Fermi surface measurements of AuTe2I by the shubnikov—de haas effect

Fermi surface measurements of the layer-compound AuTe₂I have been performed by the Shubnikov—de Haas effect. One of the Fermi surfaces, which consists of a cylinder parallel to the b-axis or an ellipsoid with anisotropy of at least 10:1, reflects the layer structure of the material. We observe in addition two other Fermi surfaces of lower anisotropy     

Fermi surface topology and the upper critical field in two-band superconductors: application to MgB2

Recent measurements of the anisotropy of the upper critical field B(c2) on MgB2 single crystals have shown a puzzling strong temperature dependence. Here, we present a calculation of the upper critical field based on a detailed modeling of band structure calculations that takes into account both the unusual Fermi surface topology and the two gap nature of the superconducting order parameter. Our results show that the strong temperature dependence of the B(c2) anisotropy can be understood as an interplay of the dominating gap on the sigma band, which possesses a small c-axis component of the Fermi velocity, with the induced superconductivity on the pi-band possessing a large c-axis component of the Fermi velocity. We provide analytic formulas for the anisotropy ratio at T=0 and T=T(c) and quantitatively predict the distortion of the vortex lattice based on our calculations.     

Deviation from Matthiessen's rule of dilute al-alloys, 3 group model

The deviation from Matthiessen's rule has been investigated. The calculations were based upon three group model, in which the Fermi surface is split up into three regions. The mean free paths over the three major regions are denoted byL ₁,L ₂ andL ₃. Besides the Fermi surface anisotropy, the anisotropy of electron scattering with phonons and impurities have also been employed in the calculations. Simple formulae for the residual resistivity, the resistivity associated with phonon scattering and the deviation from Matthiessen's rule Δ(C, T), in terms of the anisotropic parameters, were derived. The formulae were used to throw light on the deviation from Matthiessen's rule.     

Conductivity anisotropy in the antiferromagnetic state of iron pnictides

Recent experiments on iron pnictides have uncovered a large in-plane resistivity anisotropy with a surprising result: The system conducts better in the antiferromagnetic x direction than in the ferromagnetic y direction. We address this problem by calculating the ratio of the Drude weight along the x and y directions, D(x)/D(y), for the mean-field Q=(π,0) magnetic phase diagram of a five-band model for the undoped pnictides. We find that D(x)/D(y) ranges between 0.2相似文献   

Phase diagram of strongly correlated Fermi systems

Phase transitions caused by the redistribution of quasiparticle occupation numbers n(p) in homogeneous Fermi systems with particle repulsion are analyzed. The phase diagram of a strongly correlated Fermi system, when drawn in the coordinates “density ρ-dimensionless coupling constant η,” resembles a Washington pie for a rather broad class of interactions. Its upper part is “filled” with Fermi condensate, and the bottom part is filled with normal Fermi liquid. Both parts are separated by a narrow interlayer of Lifshitz phase with a multiply connected Fermi surface.     

Influence of anisotropic fluctuations on the electrical resistivity in uniaxial ferromagnetic metals near Tc

The generalized Ornstein-Zernike correlation function appropriate to a uniaxial ferromagnet is applied to study the influence of anisotropic fluctuations on the electrical resistivity in uniaxial ferromagnetic metals near T_c. It is shown that both the anisotropic fluctuations and the Fermi surface anisotropy contribute on the same footing to an anisotropy of the spin-fluctuation resistivity. The results are consistent with those obtained for the isotropic system in the same approximation and may be useful for interpreting recent spin-fluctuation resistivity data obtained experimentally for uniaxial systems.     

The superconductive transition temperature of binary and ternary solid solutions in thallium

The transition temperatures of various binary alloys and of ternary alloys in the system Tl Hg _x Sn _y were investigated. By analysing the results different anisotropy parameters were obtained. This was explained by a nonlinear valence dependence of the transition temperature, which can be caused by changes in the topology of the Fermi surface.     

In-plane torque and gap symmetry of untwinned YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript> crystals

The reversible magnetic torque of untwinned YBa₂Cu₃O₇ single crystals shows the four-fold symmetry in thea-b plane. The irreversible torque indicates evidence for a novel intrinsic pinning along thea andb axes. These facts mean that the free energy of the four-fold symmetry has a minimum when the field is applied along thea orb axis. The results are consistent with those expected from thed _x ²?y ² symmetry and rule out the possibility of thed _xy symmetry. The Fermi surface anisotropy is not responsible for the observed anisotropy. This is firstbulk evidence for thek-dependent gap anisotropy on the Fermi surface. The two-fold anisotropy parameter is found as\(\gamma _{ab} = \sqrt {{{m_a } \mathord{\left/ {\vphantom {{m_a } {m_b }}} \right. \kern-\nulldelimiterspace} {m_b }}} = 1.18 \pm 0.14\).     

The anion and cation effects in the magnetic anisotropy of rare-earth compounds: Charge screening by conduction electrons

The formation of magnetic anisotropy (MA) in rare-earth compounds with transition metals has been analyzed. The screening of the charges creating the crystal field by conduction electrons has been shown to play an important role. The calculations took into account the Friedel charge-density oscillations. The model used for RCo₅ is the point-charge crystal field including nonuniform screening by conduction electrons with an anisotropic Fermi surface. The mechanisms of strong MA due to light-element impurities (hydrogen and nitrogen) are considered. The effective charge of an impurity can heavily depend on its ionic radius and the characteristics of the Fermi surface (in particular, on the Fermi momentum k _F ) of the screening electrons. The screening of the cation and anion charge in hydrides and nitrides based on the R₂Fe₁₇ and RFe₁₁Ti intermetallic compounds is discussed.     

Application of a lindhard like model for normal state transport properties of the cuprate superconductors

We account for the transport properties of the cuprates and of organic superconductors in the normal state by a model in which, following Lindhard's theory, the velocity near the Fermi surface is increased by reduced screening. We show that the conductivity due to elastic scattering becomes temperature dependent. We account for the conductivity anisotropy, the thermoelectric power, and the Hall constant. An analysis of the London penetration depth, which takes into account the low dimensionality of the conduction, gives large Fermi velocities and small effective masses, consistent with the analysis of the transport properties.     

Relation between magnetooptical effects and the topology of the Fermi surface of ferromagnetic metals

The anomalous magnetooptical effects in transition metals are related to the structure of the Fermi surface. The anisotropy parameters for the scattering of carriers by impurity centers is derived. The anisotropy parameter is calculated for the real and imaginary parts of the anomalous complex conductivity of iron for multiply connected, spin-down cylindrical surfaces around point H of the subband, for the single spin-down electron surface around point of the subband, and for the large spin-up hole surface around point H of the subband.Translated from Izvestiya VUZ. Fizika, No. 5, pp. 23–28, May, 1971.The author thanks professor E. I. Kondorskii.     

Anisotropy effects in the A-15 superconductor Nb3Sn

Experimental B_c2(T) characteristics of two Nb₃Sn samples are analyzed using a full strong coupling theory of B_c2(T) which also includes anisotropy effects. The analysis requires a rather pronounced anisotropy of the electron-phonon interaction and of the Fermi velocity and Pauli paramagnetic limiting in order to reproduce the experimental B_c2(T) data over the whole temperature range. The analysis of the second, dirty sample shows that a pronounced change in the electron density of states at the Fermi energy is required together with reduction of the Fermi velocity anisotropy to reproduce the experimental B_c2(T) data. The analysis does not require any contributions from spin-orbit scattering processes to achieve an excellent match between theory and experiment.     

Anisotropy of electrical conductivity and hall effect in MnAs0.95P0.05

The resistivity tensor ? and the anisotropic Hall resistance ?_H are measured for 70 K ? T ? 320K. ? is assigned to the anisotropy of the Fermi surface while ?_H is related to the magnetization ellipsoid for T < T_N = 228 K (anomalous Hall effect).     

Corrugated graphene: effects of in-plane and tilted out-of-plane magnetic fields

The electronic energy level structure of the corrugated graphene electron subjected to a magnetic field tilted with respect to the graphene plane and an in-plane homogeneous magnetic field is investigated theoretically within the perturbation framework. It is shown that the anisotropy induced by the tilted magnetic field strongly modifies the Fermi velocity of the corrugated graphene electron, and the corrugated structure yields intrinsic Weiss oscillations in both Fermi velocity and the graphene Landau levels.     

Theoretical study on the superfluid density of the superconducting MNCl (M=Hf, Zr)

We consider the possibility of spin fluctuation mediated d+id′‐wave pairing for the layered nitride superconductor MMCl. Using the superconducting gap obtained within the fluctuation exchange method for the two band model, we calculate the superfluid density as a function of temperature for several doping concentrations. Reflecting the fact that the anisotropy of the d+id′‐wave gap on the Fermi surface is enhanced with the increase of the doping concentration, the overall doping dependence of the superfluid density is in qualitative agreement with the experiment.     

Multi-patch model for transport properties of cuprate superconductors

A number of normal state transport properties of cuprate superconductors are analyzed in detail using the Boltzmann equation. The momentum dependence of the electronic structure and the strong momentum anisotropy of the electronic scattering are included in a phenomenological way via a multi-patch model. The Brillouin zone and the Fermi surface are divided in regions where scattering between the electrons is strong and the Fermi velocity is low (hot patches) and in regions where the scattering is weak and the Fermi velocity is large (cold patches). We present several motivations for this phenomenology starting from various microscopic approaches. A solution of the Boltzmann equation in the case of N patches is obtained and an expression for the distribution function away from equilibrium is given. Within this framework, and limiting our analysis to the two patches case, the temperature dependence of resistivity, thermoelectric power, Hall angle, magnetoresistance and thermal Hall conductivity are studied in a systematic way analyzing the role of the patch geometry and the temperature dependence of the scattering rates. In the case of Bi-based cuprates, using ARPES data for the electronic structure, and assuming an inter-patch scattering between hot and cold states with a linear temperature dependence, a reasonable agreement with the available experiments is obtained. Received 3 August 2001 and Received in final form 1st November 2001     

Exact saturation and degeneracy of isospin bounds and zeros trajectories in pion-nucleon scattering: D. B. Ion. Joint Institute for Nuclear Research,Dubna, Moscow,P. O. Box 79, U.S.S.R.

By exploiting the analogy between the classical equation of heat conduction and the Bloch equation, the canonical density matrix is calculated exactly for noninteracting electrons in a finite step model of a metal surface. This result is then used to calculate the spatial variation of the electron density ?(z) as a function of distance z from the planar metal surface. In particular an expansion in inverse powers of $\text{V}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and V, V being the step height, is derived. The low-order terms are used to compute the electron density at the surface of Al metal: There is good agreement with a full numerical evaluation. Finally, in the limit $\text{V}\text{E}{}_{\mathrm{f}}\text{→ ∞}$, E_f being the Fermi energy, the change in the shape and size of the exchange (Fermi) hole surrounding an electron is studied as the electron moves into the vicinity of a metal surface. The anisotropy of the Fermi hole is large and the implications for one-body potential theory are briefly discussed.     

Anomalous attenuation of longitudinal ultrasound in the intermediate state of pure type I superconductor

The dependence of ultrasonic attenuation on the direction of magnetic field H (H ⊥ k) in intermediate state of very pure gallium at T = 0.5°K was found to be anomalously different from the dependence observed at the same temperature and H = H_c in normal state. This phenomenon can be explained taking into account the anisotropy of the gallium Fermi surface.     

A study of the fermi surface of α-brass

Fermi surface of α-brass was investigated up to 24.6 at.% Zn by the rotatingspecimen method in the annihilation radiation. The anisotropy in the Fermi surface of α-brass was found to be similar to that of pure copper. It was also noted that the 〈111〉-neck radius of the alloys increase monotonically with increasing content of zinc.