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Odd-parity,
spin-triplet superconductor Sr2RuO4
is so far the only known layered peroviskite that becomes
superconducting without the presence of Cu, and the only
known superconducting Ru oxide. Its electronic states are
dominated by the t2g orbitals of Ru,
forming multiple sheet Fermi surface of electron and holes.
Its normal state is strongly correlated, featuring some
rather unusual properties such as linear magnetoresistance [1].
The discovery of superconductivity in this material in 1994,
and the subsequent demonstration of spin-triplet, chiral p-wave
pairing symmetry has motivated an intensive search for novel
superconductors in 4d and 5d transition metal oxides,
leading to discoveries of many materials with novel physical
properties.
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SrRuO3 |
Sr2RuO4 |
Sr3Ru2O7 |
Sr4Ru3O10
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Ruthenates
feature a remarkably rich variety of structural types, as well
as striking structure-property relationships. We have focused
on ruthenates with a non-perovskite structure in our search
for new superconductors, and materials exhibiting novel
quantum phenomena. The
Ruddelsden-Popper homologuous series of Srn+1RunO3n+1
feature fascinating physical phenomena ranging from
ferromagnetism in the n = infinity member, SrRuO3,
to spin-triplet superconductivity in the n = 1 member Sr2RuO4,
to quantum metamagnetism to n = 2 member, Sr3Ru2O7.
In an early work on Sr3Ru2O7,
we found evidence for the coexistence of ferromagnetic and antiferromagnetic
fluctuations, and hint of metamagnetic transition in low temperature Hall effect and magnetoresistance measurements
[2]. More recently we
studied the electronic and magnetic properties of Srn+1RunO3n+1
films prepared by molecular beam epitaxy and found the
systematic behavior as the n varies
[3].
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BaRuO3(4H) |
BaRuO3(6H) |
BaRuO3(9R) |
BaRuO3(3C) |
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A dramatic example
of the striking structure-property relationship is BaRuO3. It adopts three
possible hexagonal structures, known as the 4-, the 6-, and
9-layer BaRuO3, as well as a cubic BaRuO3.
While the cubic form is fully three dimensional, with least
amount of distortion and rotation of the RuO6
octohedral, the 9-layer form has some degree of
one-dimensionality built in the structure because of the
present of short chains of face and edge sharing RuO6
octahedral. The electrical and magnetic properties of BaRuO3
vary markedly with the structure it adopts. The 9-layer BaRuO3
exhibits semiconducting behavior due to the opening of a
pseudo-gap, but is non-magnetic. However, the cubic BaRuO3
is a ferromagnetic metal. Our low-temperature measurements
revealed the remarkable variation in the physical properties
among different single crystalline 4- and 9-layer BaRuO3
[4] and tinyl magnetoresistance,
surprisingly given that the cubic BaRuO3 is
ferromagnetic.
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La4Ru6O19 |
Cubic La4Ru6O19
features a very short Ru-Ru distance (2.49 A) and an
associated metal-metal bonding. It has been observed previously
that the this material exhibit non-Fermi liquid behavior [5]
The low temperature
magnetoresistance and Hall measurements provide evidence that
the non-Fermi liquid behavior may have originated from the
presence of a ferromagnetic quantum criticality in this material
system. It is
possible that the metal-metal bonding may have led to the
formation of local moment, and the competing interactions, that
between two local moments and that between a local moment and conduction
electrons may happen to be comparable to place the material near
quantum critical point under the ambient pressure. |
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A large number
of ruthenates adopt the quasi-one-dimensional hollandite
structure featuring double chains of edge and face sharing RuO6
octahedral. We studied previously BaRu6O12 hollandite,
a weakly anisotropic metal that hosts a quantum phase
transition tuned by magnetic field. It also features a
sensitive response to disorder and a weakly localized
electronic state under high magnetic fields [6]. The
interesting question is whether the quantum phase transition
can be tuned by pressure as well, and if superconductivity can
be found in any ruthenate hollandites. |
BaRu6O12 |
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We
have been studying layered chalcogenides that exhibit
competing orders. For example, in NbSe2, both
superconductivity and charge density waves exist at low
temperatures. Furthermore, the superconductivity is band
dependent. We have prepared atomically thin flakes of NbSe2
by mechanical exfoliation so that we can studied the electric
field effect in atomically thin flakes of NbSe2,
and are exploring other layered chalcogenides hosting other
exotic quantum phenomena. |
NbSe2 |
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We
are grateful to our collaborators who have supplied us with high
quality single crystals over the years for work in this area,
especially Prof. Y. Maeno of Kyoto University, Prof. R. J. Cava
of Princeton University, Prof. Z.
Mao of Tulane University, and Prof. Z. Xu
in Zhejiang University. |
Contacts:
Yiqun Alex Ying, yzy116 @ psu.edu
Neal Staley, nes151 @ psu.edu
Publications:
[1] R.
Jin, Y. Liu, and F. Lichtenberg, “Linear-Field Dependence of the
Normal-State In-Plane Magnetoresistance of Sr2RuO4.”
Phys. Rev. B 60, 10418-104422 (1999).
[2] Y. Liu, R. Jin, Z. Q. Mao, K. D. Nelson, M. K.
Haas, and R. J.
Cava, “Electrical
transport properties of single crystal Sr3Ru2O7.:
The possible existence of an antiferromagnetic instability at low
temperatures.”
Phys. Rev. B 63, 174435 (2001).
[3]
W.
Tian, J. H. Haeni, E. Hutchinson,
B. L. Sheu, M. A. Zurbuchen, M. M. Rosario, P. Schiffer, X. Q. Pan,
Y. Liu, and D. G. Schlom, “Effect of dimensionality on magnetism
in the layered Srn+1RunO3n+1 oxide series,” Appl.
Phys. Lett.
90, 022507 (2007).
[4]
J.
T. Rijssenbeek, R. Jin, Y. Zadorozhny, Y. Liu, B. Batlogg, and R. J.
Cava, “Electrical
and magnetic properties of the two crystallographic forms of BaRuO3.”
Phys. Rev. B 59,
4561 (1999).
[5] P.
Khalifah, K. D. Nelson, R. Jin, Z. Q. Mao, Y. Liu, Q. Huang, X. P.
A. Gao, A. P. Ramirez and R. J. Cava, “Non-Fermi-Liquid Behaviour
in La4Ru6O19,” Nature 411,
669 - 671 (2001).
[6] Z.
Q. Mao, T. He, M. M. Rosario, K. D. Nelson, D. Okuno, B. Ueland, I.
G. Deac, P. Schiffer, Y. Liu, and R.J. Cava, “Quantum phase
transition in quasi-one-dimensional BaRu6O12,”
Phys. Rev. Lett. 90, 186601 (2003).
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