August 11, 2017. 10:00AM. BPB-217. Ye Li. Understanding Progenitors of Gamma-Ray Bursts with Multi-wavelength Properties.

Ph.D. Defense

Gamma-ray bursts (GRBs) are the most luminous explosions in the universe. They are generally classified into two types according to the durations of their Γ-ray emission. Progenitors of short-duration GRBs (SGRBs) are believed to be compact stars binaries (e.g., neutron stars and black holes; also known as Type I GRBs by their physical origin). For long-duration GRBs (LGRBs) they are thought to be originated from core-collapse of massive stars (Type II). However, the duration criterion is not always reliable. In this dissertation, I propose a multi-parameter method based on multi-wavelength data of GRBs to classify them by their physical nature, i.e., Type I or Type II. With a similar method, we search for SGRB-less X-ray transients with neutron star binaries as progenitors. The multi-wavelength studies of GRBs also allow for a study of the evolution of GRB population through the cosmic time.

July 10, 2017. 10:00AM. BPB-217. Jason Baker. Instrumentation and Measurement of Thermoelectric and Structural Properties of Binary Chalcogenides and Half-Heusler Alloys at Extreme Conditions Using a Paris-Edinburgh Press.

Ph.D. Defense

Understanding the high-pressure behavior of transport properties has been a driving force in the study of materials under extreme conditions for well over a century being pioneered by P.W. Bridgman in the early 20th century. Research dedicated to the study of these properties leads to a variety of important applications: exploration of insulator to semi-conductor to metal structural and electronic phase transitions, correlation of structural phase transitions and the electronic properties along phase boundaries, testing validity of theoretical models, understanding the effects of chemical pressure, among a slew of other applications. This work has designed and developed a specialized sample cell assembly for use with a Paris-Edinburgh press capable of performing high-pressure and high-temperature (HP-HT) electrical resistance, Seebeck coefficient, thermal conductivity measurements alongside energy-dispersive X-ray diffraction and X-ray radiography imagining up to 6 GPa and 500°C to fully characterize the electrical, thermal, and structural properties of materials simultaneously at extreme conditions. This system has been installed at Argonne National Laboratory at the Advanced Photon Source at the Sector 16 BM-B beamline of the High-Pressure Collaborative Access Team and is now available to general users as a measurement technique. Application of this system has been applied to thermoelectric materials: PbTe, SnTe, TiCoSb, and TiNiSn. Thermoelectric materials provide a valuable means of converting waste heat into useful electrical energy and studying their HP-HT properties allows a better understanding and identification of greater efficiency through tuning of transport properties. The detailed discussion of the design and development of this system alongside the important results on the thermoelectric materials mentioned will be presented in this dissertation.

March 30, 2017. 4:30PM. BPB-102. Prof. Isaac F. Silvera on Metalic Hydrogen

Hydrogen is the simplest and most abundant element in the Universe. Over 80 years ago Wigner and Huntington predicted that if solid molecular hydrogen was sufficiently compressed in the T=0 K limit, molecules would dissociate to form atomic metallic hydrogen. We have observed this transition at a pressure of 4.95 megabars. MH in this form has probably never existed on Earth or in the Universe; it may be a room temperature superconductor and is predicted to be metastable. Hydrogen makes up ~90% of the planet Jupiter. It is believed to occur as a layer of liquid metallic hydrogen surrounding Jupiter's core, responsible for Jupiter's magnetic field, with molecular hydrogen as the outermost layer. Descending through the atmosphere, a phase transition from liquid molecular to liquid atomic metallic hydrogen occurs as the pressure and temperature increase. This first-order phase transition to liquid metallic hydrogen is at intermediate pressures (~1-2 megabars) and temperatures of ~1000-2000 K. We have also observed this liquid-liquid transition, known as the plasma phase transition. We shall discuss the methods used to observe these phases of hydrogen at extreme conditions of static pressure in the laboratory, extending our understanding of the phase diagram of the simplest atom in the periodic table.

Prof. Isaac F. Silvera is the Thomas D. Cabot Professor of the Natural Sciences at the Lyman Laboratory of Physics at Harvard University

Prof. Lepp delivered the University Forum lecture: Supernova 1987A: Thirty Years Later

University Forum lecture looks back 30 years to the first time anyone since the days of Queen Elizabeth I was able to see a supernova with the naked eye.^*

Stephen H. Lepp is the chairman of UNLV's department of physics and astronomy. At 7:30 p.m. March 23, he will present "Supernova 1987A: Thirty Years Later" at the Marjorie Barrick Museum as part of the University Forum lecture series. Here, Lepp discusses what we learned from the first supernova visible to the naked eye in 400 years.

On Feb. 24, 1987, a supernova was discovered in the Large Magellanic Cloud. A supernova is an explosive end to a massive star. This supernova was bright enough to see with the naked eye, the first such in nearly 400 years and the first since the invention of the telescope. As such, this was the first bright supernova to be observed with modern scientific instruments. It was the first from which neutrinos were detected, the first in which molecules have been detected and the first where we have observations of the star before it blew up.

Stars begin their life in dense molecular clouds. These clouds, which exist between stars in our galaxy are basically star forming factories. Parts of the cloud can collapse to form dense objects which eventually burn hydrogen up to helium in the process of nuclear fusion. This is the most common configuration for stars, and almost all the stars you see in the night sky are undergoing this burning of hydrogen to helium.

Most of these stars are not massive enough to get temperatures high enough to burn anything beyond hydrogen, but the most massive stars will undergo a series of nuclear reactions burning elements all they way up to iron. Iron is the most tightly bound nuclei and so you cannot produce energy by converting iron to higher mass elements. The massive stars can undergo a processes called core collapse and the energy released out shines for a short time the entire galaxy. These explosions look like new stars and so were called nova or supernova.

In this talk we will cover the history of supernova observations and some interesting results from Supernova 1987A, along with some modern supernova research.

^{* From UNLV News Center}

Prof. Bing Zhang co-organized an Aspen meeting on Fast Radio Bursts

Prof. Zhang recently organized an Aspen Center for Physics conference "Fast Radio Bursts: New Probes of Fundamental Physics and Cosmology" on Feb.12-17, 2017. The conference hosted about 80 scientists from around the world to discuss the nature of Fast Radio Bursts, mysterious radio bursts discovered 10 years ago. The event was recently reported in the journals: Nature and Scientific American.

Prof. Zhaohuan Zhu Awarded Sloan Research Fellowship

Zhaohuan Zhu is the first UNLV scientist to earn fellowship for early career scholars considered ‘next generation of scientific leaders.’

From UNLV News by Shane Bevell.

Zhaohuan Zhu, assistant professor in the department of physics and astronomy, has been named a 2017 Sloan Research Fellow. He is one of 126 researchers from 60 colleges and universities in the U.S. and Canada – and the first UNLV scientist – to be awarded the prestigious fellowship.

Awarded annually since 1955 by the Alfred P. Sloan Foundation, the fellowships honor early-career scholars whose achievements mark them as the next generation of scientific leaders.

Zhu’s research focuses on the origin of Earth and other planets. By using powerful supercomputers and sophisticated numerical codes, he simulates how Earth formed billions of years ago. These computer simulations can be compared with the latest observations of young forming stars and planets to reveal the secrets of how planets are born around young stars.

“The Sloan Research Fellows are the rising stars of the academic community,” said Paul L. Joskow, President of the Alfred P. Sloan Foundation. “Through their achievements and ambition, these young scholars are transforming their fields and opening up entirely new research horizons. We are proud to support them at this crucial stage of their careers.”

Zhu received his M.S. and Ph.D. from the University of Michigan and was a postdoctoral researcher and Hubble Fellow at Princeton University before coming to UNLV last summer.

He was attracted to UNLV by its Top Tier initiative as well as the brilliant minds in the physics and astronomy department. “The research environment here is extremely active and stimulating,” Zhu said. “Additionally, my research expertise fits well in the department, and I can collaborate with almost every faculty member in the astronomy program. Another factor that attracted me here is the powerful supercomputer at the UNLV National Supercomputing Institute. My research relies on computing resources, and not every university has such powerful computers.”

The most interesting thing about the field, Zhu said, is that only recently have telescopes finally allowed researchers to image young forming planets in distant stellar systems. “The field progresses so fast and exciting new discoveries are made all the time, which I couldn’t imagine as a graduate student. Seeing planet formation in action is like seeing the birth of our Earth billions of years ago.”

Fellows receive $60,000 to further their research. The award is open to scholars in eight scientific and technical fields—chemistry, computer science, economics, mathematics, computational and evolutionary molecular biology, neuroscience, ocean sciences, and physics. Candidates must be nominated by their fellow scientists and winning fellows are selected by independent panels of senior scholars on the basis of each candidate’s independent research accomplishments, creativity, and potential to become a leader in his or her field.

“I am extremely honored to receive the Sloan Fellowship,” Zhu said. “I hope that this award will attract great students and scientists to our department and UNLV. With the support of the fellowship, I am looking forward to a productive future.”

Past Sloan Research Fellows include many towering scientific figures, including physicists Richard Feynman and Murray Gell-Mann, and game theorist John Nash. Forty-three former fellows have received a Nobel Prize in their respective field, 16 have won the Fields Medal in mathematics, 69 have received the National Medal of Science, and 16 have won the John Bates Clark Medal in economics, including every winner since 2007.

Spring 2017 Russell Frank Astronomy Lecture Announced

This semester's Russell Frank Astronomy Lecture will take place 7:30PM, Thursday March 9, 2017 in the Bigelow Physics Building room 102 at UNLV.

Professor Peter Mészáros from Penn State University will present: The Highest Energy Cosmic Rays: Messengers from the Deep Universe.

The highest energy cosmic rays are single atoms with the energy of a pistol bullet, hitting the Earth from outer space at the rate of one per century per square mile. Their energy greatly exceeds the energies reached by the largest terrestrial accelerators. These particles probably arise from giant explosive events associated with black holes. We will review the growth of cosmic ray physics from its inception to the latest multi-million dollar experiments, and discuss the recent intellectual and computational efforts which have greatly increased our understanding.

This talk is intended for a general audience including enthusiasts of all backgrounds and ages.