Scott Riggs

Two-dimensional surface state in the quantum limit of a topological insulator

The edges of graphene-based systems possess unusual electronic properties, originating from the non-trivial topological structure associated with the pseudospinorial character of the electron wavefunctions. These properties, which have no analogue for electrons described by the Schrödinger equation in conventional systems, have led to the prediction of many striking phenomena, such as gate-tunable ferromagnetism and valley-selective transport1, 2, 3. In most cases, however, the predicted phenomena are not expected to survive the strong structural and chemical disorder that unavoidably affects the edges of real graphene devices. Here, we present a theoretical investigation of the intrinsic low-energy states at the edges of electrostatically gapped bilayer graphene, and find that the contribution of edge modes to the linear conductance of realistic devices remains sizable even for highly imperfect edges. This contribution may dominate over that of the bulk for sufficiently clean devices, such as those based on suspended bilayer graphene samples. Our results illustrate the robustness of those phenomena whose origin is [...] LI, Jian, et al. Topological origin of subgap conductance in insulating bilayer graphene. Nature Physics, 2011, vol. 7, no. 1, p. 38-42 DOI : 10.1038/nphys1822

Observation of Temperature-Induced Crossover to an Orbital-Selective Mott Phase in AxFe2-ySe2 (A=K, Rb) Superconductors

Using angle-resolved photoemission spectroscopy, we observe the low-temperature state of the A(x)Fe(2-y)Se(2) (A=K, Rb) superconductors to exhibit an orbital-dependent renormalization of the bands near the Fermi level-the d(xy) bands heavily renormalized compared to the d(xz)/d(yz) bands. Upon raising the temperature to above 150 K, the system evolves into a state in which the d(xy) bands have depleted spectral weight while the d(xz)/d(yz) bands remain metallic. Combined with theoretical calculations, our observations can be consistently understood as a temperature-induced crossover from a metallic state at low temperatures to an orbital-selective Mott phase at high temperatures. Moreover, the fact that the superconducting state of A(x)Fe(2-y)Se(2) is near the boundary of such an orbital-selective Mott phase constrains the system to have sufficiently strong on-site Coulomb interactions and Hunds coupling, highlighting the nontrivial role of electron correlation in this family of iron-based superconductors.

Heat capacity through the magnetic-field-induced resistive transition in an underdoped high-temperature superconductor

for an Invited Paper for the MAR11 Meeting of The American Physical Society Quantum Oscillations and High Magnetic Field Heat Capacity in Underdoped YBCO GREGORY BOEBINGER, National High Magnetic Field Laboratory This abstract not available.

Comparative High Field Magneto-transport Of Rare Earth Oxypnictides With Maximum Transition Temperatures

We compare magnetotransport of the three iron-arsenide-based compounds ReFeAsO (Re=La, Sm, Nd) in very high DC and pulsed magnetic fields up to 45 and 54 T, respectively. Each sample studied exhibits a superconducting transition temperature near the maximum reported to date for that particular compound. While high magnetic fields do not suppress the superconducting state appreciably, the resistivity, Hall coefficient, and critical magnetic fields, taken together, suggest that the phenomenology and superconducting parameters of the oxypnictide superconductors bridges the gap between MgB{sub 2} and YBCO.

Bounding the pseudogap with a line of phase transitions in YBa2Cu3O6+δ.

Close to optimal doping, the copper oxide superconductors show ‘strange metal’ behaviour, suggestive of strong fluctuations associated with a quantum critical point. Such a critical point requires a line of classical phase transitions terminating at zero temperature near optimal doping inside the superconducting ‘dome’. The underdoped region of the temperature–doping phase diagram from which superconductivity emerges is referred to as the ‘pseudogap’ because evidence exists for partial gapping of the conduction electrons, but so far there is no compelling thermodynamic evidence as to whether the pseudogap is a distinct phase or a continuous evolution of physical properties on cooling. Here we report that the pseudogap in YBa2Cu3O6+δ is a distinct phase, bounded by a line of phase transitions. The doping dependence of this line is such that it terminates at zero temperature inside the superconducting dome. From this we conclude that quantum criticality drives the strange metallic behaviour and therefore superconductivity in the copper oxide superconductors.

Quantum oscillations in the parent pnictide BaFe2As2: Itinerant electrons in the reconstructed state

We report quantum-oscillation measurements that enable the direct observation of the Fermi surface of the low-temperature ground state of

Structure and magnetic properties of the pyrochlore iridate Y2Ir2O7

{\text{BaFe}}_{2}{\text{As}}_{2}

Measurement of the elastoresistivity coefficients of the underdoped iron arsenide Ba(Fe0.975Co0.025)2As2

. From these measurements we characterize the low-energy excitations, revealing that the Fermi surface is reconstructed in the antiferromagnetic state, but leaving itinerant electrons in its wake. The present measurements are consistent with a conventional band folding picture of the antiferromagnetic ground state, placing important limits on the topology and size of the Fermi surface.

Enhanced Fermi-surface nesting in superconducting BaFe2(As(1-x)P(x))2 revealed by the de Haas-van Alphen effect.

Neutron powder diffraction and inelastic measurements were performed examining the

Possible origin of the nonmonotonic doping dependence of the in-plane resistivity anisotropy of Ba(Fe1−xTx)2As2 (T = Co, Ni and Cu)

5d