15 research outputs found
Symmetry-mode analysis for local structure investigations using pair distribution function data
Symmetry-adapted distortion modes provide a natural way to describe distorted
structures derived from higher-symmetry parent phases. Structural refinements
using symmetry-mode amplitudes as fit variables have been used for at least 10
years in Rietveld refinements of the average crystal structure from diffraction
data; more recently, this approach has also been used for investigations of the
local structure using real-space pair distribution function (PDF) data. Here,
we further demonstrate the value of performing symmetry-mode fits to PDF data
through the successful application of this method to two topical materials:
TiSe, where we detect the subtle but long-range structural distortion
driven by the formation of a charge density wave, and MnTe, where we
characterize a large but highly localized structural distortion in terms of
symmetry-lowering displacements of the Te atoms. The analysis is performed
using fully open-source code within the DiffPy framework using two packages we
developed for this work: isopydistort, which provides a scriptable interface to
the ISODISTORT web application for group theoretical calculations, and
isopytools, which converts the ISODISTORT output into a DiffPy-compatible
format for subsequent fitting and analysis. These developments expand the
potential impact of symmetry-adapted PDF analysis by enabling high throughput
analysis and removing the need for any commercial software
Discovery of Superconductivity and Electron-Phonon Drag in the Non-Centrosymmetric Semimetal LaRhGe
We present a comprehensive study of the non-centrosymmetric semimetal
LaRhGe. Our transport measurements reveal evidence for electron-hole
compensation at low temperatures, resulting in a large magnetoresistance of
3000% at 1.8 K and 14 T. The carrier concentration is on the order of
, higher than typical semimetals. We predict theoretically
the existence of Weyl nodal lines that are protected
by the tetragonal space group. We discover superconductivity for the first time
in this compound with a of 0.39(1) K and of
2.1(1) mT, with evidence from specific heat and transverse-field muon spin
relaxation (). LaRhGe is a weakly-coupled type-I
superconductor, and we find no evidence for time-reversal symmetry breaking in
our zero-field . We study the electrical transport in the normal
state and find an unusual dependence at low temperature while
Seebeck coefficient and thermal conductivity measurements reveal a peak in the
same temperature range. We conclude that the transport properties of LaRhGe
in its normal state are strongly influenced by electron-phonon interactions.
Furthermore, we examine the temperature dependent Raman spectra of LaRhGe
and find that the lifetime of the lowest energy phonon is dominated by
phonon-electron scattering instead of anharmonic decay
Disentangling superconducting and magnetic orders in NaFe_1-xNi_xAs using muon spin rotation
Muon spin rotation and relaxation studies have been performed on a "111"
family of iron-based superconductors NaFe_1-xNi_xAs. Static magnetic order was
characterized by obtaining the temperature and doping dependences of the local
ordered magnetic moment size and the volume fraction of the magnetically
ordered regions. For x = 0 and 0.4 %, a transition to a nearly-homogeneous long
range magnetically ordered state is observed, while for higher x than 0.4 %
magnetic order becomes more disordered and is completely suppressed for x = 1.5
%. The magnetic volume fraction continuously decreases with increasing x. The
combination of magnetic and superconducting volumes implies that a
spatially-overlapping coexistence of magnetism and superconductivity spans a
large region of the T-x phase diagram for NaFe_1-xNi_xAs . A strong reduction
of both the ordered moment size and the volume fraction is observed below the
superconducting T_C for x = 0.6, 1.0, and 1.3 %, in contrast to other iron
pnictides in which one of these two parameters exhibits a reduction below TC,
but not both. The suppression of magnetic order is further enhanced with
increased Ni doping, leading to a reentrant non-magnetic state below T_C for x
= 1.3 %. The reentrant behavior indicates an interplay between
antiferromagnetism and superconductivity involving competition for the same
electrons. These observations are consistent with the sign-changing s-wave
superconducting state, which is expected to appear on the verge of microscopic
coexistence and phase separation with magnetism. We also present a universal
linear relationship between the local ordered moment size and the
antiferromagnetic ordering temperature TN across a variety of iron-based
superconductors. We argue that this linear relationship is consistent with an
itinerant-electron approach, in which Fermi surface nesting drives
antiferromagnetic ordering.Comment: 20 pages, 14 figures, Correspondence should be addressed to Prof.
Yasutomo Uemura: [email protected]
The Future of the Correlated Electron Problem
The understanding of material systems with strong electron-electron
interactions is the central problem in modern condensed matter physics. Despite
this, the essential physics of many of these materials is still not understood
and we have no overall perspective on their properties. Moreover, we have very
little ability to make predictions in this class of systems. In this manuscript
we share our personal views of what the major open problems are in correlated
electron systems and we discuss some possible routes to make progress in this
rich and fascinating field. This manuscript is the result of the vigorous
discussions and deliberations that took place at Johns Hopkins University
during a three-day workshop January 27, 28, and 29, 2020 that brought together
six senior scientists and 46 more junior scientists. Our hope, is that the
topics we have presented will provide inspiration for others working in this
field and motivation for the idea that significant progress can be made on very
hard problems if we focus our collective energies.Comment: 55 pages, 19 figure
Understanding the role of entropy in high entropy oxides
The field of high entropy oxides (HEOs) flips traditional materials science
paradigms on their head by seeking to understand what properties arise in the
presence of profound configurational disorder. This disorder, which originates
from multiple elements sharing a single lattice site, can take on a
kaleidoscopic character due to the vast numbers of possible elemental
combinations. High configurational disorder appears to imbue some HEOs with
functional properties that far surpass their non-disordered analogs. While
experimental discoveries abound, efforts to characterize the true magnitude of
the configurational entropy and understand its role in stabilizing new phases
and generating superior functional properties have lagged behind. Understanding
the role of configurational disorder in existing HEOs is the crucial link to
unlocking the rational design of new HEOs with targeted properties. In this
Perspective, we attempt to establish a framework for articulating and beginning
to address these questions in pursuit of a deeper understanding of the true
role of entropy in HEOs.Comment: 18 pages, 7 figure
Effect of interlayer coupling on the coexistence of antiferromagnetism and superconductivity in Fe pnictide superconductors: A study of Ca
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Nonsymmorphic symmetry-protected band crossings in a square-net metal PtPb4
Topological semimetals with symmetry-protected band crossings have emerged as a rich landscape to explore intriguing electronic phenomena. Nonsymmorphic symmetries in particular have been shown to play an important role in protecting the crossings along a line (rather than a point) in momentum space. Here we report experimental and theoretical evidence for Dirac nodal line crossings along the Brillouin zone boundaries in PtPb4, arising from the nonsymmorphic symmetry of its crystal structure. Interestingly, while the nodal lines would remain gapless in the absence of spin–orbit coupling (SOC), the SOC, in this case, plays a detrimental role to topology by lifting the band degeneracy everywhere except at a set of isolated points. Nevertheless, the nodal line is observed to have a bandwidth much smaller than that found in density functional theory (DFT). Our findings reveal PtPb4 to be a material system with narrow crossings approximately protected by nonsymmorphic crystalline symmetries