Overview

CURRENT RESEARCH FOCI

Quantum Materials

Ultrafast Phenomena

Ferroelectrics, Piezoelectrics, Multiferroics

Symmetry & its Applications

Semiconductor Fibers

 

Our primary experimental tools are linear and nonlinear optics, imaging, ultrafast optics, synchrotron X-ray techniques, scanning probe and transmission electron microscopies.  We closely collaborate with synthesis and theory groups around the world.

Brief motivation for each area is given below.  Some research vignettes are given in the RESEARCH tab.  Publications  in each area are listed in the PUBLICATIONS tab.  Latest publications can be accessed from the GOOGLE SCHOLAR link.

 

Quantum Materials

Goal:

To discover and probe new materials for quantum optics and quantum communications.

To predict, grow, characterize and demonstrate new nonlinear optical crystals for creating entangled photon states.

To probe quantum phenomena involving strong electron correlation, strong spin-orbit coupling, novel forms of magnetism, and non-trivial topology.

The First Quantum Revolution

Twentieth century was defined by the development of quantum mechanics and highly pure single crystal silicon that ushered in the modern era of electronics.

Simultaneously, the development of lasers (based on the quantum mechanical phenomenon of stimulated emission) and low loss optical fibers ushered in the era of optical communications and the age of Internet.

On the Threshold of the Second Quantum Revolution

In the 21st century, we are now entering an era of quantum supremacy, where quantum computers can perform computations that are beyond the reach of even the fastest supercomputers today, as recently demonstrated by Google’s Sycamore processor.  This could revolutionize the discovery of new drugs by solving quantum mechanical design problems that cannot be solved today.  It could revolutionize materials discovery for targeted applications, solve complex optimization problems, and accelerate artificial intelligence.

At the same time, quantum entanglement, that “spooky action at a distance” as Einstein called it, is now being used to transmit information through quantum teleportation.  Secure internet based on quantum cryptography is being envisioned.

Two key characteristics that new Quantum devices exploit

(1) Coherent Superposition: The classical bits make use of binary electron or photon states, |0> and |1>, which are either in the off or the on states of a switch.  The new quantum bits, or Qubits, can be in both |0> and |1> states, and can access all the infinite real numbers between 0 and 1 by creating a coherent superposition of the two states using lasers and microwave beams, e.g. sinφ|0>+cosφ|1>.  A quantum computer with such a qubit can compute a large number of potential outcomes between |0> and |1> simultaneously by varying the relative phase, φ, between them.

(2) Entanglement: Two qubits can be put a special entangled state, e.g. |0>|1>+|1>|0>.  In this state, changing one will instantaneously change the state of the other, irrespective of how far separated in space the two are.  This is what Einstein called “spooky action at a distance”. 

New Materials for Entangled Photons

We are predicting, growing, characterizing and device testing new nonlinear optical crystals that can entangle photons for quantum optics.  Discovering new nonlinear optical (NLO) crystals with large nonlinear optical coefficient, d, has been a 60-70 year challenge.  The challenge arises because it should be in the transparent optical range of the crystal where optical absorption is very low, but then d is also low typically.  Further, one requires phase-matching for an efficient entanglement, a process in which different colors of light involved in the process all travel with the same phase velocity within the crystal.  Finally high quality crystals with large optical damage threshold are needed to be grown.  As a consequence, only  a handful of nonlinear optical crystals are commercially useful today, such as LiNbO3, LiTaO3 and β-BaB2O4 (BBO).

This effort is in collaboration with Mao group in Physics (Penn State) who are experts in the crystal growth of quantum materials.  We are also collaborating with Kanatzidis group at Northwestern University on new crystals for infrared nonlinear optics. Finally, we are working with Geoffroy Hautier‘s group at Dartmouth and Gian-Marco Rignanese  group at UC Louvain to predict new nonlinear optical crystals using material genomics and machine learning.

Electronic Materials with Strong Interactions

Many body effects refers to the cases when independent electron approximation fails- in other words, there is only one electron to be considered moving in a perioidc lattice, and the effect of all other electrons is smeared into an effective field that this one electron feels.  Elements in the Groups IA and IIA (valence s-orbitals), and Groups IIIA-VIIIA (valence p-orbitals) can be described by this approximation. Believe it or not, all of modern day electronics based on p-orbital semiconductors such as Si and GaAs relies on this assumption!

However, the single electron approximation fails for many compounds with elements from the middle of the periodic table (groups IB-VIIIB). These are transition and rare earth elements.  Here, electron-electron interactions can no longer be ignored.  Going from 3d to 5d, the strength of electron-electron correlation dominates in 3d and decreases in going towards 5d while the strength of the spin-orbit coupling increases in the same direction giving rise to exotic magnetic states where J-J coupling dominates.    These are called strong interactions, namely electron-electron correlations and between electron’s total angular momenta.

We are currently probing materials with strong electron correlations that exhibit metal-insulator  transitions coupled with novel magnetic phases.  These include titanates, manganites, vanadates, nickelates, ruthenates and iridates.  An example is shown below of Ca3Ru2O7 which is polar, has two magnetic transitions, a metal-insulator transition, and a reentrant metallic phase at lower temperatures.  There is even suspicion that further hidden orders exist.We are also getting interested in superconductivity, though the efforts are just beginning in collaboration with Engel-Herbert group.

Ca3Ru2O7 is a Ruddlesden-Popper layered oxide structure (left), exhibiting polar metallicty, pseudogap (“insulating”) phase,  a paramagnetic (PM) and two antiferromagnetic (AFMa and AFMb) phases and coexisting polar and magnetic phases (multiferroicity).  Optical second harmonic generation (right panel) shows polar crystal structure at all temperatures.

 

ULTRAFAST PHENOMENA

Goal:

To use the power of modern ultrafast lasers and X-ray sources on femtosecond to picosecond time scales to probe emergent non-equillibrium phenomena in materials.

Why Ultrafast?

As the side bar illustrates, different components of a material (think of them as dancers) respond resonantly to light at different frequencies of light (think of it as music with different speeds):  electrons on femtoseconds (PetaHertz), phonons on picoseconds (order of 10-100 TeraHertz), spins on 10’s of picosecond (0.1-10 Terahertz) and so on.

Why Non-Equillibrium Phenomena?

Think of it this way:  An apple is an apple in equilibrium, where the atomic arrangement is in a local minimum energy configuration, so called the ground state.  But what if the atoms could be moved away from equilibrium in just the right way so that it is no longer an apple – perhaps it turns into an orange !   Or vice-versa? :)

This is the idea of using ultrafast lasers to perturb the electrons, lattice, spins and free carriers in such a way as to create something new that does not exist in equilibrium state.  Indeed some are some recent exciting examples of using light to create new states:  For example transient superconductivity at room temperature, light induced charge-density wave, A non-trivial topological phase in a Weyl semimetal, and light-induced switching between an antiferromagnetic insulator to a ferromagnetic metalStoica in our group has recently demonstrated the creation of an optical supercrystal using ultrafast light, one of the rare examples where the induced new order by light remains after the light is gone.

This work was performed by Stoica working closely with John Freeland and Haidan Wen at Argonne National Lab, Lane Martin and R. Ramesh in Berkeley on the oxide heterostructure synthesis, and Long-Qing Chen‘s group at Penn State on phase-field Modeling

Our team has recently (2020) been awarded a series of LCLS Campaign beamtimes at the Stanford Linear Accelerator (SLAC) to probe the creation and tuning of such supercrystals using ultrafast X-rays.

 

FERROELECTRICS, PIEZOELECTRICS AND MULTIFERROICS

Goal:

To discover new ferroelectrics and piezoelectrics with a large polarization, high temperature performance, and strong electro-optic and nonlinear optical applications.  To discover a strong room temperature ferroelectric ferromagnet, currently a holy grail, towards electrical control of magnetism.

 

Definitions and Applications:

Ferroelectrics are materials with a built-in electrical polarization arising from off-centered center of mass of the cations versus anions;  this polarization breaks inversion symmetry (or makes it acentric) and it can be reversed by an external field, to give rise to what is called a Polarization-electric-field (P-E) hysteresis loop.  Piezoelectrics respond to an external stress and generate a voltage, or conversely, strain under an applied voltage. All ferroelectrics are piezoelectrics, and both have numerous applications such as shown below.  Penn State has had historical leadership in the discover of today’s commercial piezoelectrics such as PMN-PT and others.

Multiferroics, as colloquially understood today are materials that are both ferroelectric and magnetic (though, ferroelastic, ferrodistortive, ferrotorridic orders are also historically included). The central idea is to be able to control spins with voltages.  Computers barely use magnetism other than in magnetic random access memory, because it costs more energy to create magnetic field and control spins. If spins could be controlled by voltages, it could revolutionize magnetic technologies in computing.  While a strong ferroelectric antiferromagnetic exists at room temperature (namely, BiFeO3), there is currently no strong room temperature ferroelectric ferromagnet, which remains a holy grail.

Discovery of New Polar Materials

Nature does not prefer acentric structures. Indeed they are rare (see below).  However, in layered oxides, oxygen octahedral rotations (which are ubiquitous) can, in combination with cation ordering, break inversion symmetry.

The number of layered complex oxides for the following compounds: n = 1 and 2 structures in Ruddlesden–Popper (RP), Dion–Jacobson (DJ) and Aurivillius (AV) families, and m = n = 1 perovskite (P) superlattices (PS). An exhaustive literature search (including those reported in the Inorganic Crystal Structure Database10) reveals that less than 3% of these compounds have been synthesized and less than 0.5% of those synthesized are acentric. New design ‘knobs’ and mechanisms proposed in layered oxides can dramatically expand the family of acentric oxides in the near future, with novel functional performance. Gopalan and Engel-Herbert, Nature Materials 2016

In the past 5 years (2015-2020), our group, in collaboration with theory ( Craig Fennie at Cornell, James Rondinelli at Northwestern, Nicole Benedek at Cornell, Turan Birol at the University of Minnesota) and synthesis (Tom Mallouk at Penn State (presently at UPenn), Roman-Engel Herbert at Penn State,   Darrell Schlom at Cornell,   Koji Fujita at Kyoto University,  Hirofumi Akamatsu at Kyushu University, Martha Greenblatt at Rutgers) groups, has discovered many new families of acentric and polar layered oxides including stannates, zirconates, titanates, niobates and others as indicated in the publications page.  The central idea in most of them is oxygen octahedral rotation enabling breaking inversion symmetry.

Another novel class of materials we have been exploring since 2016 is polar metals.  These are unusual materials which have a Fermi surface (the definition of a metal), as well as have a polar space group.  It may seem counter-intuitive since one may expect the free carriers to screen any tendency towards polar lattice distortions.  However, the origin has shown to be weak electron-phonon coupling; in essence one set of electrons give rise to conduction and another take part in a polar bonding, while the two sets of electrons interact with each other rather weakly.  The polar metals we have been pursuing are ruthenates, osmates, nickelates and artificial heterostructuresPossible exciting applications include polar metals as acentric electrodes to stabilize ultrathin ferroelectric devices, and to optimize both charge separation and a photovoltage in solar cell geometries. A number of polar metals are also topologically nontrivial.

Optical Second Harmonic Generation imaging of polar domains in a polar metal Ca3Ru2O7 under two different polarization conditions (left and middle panels). The right schematic illustrates the domains. (Lei, Nano Letters, 2018)

More recently, hybrid layered perovskites have become very important as high efficiency solar cell materials.  These are organic-inorganic hybrids, and are a large playing field to discover new ferroelectrics, which we have just begun exploring. Another interesting class of highly promising materials are all-organic metal-free 3D perovskite ferroelectrics which are as good as complex oxides in many respects. We have just started exploring this class of ferroelectrics.

SYMMETRY & ITS APPLICATIONS

Goal:

To discover new symmetries in nature and find their applications in materials design and property predictions.

Symmetry as a tool to understand nature

Symmetry is fundamental to all physical sciences, from the standard model of the universe to predicting the properties of materials.  From a Materials Science perspective, symmetry allows for a classification of all crystal structures. Neumann’s principle connects macroscopic physical properties to  its point group symmetry.  The following books on symmetry and properties are my favorites:

Antisymmetry

An antisymmetry operation switches between two different states of a trait, such as two time-states, position-states, charge-states, spin-states, chemical-species etc. Antisymmetry is also called two-color symmetry.  Spatial inversion, 1-bar , is an antisymmetry when considering only point groups.  Time reversal, 1′, is a well-known antisymmetry with applications in magnetic crystallography and in predicting magnetic properties with classical spins.  A new antisymmetry, distortion reversal, 1*,  introduced by our group enables predicting minimum energy pathways.  Another recent antisymmetry introduced by us, wedge reversion, 1-dagger, enables the classification of all physical quantities in arbitrary dimensions.

Color symmetry is illustrated with 2-color and 3-color Yin-Yang symbols. (a) If  is defined as an operation that exchanges black and white colors, then the 2-color Yin-Yang symbol is invariant under the symmetry operation , which is a 2-fold rotation followed by color swap. (b) If  is defined as an operation that cyclically permutes orange-blue-yellow colors, then the 3-color Yin-Yang symbol is invariant under the symmetry operation , which is a 3-fold rotation followed by a three-color permutation.

A recent review in the Annual Reviews of Materials Research by Hari Padmanabhan, Jason Munro, Ismaila Dabo and V. Gopalan overviews the fundamental concepts underlying antisymmetry, and the specific examples of antisymmetries mentioned above, namely, spatial inversion in point groups, time reversal, distortion reversal and wedge reversion.  The distinction between classical and quantum mechanical descriptions of time reversal is presented.  Applications of these antisymmetries in crystallography, diffraction, determining the form of property tensors, classifying distortion pathways in transition state theory, finding minimum energy pathways, diffusion, magnetic structures and properties, ferroelectric and multiferroic switching, classifying all physical quantities in arbitrary dimensions, and antisymmetry-protected topological phenomena are presented.

 

SEMICONDUCTOR FIBERS

Goal:

To develop optical fibers with semiconductors cores for all-fiber optoelectronics, namely to create, modulate and detect light without exiting an optical fiber.

Functional Fibers beyond glass:

Semiconductors define our current day digital reality.   John V. Badding group in Chemistry at Penn State pioneered the area of semiconductor optical fibers.  Using High-Pressure Chemical Vapor deposition, Badding group invented a way to deposit high purity semiconductors such as Si, Ge, and ZnSe into the nanoscale holes of photonic crystal fibers, thus forming fibers with semiconductor cores and silica cladding.

Badding (left) recently passed away unfortunately at a young age of 57, and at the pinnacle of his career where besides this invention of semiconductor fibers (middle and right panels), and semiconductor metalattices,  he also recently discovered a new field of chemistry by creating diamond-like nanothreads by compressing benzene.  We miss him dearly as a brilliant and creative scientist, collaborator, and a wonderful friend full of humanity and concern for others.

Such fibers can enable the field of all-fiber optoelectronics, where light generation, modulation, and detection can all be performed within an optical fiber without the light exiting the fiber, so called, fly-by information processing.  Over the years, we have collaborated with Badding group to demonstrate in-fiber p-i-n junctions for light modulation, high speed detection, fiber lasers,  endoscopic imaging in the infrared, and long single crystal semiconductors fibers of Si and Ge.  Fiber lasers in the mid-infrared was recently demonstrated here and here.  Work on semiconductor fibers is currently on hold after Badding’s untimely demise.