Nematic metamagnetic state in Sr3Ru2O7
This is an active research direction of ruthenate physics.
In the external magnetic field, the bilayer ruthenate Sr3Ru2O7
exhibits two consecutive meta-magnetic transitions.
Between these transitions resistivity measurements exhibit
spontaneous anisotropy (nematic) in the RuO2 plane.
In spite of a few years of intensive study, a satisfactory
microscopic mechanism has not been established before.
There are two key questions.
First, Sr3Ru2O7 is a t2g-band (dxz, dyz, and dxy)
system with the active 4d-orbitals of Ru.
The dxz and dyz-orbitals extend in the xz and yz plane,
respectively, thus they behave quasi-one dimensional in the ab-plane.
The dxy-orbital lies inside the ab-plane, and its band structure
is two-dimensional.
Which bands are responsible for the nematic behavior?
Second, the nematic metamagnetic states require strong exchange
interactions in the d-wave channel, while the usual exchange
interaction from Coulomb repulsion is dominate in the s-wave channel.
How to reconcile this discrepancy?
We point out that the hybridized quasi-1D dxz and dyz-bands of the t2g-orbital bands are responsible for the nematic ordering based on the following reasoning [Ref. 1] . The nematic metamagnetic state is only observed in the bilayer compound Sr3Ru2O7, but not in the monolayer compound Sr2RuO4, both of which are the t2g-orbital systems. The key difference in electronic structures between them is the bilayer splitting, which is prominent for the quasi-one dimensional bands of dxz and dyz but small for the two-dimensional bands of dxy. It is natural to expect that the spontaneous nematic behavior occurs in the bands of dxz and dyz. Furthermore, our mechanism naturally generates the exchange interaction in the d-wave channel. The orbital band hybridization between dxz and dyz shifts a significant spectra weight of the exchange interaction into the d-wave channel, thus the nematic ordering can arise from the conventional multi-band Hubbard interactions. In our theory, the nematic ordering results from the orbital ordering between the dxz and dyz-orbitals. This work is a large progress and has been widely cited by various experiment and theory groups in the community of Sr3Ru2O7.
QPI pattern on Sr3Ru2O7
We believe that the above ideas captures the essential physics observed in
Sr3Ru2O7.
Moreover, in order to make a careful comparison with experiment results,
we further refine these studies by considering the realistic band
structures of Sr3Ru2O7, which are so complicated that are seldom
seriously treated in theoretical studies.
It includes various effects from the t2g-orbital structure, the
staggered distortion of the RuO octahedra, bilayer splitting, spin-orbit
coupling, and the surface bias.
The band parameters are fitted by matching the calculated Fermi surfaces
with those measured in the ARPES measurements.
We have calculated the QPI at zero magnetic field in Sr3Ru2O7 by using a
tight-binding model which gives rise to the realistic band structure
[Ref. 2] .
The QPI exhibits a hollow square-like feature arising from the nesting
of the quasi-onedimensional dxz and dyz orbital bands, which agrees
well with the recent measurements in J. C. Davis and A. Mackenzie's
groups.
The QPI pattern in the nematic metamagnetic state is also
predicted which can be tested in future experiments.
Spectroscopic imaging scanning tunneling microscopy (STM) as a
probe to orbital ordering in transition metal oxides
Unlike charge and spin, the orbital degree of freedom of electrons
in transition metal oxides is difficult to detect.
We present a theoretical study of a new detection method
in metallic orbitally active systems by analyzing the quasi-particle
scattering interference (QPI) pattern of the spectroscopic imaging
scanning tunneling spectroscopy, which is sensitive to orbital
structures and orbital ordering
[Ref. 3] .
We generalize the theoretic framework of the scattering $T$-matrix
into the multiple orbital band systemS.
The QPIs for the dxz and dyz-orbital bands in the t2g-orbital
systems show a characteristic stripe-like feature as a consequence
of their quasi-one-dimensional nature, which is robust against
orbital hybridization. With the occurrence of orbital ordering
proposed in Sr3Ru2O7 and iron pnictides, the stripe-like
QPI patterns exhibit nematic distortion breaking the C4 symmetry.
This work provides the theoretical support of a new
detection method to orbital ordering and the nematic state.
It gives excellent explanations to features seen in the recent STM experiments
in J. C. Davis' group in both Sr3Ru2O7 and iron-pnictides.
References and talks
"Theory of unconventional metamagnetic electron states in orbital band
systems",
Phys. Rev. B 80, 104438 (2009).
See pdf file .
Phys. Rev. B 81, 184403 (2010) .
See pdf file .
Phys. Rev. Lett. 103, 176101 (2009).
See pdf file
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Last modified: July 15, 2007.