Submit Feedback

Lecture Videos

Strontium titanate is a bulk insulator that becomes superconducting at remarkably low carrier densities. Even more enigmatic properties become apparent at the strontium titanate/lanthanum aluminate (STO/LAO) interface and it is important to disentangle the effects of reduced dimensionality from the poorly-understood pairing mechanism. Recent experiments measuring the surface photoemission spectrum[1] and bulk tunneling spectrum[1] have found a cross-over, as a function of carrier density, from a polaronic regime with substantial spectral weight associated with strongly coupled phonons, to a more conventional weakly coupled Fermi liquid. Interestingly, it is only the polaronic state that becomes superconducting at low temperatures, although the properties of the superconducting phase itself appear entirely conventional. We interpret these results in a simple analytical model that extends an Engelsberg-Schrieffer theory of electrons coupled to a single longitudinal optic phonon mode to include the response of the electron liquid, and in particular phonon-plasmon hybridization. We perform a Migdal-Eliashberg calculation within our model to obtain this material's unusual superconducting phase diagram.

[1] Z. Wang et al, Nat. Mater. (2016)
[2] A.G. Swartz et al, arXiv:1608.05621

In this talk, I am going to present some of the work that I have done during my PhD. In the first part I will mostly focus on building a low-temperature Andreev reflection spectroscopy probe (which is basically a simpler version of an STM). In the second part, I will briefly talk about the observation of a new phase of matter, tip-induced superconductivity (TISC), that emerges only under mesoscopic metallic point contacts on topologically non-trivial semimetals like a 3-D Dirac semimetal Cd3As2, and a Weyl semimetal, TaAs and comment on the possible mechanism that might lead to the emergence of such a surprising phase of matter. All these experiments were done using our home-built low-temperature probe.
If time permits, I will also talk about some experiments that we did using various scanning probe based microscopic techniques, e.g., piezo-response force microscopy (PFM) and ferroelectric lithography. I will show how certain “artifacts” can limit the application of PFM in the investigation of ferroelectric materials, and how, under certain circumstances, such “artifacts” can actually turn out to be useful.

[1] L. Aggarwal, A. Gaurav, G. S. Thakur, Z. Haque, A. K. Ganguli & G. Sheet, Unconventional Superconductivity at Mesoscopic Point-contacts on the 3-Dimensional Dirac Semi-metal Cd3As2.” Nature Materials 15, 32 (2016).
[2] L. Aggarwal, S. Gayen, S. Das, R. Kumar, V. Sϋß, C. Shekhar, C. Felser & G. Sheet, “Mesoscopic superconductivity and high spin-polarization coexisting at metallic point contacts on Weyl semimetal TaAs.” Nature Communications, 8, 13974 (2017).
[3] J. S. Sekhon, L. Aggarwal & G. Sheet, “Voltage induced local hysteretic phase switching in silicon.” Applied Physics Letters 104, 162908 (2014). 

The two-dimensional diffusive metal stabilized at the interface of SrTiO3 and the Mott Insulator perovskite LaTiO3[1-2] has challenged many notions related to the formation and electronic behavior of the two-dimensional electron gas (2DEG) at the well studies LaAlO3-SrTiO3 interface. Here we discuss specifically the stability of the superconducting phase[3] at LaTiO3 – SrTiO3 interface, the nature of the superconductor – normal metal quantum phase transition (T=0 limit) driven by magnetic field, significance of the field vis-à-vis the Chandrasekhar - Clogston limit for depairing, and how the transition is initiated when the extent of Coulomb interaction amongst charge carriers is modulated by electrostatic gating[4]. The nature of the superconducting condensate is highlighted in the light of the Ti - t2g orbital driven bands and their filling in the presence of a strong Rashba spin – orbit interaction (SOI). Towards the end of the talk, we will discuss the prominent effects of Rashba SOI on normal state quantum transport and how it renormalizes a Kondo-like electronic behavior in range of temperature Tc< T < 5K[5-7]. The prominence of the Ti 3d0 and Ti 3d1 correlated electron physics in these systems will be demonstrated further from our recent studies of 2DEG in ion irradiated SrTiO3 crystals[8,9].
1. Advanced Materials 22, 4448(2010). 2. Phys. Rev. B 86, 075127(2012).
3. Nature Communications, 1, 89(2010). 4. Nature Materials 12, 542(2013).
5. Phys. Rev. B (Rapid Communication) 90, 081107(2014). 6. Phys. Rev. B 90, 075133(2014). 7. Phys. Rev. B 94, 115165 (2016).
8. Phys. Rev. B 91, 205117(2015). 9. Phys. Rev. B 92, 235115 (2015).

Advancements in nanoscale engineering of oxide interfaces and heterostructures have led to discoveries of emergent phenomena and new artificial materials. Combining the strengths of reactive molecular-beam epitaxy and pulsed-laser deposition, we show that atomic layer-by-layer laser molecular-beam epitaxy (ALL-Laser MBE) significantly advances the state of the art in constructing oxide materials with atomic layer precision. Using Sr1+xTi1-xO3 as example, we demonstrate the effectiveness of the technique in producing oxide films with stoichiometric and crystalline perfection. With the growth of La5Ni4O13, a Ruddlesden-Popper phase with n = 4 that has never been reported in the literature, we demonstrate that ALL-Laser MBE allows us to push the equilibrium thermodynamic boundary further. By growing LaAl1+yO3 films of different stoichiometry on TiO2-terminated SrTiO3 substrate at high oxygen pressure, we show that the behavior of the two-dimensional electron gas at the LaAlO3/SrTiO3interface can be quantitatively explained by the electronic reconstruction mechanism. In LaNiO3 films on LaAlOsubstrate with LaAlObuffer layer, we observed the metal insulator transition in 1.5 unit cells, which is driven by oxygen vacancies in addition to epitaxial strain and reduced dimensionality.

As Interim Chair of Temple University’s Physics Department, May 20, 2015 was a normal day for me filled with work on my class, research, promotion of colleagues, and a university task force I was chairing. I had given a public lecture for “Pint of Science,” a science festival, at an Irish pub before picking up my wife at the airport, who was returning from an overseas conference trip. My elder daughter had come home a day earlier from college for a few days. We made a plan to visit a restaurant to try their famous Korean fried chicken. All of this was suddenly and forever changed a few hours later when I was awoken by the urgent pounding on my door. I was arrested by armed FBI agents and indicted by the U.S. government for sharing protected U.S. company technology with China. The indictment was dismissed in September after it had become clear that I did not share the protected technology with China. My case has raised serious concerns about international collaborations in science and technology, civil rights, and the long-term national security and economic future of the United States.

The Kagome Lattice Heisenberg Model is one of the simplest realistic spin models with a quantum spin-liquid ground state. We discuss the current status of our understanding of this well-studied model. The precise nature of the spin-liquid state and the existence of a spin-gap in the model still remain in dispute. We also discuss experimental studies of Herbertsmithite material Kagome-antiferromagnet ZnCu_3(OH)_6Cl_2. We focus on NMR measurements by Imai and collaborators, who have presented strong evidence for a spin-gap in the excitation spectra. Through a Numerical Linked Cluster (NLC) calculation of the frequency moments, we show that despite the existence of substitutional disorder in these materials, the high temperature nuclear relaxation rates are well described by the Heisenberg model.

The 2016 Nobel Prize in Physics to Kosterlitz, Thouless and Haldane honors a new set of ideas and theoretical formalism that has gradually become the mainstay of modern condensed matter thinking. Quantum Field Theory, Topology and the Renormalization Group, lie at the heart of present day theory of condensed matter. The Kosterlitz-Thouless theory of phase transitions and Haldane's conjecture on the spin dependence of spectrum of spin-chains were two of the most influential works in bringing these ideas together. These along with the Thouless-Kohmoto-Nightingale-Den Nijs-(TKNN) work on topological invariants of band structures were duly recognized by the Nobel committee. This talk will discuss how these theories defied existing paradigms and no-go theorems. And, how they continue to play a huge role in how we address quantum phases and phase transitions today. This Nobel Prize potentially sets the stage for many more prizes in the field of Topological Matter.

We provide a perspective on the recent emergence of “topological spintronics,” which relies on the existence of helical Dirac electrons in condensed matter. Spin- and angle-resolved photoemission spectroscopy shows how the spin texture of these electronic states can be engineered using quantum tunneling [1] or by breaking time-reversal symmetry [2]. Inappropriately designed systems, broken time-reversal symmetry transforms helical Dirac states into chiral edge states, a realization of Haldane’s Chern insulator phase of matter. This is characterized by a precisely quantized Hall conductance and dissipationless edge transport without a magnetic field. We show how these edge states can be quantitatively characterized by analyzing their giant anisotropic magnetoresistance [3]. At miilikelvin temperatures, the interplay between Chern states and disordered magnetism [4] results in surprising behavior, perhaps consistent with quantum tunneling out of a ‘false vacuum’ [5]. Finally, we show how these helical Dirac electrons provide a possible pathway toward a spin device technology that works at room temperature [6,7].

[1] M. Neupane, A. Richardella et al.,Nature Communications 5, 3841 (2014).
[2] S.-Y. Xu et al., Nature Physics 8, 616 (2012).
[3] A. Kandala,A. Richardella, et al.,Nature Communications 6, 7434 (2015).
[4] E. Lachman et al., Science Advances 1, e1500740(2015).
[5] Minhao Liu et al., Science Advances 2, e1600167(2016).

The field of spintronics, or magnetic electronics, is maturing and giving rise to new subfields. An important ingredient to the vitality of magnetism research in general is the large complexity due to competitions between interactions crossing many length scales and the interplay of magnetic degrees of freedom with charge (electric currents), phonon (heat), and photons (light). One perfect example, of the surprising new concepts being generated in magnetism research is the recent discovery of magnetic skyrmions. Magnetic skyrmions are topologically distinct spin textures that are stabilized by the interplay between applied magnetic fields, magnetic anisotropies, as well as symmetric and antisymmetric exchange interactions. Due to their topology magnetic skyrmions can be stable with quasi-particle like behavior, where they can be manipulated with very low electric currents. This makes them interesting for extreme low-power information technologies, where it is envisioned that data will be encoded in topological charges, instead of electronic charges as in conventional semiconducting devices. Towards the realization of this goal we demonstrated magnetic skyrmions in magnetic heterostructures stable at room temperature, which can be manipulated using spin Hall effects. Furthermore, using inhomogeneous electric charge currents allows the generation of skyrmions in a process that is remarkably similar to the droplet formation in surface-tension driven fluid flows. However, detailed micromagnetic simulations show that depending on the electric current magnitude there are at least two regimes with different skyrmion formation mechanisms. Lastly, we demonstrated that the topological charge gives rise to a transverse motion on the skyrmions, i.e., the skyrmion Hall effect, which is in analogy to the ordinary Hall effect originating from the motion of electrically charged particles in the presence of a magnetic field.

This work was supported by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division. Lithographic patterning was carried out at the Center for Nanoscale Materials, which is supported by DOE, Office of Science, BES (#DE-AC02-06CH11357).

Two-dimensional (2D) materials are molecularly thin, layered materials held together by van der Waals forces.  Because charge moves freely in the 2D plane, these materials have potential application in electronics; however, conventional doping strategies have not been developed for 2D materials.  An alternative approach is to use electrolyte gating. Under an applied gate voltage, ions in the electrolyte create an electrostatic double layer (EDL) at the interface between the electrolyte and the semiconductor; the EDL can induce sheet carrier densities on the order of 1014 cm-2 for both electrons and holes–more than one order of magnitude larger than conventional gating techniques. I will describe our work using electrolytes to dope transistors and memory devices based on graphene and transition metal dichalcogenides (TMDs). Our group has developed a 2D electrolyte for use in memory devices based on 2D crystals, and the first device characteristics will be presented. 
This work was supported in part by the Center for Low Energy Systems Technology (LEAST), one of six SRC STARnet Centers, sponsored by MARCO and DARPA, and NSF grant #ECCSGOALI-1408425.