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RESEARCH
HIGHLIGHTS
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Strong Electron Correlations in the Normal State of the
Iron-Based FeSe0.42Te0.58 Superconductor Observed by Angle-Resolved
Photoemission SpectroscopyA. Tamai, A. Y. Ganin, E. Rozbicki,
J. Bacsa, W. Meevasana, P. D. C. King, M. Caffio, R. Schaub, S.
Margadonna, K. Prassides, M. J. Rosseinsky, and F. Baumberger
We
investigate the normal state of the “11” iron-based superconductor
FeSe0.42Te0.58 by angle-resolved photoemission. Our data reveal a
highly renormalized quasiparticle dispersion characteristic of a
strongly correlated metal. We find sheet dependent effective carrier
masses between ≈3 and 16 me corresponding to a mass enhancement over
band structure values of m*/mband ≈ 6–20. This is nearly an order of
magnitude higher than the renormalization reported previously for
iron-arsenide superconductors of the “1111” and “122” families but
fully consistent with the bulk specific heat. |
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Heavy d-electron quasiparticle interference and
real-space electronic structure of Sr3Ru2O7
Jinho Lee, M.
P. Allan, M. A. Wang, J. Farrell, S. A. Grigera, F. Baumberger, J. C.
Davis and A. P. Mackenzie
The intriguing idea that strongly interacting
electrons can generate spatially inhomogeneous electronic
liquid-crystalline phases is over a decade old, but these systems still
represent an unexplored frontier of condensed-matter physics. One
reason is that visualization of the many-body quantum states generated
by the strong interactions, and of the resulting electronic phases, has
not been achieved. Soft condensed-matter physics was transformed by
microscopies that enabled imaging of real-space structures and
patterns. A candidate technique for obtaining equivalent data in the
purely electronic systems is spectroscopic imaging scanning tunnelling
microscopy (SI-STM). The core challenge is to detect the tenuous but
‘heavy’ momentum (k)-space components of the many-body electronic state
simultaneously with its real- space constituents. Sr3Ru2O7 provides a
particularly exciting opportunity to address these issues. It possesses
a very strongly renormalized ‘heavy’ d-electron Fermi liquid and
exhibits a field-induced transition to an electronic liquid- crystalline
phase. Finally, as a layered compound, it can be cleaved to present an
excellent surface for SI-STM.
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Inhomogeneous Phase Formation on the Border of
Itinerant Ferromagnetism
G. J. Conduit, A. G. Green, and B. D. Simons
A
variety of analytical techniques suggest that quantum fluctuations lead
to a fundamental instability of the Fermi liquid that drives
ferromagnetic transitions first order at low temperatures. We present
both analytical and numerical evidence that, driven by the same quantum
fluctuations, this first order transition is preempted by the formation
of an inhomogeneous magnetic phase. This occurs in a manner that is
closely analogous to the formation of the inhomogeneous superconducting
Fulde-Ferrel-Larkin-Ovchinnikov state. We derive these results from a
field-theoretical approach supplemented with numerical quantum Monte
Carlo simulations.
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Entropy Landscape of Phase Formation Associated
with Quantum Criticality in Sr3Ru2O7
A. W. Rost, R. S. Perry, J.-F. Mercure, A. P.
Mackenzie, S. A. Grigera
Low-temperature phase transitions and the
associated quantum critical points are a major field of research, but
one in which experimental information about thermodynamics is sparse.
Thermodynamic information is vital for the understanding of quantum
many-body problems. We show that combining measurements of the
magnetocaloric effect and specific heat allows a comprehensive study of
the entropy of a system. We present a quantitative measurement of the
entropic landscape of Sr3Ru2O7, a quantum critical system in which
magnetic field is used as a tuning parameter. This allows us to track
the development of the entropy as the quantum critical point is
approached and to study the thermodynamic consequences of the formation
of a novel electronic liquid crystalline phase in its vicinity.
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Dirac Strings and Magnetic Monopoles in Spin Ice
Dy2Ti2O7
D. J. P.
Morris, D. A. Tennant, S. A. Grigera, B. Klemke, C. Castelnovo, R.
Moessner, C. Czternasty, M. Meissner, K. C. Rule, J.-U. Hoffmann, K.
Kiefer, S. Gerischer, D. Slobinsky , R. S. Perry
While
sources of magnetic fields—magnetic monopoles—have so far proven
elusive as elementary particles, several scenarios have been proposed
recently in condensed matter physics of emergent quasiparticles
resembling monopoles. A particularly simple proposition pertains to
spin ice on the highly frustrated pyrochlore lattice. The spin ice
state is argued to be well-described by networks of aligned dipoles
resembling solenoidal tubes—classical, and observable, versions of a
Dirac string. Where these tubes end, the resulting defect looks like a
magnetic monopole. We demonstrate, by diffuse neutron scattering, the
presence of such strings in the spin ice Dy2Ti2O7. This is achieved by
applying a symmetry-breaking magnetic field with which we can
manipulate density and orientation of the strings. In turn, heat
capacity is described by a gas of magnetic monopoles interacting via a
magnetic Coulomb interaction.
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Inhomogeneous Magnetic Phases: A
Fulde-Ferrell-Larkin-Ovchinnikov-Like Phase in Sr3Ru2O7
A. M. Berridge, A. G. Green, S. A. Grigera, and
B. D. Simons
The
phase diagram of Sr3Ru2O7 contains a metamagnetic transition that
bifurcates to enclose an anomalous phase with intriguing properties—a
large resistivity with anisotropy that breaks the crystal-lattice
symmetry. We propose that this is a magnetic analogue of the spatially
inhomogeneous superconducting Fulde-Ferrel-Larkin-Ovchinnikov state. We
show—through a Ginzburg-Landau expansion where the magnetization
transverse to the applied field can become spatially inhomogeneous—that
a Stoner model with electronic band dispersion can reproduce this phase
diagram and transport behavior.
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Fermi Surface and van Hove Singularities in the
Itinerant Metamagnet Sr3Ru2O7
A. Tamai, M. P. Allan, J. F. Mercure, W.
Meevasana, R. Dunkel,
D. H. Lu, R. S. Perry, A. P. Mackenzie, D. J. Singh, Z.-X. Shen, and F.
Baumberger
The low-energy electronic structure of the
itinerant metamagnet Sr3Ru2O7 is investigated by angle-resolved
photoemission and
density-functional calculations. We find
well-defined quasiparticle bands with resolution-limited linewidths and
Fermi velocities up to an order of
magnitude lower than in single layer Sr2RuO4 . The complete
topography, the cyclotron masses, and the
orbital character of the Fermi surface are determined, in agreement
with bulk sensitive de Haas – van Alphen
measurements. An analysis of the dxy band dispersion reveals a complex
density of states with van Hove
singularities near the Fermi level, a situation which is favorable for
magnetic instabilities.
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Formation of a Nematic Fluid at High Fields
in Sr3Ru2O7
R. A. Borzi, S. A. Grigera, J. Farrell, R. S.
Perry, S. J.
S. Lister, S. L. Lee, D. A. Tennant, Y. Maeno, A. P. Mackenzie
In principle, a complex assembly of strongly interacting electrons can
self-organize into a wide variety of collective states, but relatively
few such states have been identified in practice. We report that, in
the close vicinity of a metamagnetic quantum critical point,
high-purity strontium ruthenate Sr3Ru2O7 possesses a large
magnetoresistive anisotropy, consistent with the existence of an
electronic nematic fluid. We discuss a striking phenomenological
similarity between our observations and those made in high-purity
two-dimensional electron fluids in gallium arsenide devices.
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Evolution of the Fermi Surface and Quasiparticle
Renormalization through a van Hove Singularity in Sr2-yLayRuO4
K. M. Shen, N. Kikugawa, C. Bergemann, L. Balicas, F.
Baumberger, W. Meevasana, N. J. C. Ingle, Y. Maeno, Z.-X. Shen, and A.
P. Mackenzie
We employ a combination of chemical substitution and angle resolved
photoemission spectroscopy to prove that the Fermi level in the γ band
of Sr2-yLayRuO4 can be made to traverse a van Hove singularity.
emarkably, the large mass renormalization has little dependence on
either k or doping. By combining the results from photoemission with
thermodynamic measurements on the
same batches of crystals, we deduce a parametrization of the full
many-body quasiparticle dispersion
in Sr2RuO4 which extends from the Fermi level to approximately 20 meV
above it.
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Quantitative Determination of the Hubbard Model
Phase Diagram from Optical Lattice Experiments by Two-Parameter Scaling
V. L. Campo, Jr., K. Capelle, J. Quintanilla, and
C. Hooley
We propose an experiment to obtain the phase
diagram of the fermionic Hubbard model, for any dimensionality, using
cold atoms in optical lattices. It is based on measuring the total
energy for a sequence of trap profiles. It combines finite-size scaling
with an additional “finite-curvature scaling” necessary to reach the
homogeneous limit. We illustrate its viability in the 1D case,
simulating experimental data in the Bethe-ansatz local-density
approximation. Including experimental errors, the filling corresponding
to the Mott transition can be determined with better than 3% accuracy.
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Magnetothermoelectric Response at a Superfluid
–Mott-Insulator Transition
M. J. Bhaseen, A. G. Green, and S. L. Sondhi
We investigate the finite temperature
magnetothermoelectric response in the vicinity of superfluid
–Mott-insulator quantum phase transition. We focus on the particle-hole
symmetric transitions of the Bose-Hubbard model, and combine Lorentz
invariance arguments with quantum Boltzmann calculations. By means of
an epsilon expansion, we find that a nonvanishing thermoelectric tensor
and a finite thermal transport coefficient are supported in this quantum
critical regime. We comment on the singular Nernst effect in this
problem.
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