in lower panels) lies in the magnetic equatorial
plane. The reconnection potential is arbitrary
up to a constant of integration, which was chosen
to ensure that, for each run, the zero value of
reconnection potential in the magnetotail is
magnetically connected to the zero potential in
the ionosphere (18).
The reconnection potentials versus MLT with
offsets chosen in this way (Fig. 4) exhibit the same
dusk-to-dawn ratios of extrema as the ionospheric
potentials (to within 1% for the values in Fig. 3).
The minimum (duskside) potential is larger in
magnitude than maximum (dawnside) potential
for the causal and auroral depletion runs and
equal in magnitude (within numerical error) for
the uniform run. The average reconnection rate
is higher on the nightside than on the dayside
and more spatially limited. As expected from the
distribution of fast flows (Fig. 3), the reconnection
rate, per unit length x-line, averaged over a 1-hour
MLT segment to either side of FR = 0 (Table 1), is
larger in the premidnight sector, tailward of the
fast flow, in the causal-empirical run and larger
postmidnight in the Hall depletion run. The
reconnection potential difference calculated as
DFR = FR,dawn − FR,dusk exceeds the cross polar
cap potential (DFPC) in the ionosphere by 12, 6,
and 14% for the uniform, causal, and auroral depletion runs, respectively (18).
Hall currents flow antiparallel to convection
streamlines in the ionosphere (Fig. 3). If the
Hall conductance develops a gradient parallel to
a streamline—e.g., due to an enhancement in
electron precipitation—the Hall current becomes discontinuous if other effects do not intervene. Its excess current could be diverted
into a field-aligned current flowing through the
magnetosphere to alleviate charge accumulation at the discontinuity, or a charge accumulation could polarize the ionospheric plasma and
introduce a secondary electric field orthogonal to
the primary convection electric field E (directed
equatorward in a nightside auroral conductance
band and sunward poleward of it). The resulting field, when added to the primary field, locally rotates the direction of convection and the
Hall currents while driving secondary meridional
Pedersen currents. This so-called Cowling effect
produces the Harang reversal (24) in the nightside convection throat and increases the Joule
dissipation in a zonally limited channel of enhanced conductance (25).
The Cowling effect dominates the electrodynamics of M-I coupling on the 100-km-scale ionospheric resolution of the global simulations. The
sunward electric field in the polar cap produces
a CW rotation of polar convection and a duskward drift in the magnetotail lobes. The secondary Pedersen current flowing equatorward
in the nightside conductance band is supplied
by a plasmasheet dynamo that generates field-aligned currents from a tailward current system (24). The resulting duskward, bulk Lorentz
force presumably moves the otherwise symmetric plasmasheet flows toward the premidnight
sector to produce the observed asymmetry in
reconnection rates and plasmasheet fast flows.
These results demonstrate the intricate interplay
between the SW-M-I interaction and ionospheric Hall conduction in regulating magnetotail
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G. Haerendel provided helpful suggestions on the research.
Support was provided by the NASA HTP Program (grant
NNX11AO59G), NASA Geospace SR Program (grant NNX11AJ10G),
and NSF NSWP (grant AGS-1023346). Computing resources
were provided by the Computational and Information Systems
Laboratory of the National Center for Atmospheric Research
(grants 36761008 and 2761009). Simulation data are available
from W. Lotko.
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4 March 2014; accepted 5 June 2014
Fermi arcs in a doped pseudospin-1/2
Y. K. Kim,1 O. Krupin,1 J. D. Denlinger,1 A. Bostwick,1 E. Rotenberg,1 Q. Zhao,2
J. F. Mitchell,2 J. W. Allen,3 B. J. Kim2,3,4*
High-temperature superconductivity in cuprates arises from an electronic state that
remains poorly understood. We report the observation of a related electronic state in a
noncuprate material, strontium iridate (Sr2IrO4), in which the distinct cuprate fermiology
is largely reproduced. Upon surface electron doping through in situ deposition of alkali-metal
atoms, angle-resolved photoemission spectra of Sr2IrO4 display disconnected segments
of zero-energy states, known as Fermi arcs, and a gap as large as 80 millielectron volts.
Its evolution toward a normal metal phase with a closed Fermi surface as a function of doping
and temperature parallels that in the cuprates. Our result suggests that Sr2IrO4 is a useful
model system for comparison to the cuprates.
Although the mechanism of high-temperature superconductivity (HTSC) remains an open question, it is commonly believed that cer- tain distinct features of cuprates are essential to HTSC: spin-1/2 moment on a quasi–two- dimensional (2D) square lattice, Heisenberg anti- ferromagnetic coupling, and no orbital degeneracy. A minimal model based on this assumption can reproduce much of the phenomenology of the cuprates (1). Within this framework, it would be
informative to realize the key features of cuprates
in a different material (2). The 5d transition metal
oxide Sr2IrO4, with a t2g5 valence shell, is a Mott
insulator in which the orbital degeneracy is removed through strong spin-orbit coupling (3, 4).
Despite strong entanglement of spin and orbital
degrees of freedom, the resulting pseudospins
(with Jeff = 1/2 quantum number) exhibit the spin
1Advanced Light Source, Lawrence Berkeley National
Laboratory, Berkeley, CA 94720, USA. 2Materials Science
Division, Argonne National Laboratory, Argonne, IL 60439,
USA. 3Randall Laboratory of Physics, University of Michigan,
Ann Arbor, MI 48109, USA. 4Max Planck Institute for Solid
State Research, Heisenbergstraße 1, D-70569 Stuttgart,
*Corresponding author. E-mail: email@example.com