arXiv subjects

S. Doniach

Publications and source records attributed to S. Doniach.

13 recordsLinked to original sources

arXiv2002

A dynamical mean-field theory approach to superconductivity and antiferromagnetism in a strongly correlated electron system

We present the results of numerical studies of superconductivity and antiferromagnetism in a strongly correlated electron system. To do this we construct a Hubbard model on a lattice of self-consistently embedded multi-site clusters by means of a dynamical mean-field theory in which intra-cluster dynamics is treated essentially exactly. We show that a class of characteristic features which have been seen in the excitation spectra of high-$T_{c}$ cuprates (e.g., pseudogap and the spin-flip resonance), as well as their interplay with the onset of a pairing correlations, can be captured within a dynamical mean-field theory in which short-wavelength dynamics are rigorously treated. Thus we infer that the observation of the neutron scattering resonance in the superconducting state of the cuprate superconductors does not appear to be directly tied to their quasi-2D character. Although our approach is defined strictly in terms of fermion degrees of freedom, we show that we can readily identify the emergence of effective low energy bosonic degrees of freedom in the presence of a well-defined broken symmetry phase as long as their dynamics are dominated by short-range, short-wavelength fluctuations. Our results reveal that the dynamics of staggered spin degrees of freedom builds up coherence and a resonance-like sharp feature emerges as pairing correlations set in. Under conditions of superconducting broken symmetry our approach thus extends static BCS mean field theory to provide an exact treatment of quantum fluctuations of the BCS order parameter.

cond-mat.supr-con↗

arXiv2002

Three-dimensional flux states as a model for the pseudogap phase of transition metal oxides

We propose that the pseudogap state observed in the transition metal oxides can be explained by a three-dimensional flux state, which exhibits spontaneously generated currents in its ground state due to electron-electron correlations. We compare the energy of the flux state to other classes of mean field states, and find that it is stabilized over a wide range of $t$ and $δ$. The signature of the state will be peaks in the neutron diffraction spectra, the location and intensity of which are presented. The dependence of the pseudogap in the optical conductivity is calculated based on the parameters in the model.

cond-mat.str-el↗

arXiv2001

The Bose Metal: gauge field fluctuations and scaling for field tuned quantum phase transitions

In this paper, we extend our previous discussion of the Bose metal to the field tuned case. We point out that the recent observation of the metallic state as an intermediate phase between the superconductor and the insulator in the field tuned experiments on MoGe films is in perfect consistency with the Bose metal scenario. We establish a connection between general dissipation models and gauge field fluctuations and apply this to a discussion of scaling across the quantum phase boundaries of the Bose metallic state. Interestingly, we find that the Bose metal scenario implies a possible {\em two} parameter scaling for resistivity across the Bose metal-insulator transition, which is remarkably consistent with the MoGe data. Scaling at the superconductor-metal transition is also proposed, and a phenomenolgical model for the metallic state is discussed. The effective action of the Bose metal state is described and its low energy excitation spectrum is found to be $ω\propto k^{3}$.

cond-mat↗

arXiv2000

Nonlinear I-V, High Flux-Flow Velocity Instability and Evidence for Quantum Fluctuations in BSCCO Superconducting Films

The large flux-flow velocity behavior of the high temperature superconductor BSCCO in the mixed state is studied by measurement of the current-voltage characteristic as a function of temperature and magnetic field direction. We find strong evidence for vortex-antivortex pairs that are excited by quantum fluctuations and contribute to the dissipation when dissociated by a large current.

cond-mat.supr-con↗

arXiv1999

On the existence of a Bose Metal at T=0

This paper aims to justify, at a microscopic level, the existence of a two-dimensional Bose metal, i.e. a metallic phase made out of Cooper pairs at T=0. To this end, we consider the physics of quantum phase fluctuations in (granular) superconductors in the absence of disorder and emphasise the role of two order parameters in the problem, viz. phase order and charge order. We focus on the 2-d Bose Hubbard model in the limit of very large fillings, i.e. a 2-d array of Josephson junctions. We find that the algebra of phase fluctuations is that of the Euclidean group $E_{2}$ in this limit, and show that the model is equivalent to two coupled XY models in (2+1)-d, one corresponding to the phase degrees of freedom, and the other the charge degrees of freedom. The Bose metal, then, is the phase in which both these degrees of freedom are disordered(as a result of quantum frustration). We analyse the model in terms of its topological excitations and suggest that there is a strong indication that this state represents a surface of critical points, akin to the gapless spin liquid states. We find a remarkable consistency of this scenario with certain low-T_c thin film experiments.

cond-mat.supr-con↗

arXiv1998

Superinsulator Phase of Two-Dimensional Superconductors

Using path-integral Quantum Monte Carlo we study the low-temperature phase diagram of a two-dimensional superconductor within a phenomenological model, where vortices have a finite mass and move in a dissipative environment modeled by a Caldeira-Leggett term. The quantum vortex liquid at high magnetic fields exhibits superfluidity and thus corresponds to a {\em superinsulating} phase which is characterized by a nonlinear voltage-current law for an infinite system in the absence of pinning. This superinsulating phase is shifted to higher magnetic fields in the presence of dissipation.

cond-mat.supr-con↗

arXiv1998

Weakly localised bosons

In this paper, we argue that, in 2-d, the weak localisation of bosons, which occurs on the insulating side of the superconductor-insulator transition, is characterised by $ρ\sim ln(1/T)$, as compared to $σ\sim lnT$ for fermions. Such an unconventional behaviour is tied to the diffusion pole of the delocalised (uncondensed) vortices.

cond-mat.supr-con↗

arXiv1996

Dynamical Phase Transition in a Driven Disordered Vortex Lattice

Using Langevin dynamics, we have investigated the dynamics of vortices in a disordered two dimensional superconductor subjected to a uniform driving current. The results provide direct numerical evidence for a dynamical phase transition between a plastic flow regime and a moving ``hexatic glass." The simulated current-voltage characteristics are in excellent agreement with recent transport measurements on amorphous ${\rm Mo_{77}Ge_{23}}$ thin film superconductors.

cond-mat↗

arXiv1996

Field-driven topological glass transition in a model flux line lattice

We show that the flux line lattice in a model layered HTSC becomes unstable above a critical magnetic field with respect to a plastic deformation via penetration of pairs of point-like disclination defects. The instability is characterized by the competition between the elastic and the pinning energies and is essentially assisted by softening of the lattice induced by a dimensional crossover of the fluctuations as field increases. We confirm through a computer simulation that this indeed may lead to a phase transition from crystalline order at low fields to a topologically disordered phase at higher fields. We propose that this mechanism provides a model of the low temperature field--driven disordering transition observed in neutron diffraction experiments on ${\rm Bi_2Sr_2CaCu_2O_8\, }$ single crystals.

cond-mat↗

arXiv1995

Phase diagram of a semiflexible polymer chain in a $θ$ solvent: application to protein folding

We consider a lattice model of a semiflexible homopolymer chain in a bad solvent. Beside the temperature $T$, this model is described by (i) a curvature energy $\varepsilon_h$, representing the stiffness of the chain (ii) a nearest-neighbour attractive energy $\varepsilon_v$, representing the solvent (iii) the monomer density $ρ={N \over Ω}$, where $N$ and $Ω$ denote respectively the number of monomers and the number of lattice sites. This model is a simplified view of the protein folding problem, which encompasses the geometrical competition between secondary structures (the curvature term modelling helix formation) and the global compactness (modeled here by the attractive energy), but contains no side chain information...

cond-mat↗

arXiv1994

Model for FeSi, a strongly correlated insulator

Revised to reflect the differences between the second order perturbation theory method developed by Scheitzer and Czycholl, and followed by us, and the iterated perturbation theory (IPT)of Georges and Kotliar, et al. Because of our domain of interest, this does not materially change our conclusions, but it is important to realize that the method used here fails in the limit of large $U$, where IPT has had good results.

cond-mat↗

arXiv1994

Enhanced Electron Pairing in a Lattice of Berry Phase Molecules

We show that electron hopping in a lattice of molecules possessing a Berry phase naturally leads to pairing. Our building block is a simple molecular site model inspired by C$_{60}$, but realized in closer similarity with Na$_3$. In the resulting model electron hopping must be accompanied by orbital operators, whose function is to switch on and off the Berry phase as the electron number changes. The effective hamiltonians (electron-rotor and electron-pseudospin) obtained in this way are then shown to exhibit a strong pairing phenomenon, by means of 1D linear chain case studies. This emerges naturally from numerical studies of small $N$-site rings, as well as from a BCS-like mean-field theory formulation. The pairing may be explained as resulting from the exchange of singlet pairs of orbital excitations, and is intimately connected with the extra degeneracy implied by the Berry phase when the electron number is odd. The relevance of this model to fullerides, to other molecular superconductors, as well as to present and future experiments, is discussed.

cond-mat↗

arXiv1993

Effects of Columnar Pins on Flux Line Dynamics

The effects of columnar pins on the flux lines in a model High-Tc superconductor in an applied field are studied through Monte Carlo simulations. An analytic solution for a single line case is obtained and compared with the simulation results. By introducing a tilted potential, we study simulated ``IV" characteristics and the results indicate a distinct scaling behavior above and below the depinning temperature. We introduce a diverging length scale measured in terms of a ``retrapping length'' to analyze self-similar behavior across the depinning transition.

cond-mat↗