Recent Publications
Here is a sampling of my recent publications:

 "Ising
Quasiparticles
and Hidden Order in URu2Si2," P. Chandra, P. Coleman and R. Flint, Phil.
Mag. 34:3233, 38033819 (2014)
 "Hastatic
Order
in URu2Si2: Hybridization with a Twist," P. Chandra, P. Coleman and
R. Flint, PRB 91, 205103 (2015)
 "Thermodynamic
Measurements
of
Angular
Anisotropy at the Hidden Order Transition of URu2Si2,"
J. Trihn, E. Bruck, T. Siegrist, R. Flint, P. Chandra, P.
Coleman and A.P. Ramirez, PRL 117, 157201 (2016)
A New Broken Symmetry: Hidden
(Hastatic) Order in URu2Si2
The development of collective
longrange order by means of phase transitions occurs by the
spontaneous breaking of fundamental symmetries. Magnetism is a
consequence
of broken timereversal symmetry, whereas superfluidity results from
broken gauge invariance. The broken symmetry that develops
below
17.5 kelvin
in the heavyfermion compound URu2Si2 has long eluded such
identification. Here we show that the recent observation of
Ising quasiparticles
in URu2Si2 results from a spinor
order parameter that breaks double timereversal invariance, mixing
states of integer and halfinteger spin. Such "hastatic"
order
hybridizes uraniumatom conduction electrons with Ising 5f2 states
to produce Ising quasiparticles; it accounts for the large entropy
of condensation
and the
magnetic anomaly observed in torque magnetometry. Hastatic
order predicts a collosal Ising anistropy in the nonlinear
susceptbility anomaly and a
resonant,
energydependent nematicity in the tunnelling density of
states. The microscopic origin of hastatic order is identified
as a fractionalization of
threebody
boundstates into integer spin fermions and halfinteger spin bosons
and is thus an example of order parameter fractionalization. A key
prediction of
the hastatic
approach, namely the angular anisotropy of the nonlinear
susceptbitility at the hidden order transition, has been verified
experimentally and indicates that
the Ising
anisotropy is a signatory feature of the hidden order
parameter.
Order
Fractionalization
The
confluence of quantum mechanics and omplexity leads to the emergence
of rich, exotic states of quantum matter whcih demand that we expand
our ideas of
quantum
order. The twin concepts of spontaneously broken symmetry and
offdiagonal longrange order (ODLRO) are fundamental to our
understanding of phase
transitions. In electronic matter it has long been assumed
that Landau order parameters involve an even number of electron
fields, with integer spin and even charge,
that
are bosons. On the other hand, in lowdimensional magnetism,
operators are known to fractionalize so that the groundstate
excitations carry spin1/2. Motivated
by
experiment, meanfield theory and computational results, we extend
the concept of ODLRO into the time domain, proposing that in a
broken symmetry state the
order
parameter can fractionalize into halfinteger objects. Using
numerical renormalization group studies we show how such
fractionalized order can be induced
in
quantum impurity models. We then conjecture that such order
develops spontaneously in lattice quantum systems, due to positive
feedback, with predictions for
experiment.

 "Emergent
Criticality and Friedan Scaling in a 2D Frustrated
Heisenberg Antiferromagnet," P. P. Orth, P. Chandra, P. Coleman
and J. Schmalian, PRB 89, 094417 (2014) (Editor's Suggestion).
 "Emergent PowerLaw Phase in
the 2D Heisenberg Windmill Antiferromagnet: A
Computational Experiment", B. Jeevanesan, P. Chandra, P.
Coleman and P.P. Orth, PRL 115, 177201 (2015).
Sidestepping the
HohenbergMerminWagner Theorem: Finitetemperature
Criticality in a 2D Heisenberg Antiferromagnet
Computational
Experiment
confirming
the
realization of Polyakov's conjecture of algebraic order in a 2D
Heisenberg Antiferromagnet under special circumstances
A remarkable
discovery of recent years is that frustrated twodimensional
Heisenberg models can evade the HohenbergMerminWagner theorem via
the development of
longrange discrete
order driven by shortrange thermal spin fluctuations; such
discrete longrange order develops despite the persistence of a
finitespin correlation length,
leading to a finite
temperature Ising (Z2) or Potts (Z3) phase transition. This
phenomenon is wellestablished in the J1J2 Heisenberg model on the
square lattice and has
been recently
realized in ironbased superconductors; such emergent discrete
degrees of freedom occur in a range of strongly correlated
materials. Here we ask whether
an isotropic
Heisenberg spin system in twodimensions may also host a critical
phases with algebraic order and associated
BerezinskiiKosterlitzThouless transitions.
We identify and
characterize such a Heisenberg model defined on interpenetrating
honeycomb and triangular lattices with nearestneighbor
antiferromagnetic coupling.
Classically the two
sublattices decouple and "order from disorder" drives them into a
coplanar state. In the coplanar state we explicitly show that
the U(1) degrees of
freedom decouple to
form an emergent Z6 clock model with an intermediate powerlaw
phase. A novel aspect of our work is that is that we apply
Friedan's
gravitational scaling
approximately to 2D classical magnetism; this is not
just an amusing conceptual link but, with the use of Mathematica, is
a practical efficient
way to calculate
the renormalization group flows of the spin stiffnesses of a 2D
antiferromagnet without the detailed bookkeeping associated with
the
WilsonPolyakov
methodology.
Slow
(athermal)
dynamic
in a nanoscale system (usually small implies fast)!
Using a
combination of computational simulations, atomicscale resolution
imaging and phenomenological modelling, we examine the underlying
mechanism for nanodomain
restructuring
in lead zirconium titanate (PZT) nanodisks driven by electron
beams. The observed subhertz nanodomain dynamics are
identified with relaxation oscillations
where the
charging/discharging cycle time is determined by saturation of
charge traps and nanodomain wall creep. These results are
unusual in that they indicate very slow
athermal
dynamics in nanoscale systems. Though this
charging/discharging cycle here is driven by electron beams, we
believe that similar behavior could be achieved by
gating the
PZT nanodisks to make a voltagecontrolled sourcedraingate
device. Such semiconductor triodes could be important for
biological applications like medical
implants
where timescales are naturally of the order of seconds.
"Hidden Fluctuations close to a Quantum
Bicritical Point", C. Morice, P. Chandra, S.E. Rowley,
G.G. Lonzarich and S.S. Saxena, PRB 96, 245104 (2017).
"Prospects and Applications Near
Ferroelectric Quantum Phase Transitions," P. Chandra,
G.G. Lonzarich, S.E. Rowley and J.F. Scott, Reviews of
Progress
Physics 80,
112502 (2017).
Ferroelectrics near their quantum phase
transitions provide rich settings for the study of quantum
criticality; furthermore possible lowtemperature applications of
ferroelectrics include satellite memories, electrocaloric
cooling and lowloss phasedarray radar.
The emergence of
complex and fascinating states of quantum matter in the neighborhood
of zerotemperature phase transitions suggests that such quantum
phenomena
should be studied
in a variety of settings. Advanced technologies of the future
may be fabricated from materials where the cooperative behavior of
charge, spin and current
can be manipulated
at cryogenic temperatures. The propagating lattice dynamics of
displacive ferroeelectrics make them appealing for the study of
quantum critical
phenomena that is
characterized by both space and timedependent quantities. To
date, quantum criticality has been mostly studied in magnetic
systems with a goal of
exploring novel
metallic behavior and exotic superconductivity. Unlike most
magnetic cases, the ferroelectric quantum quantum critical point can
be tuned experimentally
to reside at, above
or below its upper critical dimension; this feature allows for
detailed interplay between experimental and theory using both
scaling and selfconsistent
field theory
models. Empirically the sensitivity of the ferroelectric
transition temperatures to external and to chemical pressure gives
practical access to a broad range of
temperature
behavior over several hundreds of Kelvin. Additional degrees
of freedom like charge and spin can be added and characterized
systematically. Satellite memories,
electrocaloric
cooling and lowloss phasedarray radar are among the possible
applications of lowtemperature ferroelectrics.
"Quantum Annealed Criticality" (Supplementary Material), P.
Chandra, P. Coleman, M.A. Continentino and G.G. Lonzarich, arXiv:
1805.11771.
We provide a
theoretical framework to describe compressible insulating
systems that have firstorder classical transitions and yet
display pressureinduced
quantum
criticality.
Experimentally
there exist many materials with firstorder phase transitions at
finite temperature that display quantum criticality.
Classically a strainenergy density coupling
is known to drive
firstorder transitions in compressible systems and my collaborators
and I have generalized this LarkinPikin mechanism to the quantum
case. We show that
if the T=0 system lies
above its upper critical dimension, the line of firstorder
transitions can end in a quantum annealed critical point where
zeropoint 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 latticesensitivity of these
systems.
"First Principles BulkLayer Model for
Dielectric and Piezoelectric Responses in Superlattices, J.
Bonini, J.W. Bennett, P. Chandra and K.M. Rabe, arXiv: 1809.05168.
We have extended the
firstprinciples bulklayer model, which predicts the properties
of superlattices from its bulk constituent responses to changing
electromechanical
conditions, to the prediction
of dielectric permittivity and piezoelectric response in
insulating superlattices.
In the
firstprinciples bulklayer model the superlattice structure and
polarization are determined by firstprinciples computation of the
bulk responses of the constituents to
electrical
and mechanical boundary conditions in an insulating
superlattice. In this work the model is extended to predict
functional properties, specifically dielectric, permit
tivity and
piezoelectric response. A detailed comparison between the
bulklayer model and firstprinciples calcualtions for three sets of
perovskite oxide superlattices,
PbTiO3/BaTiO3, BaTiO3/SrTiO3 and PbTiO3/SrTiO3, is presented.
The bulklayer model is shown to given an excellent first
approximation to these important functional
properties,
and to allow for the identification and investigation of additional
physics, including interface reconstruction and finitesize
effects. Technical issues in the
generation of the necessary data
for constituent compounds are addressed. These results form
the foundation for a powerful datadriven method to facilitate
discovery and
design of superlattice systems
with enhanced and tunable polarization, dielectric permittivity and
piezoelectric response.