Layered Conductors Research Articles

Interlayer Conductivity of Quasi-Two-Dimensional Layered Conductors in a Magnetic Field at Yamaji Angles

The behavior of the interlayer magnetoresistance Rzz is analyzed in quasi-two-dimensional layered metals ina magnetic field tilted at Yamaji angles at which the minimum of the interlayer conductivity is observed. Thecases of the Lorentzian line shape of Landau levels and of the shape corresponding to the self-consistent Bornapproximation are studied. At high fields, the behavior Rzz B3/2 is theoretically predicted, which agrees wellwith experimental data.

Interlayer Conductivity of Quasi-Two-Dimensional Layered Conductors in a Magnetic Field at Yamaji Angles

Interlayer Conductivity of Quasi-Two-Dimensional Layered Conductors in a Magnetic Field at Yamaji Angles

Method to Reveal and Investigate Almost 2D Fermi Surfaces in Layered Conductors: Universal Resistivity in a Parallel Magnetic Field

We suggested an original method to investigate the Fermi surfaces (FSs) in the quasi-two-dimensional conductors some time ago [A.G. Lebed and N.N. Bagmet, Phys. Rev. B 55, R8654 (1997)]. It was based on a consideration of a perpendicular conductivity in quasi-two-dimensional metals in parallel magnetic fields in the framework of the Boltzmann kinetic equation, where it was shown that the conductivity was independent on impurities. In this paper, we demonstrate that the above mentioned result is much more general than the kinetic equation and can be obtained even in a fully quantum mechanical case. We suggest to investigate this possible phenomenon in the quasi-two-dimensional organic, high-{{T}_{c}}, and some others superconductors in a metallic phase to judge if the Fermi liquid picture is valid for them or not. If the Fermi liquid picture is valid, then study of the perpendicular resistivity in the rotated parallel magnetic field allows to extract important information about the two-dimensional Fermi surfaces.

The longitudinal spin-conductivity of τ-type organic conductors

The Kubo formula is used to evaluate the real part of the longitudinal spin conductivity for τ -type organic layered conductors. These conductors are described by a four-band model, consisting of the tight binding Hamiltonian of the system. This Hamiltonian may be written as a direct sum of two two-band models, describing the spins up and down respectively. The longtudinal spin conductivity of the system is analytically evaluated by means of Kubo formula applied to the two-band model corresponding to the spin up carriers. Our results show that the regular real part of the longitudinal spin-conductivity of the system becomes zero for sufficiently large values of the spin–orbit coupling constant ( λ > 0 . 04 ) and only ballistic propagation occurs in this case. The ballistic propagation is more favourable for smaller values of the temperature in the system studied here.

Ultralow electron-surface scattering in nanoscale metals leveraging Fermi-surface anisotropy

Increasing resistivity of metal wires with reducing nanoscale dimensions is a major performance bottleneck of semiconductor computing technologies. We show that metals with suitably anisotropic Fermi velocity distributions can strongly suppress electron scattering by surfaces and outperform isotropic conductors such as copper in nanoscale wires. We derive a corresponding descriptor for the resistivity scaling of anisotropic conductors, screen thousands of metals using first-principles calculations of this descriptor and identify the most promising materials for nanoscale interconnects. Previously-proposed layered conductors such as MAX phases and delafossites show promise in thin films, but not in narrow wires due to increased scattering from side walls. We find that certain intermetallics (notably CoSn) and borides (such as YCo$_3$B$_2$) with one-dimensionally anisotropic Fermi velocities are most promising for narrow wires. Combined with first-principles electron-phonon scattering predictions, we show that the proposed materials exhibit 2-3x lower resistivity than copper at 5 nm wire dimensions.

A layer potential approach to functional and clinical brain imaging

In this work, we consider the inverse source recovery problem from sEEG, EEG and MEG point-wise data. We regard this as an inverse source recovery problem for L2 vector-fields normally oriented and supported on the grey/white matter interface, which together with the brain, skull and scalp form a non-homogeneous layered conductor. We assume that the quasistatic approximation of Maxwell’s equation holds for the electro-magnetic fields considered. The electric data is measured point-wise inside and outside the conductor while the magnetic data is measured only point-wise outside the conductor. These ill-posed problems are solved via Tikhonov regularization on triangulations of the interfaces and a piecewise linear model for the current on the triangles. Both in the continuous and discrete formulation the electric potential is expressed as a linear combination of double layer potentials while the magnetic flux density in the continuous case is a vector-surface integral whose discrete formulation features single layer potentials. A main feature of our approach is that these contributions can be computed exactly. Due to the consideration of the regularity conditions of the electric potential in the inverse source recovery problem, the Cauchy transmission problem for the electric potential is inadvertently solved as well. In the problem, we propagate only the electric potential while the normal derivatives at the interfaces of discontinuity of the electric conductivities are computed directly from the resulting solution. This reduces the computational complexity of the problem. There is a direct connection between the magnetic flux density and the electrical potential in conductors such as the one we explore, hence a coupling of the sEEG, EEG and MEG data for solving the respective inverse source recovery problems simultaneously is direct. We treat these problems in a unified approach that uses only single and/or double layer potentials. We provide numerical examples using realistic meshes of the head with synthetic data.

Evaluation of Electromechanical Properties in Multiple HTS Layered Conductors at 77 K

High engineering critical current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> ) is demanded in high-performance large-scale applications such as power cables, high-magnetic-field magnets, accelerators, and fusion reactors. In developing high current capacity and low vulnerability under a high magnetic field, high-temperature superconducting (HTS) rare-earth barium copper oxide coated conductors (CCs) have undergone various trials, and CC tape characteristics have ultimately been greatly improved. The Korea Electrotechnology Research Institute recently developed a “multiple HTS layers on one substrate” (MHOS) conductor that can carry high critical current, I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , based on the number of superconducting layers deposited on one substrate. The superconducting layers in a MHOS conductor are much thicker than in a single-layered CC tape, possibly affecting the degradation behaviors of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> under a mechanical load. To ensure the performance and reliability of MHOS conductors, it is necessary to evaluate the strain/stress response of I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . In this study, the electromechanical properties of MHOS conductors were investigated using the uniaxial tension test method as well as the single-sided bending test at 77 K and self-field and at 0.5 T using a pair of neodymium permanent magnets.

Near-surface thermal generation of longitudinal bulk acoustic waves in organic conductors

The linear thermal generation of near-surface longitudinal bulk acoustic waves (NSLBAWs) in organic conductors with layered structure is considered. These are waves that are generated and propagate at a distance of order or less than the thermal source attenuation length, i.e., are confined to the thermal skin layer of the conductor. The Green’s functions approach is applied to obtain the integral form for the solutions of the thermal conduction equation and wave equation. Analytical expressions for the temperature distribution and the wave amplitude are obtained in case when a current is applied along the least conducting direction of the quasi-two-dimensional organic conductor $ \beta - ({\rm BEDT}-{\rm TTF})_{2}{\rm IBr}_{2}$ as a thermal source. The behavior of both thermal and acoustic field is investigated near the conductor’s surface. In addition, the angular and magnetic field dependencies of the NSLBAWs amplitude are obtained and analyzed in order to emphasize the specific features of these waves. Furthermore, a comparison of the properties of both NSLBAWs and the longitudinal bulk acoustic waves that are generated and attenuate in the depth of the conductor, i.e., deep longitudinal bulk acoustic waves (DLBAWs) reveals some similarities and important differences in the process of thermal generation of the waves. We discuss the possible application of the thermal generation of NSLBAWs in organic conductors and why these waves are more preferable than the DLBAWs for studying some of the properties and/or states in layered organic conductors as well as for determining the parameters that shape the Fermi surface that cannot be obtained through the DLBAWs or transport measurements.

Topological Properties of τ-Type Organic Conductors with a Checkerboard Lattice

Although the topological phases are difficult to be realized in organic molecular crystals, we demonstrate here that they can emerge in the \tau-type organic layered conductors, \tau-(EDO-S,S-DMEDT-TTF)_2X_{1+y} and \tau-(P-S,S-DMEDT-TTF)_2X_{1+y} (X=AuBr_2, I_3, IBr_2), where EDO-S,S-DMEDT-TTF and P-S,S-DMEDT-TTF denote the planar donor molecules ethylenedioxy-S,S-dimethyl(ethylenedithio)tetrathiafulvalene and pyrazino-S,S-dimethyl(ethylenedithio)tetrathiafulvalene, respectively. The conducting layers of these conductors have a highly symmetric checkerboard structure, which can be regarded as a modified Mielke lattice. Because their electronic structure inherits that of the Mielke lattice, their conduction and valence bands exhibits the quadratic band touching. The contact point splits into a pair of Dirac cones under uniaxial strain which breaks C_4-symmetry. In \tau-type conductors, we can expect rather large spin-orbit coupling (SOC) as organic conductors. We show that the SOC in this case opens a topologically nontrivial gap at the band contact point, and the helical edge states exist in the gap. The actual \tau-type conductors could be regarded as heavily-doped topological insulators, which could exhibit finite spin Hall effect.

Simulation of temperature profile for the electron and the lattice systems in laterally structured layered conductors

Electrons in operating microelectronic semiconductor devices are accelerated by locally varying a strong electric field to acquire effective electron temperatures nonuniformly distributing in nanoscales and largely exceeding the temperature of the host crystal lattice. The thermal dynamics of electrons and the lattice are hence nontrivial and its understanding at nanoscales is decisively important for gaining a higher device performance. Here, we propose and demonstrate that in layered conductors the nonequilibrium nature between the electrons and the lattice can be explicitly pursued by simulating the conducting layer by separating it into two physical sheets representing, respectively, the electron and the lattice subsystems. We take, as an example of simulating GaAs devices, a 35 nm thick, wide U-shaped conducting channel with 15 nm radius of curvature at the inner corner of the U-shaped bend, and find a remarkable hot spot to develop due to hot-electron generation at the inner corner. The hot spot in terms of the electron temperature achieves a significantly higher temperature and is of far sharper spatial distribution when compared to the hot spot in terms of the lattice temperature. A similar simulation calculation made on a metal (NiCr) narrow lead of similar geometry shows that a hot spot shows up as well at the inner corner, but its strength and the spatial profiles are largely different from those in semiconductor devices; viz., the amplitude and the profile of the electron system are similar to those of the lattice system, indicating quasi-equilibrium between the two subsystems. The remarkable difference between the semiconductor and the metal is interpreted to be due to the large difference in the electron specific heat, rather than the difference in the electron phonon interaction. This work will provide useful hints to a deeper understanding of the nonequilibrium properties of electrical conductors, through a simple and convenient method for modeling nonequilibrium layered conductors.

Shubnikov–de Haas Thermoelectric Field Oscillations in Layered Conductors in the Vicinity of a Topological Lifshits Transition

We have studied the response of a layered conductor with a quasi-two-dimensional electron spectrum and a multisheet Fermi surface (FS) formed by two weakly corrugated cylinders and two planar sheets adjoining these cylinders to nonuniform heating. As a result of action of pressure on the conductor or its doping with impurity atoms, distance Δp between the FS cylinder and planar sheets can be reduced to such an extent that conduction electrons begin roaming over these sheets, tunneling from one FS sheet (cavity) to the other. If a conduction electron can visit several times all FS sheets during its mean free time, its motion in the plane orthogonal to the magnetic field becomes finite. In this case, Shubnikov–de Haas oscillations are excited with a period determined by the closed area circumscribed by the electron during its motion in the magnetic-breakdown trajectory in the momentum space. We have calculated the dependence of the thermoelectric field on the magnitude and orientation of a quite strong quantizing magnetic field. In a magnetic field normal to the layers, the cross sections of the cylindrical part of the FS are equidistant from both FS planar sheets. However, this equidistance is violated even for a small deviation of the field from the normal to the layers by angle ϑ, and, at a certain value ϑk of this angle, the probability of magnetic breakdown to one of the FS planar sheets can be so low that the electron cannot close the magnetic-breakdown trajectory, and its motion over the other planar sheet with a visit to the FS cylindrical part becomes infinite. In this case, the magnetic-breakdown quantum oscillations of magnetization and all kinetic characteristics of the conductor disappear, and their disappearance is repeated periodically with a change in the slope of the magnetic field to the layers as a function of tanϑ.

Shubnikov–de Haas Oscillations in the Magnetoresistance of Layered Conductors in Proximity to the Topological Lifshitz Transition

The dependence of the resistance of a layered conductor with a quasi-two-dimensional charge carrier energy spectrum on the strength and orientation of a quantizing magnetic field is studied. The case of an organic conductor with a multisheet Fermi surface consisting of a weakly warped cylinder and two adjoining planar sheets is considered. By applying an external pressure to the conductor or doping it with impurity atoms, the gap between the cylinder and the planar sheets of the Fermi surface (FS) may be reduced so that electrons start wandering on the FS, tunneling between its different parts due to magnetic breakdown. If an electron can pass through all the different sheets of the FS several times during the mean free time, its motion in the plane orthogonal to the magnetic field becomes finite. This leads to Shubnikov–de Haas oscillations with a period determined by the area enclosed by the closed breakdown orbit of an electron in momentum space. However, even at a slight tilting of the field from the normal to the layers by an angle ϑ, the equidistance is broken and at certain angles ϑk the probability of the magnetic breakdown to one of the planar FS sheets may become so low that the electron cannot complete the magnetic-breakdown orbit and its motion over the other planar sheet and the cylindrical part of the FS becomes infinite. As a result, magnetic-breakdown quantum oscillations of magnetization and all kinetic properties vanish. This vanishing repeats periodically as a function of tan ϑ with changing the tilt angle. Possibilities for experimental observation and investigation of the influence of magnetic breakdown on quantum oscillation phenomena are discussed.

In Search of Unambiguous Evidence of the Fulde–Ferrell–Larkin–Ovchinnikov State in Quasi-Low Dimensional Superconductors

In layered conductors with a sufficiently weak interlayer coupling in-plane magnetic field cause only small diamagnetic currents and the orbital depairing is strongly suppressed. Therefore, the Zeeman effect predominantly governs the spin-singlet superconductivity making the formation of the spatially modulated Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) phase possible in such materials. Despite decades of strenuous effort, this state still remains a profound mystery. In the last several years, however, there have been observed several hints indicating the experimental realization of the FFLO state in organic layered superconductors. The emergence of the FFLO phase has been demonstrated mainly based on thermodynamic quantities or microscopically with spin polarization distribution that exhibit anomalies within the superconducting state in the presence of the in-plane magnetic field. However, the direct observation of superconducting order parameter modulation is so far missing. Recently, there have been proposed theoretically several hallmark signatures for FFLO phase, which are a direct consequence of its main feature, the spatial modulation of the order parameter, and hence can provide incontrovertible evidence of FFLO. In this article, a review of these signatures and the underlying theoretical framework is given with the purpose to summarize the results obtained so far, omitting duplications, and to emphasize the ideas and physics behind them.

Anisotropic Superconductivity and Dimensional Crossover in a Layered Organic Conductor with Strong Correlation

Researchers have observed magnetotransport phenomena due to the interplay of dimensionality and gap symmetry in a highly anisotropic and strongly correlated organic superconductor.

Phase transitions in θ-(ET)4MnBr4(C6H6−nCln) (n = 1, 2) driven by ordering in anion and/or cation layers

Newly synthesized layered organic conductors θ-(ET)4MnBr4(1,2-C6H4Cl2) (1) and θ-(ET)4MnBr4(C6H5Cl) (2) show metal-like conductivity along conducting layers down to 4.3 and 60 K, respectively, while showing semiconducting properties in the perpendicular direction. Phase transitions driven by ordering in anion layers in 1 and ordering in a half of cation layers in 2 were found to occur at 297 and ~240 K, respectively. Crystal structures above and below the phase transitions were determined for 1 at 260 and 333 K and for 2 at 260 and 220 K. In the above-transition phases of both compounds, attributed to the tetragonal crystal system, all ET layers are similar in structure. In the below-transition phases, attributed to triclinic and monoclinic crystal systems, the neighboring ET layers of each phase differ in periods along the stacks and in dihedral angles θ between radical cations of the adjacent stacks.

Nodeless versus nodal scenarios of possible triplet superconductivity in the quasi-one-dimensional layered conductor Li0.9Mo6O17

We consider the problem of the orbital upper critical magnetic field, parallel to the most conducting axis of a quasi-one-dimensional layered superconductor. It is shown that superconductivity can be destroyed through orbital effects at fields much higher than the so-called Clogston-Chandrasekhar paramagnetic limiting field, $H_p$, provided that superconducting pairing of electrons are of a triplet nature. We demonstrate that the superconducting state of the quasi-one-dimensional layered conductor, $\mathrm{Li_{0.9}Mo_6O_{17}}$, is well described by the suggested theory. To this end, we consider two competing scenarios: 1: a superconducting order parameter without zeros on the Fermi surface, and 2: one with zeros on the Fermi surface - both are shown to lead to destruction of superconductivity at a magnetic field, $H^x_{c_2}$, five times higher than $H_p$. With recent experimental measurements on the $\mathrm{Li_{0.9}Mo_6O_{17}}$ favoring the nodeless order parameter, we present a strong argument supporting triplet pairing in this compound.

Possible triplet superconductivity in the quasi-one-dimensional conductor Li0.9Mo6O17

We consider a theoretical problem of the upper critical magnetic field parallel to a conducting axis of a quasi-one-dimensional layered superconductor. We show that the orbital effects against superconductivity in a magnetic field are capable of destroying the superconducting phase at low temperatures if the interplane distance is less than the corresponding coherence length. Applications of our results to recent experiments, performed in the superconductor Li${}_{0.9}$Mo${}_{6}$O${}_{17}$ [J.-F. Mercure et al., Phys. Rev. Lett. 108, 187003 (2012)], provide strong arguments in favor of a triplet superconducting pairing in this quasi-one-dimensional layered conductor.

Volume conductor models in surface electromyography: Applications to signal interpretation and algorithm test

Simulation of surface electromyogram (EMG) has provided support for the development and testing of algorithms (e.g., for the extraction of amplitude or spectral properties of the EMG or for the estimation of the muscle fiber conduction velocity) and for the interpretation of experimental results. This is the second part of a pair of papers: the first explains the methods for the development of structure based models of surface EMG; this one provides a summary of interesting applications for the interpretation of signals and testing of algorithms. Simulations from some models already introduced in the literature are compared. Moreover, estimation of indexes from an innovative layered volume conductor including a subcutaneous tissue with variable thickness in space is studied.

Observation of Angle-Dependent Stark Cyclotron Resonance in a Layered Organic Conductor

We report novel angle-dependent magnetotransport phenomena in quasi-two-dimensional layered conductors under interlayer electric fields. Interlayer conduction shows the Stark cyclotron resonance when the orbital motion of an electron becomes periodic in k -space, and the amplitude of the reasonance oscillates depending on magnetic field orientations. A conventional angle-dependent magnetoresistance oscillation switches to this oscillation at high electric fields. Using the organic conductor α-(BEDT-TTF) 2 NH 4 Hg(SCN) 4 , we have successfully observed an angle-dependent Stark cyclotron resonance for the first time.

Accelerated convergence for a normal state layered superconductor’s transverse resistance expression, measured with strip contacts

The expression of the transverse resistance for an ohmic parallelepipedic layered conductor, measured with strip contacts, extended along its width, is a slowly convergent series. This series is reworked and transformed to the sum of an analytical part and an exponentially convergent series, which reduces considerably the number of terms needed for the numerical evaluation. In addition, an asymptotic formula is obtained, valid for Γ < 2, Γ is the effective anisotropy. This formula is used to determine the room temperature resistivity anisotropy of two small Γ layered superconductors, which are 2H–NbSe 2 and 2H–TaSe 2.