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:: Honors and
Awards
:: DCHEM
Distributed Center For
Heavy Electron Materials (site under construction)
:: Current Projects
:: People (past and present)
:: Equipment
:: Recent
Publications
:: Journal Club
overview
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Organizing elements into a Periodic
Table
according to the electronic configuration of each element has been a
great scientific achievement of the late 19th century that laid the
ground for modern chemistry (http://en.wikipedia.org/wiki/Periodic_table).
Our ultimate goal is to
find similar organizing principles and framework to understand more
complex materials with strongly interacting electrons.
The quantum many body
problem is a
notoriously difficult problem and
yet condensed matter physics has proven to be one the most
technologically rewarding areas of physics. Unlike in nanoscience, the
ultimate goal is to understand not just a few but a macroscopic number
of interacting particles with more than one degrees of freedom. The
model Hamiltonians are often quite simple yet hard to solve exactly,
and have little or no predictive power. Thus, in the absence of
theoretical guidance, phenomenology coupled to experimental
trial-and-error plays an important role in making progress. In this
context, one can hardly over-emphasize the importance of being able to
grow high quality single crystals of novel materials that can be
characterized by bulk measurements (e.g. transport, magnetization, and
specific heat) and investigated in more detail with advanced
spectroscopic techniques. Our group has unique expertise in the
synthesis of new intermetallic compounds in which we investigate new
states of matter. We explore their rich phase diagrams with various
chemical substitutions, in order to understand the universality among
apparently different classes of materials. This allows us not only to
change the band filling and introduce carriers but also to tune the
delicate balance between various competing interactions. We ultimately
try to understand how the chemistry and physics conspire to give these
materials their characteristic properties, such as superconductivity
and magnetism, with the hope that this will guide in the long run
tailoring materials adjusted to the technological needs.
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The
Periodic Table (www.webelements.com) gives us a certain
freedom on the choice of elements from which the binary or ternary
alloys will be synthesized.
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current projects
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Our group's focus is currently on the single crystal growth and
characterization of new intermetallic compounds containing rare-earth
elements. Rare-earth elements have partially filled f-bands, and even
though these f-electrons don't form the chemical bonds, they
act as
local magnetic moments that are strongly coupled to the conduction band
electrons. This type of hybridization, also called Kondo coupling, is
the main reason for the spectacular mass renormalization found in some
of these materials. Indeed, conduction electrons can acquire an
effective mass a hundred times larger than the free electron mass, and
that's the reason why these compounds are dubbed heavy
fermion
systems.
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f-electrons become part of the Fermi
Surface via the Kondo coupling.
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The lattice of f-electrons, when hybridized
with conduction band,
become part of the Fermi surface, thereby transferring their magnetic
entropy to the conduction electrons. At low temperatures, various type
of ordering (superconductivity,
antiferro- or ferromagnetism occur to absorb this
excess entropy.
What combination of chemical and physical conditions favors a given
type of ordering? Are there common (universal) trends among
electronically and structurally different compounds?
The most successful attempt so far to
rationalize heavy fermions is the
phase diagram suggested by S. Doniach and it involves the competition
between two types of interactions, the Kondo coupling that screens the
local f-moments, and the magnetic exchange interaction between
neighboring f-moments, mediated by the polarized electron cloud
surrounding them. If the latter wins, there is a long range magnetic
order but if the moments are totally screened, the systems remains
paramagnetic (magnetically disordered) down to the lowest temperatures.
So, there must be an order-disorder type of transition at zero
temperature between the two ground states. Much of the current effort
is focused on understanding this so called quantum critical
point
(QCP) in the phase diagram, and its effect on finite temperature
properties. The reason why the existence of a QCP attracts so
much attention is because it is viewed as a breeding ground for
unconventional superconductivity and perhaps for more exotic phases as
well.
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Doniach's phase diagram
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A superconductor has the ability to carry electric current without
dissipation. The perfect diamagnetism of a superconductor makes it
possible to levitate magnetic objects. www.superconductors.org
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:: 115 heavy fermions
:: Kondo insulators
:: Ferromagnetism in low carrier
density and
half-metallic systems
115 Heavy Fermions
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CeMIn5 (M=Rh,Co,Ir) is a new family of heavy
fermion compound. CeRhIn5
is an ambient pressure antiferromagnet with a TNeel = 3.8K which
becomes superconductor under applied pressure. CeCoIn5 and CeIrIn5 are
ambient pressure superconductors with Tc = 2.3K and 0.4K respectively.
These compounds have recently attracted a lot of interest because they
exhibit a competition between antiferromagnetism and superconductivity
and there is compelling evidence for a quantum critical point in at
least CeRhIn5 and perhaps CeCoIn5.
The important questions we are currently
trying to address are:
How is the hybridization between f-electrons
and the conduction band
affected when the concentration of one or the other is changed?
What sets the scale for the magnetic and
superconducting transition
temperature?
What determines how large the electronic
specific heat coefficient (the
effective mass of the charge carriers) will be in the zero temperature
limit? How does it correlate with the proximity to a quantum critical
point?
Kondo Insulators
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Kondo insulators are strongly correlated
semiconductors (direct gap)
and semi-metals (indirect gap), the essential ingredient being once
again the hybridization of the f-electrons with conduction bands. The
difference with conventional semiconductors is that the magnitude of
the gap exhibit strong temperature dependence due to e-e interactions.
They can be modeled by the same Anderson lattice model that describes
the heavy fermion physics: they correspond to the half-filling of the
hybridized band. Among the most studied examples are FeSi, Ce3Bi4Pt3,
SmB6, YB12, CeNiSn. By exploring their phase diagram and by trying to
discover new materials exhibiting this type of behavior, we aim at
understanding how the gap opens and how it influences the low
temperature properties in these materials, reminiscent in many ways of
the "pseudogap" observed in the high-Tc cuprates.
Ferromagnetism in half-metallic and low carrier
density systems
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Discovery of ferromagnetism with high Curie
temperature in Mn-doped
GaAs has bolstered research on dilute magnetic semiconductors, with the
expectancy that this type of materials will have broad spintronics
applications. There are a few other cases where ferromagnetism emerges
in a lightly doped small gap semiconductor that our group is
investigating. Divalent alkaline-earth hexaborides (CaB6 and variants)
offer a spectacular example of an itinerant ferromagnetism with TCurie
of the order of 600K and saturation moments as low as 0.07
μB per
electron. The origin of ferromagnetism in such low carrier density
systems, in the absence of magnetic centers (d- or f-electron local
moments), is likely to be very different from the conventional band
ferromagnetism (Stoner model) in a metal or superexchange (RKKY)
interaction between local moments in an insulator. The important
question is how one can reconcile the high Curie temperatures (of the
order of the Fermi temperature) with the extremely weak saturating
moments.
The second case of interest is the
half-metallic systems having one
spin sub-band gapped, leading to an almost 100% spin-polarized
conduction band. The binary cubic B20 compounds seem to be a good
candidate and there are already two well-known examples of
helimagnetism : FeGe (T_Curie=280K) and MnSi (T_Curie=30K). We are
investigating if other compounds in this structure exhibit
helimagnetism.
people
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Graduate
Students
Daniel Hurt
Office: 220 Rowland Hall
e-mail: danhurt@gmail.com
Nicholas
Berry
Office: 220 Rowland Hall
e-mail: nberry@uci.edu
Dae Jeoung
Kim
Office: 220 Rowland Hall
e-mail: daejeonk@uci.edu
Postdocs
Dr. Cigdem
CAPAN
Office: 220 Rowland Hall
Phone: 949 - 824 0443
e-mail: ccapan@uci.edu
Former Group KMembers:
Dr. Andrea D. Bianchi
Assistant Professor, Universite de Montreal
514-343-6734
andrea.bianchi@umontreal.ca
equipment
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Quantum Design PPMS - The Physical
Property Measurement system is able
to measure heat capacity and resistance of samples under a wide range
of magnetic field and temperature settings. The magnetic fields
that are capable of being produced range from 0T to 9T. The
temperature ranges that we can study materials vary from 400K down to
1.8K. If we utilize the He3 attachment we can reach temperatures
of 0.4K routinely and down to 0.35K.
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Quantum
Design SQUID-VSM - The state-of-the-art Super Conducting
Quantum Interference Device with Vibrating Sample Magnetometer
(SQUID-VSM) is used to characterize our samples with magnetization
measurements. The samples can be characterized in a temperature
range of 400K to 1.8K and in magnetic fields up to 7T.
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Rigaku
X-ray Powder Diffractometer - The brand new Rigaku x-ray
machine
is used to determine lattice constants and assist with phase
identification in a rapid manner. This is a general purpose
diffractometer which can be used to characterize powders and
polycrystalline materials. All samples are run in air at room
temperature, but hermetic sample slides are possible. The JADE
data analysis software can perform search-match and pattern refinement
analysis.
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High
Temperature Furnaces - We have several types of furnaces for
material synthesis including a 3 stage temperature oven, a 1700 C
programmable oven, a vertical 1700 oven and quite a few others to cover
all the synthesis needs. Finally we have an arc melting furnace
under construction that can reach temperatures above 2000C.
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representative publications
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:: Probing
the Quantum Critical Behavior of CeCoIn5 via Hall Effect
Measurements
S. Singh, C. Capan, M. Nicklas, M. Rams, A. Gladun, H. Lee, J. F.
DiTusa, Z. Fisk, F. Steglich and S. Wirth, Phys. Rev. Lett. 98, 057001
(2007)
::
Irreversible
Dynamics of the Phase Boundary in U(Ru0.96Rh0.04)2Si2 and
Implications for Ordering
A. V. Silhanek, M. Jaime, N. Harrison, V. R. Fanelli, C. D. Batista, H.
Amitsuka, S. Nakatsuji, L. Balicas, K. H. Kim, Z. Fisk, J. L. Sarrao,
L. Civale, and J. A. Mydosh, Phys. Rev. Lett. 96, 136403 (2006)
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Fermi Surface
Changes across the Neel Phase Boundary of NdB6
R. G. Goodrich, N. Harrison, and Z. Fisk, Phys. Rev. Lett. 97, 146404
(2006)
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Magneto-Optical
Evidence of Double Exchange in a Percolating Lattice
G. Caimi, A. Perucchi, L. Degiorgi, H. R. Ott, V. M. Pereira, A. H.
Castro Neto, A. D. Bianchi, and Z. Fisk, Phys. Rev. Lett. 96, 016403
(2006)
::
Critical Phenomena
and the Quantum Critical Point of Ferromagnetic
Zr1-xNbxZn2
D. A. Sokolov, M. C. Aronson, W. Gannon, and Z. Fisk, Phys. Rev. Lett.
96, 116404 (2006)
::
Reversible Tuning
of the Heavy-Fermion Ground State in CeCoIn5
L. D. Pham, T. Park, S. Maquilon, J. D. Thompson, and Z. Fisk, Phys.
Rev. Lett. 97, 056404 (2006)
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Two Fluid
Description of the Kondo Lattice
S. Nakatsuji, D. Pines, Z. Fisk, Phys. Rev. Lett. 92, 016401 (2004)
journal club
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October 16, 2007
:: Gapped itinerant spin
excitations account for missing entropy in the hidden order state
of URu2Si2
C. R. Wiebe,J. A. Janik, G. J. MacDougall, G. M. Luke, J. D. Garrett,
H., D. Zhou,Y.-J. Jo, L. Balicas, Y. Qiu, J. R. D. Copley, Z. Yamani,
and W. J. L. Buyers, ArXiv:0710.0896
buyers_uru2si2.pdf
October11, 2007
:: Fluctuating
superconductivity in organic molecular metals close to the Mott
transition
Moon-Sun Nam, Arzhang Ardavan, Stephen J. Blundell, John A. Schlueter,
Nature 449, 584 (2007)
blundell.pdf
October 4, 2007
:: Interacting
Antiferromagnetic Droplets in Quantum Critical CeCoIn5
R. R. Urbano, B.-L. Young, N. J. Curro, J. D. Thompson, L. D. Pham, and
Z. Fisk, Phys. Rev. Lett. 99,146402 (2007)
curro_prl.pdf
September 27, 2007
:: Coexistence of Strongly
Mixed-Valence and Heavy-Fermion Character in SmOs4Sb12 Studied by Soft-
and Hard-X-Ray Spectroscopy
A. Yamasaki, S. Imada, H. Higashimichi, H. Fujiwara, T. Saita, T.
Miyamachi, A. Sekiyama, H. Sugawara,
D. Kikuchi, H. Sato, A. Higashiya, M. Yabashi, K. Tamasaku, D. Miwa, T.
Ishikawa, and S. Suga
SmOs4Sb12_PRL2007.pdf
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For
updates/corrections, please contact Alison Lara |
