SU(2) gauge theory of the Hubbard model: Emergence of an anomalous metallic phase near the Mott critical point

We propose one possible mechanism for an anomalous metallic phase appearing frequently in two spatial dimensions, that is, local pairing fluctuations. Introducing a pair-rotor representation to decompose bare electrons into collective pairing excitations and renormalized electrons, we derive an SU(2) gauge theory of the Hubbard model as an extended version of its U(1) gauge theory\cite{Florens,LeeLee} to allow only local density fluctuations. Since our effective SU(2) gauge theory admits two kinds of collective bosons corresponding to pair excitations and density fluctuations respectively, an intermediate phase appears naturally between the spin liquid Mott insulator and Fermi liquid metal of the U(1) gauge theory,\cite{Florens,LeeLee} characterized by softening of density-fluctuation modes as the Fermi liquid, but gapping of pair-excitation modes. We show that this intermediate phase is identified with an anomalous metallic phase because there are no electron-like quasiparticles although it is metallic

NuTeV Structure Function Measurement

The NuTeV experiment obtained high statistics samples of neutrino and anti-neutrino charged current events during the 1996-1997 Fermilab fixed target run. The experiment combines sign-selected neutrino and anti-neutrino beams and the upgraded CCFR iron-scintillator neutrino detector. A precision continuous calibration beam was used to determine the muon and hadron energy scales to a precision of 0.7% and 0.43% respectively. The structure functions F_2(x,Q^2) and xF_3(x,Q^2) obtained by fitting the y-dependence of the sum and the difference of the neutrino and anti-neutrino differential cross sections are presented.Comment: Proceedings of the XIII international workshop on Deep Inelastic Scattering DIS 2005, 4 pages, 4 figure

Role of disorder in the Mott-Hubbard transition

We investigate the role of disorder in the Mott-Hubbard transition based on the slave-rotor representation of the Hubbard model, where an electron is decomposed into a fermionic spinon for a spin degree of freedom and a bosonic rotor (chargon) for a charge degree of freedom. In the absence of disorder the Mott-Hubbard insulator is assumed to be the spin liquid Mott insulator in terms of gapless spinons near the Fermi surface and gapped chargons interacting via U(1) gauge fields. We found that the Mott-Hubbard critical point becomes unstable as soon as disorder is turned on. As a result, a disorder critical point appears to be identified with the spin liquid glass insulator to the Fermi liquid metal transition, where the spin liquid glass consists of the U(1) spin liquid and the chargon glass. We expect that glassy behaviors of charge fluctuations can be measured by the optical spectra in the insulating phase of an organic material $\kappa-(BEDT-TTF)_{2}Cu_{2}(CN)_{3}$. Furthermore, since the Mott-Anderson critical point depends on the spinon conductivity, universality in the critical exponents may not be found

Effect of nonmagnetic disorder on criticality in the "dirty" U(1) spin liquid

We investigate the effect of nonmagnetic disorder on the stability of the algebraic spin liquid ($ASL$) by deriving an effective field theory, nonlinear $\sigma$ model ($NL{\sigma}M$). We find that the anomalous critical exponent characterizing the criticality of the $ASL$ causes an anomalous gradient in the $NL{\sigma}M$. We show that the sign of the anomalous gradient exponent or the critical exponent of the $ASL$ determines the stability of the "dirty" $ASL$. A positive exponent results in an unstable fixed point separating delocalized and localized phases, which is consistent with our previous study [Phys. Rev. B {\bf 70}, 140405 (2004)]. We find power law suppression for the density of spinon states in contrast to the logarithmic correction in the free Dirac theory. On the other hand, a negative exponent destabilizes the $ASL$, causing the Anderson localization. We discuss the implication of our study in the pseudogap phase of high $T_c$ cuprates

Bandwidth-control vs. doping-control Mott transition in the Hubbard model

We reinvestigate the bandwidth-control and doping-control Mott transitions (BCMT and DCMT) from a spin liquid Mott insulator to a Fermi liquid metal based on the slave-rotor representation of the Hubbard model,\cite{Florens} where the Mott transitions are described by softening of bosonic collective excitations. We find that the nature of the insulating phase away from half filling is different from that of half filling in the respect that a charge density wave coexists with a topological order (spin liquid) away from half filling because the condensation of vortices generically breaks translational symmetry in the presence of "dual magnetic fields" resulting from hole doping while the topological order remains stable owing to gapless excitations near the Fermi surface. Performing a renormalization group analysis, we discuss the role of dissipative gauge fluctuations due to the Fermi surface in both the BCMT and the DCMT

Critical field theory of the Kondo lattice model in two dimensions

In the context of the U(1) slave boson theory we derive a critical field theory near the quantum critical point of the Kondo lattice model in two spatial dimensions. First we argue that strong gauge fluctuations in the U(1) slave boson theory give rise to confinement between spinons and holons, thus causing "neutralized" spinons in association with the slave boson U(1) gauge field. Second we show that critical fluctuations of Kondo singlets near the quantum critical point result in a new U(1) gauge field. This emergent gauge field has nothing to do with the slave boson U(1) gauge field. Third we find that the slave boson U(1) gauge field can be exactly integrated out in the low energy limit. As a result we find a critical field theory in terms of renormalized conduction electrons and neutralized spinons interacting via the new emergent U(1) gauge field. Based on this critical field theory we obtain the temperature dependence of specific heat and the imaginary part of the self-energy of the renormalized electrons. These quantities display non-Fermi liquid behavior near the quantum critical point

Competition between superconductivity and charge density waves

We derive an effective field theory for the competition between superconductivity (SC) and charge density waves (CDWs) by employing the SO(3) pseudospin representation of the SC and CDW order parameters. One important feature in the effective nonlinear $\sigma$ model is the emergence of Berry phase even at half filling, originating from the competition between SC and CDWs, i.e., the pseudospin symmetry. A well known conflict between the previous studies of Oshikawa\cite{Oshikawa} and D. H. Lee et al.\cite{DHLee} is resolved by the appearance of Berry phase. The Berry phase contribution allows a deconfined quantum critical point of fractionalized charge excitations with $e$ instead of $2e$ in the SC-CDW quantum transition at half filling. Furthermore, we investigate the stability of the deconfined quantum criticality against quenched randomness by performing a renormalization group analysis of an effective vortex action. We argue that although randomness results in a weak disorder fixed point differing from the original deconfined quantum critical point, deconfinement of the fractionalized charge excitations still survives at the disorder fixed point owing to a nonzero fixed point value of a vortex charge.Comment: adding a renormalization group analysis with a random fugacity term as an effect of randomness on a deconfined quantum critical poin