Spin-roton excitations in the cuprate superconductors

We identify a new kind of elementary excitations, spin-rotons, in the doped Mott insulator. They play a central role in deciding the superconducting transition temperature Tc, resulting in a simple Tc formula,Tc=Eg/6, with Eg as the characteristic energy scale of the spin rotons. We show that the degenerate S=1 and S=0 rotons can be probed by neutron scattering and Raman scattering measurements, respectively, in good agreement with the magnetic resonancelike mode and the Raman A1g mode observed in the high-Tc cuprates.Comment: 10 pages, 9 figure

Mott physics, sign structure, ground state wavefunction, and high-Tc superconductivity

In this article I give a pedagogical illustration of why the essential problem of high-Tc superconductivity in the cuprates is about how an antiferromagnetically ordered state can be turned into a short-range state by doping. I will start with half-filling where the antiferromagnetic ground state is accurately described by the Liang-Doucot-Anderson (LDA) wavefunction. Here the effect of the Fermi statistics becomes completely irrelevant due to the no double occupancy constraint. Upon doping, the statistical signs reemerge, albeit much reduced as compared to the original Fermi statistical signs. By precisely incorporating this altered statistical sign structure at finite doping, the LDA ground state can be recast into a short-range antiferromagnetic state. Superconducting phase coherence arises after the spin correlations become short-ranged, and the superconducting phase transition is controlled by spin excitations. I will stress that the pseudogap phenomenon naturally emerges as a crossover between the antiferromagnetic and superconducting phases. As a characteristic of non Fermi liquid, the mutual statistical interaction between the spin and charge degrees of freedom will reach a maximum in a high-temperature "strange metal phase" of the doped Mott insulator.Comment: 12 pages, 12 figure

Mean-Field Description of Phase String Effect in the t−Jt-J Model

A mean-field treatment of the phase string effect in the $t-J$ model is presented. Such a theory is able to unite the antiferromagnetic (AF) phase at half-filling and metallic phase at finite doping within a single theoretical framework. We find that the low-temperature occurrence of the AF long range ordering (AFLRO) at half-filling and superconducting condensation in metallic phase are all due to Bose condensations of spinons and holons, respectively, on the top of a spin background described by bosonic resonating-valence-bond (RVB) pairing. The fact that both spinon and holon here are bosonic objects, as the result of the phase string effect, represents a crucial difference from the conventional slave-boson and slave-fermion approaches. This theory also allows an underdoped metallic regime where the Bose condensation of spinons can still exist. Even though the AFLRO is gone here, such a regime corresponds to a microscopic charge inhomogeneity with short-ranged spin ordering. We discuss some characteristic experimental consequences for those different metallic regimes. A perspective on broader issues based on the phase string theory is also discussed.Comment: 18 pages, five figure

Magnetic Incommensurability in Doped Mott Insulator

In this paper we explore the incommensurate spatial modulation of spin-spin correlations as the intrinsic property of the doped Mott insulator, described by the $t-J$ model. We show that such an incommensurability is a direct manifestation of the phase string effect introduced by doped holes in both one- and two-dimensional cases. The magnetic incommensurate peaks of dynamic spin susceptibility in momentum space are in agreement with the neutron-scattering measurement of cuprate superconductors in both position and doping dependence. In particular, this incommensurate structure can naturally reconcile the neutron-scattering and NMR experiments of cuprates.Comment: 12 pages (RevTex), five postscript figure

Observation of momentum-confined in-gap impurity state in Ba0.6_{0.6}K0.4_{0.4}Fe2_2As2_2: evidence for anti-phase s±s_{\pm} pairing

We report the observation by angle-resolved photoemission spectroscopy of an impurity state located inside the superconducting gap of Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ and vanishing above the superconducting critical temperature, for which the spectral weight is confined in momentum space near the Fermi wave vector positions. We demonstrate, supported by theoretical simulations, that this in-gap state originates from weak non-magnetic scattering between bands with opposite sign of the superconducting gap phase. This weak scattering, likely due to off-plane Ba/K disorders, occurs mostly among neighboring Fermi surfaces, suggesting that the superconducting gap phase changes sign within holelike (and electronlike) bands. Our results impose severe restrictions on the models promoted to explain high-temperature superconductivity in these materials.Comment: 8 pages, 5 figures. Accepted for publication in Physical Review

Stability of antiphase line defects in nanometer-sized boron-nitride cones

We investigate the stability of boron nitride conical sheets of nanometer size, using first-principles calculations. Our results indicate that cones with an antiphase boundary (a line defect that contains either B-B or N-N bonds) can be more stable than those without one. We also find that doping the antiphase boundaries with carbon can enhance their stability, leading also to the appearance of localized states in the bandgap. Among the structures we considered, the one with the smallest formation energy is a cone with a carbon-modified antiphase boundary that presents a spin splitting of about 0.5 eV at the Fermi level.Comment: 5 two-column pages with 2 figures Accepted for publication in Physical Review B (vol 70, 15 Nov.

Cascaded acceleration of proton beams in ultrashort laser-irradiated microtubes

A cascaded ion acceleration scheme is proposed by use of ultrashort laser-irradiated microtubes. When the electrons of a microtube are blown away by intense laser pulses, strong charge-separation electric fields are formed in the microtube both along the axial and along the radial directions. By controlling the time delay between the laser pulses and a pre-accelerated proton beam injected along the microtube axis, we demonstrate that this proton beam can be further accelerated by the transient axial electric field in the laser-irradiated microtube. Moreover, the collimation of the injected proton beam can be enhanced by the inward radial electric field. Numerical simulations show that this cascaded ion acceleration scheme works efficiently even at non-relativistic laser intensities, and it can be applied to injected proton beams in the energy range from 1 to 100 MeV. Therefore, it is particularly suitable for cascading acceleration of protons to higher energy.Comment: 13 pages, 4 figure

Coexistence of Itinerant Electrons and Local Moments in Iron-Based Superconductors

In view of the recent experimental facts in the iron-pnictides, we make a proposal that the itinerant electrons and local moments are simultaneously present in such multiband materials. We study a minimal model composed of coupled itinerant electrons and local moments to illustrate how a consistent explanation of the experimental measurements can be obtained in the leading order approximation. In this mean-field approach, the spin-density-wave (SDW) order and superconducting pairing of the itinerant electrons are not directly driven by the Fermi surface nesting, but are mainly induced by their coupling to the local moments. The presence of the local moments as independent degrees of freedom naturally provides strong pairing strength for superconductivity and also explains the normal-state linear-temperature magnetic susceptibility above the SDW transition temperature. We show that this simple model is supported by various anomalous magnetic properties and isotope effect which are in quantitative agreement with experiments.Comment: 7 pages, 4 figures; an expanded versio

Geometry and the Hidden Order of Luttinger Liquids: the Universality of Squeezed Space

We present the case that Luttinger liquids are characterized by a form of hidden order which is similar, but distinct in some crucial regards, to the hidden order characterizing spin-1 Heisenberg chains. We construct a string correlator for the Luttinger liquid which is similar to the string correlator constructed by den Nijs and Rommelse for the spin chain. From a geometric prespective on the so-called `squeezed space' construction, we demonstrate that the physics at long wavelength can be reformulated in terms of a $Z_2$ gauge theory. Peculiarly, the normal spin chain lives at infinite gauge coupling where it is characterized by deconfinement. We identify the microscopic conditions required for confinement thereby identifying a novel phase of the spin-chain. We demonstrate that the Luttinger liquid can be approached in the same general framework. The difference from the spin chain is that the gauge sector is critical in the sense that the Luttinger liquid is at the phase boundary where the $Z_2$ local symmetry emerges. We evaluate the string correlator analytically and show that the squeezed space structure is present both for the strongly coupled Hubbard model and the non-interacting fermion gas. These structures are hard-wired in the mathematical structure of bosonization and this becomes obvious by considering string correlators. Numerical results are presented for the string correlator using a non-abelian version of the density matrix renormalization group algorithm, confirming in detail the expectations following from the theory. We conclude with some observations regarding the generalization of bosonization to higher dimensions.Comment: 24 pages, 14 eps figures, Revtex