38 research outputs found
Nonlinear screening theory of the Coulomb glass
A nonlinear screening theory is formulated to study the problem of gap
formation and its relation to glassy freezing in classical Coulomb glasses. We
find that a pseudo-gap ("plasma dip") in a single-particle density of states
begins to open already at temperatures comparable to the Coulomb energy. This
phenomenon is shown to reflect the emergence of short range correlations in a
liquid (plasma) phase, a process which occurs even in the absence of disorder.
Glassy ordering emerges when disorder is present, but this occurs only at
temperatures more then an order of magnitude lower, which is shown to follow
from nonlinear screening of the Coulomb interaction. Our result suggest that
the formation of the "plasma dip" at high temperatures is a process distinct
from the formation of the Efros-Shklovskii (ES) pseudo-gap, which in our model
emerges only within the glassy phase.Comment: 5 pages, 2 figures, accepted for publication to Phys. Rev. Let
Glassy dynamics in geometrically frustrated Coulomb liquids without disorder
We show that introducing long-range Coulomb interactions immediately lifts
the massive ground state degeneracy induced by geometric frustration for
electrons on quarter-filled triangular lattices in the classical limit.
Important consequences include the stabilization of a stripe-ordered
crystalline (global) ground state, but also the emergence of very many
low-lying metastable states with amorphous "stripe-glass" spatial structures.
Melting of the stripe order thus leads to a frustrated Coulomb liquid at
intermediate temperatures, showing remarkably slow (viscous) dynamics, with
very long relaxation times growing in Arrhenius fashion upon cooling, as
typical of strong glass formers. On shorter time scales, the system falls out
of equilibrium and displays the aging phenomena characteristic of supercooled
liquids above the glass transition. Our results show remarkable similarity with
the recent observations of charge-glass behavior in ultra-clean triangular
organic materials of the -(BEDT-TTF) family.Comment: 5 pages,4 figure
Critical role of electronic correlations in determining crystal structure of transition metal compounds
The choice that a solid system "makes" when adopting a crystal structure
(stable or metastable) is ultimately governed by the interactions between
electrons forming chemical bonds. By analyzing 6 prototypical binary
transition-metal compounds we demonstrate here that the orbitally-selective
strong -electron correlations influence dramatically the behavior of the
energy as a function of the spatial arrangements of the atoms. Remarkably, we
find that the main qualitative features of this complex behavior can be traced
back to simple electrostatics, i.e., to the fact that the strong -electron
correlations influence substantially the charge transfer mechanism, which, in
turn, controls the electrostatic interactions. This result advances our
understanding of the influence of strong correlations on the crystal structure,
opens a new avenue for extending structure prediction methodologies to strongly
correlated materials, and paves the way for predicting and studying
metastability and polymorphism in these systems.Comment: Main text: 8 pages, 4 figures, 1 table; Supplemental material: 2
pages, 1 figure, 2 table
The origin of Mooij correlations in disordered metals
Sufficiently disordered metals display systematic deviations from the
behavior predicted by semi-classical Boltzmann transport theory. Here the
scattering events from impurities or thermal excitations can no longer be
considered as additive independent processes, as asserted by Matthiessen's rule
following from this picture. In the intermediate region between the regime of
good conduction and that of insulation, one typically finds a change of sign of
the temperature coefficient of resistivity (TCR), even at elevated temperature
spanning ambient conditions, a phenomenology that was first identified by Mooij
in 1973. Traditional weak coupling approaches to identify relevant corrections
to the Boltzmann picture focused on long distance interference effects such as
"weak localization", which are especially important in low dimensions (1D, 2D)
and close to the zero temperature limit. Here we formulate a strong-coupling
approach to tackle the interplay of strong disorder and lattice deformations
(phonons) in bulk three-dimensional metals at high temperatures. We identify a
polaronic mechanism of strong disorder renormalization, which describes how a
lattice locally responds to the relevant impurity potential. This mechanism,
which quantitatively captures the Mooij regime, is physically distinct and
unrelated to Anderson localization, but realizes early seminal ideas of
Anderson himself, concerning the interplay of disorder and lattice
deformations
Emergent Bloch excitations in Mott matter
We develop a unified theoretical picture for excitations in Mott systems, portraying both the heavy quasiparticle excitations and the Hubbard bands as features of an emergent Fermi liquid state formed in an extended Hilbert space, which is nonperturbatively connected to the physical system. This observation sheds light on the fact that even the incoherent excitations in strongly correlated matter often display a well-defined Bloch character, with pronounced momentum dispersion. Furthermore, it indicates that the Mott point can be viewed as a topological transition, where the number of distinct dispersing bands displays a sudden change at the critical point. Our results, obtained from an appropriate variational principle, display also remarkable quantitative accuracy. This opens an exciting avenue for fast realistic modeling of strongly correlated materials
Universality of modulation length (and time) exponents
We study systems with a crossover parameter lambda, such as the temperature
T, which has a threshold value lambda* across which the correlation function
changes from exhibiting fixed wavelength (or time period) modulations to
continuously varying modulation lengths (or times). We report on a new
exponent, nuL, characterizing the universal nature of this crossover. These
exponents, similar to standard correlation length exponents, are obtained from
motion of the poles of the momentum (or frequency) space correlation functions
in the complex k-plane (or omega-plane) as the parameter lambda is varied. Near
the crossover, the characteristic modulation wave-vector KR on the variable
modulation length "phase" is related to that on the fixed modulation length
side, q via |KR-q|\propto|T-T*|^{nuL}. We find, in general, that nuL=1/2. In
some special instances, nuL may attain other rational values. We extend this
result to general problems in which the eigenvalue of an operator or a pole
characterizing general response functions may attain a constant real (or
imaginary) part beyond a particular threshold value, lambda*. We discuss
extensions of this result to multiple other arenas. These include the ANNNI
model. By extending our considerations, we comment on relations pertaining not
only to the modulation lengths (or times) but also to the standard correlation
lengths (or times). We introduce the notion of a Josephson timescale. We
comment on the presence of "chaotic" modulations in "soft-spin" and other
systems. These relate to glass type features. We discuss applications to Fermi
systems - with particular application to metal to band insulator transitions,
change of Fermi surface topology, divergent effective masses, Dirac systems,
and topological insulators. Both regular periodic and glassy (and spatially
chaotic behavior) may be found in strongly correlated electronic systems.Comment: 22 pages, 15 figure