Recent Publications

Here is a sampling of my recent publications:

          A New Broken Symmetry:  Hidden (Hastatic) Order in URu2Si2
          The development of collective long-range order by means of phase transitions occurs by the spontaneous breaking of fundamental symmetries.  Magnetism is a
          consequence of broken time-reversal symmetry, whereas superfluidity results from broken gauge invariance.  The broken symmetry that develops below
          17.5 kelvin in the heavy-fermion 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 time-reversal invariance, mixing states of integer and half-integer spin.  Such "hastatic"
          order hybridizes uranium-atom 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, energy-dependent nematicity in the tunnelling density  of states.  The microscopic origin of hastatic order is identified as a fractionalization of
          three-body bound-states into integer spin fermions and half-integer 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 off-diagonal long-range 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 low-dimensional magnetism, operators are known to fractionalize so that the ground-state excitations carry spin-1/2.  Motivated
           by experiment, mean-field 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 half-integer 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
          Sidestepping the Hohenberg-Mermin-Wagner Theorem:  Finite-temperature 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 two-dimensional Heisenberg models can evade the Hohenberg-Mermin-Wagner theorem via the development of
         long-range discrete order driven by short-range thermal spin fluctuations;  such discrete long-range order develops despite the persistence of a finite-spin correlation length,
         leading to a finite temperature Ising (Z2) or Potts (Z3) phase transition.  This phenomenon is well-established in the J1-J2 Heisenberg model on the square lattice and has
         been recently realized in iron-based 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 two-dimensions may also host a critical phases with algebraic order and associated Berezinskii-Kosterlitz-Thouless transitions. 
         We identify and characterize such a Heisenberg model defined on interpenetrating honeycomb and triangular lattices with nearest-neighbor 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 power-law 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 book-keeping associated with the
         Wilson-Polyakov methodology.

          Slow (athermal) dynamic in a nanoscale system (usually small implies fast)!

           Using a combination of computational simulations, atomic-scale 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 voltage-controlled source-drain-gate device.  Such semiconductor triodes could be important for biological applications like medical
          implants where time-scales 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 low-temperature applications of  
          ferroelectrics include satellite memories,  electrocaloric cooling and low-loss phased-array radar.

         The emergence of complex and fascinating states of quantum matter in the neighborhood of zero-temperature 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 time-dependent 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 self-consistent
         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 low-loss phased-array radar are among the possible applications of low-temperature 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 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.

        "First Principles Bulk-Layer 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 first-principles bulk-layer 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 first-principles bulk-layer model the superlattice structure and polarization are determined by first-principles 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 bulk-layer model and first-principles calcualtions for three sets of perovskite oxide superlattices,
          PbTiO3/BaTiO3, BaTiO3/SrTiO3 and PbTiO3/SrTiO3, is presented.  The bulk-layer 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 finite-size effects.  Technical issues in the
          generation of the necessary data for constituent compounds are addressed.  These results form the foundation for a powerful data-driven method to facilitate discovery and
          design of superlattice systems with enhanced and tunable polarization, dielectric permittivity and piezoelectric response.