Instability of the non-Fermi-liquid state in a metal with d or f impurities

It is shown that the non-Fermi-liquid state is unstable with respect to scattering of multiparticle excitations with different quantum numbers by one another. As a result of the scattering, a multiparticle Fermi-liquid resonance forms at the Fermi level. An anomalous increase in the conductivity occurs as a result of a transition between the non-Fermi-liquid and Fermi-liquid states. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 10, 721–726 (25 May 1999)     

Non-Fermi-liquid effects in tunneling

Non-Fermi-liquid tunneling mechanisms in a quantum structure with its own two-dimensional continuum doped with transition metal impurities are considered. New physical realizations of the two-channel Kondo orbital model with mechanisms different from those previously described in literature occur in such quantum structures. The tunneling transparency is anomalously high owing to new channels generated by multiparticle Fermi-liquid resonances near the edge of the two-dimensional energy band in the process of tunneling. The widths of new edge resonances can be much smaller than the width of the “bare” non-Fermi-liquid resonance at the Fermi level in the banks. The additional scattering due to tunneling induces a transition from the non-Fermi-liquid to the Fermi-liquid state as the separation between the Fermi level in the banks and the two-dimensional band edge in the quantum well varies. Zh. éksp. Teor. Fiz. 114, 1466–1486 (October 1998)     

The formation of heavy-fermion states in non-Fermi-liquid impurity systems

A mechanism for the occurrence of heavy-fermion states in non-Fermi-liquid (NFL) metals with f-shell impurities is proposed. The impurity with an unstable valence is suggested to have an energy spectrum consisting of a deep f-level and quasicontinuum states (narrow band) in resonance with the Fermi energy. Depending on the impurity concentration, the single-site NFL states are generated by the two-channel Kondo scattering for the low concentration (the Kondo regime) or by the screening interaction for a relatively high concentration (the X-ray-edge regime). It is shown that the NFL states are unstable against the scattering of the NFL excitations by electron states of the narrow band. This scattering generates additional narrow Fermi-liquid (FL) resonances at/near the Fermi level in the Kondo regime and in the X-ray-edge regime. The mixed-valence states are shown to be induced by new FL resonances. The mixed valence mechanism is local and is related to the instability of single-site NFL states. The FL resonances lead to the existence of additional energy scales and of pseudogaps near the Fermi level in the mixed-valence states. They also considerably narrow the region with a nearly integer valence.     

Heavy-fermion states in non-Fermi-liquid impurity metals

It is shown that a new type of instability of a non-Fermi-liquid state to the interband scattering of multiparticle excitations can dominate the formation of heavy-fermion states in non-Fermi-liquid metals doped with unstable-valence f impurities. A new mechanism is proposed for the formation of a small energy scale and pseudogaps near the Fermi level in a mixed-valent state.     

Transition between non-Fermi-liquid and Fermi-liquid states as a result of tunneling

It is shown that additional scattering due to tunneling induces a transition of the system from a non-Fermi-liquid into a Fermi-liquid state as the distance between the Fermi level in the walls and the 2D-band edge is varied in a double-barrier quantum well, doped with transition-metal impurities and having an intrinsic two-dimensional continuum. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 12, 1009–1014 (25 June 1998)     

Lifetime of charge carriers in multiwalled nanotubes

The nature of low-energy excitations in multiwalled nanotubes (MWNTs) is investigated by means of two-color time-resolved photoemission. A careful analysis of the ballistic transport, secondary excitations, and band structure effects was employed in order to extract single electron lifetimes from the observed relaxation trend. It is demonstrated that in the vicinity of the Fermi level the energy dependence of e-e scattering times is inversely proportional to approximately the square of the excitation energy. This result provides strong evidence that electron transport in MWNTs exhibits a Fermi-liquid behavior, indicating that long-range e-e interaction along the tube vanishes due to screening.     

Coulomb screening of impurity charge and anomalous tunneling transparency

This paper analyzes the effect of the screened Coulomb interaction between metallic electrons in the sidewalls, on the one hand, and a localized electron in an impurity level, on the other, on the tunneling in doped quantum structures with an intrinsic two-dimensional continuum. We show that Mahan’s non-Fermi-liquid singularity at the Fermi level is unstable against additional scattering due to tunneling. As a result, the current-voltage characteristic changes radically when the Fermi level in the sidewalls is approached by the edge of the two-dimensional band. Specifically, the peak due to the non-Fermi-liquid singularity with a section of negative differential resistance is replaced with a step-like or a two-step feature, which corresponds to a single or split Fermi-liquid resonance near the edge of the 2D band involved in the tunneling process. Zh. éksp. Teor. Fiz. 115, 1843–1859 (May 1999)     

Progress in the understanding of the normal state of the cuprates

Over the last years there has been increasing evidence that the normal state of the cuprates can not be described adequately with individual quasiparticles within Fermi-liquid theory. While the low-lying excitations in the superconducting state are nevertheless possibly of quasiparticle character, this character vanishes with the loss of superconducting phase coherence when going to the normal conducting state. Generally, this normal state is characterized by strong heterogeneity. In real space this manifests in charge and spin ordering, either static or dynamical, the so-called ‘stripes’. The spectral signatures of various models describing this unusual metallic normal state together with less exotic non-Fermi-liquid models, like the marginal Fermi liquid, will be compared to photoemission spectra with high angular and energy resolution and to results obtained by other methods. Received: 19 March 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-30/2093-7729, E-mail: christoph.janowitz@physik.hu-berlin.de     

Anomalous quasiparticle lifetime and strong electron-phonon coupling in graphite

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy on high-quality single crystals of graphite to elucidate the character of low-energy excitations. We found evidence for a well-defined quasiparticle (QP) peak in the close vicinity of the Fermi level comparable to the nodal QP in high-T(c) cuprates, together with the mass renormalization of the band at an extremely narrow momentum region around the K(H) point. Analysis of the QP lifetime demonstrates the presence of strong electron-phonon coupling and linear energy dependence of the QP scattering rate indicative of a marked deviation from the conventional Fermi-liquid theory.     

Non-Fermi-liquid behavior in the fluctuating gap model: From the pole to a zero of the Green’s function

We analyze the non-Fermi-liquid (NFL) behavior of the fluctuating gap model (FGM) of pseudogap behavior in both one and two dimensions. A detailed discussion of quasiparticle renormalization (Z-factor) is given, demonstrating a kind of marginal Fermi-liquid or Luttinger-liquid behavior and topological stability of the bare Fermi surface (the Luttinger theorem). In the two-dimensional case, we discuss the effective picture of the Fermi surface destruction both in the hot spot model of dielectric (AFM, CDW) pseudogap fluctuations and for the qualitatively different case of superconducting d-wave fluctuations, reflecting the NFL spectral density behavior and similar to that observed in ARPES experiments on copper oxides. The text was submitted by the authors in English.     

Correlations and confinement of excitations in an asymmetric Hubbard ladder

Correlation functions and low-energy excitations are investigated in the asymmetric two-leg ladder consisting of a Hubbard chain and a noninteracting tight-binding (Fermi) chain using the density matrix renormalization group method. The behavior of charge, spin and pairing correlations is discussed for the four phases found at half filling, namely, Luttinger liquid, Kondo-Mott insulator, spin-gapped Mott insulator and correlated band insulator. Quasi-long-range antiferromagnetic spin correlations are found in the Hubbard leg in the Luttinger liquid phase only. Pair-density-wave correlations are studied to understand the structure of bound pairs found in the Fermi leg of the spin-gapped Mott phase at half filling and at light doping but we find no enhanced pairing correlations. Low-energy excitations cause variations of spin and charge densities on the two legs that demonstrate the confinement of the lowest charge excitations on the Fermi leg while the lowest spin excitations are localized on the Hubbard leg in the three insulating phases. The velocities of charge, spin, and single-particle excitations are investigated to clarify the confinement of elementary excitations in the Luttinger liquid phase. The observed spatial separation of elementary spin and charge excitations could facilitate the coexistence of different (quasi-)long-range orders in higher-dimensional extensions of the asymmetric Hubbard ladder.     

Fermi systems with strong forward scattering

We review the theory of interacting Fermi systems whose low-energy physics is dominated by forward scattering, that is scattering processes generated by effective interactions with small momentum transfers. These systems include Fermi liquids as well as several important non-Fermi-liquid phases: one-dimensional Luttinger liquids, systems with long-range interactions, and fermions coupled to a gauge field. We report results for the critical dimensions separating different 'universality classes' and discuss the behaviour of physical quantities such as the momentum distribution function, the single-particle propagator and low-energy response functions in each class. The renormalization group for Fermi systems will be reviewed and applied as a link between microscopic models and effective lowenergy theories. Particular attention is paid to conservation laws, which constrain any effective low-energy theory of interacting Fermi systems. In scattering processes with small momentum transfers the velocity of each scattering particle is (almost) conserved. This asymptotic conservation law leads to non-trivial cancellations of Feynman diagrams and other simplifications, making thus possible a non-perturbative treatment of forward scattering via Ward identities or bosonization techniques.     

Real space imaging of one-dimensional standing waves: direct evidence for a Luttinger liquid

Electronic standing waves with two different wavelengths were directly mapped near one end of a single-wall carbon nanotube as a function of the tip position and the sample bias voltage with high-resolution position-resolved scanning tunneling spectroscopy. The observed two standing waves caused by separate spin and charge bosonic excitations are found to constitute direct evidence for a Luttinger liquid. The increased group velocity of the charge excitation, the power-law decay of their amplitudes away from the scattering boundary, and the suppression of the density of states near the Fermi level were also directly observed or calculated from the two different standing waves.     

Thermodynamic properties of CeCu6-xAux: Fermi-liquid vs. non-Fermi-liquid behaviour

We report on a detailed comparison of the thermodynamic properties of the heavy-fermion system CeCu₆ which can be described as a Fermi liquid at low temperatures T < 0.1 K, and CeCu_5.9Au_0.1 where strong deviations from the Fermi-liquid behaviour were found previously in the T dependence of the specific heat C, magnetization M and electrical resistivity p. The specific heat, magnetization and elastic constants are investigated in a large range of magnetic fields, corroborating the idea that the non-Fermi-liquid behaviour arises from low-lying spin excitations. For the elastic constants, a striking linear T dependence is found for CeCu_5.9Au_0.1 in contrast to the T² Fermi-liquid behaviour of CeCu₆.     

Minimal excitation states of electrons in one-dimensional wires

A strategy is proposed to excite particles from a Fermi sea in a noise-free fashion by electromagnetic pulses with realistic parameters. We show that by using quantized pulses of simple form one can suppress the particle-hole pairs which are created by a generic excitation. The resulting many-body states are characterized by one or several particles excited above the Fermi surface accompanied by no disturbance below it. These excitations carry charge which is integer for noninteracting electron gas and fractional for Luttinger liquid. The operator algebra describing these excitations is derived, and a method of their detection which relies on noise measurement is proposed.     

Instability of Non—Fermi Liquid Behavior in the Two—Channel Kondo Model

The effects of interchannel scattering of conduction electrons by the impurity and repulsion of conduction electrons at the impurity site on the two-channel Kondo model are simultaneously considered in this paper,It is shown that these two perturbations will substantially modify the usual local non-Fermi liquid behavior of the two-channel Kondo model.With bosonization and unitary transformations we find that the system can be transformed into a single channel Kondo model with anisotropy between longitudinal and transverse exchange couplings,Whatever for originally antiferromagnetic or ferromagnetic isotropic coupling,the system always flows to strong-coupling limit,which exhibits local Fermi liquid behavior at low temperatures.     

Electronic properties of the Dirac and Weyl systems with first- and higher-order dispersion in non-Fermi-liquid picture

We investigate the non-Fermi-liquid behaviors of the 2D and 3D Dirac/Weyl systems with low-order and higher order dispersion. The self-energy correction, symmetry, free energy, optical conductivity, density of states, and spectral function are studied. We found that, for Dirac/Weyl systems with higher order dispersion, the non-Fermi-liquid features remain even at finite chemical potential, and they are distinct from the ones in Fermi-liquid picture and the conventional non-Fermi-liquid picture. The power law dependence of the physical observables on the energy as well as the logarithmic renormalizations due to the long-range Coulomb interaction are showed. The Landau damping of the longitudinal excitations within random-phase-approximation (RPA) for the non-Fermi-liquid case are also discussed.     

Fano Resonance in Landau Fermi and Luttinger Liquids

With a two-channel model, we study the influence of temperature, external voltage and magnetic flux on the line shape of the Fano resonance, and show that in the Luttinger liquid case, the background transmittance and the asymmetric parameter depend strongly on the temperature and external voltage, while for the Landau Fermi liquid case they are nearly independent of these parameters in the low energy region. Moreover, we demonstrate that the asymmetric parameter changes periodically with an external magnetic flux, which is consistent with the recent experimental data.