Nematic order of spin structures of clusters in the hubbard model

The possibility of interpreting the normal pseudogap state of cuprates as a result of the formation of spin and charge structures is investigated for solutions of the Hubbard model of a finite 2D cluster based on the mean field method. The iterative self-consistency procedure reduces the initial uncorrelated spin distributions to stable structures. The Fourier components of the charge and spin distributions in such structures have peaks for characteristic incommensurate quasi-momenta depending on the doping. It is shown that for any doping, the density of states of the system has a sharp minimum (pseudogap) at the Fermi level. This emergence of the gap just at the Fermi level is a property typical of not only the superconducting state, but also the normal state of spin glasses. The characteristics of the Fermi surface averaged over the implemented structures and the properties of quasiparticles in the nodal and antinodal regions of the quasi-momentum are considered.     

Separation of orbital contributions to the optical conductivity of BaVS(3)

The correlation-driven metal-insulator transition (MIT) of BaVS(3) was studied by polarized infrared spectroscopy. In the metallic state two types of electrons coexist at the Fermi energy: the quasi-1D metallic transport of A(1g) electrons is superimposed on the isotropic hopping conduction of localized E(g) electrons. The "bad-metal" character and the weak anisotropy are the consequences of the large effective mass m(eff) approximately 7 m(e) and scattering rate Gamma > or = 160 meV of the quasiparticles in the A(1g) band. There is a pseudogap above T(MI) = 69 K, and in the insulating phase the gap follows the BCS-like temperature dependence of the structural order parameter with Delta(ch) approximately 42 meV in the ground state. The MIT is described in terms of a weakly coupled two-band model.     

Effective interactions in dilute mixtures of3He in4He

Nonlocal pseudopotentials which describe the effective interaction between³He quasiparticles, and between these quasiparticles and the background⁴He liquid, are obtained as a function of concentration and pressure by generalizing the Aldrich-Pines pseudopotentials for pure³He and⁴He to dilute mixtures. The hierarchy of physical effects which determine these pseudopotentials is established. Interaction-induced short-range correlations are the dominant physical feature; next in order of importance is the greater zero point motion associated with the replacement of a⁴He atom by a³He atom, while spin-induced Pauli principle correlations play a significantly smaller, albeit still important role. We find a consistent trend in the change of the effective direct quasiparticle interactions with increasing concentration, and show how the Aldrich-Pines pseudopotentials for pure³He quasiparticles represent a natural extension of our results for dilute mixtures. Our calculated nonlocal pseudopotential for³He quasiparticles is qualitatively similar to that proposed by Bardeen, Baym, and Pines; it changes sign at somewhat lower momentum transfers than the BBP result, varies little with concentration, and provides a physical basis for understanding the BBP result. The effective interaction between quasiparticles of parallel spin, here determined for the first time, is essentially repulsive in the very dilute limit; as the concentration increases, it becomes increasingly attractive at low momentum transfers, and resembles closely that between antiparallel spin quasiparticles at 5% concentration. The concentration-dependent transport properties calculated from these pseudopotentials (which involve only one phenomenological parameter) are in good agreement with experiment at saturated vapor pressure (SVP), 10 atm, and 20 atm. Maxima in the thermal conductivity and spin diffusion are predicted to occur at concetrations somewhat less than 4%. Because the effective quasiparticle interactions are somewhat more repulsive than those previously proposed, we find the transition of the³He quasiparticles to the superfluid state takes place at significantly lower temperatures than many previous estimates; our predicted maximum superfluid transition temperature is 2×10^–8 K (for a 0.6% mixture at 20 atm).     

Evidence for a metallic–like state in the T=0 K phase diagram of a high temperature superconductor

We examine the effects of a phenomenological pseudogap on the T=0 K phase diagram of a high temperature superconductor within a self-consistent model which exhibits a d-wave pairing symmetry. At the mean-field level the presence of a pseudogap in the normal phase of the high temperature superconductor is proved to be essential for the existence of a metallic–like state in the density versus interaction phase diagram. In the small density limit, at high attractive interaction, bosonic–like degrees of freedom are likely to emerge. Our result should be relevant for underdoped high temperature superconductors, where there is a strong evidence for the presence of a pseudogap in the excitation spectrum of the normal state quasiparticles.     

Origin of the enhanced copper spin echo decay rate in the pseudogap regime of the multilayer high-T(c) cuprates

We report measurements of the anisotropy of the spin echo decay for the inner layer Cu site of the triple layer cuprate Hg(0.8)Re(0.2)Ba(2)Ca(2)Cu(3)O(8) (T(c)=126 K). The angular dependence of the second moment (T(-2)(2M) identical with ) deduced from the decay curves indicates that T(-2)(2M) for H0 parallel c is enhanced in the pseudogap regime below T(pg) approximately 170 K, as seen in bilayer systems. Comparison of T(-2)(2M) between H0 parallel c and H0 perpendicular c indicates that this enhancement is caused by electron spin correlations between the inner and the outer CuO2 layers. The results provide the answer to the long-standing controversy regarding the opposite T dependences of (T1T)(-1) and T(-2)(2G) (T(2G): Gaussian component) in the pseudogap regime of multilayer systems.     

Commensurate spin dynamics in the superconducting state of an electron-doped cuprate superconductor

We report neutron scattering studies on two single crystal samples of the electron-doped (n-type) superconducting (SC) cuprate Nd2-xCexCuO4 (x=0.15) with T(c)=18 and 25 K. Unlike the hole-doped (p-type) SC cuprates, where incommensurate magnetic fluctuations commonly exist, the n-type cuprate shows commensurate magnetic fluctuations at the tetragonal (1/2 1/2 0) reciprocal points both in the SC and in the normal state. A spin gap opens up when the n-type cuprate becomes SC, as in the optimally doped p-type La2-xSrxCuO4. The gap energy, however, increases gradually up to about 4 meV as T decreases from T(c) to 2 K, which contrasts with the spin pseudogap behavior with a T-independent gap energy in the SC state of p-type cuprates.     

Non-commutative standard model: model building

A non-commutative version of the usual electro-weak theory is constructed. We discuss how to overcome the two major problems: (1) although we can have non-commutative U(n) (which we denote by _U* (n)) gauge theory we cannot have non-commutative SU(n) and (2) the charges in non-commutative QED are quantized to just . We show how the latter problem with charge quantization, as well as with the gauge group, can be resolved by taking the gauge group and reducing the extra U(1) factors in an appropriate way. Then we proceed with building the non-commutative version of the standard model by specifying the proper representations for the entire particle content of the theory, the gauge bosons, the fermions and Higgs. We also present the full action for the non-commutative standard model (NCSM). In addition, among several peculiar features of our model, we address the inherent CP violation and new neutrino interactions. Received: 23 January 2003, Published online: 18 June 2003     

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. 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, 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 electronlike quasiparticles although it is metallic.     

Unique nilpotent symmetry transformations for matter fields in QED: augmented superfield formalism

We derive the off-shell nilpotent (anti-)BRST symmetry transformations for the interacting U(1) gauge theory of quantum electrodynamics (QED) in the framework of the augmented superfield approach to the BRST formalism. In addition to the horizontality condition, we invoke another gauge invariant condition on the six (4,2)-dimensional supermanifold to obtain the exact and unique nilpotent symmetry transformations for all the basic fields present in the (anti-)BRST invariant Lagrangian density of the physical four (3+1)-dimensional QED. The above supermanifold is parametrized by four even space–time variables (with μ=0,1,2,3) and two odd variables (θ and ) of the Grassmann algebra. The new gauge invariant condition on the supermanifold owes its origin to the (super) covariant derivatives and leads to the derivation of unique nilpotent symmetry transformations for the matter fields. The geometrical interpretations for all the above off-shell nilpotent (anti-)BRST transformations are also discussed. PACS 11.15.-q, 12.20.-m, 03.70.+k     

Fractionalization via Z(2) gauge fields at a cold-atom quantum Hall transition

We study a single species of fermionic atoms in an "effective" magnetic field at total filling factor ν(f)=1, interacting through a p-wave Feshbach resonance, and show that the system undergoes a quantum phase transition from a ν(f)=1 fermionic integer quantum Hall state to ν(b)=1/4 bosonic fractional quantum Hall state as a function of detuning. The transition is in the (2+1)D Ising universality class. We formulate a dual theory in terms of quasiparticles interacting with a Z(2) gauge field and show that charge fractionalization follows from this topological quantum phase transition. Experimental consequences and possible tests of our theoretical predictions are discussed.     

Antiferromagnetic vortex core in Tl(2)Ba(2)CuO(6+delta) studied by nuclear magnetic resonance

Spatially resolved NMR is used to probe the magnetism in and around vortex cores of nearly optimally doped Tl(2)Ba(2)CuO(6+delta) (T(c)=85 K). The NMR relaxation rate T(-1)1 at the 205Tl site provides direct evidence that the antiferromagnetic (AF) spin correlation is significantly enhanced in the vortex core region. In the core region Cu spins show a local AF ordering with moments parallel to the layers at T(N)=20 K. Above T(N) the core region is in the paramagnetic state which is a reminiscence of the state above the pseudogap temperature (T(*) approximately 120 K), indicating that the pseudogap disappears within cores.     

Nearly antiferromagnetic Fermi liquids: a progress report

I describe recent theoretical and experimental progress in understanding the physical properties of the two dimensional nearly antiferromagnetic Fermi liquids (NAFL's) found in the normal state of the cuprate superconductors. In such NAFL's, the magnetic interaction between planar quasiparticles is strong and peaked at or near the commensurate wave vector, Q ≡ (fy fy). For the optimally doped and underdoped systems, the resulting strong antiferromagnetic correlations produce three distinct magnetic phases in the normal state: mean field above T_cr, pseudoscaling between T_cr and T_*, and pseudogap below T_*. I present arguments which suggest that the physical origin of the pseudogap found in the quasiparticle spectrum below Tcr is the formation of a precursor to a spin-densitywave- state, describe the calculations based on this scenario of the dynamical spin susceptibility, Fermi surface evolution, transport, and Hall effect, and summarize the experimental evidence in its support.     

Evolution of quasiparticle charge in the fractional quantum hall regime

The charge of quasiparticles in a fractional quantum Hall (FQH) liquid, tunneling through a partly reflecting constriction with transmission t, was determined via shot noise measurements. In the nu = 1/3 FQH state, a charge smoothly evolving from e(*) = e/3 for t(1/3) congruent with 1 to e(*) = e for t(1/3)<1 was determined, agreeing with chiral Luttinger liquid theory. In the nu = 2/5 FQH state the quasiparticle charge evolves smoothly from e(*) = e/5 at t(2/5) congruent with 1 to a maximum charge less than e(*) = e/3 at t(2/5)<1. Thus it appears that quasiparticles with an approximate charge e/5 pass a barrier they see as almost opaque.     

Spin liquid phases for spin-1 systems on the triangular lattice

Motivated by recent experiments on material Ba3NiSb2O9, we propose two novel spin liquid phases (A and B) for spin-1 systems on a triangular lattice. At the mean field level, both spin liquid phases have gapless fermionic spinon excitations with quadratic band touching; thus, in both phases the spin susceptibility and γ=C(v)/T saturate to a constant at zero temperature, which are consistent with the experimental results on Ba3NiSb2O9. On the lattice scale, these spin liquid phases have Sp(4)~SO(5) gauge fluctuation, while in the long wavelength limit this Sp(4) gauge symmetry is broken down to U(1)×Z(2) in the type A spin liquid phase, and broken down to Z(4) in the type B phase. We also demonstrate that the A phase is the parent state of the ferroquadrupole state, nematic state, and the noncollinear spin density wave state.     

Irrational charge from topological order

Topological or deconfined phases of matter exhibit emergent gauge fields and quasiparticles that carry a corresponding gauge charge. In systems with an intrinsic conserved U(1) charge, such as all electronic systems where the Coulombic charge plays this role, these quasiparticles are also characterized by their intrinsic charge. We show that one can take advantage of the topological order fairly generally to produce periodic Hamiltonians which endow the quasiparticles with continuously variable, generically irrational, intrinsic charges. Examples include various topologically ordered lattice models, the three-dimensional resonating valence bond liquid on bipartite lattices as well as water and spin ice. By contrast, the gauge charges of the quasiparticles retain their quantized values.     

Pseudogap Fermi-Bose Kondo model

We consider a magnetic impurity coupled to both fermionic quasiparticles with a pseudogap density of states and bosonic spin fluctuations. Using renormalization group and large-N calculations we investigate the phase diagram of the resulting Fermi-Bose Kondo model. We show that the Kondo temperature is strongly reduced by low-energy spin fluctuations, and make connections to experiments in cuprate superconductors. Furthermore, we derive an exact exponent for the critical behavior of the conduction electron T matrix, and propose our findings to be relevant for certain scenarios of local quantum criticality in heavy-fermion metals.     

Effect of the vortices on the nuclear spin relaxation rate in the unconventional pairing states of the organic superconductor (TMTSF)2PF6

This Letter theoretically discusses quasiparticle states and nuclear spin relaxation rates T1-1 in the quasi-one-dimensional superconductor (TMTSF)2PF6 under a magnetic field applied parallel to the conduction chains. We study the effects of Josephson-type vortices on T1(-1) by solving the Bogoliubov-de Gennes equation for p-, d- or f-wave pairing interactions. In the presence of line nodes in pairing functions, T1(-1) is proportional to T in sufficiently low temperatures because quasiparticles induced by vortices at the Fermi energy relax spins. We also try to identify the pairing symmetry of (TMTSF)2PF6.