Layered Conductors Research Articles

Field-angle dependence of interlayer off-diagonal magnetoresistance in quasi two-dimensional layered conductors

We studied field-angle effects on the interlayer off-diagonal element of the magnetoresistance in a quasi two-dimensional (Q2D) system. We numerically calculated the angle dependence of the off-diagonal resistance ρ xz in the framework of the semiclassical transport theory when the current parallel to the interlayer z-direction was applied and the magnetic field was rotated from the z-direction to the intra-layer x-direction. The calculated ρ xz showed the oscillatory structure analogous to the angle dependent magnetoresistance oscillation (AMRO). This effect which we call “the off-diagonal AMRO” can be explained as the Fermi surface topological effect in the same way as AMRO. The off-diagonal AMRO was experimentally verified in an organic conductor α-(BEDT-TTF) 2NH 4Hg(SCN) 4.

Numerical Study on AC Transport Current Loss Measurement of HTS Assembled Conductor by Pick-Up Loop Without Electrical Contacts

Numerical studies on a AC transport loss measurement method for assembled conductors of multiple HTS tapes in multi-layer polygonal arrangement by using a spiral pick-up loop without electrical contacts are presented in this paper. The previously developed numerical model to analyze the electro-magnetic field distribution and the AC loss in the HTS tape in the polygonally assembled conductor was modified. The AC loss measurement using the spiral pick-up loop for single and multi layered polygonal conductors assembled by this HTS tapes was numerically simulated. The detection properties of the AC transport loss by the spiral pick-up loop were evaluated and the validity of this measurement method was verified.

Evaluating the Permeability Distribution of a Layered Conductor by Inductance Spectroscopy

This paper presents an inverse method for determining the permeability distribution of a flat, layered conductor by using a multifrequency electromagnetic sensor. It discusses the use of spectroscopic techniques to extract depth profiles and examine more fully the internal structure of the test piece. The forward solution for a small right-cylindrical air-cored coil placed next to a layered conductor is based on the analytic solution provided by the transfer matrix approach. For the inverse solution, a modified Newton-Raphson method is used to adjust the permeability profile to fit a set of multifrequency inductances in a least-squared sense. The approximate Jacobian matrix (sensitivity matrix) is obtained by the perturbation method. The paper provides numerical results of the forward solution for cases of step and continuous permeability profiles. Inverse results based on experimental and simulated data verify the accuracy of this method, which yields good estimates for the permeability profile

Quantum (Nano-Scale) Limit in a Parallel Magnetic Field in Layered Q1D Conductors

We show that electron wave functions in a quasi-one-dimensional (Q1D) layered conductor in a parallel magnetic field are always localized on conducting layers. Wave functions and electron spectrum in a quantum limit, where the “sizes” of quasi-classical electron orbits are of the order of nanoscale distances between the layers, are determined. Possible applications of our results to physical properties of Q1D materials in their superconducting and metallic phases are discussed.

The Cyclotron Waves in Layered Conductors

We have studied theoretically the propagation of electromagnetic waves in layered conductors placed in a strong magnetic field B 0. It is shown that the wave process is very sensitive to the orientation of B 0 with respect to the layers. The weakly damping modes with the frequencies near the cyclotron resonance frequency can propagate at orientations of the magnetic field vector, for which the drift velocity of charge carriers along the B 0-direction is close to zero. The cyclotron waves spectrum is analyzed with regard to the Fermi-liquid interaction of electrons.

Magnetotransport and Magnetic-Field-Induced Density Waves in Low-Dimensional Layered Conductors

We discuss a quantum-mechanical picture for interlayer magnetotransport in quasi-one-dimensional layered conductors. We show that the lowest order contribution of interlayer coupling to interlayer conductivity, which originates from a single tunneling process between two neighboring layers, leads all of the fundamental angular effects; Lebed resonance, the Danner–Kang–Chaikin oscillations, and the third angular effect. We also discuss the mean field picture of charge density wave (CDW) and spin density wave (SDW) under magnetic fields. We see that the orbital quantization effect causes the successive transitions of field-induced SDW state. It is also discussed that the CDW state could show complicated phase transitions under magnetic fields as a result of competition between Zeeman effect and orbital effect.

Effect of the Fermi surface shape on the de Haas-van Alphen oscillations in layered conductors

In this paper, we theoretically analyze the effects of the Fermi surface (FS) shape on the magnetic oscillations in quantizing magnetic fields in quasi-two-dimensional layered conductors. The theory is developed based on a phenomenological model for the electron energy spectra introduced in the paper. The model enables various Fermi surface profiles to be taken into consideration; this gives it an advantage over the commonly used tight-binding approximation. It is shown that, when the FS curvature becomes zero at an effective cross-section with maximum/minimum cross-sectional area, this can significantly affect the amplitude, shape, and phase of the oscillations, as well as the field dependence of the amplitude.

Electric-Field Effect on the Angle-Dependent Magnetotransport Properties of Quasi-One-Dimensional Conductors

We report a novel electric field effect on angular dependent magnetotransport in quasi-one-dimensional layered conductors with a pair of sheetlike Fermi surfaces. Under tilted magnetic fields and additional interlayer electric fields, semiclassical electron orbits on two Fermi sheets become periodic at different magnetic field orientations. This causes double splitting of the Lebed's commensurability resonance in interlayer transport, and the amount of splitting allows us to estimate the Fermi velocity directly. We have successfully demonstrated this effect in the organic conductor alpha-(BEDT-TTF)2KHg(SCN)4.

Quantum (nano-scale) limit in a parallel magnetic field in layered Q1D conductors

Quantum (nano-scale) limit in a parallel magnetic field in layered Q1D conductors

Thermoelectric effect in layered conductors in a strong magnetic field

We have theoretically studied the thermoelectric effect in layered conductors with a quasi-two-dimensional electron energy spectrum of an arbitrary form in a quantizing magnetic field at low temperatures. Giant quantum oscillations of thermoelectric field versus the inverse magnetic field have been predicted, which will facilitate the experimental study of quantum oscillatory effects. Thermoelectric force in a layered conductor is shown to depend periodically upon the angle between the magnetic field direction and the normal to the layers. This orientation effect arises from the quasi-two-dimensional character of the charge carriers energy spectrum and is representative of layered conductors. Layered structures with sharply anisotropic metal type electrical conductivity have been intensively studied recently. The strong anisotropy of the kinetic coefficients of such conductors is attributed to the quasi-two-dimensional character of the energy spectrum for charge carriers. The dependence of the energy of conduction electrons on their momentum depends weakly on the momentum projection pz = pn along the normal n to the layers and can be represented in the form of a rapidly converging series

Hall effect in organic layered conductors

The Hall effect in organic layered conductors with a multisheeted Fermi surfaces was considered. It is shown that the experimental study of Hall effect and magnetoresistance anisotropy at different orientations of current and a quantizing magnetic field relative to the layers makes it possible to determine the contribution of various charge carriers groups to the conductivity, and to find out the character of Fermi surface anisotropy in the plane of layers

Quantum Limit in a Parallel Magnetic Field in Layered Conductors

We show that electron wave functions in a quasi-two-dimensional conductor in a parallel magnetic field are always localized on conducting layers. In particular, wave functions and the electron spectrum in a quantum limit, where the sizes of quasiclassical electron orbits are of the order of nanoscale distances between the layers, are determined. ac infrared measurements to investigate Fermi surfaces and to test Fermi-liquid theory in quasi-two-dimensional organic and high-Tc materials in high magnetic fields, H approximately equal 10-45 T, are suggested.

Propagation of electromagnetic waves in the electron Fermi liquid of a quasi-two-dimensional conductor under strong spatial dispersion

We have analyzed propagation of electromagnetic waves in a Fermi liquid of charge carriers in quasi-two-dimensional layered conductors placed in a magnetic field. It is shown that high-frequency collective modes, which are absent in a gas of charge carriers, can be observed even at low intensity of the Fermi-liquid interaction. The spectrum of the weakly damping waves has been obtained.

Collective modes in quasi-two-dimensional conductors under strong spatial dispersion

Propagation of electromagnetic and spin waves in layered conductors with a quasi-two-dimensional dispersion law of charge carriers is investigated theoretically in the presence of an external magnetic field with induction B 0. In layered conductors, the drift velocity v D of electrons along B 0 is an oscillatory function of the angle between the magnetic field direction and the normal to the layers. For certain orientations of the magnetic field with respect to the layers of the conductor, v D is close to zero. In these directions, there is no collision-free absorption, and weakly damped waves may propagate even under strong spatial dispersion. In the short-wave-length limit, there may exist collective modes with frequencies in the neighborhood of resonances for arbitrary orientation of the wavevector k relative to B 0. Similar types of excitations in quasi-isotropic metals are possible only when k is perpendicular to the direction of the external magnetic field.

Giant quantum oscillations of the longitudinal magnetoresistance in quasi-two-dimensional metals

We have investigated in frame of the quantum transport theory the magnetic quantum oscillations of the longitudinal magnetoresistance ρ zz in quasi-two-dimensional (2D) metals for a magnetic field perpendicular to the layers. Giant Shubnikov–de Haas oscillations are found when the cyclotron energy ℏ ω c is much larger than the interlayer transfer integral t (the 2D limit). In large magnetic fields and at low temperatures, the minima of the magnetoconductivity σ zz = ρ zz −1 exhibit a thermally activated behavior in presence of negligibly small chemical potential oscillations, as observed in the organic layered conductor β″−(BEDT−TTF) 2SF 5CH 2CF 2SO 3. The questions concerning the absence of strong chemical potential oscillations in such compound and the impurity self-energy are discussed.

Galvanomagnetic phenomena in organic layered conductors

Galvanomagnetic phenomena in organic conductors with a quasi-two-dimensional energy spectrum of an arbitrary form in the presence of several groups of charge carriers whose states belong to Fermi surface sheets with different topological structures are considered. The dependences of magnetoresistance, Shubnikov-de Haas oscillations, and Hall field on the intensity and orientation of a strong magnetic field with respect to the normal to layers n are analyzed for a Fermi surface consisting of a weakly corrugated cylinder and a plane weakly corrugated along the pz=pn plane.

Surface electromagnetic modes in layered conductors in a magnetic field

A transfer-matrix approach is developed for studies of the collective electromagnetic modes in a semi-infinite layered conductor subjected to a quantizing external magnetic field perpendicular to the layers. The dispersion relations for the surface and bulk modes are derived. It is shown that the surface mode has a gap in the long-wavelength limit and exists only if the absolute value of the in-plane wave vector q exceeds the threshold value q*=−1/(a ln|Δ|). Depending on the sign of the parameter Δ=(ε−ε0)/(ε0+ε), the frequency of the surface mode ωs(q,Δ) goes either above (for Δ>0) or below (for Δ<0) the bulk-mode frequency ω(q,k)=ω(q,k+2π/a) for any value of k. At nonzero magnetic field H the bulk mode has a singular point q0(H) at which the bulk band twists in such a way that its top and bottom bounds swap. Small variations of q near this point change dramatically the shape of the dispersion function ω(q,k) in the variable k. The surface mode has no dispersion across the layers, since its amplitude decays exponentially into the bulk of the sample. Both bulk and surface modes have in the region qa≫1 a similar asymptotic behavior ω∝q1/2, but ωs(q,Δ) lies above or below ω(q,k), respectively, for Δ>0 and Δ<0 (a is the interlayer separation; ε0 and ε stand for the dielectric constants of the media outside the sample and between the layers; q and k are the components of the wave vector in the plane and perpendicular to the layers, respectively).

Giant oscillations of the sound absorption in organic conductors in a strong magnetic field

Propagation of acoustic waves in organic layered conductors with quasi-2D electron energy spectrum in a strong magnetic field H is studied theoretically. The rate of sound attenuation G( H) as a function of the orientation and magnitude of the magnetic field and sound wave vector is analysed.

Electromagnetic waves in layered conductors and superlattices in a magnetic field

The collective electromagnetic wave excitations in layered conductors in an external quantizing magnetic field are studied theoretically. A set of coupled equations for the electric field components and the Green’s functions describing collective electromagnetic modes in such a system within the linear-response approach are obtained on the basis of a model of conducting planes embedded into a dielectric matrix. These equations, written in the layer-site representation, provide a basis for studies of the electromagnetic waves in the general case which does not imply either the spatial uniformity across the layers or the homogeneity within the conducting planes. The dispersion relations are obtained for the uniform and homogeneous layered conductors, which give a rich structure of collective electromagnetic modes in layered conductors in an external magnetic field. The dispersion and damping of helicons in layered conductors in the regimes of conventional and quantum Hall effects have been studied, both analytically and numerically. Two new modes propagating perpendicular to the magnetic field with frequencies of the order of the plasma frequency have been found.

Fabrication and electrical properties of a high-Tc superconducting layered conductor

A high-Tc superconducting layered conductor designed for a small cross-section and a relatively low current capacity is fabricated by Bi-2212 tapes and interlayer insulation. The Bi-2212 tapes are prepared by the screen-printing method and the melt-solidification process with slow cooling under monotonically reduced oxygen partial pressure. Fifteen tapes with Tc value90 K and Jc value 1 × 104 A cm-2 at 77 K and 0 T are selected to construct the conductor. Dc and ac transport measurements of the conductor show that the Ic value is 250 A which is decreased by 17% from an estimation, and the ac loss is described by the Norris equation in the elliptical approximation at the normalized current i < 0.7 even though the cross-section is a rectangle.