Inhomogeneous electron states in the systems with imperfect nesting

A brief overview of the theoretical studies on the electronic phase separation in the systems with the imperfect nesting of Fermi surface sheets is presented. Among these systems, there are chromium and its alloys, superconducting iron-based pnictides, bilayer graphene, and some other materials.     

Remnant fermi surfaces in photoemission

Recent experiments have introduced a new concept for analyzing the photoemission spectra of correlated electrons-the remnant Fermi surface (rFs), which can be measured even in systems which lack a conventional Fermi surface. Here, we analyze the rFs in a number of interacting electron models, and find that the results fall into two classes. For systems with particle-particle (pairing) instabilities, the rFs is an accurate replica of the true Fermi surface. In the presence of particle-hole (nesting) instabilities, the rFs is a map of the resulting superlattice Brillouin zone. The results suggest that the gap in Ca2CuO2Cl2 is of particle-hole origin.     

Soft fermi surfaces and breakdown of Fermi-liquid behavior

Electron-electron interactions can induce Fermi surface deformations which break the point-group symmetry of the lattice structure of the system. In the vicinity of such a "Pomeranchuk instability" the Fermi surface is easily deformed by anisotropic perturbations, and exhibits enhanced collective fluctuations. We show that critical Fermi surface fluctuations near a d-wave Pomeranchuk instability in two dimensions lead to large anisotropic decay rates for single-particle excitations, which destroy Fermi-liquid behavior over the whole surface except at the Brillouin zone diagonal.     

Pertubation theory around nonnested fermi surfaces. I. Keeping the fermi surface fixed

The perturbation expansion for a general class of many-fermion systems with a nonnested, nonspherical Fermi surface is renormalized to all orders. In the limit as the infrared cutoff is removed, the counterterms converge to a finite limit which is differentiable in the band structure. The map from the renormalized to the bare band structure is shown to be locally injective. A new classification of graphs as overlapping or nonoverlapping is given, and improved power counting bounds are derived from it. They imply that the only subgraphs that can generater factorials in ther th order of the renormalized perturbation series are indeed the ladder graphs and thus give a precise sense to the statement that ladders are the most divergent diagrams. Our results apply directly to the Hubbard model at any filling except for half-filling. The half-filled Hubbard model is treated in another place.This paper is dedicated to the memory of Ansgar Schnizer.     

Infrared singularities in one-dimensional fermi systems

We study the infrared problem in one-dimensional Fermi systems with general interactions including backward scattering. The singular structure of the ground state energy which is a function of two coupling parametersu andv is determined exactly. For attractiveu-coupling one has power law singularities inv with a characteristic singular exponentk varying continuously withu. This behavior is reminiscent of a phase transition of continuous order and is obtained here by exploiting the analogy of the 1-d Fermi system and a 2-d classical plasma with a long range logarithmic Coulomb potential and a short range cut off. We also discuss other recent approaches to the problem.Supported in part by the Army Research Office, Durham, N.C., while the author was a summer visitor at Stanford University.     

Instabilities in compensated electron ring beams

An analysis is performed of the coherent instabilities which can develop in electron ring beams when an ion component is present in the beam. Increments of the instabilities are introduced, and it is shown that the compensation of an electron beam by ions results in a weakening of the instabilities characteristic of purely electron beams. The most rapidly growing coherent instabilities are determined, and possible methods are pointed out for their stabilization.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 37–53, January, 1978.     

Thermodynamic Hartree-Fock approximation in fermi systems

Bogoliubov's variational principle is applied to a test Hamiltonian operator in Hartree-Fock approximation. The extremal conditions are proven to be identical with self-consistent equations for Green's functions.     

Instabilities in wormlike micelle systems

Shear-banding is ubiquitous in complex fluids. It is related to the organization of the flow into macroscopic bands bearing different viscosities and local shear rates and stacked along the velocity gradient direction. This flow-induced transition towards a heterogeneous flow state has been reported in a variety of systems, including wormlike micellar solutions, telechelic polymers, emulsions, clay suspensions, colloidal gels, star polymers, granular materials, or foams. In the past twenty years, shear-banding flows have been probed by various techniques, such as rheometry, velocimetry and flow birefringence. In wormlike micelle solutions, many of the data collected exhibit unexplained spatio-temporal fluctuations. Different candidates have been identified, the main ones being wall slip, interfacial instability between bands or bulk instability of one of the bands. In this review, we present experimental evidence for a purely elastic instability of the high shear rate band as the main origin for fluctuating shear-banding flows.      

Canonical and microcanonical calculations for fermi systems

Recursion relations are presented that allow exact calculation of canonical and microcanonical partition functions of degenerate Fermi systems, assuming no explicit two-body interactions. Calculations of the level density, sorted by angular momentum, are presented for 56Ni. The issue of treating unbound states is also addressed.     

Instabilities in vertically vibrated fluid-grain systems

When a bed of fluid-immersed fine grains is exposed to vertical vibration a wealth of phenomena may be observed. At low frequencies a horizontal bed geometry is generally unstable and the bed breaks spatial symmetry, acquiring a tilt. At the same time it undergoes asymmetric granular convection. Fine binary mixtures may separate completely into layers or patterns of stripes. The separated regions may exhibit instabilities in which they undergo wave-like motion or exhibit quasi-periodic oscillations. We briefly review these and a number of related behaviours, identifying the physical mechanisms behind each. Finally, we discuss the magneto-vibratory separation of binary mixtures which results from exposing each component to a different effective gravity and describe the influence of a background fluid on this process.     

Some fermi surface properties of double-exchange interaction systems

We study the photoemission spectrum of the double-exchange (DE) interaction systems. The DE Hamiltonian can be transformed into a simple form consisting of fermions and Schwinger bosons. We apply the gauge-field model and calculate the Green's function of the gauge field, fermions, and bosons. The imaginary part of the Green's function of an electron has an asymmetrical peak with strong temperature dependence. This can explain why the shape of the angle-resolved photoemission spectra of manganites near the Fermi surface is very different from that of Fermi liquid. We also show why the position of the Fermi surface is not sensitive to temperature.     

Instabilities of an electron cloud in a Penning trap

We have measured the storage instabilities of electrons in a Penning trap at low magnetic fields. These measurements are carried out as a function of the trapping voltage, for different magnetic fields. It is seen that these instabilities occur at the same positions when the trapping voltage is expressed as a percentage of the maximum voltage, given by the stability limit. The characteristic frequencies at which these instabilities occur, obey a relation that is given by n _zω _z + n ₊ω ₊ + n _-ω _- = 0, where ω _z, ω ₊ and ω _- are the axial, perturbed cyclotron and the magnetron frequencies of the trapped electrons respectively, and the n's are integers. The reason for these instabilities are attributed to higher order static perturbations in the trapping potential. Received 5 August 2002 / Received in final form 14 October 2002 Published online 17 December 2002 RID="a" ID="a"Present address: Dept. of Physics, Rampurhat College, Rampurhat, Birbhum, West Bengal, India. RID="b" ID="b"e-mail: werth@mail.uni-mainz.de     

Instabilities and coarsening of stressed crystal surfaces in aqueous solution

Strong pattern formation occurs on polished miscut surfaces of sodium chlorate (NaClO3) single crystals that are uniaxially stressed perpendicular to the step edge direction and placed in a saturated aqueous solution. The wavelength lambda of the stress-induced surface instability increased continuously in experiments up to 9 days after placed in the solution. There were three successive regimes of coarsening: (i) one-dimensional step bunching with lambda approximately t(1/4) until an undulation transition was reached, (ii) a two-dimensional coarsening mechanism with lambda approximately t(1/2), and a gradual transition to (iii) Ostwald ripening-like coarsening with lambda approximately t(1/3). The coarsening of the surface patterns towards a stable, flat surface implies the spontaneous formation of a stress-free skin on the surface of the stressed solid.     

离化靶中多脉冲强流电子束的不稳定性

多脉冲强流电子束轰击轫致辐射靶,在靶面形成等离子体层,将对后续电子束脉冲的稳定性产生影响。从基本等离子理论出发,利用成熟的等离子体粒子模拟程序计算在不同等离子条件下电子束流的稳定性。模拟显示在无外场情况下,当等离子体与电子束的密度比小于1时,能量20 MeV、束流强度2.5 kA、焦斑1.5 mm的电子束出现腊肠不稳定性,但相对靶面焦点区而言,束流稳定;当密度比在1~100时,箍缩不稳定性能够改善电子束的聚焦;当密度比在100~1000时,扭曲不稳定性起主导作用,靶面焦点区电子束流仍然稳定;当密度比大于1000后,成丝不稳定性破坏束流,电子束无法在靶面聚焦。     

离化靶中多脉冲强流电子束的不稳定性

多脉冲强流电子束轰击轫致辐射靶,在靶面形成等离子体层,将对后续电子束脉冲的稳定性产生影响。从基本等离子理论出发,利用成熟的等离子体粒子模拟程序计算在不同等离子条件下电子束流的稳定性。模拟显示在无外场情况下,当等离子体与电子束的密度比小于1时,能量20 MeV、束流强度2.5 kA、焦斑1.5 mm的电子束出现腊肠不稳定性,但相对靶面焦点区而言,束流稳定;当密度比在1~100时,箍缩不稳定性能够改善电子束的聚焦;当密度比在100~1000时,扭曲不稳定性起主导作用,靶面焦点区电子束流仍然稳定;当密度比大于1000后,成丝不稳定性破坏束流,电子束无法在靶面聚焦。     

Measurement of separate cosmic-ray electron and positron spectra with the fermi large area telescope

We measured separate cosmic-ray electron and positron spectra with the Fermi Large Area Telescope. Because the instrument does not have an onboard magnet, we distinguish the two species by exploiting Earth's shadow, which is offset in opposite directions for opposite charges due to Earth's magnetic field. We estimate and subtract the cosmic-ray proton background using two different methods that produce consistent results. We report the electron-only spectrum, the positron-only spectrum, and the positron fraction between 20 and 200?GeV. We confirm that the fraction rises with energy in the 20-100?GeV range. The three new spectral points between 100 and 200?GeV are consistent with a fraction that is continuing to rise with energy.     

Meson exchange and the theory of finite fermi systems

The spin-dependent parameters g₀ and g'₀ in Migdal's theory of finite Fermi systems are studied. Assuming their values to be such as to reproduce the M1 states in ²⁰⁸Pb at 7 and 8 MeV, the origin of these (large) values in meson-exchange contributions to the nucleon-nucleon force is studied. Pion exchange contributes little, because of cancellation between matrix elements from the spin-spin and tensor components. The largest, nearly equal, contributions to g₀ and g'₀ come from p-meson exchange. In order to explain the large values needed for g₀ and g'₀, the tensor coupling of the ρ-meson must be substantially larger than the effective ones found in current phenomenological potentials, such as the Reid soft-core and Hamada-Johnston ones. The large value of the coupling needed follows from work analyzing nucleon form factors.