VCU Department of Physics Colloquia: Spring 2018

Voltage Control of Nanoscale Magnetism: Application to Low Power Computing

Dr. Jayasimha Atulasimha

Department of Mechanical and Nuclear Engineering & Electrical and Computer Engineering, Virginia Commonwealth University

Friday, Mar. 16 at 4:00 pm in 701 W. Grace St., Room 2310

We have recently demonstrated strain and acoustic wave induced switching of magnetization in magnetostrictive Cobalt and Nickel nanomagnets [1-4] and magnetostrictive soft layer of a magnetic tunnel junction [5]. However, one common issue observed in all experiments involving magnetostrictive elements with lateral dimensions ~100-300 nm is the lack of reliability with which the magnetization is switched. This is possibly due to a combination of many factors: low magneto mechanical coupling in Cobalt and Nickel, thermal noise at room temperature and defects that pin the magnetization. This problem can potentially get worse when one scales to lateral dimensions less than 50 nm to compete with STT-RAM.

This talk will discuss the above experimental work with complementary modeling that accounts for defects and thermal noise and recent work on polarized neutron reflectometry to understand depth dependent magnetization rotation due to differential strain transfer [6] to access the potential extent to which such energy efficient strain switched nanomagnetic devices could scale to < 50nm lateral dimensions while being robust to switching error.

Furthermore, recent simulations show that direct voltage control of magnetic anisotropy (VCMA) in conjunction with magnetic magnetic skyrmion states [7-9] may offer a robust mechanism for switching nanomagnets. For example, inclusion of DMI can lead to an intermediate skyrmion state during the reversal of a perpendicular-MTJ from the ferromagnetic “up” to “down” state. Recent simulations (unpublished) that show that forcing such reversal through a specific (skyrmion) state could be more robust to both thermal noise and defects than precessional VCMA switching schemes will also be discussed.


The strain control of magnetism research was performed in J. Atulasimha’s group at VCU (N. D'Souza, M. Salehi Fashami, M. M. Al-Rashid) in collaboration with S. Bandyopadhyay group at VCU (A. Biswas, H. Ahmad) G. P. Carman group at UCLA (W. Y. Sun, P. Nordeen, A.C. Chavez), J. P. Wang group at U. of Minnesota (Z. Zhao, M. Jamali, D. Zhang) and B. Kirby and D. B. Gopman at NIST.

Simulations on VCMA switching of skyrmions/skyrmion mediated switching was performed in Atulasimha’s group (D. Bhattacharya, M. M. Al-Rashid).

Grants that supported work at VCU include:  NSF grants NEB2020: ECCS-1124714, CAREER: CCF-1253370, SHF: CCF-1216614, ECCS 1609303; SRC under NRI task 2203.001; VCU Quest Commercialization Grant and Virginia Microelectronics Seed Grant.


[1] N. D'Souza, M. Salehi Fashami, S. Bandyopadhyay and J. Atulasimha, Nano Letters, 16, 1069, 2016.

[2] V. Sampath, N. D’Souza, D. Bhattacharya, G. M. Atkinson, S. Bandyopadhyay, and J. Atulasimha,  

      Nano Letters, 16, 5681, 2016.

[3] M. Salehi-Fashami, M. Al-Rashid, W.Y. Sun, P. Nordeen, S. Bandyopadhyay, A.C. Chavez, G.P. Carman, J. 

     Atulasimha, Nanotechnology (letter), 27, 43LT01, 2016.

[4] A. Biswas, H. Ahmad, J. Atulasimha, S. Bandyopadhyay, Nano Letters, 17 (6), 3478, 2017.

[5] Z. Zhao, M. Jamali, N. D'Souza, D. Zhang, S. Bandyopadhyay, J. Atulasimha, J. P. Wang, Appl. Phys. Lett., 109,  

     102403, 2016.

[6] M. M. Al-Rashid, B. Kirby, J. Atulasimha, unpublished.

[7] D. Bhattacharya, M. M. Al-Rashid, J. Atulasimha, Scientific Reports, 6, 31272, 2016.

[8] D. Bhattacharya, M. M. Al-Rashid, J. Atulasimha, Nanotechnology, 28, 425201 2017

[9] D. Bhattacharya, J. Atulasimha, arXiv: 1707.07777v1

Tailoring Carbon Nanostructures for Energy Storage Applications

Dr. Bingqing Wei

Department of Mechanical Engineering, University of Delaware

Friday, Mar. 2 at 4:00 pm in 701 W. Grace St., Room 2310

Sustainable and renewable energy sources from hydropower, solar, and wind power are expected to release the heavy burdens on the current energy infrastructure and the environmental concerns. As these renewable energy sources such as solar and wind power are intermittent, reliable electrochemical energy storage systems, mainly including rechargeable batteries and electrochemical capacitors, are purposely explored to promote efficient utilization of these energy sources and are a growing challenge. The development of high energy storage devices has been one of the most important research areas in recent years and relies mostly on the successful engineering of electrode materials. Carbon nanostructures such as carbon nanotubes (CNTs) and graphene have been full of surprises since their emergence and are intensively investigated for use as electrode materials in energy storage devices. Utilizing CNTs, graphene, and their composites for various energy storage applications such as Li-ion and L-S batteries, and supercapacitors are under scrutiny because of their improved electrochemical activity, cost-effectiveness, benign environmental nature, and promising electrochemical performance. In this presentation, I will discuss our research strategies and efforts to employ carbon nanostructures for different energy storage applications including flexible and even stretchable electricity storage devices.

Nanoporous materials for electrochemical analysis in complex biological solutions

Dr. Maryanne Collinson

Department of Chemistry, Virginia Commonwealth University

Friday, Feb. 23 at 4:00 pm in 701 W. Grace St., Room 2310

Electrochemical measurements are difficult to make in complex biologic solutions because as soon as the electrode is placed in solution, irreversible biofouling takes place.  Recently we developed a means to make electrochemical measurements in such solutions by using nanoporous gold and/or platinum electrodes.  These electrodes are composed of nanometer sized pores that serve as a sieve, screening out large biomolecules but allowing small redox molecules to interact with the pristine surface (Collinson et al Analytical Chemistry 2013).  Such electrodes are ideal tools to make electrochemical measurements in complex solutions including blood and plasma.  The focus of this talk will be on the fabrication and characterization of different types of nanoporous electrodes and their ability to successfully make redox measurements in solutions containing known biofouling agents.


Tomography of the Atomic Nucleus

Dr. Simonetta Liuti

Department of Physics, University of Virginia

Friday, Feb. 16 at 4:00 pm in 701 W. Grace St., Room 2310

The history of our exploration of subatomic matter has witnessed a major breakthrough with every new probe being introduced. In the 1950’s Hofstadter and collaborators using elastic electron scattering measured for the first time the electromagnetic form factors of nucleons and nuclei and provided the first information on the nuclear spatial charge and magnetization distributions. In the late 1960’s and early 70’s, Friedman, Kendall and Taylor using Deep Inelastic Scattering of electrons off the nucleon, discovered its underlying quark structure displayed in their longitudinal momentum distributions.

 I will discuss probes at the next frontier that will allow us to access dynamically correlated distributions in both momentum and coordinate space -- the Wigner distributions -- at the femtoscale. Deeply Virtual Compton Scattering, namely a high energy lepton scattering off a nucleon target producing a high energy real photon and a small angle recoil proton, is one of such probes. I will explain how a detailed mapping of the quarks and gluons in the nucleon and nucleus in phase space, or a phase-space tomography, besides providing for the first time images of quarks and gluons spatial distributions, is essential for understanding the so far elusive nucleon mass and spin decompositions in terms of its quark and gluon components. 

String Compactifications for Particle Physics

Dr. James Gray, 

Department of Physics, Virginia Tech

Friday, Feb. 9 at 4:00 pm in 701 W. Grace St., Room 2310

I will review the current status of solutions to string theory which attempt to make contact with phenomenological particle physics. I will start with a description of the goals of, and approaches utilized in, this subfield of string theory. I will then go on to highlight both what is currently well understood and what the biggest outstanding problems are in the field. 

DNA ‘nanomapping’ using CRISPR/Cas9 as a programmable nanoparticle

Dr. Jason Reed, 

Department of Physics, Virginia Commonwealth University

Friday, Feb. 2 at 4:00 pm in 701 W. Grace St., Room 2310

I will discuss two technical advances in DNA nanotechnology and single molecule genomics: (1) a novel labeling technique (CRISPR-Cas9 nanoparticles) that enables high speed-AFM (HS-AFM) to map specific sequences in DNA directly, and (2) using DVD optics to image DNA molecules with high-speed AFM.  This new ‘nanoparticle’ method employs a relatively simple one-step labeling chemistry, and is sensitive enough to detect small structural variants (<30bp) in single DNA molecules.  The HS-AFM/CRISPR technique can be complementary to both sequencing and other physical mapping approaches; it is well suited to detecting structural genomic variations, as well as to aiding de novo sequence assembly using NGS sequencing data.  It also has many potential translational applications, including detecting causal mutations in cancer. 


Structural and Optical Properties of the MoTe2-WTe2 Alloy System

Dr. Patrick Vora, George Mason University

Friday, Jan. 26 at 4:00 pm in 701 W. Grace St., Room 2310

The structural polymorphism intrinsic to select transition metal dichalcogenides provides exciting opportunities for engineering novel devices. Of special interest are memory technologies that rely upon controlled changes in crystal phase, collectively known as phase change memories (PCMs). MoTe2 is ideal for PCMs as the ground state energy difference between the hexagonal (2H, semiconducting) and monoclinic (1T’, metallic) phases is minimal. This energy difference can be made arbitrarily small by substituting W for Mo on the metal sublattice, thus improving PCM performance. Therefore, understanding the properties of Mo1-xWxTe2 alloys across the entire compositional range is vital for the technological application of these versatile materials.

We combine Raman spectroscopy with aberration-corrected scanning transmission electron microscopy and x-ray diffraction to explore the MoTe2-WTe2 alloy system. The results of these studies enable the construction of the complete alloy phase diagram, while polarization-resolved Raman measurements provide phonon mode and symmetry assignments for all compositions. Temperature-dependent Raman measurements indicate a transition from 1T’- MoTeto a distorted orthorhombic phase (Td) below 250 K and facilitate identification of the anharmonic contributions to the optical phonon modes in bulk MoTeand Mo1-xWxTe2 alloys. We also identify a Raman-forbidden MoTemode that is activated by compositional disorder and find that the main WTe2 Raman peak is asymmetric for x<1. This asymmetry is well-fit by the phonon confinement model and allows the determination of the phonon correlation length. Our work is foundational for future studies of Mo1-xWxTe2 alloys and provides new insights into the impact of disorder in transition metal dichalcogenides.


Fall 2017 Physics Department Colloquium