Joerg Schmalian

The shapes of cooperatively rearranging regions in glass-forming liquids

The cooperative rearrangement of groups of many molecules has long been thought to underlie the dramatic slowing of liquid dynamics on cooling towards the glassy state. For instance, there exists experimental evidence for cooperatively rearranging regions (CRRs) on the nanometre length scale near the glass transition. The random first-order transition (RFOT) theory of glasses predicts that, near the glass-transition temperature, these regions are compact, but computer simulations and experiments on colloids suggest CRRs are string-like. Here, we present a microscopic theory within the framework of RFOT, which unites the two situations. We show that the shapes of CRRs in glassy liquids should change from being compact at low temperatures to fractal or ‘stringy’ as the dynamical crossover temperature from activated to collisional transport is approached from below. This theory predicts a correlation of the ratio of the dynamical crossover temperature to the laboratory glass-transition temperature, and the heat-capacity discontinuity at the glass transition. The predicted correlation quantitatively agrees with experimental results for 21 materials.

Anomalous Suppression of the Orthorhombic Lattice Distortion in Superconducting Ba(Fe1-xCox)2As2 Single Crystals

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.

Femtosecond Population Inversion and Stimulated Emission of Dense Dirac Fermions in Graphene

We show that strongly photoexcited graphene monolayers with 35 fs pulses quasi-instantaneously build up a broadband, inverted Dirac fermion population. Optical gain emerges and directly manifests itself via a negative conductivity at the near-infrared region for the first 200 fs, where stimulated emission completely compensates absorption loss in the graphene layer. Our experiment-theory comparison with two distinct electron and hole chemical potentials reproduce absorption saturation and gain at 40 fs, revealing, particularly, the evolution of the transient state from a hot classical gas to a dense quantum fluid with increasing the photoexcitation.

PAIRING SYMMETRY AND PAIRING STATE IN FERROPNICTIDES: THEORETICAL OVERVIEW

Abstract 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.

Momentum Dependence of the Superconducting Gap in NdFeAsO0.9F0.1 Single Crystals Measured by Angle Resolved Photoemission Spectroscopy

We use angle resolved photoemission spectroscopy to study the momentum dependence of the superconducting gap in NdFeAsO0.9F0.1 single crystals. We find that the Gamma hole pocket is fully gapped below the superconducting transition temperature. The value of the superconducting gap is 15+/-1.5 meV and its anisotropy around the hole pocket is smaller than 20% of this value-consistent with an isotropic or anisotropic s-wave symmetry of the order parameter. This is a significant departure from the situation in the cuprates, pointing to the possibility that the superconductivity in the iron arsenic based system arises from a different mechanism.

Effects of nematic fluctuations on the elastic properties of iron arsenide superconductors

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.

Quantum-critical theory of the spin-fermion model and its application to cuprates: normal state analysis

We present the full analysis of the normal state properties of the spin-fermion model near the antiferromagnetic instability in two dimensions. The 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 interaction g¯ and an energy ωsf below which the system behaves as a Fermi liquid. The ratio of the two determines the dimensionless coupling constant for spin-fermion interaction λ2 ∝ g¯/ωsf. 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 develop 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 flavours—which allows us to explicitly account for all terms which diverge as powers of λ, and treat the remaining, O(log λ) terms in the RG formalism. We apply this technique to study the properties of the spin-fermion 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 the cuprates, and argue that the spin-fermion model is capable of explaining the anomalous normal state properties of the high T c materials. We also show that the conventional {4 theory of the quantum-critical behaviour is inapplicable in 2D due to the singularity of the {4 vertex.

Competition between the pseudogap and superconductivity in the high- T c copper oxides

In a classical Bardeen–Cooper–Schrieffer superconductor, pairing and coherence of electrons are established simultaneously below the critical transition temperature (Tc), giving rise to a gap in the electronic energy spectrum. In the high-Tc copper oxide superconductors, however, a pseudogap extends above Tc. The relationship between the pseudogap and superconductivity is one of the central issues in this field. Spectral gaps arising from pairing precursors are qualitatively similar to those caused by competing electronic states, rendering a standard approach to their analysis inconclusive. The issue can be settled, however, by studying the correlation between the weights associated with the pseudogap and superconductivity spectral features. Here we report a study of two spectral weights using angle-resolved photoemission spectroscopy. The weight of the superconducting coherent peak increases away from the node following the trend of the superconducting gap, but starts to decrease in the antinodal region. This striking non-monotonicity reveals the presence of a competing state. We demonstrate a direct correlation, for different values of momenta and doping, between the loss in the low-energy spectral weight arising from the opening of the pseudogap and a decrease in the spectral weight associated with superconductivity. We therefore conclude that the pseudogap competes with the superconductivity by depleting the spectral weight available for pairing.Takeshi Kondo, Rustem Khasanov, Tsunehiro Takeuchi, 4 Joerg Schmalian, and Adam Kaminski Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland Department of Crystalline Materials Science, Nagoya University, Nagoya 464-8603, Japan EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan (Dated: February 9, 2009)

PAIRING DUE TO SPIN FLUCTUATIONS IN LAYERED ORGANIC SUPERCONDUCTORS

I show that for a \kappa-type organic (BEDT-TTF)_2-X molecular crystal, a superconducting state with T_c ~ 10 K and gap nodes on the Fermi surface can be caused by short-ranged antiferromagnetic spin fluctuations. Using a two-band description for the anti-bonding orbitals on a BEDT-TTF dimer of the \kappa-type salt, and an intermediate local Coulomb repulsion between two holes on one dimer, the magnetic interaction and the superconducting gap-function are determined self consistently within the fluctuation exchange approximation. The pairing interaction is predominantly caused by inter-band coupling and additionally affected by spin excitations of the quasi one-dimensional band.

Disentangling Cooper-pair formation above the transition temperature from the pseudogap state in the cuprates

The pseudogap state in the cuprate superconductors shows signs of electronic pair formation above the superconducting temperature. Is it just a ‘precursor’ state or a separate (and competing) state? In fact, both interpretations seem to be correct.