Dynamics and transport properties of heavy fermions: theory

The paramagnetic phase of heavy fermion systems is investigated, using a non-perturbative local moment approach to the asymmetric periodic Anderson model within the framework of dynamical mean field theory. The natural focus is on the strong coupling Kondo-lattice regime wherein single-particle spectra, scattering rates, dc transport and optics are found to exhibit w/w_L,T/w_L scaling in terms of a single underlying low-energy coherence scale w_L. Dynamics/transport on all relevant (w,T)-scales are encompassed, from the low-energy behaviour characteristic of the lattice coherent Fermi liquid, through incoherent effective single-impurity physics likewise found to arise in the universal scaling regime, to non-universal high-energy scales; and which description in turn enables viable quantitative comparison to experiment.Comment: 27 pages, 12 figure

Finite temperature dynamics of the Anderson model

The recently introduced local moment approach (LMA) is extended to encompass single-particle dynamics and transport properties of the Anderson impurity model at finite-temperature, T. While applicable to arbitrary interaction strengths, primary emphasis is given to the strongly correlated Kondo regime (characterized by the T=0 Kondo scale $\omega_{\rm K}$). In particular the resultant universal scaling behaviour of the single-particle spectrum D(\omega; T) \equiv F(\frac{\w}{\omega_{\rm K}}; \frac{T}{\omega_{\rm K}}) within the LMA is obtained in closed form; leading to an analytical description of the thermal destruction of the Kondo resonance on all energy scales. Transport properties follow directly from a knowledge of $D(\omega; T)$. The $T / \omega_{\rm K}$-dependence of the resulting resistivity $\rho(T)$, which is found to agree rather well with numerical renormalization group calculations, is shown to be asymptotically exact at high temperatures; to concur well with the Hamann approximation for the s-d model down to $T/\omega_{\rm K} \sim 1$, and to cross over smoothly to the Fermi liquid form $\rho (T) - \rho (0) \propto -(T/\omega_{\rm K})^2$ in the low-temperature limit. The underlying approach, while naturally approximate, is moreover applicable to a broad range of quantum impurity and related models

Zero-bias conductance in carbon nanotube quantum dots

We present numerical renormalization group calculations for the zero-bias conductance of quantum dots made from semiconducting carbon nanotubes. These explain and reproduce the thermal evolution of the conductance for different groups of orbitals, as the dot-lead tunnel coupling is varied and the system evolves from correlated Kondo behavior to more weakly correlated regimes. For integer fillings $N=1,2,3$ of an SU(4) model, we find universal scaling behavior of the conductance that is distinct from the standard SU(2) universal conductance, and concurs quantitatively with experiment. Our results also agree qualitatively with experimental differential conductance maps.Comment: 4 pages, 5 figure

Field-dependent dynamics of the Anderson impurity model

Single-particle dynamics of the Anderson impurity model in the presence of a magnetic field $H$ are considered, using a recently developed local moment approach that encompasses all energy scales, field and interaction strengths. For strong coupling in particular, the Kondo scaling regime is recovered. Here the frequency ($\omega/\omega_{\rm K}$) and field ($H/\omega_{\rm K}$) dependence of the resultant universal scaling spectrum is obtained in large part analytically, and the field-induced destruction of the Kondo resonance investigated. The scaling spectrum is found to exhibit the slow logarithmic tails recently shown to dominate the zero-field scaling spectrum. At the opposite extreme of the Fermi level, it gives asymptotically exact agreement with results for statics known from the Bethe ansatz. Good agreement is also found with the frequency and field-dependence of recent numerical renormalization group calculations. Differential conductance experiments on quantum dots in the presence of a magnetic field are likewise considered; and appear to be well accounted for by the theory. Some new exact results for the problem are also established

Anderson impurity in a semiconductor

We consider an Anderson impurity model in which the locally correlated orbital is coupled to a host with a gapped density of states. Single-particle dynamics are studied, within a perturbative framework that includes both explicit second-order perturbation theory and self-consistent perturbation theory to all orders in the interaction. Away from particle-hole symmetry the system is shown to be a generalized Fermi liquid (GFL) in the sense of being perturbatively connectable to the non-interacting limit; and the exact Friedel sum rule for the GFL phase is obtained. We show by contrast that the particle-hole symmetric point of the model is not perturbatively connected to the non-interacting limit, and as such is a non-Fermi liquid for all non-zero gaps. Our conclusions are in agreement with NRG studies of the problem.Comment: 7 pages, 4 figure

Single-particle dynamics of the Anderson model: a local moment approach

A non-perturbative local moment approach to single-particle dynamics of the general asymmetric Anderson impurity model is developed. The approach encompasses all energy scales and interaction strengths. It captures thereby strong coupling Kondo behaviour, including the resultant universal scaling behaviour of the single-particle spectrum; as well as the mixed valent and essentially perturbative empty orbital regimes. The underlying approach is physically transparent and innately simple, and as such is capable of practical extension to lattice-based models within the framework of dynamical mean-field theory.Comment: 26 pages, 9 figure

Requirements for Modeling and Simulation for Space Medicine Operations: Preliminary Considerations

The NASA Space Medicine program is now developing plans for more extensive use of high-fidelity medical Simulation systems. The use of simulation is seen as means to more effectively use the limited time available for astronaut medical training. Training systems should be adaptable for use in a variety of training environments, including classrooms or laboratories, space vehicle mockups, analog environments, and in microgravity. Modeling and simulation can also provide the space medicine development program a mechanism for evaluation of other medical technologies under operationally realistic conditions. Systems and procedures need preflight verification with ground-based testing. Traditionally, component testing has been accomplished, but practical means for "human in the loop" verification of patient care systems have been lacking. Medical modeling and simulation technology offer potential means to accomplish such validation work. Initial considerations in the development of functional requirements and design standards for simulation systems for space medicine are discussed

Isotope Depletion Mass Spectrometry (ID-MS) for Accurate Mass Determination and Improved Top-Down Sequence Coverage of Intact Proteins

Top-down mass spectrometry (MS) is an increasingly important technique for protein characterization. However, in many biological MS experiments, the practicality of applying top-down methodologies is still limited at higher molecular mass. In large part, this is due to the detrimental effect resulting from the partitioning of the mass spectral signal into an increasing number of isotopic peaks as molecular mass increases. Reducing the isotopologue distribution of proteins via depletion of heavy stable isotopes was first reported over 20 years ago (Marshall, A. G.; Senko, M. W.; Li, W.; Li, M.; Dillon, S., Guan, S.; Logan, T. M.. Protein Molecular Mass to 1 Da by 13C, 15N Double-Depletion and FT-ICR Mass Spectrometry. J. Am. Chem. Soc. 1997, 119, 433−434.) and has been demonstrated for several small proteins. Here we extend this approach, introducing a new highly efficient method for the production of recombinant proteins depleted in 13C and 15N and demonstrating its advantages for top-down analysis of larger proteins (up to ∼50 kDa). FT-ICR MS of isotopically depleted proteins reveals dramatically reduced isotope distributions with monoisotopic signal observed up to 50 kDa. In top-down fragmentation experiments, the reduced spectral complexity alleviates fragment-ion signal overlap, the presence of monoisotopic signals allows assignment with higher mass accuracy, and the dramatic increase in signal-to-noise ratio (up to 7-fold) permits vastly reduced acquisition times. These compounding benefits allow the assignment of ∼3-fold more fragment ions than comparable analyses of proteins with natural isotopic abundances. Finally, we demonstrate greatly increased sequence coverage in time-limited top-down experiments—highlighting advantages for top-down LC–MS/MS workflows and top-down proteomics

Local quantum phase transition in the pseudogap Anderson model: scales, scaling and quantum critical dynamics

The pseudogap Anderson impurity model provides a paradigm for understanding local quantum phase transitions, in this case between generalised fermi liquid and degenerate local moment phases. Here we develop a non-perturbative local moment approach to the generic asymmetric model, encompassing all energy scales and interaction strengths and leading thereby to a rich description of the problem. We investigate in particular underlying phase boundaries, the critical behaviour of relevant low-energy scales, and single-particle dynamics embodied in the local spectrum. Particular attention is given to the resultant universal scaling behaviour of dynamics close to the transition in both the GFL and LM phases, the scale-free physics characteristic of the quantum critical point itself, and the relation between the two.Comment: 39 pages, 19 figure

Spectral properties of a narrow-band Anderson model

We consider single-particle spectra of a symmetric narrow-band Anderson impurity model, where the host bandwidth $D$ is small compared to the hybridization strength $\Delta_{0}$. Simple 2nd order perturbation theory (2PT) in $U$ is found to produce a rich spectral structure, that leads to rather good agreement with extant Lanczos results and offers a transparent picture of the underlying physics. It also leads naturally to two distinct regimes of spectral behaviour, $\Delta_{0}Z/D\gg 1$ and $\ll 1$ (with $Z$ the quasi-particle weight), whose existence and essential characteristics are discussed and shown to be independent of 2PT itself. The self-energy $\Sigma_{i\omega}$ is also examined beyond the confines of PT. It is argued that on frequency scales of order $\omega\sim\sqrt{Delta_{0}D}$, the self-energy in {\em strong} coupling is given precisely by the 2PT result, and we point out that the resultant poles in $\Sigma_{i\omega}$ connect continuously to that characteristic of the atomic limit. This in turn offers a natural rationale for the known inability of the skeleton expansion to capture such behaviour, and points to the intrinsic dangers of partial infinite-order summations that are based on PT in $U$.Comment: 10 pages, 2 Postscript figures, uses RevTex 3.1; accepted for publication in Phys. Rev. B1