Jaime Merino
Jaime Merino
is an associate professor at the Condensed Matter Theory Department at
Universidad Autónoma de Madrid in Spain. He received his Ph.D from Universidad
Autónoma de Madrid in 1997. After postdoctoral research work at Brown
University (USA) (1997-1998), University of New South Wales (Australia)
(1998-2001) and the Max-Planck Institute for Solid State research in Stuttgart
(Germany) (2001-2003) he joined the
Condensed Matter theory department at Universidad Autónoma de Madrid (UAM) as a
Ramón y Cajal Fellow in 2003. He became a faculty member of the Condensed
Matter theory department at UAM in 2010.
Jaime Merino
is a theoretical condensed matter physicist currently working in the area of
strongly correlated systems. His main research focuses on
understanding the behavior of electronic systems in which Coulomb electron
interactions are strong. Some of the topics addressed includes the understanding of transport
properties of strongly correlated metals, out-of-equibrium quantum many-body
theory, superconductivity in strongly interacting systems, charge ordering
phenomena in low dimensional materials, deriving effective model Hamiltonians
for strongly correlated systems, quantum spin liquids, topological properties
of materials and Kondo impurities in metals.
Click here to view his publication list and recent presentations.
DESCRIPTION
OF CURRENT RESEARCH
During the
past century, the band theory of solids led to an unprecedented revolution in
the understanding of the electronic properties of simple metals, semiconductors and band insulators. In such description
electrons move independently from each other in the presence of the crystal
potential and their intermutual interaction is negligible. This is because in
such systems their Coulomb interaction is largely screened by the rest of
mobile electrons. The low energy elementary excitations consist of electrons
and holes excited across the Fermi energy of the metal.
However,
there is a whole class of materials which cannot be understood with band theory
and which have modernly refered to as: Quantum Materials. Unlike in conventional
metals, the electronic properties of these systems are dominated by collective
excitations typically arising from quantum many-body effects due to the strong
Coulomb electron-electron interaction. Examples of such behavior include: the
high temperature superconductivity in cuprates, the large effective electron
mass in heavy fermions, spin-charge separation in one-dimensional systems and
the Mott metal-insulator transition in transition metal oxides.
Some of our current research interests include:
1. Non-Fermi liquid
behavior occurring close to quantum critical points in organics and heavy
fermions: An important source of non-Fermi liquid metallic behavior is the proximity
of a metal to a magnetic order instability. Proptotypical examples are the
heavy fermions, in which by changing an external parameter such as the magnetic
field, hydrostatic pressure or doping, the material can be tuned through a Quantum Critical Point (QCP). Such kind of QCP is driven by the
critical fluctuations at a second order quantum phase transition between a
homogeneous and an ordered metal at zero 
temperature. In contrast to classical
phase transitions (such as the liquid-gas transition of water) in which thermal
fluctuations are responsible for the transition, in QCP the fluctuations of the
order parameter are strictly quantum mechanical in nature. To illustrate a
prototypical example of QCP we show below the phase diagram of YbRh2Si2
in which an applied magnetic field destroys the antiferromagnetic order of the
material (from Ref. [4]).
Within the
blue regions the resistivity behaves as in a Fermi liquid: rT2 , (T
being the temperature) whereas the orange regions indicate non-Fermi liquid
behavior with the resistivity behaving proporcional to T down to the lowest
temperaturas experimentally accessible. Such temperature dependence of the
resistivity deviates from standard Fermi liquid which is generally termed
(NFL).
The
understanding of NFL in heavy fermions has progressed
significantly in the last decade and there are two main scenarios for QCP’s. The first one is based on the destruction of a spin density
wave type of order occurring at the QCP. The second scenario assumes that a
different type of critical fluctuations destroy the Kondo cloud liberating the
screened local moments which become AF ordered. The first type of QCP falls into the standard theory of
quantum criticality of Hertz and Millis. The Ginzburg-Landau-Wilson classical
field theory of the critical modes in classical phase transitions has been
extended to include the
quantum
nature of the fluctuations [1,2] of the spin density wave order parameter.
However, Hertz and Millis theory cannot be applied to Kondo destruction type of
quantum critical points. In this case, the relevant critical modes for the
effective field theory have not yet been identified. This is due to the strong
coupling nature of the Kondo effect (as opposed to the weak coupling nature of
the spin density wave scenario).
An important question that arises is 
whether the superconductivity
observed in some of these systems arises from the quantum critical
fluctuations. The progress and knowledge acquired in heavy fermions may be
useful for understanding non Fermi liquid behavior observed in other systems
such as transition metal oxides and organics. We have currently investigated a
similar possible QCP scenario in nearly charge ordered metals in which charge
instead of magnetic fluctuations may lead to unconventional metallic behavior
around the QCP above a low energy T* scale.
[1] J. A.
Hertz, Quantum critical phenomena, Physical Review B 14, 1165 (1976).
[2] A.
Millis, Effect of a nonzero temperature on quantum critical points in itinerant
fermion systems, Physical Review B 48, 7183 (1993).
[3] T.
Moriya, Spin-fluctuations in itinerant electron magnets, Springer-Verlag,
Berlin (1985).
[4] P.
Gegenwart, Q. Si and F. Steglich, Quantum criticality in heavy-fermion metals,
Nature Physics 4, 186 (2008).
[5] H.
Lohneysen, A. Rosch, M Vojta and P. Wölfle, Fermi liquid instabilities at
magnetic quantum phase transitions, Review of Modern Physics 60, 1015 (2007).
[6] P.
Coleman, Heavy fermions: electrons at the edge of magnetism, Handbook of
Magnetism and Advanced Magnetic Materials 1, 95 (2007).
2. Mott insulating
spin liquids and superconductivity in geometrically frustrated materials: Since
Anderson's proposal [1] of the RVB theory for describing cuprate superconductors there has been intense activity searching
for materials which can display spin liquid ground states. For such spin
liquids to exist strong Coulomb interaction and geometrical frustration are
necessary ingredients which are naturally encountered in half-filled organic
materials in which molecules display
triangular lattice arrangements. Recent experimental evidence for a spin liquid
has come from spin susceptibility and NMR measurements on the
quasi-two-dimensional Mott insulator kappa-(ET)2Cu2(CN)3 [2] which show the
absence of long range magnetic order down to very low temperatures of about 32
mK. The data has been fitted to a Heisenberg model on an anisotropic triangular
lattice through series expansions [3] and the properties of the spin liquid
sate investigated [4] through variational wavefunctions. It is yet unclear if
the spin liquid actually exists in the organic compound, the nature of the spin
liquid state and, in particular, if its excitations are gapped or not. An
important issue is the origin of the superconductivity appearing when pressure
is applied to the spin liquid insulator which remains an open issue [5].
[1] P. W.
Anderson, Resonating valence bonds: A new kind of insulator?, Materials
Research Bulletin 8, 153 (1973).
[2] Y.
Shimizu, et. al., Spin liquid state in an organic Mott insulator with a
triangular lattice, Physical Review Letters, 91, 107001 (2003).
[3] W.
Zheng, et. al., Temperature dependence of the magnetic susceptibility for
triangular antiferromagnets with anisotropic exchange constants, Physical
Review B, 59,14367 (1999).
[4] O. I.
Motrunich, Variational study of triangular lattice spin-1/2 model with ring
exchanges and spin liquid state in
\kappa-(ET)_{2}Cu_{2}(CN)_{3},
Phys. Rev. B 72, 045105 (2005)
[5]
Kurosaki et. al., Mott transition from a spin liquid to a Fermi liquid in the
Spin-frustrated compound kappa-(ET)2Cu2(CN)3, Phys. Rev. Lett. 95, 177001
(2005).
3. Pseudogap phase
in organic and cuprate superconductors: Understanding the mechanism of
high-Tc superconductivity in cuprate materials is a fundamental challenge in
condensed matter theory. The 'normal' metallic phase of these systems is highly
unconventional displaying strong deviations from Landau-Fermi liquid behavior
particularly in the underdoped regime in which a pseudogap phase with no
apparent broken symmetry occurs. The most 'anomalous' observation in this phase
is that the Fermi surface consists of disconnected arcs along the Brillouin
zone diagonals as shown below (from Kanigel et. al., Nature Physics (2006))
A pseudogap
phase has also been observed in the metallic phase of layered organic materials
which are in close proximity to a Mott insulating phase. The common existence
of a pseudogap state in the doping driven Mott insulators (cuprates) and in the
bandwidth Mott transition (organics) suggests that the pseudogap is inherent to
the properties of the Mott insulator in two-dimensional systems. We are currently
trying to understand the microscopic origin of the pseudogap phase by exploring
the evolution of the one-electron properties across the Mott metal-
insulator
transition based on Dynamical Cluster Approximation (DCA) combined with QMC
methods for the quantum impurity problem.
4. Field induced
metallic states in correlated insulators: In the last decade the response
of correlated insulators to external electric fields has been explored.
Non-linear response currents (different to Ohms law) and switching phenomena
have been recently observed in manganites [1], charge ordered organic layered
compounds [2,3], the Mott insulator VO2 [4].Correlated insulators
such as Mott insulators behave very differently from band insulators in the
presence of applied fields. The external applied electric field can control the
size of the one-electron gap directly in contrast to the situation in
conventional semiconductors. Dynamical mechanisms such as the unbinding of doublons and holons have been proposed to decribe the
metallic state induced by the external electric field. At present the understanding of these processes is
rather poor because systematic theoretical approaches which include the strong
Coulomb correlation effects as well as the steady state situation need to be
developed. Only some recent attempts for describing such phenomena have
appeared although based on mean-field approximations for the Coulomb
interaction [5]. Our aim is to understand such field induced metal-insulator
transitions in correlated insulators.
[1] A.
Asamitsu, Y. Tomioka, H. Kuwahara, and Y. Tokura, Nature 388, 50 (1997).
[2] F.
Sawano, et. al., An organic thyristor, Nature 437, 522 (2005).
[3] Y.
Takahide, T. Konoike, K. Enomoto, M. Nishimura, Phys. Rev. Lett. 96, 136602 (2006);
Phys. Rev. Lett. 98, 116602 (2007).
[4]
Hyung-Tak Kim, et. al., Jour. Appl. Phys. 107, 023702 (2010).
[5] E.
Yukawa and M. Ogata, Journal of the Physical Society of Japan 79, 023705
(2010).
TEACHING
PUBLICATIONS
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