superconductivity are introduced in Chapter 4. The book is mainly addressed to specialists in materials science and in the field of superconductivity. At the same time, students will also benefit from reading the first nine chapters of the book.

cond-mat.supr-con↗
arXiv2005

Mechanism of High-Tc Superconductivity Based Mainly on Tunneling Measurements in Cuprates

The main purpose of this brief review is to present a sketch of the mechanism of high-Tc superconductivity based mainly on tunneling measurements in cuprates. In the review, we shall mostly discuss tunneling spectroscopy in Bi2212. Analysis of the data shows that the Cooper pairs in cuprates are topological excitations (e.g. quasi-one dimensional bisolitons, discrete breathers etc.), and the phase coherence among the Cooper pairs appears due to spin fluctuations.

cond-mat.supr-con↗
arXiv2004

Nonlinear excitations: Solitons

The main purpose of this chapter is to present a brief introduction to nonlinear excitations, and to underline the soliton concept. This chapter is Chapter 5 in the book High-Temperature Superconductivity: The Nonlinear Mechanism and Tunneling Measurements (Kluwer Academic, Dordrecht, 2002), pages 101-142.

cond-mat.supr-con↗
arXiv2004

Principles of Superconductivity

The purpose of this chapter is to discuss the main principles of superconductivity as a phenomenon, valid for every superconductor independently of its characteristic properties and material. The underlying mechanisms of superconductivity can be different for various materials, but certain principles must be satisfied. The chapter introduces four principles of superconductivity. (The chapter is slightly nodified from the original one in order to be self-contained.)

cond-mat.supr-con↗
arXiv2004

Determination of the Coherence Length and the Cooper-Pair Size in Unconventional Superconductors by Tunnelling Spectroscopy

The main purpose of the paper is to discuss a possibility of the determination of the values of the coherence length and the Cooper-pair size in unconventional superconductors by using tunnelling spectroscopy. In the mixed state of type-II superconductors, an applied magnetic field penetrates the superconductor in the form of vortices which form a regular lattice. In unconventional superconductors, the inner structure of a vortex core has a complex structure which is determined by the order parameter of the superconducting state and by the pairing wavefunction of the Cooper pairs. In clean superconductors, the spatial variations of the order parameter and the pairing wavefunction occur over the distances of the order of the coherence length and the Cooper-pair size, respectively. Therefore, by performing tunnelling spectroscopy along a line passing through a vortex core, one is able, in principle, to estimate the values of the coherent length and the Cooper-pair size.

cond-mat.supr-con↗
arXiv2004

The oxygen isotope effect on critical temperature in superconducting copper oxides

The isotope effect provided a crucial key to the development of the BCS (Bardeen-Cooper-Schrieffer) microscopic theory of superconductivity for conventional superconductors. In superconducting cooper oxides (cuprates) showing an unconventional type of superconductivity, the oxygen isotope effect is very peculiar: the exponential coefficient strongly depends on doping level. No consensus has been reached so far on the origin of the isotope effect in the cuprates. Here we show that the oxygen isotope effect in cuprates is in agreement with the bisoliton theory of superconductivity.

cond-mat.supr-con↗
arXiv2003

Anomaly in the Tunneling I(V) Characteristics of Bi2212

Tunneling measurements have been carried out on slightly overdoped Bi2212 single crystals below and above the critical temperature by break-junctions and in-plane point-contacts. An anomaly was found in the tunneling I(V) characteristics. Analysis of the data shows that the anomaly is caused by the superconducting condensate. In the extracted I(V) characteristics of the condensate, the constant asymptotics points to the presence of one-dimensionality in Bi2212. The anomaly found here puts additional constraints on the final theory of high-T_c superconductivity.

cond-mat.supr-con↗
arXiv2002

Seven energy/temperature scales in hole-doped cuprates

The purpose of this paper is to discuss a phase diagram of hole-doped cuprates. Hole-doped cuprates have a very rich phase diagram as a function of doping. This is due to a charge inhomogeneity in CuO_2 planes: the charge distribution in CuO_2 planes is inhomogeneous both on a nanoscale and macroscale. Depending on doping level, the CuO_2 planes comprise at least four different types of clusters having a size of a few nanometers. Each type of clusters has its own features. As a consequence, the common phase diagram consists of, at least, seven different energy/temperature scales. As an example, we consider a phase diagram of Bi2212.

cond-mat.supr-con↗
arXiv2001

The Mechanism of High-Tc Superconductivity: "Nonlinear" Superconductivity

The main purpose of the paper is to present an overview of the current situation in the development of understanding of the mechanism of high-Tc superconductivity which arises due to moderately strong, nonlinear electron-phonon interactions and due to magnetic (spin) fluctuations.

cond-mat.supr-con↗
arXiv2001

The Distribution of the Energy Gap and Josephson IcRn Product in Bi2212 by Tunneling Spectroscopy

We present direct measurements of the density of states by tunneling spectroscopy on slightly overdoped Bi2212 single crystals at low temperature using break-junction and point-contact techniques. We find that (i) the variation of the gap magnitude, Delta, between 20 and 36 meV is likely to be intrinsic to the Bi2212, and (ii) there is a correlation between the maximum value of the Josephson IcRn product and the gap magnitude: IcRn decreases with the increase of Delta. The maximum IcRn value of 26 mV is observed at Delta = 20.5 meV. For Delta = 36.5 meV, the maximum measured value of IcRn is 7.3 mV. We conclude that (i) the distribution of the Josephson IcRn product as a function of gap magnitude can not be explained by the presence of a single energy gap in Bi2212, and (ii) the coherence energy scale in Bi2212 has the maximum Josephson strength.

cond-mat.supr-con↗
arXiv2001

Quasi-One-Dimensional Topological-Excitation Liquid in Bi2212 from Tunneling Spectroscopy

Tunneling measurements have been carried out on heavily underdoped, slightly overdoped and partially Ni-substituted Bi2212 single crystals by using a break-junction technique. We find that in-plane tunneling spectra below Tc are the combination of incoherent part from the pseudogap and coherent quasiparticle peaks. There is a clear correlation between the magnitude of the pseudogap and the magnitude of the superconducting gap in Bi2212. The analysis of the data suggests that the tunneling pseudogap in Bi2212 is predominantly a charge-density-wave gap on dynamical charge stripes. The tunneling characteristics corresponding to the quasiparticle peaks are in excellent agreement with theoretical predictions made for a quasi-one dimensional topological-excitation liquid. In addition, the analysis of data measured by different techniques shows that the phase coherence along the c-axis is established at Tc due to spin fluctuations in local antiferromagnetic domains of CuO2 planes.

cond-mat.supr-con↗