Theory of quantum metal to superconductor transitions in highly
conducting systems.
Background:
Phase transitions occur at critical points at which some macroscopic
properties of a material change abruptly from those characteristic of
one phase to those of another. Classical phase transitions occur at a
critical temperature which separates a high temperature phase from a low
temperature one. Quantum phase transitions occur at zero temperature, as
a macroscopic parameter is varied, such as the pressure, the magnetic
field, or amount of disorder. In many cases, quantum phase transitions
are similar to classical phase transitions, although with subtle
differences such as different critical exponents. However, there is
always one fundamental difference between classical and quantum
transitions; in quantum transitions, the dynamics and the thermodynamics
are inexoriably linked, whereas classically two systems could exhibit
identical thermodynamic behaviors in the neighborhood of the transition,
but quite distinct dynamical behavior. For this reason, quantum phase
transitions in metallic systems are quite different from classical ones,
and remain mysterious and incompletely understood. The specific
dynamical character of a metal, which is responsible for the existence
of a finite conductivity in the zero temperature limit, impresses itself
on all aspects of the quantum physics. A broad array of the most
intensively studied problems in condensed matter physics are related to
this problem.
Another important issue in phase transitions is the role of ¿quenched
disorder.¿ For instance, for a classical transition, one might imagine
that the transition would ¿broadened,¿ in the sense that it occurs at
one T_c in one part of the system and a different temperature elsewhere.
However, for classical critical phenomena, the emergence of a long
correlation length in the vicinity of the critical point assures that
local fluctuations of the properties of the system are averaged out,
leading to a sharp, more or less homogeneous transition. In quantum
systems, however, and especially in metals, the long range quantum
coherence of the system calls this general line of analysis into question.
Results:
We have undertaken a systematic study of the zero temperature quantum
phase transition from a superconducting to a metallic state, under a
broad range of conditions. We have found that a generic characteristic
of this transition is that the small inhomogeneities of the material are
amplified in the vicinity of the critical point, so that the system
inevitably resembles an array rare ¿puddles,¿ which are locally
superconducting, weakly coupled to each other through large regions of
intervening normal metal. This leads to an anomalously large regime in
the phase diagram, especially at low but non-zero temperatures, in which
the system is neither a superconductor nor a normal metal, but exhibits
new, highly quantum behavior which interpolates between the two. It
leads to the existence of a superconductor to metal transition in
situations in which it was believed that only a superconductor to
insulator transition was possible. Finally, in the case of a d-wave
superconductor, we find that near the superconductor to metal
transition, there occurs at lest one additional new phase, an
inhomogeneous superconducting phase with global s-wave symmetry. This
result has important implications for the behavior of overdoped cuprate
high temperature superconductors near the point at which
superconductivity is quenched.
***
LQ11850
We describe a new phenomenon concerned with the water response to a
temperature gradient. We show that water molecules reorient
their dipole moment along the direction of the gradient. The
polarization of water can be efficiently tuned by varying the
temperature
gradient strength.
Temperature gradients are omnipresent. They can be exploited to design
smart materials that generate electricity from waste heat.
Seebeck observed such phenomenon in the 19th century by putting in
contact two dissimilar metals. Similar phenomena are
observed in solutions containing water and salt. Using non equilibrium
thermodynamics theory and computer simulations,
we have shown that pure water will "react" to a temperature gradient
by polarizing itself. This response is driven by the desire of
water to minimize the production of entropy.
Our work shows that large temperature gradients, typically 106 – 108
K/m, can strongly polarize water. These gradients can be
created in the lab by heating gold nanoparticles with lasers, and they
may occur naturally in our bodies, as a by product of the
operation of small molecular motors, P-ATPase, which plays a major
role in enabling muscular activity. These and other biological
processes occur in aqueous solutions. The effect described here should
be important to understand non equilibrium phenomena of
relevance in biology, biochemistry and biophysics.
***
156805PRA
Enhancement of photon correlation with chirped laser pulses
Photons can be produced like twins and correlated in a controllable manner, thanks to the quantum optical techniques using lasers. To what extend can we control the quantum correlation of photons? How large correlation can we produce? By exciting an atom with laser pulses that are chirped in frequencies, it is possible to produce as substantially large nonclassical correlation. The advances in femtosecond laser pulses has benefited the study of molecular dynamics. Raymond Ooi of Korea University believes that this finding opens up a new route to engineer the production of nonclassical photon pairs using shaped laser pulses.
***
EK10395
Anderson localization as a measure of structure properties
Anderson localization in disordered system is an ubiquitous phenomenon which occurs both in quantum systems such as electron and spin in disordred systems and classical systems such as optical wave and acoustical waves in random media.
This kind of localization happens in configuration space and has been studied in depth in the past five decades.
Recent years' studies in complex networks have discovered that the disorder can happen in topological space.
In a recent work published in Physical Review E, Zhu and co-workers, from National University of Singapore, proposed to use localization as a measure of structure properties. This is possible by mapping networks to large clusters, namely, the nodes and edges to atoms and bonds between them, respectively.
Zhu et al focused on the special localizations come from the connection distribution disorders in the topological structure. And due to the lack of Euclidean distance, Zhu et al extend the localization definition by using the probability distribution function of the occurring probabilities at the nodes.
Several techniques such as participation ratio, structure entropy, distribution of nearest neighbor level spacing, and wavelet analysis are employed to describe the localization properties in detail. Interestingly, Zhu et al find multi-fractal structures embedded in the ranked occurring probabilities. It is well-known that the fractal structure of a lattice can lead the self-similar properties in wave functions. Zhu et al's finding can be regarded as an evidence and quantitative measure of self-similar structures of networks.
The structure-induced localization may have potential applications in understanding the electronic and heat transfer properties of materials such as conductive polymers and carbon nanonets.
***
BR10698
Fractal worlds ruled by Fermi-Dirac quantum statistics
Neutron stars are protected against a gravitational collapse because
of Fermi-Dirac quantum statistics causing
a degeneracy pressure. The state of electrons in metals like copper is
understood by the same principle of filling
up single particle states employing the Pauli exclusion rule.
Intriguingly, this paradigm has been shattered by
the observation that the electron systems found in some heavy fermion
metals and high Tc superconductors lack
a Fermi degeneracy scale, while they show instead a scale invariant
quantum dynamics. The lack of understanding
of these fermionic quantum critical states is rooted in a deep
theoretical problem: matter formed from bosons is quite
well understood employing the analogy with classical matter following
from Feynman’s path integral, but the infamous
‘minus signs’ associated with Fermi-Dirac statistics completely block
this alley.
Krueger and Zaanen demonstrate for the first time how to reconcile
theoretically the emergence of scale invariance
with the workings of Fermi-Dirac statistics. In the alternative
‘Ceperley’ path integral the effects of fermion statistics
are encoded in a geometrical ‘nodal structure’ which is a smooth
manifold in the normal Fermi-liquid case but turns
into a fractal when the fermions become quantum critical. Employing a
wavefunction Ansatz dating back to
Feynman they present an explicit example of a such a critical state
(see Fig.) that appears to be consistent with
experimental observations.
***
BR10547
zero temperature quantum phase transition from a superconducting to a metallic state
Phase transitions occur at critical points at which some macroscopic properties of a material change abruptly from those characteristic of one phase to those of another. Classical phase transitions occur at a critical temperature which separates a high temperature phase from a low temperature one. Quantum phase transitions occur at zero temperature, as a macroscopic parameter is varied, such as the pressure, the magnetic field, or amount of disorder. In many cases, quantum phase transitions are similar to classical phase transitions, although with subtle differences such as different critical exponents. However, there is always one fundamental difference between classical and quantum transitions; in quantum transitions, the dynamics and the thermodynamics are inexoriably linked, whereas classically two systems could exhibit identical thermodynamic behaviors in the neighborhood of the transition, but quite distinct dynamical behavior.
For this reason, quantum phase transitions in metallic systems are quite different from classical ones, and remain mysterious and incompletely understood. The specific dynamical character of a metal, which is responsible for the existence of a finite conductivity in the zero temperature limit, impresses itself on all aspects of the quantum physics. A broad array of the most intensively studied problems in condensed matter physics are related to this problem.
Another important issue in phase transitions is the role of “quenched disorder.” For instance, for a classical transition, one might imagine that the transition would “broadened,” in the sense that it occurs at one Tc in one part of the system and a different temperature elsewhere. However, for classical critical phenomena, the emergence of a long correlation length in the vicinity of the critical point assures that local fluctuations of the properties of the system are averaged out, leading to a sharp, more or less homogeneous transition. In quantum systems, however, and especially in metals, the long range quantum coherence of the system calls this general line of analysis into question.
Results:
We have undertaken a systematic study of the zero temperature quantum phase transition from a superconducting to a metallic state, under a broad range of conditions. We have found that a generic characteristic of this transition is that the small inhomogeneities of the material are amplified in the vicinity of the critical point, so that the system inevitably resembles an array rare “puddles,” which are locally superconducting, weakly coupled to each other through large regions of intervening normal metal. This leads to an anomalously large regime in the phase diagram, especially at low but non-zero temperatures, in which the system is neither a superconductor nor a normal metal, but exhibits new, highly quantum behavior which interpolates between the two. It leads to the existence of a superconductor to metal transition in situations in which it was believed that only a superconductor
to insulator transition was possible. Finally, in the case of a d-wave superconductor, we find that near the superconductor to metal transition, there occurs at lest one additional new phase, an inhomogeneous superconducting phase with global s-wave symmetry. This result has important implications for the behavior of overdoped cuprate high temperature superconductors near the point at which superconductivity is quenched.
