Under pressure, 'squishy' compound reacts in remarkable ways

Date:
July 27, 2021
Source:
University of Rochester
Summary:
When a compound of manganese and sulfide (MnS2) is compressed
in a diamond anvil, it transitions from an insulator into a
metallic state and back into an insulator. This is accompanied
by unprecedented decreases in resistance and volume across an
extremely narrow range of pressure changes at room temperatue,
say researchers.



FULL STORY ========================================================================== Remarkable things happen when a "squishy" compound of manganese and
sulfide (MnS2) is compressed in a diamond anvil, say researchers from the University of Rochester and the University of Nevada, Las Vegas (UNLV).


========================================================================== "This is a new type of charge transfer mechanism, and so from a science community point of view this is very, very exciting. We are showing
remarkable physical transformations over a very, very short range of parameters, in this case pressure," says Ashkan Salamat, associate
professor of physics at UNLV.

For example, as the pressure increases, MnS2, a soft insulator,
transitions into a metallic state and then into an insulator again,
the researchers describe in a paper flagged as an editor's choice in
Physical Review Letters.

"Metals usually remain metals; it is highly unlikely that they can then
be changed back to an insulator," says Ranga Dias, assistant professor of mechanical engineering and of physics and astronomy at Rochester. "The
fact that this material goes from an insulator to a metal and back to
an insulator is very rare." Moreover, the transitions are accompanied
by unprecedented decreases in resistance and volume across an extremely
narrow range of pressure change - - all occurring at about 80 degrees Fahrenheit. The relatively low temperature enhances the chances that the
metal transition process might eventually be harnessed for technology,
Salamat says.

In previous papers in Nature and Physical Review Letters, the
Dias and Salamat collaboration set new benchmarks toward achieving superconductivity at room temperatures. A common denominator of their
work is exploring the "remarkably bizarre" ways transition metals and
other materials behave when they are paired with sulfides, and then
compressed in a diamond cell anvil.



==========================================================================
"The new phenomena we are reporting is a fundamental example of responses
under high pressure -- and will find a place in physics textbooks,"
Salamat says.

"There's something very intriguing about how sulfur behaves when
it is attached to other elements. This has led to some remarkable breakthroughs." The breakthroughs achieved by the Dias and Salamat labs
have involved compressing mere picoliters of material -- about the size
of a single inkjet particle.

Spin and pressure underlie dramatic metal transition Underlying the
transitions described in this paper are the way the spin states (angular momentum) of individual electrons interact as pressure is applied,
Dias and Salamat explain.

When MnS2 is in its normal insulator state, electrons are primarily in unpaired, "high spin" orbitals, causing atoms to actively bounce back
and forth. This results in the material having higher resistance to
an electrical charge because there is less free space for individual
electrons trying to pass through the material.



==========================================================================
But as pressure is applied -- and the material is compressed toward
a metallic state -- the electron orbitals "start to see each other,
immediately come toward each other, and pairs of electrons start linking
up as one," Salamat says.

This opens up more space for individual electrons to move through the
material -- so much so that resistance drops dramatically by 8 orders
of magnitude, as pressure is increased from 3 gigapascals (435,000 psi)
to 10 gigapascals. This is a relative "nudge" compared to the 182 to
268 gigapascals required for superconducting materials.

"Given the small range of pressure involved, a drop in resistance of
this magnitude is really enormous," Dias says.

Low resistance is maintained even in the final phase -- when the MnS2
reverts to an insulator -- because the electrons remain in a "low spin"
state.

Basic materials science, future technological advances As often occurs
with new discoveries in basic science, the possible applications have
yet to be explored.

However, Salamat says, a transition metal which, with a relatively
small amount of strain, can jump from one state to another -- at room temperature, no less - - is likely to be useful.

"You could imagine having a logic switch or writing hard disk, where a
very, very small permutation in strain or voltage could make something
jump from one electronic state to another. New versions of flash memory,
or solid state memory, could permutate and take on a new approach using
these types of materials," Salamat says.

"You can do quite aggressive maneuvers to drive these materials at 300
kelvin, making them potentially useful for technology." Lead author
Dylan Durkee, a former undergraduate researcher in the Salamat lab,
is now working as a graduate student with Dias. Other coauthors include
Nathan Dasenbrock-Gammon and Elliot Snider at Rochester; Keith Lawler, Alexander Smith, and Christian Childs at UNLV; Dean Smith at Argonne
National Laboratory, and Simon A.J. Kinder at University of Bourgogne.

The National Science Foundation and the Department of Energy supported
the research with funding. The UNLV National Supercomputing Institute
provided computational resources, and portions of the work were performed
at Argonne National Laboratory and University of Bourgogne.

========================================================================== Story Source: Materials provided by University_of_Rochester. Original
written by Bob Marcotte. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Dylan Durkee, Nathan Dasenbrock-Gammon, G. Alexander Smith, Elliot
Snider, Dean Smith, Christian Childs, Simon A. J. Kimber,
Keith V.

Lawler, Ranga P. Dias, Ashkan Salamat. Colossal Density-Driven
Resistance Response in the Negative Charge Transfer
Insulator MnS2. Physical Review Letters, 2021; 127 (1) DOI:
10.1103/PhysRevLett.127.016401 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210727145247.htm

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