Physicist joins international effort to unveil the behavior of 'strange metals'

Date:
October 19, 2020
Source:
The University of Hong Kong
Summary:
Physicists have solved the puzzle of the NFL behavior in interacting
electrons systems, and provided a protocol for the establishment
of new paradigms in quantum metals, through quantum many-body
computation and analytical calculations.



FULL STORY ==========================================================================
The Landau's theory of Fermi liquid (FL) (Note 1), established in the
first half of the 20th century, is the foundation of the scientific
and industrial usage of the metallic materials in our society. It is
also the basis of our current understanding of metals. However, in the
second half of the 20th century, more and more metallic materials were discovered which behave very differently. The non-Fermi liquid (NFL)
behaviour of these "strange metals" remains a puzzle to physicists,
and there is no established theory to explain them.


========================================================================== Recently, a joint research team comprising members including Dr Zi Yang
MENG, Associate Professor of Department of Physics at the University
of Hong Kong (HKU), Dr Avraham KLEIN and Professor Andrey CHUBUKOV
from the University of Minnesota, Dr Kai SUN, Associate Professor from
the University of Michigan, and Dr Xiao Yan XU from the University of California at San Diego, has solved the puzzle of the NFL behaviour
in interacting electrons systems, and provided a protocol for the
establishment of new paradigms in quantum metals, through quantum
many-body computation and analytical calculations. The findings have
recently been published in Npj Quantum Materials. The work was supported
by the Research Grants Council of HKSAR, and the Ministry of Science
and Technology of China.

Breaking discoveries of mysterious NFL behaviour The Landau's theory of
Fermi liquid (FL) successfully explained many features of simple metals
like Copper, Silver, Gold and Iron, such as when temperature changes,
their resistivity, heat capacity and other properties follow simple
function form with respect to temperature T (for example, resistivity
follows r~T2 and heat capacity follows C~T, independent of material
details). The success of the Fermi liquid theory lies in the central
assumption that the electrons, the droplets in the Fermi liquid are not interacting with each other, but behave identically in the material.

However, many metallic materials which were discovered after FL was established, do NOT behave as FL. For example, in the so-called high- temperature superconductor compounds -- copper oxides and iron pnictides
- - their resistivities are linear in temperature r~T before the system
becomes superconducting (resistivity is then zero), and such systems are
in general dubbed Non-Fermi-Liquids (NFL). Different from the simple FL,
the electrons of NFL, the droplets, strongly interact with each other.

NFLs have potential application in solving the energy crisis The
physicists still do not have much clue about NFL, which makes it very
difficult to make concrete predictions. Still, these systems are essential
for the continued prosperity of human society, as NFLs hold the key in
making use of high-temperature superconducting material that will solve
the energy crisis.

Currently, the so-called high-temperature superconducting materials
still only work at temperature scale of-100 Celsius -- they are called high-temperature in comparison with the FL superconductors, which work
at temperature scale of -200 Celsius -- so it is still hard to put high-temperature superconductors into daily usage at room temperature,
but only then can we enjoy the nice properties of such material that
the electronic power will not be loss in heat due to resistivity. Only
when we understand how the NFL in high-temperature superconductor works
at -100 Celsius, can we then design the ultimate material to work at
room temperature. Therefore, the complete understanding of NFL is of
vital importance.



========================================================================== Physicists from analytical background have been trying to understand
NFL for about half a century. The problem of analytical calculation is
that, due to the quantum many-body nature of the NFL, the convergence
and accuracy of many theoretical predictions cannot be controlled or guaranteed; one would need unbiased quantum computation to verify these prepositions.

Key revelation to the puzzle is computation At the numerical front,
there have been many previous attempts, but the problem is that the
results obtained are always different from the analytical prediction. For example, the most important quantity of the NFL, the self- energy S ,
which describes the level of the electron interactions in the material,
is expected to have a power-law frequency dependence such as S~o2/3.

However, the computed self-energy doesn't follow such as power-law,
it shows a slow diverging behaviour, that is the self-energy computed
doesn't go to zero as frequency is reduced, but instead gets larger and
large. Such difference makes the situation even more perplexing.

After a very inspirational discussion between Dr Meng, Professor Chubukov,
and Dr Klein, they realized that the setting of the numerical simulation
is actually different from that of the analytical calculation. Such
subtlety comes from the fact that the model simulations are performed
on the finite system at finite temperature, that is T!=0, whereas the analytical expectations are strictly at the zero temperature T=0. In other words, the numerical data actually contain both the zero temperature
NFL contribution and the contribution from the fluctuations at finite temperature. To be able to reveal the NFL behaviour from the lattice
model simulation such as the setting, one would need to deduce the finite temperature contribution.

This turns out to be the key revelation to the puzzle of NFL. Dr Klein,
Dr Sun and Prof Chubukov derived the analytical form of the finite
temperature contribution (with the input from the lattice model designed
by Dr Meng and Dr Xu) for Dr Meng and Dr Xu to employ and deduce from
the numerical data, the results are shown as the black dashed line
and the data round it. To everyone's surprise and ecstasy, the results
after the deduction perfectly exhibit the expected NFL behaviour, from
finite temperature all the way to zero temperature, the power-law is
revealed. It is the first time that such clear NFL behaviour has been
obtained from unbiased numerical simulation.

Bring a better future to the society Dr Meng said it is expected that this
work will inspire many follow-up theoretical and experimental researches,
and in fact, promising results for further identification of NFL behaviour
in another model system have been obtained by the further investigations,
he said: "This research work reveals the puzzle of Non-Fermi-liquid for
several decades and paves the avenue for the establishment of new paradigm
of quantum metals beyond those more than half-a- century ago. Eventually,
we will be able to understand the NFL materials such as high-temperature superconductors as we understand simple metals such as Cooper and Sliver
now, and such new understanding will solve the energy crisis and bring
better industrial and personal applications to the society."

========================================================================== Story Source: Materials provided by The_University_of_Hong_Kong. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Xiao Yan Xu, Avraham Klein, Kai Sun, Andrey V. Chubukov, Zi
Yang Meng.

Identification of non-Fermi liquid fermionic self-energy from
quantum Monte Carlo data. npj Quantum Materials, 2020; 5 (1) DOI:
10.1038/s41535- 020-00266-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201019103445.htm

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