Recent Publications
Here is a sampling of my recent publications:
Emergent Potts Order in a Coupled
Hexatic-Nematic XY Model, V. Drouin-Touchette, P.P.
Orth, P. Coleman, P. Chandra and T. C. Lubensky,
PRX 12, 011043 (2022)
We address a long-standing mystery in the liquid
crystals community, proposing that the "mystery phase" above the
nematic KT transition is one of
composite Potts order resulting from
the confinement of fractionalized vortices.
Addressing the nature of an unexpected smectic-A' phase
in liquid crystal 54COOBC films, we perform large-scale Monte
Carlo simulations of a coupled hexatic-nematic
XY model. The
resulting finite-temperature phase diagram reveals a small region
with composite Potts Z_3 order above the vortex binding
transition; this phase is
characterized by
relative hexatic-nematic ordering though both variables are
disordered. The system develops algebraic hexatic and
nematic order only at a lower
temperature. This
multi-step melting scenario agrees well with the experimental
observations of a sharp specific heat anomaly that emerges above
the onset of
hexatic positional
order. We therefore propose that the smectic-A' phase is
characterized by composite Potts order and bound-states of
fractional vortices.
Quantum Annealed Criticality: A
Scaling Description, P. Chandra, P. Coleman,
M.A. Continentino and G.G. Lonzarich, Phys. Rev. Res. 2, 043440
(2020).
We provide a theoretical
framework to describe compressible insulating systems
that have first-order classical transitions and yet display
pressure-induced
quantum
criticality.
Experimentally
there exist many materials with first-order phase transitions at
finite temperature that display quantum criticality.
Classically a strain-energy density coupling
is known to drive
first-order transitions in compressible systems and my
collaborators and I have generalized this Larkin-Pikin mechanism
to the quantum case. We show that
if the T=0 system lies
above its upper critical dimension, the line of first-order
transitions can end in a quantum annealed critical point where
zero-point fluctuations restore the
underlying criticality
of the order parameter. The possibility of quantum annealed
criticality in compressible materials, magnetic and ferroelectric,
provides new settings for the
exploration of exotic
quantum phases where a broad temperature range can be probed with
easily accesible pressures due to the lattice-sensitivity of these
systems.
Multiband
Quantum Criticality of Polar Metals, P.A. Volkov and
P. Chandra, Phys. Rev. Lett. 124, 237601
(2020).
Here we
demonstrate that multiband metals near inversion-symmetry breaking
(polar) QCPs provide rich new platforms for the exploration of
strongly
correlated
physics including non-Fermi liquid phases.
Motivated
by recent experimental realizations of polar metals with broken
inversion symmetry, we explore the emergence of strong correlations
driven by criticality
when the
polar transition temperature is tuned to zero. Overcoming
previously discussed challenges, we demonstrate a robust
mechanism for coupling between the
critical mode
and electrons in multiband metals. We identify and
characterize several novel interacting phases, including non-Fermi
liquids, when band crossings are
close to the
Fermi level, and present their experimental signatures for three
generic types of band crossings.
We
provide a spectroscopic signature of spin-orbit assisted
electron-phonon coupling in disordered polar materials under
applied magnetic field.
In
metals in the vicinity of a polar transition, interactions between
electrons and soft phonon modes remain to be determined. Here
we explore the consequences of
spin-orbit
assisted electron-phonon coupling on the collective modes of such
nearly polar metals in the presence of magnetic field. We find
that the soft polar phonon
hybridizes
with spin-flip electronic excitations of the Zeeman-split bands
leading to an anticrossing. The associated energy splitting
allows for an unambiguous
determination
of the spin-orbit mediated coupling to soft modes in polar metals by
spectroscopic experiments. The approach to the polar
transition is reflected by
the softening
of the effective g-factor of the hybridized spin-flip mode.
Analyzing the static limit, we find that the polar order parameter
can be oriented by magnetic
field.
This provides possibilities for new switching protocols in polar
metallic materials. We demonstrate that the effects we predict
can be observed with current
experimental
techniques and discuss promising material candidates.
Superconductivity from Energy
Fluctuations in Dilute Quantum Critical Polar Metals,
P.A. Volkov, P. Chandra and P. Coleman,
arXiv:2106.11295
(2021).
We study a mechanism
for superconductivity in dilute quantum critical polar metals
where the electron pairing is mediated by energy fluctuations.
Superconductivity in
low carrier density metals challenges the conventional
electron-phonon theory due to the absence of retardation required to
overcome
Coulomb repulsion. Here we
demonstrate that pairing mediated by energy fluctuations,
ubiquitously present close to continuous phase transitions, occurs
in
dilute quantum critical
polar metals and results in a dome-like dependence of the
superconducting Tc on carrier density, characteristic of
non-BCS
superconductors. In
quantum critical polar metals the Coulomb repulsion is heavily
screened, while the critical transverse interactions emerge from the
energy
fluctuations of the critical
phonons, resembling the gravitational interactions of a
chargeless dark matter universe. Our estimate show us that
this mechanism
may explain the critical
temperatures observed in doped SrTiO3. We provide predictions
for the enhancement of superconductivity near polar quantum
criticality in two- and
three-dimensional materials that can be used to test our theory.
Observation
of a Critical Change Mode in a Strange Metal, H.
Kobayashi, Y. Saskaguchi, H. Kitagawa, M. Oura, S. Ikeda, K.
Kuga, S. Suzuki,
S. Nakatsjui, R. Masuda,
Y. Kobayashi, M. Seto, Y. Yoda, K. Tamasaku, Y. Komijani, P.
Chandra and P. Coleman, arXiv:2202:12464 (2022).
We report the first direct
observation of slow critical charge fluctuations in a strange
metal using synchrotron Mossbauer spectroscopy.
Quantum electronic
matter has long been understood in terms of two limiting behaviors
of electrons: one of delocalized metallic states, and the
other of
localized magnetic
states. Understanding the strange metallic behavior which
develops at the brink of localization demands new probes of the
underlying
electronic charge
dynamics. Using a state-of-the-art technique,
synchrotron-radiation-based Mossbauer spectroscopy, we have
studied teh longitundinal
charge fluctuations of the
strange metal phase of beta-YbAlB4 as a function of temperature
and pressure. We find that the usual single absorption peak
in the Fermi liquid regime
splits into two peaks upon entering the critical regime.
This spectrum is naturally interpreted as a single nuclear
transition,
modulated by nearby
electronic valence fluctuations, whose long-time scales are
further enhanced due to the formation of charged polarons.
Our results
represent a direct
observation of critical charge fluctuations as a new signature of
strange metals.