HiPSEC Students Seminar

The High Pressure Science and Engineering Center meets every Wednesday in the BPB conference room at 11:30 AM. We typically have a scientific talk, reports on students’ attendance of conferences and workshops and general discussions. We welcome anyone interested in learning about high pressure science and about HiPSEC to attend our meetings. Please e-mail lavina@physics.unlv.edu if you wish to receive the seminar alert.

2017 


Wednesday, Apr 26, 11:30 am, BPB 217

Calorimetry Study of the Phase Diagrams of EuNi2Ge2 and Eu2Ni3Ge5Under Pressure

Dr. S. Esakki Muthu

In this talk I will present the phase diagrams of EuNi2Ge2 and Eu2Ni3Ge5studied by ac calorimetry under pressure using a diamond anvil cell. In EuNi2Gethe antiferromagnetic transition exists up to 1.5 GPa. The sudden disappearance of magnetic order around 2 GPa is confirmed, consistent with the probable occurrence of a first order valence transition near that pressure. The ac calorimetry results on Eu2Ni3Geclearly show 2 antiferromagnetic transitions, and suggest that magnetic order persists up to higher pressure than previously expected. At high pressure, where heavy fermion behavior has previously been reported, the Néel temperature is decreasing, and magnetic order is expected to disappear at an extrapolated pressure of 12-14 GPa. A semi-quantitative analysis of the pressure dependence of the specific heat does not show any large changes, but is compatible with a moderate enhancement of g. The similarities with the phase diagrams of Yb and Ce heavy fermion systems will be discussed.


Wednesday, Apr 19, 11:30 am, BPB 217

Laser annealing after a kinetically hindered phase transition in the pyrochlore La2Sn2O7

Christian Childs

HiPSEC, UNLV

There is interest in identifying materials for use in radioactive waste applications and studying their structural properties. Studies has shown that pyrochlore (A2B2O7) compounds might be promising candidates for such purposes. In an attempt to obtain a broader understanding of the structural stability of these compounds, we have investigated the pyrochlore La2Sn2O7 under extreme conditions: high temperature isobaric and high-pressure isothermal study. Both quasi-hydrostatic and non-hydrostatic isothermal high pressure compression runs were undertaken to reveal alternative structural transformations and studied using synchrotron angle-dispersive X-ray diffraction and Raman scattering techniques, supported by ab-initio random structure searching (AIRSS). Compression using He as a quasi-hydrostatic pressure transmitting medium permitted the observation of a new high pressure phase via a kinetically hindered first-order phase transition that begins at 49 GPa and finally goes to completion at 61 GPa. Alternatively, the non-hydrostatic compression, using no pressure-transmitting medium, revealed the emergence of a phase transition at 46 GPa. However, it appears that this process occurs via a stress-induced pathway and in fact transforms into an amorphized phase by 67 GPa. Annealing the amorphized phase, through laser heating, at 70 GPa allows us to crystallize the high-pressure phase which is recoverable to ambient conditions. Our AIRSS calculations reveal three competing metastable structures, close in energy at 70 GPa.


Wednesday, Mar 22, 11:30 am, BPB 217

Unexpected Structural and Electronic Behavior in Sn3N4

John Kearney

HiPSEC, UNLV

Group 14 nitrides have been a topic of recent scientific focus due to their interesting  mechanical and electronic properties.  My talk will describe our research into the behavior of the nitrogen-rich nitride Sn3N4 under high pressure-temperature conditions.  Using in-house experimental optical techniques and in situ X-ray diffraction, coupled with laser heating in the diamond anvil cell we have observed exotic electronic behavior with pressure. The band gap in this optoelectronic material reveals an opening from 0.5 to 3 eV with pressure between 0 and 130 GPa, after which the band gap slowly closes. A component of this talk will be focused on the possible mechanisms for this phenomenon.  The structure of Sn3N4 at these extreme pressures (measured up to 230 GPa) has not yet been solved, and I will discuss the methods and difficulties faced in identifying the new phases predicted by ab initio random structure searching (AIRSS).


Wednesday, Mar 15, 11:30 am, BPB 217

Quantum Critical Heavy Fermions

Brian Light

HiPSEC, UNLV

High pressure techniques can be used to tune heavy fermion materials to a quantum critical point.   Presented here is the basic theory behind this.   The Kondo effect, the Anderson model, the Kondo lattice and the RKKY interaction are discussed qualitatively.  Selected publications from experimental work performed at HiPSEC that have demonstrated this as well as potential future work are also discussed.


Wednesday, Mar 1, 11:30 am, BPB 217

Shock Recovery of Bismuth

Zachary Fussell

HiPSEC, UNLV

Between 0 and 10 GPa there are five different bismuth phases. High-pressure bismuth (Bi) phases have been examined in static compression experiments; however, none could be recovered to ambient conditions. Here we report Bi-III recovery (stable above 3 GPa) to ambient conditions from a shock compression experiment to 5.7 GPa. Bi-III was identified by synchrotron micro-diffraction and backscatter electron imaging. Our work shows shock-compression provides a tool for recovering high-pressure phases that otherwise elude decompression. DOE/NV/25946–3140


Wednesday, Feb 8, 11:30 am, BPB 217

Recreating planetary cores in the laboratory

Daniel Sneed

HiPSEC, UNLV

Of the different planets in our solar system, our giant icy neighbors Neptune and Uranus are extremely fascinating, Uranus being the more peculiar of the two. Though Uranus is almost 2 billion kilometers closer to the sun than Neptune, its average surface temperature can dip as low as 50 K, actually making it colder than Neptune. The primary belief for this is that the core of Uranus, which is believed to be made of diamond, silicates, iron, and nickel, has shed most of its energy; cooling down to a point where it no longer radiates much heat. Despite having a cold core, Uranus has a very active and unique magnetic field. Unlike Earth, whose magnetic field is driven by a very hot active core, it is believed that Uranus’s unique magnetic field is actually driven by mixtures of super ionic and metallic molecular compounds. It has been measured that the primary components of its mantle are water, ammonia, and methane, which have all been predicted to show superionic properties at the conditions present within the mantle. There have recently been efforts in attempting to recreate the conditions necessary to verify these predicted phases, primarily in the area of shock compression. This is difficult as the region of P-T space that these superionic phases exist cannot be reached by traditional shock compression. In a traditional shock experiment the path the hugoniot takes overshoots the targeted region due to high amounts of entropy generated, so very high temperatures are reached along with very high pressures. One way around this is by compressing the sample to a specific density prior to shocking in order to take off hugoniot shock paths, reaching the pressures necessary at much lower temperatures. I will discuss some details of how these pre-compressed experiments are performed as well as how Velocity Interferometry for Any Reflector (VISAR) is used to interpret the results.


Wednesday, Jan 18, 11:30 am, BPB 217

Adventures with 5d orbitals at high pressure

Daniel Haskel

Advanced Photon Source, Argonne National Laboratory

While first-row (3d) transition metal (TM) oxides continue to provide a rich playground for studies of electron correlations, recent focus has shifted to third-row (5d) TM oxides in the search for novel quantum states. The sizable spin-orbit interaction in heavy 5d ions, coupled with reduced on-site Coulomb interactions as a result of the large spatial extent of 5d orbitals, create unique experimental and theoretical opportunities for discovery of new electronic phases of matter.

We have studied some of the consequences of enhanced S-O coupling and spatial extent of 5d orbitals on the electronic structure and magnetic (exchange) interactions in a novel “iridate” magnetic insulator, Sr2IrO4. The high-brilliance, penetration power, and polarization/energy tunability of synchrotron radiation enable the use of x-ray absorption spectroscopy (including circular dichroism) and resonant magnetic scattering techniques in the diamond anvil cell providing exquisite sensitivity to the evolution of electronic correlations at high pressure. Among other findings, we discovered that pressure leads to frustration of exchange interactions in the square lattice of entangled spin-orbital Iridium moments and emergence of quantum paramagnetism, possibly a quantum spin liquid phase.

Higher brilliance, 4th generation synchrotron light sources now in development around the globe will bring unprecedented opportunities for studies of electronic/magnetic order at the limit of static high-pressure generation technologies.

Work at Argonne is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC-02-06CH11357.

 

Attendance sheet