Influence of magnetic quantization on the effective mass in semiconductor superlattices

An attempt is made to study the effect of a quantizing magnetic field on the effective electron mass in a semiconductor superlattice at low temperatures. It is found on the basis of the tight-binding approximation, taking GaAs-Ga_1–x Al _x As an example, that the effective mass at the Fermi level depends on the magnetic quantum number due to the cosine dependence of the wave-vector in the superlattice direction. The mass also exhibits oscillatory features in the presence of magnetic quantization because of its dependence on Fermi energy which oscillates with changing magnetic field.     

Non-Fermi liquid behavior in the stripe phase of the extended Hubbard model

Within a strong-coupling perturbative approach, based on a Cumulant Expansion of the extended single-band Hubbard model, we show that the on-shell inverse scattering time deviates from the normal Fermi-liquid behavior near the points of the Fermi surface connected by the characteristic wave-vector of an incommensurate charge density wave. The violation of the Fermi liquid behavior is associated with a square root behavior of the inverse quasiparticle lifetime in proximity of a stripe phase. Some relevant features observed in ARPES experiments on Bi2212 are qualitatively reproduced. Received 28 February 2002 / Received in final form 7 May 2002 Published online 9 July 2002     

Common features in high Tc cuprates and diborides

The normal state of magnesium diboride, the simplest metallic system showing the macroscopic quantum coherence at high temperature (high T_c superconductivity––HTcS) shows some features that are common with the cuprates, shedding light on the pairing mechanism for T_c amplification in HTcS: (1) two or more electronic components associated with multiple Fermi surface portions; (2) crossing of electronic topological transitions (ETT) where the Fermi surface changes its topology and (3) phonon softening in extended Kohn anomalies (EKA) over a large wave-vector range.     

Photoemission spectroscopy in metals:: band structure-Fermi surface–spectral function

Angular resolved photoelectron spectroscopy plays a key role in the study of the electronic structure of solids. We discuss recent methodical developments in its application to metallic systems. These include a new procedure for absolute E(k) band structure determination, which allows complete control of the three-dimensional wave-vector k, as well as a method for Fermi surface mapping based on measurements of the angular photoelectron intensity distribution. Going beyond a simple one-electron picture, we examine under which conditions the photoemission signal can be interpreted in terms of the electron removal spectrum of an interacting electron system and discuss an experimental test on a suitable Fermi liquid metal, which supports this many-body interpretation.     

非晶碳结构建模和电子结构的第一性原理研究

基于密度泛函理论,采用了一种更为精确的交换相关泛函OLYP(OPTX+LYP),对密度范围从2.0到3.2 g/cm³的非晶碳进行结构建模. 模拟得到的5个碳网络结构无论从径向分布函数还是sp³含量都与实验符合得很好. 对非晶碳电子结构的研究表明费米能级附近的电子态密度主要是sp²碳原子的贡献. 随着密度的增加,sp³碳原子增加,费米能级附近的态密度越来越小. 小环结构增加了费米能级附近的电子态密度,缺陷态在费米能级形 关键词： 非晶碳 密度泛函理论 电子结构     

非晶碳结构建模和电子结构的第一性原理研究

基于密度泛函理论，采用了一种更为精确的交换相关泛函OLYP(OPTX+LYP)，对密度范围从2.0到3.2 g/cm³的非晶碳进行结构建模. 模拟得到的5个碳网络结构无论从径向分布函数还是sp³含量都与实验符合得很好. 对非晶碳电子结构的研究表明费米能级附近的电子态密度主要是sp²碳原子的贡献. 随着密度的增加，sp³碳原子增加，费米能级附近的态密度越来越小. 小环结构增加了费米能级附近的电子态密度，缺陷态在费米能级形     

Resonance of the Fermi velocity in weakly doped graphene

It has been revealed that the dependence of the Fermi velocity on the number of charge carriers in doped graphene contains a sharp peak at which the Fermi velocity can increase by a factor of more than 1.5. It has been shown that this peak, which appears when the Fermi energy coincides with the energy of single-impurity resonance, exists when the concentration of impurities is lower than the corresponding critical concentration at which the spectrum of the system is rearranged.     

Strongly enhanced magnetic fluctuations in a large-mass layered ruthenate

We have studied the magnetic excitations in Ca2-xSrxRuO4, x=0.52 and 0.62, which exhibit an anomalously high susceptibility and heavy mass Fermi liquid behavior. Our inelastic neutron scattering experiments reveal strongly enhanced magnetic fluctuations around an incommensurate wave-vector (0.22,0,0) pointing to a magnetic instability. The magnetic fluctuations show no correlation in the c direction and also along the RuO2 planes the signal is extremely broad, Deltaq=0.45 A(-1). These fluctuations can quantitatively account for the high specific heat coefficient and relate to the high macroscopic susceptibility. The magnetic scattering is attributed to the d(xy) band, the active band for spin triplet superconductivity in Sr2RuO4.     

Damping of zero sound in Luttinger liquids

We calculate the damping γ_q of collective density oscillations (zero sound) in a one-dimensional Fermi gas with dimensionless forward scattering interaction F and quadratic energy dispersion k² / 2 m at zero temperature. Using standard many-body perturbation theory, we obtain γ_q from the expansion of the inverse irreducible polarization to first order in the effective screened (RPA) interaction. For wave-vectors | q| /k_F ≪F (where k_F = m v_F is the Fermi wave-vector) we find to leading order γ_q ∝| q |³ /(v_F m²). On the other hand, for F ≪| q| /k_F most of the spectral weight is carried by the particle-hole continuum, which is distributed over a frequency interval of the order of q²/m. We also show that zero sound damping leads to a finite maximum proportional to |k - k_F | ^{-2 + 2 η} of the charge peak in the single-particle spectral function, where η is the anomalous dimension. Our prediction agrees with photoemission data for the blue bronze K_0.3MoO₃. We comment on other recent calculations of γ_q.     

Asymmetric zero-bias anomaly for strongly interacting electrons in one dimension

We study a system of one-dimensional electrons in the regime of strong repulsive interactions, where the spin exchange coupling J is small compared with the Fermi energy, and the conventional Tomonaga-Luttinger theory does not apply. We show that the tunneling density of states has a form of an asymmetric peak centered near the Fermi level. In the spin-incoherent regime, where the temperature is large compared to J, the density of states falls off as a power law of energy epsilon measured from the Fermi level, with the prefactor at positive energies being twice as large as that at the negative ones. In contrast, at temperatures below J the density of states forms a split peak with most of the weight shifted to negative epsilon.     

Evidence for an instability near twice the fermi wave vector in the low electronic density liquid metal Li(NH3)4

We report high-resolution inelastic x-ray scattering measurements in the metallic liquid Li(NH3)4, which to a good approximation can be treated as a dilute alkali metal. We see a well-defined excitation out to large momentum transfers. This excitation shows a strong softening at wave vectors near the first peak in the structure factor, which occurs near twice the Fermi momentum.     

Measurement of the interaction energy near a Feshbach resonance in a 6Li Fermi gas

We investigate the strongly interacting regime in an optically trapped 6Li Fermi mixture near a Feshbach resonance. The resonance is found at 800(40) G in good agreement with theory. Anisotropic expansion of the gas is interpreted by collisional hydrodynamics. We observe an unexpected and large shift (80 G) between the resonance peak and both the maximum of atom loss and the change of sign of the interaction energy.     

A study of local density of states of atom clusters on the W(110) surface

The kinetic energy distributions of field desorbed He ions from tungsten clusters of one to five atoms on a W(110) surface are measured using a high resolution pulsed-laser time-of-flight atom probe. The He field ion energy distribution from the single W adatom shows an extra peak-like feature centered at 2.7 eV above the Fermi level. It has a full width at half maximum (fwhm) of 2.3 eV. The data from two tungsten adatoms separated by two lattice constants have nearly the same feature with the extra peak located at 2.5 eV above the Fermi level. These peaks arise from resonance tunneling with the adatom local density of states (LDOS). The He ion energy distribution of a tungsten dimer has an extra peak centered at 1.5 eV above the Fermi level. The fwhm is about 4 eV. The spectra from four and five tungsten adatom clusters show only one peak each, similar to that from a flat plane.     

Bound states in optical absorption of semiconductor quantum wells containing a two-dimensional electron Gas

The dependence of the optical absorption spectrum of a semiconductor quantum well on two-dimensional electron concentration n(e) is studied using CdTe samples. The trion peak (X-) seen at low n(e) evolves smoothly into the Fermi edge singularity at high n(e). The exciton peak (X) moves off to high energy, weakens, and disappears. The X,X- splitting is linear in n(e) and closely equal to the Fermi energy plus the trion binding energy. For Cd0.998Mn0.002Te quantum wells in a magnetic field, the X,X- splitting reflects unequal Fermi energies for M = +/-1/2 electrons. The data are explained by Hawrylak's theory of the many-body optical response including spin effects.     

Local wave-vector, Shannon and Fisher information

It is shown that the local wave-vector gives the connection between the Shannon and Fisher information: the local wave-vector is the gradient of the Shannon information per particle and the square of the local wave-vector is the Fisher information per particle.     

Bounds to the wave-vector dependent susceptibility of the ising model

It is shown that rigorous upper and lower bounds to the wave-vector dependent susceptibility of the Ising Model are obtained either when an upper bound to the spin pair correlation function and a lower bound to the susceptibility at zero wave-vector are given, or when a lower bound to the former and an upper bound to the latter are given. An example of the numerical computation of the bounds is presented for the Ising model on the sc lattice.     

Engineering of multicolor spatial solitons with chirped-period quasi-phase-matching gratings in optical parametric amplification

We demonstrate the excitation of solitons in a parametric amplifier with enhanced signal content through the use of a chirped-period quasi-phase-matching grating. This technique affords a low soliton threshold at the input end of a parametric amplifier, and the subsequent transformation to a desired soliton that exists at nonzero wave-vector mismatch through the use of a linearly chirped quasi-phase-matching grating. This approach has an advantage over direct excitation of solitons at nonzero wave-vector mismatch in uniform nonlinear materials and holds potential for improving the efficiency and mode quality of high-gain parametric amplifiers.     

Tomographic rf spectroscopy of a trapped Fermi gas at unitarity

We present spatially resolved radio-frequency spectroscopy of a trapped Fermi gas with resonant interactions and observe a spectral gap at low temperatures. The spatial distribution of the spectral response of the trapped gas is obtained using in situ phase-contrast imaging and 3D image reconstruction. At the lowest temperature, the homogeneous rf spectrum shows an asymmetric excitation line shape with a peak at 0.48(4)epsilonF with respect to the free atomic line, where epsilonF is the local Fermi energy.     

Third-harmonic generation and self-channeling in air using high-power femtosecond laser pulses

It is shown, both theoretically and experimentally, that during laser pulse filamentation in air an intense ultrashort third-harmonic pulse is generated forming a two-colored filament. The third-harmonic pulse maintains both its peak intensity and energy over distances much longer than the characteristic coherence length. We argue that this is due to a nonlinear phase-locking mechanism between the two pulses in the filament and is independent of the initial material wave-vector mismatch. A rich spatiotemporal propagation dynamics of the third-harmonic pulse is predicted. Potential applications of this phenomenon to other parametric processes are discussed.