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A. W. W. Ludwig

Publications and source records attributed to A. W. W. Ludwig.

28 records · Page 2Linked to original sources
arXiv1999

gl(N|N) Super-Current Algebras for Disordered Dirac Fermions in Two Dimensions

We consider the non-hermitian 2D Dirac Hamiltonian with (A): real random mass, imaginary scalar potential and imaginary gauge field potentials, and (B) arbitrary complex random potentials of all three kinds. In both cases this Hamiltonian gives rise to a delocalization transition at zero energy with particle-hole symmetry in every realization of disorder. Case (A) is in addition time-reversal invariant, and can also be interpreted as the random-field XY Statistical Mechanics model in two dimensions. The supersymmetric approach to disorder averaging results in current-current perturbations of $gl(N|N)$ super-current algebras. Special properties of the $gl(N|N)$ algebra allow the exact computation of the beta-functions, and of the correlation functions of all currents. One of them is the Edwards-Anderson order parameter. The theory is `nearly conformal' and possesses a scale-invariant subsector which is not a current algebra. For N=1, in addition, we obtain an exact solution of all correlation functions. We also study the delocalization transition of case (B), with broken time reversal symmetry, in the Gade-Wegner (Random-Flux) universality class, using a GL(N|N;C)/U(N|N) sigma model, as well as its PSL(N|N) variant, and a corresponding generalized random XY model. For N=1 the sigma model is shown to be identical to the current-current perturbation. For the delocalization transitions (case (A) and (B)) a density of states, diverging at zero energy, is found.

cond-mat.mes-hall↗
arXiv1998

Two-Leg Ladders and Carbon Nanotubes: Exact Properties at Finite Doping

Recently Lin, Balents, and Fisher have demonstrated that two-leg Hubbard ladders and armchair carbon nanotubes renormalize onto the integrable SO(8) Gross-Neveu model. We exploit this integrability to examine these systems in their doped phase. Using thermodynamic Bethe ansatz, we compute exactly both the spin and single particle gaps and the Luttinger parameter describing low energy excitations. We show both the spin and particle gap do not vanish at finite doping, while the Luttinger parameter remains close to its free fermionic value of 1. A similar set of conclusions is drawn for the undoped systems' behaviour in a finite magnetic field. We also comment on the exisitence in these systems of the $π$-resonance, a hallmark of Zhang's SO(5) theory of high $T_c$ superconductivity.

cond-mat.str-el↗
arXiv1998

The 2-Channel Kondo Model I: Review of Experimental Evidence for its Realization in Metal Nanoconstrictions

Certain zero-bias anomalies (ZBAs) in the voltage, temperature and magnetic field dependence of the conductance $G(V,T,H)$ of quenched Cu point contacts have previously been interpreted to be due to non-magnetic 2-channel Kondo (2CK) scattering from near-degenerate atomic two-level tunneling systems (Ralph and Buhrman, 1992; Ralph et al. 1994), and hence to represent an experimental realization of the non-Fermi-liquid physics of the T=0 fixed point of the 2-channel Kondo model. In this, the first in a series of three papers (I,II,III) devoted to 2-channel Kondo physics, we present a comprehensive review of the quenched Cu ZBA experiments and their 2CK interpretation, including new results on ZBAs in constrictions made from Ti or from metallic glasses. We first review the evidence that the ZBAs are due to electron scattering from stuctural defects that are not static, but possess internal dynamics. In order to distinguish between several mechanisms proposed to explain the experiments, we then analyze the scaling properties of the conductance at low temperature and voltage and extract from the data a universal scaling function $Γ(v)$. The theoretical calculation of the corresponding scaling function within the 2CK model is the subject of papers II and III. The main conclusion of our work is that the properties of the ZBAs, and most notably their scaling behavior, are in good agreement with the 2CK model and clearly different from several other proposed mechanisms.

cond-mat↗
arXiv1997

Resonant Multi-Lead Point-Contact Tunneling

We analyze a model of resonant point-contact tunneling between multiple Luttinger liquid leads. The model is a variant of the multi-channel Kondo model and can be related to the quantum Brownian motion of a particle on lattices with $π$-flux through each plaquette (in the 3-lead case, it is a honeycomb lattice with $π$-flux). By comparing the perturbative and instanton gas expansions, we find a duality property of the model. At the boundary, this duality exchanges Neumann and Dirichlet boundary conditions on the Tomonaga-Luttinger bosons which describe the leads; in the bulk, it exchanges the `momentum' and `winding' modes of these bosons. Over a certain range of Luttinger liquid parameter, $g$, a novel non-trivial intermediate coupling fixed point controls the low-energy physics. The finite conductance at this fixed point can be exactly computed for two special values of $g$. For larger values of $g$, there is a stable fixed point at strong coupling which has enhanced conductance resulting from an analogue of Andreev reflection at the point contact.

cond-mat.mes-hall↗
arXiv1997

Reply to Skorik's Comment

In a series of papers we combined the Bethe ansatz with a kinetic (Boltzmann) equation in order to compute exactly transport properties in a Luttinger liquid with an impurity. In a recent comment [cond-mat/9708163], Skorik claimed that while our results were correct in linear response, there was a ``serious flaw'' in our calculation at finite voltage. We explain here why Skorik's arguments are inappropriate, and that there is no flaw in our work [cond-mat/9408068; cond-mat/9505031; cond-mat/9503172].

cond-mat.mes-hall↗
arXiv1997

Minimal Models with Integrable Local Defects

We describe a general way of constructing integrable defect theories as perturbations of conformal field theory by local defect operators. The method relies on folding the system onto a boundary field theory of twice the central charge. The classification of integrable defect theories obtained in this way parallels that of integrable bulk theories which are a perturbation of the tensor product of two conformal field theories. These include local defect perturbations of all $c<1$ minimal models, as well as of the coset theories based on SO(2n), obtained in this way. We discuss in detail the former case of all the Virasoro minimal models. In the Ising case our construction corresponds to having a spin field as a defect operator; in the folded formulation this is mapped onto an orbifolding of the boundary sine-Gordon theory at $β^2/8π= 1/8$, or a version of the anisotropic Kondo model.

hep-th↗
arXiv1995

Exact non-equilibrium transport through point contacts in quantum wires and fractional quantum Hall devices

We have recently calculated exact non-equilibrium quantum transport properties through a point contact in a Luttinger liquid. Using a particular quasiparticle basis of the Hilbert space dictated by integrability, we here compute explicitly the exact $I(V)$ characteristic and conductance out of equilibrium as a function of driving voltage $V$ and temperature $T$. These are described by universal scaling functions of two variables, the scaled point-contact interaction strength, and $V/T$. The differential-conductance curve as a function of the interaction strength broadens significantly as $V/T$ is increased, and develops a pronounced maximum at a (universal) critical value $(e V/k_B T)=7.18868...$. In addition, we derive an exact duality between strong and weak backscattering. The theory presented here has recently been realized experimentally in resonant tunneling-transport experiments between edge states in fractional quantum Hall effect devices. In this context the exact duality is between electron tunneling and Laughlin-quasiparticle tunneling.

cond-mat↗
arXiv1995

Exact non-equilibrium DC shot noise in Luttinger liquids and fractional quantum Hall devices

A point contact in a Luttinger liquid couples the left- and right-moving channels, producing shot noise. We calculate exactly the DC shot noise at zero temperature in the out-of-equilibrium steady state where current is flowing. Integrability of the interaction ensures the existence of a quasiparticle basis where quasiparticles scatter ``one by one'' off the point contact. This enables us to apply a direct generalization of the Landauer approach to shot noise to this interacting model. We find a simple relation of the noise to the current and the differential conductance. Our results should be experimentally-testable in a fractional quantum Hall effect device, providing a clear signal of the fractional charge of the Laughlin quasiparticles.

cond-mat↗
arXiv1994

Spinon Bases, Yangian Symmetry and Fermionic Representations of Virasoro Characters in Conformal Field Theory

We study the description of the $SU(2)$, level $k=1$, Wess-Zumino-Witten conformal field theory in terms of the modes of the spin-1/2 affine primary field $ϕ^α$. These are shown to satisfy generalized `canonical commutation relations', which we use to construct a basis of Hilbert space in terms of representations of the Yangian $Y(sl_2)$. Using this description, we explicitly derive so-called `fermionic representations' of the Virasoro characters, which were first conjectured by Kedem et al.~\cite{kedem}. We point out that similar results are expected for a wide class of rational conformal field theories.

hep-th↗