Physics and Astronomy Colloquia

# Department of Physics & Astronomy Colloquia

## 2017-2018 Program

NB: Unless noted otherwise, Physics Colloquia are held as mentioned above.

•   Jan. 22, 2018 Dr. Maciej M. Maska, Institute of Physics, University of Silesia

• Majorana quasiparticles in nanowires: the role of disorder and electron correlations

Host: Dr. Mark Novotny

Abstract:

In 1937 Ettore Majorana predicted existence of particles, now known as Majorana fermions, identical to their own antiparticles. While Majorana fermions have remained undetected in high energy physics for 80 years, in the last decade the search for Majorana quasiparticles in condensed matter has become one of the hottest topics in physics. Since they are expected to obey unconventional exchange statistics, a lot of hopes have been pinned down on Majorana particles as building blocks of a topological quantum computer. One of the systems where these (quasi)particles were observed is a semiconductor nanowire with spin–orbit interaction coupled to s–wave superconductor. It is known that in low–dimensional systems disorder and Coulomb interactions are crucial and can drastically affect their properties. It turns out that while disorder always reduces stability of Majorana fermions in a nanowire, weak interactions can enhance it. In this talk, I will first review the idea of Majorana fermions in condensed matter, in particular in nanowires, and then discuss both these effects.

•   Nov. 20, 2017 Dr. Erin E. Peters, Department of Chemistry, University of Kentucky

• A Tale of Two Structures: Nuclear and Material

Host: Dr. Benjamin Crider

Abstract:

At the University of Kentucky Accelerator Laboratory, inelastic neutron scattering (INS) with the detection of emitted γ rays is a well-used tool for studying the structure of stable nuclei. In particular, lifetimes in the femtosecond region of excited states in a nucleus can be measured with the Doppler-shift attenuation method following inelastic neutron scattering (DSAM-INS). Several years ago, the structure of 94Zr was investigated by INS from an enriched $94ZrO$2 scattering sample. The measured level lifetimes revealed anomalous behavior unobserved in any other nucleus. These novel results were published, but later called into question. In order to confirm the anomaly, further measurements were performed, but the results differed significantly, deepening the mystery. An investigation of the scattering sample was required to reveal the source of the discrepancy; X ray diffraction, scanning electron microscopy, and even some chemistry was performed to characterize the material structure and to provide clues to the solution. Although we learned a few lessons in the process, our story still has a happy ending, as the original anomalous result had been used to motivate additional nuclear structure studies at TRIUMF laboratory in Vancouver, Canada. These results, when combined with the new Kentucky data, led to PRL-worthy nuclear physics.

•   Nov. 13, 2017 Ms. Manpreet Kaur, Sri Guru Granth Sahib World University

• Clustering effects and decay mechanisms in light mass alpha and non-alpha conjugate nuclei formed at sub-barrier and above barrier energies

Host: Dr. Lamiaa El Fassi

Abstract:

The discovery of $\alpha$-decay in the heavy nuclei prompted the idea that nucleus can be visualized as being composed of $\alpha$-particles as building block. Since then the tendency of nucleons to conglomerate within the nucleus has captured a central attention in the nuclear structure and nuclear reaction studies for last many decades. Quite interestingly, low energy reactions act as an indispensable tool to explore the cluster structure within the nucleus. Several attempts have been undertaken to study the role of cluster structure on the dynamics of reactions involving light mass $\alpha$-cluster nuclei. It makes an interesting case to look into the reaction dynamics of light mass $\alpha$-conjugate ($20Ne$, $28Si$, $40Ca$) and non-$\alpha$ conjugate nuclei ($20,21,22Ne$, $39K$), which has been explored along with clustering aspects in the present work, within the quantum mechanical fragmentation theory (QMFT) based Dynamical Cluster-decay Model (DCM). The clustering effects have been investigated in their intrinsic excited state at resonant-state energies given by the Ikeda diagram [1], by taking into account the proper temperature-dependent pairing-energy term in liquid drop energies. Also, the clustering features have been studied in $N= Z,40Ca*$ and $N # Z,39K*$ composite systems in reference to experimentally available data. The results present that clustering scenario changes with increasing temperature due to decrease in pairing strength at high energies [2,3]. Also, the clustering aspects in $20Ne$ nucleus using microscopic and macroscopic approaches of relativistic mean field theory (RMFT) and QMFT have been studied comparatively, which show remarkable agreement. These results are in line with that of density functional approach for $20Ne$ [4]. Furthermore, the emission of different IMFs/clusters has been studied in reference to available Z-distribution data at above barrier energies. The study shows the co-existence of competing reaction mechanisms (FF and DIO) in their decay. The DCM calculated cross-sections are in good comparison with experimental data [5].
The availability of radioactive ion beams due to advanced accelerator technology, provides the opportunity to study the fusion process with neutron rich nuclei. The experimental studies with neutron rich projectile show the fusion enhancement compared to the case of $\beta$-stable projectile. The experimental study of near barrier and sub-barrier fusion reaction of exotic $18O$ beam with $12C$ target shows the fusion enhancement [6]. The fusion enhancement for this reaction leading to composite system $30Si*$ has been explored through study of dynamical aspects such as potential energy surface, preformation profile of fragments and decay barrier characteristics at sub-barrier and near energies in comparison with $28Si*$ formed in $16O +12C$ reaction, within the QMFT based dynamical cluster decay model (DCM).

[1] K. Ikeda et al., Prog. Theor. Phys. Suppl. E 68,464 (1968); W. Von Oertzen et al., Phys. Rep. 432, 43 (2006).
[2] R.K. Gupta et al., Int. Rev. Phys 2, 369 (2008); Lecture Notes in Physics 818, 223 (2010).
[3] M. Kaur, B.B. Singh, S.K. Patra, R.K. Gupta, PRC 95, 014611 (2017).
[4] J. P. Ebran, E. Khan, T. Niksic, and D.Vretenar, Nature (London) 487, 341 (2012).
[5] M.M. Coimbra et al., NPA 535, 161 (1991); S. Kundu et al., PRC 78, 044601 (2008); Parmana 82, 727 (2014).
[6] T.K. Steinbach, J. Vadas, J. Schmidt, C. Haycraft, S. Hudan, R. T. deSouza et al., PRC 90, 041603(R) (2014).

•   Nov. 6, 2017 Dr. Mark Novotny, Mississippi State University

• Quantum Supremacy in 2018? Adiabatic Quantum Computers: Huge Advance or All Hype?

Host: Dr. Benjamin Crider

Abstract:

This colloquium is suitable for non-physicists. The availability of near-ideal quantum annealing machines, also known as Adiabatic Quantum Computers (AQC), with about N>50 qubits would be an extremely disruptive technology (see attached picture). A qubit is a quantum superposition of the 0 and the 1 bit at the heart of all binary technology. The ability of an ideal AQC to perform calculations impractical for any binary computer is why governments and companies (including Google) are making substantial investments in AQC. Google has as a stated goal to achieve quantum supremacy in 2018 -- - what will that mean? D-Wave produces a quantum annealing machine with N>2000 qubits. An introduction to AQC machines will be presented. Questions addressed will include whether current AQC technologies: are adiabatic? are quantum? are a computer? If AQC are not all hype, it is an impactful new tool. As with any new tool three things should be done: 1) test the current tool, 2) understand applications enabled by the availability of the current tool and future advanced tools, 3) work to improve next generations of the tool. All three will be touched on in this lecture, including tests and applications of the D-Wave 2000Q with N>2000 qubits.

•   Oct. 30, 2017** Mr. Prajwal Mohanmurthy, Massachusetts Institute of Technology

• NStar: Searching for Mirror Neutron - Neutron Oscillations

Host: Dr. Dipangkar Dutta

Abstract:

The corner stone of standard model of particle physics is the Lorentz symmetry (a special result of which is Einstein's special theory of relativity). It was shown by G. L$ü$ders and Pauli that Lorentz symmetry translates to the join conservation of the three discrete symmetries of Charge inversion, Parity inversion and Time inversion [1, 2]. This equivalence is known as CPT theorem. Weak nuclear force mediated neutral Kaon (particle) decay (to 2 $\pi 0$ or to 3 $\pi 0$) showed that CP symmetry is violated [3]. Violation of CP symmetry is allowed by the CPT theorem, if T-symmetry is also violated. But to date no CP or T-symmetry violation has been observed in any strong force mediated process. This is known as the Strong-CP problem [4]. It was pointed by Ref. [5] that introduction of a mirror realm (which does not interact with our real realm) could solve the Strong-CP problem and that neutral particles such as neutrons may spontaneously oscillate to their mirror universe counterpart ($n ↔ n*$) [6]. Consequently, two separate groups performed their experiments in search of such neutron - mirror neutron oscillations and reported having found no evidence of such oscillations [7, 8]. This in-turn set limits on the oscillation time, $\tau$nn* > 414 s. Soon after, Ref. [9] pointed out inconsistencies in the results obtained by these two experiments. Furthermore, Ref. [9] showed that when the results of these two experiments are combined, the inconsistencies can be explained by introducing a mirror neutron oscillation in presence of a magnetic field in the mirror realm. Indeed, the two prior experiments had assumed the absence of any magnetic fields in the mirror realm and only considered applied real magnetic fields. Therefore we need a new experiment to verify or exclude these spurious results.

[1] G. L$ü$ders, Det. Kong. Danske Videnskabernes Selskab, Mat.-fys. Medd., 28, No. 5 (1954).
[2] W. Pauli, Niels Bohr and the Development of Physics, McGraw-Hill, New York (1955): 30-51.
[3] J. H. Christenson, J. W. Cronin, V. L. Fitch, and R. Turlay, Phys. Rev. Lett. 13 (1964): 138.
[4] Mannel, Thomas, Theory and Phenomenology of CP Violation, Nuclear Physics B, 167 (2006): 170174.
[5] Z. Berezhiani, L. Gianfagna, M. Giannotti, Strong CP problem and mirror world: the Weinberg Wilczek axion revisited, Nuclear Physics B, Vol. 500, Issue 34, 22 (2001): 286-296.
[6] Z. Berezhiani and L. Bento, NeutronMirror-Neutron Oscillations: How Fast Might They Be?, Phys. Rev. Lett. 96 (2006):081801.
[7] G. Ban et al., Phys. Rev. Lett. 99 (2007): 161603.
[8] A.P. Serebrov et al., Phys. Lett. B 663 (2008): 181.
[9] Z. Berezhiani, More about neutron - mirror neutron oscillation, Eur. Phys. J. C 64 (2009): 421-431.

** Note the special colloqium time, 3:40 PM!

• Oct. 23, 2017** Dr. Dipangkar Dutta, Mississippi State University

• Electrons with a twist: a new tool for nuclear physics

Host: Dr. Lamiaa El Fassi

Abstract:

The recent demonstration of electron beams carrying quantized orbital angular momentum (OAM), also known as twisted or vortex electron beams, provides a new and unexplored degree of freedom for use in nuclear and particle physics. For example, it could be used to probe fundamental questions about the origin of the proton's spin, such as, the contribution due to the orbital angular momentum of quarks and gluons in the proton. We will discuss how vortex electrons carry OAM and how they are generated, and possible scattering observables to monitor their twistedness? We will also discuss efforts underway at Jefferson Lab (JLab) to develop a new vortex electron sources in order to explore the use of Mott scattering to monitor its twistedness as well as verify the OAM preserving acceleration of the vortex electrons. If successful it could eventually lead to high energy electron beams carrying quantized OAM and open up a new frontier in nuclear physics.

** Note the special colloqium time, 3:40 PM!

• Sept. 20, 2017* Dr. Mina Yoon, Oak Ridge National Laboratory jointly with University of Tennessee

• First-principles Materials by Design for Thermodynamically Stable Low-dimensional Electrides

Host: Dr. Seong-Gon Kim

Abstract:

Two-dimensional (2D) electrides, emerging as a new type of layered material whose electrons are confined in interlayer spaces instead of at atomic proximities, are receiving interest for their high performance in various (opto)electronics and catalytic applications. A realization of electrides containing anionic electrons has been a great challenge because of their thermodynamic stability. For example, experimentally, only a couple of layered nitrides and carbides have been identified as 2D electrides. We developed a materials by design scheme and applied it to the computational exploration of new low-dimensional electrides. Our approach here offers an important alternative that overcomes the current limitation on discovery of new 2D inorganic electrides. By combining the global structure optimization method and first-principles calculations, we identified new thermodynamically stable electrides that are experimentally accessible. Most remarkably, we, for the first time, reveal an effective design rule for 2D electrides [1]. We then discover another new class of electrides based on 1D building blocks by coupling materials database searches and first-principlescalculations-based analysis. This new class of electrides, composed of 1D nanorod building blocks, has crystal structures that mimic $\beta -TiCl$3 with the position of anions and cations exchanged. Unlike the weakly coupled nanorods of $\beta -TiCl$3, $Cs$3O and $Ba$3N retain 1D anionic electrons along the hollow inter-rod sites; additionally, strong inter-rod interaction in $C$3O and $Ba$3N induces band inversion in a 2D superatomic triangular lattice, resulting in Dirac nodal lines [2]. Our work [1, 2] represents an important scientific advancement over previous knowledge of realizing electrides in terms of both materials and design principles, and should interest the communities of catalytic chemistry, surface physics, and structural chemistry, as well as the related engineering disciplines.

[1] First-Principles Prediction of Themodynamically Stable Two-Dimensional Electrides, W. Ming, M. Yoon, M.-H. Du, F. Liu, K. Lee, and S. W. Kim, J. Am. Chem. Soc. 138, 15336 (2016).
[2]. New electrides based on one-dimensional building blocks, Changwon Park, Sung Wng Kim, Mina Yoon (2017, submitted to Phys. Rev. Lett.).

* Note the special colloqium date, Wednesday afternoon!

### 2017-2018 Committee

Lamiaa El Fassi (Chair) (325-0627, le334@msstate.edu email)
Benjamin Crider (325-4017, bpc135@msstate.edu email)
Ariunbold Gombojav (325-2927, ag2372@msstate.edu email)
Jinwu Ye (325-2926, jy306@msstate.edu email)
Secretary: Susan Galloway (325-2806, srg133@msstate.edu email)

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