We describe electrical transport in ideal single-layer graphene at zero applied bias. There is a crossover from collisionless transport at frequencies larger than k_B T/hbar (T is the temperature) to collision-dominated transport at lower frequencies. The d.c. conductivity is computed by the solution of a quantum Boltzmann equation. Due to a logarithmic singularity in the collinear scattering amplitude (a consequence of relativistic dispersion in two dimensions) quasi-particles and -holes moving in the same direction tend to an effective equilibrium distribution whose parameters depend on the direction of motion. This property allows us to find the non-equilibrium distribution functions and the quantum critical conductivity exactly to leading order in 1/|ln(alpha)| where alpha is the coupling constant characterizing the Coulomb interactions.Comment: 19 pages, 1 figure; (v2) added comment on hydrodynamic long-time tail

High-resolution x-ray diffraction measurements reveal an unusually strong response of the lattice to superconductivity in Ba(Fe1-xCox)2As2. The orthorhombic distortion of the lattice is suppressed and, for Co doping near x=0.063, the orthorhombic structure evolves smoothly back to a tetragonal structure. We propose that the coupling between orthorhombicity and superconductivity is indirect and arises due to the magnetoelastic coupling, in the form of emergent nematic order, and the strong competition between magnetism and superconductivity.

We present the full analysis of the normal state of the spinfermion model near the antiferromagnetic instability in two dimensions. This model describes low-energy fermions interacting with their own collective spin fluctuations, which soften at the antiferromagnetic transition. We argue that in 2D, the system has two typical energies -an effective spin-fermion interactionḡ and an energy ω sf below which the system behaves as a Fermi liquid. The ratio of the two determines the dimensionless coupling constant for spin-fermion interaction λ 2 ∝ḡ/ω sf . We show that λ scales with the spin correlation length and diverges at criticality. This divergence implies that the conventional perturbative expansion breaks down. We developed a novel approach to the problem -the expansion in either the inverse number of hot spots in the Brillouin zone, or the inverse number of fermionic flavors -which allowed us to explicitly account for all terms which diverge as powers of λ, and treat the remaining, O(λ) terms in the RG formalism. We applied this technique to study the properties of the spinfermion model in various frequency and temperature regimes. We present the results for the fermionic spectral function, spin susceptibility, optical conductivity and other observables. We compare our results in detail with the normal state data for cuprates, and argue that the spin-fermion model is capable to explain the anomalous normal state properties of high Tc materials. We also discuss the non -applicability of the conventional φ 4 theory of the quantum-critical behavior in 2D.

We demonstrate that the changes in the elastic properties of the FeAs systems, as seen in our resonant ultrasound spectroscopy data, can be naturally understood in terms of fluctuations of emerging nematic degrees of freedom. Both the softening of the lattice in the normal, tetragonal phase as well as its hardening in the superconducting phase are consistently described by our model. Our results confirm the view that structural order is induced by magnetic fluctuations.

We review the main ingredients for an unconventional pairing state in the ferropnictides, with particular emphasis on interband pairing due to magnetic fluctuations. Summarizing the key experimental prerequisites for such pairing, the electronic structure and nature of magnetic excitations, we discuss the properties of the s ± state that emerges as a likely candidate pairing state for these materials and survey experimental evidence in favor of and against this novel state of matter.One fist of iron, the other of steel If the right one don't get you, then the left one will Merle Travis, 16 tons

No abstract

Hydrodynamics and collision dominated transport are crucial to understand the slow dynamics of many correlated quantum liquids. The ratio η/s of the shear viscosity η to the entropy density s is uniquely suited to determine how strongly the excitations in a quantum fluid interact. We determine η/s in clean undoped graphene using a quantum kinetic theory. As a result of the quantum criticality of this system the ratio is smaller than in many other correlated quantum liquids and, interestingly, comes close to a lower bound conjectured in the context of the quark gluon plasma. We discuss possible consequences of the low viscosity, including pre-turbulent current flow. PACS numbers: 67.90.+z,81.05.Uw Graphene [1,2], attracts a lot of attention due to the massless relativistic dispersion of its quasiparticles and their high mobility. Recently, it was shown that this material offers a unique opportunity to observe transport properties of a plasma of ultrarelativistic particles at moderately high temperatures [3]. Undoped graphene is located at a special point in parameter space where the Fermi surface shrinks to two points, and in many respects it behaves similarly as other systems close to more complex quantum critical points [4]. Due to its massless Dirac particles graphene also shares interesting properties with the ultrarelativistic quark gluon plasma. The latter, surprisingly, has an unexpectedly low shear viscosity, as was observed in the dense matter balls created at the relativistic heavy ion collider RHIC [5]. We show here that an analogous property can be found in undoped graphene, reflecting its quantum criticality.The shear viscosity η measures the resistance of a fluid to establishing transverse velocity gradients, see Fig. 1. The smaller the viscosity, the higher the tendency to turbulent flow dynamics. Viscosity, similarly as resistivity in a conductor, leads to entropy production by degrading inhomogeneities in the velocity field. While ideal fluids with η = 0 cannot exist, it is interesting to seek for perfect fluids which come as close as possible to this ideal.Viscosity has the units of n where n is some density. To quantify the magnitude of the viscosity, it is natural to compare η/ to the density of thermal excitations, n th , which can be estimated by the entropy density, s ∼ k B n th . Motivated by the nearly perfect fluid behavior seen in the RHIC experiments, Kovtun et al. have recently postulated a lower bound for the ratio of η and s for a wide class of systems [6]:Equality was obtained for an infinitely strongly coupled conformal field theory by mapping it to weakly coupled gravity using the AdS-CFT correspondence. While examples violating the bound (1)were found (see [7]), the existence of some lower bound with k B η/ s of order unity for a given family of fluids is not unexpected. It is analogous to the Mott-Ioffe-Regel limit for the minimum conductivity of poor metals [8,9], and to the saturation of the relaxation rate at τ −1 rel = k B T / · O(1) close to strongly coupled quantum cri...

A pairing gap and coherence are the two hallmarks of superconductivity. In a classical BCS superconductor they are established simultaneously at T c . In the cuprates, however, an energy gap (pseudogap) extends above T c [1, 2, 3,4,5,6,7,8]. The origin of this gap is one of the central issues in high temperature superconductivity. Recent experimental evidence demonstrates that the pseudogap and the superconducting gap are associated with different energy scales [9,10,11,12,13,14]. It is however not clear whether they coexist independently or compete [9,12,14,15]. In order to understand the physics of cuprates and improve their superconducting properties it is vital to determine whether the pseudogap is friend or foe of high temperature supercondctivity [16]. Here we report evidence from angle resolved photoemission spectroscopy (ARPES) that the pseudogap and high temperature superconductivity represent two competing orders. We find that there is a direct correlation between a loss in the low energy spectral weight due to the pseudogap and a decrease of the coherent fraction of paired electrons. Therefore, the pseudogap competes with the superconductivity by depleting the spectral weight available for pairing in the region of momentum space where the superconducting gap is largest. This leads to a very unusual state in the underdoped cuprates, where only part of the Fermi surface develops coherence.Coherence in the superconducting state of the cuprates manifests itself by the appearance of a narrow peak in the ARPES lineshape [17], while the pseudogap [2, 3,4,12] depletes the low energy spectral weight below the pseudogap energy. The simplicity of the Bi 2 Sr 2 CuO 6+δ (Bi2201) spectra, as measured by ARPES, permits us to perform a straight forward quantitative analysis of the two features because the energy distribution curves (EDCs) in this single layer material lack the large renormalization effects (e.g. peak-hump-dip structure) and bilayer splitting that are present [6,7] in double layered Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212). This feature, however, means the spectral changes associated with the superconducting transition in Bi2201 are much more difficult to observe [18]. By acquiring very high resolution and stable ARPES data with high statistics, we are able to study the temperature and momentum dependence of the spectral weight near the chemical potential, with unprecedented accuracy. Experimental and sample preparation details are provided in the Supplementary Information. In Fig. 1 we examine the temperature dependence of the spectral lineshape in overdoped Bi2201 (T c =29K). Above the pseudogap temperature (T * ) (∼110K for this sample), the symmetrized EDCs [4] (see Supplementary Information) show a peak centered at the chemical potential -consistent with the metallic state of the sample. Upon cooling below T * , the low energy spectral weight decreases (within ∼20 meV), leading to a characteristic dip and very broad spectral peaks that signify the opening of an energy gap, as shown in Fig. 1(d). The loss...